The innervation of the mystacial pad in the adult rat studied by anterograde transport of HRP conjugates.

Peripheral and central terminations of mystacial pad afferents in rats were labeled by anterograde transport of wheat germ agglutinin-HRP (WGA-HRP) or choleragenoid HRP (B-HRP). Tracer was injected in the trigeminal ganglion and survival times were 6-24 h. Most of the innervation previously observed with other techniques in the mystacial pad were labeled by at least one of the tracers. This included extensive reticular endings from large-caliber afferents and a loose network of fine-caliber axons in vibrissal follicle-sinus complexes (F-SCs). Also included were individual highly branching bush-like profiles in the intervibrissal epidermis that arose from fine to medium caliber afferents. Other endings were revealed, such as beaded endings affiliated with tylotrich hairs and presumptive encapsulated lamellated endings affiliated with both vibrissae and small sinus hairs. Finally, the anterograde labeling also revealed differences in the branching pattern of Merkel afferents to the rete ridge collars and ring sinuses of F-SCs. Each tracer produced different patterns of labeling related to the survival time in the mystacial pad which corresponded to particular patterns of labeling in the trigeminal nucleus caudalis. WGA-HRP produced dense labeling of all types of afferents and peripheral endings as well as all laminae of nucleus caudalis after short survivals, but the labeling diffused as the survival times were increased. B-HRP preferentially filled the largest afferents and endings after shorter survivals, while smaller profiles became progressively labeled after longer survivals. In nucleus caudalis, profiles extending into laminae III, IV and inner part of lamina II were labeled with B-HRP after shorter survivals, but the outer part of lamina II also became labeled with longer survivals. This has not been previously observed with B-HRP. Along with other recent findings, these results reveal that the innervation of the mystacial pad especially by fine-caliber axons is far more extensive and complex than previously described. Also, depending on the survival time, the central and peripheral labeling patterns differ, which must be taken into account when interpreting results using these two tracers.