Differential involvement of gustatory insular cortex and amygdala in the acquisition and retrieval of conditioned taste aversion in rats.

Lesion studies of the role of the gustatory insular cortex (GC) and amygdala (Am) in conditioned taste aversion (CTA) are confounded by the irreversibility of the intervention. Functional ablation methods allow more specific influencing of different phases of CTA acquisition and retrieval. Bilateral tetrodotoxin (TTX) blockade of GC (10 ng) or Am (3 ng) before or after saccharin drinking in rats with chronically implanted intracerebral cannulae showed that GC is indispensable for the initial processing of the taste stimulus but not for the association of the gustatory trace with the symptoms of LiCl poisoning. Gustatory signals can by-pass the blocked Am, the inactivation of which, however, impairs the gustatory trace-poisoning association. TTX injection into both GC and Am impairs CTA retrieval more than isolated blockade of either of these structures. It is argued that GC and Am implement processing of gustatory and visceral signals, respectively, but that formation and consolidation of the CTA engram proceeds outside forebrain, probably at the level of the brainstem.     

Chelerythrine, a specific PKC inhibitor, blocks acquisition but not consolidation and retrieval of conditioned taste aversion in rat

Association of the short-term memory of the gustatory conditioned stimulus (CS) with visceral malaise (unconditioned stimulus, US) in conditioned taste aversion (CTA) paradigm takes place in the parabrachial nuclei (PBN) of brainstem. In order to ascertain the role of protein-kinase C (PKC) during different phases of CTA acquisition and retrieval, four experimental series were carried out. In Experiment 1, 1 μl of 10 mM of PKC inhibitor chelerythrine prevented CTA acquisition when applied into PBN in the CS-US interval. In Experiment 2, the necessity of PKC activity in different phases of CTA acquisition was tested by prolonging the time interval between PBN administration of chelerythrine and i.p. LiCl. CTA acquisition was prevented when chelerythrine-induced blockade of PKC coincided with GSTM persistence but not with CTA consolidation. In Experiment 3, the interval between saccharin drinking and LiCl injection was prolonged to 120 min. Again, chelerythrine blockade of PKC activity prevented CTA formation when it interfered with GSTM persistence. In Experiment 4, the possibility that PKC activity is necessary also for CTA retrieval was tested by chelerythrine application into PBN 5 min before retrieval testing. In this case, the chelerythrine-induced PKC blockade did not impair CTA retrieval. It is concluded that PKC is important for GSTM formation and persistence but not for CTA consolidation or retrieval.     

Differential participation of temporal structures in the consolidation and reconsolidation of taste aversion extinction

The extinction process has been described as the decline in the frequency or intensity of the conditioned response following the withdrawal of reinforcement. Hence, experimental extinction does not reflect loss of the original memory, but rather reflects new learning, which in turn requires consolidation in order to be maintained in the long term. During extinction of conditioned taste aversion (CTA), a taste previously associated with aversive consequences acquires a safe status through continuous presentations of the flavor with no aversive consequence. In addition, reconsolidation has been defined as the labile state of a consolidated memory after its reactivation by the presentation of relevant information. In this study, we analyzed structures from the temporal lobe that could be involved in consolidation and reconsolidation of extinction of CTA by means of new protein synthesis. Our results showed that protein synthesis in the hippocampus (HC), the perirhinal cortex (PR) and the insular cortex (IC) of rats participate in extinction consolidation, whereas the basolateral amygdala plays no part in this phenomenon. Furthermore, we found that inhibition of protein synthesis in the IC in a third extinction trial had an effect on reconsolidation of extinction. The participation of the HC in taste memory has been described as a downmodulator for CTA consolidation, and has been related to a context–taste association. Altogether, these data suggest that extinction of aversive taste memories are subserved by the IC, HC and PR, and that extinction can undergo reconsolidation, a process depending only on the IC.     

