Input from the amygdala to the rat nucleus accumbens: its relationship with tyrosine hydroxylase immunoreactivity and identified neurons

Chronic social defeat stress gives rise to social avoidance through fear learning

Chronic social defeat stress (CSDS), a widely used rodent model of stress, reliably leads to decreased social interaction in stress susceptible animals. Here, we investigate a role for fear learning in this response using male 129 Sv/Ev mice, a strain that is more vulnerable to CSDS than the commonly used C57BL/6 strain. We first demonstrate that defeated 129 Sv/Ev mice avoid a CD-1 mouse, but not a conspecific, indicating that motivation to socialize is intact in this strain. CD-1 avoidance is characterized by approach behavior that results in running in the opposite direction, activity that is consistent with a threat response. We next test whether CD-1 avoidance is subject to the same behavioral changes found in traditional models of Pavlovian fear conditioning. We find that associative learning occurs across 10 days CSDS, with defeated mice learning to associate the color of the CD-1 coat with threat. This leads to the gradual acquisition of avoidance behavior, a conditioned response that can be extinguished with 7 days of repeated social interaction testing (5 tests/day). Pairing a CD-1 with a tone leads to second-order conditioning, resulting in avoidance of an enclosure without a social target. Finally, we show that social interaction with a conspecific is a highly variable response in defeated mice that may reflect individual differences in generalization of fear to other social targets. Our data indicate that fear conditioning to a social target is a key component of CSDS, implicating the involvement of fear circuits in social avoidance.

Chronic Social Defeat Stress Gives Rise to Social Avoidance Through Fear Learning

Chronic social defeat stress (CSDS), a widely used rodent model of stress, reliably leads to decreased social interaction in stress susceptible animals. Here, we investigate a role for fear learning in this response using 129Sv/Ev mice, a strain that is more vulnerable to CSDS than the commonly used C57BL/6 strain. We first demonstrate that defeated 129Sv/Ev mice avoid a CD-1 mouse, but not a conspecific, indicating that motivation to socialize is intact in this strain. CD-1 avoidance is characterized by approach behavior that results in running in the opposite direction, activity that is consistent with a threat response. We next test whether CD-1 avoidance is subject to the same behavioral changes found in traditional models of Pavlovian fear conditioning. We find that associative learning occurs across 10 days CSDS, with defeated mice learning to associate the color of the CD-1 coat with threat. This leads to the gradual acquisition of avoidance behavior, a conditioned response that can be extinguished with 7 days of repeated social interaction testing (5 tests/day). Pairing a CD-1 with a tone leads to second-order conditioning, resulting in avoidance of an enclosure without a social target. Finally, we show that social interaction with a conspecific is a highly variable response in defeated mice that may reflect individual differences in generalization of fear to other social targets. Our data indicate that fear conditioning to a social target is a key component of CSDS, implicating the involvement of fear circuits in social avoidance.

Cocaine induces input and cell-type-specific synaptic plasticity in ventral pallidum-projecting nucleus accumbens medium spiny neurons

Cocaine use and abstinence induce long-term synaptic alterations in the excitatory input to nucleus accumbens (NAc) medium spiny neurons (MSNs). The NAc regulates reward-related behaviors through two parallel projections to the ventral pallidum (VP)-originating in D1 or D2-expressing MSNs (D1-MSNs_→VP; D2-MSNs_→VP). The activity of these projections depends on their excitatory synaptic inputs, but it is not known whether and how abstinence from cocaine affects the excitatory transmission to D1-MSNs_→VP and D2-MSNs_→VP. Here we examined different forms of cocaine-induced synaptic plasticity in the inputs from the basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) to NAc D1-MSNs_→VP and putative D2-MSNs_→VP (pD2-MSNs_→VP) in the core and shell subcompartments of the NAc. We used the whole-cell patch-clamp technique to record excitatory postsynaptic currents from D1-tdTomato mice injected with ChR2 in either the BLA or the mPFC and retrograde tracer (RetroBeads) in the VP. We found that cocaine conditioned place preference (CPP) followed by abstinence potentiated the excitatory input from the BLA and mPFC to both D1-MSNs_→VP and pD2-MSNs_→VP. Interestingly, while the strengthening of the inputs to D1-MSNs_→VP was of postsynaptic origin and manifested as increased AMPA to NMDA ratio, in pD2-MSNs_→VP plasticity was predominantly presynaptic and was detected as changes in the paired-pulse ratio and coefficient of variation. Lastly, some of the changes were sex-specific. Overall our data show that abstinence from cocaine changes the excitatory inputs to both D1-MSNs_→VP and pD2-MSNs_→VP but with different mechanisms. This may help understand how circuits converging into the VP change after cocaine exposure.

Involvement of the dopaminergic system in the reward-related behavior of pregabalin

There has been an increase in cases of drug addiction and prescription drug abuse worldwide. Recently, pregabalin abuse has been a focus for many healthcare agencies, as highlighted by epidemiological studies. We previously evaluated the possibility of pregabalin abuse using the conditioned place preference (CPP) paradigm. We observed that a 60 mg/kg dose could induce CPP in mice and that pregabalin-rewarding properties were mediated through glutamate neurotransmission. Notably, the dopaminergic reward circuitry is also known to play a crucial role in medication-seeking behavior. Therefore, this study aimed to explore the possible involvement of dopaminergic receptor-1 in pregabalin-induced CPP. Mice were randomly allocated to receive saline or the dopamine-1 receptor antagonist SKF-83566 (0.03 mg/kg, intraperitoneal). After 30 min, the mice received either saline or pregabalin (60 mg/kg) during the conditioning phase. Among the control groups that received saline or SKF-83566, the time spent in the two conditioning chambers was not significantly altered. However, among the pregabalin-treated group, there was a marked increase in the time spent in the drug-paired chamber compared to the time spent in the vehicle-paired chamber. Notably, blocking dopamine-1 receptors with SKF-83566 completely prevented pregabalin-induced place preference, thus demonstrating the engagement of the dopaminergic system in pregabalin-induced reward-related behavior.

Nerve injury alters restraint-induced activation of the basolateral amygdala in male rats

The amygdala is critical for the production of appropriate responses towards emotional or stressful stimuli. It has a characteristic neuronal activation pattern to acute stressors. Chronic pain and acute stress have each been shown to independently modulate the activity of the amygdala. Few studies have investigated the effect of pain or injury, on amygdala activation to acute stress. This study investigated the effects of a neuropathic injury on the activation response of the amygdala to an acute restraint stress. Chronic constriction injury of the right sciatic nerve (CCI) was used to create neuropathic injury and a single brief 15-min acute restraint was used as an emotional/psychological stressor. All rats received cholera toxin B (CTB) retrograde tracer injections into the medial prefrontal cortex (mPFC) to assess if the amygdala to mPFC pathway was specifically regulated by the combination of neuropathic injury and acute stress. To assess differential patterns of activity in amygdala subregions, cFos expression was used as a marker for "acute", restraint triggered neuronal activation, and FosB/ΔFosB expression was used to reveal prolonged neuronal activation/sensitisation triggered by CCI. Restraint resulted in a characteristic increase in cFos expression in the medial amygdala, which was not altered by CCI. Rats with a CCI showed increased cFos expression in the basolateral amygdala (BLA), in response to an acute restraint stress, but not in neurons projecting to the prefrontal cortex. Further, CCI rats showed an increase in FosB/ΔFosB expression which was exclusive to the BLA. This increase likely reflects sensitisation of the BLA as a consequence of nerve injury which may contribute to heightened sensitivity of BLA neurons to acute emotional/ psychological stressors.

