Novelty seeking behavior in the rat is dependent upon the integrity of the noradrenergic system

Curiosity-driven exploration: foundations in neuroscience and computational modeling

Curiosity refers to the intrinsic desire of humans and animals to explore the unknown, even when there is no apparent reason to do so. Thus far, no single, widely accepted definition or framework for curiosity has emerged, but there is growing consensus that curious behavior is not goal-directed but related to seeking or reacting to information. In this review, we take a phenomenological approach and group behavioral and neurophysiological studies which meet these criteria into three categories according to the type of information seeking observed. We then review recent computational models of curiosity from the field of machine learning and discuss how they enable integrating different types of information seeking into one theoretical framework. Combinations of behavioral and neurophysiological studies along with computational modeling will be instrumental in demystifying the notion of curiosity.

Cell-type-specific optogenetic stimulation of the locus coeruleus induces slow-onset potentiation and enhances everyday memory in rats

Memory formation is typically divided into phases associated with encoding, storage, consolidation, and retrieval. The neural determinants of these phases are thought to differ. This study first investigated the impact of the experience of novelty in rats incurred at a different time, before or after, the precise moment of memory encoding. Memory retention was enhanced. Optogenetic activation of the locus coeruleus mimicked this enhancement induced by novelty, both when given before and after the moment of encoding. Optogenetic activation of the locus coeruleus also induced a slow-onset potentiation of field potentials in area CA1 of the hippocampus evoked by CA3 stimulation. Despite the locus coeruleus being considered a primarily noradrenergic area, both effects of such stimulation were blocked by the dopamine D1/D5 receptor antagonist SCH 23390. These findings substantiate and enrich the evidence implicating the locus coeruleus in cellular aspects of memory consolidation in hippocampus.

Altered norepinephrine transmission after spatial learning impairs sleep-mediated memory consolidation in rats

The therapeutic use of noradrenergic drugs makes the evaluation of their effects on cognition of high priority. Norepinephrine (NE) is an important neuromodulator for a variety of cognitive processes and may importantly contribute to sleep-mediated memory consolidation. The NE transmission fluctuates with the behavioral and/or brain state and influences associated neural activity. Here, we assessed the effects of altered NE transmission after learning of a hippocampal-dependent task on neural activity and spatial memory in adult male rats. We administered clonidine (0.05 mg/kg, i.p.; n = 12 rats) or propranolol (10 mg/kg, i.p.; n = 11) after each of seven daily learning sessions on an 8-arm radial maze. Compared to the saline group (n = 9), the drug-treated rats showed lower learning rates. To assess the effects of drugs on cortical and hippocampal activity, we recorded prefrontal EEG and local field potentials from the CA1 subfield of the dorsal hippocampus for 2 h after each learning session or drug administration. Both drugs significantly reduced the number of hippocampal ripples for at least 2 h. An EEG-based sleep scoring revealed that clonidine made the sleep onset faster while prolonging quiet wakefulness. Propranolol increased active wakefulness at the expense of non-rapid eye movement (NREM) sleep. Clonidine reduced the occurrence of slow oscillations (SO) and sleep spindles during NREM sleep and altered the temporal coupling between SO and sleep spindles. Thus, pharmacological alteration of NE transmission produced a suboptimal brain state for memory consolidation. Our results suggest that the post-learning NE contributes to the efficiency of hippocampal-cortical communication underlying memory consolidation.

The Hole-Board Test in Mutant Mice

First described by Boissier and Simon in (Ther Recreat J 17:1225-1232, 1962), the hole-board has become a recognized test of anxiety and spatial memory. Benzodiazepines acting at the GABA_A-BZD site increase hole-pokes in rats and mice, indicating a loss in behavioral inhibition concordant with the behavior of mutant mice deficient in the GABA transporter. Hole-poking also depends on arousal mechanisms dependent on dopaminergic transmission, as indicated by drug and null mutant studies. In addition, the behavior is modified in natural and null mutants affecting the cerebellum as well as null mutants affecting neuropeptides, growth factors, cell adhesion, and inflammation. Further research is required to determine convergences between genetic and pharmacological effects.

Negative relationship between brain α1A-AR neurotransmission and βArr2 levels in anxious adolescent rats subjected to early life stress

Early-life stress is correlated with the development of anxiety-related behavior in adolescence, but underlying mechanisms remain poorly known. The α_1A-adrenergic receptor (AR) is linked to mood regulation and its function is assumed to be regulated by β-arrestins (βArrs) via desensitization and downregulation. Here, we investigated correlation between changes in α_1A-AR and βArr2 levels in the prefrontal cortex (PFC) and hippocampus of adolescent and adult male rats subjected to maternal separation (MS) and their relationship with anxiety-like behavior in adolescence. MS was performed 3 h per day from postnatal days 2-11 and anxiety-like behavior was evaluated in the elevated plus-maze and open field tests. The protein levels were examined using western blot assay. MS decreased α_1A-AR expression and increased βArr2 expression in both brain regions of adolescent rats, while induced reverse changes in adulthood. MS adolescent rats demonstrated higher anxiety-type behavior and lower activity in behavioral tests than controls. Decreased α_1A-AR levels in MS adolescence strongly correlated with reduced time spent in the open field central area, consistent with increased anxiety-like behavior. An anxiety-like phenotype was mimicked by acute and chronic treatment of developing rats with prazosin, an α_1A-AR antagonist, suggesting α_1A-AR downregulation may facilitate anxiety behavior in MS adolescent rats. Together, our results indicate a negative correlation between α_1A-AR neurotransmission and βArr2 levels in both adults and anxious-adolescent rats and suggest that increased βArr2 levels may contribute to posttranslational regulation of α_1A-AR and modulation of anxiety-like behavior in adolescent rats. This may provide a path to develop more effective anxiolytic treatments.

Noradrenergic circuits in the forebrain control affective responses to novelty

RATIONALE

In rodents, exposure to novel environments elicits initial anxiety-like behavior (neophobia) followed by intense exploration (neophilia) that gradually subsides as the environment becomes familiar. Thus, innate novelty-induced behaviors are useful indices of anxiety and motivation in animal models of psychiatric disease. Noradrenergic neurons are activated by novelty and implicated in exploratory and anxiety-like responses, but the role of norepinephrine (NE) in neophobia has not been clearly delineated.

OBJECTIVE

We sought to define the role of central NE transmission in neophilic and neophobic behaviors.

METHODS

We assessed dopamine β-hydroxylase knockout (Dbh -/-) mice lacking NE and their NE-competent (Dbh +/-) littermate controls in neophilic (novelty-induced locomotion; NIL) and neophobic (novelty-suppressed feeding; NSF) behavioral tests with subsequent quantification of brain-wide c-fos induction. We complimented the gene knockout approach with pharmacological interventions.

RESULTS

Dbh -/- mice exhibited blunted locomotor responses in the NIL task and completely lacked neophobia in the NSF test. Neophobia was rescued in Dbh -/- mice by acute pharmacological restoration of central NE with the synthetic precursor L-3,4-dihydroxyphenylserine (DOPS), and attenuated in control mice by the inhibitory α2-adrenergic autoreceptor agonist guanfacine. Following either NSF or NIL, Dbh -/- mice demonstrated reduced c-fos in the anterior cingulate cortex, medial septum, ventral hippocampus, bed nucleus of the stria terminalis, and basolateral amygdala.

