Stress Impairs Prefrontal Cortical Function via D1 Dopamine Receptor Interactions With Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels

The meso-connectomes of mouse, marmoset, and macaque: network organization and the emergence of higher cognition

The recent publications of the inter-areal connectomes for mouse, marmoset, and macaque cortex have allowed deeper comparisons across rodent vs. primate cortical organization. In general, these show that the mouse has very widespread, "all-to-all" inter-areal connectivity (i.e. a "highly dense" connectome in a graph theoretical framework), while primates have a more modular organization. In this review, we highlight the relevance of these differences to function, including the example of primary visual cortex (V1) which, in the mouse, is interconnected with all other areas, therefore including other primary sensory and frontal areas. We argue that this dense inter-areal connectivity benefits multimodal associations, at the cost of reduced functional segregation. Conversely, primates have expanded cortices with a modular connectivity structure, where V1 is almost exclusively interconnected with other visual cortices, themselves organized in relatively segregated streams, and hierarchically higher cortical areas such as prefrontal cortex provide top-down regulation for specifying precise information for working memory storage and manipulation. Increased complexity in cytoarchitecture, connectivity, dendritic spine density, and receptor expression additionally reveal a sharper hierarchical organization in primate cortex. Together, we argue that these primate specializations permit separable deconstruction and selective reconstruction of representations, which is essential to higher cognition.

Interaction Between HCN and Slack Channels Regulates mPFC Pyramidal Cell Excitability in Working Memory Circuits

The ability of monkeys and rats to carry out spatial working memory tasks has been shown to depend on the persistent firing of pyramidal cells in the prefrontal cortex (PFC), arising from recurrent excitatory connections on dendritic spines. These spines express hyperpolarization-activated cyclic nucleotide-gated (HCN) channels whose open state is increased by cAMP signaling, and which markedly alter PFC network connectivity and neuronal firing. In traditional neural circuits, activation of these non-selective cation channels leads to neuronal depolarization and increased firing rate. Paradoxically, cAMP activation of HCN channels in PFC pyramidal cells reduces working memory-related neuronal firing. This suggests that activation of HCN channels may hyperpolarize rather than depolarize these neurons. The current study tested the hypothesis that Na+ influx through HCN channels activates Slack Na+-activated K+ (KNa) channels to hyperpolarize the membrane. We have found that HCN and Slack KNa channels co-immunoprecipitate in cortical extracts and that, by immunoelectron microscopy, they colocalize at postsynaptic spines of PFC pyramidal neurons. A specific blocker of HCN channels, ZD7288, reduces KNa current in pyramidal cells that express both HCN and Slack channels, but has no effect on KNa currents in an HEK cell line expressing Slack without HCN channels, indicating that blockade of HCN channels in neurons reduces K+ current indirectly by lowering Na+ influx. Activation of HCN channels by cAMP in a cell line expressing a Ca2+ reporter results in elevation of cytoplasmic Ca2+, but the effect of cAMP is reversed if the HCN channels are co-expressed with Slack channels. Finally, we used a novel pharmacological blocker of Slack channels to show that inhibition of Slack in rat PFC improves working memory performance, an effect previously demonstrated for blockers of HCN channels. Our results suggest that the regulation of working memory by HCN channels in PFC pyramidal neurons is mediated by an HCN-Slack channel complex that links activation HCN channels to suppression of neuronal excitability.

A Molecular Framework for Delirium

Over 2.6 million adults over the age of 65 develop delirium each year in the United States (US). Delirium is associated with a significant increase in mortality and the US health care costs associated with delirium are estimated at over $164 billion annually. Despite the prevalence of the condition, the molecular pathophysiology of delirium remains unexplained, limiting the development of pharmacotherapies. Delirious patients can be identified by prominent impairments in attention and working memory (WM), two cognitive domains that localize to the dorsolateral prefrontal cortex (dlPFC). The dlPFC is also a key site for Alzheimer's disease (AD) pathology, and given the high risk of delirium in AD patients, suggests that efforts at understanding delirium might focus on the dlPFC as a final common endpoint for cognitive changes. Preclinical studies of the dlPFC reproduce many of the pharmacological observations made of delirious patients, including sensitivity to anticholinergics and an 'inverted U' pattern of dependence on monoaminergic input, with diminished performance outside a narrow range of signaling. Medications like guanfacine, which influence the dlPFC in the context of attention-deficit/hyperactivity disorder (ADHD), have emerged as therapies for delirium and motivate a detailed understanding of the influence of α-2 agonists on WM. In this review, I will discuss the neural circuitry and molecular mechanisms underlying WM and the function of the dlPFC. Localizing the cognitive deficits that are commonly seen in delirious patients may help identify new molecular targets for this highly prevalent disease.

Dynamic Network Connectivity: from monkeys to humans

Human brain imaging research using functional MRI (fMRI) has uncovered flexible variations in the functional connectivity between brain regions. While some of this variability likely arises from the pattern of information flow through circuits, it may also be influenced by rapid changes in effective synaptic strength at the molecular level, a phenomenon called Dynamic Network Connectivity (DNC) discovered in non-human primate circuits. These neuromodulatory molecular mechanisms are found in layer III of the macaque dorsolateral prefrontal cortex (dlPFC), the site of the microcircuits shown by Goldman-Rakic to be critical for working memory. This research has shown that the neuromodulators acetylcholine, norepinephrine, and dopamine can rapidly change the strength of synaptic connections in layer III dlPFC by (1) modifying the depolarization state of the post-synaptic density needed for NMDA receptor neurotransmission and (2) altering the open state of nearby potassium channels to rapidly weaken or strengthen synaptic efficacy and the strength of persistent neuronal firing. Many of these actions involve increased cAMP-calcium signaling in dendritic spines, where varying levels can coordinate the arousal state with the cognitive state. The current review examines the hypothesis that some of the dynamic changes in correlative strength between cortical regions observed in human fMRI studies may arise from these molecular underpinnings, as has been seen when pharmacological agents or genetic alterations alter the functional connectivity of the dlPFC consistent with the macaque physiology. These DNC mechanisms provide essential flexibility but may also confer vulnerability to malfunction when dysregulated in cognitive disorders.

