Effects of morphine on brain-stimulation reward thresholds in young and aged rats

Mesolimbic opioid systems are altered with aging; however, the effects of these changes on the rewarding actions of opioids have not been examined. The present experiment assessed differences in the responsiveness of brain reward pathways in young and aged rats to the effects of morphine using the brain-stimulation reward (BSR) model. Aged (24 months) and young (5 months) male F344/BNF1 rats were stereotaxically implanted with a bipolar stainless steel electrode into the lateral hypothalamic (LH) region of the medial forebrain bundle. Thresholds were determined using the rate-independent psychophysical method. Each animal was tested after the administration of saline or morphine at 0.5, 1, 2.5, 5 and 10 mg/kg doses. A significant difference in the mean baseline threshold between aged (99.8+/-6 microA) and young rats (149.1+/-14 microA) was observed. Although in both groups morphine lowered the BSR threshold, there were no significant differences between the groups except at the 10-mg/kg dose, the difference did approach significance. This study indicates that there are baseline differences in the rewarding threshold in the two groups, that morphine lowers the threshold in young and aged animals and that the hedonic effects produced by morphine, for the most, part remain preserved in aged animals.