Complementary roles for the amygdala and hippocampus in aversive conditioning to explicit and contextual cues

Expression of Fos-related antigens in the nucleus accumbens and associated regions following exposure to a cocaine-paired environment

This study examined whether conditioned hyperactivity measured in a cocaine-paired environment was associated with increased expression of Fos-related antigens (FRA) within the nucleus accumbens (NAc) and associated forebrain regions of rats. Three groups of rats were given repeated injections of either cocaine in the test environment and saline in the colony room (group Paired), saline in the test environment and cocaine in the colony room (group Unpaired), or saline in both environments (group Control). All rats were subsequently given a drug-free test for conditioned hyperactivity in the test environment, and their brains were removed so that FRA immunohistochemistry could be conducted. Rats in the Paired group showed conditioned hyperactivity during the conditioning test, and this behavioural response was associated with increased FRA expression within the caudal NAc, the medial prefrontal cortex and the lateral septum relative to the Unpaired and Control groups. Paired rats also showed increased FRA expression within the orbital prefrontal cortex, the claustrum, the caudal amygdala (basolateral and central regions), the paraventricular thalamic nucleus, the subiculum of the hippocampus, and the lateral habenula relative to the Control group. However, the FRA levels in these latter sites were not significantly increased relative to those of Unpaired rats, indicating that genomic responses in these regions were not entirely context dependent. The correspondence between conditioned hyperactivity and enhanced FRA expression within the caudal NAc, the medial prefrontal cortex and lateral septum suggests that these regions may participate in the expression of conditioned responses to cocaine-related stimuli.

The effects of neurotoxic hippocampal lesions on two effects of context after fear extinction

Three conditioned suppression experiments with rats examined the role of the hippocampus in 2 effects of context after extinction. Reinstatement is the context-specific recovery of fear to an extinguished conditioned stimulus (CS) that occurs following independent presentations of the unconditioned stimulus (US), after extinction. Renewal is the recovery of fear when the CS is presented in the context in which it was conditioned, after extinction in a different context. Results indicated that neurotoxic lesions of the hippocampus, performed before conditioning, abolished reinstatement, which depends on context-US associations, but not renewal, which does not. This dissociation is not the result of differences in the recentness of context learning that ordinarily governs the 2 effects. The results suggest that the hippocampus is necessary for some, but not all, types of contextual learning.

Influence of the ventral hippocampal formation on plasma vasopressin, hypothalamic-pituitary-adrenal axis, and behavioral responses to novel acoustic stress

The ventral hippocampal formation (vHF) seems to constrain diverse responses to psychological stimuli, and disruption of this function may underlie severe neuropsychiatric diseases. In particular, the ventral subiculum inhibits hypothalamic-pituitary-adrenal axis (HPA) activity following psychological, but not systemic, stressors. Despite the difficulty in interpreting such HPA responses, they have been relied upon to further characterize vHF function, because increased HPA axis activity is implicated in neuropsychiatric disturbances, and reliance on behavioral and cognitive data is even more problematic. Plasma arginine vasopressin (pAVP), which is inhibited by psychological stimuli and is also implicated in diverse neuropsychiatric diseases, provides a less ambiguous measure of CNS function. To test if its inhibition by psychological stress is also mediated by the vHF, we conducted two studies. In the first, pAVP and behavioral responses to novel acoustic stress were assessed in rats with bilateral excitotoxic lesions of the ventral subiculum and the ventral hippocampus. The subiculum lesions blocked the fall in pAVP and enhanced escape behaviors, whereas the hippocampal lesions produced responses intermediate to those in the subiculum-lesioned and control rats. In the second study, the pAVP response was similarly blocked by small lesions restricted to those vHF subfields which project to the neuroendocrine hypothalamus, compared to the response in animals with lesions in other vHF subfields. These results indicate that discrete projections from the vHF inhibit the pAVP response to psychological stimuli, and suggest that pAVP may provide a reliable probe of vHF activity.

Contextual fear conditioning is associated with an increase of acetylcholine release in the hippocampus of rat

The effects of contextual fear conditioning on the release of acetylcholine (ACh) in the hippocampus of freely moving rats was assessed using microdialysis. Measures were carried out during both acquisition and retention testing (re-exposure to the conditioning chamber) and compared between animals that either received foot-shocks as unconditioned stimulus (conditioned group) or no foot-shocks (control group) during acquisition. Results showed that during acquisition, hippocampal ACh extracellular level was increased with respect to baseline but that this increase was of similar magnitude in both groups. By contrast, re-exposure to the conditioning chamber the day after (retention testing) produced a significantly greater increase in ACh extracellular level in the conditioned (that, otherwise, displayed conditioned freezing behavior to contextual cues), than in the control group (which displayed virtually no freezing). This enhanced hippocampal ACh release seems to result from the greater hippocampal processing of contextual stimuli in conditioned animals with respect to controls.

Catecholaminergic mechanisms underlying neurohypophysial hormone responses to unconditioned or conditioned aversive stimuli in rats

Oxytocin release from the neurohypophysis is facilitated by systemic cholecystokinin octapeptide (CCK) administration and noxious stimuli. Oxytocin release after CCK administration is mediated by A2 noradrenergic neurones while the release after noxious stimuli appears to be mediated by A1 noradrenergic neurones. On the other hand, facilitation of vasopressin release after noxious stimuli is not dependent upon noradrenergic neurones but on dopamine receptors. Environmental stimuli previously paired with noxious stimuli (conditioned fear stimuli) or novel environmental stimuli facilitate oxytocin release and suppress vasopressin release. These neuroendocrine responses to conditioned fear stimuli, but not to novel stimuli, are impaired by central noradrenaline depletion or i.c.v. adrenoceptor antagonists. These data suggest that there are at least two types of stress responses in neuroendocrine systems, one noradrenaline dependent, and one noradrenaline independent. It is also suggested that noradrenergic neurones are functionally heterogeneous in the control of oxytocin release.

On imputing function to structure from the behavioural effects of brain lesions

What is the link, if any, between the patterns of connections in the brain and the behavioural effects of localized brain lesions? We explored this question in four related ways. First, we investigated the distribution of activity decrements that followed simulated damage to elements of the thalamocortical network, using integrative mechanisms that have recently been used to successfully relate connection data to information on the spread of activation, and to account simultaneously for a variety of lesion effects. Second, we examined the consequences of the patterns of decrement seen in the simulation for each type of inference that has been employed to impute function to structure on the basis of the effects of brain lesions. Every variety of conventional inference, including double dissociation, readily misattributed function to structure. Third, we tried to derive a more reliable framework of inference for imputing function to structure, by clarifying concepts of function, and exploring a more formal framework, in which knowledge of connectivity is necessary but insufficient, based on concepts capable of mathematical specification. Fourth, we applied this framework to inferences about function relating to a simple network that reproduces intact, lesioned and paradoxically restored orientating behaviour. Lesion effects could be used to recover detailed and reliable information on which structures contributed to particular functions in this simple network. Finally, we explored how the effects of brain lesions and this formal approach could be used in conjunction with information from multiple neuroscience methodologies to develop a practical and reliable approach to inferring the functional roles of brain structures.

