Chapter 28 Theories of basal forebrain organization and the “emotional motor system”

Medial preoptic circuits governing instinctive social behaviors

The medial preoptic area (MPOA) has long been implicated in maternal and male sexual behavior. Modern neuroscience methods have begun to reveal the cellular networks responsible, while also implicating the MPOA in other social behaviors, affiliative social touch, and aggression. The social interactions rely on input from conspecifics whose most important modalities in rodents are olfaction and somatosensation. These inputs bypass the cerebral cortex to reach the MPOA to influence the social function. Hormonal inputs also directly act on MPOA neurons. In turn, the MPOA controls social responses via various projections for reward and motor output. The MPOA thus emerges as one of the major brain centers for instinctive social behavior. While key elements of MPOA circuits have been identified, a synthesis of these new data is now provided for further studies to reveal the mechanisms by which the area controls social interactions.

Over-activation of primate subgenual cingulate cortex enhances the cardiovascular, behavioral and neural responses to threat

Stress-related disorders such as depression and anxiety are characterized by enhanced negative emotion and physiological dysfunction. Whilst elevated activity within area 25 of the subgenual anterior cingulate cortex (sgACC/25) has been implicated in these illnesses, it is unknown whether this over-activity is causal. By combining targeted intracerebral microinfusions with cardiovascular and behavioral monitoring in marmosets, we show that over-activation of sgACC/25 reduces vagal tone and heart rate variability, alters cortisol dynamics during stress and heightens reactivity to proximal and distal threat. ¹⁸F-FDG PET imaging shows these changes are accompanied by altered activity within a network of brain regions including the amygdala, hypothalamus and dorsolateral prefrontal cortex. Ketamine, shown to have rapid antidepressant effects, fails to reverse elevated arousal to distal threat contrary to the beneficial effects we have previously demonstrated on over-activation induced reward blunting, illustrating the symptom-specificity of its actions.

Alexander et al. causally implicate over-activity in primate subgenual cingulate in affective and cardiovascular dysfunction relevant to anxiety and depression. Over-activation led to elevated activity in a stress-related network whilst decreasing activity in higher-order prefrontal cognitive regions.

Fos expression induced by olanzapine and risperidone in the central extended amygdala

The extended amygdala has been proposed to play an essential role in cognitive and affective processes and in neuropsychiatric disorders. In the present study, we examined the induction of Fos-like nuclei in the central amygdaloid nucleus (CeA), sublenticular extended amygdala (SLEA), interstitial nucleus of the posterior limb of the anterior commissure (IPAC), and bed nucleus of the stria terminalis (BSTL) of rodents to improve the knowledge regarding the pharmacological profile, therapeutic efficacy, and side-effects of olanzapine, an atypical antipsychotic drug and risperidone, a mixed atypical/typical antipsychotic drug in the rat brain. In addition, we evaluated the induction of Fos-like-nuclei in areas connected with these structures such as prefrontal cortex (PFCx), and nucleus accumbens shell, and in other important areas including the lateral septum and caudate-putamen that are involved in the therapeutic efficacy or side-effects of antipsychotic drugs. Fos-like-immunoreactivity induced by olanzapine and risperidone was compared with that by the atypical antipsychotic clozapine and typical antipsychotic haloperidol. Regarding the extended amygdala, and similarly to clozapine, olanzapine (5-10 mg/kg) and, with a lower efficacy, risperidone (1-3 mg/kg), induced Fos-like-nuclei in CeA, IPAC, SLEA, and BSTL. Both these drugs increased the induction of Fos-like-nuclei in PFCx, nucleus accumbens shell, lateral septum, and caudate-putamen. On the contrary, the increase of Fos-like-nuclei in the extended amygdala by haloperidol was restricted to IPAC only. These findings, consistent with the important role of extended amygdala in neuropsychiatric disorders characterized by affective disturbances, showed that olanzapine and risperidone, contrary to haloperidol, preferentially activated Fos-expression in these brain areas.

The value of extended amygdala structures in emotive effects of narcogenic with diverse chemical structure

Introduction: Studies on the mechanisms of the reinforcing action of opioid and non-opioid narcotics confirmed the existence in the brain of a specialized system named the extended amygdala.

Materials and methods: To clarify the value of the extended amygdala structures (bed nucleus, central nucleus of the amygdala and nucleus accumbens shell) in the mechanisms of unconditioned and conditioned reinforcement activated by various narcogenic, this paper carried out a neuropharmacological analysis of these effects, using blockade of dopamine receptors, GABA, opioids and CRF receptors within these brain structures, as well as an analysis of behavioral responses by self-stimulation (unconditioned reinforcement) and conditioned place preference (CPP) (conditioned reinforcement).

Results and discussion: The central amygdala and the bed nucleus have a controlling influence on the hypothalamus, which is predominantly of CRF-, GABA- and dopaminergic nature. Through D1 dopamine receptors,, a direct positive (activating) effect on the lateral hypothalamus is made. The D2 receptor blockade of the nucleus accumbens prevents narcogenic from exerting the reinforcing properties, which are primarily stimulating. The blockade of the D1 receptors of the nucleus accumbens by SCH-23390 prevents the expression of unconditioned and conditioned reinforcing properties of predominantly opiates and opioids. The blockade of GABAA receptors in the nucleus accumbens with bicuculline prevents the manifestation of the primary and secondary reinforcing properties (CPP) of psychostimulant drugs (amphetamine), without affecting the effects of opiates and opioids (fentanyl and leu-enkephalin).

Conclusion: The pharmacological analysis proves that CRF, dopamine and GABA receptors are most important for the correction of reinforcement activated by various narcogenic.

A Focus on the Functions of Area 25

Subcallosal area 25 is one of the least understood regions of the anterior cingulate cortex, but activity in this area is emerging as a crucial correlate of mood and affective disorder symptomatology. The cortical and subcortical connectivity of area 25 suggests it may act as an interface between the bioregulatory and emotional states that are aberrant in disorders such as depression. However, evidence for such a role is limited because of uncertainty over the functional homologue of area 25 in rodents, which hinders cross-species translation. This emphasizes the need for causal manipulations in monkeys in which area 25, and the prefrontal and cingulate regions in which it is embedded, resemble those of humans more than rodents. In this review, we consider physiological and behavioral evidence from non-pathological and pathological studies in humans and from manipulations of area 25 in monkeys and its putative homologue, the infralimbic cortex (IL), in rodents. We highlight the similarities between area 25 function in monkeys and IL function in rodents with respect to the regulation of reward-driven responses, but also the apparent inconsistencies in the regulation of threat responses, not only between the rodent and monkey literatures, but also within the rodent literature. Overall, we provide evidence for a causal role of area 25 in both the enhanced negative affect and decreased positive affect that is characteristic of affective disorders, and the cardiovascular and endocrine perturbations that accompany these mood changes. We end with a brief consideration of how future studies should be tailored to best translate these findings into the clinic.

Toward a systems-oriented approach to the role of the extended amygdala in adaptive responding

Research into the structure and function of the basal forebrain macrostructure called the extended amygdala (EA) has recently seen considerable growth. This paper reviews that work, with the objectives of identifying underlying themes and developing a common goal towards which investigators of EA function might work. The paper begins with a brief review of the structure and the ontological and phylogenetic origins of the EA. It continues with a review of research into the role of the EA in both aversive and appetitive states, noting that these two seemingly disparate avenues of research converge on the concept of reinforcement - either negative or positive - of adaptive responding. These reviews lead to a proposal as to where the EA may fit in the organization of the basal forebrain, and an invitation to investigators to place their findings in a unifying conceptual framework of the EA as a collection of neural ensembles that mediate adaptive responding.

