Bielajew C, Shizgal P. Evidence implicating descending fibers in self stimulation of the medial forebrain bundle. J Neurosci 6: 919-929 (original) (raw)
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Evidence implicating descending fibers in self-stimulation of the medial forebrain bundle
The Journal of neuroscience : the official journal of the Society for Neuroscience, 1986
The role of ascending and descending fibers in self-stimulation of the lateral hypothalamus and ventral tegmental area in the rat was assessed by noting whether anodal hyperpolarization of one of these sites could reduce the rewarding effect of stimulating the other site. Strength-duration curves were obtained by psychophysical means, with one of the depth electrodes serving as the cathode and the other as the anode. It was anticipated that at long pulse durations, conduction in some of the fibers stimulated at the cathode would be blocked at the anode. At shorter durations, the anodal hyperpolarization should have dissipated before the arrival of the action potentials triggered by the cathode. Thus, the predicted effect of the block was to bend the strength-duration curves obtained with two depth electrodes upward at long pulse durations, provided that the anode lay between the cathode and the efferent stages of the pathway responsible for the rewarding effect. To control for possi...
Behavioral methods for inferring anatomical linkage between rewarding brain stimulation sites
Journal of comparative and physiological psychology, 1980
Rats lever pressed for concurrent electrical stimulation of the lateral hypothalamus and ventral tegmentum. The pulse-pair stimulation technique was used, with the first pulse of each pair applied to one electrode and the second to the other electrode; the intrapair interval was varied. The effectiveness of stimulation, measured behaviorally, increased abruptly (within .4 msec) as the intrapair interval was increased in the range from 1.0 to 2.0 msec. These results, which do not resemble single-electrode refractory period results, are interpreted as evidence of collision in the directly stimulated, reward-related neurons linking the two sites. We conclude that self-stimulation of the medial forebrain bundle involves the direct activation of long-axon, longitudinal pathways. Estimates of the conduction velocity in the fibers subserving the collision-like effects are consistent with the properties of small myelinated axons but not central monoaminergic fibers.
Behavioural Brain Research, 1994
A behavioral version of the collision test was used to determine whether reward-relevant neurons directly link self-stimulation sites in the lateral hypothalamic (LH) and ventral tegmental (VTA) areas. Five male rats served as subjects. Trains of conditioning (C) and test (T) pulses were delivered to the two stimulation sites, each site receiving one of the pulses from each pair. The C-T interval was varied from 0.2-17.3 ms, and the effectiveness of the paired pulse stimulation was estimated by comparing the rate-number curve obtained at each C-T interval to rate-number curves obtained with trains of evenly spaced single pulses delivered via one electrode. For 4 of the subjects, stimulation effectiveness increased with the C-T interval, and the form of this increase was similar regardless of which electrode delivered the C-pulses. These increases in effectiveness are consistent with recovery' from collision block in reward-relevant fibers stimulated at both sites. The domain of the rising portion of the effectiveness versus C-T interval curve spanned 2.2-7.7 ms. Such a gradual rise suggests that the directly stinmlated substrate is composed of fibers with a wide range of conduction velocities and/or refractory periods. The discrepancy between these gradually rising collision curves and the steeply rising curves obtained in previous collision studies may have been due to inadequate sampling of the rate-number function in the earlier studies.
A portrait of the substrate for self-stimulation
Psychological Review, 1981
Quantitative properties of the neural system mediating the rewarding and priming effects of medial forebrain bundle (MFB) stimulation in the rat have been determined by experiments that trade one parameter of the electrical stimulus against another. The first-order neurons in this substrate are for the most part long, thin, myelinated axons, coursing in the MFB and ventral tegmentum, with absolute refractory periods in the range .5-1.2 msec and conduction velocities of 2-8 m/sec. Local potentials in these axons decay with a time constant of about .1 msec. A supernormal period follows the recovery from refractoriness. These axons integrate current over exceptionally long intervals, accommodate slowly, and fire on the break of prolonged anodal pulses. These properties rule out the hypothesis that catecholamine pathways constitute the first-order axons. The second-order (postsynaptic) part of the substrate shows surprisingly simple spatial and temporal integrating characteristics. We examine the logic that permits conclusions of this sort to be derived from behavioral data and the role of these derivations in establishing neurobehavioral linkage hypotheses.
Brain Research, 1989
Psychophysically derived estimates of recovery from refractoriness were obtained at self-stimulation sites in the lateral hypothalamus and ventral tegmental area. The refractory periods of single units driven by the same stimulation electrodes and stimulation fields were then measured electrophysiologically. Antidromically driven units with refractory periods longer than those of the neurons responsible for the rewarding effect were concentrated in the septal complex. Units with refractory periods that overlapped the estimates for the reward-related neurons were found in this region as well but were also encountered in neighboring structures lateral, ventral, and/or caudal to the septal nuclei. It is argued that this latter class of units should be considered as possible constituents of the directly stimulated substrate for the rewarding effect because they are driven by rewarding stimulation, have refractory periods similar to those of the reward-related neurons and arise in or near regions in which lesions have been effective in decreasing the rewarding effect of stimulating the medial forebrain bundle.
