Temporal and qualitative dynamics of conditioned taste aversion processing: combined generalization testing and licking microstructure analysis

The pattern of licking microstructure during various phases of a conditioned taste aversion (CTA) was evaluated. In Experiment 1, rats ingested lithium chloride (LiCl) for 3 trials and were then offered sodium chloride (NaCl) or sucrose on 3 trials. A CTA to LiCl developed and generalized to NaCl but not to sucrose. CTA intake suppression was characterized by reductions in burst size, average ingestion rate, and intraburst lick rate, and increases in brief pauses and burst counts. Compared with previous studies, LiCl licking shifted from a pattern initially matching that for normally accepted NaCl to one matching licking for normally avoided quinine hydrochloride by the end of the 1st acquisition trial. In Experiment 2, a novel paradigm was developed to show that rats expressed CTA generalization within 9 min of their first LiCl access. These results suggest that licking microstructure analysis can be used to assay changes in hedonic evaluation caused by treatments that produce aversive states.     

Conditioned aversion to a taste perceived while grooming

Four experiments were conducted to determine how the characteristics of conditioned taste aversion (CTA), as described from studies conducted in the drinking and feeding contexts, applied in the grooming context. In Experiment 1, sodium saccharin was mixed with a "neutral-tasting" jelly and applied to the fur of male Sprague-Dawley rats. Rats injected with LiCl after the applications strongly avoided saccharin solutions in subsequent 1-hr, 2-choice (Saccharin solution vs water) drinking tests, whereas rats injected with NaCl or given plain jelly on the fur showed only an initial neophobic response to the saccharin solution. Thus, the taste of saccharin was perceived while grooming and the CTA formed in the grooming context generalized to drinking. In experiments 2-4, we obtained evidence that: (a) rats discriminated between one intensity of saccharin applied to the fur and another used in the test solution; and (b) rats differentiated between qualities of the two tastants applied to the fur in that saccharin overshadowed NaCl; and (c) taste qualities were more important than toxic properties when two stimuli (Saccharin, LiCl) were used (saccharin overshadowed NaCl in subsequent drinking tests). We speculate that taste, while grooming might play a role in social communication in some vertebrates. Further, CTA and grooming might have uses in rodent control (e.g., in agricultural situations) not previously considered such as in delivering a non-attractive, low-salience toxin so that the taste of the crop overshadows that of the bait, and induces crop aversion.     

Taste reactivity as a dependent measure of the rapid formation of conditioned taste aversion: a tool for the neural analysis of taste-visceral associations

Several explanations may account for deficits in the ability of animals to form taste aversions following neural manipulations. These encompass impairments in conditioned stimulus (CS) and unconditioned stimulus (US) processing, conditioned response (CR) measurement, and expression, memory, and taste-visceral integration. A behavioral procedure that aids in the distinction between some of these possibilities is presented. In Experiment 1, 10 rats received seven intraoral (IO) infusions of sucrose (30 s, 0.55 ml) spaced every 5 min starting immediately after the injection of 3.0 mEq/kg of lithium chloride (LiCl). Control rats (n = 12) were treated identically except that they were injected with sodium chloride (NaCl). Oromotor and somatic taste reactivity behaviors were videotaped and analyzed. Lithium-injected rats systematically decreased their ingestive taste reactivity behavior over time, whereas aversive behavior increased. Control rats maintained high and stable levels of ingestive responding and demonstrated virtually no aversive behavior over the 30-min period following sodium injection. Rats were tested several days later for the presence of a conditioned taste aversion (CTA). Rats previously injected with lithium during sucrose infusions demonstrated significantly more aversive behavior than the control group, which demonstrated none. There were no differences in the level of ingestive behavior displayed by the two groups on the CTA test. Experiment 3 revealed that when similarly treated rats were tested for a CTA while in a lithium-induced state, a difference in the ingestive behavior between the two groups was observed. In Experiment 2, naive rats were injected with either NaCl or LiCl but did not receive their first sucrose infusion until 20 min later. These rats also received sucrose infusions at 25 and 30 min postinjection. There were no differences in the taste reactivity behavior displayed by lithium- or sodium-injected rats during any of the sucrose infusions. Collectively, these findings indicate that rats dramatically change their oromotor responses to sucrose during the period following LiCl administration, provided that the infusions start immediately after injection. Furthermore, this time-related behavioral change is predominantly attributable to associative processes. This paradigm can be useful in distinguishing between neural manipulations that affect the establishment of taste-visceral associations from others that affect the animal's ability to retain such associations over the commonly employed 24-hr conditioning-test interval.     

