EMOTIONAL AND FOOTSHOCK STIMULI INDUCE DIFFERENTIAL LONG-LASTING BEHAVIORAL-EFFECTS IN RATS - INVOLVEMENT OF OPIOIDS

Rats were exposed to either a footshock stimulus (FS) or emotional sti mulus (ES, forced perception of another rat receiving footshocks) duri ng a daily 10-min session for 5 consecutive days. The consequences of FS and ES on their behavioural responsiveness were assessed at differe nt post-stress intervals using a small open-field. FS induced a decrea se in ambulation, rearing and sniffing and an increased immobility in the small open field. These effects were present in rats tested immedi ately after the last session and remained present for at least 15 days . In contrast, ES induced a transient decrease in ambulation and reari ng immediately after the last session, but in the period from half an hour until at least 15 days after the stimulus experience, an increase in ambulation, rearing and sniffing was observed. Exposure to one foo tshock per session for 5 consecutive days or to 10 footshocks in a sin gle session also resulted in a long-lasting reduction in ambulation an d sniffing and an increase in immobility. The former regime did not in fluence the behavioural response of ES rats, but the latter resulted i n an increase in ambulation, rearing and sniffing in ES rats. Naloxone (1 mg/kg s.c.) pretreatment antagonized the increased behavioural act ivity of the ES rats whereas the activity of control and FS animals wa s not affected, suggesting an involvement of endogenous opioid systems in the behavioural responses observed in ES rats. It is suggested tha t the behavioural responses of the ES and FS animals are regulated by different mechanisms. (C) 1998 Elsevier Science B.V. All rights reserv ed.