ELECTROPHYSIOLOGICAL PROPERTIES OF PARAVENTRICULAR MAGNOCELLULAR NEURONS IN RAT-BRAIN SLICES - MODULATION OF I-A BY ANGIOTENSIN-II

Whole-cell patch-clamp recordings obtained from magnocellular neurons of the hypothalamic paraventricular nucleus in brain slice preparation s of adult Sprague-Dawley rats have been utilized to examine three out ward potassium conductances and the ionic mechanisms through which ang iotensin II exerts its neurotransmitter actions within this region. Lu cifer Yellow fills showed that neurons from which we recorded had larg e ovoid cell bodies 11-17 mu m wide and 22-35 mu m long, as well as 1- 3 minimally branched processes, anatomical features in accordance with those previously described for magnocellular neuroendocrine neurons. These neurons had an average resting membrane potential of -58.3 +/- 0 .9 (mean +/- S.E.M.) mV, spike amplitude of 92.8 +/- 1.4 mV, and input resistance of 788.9 +/- 50.4 M Ohm. Most of these cells displayed irr egular or continuous spontaneous activity with a mean frequency of 2.4 4 +/- 0.33 Hz. Voltage-clamp recordings revealed three outward potassi um currents; (1) a delayed outward current (I-K), (2) a Ca2+-dependent outward current (I-K(Ca)) and (3) a transient outward current (I-A). These currents were classified according to their voltage dependence, inactivation, Ca2+ dependence and pharmacology. The I-K was activated by depolarization beyond -40 mV and its amplitude consistently increas ed with depolarizing steps. The membrane conductance underlying this c urrent was 27.3 +/- 3.8 nS for depolarization to +50 mV. In medium con taining 2 mM Ca2+, depolarization to above -20 mV evoked a slowly-acti vating I-K(Ca) which showed minimal inactivation. This current was sup pressed in Ca2+-free/Co2+ medium and its membrane conductance was also smaller (19.4 +/- 3.5 nS at +50 mV) than that of I-K. The I-A demonst rated both fast activation and inactivation and was evoked only if dep olarizing pulse steps were preceded by conditioning hyperpolarization. The activation threshold was approximately -65 mV and I-A amplitude i ncreased in non-linear fashion as test voltage steps became more posit ive. The 90% maximum of I-A conductance was 15.7 +/- 1.1 nS, and was o bserved at membrane potentials around -15 mV. The reversal potentials of these currents were in accordance with the K+ equilibrium potential . Tetra-ethylammonium reversibly inhibited both the peak and steady-st ate currents of the I-K, while 4-aminopyridine suppressed the I-A. Rep lacement of 2 mM Ca2+ with 2 mM Co2+ in our bath solution or addition of Co2+ into Ca2+-free medium reduced the magnitude of I-A, revealing the existence of a Co2+-sensitive I-A. Bath administration of 10(-7) M angiotensin was without significant effect on I-K, but resulted in a statistically significant reduction in I-A (-31.0 +/- 4.1%) in 12 of 1 4 paraventricular nucleus cells tested, effects which were not observe d following pretreatment with the AT(1) receptor antagonist losartan. We conclude that in paraventricular nucleus magnocellular cells, like other CNS neurons, at least three sets of potassium channels contribut e to the outward current evoked by depolarization. Our data also demon strate ionic mechanisms through which angiotensin may act at AT(1) rec eptors to influence the excitability of hypothalamic neuroendocrine ce lls.