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.     

Two novel paradigms for the simultaneous assessment of conditioned taste aversion and food intake effects of anorexic agents

The conditioned taste aversion (CTA) is routinely used to assess the aversive consequences of anorexic agents, including potential pharmacological therapies for obesity. In a typical CTA paradigm, rats briefly sampling a novel tastant (e.g., saccharin) are acutely administered with toxin (e.g., lithium chloride, LiCl). After as few as one taste-toxin pairing, rats will reliably avoid the novel tastant. This paradigm is frequently used for the assessment of possible aversive consequences of drugs that are candidates for pharmacological therapies. The degree to which the drug supports development of a CTA is interpreted as an index of its aversive properties. Difficulties with previous work include the inability to assess affects on food intake and CTA simultaneously, particularly during chronic drug administration. We report here two novel CTA paradigms for the assessment of appetitive and aversive consequences of anorexic agents, simultaneously. In the first experiment, animals receive an intraoral infusion of a novel and highly palatable tastant immediately prior to administration of increasing doses of LiCl. In the second experiment, rats were implanted intraperitoneally with osmotic minipumps that chronically delivered a low dose of LiCl for 7 days. LiCl did not affect short or long term food intake in either experiment. However, LiCl did support the development of a CTA in both paradigms. These results suggest that both the appetitive and aversive consequences of anorexic agents can be assessed simultaneously during either acute or chronic drug administration.     

Disruption of conditioned taste aversion by ECS: the role of lithium chloride

Rats were taught a conditioned taste aversion (CTA) by pairing a 10% sucrose solution (CS) with lithium chloride (LiCl)-induced poisoning (UCS). The CS-UCS interval was 30 min. The LiCl dose (20 ml/kg) was either strong (0.15 M) or weaker (0.075 M). Electroconvulsive shock (ECS) (80 mA for 600 msec) was interpolated within the CS-UCS interval at either 15 or 30 min. ECS caused a significant disruption of CTA only when the aversion was established with the weaker dose of LiCl. There was also no indication that interference with CTA was dependent upon close temporal contiguity between the ECS and LiCl. In a second experiment a CTA was established with LiCl (0.15 M) which was heated to 45 degrees C. Under these conditions ECS produced a similar disruption of learning to that when the UCS was the weaker dose of LiCl (0.075 M). The results suggest that an apparent differential loss of learning within the CS-UCS interval described in a previous report was accidentally created when some groups of animals were poisoned with warm and others with cold LiCl.     

Conditioned taste aversion induced by estradiol pellets.

In two experiments, ovariectomized rats were given a novel diet prior to the implantation of a fused pellet of estradiol (E2 pellet). In short-term (3 weeks) ovariectomized rats, the suppression of food intake induced by estrogen was not affected by the introduction of the novel diet. However, a sensitive two-choice preference test revealed that subjects implanted with the E2 pellet had a lesser preference for the novel diet than controls implanted with the vehicle pellet. In long-term (18 weeks) ovariectomized rats, implantation of the E2 pellet had a large effect on the consumption of the novel diet. Intake was reduced to less than 1 g in all subjects on Days 3-7 after E2 pellet implantation. A subsequent two-choice preference test indicated the presence of a strong aversion to the novel diet in the estradiol-treated rats relative to the controls. These experiments show that estradiol can induce conditioned taste aversions that have either no effect on intake or totally suppress food intake, depending upon postovariectomy time.     

Forebrain structures specifically activated by conditioned taste aversion

This study investigates which forebrain structures show Fos protein expression during conditioned taste aversion (CTA) acquisition and whether Fos expression depends on the aversion strength. A novel taste paired with an intraperitoneal injection of a low dose of the malaise-inducing agent lithium chloride (LiCl) induced a weak CTA, whereas associating this novel taste with a high dose of LiCl induced a strong CTA. Increasing the strength of the gastric malaise alone enhanced Fos expression in central, basal, and lateral amygdala nuclei and decreased Fos expression in the nucleus accumbens core. Taste-malaise association induced specific Fos activation in the insular cortex (with both the low and the high doses of LiCl) and the nucleus accumbens shell (with the high LiCl dose only). No significant variation of Fos expression was measured in the perirhinal cortex. Several forebrain areas may be sites of taste-malaise convergence during CTA acquisition depending on the strength of the aversion.     

