Serotonin(1A) receptor agonism in the expression of behavioral dopaminergic supersensitivity in subchronic haloperidol treated rats.

The idea that serotonin (5-hydroxytryptamine; 5-HT) is contributed in schizophrenia has long been advocated and alterations in 5-HT neurotransmission has been hypothesized to modulate both the therapeutic and extrapyramidal symptoms (EPS) liability of conventional neuroleptics. The 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a preferential 5-HT(1A) ligand, has been reported to attenuate EPS functions of haloperidol in animals. In view of a possible role of 5-HT(1A) receptors in the management of EPS functions of a neuroleptic drug, the present study was designed to investigate behavioral responses of 8-OH-DPAT at a challenge dose of 0.5mg/Kg in rats with subchronic haloperidol administration at a dose of 5mg/Kg twice daily for 5 days. The intensity of 5-HT syndrome provoked by 8-OH-DPAT was taken as a measure of postsynaptic responses. In the present study administration of haloperidol at a dose of 5mg/Kg twice daily for 5 days decreased locomotion significantly (p<0.01) in familiar (home cage) environment. Subchronic administration of haloperidol at the same dose elicited significant (p<0.01) cataleptic responses in rats when compared with saline treated rats. Results revealed that 8-OH-DPAT-induced hyperlocomotion (p<0.05) and forepaw treading (p<0.1) were significantly smaller in rats pre-treated with haloperidol for 5 days than repeatedly saline injected rats. Conversely, the other components of the syndrome i.e. flat body posture (p<0.001), hind limb abduction (p<0.001) and straub tail (p<0.01) were significantly greater in repeated haloperidol treated rats when compared with repeated saline injected rats. These findings help to demonstrate a causal link between the upregulation of DA-D(2) receptors and the decrease in the effectiveness of presynaptic 5-HT(1A) receptors following subchronic haloperidol administration and this may further help to yield an antipsychotic agent with an improved profile of efficacy to EPS, thereby widening its therapeutic window.