Glossopharyngeal nerve transection impairs unconditioned avoidance of diverse bitter stimuli in rats.

There is growing evidence of heterogeneity among responses to bitter stimuli at the peripheral, central and behavioral levels. For instance, the glossopharyngeal (GL) nerve and neurons receiving its projections are more responsive to bitter stimuli than the chorda tympani (CT) nerve, and this is particularly true for some bitter stimuli like PROP & cycloheximide that stimulate the GL to a far greater extent. Given this information, we hypothesized that cutting the GL would have a greater effect on behavioral avoidance of cycloheximide and PROP than quinine and denatonium, which also stimulate the CT, albeit to a lesser degree than salts and acids. Forty male SD rats were divided into four surgery groups: bilateral GL transection (GLX), chorda tympani transection (CTX), SHAM surgery, and combined transection (CTX + GLX). Postsurgical avoidance functions were generated for the four bitter stimuli using a brief-access test. GLX significantly compromised avoidance compared to both CTX and SHAM groups for all stimuli (p < .02), while CTX and SHAM groups did not differ. Contrary to our hypothesis, GLX had a greater effect on quinine than cycloheximide (mean shift of 1.02 vs. 0.27 log10 units). Moreover, combined CTX + GLX transection shifted the concentration-response function further than GLX alone for every stimulus except cycloheximide (ps < .03), suggesting that the GSP nerve is capable of maintaining avoidance of this stimulus to a large degree. This hypothesis is supported by reports of cycloheximide-responsive cells with GSP-innervated receptive fields in the NST and PBN.

Bitter-responsive brainstem neurons: characteristics and functions.

The sensation that humans describe as "bitter" is evoked by a large group of chemically diverse ligands. Bitter stimuli are avoided by a range of species and elicit reflex rejection, behaviors considered adaptations to the toxicity of many of these compounds. We review novel evidence for neurons that are narrowly tuned to bitter ligands at the initial stages of central processing. These "B-best" neurons in the nucleus of the solitary tract (NST) and parabrachial nucleus (PBN) respond to multiple types of bitter stimuli and exhibit average responses to bitter tastants that are 6-8 times larger than to moderate concentrations of compounds representing other qualities. However, in the PBN B-best units are appreciably activated by intense salt and acid. Neurons broadly sensitive to salts and acids ("AN" neurons) also responded to bitter stimuli. This sensitivity appeared restricted to stronger intensities of ionic bitters, as cycloheximide remained ineffective across concentrations. In addition to chemosensitive profile, B-best neurons were also distinctive with regard to their posterior receptive fields, long latencies, slow firing rates and projection status. Compared to B-best NST cells, those in the PBN received increased convergence from anterior and posterior receptive fields and responded to a greater number of bitter stimuli. We conclude that B-best neurons likely contribute to pathways underlying gaping, aversive hedonic quality and taste coding. The differential responsiveness of B-best and AN neurons to ionic and nonionic bitter ligands also suggests a potential substrate for discrimination within this quality.

Bitter-responsive gustatory neurons in the rat parabrachial nucleus.

Bitterness is a distinctive taste sensation, but central coding for this quality remains enigmatic. Although some receptor cells and peripheral fibers are selectively responsive to bitter ligands, central bitter responses are most typical in broadly tuned neurons. Recently we reported more specifically tuned bitter-best cells (B-best) in the nucleus of the solitary tract (NST). Most had glossopharyngeal receptive fields and few projected to the parabrachial nucleus (PBN), suggesting a role in reflexes. To determine their potential contribution to other functions, the present study investigated whether B-best neurons occur further centrally. Responses from 90 PBN neurons were recorded from anesthetized rats. Stimulation with four bitter tastants (quinine, denatonium, propylthiouracil, cycloheximide) and sweet, umami, salty, and sour ligands revealed a substantial proportion of B-best cells (22%). Receptive fields for B-best NST neurons were overwhelmingly foliate in origin, but in PBN, about half received foliate and nasoincisor duct input. Despite convergence, most B-best PBN neurons were as selectively tuned as their medullary counterparts and response profiles were reliable. Regardless of intensity, cycloheximide did not activate broadly tuned acid/sodium (AN) neurons but did elicit robust responses in B-best cells. However, stronger quinine activated AN neurons and concentrated electrolytes stimulated B-best cells, suggesting that B-best neurons might contribute to higher-order functions such as taste quality coding but work in conjunction with other cell types to unambiguously signal bitter-tasting ligands. In this ensemble, B-best neurons would help discriminate sour from bitter stimuli, whereas AN neurons might be more important in differentiating ionic from nonionic bitter stimuli.

Amiloride-insensitive units of the chorda tympani nerve are necessary for normal ammonium chloride detectability in the rat.

In spite of its common use as a standard stimulus in peripheral nerve recordings, relatively little is known about the psychophysics of NH-sub-4Cl taste. Rats' detection threshold for this salt was tested under a variety of conditions, including amiloride (100 muM) treatment and bilateral chorda tympani (CT) nerve transection. Detectability was measured with a 2-lever operant discrimination procedure used previously to measure detection thresholds for NaCl and KCl. Although NH-sub-4Cl and KCl appear to share a common taste quality and transduction mechanism, the logistic function and threshold for NH-sub-4Cl were found to be more similar to those of NaCl than to those of KCl. Like that of KCl, however, the detection threshold for NH4Cl increased significantly with CT transection (0.54 log-sub-1-sub-0 units, p < .004), but not with amiloride adulteration. This finding supports the hypothesis that the CT is necessary for normal salt detection regardless of stimulus, and suggests that amiloride does not appreciably impact responses to nonsodium salts at the behavioral level.

