Sex differences in electrophysiological and behavioral responses to NaCl taste

We tested the hypothesis that sex differences in preference for NaCl are attributable to estrogen-mediated alterations in gustatory processing. Electrophysiological responses of the chorda tympani nerve to NaCl were blunted by estrogen treatment in ovariectomized female rats, suggesting that females are less sensitive to concentrated NaCl solutions during high estrogen conditions. In contrast, after a taste aversion was conditioned to 150-mM NaCl, estrogen- and oil-treated ovariectomized rats generalized the aversion to a lower concentration of NaCl than did males, suggesting that females are more sensitive to the taste of dilute NaCl solutions regardless of estrogen. Thus, sex differences in NaCl preferences may be attributable to differences in NaCl taste processing that involve both acute and developmental effects of estrogen.     

Sweet and Bitter Taste of Ethanol in C57BL/6J and DBA2/J Mouse Strains

Studies of inbred strains of rats and mice have suggested a positive association between strain variations in sweet taste and ethanol intake. However, strain associations by themselves are insufficient to support a functional link between taste and ethanol intake. We used conditioned taste aversion (CTA) to explore the sweet and bitter taste of ethanol and ability to detect sucrose, quinine and ethanol in C57BL/6J (B6) and DBA/2J (D2) mouse strains that are frequently used in alcohol research. The present study showed that C57BL/6J mice generalized taste aversions from sucrose and quinine solutions to 10% ethanol and, reciprocally, aversions to 10% ethanol generalized to each of these solutions presented separately. Only conditioned aversions to quinine generalized to ethanol in the DBA/2J strain but an aversion conditioned to ethanol did not generalize reciprocally to quinine. Thus, considering these two gustatory qualities, 10% ethanol tastes both sweet and bitter to B6 mice but only bitter to D2. Both strains were able to generalize taste aversions across different concentrations of the same compound. B6 were able to detect lower concentrations of quinine than D2 but both strains were able to detect sucrose and (in contrast to previous findings) ethanol at similar concentrations. The strain-dependent gustatory profiles for ethanol may make an important contribution to the understanding of the undoubtedly complex mechanisms influencing high ethanol preference of B6 and pronounced ethanol avoidance of D2 mice.     

Comparative effects of lesion of the basolateral nucleus and lateral nucleus of the amygdaloid body on neophobia and conditioned taste aversion in the rat

The effects of bilateral lesions of the basolateral and lateral nuclei of the amygdala on the neophobic response and LiCl-conditioned taste aversion to a saccharin solution were studied in rats. Compared to intact animals, rats with basolateral lesions did not exhibit neophobia to the novel stimulus, while rats with lateral lesions demonstrated an initial preference to the sweet solution over water. The LiCl-induced aversion was suppressed after basolateral lesions and was unchanged after lateral lesions. It is concluded that these two amygdaloid nuclei play an important but distinct role in neophobia and conditioned taste aversion.     

Memory-dependent c-Fos expression in the nucleus accumbens and extended amygdala following the expression of a conditioned taste aversive in the rat

Retrieving the memory of a conditioned taste aversion involves multiple forebrain areas. Although the amygdala clearly plays a role in the expression of a conditioned taste aversion, critical regions, downstream from the amygdala remain to be defined. To this end, Fos immunoreactivity was used in the rat to explore forebrain structures associated with retrieval that have an anatomical relationship with the amygdala. The results showed that expression of a conditioned taste aversion to 0.5 M sucrose elicited neuronal activation in the nucleus accumbens and in a complex of structures collectively referred to as the extended amygdala. The posterior hypothalamus and parasubthalamic nucleus, which receive inputs from the extended amygdala, were also activated upon re-exposure to the sucrose conditioned stimulus. Fos immunoreactivity did not increase in these regions in response to an innately aversive tastant, quinine hydrochloride (conditioned stimulus control), nor to LiCl-induced visceral stimulation in unconditioned animals (unconditioned stimulus control). In addition, these regions did not respond to the sucrose conditioned stimulus in sham-conditioned animals. These results suggest that conditioned and innately aversive tastes are differentially processed in the forebrain circuitry that includes the nucleus accumbens and extended amygdala.     

