Multiple Factors Influencing the In Vitro Release of [Met5]-Enkephalin from Rat Hypothalamic Slices

This study examined several in vivo and in vitro factors which influence the release of Met5]-enkephalin (Met-ENK) from male rat hypothalamic slices superfused in vitro. Met-ENK release was significantly stimulated by corticotropin-releasing hormone (CRH; 10(-12)-10(-8) M), an effect which was abolished in the presence of the CRH-receptor antagonist, alpha-helical CRF9-41 (10(-6) M). The amount of Met-ENK release diminished with time in experiments in which the slices were continuously exposed to CRH. The opioid receptor antagonist naloxone (10(-6) M) stimulated Met-ENK release, even in the presence of the Na+ -channel blocker tetrodotoxin (10(-6) M), a result indicating presynaptic opioid feedback inhibition of Met-ENK release. The role of gonadal steroids in the control of Met-ENK release in vitro was also examined. It was found that the basal and CRH-induced release of Met-ENK was not changed 1 week after castration. However, a significant increase in the basal release of this peptide was observed 4 weeks after gonadectomy, and the Met-ENK-releasing efficacy of CRH was found to be reduced. The Met-ENK content of hypothalami from 1-week castrates was not significantly changed from control levels, but was significantly reduced in those from 4-week castrates. These long-term effects of castration could be overcome by the subcutaneous implantation of testosterone-containing capsules at the time of castration.