Brain monoamine asymmetry in chicks subjected to a separation-stress procedure with litter substrate

Abstract

Activation of the right hemispheric neurotransmitter systems is related to negative emotion and stress in mammals, but this relationship is not fully known in birds. The effect of the presence of sawdust litter on behavior and brain monoamine laterality in isolation stressed chicks (6 days old) was investigated. Although there was no significant difference in peep, movement distance in litter group chicks (n＝7) was significantly longer than that in non-litter group chicks (n＝6) during 15 min isolation test (P＜0.01). The laterality index (a measure of central monoamine laterality indicating the predominance of right over left hemispheric activity) of serotonin (5-HT) in intact chicks (no stress manipulation; n＝8) was higher than those in chicks with and without litter substrate (P＜0.01). The value of the dopamine (DA) metabolite ratio in intact chicks was lower than that in the non-litter group chicks (P＜ 0.05), but was not low in the litter group chicks. In contrast, there was no significant difference in the levels of the 5- HT metabolite ratio among the groups. In addition, the laterality index of the ratio correlated negatively with the number of peeps in chicks with sawdust litter (P＜0.01). It has been suggested that emotional processing in the right hemisphere might be optimal with DAergic and/or serotonergic lateralization, whereas either a decrease or an increase in this lateralization might be reinstated in animals suffering stress. In addition, the present results imply that sawdust litter may attenuate isolation stress that induces anxiety in chicks, and that the laterality index of the DA metabolite ratio may be effective in estimating positive emotional change.