Differential effects of 192IgG-saporin and NMDA-induced lesions into the basal forebrain on cholinergic activity and taste aversion memory formation

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

Opposing Roles of Cholinergic and GABAergic Activity in the Insular Cortex and Nucleus Basalis Magnocellularis during Novel Recognition and Familiar Taste Memory Retrieval

Acetylcholine (ACh) is thought to facilitate cortical plasticity during memory formation and its release is regulated by the nucleus basalis magnocellularis (NBM). Questions remain regarding which neuronal circuits and neurotransmitters trigger activation or suppression of cortical cholinergic activity. During novel, but not familiar, taste consumption, there is a significant increase in ACh release in the insular cortex (IC), a highly relevant structure for taste learning. Here, we evaluate how GABA inhibition modulates cholinergic transmission and its involvement during taste novelty processing and familiar taste memory retrieval. Using saccharin as a taste stimulus in a taste preference paradigm, we examined the effects of injecting the GABAA receptor agonist muscimol or the GABAA receptor antagonist bicuculline into the IC or NBM during learning or retrieval of an appetitive taste memory on taste preference in male Sprague Dawley rats. GABAA receptor agonism and antagonism had opposite effects on cortical ACh levels in novel taste presentation versus familiar taste recognition and ACh levels were associated with the propensity to acquire or retrieve a taste memory. These results indicate that the pattern of cortical cholinergic and GABAergic neuroactivity during novel taste exposure is the opposite of that which occurs during familiar taste recognition and these differing neurotransmitter system states may enable different behavioral consequences. Divergences in ACh and GABA levels may produce differential alterations in excitatory and inhibitory neural processes within the cortex during acquisition and retrieval.

SIGNIFICANCE STATEMENT

During learning and recall, several brain structures act together. This work demonstrates interactions between cortical cholinergic and GABAergic systems during taste learning and memory retrieval. We found that the neuroactivity pattern during novel taste exposure is opposite that which occurs during familiar taste recognition. GABAA receptors must be inactive during novel tasting to enable new memory formation, but must be active and inhibiting acetylcholine release in the cortex to allow memory retrieval. These findings indicate that GABA inhibition modulates cholinergic transmission and that cholinergic-GABAergic system interactions are important during the transition from novel to familiar memory.

Effect of acupuncture-like stimulation on cortical cerebral blood flow in aged rats

This study aimed to examine the effect of acupuncture-like stimulation on cortical cerebral blood flow (CBF) in aged rats and the contribution of the intracranial cholinergic vasodilatory system on its response. In urethane-anesthetized rats of 30-37 months of age, manual acupuncture-like stimulation of a forepaw produced an increase in the CBF, independent of systemic arterial pressure. The increase in the CBF induced by forepaw stimulation was abolished by intravenous administration of cholinergic receptor antagonists. Manual acupuncture-like stimulation of a forepaw increased extracellular acetylcholine release in the cerebral cortex. These results suggest that natural somatic afferent stimulation, such as acupuncture-like stimulation, activates the intracranial - most likely, basal forebrain - cholinergic vasodilatory system in the cerebral cortex, even in extremely aged rats.

Protein degradation by ubiquitin-proteasome system in formation and labilization of contextual conditioning memory

The ubiquitin–proteasome system (UPS) of protein degradation has been evaluated in different forms of neural plasticity and memory. The role of UPS in such processes is controversial. Several results support the idea that the activation of this system in memory consolidation is necessary to overcome negative constrains for plasticity. In this case, the inhibition of the UPS during consolidation impairs memory. Similar results were reported for memory reconsolidation. However, in other cases, the inhibition of UPS had no effect on memory consolidation and reconsolidation but impedes the amnesic action of protein synthesis inhibition after retrieval. The last finding suggests a specific action of the UPS inhibitor on memory labilization. However, another interpretation is possible in terms of the synthesis/degradation balance of positive and negative elements in neural plasticity, as was found in the case of long-term potentiation. To evaluate these alternative interpretations, other reconsolidation-interfering drugs than translation inhibitors should be tested. Here we analyzed initially the UPS inhibitor effect in contextual conditioning in crabs. We found that UPS inhibition during consolidation impaired long-term memory. In contrast, UPS inhibition did not affect memory reconsolidation after contextual retrieval but, in fact, impeded memory labilization, blocking the action of drugs that does not affect directly the protein synthesis. To extend these finding to vertebrates, we performed similar experiments in contextual fear memory in mice. We found that the UPS inhibitor in hippocampus affected memory consolidation and blocked memory labilization after retrieval. These findings exclude alternative interpretations to the requirement of UPS in memory labilization and give evidence of this mechanism in both vertebrates and invertebrates.