Ontogenesis of learning: IV. Dissociation of memory and perceptual-altering processes mediating taste neophobia in the rat

The rat's neophobic response to tastes can be conceptualized as depending on two processes: memory processes that store a representation of an experienced taste so that it can be recognized as familiar on subsequent encounters, and perceptual-altering processes that respond differentially to novel versus familiar tastes. We investigated the ontogeny of these processes by studying the preweanling rat's behavioral reaction to a 10% sucrose solution. Experiments I and II suggest that the memory processes mature earlier (by 7-8 days of age) than the perceptual-altering processes (about 11 days of age). Experiment III suggests that pups do not acquire an aversion to sucrose paired with illness until they are 12-days-old, implying a close correspondence between the maturation of processes mediating neophobia and taste aversion learning. These findings are consistent with our previous work (Vogt & Rudy, 1984), and our view that ontogenetic dissociations in taste-guided behavior reflect caudal-to-rostral maturation in the ascending gustatory system.     

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 effect of taste familiarity on intake and taste reactivity in infant rats

With infant rats, unlike with adults, increased intake of a taste after mere exposure to this stimulus is not consistently found; this has sometimes been interpreted as a failure by the immature subject to recognize tastes as familiar. We studied the effect of preexposure to a tastant, measuring taste reactivity and intake in 14‐day‐old rats. Familiarity increased hedonic response to sucrose, but also increased aversive response to quinine and ethanol. With the sucrose‐quinine compound, familiarity increased both the hedonic and the aversive reaction to the stimulus. In no case was a differential reactivity to water observed. Significant increased intake after familiarization was only found with quinine or the sucrose‐quinine compound. Results indicate that in infant rats, and with the present parameters, taste familiarity enhances responsiveness to these stimuli, an effect not always accompanied by detectable changes in intake. © 2009 Wiley Periodicals, Inc. Dev Psychobiol 52:109–120, 2010     

Taste avoidance, but not aversion, learning in rats lacking gustatory cortex.

Control rats rapidly learned to avoid drinking either a sucrose solution (Experiment 1) or a NaCl solution (Experiment 2) when the taste was paired with illness. These rats also produced aversive reactivity to each of these solutions in a taste reactivity test. Rats that lacked gustatory cortex (GC) learned to avoid drinking sucrose and NaCl, albeit at a slower rate than control rats. GC rats failed to display aversive reactivity to these tastes. The GC rats did show normal aversive reactivity to a strong quinine HCl solution during additional tests. It is suggested that the avoidance developed by GC rats did not entail a palatability shift of the conditional stimulus as it did in control rats. This altered learning strategy may account for the consistent learning deficits found in GC rats trained to avoid tastes.     

Conditioned taste aversion in lean and obese rats with ventromedial hypothalamic knife cuts

The development of a conditioned taste aversion (CTA) was assessed in rats made hyperphagic with parasagittal knife cuts in the ventromedial hypothalamus (VMH). The animals were water deprived and presented with a .1% saccharin solution paired with injections of either lithium chloride or sodium chloride. In Experiment 1, VMH rats tested at a nonobese weight level did not differ from sham-operated control rats in the acquisition and extinction of the CTA. In Experiment 2, moderately obese VMH rats displayed a stronger CTA than did sham-operated control rats as evidenced by a slower rate of extinction. This effect was not due to the higher absolute dose of LiCl given to the obese VMH rats. A second group of obese VMH rats given an amount of LiCl equivalent to that given to the control rats also displayed retarded extinction of the CTA. The results of these experiments demonstrate that hyperphagia-inducing knife cuts do not alter aversive taste conditioning in rats but that hypothalamic obesity does enhance conditioned taste aversions. This may reflect an obesity-induced suppression in appetitive motivation.     