Cortical and Thalamic Interaction with Amygdala-to-Accumbens Synapses

The nucleus accumbens shell (NAcSh) regulates emotional and motivational responses, a function mediated, in part, by integrating and prioritizing extensive glutamatergic projections from limbic and paralimbic brain regions. Each of these inputs is thought to encode unique aspects of emotional and motivational arousal. The projections do not operate alone, but rather are often activated simultaneously during motivated behaviors, during which they can interact and coordinate in shaping behavioral output. To understand the anatomic and physiological bases underlying these interprojection interactions, the current study in mice of both sexes focused on how the basolateral amygdala projection (BLAp) to the NAcSh regulates, and is regulated by, projections from the medial prefrontal cortex (mPFCp) and paraventricular nucleus of the thalamus (PVTp). Using a dual-color SynaptoTag technique combined with a backfilling spine imaging strategy, we found that all three afferent projections primarily targeted the secondary dendrites of NAcSh medium spiny neurons, forming putative synapses. We detected a low percentage of BLAp contacts closely adjacent to mPFCp or PVTp presumed synapses, and, on some rare occasions, the BLAp formed heterosynaptic interactions with mPFCp or PVTp profiles or appeared to contact the same spines. Using dual-rhodopsin optogenetics, we detected signs of dendritic summation of BLAp with PVTp and mPFCp inputs. Furthermore, high-frequency activation of BLAp synchronous with the PVTp or mPFCp resulted in a transient enhancement of the PVTp, but not mPFCp, transmission. These results provide anatomic and functional indices that the BLAp interacts with the mPFCp and PVTp for informational processing within the NAcSh.SIGNIFICANCE STATEMENT The nucleus accumbens regulates emotional and motivational responses by integrating extensive glutamatergic projections, but the anatomic and physiological bases on which these projections integrate and interact remain underexplored. Here, we used dual-color synaptic markers combined with backfilling of nucleus accumbens medium spiny neurons to reveal some unique anatomic alignments of presumed synapses from the basolateral amygdala, medial prefrontal cortex, and paraventricular nucleus of thalamus. We also used dual-rhodopsin optogenetics in brain slices, which reveal a nonlinear interaction between some, but not all, projections. These results provide compelling anatomic and physiological mechanisms through which different glutamatergic projections to the nucleus accumbens, and possibly different aspects of emotional and motivational arousal, interact with each other for final behavioral output.

Molecular changes in the nucleus accumbens and prefrontal cortex associated with the locomotor sensitization induced by coca paste seized samples

RATIONALE

In previous studies, we have demonstrated that seized samples of a smokable form of cocaine, also known as coca paste (CP), induced behavioral sensitization in rats. Interestingly, this effect was accelerated and enhanced when the samples were adulterated with caffeine. While the cocaine phenomenon is associated with persistent functional and structural alterations in the prefrontal cortex (PFC) and nucleus accumbens (NAc), the molecular mechanisms underlying the CP sensitization and the influence of caffeine remains still unknown.

OBJECTIVE

We examined the gene expression in NAc and mPFC after the expression caffeine-adulterated and non-adulterated CP locomotor sensitization.

METHODS

The locomotor sensitization was established in C57BL/6 mice, repeatedly treated with a CP-seized sample adulterated with caffeine (CP-2) and a non-adulterated one (CP-1). We then assessed the mRNA expression of receptor subunits of the dopaminergic and glutamatergic systems in the medial PFC (mPFC) and NAc. Other molecular markers (e.g., adenosinergic, endocannabinoid receptor subunits, and synaptic plasticity-associated genes) were also analyzed.

RESULTS

Only CP-2-treated mice expressed locomotor sensitization. This phenomenon was associated with increased Drd1a, Gria1, Cnr1, and Syn mRNA expression levels in the NAc. Drd3 mRNA expression levels were only significantly increased in mPFC of CP-2-treated group.

CONCLUSIONS

Our results demonstrated that caffeine actively collaborates in the induction of the molecular changes underlying CP sensitization. The present study provides new knowledge on the impact of active adulterants to understand the early dependence induced by CP consumption.

Inter-individual differences in serotonin and glutamate co-transmission reflect differentiation in context-induced conditioned 50-kHz USVs response after morphine withdrawal

A growing body of research provides compelling evidence that in rats 50-kHz USVs are a form of expression of positive emotions. Context-induced 50-kHz USVs emission is variable among rats, indicating individual differences in contextual response bound up with pharmacological reward. The aims of this study were to: extract the most important neurotransmitters related to context-induced conditioned 50-kHz USVs response; find biological basis of existing inter-individual differences in context-induced conditioned 50-kHz USVs response; create a model of all-to-all neurotransmitters correlations. The data collected here confirms that re-exposure to the context of morphine administration after the withdrawal period increases the level of 50-kHz USVs and this contextual response is associated with elevated serotonin concentrations in amygdala, hippocampus and mPFC and with increased Glu/Gln ratio in nucleus accumbens. The concentration of serotonin increases simultaneously in amygdala, nucleus accumbens and hippocampus. Moreover, 5-HT concentration in amygdala is bound up with glutamate level in this structure as well as in hippocampus. Furthermore, Glu/Gln ratio in nucleus accumbens has strong associations with Glu/Gln ratio simultaneously in VTA, amygdala, striatum and hippocampus. All-to-all-analysis indicate that concentration of glutamate in hippocampus is proportional to glutamate in VTA and GABA concentration in the hippocampus. We have also demonstrated that Glu/GABA ratio in VTA and amygdala was elevated after post withdrawal re-exposure to the pharmacological reward paired context. Presented analysis indicates a strong correlation between serotonergic and glutamatergic systems in context-induced conditioned response. The strength of this co-transmission correlates with the number of 50-kHz USVs emitted in response to morphine-paired context.

