Neural topography and chronology of memory consolidation: a review of functional inactivation findings

The development of psychopathy

The current review focuses on the construct of psychopathy, conceptualized as a clinical entity that is fundamentally distinct from a heterogeneous collection of syndromes encompassed by the term 'conduct disorder'. We will provide an account of the development of psychopathy at multiple levels: ultimate causal (the genetic or social primary cause), molecular, neural, cognitive and behavioral. The following main claims will be made: (1) that there is a stronger genetic as opposed to social ultimate cause to this disorder. The types of social causes proposed (e.g., childhood sexual/physical abuse) should elevate emotional responsiveness, not lead to the specific form of reduced responsiveness seen in psychopathy; (2) The genetic influence leads to the emotional dysfunction that is the core of psychopathy; (3) The genetic influence at the molecular level remains unknown. However, it appears to impact the functional integrity of the amygdala and orbital/ventrolateral frontal cortex (and possibly additional systems); (4) Disruption within these two neural systems leads to impairment in the ability to form stimulus-reinforcement associations and to alter stimulus-response associations as a function of contingency change. These impairments disrupt the impact of standard socialization techniques and increase the risk for frustration-induced reactive aggression respectively.

The role of the amygdala and rostral anterior cingulate in encoding expected outcomes during learning

Successful passive avoidance learning is thought to require the use of learned stimulus-reinforcement associations to guide decision making [Baxter, M.G., Murray, E.A., 2002. The amygdala and reward. Nature Reviews. Neuroscience 3, 563-573]. The current experiment investigated the neural correlates of successful passive avoidance learning in 19 healthy adults. Behaviorally, subjects showed a distinct pattern of performance: early indiscriminate responding to stimuli (pre-criterion performance), followed by relatively rapid learning before a plateau of successful performance (post-criterion performance). Neural responses to post-criterion correct responses were compared with neural responses to both incorrect responses and pre-criterion correct responses. Post-criterion correct responding was associated with increased activation in regions including rostral anterior cingulate, insula, caudate, hippocampal regions, and the amygdala.

Reversible inactivation of the dorsal hippocampus by tetrodotoxin or lidocaine: A comparative study on cerebral functional activity and motor coordination in the rat

Reversible inactivation of the hippocampus by lidocaine or tetrodotoxin is used to investigate implications of this structure in memory processes. Crucial points related to such inactivation are the temporal and spatial extents of the blockade. We compared effects of intrahippocampal infusions of commonly-used doses of lidocaine (5 or 10 mug) or tetrodotoxin (5 or 10 ng) in rats at two post-infusion delays (5 or 30 min), using 2-deoxyglucose autoradiography to visualize local cerebral glucose metabolism, and beam-walking performance to assess motor coordination. In addition, memory retrieval was evaluated in a water maze after bilateral infusions of 10 mug lidocaine. A unilateral tetrodotoxin infusion induced dose- and time-dependent reductions of 2-deoxyglucose uptake in the vicinity of the infusion site (dorsal hippocampus: -29% to -67%) and in other ipsi- and contralateral brain regions (ventral hippocampus, lateral thalamus, cortical regions). The maximal effect was at 10 ng, at the delay of 30 min between the tetrodotoxin infusion and the 2-deoxyglucose injection. Uni- and bilateral infusions of tetrodotoxin induced dramatic motor coordination deficits. Conversely, lidocaine reduced 2-deoxyglucose uptake (-19%) in the dorsal hippocampus only at 10 mug, with weak extrahippocampal effects. Whether infused uni- or bilaterally and regardless of the dose, lidocaine did not alter motor coordination. When infused bilaterally, however, 10 microg of lidocaine impaired short-term retrieval of spatial information in a water maze. Because lidocaine i) induced a weak though significant functional blockade mainly restricted to the infusion site, ii) had no consequences on motor coordination and, nevertheless iii) altered short-term spatial memory retrieval, we conclude that acute intrahippocampal infusions of lidocaine may offer some advantages over tetrodotoxin at the doses used herein.

