Effects of Verapamil on Dopamine Dependent Behaviours in Rats.

Abstract : Verapamil at 5, 10 and 20 mg/kg ip did not inhibit the conditioned avoidance response, neither induced catalepsy nor antagonised apomorphine stereotypy in rats indicating that these doses do not block the postsynaptic striatal D 2 and D 1 dopamine (DA) receptors. However, pretreatment with 10 and 20 mg/kg ip verapamil potentiated methamphetamine stereotypy and antagonised catalepsy induced by small doses (0.05 and 0.1 mg/kg ip) of apomorphine. Antagonism of small dose apomorphine-induced catalepsy suggests that at these doses verapamil blocks the presynaptic D2 DA autoreceptors. Further, pretreatment with 10 mg/kg verapamil antagonised, while pretreatment with 20 mg/kg potentiated haloperidol catalepsy. Potentiation of methamphetamine stereotypy by 10 and 20 mg/kg verapamil is explained on the basis of blockade of presynaptic D2 DA autoreceptors by these doses of verapamil and its reported DA neuronal uptake blocking activity. Antagonism of haloperidol catalepsy by 10 mg/kg verapamil is also explained on the basis of presynaptic D2 DA autoreceptor induced blockade by 10 mg/kg verapamil whereas potentiation of haloperidol catalepsy by 20 mg/kg verapamil is explained on the basis of its calcium channel blocking activity.