The convergence of axon terminals from the mediodorsal thalamic nucleus and ventral tegmental area on pyramidal cells in layer V of the rat prelimbic cortex

Heroin Self-Administration and Extinction Increase Prelimbic Cortical Astrocyte-Synapse Proximity and Alter Dendritic Spine Morphometrics That Are Reversed by N-Acetylcysteine

Clinical and preclinical studies indicate that adaptations in corticostriatal neurotransmission significantly contribute to heroin relapse vulnerability. In animal models, heroin self-administration and extinction produce cellular adaptations in both neurons and astrocytes within the nucleus accumbens (NA) core that are required for cue-induced heroin seeking. Specifically, decreased glutamate clearance and reduced association of perisynaptic astrocytic processes with NAcore synapses allow glutamate release from prelimbic (PrL) cortical terminals to engage synaptic and structural plasticity in NAcore medium spiny neurons. Normalizing astrocyte glutamate homeostasis with drugs like the antioxidant N-acetylcysteine (NAC) prevents cue-induced heroin seeking. Surprisingly, little is known about heroin-induced alterations in astrocytes or pyramidal neurons projecting to the NAcore in the PrL cortex (PrL-NAcore). Here, we observe functional adaptations in the PrL cortical astrocyte following heroin self-administration (SA) and extinction as measured by the electrophysiologically evoked plasmalemmal glutamate transporter 1 (GLT-1)-dependent current. We likewise observed the increased complexity of the glial fibrillary acidic protein (GFAP) cytoskeletal arbor and increased association of the astrocytic plasma membrane with synaptic markers following heroin SA and extinction training in the PrL cortex. Repeated treatment with NAC during extinction reversed both the enhanced astrocytic complexity and synaptic association. In PrL-NAcore neurons, heroin SA and extinction decreased the apical tuft dendritic spine density and enlarged dendritic spine head diameter in male Sprague-Dawley rats. Repeated NAC treatment during extinction prevented decreases in spine density but not dendritic spine head expansion. Moreover, heroin SA and extinction increased the co-registry of the GluA1 subunit of AMPA receptors in both the dendrite shaft and spine heads of PrL-NAcore neurons. Interestingly, the accumulation of GluA1 immunoreactivity in spine heads was further potentiated by NAC treatment during extinction. Finally, we show that the NAC treatment and elimination of thrombospondin 2 (TSP-2) block cue-induced heroin relapse. Taken together, our data reveal circuit-level adaptations in cortical dendritic spine morphology potentially linked to heroin-induced alterations in astrocyte complexity and association at the synapses. Additionally, these data demonstrate that NAC reverses PrL cortical heroin SA-and-extinction-induced adaptations in both astrocytes and corticostriatal neurons.

Cocaine and habit training cause dendritic spine rearrangement in the prelimbic cortex

Successfully navigating dynamic environments requires organisms to learn the consequences of their actions. The prelimbic prefrontal cortex (PL) formulates action-consequence memories and is modulated by addictive drugs like cocaine. We trained mice to obtain food rewards and then unexpectedly withheld reinforcement, triggering new action-consequence memory. New memory was disrupted by cocaine when delivered immediately following non-reinforcement, but not when delayed, suggesting that cocaine disrupted memory consolidation. Cocaine also rapidly inactivated cofilin, a primary regulator of the neuronal actin cytoskeleton. This observation led to the discovery that cocaine also within the time of memory consolidation elevated dendritic spine elimination and blunted spine formation rates on excitatory PL neurons, culminating in thin-type spine attrition. Training drug-naive mice to utilize inflexible response strategies also eliminated thin-type dendritic spines. Thus, cocaine may disrupt action-consequence memory, at least in part, by recapitulating neurobiological sequalae occurring in the formation of inflexible habits.

Novel Cerebello-Amygdala Connections Provide Missing Link Between Cerebellum and Limbic System

The cerebellum is emerging as a powerful regulator of cognitive and affective processing and memory in both humans and animals and has been implicated in affective disorders. How the cerebellum supports affective function remains poorly understood. The short-latency (just a few milliseconds) functional connections that were identified between the cerebellum and amygdala—a structure crucial for the processing of emotion and valence—more than four decades ago raise the exciting, yet untested, possibility that a cerebellum-amygdala pathway communicates information important for emotion. The major hurdle in rigorously testing this possibility is the lack of knowledge about the anatomy and functional connectivity of this pathway. Our initial anatomical tracing studies in mice excluded the existence of a direct monosynaptic connection between the cerebellum and amygdala. Using transneuronal tracing techniques, we have identified a novel disynaptic circuit between the cerebellar output nuclei and the basolateral amygdala. This circuit recruits the understudied intralaminar thalamus as a node. Using ex vivo optophysiology and super-resolution microscopy, we provide the first evidence for the functionality of the pathway, thus offering a missing mechanistic link between the cerebellum and amygdala. This discovery provides a connectivity blueprint between the cerebellum and a key structure of the limbic system. As such, it is the requisite first step toward obtaining new knowledge about cerebellar function in emotion, thus fundamentally advancing understanding of the neurobiology of emotion, which is perturbed in mental and autism spectrum disorders.