The parabrachial nucleus and conditioned taste aversion

The parabrachial nucleus (PBN) surrounds the brachium conjunctivum in the dorsolateral pons. Although composed of numerous subnuclei, the PBN is typically organized into medial and lateral subdivisions according to their location relative to the brachium. In rodents, the medial PBN is part of the central gustatory system, whereas the lateral PBN is a component of the visceral sensory system. Lesions of the PBN disrupt conditioned taste aversion, a critically important learning mechanism that prevents the repeated ingestion of toxic food. Relevant neurobehavioral literature is reviewed to elucidate the role of the PBN in taste aversion learning.     

Functional inactivation of orexin 1 receptors in CA1 region impairs acquisition, consolidation and retrieval in Morris water maze task

Orexin containing neurons in the lateral hypothalamic area (LHA) produce orexin-A (hypocretin-1) and orexin-B (hypocretin-2) and send their axons to the hippocampus, which predominantly expresses orexin 1 receptors (OX1Rs) showing a higher affinity to orexin-A. Recent studies have shown that central administration of orexin-A has an effect on learning and memory but literature concerning the role of orexinergic system in cognition remains controversial. Therefore, we examined the effect of pre-training, post-training and pre-probe trial intrahippocampal CA1 administration of a selective OX1R the orexin 1 receptor antagonist SB-334867-A (1.5, 3, 6 microg/0.5 microl) on acquisition, consolidation and retrieval in a single-day testing version of Morris water maze (MWM) task. Our results show that, SB-334867-A impaired acquisition, consolidation and retrieval of MWM task as compared with the control group. This drug had no effect on escape latency of a non-spatial visual discrimination task. Therefore, it seems that endogenous orexins, especially orexin-A, play an important role in spatial learning and memory in the rat.     

Mechanisms of radiation-induced conditioned taste aversion learning

The literature on taste aversion learning is reviewed and discussed, with particular emphasis on those studies that have used exposure to ionizing radiation as an unconditioned stimulus to produce a conditioned taste aversion. The primary aim of the review is to attempt to define the mechanisms that lead to the initiation of the taste aversion response following exposure to ionizing radiation. Studies using drug treatments to produce a taste aversion have been included to the extent that they are relevant to understanding the mechanisms by which exposure to ionizing radiation can affect the behavior of the organism.     

Attenuation of neophobia and conditioned taste aversion in the rabbit

The phenomenon of taste neophobia to apple juice does exist in the rabbit. It was found that malic acid was the main stimulus source. Attenuation of taste neophobia was due to neuronal processes which took place between rather than during drinking sessions. Conditioned taste aversion learning took place if apomorphine was injected after apple juice intake.     

Impaired acquisition of the Morris water maze following global ischemic damage in the gerbil.

Five minutes of global ischemia in the Mongolian gerbil impaired acquisition of a Morris water maze task when testing began 72 h after surgery. In spite of extensive damage to CA1 pyramidal cells, ischemic animals eventually learned to locate a submerged platform and performed normally on a subsequent retention test. Animals that were allowed a more protracted recovery period (21 days) acquired the task as readily as control gerbils. These results suggest that undamaged structures within and external to the hippocampal formation allow spatial learning to proceed at a somewhat reduced rate.     

Insular cortex and amygdala lesions differentially affect acquisition on inhibitory avoidance and conditioned taste aversion

These experiments examined the effects of NMDA-induced lesions of the amygdala and insular (gustatory) cortex (IC) on inhibitory avoidance learning and conditioned taste aversion (CTA) in rats. IC lesions, but not amygdala lesions, disrupted CTA. In contrast, lesions of either brain region disrupted inhibitory avoidance learning. These findings support the view that the IC is strongly involved in the acquisition of external as well as visceral aversively motivated behavior. Despite extensive functional interconnections, these 2 brain regions appear to have different roles in mediating different forms of aversively based learning.     

The effects of trimethyltin chloride on the acquisition of long delay conditioned taste aversion learning in the rat

Naive rats injected with LiCl at various times following consumption of a novel saccharin solution subsequently avoided the ingestion of saccharin with the degree of the aversion related to the interval between ingestion and LiCl administration. Although a similar relationship was also evident in animals which had received a single intragastric administration of trimethyltin chloride 21 days prior to the pairing of saccharin and LiCl, the trimethyltin-pretreated subjects receiving delayed injections of LiCl displayed weaker taste aversions than those not exposed to trimethyltin. This disruption in the acquisition of taste aversions over long delays is consistent with other work suggesting that trimethyltin disrupts tasks involving short-term memory. The utility of the conditioned taste aversion paradigm in detecting and characterizing drug toxicity was discussed.     

