Collateral innervation of the medial and lateral prefrontal cortex by amygdaloid, thalamic, and brain-stem neurons

Excitatory synaptic structural abnormalities produced by templated aggregation of α-syn in the basolateral amygdala

Parkinson's disease (PD) and Dementia with Lewy bodies (DLB) are characterized by neuronal α-synuclein (α-syn) inclusions termed Lewy Pathology, which are abundant in the amygdala. The basolateral amygdala (BLA), in particular, receives projections from the thalamus and cortex. These projections play a role in cognition and emotional processing, behaviors which are impaired in α-synucleinopathies. To understand if and how pathologic α-syn impacts the BLA requires animal models of α-syn aggregation. Injection of α-syn pre-formed fibrils (PFFs) into the striatum induces robust α-syn aggregation in excitatory neurons in the BLA that corresponds with reduced contextual fear conditioning. At early time points after aggregate formation, cortico-amygdala excitatory transmission is abolished. The goal of this project was to determine if α-syn inclusions in the BLA induce synaptic degeneration and/or morphological changes. In this study, we used C57BL/6 J mice injected bilaterally with PFFs in the dorsal striatum to induce α-syn aggregate formation in the BLA. A method was developed using immunofluorescence and three-dimensional reconstruction to analyze excitatory cortico-amygdala and thalamo-amygdala presynaptic terminals closely juxtaposed to postsynaptic densities. The abundance and morphology of synapses were analyzed at 6- or 12-weeks post-injection of PFFs. α-Syn aggregate formation in the BLA did not cause a significant loss of synapses, but cortico-amygdala and thalamo-amygdala presynaptic terminals and postsynaptic densities with aggregates of α-syn show increased volumes, similar to previous findings in human DLB cortex, and in non-human primate models of PD. Transmission electron microscopy showed that asymmetric synapses in mice with PFF-induced α-syn aggregates have reduced synaptic vesicle intervesicular distances, similar to a recent study showing phospho-serine-129 α-syn increases synaptic vesicle clustering. Thus, pathologic α-syn causes major alterations to synaptic architecture in the BLA, potentially contributing to behavioral impairment and amygdala dysfunction observed in synucleinopathies.

Dysregulated neuromodulation in the anterior cingulate cortex in chronic pain

Chronic pain is a significant global socioeconomic burden with limited long-term treatment options. The intractable nature of chronic pain stems from two primary factors: the multifaceted nature of pain itself and an insufficient understanding of the diverse physiological mechanisms that underlie its initiation and maintenance, in both the peripheral and central nervous systems. The development of novel non-opioidergic analgesic approaches is contingent on our ability to normalize the dysregulated nociceptive pathways involved in pathological pain processing. The anterior cingulate cortex (ACC) stands out due to its involvement in top-down modulation of pain perception, its abnormal activity in chronic pain conditions, and its contribution to cognitive functions frequently impaired in chronic pain states. Here, we review the roles of the monoamines dopamine (DA), norepinephrine (NE), serotonin (5-HT), and other neuromodulators in controlling the activity of the ACC and how chronic pain alters their signaling in ACC circuits to promote pathological hyperexcitability. Additionally, we discuss the potential of targeting these monoaminergic pathways as a therapeutic strategy for treating the cognitive and affective symptoms associated with chronic pain.

Evolution of prefrontal cortex

Subdivisions of the prefrontal cortex (PFC) evolved at different times. Agranular parts of the PFC emerged in early mammals, and rodents, primates, and other modern mammals share them by inheritance. These are limbic areas and include the agranular orbital cortex and agranular medial frontal cortex (areas 24, 32, and 25). Rodent research provides valuable insights into the structure, functions, and development of these shared areas, but it contributes less to parts of the PFC that are specific to primates, namely, the granular, isocortical PFC that dominates the frontal lobe in humans. The first granular PFC areas evolved either in early primates or in the last common ancestor of primates and tree shrews. Additional granular PFC areas emerged in the primate stem lineage, as represented by modern strepsirrhines. Other granular PFC areas evolved in simians, the group that includes apes, humans, and monkeys. In general, PFC accreted new areas along a roughly posterior to anterior trajectory during primate evolution. A major expansion of the granular PFC occurred in humans in concert with other association areas, with modifications of corticocortical connectivity and gene expression, although current evidence does not support the addition of a large number of new, human-specific PFC areas.

Translating Fear Circuitry: Amygdala Projections to Subgenual and Perigenual Anterior Cingulate in the Macaque

Rodent fear-learning models posit that amygdala–infralimbic connections facilitate extinction while amygdala–prelimbic prefrontal connections mediate fear expression. Analogous amygdala–prefrontal circuitry between rodents and primates is not established. Using paired small volumes of neural tracers injected into the perigenual anterior cingulate cortex (pgACC; areas 24b and 32; a potential homologue to rodent prelimbic cortex) and subgenual anterior cingulate cortex (sgACC, areas 25 and 14c; a potential homologue to rodent infralimbic cortex) in a single hemisphere, we mapped amygdala projections to the pgACC and sgACC within single subjects. All injections resulted in dense retrograde labeling specifically within the intermediate division of the basal nucleus (Bi) and the magnocellular division of the accessory basal nucleus (ABmc). Areal analysis revealed a bias for connectivity with the sgACC, with the ABmc showing a greater bias than the Bi. Double fluorescence analysis revealed that sgACC and pgACC projections were intermingled within the Bi and ABmc, where a proportion were double labeled. We conclude that amygdala inputs to the ACC largely originate from the Bi and ABmc, preferentially connect to the sgACC, and that a subset collaterally project to both sgACC and pgACC. These findings advance our understanding of fear extinction and fear expression circuitry across species.

Neurocircuit differences between memory traces of persistent hypoactivity and freezing following fear conditioning among the amygdala, hippocampus, and prefrontal cortex

We aimed to investigate the persistent trace of one traumatic event on neurocircuit controls in rats. Conditioning was reflected by reductions in rates of 'freezing' and 'other-than-freezing' motor activities, between which rats could alternate on delivery of pulsed footshocks of intensity 0.5 mA but not 1.0 mA. At the latter intensity, freezing began to suppress motor activity. The conditional responses evident during both the context and tone sessions persisted when the tests were repeated on post-conditioning days 7 and 8. Thus, difficulties with fear extinction/reduction remained. However, persistence was not evident on post-conditioning days 1 and 2. One day after the 1.0 mA pulsed footshock, ibotenate lesions and corresponding sham surgeries were performed in unilateral and bilateral hemispheres of the amygdala, hippocampus, and prefrontal cortex, as well as three different disconnections (one unilateral and another contralateral lesions out of three regions, a total of nine groups), and were tested on days 7-8. The drastic restoration of freezing following bilateral amygdala lesions was also evident in animals with three types of disconnection; however, this was not the case for hypoactivity. These results imply that a serious experience can drive different neurocircuits that all involve the amygdala, forming persistent concurrent memories of explicit (e.g., 'freezing') or implicit (e.g., 'other-than-freezing' motor activity) emotions, which may exhibit mutual interference.

Organization of afferents to the orbitofrontal cortex in the rat

The prefrontal cortex (PFC) is usually defined as the frontal cortical area receiving a mediodorsal thalamic (MD) innervation. Certain areas in the medial wall of the rat frontal area receive a MD innervation. A second frontal area that is the target of MD projections is located dorsal to the rhinal sulcus and often referred to as the orbitofrontal cortex (OFC). Both the medial PFC and OFC are comprised of a large number of cytoarchitectonic regions. We assessed the afferent innervation of the different areas of the OFC, with a focus on projections arising from the mediodorsal thalamic nucleus, the basolateral nucleus of the amygdala, and the midbrain dopamine neurons. Although there are specific inputs to various OFC areas, a simplified organizational scheme could be defined, with the medial areas of the OFC receiving thalamic inputs, the lateral areas of the OFC being the recipient of amygdala afferents, and a central zone that was the target of midbrain dopamine neurons. Anterograde tracer data were consistent with this organization of afferents, and revealed that the OFC inputs from these three subcortical sites were largely spatially segregated. This spatial segregation suggests that the central portion of the OFC (pregenual agranular insular cortex) is the only OFC region that is a prefrontal cortical area, analogous to the prelimbic cortex in the medial prefrontal cortex. These findings highlight the heterogeneity of the OFC, and suggest possible functional attributes of the three different OFC areas.

