Selective activation of medial prefrontal-to-accumbens projection neurons by amygdala stimulation and Pavlovian conditioned stimuli

Chronic restraint stress induces depression-like behaviors and alterations in the afferent projections of medial prefrontal cortex from multiple brain regions in mice

INTRODUCTION

The medial prefrontal cortex (mPFC) forms output pathways through projection neurons, inversely receiving adjacent and long-range inputs from other brain regions. However, how afferent neurons of mPFC are affected by chronic stress needs to be clarified. In this study, the effects of chronic restraint stress (CRS) on the distribution density of mPFC dendrites/dendritic spines and the projections from the cortex and subcortical brain regions to the mPFC were investigated.

METHODS

In the present study, C57BL/6 J transgenic (Thy1-YFP-H) mice were subjected to CRS to establish an animal model of depression. The infralimbic (IL) of mPFC was selected as the injection site of retrograde AAV using stereotactic technique. The effects of CRS on dendrites/dendritic spines and afferent neurons of the mPFC IL were investigaed by quantitatively assessing the distribution density of green fluorescent (YFP) positive dendrites/dendritic spines and red fluorescent (retrograde AAV recombinant protein) positive neurons, respectively.

RESULTS

The results revealed that retrograde tracing virus labeled neurons were widely distributed in ipsilateral and contralateral cingulate cortex (Cg1), second cingulate cortex (Cg2), prelimbic cortex (PrL), infralimbic cortex, medial orbital cortex (MO), and dorsal peduncular cortex (DP). The effects of CRS on the distribution density of mPFC red fluorescence positive neurons exhibited regional differences, ranging from rostral to caudal or from top to bottom. Simultaneously, CRS resulted a decrease in the distribution density of basal, proximal and distal dendrites, as well as an increase in the loss of dendritic spines of the distal dendrites in the IL of mPFC. Furthermore, varying degrees of red retrograde tracing virus fluorescence signals were observed in other cortices, amygdala, hippocampus, septum/basal forebrain, hypothalamus, thalamus, mesencephalon, and brainstem in both ipsilateral and contralateral brain. CRS significantly reduced the distribution density of red fluorescence positive neurons in other cortices, hippocampus, septum/basal forebrain, hypothalamus, and thalamus. Conversely, CRS significantly increased the distribution density of red fluorescence positive neurons in amygdala.

CONCLUSION

Our results suggest a possible mechanism that CRS leads to disturbances in synaptic plasticity by affecting multiple inputs to the mPFC, which is characterized by a decrease in the distribution density of dendrites/dendritic spines in the IL of mPFC and a reduction in input neurons of multiple cortices to the IL of mPFC as well as an increase in input neurons of amygdala to the IL of mPFC, ultimately causing depression-like behaviors.

A Computational Model for the Simulation of Prepulse Inhibition and Its Modulation by Cortical and Subcortical Units

The sensorimotor gating is a nervous system function that modulates the acoustic startle response (ASR). Prepulse inhibition (PPI) phenomenon is an operational measure of sensorimotor gating, defined as the reduction of ASR when a high intensity sound (pulse) is preceded in milliseconds by a weaker stimulus (prepulse). Brainstem nuclei are associated with the mediation of ASR and PPI, whereas cortical and subcortical regions are associated with their modulation. However, it is still unclear how the modulatory units can influence PPI. In the present work, we developed a computational model of a neural circuit involved in the mediation (brainstem units) and modulation (cortical and subcortical units) of ASR and PPI. The activities of all units were modeled by the leaky-integrator formalism for neural population. The model reproduces basic features of PPI observed in experiments, such as the effects of changes in interstimulus interval, prepulse intensity, and habituation of ASR. The simulation of GABAergic and dopaminergic drugs impaired PPI by their effects over subcortical units activity. The results show that subcortical units constitute a central hub for PPI modulation. The presented computational model offers a valuable tool to investigate the neurobiology associated with disorder-related impairments in PPI.

The medial orbitofrontal cortex governs reward-related circuits in an age-dependent manner

Nucleus accumbens (NAc) neurons integrate excitatory inputs from cortical and limbic structures, contributing to critical cognitive functions, including decision-making. As these afferents mature from adolescence through adulthood, incoming signals to the NAc may summate differently between age groups. Decision-making evaluates both reward and risk before action selection, suggesting an interplay between reward- and risk-related circuits. Medial orbitofrontal cortex (MO)-NAc circuits permit risk assessment behaviors and likely underlie risk information incorporation. As adolescents make reward-centric choices regardless of risk, we hypothesized the impact of MO activity alters reward-related NAc circuits in an age-dependent manner. To test this hypothesis, we used single-unit electrophysiology to measure MO train stimulation's effect on reward-related pathways, specifically the basolateral amygdala (BLA)-NAc circuit, in adult and adolescent rats. MO train stimulation altered the strength but not the timing of BLA-NAc interactions in a frequency-dependent manner. In adults, MO train stimulation produced a frequency-dependent, bidirectional effect on BLA-evoked NAc AP probability. Contrastingly, MO train stimulation uniformly attenuated BLA-NAc interactions in adolescents. While the mature MO can govern reward-related circuits in an activity-dependent manner, perhaps to adapt to positive or negative decision-making outcomes, the adolescent MO may be less able to bidirectionally impact reward-related pathways resulting in biased decision-making.

Coordinated Prefrontal State Transition Leads Extinction of Reward-Seeking Behaviors

Extinction learning suppresses conditioned reward responses and is thus fundamental to adapt to changing environmental demands and to control excessive reward seeking. The medial prefrontal cortex (mPFC) monitors and controls conditioned reward responses. Abrupt transitions in mPFC activity anticipate changes in conditioned responses to altered contingencies. It remains, however, unknown whether such transitions are driven by the extinction of old behavioral strategies or by the acquisition of new competing ones. Using in vivo multiple single-unit recordings of mPFC in male rats, we studied the relationship between single-unit and population dynamics during extinction learning, using alcohol as a positive reinforcer in an operant conditioning paradigm. To examine the fine temporal relation between neural activity and behavior, we developed a novel behavioral model that allowed us to identify the number, onset, and duration of extinction-learning episodes in the behavior of each animal. We found that single-unit responses to conditioned stimuli changed even under stable experimental conditions and behavior. However, when behavioral responses to task contingencies had to be updated, unit-specific modulations became coordinated across the whole population, pushing the network into a new stable attractor state. Thus, extinction learning is not associated with suppressed mPFC responses to conditioned stimuli, but is anticipated by single-unit coordination into population-wide transitions of the internal state of the animal.SIGNIFICANCE STATEMENT The ability to suppress conditioned behaviors when no longer beneficial is fundamental for the survival of any organism. While pharmacological and optogenetic interventions have shown a critical involvement of the mPFC in the suppression of conditioned responses, the neural dynamics underlying such a process are still largely unknown. Combining novel analysis tools to describe behavior, single-neuron response, and population activity, we found that widespread changes in neuronal firing temporally coordinate across the whole mPFC population in anticipation of behavioral extinction. This coordination leads to a global transition in the internal state of the network, driving extinction of conditioned behavior.

