Footshock-induced responses in ventral subiculum neurons are mediated by locus coeruleus noradrenergic afferents

The ventral subiculum (vSub) of the hippocampus is critically involved in mediating the forebrain's response to stress, particularly with regard to psychogenic stressors. Stress, in turn, is known to aggravate many psychiatric conditions including schizophrenia, depression, anxiety, and drug abuse. Pathological alterations in hippocampal function have been identified in all these disorders; thus, it is of interest to understand how stress affects this brain region. The vSub receives dense projections from the stress-related locus coeruleus (LC); however, it is not known what role this input plays in signaling stressful stimuli. In this study, the direct LC innervation of the vSub was investigated as a potential mediator of stress responses in this region. To examine responses to an acute stressor, the effect of footshock on single vSub neurons was tested in rats. Footshock inhibited 13%, and activated 48% of neurons in this region. Importantly, responses to footshock were correlated with LC stimulation-evoked responses in single neurons, and LC inactivation blocked these responses. Furthermore, prazosin, an alpha-1 antagonist, reversed footshock-evoked inhibition, revealing an underlying activation. Inactivation of the basolateral amygdala (BLA) did not block phasic footshock-evoked activation; however, it reduced tonic activity in the vSub. These results suggest that the LC NE system plays an important role in mediating stress responses in the vSub. Footshock evokes both inhibition and excitation in the vSub, by activating noradrenergic inputs from the LC. These responses may contribute to stress adaptation; while an imbalance of this system may lead to pathological stress responses in mental disorders.

Differential effects of acute and repeated stress on hippocampus and amygdala inputs to the nucleus accumbens shell

The basolateral amygdala (BLA) and ventral subiculum (vSub) of the hippocampus convey emotion and context information, respectively, to the nucleus accumbens (NAc). Using in vivo extracellular recordings from NAc neurons, we examined how acute and repeated restraint stress alters the plasticity of the vSub and BLA afferent pathways. High-frequency (HFS) and low-frequency (LFS) stimulation was applied to the vSub to assess the impact on NAc responses to vSub and BLA inputs. In addition, iontophoretic application of the dopamine D2-antagonist sulpiride was used to explore the role of dopamine in the NAc in mediating the effects of stress on plasticity. Acute and repeated restraint caused disparate effects on BLA- and vSub-evoked responses in the NAc. Following repeated restraint, but not after acute restraint, HFS of the vSub failed to potentiate the vSub–NAc pathway while instead promoting a long-lasting reduction of the BLA–NAc pathway and these effects were independent of D2-receptor activity. In contrast, LFS to the vSub pathway after acute restraint resulted in potentiation in the vSub–NAc pathway while BLA-evoked responses were unchanged. When sulpiride was applied prior to LFS of the vSub after acute stress, there was a pronounced decrease in vSub-evoked responses similar to control animals. This work provides new insight into the impact of acute and repeated stress on the integration of context and emotion inputs in the NAc. These data support a model of stress whereby the hippocampus is inappropriately activated and dominates the information processing within this circuit via a dopaminergic mechanism after acute bouts of stress.

Abnormal stress responsivity in a rodent developmental disruption model of schizophrenia

Although numerous studies have implicated stress in the pathophysiology of schizophrenia, less is known about how the effects of stress interact with genetic, developmental, and/or environmental determinants to promote disease progression. In particular, it has been proposed that in humans, stress exposure in adolescence could combine with a predisposition towards increased stress sensitivity, leading to prodromal symptoms and eventually psychosis. However, the neurobiological substrates for this interaction are not fully characterized. Previous work in our lab has demonstrated that rats born to dams administered with the DNA-methylating agent methylazoxymethanol acetate (MAM) at gestational day 17 exhibit as adults behavioral and anatomical abnormalities consistent with those observed in patients with schizophrenia. Here, we examined behavioral and neuroendocrine responses to stress in the MAM model of schizophrenia. MAM-treated male rats were exposed to acute and repeated footshock stress at prepubertal, peripubteral, and adult ages. Ultrasonic vocalizations (USVs), freezing, and corticosterone responses were quantified. We found that juvenile MAM-treated rats emitted significantly more calls, spent more time vocalizing, emitted calls at a higher rate, and showed more freezing in response to acute footshock stress when compared with their saline (SAL) treated counterparts, and that this difference is not present in older animals. In addition, adolescent MAM-treated animals displayed a blunted HPA axis corticosterone response to acute footshock that did not adapt after 10 days of stress exposure. These data demonstrate abnormal stress responsivity in the MAM model of schizophrenia and suggest that these animals are more sensitive to the effects of stress in youth.