Functional (dissociative) retrograde amnesia

Retrograde amnesia is described as condition which can occur after direct brain damage, but which occurs more frequently as a result of a psychiatric illness. In order to understand the amnesic condition, content-based divisions of memory are defined. The measurement of retrograde memory is discussed and the dichotomy between "organic" and "psychogenic" retrograde amnesia is questioned. Briefly, brain damage-related etiologies of retrograde amnesia are mentioned. The major portion of the review is devoted to dissociative amnesia (also named psychogenic or functional amnesia) and to the discussion of an overlap between psychogenic and "brain organic" forms of amnesia. The "inability of access hypothesis" is proposed to account for most of both the organic and psychogenic (dissociative) patients with primarily retrograde amnesia. Questions such as why recovery from retrograde amnesia can occur in retrograde (dissociative) amnesia, and why long-term new learning of episodic-autobiographic episodes is possible, are addressed. It is concluded that research on retrograde amnesia research is still in its infancy, as the neural correlates of memory storage are still unknown. It is argued that the recollection of episodic-autobiographic episodes most likely involves frontotemporal regions of the right hemisphere, a region which appears to be hypometabolic in patients with dissociative amnesia.

The Role of the Medial Prefrontal Cortex in the Conditioning and Extinction of Fear

Once acquired, a fearful memory can persist for a lifetime. Although learned fear can be extinguished, extinction memories are fragile. The resilience of fear memories to extinction may contribute to the maintenance of disorders of fear and anxiety, including post-traumatic stress disorder (PTSD). As such, considerable effort has been placed on understanding the neural circuitry underlying the acquisition, expression, and extinction of emotional memories in rodent models as well as in humans. A triad of brain regions, including the prefrontal cortex, hippocampus, and amygdala, form an essential brain circuit involved in fear conditioning and extinction. Within this circuit, the prefrontal cortex is thought to exert top-down control over subcortical structures to regulate appropriate behavioral responses. Importantly, a division of labor has been proposed in which the prelimbic (PL) and infralimbic (IL) subdivisions of the medial prefrontal cortex (mPFC) regulate the expression and suppression of fear in rodents, respectively. Here, we critically review the anatomical and physiological evidence that has led to this proposed dichotomy of function within mPFC. We propose that under some conditions, the PL and IL act in concert, exhibiting similar patterns of neural activity in response to aversive conditioned stimuli and during the expression or inhibition of conditioned fear. This may stem from common synaptic inputs, parallel downstream outputs, or cortico-cortical interactions. Despite this functional covariation, these mPFC subdivisions may still be coding for largely opposing behavioral outcomes, with PL biased towards fear expression and IL towards suppression.

Stress as a one-armed bandit: Differential effects of stress paradigms on the morphology, neurochemistry and behavior in the rodent amygdala

Neuroplasticity may be defined as the ability of the central nervous system (CNS) to respond to changes in the internal and external environment and it is well established that some stimuli have the ability to facilitate or impair neuroplasticity depending on the pre-existing milieu. A classic example of a stimulus that can both facilitate and impair neuroplasticity is stress. Indeed, the ability of CNS to respond to acute stress is often dependent upon the prior stress history of the individual. While responses to acute stress are often viewed as adaptive in nature, stress reactivity in subjects with prior chronic stress experiences are often linked to neuropsychiatric disorders, including major depressive disorder, post-traumatic stress disorder (PTSD) and anxiety. In rodent studies, chronic stress exposure produces structural and functional alterations in the hippocampus and medial prefrontal cortex that are consistent across different types of stress paradigms. Conversely, the amygdala appears to exhibit differential structural and functional responses to stress that are dependent on a variety of factors, including the type of stressor performed and the duration of the stress paradigm. This is most evident in output measures including morphological analysis of amygdala neurons, measurement of glutamatergic tone in amygdalar subdivisions and the analysis of amygdala-centric behaviors. Accordingly, this review will provide an overview of the effects of stress on the structural and functional plasticity of the rodent amygdala, especially in relation to the differential effects of repeated or chronic stress paradigms on dendritic architecture, neurochemistry of the glutamatergic system and behavior.

Distribution of VGLUT3 in highly collateralized axons from the rat dorsal raphe nucleus as revealed by single-neuron reconstructions

This study aimed at providing the first detailed morphological description, at the single-cell level, of the rat dorsal raphe nucleus neurons, including the distribution of the VGLUT3 protein within their axons. Electrophysiological guidance procedures were used to label dorsal raphe nucleus neurons with biotinylated dextran amine. The somatodendritic and axonal arborization domains of labeled neurons were reconstructed entirely from serial sagittal sections using a computerized image analysis system. Under anaesthesia, dorsal raphe nucleus neurons display highly regular (1.72±0.50 Hz) spontaneous firing patterns. They have a medium size cell body (9.8±1.7 µm) with 2–4 primary dendrites mainly oriented anteroposteriorly. The ascending axons of dorsal raphe nucleus are all highly collateralized and widely distributed (total axonal length up to 18.7 cm), so that they can contact, in various combinations, forebrain structures as diverse as the striatum, the prefrontal cortex and the amygdala. Their morphological features and VGLUT3 content vary significantly according to their target sites. For example, high-resolution confocal analysis of the distribution of VGLUT3 within individually labeled-axons reveals that serotonin axon varicosities displaying VGLUT3 are larger (0.74±0.03 µm) than those devoid of this protein (0.55±0.03 µm). Furthermore, the percentage of axon varicosities that contain VGLUT3 is higher in the striatum (93%) than in the motor cortex (75%), suggesting that a complex trafficking mechanism of the VGLUT3 protein is at play within highly collateralized axons of the dorsal raphe nucleus neurons. Our results provide the first direct evidence that the dorsal raphe nucleus ascending projections are composed of widely distributed neuronal systems, whose capacity to co-release serotonin and glutamate varies from one forebrain locus to the other.

A translational neuroscience framework for the development of socioemotional functioning in health and psychopathology

The development of socioemotional functioning is a complex process that occurs over a protracted time period and requires coordinating affective, cognitive, and social faculties. At many points in development, the trajectory of socioemotional development can be deleteriously altered due to a combination of environmental insults and individual vulnerabilities. The result can be psychopathology. However, researchers are just beginning to understand the neural and genetic mechanisms involved in the development of healthy and disordered socioemotional functioning. We propose a translational developmental neuroscience framework to understand the transactional process that results in socioemotional functioning in both healthy and disordered populations. We then apply this framework to healthy socioemotional development, pediatric anxiety, pediatric depression, and autism spectrum disorder, selectively reviewing current literature in light of the framework. Finally, we examine ways that the framework can help to frame future directions of research on socioemotional development and translational implications for intervention.

Neuropharmacological modulation of the P3-like event-related potential in a rat two-tone auditory discrimination task with modafinil and NS9283, a positive allosteric modulator of α4β2 nAChRs

The P300 (P3) event-related potential (ERP) is a neurophysiological signal believed to reflect cognitive processing of salient cues, and is thus used as a measure of attention and working memory. Additionally, P3 amplitude and latency is altered in neurological diseases and can be pharmacologically modulated. As P3-like ERPs can be recorded in rodents, it may serve as a potential translational biomarker of value for drug discovery. Here we investigated whether a positive allosteric modulator of α4β2 nicotinic acetylcholine receptors, NS9283, and the psychostimulant modafinil could modulate P3-like ERPs in healthy adult rats performing an auditory oddball discrimination task. ERPs were recorded with electroencephalography electrodes implanted into mediodorsal (MD) thalamus, medial prefrontal cortex, hippocampus and auditory cortex (AC). P3-like ERPs were detected in all brain regions, displaying larger amplitudes in target trials compared to non-target trials. Administration of modafinil (64 mg/kg) decreased P3-like ERP latency in MD thalamus and AC, whereas NS9283 augmented P3-like ERP amplitude in MD thalamus at 0.3 mg/kg and in AC at 3.0 mg/kg. Additionally, N1 pre-attention peak amplitude in MD thalamus was increased with 0.3 mg/kg NS9283. Neither of the compounds enhanced task performance. Rather, modafinil lowered correct rejections in non-target trials. In summary, our findings reveal pharmacological modulation of the rat P3-like ERP in cortical and subcortical regions by modafinil and NS9283. These findings encourage further exploration of the rat P3-like ERP in order to promote the understanding of its meaning within cognition, as well as its applicability as a translatable biomarker in drug development.

