MEMBRANE-PROPERTIES OF IDENTIFIED GUINEA-PIG PARAVENTRICULAR NEURONS AND THEIR RESPONSE TO AN OPIOID MU-RECEPTOR AGONIST - EVIDENCE FOR AN INCREASE IN K+ CONDUCTANCE

Intracellular recordings were obtained from neurons in the paraventricular nucleus (PVN) of guinea-pig hypothalamic slices. Passive and active properties of the neurons were determined, and when possible, recorded neurons were injected with biocytin. The effects of a mu-receptor opioid agonist [D-Ala2, Nme-Phe4, Gly5-ol]enkephalin (DAGO) were studied in order to determine which types of cells have mu receptors and to test the hypothesis that an increase in K+ conductance causes mu-receptor-mediated inhibition in the PVN. The recorded cells inside the PVN were divided into two groups, primarily on the basis of the presence or absence of a low threshold Ca2+ spike (LTS). In one group of neurons, LTS potentials could not be evoked (non-LTS cells, n=42). In another group of neurons (n = 35), LTS potentials with one or two Na+ spikes could be initiated with depolarizing pulses superimposed on steady hyperpolarizing currents; however, these neurons did not fire robust bursts (i.e. non-bursting LTS cells). The mean time constant of non-LTS cells (19.9+/-1.6 ms; mean+/-SEM) was significantly shorter than that of non-bursting LTS cells (26.7+/-2.1 ms). There were no differences in the mean resting membrane potential, spike amplitude, spike duration, input resistance, spike threshold and pattern of synaptic inputs between the two groups. Intracellular labeling with biocytin combined with cresyl violet counter-staining demonstrated that the two types of cells were located in the PVN. The mean diameters of non-LTS cells along their long axis (25.4+/-1.7 mum) and short axis (17.5+/-2.2 mum, n = 7) were larger than those of non-bursting LTS cells (22.1+/-1.0 mum, 15.2+/-1.1 mum, respectively, n = 12); this suggests that non-LTS cells are magnocellular neurons and non-bursting LTS cells are parvocellular neurons, as previously described in the rat. Bath application of DAGO at a concentration of 1 muM inhibited 7 of 25 (28%) non-LTS cells tested, and 4 of 21 (19%) non-bursting LTS cells. The main effect of DAGO on PVN cells was a hyperpolarization of membrane potential (4 to 15 mV) with a decrease of input resistance (20% to 38%). Quinine (100 muM to 1 mM), a K+ channel blocker, eliminated or reduced inhibitions mediated by DAGO. The apparent reversal potential of the hyperpolarization induced by the mu-receptor agonist was about -85 mV. These data in the guinea-pig PVN suggest that non-LTS cells are putative magnocellular neurons, and non-bursting LTS cells are parvocellular neurons. The mu-receptor agonist, DAGO, directly hyperpolarizes approximately equal numbers of each of these cell types. Experiments with quinine and injected currents support the hypothesis that DAGO activation of mu receptors on neurons in the region of the PVN leads to an increase in K+ conductance.