Male rats put in the presence of a receptive female rat that they can see, hear and smell, but cannot touch, show penile erection episodes. These non-contact erections occur concomitantly with an increase in nitric oxide production in the paraventricular nucleus of the hypothalamus, as detected by the increase in the NO2– and NO3– concentration in the paraventricular dialysate obtained from these males by in vivo microdialysis.
NO2– concentration increased from 0.81 ± 0.12 to 2.51 ± 0.43 μm and that of NO3– from 4.50 ± 0.73 to 8.31 ± 2.3 μm. The NO2– increase was prevented by the nitric oxide synthase inhibitor NG–nitro-l-arginine methylester (20 μg) given unilaterally in the paraventricular nucleus, which also prevented non-contact erections. In contrast, the nitric oxide scavenger haemoglobin (20 μg) prevented the NO2– increase, but not non-contact erections; while the guanylate cyclase inhibitor methylene blue (20 μg) was ineffective on either response. NO2– and NO3– concentration was also increased in the paraventricular dialysate of male rats during in copula penile erections, that is, when sexual activity was allowed with the receptive females. As found with non-contact erections, NG–nitro-l-arginine methylester prevented NO2– increase and impaired copulatory behaviour; haemoglobin prevented NO2– increase only; and methylene blue was ineffective on either response. The present results confirm that nitric oxide is a physiological mediator of penile erection at the level of the paraventricular nucleus of the hypothalamus.