Peripubertal diazepam administration prevents the emergence of dopamine system hyperresponsivity in the MAM developmental disruption model of schizophrenia

Schizophrenia is believed to arise from an interaction of genetic predisposition and adverse environmental factors, with stress being a primary variable. We propose that alleviating anxiety produced in response to stress during a sensitive developmental period may circumvent the dopamine (DA) system alterations that may correspond to psychosis in adults. This was tested in a developmental rat model of schizophrenia based on prenatal administration of the mitotoxin methyl azoxymethanol acetate (MAM). MAM administration leads to a hyperdopaminergic state consisting of an increase in the number of DA neurons firing spontaneously, which correlates with an increased behavioral response to amphetamine. MAM-treated rats exhibited a heightened level of anxiety during adolescence. Peripubertal administration of the antianxiety agent diazepam was found to prevent the increase in DA neuron activity and blunt the behavioral hyperresponsivity to amphetamine in these rats. These data suggest that the pathophysiological factors leading to the onset of psychosis in early adulthood may be circumvented by controlling the response to stress during the peripubertal period.

Afferent drive of medial prefrontal cortex by hippocampus and amygdala is altered in MAM-treated rats: evidence for interneuron dysfunction

Evidence indicates that the prefrontal cortex and its regulation by afferent inputs are disrupted in schizophrenia. Using a validated rat model of schizophrenia based on prenatal administration of the mitotoxin methyl azoxymethanol acetate (MAM), we examined the convergent projections from the ventral hippocampus (vHipp) and the basolateral amygdala (BLA) in the medial prefrontal cortex (mPFC). In vivo extracellular recordings were done in anesthetized rats to assess how prior stimulation of the BLA or vHipp input to the mPFC affected mPFC responses to subsequent stimulation of these regions. The interstimulus interval (ISI) of the BLA and vHipp pulse stimulation was varied randomly between 0 and 130 ms, and the probability of evoked spike response in the mPFC measured. We found that BLA input increased vHipp-evoked spike probability at ISIs 40-130 ms, but decreased spike probability at ISIs 10-20 ms. This would be consistent with activation of inhibitory interneurons at shorter ISIs by BLA stimulation. In contrast, in MAM-treated rats BLA stimulation increased vHipp-evoked spike probability in mPFC at all ISIs tested. Given that interneurons are driven primarily by N-methyl-D-aspartate (NMDA) channel activation, the effects of the NMDA channel blocker, phencyclidine (PCP), were tested. PCP was found to completely attenuate the inhibitory effect of BLA input on vHipp-evoked responses in mPFC at shorter ISIs, causing the response in control rats treated with PCP to resemble that observed in the MAM rat. In contrast to the effects of BLA stimulation on vHipp-mPFC-evoked responses, there was no inhibitory period when examining the effects of vHipp stimulation on BLA-mPFC-evoked responses in control rats, but in MAM-treated rats there was a significant inhibition at short intervals. Thus, both affective input arising from the BLA and context-dependent input from the vHipp exert a modulatory effect on mPFC neural activity in response to these inputs. Whereas the BLA potentiated vHipp input to the mPFC at long intervals, there was a short-interval inhibitory period that appeared to be mediated by an NMDA-dependent drive of interneurons. This inhibitory modulation was absent in the model of schizophrenia and following PCP, which is consistent with an interneuron disruption in this disorder.

An inexpensive, charge-balanced rodent deep brain stimulation device: a step-by-step guide to its procurement and construction

BACKGROUND

Despite there being a relatively large number of methods papers which detail specifically the development of stimulation devices, only a small number of reports involve the application of these devices in freely moving animals. To date multiple preclinical neural stimulators have been designed and described but have failed to make an impact on the methods employed by the majority of laboratories studying DBS. Thus, the overwhelming majority of DBS studies are still performed by tethering the subject to an external stimulator. We believe that the low adoption rate of previously described methods is a result of the complexity of replicating and implementing these methods.

NEW METHOD

Here were describe both the design and procurement of a simple and inexpensive stimulator designed to be compatible with commonly used, commercially available electrodes (Plastics 1).

RESULTS

This system is initially programmable in frequency, pulsewidth and current amplitude, and delivers biphasic, charge-balanced output to two independent electrodes.

