Idazoxan, an alpha-2 antagonist, facilitates memory retrieval in the rat

Effects of intracerebral noradrenaline on cognitive decline associated with the loss of occlusal support

PURPOSE

The lack of occlusal support reportedly reduces cognitive function; however, the underlying mechanisms remain unclear. The locus coeruleus, which is located adjacent to the trigeminal mesencephalic nucleus, secretes noradrenaline throughout the brain. In this study, we evaluated the effects of noradrenaline in the hippocampus and cerebral cortex on cognitive decline following tooth extraction in rats.

METHODS

We performed passive avoidance experiments on male Wistar rats with extracted maxillary molars and determined the neuron density in the locus coeruleus and trigeminal mesencephalic nucleus using immunostaining and Nissl staining, respectively. We also assessed noradrenaline concentrations in the hippocampus and cerebral cortex using enzyme-linked immunosorbent assay.

RESULTS

In the passive avoidance experiment, the latency in the bright compartment was significantly shorter (P < 0.05) in the extraction group than in the control group. The numbers of cells in the locus coeruleus and trigeminal nucleus were significantly lower (P < 0.05) in the extraction group compared to those in the control group. The noradrenaline levels in the hippocampus and cerebral cortex were also significantly lower (P < 0.05) in the extraction group than those in the control group.

CONCLUSIONS

The lack of occlusal support associated with tooth extraction reduces the number of cells in the trigeminal mesencephalic nucleus and locus coeruleus, which may reduce the supply of noradrenaline to the cerebral cortex and hippocampus, leading to a decline in cognitive function.

Contextual memory engrams, and the neuromodulatory influence of the locus coeruleus

Here, we review the basis of contextual memory at a conceptual and cellular level. We begin with an overview of the philosophical foundations of traversing space, followed by theories covering the material bases of contextual representations in the hippocampus (engrams), exploring functional characteristics of the cells and subfields within. Next, we explore various methodological approaches for investigating contextual memory engrams, emphasizing plasticity mechanisms. This leads us to discuss the role of neuromodulatory inputs in governing these dynamic changes. We then outline a recent hypothesis involving noradrenergic and dopaminergic projections from the locus coeruleus (LC) to different subregions of the hippocampus, in sculpting contextual representations, giving a brief description of the neuroanatomical and physiological properties of the LC. Finally, we examine how activity in the LC influences contextual memory processes through synaptic plasticity mechanisms to alter hippocampal engrams. Overall, we find that phasic activation of the LC plays an important role in promoting new learning and altering mnemonic processes at the behavioral and cellular level through the neuromodulatory influence of NE/DA in the hippocampus. These findings may provide insight into mechanisms of hippocampal remapping and memory updating, memory processes that are potentially dysregulated in certain psychiatric and neurodegenerative disorders.

Noradrenergic projections regulate the acquisition of classically conditioned eyelid responses in wild-type and are impaired in kreisler mice

During embryonic development, heterozygous mutant kreisler mice undergo ectopic expression of the Hoxa3 gene in the rostral hindbrain, affecting the opioid and noradrenergic systems. In this model, we have investigated behavioral and cognitive processes in their adulthood. We confirmed that pontine and locus coeruleus neuronal projections are impaired, by using startle and pain tests and by analyzing immunohistochemical localization of tyrosine hydroxylase. Our results showed that, even if kreisler mice are able to generate eyelid reflex responses, there are differences with wild-types in the first component of the response (R1), modulated by the noradrenergic system. The acquisition of conditioned motor responses is impaired in kreisler mice when using the trace but not the delay paradigm, suggesting a functional impairment in the hippocampus, subsequently confirmed by reduced quantification of alpha2a receptor mRNA expression in this area but not in the cerebellum. Moreover, we demonstrate the involvement of adrenergic projection in eyelid classical conditioning, as clonidine prevents the appearance of eyelid conditioned responses in wild-type mice. In addition, hippocampal motor learning ability was restored in kreisler mice by administration of adrenergic antagonist drugs, and a synergistic effect was observed following simultaneous administration of idazoxan and naloxone.

The role of hippocampus in memory reactivation: an implication for a therapeutic target against opioid use disorder

Purpose of the review

The abuse of opioids induces many terrible problems in human health and social stability. For opioid-dependent individuals, withdrawal memory can be reactivated by context, which is then associated with extremely unpleasant physical and emotional feelings during opioid withdrawal. The reactivation of withdrawal memory is considered one of the most important reasons for opioid relapse, and it also allows for memory modulation based on the reconsolidation phenomenon. However, studies exploring withdrawal memory modulation during the reconsolidation window are lacking. By summarizing the previous findings about the reactivation of negative emotional memories, we are going to suggest potential neural regions and systems for modulating opioid withdrawal memory.

Recent findings

Here, we first present the role of memory reactivation in its modification, discuss how the hippocampus participates in memory reactivation, and discuss the importance of noradrenergic signaling in the hippocampus for memory reactivation. Then, we review the engagement of other limbic regions receiving noradrenergic signaling in memory reactivation. We suggest that noradrenergic signaling targeting hippocampus neurons might play a potential role in strengthening the disruptive effect of withdrawal memory extinction by facilitating the degree of memory reactivation.

Summary

This review will contribute to a better understanding of the mechanisms underlying reactivation-dependent memory malleability and will provide new therapeutic avenues for treating opioid use disorders.

Locus coeruleus in memory formation and Alzheimer's disease

Catecholamine neurons of the locus coeruleus (LC) in the dorsal pontine tegmentum innervate the entire neuroaxis, with signaling actions implicated in the regulation of attention, arousal, sleep-wake cycle, learning, memory, anxiety, pain, mood, and brain metabolism. The co-release of norepinephrine (NE) and dopamine (DA) from LC terminals in the hippocampus plays a role in all stages of hippocampal-memory processing. This catecholaminergic regulation modulates the encoding, consolidation, retrieval, and reversal of hippocampus-based memory. LC neurons in awake animals have two distinct firing modes: tonic firing (explorative) and phasic firing (exploitative). These two firing modes exert different modulatory effects on post-synaptic dendritic spines. In the hippocampus, the firing modes regulate long-term potentiation (LTP) and long-term depression, which differentially regulate the mRNA expression and transcription of plasticity-related proteins (PRPs). These proteins aid in structural alterations of dendritic spines, that is, structural long-term potentiation (sLTP), via expansion and structural long-term depression (sLTD) via contraction of post-synaptic dendritic spines. Given the LC's role in all phases of memory processing, the degeneration of 50% of the LC neuron population occurring in Alzheimer's disease (AD) is a clinically relevant aspect of disease pathology. The loss of catecholaminergic regulation contributes to dysfunction in memory processes along with impaired functions associated with attention and task completion. The multifaceted role of the LC in memory and general task performance and the close correlation of LC degeneration with neurodegenerative disease progression together implicate it as a target for new clinical assessment tools.

