Taste does not determine daily intake of dilute sugar solutions in mice.

When a rodent licks a sweet-tasting solution, taste circuits in the central nervous system that facilitate stimulus identification, motivate intake, and prepare the body for digestion are activated. Here, we asked whether taste also determines daily intake of sugar solutions in C57BL/6 mice. We tested several dilute concentrations of glucose (167, 250, and 333 mM) and fructose (167, 250, and 333 mM). In addition, we tested saccharin (38 mM), alone and in binary mixture with each of the sugar concentrations, to manipulate sweet taste intensity while holding caloric value constant. In experiment 1, we measured taste responsiveness to the sweetener solutions in two ways: chorda tympani nerve responses and short-term lick tests. For both measures, the mice exhibited the following relative magnitude of responsiveness: binary mixtures > saccharin > individual sugars. In experiment 2, we asked whether the taste measures reliably predicted daily intake of the sweetener solutions. No such relationship was observed. The glucose solutions elicited weak taste responses but high daily intakes, whereas the fructose solutions elicited weak taste responses and low daily intakes. On the other hand, the saccharin + glucose solutions elicited strong taste responses and high daily intakes, while the saccharin + fructose solutions elicited strong taste responses but low daily intakes. Overall, we found that 1) daily intake of the sweetener solutions varied independently of the magnitude of the taste responses and 2) the solutions containing glucose stimulated substantially higher daily intakes than did the solutions containing isomolar concentrations of fructose. Given prior work demonstrating greater postoral stimulation of feeding by glucose than fructose, we propose that the magnitude of postoral nutritive stimulation plays a more important role than does taste in determining daily intake of dilute sugar solutions.

Selective adaptation to noxious foods by a herbivorous insect.

When animals repeatedly sample a noxious food over a period of 1-4 days, they can markedly reduce their aversive behavioral response to the diet's unpleasant taste (e.g. 'bitterness') or toxic effects. This long-term adaptation process is selective, however, permitting insects to adapt physiologically to some but not all noxious foods. We hypothesized (i) that the selective nature of this adaptation process stems from the fact that some unpalatable foods are toxic while others are harmless and (ii) that insects have more difficulty adapting to foods that are both unpalatable and toxic. Our model system consisted of Manduca sexta caterpillars and two compounds that taste bitter to humans and elicit an aversive behavioral response in this insect (salicin and aristolochic acid). We found that 2 days of exposure to a salicin diet completely adapted the aversive response of the caterpillars to salicin, but that exposure to an aristolochic acid diet failed to adapt the aversive response to aristolochic acid. We determined that M. sexta could not adapt to the aristolochic acid diet because it lacked mechanisms for reducing the compound's toxicity. In contrast, the salicin diet did not produce any apparent toxic effects, and the caterpillars adapted to its aversive taste within 12 h of exposure. We also found that the salicin adaptation phenomenon (i) was mediated by the central gustatory system, (ii) generalized to salicin concentrations that were twice those in the adapting diet and (iii) offset spontaneously when the caterpillar was transferred to a salicin-free diet. We propose that toxicity is a more significant barrier to dietary adaptation than 'bitterness' in this insect.

A peripheral mechanism for behavioral adaptation to specific "bitter" taste stimuli in an insect.

Animals have evolved several chemosensory systems for detecting potentially dangerous foods in the environment. Activation of specific sensory cells within these chemosensory systems usually elicits an aversive behavioral response, leading to avoidance of the noxious foods. Although this aversive behavioral response can be adaptive, there are many instances in which it generates "false alarms," causing animals to reject harmless foods. To minimize the number of false alarms, animals have evolved a variety of physiological mechanisms for selectively adapting their aversive behavioral response to harmless noxious compounds. We examined the mechanisms underlying exposure-induced adaptation to specific "bitter" compounds in Manduca sexta caterpillars. M. sexta exhibits an aversive behavioral response to many plant-derived compounds that taste bitter to humans, including caffeine and aristolochic acid. This aversive behavioral response is mediated by three pairs of bitter-sensitive taste cells: one responds vigorously to aristolochic acid alone, and the other two respond vigorously to both caffeine and aristolochic acid. We found that 24 hr of exposure to a caffeinated diet desensitized all of the caffeine-responsive taste cells to caffeine but not to aristolochic acid. In addition, we found that dietary exposure to caffeine adapted the aversive behavioral response of the caterpillar to caffeine, but not to aristolochic acid. We propose that the adapted aversive response to caffeine was mediated directly by the desensitized taste cells and that the adapted aversive response did not generalize to aristolochic acid because the signaling pathway for this compound was insulated from that for caffeine.