Stress-related cellular pathophysiology as a crosstalk risk factor for neurocognitive and psychiatric disorders

In this narrative review, we examine biological processes linking psychological stress and cognition, with a focus on how psychological stress can activate multiple neurobiological mechanisms that drive cognitive decline and behavioral change. First, we describe the general neurobiology of the stress response to define neurocognitive stress reactivity. Second, we review aspects of epigenetic regulation, synaptic transmission, sex hormones, photoperiodic plasticity, and psychoneuroimmunological processes that can contribute to cognitive decline and neuropsychiatric conditions. Third, we explain mechanistic processes linking the stress response and neuropathology. Fourth, we discuss molecular nuances such as an interplay between kinases and proteins, as well as differential role of sex hormones, that can increase vulnerability to cognitive and emotional dysregulation following stress. Finally, we explicate several testable hypotheses for stress, neurocognitive, and neuropsychiatric research. Together, this work highlights how stress processes alter neurophysiology on multiple levels to increase individuals' risk for neurocognitive and psychiatric disorders, and points toward novel therapeutic targets for mitigating these effects. The resulting models can thus advance dementia and mental health research, and translational neuroscience, with an eye toward clinical application in cognitive and behavioral neurology, and psychiatry.

Localization of PDE4D, HCN1 channels, and mGluR3 in rhesus macaque entorhinal cortex may confer vulnerability in Alzheimer's disease

Alzheimer's disease cortical tau pathology initiates in the layer II cell clusters of entorhinal cortex, but it is not known why these specific neurons are so vulnerable. Aging macaques exhibit the same qualitative pattern of tau pathology as humans, including initial pathology in layer II entorhinal cortex clusters, and thus can inform etiological factors driving selective vulnerability. Macaque data have already shown that susceptible neurons in dorsolateral prefrontal cortex express a "signature of flexibility" near glutamate synapses on spines, where cAMP-PKA magnification of calcium signaling opens nearby potassium and hyperpolarization-activated cyclic nucleotide-gated channels to dynamically alter synapse strength. This process is regulated by PDE4A/D, mGluR3, and calbindin, to prevent toxic calcium actions; regulatory actions that are lost with age/inflammation, leading to tau phosphorylation. The current study examined whether a similar "signature of flexibility" expresses in layer II entorhinal cortex, investigating the localization of PDE4D, mGluR3, and HCN1 channels. Results showed a similar pattern to dorsolateral prefrontal cortex, with PDE4D and mGluR3 positioned to regulate internal calcium release near glutamate synapses, and HCN1 channels concentrated on spines. As layer II entorhinal cortex stellate cells do not express calbindin, even when young, they may be particularly vulnerable to magnified calcium actions and ensuing tau pathology.

Interaction Between HCN and Slack Channels Regulates mPFC Pyramidal Cell Excitability and Working Memory

The ability of monkeys and rats to carry out spatial working memory tasks has been shown to depend on the persistent firing of pyramidal cells in the prefrontal cortex (PFC), arising from recurrent excitatory connections on dendritic spines. These spines express hyperpolarization-activated cyclic nucleotide-gated (HCN) channels whose open state is increased by cAMP signaling, and which markedly alter PFC network connectivity and neuronal firing. In traditional neural circuits, activation of these non-selective cation channels leads to neuronal depolarization and increased firing rate. Paradoxically, cAMP activation of HCN channels in PFC pyramidal cells reduces working memory-related neuronal firing. This suggests that activation of HCN channels may hyperpolarize rather than depolarize these neurons. The current study tested the hypothesis that Na+ influx through HCN channels activates Slack Na+-activated K+ (KNa) channels to hyperpolarize the membrane. We have found that HCN and Slack KNa channels coimmunoprecipitate in cortical extracts and that, by immunoelectron microscopy, they colocalize at postsynaptic spines of PFC pyramidal neurons. A specific blocker of HCN channels, ZD7288, reduces KNa current in pyramidal cells that express both HCN and Slack channels, but has no effect on KNa currents in an HEK cell line expressing Slack without HCN channels, indicating that blockade of HCN channels in neurons reduces K+ +current indirectly by lowering Na+ influx. Activation of HCN channels by cAMP in a cell line expressing a Ca2+ reporter results in elevation of cytoplasmic Ca2+, but the effect of cAMP is reversed if the HCN channels are co-expressed with Slack channels. Finally, we used a novel pharmacological blocker of Slack channels to show that inhibition of Slack in rat PFC improves working memory performance, an effect previously demonstrated for blockers of HCN channels. Our results suggest that the regulation of working memory by HCN channels in PFC pyramidal neurons is mediated by an HCN-Slack channel complex that links activation HCN channels to suppression of neuronal excitability.

