Do rats have a prefrontal cortex?

The emerging neuroscience of appetitive and drug cue extinction in humans

Fear extinction has been extensively studied in both humans and non-human animals, and this work has contributed greatly to our understanding and treatment of anxiety disorders. Yet other psychopathologies like addiction might be associated with impairments selectively in extinction of non-fear based, appetitive and drug cue associations, and these processes have been underexplored in clinical translational neuroscience. Important questions regarding similarities and differences in the neurobiological mechanisms underlying aversive and appetitive extinction remain unanswered, particularly those pertaining to cross-species evidence for the role of the ventromedial prefrontal cortex and, to some extent, the striatum. Here, we aim to draw attention to the paucity of studies investigating non-fear based extinction in humans, summarize emerging findings from the available literature, and highlight important directions for future research. We argue that closing these gaps in our understanding could help inform the development of more targeted, and perhaps more durable, forms of extinction-based treatments for addiction and related psychopathologies characterized by abnormally persistent appetitive and drug cue associations.

Thalamic afferents to prefrontal cortices from ventral motor nuclei in decision-making

The focus of this literature review is on the three interacting brain areas that participate in decision‐making: basal ganglia, ventral motor thalamic nuclei, and medial prefrontal cortex, with an emphasis on the participation of the ventromedial and ventral anterior motor thalamic nuclei in prefrontal cortical function. Apart from a defining input from the mediodorsal thalamus, the prefrontal cortex receives inputs from ventral motor thalamic nuclei that combine to mediate typical prefrontal functions such as associative learning, action selection, and decision‐making. Motor, somatosensory and medial prefrontal cortices are mainly contacted in layer 1 by the ventral motor thalamic nuclei and in layer 3 by thalamocortical input from mediodorsal thalamus. We will review anatomical, electrophysiological, and behavioral evidence for the proposed participation of ventral motor thalamic nuclei and medial prefrontal cortex in rat and mouse motor decision‐making.

What, If Anything, Is Rodent Prefrontal Cortex?

Prefrontal cortex (PFC) means different things to different people. In recent years, there has been a major increase in publications on the PFC, especially using mice. However, inconsistencies in the nomenclature and anatomical boundaries of PFC areas has made it difficult for researchers to compare data and interpret findings across species. We conducted a meta-analysis of publications on the PFC of humans and rodents and found dramatic differences in the focus of research on these species. In addition, we compared anatomical terms and criteria across several common rodent brain atlases and found inconsistencies among, and even within, leading atlases. To assess the impact of these issues on the research community, we conducted a survey of established PFC researchers on their use of anatomical terms and found little consensus. We report on the results of the survey and propose an alternative scheme for interpreting data from rodent studies, based on structural analysis of the corpus callosum and nomenclature used in research on the anterior cingulate cortex (ACC) of primates.

Feedforward- and motor effort-dependent increase in prefrontal oxygenation during voluntary one-armed cranking

KEY POINTS

Some cortical areas are believed to transmit a descending signal in association with motor intention and/or effort that regulates the cardiovascular system during exercise (termed central command). However, there was no evidence for the specific cortical area responding prior to arbitrary motor execution and in proportion to the motor effort. Using a multichannel near-infrared spectroscopy system, we found that the oxygenation of the dorsolateral and ventrolateral prefrontal cortices on the right side increases in a feedforward- and motor effort-dependent manner during voluntary one-armed cranking with the right arm. This finding may suggest a role of the dorsolateral and ventrolateral prefrontal cortices in triggering off central command and may help us to understand impaired regulation of the cardiovascular system in association with lesion of the prefrontal cortex.

ABSTRACT

Output from higher brain centres (termed central command) regulates the cardiovascular system during exercise in a feedforward- and motor effort-dependent manner. This study aimed to determine a cortical area responding prior to arbitrarily started exercise and in proportion to the effort during exercise. The oxygenation responses in the frontal and frontoparietal areas during one-armed cranking with the right arm were measured using multichannel near-infrared spectroscopy, as indexes of regional blood flow responses, in 20 subjects. The intensity of voluntary exercise was 30% and 60% of the maximal voluntary effort (MVE). At the start period of both voluntary cranking tasks, the oxygenation increased (P < 0.05) only in the lateral and dorsal part of the dorsolateral prefrontal cortex (DLPFC), ventrolateral prefrontal cortex (VLPFC) and sensorimotor cortices. Then, the oxygenation increased gradually in all cortical areas during cranking at 60% MVE, while oxygenation increased only in the frontoparietal area and some of the frontal area during cranking at 30% MVE. The rating of perceived exertion to the cranking tasks correlated (P < 0.05) with the oxygenation responses on the right side of the lateral-DLPFC (r = 0.46) and VLPFC (r = 0.48) and the frontopolar areas (r = 0.47-0.49). Motor-driven passive one-armed cranking decreased the oxygenation in most cortical areas, except the contralateral frontoparietal areas. Accordingly, the lateral-DLPFC and VLPFC on the right side would respond in a feedforward- and motor effort-dependent manner during voluntary exercise with the right arm. Afferent inputs from mechanosensitive afferents may decrease the cortical oxygenation.

Neuropharmacology of attention

Early philosophers and psychologists defined and began to describe attention. Beginning in the 1950's, numerous models of attention were developed. This corresponded with an increased understanding of pharmacological approaches to manipulate neurotransmitter systems. The present review focuses on the knowledge that has been gained about these neurotransmitter systems with respect to attentional processing, with emphasis on the functions mediated within the medial prefrontal cortex. Additionally, the use of pharmacotherapies to treat psychiatric conditions characterized by attentional dysfunction are discussed. Future directions include developing a more comprehensive understanding of the neural mechanisms underlying attentional processing and novel pharmacotherapeutic targets for conditions characterized by aberrant attentional processing.

Prefrontal circuit organization for executive control

The essential role of executive control is to select the most appropriate behavior among other candidates depending on the sensory information (exogenous information) and on the subject's internal state (endogenous information). Here I review series of the evidence implicating that the rodent prefrontal cortex (PFC) evaluates and compares the expected outcome for candidate actions that are automatically primed by exogenous and endogenous information, and selects the most appropriate action while inhibiting the others, with different PFC subregions contributing to distinct aspects of the computation via differential recruitments of the distributed networks. The recurrent nature of the PFC networks further facilitates the computation by integrating bottom-up signals over a long timescale. I also overview the local circuit organization in the PFC, where vasoactive intestinal peptide-positive (VIP) GABAergic interneurons are tightly linked with the cholinergic system and play significant roles in regulating executive control signals. The empirical evidence inspires the disinhibitory module hypothesis of the PFC organization that a group of pyramidal neurons and interneurons forms a disinhibitory module with similar task-variable selectivity in the PFC, and long-range inputs and neuromodulations in these modules exert a distributed gain modulation of the ongoing executive control signals by adjusting VIP neuron activity.