Involvement of neurotransmitters in urocortin-induced passive avoidance learning in mice

The action of urocortin on one-way passive avoidance learning was tested in mice. Urocortin was administered into the lateral brain ventricle and the latency of the passive avoidance response was measured 24 h later. For study of the roles of various neurotransmitters in mediating the action of urocortin on the consolidation of memory, the animals were pretreated with different receptor antagonists. Urocortin facilitated the acquisition, consolidation and also retrieval of the passive avoidance response. The following receptor antagonists blocked the action of urocortin on consolidation: haloperidol, atropine, phenoxybenzamine, bicuculline, the CRF antagonist CRF9-41 and methysergide. Propranolol attenuated, but did not fully block the action of urocortin, while naloxone and nitro-L-arginine were ineffective. The results obtained demonstrate that urocortin is able to improve learning and memory and also retrieval processes in a passive avoidance learning in mice. D2, muscarinic cholinergic, alfa-adrenergic, CRF, serotonergic (5HT 1-2), GABA B receptors are involved in the consolidation of the passive avoidance response.

Responding to the emotions of others: dissociating forms of empathy through the study of typical and psychiatric populations

Empathy is a lay term that is becoming increasingly viewed as a unitary function within the field of cognitive neuroscience. In this paper, a selective review of the empathy literature is provided. It is argued from this literature that empathy is not a unitary system but rather a loose collection of partially dissociable neurocognitive systems. In particular, three main divisions can be made: cognitive empathy (or Theory of Mind), motor empathy, and emotional empathy. The two main psychiatric disorders associated with empathic dysfunction are considered: autism and psychopathy. It is argued that individuals with autism show difficulties with cognitive and motor empathy but less clear difficulties with respect to emotional empathy. In contrast, individuals with psychopathy show clear difficulties with a specific form of emotional empathy but no indications of impairment with cognitive and motor empathy.

Mechanisms of memory stabilization: are consolidation and reconsolidation similar or distinct processes?

Consolidation of new memories depends on a crucial phase of protein synthesis. It is widely held that, once consolidated, memories are stable and resilient to disruption. However, established memories become labile when recalled and require another phase of protein synthesis to be maintained. Therefore, it has been proposed that when a memory is reactivated it must undergo additional consolidation (reconsolidation) to persist. To determine whether reconsolidation recapitulates consolidation, in the past few years several groups have investigated whether the same molecules and pathways mediate the formation of a memory and its maintenance after reactivation. At first glance, the results appear conflicting: although both processes appear to engage the same molecules and mechanisms, brain areas involved in consolidation after initial training are not required for reconsolidation. In addition, the formation of a memory and its maintenance after reactivation seem to have distinctive temporal molecular requirements. This review concludes with a working model that could explain the apparent controversy of memory vulnerability after reactivation.

Learning and memory

Learning and memory processes are thought to underlie a variety of human psychiatric disorders, including generalised anxiety disorder and post-traumatic stress disorder. Basic research performed in laboratory animals may help to elucidate the aetiology of the respective diseases. This chapter gives a short introduction into theoretical and practical aspects of animal experiments aimed at investigating acquisition, consolidation and extinction of aversive memories. It describes the behavioural paradigms most commonly used as well as neuroanatomical, cellular and molecular correlates of aversive memories. Finally, it discusses clinical implications of the results obtained in animal experiments in respect to the development of novel pharmacotherapeutic strategies for the treatment of human patients.

Expression of the 5-HT receptors in rat brain during memory consolidation

Serotonin (5-hydroxytryptamine, 5-HT) system displays more than 14 receptors subtypes on brain areas involved in learning and memory processes, and pharmacological manipulation of specific receptors selectively affects memory formation. In order to begin the search of 5-HT receptors expression during memory formation, in this work, we aimed to determine, by autoradiography (using 3H 5-HT as ligand, 2 nM, specific activity 123 Ci/mmol), 5-HT receptors (5-HTR) expression in passive (untrained) and autoshaping trained (3 sessions) adult (3 months) and old (9 months) male rats. Thus, trained adult rats had better retention than old animals. Raphe nuclei of adult and old trained rats expressed less receptors on medial and dorsal, respectively. Hippocampal CA1 area and dentate gyrus of adult trained rats expressed less 5-HTR, while dentate gyrus of old increased them. Basomedial amygdaloid nucleus in old trained rats expressed more 5-HTR; while in the basolateral amygdaloid nucleus they were augmented in both groups. Training decreased or did not change 5-HTR in caudate-putamen of adult or old animals. The above profile of 5-HTR expression is consistent with previous reports, and suggests that memory formation and aging modulates 5-HTR expression in brain areas relevant to memory systems.