Interactions of temperature and taste in conditioned aversions

The influence of temperature on taste cues and the ability to discriminate and learn about different temperatures of foods are important factors regulating ingestion. The goal of this research was to demonstrate that thermal orosensory input can serve as a salient stimulus to guide ingestive behavior in the rat, and also that it interacts with gustatory input during choice and conditioned aversion experiments. A novel apparatus with Peltier refrigerators was used to control the temperature of solutions in 10-min, 2-bottle tests. It was determined that naive rats preferred cold water (10 °C) to warm water (40°). When cold water was paired with a toxic LiCl injection, rats avoided cold water and drank warm water, thus demonstrating that cold water could serve as the conditioned stimulus in a conditioned temperature aversion. Rats conditioned against cold water could discriminate 10 °C water from 16 °C water, but not from 13 °C water, thus showing an ability to discriminate orosensory thermal cues to within 3-6 °C. Rats also generalized conditioned aversions from cold water to cold saccharin and cold sucrose solutions. However, if rats were conditioned against a compound taste and thermal stimulus (10 °C, 0.125% saccharin), the rats could distinguish and avoid each component individually, i.e., by avoiding cold water or warm saccharin. Finally, daily 2-bottle extinction tests were used to assess the strength of aversions conditioned against a taste cue (0.25 M sucrose), a thermal cue (10 °C water), or the combination. Aversions to taste or temperature alone persisted for 7-14 days of extinction testing, but the combined taste-temperature aversion was stronger and did not extinguish after 20 days of extinction testing. These results demonstrate that temperature can serve as a salient cue in conditioned aversions that affect ingestion independent of taste cues or by potentiating taste cues.     

Behavioral comparison of sucrose and l-2-amino-4-phosphonobutyrate (l-AP4) tastes in rats: Does l-AP4 have a sweet taste?

Even though it is generally thought that umami stimuli such as monosodium glutamate (MSG) and sweet stimuli such as sucrose are detected by different taste receptors, these stimuli appear to share taste qualities when amiloride (a sodium channel blocker) is present to reduce the sodium taste. Single fiber recording studies of the facial and glossopharyngeal nerves have shown that encoding of l-2-amino-4-phosphonobutyrate (l-AP4), a potent mGluR4 agonist that elicits a taste quite similar to MSG, may occur in the same fibers that also encode sweet stimuli. This suggests that l-AP4 and sweet substances may activate common receptors or afferent signaling mechanisms. We report results of behavioral experiments that test this hypothesis. In the first study, rats conditioned to avoid sucrose or l-AP4 generalized the aversion to the opposite substance, indicating that both substances elicited similar tastes. However, two taste discrimination experiments showed that rats easily discriminated between sucrose and l-AP4 over a wide range of concentrations, even when the cue function of sodium associated with l-AP4 was reduced by amiloride and neutralized by adding equimolar concentrations of NaCl to sucrose. These data suggest that even though l-AP4 and sucrose elicit similar taste qualities, one or both substances also elicit other taste qualities not shared by the opposite substance. They also suggest that the taste-mGluR4 receptor and the signal pathway activated by l-AP4 are not the same as those activated by sucrose. These data, when combined with fiber recording data, suggest that there is convergence of l-AP4 and sucrose signals at some point early in the gustatory pathway.     

Contingent and non-contingent actions of sucrose and saccharin reinforcers: effects on taste preference and memory

Preference curves were generated by comparing 14 concentrations each of sucrose and saccharin in a 20-minute test in which rats were presented with a choice of a sweet solution and water. The most preferred concentration and one concentration above and one below the most preferred for both substances were studied further. The sucrose and saccharin solutions were contingently paired with novel flavors in a conditioned taste preference (CTP) paradigm. All of the sweet solutions enhanced the animals' subsequent conditioned taste preferences for the flavors. The lack of difference between the effects of the solutions in this paradigm suggest that they had similar rewarding values and that CTP's are established mainly on the basis of taste cues. In another experiment, post-training ingestion of sucrose solutions, injection of glucose and, to a much lesser extent, ingestion of saccharin solutions retroactively and non- contingently improved retention of a previously formed, classically conditioned association. The results indicated that this effect was mainly due to the post- ingestional effects of the sucrose solutions, although taste factors also had a slight influence. This series of experiments parallels previous findings with self-stimulation as the reinforcer. The results support the hypothesis that reinforcers have a dual action on behavior: the elicitation of affective states that, when paired with environmental stimuli, can influence future behavior towards those stimuli; and a non-contingent retroactive enhancement of retention of previously formed associations.     

