COMPARISON OF RESPONSE PROPERTIES OF NEURONS IN THE INFERIOR COLLICULUS OF GUINEA-PIGS UNDER DIFFERENT ANESTHETICS

Responses of the inferior colliculus (IC) neurons to acoustical stimul ation were recorded in anesthetized guinea pigs Three kinds of anesthe sia were used: (1) urethane (8 ml/kg b.w of 20 per cent solution i.p.) ; (2) ketamine-xylazine combination (1 ml/kg b.w of mixture 2:1); and (3) pentobarbital (25 mg/kg i.p.) combined with intramuscular injectio n of fentanyl (0.5 mi) and droperidol (1 mi). The frequency tuning of neurons evaluated on the basis of Q(10) values and the composite neura l audiogram represented by points of lowest thresholds of individual I C neurons were similar for guinea pigs treated with any of the anesthe tics. The number of spontaneously active IC neurons was significantly larger with ketamine than with urethane or pentobarbital. The response latencies to tone bursts at characteristic frequency (CF) were shorte st in animals anesthetized with pentobarbital. Whereas with ketamine a nd urethane many neurons were recorded in which response latencies wer e longer than 40 ms, in pentobarbital-anesthetized animals the latenci es usually did not exceed 25 ms. The occurrence of neurons with an ons et type of response was significantly larger with pentobarbital than w ith the other two anesthetics. In ketamine and urethane anesthesia, th resholds of units with sustained response were significantly lower tha n thresholds of units with onset response. Our results suggest that in experiments where the level of spontaneous activity, latency and type of responses are important parameters, the kind of anesthesia should be taken into account.