Effects of early amygdaloid lesions on the development of reactivity in the rat

Abstract

When carried out in 7–8 day old rats housed in grouped condition following weaning, bilateral amygdaloid lesions provoke a lasting hyperreactivity in terms of increased locomotor activity and higher rearing scores in the openfield. The lesion-induced hyperreactivity which is clearly apparent at 40 days and which persists till adult age, is less marked if the weaned rats are housed in isolation. When carried out at the age of 25 days, amygdaloid lesions enhance even more the rat's responsiveness to a novel environment. Early amygdaloid lesions provoke a marked increase in the proportion of rats that display mouse-killing behavior when adult, a behavioral effect that can be linked up with their hyperreactivity. On the contrary, bilateral amygdaloid lesions carried out in adult rats (at 3 months of age) do not change their responsiveness in the open-field, and only a small proportion of the lesioned rats (35 percent) spontaneously display mouse-killing behavior. It is concluded from the data obtained that the amygdala is involved in two closely related processes which play an important role in the ontogenetic development of the rat's reactivity and social responses. On the one hand, the amygdala takes an essential part in the setting up of a stable inhibition of aggressive responses on the basis of social interactions; on the other hand, it is involved in mechanisms which progressively lessen in a more general way the organism's reactivity to its environment.