The neurobiology of human fear generalization: meta-analysis and working neural model

Fear generalization to stimuli resembling a conditioned danger-cue (CS+) is a fundamental dynamic of classical fear-conditioning. Despite the ubiquity of fear generalization in human experience and its known pathogenic contribution to clinical anxiety, neural investigations of human generalization have only recently begun. The present work provides the first meta-analysis of this growing literature to delineate brain substrates of conditioned fear-generalization and formulate a working neural model. Included studies (K = 6, N = 176) reported whole-brain fMRI results and applied generalization-gradient methodology to identify brain activations that gradually strengthen (positive generalization) or weaken (negative generalization) as presented stimuli increase in CS+ resemblance. Positive generalization was instantiated in cingulo-opercular, frontoparietal, striatal-thalamic, and midbrain regions (locus coeruleus, periaqueductal grey, ventral tegmental area), while negative generalization was implemented in default-mode network nodes (ventromedial prefrontal cortex, hippocampus, middle temporal gyrus, angular gyrus) and amygdala. Findings are integrated within an updated neural account of generalization centering on the hippocampus, its modulation by locus coeruleus and basolateral amygdala, and the excitation of threat- or safety-related loci by the hippocampus.

Acetylcholine from the nucleus basalis magnocellularis facilitates the retrieval of well-established memory

Retrieval deficit of long-term memory is a cardinal symptom of dementia and has been proposed to associate with abnormalities in the central cholinergic system. Difficulty in the retrieval of memory is experienced by healthy individuals and not limited to patients with neurological disorders that result in forgetfulness. The difficulty of retrieving memories is associated with various factors, such as how often the event was experienced or remembered, but it is unclear how the cholinergic system plays a role in the retrieval of memory formed by a daily routine (accumulated experience). To investigate this point, we trained rats moderately (for a week) or extensively (for a month) to detect a visual cue in a two-alternative forced-choice task. First, we confirmed the well-established memory in the extensively trained group was more resistant to the retrieval problem than recently acquired memory in the moderately trained group. Next, we tested the effect of a cholinesterase inhibitor, donepezil, on the retrieval of memory after a long no-task period in extensively trained rats. Pre-administration of donepezil improved performance and reduced the latency of task initiation compared to the saline-treated group. Finally, we lesioned cholinergic neurons of the nucleus basalis magnocellularis (NBM), which project to the entire neocortex, by injecting the cholinergic toxin 192 IgG-saporin. NBM-lesioned rats showed severely impaired task initiation and performance. These abilities recovered as the trials progressed, though they never reached the level observed in rats with intact NBM. These results suggest that acetylcholine released from the NBM contributes to the retrieval of well-established memory developed by a daily routine.

Hippocampal Theta-Gamma Coupling Reflects State-Dependent Information Processing in Decision Making

During decision making, hippocampal activity encodes information sometimes about present and sometimes about potential future plans. The mechanisms underlying this transition remain unknown. Building on the evidence that gamma oscillations at different frequencies (low gamma [LG], 30–55 Hz; high gamma [HG], 60–90 Hz; and epsilon, 100–140 Hz) reflect inputs from different circuits, we identified how changes in those frequencies reflect different information-processing states. Using a unique noradrenergic manipulation by clonidine, which shifted both neural representations and gamma states, we found that future representations depended on gamma components. These changes were identifiable on each cycle of theta as asymmetries in the theta cycle, which arose from changes within the ratio of LG and HG power and the underlying phases of those gamma rhythms within the theta cycle. These changes in asymmetry of the theta cycle reflected changes in representations of present and future on each theta cycle.

Association between pupil dilation and implicit processing prior to object recognition via insight

Insight refers to the sudden conscious shift in the perception of a situation following a period of unconscious processing. The present study aimed to investigate the implicit neural mechanisms underlying insight-based recognition, and to determine the association between these mechanisms and the extent of pupil dilation. Participants were presented with ambiguous, transforming images comprised of dots, following which they were asked to state whether they recognized the object and their level of confidence in this statement. Changes in pupil dilation were not only characterized by the recognition state into the ambiguous object but were also associated with prior awareness of object recognition, regardless of meta-cognitive confidence. Our findings indicate that pupil dilation may represent the level of implicit integration between memory and visual processing, despite the lack of object awareness, and that this association may involve noradrenergic activity within the locus coeruleus-noradrenergic (LC-NA) system.

The Longevity of Hippocampus-Dependent Memory Is Orchestrated by the Locus Coeruleus-Noradrenergic System

The locus coeruleus is connected to the dorsal hippocampus via strong fiber projections. It becomes activated after arousal and novelty, whereupon noradrenaline is released in the hippocampus. Noradrenaline from the locus coeruleus is involved in modulating the encoding, consolidation, retrieval, and reversal of hippocampus-based memory. Memory storage can be modified by the activation of the locus coeruleus and subsequent facilitation of hippocampal long-term plasticity in the forms of long-term depression and long-term potentiation. Recent evidence indicates that noradrenaline and dopamine are coreleased in the hippocampus from locus coeruleus terminals, thus fostering neuromodulation of long-term synaptic plasticity and memory. Noradrenaline is an inductor of epigenetic modifications regulating transcriptional control of synaptic long-term plasticity to gate the endurance of memory storage. In conclusion, locus coeruleus activation primes the persistence of hippocampus-based long-term memory.

Memory under stress: from single systems to network changes

Stressful events have profound effects on learning and memory. These effects are mainly mediated by catecholamines and glucocorticoid hormones released from the adrenals during stressful encounters. It has been known for long that both catecholamines and glucocorticoids influence the functioning of the hippocampus, a critical hub for episodic memory. However, areas implicated in other forms of memory, such as the insula or the dorsal striatum, can be affected by stress as well. Beyond changes in single memory systems, acute stress triggers the reconfiguration of large scale neural networks which sets the stage for a shift from thoughtful, 'cognitive' control of learning and memory toward more reflexive, 'habitual' processes. Stress-related alterations in amygdala connectivity with the hippocampus, dorsal striatum, and prefrontal cortex seem to play a key role in this shift. The bias toward systems proficient in threat processing and the implementation of well-established routines may facilitate coping with an acute stressor. Overreliance on these reflexive systems or the inability to shift flexibly between them, however, may represent a risk factor for psychopathology in the long-run.

Interactions of nitric oxide with α2 -adrenoceptors within the locus coeruleus underlie the facilitation of inhibitory avoidance memory by agmatine

BACKGROUND AND PURPOSE

Agmatine, a putative neurotransmitter, plays a vital role in learning and memory. Although it is considered an endogenous ligand of imidazoline receptors, agmatine exhibits high affinity for α-adrenoceptors, NOS and NMDA receptors. These substrates within the locus coeruleus (LC) are critically involved in learning and memory processes.

EXPERIMENTAL APPROACH

The hippocampus and LC of male Wistar rat were stereotaxically cannulated for injection. Effects of agmatine, given i.p. or intra-LC, on acquisition, consolidation and retrieval of inhibitory avoidance (IA) memory were measured. The NO donor S-nitrosoglutathione, non-specific (L-NAME) and specific NOS inhibitors (L-NIL, 7-NI, L-NIO), the α2 -adrenoceptor antagonist (yohimbine) or the corresponding agonist (clonidine) were injected intra-LC before agmatine. Intra-hippocampal injections of the NMDA antagonist, MK-801 (dizocilpine), were used to modify the memory enhancing effects of agmatine, SNG and yohimbine. Expression of tyrosine hydroxylase (TH) and eNOS in the LC was assessed immunohistochemically.

