Steroid anaesthesia: alphadione depresses multiunit activity in the mesencephalic reticular formation

Cortical EEG and multiunit activity (MUA) of the mesencephalic reticular formation (MRF), area hypothalami anterior (AH) and the nucleus amygdalae basalis (AMY) were studied before and after different doses of alphadione (Althesin) and hexobarbitone (Evipan-Natrium) given to cats with chronically implanted electrodes. Non-anaesthetic doses of alphadione (0.15 ml/kg; 0.3 ml/kg; 0.6 ml/kg and 1.2 ml/kg i.p.) had sedative effects decreasing selectively the MUA in the MRF. In doses of 2.0 ml/kg, 2.4 ml/kg and 3.0 ml/kg i.p., alphadione induced anaesthesia which was associated with a rapid decrease of MUA in the MRF and by a gradual decrease of activity in the AH and AMY. The i.p. dose of 3.0 ml/kg abolished MUA responses of the reticular formation to acoustic, visual and somatic stimulation but failed to block responses to pain. Deep anaesthesia with lasting analgesia could be maintained by i.v. infusion (0.075 ml/kg/min). This procedure blocked the responsiveness to painful stimulation while pharyngeal and laryngeal reflexes were maintained. Hexobarbitone in a dose of 20.0 mg/kg i.p. did not produce anaesthesia in the cat. Administration of 40.0 mg/kg i.p. resulted in a rapid decrease of MUA in the MRF, AH and AMY, MUA responses to each stimulation were abolished and the pharyngeal reflex was blocked.