GABAergic neurons in nucleus accumbens are correlated to resilience and vulnerability to chronic stress for major depression

Abstract

Background: Major depression, persistent low mood, is one of common psychiatric diseases. Chronic stressful life is believed to be a major risk factor that leads to dysfunctions of the limbic system. However, a large number of the individuals with experiencing chronic stress do not suffer from major depression, called as resilience. Endogenous mechanisms underlying neuronal invulnerability to chronic stress versus major depression are largely unknown. As GABAergic neurons are vulnerable to chronic stress and their impairments is associated with major depression, we have examined whether the invulnerability of GABAergic neurons in the limbic system is involved in resilience.

Results: GABAergic neurons in the nucleus accumbens from depression-like mice induced by chronic unpredictable mild stress appear the decreases in their GABA release, spiking capability and excitatory input reception, compared with those in resilience mice. The levels of decarboxylase and vesicular GABA transporters decrease in depression-like mice, but not resilience.

Materials and methods: Mice were treated by chronic unpredictable mild stress for three weeks. Depression-like behaviors or resilience was confirmed by seeing whether their behaviors change significantly in sucrose preference, Y-maze and forced swimming tests. Mice from controls as well as depression and resilience in response to chronic unpredictable mild stress were studied in terms of GABAergic neuron activity in the nucleus accumbens by cell electrophysiology and protein chemistry.

Conclusions: The impairment of GABAergic neurons in the nucleus accumbens is associated with major depression. The invulnerability of GABAergic neurons to chronic stress may be one of cellular mechanisms for the resilience to chronic stress.

Keywords: depression; neuron; nucleus accumbens; resilience; synapse.

Figure 1. Chronic unpredictable mild stress (CUMS) leads mice to express depression-like behaviors or resilience

(A) shows the SPT values (%) in the mice from CUMS-induced depression (red bar), CUMS resilience (orange bar) and control group (blue bar). (B) illustrates the ratios of stay time in M-arm to stay time in three arms by the YMT in the mice from CUMS-induced depression (red bar), CUMS resilience (orange) and control group (blue). (C) illustrate immobile time of staying in the water cylinder by the FST in the mice from CUMS-induced depression (red bar), CUMS resilience (orange) and control group (blue). Three asterisks show p < 0.001, in which one-way ANOVA was used for the comparisons among CUMS-induced depression, CUMS resilience and control mice, while paired-t test was for the comparisons before and after the CUMS.

Figure 2. Inhibitory synaptic transmission is downregulated in GABAergic neurons of the nucleus accumbens from CUMS-induced depression mice, but not resilience

sIPSCs were recorded under voltage-clamp in the brain slices from control, resilience and depression-like mice in presence of 10 μM CNQX and 40 μM D-AP5. (A) Left panel shows sIPSCs from a control mouse (blue traces), middle panel shows sIPSCs from a depression-like mouse (reds) and right panel shows sIPSCs from a resilience mouse (orange). Calibration bars are 4 pA in vertical bar as well as 2 seconds (top traces) and 100 milliseconds (bottoms) in horizontal. (B) shows cumulative probability versus sIPSC amplitudes from depression-like mice (red symbols), resilience mice (orange) and control mice (blue). Dash-lines indicate sIPSC amplitudes at cumulative probability to 67% (CP₆₇). (C) shows cumulative probability versus inter-sIPSC intervals from the depression-like mice (red symbols), resilience mice (orange) and control (blue). Dash-lines indicate sIPSC intervals at CP₆₇ in control (blue line; n = 12 cells), resilience (orange; n = 14 cells) and depression-like mice (red; n = 14 cells). The inserted figure shows a comparison of sIPSC intervals at CP₆₇ from the mice of CUMS-induced depression (red bar), resilience (orange) and control (blue), in which two asterisks show p < 0.01, one-way ANOVA).

Figure 3. GABA synthesis, uptake and release are impaired in the nucleus accumbens of CUMS-induced depression mice, but not resilience mice

The expression and relative quantity of proteins GAD-67 and VGAT were studied by western-blot. (A) Left panel shows GAD-67 expressions from the mice of control, CUMS resilience, and CUMS-induced depression, where internal control is done with β-actin. Right panel shows VGAT expressions from the mice of control, CUMS resilience and CUMS-induced depression. (B) illustrates the normalized ratios of GAD67 and VGAT to β-actin from control mice (dark-gray bars, n = 8), resilience mice (gray, n = 4) and CUMS-induced depression mice (light-grays, n = 8). The relative ratios for control mice are normalized to be one. An asterisk presents p < 0.05, two asterisks denote p < 0.01, and three asterisks denote p < 0.001 (one-way ANOVA).

Figure 4. The ability to produce the sequential spikes on GABAergic neurons of the nucleus accumbens decreases in the depression-like mice, but not resilience

Sequential spikes induced by various stimulus intensities were recorded on GABAergic neurons in the nucleus accumbens in brain slices under current-clamp. (A) illustrates depolarization-induced the sequential spikes on GABAergic neurons from a CUMS-induced depression mouse. (B) illustrates depolarization-induced the sequential spikes on GABAergic neurons from a CUMS resilience mouse. (C) illustrates depolarization-induced the sequential spikes on GABAergic neurons from a control mouse. (D) illustrates spikes per second versus normalized stimuli in GABAergic neurons from the depression-like mice (red symbols, n = 25 cells), resilience mice (orange, n = 21 cells) and control mice (blue, n = 24 cells). Inserted figure shows spikes per second at 1.8 normalized stimuli from depression-like mice (red bar), resilience mice (orange) and control mice (blue), in which two asterisks indicate p < 0.01 and three asterisks are p < 0.001 (one-way ANOVA).

Figure 5. Excitatory synaptic transmission is downregulated in GABAergic neurons of the nucleus accumbens from CUMS-induced depression mice, but not resilience

sEPSCs were recorded under voltage-clamp in the brain slices from control, resilience and depression-like mice in presence of 10 mM bicuculline. (A) Left panel shows sEPSCs from a control mouse (blue traces), middle panel shows sEPSCs from a depression-like mouse (reds) and right panel shows sEPSCs from a resilience mouse (orange). Calibration bars are 6 pA in vertical bar as well as 3 seconds (top traces) and 100 milliseconds (bottoms) in horizontal. (B) shows cumulative probability versus sEPSC amplitudes from depression-like mice (red symbols), resilience mice (orange) and control mice (blue). Dash-lines indicate sEPSC amplitudes at cumulative probability to 67% (CP₆₇). (C) shows cumulative probability versus inter-sEPSC intervals from the depression-like mice (red symbols), resilience mice (orange) and control mice (blue). Dash-lines indicate sEPSC intervals at CP₆₇ in control (blue line; n = 23 cells), resilience (orange; n = 16 cells) and depression-like mice (red; n = 39 cells). The inserted figure shows a comparison of sEPSC interval at CP₆₇ from the mice of CUMS-induced depression (red bar), resilience (orange) and control (blue), in which an asterisk shows p < 0.05 and two asterisks show p < 0.01, one-way ANOVA).

Figure 6. Pathological changes at GABAergic neurons in the nucleus accumbens of CUMS-induced depression mice, compared with those in CUMS-resilience mice

Left panel shows the coronal section of the mouse brain including nucleus accumbens. Right-top panel shows the functional downregulation of GABAergic neurons, such as GABA release, neuronal excitability and excitatory input reception. Right-bottom panel illustrates the functional state of GABAergic neurons in the nucleus accumbens of CUMS-resilience mice.