Histopathological and behavioral characterization of a novel model of cardiac arrest and cardiopulmonary resuscitation in mice

Cardiac arrest is associated with high mortality and poor neurological outcome. We characterized functional and histological outcome in a novel mouse model of cardiac arrest and cardiopulmonary resuscitation (CPR) in order to study neuroprotective mechanisms. Cardiac arrest was induced in male C57Bl/6 and 129SVEV mice by i.v. injection of KCl. After 10 min cardiac standstill, CPR was initiated by administration of epinephrine, ventilation with 100% oxygen and chest compressions. Twenty-four hours before and 3 or 7 days after CPR, mice were subjected to behavioral testing using a passive avoidance task, locomotor activity in an open field, and spontaneous alternation in a T-maze. Hippocampal and caudoputamen injury was quantified 3 or 7 days after CPR. At both time points, caudoputamen injury was worse in 129SVEV mice. Post-ischemic mice of both strains showed a reduced number of correct choices in the T-maze up to 7 days after CPR, and were temporarily impaired in learning the passive avoidance task with a retention deficit on day 3 but not on day 7. Locomotor activity showed strain differences with C57Bl/6 mice being more active, but little ischemia-related effects. A dissociation between functional and histological outcome was found emphasizing the importance of combining both outcome measures for evaluation of neuroprotective strategies.

The roles of orbital frontal cortex in the modulation of antisocial behavior

This article considers potential roles of orbital frontal cortex in the modulation of antisocial behavior. Two forms of aggression are distinguished: reactive aggression elicited in response to frustration/threat and goal directed, instrumental aggression. It is suggested that orbital frontal cortex is directly involved in the modulation of reactive aggression. It is argued that orbital frontal cortex does not "inhibit" reactive aggression but rather may both increase or decrease its probability as a function of social cues present in the environment. Early dysfunction in this function of orbital frontal cortex may be linked to the development of Borderline Personality Disorder. Instrumental aggression is linked to a fundamental failure in moral socialization. However, the available data suggest that the amygdala, but not orbital frontal cortex, is required for functions such as aversive conditioning and passive avoidance learning that are necessary for moral socialization. Psychopathic individuals who present with significant instrumental aggression, are impaired in aversive conditioning and passive avoidance learning and show evidence of amygdala dysfunction. Orbital frontal cortex and the amygdala are involved in response reversal where instrumental responses must be reversed following contingency change. Impairments in response reversal are also seen in psychopathic individuals. However, it remains unclear whether impairment in response reversal per se is associated with antisocial behavior.

Reversible inactivation of the dorsal vagal complex blocks lipopolysaccharide-induced social withdrawal and c-Fos expression in central autonomic nuclei

Peripheral administration of lipopolysaccharide (LPS), a potent activator of the immune system, induces symptoms of behavioral depression, such as social withdrawal, concommitant with increases in c-Fos expression in central autonomic network nuclei. Previous studies implicated vagal visceral sensory nerves in transduction of immune-related signals relevant to for the induction of social withdrawal, a symptom of behavioral depression. Vagal sensory nerves terminate in the dorsal vagal complex (DVC) of the brainstem, a region that functions to integrate visceral signals and may also play a role in modulating arousal and affect. The objective of the current study was to determine whether the DVC contributes to immunosensory pathways driving symptoms of social withdrawal associated with LPS-induced behavioral depression, using a reversible lesion technique to temporarily inactivate the DVC. To assess the effects of DVC inactivation on LPS-induced social withdrawal and the subsequent changes in brain activation, we used behavioral assessment of social withdrawal, and analyzed c-Fos expression, a marker of neuronal activation, in the central nucleus of the amygdala (CEA), bed nucleus of the stria terminalis (BST), hypothalamic paraventricular nucleus (PVN), and ventromendial preoptic area (VMPO). Two hours following intraperitoneal LPS injection, there was a significant increase in c-Fos immunoreactivity in forebrain regions in animals treated with LPS. DVC inactivation completely blocked LPS-induced social withdrawal and dramatically reduced LPS-induced Fos expression in all four forebrain regions assessed. Collectively, these findings support the idea that the DVC acts as an immune-behavior interface between the peripheral stimuli and brain areas involved in modulating social behavior.

