The pontine micturition center in rat receives direct lumbosacral input. An ultrastructural study.

The act of micturition differs strongly among species. For example, adult cats and humans urinate primarily in a safe environment ('guarded urination'), while rats urinate more reflexively ('reflex urination'). This study in adult rats investigates the existence of direct lumbosacral cord projections to spinally projecting neurons in the pontine micturition center (PMC). Bilateral injections of wheat germ agglutinin horseradish peroxidase in the caudal lumbar and rostral sacral cord resulted in labeled profiles, including retrogradely labeled neurons in the PMC. At the ultrastructural level, anterogradely labeled terminals in the PMC were found, which were filled with many round and some pleiomorphic and flat vesicles. About eleven percent of the terminals contacted retrogradely labeled dendrites. Of the labeled terminals 80% contained asymmetric synaptic clefts, and 20% symmetric synaptic clefts. The results provide evidence that in the rat, unlike the cat, a direct lumbosacral pathway to the PMC exists, which might explain the differences in micturition behavior between rats and cats.