Bi-directional modulation of hyperpolarization-activated cation currents (Ih) by ethanol in rat hippocampal CA3 pyramidal neurons

It is widely acknowledged that ethanol (EtOH) can alter many neuronal functions, including synaptic signaling, firing discharge, and membrane excitability, through its interaction with multiple membrane proteins and intracellular pathways. Previous work has demonstrated that EtOH enhances the firing rate of hippocampal GABAergic interneurons and thus the presynaptic GABA release at CA1 and CA3 inhibitory synapses through a positive modulation of the hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels. Activation of HCN channels produce an inward current, commonly called I_h, which plays an essential role in generating/regulating specific neuronal activities in GABAergic interneurons and principal glutamatergic pyramidal neurons such as those in the CA3 subregion. Since the direct effect of EtOH on HCN channels expressed in CA3 pyramidal neurons was not thoroughly elucidated, we investigated the possible interaction between EtOH and HCN channels and the impact on excitability and postsynaptic integration of these neurons. Patch-clamp recordings were performed in single CA3 pyramidal neurons from acute male rat coronal hippocampal slices. Our results show that EtOH modulates HCN-mediated I_h in a concentration-dependent and bi-directional manner, with a positive modulation at lower (20 mM) and an inhibitory action at higher (60-80 mM) concentrations. The modulation of I_h by EtOH was mimicked by forskolin, antagonized by different drugs that selectively interfere with the AC/cAMP/PKA intracellular pathway, as well as by the selective HCN inhibitor ZD7288. Altogether, these data further support the evidence that HCN channels may represent an important molecular target through which EtOH may regulate neuronal activity.

Elevated prefrontal dopamine interferes with the stress-buffering properties of behavioral control in female rats

Stress-linked disorders are more prevalent in women than in men and differ in their clinical presentation. Thus, investigating sex differences in factors that promote susceptibility or resilience to stress outcomes, and the circuit elements that mediate their effects, is important. In male rats, instrumental control over stressors engages a corticostriatal system involving the prelimbic cortex (PL) and dorsomedial striatum (DMS) that prevent many of the sequelae of stress exposure. Interestingly, control does not buffer against stress outcomes in females, and here, we provide evidence that the instrumental controlling response in females is supported instead by the dorsolateral striatum (DLS). Additionally, we used in vivo microdialysis, fluorescent in situ hybridization, and receptor subtype pharmacology to examine the contribution of prefrontal dopamine (DA) to the differential impact of behavioral control. Although both sexes preferentially expressed D1 receptor mRNA in PL GABAergic neurons, there were robust sex differences in the dynamic properties of prefrontal DA during controllable stress. Behavioral control potently attenuated stress-induced DA efflux in males, but not females, who showed a sustained DA increase throughout the entire stress session. Importantly, PL D1 receptor blockade (SCH 23390) shifted the proportion of striatal activity from the DLS to the DMS in females and produced the protective effects of behavioral control. These findings suggest a sex-selective mechanism in which elevated DA in the PL biases instrumental responding towards prefrontal-independent striatal circuitry, thereby eliminating the protective impact of coping with stress.

The Aversive Lens: Stress effects on the prefrontal-cingulate cortical pathways that regulate emotion

ARNSTEN, A.F.T., M.K.P. Joyce and A.C. Roberts. The Aversive Lens: Stress effects on the prefrontal-cingulate cortical pathways that regulate emotion. NEUROSCI BIOBEHAV REV XXX-XXX, 2022. The symptoms of major-depressive-disorder include psychic pain and anhedonia, i.e. seeing the world through an "aversive lens". The neurobiology underlying this shift in worldview is emerging. Here these data are reviewed, focusing on how activation of subgenual cingulate (BA25) induces an "aversive lens", and how higher prefrontal cortical (PFC) areas (BA46/10/32) provide top-down regulation of BA25 but are weakened by excessive dopamine and norepinephrine release during stress exposure, and dendritic spine loss with chronic stress exposure. These changes may generate an attractor state, which maintains the brain under the control of BA25, requiring medication or neuromodulatory treatments to return connectivity to a more flexible state. In line with this hypothesis, effective anti-depressant treatments reduce the activity of BA25 and restore top-down regulation by higher circuits, e.g. as seen with SSRI medications, ketamine, deep brain stimulation of BA25, or rTMS to strengthen dorsolateral PFC. This research has special relevance in an era of chronic stress caused by the COVID19 pandemic, political unrest and threat of climate change.

Methylation pattern and mRNA expression of synapse-relevant genes in the MAM model of schizophrenia in the time-course of adolescence

Schizophrenia is highly heritable and aggregating in families, but genetics alone does not exclusively explain the pathogenesis. Many risk factors, including childhood trauma, viral infections, migration, and the use of cannabis, are associated with schizophrenia. Adolescence seems to be the critical period where symptoms of the disease manifest. This work focuses on studying an epigenetic regulatory mechanism (the role of DNA methylation) and its interaction with mRNA expression during development, with a particular emphasis on adolescence. The presumptions regarding the role of aberrant neurodevelopment in schizophrenia were tested in the Methyl-Azoxy-Methanol (MAM) animal model. MAM treatment induces neurodevelopmental disruptions and behavioral deficits in off-springs of the treated animals reminiscent of those observed in schizophrenia and is thus considered a promising model for studying this pathology. On a gestational day-17, adult pregnant rats were treated with the antimitotic agent MAM. Experimental animals were divided into groups and subgroups according to substance treatment (MAM and vehicle agent [Sham]) and age of analysis (pre-adolescent and post-adolescent). Methylation and mRNA expression analysis of four candidate genes, which are often implicated in schizophrenia, with special emphasis on the Dopamine hypothesis i.e., Dopamine receptor D₂ (Drd2), and the "co-factors" Disrupted in schizophrenia 1 (DISC1), Synaptophysin (Syp), and Dystrobrevin-binding protein 1 (Dtnbp1), was performed in the Gyrus cingulum (CING) and prefrontal cortex (PFC). Data were analyzed to observe the effect of substance treatment between groups and the impact of adolescence within-group. We found reduced pre-adolescent expression levels of Drd2 in both brain areas under the application of MAM. The "co-factor genes" did not show high deviations in mRNA expression levels but high alterations of methylation rates under the application of MAM (up to ~20%), which diminished in the further time course, reaching a comparable level like in Sham control animals after adolescence. The pre-adolescent reduction in DRD2 expression might be interpreted as downregulation of the receptor due to hyperdopaminergic signaling from the ventral tegmental area (VTA), eventually even to both investigated brain regions. The notable alterations of methylation rates in the three analyzed co-factor genes might be interpreted as attempt to compensate for the altered dopaminergic neurotransmission.

