Latent Inhibition and Conditioning in Rat Strains Which Show Differential Prepulse Inhibition

Latent inhibition (LI) is the retardation of associative conditioning resulting from preexposure of the conditioned stimulus (CS) alone prior to conditioning. Schizophrenic patients show deficient prepulse inhibition (PPI) and, at least acutely, deficient LI as well. We recently found that Brown Norway (BN) rats show a PPI deficit compared to Wistar-Kyoto (WKY) rats. If PPI and LI depend on neural processes with common genetic substrates, then LI should be deficient in BN rats as well. Here, LI of a conditioned taste aversion was examined in BN and WKY rats. One group from each strain was preexposed to a saccharin-flavored solution (CS) the day prior to conditioning. For taste aversion conditioning, these two groups again consumed saccharin and were injected with lithium chloride (unconditioned stimulus) 10 min later. A second group from each strain was not preexposed to the CS and was treated identically during conditioning, while a third group was not conditioned (injected with sodium chloride). To test for taste aversion conditioning, saccharin was offered for 20 min/day for 3 days. Nonconditioned BN and WKY rats consumed equal amounts of saccharin on test days. In both strains, conditioned rats showed a saccharin aversion. However, conditioning was less robust in BN than in WKY rats. WKY rats showed good LI of the conditioned taste aversion in that preexposed WKY rats consumed significantly more saccharin on test days than conditioned, nonpreexposed WKY rats. Preexposed BN rats did not consume significantly more saccharin on test days than conditioned, nonpreexposed BN rats. The previously reported deficiency in PPI in the BN rats was confirmed here 1 week after the taste aversion experiment. These results suggest that BN rats show deficient LI as well as PPI and display poor associative learning, a trait also reported in schizophrenia.     

Modulation of body temperature through taste aversion conditioning.

Injection of rats with bacterial lipopolysaccharide (LPS) results in an initial fall in body temperature followed by a fever. Lithium chloride (LiCl) injection induces a fall in body temperature without subsequent fever production. When these substances were incorporated as unconditioned stimuli in a taste aversion conditioning paradigm, using saccharin flavour as the conditioning stimulus, these differential effects on body temperature were reenlisted on reexposure to saccharin alone 7 days after conditioning. The changes in body temperature on reexposure were similar in direction and kinetics as on the conditioning day although reduced in magnitude. The finding of a true conditioned effect in these studies is in contrast to the paradoxical or compensatory conditioned body temperature responses described elsewhere using different conditioning models. This apparent conflict may be explained on the basis of different unconditioned stimuli acting on efferent versus afferent arms of a negative feedback system. Since body temperature changes often occur in association with immune responses, these findings may have implications to behavioural conditioning of immunity, the outcome of which may reflect the indirect effects on immunity of inadvertent conditioning of thermoregulatory changes.     

Backward conditioning of tumor necrosis factor-α in a single trial: changing intervals between exposures to lipopolysaccharide and saccharin taste

The current study examined the effect of backward conditioning with three different time intervals between exposures to lipopolysaccharide (LPS) as the unconditioned stimulus (US) and saccharin taste in water as the potential conditioned stimulus (CS). Forty-eight naïve female BALB/c mice at three months of age served as subjects, divided into six groups. Four groups were assigned to Experiment 1 for the tumor necrosis factor alpha (TNF-α) measure, and the remaining two groups were used in Experiment 2 to measure taste aversion behavior. Both experiments employed a single trial. The timing of introduction to the saccharin taste varied between 3 min, 7 h, and 24 h following an LPS injection in Experiment 1. Experiment 2 employed the three-minute interval only. These intervals correspond to distinct immunological, physiological, and behavioral events induced by LPS. On the day after re-exposure to the saccharin taste, the TNF-α groups were challenged with LPS to test the LPS tolerance response. While backward conditioning of taste aversion behavior was not observed, some evidence of conditioned TNF-α response and subsequent development of LPS tolerance was observed with backward conditioning in a single trial. This exploratory study demonstrated that the effect of backward conditioning on conditioned TNF-α response and LPS tolerance response in a single trial depended on the timing of when a CS is presented after LPS exposure.     

Unconditioned Stimulus Intensity and Retention Interval Effects

In single-element taste-aversion learning, retention interval effects are seen if taste aversions are paradoxically weak when they are tested 1 day after conditioning than when they are tested 3 or more days after conditioning. One explanation of this phenomenon is that weaker taste aversions may increase in strength across a retention interval. To test this possibility, rats were given saccharin followed by an unconditioned stimulus (US) of weak, medium, or high intensity; testing occurred after a 1-day or a 5-day retention interval. The results showed retention-interval effects only at medium and high dosage levels, but not following a weak-intensity US. Furthermore, at the 5-day retention interval, aversion strength increased as the intensity of the US increased. However, at the 1-day retention interval, there were no significant differences due to US intensity. In accordance with previous experiments, this outcome suggests that nonassociative factors, such as US novelty, and not associative factors (e.g., US intensity), modulate taste aversion performance on a 1-day test.     