Genetically induced cholinergic hyper-innervation enhances taste learning

Acute inhibition of acetylcholine (ACh) has been shown to impair many forms of simple learning, and notably conditioned taste aversion (CTA). The most adhered-to theory that has emerged as a result of this work – that ACh increases a taste’s perceived novelty, and thereby its associability – would be further strengthened by evidence showing that enhanced cholinergic function improves learning above normal levels. Experimental testing of this corollary hypothesis has been limited, however, by side-effects of pharmacological ACh agonism and by the absence of a model that achieves long-term increases in cholinergic signaling. Here, we present this further test of the ACh hypothesis, making use of mice lacking the p75 pan-neurotrophin receptor gene, which show a resultant over-abundance of cholinergic neurons in sub-regions of the basal forebrain (BF). We first demonstrate that the p75−/− abnormality directly affects portions of the CTA circuit, locating mouse gustatory cortex (GC) using a functional assay and then using immunohistochemisty to demonstrate cholinergic hyper-innervation of GC in the mutant mice – hyper-innervation that is unaccompanied by changes in cell numbers or compensatory changes in muscarinic receptor densities. We then demonstrate that both p75−/− and wild-type (WT) mice learn robust CTAs, which extinguish more slowly in the mutants. Further testing to distinguish effects on learning from alterations in memory retention demonstrate that p75−/− mice do in fact learn stronger CTAs than WT mice. These data provide novel evidence for the hypothesis linking ACh and taste learning.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

A cholinergic-dependent role for the entorhinal cortex in trace fear conditioning

Trace conditioning is considered a model of higher cognitive involvement in simple associative tasks. Studies of trace conditioning have shown that cortical areas and the hippocampal formation are required to associate events that occur at different times. However, the mechanisms that bridge the trace interval during the acquisition of trace conditioning remain unknown. In four experiments with fear conditioning in rats, we explored the involvement of the entorhinal cortex (EC) in the acquisition of fear under a trace-30 s protocol. We first determined that pretraining neurotoxic lesions of the EC selectively impaired trace-, but not delay-conditioned fear as evaluated by freezing behavior. A local cholinergic deafferentation of the EC using 192-IgG-saporin did not replicate this deficit, presumably because cholinergic interneurons were spared by the toxin. However, pretraining local blockade of EC muscarinic receptors with the M1 antagonist pirenzepine yielded a specific and dose-dependent deficit in trace-conditioned responses. The same microinjections performed after conditioning were without effect on trace fear responses. These effects of blocking M1 receptors are consistent with the notion that conditioned stimulus (CS)-elicited, acetylcholine-dependent persistent activities in the EC are needed to maintain a representation of a tone CS across the trace interval during the acquisition of trace conditioning. This function of the EC is consistent with recent views of this region as a short-term stimulus buffer.

Brain cholinergic vulnerability: Relevance to behavior and disease

The major populations of cholinergic neurons in the brain include two "projection" systems, located in the pontine reticular formation and in the basal forebrain. These two complexes comprise, in part, the anatomical substrates for the "ascending reticular activating system" (ARAS). The pontine cholinergic system relays its rostral influences mainly through thalamic intralaminar nuclei, but it also connects to the basal forebrain and provides a minor innervation of cortex. The basal forebrain cholinergic complex (BFCC) projects directly to cortex and hippocampus, and has a minor connection with the thalamus. Recent data reveal that a parallel system of basal forebrain GABAergic projection neurons innervates cortex/hippocampus in a way that seems to complement the BFCC. Generally, the picture developed from more than 50 years of research is consistent with a "global" influence of these two ascending cholinergic projections on cortical and hippocampal regions. Seemingly, the BFCC acts in tandem or in parallel with the pontine cholinergic projection to activate the electro-encephalogram, increase cerebral blood flow, regulate sleep-wake cycling, and modulate cognitive function. There are quite a number and variety of human brain conditions, notably including Alzheimer's disease, in which degeneration of basal forebrain cholinergic neurons has been documented. Whether the corticopetal GABA system is affected by disease has not been established. Studies of degeneration of the pontine projection are limited, but the available data suggest that it is relatively preserved in Alzheimer's disease. Hypotheses of BFCC degeneration include growth factor deprivation, intracellular calcium dysfunction, amyloid excess, inflammation, and mitochondrial abnormalities/oxidative stress. But, despite considerable research conducted over several decades, the exact mechanisms underlying brain cholinergic vulnerability in human disease remain unclear.