Unusual Molecular Regulation of Dorsolateral Prefrontal Cortex Layer III Synapses Increases Vulnerability to Genetic and Environmental Insults in Schizophrenia

Schizophrenia is associated with reduced numbers of spines and dendrites from layer III of the dorsolateral prefrontal cortex (dlPFC), the layer that houses the recurrent excitatory microcircuits that subserve working memory and abstract thought. Why are these synapses so vulnerable, while synapses in deeper or more superficial layers are little affected? This review describes the special molecular properties that govern layer III neurotransmission and neuromodulation in the primate dlPFC and how they may render these circuits particularly vulnerable to genetic and environmental insults. These properties include a reliance on NMDA receptor rather than AMPA receptor neurotransmission; cAMP (cyclic adenosine monophosphate) magnification of calcium signaling near the glutamatergic synapse of dendritic spines; and potassium channels opened by cAMP/PKA (protein kinase A) signaling that dynamically alter network strength, with built-in mechanisms to take dlPFC "offline" during stress. A variety of genetic and/or environmental insults can lead to the same phenotype of weakened layer III connectivity, in which mechanisms that normally strengthen connectivity are impaired and those that normally weaken connectivity are intensified. Inflammatory mechanisms, such as increased kynurenic acid and glutamate carboxypeptidase II expression, are especially detrimental to layer III dlPFC neurotransmission and modulation, mimicking genetic insults. The combination of genetic and inflammatory insults may cross the threshold into pathology.

Excitatory and inhibitory effects of HCN channel modulation on excitability of layer V pyramidal cells

Dendrites of cortical pyramidal cells are densely populated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, a.k.a. Ih channels. Ih channels are targeted by multiple neuromodulatory pathways, and thus are one of the key ion-channel populations regulating the pyramidal cell activity. Previous observations and theories attribute opposing effects of the Ih channels on neuronal excitability due to their mildly hyperpolarized reversal potential. These effects are difficult to measure experimentally due to the fine spatiotemporal landscape of the Ih activity in the dendrites, but computational models provide an efficient tool for studying this question in a reduced but generalizable setting. In this work, we build upon existing biophysically detailed models of thick-tufted layer V pyramidal cells and model the effects of over- and under-expression of Ih channels as well as their neuromodulation. We show that Ih channels facilitate the action potentials of layer V pyramidal cells in response to proximal dendritic stimulus while they hinder the action potentials in response to distal dendritic stimulus at the apical dendrite. We also show that the inhibitory action of the Ih channels in layer V pyramidal cells is due to the interactions between Ih channels and a hot zone of low voltage-activated Ca2+ channels at the apical dendrite. Our simulations suggest that a combination of Ih-enhancing neuromodulation at the proximal part of the apical dendrite and Ih-inhibiting modulation at the distal part of the apical dendrite can increase the layer V pyramidal excitability more than either of the two alone. Our analyses uncover the effects of Ih-channel neuromodulation of layer V pyramidal cells at a single-cell level and shed light on how these neurons integrate information and enable higher-order functions of the brain.

Evidence for the contribution of HCN1 gene polymorphism (rs1501357) to working memory at both behavioral and neural levels in schizophrenia patients and healthy controls

Gene HCN1 polymorphism (rs1501357) has been proposed to be one of the candidate risk genes for schizophrenia in the second report of the Psychiatric Genomics Consortium–Schizophrenia Workgroup. Although animal studies repeatedly showed a role of this gene in working memory, its contribution to working memory in human samples, especially in schizophrenia patients, is still unknown. To explore the association between rs1501357 and working memory at both behavioral (Study 1) and neural (Study 2) levels, the current study involved two independent samples. Study 1 included 876 schizophrenia patients and 842 healthy controls, all of whom were assessed on a 2-back task, a dot pattern expectancy task (DPX), and a digit span task. Study 2 included 56 schizophrenia patients and 155 healthy controls, all of whom performed a 2-back task during functional magnetic resonance imaging (fMRI) scanning. In both studies, we consistently found significant genotype-by-diagnosis interaction effects. For Study 1, the interaction effects were significant for the three tasks. Patients carrying the risk allele performed worse than noncarriers, while healthy controls showed the opposite pattern. For Study 2, the interaction effects were observed at the parietal cortex and the medial frontal cortex. Patients carrying the risk allele showed increased activation at right parietal cortex and increased deactivation at the medial frontal cortex, while healthy controls showed the opposite pattern. These results suggest that the contributions of rs1501357 to working memory capability vary in different populations (i.e., schizophrenia patients vs. healthy controls), which expands our understanding of the functional impact of the HCN1 gene. Future studies should examine its associations with other cognitive functions.