CONCLUSION

These findings indicate that central NE signaling is required for the expression of both neophilic and neophobic behaviors. Further, we describe a putative noradrenergic novelty network as a potential therapeutic target for treating anxiety and substance abuse disorders.

The functional networks of a novel environment: Neural activity mapping in awake unrestrained rats using positron emission tomography

INTRODUCTION

Novel environment stimulation is thought to have an important role in cognitive development and has been shown to encourage exploratory behavior in rats. However, psychopathology or perceived danger or stress can impede this exploratory drive. The balance between brain circuits controlling the exploratory drive elicited by a novel environment, and the avoidance response to stressors, is not well understood.

METHODS

Using positron emission tomography (PET) and the glucose analog [¹⁸ F]fluorodeoxyglucose (18F-FDG), we assessed awake brain glucose metabolism (BGluM) in rats while in a novel environment (cage of an unfamiliar male rat) and non-novel environment (the animal's home cage).

RESULTS

Exposure to the novel environment increased BGluM in regions associated with vision (visual cortex), motor function and motivated behavior (striatum and motor cortex), and anxiety (stria terminalis), and decreased BGluM in regions associated with auditory processing (auditory cortex, insular cortex, inferior colliculus), locomotor activity (globus pallidus, striatum, motor cortex, ventral thalamic nucleus), spatial navigation (retrosplenial cortex), and working memory (hippocampus, cingulate cortex, prelimbic cortex, orbitofrontal cortex).

CONCLUSION

These results suggest that the novel cage is a stressful environment that inhibits activity in brain regions associated with exploratory behavior. Patterns of inhibition in the novel cage also support the proposed rat default mode network, indicating that animals are more cognitively engaged in this environment. Additionally, these data support the unique capability of combining FDG-PET with psychopharmacology experiments to examine novelty seeking and brain activation in the context of decision making, risk taking, and cognitive function more generally, along with response to environmental or stress challenges.

Mice lacking paternal expression of imprinted Grb10 are risk-takers

The imprinted genes Grb10 and Nesp influence impulsive behavior on a delay discounting task in an opposite manner. A recently developed theory suggests that this pattern of behavior may be representative of predicted effects of imprinted genes on tolerance to risk. Here we examine whether mice lacking paternal expression of Grb10 show abnormal behavior across a number of measures indicative of risk‐taking. Although Grb10^+/p mice show no difference from wild type (WT) littermates in their willingness to explore a novel environment, their behavior on an explicit test of risk‐taking, namely the Predator Odor Risk‐Taking task, is indicative of an increased willingness to take risks. Follow‐up tests suggest that this risk‐taking is not simply because of a general decrease in fear, or a general increase in motivation for a food reward, but reflects a change in the trade‐off between cost and reward. These data, coupled with previous work on the impulsive behavior of Grb10^+/p mice in the delayed reinforcement task, and taken together with our work on mice lacking maternal Nesp, suggest that maternally and paternally expressed imprinted genes oppositely influence risk‐taking behavior as predicted.

Structured inhibitory activity dynamics in new virtual environments

Inhibition plays a powerful role in regulating network excitation and plasticity; however, the activity of defined interneuron types during spatial exploration remain poorly understood. Using two-photon calcium imaging, we recorded hippocampal CA1 somatostatin- and parvalbumin-expressing interneurons as mice performed a goal-directed spatial navigation task in new visual virtual reality (VR) contexts. Activity in both interneuron classes was strongly suppressed but recovered as animals learned to adapt the previously learned task to the new spatial context. Surprisingly, although there was a range of activity suppression across the population, individual somatostatin-expressing interneurons showed consistent levels of activity modulation across exposure to multiple novel environments, suggesting context-independent, stable network roles during spatial exploration. This work reveals population-level temporally dynamic interneuron activity in new environments, within which each interneuron shows stable and consistent activity modulation.

Modulation of locus coeruleus activity by novel oddball stimuli

It has long been known from animal literature that the locus coeruleus (LC), the source region of noradrenergic neurons in the brain, is sensitive to unexpected, novel, and other salient events. In humans, however, direct assessment of LC activity has proven to be challenging due to its small size and difficult localization, which is why noradrenergic activity has often been assessed using more indirect measures such as electroencephalography (EEG) and pupil recordings. Here, we combined high-resolution functional magnetic resonance imaging (fMRI) with a special anatomical sequence to assess neural activity in the LC in response to different types of salient stimuli in an oddball paradigm (novel neutral oddballs, novel emotional oddballs, and familiar target oddballs). We found a significant linear increase of LC activity from standard trials, over familiar target oddballs, to novel neutral and novel emotional oddballs. Importantly, when breaking down this linear trend, only novel oddball stimuli led to robust activity increases as compared to standard trials, with no statistical difference between neutral and emotional ones. This pattern suggests that activity modulations in the LC in the present study were mainly driven by stimulus novelty, rather than by emotional saliency, task relevance, or contextual novelty alone. Moreover, the absence of significant activity modulations in response to target oddballs (which were reported in a recent study) suggests that the LC represents relative rather than absolute saliency of a stimulus in its respective context.

Curiosity in old age: A possible key to achieving adaptive aging

Curiosity is a fundamental part of human motivation that supports a variety of human intellectual behaviors ranging from early learning in children to scientific discovery. However, there has been little attention paid to the role of curiosity in aging populations. By bringing together broad but sparse neuroscientific and psychological literature on curiosity and related concepts (e.g., novelty seeking in older adults), we propose that curiosity, although it declines with age, plays an important role in maintaining cognitive function, mental health, and physical health in older adults. We identify the dopaminergic reward system and the noradrenergic system as the key brain systems implicated in curiosity processing and discuss how these brain systems contribute to the relationship between curiosity and adaptive aging.

Dual contributions of noradrenaline to behavioural flexibility and motivation

INTRODUCTION

While several theories have highlighted the importance of the noradrenergic system for behavioral flexibility, a number of recent studies have also shown a role for noradrenaline in motivation, particularly in effort processing. Here, we designed a novel sequential cost/benefit decision task to test the causal influence of noradrenaline on these two functions in rhesus monkeys.

METHODS

We manipulated noradrenaline using clonidine, an alpha-2 noradrenergic receptor agonist, which reduces central noradrenaline levels and examined how this manipulation influenced performance on the task.

RESULTS

Clonidine had two specific and distinct effects: first, it decreased choice variability, without affecting the cost/benefit trade-off; and second, it reduced force production, without modulating the willingness to work.

CONCLUSIONS

Together, these results support an overarching role for noradrenaline in facing challenging situations in two complementary ways: by modulating behavioral volatility, which would facilitate adaptation depending on the lability of the environment, and by modulating the mobilization of resources to face immediate challenges.

