Opioids act at μ-receptors to hyperpolarize arcuate neurons via an inwardly rectifying potassium conductance

Abstract

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 ( R in), determined in the potential range between −60 and −80 mV, was 358 ± 30 M μ ( n = 38) and ARC neurons showed inward rectification at potentials negative to the equilirbium potential for potassium. The selective μ-opioid agonist Tyr- D-Ala-Gly-MePhe-Gly-ol (DAGO) was applied by pressure pipette application at concentrations of 10 or 20 μM. 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 R in of 32±3, and the reversal potential, measured from current-voltage plots, was −94 ± 2 mV. These effects wre mimicked bny batgh concentrations of 0.5–1.0 μM DAGO. In voltage clamp, DAGO caused an outward current to flow at −60 mV (range 50–185 pA, n=6). This current reversal 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 μM) had no effect on the ability of DAGO to hyperpolarize the membrane. The hyperpolarization and decrease in R in induced by DAGO were blocked by the opioid antagonist naloxone (100 nM−1 μM). DAGO responsive cells were unaffected by a κ-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 σ-receptor opioid agonist (Tyr- D-Pen-Gly-Phe- D-Pen; DPDPE). The effects of DPDPE, but not DAGO, were blocked by a σ-antagonist (ICI 174,864; 1 μM). The present results indicate that activation of ARC μ-receptors leads to an increase in an inwardly rectifying potassium conductance and a subsequent hyperopolarization of most ARC neurons. We suggest that this μ-receptor-induced hyperpolarization of ARC neurons may inderlie the opiod inhibition of reproductive events in the mammal.