Basolateral amygdala, nicotinic cholinergic receptors, and nicotine: Pharmacological effects and addiction in animal models and humans

The amygdala is involved in processing incoming information about rewarding stimuli and emotions that denote danger such as anxiety and fear. Bi-directional neural connections between basolateral amygdala (BLA) and brain regions such as nucleus accumbens, prefrontal cortex, hippocampus, and hindbrain regions regulate motivation, cognition, and responses to stress. Altered local regulation of BLA excitability is pivotal to the behavioral disturbances characteristic of posttraumatic stress disorder, and relapse to drug use induced by stress. Herein, we review the physiological regulation of BLA by cholinergic inputs, emphasizing the role of BLA nicotinic receptors. We review BLA-dependent effects of nicotine on cognition, motivated behaviors, and emotional states, including memory, taking and seeking drugs, and anxiety and fear in humans and animal models. The alterations in BLA activity observed in animal studies inform human behavioral and brain imaging research by enabling a more exact understanding of altered BLA function. Converging evidence indicates that cholinergic signaling from basal forebrain projections to local nicotinic receptors is an important physiological regulator of BLA and that nicotine alters BLA function. In essence, BLA is necessary for behavioral responses to stimuli that evoke anxiety and fear; reinstatement of cue-induced drug seeking; responding to second-order cues conditioned to abused drugs; reacquisition of amplified nicotine self-administration due to chronic stress during abstinence; and to promote responding for natural reward.

Cocaine-induced synaptic structural modification is differentially regulated by dopamine D1 and D3 receptors-mediated signaling pathways

Synaptic plasticity plays a critical role in cocaine addiction. The dopamine D1 and D3 receptors differentially regulate the cocaine-induced gene expression, structural remodeling and behavioral response. However, how these two receptors coordinately mediate the ultra-structural changes of synapses after cocaine exposure and whether these changes are behaviorally relevant are still not clear. Here, using quantitative electron microscopy, we show that D1 and D3 receptors have distinct roles in regulating cocaine-induced ultra-structural changes of synapses in the nucleus accumbens and caudoputamen. Pre-treatment of cocaine-treated mice with D3 receptor antagonist NGB2904 resulted in an increase in the ratio of total and asymmetric synapse to neuron and in the length of postsynaptic densities, compared with cocaine treatment alone. In contrast, pre-treatment of cocaine-treated mice with D1 receptor antagonist SCH23390 caused a reduction in synapse-to-neuron ratio and in postsynaptic densities length. Similarly, NGB2904 and SCH23390 showed opposite/differential effects on cocaine-induced structural plasticity, conditioned place preference and locomotor activity and signaling activation, including the activation of ERK, CREB and NR1 and the expression of c-fos and Cdk5. Therefore, we provide direct electron microscopy evidence that dopamine D1 and D3 receptors reciprocally regulate the ultra-structural changes of synapses following chronic exposure to cocaine. In addition, our data suggest that D1 and D3 receptors may regulate cocaine-induced ultra-structural changes and behavior responses by impact on structural plasticity and signaling transduction.

Progressive modulation of accumbal neurotransmission and anxiety-like behavior following protracted nicotine withdrawal

Due to the highly addictive properties of nicotine, a low percentage of users successfully maintain cessation for longer periods of time. This might be linked to neuroadaptations elicited by the drug, and understanding progressive changes in neuronal function might provide critical insight into nicotine addiction. We have previously shown that neurotransmission in the nucleus accumbens (nAc), a key brain region with respect to drug reinforcement and relapse, is suppressed for as long as seven months after a brief period of nicotine treatment. Studies were therefore performed to define the temporal properties of these effects, and to assess behavioral correlates to altered neurotransmission. Ex vivo electrophysiology revealed progressive depression of synaptic efficacy in the nAc of rats previously receiving nicotine. In addition, following three months of nicotine withdrawal, the responses to GABA_A receptor modulating drugs were blunted together with downregulation of several GABA_A receptor subunits. In correlation to reduced accumbal neurotransmission, a reduced anxiety-like behavior; assessed in the elevated plus-maze and marble burying tests, were identified in animals pre-treated with nicotine. Lastly, to test the causal relationship between suppressed excitability in the nAc and reduced anxiety-like behavior, rats received local administration of diazepam in the nAc while monitoring behavioral effects on the elevated plus-maze. These results show that nicotine produces long-lasting changes in the GABAergic system, which are observed first after extended withdrawal. Our data also suggest that nicotine produces a progressive suppression of accumbal excitability, which could result in behavioral alterations that may have implications for further drug intake.

Basolateral amygdala and stress-induced hyperexcitability affect motivated behaviors and addiction

The amygdala integrates and processes incoming information pertinent to reward and to emotions such as fear and anxiety that promote survival by warning of potential danger. Basolateral amygdala (BLA) communicates bi-directionally with brain regions affecting cognition, motivation and stress responses including prefrontal cortex, hippocampus, nucleus accumbens and hindbrain regions that trigger norepinephrine-mediated stress responses. Disruption of intrinsic amygdala and BLA regulatory neurocircuits is often caused by dysfunctional neuroplasticity frequently due to molecular alterations in local GABAergic circuits and principal glutamatergic output neurons. Changes in local regulation of BLA excitability underlie behavioral disturbances characteristic of disorders including post-traumatic stress syndrome (PTSD), autism, attention-deficit hyperactivity disorder (ADHD) and stress-induced relapse to drug use. In this Review, we discuss molecular mechanisms and neural circuits that regulate physiological and stress-induced dysfunction of BLA/amygdala and its principal output neurons. We consider effects of stress on motivated behaviors that depend on BLA; these include drug taking and drug seeking, with emphasis on nicotine-dependent behaviors. Throughout, we take a translational approach by integrating decades of addiction research on animal models and human trials. We show that changes in BLA function identified in animal addiction models illuminate human brain imaging and behavioral studies by more precisely delineating BLA mechanisms. In summary, BLA is required to promote responding for natural reward and respond to second-order drug-conditioned cues; reinstate cue-dependent drug seeking; express stress-enhanced reacquisition of nicotine intake; and drive anxiety and fear. Converging evidence indicates that chronic stress causes BLA principal output neurons to become hyperexcitable.

Amygdala-ventral striatum circuit activation decreases long-term fear

In humans, activation of the ventral striatum, a region associated with reward processing, is associated with the extinction of fear, a goal in the treatment of fear-related disorders. This evidence suggests that extinction of aversive memories engages reward-related circuits, but a causal relationship between activity in a reward circuit and fear extinction has not been demonstrated. Here, we identify a basolateral amygdala (BLA)-ventral striatum (NAc) pathway that is activated by extinction training. Enhanced recruitment of this circuit during extinction learning, either by pairing reward with fear extinction training or by optogenetic stimulation of this circuit during fear extinction, reduces the return of fear that normally follows extinction training. Our findings thus identify a specific BLA-NAc reward circuit that can regulate the persistence of fear extinction and point toward a potential therapeutic target for disorders in which the return of fear following extinction therapy is an obstacle to treatment.