Glutamatergic Systems and Memory Mechanisms Underlying Opioid Addiction

Glutamate is the main excitatory neurotransmitter in the brain and is of critical importance for the synaptic and circuit mechanisms that underlie opioid addiction. Opioid memories formed over the course of repeated drug use and withdrawal can become powerful stimuli that trigger craving and relapse, and glutamatergic neurotransmission is essential for the formation and maintenance of these memories. In this review, we discuss the mechanisms by which glutamate, dopamine, and opioid signaling interact to mediate the primary rewarding effects of opioids, and cover the glutamatergic systems and circuits that mediate the expression, extinction, and reinstatement of opioid seeking over the course of opioid addiction.

Maturation of amygdala inputs regulate shifts in social and fear behaviors: A substrate for developmental effects of stress

Stress can negatively impact brain function and behaviors across the lifespan. However, stressors during adolescence have particularly harmful effects on brain maturation, and on fear and social behaviors that extend beyond adolescence. Throughout development, social behaviors are refined and the ability to suppress fear increases, both of which are dependent on amygdala activity. We review rodent literature focusing on developmental changes in social and fear behaviors, cortico-amygdala circuits underlying these changes, and how this circuitry is altered by stress. We first describe changes in fear and social behaviors from adolescence to adulthood and parallel developmental changes in cortico-amygdala circuitry. We propose a framework in which maturation of cortical inputs to the amygdala promote changes in social drive and fear regulation, and the particularly damaging effects of stress during adolescence may occur through lasting changes in this circuit. This framework may explain why anxiety and social pathologies commonly co-occur, adolescents are especially vulnerable to stressors impacting social and fear behaviors, and predisposed towards psychiatric disorders related to abnormal cortico-amygdala circuits.

Prefrontal cortical and nucleus accumbens contributions to discriminative conditioned suppression of reward-seeking

Fear can potently inhibit ongoing behavior, including reward-seeking, yet the neural circuits that underlie such suppression remain to be clarified. Prior studies have demonstrated that distinct subregions of the rodent medial prefrontal cortex (mPFC) differentially affect fear behavior, whereby fear expression is promoted by the more dorsal prelimbic cortex (PL) and inhibited by the more ventral infralimbic cortex (IL). These mPFC regions project to subregions of the nucleus accumbens, the core (NAcC) and shell (NAcS), that differentially contribute to reward-seeking as well as affective processes that may be relevant to fear expression. Here, we investigated how these mPFC and NAc subregions contribute to discriminative fear conditioning, assessed by conditioned suppression of reward-seeking. Bilateral inactivation of the NAcS or PL reduced the expression of conditioned suppression to a shock-associated CS+, whereas NAcC inactivation reduced reward-seeking without affecting suppression. IL inactivation caused a general reduction in conditioned suppression following discriminative conditioning, but not when using a single-stimulus design. Pharmacological disconnection of the PL → NAcS pathway revealed that this projection mediates conditioned suppression. These data add to a growing literature implicating discrete cortico-striatal pathways in the suppression of reward-seeking in response to aversive stimuli. Dysfunction within related structures may contribute to aberrant patterns of behavior in psychiatric illnesses including substance use disorders.

Reciprocal amygdala-prefrontal interactions in learning

Animals constantly evaluate their environment in order to avoid potential threats and obtain reward in the form of food, shelter and social interactions. In order to appropriately respond to sensory cues from the environment, the brain needs to form and store multiple cue-outcome associations. These can then be used to form predictions of the valence of sounds, smells and other sensory inputs arising from the surroundings. However, these associations must be subject to constant update, as the environment can rapidly change. Failing to adapt to such change can be detrimental to survival. Several systems in the mammalian brain have evolved to perform these important behavioral functions. Among these systems, the amygdala and prefrontal cortex are prominent players. Although the amygdala has been shown to form strong cue-outcome associations, the prefrontal cortex is essential for modifying these associations through extinction and reversal learning, and synaptic plasticity occurring in the strong reciprocal connections between these structures is thought to underlie both adaptive and maladaptive learning. Here we review the synaptic organization of the amygdala-prefrontal circuit, and summarize the physiological and behavioral evidence for its involvement in appetitive and aversive learning.

Long-Lasting Alterations in Gene Expression of Postsynaptic Density 95 and Inotropic Glutamatergic Receptor Subunit in the Mesocorticolimbic System of Rat Offspring Born to Morphine-Addicted Mothers

Prenatal exposure to morphine causes altered glutamatergic neurotransmission, which plays an important pathophysiological role for neurobiological basis of opiate-mediated behaviors in such offspring. However, it is still not clear whether such alteration involves gene expression of ionotropic glutamate receptor subunits. In this study, we further studied whether prenatal morphine exposure resulted in long-term changes in the gene expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, N-methyl-d-aspartate (NMDA) receptor, and postsynaptic density 95 in the mesocorticolimbic area (an essential integration circuitry for drug craving behavior), nucleus accumbens (NAc), ventral tegmental area (VTA), and prefrontal cortex (PFC), of rat offspring from morphine-addicted mothers. Experimental results showed that prenatal morphine exposure led to a persistent downregulation of gene expression in the AMPA and NMDA receptor subunit, with a differential manner of decreased magnitudes, at the age of postnatal days 14 (P14) and P30. However, in PFC, the gene expression of the AMPA receptor subunit was not synchronized in observed rat offspring subjected to prenatal morphine exposure. An upregulation of gene expression in the AMPA receptor subunit 3 (GluR3) was persistently observed at P14 and P30. Furthermore, the gene expressions of PSD-95 in NAc, VTA, and PFC were all decreased concurrently. Collectively, the results suggest that prenatal exposure to morphine may initiate molecular mechanisms leading to a long-lasting, differential alteration in gene expression of the inotropic glutamate receptor subunit and PSD-95 in the mesocorticolimbic circuitry in rat offspring. This study raises a possibility in which differential changes in gene expression with a long-lasting manner may play a role for the development of nearly permanent changes in opiate-mediated behaviors, at least in part for the neurobiological pathogenesis in offspring.

