Comprehensive immunofluorescence and lectin binding analysis of intervibrissal fur innervation in the mystacial pad of the rat.
The innervation of the intervibrissal fur in the mystacial pad of the rat and mouse was examined by immunofluorescence with a wide variety of antibodies for neuronal related structural proteins, enzymes, and peptides as well as for lectin binding histofluorescence with Griffonia simplicifolia (GSA). Anti-protein gene product 9.5 (PGP) immunofluorescence labeled all sets of axons and endings. The innervation in the upper dermis and epidermis was distributed through a four tiered dermal plexus. From deep to superficial, the second tier was the source of all apparent myelinated mechanoreceptors, the third tier of nearly all the peptidergic and GSA binding innervation, and the fourth tier of nonpeptidergic GSA negative innervation (peptide-/GSA-). Three types of mechanoreceptors-Merkel, transverse lanceolate, and longitudinal lanceolate endings-innervated guard hair follicles. All had similar labeling characteristics for 160 kDa and 200 kDa neurofilament subunits, peripherin, carbonic anhydrase, synaptophysin, and S100. Palisades of longitudinal lanceolate endings were part of piloneural complexes along circumferentially oriented sets of transverse lanceolate endings, peptidergic free nerve endings (FNEs), and peptide-/GSA- FNEs. The longitudinal lanceolate endings were the only mechanoreceptors in the mystacial pad that had detectable calcitonin gene-related peptide. The epidermis contained four types of unmyelinated endings: simple free nerve endings (FNEs), penicillate endings, cluster endings and bush endings. Only the simple FNEs were clearly peptidergic. Virtually all others were peptide-/ GSA-. Each bush ending was actually an intermingled cluster of endings formed by several unmyelinated axons and occasionally an Adelta axon. In contrast to the other unmyelinated innervation to the epidermis, bush endings labeled with an antibody against the Schwann cell protein S100. The necks and mouths of follicles, as well as superficial vasculature, were innervated by a mixture of unmyelinated peptidergic and/or GSA labeled sensory and sympathetic axons. Small presumptive sweat glands were innervated by three sets of peptidergic axons of which one was immunoreactive for somatostatin. Potential functions of the various sets of innervation are discussed.