How do inositol and glucose modulate feeding in Manduca sexta caterpillars?

Many species of caterpillar possess taste cells that respond exceptionally vigorously to the sugar alcohol myo-inositol. We examined the functional significance of these inositol-sensitive taste cells in Manduca sexta caterpillars through an integrated series of electrophysiological and behavioral studies. Neural recordings from all the gustatory chemosensilla revealed that M. sexta have only two pairs of inositol-sensitive taste cells, which respond strongly and selectively to myo-inositol, and two pairs of sugar-sensitive taste cells, which respond relatively weakly to sugars (glucose and sucrose). Behavioral studies established that myo-inositol incites feeding and counteracts the inhibitory effects of aversive taste stimuli (e.g. caffeine) on feeding, but does not promote increased consumption once feeding has been initiated. In contrast, glucose and sucrose did not produce any robust effects on feeding. We failed to obtain any evidence of sensory inhibition between taste cells that responded to myo-inositol and caffeine, indicating that myo-inositol counteracts the inhibitory effects of caffeine on feeding through a central gustatory mechanism. We conclude that sensory input from the inositol-sensitive taste cells, but not the sugar-sensitive taste cells, plays an important role in regulating feeding in M. sexta.

Contribution of different bitter-sensitive taste cells to feeding inhibition in a caterpillar (Manduca sexta).

Many compounds that taste bitter to humans also inhibit feeding in insects. Caterpillars (e.g., Manduca sexta) detect these compounds with a few bitter-sensitive taste cells. This study examined the role of these taste cells in feeding inhibition. Behavioral studies demonstrated that 3 bitter compounds (caffeine, salicin, and aristolochic acid) all inhibited feeding rapidly in Manduca sexta. Electrophysiological studies revealed that each pair of bitter-sensitive taste cell differs in responsiveness to the bitter compounds. Ablation studies indicated that (a) those pairs of bitter-sensitive taste cells that responded vigorously to a particular bitter compound were sufficient to inhibit feeding on diets containing the same compound, but that (b) no pair of bitter-sensitive taste cells was necessary for inhibiting feeding. Thus, the different pairs of bitter-sensitive taste cells appear to make partially redundant contributions to feeding inhibition.

Diet-induced plasticity in the taste system of an insect: localization to a single transduction pathway in an identified taste cell.

We studied exposure-induced sensitivity changes in an identified taste cell from Manduca sexta, a herbivorous caterpillar. This taste cell occurs within the lateral styloconic sensillum and responds selectively to compounds that humans characterize as bitter (e.g. caffeine, salicin and aristolochic acid). We made extracellular recordings from several classes of identified taste cell within the lateral sensillum, both before and after dietary exposure (for 48 h) to a suprathreshold concentration of caffeine, salicin or aristolochic acid. Our results revealed (1) that dietary exposure to caffeine desensitized the bitter-sensitive taste cell to caffeine, whereas dietary exposure to salicin or aristolochic acid did not desensitize the same taste cell to salicin or to aristolochic acid; (2) that dietary exposure to caffeine failed to alter the responsiveness of the sugar-, salt- or inositol-sensitive taste cells within the same sensillum; (3) that the caffeine-induced desensitization phenomenon generalized to salicin, a compound that stimulates the same transduction pathway as caffeine, but not to aristolochic acid, a compound that stimulates a different pathway; and (4) that chronically stimulating the lateral sensillum with caffeine, in the absence of ingestion, was sufficient to induce desensitization. We conclude that caffeine causes desensitization through a direct effect on a single transduction pathway within the bitter-sensitive taste cell.

Maxillary palps can mediate taste rejection of plant allelochemicals by caterpillars.