Layer- and subregion-specific differences in the neurophysiological properties of rat medial prefrontal cortex pyramidal neurons

Medial prefrontal cortex (mPFC) is critical for the expression of long-term conditioned fear. However, the neural circuits involving fear memory acquisition and retrieval are still unclear. Two subregions within mPFC that have received a lot of attention are the prelimbic (PL) and infralimbic (IL) cortices (e.g., Santini E, Quirk GJ, Porter JT. J Neurosci 28: 4028-4036, 2008; Song C, Ehlers VL, Moyer JR Jr J Neurosci 35: 13511-13524, 2015). Interestingly, PL and IL may play distinct roles during fear memory acquisition and retrieval but the underlying mechanism is poorly understood. One possibility is that the intrinsic membrane properties differ between these subregions. Thus, the current study was carried out to characterize the basic membrane properties of mPFC neurons in different layers and subregions. We found that pyramidal neurons in L2/3 were more hyperpolarized and less excitable than in L5. This was observed in both IL and PL and was associated with an enhanced h-current in L5 neurons. Within L2/3, IL neurons were more excitable than those in PL, which may be due to a lower spike threshold and higher input resistance in IL neurons. Within L5, the intrinsic excitability was comparable between neurons obtained in IL and PL. Thus, the heterogeneity in physiological properties of mPFC neurons may underlie the observed subregion-specific contribution of mPFC in cognitive function and emotional control, such as fear memory expression. NEW & NOTEWORTHY This is the first study to demonstrate that medial prefrontal cortical (mPFC) neurons are heterogeneous in both a layer- and a subregion-specific manner. Specifically, L5 neurons are more depolarized and more excitable than those neurons in L2/3, which is likely due to variations in h-current. Also, infralimbic neurons are more excitable than those of prelimbic neurons in layer 2/3, which may be due to differences in certain intrinsic properties, including input resistance and spike threshold.

Limbic-motor integration by neural excitations and inhibitions in the nucleus accumbens

The nucleus accumbens (NAc) has often been described as a "limbic-motor interface," implying that the NAc integrates the value of expected rewards with the motor planning required to obtain them. However, there is little direct evidence that the signaling of individual NAc neurons combines information about predicted reward and behavioral response. We report that cue-evoked neural responses in the NAc form a likely physiological substrate for its limbic-motor integration function. Across task contexts, individual NAc neurons in behaving rats robustly encode the reward-predictive qualities of a cue, as well as the probability of behavioral response to the cue, as coexisting components of the neural signal. In addition, cue-evoked activity encodes spatial and locomotor aspects of the behavioral response, including proximity to a reward-associated target and the latency and speed of approach to the target. Notably, there are important limits to the ability of NAc neurons to integrate motivational information into behavior: in particular, updating of predicted reward value appears to occur on a relatively long timescale, since NAc neurons fail to discriminate between cues with reward associations that change frequently. Overall, these findings suggest that NAc cue-evoked signals, including inhibition of firing (as noted here for the first time), provide a mechanism for linking reward prediction and other motivationally relevant factors, such as spatial proximity, to the probability and vigor of a reward-seeking behavioral response.NEW & NOTEWORTHY The nucleus accumbens (NAc) is thought to link expected rewards and action planning, but evidence for this idea remains sparse. We show that, across contexts, both excitatory and inhibitory cue-evoked activity in the NAc jointly encode reward prediction and probability of behavioral responding to the cue, as well as spatial and locomotor properties of the response. Interestingly, although spatial information in the NAc is updated quickly, fine-grained updating of reward value occurs over a longer timescale.

Modulation of the magnitude of conditioned taste aversion in rats with excitotoxic lesions of the basolateral amygdala

The amygdala is one of the structures involved in the acquisition of conditioned taste aversion (CTA). Nevertheless, the specific roles that the nuclei of this structure play in CTA learning are controversial. Electrolytic lesions applied to the basolateral nucleus of the amygdala can eliminate or reduce the acquisition of this learning. This effect has been attributed to the involvement of fibers that pass through this nucleus and connect with other structures that are critical for CTA. Excitotoxic lesions may allow a clearer insight as to the potential involvement of this nucleus in the acquisition of CTA. The few studies to date that have used this paradigm have shown effects on taste aversion learning after applying extensive lesions to the amygdala. Thus, the aim of the present study was to determine the effect of selective excitotoxic lesions of the basolateral amygdala on the acquisition of CTA. The effects of these lesions on learning were compared with the effects observed in animals with sham lesions and animals with lesions of the hippocampus, which is a structure apparently not involved in CTA. The results revealed a decreased aversion in animals with basolateral lesions compared with both the sham and hippocampus-lesioned groups. Based on these findings, the role of this specific nucleus of the amygdala in the acquisition of taste aversion is briefly discussed.

Elucidating neural network functional connectivity abnormalities in bipolar disorder: toward a harmonized methodological approach

Bipolar disorder (BD), a mood disorder characterized by emotional lability and dysregulation, is associated with alterations in functional connectivity, particularly as assessed using functional MRI. Here, we provide an overview of the extant literature, and themes that have emerged within it. We identified published research describing functional connectivity in BD using PubMed and follow-up searches. The most consistent evidence favors abnormally heightened functional connectivity between the amygdala and the lateral regions of the ventral prefrontal cortex (PFC), both during rest or emotional processing. Altered interactions between the amygdala and more medial PFC regions have been implicated in BD, but are less consistently related to core symptoms and are sometimes associated with mood state or psychosis. Interactions between medial and lateral ventral PFC have also been reported to be altered in BD, and may mediate estimates of amygdala/vlPFC connectivity. We also describe other themes, including an emerging literature examining reward circuitry, which has highlighted abnormal functional interactions between the ventral striatum and medial prefrontal cortex, as well as the advent of examining global network abnormalities in BD. Functional connectivity studies in BD have established altered interactions between PFC and the amygdala. To address the inconsistencies in the literature, we suggest avenues for the adoption of large scale, and network-based analysis of connectivity, the integration of structural connectivity and the acknowledgement of dynamic and context-related shifts in functional connectivity as a means of clarifying the abnormal neural circuitry in the disorder.

Low- and high-gamma oscillations deviate in opposite directions from zero-phase synchrony in the limbic corticostriatal loop

The loop structure of cortico-striatal anatomy in principle enables both descending (cortico-striatal) and ascending (striato-cortical) influences, but the factors that regulate the flow of information in these loops are not known. We report that low- and high-gamma oscillations (∼50 and ∼80 Hz, respectively) in the local field potential of freely moving rats are highly synchronous between the infralimbic region of the medial prefrontal cortex (mPFC) and the ventral striatum (vStr). Strikingly, high-gamma oscillations in mPFC preceded those in vStr, whereas low-gamma oscillations in mPFC lagged those in vStr, with short (∼1 ms) time lags. These systematic deviations from zero-phase synchrony were consistent across measures based on amplitude cross-correlation and phase slopes and were robustly maintained between behavioral states and different individual subjects. Furthermore, low- and high-gamma oscillations were associated with distinct ensemble spiking patterns in vStr, even when controlling for overt behavioral differences and slow changes in neural activity. These results imply that neural activity in vStr and mPFC is tightly coupled at the gamma timescale and raise the intriguing possibility that frequency-specific deviations from this coupling may signal transient leader-follower switches.