Amyloid Beta Alters Prefrontal-dependent Functions Along with its Excitability and Synaptic Plasticity in Male Rats

Prefrontal cortex (PFC)-related functions, such as working memory (WM) and cognitive flexibility (CF), are among the first to be altered at early stages of Alzheimer's disease (AD). Likewise, transgenic AD models carrying different AD-related mutations, mostly linked to the overproduction of amyloid beta (Aβ) and other peptides, show premature behavioral and functional symptoms associated with PFC alterations. However, little is known about the effects of intracerebral or intra-PFC Aβ infusion on WM and CF, as well as on pyramidal cell excitability and plasticity. Thus, here we evaluated the effects of a single Aβ injection, directly into the PFC, or its intracerebroventricular (icv) application, on PFC-dependent behaviors and on the intrinsic and synaptic properties of layer V pyramidal neurons in PFC slices. We found that a single icv Aβ infusion reduced learning and performance of a delayed non-matching-to-sample WM task and prevented reversal learning in a matching-to-sample version of the task, several weeks after its infusion. The inhibition of WM performance was reproduced more potently by a single PFC Aβ infusion and was associated with Aβ accumulation. This behavioral disruption was related to increased layer V pyramidal cell firing, larger sag membrane potential, increased fast after-hyperpolarization and a failure to sustain synaptic long-term potentiation, even leading to long-term depression, at both the hippocampal-PFC pathway and intracortical synapses. These findings show that Aβ can affect PFC excitability and synaptic plasticity balance, damaging PFC-dependent functions, which could constitute the foundations of the early alterations in executive functions in AD patients.

Dopamine D1 receptor-mediated β-arrestin signaling: Insight from pharmacology, biology, behavior, and neurophysiology

The awareness of the potential importance of functional selectivity/biased signaling has led to the discovery of biased compounds as both research tools and novel drugs. A major pan-receptor focus has been to identify GPCR-selective ligands that have bias in G protein-dependent vs. β-arrestin related signaling. Although this field has exploded during the past two decades, it is only recently that highly β-arrestin biased ligands for the dopamine D1 receptor were reported. We now summarize important pharmacological, molecular, and cellular studies relevant to D1-mediated β-arrestin-related signaling. It is intriguing that many results emerged from behavioral and physiological studies implying that bias toward or against D1-mediated β-arrestin either can improve or impair functional outcomes. We discuss the importance of understanding the translatability of cell and animal models to have more precise functional targeting to harness the value of this signaling pathway.

Dose-Dependent Regulation on Prefrontal Neuronal Working Memory by Dopamine D1 Agonists: Evidence of Receptor Functional Selectivity-Related Mechanisms

Low doses of dopamine D1 agonists improve working memory-related behavior, but high doses eliminate the improvement, thus yielding an ‘inverted-U’ dose-response curve. This dose-dependency also occurs at the single neuron level in the prefrontal cortex where the cellular basis of working memory is represented. Because signaling mechanisms are unclear, we examined this process at the neuron population level. Two D1 agonists (2-methyldihydrexidine and CY208,243) having different signaling bias were tested in rats performing a spatial working memory-related T-maze task. 2-Methyldihydrexidine is slightly bias toward D1-mediated β-arrestin-related signaling as it is a full agonist at adenylate cyclase and a super-agonist at β-arrestin recruitment, whereas CY208,243 is slightly bias toward D1-mediated cAMP signaling as it has relatively high intrinsic activity at adenylate cyclase, but is a partial agonist at β-arrestin recruitment. Both compounds had the expected inverted U dose-dependency in modulating prefrontal neuronal activities, albeit with important differences. Although CY208,243 was superior in improving the strength of neuronal outcome sensitivity to the working memory-related choice behavior in the T-maze, 2-methyldihydrexidine better reduced neuron-to-neuron variation. Interestingly, at the neuron population level, both drugs affected the percentage, uniformity, and ensemble strength of neuronal sensitivity in a complicated dose-dependent fashion, but the overall effect suggested higher efficiency and potency of 2-methyldihydrexidine compared to CY208,243. The differences between 2-methyldihydrexidine and CY208,243 may be related to their specific D1 signaling. These results suggest that D1-related dose-dependent regulation of working memory can be modified differentially by functionally selective ligands, theoretically increasing the balance between desired and undesired effects.

A dopamine D1 agonist vs. methylphenidate in modulating prefrontal cortical working memory

Methylphenidate is used widely to treat symptoms of attention-deficit/hyperactivity disorder (ADHD), but like other stimulants has significant side effects. This study utilized a rodent model (spontaneously hypertensive rat) of spatial working memory (sWM) to compare the effects of methylphenidate with the novel dopamine D₁-like receptor agonist 2-methyldihydrexidine. Acute oral administration of methylphenidate (1.5 mg/kg) caused sWM improvement in half of the tested rats, but impairment in the others. Both improvement or impairment were eliminated by administration of the D₁ antagonist SCH39266 directly into the prefrontal cortex (PFC). Conversely, 2-methyldihydrexidine showed greater sWM improvement compared to methylphenidate without significant impairment in any subject. Its effects correlated negatively with vehicle-treated baseline performance (i.e., rats with lower baseline performance improved more than rats with higher baseline performance). These behavioral effects were associated with neural activities in the PFC. Single neuron firing rate was changed, leading to the alteration in neuronal preference to correct or error behavioral responses. Overall, 2-methyldihydrexidine was superior to methylphenidate in decreasing the neuronal preference, prospectively, in the animals whose behavior was improved. In contrast, methylphenidate, but not 2-methyldihydrexidine, significantly decreased neuronal preference, retrospectively, in those animals who had impaired performance. These results suggest that a D₁ agonist may be more effective than methylphenidate in regulating sWM-related behavior through neural modulation of the PFC, and thus may be superior to methylphenidate or other stimulants as ADHD pharmacotherapy. Significance Statement Methylphenidate is effective in ADHD by its indirect agonist stimulation of dopamine and/or adrenergic receptors, but the precise effects on specific targets are unclear. We compared methylphenidate to a dopamine D₁ receptor-selective agonist by investigating effects on working memory occurring via neural modulation in the prefrontal cortex. The data suggest that pharmacological treatment selectively targeting the dopamine D₁ may offer a superior approach to ADHD pharmacotherapy.