Prolonged Consumption of Sucrose in a Binge-Like Manner, Alters the Morphology of Medium Spiny Neurons in the Nucleus Accumbens Shell

The modern diet has become highly sweetened, resulting in unprecedented levels of sugar consumption, particularly among adolescents. While chronic long-term sugar intake is known to contribute to the development of metabolic disorders including obesity and type II diabetes, little is known regarding the direct consequences of long-term, binge-like sugar consumption on the brain. Because sugar can cause the release of dopamine in the nucleus accumbens (NAc) similarly to drugs of abuse, we investigated changes in the morphology of neurons in this brain region following short- (4 weeks) and long-term (12 weeks) binge-like sucrose consumption using an intermittent two-bottle choice paradigm. We used Golgi-Cox staining to impregnate medium spiny neurons (MSNs) from the NAc core and shell of short- and long-term sucrose consuming rats and compared these to age-matched water controls. We show that prolonged binge-like sucrose consumption significantly decreased the total dendritic length of NAc shell MSNs compared to age-matched control rats. We also found that the restructuring of these neurons resulted primarily from reduced distal dendritic complexity. Conversely, we observed increased spine densities at the distal branch orders of NAc shell MSNs from long-term sucrose consuming rats. Combined, these results highlight the neuronal effects of prolonged binge-like intake of sucrose on NAc shell MSN morphology.

Presynaptic D1 heteroreceptors and mGlu autoreceptors act at individual cortical release sites to modify glutamate release

The aim of this work was to study release of glutamic acid (GLU) from one-axon terminal or bouton at-a-time using cortical neurons grown in vitro to study the effect of presynaptic auto- and heteroreceptor stimulation. Neurons were infected with release reporters SypHx2 or iGluSnFR at 7 or 3 days-in-vitro (DIV) respectively. At 13-15 DIV single synaptic boutons were identified from images obtained from a confocal scanning microscope before and after field electrical stimulation. We further stimulated release by raising intracellular levels of cAMP with forskolin (10µM). Forskolin-mediated effects were dependent on protein kinase A (PKA) and did not result from an increase in endocytosis, but rather from an increase in the size of the vesicle readily releasable pool. Once iGluSnFR was confirmed as more sensitive than SypHx2, it was used to study the participation of presynaptic auto- and heteroreceptors on GLU release. Although most receptor agonizts (carbamylcholine, nicotine, dopamine D2, BDNF) did not affect electrically stimulated GLU release, a significant increase was observed in the presence of metabotropic D1/D5 heteroreceptor agonist (SKF38393 10µM) that was reversed by PKA inhibitors. Interestingly, stimulation of group II metabotropic mGLU2/3 autoreceptors (LY379268 50nM) induced a decrease in GLU release that was reversed by the specific mGLU2/3 receptor antagonist (LY341495 1µM) and also by PKA inhibitors (KT5720 200nM and PKI14-22 400nM). These changes in release probability at individual release sites suggest another level of control of the distribution of transmitter substances in cortical tissue.

Mesocorticolimbic dopamine functioning in primary psychopathy: A source of within-group heterogeneity

Despite similar emotional deficiencies, primary psychopathic individuals can be situated on a continuum that spans from controlled to disinhibited. The constructs on which primary psychopaths are found to diverge, such as self-control, cognitive flexibility, and executive functioning, are crucially regulated by dopamine (DA). As such, the goal of this review is to examine which specific alterations in the meso-cortico-limbic DA system and corresponding genes (e.g., TH, DAT, COMT, DRD2, DRD4) might bias development towards a more controlled or disinhibited expression of primary psychopathy. Based on empirical data, it is argued that primary psychopathy is generally related to a higher tonic and population activity of striatal DA neurons and lower levels of D2-type DA receptors in meso-cortico-limbic projections, which may boost motivational drive towards incentive-laden goals, dampen punishment sensitivity, and increase future reward-expectancy. However, increasingly higher levels of DA activity in the striatum (moderate versus pathological elevations), lower levels of DA functionality in the prefrontal cortex, and higher D1-to-D2-type receptor ratios in meso-cortico-limbic projections may lead to increasingly disinhibited and impetuous phenotypes of primary psychopathy. Finally, in order to provide a more coherent view on etiological mechanisms, we discuss interactions between DA and serotonin that are relevant for primary psychopathy.

Role of Dopamine Neurons in Reward and Aversion: A Synaptic Plasticity Perspective

The brain is wired to predict future outcomes. Experience-dependent plasticity at excitatory synapses within dopamine neurons of the ventral tegmental area, a key region for a broad range of motivated behaviors, is thought to be a fundamental cellular mechanism that enables adaptation to a dynamic environment. Thus, depending on the circumstances, dopamine neurons are capable of processing both positive and negative reinforcement learning strategies. In this review, we will discuss how changes in synaptic plasticity of dopamine neurons may affect dopamine release, as well as behavioral adaptations to different environmental conditions falling at opposite ends of a saliency spectrum ranging from reward to aversion.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Immobility behavior during the forced swim test correlates with BNDF levels in the frontal cortex, but not with cognitive impairments

The forced swim test (FST) is widely used to evaluate the antidepressant-like activity of compounds and is sensitive to stimuli that cause depression-like behaviors in rodents. The immobility behavior observed during the test has been considered to represent behavioral despair. In addition, some studies suggest that the FST impairs rats' performance on cognitive tests, but these findings have rarely been explored. Thus, we investigated the effects of the FST on behavioral tests related to neuropsychiatric diseases that involve different cognitive components: novel object recognition (NOR), the object location test (OLT) and prepulse inhibition (PPI). Brain-derived neurotrophic factor (BDNF) levels in the frontal cortex and hippocampus were evaluated. The rats were forced to swim twice (15-min session followed by a 5-min session 24h later) and underwent cognitive tests 24h after the last swimming exposure. The FST impaired the rats' performance on the OLT and reduced the PPI and acoustic startle responses, whereas the NOR was not affected. The cognitive impairments were not correlated with an immobility behavior profile, but a significant negative correlation between the frontal BDNF levels and immobility behavior was identified. These findings suggest a protective role of BDNF against behavioral despair and demonstrate a deleterious effect of the FST on spatial memory and pre-attentive processes, which point to the FST as a tool to induce cognitive impairments analogous to those observed in depression and in other neuropsychiatric disorders.

The "addicted" spine

Units of dendritic branches called dendritic spines represent more than simply decorative appendages of the neuron and actively participate in integrative functions of “spinous” nerve cells thereby contributing to the general phenomenon of synaptic plasticity. In animal models of drug addiction, spines are profoundly affected by treatments with drugs of abuse and represent important sub cellular markers which interfere deeply into the physiology of the neuron thereby providing an example of the burgeoning and rapidly increasing interest in “structural plasticity”. Medium Spiny Neurons (MSNs) of the Nucleus Accumbens (Nacc) show a reduced number of dendritic spines and a decrease in TH-positive terminals upon withdrawal from opiates, cannabinoids and alcohol. The reduction is localized “strictly” to second order dendritic branches where dopamine (DA)-containing terminals, impinging upon spines, make synaptic contacts. In addition, long-thin spines seems preferentially affected raising the possibility that cellular learning of these neurons may be selectively hampered. These findings suggest that dendritic spines are affected by drugs widely abused by humans and provide yet another example of drug-induced aberrant neural plasticity with marked reflections on the physiology of synapses, system structural organization, and neuronal circuitry remodeling.

