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 slices 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-dependent inward rectification (I_h) that was blocked by CsCl (3 mM) or by ZD 7288 (30 μM). In addition, 51% of the cells expressed a low-threshold spike (LTS) associated with a transient inward current (I_T) that was blocked by NiCl₂ (200 μM). A smaller percentage of POA neurons (29%) expressed a transient outward, A-type K⁺ current that was antagonized by a high concentration of 4-aminopyridine (3 mM). Moreover, POA neurons responded to bath application of the μ-opioid receptor agonist DAMGO (93%) or the GABA(B) receptor agonist baclofen (83%) with a membrane hyperpolarization or an outward current. These responses were accompanied by a decrease in input resistance or an increase in conductance, respectively, and were attenuated by BaCl₂ (100 μM). In addition, the reversal potential for these responses closely approximated the Nernst equilibrium potential for K⁺. These results suggest that 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 inhibited upon μ-opioid or GABA(B) receptor stimulation via the activation of an inwardly-rectifying K⁺ conductance. Such intrinsic and transmitter-activated conductances likely serve as important determinants of the firing patterns of POA neurons.