Neuromodulatory Systems and Their Interactions: A Review of Models, Theories, and Experiments

Neuromodulatory systems, including the noradrenergic, serotonergic, dopaminergic, and cholinergic systems, track environmental signals, such as risks, rewards, novelty, effort, and social cooperation. These systems provide a foundation for cognitive function in higher organisms; attention, emotion, goal-directed behavior, and decision-making derive from the interaction between the neuromodulatory systems and brain areas, such as the amygdala, frontal cortex, hippocampus, and sensory cortices. Given their strong influence on behavior and cognition, these systems also play a key role in disease states and are the primary target of many current treatment strategies. The fact that these systems interact with each other either directly or indirectly, however, makes it difficult to understand how a failure in one or more systems can lead to a particular symptom or pathology. In this review, we explore experimental evidence, as well as focus on computational and theoretical models of neuromodulation. Better understanding of neuromodulatory systems may lead to the development of novel treatment strategies for a number of brain disorders.

Regulation of Trophic Factor Expression by Innervating Target Regions in Intraocular Double Transplants

Trophic factors have been found to play a significant role both in long-term survival processes and in more rapid and dynamic processes in the brain and spinal cord. However, little is known regarding the regulation of expression of growth factors, and how these proteins interact on a cell-to-cell basis. We have studied protein levels of one growth factor known to affect the noradrenergic innervation of the hippocampal formation, namely brain-derived neurotrophic factor (BDNF). The purpose of the present study was to determine if appropriate innervation or contact between the LC noradrenergic neurons and their target, the hippocampus, affects expression of this growth factor in either brain region. Fetal brain stem tissue, containing the LC, and hippocampal formation were dissected from embryonic day 17 rat fetuses and transplanted together or alone into the anterior chamber of the eye of adult Fisher 344 rats. The tissue was grown together for 6 weeks, after which the animals were sacrificed and ELISAs for BDNF were undertaken. Transplantation to the anterior chamber of the eye increased the expression of BDNF in the hippocampal but not the brain stem tissue, compared with levels observed in fetal and adult rats in vivo. In addition, double grafting with hippocampal tissue more than tripled BDNF levels in brain stem grafts and doubled BDNF levels in the hippocampal portion of double grafts compared with hippocampal single grafts. Triple grafts containing basal forebrain, hippocampus, and brain stem LC tissue increased brain stem and hippocampal BDNF levels even further. Colchicine treatment of LC-hippocampal double grafts gave rise to a significant decrease in hippocampal BDNF levels to levels seen in single hippocampal grafts, while only a partial reduction of BDNF levels was seen in the brain stem portion of the same double grafts treated with colchicine. The findings suggest that an appropriate hippocampal innervation or contact with its target tissues is essential for regulation of BDNF expression in the brain stem, and that retrograde transport of BDNF can occur between double grafted fetal tissues in oculo.

Ecologically relevant neurobehavioral assessment of the development of threat learning

As altricial infants gradually transition to adults, their proximate environment changes. In three short weeks, pups transition from a small world with the caregiver and siblings to a complex milieu rich in dangers as their environment expands. Such contrasting environments require different learning abilities and lead to distinct responses throughout development. Here, we will review some of the learned fear conditioned responses to threats in rats during their ontogeny, including behavioral and physiological measures that permit the assessment of learning and its supporting neurobiology from infancy through adulthood. In adulthood, odor-shock conditioning produces robust fear learning to the odor that depends upon the amygdala and related circuitry. Paradoxically, this conditioning in young pups fails to support fear learning and supports approach learning to the odor previously paired with shock. This approach learning is mediated by the infant attachment network that does not include the amygdala. During the age range when pups transition from the infant to the adult circuit (10-15 d old), pups have access to both networks: odor-shock conditioning in maternal presence uses the attachment circuit but the adult amygdala-dependent circuit when alone. However, throughout development (as young as 5 d old) the attachment associated learning can be overridden and amygdala-dependent fear learning supported, if the mother expresses fear in the presence of the pup. This social modulation of the fear permits the expression of defense reactions in life threatening situations informed by the caregiver but prevents the learning of the caregiver itself as a threat.

Manipulating Decisiveness in Decision Making: Effects of Clonidine on Hippocampal Search Strategies

Decisiveness is the ability to commit to a decision quickly and efficiently; in contrast, indecision entails the repeated consideration of multiple alternative possibilities. In humans, the α2-adrenergic receptor agonist clonidine increases decisiveness in tasks that require planning through unknown neural mechanisms. In rats, indecision is manifested as reorienting behaviors at choice points (vicarious trial and error [VTE]), during which hippocampal representations alternate between prospective options. To determine whether the increase in decisiveness driven by clonidine also entails changes in hippocampal search processes, we compared the effect of clonidine on spatial representations in hippocampal neural ensembles as rats passed through a T-shaped decision point. Consistent with previous experiments, hippocampal representations reflected both chosen and unchosen paths during VTE events under saline control conditions. Also, consistent with previous experiments, hippocampal representations reflected the chosen path more than the unchosen path when the animal did not show VTE at the choice point. Injection of clonidine suppressed the spatial representation of the unchosen path at the choice point on VTE laps and hastened the differentiation of spatial representations of the chosen path from the unchosen path on non-VTE laps to appear before reaching the choice point. These results suggest that the decisiveness seen under clonidine is due to limited exploration of potential options in hippocampus, and suggest novel roles for noradrenaline as a modulator of the hippocampal search processes. Significance statement: Clonidine, an α2-adrenergic receptor agonist, which decreases the level of noradrenaline in vivo, has an interesting effect in humans and other animals: it makes them more decisive. However, the mechanisms by which clonidine makes them more decisive remain unknown. Researchers have speculated that clonidine limits the amount of mental search that subjects do when planning options. We test this hypothesis by measuring the mental search strategy in rats through hippocampal recordings. We find that clonidine limits the options searched by rats, suggesting that noradrenaline also plays a role in balancing exploration and exploitation in internally simulated behaviors, similar to its role in balancing exploration and exploitation in external behaviors.

Novelty seeking is related to individual risk preference and brain activation associated with risk prediction during decision making

Novelty seeking (NS) is a personality trait reflecting excitement in response to novel stimuli. High NS is usually a predictor of risky behaviour such as drug abuse. However, the relationships between NS and risk-related cognitive processes, including individual risk preference and the brain activation associated with risk prediction, remain elusive. In this fMRI study, participants completed the Tridimensional Personality Questionnaire to measure NS and performed a probabilistic decision making task. Using a mathematical model, we estimated individual risk preference. Brain regions associated with risk prediction were determined via fMRI. The NS score showed a positive correlation with risk preference and a negative correlation with the activation elicited by risk prediction in the right posterior insula (r-PI), left anterior insula (l-AI), right striatum (r-striatum) and supplementary motor area (SMA). Within these brain regions, only the activation associated with risk prediction in the r-PI showed a correlation with NS after controlling for the effect of risk preference. Resting-state functional connectivity between the r-PI and r-striatum/l-AI was negatively correlated with NS. Our results suggest that high NS may be associated with less aversion to risk and that the r-PI plays an important role in relating risk prediction to NS.

