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The effect of i.v. sulpiride on the firing rate of dopaminergic neurons in the substantia nigra, pars compacta (SN-DA cells) and tyrosine hydroxylase (TH) activity in the caudate nucleus was studied. In rats, paralyzed with succinylcholine and artificially respirated, (-)-sulpiride (10-50 mg/kg) produced a dose-related increase in the firing rate of SN-DA cells and in TH activity. On the contrary, in rats anesthetized with halothane, (-)-sulpiride (up to 50 mg/kg) activated neither dopaminergic firing nor TH activity. However, (-)-sulpiride (10-25 mg/kg) readily reversed the inhibitory effect of i.v. apomorphine (25 micrograms/kg) on dopaminergic firing in both anesthetized and unanesthetized rats. Since sulpiride fails to inhibit DA-sensitive adenylate cyclase, it may be concluded that DA receptors, whose blockade results in increased dopaminergic firing and TH activation, are not coupled with this enzyme. Moreover, the results indicate that the mechanism responsible for firing and TH stimulation is inhibited by halothane anesthesia. The latter significantly decreased also the stimulant effect of i.v. haloperidol on striatal TH activity.
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