Contradictory effects of erythropoietin on inhibitory synaptic transmission in left and right prelimbic cortex of mice

Erythropoietin (EPO) has been shown to improve cognitive function in mammals as well as in patients of psychiatric diseases by directly acting on the brain. In addition, EPO attenuates the synaptic transmission and enhances short- and long-term synaptic plasticity in hippocampus of mice, although there are still many discrepancies between different studies. It has been suggested that the divergences of different studies take root in different in-vivo application schemata or in long-term trophic effects of EPO. In the current study, we investigated the direct effects of EPO in slices of prelimbic cortex (PrL) by acute ex-vivo application of EPO, so that the erythropoietic or other trophic effects could be entirely excluded. Our results showed that the EPO effects were contradictory between the left and the right PrL. It enhanced the inhibitory transmission in the left and depressed the inhibitory transmission in the right PrL. Strikingly, this lateralized effect of EPO could be consistently found in individual bi-lateral PrL of all tested mice. Thus, our data suggest that EPO differentially modulates the inhibitory synaptic transmission of neuronal networks in the left and the right PrL. We hypothesize that such lateralized effects of EPO contribute to the development of the lateralization of stress reaction in PFC and underlie the altered bilateral GAGAergic synaptic transmission and oscillation patterns under stress that impact the central emotional and cognitive control in physiology as well as in pathophysiology.

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EPO showed fast effects on inhibitory transmission in the prefrontal cortex of mice.

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EPO enhanced the inhibitory transmission in the left and depressed it in the right prelimbic cortex of mice.

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The expression of EPOR in GAD⁺-neurons is different between the left and right PFC.

Basal Forebrain Nuclei Display Distinct Projecting Pathways and Functional Circuits to Sensory Primary and Prefrontal Cortices in the Rat

Recent evidence supports that specific projections between different basal forebrain (BF) nuclei and their cortical targets are necessary to modulate cognitive functions in the cortex. We tested the hypothesis of the existence of specific neuronal populations in the BF linking with specific sensory, motor, and prefrontal cortices in rats. Neuronal tracing techniques were performed using retrograde tracers injected in the primary somatosensory (S1), auditory (A1), and visual (V1) cortical areas, in the medial prefrontal cortex (mPFC) as well as in BF nuclei. Results indicate that the vertical and horizontal diagonal band of Broca (VDB/HDB) nuclei target specific sensory cortical areas and maintains reciprocal projections with the prelimbic/infralimbic (PL/IL) area of the mPFC. The basal magnocellular nucleus (B nucleus) has more widespread targets in the sensory-motor cortex and does not project to the PL/IL cortex. Optogenetic stimulation was used to establish if BF neurons modulate whisker responses recorded in S1 and PL/IL cortices. We drove the expression of high levels of channelrhodopsin-2, tagged with a fluorescent protein (ChR2-eYFP) by injection of a virus in HDB or B nuclei. Blue-light pulses were delivered to the BF through a thin optic fiber to stimulate these neurons. Blue-light stimulation directed toward the HDB facilitated whisker responses in S1 cortex through activation of muscarinic receptors. The same optogenetic stimulation of HDB induced an inhibition of whisker responses in mPFC by activation of nicotinic receptors. Blue-light stimulation directed toward the B nucleus had lower effects than HDB stimulation. Our findings pointed the presence of specific neuronal networks between the BF and the cortex that may play different roles in the control of cortical activity.

N-methyl-d-aspartate receptor dysfunction in the prefrontal cortex of stroke-prone spontaneously hypertensive rat/Ezo as a rat model of attention deficit/hyperactivity disorder

AIM

We previously reported that stroke-prone spontaneously hypertensive rat/Ezo (SHRSP/Ezo) has high validity as an attention deficit/hyperactivity disorder (AD/HD) animal model, based on its behavioral phenotypes, such as inattention, hyperactivity, and impulsivity. Fronto-cortical dysfunction is implicated in the pathogenesis of AD/HD. In this study, we investigated prefrontal cortex (PFC) function in SHRSP/Ezo rats by electrophysiological methods and radioreceptor assay.

METHODS

We recorded excitatory postsynaptic potential in layer V pyramidal neurons in the PFC by intracellular recording method to assess synaptic plasticity in the form of long-term potentiation (LTP). We also performed N-methyl-d-aspartate acid (NMDA) receptor binding assay in the PFC and hippocampus using radiolabeled NMDA receptor antagonist [³ H]MK-801.

RESULTS

Theta-burst stimulation induced LTP in the PFC of genetic control, WKY/Ezo, whereas failed to induce LTP in that of SHRSP/Ezo. The K_d value of [³ H]MK-801 binding for NMDA receptors in the PFC of SHRSP/Ezo was higher than in the WKY/Ezo. Neither the B_max nor K_d of [³ H]MK-801 binding in the SHRSP/Ezo hippocampus was significantly different to WKY/Ezo.

CONCLUSION

These results suggest that the AD/HD animal model SHRSP/Ezo has NMDA receptor dysfunction in the PFC.

Dissecting executive control circuits with neuron types

Executive control supports our ability to behave flexibly in accordance with a given situation. In order to fully understand how cortical circuits achieve this task, we need to determine the intrinsic physiological and connection profiles of neuron types and analyze their functional roles during behavior. This article introduces current knowledge regarding neuron type classification in the cortex and reviews our understanding of how each neuron type is incorporated in the functional cortical circuit to implement executive control. Recent work using neuron-type specific imaging/recording has begun to reveal significant functional organizations of pyramidal neurons and their subtypes depending on the layers and long-range projection targets. GABAergic interneurons also make crucial contributions to executive control in a subtype-specific manner. Vasoactive intestinal peptide (VIP)-positive interneurons are preferentially recruited by top-down inputs from higher-order cortical regions and amplify the signals in pyramidal neurons by inhibiting other interneuron subtypes. Particularly in the prefrontal cortex, one of the hierarchically highest cortices, executive control signals are regulated by the VIP neuron-mediated disinhibition and robustly maintained through recurrent connections at a long time scale. The differences and commonalities in the functional organization between sensory areas and the prefrontal cortex are discussed.

Microglial activation is inversely associated with cognition in individuals living with HIV on effective antiretroviral therapy

OBJECTIVE

Despite viral suppression, HIV-associated cognitive impairment persists and may be partially due to persistent immune signalling by cells of the myeloid-lineage. Here, we aimed to understand the contribution of activated microglia located in vulnerable brain regions (e.g. frontal, subcortical) of HIV-infected, virally suppressed (HIV+VS) individuals in relation to cognitive and motor function.

