Neurons in the rat arcuate nucleus are hyperpolarized by gaba_b and μ-opioid receptor agonists: Evidence for convergence at a ligand-gated potassium conductance

Both γ-aminobutyric acid (GABA) and the endogenous opioid peptides have pervasive effects on neuroendocrine function. This study examined the effects of selective activation of GABA_B and/or μ-opioid receptors on neurons of the arcuate nucelus (ARC) of the rat hypothalamus using intracellular recording of cells in a hypothalamic slice. Some recorded neurons were filled with biocytin allowing subsequent identification and immunocytochemical evaluation for the presence of β-endorphin. ARC neurons exhibited a broad array of active and passive conductances. Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGOL), a μ-opioid receptor agonist, inhibited spontaneous firing, hyperpolarized 68% of ARC cells in a dose-dependent manner and increased cell conductance. Baclofen, a GABA_B receptor agonist, hyperpolarized all cells tested. The reversal potentials for both the DAGOL- and baclofen-induced currents were near that of a potassium conductance. Maximal activation by either of the agonists blocked the effects of the other agonist. Identified β-endorphin cells were inhibited by both DAGOL and baclofen. The results of these in vitro studies suggest that GABA_B and μ-opioid receptors are coupled to the same set of potassium channels and that these channels directly and powerfully inhibit most ARC cells, including β-endorphin neurons. We propose that convergence of inhibitory influences at the ligand-gated potassium conductance described here may be an important site of interaction for opioidergic, GABAergic and other putative neurotransmitter systems in the control of neuroendocrine circuits by the ARC.