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 (W GA-HRP) or choleragenoid HRP (B-HRP). Tracer was injected in the trige minal ganglion and survival times were 6-24 h. Most of the innervation previously observed with other techniques in the mystacial pad were l abeled by at least one of the tracers. This included extensive reticul ar endings from large-caliber afferents and a loose network of fine-ca liber axons in vibrissal follicle-sinus complexes (F-SCs). Also includ ed were individual highly branching bush-like profiles in the intervib rissal epidermis that arose from fine to medium caliber afferents. Oth er endings were revealed, such as beaded endings affiliated with tylot rich hairs and presumptive encapsulated lamellated endings affiliated with both vibrissae and small sinus hairs. Finally, the anterograde la beling also revealed differences in the branching pattern of Merkel af ferents to the rete ridge collars and ring sinuses of F-SCs. Each trac er produced different patterns of labeling related to the survival tim e in the mystacial pad which corresponded to particular patterns of la beling in the trigeminal nucleus caudalis. WGA-HRP produced dense labe ling of all types of afferents and peripheral endings as well as all l aminae of nucleus caudalis after short survivals, but the labeling dif fused as the survival times were increased. B-HRP preferentially fille d the largest afferents and endings after shorter survivals, while sma ller profiles became progressively labeled after longer survivals. In nucleus caudalis, profiles extending into laminae III, IV and inner pa rt of lamina II were labeled with B-HRP after shorter survivals, but t he outer part of lamina II also became labeled with longer survivals. This has not been previously observed with B-HRP. Along with other rec ent findings, these results reveal that the innervation of the mystaci al pad especially by fine-caliber axons is far more extensive and comp lex than previously described. Also, depending on the survival time, t he central and peripheral labeling patterns differ, which must be take n into account when interpreting results using these two tracers.