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Papers by carlos bolaños

Psychopharmacology, 1995

The ability of kappa opioid agonists to modulate dopamine-mediated behavior and Fos immunore-acti... more The ability of kappa opioid agonists to modulate dopamine-mediated behavior and Fos immunore-activity was assessed in adult rats. It was predicted that kappa agonist treatment would block the unconditioned and conditioned behaviors produced by cocaine (an indirect dopamine agonist). In the initial experiments, cocaine-induced locomotor activity was assessed after either acute or chronic injections of the kappa receptor agonist U-50,488 (5 mg/kg, SC). As expected, U-50,488 decreased cocaine-induced activity, while leaving baseline activity levels unaffected. Interestingly, chronic treatment with U-50,488 did not induce behavioral tolerance. The conditioned effects of cocaine (20 mg/kg, IP) were assessed using the conditioned place preference (CPP) paradigm. As expected, rats showed a preference for the cocaine-paired compartment, an effect blocked by U-50,488 (5 mg/kg, SC). One hour after CPP testing, rats were killed and Fos immunoreactivity was assessed. Rats conditioned with cocaine, but not U-50,488, showed increased Fos activity in the anterior cingulate cortex, piriform cortex, lateral septal area, and olfactory tubercles. When considered together, these results suggest that U-50,488 was effective at blocking the unconditioned and conditioned effects of cocaine, as well as cocaine-induced neuronal activity (as measured by Fos induction).

Behavioral Neuroscience, 1997

The kappa-opioid agonist U-50,488 increases the locomotor activity of preweanling rats. The autho... more The kappa-opioid agonist U-50,488 increases the locomotor activity of preweanling rats. The authors attempted to determine whether this effect was modulated by dopamine (DA) system functioning. Surprisingly, U-50,488's locomotor activating effects were attenuated by both the DA receptor antagonist flupenthixol and the DA receptor agonist R(-)-propylnorapomorphine (NPA). In order to determine those brain areas stimulated by U-50,488, Fos immunoreactivity was assessed in 17- and 80-day-old rats. U-50,488 not only enhanced the locomotor activity of the younger rats, but it also enhanced Fos expression in various brain areas, including the nucleus accumbens and medial striatum. NPA blocked U-50,488-induced Fos expression in the latter region. When considered together, these results indicate that U-50,488 does not increase locomotion by stimulating a DA mechanism. Instead, either agonizing or antagonizing DA receptors is sufficient to disrupt U-50,488's locomotor activating effects in the preweanling rat.

European Journal of Pharmacology, 1996

methanesulfonate (U-50,488) to modulate morphine-induced reward was assessed in preweanling (10-a... more methanesulfonate (U-50,488) to modulate morphine-induced reward was assessed in preweanling (10-and 17-day-old) and periadolescent (35-day-old) rats using the conditioned place preference paradigm. Conditioning and testing were conducted in a three compartment chamber, with each end compartment having its own distinct tactile and odor cues (almond or lemon). An abbreviated conditioned place preference procedure was used in which rats received two saline-odor pairings on the first conditioning day, and two saline-or morphine-odor pairings on the second day. In some experiments, rats were given U-50,488 (2-10 mg/kg, s.c.) 30 min prior to being conditioned with morphine (0.1-8 mg/kg, i.p.). On the third day, rats were allowed free access to the entire chamber for 900 s and compartment preferences were determined. Similar to adult rats, morphine (0.5 mg/kg) was consistently able to induce conditioned place preferences in the two preweanling age groups. This effect was attenuated by K-opioid receptor agonist pretreatment, as U-50,488 not only enhanced the locomotor activity of 10-and 17-day-old rats, but it blocked the morphine-induced place preference conditioning of these younger animals. In contrast, periadolescent (35-day-old) rats did not exhibit morphine-induced place preferences, nor did they show enhanced locomotor activity after U-50,488 treatment; however, using the same procedure, a different group of similarly aged rats showed conditioned preference produced by 20 mg/kg cocaine (i.p.). Therefore, these results suggest that reward processes are functionally mature in the preweanling rat (at least by 10 days of age), but that periadolescent rats are generally unresponsive to /x-and •-opioid drugs.

European Journal of Pharmacology, 1995

The effects of dopamine D 1 and D 2 receptor antagonists on the reward processes of 10-and 17-day... more The effects of dopamine D 1 and D 2 receptor antagonists on the reward processes of 10-and 17-day-old rats were assessed using the conditioned place preference paradigm. Conditioning and testing were conducted in a three-compartment chamber, with each end compartment having its own distinct tactile and odor cues (almond and lemon). During six experiments, 10-and 17-day-old rats (age at initial conditioning) were injected intraperitoneally with either saline, the dopamine D 1 receptor antagonist R(+)-SCH 23390 hydrochloride (0.01-1.0 mg/kg), or the dopamine D e receptor antagonists (+)-sulpiride (1-100 mg/kg) or S(-)-eticlopride hydrochloride (0.1-0.5 mg/kg) 30 min prior to being injected with cocaine hydrochloride (20 mg/kg) or saline. After the latter injections, rats were immediately confined in the lemon-scented (nonpreferred) compartment for 30 min. On the alternate conditioning day, rats were injected with saline and confined in the almond-scented compartment. On the third day (i.e., the test day), rats were given saline and allowed free access to the entire chamber for 15 min. The results showed that the dopamine D 1 receptor antagonist SCH 23390 blocked the cocaine-induced place preference conditioning of both 10-and 17-day-old rats. Surprisingly, the dopamine D 2 receptor antagonists sulpiride and eticlopride blocked the place preference conditioning of 10-day-old rats, while leaving the 17-day-old rats unaffected. These results indicate that dopamine D~ receptors are critically involved in the reward processes of preweanling rats, but that the importance of dopamine D 2 receptors changes across ontogeny.

