Food restriction increases NMDA receptor-mediated calcium—calmodulin kinase II and NMDA receptor/extracellular signal-regulated kinase 1/2-mediated cyclic amp response element-binding protein phosphorylation in nucleus accumbens upon D-1 dopamine receptor stimulation in rats

Abstract

Biological drive states exert homeostatic control in part by increasing the reinforcing effects of environmental incentive stimuli. An apparent by-product of this adaptive response is the enhanced acquisition of drug self-administration behavior in food-restricted (FR) animals. While previous research has demonstrated increased central sensitivity to rewarding effects of abused drugs and direct dopamine (DA) receptor agonists in FR subjects, the underlying neurobiology is not well understood. Recently, it was demonstrated that intracerebroventricular (i.c.v.) injection of the D-1 DA receptor agonist, SKF-82958 produces a stronger activation of striatal extracellular signal-regulated kinase (ERK) 1/2 and cyclic AMP response element-binding protein (CREB) in FR relative to ad libitum (AL) fed rats. The main purpose of the present study was to characterize the involvement and mechanisms of interaction between NMDA receptor function and the augmented cellular responses to D-1 DA receptor stimulation in nucleus accumbens (NAc) of FR rats. In experiment 1, Western immunoblotting was used to demonstrate that i.c.v. injection of SKF-82958 (20 μg) produces greater phosphorylation of the NMDA NR1 subunit and calcium-calmodulin kinase II (CaMK II) in NAc of FR as compared with AL rats. In experiment 2, pretreatment of subjects with the NMDA antagonist, MK-801 (1.0 mg/kg, i.p.) decreased SKF-82958-induced activation of CaMK II, ERK1/2 and CREB, and reversed the augmenting effect of FR on activation of all three proteins. In experiment 3, pretreatment with the mitogen-activated protein kinase/ERK kinase inhibitor SL-327 (60 mg/kg, i.p.) suppressed SKF-82958- induced activation of ERK1/2 and reversed the augmenting effect of FR on CREB activation. These results point to specific neuroadaptations in the NAc of FR rats whereby D-1 DA receptor stimulation leads to increased NMDA NR1 subunit phosphorylation and consequent increases in NMDA receptor-dependent CaMK II and ERK1/2 signaling, and increased NMDA receptor/ERK1/2-dependent phosphorylation of the nuclear transcription factor, CREB. The upregulated cellular responses to D-1 DA agonist challenge may play a role in the augmentation of drug reward and appetitive instrumental learning during periods of food restriction.