Activation of corticotropin releasing factor-containing neurons in the rat central amygdala and bed nucleus of the stria terminalis following exposure to two different anxiogenic stressors

Rats exposed to the odor of a predator or to the elevated plus maze (EPM) express unique unconditioned fear behaviors. The extended amygdala has previously been demonstrated to mediate the response to both predator odor and the EPM. We seek to determine if divergent amygdalar microcircuits are associated with the different behavioral responses. The current experiments compared activation of corticotropin-releasing factor (CRF)-containing neuronal populations in the central amygdala and bed nucleus of the stria terminalis (BNST) of rats exposed to either the EPM (5 min) versus home cage controls, or predator (ferret) odor versus butyric acid, or no odor (30 min). Sections of the brains were prepared for dual-labeled immunohistochemistry and counts of c-Fos co-localized with CRF were made in the centrolateral and centromedial amygdala (CLA and CMA) as well as the dorsolateral (dl), dorsomedial (dm), and ventral (v) BNST. Ferret odor-exposed rats displayed an increase in duration and a decrease in latency of defensive burying versus control rats. Exposure to both predator stress and EPM induced neuronal activation in the BNST, but not the central amygdala, and similar levels of neuronal activation were seen in both the high and low anxiety groups in the BNST after EPM exposure. Dual-labeled immunohistochemistry showed a significant increase in the percentage of CRF/c-Fos co-localization in the vBNST of ferret odor-exposed rats compared to control and butyric acid-exposed groups as well as EPM-exposed rats compared to home cage controls. In addition, an increase in the percentage of CRF-containing neurons co-localized with c-Fos was observed in the dmBNST after EPM exposure. No changes in co-localization of CRF with c-Fos was observed with these treatments in either the CLA or CMA. These results suggest that predator odor and EPM exposure activates CRF neurons in the BNST to a much greater extent than CRF neurons of the central amygdala, and indicates unconditioned anxiogenic stimuli may activate unique anatomical circuits in the extended amygdala.

Sexually dimorphic patterns of neural activity in response to juvenile social subjugation

After experiencing juvenile social subjugation (JSS), adult female rats display more severe depression- and anxiety-like behaviors than adult males, suggesting that JSS is encoded in a sex-specific manner. To test this hypothesis, prepubertal rats (P28-33) were subjected to 10 aggressive acts in ≤10 min from an aggressive adult male, a 10 min encounter with a non-aggressive adult male, or to 10 min in an empty, clean cage (handled control) and were sacrificed one hour later. We then used unbiased stereology to estimate the total number and proportion of neurons immunoreactive for the immediate early gene product Fos bilaterally in the basolateral amygdala (BLA), the anterior and posterior subdivisions of the bed nucleus of the stria terminalis, and the paraventricular nucleus of the hypothalamus (PVN). Overall, females' Fos responses were less selective than males'. The BLA in males displayed a selective Fos response to the non-aggressive male, whereas no such selectivity occurred in the BLA of females. Additionally, there were more neurons overall in the left BLA than the right and this lateralization was specific to males. The principal subdivision of the BST (BSTpr) in males responded selectively to JSS, whereas the BSTpr in females responded to both the non-aggressive and aggressive males. We also found that the regional volume and neuron number of the BSTpr is greater in males than in females. Finally, the PVN in males was, like the BLA, selective for the non-aggressive male, whereas none of the experiences elicited a selective response in females. The greater selectivity for non-threatening stimuli in males in three stress-responsive brain regions may be a clue as to why males are less susceptible to the anxiogenic effects of JSS.

D(1)-like receptors in the nucleus accumbens shell regulate the expression of contextual fear conditioning and activity of the anterior cingulate cortex in rats

Although dopamine-related circuits are best known for their roles in appetitive motivation, consistent data have implicated this catecholamine in some forms of response to stressful situations. In fact, projection areas of the ventral tegmental area, such as the amygdala and hippocampus, are well established to be involved in the acquisition and expression of fear conditioning, while less is known about the role of the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) in these processes. In the present study, we initially investigated the involvement of the mPFC and NAc in the expression of conditioned fear, assessing freezing behaviour and Fos protein expression in the brains of rats exposed to a context, light or tone previously paired with footshocks. Contextual and cued stimuli were able to increase the time of the freezing response while only the contextual fear promoted a significant increase in Fos protein expression in the mPFC and caudal NAc. We then examined the effects of specific dopaminergic agonists and antagonists injected bilaterally into the posterior medioventral shell subregion of the NAc (NAcSh) on the expression of contextual fear. SKF38393, quinpirole and sulpiride induced no behavioural changes, but the D1-like receptor antagonist SCH23390 increased the freezing response of the rats and selectively reduced Fos protein expression in the anterior cingulate cortex and rostral NAcSh. These findings confirm the involvement of the NAcSh in the expression of contextual fear memories and indicate the selective role of NAcSh D1-like receptors and anterior cingulate cortex in this process.

Increased coherence among striatal regions in the theta range during attentive wakefulness

The striatum, the largest component of the basal ganglia, is usually subdivided into associative, motor and limbic components. However, the electrophysiological interactions between these three subsystems during behavior remain largely unknown. We hypothesized that the striatum might be particularly active during exploratory behavior, which is presumably associated with increased attention. We investigated the modulation of local field potentials (LFPs) in the striatum during attentive wakefulness in freely moving rats. To this end, we implanted microelectrodes into different parts of the striatum of Wistar rats, as well as into the motor, associative and limbic cortices. We then used electromyograms to identify motor activity and analyzed the instantaneous frequency, power spectra and partial directed coherence during exploratory behavior. We observed fine modulation in the theta frequency range of striatal LFPs in 92.5 ± 2.5% of all epochs of exploratory behavior. Concomitantly, the theta power spectrum increased in all striatal channels (P < 0.001), and coherence analysis revealed strong connectivity (coefficients >0.7) between the primary motor cortex and the rostral part of the caudatoputamen nucleus, as well as among all striatal channels (P < 0.001). Conclusively, we observed a pattern of strong theta band activation in the entire striatum during attentive wakefulness, as well as a strong coherence between the motor cortex and the entire striatum. We suggest that this activation reflects the integration of motor, cognitive and limbic systems during attentive wakefulness.

Reproduction: a new venue for studying function of adult neurogenesis?

Adult neurogenesis has been a focus within the past few years because it is a newly recognized form of neuroplasticity that may play significant roles in behaviors and recovery process after disease. Mammalian adult neurogenesis could be found in two brain regions: hippocampus and subventricular zone (SVZ). While it is well established that hippocampal neurogenesis participates in memory formation and anxiety, the physiological function of SVZ neurogenesis is still under intense investigation. Recent studies disclose that SVZ neurogenesis is under regulation of reproductive cues like pheromones. Reciprocally, the newborn neurons may exert their effect on reproductive and maternal behaviors. This review discusses recent understanding of the interrelationship between neurogenesis and reproduction. The studies highlighted in this review illustrate the potential importance of neurogenesis in reproductive function and will provide new insights for the significance of adult neurogenesis.

