Cracking taste codes by tapping into sensory neuron impulse traffic

Abstract

Insights into the biological basis for mammalian taste quality coding began with electrophysiological recordings from "taste" nerves and this technique continues to produce essential information today. Chorda tympani (geniculate ganglion) neurons, which are particularly involved in taste quality discrimination, are specialists or generalists. Specialists respond to stimuli characterized by a single taste quality as defined by behavioral cross-generalization in conditioned taste tests. Generalists respond to electrolytes that elicit multiple aversive qualities. Na(+)-salt (N) specialists in rodents and sweet-stimulus (S) specialists in multiple orders of mammals are well characterized. Specialists are associated with species' nutritional needs and their activation is known to be malleable by internal physiological conditions and contaminated external caloric sources. S specialists, associated with the heterodimeric G-protein coupled receptor T1R, and N specialists, associated with the epithelial sodium channel ENaC, are consistent with labeled line coding from taste bud to afferent neuron. Yet, S-specialist neurons and behavior are less specific than T1R2-3 in encompassing glutamate and E generalist neurons are much less specific than a candidate, PDK TRP channel, sour receptor in encompassing salts and bitter stimuli. Specialist labeled lines for nutrients and generalist patterns for aversive electrolytes may be transmitting taste information to the brain side by side. However, specific roles of generalists in taste quality coding may be resolved by selecting stimuli and stimulus levels found in natural situations. T2Rs, participating in reflexes via the glossopharynygeal nerve, became highly diversified in mammalian phylogenesis as they evolved to deal with dangerous substances within specific environmental niches. Establishing the information afferent neurons traffic to the brain about natural taste stimuli imbedded in dynamic complex mixtures will ultimately "crack taste codes."

Figure 1. Electrophysiological recordings are obtained from single afferent neurons and whole afferent nerves

A. By teasing fascicles from a de-sheathed afferent nerve cut before it enters the brain, action potentials of single fibers are recorded. The example illustrated is from the rat glossopharyngeal nerve (Frank, 1991); stimuli were applied from arrow to end of record. B. The overlapping action potentials from hundreds of fibers in a whole nerve are rectified and integrated to monitor total neural activity. The example illustrated is from the hamster chorda tympani nerve (Hettinger and Frank, 1987); periods of application of NaCl and amiloride are designated by horizontal lines.

Figure 2. Specialist hamster chorda tympani neurons with sucrose-best response profiles transmit information to the brain about sweeteners

Mean (±se) neural response rates to 0.1 M sucrose, 30 mM NaCl, 3 mM HCl, 1mM quinine HCl and control spontaneous neural spiking rates for 10 hamster chorda tympani (CT) S neurons are plotted. Note the high variation in sampled means. Data, ¼^th of a sample of 40 taste-responsive CT neurons, are from Frank et al., 1988.

Figure 3. Hamsters’ learned aversions to the other sweeteners saccharin and fructose generalize to 100 mM sucrose

Aversions learned to ionic stimuli do not generalize to sucrose. CT S-fibers respond to sweeteners but do not respond consistently to ionic stimuli and likely provide the “sweet” cue for hamsters. The CS groups learned aversions to 100 mM sucrose, NaNO₃, NaCl and MgSO₄, 300 mM fructose, NH₄Cl and KCl, 1 mM sodium saccharin (saccha), 410 mM Na₂SO₄, 3 mM citric acid (H), 10 mM acetic (Acet) and hydrochloric acids, and 1 mM quinine HCl. Mean data are from Frank and Nowlis, 1989.

Figure 4. N Specialist (NaCl-best) and E generalist (HCl-best) hamster chorda tympani nerve fibers transmit distinct information to the hamster brain about ionic taste stimuli

Mean (±se) neural response rates to 100 mM sucrose and KCl; 30 mM NaCl, NaNO₃, MgSO₄ and NH₄Cl; 3 mM HCl, acetic (Acet) and citric acid (H), 1 mM quinine HCl; and control spontaneous neural spiking rates for 21 N and 9 E neurons are plotted. Most neurons responding best to ionic taste stimuli responded to NaCl. Data, 3/4^ths of a sample of 40 taste-responsive CT neurons, are from Frank et al., 1988.