Forebrain GABAergic projections from the dorsal raphe nucleus identified by using GAD67-GFP knock-in mice

The dorsal raphe nucleus (DR) contains serotonergic (5-HT) neurons that project widely throughout the forebrain. These forebrain regions also receive innervation from non-5-HT neurons in the DR. One of the main groups of non-5-HT neurons in the DR is γ-aminobutyric acid (GABA)ergic, but their projections are poorly understood due to the difficulty of labeling these neurons immunohistochemically. To identify GABAergic projection neurons within the DR in the current study, we used a knock-in mouse line in which expression of green fluorescent protein (GFP) is controlled by the glutamic acid decarboxylase (GAD)67 promotor. Projections of GAD67-GFP neurons to the prefrontal cortex (PFC), nucleus accumbens (NAC), and lateral hypothalamus (LH) were evaluated by using retrograde tract tracing. The location of GAD67-GFP neurons projecting to each of these areas was mapped by rostrocaudal and dorsoventral location within the DR. Overall, 16% of DR neurons projecting to either the PFC or NAC were identified as GAD67-GFP neurons. GAD67-GFP neurons projecting to the PFC were most commonly found ventrally, in the rostral two-thirds of the DR. NAC-projecting GAD67-GFP neurons had an overlapping distribution that extended dorsally. GAD67-GFP neurons made a larger contribution to the projection of the DR to the LH, accounting for 36% of retrogradely labeled neurons, and were widespread throughout the DR. The current data indicate that DR GABAergic neurons not only may have the capacity to influence local network activity, but also make a notable contribution to DR output to multiple forebrain targets. J. Comp. Neurol. 520:4157-4167, 2012. © 2012 Wiley Periodicals, Inc.

Pattern of distribution of serotonergic fibers to the orbitomedial and insular cortex in the rat

As is well recognized, serotonergic (5-HT) fibers distribute widely throughout the brain, including the cerebral cortex. Although some early reports described the 5-HT innervation of the prefrontal cortex (PFC) in rats, the focus was on sensorimotor regions and not on the 'limbic' PFC - or on the medial, orbital and insular cortices. In addition, no reports have described the distribution of 5-HT fibers to PFC in rats using antisera to the serotonin transporter (SERT). Using immunostaining for SERT, we examined the pattern of distribution of 5-HT fibers to the medial, orbital and insular cortices in the rat. We show that 5-HT fibers distribute massively throughout all divisions of the PFC, with distinct laminar variations. Specifically, 5-HT fibers were densely concentrated in superficial (layer 1) and deep (layers 5/6) of the PFC but less heavily so in intermediate layers (layers 2/3). This pattern was most pronounced in the orbital cortex, particularly in the ventral and ventrolateral orbital cortices. With the emergence of granular divisions of the insular cortex, the granular cell layer (layer 4) was readily identifiable by a dense band of labeling confined to it, separating layer 4 from less heavily labeled superficial and deep layers. The pattern of 5-HT innervation of medial, orbital and insular cortices significantly differed from that of sensorimotor regions of the PFC. Serotonergic labeling was much denser overall in limbic compared to non-limbic regions of the PFC, as was striking demonstrated by the generally weaker labeling in layers 1-3 of the primary sensory and motor cortices. The massive serotonergic innervation of the medial, orbital and insular divisions of the PFC likely contributes substantially to well established serotonergic effects on affective and cognitive functions, including a key role in many neurological and psychiatric diseases.

Leveraging the cortical cholinergic system to enhance attention

Attentional impairments are found in a range of neurodegenerative and neuropsychiatric disorders. However, the development of procognitive enhancers to alleviate these impairments has been hindered by a lack of comprehensive hypotheses regarding the circuitry mediating the targeted attentional functions. Here we discuss the role of the cortical cholinergic system in mediating cue detection and attentional control and propose two target mechanisms for cognition enhancers: stimulation of prefrontal α4β2* nicotinic acetylcholine receptors (nAChR) for the enhancement of cue detection and augmentation of tonic acetylcholine levels for the enhancement of attentional control. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Transient inactivation of the neonatal ventral hippocampus impairs attentional set-shifting behavior: reversal with an α7 nicotinic agonist

Cognitive deficits represent a core symptom cluster in schizophrenia that are thought to reflect developmental dysregulations within a neural system involving the ventral hippocampus (VH), nucleus accumbens (NAC), and prefrontal cortex (PFC). The present experiments determined the cognitive effects of transiently inactivating VH in rats during a sensitive period of development. Neonatal (postnatal day 7, PD7) and adolescent (PD32) male rats received a single bilateral infusion of saline or tetrodotoxin (TTX) within the VH to transiently inactivate local circuitry and efferent outflow. Rats were tested as adults on an attentional set-shifting task. Performance in this task depends upon the integrity of the PFC and NAC. TTX infusions did not affect the initial acquisition or ability to learn an intra-dimensional shift. However, TTX rats required a greater number of trials than did controls to acquire the first reversal and extra-dimensional shift (ED) stages. These impairments were age and region-specific as rats infused with TTX into the VH at PD32, or into the dorsal hippocampus at PD7, exhibited performance in the task similar to that of controls. Finally, acute systemic administration of the partial α7 nicotinic acetylcholine receptor (nAChR) agonist SSR 180711 (3.0 mg/kg) eliminated the TTX-induced performance deficits. Given that patients with schizophrenia exhibit hippocampal pathophysiology and deficits in the ED stages of set-shifting tasks, our results support the significance of transient hippocampal inactivation as an animal model for studying the cognitive impairments in schizophrenia as well as the pro-cognitive therapeutic potential of α7 nAChR agonists.

Transient inactivation of the neonatal ventral hippocampus permanently disrupts the mesolimbic regulation of prefrontal cholinergic transmission: implications for schizophrenia

These experiments determined the mesolimbic modulation of cortical cholinergic transmission in a neurodevelopmental model of schizophrenia. Mesolimbic-cholinergic abnormalities are hypothesized to contribute to the cognitive deficits seen in schizophrenia. Stimulation of NMDA receptors in nucleus accumbens (NAC) increases acetylcholine (ACh) release in the prefrontal cortex (PFC), a mechanism recently demonstrated to contribute to the control of attentional performance. We determined the ability of intra-NAC administration of NMDA to increase prefrontal ACh levels in adult rats that had received bilateral infusions of tetrodotoxin (TTX) to transiently interrupt impulse flow in the ventral hippocampus (VH) during development. Rats received infusions of TTX or saline on postnatal day 7 (PD7) or day 32 (PD32), and the effects of NAC NMDA receptor stimulation on prefrontal cholinergic neurotransmission were assessed in adulthood. In animals treated as controls on PD7, NMDA increased prefrontal ACh levels by 121% above baseline. In contrast, PD7 infusions of TTX into the VH abolished the ability of NAC NMDA to activate prefrontal cholinergic neurotransmission (7% increase). In animals that received TTX infusions on PD32, NMDA-evoked cholinergic activity did not differ from controls, indicating a restricted, neonatal critical period during which VH TTX impacts the organization of mesolimbic-basal forebrain-cortical circuitry. Importantly, the failure of NAC NMDA to evoke cholinergic activity in rats treated with TTX on PD7 did not reflect a reduced excitability of corticopetal cholinergic neurons because administration of amphetamine produced similar elevations of prefrontal ACh levels in PD7 TTX and PD7 control animals. A third series of experiments demonstrated that the effects of PD7 TTX are a specific consequence of transient disruption of impulse flow in the VH. Intra-NAC NMDA evoked prefrontal ACh release in rats receiving TTX, on PD7, into the dorsal hippocampus (DH), basolateral amygdala, or NAC. Thus, impulse flow specifically within the VH, during a sensitive period of development, is necessary for the functional organization of a mesolimbic-cortical circuit known to mediate attentional control processes. Therefore, neonatal inactivation of VH represents an effective animal model for studying the basis of certain cognitive symptoms of schizophrenia.

Collateralized dorsal raphe nucleus projections: a mechanism for the integration of diverse functions during stress

The midbrain dorsal raphe nucleus (DR) is the origin of the central serotonin (5-HT) system, a key neurotransmitter system that has been implicated in the expression of normal behaviors and in diverse psychiatric disorders, particularly affective disorders such as depression and anxiety. One link between the DR-5-HT system and affective disorders is exposure to stressors. Stress is a major risk factor for affective disorders, and stressors alter activity of DR neurons in an anatomically specific manner. Stress-induced changes in DR neuronal activity are transmitted to targets of the DR via ascending serotonergic projections, many of which collateralize to innervate multiple brain regions. Indeed, the collateralization of DR efferents allows for the coordination of diverse components of the stress response. This review will summarize our current understanding of the organization of the ascending DR system and its collateral projections. Using the neuropeptide corticotropin-releasing factor (CRF) system as an example of a stress-related initiator of DR activity, we will discuss how topographic specificity of afferent regulation of ascending DR circuits serves to coordinate activity in functionally diverse target regions under appropriate conditions.

