Median preoptic nucleus neurons: an in vitro patch-clamp analysis of their intrinsic properties and noradrenergic receptors in the rat.

The median preoptic nucleus is recognized as an important forebrain site involved in hydromineral and cardiovascular homeostasis. In the present study, whole cell patch-clamp recordings in parasagittal slices of adult rat brain were used to obtain information on the properties of median preoptic neurons. Lucifer Yellow-labelled cells demonstrated small ovoid somata with two to three aspiny main dendrites and axons that branched sparingly. Median preoptic neurons displayed varying degrees of hyperpolarization-activated time-dependent and/or time-independent inward rectification, and 86% of cells demonstrated low threshold spikes. Median preoptic nucleus is known to receive a prominent noradrenergic innervation from the medulla, and 59% of 156 tested neurons were found to respond to bath applied noradrenaline (1-100 microM). In the majority (n = 62) of cells, the response was an alpha 2 adrenoreceptor-mediated, tetrodotoxin-resistant, membrane hyperpolarization that was associated with a 43 +/- 6% increase in membrane conductance. The net noradrenaline-induced current (5-45 pA) was inwardly rectifying, cesium-resistant but barium sensitive. Current reversal at -102 +/- 4 mV in 3.1 mM [K]o and -62 +/- 3 mV in 10 mM [K]o implied opening of potassium channels. By contrast, a minority (n = 27) of cells responded to noradrenaline with an alpha 1-mediated, tetrodotoxin-resistant membrane depolarization. These observations imply a functional diversity among median preoptic neurons, and the prevalence of hyperpolarizing alpha 2 and, to a lesser extent, depolarizing alpha 1 adrenoreceptors on median preoptic neurons suggests that noradrenergic inputs can exert a prominent influence on their cellular excitability.