NPY+-, but not PV+- GABAergic neurons mediated long-range inhibition from infra- to prelimbic cortex

Anxiety disorders are thought to reflect deficits in the regulation of fear memories. While the amygdala has long been considered a site of storage of fear memories, newer findings suggest that the prefrontal cortex (PFC) is essential in the regulation of amygdala-dependent memories and fear expression. Here, activation of the prelimbic cortex (PrL) enhances the expression of fear, while an elevated activity in the infralimbic cortex (IL) enhances fear extinction. Despite the presence of these facts, we still know very little about the synaptic interconnectivity within the PFC. The aim of the present study was to investigate the inhibitory circuits between prelimbic and IL using morphological and electrophysiological methods. Our immunohistochemical analysis revealed that the distribution of PV(+)- and NPY(+)-GABAergic neurons was strikingly different within the PFC. In addition, we provided the first experimental evidence that the pyramidal neurons in the PrL received a direct inhibitory input mediated by bipolar NPY(+)-GABAergic projection neurons in the IL. Deletion of the anxiety-related neuroligin 2 gene caused a decrease of this direct synaptic inhibition that originated from the IL. Thus, our data suggested that activation of the IL might not only directly activate the corresponding downstream anxiolytic pathway, but also suppress the PrL-related anxiogenic pathway and thus could differentially bias the regulation of fear expression and extinction.

Basolateral and central amygdala differentially recruit and maintain dorsolateral striatum-dependent cocaine-seeking habits

In the development of addiction, drug seeking becomes habitual and controlled by drug-associated cues, and the neural locus of control over behaviour shifts from the ventral to the dorsolateral striatum. The neural mechanisms underlying this functional transition from recreational drug use to drug-seeking habits are unknown. Here we combined functional disconnections and electrophysiological recordings of the amygdalo-striatal networks in rats trained to seek cocaine to demonstrate that functional shifts within the striatum are driven by transitions from the basolateral (BLA) to the central (CeN) amygdala. Thus, while the recruitment of dorsolateral striatum dopamine-dependent control over cocaine seeking is triggered by the BLA, its long-term maintenance depends instead on the CeN. These data demonstrate that limbic cortical areas both tune the function of cognitive territories of the striatum and thereby underpin maladaptive cocaine-seeking habits.

Trace Fear Conditioning Differentially Modulates Intrinsic Excitability of Medial Prefrontal Cortex-Basolateral Complex of Amygdala Projection Neurons in Infralimbic and Prelimbic Cortices

Neuronal activity in medial prefrontal cortex (mPFC) is critical for the formation of trace fear memory, yet the cellular mechanisms underlying these memories remain unclear. One possibility involves the modulation of intrinsic excitability within mPFC neurons that project to the basolateral complex of amygdala (BLA). The current study used a combination of retrograde labeling and in vitro whole-cell patch-clamp recordings to examine the effect of trace fear conditioning on the intrinsic excitability of layer 5 mPFC-BLA projection neurons in adult rats. Trace fear conditioning significantly enhanced the intrinsic excitability of regular spiking infralimbic (IL) projection neurons, as evidenced by an increase in the number of action potentials after current injection. These changes were also associated with a reduction in spike threshold and an increase in h current. In contrast, trace fear conditioning reduced the excitability of regular spiking prelimbic (PL) projection neurons, through a learning-related decrease of input resistance. Interestingly, the amount of conditioned freezing was (1) positively correlated with excitability of IL-BLA projection neurons after conditioning and (2) negatively correlated with excitability of PL-BLA projection neurons after extinction. Trace fear conditioning also significantly enhanced the excitability of burst spiking PL-BLA projection neurons. In both regions, conditioning-induced plasticity was learning specific (observed in conditioned but not in pseudoconditioned rats), flexible (reversed by extinction), and transient (lasted <10 d). Together, these data suggest that intrinsic plasticity within mPFC-BLA projection neurons occurs in a subregion- and cell-type-specific manner during acquisition, consolidation, and extinction of trace fear conditioning. Significance statement: Frontal lobe-related function is vital for a variety of important behaviors, some of which decline during aging. This study involves a novel combination of electrophysiological recordings from fluorescently labeled mPFC-to-amygdala projection neurons in rats with acquisition and extinction of trace fear conditioning to determine how specific neurons change during behavior. This is the first study to demonstrate that trace fear conditioning significantly alters the intrinsic excitability of mPFC-to-amygdala projection neurons in a subregion- and cell-type-specific manner, which is also transient and reversed by extinction. These data are of broad interest to the neuroscientific community, and the results will inspire additional studies investigating the cellular mechanisms underlying circuit-specific changes within the brain as a result of associative learning and memory.

Distinct ensembles of medial prefrontal cortex neurons are activated by threatening stimuli that elicit excitation vs. inhibition of movement

Neural circuits controlling defensive behavior were investigated by recording single units in medial prefrontal cortex (mPFC) and dorsolateral periaqueductal gray (dlPAG) while rats expressed conditioned fear responses to an auditory conditioned stimulus (CS; 20-s train of white noise pips) previously paired with an aversive unconditioned stimulus (US; 2-s train of periorbital shocks). The CS elicited conditioned movement inhibition (CMI; characterized by decreased movement speed and freezing) when rats had not recently encountered the US, whereas the CS elicited conditioned movement excitation (CME; characterized by increased movement speed and flight behavior) after recent US encounters. Many mPFC neurons were "strategy-selective" cells that changed their firing rates only when the CS elicited CME (15/71) or CMI (13/71) responses, whereas few mPFC cells (4/71) responded nonselectively to the CS during either response. By contrast, many dlPAG neurons (20/74) responded nonselectively to the CS, but most (40/74) were excited by the CS selectively during CME trials (and none during CMI trials). CME-selective neurons in dlPAG responded phasically after CS pips that elicited CME responses, whereas CME-selective neurons in mPFC showed tonically elevated activity before and after pips that evoked CME responses. These findings suggest that, at the time when the CS occurs, tonic firing rates of CME- and CMI-selective mPFC neurons may bias the rat's choice of whether to express CME vs. CMI responses, perhaps via projections to downstream structures (such as amygdala and PAG) that influence how sensory stimuli are mapped onto motor circuits that drive the expression of competing behaviors.