Psychopharmacology, 1994

In the present study, the abilities of NPA (a direct DA receptor agonist) and amphetamine (an ind... more In the present study, the abilities of NPA (a direct DA receptor agonist) and amphetamine (an indirect DA receptor agonist) to induce short- and long-term behavioral sensitization were assessed in 11- and 17-day-old rats (age at initial injection). Rats were injected on 4 consecutive days with amphetamine (1.0, 2.5, or 5.0 mg/kg), NPA (1.0 mg/kg), or saline. A final test day occurred either 2 days (experiment 1) or 8 days (experiment 2) later. On the test day, rats given successive agonist injections received a single injection of the same agonist again; whereas rats given successive saline injections received either amphetamine or NPA for the first time. Five minutes after injection, locomotor activity (line-crosses), stereotyped sniffing, and vertical activity were measured during a 30-min testing session. The results showed that 11- and 17-day-old rats exhibited behavioral sensitization when tested with NPA or amphetamine after a 2-day interval. In contrast, neither NPA nor amphetamine was able to sensitize the behaviors of preweanling rats when measured 8 days after initial drug treatments. Therefore, these results show that both direct and indirect DA agonists are able to induce short-term behavioral sensitization in preweanling rats, but that the mechanisms responsible for mediating long-term behavioral sensitization have not yet matured.

Proceedings of The National Academy of Sciences, 2005

Sexual deficits and other behavioral disturbances such as anxietylike behaviors can be observed i... more Sexual deficits and other behavioral disturbances such as anxietylike behaviors can be observed in animals that have undergone social isolation, especially in species having important social interactions. Using a model of protracted social isolation in adult rats, we observed increased anxiety-like behavior and deficits in both the latency to initiate sexual behavior and the latency to ejaculate. We show, using transgenic cAMP response element (CRE)-LacZ reporter mice, that protracted social isolation also reduces CREdependent transcription within the nucleus accumbens. This decrease in CRE-dependent transcription can be mimicked in nonisolated animals by local viral gene transfer of a dominant negative mutant of CRE-binding protein (CREB). We previously showed that this manipulation increases anxiety-like behavior. We show here that it also impairs initiation of sexual behavior in nonisolated animals, a deficit that can be corrected by anxiolytic drug treatment. This local reduction in CREB activity, however, has no influence on ejaculation parameters. Reciprocally, we used the viral transgenic approach to overexpress CREB in the nucleus accumbens of isolated animals. We show that this local increase in CREB activity completely rescued the anxiety phenotype of the isolated animals, as well as their deficit in initiating sexual behavior, but failed to rescue the deficit in ejaculation. Our data suggest a role for the nucleus accumbens in anxiety responses and in specific aspects of sexual behavior. The results also provide insight into the molecular mechanisms by which social interactions affect brain plasticity and behavior.

Neurotrophic factor signaling pathways modulate cellular and behavioral responses to drugs of abu... more Neurotrophic factor signaling pathways modulate cellular and behavioral responses to drugs of abuse. In addition, chronic exposure to morphine increases expression of phospholipase C␥1 (PLC␥1) (a protein involved in neurotrophic signaling) in the ventral tegmental area (VTA), a neural substrate for many drugs of abuse. Using viral-mediated gene transfer to locally alter the activity of PLC␥1, we show that overexpression of PLC␥1 in rostral portions of the VTA (R-VTA) results in increased morphine place preference, whereas PLC␥1 overexpression in the caudal VTA (C-VTA) results in avoidance of morphine-paired compartments. In addition, overexpression of PLC␥1 in R-VTA causes increased preference for sucrose and increased anxiety-like behavior but does not affect responses to stress or nociceptive stimuli. In contrast, overexpression of PLC␥1 in C-VTA decreases preference for sucrose and increases sensitivity to stress and nociceptive stimuli, although there was a tendency for increased anxiety-like behavior as seen for the R-VTA. These results show that levels of PLC␥1 in the VTA regulate responsiveness to drugs of abuse, natural rewards, and aversive stimuli and point to the possibility that distinct topographical regions within the VTA mediate generally positive versus negative responses to emotional stimuli. Moreover, these data also support a role for drug-induced elevations in PLC␥1 expression in the VTA in mediating long-term adaptations to drugs of abuse and aversive stimuli.

