The sweet-bitter taste of alcohol: aversion generalization to various sweet-quinine mixtures in the rat

Rats were trained to avoid a 5% alcohol solution and then testedwith either sweet + quinine hydrochloride solutions (Experiment1) or sweet + hydrochloric acid solutions (Experiment 2). Thesweet stimuli used were sucrose, glucose, fructose and saccharin.Significant aversion generalization was found only in Experiment1 where trained rats generalized to all four test stimuli, thussuggesting that alcohol has a sweet taste (in combination withbitter) not specific to one sweetener. No significant aversiongeneralization was noted in Experiment 2 when sweet + hydrochloricacid solutions were tested. In Experiment 3, rats were trainedto avoid 6% alcohol and tested with sucrose + quinine hydrochloridemixtures with varying concentrations of each component. In general,rats showed generalization of the alcohol aversions across thevarious concentrations of sucrose and quinine hydrochloridetested.     

Monosodium glutamate and sweet taste: generalization of conditioned taste aversion between glutamate and sweet stimuli in rats

Even though monosodium glutamate (MSG) is a prototypical umami substance, previous studies reported that a conditioned taste aversion (CTA) to MSG, mixed with amiloride to block the taste of sodium, generalizes to sucrose. These findings suggest that the taste of glutamate mimics the taste of sucrose and raise the question of whether glutamate has a broadly tuned sweet taste component. To test this hypothesis, CTA experiments were conducted to test for generalization between MSG and several sweet stimuli: sucrose, glucose, maltose, saccharin and SC-45647. Strong bidirectional generalization was seen between MSG mixed with amiloride and sucrose, glucose, saccharin and SC-45647. Weak generalization was seen between MSG and maltose, and sucrose and maltose. None of the CTAs generalized to NMDA. These findings support the hypothesis that the taste of MSG has broadly tuned, sweet-like characteristics, possibly due to the convergence of afferent signals for MSG, natural sugars and artificial sweeteners.     

Conditioned taste aversion elicited in anaesthetized rats by scorpion venom

Conditioned saccharin aversion was elicited in rats by the use of scorpion venom. After 15 min of saccharin drinking, groups of rats were injected with venom, Nembutal, LiCl or isotonic saline. Two groups were anaesthetized 10 min after saccharin drinking and injected with venom or with LiCl. During retention test saccharin aversion was observed in the venom and LiCl groups. It is concluded that anaesthesia does not prevent conditioned taste aversion engram formation.     

Overshadowing of running-based taste aversion learning by another taste cue

Training rats with serial presentations of two taste solutions before confinement in an activity wheel (X → A → running) resulted in weak aversion to taste X, compared to a training procedure without the presentation of A. Demonstration of the overshadowing effect in the present study provides another parallel feature between running-based taste aversion learning and Pavlovian conditioning preparations including poison-based taste aversion learning. It also indirectly supports the claim that cue competition causes degraded contingency effect and cover-cue effect in rats’ running-based taste aversion (Nakajima, 2008).     

Brain regions responsible for the expression of conditioned taste aversion in rats

Conditioned taste aversion (CTA) is acquired when the ingestion of a food is followed by malaise. CTA is a kind of fear learning making animals avoid subsequent intake of the food and show aversive behavior to the taste of the food. To elucidate the brain regions responsible for the expression of CTA, our previous electrophysiological and recent c-fos immunohistochemical studies have been reviewed. Among a variety of brain regions including the parabrachial nucleus, amygdala, insular cortex, supramammillary nucleus, nucleus accumbens, and ventral pallidum that are involved in different phases of CTA expression, the enhanced taste sensitivity to facilitate detection of the conditioned stimulus may originate in the central nucleus of the amygdala and the hedonic shift, from positive to negative, may originate in the basolateral nucleus of the amygdala.     