The mesopontine rostromedial tegmental nucleus: A structure targeted by the lateral habenula that projects to the ventral tegmental area of Tsai and substantia nigra compacta

Prior studies revealed that aversive stimuli and psychostimulant drugs elicit Fos expression in neurons clustered above and behind the interpeduncular nucleus that project strongly to the ventral tegmental area (VTA) and substantia nigra (SN) compacta (C). Other reports suggest that these neurons modulate responses to aversive stimuli. We now designate the region containing them as the "mesopontine rostromedial tegmental nucleus" (RMTg) and report herein on its neuroanatomy. Dense micro-opioid receptor and somatostatin immunoreactivity characterize the RMTg, as do neurons projecting to the VTA/SNC that are enriched in GAD67 mRNA. Strong inputs to the RMTg arise in the lateral habenula (LHb) and, to a lesser extent, the SN. Other inputs come from the frontal cortex, ventral striatopallidum, extended amygdala, septum, preoptic region, lateral, paraventricular and posterior hypothalamus, zona incerta, periaqueductal gray, intermediate layers of the contralateral superior colliculus, dorsal raphe, mesencephalic, pontine and medullary reticular formation, and the following nuclei: parafascicular, supramammillary, mammillary, ventral lateral geniculate, deep mesencephalic, red, pedunculopontine and laterodorsal tegmental, cuneiform, parabrachial, and deep cerebellar. The RMTg has meager outputs to the forebrain, mainly to the ventral pallidum, preoptic-lateral hypothalamic continuum, and midline-intralaminar thalamus, but much heavier outputs to the brainstem, including, most prominently, the VTA/SNC, as noted above, and to medial tegmentum, pedunculopontine and laterodorsal tegmental nuclei, dorsal raphe, and locus ceruleus and subceruleus. The RMTg may integrate multiple forebrain and brainstem inputs in relation to a dominant LHb input. Its outputs to neuromodulatory projection systems likely converge with direct LHb projections to those structures.

The lateral habenula: no longer neglected

In this contribution to the CNS Spectrums neuroanatomy series, Stefanie Geisler, MD, discusses the lateral habenula (LHb). This nuclear complex is one of the areas of the brain that forms part of the cross-talk between limbic fore-brain and some important ascending modulatory pathways. Situated at the caudal end of the dorsal diencephalon and classically regarded as projecting largely to the brainstem, including the serotoninergic raphe nuclei, the LHb receives afferents from widespread forebrain areas. Therefore, the LHb is able to influence serotonin tone in the brain, and has long interested neuroanatomists as a potential limbic-motor interface. Nonetheless, the LHb was not much discussed outside neuroanatomical circles until recently, when it was discovered that its impact on the mesotelencephalic dopamine system is probably much greater than had been assumed. The LHb has become a hot topic. This article-addresses these developments and emphasizes the clinical relevance of this interesting brain structure.

The structural basis for mapping behavior onto the ventral striatum and its subdivisions

The striatum can be divided into dorsal (caudate-putamen) and ventral parts. In the ventral division, the nucleus accumbens, which subserves adaptive and goal-directed behaviors, is further subdivided into shell and core. Accumbal neurons show different types of experience-dependent plasticity: those in the core seem to discriminate the motivational value of conditioned stimuli, features that rely on the integration of information and enhanced synaptic plasticity at the many spines on these cells, whereas shell neurons seem to be involved with the release of predetermined behavior patterns in relation to unconditioned stimuli, and the behavioral consequences of repeated administration of addictive drugs. In the core, the principal neurons are medium sized and densely spiny, but in the medial shell, these same neurons are much smaller and their dendrites, significantly less spiny, suggesting that morphological differences could mediate unique neuroadaptations associated with each region. This review is focused on evaluating the structural differences in nucleus accumbens core and shell neurons and discusses how such different morphologies could underlie distinguishable behavioral processes.

Toward an integrative perspective on hippocampal function: from the rapid encoding of experience to adaptive behavior

The mammalian hippocampus has been associated with learning and memory, as well as with many other behavioral processes. In this article, these different perspectives are brought together, and it is pointed out that integration of diverse functional domains may be a key feature enabling the hippocampus to support not only the encoding and retrieval of certain memory representations, but also their translation into adaptive behavior. The hippocampus appears to combine: (i) sensory afferents and synaptic mechanisms underlying certain types of rapid learning; and (ii) links to motivational, emotional, executive, and sensorimotor functions. Recent experiments are highlighted, indicating that the induction of hippocampal synaptic plasticity is required to encode rapidly aspects of experience, such as places, into memory representations; subsequent retrieval of these representations requires transmission through the previously modified hippocampal synapses, but no further plasticity. In contrast, slow incremental place learning may not absolutely require hippocampal contributions. The neocortical sensory inputs, especially visuo-spatial information, necessary for hippocampus-dependent rapid learning, are preferentially associated with the septal to intermediate hippocampus. In contrast, connectivity with the prefrontal cortex and subcortical sites, which link the hippocampus to motivational, emotional, executive, and sensorimotor functions, is primarily associated with the intermediate to temporal hippocampus. A model of functional differentiation and integration along the septo-temporal axis of the hippocampus is proposed, describing key hippocampal contributions to adaptive behavior based on information encoded during a single or a few past experiences.

Lipopolysaccharide induces delayed FosB/DeltaFosB immunostaining within the mouse extended amygdala, hippocampus and hypothalamus, that parallel the expression of depressive-like behavior

Proinflammatory cytokines induce both sickness behavior and depression, but their respective neurobiological correlates are still poorly understood. The aim of the present study was therefore to identify in mice the neural substrates of sickness and depressive-like behavior induced by lipopolysaccharide (LPS, 830 microg/kg, intraperitoneal). LPS-induced depressive-like behavior was dissociated from LPS-induced sickness by testing mice either at 6 h (at which time sickness was expected to be maximal) or at 24 h post-LPS (at which time sickness was expected to be minimal and not to bias the measurement of depressive-like behavior). Concurrently, the expression of acute and chronic cellular reactivity markers (c-Fos and FosB/DeltaFosB, respectively) was mapped by immunohistochemistry at these two time points. In comparison to saline, LPS decreased motor activity in a new cage at 6 h but not at 24 h. In contrast, the duration of immobility in the tail suspension test was increased at both 6 and 24 h. This dissociation between decreased motor activity and depressive-like behavior was confirmed at 24 h post-LPS in the forced swim test. LPS also decreased sucrose consumption at 24 and 48 h, despite normal food and water consumption by that time. At 24 h post-LPS, LPS-induced depressive-like behavior was associated with a delayed cellular activity (as assessed by FosB/DeltaFosB immunostaining) in specific brain structures, particularly within the extended amygdala, hippocampus and hypothalamus, whereas c-Fos labeling was markedly decreased by that time in all the brain areas at 6 h post-LPS. These results provide the first evidence in favor of a functional dissociation between the brain structures that underlie cytokine-induced sickness behavior and cytokine-induced depressive-like behavior, and provide important cues about the neuroanatomical brain circuits through which cytokines could have an impact on affect.

