Figure 1.

Chronic social defeat stress increases the firing rates and bursting events in VTA DA neurons in susceptible, not resilient (unsusceptible), mice. A, Timeline of chronic social defeat, behavioral testing, and in vivo recording protocols. B, The time spent in the interaction zone during a social interaction test (***p < 0.0001 vs control or unsusceptible; 8–20 mice/group). C, Sample traces, bursting events, and spikes for in vivo recordings from VTA DA neurons from control, susceptible, and unsusceptible mice. D, Overall VTA firing rates in control, susceptible, and unsusceptible mice (***p < 0.0001 vs control or unsusceptible mice, n = 58–72 cells, 15 mice/group). E, Average VTA firing rate for each mouse is significantly negatively correlated with the time spent in interaction zone measured on day 11 (p < 0.05, n = 35 mice). F, H, J, Burst-firing events of VTA DA neurons, including percentage of burst-firing cells, percentage of spikes in bursts, and number of spikes in a burst, were increased in susceptible, not unsusceptible, mice (**p < 0.01, ***p < 0.001 vs control or unsusceptible mice; n = 23–49 cells, 8–15 mice/group). G, I, K, The average changes of these bursting events for each animal also inversely correlated with its social interaction time (p < 0.05; n = 35 mice). L, Scatter plot of the bursting activity and firing rates of all neurons recorded from control, susceptible, and unsusceptible mice. Note the large number of VTA DA neurons in susceptible mice with high firing rate and more bursting activity. Firing rates are significantly positively correlated with percentage of spikes in bursts in all three groups, suggesting the increased firing rates were associated with the increase of bursting events (control: r ² = 0.127, n = 65 cells/15 mice, p < 0.001; susceptible: r ² = 0.0952, n = 72 cells/12 mice, p < 0.001; unsusceptible: r ² = 0.0758, n = 58 cells/8 mice, p < 0.05).