Dissociable roles of the central and basolateral amygdala in appetitive emotional learning

The amygdala is considered to be a core component of the brain's fear system. Data from neuroimaging studies of normal volunteers and brain-damaged patients perceiving emotional facial expressions, and studies of conditioned freezing in rats, all suggest a specific role for the amygdala in aversive motivation. However, the amygdala may also be critical for emotional processing in positive or appetitive settings. Using an appetitive Pavlovian approach procedure we show a theoretically important dissociation in the effects of excitotoxic lesions of the central nucleus and basolateral area of the amygdala, in the rat. Whilst central nucleus lesions impair appetitive Pavlovian conditioning, basolateral lesions do not. Together with other data, these results not only support the hypothesis that the amygdala is critical for appetitive as well as aversive learning, but are also consistent with amygdala subsystems subserving distinct aspects of emotional learning. Lesions of the dorsal or ventral subiculum were without effect on autoshaping, indicating the lack of involvement of hippocampal processing in this form of emotional behaviour and emphasizing further the neural specificity of the effects seen following central amygdala lesions.

Paradoxical facilitatory effect of fornix lesions on acquisition of contextual fear conditioning in mice

The present study examined the effect of fornix lesions on freezing behavior elicited by contextual and phasic conditioned stimuli. Male mice of the C57Bl/6 strain received electrolytic lesions of the fornix. Ten days following the lesion, they were submitted to acquisition of one-trial classical fear conditioning involving the pairing of an auditory conditioned stimulus (CS) with a footshock unconditioned stimuli (US). Analysis of conditioned fear responses showed that fornix lesions enhanced the freezing response elicited by exposure to the conditioning chamber 24 h after a single tone-shock pairing. In contrast, the two groups did not differ on their fear responses during the auditory cue test. Analysis of the time-course of freezing behavior during re-exposure to the conditioning chamber suggests, however, that the observed fornix lesion-induced facilitation of freezing to the conditioning chamber is more likely due to a facilitation of the processing of a simple (unimodal) rather than polymodal (contextual) CS-US association.

Effect of sex on fear conditioning is similar for context and discrete CS in Wistar, Lewis and Fischer rat strains

Enhanced fear in males relative to females, both innate and conditioned, is a well-described characteristic of behavior in the laboratory rat. In the case of aversive conditioning to foot shock in Long-Evans rats, it has been described that conditioning to general (nondiscrete) contextual cues is greater in male rats relative to female rats, whereas conditioning to a discrete, predictive stimulis (CS) is not. These findings have been combined with evidence for greater levels of hippocampal LTP in males in Sprague-Dawley rats to derive a model of hippocampal-LTP-mediated contextual and not CS, fear conditioning. The present study reports on an analysis of the effect of sex in contextual and discrete CS conditioning to foot shock, assessed via measurement of freezing behavior in a novel automated paradigm, in three rat strains: Wistar, Fischer, and Lewis. In Wistar rats, there was a consistent but nonsignificant tendency for males to demonstrate both more contextual and more CS conditioning than females; in Fischer rats, males demonstrated both more contextual and more CS conditioning than females; in Lewis rats, a markedly enhanced acquisition of freezing in males did not translate into a sex difference in either context or CS conditioning at expression. Therefore, within each strain the effect of sex was consistent between context and CS conditioning. These findings, taken together with the hippocampal LTP evidence, suggest that the latter mediates both contextual and discrete CS aversive conditioning, and contributes to sex differences in both these forms of conditioning, in those strains where these sex differences exist.

Vasopressin in the lateral septum promotes elemental conditioning to the detriment of contextual fear conditioning in mice

Previous experiments using a classical fear conditioning paradigm have provided evidence that the processing of contextual conditional stimuli (CSs) by the hippocampus would be controlled by the amygdala through a modulation of hippocampal-lateral septal (H-LS) excitability. More specifically, our suggestion was that vasopressin release into the LS would occur in an elemental conditioning case [pairing CS-US (unconditional stimulus) procedure] and would result in less hippocampal-dependent contextual stimuli processing (i.e. overshadowing of CSs by the simple CS). Conversely, when an unpairing CS-US procedure is used, this would result in more contextual stimuli processing through a decrease in vasopressin release into the LS. The aim of the present experiment was to test this hypothesis using intraseptal injection of vasopressin or its V1/V2 antagonist. In agreement with this hypothesis, results suggest that vasopressin release into the LS would constitute a device by which priority is given to the more salient simple stimulus to the detriment of contextual information.

Neurotoxic basolateral amygdala lesions impair learning and memory but not the performance of conditional fear in rats

We examined the influence of extensive overtraining (75 trials) on the impact of neurotoxic basolateral amygdala (BLA) lesions on Pavlovian fear conditioning in rats. As we have shown previously, pretraining BLA lesions yielded severe deficits in the acquisition of conditional freezing in rats trained with either 1 or 25 conditioning trials. However, extensive overtraining (50 or 75 trials) mitigated deficits in conditional freezing. Under these conditions the rats with BLA lesions expressed normal and robust freezing behavior, although they required at least 10 times as much training as control rats to reach this level of performance. The ability of rats with BLA lesions to acquire and express conditional freezing after extensive overtraining was modality-specific; conditional freezing in individual rats was acquired to contextual, but not acoustic, conditional stimuli. These results suggest that neural circuitry outside of the amygdala can mediate contextual fear conditioning under some conditions. In contrast to pretraining lesions, post-training BLA lesions eradicated the memory for Pavlovian fear in rats trained with either 1 or 75 trials; this deficit was not modality-specific. Together, these results reveal that impairments in the acquisition and expression of conditional fear in rats with BLA lesions are not attributable to deficits in the performance of the freezing response but are attributable to disruptions in the learning and memory of Pavlovian fear conditioning.

The role of the hippocampal formation in controlling GABA release in the nucleus accumbens during an emotional conditioned response

Intracerebral dialysis was used in living hooded rats in combination with high-performance liquid chromatography with electrochemical detection to study the effects of lesions to the hippocampal formation on GABA release into the intercellular space of the medial part of the nucleus accumbens during an emotional conditioned response to situational stimuli. These experiments showed that the procedures of learning and performing the emotional conditioned response were accompanied by increases in GABA levels in the intercellular space of the nucleus accumbens. Lesioning of the hippocampal formation with ibotenic acid interfered with the performance of the emotional conditioned response and decreased the release of GABA in the nucleus accumbens induced by the behavioral test to levels not significantly different from those due to background release. These data suggest that the hippocampal formation has a role in reproducing memory traces produced by situational stimuli and that this role is mediated by its influence on the GABAergic system of the nucleus accumbens.