Muscimol inactivation of the septo-preoptic complex affects medial forebrain bundle self-stimulation only when directed at the complex's ventrolateral components

Elements of the septo-preoptic basal forebrain complex, particularly the lateral and medial septum, the diagonal band of Broca, and the magnocellular preoptic area, have been linked to medial forebrain bundle (MFB) self-stimulation. This study examines the roles of these areas in MFB self-stimulation by temporarily inactivating them with 25 and 50ng doses of the GABA(A) receptor agonist muscimol. Changes in performance capacity and stimulation reward effectiveness were evaluated with the rate-frequency curve shift paradigm. When infused into the lateral and medial septum and the vertical limb of the diagonal band of Broca, both doses of muscimol were as ineffective as saline in altering either the rats' maximum rate of response for stimulation or the frequency required to maintain half-maximal response rate (required frequency). However, when infused into the horizontal limb of the diagonal band of Broca or the magnocellular preoptic area, muscimol substantially decreased maximal response rate and modestly increased required frequency. Changes in maximum rate were dose-dependent, but changes in required frequency were not. Muscimol infusions contralateral to the stimulated hemisphere were as effective as ipsilateral infusions; bilateral infusions tended to so suppress responding that resulting rate-frequency curves were often invalid. These results suggest a role in MFB self-stimulation for only the ventrolateral components of the septo-preoptic complex, and support past observations of considerable bilaterality in the neural systems that support self-stimulation.

Sequence of information processing for emotions based on the anatomic dialogue between prefrontal cortex and amygdala

The prefrontal cortex and the amygdala have synergistic roles in regulating purposive behavior, effected through bidirectional pathways. Here we investigated the largely unknown extent and laminar relationship of prefrontal input-output zones linked with the amygdala using neural tracers injected in the amygdala in rhesus monkeys. Prefrontal areas varied vastly in their connections with the amygdala, with the densest connections found in posterior orbitofrontal and posterior medial cortices, and the sparsest in anterior lateral prefrontal areas, especially area 10. Prefrontal projection neurons directed to the amygdala originated in layer 5, but significant numbers were also found in layers 2 and 3 in posterior medial and orbitofrontal cortices. Amygdalar axonal terminations in prefrontal cortex were most frequently distributed in bilaminar bands in the superficial and deep layers, by columns spanning the entire cortical depth, and less frequently as small patches centered in the superficial or deep layers. Heavy terminations in layers 1-2 overlapped with calbindin-positive inhibitory neurons. A comparison of the relationship of input to output projections revealed that among the most heavily connected cortices, cingulate areas 25 and 24 issued comparatively more projections to the amygdala than they received, whereas caudal orbitofrontal areas were more receivers than senders. Further, there was a significant relationship between the proportion of 'feedforward' cortical projections from layers 2-3 to 'feedback' terminations innervating the superficial layers of prefrontal cortices. These findings indicate that the connections between prefrontal cortices and the amygdala follow similar patterns as corticocortical connections, and by analogy suggest pathways underlying the sequence of information processing for emotions.

The central extended amygdala network as a proposed circuit underlying reward valuation

The phenomenon of medial forebrain bundle self-stimulation offers a powerful model of reward-based behavior. In particular, it appears to activate a neural system whose natural function is to compute the survival value or utility of present stimuli and to help orchestrate responses toward those inputs. Although the anatomical identity of this system is as yet unknown, recent descriptions of anatomical macrosystems within the basal forebrain lead to the proposal that it may be largely contained within the central extended amygdala network. This paper reviews decades' worth of behavioral and neurophysiological investigations of brain stimulation reward that support or are at least consistent with this idea. The proposed network circuitry underlying self-stimulation is also placed into the larger context of basal forebrain function, specifically, the role of the ventral striatopallidum in linking motivation to behavior, the role of the amygdala in detecting motivationally significant inputs, and the role of the magnocellular complex in communicating reward information to cortical and hippocampal targets.

Memory-dependent c-Fos expression in the nucleus accumbens and extended amygdala following the expression of a conditioned taste aversive in the rat

Retrieving the memory of a conditioned taste aversion involves multiple forebrain areas. Although the amygdala clearly plays a role in the expression of a conditioned taste aversion, critical regions, downstream from the amygdala remain to be defined. To this end, Fos immunoreactivity was used in the rat to explore forebrain structures associated with retrieval that have an anatomical relationship with the amygdala. The results showed that expression of a conditioned taste aversion to 0.5 M sucrose elicited neuronal activation in the nucleus accumbens and in a complex of structures collectively referred to as the extended amygdala. The posterior hypothalamus and parasubthalamic nucleus, which receive inputs from the extended amygdala, were also activated upon re-exposure to the sucrose conditioned stimulus. Fos immunoreactivity did not increase in these regions in response to an innately aversive tastant, quinine hydrochloride (conditioned stimulus control), nor to LiCl-induced visceral stimulation in unconditioned animals (unconditioned stimulus control). In addition, these regions did not respond to the sucrose conditioned stimulus in sham-conditioned animals. These results suggest that conditioned and innately aversive tastes are differentially processed in the forebrain circuitry that includes the nucleus accumbens and extended amygdala.

Ionotropic glutamatergic neurotransmission in the ventral tegmental area modulates ΔFosB expression in the nucleus accumbens and abstinence syndrome in morphine withdrawal rats

The present study sought to assess whether the blockade of ionotropic glutamate receptors in the ventral tegmental area could modulate morphine withdrawal in morphine-dependent rats and the expression of stable DeltaFosB isoforms in the nucleus accumbens during morphine withdrawal. Rats were injected (i.p.) with increasing doses of morphine for 1 week to develop physical dependence, and withdrawal was then precipitated by one injection of naloxone (2 mg/kg, i.p.). Abstinence signs such as jumping, wet-dog shake, writhing posture, weight loss, and Gellert-Holtzman scale score were recorded to evaluate naloxone-induced morphine withdrawal. Two ionotropic glutamate receptor antagonists, dizocilpine (MK-801) and 6, 7-dinitroquinnoxaline-2, 3-dione (DNQX), were microinjected unilaterally into the ventral tegmental area 30 min before naloxone precipitation. A second injection of naloxone (2 mg/kg i.p.) was given 1 h after the first naloxone injection to sustain a maximal level of withdrawal so that the expression of stable DeltaFosB isoforms in the nucleus accumbens could be measured. This would enable determination of the correlation between the MK-801 or DNQX-induced decrease in somatic withdrawal signs and the change in neuronal activity in the nucleus accumbens. The results showed that both MK-801 and DNQX significantly alleviated all symptoms of morphine withdrawal except for weight loss and reduced the expression of stable DeltaFosB isoforms within the nucleus accumbens. These data suggest that ionotropic glutamatergic neurotransmission in the ventral tegmental area regulates the levels of stable DeltaFosB isoforms in the nucleus accumbens, which play a very important role in modulating opiate withdrawal.

Altered mesencephalic dopaminergic populations in adulthood as a consequence of brief perinatal glucocorticoid exposure

Early exposure to stressors is strongly associated with enduring effects on central nervous system function, but the mechanisms and neural substrates involved in this biological 'programming' are unclear. This study tested the hypothesis that inappropriate exposure to glucocorticoid stress hormones (GCs) during critical periods of development permanently alters the mesencephalic dopaminergic populations in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Using a rat model, the synthetic GC dexamethasone was added to the maternal drinking water during gestational days 16-19 or over the first week of postnatal life. In adulthood, the effects upon tyrosine hydroxylase immunopositive (TH+) cell numbers in the midbrain, and monoamine levels in the forebrain, of the adult offspring were assessed and compared with control offspring whose dams received normal drinking water. In the VTA, both prenatal and postnatal dexamethasone treatment increased TH+ cell numbers by approximately 50% in males and females. Although prenatal dexamethasone treatment also increased TH+ cell numbers in the SNc by 40-50% in males and females, postnatal treatment affected females only by increasing TH+ cell numbers by approximately 30%. In comparison, similar changes were not detected in the monoamine levels of the dorsolateral striatum, nucleus accumbens or infralimbic cortex of either males or females, which is a feature likely to reflect adaptive changes in these pathways. These studies demonstrate that the survival or phenotypic expression of VTA and SNc dopaminergic neurones is profoundly influenced by brief perinatal exposure to GCs at times when endogenous levels are normally low. These findings are the first to demonstrate permanent changes in the cytoarchitecture within midbrain dopamine nuclei after perinatal exposure to stress hormones and implicate altered functionality. Thus, they have significance for the increasing use of GCs in perinatal medicine and indicate potential mechanisms whereby perinatal distress may predispose to the development of a range of psychiatric conditions in later life.