KEY RESULTS

Agmatine (intra-LC or i.p.) facilitated memory retrieval in the IA test. S-nitrosoglutathione potentiated, while L-NAME and L-NIO decreased, these effects of agmatine. L-NIL and 7-NI did not alter the effects of agmatine. Yohimbine potentiated, whereas clonidine attenuated, effects of agmatine within the LC. The effects of agmatine, S-nitrosoglutathione and yohimbine were blocked by intra-hippocampal MK-801. Agmatine increased the population of TH- and eNOS-immunoreactive elements in the LC.

CONCLUSIONS AND IMPLICATIONS

The facilitation of memory retrieval in the IA test by agmatine is probably mediated by interactions between eNOS, NO and noradrenergic pathways in the LC.

Neuromodulatory signaling in hippocampus-dependent memory retrieval

Considerable advances have been made toward understanding the molecular signaling events that underlie memory acquisition and consolidation. In contrast, less is known about memory retrieval, despite its necessity for utilizing learned information. This review focuses on neuromodulatory and intracellular signaling events that underlie memory retrieval mediated by the hippocampus, for which the most information is currently available. Among neuromodulators, adrenergic signaling is required for the retrieval of various types of hippocampus-dependent memory. Although they contribute to acquisition and/or consolidation, cholinergic and dopaminergic signaling are generally not required for retrieval. Interestingly, while not required for retrieval, serotonergic and opioid signaling may actually constrain memory retrieval. Roles for histamine and non-opioid neuropeptides are currently unclear but possible. A critical effector of adrenergic signaling in retrieval is reduction of the slow afterhyperpolarization mediated by β1 receptors, cyclic AMP, protein kinase A, Epac, and possibly ERK. In contrast, stress and glucocorticoids impair retrieval by decreasing cyclic AMP, mediated in part by the activation of β2 -adrenergic receptors. Clinically, alterations in neuromodulatory signaling and in memory retrieval occur in Alzheimer's disease, Down syndrome, depression, and post-traumatic stress disorder, and recent evidence has begun to link changes in neuromodulatory signaling with effects on memory retrieval.

Memory of a drug lapse: Role of noradrenaline

Memory processes may be involved in the transition from drug lapses to relapse. This study explored the role of noradrenaline (NA) in reacquisition of place preference, an animal model of relapse that involves the updating of memories about drugs and associated stimuli. Experiments involved 7 phases: habituation, conditioning (1 mg/kg heroin and vehicle; 4 pairings each), test of conditioning (Test I), extinction (vehicle and vehicle; 4 pairings each), test of extinction (Test II), reconditioning (1 mg/kg heroin and vehicle; 1 re-pairing each), and test of reconditioning (Test III). To target memory stabilization processes, various treatments were administered post-reconditioning: systemic clonidine (0, 10, 40, 100 μg/kg; α2 adrenergic receptor agonist); intra-locus coeruleus (LC) clonidine (0, 4.5, 18 nmol); and intra-basolateral amygdala (BLA) propranolol/prazosin (0, 34/2.4 nmol; β and α1 adrenergic receptor antagonists, respectively). The effect of post-reconditioning systemic clonidine on BLA c-fos expression was also assessed. It was found that systemic clonidine dose-dependently blocked heroin reacquisition when given immediately or 4 h post-reconditioning, but not 8 h later or 4 h prior to Test III. Similar effects were observed following intra-LC clonidine infusions. Post-reconditioning systemic clonidine also blocked reacquisition of cocaine place preference (20 mg/kg). Finally, BLA c-fos expression was reduced by clonidine, and blockade of BLA β and α1 receptors prevented heroin reacquisition. These findings in rats support the hypothesis that relapse involves memory stabilization processes that can be disrupted by suppression of central NA activity.

Functional polymorphism (C-824T) of the tyrosine hydroxylase gene affects IQ in schizophrenia

AIMS

Progressive cognitive decline has been an important issue in the treatment and care of patients with schizophrenia. Tyrosine hydroxylase (TH) is the rate-limiting enzyme for the biosynthesis of catecholamine, including dopamine and noradrenaline. In this report, we examined a possible association of a genetic variant in the TH promoter region.

METHODS

Association of a genetic variant in the TH promoter region, C-824T (rs10770141), with intellectual ability in 132 patients with schizophrenia and 282 healthy subjects was examined. The transcriptional activity of the plasmids harboring the TH promoter region with either C or T nucleotide at -824 was assayed using a luciferase gene as a reporter.

RESULTS

We found significant effects of the genotype on the full-scale IQ, verbal IQ, and performance IQ, in patients with schizophrenia. IQ was lower in individuals with the C/C genotype than those with T carriers. The plasmid with the T allele at -824 showed higher transcriptional activity than that with the C allele in a transient transfection experiment using a luciferase gene as a reporter, implying that the T carriers may have higher TH activities and retain higher levels of catecholamines in the brain.

CONCLUSIONS

The present data suggest that the biosynthesis of catecholamine by the action of TH should be deeply involved in decreased intellectual ability in patients with schizophrenia. This is the first report, as far as we know, showing a correlation between TH expression and IQ in humans.

Noradrenergic modulation of vicarious trial-and-error behavior during a spatial decision-making task in rats

Deliberation between possible options before making a decision is crucial to responding with an optimal choice. However, the neural mechanisms regulating this deliberative decision-making process are still unclear. Recent studies have proposed that the locus coeruleus-noradrenaline (LC-NA) system plays a role in attention, behavioral flexibility, and exploration, which contribute to the search for an optimal choice under uncertain situations. In the present study, we examined whether the LC-NA system relates to the deliberative process in a T-maze spatial decision-making task in rats. To quantify deliberation in rats, we recorded vicarious trial-and-error behavior (VTE), which is considered to reflect a deliberative process exploring optimal choices. In experiment 1, we manipulated the difficulty of choice by varying the amount of reward pellets between the two maze arms (0 vs. 4, 1 vs. 3, 2 vs. 2). A difficulty-dependent increase in VTE was accompanied by a reduction of choice bias toward the high reward arm and an increase in time required to select one of the two arms in the more difficult manipulation. In addition, the increase of c-Fos-positive NA neurons in the LC depended on the task difficulty and the amount of c-Fos expression in LC-NA neurons positively correlated with the occurrence of VTE. In experiment 2, we inhibited LC-NA activity by injection of clonidine, an agonist of the alpha2 autoreceptor, during a decision-making task (1 vs. 3). The clonidine injection suppressed occurrence of VTE in the early phase of the task and subsequently impaired a valuable choice later in the task. These results suggest that the LC-NA system regulates the deliberative process during decision-making.

Orienting and reorienting: the locus coeruleus mediates cognition through arousal

Mood, motivation, attention, and arousal are behavioral states having a profound impact on cognition. Behavioral states are mediated though the peripheral nervous system and neuromodulatory systems in the brainstem. The noradrenergic nucleus locus coeruleus is activated in parallel with the autonomic system in response to biological imperatives. These responses can be spontaneous, to unexpected salient or threatening stimuli, or they can be conditioned responses to awaited behaviorally relevant stimuli. Noradrenaline, released in forebrain structures, will facilitate sensory processing, enhance cognitive flexibility and executive function in the frontal cortex, and promote offline memory consolidation in limbic structures. Central activation of neuromodulatory neurons and peripheral arousal, together, prepare the organism for a reorientation or reset of cortical networks and an adaptive behavioral response.

