Prefrontal cortical dynorphin peptidergic transmission constrains threat-driven behavioral and network states.

Prefrontal cortical (PFC) circuits provide top-down control of threat reactivity. This includes ventromedial PFC (vmPFC) circuitry, which plays a role in suppressing fear-related behavioral states. Dynorphin (Dyn) has been implicated in mediating negative affect and maladaptive behaviors induced by severe threats and is expressed in limbic circuits, including the vmPFC. However, there is a critical knowledge gap in our understanding of how vmPFC Dyn-expressing neurons and Dyn transmission detect threats and regulate expression of defensive behaviors. Here, we demonstrate that Dyn cells are broadly activated by threats and release Dyn locally in the vmPFC to limit passive defensive behaviors. We further demonstrate that vmPFC Dyn-mediated signaling promotes a switch of vmPFC networks to a fear-related state. In conclusion, we reveal a previously unknown role of vmPFC Dyn neurons and Dyn neuropeptidergic transmission in suppressing defensive behaviors in response to threats via state-driven changes in vmPFC networks.

The transcriptional response of cortical neurons to concussion reveals divergent fates after injury.

Traumatic brain injury (TBI) is a risk factor for neurodegeneration, however little is known about how different neuron types respond to this kind of injury. In this study, we follow neuronal populations over several months after a single mild TBI (mTBI) to assess long ranging consequences of injury at the level of single, transcriptionally defined neuronal classes. We find that the stress responsive Activating Transcription Factor 3 (ATF3) defines a population of cortical neurons after mTBI. We show that neurons that activate ATF3 upregulate stress-related genes while repressing many genes, including commonly used markers for these cell types. Using an inducible reporter linked to ATF3, we genetically mark damaged cells to track them over time. Notably, we find that a population in layer V undergoes cell death acutely after injury, while another in layer II/III survives long term and retains the ability to fire action potentials. To investigate the mechanism controlling layer V neuron death, we genetically silenced candidate stress response pathways. We found that the axon injury responsive kinase MAP3K12, also known as dual leucine zipper kinase (DLK), is required for the layer V neuron death. This work provides a rationale for targeting the DLK signaling pathway as a therapeutic intervention for traumatic brain injury. Beyond this, our novel approach to track neurons after a mild, subclinical injury can inform our understanding of neuronal susceptibility to repeated impacts.

The guanine nucleotide exchange factor RapGEF2 is required for ERK-dependent immediate-early gene (Egr1) activation during fear memory formation.

The MAP kinase ERK is important for neuronal plasticity underlying associative learning, yet specific molecular pathways for neuronal ERK activation are undetermined. RapGEF2 is a neuron-specific cAMP sensor that mediates ERK activation. We investigated whether it is required for cAMP-dependent ERK activation leading to other downstream neuronal signaling events occurring during associative learning, and if RapGEF2-dependent signaling impairments affect learned behavior. Camk2α-cre+/-::RapGEF2fl/fl mice with depletion of RapGEF2 in hippocampus and amygdala exhibit impairments in context- and cue-dependent fear conditioning linked to corresponding impairment in Egr1 induction in these two brain regions. Camk2α-cre+/-::RapGEF2fl/fl mice show decreased RapGEF2 expression in CA1 and dentate gyrus associated with abolition of pERK and Egr1, but not of c-Fos induction, following fear conditioning, impaired freezing to context after fear conditioning, and impaired cAMP-dependent long-term potentiation at perforant pathway and Schaffer collateral synapses in hippocampal slices ex vivo. RapGEF2 expression is largely eliminated in basolateral amygdala, also involved in fear memory, in Camk2α-cre+/-::RapGEF2fl/fl mice. Neither Egr1 nor c-fos induction in BLA after fear conditioning, nor cue-dependent fear learning, are affected by ablation of RapGEF2 in BLA. However, Egr1 induction (but not that of c-fos) in BLA is reduced after restraint stress-augmented fear conditioning, as is freezing to cue after restraint stress-augmented fear conditioning, in Camk2α-cre+/-::RapGEF2fl/fl mice. Cyclic AMP-dependent GEFs have been genetically associated as risk factors for schizophrenia, a disorder associated with cognitive deficits. Here we show a functional link between one of them, RapGEF2, and cognitive processes involved in associative learning in amygdala and hippocampus.

