Aim of the present study was to verify, by means of double retrograde neuronal tracers technique, the hypothesis that a subpopulation of sensory and autonomic neurons send collateral axons to both smooth and striated genital muscles. We also wanted to define the neurochemical content of the eventually retrogradelly double labeled (RDL) neurons in the sympathetic trunk ganglia (STG). We used six intact pigs and we injected the tracer Diamidino Yellow (DY) in the smooth left retractor penis muscle (RPM) and the tracer Fast Blue (FB) in the striated left bulbospongiosus muscle (BSM). Rare (2 +/- 0.6) RDL neurons were found in the ipsilateral S2 spinal ganglion (SG), 220 +/- 42 in the ipsilateral STGs, from L3 to S3, 19 +/- 15 in the contralateral S1–S2 ones and 22 +/- 5 in the bilateral caudal mesenteric ganglia (CMG). The RDL neurons of the STG were IR for TH (85 +/- 13%), DbH (69 +/- 17%), NPY (69 +/- 23%), nNOS (60 +/- 11%), LENK (54 +/- 19%), VIP (53+/-26%), SOM (40 +/-8%), CGRP (34 +/- 12%), SP (31 +/- 16%), and VAChT (28 +/- 3%). Our research highlights the presence of sensory and sympathetic neurons with qualitatively different neurochemical content sending axons both to the smooth RPM and to the striated BSM of the pig. These RDL neurons are likely to project to the smooth vasal musculature to create the ideal physiological conditions in which these muscles can optimize the erectile function.
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