Evidence of a central inhibition of growth hormone secretion by calcitonin gene-related peptide.

The effects of intracerebroventricular or intracarotid injection of synthetic rat calcitonin gene-related peptide (CGRP) on growth hormone (GH) secretion induced by various stimuli in male rats were examined. CGRP (2.5 or 5 micrograms/rat intracerebroventricularly, i.c.v.) was administered 10 min before beta-endorphin (0.5 microgram/rat i.c.v.) or morphine (1 mg/kg intracarotidly, i.a.) or clonidine (0.25 mg/kg i.a.) or GH-releasing hormone (GHRH1-40; 2 micrograms/kg i.a.). When injected peripherally, CGRP (10 micrograms/rat, i.a.) was administered 5 min before morphine or GHRH. To investigate the possible involvement of somatostatin (SRIF) in the inhibition of GH secretion by CGRP, the effect of the peptide on GHRH-induced GH release was also examined in rats with hypothalamic SRIF depletion obtained by pretreatment (4 h before) with cysteamine (300 mg/kg subcutaneously). Blood samples for hormone determination were drawn from freely moving rats at various times before and after drug treatment. The intracerebroventricular administration of CGRP (5 micrograms/rat) significantly inhibited GH secretion induced by all the stimuli used. In rats with SRIF depletion CGRP did not modify the stimulation of GH by GHRH. When CGRP was administered intracarotidly, even the dose of 10 micrograms/rat did not reduce the GH release induced by GHRH or morphine. The effects of intracerebroventricular CGRP on basal or beta-endorphin-induced prolactin release were also examined. When given intracerebroventricularly, the peptide did not modify prolactin secretion. The present results indicate that CGRP has a central inhibitory role in the control of GH secretion, probably through a stimulation of SRIF release.