Specificity of amygdalostriatal interactions in the involvement of mesencephalic dopaminergic neurons in affective perception

Understanding the role of dopamine in conditioned and unconditioned fear

Pharmacological and molecular imaging studies in anxiety disorders have primarily focused on the serotonin system. In the meantime, dopamine has been known as the neurotransmitter of reward for 60 years, particularly for its action in the nervous terminals of the mesocorticolimbic system. Interest in the mediation by dopamine of the well-known brain aversion system has grown recently, particularly given recent evidence obtained on the role of D2 dopamine receptors in unconditioned fear. However, it has been established that excitation of the mesocorticolimbic pathway, originating from dopaminergic (DA) neurons from the ventral tegmental area (VTA), is relevant for the development of anxiety. Among the forebrain regions innervated by this pathway, the amygdala is an essential component of the neural circuitry of conditioned fear. Current findings indicate that the dopamine D2 receptor-signaling pathway connecting the VTA to the basolateral amygdala modulates fear and anxiety, whereas neural circuits in the midbrain tectum underlie the expression of innate fear. The A13 nucleus of the zona incerta is proposed as the origin of these DA neurons projecting to caudal structures of the brain aversion system. In this article we review data obtained in studies showing that DA receptor-mediated mechanisms on ascending or descending DA pathways play opposing roles in fear/anxiety processes. Dopamine appears to mediate conditioned fear by acting at rostral levels of the brain and regulate unconditioned fear at the midbrain level.

Acupuncture – Self-Appraisal and the Reward System

Acupuncture is an ancient therapy with a variety of different explanatory models. A cascade of physiological effects has been reported, both in the peripheral and the central nervous system, following the insertion of a needle or light tapping of the skin. Clinical trials testing the specific claims of acupuncture have generally tried to focus on testing the efficacy of applying specific techniques and/or specified points. However, different conditions may respond differently to different modes of stimulation.

Recently, it was demonstrated that both superficial and deep needling (with de qi/Hibiki) resulted in amelioration of patellofemoral pain and unpleasantness. The pleasurable aspect of the acupuncture experience has largely been ignored as it has been considered secondary to its pain alleviating effects. This aspect of acupuncture treatment is likely to be related to activation of self-appraisal and the reward system.

When a patient seeks a therapist there are expectations of a specific effect. These expectations are partly based on self-relevant phenomena and self-referentia introspection and constitute the preference. Also, when asked about the effect of the treatment, processes that orientate pre-attentive anticipatory or mnemonic information and processes that mediate self-reflection and recollection are integrated together with sensory detection to enable a decision about the patient's perception of the effect of acupuncture treatment. These ‘self-appraisal’ processes are dependent on two integrated networks: a ventral medial prefrontal cortex paralimbic limbic ‘affective’ pathway and a dorsal medial prefrontal cortex cortical hippocampal ‘cognitive’ pathway.

The limbic structures are implicated in the reward system and play a key role in most diseases and illness responses including chronic pain and depression, regulating mood and neuromodulatory responses (eg sensory, autonomic, and endocrine). The pleasurable and neuromodulatory aspects of acupuncture as well as ‘placebo needling’ may partly be explained by the activation or deactivation of limbic structures including the hippocampus, amygdala, and their connections with the hypothalamus.

In patients with patellofemoral pain, the effects of superficial and deep needling remained for six months. These long term pain-alleviating effects have been attributed to activation of pain inhibiting systems in cortical and subcortical pathways. When considering long term effects the cortical cerebellar system needs to be taken into account. The cortical cerebellar system is probably central to the development of neural models that learn and eventually stimulate routinely executed (eg motor skills) and long term (eg pain alleviation) cognitive processes. These higher order cognitive processes are initially mediated in prefrontal cortical loci but later shift control iteratively to internal cerebellar representations of these processes. Possibly part of the long term healing effects of acupuncture may be attributed to changes in the cerebellar system thereby sparing processing load in cortical and subcortical areas.

As cortical and subcortical structures are activated and/or de-activated following stimulation of receptors in the skin, disregarding site, ‘placebo or sham needling’ does not exist and conclusions drawn on the basis that it is an inert control are invalid.

‘Self’ may be seen as a shifting illusion, ceaselessly constructed and deconstructed, and the effect of acupuncture may reflect its status (as well as that of the therapist).

Different effects of levodopa and subthalamic stimulation on emotional conflict in Parkinson's disease

Parkinson's disease impairs the decoding of emotional stimuli reflecting alterations of the limbic cortico-subcortical network. The objective of this study was to assess and compare the behavioral and electrophysiological effects of both levodopa and subthalamic stimulation on emotional processing in Parkinson's disease. Operated patients (n =16) and matched healthy subjects performed an emotional Stroop task, in which the emotion expressed by a face must be recognized while ignoring an emotional distractive word and that includes a neutral control sub-task. Patients were tested in the four possible treatment conditions (off stim/off med; on stim/off med; off stim/on med; and on stim/on med). High-resolution electroencephalography was recorded while performing the task. Patients made significantly more mistakes in facial emotion recognition than healthy subjects (p < .005). Untreated patients performed worse in the emotional trials than in the control sub-task (p < .05). Fearful faces induced significantly slower reaction times than happy faces in patients (p = .0002), but not in the healthy subjects. The emotional Stroop effect with levodopa was significantly higher than with subthalamic stimulation when fearful faces were assessed (p = .0243). Conversely, treatments did not modulate the Stroop effect of the control sub-task. EEG demonstrated that, compared with the untreated state, levodopa but not subthalamic stimulation significantly increases the amplitude of the event-related potential N170 (p = .002 vs. p = .1, respectively), an electrophysiological biomarker of early aspects of facial processing. The activity of the N170 cortical sources within the right fusiform gyrus was increased by levodopa (p < .05) but not by stimulation. While levodopa normalizes the recognition of emotional facial expression and early EEG markers of emotional processing, subthalamic stimulation does not. Thus, operated patients require dopaminergic medication in addition to stimulation to treat emotional symptoms of Parkinson's disease.

Endogenous nociceptin system involvement in stress responses and anxiety behavior

The mechanisms underpinning stress-related behavior and dysfunctional events leading to the expression of neuropsychiatric disorders remain incompletely understood. Novel candidates involved in the neuromodulation of stress, mediated both peripherally and centrally, provide opportunities for improved understanding of the neurobiological basis of stress disorders and may represent targets for novel therapeutic development. This chapter provides an overview of the mechanisms by which the opioid-related peptide, nociceptin, regulates the neuroendocrine stress response and stress-related behavior. In our research, we have employed nociceptin receptor antagonists to investigate endogenous nociceptin function in tonic control over stress-induced activity of the hypothalamo-pituitary-adrenal axis. Nociceptin demonstrates a wide range of functions, including modulation of psychological and inflammatory stress responses, modulation of neurotransmitter release, immune homeostasis, in addition to anxiety and cognitive behaviors. Greater appreciation of the complexity of limbic-hypothalamic neuronal networks, together with attention toward gender differences and the roles of steroid hormones, provides an opportunity for deeper understanding of the importance of the nociceptin system in the context of the neurobiology of stress and behavior.

