Stress and Cocaine Trigger Divergent and Cell Type-Specific Regulation of Synaptic Transmission at Single Spines in Nucleus Accumbens

Abstract

Background: Repeated exposure to cocaine or social stress leads to lasting structural and functional synaptic alterations in medium spiny neurons (MSNs) of nucleus accumbens (NAc). Although cocaine-induced and stress-induced structural changes in dendritic spines have been well documented, few studies have investigated functional consequences of cocaine and stress at the level of single spines.

Methods: We exposed mice to chronic cocaine or chronic social defeat stress and used two-photon laser scanning microscopy with glutamate photo-uncaging and whole-cell recording to examine synaptic strength at individual spines on two distinct types of NAc MSNs in acute slices after 24 hours of cocaine withdrawal and after chronic social defeat stress.

Results: In animals treated with cocaine, average synaptic strength was reduced specifically at large mushroom spines of MSNs expressing dopamine receptor type 1 (D1-MSNs). In contrast, cocaine promoted a rightward shift in the distribution of synaptic weights toward larger synaptic responses in MSNs expressing dopamine receptor type 2 (D2-MSNs). After chronic social defeat stress, resilient animals displayed an upregulation of synaptic strength at large mushroom spines of D1-MSNs and a concomitant downregulation in D2-MSNs. Although susceptible mice did not exhibit a significant overall change in synaptic strength on D1-MSNs or D2-MSNs, we observed a slight leftward shift in cumulative distribution of large synaptic responses in both cell types.

Conclusions: This study provides the first functional cell type-specific and spine type-specific comparison of synaptic strength at a single spine level between cocaine-induced and stress-induced neuroadaptations and demonstrates that psychoactive drugs and stress trigger divergent changes in synaptic function in NAc.

Keywords: Chronic social defeat stress; Cocaine addiction; Dendritic spine; Glutamate uncaging; Nucleus accumbens; Synaptic transmission.

Figure 1

Uncaging-evoked uEPSCs at single mushroom and thin spines of D1- and D2-MSNs. (A) Schematic of a coronal cross-section of the mouse brain with Nissl stain. A red star depicts the middle of the shell region of NAc where all of the recordings were made. (B) A representative two-photon image of an individual MSN (medium spiny neuron) in the NAc shell. (C) A DIC (differential interference contrast) image of putative MSNs in NAc shell. (D) The DIC image is overlaid with a D2-GFP image, where green fluorescence points to a GFP⁺/putative D2-MSN (white arrow), while a cell lacking green fluorescence is a putative D1-MSN (white asterisk). (E) A segment of an MSN proximal dendritic branch showing representative mushroom spines (red) and thin spines (blue). (F) Representative uncaging-evoked responses from mushroom and thin spines. Arrows indicate the uncaging pulse.

Figure 2

Bidirectional effect of cocaine on evoked uEPSCs at single spines of D1- and D2-MSNs. (A) Schematic outlining the experimental timeline for cocaine administration and MNI-glutamate uncaging. (B) A summary plot of the effect of cocaine on the percent of responsive and unresponsive thin spines. (C, D) Representative traces with the summary of average data points of uncaging-evoked uEPSC amplitudes in mushroom and thin spines after saline or cocaine treatment in D1-MSNs (C) and D2-MSNs (D). (E) Cumulative frequency plot of uncaging-evoked uEPSCs from mushroom spines of D1-MSNs after a saline or a cocaine treatment. (F) Cumulative frequency plot of uncaging-evoked uEPSCs from mushroom spines of D2-MSNs after a saline or a cocaine treatment.

Figure 3

Bidirectional effect of stress on evoked uEPSCs at single spines of D1-MSNs and D2-MSNs. (A) Schematic outlining the experimental timeline for CSDS and uncaging experiment. (B) A summary plot of the effect of stress on the percent of responsive and unresponsive thin spines. (C, D) Representative traces with the summary of average data points of uncaging-evoked uEPSC amplitudes in mushroom and thin spines after CSDS in D1-MSNs (C) and D2-MSNs (D). (E) Cumulative frequency plot of uncaging-evoked uEPSCs from mushroom spines of D1-MSNs in control and post-CSDS conditions. (F) Cumulative frequency plot of uncaging-evoked uEPSCs from mushroom spines of D2-MSNs in control and post-CSDS conditions.