Naloxone stimulation of luteinizing hormone release in prepubertal female rats; role of serotonergic system

The purpose of this study was to determine whether naloxone stimulated LH release via a serotonergic mechanism. Injection of naloxone hydrochloride (2 mg/kg B.W.) into 25-day old female prepubertal rats resulted in a significant elevation in serum LH 30 min later. Injection of this dose of naloxone together with morphine sulfate (2 or 5 mg/kg B.E.) resulted in inhibition of naloxone-induced LH release. When rats were first injected with 5-hydroxytryptophan (5-HTP) to increase hypothalamic serotonin content, naloxone failed to increase serum LH levels. On the other hand, when parachlorophenylalanine (PCPA) was given first to reduce hypothalamic serotonin content, naloxone-induced LH release was potentiated. Morphine failed to inhibit the naloxone-induced rise in serum LH when PCPA was first administered. Neither 5-HTP nor PCPA, when injected alone, altered serum LH values. These results suggest that naloxone promotes LH release by reducing hypothalamic serotonergic activity, and morphine inhibits LH release by increasing hypothalamic serotonergic activity. This does not exclude possible involvement of other neurotransmitters.