Kindling increases aversion to saccharin in taste aversion learning

Kindling is a model in which an initially subconvulsive electrical stimulation of certain brain areas eventually develops a generalized seizure that produces behavioral and long term neuronal changes. In the present study we evaluated if kindling can modify conditioning taste aversion (CTA). In this paradigm animals acquire aversion to saccharin when it is presented as the conditioned stimulus (CS) followed by an injection of lithium chloride (LiCl) that induces a gastric irritation as the unconditioned stimulus (US). Male Wistar rats were implanted with bipolar electrodes aimed at the right amygdala (AMG) or at the right insular cortex (IC). The animals were stimulated daily until they reached stages 2-4 (intermediate) or until kindling was fully established (three consecutive stage 5 seizures). At least two weeks after kindling stimulation had ceased the animals were deprived of water for 24 h and given 10-min drinking sessions twice a day for 4 days. On day 5 (morning session) tap water was replaced by saccharin solution (0.1%), 20 min later the animals were injected with LiCl (7.5 ml/kg i.p., 0.2 M) to induce gastric malaise or taste aversion. After three more days of baseline consumption, water was substituted by a fresh 0.1% saccharin solution to test the aversion. AMG-kindling delayed the extinction of CTA. Animals with kindling in the IC had a higher retention than the sham kindling group; that is, they drank significantly less saccharin solution than the other groups. The results of the present experiment show that local modification of brain function induced by kindling stimulation can prolong the aversive effects of CTA.     

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.     

Effects of taste aversion conditioning on the primary antibody response to sheep red blood cells and Brucella abortus in the albino rat

It has been shown that use of the saccharin/cyclophosphamide taste aversion paradigm produced conditioned immunosuppression as well as saccharin avoidance after two post-conditioning exposures to the aversive stimulus [3,18]. In the present study the effects of saccharin/cyclophosphamide conditioning on the primary humoral antibody response to two antigens: sheep red blood cells (SRBC), a T-cell dependent antigen and Brucella abortus (B. abortus), a T-cell independent antigen, were examined. In addition the effects of a third exposure to the aversive stimulus, saccharin, on conditioned immunosuppression were examined. The results indicate that the target cell of taste aversion conditioning, in relation to immune dysfunction, could be the T-lymphocyte and that conditioned immunosuppression is dependent on the presence of the behavioral response. These results constitute a replication of previous studies [3,18] and provide evidence which indicates that behaviorally conditioned immunosuppression is a consequence of the parameters of taste aversion conditioning.     

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.     

Effects of distribution of the drug unconditioned stimulus on taste-aversion learning

Rats injected with lithium chloride after exposure to a taste or olfactory stimulus learn stronger aversions to these cues if the drug is administered in two small injections 35 min apart than if all of the drug is given in a single injection. This facilitation of conditioning produced by distribution of the drug unconditioned stimulus occurs with both low and high lithium doses (Experiments 1 and 2), is more evident in male than in female rats (Experiment 1), and is directly related to the amount of the flavored solution consumed prior to drug treatment (Experiment 4). Increasing the interval between two small drug injections beyond an optimal value results in a progressive loss of the facilitation of conditioning (Experiments 2 and 3), and the optimal drug distribution interval may be shorter for olfactory cues (Experiment 3) than for taste stimuli (Experiments 1 and 2). Control observations (Experiments 5A and 5B) showed that the drug distribution effect is not due to handling or other non-drug factors involved in giving two rather than only one injection. The phenomenon is consistent with recently-proposed models of conditioning and suggests that the differential effectiveness of various drugs in taste aversion conditioning may be related to differences in the time course of the unconditioned drug effects.     

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.     

Further examination of ontogenetic limitations on conditioned taste aversion

This study was to resolve a discrepancy in the literature as to the capability of infant rats in acquiring conditioned taste aversion. Previous studies had indicated that during the 1st postnatal week, an aversion to saccharin could be conditioned when paired with lithium chloride (LiCl). Analogous conditioning with sucrose did not seem to occur until the end of the 2nd postnatal week, however, even though sucrose is discriminated from water and preferred before then. We observed that 5- and 9-day old pups express conditioned taste aversion to both saccharin and sucrose flavors that previously were paired with illness induced by LiCl. This learning occurred only when several hours separated cannulation and conditioning. A number of other factors that seemed likely to determine this early learning were found to have no effect. Thus it appears that rats can learn taste aversions very early in life, but only under certain circumstances. The results are discussed with reference to Vogt and Rudy's (1984) conclusions on the ontogeny of taste guided behaviors in the rat.     