COMPARISON WITH EXISTING METHOD(S)

It is easy to implement requiring neither subcutaneous implantation nor custom-made electrodes and has been optimized for either direct mounting to the head or for use with rodent jackets.

CONCLUSIONS

This device is inexpensive and universally accessible, facilitating high throughput, low cost, long-term rodent deep brain stimulation experiments.

Evaluation of animal models of obsessive-compulsive disorder: correlation with phasic dopamine neuron activity

Obsessive compulsive disorder (OCD) is a psychiatric condition defined by intrusive thoughts (obsessions) associated with compensatory and repetitive behaviour (compulsions). However, advancement in our understanding of this disorder has been hampered by the absence of effective animal models and correspondingly analysis of the physiological changes that may be present in these models. To address this, we have evaluated two current rodent models of OCD; repeated injection of dopamine D2 agonist quinpirole and repeated adolescent injection of the tricyclic agent clomipramine in combination with a behavioural paradigm designed to produce compulsive lever pressing. These results were then compared with their relative impact on the state of activity of the mesolimbic dopaminergic system using extracellular recoding of spontaneously active dopamine neurons in the ventral tegmental area (VTA). The clomipramine model failed to exacerbate compulsive lever pressing and VTA dopamine neurons in clomipramine-treated rats had mildly diminished bursting activity. In contrast, quinpirole-treated animals showed significant increases in compulsive lever pressing, which was concurrent with a substantial diminution of bursting activity of VTA dopamine neurons. Therefore, VTA dopamine activity correlated with the behavioural response in these models. Taken together, these data support the view that compulsive behaviours likely reflect, at least in part, a disruption of the dopaminergic system, more specifically by a decrease in baseline phasic dopamine signalling mediated by burst firing of dopamine neurons.

Disruption of prefrontal cortical-hippocampal balance in a developmental model of schizophrenia: reversal by sulpiride

The nucleus accumbens (NAc) receives converging inputs from the medial prefrontal cortex (mPFC) and the hippocampus which have competitive interactions in the NAc to influence motivational drive. We have previously shown altered synaptic plasticity in the hippocampal-NAc pathway in the methylazoxymethanol acetate (MAM) developmental model of schizophrenia in rodents that is dependent on cortical inputs. Thus, because mPFC-hippocampal balance is known to be partially altered in this model, we investigated potential pathological changes in the hippocampal influence over cortex-driven NAc spike activity. Here we show that the reciprocal interaction between the hippocampus and mPFC is absent in MAM animals but is able to be reinstated with administration of the antipsychotic drug, sulpiride. The lack of interaction between these structures in this model could explain the attentional deficits in schizophrenia patients and shed light onto their inability to focus on a single task.

Deep brain stimulation of the ventral hippocampus restores deficits in processing of auditory evoked potentials in a rodent developmental disruption model of schizophrenia

Existing antipsychotic drugs are most effective at treating the positive symptoms of schizophrenia but their relative efficacy is low and they are associated with considerable side effects. In this study deep brain stimulation of the ventral hippocampus was performed in a rodent model of schizophrenia (MAM-E17) in an attempt to alleviate one set of neurophysiological alterations observed in this disorder. Bipolar stimulating electrodes were fabricated and implanted, bilaterally, into the ventral hippocampus of rats. High frequency stimulation was delivered bilaterally via a custom-made stimulation device and both spectral analysis (power and coherence) of resting state local field potentials and amplitude of auditory evoked potential components during a standard inhibitory gating paradigm were examined. MAM rats exhibited alterations in specific components of the auditory evoked potential in the infralimbic cortex, the core of the nucleus accumbens, mediodorsal thalamic nucleus, and ventral hippocampus in the left hemisphere only. DBS was effective in reversing these evoked deficits in the infralimbic cortex and the mediodorsal thalamic nucleus of MAM-treated rats to levels similar to those observed in control animals. In contrast stimulation did not alter evoked potentials in control rats. No deficits or stimulation-induced alterations were observed in the prelimbic and orbitofrontal cortices, the shell of the nucleus accumbens or ventral tegmental area. These data indicate a normalization of deficits in generating auditory evoked potentials induced by a developmental disruption by acute high frequency, electrical stimulation of the ventral hippocampus.

