Dopamine neurons modulate neural encoding and expression of depression-related behaviour

Abstract

Major depression is characterized by diverse debilitating symptoms that include hopelessness and anhedonia. Dopamine neurons involved in reward and motivation are among many neural populations that have been hypothesized to be relevant, and certain antidepressant treatments, including medications and brain stimulation therapies, can influence the complex dopamine system. Until now it has not been possible to test this hypothesis directly, even in animal models, as existing therapeutic interventions are unable to specifically target dopamine neurons. Here we investigated directly the causal contributions of defined dopamine neurons to multidimensional depression-like phenotypes induced by chronic mild stress, by integrating behavioural, pharmacological, optogenetic and electrophysiological methods in freely moving rodents. We found that bidirectional control (inhibition or excitation) of specified midbrain dopamine neurons immediately and bidirectionally modulates (induces or relieves) multiple independent depression symptoms caused by chronic stress. By probing the circuit implementation of these effects, we observed that optogenetic recruitment of these dopamine neurons potently alters the neural encoding of depression-related behaviours in the downstream nucleus accumbens of freely moving rodents, suggesting that processes affecting depression symptoms may involve alterations in the neural encoding of action in limbic circuitry.

Figure 1. Selective inhibition of VTA dopamine neurons induces a depression-like phenotype

a, Cre-dependent AAV. b, Confocal images of midbrain dopamine neurons. Orange dotted lines, location of fibre optic. Bottom, images of VTA neurons directly below fibre track. c, Photoinhibition of VTA dopamine neurons acutely reduces escape-related behaviour. Two-way ANOVA demonstrates the group-by-light epoch interaction (interaction of the experimental-group factor and light-condition factor in the test), F_4,38 = 3.95, P = 0.00089; Bonferroni post-hoc test shows reduced struggling in the eNpHR3.0 group relative to the eYFP group, *P < 0.05. Error bars, s.e.m. d, Inhibition of VTA dopamine neurons does not produce a significant difference in open-field locomotion; two-way ANOVA did not demonstrate a significant group-by-light epoch interaction, F_3,48 = 1.76, P = 0.17. Error bars, s.e.m. e, Schematic and results of the 90-min sucrose-preference test. Photoinhibition of VTA dopamine neurons acutely reduces sucrose preference; two-way ANOVA revealed that opsin expression has a significant effect, F_1,42 = 6.31, P = 0.016; Bonferroni post-hoc test revealed significant differences between groups only in the light-on epoch, *P < 0.05. Error bars, s.e.m. Off, light off; On, light on; WPRE; woodchuck hepatitis virus post-transcriptional regulatory element.

Figure 2. Temporally sparse phasic photoactivation of VTA dopamine neurons rescues stress-induced depression-like phenotype

a, Experimental groups. b, Schematic of illumination pattern; 473-nm light used to elicit phasic bursts of activity in ChR2-expressing VTA dopamine neurons. c, Phasic illumination of VTA dopamine neurons rescues stress-induced reduction in struggling on TST in ChR2 CMS but not eYFP CMS mice (**P < 0.001; eYFP CMS = 26.8 s ± 3.5 s; ChR2 CMS = 28.0 s ± 2.7 s; ChR2 non-CMS = 55.63 s ± 4.4 s; eYFP non-CMS = 56.0 s ± 4.7 s). Two-way ANOVA revealed a significant group-by-light epoch interaction (F_6,134 = 6.04, P < 0.0001) and a significant influence of experimental condition on performance as revealed by one-way ANOVA (F_3,67 = 6.20, P = 0.0009; Bonferroni post-hoc test). Error bars, s.e.m. d, Illumination parameters, as used in the TST, did not change locomotor activity in the open field, with no significant group-by-epoch interaction in two-way ANOVA (F_9,152 = 0.99, P = 0.4493), and no detectable differences revealed by Bonferroni post-hoc tests on the same timescale. Error bars, s.e.m. e, Phasic activation of VTA dopamine neurons rescued the stress-induced decrease in sucrose preference in ChR2 CMS, but not eYFP CMS mice; one-way ANOVA, Dunn’s post-hoc test comparing baseline to light-on epoch, P < 0.01 for ChR2 CMS mice, P = 0.2851 for eYFP CMS mice. Two-way ANOVA revealed a significant group-by-light epoch interaction (F_6,62 = 4.33, P = 0.001), and a significant influence of experimental condition on performance was revealed by one-way ANOVA (F_3,31 = 3.40, P = 0.0299); **P < 0.01 for ChR2 CMS mice. Error bars, s.e.m.

Figure 3. Phasic activation of VTA dopamine neurons modulates escape-related behaviour in TH::Cre rats

a, In vivo electrophysiology sessions. b, Integration of recordings in the NAc, illumination of ChR2-expressing VTA dopamine neurons, and precision measurement of swimming behaviour in five TH::Cre rats treated with CMS. c, Light-induced modulation of swimming; phasic illumination of ChR2-expressing VTA dopamine neurons increases escape behaviour of TH::Cre rats in FST. Thick red line, mean kick rate; dotted lines, s.e.m. d, Phasic illumination of ChR2-expressing VTA dopamine neurons increases kick rate in FST (paired t-test; **P = 0.0088; mean increase in kick rate of 73.9% ± 21.2% relative to light-off epochs) but not ambulation rate in OFT (mean decrease of 60.2% ± 22.9%; mean ± s.e.m.). e, Peri-event raster histogram showing kick events referenced to the train of 8 light pulses at 30 Hz every 5 s (blue); kick events are not time-locked to light pulses (bin width of 0.175 s for all histograms). f, Scatter-plot relating the degree to which behaviour of each rat was modulated by light-driven VTA dopamine neuron activation, to the proportion of all light-responsive units recorded from the same subject (Pearson’s correlation Test, P = 0.0271, r = 0.9195).

Figure 4. Phasic activation of VTA dopamine neurons modulates NAc encoding of escape-related behaviour

Recording of 123 NAc neurons from 5 CMS TH::Cre rats expressing ChR2 in VTA dopamine neurons. a, Peri-event raster histograms for representative neurons showing phasic excitation associated with both light pulses and kick events (Example cell 1; 15 out of 24 cells) or for representative neurons showing different phasic responses with light pulses versus kick events (Example cell 2; 9 out of 24 cells). b, Peri-event raster histograms for representative neurons that selectively encoded kick events during either the light-on epoch (Example cell 3; 13 cells) or the light-off epoch (Example cell 4; 16 cells), or encoded kick events in both light-on and light-off epochs differentially (Example cell 5; 1 out of 14 cells) or similarly (Example cell 6; 13 out of 14 cells).