Propranolol reverses open field effects on frustration

Reactivity to a reward is affected by prior experience with different reinforcer values of that reward, a phenomenon known as incentive relativity. Incentive relativity can be studied via the consummatory successive negative contrast (cSNC) paradigm, in which acceptance of 4% sucrose is assessed in animals that had been exposed to 32% sucrose. These downshifted animals usually exhibit significantly less sucrose acceptance than animals that always received the 4% sucrose solution. In previous work, we found that exploration of a novel open field (OF) before the first trial with the downshifted solution attenuated the contrast effect. The goal of the present experiments was to expand the knowledge on the effects of OF exposure on cSNC. We evaluated the effect OF exposure before the second downshift trial and assessed the mediational role of the adrenergic system in the effects of OF during the first and second trial of cSNC. The results indicate that OF applied before the first or second downshift trials exert opposite effects and that the adrenergic system is involved in the acquisition and consolidation of the OF information.

Individual differences in novelty-seeking behavior in spontaneously hypertensive rats: Enhanced sensitivity to the reinforcing effect of methylphenidate in the high novelty-preferring subpopulation

BACKGROUND

High novelty seeking has been assumed to predict vulnerability to use addictive drugs. Notably, it is also a symptom associated with attention-deficit/hyperactivity disorder (ADHD). The aim of this study was to identify whether spontaneously hypertensive rats (SHRs), putative animal models of ADHD, display individual differences in novelty-seeking behavior, and whether high novelty-seeking SHRs show enhanced sensitivity to the reinforcing effect of methylphenidate, the most commonly prescribed stimulant ADHD medication.

METHODS

First, we established that SHRs show higher levels of novelty-seeking behavior than their normotensive control strain, Wistar Kyoto (WKY) rats. Novelty seeking was measured in two tests: open field test in a novel test arena, and novel object preference tests. Thereafter, SHRs were classified into high responders (HR) or low responders (LR), high novelty-preferring (HNP) or low novelty-preferring (LNP) rats, based on individual scores in the two behavioral assays. Methylphenidate self-administration was assessed thereafter.

RESULTS

SHRs showed higher levels of novelty-seeking behavior than WKY rats. HR/LR and HNP/LNP subgroups were identified. HR and LR rats showed comparable rates of methylphenidate self-administration. However, HNP SHRs worked more for methylphenidate infusions than their LNP counterparts.

CONCLUSIONS

We showed some evidence on inter-individual variations in novelty seeking in SHRs. Importantly, we demonstrated enhanced sensitivity of HNP SHRs to the reinforcing effect of methylphenidate, indicating a "drug-vulnerable" SHR subpopulation. These findings are important as they may provide basis for a potential screening tool to identify a subset of ADHD patients (i.e. high novelty seekers) who may be at risk for misusing/abusing methylphenidate.

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.

Withdrawal symptoms and rebound syndromes associated with switching and discontinuing atypical antipsychotics: theoretical background and practical recommendations

With the widespread use of atypical or second-generation antipsychotics, switching treatment has become current practice and more complicated, as the pharmacological profiles of these agents differ substantially despite their similarity in being 'atypical'. All share the ability to block dopamine D₂ receptors, and most of them also block serotonin 5-HT2A receptors. Apart from these common features, some atypical antipsychotics are also able to block or stimulate other dopamine or serotonin receptors, as well as histaminergic, muscarinergic or adrenergic receptors. As a result of the varying receptor affinities, in switching or discontinuing compounds several possible pitfalls have to be considered, including the occurrence of withdrawal and rebound syndromes. This article reviews the pharmacological background of functional blockade or stimulation of receptors of interest in regard to atypical antipsychotics and the implicated potential withdrawal and rebound phenomena. A MEDLINE search was carried out to identify information on withdrawal or rebound syndromes occurring after discontinuation of atypical antipsychotics. Using the resulting literature, we first discuss the theoretical background to the functional consequences of atypical antipsychotic-induced blockade or stimulation of neurotransmitter receptors and, secondly, we highlight the clinical consequences of this. We then review the available clinical literature on switching between atypical antipsychotics, with respect to the occurrence of withdrawal or rebound symptoms. Finally, we offer practical recommendations based on the reviewed findings. The systematic evaluation of withdrawal or rebound phenomena using randomized controlled trials is still understudied. Knowledge of pharmacological receptor-binding profiles may help clinicians in choosing adequate switching or discontinuation strategies for each agent. Results from large switching trials indicate that switching atypical antipsychotics can be performed in a safe manner. Treatment-emergent adverse events during or after switching are not always considered to be, at least in part, associated with the pre-switch antipsychotic. Further studies are needed to substantiate the evidence gained so far on different switching strategies. The use of concomitant medication, e.g., benzodiazepines or anticholinergic drugs, may help to minimize symptoms arising from the discontinuation or switching of antipsychotic treatment.

Synergistic tonic and phasic activity of the locus coeruleus norepinephrine (LC-NE) arousal system is required for optimal attentional performance

A certain level of arousal is required for an individual to perform optimally, and the locus coeruleus norepinephrine (LC-NE) system plays a central role in optimizing arousal. Tonic firing of LC-NE neurons needs to be held within a narrow range of 1-3 Hz to facilitate phasic firing of the LC-NE neurons; these two modes of activity act synergistically, to allow the individual to perform attentional tasks optimally. How this information can be applied to further our understanding of psychiatric disorders has not been fully elucidated. Here we propose two models of altered LC-NE activity that result in attentional deficits characteristic of psychiatric disorders: 1) 'hypoaroused' individuals with e.g. attention-deficit/hyperactivity disorder (ADHD) have decreased tonic firing of the LC-NE system, resulting in decreased cortical arousal and poor attentional performance and 2) 'hyperaroused' individuals with e.g. anxiety disorders have increased tonic firing of the LC-NE system, resulting in increased cortical arousal and impaired attentional performance. We argue that hypoarousal (decreased tonic firing of LC-NE neurons) and hyperarousal (increased tonic firing of LC-NE neurons) are suboptimal states in which phasic activity of LC-NE neurons is impeded. To further understand the neurobiology of attentional dysfunction in psychiatric disorders a translational approach that integrates findings on the LC-NE arousal system from animal models and human imaging studies may be useful.

β-Adrenoreceptor stimulation mediates reconsolidation of social reward-related memories

Background

In recent years, the notion that consolidated memories become transiently unstable after retrieval and require reconsolidation to persist for later use has received strong experimental support. To date, the majority of studies on reconsolidation have focused on memories of negative emotions, while the dynamics of positive memories have been less well studied. Social play, the most characteristic social behavior displayed by young mammals, is important for social and cognitive development. It has strong rewarding properties, illustrated by the fact that it can induce conditioned place preference (CPP). In order to understand the dynamics of positive social memories, we evaluated the effect of propranolol, a β-adrenoreceptor antagonist known to influence a variety of memory processes, on acquisition, consolidation, retrieval and reconsolidation of social play-induced CPP in adolescent rats.