Monoaminergic Neuromodulation of Sensory Processing

All neuronal circuits are subject to neuromodulation. Modulatory effects on neuronal processing and resulting behavioral changes are most commonly reported for higher order cognitive brain functions. Comparatively little is known about how neuromodulators shape processing in sensory brain areas that provide the signals for downstream regions to operate on. In this article, we review the current knowledge about how the monoamine neuromodulators serotonin, dopamine and noradrenaline influence the representation of sensory stimuli in the mammalian sensory system. We review the functional organization of the monoaminergic brainstem neuromodulatory systems in relation to their role for sensory processing and summarize recent neurophysiological evidence showing that monoamines have diverse effects on early sensory processing, including changes in gain and in the precision of neuronal responses to sensory inputs. We also highlight the substantial evidence for complementarity between these neuromodulatory systems with different patterns of innervation across brain areas and cortical layers as well as distinct neuromodulatory actions. Studying the effects of neuromodulators at various target sites is a crucial step in the development of a mechanistic understanding of neuronal information processing in the healthy brain and in the generation and maintenance of mental diseases.

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.

Input Convergence, Synaptic Plasticity and Functional Coupling Across Hippocampal-Prefrontal-Thalamic Circuits

Executive functions and working memory are long known to involve the prefrontal cortex (PFC), and two PFC-projecting areas: midline/paramidline thalamus (MLT) and cornus ammonis 1 (CA1)/subiculum of the hippocampal formation (HF). An increasing number of rodent electrophysiology studies are examining these substrates together, thus providing circuit-level perspectives on input convergence, synaptic plasticity and functional coupling, as well as insights into cognition mechanisms and brain disorders. Our review article puts this literature into a method-oriented narrative. As revisited throughout the text, limbic thalamic and hippocampal afferents to the PFC gate one another’s inputs, which in turn are modulated by PFC interneurons and ascending monoaminergic projections. In addition, long-term synaptic plasticity, paired-pulse facilitation (PPF), and event-related potentials (ERP) dynamically vary across PFC-related circuits during learning paradigms and drug effects. Finally, thalamic-prefrontal loops, which have been shown to amplify both cognitive processes and limbic seizures, are also being implicated as relays in the prefrontal-hippocampal feedback, contributing to spatial navigation and decision making. Based on these issues, we conclude the review with a critical synthesis and some research directions.

Inhibiting Rho kinase promotes goal-directed decision making and blocks habitual responding for cocaine

The prelimbic prefrontal cortex is necessary for associating actions with their consequences, enabling goal-directed decision making. We find that the strength of action–outcome conditioning correlates with dendritic spine density in prelimbic cortex, suggesting that new action–outcome learning involves dendritic spine plasticity. To test this, we inhibited the cytoskeletal regulatory factor Rho kinase. We find that the inhibitor fasudil enhances action–outcome memory, resulting in goal-directed behavior in mice that would otherwise express stimulus-response habits. Fasudil transiently reduces prelimbic cortical dendritic spine densities during a period of presumed memory consolidation, but only when paired with new learning. Fasudil also blocks habitual responding for cocaine, an effect that persists over time, across multiple contexts, and depends on actin polymerization. We suggest that Rho kinase inhibition promotes goal-oriented action selection by augmenting the plasticity of prelimbic cortical dendritic spines during the formation of new action–outcome memories.

Action-outcome learning requires the prelimbic prefrontal cortex. Here the authors report that fasudil, a Rho kinase inhibitor, reduces dendritic spine densities on prelimbic neurons in an activity-dependent manner, stimulating goal-directed actions, and reducing habitual responding for cocaine.

Multiplexed neurochemical signaling by neurons of the ventral tegmental area

The ventral tegmental area (VTA) is an evolutionarily conserved structure that has roles in reward-seeking, safety-seeking, learning, motivation, and neuropsychiatric disorders such as addiction and depression. The involvement of the VTA in these various behaviors and disorders is paralleled by its diverse signaling mechanisms. Here we review recent advances in our understanding of neuronal diversity in the VTA with a focus on cell phenotypes that participate in 'multiplexed' neurotransmission involving distinct signaling mechanisms. First, we describe the cellular diversity within the VTA, including neurons capable of transmitting dopamine, glutamate or GABA as well as neurons capable of multiplexing combinations of these neurotransmitters. Next, we describe the complex synaptic architecture used by VTA neurons in order to accommodate the transmission of multiple transmitters. We specifically cover recent findings showing that VTA multiplexed neurotransmission may be mediated by either the segregation of dopamine and glutamate into distinct microdomains within a single axon or by the integration of glutamate and GABA into a single axon terminal. In addition, we discuss our current understanding of the functional role that these multiplexed signaling pathways have in the lateral habenula and the nucleus accumbens. Finally, we consider the putative roles of VTA multiplexed neurotransmission in synaptic plasticity and discuss how changes in VTA multiplexed neurons may relate to various psychopathologies including drug addiction and depression.

Modulation of excitatory neurotransmission by neuronal/glial signalling molecules: interplay between purinergic and glutamatergic systems

Glutamate is the main excitatory neurotransmitter of the central nervous system (CNS), released both from neurons and glial cells. Acting via ionotropic (NMDA, AMPA, kainate) and metabotropic glutamate receptors, it is critically involved in essential regulatory functions. Disturbances of glutamatergic neurotransmission can be detected in cognitive and neurodegenerative disorders. This paper summarizes the present knowledge on the modulation of glutamate-mediated responses in the CNS. Emphasis will be put on NMDA receptor channels, which are essential executive and integrative elements of the glutamatergic system. This receptor is crucial for proper functioning of neuronal circuits; its hypofunction or overactivation can result in neuronal disturbances and neurotoxicity. Somewhat surprisingly, NMDA receptors are not widely targeted by pharmacotherapy in clinics; their robust activation or inhibition seems to be desirable only in exceptional cases. However, their fine-tuning might provide a promising manipulation to optimize the activity of the glutamatergic system and to restore proper CNS function. This orchestration utilizes several neuromodulators. Besides the classical ones such as dopamine, novel candidates emerged in the last two decades. The purinergic system is a promising possibility to optimize the activity of the glutamatergic system. It exerts not only direct and indirect influences on NMDA receptors but, by modulating glutamatergic transmission, also plays an important role in glia-neuron communication. These purinergic functions will be illustrated mostly by depicting the modulatory role of the purinergic system on glutamatergic transmission in the prefrontal cortex, a CNS area important for attention, memory and learning.

