Electrophysiological properties of paraventricular magnocellular neurons in rat brain slices: modulation of IA by angiotensin II.

Abstract

Whole-cell patch-clamp recordings obtained from magnocellular neurons of the hypothalamic paraventricular nucleus in brain slice preparations of adult Sprague-Dawley rats have been utilized to examine three outward potassium conductances and the ionic mechanisms through which angiotensin II exerts its neurotransmitter actions within this region. Lucifer Yellow fills showed that neurons from which we recorded had large ovoid cell bodies 11-17 microns wide and 22-35 microns 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 omega. Most of these cells displayed irregular or continuous spontaneous activity with a mean frequency of 2.44 +/- 0.33 Hz. Voltage-clamp recordings revealed three outward potassium currents; (1) a delayed outward current (IK), (2) a Ca(2+)-dependent outward current (IK(Ca)) and (3) a transient outward current (IA). These currents were classified according to their voltage dependence, inactivation, Ca2+ dependence and pharmacology. The IK was activated by depolarization beyond -40 mV and its amplitude consistently increased with depolarizing steps. The membrane conductance underlying this current was 27.3 +/- 3.8 nS for depolarization to +50 mV. In medium containing 2 mM Ca2+, depolarization to above -20 mV evoked a slowly-activating IK(Ca) which showed minimal inactivation. This current was suppressed in Ca(2+)-free/Co2+ medium and its membrane conductance was also smaller (19.4 +/- 3.5 nS at +50 mV) than that of IK. The IA demonstrated both fast activation and inactivation and was evoked only if depolarizing pulse steps were preceded by conditioning hyperpolarization. The activation threshold was approximately -65 mV and IA amplitude increased in non-linear fashion as test voltage steps became more positive. The 90% maximum of IA conductance was 15.7 +/- 1.1 nS, and was observed 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-state currents of the IK, while 4-aminopyridine suppressed the IA. Replacement of 2 mM Ca2+ with 2 mM Co2+ in our bath solution or addition of Co2+ into Ca(2+)-free medium reduced the magnitude of IA, revealing the existence of a Co(2+)-sensitive IA. Bath administration of 10(-7) M angiotensin was without significant effect on IK, but resulted in a statistically significant reduction in IA (-31.0 +/- 4.1%) in 12 of 14 paraventricular nucleus cells tested, effects which were not observed following pretreatment with the AT1 receptor antagonist losartan. We conclude that in paraventricular nucleus magnocellular cells, like other CNS neurons, at least three sets of potassium channels contribute to the outward current evoked by depolarization. Our data also demonstrate ionic mechanisms through which angiotensin may act at AT1 receptors to influence the excitability of hypothalamic neuroendocrine cells.