Importance of norepinephrine alpha 2-receptor activation for morphine-induced rat growth hormone secretion

The normal pulsatile secretion of rat growth hormone (rGH) requires intact function in monoaminergic neurons. The importance of norepinephrine (NE) for the secretion is well documented, while the roles of dopamine (DA) and 5-hydroxytryptamine (5-HT) are still a matter of controversy. Morphine, as well as endogenous opioid peptides, are known to stimulate the secretion of GH. Whether the opiate-induced GH release is dependent on monoamines was investigated in the present study. Administration of morphine (10 mg/kg) resulted within 30 min in elevations of plasma rGH exceeding 40 ng/ml. This effect was almost completely antagonized by reserpine (10 mg/kg), given 5 h before morphine and partially antagonized by reserpine (2 mg/kg) administered 25 h before morphine. Administration of tetrabenazine (75 mg/kg) protects monoamine granules from irreversible destruction and counteracted the morphine antagonistic effects of reserpine (2 mg/dg) on rGH secretion indicating that the latter effect is due to blockade of monoaminergic neurotransmission. Pretreatment with either haloperidol (1 mg/kg), p-chlorophenylalanine (PCPA) (300 mg/kg x III) or phenoxybenzamine (10 mg/kg) did not reduce the effect of morphine on rGH release whereas yohimbine (3 mg/kg) effectively prevented it. In reserpine-pretreated animals administration of clonidine (0.5 mg/kg) potentiated the GH releasing effect of morphine. These results indicate that morphine-induced GH release is dependent on activation of postsynaptic NE receptors of the alpha 2-subtype.