Enhancement of some 5-HT-dependent behavioural responses following repeated immobilization in rats

Responses to drugs affecting 5-hydroxytryptamine (5-HT) and dopaminergic (DA) system have been examined in rats after repeated immobilization. Groups of rats were immobilized for 2 h per day for up to 7 days. Twenty-four hours later their behavioural responses to various drugs were tested. Rats immobilized for 7 days showed decreased sniffing and increased grooming and body shakes. When given amphetamine (3 mg/kg, i.p.) the intensity of classical dopamine-dependent behaviours was similar to that of non-immobilized controls. Some responses to the 5-HT releaser p-chloroamphetamine (PCA) (4 and 10 mg/kg, i.p.) and the 5-HT agonist 5-methoxy-N,N-dimethyltryptamine (5-MEODMT) (5 mg/kg, i.p.) (forepaw treading and tremor) were enhanced after 7 days immobilization but others (limb abduction and headweaving) were not. These responses were not enhanced after 1 or 3 days immobilization. Backward walking and body shakes induced by PCA were also enhanced after 7 days immobilization. Concentrations of 5-HT, DA and their metabolites in striatum, cortex, hippocampus, hypothalamus and midbrain of non-drug-treated control and immobilized groups were comparable. Brain PCA concentrations 30 min after injection were also comparable. The above biochemical and behavioural data suggest that repeated immobilization increases some 5-HT postsynaptic functions. These results are discussed in relation to non-drug-provoked behavioural abnormalities occurring 24 h after the first immobilization but no longer evident after 7 periods of immobilization.