Fine structural and histochemical study of lingual taste organs ofRana catesbeiana (Anura: Ranidae) transplanted to liver

Membrane excitability of wing and rod cells in frog taste discs following denervation

The frog tongue has a disc-shaped taste organ (taste disc) on the top of fungiform papillae. The taste disc contains two types of cells, wing cells with a sheet-like apical process and rod cells with a rod-like apical process. Both wing and rod cells can produce action potentials. Unlike the taste buds of mammals, frog taste discs do not degenerate over a long period after denervation. Here we report that the shapes of wing and rod cells isolated from taste discs in the bullfrog (Rana catesbeiana) remained unchanged 1 month after cutting bilateral glossopharyngeal nerves. By applying the whole cell patch-clamp technique to isolated wing and rod cells, we found voltage-dependent inward currents and outward currents and action potentials in denervated frogs as seen in normal frogs. These results suggest that the maintenance of morphological integrity and electrical excitability of taste cells does not require a nerve supply in frogs.

Merkel cells are responsible for the initiation of taste organ morphogenesis in the frog

Taste organs in the frog have a distinctive cell type located exclusively in the basal portion. In the same fashion as type III cells in mammalian taste buds, these basal cells show immunoreactivity for serotonin antibody. Further, these cells are morphologically similar to epidermal Merkel cells. To determine the significance of these serotonergic basal cells, we examined the early development of taste organs during metamorphosis of the frog by focusing on the origin and possible roles of serotonergic basal cells. For convenience of description, five stages of development (metamorphic stage to climax stages A-D) are defined. In the metamorphic stage, a few noninnervated Merkel cells appear at the upper layer of the lingual epithelium. No neuronal elements are seen in the epithelium at this stage. At climax stages A-B, immature fungiform papillae become discernible in the dorsal surface of the tongue, where the Merkel cells are located. Merkel cells then move downward and extend their cytoplasmic processes toward the basal lamina. These cells are identified by their intense immunoreactivity for serotonin. During the later stages, many nerve fibers in the subepithelial connective tissue approach the epithelium containing Merkel cells. At climax stages C-D, Merkel cells extend cytoplasmic processes along the basal lamina toward the center of the newly forming fungiform papillae. The morphology of these Merkel cells exactly coincides with that of serotonergic basal cells in adult taste organs. Profuse exocytotic release of dense-cored granules of Merkel cells toward the nerve fibers through the basal lamina is frequently seen in these stages. The present study indicates that serotonergic basal cells are derived from intraepithelial Merkel cells, which act as target sites for growing nerves and may be responsible for the initiation of taste organ morphogenesis.

Nerve dependency of developing and mature sensory receptor cells

Old and recent data concerning development of sensory cells and trophic interdependency of sensory neurons and sensory cells is reviewed for the ear, the lateral line system, the electroreceptive system, and the taste system. All sensory neurons originate from placodes. However, only most ear, lateral line and electrosensory cells derive from placodes, while the taste sensory cell originate locally. All sensory cells apparently are nerve independent for their formation, and at least sensory cells in the ear and the taste system share the neurotrophic support for their specific sensory neurons. Later, most of these sensory cells appear to depend, to a variable degree, on some innervation for maintenance. While the molecular nature of the signal cascade from sensory cells to sensory neurons is known in at least two systems, nothing is known about the molecular nature of the signal cascade from the sensory neurons back to the sensory cells.

Antagonism between the suppressive effects of NH3 and CO2 on bullfrog taste nerve responses to quinine

The suppression mechanism of NH3 and CO2 on bullfrog taste nerve responses to 0.1 mM quinine was investigated by applying them directly on the tongue surface (surface application) or at the tongue interstices through the lingual artery (interstitial application). The surface application of NH3 and CO2 reversibly suppressed the taste nerve responses with IC50 values of 0.37 mM and 2.2 mM, respectively, whereas their hydrates were ineffective. The interstitial application of NH3 reversibly suppressed the taste nerve responses. The surface application of CO2 recovered the suppressed responses to quinine. The 4 s application of CO2 prior to that of 0.1 mM quinine had the maximum effect. These results show that NH3-induced alkalosis and CO2-induced acidosis of taste cells, taste nerve endings, or tongue interstices surrounding them suppressed the taste nerve responses, and that the neutralization of their intracellular pH recovered these responses. The time-dependent recovery suggests that the neutralization at a layer inside taste cells, taste nerve endings, or the interstices surrounding them is critical in taste transduction.