Allopregnanolone inhibits learning in the Morris water maze

The progesterone metabolite allopregnanolone (3alpha-OH-5alpha-pregnane-20-one) inhibits neural functions, enhancing the GABA induced GABA(A) receptor activation. This effect is benzodiazepine like and benzodiazepines are known to impair memory. Acute effects of allopregnanolone on the hippocampus dependent spatial learning in the Morris water maze have not been studied. Adult male Wistar rats where injected (i.v.) with allopregnanolone (2 mg/kg), or vehicle, daily for 11 days. At 8 or 20 min after each injection, studies of place navigation were performed in the Morris water maze. Allopregnanolone concentrations in plasma and in nine different brain areas where analyzed by radioimmunoassay. The latency to find the platform was increased 8 min after the allopregnanolone injection, while normal learning was seen after 20 min. Swim speed did not differ between groups. A higher number of rats were swimming close to the pool wall (thigmotaxis) in the 8 min allopregnanolone group compared to the other groups. Allopregnanolone concentrations in the brain tissue at 8 min were 1.5 to 2.5 times higher then at 20 min after the allopregnanolone injections. After vehicle injections the brain concentrations of allopregnanolone were at control levels. Plasma concentrations of allopregnanolone followed the same pattern as in the brain, with the exception of an increase 8 min after vehicle injections. The natural progesterone metabolite allopregnanolone can inhibit learning in the Morris water maze, an effect not caused by motor impairment. The learning impairment might be due to a combination of changed swimming behavior and difficulties in navigation.     

Anaphylactic shock-induced conditioned taste aversion. II. Correlation between taste aversion and indicators of anaphylactic shock

Previous studies (V. J. Djuri?, B. M. Markovi?, M. Lazarevi?, & B. D. Jankovi?, 1987, in B. D. Jankovi?, B. M. Markovi?, & N. H. Spector (Eds.), Neuroimmune interactions, pp. 561-568, New York: New York Acad. Sci.; B. M. Markovi?, V. J. Djuri?, M. Lazarevi?, & B. D. Jankovi?, 1988, Brain Behav. Immun. 2, 11-23) have shown that rats learn to associate the taste of saccharin with the induction of anaphylactic shock, thus exhibiting conditioned taste aversion (CTA) toward an otherwise preferred saccharin solution. The present experiment investigates the effect of unconditioned stimulus intensity (the amount of antigen used for the induction of shock) on CTA. Rats were sensitized to ovalbumin and subjected to a conditioning trial in which the conditioned stimulus (CS; saccharin solution given orally) signaled the presentation of the unconditioned stimulus (US; shocking doses of ovalbumin ranging from 0.5 to 3 mg given intraperitoneally). Behavioral signs, hematocrit, and rectal temperature were used for evaluation of anaphylactic shock. Twenty-four hours after the conditioning trial, rats were subjected to a two-bottle preference test between saccharin solution and water. Multiple regression statistical analysis revealed significant correlations among saccharin preference ratio, dose of antigen used for the induction of shock, behavioral signs of shock, rise in hematocrit, and fall in rectal temperature. A dose-dependent relation among saccharin preference ratio and physiological indicators of shock suggests that conditioned anaphylactic shock-induced avoidance behavior is functionally related to homeostatic factors involved in immune reactivity.     