The functional DRD3 Ser9Gly polymorphism (rs6280) is pleiotropic, affecting reward as well as movement

Abnormalities of motivation and behavior in the context of reward are a fundamental component of addiction and mood disorders. Here we test the effect of a functional missense mutation in the dopamine 3 receptor (DRD3) gene (ser9gly, rs6280) on reward-associated dopamine (DA) release in the striatum. Twenty-six healthy controls (HCs) and 10 unmedicated subjects with major depressive disorder (MDD) completed two positron emission tomography (PET) scans with [¹¹C]raclopride using the bolus plus constant infusion method. On one occasion subjects completed a sensorimotor task (control condition) and on another occasion subjects completed a gambling task (reward condition). A linear regression analysis controlling for age, sex, diagnosis, and self-reported anhedonia indicated that during receipt of unpredictable monetary reward the glycine allele was associated with a greater reduction in D2/3 receptor binding (i.e., increased reward-related DA release) in the middle (anterior) caudate (p<0.01) and the ventral striatum (p<0.05). The possible functional effect of the ser9gly polymorphism on DA release is consistent with previous work demonstrating that the glycine allele yields D3 autoreceptors that have a higher affinity for DA and display more robust intracellular signaling. Preclinical evidence indicates that chronic stress and aversive stimulation induce activation of the DA system, raising the possibility that the glycine allele, by virtue of its facilitatory effect on striatal DA release, increases susceptibility to hyperdopaminergic responses that have previously been associated with stress, addiction, and psychosis.

SaBer DBS: a fully programmable, rechargeable, bilateral, charge-balanced preclinical microstimulator for long-term neural stimulation

To effectively study the mechanisms by which deep brain stimulation (DBS) produces its therapeutic benefit and to evaluate new therapeutic indications, it is vital to administer DBS over an extended period of time in awake, freely behaving animals. To date multiple preclinical stimulators have been designed and described. However, these stimulators have failed to incorporate some of the design criteria necessary to provide a system analogous to those used clinically. Here we define these design criteria and propose an improved and complete preclinical DBS system. This system is fully programmable in frequency, pulse-width and current amplitude, has a rechargeable battery and delivers biphasic, charge-balanced output to two independent electrodes. The system has been optimized for either implantation or for use externally via attachment to rodent jackets.

Blinded prospective evaluation of computer-based mechanistic schizophrenia disease model for predicting drug response

The tremendous advances in understanding the neurobiological circuits involved in schizophrenia have not translated into more effective treatments. An alternative strategy is to use a recently published ‘Quantitative Systems Pharmacology’ computer-based mechanistic disease model of cortical/subcortical and striatal circuits based upon preclinical physiology, human pathology and pharmacology. The physiology of 27 relevant dopamine, serotonin, acetylcholine, norepinephrine, gamma-aminobutyric acid (GABA) and glutamate-mediated targets is calibrated using retrospective clinical data on 24 different antipsychotics. The model was challenged to predict quantitatively the clinical outcome in a blinded fashion of two experimental antipsychotic drugs; JNJ37822681, a highly selective low-affinity dopamine D₂ antagonist and ocaperidone, a very high affinity dopamine D₂ antagonist, using only pharmacology and human positron emission tomography (PET) imaging data. The model correctly predicted the lower performance of JNJ37822681 on the positive and negative syndrome scale (PANSS) total score and the higher extra-pyramidal symptom (EPS) liability compared to olanzapine and the relative performance of ocaperidone against olanzapine, but did not predict the absolute PANSS total score outcome and EPS liability for ocaperidone, possibly due to placebo responses and EPS assessment methods. Because of its virtual nature, this modeling approach can support central nervous system research and development by accounting for unique human drug properties, such as human metabolites, exposure, genotypes and off-target effects and can be a helpful tool for drug discovery and development.

Long-term high frequency deep brain stimulation of the nucleus accumbens drives time-dependent changes in functional connectivity in the rodent limbic system

Deep brain stimulation of the ventral striatum is an effective treatment for a variety of treatment refractory psychiatric disorders yet the mechanism of action remains elusive. We examined how five days of stimulation affected rhythmic brain activity in freely moving rats in terms of oscillatory power within, and coherence between, selected limbic regions bilaterally. Custom made bipolar stimulating/recording electrodes were implanted, bilaterally, in the nucleus accumbens core. Local field potential (LFP) recording electrodes were implanted, bilaterally in the prelimbic and orbitofrontal cortices and mediodorsal thalamic nucleus. Stimulation was delivered bilaterally with 100 μs duration constant current pulses at a frequency of 130 Hz delivered at an amplitude of 100 μA using a custom-made stimulation device. Synchronized video and LFP data were collected from animals in their home cages before, during and after stimulation. Signals were processed to remove movement and stimulation artifacts, and analyzed to determine changes in spectral power within, and coherence between regions. Five days stimulation of the nucleus accumbens core yielded temporally dynamic modulation of LFP power in multiple bandwidths across multiple brain regions. Coherence was seen to decrease in the alpha band between the mediodorsal thalamic nucleus and core of the nucleus accumbens. Coherence between each core of the nucleus accumbens bilaterally showed rich temporal dynamics throughout the five day stimulation period. Stimulation cessation revealed significant "rebound" effects in both power and coherence in multiple brain regions. Overall, the initial changes in power observed with short-term stimulation are replaced by altered coherence, which may reflect the functional action of DBS.