Figure 5. Hamsters distinguish sodium salts from acids and bitter salts better than they distinguish among acids and bitter salts

CTAs to Na⁺ salts generalized specifically to 100 mM NaCl (blue), CTAs to acids and NH₄Cl generalized strongly to 10 mM HCl (red), and CTAs to non-sodium salts generalized strongly to 1 mM quinine HCl (gray). The CS groups learned aversions to 410 mM Na₂SO₄, 100 mM NaNO₃, NaCl and MgSO₄, 3 mM citric acid (H), 10 mM acetic (Acet) and hydrochloric acids, 300 mM NH₄Cl and KCl, and 1 mM quinine HCl. The mean data are from Frank and Nowlis, 1989.

Figure 6. Differences in the tastes of NaCl and NH₄Cl or KCl disappear after treating the tongue with amiloride in rats

An aversion learned to the conditioned stimulus (CS) 500 mM NaCl was specific to NaCl in control rats (Water); but after treating with 100 µM amiloride (Amiloride), sodium, ammonium and potassium chlorides tasted alike as measured by percent suppression of intake. Values are derived from mean data in Hill et al., 1990.

Figure 7. Blocking ENaC mutes NaCl and LiCl responses of CT N fibers but not CT E fibers, which also detect NaCl and LiCl

After treating the tongue with 100 µM amiloride (+Amiloride), the remaining amiloride-unsusceptible NaCl response elicits a taste perceptually like non-Na⁺ salts (FIG. 6). In effect, amiloride serves as a pharmacological “knockout” of one of several rodent CT ionic taste systems. The specific taste of Na⁺/Li⁺ salts is eliminated when the tongue is treated with amiloride and N neurons are muted. Mean 5-s response data from Ninomiya et al. 1988.

Figure 8. The hamster chorda tympani nerve responds to ionic stimuli that are bitter to people, but does not respond to behaviorally avoided non-ionic bitter stimuli

Quinine and denatonium are avoided behaviorally at 0.3 –1 mM (plum arrow); SOA, n-propylthiouracil and caffeine avoided at 1–3 mM (light purple arrow) and cycloheximide avoided at 1 µM, well below the graph’s scale. Mean data (±se) are from Frank et al., 2004 and Hettinger et al., 2007.

Figure 9. Stimuli that are bitter to humans form distinct perceptual categories in hamsters

Ionic bitter stimuli: denatonium, quinine and MgSO₄, and non-ionic sucrose octaacetate (SOA) and caffeine are perceptually distinct to hamsters. Test stimuli were water (control), denatonium and SOA. Quinine and MgSO₄ aversions (CS group) generalized strongly to denatonium but did not generalize to SOA. Caffeine and SOA aversions did not generalize to denatonium, nor did caffeine and SOA aversions cross-generalize. Stimuli were 3 mM denatonium benzoate, 1 mM quinine HCl, 180 mM MgSO₄, 30–100 mM caffeine and 1–1.5 mM SOA. Mean data (±se) are from Frank et al. 2004.

Figure 10. Hamster N neurons respond transiently to an anion shift from acetate to chloride with an increase in neural activity after 25-sec adaptation

The reciprocal shift from chloride to acetate did not increase the neural response. Stimuli were 30 mM sodium chloride and sodium acetate (N = 3). Mean data are from Rehnberg et al. 1993.

Figure 11. Responses of CT Na⁺ specialists to NaCl are stronger in sodium-replete than in sodium-deprived rats

Mean (+ se) response rates for 10 seconds are plotted. The dashed lines indicate mean (+ 2 se) control response levels in sodium-deprived and sodium-replete animals. Test stimuli were 500 mM sucrose, 100 mM NaCl, 10 mM HCl, and 20 mM quinine HCl. Data are from Contreras and Frank, 1979.

Figure 12. Responses of hamster sucrose-best specialists to 100 mM sucrose contaminated with increasing levels of bitter quinine

Means ±se are plotted. The dashed line marks control mean + 2 se, a response level considered to be above background. Data are from Frank, Formaker and Hettinger, 2005.