Single neurons in the nucleus of the solitary tract respond selectively to bitter taste stimuli.

Molecular data suggest that receptors for all bitter ligands are coexpressed in the same taste receptor cells (TRCs), whereas physiological results indicate that individual TRCs respond to only a subset of bitter stimuli. It is also unclear to what extent bitter-responsive neurons are stimulated by nonbitter stimuli. To explore these issues, single neuron responses were recorded from the rat nucleus of the solitary tract (NST) during whole mouth stimulation with a variety of bitter compounds: 10 microM cycloheximide, 7 mM propylthiouracil, 10 mM denatonium benzoate, and 3 mM quinine hydrochloride at intensities matched for behavioral effectiveness. Stimuli representing the remaining putative taste qualities were also tested. Particular emphasis was given to activating taste receptors in the foliate papillae innervated by the quinine-sensitive glossopharyngeal nerve. This method revealed a novel population of bitter-best (B-best) cells with foliate receptive fields and significant selectivity for bitter tastants. Across all neurons, multidimensional scaling depicted bitter stimuli as loosely clustered yet clearly distinct from nonbitter tastants. When neurons with posterior receptive fields were analyzed alone, bitter stimuli formed a tighter cluster. Nevertheless, responses to bitter stimuli were variable across B-best neurons, with cycloheximide the most, and quinine the least frequent optimal stimulus. These results indicate heterogeneity for the processing of ionic and nonionic bitter tastants, which is dependent on receptive field. Further, they suggest that neurons selective for bitter substances could contribute to taste coding.

Nerve regeneration-induced recovery of quinine avoidance after complete gustatory deafferentation of the tongue.

The concentration-dependent decrease in quinine licking by rats is substantially attenuated by combined bilateral transection of the chorda tympani (CT) and glossopharyngeal (GL) nerves, but transection of either nerve alone produces marginal impairments at most. Here we tested whether regeneration of one or both of these nerves after combined transection would result in recovery of taste avoidance. Water-restricted rats were presented with a series of brief-access (5 s) taste trials (water and 0.003-3.0 mM quinine-HCl) in a 5-day test block of 40-min sessions both before nerve transection and starting 75-77 days after transection. Licking avoidance returned to presurgical levels when both nerves were allowed to regenerate. When only the GL was allowed to regenerate, performance did not differ from that of sham-transected animals. This suggests that even after considerable gustatory deafferentation, regeneration has the capacity to restore normal taste-guided behavior. Surprisingly, when only the CT was allowed to regenerate, avoidance behavior was severely impaired and was not different from that of rats in which regeneration of both nerves was prevented. Taking into account prior findings, it appears that the absence of the GL in the presence of an intact CT is fundamentally different from the absence of the GL in the presence of a regenerated CT with respect to some taste functions. This represents the first reported instance to our knowledge in which the capacity of a regenerated nerve to maintain taste-guided behavior was distinctly different from that of an intact nerve in a rodent model.

Anion size does not compromise sodium recognition by rats after acute sodium depletion.

Amiloride-insensitive sodium taste transduction is severely limited by large anions (i.e., gluconate). We found that in a brief-access taste test, sodium-depleted rats exhibited similar levels of increased licking to several sodium salts regardless of anion but did not increase licking to nonsodium salts compared with water. The enhanced licking of sodium salts was abolished in the presence of amiloride. These results suggest that the amiloride-sensitive taste transduction pathway is not only necessary but that it is also sufficient for sodium identification in rats. Sodium-depleted rats tested with amiloride initiated significantly more trials than nondepleted rats; hence, appetitive behavior was mildly potentiated by depletion, even in the absence of a sodium taste cue. Overall, these findings provide compelling support for the primacy of the amiloride-sensitive taste transduction mechanism and its associated neural pathway in the recognition of the sodium cation.

Transecting the gustatory branches of the facial nerve impairs NH(4)Cl vs. KCl discrimination in rats.

Ammonium and potassium chloride share a common taste quality and an amiloride-insensitive route of transduction. An amiloride-sensitive pathway might also be partially activated by these salts, although very few studies have reported effects of amiloride on nonsodium salt perception. This experiment was designed to determine 1) whether rats could discriminate KCl from NH(4)Cl and, if discrimination was evident, whether performance was impaired with 2) amiloride or 3) gustatory nerve transection. Rats were trained to discriminate KCl from NH(4)Cl (n = 8) and NaCl from NH(4)Cl (n = 8). Amiloride (100 microM) impaired NaCl vs. NH(4)Cl but not KCl vs. NH(4)Cl performance, whereas both groups showed significant impairments after transection of the chorda tympani (CT) and greater superficial petrosal (GSP) branches of the facial nerve. This suggests that rats can discriminate between KCl and NH(4)Cl and that this discrimination does not rely on an amiloride-sensitive mechanism but does depend on the CT and/or GSP nerves. This experiment supports the hypothesis that the facial nerve is important for salt taste recognition and discrimination.