European Journal of Neuroscience, 2002

Adenylyl cyclase (AC) type VIII has been implicated in several forms of neural plasticity, includ... more Adenylyl cyclase (AC) type VIII has been implicated in several forms of neural plasticity, including drug addiction and learning and memory. In the present study, we directly examined the role for the transcription factor CREB (cAMP response element binding protein) in regulating ACVIII expression by cloning a 5.2 kilobase region upstream of the translation start site of the mouse ACVIII gene. Analysis of this fragment revealed consensus elements for several transcription factors, including a canonical cAMP response element (CRE) in close proximity to the transcription initiation region. Next, ACVIII promoter activity was studied in two neural-derived cell lines and in primary cultures of rat striatal neurons. Activation of the cAMP pathway by forskolin treatment increased promoter activity, and a series of deletion and point mutants demonstrated that this activation is mediated speci®cally via the canonical CRE site. Gel shift assays con®rmed that this site can bind CREB and several CREB family proteins. Further, activation of the ACVIII promoter by forskolin was potentiated by expression of a constitutively active form of CREB, CREB-VP16, whereas it was inhibited by expression of a dominant-negative form of CREB, A-CREB. Finally, over-expression of CREB in vivo, by viral-mediated gene transfer, induced ACVIII promoter activity in the brains of ACVIII-LacZ transgenic mice. These results suggest that the ACVIII gene is regulated by CREB in vitro and in vivo and that this regulation may contribute to CREB-dependent neural plasticity.

European Journal of Neuroscience, 2005

The transcription factor ΔFosB is induced in the nucleus accumbens and dorsal striatum by chronic... more The transcription factor ΔFosB is induced in the nucleus accumbens and dorsal striatum by chronic exposure to several drugs of abuse, and increasing evidence supports the possibility that this induction is involved in the addiction process. However, to date there has been no report of ΔFosB induction by drugs of abuse in the ventral tegmental area (VTA), which is also a critical brain reward region. In the present study, we used immunohistochemistry to demonstrate that chronic forced administration of cocaine induces ΔFosB in the rat VTA. This induction occurs selectively in a γ-aminobutyric acid (GABA) cell population within the posterior tail of the VTA. A similar effect is seen after chronic cocaine self-administration. Induction of ΔFosB in the VTA occurs after psychostimulant treatment only: it is seen with both chronic cocaine and amphetamine, but not with chronic opiates or stress. The expression of ΔFosB appears to be mediated by dopamine systems, as repeated administration of a dopamine uptake inhibitor induced ΔFosB in the VTA, while administration of serotonin or norepinephrine uptake inhibitors failed to produce this effect. Time course analysis showed that, following 14 days of cocaine administration, ΔFosB persists in the VTA for almost 2 weeks after cocaine withdrawal. This accumulation and persistence may account for some of the long-lasting changes in the brain associated with chronic drug use. These results provide the first evidence of ΔFosB induction in a discrete population of GABA cells in the VTA, which may regulate the functioning of the brain's reward mechanisms.

Science, 2006

Mice experiencing repeated aggression develop a long-lasting aversion to social contact, which ca... more Mice experiencing repeated aggression develop a long-lasting aversion to social contact, which can be normalized by chronic, but not acute, administration of antidepressant. Using viralmediated, mesolimbic dopamine pathway-specific knockdown of brain-derived neurotrophic factor (BDNF), we showed that BDNF is required for the development of this experience-dependent social aversion. Gene profiling in the nucleus accumbens indicates that local knockdown of BDNF obliterates most of the effects of repeated aggression on gene expression within this circuit, with similar effects being produced by chronic treatment with antidepressant. These results establish an essential role for BDNF in mediating long-term neural and behavioral plasticity in response to aversive social experiences.

Biological Psychiatry, 2008

Background: Methylphenidate (MPH) is prescribed for the treatment of attention-deficit/hyperactiv... more Background: Methylphenidate (MPH) is prescribed for the treatment of attention-deficit/hyperactivity disorder. Exposure to MPH before adulthood causes behavioral deficits later in life, including anxiety-and depression-like behaviors and decreased responding to natural and drug rewards. We examined the ability of fluoxetine (FLX), a selective serotonin reuptake blocker, to normalize these MPH-induced behavioral deficits.

Biological Psychiatry, 2008

Background-The neurobiological mechanisms by which only a minority of stress-exposed individuals ... more Background-The neurobiological mechanisms by which only a minority of stress-exposed individuals develop psychiatric diseases remain largely unknown. Recent evidence suggests that dopaminergic neurons of the ventral tegmental area (VTA) play a key role in the manifestation of stress vulnerability.

Biological Psychiatry, 2003

Background: Methylphenidate (MPH) is a psychomotor stimulant medication widely used for the treat... more Background: Methylphenidate (MPH) is a psychomotor stimulant medication widely used for the treatment of attention-deficit/hyperactivity disorder (ADHD). Given the extent of prescribed use of MPH, and because MPH interacts with the same brain pathways activated by drugs of abuse, most research has focused on assessing MPH's potential to alter an individual's risk for adult drug addiction. Data examining other potential long-term behavioral consequences of early MPH administration are lacking, however. Methods: We investigated the long-term behavioral consequences of chronic administration of MPH (2.0 mg/kg) during pre-and periadolescent development in adult rats by assessing their behavioral reactivity to a variety of emotional stimuli. Results: The MPH-treated animals were significantly less responsive to natural rewards such as sucrose, noveltyinduced activity, and sex compared with vehicle-treated control animals. In contrast, MPH-treated animals were significantly more sensitive to stressful situations, showed increased anxiety-like behaviors, and had enhanced plasma levels of corticosterone. Conclusions: Chronic exposure to MPH during development leads to decreased sensitivity to rewarding stimuli and results in enhanced responsivity to aversive situations. These results highlight the need for further research to improve understanding of the effects of stimulants on the developing nervous system and the potential enduring effects resulting from early-life drug exposure. Biol Psychiatry 2003;54:1317-1329