Dopamine preferentially inhibits NMDA receptor-mediated EPSCs by acting on presynaptic D1 receptors in nucleus accumbens during postnatal development

Nucleus accumbens (nAcb), a major site of action of drugs of abuse and dopamine (DA) signalling in MSNs (medium spiny neurons), is critically involved in mediating behavioural responses of drug addiction. Most studies have evaluated the effects of DA on MSN firing properties but thus far, the effects of DA on a cellular circuit involving glutamatergic afferents to the nAcb have remained rather elusive. In this study we attempted to characterize the effects of dopamine (DA) on evoked glutamatergic excitatory postsynaptic currents (EPSCs) in nAcb medium spiny (MS) neurons in 1 to 21 day-old rat pups. The EPSCs evoked by local nAcb stimuli displayed both AMPA/KA and NMDA receptor-mediated components. The addition of DA to the superfusing medium produced a marked decrease of both components of the EPSCs that did not change during the postnatal period studied. Pharmacologically isolated AMPA/KA receptor-mediated response was inhibited on average by 40% whereas the isolated NMDA receptor-mediated EPSC was decreased by 90%. The effect of DA on evoked EPSCs were mimicked by the D₁-like receptor agonist SKF 38393 and antagonized by the D₁-like receptor antagonist SCH 23390 whereas D₂-like receptor agonist or antagonist respectively failed to mimic or to block the action of DA. DA did not change the membrane input conductance of MS neurons or the characteristics of EPSCs produced by the local administration of glutamate in the presence of tetrodotoxin. In contrast, DA altered the paired-pulse ratio of evoked EPSCs. The present results show that the activation D₁-like dopaminergic receptors modulate glutamatergic neurotransmission by preferentially inhibiting NMDA receptor-mediated EPSC through presynaptic mechanisms.

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.

Stimulus-specific and differential distribution of activated extracellular signal-regulated kinase in the nucleus accumbens core and shell during Pavlovian-instrumental transfer

The ability of reward-predictive cues to potentiate reward-seeking behavior--a phenomenon termed Pavlovian--instrumental transfer (PIT)--depends on the activation of extracellular signal-regulated kinase (ERK) in the nucleus accumbens (NAc). Here, we utilized immunohistochemistry to investigate the subregional pattern of ERK activation during PIT, and the contribution of different elements in the PIT condition to the distribution of ERK signaling in the NAc of rats. We found that the occurrence of reward-seeking behavior (lever pressing) did not affect ERK activation in either the core or the shell of the NAc. In contrast, presentation of the reward-predictive cue (auditory conditioned stimulus) caused a significant increase in ERK activation in both subregions of the NAc, with the effect being slightly more robust in the core than the shell. Different from the pattern evoked by the reward-predictive cue, presentation of the reward itself (food pellets) had no effect on ERK activation in the core but caused a pronounced increase in ERK activation in the shell. Taken together, our results demonstrate that ERK signaling in the NAc during PIT involves both the core and the shell and is driven by the conditioned cue irrespective of whether the situation permits engagement in reward-seeking behavior. Furthermore, our results show that the subregional distribution of ERK signaling in the NAc evoked by rewards differs from that evoked by cues that predict them. The stimulus-specific differential pattern of ERK signaling described here may present the molecular complement to stimulus-specific increases in NAc cell firing reported previously.

Cocaine-induced reinstatement of a conditioned place preference in developing rats: involvement of the d2 receptor

Reinstatement of conditioned place preferences have been used to investigate physiological mechanisms mediating drug-seeking behavior in adolescent and adult rodents; however, it is still unclear how psychostimulant exposure during adolescence affects neuron communication and whether these changes would elicit enhanced drug-seeking behavior later in adulthood. The present study determined whether the effects of intra-ventral tegmental area (VTA) or intra-nucleus accumbens septi (NAcc) dopamine (DA) D2 receptor antagonist infusions would block (or potentiate) cocaine-induced reinstatement of conditioned place preferences. Adolescent rats (postnatal day (PND 28–39)) were trained to express a cocaine place preference. The involvement of D2 receptors on cocaine-induced reinstatement was determined by intra-VTA or intra-NAcc infusion of the DA D2 receptor antagonist sulpiride (100 μM) during a cocaine-primed reinstatement test (10 mg/kg cocaine, i.p.). Infusion of sulpiride into the VTA but not the NAcc blocked reinstatement of conditioned place preference. These data suggest intrinsic compensatory mechanisms in the mesolimbic DA pathway mediate responsivity to cocaine-induced reinstatement of a conditioned place preference during development.

Amygdalofugal axon terminals immunoreactive for L-aspartate or L-glutamate in the nucleus accumbens of rats and domestic chickens: a comparative electron microscopic immunocytochemical study combined with anterograde pathway tracing

Several studies have shown that L-aspartate (Asp) is present in synaptic vesicles and released exocytotically from presynaptic terminals, possibly by Ca(2+)-dependent corelease of Asp and L-glutamate (Glu). It has been demonstrated that both excitatory amino acids (EAAs) are released from the rat striatum as part of corticostriatal neurotransmission. The single or colocalized occurrence of Asp and Glu in specific synaptic boutons of the chicken medial striatum/nucl. accumbens has been demonstrated by our group using ultrastructural immunocytochemistry. However, evidence for the presence of EAAs in any specific striatal pathway was only circumstantial. Here, we report on the distribution of Asp and Glu in specific synaptic terminals of the amygdalostriatal pathway, both in rat and chicken brains, combining anterograde tracing with postembedding immunogold labeling of Asp or Glu. Immunoreactivity for Asp and Glu was observed in amygdalofugal terminals with asymmetrical synaptic junctions (morphologically representing excitatory synapses) in both species. The postsynaptic targets were either dendritic spines or small dendrites, whereas axosomatic or axo-axonic connections were not observed. Ultrastructurally, the synaptic terminals immunoreactive for Asp were indistinguishable from those immunoreactive for Glu. The findigs are consistent with an Asp-Glu corelease mechanism, with a distinct synaptic contingent, evolutionarily conserved in the amygdalostriatal pathway.

After-training emotional interference may modulate sequence awareness in a serial reaction time task

The purpose of the present experiment was to investigate the effects of emotional interference on consolidation of sequential learning. In different sessions, 6 groups of subjects were initially trained on a serial reaction time task (SRTT). To modulate consolidation of the newly learned skill, subjects were exposed, after the training, to 1 of 3 (positive, negative or neutral) different classes of emotional stimuli which consisted of a set of emotional pictures combined with congruent emotional musical pieces or neutral sound. Emotional intervention for each subject group was done in 2 different time intervals (either directly after the training session or 6 h later). After a 72 h post-training interval, each group was retested on the SRTT. Re-test performance was evaluated in terms of response times and accuracy during execution of a target sequence. Emotional intervention did not influence either response times or accuracy of re-testing SRTT target task performance, both variables sensitive to implicit knowledge acquired during SRTT training. However, explicit awareness of sequence knowledge after 72 h was enhanced when negative stimuli had been applied at 0 h after training. These findings suggest that consolidation of explicit aspects of procedural learning may be more responsive toward emotional interference than implicit aspects.