Thalamic neuromodulation and its implications for executive networks

The thalamus is a key structure that controls the routing of information in the brain. Understanding modulation at the thalamic level is critical to understanding the flow of information to brain regions involved in cognitive functions, such as the neocortex, the hippocampus, and the basal ganglia. Modulators contribute the majority of synapses that thalamic cells receive, and the highest fraction of modulator synapses is found in thalamic nuclei interconnected with higher order cortical regions. In addition, disruption of modulators often translates into disabling disorders of executive behavior. However, modulation in thalamic nuclei such as the midline and intralaminar groups, which are interconnected with forebrain executive regions, has received little attention compared to sensory nuclei. Thalamic modulators are heterogeneous in regards to their origin, the neurotransmitter they use, and the effect on thalamic cells. Modulators also share some features, such as having small terminal boutons and activating metabotropic receptors on the cells they contact. I will review anatomical and physiological data on thalamic modulators with these goals: first, determine to what extent the evidence supports similar modulator functions across thalamic nuclei; and second, discuss the current evidence on modulation in the midline and intralaminar nuclei in relation to their role in executive function.

Altered short-term plasticity in the prefrontal cortex after early life seizures

Seizures during development are a relatively common occurrence and are often associated with poor cognitive outcomes. Recent studies show that early life seizures alter the function of various brain structures and have long-term consequences on seizure susceptibility and behavioral regulation. While many neocortical functions could be disrupted by epileptic seizures, we have concentrated on studying the prefrontal cortex (PFC) as disturbance of PFC functions is involved in numerous co-morbid disorders associated with epilepsy. In the present work we report an alteration of short-term plasticity in the PFC in rats that have experienced early life seizures. The most robust alteration occurs in the layer II/III to layer V network of neurons. However short-term plasticity of layer V to layer V network was also affected, indicating that the PFC function is broadly influenced by early life seizures. These data strongly suggest that repetitive seizures early in development cause substantial alteration in PFC function, which may be an important component underlying cognitive deficits in individuals with a history of seizures during development.

Arg kinase regulates prefrontal dendritic spine refinement and cocaine-induced plasticity

Adolescence is characterized by vulnerability to the development of neuropsychiatric disorders including drug addiction, as well as prefrontal cortical refinement that culminates in structural stability in adulthood. Neuronal refinement and stabilization are hypothesized to confer resilience to poor decision making and addictive-like behaviors, although intracellular mechanisms are largely unknown. We characterized layer V prefrontal dendritic spine development and refinement in adolescent wild-type mice and mice lacking the cytoskeletal regulatory protein Abl-related gene (Arg) kinase. Relative to hippocampal CA1 pyramidal neurons, which exhibited a nearly linear increase in spine density up to postnatal day 60 (P60), wild-type prefrontal spine density peaked at P31, and then declined by 18% by P56-P60. In contrast, dendritic spines in mice lacking Arg destabilized by P31, leading to a net loss in both structures. Destabilization corresponded temporally to the emergence of exaggerated psychomotor sensitivity to cocaine. Moreover, cocaine reduced dendritic spine density in wild-type orbitofrontal cortex and enlarged remaining spine heads, but arg(-/-) spines were unresponsive. Local application of Arg or actin polymerization inhibitors exaggerated cocaine sensitization, as did reduced gene dosage of the Arg substrate, p190RhoGAP. Genetic and pharmacological Arg inhibition also retarded instrumental reversal learning and potentiated responding for reward-related cues, providing evidence that Arg regulates both psychomotor sensitization and decision-making processes implicated in addiction. These findings also indicate that structural refinement in the adolescent orbitofrontal cortex mitigates psychostimulant sensitivity and support the emerging perspective that the structural response to cocaine may, at any age, have behaviorally protective consequences.

The effects of electrical stimulation or an electrolytic lesion in the mediodorsal thalamus of the rat on survival, body weight, food intake and running activity in the activity-based anorexia model

The glucose metabolism in the mediodorsal thalamus (MD) is increased in rats in the activity-based anorexia (ABA) model. In patients, electrical stimulation in hyperactive brain regions reduced symptoms in e.g. major depressive disorder and cluster headache. In two blinded randomised controlled experiments, we therefore examined the effects of high-frequency electrical stimulation and an electrolytic lesion in the MD in a validated rat model for anorexia nervosa. The ABA model was successfully replicated in all our experiments, with a reduction in body weight, food intake, and survival time and an increase in running activity. In a first experiment, we evaluated the effect of electrical stimulation or a curative lesion in the MD on survival, body weight, food intake and locomotor activity in ABA rats. Electrical MD stimulation or an electrolytic MD lesion did not improve the symptoms of rats in the ABA model, compared to control groups. In a second experiment, we investigated the effect of a preventive electrolytic lesion in the MD on rats in the ABA model. Although there was no significant improvement of survival, body weight and food intake, locomotor activity was significantly reduced in the lesion group compared to the control group. Apart from this positive effect on running activity, we found no convincing evidence for the suitability of the MD as a neuromodulation target for anorexia nervosa patients.