Memory reactivation effects independent of reconsolidation

Memory reactivation is an important process resulting from reexposure to salient training-related information whereby a memory is brought from an inactive to an active state. Reactivation is the first stage of memory retrieval but can result from the exposure to salient cues without any behavioral output. Such cue-induced reactivation, although frequently used by neuroscientists to study reconsolidation, has seldom been considered as a process in its own right and studied as such. This review presents arguments indicating that memory reactivation has two main consequences: (1) to enhance the accessibility of the target memory and (2) to make the memory malleable. Accordingly, reactivation creates a transient state during which the content of the memory is easily accessible and can be modified and/or updated. As both of these aspects can be observed shortly after memory reactivation, this review emphasizes that reconsolidation is not necessarily required for these processes and calls attention to reactivation as a factor in the dynamics of the memory.

Knockdown of α2C-adrenoceptors in the occipital cortex rescued long-term potentiation in hidden prenatally malnourished rats

Moderate reduction in the protein content of the mother's diet calorically compensated by carbohydrates (the so-called "hidden" prenatal malnutrition) leads to increased neocortical expression of the α(2C)-adrenoceptor subtype, together with decreased cortical release of noradrenaline and impaired long-term potentiation (LTP) and visuospatial memory performance during the rat postnatal life. In order to study whether overexpression of the α(2C)-adrenoceptor subtype is causally related to the decreased indices of neocortical plasticity found in prenatally malnourished rats, we evaluated the effect of intracortical (occipital cortex) administration of an antisense oligodeoxynucleotide (ODN) raised against the α(2C)-adrenoceptor mRNA on the LTP elicited in vivo in the occipital cortex of hidden prenatally malnourished rats. In addition, we compare the effect of the antisense ODN to that produced by systemical administration of the subtype-nonselective α(2)-adrenoceptor antagonist atipamezole. Prenatal protein malnutrition led to impaired occipital cortex LTP together with increased expression of α(2C)-adrenoceptors (about twice Bmax) in the same cortical region. [(3)H]-rauwolscine binding assay showed that a 7-day intracortical antisense ODN treatment in the malnourished rats resulted in 50% knockdown of α(2C)-adrenoceptor expression and, in addition, completely rescued the ability of the occipital cortex to develop and maintain long-term potentiation. Atipamezole (0.3 mg/kg i.p.) also led to full recovery of neocortical LTP in malnourished rats. The present results argue in favor of our original hypothesis that the deleterious effect of prenatal malnutrition on neocortical plasticity in the adult progeny is in part consequence of increased neocortical α(2C)-adrenoceptor expression. This receptor subtype is known to be involved in the presynaptic control of noradrenaline release from central neurons, a neurotransmitter that critically influences LTP and memory formation.

Physiology of quantal norepinephrine release from somatodendritic sites of neurons in locus coeruleus

Norepinephrine (NE) released from the nerve terminal of locus coeruleus (LC) neurons contributes to about 70% of the total extracellular NE in primates brain. In addition, LC neurons also release NE from somatodendritic sites. Quantal NE release from soma of LC neurons has the characteristics of long latency, nerve activity-dependency, and autoinhibition by α₂-adrenergic autoreceptor. The distinct kinetics of stimulus-secretion coupling in somata is regulated by action potential patterns. The physiological significance of soma and dendritic release is to produce negative-feedback and to down-regulate neuronal hyperactivity, which consequently inhibit NE release from axon terminal of LC projecting to many brain areas. Recent discoveries about the LC somatodendritic release may provide new insights into the pathogenesis of clinic disease involving LC-NE system dysfunction, and may help developing remedy targeted to the LC area.

Inhibition of β-adrenergic receptors induces a persistent deficit in retrieval of a cocaine-associated memory providing protection against reinstatement

Drug-seeking behavior is maintained by encounters with drug-associated cues. Preventing retrieval of drug-associated memories that these cues provoke would therefore limit relapse susceptibility; however, little is known regarding the mechanisms of retrieval. Here, we show that β-adrenergic receptor activation is necessary for the retrieval of a cocaine-associated memory. Using a conditioned place preference (CPP) procedure, rats were conditioned to associate one chamber, but not another, with cocaine. When administered before a CPP trial, propranolol, but not saline, prevented retrieval of a cocaine-associated CPP. In subsequent drug-free trials, rats previously treated with propranolol continued to show a retrieval deficit, as no CPP was evident. This retrieval deficit was long lasting and robust, as the CPP did not re-emerge during a test for spontaneous recovery 14 days later or reinstate following a priming injection of cocaine. Moreover, the peripheral β-adrenergic receptor antagonist sotalol did not affect retrieval. Thus, retrieval of cocaine-associated memories is mediated by norepinephrine acting at central β-adrenergic receptors. Our findings support the use of propranolol, a commonly prescribed β-blocker, as an adjunct to exposure therapy for the treatment of addiction by preventing retrieval of drug-associated memories during and long after treatment, and by providing protection against relapse.

Reactivation, retrieval, replay and reconsolidation in and out of sleep: connecting the dots

The neurobiology of memory has taken on a new look over the past decade. Re-discovery of cue-dependent amnesia, wide availability of functional imaging tools and increased dialog among clinicians, cognitive psychologists, behavioral neuroscientists, and neurobiologists have provided impetus for the search for new paradigms for the study of memory. Memory is increasingly viewed as an open-ended process, with retrieval being recognized as an intricate part of the encoding process. New memories are always made on the background of past experience, so that every consolidation is, in fact reconsolidation, serving to update and strengthen memories after retrieval. Spontaneous reactivation of memory circuits occurs during sleep and there is converging evidence from rodent and human studies that this is an important part of the extended off-line memory processing. The noradrenergic neuromodulatory system is engaged at retrieval, facilitating recall. The noradrenergic system is also activated during sleep after learning and noradrenergic neurons fire in concert with cortical oscillations that are associated with reactivation of memory circuits. We suggest that the noradrenergic system and perhaps other neuromodulatory systems, may be a key to linking off-line memory reactivation, retrieval, and memory reconsolidation processes at both synaptic and systems levels, in and out of sleep.

Reboxetine enhances the olanzapine-induced antipsychotic-like effect, cortical dopamine outflow and NMDA receptor-mediated transmission

Preclinical data have shown that addition of the selective norepinephrine transporter (NET) inhibitor reboxetine increases the antipsychotic-like effect of the D(2/3) antagonist raclopride and, in parallel, enhances cortical dopamine output. Subsequent clinical results suggested that adding reboxetine to stable treatments with various antipsychotic drugs (APDs) may improve positive, negative and depressive symptoms in schizophrenia. In this study, we investigated in rats the effects of adding reboxetine to the second-generation APD olanzapine on: (i) antipsychotic efficacy, using the conditioned avoidance response (CAR) test, (ii) extrapyramidal side effect (EPS) liability, using a catalepsy test, (iii) dopamine efflux in the medial prefrontal cortex and the nucleus accumbens, using in vivo microdialysis in freely moving animals and (iv) cortical N-methyl-D-aspartate (NMDA) receptor-mediated transmission, using intracellular electrophysiological recording in vitro. Reboxetine (6 mg/kg) enhanced the suppression of CAR induced by a suboptimal dose (1.25 mg/kg), but not an optimal (2.5 mg/kg) dose of olanzapine without any concomitant catalepsy. Addition of reboxetine to the low dose of olanzapine also markedly increased cortical dopamine outflow and facilitated prefrontal NMDA receptor-mediated transmission. Our data suggest that adjunctive treatment with a NET inhibitor may enhance the therapeutic effect of low-dose olanzapine in schizophrenia without increasing EPS liability and add an antidepressant action, thus in principle allowing for a dose reduction of olanzapine with a concomitant reduction of dose-related side effects, such as EPS and weight gain.