Ventral pallidum mediates amygdala-evoked deficits in prepulse inhibition

Prepulse inhibition (PPI) is an operational measure of sensorimotor gating. It is defined as a reduction in magnitude of a startle response when a startling stimulus is preceded by a weaker "prepulse." PPI has been found to be altered in patients with schizophrenia, autism spectrum disorders, and other neuropsychiatric illnesses. As such, the neural substrates regulating PPI are of particular interest. Previous studies using lesions, selective blockade of N-methyl-d-aspartate (NMDA) receptors, and pharmacological disinhibition have demonstrated that impairment of the function of the basolateral and lateral nuclei of the amygdala (BLA) disrupts PPI. However, transient gamma aminobutyric acid-mediated (GABA-mediated) inactivation of BLA has not been evaluated for effects on PPI. Furthermore, the downstream projection targets that mediate BLA-evoked disruptions of PPI have not been elucidated. Thus, in the present study, we evaluated the effect on PPI of bilateral and unilateral inactivation of BLA, by microinfusion of the GABA-A receptor agonist, muscimol. We found that either bilateral or unilateral inactivation impaired PPI. Because unilateral inactivation was sufficient to impair PPI, we hypothesized that this was due to an indirect activation of a downstream target of BLA, the ventral pallidum (VP). Because VP inhibition normalizes PPI deficits evoked from nucleus accumbens (Kodsi & Swerdlow, 1994), we next tested the degree to which VP inhibition would normalize PPI deficits evoked from BLA. We unilaterally inactivated BLA with concurrent inactivation of VP and found that VP inactivation blocked BLA-evoked deficits in PPI. We suggest that BLA inactivation disrupts PPI through disinhibition of VP.

Insights to drug addiction derived from ultrastructural views of the mesocorticolimbic system

Drugs of abuse increase the release of dopamine from mesocorticolimbic neurons in the ventral tegmental area. Thus, insights into the cytoarchitecture and the synaptic circuitry affecting the activity of dopaminergic neurons in this area are fundamental for understanding the commonalities produced by mechanistically distinct drugs of abuse. Electron microscopic immunolabeling has provided these insights and also shown the critical relationships between the dopaminergic axon terminals and their targeted neurons in the prefrontal cortex and in the both the dorsal and ventral striatum. These brain regions are among those where dopamine and associated neurotransmitters are most implicated in the transition from recreational to compulsive consumption of reinforcing drugs. Thus, the synaptic circuitry and drug-induced plasticity occurring in the ventral tegmental area and in dopamine-targeted regions are reviewed, as both are essential for understanding the long-lasting changes produced by addictive substances.

Neurons of human nucleus accumbens

BACKGROUND/AIM

Nucleus accumbens is a part of the ventral striatum also known as a drug active brain region, especially related with drug addiction. The aim of the study was to investigate the Golgi morphology of the nucleus accumbens neurons.

METHODS

The study was performed on the frontal and sagittal sections of 15 human brains by the Golgi Kopsch method. We classified neurons in the human nucleus accumbens according to their morphology and size into four types: type I--fusiform neurons; type II--fusiform neurons with lateral dendrite, arising from a part of the cell body; type III--pyramidal-like neuron; type IV--multipolar neuron. The medium spiny neurons, which are mostly noted regarding to the drug addictive conditions of the brain, correspond to the type IV--multipolar neurons.

RESULTS

Two regions of human nucleus accumbens could be clearly recognized on Nissl and Golgi preparations each containing different predominant neuronal types. Central part of nucleus accumbens, core region, has a low density of impregnated neurons with predominant type III, pyramidal-like neurons, with spines on secondary branches and rare type IV, multipolar neurons. Contrary to the core, peripheral region, shell of nucleus, has a high density of impregnated neurons predominantly contained of type I and type IV--multipolar neurons, which all are rich in spines on secondary and tertiary dendritic branches.

CONCLUSION

Our results indicate great morphological variability of human nucleus accumbens neurons. This requires further investigations and clarifying clinical significance of this important brain region.

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.

Ventral tegmental area-basolateral amygdala-nucleus accumbens shell neurocircuitry controls the expression of heroin-conditioned immunomodulation

The present investigations sought to determine whether the ventral tegmental area (VTA), basolateral amygdala (BLA), and nucleus accumbens shell (NAC) comprise a circuitry that mediates heroin-induced conditioned immunomodulation. Rats were given conditioning trials in which they received an injection of heroin upon placement into a distinctive environment. Prior to testing, rats received unilateral intra-BLA microinfusion of a D(1) antagonist concomitantly with unilateral intra-NAC shell microinfusion of an NMDA antagonist. Disconnection of the VTA-BLA-NAC circuit impaired the ability of the heroin-paired environment to suppress lipopolysaccharide-induced immune responses, defining for the first time a specific neural circuit involved in conditioned neural-immune interactions.

Pilot trial of Melissa officinalis L. leaf extract in the treatment of volunteers suffering from mild-to-moderate anxiety disorders and sleep disturbances

Botanicals are an alternative option to prescription drugs for the alleviation of symptoms due to anxiety disorders and insomnia. Melissa officinalis L. has been shown as an anti-stress and anxiolytic agent. We previously reported moderate stress improvement in mice in which Cyracos(®), a standardized Melissa officinalis L. extract, was administrated. Cyracos(®) contains phytochemicals that inhibit gamma-aminobutyric acid catabolism. This was a prospective, open-label, 15-day study to evaluate the efficacy of Cyracos(®) on stressed volunteers, who have mild-to-moderate anxiety disorders and sleep disturbances. Using clinician rating criteria, primary outcomes showed improvement of symptoms. Cyracos(®) reduced anxiety manifestations by 18% (p < 0.01), ameliorated anxiety-associated symptoms by 15% (p < 0.01) and lowered insomnia by 42% (p < 0.01). As much as 95% of subjects (19/20) responded to treatment, of which 70% (14/20) achieved full remission for anxiety, 85% (17/20) for insomnia, and 70% (14/20) for both. Our study demonstrates, for the first time that chronic administration of Melissa officinalis L. relieves stress-related effects. It is critical that further studies incorporate a placebo and investigate physiological stress markers.

Cortico-basal ganglia circuitry: a review of key research and implications for functional connectivity studies of mood and anxiety disorders

There is considerable evidence that dysfunction of the cortico-basal ganglia circuits may be associated with several mood and anxiety disorders. However, it is unclear whether circuit abnormalities contribute directly either to the neurobiology of these conditions or to the manifestation of symptoms. Understanding the role of these pathways in psychiatric illness has been limited by an incomplete characterization of normal function. In recent years, studies using animal models and human functional imaging have greatly expanded the literature describing normal cortico-basal ganglia circuit function. In this paper, recent key studies of circuit function using human and animal models are reviewed and integrated with findings from other studies conducted over the previous decades. The literature suggests several hypotheses of cortico-basal ganglia circuitry function in mood and anxiety disorders that warrant further exploration. Hypotheses are proposed herein based upon the cortico-basal ganglia mechanisms of: (1) feedforward and feedback control, (2) circuit integration and (3) emotional control. These are presented as models of circuit function, which may be particularly relevant to future investigations using neuroimaging and functional connectivity analyses.

