Inhibition of sexual reflexes by lumbosacral injection of a GABAB agonist in the male rat.

The effects of gamma-aminobutyric acid (GABA) agonists on penile reflexes were investigated. An intrathecal injection of baclofen (0.2, 0.4, or 0.8 microgram), a GABAB receptor agonist, into the subarachnoid space of the lumbosacral spinal cord (L5-S1), resulted in a dose-related decrease in the number of animals responding in a penile reflex test. Doses of 0.2 and 0.4 microgram of baclofen decreased the number of erections; 0.4 microgram also increased the latency to the first glans erection. The highest dose of baclofen (0.8 microgram) completely inhibited penile responses in these tests. None of these doses, however, prevented rats from copulating to ejaculation. Antecedent ejaculation, which facilitated the onset of penile reflexes in saline controls, also blocked the inhibitory effects on penile responses by the lower doses (0.2 and 0.4 microgram) of baclofen, but was ineffective in animals treated with 0.8 microgram baclofen. In contrast to the inhibitory effects of baclofen in the lumbosacral cord, an intrathecal injection of baclofen (0.8 microgram) at thoracic segments (T8-T10) did not affect penile erections elicited following an ejaculation. The role of spinal GABAA receptors in sexual reflexes was assessed by intrathecal injection of a GABAA agonist. THIP (0.5.1. or 2 micrograms), onto the lumbosacral cord. Only at the largest dose of THIP were slight inhibitory effects on penile reflexes observed. Together, these data indicate that stimulation of GABAB receptors in the lumbosacral spinal cord inhibits erectile mechanisms ex copula.