Symmetrical effect of pre-exposure between alcohol and morphine on conditioned taste aversion

It has previously been reported that pre-exposure to a psychoactive drug can block the conditioned taste aversion associated with that drug. This study was an attempt to investigate alcohol-morphine interactions using this pre-exposure paradigm. After two weeks of adaptation to a schedule of daily 30-minute access to water, rats were pre-exposed to morphine, ethanol, or the respective vehicle control every second day for three days before (Days 1, 3, 5) and after the first conditioning day (Days 8, 10, 12). On conditioning days (Days 7, 14), animals were first presented with a saccharin solution for 30 minutes following which animals that were pre-exposed to morphine were injected with ethanol while those pre-exposed to ethanol were administered with morphine. Saccharin was again presented on three more occasions (Days 21, 28, 35) without drug injection. Using the percent change in saccharin consumed from the first presentation as a measure of aversion, it was found that pre-exposure to morphine blocked ethanol conditioned taste aversion. Similarly, animals pre-exposed to ethanol showed less aversion to the saccharin paired with morphine. This is the first demonstration of a symmetrical relationship between alcohol and the opiates.     

Effect of extra running on running-based taste aversion in rats

Voluntary running in an activity wheel endowed rats with aversion to a taste solution consumed before the running. This running-based taste aversion was attenuated by extra running opportunities interspersed among the taste–running pairings, but the attenuating effect was reduced by signaling the extra running by another taste cue. These results correspond to the so-called degraded contingency effect and cover-cue effect in the traditional preparations of Pavlovian conditioning.     

Gamma-L-glutamyl-taurine (Litoralon) affects conditioned taste aversion in rats

The dipeptide gamma-L-glutamyl-taurine (Litoralon) reduced neophobia of rats at a dose of 5.0 mg/kg (i.p.) in a "one-bottle forced choice paradigm" for conditioned taste aversion (CTA), but did not significantly affect the rats' "memory" of intoxication following chronic treatment at doses of 0.05, 0.50 and 5.00 mg/kg (i.p.). Acute treatment with Litoralon (10-1000 micrograms/kg, i.p.) did not affect CTA checked in a "two-bottle test", when administered immediately following the unconditioned stimulus (LiCl injection). In contrast, when given 90 min prior to the retention test, the injection of Litoralon (50.0 micrograms/kg) and gamma-aminobutyryl ethanolamine phosphate (100 and 500 micrograms/kg) resulted in a significantly higher intake of saccharin solution by the rats. This effect is comparable to the action of diazepam tested in the same experimental procedure. The results support our hypothesis about the anti-conflict potencies of these dipeptides, exerted by reducing aversion of phobia and/or the anxiety level of the animals in the experimental situation.     

Conditioned taste aversion elicited by intracerebral administration of drugs

Conditioned taste aversion (CTA) is a vital adaptive reaction governed by highly reliable but poorly understood central mechanisms. In an attempt to elucidate the site of action of various CTA eliciting drugs, equipotent dosages were applied by the systemic (i.p.) and intracerebral (i.c.) route. Rats were offered water on days 1 and 2. On day 3 they received 0.1% sodium saccharin (CS) followed by pentobarbital anaesthesia and i.p. or i.c. injection of the drug (US). After water on day 4, the rats were allowed to choose between water and saccharin on day 5. The putative central action of amphetamine was not confirmed by this experimental arrangement, since CTA was evoked by only moderately (about 10 times) lower i.c. than i.p. dosages. Similar ratio of the i.c. to i.p. effective dosages was obtained with carbachol. On the other hand, CTA of clearly central origin was caused by harmaline and by other monoamine oxidase inhibitors, pargyline and clorgyline, which elicited comparable aversion using 500, 400 and 250 times lower i.c. than i.p. dosages, respectively. The intracerebral gradient of the effect pointed to the lower medulla (inferior olive, raphe nuclei) as the critical brain region and to serotonin as the transmitter participating in the aversive labeling of the gustatory stimulus. The CTA-forming mechanism can also be studied by analysing the action of drugs, e.g. convulsants, which do not produce CTA even when applied at highly toxic dosages (LD 50) eliciting long lasting convulsions (picrotoxin, 5 mg/kg; bicuculline, 5 mg/kg). It is concluded that comparison of brain events elicited by drugs which can or cannot serve as unconditioned stimuli in the CTA paradigm may substantially contribute to the exploration of the underlying neural mechanisms.     