Glutamate in the parabrachial nucleus of rats during conditioned taste aversion

Brain microdialysis combined with HPLC and spectroscopic detection was used to monitor extracellular glutamate in the parabrachial nucleus (PBN) of rats during acquisition of a conditioned taste aversion (CTA). Microdialysis fractions taken every 20 min were used to assess the effects of presentation of the conditioned stimulus alone (CS, consumption of 0.1% saccharin), the unconditioned stimulus alone (US, intraperitoneal injection of 0.15 M LiCl, 2% b.w. induced malaise after water drinking) as well as that of CS-US pairing. After 15 min of saccharin drinking, the glutamate concentration in the eluate (20 microl/20 min) reached 80% above the baseline but returned to the basal value in the next fraction. LiCl alone (applied 1 h after 15 min drinking of water) increased glutamate only following some delay, i.e. in the second and third post-lithium fraction by 90 and 67%, respectively. However, when LiCl was injected 1 h after the onset of saccharin intake, the glutamate concentration rose significantly (by 95%) already in the first post-LiCl fraction and by 120% in the second one. It appears, therefore, that the 'saccharin trace' facilitates the effect of lithium on extracellular concentration of glutamate in PBN during acquisition of CTA.     

Differential deficits in the Morris water maze following cytotoxic lesions of the anterior thalamus and fornix transection

Rats with complete fornix lesions or cytotoxic lesions placed in the anterior thalamic region were trained on an allocentric spatial memory test (the Morris water maze). While both lesions led to impairments in locating the hidden platform in this test of reference memory, the thalamic lesions led to a significantly greater deficit than that observed after fornix transection as measured by a number of performance indices. The lesions also led to different patterns of swim behaviour in the pool. The severity of the thalamic lesion deficit was associated with anterior thalamic nuclei damage but not with damage to the nucleus medialis dorsalis. Both the fornix and the thalamic lesions also severely impaired T-maze alternation. In contrast, neither set of lesions appeared to affect the recognition of small or large objects. While the study provides further evidence of a close functional relationship between the hippocampus and the anterior thalamic nuclei, it also shows that disconnection of the fornical inputs to the anterior thalamic nuclei does not provide a full explanation of the thalamic deficit.     

A test of conditioned taste aversion with mouse interferon-alpha

Interferon-alpha treatment induces anorexia in cancer patients and in mice. The ability of interferon-alpha to induce a conditioned taste aversion (CTA) in mice was explored. Mice were injected with doses of interferon-alpha that had been shown to induce anorexia (M. A. Segall & L. S. Crnic, Behav. Neurosci., 1990; 1600 or 800 U/g of mouse) after being given a novel solution (chocolate milk). Neither dose of interferon resulted in a decrease of milk intake on the test day. To determine whether the known cognitive effects of interferon interfere with the ability of a mouse to learn CTA, a combined injection of interferon-alpha and LiCl (an illness-inducing agent) was given. The combination of interferon-alpha with LiCl did not interfere with the LiCl-induced CTA. Thus, any possible cognitive effects of interferon did not interfere with conditioned taste aversion. To test the possibility that repeated trials might establish a conditioned taste aversion to interferon, mice were given three conditioning trials spaced 3 days apart. No aversion developed. Doses of interferon-alpha that produce anorexia do not produce a conditioned taste aversion, indicating that separate mechanisms are activated in these two behaviors.     

Memantine affects cognitive flexibility in the Morris water maze

Alzheimer's disease (AD) is multi-factorial mental disorder characterized by a copious array of congruent features cumulating in disrupted memory and dysthymia. Though the mechanism remains elusive, the highly unspecific pharmaceutical, memantine, provides modest benefits for patients with moderate-to-severe AD. A greater understanding of how memantine affects cognitive function promises to facilitate the search for better therapeutics. We therefore examined cognitive flexibility of mice following 5 and 10 mg/kg memantine administration using a platform re-location water maze. Strikingly, subjects receiving memantine demonstrated memory impairment relative to controls when re-trained off drug, revealing a novel and unusual disruption of cognitive flexibility.     

Long-term potentiation in the insular cortex enhances conditioned taste aversion retention

Long-lasting changes in synaptic strength, such as long-term potentiation (LTP), are thought to underlie memory formation. Recent studies on the insular cortex (IC), a region of the temporal cortex implicated in the acquisition and retention of conditioned taste aversion (CTA), have demonstrated that tetanic stimulation of the basolateral nucleus of the amygdala (Bla) induce LTP in the IC of adult rats in vivo, as well as, that blockade of N-methyl-D-aspartate (NMDA) receptors disrupts CTA and IC-LTP induction in vivo. Here, we present experimental data showing that induction of LTP in the Bla-IC projection previous to CTA training enhances the retention of this task. These findings are of particular interest since they provide support for the view that the neural mechanisms underlying neocortical LTP may contribute to memory related functions performed by the IC.     