Prefrontal beta2 subunit-containing and alpha7 nicotinic acetylcholine receptors differentially control glutamatergic and cholinergic signaling

One-second-long increases in prefrontal cholinergic activity ("transients") were demonstrated previously to be necessary for the incorporation of cues into ongoing cognitive processes ("cue detection"). Nicotine and, more robustly, selective agonists at alpha4beta2* nicotinic acetylcholine receptors (nAChRs) enhance cue detection and attentional performance by augmenting prefrontal cholinergic activity. The present experiments determined the role of beta2-containing and alpha7 nAChRs in the generation of prefrontal cholinergic and glutamatergic transients in vivo. Transients were evoked by nicotine, the alpha4beta2* nAChR agonist ABT-089 [2-methyl-3-(2-(S)-pyrrolindinylmethoxy) pyridine dihydrochloride], or the alpha7 nAChR agonist A-582941 [2-methyl-5-(6-phenyl-pyridazin-3-yl)-octahydro-pyrrolo[3,4-c]pyrrole]. Transients were recorded in mice lacking beta2 or alpha7 nAChRs and in rats after removal of thalamic glutamatergic or midbrain dopaminergic inputs to prefrontal cortex. The main results indicate that stimulation of alpha4beta2* nAChRs evokes glutamate release and that the presence of thalamic afferents is necessary for the generation of cholinergic transients. ABT-089-evoked transients were completely abolished in mice lacking beta2* nAChRs. The amplitude, but not the decay rate, of nicotine-evoked transients was reduced by beta2* knock-out. Conversely, in mice lacking the alpha7 nAChR, the decay rate, but not the amplitude, of nicotine-evoked cholinergic and glutamatergic transients was attenuated. Substantiating the role of alpha7 nAChR in controlling the duration of release events, stimulation of alpha7 nAChR produced cholinergic transients that lasted 10- to 15-fold longer than those evoked by nicotine. alpha7 nAChR-evoked cholinergic transients are mediated in part by dopaminergic activity. Prefrontal alpha4beta2* nAChRs play a key role in evoking and facilitating the transient glutamatergic-cholinergic interactions that are necessary for cue detection and attentional performance.

Mediodorsal thalamic lesions block the stress-induced inversion of serial memory retrieval pattern in mice

This study examines the effects of ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD) on serial contextual memory retrieval in non-stress and stress conditions. Independent groups of mice learned two successive contextual serial discriminations (D1 and D2) in a four-hole board. The discriminations differed each by the color and texture of the floor. Twenty-four hours later, memory testing occurred in independent groups of mice on one of the two floors of the initial acquisition session. Half of the subjects received three electric footschocks (0.9mA, 2s) 5min prior to testing. Results showed that (i) stress induced a plasma corticosterone rise of same magnitude in sham-operated and MD-lesioned mice; (ii) non-stressed sham-operated mice accurately remembered D1 but not D2, whereas stressed sham-operated animals remembered D2 but not D1; (iii) non-stressed MD-lesioned mice exhibited a memory retrieval pattern similar to that observed in non-stressed sham-operated mice; (iv) however, the stress-induced inversion of the memory retrieval pattern was not observed in MD animals. The effects of MD lesions on memory retrieval in this task are similar to those observed in earlier studies in prefrontal cortex or amygdala-lesioned mice [Chauveau F, Piérard C, Coutan M, Drouet I, Liscia P, Béracochéa D. Prefrontal cortex or basolateral amygdala lesions blocked the stress-induced inversion of serial memory pattern in mice. Neurobiol Learn Mem 2008;90:395-403]; they are however in sharp contrast with mice exhibiting hippocampal lesions [Chauveau F, Pierard C, Tronche C, Coutan M, Drouet I, Liscia P, et al. The hippocampus and prefrontal cortex are differentially involved in serial memory retrieval in non-stress and stress condition. Neurobiol Learn Mem; in press; Chauveau F, Pierard C, Tronche C, Coutan M, Drouet I, Liscia P, et al. Rapid stress-induced corticosterone rise in the hippocampus reverses serial memory retrieval pattern. Hippocampus; in press]. Overall, the present findings highlight the involvement of the MD in an AMG/PFC system mediating the rapid effects of stress on serial memory retrieval.

nAChR agonist-induced cognition enhancement: integration of cognitive and neuronal mechanisms

The identification and characterization of drugs for the treatment of cognitive disorders has been hampered by the absence of comprehensive hypotheses. Such hypotheses consist of (a) a precisely defined cognitive operation that fundamentally underlies a range of cognitive abilities and capacities and, if impaired, contributes to the manifestation of diverse cognitive symptoms; (b) defined neuronal mechanisms proposed to mediate the cognitive operation of interest; (c) evidence indicating that the putative cognition enhancer facilitates these neuronal mechanisms; (d) and evidence indicating that the cognition enhancer facilitates cognitive performance by modulating these underlying neuronal mechanisms. The evidence on the neuronal and attentional effects of nAChR agonists, specifically agonists selective for alpha4beta2* nAChRs, has begun to support such a hypothesis. nAChR agonists facilitate the detection of signals by augmenting the transient increases in prefrontal cholinergic activity that are necessary for a signal to gain control over behavior in attentional contexts. The prefrontal microcircuitry mediating these effects include alpha4beta2* nAChRs situated on the terminals of thalamic inputs and the glutamatergic stimulation of cholinergic terminals via ionotropic glutamate receptors. Collectively, this evidence forms the basis for hypothesis-guided development and characterization of cognition enhancers.

The development of emotion-related neural circuitry in health and psychopathology

Disturbances in the detection of, response to, and interpretation of emotion are common in many forms of psychopathology. The amygdala, striatum, and structures within the prefrontal cortex are highly involved in mediating these stages of emotion processing, and evidence indicates that these regions show structural and functional alterations in different types of psychopathology, including anxiety, depression, and autism spectrum disorders. However, we do not know how genes and the environment interact to alter development of these brain regions in ways that give rise to emotion-related psychopathology. This review discusses the current understanding of brain regions that are involved in emotional functioning, how they develop, and how they are altered in three forms of psychopathology: anxiety, depression, and autism spectrum disorders. Following this, a framework is described that may facilitate the integration of investigations of genetic variation and brain function with symptom and diagnostic measures. The framework involves three components: (a) a greater emphasis on simultaneously analyzing multiple levels (genes, brain function, behavior, symptoms, and diagnoses); (b) further integration of developmental considerations, including timing of environmental events, adaptations (or maladaptations), and disorder-related trajectories that guide some children toward atypical experiences; and (c) greater cross-talk between animal and human investigations to take advantage of biological measures that cannot be acquired in humans.

Glutamatergic contributions to nicotinic acetylcholine receptor agonist-evoked cholinergic transients in the prefrontal cortex

Because modulation of cortical cholinergic neurotransmission has been hypothesized to represent a necessary mechanism mediating the beneficial cognitive effects of nicotine and nicotinic acetylcholine receptor (nAChR) subtype-selective agonists, we used choline-sensitive microelectrodes for the real-time measurement of ACh release in vivo, to characterize cholinergic transients evoked by nicotine and the alpha4beta2*-selective nAChR partial agonist 2-methyl-3-(2-(S)-pyrrolindinylmethoxy)pyridine dihydrochloride (ABT-089), a clinically effective cognition enhancer. In terms of cholinergic signal amplitudes, ABT-089 was significantly more potent than nicotine in evoking ACh cholinergic transients. Moreover, cholinergic signals evoked by ABT-089 were characterized by faster signal rise time and decay rate. The nAChR antagonist mecamylamine attenuated the cholinergic signals evoked by either compound. Cholinergic signals evoked by ABT-089 were more efficaciously attenuated by the relatively beta2*-selective nAChR antagonist dihydro-beta-erythroidine. The alpha7 antagonist methyllycaconitine did not affect choline signal amplitudes but partly attenuated the relatively slow decay rate of nicotine-evoked cholinergic signals. Furthermore, the AMPA receptor antagonist DNQX as well as the NMDA receptor antagonist APV more potently attenuated cholinergic signals evoked by ABT-089. Using glutamate-sensitive microelectrodes to measure glutamatergic transients, ABT-089 was more potent than nicotine in evoking glutamate release. Glutamatergic signals were highly sensitive to tetrodotoxin-induced blockade of voltage-regulated sodium channels. Together, the present evidence indicates that compared with nicotine, ABT-089 evokes more potent and sharper cholinergic transients in prefrontal cortex. Glutamatergic mechanisms necessarily mediate the cholinergic effects of nAChR agonists in the prefrontal cortex.

Involvement of the medial prefrontal cortex in central cardiovascular modulation in the rat

The medial prefrontal cortex (MPFC) and specifically its ventral portion (vMPFC) have been reported to modulate autonomic responses. On the cardiovascular system, this modulation is characterized by an influence on arterial blood pressure, regional blood flow as well as cardiac sympathetic and parasympathetic responses. The vMPFC also modulates baroreflex activity. Several neurotransmitters are present in the vMPFC. Among them L-glutamate, acetylcholine and noradrenaline are involved with cardiovascular modulation. In the present review, we describe evidences on the mechanisms involved in the vMPFC-related cardiovascular modulation.