Prefrontal cortical dynorphin peptidergic transmission constrains threat-driven behavioral and network states.

Prefrontal cortical (PFC) circuits provide top-down control of threat reactivity. This includes ventromedial PFC (vmPFC) circuitry, which plays a role in suppressing fear-related behavioral states. Dynorphin (Dyn) has been implicated in mediating negative affect and mal-adaptive behaviors induced by severe threats and is expressed in limbic circuits, including the vmPFC. However, there is a critical knowledge gap in our understanding of how vmPFC Dyn-expressing neurons and Dyn transmission detect threats and regulate expression of defensive behaviors. Here, we demonstrate that Dyn cells are broadly activated by threats and release Dyn locally in the vmPFC to limit passive defensive behaviors. We further demonstrate that vmPFC Dyn-mediated signaling promotes a switch of vmPFC networks to a fear-related state. In conclusion, we reveal a previously unknown role of vmPFC Dyn neurons and Dyn neuropeptidergic transmission in suppressing defensive behaviors in response to threats via state-driven changes in vmPFC networks.

Risk Assessment of Maladaptive Behaviors in Adolescents: Nutrition, Screen Time, Prenatal Exposure, Childhood Adversities - Adolescent Brain Cognitive Development Study.

PURPOSE: We aimed to identify significant contributing factors to the risk of maladaptive behaviors, such as alcohol use disorder or obesity, in children. To achieve this, we utilized the extensive adolescent brain cognitive development data set, which encompasses a wide range of environmental, social, and nutritional factors. METHODS: We divided our sample into equal sets (test, validation; n = 3,415 each). On exploratory factor analysis, six factor domains were identified as most significant (fat/sugar intake, screen time, and prenatal alcohol exposure, parental aggressiveness, hyperactivity, family violence, parental education, and family income) and used to stratify the children into low- (n = 975), medium- (n = 967), high- (n = 977) risk groups. Regression models were used to analyze the relationship between identified risk groups, and differences in reward sensitivity, and behavioral problems at 2-year follow-up. RESULTS: The functional magnetic resonance imaging analyses showed reduced activation in several brain regions during reward or loss anticipation in high/medium-risk (vs. low-risk) children on a monetary incentive delay task. High-risk children exhibited heightened middle frontal cortex activity when receiving large rewards. They also displayed increased impulsive and motivated reward-seeking behaviors, along with behavioral problems. These findings replicated in our validation set, and a negative correlation between middle frontal cortexthickness and impulsivity behavior was observed in high-risk children. DISCUSSION: Our findings show altered reward function and increased impulsiveness in high-risk adolescents. This study has implications for early risk identification and the development of prevention strategies for maladaptive behaviors in children, particularly those at high risk.

Dynorphin / kappa-opioid receptor regulation of excitation-inhibition balance toggles afferent control of prefrontal cortical circuits in a pathway-specific manner.