Cocaine- and morphine-induced synaptic plasticity in the nucleus accumbens

The critical brain areas and molecular mechanisms involved in drug abuse and dependence have been extensively studied. Drug-induced persistent behaviors such as sensitization, tolerance, or relapse, however, far outlast any previously reported mechanisms. A challenge in the field of addiction, therefore, has been to identify drug-induced changes in brain circuitry that may subserve long-lasting changes in behavior. This study examined behavioral changes and electron microscopic evidence of altered synaptic connectivity within the nucleus accumbens (NAc) following repeated administration of cocaine or morphine. The unbiased quantitative stereological physical disector method was used to estimate the number of synapses per neuron. Increases in the synapse-to-neuron ratio were found in the NAc shell of cocaine-treated (49.1%) and morphine-treated (55.1%) rats and in the NAc core of cocaine-treated animals (49.1%). This study provides direct ultrastructural evidence of drug-induced synaptic plasticity and identifies synaptic remodeling as a potential neural substrate underlying drug-induced behavioral sensitization.

Sub-region specific contribution of the ventral hippocampus to drug context-induced reinstatement of cocaine-seeking behavior in rats

The ventral hippocampus (VH) plays critical roles in cue-induced and cocaine-primed reinstatement of cocaine seeking [Rogers JL, See RE (2007) Neurobiol Learn Mem 87:688-692]. Subregions of the VH make distinct projections to elements of the brain relapse circuitry that mediate drug context-induced reinstatement. Thus, the VH may also critically contribute to this form of cocaine seeking in a subregion-specific manner. Accordingly, this study evaluated the hypothesis that functional inactivation of the ventral hippocampus proper (VHp)-but not of the dentate gyrus (DG)-impairs cocaine seeking elicited by re-exposure to a drug-paired environmental context. Rats were trained to lever press for un-signaled i.v. cocaine infusions (0.15 mg/infusion) in a distinct environmental context (cocaine-paired context) followed by extinction training in a distinctly different context (extinction context). Subsequently, cocaine-seeking behavior (i.e., non-reinforced active lever responding) was assessed in either the previously cocaine-paired context or the extinction context. Rats received bilateral microinfusions of the GABA agonist cocktail, baclofen+muscimol (BM: 1.0/.01 mM), or vehicle into the VHp, DG, or the posterior dorsal hippocampus (pDH; extra-VH control) immediately before each test session. Exposure to the previously cocaine-paired context, but not the extinction context, reinstated extinguished cocaine-seeking behavior following vehicle pretreatment. BM pretreatment administered into the VHp, but not the DG or pDH, significantly attenuated drug context-induced cocaine seeking. These results indicate that the VH contributes to drug context-induced cocaine seeking in a subregion-specific manner, with the functional integrity of the VHp being necessary for memory or motivational aspects of drug-paired environmental stimuli that sustain stimulus control over goal-directed behavior.

Dopamine D1 receptors within the basolateral amygdala mediate heroin-induced conditioned immunomodulation

This study investigates the role of basolateral amygdala (BLA) dopamine in heroin-induced conditioned immunomodulation. Animals underwent conditioning in which heroin administration was repeatedly paired with placement into a conditioning chamber. Six days after the final conditioning session animals were returned to the chamber and received intra-BLA microinfusions of dopamine, D(1) or D(2), antagonist. Antagonism of D(1), but not D(2), receptors within the BLA blocked the suppressive effect of heroin-associated environmental stimuli on iNOS, TNF-α and IL-1β. This study is the first to demonstrate that the expression of heroin's conditioned effects on proinflammatory mediators require dopamine D(1) receptors within the BLA.

Quantitative analysis of pre- and postsynaptic sex differences in the nucleus accumbens

The nucleus accumbens (NAc) plays a central role in motivation and reward. While there is ample evidence for sex differences in addiction-related behaviors, little is known about the neuroanatomical substrates that underlie these sexual dimorphisms. We investigated sex differences in synaptic connectivity of the NAc by evaluating pre- and postsynaptic measures in gonadally intact male and proestrous female rats. We used DiI labeling and confocal microscopy to measure dendritic spine density, spine head size, dendritic length, and branching of medium spiny neurons (MSNs) in the NAc, and quantitative immunofluorescence to measure glutamatergic innervation using pre- (vesicular glutamate transporter 1 and 2) and postsynaptic (postsynaptic density 95) markers, as well as dopaminergic innervation of the NAc. We also utilized electron microscopy to complement the above measures. Clear but subtle sex differences were identified, namely, in distal dendritic spine density and the proportion of large spines on MSNs, both of which are greater in females. Sex differences in spine density and spine head size are evident in both the core and shell subregions, but are stronger in the core. This study is the first demonstration of neuroanatomical sex differences in the NAc and provides evidence that structural differences in synaptic connectivity and glutamatergic input may contribute to behavioral sex differences in reward and addiction.

Nicotinic acetylcholine receptors of the dorsal hippocampus and the basolateral amygdala are involved in ethanol-induced conditioned place preference

The purpose of this study was to evaluate whether nicotinic acetylcholine receptors of the dorsal hippocampus and the basolateral amygdala (BLA) can potentiate ethanol response in the conditioned place preference (CPP) paradigm. I.p. administration of different doses of ethanol (0.25-1 g/kg) did not induce CPP. However, the higher dose of the drug (1.5 g/kg i.p.) induced place aversion. Furthermore, microinjection of nicotine (0.5-1 microg/rat) into both CA1 regions (intra-CA1) and the BLA (intra-BLA) did not produce a significant CPP. Interestingly, intra-CA1 or -BLA administration of nicotine plus ethanol (0.5 g/kg) during conditioning phase significantly induced a strong CPP. Microinjection of mecamylamine, the nicotinic acetylcholine receptor antagonist, into the CA1 regions or into the BLA did not alter CPP. However, intra-CA1 or -BLA microinjection of mecamylamine (1-4 microg/rat) reversed the response induced by the microinjection of nicotine (1 microg/rat, intra-CA1 or -BLA) plus ethanol (0.5 g/kg i.p.) in the CPP paradigm. On the other hand, the microinjection of nicotine (0.5-1.5 microg/rat) into the BLA, but not into the CA1 regions before the testing phase potentiated the response of ethanol on the expression of conditioned place preference. Moreover, intra-CA1 administration of nicotine plus ethanol increased the locomotor activity on the test day which was reversed by pretreatment with mecamylamine, while other treatments had no effect on locomotor activity. It can be concluded that the activation of nicotinic acetylcholine receptors of the dorsal hippocampus and the basolateral amygdala can potentiate the ethanol response in the CPP paradigm.

