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Metoclopramide (5 to 40 mg/kg, i.p.) induces catalepsy and antagonised apomorphine induced cage climbing behaviour in mice. This further indicate its postsynaptic striatal and mesolimbic D 2 dopamine (DA) receptor blocking activity. Metoclopramide at 1.25 and 2.5 mg/kg, i.p. induced stereotyped cage climbing behaviour in mice. Pretreatment with haloperidol and alpha-methyl-p-tyrosine significantly antagonised metoclopramide (1.25 and 2.5 mg/kg)-induced stereotyped cage climbing behaviour. Metoclopramide at these doses induces stereotyped cage climbing behaviour by releasing DA from the mesolimbic dopaminergic neurons with resultant activation of the postsynaptic mesolimbic D 2 DA receptors by the released DA. DA releasing action of metoclopramide (1.25 and 2.5 mg/kg, i.p.) and the subsequent induction of the stereotyped cage climbing behaviour by these doses of metoclopramide is explained on the basis of selective blockade of the presynaptic D 2 DA autoreceptors by these doses of metoclopramide.

Pentazocine, a kappa opioid receptor agonist, induced catalepsy in mice suggesting thereby that it might possess postsynaptic striatal D 2 dopamine (DA) receptor blocking activity. However, our other findings, that pentazocine pretreatment did not antagonise the cage climbing behaviour induced by the directly acting DA agonist apomorphine in mice and actually potentiated the stereotyped behaviour induced by the indirectly acting DA agonist methamphetamine in mice, indicate that pentazocine does not possess postsynaptic striatal and mesolimbic D 2 DA receptor blocking activity. Pretreatment with naloxone, an antagonist of opioid receptors, antagonised pentazocine-induced catalepsy. This suggests the possible involvement of opioid mechanisms in the induction of catalepsy by pentazocine in mice.

Racemate pentazocine was found to induce stereotyped behaviour (SB) in rats. Pretreatment with haloperidol and alpha-methyl-p-tyrosine significantly antagonised dl-pentazocine induced SB. This indicates that dl-pentazocine induces SB by releasing dopamine (DA) from the nigrostriatal and mesolimbic dopaminergic neurones with resultant activation of the postsynaptic striatal and mesolimbic D2 DA receptors by the released DA. However, pretreatment with naloxone failed to significantly modify dl-pentazocine induced SB indicating thereby that opioid mechanisms are not involved in the DA releasing action of dl-pentazocine. Our findings are explained on the basis of recent reports that the d-isomer of pentazocine releases DA by stimulating sigma receptors located on the nigrostriatal and mesolimbic dopaminergic neurones.