Development of chorda tympani nerve taste responses in the hamster

Selective Removal of Sodium Salt Taste Disrupts the Maintenance of Dendritic Architecture of Gustatory Relay Neurons in the Mouse Nucleus of the Solitary Tract

Neuronal activity plays critical roles in the development of sensory circuits in the mammalian brain. Experimental procedures are now available to alter the function of specific taste transduction pathways and have been especially useful in studying how stimulus-specific taste activity influences the development of central gustatory circuits. We previously used a mouse knock-out (KO) model in which the transduction channel necessary for sodium taste is removed from taste bud cells throughout life. In these KO mice, the terminal fields that carry taste information from taste buds into the nucleus of the solitary tract (NST) fail to mature, suggesting that sodium-elicited taste activity is important for the proper development of central gustatory circuits. Here, we tested the hypothesis that the development and maintenance of the dendritic architecture of NST relay cells, the primary postsynaptic partner of gustatory nerve terminal fields, are similarly dependent on sodium-elicited taste activity. The dendritic fields of NST relay cells, from adult male and female mice in which the α-subunit of the epithelial sodium channel (αENaC) was conditionally deleted in taste bud cells throughout life, were up to 2.4× larger and more complex than that of age-matched control mice. Interestingly, these differences in dendritic architecture did not appear until after the age when terminal fields begin “pruning,” after postnatal day (P)20. Overall, our results suggest that ENaC-mediated sodium taste activity is necessary for the maintenance of dendritic fields of relay cells in the gustatory NST.

Cracking taste codes by tapping into sensory neuron impulse traffic

Insights into the biological basis for mammalian taste quality coding began with electrophysiological recordings from "taste" nerves and this technique continues to produce essential information today. Chorda tympani (geniculate ganglion) neurons, which are particularly involved in taste quality discrimination, are specialists or generalists. Specialists respond to stimuli characterized by a single taste quality as defined by behavioral cross-generalization in conditioned taste tests. Generalists respond to electrolytes that elicit multiple aversive qualities. Na(+)-salt (N) specialists in rodents and sweet-stimulus (S) specialists in multiple orders of mammals are well characterized. Specialists are associated with species' nutritional needs and their activation is known to be malleable by internal physiological conditions and contaminated external caloric sources. S specialists, associated with the heterodimeric G-protein coupled receptor T1R, and N specialists, associated with the epithelial sodium channel ENaC, are consistent with labeled line coding from taste bud to afferent neuron. Yet, S-specialist neurons and behavior are less specific than T1R2-3 in encompassing glutamate and E generalist neurons are much less specific than a candidate, PDK TRP channel, sour receptor in encompassing salts and bitter stimuli. Specialist labeled lines for nutrients and generalist patterns for aversive electrolytes may be transmitting taste information to the brain side by side. However, specific roles of generalists in taste quality coding may be resolved by selecting stimuli and stimulus levels found in natural situations. T2Rs, participating in reflexes via the glossopharynygeal nerve, became highly diversified in mammalian phylogenesis as they evolved to deal with dangerous substances within specific environmental niches. Establishing the information afferent neurons traffic to the brain about natural taste stimuli imbedded in dynamic complex mixtures will ultimately "crack taste codes."

Stimulus-induced increase of taste responses in the hamster chorda tympani by repeated exposure to 'novel' tastants

Variations in amplitude of responses of the chorda tympani to repeated application of various novel tastants were measured in familiarized and control groups of adult hamsters. Three groups of 10 hamsters were pre-exposed to 5 mM dulcin, 50 mM potassium L-glutamate (KGlu) or 1 mM 5'guanosine monophosphate (5'GMP). In the fourth group, the tongue was rinsed with 5'GMP for 20 min just prior to recording from the chorda tympani. The tastants were novel to the fifth group (naïve control). A series of 17 stimuli was repeated six times and responses were quantified relative to the initial response of each of the 50 hamsters. The responses of the chorda tympani increased with repetition in the control group. In contrast, no increase in amplitude of response to the pre-exposed tastants or to stimuli with qualitatively related tastes was observed in the group familiarized with either KGlu or 5'GMP. These results indicate that the response of the chorda tympani depends on previous exposure to a tastant. The sensitivity of taste cells appears to be modulated, possibly by stimulus-induced supplementary receptors.