Chorda tympani nerve transection alters linoleic acid taste discrimination by male and female rats

Taste is intimately associated with food choice, yet little is known about the role of taste in preferences for dietary fat, a major component of many foods. We measured the taste threshold for linoleic acid (LA), an essential free fatty acid found in dietary fat, before and after bilateral transections of the chorda tympani nerve (CTX) in adult male and female rats. We conditioned a taste aversion to 88 microM LA and assessed the generalization of the aversion to lower LA concentrations to determine LA discrimination thresholds. We discovered that female rats had a lower LA discrimination threshold (approximately 2.75 microM LA) than did male rats (approximately 11 microM LA). In another set of animals, we performed CTX and found that CTX elevated LA threshold to the same level (approximately 22 microM LA) in male and female rats. Finally, we evaluated licking responses to 11, 22, 44 and 88 microM LA mixed in sucrose by male rats and ovariectomized (OVX) female rats treated with estradiol benzoate or oil vehicle. All rats increased licking to increasing LA concentrations, but OVX rats responded to a lower LA concentration (22 microM) than did males (44 microM) in 10-s trials. However, estradiol did not affect this outcome. Collectively, these experiments show that male and female rats use taste to discriminate LA and that the chorda tympani nerve, which innervates taste buds on the anterior tongue, plays a role in this discrimination. Furthermore, sex differences in fat preferences may depend on differences in fatty acid taste thresholds as well as on the taste stimuli with which fat is combined.     

Perceptual characteristics of the amiloride-suppressed sodium chloride taste response in the rat

The contribution of amiloride-sensitive membrane components to the perception of NaCl taste was assessed by using a conditioned taste aversion procedure. Eight independent groups of adult rats were conditioned to avoid either 0.1M NaCl, 0.5M NaCl; 0.1M NH4Cl, or 1.0M sucrose while their tongues were exposed either to water or to the sodium transport blocker amiloride hydrochloride. In contrast to rats exposed to water during conditioning, rats exposed to amiloride were unable to acquire a conditioned taste aversion to 0.1M NaCl. Differences in the acquisition of taste aversions between the amiloride- and nonamiloride-treated groups were not apparent when the conditioned stimulus (CS) was 0.5M NaCl, 0.1M NH4Cl, or 1.0M sucrose. Although the magnitude of the 0.5M NaCl aversion was similar between amiloride- and non-amiloride-treated rats, the perceptual characteristics of the CS differed between groups. Analyses of stimulus generalization gradients revealed that amiloride-treated rats generally avoided all monochloride salts after conditioning to 0.5M NaCl but not nonsodium salts or nonsalt stimuli. In contrast, rats not treated with amiloride only generalized the 0.5M NaCl aversion to sodium salts. No differences in generalization gradients occurred between groups when the CS was 0.1M NH4Cl or 1.0M sucrose. These findings suggest that the "salty" taste of NaCl is primarily related to the amiloride-sensitive portion of the functional taste response in rats. Conversely, the portion of the NaCl response insensitive to amiloride appears to have "sour-salty" perceptual characteristics and does not appear to be perceived as being salty.     

Taste effectiveness of some D- and L-amino acids in mice

To measure the hedonic effectiveness of some D-amino acids two-bottle preference tests were carried out on ddy mice, and subsequently conditioned taste aversion (CTA) experiments were performed to determine to what degree hedonic responses to D- and L-amino acids are related to sweet taste in mice. Mice showed preferences for D-Trp, D-His, and D-Leu over certain concentration ranges, but were behaviorally neutral to D-Ser and D-Val up to 0.1 M. The preference magnitudes for D-amino acids increased with increasing molecular weight (MW) or the bulk of the side chain. CTA experiments using 0.2 M sucrose as a conditioning stimulus and 4 D- and 6 L-amino acids as test stimuli indicated that generalization of sucrose taste became stronger with increasing MW of D-amino acids and decreasing MW of L-amino acids. There was a highly significant positive correlation between the degree of generalization of sucrose taste and the preference magnitude for D- and L-amino acids. The results indicate that preferences for D- and L-amino acids in mice result from sweet taste produced by these chemicals. Comparison of the results with the human psychophysical data reveal similarity between humans and ddy mice in taste sensitivity to amino acids.     

