Bruce Oakley

BDNF is required for the normal development of taste neurons in vivo.

THE vallate gustatory epithelium of neonatal trkB null mutant mice (−/−) lacked innervation. This prompted the evaluation of null mutant mice corresponding to the three neurotrophin ligands for tyrosine kinase receptor B (TrkB): brain-derived neurotrophic factor (BDNF), neurotrophin (NT)3, NT4. The vallate gustatory epithelium of nt3−/− mice and of nt4−/− mice appeared normal. Only bdnf−/− mice had a vallate papilla that was stunted, sparsely innervated, and lacked up to 98% of its taste buds. All three defects persisted. For example, the vallate papilla of 12-day-old bdnf−/− mice remained markedly less well innervated than the vallate of 7-day-old or newborn bdnf+/+ mice. The foliate taste papillae of neonatal bdnf−/− mice had similar defects. We conclude that the normal development of taste neurons requires BDNF.

SINGLE UNIT DISCRIMINATION OF FISH ODOURS RELEASED BY CHAR (SALMO ALPINUS L.) POPULATIONS

Abstract 1. 1. Dilute fish odours from four distinct char populations elicited differential responses from char olfactory bulb cells. 2. 2. Differential sensory responses produced by odours from migratory and non-migratory populations of char suggest that fish odours could act as pheromones which help guide homing in these salmonids. 3. 3. Skin mucous may well be the principal source of odorants released by the char.

Transduction physiology of olfactory receptor cilia

Electro-olfactograms (EOGs) evoked by 8 odorants from frog olfactory epithelia during ciliary regrowth and during epithelial regeneration were analyzed. During ciliary regrowth following detergent-induced ciliary removal, EOG amplitudes initially increase proportionately with ciliary length. EOGs reach maximal amplitudes after 2 days of growth, when cilia are 40 micron long. Therefore olfactory transduction sites are located primarily on cilia rather than on the dendrite terminal and most of the receptor current enters through the proximal portion of the cilium. Zinc sulfate lavage of the nasal cavity causes selective necrosis of the receptor epithelium. During epithelial regeneration, EOGs increase linearly with time from 13 days after zinc lavage, the time of first cilium emergence, through 30 days. The rate of increase is different for different odorants. At 30 days and within a period of a few days, EOG amplitudes increase abruptly, then asymptote. Thus the development of receptors for different substances occurs at different rates and occurs in two steps. The transition between the two developmental states is coincident with arrival of receptor axon terminals at the central nervous system and with the immobilization of the ciliary contractile apparatus. Since there is continual generation of new receptor neurons throughout life, EOGs recorded in a normal nose reflect a complex combination of the differing receptor processes of cells of differing developmental stages.

THE MORPHOGENESIS OF MOUSE VALLATE GUSTATORY EPITHELIUM AND TASTE BUDS REQUIRES BDNF-DEPENDENT TASTE NEURONS

The developmental absence of brain-derived neurotrophic factor (BDNF) in null mutant mice caused three interrelated defects in the vallate gustatory papilla: sparse innervation, a reduction in the area of the gustatory epithelium, and fewer taste buds. On postnatal day 7, the stunted vallate papilla of bdnf null mutant mice was 30% narrower, the trench walls 35% reduced in area, and the taste buds 75% less abundant compared with wild-type controls. Quantitative assessment of innervation density was carried out to determine if the small trench walls and shortage of taste buds could be secondary consequences of the depletion of gustatory neurons. The diminished gustatory innervation was linearly associated with a reduced trench wall area (r=+0.94) and fewer taste buds (r=+0.96). Residual taste buds were smaller than normal and were innervated by a few surviving taste neurons. We conclude that BDNF-dependent taste neurons contribute to the morphogenesis of lingual gustatory epithelia and are necessary for both prenatal and postnatal mammalian taste bud formation. The gustatory system provides a conspicuous example of impaired sense organ morphogenesis that is secondary to sensory neuron depletion by neurotrophin gene null mutation.

Neural control of ectopic filiform spines in adult tongue.