Serotonin and Dopamine Interactions in Rodents and Primates: Implications for Psychosis and Antipsychotic Drug Development

Since the late 1950s, appreciation of dopamine receptor blockade has played a primary role in understanding the mechanism underlying the therapeutic effects of antipsychotic drugs in schizophrenic patients in treating the positive symptoms of schizophrenia (e.g., delusions and hallucinations). Development of the second generation of antipsychotic drugs, otherwise known as atypical antipsychotic drugs, has resulted in treatments with improved subjective tolerability but relatively modest improvements in the negative symptoms of schizophrenia such as avolition, flat affect, and anhedonia. The major current challenge is to develop medications which can further improve negative symptoms treatment and also tackle the intractable clinical problems of cognitive impairment associated with schizophrenia. Further advances along these lines with respect to the dopaminergic and serotonergic neurostransmitter systems will be aided by an appreciation of the interaction between dopamine and serotonin receptor subtypes in a range of key brain structures, such as the prefrontal cortex, thalamus, striatum, amygdala, hippocampus, and the brain stem nuclei, from which the cell bodies of monoaminergic-containing neurons originate. Increasing emphasis on the use of animal models which are homologous to critical aspects of the pathophysiology in the brains of schizophrenic patients will also be required, especially as negative symptoms and cognitive impairment become an important focus for generating novel therapeutics.

Mediodorsal thalamic afferents to layer III of the rat prefrontal cortex: synaptic relationships to subclasses of interneurons

The mediodorsal nucleus of the thalamus (MD) represents the main subcortical structure that projects to the prefrontal cortex (PFC) and it regulates key aspects of the cognitive functions of this region. Within the PFC, GABA local circuit neurons shape the activity patterns and hence the "memory fields" of pyramidal cells. Although the connections between the MD and PFC are well established, the ultrastructural relationships between projecting fibers from the MD and different subclasses of GABA cells in the PFC are not known. In order to address this issue in the rat, we examined MD axons labeled by tract-tracing in combination with immunogold-silver to identify different calcium-binding proteins localized within separate populations of interneurons. Electron micrographic examination of PFC sections from these animals revealed that MD terminals made primarily asymmetric synapses onto dendritic spines and less commonly onto dendritic shafts. Most of the dendrites receiving MD synaptic input were immunoreactive for parvalbumin (ParV), whereas MD synapses onto dendrites labeled for calretinin or calbindin were less frequent. We also observed that some MD terminals were themselves immunoreactive for calcium-binding proteins, again more commonly for ParV. These results suggest that the MD exerts a dual influence on PFC pyramidal cells: direct inputs onto spines and an indirect influence mediated via synapses onto each subclass of interneurons. The apparent preferential input to ParV cells endows MD afferents with a strong indirect inhibitory influence on pyramidal neuron activity by virtue of ParV cell synapses onto soma, proximal dendrites, and axon initial segments.

Synaptic relationships between axon terminals from the mediodorsal thalamic nucleus and gamma-aminobutyric acidergic cortical cells in the prelimbic cortex of the rat

Although the reciprocal interconnections between the prefrontal cortex and the mediodorsal nucleus of the thalamus (MD) are well known, the involvement of inhibitory cortical interneurons in the neural circuit has not been fully defined. To address this issue, we conducted three combined neuroanatomical studies on the rat brain. First, the frequency and the spatial distribution of synapses made by reconstructed dendrites of nonpyramidal neurons were identified by impregnation of cortical cells with the Golgi method and identification of thalamocortical terminals by degeneration following thalamic lesions. Terminals from MD were found to make synaptic contacts with small dendritic shafts or spines of Golgi-impregnated nonpyramidal cells with very sparse dendritic spines. Second, a combined study that used anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and postembedding gamma-aminobutyric acid (GABA) immunocytochemistry indicated that PHA-L-labeled terminals from MD made synaptic junctions with GABA-immunoreactive dendritic shafts and spines. Nonlabeled dendritic spines were found to receive both axonal inputs from MD with PHA-L labelings and from GABAergic cells. In addition, synapses were found between dendritic shafts and axon terminals that were both immunoreactive for GABA. Third, synaptic connections between corticothalamic neurons that project to MD and GABAergic terminals were investigated by using wheat germ agglutinin conjugated to horseradish peroxidase and postembedding GABA immunocytochemistry. GABAergic terminals in the prelimbic cortex made symmetrical synaptic contacts with retrogradely labeled corticothalamic neurons to MD. All of the synapses were found on cell somata and thick dendritic trunks. These results provide the first demonstration of synaptic contacts in the prelimbic cortex not only between thalamocortical terminals from MD and GABAergic interneurons but also between GABAergic terminals and corticothalamic neurons that project to MD. The anatomical findings indicate that GABAergic interneurons have a modulatory influence on excitatory reverberation between MD and the prefrontal cortex.

Behavioral effects of neonatal and adult excitotoxic lesions of the mediodorsal thalamus in the adult rat

We examined in the rat, the effects of neonatal (postnatal Day 7) and adult excitotoxic lesions of the mediodorsal thalamus (MDT), a brain area innervating the prefrontal cortex and implicated as a site of neuropathology in schizophrenia. Previous studies showed that rats with neonatal excitotoxic damage of the ventral hippocampus (VH), used as an animal model of this disorder, display in young adulthood a variety of abnormalities reminiscent of schizophrenia, including hyperactivity to stressful stimuli and amphetamine. It has been speculated that behavioral abnormalities of the neonatally VH lesioned animals are mediated through MDT projections to the prefrontal cortex. We tested if rats with ibotenic acid (1.5 microg per hemisphere in neonates, 2 microg in adults) lesions of MDT exhibited motor hyperactivity in the same experimental conditions (i.e. in response to novelty, saline injections and amphetamine administration) as rats with the VH lesions. We found that, in contrast to rats with VH lesions, neonatally lesioned MDT rats showed reduced vertical activity in response to amphetamine and no changes in locomotor activity to novelty, saline or amphetamine injections 7 weeks postlesion. Adult lesioned MDT rats exhibited no changes in motor activity as compared to controls at 7 weeks postlesion. These results indicate that neonatal or adult excitotoxic lesions of MDT do not produce behavioral changes analogous to those seen after neonatal VH lesions and do not appear to reproduce animal model-like features of schizophrenia.