Inactivation of Dorsolateral Striatum Impairs Acquisition of Response Learning in Cue-Deficient, but Not Cue-Available, Conditions

Rats received bilateral injections of lidocaine or artificial cerebrospinal fluid (aCSF) into the doisolateral striatum 6 min prior to training in either a plus- or T-shaped maze under cue-poor or cue-available conditions. Lidocaine injections significantly impaired acquisition in the cue-poor environments, but not in the cue-available environments. In addition, aCSF control rats trained in a plus-maze in a cue-poor environment reached criterion much more rapidly than did rats trained in a cue-available environment. These findings suggest that cue availability can permit acquisition of response learning in a manner that is not dependent on activity of the striatum. However, in a cue-poor environment, alternate strategies may be less readily available, revealing more efficient striatal involvement in response learning.

A pharmacological analysis of an associative learning task: 5-HT(1) to 5-HT(7) receptor subtypes function on a pavlovian/instrumental autoshaped memory

Recent studies using both invertebrates and mammals have revealed that endogenous serotonin (5-hydroxytryptamine [5-HT]) modulates plasticity processes, including learning and memory. However, little is currently known about the mechanisms, loci, or time window of the actions of 5-HT. The aim of this review is to discuss some recent results on the effects of systemic administration of selective agonists and antagonists of 5-HT on associative learning in a Pavlovian/instrumental autoshaping (P/I-A) task in rats. The results indicate that pharmacological manipulation of 5-HT1-7 receptors or 5-HT reuptake sites might modulate memory consolidation, which is consistent with the emerging notion that 5-HT plays a key role in memory formation.

Intravenous versus intrastriatal cord blood administration in a rodent model of stroke

Human umbilical cord blood (hUCB) is a rich source of hematopoietic stem cells that have been used to reconstitute immune cells and blood lineages. Cells from another hematopoietic source, bone marrow, have been found to differentiate into neural cells and are effective in the treatment of stroke. In this study, we administered hUCB cells intravenously into the femoral vein or directly into the striatum and assessed which route of cell administration produced the greatest behavioral recovery in rats with permanent middle cerebral artery occlusion (MCAO). All animals were immunosuppressed with cyclosporine (CSA). When spontaneous activity was measured using the Digiscan automated system, it was found to be significantly less when hUCB was transplanted 24 hr after stroke compared with nontransplanted, stroked animals (P < 0.01). Furthermore, behavioral recovery was similar with both striatal and femoral hUCB delivery. This is in contrast to the step test, in which significant improvements were found only after femoral delivery of the hUCB cells. In the passive avoidance test, transplanted animals learned the task faster than nontransplanted animals (P < 0.05). Together, these results suggest that hUCB transplantation may be an effective treatment for brain injuries, such as stroke, or neurodegenerative disorders. In addition, intravenous delivery may be more effective than striatal delivery in producing long-term functional benefits to the stroked animal.

Drug addiction, relapse, and the amygdala

Evidence has extensively implicated the amygdala in the associative learning process for appetitive reinforcers. Recent interest has focused on the role of the amygdala in the learned associations that occur during the process of drug addiction and relapse. Using an animal model of relapse after chronic cocaine self-administration, we found that rats reinstate extinguished lever responding for conditioned stimuli (tone + light) previously paired with cocaine or heroin ("conditioned-cued reinstatement"). The basolateral amygdala (BLA) complex plays a critical role in this behavior, because permanent lesions or reversible pharmacologic inactivation of the BLA attenuates conditioned-cued reinstatement without affecting cocaine self-administration or cocaine-primed reinstatement. Conditioned-cued reinstatement appears to be mediated in part by dopamine inputs to the BLA, as intra-BLA infusion of a dopamine D1 receptor antagonist blocks reinstatement, whereas intra-BLA infusion of amphetamine potentiates reinstatement. Furthermore, the BLA is also necessary for acquisition of associative learning with cocaine-paired stimuli. Disruption of neural activity within the BLA by sodium channel blockade or muscarinic receptor blockade just before acquisition of stimulus-cocaine associations blocks the ability of conditioned stimuli to elicit conditioned-cued reinstatement after extinction. Together, these results reveal the importance of the amygdala as part of a corticolimbic circuit mediating both the acquisition and the expression of conditioning that plays a critical role in relapse to drug-seeking behavior.

