Prenatal development of peptidergic primary afferent projections to mouse lumbosacral autonomic preganglionic cell columns

To examine the prenatal development of spinal visceral reflexes, primary sensory nerve fibers immunoreactive for calcitonin gene-related peptide (CGRP) were examined in the spinal cord, particularly in the autonomic preganglionic nuclei of mouse embryos. On embryonic day 16 (E16), CGRP-immunoreactive fibers were first observed in the sacral intermediolateral nucleus (IML) of the parasympathetic division as well as in the lumbar central autonomic nucleus (CA) of the sympathetic division, where they appeared in proximity to preganglionic neuronal perikarya immunoreactive for choline acetyltransferase or nitric oxide synthase. Most of the CGRP-immunoreactive varicosities were negative for substance P. Substance P-immunoreactive varicosities were scattered in these nuclei, but no appositions were seen on the preganglionic neuronal perikarya. On E18, CGRP-immunoreactive fibers were more abundant in the sacral IML and the lumbar CA. Co-expression of substance P and CGRP was frequently observed in the varicosities very close to the preganglionic neuronal perikarya on E18. CGRP-immunoreactive fibers were also observed in the lumbar IML on E18, although significantly fewer were found in this nucleus compared with the sacral IML. In contrast to the upper lumbar level, no fibers immunoreactive for CGRP were observed in the IML at the thoracic level. These results suggest that peptidergic primary sensory fibers grow to project to the selective targets of autonomic preganglionic neurons during the embryonic period. The potential direct connections between the peptidergic primary sensory fibers and preganglionic neurons innervating the pelvic viscera might provide a circuit for spinal visceral reflexes active in embryos.