The medial prefrontal cortex (mPFC) controls behavior via connections with limbic excitatory afferents that engage various inhibitory motifs to shape mPFC circuit function. The dynorphin (Dyn) / kappa-opioid receptor (KOR) system is highly enriched in the mPFC, and its dysregulation is implicated in neuropsychiatric disorders. However, it is unclear how the Dyn / KOR system modulates excitatory and inhibitory circuits that are integral for mPFC information processing and behavioral control. Here, we provide a circuit-based framework wherein mPFC Dyn / KOR signaling regulates excitation-inhibition balance by toggling which afferents drive mPFC neurons. Dyn / KOR regulation of afferent inputs is pathway-specific. Dyn acting on presynaptic KORs inhibits glutamate release from afferent inputs to the mPFC, including the basolateral amygdala (BLA), paraventricular nucleus of the thalamus, and contralateral cortex. The majority of excitatory synapses to mPFC neurons, including those from the ventral hippocampus (VH), do not express presynaptic KOR, rendering them insensitive to Dyn / KOR modulation. Dyn / KOR signaling also suppresses afferent-driven recruitment of specific inhibitory sub-networks, providing a basis for Dyn to disinhibit mPFC circuits. Specifically, Dyn / KOR signaling preferentially suppresses SST interneuron- relative to PV interneuron-mediated inhibition. Selective KOR action on afferents or within mPFC microcircuits gates how distinct limbic inputs drive spiking in mPFC neurons. Presynaptic Dyn / KOR signaling decreases KOR-positive input-driven (e.g. BLA) spiking of mPFC neurons. In contrast, KOR-negative input recruitment of mPFC neurons is enhanced by Dyn / KOR signaling via suppression of mPFC inhibitory microcircuits. Thus, by acting on distinct circuit elements, Dyn / KOR signaling shifts KOR-positive and negative afferent control of mPFC circuits, providing mechanistic insights into the role of neuropeptides in shaping mPFC function. Together, these findings highlight the utility of targeting the mPFC Dyn / KOR system as a means to treat neuropsychiatric disorders characterized by dysregulation in mPFC integration of long-range afferents with local inhibitory microcircuits.

Dissociable control of motivation and reinforcement by distinct ventral striatal dopamine receptors.

Dopamine release in striatal circuits, including the nucleus accumbens (NAc), tracks separable features of reward such as motivation and reinforcement. However, the cellular and circuit mechanisms by which dopamine receptors transform dopamine release into distinct constructs of reward remain unclear. Here, we show that dopamine D3 receptor (D3R) signaling in the NAc drives motivated behavior by regulating local NAc microcircuits. Furthermore, D3Rs co-express with dopamine D1 receptors (D1Rs), which regulate reinforcement, but not motivation. Paralleling dissociable roles in reward function, we report non-overlapping physiological actions of D3R and D1R signaling in NAc neurons. Our results establish a novel cellular framework wherein dopamine signaling within the same NAc cell type is physiologically compartmentalized via actions on distinct dopamine receptors. This structural and functional organization provides neurons in a limbic circuit with the unique ability to orchestrate dissociable aspects of reward-related behaviors that are relevant to the etiology of neuropsychiatric disorders.

Integrative multi-dimensional characterization of striatal projection neuron heterogeneity in adult brain.

Striatal projection neurons (SPNs) are traditionally segregated into two subpopulations expressing dopamine (DA) D1-like or D2-like receptors. However, this dichotomy is challenged by recent evidence. Functional and expression studies raise important questions: do SPNs co-express different DA receptors, and do these differences reflect unique striatal spatial distributions and expression profiles? Using RNAscope in mouse striatum, we report heterogenous SPN subpopulations distributed across dorsal-ventral and rostral-caudal axes. SPN subpopulations co-express multiple DA receptors, including D1 and D2 (D1/2R) and D1 and D3. Our integrative approach using single-nuclei multi-omics analyses provides a simple consensus to describe SPNs across diverse datasets, connecting it to complementary spatial mapping. Combining RNAscope and multi-omics shows D1/2R SPNs further separate into distinct subtypes according to spatial organization and conserved marker genes. Each SPN cell type contributes uniquely to genetic risk for neuropsychiatric diseases. Our results bridge anatomy and transcriptomics to offer new understandings of striatal neuron heterogeneity.

A systematic review of the biological mediators of fat taste and smell.