Memory trace in prefrontal cortex: theory for the cognitive switch

The dorsolateral prefrontal cortex in human and non-human primates functions as the highest-order executor for the perception-action cycle. According to this view, when perceptual stimuli from the environment are novel or complex, the dorsolateral prefrontal cortex serves to set consciously a goal-directed scheme which broadly determines an action repertory to meet the particular demand from the environment. In this respect, the dorsolateral prefrontal cortex is a short-term activation device with the properties of a cognitive switch', because it couples a particular set of perceptual stimuli to a particular set of actions. Here, I suggest that, in order for the organism to react systematically to the environment, neural traces for the switch function must be stored in the brain. Thus, the highest-order, perception-action interface function of the dorsolateral prefrontal cortex per se depends on permanently stored neural traces in the dorsolateral prefrontal cortex and related structures. Such a memory system may be located functionally between two of the well-documented memory systems in the brain: the declarative memory system and the procedural memory system. Finally, based on available neurophysiological data, the possible mechanisms underlying the formation of cognitive switch traces are proposed.

Activation of presynaptic D1 dopamine receptors by dopamine increases the frequency of spontaneous excitatory postsynaptic currents through protein kinase A and protein kinase C in pyramidal cells of rat prelimbic cortex

To determine the effect of dopamine on the frequency of spontaneous excitatory postsynaptic currents (EPSCs) in pyramidal cells of layers V-VI of the prelimbic cortex, whole-cell patch-clamp recordings were made from 92 pyramidal cells of layers V-VI of the rat prelimbic cortex. In normal buffer, dopamine 100 microM apparently increased the frequency of spontaneous EPSCs. Decreasing the concentration of dopamine from 100 to 50 microM was accompanied by a decreased effect of dopamine. Further decreasing the dopamine concentration to 10 and 1 microM had no effects on the frequency of spontaneous EPSCs. In the presence of tetrodotoxin or cadmium, the increasing effect of dopamine was eliminated. The increasing effect of dopamine was blocked by the dopamine D1 receptor antagonist SCH23390, but not by the dopamine D2 receptor antagonist sulpiride. The D1 receptor agonist SKF38393 partially mimicked the increasing effect, but the D2 receptor agonist quinpirole did not. The alpha(1)-adrenoceptor antagonist prazosin could not block the increasing effect of dopamine on the frequency of spontaneous EPSCs in most cells tested. The protein kinase A inhibitor H-89 and the protein kinase C inhibitor chelerythrine could antagonize the effect of dopamine. The protein kinase A activator forskolin and the protein kinase C activator phorbol 12,13-dibutyrate could mimic the effect of dopamine. These results indicate that dopamine, presynaptically acting on dopamine D1 receptors, increases the frequency of spontaneous EPSCs via intracellular protein kinase A and protein kinase C signaling pathways in pyramidal cells of layers V-VI of the prelimbic cortex.

Interaction between P2Y and NMDA receptors in layer V pyramidal neurons of the rat prefrontal cortex

In the first part of this study, monosynaptic excitatory postsynaptic potentials (EPSPs) in layer V of the rat prefrontal cortex were evoked by electrical stimulation of layer I. Recordings by intracellular sharp microelectrodes showed that EPSPs were concentration-dependently facilitated by the P2 receptor antagonistic ATP analogue 2-methylthio ATP (2-MeSATP), while ATP itself depressed the synaptic potentials. The inhibitory effect of ATP turned into facilitation in the presence of the adenosine A(1) receptor antagonist DPCPX. The 2-MeSATP-induced potentiation of EPSP amplitudes were prevented by the P2 receptor antagonists PPADS and Suramin. The EPSP was almost abolished by coapplication of the NMDA receptor antagonist AP-5 and the AMPA/kainate receptor antagonist CNQX. After blockade of the NMDA receptor-mediated part of the EPSP by AP-5, the stimulatory effect of 2-MeSATP disappeared. When NMDA or AMPA were pressure-applied onto pyramidal cells, only the NMDA-induced depolarization was potentiated by 2-MeSATP. In the second part of the study, NMDA-induced currents were measured by whole-cell patch-clamp pipettes. ATP, 2-MeSATP, UDP and UTP potentiated the response to NMDA, while ADP-beta-S was inactive. PPADS antagonized the effect of ATP. Synaptic isolation of pyramidal neurons by a Ca(2+)-free medium or tetrodotoxin did not alter the effect of ATP which, however, was markedly depressed when GTP in the micropipette was replaced by GDP-beta-S. These observations suggest that in layer V pyramidal neurons of the prefrontal cortex postsynaptically localized P2Y receptors interact with NMDA receptor-channels.