Associative processes in addiction and reward. The role of amygdala-ventral striatal subsystems

Only recently have the functional implications of the organization of the ventral striatum, amygdala, and related limbic-cortical structures, and their neuroanatomical interactions begun to be clarified. Processes of activation and reward have long been associated with the NAcc and its dopamine innervation, but the precise relationships between these constructs have remained elusive. We have sought to enrich our understanding of the special role of the ventral striatum in coordinating the contribution of different functional subsystems to confer flexibility, as well as coherence and vigor, to goal-directed behavior, through different forms of associative learning. Such appetitive behavior comprises many subcomponents, some of which we have isolated in these experiments to reveal that, not surprisingly, the mechanisms by which an animal sequences responding to reach a goal are complex. The data reveal how the different components, pavlovian approach (or sign-tracking), conditioned reinforcement (whereby pavlovian stimuli control goal-directed action), and also more general response-invigorating processes (often called "activation," "stress," or "drive") may be integrated within the ventral striatum through convergent interactions of the amygdala, other limbic cortical structures, and the mesolimbic dopamine system to produce coherent behavior. The position is probably not far different when considering aversively motivated behavior. Although it may be necessary to employ simplified, even abstract, paradigms for isolating these mechanisms, their concerted action can readily be appreciated in an adaptive, functional setting, such as the responding by rats for intravenous cocaine under a second-order schedule of reinforcement. Here, the interactions of primary reinforcement, psychomotor activation, pavlovian conditioning, and the control that drug cues exert over the integrated drug-seeking response can be seen to operate both serially and concurrently. The power of our analytic techniques for understanding complex motivated behavior has been evident for some time. However, the crucial point is that we are now able to map these components with increasing certainty onto discrete amygdaloid, and other limbic cortical-ventral striatal subsystems. The neural dissection of these mechanisms also serves an important theoretical purpose in helping to validate the various hypothetical constructs and further developing theory. Major challenges remain, not the least of which is an understanding of the operation of the ventral striatum together with its dopaminergic innervation and its interactions with the basolateral amygdala, hippocampal formation, and prefrontal cortex at a more mechanistic, neuronal level.

Intraseptal injection of scopolamine increases the effect of systemic diazepam on passive avoidance learning and emotionality in rats

This experiment was designed to assess the role of the septo-hippocampal cholinergic (ACh) system in the deleterious effects produced by systemic benzodiazepine injection on learning processes in rats. Retention of a step through passive avoidance task was analysed after systemic injection of increasing doses of either scopolamine or diazepam applied alone 30 min before the acquisition phase. Results indicated a dose related impairment of retention by each drug: in addition, sub-threshold doses of scopolamine and diazepam applied in combination (diazepam: 2mg/kg plus scopolamine: 0.3mg/kg) produced a decrease of retention latencies, thus showing an additive effect of the combined treatment. Secondly, a sub-threshold dose of scopolamine (15microg/0.5microl) was also administered into the medial septal area, together with an i.p. injection of 2mg/kg of diazepam. This combined treatment produced a severe impairment of retention, in parallel with a large reduction in emotionality (number of faeces). The data are consistent with the hypothesis that peripheral administration of behaviorally effective doses of diazepam on passive avoidance learning might act partially via a septal ACh-GABA/benzodiazepine mechanism. It is also suggested that this mechanism subserves both anxiety and the memorisation of contextual stimuli associated with passive avoidance acquisition, through the modification of the septo-hippocampal activity.

Discriminative fear conditioning to context expressed by multiple measures of fear in the rat

There has been a renewed interest in the neural basis of fear conditioning to context. These current approaches are accompanied by some limitations including the use of short testing windows, non-discriminative paradigms, and unitary fear response assessment. In an attempt to circumvent these limitations, a discriminative context procedure assessing multiple response measures of fear was used in the present study. Conditioning consisted of three training sessions and each session consisted of 2 days. On day one, the animals were placed in the paired context and received three foot shocks. On the other day, they were placed in the unpaired chamber in the absence of any aversive event. Animals were tested after each training session and the response measures of fear recorded included: preference, freezing, heart rate, ultrasonic vocalizations, defecation, body temperature, urination and locomotion. The results suggest that behavioral, as well as physiological changes evoked by fearful stimuli become associated with the context in which the aversive event occurred. In general these findings also suggest that there are different learning parameters for the measures of fear examined in this paradigm.

Lesions of the medial or lateral perforant path have different effects on hippocampal contributions to place learning and on fear conditioning to context

The axons of the neurons in the medial and lateral components of the entorhinal cortex (MEC and LEC) form the medial and lateral perforant paths (MPP and LPP) which represent the major source of cortical input to the hippocampus. Anatomical, physiological, and pharmacological studies have shown that MPP and LPP are distinct. Unfortunately, assessment of the functional significance of damage to either of these pathways has not used tasks known to be sensitive to hippocampal function in the rodent. In this study, we performed dissociated lesions of MPP and LPP using a combined physiological and anatomical method. Rats with lesions of either the MPP or the LPP were tested on place learning in the water task and on a discriminative fear conditioning to context task. The results indicated that the MPP, but not LPP, lesions resulted in impaired place learning. The context discrimination data revealed an amygdala-like, reduced fear effect of MPP lesions and an enhanced discriminative fear conditioning to context effect of LPP lesions. Consistent with a two-stage model of spatial learning proposed by Buzsaki (Buzsaki G. Two-stage model of memory trace formation: a role for 'noisy' brain states. Neuroscience 1989;31(3):551-570), the impairment in the water task can be interpreted as reflecting the higher efficiency of the MPP synapses in activating hippocampal neurons. The context discrimination results can be explained by either a dissociation of sensory information that reaches the MEC and LEC, or alternatively, by a dissociation between the limbic nature of the MEC and the sensory nature of the LEC.

Effects of mild traumatic brain injury on immunoreactivity for the inducible transcription factors c-Fos, c-Jun, JunB, and Krox-24 in cerebral regions associated with conditioned fear responding

We have previously demonstrated that mild traumatic brain injury (TBI) of the right parietal cortex results in a relatively selective deficit in conditioned fear responding. However, this behavioural deficit is very consistent and unrelated to the extent of the cortical necrotic lesion. We were therefore interested in determining if other brain regions might show a consistent response to mild TBI, and therefore, more reliably relate to the behavioural change. Increased expression of inducible transcription factors (ITFs) has been used to study which brain regions respond to a variety of events. In the present study, we examined the expression patterns of immunoreactivity (IR) for four ITFs (c-Fos, c-Jun, JunB, and Krox-24) at 3 h after mild fluid percussion TBI. Changes in ITF expression were only observed ipsilateral to the side of TBI. The clearest changes were observed in brain regions known to be involved in conditioned fear responding, such as the amygdala complex and hippocampal formation and several cortical regions. In contrast, no changes in IR for any of the ITFs were observed in the striatum, nucleus accumbens, nucleus basalis magnocellularis, septum or periacqueductal grey. Unlike the extent of visible damage to the cortex at the site of impact, the overexpression of ITFs showed a notable consistency between animals subjected to TBI. This consistency in regions known to be involved in conditioned fear responding (i.e., amygdala complex and hippocampal formation) lead us to suggest that it is these changes, rather than the more variable cortical necrotic lesion, that is responsible for the behavioural deficits we observe following mild TBI. Importantly, our results demonstrate that like the hippocampus, the amygdala is a sub-cortical structure particularly sensitive to the effects of mild brain trauma and underline the fact that cerebral regions distant from the location of the fluid impact can be affected.