Methodology/Principal Findings

Systemic treatment with propranolol, immediately before or after a CPP test (i.e. retrieval session), attenuated CPP 24 h later. Following extinction, CPP could be reinstated in saline- but not in propranolol-treated rats, indicating that propranolol treatment had persistently disrupted the CPP memory trace. Propranolol did not affect social play-induced CPP in the absence of memory retrieval or when administered 1 h or 6 h after retrieval. Furthermore, propranolol did not affect acquisition, consolidation or retrieval of social play-induced CPP.

Conclusions/Significance

We conclude that β-adrenergic neurotransmission selectively mediates the reconsolidation, but not other processes involved in the storage and stability of social reward-related memories in adolescent rats. These data support the notion that consolidation and reconsolidation of social reward-related memories in adolescent rats rely on distinct neural mechanisms.

Learning-facilitated long-term depression and long-term potentiation at mossy fiber-CA3 synapses requires activation of β-adrenergic receptors

Learning-facilitated plasticity refers to hippocampal synaptic plasticity that is facilitated by novel spatial learning events. Both long-term potentiation (LTP) and long-term depression (LTD) are facilitated by novel hippocampus-dependent learning. This has important ramifications for our understanding of how the hippocampus encodes memory. One structure that is rarely studied in vivo, but is believed to be crucially important for working and long-term memory processing is the hippocampal CA3 region. Whereas learning-facilitated plasticity has been described in this structure, the mechanisms underlying this phenomenon have not been explored. The noradrenergic system plays an important role in arousal and qualification of new information as salient. It regulates synaptic plasticity in the dentate gyrus and CA1, but nothing is known about the regulation by the noradrenergic system of synaptic plasticity in the CA3 region. We explored whether β-adrenergic receptors contribute to learning-facilitated plasticity at mossy fiber (mf)-CA3 synapses of behaving rats. We found that receptor antagonism had no effect on basal synaptic transmission, short-term potentiation (STP), short-term depression, LTP, or LTD, that were electrically induced by patterned afferent stimulation. We found, however, that both learning-facilitated LTP and LTD were prevented by antagonism of β-adrenergic receptors, whereas the agonist isoproterenol facilitated STP into LTP. Thus, learning-facilitated and electrically-induced plasticity may not share the same prerequisites. These results support that the mf synapse engages in a distinct aspect of encoding of spatial information that involves both LTP and LTD. Furthermore, changes in arousal that are coupled to new learning are associated with activation of hippocampal β-adrenergic receptors that in turn comprise a key element in this type of information acquisition and processing by the CA3 region.

α- and β-Adrenergic receptors differentially modulate the emission of spontaneous and amphetamine-induced 50-kHz ultrasonic vocalizations in adult rats

Amphetamine (AMPH) increases adult rat 50-kHz ultrasonic vocalizations, preferentially promoting frequency-modulated (FM) calls that have been proposed to reflect positive affect. The main objective of this study was to investigate a possible noradrenergic contribution to AMPH-induced calling. Adult male Long-Evans rats were tested with AMPH (1 mg/kg intraperitoneal) or saline combined with various systemic pretreatments: clonidine (α2 adrenergic agonist), prazosin (α1 antagonist), atipamezole (α2 antagonist), propranolol, betaxolol, and/or ICI 118,551 (β1/β2, β1, and β2 antagonists, respectively), nadolol (β1/β2 antagonist, peripheral only), or NAD-299 (5HT(1A) antagonist). In addition, effects of cirazoline (α1 adrenergic agonist) and cocaine (0.25-1.5 mg/kg intravenous) were studied alone. AMPH-induced calling was suppressed by low-dose clonidine and prazosin. Cirazoline and atipamezole did not significantly affect calling rate. Propranolol, without affecting the call rate, dose dependently promoted 'flat' calls under AMPH while suppressing 'trills,' thus reversing the effects of AMPH on the 'call subtype profile.' This effect of propranolol seemed to be mediated by simultaneous inhibition of CNS β1 and β2 rather than by 5HT(1A) receptors. Finally, cocaine elicited fewer calls than did AMPH, but produced the same shift in the call subtype profile. Taken together, these results reveal differential drug effects on flat vs trill vs other FM 50-kHz calls. These findings highlight the value of detailed call subtype analyses, and show that 50-kHz calls are associated with adrenergic α1- and β-receptor mechanisms. These preclinical findings suggest that noradrenergic contributions to psychostimulant subjective effects may warrant further investigation.

Chronic administration of THC prevents the behavioral effects of intermittent adolescent MDMA administration and attenuates MDMA-induced hyperthermia and neurotoxicity in rats

Most recreational users of 3, 4-methylenedioxymethamphetamine (MDMA or "ecstasy") also take cannabis, in part because cannabis can reduce the dysphoric symptoms of the ecstasy come-down such as agitation and insomnia. Although previous animal studies have examined the acute effects of co-administering MDMA and Δ(9)-tetrahydrocannabinol (THC), which is the major psychoactive ingredient in cannabis, research on chronic exposure to this drug combination is lacking. Therefore, the present study was conducted to investigate the effects of chronic adolescent administration of both THC and MDMA on behavior and on regional serotonin transporter (SERT) binding and serotonin (5-HT) concentrations as indices of serotonergic system integrity. Male Sprague-Dawley rats were divided into four drug administration groups: (1) MDMA alone, (2) THC alone, (3) MDMA plus THC, and (4) vehicle controls. MDMA (2 × 10 mg/kg × 4 h) was administered every fifth day from postnatal day (PD) 35 to 60 to simulate intermittent recreational ecstasy use, whereas THC (5mg/kg) was given once daily over the same time period to simulate heavy cannabis use. THC unexpectedly produced a modest hyperthermic effect when administered alone, but in animals co-treated with both THC and MDMA, there was an attenuation of MDMA-induced hyperthermia on dosing days. Subsequent testing conducted after a drug washout period revealed that THC reduced MDMA-related behavioral changes in the emergence and social interaction tests of anxiety-like behavior and also blunted the MDMA-induced decrease in exploratory behavior in the hole-board test. THC additionally attenuated MDMA -induced decreases in 5-HT levels and in SERT binding in the frontal cortex, parietal cortex, and striatum, but not in the hippocampus. These results suggest that chronic co-administration of THC during adolescence can provide some protection against various adverse physiological, behavioral, and neurochemical effects produced by MDMA.

Electromagnetic field effect or simply stress? Effects of UMTS exposure on hippocampal longterm plasticity in the context of procedure related hormone release

Harmful effects of electromagnetic fields (EMF) on cognitive and behavioural features of humans and rodents have been controversially discussed and raised persistent concern about adverse effects of EMF on general brain functions. In the present study we applied radio-frequency (RF) signals of the Universal Mobile Telecommunications System (UMTS) to full brain exposed male Wistar rats in order to elaborate putative influences on stress hormone release (corticosteron; CORT and adrenocorticotropic hormone; ACTH) and on hippocampal derived synaptic long-term plasticity (LTP) and depression (LTD) as electrophysiological hallmarks for memory storage and memory consolidation. Exposure was computer controlled providing blind conditions. Nominal brain-averaged specific absorption rates (SAR) as a measure of applied mass-related dissipated RF power were 0, 2, and 10 W/kg over a period of 120 min. Comparison of cage exposed animals revealed, regardless of EMF exposure, significantly increased CORT and ACTH levels which corresponded with generally decreased field potential slopes and amplitudes in hippocampal LTP and LTD. Animals following SAR exposure of 2 W/kg (averaged over the whole brain of 2.3 g tissue mass) did not differ from the sham-exposed group in LTP and LTD experiments. In contrast, a significant reduction in LTP and LTD was observed at the high power rate of SAR (10 W/kg). The results demonstrate that a rate of 2 W/kg displays no adverse impact on LTP and LTD, while 10 W/kg leads to significant effects on the electrophysiological parameters, which can be clearly distinguished from the stress derived background. Our findings suggest that UMTS exposure with SAR in the range of 2 W/kg is not harmful to critical markers for memory storage and memory consolidation, however, an influence of UMTS at high energy absorption rates (10 W/kg) cannot be excluded.

