Chronic L-DOPA treatment of mice: a behavioural and biochemical study.

Mice were pretreated once daily with L-DOPA (200 mg/kg) plus benserazide (B) (50 mg/kg) for ten days and challenged with various doses of L-DOPA + B on the first, fourth or sixteenth days of withdrawal. L-DOPA + B-pretreated mice were more sensitive the locomotor stimulant effect of L-DOPA + B challenge one and four days, but not sixteen days after withdrawal. The enhanced response was most marked on the first day of withdrawal. Other mice, pretreated once daily with B (50 mg/kg), responded one day after the tenth dose with a slightly enhanced response to L-DOPA + B challenge compared to the response to vehicle-pretreated animals. Moreover, vehicle-pretreated mice challenged with B alone, were significantly less active than those challenged with vehicle. On the first day of withdrawal, the L-DOPA + B-pretreated animals were supersensitive to locomotor stimulant effects of apomorphine but subsensitive to dexamphetamine (Bailey et al., 1979). On the fourth day of withdrawal, there were no differences in the responses of the L-DOPA + B-pretreated mice compared to the vehicle-pretreated mice, to apomorphine or apomorphine plus clonidine, but L-DOPA + B-pretreated mice were still subsensitive to the locomotor stimulant effects of dexamphetamine. Clonidine produced a dose-dependent, but similar, degree of hypothermia in both pretreatment groups. On the first and fourth days of withdrawal L-DOPA + B-pretreated mice exhibited higher brain levels of dopamine (DA) and DOPA than vehicle-pretreated mice in response to an acute dose of L-DOPA + B. The biochemical results suggest that the enhanced locomotor response to L-DOPA + B in L-DOPA + B-pretreated mice is probably dependent on changes in the amount of L-DOPA (and DA) available in the brain. Moreover, it is not ruled out that some of the effects of L-DOPA + B pretreatment were due to the B alone. Some of the enhanced response to L-DOPA + B on the first day of withdrawal may have been dependent on the same mechanism as that underlying the apparent supersensitivity to apomorphine. The subsensitive response to dexamphetamine would appear to be independent of changes in post-synaptic DA and alpha-adrenergic receptor sensitivity.