Mechanism of opiate-induced release of GH in the rat

Morphine (M) is known to be a powerful releaser of both growth hormone (GH) and prolactin (PRL) in the rat. More recently, endogenous opiate peptides such as β-endorphin (E) have also been found to stimulate the release of both hormones. Although the mechanism of the release is unknown, it clearly involves the central nervous system (CNS). We have recently performed several experiments to verify whether CNS serotonergic pathways are implicated in such an effect. Pretreatment of rats with the serotonin (SER) depletor PCPA significantly enhanced GH release induced by M and by E, while PRL release was unaffected. The SER precursor 5-hydroxytryptophan antagonized M-induced GH release. However, acute treatment with E, or chronic treatment with M did not modify the activity of raphe nuclei tryptophan hydroxylase. The hypothalamic hormone TRH was capable of antagonizing M- and E- induced GH release through a CNS serotonergic pathway. It is suggested that opiates stimulate GH release by suppression of an inhibitory serotonergic tonus.