The tongue surface directly above a fungiform taste bud is flat, thinly keratinized, and free of filiform spines. We examined fungiform papillae in serial sections of rat and gerbil tongues after unilateral transection of the chorda-lingual nerve had caused many fungiform taste buds to degenerate. Such empty fungiform papillae often formed a solitary keratinized outgrowth that closely resembled the spine of an ordinary filiform papilla. By six months an ectopic spine was found on 61% of empty fungiform papillae, but never on fungiform papillae that contained a taste bud. Experimental innervation of the tongue reduced the incidence of ectopic filiform spines in proportion to the cross-sectional area of the trigeminal nerve branches tested (the mylohyoid nerve, the lingual nerve, lingual + mylohyoid or lingual + auriculotemporal nerves). The chorda tympani nerve was 60 times more effective than trigeminal nerves in preventing ectopic filiform spines. We suggest that positive and negative trophic actions are normal characteristics of taste axons, for they promote the formation of taste buds and prevent the expression of ectopic filiform spines. By preventing the outgrowth of ectopic spines on fungiform papillae, taste axons maintain a thinly keratinized apical surface that can be breached by the taste receptor cells.

On the specification of taste neurons in the rat tongue

SUMMARY It is known in mammals that regenerating taste fibers will reform, innervate and maintain new taste buds in characteristic tongue locations--the taste papillae. The ability of non-taste sensory and motor nerve fibers (auriculo-temporal, mylohyoid and hypoglossal nerves) to substitute for a taste (IXth) nerve was evaluated by whole nerve recording and anatomical examination of experimentally innervated rat tongues. The non-taste nerve endings grew into the tongue and were physiologically responsive to cooling and to localized mechanical stimulation of the foliate and circumvallate taste papillae on the tongue. However, unlike the control regenerated IXth nerve these non-taste nerves were unable to (1) reform new taste buds, or (2) innervate, and (3) maintain existing taste buds. It was suggested that taste fibers have physico-chemical properties which permit interaction with groups of specialized cells in the dorsal epithelium of the tongue (taste receptor cell precursors) by a process involving recognition between matched taste fiber and nascent receptor cell.

The gustatory competence of the lingual epithelium requires neonatal innervation

The rat vallate papilla is bilaterally innervated by the IXth nerve whose axons are required for the normal development of its several hundred taste buds. Temporary denervation during the developmental sensitive period for taste buds prevented most vallate taste buds from forming. Specifically, removing one IXth nerve and crushing the other in 3 day old neonates eliminated axons from the vallate papilla for about 10 days and by adulthood resulted in a mean +/- 1 S.E.M. of 48 +/- 12 vallate taste buds. Two explanations for the shortfall of adult vallate taste buds were evaluated: either 10 days of neonatal denervation impaired the gustatory competence of the vallate papilla, or the IXth nerves trophic support of taste buds failed to recover after nerve crush on day 3. In adults, it was found that a IXth nerve previously crushed on day 3 would support numerous vallate taste buds (183 +/- 27), provided that the vallate papilla had been continuously innervated by the contralateral IXth nerve during neonatal development. Consequently, taste neurons, whose axons had been crushed on day 3, seemed to survive and retain their trophic capacity to support taste buds in adults. To test for diminished competence of the gustatory epithelium, one IXth nerve was crushed on day 3 while the contralateral IXth nerve was removed. Beginning on day 75, the chorda tympani nerve was substituted for the re-innervating axons of the crushed IXth nerve. The cross-innervating chorda tympani ultimately supported only 51 +/- 10 vallate taste buds. In contrast, in vallate papillae that developed without interruption of the contralateral IXth nerve during the sensitive period, the cross-innervating chorda tympani by itself supported more than four times as many vallate taste buds (214 +/- 22). Evidently, a neonatal period of denervation permanently restricts the gustatory competence of the vallate epithelium; nerve-dependent precursors of taste receptor cells probably died or permanently changed their fate.