Recovery of amiloride-sensitive neural coding during regeneration of the gustatory nerve: behavioral-neural correlation of salt taste discrimination

The chorda tympani (CT) nerve innervating the anterior tongue contains two types of NaCl-responsive fibers: one, the N-type, receives input from receptor cells, the NaCl responses of which are strongly inhibited by amiloride, whereas the other, the E-type, receives input from cells poorly sensitive or insensitive to amiloride. To investigate the formation of this differentially responsive neural system, we crushed the mouse CT nerve and examined the subsequent recovery of NaCl responses and amiloride sensitivity of the regenerated nerve and behavioral discrimination between NaCl and KCl. At 2 weeks after the nerve crush, no significant response of the nerve to chemical stimuli was observed. At 3 weeks, responses to salts gradually reappeared. In this period, almost all single fibers responding to NaCl were insensitive to amiloride (E-type). At 4 weeks, some of the single fibers showed amiloride sensitivity (N-type) and behavioral discrimination between NaCl and KCl reappeared. After >or=5 weeks, the number of N-type fibers had reached the control level and became approximately equal to that of E-type fibers. During the course of recovery, N-type and E-type fibers were clearly distinguishable on the basis of their amiloride sensitivities, their KCl/NaCl response ratios, and their concentration-response relationships to NaCl. These results suggest that two salt-responsive systems are independently reformed after the nerve crush. The selective synapse reformation may account for recovery of behavioral discrimination between NaCl and KCl after taste nerve crush and regeneration. It may also explain stable sensory coding for taste quality during the continuous turnover of receptor cells in the healthy animal.

Age-dependency of analgesia elicited by intraoral sucrose in acute and persistent pain models

Treatment of pain in newborns is associated with problematic drug side effects. Previous studies demonstrate that an intraoral infusion of sucrose and other sweet components of mother's milk are effective in alleviating pain in infant rats and humans. These findings are of considerable significance, as sweet tastants are used in pain and stress management in a number of clinical procedures performed in human infants. The ability of sweet stimuli to induce analgesia is absent in adult rats, suggesting that this is a developmentally transient phenomenon. However, the age range over which intraoral sucrose is capable of producing analgesia is not known. We investigated the effects of intraoral sucrose (7.5%) on nocifensive withdrawal responses to thermal and mechanical stimuli in naive and inflamed rats at postnatal days (P) P0-21. In some rats, Complete Freund's adjuvant (CFA) was injected in a fore- or hindpaw to produce inflammation. In non-inflamed animals, for noxious thermal stimuli, sucrose-induced analgesia emerged at P3, peaked at P7-10, then progressively declined and was absent at P17. For mechanical forepaw stimuli, sucrose-induced analgesia emerged, and was maximal at approximately P10, then declined and was absent at P17. By contrast, maximal sucrose-induced analgesia for mechanical hindpaw stimuli was delayed (P13) compared to that for the forepaw, although it was also absent at P17. In inflamed animals, sucrose reduced hyperesthesia and hyperalgesia assessed with mechanical stimuli. Sucrose-induced analgesia in inflamed animals was initially present at P3 for the forepaw and P13 for the hindpaw, and was absent by P17 for both limbs. Intraoral sucrose produced significantly greater effects on responses in fore- and hindpaws in inflamed rats than in naive rats indicating that it reduces hyperalgesia and allodynia beyond its effects on responses in naive animals. These findings support the hypothesis that sucrose has a selective influence on analgesic mechanisms and that an enhanced sucrose effect takes place in hyperalgesic, inflamed animals as compared to naive animals. Taken together, these results indicate that intraoral sucrose alleviates transient pain in response to thermal and mechanical stimuli, and also effectively reduces inflammatory hyperalgesia and allodynia. Sucrose-induced analgesia is age-dependent and limited to the pre-weaning period in rats. The age-dependency of sucrose-induced analgesia and its differential maturation for the fore- and hindpaw may be due to developmental changes in endogenous analgesic mechanisms and developmental modulation of the interaction between gustatory and pain modulatory pathways.