Contrasting Fos expression induced by acute reboxetine and fluoxetine in the rat forebrain: neuroanatomical substrates for the antidepressant effect

RATIONALE

Antidepressants preferentially facilitating serotonin seem to be particularly effective for treating the anxiety and aggressive component of the depressive syndrome, whereas those with a noradrenergic profile seem to be more effective in reducing psychomotor retardation, although their overall antidepressant effects are about the same. However, the mechanism of this difference remains unknown.

OBJECTIVES

To investigate the neural substrate for the different therapeutic efficacies of fluoxetine and reboxetine, we examined the regional Fos immunoreactivity (Fos-ir) induced by the two agents.

METHODS

Male Wistar rats (290-330 g) were given a subcutaneous injection of fluoxetine (5 or 10 mg/kg), reboxetine (5 or 10 mg/kg) or saline. Two hours later, rats were perfused through the ascending aorta and their brains were processed for Fos immunohistochemistry. Fos-ir was quantified by counting the number of Fos-ir-positive nuclei in six areas of the forebrain.

RESULTS

The shell of the nucleus accumbens was the only region in which both fluoxetine and reboxetine equally increased Fos-ir expression. Fluoxetine particularly induced Fos-ir in the central nucleus of the amygdala. In contrast, reboxetine induced Fos-ir in the cingulate cortex area 3 and the lateral orbital cortex.

CONCLUSIONS

These results suggest that the shell region may be one possible target for the antidepressant effects of fluoxetine and reboxetine. Furthermore, the difference in their clinical effects may depend on their different target sites of action.

Hippocampal modulation of sensorimotor processes

While the hippocampus makes unique contributions to memory, it has also long been associated with sensorimotor processes, i.e. innate processes involving control of motor responses to sensory stimuli. Moreover, hippocampal dysfunction has been implicated in neuropsychiatric diseases, such as schizophrenia and anxiety disorders, primarily characterized by non-mnemonic deficits in the processing of and responding to sensory information. This review is concerned with the hippocampal modulation of three sensorimotor processes in rats-locomotor activity, prepulse inhibition (PPI) of the startle reflex, and the startle reflex itself-whose alterations are related to human psychosis or anxiety disorders. Its main purpose is to present and discuss the picture emerging from studies examining the effects of pharmacological manipulations of the dorsal and ventral hippocampus by local drug microinfusions. While a role of the hippocampus in regulating locomotor activity, PPI, and startle reactivity has also been suggested based on the effects of hippocampal lesions, the microinfusion studies have revealed additional important details of this role and suggest modifications of notions based on lesion studies. In summary, the microinfusion studies corroborate that hippocampal mechanisms can directly influence locomotor activity, PPI, and startle reactivity, and that aberrant hippocampal function may contribute to neuropsychiatric diseases, in particular psychosis. The relation between different sensorimotor processes and hippocampal neurotransmission, the role of ventral and dorsal hippocampus, and the extrahippocampal mechanisms mediating the hippocampal modulation of different sensorimotor processes can partly be dissociated. Thus, the hippocampal modulation of these sensorimotor processes appears to reflect multiple operations, rather than one unitary operation.

Dissociation between mesocortical dopamine release and fear-related behaviours in two psychogenetically selected lines of rats that differ in coping strategies to aversive conditions

The mesocortical and mesolimbic dopaminergic (DAergic) pathways are activated by either aversive or rewarding stimuli. The functional tone of these DAergic neurons also increases during the execution of cognitive tasks. The present study was designed to examine the relationship between mesocortical and mesolimbic DAergic function and the expression of fear-related behaviours as compared with attention- and cognition-related mechanisms (e.g. coping strategies), in response to aversive conditions. To this aim, we used two psychogenetically selected rat lines, Roman high-avoidance (RHA/Verh) and Roman low-avoidance (RLA/Verh), which display drastically different emotion- and coping-related behaviours in response to stressors: RLA/Verh rats are 'reactive copers' and more fearful than RHA/Verh rats, which are 'proactive copers'. Brain dialysis experiments demonstrated that tail-pinch (TP) and the anxiogenic compounds pentylenetetrazol (PTZ) and ZK 93426 increased DA output in the medial prefrontal cortex (PFCX) of RHA/Verh but not RLA/Verh, rats. In contrast, in the shell compartment of the nucleus accumbens (NAC shell), TP caused a small increase in DA output only in RLA/Verh rats, whereas PTZ and ZK 93426 had no significant effect on either line. RHA/Verh rats displayed more robust and longer lasting coping activity and less frequent freezing and self-grooming episodes than did RLA/Verh rats after TP, PTZ or ZK 93426. This dissociation between fear-related behaviour and cortical DAergic activation argues against the view that the latter may be involved in the control of fear-like responses. We therefore propose that the activation of mesocortical DAergic projections by aversive stimuli underlies the cognitive mechanisms that are triggered in an attempt to gain control over the stressor.

Extended amygdala and basal forebrain

The basal forebrain is a confluence of systems that are crucial to understanding some of the most important functions of the brain, including reward and punishment, learning and cognition, and feeding and reproduction. Basic to understanding this broad spectrum of behavior is untangling the interwoven functional systems in basal forebrain. This has been grounded by the appreciation that the major nearby structures, that is, amygdala and basal ganglia, provide a context for interpreting basal forebrain areas that are best viewed as extensions of either of these larger regions. The components of basal forebrain, the ventral striatopallidal system and the medial and central divisions of extended amygdala, are subcortical relays for information garnered from brain stem, thalamus, and cortical areas. With respect to the classically defined amygdala of the temporal lobe, the lateral-basolateral complex, and the superficial amygdaloid nuclei may tentatively be viewed as specialized cortical regions. Their output targets both the striatopallidal complex and the extended amygdala, with some of the most massive basal forebrain efferents originating in the basolateral amygdaloid complex. The subcortical projections of the basolateral nucleus, at least in the rat, appear to be dichotomous, with anterior (or magnocellular) portions of the nucleus preferentially targeting striatum and ventral striatum (including the core of the nucleus accumbens), while the posterior (small-celled) portions of the basolateral nucleus target the extended amygdala as well as the shell of the nucleus accumbens. This divergence represents a particular opportunity for behavioral neuroscientists analyzing basal forebrain functions. Studies exploiting the dual subcortical projection of basolateral amygdala indicate distinct functional roles for striatum versus extended amygdala. These reinforce the identification of extended amygdala as a functional-anatomical entity distinct from the striatopallidal system.