Effects of dose and of partial body ionizing radiation on taste aversion learning in rats with lesions of the area postrema

The effect of area postrema lesions on the acquisition of a conditioned taste aversion following partial body exposure to ionizing radiation was investigated in rats exposed to head-only irradiation at 100, 200 and 300 rad or to body-only irradiation at 100 and 200 rad. Following head-only irradiation area postrema lesions produced a significant attenuation of the radiation-induced taste aversion at all dose levels, although the rats still showed a significant reduction in sucrose preference. Following body-only exposure, area postrema lesions completely disrupted the acquisition of the conditioned taste aversion. The results are interpreted as indicating that: (a) the acquisition of a conditioned taste aversion following body-only exposure is mediated by the area postrema; and (b) taste aversion learning following radiation exposure to the head-only is mediated by both the area postrema and a mechanism which is independent of the area postrema.     

Enhanced sucrose and Polycose preference in sweet "sensitive" (C57BL/6J) and "subsensitive" (129P3/J) mice after experience with these saccharides

Prior research with inbred mouse strains indicates that C57BL/6J (B6) mice display stronger preference and acceptance for various sweeteners than do 129P3/J (129) mice. Experiment 1 examined the extent to which this strain difference could be modified by repeated exposure to sucrose. Sucrose-naive 129 mice displayed weaker preferences than did B6 mice for 0.5% to 4% sucrose solutions during 23h/day sugar vs. water tests. Sucrose preference did not differ at 8-32% concentrations. Yet, when retested with sucrose, the 129 and B6 mice showed identical robust preferences (>90%) for 0.5-32% solutions. The strains also did not differ in sucrose preference in tests with descending sucrose concentrations (0.5-0.0625%). Sucrose-experienced 129 mice also showed enhanced preference for dilute saccharin solutions suggesting that their sweet taste responsivity was enhanced. Experiment 2 revealed that preference for dilute saccharin solutions was enhanced by prior saccharin experience in B6 but not 129 mice. Experiment 3 tested the strains with Polycose which has a palatable taste different from that of sucrose. Polycose-naive 129 mice displayed weaker preferences for dilute (0.5-4%) but not concentrated (8-32%) Polycose solutions relative to B6 mice. In the second test series Polycose preferences were nearly identical in the two strains. In Experiments 1 and 3, prior sucrose or Polycose experience also reduced or eliminated strain differences in saccharide acceptance (absolute intake) at higher but not lower concentrations. Thus, exposure to the oral and post-oral actions of sucrose and Polycose increased saccharide preference in B6 mice and even more in 129 mice so that the strain difference virtually disappeared. Whether the 129 mice responded to the taste or other properties (e.g., odor) of the dilute saccharide solutions is not certain but their gustatory sensitivity needs to be reconsidered.     

Estrogen-induced suppression of intake is not mediated by taste aversion in female rats

Estrogen treatment can suppress the intake of a previously presented gustatory conditioned stimulus (CS). This finding has been interpreted as an estrogen-induced conditioned taste aversion. However, a distinction must be made between taste aversion and taste avoidance. In particular, tastes are only considered aversive if they elicit a stereotypic behavioral response, otherwise the reduction in intake is classified as an avoidance. Although aversive orofacial responses have been reported in male rats after taste-estrogen pairings, they have not been examined in ovariectomized female rats. The goal of the present investigation, then, was to use similar procedures to determine whether conditioned aversion also mediates the estrogen-induced reduction of intake in female rats. Animals were introduced to a novel 0.1% saccharin solution and immediately thereafter were given a subcutaneous injection of vehicle or estradiol benzoate (10 microg). Responses were assessed using a two-bottle preference test, a one-bottle acceptance test, and a taste reactivity (TR) test. The results confirmed previous reports of a reduced preference for saccharin after saccharin-estradiol pairing using the two-bottle test. The reduction in intake during the one-bottle test, however, was not accompanied by stereotypic aversive responses, such as gaping. Surprisingly, a similar reduction in intake also occurred when using a backward conditioning procedure in which estrogen was injected before, rather than after, CS access. Thus, the present results show that the suppressive effects of estrogen reflect an avoidance, rather than aversion and, moreover, that the reduced intake may be due to an unconditioned, rather than a conditioned, response.     