Fornix, medial prefrontal cortex, nucleus accumbens, and mediodorsal thalamic nucleus: Roles in a fear-based context discrimination task

The goal of the present study was to evaluate the contributions of various brain structures anatomically and functionally linked to the hippocampus and amygdala in a fear-based context discrimination task. The brain areas of interest included the fornix, medial prefrontal cortex, mediodorsal (MD) thalamic nucleus, and nucleus accumbens. Damage to the MD thalamic nucleus and medial prefrontal cortex produced the largest impairment in context-specific fear responses. Damage to the fornix impaired some fear responses (freezing, ultrasonic vocalizations, defecation, and approach/avoidance) while leaving conditioned fear expression of heart rate and urination unaltered. Damage to the nucleus accumbens was also coupled with deficits in the discriminative expression of some (heart rate, urination, and ultrasonic vocalizations) but sparing of context-appropriate freezing, defecation, and approach/avoidance behaviors.

Vglut2 afferents to the medial prefrontal and primary somatosensory cortices: a combined retrograde tracing in situ hybridization study [corrected]

Glutamate transmission is critical for controlling cortical activity, but the specific contribution of the different isoforms of vesicular glutamate transporters in subcortical pathways to the neocortex is largely unknown. To determine the distribution and neocortical projections of vesicular glutamate transporter2 (Vglut2)-containing neurons, we used in situ hybridization and injections of the retrograde tracer Fluoro-Gold into the medial prefrontal and primary somatosensory cortices. The thalamus contains the majority of Vglut2 cells projecting to the neocortex (approximately 90% for the medial prefrontal cortex and 96% for the primary somatosensory cortex) followed by the hypothalamus and basal forebrain, the claustrum, and the brainstem. There are significantly more Vglut2 neurons projecting to the medial prefrontal cortex than to the primary somatosensory cortex. The medial prefrontal cortex also receives a higher percentage of Vglut2 projection from the hypothalamus than the primary somatosensory cortex. About 50% of thalamic Vglut2 projection to the medial prefrontal cortex and as much as 80% of the thalamic projection to primary somatosensory cortex originate in various relay thalamic nuclei. The remainder arise from different midline and intralaminar nuclei traditionally thought to provide nonspecific or diffuse projection to the cortex. The extrathalamic Vglut2 corticopetal projections, together with the thalamic intralaminar-midline Vglut2 corticopetal projections, may participate in diffuse activation of the neocortex.

Modifications of local cerebral glucose utilization in thalamic structures following injection of a dopaminergic agonist in the nucleus accumbens—involvement in antiepileptic effects?

Dopaminergic transmission in the nucleus accumbens (NAcc) is implicated in different aspects of reward and motivational mechanisms. More recently, it has been suggested that this nucleus could also be involved in the modulation of generalized epileptic seizures. In particular, microinjection of dopaminergic agonists in the NAcc suppresses the occurrence of epileptic seizures in a model of absence seizures, the GAERS (generalized absence epileptic rats from Strasbourg). The aim of this study was to identify the structures involved in this effect. Local cerebral metabolic rates for glucose utilization (LCMRglc) were measured in different parts of the basal ganglia and output structures after apomorphine injection in the NAcc in GAERS and in the inbred non-epileptic rats (NE), concomitantly with seizure suppression. Apomorphine injection in the NAcc induced a significant increase of glucose intake in the anteromedial, mediodorsal and ventrolateral nuclei of the thalamus in NE rats, while no significant changes were observed in the basal ganglia structures (globus pallidus, subthalamic nucleus, substantia nigra). Furthermore, microinjections of muscimol (100 and 200 pmol/side) in the mediodorsal nucleus of the thalamus in GAERS rats suppressed seizures. These results suggest that the mediodorsal nucleus of the thalamus could be involved in absence seizures modulation. Along with data from the literature, our data suggest that this nucleus could participate in the control of the basal ganglia over generalized epileptic seizures.

Effect of mediodorsal thalamic nucleus lesion on contextual fear conditioning in rats

Much evidence from animal and clinical studies has shown that the mediodorsal nucleus of the thalamus (MD) is related to various types of memory, such as visual recognition, object-reward association, spatial working, and reference memory; however, few studies have investigated its role in emotion-related learning and memory processes. This study compared the effect of pre- and posttraining bilateral lesions of the mediodorsal thalamic nucleus with those of the amygdala on contextual conditioned fear. Both pre- and posttraining amygdala lesions almost eliminated conditioned freezing, and significantly blocked postshock freezing when behavioral tests were performed immediately after footshocks, reconfirming previous studies that the amygdala is implicated in the learning of Pavlovian conditioning. Both pre- and posttraining lesions of the mediodorsal nucleus of the thalamus significantly attenuated conditioned freezing but had no effect on postshock freezing. In contrast to lesions of the amygdala, those of the mediodorsal thalamic nucleus failed to alter the increased defecation induced by conditioned fear stress. Our results suggest that the mediodorsal nucleus of the thalamus has an important role in acquisition, consolidation or retrieval in Pavlovian contextual fear conditioning. Possible neural circuits, incorporating the amygdala, MD, and hippocampus, and the functional similarity of the MD and hippocampus in contextual fear conditioning, are also discussed.

Juvenile emotional experience alters synaptic composition in the rodent cortex, hippocampus, and lateral amygdala

A quantitative anatomical study in the rodent anterior cingulate and somatosensory cortex, hippocampus, and lateral amygdala revealed region-, cell-, and dendrite-specific changes of spine densities in 3-week-old Octodon degus after repeated parental separation. In parentally separated animals significantly higher spine densities were found on the apical and basal dendrites of the cingulate cortex (up to 143% on apical and 138% on basal dendrite). Branching order analysis revealed that this effect is seen on all segments of the apical dendrite, whereas on the basal dendrites significantly higher spine densities were seen only on the outer branches (third to fifth dendritic segments). Increased spine densities were also observed on the hippocampal CA1 pyramidal neurons (up to 109% on the distal apical segments and up to 106% on the basal segment) compared with the control group. In contrast, significantly reduced spine densities were observed on the granule cell dendrites in the dentate gyrus (down to 92%) and on the apical dendrites in the medial nucleus of the amygdala (down to 95%). No significant changes of spine densities were seen in the somatosensory cortex (except for an increase in the proximal apical segments) and in the lateral nucleus of the dorsal amygdala (except for an increase in the proximal basal dendritic segments). These results demonstrate that repeated stressful emotional experience alters the balance of presumably excitatory synaptic inputs of pyramidal neurons in the limbic system. Such experience-induced modulations of limbic circuits may determine psychosocial and cognitive capacities during later life.

The medial prefrontal cortex in the rat: evidence for a dorso-ventral distinction based upon functional and anatomical characteristics

The prefrontal cortex in rats can be distinguished anatomically from other frontal cortical areas both in terms of cytoarchitectonic characteristics and neural connectivity, and it can be further subdivided into subterritories on the basis of such criteria. Functionally, the prefrontal cortex of rats has been implicated in working memory, attention, response initiation and management of autonomic control and emotion. In humans, dysfunction of prefrontal cortical areas with which the medial prefrontal cortex of the rat is most likely comparable is related to psychopathology including schizophrenia, sociopathy, obsessive-compulsive disorder, depression, and drug abuse. Recent literature points to the relevance of conducting a functional analysis of prefrontal subregions and supports the idea that the area of the medial prefrontal cortex in rats is characterized by its own functional heterogeneity, which may be related to neuroanatomical and neurochemical dissociations. The present review covers recent findings with the intent of correlating these distinct functional differences in the dorso-ventral axis of the rat medial prefrontal cortex with anatomical and neurochemical patterns.

Chronic stress effects on memory: sex differences in performance and monoaminergic activity

Increasing evidence suggests that the time course of advantageous versus deleterious effects of stress on physiologic function is also apparent in some brain functions, including learning and memory. This article reviews the effects of chronic stress on behavioral performance and, more importantly, shows that sex of the subject, as well as duration and intensity of stress, is an important determinant of the functional/behavioral, neurochemical, and anatomical consequences of the stress. Following chronic stress (7-28 days of restraint, 6 h/day), male and female rats were tested on a visual memory task (object recognition) and two spatial memory tasks (object placement and radial arm maze). At 21 days, stress impaired males on all tasks while females were either enhanced (spatial memory tasks) or not impaired (nonspatial memory tasks). Additionally, the influence of the hypothalamic-pituitary-adrenocortical axis in mediating the sex-specific responses to stress is considered. Behavioral and neurochemical assessments following chronic stress in ovariectomized females, with and without estradiol, suggest that estrogen exerts both organizational and activational influences on the observed sex differences in response to stress. Furthermore, stress differentially affected central transmitter levels in the frontal cortex, hippocampus, and amygdala depending on sex. The possible role of these sex-specific changes in neurotransmitter levels in mediating behavioral differences in response to stress is discussed. While these results are thus far limited to a few studies and require both further investigation and verification, chronic stress appears to be associated with distinct, sex-differentiated behavioral/cognitive and neurochemical responses. We conclude that sex differences must be taken into account when investigating or describing stress and associated sequalae.

