Somatostatin Release from Rat Hypothalamus in Vitro: Effects of Melatonin and Serotonin*

Abstract

The release of immunorective somatostatin (SRIF) from explants of rat medial basal hypothalamus, which were maintained in culture for 24 h, was quantitated by a sensitive RIA. Validity of the specific SRIF assay has been previously established by chromatographic criteria using gel and high pressure systems and by the demonstration of immunological parallelism. After 24 h of culture in medium containing heatinactivated fetal calf serum, hypothalamic fragments were incubated in serum-free medium, and the release of immunoreactive SRIF was quantitated. Melatonin at concentrations of 10^-8 and 10^-7 M stimulated SRIF release, and no significant increases were observed at concentrations of 10^-9 M or less or at concentrations of 10^-6 M or greater. Serotonin oxalate at concentrations of 10^-8-10^-5 M significantly inhibited SRIF release. The serotonin antagonist cyproheptadine at a concentration of 1CT5 M had no effect on basal SRIF release but abolished the inhibitory effect of 10^-7 M serotonin. Finally, when hypothalami were incubated with melatonin and serotonin, each at 10^-7 M, SRIF release was unchanged compared to control values. The results suggest that the brain indoleamines, melatonin and serotonin, may modulate GH secretion by effects on SRIF release at a hypothalamic level.