DESIGN

Twenty-one HIV+VS individuals underwent PET with [11C]DPA-713 to image the translocator protein 18 kDa (TSPO), a marker of microglial activation, and completed a comprehensive neuropsychological test battery.

METHODS

Multivariable linear regressions were used to examine the contribution of [11C]DPA-713 binding to cognitive performance.

RESULTS

Higher [11C]DPA-713 binding was associated with lower cognition among HIV+VS individuals. [11C]DPA-713 binding in middle frontal gyrus/frontal cortex, hippocampus/temporal cortex and occipital cortex was inversely associated with performance on a number of cognitive domains, including verbal memory, processing speed/attention/concentration, executive function, working memory and motor function. [C]DPA-713 binding in parietal cortex, cerebellum and thalamus was associated with only specific cognitive domains including visual construction and verbal memory. Binding was not associated with global cognitive performance.

CONCLUSION

The findings add to the growing body of evidence that immune-mediated brain injury may contribute to domain specific, HIV-associated, cognitive vulnerabilities despite viral suppression.

Functional heterogeneity within the rodent lateral orbitofrontal cortex dissociates outcome devaluation and reversal learning deficits

The orbitofrontal cortex (OFC) is critical for updating reward-directed behaviours flexibly when outcomes are devalued or when task contingencies are reversed. Failure to update behaviour in outcome devaluation and reversal learning procedures are considered canonical deficits following OFC lesions in non-human primates and rodents. We examined the generality of these findings in rodents using lesions of the rodent lateral OFC (LO) in instrumental action-outcome and Pavlovian cue-outcome devaluation procedures. LO lesions disrupted outcome devaluation in Pavlovian but not instrumental procedures. Furthermore, although both anterior and posterior LO lesions disrupted Pavlovian outcome devaluation, only posterior LO lesions were found to disrupt reversal learning. Posterior but not anterior LO lesions were also found to disrupt the attribution of motivational value to Pavlovian cues in sign-tracking. These novel dissociable task- and subregion-specific effects suggest a way to reconcile contradictory findings between rodent and non-human primate OFC research.

Contributions of medial prefrontal cortex to decision making involving risk of punishment

The prefrontal cortex (PFC) plays an important role in several forms of cost-benefit decision making. Its contributions to decision making under risk of explicit punishment, however, are not well understood. A rat model was used to investigate the role of the medial PFC (mPFC) and its monoaminergic innervation in a Risky Decision-making Task (RDT), in which rats chose between a small, "safe" food reward and a large, "risky" food reward accompanied by varying probabilities of mild footshock punishment. Inactivation of mPFC increased choice of the large, risky reward when the punishment probability increased across the session ("ascending RDT"), but decreased choice of the large, risky reward when the punishment probability decreased across the session ("descending RDT"). In contrast, enhancement of monoamine availability via intra-mPFC amphetamine reduced choice of the large, risky reward only in the descending RDT. Systemic administration of amphetamine reduced choice of the large, risky reward in both the ascending and descending RDT; however, this reduction was not attenuated by concurrent mPFC inactivation, indicating that mPFC is not a critical locus of amphetamine's effects on risk taking. These findings suggest that mPFC plays an important role in adapting choice behavior in response to shifting risk contingencies, but not necessarily in risk-taking behavior per se.

Anterior cingulate cortex and its input to the basolateral amygdala control innate fear response

Prefrontal brain areas are implicated in the control of fear behavior. However, how prefrontal circuits control fear response to innate threat is poorly understood. Here, we show that the anterior cingulate cortex (ACC) and its input to the basolateral nucleus of amygdala (BLA) contribute to innate fear response to a predator odor in mice. Optogenetic inactivation of the ACC enhances freezing response to fox urine without affecting conditioned freezing. Conversely, ACC stimulation robustly inhibits both innate and conditioned freezing. Circuit tracing and slice patch recordings demonstrate a monosynaptic glutamatergic connectivity of ACC-BLA but no or very sparse ACC input to the central amygdala. Finally, our optogenetic manipulations of the ACC-BLA projection suggest its inhibitory control of innate freezing response to predator odors. Together, our results reveal the role of the ACC and its projection to BLA in innate fear response to olfactory threat stimulus.

Brain circuits that control innate fear response are essential for an animal’s survival. Here, the authors report how the anterior cingulate cortex and its projection to amygdala control the innate fear response in mice.

Mouse MRI shows brain areas relatively larger in males emerge before those larger in females

Sex differences exist in behaviors, disease and neuropsychiatric disorders. Sexual dimorphisms however, have yet to be studied across the whole brain and across a comprehensive time course of postnatal development. Here, we use manganese-enhanced MRI (MEMRI) to longitudinally image male and female C57BL/6J mice across 9 time points, beginning at postnatal day 3. We recapitulate findings on canonically dimorphic areas, demonstrating MEMRI’s ability to study neuroanatomical sex differences. We discover, upon whole-brain volume correction, that neuroanatomical regions larger in males develop earlier than those larger in females. Groups of areas with shared sexually dimorphic developmental trajectories reflect behavioral and functional networks, and expression of genes involved with sex processes. Also, post-pubertal neuroanatomy is highly individualized, and individualization occurs earlier in males. Our results demonstrate the ability of MEMRI to reveal comprehensive developmental differences between male and female brains, which will improve our understanding of sex-specific predispositions to various neuropsychiatric disorders.

Sex differences occur in various aspects of neurodevelopment. Here the authors use manganese-enhanced MRI at nine different postnatal stages to detail the development of structural sex differences in the mouse brain.

Pyramidal Cell Subtypes and Their Synaptic Connections in Layer 5 of Rat Frontal Cortex

The frontal cortical areas make a coordinated response that generates appropriate behavior commands, using individual local circuits with corticostriatal and corticocortical connections in longer time scales than sensory areas. In secondary motor cortex (M2), situated between the prefrontal and primary motor areas, major subtypes of layer 5 corticostriatal cells are crossed-corticostriatal (CCS) cells innervating both sides of striatum, and corticopontine (CPn) cells projecting to the ipsilateral striatum and pontine nuclei. CCS cells innervate CPn cells unidirectionally: the former are therefore hierarchically higher than the latter among L5 corticostriatal cells. CCS cells project directly to both frontal and nonfrontal areas. On the other hand, CPn cells innervate the thalamus and layer 1a of frontal areas, where thalamic fibers relaying basal ganglia outputs are distributed. Thus, CCS cells can make activities of frontal areas in concert with those of nonfrontal area using corticocortical loops, whereas CPn cells are more involved in closed corticostriatal loops than CCS cells. Since reciprocal connections between CPn cells with facilitatory synapses may be related to persistent activity, CPn cells play a key role of longer time constant processes in corticostriatal as well as in corticocortical loops between the frontal areas.

