Endogenous dopamine and endocannabinoid signaling mediate cocaine-induced reversal of AMPAR synaptic potentiation in the nucleus accumbens shell

Abstract

Repeated exposure to drugs of abuse alters the structure and function of neural circuits mediating reward, generating maladaptive plasticity in circuits critical for motivated behavior. Within meso-corticolimbic dopamine circuitry, repeated exposure to cocaine induces progressive alterations in AMPAR-mediated glutamatergic synaptic transmission. During a 10-14 day period of abstinence from cocaine, AMPAR signaling is potentiated at synapses on nucleus accumbens (NAc) medium spiny neurons (MSNs), promoting a state of heightened synaptic excitability. Re-exposure to cocaine during abstinence, however, rapidly reverses and depotentiates enhanced AMPAR signaling. To understand how re-exposure to cocaine alters AMPAR synaptic transmission, we investigated the roles of dopamine and endocannabinoid (eCB) signaling in modifying synaptic strength in the NAc shell. Using patch-clamp recordings from NAc slices prepared after 10-14 days of abstinence from repeated cocaine, we found that AMPAR-mediated depotentiation is rapidly induced in the NAc shell within 20 min of cocaine re-exposure ex vivo, and persists for up to five days before synapses return to levels of potentiation observed during abstinence. In cocaine-treated animals, global dopamine receptor activation was both necessary and sufficient for the cocaine-evoked depotentiation of AMPAR synaptic function. Additionally, we identified that CB1 receptors are engaged by endogenous endocannabinoids (eCBs) during re-exposure to cocaine ex vivo. Overall, these results indicate the central role that dopamine and eCB signaling mechanisms play in modulating cocaine-induced AMPAR plasticity in the NAc shell.

Keywords: AMPA receptor; Cocaine; Dopamine; Endocannabinoid; Nucleus accumbens; Synaptic plasticity.

Figure 1.. Cocaine-induced potentiation of AMPAR synaptic transmission in the NAc shell is reversed by in vivo cocaine challenge and persists for up to 4 days post challenge.

(A) Experimental timeline. Following 5 days of saline or cocaine (5 mg/kg; i.p.) injections and a 10-14 d abstinence period, animals received a single challenge injection of cocaine. Electrophysiological recordings were performed either during abstinence, 24 h following a challenge injection, or 5 d following a challenge injection. (B) Representative miniature excitatory postsynaptic current (mEPSC) traces from NAc shell neurons from saline + no challenge (Sal), cocaine + no challenge (Coc), cocaine + cocaine challenge at 24h (Coc – 24h), and cocaine + cocaine challenge at 5d (Coc – 5d); scale bars, 20 pA/200 ms. (C) Mean mEPSC amplitude (pA; left) and frequency (Hz; right) in the NAc shell from saline + no challenge (Sal, white; n=7, N=3), cocaine + no challenge (Coc, gray; n=8, N=5), cocaine + cocaine challenge at 24h (Coc-coc 24h, black; n=9, N=3); cocaine + cocaine challenge at 5d (Coc-coc 5d, green; n=9, N=3). (D) Cumulative probability distributions of mEPSC amplitude (pA; left) and inter-event interval (ms; right) from saline + no challenge (Sal, medium gray), cocaine + no challenge (Coc-sal, light gray), cocaine + cocaine challenge at 24h (Coc-coc 24h, black), cocaine + cocaine challenge at 5d (Coc-coc 5d, green). All data are presented as mean ± SEM. n, number of cells; N, number of animals. * p≤0.05 vs. Coc, ** p≤0.01 vs Coc; # p≤0.05 vs Coc-coc 5d, ## p≤0.01 vs Coc-coc 5d.

Figure 2.. Ex vivo cocaine re-exposure induces depotentiation of AMPAR signaling in the NAc shell that is maximally expressed within 20 minutes post challenge.

(A) Experimental timeline. Following 10-14 days of abstinence from repeated saline or cocaine (5 mg/kg; i.p.) injections, electrophysiological recordings were performed in acute slices receiving 1) no bath challenge (ACSF only) or 2) ex vivo cocaine (10 μM; 10 min), with recordings taking place at 20, 40, or 60 min following the initial 10 min bath challenge. (B) Representative miniature excitatory postsynaptic current (mEPSC) traces from NAc shell neurons from saline + no challenge (Sal), cocaine + no challenge (Coc), cocaine + cocaine challenge at 20 min (Coc-coc + 20), cocaine + cocaine challenge at 40 min (Coc-coc + 40), and cocaine + cocaine challenge at 60 min (Coc-coc + 60). (C) Mean mEPSC amplitude (pA; left) and frequency (Hz; right) in the NAc shell from cocaine + no challenge (Coc, gray; n=23, N=19), cocaine + cocaine 20 min post challenge (Coc-coc 20, green; n=7, N=7), cocaine + cocaine 40 min post challenge (Coc-coc 40, green; n=6, N=6), cocaine + cocaine 60 min post challenge (Coc-coc 60, green; n=6, N=6). (D) Cumulative probability distributions of mEPSC amplitude (pA; left) and inter-event interval (ms; right) from cocaine + no challenge (Coc-sal, gray), cocaine + cocaine 20 min post challenge (Coc-coc + 20, light green), cocaine + cocaine 40 min post challenge (Coc-coc + 40, medium green), cocaine + cocaine 60 min post challenge (Coc-coc + 60). All data are presented as mean ± SEM. n, number of cells; N, number of animals. ** p≤0.01 vs. Coc, *** p≤0.001 vs Coc.