Biological Psychiatry, 2006

The neural consequences of early-life exposure to methylphenidate (MPH; Ritalin) are of great int... more The neural consequences of early-life exposure to methylphenidate (MPH; Ritalin) are of great interest given the widespread, and sometimes inappropriate, use in children. Here we examine the impact of juvenile MPH exposure on adult hippocampal neurogenesis. Methods: Rats received MPH (2.0 mg/kg, intraperitoneal, twice daily) or saline (SAL) during preadolescence (postnatal days 20 -35). Hippocampal cell proliferation (Experiment 1), neurogenesis (Experiment 2), and stress-induced changes in cell proliferation (Experiment 3) were assessed at several developmental stages including adulthood. Results: Juvenile exposure to MPH did not alter proliferation at any developmental time point relative to control rats; however, exposure to MPH significantly decreased the long-term survival of newborn cells in adult rats, particularly in the temporal hippocampus. Although MPH-treated rats had higher levels of corticosterone after restraint stress, they did not show the expected greater decrease in hippocampal cell proliferation relative to control animals. Conclusions: Early-life exposure to MPH inhibits the survival of adult-generated neurons in the temporal hippocampus and may reduce progenitor sensitivity to corticosterone-induced decreases in proliferation. These findings suggest that decreased adult neurogenesis is an enduring consequence of early-life exposure to MPH and are discussed for their relevance to humans.

Nature Neuroscience, 2009

Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult ro... more Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult rodents. Social isolation induced both anxiety- and anhedonia-like symptoms and decreased cAMP response element-binding protein (CREB) activity in the nucleus accumbens shell (NAcSh). All of these abnormalities were reversed by chronic, but not acute, antidepressant treatment. However, although the anxiety phenotype and its reversal by antidepressant treatment were CREB-dependent, the anhedonia-like symptoms were not mediated by CREB in NAcSh. We found that decreased CREB activity in NAcSh correlated with increased expression of certain K(+) channels and reduced electrical excitability of NAcSh neurons, which was sufficient to induce anxiety-like behaviors and was reversed by chronic antidepressant treatment. Together, our results describe a model that distinguishes anxiety- and depression-like behavioral phenotypes, establish a selective role of decreased CREB activity in NAcSh in anxiety-like behavior, and provide a mechanism by which antidepressant treatment alleviates anxiety symptoms after social isolation.

Nature Neuroscience, 2006

Chronic morphine administration (via subcutaneous pellet) decreases the size of dopamine neurons ... more Chronic morphine administration (via subcutaneous pellet) decreases the size of dopamine neurons in the ventral tegmental area (VTA), a key reward region in the brain, yet the molecular basis and functional consequences of this effect are unknown. In this study, we used viral-mediated gene transfer in rat to show that chronic morphine-induced downregulation of the insulin receptor substrate 2 (IRS2)-thymoma viral proto-oncogene (Akt) signaling pathway in the VTA mediates the decrease in dopamine cell size seen after morphine exposure and that this downregulation diminishes morphine reward, as measured by conditioned place preference. We further show that the reduction in size of VTA dopamine neurons persists up to 2 weeks after morphine withdrawal, which parallels the tolerance to morphine's rewarding effects caused by previous chronic morphine exposure. These findings directly implicate the IRS2-Akt signaling pathway as a critical regulator of dopamine cell morphology and opiate reward.

Biological Psychiatry, 2003

Previous work has shown that brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine ... more Previous work has shown that brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), are involved in appetitive behavior. Here we show that BDNF in the ventral tegmental area-nucleus accumbens (VTA-NAc) pathway is also involved in the development of a depression-like phenotype.

Neuromolecular Medicine, 2004

The involvement of neurotrophic factors in neuronal survival and differentiation is well establis... more The involvement of neurotrophic factors in neuronal survival and differentiation is well established. The more recent realization that these factors also play pivotal roles in the maintenance and activity-dependent remodeling of neuronal functioning in the adult brain has generated excitement in the neurosciences. Neurotrophic factors have been implicated in the modulation of synaptic transmission and in the mechanisms underlying learning and memory, mood disorders, and drug addiction. Here the evidence for the role of neurotrophins and other neurotrophic factors—and the signaling pathways they activate—in mediating long-term molecular, cellular, and behavioral adaptations associated with drug addiction is reviewed.

Nature Neuroscience, 2006

The transcription factor DFosB is induced in the nucleus accumbens (NAc) and dorsal striatum by t... more The transcription factor DFosB is induced in the nucleus accumbens (NAc) and dorsal striatum by the repeated administration of drugs of abuse. Here, we investigated the role of DFosB in the NAc in behavioral responses to opiates. We achieved overexpression of DFosB by using a bitransgenic mouse line that inducibly expresses the protein in the NAc and dorsal striatum and by using viral-mediated gene transfer to specifically express the protein in the NAc. DFosB overexpression in the NAc increased the sensitivity of the mice to the rewarding effects of morphine and led to exacerbated physical dependence, but also reduced their sensitivity to the analgesic effects of morphine and led to faster development of analgesic tolerance. The opioid peptide dynorphin seemed to be one target through which DFosB produced this behavioral phenotype. Together, these experiments demonstrated that DFosB in the NAc, partly through the repression of dynorphin expression, mediates several major features of opiate addiction.

