Influence of increasing doses of pentobarbital on the mesencephalic reticular formation in rats. Spontaneous firing of neuronal pairs and activity evoked by polarization.

In rats immobilized by D-tubocurarine the spontaneous activity in pairs of mesencephalic reticular neurons was recorded by means of one microelectrode and differentiated by amplitude discriminators. Spontaneous activity of neuronal pairs and activity evoked by electrical polarization of cells (5-235 nA) through the recording microelectrode was evaluated in the curarized state and after cumulative doses of 15 mg/kg pentobarbital. No correlation between mean interspike interval duration in pairs of adjacent neurons was found in the unanesthetized state. After pentobarbital a slight dependence was observed between the values. Furthermore, it is probable that adjacent neurons will both cease firing at the same dose of pentobarbital. Cross-correlation of the spike trains of two adjacent neurons was found in the range of minus 10 msec to plus 10 msec (which is probably an expression of direct interaction or of a common input) in 6 of 50 pairs analyzed in the unanesthetized state. This dependence fully disappeared after 15-30 mg/kg of pentobarbital. After 15 mg/kg of pentobarbital in 17 recorded pairs (out of 50 analyzed) peaks in the cross-correlograms appeared in a range of about 100 msec and at a distance of 150-300 msec. Such a relationship was already present in 4 pairs of neurons in the unanesthetized state. The similarity of autocorrelation and cross-correlation histograms in this case favors the hypothesis that spontaneous activity of many mesencephalic reticular neurons is synchronized after pentobarbital administration. In many reticular neurons the firing may be evoked by direct cell polarization through the recording microelectrode even at cumulative doses of pentobarbital much higher than those sufficient to block spontaneous activity.