Effects of excitotoxic lesions of the basolateral amygdala on conditional discrimination learning with primary and conditioned reinforcement

Rats with excitotoxic lesions of the basolateral amygdala (BLA) were not impaired in the acquisition of an appetitive visuospatial conditional discrimination between stimuli varying in temporal frequency that has previously been shown to be sensitive to the effects of lesions of the striatum and cingulate cortex. After asymptotic performance was attained, discrimination was reinforced according to a fixed ratio (FR) schedule under which n presentations of sucrose were provided following n correct responses; each correct response also being reinforced immediately by a light acting as a conditioned reinforcer. Under these conditions of reinforcement when FRn=5, BLA-lesioned rats initially showed transient impairments in several aspects of performance, but rapidly attained control levels over subsequent test sessions. No further impairments occurred when FRn=10/20. However, in various conditions of extinction, further differences in performance were revealed between the BLA-lesioned and control groups, notably a significantly enhanced resistance to extinction when both sucrose and conditioned reinforcement were omitted. The results are discussed in terms of limbic-striatal mechanisms in the control of discrimination learning and the possible role of the amygdala in the mediation of different aspects of conditioned reinforcement.

Metabotropic glutamate receptor subtype 7 ablation causes deficit in fear response and conditioned taste aversion

Metabotropic glutamate receptors (mGluRs) consist of eight different subtypes and exert their effects on second messengers and ion channels via G-proteins. The function of individual mGluR subtypes in the CNS, however, largely remains to be clarified. We examined the fear response of freezing after electric shock in wild-type and mGluR7(-/-) knockout littermates. Wild-type mice displayed freezing immediately after and 1 d after footshock. In comparison, mGluR7(-/-) knockout mice showed significantly reduced levels in both immediate postshock and delayed freezing responses. However, the knockout mice exhibited no abnormalities in pain sensitivity and locomotor activity. To further examine amygdala-dependent behavior, we performed conditioned taste aversion (CTA) experiments. In wild-type mice, the administration of saccharin followed by intraperitoneal injection of the malaise-inducing agent LiCl resulted in an association between saccharin and LiCl. This association caused strong CTA toward saccharin. In contrast, mGluR7(-/-) knockout mice failed to associate between the taste and the negative reinforcer in CTA experiments. Again, the knockout mice showed no abnormalities in taste preference and in the sensitivity to LiCl toxicity. These results indicate that mGluR7 deficiency causes an impairment of two distinct amygdala-dependent behavioral paradigms. Immunohistochemical and immunoelectron-microscopic analyses showed that mGluR7 is highly expressed in amygdala and preferentially localized at the presynaptic axon terminals of glutamatergic neurons. Together, these findings strongly suggest that mGluR7 is involved in neural processes subserving amygdala-dependent averse responses.

The effect of excitotoxic hippocampal lesions on simple and conditional discrimination learning in the rat

The effect of excitotoxic lesions of the hippocampus on acquisition and reversal of simple and conditional tasks was investigated using a Y-maze. Hippocampal-lesioned rats were severely impaired on acquisition and reversal of a conditional visuo-spatial task (where different pairs of visually distinctive choice arms indicated whether a left or right arm choice was correct on that trial) and were unable to acquire a visuo-visual conditional discrimination (where the appearance of the start arm indicated which of the visually distinctive choice arms was correct irrespective of their left/right position). They were not impaired on acquisition or reversal of a simple spatial left/right discrimination task (where all arms had the same visual appearance) nor on acquisition of a visual discrimination (where the correct, visually distinctive, choice arm varied in its left/right position). Hippocampal-lesioned rats were, however, impaired on reversal of this visual discrimination task and on acquisition and reversal of another visual discrimination task in which the visually distinctive choice arms were less different from each other than in the first version of this task. The degree of impairment in the lesioned rats was related to task difficulty for the sham-operated rats and was not specific to tasks requiring spatial choices, visual discrimination or conditional responding. The impairment on conditional tasks was greater than the impairment on those non-conditional tasks which happened to be matched for task difficulty for the sham-operated rats, suggesting that the conditional demand may target the function of the hippocampus rather closely. Statistically worse than chance performance by hippocampal-lesioned (and sham-operated) rats at the beginning of reversal testing, which was given 24 h after achieving criterion on acquisition of that task, indicated that hippocampal-lesioned rats simultaneously exhibited good memory but impaired learning for the type of information required for those tasks.

Ibotenate lesions of the hippocampus impair spatial learning but not contextual fear conditioning in mice

Recently, gene targeting and other mouse transgenic techniques have been used to study the cellular mechanisms underlying learning and memory mechanisms in the hippocampus. A key assumption of many of these studies is that lesions of the hippocampus have a similar impact on learning and memory in mice and in rats. Here, we used axon-sparing ibotenate lesions to determine whether damage to the hippocampus disrupts spatial learning and contextual conditioning in mice, as it is known to do in rats. Our results demonstrated that hippocampal lesions impair performance in the hidden-platform version of the water maze under a variety of experimental conditions. Neither keeping the start site constant, nor prior training with the visible-platform task fully rescued the spatial learning deficits of the lesioned mice. As previously shown in rats, the lesions left the performance of the mice intact in the visible-platform version of the water maze, indicating that they do not affect all types of learning, and that disruptions of sensory processing or motivation probably did not account for their deficits in the hidden-platform task. In contrast, the very same lesions did not affect either cued or contextual fear conditioning. These results confirm the involvement of the hippocampus in spatial learning in mice, and they also demonstrate that hippocampal-lesioned mice can show contextual fear conditioning. Thus, the behavioral findings presented here are crucial for the interpretation of transgenic experiments with the widely used water maze and fear-conditioning paradigms.

The medullary dorsal reticular nucleus facilitates pain behaviour induced by formalin in the rat

The influence of the dorsal reticular nucleus (DRt) on pain behaviour during the formalin test was studied in the rat by lesioning the nucleus through local application of electrical current or quinolinic acid. Animals in which the DRt was lesioned ipsilaterally to the paw injected with formalin spent less time in focused (licking, biting or scratching the injected paw) and total (focused pain behaviour plus protection of the injected paw during movements) pain behaviour, and showed paw-jerks less frequently than non-lesioned animals in both phases 1 and 2 of the test. Animals in which the DRt was lesioned contralaterally to the injected paw presented a decrease in total pain behaviour and number of paw-jerks only during phase 2. The number of superficial (laminae I-II) and deep (laminae III-VI) spinal dorsal horn cells expressing the c-fos proto-oncogene 2 h after subcutaneous injection of formalin was reduced by 34% and 50%, respectively, in animals with an ipsilateral DRt lesion as compared to non-lesioned rats. No differences in c-fos expression were observed after lesioning the DRt contralateral to the formalin injection. The results indicate that the DRt is involved in the facilitation of nociception during the formalin test by enhancing the response capacity of dorsal horn neurons to noxious stimulation. It is suggested that the pronociceptive action of the DRt is mediated by the reciprocal connections it establishes with the spinal dorsal horn.

