Laura C. Geran

Amiloride increases sodium chloride taste detection threshold in rats

The epithelial sodium-channel blocker amiloride has been shown to inhibit sodium responses in the 7th cranial nerve of the rat. In the signal detection task used in this study, amiloride (100 microM) treatment raised the NaCl threshold by approximately 1 log10 unit. The inhibition constant for amiloride was 1 microM at 0.013 M NaCl. Because the NaCl intake of adult rats has been shown to be related to the level of dietary NaCl exposure early in development, rats were exposed by way of maternal diet to 1 of 3 diets (0.1% NaCl, n = 8; 1.0% NaCl, n = 8; 3.0% NaCl, n = 9) from conception through weaning, to determine whether this treatment affects taste sensitivity. At Postnatal Day 30, rats were placed on 1.0% NaCl chow. This treatment did not affect NaCl detection or amiloride sensitivity in adulthood. The amiloride-induced shifts in NaCl sensitivity functions imply that the transcellular sodium transduction pathway is necessary for normal NaCl detection in the rat.

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.

Sodium taste detectability in rats is independent of anion size: the psychophysical characteristics of the transcellular sodium taste transduction pathway.

There are two known sodium transduction pathways in the rat gustatory system. The transcellular pathway is blocked by amiloride, and the paracellular pathway is limited by the anion gluconate. The contribution of each pathway to sodium detection was assessed. Sodium gluconate (NaGlu) and NaCl thresholds did not differ, implying that the paracellular pathway is not necessary for normal sodium detection. Adding 100 microM amiloride raised both NaCl and NaGlu thresholds but did not abolish all performance to NaGlu, indicating that some chemical cue was present at high concentrations. Rats were also exposed to one of three NaCl diets (0.12%, 1.0%, or 6.0% NaCl) through maternal and ad lib intake from Embryonic Day 1 through testing in adulthood. No differences across dietary groups were found for NaCl or NaGlu threshold with or without amiloride. Thus, this developmental dietary treatment does not appear to affect taste sensitivity to sodium subserved through either transduction pathway. Collectively, these data suggest that the transcellular transduction pathway is both necessary and sufficient for normal sodium detection.

Chorda tympani nerve transection, but not amiloride, increases the KCl taste detection threshold in rats.

Water-restricted rats were trained to press one lever after KCl presentation and the other lever after distilled water. Water reinforcement was given after each correct response, and a time-out followed each incorrect response. Rats were trained and tested on KCl stimuli of varying concentrations. Threshold was defined as the KCl concentration corresponding to 1/2 the maximum asymptote of performance for each rat. The geometric mean KCl detection threshold for all rats was 0.033 M KCl. Rats that had the chorda tympani nerve (CT) bilaterally transected showed an average increase in KCl threshold of approximately 0.60 log10 units, whereas sham-operated rats showed no change. Control rats retested with 100 microM amiloride added to all KCl concentrations and water displayed no change in threshold. These results suggest that although the CT contributes significantly to the rats sensitivity to KCl, amiloride-sensitive taste transduction pathways do not.

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.

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.

Temporal characteristics of gustatory responses in rat parabrachial neurons vary by stimulus and chemosensitive neuron type.

It has been demonstrated that temporal features of spike trains can increase the amount of information available for gustatory processing. However, the nature of these temporal characteristics and their relationship to different taste qualities and neuron types are not well-defined. The present study analyzed the time course of taste responses from parabrachial (PBN) neurons elicited by multiple applications of “sweet” (sucrose), “salty” (NaCl), “sour” (citric acid), and “bitter” (quinine and cycloheximide) stimuli in an acute preparation. Time course varied significantly by taste stimulus and best-stimulus classification. Across neurons, the ensemble code for the three electrolytes was similar initially but quinine diverged from NaCl and acid during the second 500ms of stimulation and all four qualities became distinct just after 1s. This temporal evolution was reflected in significantly broader tuning during the initial response. Metric space analyses of quality discrimination by individual neurons showed that increases in information (H) afforded by temporal factors was usually explained by differences in rate envelope, which had a greater impact during the initial 2s (22.5% increase in H) compared to the later response (9.5%). Moreover, timing had a differential impact according to cell type, with between-quality discrimination in neurons activated maximally by NaCl or citric acid most affected. Timing was also found to dramatically improve within-quality discrimination (80% increase in H) in neurons that responded optimally to bitter stimuli (B-best). Spikes from B-best neurons were also more likely to occur in bursts. These findings suggest that among PBN taste neurons, time-dependent increases in mutual information can arise from stimulus- and neuron-specific differences in response envelope during the initial dynamic period. A stable rate code predominates in later epochs.

The 15th International Symposium on Olfaction and Taste

Timothy S.McClintock, DonaldA.Wilson, StevenD.Munger, LauraGeran, andScottHerness Department of Physiology, University of Kentucky, Emotional Brain Institute, Nathan Kline Institute, NYU School of Medicine, New York, NY, Department of Anatomy and Neurobiology, University ofMaryland School ofMedicine, Ohio State University, College of Dentistry, Division of Oral Biology, College of Dentistry, Ohio State University