Consequences at adulthood of transient inactivation of the parahippocampal and prefrontal regions during early development: new insights from a disconnection animal model for schizophrenia

The psychic disintegration characteristic of schizophrenia is thought to result from a defective connectivity, of neurodevelopmental origin, between several integrative brain regions. The parahippocampal region and the prefrontal cortex are described as the main regions affected in schizophrenia. Interestingly, latent inhibition (LI) has been found to be reduced in patients with schizophrenia, and the existence of a dopaminergic dysfunction is also generally well accepted in this disorder. In the present review, we have integrated behavioral and neurochemical data obtained in a LI protocol involving adult rats subjected to neonatal functional inactivation of the entorhinal cortex, the ventral subiculum or the prefrontal cortex. The data discussed suggest a subtle and transient functional blockade during early development of the aforementioned brain regions is sufficient to induce schizophrenia-related behavioral and dopaminergic abnormalities in adulthood. In summary, these results support the view that our conceptual and methodological approach, based on functional disconnections, is valid for modeling some aspects of the pathophysiology of schizophrenia from a neurodevelopmental perspective.

Sensory, hormonal, and neural basis of maternal aggression in rodents

We review existing knowledge of the neural, hormonal, and sensory basis of maternal aggression in the female rat. Although females may express different kinds of aggression, such as defense or dominance, the most frequent and conspicuous form of aggressive behavior among females is the one associated with motherhood. Maternal aggression occurs in various vertebrate and invertebrate species; however, our emphasis will be on maternal aggression in rats because most of the physiological investigations have been performed in this species. Firstly, we address those factors that predispose the female to attack, such as the endocrine profile, the maternal state, and the stimulation provided by the pups, as well as those that trigger the aggressive response, as the intruder's characteristics and the context. As the postpartum aggression is a fundamental component of the maternal repertoire, we emphasize its association with maternal motivation and the reduction of fear and anxiety in dams. Finally, we outline the neurocircuitry involved in the control of maternal aggression, stressing the role of the ventro-orbital region of prefrontal cortex and the serotoninergic system.

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.

Memory reconsolidation in aversive and appetitive settings

Memory reconsolidation has been observed across species and in a number of behavioral paradigms. The majority of memory reconsolidation studies have been carried out in Pavlovian fear conditioning and other aversive memory settings, with potential implications for the treatment of post-traumatic stress disorder. However, there is a growing literature on memory reconsolidation in appetitive reward-related memory paradigms, including translational models of drug addiction. While there appears to be substantial similarity in the basic phenomenon and underlying mechanisms of memory reconsolidation across unconditioned stimulus valence, there are also notable discrepancies. These arise both when comparing aversive to appetitive paradigms and also across different paradigms within the same valence of memory. We review the demonstration of memory reconsolidation across different aversive and appetitive memory paradigms, the commonalities and differences in underlying mechanisms and the conditions under which each memory undergoes reconsolidation. We focus particularly on whether principles derived from the aversive literature are applicable to appetitive settings, and also whether the expanding literature in appetitive paradigms is informative for fear memory reconsolidation.

Early prefrontal functional blockade in rats results in schizophrenia-related anomalies in behavior and dopamine

Growing evidence suggests schizophrenia may arise from abnormalities in early brain development. The prefrontal cortex (PFC) stands out as one of the main regions affected in schizophrenia. Latent inhibition, an interesting cognitive marker for schizophrenia, has been found in some studies to be reduced in acute patients. It is generally widely accepted that there is a dopaminergic dysfunctioning in schizophrenia. Moreover, several authors have reported that the psychostimulant, D-amphetamine (D-AMP), exacerbates symptoms in patients with schizophrenia. We explored in rats the effects in adulthood of neonatal transient inactivation of the PFC on behavioral and neurochemical anomalies associated with schizophrenia. Following tetrodotoxin (TTX) inactivation of the left PFC at postnatal day 8, latent inhibition-related dopaminergic responses and dopaminergic reactivity to D-AMP were monitored using in vivo voltammetry in the left core part of the nucleus accumbens in adult freely moving rats. Dopaminergic responses and behavioral responses were followed in parallel. Prefrontal neonatal inactivation resulted in disrupted behavioral responses of latent inhibition and latent inhibition-related dopaminergic responses in the core subregion. After D-AMP challenge, the highest dose (1.5 mg/kg i.p.) induced a greater dopamine increase in the core in rats microinjected with TTX, and a parallel increase in locomotor activity, suggesting that following prefrontal neonatal TTX inactivation animals display a greater behavioral and dopaminergic reactivity to D-AMP. Transitory inactivation of the PFC early in the postnatal developmental period leads to behavioral and neurochemical changes in adulthood that are meaningful for schizophrenia modeling. The data obtained may help our understanding of the pathophysiology of this disabling disorder.

Amphetamine exposure selectively enhances hippocampus-dependent spatial learning and attenuates amygdala-dependent cue learning

Behaviorally sensitizing regimen of amphetamine (AMPH) exposure has diverse effects on learning, memory, and cognition that are likely to be a consequence of long-term neural adaptations occurring in the cortico-limbic-striatal circuitry. In particular, altered dopamine signaling in the nucleus accumbens and medial prefrontal cortex has been implicated to underlie AMPH-induced changes in behavior. This study sought to test the hypothesis that repeated AMPH exposure disrupts the regulation of limbic information processing and the balance of competing limbic control over appetitive behavior. Mice received seven intraperitoneal injections of D-AMPH (2.5 mg/kg or 5 mg/kg) or vehicle solution (saline) and were trained in (1) a simultaneous conditioned cue and place preference task using a six-arm radial maze, found to depend on the integrity of the hippocampus (HPC) and basolateral amygdala (BLA), respectively and (2) a conditional BLA-dependent cue, and HPC-dependent place learning task using an elevated T-maze. In both tasks, the vehicle pretreatment group initially acquired cue learning, followed by the emergence of significant place/spatial learning. In contrast, pretreatment with repeated AMPH caused marked deviations from normal acquisition patterns of place and cue conditioning, significantly facilitating HPC-dependent place conditioning in the first task while attenuating BLA-dependent cue conditioning in both tasks. These findings provide the first demonstration of an aberrant regulation of HPC- and BLA-dependent learning as a result of AMPH exposure, highlighting the importance of the meso-coticolimbic dopamine system in maintaining the balance of limbic control over appetitive behavior.

Basolateral amygdala modulates terminal dopamine release in the nucleus accumbens and conditioned responding

BACKGROUND

Dopamine signaling in the nucleus accumbens (NAc) is essential for goal-directed behaviors and primarily arises from burst firing of ventral tegmental area neurons. However, the role of associative neural substrates such as the basolateral amygdala (BLA) in regulating phasic dopamine release in the NAc, particularly during reward seeking, remains unknown.

METHODS

Male Sprague-Dawley rats learned to discriminate two cues: a discriminative stimulus (DS) that predicted sucrose reinforcement contingent upon a lever press and a nonassociated stimulus (NS) that predicted a second lever never reinforced with sucrose. Following training, a test session was completed in which NAc dopamine was measured using fast-scan cyclic voltammetry in conjunction with inactivation of the ipsilateral BLA (gamma-aminobutyric acid agonists; baclofen/muscimol) to determine the contribution of BLA activity to dopamine release in the NAc core during the task.

RESULTS

Under vehicle conditions, DS and NS presentation elicited dopamine release within the NAc core. The DS evoked significantly more dopamine than the NS. Inactivation of the BLA selectively attenuated the magnitude of DS-evoked dopamine release, concurrent with an attenuation of DS-evoked conditioned approaches. Other behavioral responses (e.g., lever pressing) and dopamine release concomitant with those events were unaltered by BLA inactivation. Furthermore, neither ventral tegmental area electrically stimulated dopamine release nor the probability of high concentration dopamine release events was altered following BLA inactivation.