A computer controlled analysis of freezing behaviour

The conditioned freezing response in rats has been much used both by psychologists and neuroscientists to investigate the behavioural effects of brain lesions and of changes in motivational state. The primary advantage of the freezing response is that it can be used without motivational manipulations such as food or water deprivation. Previously, freezing has been measured by a human observer either from video recordings or during the test sessions themselves. But these methods of data collection have potential disadvantages. In the present paper, we describe a new, computer controlled, automated procedure for assessing conditioned freezing. Each conditioning chamber contains a mini-video camera. Behaviour is analysed on-line by means of a programme which compares every two adjacent seconds of video tape to generate a screen representing the percentage difference between them. A difference of <0.05% (50 pixels) is classified as a freezing response. Experiments are described in which we measure conditioned freezing and its development over time, in response to contextual cues and to a discrete tone which had been paired with foot shock. We demonstrate our apparatus and methods of data analysis to be sensitive to: number of tone-shock pairings, rat strain and tone pre-exposure.

Fear and the brain: where have we been, and where are we going?

In recent years, there has been an explosion of interest in the neural basis of emotion. Much of this enthusiasm has been triggered by studies of the amygdala and its contribution to fear. This work has shown that the amygdala detects and organizes responses to natural dangers (like predators) and learns about novel threats and the stimuli that predict their occurrence. The latter process has been studied extensively using a procedure called classical fear conditioning. This article surveys the progress that has been made in understanding the neural basis of fear and its implications for anxiety disorders, as well as the gaps in our knowledge.

Perturbed dentate gyrus function in serotonin 5-HT2C receptor mutant mice

Serotonin systems have been implicated in the regulation of hippocampal function. Serotonin 5-HT2C receptors are widely expressed throughout the hippocampal formation, and these receptors have been proposed to modulate synaptic plasticity in the visual cortex. To assess the contribution of 5-HT2C receptors to the serotonergic regulation of hippocampal function, mice with a targeted 5-HT2C-receptor gene mutation were examined. An examination of long-term potentiation at each of four principal regions of the hippocampal formation revealed a selective impairment restricted to medial perforant path-dentate gyrus synapses of mutant mice. This deficit was accompanied by abnormal performance in behavioral assays associated with dentate gyrus function. 5-HT2C receptor mutants exhibited abnormal performance in the Morris water maze assay of spatial learning and reduced aversion to a novel environment. These deficits were selective and were not associated with a generalized learning deficit or with an impairment in the discrimination of spatial context. These results indicate that a genetic perturbation of serotonin receptor function can modulate dentate gyrus plasticity and that plasticity in this structure may contribute to neural mechanisms underlying hippocampus-dependent behaviors.

Basolateral amygdala is not critical for cognitive memory of contextual fear conditioning

Evidence that lesions of the basolateral amygdala complex (BLC) impair memory for fear conditioning in rats, measured by lack of "freezing" behavior in the presence of cues previously paired with footshocks, has suggested that the BLC may be a critical locus for the memory of fear conditioning. However, evidence that BLC lesions may impair unlearned as well as conditioned freezing makes it difficult to interpret the findings of studies assessing conditioned fear with freezing. The present study investigated whether such lesions prevent the expression of several measures of memory for contextual fear conditioning in addition to freezing. On day 1, rats with sham lesions or BLC lesions explored a Y maze. The BLC-lesioned rats (BLC rats) displayed a greater exploratory activity. On day 2, each of the rats was placed in the "shock" arm of the maze, and all of the sham and half of the BLC rats received footshocks. A 24-hr retention test assessed the freezing, time spent per arm, entries per arm, and initial entry into the shock arm. As previously reported, shocked BLC rats displayed little freezing. However, the other measures indicated that the shocked BLC rats remembered the fear conditioning. They entered less readily and less often and spent less time in the shock arm than did the control nonshocked BLC rats. Compared with the sham rats, the shocked BLC rats entered more quickly and more often and spent more time in the shock arm. These findings indicate that an intact BLC is not essential for the formation and expression of long-term cognitive/explicit memory of contextual fear conditioning.

Traumatic amnesia, repression, and hippocampus injury due to emotional stress, corticosteroids and enkephalins

The role of stress, arousal, emotional trauma, and corticosteroid and enkephalin secretion on memory and the hippocampus, and the development of traumatic amnesia and repressed memory syndrome are detailed. Animal and human studies are reviewed. Trauma-induced memory deficits appear to be secondary to abnormal neocortical and hippocampal arousal, and corticosteroid and enkephalin secretion which can induce atrophy or seizures within the hippocampus, suppress hippocampal theta activity and long term potentiation, as well as injure hippocampal pyramidal cells. Predisposing factors include individual, age, and sex differences in arousal, and previous emotional trauma or temporal lobe or hippocampal injury. However, as the amygdala processes and stores emotional experiences in memory, patients may also demonstrate trauma related symptoms, including flashbacks as well as shrinking retrograde amnesia.

Relationship between fos production and classical fear conditioning: effects of novelty, latent inhibition, and unconditioned stimulus preexposure

The relationship between FOS production in the sensory cortex and limbic system and the ability of C57BL/6N mice to acquire context- and tone-dependent freezing were investigated after fear conditioning, which was achieved by exposure of mice to context only or context and tone (10 kHz, 75 dB) as conditioned stimuli (Cs) paired with an electric footshock (0.7 mA, constant) as unconditioned stimulus (Us). The effect of preexposure to Cs or Cs paired with Us on FOS production and learning was also tested. It was demonstrated that high simultaneous FOS production in the parietal cortex, hippocampus, and amygdala paralleled the ability of mice to acquire strong freezing responses to novel Cs. After contextual preexposure (latent inhibition), FOS production could be elicited in the central amygdala only by shock and in the basolateral amygdala only by tone. Under these conditions, the ability of mice to acquire contextual freezing was almost abolished, whereas tone-dependent freezing was reduced. Lacking FOS production in the central amygdala after preexposure to context followed by shock (Us preexposure effect) paralleled the inability of mice to acquire tone-dependent freezing, although the tone elicited FOS production in the basolateral amygdala. On the basis of these findings it was concluded that synchronous Cs- and Us-induced FOS production in several defined forebrain areas was accompanied with associative learning of novel stimuli, and that a subsequent low level of FOS production might have been responsible or indicative for delayed conditioning to those stimuli.

Selective effects of peripheral lipopolysaccharide administration on contextual and auditory-cue fear conditioning

The reported experiments explore the effects of peripheral LPS administration on learning and memory processes. As measured by the conditioned freezing response, intraperitoneal LPS administration given after conditioning impaired contextual but not auditory-cue fear conditioning in both juvenile (hooded Long Evans) and adult rats (albino Sprague Dawley) of two different strains. This impairment in contextual fear conditioning was not dependent on the presence of the tone. Preexposure to the context eliminated the effect of LPS on contextual fear conditioning, and in addition, LPS given after context preexposure negated the beneficial effects of preexposure on contextual fear. These results suggest that LPS disrupts posttrial memory consolidation processes. In support of the hypothesis that LPS-induced proinflammatory cytokine release is involved in producing the impairment in contextual fear caused by LPS, peripheral interleukin-1 receptor antagonist (IL-1ra) administered subcutaneously at a dose of 100 mg/kg prevented the impairment in contextual fear caused by LPS. These experiments provide evidence for a role of immune activation and cytokine activity in learning and memory processes.