Modulation of memory and visuospatial processes by biperiden and rivastigmine in elderly healthy subjects

RATIONALE

The central cholinergic system is implicated in cognitive functioning. The dysfunction of this system is expressed in many diseases like Alzheimer's disease, dementia of Lewy body, Parkinson's disease and vascular dementia. In recent animal studies, it was found that selective cholinergic modulation affects visuospatial processes even more than memory function.

OBJECTIVE

In the current study, we tried to replicate those findings. In order to investigate the acute effects of cholinergic drugs on memory and visuospatial functions, a selective anticholinergic drug, biperiden, was compared to a selective acetylcholinesterase-inhibiting drug, rivastigmine, in healthy elderly subjects.

METHODS

A double-blind, placebo-controlled, randomised, cross-over study was performed in 16 healthy, elderly volunteers (eight men, eight women; mean age 66.1, SD 4.46 years). All subjects received biperiden (2 mg), rivastigmine (3 mg) and placebo with an interval of 7 days between them. Testing took place 1 h after drug intake (which was around Tmax for both drugs). Subjects were presented with tests for episodic memory (wordlist and picture memory), working memory tasks (N-back, symbol recall) and motor learning (maze task, pursuit rotor). Visuospatial abilities were assessed by tests with high visual scanning components (tangled lines and Symbol Digit Substitution Test).

RESULTS

Episodic memory was impaired by biperiden. Rivastigmine impaired recognition parts of the episodic memory performance. Working memory was non-significantly impaired by biperiden and not affected by rivastigmine. Motor learning as well as visuospatial processes were impaired by biperiden and improved by rivastigmine.

CONCLUSIONS

These results implicate acetylcholine as a modulator not only of memory but also of visuospatial abilities.

Attentional functions of cortical cholinergic inputs: what does it mean for learning and memory?

The hypothesis that cortical cholinergic inputs mediate attentional functions and capacities has been extensively substantiated by experiments assessing the attentional effects of specific cholinotoxic lesions of cortical cholinergic inputs, attentional performance-associated cortical acetylcholine release, and the effects of pharmacological manipulations of the excitability of basal forebrain corticopetal cholinergic projections on attentional performance. At the same time, numerous animal experiments have suggested that the integrity of cortical cholinergic inputs is not necessary for learning and memory, and a dissociation between the role of the cortical cholinergic input system in attentional functions and in learning and memory has been proposed. We speculate that this dissociation is due, at least in part, to the use of standard animal behavioral tests for the assessment of learning and memory which do not sufficiently tax defined attentional functions. Attentional processes and the allocation of attentional capacities would be expected to influence the efficacy of the acquisition and recall of declarative information and therefore, persistent abnormalities in the regulation of the cortical cholinergic input system may yield escalating impairments in learning and memory. Furthermore, the cognitive effects of loss of cortical cholinergic inputs are augmented by the disruption of the top-down regulation of attentional functions that normally acts to optimize information processing in posterior cortical areas. Because cortical cholinergic inputs play an integral role in the mediation of attentional processing, the activity of cortical cholinergic inputs is hypothesized to also determine the efficacy of learning and memory.

Diencephalic damage decreases hippocampal acetylcholine release during spontaneous alternation testing

A rodent model of diencephalic amnesia, pyrithiamine-induced thiamine deficiency (PTD), was used to investigate diencephalic-hippocampal interactions. Acetylcholine (ACh) release, a marker of memory-related activation, was measured in the hippocampus of PTD-treated and control rats prior to, during, and after spontaneous alternation test. During behavioral testing, all animals displayed increases in ACh release. However, both the percent increase of ACh release during spontaneous alternation testing and the alternation scores were higher in control rats relative to PTD-treated rats. Thus, when rats are tested on a task with demands dependent on the hippocampus, it appears that the hippocampus is not fully activated after diencephalic damage.

Lesions of the Basal forebrain cholinergic system impair task acquisition and abolish cortical plasticity associated with motor skill learning

The contribution of the basal forebrain cholinergic system in mediating plasticity of cortical sensorimotor representations was examined in the context of normal learning. The effects of specific basal forebrain cholinergic lesions upon cortical reorganization associated with learning a skilled motor task were investigated, addressing, for the first time, the functional consequences of blocking cortical map plasticity. Results demonstrate that disrupting basal forebrain cholinergic function disrupts cortical map reorganization and impairs motor learning. Cholinergic lesions do not impair associative fear learning or overall sensorimotor function. These results support the hypothesis that the basal forebrain cholinergic system may be specifically implicated in forms of learning requiring plasticity of cortical representations.

