ELECTROPHYSIOLOGICAL PROPERTIES OF RAT PONTINE NUCLEI NEURONS IN-VITRO - I - MEMBRANE-POTENTIALS AND FIRING PATTERNS

We used a new slice preparation of rat brain stem to establish the bas ic membrane properties of neurons in the pontine nuclei (PN). Using st andard intracellular recordings, we found that pontine cells displayed a resting membrane potential of -63 +/- 6 mV (mean +/- SD), an input resistance of 53 +/- 21 M Omega, a membrane time constant of 5.3 +/- 2 .4 ms and were not spontaneously active. The current-voltage relations hip of most of the PN neurons showed the characteristics of inward rec tification in both depolarizing and hyperpolarizing directions. A prom inent feature of the firing of pontine neurons was a marked firing rat e adaptation, which eventually caused the cells to cease firing. Sever al types of membrane conductances possibly contribute to this feature. For one, a medium and a slow type of afterhyperpolarization (AHP) con trol the pattern of firing. The medium AHP was partly susceptible to b lockade of calcium influx, whereas it was abolished completely by bloc kade of potassium channels with tetraethylammonium, indicating that it is based on at least two conductances: a calcium-dependent and a calc ium-independent one. The slow AHP was carried by potassium ions and co uld be blacked effectively by preventing calcium influx into the cell. It was present after single spikes but was strongest after a high-fre quency spike train. Calcium entry into the cell was mediated by high-t hreshold calcium channels that were detected by the generation of calc ium spikes under blockade of potassium channels. Furthermore, the earl y phase of the firing rate adaptation was shown to be related to the t ime course of a slow, tetrodotoxin (TTX)-sensitive, persistent sodium potential, which was activated already in the subthreshold range of me mbrane potentials. This potential was time dependent and imposed as a depolarizing ''hump'' with a maximum occurring in most cases between 5 0 and 100 ms after stimulus onset. In the suprathreshold range, it gen erated plateau potentials following fast spikes, if potassium channels were blocked. After the complete adaptation of the firing rate, PN ne urons were observed to display irregular fluctuations of the membrane potential, which sometimes reached firing threshold thereby eliciting an irregular low-frequency spike train. As these fluctuations could be blocked with TTX, they probably are based on the persistent sodium cu rrents. The opposing drive in hyperpolarizing direction may be provide d by strong outward currents that generated a marked outward rectifica tion in the current-voltage relationship under TTX. In conclusion, PN neurons show complex membrane properties that are reminiscent in many ways to cerebrocortical ''regular firing'' neurons.