Pathways for emotion: interactions of prefrontal and anterior temporal pathways in the amygdala of the rhesus monkey

The amygdala has been implicated in processing information about the emotional significance of the environment and in the expression of emotions, through robust pathways with prefrontal, anterior temporal areas, and central autonomic structures. We investigated the anatomic organization and intersection of these pathways in the amygdala in rhesus monkeys with the aid of bidirectional, retrograde and anterograde tracers. Connections of the amygdala with orbitofrontal and medial prefrontal areas were robust and bidirectional, whereas connections with lateral prefrontal areas were sparse, unidirectional and ascending. Orbitofrontal axons terminated densely in a narrow band around the borders of the magnocellular basolateral nucleus, surrounded by projection neurons along a continuum through the nuclei of the basal complex. In contrast, the input and output zones of medial prefrontal areas were intermingled in the amygdala. Moreover, medial prefrontal axonal terminations were expansive, spreading into the parvicellular basolateral nucleus, which is robustly connected with hypothalamic autonomic structures, suggesting that they may influence the expressive emotional system of the amygdala. On the other hand, orbitofrontal axons heavily targeted the intercalated masses, which issue inhibitory projections to the central nucleus, at least in rats and cats. The central nucleus, in turn, issues a significant inhibitory projection to hypothalamic and brainstem autonomic structures. This evidence suggests that orbitofrontal areas exercise control on the internal processing of the amygdala. In addition, the results provided direct evidence that the connections of anterior temporal visual and auditory association cortices occupy overlapping territories with the orbitofrontal cortices particularly in the posterior half of the amygdala, and specifically within the intermediate sector of the basolateral nucleus and in the magnocellular part of the basomedial nucleus (also known as accessory basal), suggesting a closely linked triadic network. This intricate network may be recruited in cognitive tasks that are inextricably linked with emotional associations.

A differential involvement of the shell and core subterritories of the nucleus accumbens of rats in memory processes

The role of the core and the shell subterritories of the nucleus accumbens in conditioned freezing and spatial learning was investigated by means of selective N-methyl-D-aspartate lesions. Shell-lesioned rats showed reduced conditioned freezing to context and a tendency toward reduced freezing to the discrete stimulus compared with controls. However, lesions of the core did not modify the freezing response either to the context or to the discrete stimuli. Although spatial memory, as assessed by a water-maze paradigm, was not disrupted by the lesions, in a 4-arm baited, 4-arm unbaited radial-arm maze paradigm, the shell-lesioned rats showed selective deficits in working memory, but not in reference memory. In contrast, core-lesioned rats showed no memory deficits.

Dopamine receptor blockade in the rat medial prefrontal cortex reduces spontaneous and amphetamine-induced activity and does not affect prepulse inhibition

The functions and interactions of cortical and subcortical dopamine systems are of interest because alterations in these systems have been implicated in neuropsychiatric diseases, such as schizophrenia. It has been proposed that prefrontal dopamine transmission may oppose dopamine transmission in subcortical sites, such as the nucleus accumbens. Accordingly, reduced prefrontal dopamine transmission would be expected to enhance or induce behavioral effects that have been associated with stimulation of accumbal dopamine receptors. In rats, spontaneous and amphetamine-induced activity is supported by dopamine receptor stimulation in the nucleus accumbens, while prepulse inhibition (PPI) of the acoustic startle response, which is used to measure sensorimotor gating and is disrupted in schizophrenia, is reduced by increased accumbal dopamine receptor stimulation. In the present experiments, we found that bilateral infusion of the dopamine D1/D2 receptor antagonist cis-flupenthixol dihydrochloride into the medial prefrontal cortex of Wistar rats (25 microg each side) reduced spontaneous activity and completely blocked induction of hyperactivity by systemic administration of D-amphetamine sulfate (1 mg/kg), while not affecting PPI. These findings do not support an antagonism between prefrontal and accumbal dopamine in the control of behavior. Rather, our data demonstrate that prefrontal dopamine transmission may modulate some behavioral processes in a similar way to accumbal dopamine.

A differential involvement of the shell and core subterritories of the nucleus accumbens of rats in attentional processes

The nucleus accumbens comprises of two anatomically distinct subterritories: an inner core and an outer shell region. The distinct pattern of the core and shell input and output targets suggests that these two regions may mediate different behavioral processes. Using N-methyl-D-aspartate excitotoxic lesions in either the core or shell region, we investigated whether we can dissociate functionally these two subterritories. N-Methyl-D-aspartate-lesioned, sham-lesioned and non-operated animals were tested for locomotor activity in an open field and in two behavioral paradigms known to evaluate attentional deficits, namely the pre-pulse inhibition of the acoustic startle reflex and latent inhibition, measured in a two-way active avoidance paradigm. The shell-lesioned animals showed a small but significant hyperactivity in the open field when compared to the core-lesioned and to control animals. In the pre-pulse inhibition paradigm, core-lesioned animals demonstrated reduced pre-pulse inhibition to the two high pre-pulse intensities (80 dB[A], 84 dB[A]). In the active avoidance paradigm, whereas no lesion effects were detected in the non-pre-exposed groups, clear attenuation of latent inhibition was found in the shell-lesioned rats, in comparison to both core-lesioned and control rats, due to improved avoidance performance of the shell-pre-exposed group. From these results we suggest that the two subterritories of the nucleus accumbens are differentially involved in attention-related processes: the core lesion leads to significant disruption of pre-pulse inhibition while the shell lesion leads to heightened activity and significant attenuation of latent inhibition.

Selective responding of nucleus accumbens core and shell dopamine to aversively conditioned contextual and discrete stimuli

Dopamine transmission within the nucleus accumbens has been implicated as a neurochemical substrate of associative learning processes. It has been suggested that the acquisition of classically conditioned fear to a specific environment, or context, differs fundamentally from the development of conditioned fear to a discrete stimulus, such as a light or a tone. In this study, we assessed extracellular dopamine in the rat nucleus accumbens shell and core during the expression of a conditioned fear response. Animals were aversively conditioned to either a context or a tone and extracellular dopamine was measured in the nucleus accumbens shell and core by in vivo microdialysis over the next 2 days as animals were returned first to the conditioning chamber (day 1: context test), and subsequently as animals were again returned to the chamber and presented with the conditioned tone stimulus (day 2: tone test). Dopamine levels in the core were significantly higher in the Context-Shock group compared to the Tone-Shock group during the 30-min exposure to context while dopamine levels in the nucleus accumbens shell did not differ significantly during the context test between groups. In contrast, extracellular dopamine in the shell but not the core of Tone-Shock animals increased significantly during presentation of the tone. Dopamine in both the shell and core remained unchanged during the tone test in the Context-Shock groups.These data suggest distinct roles for shell and core dopamine transmission in the expression of a conditioned emotional response. While dopamine increased in the shell primarily during the presentation of a discrete tone conditioned stimulus, core dopamine responded more to a contextual conditioned stimulus. These results may reflect differences in either the type of information acquired or the salience of the learned associations which are formed to a context vs. a discrete tone cue.

Photoperiod and social cues influence the medial amygdala but not the bed nucleus of the stria terminalis in the Siberian hamster

We investigated whether the posterodorsal nucleus of the medial amygdala (MePD) and the posteromedial nucleus of the bed nucleus of the stria terminalis (BSTpm) undergo structural changes in response to photoperiod or social environment in the Siberian hamster, a seasonally breeding rodent. Adult male hamsters were either kept in long days (LD; 15:9 h light:dark) from birth or were transferred at 12-16 weeks of age to short days (SD; 8:16) and housed with a male conspecific for 11 weeks. Other males were transferred to SD but were housed with an unrelated female conspecific from LD. Males transferred to SD without a female cagemate displayed testicular regression, but males transferred to SD with a female cagemate did not. The regional volume and average soma size of the BSTpm and the MePD were estimated using Nissl-stained brain sections. Neither photoperiod nor social condition modified either of the BSTpm measures. Among males housed in same-sex groups, the average soma size in the MePD was significantly smaller in SD males than in LD males. Cohabitation with a female resulted in MePD volumes indistinguishable from LD males. These results indicate that the MePD, a nucleus implicated in socio-sexual behavior, can respond to photoperiodic as well as to social cues.