Switching memory systems during learning: changes in patterns of brain acetylcholine release in the hippocampus and striatum in rats

This experiment measured acetylcholine (ACh) release simultaneously in the hippocampus and striatum while rats were trained in a cross maze. Consistent with past findings, rats initially showed learning on the basis of place (i.e., turning to the correct position relative to the room), but after extensive training, rats shifted to learning on the basis of response (i.e., turning to the right/left to find the food). Profiles of ACh release in the hippocampus and striatum were markedly different during training. In the hippocampus, ACh release increased by approximately 60% at the onset of training and remained at that level of release throughout training, even after the rats began to show learning on the basis of turning rather than place. In the striatum, increases in ACh release occurred later, reaching asymptotic increases of 30-40%, coincident with a transition from expressing place learning to expressing response learning. These findings suggest that the hippocampal and striatal systems both participate in learning in this task, but in a manner characterized by differential activation of the neural systems. The hippocampal system is apparently engaged first before the striatum is activated and, to the extent the hippocampus is important for place learning, promotes the use of a place solution to the maze. Later in training, although the hippocampus remains activated, the striatum is also activated in a manner that may enable the use of a response strategy to solve the maze. These findings may offer a neurobiological marker of a transition during skill learning from declarative to procedural learning.

Blockade of cortical muscarinic but not NMDA receptors prevents a novel taste from becoming familiar

Exposure to a novel taste solution in the rat is followed by a decrease in its intake known as neophobia. This effect gradually disappears, and consumption increases from the second presentation of the taste (attenuation of neophobia), reflecting that the animal learned that it is safe to drink it. Conversely, if gastric malaise is induced after first intake, the rat will develop a long-lasting aversion (conditioned taste aversion). Previous attempts to elucidate the physiological nature of taste memory trace stems only from procedures that require malaise to measure taste memory. Here we assess the relevance of both muscarinic and N-methyl-d-aspartate receptors, known to be involved in conditioned taste aversion, on taste memory using a nonaversive procedure (attenuation of neophobia learning). Attenuation of neophobia was impaired by the muscarinic receptor antagonist, scopolamine, microinjected 20 min before, immediately after or up to 2 h after the first taste experience, suggesting that muscarinic receptors are involved in the acquisition and consolidation of attenuation of neophobia learning. However, the N-methyl-d-aspartate receptor antagonist, d,l-2-amino-5-phosphonovaleric acid, did not affect attenuation of neophobia even when the same dose of the drug was able to disrupt conditioned taste aversion learning, which suggests that attenuation of neophobia learning would be independent of N-methyl-d-aspartate receptors activity in the insular cortex. The neophobic response induced by strong saccharin presentation was not affected by either of the treatments given, which rules out any impairment in taste perception. These results indicate that while cortical muscarinic receptors are important in the formation and consolidation of safe memory trace, N-methyl-d-aspartate receptor activity appears to be noncritical.

The role of cortical cholinergic pre- and post-synaptic receptors in taste memory formation

A number of studies have implicated cholinergic activity in the mediation of learning and memory processes. However, the specific role of muscarinic receptors in memory formation mechanisms is less known. The aim of the present study is to evaluate the effects of muscarinic antagonist M2 presynaptic receptor, AFDX-116 (0.5mM) and M1 and M3 post-synaptic receptor pirenzepine (100mM), as well as a non-selective muscarinic antagonist, scopolamine (136mM), in the insular cortex (IC) during acquisition and retrieval of conditioned taste aversion (CTA). In addition, we evaluate the effects of those antagonists in cortical ACh release by in vivo microdialysis and the effects on the induction of in vivo LTP in the BLA-IC projection. The results showed that the cortical microinjections of scopolamine and pirenzepine, but not AFDX-116, produced significant disruption in the acquisition of CTA, without effects during retrieval. Microinjections of scopolamine and AFDX-116 produced significant cortical ACh release, while infusions of pirenzepine did not produce any release. Application of scopolamine and pirenzepine diminished induction of LTP in the BLA-IC projection, but not AFDX-116, as compared with vehicle. The induction of BLA-CI LTP seems to be modulated by post-synaptic muscarinic acetylcholine receptors and not by pre-synaptic muscarinic receptors. These results suggest a differential involvement of cholinergic receptors during acquisition and retrieval of aversive memory formation, as well as a differential role of muscarinic receptors in the biochemical and electrophysiological processes that may underlay aversive memory.

