Activation of a subset of lumbar spinothalamic neurons after copulatory behavior in male but not female rats

The precise pathways that convey copulation-related information to forebrain regions activated during male and female sexual behavior are poorly understood. Previous work from our laboratory and others has demonstrated the existence of a spinothalamic pathway that is a candidate to relay information to these areas. This pathway originates from a population of spinothalamic neurons in the lumbar spinal cord containing several neuropeptides including galanin, located in laminas 7 and 10 of the lumbar segments 3 and 4. To investigate the involvement of these lumbar spinothalamic neurons in conveying copulation-related information, we tested the hypothesis that these cells are activated after ejaculation in male rats and vaginocervical stimulation in female rats. This was assessed using galanin or cholecystokinin as a marker for this subset of spinothalamic neurons and Fos-immunoreactivity as a marker for neuronal activation. The results demonstrated that activation of these spinothalamic neurons is triggered by stimuli associated with ejaculation. Fos induction was specifically associated with ejaculation, because mounts or intromissions did not trigger expression. Moreover, these spinothalamic neurons were not activated by vaginocervical stimulation in female rats. Spinothalamic neurons have generally been associated with signaling pain and temperature information. The present findings demonstrate that a specific subpopulation of spinothalamic neurons signals information associated with ejaculation.