Locus ceruleus and anterior cingulate cortex sustain wakefulness in a novel environment

Locus ceruleus (LC) neuronal activity is correlated with the waking state, yet LC lesions produce only minor alterations in daily wakefulness. Here, we report that sustained elevations in neurobehavioral and EEG arousal in response to exposure to an environment with novel stimuli, including social interaction, are prevented by selective chemical lesions of the LC in rats. Similar results are seen when the anterior cingulate cortex (ACC), which receives especially dense LC innervation, is selectively denervated of LC input or is ablated by the cell-specific neurotoxin ibotenic acid. Anterograde tracing combined with tyrosine hydroxylase immunohistochemistry demonstrates ACC terminals in apposition with the distal dendrites of LC neurons. Our data implicate the ACC as both a source of input to the LC as well as one of its targets and suggests that the two structures engage in a dialog that may provide a critical neurobiological substrate for sustained attention.

Disruption of Long-Term Alcohol-Related Memory Reconsolidation: Role of β-Adrenoceptors and NMDA Receptors

Disrupting reconsolidation of drug-related memories may be effective in reducing the incidence of relapse. In the current study we examine whether alcohol-related memories are prone to disruption by the β-adrenergic receptor antagonist propranolol (10 mg/kg) and the NMDA receptor antagonist MK801 (0.1 mg/kg) following their reactivation. In operant chambers, male Wistar rats were trained to self-administer a 12% alcohol solution. After 3 weeks of abstinence, the animals were placed in the self-administration cages and were re-exposed to the alcohol-associated cues for a 20-min retrieval period, immediately followed by a systemic injection of either propranolol, MK801 or saline. Rats were tested for cue-induced alcohol seeking on the following day. Retrieval session, injection and test were repeated on two further occasions at weekly intervals. Both propranolol and MK801 administration upon reactivation did not reduce alcohol seeking after the first reactivation test. However, a significant reduction of alcohol seeking was observed over three post-training tests in propranolol treated animals, and MK801 treated animals showed a strong tendency toward reduced alcohol seeking (p = 0.06). Our data indicate that reconsolidation of alcohol-related memories can be disrupted after a long post-training interval and that particularly β-adrenergic receptors may represent novel targets for pharmacotherapy of alcoholism, in combination with cue-exposure therapies.

Agonist-induced internalization of κ-opioid receptors in noradrenergic neurons of the rat locus coeruleus

Kappa-opioid receptors (κOR) are positioned to modulate pre- and post-synaptic responses of norepinephrine-containing neurons in the rat locus coeruleus (LC). The ability of an acute systemic injection of a long acting κOR agonist, U50,488, to induce trafficking of κOR was assessed in the LC using immunogold-silver detection in male Sprague-Dawley rats. U50,488 administration shifted immunogold-silver labeling indicative of κOR from primarily plasmalemmal sites to intracellular sites when compared to vehicle-treated subjects. This translocation from the plasma membrane to the cytoplasmic compartment was prevented by pre-treatment with the κOR antagonist, norbinaltorphimine (norBNI). To determine whether agonist stimulation could induce adaptations in the expression of the noradrenergic synthesizing enzyme, dopamine beta hydroxylase (DβH), and κOR expression, Western blot analysis was used to compare expression levels of DβH and κOR following U50,488 administration. Expression levels for DβH and κOR were significantly increased following U50,488 administration when compared to controls. These data indicate that a systemic injection of a κOR agonist stimulates internalization of κORs in noradrenergic neurons and can impact κOR and DβH expression levels in this stress-sensitive brain region.

Limited efficacy of propranolol on the reconsolidation of fear memories

Previous studies suggested that the beta-adrenergic receptor antagonist propranolol might be a novel, potential treatment for post-traumatic stress disorder (PTSD). This hypothesis stemmed mainly from rodent studies showing that propranolol interferes with the reconsolidation of Pavlovian fear conditioning (FC). However, subsequent investigations in humans have produced controversial evidence about the effect of propranolol on fear memories and an effect on PTSD symptomatology has yet to be reported. Thus, it remains to be established whether propranolol interferes with the reconsolidation of fear memories at large. To address this question, we tested the effect of systemic injections of propranolol administered before or after the retrieval of an inhibitory avoidance (IA) memory elicited with different footshock intensities. In parallel, the same treatment was tested on the reconsolidation of Pavlovian FC. Propranolol showed no effect on the reconsolidation of IA, although the pre-retrieval administration resulted in a significant retrieval impairment. This impairment was transient, and memory returned to control levels at later times. In agreement with previous studies, we found that systemic administration of propranolol disrupts the reconsolidation of Pavlovian FC and that its injection following a retrieval elicited by cue exposure also interferes with the reconsolidation of contextual FC. Hence, propranolol disrupts the reconsolidation of Pavlovian FC, but has no effect on the reconsolidation of IA. The results indicate that the efficacy of systemic administration of propranolol in disrupting the reconsolidation of fear memories is limited.

Cross-species assessments of motor and exploratory behavior related to bipolar disorder

Alterations in exploratory behavior are a fundamental feature of bipolar mania, typically characterized as motor hyperactivity and increased goal-directed behavior in response to environmental cues. In contrast, abnormal exploration associated with schizophrenia and depression can manifest as prominent withdrawal, limited motor activity, and inattention to the environment. While motor abnormalities are cited frequently as clinical manifestations of these disorders, relatively few empirical studies have quantified human exploratory behavior. This article reviews the literature characterizing motor and exploratory behavior associated with bipolar disorder and genetic and pharmacological animal models of the illness. Despite sophisticated assessment of exploratory behavior in rodents, objective quantification of human motor activity has been limited primarily to actigraphy studies with poor cross-species translational value. Furthermore, symptoms that reflect the cardinal features of bipolar disorder have proven difficult to establish in putative animal models of this illness. Recently, however, novel tools such as the human behavioral pattern monitor provide multivariate translational measures of motor and exploratory activity, enabling improved understanding of the neurobiology underlying psychiatric disorders.

GBR 12909 administration as a mouse model of bipolar disorder mania: mimicking quantitative assessment of manic behavior

RATIONALE

Mania is a core feature of bipolar disorder (BD) that traditionally is assessed using rating scales. Studies using a new human behavioral pattern monitor (BPM) recently demonstrated that manic BD patients exhibit a specific profile of behavior that differs from schizophrenia and is characterized by increased motor activity, increased specific exploration, and perseverative locomotor patterns as assessed by spatial d.