Psychopharmacology, 1995

The ability of kappa opioid agonists to modulate dopamine-mediated behavior and Fos immunore-acti... more The ability of kappa opioid agonists to modulate dopamine-mediated behavior and Fos immunore-activity was assessed in adult rats. It was predicted that kappa agonist treatment would block the unconditioned and conditioned behaviors produced by cocaine (an indirect dopamine agonist). In the initial experiments, cocaine-induced locomotor activity was assessed after either acute or chronic injections of the kappa receptor agonist U-50,488 (5 mg/kg, SC). As expected, U-50,488 decreased cocaine-induced activity, while leaving baseline activity levels unaffected. Interestingly, chronic treatment with U-50,488 did not induce behavioral tolerance. The conditioned effects of cocaine (20 mg/kg, IP) were assessed using the conditioned place preference (CPP) paradigm. As expected, rats showed a preference for the cocaine-paired compartment, an effect blocked by U-50,488 (5 mg/kg, SC). One hour after CPP testing, rats were killed and Fos immunoreactivity was assessed. Rats conditioned with cocaine, but not U-50,488, showed increased Fos activity in the anterior cingulate cortex, piriform cortex, lateral septal area, and olfactory tubercles. When considered together, these results suggest that U-50,488 was effective at blocking the unconditioned and conditioned effects of cocaine, as well as cocaine-induced neuronal activity (as measured by Fos induction).

Behavioral Neuroscience, 1997

The kappa-opioid agonist U-50,488 increases the locomotor activity of preweanling rats. The autho... more The kappa-opioid agonist U-50,488 increases the locomotor activity of preweanling rats. The authors attempted to determine whether this effect was modulated by dopamine (DA) system functioning. Surprisingly, U-50,488's locomotor activating effects were attenuated by both the DA receptor antagonist flupenthixol and the DA receptor agonist R(-)-propylnorapomorphine (NPA). In order to determine those brain areas stimulated by U-50,488, Fos immunoreactivity was assessed in 17- and 80-day-old rats. U-50,488 not only enhanced the locomotor activity of the younger rats, but it also enhanced Fos expression in various brain areas, including the nucleus accumbens and medial striatum. NPA blocked U-50,488-induced Fos expression in the latter region. When considered together, these results indicate that U-50,488 does not increase locomotion by stimulating a DA mechanism. Instead, either agonizing or antagonizing DA receptors is sufficient to disrupt U-50,488's locomotor activating effects in the preweanling rat.

European Journal of Pharmacology, 1996

methanesulfonate (U-50,488) to modulate morphine-induced reward was assessed in preweanling (10-a... more methanesulfonate (U-50,488) to modulate morphine-induced reward was assessed in preweanling (10-and 17-day-old) and periadolescent (35-day-old) rats using the conditioned place preference paradigm. Conditioning and testing were conducted in a three compartment chamber, with each end compartment having its own distinct tactile and odor cues (almond or lemon). An abbreviated conditioned place preference procedure was used in which rats received two saline-odor pairings on the first conditioning day, and two saline-or morphine-odor pairings on the second day. In some experiments, rats were given U-50,488 (2-10 mg/kg, s.c.) 30 min prior to being conditioned with morphine (0.1-8 mg/kg, i.p.). On the third day, rats were allowed free access to the entire chamber for 900 s and compartment preferences were determined. Similar to adult rats, morphine (0.5 mg/kg) was consistently able to induce conditioned place preferences in the two preweanling age groups. This effect was attenuated by K-opioid receptor agonist pretreatment, as U-50,488 not only enhanced the locomotor activity of 10-and 17-day-old rats, but it blocked the morphine-induced place preference conditioning of these younger animals. In contrast, periadolescent (35-day-old) rats did not exhibit morphine-induced place preferences, nor did they show enhanced locomotor activity after U-50,488 treatment; however, using the same procedure, a different group of similarly aged rats showed conditioned preference produced by 20 mg/kg cocaine (i.p.). Therefore, these results suggest that reward processes are functionally mature in the preweanling rat (at least by 10 days of age), but that periadolescent rats are generally unresponsive to /x-and •-opioid drugs.

European Journal of Pharmacology, 1995

The effects of dopamine D 1 and D 2 receptor antagonists on the reward processes of 10-and 17-day... more The effects of dopamine D 1 and D 2 receptor antagonists on the reward processes of 10-and 17-day-old rats were assessed using the conditioned place preference paradigm. Conditioning and testing were conducted in a three-compartment chamber, with each end compartment having its own distinct tactile and odor cues (almond and lemon). During six experiments, 10-and 17-day-old rats (age at initial conditioning) were injected intraperitoneally with either saline, the dopamine D 1 receptor antagonist R(+)-SCH 23390 hydrochloride (0.01-1.0 mg/kg), or the dopamine D e receptor antagonists (+)-sulpiride (1-100 mg/kg) or S(-)-eticlopride hydrochloride (0.1-0.5 mg/kg) 30 min prior to being injected with cocaine hydrochloride (20 mg/kg) or saline. After the latter injections, rats were immediately confined in the lemon-scented (nonpreferred) compartment for 30 min. On the alternate conditioning day, rats were injected with saline and confined in the almond-scented compartment. On the third day (i.e., the test day), rats were given saline and allowed free access to the entire chamber for 15 min. The results showed that the dopamine D 1 receptor antagonist SCH 23390 blocked the cocaine-induced place preference conditioning of both 10-and 17-day-old rats. Surprisingly, the dopamine D 2 receptor antagonists sulpiride and eticlopride blocked the place preference conditioning of 10-day-old rats, while leaving the 17-day-old rats unaffected. These results indicate that dopamine D~ receptors are critically involved in the reward processes of preweanling rats, but that the importance of dopamine D 2 receptors changes across ontogeny.

