Intracellular recordings were made from 48 hypothalamic arcuate (ARC) neurons under current- and voltage-clamp in slices prepared from female guinea pigs which had been ovariectomized and pretreated with estradiol. Twenty ARC neurons were silent (RMP: -62 +/- 2 mV) and 28 cells were spontaneously active (7.3 +/- 1.1 Hz; threshold -57 +/- 1 mV). The input resistance (Rin), determined in the potential range between -60 and -80 mV, was 358 +/- 30 M omega (n = 38) and ARC neurons showed inward rectification at potentials negative to the equilibrium potential for potassium. The selective mu-opioid agonist Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO) was applied by pressure pipette application at concentrations of 10 or 20 microM. DAGO decreased spontaneous firing and it hyperpolarized 26 of 31 neurons (9.6 +/- 0.8 mV; range 3-21 mV). Concomitant with the hyperpolarization, DAGO caused a decrease in Rin of 32 +/- 3, and the reversal potential, measured from current-voltage plots, was -94 +/- 2 mV. These effects were mimicked by bath concentrations of 0.5-1.0 microM DAGO. In voltage clamp, DAGO caused an outward current to flow at -60 mV (range 50-185 pA, n = 6). This current reversed at -92 +/- 2 mV (n = 6) and exhibited inward rectification. An additional 6 ARC neurons were tested with DAGO in varying extracellular concentrations of K+ (2.5, 5 and 10 mM) and the reversal potential for the effect of DAGO shifted by 58 mV per decade change in extracellular K+ concentration. DAGO decreased spontaneous postsynaptic potentials in some cells, but TTX (1 microM) had no effect on the ability of DAGO to hyperpolarize the membrane. The hyperpolarization and decrease in Rin induced by DAGO were blocked by the opioid antagonist naloxone (100 nM-1 microM). DAGO responsive cells were unaffected by a kappa-opioid agonist (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1- pyrrolidinyl)cyclohexyl]benzeneacetamide methanesulphonate; U50,488H), however, 2 of 5 cells also were hyperpolarized by a selective delta-receptor opioid agonist (Tyr-D-Pen-Gly-Phe-D-Pen; DPDPE). The effects of DPDPE, but not DAGO, were blocked by a delta-antagonist (ICI 174,864; 1 microM). The present results indicate that activation of ARC mu-receptors leads to an increase in an inwardly rectifying potassium conductance and a subsequent hyperpolarization of most ARC neurons. We suggest that this mu-receptor-induced hyperpolarization of ARC neurons may underlie the opioid inhibition of reproductive events in the mammal.