Feeding, body weight, and sensitivity to non-ingestive reward stimuli during and after 12-day continuous central infusions of melanocortin receptor ligands.

The brain melanocortin system mediates downstream effects of hypothalamic leptin and insulin signaling. Yet, there have been few studies of chronic intracerebroventricular (i.c.v.) melanocortin receptor (MCR) agonist or antagonist infusion. Although there is evidence of interaction between melanocortin and dopamine (DA) systems, effects of chronic MCR ligand infusion on behavioral sensitivity to non-ingestive reward stimuli have not been investigated. The objective of this study was to investigate effects of chronic i.c.v. infusion of the MCR agonist, MTII, and the MCR antagonist, SHU9119, on food intake, body weight, and sensitivity to rewarding lateral hypothalamic electrical stimulation (LHSS) and the reward-potentiating (i.e., threshold-lowering) effect of D-amphetamine. The MCR antagonist, SHU9119 (0.02 microg/h) produced sustained hyperphagia and weight gain during the 12-day infusion period, followed by compensatory hypophagia and an arrest of body weight gain during the 24-day post-infusion period. At no point during the experiment was sensitivity to LHSS or D-amphetamine (0.25mg/kg, i.p.) altered. The MCR agonist, MTII (0.02 microg/h) produced a brief hypophagia (3 days) followed by a return to control levels of daily intake, but with body weight remaining at a reduced level throughout the 12-day infusion period. This was followed by compensatory hyperphagia and weight gain during the 24-day post-infusion period. There was no change in sensitivity to non-ingestive reward stimuli during the infusion of MTII. However, sensitivity to D-amphetamine was increased during the 24-day post-infusion period. It therefore seems that changes in ingestive behavior that occur during chronic MCR ligand infusion may not affect the response to non-ingestive reward stimuli. However, it is possible that the drive to re-feed and restore body weight following MCR agonist treatment includes neuroadaptations that enhance the incentive effects of drug stimuli.

Effects of the D(3) dopamine receptor antagonist, U99194A, on brain stimulation and d-amphetamine reward, motor activity, and c-fos expression in ad libitum fed and food-restricted rats.

RATIONALE: Previous studies indicate that the D(3) dopamine (DA) receptor is preferentially expressed in limbic forebrain DA terminal areas and may mediate functional effects opposite those of the D(1) and D(2) receptor types. However, the locations of the D(3) receptors that regulate behavior, and the range of behavioral functions regulated, are not clear. OBJECTIVE: The objective of this study was to evaluate behavioral and cellular effects of the preferential D(3) dopamine receptor antagonist, U99194A. METHODS: In experiment 1, the rewarding effect of U99194A (5.0, 10.0 and 20.0 mg/kg, SC) was measured in terms of its ability to lower the threshold for lateral hypothalamic self-stimulation (LHSS) in ad libitum fed rats. To amplify a possibly weak reward signal, testing was also conducted in food-restricted rats. The ability of U99194A to alter the threshold-lowering effect of d-amphetamine was also assessed. In experiment 2, effects of U99194A on horizontal and vertical motor activity were compared in ad libitum fed and food-restricted rats. In experiment 3, effects of a behaviorally active dose of U99194A (5.0 mg/kg) on brain c-fos expression were measured and compared to those produced by d-amphetamine (0.5 mg/kg, IP). In experiment 4, the motor and cellular activating effects of U99194A were challenged with the D(1) dopamine receptor antagonist, SCH-23390 (0.1 mg/kg). RESULTS: U99194A displayed no rewarding efficacy in the LHSS paradigm. U99194A did, however, augment the rewarding effect of d-amphetamine. U99194A also produced a motor activating effect, reversible by SCH-23390, which was greater in food-restricted than ad libitum fed rats. The pattern and intensity of fos-like immunoreactivity (FLI) induced by U99194A was similar to that produced by d-amphetamine and was blocked, in caudate-putamen and nucleus accumbens, by SCH-23390. CONCLUSIONS: These results indicate that U99194A has psychostimulant-like effects on motor activity and striatal c-fos expression that are dependent upon the D(1) DA receptor. However, doses of U99194A that are adequate to stimulate motor activity and c-fos expression in striatal and limbic structures do not possess direct rewarding effects in the LHSS paradigm. Overall, these results seem consistent with the hypothesis that D(3) antagonism enhances D(1)/D(2) mediated signaling with behavioral effects dependent on both the density of D(3) receptors and the prevailing level of DA transmission in particular brain regions.