Electrophysiological properties of guinea‐pig thalamic neurones: an in vitro study.

Abstract

The electroresponsive properties of guinea‐pig thalamic neurones were studied using an in vitro slice preparation. A total of 650 cells were recorded intracellularly comprising all regions of the thalamus; of these 229 fulfilled our criterion for recording stability and were used as the data base for this report. The resting membrane potential for thirty‐four representative neurones which were analysed in detail was ‐64 +/‐ 5 mV (mean +/‐ S.D.), input resistance 42 +/‐ 18 M omega, and action potential amplitude 80 +/‐ 7 mV. Intracellular staining with horseradish peroxidase and Lucifer Yellow revealed that the recorded cells had different morphology. In some their axonal trajectory characterized them as thalamo‐cortical relay cells. Two main types of neuronal firing were observed. From a membrane potential negative to ‐60 mV, anti‐ or orthodromic and direct activation generated a single burst of spikes, consisting of a low‐threshold spike (l.t.s.) of low amplitude and a set of fast superimposed spikes. Tonic repetitive firing was observed if the neurones were activated from a more positive membrane potential; this was a constant finding in all but two of the cells which fulfilled the stability criteria. The l.t.s. response was totally inactivated at membrane potentials positive to ‐55 mV. As the membrane was hyperpolarized from this level the amplitude of the l.t.s. increased and became fully developed at potentials negative to ‐70 mV. This increase is due to a de‐inactivation of the ionic conductance generating this response. After activation the l.t.s. showed refractoriness for approximately 170 ms. Deinactivation of l.t.s. is a voltage‐ and time‐dependent process; full de‐inactivation after a step hyperpolarization to maximal l.t.s. amplitude (‐75 to ‐80 mV) requires 150‐180 ms. Membrane depolarization positive to ‐55 mV generated sudden sustained depolarizing 'plateau potentials', capable of supporting repetitive firing (each action potential being followed by a marked after‐hyperpolarization, a.h.p.). The a.h.p. and the plateau potential controlled the voltage trajectory during the interspike interval and, with the fast spike, constitute a functional state where the thalamic neurone displayed oscillatory properties. Frequency‐current (f‐I) plots from different initial levels of membrane potential were obtained by the application of square current pulses of long duration (2s). From resting membrane potential and from hyperpolarized levels a rather stereotyped onset firing rate was observed due to the presence of the l.t.s.(ABSTRACT TRUNCATED AT 400 WORDS)