CONCLUSIONS

These results demonstrate that the BLA terminally modulates dopamine signals within the NAc core under specific, behaviorally relevant conditions, illustrating a functional mechanism by which the BLA selectively facilitates responding to motivationally salient environmental stimuli.

Cocaine- and amphetamine-regulated transcript peptide immunoreactivity in the brain of the CCK-1 receptor deficient obese OLETF rat

Cocaine- and amphetamine-regulated transcript (CART) peptide is expressed in brain areas involved in homeostatic regulation and reward. CART has been shown to reduce food intake, but the underlying mechanisms and the relevance of this effect on obesity yet remain unknown. Therefore, we used immunohistochemistry to investigate the expression of CART peptide in various brain regions of the obese Otsuka Long Evans Tokushima Fatty (OLETF) rats lacking the CCK-1 receptor. Analysis revealed that whereas the distribution of CART-peptide immunoreactive neurons and axonal networks was identical in OLETF rats and lean controls, the intensity of CART immunoreactivity was significantly reduced in the rostral part of the nucleus accumbens (p < 0.01), the basolateral complex of the amygdala (p < 0.05) and the rostro-medial nucleus of the solitary tract (p < 0.001) of the OLETF rats. These areas are involved in reward and integration of taste and viscerosensory information and have been previously associated with altered functions in this strain. The findings suggest that in addition to previously described deficits in peripheral satiety signals and augmented orexigenic regulation, the anorectic effect of CART peptide may also be diminished in OLETF rats.

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.

Functional interaction between the basolateral amygdala and the nucleus accumbens underlies incentive motivation for food reward on a fixed ratio schedule

The ability for incentive properties of reward stimuli to maintain motivated behavior in the absence of the rewards themselves may be reliant in part on a glutamatergic projection from the basolateral (BLA) amygdala to the nucleus accumbens septi (NAS). The present work examined this idea in regard to food reward. In the first part of this study, lever pressing by rats on a fixed ratio 16 (FR16) schedule of food reinforcement was suppressed in a dose-dependent manner following bilateral infusion of the GABA(A) agonist muscimol to the BLA. Consumption of food when freely available was unaffected by the highest dose of muscimol, suggesting no change in the primary reward value of the food. Bilateral infusion of the broad-spectrum dopamine (DA) receptor antagonist flupenthixol to the NAS also resulted in a significant decrease in FR16 performance. As with the amygdala, consumption of freely available food was not affected by flupenthixol injections into the NAS. When unilateral injection of flupenthixol to the NAS was combined with contralateral injection of muscimol to the BLA, FR16 performance was suppressed. No significant change in lever press performance was observed following unilateral NAS injection of flupenthixol combined with ipsilateral injection of muscimol to the BLA. The results of this study support the idea that a functional connection between the BLA and NAS transmits incentive information necessary for the maintenance of responding in the absence of primary reward.

Differential influence of the ventral subiculum on dopaminergic responses observed in core and dorsomedial shell subregions of the nucleus accumbens in latent inhibition

It has previously been reported that dopamine (DA) responses observed in the core and dorsomedial shell parts of the nucleus accumbens (Nacc) in latent inhibition (LI) are dependent on the left entorhinal cortex (ENT). The present study was designed to investigate the influence of the left ventral subiculum (SUB) closely linked to the ENT on the DA responses obtained in the Nacc during LI, using an aversive conditioned olfactory paradigm and in vivo voltammetry in freely moving rats. In the first (pre-exposure) session, functional blockade of the left SUB was achieved by local microinjection of tetrodotoxin (TTX). In the second session, rats were aversively conditioned to banana odor, the conditional stimulus (CS). In the retention (test) session the results were as follows: (1) pre-exposed (PE) conditioned animals microinjected with TTX, displayed aversion toward the CS; (2) in the core part of the Nacc, for PE-TTX-conditioned rats as for non-pre-exposed (NPE) conditioned animals, DA levels remained close to the baseline whereas DA variations in both groups were significantly different from the DA increases observed in PE-conditioned rats microinjected with the solvent (phosphate-buffered saline (PBS)); (3) in the shell part of the Nacc, for PE-TTX-conditioned rats, DA variations were close to or above the baseline. They were situated between the rapid DA increases observed in NPE-conditioned animals and the transient DA decreases obtained in PE-PBS-conditioned animals. These findings suggest that, in parallel to the left ENT, the left SUB controls DA LI-related responses in the Nacc. The present data may also offer new insight into the pathophysiology of schizophrenia.

Neonatal functional blockade of the entorhinal cortex results in disruption of accumbal dopaminergic responses observed in latent inhibition paradigm in adult rats

Latent inhibition (LI) has been found to be disrupted in non-treated patients with schizophrenia. Dopaminergic (DAergic) dysfunctioning is generally acknowledged to occur in schizophrenia. Various abnormalities in the entorhinal cortex (ENT) have been described in patients with schizophrenia. Numerous data also suggest that schizophrenia has a neurodevelopmental origin. The present study was designed to test the hypothesis that reversible inactivation of the ENT during neonatal development results in disrupted DA responses characteristic of LI in adult rats. Tetrodotoxin (TTX) was microinjected locally in the left ENT at postnatal day 8 (PND8). DA variations were recorded in the dorsomedial shell and core parts of the nucleus accumbens (Nacc) using in vivo voltammetry in freely-moving grown-up rats in a LI paradigm. In the first session the animals were pre-exposed (PE) to the conditional stimulus (banana odour) alone. In the second they were aversively conditioned to banana odour. In the third (test) session the following results were obtained in PE animals subjected to temporary inactivation of the ENT at PND8: (1) aversive behaviour was observed in TTX-PE conditioned animals; (2) DA variations in the dorsomedial shell and core parts of the Nacc were similar in TTX-PE and non-pre-exposed conditioned rats. These findings strongly suggest that neonatal disconnection of the ENT disrupts LI in adult animals. They may further our understanding of the pathophysiology of schizophrenia.

Dendritic distributions of dopamine D1 receptors in the rat nucleus accumbens are synergistically affected by startle-evoking auditory stimulation and apomorphine

Prepulse inhibition of the startle response to auditory stimulation (AS) is a measure of sensorimotor gating that is disrupted by the dopamine D1/D2 receptor agonist, apomorphine. The apomorphine effect on prepulse inhibition is ascribed in part to altered synaptic transmission in the limbic-associated shell and motor-associated core subregions of the nucleus accumbens (Acb). We used electron microscopic immunolabeling of dopamine D1 receptors (D1Rs) in the Acb shell and core to test the hypothesis that region-specific redistribution of D1Rs is a short-term consequence of AS and/or apomorphine administration. Thus, comparisons were made in the Acb of rats killed 1 h after receiving a single s.c. injection of vehicle (VEH) or apomorphine (APO) alone or in combination with startle-evoking AS (VEH+AS, APO+AS). In both regions of all animals, the D1R immunoreactivity was present in somata and large, as well as small, presumably more distal dendrites and dendritic spines. In the Acb shell, compared with the VEH+AS group, the APO+AS group had more spines containing D1R immunogold particles, and these particles were more prevalent on the plasma membranes. This suggests movement of D1Rs from distal dendrites to the plasma membrane of dendritic spines. Small- and medium-sized dendrites also showed a higher plasmalemmal density of D1R in the Acb shell of the APO+AS group compared with the APO group. In the Acb core, the APO+AS group had a higher plasmalemmal density of D1R in medium-sized dendrites compared with the APO or VEH+AS group. Also in the Acb core, D1R-labeled dendrites were significantly smaller in the VEH+AS group compared with all other groups. These results suggest that alerting stimuli and apomorphine synergistically affect distributions of D1R in Acb shell and core. Thus adaptations in D1R distribution may contribute to sensorimotor gating deficits that can be induced acutely by apomorphine or develop over time in schizophrenia.