Taste and smell play a key role in our ability to perceive foods. Overconsumption of highly palatable energy-dense foods can lead to increased caloric intake and obesity. Thus there is growing interest in the study of the biological mediators of fat taste and associated olfaction as potential targets for pharmacologic and nutritional interventions in the context of obesity and health. The number of studies examining mechanisms underlying fat taste and smell has grown rapidly in the last 5 years. Therefore, the purpose of this systematic review is to summarize emerging evidence examining the biological mechanisms of fat taste and smell. A literature search was conducted of studies published in English between 2014 and 2021 in adult humans and animal models. Database searches were conducted using PubMed, EMBASE, Scopus, and Web of Science for key terms including fat/lipid, taste, and olfaction. Initially, 4,062 articles were identified through database searches, and a total of 84 relevant articles met inclusion and exclusion criteria and are included in this review. Existing literature suggests that there are several proteins integral to fat chemosensation, including cluster of differentiation 36 (CD36) and G protein-coupled receptor 120 (GPR120). This systematic review will discuss these proteins and the signal transduction pathways involved in fat detection. We also review neural circuits, key brain regions, ingestive cues, postingestive signals, and genetic polymorphism that play a role in fat perception and consumption. Finally, we discuss the role of fat taste and smell in the context of eating behavior and obesity.

Prefrontal Interneurons: Populations, Pathways, and Plasticity Supporting Typical and Disordered Cognition in Rodent Models.

Prefrontal cortex (PFC) inhibitory microcircuits regulate the gain and timing of pyramidal neuron firing, coordinate neural ensemble interactions, and gate local and long-range neural communication to support adaptive cognition and contextually tuned behavior. Accordingly, perturbations of PFC inhibitory microcircuits are thought to underlie dysregulated cognition and behavior in numerous psychiatric diseases and relevant animal models. This review, based on a Mini-Symposium presented at the 2022 Society for Neuroscience Meeting, highlights recent studies providing novel insights into: (1) discrete medial PFC (mPFC) interneuron populations in the mouse brain; (2) mPFC interneuron connections with, and regulation of, long-range mPFC afferents; and (3) circuit-specific plasticity of mPFC interneurons. The contributions of such populations, pathways, and plasticity to rodent cognition are discussed in the context of stress, reward, motivational conflict, and genetic mutations relevant to psychiatric disease.

Prenatal Caffeine Exposure Is Linked to Elevated Sugar Intake and BMI, Altered Reward Sensitivity, and Aberrant Insular Thickness in Adolescents: An ABCD Investigation.

Prenatal caffeine exposure (PCE) has been positively associated with elevated body mass index (BMI) in children. Why this association occurs is unclear, but it is possible that PCE alters the in utero development of brain structures associated with food preference, leading to more total sugar intake (TSI, grams) later in childhood. To test this hypothesis, we investigated if PCE (daily/weekly/ 0.01) of excessive PCE (vs. no exposure) with elevated BMI (daily/weekly/daily limit; consistent in boys and girls), increased TSI (daily) and insular thickness (daily/weekly), as well as low middle frontal cortex (MFC) activation (daily). Our sub-analysis revealed an association of daily/weekly PCE (vs. no exposure) with increased gram sugar intake from soft drinks. We also identified a positive relationship of excessive PCE with elevated TSI and increased insular thickness (a key gustatory region), while in a Sobel test, reward sensitivity (reduced brain reactivity to reward anticipation in MFC; tracks reward outcomes) mediated (Test statistic = 2.23; p = 0.02) the PCE-linked BMI changes in adolescents. Our findings suggest that excessive PCE might be detrimental to frontal lobe development and altered reward sensitivity to food, thereby increasing risk for elevated TSI and obesity. Our results support recommendations to limit caffeine intake during pregnancy.

Dynorphin/kappa opioid receptor system regulation on amygdaloid circuitry: Implications for neuropsychiatric disorders.