Functional and structural deficits at accumbens synapses in a mouse model of Fragile X

Fragile X is the most common cause of inherited intellectual disability and a leading cause of autism. The disease is caused by mutation of a single X-linked gene called fmr1 that codes for the Fragile X mental retardation protein (FMRP), a 71 kDa protein, which acts mainly as a translation inhibitor. Fragile X patients suffer from cognitive and emotional deficits that coincide with abnormalities in dendritic spines. Changes in spine morphology are often associated with altered excitatory transmission and long-term plasticity, the most prominent deficit in fmr1-/y mice. The nucleus accumbens, a central part of the mesocortico-limbic reward pathway, is now considered as a core structure in the control of social behaviors. Although the socio-affective impairments observed in Fragile X suggest dysfunctions in the accumbens, the impact of the lack of FMRP on accumbal synapses has scarcely been studied. Here we report for the first time a new spike timing-dependent plasticity paradigm that reliably triggers NMDAR-dependent long-term potentiation (LTP) of excitatory afferent inputs of medium spiny neurons (MSN) in the nucleus accumbens core region. Notably, we discovered that this LTP was completely absent in fmr1-/y mice. In the fmr1-/y accumbens intrinsic membrane properties of MSNs and basal excitatory neurotransmission remained intact in the fmr1-/y accumbens but the deficit in LTP was accompanied by an increase in evoked AMPA/NMDA ratio and a concomitant reduction of spontaneous NMDAR-mediated currents. In agreement with these physiological findings, we found significantly more filopodial spines in fmr1-/y mice by using an ultrastructural electron microscopic analysis of accumbens core medium spiny neuron spines. Surprisingly, spine elongation was specifically due to the longer longitudinal axis and larger area of spine necks, whereas spine head morphology and postsynaptic density size on spine heads remained unaffected in the fmr1-/y accumbens. These findings together reveal new structural and functional synaptic deficits in Fragile X.

Bidirectional modulation of incubation of cocaine craving by silent synapse-based remodeling of prefrontal cortex to accumbens projections

Glutamatergic projections from the medial prefrontal cortex (mPFC) to nucleus accumbens (NAc) contribute to cocaine relapse. Here we show that silent synapse-based remodeling of the two major mPFC-to-NAc projections differentially regulated the progressive increase in cue-induced cocaine seeking after withdrawal (incubation of cocaine craving). Specifically, cocaine self-administration in rats generated AMPA receptor-silent glutamatergic synapses within both infralimbic (IL) and prelimbic mPFC (PrL) to NAc projections, measured after 1 day of withdrawal. After 45 days of withdrawal, IL-to-NAc silent synapses became unsilenced/matured by recruiting calcium-permeable (CP) AMPARs, whereas PrL-to-NAc silent synapses matured by recruiting non-CP-AMPARs, resulting in differential remodeling of these projections. Optogenetic reversal of silent synapse-based remodeling of IL-to-NAc and PrL-to-NAc projections potentiated and inhibited, respectively, incubation of cocaine craving on withdrawal day 45. Thus, pro- and antirelapse circuitry remodeling is induced in parallel after cocaine self-administration. These results may provide substrates for utilizing endogenous antirelapse mechanisms to reduce cocaine relapse.

Optogenetic dissection of basolateral amygdala projections during cue-induced reinstatement of cocaine seeking

Stimuli previously associated with drugs of abuse can become triggers that elicit craving and lead to drug-seeking behavior. The basolateral amygdala (BLA) is a key neural structure involved in cue-induced reinstatement of cocaine seeking. Previous studies have also implicated projections from the BLA directly to the nucleus accumbens (NAc) in these behaviors. However, other structures critically involved in cocaine seeking are targets of BLA innervation, including the prelimbic prefrontal cortex (PL). It has been shown that BLA or PL innervation direct to the NAc can modulate reward-related behaviors but the BLA also projects to the PL, and given the importance of the PL projection to the NAc for reinstated drug seeking, we hypothesized the BLA to PL projection may indirectly influence behavior via PL innervation to the NAc. We delivered a virus expressing the inhibitory optogenetic construct ArchT into the BLA and implanted fiber optics above the injection site or axon terminal fields in either the NAc or PL. Rats then went through 12 days of cocaine self-administration followed by extinction training. Following extinction, animals underwent cue-induced reinstatement sessions in the presence or absence of optical inhibition. Inactivation of the BLA and either the BLA core subcompartment of the NAc (BLA-to-NAcore) BLA-to-PL projections inhibited cue-induced reinstatement. These data demonstrate that the BLA projection either directly into the NAc, or indirectly via the PL, is a necessary regulator of drug-seeking behavior.

Amygdalostriatal projections in the neurocircuitry for motivation: a neuroanatomical thread through the career of Ann Kelley

In MacLean's triune brain, the amygdala putatively subserves motivated behavior by modulating the "reptilian" basal ganglia. Accordingly, Ann Kelley, with Domesick and Nauta, influentially showed that amygdalostriatal projections are much more extensive than were appreciated. They highlighted that amygdalar projections to the rostral ventromedial striatum converged with projections from the ventral tegmental area and cingulate cortex, forming a "limbic striatum". Caudal of the anterior commissure, the entire striatum receives afferents from deep basal nuclei of the amygdala. Orthologous topographic projections subsequently were observed in fish, amphibians, and reptiles. Subsequent functional studies linked acquired value to action via this neuroanatomical substrate. From Dr. Kelley's work evolved insights into components of the distributed, interconnected network that subserves motivated behavior, including the nucleus accumbens shell and core and the striatal-like extended amygdala macrostructure. These heuristic frameworks provide a neuroanatomical basis for adaptively translating motivation into behavior. The ancient amygdala-to-striatum pathways remain a current functional thread not only for stimulus-response valuation, but also for the psychopathological plasticity that underlies addiction-related memory, craving and relapse.

Heteromeric nicotinic receptors are involved in the sensitization and addictive properties of MDMA in mice

We have investigated the effect of nicotinic receptor ligands in the behavioral sensitization (hyperlocomotion) and rewarding properties (conditioned place preference paradigm, CPP) of 3,4-methylenedioxy-methamphetamine (MDMA) in mice. Each animal received intraperitoneal pretreatment with either saline, dihydro-β-erythroidine (DHβE, 1 mg/kg) or varenicline (VAR, 0.3 mg/kg), 15 min prior to subcutaneous saline or MDMA (5 mg/kg), for 10 consecutive days. On day 1, both DHβE and VAR inhibited the MDMA-induced hyperlocomotion. After 10 days of treatment, MDMA induced a hyperlocomotion that was not reduced (rather enhanced) in antagonist-pretreated animals. This early hyperlocomotion was accompanied by a significant increase in heteromeric nicotinic receptors in cortex that was not blocked by DHβE or VAR. Behavioral sensitization to MDMA was highest 2 weeks after the discontinuation of MDMA treatment. This additional increase in sensitivity was prevented in animals pretreated with DHβE or VAR. At this time, MDMA-treated mice showed a significant increase in heteromeric receptors in cortex that was prevented by DHβE and VAR. An involvement of α7 nicotinic receptors in this effect is ruled out. MDMA (10 mg/kg) induced positive CPP that was abolished by DHβE (2 mg/kg) and VAR (2 mg/kg). Moreover, chronic nicotine pretreatment (2 mg/kg, ip, b.i.d., for 14 days) caused MDMA, administered at a low dose (3 mg/kg), to induce CPP, which would otherwise not occur. Finally, present results point out that heteromeric nicotinic receptors are involved in locomotor sensitization and addictive potential induced by MDMA. Thus, varenicline might be a useful drug to treat both tobacco and MDMA abuse at once.

