Rapid kinetic studies of the unfolding of the small protein barstar by urea have been used to demonstrate the presence of at least two unfolding intermediates on two competing unfolding pathways. One intermediate has native-like secondary structure but has a partially solvated hydrophobic core, while the other is devoid of considerable secondary structure but has an intact hydrophobic core. It is shown that the transition states on the two pathways are very dissimilar structurally, but very similar energetically.

No abstract

The peripheral taste system likely maintains a specific relationship between ganglion cells that signal a particular taste quality and taste bud cells responsive to that quality. We have explored a measure of the receptoneural relationship in the mouse. By injecting single fungiform taste buds with lipophilic retrograde neuroanatomical markers, the number of labeled geniculate ganglion cells innervating single buds on the tongue were identified. We found that three to five ganglion cells innervate a single bud. Injecting neighboring buds with different color markers showed that the buds are primarily innervated by separate populations of geniculate cells (i.e., multiply labeled ganglion cells are rare). In other words, each taste bud is innervated by a population of neurons that only connects with that bud. Palate bud injections revealed a similar, relatively exclusive receptoneural relationship. Injecting buds in different regions of the tongue did not reveal a topographic representation of buds in the geniculate ganglion, despite a stereotyped patterned arrangement of fungiform buds as rows and columns on the tongue. However, ganglion cells innervating the tongue and palate were differentially concentrated in lateral and rostral regions of the ganglion, respectively. The principal finding that small groups of ganglion cells send sensory fibers that converge selectively on a single bud is a new-found measure of specific matching between the two principal cellular elements of the mouse peripheral taste system. Repetition of the experiments in the hamster showed a more divergent innervation of buds in this species. The results indicate that whatever taste quality is signaled by a murine geniculate ganglion neuron, that signal reflects the activity of cells in a single taste bud.

The present study evaluates the central circuits that are synaptically engaged by very small subsets of the total population of geniculate ganglion cells to test the hypothesis that taste ganglion cells are heterogeneous in terms of their central connections. We used trans-synaptic anterograde pseudorabies virus labeling of fungiform taste papillae to infect single or small numbers of geniculate ganglion cells, together with the central neurons with which they connect, to define differential patterns of synaptically linked neurons in the taste pathway. Labeled brain cells were localized within known gustatory regions, including the rostral central subdivision (RC) of the nucleus of the solitary tract (NST), the principal site where geniculate axons synapse, and the site containing most of the cells that project to the parabrachial nucleus (PBN) of the pons. Cells were also located in the rostral lateral NST subdivision (RL), a site of trigeminal and sparse geniculate input, and the ventral NST (V) and medullary reticular formation (RF), a caudal brainstem pathway leading to reflexive oromotor functions. Comparisons among cases, each with a random, very small subset of labeled geniculate neurons, revealed "types" of central neural circuits consistent with a differential engagement of either the ascending or the local, intramedullary pathway by different classes of ganglion cells. We conclude that taste ganglion cells are heterogeneous in terms of their central connectivity, some engaging, predominantly, the ascending "lemniscal," taste pathway, a circuit associated with higher order discriminative and homeostatic functions, others engaging the "local," intramedullary "reflex" circuit that mediates ingestion and rejection oromotor behaviors. Indexing termstaste; nucleus of the solitary tract; gustatory system; taste bud; virus; synaptic organization; pathways; geniculate ganglion; connections; tract tracing; cell typesThe taste system begins with specialized cells of the taste buds. These receptor cells are innervated by sensory ganglion cells that transmit taste information to the brain; specifically, in mammals, to the nucleus of the solitary tract (NST) of the medulla. This nucleus, which extends for the length of the medulla, consists of a complex of cytoarchitectonic subdivisions. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptPrevious anatomical studies have shown that the subdivisions have different connections. For example, one subdivision receives most of the input from the gustatory ganglion cells. That subdivision and others engage in various output connections, including ascending projections to higher centers (e.g., the pontine parabrachial nucleus), local intramedullary projections (e.g., connecting with the preoromotor reticular formation), and intra-NST projections (e.g., linking rostral gustatory subdivisions with caudal, general viscerosensory subdivisions; e.g., for review see Whitehead and Finger, 2008). However, these various connections have been described ...