Amygdaloid circuits are involved in a variety of emotional and motivation-related behaviors and are impacted by stress. The amygdala expresses several neuromodulatory systems, including opioid peptides and their receptors. The Dynorphin (Dyn)/kappa opioid receptor (KOR) system has been implicated in the processing of emotional and stress-related information and is expressed in brain areas involved in stress and motivation. Dysregulation of the Dyn/KOR system has also been implicated in various neuropsychiatric disorders. However, there is limited information about the role of the Dyn/KOR system in regulating amygdala circuitry. Here, we review the literature on the (1) basic anatomy of the amygdala, (2) functional regulation of synaptic transmission by the Dyn/KOR system, (3) anatomical architecture and function of the Dyn/KOR system in the amygdala, (4) regulation of amygdala-dependent behaviors by the Dyn/KOR system, and (5) future directions for the field. Future work investigating how the Dyn/KOR system shapes a wide range of amygdala-related behaviors will be required to increase our understanding of underlying circuitry modulation by the Dyn/KOR system. We anticipate that continued focus on the amygdala Dyn/KOR system will also elucidate novel ways to target the Dyn/KOR system to treat neuropsychiatric disorders.

Neuropeptide System Regulation of Prefrontal Cortex Circuitry: Implications for Neuropsychiatric Disorders.

Neuropeptides, a diverse class of signaling molecules in the nervous system, modulate various biological effects including membrane excitability, synaptic transmission and synaptogenesis, gene expression, and glial cell architecture and function. To date, most of what is known about neuropeptide action is limited to subcortical brain structures and tissue outside of the central nervous system. Thus, there is a knowledge gap in our understanding of neuropeptide function within cortical circuits. In this review, we provide a comprehensive overview of various families of neuropeptides and their cognate receptors that are expressed in the prefrontal cortex (PFC). Specifically, we highlight dynorphin, enkephalin, corticotropin-releasing factor, cholecystokinin, somatostatin, neuropeptide Y, and vasoactive intestinal peptide. Further, we review the implication of neuropeptide signaling in prefrontal cortical circuit function and use as potential therapeutic targets. Together, this review summarizes established knowledge and highlights unknowns of neuropeptide modulation of neural function underlying various biological effects while offering insights for future research. An increased emphasis in this area of study is necessary to elucidate basic principles of the diverse signaling molecules used in cortical circuits beyond fast excitatory and inhibitory transmitters as well as consider components of neuropeptide action in the PFC as a potential therapeutic target for neurological disorders. Therefore, this review not only sheds light on the importance of cortical neuropeptide studies, but also provides a comprehensive overview of neuropeptide action in the PFC to serve as a roadmap for future studies in this field.

Fundamentals of the Dynorphins/Kappa Opioid Receptor System: From Distribution to Signaling and Function.

This chapter provides a general introduction to the dynorphins (DYNs)/kappa opioid receptor (KOR) system, including DYN peptides, neuroanatomy of the DYNs/KOR system, cellular signaling, and in vivo behavioral effects of KOR activation and inhibition. It is intended to serve as a primer for the book and to provide a basic background for the chapters in the book.

Dynorphin/Kappa-Opioid Receptor System Modulation of Cortical Circuitry.

Cortical circuits control a plethora of behaviors, from sensation to cognition. The cortex is enriched with neuropeptides and receptors that play a role in information processing, including opioid peptides and their cognate receptors. The dynorphin (DYN)/kappa-opioid receptor (KOR) system has been implicated in the processing of sensory and motivationally-charged emotional information and is highly expressed in cortical circuits. This is important as dysregulation of DYN/KOR signaling in limbic and cortical circuits has been implicated in promoting negative affect and cognitive deficits in various neuropsychiatric disorders. However, research investigating the role of this system in controlling cortical circuits and computations therein is limited. Here, we review the (1) basic anatomy of cortical circuits, (2) anatomical architecture of the cortical DYN/KOR system, (3) functional regulation of cortical synaptic transmission and microcircuit function by the DYN/KOR system, (4) regulation of behavior by the cortical DYN/KOR system, (5) implications for the DYN/KOR system for human health and disease, and (6) future directions and unanswered questions for the field. Further work elucidating the role of the DYN/KOR system in controlling cortical information processing and associated behaviors will be of importance to increasing our understanding of principles underlying neuropeptide modulation of cortical circuits, mechanisms underlying sensation and perception, motivated and emotional behavior, and cognition. Increased emphasis in this area of study will also aid in the identification of novel ways to target the DYN/KOR system to treat neuropsychiatric disorders.