The suppressive effects of sucrose and cocaine, but not lithium chloride, are greater in Lewis than in Fischer rats: evidence for the reward comparison hypothesis

Rats suppress intake of a saccharin conditioned stimulus (CS) when it is paired with an aversive unconditioned stimulus (US), an appetitive US, or a drug of abuse such as morphine or cocaine. It is unclear, however, whether the reduction in intake induced by these drugs is mediated by their aversive or their rewarding properties. The present set of experiments addressed this question by comparing the suppressive effects of a known aversive US (LiCl), a known reinforcing US (sucrose), and a drug of abuse (cocaine) in two strains of rats (i.e., Lewis and Fischer 344 rats) that differ in their preference for rewarding stimuli. The results show that, although both strains readily acquired a LiCl-induced conditioned taste aversion (CTA), the suppressive effects of sucrose and cocaine were robust in the drug-preferring Lewis rats and absent in the Fischer rats. These data argue against a CTA account and in favor of the reward comparison hypothesis.     

Sex differences in behavioral taste responses to and ingestion of sucrose and NaCl solutions by rats

Sex differences in the ingestion of food and concentrated NaCl solutions by rats have been investigated for more than a quarter of a century, though the underlying mechanism(s) and the role of reproductive hormones remain the subject of debate. We hypothesized that sex differences in the ingestion of sucrose and NaCl solutions are attributable, in part, to sex differences in taste responses/taste perception. We employed short-access, 10-s tests along with 18-h, two-bottle preference tests to examine sex differences in sensitivity to and ingestion of sucrose and NaCl solutions. To evaluate the role of estrogen, we ovariectomized (OVX) female rats and then used an estrogen-replacement schedule that mimics the pattern of fluctuation of estrogen levels in intact female rats. We observed striking sex differences in the rate of licking NaCl mixed in a dilute sucrose solution. Compared to males, OVX rats with or without estrogen licked at higher rates to more concentrated NaCl solutions, suggesting that female rats are less sensitive to concentrated NaCl solutions. Although less pronounced, we also observed sex differences in the rate of licking to sucrose, particularly at lower concentrations. Compared to males, OVX rats with or without estrogen licked less, suggesting that female rats are less sensitive to lower concentrations of sucrose. Estrogen appeared to play, at most, a small role in mediating taste responses to specific concentrations of sucrose in these testing procedures. Nonetheless, sex differences in taste responses were clear, and it seems likely that such differences underlie, in part, observed differences in ingestion.     

High doses of vasopressin delay the onset of extinction and strengthen acquisition of LiCl-induced conditioned taste avoidance

When low doses of vasopressin are given 50 min after pairing sucrose consumption with a high dose of LiCl, extinction of the LiCl-induced conditioned taste avoidance is accelerated. These low doses of vasopressin do not themselves induce conditioned taste avoidance when paired with sucrose consumption. Predicated on previous studies administering two avoidance-inducing agents after sucrose consumption, studies were designed to determine whether high doses of vasopressin capable of inducing conditioned taste avoidance would (1) delay rather than accelerate extinction of a conditioned taste avoidance induced by a high dose of LiCl and (2) strengthen acquisition of a conditioned taste avoidance induced by a low dose of LiCl. The results of three studies showed that doses of 9 and 18 microg/kg of vasopressin induced a conditioned taste avoidance when injected 50 min after sucrose consumption, delayed the onset of extinction when injected 50 min after pairing sucrose consumption with a high dose of LiCl, and strengthened acquisition of a conditioned taste avoidance when injected 50 min after pairing sucrose consumption with a low dose of LiCl. Taken together, these data suggest that the delay in onset of extinction is due to a strengthening of acquisition. It has been suggested that vasopressin is a mnemonic neuropeptide that delays extinction of learned tasks. However, for conditioned taste avoidance, the evidence for the effects of low doses of vasopressin on extinction do not support this hypothesis and the evidence for high doses of vasopressin can be accounted for by the avoidance-inducing properties of vasopressin.     