CS-specific gamma, theta, and alpha EEG activity detected in stimulus generalization following induction of behavioral memory by stimulation of the nucleus basalis

Tone paired with stimulation of the nucleus basalis (NB) induces behavioral memory that is specific to the frequency of the conditioned stimulus (CS), assessed by cardiac and respiration behavior during post-training stimulus generalization testing. This paper focuses on CS-specific spectral and temporal features of conditioned EEG activation. Adult male Sprague-Dawley rats, chronically implanted with a stimulating electrode in the NB and a recording electrode in the ipsilateral auditory cortex, received either tone (6kHz, 70dB, 2s) paired with co-terminating stimulation of the nucleus basalis (0.2s, 100Hz, 80-105 microA, ITI approximately 45s) or unpaired presentation of the stimuli (approximately 200 trials/day for approximately 14 days). CS-specificity was tested 24h post-training by presenting test tones to obtain generalization gradients for the EEG, heart rate, and respiration. Behavioral memory was evident in cardiac and respiratory responses that were maximal to the CS frequency of 6kHz. FFT analyses of tone-elicited changes of power in the delta, theta, alpha, beta1, beta2, and gamma bands in the paired group revealed that conditioned EEG activation (shift from lower to higher frequencies) was differentially spectrally and temporally specific: theta, and alpha to a lesser extent, decreased selectively to 6kHz during and for several seconds following tone presentation while gamma power increased transiently during and after 6kHz. Delta exhibited no CS-specificity and the beta bands showed transient specificity only after several seconds. The unpaired group exhibited neither CS-specific behavioral nor EEG effects. Thus, stimulus generalization tests reveal that conditioned EEG activation is not unitary but rather reflects CS-specificity, with band-selective markers for specific, associative neural processes in learning and memory.

Differential involvement of cortical muscarinic and NMDA receptors in short- and long-term taste aversion memory

In conditioned taste aversion, an animal avoids a taste previously associated with toxic effects, and this aversive memory formation requires an intact insular cortex. In this paper, we investigated the possible differential involvement of cholinergic and glutamatergic receptors in the insular cortex in short-term memory (STM) and long-term memory (LTM) of taste aversion in rats. Taste aversion was induced by intraperitoneal administration of lithium chloride (a malaise-inducing drug) 15 min after experience with an unfamiliar taste. In order to test STM and LTM of taste aversion, taste stimulus was again presented 4 h and 72 h after lithium injection, respectively. During the acquisition, microinjection of the muscarinic antagonist, scopolamine, in the insular cortex before, but not after, the presentation of the new taste, abolished STM as well as LTM. Blockade of the NMDA receptor, in the insular cortex, by AP5 before, but not after, the presentation of the taste stimulus, impaired LTM but left STM intact. Moreover, when injected 1 h after malaise induction (i.e., during taste-illness association), AP5 disrupted both STM and LTM. These results suggest that activation of muscarinic receptors in the insular cortex is involved in the acquisition of taste memory, whereas NMDA receptors participate in taste memory consolidation. These data demonstrate that different neurochemical mechanisms subserve different memory phases. NMDA receptors are also probably involved in processing the visceral input, thus allowing subsequent taste-illness association. This indicates that in the same cortical area the same neurotransmitter system can be involved in distinct processes: taste memory consolidation vs. taste-illness association.

Differential effects of bicuculline and muscimol microinjections into the nucleus basalis magnocellularis in taste and place aversive memory formation

The role of the nucleus basalis magnocellularis (NBM) in learning and memory has been demonstrated in different learning paradigms such as conditioned taste aversion (CTA) and inhibitory avoidance (IA). This participation has been related to the cholinergic system, but recent studies have reported the potential role of other neurotransmitters such as GABA. The effects of acute intracerebral administration of the GABAergic antagonist bicuculline (0.05 microg) and the GABAergic agonist muscimol (0.05 microg) into the NBM of male Wistar rats were assessed in CTA and IA learning. In both learning tasks, the drug administration was performed before the acquisition. Taste aversion learning was not affected by the infusion of any of the drugs administered. IA acquisition was not affected by the administration of bicuculline or muscimol, requiring similar number of trials to reach the learning criterion. However, when the rats were tested 24 h later, those injected with bicuculline or muscimol showed an impairment of the IA learning. The present results support a role of the GABAergic system in the consolidation process of IA learning.

