DISSOCIATION OF SEROTONINERGIC AND DOPAMINERGIC COMPONENTS IN ACUTE EFFECTS OF 1-METHYL-4-PHENYL-1,2,3,6-TETRAHYDROPYRIDINE IN MICE

Behavioural and neurochemical effects of acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment in mice have been studied in order to determine the change in the neurotransmitter profile of the following areas of the brain: substantia nigra (SN), nucleus caudatus putamen (NCP), limbic system (LS; tuberculum olfactorium and nucleus accumbens), medulla oblongata (MO) and cerebellum (CER). Subcutaneous administration of MPTP (40 mg/kg) caused behavioural syndromes including restlessness, straub tail, hindlimb abduction, tremor, jumping, bradvkinesia and akinesia in Balb/c mice. There existed a well-defined biphasic profile of motor activity comprising of an initial excitatory phase followed by an inhibitory phase tasting about two and a half and five hours, respectively. A significant rise in 5-hydroxytryptamine (5-HT) content together with a decreased 5-HT utilization as evidenced by lower 5-hydroxyindole acetic acid (5-HIAA) to 5-HT ratio in the above brain areas demarcated the excitatory phase, whereas the inhibitory phase was distinguished by a significant decrease in dopamine (DA) content along with an increased turnover of the amine as shown by a higher homovanillic acid (HVA) to DA ratio in the functionally important nuclei of the extrapyramidal system like SN, NCP and LS. Methysergide, a nonspecific 5-HT receptor blocker, but not ketanserin, a specific 5-HT2 antagonist, prevented the occurrence of the initial excitatory phase without affecting the depressive phase. Administration of apomorphine. a dopamine agonist, 30 minutes prior to MPTP was ineffective, whereas its application 90 minutes after MPTP prevented the occurrence of bradykinesia and akinesia. Interestingly, treatment with haloperidol, the dopamine (D1/D2) antagonist, before and after MPTP administration caused an early onset and prolongation of the inhibitory phase without affecting the initial hyperexcitement. The results provide direct evidence for the involvement of serotoninergic and dopaminergic mechanisms in the genesis of the early and late syndromes of acute MPTP poisoning respectively.