All caterpillars possess a pair of maxillary palps that "drum" the surface of foods during feeding. These chemosensory organs contain over 65% of a caterpillar's taste receptor cells, but their functional significance remains largely unknown. We examined their role in rejection of plant allelochemicals, using the tobacco hornworm (Manduca sexta) as a model insect and an extract from a plant species (Grindelia glutinosa) as a model stimulus. We selected this system because hornworms reject foods containing Grindelia extract, and because preliminary studies indicated that their maxillary palps respond to this extract. We hypothesized that Grindelia extract elicits rejection through stimulating: (1) olfactory receptor cells, (2) taste receptor cells, (3) oral mechanoreceptors, and/or (4) a postingestive response mechanism. Our results were consistent only with hypothesis 2: caterpillars approached Grindelia-treated diets without apparent hesitation, but rejected it within 6 s of initiating biting; Grindelia-treated solutions stimulated taste receptor cells in the maxillary palp, but not the other gustatory chemosensilla; and ablating the maxillary palps eliminated rejection of Grindelia-treated diets. Our results demonstrate that taste receptor cells in the maxillary palps mediate rejection of Grindelia extract, and provide the first direct evidence for the role of maxillary palps in rejection of plant allelochemicals.

Electrophysiological evidence for two transduction pathways within a bitter-sensitive taste receptor.

Among the sapid stimuli, those that elicit bitter taste are the most abundant and structurally diverse. To accommodate this diversity, animals are thought to use multiple bitter transduction pathways. We examined the role of individual taste receptor cells (TRCs) in this transduction process by focusing on one of the taste organs, or chemosensilla, of a caterpillar (Manduca sexta). This chemosensillum (the lateral styloconicum) contains four functionally distinct TRCs: the salt, sugar, inositol, and deterrent TRCs, which are known to respond strongly to, in respective order, salts, sugars, inositol, and compounds humans describe as bitter. Using an extracellular recording technique, we tested three hypotheses for how a structurally diverse array of bitter compounds (salicin, caffeine, and aristolochic acid) could excite the same chemosensillum: several TRCs within the lateral styloconica respond to the bitter compounds; only the deterrent TRC responds to the bitter compounds, through a single transduction pathway; and only the deterrent TRC responds to the bitter compounds, but through multiple transduction pathways. To discriminate among these hypotheses, we tested five predictions. The first addressed how many TRCs within the lateral styloconica responded to the bitter compounds. Subsequent predictions were based on the results of the test of the first prediction and assumed that only the deterrent TRC responded to these compounds. These latter predictions addressed whether the bitter compounds acted through one or multiple transduction pathways. We obtained evidence consistent with the third hypothesis: only the deterrent TRC responded to the bitter compounds; the temporal patterns of firing and concentration-response curves elicited by caffeine and salicin were similar to each other, but different from those elicited by aristolochic acid; the patterns of sensory adaptation and disadaptation elicited by caffeine and salicin were similar to each another, but different from those elicited by aristolochic acid; reciprocal cross-adaptation occurred between caffeine and salicin, but not between aristolochic acid and caffeine or aristolochic acid and salicin; and the responsiveness of individual deterrent TRCs to caffeine and salicin correlated significantly, whereas that to aristolochic acid and caffeine or aristolochic acid and salicin did not. Taken together, these results indicate that the deterrent TRC contains at least two excitatory transduction pathways: one responds to caffeine and salicin and the other to aristolochic acid. To our knowledge, this is the first direct support for the existence of two bitter transduction pathways within a single TRC.

Causal connection between detoxification enzyme activity and consumption of a toxic plant compound.

Insect herbivores can increase their detoxification activities against a particular plant poison in response to prolonged ingestion of the same compound. For example, larval tobacco hornworms (Manduca sexta) experience a dramatic increase in cytochrome P450 activity against nicotine after ingesting nicotine. While it is generally assumed that this induction process permits increased consumption of toxic plant tissues, we are not aware of any direct experimental support for this assumption. Using a two-tiered approach, we examined the functional significance of P450 induction to M. sexta larvae ingesting a toxic but non-deterrent concentration of nicotine. First, we related the time-course of P450 induction in midgut microsomes to changes in nicotine consumption. When offered a nicotine diet, larvae failed to show a significant increase in consumption before 36 h, which was coincident with the time-course of the induction of midgut P450 activities against aldrin and nicotine. Second, we determined whether inhibiting the induced P450 activities affected nicotine consumption. We found that the increase in nicotine consumption following the induction of nicotine metabolism could be strongly inhibited by treatment with piperonyl butoxide, which by itself did not inhibit consumption. These results provide direct evidence for a causal connection between P450-mediated detoxification activity and consumption of a toxic plant compound.