p53 and Bax: Putative death factors in taste cell turnover

The turnover of cells in renewing epithelia presents an opportunity to examine cell death pathways in adult vertebrates. In mouse lingual epithelium a typical taste receptor cell survives for 9 days, until it is killed by an unknown cascade of death factors. Apoptosis was implicated by the presence of fragmented DNA in about 8% of taste receptor cells in the vallate papilla. In using immunocytochemistry to seek putative death factors, we observed that squamous epithelial cells of the tongue were negative for Bax, a death factor in the Bcl‐2 family of survival/death factors, and were also negative for p53, a tumor‐suppressor protein linked to apoptosis and Bax transcription. In contrast, 8–10% of the taste receptor cells were Bax‐positive, and 9–11% were p53 positive. These immunopositive taste receptor cells were more likely to display death‐related morphologic defects than other receptor cells, and they frequently coexpressed p53 and Bax. In both neonatal and adult mice, the labeling of dividing cells with 5‐bromo‐2′‐deoxyuridine indicated that all Bax‐positive taste cells were at least 5 days old. On postnatal day 7, when few taste cells were old, no more than 1% of taste cells were immunopositive for either p53 or Bax. We inferred that old taste receptor cells employ p53 and Bax as part of their apoptotic death pathway. The routine expression of p53 by postmitotic, aged taste cells broadens the conventional view that p53 is restricted to mitotic cells that have stress‐damaged DNA. Furthermore, the scattered distribution of aged receptor cells within the taste bud excludes some explanations for stable taste signals during receptor cell turnover. J. Comp. Neurol. 413:168–180, 1999.

Comparative gustatory responses in four species of gerbilline rodents

SummaryIntegrated taste responses to chemical stimulation of the tongue were recorded from the intact chorda tympani nerve in four species of gerbils (Meriones libycus, M. shawi, M. unguiculatus andPsammomys obesus). 1.Sucrose was one of the most effective stimulants.2.In comparison with sucrose, NaCl was less effective in gerbils than in other rodents. The smallest NaCl responses were found in those gerbil species whose native habitat and food subject them to the greatest osmotic stress.3.The non-linearity of the NaCl concentration-response data suggested there may be more than one type of NaCl receptor site.4.Two groups of cations were recognized as stimulants inMeriones species: (Na+, Li+) and (K+, Rb+, Cs+).5.The stimulating effectiveness of methylα andβ-D-glucopyranoside was compared with several naturally occuring sugars. From this analysis, it was concluded that the stimulating potency of glucopyranoside is increased by an alpha linked substituent, as in sucrose.6.The only reliable neural responses to water rinse occured with the divalent cations in the two species not found in extreme desert. It is improbable that gustatory water responses play a role in the water balance of desert rodents.7.Sunflower seeds contain 4.28% sucrose which is the dominant sapid substance. Lipid soluble chemicals did not stimulate the taste receptors.8.Some aspects of the evolution of gustatory systems are discussed.

Functional redundancy and gustatory development in bdnf null mutant mice

In the mouse nasopalate papilla and in the trenches of the foliate and vallate papillae, taste buds accumulated primarily during the first 2 weeks after birth. Null mutation for brain-derived neurotrophic factor caused extensive death of embryonic taste neurons, with the secondary outcome that most taste buds failed to form. However not all taste neurons died; functional redundancy rescued a variable number. The primary research objective was to identify the likely site of the taste neuron rescue factor that substituted for BDNF. In this quest taste bud abundance served as a useful gauge of taste neuron abundance. The proportion of taste buds that developed was variable and uncorrelated among the nasopalate, vallate, and foliate gustatory papillae within each bdnf null mutant mouse. Thus, in spite of shared IXth nerve innervation, the vallate and foliate papillae independently varied in residual gustatory innervation. This variation rules against the rescue of gustatory neurons by system-wide factors or by factors acting on the IXth ganglion or nerve trunk. Therefore it is likely that surviving BDNF-deprived taste neurons were stochastically rescued by a redundant neurotrophic factor at the level of the local gustatory epithelium. These findings broaden the classic expectation that target tissue supplies only a single neurotrophic factor that can sustain sensory (taste) neurons.