OBJECTIVES

It was hypothesized that disrupting dopaminergic homeostasis by inhibiting dopamine transporter (DAT) function would produce a BD mania-like phenotype in mice as assessed by the mouse BPM.

METHODS

We compared the spontaneous locomotor and exploratory behavior of C57BL/6J mice treated with the catecholamine transporter inhibitor amphetamine or the selective DAT inhibitor GBR 12909 in the mouse BPM. We also assessed the duration of the effect of GBR 12909 by testing mice in the BPM for 3 h and its potential strain dependency by testing 129/SvJ mice.

RESULTS

Amphetamine produced hyperactivity and increased perseverative patterns of locomotion as reflected in reduced spatial d values but reduced exploratory activity in contrast to the increased exploration observed in BD patients. GBR 12909 increased activity and reduced spatial d in combination with increased exploratory behavior, irrespective of inbred strain. These effects persisted for at least 3 h.

CONCLUSIONS

Thus, selectively inhibiting the DAT produced a long-lasting cross-strain behavioral profile in mice that was consistent with that observed in manic BD patients. These findings support the use of selective DAT inhibition in animal models of the impaired dopaminergic homeostasis putatively involved in the pathophysiology of BD mania.

Cross-fostering does not alter the neurochemistry or behavior of spontaneously hypertensive rats

Background

Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable developmental disorder resulting from complex gene-gene and gene-environment interactions. The most widely used animal model, the spontaneously hypertensive rat (SHR), displays the major symptoms of ADHD (deficits in attention, impulsivity and hyperactivity) and has a disturbance in the noradrenergic system when compared to control Wistar-Kyoto rats (WKY). The aim of the present study was to determine whether the ADHD-like characteristics of SHR were purely genetically determined or dependent on the gene-environment interaction provided by the SHR dam.

Methods

SHR/NCrl (Charles River, USA), WKY/NCrl (Charles River, USA) and Sprague Dawley rats (SD/Hsd, Harlan, UK) were bred at the University of Cape Town. Rat pups were cross-fostered on postnatal day 2 (PND 2). Control rats remained with their birth mothers to serve as a reference for their particular strain phenotype. Behavior in the open-field and the elevated-plus maze was assessed between PND 29 and 33. Two days later, rats were decapitated and glutamate-stimulated release of [³H]norepinephrine was determined in prefrontal cortex and hippocampal slices.

Results

There was no significant effect of "strain of dam" but there was a significant effect of "pup strain" on all parameters investigated. SHR pups travelled a greater distance in the open field, spent a longer period of time in the inner zone and entered the inner zone of the open-field more frequently than SD or WKY. SD were more active than WKY in the open-field. WKY took longer to enter the inner zone than SHR or SD. In the elevated-plus maze, SHR spent less time in the closed arms, more time in the open arms and entered the open arms more frequently than SD or WKY. There was no difference between WKY and SD behavior in the elevated-plus maze. SHR released significantly more [³H]norepinephrine in response to glutamate than SD or WKY in both hippocampus and prefrontal cortex while SD prefrontal cortex released more [³H]norepinephrine than WKY. SHR were resilient, cross-fostering did not reduce their ADHD-like behavior or change their neurochemistry. Cross-fostering of SD pups onto SHR or WKY dams increased their exploratory behavior without altering their anxiety-like behavior.

Conclusion

The ADHD-like behavior of SHR and their neurochemistry is genetically determined and not dependent on nurturing by SHR dams. The similarity between WKY and SD supports the continued use of WKY as a control for SHR and suggests that SD may be a useful additional reference strain for SHR. The fact that SD behaved similarly to WKY in the elevated-plus maze argues against the use of WKY as a model for anxiety-like disorders.

A dose-response study of the effects of pre-test administration of beta-adrenergic receptor antagonist propranolol on the learning of active place avoidance, a spatial cognition task, in rats

The involvement of various neurotransmitter receptors in the brain in the regulation of spatial behavior is a focus of interest for many cognitive neuroscientists. Active allothetic place avoidance (AAPA) task have been demonstrated to require spatial mapping and cognitive coordination and is highly dependent upon hippocampus. The present study was designed to evaluate the role of beta-adrenergic receptors in the modulation of locomotor and spatial behavior in this task. Four doses of centrally active beta-adrenergic antagonist propranolol (5, 20, 25 and 30 mg/kg) were administered intraperitoneally 30 min prior to testing in the place avoidance task. Four daily sessions were pursued, each lasting 20 min. A dose of 25 mg/kg was found to induce a deficit in spatial behavior (measured by number of entrances into the shock sector) without altering locomotion; lower doses were without effect. The highest dose (30 mg/kg) impaired both locomotion and avoidance behavior. The results suggest that beta-adrenoceptors are involved in the regulation of behavior in the place avoidance task and that it is possible to dissociate the effect of propranolol on the spatial performance and locomotion in the AAPA using dose-selection.

Rhythmic oscillations in quantitative EEG measured during a continuous performance task

The objective of the present investigation was to determine if cyclic variations in human performance recorded during a 30 min continuous performance task would parallel cyclic variations in right-hemisphere beta-wave activity. A fast fourier transformation was performed on the quantitative electroencephalogram (qEEG) and the performance record of each participant (N = 62), producing an individual periodogram for each outcome measure. An average periodogram was then produced for both qEEG and performance by combining (averaging) the amplitudes associated with each periodicity in the 62 original periodograms. Periodicities ranging from 1.00 to 2.00 min and from 4.70 to 5.70 min with amplitudes greater than would be expected due to chance were retained (Smith et al. 2003). The results of the present investigation validate the existence of cyclic variations in human performance that have been identified previously (Smith et al. 2003) and extend those findings by implicating right-hemisphere mediated arousal in the process (Arruda et al. 1996, 1999, 2007). Significant cyclic variations in left-hemisphere beta-wave activity were not observed. Taken together, the findings of the present investigation support a model of sustained attention that predicts cyclic changes in human performance that are the result of cyclic changes in right-hemisphere arousal.

Cannabinoids and novelty investigation: influence of age and duration of exposure

Administration of the synthetic cannabinoid receptor agonist WIN 55,212-2 has been shown to increase indices of noradrenergic activity. Neuroanatomical, neurochemical and behavioral studies have provided evidence supporting a marked impact of cannabinoids on the rat coeruleo-cortical pathway. As activity of this pathway is implicated in setting specific attentional modes, the present study assessed the influence of acute and repeated systemic administration of WIN 55,212-2 on novelty investigation in adolescent and adult male rats by using the hole board behavioral paradigm. Animals were individually acclimated to the hole board for 10-min sessions over 3 days, and novel objects were introduced on the fourth day. Novelty-seeking behavior was measured by comparison of the average number of return visits to a hole containing a novel object versus the average number of return visits to an empty hole. While attenuation of novelty preference was observed in adult rats acutely treated with WIN 55,212-2, both acutely treated adolescent groups retained their preference for novelty. All groups treated with repeated administration of either drug or vehicle demonstrated novelty preference, and no differences were found in the measure of novelty investigation between the groups. Furthermore, this study reproduced findings showing significant differences in locomotor activity that did not coincide with differences in novelty-seeking behavior. These data thus suggest a complex effect of CB1 receptor modulation on novelty preference in the male rat that is modulated by age and treatment.