Role of glutamate in the amygdala and lateral hypothalamus in conditioned taste aversion

The role of glutamate in conditioned taste aversion was investigated. Both, in the amygdala (AMYG) and in the lateral hypothalamus (LH) extracellular levels of glutamate were assessed by microdialysis and capillary electrophoresis with laser induced fluorescence detection. Rats were conditioned by pairing a novel flavor (strawberry flavor) with an intraperitoneal injection of lithium chloride. When the conditioned stimulus (strawberry flavored solution) was injected into the mouth of conditioned rats, there was an increase of glutamate release in the AMYG, and a decrease in glutamate release in the LH. These results predicted that glutamate release in the AMYG and the LH was involved in CTA. This possibility was tested by MK-801 (glutamate antagonist) and glutamate microinjections. MK-801 injections in AMYG attenuated the rejection of the novel flavor, and in the LH did not cause any effect on CTA. Glutamate microinjections in the AMYG caused CTA. These results suggest that glutamatergic activity in the AMYG might be a relevant neurochemical correlate and cause of conditioned taste aversion.     

Effects of midazolam on the expression of conditioned taste aversion in rats

In conditioned taste aversion (CTA), the animals learn to avoid a taste substance (conditioned stimulus, CS) which was previously associated with visceral distress (unconditioned stimulus, US). The present study examined the effects of administration of midazolam (MDZ), a benzodiazepine agonist, after the acquisition of CTA on the expression of CTA. After ingestion of 0.5 M sucrose (CS) was paired with an intraperitoneal (i.p.) injection of 0.15 M LiCl (US), control rats showed strong CTA to the CS. However, a systemic injection of MDZ (1.5 mg/kg, i.p.) before the retention test prevented conditioned animals from rejecting the CS, but in the subsequent retention tests where the drug was not administrated, those animals again showed strong aversions to the CS. Aversive taste reactivity patterns to the intraorally infused sucrose and 0.3 M dl-alanine in the conditioned animals were also diminished by the similar injection of MDZ, but not by a serotonergic anxiolytic agent, buspirone (2.5 or 5.0 mg/kg, i.p.). General taste sensory deficit might not be induced by MDZ because the drug injection did not impair conditioned aversions to 0.2 M NaCl and 0.01 M HCl. Infusion of MDZ into the basolateral nucleus of the amygdala (BLA) also attenuated conditioned aversions to sucrose. These results suggest that systemic or intra-BLA administrations of MDZ impair the expression of CTA selectively to sweet-tasting substances, implying that a transient MDZ-induced CTA expression deficit is due to the enhancement of palatability of CSs with preferable tastes rather than general anxiolytic or amnesic effects of MDZ.     

Automatic recognition of explorative strategies in the Morris water maze

Notwithstanding the development of reliable tracking systems, the quantification methodology of the Morris water maze (MWM) has witnessed an operational mismatch between the indexes used to quantify MWM performance and the cognitive concepts derived from these indexes. Indeed, escape latency is the main, and often unique, performance measure used for the quantification of behavior. Aim of the present work was to overcome this limitation by presenting a methodology that allows for automatic categorical pattern recognition of the behavioral strategies performed in the MWM. By selecting few a priori and user-defined behavioral categories, many quantitative variables and regions of interest (ROIs), we used discriminant analysis (DA) to obtain 97.9% of correct automatic recognition of categories. The developed discriminant model (DM) also allowed to predict category membership of newly recorded swim paths with the same statistical efficacy (96%), and to identify the variables that better discriminate between adjacent categories. The combination of DA with a tracking system, a selection of many variables, different ROIs and qualitative categorization, reduces the gap between the measurement process and the categories used to describe a given behavior, and offers a methodology to computationally reproduce the human categorization of behaviors in the MWM.