Saccharin's rewarding, conditioned reinforcing, and memory-improving properties: mediation by isomorphic or independent processes?

Unconditioned reward and conditioned reinforcing effects may reflect an isomorphic motivational process because increased conditioned reinforcing effects were seen with increased amounts of saccharin consumed in taste and place conditioning. Reinforcing effects in place conditioning leveled off as saccharin unconditioned consumption reached maximum amounts of approximately 140 mg/rat. Posttrial consumption, but not intraperitoneal injection, of saccharin significantly enhanced conditioned place and taste preferences as well as conditioned taste aversions. Saccharin's memory-improving effects in both aversive and appetitive conditioning suggest a process separate from the reward-reinforcement process. Independent of effects on blood glucose, the motivational property of saccharin's sweet taste undergoes differential central processing to mediate reward-reinforcement versus memory improvement processes.     

Taste aversions conditioned with partial body radiation exposures

Radiation-induced taste aversion was compared in rats which received partial body exposure to the head or abdomen with rats receiving whole body irradiation. Exposure levels ranged from 25 to 300 roentgens (R). In additional groups, saccharin aversion to partial body gamma ray exposures of the abdomen were conditioned in animals which had prior experience with the saccharin solution. Aversion was measured with a single-bottle short-term test, a 23-hour preference test and by the number of days taken to recover from the aversion. Whole body exposure was most effective in conditioning the aversion, and exposure of the abdominal area was more effective than exposure of the head. Also, the higher the exposure, the stronger the aversion. Rats receiving prior experience with the saccharin did not condition as well as control rats with no prior saccharin experience. The possible role of radiation-induced taste aversion in human radiotherapy patients was discussed.     

Duodenal pain and spinal morphine induce conditioned taste aversion in rats

Conditioned taste aversion (CTA) is a behavioural response essential to the survival of an individual. The combination of taste and odour of most foods provides a strong conditioned stimulus (CS) for an animal to respond in an appropriate way to any harmful unconditioned stimuli (US) that follow. The most widely used conditioned stimuli are drinkable sweet solutions, such as saccharin and sucrose. CTA-like responses are also found for environmental unconditioned stimuli, but these usually take longer training. In the present study, the aversive nature of a duodenal distention with an implanted balloon catheter was studied in freely moving rats using either CTA against a sucrose solution, or a light-dark passive avoidance (PA) paradigm. In addition, the effect of spinal morphine on CTA and the cardiovascular response to duodenal distention were studied. CTA could be induced by a single, but long-lasting 20-minute duodenal distention, which did not induce PA behaviour in a light-dark box. Spinal infusion of morphine alone induced CTA, suggesting that the model is unsuitable to investigate spinal pharmacological modulation of visceral pain. Spinal morphine did reduce the cardiovascular response to duodenal distention, strengthening its validity as a visceral pain model. Since CTA is a complicating factor in the field of chemotherapy in cancer patients and spinal morphine causes nausea and vomiting in humans, CTA may also complicate spinal drug treatment or anaesthesia.     

Behaviorally conditioned immunosuppression.

An illness-induced taste aversion was conditioned in rats by pairing saccharin with cyclophosphamide, an immunosuppressive agent. Three days after conditioning, all animals were injected with sheep erythrocytes. Hemagglutinating antibody titers measured 6 days after antigen administration were high in placebo-treated rats. High titers were also observed in nonconditioned animals and in conditioned animals that were nor subsequently exposed to saccharin. No agglutinating antibody was detected in conditioned animals treated with cyclophosphamide at the time of antigen administration. Conditioned animals exposed to saccharin at the time of or following the injection of antigen were significantly immunosuppressed. An illness-induced taste aversion was also conditioned using LiCl, a nonimmunosuppressive agent. In this instance, however, there was no attenuation of hemagglutinating antibody titers in response to injection with antigen.     