Neuronal and behavioral correlations in the medial prefrontal cortex and nucleus accumbens during cocaine self-administration by rats

Up to 31 neurons per animal were simultaneously recorded from the medial prefrontal cortex and nucleus accumbens in 15 rats during i.v. cocaine self-administration sessions, using a multi-channel, single-unit recording technique. Alterations of neuronal activity (both excitatory and inhibitory) were found a few seconds before each lever press for cocaine infusion; we have called these pre-lever press neuronal activations "anticipatory responses". A detailed video analysis revealed that these neuronal firing alterations were associated with specific portions of the behavioral sequence performed before each lever press in both recording areas. Some of the simultaneously recorded neurons displayed similar firing patterns in relation to a given behavioral episode within the behavioral sequence (turning, raising head, etc.), while others fired at different times relative to each behavioral event. Cross-correlational analyses revealed inter-regional and intra-regional correlated firing patterns between pairs of simultaneously recorded medial prefrontal cortex and nucleus accumbens neurons. This correlated firing occurred in the neurons with and without anticipatory responses, although the incidence of correlations between anticipatory neuron pairs was much higher than that between non-anticipatory neuron pairs (18.4% vs 3.6%). Many correlated neuron pairs displayed a time lag in the peak of correlational activity that indicated a temporal sequence in correlated activity. In contradiction to our hypothesis, the temporal pattern of correlation reveals that there are more cases in which nucleus accumbens neurons fired ahead of medial prefrontal cortex neurons. The results suggest that multiple mesocorticolimbic neuronal circuits may code sequential steps during the behavioral sequence performed to obtain an infusion of cocaine. The observed correlated firing between the medial prefrontal cortex and the nucleus accumbens indicates that dynamic, coherent activity occurs within the mesocorticolimbic circuit. Because this circuit is hypothesized to drive drug-seeking behavior, we suggest that this correlated firing between the nucleus accumbens and the medial prefrontal cortex may participate in the control of cocaine self-administration. In addition, the finding that correlated activity within the nucleus accumbens more often precedes that of the medial prefrontal cortex suggests that the nucleus accumbens may play a prime role in the initiation of cocaine self-administration.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Anatomy, physiology, and synaptic responses of rat layer V auditory cortical cells and effects of intracellular GABA(A) blockade

The varied extracortical targets of layer V make it an important site for cortical processing and output, which may be regulated by differences in the pyramidal neurons found there. Two populations of projection neurons, regular spiking (RS) and intrinsic bursting (IB), have been identified in layer V of some sensory cortices, and differences in their inhibitory inputs have been indirectly demonstrated. In this report, IB and RS cells were identified in rat auditory cortical slices, and differences in thalamocortical inhibition reaching RS and IB cells were demonstrated directly using intracellular GABA(A) blockers. Thalamocortical synaptic input to RS cells was always a combination of excitation and both GABA(A) and GABA(B) inhibition. Stimulation seldom triggered a suprathreshold response. IB cell synaptic responses were mostly excitatory, and stimulation usually triggered action potentials. This apparent difference was confirmed directly using intracellular chloride channel blockers. Before intracellular diffusion, synaptic responses were stable and similar to control conditions. Subsequently, GABA(A) was blocked, revealing a cell's total excitatory input. On GABA(A) blockade, RS cells responded to synaptic stimulation with large, suprathreshold excitatory events, indicating that excitation, while always present in these cells, is masked by GABA(A). In IB cells that had visible GABA(A) input, it often masked an excitatory postsynaptic potential (EPSP) that could lead to additional suprathreshold events. These findings indicate that IB cells receive less GABA(A)-mediated inhibitory input and are able to spike or burst in response to thalamocortical synaptic stimulation far more readily than RS cells. Such differences may have implications for the influence each cell type exerts on its postsynaptic targets.

Dopamine-mediated stabilization of delay-period activity in a network model of prefrontal cortex

The prefrontal cortex (PFC) is critically involved in working memory, which underlies memory-guided, goal-directed behavior. During working-memory tasks, PFC neurons exhibit sustained elevated activity, which may reflect the active holding of goal-related information or the preparation of forthcoming actions. Dopamine via the D1 receptor strongly modulates both this sustained (delay-period) activity and behavioral performance in working-memory tasks. However, the function of dopamine during delay-period activity and the underlying neural mechanisms are only poorly understood. Recently we proposed that dopamine might stabilize active neural representations in PFC circuits during tasks involving working memory and render them robust against interfering stimuli and noise. To further test this idea and to examine the dopamine-modulated ionic currents that could give rise to increased stability of neural representations, we developed a network model of the PFC consisting of multicompartment neurons equipped with Hodgkin-Huxley-like channel kinetics that could reproduce in vitro whole cell and in vivo recordings from PFC neurons. Dopaminergic effects on intrinsic ionic and synaptic conductances were implemented in the model based on in vitro data. Simulated dopamine strongly enhanced high, delay-type activity but not low, spontaneous activity in the model network. Furthermore the strength of an afferent stimulation needed to disrupt delay-type activity increased with the magnitude of the dopamine-induced shifts in network parameters, making the currently active representation much more stable. Stability could be increased by dopamine-induced enhancements of the persistent Na(+) and N-methyl-D-aspartate (NMDA) conductances. Stability also was enhanced by a reduction in AMPA conductances. The increase in GABA(A) conductances that occurs after stimulation of dopaminergic D1 receptors was necessary in this context to prevent uncontrolled, spontaneous switches into high-activity states (i.e., spontaneous activation of task-irrelevant representations). In conclusion, the dopamine-induced changes in the biophysical properties of intrinsic ionic and synaptic conductances conjointly acted to highly increase stability of activated representations in PFC networks and at the same time retain control over network behavior and thus preserve its ability to adequately respond to task-related stimuli. Predictions of the model can be tested in vivo by locally applying specific D1 receptor, NMDA, or GABA(A) antagonists while recording from PFC neurons in delayed reaction-type tasks with interfering stimuli.

