Dopaminergic drugs in the medial preoptic area and nucleus accumbens: effects on motor activity, sexual motivation, and sexual performance.

In two experiments, dopamine agonists and/or antagonists were injected into the medial preoptic area (MPOA) or the nucleus accumbens (NAcc) of male rats. The animals were then tested in an X-mase with four goal boxes, which contained a receptive female, a male, or were empty. In Experiment 1, the D1 antagonist SCH-23390 and the D2 antagonist raclopride in the MPOA decreased the percentage of trials on which the female's chamber was chosen, a measure of sexual motivation. Raclopride also decreased the number of animals that copulated after choosing the female's chamber. The 10-micrograms dose of the D3/D2 agonist quinelorane increased the latency to reach the female's chamber, slowed the onset of copulation, and decreased the number of intromissions preceding an ejaculation. In Experiment 2, 1- and 5-micrograms doses of quinelorane and of the mixed D1/D2 agonist apomorphine were injected bilaterally into the NAcc. Both doses of quinelorane increased the number of times that the subject did not select a chamber within 60 s. No drug in the NAcc affected specifically sexual motivation or performance. The results are consistent with differential influence of the MPOA and the NAcc on motor activity, sexual motivation, and sexual performance.

Opposite influence of medial preoptic D1 and D2 receptors on genital reflexes: implications for copulation.

Dopamine D1 and D2 receptors may synergize with or oppose each other's effects. We suggest that stimulation of D1 and D2 receptors in the medial preoptic area (MPOA) of male rats have opposing effects on genital reflexes. In Experiment 1 a D1 agonist injected into the MPOA increased the number of ex copula erections but decreased the number of seminal emissions. In Experiment 2 a D1 antagonist had the opposite effects (decreased erections and increased seminal emissions), as had a D2 agonist previously. We also suggest that D1 and D2 mechanisms in the MPOA have different thresholds of activation. In Experiment 3 a low dose of the mixed D1/D2 agonist apomorphine increased erections and anteroflexions, an effect blocked by the D1 antagonist. In Experiments 3 and 4 a high dose of apomorphine increased seminal emissions, an effect blocked by the D2 antagonist. Thus, low levels of dopaminergic stimulation may facilitate erections and anteroflexions (controlled by the parasympathetic system and striated muscles) via D1 receptors; higher or more prolonged stimulation may shift to seminal emission (controlled by the sympathetic system) via D2 receptors. This may explain the progression from erectile to ejaculatory mechanisms during copulation.

Microinjection of the dopamine antagonist cis-flupenthixol into the MPOA impairs copulation, penile reflexes and sexual motivation in male rats.

Microinjection of the dopamine antagonist cis-flupenthixol into the medial preoptic area was previously shown to impair male rat copulatory behavior. The present experiments provide further evidence of cis-flupenthixol's inhibitory effects on male sexual behavior. Following microinjections of moderate to high doses of cis-flupenthixol, males exhibited slower copulatory rates and fewer ejaculations in copula, fewer ex copula erections and penile movements, and reduced sexual motivation in an X-maze. Locomotion in the X-maze was not significantly affected. Microinjections of the inactive isomer trans-flupenthixol produced no change in any behavioral measure, indicating that cis-flupenthixol's effects were receptor mediated. We suggest that dopamine receptors in the MPOA influence copulation primarily by regulating reflexive and motivational factors, but not locomotion.