A comparison between taste avoidance and conditioned disgust reactions induced by ethanol and lithium chloride in preweanling rats

Adult rats display taste avoidance and disgust reactions when stimulated with gustatory stimuli previously paired with aversive agents such as lithium chloride (LiCl). By the second postnatal week of life, preweanling rats also display specific behaviors in response to a tastant conditioned stimulus (CS) that predicts LiCl‐induced malaise. The present study compared conditioned disgust reactions induced by LiCl or ethanol (EtOH) in preweanling rats. In Experiment 1 we determined doses of ethanol and LiCl that exert similar levels of conditioned taste avoidance. After having equated drug dosage in terms of conditioned taste avoidance, 13‐day‐old rats were given a single pairing of a novel taste (saccharin) and either LiCl or ethanol (2.5 g/kg; Experiment 2). Saccharin intake and emission of disgust reactions were assessed 24 and 48 hr after training. Pups given paired presentations of saccharin and the aversive agents (ethanol or LiCl) consumed less saccharin during the first testing day than controls. These pups also showed more aversive behavioral reactions to the gustatory CS than controls. Specifically, increased amounts of grooming, general activity, head shaking, and wall climbing as well as reduced mouthing were observed in response to the CS. Conditioned aversive reactions but not taste avoidance were still evident on the second testing day. In conclusion, a taste CS paired with postabsorptive effects of EtOH and LiCl elicited a similar pattern of conditioned rejection reactions in preweanling rats. These results suggest that similar mechanisms may be underlying CTAs induced by LiCl and a relatively high EtOH dose. © 2010 Wiley Periodicals, Inc. Dev Psychobiol 52: 545–557, 2010.     

Alcohol aversion generalization in rats: specific disruption of taste and odor cues with gustatory neocortex or olfactory bulb ablations

Rats with ablations of the gustatory neocortex (Experiment 1) and rats with olfactory bulb ablations (Experiment 2) were compared with normal rats for aversion generalization to both single taste solutions (sucrose, sodium chloride, quinine hydrochloride, hydrochloric acid) and compound taste solutions (pairs of the four single tastants) following alcohol aversion training. All rats acquired equal and strong alcohol aversions. Control rats showed consistent aversion generalization to both the sucrose + quinine and the sucrose + hydrochloric acid solutions; no significant generalization occurred to the single tastants except a weak generalization to sucrose in Experiment 2. Rats with gustatory neocortical ablations failed to show aversion generalization to any of the taste solutions. Rats with olfactory bulbectomies displayed the same aversion generalization functions as control rats but exhibited significantly faster extinction of the alcohol aversion than did the trained control rats. Results from the present experiments suggest that during alcohol aversion learning, rats lacking gustatory neocortex use odor cues (no taste generalization), whereas rats lacking olfactory bulbs utilize taste cues (normal taste generalization).     

Prenatal psychological stress effects on taste neophobia

Pregnant mothers were subjects to a novel environment, a treatment which is known to induce an 11-hydroxycorticosterone (stress) response, for 30 min daily for five successive days while control mothers were control handled. The novelty and control treatments were given to the pregnant mothers either during terms 1, 2 or 3 of pregnancy. After parturition, all offspring were fostered to neutral foster mothers. Testing for the neophobic taste response was carried out after the offspring were 90 days old. It was found that prenatally stressed animals responded with increased neophobia to a novel saccharin solution. Female offspring drank significantly more of the test solution than male offspring. The neophobic response appears to be an index of exploration. The neophobic response of prenatally treated rats was found to be in the opposite direction of postnatally treated rats. However, both findings follow similar response patterns of pre- and postnatal stressed rats tested in an open field.     

