The reduction of water intake in rats caused by a low dose of apomorphine is unaltered by alpha-methyl-p-tyrosine: are autoreceptors not involved?

Low doses of the dopamine (DA) agonist apomorphine (APO) induces a behavioural syndrome characterized by reduced spontaneous activity, reduced food and water intake and induction of yawning and penile erections. Traditionally these effects of APO have been considered to be caused by a preferential stimulation of DA autoreceptors, causing a decreased amount of transmitter at the postsynaptic receptors. If this is so, it could be hypothesized that 1) the same behavioural effects should be obtained if DA transmission is decreased by some other means, for example by synthesis inhibition, and that 2) the response to APO should be altered if DA transmission is already lowered. It was found that high doses of alpha-methyl-p-tyrosine (alpha-MPT; 50-200 mg/kg) did not reduce water intake in thirsty rats, which low doses of APO do. It was further found that pretreatment with alpha-MPT did not alter the response to APO. These results are difficult to reconcile with the DA autoreceptor hypothesis claiming that behavioural effects of low doses of APO are caused by a decreased release of DA. An alternative interpretation is that low doses of APO stimulates a certain population of sensitive postsynaptic D-2 receptors.