Psychopharmacology, 1994

In the present study, the abilities of NPA (a direct DA receptor agonist) and amphetamine (an ind... more In the present study, the abilities of NPA (a direct DA receptor agonist) and amphetamine (an indirect DA receptor agonist) to induce short- and long-term behavioral sensitization were assessed in 11- and 17-day-old rats (age at initial injection). Rats were injected on 4 consecutive days with amphetamine (1.0, 2.5, or 5.0 mg/kg), NPA (1.0 mg/kg), or saline. A final test day occurred either 2 days (experiment 1) or 8 days (experiment 2) later. On the test day, rats given successive agonist injections received a single injection of the same agonist again; whereas rats given successive saline injections received either amphetamine or NPA for the first time. Five minutes after injection, locomotor activity (line-crosses), stereotyped sniffing, and vertical activity were measured during a 30-min testing session. The results showed that 11- and 17-day-old rats exhibited behavioral sensitization when tested with NPA or amphetamine after a 2-day interval. In contrast, neither NPA nor amphetamine was able to sensitize the behaviors of preweanling rats when measured 8 days after initial drug treatments. Therefore, these results show that both direct and indirect DA agonists are able to induce short-term behavioral sensitization in preweanling rats, but that the mechanisms responsible for mediating long-term behavioral sensitization have not yet matured.

Proceedings of The National Academy of Sciences, 2005

Sexual deficits and other behavioral disturbances such as anxietylike behaviors can be observed i... more Sexual deficits and other behavioral disturbances such as anxietylike behaviors can be observed in animals that have undergone social isolation, especially in species having important social interactions. Using a model of protracted social isolation in adult rats, we observed increased anxiety-like behavior and deficits in both the latency to initiate sexual behavior and the latency to ejaculate. We show, using transgenic cAMP response element (CRE)-LacZ reporter mice, that protracted social isolation also reduces CREdependent transcription within the nucleus accumbens. This decrease in CRE-dependent transcription can be mimicked in nonisolated animals by local viral gene transfer of a dominant negative mutant of CRE-binding protein (CREB). We previously showed that this manipulation increases anxiety-like behavior. We show here that it also impairs initiation of sexual behavior in nonisolated animals, a deficit that can be corrected by anxiolytic drug treatment. This local reduction in CREB activity, however, has no influence on ejaculation parameters. Reciprocally, we used the viral transgenic approach to overexpress CREB in the nucleus accumbens of isolated animals. We show that this local increase in CREB activity completely rescued the anxiety phenotype of the isolated animals, as well as their deficit in initiating sexual behavior, but failed to rescue the deficit in ejaculation. Our data suggest a role for the nucleus accumbens in anxiety responses and in specific aspects of sexual behavior. The results also provide insight into the molecular mechanisms by which social interactions affect brain plasticity and behavior.

Neurotrophic factor signaling pathways modulate cellular and behavioral responses to drugs of abu... more Neurotrophic factor signaling pathways modulate cellular and behavioral responses to drugs of abuse. In addition, chronic exposure to morphine increases expression of phospholipase C␥1 (PLC␥1) (a protein involved in neurotrophic signaling) in the ventral tegmental area (VTA), a neural substrate for many drugs of abuse. Using viral-mediated gene transfer to locally alter the activity of PLC␥1, we show that overexpression of PLC␥1 in rostral portions of the VTA (R-VTA) results in increased morphine place preference, whereas PLC␥1 overexpression in the caudal VTA (C-VTA) results in avoidance of morphine-paired compartments. In addition, overexpression of PLC␥1 in R-VTA causes increased preference for sucrose and increased anxiety-like behavior but does not affect responses to stress or nociceptive stimuli. In contrast, overexpression of PLC␥1 in C-VTA decreases preference for sucrose and increases sensitivity to stress and nociceptive stimuli, although there was a tendency for increased anxiety-like behavior as seen for the R-VTA. These results show that levels of PLC␥1 in the VTA regulate responsiveness to drugs of abuse, natural rewards, and aversive stimuli and point to the possibility that distinct topographical regions within the VTA mediate generally positive versus negative responses to emotional stimuli. Moreover, these data also support a role for drug-induced elevations in PLC␥1 expression in the VTA in mediating long-term adaptations to drugs of abuse and aversive stimuli.