Modulation of memory consolidation for olfactory learning by reversible inactivation of the basolateral amygdala

The role of the basolateral amygdala (BLA) in the consolidation of an association between an olfactory stimulus and footshock was investigated with a reversible lesion technique of post-training intra-BLA infusions of tetrodotoxin. Rats receiving tetrodotoxin infusions following paired odor-shock presentations spent more time near the odor, and reacted differently on contact with the odor when tested 24 hr after training, than did rats receiving paired presentations and saline infusions, but they did not differ from rats receiving unpaired presentations and saline infusions. The results indicate that the BLA plays a similar role in influencing consolidation of olfactory-based memory as it does for memory based on other modalities. Thus, these findings strengthen the view that the BLA plays a general role in modulation of memory storage for emotionally arousing events.

Temporally graded requirement for protein synthesis following memory reactivation

Learning of new information is transformed into long-lasting memory through a process known as consolidation, which requires protein synthesis. Classical theory held that once consolidated, memory was insensitive to disruption. However, old memories that are insensitive to protein synthesis inhibitors can become vulnerable if they are recalled (reactivated). These findings led to a new hypothesis that when an old memory is reactivated, it again becomes labile and, similar to a newly formed memory, requires a process of reconsolidation in order to be maintained. Here, we show that the requirement for protein synthesis of a reactivated memory is evident only when the memory is recent. In fact, memory vulnerability decreases as the time between the original training and the recall increases.

Urocortin shares the memory modulating effects of corticotropin-releasing factor (CRF): mediation by CRF1 receptors

Intracerebroventricular (i.c.v.) administration of corticotropin-releasing factor (CRF) biphasically affects performance in tests of learning and memory. In the present study, we used CRF, urocortin (Ucn), a recently cloned CRF homologue, and CRF receptor antagonists, to determine which CRF receptor subtype(s) mediate the memory modulating effects of CRF receptor agonists in male Wistar rats. Under difficult learning conditions (massed trials), i.c.v. pretreatment with CRF or Ucn facilitated the acquisition of spatial navigation in the Morris water maze in a non-dose-dependent fashion (optimal doses of 0.1 and 0.03 microg, respectively). Under less difficult learning conditions (spaced trials), both peptides impaired water maze performance. In addition, with i.c.v. posttraining treatment, the peptides were equipotent (1.0 microg) in facilitating the consolidation of passive avoidance learning. The performance-enhancing effects of Ucn in both water maze and passive avoidance paradigms were reversed by i.c.v. pretreatment with D-Phe CRF(12-41) (2.5, 5 microg), a broad CRF(1)/CRF(2) receptor antagonist, or antalarmin (10 microg), a potent, nonpeptide, CRF(1) selective receptor antagonist. Thus, Ucn shares CRF's memory-modulating effects, and these effects appear to be mediated via the CRF(1) receptor. These findings are consistent with the hypothesis that CRF receptor agonists affect performance in tests of learning and memory by increasing arousal.

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.

Cerebellar role in fear-conditioning consolidation

Some cerebellar structures are known to be involved in the memorization of several conditioned responses. The role of the interpositus nucleus (IN) and the vermis (VE) in fear-conditioning consolidation was investigated by means of a combined behavioral and neurophysiological technique. The IN and VE were subjected to fully reversible tetrodotoxin (TTX) inactivation during consolidation in adult male Wistar rats that underwent acoustic conditioned stimulus (CS) and context fear training. TTX was injected in different groups of rats at increasing intervals after the acquisition session. Memory was assessed as conditioned freezing duration measured during retention testing, always performed 72 and 96 h after the stereotaxic TTX administration. This schedule ensures that there is no interference with normal cerebellar function during either the acquisition or the retrieval phase so that any amnesic effect may be due only to consolidation disruption. Our results show that IN functional integrity is necessary for acoustic CS fear response memory formation up to the 96-h after-acquisition delay. VE functional integrity was shown to be necessary for memory formation of both context (up to the 96-h after-acquisition delay) and acoustic CS (up to the 192-h after-acquisition delay) fear responses. The present findings help to elucidate the role of the cerebellum in memory consolidation and better define the neural circuits involved in fear memories.