Mice with a targeted disruption of the neurotrophin receptor trkB lose their gustatory ganglion cells early but do develop taste buds

The alleged ability of taste afferents to induce taste buds in developing animals is investigated using a mouse model with a targeted deletion of the tyrosine kinase receptor trkB for the neurotrophin BDNF. This neurotrophin was recently shown to be expressed in developing taste buds and the receptor trkB has been shown to be expressed in the developing ganglion cells that innervate the taste buds. Our data show a reduction of geniculate ganglion cells to about 5% of control animals in neonates. Degeneration of ganglion cells starts when processes reach the central target (solitary tract) but before they reach the peripheral target (taste buds). Degeneration of ganglion cells is almost completed in trkB knockout mice before taste afferents reach in control animals the developing fungiform papillae. Four days later the first taste buds can be identified in fungiform papillae of both control and trkB knockout mice in about equal number and density. Many taste buds undergo a normal maturation compared to control animals. However, the more lateral and caudal fungiform papillae grow less in size and become less conspicuous in older trkB knockout mice. No intragemmal innervation can be found in trkB knockout taste buds but a few extragemmal fibers enter the apex and end between taste had cells without forming specialized synapses. Taste buds of trkB knockout mice appear less well organized than those of control mice, but some cells show similar vesicle accumulations as control taste bud cells in their base but no synaptic contact to an afferent. These data strongly suggest that the initial-development of many fungiform papillae and taste buds is independent of the specific taste innervation. It remains to be shown why others appear to be more dependent on proper innervation.

The frog taste disc: a prototype of the vertebrate gustatory organ

The frog taste disc (TD) is apparently the largest gustatory organ found in vertebrates and seems to differentiate into a specialized variety of the prototypic scheme of the taste bud. An explanation for this unusual organization is lacking although it is possible to speculate the existence of environmental and nutritional requirements. Up to the present time, the most common model of the TD was based on two main cell types (sensory and sustentacular). This model may oversimplify the morphology of this structure since more numerous cell types have been described. We now propose a new model of the TD, based on comprehensive data on the ultrastructure of the organ obtained in the last 20 years. The main conclusions are the following: (1) the TD is a pluristratified epithelium with a general organization similar to that of the olfactory and vomeronasal epithelium; (2) it has skeleton composed of three different types of epithelial cells; (3) the chemoreceptorial surface is covered by different microenvironments; (4) three different types of neuro-epithelial systems are present; the type II is an 'open' sensory cell with axonal contacts devoid of vesicles; the type III is an 'open' sensory cell with synaptic-like junctions; the type i.v. is a 'closed' sensory cell with a 'Merkel-neurite complex'; (5) the nerve fibers in the basal plexus are mostly cholinergic while the peridiscal nerve fibers are mostly peptidergic. The presence of several cell types in the TD must be considered using these large receptors in electrophysiological studies or as a source of isolated cells, and their complexity must induce caution in the interpretation of the data. Text books of histology usually describe the peripheral structures associated with taste as very simple: an idea that probably must be revised. A taste organ is a highly complex structure composed of several sensory systems and a comparative approach can aid comprehension of its general organization. The study of the 'large taste organs' present in some species of amphibians can provide useful data for knowledge of the gustatory system of vertebrates.

Persistence and calcium-dependent ATPase staining of denervated fungiform taste buds in the hamster

Some fungiform taste buds in the hamster have been previously shown to persist for indefinite periods when deprived of their gustatory, chorda tympani (CT), innervation or both their CT and their trigeminal, lingual nerve, innervation (CT-L). The properties and numbers of persisting fungiform taste buds were examined 1 or 3 weeks after permanent CT or combined CT-L nerve cuts. The purpose was to reveal the status of taste buds at a time (3 weeks) when regenerating nerve fibres would normally be expected to reinnervate the epithelium. Denervated taste buds retain many normal characteristics including the pattern of histochemical staining for ectocalcium-dependent ATPase (Ca-ATPase). Taste-bud cells (including basal cells) have an intensely Ca-ATPase stained core surrounded by lightly stained peripheral cells. The Ca-ATPase stain was used to help identify and to define the size of the taste-bud core in denervated taste buds. Following CT-L or CT denervation most taste buds persisted; however the size of the taste-bud core was dramatically reduced. Fungiform taste buds differed in size based on their location in one of three tongue regions. The percentage decrease in size after denervation was also region specific and about the same for CT-L or CT cuts, suggesting that trigeminal fibres have no trophic effect on taste buds. However, trigeminal denervation caused a reduction in the number of persisting taste buds relative to CT denervation alone, which may be due to damage because of the loss of somatosensation.

Response of the gustatory system to peripheral nerve injury

Peripheral taste nerve damage occurs as a result of disease and surgery. The response depends on the taste field affected and the species. Nerve regeneration is robust after the nerve is crushed or after it is cut if the severed ends are anastomosed. Taste buds, which appear after nerve regeneration, may be derived from dormant stem cells outside of taste buds or from remnants of taste buds that persist following denervation. Sprouting by intact taste nerves into denervated fields apparently does not occur. Regenerated primary afferents have taste response specificity, but it is unknown if neural response types are retained peripherally or centrally. Recent behavioral studies show specific deficits following loss of restricted taste fields in rodents, but little is known about recovery after nerve regeneration. Specific deficits have not been demonstrated in humans, although taste sensitivity has been correlated with numbers of taste buds. Enigmas such as these may be solved once the response of the central nervous system to gustatory nerve injury is defined.