The Lateral Habenula as a Relay of Cortical Information to Process Working Memory

Working memory is a cognitive ability allowing the temporary storage of information to solve problems or adjust behavior. While working memory is known to mainly depend on the medial prefrontal cortex (mPFC), very few is known about how cortical information are relayed subcortically. By its connectivity, the lateral habenula (lHb) might act as a subcortical relay for cortical information. Indeed, the lHb receives inputs from several mPFC subregions, and recent findings suggest a role for the lHb in online processing of spatial information, a fundamental aspect of working memory. In rats, in a delayed non-matching to position paradigm, using focal microinjections of the GABAA agonist muscimol we showed that inactivation of the lHb (16 ng in 0.2 µL per side), as well as disconnection between the prelimbic region of the mPFC (mPFC/PrL, 32 ng in 0.4 µL in one hemisphere) and the lHb (16 ng in 0.2 µL in the lHb in the contralateral hemisphere) impaired working memory. The deficits were unlikely to result from motivational or motor deficits as muscimol did not affect reward collection or cue responding latencies, and did not increase the number of omissions. These results show for the first time the implication of the lHb in mPFC-dependent memory processes, likely as a relay of mPFC/PrL information. They also open new perspectives in the understanding of the top-down processing of high-level cognitive functions.

Coordinated prefrontal-hippocampal activity and navigation strategy-related prefrontal firing during spatial memory formation

Learning the location of relevant places in the environment is crucial for survival. Such capacity is supported by a distributed network comprising the prefrontal cortex and hippocampus, yet it is not fully understood how these structures cooperate during spatial reference memory formation. Hence, we examined neural activity in the prefrontal-hippocampal circuit in mice during acquisition of spatial reference memory. We found that interregional oscillatory coupling increased with learning, specifically in the slow-gamma frequency (20 to 40 Hz) band during spatial navigation. In addition, mice used both spatial and nonspatial strategies to navigate and solve the task, yet prefrontal neuronal spiking and oscillatory phase coupling were selectively enhanced in the spatial navigation strategy. Lastly, a representation of the behavioral goal emerged in prefrontal spiking patterns exclusively in the spatial navigation strategy. These results suggest that reference memory formation is supported by enhanced cortical connectivity and evolving prefrontal spiking representations of behavioral goals.

Three-dimensional intact-tissue sequencing of single-cell transcriptional states

Retrieving high-content gene-expression information while retaining three-dimensional (3D) positional anatomy at cellular resolution has been difficult, limiting integrative understanding of structure and function in complex biological tissues. We developed and applied a technology for 3D intact-tissue RNA sequencing, termed STARmap (spatially-resolved transcript amplicon readout mapping), which integrates hydrogel-tissue chemistry, targeted signal amplification, and in situ sequencing. The capabilities of STARmap were tested by mapping 160 to 1020 genes simultaneously in sections of mouse brain at single-cell resolution with high efficiency, accuracy, and reproducibility. Moving to thick tissue blocks, we observed a molecularly defined gradient distribution of excitatory-neuron subtypes across cubic millimeter-scale volumes (>30,000 cells) and a short-range 3D self-clustering in many inhibitory-neuron subtypes that could be identified and described with 3D STARmap.

Bilateral ischaemic lesions of the medial prefrontal cortex are anxiogenic in the rat

OBJECTIVE

Stroke patients often suffer from delayed disturbances of mood and cognition. In rodents, the prefrontal cortex (PFC) is involved in both higher order cognition and emotion. Our objective was to determine if bilateral focal ischaemic lesions restricted to the medial prefrontal cortex (mPFC) could be used to model post-stroke anxiety and/or cognitive deficits.

METHODS

Groups of adult male Sprague-Dawley rats (n=9) received bilateral injections of either endothelin-1 (ET-1) (400 pmol) or vehicle (artificial cerebrospinal fluid) into the mPFC and were tested at various times using both a test of temporal order memory and in an elevated plus maze. Lesions were verified histologically.

RESULTS

ET-1 lesioned rats had reduced mobility on post-surgery day 8 that had resolved by day 29 at which time they spent significantly more time in the closed arm of the plus maze CONCLUSION: We conclude that ischaemic lesions localised to the mPFC can be used to model post-stroke anxiety in rats.

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.

Dopamine is Required for Activity-Dependent Amplification of Arc mRNA in Developing Postnatal Frontal Cortex

The activity-regulated gene Arc/Arg3.1 encodes a postsynaptic protein crucially involved in glutamatergic synaptic plasticity. Genetic mutations in Arc pathway and altered Arc expression in human frontal cortex have been associated with schizophrenia. Although Arc expression has been reported to vary with age, what mechanisms regulate Arc mRNA levels in frontal cortex during postnatal development remains unclear. Using quantitative mRNA analysis of mouse frontal cortical tissues, we mapped the developmental profiles of Arc expression and found that its mRNA levels are sharply amplified near the end of the second postnatal week, when mouse pups open their eyes for the first time after birth. Surprisingly, electrical stimulation of the frontal cortex before eye-opening is not sufficient to drive the amplification of Arc mRNA. Instead, this amplification needs both electrical stimulation and dopamine D1-type receptor (D1R) activation. Furthermore, visual stimuli-driven amplification of Arc mRNA is also dependent on D1R activation and dopamine neurons located in the ventral midbrain. These results indicate that dopamine is required to drive activity-dependent amplification of Arc mRNA in the developing postnatal frontal cortex and suggest that joint electrical and dopaminergic activation is essential to establish the normal expression pattern of a schizophrenia-associated gene during frontal cortical development.