Neonatal development of projections from the basolateral amygdala to prefrontal, striatal, and thalamic structures in the rat

Recently, an animal model for neurodevelopmental disorders has been developed. In this model, the effects of an early neonatal (postnatal day 7 [Pd 7]) basolateral amygdala lesion are compared with the effects of a lesion later in life (Pd 21). The reported data indicate that amygdala damage at a specific point early in life results in enduring behavioral disturbances that become more manifest after puberty, for example, only an early lesion resulted in a disruption of the prepulse inhibition, which is also observed in people suffering from schizophrenia. Accordingly, it was postulated that the early damage may affect the neuroanatomic and neurochemical organization and functioning of other brain structures. This was studied by use of the anterograde tracers biotinylated dextran amine and Phaseolus vulgaris-leucoagglutinin. At neonatal days 7, 9, 11, 13, and 26, amygdaloid fibers were in particular present in the mediodorsal thalamus (MDT), nucleus accumbens (Acb), and prefrontal cortex (PFC). The development of the topography of the amygdaloid innervation, however, differed markedly for the MDT and Acb compared with the PFC. For the MDT and Acb, no major changes in innervation were observed between Pd 7 and Pd 26, whereas the innervation of the PFC reorganized from a neonatal diffuse (Pd 7 and 9) to a restricted pattern (Pd 11, 13, and 26). In addition, the innervation changed to an adult-like bilaminar pattern. These data provide information on the circuitry that may be involved in the aberrant neurodevelopment of neonatally amygdala-lesioned rats, which have been proposed as an animal model for neurodevelopmental psychopathological disorders.

Involvement of the serotonergic type 1A (5-HT1A) receptor in the agranular insular cortex in the consolidation of memory for inhibitory avoidance in rats

Adult male Wistar rats were bilaterally implanted with indwelling cannulae in the agranular insular cortex of the prefrontal cortex. After recovery, animals were trained in a step-down inhibitory avoidance task (3.0-s, 0.4-mA footshock) and received, immediately after training, a 0.5-microl infusion of the serotonergic type 1A (5-HT1A) receptor agonist dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT) or of the 5- HT1A receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine hydrobromide (NAN-190), or of vehicle alone (20% DMSO). Retention testing was carried out 24 h after training. 8-OH-DPAT (1.25 and 6.25 microg but not 0.0125 or 0.125 microg) was amnesic. NAN-190 was not effective at 0.125 or 1.25 microg any dose but reversed amnesia when given at 1.250 microg simultaneously with both effective doses of 8-OH-DPAT. These results show that an overactivation of 5-HT1A receptors in the agranular insular cortex impairs memory consolidation of inhibitory avoidance, in rats, immediately after training. This suggests that these receptors of the insular cortex may modulate memory consolidation.

Systemic and intra-accumbens administration of amphetamine differentially affects cortical acetylcholine release

The present experiments tested the hypothesis that the amphetamine-induced increase in dopamine release in the nucleus accumbens represents a necessary and sufficient component of the ability of systemically administered amphetamine to stimulate cortical acetylcholine release. The effects of systemic or intra-accumbens administration of amphetamine on accumbens dopamine release and cortical acetylcholine release were assessed simultaneously in awake animals equipped with dialysis probes inserted into the shell of the nucleus accumbens and the medial prefrontal cortex. Additionally, the ability of intra-accumbens administration of dopamine D(1) and D(2) receptor antagonists to attenuate the effects of systemic amphetamine on cortical acetylcholine was tested. The effects of all treatments were assessed in interaction with a stimulus-induced activation of cortical acetylcholine release to account for the possibility that the demonstration of the trans-synaptic effects of accumbens dopamine requires pre-activation of basal forebrain circuits. Systemic amphetamine resulted in increases in basal cortical acetylcholine and accumbens dopamine efflux. Intra-accumbens administration of amphetamine substantially increased accumbens dopamine efflux, but did not significantly affect cortical acetylcholine efflux. Furthermore, intra-accumbens administration of sulpiride or SCH 23390 did not attenuate the systemic amphetamine-induced increase in cortical acetylcholine efflux. Collectively, the present data suggest that increases in accumbens dopamine release are neither sufficient nor necessary for the effects of systemically administered amphetamine on cortical acetylcholine release. The systemic amphetamine-induced increase in cortical acetylcholine may be mediated via multiple, parallel pathways and may not be attributable to a single afferent pathway of the basal forebrain.

Cortical cholinergic inputs mediating arousal, attentional processing and dreaming: differential afferent regulation of the basal forebrain by telencephalic and brainstem afferents

Basal forebrain corticopetal neurons participate in the mediation of arousal, specific attentional functions and rapid eye movement sleep-associated dreaming. Recent studies on the afferent regulation of basal forebrain neurons by telencephalic and brainstem inputs have provided the basis for hypotheses which, collectively, propose that the involvement of basal forebrain corticopetal projections in arousal, attention and dreaming can be dissociated on the basis of their regulation via major afferent projections. While the processing underlying sustained, selective and divided attention performance depends on the integrity of the telencephalic afferent regulation of basal forebrain corticopetal neurons, arousal-induced attentional processing (i.e. stimulus detection, selection and processing as a result of a novel, highly salient, aversive or incentive stimuli) is mediated via the ability of brainstem ascending noradrenergic projections to the basal forebrain to activate or "recruit" these telencephalic afferent circuits of the basal forebrain. In rapid eye movement sleep, both the basal forebrain and thalamic cortiocopetal projections are stimulated by cholinergic afferents originating mainly from the pedunculopontine and laterodorsal tegmenta in the brainstem. Rapid eye movement sleep-associated dreaming is described as a form of hyperattentional processing, mediated by increased activity of cortical cholinergic inputs and their cortical interactions with activated thalamic efferents. In this context, long-standing speculations about the similarities between dreaming and psychotic cognition are substantiated by describing the role of an over(re)active cortical cholinergic input system in either condition. Finally, while determination of the afferent regulation of basal forebrain corticopetal neurons in different behavioral/cognitive states assists in defining the general cognitive functions of cortical acetylcholine, this research requires a specification of the precise anatomical organization of basal forebrain afferents and their interactions in the basal forebrain. Furthermore, the present hypotheses remain incomplete because of the paucity of data concerning the regulation and role of basal forebrain non-cholinergic, particularly GABAergic, efferents.

Cholinergic inputs to the rat medial prefrontal cortex mediate potentiation of the cardiovascular defensive response by the anxiogenic benzodiazephine receptor partial inverse agonist FG 7142

Consistent with its putative anxiogenic actions, administration of the benzodiazepine receptor partial inverse agonist FG 7142 has been shown to potentiate defensive-like cardiovascular reactivity to an acoustic stimulus in the rat, an effect that appears to be mediated by the basal forebrain cholinergic system. The present studies tested the hypothesis that the basal forebrain cholinergic projections to the medial prefrontal cortex, an area that has been implicated in both anxiety and autonomic control, may be a relevant pathway underlying this response potentiation. Infusions of the muscarinic receptor agonist carbachol into the medial prefrontal cortex, but not into the lateral prefrontal cortex or the basolateral amygdala, mimicked the effects of systemically administered FG 7142 on the cardioacceleratory response. Infusions of the muscarinic antagonist atropine blocked this effect, as well as the response-potentiating actions of FG 7142. The effects of FG 7142 were also blocked by lesions of the cholinergic inputs to the medial prefrontal cortex produced by local infusions of the immunotoxin 192 immunoglobulin G-saporin into this area. These findings indicate that cholinergic activation of the medial prefrontal cortex is sufficient to enhance the cardioacceleratory defensive response, and that cholinergic inputs to the medial prefrontal cortex are necessary for the response-potentiating effects of FG 7142. These results are consistent with a recent neurobiological model of anxiety and autonomic control that attributes the enhanced processing of anxiety-related stimuli and contexts to increases in activity in cortical cholinergic inputs.

Contribution of ventrolateral prefrontal cortex to the acquisition and extinction of conditioned fear in rats

The ventrolateral, agranular insular portion of prefrontal cortex (PFC) in rats is involved in visceral functions and has been shown to be involved in emotional processes. However, its contribution to aversive learning has not been well defined. Classical fear conditioning has been a powerful tool for illuminating some of the primary neural structures involved in aversive emotional learning. We measured both the acquisition and the extinction of conditioned fear following lesions of the ventrolateral PFC of rats. Lesions reduced fear reactivity to contextual stimuli associated with conditioning without affecting CS acquisition, and had no effect on response extinction. Ventrolateral PFC may normally be involved in the processing of contextual information while not being directly involved in extinction processes within the aversive domain.