Conditioned taste aversion from neostigmine or methyl-naloxonium in the nucleus accumbens

Taylor, K.M. Mark, G.P. Hoebel, B.G. Conditioned taste aversion from neostigmine or methyl-naloxonium in the nucleus accumbens Physiol Behav 00(00):000-000, 2011. An opioid antagonist injected in the nucleus accumbens of a morphine-dependent rat will lower extracellular dopamine and release acetylcholine (ACh), as also seen in opiate withdrawal. It was hypothesized that raising extracellular ACh experimentally would be aversive as reflected by the induction of a conditioned taste aversion. Rats were implanted with cannulas aimed above the nucleus accumbens (NAc) for injection of the opiate antagonist methyl-naloxonium in morphine-dependent animals or neostigmine to increase ACh in drug naïve animals. Experiment 1 in addicted rats showed that local morphine withdrawal by local injection of methyl-naloxonium paired with the taste of saccharin induces a conditioned taste aversion. Experiment 2 in non-addicted rats demonstrated the same learned aversion after local administration of the cholinergic agonist neostigmine in the NAc. These results suggest that ACh released in the NAc during opiate withdrawal contributes to the dysphoric, aversive state characteristic of withdrawal. This accumbens system is implicated in the mechanism for generating the memory of an aversive event that is expressed as learned taste aversion.     

Alterations of salt taste perception in the developing rat

Behavioral correlates of changing neurophysiological taste sensitivities during development were assessed with a conditioned taste aversion procedure. Young rats (age 25-30 days) avoided 0.1M monochloride salts and 1.0M sucrose reliably less than adults (age 90-105 days), but the two groups did not differ when the conditioned stimulus (CS) was 0.1M citric acid. Analyses of generalization gradients revealed that young rats were unable to discriminate among the tastes of NaCl, NH4Cl, and KCl, whereas adults readily made such discriminations. Both age groups had similar generalization gradients when the CS was 1.0M sucrose or 0.1M citric acid. These data indicate that quantitative and qualitative aspects of salt taste perception alter with age. Furthermore, the behavioral changes noted in the present study correspond closely with previous findings from developmental studies of neurophysiological taste responses.     

Role of the perirhinal cortex in rats' conditioned taste aversion response memorization

The role of the perirhinal cortex (PC) in conditioned taste aversion (CTA) learning was investigated in Long Evans rats. CTA was induced by the intraperitoneal administration of LiCl 60 min after saccharin-sweetened water drinking. The PC was reversibly inactivated by the stereotaxic administration of tetrodotoxin (TTX) 60 min before saccharin drinking, immediately after saccharin drinking (Experiment 1), 6 or 24 hr after LiCl administration (Experiment 2), and 60 min before CTA retrieval testing (Experiment 3). Only pre-saccharin drinking PC inactivation disrupted CTA. Thus, PC integrity is necessary only during the earliest phases of CTA mnemonic processing, that is, taste information acquisition and early gustatory memory elaboration. The results are discussed in relation to PC connectivity and PC temporal involvement in the memorization process of other aversive responses.     

Intraoral intake and taste reactivity responses elicited by sucrose and sodium chloride in chronic decerebrate rats

The oral stimulating effects of sucrose and sodium chloride (NaCl) were assessed in chronic decerebrate and pair-fed intact control rats by measuring both oral motor taste-reactivity responses and intraoral intake volume. Taste-reactivity responses were videotaped during the first minute of the intraoral taste infusion. The infusion continued until the taste solution was rejected from the mouth, and the intake volume was computed accordingly. The number of ingestive taste-reactivity responses and the volume of intraoral intake consumed by pair-fed control and decerebrate rats increased with increasing sucrose concentration. Sucrose intake increased as concentration increased to 0.1 M, then plateaued between 0.3, 1.3, and 2.0 M sucrose for both groups. For control rats, intraoral NaCl elicited an inverted U-shaped function for both taste-reactivity responses and intake. Taste-reactivity responses of chronic decerebrate rats varied with NaCl concentration. In contrast to control rats, intake of NaCl did not differ from that of water for decerebrate rats. These data indicate that caudal brain-stem mechanisms are sufficient to control sucrose intake but are not adequate for the concentration dependent intake of NaCl. Second, these data also indicate that it is possible for taste-elicited oral motor responses to be dissociated from intake. The different roles of taste and postingestive factors in sucrose and NaCl intake are discussed.     

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.     

