Oral dyskinesias and morphological changes in rat striatum during long-term haloperidol administration

Rationale: Neuroleptic-induced oral dyskinesias in rats, a putative analogue to human tardive dyskinesia, may be due to increased glutamate release within the striatum. This may lead to excitotoxic degeneration and, as a consequence, persistent motor side effects. Objectives: To investigate whether alterations in glutamatergic synapses within the striatum are associated with the development of neuroleptic-induced oral dyskinesia. Methods: Haloperidol was administered for 20 weeks, and rats with high and low levels of vacuous chewing movements (VCM) were analyzed for morphological changes with electron microscopy at three time points. Results: At week 8, the high VCM rats had a larger nerve terminal area and lower density of nerve terminal glutamate immunoreactivity than the other groups. After 18 weeks of treatment, the nerve terminal area was increased relative to controls in both the high and low VCM groups. After discontinuation of treatment, there were no significant morphological differences between the groups, but the level of VCM was still significantly increased in the high VCM group. Conclusions: These results show that striatal glutamatergic transmission is affected during haloperidol treatment and the nerve terminal area and the density of nerve terminal glutamate immunoreactivity are important in determining the VCM response to haloperidol treatment. This indicates that increased glutamatergic synaptic activity in the striatum contributes to the development of human tardive dyskinesia.