PV Interneurons: Critical Regulators of E/I Balance for Prefrontal Cortex-Dependent Behavior and Psychiatric Disorders

Elucidating the prefrontal cortical microcircuit has been challenging, given its role in multiple complex behaviors, including working memory, cognitive flexibility, attention, social interaction and emotional regulation. Additionally, previous methodological limitations made it difficult to parse out the contribution of certain neuronal subpopulations in refining cortical representations. However, growing evidence supports a fundamental role of fast-spiking parvalbumin (PV) GABAergic interneurons in regulating pyramidal neuron activity to drive appropriate behavioral responses. Further, their function is heavily diminished in the prefrontal cortex (PFC) in numerous psychiatric diseases, including schizophrenia and autism. Previous research has demonstrated the importance of the optimal balance of excitation and inhibition (E/I) in cortical circuits in maintaining the efficiency of cortical information processing. Although we are still unraveling the mechanisms of information representation in the PFC, the E/I balance seems to be crucial, as pharmacological, chemogenetic and optogenetic approaches for disrupting E/I balance induce impairments in a range of PFC-dependent behaviors. In this review, we will explore two key hypotheses. First, PV interneurons are powerful regulators of E/I balance in the PFC, and help optimize the representation and processing of supramodal information in PFC. Second, diminishing the function of PV interneurons is sufficient to generate an elaborate symptom sequelae corresponding to those observed in a range of psychiatric diseases. Then, using this framework, we will speculate on whether this circuitry could represent a platform for the development of therapeutic interventions in disorders of PFC function.

Quantitative assessment of prefrontal cortex in humans relative to nonhuman primates

A longstanding controversy in neuroscience pertains to differences in human prefrontal cortex (PFC) compared with other primate species; specifically, is human PFC disproportionately large? Distinctively human behavioral capacities related to higher cognition and affect presumably arose from evolutionary modifications since humans and great apes diverged from a common ancestor about 6–8 Mya. Accurate determination of regional differences in the amount of cortical gray and subcortical white matter content in humans, great apes, and Old World monkeys can further our understanding of the link between structure and function of the human brain. Using tissue volume analyses, we show a disproportionately large amount of gray and white matter corresponding to PFC in humans compared with nonhuman primates.

Humans have the largest cerebral cortex among primates. The question of whether association cortex, particularly prefrontal cortex (PFC), is disproportionately larger in humans compared with nonhuman primates is controversial: Some studies report that human PFC is relatively larger, whereas others report a more uniform PFC scaling. We address this controversy using MRI-derived cortical surfaces of many individual humans, chimpanzees, and macaques. We present two parcellation-based PFC delineations based on cytoarchitecture and function and show that a previously used morphological surrogate (cortex anterior to the genu of the corpus callosum) substantially underestimates PFC extent, especially in humans. We find that the proportion of cortical gray matter occupied by PFC in humans is up to 1.9-fold greater than in macaques and 1.2-fold greater than in chimpanzees. The disparity is even more prominent for the proportion of subcortical white matter underlying the PFC, which is 2.4-fold greater in humans than in macaques and 1.7-fold greater than in chimpanzees.

Morphological and functional evidence of increased excitatory signaling in the prelimbic cortex during ethanol withdrawal

Excessive alcohol consumption in humans induces deficits in decision making and emotional processing, which indicates a dysfunction of the prefrontal cortex (PFC). The present study aimed to determine the impact of chronic intermittent ethanol (CIE) inhalation on mouse medial PFC pyramidal neurons. Data were collected 6-8 days into withdrawal from 7 weeks of CIE exposure, a time point when mice exhibit behavioral symptoms of withdrawal. We found that spine maturity in prelimbic (PL) layer 2/3 neurons was increased, while dendritic spines in PL layer 5 neurons or infralimbic (IL) neurons were not affected. Corroborating these morphological observations, CIE enhanced glutamatergic transmission in PL layer 2/3 pyramidal neurons, but not IL layer 2/3 neurons. Contrary to our predictions, these cellular alterations were associated with improved, rather than impaired, performance in reversal learning and strategy switching tasks in the Barnes maze at an earlier stage of chronic ethanol exposure (5-7 days withdrawal from 3 to 4 weeks of CIE), which could result from the anxiety-like behavior associated with ethanol withdrawal. Altogether, this study adds to a growing body of literature indicating that glutamatergic activity in the PFC is upregulated following chronic ethanol exposure, and identifies PL layer 2/3 pyramidal neurons as a sensitive target of synaptic remodeling. It also indicates that the Barnes maze is not suitable to detect deficits in cognitive flexibility in CIE-withdrawn mice.

Rat anterior cingulate cortex recalls features of remote reward locations after disfavoured reinforcements

The anterior cingulate cortex (ACC) encodes information supporting mnemonic and cognitive processes. We show here that a rat’s position can be decoded with high spatiotemporal resolution from ACC activity. ACC neurons encoded the current state of the animal and task, except for brief excursions that sometimes occurred at target feeders. During excursions, the decoded position became more similar to a remote target feeder than the rat’s physical position. Excursions recruited activation of neurons encoding choice and reward, and the likelihood of excursions at a feeder was inversely correlated with feeder preference. These data suggest that the excursion phenomenon was related to evaluating real or fictive choice outcomes, particularly after disfavoured reinforcements. We propose that the multiplexing of position with choice-related information forms a mental model isomorphic with the task space, which can be mentally navigated via excursions to recall multimodal information about the utility of remote locations.

Reciprocal Circuits Linking the Prefrontal Cortex with Dorsal and Ventral Thalamic Nuclei

Reciprocal interactions between the prefrontal cortex (PFC) and thalamus play a critical role in cognition, but the underlying circuits remain poorly understood. Here we use optogenetics to dissect the specificity and dynamics of cortico-thalamo-cortical networks in the mouse brain. We find that cortico-thalamic (CT) neurons in prelimbic PFC project to both mediodorsal (MD) and ventromedial (VM) thalamus, where layer 5 and 6 inputs activate thalamo-cortical (TC) neurons with distinct temporal profiles. We show that TC neurons in MD and VM in turn make distinct connections in PFC, with MD preferentially and strongly activating layer 2/3 cortico-cortical (CC) neurons. Finally, we assess local connections from superficial CC to deep CT neurons, which link thalamo-cortical and cortico-thalamic networks within the PFC. Together our findings indicate that PFC strongly drives neurons in the thalamus, whereas MD and VM indirectly influence reciprocally connected neurons in the PFC, providing a mechanistic understanding of these circuits.