Development of rat chorda tympani sodium responses: evidence for age-dependent changes in global amiloride-sensitive Na(+) channel kinetics

In rat, chorda tympani nerve taste responses to Na(+) salts increase between roughly 10 and 45 days of age to reach stable, mature magnitudes. Previous evidence from in vitro preparations and from taste nerve responses using Na(+) channel blockers suggests that the physiological basis for this developmental increase in gustatory Na(+) sensitivity is the progressive addition of functional, Na(+) transduction elements (i.e., amiloride-sensitive Na(+) channels) to the apical membranes of fungiform papilla taste receptor cells. To avoid potential confounding effects of pharmacological interventions and to permit quantification of aggregate Na(+) channel behavior using a kinetic model, we obtained chorda tympani nerve responses to NaCl and sodium gluconate (NaGlu) during receptive field voltage clamp in rats aged from 12-14 to 60 days and older (60+ days). Significant, age-dependent increases in chorda tympani responses to these stimuli occurred as expected. Importantly, apical Na(+) channel density, estimated from an apical Na(+) channel kinetic model, increased monotonically with age. The maximum rate of Na(+) response increase occurred between postnatal days 12-14 and 29-31. In addition, estimated Na(+) channel affinity increased between 12-14 and 19-23 days of age, i.e., on a time course distinct from that of the maximum rate of Na(+) response increase. Finally, estimates of the fraction of clamp voltage dropped across taste receptor apical membranes decreased between 19-23 and 29-31 days of age for NaCl but remained stable for NaGlu. The stimulus dependence of this change is consistent with a developmental increase in taste bud tight junctional Cl(-) ion permeability that lags behind the developmental increase in apical Na(+) channel density. A significant, indirect anion influence on apical Na(+) channel properties was present at all ages tested. This influence was evident in the higher apparent apical Na(+) channel affinities obtained for NaCl relative to NaGlu. This stimulus-dependent modulation of apical Na(+) channel apparent affinity relies on differences in the transepithelial potentials between NaCl and NaGlu. These originate from differences in paracellular anion permeability but act also on the driving force for Na(+) through apical Na(+) channels. Detection of such an influence on taste depends fundamentally on the preservation of taste bud polarity and on a direct measure of sensory function, such as the response of primary afferents.

New perspectives in a gustatory physiology: transduction, development, and plasticity

Major advances in the understanding of mammalian gustatory transduction mechanisms have occurred in the past decade. Recent research has revealed that a remarkable diversity of cellular mechanisms are involved in taste stimulus reception. These mechanisms range from G protein-and second messenger-linked receptor systems to stimulus-gated and stimulus-admitting ion channels. Contrary to widely held ideas, new data show that some taste stimuli interact with receptive sites that are localized on both the apical and basolateral membranes of taste cells. Studies of taste system development in several species indicate that the transduction pathways for some stimuli are modulated significantly during the early postnatal period. In addition, recent investigations of adult peripheral gustatory system plasticity strongly suggest that the function of the Na+ sensing system can be modulated by circulating hormones, growth factors, or cytokines.

Development of fungiform papillae, taste buds, and their innervation in the hamster

Fungiform taste buds in mature hamsters are less subject to neurotrophic influences than those of other species. This study evaluates taste-bud neurotrophism during development in hamsters by examining the relation between growing nerves and differentiating fungiform papillae. Chorda tympani (CT) or lingual (trigeminal) nerve (LN) fibers were labelled with Lucifer Yellow as they grew into (CT fibers) or around (LN fibers) developing taste buds. Developing fungiform papillae and taste pores were counted with the aid of a topical tongue stain. The tongue forms on embryonic days (E) 10.5-11 and contains deeply placed CT and LN fibers but no papillae. By E12, the tongue epithelium develops scattered elevations. These "eminences" selectively become innervated by LN fibers that grow to the epithelium earlier and in larger numbers than CT fibers. Definitive fungiform papillae form rapidly during E13-14 and become heavily innervated by LN fibers. Intraepithelial CT fibers, rare at E13, invariably innervate fungiform papillae containing nascent taste buds at E14. During E14-15 (birth = E15-16), most papillae contain taste buds with pores, extensive perigemmal LN innervation, and extensive intragemmal CT innervation. At birth, numbers of fungiform papillae and taste pores are adultlike. The results show that fungiform eminences begin forming in the absence of innervation. The subsequent differentiation of definitive fungiform papillae and their innervation by LN fibers occur synchronously, prior to the differentiation of taste buds and their CT innervation. The hamster is precocious (e.g., compared to rat) in terms of LN development and the structural maturity of the anterior tongue at birth.