Developmental changes in responsivity to threat are stimulus-specific in rats

During early ontogeny, stimuli that pose a threat to an animal change. Unrelated adult male rats may kill young rats, but infanticide ends around weaning. Predation, on the other hand, may increase during early ontogeny when rats begin to extend their activity range. We investigated the developmental course of two defensive responses, immobility and analgesia, in young rats exposed to an adult male rat or to predator cues. Preweaning 14-day-old rats became immobile and analgesic when exposed to the male and showed immobility but not analgesia when exposed to cat odor. On Day 26, around weaning, the presence of the male rat no longer induced immobility and analgesia whereas cat odor produced higher levels of immobility and analgesia compared to control and male-exposed animals. This developmental change in responsivity may reflect the differences in the risk of being harmed by a male or a cat during different periods of ontogeny.

Neural interaction between the basal forebrain and functionally distinct prefrontal cortices in the rhesus monkey

The prefrontal cortex in rhesus monkeys is a heterogeneous region by structure, connections and function. Caudal medial and orbitofrontal cortices receive input from cortical and subcortical structures associated with emotions, autonomic function and long-term memory, while lateral prefrontal cortices are linked with structures associated with working memory. With the aid of neural tracers we investigated whether functionally distinct orbitofrontal, medial and lateral prefrontal cortices have specific or common connections with an ascending modulatory system, the basal forebrain. Ascending projections originated in the diagonal band and the basalis nuclei of the basal forebrain in regions demarcated by choline acetyltransferase. Although the origin of projections from the basal forebrain to lateral, medial and orbitofrontal cortices partially overlapped, projections showed a general topography. The posterior part of the nucleus basalis projected preferentially to lateral prefrontal areas while its rostrally adjacent sectors projected to medial and orbitofrontal cortices. The diagonal band nuclei projected to orbitofrontal and medial prefrontal areas. Cortical and subcortical structures that are interconnected appear to have a similar pattern of connections with the basal forebrain. In comparison to the ascending projections, the descending projections were specific, originating mostly in the posterior (limbic) component of medial and orbitofrontal cortices and terminating in the diagonal band nuclei and in the anterior part of the nucleus basalis. In addition, prefrontal limbic areas projected to two other systems of the basal forebrain, the ventral pallidum and the extended amygdala, delineated with the striatal-related markers dopamine, adenosine 3':5'-monophosphate regulated phosphoprotein of M(r) 32kDa, and the related phosphoprotein Inhibitor-1. These basal forebrain systems project to autonomic nuclei in the hypothalamus and brainstem. We interpret these results to indicate that lateral prefrontal areas, which have a role in working memory, receive input from, but do not issue feedback projections to the basal forebrain. In contrast, orbitofrontal and medial prefrontal areas, which have a role in emotions and long-term memory, have robust bidirectional connections with the basal forebrain. Moreover, orbitofrontal and medial prefrontal cortices target the ventral pallidum and the extended amygdala, through which high-order association areas may activate motor autonomic structures for the expression of emotions.

Increased alcohol intake and behavioral disinhibition in rats with ventral striatal neuron loss

A previous study of ours reported excessive alcohol intake, enhanced defensive aggressiveness (hyperreactivity towards the experimenter), impulsive behavior, and reduced cortical serotonin levels in rats following extensive basal forebrain axon-sparing lesions involving the septal area and the ventral striatum. This constellation of signs resembles that seen clinically in "Dionysian" alcoholics. The present investigation aimed at examining the effect of ibotenic acid lesions restricted to the septal area or the ventral striatum on this behavioral profile. Experiment 1 indicated that medium-sized lesions (induced by infusing 0.35 microl ibotenic acid in each hemisphere) encompassing the septal area or the ventral striatum elicited a qualitatively similar behavioral profile. Both lesion types markedly enhanced the intake of 6% ethanol, and both groups were significantly more hyperreactive towards the experimenter. A brief doorbell signal elicited significantly more fleeing in rats with basal forebrain lesions, and licking from an electrified waterspout in the punished drinking test caused lesser suppression of locomotor activity than normal. Both groups also showed significant deficits in food hoarding. Histological examination revealed that the posterior portion of the ventral striatal lesion typically overlapped with the anterior portion of the septal lesion. Experiment 2 avoided this neuropathological overlap, and examined groups bearing small discrete lesions (induced by infusing 0.15 microl ibotenic acid in each hemisphere) restricted to either the accumbens part of the ventral striatum or the dorsal septal area. Lesions to the nucleus accumbens were associated with an increase in home-cage alcohol drinking, no hyperreactivity towards the experimenter, potentiation of fleeing at the expense of freezing in response to a sudden auditory signal, and disinhibited behavior in the punished drinking test with increased punished responding and reduced behavioral suppression. Rats with small septal lesions showed a weak enhancement of defensive aggression, but no other behavioral alterations. Our results suggest that ventral striatal neuron loss gives rise to excessive alcohol drinking and enhanced impulsivity.

Possible pathways through which neurons of the shell of the nucleus accumbens influence the outflow of the core of the nucleus accumbens

The nucleus accumbens (Acb), a major sector of the ventral striatum, is considered to be an integral part of the striatal complex. The Acb has been shown to be composed of two subdivisions, core and shell, which are distinguishable in several aspects, suggesting that these two subdivisions play different functional roles. The aim of this study was to identify pathways of the efferents of the shell of the Acb to influence the outflow of the core of the Acb. Potential disynaptic projections of the shell to the core of the Acb were investigated in chloral hydrate-anesthetized male Sprague-Dawley rats. Following ipsilateral injections of biotinylated dextran amine (BDA) into the shell of the Acb and cholera toxin B subunit (CT-B) into the core, strong overlapping distributions of BDA-labeled terminals and CT-B-labeled neuronal cell somata were found in the medial part of the ventral tegmental area, medial part of the lateral hypothalamic area, and dorsolateral part of the basolateral amygdaloid nucleus. The significance of multiple sites of relay between the efferents of the shell and the afferents of the core of the Acb at different levels of the neuraxis may be related to the functional specificity of each relay site.

Influence of the ventral hippocampal formation on plasma vasopressin, hypothalamic-pituitary-adrenal axis, and behavioral responses to novel acoustic stress

The ventral hippocampal formation (vHF) seems to constrain diverse responses to psychological stimuli, and disruption of this function may underlie severe neuropsychiatric diseases. In particular, the ventral subiculum inhibits hypothalamic-pituitary-adrenal axis (HPA) activity following psychological, but not systemic, stressors. Despite the difficulty in interpreting such HPA responses, they have been relied upon to further characterize vHF function, because increased HPA axis activity is implicated in neuropsychiatric disturbances, and reliance on behavioral and cognitive data is even more problematic. Plasma arginine vasopressin (pAVP), which is inhibited by psychological stimuli and is also implicated in diverse neuropsychiatric diseases, provides a less ambiguous measure of CNS function. To test if its inhibition by psychological stress is also mediated by the vHF, we conducted two studies. In the first, pAVP and behavioral responses to novel acoustic stress were assessed in rats with bilateral excitotoxic lesions of the ventral subiculum and the ventral hippocampus. The subiculum lesions blocked the fall in pAVP and enhanced escape behaviors, whereas the hippocampal lesions produced responses intermediate to those in the subiculum-lesioned and control rats. In the second study, the pAVP response was similarly blocked by small lesions restricted to those vHF subfields which project to the neuroendocrine hypothalamus, compared to the response in animals with lesions in other vHF subfields. These results indicate that discrete projections from the vHF inhibit the pAVP response to psychological stimuli, and suggest that pAVP may provide a reliable probe of vHF activity.