Effects of middle cerebral artery occlusion on spontaneous activity and cognitive function in rats

The middle cerebral artery occlusion (MCAO) in the rat is a commonly used model to evaluate new therapeutic strategies for the treatment of ischemic stroke. However, many such studies rely on short-term neurological examination and infarct volume as endpoint measures while neglecting more long-term functional assessments. In this study, we examined whether there were changes in passive avoidance behavior, spontaneous behavioral patterns across the light-dark cycle, and motor coordination as measured on the rotorod test 1 month after Sprague-Dawley rats had undergone MCAO. Compared to age-matched controls, fewer animals in the MCAO group remained on the platform during the passive avoidance retention test (p < .03). Significant differences between the groups were observed in the spontaneous activity during the initial portions of both the light and dark testing periods, when the test situation was new (p < .01 to .05, depending on the variable examined). Any differences on the rotorod test failed to gain statistical significance. These results suggest that at least the passive avoidance test and measures of spontaneous activity are sensitive to ischemia-induced damage over a more prolonged survival period and therefore may be appropriate measures for long-term effectiveness of new treatments.

Neural substrates of conditioned-cued relapse to drug-seeking behavior

Relapse to drug use following abstinence is a significant impediment in the long-term treatment of drug abuse and dependence. Conditioned stimuli are believed to be critically involved in activating drug craving and relapse to compulsive drug-taking behavior. Studies in humans and animal models have recently begun to identify the fundamental neural circuitry that mediates relapse following withdrawal from chronic drug self-administration. The current review summarizes key findings in this area that have converged on the amygdalar complex and regions of the frontal lobe as critical structures in conditioned-cued relapse. It is proposed that the amygdala is a key regulator of discrete stimulus-reinforcer associations, while the anterior cingulate and orbitofrontal cortex are critical regulators of relapse evoked by conditioned stimuli that predict drug availability. This corticolimbic circuitry may form the neural basis of multiple long-term conditioned associations produced by a variety of drugs of abuse ranging from psychostimulants to opiates. Future studies aimed at discerning the functional roles of these pathways will provide critical direction for the development of treatments for the prevention of relapse.

Time-dependent inhibition of hippocampal LTP in vitro following contextual fear conditioning in the rat

The effects of contextual fear-learning on hippocampal synaptic excitability were investigated by means of high frequency tetanic stimulation (HFS) in rat brain slices (hippocampal CA1 region), prepared at different intervals (immediately, 24 h or 7 days) after a one-trial contextual fear conditioning paradigm session. In the latter, rats that had previously received aversive electrical footshocks in the experimental apparatus exhibited freezing (the conditioned response) when placed again in the same apparatus (retrieval test). It was shown that contextual fear-learning affects the hippocampal synaptic response. In fact, the HFS produced a decrease in the amplitude of short-term (STP) and long-term potentiation (LTP) when compared to control "naïve" subject values. This decrease in STP amplitude could be observed only in slices prepared immediately after the training session. A decrease in the amplitude of long-term but not short-term potentiation was also observed at 24 h. At 7 days, no decreases in amplitude were observed. These modifications may be thought of as specifically associated with the learning process as they were not recorded in brain preparations from "shock-only" rats (i.e. those that received the same number of aversive stimuli of equal intensity as the conditioned group but with the shocks compressed temporally so that the shocked subjects could not associate nociceptive stimulation and surroundings - no conditioned freezing during retention testing). In "exploration" preparations (brain slices from rats having only freely explored the experimental apparatus without receiving any adverse stimulation) a decrease in LTP amplitude was recorded only immediately after the training session, and STP was never modified. The synaptic response modifications do not appear to be due to presynaptic events, as they are not associated with paired-pulse facilitation curve (PPF) modifications. The present results show that contextual fear conditioning and exploration of a novel environment (i) reduce the ability to induce synaptic plasticity; (ii) differentially influence STP and LTP and that (iii) the persistence of synaptic modifications depends on an animal's prior experience.

