Postnatal Maturation of Rat hypothalamo-neurohypophysial Neurons: Evidence for a developmental decrease in calcium entry during Action potentials. H. Widmer, Amerdeil, H., P. Fontanaud and Desarm[acute]enien, M. G.. Biologie des Neurones Endocrines, CNRS UPR 9055, CCIPE 141 Rue de la Cardonille, 34094 Montpellier Cedex 5, France.
APStracts 3:0191N, 1996.
Action potentials and voltage-gated currents were studied in acutely dissociated neurosecretory cells from rat supraoptic nucleus during the three first postnatal weeks (PW1-PW3), a period corresponding to the final establishment of neuroendocrine relationships. Action potential duration (at half maximum) decreased from 2.7 to 1.8 ms; this was attributable to a decrease in decay time. Cadmium application (250 M) reduced the decay time by 43 % at PW1 and 21 % at PW3, indicating that the contribution of calcium currents to action potentials decreased during postnatal development. The density of high voltage-activated calcium currents increased from 4.4 to 10.1 pA/pF at postnatal days 1-5 and 11-14, respectively. The conductance density of sustained potassium current, measured at +20 mV, increased from 0.35 (PW1) to 0.53 nS/pF (PW3). The time to half maximal amplitude did not change. Conductance density, time- and voltage-dependent inactivation of the transient potassium current were stable from birth. At PW1, the density and time constant of decay (measured at 0 mV) were 0.29 nS/pF and 17.9 ms respectively. Voltage-dependent properties and density (1.1 nS/pF) of the sodium current did not change postnatally. During PW1, fitting the mean activation data with a Boltzmann function gave a half activation potential of -25 mV. A double Boltzman equation was necessary to adequately fit the inactivation data, suggesting the presence of two populations of sodium channels. One population accounted for about 14 % of the channels, with a half inactivation potential at -86 mV; the other population showed a half inactivation potential of -51 mV. Amathematical model, based on Hodgkin-Huxley equations, was used to assess the respective contribution of individual currents to action potentials. When the densities of calcium and sustained potassium currents were changed from immature to mature values, the decay time of the action potentials generated with the model decreased from 2.85 to 1.95 ms. A similar reduction was obtained when only the density of the potassium current was increased. Integration of the calcium currents generated during mature and immature action potentials demonstrated a significant decrease in calcium entry during development. We conclude that the developmental reduction of the action potential duration a) is a consequence of the developmentally regulated increase in a sustained potassium current, and b) leads to a reduction of the participation of calcium currents to the action potential, resulting in a decreased amount of calcium entering the cell during each action potential.

Received 7 December 1996; accepted in final form 3 September 1996.
APS Manuscript Number J831-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996