Choice for Drug or Natural Reward Engages Largely Overlapping Neuronal Ensembles in the Infralimbic Prefrontal Cortex

Cue-reward associations form distinct memories that can drive appetitive behaviors and are involved in craving for both drugs and natural rewards. Distinct sets of neurons, so-called neuronal ensembles, in the infralimbic area (IL) of the medial prefrontal cortex (mPFC) play a key role in alcohol seeking. Whether this ensemble is specific for alcohol or controls reward seeking in general remains unclear. Here, we compared IL ensembles formed upon recall of drug (alcohol) or natural reward (saccharin) memories in male Wistar rats. Using an experimental framework that allows identification of two distinct reward-associated ensembles within the same animal, we found that cue-induced seeking of either alcohol or saccharin activated ensembles of similar size and organization, whereby these ensembles consist of largely overlapping neuronal populations. Thus, the IL seems to act as a general integration hub for reward seeking behavior, but also contains subsets of neurons that encode for the different rewards.SIGNIFICANCE STATEMENT Cue-reward associations form distinct memories that can act as drivers of appetitive behaviors and are involved in craving for natural rewards as well as for drugs. Distinct sets of neurons, so-called neuronal ensembles, in the infralimbic area of the mPFC play a key role in cue-triggered reward seeking. However, it is unclear whether these ensembles act as broadly tuned controllers of approach behavior or represent the learned associations between specific cues and rewards. Using an experimental framework that allows identification of two distinct reward-associated ensembles within the same animal we find largely overlapping neuronal populations. Repeated activation by two distinct events could reflect the linking of the two memory traces within the same neuron.

Dopamine D1 receptor density in the mPFC responds to cognitive demands and receptor turnover contributes to general cognitive ability in mice

In both humans and mice, performance on tests of intelligence or general cognitive ability (GCA) is related to dopamine D1 receptor-mediated activity in the prelimbic cortex, and levels of DRD1 mRNA predict the GCA of mice. Here we assessed the turnover rate of D1 receptors as well as the expression level of the D1 chaperone protein (DRiP78) in the medial PPC (mPFC) of mice to determine whether rate of receptor turnover was associated with variations in the GCA of genetically heterogeneous mice. Following assessment of GCA (aggregate performance on four diverse learning tests) mice were administered an irreversible dopamine receptor antagonist (EEDQ), after which the density of new D1 receptors were quantified. GCA was positively correlated with both the rate of D1 receptor recovery and levels of DRiP78. Additionally, the density of D1 receptors was observed to increase within 60 min (or less) in response to intense demands on working memory, suggesting that a pool of immature receptors was available to accommodate high cognitive loads. These results provide evidence that innate general cognitive abilities are related to D1 receptor turnover rates in the prefrontal cortex, and that an intracellular pool of immature D1 receptors are available to accommodate cognitive demands.

Contrasting effects of pre-training on acquisition of operant and radial arm maze tasks in rats

Performing multiple tasks either simultaneously, in rapid alternation or in succession, is routine in daily life. Further, testing rodents in a battery of tests is common both in drug discovery and behavioral phenotyping research. However, learning of new tasks can be influenced by prior experience(s). There has been some research on 'switching cost' involved in the transition from one behavior to another. However, there has been no specific assessment of the effect of learning an operant paradigm on performance in a spatial memory task and vice versa. Accordingly, we evaluated task switching between two forms of learning paradigms, operant conditioning and radial arm maze (RAM) tasks. In experiment 1, rats were trained for operant conditioning with food reward followed by a partially baited RAM task. In experiment 2, rats were trained first on a RAM task followed by operant learning. Pre-training on the operant task, impaired the acquisition of the RAM. On the contrary, pre-training on the RAM enhanced operant performance. Our study reveals significant effects of the test order on task-switching in rats. This knowledge can be useful when framing test sequences in test batteries for drug discovery research and screening genetically modified mice.

Prefrontal cortical-specific differences in behavior and synaptic plasticity between adolescent and adult mice

Adolescence is a highly vulnerable period for the emergence of major neuropsychological disorders and is characterized by decreased cognitive control and increased risk-taking behavior and novelty-seeking. The prefrontal cortex (PFC) is involved in the cognitive control of impulsive and risky behavior. Although the PFC is known to reach maturation later than other cortical areas, little information is available regarding the functional changes from adolescence to adulthood in PFC, particularly compared with other primary cortical areas. This study aims to understand the development of PFC-mediated, compared with non-PFC-mediated, cognitive functions. Toward this aim, we performed cognitive behavioral tasks in adolescent and adult mice and subsequently investigated synaptic plasticity in two different cortical areas. Our results showed that adolescent mice exhibit impaired performance in PFC-dependent cognitive tasks compared with adult mice, whereas their performance in non-PFC-dependent tasks is similar to that of adults. Furthermore, adolescent mice exhibited decreased long-term potentiation (LTP) within upper-layer synapses of the PFC but not the barrel cortex. Blocking GABAA receptor function significantly augments LTP in both the adolescent and adult PFC. No change in intrinsic excitability of PFC pyramidal neurons was observed between adolescent and adult mice. Finally, increased expression of the NR2A subunit of the N-methyl-d-aspartate receptors is found only in the adult PFC, a change that could underlie the emergence of LTP. In conclusion, our results demonstrate physiological and behavioral changes during adolescence that are specific to the PFC and could underlie the reduced cognitive control in adolescents.

NEW & NOTEWORTHY This study reports that adolescent mice exhibit impaired performance in cognitive functions dependent on the prefrontal cortex but not in cognitive functions dependent on other cortical regions. The current results propose reduced synaptic plasticity in the upper layers of the prefrontal cortex as a cellular correlate of this weakened cognitive function. This decreased synaptic plasticity is due to reduced N-methyl-d-aspartate receptor expression but not due to dampened intrinsic excitability or enhanced GABAergic signaling during adolescence.

Expression of the ADHD candidate gene Diras2 in the brain

The distinct subgroup of the Ras family member 2 (DIRAS2) gene has been found to be associated with attention-deficit/hyperactivity disorder (ADHD) in one of our previous studies. This gene is coding for a small Ras GTPase with unknown function. DIRAS2 is highly expressed in the brain. However, the exact neural expression pattern of this gene was unknown so far. Therefore, we investigated the expressional profile of DIRAS2 in the human and murine brain. In the present study, qPCR analyses in the human and in the developing mouse brain, immunocytological double staining on murine hippocampal primary cells and RNA in situ hybridization (ISH) on brain sections of C57BL/6J wild-type mice, have been used to reveal the expression pattern of DIRAS2 in the brain. We could show that DIRAS2 expression in the human brain is the highest in the hippocampus and the cerebral cortex, which is in line with the ISH results in the mouse brain. During mouse brain development, Diras2 levels strongly increase from prenatal to late postnatal stages. Co-expression studies indicate Diras2 expression in glutamatergic and catecholaminergic neurons. Our findings support the idea of DIRAS2 as a candidate gene for ADHD as the timeline of its expression as well as the brain regions and cell types that show Diras2 expression correspond to those assumed to underlie the pathomechanisms of the disease.