Intake and behavioral responsiveness to taste stimuli in infant rats from 1 to 15 days of age

Infant rats from 1 to 15 days of age received oral infusions of various taste solutions, and their intake of and behavioral responses to the infusions were observed. From 1 day of age on, pups responded to strong acid and quinine solutions with certain characteristic aversive responses and suppressed intake (Experiment 1). Although even very young (1- and 3-day-old) pups suppressed their intake of strong quinine and acid solutions, and rejected them by showing certain aversive responses (gaping and forelimb flailing), other components of the adultlike aversion response sequence (chin scraping and paw treading) did not appear until approximately 12 days of age. This suggests that the failure of pups to engage in chin scraping and paw treading prior to 12 days is not due to an inability to detect aversive solutions, but rather may be due to an immaturity in pups younger than 12 days of the neural substrates subserving these aversive responses. The emergence of chin scraping and paw treading to strong acid or quinine solutions was not influenced to any marked extent by either the deprivational state of the pup (Experiment 2a) or the site of the infusion (Experiment 2b).     

Neonatal ablations of the gustatory neocortex in the rat: taste aversion learning and taste reactivity

Rats sustaining ablations of gustatory neocortex (GN) at 2, 10, 20, or 60 days of age were compared with control rats in the acquisition and extinction of a learned taste aversion; in addition, these rats were tested for taste preference across five concentrations of sodium chloride solution. Results indicated that GN ablation disrupted aversion acquisition and extinction regardless of age at surgery. Taste response functions for the sodium chloride solutions shown by all GN groups of rats mirrored those of control rats: preference (relative to water baseline) for middle concentrations and rejection of the strongest salt concentration. There was a suggestion that the 20- and 60-day-old GN rats were hyperresponsive to the suprathreshold concentrations of NaCl (except the strongest concentration). The increased response to salt solutions in the 20- and 60-day GN rats may have been related to the significant decreases in water consumption relative to that of normal rats. Water consumption of control rats and GN rats in the 2-day and 10-day groups was essentially equal. It is concluded that infant ablation of the GN does not spare normal taste aversion learning and that rats with GN ablations, regardless of age at surgery, respond in a normal manner to the hedonic aspects of sodium chloride solutions.     

Effects of stimulus preexposure on the generalization of conditioned taste aversions in infant rats

Generalization of a conditioned taste aversion in infant rats and how this is affected by stimulus preexposure was investigated in a series of experiments. In Experiment 1 generalization of a conditioned aversion between two tastes (sweet and salty) was found, and the effect of tastes preexposure was a reduction in generalization (Experiment 2). However, when these tastes were combined with a common taste (acid) that was less (Experiment 3) or more intense (Experiment 3b), the effect of stimulus preexposure was a stronger generalization of the conditioned aversion. In this case, a reduction on generalization was again observed by increasing the number of preexposure trials to the taste compounds (Experiment 4). In all cases the generalization levels were directly related to the effect of stimulus preexposure on the acquisition rate of conditioning. It can be concluded that, with the appropriate parameters, a reduction of generalization of a conditioned taste aversion can be obtained after taste exposure in preweanling rats.     

Altered NaCl taste responses precede increased NaCl ingestion during Na(+) deprivation

It has been suggested that Na(+) deficiency alters the sensitivity of taste receptors, thereby rendering NaCl solutions more palatable or preferred and more likely to be ingested. Increased ingestion of concentrated NaCl solutions by rats during dietary Na(+) deprivation occurs only after approximately 8-10 days. To determine whether changes in gustatory responses mediate the deprivation-induced NaCl ingestion (salt appetite), we evaluated taste responses to a range of NaCl concentrations before, during, and after dietary Na(+) deprivation. Rats were trained to lick rapidly in short-duration (10 s) tests by mixing NaCl solutions in a dilute sucrose solution. This method elicited consistent, interpretable rates of licking, even of normally avoided NaCl concentrations, without the necessity of depriving the rats of water. The licking rate increased after dietary Na(+) deprivation of only 2 days, increased further after 5 days of Na(+) deprivation and, after 10 days, was not different from that after 2 days. These results suggest that a change in the response to NaCl taste, as evidenced by increased rates of licking during short-access tests, occurred after 2 days of dietary Na(+) deprivation. In contrast, a significant increase in the 24-h ingestion of a concentrated NaCl solution occurred only after approximately 1 week of maintenance on Na(+)-deficient chow. Thus, it is unlikely that a delayed change in the response to NaCl taste to more palatable or preferred underlies the delayed increase in 24-h NaCl intake during dietary Na(+) deprivation.