Transient overexpression of alpha-Ca2+/calmodulin-dependent protein kinase II in the nucleus accumbens shell enhances behavioral responding to amphetamine

Abstract

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is known to contribute to the expression of psychostimulant sensitization by regulating dopamine (DA) overflow from DA neuron terminals in the nucleus accumbens (NAcc). The present experiments explored the contribution of CaMKII in NAcc neurons postsynaptic to these terminals where it is known to participate in a number of signaling pathways that regulate responding to psychostimulant drugs. Exposure to amphetamine transiently increased alphaCaMKII levels in the shell but not the core of the NAcc. Thus, HSV (herpes simplex viral) vectors were used to transiently overexpress alphaCaMKII in NAcc neurons in drug-naive rats, and behavioral responding to amphetamine was assessed. Transiently overexpressing alphaCaMKII in the NAcc shell led to long-lasting enhancement of amphetamine-induced locomotion and self-administration manifested when alphaCaMKII levels were elevated and persisting long after they had returned to baseline. Enhanced locomotion was not observed after infection in the NAcc core or sites adjacent to the NAcc. Transient elevation of NAcc shell alphaCaMKII levels also enhanced locomotor responding to NAcc AMPA and increased phosphorylation levels of GluR1 (Ser831), a CaMKII site, both soon and long after infection. Similar increases in pGluR1 (Ser831) were observed both soon and long after exposure to amphetamine. These results indicate that the transient increase in alphaCaMKII observed in neurons of the NAcc shell after viral-mediated gene transfer and likely exposure to amphetamine leads to neuroadaptations in AMPA receptor signaling in this site that may contribute to the long-lasting maintenance of behavioral and incentive sensitization by psychostimulant drugs like amphetamine.

Figure 1.

Exposure to amphetamine increases αCaMKII and pGluR1 in NAcc shell. Rats were exposed to amphetamine (AMPH) or saline (SAL) and killed 4 d or 2–3 weeks later. Protein levels obtained in the NAcc shell or core are shown as group mean (+SEM) percentage change from controls with representative immunoblots. In the NAcc shell, amphetamine exposure produced a transient increase in αCaMKII (A, B) and pGluR1 (Ser845) (E, F). Long-lasting increases in pGluR1 (Ser831) were observed in this site (C, D). No significant effects were detected in the NAcc core. *p < 0.05, **p < 0.01, AMPH-exposed (black bars) versus SAL-exposed (cross-hatched bars). n = 4–8/group.

Figure 2.

Viral-mediated transient overexpression of αCaMKII in the NAcc shell. A, Western blots of αCaMKII obtained 4 d, 8 d, and 2–3 weeks postinfection (PI). αCaMKII was maximally and only elevated 4 d PI. Protein levels are expressed as group mean (+SEM) percentage change from controls (n = 3–6/group). *p < 0.05, αCaMKII (black bars) versus control (cross-hatched bars). The schema illustrates the components of the HSV amplicon used to overexpress αCaMKII. B, Photomicrographs of the NAcc obtained 4 d after infection with HSV-αCaMKII-GFP (top) or HSV-LacZ (bottom) illustrating GFP- or β-galactosidase-positive neurons in close proximity to the injection cannula tips (arrows).

Figure 3.

Transient αCaMKII overexpression in the NAcc shell enhances locomotor responding to amphetamine. A, Data are shown as group mean (+SEM) 2 h total locomotor counts observed after the amphetamine injections on the different test days postinfection (n/group = 7–10). *p < 0.05; **p < 0.01; ***p < 0.001, significantly different from controls at specified day. B, Time course of the enhanced locomotor response to amphetamine observed 4 and 34 d after infection. Data are shown as group mean locomotor counts (±SEM) before and after the amphetamine injection (arrows at abscissas). C, Location of the injection cannula tips for rats included in the data analyses. The symbols refer to group affiliation (○, controls; •, αCaMKII). The line drawings are from the work by Paxinos and Watson (1997). The numbers to the right indicate millimeters from bregma.

Figure 4.

Transient αCaMKII overexpression in the NAcc shell enhances locomotor responding to NAcc AMPA. Data are shown as group mean locomotor counts (±SEM) before and after NAcc AMPA 4 d (A) or 2–3 weeks after infection (B). *p < 0.05, HSV-αCaMKII versus control. n/group = 7–13. The insets show group mean (+SEM) 2 h total locomotor counts obtained after AMPA for each day. C, Location of the injection cannula tips for rats included in the data analyses illustrated as in Figure 3.

Figure 5.

Transient αCaMKII overexpression in the NAcc shell enhances amphetamine self-administration. A, Data are shown as group mean (+SEM) number of amphetamine infusions obtained averaged over the first 4 d before infection, on the fourth day after infection (when protein levels were maximal), and averaged over days 4–14 after infection. The cumulative number of presses required to obtain these infusions is also shown. *p < 0.05, significantly different from controls. n/group = 7–9. B, Time course for amphetamine self-administration on the 4 d before infection (arrow) and the 14 d after infection. Data are shown as group mean (±SEM) number of amphetamine infusions obtained. C, Location of the injection cannula tips for rats included in the data analyses illustrated as in Figure 3.

Figure 6.

Transient αCaMKII overexpression increases pGluR1 in the NAcc shell. Protein levels and representative immunoblots obtained 4 d or 2–3 weeks after infection are shown as in Figure 1. Long-lasting increases in pGluR1 (Ser831) (A) were observed. Levels of pGluR1 (Ser845) were not significantly increased either soon or long after infection (B). *p < 0.05, HSV-αCaMKII (black bars) versus control (cross-hatched bars). n/group = 5–9.