Figure 3.. Dopamine receptor activation is required for depotentiation of NAc shell AMPAR signaling induced by ex vivo cocaine re-exposure.

(A) Experimental timeline. Following 10-14 days of abstinence from repeated saline or cocaine (5 mg/kg; i.p.) injections, electrophysiological recordings were performed in acute slices receiving 1) no bath challenge (ACSF only); 2) ex vivo cocaine challenge (10 μM; 10 min); 3) ex vivo bath application of the dopamine receptor antagonist flupenthixol (20 μM; 10 min) followed by 10 μM cocaine + flupenthixol (10 min); or 4) ex vivo bath application of the dopamine receptor agonist apomorphine (1 μM). (B) Representative miniature excitatory postsynaptic current (mEPSC) traces from NAc shell neurons from saline + no challenge (Sal), cocaine + no challenge (Coc-sal), cocaine + cocaine challenge (Coc-coc), cocaine + flupenthixol/cocaine (Coc-Flp-coc), cocaine + apomorphine (Coc-Apo). C) Mean mEPSC amplitude (pA; left) and frequency (Hz; right) in the NAc shell from saline + no challenge (Sal, white; n=8; N=6), cocaine + no challenge (Coc, gray; n=10, N=11), cocaine + cocaine challenge (Coc-coc, red; n=9, N=7), cocaine + flupenthixol/cocaine (Coc-Flp-coc, blue; n=7, N=5), cocaine + apomorphine (Coc-Apo, black; n=6, N=4). (D) Cumulative probability distributions of mEPSC amplitude (pA; left) and inter-event interval (ms; right) from cocaine + no challenge (Coc-sal, gray), cocaine + cocaine challenge (Coc-coc, red), cocaine + flupenthixol/coc (Coc-Flp-coc, blue), cocaine + apomorphine (Coc-Apo, black). All data are presented as mean ± SEM. n, number of cells; N, number of animals. * p≤0.05 vs. Coc, ** p≤0.01 vs. Coc, *** p≤0.001 vs Coc; # p≤0.06 vs. Coc-Flp-coc, ## p≤0.01 vs. Coc-Flp-coc.

Figure 4.. Activation of CB1 receptors is required for reversal of NAc shell AMPAR potentiation induced by ex vivo cocaine re-exposure.

(A) Experimental timeline. Following 10-14 days of abstinence from repeated saline or cocaine (5 mg/kg; i.p.) injections, electrophysiological recordings were performed in acute slices receiving 1) no bath challenge (ACSF only); 2) ex vivo cocaine challenge (10 μM; 10 min); 3) ex vivo bath application of the CB1 receptor antagonist SR141716A (1 μM; 10 min) followed by 10 μM cocaine + SR141716A (10 min); or 4) ex vivo bath application of CB1 receptor agonist WIN 55,212-2 (1 μM). (B) Representative miniature excitatory postsynaptic current (mEPSC) traces from NAc shell neurons from saline + no challenge (Sal), cocaine + no challenge (Coc-sal), cocaine + cocaine challenge (Coc-coc), cocaine + SR141716A/cocaine (Coc-SR-coc), cocaine + WIN 55,212-2 (Coc-Win). C) Mean mEPSC amplitude (pA; left) and frequency (Hz; right) in the NAc shell from saline + no challenge (Sal, white; n=8, N=7), cocaine + no challenge (Coc, gray; n=12, n=9), cocaine + cocaine challenge (Coc-coc, red; n=10, N=6), cocaine + SR141716A/cocaine (Coc-SR-coc, blue; n=15, N=9), cocaine + WIN 55,212-2 (Coc-Win, black; n=8, N=4). (D) Cumulative probability distributions of mEPSC amplitude (pA; left) and interevent interval (ms; right) from cocaine + no challenge (Coc-sal, gray), cocaine + cocaine challenge (Coc-coc, red), cocaine + SR141716A/coc (Coc-SR-coc, blue), cocaine + WIN 55,212-2 (Coc-Win, black). All data are presented as mean ± SEM. n, number of cells; N, number of animals. * p≤0.05 vs. Coc, ** p≤0.01 vs. Coc, *** p≤0.001 vs Coc, **** p≤0.0001; # p≤0.06 vs. Coc-SR-coc, ## p≤0.01 vs. Coc-SR-coc.