Acute and chronic cocaine-induced potentiation of synaptic strength in the ventral tegmental area: electrophysiological and behavioral correlates in individual rats

Abstract

The initiation of the psychostimulant sensitization process depends on the mesolimbic system, which projects from the ventral tegmental area (VTA) to the nucleus accumbens. Although such initiation is primarily dependent on glutamatergic activity in VTA neurons, the exact role VTA excitatory synapses play in this process is poorly understood. Here, we examine the effects of repeated in vivo injections of cocaine on the magnitude and duration of the increase in strength at VTA excitatory synapses reported previously to be elicited by a single in vivo exposure to cocaine (Ungless et al., 2001; Saal et al., 2003). We also compare the synaptic modifications induced by cocaine with its effects on locomotor activity. Surprisingly, repeated cocaine exposure potentiated the ratio of AMPA receptor-mediated to NMDA receptor-mediated EPSCs to a similar extent and duration as a single in vivo cocaine exposure. In naive animals, the magnitude of the cocaine-induced locomotor activity after a single injection of cocaine correlated with the magnitude of the accompanying synaptic enhancement. This correlation was lost on the seventh day of repeated cocaine administration, as well as when a challenge injection was given 10 d after the cessation of repeated cocaine administration. These results suggest that the cocaine-induced synaptic plasticity at VTA excitatory synapses is transient, and its duration depends on the last exposure to cocaine. Furthermore, chronic cocaine exposure disrupts the normal, presumably adaptive relationship between synaptic enhancement in the VTA and behavior.

Figure 1.

Multiple injections of cocaine potentiate AMPAR/NMDAR ratio to a similar extent as a single injection of cocaine. A, Diagram of injection protocol for single or multiple injections. B, Example recordings of evoked NMDA and AMPA EPSCs from VTA neurons of rats 24 hr after treatment with seven consecutive injections of cocaine or saline. C, Rats were given a single injection of cocaine (15 mg/kg; dark shaded bar) or saline (light shaded bar), multiple injections of cocaine (filled bars) or saline (open bars), or no treatment (dotted bars). The AMPAR/NMDAR ratio was taken as the peak amplitude of AMPAR current over peak amplitude of NMDAR current. AMPAR/NMDAR ratio of cells from rats treated with a single injection (n = 19) or multiple injections (n = 30) of cocaine was significantly larger than saline (n = 17; p < 0.05; n = 27; p < 0.01, respectively) or naive (n = 8) controls. Bars represent mean AMPAR/NMDAR ratio and SEM.

Figure 2.

Potentiation of AMPAR/NMDAR ratio after multiple injections of cocaine is attributable to increases in AMPA current. A, Bath application of AMPA (10 μm) with cyclothiazide (100 μm) for 30 sec elicited a greater inward current (10 sec bins) in chronic cocaine-treated animals (n = 4) than in saline-treated animals (n = 5; p < 0.05). B, In neurons voltage-clamped at 40 mV, bath application of NMDA (10 μm) for 30 sec elicited similar current (10 sec bins) in both chronic cocaine- and saline-treated animals (n = 5 and 6; p < 0.05). C, Maximal current change after 30 sec AMPA or NMDA application from chronic cocaine- or saline-treated rats. Bars represent means and SEM.

Figure 3.

Multiple injections of cocaine potentiate AMPAR/NMDAR ratio lasting for 5 but not 10 d. A, Diagram of injection protocol for single or multiple injections. Rats were treated with single (B) or multiple (C) injections of saline (open bars) or cocaine (filled bars) as previously described. Recordings of AMPAR/NMDAR ratio made 5 d after the final cocaine injection (single injection, n = 9; multiple injections, n = 8) were significantly larger than the saline controls (^*p < 0.05; single, n = 6; ^**p < 0.01; multiple, n = 10). However, AMPAR/NMDAR ratio 10 d after the final cocaine injection (n = 10) was not significantly different from that of the saline controls (n = 10). Bars represent mean AMPAR/NMDAR ratio and SEM.

Figure 4.

Locomotor activity positively correlates with potentiated AMPAR/NMDAR ratio after a single injection of cocaine. Rats were monitored for locomotor activity for 60 min immediately after a single cocaine (15 mg/kg) or saline injection. A, B, Distance traveled (A) and stereotypic counts (B) increased after cocaine (n = 11) compared with saline (n = 10; p < 0.01). Bars represent mean AMPAR/NMDAR ratio and SEM. C, D, Distance traveled (C) and stereotypic counts (D) correlated positively with the AMPAR/NMDAR ratio of cocaine-treated rats (distance traveled, r = 0.85; p < 0.01; n = 8; stereotypic counts, r = 0.62; p = 0.1; n = 8; filled squares). E, F, In saline-treated rats, distance traveled (E) and stereotypy (F) did not correlate with AMPAR/NMDAR ratio (distance traveled, r = 0.35; n = 5; stereotypy, r = -0.27; n = 5; open squares). The mean of AMPAR/NMDAR ratios per rat was compared with its locomotor activity.