Glutamatergic activity in the amygdala signals visceral input during taste memory formation

Conditioned taste aversion (CTA) is a learning paradigm in which an animal avoids a taste (conditioned stimulus) previously associated with visceral toxic effects [or unconditioned stimulus (US)]. Although many studies have implicated glutamate-mediated neurotransmission in memory consolidation of different types of learning tasks, including CTA, the exact role of this neurotransmitter system in memory formation is not known. Thus, we set out to determine whether glutamate mediates signaling of the US in CTA. We present evidence obtained by in vivo microdialysis that the US (i.p. injection of lithium chloride) induced a dramatic increase in glutamate release in the amygdala and a modest but significant release in the insular cortex. Moreover, CTA can be elicited by intra-amygdalar microinjections of glutamate; consequently, when glutamate is administered just before the presentation of a weak US, a clear CTA is induced. In contrast, the injection of glutamate alone or glutamate 2 h after the suboptimal US did not have any effect on the acquisition of CTA. These results indicate that glutamate activation of the amygdala can partially substitute the US in CTA, thus providing a clear indication that the amygdala conveys visceral information for this kind of memory.

Effects of conditioned food aversions on nutritional behavior in humans

Conditioned food aversion (CFA) and taste aversion (CTA) are widely occurring phenomena mediating rejection of solids or liquids, the ingestion of which has induced the onset of post-ingestional malaise. It is a powerful and durable imprint learning that may influence food choice and intake in all animals, including humans. For ethical reasons, CTA has been extensively investigated in a wide variety of laboratory animal's species but only incidentally in humans. Nevertheless, convincing evidence has been provided that CFA and CTA learning are possible in a wide range of human subjects. The results in humans may have some limitations in accuracy since data are sparse, sometimes indirect, and poorly controlled. There is only limited information on the extent of CFA in the elderly since most studies have employed questionnaire and/or interview methods on young people (i.e. college students). The present review evaluates the literature derived both from laboratory animals and humans. In the first instance, the salient features of food and taste aversion learning and the neural mechanisms involved in this learning behavior will be examined. Then, the problems encountered when trying to assess the role of learned food and taste aversions in the nutritional status of healthy as well as sick young or elderly people will be considered. In particular, the importance of CFA on the nutritional status of cancer patients and treatment of alcoholism will be examined. It is concluded that the data are compelling enough to warrant further research and, some indications and recommendations are suggested.

Selective immunolesions of CH4 cholinergic neurons do not disrupt spatial memory in rats

Adult male Long-Evans rats were subjected to bilateral lesions of the cholinergic neurons in the nucleus basalis magnocellularis (NBM) by injection of 0.2 or 0.4 microg 192-IgG-saporin in 0.4 microl phosphate-buffered saline. Control rats received an equivalent amount of phosphate-buffered saline. Starting 2 weeks after surgery, all rats were tested for locomotor activity in their home cage, beam-walking performance, T-maze alternation rates (working memory), reference and working memory performance in a water-maze task, and memory capabilities in the eight-arm radial maze task using uninterrupted and interrupted (delay of 2 min, 2 h and 6 h after four arms had been visited) testing procedures. Histochemical analysis showed a significant decrease of acetylcholinesterase (AChE)-positive reaction products (30-66%) in various cortical regions at the 0.2-microg dose. At the dose of 0.4 microg, there was an additional, although weak, damage to the hippocampus (17-30%) and the cingulate cortex (34%). The behavioral results showed only minor impairments in spatial memory tasks, and only during initial phases of the tests (reference memory in the water maze, working memory in the radial maze). The behavioral effects of the dramatic cholinergic lesions do not support the idea of a substantial implication of cholinergic projections from the NBM to the cortex in the memory processes assessed in this study, but they remain congruent with an involvement of these projections in attentional functions.