5-HT2- and D1-mechanisms of the basolateral nucleus of the amygdala enhance conditioned fear and impair unconditioned fear

The inferior colliculus (IC) is involved in processing of auditory information, but also integrates acoustic information of aversive nature. In fact, chemical stimulation of the IC with semicarbazide (SMC) - an inhibitor of the GABA synthesizing enzyme glutamic acid decarboxylase - has been found to cause defensive behavior in an open-field test and functions as an unconditioned stimulus in the place conditioned aversion test (PCA). A question has arisen regarding whether the basolateral nucleus of the amygdala (BLA) is involved in the acquisition of the aversive information ascending from the IC and whether dopaminergic and serotoninergic mechanisms of the BLA regulate this process. Recent evidence has shown that inactivation of the BLA with muscimol inhibits the PCA and causes an increase in the aversiveness of the chemical stimulation of the IC. Based on this, we examined the effects of ketanserin and SCH-23390, antagonists of the 5HT(2) and D(1) receptors, respectively, on the conditioned and unconditioned fear elicited by IC stimulation with SMC. The results obtained confirm the crucial role of 5-HT(2)- and D(1)-mechanisms of the BLA on conditioned fear in that ketanserin and SCH-23390 injections into the BLA caused a reduction in the PCA. On the other hand, ketanserin and SCH-23390 injections into the BLA enhanced the aversiveness of the IC injections of SMC. These findings suggest that while 5-HT(2) and DA(1) mechanisms in the BLA appear to facilitate the conditioned fear they inhibit the unconditioned fear triggered by IC activation.

Taste Memory Formation: Role of Nucleus Accumbens

When a novel taste has been associated with postingestive malaise, animals recognize this taste as aversive. This associative learning is known as conditioned taste aversion. However, when an animal consumes a novel taste and no aversive consequences follow, it becomes recognized as a safe signal, leading to an increase in its consumption in subsequent presentations. In this review, we will discuss the results related to the taste memory formation focusing particularly on the nucleus accumbens (NAcc). The NAcc keeps projections with amygdala, insular cortex, parabrachial nucleus, and nucleus of the solitary tract areas important for taste memory formation. We will review the evidence relating to how the NAcc could be involved in taste memory formation, due to its role in the taste memory trace formation and its role in the association of the conditioned stimulus-unconditioned stimulus, and finally the retrieval of taste memory. In this context, we will review the participation of the cholinergic, dopaminergic, and glutamatergic systems in the NAcc during taste memory formation.

The mesolimbic dopamine reward circuit in depression

The neural circuitry that mediates mood under normal and abnormal conditions remains incompletely understood. Most attention in the field has focused on hippocampal and frontal cortical regions for their role in depression and antidepressant action. While these regions no doubt play important roles in these phenomena, there is compelling evidence that other brain regions are also involved. Here we focus on the potential role of the nucleus accumbens (NAc; ventral striatum) and its dopaminergic input from the ventral tegmental area (VTA), which form the mesolimbic dopamine system, in depression. The mesolimbic dopamine system is most often associated with the rewarding effects of food, sex, and drugs of abuse. Given the prominence of anhedonia, reduced motivation, and decreased energy level in most individuals with depression, we propose that the NAc and VTA contribute importantly to the pathophysiology and symptomatology of depression and may even be involved in its etiology. We review recent studies showing that manipulations of key proteins (e.g. CREB, dynorphin, BDNF, MCH, or Clock) within the VTA-NAc circuit of rodents produce unique behavioral phenotypes, some of which are directly relevant to depression. Studies of these and other proteins in the mesolimbic dopamine system have established novel approaches to modeling key symptoms of depression in animals, and could enable the development of antidepressant medications with fundamentally new mechanisms of action.

NMDA and muscarinic receptors of the nucleus accumbens have differential effects on taste memory formation

Animals recognize a taste cue as aversive when it has been associated with post-ingestive malaise; this associative learning is known as conditioned taste aversion (CTA). When an animal consumes a new taste and no negative consequences follow, it becomes recognized as a safe signal, leading to an increase in its consumption in subsequent presentations (attenuation of neophobia, AN). It has been shown that the nucleus accumbens (NAcc) has an important role in taste learning. To elucidate the involvement of N-methyl-D-aspartate (NMDA) and muscarinic receptors in the NAcc during safe and aversive taste memory formation, we administrated bilateral infusions of DL-2-amino-5-phosphonopentanoic acid (APV) or scopolamine in the NAcc shell or core respectively. Our results showed that pre-training injections of APV in the NAcc core and shell disrupted aversive but not safe taste memory formation, whereas pre-training injections of scopolamine in the NAcc shell, but not core, disrupted both CTA and AN. These results suggest that muscarinic receptors seem to be necessary for processing taste stimuli for either safe or aversive taste memory, whereas NMDA receptors are only involved in the aversive taste memory trace formation.

Striatal dopaminergic responses observed in latent inhibition are dependent on the hippocampal ventral subicular region

We showed recently that behavioural and striatal dopaminergic (DA) responses obtained in latent inhibition are crucially dependent on the parahippocampal region, the entorhinal cortex. In the present study, we investigated the influence exerted by the hippocampal ventral subicular region (SUB) on the DA responses in the anterior part of the dorsal striatum using in vivo voltammetry in freely moving rats and the same latent inhibition paradigm. To that end, the left SUB was temporarily blocked with tetrodotoxin (TTX) during pre-exposure to a new olfactory stimulus (banana odour). During the second session the animals were aversively conditioned to banana odour. With respect to the results obtained during the test session (third presentation of banana odour), similar changes in behaviour and DA levels were obtained in control and conditioned rats microinjected with the solvent, phosphate-buffered saline (PBS), in the SUB, consistently with a latent inhibition phenomenon. In contrast, after reversible inactivation of the SUB during the pre-exposure session, TTX-pre-exposed conditioned animals displayed aversive behaviour in the test session, and anterior striatal DA variations in these animals differed significantly from those obtained in pre-exposed rats injected locally with PBS. Striatal DA variations obtained in conditioned animals microinjected with TTX were also significantly different from those observed in conditioned non-pre-exposed animals. The present data suggest that, in parallel to the entorhinal cortex, the SUB regulates the latent inhibition-related behavioural and DA responses in the anterior part of the dorsal striatum. These data may provide new insight into the pathophysiology of schizophrenic psychoses.

Role of stress, corticotrophin releasing factor (CRF) and amygdala plasticity in chronic anxiety

Stress initiates a series of neuronal responses that prepare an organism to adapt to new environmental challenges. However, chronic stress may lead to maladaptive responses that can result in psychiatric syndromes such as anxiety and depressive disorders. Corticotropin-releasing factor (CRF) has been identified as a key neuropeptide responsible for initiating many of the endocrine, autonomic and behavioral responses to stress. The amygdala expresses high concentrations of CRF receptors and is itself a major extrahypothalamic source of CRF containing neurons. Within the amygdala, the basolateral nucleus (BLA) has an important role in regulating anxiety and affective responses. During periods of stress, CRF is released into the amygdala and local CRF receptor activation has been postulated as a substrate for stress-induced alterations in affective behavior. Previous studies have suggested that synaptic plasticity in the BLA contributes to mechanisms underlying long-term changes in the regulation of affective behaviors. Several studies have shown that acute glutamate receptor-mediated activation, by either GABA-mediated disinhibition or CRF-mediated excitation, induces long-term synaptic plasticity and increases the excitability of BLA neurons. This review summarizes some of the data supporting the hypotheses that stress induced plasticity within the amygdala may be a critical step in the pathophysiology of the development of chronic anxiety states. It is further proposed that such a change in the limbic neural circuitry is involved in the transition from normal vigilance responses to pathological anxiety, leading to syndromes such as panic and post-traumatic stress disorders.