Is chemosensory input essential for the rapid rejection of toxic foods?

Herbivorous insects often rapidly reject foods containing toxic plant compounds. While the functional significance of this rejection response is clear, the mechanistic basis is not. The role of peripheral chemoreceptors in the rapid rejection of toxic foods was examined using a model system consisting of nicotine and the tobacco hornworm (Manduca sexta), which is a pest of tobacco plants. When offered diets containing naturally occurring concentrations of nicotine, larvae initially fed readily, but abruptly stopped feeding within 30s. A high percentage of larvae also exhibited toxic responses mediated by the central nervous system (twitching and writhing) to the ingested nicotine within 30s, indicating that nicotine could have been absorbed within the same time as the rejection response. Two lines of evidence are provided against a role of peripheral chemoreceptors in this rapid rejection response. First, all mouthpart chemoreceptors were ablated from the larvae, and they were then subjected to feeding tests with diets containing either nicotine or a compound (caffeine) that is known to stimulate deterrent taste receptors in M. sexta. Whereas the ablations virtually eliminated the rejection response to caffeine, they had no measurable impact on the rejection response to nicotine. Second, sensory recordings from two important gustatory sensilla (the medial and lateral styloconica) failed to demonstrate a plausible role of sensory input from either sensillum in the rapid rejection of nicotine. The most parsimonious interpretation of these results is that the nicotine rejection response was mediated by a rapidly acting post-ingestive mechanism.

Is the bitter rejection response always adaptive?

The bitter rejection response consists of a suite of withdrawal reflexes and negative affective responses. It is generally assumed to have evolved as a way to facilitate avoidance of foods that are poisonous because they usually taste bitter to humans. Using previously published studies, the present paper examines the relationship between bitterness and toxicity in mammals, and then assesses the ecological costs and benefits of the bitter rejection response in carnivorous, omnivorous, and herbivorous (grazing and browsing) mammals. If the bitter rejection response accurately predicts the potential toxicity of foods, then one would expect the threshold for the response to be lower for highly toxic compounds than for nontoxic compounds. The data revealed no such relationship. Bitter taste thresholds varied independently of toxicity thresholds, indicating that the bitter rejection response is just as likely to be elicited by a harmless bitter food as it is by a harmful one. Thus, it is not necessarily in an animal's best interest to have an extremely high or low bitter threshold. Based on this observation, it was hypothesized that the adaptiveness of the bitter rejection response depends upon the relative occurrence of bitter and potentially toxic compounds in an animal's diet. Animals with a relatively high occurrence of bitter and potentially toxic compounds in their diet (e.g., browsing herbivores) were predicted to have evolved a high bitter taste threshold and tolerance to dietary poisons. Such an adaptation would be necessary because a browser cannot "afford" to reject all foods that are bitter and potentially toxic without unduly restricting its dietary options. At the other extreme, animals that rarely encounter bitter and potentially toxic compounds in their diet (e.g., carnivores) were predicted to have evolved a low bitter threshold. Carnivores could "afford" to utilize such a stringent rejection mechanism because foods containing bitter and potentially toxic compounds constitute a small portion of their diet. Since the low bitter threshold would reduce substantially the risk of ingesting anything poisonous, carnivores were also expected to have a relatively low tolerance to dietary poisons. This hypothesis was supported by a comparison involving 30 mammal species, in which a suggestive relationship was found between quinine hydrochloride sensitivity and trophic group, with carnivores > omnivores > grazers > browsers. Further support for the hypothesis was provided by a comparison across browsers and grazers in terms of the production of tannin-binding salivary proteins, which probably represent an adaptation for reducing the bitterness and astringency of tannins.(ABSTRACT TRUNCATED AT 400 WORDS)

Consistency of meal patterns in laboratory rats.