Figure 5.

After multiple injections of cocaine, resulting in sensitization, locomotor activity no longer correlates with AMPAR/NMDAR ratio. Rats were monitored for locomotor activity on the first (day 1) and the last (day 7) day of injection regimen. A, B, Distance traveled (A) (^*p < 0.05) and stereotypic counts (B) (^**p < 0.01) were greater on days 1 and 7 in cocaine-treated rats compared with saline-treated rats (cocaine, n = 11; saline, n = 12; p < 0.01). C, D, Correlation of distance traveled (C) and stereotypic counts (D) on the last injection day (day 7) and AMPAR/NMDAR ratio in rats treated with multiple injections of cocaine (n = 11). E, F, Correlation of the relative sensitization, the ratio of distance traveled (E) and stereotypic counts (F) on days 7 and 1, and AMPAR/NMDAR ratio in cocaine-treated rats (n = 11). G, H, Correlation of distance traveled (G) and stereotypic counts (H) on day 7, and AMPAR/NMDAR ratio in saline-treated rats (n = 12). Bars represent mean AMPAR/NMDAR ratio and SEM. The mean of AMPAR/NMDAR ratios per rat was compared with its locomotor activity.

Figure 6.

After 10 d withdrawal from multiple cocaine injections, a challenge injection of cocaine reinstates potentiated AMPAR/NMDAR ratio in sensitized rats. A, Diagram of injection regimen. Distance traveled was measured on the first (day 1) and last (day 7) days of the multiple injection regimen, and then distance traveled was measured again immediately after a challenge injection of cocaine (15 mg/kg) or saline 10 d later (day 17). AMPAR/NMDAR ratios were measured 24 hr after the challenge injection (day 18). B, Rats pretreated with cocaine and given a cocaine challenge injection (filled bar; n = 12) and rats pretreated with saline and given a cocaine challenge injection (light shaded bar; n = 9) had potentiated AMPAR/NMDAR ratios compared with rats pretreated with cocaine and given a saline challenge injection (dark shaded bar; n = 7) and rats pretreated with saline and given a saline challenge injection (open bar; n = 11). C, Rats pretreated with multiple injections of cocaine and a challenge injection of cocaine after 10 d of withdrawal (filled circles; n = 6), had elevated locomotor activity compared with rats pretreated with saline and challenged with a cocaine injection (open circles; n = 8; p < 0.01). Rats pretreated with cocaine did not show enhanced locomotor response to a saline challenge (filled squares; n = 6). Rats pretreated with saline and given a challenge injection of saline (open squares; n = 8) represented baseline locomotor activity. D, Locomotor activity on day 17 in rats pretreated with cocaine receiving a cocaine (filled bar; n = 6) or saline (dark shaded bar; n = 6) challenge injection and rats pretreated with saline receiving a cocaine (light shaded bar; n = 8) or saline (open bar; n = 8) challenge injection. Bars represent mean cumulative distance traveled and SEM. p values were calculated using a two-way ANOVA.

Figure 7.

AMPAR/NMDAR ratios correlate with locomotor activity measured after a cocaine challenge injection in saline-pretreated rats but not in cocaine-pretreated rats. Rats were pretreated for 7 d with saline or cocaine and given a saline or cocaine (15 mg/kg) challenge injection after 10 d of withdrawal. Circles represent the correlate of distance traveled, and squares represent the correlate of stereotypic counts measured immediately after challenge injection to AMPAR/NMDAR ratio recorded 24 hr later. A, B, Correlation of distance traveled (A) (r = 0.75; p < 0.05; n = 12) and stereotypic counts (B) (r = 0.63; p < 0.05; n = 12) and AMPAR/NMDAR ratio in saline-pretreated rats given a cocaine challenge injection. C, D, Correlation of distance traveled (C) [n = 9 (1 point is occluded); r = -0.16] or stereotypic counts (D) (n = 9; r = 0.05) and AMPAR/NMDAR ratio in cocaine-pretreated rats given a cocaine challenge injection. The mean of AMPAR/NMDAR ratios per rat was compared with its locomotor activity.