Somatostatin release from rat hypothalamus in vitro: effects of melatonin and serotonin

The release of immunoreactive 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 heat-inactivated 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 10(--5)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.