Meal pattern measures (e.g., bout length and bout number) provide a detailed description of the elements of ingestion, and as a result they are theoretically more sensitive to experimental manipulation than simple measurements of total consumption over a fixed time period. For this reason, meal pattern measures are often used as a way to infer how rodents respond to tastants or how pharmacological or surgical treatments modify ingestive behavior. This approach relies on the assumption that under normal conditions, the meal patterns of individual rats are consistent across consecutive light-dark (L-D) cycles. If these patterns are inconsistent, then experimental designs involving meal patterns would have limited power, and thus require relatively large sample sizes. The present study critically evaluates the consistency assumption in Sprague-Dawley and Fisher-344 rats by monitoring meal patterns over 10 consecutive L-D cycles. For each rat, the following feeding and drinking measures were determined: total daily intake, duration of night bouts, number of night bouts, and number of licks per night bout. The analysis excluded daytime ingestive measures owing to their infrequent occurrence. Ingestive measures were highly consistent across time in all Sprague-Dawleys, but in only a minority of the Fisher-344s. The distribution of feeding and drinking activity throughout each night was also determined in the same rats. Whereas Sprague-Dawleys displayed lights-off and lights-on peaks of ingestive activity, only a minority of Fisher-344s displayed a consistent lights-on peak of ingestive activity. It is concluded that rat strains can differ with respect to the consistency of meal patterning, and that such strain differences should be considered in future comparative studies of meal patterns in rats.

Preference and aversion for deterrent chemicals in two species of Peromyscus mouse.

Deterrent chemicals such as quinine hydrochloride (QHC1) are generally considered to be aversive to mammals at all detectable concentrations. However, several species contain individuals that drink solutions containing low concentrations of deterrents in preference to plain water. The present study examines this paradoxical preference in two species of mouse, Peromyscus melanotis and P. aztecus. Preliminary findings had suggested that whereas some P. aztecus prefer low concentrations of QHC1, no P. melanotis prefer any concentration of QHC1. Experiment 1 tested the hypothesis that individual mice that prefer low concentrations of QHC1 would respond similarly to four other deterrents described by humans as bitter and/or astringent (ouabain, hop extract, sucrose octaacetate, and tannic acid) in 48-h, two-bottle choice tests. Peromyscus aztecus displayed a large amount of intraspecific variation in response to all five deterrents. Those P. aztecus that drank low concentrations of QHC1 in preference to plain water were significantly more likely to respond similarly to low concentrations of the other deterrents. No P. melanotis displayed a preference for any concentration of either deterrent. Experiment 2 examined the temporal stability of the response to 0.1 mM QHC1 in P. aztecus over six consecutive choice tests. Mice were divided into three groups based on their initial response to the QHC1 solution (preference, no response, or rejection) and then subjected to the 12-day test. The response of mice within each of the groups did not change significantly over time.(ABSTRACT TRUNCATED AT 250 WORDS)

Effectiveness of cardenolides as feeding deterrents toPeromyscus mice.

I compared the feeding responses of five species ofPeromyscus mice (aztecus, polionotus, melanotis, leucopus, andmaniculatus) to three bitter-tasting cardenolides (ouabain, digoxin, and digitoxin) that differ greatly in lipophilic character.Peromyscus, like other muroid rodents, are unusual in that they can ingest relatively large amounts of cardenolides without adverse physiologic effects. In experiment 1, I determined avoidance thresholds for the three cardenolides with 48 hr, two-choice tests. Mice exhibited large interspecific differences in avoidance threshold, and the interspecific ranking of the thresholds (maniculatus=leucopus >melanotis >polionotus >aztecus) was the same for each of the cardenolides. In experiment 2, I reevaluated the avoidance thresholds, but this time monitored the pattern of intake (i.e., bout lengths) during initial feeding encounters with cardenolidelaced diets. For each cardenolide, mice were subjected to three tests. In test 1, they received a control diet; in test 2, a diet containing the cardenolide at a concentration 1 log, unit below the avoidance threshold (as determined in experiment 1); and in test 3, a diet containing the cardenolide at the avoidance threshold concentration. Results were similar across all species and cardenolide types: Bout lengths in tests 1 and 2 were statistically equal, whereas those in test 3 were significantly shorter than those in test 1. The rapid rejection of cardenolide-laced diets in test 3 is consistent with a preingestive (i.e., gustatory) mechanism underlying the avoidance thresholds. I conclude (1) thatPeromyscus species differ substantially in taste sensitivity to cardenolides and that these differences may influence each species' respective ability to eat cardenolide-laced insects; and (2) that a species' relative taste sensitivity to one cardenolide predicts its sensitivity to other cardenolides.