The role of intrinsic and agonist-activated conductances in determining the firing patterns of preoptic area neurons in the guinea pig

Whole-cell and intracellular recordings were made in coronal hypothalamic s lices prepared from ovariectomized female guinea pigs. 62% of preoptic area (POA) neurons fired action potentials in a bursting manner, and exhibited a significantly greater afterhyperpolarization (AHP) than did non-bursting POA neurons. The majority (70%) of POA neurons (n=76) displayed a time-depe ndent inward rectification (I-h) that was blocked by CsCl (3 mM) or by ZD 7 288 (30 muM) In addition, 51% of the cells expressed a low-threshold spike (LTS) associated with a transient inward current (I-h) that was blocked by NiCl2 (200 muM) A smaller percentage of POA neurons (29%) expressed a trans ient outward, A-type K+ current that was antagonized by a high concentratio n of 4-aminopyridine (3 mM). Moreover, POA neurons responded to bath applic ation of the mu -opioid receptor agonist DAMGO (93%) or the GABA(B) recepto r agonist baclofen (83%) with a membrane hyperpolarization or an outward cu rrent. These responses were accompanied by a decrease in input resistance o r an increase in conductance, respectively, and were attenuated by BaCl2 (1 00 muM). In addition, the reversal potential for these responses closely ap proximated the Nernst equilibrium potential for K+. These results suggest t hat POA neurons endogenously express to varying degrees an AHP, an I-h, an I-T and an A-type K+ current. The vast majority of these neurons also are i nhibited upon mu -opioid or GABA(B) receptor stimulation via the activation of an inwardly-rectifying K+ conductance. Such intrinsic and transmitter-a ctivated conductances likely serve as important determinants of the firing patterns of POA neurons. (C) 2000 Elsevier Science B.V. All rights reserved .