Postnatal Development Of K～+ Currents Underlying Changes In Excitement Of SD Rat Hypothalamic Neurons

The hypothalamic neurons of postnatal rats show temperature-sensitive gradually in their firing rate activity. In this study, we test the effects of K+ currents kinetics on the development of thermosensitivity from postnatal day 1(P1) to P18 neurons measured via current-clamp and voltage-clamp techniques using acutely isolated neurons over the same period. The research including four parts described here explores the excitability and K+ currents activity at different temperature. The main results are as follows:1. Around the time of birth, the membrane passive characteristics changed remarkably. The resting membrane potential shifted negatively from -42.6?.1 at P1 to- 61.3 ?.4 at P18(P<0.05) and temperature had no effect on them. On the other hand, the membrane input resistance decreased from 2721 ?367M to 605 ?77M(P<0.05) while raising temperature further decreased it.2. Spontaneous action potential can not be recorded from isolated neurons, while single action potential was recorded by a majority of neurons after current injection. The threshold of action potential didn't noticeably change with age. However, action potential amplitudes significantly increased after birth. Concurrently, the duration at half-amplitude declined obviously. The shorter duration resulted from significant increases in action potential maximum rates of rise and repolarization. On the other hand, an increase in rate of prepotential depolarization. And fAHP were observed.Similar increases were apparent during temperature raising.3. Potassium currents were essential elements for the generation of AP. Delayed rectifier whole-cell currents were substantially reduced by 4-AP(51.5%, 0.2mM) of P1 and by 4-AP(72.4%,0.2mM) of p18. There were correlated with increased neurons excitability that prolonged the duration of half-amplitude of AP 24% and 33% respectively.4. To test the single channel kinetics of delayed rectifier ppotassium currents, the results indicated that the development and membrane temperature had permeability had no change that was 80Ps or so, but the probability of channel opening(Po) increased. The open density of channel increased from 0.1channel/m2 in P1 to 0.26 channel 1/ m2 in P9 and kept stable afterwards, so its kinitic showed "S" curve. The Po vs V curve that suggested the voltage-dependent shifted to the left. An increase in temperature significant raised the ion peameability and also prolonged upon time of channel(P<0.05), but decreased the voltage that made Po increased e-fold. In contrast to modulate the open time with raising temperature of burst of channels, the number of long-burst open was largely increased with age. Meanwhile, there was almost no change of the duration of between-burst in the process of the development and largely shorter with increasing temperature that showed to modulate the mode of channel open.Taken together, thermosensitivity was considered to be a dynamic process which could influenced by synaptic input in the course of hypothalamic neurons postnatal development. The change of K+ current kinetic activity play an important role with age that had effects on the rate of repolorization of AP and pre-(post-)potential. It is necessary to explore the further machnisms of neuronal thermosensitivity development and differentiation on brain slices that kept synaptic connectivity by patch-clamp and single-cell-reverse transccriptase polymerase chain reaction.