Interhemispheric links in brain stimulation reward
Behavioural Brain Research, 1995
The MFB substrate of self-stimulation (SS) has generally been viewed as a unilateral system. We re-examined this belief with pairs of moveable SS electrodes placed bilaterally in the MFB. Rats barpressed for trains of single or twin cathodal pulses of fixed intensity and width and of variable frequency. The first (C) and second (T) pulse of each pair was delivered through the left and right electrode or inversely. C-T intervals ranging from 0.2 to 5.0 ms were tested. The frequency of C pulses required for criterial bar-pressing was used to plot the stimulation efficacy (SE), as a function of the C-T interval and pulse presentation order. The electrodes were subsequently moved and the same procedure repeated for more ventral sites. With some pairs of contralateral hypothalamic (H) sites, the SE was independent of the C-T interval. However, with other pairs of contralateral H sites, the SE increased with C-T interval in a manner resembling a collision effect, with the important exception that no conduction time (CT) was apparent in the data. The absence of CT excludes the presence of a genuine collision effect. When one pulse was sent to the H and another to the contralateral ventral tegmentum (VT), the H-VT curve rose always earlier than the VT-H curve, thus resembling a transynaptic collision effect. However, the C-T interval at which the VT-H curve began rising (always 1.0 ms or less) fails to support the contention that the electrodes activated fibers separated by a synapse. Finally, a typical collision effect was noted with ipsilateral H-VT electrode placements, confirming the presence of direct linkage between ipsilateral MFB sites. Computer-generated data based on two parsimonious assumptions were found to match the empirical results. These assumptions were that each electrode activated a different branch of the same reward neuron and that conduction failure occurred at the branchpoint. The model, which posits that a large number of MFB reward neurons send branches to the other hemisphere, is testable and makes clear-cut predictions about the effects of lesions. In a preliminary test, we recorded the H and contralateral VT threshold frequencies before and after lesioning the H. The H threshold increased more when using small pulse current and remained constant throughout the 4-week testing period. The VT threshold was elevated more for intermediate pulse current and kept increasing with time. The VT changes suggest that the H lesion induced retrograde degeneration which affected the output of fibers coursing through a short, identifiable distance from the VT electrode. A model based on decussating branches predicts that lesions of the MFB should have variable effects on the SS of distal ipsilateral sites, a prediction matched by a substantial body of contradictory data.
Attenuation of medial forebrain bundle reward by anterior lateral hypothalamic lesions
Behavioural Brain Research, 1996
Psychophysical data consistent with rostro-caudal conduction along reward-relevant neurons linking the lateral hypothalamus (LH) and ventral tegmental area (VTA) have lead to the hypothesis that some of the directly activated neurons responsible for medial forebrain bundle (MFB) self-stimulation arise anterior to the level of the LH. This hypothesis has been challenged on the grounds that lesions to the anterior LH (ALH) often fail to degrade the rewarding value of stimulating more posterior MFB sites. The present study was aimed at investigating the effect of lesion location and stimulation current on the efficacy of ALH lesions in an effort to account for the inconsistencies in the earlier data. Self-stimulation thresholds were obtained for LH and VTA sites by estimating the number of pulses per stimulation train required for half-maximal responding at each of 3 currents. Electrolytic lesions (anodal, 1.0 mA for 10 s) were then made to the ALH at varying medial-lateral coordinates. In 7 of the 14 rats with MFB stimulation sites, lesions to the ALH produced increases in threshold which often declined over the next several days to weeks; in 5 cases thresholds remained elevated by 0.1 to 0.25 loglo units above baseline up to the end of testing. In all but one case, the effective lesions were centered in the lateral ALH. Increases in threshold were more likely to be detected when stimulating at low currents; at low currents fewer neurons are recruited and the lesion can have a greater proportional effect on threshold. These data support the hypothesis that cell bodies, terminals, or fibers of passage in the ALH contribute to the rewarding effect of stimulating more posterior MFB sites.
Physiology & Behavior, 1981
SHIZGAL. Refractory periods and anatomical linkage of the substrates for lateral hypothalamic and periaqueductal gray self-stimulation. PHYSIOL. BEHAV. 27(1) [95][96][97][98][99][100][101][102][103][104] 1981.--The pulse-pair technique was employed (a) to investigate the refractory periods of the reward-related neurons activated by lateral hypothalamic (LH) and periaqueductal gray (PAG) stimulation, and (b) to examine whether these two sites are linked by the same reward-related neurons. These properties of the reward substrate were inferred from selfstimulation data. In eight out of ten subjects, recovery from refractoriness began at about the same time at both sites (0.4--0.6 msec). However, the time course of recovery was consistently slower at the PAG placements. A behavioral adaptation of the collision test failed to reveal reward-related fibers that directly link the LH and PAG. Nonetheless, the rewarding effects of stimulation at the two sites summated with roughly 60% efficiency. These data are consistent with the notion that the rewarding effects of LH and PAG stimulation are mediated by different pathways that ultimately converge.