Interaction between nicotinic cholinergic receptors and alpha-2 adrenergic systems in regulating growth hormone secretion in conscious rats.

The possible role and mechanism of central nicotinic cholinergic receptors in regulating growth hormone (GH) secretion were investigated in conscious, unrestrained male rats. The administration of galanin (GAL), a neuropeptide, GH-releasing hormone (GHRH) and UK 14304, an alpha-2 adrenergic agonist, caused increases in plasma GH levels. Pretreatment of the animals with hexamethonium significantly reduced GAL- and GHRH-induced GH secretion, but UK 14304-induced GH release was not affected. Neosurugatoxin, a nicotinic cholinergic neurotoxin, partially inhibited GAL-induced GH release. Pretreatment with neostigmine elevated basal plasma GH levels and partially inhibited GAL-induced GH secretion. However, GHRH- and UK 14304-induced GH release was additive to the effect of neostigmine. Yohimbine inhibited GH release induced by UK 14304, GAL and GHRH. Neostigmine-induced GH secretion was partially inhibited by yohimbine and phenoxybenzamine. These findings suggest that central nicotinic receptors have a stimulatory role in regulating GH secretion and that the nicotinic cholinergic system coupled with the alpha-2 adrenergic mechanism is involved in GH secretion induced by galanin in conscious rats.