Involvement of the anterior insular gustatory neocortex in taste-potentiated odor aversion learning

When an odor conditioned stimulus (CS) precedes illness (unconditioned stimulus; UCS), rats acquire relatively weak odor aversions. Conversely, when a compound odor-taste (flavor) CS precedes illness, rats acquire robust aversions both to the odor and to the taste components of a compound flavor CS. Thus, tastes potentiate odor-illness aversions during toxiphobic conditioning. Such conditioning effects have been referred to as taste-potentiated odor aversion learning (POA). Previous neurobehavioral experiments have shown that the anterior insular gustatory neocortex contributes to conditioned taste aversion (CTA) learning. The present experiment examined the involvement of the anterior insular gustatory neocortex in CTA learning and POA learning. To that end, four distinct groups of rats received bilateral electrolytic lesion placements in the orbitofrontal neocortex, the "somatic" gustatory neocortex, the anterior insular gustatory neocortex or the posterior insular neocortex. Control animals received anesthesia only. Subgroups of animals thereafter received aversion conditioning using either an odor (almond) CS or a compound odor-taste (almond-saccharin) CS. Aversions to the almond odorant and/or saccharin tastant were evaluated during extinction. Results indicated that animals lacking orbitofrontal neocortex or posterior insular neocortex acquired normal CTAs and POAs. Animals lacking somatic gustatory neocortex exhibited impaired CTA learning, yet those animals showed normal POA learning. Lesions centered in the anterior insular neocortex impaired both CTA learning and POA learning. These results demonstrate that the insular gustatory neocortex is uniquely involved in the higher-order integration of odors, tastes and illness.     

Multiple remote-US preexposures and the blocking effect produced by a proximal-US

Weanling, young-adult, and old-age Wistar albino rats were used to determine whether number of unconditioned stimulus (US) presentations, given 24 h or more (remote preexposure) prior to conditioning, alters the blocking effect of a single-US preexposure given 2 h before (proximal) taste aversion conditioning. As the number of remote-US preexposures increased from 0 to 6, the ability of the proximal-US preexposure to block conditioning initially increased then decreased for all age groups. Of the models put forth to explain US preexposure effects on conditioned taste aversion (CTA), only Wagner's information processing model adequately explained the reduction of the blocking effect of the proximal-US preexposure produced as a result of increasing remote-US preexposures.     

When pentobarbital is the conditioned stimulus and amphetamine is the unconditioned stimulus, conditioning depends on the type of conditioned response

Injections of drugs into rats were used as conditioned stimuli (CSs) and as unconditioned stimuli (USs). With heart rate (HR) conditioning, the pentobarbital CS produces a higher HR than under control conditions. With avfail (aversion failure) conditioning, the pentobarbital CS loses much of its capacity to induce a conditioned taste aversion. HR conditioning was obtained with forward delays of up to 30 min and backward delays of up to 270 min, where the delays are defined by the interinjection interval. Avfail was obtained with forward delays of up to 270 min but not with backward delays. Neither HR conditioning nor avfail were context specific but could be demonstrated in a test apparatus after pairings that occurred in the home cage. This indicated that the external environment was not an important part of the effective stimulus complex. When HR conditioning was obtained, its latency and duration was not related to the delay between the CS and US injections or whether they were forward or backward.     

Differential involvement of dopamine D1 receptors in morphine- and lithium-conditioned saccharin avoidance

Conditioned saccharin avoidance (CSA) can be produced when either lithium chloride (LiCl) or a reinforcing drug, such as morphine, is administered following exposure to the taste of saccharin. In this study we investigated the involvement of dopamine (DA) transmission in the acquisition of morphine and LiCl-CSA. CSA was evaluated in a two-bottle choice paradigm with two conditioning pairings between saccharin and morphine or LiCl as unconditioned stimulus (US). Morphine hydrochloride (7.5 mg/kg s.c.) or LiCl (40 mg/kg i.p.), administered 45 and 120′ respectively after saccharin-drinking session, induced strong CSA. The DA D₁ receptor antagonist, SCH 39166 (0.1 mg/kg s.c.), impaired morphine-CSA if administered 15′ and, to a lesser extent, 30′ but not 45′ before the drug (i.e immediately after saccharin drinking). In contrast SCH 39166 reduced LiCl-CSA when administered 45′ before the drug and even more so when administered 105′ before LiCl i.e. immediately after saccharin drinking. Therefore SCH 39166 impaired morphine-CSA when given shortly before the drug, while it impaired LiCl-CSA when given shortly after saccharin. Raclopride, a specific antagonist of D₂ receptors, failed to affect LiCl- and morphine-CSA. These results are consistent with the idea that DA, acting on D₁ receptors, plays a differential role in morphine- and LiCl-CSA. In LiCl-CSA DA is necessary for the processing (consolidation) of the short-term memory trace of the saccharin taste to be associated with the lithium-induced aversive state, while in morphine CSA contributes to mediate the appetitive properties of the drug.