Central control of penile erection: a re-visitation of the role of oxytocin and its interaction with dopamine and glutamic acid in male rats

Oxytocin is a potent inducer of penile erection when injected into the central nervous system. In male rats, the most sensitive brain area for the pro-erectile effect of oxytocin is the paraventricular nucleus of the hypothalamus. This nucleus and surrounding regions contain the cell bodies of all oxytocinergic neurons projecting to extra-hypothalamic brain areas and the spinal cord. This review shows that oxytocin induces penile erection also when injected in some of these areas (e.g., ventral tegmental area, ventral subiculum of the hippocampus, posteromedial cortical nucleus of the amygdala and thoraco-lumbar spinal cord). Microinjection studies combined with intra-cerebral microdialysis and double immuno-fluorescence studies suggest that oxytocin in these areas activates directly or indirectly (mainly through glutamic acid) mesolimbic dopaminergic neurons. Dopamine released in the nucleus accumbens in turn activates neural pathways leading to the activation of incerto-hypothalamic dopaminergic neurons in the paraventricular nucleus. This activates not only oxytocinergic neurons projecting to the spinal cord and mediating penile erection, but also those projecting to the above extra-hypothalamic areas, modulating directly or indirectly (through glutamic acid) the activity of mesolimbic dopaminergic neurons controlling motivation and reward. Together these neural pathways may constitute a complex hypothetical circuit, which plays a role not only in the consummatory phase of sexual activity (erectile function and copulation), but also in the motivational and rewarding aspects of the anticipatory phase of sexual behaviour.