Background: Neuroanatomical and lesion studies have identified lumbar spinothalamic (LSt) neurons to be essential for ejaculation, but their precise role remains elusive.
Objective: To assess the role of LSt neurons as a spinal pattern generator for ejaculation (SGE) and their action on anatomical structures involved in the two ejaculation phases, the emission and expulsion of semen.
Design: The bulbospongiosus muscle (BSM) was implanted with electrodes and the seminal vesicle (SV) or vas deferens (VD) lumen catheterized in adult anaesthetized rats. Spinal exposure at the fourth lumbar segment (L4) allowed lowering an electrode stereotaxically into area VII/X for brief (300-500ms) electrical stimulation of LSt neurons, while recording BSM-EMG and intraluminal SV or VD pressure.
Results: Brief electrical microstimulation in the LSt neuron area evoked the expulsion of semen in 17 of 17 rats, with motile spermatozoa in 10 of 17 rats. After stimulation, SV/VD luminal pressure directly rose and fell, followed by rhythmic BSM contractions lasting approximately 25s. Acute T8-T9 spinalization (n=4) did not alter the activation pattern of the BSM-EMG response. Injection of the GABA(A)-receptor agonist muscimol, inhibiting neuronal activity into the LSt neuron area after LSt neuron microstimulation (n=5), stopped BSM contractions in midstream.
Conclusions: Electrical microstimulation of LSt neurons activates the entire sequence of ejaculation in rats in a coordinated fashion, ie the emission (SV/VD contraction) followed by expulsion (rhythmic BSM contractions) of living spermatozoa. Midcourse interruption of ejaculation following intraspinal muscimol injection establishes that LSt neurons are the SGE. This could help to identify spinal pharmacological targets for the treatment of ejaculatory disorders and provide the rationale for intraspinal stimulation to treat anejaculation in infertile spinal cord injured (SCI) patients.