Rapid regulation of depression-related behaviours by control of midbrain dopamine neurons

Abstract

Ventral tegmental area (VTA) dopamine neurons in the brain's reward circuit have a crucial role in mediating stress responses, including determining susceptibility versus resilience to social-stress-induced behavioural abnormalities. VTA dopamine neurons show two in vivo patterns of firing: low frequency tonic firing and high frequency phasic firing. Phasic firing of the neurons, which is well known to encode reward signals, is upregulated by repeated social-defeat stress, a highly validated mouse model of depression. Surprisingly, this pathophysiological effect is seen in susceptible mice only, with no apparent change in firing rate in resilient individuals. However, direct evidence--in real time--linking dopamine neuron phasic firing in promoting the susceptible (depression-like) phenotype is lacking. Here we took advantage of the temporal precision and cell-type and projection-pathway specificity of optogenetics to show that enhanced phasic firing of these neurons mediates susceptibility to social-defeat stress in freely behaving mice. We show that optogenetic induction of phasic, but not tonic, firing in VTA dopamine neurons of mice undergoing a subthreshold social-defeat paradigm rapidly induced a susceptible phenotype as measured by social avoidance and decreased sucrose preference. Optogenetic phasic stimulation of these neurons also quickly induced a susceptible phenotype in previously resilient mice that had been subjected to repeated social-defeat stress. Furthermore, we show differences in projection-pathway specificity in promoting stress susceptibility: phasic activation of VTA neurons projecting to the nucleus accumbens (NAc), but not to the medial prefrontal cortex (mPFC), induced susceptibility to social-defeat stress. Conversely, optogenetic inhibition of the VTA-NAc projection induced resilience, whereas inhibition of the VTA-mPFC projection promoted susceptibility. Overall, these studies reveal novel firing-pattern- and neural-circuit-specific mechanisms of depression.

Figure 1. Phasic, but not tonic, optical stimulation of VTA DA neurons during a subthreshold social defeat induces a susceptible phenotype

Confocal image showing co-expression of AAV-DIO-ChR2 in TH+ DA cells from TH-Cre mice. b, Quantification shows that ChR2-expressing TH+ cells are 62±4% of total TH+ neurons in the VTA and there was no expression of ChR2 in TH- neurons (n=2–3 sections from n=4 animals). c, Optical stimulation protocols for mimicking tonic (0.5 Hz) or phasic (20 Hz) firing. Note that for both stimulating protocols, five spikes are induced over each ten second period. d1, Experimental timeline. d2, Detailed schematic of the subthreshold paradigm showing laser stimulation during social defeat. e, Social interaction data in control, tonic, and phasic groups. (F₂,₃₀ = 4.70, p<0.05; post-hoc test: * p<0.05; n=7–14). f. Sucrose preference measured over a 12-hr period after the social interaction test (F₂,₂₂ = 5.22, p<0.05; post-hoc test, * p<0.05; n=7–10). All bar graphs depict ± s.e.m.

Figure 2. Phasic optical stimulation of VTA DA neurons during the social interaction test instantly induces a susceptible phenotype in two social defeat paradigms

a, Social interaction data in control, tonic, and phasic groups (F₂,₂₂ = 4.00, p<0.05; post-hoc test: * p<0.05; n=7–11). b, Sucrose preference measured over a 12-hr period after the social interaction test (F₂,₂₅ = 3.47, p<0.05; post-hoc test: * denotes p<0.05; n=8–11). c, Social interaction data measured on day 17 (t₁₅ = 2.72, * denotes p<0.05; two tailed t-test, n=11–18). d, Sucrose preference measured over a 12-hr period after the social interaction test (t₁₇ = 2.34, * p<0.05; two tailed t-test, n=6–12). e, Sample traces: showing in vitro spontaneous activity of VTA DA neurons from TH-Cre mice that underwent tonic and phasic stimulation during the social interaction test 24 hr after subthreshold social defeat (see Supplementary Fig. 5j for the experimental timeline). Bar graph: Comparison of spontaneous firing in VTA DA neurons from EYFP-control, tonic, and phasic-stimulated mice (F_2,50 = 3.19, p<0.05; post-hoc test, * p<0.05; n=17–19). f, Significantly less current was required to evoke a single spike in phasic stimulated mice compared to EYFP control mice (t₂₁ = 1.8, * p<0.05; one tailed t-test, n=12–16). g, VTA DA cells from phasic stimulated mice display overall increased cell excitability to incremental steps in current injections (50, 100, 150 and 200pA) compared with EYFP control and tonic stimulated mice (F_2,140 = 16.13, p<0.001; post-hoc test, * p<0.05 ** p<0.005; n=5–17). All bar graphs depict ± s.e.m.

Figure 3. Bidirectional effect of modulating the VTA-NAc pathway on susceptibility to social defeat

a, Sample traces recorded from VTA-NAc neurons in VTA slices. b, Firing rates of VTA-NAc neurons from control, resilient, and susceptible mice (F₂,₈₉ = 15.77, p<0.001; post-hoc test, * p<0.001; n=12–52). c, Social interaction data obtained during optical stimulation of VTA-NAc neurons in control, tonic, and phasic groups. (F₂,₂₀ = 4.43, p<0.05; post-hoc test: * p<0.05; n=5–10). d, Sucrose preference data measured over a 12-hr period after the social interaction test. (F₂,₁₈ = 4.80, p<0.05; post-hoc test, * p<0.05; n=5–9). e, Social interaction during optical inhibition of VTA-NAc neurons in previously susceptible mice (No target: t₁₅ = 4.2, * p<0.001; two tailed t-test, n=8-9; Target: t₁₅ = 2.6, * p<0.05; two tailed t-test, n=8–9). f, Sucrose preference measured over a 12-hr period after the social interaction test (t₁₄ = 2.3, * p<0.05; two tailed t-test, n=8–9). All bar graphs depict ± s.e.m.

Figure 4. Effect of modulating the VTA-mPFC pathway on susceptibility to social defeat

a, Sample traces recorded from VTA-mPFC neurons in VTA slices. b, Firing rates of VTA-mPFC neurons from control, resilient, and susceptible mice (F₂,_.38 = 15.07, p=0.0005; post-hoc test, * p<0.005; n=11–15). c, Social interaction data obtained by optical stimulation of VTA-mPFC neurons in control, tonic, and phasic groups (F₂,₃₉ = 1.29, p=0.29; n=11–17). d, Sucrose preference data measured over a 12-hr period after the social interaction test (F₂,₃₃ = 0.19, p=0.82; n=10–16). e, Social interaction data obtained during optical inhibition of VTA-mPFC neurons (t₂₉ = 2.5, * p<0.05; two tailed t-test, n=12–19). f, Sucrose preference measured over a 12-hr period after the social interaction test (t₂₉=0.38, p>0.05, n=12–19). All bar graphs depict ± s.e.m.