Personality predicts activity in reward and emotional regions associated with humor

Previous research and theory suggest that two stable personality dimensions, extroversion and neuroticism, differentially influence emotional reactivity to a variety of pleasurable phenomena. Here, we use event-related functional MRI to address the putative neural and behavioral associations between humor appreciation and the personality dimensions of introversion-extroversion and emotional stability-neuroticism. Our analysis showed extroversion to positively correlate with humor-driven blood oxygenation level-dependent signal in discrete regions of the right orbital frontal cortex, ventrolateral prefrontal cortex, and bilateral temporal cortices. Introversion correlated with increased activation in several regions, most prominently the bilateral amygdala. Although neuroticism did not positively correlate with any whole-brain activation, emotional stability (i.e., the inverse of neuroticism) correlated with increased activation in the mesocortical-mesolimbic reward circuitry encompassing the right orbital frontal cortex, caudate, and nucleus accumbens. Our findings tie together existing neurobiological studies of humor appreciation and are compatible with the notion that personality style plays a fundamental role in the neurobiological systems subserving humor appreciation.

The role of dopamine in conditioning and latent inhibition: what, when, where and how?

It is well established that dopamine is released in the nucleus accumbens (NAC) in animals in rewarding or reinforcing situations, and widely believed that this release is the substrate of, or at least closely related to, the experience of reward. The demonstration of conditioned release of dopamine by stimuli conditioned to primary rewards has reinforced this view. However, a number of observations do not sit comfortably with this interpretation, most notably that dopamine is released equally effectively in NAC by aversive stimuli, and stimuli conditioned to them. Furthermore, additional release of dopamine is seen during conditioning, even if motivational stimuli of either type are not involved. It is suggested here that one important action of NAC dopamine release is to restore the salience of potential conditioned stimuli, when this has been reduced by prior un-reinforced experience. The paradigm of latent inhibition (LI) demonstrates a behavioural effect of this type, and extensive studies on the role of dopamine in LI have been undertaken by us and others. Those studies are reviewed here, together with some previously unpublished data, to demonstrate that (1) amphetamine disruption of LI is indeed a function of calcium-dependant dopamine release in the NAC at the time of conditioning; (2) other drugs acting on LI via changes in dopamine transmission act at the same locus; (3) the disruptive effect of indirect dopamine agonists on LI can be prevented by either D-1 selective receptor antagonists, or D-2 selective receptor antagonists. It is concluded that dopamine release in these very varied behavioural contexts (reward, punishment, conditioning, modulation of salience) must be differentiated in some way, and that this should be investigated. An alternative explanation, if they are not differentiated, would be that the release in fact does have the same functional significance in each case. We suggest that this common significance might be the broadening of attention to take in potentially conditionable stimuli, which have previously been devalued.

Latent inhibition is disrupted by nucleus accumbens shell lesion but is abnormally persistent following entire nucleus accumbens lesion: The neural site controlling the expression and disruption of the stimulus preexposure effect

Latent inhibition (LI) is the proactive interference of repeated nonreinforced preexposure to a stimulus with subsequent performance on a learning task involving that stimulus. The present experiments investigated the role of the nucleus accumbens (NAC) in LI. LI was measured in a thirst motivated conditioned emotional response procedure with low or high number of conditioning trials, and in two-way active avoidance procedure with the stages of preexposure and conditioning taking place in the same or different contexts. Sham-lesioned rats showed LI with low but not high number of conditioning trials and if preexposure and conditioning took place in the same context but not if the context was changed between the stages. Lesion to the shell subregion of the NAC disrupted LI but LI was preserved in rats with a combined lesion to the NAC shell and core subregions. Moreover, rats with a combined shell-core lesion persisted in showing LI in spite of high number of conditioning trials and in spite of context change. These results show that the NAC is not essential for the acquisition of LI but rather plays a key role in regulating the expression of LI. Moreover, they suggest that the two subregions of the NAC contribute competitively and cooperatively to this process, selecting the response appropriate to the stimulus-no event or the stimulus-reinforcement association in conditioning.

Increases in extracellular levels of 5-HT and dopamine in the basolateral, but not in the central, nucleus of amygdala induced by aversive stimulation of the inferior colliculus

Consistent evidence has shown that dopamine release in the prefrontal cortex is increased by electrical stimulation of the inferior colliculus (IC) as unconditioned stimulus. Recent reports have also demonstrated that inactivation of the basolateral nucleus of the amygdala (BLA) with muscimol enhances the behavioural consequences of the aversive stimulation of the IC and reduces the dopamine release in the prefrontal cortex. Moreover, neurotoxic lesions of the BLA enhance whereas those of the central nucleus of the amygdala (CeA) reduce the aversiveness of the electrical stimulation of the IC. Based on these findings the present study examined the effects of the electrical stimulation of the IC on the extracellular levels of serotonin and dopamine in the BLA and CeA. To this end, rats implanted with a stimulation electrode in the IC also bore a microdialysis probe in the BLA or CeA for determination of the release of dopamine and serotonin. IC electrical stimulation at the freezing and escape thresholds increased the levels of serotonin ( approximately 70%) and dopamine ( approximately 60%) in the BLA related to the basal values. Similarly, the metabolites DOPAC and 5-HIAA increased in a parallel fashion in BLA. No significant changes could be detected in these biogenic amines and metabolites in CeA following IC aversive stimulation. These findings point to a differential role of serotonergic and dopaminergic mechanisms of the BLA and CeA in the setting up of adaptive responses to fear states generated at the inferior colliculus level.

Aversive stimulation of the inferior colliculus changes dopamine and serotonin extracellular levels in the frontal cortex: modulation by the basolateral nucleus of amygdala

We have shown that stimulation of the neural substrates in the inferior colliculus (IC) causes a significant increase in the extracellular levels of dopamine (DA) in frontal cortex (FC). Also, it has been reported that the basolateral complex of the amygdala (BLA) serves as a filter for unconditioned and conditioned aversive information that ascend to higher structures from the brainstem. Linking these two kinds of information, this work examines whether inactivation of BLA interferes with the activation of cortical dopaminergic outputs produced by aversive stimulation of the IC. To this end, rats were implanted with an electrode in the IC for the determination of the threshold of escape responses. Each rat also bore a cannula implanted in the BLA for injections of lidocaine (10 microg/0.5 microL), muscimol (0.5 microg/0.5 microL), or its vehicle and a microdialysis probe in the FC for determination of the amount of DA and serotonin (5-HT). The data obtained show that IC electrical stimulation caused an increase in the DA release while it reduced the 5-HT release in the FC. BLA inactivation with both lidocaine or muscimol enhanced the aversiveness of the electrical stimulation of the IC and attenuated the increase in DA, while the reduction in 5-HT release in the FC remained unaffected. These findings suggest that ascending aversive information from IC on their way up to higher structures in the SNC courses with opposite modulation by DA/5-HT mechanisms in the FC. These processes are regulated by filters located in the BLA. It is proposed that the loss of these BLA regulatory mechanisms prevents the expression of these modulatory mechanisms in the FC that are adaptive responses in order to cope with unconditioned aversive stimulus triggered at the brainstem level.

