Immunocytochemical evidence of plasticity in the nervous structures of therat lower lip

In this immunocytochemical study we investigated the distribution of nervou s structures in the lower lip of adult rats. The region is characterized by a rich cutaneous and mucosal sensory innervation originating from terminal branches of the trigeminal system. Lower lip innervation was investigated by detection of the general neuronal marker protein gene product 9.5 (PGP 9 .5) and the growth-associated protein 43 (GAP-43), a neurochemical marker o f neuronal plasticity. The entire neural network of both cutaneous and muco sal aspects was stained by the antibody to PGP 9.5. In particular, nerve fi bers were observed in the submucosal and the subepithelial plexuses. Thin i mmunoreactive fibers were observed within the epithelial layers ending as f ree fibers or as fibers associated with immunopositive Merkel cells. Well-i dentified anatomical structures receiving sensory or autonomic innervation were also surrounded by PGP 9.5-ir nerve fibers, in particular, hair follic les, vibrissae, glands, and blood vessels. GAP-43-immunostained nerve fiber s were observed in all these structures; however, they were generally less numerous than the PGP 9.5-immunoreactive elements. An equal amount of PGP 9 .5 and GAP-43 immunoreactivity occurred, in contrast, in the subepidermal a nd the submucosal plexuses, or in the epidermis and the mucosal epithelium. The present results show that GAP-43 is normally expressed in the mature t rigeminal sensory system of the rat. Skin and oral mucosa are characterized by continuous remodeling that may also involve the sensory nervous apparat us. Continuous neural remodeling, regeneration and sprouting may be the rea son for the observed expression of GAP-43.