Localization of calcitonin gene-related peptide within the genitofemoral nerve in immature rats

Calcitonin gene-related peptide (CGRP) has been proposed to influence migration and testicular descent by release from the genitofemoral nerve. The site of CGRP within the nerve has been controversial, with conflicting views on whether CGRP is synthesised and released from the motor nerves.

The genitofemoral nerve (GFN) was retrogradely labelled by fluorescent dye (DAPI) in 25 Sprague-Dawley rats (days 5, 16, and 31, n = 8 in each group; day 35, n = 1). Spinal cords and dorsal root ganglia (DRG) were removed two to three days later and sectioned for immunofluorescence. Substance P and CGRP-containing cells were labelled with fluorescein-linked antibodies. Specimens were examined by double fluorescence to identify cells with both markers.

The motor nucleus of the GFN contained 119 cells on day 7 and 284 cells by days 19 through 34. A prominent band of CGRP-containing fibers, arising from the dorsal horn, synapsed with the GFN motor nucleus itself. CGRP-labelled GFN cells were found in the DRG by double labelling.

CGRP from the GFN may affect gubernacular migration by release from the sensory nerves, rather than motor nerves as previously thought. The GFN motor nucleus receives CGRP-containing innervation from the dorsal horn, which may form part of the cremasteric reflex.