What's in a "smile?" Intra-operative observations of contralateral smiles induced by deep brain stimulation

OBJECTIVE

To describe smiling and euphoria induced by deep brain stimulation (DBS).

BACKGROUND AND SIGNIFICANCE

The brain systems inducing emotional experiences and displays are not entirely known, but the ventral striatum including the nucleus accumbens has been posited to play a critical role in mediating emotions with positive valence. DBS has been successfully employed for the treatment of movement disorders, and most recently obsessive compulsive disorder (OCD). The purpose of this report is to describe the emotional changes associated with stimulation of the ventral striatum.

METHODS

A single patient with intractable OCD had electrode arrays placed in the right and left anterior limbs of the internal capsule and region of the nucleus accumbens. Changes in facial movement during stimulation were quantified by video recording. Ten video segments, time locked to the onset of stimulation, were digitized and changes in pixel intensity that occurred over both sides of the lower face, on a frame by frame basis, following stimulation onset were computed. These summed changes in pixel intensity represented the dependent variable of "entropy" and directly corresponded to changes in light reflectance that occur during facial movement.

RESULTS

During stimulation on both the right and left side, the patient consistently developed a half smile on the side of the face contralateral to the stimulating electrode, and also became euphoric. The effect ceased when DBS was discontinued.

CONCLUSIONS

DBS in the region of the nucleus accumbens produced smile and euphoria suggesting that alterations in the ventral striatum may result in emotional experience and displays. We hypothesize the existence of a limbic-motor network responsible for such changes. This observation suggests that DBS may be useful as a therapy for mood disorders.

Influence of the entorhinal cortex on accumbal and striatal dopaminergic responses in a latent inhibition paradigm

The use of latent inhibition paradigms is one means of investigating the involvement of mesencephalic dopaminergic (DA) neurons in cognitive processes. We have shown recently that DA neurons reaching the core and the dorsomedial shell parts of the nucleus accumbens and the anterior part of the striatum are differentially involved in latent inhibition. In other respects, theoretical, behavioral and anatomo-functional data suggest that the entorhinal cortex (ENT) may control latent inhibition expression. In this study, using in vivo voltammetry in freely moving rats, we investigated the influence of the ENT on the DA responses obtained in the core and dorsomedial shell parts of the nucleus accumbens and the anterior part of the striatum. For this purpose a reversible inactivation of the left ENT was achieved by the local microinjection of tetrodotoxin, 3 h before pre-exposure to the conditional stimulus (banana odour). During the second session, animals were aversively conditioned to banana odour. Results obtained during the third session (test session), in animals submitted to the reversible blockade of the ENT before the first session were as follows: (1) pre-exposed conditioned animals displayed behavioral aversive responses; (2) where core DA responses were concerned, responses were situated between those observed in pre-exposed and non-pre-exposed conditioned animals; (3) by contrast, where the dorsomedial shell part of the nucleus accumbens and the anterior striatum were concerned, DA variations were not statistically different in pre-exposed and non-pre-exposed conditioned rats. These data suggest that the left ENT exerts a crucial influence over the latent-inhibition-related DA responses in the left dorsomedial shell part of the nucleus accumbens and the left anterior part of the striatum, whereas one or more other brain regions control DA variations in the left core part of the nucleus accumbens. These data may help us to understand the pathophysiology of schizophrenic psychoses.

Detecting subsecond dopamine release with fast-scan cyclic voltammetry in vivo

BACKGROUND

Dopamine is a potent neuromodulator in the brain, influencing a variety of motivated behaviors and involved in several neurologic diseases. Measurements of extracellular dopamine in the brains of experimental animals have traditionally focused on a tonic timescale (minutes to hours). However, dopamine concentrations are now known to fluctuate on a phasic timescale (subseconds to seconds).

APPROACH

Fast-scan cyclic voltammetry provides analytical chemical measurements of phasic dopamine signals in the rat brain.

CONTENT

Procedural aspects of the technique are discussed, with regard to appropriate use and in comparison with other methods. Finally, examples of data collected using fast-scan cyclic voltammetry are summarized, including naturally occurring dopamine transients and signals arising from electrical stimulation of dopamine neurons.

SUMMARY

Fast-scan cyclic voltammetry offers real-time measurements of changes in extracellular dopamine concentrations in vivo. With its subsecond time resolution, micrometer-dimension spatial resolution, and chemical selectivity, it is the most suitable technique currently available to measure transient concentration changes of dopamine.

The "two-headed" latent inhibition model of schizophrenia: modeling positive and negative symptoms and their treatment

RATIONALE

Latent inhibition (LI), namely, poorer performance on a learning task involving a previously pre-exposed non-reinforced stimulus, is disrupted in the rat by the dopamine (DA) releaser amphetamine which produces and exacerbates psychotic (positive) symptoms, and this is reversed by treatment with typical and atypical antipsychotic drugs (APDs) which on their own potentiate LI. These phenomena are paralleled by disrupted LI in normal amphetamine-treated humans, in high schizotypal humans, and in schizophrenia patients in the acute stages of the disorder, as well as by potentiated LI in normal humans treated with APDs. Consequently, disrupted LI is considered to provide an animal model of positive symptoms of schizophrenia with face, construct and predictive validity.

OBJECTIVES

To review most of the rodent data on the neural substrates of LI as well as on the effects of APDs on this phenomenon with an attempt to interpret and integrate these data within the framework of the switching model of LI; to show that there are two distinct LI models, disrupted and abnormally persistent LI; to relate these findings to the clinical condition.

RESULTS

The nucleus accumbens (NAC) and its DA innervation form a crucial component of the neural circuitry of LI, and are involved at the conditioning stage. There is a clear functional differentiation between the NAC shell and core subregions whereby damage to the shell disrupts LI and damage to the core renders LI abnormally persistent under conditions that disrupt LI in normal rats. The effects of shell and core lesions parallel those produced by lesions to the major sources of input to the NAC: entorhinal cortex lesion, like shell lesion, disrupts LI, whereas hippocampal lesion, like core lesion, produces persistent LI with changes in context, and basolateral amygdala (BLA) lesion, like core lesion, produces persistent LI with extended conditioning. Systemically induced blockade of glutamatergic as well as DA transmission produce persistent LI via effects exerted at the conditioning stage, whereas enhancement of DA transmission disrupts LI via effects at the conditioning stage. Serotonergic manipulations can disrupt or potentiate LI via effects at the pre-exposure stage. Both typical and atypical APDs potentiate LI via effects at conditioning whereas atypical APDs in addition disrupt LI via effects at pre-exposure. Schizophrenia patients can exhibit disrupted or normal LI as a function of the state of the disorder (acute versus chronic), as well as persistent LI.

CONCLUSIONS

Different drug and lesion manipulations produce two poles of abnormality in LI, namely, disrupted LI under conditions which lead to LI in normal rats, and abnormally persistent LI under conditions which disrupt it in normal rats. Disrupted and persistent LI are differentially responsive to APDs, with the former reversed by both typical and atypical APDs and the latter selectively reversed by atypical APDs. It is suggested that this "two-headed LI model" mimics two extremes of deficient cognitive switching seen in schizophrenia, excessive and retarded switching between associations, mediated by dysfunction of different brain circuitries, and can serve to model positive symptoms of schizophrenia and typical antipsychotic action, as well as negative symptoms of schizophrenia and atypical antipsychotic action.

