Altered sensitivity to rewarding and aversive drugs in mice with inducible disruption of cAMP response element-binding protein function within the nucleus accumbens

Abstract

The transcription factor cAMP response element-binding protein (CREB) within the nucleus accumbens (NAc) plays an important role in regulating mood. In rodents, increased CREB activity within the NAc produces depression-like signs including anhedonia, whereas disruption of CREB activity by expression of a dominant-negative CREB (mCREB, which acts as a CREB antagonist) has antidepressant-like effects. We examined how disruption of CREB activity affects brain reward processes using intracranial self-stimulation (ICSS) and inducible bitransgenic mice with enriched expression of mCREB in forebrain regions including the NAc. Mutant mice or littermate controls were prepared with lateral hypothalamic stimulating electrodes, and trained in the ICSS procedure to determine the frequency at which the stimulation becomes rewarding (threshold). Inducible expression of mCREB did not affect baseline sensitivity to brain stimulation itself. However, mCREB-expressing mice were more sensitive to the rewarding (threshold-lowering) effects of cocaine. Interestingly, mCREB mice were insensitive to the depressive-like (threshold-elevating) effects of the kappa-opioid receptor agonist U50,488. These behavioral differences were accompanied by decreased mRNA expression of G-protein receptor kinase-3 (GRK3), a protein involved in opioid receptor desensitization, within the NAc of mCREB mice. Disruption of CREB or GRK3 activity within the NAc specifically by viral-mediated gene transfer enhanced the rewarding impact of brain stimulation in rats, establishing the contribution of functional changes within this region. Together with previous findings, these studies raise the possibility that disruption of CREB in the NAc influences motivation by simultaneously facilitating reward and reducing depressive-like states such as anhedonia and dysphoria.

Figure 1.

Effect of inducible expression of mCREB on brain stimulation reward. a, Stimulating electrodes were located in the MFB. b, There were no differences between mCREB transgenic mice and controls in the minimal current (mean ± SEM) required for reliable ICSS at 3 weeks (n = 19–21) or 6 weeks (n = 7–10) after DOX discontinuation.

Figure 2.

Effect of inducible expression of mCREB on sensitivity to cocaine (COC) in the ICSS test 6 weeks after DOX discontinuation. a, b, Rate of responding as a function of stimulation frequency during preinjection baseline, and after COC (10 mg/kg, i.p.). COC induced parallel leftward shifts and reductions in ICSS thresholds in control (a) and mCREB (b) mice, but the effects were larger in the mutants. Data are from representative mice. c, Time course of COC effects (10 mg/kg, i.p.); drug effects were evident over the 1 h test period. d, When data are expressed as mean (±SEM) percentage of pretreatment threshold for this test period, COC had significant threshold-lowering effects at a lower dose in mCREB mice (2.5 mg/kg) than in controls (5.0 mg/kg). Furthermore, the threshold-lowering effects of 10 mg/kg COC were significantly larger in mCREB mice. *p < 0.05, **p < 0.01 for within-group comparisons with saline (SAL; 0.0 dose) treatment, ^p < 0.05, ^^p < 0.01 for between-genotype comparisons, n = 9 mice/group.

Figure 3.

Effect of inducible expression of mCREB on sensitivity to U50 in the ICSS test 6 weeks after DOX discontinuation. a, b, Rate of responding as a function of stimulation frequency during preinjection baseline, and after U50 (3.0 mg/kg, i.p.). U50 induced parallel rightward shifts and increases in ICSS thresholds in control mice (a), but had negligible effects in mCREB mice (b). Data are from representative mice. c, Time course of U50 effects (3.0 mg/kg, i.p.); drug effects were evident from 30 to 75 min of the 90 min test period. d, When data are expressed as mean (±SEM) percentage of pretreatment threshold for the 30–75 min period, U50 had significant threshold-elevating effects in control mice (3.0 and 5.5 mg/kg) without affecting mCREB mice. *p < 0.05, **p < 0.01 for within-group comparisons with saline (SAL; 0.0 dose) treatment; ^p < 0.05, ^^p < 0.01 for between-genotype comparisons, n = 10–12 mice/group.

Figure 4.

Effects of HSV-mediated transgene expression in the NAc shell on ICSS. Expression of mCREB and dnGRK3 reduced mean (±SEM) ICSS thresholds in 30 min test on days 3 and 5. Data are expressed as percentage pre-gene transfer baselines. *p < 0.05, n = 9–14 rats/group.