Taste responses to dilute sucrose solutions are modulated by stage of the estrous cycle and fenfluramine treatment in female rats

Meal size is decreased during the estrous stage of the rat's ovarian reproductive cycle. This is mediated, in part, by estradiol's ability to increase the strength by which negative-feedback signals function to inhibit meal size. For example, we recently reported that the anorectic effect of fenfluramine, a serotonin agonist, is enhanced during estrus. Here, we investigated whether a decrease in the strength of positive-feedback signals, like those related to the taste of food, contributes to the decrease in meal size observed either in estrous rats or following fenfluramine treatment. Rats were given brief access to six sucrose solutions (0.0, 0.025, 0.05, 0.1, 0.2, and 0.4 M) and the mean number of licks to these solutions was monitored in diestrous and estrous rats treated with 1 mg/kg fenfluramine or saline vehicle. Following saline treatment, estrous rats displayed fewer licks than diestrous rats to the 0.025 M sucrose solution. Following fenfluramine treatment, a decrease in the number of licks to 3 of the 5 sucrose solutions was observed in diestrous rats only. This decrease in sucrose palatability was limited to brief access tests, as overnight preference for the 0.025 M sucrose solution was not decreased by fenfluramine in either diestrous or estrous rats. Our findings suggest that estrous rats experience a decrease in the strength of positive-feedback signals elicited by a dilute sucrose solution and that the anorectic effect of fenfluramine is associated with a decline in positive-feedback signaling in the diestrous rat.     

Recall of a previously acquired conditioned taste aversion in rats following lesions of the area postrema

A conditioned taste aversion was produced by pairing a novel sucrose solution with either 3 mEq lithium chloride or with 100 rad gamma radiation in rats with the area postrema intact. Lesions of the area postrema were then made in half of the rats exposed to each treatment and in rats that were not treated with the unconditioned stimulus. When tested for a conditioned taste aversion, all treated rats showed a significant aversion to the sucrose solution compared to the untreated control rats. There were no significant differences between rats with area postrema lesions and those with the area postrema intact, indicating that the lesions had no effect on the recall of the previously acquired aversion. The results are interpreted as being consistent with the hypothesis that the role of the area postrema in taste aversion learning is to monitor blood and cerebrospinal fluid for potential toxins and to transmit that information to the central nervous system.     

Sucrose taste but not Polycose taste conditions flavor preferences in rats

Rats have an inborn preference for sweet taste and learn to prefer flavors associated with sweetness. They are also strongly attracted to the taste of glucose polymers (e.g., Polycose). This "poly" taste differs in quality from the sweet taste of sugar. To determine if poly taste, like sweet taste, conditions flavor preferences rats were trained with a distinctive flavor (CS+) added to 2% Polycose solution and a different flavor (CS-) added to plain water. In a subsequent two-bottle test the rats did not prefer the CS+ to CS- when both flavors were presented in water. In contrast, other rats significantly preferred a CS+ flavor that had been paired with 2% sucrose. Adding saccharin to a flavored Polycose solution did not improve CS+ flavor learning; rather, Polycose appeared to overshadow saccharin-induced conditioning. Flavor conditioning by a 16% Polycose solution was assessed using a sham-feeding procedure to prevent post-oral reinforcement. Although the rats sham-fed substantial amounts of the CS+ flavored Polycose solution, they failed to prefer the CS+ to the CS- flavor. This contrasts with the preference other rats displayed for a CS+ paired with sham-fed sucrose. Why attractive sweet and poly tastes differ in their ability to condition flavor preferences is not certain, although some findings suggest that they differentially activate dopamine and/or serotonin circuits involved in flavor learning.     

Postconditioning recovery from the latent inhibition effect in conditioned taste aversion

Two experiments were conducted examining the effects of flavor (CS) preexposure on the retention of conditioned taste aversion. In Experiment 1, rats received preexposure to sucrose solution followed by a sucrose-illness pairing. The expected “latent inhibition” effect was obtained when testing occurred after a two-day but not an eleven-day training-to-test interval. Experiment 2 extended these results by employing five- and twenty-one-day training-to-test interval parameters and provided evidence that the stronger taste aversion displayed by preexposed subjects following long retention intervals is not attributable to differences in training consumption of sucrose solution. This posttraining increase in conditioned taste aversion (CTA) suggests that preexposure blocks expression of memory.