Beta-adrenergic blockade during memory retrieval in humans evokes a sustained reduction of declarative emotional memory enhancement

Memory enhancement for emotional events is dependent on amygdala activation and noradrenergic modulation during learning. A potential role for noradrenaline (NE) during retrieval of emotional memory is less well understood. Here, we report that administration of the beta-adrenergic receptor antagonist propranolol at retrieval abolishes a declarative memory enhancement for emotional items. Critically, this effect persists at a subsequent 24 h memory test, in the absence of propranolol. Thus, these findings extend our current understanding of the role of NE in emotional memory to encompass effects at retrieval, and provide face validity to clinical interventions using beta-adrenergic antagonists in conjunction with reactivation of unwanted memories in anxiety-related disorders.

Orexin-A and orexin-B during the postnatal development of the rat brain

Orexin-A and orexin-B are hypothalamic neuropeptides isolated from a small group of neurons in the hypothalamus, which project their axons to all major parts of the central nervous system. Despite the extensive information about orexin expression and function at different parts of the nervous system in adults, data about the development and maturation of the orexin system in the brain are a bit contradictory and insufficient. A previous study has found expression of orexins in the hypothalamus after postnatal day 15 only, while others report orexins detection at embryonic stages of brain formation. In the present study, we investigated the distribution of orexin-A and orexin-B neuronal cell bodies and fibers in the brain at three different postnatal stages: 1-week-, 2-week-old and adult rats. By means of immunohistochemical techniques, we demonstrated that a small subset of cells in the lateral hypothalamus, and the perifornical and periventricular areas were orexin-A and orexin-B positive not only in 2-week-old and adult rats but also in 1-week-old animals. In addition, orexin-A and orexin-B expressing neuronal varicosities were found in many other brain regions. These results suggest that orexin-A and orexin-B play an important role in the early postnatal brain development. The widespread distribution of orexinergic projections through all these stages may imply an involvement of the two neurotransmitters in a large variety of physiological and behavioral processes also including higher brain functions like learning and memory.

Long latency of evoked quantal transmitter release from somata of locus coeruleus neurons in rat pontine slices

The locus coeruleus (LC) harbors a compact group of noradrenergic cell bodies projecting to virtually all parts of the central nervous system. By using combined measurements of amperometry and patch-clamp, quantal vesicle release of noradrenaline (NA) was detected as amperometric spikes, after depolarization of the LC neurons. After a pulse depolarization, the average latency of amperometric spikes was 1,870 ms, whereas the latency of glutamate-mediated excitatory postsynaptic currents was 1.6 ms. A substantial fraction of the depolarization-induced amperometric spikes originated from the somata. In contrast to glutamate-mediated excitatory postsynaptic currents, NA secretion was strongly modulated by the action potential frequency (0.5-50 Hz). Somatodendritic NA release from LC upon enhanced cell activity produced autoinhibition of firing and of NA release. We conclude that, in contrast to classic synaptic transmission, quantal NA release from LC somata is characterized by a number of distinct properties, including long latency and high sensitivity to action potential frequency.

Norepinephrine and the dentate gyrus

Norepinephrine's role in the dentate gyrus is assessed based on a review of what is known about its innervation and receptor patterns and its functional effects at both cellular and behavioral levels. The data support seven hypotheses: (1) Norepinephrine's functional actions are primarily mediated by beta adrenoceptors and include electrophysiological enhancement of responses to excitatory input and glycogenolytic metabolic support of excitatory synaptic activity. (2) At the cellular level, locus coeruleus burst release of norepinephrine transiently inhibits feedforward interneurons and either excites or inhibits subpopulations of feedback interneurons. Consistent with reduced feedforward inhibition, granule cell firing is transiently increased. Concomitant EEG effects include transient increases in theta power and decreases in beta and gamma power. (3) Norepinephrine selectively promotes the processing of medial perforant path spatial input. This effect is mediated both through short- and long-term potentiation of cell excitability and through delayed potentiation of synaptic input. A critical level of norepinephrine release is required for long-term effects to norepinephrine alone. Norepinephrine release switches early phase frequency-induced long-term potentiation of perforant path input to an enduring late phase form and can reinstate decayed long-term potentiation. Norepinephrine also promotes frequency-induced potentiation of granule cell output at the mossy fiber to CA3 connection. (4) Local increases in norepinephrine accompany glutamate release and release of other neurotransmitters providing a mechanism for norepinephrine enhancement effects independent of locus coeruleus firing. (5) Stimuli, such as novelty and reward and punishment, which activate locus coeruleus neurons, enhance responses to medial perforant path input and engage late phase frequency-induced long-term potentiation through beta adrenoceptor activation. (6) Behavioral studies are consistent with the mechanistic evidence for a norepinephrine role in promoting learning and memory and assisting retrieval. (7) The overall profile suggests lower levels of norepinephrine may facilitate pattern completion or memory retrieval while higher levels would recruit global remapping and promote long-term episodic memory.

Locus coeruleus activation by foot shock or electrical stimulation inhibits amygdala neurons

The noradrenergic nucleus locus coeruleus (LC) has a direct projection to the basal lateral amygdala (BLA). Behavioral, lesion and pharmacological studies suggest that this pathway has an important role in mediating responses to emotional stimuli and in the formation of long term memory. The effect of LC activation on the activity of BLA neurons in vivo is not known. Therefore, in the present experiments, simultaneous extracellular unit recordings were made in the two regions while the anesthetized rat received electrical stimulation of the paw to simulate a real-life acute stressor, commonly used as an aversive reinforcer in conditioning experiments. All LC neurons exhibited a multiphasic excitatory response followed by prolonged inhibition. Responses of BLA cells were more heterogeneous, but predominantly inhibitory, with a release from inhibition during the refractory phase of LC. Direct electrical stimulation of the LC with a single pulse also elicited an inhibitory response in BLA. BLA response to both footshock and LC stimulation was partially blocked by the beta adrenergic receptor antagonist, timolol, infused into the BLA. These experiments are the first to report in vivo effects of activation of the noradrenergic system on neuronal activity in the BLA.