cAMP-PKA cascade: An outdated topic for depression?

Depression is a common neuropsychiatric disorder characterized by persistent depressed mood and causes serious socioeconomic burden worldwide. Hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, deficiency of monoamine transmitters, neuroinflammation and abnormalities of the gut flora are strongly associated with the onset of depression. The cyclic AMP (cAMP)/protein kinase A (PKA) cascade, a major cross-species cellular signaling pathway, is supposed as important player and regulator of depression onset by controlling synaptic plasticity, cytokinesis, transcriptional regulation and HPA axis. In the central nervous system, the cAMP-PKA cascade can dynamically shape neural circuits by enhancing synaptic plasticity, and affect K⁺ channels by phosphorylating Kir4.1, thereby regulating neuronal excitation. The reduction of cAMP-PKA cascade affects neuronal excitation as well as synaptic plasticity, ultimately leading to pathological outcome of depression, while activation of cAMP-PKA cascade would provide a rapid antidepressant effect. In this review, we proposed to reconsider the function of cAMP-PKA cascade, especially in the rapid antidepressant effect. Local activation or indirect activation of PKA through adjusting anchor proteins would provide new idea for acute treatment of depression.

Intercellular Communication in the Central Nervous System as Deduced by Chemical Neuroanatomy and Quantitative Analysis of Images: Impact on Neuropharmacology

In the last decades, new evidence on brain structure and function has been acquired by morphological investigations based on synergic interactions between biochemical anatomy approaches, new techniques in microscopy and brain imaging, and quantitative analysis of the obtained images. This effort produced an expanded view on brain architecture, illustrating the central nervous system as a huge network of cells and regions in which intercellular communication processes, involving not only neurons but also other cell populations, virtually determine all aspects of the integrative function performed by the system. The main features of these processes are described. They include the two basic modes of intercellular communication identified (i.e., wiring and volume transmission) and mechanisms modulating the intercellular signaling, such as cotransmission and allosteric receptor-receptor interactions. These features may also open new possibilities for the development of novel pharmacological approaches to address central nervous system diseases. This aspect, with a potential major impact on molecular medicine, will be also briefly discussed.

Integrative Brain Dynamics in Childhood Bullying Victimization: Cognitive and Emotional Convergence Associated With Stress Psychopathology

Bullying victimization is a form of psychological stress that is associated with poor outcomes in the areas of mental health and learning. Although the emotional maladjustment and memory impairment following interpersonal stress are well documented, the mechanisms of complex cerebral dysfunctions have neither been outlined nor studied in depth in the context of childhood bullying victimization. As a contribution to the cross-disciplinary field of developmental psychology and neuroscience, we review the neuropathophysiology of early life stress, as well as general psychological stress to synthesize the data and clarify the versatile dynamics within neuronal networks linked to bullying victimization. The stress-induced neuropsychological cascade and associated cerebral networks with a focus on cognitive and emotional convergence are described. The main findings are that stress-evoked neuroendocrine reactivity relates to neuromodulation and limbic dysregulation that hinder emotion processing and executive functioning such as semantic cognition, cognitive flexibility, and learning. Developmental aspects and interacting neural mechanisms linked to distressed cognitive and emotional processing are pinpointed and potential theory-of-mind nuances in targets of bullying are presented. The results show that childhood stress psychopathology is associated with a complex interplay where the major role belongs to, but is not limited to, the amygdala, fusiform gyrus, insula, striatum, and prefrontal cortex. This interplay contributes to the sensitivity toward facial expressions, poor cognitive reasoning, and distress that affect behavioral modulation and emotion regulation. We integrate the data on major brain dynamics in stress neuroactivity that can be associated with childhood psychopathology to help inform future studies that are focused on the treatment and prevention of psychiatric disorders and learning problems in bullied children and adolescents.

Direct Regulation of Hyperpolarization-Activated Cyclic-Nucleotide Gated (HCN1) Channels by Cannabinoids

Cannabinoids are a broad class of molecules that act primarily on neurons, affecting pain sensation, appetite, mood, learning, and memory. In addition to interacting with specific cannabinoid receptors (CBRs), cannabinoids can directly modulate the function of various ion channels. Here, we examine whether cannabidiol (CBD) and Δ9-tetrahydrocannabinol (THC), the most prevalent phytocannabinoids in Cannabis sativa, can regulate the function of hyperpolarization-activated cyclic-nucleotide-gated (HCN1) channels independently of CBRs. HCN1 channels were expressed in Xenopus oocytes since they do not express CBRs, and the effects of cannabinoid treatment on HCN1 currents were examined by a two-electrode voltage clamp. We observe opposing effects of CBD and THC on HCN1 current, with CBD acting to stimulate HCN1 function, while THC inhibited current. These effects persist in HCN1 channels lacking the cyclic-nucleotide binding domain (HCN1ΔCNBD). However, changes to membrane fluidity, examined by treating cells with TX-100, inhibited HCN1 current had more pronounced effects on the voltage-dependence and kinetics of activation than THC, suggesting this is not the primary mechanism of HCN1 regulation by cannabinoids. Our findings may contribute to the overall understanding of how cannabinoids may act as promising therapeutic molecules for the treatment of several neurological disorders in which HCN function is disturbed.

