Endocrine effects of centrally injected nociceptin in the rat

In the present study we investigated the mechanisms involved in the endocrine effect of nociceptin/orphanin FQ (OFQ) in the rat and the possible interaction between OFQ and morphine in the control of growth hormone (GH) secretion. The intracerebroventricular administration of OFQ (2.3 or 23 microg/rat, i.c.v.) in freely moving male rats caused an increase in the secretion of both GH and prolactin (PRL). The possible involvement of the catecholaminergic (CA) system was studied by administering OFQ to CA-depleted rats (rats given 200 mg/kg of alpha-methyl-p-tyrosine subcutaneously 2 h before the i.c.v. dose of OFQ). In these CA-depleted rats, administration of OFQ (23 microg/rat, i.c.v.) did not stimulate GH secretion, whereas it significantly enhanced PRL secretion. In rats anesthetized with ketamine, which induces a significant increase of GH, PRL and corticosterone secretion by activating the sympathetic tone, OFQ (23 microg/rat, i.c.v.) did not modify GH and corticosterone levels, whereas again it significantly potentiated PRL secretion. Overall these results indicate that CA system is involved in the stimulatory action of OFQ on GH but not on PRL secretion. In fact the stimulation of PRL, but not that of GH, was still evident after impairment of the CA system. Pretreatment with OFQ (23 microg/rat, i.c.v.) attenuated the GH secretion induced by morphine (1 mg/kg, given by intra-arterial injection), thus showing a negative interaction between OFQ and morphine in the control of GH secretion.