Combining pharmacotherapy with cognitive behavioral therapy: traditional and new approaches

Given the ever-increasing sources of trauma both nationally and globally, it is imperative to develop new and better treatments for anxiety disorders such as posttraumatic stress disorder (PTSD). This review is a collection of presentations that seek to do just that, either by using pharmacotherapy to try to prevent or erase the formation of traumatic fear memories, or to enhance exposure-based cognitive-behavioral therapy using pharmacological agents that have been effective in enhanced extinction of fear in rodents.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Mild prenatal protein malnutrition increases α2C-adrenoceptor expression in the rat cerebral cortex during postnatal life

Mild reduction in the protein content in the diet of pregnant rats from 25 to 8% casein, calorically compensated by carbohydrates, does not alter body and brain weights of rat pups at birth, but results in significant changes of the concentration and release of cortical noradrenaline during postnatal life, together with impaired long-term potentiation and memory formation. Since some central noradrenergic receptors are critically involved in neuroplasticity, the present study evaluated, by utilizing immunohistochemical methods, the effect of mild prenatal protein malnutrition on the alpha 2C-adrenoceptor expression in the frontal and occipital cortices of 8- and 60-day-old rats. At day 8 of postnatal age, prenatally malnourished rats exhibited a three-fold increase of alpha 2C-adrenoceptor expression in both the frontal and the occipital cortices, as compared to well-nourished controls. At 60 days of age, prenatally malnourished rats showed normal expression levels scores of alpha 2C-adrenoceptor in the neocortex. Results suggest that overexpression of neocortical alpha 2C-adrenoceptors during early postnatal life, subsequent to mild prenatal protein malnutrition, could in part be responsible for neural and behavioral disturbances showing prenatally malnourished animals during the postnatal life.

Beta1-adrenoceptor or alpha1-adrenoceptor activation initiates early odor preference learning in rat pups: support for the mitral cell/cAMP model of odor preference learning

We proposed that mitral cell beta1-adrenoceptor activation mediates rat pup odor preference learning. Here we evaluate beta1-, beta2-, alpha1-, and alpha2-adrenoceptor agonists in such learning. The beta1-adrenoceptor agonist, dobutamine, and the alpha1-adrenoceptor agonist, phenylephrine, induced learning, and both exhibited an inverted U-curve dose-response relationship to odor preference learning. Phenylephrine-induced learning occurred in the presence of propranolol to prevent indirect activation of beta-adrenoceptors. Alpha1-adrenoceptor mediation may represent a novel mechanism inducing learning or may increase cAMP in mitral cells via indirect activation of GABA(B) receptors. Neither the beta2-adrenoceptor agonist, salbutamol, nor the alpha2-adrenoceptor agonist, clonidine, induced learning.

Noradrenergic modulation of cognitive function in rat medial prefrontal cortex as measured by attentional set shifting capability

The brain noradrenergic system is thought to facilitate neuronal processes that promote behavioral activation, alertness, and attention. One region in which norepinephrine may exert such effects is the medial prefrontal cortex, which has been implicated in many cognitive functions including arousal, attention, motivation, working memory, response inhibition, and behavioral flexibility. The present study addressed the modulatory influence of noradrenergic neurotransmission in medial prefrontal cortex on cognitive function in rats, as measured by performance in an attentional set shifting task. In experiment 1, we tested effects of increasing and decreasing brain noradrenergic neurotransmission by systemic administration of the alpha2-adrenergic autoreceptor antagonist and agonist drugs, atipamezole and clonidine, respectively. Atipamezole pretreatment significantly improved performance on the stages of the attentional task requiring an extradimensional shift in attention, and those involving stimulus reversals, whereas clonidine had no effect at any stage. In experiment 2, we then tested effects of microinjecting alpha1- or beta-adrenergic receptor antagonists into medial prefrontal cortex on the enhancement of performance on the extradimensional task produced by atipamezole. The atipamezole-induced enhancement of performance on the extradimensional set shifting task was blocked by alpha1-, but not beta-adrenergic receptor antagonists in medial prefrontal cortex. Neither antagonist alone had any effect on extradimensional set shift performance in the absence of atipamezole-induced enhancement. These results indicate that elevating noradrenergic activity at alpha1-receptors in medial prefrontal cortex facilitates cognitive performance of rats in an attentional set-shifting task, which may contribute to the role of norepinephrine in behavioral state changes such as arousal, or to the beneficial cognitive effects of psychotherapeutic drugs that target noradrenergic neurotransmission.

Neurotransmitter deficits in behavioural and psychological symptoms of Alzheimer's disease

Behavioural and psychological symptoms of dementia (BPSD) occur in 50-90% of Alzheimer's disease (AD) patients. Imbalance of different neurotransmitters (acetylcholine, dopamine, noradrenaline and serotonin), involvement of specific brain regions responsible for emotional activities (parahippocampal gyrus, dorsal raphe and locus coeruleus) and cortical hypometabolism have been proposed as neurobiological substrate of BPSD. Compared to with respect to the neurochemical component, the cholinergic dysfunction seems to play a major role in contributing to BPSD occurrence. This view is also supported by the findings of recent trials with cholinesterase inhibitors, showing that these drugs are effective in controlling and/or improving BPSD, independent on effects on cognitive dysfunction. On the site of psychotropic drugs, atypical or novel antipsychotics represent the reference drugs for treating BPSD, whereas classic antipsychotic drugs for their profile and the potential side effects should be avoided.

Idazoxan activates rat forebrain glycogen phosphorylase in vivo: A histochemical study

In vitro experiments show norepinephrine activates glycogen phosphorylase and glycogenolysis in forebrain glia. The present study used idazoxan (5 mg/kg) to elevate NE in vivo and examined patterns of active (aGP) and total (tGP) glycogen phosphorylase reactivity in selected neocortical, hippocampal, diencephalic, and striatal sites using a histochemical method. In somatosensory neocortex, aGP reactivity was highest in Layer 4 with consistent reactivity in the barrel fields in vehicle-treated brains. In the hippocampus, the stratum lacunosum moleculare was highly reactive, while cell layers were least reactive. The dentate gyrus and CA3 were more reactive for aGP than CA1. In the diencephalon, the medial habenula was most reactive followed by the reticular nucleus of the thalamus. In the striatum, globus pallidus was most reactive. Reactivity patterns for tGP were similar to those for aGP, but more intense. The neocortex had the highest overall reactivity for tGP. An estimate of the percentage of aGP relative to tGP suggested the regions sampled had similar levels of median basal activation (approximately 65%). Idazoxan increased aGP reactivity in all regions of the neocortex assessed (layers 3-6 of primary and secondary somatosensory cortex and the barrel fields). The neuropil layers, but not the cell layers, of hippocampus were more reactive following idazoxan treatment. Idazoxan also increased aGP reactivity in the laterodorsal, paraventricular, and reticular nuclei of the thalamus. The largest idazoxan-induced changes, as an estimated percentage of tGP, occurred in the hippocampus (approximately 16% for stratum lacunosum moleculare and for CA1 stratum oriens). Increases ranged from approximately 3 to 6% in neocortex and were less than 3% in the diencephalic and striatal areas. These effects of idazoxan are consistent with a role for norepinephrine in activating forebrain glycogenolyis in vivo and supporting increased brain metabolism. They contrast with earlier evidence showing that idazoxan reduces 2-deoxyglucose uptake in these brain areas. Idazoxan, and norepinephrine, may preferentially recruit glycolytic over oxidative metabolism in the rat forebrain.