Intranasal administration of oxytocin: behavioral and clinical effects, a review

•

The mechanisms behind the effects of IN-applied substances need more attention.

•

The mechanisms involved in the brain-distribution of IN-OT are completely unexplored.

•

The possibly cascading effects of IN-OT on the intrinsic OT-system require serious investigation.

•

IN-OT induces clear and specific changes in neural activation.

•

IN-OT is a promising approach to treat certain clinical symptoms.

The intranasal (IN-) administration of substances is attracting attention from scientists as well as pharmaceutical companies. The effects are surprisingly fast and specific. The present review explores our current knowledge about the routes of access to the cranial cavity. ‘Direct-access-pathways’ from the nasal cavity have been described but many additional experiments are needed to answer a variety of open questions regarding anatomy and physiology.

Among the IN-applied substances oxytocin (OT) has an extensive history. Originally applied in women for its physiological effects related to lactation and parturition, over the last decade most studies focused on their behavioral ‘prosocial’ effects: from social relations and ‘trust’ to treatment of ‘autism’.

Only very recently in a microdialysis study in rats and mice, the ‘direct-nose-brain-pathways’ of IN-OT have been investigated directly, implying that we are strongly dependent on results obtained from other IN-applied substances. Especially the possibility that IN-OT activates the ‘intrinsic’ OT-system in the hypothalamus as well needs further clarification.

We conclude that IN-OT administration may be a promising approach to influence human communication but that the existing lack of information about the neural and physiological mechanisms involved is a serious problem for the proper understanding and interpretation of the observed effects.

Research Review: altered reward function in adolescent depression: what, when and how?

BACKGROUND

Conceptual models and recent evidence indicate that neural response to reward is altered in depression. Taking a developmental approach to investigating reward function in adolescent depression can elucidate the etiology, pathophysiology and course of depression, a disorder that typically begins during adolescence and has high rates of recurrence.

METHODS

This conceptual review describes the what, when and how of altered reward function in adolescent depression. With the goal of generating new, testable hypotheses within a developmental affective neuroscience framework, we critically review findings and suggest future directions. Peer-reviewed empirical papers for inclusion in this critical review were obtained by searching PubMed, PsycInfo and ScienceDirect for the years 1990-2010.

RESULTS

A pattern of low striatal response and high medial prefrontal response to reward is evident in adolescents and adults with depression. Given the salience of social stimuli for positive affect and depression, reward function might be especially disrupted in response to social rewards. Because of changes in the dopamine system and reward function with aging, altered reward function in depression might be more evident during adolescence than later in life; however, low reward function may also be a stable characteristic of people who experience depression. Mechanisms of altered reward function in depression could include disrupted balance of corticostriatal circuit function, with disruption occurring as aberrant adolescent brain development.

CONCLUSIONS

Future studies should examine responses to social rewards; employ longitudinal and prospective designs; and investigate patterns of functional connectivity in reward circuits. Understanding altered reward function in depression has potential implications for treatment development. A more rigorous approach to investigating anhedonia, threat-reward interactions and comorbid anxiety will be valuable to future progress in describing the role of reward function in the pathophysiology of depression.

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.

The contribution of NMDA receptor signaling in the corticobasal ganglia reward network to appetitive Pavlovian learning

NMDA receptors (NMDARs) contribute to phasic transmission and synaptic plasticity and are thought to be important for learning. To better understand where NMDAR signaling is necessary for learning, we combined viral genetic strategies with genetic mouse models to investigate the contribution of NMDARs in the dopamine system to appetitive Pavlovian conditioning. NMDAR signaling in dopamine neurons was not required for Pavlovian conditioning; however, NMDARs in D(1) dopamine receptor (D(1)R)-expressing medium spiny neurons (MSNs), which receive input from dopamine neurons, were critical for this type of learning. NMDAR signaling was also required in brain regions that project to dopamine neurons, because removing NMDARs from afferent neurons to the ventral tegmental area (VTA) also prevented learning. This effect was likely attributable to loss of NMDAR signaling in the neurons of the prefrontal cortex (PFC), because learning could be restored in these animals by rescuing NMDAR expression in the PFC. Moreover, removing NMDARs exclusively from the PFC also prevented learning. Our findings suggest that NMDARs in neurons that project to and receive projections from the VTA are necessary for Pavlovian conditioning and specifically implicate the PFC and D(1)R-expressing MSNs in associative learning.

At the limbic-motor interface: disconnection of basolateral amygdala from nucleus accumbens core and shell reveals dissociable components of incentive motivation

Although it has long been hypothesized that the nucleus accumbens (NAc) acts as an interface between limbic and motor regions, direct evidence for this modulatory role on behavior is lacking. Using a disconnection procedure in rats, we found that basolateral amygdala (BLA) input to the core and medial shell of the NAc separately mediate two distinct incentive processes controlling the performance of goal-directed instrumental actions, respectively: (i) the sensitivity of instrumental responding to changes in the experienced value of the goal or outcome, produced by specific satiety-induced outcome devaluation; and (ii) the effect of reward-related cues on action selection, observed in outcome-specific Pavlovian-instrumental transfer. These results reveal, therefore, that dissociable neural circuits involving BLA inputs to the NAc core and medial shell mediate distinct components of the incentive motivational processes controlling choice and decision-making in instrumental conditioning.

The basolateral amygdala differentially regulates conditioned neural responses within the nucleus accumbens core and shell

The ability to process information regarding reward-predictive cues involves a diverse network of neural substrates. Given the importance of the nucleus accumbens (NAc) and the basolateral amygdala (BLA) in associative reward processes, recent research has examined the functional importance of BLA-NAc interactions. Here, multi-neuron extracellular recordings of NAc neurons coupled to microinfusion of GABAA and GABAB agonists into the BLA were employed to determine the functional contribution of the BLA to phasic neural activity across the NAc core and shell during a cued-instrumental task. NAc neural response profiles prior to BLA inactivation exhibited largely indistinguishable activity across the core and shell. However, for NAc neurons that displayed cue-related increases in firing rates during the task, BLA inactivation significantly reduced this activity selectively in the core (not shell). Additionally, phasic increases in firing rate in the core (not shell) immediately following the lever press response were also significantly reduced following BLA manipulation. Concurrent with these neural changes, BLA inactivation caused a significant increase in latency to respond for rewards and a decrease in the percentage of trials in which animals made a conditioned approach to the cue. Together, these results suggest that an excitatory projection from the BLA provides a selective contribution to conditioned neural excitations of NAc core neurons during a cued-instrumental task, providing insight into the underlying neural circuitry that mediates responding to reward-predictive cues.

