Chorda Tympani Responses to Gustatory Stimuli in Developing Rats

Abstract

The developmental change in rat taste sensitivity during the period from the 2nd to the 12th postnatal week was studied by recording the responses of the chorda tympani nerve to lingual stimulation by NaCl, LiCl,KCl,NH4Cl,HCl, quinine hydrochloride(Q-HCl),and sucrose. The threshold for all stimulants except sucrose was lower in the 3-weekold rats than in the 12-week-old ones.The order of the magnitudes of the normalized neural responses to the 0.1 M monochloride salts of the gustatory nerves was NH4Cl>NaCl=LiCl>KCl in the 3-week-old rats, and NaCl=LiCl>NH4Cl>KCl in the 12-week-old rats.The magnitude of the normalized responses to NH4Cl, HC1, and Q-HCl at nearly all concentrations was greater at 3 weeks than at 12 weeks,while the magnitude of the responses to NaCl and LiCl at high concentrations was lesser at 3 weeks than at 12 weeks.Changes in the gustatory sensitivity to various taste stimuli during development can be classified into the following two types:(1)sensitivity-decreasing type the magnitude of the response to NH4Cl,Q-HCl,and HCl was greatest during the first few weeks and decreased thereafter,and(2)sensitivity-increasing type the magnitude of the response to NaCl, LiCl,and sucrose showed an increase accompanying the growth of rats from around the 2nd week after birth.Many psychological and behavioral studies on the development of the gustatory system have been performed on human fetuses (LILEY,1972),newborn babies (MALLER and DESOR,1974;),infants(DESOR et al.,1973,adults and elderly people (BYRD and GERTMAN,1959;COOPER et al.,1959),and rats (TEITELBAUM et al.,1969;JACOBS et al.,1977;OHI,1979). An electrophysiological study on the postnatal development of the gustatory system has also been thought necessary for full understanding of the ontogeny of gustatory function.However,neurophysiological studies on the development of taste receptors have been undertaken only by BRADLEY and MISTRETTA(1973),who

Age-related decrease of the chorda tympani nerve terminal field in the nucleus of the solitary tract is prevented by dietary sodium restriction during development

“…This is when stimulus-induced responses reach maturity in control rats (Hill and Almli, 1980;Yamada, 1980;Ferrell et al, 1981). Therefore, the afferent activity responsible for the reorganization of the terminal field in controls may not be present in sodium-restricted rats.…”

“…Both sodium-restricted and control rats were studied at 15-17 days postnatal (P15-17), 25-27 days postnatal (P25-27), 35-37 days postnatal, and 43-200 days postnatal (adult). These ages were selected to correspond to important periods during normal development: during normal increased sensitivity to Na + salts in the NTS (P15-17) (Hill et al, 1983); during the end of the period when the chorda tympani nerve terminal field is adult-like in size within the NTS and when peripheral responses to salts are nearly mature (P25-27) (Hill and Almli, 1980;Yamada, 1980;Ferrell et al, 1981;Lasiter et al, 1989;Lasiter, 1992); before taste responses in NTS neurons mature (P35-37) (Hill et al, 1983), and at adulthood (adult; >50 days) when adult morphology and function occur (Hill and Almli, 1980;Yamada, 1980;Ferrell et al, 1981;Lasiter et al, 1989;Lasiter, 1992). The adult period also corresponds with the age used in our prior studies of dietary-induced changes in chorda tympani nerve terminal field (King and Hill, 1991).…”

“…During normal development, significant stimulus-elicited responses are not seen until about the second post-natal week (Hill and Almli, 1980;Yamada, 1980;Ferrell et al, 1981).…”

Neural Plasticity in the Gustatory System

“…[24][25][26] In fact, responses to some concentrations increase three-fold from the first through the third week postnatally. [24][25][26] A corresponding increase in ENaC function is the underlying mechanism responsible for such a large increase. 27 Therefore, there is an increase in a single transduction pathway accounting for the largest age-related increase in taste function.…”

Myelination in the Chorda Tympani of the Postnatal Rat: A Quantitative Electron Microscope Study

“…Rapid de velopmental changes in electrophysiological responses of the rat chorda tympani to tongue stimulation with salts and acids have recently been described indepen dently by researchers from three different laboratories [9][10][11]. Those studies indicate that the stimulatory effec tiveness of various compounds, relative to each other, changes over time, with changes occurring quite rapidly during the first 20 days after birth, and more slowly after that, until responses at about 50 days are essentially the same as for older adults.…”