Phthalic acid amygdalopetal lesion of the nucleus basalis magnocellularis induces reversible memory deficits in rats

The basolateral amygdala (BLA) is extensively implicated in emotional learning and memory. The current study investigated the contribution of cholinergic afferents to the BLA from the nucleus basalis magnocellularis in influencing aversive learning and memory. Sprague-Dawley rats were given permanent unilateral phthalic acid (300 ng) lesions of the nucleus basalis magnocellularis and were chronically implanted with cannulas aimed at the ipsilateral BLA. Lesioned rats showed a pronounced inhibitory avoidance task retention deficit that was attenuated by acute posttraining infusions of the muscarinic cholinergic agonist oxotremorine (4 ng) or the indirect agonist physostigmine (1 microg) into the BLA. Continuous multiple-trial inhibitory avoidance training and testing revealed that lesioned rats have a mild acquisition deficit, requiring approximately 1 additional shock to reach the criterion, and a pronounced consolidation deficit as indicated by a shorter latency to enter the shock compartment on the retention test. Because lesioned rats did not differ from sham-operated controls in performance on a spatial water maze task or in shock sensitivity, it is not likely that the memory impairments produced by the phthalic acid lesions are due to any general sensory or motor deficits. These findings suggest that the dense cholinergic projection from the nucleus basalis magnocellularis to the BLA is involved in both the acquisition and the consolidation of the aversive inhibitory avoidance task.

Impairments in negative patterning, but not simple discrimination learning, in rats with 192 IgG-saporin lesions of the nucleus basalis magnocellularis

Rats with 192 IgG-saporin lesions of the nucleus basalis magnocellularis (NBM) and sham-operated rats were trained in either a simple discrimination paradigm assessing simple association learning or a negative patterning paradigm assessing configural association learning. In the simple discrimination task, rats were reinforced for responding to a light but were not reinforced for responding to a tone. In the negative patterning discrimination task, rats were reinforced for responding to either a light or a tone presented alone but were not reinforced for responding to both stimuli presented simultaneously. Simple discrimination learning was not affected, whereas acquisition of negative patterning was impaired by NBM lesions. Impaired configural association learning may reflect a loss in the ability of rats with NBM lesions to attend to multiple sensory stimuli or to cope with conflicting response strategies.

Induction of behavioral associative memory by stimulation of the nucleus basalis

The nucleus basalis (NB) has been implicated in memory formation indirectly, by lesions, pharmacological manipulations, and neural correlates of learning. Prior findings imply that engagement of the NB during learning promotes memory storage. We directly tested this NB-memory hypothesis by determining whether stimulation of the NB induces behavioral associative memory. Rats were trained either with paired tone (6 kHz) and NB stimulation or with the two stimuli unpaired. We later determined the specificity of cardiac and respiratory behavioral responses to the training tone and several other acoustic frequencies. Paired subjects exhibited frequency generalization gradients with a peak of 6 kHz for both cardiac and respiratory behavior. Unpaired subjects exhibited no generalization gradient. The development of such specific, associative behavioral responses indicates that tone paired with NB stimulation induced behavioral associative memory. The discovery of memory induction by direct activation of the NB supports the NB-memory hypothesis and provides a potentially powerful way to control and investigate neural mechanisms of memory.

Extensive immunolesions of basal forebrain cholinergic system impair offspring recognition in sheep

The involvement of the basal forebrain cholinergic system has been extensively investigated in instrumental learning but little is known of its participation in social memory, especially in the memorization of individual traits of a conspecific. The present study tested in sheep its contribution to both instrumental learning and individual offspring recognition. Six weeks before parturition, ewes received injections of a specific cholinergic immunotoxin (ME20.4 IgG-saporin) into the lateral ventricles (150 microg) and in some cases additional immunotoxin injections into the nucleus basalis (11 microg/side). After 3 weeks of recovery, ewes were trained on a classical instrumental visual discrimination task known to be sensitive to cholinergic deficits. The formation of memory of offspring was assessed through both olfactory and visual/auditory recognition tasks. Olfactory recognition was tested by presenting at suckling successively an alien and the familiar lamb at 2 and 4 h after parturition. Visual/auditory recognition of the lamb was performed using a non-olfactory discrimination test between the familiar and an alien lamb after 12 h of mother-young contact. The lesion extent was assessed by counting choline acetyltransferase-immunopositive neurons in the basal forebrain and measuring the density of acetylcholinesterase fibers in different target areas. Results showed that immunotoxic lesions delayed acquisition of the instrumental visual discrimination. Moreover, olfactory recognition of the lamb was severely impaired while visual/auditory lamb recognition was marginally altered. There was no evidence for sensorimotor or motivational deficits. Importantly, impairment was observed in animals for which loss of basal forebrain cholinergic neurons and their efferent fibers was higher than 75%, while striatal cholinergic neurons and Purkinje cells were unaffected. This study provides evidence that the basal forebrain cholinergic system contributes not only to instrumental but also to social learning. In addition, the cholinergic modulation seems of importance for processing visual and olfactory modalities. However, since only extensive lesions affect performance, this indicates that the basal forebrain cholinergic system possesses substantial reserve capacity to sustain cognitive functions.