Chronic psychosocial stress reduces the density of dopamine transporters

The effect of different types of physical stress on brain dopaminergic function has been well established in rodents; however, the role of the dopaminergic system in more naturalistic stress situations is poorly understood. Therefore, the aim of the current study was to investigate the effect of chronic psychosocial stress on the dopamine transporter, which is an important component in the regulation of dopaminergic neurotransmission. For this purpose, we used the well-characterized paradigm of subordination stress in male tree shrews (Tupaia belangeri). In the present study, the animals were subjected to psychosocial stress for 28 days. Animals were daily videotaped and locomotor activity was quantified. In subordinate animals, urinary cortisol and noradrenaline, as well as adrenal weight, were increased, whereas body weight, locomotor activity and testicular function were decreased. Brain dopamine transporter binding sites were quantified by in vitro autoradiography using [3H] WIN 35,428 as ligand. Chronic stress reduced the number of binding sites (Bmax) in the caudate nucleus and the putamen without affecting the affinity (Kd). Stress did not influence the binding parameters in the nucleus accumbens, the substantia nigra or the ventral tegmental area. Furthermore, we found a positive correlation between locomotor activity and the Bmax values for [3H] WIN 35,428 binding in the caudate nucleus, the putamen and the nucleus accumbens. The present study shows that a naturalistic stressor, such as chronic psychosocial conflict, decreases dopamine transporter binding sites in motor-related brain areas, suggesting that the reduction in locomotor activity in subordinate tree shrews is related to the downregulation of dopamine transporter binding sites.

A low dose of lithium chloride selectively induces Fos protein in the central nucleus of the amygdala of rat brain

1. Lithium is a very effective treatment for mood disorders. To elucidate the neural substrates of the mood stabilizing actions of lithium, in the present study the authors investigated the effects of a low dose of lithium on regional expression of Fos protein. 2. The administrations of a high dose of lithium chloride (100 mg/kg) induced Fos in widespread areas of the rat brain. In contrast, administration of a low dose of lithium chloride, equivalent to a therapeutic dose in humans, induced Fos only in the central nucleus of the amygdala. 3. These results demonstrate that the central nucleus of the amygdala plays important role in the neural framework that is responsible for the mood-stabilizing effect of lithium.

Defensive conditioning-related functional heterogeneity among nuclei of the rat amygdala revealed by c-Fos mapping

The amygdala is a complex forebrain structure proposed to play a pivotal role in fear conditioning circuitry. In this study, c-Fos immunomapping was applied to investigate the functional activation of particular amygdalar nuclei following a 50-trial training session of two-way active avoidance reaction. To dissect distinctive responses displayed by the animals and to cluster them into groups of correlated behaviors, factor analysis was employed. The training procedure resulted in an increase of c-Fos expression within the cortical, medial, lateral and basolateral, but not central, nuclei. The expression in the cortical nucleus correlated negatively with grooming behavior, whereas c-Fos immunolabeling of the other three subdivisions of the amygdala could be associated with the number of intertrial responses. No correlation was observed between c-Fos expression and avoidance reactions performed or the amount of shock received by the animal. The results obtained with c-Fos mapping of various regions of rat amygdala, combined with a fine dissection of behavioral repertoire, imply that there are specific functional links between particular parts of the structure and distinctive behaviors that reflect various emotional states of the animal.

Behavioral disinhibition following basal forebrain excitotoxin lesions: alcohol consumption, defensive aggression, impulsivity and serotonin levels

Research on alcoholism have identified a subgroup in which the drinking problem is associated with high rates of violence, an impulsive disposition and signs of reduced serotonin functioning in the brain. The present study reports that male Wistar rats with ibotenic acid-induced (5 micrograms/0.5 microliter) neuron loss in the basal forebrain (ventral striatum, septal area and adjacent structures) showed behavioral and neurochemical signs not unlike this subtype of alcoholics. Thus, rats with this lesion exhibited excessive 6% alcohol drinking in a two-bottle choice test and showed augmentation of certain defensive behaviors, including defensive aggression and increased activity-during signal. In the punished drinking test, a passive avoidance task which taps psychological mechanisms underlying impulsivity [56], experimental rats were not different from sham-operated controls with regard to the number of punished licks, but punishment evoked less disruption of ongoing behavior in subjects with basal forebrain damage. The virtual absence of food hoarding in the face of normal feeding may constitute yet another sign of increased impulsivity, indicating as it does a diminished influence of future rewards on behavior. As expected, in view of ibotenic acid's selectivity for neuronal perikarya, the concentrations of dopamine, serotonin and norepinephrine were normal in the lesioned area. However, the levels of serotonin and norepinephrine in the cortex were reduced. A separate experiment examined the extent to which serotonin depletion alone reproduced the behavioral profile induced by basal forebrain neuron loss. However, measures of alcohol consumption, defensive behavior and impulsivity were not different from controls in rats given intracerebroventricular 5,7-DHT (150 micrograms/20 microliters), except for a modest increase in defensive aggression.

Basal forebrain afferent projections modulating cortical acetylcholine, attention, and implications for neuropsychiatric disorders

Cortical acetylcholine (ACh) mediates the detection, selection, and processing of stimuli and associations, and the allocation of processing resources for these attentional functions. For example, loss of cortical cholinergic inputs impairs the performance of rats in tasks designed to assess sustained or divided attention. Intrabasalis infusions of benzodiazepine receptor (BZR) agonists block increases in cortical ACh efflux and impair attentional abilities. Studies on the regulation of cortical ACh efflux by nucleus accumbens (NAC) dopamine (DA) demonstrate that increases in cortical ACh efflux are attenuated by intra-accumbens administration of D1 and, more potently, D2 receptor antagonists. These and other data support the hypothesis that NAC DA, via GABAergic projections to the basal forebrain, controls the excitability of basal forebrain cholinergic neurons. As increases in NAC DA have been hypothesized to represent a major neuronal mediator of schizophrenia and the compulsive use of addictive drugs, the data predict that the abnormal regulation of cortical ACh release represents a crucial neuronal mechanism mediating the cognitive components of these psychopathological disorders.

The concepts of the ventral striatopallidal system and extended amygdala

The concepts of the ventral striatopallidal system and extended amygdala have significantly improved our understanding of basal forebrain organization. As a result of these and other advances during the last twenty years, many of the most prominent basal forebrain structures, including the nucleus accumbens, olfactory tubercle, and amygdaloid body, have all but lost their relevance as independent functional anatomical units. In order to appreciate the distinct differences that exist between the ventral striatopallidal system and the extended amygdala, and as a way of explaining the choice of the terms ventral striatopallidal system and extended amygdala, we will review the discovery and subsequent elaboration of these two systems. On the background of these discussions, we will then proceed to dispel some recently published misgivings regarding the usefulness of the extended amygdaloid concept.