Effects of meprobamate, chlordiazepoxide, diazepam, and sodium pentobarbital on visually evoked responses in the tectotegmental area of the rat

Abstract

The effects of meprobamate, chlordiazepoxide, diazepam and sodium pentobarbital on sensory input to the boundary area between the dorsal midbrain tegmentum and ventral tectum of the rat have been investigated in order to find out whether effects obtained with these compounds in the hypothalamus can also be observed at the mesencephalic level. The following results have been obtained. 1. (1) At a dose of 2[middle dot]5 mg/kg i.p., chlordiazepoxide had no effect; at 5, 10, 15, and 20 mg/kg, this compound decreased the amplitude of the initial components of the visually evoked responses in the rat. There was an increased effect at higher doses.2. (2) Diazepam had effects similar to those produced by administration of chlordiazepoxide. The evoked responses were of smaller amplitude after injection of 5, 10, and 20 mg/kg i.p. At the highest dose, diazepam caused larger reductions in amplitude than chlordiazepoxide at 20 mg/kg.3. (3) Meprobamate at 40, 80 and 100 mg/kg i.p. increased the amplitude of the evoked responses. There was more individual variation with this compound than with chlordiazepoxide, and more waxing and waning of the effects, but little difference in the magnitude of facilitation at the various doses. At a dose of 120 mg/kg, the size of the evoked responses decreased.4. (4) At doses of 5, 10, and 20 mg/kg i.p., sodium pentobarbital caused increases in the amplitude of the evoked responses. Only at a dose of 20 mg/kg was there a slight decrease in amplitude, lasting only 25 min after drug administration. Thereafter, responses to this dose were larger than control responses, but not as large as responses to the two smaller doses.The significance of these results is discussed in the context of earlier data on the effects of these compounds and the barbiturates on sensory input to the midbrain reticular formation. A difference in mode of action, at low doses, is shown by the data. A relationship between the effects of meprobamate and the barbiturates on sensory input in the rat finds support in these results.