Postpartum and depression status are associated with lower [[¹¹C]raclopride BP(ND) in reproductive-age women

The early postpartum period is associated with increased risk for affective and psychotic disorders. Because maternal dopaminergic reward system function is altered with perinatal status, dopaminergic system dysregulation may be an important mechanism of postpartum psychiatric disorders. Subjects included were non-postpartum healthy (n=13), postpartum healthy (n=13), non-postpartum unipolar depressed (n=10), non-postpartum bipolar depressed (n=7), postpartum unipolar (n=13), and postpartum bipolar depressed (n=7) women. Subjects underwent 60 min of [¹¹C]raclopride-positron emission tomography imaging to determine the nondisplaceable striatal D₂/₃ receptor binding potential (BP(ND)). Postpartum status and unipolar depression were associated with lower striatal D₂/₃ receptor BP(ND) in the whole striatum (p=0.05 and p=0.02, respectively) that reached a maximum of 7-8% in anteroventral striatum for postpartum status (p=0.02). Unipolar depression showed a nonsignificant trend toward being associated with 5% lower BP(ND) in dorsal striatum (p=0.06). D₂/₃ receptor BP(ND) did not differ significantly between unipolar depressed and healthy postpartum women or between bipolar and healthy subjects; however, D₂/₃ receptor BP(ND) was higher in dorsal striatal regions in bipolar relative to unipolar depressives (p=0.02). In conclusion, lower striatal D₂/₃ receptor BP(ND) in postpartum and unipolar depressed women, primarily in ventral striatum, and higher dorsal striatal D₂/₃ receptor BP(ND) in bipolar relative to unipolar depressives reveal a potential role for the dopamine (DA) system in the physiology of these states. Further studies delineating the mechanisms underlying these differences in D₂/₃ receptor BP(ND), including study of DA system responsivity to rewarding stimuli, and increasing power to assess unipolar vs bipolar-related differences, are needed to better understand the affective role of the DA system in postpartum and depressed women.

Different stressors produce excitation or inhibition of mesolimbic dopamine neuron activity: response alteration by stress pre-exposure

Stressors can exert a wide variety of responses, ranging from adaptive responses to pathological changes; moreover, recent studies suggest that mild stressors can attenuate the response of a system to major stressful events. We have previously shown that 2-week exposure to cold, a comparatively mild inescapable stressor, induced a pronounced reduction in ventral tegmental area (VTA) dopamine (DA) neuron activity, whereas restraint stress increases DA neuron activity. However, it is not known if these stressors differentially impact the VTA in a region-specific manner, if they differentially impact behavioral responses, or whether the effects of such different stressors are additive or antagonistic with regard to their impact on DA neuron firing. To address these questions, single-unit extracellular recordings were performed in anesthetized control rats and rats exposed to chronic cold, and tested after delivery of a 2-h restraint session. Chronic cold stress strongly attenuated the number of DA neurons firing in the VTA, and this effect occurred primarily in the medial and central VTA regions that preferentially project to reward-related ventral striatal regions. Chronic cold exposure also prevented the pronounced increase in DA neuron population activity without affecting the behavioral sensitization to amphetamine produced by restraint stress. Taken together, these data show that a prolonged inescapable mild stressor can induce plastic changes that attenuate the DA system response to acute stress.

