EFFECT OF ISOLATION REARING ON 5-HT AGONIST-INDUCED RESPONSES IN THE RAT

Rats were reared from weaning (21 days of age) either in isolation or in social groups of five for 30 days and were then tested for spontaneous locomotor activity and 7 days later for 5-hydroxytryptamine (5-HT) agonist-induced behaviour. Isolation-reared animals displayed locomotor hyperactivity when placed in a novel environment. 5-methoxy-N,N-dimenthyltryptamine (5-MeODMT) (2 mg/kg IP) and 8-hydroxy-2-(di-n-propyl-amino) tetralin (8-OH-DPAT) (0.32 mg/kg SC) elicited various components of the "5-HT behavioural syndrome" in both groups of animals, with forepaw treading and flat body posture being significantly more pronounced in isolation-reared animals. 1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (2.5 mg/kg IP), a 5-HT2 selective agonist, produced a significantly greater number of back muscle contractions in isolation-reared animals but there was no difference between the two groups in the number of wet-dog shakes produced. Forepaw treading and flat body posture are thought to be mediated by 5-HT1A receptor activation, and stimulation of this receptor by either 5-MeODMT or 8-OH-DPAT produced greater responding in isolation-reared rats, suggesting supersensitivity of the post-synaptic 5-HT1A receptor. Wet-dog shakes are thought to be mediated by 5-HT2 and other (none-5-HT) receptors while back muscle contractions have been shown to be mediated by 5-HT2 receptors, indicating that there is also an increase in 5-HT2 receptor responsiveness in the socially-isolated animals. In general, the results indicate post-synaptic 5-HT receptor supersensitivity in isolation-reared rats and these receptor changes may be involved in the behavioural profile observed in such rats.