Uterine position and conditioned taste aversion

Three experiments were designed to determine the influence of uterine position on the performance of female rats in a conditioned taste aversion paradigm. The first and second experiments confirmed a differential behavioral response by males and females during acquisition and extinction of the conditioned taste aversion. However, no differences were found between females that had caudal male littermates in utero (MF) and females that had no caudal male littermates (FF). In the third experiment, in which testosterone was administered to females throughout testing, MF females showed an increased sensitivity to testosterone and a more prolonged rate of extinction than FF females. Exposure to testosterone during prenatal development heightened postnatal responsiveness to testosterone in female rats. The results are discussed in terms of the organizational and activational effects of testosterone on behavior in a conditioned taste aversion situation.     

Acid taste thresholds assessed by conditioned taste aversion and two-bottle preference in rats

The conditioned taste aversion (CTA) threshold for either citric acid (CA) or HCl solutions and the two-bottle taste preference (TBP) threshold were determined in rats that are familiarized to the odor of conditioning solutions or that are naive. The CTA method appeared to be more sensitive than the TBP test, particularly when rats were not familiarized to the odor of the conditioning solution. The CTA threshold for HCl-conditioned rats and familiarized to the odor of conditioning solution lies between 1.00 and 2.00 mmol; in unconditioned rats, it lies between 4.00 and 5.00 mmol. In CA-conditioned and odor-familiarized rats, the threshold lies between 0.09 and 0.20 mmol; in unconditioned rats, it lies between 7.00 and 10.00 mmol. In rats not familiarized to the odor of the conditioning solution, the threshold for HCl-conditioned rats lies between 0.90 and 1.00 mmol; in unconditioned rats, it lies between 2.00 and 3.00 mmol. In CA-conditioned rats, the CTA threshold lies between 0.03 and 0.05 mmol; in unconditioned rats, it lies between 4.00 and 7.00 mmol. The two-bottle test is less sensitive than the CTA method. The TBP threshold lies between HCl 4.00 and 5.00 mmol, and between CA 4.00 and 7.00 mmol. The odor of a solution may potentiate the ability of rats to detect the concentration of CA and HCl solutions.     

Protection against radiation-induced conditioned taste aversion by Centella asiatica

Radiations are known to cause behavioural perturbations like conditioned taste aversion (CTA), performance decrement, learning, etc., even at very low doses. The manifestation of radiation-induced behavioural degradation has not been understood well and requires further studies. Therefore, the effects of low-dose whole-body 60Co gamma-irradiation in male rats were studied in terms of body weight and CTA learning. For CTA, the consumption of saccharin solution was considered as a parameter. To protect against the adverse effects of radiation, Centella asiatica (aqueous extract) was tested and compared with ondansetron, a standard antiemetic drug. A dose of 2 Gy incurred significant body weight loss [t(9)=9.00, P<.05] and induced CTA in rats [t(26)=9.344, P<.01]. Administration of C. asiatica (100 mg/kg bw ip, 2 Gy, -1 h) rendered significant radioprotection against radiation-induced body weight loss and CTA that became evident on the second postirradiation day [t(7)=0.917, P>.05; t(7)=4.016, P>.05]. Ondansetron (1 mg/kg bw) elicited higher degree of protection against CTA [t(7)=3.641, P>.05] than C. asiatica [t(7)=7.196, P>.05] on the first postirradiation day, but on the second postirradiation day, both were equally effective [t(7)=3.38, P>.05; t(7)=4.01, P>.05]. In case of C. asiatica-treated animals, however, there was a consistently declining CTA from the second to the fifth postirradiation day whereas in ondansetron-treated animals it was inconsistent. Present investigation suggests that C. asiatica could be useful in preventing radiation-induced behavioural changes during clinical radiotherapy.     