Emotional imagery: assessing pleasure and arousal in the brain's reward circuitry

Research on emotional perception and learning indicates appetitive cues engage nucleus accumbens (NAc) and medial prefrontal cortex (mPFC), whereas amygdala activity is modulated by the emotional intensity of appetitive and aversive cues. This study sought to determine patterns of functional activation and connectivity among these regions during narrative emotional imagery. Using event-related fMRI, we investigate activation of these structures when participants vividly imagine pleasant, neutral, and unpleasant scenes. Results indicate that pleasant imagery selectively activates NAc and mPFC, whereas amygdala activation was enhanced during both pleasant and unpleasant imagery. NAc and mPFC activity were each correlated with the rated pleasure of the imagined scenes, while amygdala activity was correlated with rated emotional arousal. Functional connectivity of NAc and mPFC was evident throughout imagery, regardless of hedonic content, while correlated activation of the amygdala with NAc and mPFC was specific to imagining pleasant scenes. These findings provide strong evidence that pleasurable text-driven imagery engages a core appetitive circuit, including NAc, mPFC, and the amygdala.

Prefrontal control of fear: more than just extinction

Although fear research has largely focused on the amygdala, recent findings highlight cortical control of the amygdala in the service of fear regulation. In rodent models, it is becoming well established that the infralimbic (IL) prefrontal cortex plays a key role in extinction learning, and recent findings are uncovering molecular mechanisms involved in extinction-related plasticity. Furthermore, mounting evidence implicates the prelimbic (PL) prefrontal cortex in the production of fear responses. Both IL and PL integrate inputs from the amygdala, as well as other structures to gate the expression of fear via projections to inhibitory or excitatory circuits within the amygdala. We suggest that dual control of the amygdala by separate prefrontal modules increases the flexibility of an organism's response to danger cues.

Amygdala mechanisms of Pavlovian psychostimulant conditioning and relapse

Psychostimulant addiction often consists of periods of sustained drug abstinence disrupted by periods of relapse and renewed heavy drug use. Prevention of relapse remains the greatest challenge to the successful treatment of drug addiction. Drug-associated cues are a primary trigger for relapse, as they can elicit intense craving for the drug. These cues become associated with the drug reward through Pavlovian learning processes that develop over multiple drug-cue pairings. The amygdala (AMY) is critical for such drug-related learning. Intrinsic and extrinsic circuitry position the AMY to integrate cue and drug-related information and influence drug-seeking and drug-taking behaviors. Animal models of conditioned drug reward, drug use, and relapse have confirmed the necessary role of the AMY for drug conditioned cues to control motivated behavior. Neurons within the AMY are responsive to the primary effects of psychostimulants, and more critically, they also respond to the presentation of drug-associated cues. The mechanisms by which conditioned cues come to influence drug-seeking behavior likely involve long-term plasticity and neuroadaptations within the AMY. A greater understanding of the associative learning mechanisms that depend upon the AMY and related limbic and cortical structures, and the process by which drug cues come to gain control over behavior that maintains the addictive state, will facilitate the development of more effective addiction treatments.

Cortico-Basal Ganglia reward network: microcircuitry

Many of the brain's reward systems converge on the nucleus accumbens, a region richly innervated by excitatory, inhibitory, and modulatory afferents representing the circuitry necessary for selecting adaptive motivated behaviors. The ventral subiculum of the hippocampus provides contextual and spatial information, the basolateral amygdala conveys affective influence, and the prefrontal cortex provides an integrative impact on goal-directed behavior. The balance of these afferents is under the modulatory influence of dopamine neurons in the ventral tegmental area. This midbrain region receives its own complex mix of excitatory and inhibitory inputs, some of which have only recently been identified. Such afferent regulation positions the dopamine system to bias goal-directed behavior based on internal drives and environmental contingencies. Conditions that result in reward promote phasic dopamine release, which serves to maintain ongoing behavior by selectively potentiating ventral subicular drive to the accumbens. Behaviors that fail to produce an expected reward decrease dopamine transmission, which favors prefrontal cortical-driven switching to new behavioral strategies. As such, the limbic reward system is designed to optimize action plans for maximizing reward outcomes. This system can be commandeered by drugs of abuse or psychiatric disorders, resulting in inappropriate behaviors that sustain failed reward strategies. A fuller appreciation of the circuitry interconnecting the nucleus accumbens and ventral tegmental area should serve to advance discovery of new treatment options for these conditions.

Naloxone-precipitated withdrawal enhances ERK phosphorylation in prefrontal association cortex and accumbens nucleus of morphine-dependent mice

Mitogen-activated protein kinases (MAPK) can be activated by opioids such as morphine via opioid receptor, and their activations have been observed in synaptic plasticity, learning, memory and addiction. Long-term exposure to morphine may induce physical dependence, manifested as somatic withdrawal symptoms such as diarrhea, body weight loss, jumping and headshaking, when drug is deprived. Though morphine dependence and withdrawal have been extensively studied, their molecular mechanisms have not been fully elucidated. In the present study, the physical dependence on morphine was developed in mice by an intermittent, escalating procedure of morphine injections, and was measured by the body weight loss and the behavioral signs (jumping and headshaking). We found that the mice with chronic morphine administration experienced dramatic body weight loss, compared with the saline-treated controls. Naloxone-precipitated withdrawal led to more body weight loss, compared with spontaneous withdrawal. Naloxone-precipitated withdrawal mice showed significantly aggravated morphine-withdrawal symptoms (including jumping and heading shaking), compared with spontaneous withdrawal mice. MAPK pathway activities in the frontal association cortex (FrA), accumbens nucleus (Acb) and caudate putamen (CPu) were examined to probe into molecular mechanism for morphine dependence and withdrawal. Compared with saline-treated mice, morphine-dependent mice and spontaneous withdrawal mice, naloxone-precipitated withdrawal mice showed a significantly increased ERK phosphorylation in FrA and Acb, but not in CPu. However, the activities of other protein kinases in the MAPK pathway, including p38 and JNK, showed no changes in FrA, Acb and CPu of the mice during the chronic morphine dependence and withdrawal phases. These results suggest that the ERK phosphorylation in FrA and Acb may be associated with naloxone-precipitated withdrawal syndrome.

Integrated cannabinoid CB1 receptor transmission within the amygdala-prefrontal cortical pathway modulates neuronal plasticity and emotional memory encoding

The cannabinoid CB1 receptor system is functionally involved in the processing and encoding of emotionally salient sensory information, learning and memory. The CB1 receptor is found in high concentrations in brain structures that are critical for emotional processing, including the basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC). In addition, synaptic plasticity in the form of long-term potentiation (LTP) within the BLA > mPFC pathway is an established correlate of exposure to emotionally salient events. We performed a series of in vivo LTP studies by applying tetanic stimulation to the BLA combined with recordings of local field potentials within prelimbic cortical (PLC) region of the rat mPFC. Systemic pretreatment with AM-251 dose dependently blocked LTP along the BLA-PLC pathway and also the behavioral acquisition of conditioned fear memories. We next performed a series of microinfusion experiments wherein CB1 receptor transmission within the BLA > PLC circuit was pharmacologically blocked. Asymmetrical, interhemispheric blockade of CB1 receptor transmission along the BLA > PLC pathway prevented the acquisition of emotionally salient associative memory. Our results indicate that coordinated CB1 receptor transmission within the BLA > PLC pathway is critically involved in the encoding of emotional fear memories and modulates neural plasticity related to the encoding of emotionally salient associative learning.