European Journal of Neuroscience, 2002

Adenylyl cyclase (AC) type VIII has been implicated in several forms of neural plasticity, includ... more Adenylyl cyclase (AC) type VIII has been implicated in several forms of neural plasticity, including drug addiction and learning and memory. In the present study, we directly examined the role for the transcription factor CREB (cAMP response element binding protein) in regulating ACVIII expression by cloning a 5.2 kilobase region upstream of the translation start site of the mouse ACVIII gene. Analysis of this fragment revealed consensus elements for several transcription factors, including a canonical cAMP response element (CRE) in close proximity to the transcription initiation region. Next, ACVIII promoter activity was studied in two neural-derived cell lines and in primary cultures of rat striatal neurons. Activation of the cAMP pathway by forskolin treatment increased promoter activity, and a series of deletion and point mutants demonstrated that this activation is mediated speci®cally via the canonical CRE site. Gel shift assays con®rmed that this site can bind CREB and several CREB family proteins. Further, activation of the ACVIII promoter by forskolin was potentiated by expression of a constitutively active form of CREB, CREB-VP16, whereas it was inhibited by expression of a dominant-negative form of CREB, A-CREB. Finally, over-expression of CREB in vivo, by viral-mediated gene transfer, induced ACVIII promoter activity in the brains of ACVIII-LacZ transgenic mice. These results suggest that the ACVIII gene is regulated by CREB in vitro and in vivo and that this regulation may contribute to CREB-dependent neural plasticity.

European Journal of Neuroscience, 2005

The transcription factor ΔFosB is induced in the nucleus accumbens and dorsal striatum by chronic... more The transcription factor ΔFosB is induced in the nucleus accumbens and dorsal striatum by chronic exposure to several drugs of abuse, and increasing evidence supports the possibility that this induction is involved in the addiction process. However, to date there has been no report of ΔFosB induction by drugs of abuse in the ventral tegmental area (VTA), which is also a critical brain reward region. In the present study, we used immunohistochemistry to demonstrate that chronic forced administration of cocaine induces ΔFosB in the rat VTA. This induction occurs selectively in a γ-aminobutyric acid (GABA) cell population within the posterior tail of the VTA. A similar effect is seen after chronic cocaine self-administration. Induction of ΔFosB in the VTA occurs after psychostimulant treatment only: it is seen with both chronic cocaine and amphetamine, but not with chronic opiates or stress. The expression of ΔFosB appears to be mediated by dopamine systems, as repeated administration of a dopamine uptake inhibitor induced ΔFosB in the VTA, while administration of serotonin or norepinephrine uptake inhibitors failed to produce this effect. Time course analysis showed that, following 14 days of cocaine administration, ΔFosB persists in the VTA for almost 2 weeks after cocaine withdrawal. This accumulation and persistence may account for some of the long-lasting changes in the brain associated with chronic drug use. These results provide the first evidence of ΔFosB induction in a discrete population of GABA cells in the VTA, which may regulate the functioning of the brain's reward mechanisms.

Science, 2006

Mice experiencing repeated aggression develop a long-lasting aversion to social contact, which ca... more Mice experiencing repeated aggression develop a long-lasting aversion to social contact, which can be normalized by chronic, but not acute, administration of antidepressant. Using viralmediated, mesolimbic dopamine pathway-specific knockdown of brain-derived neurotrophic factor (BDNF), we showed that BDNF is required for the development of this experience-dependent social aversion. Gene profiling in the nucleus accumbens indicates that local knockdown of BDNF obliterates most of the effects of repeated aggression on gene expression within this circuit, with similar effects being produced by chronic treatment with antidepressant. These results establish an essential role for BDNF in mediating long-term neural and behavioral plasticity in response to aversive social experiences.

Biological Psychiatry, 2008

Background: Methylphenidate (MPH) is prescribed for the treatment of attention-deficit/hyperactiv... more Background: Methylphenidate (MPH) is prescribed for the treatment of attention-deficit/hyperactivity disorder. Exposure to MPH before adulthood causes behavioral deficits later in life, including anxiety-and depression-like behaviors and decreased responding to natural and drug rewards. We examined the ability of fluoxetine (FLX), a selective serotonin reuptake blocker, to normalize these MPH-induced behavioral deficits.

Biological Psychiatry, 2008

Background-The neurobiological mechanisms by which only a minority of stress-exposed individuals ... more Background-The neurobiological mechanisms by which only a minority of stress-exposed individuals develop psychiatric diseases remain largely unknown. Recent evidence suggests that dopaminergic neurons of the ventral tegmental area (VTA) play a key role in the manifestation of stress vulnerability.

Biological Psychiatry, 2003

Background: Methylphenidate (MPH) is a psychomotor stimulant medication widely used for the treat... more Background: Methylphenidate (MPH) is a psychomotor stimulant medication widely used for the treatment of attention-deficit/hyperactivity disorder (ADHD). Given the extent of prescribed use of MPH, and because MPH interacts with the same brain pathways activated by drugs of abuse, most research has focused on assessing MPH's potential to alter an individual's risk for adult drug addiction. Data examining other potential long-term behavioral consequences of early MPH administration are lacking, however. Methods: We investigated the long-term behavioral consequences of chronic administration of MPH (2.0 mg/kg) during pre-and periadolescent development in adult rats by assessing their behavioral reactivity to a variety of emotional stimuli. Results: The MPH-treated animals were significantly less responsive to natural rewards such as sucrose, noveltyinduced activity, and sex compared with vehicle-treated control animals. In contrast, MPH-treated animals were significantly more sensitive to stressful situations, showed increased anxiety-like behaviors, and had enhanced plasma levels of corticosterone. Conclusions: Chronic exposure to MPH during development leads to decreased sensitivity to rewarding stimuli and results in enhanced responsivity to aversive situations. These results highlight the need for further research to improve understanding of the effects of stimulants on the developing nervous system and the potential enduring effects resulting from early-life drug exposure. Biol Psychiatry 2003;54:1317-1329

