Neurotransmitter involvement in naloxone-induced stimulation of pulsatile LH release on day 8 of pregnancy in the rat

Naloxone, an endogenous opioid peptide (EOP) receptor antagonist, increases LH pulse frequency and amplitude in early gestation in the rat (7). The object of this study was to further explore the suppression of pulsatile LH release by EOPs on day 8 of pregnancy by examining whether inhibition of norepinephrine (NE) or epinephrine (EPIN) synthesis, or stimulation of μ-aminobutyric acid (GABA)-B receptors, modified the ability of naloxone infusion (0.5 mg/kg/hr for 3.5 hr) to stimulate pulsatile LH secretion. Blood sampling (50 μl whole blood/5 min) began 0.5 hr after the onset of infusion. Three studies were conducted. 1) LY 134046 (PNMT inhibitor, 50 mg/kg IP), given at − 27, − 20, and − 3 hr relative to the onset of a 3-hr blood sampling period, produced no change in hypothalamic-preoptic area (HPOA) levels of NE, and a 76% decline in HPOA-EPIN levels, as determined by HPLC. Although basal LH pulse frequency was reduced, this treatment had no effect on the stimulatory action of naloxone on pulsatile LH release. 2) FLA-63 (DBH inhibitor, 25 mg/kg IP) given at − 3 hr produced a 72% decline in HPOA-NE levels, a 44% decrease in HPOA-EPIN values, and blocked the stimulatory action of naloxone on LH pulse frequency. Since depletion of HPOA-EPIN by LY 134046 did not compromise the LH response to naloxone, this effect of FLA-63 is due to depletion of HPOA-NE levels. 3) While saline had no effect on the increased pulsatile LH release caused by naloxone, administration of baclofen (GABA-B receptor agonist, 6 mg/kg IV) suppressed the pulsatile LH secretory response to naloxone infusion. These data demonstrate that inhibition of NE but not EPIN synthesis, or stimulation of GABA-B receptors, are both capable of interfering with naloxone-induced stimulation of pulsatile LH release on day 8 of pregnancy in the rat.