Recovery of salt taste responses and PGP 9.5 immunoreactive taste bud cells during regeneration of the mouse chorda tympani nerve.

Taste receptor cells are replaced with an average life span of ~10 days in mammals. This turnover is accompanied by continuing synaptic reconnection between newly formed taste cells and gustatory fibers. However, little is known of how sensory information of the fibers is maintained during the synaptic reconnection. Our previous study examined taste responses of regenerated mouse chorda tympani (CT) fibers and revealed that each fiber type classified based on sensitivity to amiloride maintains its characteristics after synapse reformation between regenerated taste axons and receptor cells. That is, we found that there are approximately equal numbers of two types of NaCl-responsive neurons; one type showed strong suppression by amiloride [amiloride-sensitive (AS), N-type fiber], and the other type showed only weak or no suppression by amiloride [amiloride-insensitive (AI), E-type fiber] in intact, regenerated and cross-regenerated taste nerve (Ninomiya, 1998). Our subsequent study investigated the processes of reformation of AI and AS neural systems during the CT regeneration. We postulated and verified whether incoming regenerated CT axons would (i) induce AI and AS properties after synapse formation with identical progenitors; (ii) innervate taste cells randomly followed by elimination of mismatched branches; or (iii) selectively innervate AS or AI taste progenitor cells (Yasumatsu et al., 2003). The results revealed that NaCl responses of the CT recovered from 3 weeks after the nerve crush and most NaCl responsive fibers showed AI (E-type). The number of fibers responding to NaCl after amiloride formed a bimodal distribution from 4 weeks and there were no clusters of fibers with intermediate sensitivity to amiloride (Yasumatsu et al., 2003, figure 6). Moreover, N-type and E-type were clearly different in Kd value and response selectivity (KCl/NaCl response ratios) right from the beginning of their appearance. Thus, these findings from our electrophysiological studies are consistent with the view that regenerating taste axons selectively innervate their corresponding classes of taste progenitor cells. In the present study, to test the possibilities further, we examined PGP9.5, a maker of neurons (Thompson et al., 1983) and sensory paraneurons (Iwanaga et al., 1992), immunoreactive (IR) taste bud cells and the number of taste buds after crushing the mouse CT nerve.