Efferent connections of the anteromedial nucleus of the thalamus of the rat

The projections from the anteromedial nucleus of the thalamus (AM) were investigated using anterograde and retrograde tracing techniques. AM projects to nearly the entire rostrocaudal extent of limbic cortex and to visual cortex. Anteriorly, AM projects to medial orbital, frontal polar, precentral agranular, and infraradiata cortices. Posteriorly, AM projects to retrosplenial granular, entorhinal, perirhinal and presubicular cortices, and to the subiculum. Further, AM projects to visual cortical area 18b, and to the lateral and basolateral nuclei of the amygdala. AM projections are topographically organized, i.e., projections to different cortical areas arise from distinct parts of AM. The neurons projecting to rostral infraradiata cortex (IRalpha) are more caudally located in AM than the neurons projecting to caudal infraradiata cortex (IRbeta). The neuronal cell bodies that project to the terminal field in area 18b are located primarily in ventral and lateral parts of AM, whereas neurons projecting to perirhinal cortex and amygdala are more medially located in AM. Injections into the most caudal, medial part of AM (i.e., the interanteromedial [IAM] nucleus) label terminals in the rostral precentral agranular, caudal IRbeta, and caudal perirhinal cortices. Whereas most AM axons terminate in layers I and V-VI, exceptions to this pattern include area 18b (axons and terminals in layers I and IV-V), the retrosplenial granular cortex (axons and terminals in layers I and V), and the presubicular, perirhinal, and entorhinal cortices (axons and terminals predominantly in layer V). Together, these findings suggest that AM influences a widespread area of limbic cortex.

Restricted lesions to ventral prefrontal subareas block reversal learning but not visual discrimination learning in rats

Previous studies have shown that extensive damage to the medial prefrontal cortex (mPFC) of rats causes reversal learning deficits. The mPFC of rats, however, consists of several subareas that are different from each other in both cytoarchitecture and neural connectivity, suggesting a functional dissociation among the mPFC subareas. In the present study, selective lesions of the mPFC of rats were made with a specially designed microknife whose intracranial placement could be controlled stereotaxically. Restricted lesions were made to each of the 3 parts of the mPFC: the anterior cingulate area (AC) (including the medial precentral area, PrCm), the prelimbic area (PL), and the infralimbic area (IL). One week after surgery, rats were trained in an aversively motivated visual discrimination task in a novel rotating T-maze. After reaching the acquisition criterion, rats were trained in a reversal task in the same maze. No difference was found in acquisition between control and mPFC lesioned rats. However, lesions of either the PL or the IL produced a marked deficit in the reversal task. This behavioral deficit was not found in rats with lesions of the AC. The results indicate that the mPFC of rats is not essential for discrimination learning, but that each of the 2 ventral subareas of the mPFC, PL, and IL, plays a critical role in reversal learning.

Anxiety and cardiovascular reactivity: the basal forebrain cholinergic link

The relations between anxiety states and autonomic functions are considered from the vantage of a model of the neural systems underlying anxiety and autonomic control. An important component of this model is the involvement of the basal forebrain cortical cholinergic system that is seen to play a crucial role in the cognitive aspects of anxiety, and the links between anxiety and autonomic regulation. An additional aspect of the model is the detailing of the routes by which autonomic reactivity and associated visceral afference can modulate more rostral components of the system. The proposed model offers a more comprehensive framework for research on the neurobiology of anxiety and autonomic control.

Effects of ibotenic acid-induced loss of neurons in the medial prefrontal cortex of rats on behavioral vigilance: evidence for executive dysfunction

Rats were trained in a previously validated task for the assessment of sustained attention, or vigilance. This task required the animals to discriminate between signals of variable lengths and non-signal events by making an appropriate lever press. The performance of sham-lesioned animals in this task was characterized by a signal-length dependent number of hits. Also, approximately 70 percent of the non-signals were correctly rejected. Ibotenic acid-induced lesions of the medial prefrontal cortex decreased the relative number of hits and correct rejections and, in essence, resulted in random lever selection. The lesion did not affect the number of omissions or side bias. Furthermore, the performance of lesioned animals was insensitive to the detrimental effects of distractors. The effects of the lesions do not support an interpretation in terms of sustained attention. Rather, the pattern of the lesioned animals' performance is speculated to reveal a fundamental disruption of decisional processes, reminiscent of the executive dysfunctions observed in patients with damage to ventromedial prefrontal areas or with schizophrenia.

Emotional and behavioral correlates of mediodorsal thalamic neurons during associative learning in rats

Neuronal activity was recorded from the mediodorsal thalamic nucleus (MD) of behaving rats that were trained to lick a protruding spout just after a conditioned stimulus to obtain reward or to avoid shock. Conditioned stimuli included both elemental (auditory or visual stimuli) and configural (simultaneous presentation of auditory and visual stimuli predicting reward outcome opposite that predicted by each stimulus presented alone) stimuli. Of 122 MD neurons responding during the task, the activity of 13 increased just before licking only during the task, but not before spontaneous licking during the intertrial interval (conditioned behavior related). These conditioned behavior-related neurons were located mainly in the lateral MD, which has intimate anatomical connections with motor-related areas such as anterior cingulate and striatum. The activity of the other 109 neurons was related to conditioned stimulation (conditioned stimulus related). Most of these neurons responded differentially to both elemental and configural stimuli in terms of reward contingency, and also changed their responses during extinction and relearning trials. Conditioned stimulus-related neurons with latencies < 300 msec were located mainly in the rostromedial MD, which receives afferents from the basolateral nucleus of the amygdala in which sensory information from various sources converge. Furthermore, most differential neurons that were tested responded during the delay period in a reward task in which a delay was imposed between the conditioned stimulus and reward delivery. The present results, along with previous anatomical studies, suggest the existence of two limbic circuits: anterior cingulate-striatum-lateral MD (motor) and amygdala-medial MD-orbital prefrontal cortex (short-term memory/emotion).

Postnatal development of amygdaloid projections to the prefrontal cortex in the rat studied with retrograde and anterograde tracers

The prefrontal cortex (PFC) and the amygdala are involved in a number of common functions, such as emotional and social behavior, stress, visceral functions, ingestive behavior, self-stimulation, and certain aspects of learning and memory. The amygdala massively projects to the PFC and may play a role in the developmental plasticity reported for several of these functions. We have studied the normal postnatal development of the amygdaloid projections to the rat prefrontal cortex by using the retrogradely transported fluorescent dye fast blue and the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Shortly after birth some fibers were observed in the frontal pole of the rat brain. These fibers were scattered throughout all prefrontal cortical areas. The majority of the amygdaloid cells contributing to this pattern at that stage of development were located in the anterior and ventral basolateral nuclei, whereas a minority were located in the posterior basolateral nucleus. The transition from a diffuse fiber distribution to a characteristic bilaminar pattern occurred around postnatal day 12 in the lateral and rostral medial PFC. The PHA-L injections confirmed the existence of a topographical organization of the amygdalo-prefrontocortical projections. Our observations suggest that the development of amygdala innervation of the PFC parallels the emergence of PFC cytoarchitectural organization.

Telencephalic afferents to the caudolateral neostriatum of the pigeon

The pigeon caudolateral neostriatum (NCL) shares a dopaminergic innervation with mammalian frontal cortical areas and is implicated in the regulation of avian cognitive behavior. Retrograde tracing methods were used to identify forebrain projections to NCL and to suggest a possible role of this area in mediating spatial behavior. NCL receives telencephalic projections from the hyperstriatum accessorium, cells along the border of hyperstriatum dorsale and hyperstriatum ventrale, anterolateral hyperstriatum adjacent to the vallecula, confined cell groups within the anterior neostriatum, and subdivisions of the archistriatum. In addition, labeling of a small number of large cells near the fasciculus prosencephali lateralis was observed at the level of the anterior commissure. In accordance with previous studies, projections of subtelencephalic areas were revealed to originate from the thalamic posterior dorsolateral nucleus and nucleus subrotundus, as well as from the tegmental nucleus pedunculopontinus and locus coeruleus. Forebrain connections of NCL show that somatosensory, visual, and olfactory information can combine in this division of the neostriatum. NCL is therefore suited to participate in a neural circuit that regulates spatial behavior. Moreover, the present study reveals that NCL is reached by a limbic projection from the nucleus taeniae. This projection also suggests similarity between NCL and mammalian frontal cortical areas.

