The effects of spinal or mesencephalic transections on sleep-related erections and ex-copula penile reflexes in the rat

The neural mechanisms of penile erections during paradoxical sleep (PS) rem ain unknown since it has yet to be the subject of neurophysiological invest igation. Using a new experimental model for sleep-related erection research in freely behaving rats, neural transections were undertaken to definitive ly elucidate the effects of paraplegia on PS-related erections and to deter mine at which brain level the mechanisms underlying PS erectile activity ar e generated, Continuous polygraphic recordings, as well as ex-copula penile reflexes, were performed in male Sprague Dawley rats before and after spin al (n = 4) or mesencephalic (n = 6) transections. Spinal transections virtu ally eliminated PS-related erections. Following mesencephalic transections, PS remained qualitatively intact in all rats. PS erectile activity, howeve r, was severely disrupted, as shown by a significant decrease in the total number of erections, the number of erections per hour, and the percentage o f PS phases exhibiting an erectile event. Finally, spinal and mesencephalic transections had contrasting effects on ex-copula penile reflexes. Spinal transections significantly shortened the latency to reflex induction and in creased the percentage of tests eliciting an erectile event, whereas mesenc ephalic transections significantly increased the latency to reflex inductio n without affecting the percentage of tests eliciting an erectile event. Th ese data suggest that the brainstem is not sufficient for the generation of PS erectile activity even though it is sufficient for the generation of ot her classic PS phenomena. We conclude that neural structures rostral to the mesencephalopn (i.e., the forebrain) are essential for the maintenance and integrity of PS related-erections. The reflex erection data suggest that s pinal transection removes a tonic descending inhibition of erections, where as such an inhibition not only remains intact, but appears enhanced followi ng mesencephalic transection. We hypothesize that the forebrain plays a fac ilitatory role in erectile control, at least in part, through disinhibition of brainstem tonic anti-erectile mechanisms.