Inhibition by dopamine of the neurotransmission in the rat vas deferens.

The effects of alpha-adrenoceptor and dopaminoceptor agonists and antagonists were investigated on presynaptic receptors in the prostatic portion of rat vas deferens. The variable studied was the early component (250 msec) of the motor response elicited by field stimulation (single pulses). All the experiments were carried out in the presence of cocaine 30 mumol/l and hydrocortisone 28 mumol/l so as to block the sites of amines loss and 1-propranolol 0.3 mumol/l to block beta-adrenoceptors. Clonidine, noradrenaline (NA) and dopamine (DA) inhibited the motor response in a concentration-dependent manner. DA was 10 and 10(4) times less potent than NA and clonidine respectively. The selective D2 agonist, LY 141865, failed to inhibit the motor response even at a high concentration (30 mumol/l). Yohimbine (0.1, 0.3 and 1 mumol/l) antagonized competitively the effect of clonidine, NA and DA showing similar - log KB values (7.57; 7.68 and 7.09 respectively). Likewise, idaxozan (0.03 mumol/l) blocked the inhibitory effect of DA in the same order of potency (- log KB = 7.81). On the other hand, pimozide 0.21 mumol/l and Schering 23390 3 mumol/l antagonized the inhibitory effect of DA, showing a lower potency than the other antagonists. Taken together, these findings do not support the hypothesis that DA activates a specific population of prejunctional dopaminoceptors to inhibit the motor response elicited by field stimulation in the presence of cocaine, hydrocortisone and 1-propranolol in the prostatic portion of the rat vas deferens. Instead, the population of prejunctional alpha 2-adrenoceptor may be involved.