Calcineurin knockout mice show a selective loss of small spines

Calcineurin is required for long-term depression and activity-dependent spine shrinkage, and calcineurin mutations have been identified in patients with schizophrenia. Moreover, mice with conditional knockout of calcineurin B (CNB-KO) exhibit behavioral abnormalities suggestive of schizophrenia. Changes in the dendritic spines of these mice, however, have not been investigated. We therefore examined the dendritic spines of CNB-KO mice, and observed a significant reduction in small spines and an increase in large spines in the prefrontal and visual cortices. The effect of CNB-KO on the spine sizes was relatively moderate, possibly due to the presence of spontaneous fluctuations (dynamics) in the dendritic spines themselves. Thus, CNB-KO mice showed a spine phenotype similar to those recently reported in patients with schizophrenia.

Cognitive Functions and Neurodevelopmental Disorders Involving the Prefrontal Cortex and Mediodorsal Thalamus

The mediodorsal nucleus of the thalamus (MD) has been implicated in executive functions (such as planning, cognitive control, working memory, and decision-making) because of its significant interconnectivity with the prefrontal cortex (PFC). Yet, whilst the roles of the PFC have been extensively studied, how the MD contributes to these cognitive functions remains relatively unclear. Recently, causal evidence in monkeys has demonstrated that in everyday tasks involving rapid updating (e.g., while learning something new, making decisions, or planning the next move), the MD and frontal cortex are working in close partnership. Furthermore, researchers studying the MD in rodents have been able to probe the underlying mechanisms of this relationship to give greater insights into how the frontal cortex and MD might interact during the performance of these essential tasks. This review summarizes the circuitry and known neuromodulators of the MD, and considers the most recent behavioral, cognitive, and neurophysiological studies conducted in monkeys and rodents; in total, this evidence demonstrates that MD makes a critical contribution to cognitive functions. We propose that communication occurs between the MD and the frontal cortex in an ongoing, fluid manner during rapid cognitive operations, via the means of efference copies of messages passed through transthalamic routes; the conductance of these messages may be modulated by other brain structures interconnected to the MD. This is similar to the way in which other thalamic structures have been suggested to carry out forward modeling associated with rapid motor responding and visual processing. Given this, and the marked thalamic pathophysiology now identified in many neuropsychiatric disorders, we suggest that changes in the different subdivisions of the MD and their interconnections with the cortex could plausibly give rise to a number of the otherwise disparate symptoms (including changes to olfaction and cognitive functioning) that are associated with many different neuropsychiatric disorders. In particular, we will focus here on the cognitive symptoms of schizophrenia and suggest testable hypotheses about how changes to MD-frontal cortex interactions may affect cognitive processes in this disorder.

Translational evaluation of translocator protein as a marker of neuroinflammation in schizophrenia

Positron emission tomography (PET) imaging with radiotracers that target translocator protein 18 kDa (TSPO) has become a popular approach to assess putative neuroinflammatory processes and associated microglia activation in psychotic illnesses. It remains unclear, however, whether TSPO imaging can accurately capture low-grade inflammatory processes such as those present in schizophrenia and related disorders. Therefore, we evaluated the validity of TSPO as a disease-relevant marker of inflammation using a translational approach, which combined neurodevelopmental and neurodegenerative mouse models with PET imaging in patients with recent-onset schizophrenia and matched controls. Using an infection-mediated neurodevelopmental mouse model, we show that schizophrenia-relevant behavioral abnormalities and increased inflammatory cytokine expression are associated with reduced prefrontal TSPO levels. On the other hand, TSPO was markedly upregulated in a mouse model of acute neurodegeneration and reactive gliosis, which was induced by intrahippocampal injection of kainic acid. In both models, the changes in TSPO levels were not restricted to microglia but emerged in various cell types, including microglia, astrocytes and vascular endothelial cells. Human PET imaging using the second-generation TSPO radiotracer [11C]DPA-713 revealed a strong trend towards reduced TSPO binding in the middle frontal gyrus of patients with recent-onset schizophrenia, who were previously shown to display increased levels of inflammatory cytokines in peripheral and central tissues. Together, our findings challenge the common assumption that central low-grade inflammation in schizophrenia is mirrored by increased TSPO expression or ligand binding. Our study further underscores the need to interpret altered TSPO binding in schizophrenia with caution, especially when measures of TSPO are not complemented with other markers of inflammation. Unless more selective microglial markers are available for PET imaging, quantification of cytokines and other inflammatory biomarkers, along with their molecular signaling pathways, may be more accurate in attempts to characterize inflammatory profiles in schizophrenia and other mental disorders that lack robust reactive gliosis.

Fluid network dynamics in the prefrontal cortex during multiple strategy switching

Coordinated shifts of neuronal activity in the prefrontal cortex are associated with strategy adaptations in behavioural tasks, when animals switch from following one rule to another. However, network dynamics related to multiple-rule changes are scarcely known. We show how firing rates of individual neurons in the prelimbic and cingulate cortex correlate with the performance of rats trained to change their navigation multiple times according to allocentric and egocentric strategies. The concerted population activity exhibits a stable firing during the performance of one rule but shifted to another neuronal firing state when a new rule is learnt. Interestingly, when the same rule is presented a second time within the same session, neuronal firing does not revert back to the original neuronal firing state, but a new activity-state is formed. Our data indicate that neuronal firing of prefrontal cortical neurons represents changes in strategy and task-performance rather than specific strategies or rules.

Population activity of neurons in the prefrontal cortex has been shown to represent cognitive strategy and rules. Here the authors report that when the same rule is repeated on multiple occasions in the task, it is accompanied each time by a new prefrontal firing rate state.

Dopamine Development in the Mouse Orbital Prefrontal Cortex Is Protracted and Sensitive to Amphetamine in Adolescence

The prefrontal cortex (PFC) is divided into subregions, including the medial and orbital prefrontal cortices. Dopamine connectivity in the medial PFC (mPFC) continues to be established throughout adolescence as the result of the continuous growth of axons that innervated the nucleus accumbens (NAcc) prior to adolescence. During this period, dopamine axons remain vulnerable to environmental influences, such as drugs used recreationally by humans. The developmental trajectory of the orbital prefrontal dopamine innervation remains almost completely unstudied. Nonetheless, the orbital PFC (oPFC) is critical for some of the most complex functions of the PFC and is disrupted by drugs of abuse, both in adolescent humans and rodents. Here, we use quantitative neuroanatomy, axon-initiated viral-vector recombination, and pharmacology in mice to determine the spatiotemporal development of the dopamine innervation to the oPFC and its vulnerability to amphetamine in adolescence. We find that dopamine innervation to the oPFC also continues to increase during adolescence and that this increase is due to the growth of new dopamine axons to this region. Furthermore, amphetamine in adolescence dramatically reduces the number of presynaptic sites on oPFC dopamine axons. In contrast, dopamine innervation to the piriform cortex is not protracted across adolescence and is not impacted by amphetamine exposure during adolescence, indicating that dopamine development during adolescence is a uniquely prefrontal phenomenon. This renders these fibers, and the PFC in general, particularly vulnerable to environmental risk factors during adolescence, such as recreational drug use.