An integrative theory of locus coeruleus-norepinephrine function: adaptive gain and optimal performance

Historically, the locus coeruleus-norepinephrine (LC-NE) system has been implicated in arousal, but recent findings suggest that this system plays a more complex and specific role in the control of behavior than investigators previously thought. We review neurophysiological and modeling studies in monkey that support a new theory of LC-NE function. LC neurons exhibit two modes of activity, phasic and tonic. Phasic LC activation is driven by the outcome of task-related decision processes and is proposed to facilitate ensuing behaviors and to help optimize task performance (exploitation). When utility in the task wanes, LC neurons exhibit a tonic activity mode, associated with disengagement from the current task and a search for alternative behaviors (exploration). Monkey LC receives prominent, direct inputs from the anterior cingulate (ACC) and orbitofrontal cortices (OFC), both of which are thought to monitor task-related utility. We propose that these frontal areas produce the above patterns of LC activity to optimize utility on both short and long timescales.

Mild prenatal protein malnutrition increases α2C-adrenoceptor density in the cerebral cortex during postnatal life and impairs neocortical long-term potentiation and visuo-spatial performance in rats

Mild reduction in the protein content of the mother's diet from 25 to 8% casein, calorically compensated by carbohydrates, does not alter body and brain weights of rat pups at birth, but leads to significant enhancements in the concentration and release of cortical noradrenaline during early postnatal life. Since central noradrenaline and some of its receptors are critically involved in long-term potentiation (LTP) and memory formation, this study evaluated the effect of mild prenatal protein malnutrition on the alpha2C-adrenoceptor density in the frontal and occipital cortices, induction of LTP in the same cortical regions and the visuo-spatial memory. Pups born from rats fed a 25% casein diet throughout pregnancy served as controls. At day 8 of postnatal age, prenatally malnourished rats showed a threefold increase in neocortical alpha2C-adrenoceptor density. At 60 days-of-age, alpha2C-adrenoceptor density was still elevated in the neocortex, and the animals were unable to maintain neocortical LTP and presented lower visuo-spatial memory performance. Results suggest that overexpression of neocortical alpha2C-adrenoceptors during postnatal life, subsequent to mild prenatal protein malnutrition, could functionally affect the synaptic networks subserving neocortical LTP and visuo-spatial memory formation.

Glucocorticoid effects on memory retrieval require concurrent noradrenergic activity in the hippocampus and basolateral amygdala

Previous findings indicate that administration of abeta-adrenoceptor antagonist systemically blocks glucocorticoid impairment of memory retrieval. Here, we report that beta-adrenoceptor activation in the hippocampus and the basolateral complex of the amygdala (BLA) is implicated in the impairing effects of glucocorticoids on memory retrieval. The specific glucocorticoid receptor (GR) agonist 11beta,17beta-dihydroxy-6,21-dimethyl-17alpha-pregna-4,6-trien-20yn-3-one (RU 28362) (15 ng) infused into the hippocampus of male Sprague Dawley rats 60 min before water maze retention testing, 24 hr after training, impaired probe trial retention performance, as assessed by quadrant search time and initial latency to cross the platform location. Because we found previously that RU 28362 infused into the hippocampus does not affect water maze acquisition or immediate recall, the findings suggest that the GR agonist-induced retention impairment was attributable to a selective influence on long-term memory retrieval. Likewise, systemic injections of the beta1-adrenoceptor partial agonist xamoterol (3.0 or 10.0 mg/kg, s.c.) 60 min before the probe trial dose-dependently impaired retention performance. The beta-adrenoceptor antagonist propranolol (2.0 mg/kg) administered subcutaneously before retention testing did not affect retention performance alone, but blocked the memory retrieval impairment induced by concurrent intrahippocampal infusions of RU 28362. Pretest infusions of the beta1-adrenoceptor antagonist atenolol into either the hippocampus (1.25 microg in 0.5 microl) or the BLA (0.5 microg in 0.2 microl) also prevented the GR agonist-induced memory retrieval impairment. These findings suggest that glucocorticoids impair retrieval of long-term spatial memory by facilitating noradrenergic mechanisms in the hippocampus, and additionally, that norepinephrine-mediated BLA activity is critical in enabling hippocampal glucocorticoid effects on memory retrieval.

α2-Adrenoceptor modulation of long-term potentiation elicited in vivo in rat occipital cortex

Pretreatment with the alpha(2)-adrenoceptor agonist clonidine (31.25, 62.5, or 125 microg/kg, i.p.) dose-dependently reduced long-term potentiation (LTP) elicited in vivo in the occipital cortex of anesthetized rats, whereas pretreatment with the alpha(2)-adrenoceptor antagonist yohimbine (0.133, 0.4, or 1.2 mg/kg, i.p.) increased neocortical LTP in a dose-dependent fashion. These effects could be related to the reported disruptive and facilitatory actions induced on memory formation by pretreatment with alpha(2)-adrenoceptor agonists and antagonists, respectively.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

Alpha 2-adrenoceptors in the basal ganglia have a role in memory consolidation and reinforcement

This study demonstrates a role for alpha(2)-adrenoceptors in the basal ganglia in the consolidation of memory using weakly and strongly reinforced models of discriminated avoidance learning in the chick. The memory enhancing action of noradrenaline injected into the basal ganglia (lobus parolfactorius-LPO) was reduced in the presence of the alpha(2)-adrenoceptor antagonist yohimbine, but when noradrenaline was injected into the multi-modal association area (intermediate medial hyperstriatum ventrale-IMHV), yohimbine failed to prevent memory enhancement. Yohimbine injected into the LPO prevented, whereas the alpha(2)-adrenoceptor agonists oxymetazoline and clonidine enhanced, consolidation of memory. The timing of the inhibitory effect of yohimbine in the LPO suggested that alpha(2)-adrenoceptor involvement occurs 10-15 min after training, and that stimulation of alpha(2)-ARs in LPO is necessary for subsequent consolidation of memory. Oxymetazoline, being hydrophilic, was ineffective injected into IMHV, whereas the action of the lipophilic alpha(2)-adrenoceptor agonist clonidine in the IMHV was interpreted as an action at a site more distal in the brain, probably the LPO. The results suggest that noradrenaline release in the basal ganglia in the chick stimulates alpha(2)-adrenoceptors, which modulate and consolidate memory formation mediated by beta(2)- or beta(3)-ARs in the association area. The LPO may be responsible for the reinforcement of memory in the IMHV.

Exposure to a retrieval cue in rats induces changes in regional brain glucose metabolism in the amygdala and other related brain structures

Pre-test exposure to training-related cues is known to improve subsequent retention performance. To identify brain regions engaged in processes promoted by retrieval cues, a brain imaging approach using the [6-14C]glucose autoradiographic technique was used. Sprague-Dawley rats trained in a brightness discrimination avoidance task were submitted to different cueing conditions after a 1- or a 21-day training-to-test interval (TTI). Animals were either non-cued, cued with a box, or cued with a box and the light that served as a discriminative stimulus. Effects of the different cueing conditions on retention performance or on metabolic activity in 58 different brain regions were investigated. Rats cued with the light exhibited a subsequent improvement of their retention performance relative to controls, when tested at the 1- but not 21-days TTI, confirming our previous results. At the 1-day retention interval, a comparison between rats cued with the box and rats cued with the box and the light showed that the light cue significantly increased glucose uptake in a neuronal network composed of the lateral, basal, and central nuclei of the amygdala, the anterior and suprachiasmatic hypothalamic nuclei, the nucleus accumbens, the medial septum, and the insular cortex. In contrast, at the 21-day retention interval, both groups demonstrated similar cerebral metabolic activity. The present results indicate that exposure to a light cue increased metabolic activity in the previously mentioned brain structures only when the light acted as an effective retrieval cue, suggesting an involvement of this network in the processes triggered by a retrieval cue. Arguments are provided supporting the notion that the amygdala may play a key role in these processes. Whether the amygdala is a part of a neural network involved in retrieval processes or in neuromodulating systems that favour the efficacy of retrieval processes is also discussed.