Differential involvement of dopamine in the anterior and posterior parts of the dorsal striatum in latent inhibition

The involvement of mesostriatal dopaminergic neurons in cognitive operations is not well understood, and needs to be further clarified. The use of latent inhibition paradigms is a means of investigating cognitive processes. In this study, we investigated the involvement in latent inhibition of dopaminergic inputs in the anterior part and posterior part of the dorsal striatum. The latent inhibition phenomenon was observed in a conditioned olfactory aversion paradigm. Changes in extracellular dopamine levels induced by the conditioned olfactory stimulus (banana odor) were monitored in the two parts of the dorsal striatum in the left hemisphere after pre-exposure to the olfactory stimulus using in vivo voltammetry in freely moving rats. During the conditioning session animals received either an i.p. injection of NaCl (0.9%) (control groups) or an i.p. injection of LiCl (0.15 M) (conditioned groups). Dopamine variations and place preference or aversion toward the stimulus were analyzed simultaneously in pre-exposed and non-pre-exposed animals. Data collected during the retention (test) session were as follows. Where the anterior part of the striatum was concerned, similar enhancements in dopamine levels (+100%) were obtained in pre-exposed and non-pre-exposed control animals, as well as in the pre-exposed experimental animals. In contrast, dopamine levels in the non-pre-exposed experimental group (conditioned animals) remained fairly consistently close to the baseline after the presentation of the olfactory stimulus. Where the posterior part of the striatum was concerned, increases in extracellular dopamine levels were similar (+50%) for the different groups. The present results suggested that dopaminergic neurons reaching the anterior part of the dorsal striatum are implicated in the latent inhibition phenomenon and affective perception, whereas dopaminergic terminals in the posterior part of the dorsal striatum appeared to be involved neither in latent inhibition nor in affective perception of the stimulus, seeming only to be affected by the intrinsic properties of the stimulus. Cognitive as well as affective deficits have been reported in patients with schizophrenia. Thus the present data may be considered in the context of the pathophysiology of schizophrenic psychoses.

The extended amygdala and the dopamine system: another piece of the dopamine puzzle

The dopamine (DA) system has long been associated with the pathophysiology of psychosis. The DA theory of schizophrenia continues to find support in neuroreceptor imaging and ligand-binding studies that show excess DA transmission in patients, as compared to controls. The pathways that regulate the primate DA system, however, have yet to be fully elucidated. The amygdala, including its extended amygdala component, is involved in evaluating the emotional value of sensory stimuli. Since emotionally relevant sensory stimuli are distorted during psychotic episodes, we hypothesize that amygdaloid influences are likely to be significant modulators of the DA system. We reviewed evidence for direct projections from the central extended amygdala to specific subpopulations of DA neurons, and we discuss how these pathways may serve as important conduits of emotionally relevant information that can have immediate and long-term effects on DA regulation.

Dopamine modulates the response of the human amygdala: a study in Parkinson's disease

In addition to classic motor signs and symptoms, Parkinson's disease (PD) is characterized by neuropsychological and emotional deficits, including a blunted emotional response. In the present study, we explored both the neural basis of abnormal emotional behavior in PD and the physiological effects of dopaminergic therapy on the response of the amygdala, a central structure in emotion processing. PD patients and matched normal controls (NCs) were studied with blood oxygenation level-dependent functional magnetic resonance imaging during a paradigm that involved perceptual processing of fearful stimuli. PD patients were studied twice, once during a relatively hypodopaminergic state (i.e., > or =12 hr after their last dose of dopamimetic treatment) and again during a dopamine-replete state. The imaging data revealed a robust bilateral amygdala response in NCs that was absent in PD patients during the hypodopaminergic state. Dopamine repletion partially restored this response in PD patients. Our results demonstrate an abnormal amygdala response in PD that may underlie the emotional deficits accompanying the disease. Furthermore, consistent with findings in experimental animal paradigms, our results provide in vivo evidence of the role of dopamine in modulating the response of the amygdala to sensory information in human subjects.

Dissociation in the involvement of dopaminergic neurons innervating the core and shell subregions of the nucleus accumbens in latent inhibition and affective perception

Mesencephalic dopaminergic neurons have been found to be involved in affective processes. Their implication in cognitive processes appears less well understood. The use of latent inhibition paradigms is a means of studying these kinds of processes. In this study, we investigated the involvement of dopaminergic projections in the core, the dorsomedial shell and the ventromedial shell of the nucleus accumbens, in latent inhibition in olfactory aversive learning. Variations in extracellular dopamine levels induced by an aversively conditioned olfactory stimulus were monitored in the three parts of the nucleus accumbens in the left hemisphere, after pre-exposure to the olfactory stimulus using in vivo voltammetry in freely moving rats. The parallel between dopamine changes and place preference or aversion toward the stimulus were analyzed in pre-exposed and non-pre-exposed animals. Results showed that dopaminergic neurons innervating the nucleus accumbens are differentially involved in the latent inhibition phenomenon. Dopaminergic neurons innervating the core and the dorsomedial shell subregions of the nucleus accumbens appeared to be involved in latent inhibition processes, unlike those reaching the ventromedial shell. Nonetheless dopamine in the ventromedial shell was found to be involved in affective perception of the stimulus.The present data suggest that dopaminergic neurons innervating the three nucleus accumbens subregions are functionally related to networks involved in parallel processing of the cognitive and affective values of environmental information, and that interaction between these systems, at some levels, may lead to a given behavioral output. These data may provide new insights into the pathophysiology of schizophrenic psychoses.

A 6-hydroxydopamine lesion of the mesostriatal dopamine system decreases the expression of corticotropin releasing hormone and neurotensin mRNAs in the amygdala and bed nucleus of the stria terminalis

The mesostriatal dopamine (DA) system is known to play a vital role in extrapyramidal motor responses, and animals with a unilateral 6-hydroxydopamine (6-OHDA) lesion of this system have proved useful in studying the behavioral and neurobiological effects of DA depletion. Less is known about the role of this system in modulating emotional responses, although a number of lines of evidence suggest that dopamine influences emotional behavior. During the course of a study involving rats that had a unilateral 6-OHDA lesion, we discovered a hemispheric asymmetry in the levels of corticotropin releasing hormone (CRH) mRNA in the central nucleus of the amygdala (CEA). The present study was performed in order to determine (1) if the lesion resulted in a decrease in CRH mRNA, and/or if there was upregulation on the intact side, (2) if a similar imbalance in CRH mRNA was observed in other brain regions and (3) if levels of other neuropeptide mRNAs were affected by the lesion. Adult male Sprague-Dawley rats were left unoperated or were pretreated with desipramine and then injected unilaterally with 6-OHDA into the medial forebrain bundle to lesion the ascending mesostriatal DA neurons. Animals were killed 15-31 days following surgery and brain sections processed for CRH, neurotensin and enkephalin mRNAs by in situ hybridization. Levels of CRH and neurotensin mRNAs were decreased on the lesioned side in the CEA and oval nucleus of the BST (BSTov) relative to the intact side and to unoperated controls. Levels of enkephalin mRNA in these regions were not affected by the lesion. These effects appeared specific, because the lesion did not alter CRH mRNA expression in the ventral BST, paraventricular nucleus of the hypothalamus or cortex or neurotensin mRNA expression in the CA1 region of the hippocampus. In contrast, and consistent with previous reports, levels of neurotensin and enkephalin mRNAs were upregulated on the lesioned side of the striatum. This study provides evidence that the mesostriatal DA system regulates CRH and neurotensin mRNA in the BSTov and CEA, suggesting that dopamine may be an important modulator of CRH and neurotensin function within these nuclei. Although the precise mechanisms are not clear, and the involvement of noradrenergic systems cannot be precluded, data are consistent with the idea that dopamine, released in response to a stressful experience for example, interacts with CRH and neurotensin in the extended amygdala to affect emotional responsiveness.