Serotonergic inhibition of responding for conditioned but not primary reinforcers.

Serotonin is widely implicated as a modulator of brain reward function. However, laboratory studies have not yielded a consensus on which specific reward-related processes are influenced by serotonin and in what manner. Here we explored the role of serotonin in cue-reward learning in mice. In a first series of experiments, we found that acute administration of the serotonin reuptake inhibitors citalopram, fluoxetine, or duloxetine all reduced lever pressing reinforced on an FR1 schedule with presentation of a cue that had been previously paired with delivery of food. However, citalopram had no effect on responding that was reinforced with both cue and food on an FR1 schedule. Furthermore, citalopram did not affect nose poke responses that produced no auditory, visual, or proprioceptive cues but were reinforced with food pellets on a progressive ratio schedule. We next performed region-specific knock out of tryptophan hydroxylase-2 (Tph2), the rate-limiting enzyme in serotonin synthesis. Viral delivery of Cre recombinase was targeted to dorsal or median raphe nuclei (DRN, MRN), the major sources of ascending serotonergic projections. MRN but not DRN knockouts were impaired in development of cue-elicited approach during Pavlovian conditioning; both groups were subsequently hyper-responsive when lever pressing for cue presentation. The inhibitory effect of citalopram was attenuated in DRN but not MRN knockouts. Our findings are in agreement with prior studies showing serotonin to suppress responding for conditioned reinforcers. Furthermore, these results suggest an inhibitory role of MRN serotonin neurons in the initial attribution of motivational properties to a reward-predictive cue, but not in its subsequent maintenance. In contrast, the DRN appears to promote the reduction of motivational value attached to a cue when it is presented repeatedly in the absence of primary reward.

Microglial activation elicits a negative affective state through prostaglandin-mediated modulation of striatal neurons.

Microglia are activated in many neurological diseases and have been suggested to play an important role in the development of affective disorders including major depression. To investigate how microglial signaling regulates mood, we used bidirectional chemogenetic manipulations of microglial activity in mice. Activation of microglia in the dorsal striatum induced local cytokine expression and a negative affective state characterized by anhedonia and aversion, whereas inactivation of microglia blocked aversion induced by systemic inflammation. Interleukin-6 signaling and cyclooxygenase-1 mediated prostaglandin synthesis in the microglia were critical for the inflammation-induced aversion. Correspondingly, microglial activation led to a prostaglandin-dependent reduction of the excitability of striatal neurons. These findings demonstrate a mechanism by which microglial activation causes negative affect through prostaglandin-dependent modulation of striatal neurons and indicate that interference with this mechanism could milden the depressive symptoms in somatic and psychiatric diseases involving microglial activation.

Cooperative synaptic and intrinsic plasticity in a disynaptic limbic circuit drive stress-induced anhedonia and passive coping in mice.

Stress promotes negative affective states, which include anhedonia and passive coping. While these features are in part mediated by neuroadaptations in brain reward circuitry, a comprehensive framework of how stress-induced negative affect may be encoded within key nodes of this circuit is lacking. Here, we show in a mouse model for stress-induced anhedonia and passive coping that these phenomena are associated with increased synaptic strength of ventral hippocampus (VH) excitatory synapses onto D1 medium spiny neurons (D1-MSNs) in the nucleus accumbens medial shell (NAcmSh), and with lateral hypothalamus (LH)-projecting D1-MSN hyperexcitability mediated by decreased inwardly rectifying potassium channel (IRK) function. Stress-induced negative affective states are prevented by depotentiation of VH to NAcmSh synapses, restoring Kir2.1 function in D1R-MSNs, or disrupting co-participation of these synaptic and intrinsic adaptations in D1-MSNs. In conclusion, our data provide strong evidence for a disynaptic pathway controlling maladaptive emotional behavior.