Differential taste responses of mouse chorda tympani and glossopharyngeal nerves to sugars and amino acids

The differential taste responses of the chorda tympani (CT) and the glossopharyngeal (GL) nerves in preweanling and adult mice were examined by comparing magnitudes of responses to six sugars and 10 amino acids. The results indicate that for sugars the responses of the CT nerve are greater than those of the GL nerve while for umami and essential amino acids the responses of the GL nerve are greater than those of the CT nerve. Such differential taste responses of the CT and GL nerves does not prominently change during development.

Developmental neurobiology of salt taste sensation

A principal process in the homeostatic control of sodium levels is salt intake, and the sense of taste has a primary role in regulating ingestion. Because ingestion of sodium chloride (NaCl) is essential for life, the taste system for salt sensation might be expected to exhibit mature functional characteristics from very early development. However, major changes in gustatory nerve responses to NaCl take place during development. In sheep and rat, the peripheral nerve responses to NaCl are of low magnitude during early development. Progressively, the taste system acquires an increasing proportion of fibers that respond maximally to NaCl. The sodium responsiveness emerges in the context of shifting peripheral innervation patterns and the apparent addition of functional receptor membrane channels sensitive to the sodium transport blocker, amiloride. These developmental processes can be altered by early manipulation of sodium in the diet.

Postnatal development of palatal and laryngeal taste buds in the hamster

Mammalian taste buds are distributed within several distinct subpopulations, innervated by branches of three cranial nerves. These taste bud populations originate and mature at different times in various mammalian species and are thought to play differential roles in the control of taste-mediated behaviors. The hamster is a common animal for the electrophysiological study of the gustatory system, and it has been shown that taste buds innervated by the IXth nerve develop postnatally in this species. To delineate further the development of the gustatory system of hamsters, we quantified the number of taste buds appearing on the palatal, nasopharyngeal, and laryngeal epithelium from birth through 120 days of age. Taste buds are present in almost adult numbers on the soft palate at birth, but only 39% of these are mature. Distinct taste pores, indicative of mature taste buds, increase in number until about 20-30 days of life, at which time all of the taste buds on the soft palate and on the nasoincisive papillae are fully developed. Taste buds are concentrated primarily on the posterior and medial portions of the soft palate. Taste buds located on the laryngeal surface of the epiglottis and the aryepiglottal folds are absent at birth and originate and mature over the following 120 days. Laryngeal taste buds are more concentrated on the aryepiglottal folds than on the epiglottis. On the soft palate and in the epiglottal region, the maturation of taste buds is well characterized by a logarithmic function (Y = a log X + B) relating the number of mature taste buds to postnatal age. On the soft palate, the length of the taste buds from base to apex correlates with the thickness of the epithelium, which increases with development. The diameter of mature taste buds on the soft palate does not change with age. In contrast to many mammalian species, in rodents taste bud development occurs mostly after birth. Rapid postnatal development progresses at a time when ingestive behavior is undergoing a number of significant changes. Taste buds in the larynx have been implicated in a number of laryngeal reflexes (i.e., apnea, swallowing) in several nonrodent species. The electrophysiological properties of superior laryngeal nerve fibers would suggest a similar function for epiglottal taste buds in the hamster.

Behavioral reactions to gustatory stimuli in young chicks (Gallus gallus domesticus)

Freely-moving, posthatch chicks were individually presented 2 concentrations each of quinine, citric acid, fructose, sucrose, sodium saccharin, and distilled water and their behavioral reactions were videotaped and analyzed. Already during the first posthatch day distinct rejection responses to quinine and citric acid could be recognized. Prolonged head shaking and beak clapping episodes were the most dominant features of these reactions. While responses to water and sweet stimuli could be interpreted as acceptance behaviors, the resolution was generally not fine enough to discriminate between reactions to the 2 different sweet concentrations of these stimuli or between them and water. When only water or sugar solutions were presented to other hatchlings in a single session, there was a suggestion of more definite acceptance behavior to some sweet stimuli as compared to water. It is concluded that the systems mediating aversive gustatory responses are present and functioning in posthatching chicks while acceptance responses, though present, are less discriminative among stimuli.