Ventral striatal prediction error signaling is associated with dopamine synthesis capacity and fluid intelligence

Fluid intelligence represents the capacity for flexible problem solving and rapid behavioral adaptation. Rewards drive flexible behavioral adaptation, in part via a teaching signal expressed as reward prediction errors in the ventral striatum, which has been associated with phasic dopamine release in animal studies. We examined a sample of 28 healthy male adults using multimodal imaging and biological parametric mapping with (1) functional magnetic resonance imaging during a reversal learning task and (2) in a subsample of 17 subjects also with positron emission tomography using 6-[(18) F]fluoro-L-DOPA to assess dopamine synthesis capacity. Fluid intelligence was measured using a battery of nine standard neuropsychological tests. Ventral striatal BOLD correlates of reward prediction errors were positively correlated with fluid intelligence and, in the right ventral striatum, also inversely correlated with dopamine synthesis capacity (FDOPA K inapp). When exploring aspects of fluid intelligence, we observed that prediction error signaling correlates with complex attention and reasoning. These findings indicate that individual differences in the capacity for flexible problem solving relate to ventral striatal activation during reward-related learning, which in turn proved to be inversely associated with ventral striatal dopamine synthesis capacity.

Heterogeneous processing of amygdala and hippocampal inputs in the rostral and caudal subregions of the nucleus accumbens

The nucleus accumbens (NAc) receives converging input from a number of structures proposed to play a role in affective disorders. In particular, the basolateral amygdala (BLA) provides an affective input that overlaps with context-related information derived from the ventral subiculum of the hippocampus (vSub). We examined how stimulation of the BLA is modulated by, and in turn affects, vSub inputs to this region. In-vivo extracellular recordings were performed in the NAc of anaesthetized rats. The effect of high-frequency (theta-burst) stimulation (HFS) of the BLA on both BLA and vSub-evoked responses was tested. In addition, the involvement of dopamine D2 receptors in BLA-induced plasticity in the NAc was examined by pre-treatment with sulpiride (5 mg/kg i.v.). Finally, tetrodotoxin (TTX) was used to inactivate the vSub and the effect on BLA-evoked responses was assessed. We found that HFS of the BLA causes hetereogeneous patterns of plasticity, depression and potentiation, respectively, in the rostral and caudal subregions of the NAc that are disrupted following D2 receptor antagonist treatment. In addition, inactivating the vSub with TTX attenuates the ability of the BLA to drive spike firing in the NAc. Thus, the vSub is required for activation of the NAc by the BLA. These data support a model whereby the amygdala can coordinate reward-seeking and fear-related behaviours via its differential regulation of NAc output. In addition, the hippocampus inappropriately dominates information processing within this circuit, potentially contributing to the overwhelming focus on internal emotional states in disorders such as depression.

Reorganization of striatal inhibitory microcircuits leads to pathological synchrony in the Basal Ganglia

Neural synchronization plays an important role in information flow in the nervous system under healthy and pathological conditions. In this issue of Neuron, Gittis et al. show that reorganization of striatal microcircuits promotes synchronous activity and may underlie the pathological network oscillations at the root of motor symptoms described in Parkinson's disease.

Hippocampal dysregulation of dopamine system function and the pathophysiology of schizophrenia

Substantial evidence suggests that psychosis in schizophrenia is associated with dysregulation of subcortical dopamine system function. Here we examine evidence that this dysregulation is secondary to hyperactivity within hippocampal subfields. Enhanced hippocampal activity has been reported in preclinical models and in schizophrenia patients. Moreover, this hippocampal hyperactivity is correlated with enhanced dopamine neuron activity and positive symptoms, respectively. Thus, restoration of hippocampal function could provide a more effective therapeutic approach than current therapeutics based on blockade of dopamine D2 receptors. Indeed, initial studies demonstrate that allosteric modulation of the α5GABA(A) receptor can decrease aberrant dopamine signaling and associated behaviors in a verified rodent model of psychosis.

Hippocampus, amygdala, and stress: interacting systems that affect susceptibility to addiction

Stress is one of the major factors in drug abuse, particularly in relapse and drug-seeking behavior. However, the mechanisms underlying the interactions between stress and drug abuse are unclear. For many years, studies have focused on the role of the dopaminergic reward system in drug abuse. Our results, for example, show that increased dopaminergic activity is induced by drug sensitization and different stressors via potentiation of the ventral subiculum-nucleus accumbens (NAc) pathway. Although the role of the norepinephrine (NE) system in stress is well known, its involvement in drug abuse has received less attention. This review explores the different mechanisms by which stressors can modulate the ventral subiculum-accumbens pathway, and how these modulations can induce alterations in the behavioral response to drug administration. In particular, we will focus on two main afferents to the NAc, the basolateral amygdala and the ventral subiculum of the hippocampus, and their interactions with the locus coeruleus-norepinephrine system.