Activation of dopamine neurons is critical for aversive conditioning and prevention of generalized anxiety - PubMed (original) (raw)

Activation of dopamine neurons is critical for aversive conditioning and prevention of generalized anxiety

Larry S Zweifel et al. Nat Neurosci. 2011 May.

Abstract

Generalized anxiety is thought to result, in part, from impairments in contingency awareness during conditioning to cues that predict aversive or fearful outcomes. Dopamine neurons of the ventral midbrain exhibit heterogeneous responses to aversive stimuli that are thought to provide a critical modulatory signal to facilitate orientation to environmental changes and assignment of motivational value to unexpected events. Here we describe a mouse model in which activation of dopamine neurons in response to an aversive stimulus is attenuated by conditional genetic inactivation of functional NMDA receptors on dopamine neurons. We discovered that altering the magnitude of excitatory responses by dopamine neurons in response to an aversive stimulus was associated with impaired conditioning to a cue that predicts an aversive outcome. Impaired conditioning by these mice was associated with the development of a persistent, generalized anxiety-like phenotype. These data are consistent with a role for dopamine in facilitating contingency awareness that is critical for the prevention of generalized anxiety.

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Conflict of interest statement

Competing Interests Statement

The authors declare that they have no competing financial interests.

Figures

Figure 1

Figure 1

NMDARs control amplitude of activation of dopamine neurons in response to tail pinch. (a–d) Assessment of NMDAR–dependent activation of dopamine neurons during tail pinch. (a,b) Peri–event time histograms (PETHs) of representative dopamine neurons illustrating inhibitory (b) and excitatory (c) responses to tail pinch (n = 5 control mice and n = 6 knockout mice). Inserts for a and b, left: average waveform for representative neurons, right: pie charts show proportion of QS neurons inhibited or activated by tail pinch (black shade) for control and knockout mice. (c,d) Average Z–score corrected (see Supporting Online Material) inhibitory (c) and excitatory (d) responses to tail pinch (dashed lines represent standard error of the mean, s.e.m.). Excitatory responses are significantly reduced in KO mice compared to controls (Bonferroni posttests ***P < 0.001).

Figure 2

Figure 2

Cue–dependent fear conditioning is impaired in knockout mice. (a) Startle amplitude is enhanced 10 min after cue–footshock pairings to a greater extent in knockout (n=8) compared than control (n=9) mice in both the presence and absence of the cue (Bonferroni posttests, #P < 0.05 and ##P < 0.01 post–conditioning compared to pre–conditioning in the presence or absence of the cue, *P < 0.05 knockout compared to control no cue and cue tests). (b) Amplitude of startle response in conditioned knockout mice at 105 dB is lower than the startle response of unconditioned knockout mice at 120 dB (*P < 0.05). Startle amplitude to footshock during conditioning trials is not different between groups. Error bars represent s.e.m.

Figure 3

Figure 3

Sensitization of ASR following fear conditioning in knockout mice is context independent. (a) ASR before (dashed line) and 10 min following footshock conditioning (solid line) in a distinct environmental context (n = 13, control and n = 11, knockout; Bonferoni posttests *P < 0.05, knockout post–shock vs. knockout pre–shock and control pre– and post–shock). (b) ASR (n = 14, control and n = 14, knockout) before (dashed line) and 1 day post–shock (solid line) is elevated in knockout mice following shock (Bonferoni posttests ***P < 0.001 and *P < 0.05, knockout post–shock vs. knockout pre–shock and control pre– and post–shock). (c) Same groups of mice as in (b) 1 week following conditioning in novel context demonstrating persistent elevation of the ASR in knockout mice following footshock (Bonferoni posttests ***P < 0.001, knockout post–shock vs. knockout pre–shock and control pre– and post–shock). (d) ASR following repeated exposure to ASR chamber without conditioning (n = 8, control and n = 7, knockout) is not different between groups. (e) Average ASR at 105 dB across all groups of mice pre– and post–conditioning (NS = no shock, repeated exposure group; Bonferoni posttests *P < 0.05, knockout post–shock vs. knockout pre–shock and knockout postshock vs. control pre– and post–shock). Error bars represent s.e.m.

Figure 4

Figure 4

Anxiety–related behavior is enhanced in knockout mice following footshock conditioning. (a) Frequency of open–arm entries in an elevated–plus maze following footshock is significantly reduced in knockout (n = 14) compared to control mice (n = 13) or mice repeatedly exposed (RE) to the elevated–plus maze (Bonferoni posttests, **P < 0.01, knockout post–shock vs. all other groups). (b) Representative activity traces from control (left) and knockout mice (right) in elevated–plus maze test following footshock (C, closed arm and O, open arm). (c) Frequency of center crossings in an open–field apparatus by knockout mice (n=14) following footshock conditioning is reduced compared to control mice (n=13) or mice repeatedly exposed to the open field (knockout n=13, control n=10; Bonferoni posttests, *P < 0.05, knockout post–shock vs. all other groups). (d) Representative activity traces from control (left) and knockout mice (right) in open field following footshock conditioning. (e) Distance traveled in the open arm of the elevated–plus maze is significantly reduced in knockout mice following footshock compared to other groups (Bonferoni posttests, *P < 0.05, knockout post–shock vs. all other groups). (f) Distance traveled in closed arm of elevated–plus maze is not significantly reduced in knockout mice that received footshock. Error bars represent s.e.m.

Figure 5

Figure 5

Sensory motor gating, peripheral stress response, and monoamine levels are not altered following footshock conditioning. (a) Increasing pre–pulse intensities leads to greater PPI of ASRs but is not altered in knockout mice following footshock (pre–pulse effect, *** P < 0.0001). (b) Cort levels are increased immediately (t=0) after exposure to the startle chamber with shock (S) or without shock (NS) and are elevated 1 h following shock, but not 1 d or 1 wk later. There is no significant difference between genotypes. (c) Whole–brain monoamine levels as measured by HPLC are not different in knockout or control mice that had received shock. (d) Monoamine metabolites are unaltered by footshock conditioning. Error bars represent s.e.m.

Figure 6

Figure 6

Conditional restoration of NMDAR signaling to ventral midbrain dopamine neurons prevents generalized anxiety–like behavior. (a) Low magnification of ventral midbrain (top) demonstrating HA–NR1 (green) is predominantly localized to the tyrosine hydroxylase (TH)–positive region of the VTA and not the SNc (scale bar = 500 μm). High magnification (bottom) shows HA–NR1 co–localized with TH–positive neurons (scale bar = 25 μm). (b) Evoked AMPAR and NMDAR–mediated EPSCs from control, knockout, and virally rescued knockout mice. (c–e) Expression of HA–NR1 in knockout mice prevents generalized anxiety–like behavior: (c) ASR, (d) frequency of open arm entries in the elevated–plus maze, and (e) frequency of center crossing in the open–field before and after foot shock conditioning in novel context (control, n = 11 and virally rescued knockout, n = 11). Error bars represent s.e.m.

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