Taste aversion learning in preweanling rats exposed to alcohol prenatally

The effects of prenatal alcohol exposure on the development of a conditioned taste aversion were examined in preweanling rat pups. Mothers of these pups were fed isocaloric liquid diets containing either 35 or 0% ethanol-derived calories (EDC) from gestation days 6 through 20. A pair-feeding procedure was employed, and an ad lib lab chow control group was also included. At 5, 10, or 15 days of age, pups were infused with a saccharin solution through a cannula implanted in the oral cavity. Half of the pups in each group were then injected with lithium chloride (LiCl), which served as the poisoning agent, and the other half with sodium chloride (NaCl) as a control. Animals were subsequently tested for a conditioned aversion to the saccharin solution. At 15 days of age, all of the pups in the LiCl-poisoned group demonstrated a conditioned taste aversion to the saccharin solution, but the degree of this aversion was less in alcohol-exposed offspring. At 10 days of age, a taste aversion was learned, although it was not as strong as that shown by 15-day-old pups, and it appeared to be learned equally well by all of the prenatal treatment groups. At 5 days of age, there was marginal support for taste aversion learning. Again, it did not interact with prenatal treatment. The ontogenic differences in taste aversion learning exhibited by alcohol-exposed offspring relative to controls are discussed in terms of altered hippocampal development.     

Blocking of acquisition of a taste aversion by a context experienced prior to the taste

This experiment tested the proposal that events taking place before a rat has access to a taste can proactively interfere with acquisition of an aversion to the taste when this has been followed by lithium chloride injection. Rats were initially given context discrimination whereby placement in one distinctive context (target) was followed by lithium injection, while placement in a second context (safe) was followed by saline injection. In the subsequent 1-trial taste conditioning session, rats were first placed in either their target context (Blocking group), their safe context (Control-Safe group) or a neutral context (Control-Neutral group), then given access to sucrose and 30 min later were injected with lithium. Subsequent tests of sucrose intakes revealed a blocking effect. These results indicate that proactive interference with taste aversion learning by a context can occur that is unlikely to be based on generalization decrement.     

Hedonic taste in Drosophila revealed by olfactory receptors expressed in taste neurons

Taste and olfaction are each tuned to a unique set of chemicals in the outside world, and their corresponding sensory spaces are mapped in different areas in the brain. This dichotomy matches categories of receptors detecting molecules either in the gaseous or in the liquid phase in terrestrial animals. However, in Drosophila olfactory and gustatory neurons express receptors which belong to the same family of 7-transmembrane domain proteins. Striking overlaps exist in their sequence structure and in their expression pattern, suggesting that there might be some functional commonalities between them. In this work, we tested the assumption that Drosophila olfactory receptor proteins are compatible with taste neurons by ectopically expressing an olfactory receptor (OR22a and OR83b) for which ligands are known. Using electrophysiological recordings, we show that the transformed taste neurons are excited by odor ligands as by their cognate tastants. The wiring of these neurons to the brain seems unchanged and no additional connections to the antennal lobe were detected. The odor ligands detected by the olfactory receptor acquire a new hedonic value, inducing appetitive or aversive behaviors depending on the categories of taste neurons in which they are expressed i.e. sugar- or bitter-sensing cells expressing either Gr5a or Gr66a receptors. Taste neurons expressing ectopic olfactory receptors can sense odors at close range either in the aerial phase or by contact, in a lipophilic phase. The responses of the transformed taste neurons to the odorant are similar to those obtained with tastants. The hedonic value attributed to tastants is directly linked to the taste neurons in which their receptors are expressed.     

Conditioned taste aversion by lithium and saline: a methodological evaluation

Conditioned taste aversion (CTA) to saccharin induced by orallithium chloride and normal saline were investigated in 270Wistar rats in a CTA paradigm. They were trained to drink for20-min sessions per day in a two-bottle choice, providing saccharinand water simultaneously. Saccharinn neophobia was replacedby a preference with two more exposures. Saccharin was withheldfor 2 days following force-feeding of lithium or saline in orderto assess lithium-illness. Lithium-fed rats showed signs ofillness followed by a strong CTA to saccharin persisting for3–4 days. Saline-fed rats did not suffer from any illnessbut showed a weak and inconsistent CTA lasting for a day ortwo. Scopolamine injections prior to force-feeding did not affectthe pattern of aversion in either group, but significantly suppressedthe revocation of CTA in the saline-fed rats by a second oraldose of saline.     

A non-taste cue of sucrose in short-term taste tests in rats

Data are presented showing that rats can discriminate amongsucrose solution concentrations by some cue other than taste,possibly by olfaction. Non-taste factors need to be consideredin taste discrimination studies.