Conditioned reflex release of dopamine in the nucleus accumbens after disruption of the hippocampal formation in rats

Vital intracerebral microdialysis combined with HPLC with electrochemical detection was used to study changes in dopamine release in the nucleus accumbens during the development and realization of an emotional conditioned response in hooded rats with lesions to the hippocampal formation. These studies showed that one month after bilateral administration of ibotenic acid into the hippocampal formation, rats had weakened emotional responses to contextual stimuli. The process of development of the conditioned reflex was accompanied by higher-level and longer-lasting release of dopamine in the nucleus accumbens than in sham-operated rats. Dopamine release levels in the nucleus accumbens during realization of the conditioned reflex to contextual stimuli in rats with hippocampal lesions and sham-operated rats were identical.

Mild traumatic lesion of the right parietal cortex in the rat: characterisation of a conditioned freezing deficit and its reversal by dizocilpine

We have previously demonstrated that traumatic injury of the lateral aspect of the right parietal cortex results in reduced acquisition of the passive avoidance task but enhanced learning in an active avoidance procedure. In order to try to explain the apparent dichotomy between these findings a series of experiments examined the effect of fluid percussion-induced traumatic brain injury (FP-TBI) on the conditioned freezing response to a context previously paired with an aversive stimulus. Rats subjected to FP-TBI displayed less conditioned freezing than the sham-operated controls. This effect was particularly marked when the delay between context exposure and footshock was short (< or = 30 s) and was no longer significant when this delay was 3 min, indicating that the injured animals did not have an impaired freezing response per se. This phenomenon was enduring such that it could still be observed 2 months following the surgery. There was no significant freezing deficit after FP-TBI of the motor cortex, demonstrating that the site of injury is important and that the freezing deficit is not a general response to CNS trauma. The NMDA receptor antagonist dizocilpine (MK-801, 1 mg/kg i.v.) significantly reduced the trauma-induced freezing deficit when administered as a single bolus 15 min prior to the surgery, or as three repeated treatments (3 x 0.33 mg/kg) 15 min, and 6 and 24 h following lesion. The trauma-induced deficit in conditioned freezing can explain the differences in active and passive avoidance behaviours and appears to be specific to lesion of the lateral parietal cortex. In addition, the behavioural deficit can be attenuated using the neuroprotective agent dizocilpine, suggesting that it may prove useful as a sensitive and specific measure of cortical damage following traumatic injury.

Differential effects of amygdala lesions on early and late plastic components of auditory cortex spike trains during fear conditioning

In auditory fear conditioning, pairing of a neutral acoustic conditioned stimulus (CS) with an aversive unconditioned stimulus (US) results in an enhancement of neural responses to the CS in the amygdala and auditory cortex. It is not clear, however, whether cortical plasticity governs neural changes in the amygdala or vice versa, or whether learning in these two structures is determined by independent processes. We examined this issue by recording single-cell activity in the auditory cortex (areas Te1, Te1v, and Te3) of freely behaving, amygdalectomized rats using a movable bundle of microwires. Amygdala damage did not affect short-latency (0-50 msec) tone responses, nor did it interfere with conditioning-induced increases of these onset responses. In contrast, lesions of the amygdala interfered with the development of late (500-1500 msec) conditioned tone responses that were not present before conditioning. Furthermore, whereas onset conditioned responses in the control group remained elevated after 30 extinction trials (presentation of CS alone), onset responses in lesioned animals returned to their preconditioning firing level after approximately 10 extinction trials. These results suggest that the amygdala enables the development of long-latency (US anticipatory) responses and prevents the extinction of short-latency onset responses to threatening stimuli. The findings further suggest that auditory cortex cells may participate differently in explicit and implicit memory networks.

Amygdala modulation of multiple memory systems: hippocampus and caudate-putamen

A series of five experiments examined the differential mnemonic roles of the hippocampus and caudate-putamen and the modulatory influence of the amygdala on hippocampal and caudate-putamen memory processes. Findings indicate that (a) posttraining intrahippocampal injections of amphetamine selectively enhance memory in a hidden platform water maze task, (b) posttraining intracaudate injections of amphetamine selectively enhance memory in a visible platform water maze task, (c) posttraining intra-amygdala injections of amphetamine enhance memory in both water maze tasks, (d) preretention intrahippocampal lidocaine injections block expression of the memory enhancing effects of posttraining intrahippocampal amphetamine injections in the hidden platform task, (e) preretention intracaudate lidocaine injections block expression of the memory enhancing effects of posttraining intracaudate amphetamine injections in the visible platform task, (f) preretention intra-amygdala lidocaine injections do not block the memory enhancing effect of posttraining intra-amygdala amphetamine injections on either task, (g) in the hidden platform task, posttraining intrahippocampal, but not intracaudate, lidocaine injections block the memory enhancing effects of posttraining intra-amygdala amphetamine, (h) in the visible platform task, posttraining intracaudate, but not intrahippocampal, lidocaine injections block the memory enhancing effects of posttraining intra-amygdala amphetamine. The findings indicate a double dissociation between the roles of the hippocampus and caudate-putamen in memory and suggest that the amygdala exerts a modulatory influence on both the hippocampal and caudate-putamen memory systems.

The effects of hippocampal aspiration lesions on conditioning to the CS and to a background stimulus in trace conditioned suppression

Rats with hippocampal aspiration lesions or cortical control lesions were compared to sham operated controls in a trace conditioned suppression task, in which a long-lasting background stimulus played the role of more conventional contextual cues. In all three surgical treatment groups, conditioning to the explicit conditioned stimulus (CS) decreased, but conditioning to the background cue increased, when a longer trace interval was used. There was thus no evidence of a differential partitioning of associative conditioning as a result of the lesion, despite the evident sensitivity of the behavioural paradigm to variations in the CS-->unconditioned stimulus (US) interval. This result contrasts with earlier reports using conventional contextual cues in analogous experimental designs, and so suggests that the sensitivity of contextual conditioning to hippocampal dysfunction depends at least in part on the physical nature of conventional contextual cues, and not solely on the less precise predictive information that such cues typically provide.

Recognition memory in rats--I. Concepts and classification

Recognition is the process by which a subject is aware that a stimulus has been previously experienced. It requires that the characteristics of events are perceived, discriminated, identified and then compared (matched) against a memory of the characteristics of previously experienced events. Understanding recognition memory, its underlying neuronal mechanisms, its dysfunction and alleviation of the latter by putative cognition enhancing drugs is a major research target and has triggered a wealth of animal studies. One of the most widely used animals for this purpose is the rat, and it is the rat's recognition memory which is the focus of this review. In this first part, concepts of recognition memory, stages of mnemonic processing and paradigms for the measurement of the rat's recognition memory will be discussed. In two subsequent articles (parts II and III) we will focus on the neuronal mechanisms underlying recognition memory in rats. Three major points arise from the comparison of paradigms that have in the past been used to assess recognition memory in rats. First, it should be realized that some tasks which, at face value, can all be considered to measure recognition memory in rats, may not assess recognition memory at all but may, for example, be based on recall rather than recognition. Second, it is evident that different types of recognition memory can be distinguished and that tasks differ in the type of recognition memory taxed. Some paradigms, for example, measure familiarity, whereas others assess recency. Furthermore, paradigms differ as to whether spatial stimuli or items are employed. Third, different processes, ranging from stimulus-response learning to the formation of concepts, may be involved to varying extent in different tasks. These are important considerations and question the predictive validity of the results obtained from studies examining, for example, the effects of putative cognition enhancing drugs.