Selective antibody-induced cholinergic cell and synapse loss produce sustained hippocampal and cortical hypometabolism with correlated cognitive deficits

The physiological interrelationships between cognitive impairments, neurotransmitter loss, amyloid processing and energy metabolism changes in AD, cholinergic dementia and Down's syndrome are largely unknown to date. This report contains novel studies into the association between cognitive function and cerebral metabolism after long-term selective CNS cholinergic neuronal and synaptic loss in a rodent model. We measured local cerebral rates of glucose utilization ((14)C-2-deoxyglucose) throughout the brains of awake rats 4.5 months after bilateral intraventricular injections of a cholinotoxic antibody directed against the low-affinity NGF receptor (p75 NGF) associated with cholinergic neurons (192 IgG-saporin). Permanent cholinergic synapse loss was demonstrated by [(3)H]-vesamicol in vitro autoradiography defining presynaptic vesicular acetylcholine (ACh) transport sites. While other metabolic studies have defined acute and transient glucose use changes after relatively nonspecific lesions of anatomical regions containing cholinergic neurons, our results show sustained reductions in glucose utilization in brain regions impacted by cholinergic synapse loss, including frontal cortical and hippocampal regions, relative to glucose use levels in control rats. In the same animals, impaired cognitive spatial performance in a Morris water maze was correlated with reduced glucose use rates in the cortex and hippocampus at this time point, which is consistent with increased postmortem cortical and hippocampal amyloid precursor protein (APP) levels (45, 46). These results are consistent with the view of cholinergic influence over metabolism, APP processing, and cognition in the cortex and hippocampus.

Differential participation of the NBM in the acquisition and retrieval of conditioned taste aversion and Morris water maze

Deficits in both learning and memory after lesions of the cholinergic basal forebrain, in particular the nucleus basalis magnocellularis (NBM), have been widely reported. However, the participation of the cholinergic system in either acquisition or retrieval of memory process is still unclear. In this study, we tested the possibility that excitotoxic lesions of the NBM affect either acquisition or retrieval of two tasks. In the first experiment, animals were trained for two conditioned taste aversion tasks using different flavors, saccharine and saline. The acquisition of the first task was before NBM lesions (to test retrieval) and the acquisition of the second task was after the lesions (to test acquisition). Accordingly, in the first part of the second experiment, animals were trained in the Morris water maze (MWM), lesioned and finally tested. In the final part of this experiment, another set of animals was lesioned, then trained in the MWM and finally tested. All animals were able to retrieve conditioned taste aversion (CTA) and MWM when learned before NBM lesions; however, lesions disrupted the acquisition of CTA and MWM. The results suggest that the NBM and cholinergic system may play an important role in acquisition but not during retrieval of aversive memories.

Redundant basal forebrain modulation in taste aversion memory formation

Mnemonic deficits resulting from excitotoxic lesion of the basal forebrain have been classically attributed to the resulting depletion of cortical acetylcholine activity. It has been demonstrated that in spite of the strong cholinergic depletion after injections into the basal forebrain of the immunotoxin 192IgG-saporin, no detectable deficit can be found in the acquisition of several learning tasks, including conditioned taste aversion. Conversely, NMDA-induced lesions of the basal forebrain strongly impair taste aversion learning. In this study we show that 192IgG-saporin produces an efficient and selective cholinergic deafferentation of the rat neocortex but not the amygdala. Furthermore, a stronger relationship between severity of memory impairment after NMDA lesions and basoamygdaloid cholinergic deafferentation was found. Therefore, in a second experiment, we show that combining NMDA-induced lesions into the basolateral amygdala with 192IgG-saporin injections into the basal forebrain results in a strong disruption of taste aversion learning, whereas none of these treatments were by themselves capable of producing any detectable impairment in this learning task. The double lesion effect was only paralleled by simple NMDA lesions into the basal forebrain, suggesting that the learning deficits associated to excitotoxic lesions of the basal forebrain are the result of the simultaneous destruction of the corticopetal and basoamygdaloid interaction. A model is proposed, according to which the modulation of learning processes exerted by the basal forebrain can be redundantly performed by both the basocortical and basoamygdaloid pathway.