Biological Psychiatry, 2006

The neural consequences of early-life exposure to methylphenidate (MPH; Ritalin) are of great int... more The neural consequences of early-life exposure to methylphenidate (MPH; Ritalin) are of great interest given the widespread, and sometimes inappropriate, use in children. Here we examine the impact of juvenile MPH exposure on adult hippocampal neurogenesis. Methods: Rats received MPH (2.0 mg/kg, intraperitoneal, twice daily) or saline (SAL) during preadolescence (postnatal days 20 -35). Hippocampal cell proliferation (Experiment 1), neurogenesis (Experiment 2), and stress-induced changes in cell proliferation (Experiment 3) were assessed at several developmental stages including adulthood. Results: Juvenile exposure to MPH did not alter proliferation at any developmental time point relative to control rats; however, exposure to MPH significantly decreased the long-term survival of newborn cells in adult rats, particularly in the temporal hippocampus. Although MPH-treated rats had higher levels of corticosterone after restraint stress, they did not show the expected greater decrease in hippocampal cell proliferation relative to control animals. Conclusions: Early-life exposure to MPH inhibits the survival of adult-generated neurons in the temporal hippocampus and may reduce progenitor sensitivity to corticosterone-induced decreases in proliferation. These findings suggest that decreased adult neurogenesis is an enduring consequence of early-life exposure to MPH and are discussed for their relevance to humans.

Nature Neuroscience, 2009

Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult ro... more Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult rodents. Social isolation induced both anxiety- and anhedonia-like symptoms and decreased cAMP response element-binding protein (CREB) activity in the nucleus accumbens shell (NAcSh). All of these abnormalities were reversed by chronic, but not acute, antidepressant treatment. However, although the anxiety phenotype and its reversal by antidepressant treatment were CREB-dependent, the anhedonia-like symptoms were not mediated by CREB in NAcSh. We found that decreased CREB activity in NAcSh correlated with increased expression of certain K(+) channels and reduced electrical excitability of NAcSh neurons, which was sufficient to induce anxiety-like behaviors and was reversed by chronic antidepressant treatment. Together, our results describe a model that distinguishes anxiety- and depression-like behavioral phenotypes, establish a selective role of decreased CREB activity in NAcSh in anxiety-like behavior, and provide a mechanism by which antidepressant treatment alleviates anxiety symptoms after social isolation.

Nature Neuroscience, 2006

Chronic morphine administration (via subcutaneous pellet) decreases the size of dopamine neurons ... more Chronic morphine administration (via subcutaneous pellet) decreases the size of dopamine neurons in the ventral tegmental area (VTA), a key reward region in the brain, yet the molecular basis and functional consequences of this effect are unknown. In this study, we used viral-mediated gene transfer in rat to show that chronic morphine-induced downregulation of the insulin receptor substrate 2 (IRS2)-thymoma viral proto-oncogene (Akt) signaling pathway in the VTA mediates the decrease in dopamine cell size seen after morphine exposure and that this downregulation diminishes morphine reward, as measured by conditioned place preference. We further show that the reduction in size of VTA dopamine neurons persists up to 2 weeks after morphine withdrawal, which parallels the tolerance to morphine's rewarding effects caused by previous chronic morphine exposure. These findings directly implicate the IRS2-Akt signaling pathway as a critical regulator of dopamine cell morphology and opiate reward.

Biological Psychiatry, 2003

Previous work has shown that brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine ... more Previous work has shown that brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), are involved in appetitive behavior. Here we show that BDNF in the ventral tegmental area-nucleus accumbens (VTA-NAc) pathway is also involved in the development of a depression-like phenotype.

Neuromolecular Medicine, 2004

The involvement of neurotrophic factors in neuronal survival and differentiation is well establis... more The involvement of neurotrophic factors in neuronal survival and differentiation is well established. The more recent realization that these factors also play pivotal roles in the maintenance and activity-dependent remodeling of neuronal functioning in the adult brain has generated excitement in the neurosciences. Neurotrophic factors have been implicated in the modulation of synaptic transmission and in the mechanisms underlying learning and memory, mood disorders, and drug addiction. Here the evidence for the role of neurotrophins and other neurotrophic factors—and the signaling pathways they activate—in mediating long-term molecular, cellular, and behavioral adaptations associated with drug addiction is reviewed.

Nature Neuroscience, 2006

The transcription factor DFosB is induced in the nucleus accumbens (NAc) and dorsal striatum by t... more The transcription factor DFosB is induced in the nucleus accumbens (NAc) and dorsal striatum by the repeated administration of drugs of abuse. Here, we investigated the role of DFosB in the NAc in behavioral responses to opiates. We achieved overexpression of DFosB by using a bitransgenic mouse line that inducibly expresses the protein in the NAc and dorsal striatum and by using viral-mediated gene transfer to specifically express the protein in the NAc. DFosB overexpression in the NAc increased the sensitivity of the mice to the rewarding effects of morphine and led to exacerbated physical dependence, but also reduced their sensitivity to the analgesic effects of morphine and led to faster development of analgesic tolerance. The opioid peptide dynorphin seemed to be one target through which DFosB produced this behavioral phenotype. Together, these experiments demonstrated that DFosB in the NAc, partly through the repression of dynorphin expression, mediates several major features of opiate addiction.