Estrous cycle and food availability affect feeding induced by amygdala 5-HT receptor blockade

We have recently reported that bilateral infusions of the 5-HT receptor antagonist metergoline (MET) into the posterior basolateral amygdala (pBLA) elicit feeding in female rats tested at mid-light cycle. The present study was performed to determine whether (1) testing at two different phases of the estrous cycle, and/or (2) the palatability of the food might modify this effect. Subjects were 18 adult females with bilateral pBLA cannulae. Following familiarization with Froot Loops cereal, a within-subjects design tested all animals for 1- and 2-h food intake under 2 Drug (0.3 nmol MET vs. Vehicle), 2 Estrous Cycle (diestrus vs. estrus) and 2 Food (lab chow vs. Froot Loops) conditions. Rats weighed more at diestrus than at proestrus (P<.05) or estrus (P<.005). Multivariate analyses of variance (MANOVAs) revealed a preference for Froot Loops over lab chow (P<.0001). MET increased feeding regardless of food type (P<.0001). Rats ate more Froot Loops (P<.01), but not lab chow, at diestrus vs. estrus. A three-way interaction (P<.05) showed rats ate more during the first hour in estrus than in diestrus to lab chow but not Froot Loops. These data suggest pBLA MET differentially affects feeding over the estrous cycle depending on the palatability of food available.

Increased conditioned fear response and altered balance of dopamine in the shell and core of the nucleus accumbens during amphetamine withdrawal

It has been suggested that neuroadaptations within the nucleus accumbens (NAC) dopaminergic (DA) projection contribute to the negative affect associated with psychostimulant withdrawal. The present study assessed the effects of amphetamine (AMPH) withdrawal on behavioral and NAC DA responses to conditioned fear stress. Animals injected with escalating-dose AMPH (1-5mg/kg, three injections/day, 6 days) or saline (SAL) acquired a tone-shock association on withdrawal day 3 and were tested for extinction of conditioned freezing to the tone on withdrawal day 4. Extracellular levels of NAC shell and core DA were monitored using in vivo microdialysis on both days. AMPH-withdrawn animals exhibited more conditioned freezing than SAL animals during both acquisition and extinction. During acquisition, DA increased more in the shell than the core of the NAC in both AMPH and SAL groups. During extinction to the tone, shell DA increased in SAL- but not AMPH-treated animals, whereas core DA activity was greater in AMPH than SAL animals. These data demonstrate that AMPH withdrawal alters the balance between shell and core DA transmission while increasing the behavioral expression of conditioned fear. Such drug-induced neuroadaptations in the NAC stress response may be involved in the exacerbation of negative emotions associated with drug withdrawal and stimulant-induced psychosis.

Distribution of CB1 cannabinoid receptors in the amygdala and their role in the control of GABAergic transmission

Cannabinoids are the most popular illicit drugs used for recreational purposes worldwide. However, the neurobiological substrate of their mood-altering capacity has not been elucidated so far. Here we report that CB1 cannabinoid receptors are expressed at high levels in certain amygdala nuclei, especially in the lateral and basal nuclei, but are absent in other nuclei (e.g., in the central nucleus and in the medial nucleus). Expression of the CB1 protein was restricted to a distinct subpopulation of GABAergic interneurons corresponding to large cholecystokinin-positive cells. Detailed electron microscopic investigation revealed that CB1 receptors are located presynaptically on cholecystokinin-positive axon terminals, which establish symmetrical GABAergic synapses with their postsynaptic targets. The physiological consequence of this particular anatomical localization was investigated by whole-cell patch-clamp recordings in principal cells of the lateral and basal nuclei. CB1 receptor agonists WIN 55,212-2 and CP 55,940 reduced the amplitude of GABA(A) receptor-mediated evoked and spontaneous IPSCs, whereas the action potential-independent miniature IPSCs were not significantly affected. In contrast, CB1 receptor agonists were ineffective in changing the amplitude of IPSCs in the rat central nucleus and in the basal nucleus of CB1 knock-out mice. These results suggest that cannabinoids target specific elements in neuronal networks of given amygdala nuclei, where they presynaptically modulate GABAergic synaptic transmission. We propose that these anatomical and physiological features, characteristic of CB1 receptors in several forebrain regions, represent the neuronal substrate for endocannabinoids involved in retrograde synaptic signaling and may explain some of the emotionally relevant behavioral effects of cannabinoid exposure.

Amygdala regulation of nucleus accumbens dopamine output is governed by the prefrontal cortex

A dynamic interaction between the prefrontal cortex (PFC), amygdala, and nucleus accumbens (NAc) may be fundamental to regulation of goal-directed behavior by affective and cognitive processes. This study demonstrates that a mechanism for this triadic relationship is an inhibitory control by prefrontal cortex on accumbal dopamine release during amygdala activation. In freely moving rats, microstimulation of basolateral amygdala at intensities that produced mild behavioral activation produced an expected rapid increase in glutamate efflux in the prefrontal cortex and the nucleus accumbens shell region of the ventral striatum. However, during the stimulation, dopamine release increased only in the prefrontal cortex, not in the nucleus accumbens. An increase in accumbal dopamine release was observed during the stimulation if glutamate activation in the prefrontal cortex was inhibited at either presynaptic or postsynaptic levels. Some behaviors expressed during the stimulation were intensified in animals in which prefrontal cortex glutamate activation was blocked. In addition, these animals continued to express stimulus-induced behaviors after the termination of stimulation, whereas normal poststimulus behaviors such as ambulation and grooming were not displayed as frequently. Considering that dopamine neurotransmission in the nucleus accumbens is thought to play an integral role in goal-directed motor behavior, these findings suggest that the prefrontal cortex influences the behavioral impact of amygdala activation via a concomitant active suppression of accumbal dopamine release. Absence of this cortical influence appears to result in an aberrant pattern of behavioral expression in response to amygdala activation, including behavioral perseveration after stimulus termination.

Posterodorsal amygdala lesions reduce feeding stimulated by 8-OH-DPAT

Injections of the serotonin (5-HT)(1A) agonist, 8-hydroxy-2(di-n-propylamino)tetralin, (8-OH-DPAT), either systemically or into the midbrain raphe nuclei, elicit food intake in otherwise satiated rats. Lesions of the paraventricular nucleus of the hypothalamus are well known for producing long-term overeating, but past research has excluded this site as a potential locus for short-term 8-OH-DPAT feeding effects. More recent work shows that small lesions of the posterodorsal amygdala (PDA) elicit overeating in their own right. Since this and related regions of the amygdala receive 5-HT innervations from the dorsal raphe nucleus (DRN), we determined if PDA lesions might alter feeding after injecting 8-OH-DPAT into this midbrain region. Adult female rats received either bilateral electrolytic lesions of the PDA or sham lesions. After recording weight gains for over 1 month, all rats were implanted with DRN cannulae, then randomly tested every 3-4 days for 1 h intake of standard lab chow after 0, 0.4, 0.8 or 1.6 nmol injections of 8-OH-DPAT. Additional 90 min measures of intake were also made after 0 vs. 250 microg (760 nmol) 8-OH-DPAT s.c. At the two highest DRN doses tested, lesioned rats showed 50% less intake compared to shams. A similar profile emerged after the single s.c. dose. These results suggest that the PDA may be an important locus at which reduced release of endogenous 5-HT stimulates feeding. Alternatively, the PDA may represent part of a larger brain circuit whose integrity is necessary for eliciting intake in response to a variety of feeding stimuli.