Re-evaluation of the spatial memory deficits induced by hippocampal short lasting inactivation reveals the need for cortical co-operation

Evidence has accumulated that the rat hippocampus plays a central role in spatial memory. In complement to lesion studies, reversible lidocaïne-induced inactivations have been used to investigate the time-course of the memory processes mediated by the hippocampus. A number of studies suggest that, in some conditions, the hippocampus is not necessary for online acquisition of spatial information. To test this hypothesis, we examined the effects of bilateral lidocaïne-induced inactivations of the dorsal hippocampus in the acquisition of new spatial information. After initial learning of a place navigation task in the water maze, rats were tested for acquisition of a new platform location and received injections of lidocaïne in the hippocampus prior to each daily four-trial block. The training blocks were separated by a 24-h period allowing the hippocampus to recover from inactivation. The results show that lidocaïne-injected rats were able to learn the new platform location like controls. Inactivations, however, was found to induce a within-block learning impairment. This suggests that the hippocampus can perform off-line processing and that another structure is able to handle spatial information during hippocampal inactivations. Parietal-lesioned rats that received an injection of lidocaïne were still able to learn the new platform location suggesting that the parietal cortex does not sustain this role. Overall, our results suggest that the hippocampus is not necessary for all stages of memory formation and co-operates with other brain, possibly cortical, structures which remain to be determined.

Pituitary adenylate cyclase-activating polypeptide hormone (PACAP) at very low dosages improves memory in the rat

To ascertain whether very low dosages of pituitary adenylate cyclase-activating polypeptide (PACAP) influence learning in mammals, immediately after the acquisition trial of a passive avoidance response (PAR) paradigm, PACAP-38 was administered intracerebroventricularly at increasing dosages (0, 0.02, 0.2, 2, 20, and 200 ng in 10 microl saline) to different groups of rats. The mnemonic effects were measured by means of retention testing 48 and 96 h later. At intermediate PACAP-38 concentrations there was a significant mnemonic improvement of the PAR. The maximal effect was observed after the 0.2-ng PACAP-38 administration (longer step-through latencies). There was a lesser effect at the subsequent higher concentration, 2 ng. Higher dosages had no effects. It is concluded that PACAP-38 acts as an enhancer of mammalian mnemonic processes even at very low dosages. The positive effect follows an inverted U-shaped dose-response curve. The results may be of interest for the therapy of some neuropathological conditions.

Persistent and delayed behavioral changes after nicotine treatment in adolescent rats

Despite the increasing use of tobacco by adolescents, few animal studies have addressed the neurobehavioral consequences of nicotine exposure during this period. We administered nicotine to adolescent rats via continuous infusion on postnatal days (PN) 30 through 47.5, using a dosage regimen that maintains plasma levels similar to those found in smokers or in users of the transdermal nicotine patch. Behavior in a novel open field and learning a passive avoidance task were assessed during nicotine treatment and for 2 weeks post-treatment. On PN44, during nicotine exposure, female rats showed decreased grooming, an effect not seen in males; this effect is opposite to the effects of nicotine in adult rats. Two weeks after cessation of nicotine administration, females showed deficits in locomotor activity and rearing, whereas males again were unaffected; the behavioral deficits appeared at the same age at which gender-selective brain cell damage emerges. In contrast, nicotine exposure enhanced passive avoidance, with the effect intensifying and persisting throughout the post-treatment period. These results reinforce the concept that developmental vulnerability to nicotine extends into adolescence, with patterns of drug effects different from those in earlier or later periods. The correlation of neurochemical with behavioral effects strengthens the connection between adolescent nicotine exposure and persistent functional changes that may influence drug habituation, learning and memory.

