Distribution and peptide content of sympathetic axons innervating different regions of the cutaneous venous bed in the pinna of the guinea pig ear.

This study utilised catecholamine fluorescence histochemistry, multiple-labelling immunohistochemistry and retrograde axonal transport to determine the distribution and neuropeptide content of sympathetic neurons innervating veins in the dorsal skin of the guinea pig pinna. There was a dense plexus of sympathetic axons innervating the large central vein at the base of the ear and the adjoining maxillary vein. The density of the plexus decreased towards the distal margin of the ear and in smaller veins, and was very sparse in the lateral veins at the base of the ear. Small venules < 50-80 microns diameter were not innervated. Probably all noradrenergic axons in the large veins near the base of the ear contained immunoreactivity (IR) to neuropeptide Y (NPY), but the proportion of axons with NPY-IR decreased in large veins in more lateral regions of the ear, and in smaller veins (80-100 microns diameter). Injections of Fast Blue or DiI close to large veins in the ear resulted in retrograde labelling of tyrosine hydroxylase-IR neurons in the ipsilateral superior cervical ganglion, 64% of which contained NPY-IR. Nearly all (> 90%) of the TH-IR axons in small and large veins also contained IR to dynorphin-related peptides. Thus, sympathetic venoconstriction in the thermoregulatory bed of the guinea pig pinna is likely to occur predominantly in the large central veins towards the base of the ear. Furthermore, sympathetic constriction is likely to be qualitatively different in large cutaneous veins from that occurring in small veins, and absent altogether from small venules and some lateral veins.