Blockage of the effects of testosterone on extinction of a conditioned taste aversion by estradiol: time of action

Testosterone (T) prolongs the extinction of a conditioned taste aversion only if it is present during extinction. Experiments were conducted to determine whether estradiol (E) blocks the effects of T by acting during acquisition or extinction. In the first experiment, gonadectomized male and female rats injected with estradiol dipropionate (EP) and testosterone propionate (TP) during extinction had significantly faster extinction rates than those only injected with TP. Treating gonadectomized rats with TP prior to as well as during extinction did not prevent EP from blocking the effects of T. In Experiment 2, E was equally effective in preventing T from prolonging extinction when it was implanted in gonadectomized males during acquisition, extinction, or both acquisition and extinction. Thus, E does not have to be present concurrently with T during extinction to be effective. This suggests that E does not act on a T-related mechanism but rather acts independently of T.     

Odor of taste stimuli in conditioned "taste" aversion learning

The present research addresses whether rats can express odor aversions to the odor of taste stimuli. In Experiment 1, saccharin or salt were either mixed in distilled water, so the rats could taste and smell them, or presented on disks attached to the tubes' metal spouts so the rats could only smell them. Aversions were established to taste stimuli under both conditions. The results of Experiment 2 indicate that conditioning was to the odor of the tastes when they were presented on disks in Experiment 1, hence both taste and odor aversions were established by means of "taste" stimuli. Taste aversion learning thus may more properly be termed flavor aversion learning, with flavor referring to both taste and odor components.     

The role of drinking in the suppression of food intake by recent activity.

The standard activity-based anorexia procedure provides rats with access to a running wheel while restricting their access to dry food. This can produce reduced food intake and progressive weight loss. Using this procedure, in the present study (Experiment 1) the authors found changes in drinking patterns both in the period of high activity preceding food access and during the feeding period. Varying the procedure by providing wet mash (Experiment 2) or by prior adaptation to a drinking schedule (Experiment 3) prevented the self-starvation effect. These results indicate the importance of drinking when analyzing the effect of recent activity on food intake.     

The role of a noxious taste in determining food intake in the rat

Adult female rats were given access to cornstarch, fat, and 2 cups of casein ad lib. Sucrose octaacetate was added to one or both casein cups in concentrations of 0, 0.1, or 2.5%. The lower concentrations of SOA had no effect on total caloric intake or food choices, although a taste aversion experiment indicated that the rats could detect SOA at this level. The 2.5% concentration had no effect on total caloric intake. If only one casein cup was treated with SOA, the animals consumed 70% of their protein from the unadulterated casein supply, but total protein intake was unchanged. If both cups were treated with 2.5% SOA, protein intake decreased, but carbohydrate consumption showed a compensatory increase. Fat intake was unchanged.     

Further evidence for conditioned taste aversion induced by forced swimming

A series of experiments with rats reported that aversion to a taste solution can be established by forced swimming in a water pool. Experiment 1 demonstrated that correlation of taste and swimming is a critical factor for this phenomenon, indicating associative (i.e., Pavlovian) nature of this learning. Experiment 2 showed that this learning obeys the Pavlovian law of strength, by displaying a positive relationship between the duration of water immersion in training and the taste aversion observed in subsequent testing. Experiment 3 revealed that swimming rather than being wet is the critical agent, because a water shower did not endow rats with taste aversion. Experiment 4 found that taste aversion was a positive function of water level of the pools in training (0, 12 or 32 cm). These results, taken together, suggest that energy expenditure caused by physical exercise might be involved in the development of taste aversion.     

Impaired conditioned taste aversion learning in APP transgenic mice

Cognition in transgenic mouse models of Alzheimer's disease (AD) has been predominantly characterized in explicit spatial orientation tasks. However, dementia in AD encompasses also implicit memory systems. In the present study a line of transgenic mice (TgCRND8) encoding a double mutated allele of the human amyloid precursor protein (APP) genes was evaluated in an implicit associative learning task of conditioned taste aversion (CTA). CTA is a form of Pavlovian classical conditioning, in which a mouse learns to avoid a novel taste of saccharine (conditioned stimulus) paired with an experimentally induced (systemic injection of lithium chloride) nausea (unconditioned stimulus). In contrast to conditioned non-Tg mice, TgCRND8 APP mice developed weaker aversion against saccharine and quickly increased its consumption in repeated tests. These results indicate that TgCRND8 mice show a significant impairment not only in explicit spatial memory, as has been previously shown [Nature 408 (2000) 979], but also in implicit memory. Control experiments confirmed that TgCRND8 and non-Tg mice had comparable taste sensitivities in response to appetitive as well as aversive tastes. The study suggests that the CTA paradigm can be a sensitive tool to evaluate deficits in implicit associative learning in APP transgenic mouse models of AD.     