Different hippocampal molecular requirements for short- and long-term retrieval of one-trial avoidance learning

Rats were trained in one-trial step-down inhibitory avoidance and tested either 3 h or 31 days later. Ten minutes prior to the retention test, through indwelling cannulae placed in the CA1 region of the dorsal hippocampus, they received 0.5 microl infusions of: saline, a vehicle (2% dimethylsulfoxide in saline), the glutamate NMDA receptor blocker, aminophosphonopentanoic acid (AP5) (5.0 microg), the AMPA/kainate receptor blocker, cyanonitroquinoxaline dione (CNQX) (0.25 or 1.25 microg), the metabotropic receptor antagonist, methylcarboxyphenylglycine (MCPG) (0.5 or 2.5 microg), the inhibitor of calcium/calmodulin-dependent protein kinase II (KN62) (3.5 microg), the inhibitor of cAMP-dependent protein kinase (PKA), Rp-cAMPs (0.1 or 0.5 microg), the stimulant of the same enzyme, Sp-cAMPs (0.1 or 0.5 microg), or the inhibitor of the mitogen-activated protein kinase (MAPK) kinase, PD098059 (10 or 50 microM). CNQX, KN62 and PD098059 were dissolved in the vehicle; the other drugs were dissolved in saline. All these drugs, at the same doses, had been previously found to affect short- and long-term memory formation of this task. Retrieval measured 3 h after training (short-term memory) was blocked by CNQX and MCPG, and was unaffected by all the other drugs. In contrast, retrieval measured at 31 days was blocked by MCPG, Rp-cAMPs and PD098059, enhanced by Sp-cAMPs, and unaffected by CNQX, AP5 or KN62. The results indicate that, in CA1, glutamate metabotropic receptors are necessary for the retrieval of both short- and long-term memory; AMPA/kainate receptors are necessary for short-term but not long-term memory retrieval, and NMDA receptors are uninvolved in retrieval. Both the PKA and MAPK signalling pathways are required for the retrieval of long-term but not short-term memory.

Auditory thalamus, dorsal hippocampus, basolateral amygdala, and perirhinal cortex role in the consolidation of conditioned freezing to context and to acoustic conditioned stimulus in the rat

On the basis of previous experimental evidence, it is known that the auditory thalamus (AT), the dorsal hippocampus (DH), the basolateral amygdala (BLA), and the perirhinal cortex (PC) are involved in the mnemonic processing of conditioned freezing. In particular, BLA and PC appear to be involved both in conditioned stimulus (CS) and context conditioned freezing. Through AT, the auditory CS is sent to other sites, whereas DH is involved in context conditioning. Nevertheless, the existing evidence does not make it possible to assess AT, DH, BLA, and PC involvement during the consolidation phase of conditioned freezing. To address this question, fully reversible tetrodotoxin (TTX) inactivation was performed on adult male Wistar rats having undergone CS and context fear training. Anesthetized animals were injected stereotaxically with TTX (either 5 or 10 ng in 0.5 or 1.0 microliter of saline, according to site dimensions) at increasing post-acquisition delays. Context and CS freezing durations were measured during retention testing, always performed 48 and 72 hr after TTX administration. The results showed that AT inactivation does not disrupt consolidation of either contextual or auditory fear memories. In contrast, inactivation of the other three structures disrupted consolidation. For the DH, this disruption was specific to contextual cues and only occurred when inactivation was performed early (up to 1.5 hr) after training. The BLA and PC were shown to be involved in the consolidation of both contextual and auditory fear. Their involvement persisted for longer periods of time (2d for BLA and 8 d for PC). These findings provide information to build a temporal profile for the post-training processing of fear memories in structures known to be important for this form of learning. The results are discussed in relation to previous studies on conditioned freezing and other aversive conditioned response neural correlates.

Reversible neural inactivation reveals hippocampal participation in several memory processes

Studies of patients and animals with brain lesions have implicated the hippocampal formation in spatial, declarative/relational and episodic types of memory. These and other types of memory consist of a series of interdependent but potentially dissociable memory processes-encoding, storage, consolidation and retrieval. To identify whether hippocampal activity contributes to these processes independently, we used a novel method of inactivating synaptic transmission using a water-soluble antagonist of AMPA/kainate glutamate receptors. Once calibrated using electrophysiological and two-deoxyglucose techniques in vivo, drug or vehicle was infused chronically or acutely into the dorsal hippocampus of rats at appropriate times during or after training in a water maze. Our findings indicate that hippocampal neural activity is necessary for both encoding and retrieval of spatial memory and for either trace consolidation or long-term storage.