The Signaling and Pharmacology of the Dopamine D1 Receptor

The dopamine D1 receptor (D1R) is a Gα_s/olf-coupled GPCR that is expressed in the midbrain and forebrain, regulating motor behavior, reward, motivational states, and cognitive processes. Although the D1R was initially identified as a promising drug target almost 40 years ago, the development of clinically useful ligands has until recently been hampered by a lack of suitable candidate molecules. The emergence of new non-catechol D1R agonists, biased agonists, and allosteric modulators has renewed clinical interest in drugs targeting this receptor, specifically for the treatment of motor impairment in Parkinson's Disease, and cognitive impairment in neuropsychiatric disorders. To develop better therapeutics, advances in ligand chemistry must be matched by an expanded understanding of D1R signaling across cell populations in the brain, and in disease states. Depending on the brain region, the D1R couples primarily to either Gα_s or Gα_olf through which it activates a cAMP/PKA-dependent signaling cascade that can regulate neuronal excitability, stimulate gene expression, and facilitate synaptic plasticity. However, like many GPCRs, the D1R can signal through multiple downstream pathways, and specific signaling signatures may differ between cell types or be altered in disease. To guide development of improved D1R ligands, it is important to understand how signaling unfolds in specific target cells, and how this signaling affects circuit function and behavior. In this review, we provide a summary of D1R-directed signaling in various neuronal populations and describe how specific pathways have been linked to physiological and behavioral outcomes. In addition, we address the current state of D1R drug development, including the pharmacology of newly developed non-catecholamine ligands, and discuss the potential utility of D1R-agonists in Parkinson's Disease and cognitive impairment.

Neuromodulation of prefrontal cortex cognitive function in primates: the powerful roles of monoamines and acetylcholine

The primate prefrontal cortex (PFC) subserves our highest order cognitive operations, and yet is tremendously dependent on a precise neurochemical environment for proper functioning. Depletion of noradrenaline and dopamine, or of acetylcholine from the dorsolateral PFC (dlPFC), is as devastating as removing the cortex itself, and serotonergic influences are also critical to proper functioning of the orbital and medial PFC. Most neuromodulators have a narrow inverted U dose response, which coordinates arousal state with cognitive state, and contributes to cognitive deficits with fatigue or uncontrollable stress. Studies in monkeys have revealed the molecular signaling mechanisms that govern the generation and modulation of mental representations by the dlPFC, allowing dynamic regulation of network strength, a process that requires tight regulation to prevent toxic actions, e.g., as occurs with advanced age. Brain imaging studies in humans have observed drug and genotype influences on a range of cognitive tasks and on PFC circuit functional connectivity, e.g., showing that catecholamines stabilize representations in a baseline-dependent manner. Research in monkeys has already led to new treatments for cognitive disorders in humans, encouraging future research in this important field.

Mechanisms underlying dorsolateral prefrontal cortex contributions to cognitive dysfunction in schizophrenia

Kraepelin, in his early descriptions of schizophrenia (SZ), characterized the illness as having "an orchestra without a conductor." Kraepelin further speculated that this "conductor" was situated in the frontal lobes. Findings from multiple studies over the following decades have clearly implicated pathology of the dorsolateral prefrontal cortex (DLPFC) as playing a central role in the pathophysiology of SZ, particularly with regard to key cognitive features such as deficits in working memory and cognitive control. Following an overview of the cognitive mechanisms associated with DLPFC function and how they are altered in SZ, we review evidence from an array of neuroscientific approaches addressing how these cognitive impairments may reflect the underlying pathophysiology of the illness. Specifically, we present evidence suggesting that alterations of the DLPFC in SZ are evident across a range of spatial and temporal resolutions: from its cellular and molecular architecture, to its gross structural and functional integrity, and from millisecond to longer timescales. We then present an integrative model based upon how microscale changes in neuronal signaling in the DLPFC can influence synchronized patterns of neural activity to produce macrocircuit-level alterations in DLPFC activation that ultimately influence cognition and behavior. We conclude with a discussion of initial efforts aimed at targeting DLPFC function in SZ, the clinical implications of those efforts, and potential avenues for future development.

Mechanisms of synaptic transmission dysregulation in the prefrontal cortex: pathophysiological implications

The prefrontal cortex (PFC) serves as the chief executive officer of the brain, controlling the highest level cognitive and emotional processes. Its local circuits among glutamatergic principal neurons and GABAergic interneurons, as well as its long-range connections with other brain regions, have been functionally linked to specific behaviors, ranging from working memory to reward seeking. The efficacy of synaptic signaling in the PFC network is profundedly influenced by monoaminergic inputs via the activation of dopamine, adrenergic, or serotonin receptors. Stress hormones and neuropeptides also exert complex effects on the synaptic structure and function of PFC neurons. Dysregulation of PFC synaptic transmission is strongly linked to social deficits, affective disturbance, and memory loss in brain disorders, including autism, schizophrenia, depression, and Alzheimer's disease. Critical neural circuits, biological pathways, and molecular players that go awry in these mental illnesses have been revealed by integrated electrophysiological, optogenetic, biochemical, and transcriptomic studies of PFC. Novel epigenetic mechanism-based strategies are proposed as potential avenues of therapeutic intervention for PFC-involved diseases. This review provides an overview of PFC network organization and synaptic modulation, as well as the mechanisms linking PFC dysfunction to the pathophysiology of neurodevelopmental, neuropsychiatric, and neurodegenerative diseases. Insights from the preclinical studies offer the potential for discovering new medical treatments for human patients with these brain disorders.