Medial prefrontal cortical output neurons to the ventral tegmental area (VTA) and their responses to burst-patterned stimulation of the VTA: neuroanatomical and in vivo electrophysiological analyses

During a delayed period in a delayed-response task, prefrontal cortical neurons show a change in neuronal firing rate that is dependent on a functional mesocortical dopamine input. This change in firing rate has been attributed to be part of the cellular processes underlying working memory. However, it is unclear what neural mechanisms activate mesocortical dopamine neurons to provide an optimal level of dopamine to modulate the firing of the medial prefrontal cortical (mPFC) neurons. This study examined the possibility of whether mPFC neurons that project to the ventral tegmental area (VTA) might activate the ascending mesocortical dopamine neurons. To determine the locations of the mPFC-->VTA neurons, cholera toxin subunit B was microinjected into the VTA. Retrogradely labeled mPFC neurons mainly reside in the deep lamina V and VI. In vivo single unit recording in urethane-anesthetized rats were also used to determine the responses of some of these neurons to burst-patterned stimulation of the VTA. Single-pulse stimulation (1 Hz) of the VTA antidromically activated burst firing mPFC-->VTA neurons. In response to burst-patterned stimulation of the VTA, which mimicked burst firing of VTA dopamine neurons (4-10 pulses at 10-15 Hz cycled at 0.5-3 Hz), the temporal structure of spontaneous burst firing patterns of these neurons but not their mean firing rate were changed. However, the mean firing rate of the non-VTA projecting neurons (i.e., no antidromic response to VTA stimulations) was either increased or decreased by similar burst-patterned stimulation of the VTA. These data suggest that burst-patterned stimulation of the ascending VTA-->mPFC or putative mesocortical dopamine neurons might have released dopamine and/or other neuromodulators to modulate the temporal code, rather than the rate code, of mPFC-->VTA neurons. Medial PFC neurons that project elsewhere (e.g., nucleus accumbens or mediodorsal thalamus) may mediate the sustained firing rate changes during, e.g., short-term working memory.

Dopamine receptors and groups I and II mGluRs cooperate for long-term depression induction in rat prefrontal cortex through converging postsynaptic activation of MAP kinases

Tetanic stimuli to layer I-II afferents in rat prefrontal cortex induced long-term depression (LTD) of layer I-II to layer V pyramidal neuron glutamatergic synapses when tetani were coupled to bath application of dopamine. This LTD was blocked by the following metabotropic glutamate receptor (mGluR) antagonists coapplied with dopamine: (S)-alpha-methyl-4-carboxyphenylglycine (MCPG; group I and II antagonist), (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; group I antagonist), or (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE; group II antagonist). This suggests that the dopamine-facilitated LTD requires synaptic activation of groups I and II mGluRs during tetanus. LTD could also be induced by coupling tetani to bath application of groups I and II mGluR agonist (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). In the next series of experiments, coapplication of dopamine and 1S,3R-ACPD, but not application of either drug alone, consistently induced LTD without tetani or even single test stimuli during drug application, suggesting that coactivation of dopamine receptors and the mGluRs is sufficient for LTD induction. Immunoblot analyses with anti-active mitogen-activated protein kinases (MAP-Ks) revealed that D1 receptors, D2 receptors, group I mGluRs, and group II mGluRs all contribute to MAP-K activation in prefrontal cortex, and that combined activation of dopamine receptors and mGluRs synergistically or additively activate MAP-Ks. Consistently, LTD by dopamine + 1S, 3R-ACPD coapplication, as well as the two other forms of LTD (LTD by dopamine + tetani and LTD by 1S,3R-ACPD + tetani), was blocked by bath application of MAP-K kinase inhibitor PD98059. LTD by dopamine + 1S,3R-ACPD coapplication was also blocked by postsynaptic injection of synthetic MAP-K substrate peptide. Our results suggest that dopamine receptors and groups I and II mGluRs cooperate to induce LTD through converging postsynaptic activation of MAP-Ks.

Opposite modulation of cortical N-methyl-D-aspartate receptor-mediated responses by low and high concentrations of dopamine

To examine whether dopamine modulates cortical N-methyl-D-aspartate receptor-mediated glutamate transmission, whole-cell recordings were made from identified pyramidal cells located in layers V and VI of the medial prefrontal cortex of the rat using a slice preparation. In the presence of tetrodotoxin and the absence of Mg2+, a brief local application of N-methyl-D-aspartate evoked an inward current which was blocked by the N-methyl-D-aspartate antagonist dizocilpine maleate but not affected by the non-N-methyl-D-aspartate antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline, suggesting that the observed current is mediated by N-methyl-D-aspartate receptors located on recorded cells. Bath application of dopamine produced opposite effects on the N-methyl-D-aspartate current depending on the concentrations of dopamine applied. At low concentrations (<50 microM), dopamine enhanced the N-methyl-D-aspartate current, whereas at higher concentrations, dopamine suppressed the current. The same concentrations of dopamine did not significantly affect the inward current induced by the non-N-methyl-D-aspartate agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. The enhancing effect of dopamine on the N-methyl-D-aspartate response was mimicked by the D1 agonist SKF38393 and blocked by the D1 antagonist SCH31966, whereas the suppressing effect was mimicked by the D2 agonist quinpirole and blocked by the D2 antagonist eticlopride. The above results suggest that dopamine at low concentrations acts preferentially on D1-like receptors to promote N-methyl-D-aspartate receptor-mediated transmission, while at high concentrations dopamine also activates D2-like receptors, leading to a suppression of the N-methyl-D-aspartate function. This differential modulation of N-methyl-D-aspartate function may have significant implications for understanding behaviors and disorders involving both cortical dopamine- and glutamate-mediated neurotransmission.