Peripheral endotoxin causes long-lasting changes in locus coeruleus activity via IL-1 in the brain

Activity of locus coeruleus (LC) neurons, the major noradrenergic cell-body group in the brain whose axons give rise to approximately 70% of norepinephrine (NE) in the brain, is believed to play an important role in attention/vigilance, cognitive functions and behavioral disorders, particularly depression. Results described here show that in the rat, intraperitoneal (i.p.) injection of lipopolysaccharide (LPS, a bacterial endotoxin) causes long-lasting changes in electrophysiological activity of LC neurons that are mediated by interleukin-1 (IL-1) acting locally in the LC region. First, it was found that IL-1, when microinjected into the LC region or stimulated/expressed in that brain region, increased activity of LC neurons. The only exception to this was that a very low dose of microinjected IL-1 (5 pg) decreased LC activity, which could be blocked by an antagonist to corticotropin-releasing hormone (CRH), thus suggesting that the decrease was due to IL-1 stimulation of CRH release. All of these effects could be blocked by injection and/or infusion of IL-1 receptor antagonist (IL-1RA) specifically into the LC region. Next, intraperitoneal (i.p.) injection of a low dose of LPS(10 µg/kg or 100 ng/kg) was also found to increase LC activity. The excitation of LC produced by 10 µg/kg i.p. LPS increased progressively for at least 1 week, with LC neurons firing at more than twice their normal rate at 1 week after the i.p. LPS injection. Alteration of LC activity lasted for 3 weeks after a single i.p. injection of 10 µg/kg LPS. The effects of i.p. LPS on LC activity at any time after i.p. injection could be blocked by a brief microinfusion of IL-1RA into the LC region, thereby indicating that changes in LC activity seen after the i.p. LPS were caused by IL-1 acting in the LC region. Finally, i.p. injection of peptidoglycan, representing gram-positive bacteria, and polyinsinic-polycytidylic acid [poly(I):(C)], representing viral infection, also caused increases in LC activity, and the effects of peptidoglycan [but not those of poly(I):(C)] were blocked by microinfusion of IL-1RA into LC. These findings suggest that bacterial infections can give rise to prolonged changes in brain activity through cytokine action in brain.

Role of adrenoceptor subtypes in memory consolidation

Noradrenaline release in areas within the forebrain occurs following activation of noradrenergic cells in the locus coeruleus (LoC). Release of noradrenaline by attentional/arousal/vigilance factors appears to be essential for learning and is responsible for the consolidation of memory. Noradrenaline can activate any of nine different adrenoceptor (AR) subtypes in the brain and selectivity of action may be achieved by the spatial location and relative density of the AR subtypes, by different affinities of the different subtypes and by temporal selectivity in terms of when the different ARs are activated in the memory formation process. This review examines the use of selective agonists and antagonists to determine the roles of the AR subtypes in the one-trial discriminated avoidance learning paradigm in the chick. A model is developed that integrates noradrenergic activity in basal ganglia (lobus parolfactorius (LPO)) and association cortex (intermediate medial hyperstriatum ventrale (IMHV)) leading to the consolidation of memory 30 min after training. There is evidence that beta(2)- and beta(3)-ARs are important in the association area but require input from alpha(2)-AR stimulated activity in the basal ganglia for consolidation. On the other hand, alpha(1)-AR activation in the IMHV is inhibitory and prevents consolidation. While there is no role for beta(1)-ARs in memory consolidation, they play a role in short-term memory (STM). The use of the precocial chick has clear advantages in having a temporally discrete learning task which allows for discrimination memory and whose development can be followed at discrete intervals after learning. These studies reveal clear roles for AR subtypes in the formation and consolidation of memory in the chick, which have allowed the development of a model that can now be tested in mammalian systems.

Noradrenergic regulation of hippocampal place cells

The influence of noradrenergic input to the hippocampus was assessed by recording hippocampal place cells and by modulating the noradrenergic tone with a selective agonist and antagonist of the alpha2-autoreceptors. The rats received intraperitoneal injection of 5 microg/kg of dexmedetomidine (DEX), an alpha2-agonist, 0.2 mg/kg of atipamezole (ATI), an alpha2-antagonist, or saline. Hippocampal place cells were recorded in a familiar rectangular environment and in three types of new environments (rectangle, square, and cylinder). Recordings in the familiar environment were conducted in two phases, either before (early phase) or after (late phase) the exposure to new environments. In the familiar environment, DEX significantly increased the mean firing rate of hippocampal interneurons, while ATI increased the mean firing rate of pyramidal cells. Only ATI in the early phase of the experiment impaired spatial selectivity. Both drugs induced field rotations in the early phase of the study, but in the late phase DEX decreased, while ATI increased field stability in the familiar environment. The drug effects in the new environment were dependent on degree of novelty. No difference between treatments was observed in the new rectangle, but in the square and cylinder, ATI increased the mean firing rate, number of fields, and field area compared to other treatments. Stability of the original firing fields in the familiar rectangle was dependent on type of interfering environment and drug treatment. Exposure to another rectangle had the smallest impact, and exposure to a square the largest impact, on the original field pattern. ATI impaired stability of the original field after exposure to a rectangular and cylinder, while the impairing effect of DEX was only observed after exposure to a cylinder. In conclusion, increased noradrenergic tone increases the firing rate of hippocampal place cells, especially when the experimental situation and environment are new, but this increase is spatially nonselective. Furthermore, manipulation of the noradrenergic tone in either direction leads to instability of firing fields.

Noradrenergic changes, aggressive behavior, and cognition in patients with dementia

BACKGROUND

We wished to examine the integrity of the noradrenergic system in patients with Alzheimer's disease, mixed/other dementias and controls, and possible relationships between changes in the noradrenergic system and the presence of behavioral and psychiatric signs and symptoms in dementia.

METHODS

Alpha(2) adrenoceptor sites were measured by radioligand binding in three cortical regions of 46 individuals with dementia and 33 elderly normal controls together with cortical noradrenaline concentration and locus coeruleus cell and neurofibrillary tangle counts.

RESULTS

The alpha(2) adrenergic receptor density was unaltered in patients with Alzheimer's disease, mixed/other dementias compared with controls; however, there was a loss of locus coeruleus cells in subjects with dementia, reaching 50% within the rostral nucleus. In addition, a significant reduction was seen in the midtemporal cortical noradrenaline concentration (31% decrease) in patients with Alzheimer's disease. In subjects with dementia, there was a positive correlation between aggressive behavior and magnitude of rostral locus coeruleus cell loss, while the reduction in noradrenaline concentration correlated with cognitive impairment.

CONCLUSIONS

Subgroups of patients with Alzheimer's disease may have different neurochemical changes from patients lacking these changes. Therefore, this study may have implications for the treatment of behavioral and psychiatric signs and symptoms in dementia, particularly aggressive behavior in patients with dementia.