Dissociation of the neurochemical and behavioral toxicology of MDMA ('Ecstasy') by citalopram

High or repeated doses of the recreational drug 3,4-methylenedioxymethamphetamine (MDMA, or 'Ecstasy') produce long-lasting deficits in several markers of serotonin (5-HT) system integrity and also alter behavioral function. However, it is not yet clear whether MDMA-induced serotonergic neurotoxicity is responsible for these behavioral changes or whether other mechanisms are involved. The present experiment tested the hypothesis that blocking serotonergic neurotoxicity by pretreatment with the selective 5-HT reuptake inhibitor citalopram will also prevent the behavioral and physiological consequences of an MDMA binge administration. Male, Sprague-Dawley rats (N=67) received MDMA (4 x 10 mg/kg) with or without citalopram (10 mg/kg) pretreatment. Core temperature, ejaculatory response, and body weight were monitored during and immediately following drug treatments. A battery of tests assessing motor, cognitive, exploratory, anxiety, and social behaviors was completed during a 10-week period following MDMA administration. Brain tissue was collected at 1 and 10 weeks after drug treatments for measurement of regional 5-HT transporter binding and (for the 1-week samples) 5-HT and 5-HIAA concentrations. Citalopram pretreatment blocked MDMA-related reductions in aggressive and exploratory behavior measured in the social interaction and hole-board tests respectively. Such pretreatment also had the expected protective effect against MDMA-induced 5-HT neurotoxicity at 1 week following the binge. In contrast, citalopram did not prevent most of the acute effects of MDMA (eg hyperthermia and weight loss), nor did it block the decreased motor activity seen in the binge-treated animals 1 day after dosing. These results suggest that some of the behavioral and physiological consequences of a high-dose MDMA regimen in rats are mediated by mechanisms other than the drug's effects on the serotonergic system. Elucidation of these mechanisms requires further study of the influence of MDMA on other neurotransmitter systems.

Noradrenergic modulation of electrical coupling in GABAergic networks of the hippocampus

Noradrenergic modulation of cortical circuits is involved in information processing, regulation of higher functions, and prevention of epileptic activity. Here, we studied the effects of noradrenaline on the functional connectivity of GABAergic networks of the hippocampus and show that electrical synapses between interneurons are a novel target of noradrenergic modulation in vitro. Application of noradrenaline or of the selective beta-adrenergic agonist isoproterenol decreased gap junction-based coupling in paired recordings from stratum lacunosum-moleculare interneurons by approximately 40%. Similar results were obtained after pharmacological stimulation of the adenylyl cyclase with forskolin. In contrast, the adenylyl cyclase antagonist MDL12330A [cis-N-(2-phenylcyclopentyl)azacyclotridec-1-en-2-amine] or the specific protein kinase A (PKA) inhibitor H89 (N-[2-(p-bromocinnamyl-amino)ethyl]-5-isoquinolinesulfonamide dihydrochloride) enhanced the basal strength of coupling by approximately 30%. In addition, PKA-mediated phosphorylation was critical for both isoproterenol- and forskolin-dependent regulation of coupling, because inclusion of the PKA antagonist KT5720 [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylicacid hexyl ester] in the recording pipettes prevented modulation. Lastly, we studied the effects of beta-adrenergic modulation on mixed polysynaptic transmission within the GABAergic network. Isoproterenol depressed propagation of GABA(A) receptor-mediated synaptic currents, but did not change significantly direct GABAergic input, indicating that regulation of electrical coupling adds flexibility to the information flow generated by chemical synapses. In conclusion, activation of beta-adrenergic receptors in stratum lacunosum-moleculare GABAergic networks reduces electrical synaptic transmission via a cAMP/PKA signaling cascade, and affects the degree of synaptic divergence within the circuit. We propose that this dynamic modulation and interplay between electrical and chemical synaptic transmission in GABAergic networks contributes to the tuning of memory processes in vivo, and prevents hypersynchronous activity.

-Adrenoreceptors Comprise a Critical Element in Learning-Facilitated Long-Term Plasticity

A novel spatial environment consists of several different types of information that may be encoded by cellular information storage mechanisms such as long-term potentiation (LTP) and long-term depression (LTD). Arousal, mediated, for example, by activation of the noradrenergic system, is a critical factor in information acquisition and may enhance the encoding of novel spatial information. Using electrophysiological recordings of hippocampal responses in freely moving rats during spatial learning, we investigated the role of the beta-adrenoreceptor in Schaeffer collateral-CA1 synaptic plasticity. We found that novel exploration of spatial context facilitates induction of LTD that is inhibited by intracerebroventricular application of the beta-adrenoreceptor antagonist, propranolol. Long-lasting homosynaptic LTD, that was electrically induced by low-frequency stimulation, was unaffected by the antagonist. Although application of a beta-adrenoreceptor agonist (isoproterenol) did not affect electrically induced LTD, agonist application facilitated short-term depression (STD) into LTD and mimicked the augmentation, through spatial exploration, of STD into LTD. Exploration of a novel empty environment facilitated LTP that was prevented by application of propranolol. These results suggest that beta-adrenoreceptors may facilitate encoding of spatial information through synaptic plasticity in the hippocampus and that noradrenaline is a key factor in effective information acquisition.

Early life stress and novelty seeking behavior in adolescent monkeys

Recent evidence suggests that early exposure to mild stress promotes the development of novelty seeking behavior. Here we test this hypothesis in squirrel monkeys and investigate whether novelty seeking behavior is associated with differences in cerebrospinal fluid (CSF) levels of the serotonin metabolite 5-hydroxyindoleacetic acid (5HIAA), the dopamine metabolite homovanillic acid (HVA), the norepinephrine metabolite 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), and the neuropeptide corticotrophin-releasing factor (CRF). Monkeys were randomized early in life to either mild intermittent stress (IS) or no stress (NS) conditions, and subsequently presented with opportunities to interact with a familiar or novel object in a test box that was connected to each monkey's home cage. To further minimize the potentially stressful nature of the test situation, monkeys were acclimated to the test procedures prior to study initiation. Post-test plasma levels of cortisol in IS and NS monkeys did not differ significantly from baseline levels measured in undisturbed conditions. During testing, more IS than NS monkeys voluntarily left the home cage, and IS monkeys spent more time in the test box compared to NS monkeys. More IS than NS monkeys engaged in object exploration in the test box, and IS monkeys preferred to interact with the novel vs. familiar object. Novelty seeking was not associated with differences in 5HIAA, HVA, MHPG, or CRF, but correlated with differences in object exploration observed in a different test situation at an earlier age. These trait-like differences in novelty seeking appear to reflect mild early stress-induced adaptations that enhance curiosity and resilience.