Differential modulation of changes in hippocampal-septal synaptic excitability by the amygdala as a function of either elemental or contextual fear conditioning in mice

Recent data obtained using a classic fear conditioning paradigm showed a dissociation between the retention of associations relative to contextual information (dependent on the hippocampal formation) and the retention of elemental associations (dependent on the amygdala). Furthermore, it was reported that conditioned emotional responses (CERs) could be dissociated from the recollection of the learning experience (declarative memory) in humans and from modifications of the hippocampal-septal excitability in animals. Our aim was to determine whether these two systems ("behavioral expression" system and "factual memory" system) interact by examining the consequences of amygdalar lesions (1) on the modifications of hippocampal-septal excitability and (2) on the behavioral expression of fear (freezing) resulting from an aversive conditioning during reexposure to conditional stimuli (CSs). During conditioning, to modulate the predictive nature of the context and of a discrete stimulus (tone) on the unconditional stimulus (US) occurrence, the phasic discrete CS was paired with the US or randomly distributed with regard to the US. After the lesion, the CER was dramatically reduced during reexposure to the CSs, whatever the type of acquisition. However, the changes in hippocampal-septal excitability persisted but were altered. For controls, a decrease in septal excitability was observed during reexposure to the conditioning context only for the "unpaired group" (predictive context case). Conversely, among lesioned subjects this decrease was observed in the "paired group" (predictive discrete CS case), whereas this decrease was significantly reduced in the unpaired group with respect to the matched control group. The amplitude and the direction of these modifications suggest a differential modulation of hippocampal-septal excitability by the amygdala to amplify the contribution of the more predictive association signaling the occurrence of the aversive event.

Pain processing during three levels of noxious stimulation produces differential patterns of central activity

Previous functional imaging studies have demonstrated a number of discrete brain structures that increase activity with noxious stimulation. Of the commonly identified central structures, only the anterior cingulate cortex shows a consistent response during the experience of pain. The insula and thalamus demonstrate reasonable consistency while all other regions, including the lentiform nucleus, somatosensory cortex and prefrontal cortex, are active in no more than half the current studies. The reason for such discrepancy is likely to be due in part to methodological variability and in part to individual variability. One aspect of the methodology which is likely to contribute is the stimulus intensity. Studies vary considerably regarding the intensity of the noxious and non-noxious stimuli delivered. This is likely to produce varying activation of central structures coding for the intensity, affective and cognitive components of pain. Using twelve healthy volunteers and positron emission tomography (PET), the regional cerebral blood flow (rCBF) responses to four intensities of stimulation were recorded. The stimulation was delivered by a CO2 laser and was described subjectively as either warm (not painful), pain threshold just painful), mildly painful or moderately painful. The following group subtractions were made to examine the changing cerebral responses as the stimulus intensity increased: (1) just painful - warm; (2) mild pain - warm; and (3) moderate pain - warm. In addition, rCBF changes were correlated with the subjective stimulus ratings. The results for comparison '1' indicated activity in the contralateral prefrontal (area 10/46/44), bilateral inferior parietal (area 40) and ipsilateral premotor cortices (area 6), possibly reflecting initial orientation and plans for movement. The latter comparisons and correlation analysis indicated a wide range of active regions including bilateral prefrontal, inferior parietal and premotor cortices and thalamic responses, contralateral hippocampus, insula and primary somatosensory cortex and ipsilateral perigenual cingulate cortex (area 24) and medial frontal cortex (area 32). Decreased rCBF was observed in the amygdala region. These responses were interpreted with respect to their contribution to the multidimensional aspects of pain including fear avoidance, affect, sensation and motivation or motor initiation. It is suggested that future studies examine the precise roles of each particular region during the central processing of pain.

Emotional memory and psychopathology

A leading model for studying how the brain forms memories about unpleasant experiences is fear conditioning. A cumulative body of work has identified major components of the neural system mediating this form of learning. The pathways involve transmission of sensory information from processing areas in the thalamus and cortex to the amygdala. The amygdala's lateral nucleus receives and integrates the sensory inputs from the thalamic and cortical areas, and the central nucleus provides the interface with motor systems controlling specific fear responses in various modalities (behavioural, autonomic, endocrine). Internal connections within the amygdala allow the lateral and central nuclei to communicate. Recent studies have begun to identify some sites of plasticity in the circuitry and the cellular mechanisms involved in fear conditioning. Through studies of fear conditioning, our understanding of emotional memory is being taken to the level of cells and synapses in the brain. Advances in understanding emotional memory hold out the possibility that emotional disorders may be better defined and treatment improved.

Neurotoxic lesions of the dorsal hippocampus and Pavlovian fear conditioning in rats

Electrolytic lesions of the dorsal hippocampus (DH) produce deficits in both the acquisition and expression of conditional fear to contextual stimuli in rats. To assess whether damage to DH neurons is responsible for these deficits, we performed three experiments to examine the effects of neurotoxic N-methyl-D-aspartate (NMDA) lesions of the DH on the acquisition and expression of fear conditioning. Fear conditioning consisted of the delivery of signaled or unsignaled footshocks in a novel conditioning chamber and freezing served as the measure of conditional fear. In Experiment 1, posttraining DH lesions produced severe retrograde deficits in context fear when made either 1 or 28, but not 100, days following training. Pretraining DH lesions made 1 week before training did not affect contextual fear conditioning. Tone fear was impaired by DH lesions at all training-to-lesion intervals. In Experiment 2, posttraining (1 day), but not pretraining (1 week), DH lesions produced substantial deficits in context fear using an unsignaled shock procedure. In Experiment 3, pretraining electrolytic DH lesions produced modest deficits in context fear using the same signaled and unsignaled shock procedures used in Experiments 1 and 2, respectively. Electrolytic, but not neurotoxic, lesions also increased pre-shock locomotor activity. Collectively, this pattern of results reveals that neurons in the DH are not required for the acquisition of context fear, but have a critical and time-limited role in the expression of context fear. The normal acquisition and expression of context fear in rats with neurotoxic DH lesions made before training may be mediated by conditioning to unimodal cues in the context, a process that may rely less on the hippocampal memory system.