Activity-dependent depression of medial prefrontal cortex inputs to accumbens neurons by the basolateral amygdala

The encoding of reward-predictive stimuli by neurons in the nucleus accumbens (NAcc) depends on integrated synaptic activity from the basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) afferent inputs. In a previous study, we found that single electrical stimulation pulses applied to the BLA facilitate mPFC-evoked spiking in NAcc neurons in a timing-dependent manner, presumably by a fast glutamatergic mechanism. In the present study, the ability of repetitive BLA activation to modulate synaptic inputs to NAcc neurons through dopamine- or N-methyl-D-aspartate (NMDA)-dependent mechanisms is characterized. NAcc neurons receiving excitatory input from both mPFC and BLA were recorded in urethane-anesthetized rats. Train stimulation of the BLA depressed mPFC-evoked spiking in these neurons. This was not attributable to mechanisms involving NMDA or dopamine D1, D2, D3 or D5 receptors, since blockade of these receptors did not affect the BLA-mediated depression. BLA-mediated depression was only evident when the BLA stimulation evoked spikes in the recorded neuron; thus, depolarization of the recorded neuron may be critical for this effect. The ability of the BLA to suppress mPFC-to-NAcc signaling may be a mechanism by which normal or pathologically heightened emotional states disrupt goal-directed behavior in favor of emotionally-driven responses.

Divergent effects of genetic variation in endocannabinoid signaling on human threat- and reward-related brain function

BACKGROUND

Fatty acid amide hydrolase (FAAH) is a key enzyme in regulating endocannabinoid (eCB) signaling. A common single nucleotide polymorphism (C385A) in the human FAAH gene has been associated with increased risk for addiction and obesity.

METHODS

Using imaging genetics in 82 healthy adult volunteers, we examined the effects of FAAH C385A on threat- and reward-related human brain function.

RESULTS

Carriers of FAAH 385A, associated with reduced enzyme and possibly increased eCB signaling, had decreased threat-related amygdala reactivity but increased reward-related ventral striatal reactivity in comparison with C385 homozygotes. Similarly divergent effects of FAAH C385A genotype were manifest at the level of brain-behavior relationships. The 385A carriers showed decreased correlation between amygdala reactivity and trait anxiety but increased correlation between ventral striatal reactivity and delay discounting, an index of impulsivity.

CONCLUSIONS

Our results parallel pharmacologic and genetic dissection of eCB signaling, are consistent with the psychotropic effects of Delta(9)-tetrahydrocannabinol, and highlight specific neural mechanisms through which variability in eCB signaling impacts complex behavioral processes related to risk for addiction and obesity.

Timing-dependent regulation of evoked spiking in nucleus accumbens neurons by integration of limbic and prefrontal cortical inputs

Single nucleus accumbens (NAcc) neurons receive excitatory synaptic input from cortical and limbic structures, and the integration of converging goal- and motivation-related signals in these neurons influences reward-directed actions. While limbic/cortical synaptic input summation has been characterized at subthreshold intensities, the manner in which multiple inputs govern NAcc neuron spike discharge has not been measured and is poorly understood. Single NAcc neurons were recorded in urethane-anesthetized rats, and spiking was evoked by coincident stimulation of two major NAcc afferent regions: the basolateral amygdala (BLA) and medial prefrontal cortex (mPFC). BLA input increased NAcc spiking elicited by mPFC stimulation depending on the timing of the stimulation pulses, consistent with the summation of monosynaptically evoked excitatory activity. When mPFC input intensity was below threshold for evoked spiking, the addition of BLA input produced the largest facilitation of evoked spiking, and the latency of the evoked spikes reflected the latency of the individual inputs. When mPFC inputs were stimulated at higher intensities, BLA-mediated facilitation was weaker, and the spike latency reflected only the mPFC input. Thus NAcc neurons integrate both the magnitude and timing of afferent synaptic activity, suggesting that NAcc neuron output is strongly dependent on the comparative magnitude of synaptic activity in its afferent structures. These interactions may be crucial integrative mechanisms that allow motivational and cognitive information to produce appropriate reward-directed actions.

Cue-elicited drug craving represses ERK activation in mice prefrontal association cortex

Morphine addiction is characterized by compulsive drug-taking behavior and high rates of relapse that reflect reward-controlled learning, consolidation and reconsolidation of drug cues. Extracellular signal-regulated protein kinase (ERK) is one of the cellular molecules that have been highly implicated in the synaptic plasticity processes of learning and memory in cocaine addiction. However, the roles of ERK in the morphine-paired conditioned place preference (CPP) are not clear. In the present study, we found that compared to the morphine-unpaired and saline-paired and saline-unpaired groups, morphine-paired mice showed depressed ERK2 activity in the Frontal Association Cortex (FrA), whereas ERK1 activity was not changed in the same region. In the Accumbens Nucleus (Acb) and Caudate Putamen (CPu) that are associated with cocaine addiction, the activities of ERK1 and ERK2 among four groups showed no difference. These results suggest that the FrA plays an important role in morphine craving and that ERK2 is involved in eliciting the environment-related morphine craving, which is totally different from those induced by morphine itself.

Basolateral amygdala neurons facilitate reward-seeking behavior by exciting nucleus accumbens neurons

Both the nucleus accumbens (NAc) and basolateral amygdala (BLA) contribute to learned behavioral choice. Neurons in both structures that encode reward-predictive cues may underlie the decision to respond to such cues, but the neural circuits by which the BLA influences reward-seeking behavior have not been established. Here, we test the hypothesis that the BLA drives NAc neuronal responses to reward-predictive cues. First, using a disconnection experiment, we show that the BLA and dopamine projections to the NAc interact to promote the reward-seeking behavioral response. Next, we demonstrate that BLA neuronal responses to cues precede those of NAc neurons and that cue-evoked excitation of NAc neurons depends on BLA input. These results indicate that BLA input is required for dopamine to enhance the cue-evoked firing of NAc neurons and that this enhanced firing promotes reward-seeking behavior.

A neural model of voluntary and automatic emotion regulation: implications for understanding the pathophysiology and neurodevelopment of bipolar disorder

The ability to regulate emotions is an important part of adaptive functioning in society. Advances in cognitive and affective neuroscience and biological psychiatry have facilitated examination of neural systems that may be important for emotion regulation. In this critical review we first develop a neural model of emotion regulation that includes neural systems implicated in different voluntary and automatic emotion regulatory subprocesses. We then use this model as a theoretical framework to examine functional neural abnormalities in these neural systems that may predispose to the development of a major psychiatric disorder characterized by severe emotion dysregulation, bipolar disorder.