Developmental changes in taste response characteristics of rat single chorda tympani fibers.

Integrated records show that multifiber responses from the chorda tympani nerve to NaCl and LiCl are of very small magnitude in young postnatal rats. When compared with NH&l, KCl, and citric acid, the integrated responses to NaCl and LiCl gradually increase in magnitude as a function of development. To clarify the basis of changes in multifiber responses, we recorded from single fibers in the chorda tympani nerve in postnatal rats aged: 14 to 20 days, 24 to 35 days, and adult. Taste stimuli applied to the anterior tongue were: 0.1 and 0.5 M NaCl, LiCl, NH4C1, and KC1 and 0.1 M citric acid. Even in the youngest rats, at least 89% of the single fibers responded to all salts. Therefore, the small magnitude of the multifiber responses to NaCl and LiCl in young animals cannot be explained by an inability of young receptors to respond to these salts. However, single fiber response frequencies during lingual stimulation with NaCl and LiCl were low in young rats, and during development, the frequencies increased to approximately twice the youngest values. Concomitantly, the response frequencies for NH&l did not change and those for KC1 remained the same or progressively increased, depending on the concentration. The responses to citric acid were different from those to any of the salts because response frequencies decreased during development. We also found that more fibers responded maximally to NaCl and LiCl as a function of age and fewer fibers were maximally responsive to NH&l. For all salts and citric acid, a developmental decrease in response latencies was observed. Single fiber data have revealed that the previously observed changes in multifiber salt responses are attributable to differential changes in response frequencies and in the ranking of salts that are maximally effective. Developmental changes in taste response frequencies and latencies must relate to one or more processes at various levels of the peripheral gustatory system, including the taste pore, the receptor membrane, the synapses between receptor cells and chorda tympani afferents, and the chorda tympani fibers themselves. Neurophysiological changes associated with development have been documented extensively in the visual and auditory systems (e.g., Movshon and Van Sluyters, 1981; Saunders and Brock, 1978). Very recently, it has become apparent that developmental changes also occur ’ This work was supported by National Institutes of Health National Research Postdoctoral Service Award NS06423 to D. L. H., National Science Foundation Grant BNS 8015737 to C. M. M. and R. M. B., and National Institute of Dental Research Research Career Development Award DE 00066 to C. M. M. ’ To whom correspondence should be addressed. ,’ Also from the Center for Human Growth and Development and Research Area, School of Nursing, University of Michigan, Ann Arbor, MI 48109. in the gustatory sense. Studies in sheep have demonstrated that neurophysiological taste responses in the peripheral and central nervous systems alter substantially during fetal development and after birth (Bradley and Mistretta, 1980; Mistretta and Bradley, 1978, 1980). Changes in peripheral taste responses now have been observed in the rat also (Ferrell et al., 1981; Hill and Almli, 1980; Yamada, 1980), a mammal with a much shorter developmental time course than sheep. The sheep has a lengthy gestation (150 days) and structural development of the peripheral taste system is primarily a prenatal event (Bradley and Mistretta, 1973). In rats, taste buds appear toward the end of the short gestation (21 days) and major morphological changes occur postnatally (Farbman, 1965; Mistretta, 1972). ’ Also from the Department of Physiology, University of Michigan, Developmental studies of the rat taste system have Ann Arbor, MI 48109. included multifiber recordings of neural responses to