WAY100635 and latent inhibition in the rat: selective effects at preexposure

The influence of the selective, silent 5HT1a antagonist WAY100635 (Wyeth Research Ltd) on the latent inhibition effect was examined in a within-subject, on-baseline conditioned suppression procedure in rats. WAY100635 was found to enhance the latent inhibition effect, producing a retardation in the acquisition of conditioned suppression following a level of stimulus preexposure known to be insufficient to produce a latent inhibition effect in control animals. This influence of the drug was restricted to its actions during the preexposure phase of the experiment, and the drug also abolished the unconditioned suppression of lever pressing that occurs on the first presentation of a novel auditory stimulus. These findings are discussed in terms of the possible influence of serotonergic manipulations on contextual processing, and also have important implications for current animal models of schizophrenia which stress the role of dopaminergic mechanisms in latent inhibition.

Lesions of the basolateral amygdala abolish the ability of drug associated cues to reinstate responding during withdrawal from self-administered cocaine

This study investigated the ability of bilateral excitotoxic lesions of the basolateral amygdala (BLA) to disrupt cocaine self-administration, responding during extinction sessions, and stimulus cued recovery of extinguished responding in rats. BLA and sham lesions following 7 days of 3 h limited access cocaine self-administration sessions (0.33 mg/infusion) on a fixed ratio 1 (FR1) schedule of reinforcement failed to alter cocaine intake and responding on 7 subsequent days of self-administration. This lack of effect suggests that the BLA is not paramount for the maintenance of cocaine's reinforcing effects. In contrast, BLA lesions, but not sham lesions, following 7 to 14 days of cocaine self-administration attenuated responding on a lever associated with cocaine infusions on the first day of extinction sessions and blocked the ability of drug associated stimuli to reinstate extinguished responding following 20 daily extinction sessions. However, lesions of the BLA did not attenuate stimulus cued recovery of responding following 43 days of withdrawal. These results are consistent with the hypothesis that the BLA is important for the conditioned incentive properties of reinforcers, but not primary reinforcement itself.

Different types of fear-conditioned behaviour mediated by separate nuclei within amygdala

The amygdala has long been thought to be involved in emotional behaviour, and its role in anxiety and conditioned fear has been highlighted. Individual amygdaloid nuclei have been shown to project to various cortical and subcortical regions implicated in affective processing. Here we show that some of these nuclei have separate roles in distinct mechanisms underlying conditioned fear responses. Rats with lesions of the central nucleus exhibited reduction in the suppression of behaviour elicited by a conditioned fear stimulus, but were simultaneously able to direct their actions to avoid further presentations of this aversive stimulus. In contrast, animals with lesions of the basolateral amygdala were unable to avoid the conditioned aversive stimulus by their choice behaviour, but exhibited normal conditioned suppression to this stimulus. This double dissociation demonstrates that distinct neural systems involving separate amygdaloid nuclei mediate different types of conditioned fear behaviour. We suggest that theories of amygdala function should take into account the roles of discrete amygdala subsystems in controlling different components of integrated emotional responses.

Electrolytic lesions of the fimbria/fornix, dorsal hippocampus, or entorhinal cortex produce anterograde deficits in contextual fear conditioning in rats

Recent data indicate that dorsal hippocampal (DH) lesions disrupt Pavlovian fear conditioning to contextual cues in rats. In the present study, we examined the effects of electrolytic lesions of the fimbria/fornix (FX) or entorhinal cortex (EC), the primary afferent projection systems to the DH, on contextual fear conditioning in rats. Conditioning consisted of the delivery of unsignaled footshocks in a novel observation chamber, and freezing served as the measure of conditional fear. Electrolytic lesions of the FX, DH, or EC made 1 week before training produced anterograde impairments in both immediate postshock freezing on the conditioning day and freezing during the context extinction test 24 h following training. The deficits in conditional freezing produced by FX, DH, and EC lesions were not statistically different, although the deficits in rats with FX or EC lesions tended to be more severe than those in rats with DH lesions. In addition to producing deficits in conditional freezing, FX, DH, or EC lesions produced a pronounced locomotor hyperactivity. Within the lesion and sham groups, however, locomotor activity was not significantly correlated with conditional freezing. These results indicate that contextual fear deficits in rats with hippocampal formation damage are equivalent following either FX, DH, or EC lesions. The relationship of freezing deficits and locomotor hyperactivity in rats with hippocampal formation lesions is discussed.

Central cholinergic systems and cognition

The organization and possible functions of basal forebrain and pontine cholinergic systems are reviewed. Whereas the basal forebrain cholinergic neuronal projections likely subserve a common electrophysiological function, e.g. to boost signal-to-noise ratios in cortical target areas, this function has different effects on psychological processes dependent upon the neural network operations within these various cortical domains. Evidence is presented that (a) the nucleus basalis-neocortical cholinergic system contributes greatly to visual attentional function, but not to mnemonic processes per se; (b) the septohippocampal projection is involved in the modulation of short-term spatial (working) memory processes, perhaps by prolonging the neural representation of external stimuli within the hippocampus; and (c) the diagonal band-cingulate cortex cholinergic projection impacts on the ability to utilize response rules through conditional discrimination. We also suggest that nucleus basalis-amygdala cholinergic projections have a role in the retention of affective conditioning while brainstem cholinergic projections to the thalamus and midbrain dopamine neurons affect basic arousal processes (e.g. sleep-wake cycle) and behavioral activation, respectively. The possibilities and limitations of therapeutic interventions with procholinergic drugs in patients with Alzheimer's disease and other neurodegenerative disorders in which basal forebrain cholinergic neurons degenerate are also discussed.

Ibotenic acid lesions of the amygdala basolateral complex or central nucleus differentially effect the response to reductions in reward

The present study examined the role of the amygdala in the acquisition and expression of the Crespi effect (Crespi, L.P., Quantitative variation in incentive and performance in the white rat, Am. J. Psychol., 55 (1942) 467-517), also known as successive negative behavioral contrast. In Experiment One rats with bilateral amygdala cannulae were trained to run a straight alley for either a large (ten pellet) or small (one pellet) food reward. After 8 days of training, half of the rats in each reward condition received vehicle or ibotenic acid administered bilaterally into the amygdala. After 4 days of recovery from the induction of the lesions, training resumed. On Day 12 of training, the reward for rats in the large reward condition was shifted to one pellet and this reward level was maintained for the next 4 days of training. Both the lesioned and unlesioned shifted rats exhibited increased latencies to the reduction. However, shifted lesioned rats displayed a more persistent increase in latencies than shifted unlesioned rats, exhibiting significantly longer latencies than those of unlesioned rats by Day 15. This finding suggests that large amygdala lesions may impair learning of the appetitive value of the small reward. Experiment Two examined the effects of discrete ibotenic acid lesions of either the central nucleus or basolateral/lateral complex of the amygdala. Lesions of the central nucleus produced results similar to those of Experiment One. However, in Experiment Two the performance of shifted unlesioned and lesioned groups diverged significantly 1 day earlier, on Day 14. In contrast, lesions of the basolateral/lateral complex reduced the duration of the contrast effect. Shifted lesioned rats exhibited significantly lower latencies than shifted unlesioned rats by the first postshift day, Day 13. This finding suggests that the basolateral/lateral complex may be involved in learning about, or expressing the response to, the aversiveness of reward reduction.