An ultrastructural study of the neural circuit between the prefrontal cortex and the mediodorsal nucleus of the thalamus

Synaptic connectivity between the prefrontal cortex (PFC) and the mediodorsal thalamic nucleus (MD) of the rat has been investigated with the electron microscope after labeling both the pre- and postsynaptic elements. Prefrontal corticothalamic fibers end exclusively as small axon terminals with round synaptic vesicles (SR boutons), which make asymmetrical synaptic contacts with distal dendritic segments of MD neurons. Thalamocortical terminals from MD in PFC are also of the SR type and form asymmetrical synaptic contacts predominantly with dendritic spines arising from the apical or basal dendrites of pyramidal cells whose somata reside in layers III, V and VI. At least some pyramidal cells in layer III that receive MD afferents are callosal cells, whereas deep layer pyramidal cells projecting to MD receive directly some of the thalamocortical terminations from MD, suggesting that the recurrent loop to MD is monosynaptically mediated. Thus, taken together with recent evidence that both the PFC-MD and MD-PFC pathways are glutamatergic and excitatory, the cortical excitation exerted by afferent fibers from MD is transferred, not only back to MD itself through deep pyramidal cells, but also the contralateral prefrontal cortex via pyramidal cells in layer III of the ipsilateral prefrontal cortex. Concerning modulatory and inhibitory inputs, fibers to MD from the ventral pallidum and substantia nigra pars reticulata have been shown to be inhibitory and GABAergic. In addition, fibers from the ventral tegmental area preferentially make symmetrical membrane thickenings (i.e. inhibitory synapses) on deep pyramidal cells in PFC that receive synaptic endings from MD. From these morphological grounds, therefore, cells in the ventral pallidum, the substantia nigra pars reticulata and the ventral tegmental area may mediate, to some extent, an inhibitory effect on the reverberatory excitation between PFC and MD.

Inhibitory effect of A10 dopaminergic neurons of the ventral tegmental area on the orienting response evoked by acoustic stimulation in the cat

The effect of bilateral electric stimulation of A10 dopaminergic neurons of the ventral tegmental area (80-300 microA, 20-50 Hz, 0.1-0.5 ms, 2 s duration) on latency and duration of the orienting response, evoked by acoustic stimuli (4500-8000 Hz, 2 s), was studied in the cat. A10 neuron stimulation, simultaneous with the acoustic one, was performed with threshold parameters inducing minimal behavioral signs (head searching movement, sniffing, increase in alertness). By means of a videoanalysis system, a statistically significant increase, both of latency and duration of the response, was observed. The possible role of dopamine was studied administrating sulpiride (20 mg/kg i.p.), a dopaminergic antagonist prevalently acting on the mesolimbic-mesocortical system. In this condition, the disappearance of A10 neuron effect occurred. Sulpiride injection did not affect the parameters of the orienting response to acoustic stimulus alone, suggesting a direct effect on A10 dopaminergic neurons. Moreover, when saline administration was carried out, no significant modification of the effects, obtained following A10 neuron activation, was observed. The data suggest that A10 dopaminergic neurons, origin of the mesolimbic-mesocortical system, may be involved in the control of the response to sensory stimuli, likely by influencing sensorimotor integration processes. An involvement in the inhibitory regulation of the switching of attention is also discussed.

Local-circuit neurones in the medial prefrontal cortex (areas 25, 32 and 24b) in the rat: morphology and quantitative distribution

This paper is a light microscopical study describing the detailed morphology and quantitative distribution of local circuit neurones in areas 25, 32, and 24b of the medial prefrontal cortex (mPFC) in the rat. Cortical interneurones were identified immunocytochemically by their expression of calretinin (CR), parvalbumin (PV), and calbindin D-28k (CB) immunoreactivity. Neurones immunoreactive for gamma-aminobutyric acid (GABA) were also investigated, as were interneurones containing reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase activity. Several distinct classes of CR+, PV+, and CB+ neurones were identified; the most frequent were: bipolar/bitufted CR+ cells in upper layer 3; multipolar PV+ neurones in layers 3 and 5; and bitufted/multipolar CB+ neurones in lower layer 3. CB+ neurones resembling Martinotti and neurogliaform cells were also present in layers 5/6. The morphologies and depth distributions of each cell type were consistent across the three areas of mPFC studied. Seven classes of diaphorase-reactive mPFC neurone are described; these cells were composed about 0.8% of the total neurone population and had a peak distribution located in mid- to lower layer 5 in each area. In areas 32 and 25, three defined bands of diffuse NADPH diaphorase staining were located in layer 2 and in upper and deep layer 5. Diaphorase reactivity was very infrequently colocalised with either CR, PV, or CB immunoreactivities. The numerical densities of neurones (N(V), number of cells per mm3) in each layer were calculated stereologically. The mean total neuronal N(V) estimate for areas 25, 32, and 24b was 51,603 +/- 3,324 (mean +/- S.D.; n = 8). Significant interareal differences were detected. From cortical thickness data and neuronal N(V) estimates, the absolute number of neurones under 1 mm2 of cortical surface (N(C)) have been derived. The mean N(C) value for areas 25, 32, and 24b was 57,328 +/- 7,505 neurones. In immunolabelled Nissl-stained sections, CR+ neurones constituted an overall 4.0%, PV+ cells 5.6%, and CB+ 3.4% of the total neurone populations in mPFC. GABA+ cells represented a mean of 16.2% (14.8-17.2%) of neurones in areas 25, 32 and 24b. The absolute numbers of CR+, PV+, CB+, and GABA+ neurones within individual layers in a column of cortex under 1 mm2 of cortical surface (N(L)) have also been derived, with significant interareal differences in N(L) values being detected. The data provide the structural basis for a qualitative and quantitative definition of local cortical circuits in the rat mPFC.