Stimulation of the LH release by naloxone in anaesthetized cats after ovariectomy

The effect of intravenous injections or infusions of the opioid receptor antagonist naloxone on the secretion of luteinizing hormone (LH) was studied in 18 spayed cats anaesthetized with Althesin. Effective injections significantly increased the LH concentration of plasma samples (taken every 10-15 min and measured by radio-immunoassay) to a peak 20-30 min after injection. The concentration thereafter declined exponentially (ke = 0.42), and, in 4/8 trials rose again significantly and declined again without further injection. The threshold dose was between 0.4 and 0.5 mg/kg. There did not appear to be a dose dependence of the effect above threshold. Infusion of naloxone at levels up to 5 mg/kg/h was effective in producing a pulsatile release of LH and repeated injections of threshold doses (0.5 mg/kg) could produce a maintained plateau and pulsatile release at frequencies comparable to pulse frequencies in vivo.