Functional anatomy of human hippocampal formation and related structures

Data on the internal organization, and neuronal connections of the human hippocampal formation and related structures of the limbic system are briefly reviewed. In the healthy brain, somatosensory, visual, and auditory input proceeds through neocortical core and belt fields to a variety of association areas, and from here the data is transported via long corticocortical pathways to the extended prefrontal association cortex. Tracts generated from this highest organization level of the brain guide the data via the frontal belt (premotor cortex) to the frontal core (primary motor area). The striatal and cerebellar loops provide the major routes for this data transfer. The main components of the limbic system (the hippocampal formation, the entorhinal region, and the amygdala) maintain a strategic position between the sensory and the motor association areas. Part of the stream of data from the sensory association areas to the prefrontal cortex branches off and eventually converges on the entorhinal region and the amygdala, These connections establish the afferent leg of the limbic loop. In addition, the limbic centers receive substantial input from nuclei processing viscerosensory information. The entorhinal region, the hippocampal formation, and the amygdala are densely interconnected. Important among these connections is the perforant path, which originates in the entorhinal cortex and projects to the hippocampal formation (fascia dentata, Ammon's horn, and subiculum). The subiculum projects to the amygdala, entorhinal region, mamillary nuclei, and anterior and midline thalamic nuclei. The hippocampal formation, the entorhinal region, and the amygdala generate the efferent leg of the limbic loop, which is directed toward the prefrontal cortex. Additional projections reach the key nuclei that control endocrine and autonomic functions. Furthermore, the amygdala exerts influence on all nonthalamic nuclei projecting in a nonspecific manner to the cerebral cortex (ie, the cholinergic magnocellular forebrain nuclei, the histaminergic tuberomamillary nucleus, the dopaminergic nuclei of the ventral tegmentum, the serotonergic anterior raphe nuclei, and the noradrenergic locus ceruleus). The limbic loop centers thus are in the unique strategic position to perform integration of exteroceptive sensory data of various sources with interoceptive stimuli from autonomic centers. Their efferent projections exert influence on both the prefrontal association cortex and the key centers controlling endocrine and autonomic functions.

Mapping of limbic seizure progressions utilizing the electrogenic status epilepticus model and the 14C-2-deoxyglucose method

We have previously described a model of limbic status epilepticus in which chronic prolonged seizure states of immobile, exploratory, minor convulsive or clonic convulsive behavior are induced by intracerebral electrical stimulation; these states appear to belong to the same behavioral progression as kindled seizures. We postulated that the underlying seizure substrates, as mapped by the 14C-2-deoxyglucose method, should reflect a corresponding anatomic progression of discharge spread. Status epilepticus was induced in rat by pulsed-train current delivered for up to 90 min to one of several subcortical areas. Autoradiographs revealed that most of the observed patterns of seizure-induced metabolic activation comprised a hierarchical sequence, such that progressively more extensive patterns subsumed anatomic territories activated in less extensive patterns, thus allowing inferences as to the progression of discharge spread. In this sequence, the basolateral amygdala ipsilateral to the induction electrode was among the first structures to be activated. In successively larger activation patterns a small unilateral network related to basolateral amygdala was involved; this evolved through a transitional state to a unilateral extensive limbic pattern; which in turn was succeeded by bilateral extensive limbic activation. This hierarchical sequence culminated in a neocortical activation pattern, in which most of the forebrain was involved in intense seizure-induced activation. Seizure behaviors increased in severity in correspondence with the underlying seizure-activated anatomic substrate. In contrast, patterns of seizure activation were observed which did not fit within the early stages of the above sequence, although analysis indicates that the later stages of spread may be shared. The study of these patterns and those reported in the literature indicates that although limbic seizure networks may be anatomically distinct at their origination, further expansion is characterized by overlap; upon assumption of extensive patterns of activation the number of nuclei participating is so vast that the identity of the limbic originator is lost and common convulsive manifestations occur.

Topographic organization of collateral projections from the basolateral amygdaloid nucleus to both the prefrontal cortex and nucleus accumbens in the rat

The basolateral amygdaloid nucleus, a limbic/autonomic center in the basal forebrain, has been known to send projection fibers to the prelimbic and dorsal agranular insular areas in the prefrontal cortex, as well as to the nucleus accumbens. In the present study, we investigated single basolateral amygdaloid nucleus neurons sending their axons to both the prefrontal cortex and nucleus accumbens. The fluorescent retrograde double-labeling technique was employed in the rat; True Blue was injected into the prelimbic or dorsal agranular insular cortex, and Diamidino Yellow into the medial or lateral part of the nucleus accumbens. The majority of basolateral amygdaloid nucleus neurons projecting to the dorsal agranular insular cortex or prelimbic cortex were located, respectively, in the rostral two-thirds or caudal two-thirds of the nucleus, while those projecting to the medial or lateral part of the nucleus accumbens were diffusely distributed in the nucleus. Almost 50% of basolateral amygdaloid nucleus neurons projecting to the prelimbic cortex sent their axon collaterals to the medial part of the nucleus accumbens. About 30-40% of basolateral amygdaloid nucleus neurons projecting to the dorsal agranular insular cortex or prelimbic cortex issued their axon collaterals to the lateral part of the nucleus accumbens. The axons bifurcating to both the dorsal agranular insular cortex and lateral part of the nucleus accumbens, those bifurcating to both the prelimbic cortex and lateral part of the nucleus accumbens, or those bifurcating to both the prelimbic cortex and medial part of the nucleus accumbens arose preferentially from the rostral, middle or caudal parts of the basolateral amygdaloid nucleus, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Topographic organization of subcortical projections to the anterior thalamic nuclei in the rat

Subcortical projections to the anterior thalamic nuclei were studied in the rat, with special reference to projections from the mammillary nuclei, by retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. The medial mammillary nucleus (MM) projects predominantly ipsilaterally to the entire anterior thalamic nuclei, whereas the lateral mammillary nucleus projects bilaterally to the anterodorsal nucleus (AD) of the anterior thalamic nuclei. A topographic relationship was recognized between the MM and the anterior thalamic nuclei. The dorsal region of the pars mediana of the MM projects to the interanteromedial nucleus (IAM), whereas the ventral region projects to the rostral part of the anteromedial nucleus (AM). The dorsal and the ventral regions of the pars medialis project to the dorsomedial part of the AM at its caudal and rostral levels, respectively. The dorsomedial region of the pars lateralis projects to the ventral AM. The ventrolateral region of the pars lateralis projects to the ventral part of the anteroventral nucleus (AV) in such a manner that rostral cells project rostrally and caudal cells project caudally. The pars basalis projects predominantly ipsilaterally to the dorsolateral AV and bilaterally to the AD. The rostrolateral region of the pars posterior projects to the lateral AV, whereas the medial and the caudal regions of the pars posterior project to the dorsomedial AV. The rostrodorsal part of the nucleus reticularis thalami was found to project to the anterior thalamic nuclei; cells located rostrally in this part project to the IAM and AM, whereas cells located caudodorsally project to the AV and AD. The laterodorsal tegmental nucleus projects predominantly ipsilaterally to the AV, especially to its dorsolateral part. The present study demonstrates that subdivisions of the subcortical structures are connected to the subnuclei of the anterior thalamic nuclei, with a clear-cut topography arranged in the dorsoventral and the rostrocaudal dimensions.

The dorsal, posterodorsal, and ventral tegmental nuclei: a cyto- and chemoarchitectonic study in the human

In order to verify the existence of the ventral and posterodorsal tegmental nuclei and to extend previous findings regarding the dorsal tegmental nucleus in the human brainstem, studies were conducted using cyto- and chemoarchitectonics, and computer reconstruction techniques. Serial sections of five brainstems from adults with no known neurological disorders were stained for Nissl substance, acetylcholinesterase, and substance P. The topography, cytoarchitecture, and acetylcholinesterase reactivity of the tegmental nuclei were presented in a mini-atlas depicting sections cut in transverse and sagittal planes. The dorsal and posterodorsal tegmental nuclei were identified fully within the central grey matter while the ventral tegmental nucleus extended across the medial longitudinal fasciculus into the pontine reticular formation. The dorsal tegmental nucleus featured a cell-poor pericentral part, strongly positive for acetylcholinesterase, and a central part comprised of densely packed small neurons that displayed moderate acetylcholinesterase reactivity and strong substance P-like immunoreactivity. The posterodorsal tegmental nucleus, located in the same transverse plane as the rostral part of the motor nucleus of the trigeminal nerve, was composed of diffusely arranged small to medium neurons with its neuropil displaying moderate acetylcholinesterase reactivity and strong substance P-like immunoreactivity. The ventral tegmental nucleus, identified as a prominent structure in the pontine tegmentum immediately rostral to the genu of the facial nerve, contained predominantly large neurons and displayed intensive acetylcholinesterase reactivity and substance P-like immunoreactivity. These studies showed that the tegmental nuclei, which displayed distinctive cyto- and chemoarchitectonic features, were fully present in adult human brainstem.