Development of inhibitory synaptic inputs on layer 2/3 pyramidal neurons in the rat medial prefrontal cortex

Inhibitory control of pyramidal neurons plays a major role in governing the excitability in the brain. While spatial mapping of inhibitory inputs onto pyramidal neurons would provide important structural data on neuronal signaling, studying their distribution at the single cell level is difficult due to the lack of easily identifiable anatomical proxies. Here, we describe an approach where in utero electroporation of a plasmid encoding for fluorescently tagged gephyrin into the precursors of pyramidal cells along with ionotophoretic injection of Lucifer Yellow can reliably and specifically detect GABAergic synapses on the dendritic arbour of single pyramidal neurons. Using this technique and focusing on the basal dendritic arbour of layer 2/3 pyramidal cells of the medial prefrontal cortex, we demonstrate an intense development of GABAergic inputs onto these cells between postnatal days 10 and 20. While the spatial distribution of gephyrin clusters was not affected by the distance from the cell body at postnatal day 10, we found that distal dendritic segments appeared to have a higher gephyrin density at later developmental stages. We also show a transient increase around postnatal day 20 in the percentage of spines that are carrying a gephyrin cluster, indicative of innervation by a GABAergic terminal. Since the precise spatial arrangement of synaptic inputs is an important determinant of neuronal responses, we believe that the method described in this work may allow a better understanding of how inhibition settles together with excitation, and serve as basics for further modelling studies focusing on the geometry of dendritic inhibition during development.

Opposing Cholinergic and Serotonergic Modulation of Layer 6 in Prefrontal Cortex

Prefrontal cortex is a hub for attention processing and receives abundant innervation from cholinergic and serotonergic afferents. A growing body of evidence suggests that acetylcholine (ACh) and serotonin (5-HT) have opposing influences on tasks requiring attention, but the underlying neurophysiology of their opposition is unclear. One candidate target population is medial prefrontal layer 6 pyramidal neurons, which provide feedback modulation of the thalamus, as well as feed-forward excitation of cortical interneurons. Here, we assess the response of these neurons to ACh and 5-HT using whole cell recordings in acute brain slices from mouse cortex. With application of exogenous agonists, we show that individual layer 6 pyramidal neurons are bidirectionally-modulated, with ACh and 5-HT exerting opposite effects on excitability across a number of concentrations. Next, we tested the responses of layer 6 pyramidal neurons to optogenetic release of endogenous ACh or 5-HT. These experiments were performed in brain slices from transgenic mice expressing channelrhodopsin in either ChAT-expressing cholinergic neurons or Pet1-expressing serotonergic neurons. Light-evoked endogenous neuromodulation recapitulated the effects of exogenous neurotransmitters, showing opposing modulation of layer 6 pyramidal neurons by ACh and 5-HT. Lastly, the addition of 5-HT to either endogenous or exogenous ACh significantly suppressed the excitation of pyramidal neurons in prefrontal layer 6. Taken together, this work suggests that the major corticothalamic layer of prefrontal cortex is a substrate for opposing modulatory influences on neuronal activity that could have implications for regulation of attention.

How can preclinical mouse models be used to gain insight into prefrontal cortex dysfunction in obsessive-compulsive disorder?

Obsessive-compulsive disorder is a debilitating psychiatric disorder that is characterised by perseverative thoughts and behaviours. Cognitive and affective disturbances play a central role in this illness, and it is therefore not surprising that clinical neuroimaging studies have demonstrated widespread alterations in prefrontal cortex functioning in patients. Preclinical mouse experimental systems provide the opportunity to gain mechanistic insight into the neurobiological changes underlying prefrontal cortex dysfunction through new technologies that allow measurement and manipulation of activity in discrete neural populations in awake, behaving mice. However, recent preclinical research has focused on striatal dysfunction, and has therefore provided relatively little insight regarding the role of the prefrontal cortex in obsessive-compulsive disorder-relevant behaviours. Here, we will discuss a number of translational prefrontal cortex-dependent paradigms, including obsessive-compulsive disorder-relevant tasks that produce compulsive responding, and how they can be leveraged in this context. Drawing on recent examples that have led to mechanistic insight about specific genes, cell types and circuits that mediate prefrontal cortex contributions to distinct aspects of cognition, we will provide a framework for applying similar strategies to identify neural mechanisms underlying obsessive-compulsive disorder-relevant behavioural domains. We propose that research using clinically relevant paradigms will accelerate translation of findings from preclinical mouse models, thus supporting the development of novel therapeutics targeted to specific pathophysiological mechanisms.

NLGN1 and NLGN2 in the prefrontal cortex: their role in memory consolidation and strengthening

The prefrontal cortex (PFC) is critical for memory formation, but the underlying molecular mechanisms are poorly understood. Clinical and animal model studies have shown that changes in PFC excitation and inhibition are important for cognitive functions as well as related disorders. Here, we discuss recent findings revealing the roles of the excitatory and inhibitory synaptic proteins neuroligin 1 (NLGN1) and NLGN2 in the PFC in memory formation and modulation of memory strength. We propose that shifts in NLGN1 and NLGN2 expression in specific excitatory and inhibitory neuronal subpopulations in response to experience regulate the dynamic processes of memory consolidation and strengthening. Because excitatory/inhibitory imbalances accompany neuropsychiatric disorders in which strength and flexibility of representations play important roles, understanding these mechanisms may suggest novel therapies.

Prefrontal Cortical Opioids and Dysregulated Motivation: A Network Hypothesis

Loss of inhibitory control over appetitively motivated behavior occurs in multiple psychiatric disorders, including drug abuse, behavioral addictions, and eating disorders with binge features. In this opinion article, novel actions of μ-opioid peptides in the prefrontal cortex (PFC) that could contribute to inhibitory control deficits will be discussed. Evidence has accrued to suggest that excessive intra-PFC μ-opioid receptor (μ-OR) signaling alters the PFC response to excitatory drive, resulting in supernormal and incoherent recruitment of multiple PFC output pathways. Affected pathways include functionally opposed PFC→hypothalamus 'appetitive driver' and PFC→striatum 'appetitive limiter' projections. This network perturbation engenders disorganized, impulsive appetitive responses. Evidence supporting this hypothesis from human imaging and animal studies will be discussed, and combinatorial drug treatments targeting μ-ORs and specific PFC subcortical targets will be explored.