Involvement of Hypothalamic Growth Hormone (GH)-Releasing Factor in GH Secretion Induced by Intracerebroventricular Injection of Somatostatin in Rats*

Intracerebroventricular (icv) injection of somatostatin-14 (SRIF-14, 5 micrograms/rat) caused an increase in plasma GH in urethane-anesthetized rats and in conscious freely moving rats. Antiserum specific for rat GH-releasing factor (GRF) (0.5 ml/rat, iv) blunted GH release induced by SRIF-14 in these animals. The antiserum also suppressed spontaneous GH surges in conscious rats. In contrast, GH release induced by prostaglandin E2 (5 micrograms/100 g BW, iv) was not affected by the antiserum. SRIF-14 (5 micrograms/rat, icv) also raised plasma GH levels in conscious rats during the constant iv infusion of SRIF-14 (55 ng/55 microliter X min) which suppressed spontaneous GH secretion. Neither plasma TSH levels nor TSH release induced by a TRH analog were affected by icv injection of SRIF-14. These results suggest that the central stimulating effect of SRIF-14 on rat GH secretion is mediated, at least in part, by hypothalamic GRF and not due to a direct action on the pituitary.