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J Neurophysiol 84: 1814-1825, 2000;
0022-3077/00 $5.00
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The Journal of Neurophysiology Vol. 84 No. 4 October 2000, pp. 1814-1825
Copyright ©2000 by the American Physiological Society

A Slow Transient Potassium Current Expressed in a Subset of Neurosecretory Neurons of the Hypothalamic Paraventricular Nucleus

Jason A. Luther,1 Katalin Cs. Halmos,2 and Jeffrey G. Tasker1,2

 1Neuroscience Program and  2Department of Cell and Molecular Biology, Tulane University, New Orleans, Louisiana 70118

Luther, Jason A., Katalin Cs. Halmos, and Jeffrey G. Tasker. A Slow Transient Potassium Current Expressed in a Subset of Neurosecretory Neurons of the Hypothalamic Paraventricular Nucleus. J. Neurophysiol. 84: 1814-1825, 2000. Type I putative magnocellular neurosecretory cells of the hypothalamic paraventricular nucleus (PVN) express a prominent transient outward rectification generated by an A-type potassium current. Described here is a slow transient outward current that alters cell excitability and firing frequency in a subset of type I PVN neurons (38%). Unlike most of the type I neurons (62%), the transient outward current in these cells was composed of two kinetically separable current components, a fast activating, fast inactivating component, resembling an A-type potassium current, and a slowly activating [10-90% rise time: 20.4 ± 12.8 (SE) ms], slowly inactivating component (time constant of inactivation: tau  = 239.0 ± 66.1 ms). The voltage dependence of activation and inactivation and the sensitivity to block by 4-aminopyridine (5 mM) and tetraethylammonium chloride (10 mM) of the fast and slow components were similar. Compared to the other type I neurons, the neurons that expressed the slow transient outward current were less excitable when hyperpolarized, requiring larger current injections to elicit an action potential (58.5 ± 13.2 vs. 15.4 ± 2.4 pA; 250-ms duration; P < 0.01), displaying a longer delay to the first spike (184.9 ± 15.7 vs. 89.7 ± 8.8 ms with 250- to 1,000-ms, 50-pA current pulses; P < 0.01), and firing at a lower frequency (18.7 ± 4.6 vs. 37.0 ± 5.5 Hz with 100-pA current injections; P < 0.05). These data suggest that a distinct subset of type I PVN neurons express a novel slow transient outward current that leads to a lower excitability. Based on double labeling following retrograde transport of systemically administered fluoro-gold and intracellular injection of biocytin, these cells are neurosecretory and are similar morphologically to magnocellular neurosecretory cells, although it remains to be determined whether they are magnocellular neurons.




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