Peripheral gustatory processing of sweet stimuli by golden hamsters

https://doi.org/10.1016/j.brainresbull.2005.04.004Get rights and content

Abstract

Behaviors and taste-nerve responses to bitter stimuli are linked to compounds that bind T2 receptors expressed in one subset of taste-bud receptor cells (TRCs); and behavioral and neural responses to sweet stimuli are linked to chemical compounds that bind a T1 receptor expressed in a different TRC subset. Neural and behavioral responses to bitter-sweet mixtures, however, complicate the ostensible bitter and sweet labeled lines. In the golden hamster, Mesocricetus auratus, quinine hydrochloride, the bitter prototype, suppresses chorda tympani (CT) nerve responses to the sweet prototype: sucrose. This bitter-sweet inhibition was tested with concentration series of sucrose and dulcin, a hydrophobic synthetic sweetener that hamsters behaviorally cross-generalize with sucrose. Dulcin, sucrose and other sweeteners activate one subset of CT fibers: S neurons; whereas, quinine activates a separate subset of CT fibers: E neurons. Whole-nerve and S-neuron CT responses to a sweetener concentration series, mixed with 0, 1, 3 and 10 mM quinine, were measured for 0–2.5 s transient and/or 2.6–10 s steady-state response periods. Ten-sec total single-fiber records, aligned at response onset, were averaged for 100 ms bins to identify response oscillations. Quinine inhibition of dulcin and sucrose responses was identical. Each log molar increment in quinine resulted in equivalent declines in response to either sweetener. Furthermore, sucrose response decrements paralleled response increments in quinine-sensitive CT neurons to the same quinine increases. A 1.43 Hz bursting rhythm to the sweeteners was unchanged by quinine inhibition or decreases in sweetener concentration. Taste-bud processing, possibly between-cell inhibition and within-cell negative feedback, must modify signals initiated by T1 receptors before they are transmitted to the brain.

Introduction

S neurons, a subset of chorda tympani (CT) neurons well-defined in the golden hamster (Mesocricetus auratus), are quite specific for stimuli that are sweet and respond with a characteristic rhythmic bursting pattern [4], [13], [14], [45], [47]. The CT innervates taste buds within fungiform papillae in the epithelium of the anterior two-thirds of the tongue. Taste-nerve and behavioral responses of transgenic mice, and the distribution of candidate sweet and bitter G-protein coupled receptors (GPCR) among taste receptor cells (TRCs) are consistent with labeled-line codes for the sweet and bitter tastes. A labeled line transmits information about one taste quality from TRCs to brain [34], [50], [66]. Bitter and sweet GPCR determine chemical selectivity and are expressed in separate TRC subsets that initiate, respectively, behavioral aversion or preference [28], [38], [63], [65]. However, transmission of information about sweet stimuli to the brain by S neurons [17] occurs after initial processing among TRCs in taste buds [24], [35]. Studies of taste-nerve responses to stimulus mixtures suggest possible functions of the taste-bud processing.

Candidate peripheral labeled-line codes are based on studies using single-compound, uni-quality stimuli. However, the gustatory system most often deals with heterogeneous mixtures composed of stimuli with different taste qualities. In mixtures, quinine hydrochloride, a prototype bitter stimulus, inhibits CT afferent signals elicited by a prototype sweet stimulus: sucrose. Primary evidence for this interaction comes from the hamster CT, which shows response suppression exclusively in S neurons. In the CT there is no reciprocal suppression of quinine responses by sucrose [9], [10], a reciprocity observed in third order taste neurons of the parabrachial nucleus [59], [60]. How across-quality inhibition relates to proposed peripheral labeled-line codes has not been addressed.

Hamsters behaviorally prefer and cross-generalize with sucrose many, but by no means all, stimuli that are sweet to humans [4], [52]. Such stimuli include the synthetic sweetener, dulcin (p-ethoxyphenyl urea), and the amino acid, d-phenylalanine [30], [40]. Hamsters do not behaviorally cross-generalize the sweeteners with quinine. Besides inhibiting sucrose responses in CT S neurons, quinine, along with other ionic stimuli that hamsters cross-generalize with quinine, activates electrolyte-sensitive E neurons [15], [18], [39]. E neurons are generalists that are also activated by non-bitter stimuli [14], and, thus, are unlikely bitter-labeled lines.

To determine whether quinine inhibition is specific for disaccharides like sucrose or may be as broad as the stimulus set activating S neurons, quinine–dulcin mixtures were studied. Dulcin, an aryl urea, is structurally similar to the sweet amino acid d-phenylalanine. Importantly, like sucrose, dulcin specifically activates CT S neurons [4], [12], [47]; and both sucrose and dulcin are ligands for the rat candidate T1 sweet receptor, heterodimeric GPCR, T1R2/T1R3 [28]. Analysis of CT whole-nerve and single-unit data on quinine–dulcin and quinine–sucrose mixtures is straightforward because quinine and sweeteners activate distinct subsets CT neurons.

Section snippets

Subjects

Electrophysiological responses were recorded from the right chorda tympani nerve (CT) of 12 adult, male, 110–180 g, golden hamsters (Mesocricetus auratus).

Surgical procedure

Hamsters were anesthetized by intraperitoneal injection of sodium pentobarbital (Abbot Labs, N. Chicago, IL, USA; initial dose: 80 mg/kg, subsequent doses: 40 mg/kg) to maintain a surgical level of anesthesia, and terminated at the end of the experiment. Body temperature was regulated at ∼37 °C with a Deltaphase® isothermal pad. A tracheal

Quinine–dulcin binary mixtures, whole CT nerve responses

Fig. 3 presents the average results for concentration series of quinine alone and quinine mixed with 3 or 10 mM dulcin. The 1 mM dulcin did not elicit reliable CT responses and when mixed with the series of quinine concentrations, elicited response levels indistinguishable from responses to quinine. It was thus omitted from further analysis.

To see if quinine inhibited responses to dulcin as hypothesized, data from six nerves for 3 and 10 mM dulcin mixed with 0, 1, 3 and 10 mM quinine (two upper

Discussion

Inhibitory effects of aversive compounds on neural taste responses to palatable compounds appear in species as diverse invertebrate flies: Phormia regina [5] and leeches: Hirudo medicinalis [27] and vertebrate rodents: Mesocricetus auratus [9], [10]. The inhibition is present in the gustatory periphery and not generated by circuitry in the central nervous system (CNS). Multiple aversive compounds with little structural similarity inhibit effects of palatable sugars in blowflies and palatable

Conclusion

We demonstrate “bitter-sweet” inhibition and a “sweet” rhythm in hamster chorda tympani responses that are indicative of the kinds of processing of gustatory information that occur in taste buds, and demonstrate that signaling of sweet taste by GPCRs is not transmitted unchanged via CT afferents to the brainstem.

Acknowledgements

This work was supported by a grant from the National Institutes of Health (USA), National Institute on Deafness and other Communicative Disorders: DC 004099.

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