Autonomic responses to periaqueductal grey stimulation during inhibition of tail flick response in rats

The analgesic effects evoked by stimulating midbrain sites are well known but electrical stimulation in the periaqueductal grey matter (PAG) can also produce aggressive or defensive behaviour which is accompanied by a characteristic pattern of autonomic changes termed the ‘defence reaction’ (Abrahams et al. 1960) or ‘visceral alerting’ (Hilton et al. 1983).