A study of the amygdaloid defence reaction showing the value of Althesin anaesthesia in studies of the functions of the fore-brain in cats.

1. In cats under Althesin (alphaxalone-alphadalone) anaesthesia, sites in the amygdala and brain-stem defence areas have been electrically stimulated by means of monopolar, semi-microelectrodes. 2. Such stimulation evoked a consistent pattern of visceral changes characteristic of the alerting stage of the defence reaction as it has been described by previous workers. This "visceral alerting reaction" included increases in arterial blood pressure, heart-rate and cardiac output with vasoconstriction in kidney, intestines and skin but vasodilatation in the hind limbs. 3. These results differ strikingly from those reported previously n that, under conventional anaesthetics, such as chloralose or barbiturate, the full visceral alerting reaction cannot be evoked by amygdala stimulation, or any other manoeuvre which involves transmission through the brain stem defence areas. 4. The area of the amygdala from which such responses can be elicited under althesin closely resembles that which has been reported to evoke defence reactions in conscious animals. 5. It is concluded that Althesin, used in the manner described, does not distort synaptic transmission in the forebrain in the way that conventional anaesthetics do. It is suggested that this steroid anaesthetic may be invaluable in any studies of fore-brain physiology in the cat.