Ethanol-induced conditioned taste aversion in 15 inbred mouse strains

This study used a genetic correlational strategy to characterize the neurobiological basis of ethanol's (0, 2, or 4 g/kg) aversive effects as indexed by conditioned taste aversion. Substantial strain differences in taste aversion and hypothermia were observed, but the genetic correlation between these phenotypes was not significant. However, significant genetic correlations were observed between taste aversion and ethanol-related behaviors measured in previous studies, including home-cage ethanol preference (r = .68) and ethanol withdrawal severity (r = -.69). Strains showing stronger taste aversion tended to show lower ethanol preference and higher withdrawal severity. This pattern of findings is consistent with previous studies suggesting a commonality in neurobiological mechanisms underlying these phenotypes. These results do not support the hypothesis that ethanol-induced taste aversion is mediated by the drug's rewarding properties.     

Profound conditioned taste aversion induced by oral consumption of 2-deoxy-D-glucose

In a previous experiment rats avidly avoided a solution of 2-deoxy-d-glucose in 0.2% saccharin (2-DG+S), drinking less than 1 mL over 3 days. The present study investigated taste avoidance and conditioned taste aversion (CTA) to orally presented 2-DG+S as well as CTA in response to i.p. injections of 2-DG. In Experiment 1, rats were given either a glucose/saccharin(G +S) solution or 2-DG+S for eight 30-s test periods. Four seconds into the first 30-s test, rats in the 2-DG+S group licked significantly less than rats in the G+S group, and licking almost totally ceased in the remaining seven tests. Overnight water intake was not different between the groups, but when offered a G+S solution, rats in the 2-DG+S group almost totally avoided the solution and still showed a significant aversion to G+S when retested 6 days later. In Experiment 2, rats were allowed to drink G +S, and were then injected i.p. with 500 mg 2-DG/kg, 500 mg D-glucose/kg body weight in 0.3 mL water, or with 0.3 mL saline. When tested with a G +S solution the next day, rats in the 2-DG group showed a highly significant avoidance, while rats in the glucose group were not different from those in the saline group. The results of the second experiment are consistent with earlier studies of CTA induced by i.p. injections of 2-DG. The present study indicates that small amounts of orally ingested 2-DG produce a CTA as strong or even stronger than that following injected 2-DG, most likely by inducing malaise. Whether the onset of malaise is fast enough to account for the rapid initial avoidance of this solution or a taste factor is also involved is not yet clear.     

Differential effect of free intake versus oral perfusion of sucrose in conditioned taste aversion in rats.

Five experiments were designed to investigate LiCl-induced conditioned taste aversion (CTA) obtained in rats whether after free intake of a sucrose solution (active mode) or after forced administration through an intraoral cannula (passive mode). It was found in Experiment 1 that actively conditioned rats showed a slower extinction rate as revealed by repeated two-bottle tests (active testing) as opposed to passively conditioned ones. As these rats underwent a mode change between conditioning and testing, the differential extinction rate might have arisen from this change inducing a generalization decrement effect or acting as a contextual shift. In Experiment 2, no evidence for any generalization decrement was found. The possibility that the mode of sucrose delivery could have contextual properties in CTA through a "renewal test" after extinction and a latent inhibition experiment was further tested in Experiments 3 and 4. When active testing followed passive extinction, a CTA was afresh obtained in rats actively conditioned in active conditions. Latent inhibition was attenuated in rats preexposed in passive conditions and conditioned in active conditions (i.e., when a shift in the drinking mode occurred between preexposure and conditioning). In Experiment 5, intraoral perfusion was used in both groups. The active subjects had to nose poke for intraoral administration of sucrose. The yoked control passive subjects received simultaneously the same amount of sucrose. The levels of CTA differed also from the actively to the passively conditioned subjects. Results are discussed in terms of free intake activity acting as a contextual modulator of CTA.