Inhibition of CaMKII in the nucleus accumbens shell decreases enhanced amphetamine intake in sensitized rats

Abstract

Microinjection of the calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 into the nucleus accumbens (NAcc) shell impairs expression of the sensitized locomotion and NAcc dopamine (DA) overflow normally observed in psychostimulant-exposed rats. Based on these results, we investigated the effect of NAcc shell KN-93 on the enhanced amphetamine (AMPH) intake normally observed in AMPH- relative to saline-exposed rats. Rats were administered five injections of either AMPH (1.5mg/kg, i.p.) or saline, one injection every 2-3 days. Fourteen days following the last injection, they were trained to self-administer AMPH (200 microg/kg/infusion, i.v.) first on fixed ratio schedules (FR) and then on a progressive ratio schedule of reinforcement (PR). As expected, AMPH-exposed rats worked harder and obtained significantly more drug infusions than saline-exposed rats on the PR schedule. After 4 days of stable responding, all rats were bilaterally microinjected with KN-93 (1 or 10 nmol/0.5 microl/side) into the NAcc shell, 2 min prior to the beginning of the self-administration session. Inhibiting CaMKII in this site reduced the enhanced drug intake observed in AMPH-exposed rats to levels no longer significantly different from those of saline-exposed rats. Responding in these latter controls was not affected by KN-93 nor did KN-93 affect responding in AMPH-exposed rats when it was infused into the NAcc core. Thus, in a manner similar to what has been reported for sensitized locomotion and NAcc DA overflow, these results suggest that inhibiting CaMKII in the NAcc shell attenuates the enhanced motivation to obtain a drug reinforcer that is normally displayed in AMPH-exposed rats.

Figure 1. Injection cannula tip placements in the NAcc shell and core

Black circles represent placements in the NAcc shell; open circles represent placements in the NAcc core. Line drawings are from Paxinos and Watson [13]. Numbers to the right indicate mm from bregma.

Figure 2. Previous exposure to AMPH enhances AMPH self-administration under a PR schedule of reinforcement

Data are shown as mean (±SEM) number of AMPH infusions obtained on each of the first 4 PR test days. The cumulative number of presses required to obtain these infusions is also shown. AMPH-exposed rats worked more and consequently obtained significantly more infusions compared to saline-exposed controls over all 4 PR test days. n=8–9/group.

Figure 3. Microinjection of KN-93 into the NAcc shell reduces the enhanced AMPH self-administration observed in sensitized rats

Data are shown as mean (+SEM) number of AMPH infusions obtained on PR test day 4 (PR4) and the 10 nmol/side KN-93 test day (KN-93). The cumulative number of presses required to obtain these infusions is also shown. Following the KN-93 microinjection, AMPH-exposed rats showed a significant reduction in drug intake compared to that observed on PR test day 4. Inhibiting CaMKII produced no effect in saline-exposed controls. n=8–9/group. ^*, P<0.05, significantly different from saline-exposed controls at PR test day 4. ^†††, P<0.001, significantly different from PR test day 4. INSET: Microinjection of 1.0 nmol/side KN-93 into the NAcc shell or 10 nmol/side KN-93 into the NAcc core had no effect on drug intake in AMPH-exposed rats. Data are shown as the difference in the number of infusions obtained on the KN-93 test day from that obtained on PR test day 4 in AMPH-exposed rats tested with KN-93 in the NAcc core (10 nmol/side; n=7), NAcc shell (1.0 nmol/side; n=5), and NAcc shell (10 nmol/side; n=9). ^†††, P<0.001, significantly different from PR test day 4.