Limited convergence of rhinal cortical and dopaminergic inputs in the rat basolateral amygdala: an ultrastructural analysis

The basolateral nuclear complex of the amygdala (BLC) receives robust sensory inputs from the rhinal cortices (RCx) that are important for the generation of emotional behavior. The BLC is also one of the main targets of the mesolimbic dopamine (DA) system. DA potentiates cortical sensory inputs to the BLC, which leads to an increase in the excitability of BLC pyramidal cells. These findings suggest that there may be convergence of RCx and DA inputs onto the dendrites of pyramidal cells in the BLC. In the present study we used dual-labeling immunohistochemistry and anterograde tract-tracing at the ultrastructural level to test this hypothesis in the rat brain. RCx axons were labeled by Phaseolus vulgaris leucoagglutinin (PHA-L) injections, whereas tyrosine hydroxylase (TH) was used as a marker for DA axons. The extent of convergence of these axons was analyzed in the posterior subdivision of the basolateral nucleus (BLp), which is densely innervated by both inputs. RCx synapses were asymmetrical and mainly contacted dendritic spines (86.4%) and dendritic shafts (12.1%). TH-positive (TH+) terminals also mainly formed synapses (symmetrical) and appositions with spines and shafts of dendrites. However, ultrastructural analysis found a very low percentage of RCx terminals converging with DA terminals onto unlabeled dendrites (9.4%) and axons (7.5 %), or exhibiting direct contacts with TH+ terminals (3.8%). These findings suggest that the association of specific behaviorally salient sensory stimuli with dopamine release in the BLC is not dependent on a point-to-point spatial relationship of cortical and DA inputs.

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.

The uncertainty processing theory of motivation

Most theories describe motivation using basic terminology (drive, 'wanting', goal, pleasure, etc.) that fails to inform well about the psychological mechanisms controlling its expression. This leads to a conception of motivation as a mere psychological state 'emerging' from neurophysiological substrates. However, the involvement of motivation in a large number of behavioural parameters (triggering, intensity, duration, and directedness) and cognitive abilities (learning, memory, decision, etc.) suggest that it should be viewed as an information processing system. The uncertainty processing theory (UPT) presented here suggests that motivation is the set of cognitive processes allowing organisms to extract information from the environment by reducing uncertainty about the occurrence of psychologically significant events. This processing of information is shown to naturally result in the highlighting of specific stimuli. The UPT attempts to solve three major problems: (i) how motivations can affect behaviour and cognition so widely, (ii) how motivational specificity for objects and events can result from nonspecific neuropharmacological causal factors (such as mesolimbic dopamine), and (iii) how motivational interactions can be conceived in psychological terms, irrespective of their biological correlates. The UPT is in keeping with the conceptual tradition of the incentive salience hypothesis while trying to overcome the shortcomings inherent to this view.

Distinct opioid circuits determine the palatability and the desirability of rewarding events

It generally is assumed that a common neural substrate mediates both the palatability and the reward value of nutritive events. However, recent evidence suggests this assumption may not be true. Whereas opioid circuitry in both the nucleus accumbens and ventral pallidum has been reported to mediate taste-reactivity responses to palatable events, the assignment of reward or inventive value to goal-directed actions has been found to involve the basolateral amygdala. Here we found that, in rats, the neural processes mediating palatability and incentive value are indeed dissociable. Naloxone infused into either the ventral pallidum or nucleus accumbens shell blocked the increase in sucrose palatability induced by an increase in food deprivation without affecting the performance of sucrose-related actions. Conversely, naloxone infused into the basolateral amygdala blocked food deprivation-induced changes in sucrose-related actions without affecting sucrose palatability. This double dissociation of opioid-mediated changes in palatability and incentive value suggests that the role of endogenous opioids in reward processing does not depend on a single neural circuit. Rather, changes in palatability and in the incentive value assigned to rewarding events seem to be mediated by distinct neural processes.

Involvement of dopaminergic mechanisms in the nucleus accumbens core and shell subregions in the expression of fear conditioning

The involvement of dopamine (DA) mechanisms in the nucleus accumbens (NAC) in fear conditioning has been proposed by many studies that have challenged the view that the NAC is solely involved in the modulation of appetitive processes. However, the role of the core and shell subregions of the NAC in aversive conditioning remains unclear. The present study examined DA release in these NAC subregions using microdialysis during the expression of fear memory. Guide cannulae were implanted in rats in the NAC core and shell. Five days later, the animals received 10 footshocks (0.6 mA, 1 s duration) in a distinctive cage A (same context). On the next day, dialysis probes were inserted through the guide cannulae into the NAC core and shell subregions, and the animals were behaviorally tested for fear behavior either in the same context (cage A) or in a novel context (cage B). Dialysates were collected every 5 min for 90 min and analyzed by high-performance liquid chromatography. The rats exhibited a significant fear response in cage A but not in cage B. Moreover, increased DA levels in both NAC subregions were observed 5-25 min after the beginning of the test when the animals were tested in the same context compared with accumbal DA levels from rats tested in the different context. These findings suggest that DA mechanisms in both the NAC core and shell may play an important role in the expression of contextual fear memory.

A developmental neurobiological model of motivated behavior: anatomy, connectivity and ontogeny of the triadic nodes

Adolescence is the transition period that prepares individuals for fulfilling their role as adults. Most conspicuous in this transition period is the peak level of risk-taking behaviors that characterize adolescent motivated behavior. Significant neural remodeling contributes to this change. This review focuses on the functional neuroanatomy underlying motivated behavior, and how ontogenic changes can explain the typical behavioral patterns in adolescence. To help model these changes and provide testable hypotheses, a neural systems-based theory is presented. In short, the Triadic Model proposes that motivated behavior is governed by a carefully orchestrated articulation among three systems, approach, avoidance and regulatory. These three systems map to distinct, but overlapping, neural circuits, whose representatives are the striatum, the amygdala and the medial prefrontal cortex. Each of these system-representatives will be described from a functional anatomy perspective that includes a review of their connectivity and what is known of their ontogenic changes.

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.

Contributions of the amygdala and medial prefrontal cortex to incentive cue responding

Reward-seeking behavior is controlled by neuronal circuits that include the basolateral nucleus of amygdala (BLA), medial prefrontal cortex (mPFC), nucleus accumbens (NAc) and ventral tegmental area. Using a discriminative stimulus (DS) task in which an intermittently presented cue (DS) directs rats to make an operant response for sucrose, we previously demonstrated that dopamine receptor antagonism in the NAc reduced reinforced cue responding, whereas general inactivation of the NAc increased behavioral responding in the absence of the cue. Because they send major glutamatergic projections to the NAc, the BLA and mPFC may also contribute to reward-seeking behaviors modulated by the NAc. In this study we compare the effects of BLA and mPFC inactivation on rats' performance of a DS task. BLA inactivation by combined GABA(A) and GABA(B) agonists impaired cue responding with minimal effects on operant behavior in the absence of cues. Dorsal medial prefrontal cortex (dmPFC) inactivation also inhibited cue-evoked reward-seeking. In contrast, ventral medial prefrontal cortex (vmPFC) inactivation disinhibited responding to unrewarded cues with less influence on reinforced cue responding. These findings demonstrate that the BLA and dmPFC facilitate cue-evoked reward-seeking, whereas, in the same task the vmPFC exerts inhibitory control over unrewarded behaviors.