A dopamine gradient controls access to distributed working memory in the large-scale monkey cortex

Dopamine is required for working memory, but how it modulates the large-scale cortex is unknown. Here, we report that dopamine receptor density per neuron, measured by autoradiography, displays a macroscopic gradient along the macaque cortical hierarchy. This gradient is incorporated in a connectome-based large-scale cortex model endowed with multiple neuron types. The model captures an inverted U-shaped dependence of working memory on dopamine and spatial patterns of persistent activity observed in over 90 experimental studies. Moreover, we show that dopamine is crucial for filtering out irrelevant stimuli by enhancing inhibition from dendrite-targeting interneurons. Our model revealed that an activity-silent memory trace can be realized by facilitation of inter-areal connections and that adjusting cortical dopamine induces a switch from this internal memory state to distributed persistent activity. Our work represents a cross-level understanding from molecules and cell types to recurrent circuit dynamics underlying a core cognitive function distributed across the primate cortex.

Functional Selectivity of Dopamine D1 Receptor Signaling: Retrospect and Prospect

Research progress on dopamine D₁ receptors indicates that signaling no longer is limited to G protein-dependent cyclic adenosine monophosphate phosphorylation but also includes G protein-independent β-arrestin-related mitogen-activated protein kinase activation, regulation of ion channels, phospholipase C activation, and possibly more. This review summarizes recent studies revealing the complexity of D₁ signaling and its clinical implications, and suggests functional selectivity as a promising strategy for drug discovery to magnify the merit of D₁ signaling. Functional selectivity/biased receptor signaling has become a major research front because of its potential to improve therapeutics through precise targeting. Retrospective pharmacological review indicated that many D₁ ligands have some degree of mild functional selectivity, and novel compounds with extreme bias at D₁ signaling were reported recently. Behavioral and neurophysiological studies inspired new methods to investigate functional selectivity and gave insight into the biased signaling of several drugs. Results from recent clinical trials also supported D₁ functional selectivity signaling as a promising strategy for discovery and development of better therapeutics.

The genie in the bottle-magnified calcium signaling in dorsolateral prefrontal cortex

Neurons in the association cortices are particularly vulnerable in cognitive disorders such as schizophrenia and Alzheimer's disease, while those in primary visual cortex remain relatively resilient. This review proposes that the special molecular mechanisms needed for higher cognitive operations confer vulnerability to dysfunction, atrophy, and neurodegeneration when regulation is lost due to genetic and/or environmental insults. Accumulating data suggest that higher cortical circuits rely on magnified levels of calcium (from NMDAR, calcium channels, and/or internal release from the smooth endoplasmic reticulum) near the postsynaptic density to promote the persistent firing needed to maintain, manipulate, and store information without "bottom-up" sensory stimulation. For example, dendritic spines in the primate dorsolateral prefrontal cortex (dlPFC) express the molecular machinery for feedforward, cAMP-PKA-calcium signaling. PKA can drive internal calcium release and promote calcium flow through NMDAR and calcium channels, while in turn, calcium activates adenylyl cyclases to produce more cAMP-PKA signaling. Excessive levels of cAMP-calcium signaling can have a number of detrimental effects: for example, opening nearby K+ channels to weaken synaptic efficacy and reduce neuronal firing, and over a longer timeframe, driving calcium overload of mitochondria to induce inflammation and dendritic atrophy. Thus, calcium-cAMP signaling must be tightly regulated, e.g., by agents that catabolize cAMP or inhibit its production (PDE4, mGluR3), and by proteins that bind calcium in the cytosol (calbindin). Many genetic or inflammatory insults early in life weaken the regulation of calcium-cAMP signaling and are associated with increased risk of schizophrenia (e.g., GRM3). Age-related loss of regulatory proteins which result in elevated calcium-cAMP signaling over a long lifespan can additionally drive tau phosphorylation, amyloid pathology, and neurodegeneration, especially when protective calcium binding proteins are lost from the cytosol. Thus, the "genie" we need for our remarkable cognitive abilities may make us vulnerable to cognitive disorders when we lose essential regulation.

Physiology and Therapeutic Potential of SK, H, and M Medium AfterHyperPolarization Ion Channels

SK, HCN, and M channels are medium afterhyperpolarization (mAHP)-mediating ion channels. The three channels co-express in various brain regions, and their collective action strongly influences cellular excitability. However, significant diversity exists in the expression of channel isoforms in distinct brain regions and various subcellular compartments, which contributes to an equally diverse set of specific neuronal functions. The current review emphasizes the collective behavior of the three classes of mAHP channels and discusses how these channels function together although they play specialized roles. We discuss the biophysical properties of these channels, signaling pathways that influence the activity of the three mAHP channels, various chemical modulators that alter channel activity and their therapeutic potential in treating various neurological anomalies. Additionally, we discuss the role of mAHP channels in the pathophysiology of various neurological diseases and how their modulation can alleviate some of the symptoms.

The Role of G Protein-Coupled Receptors (GPCRs) and Calcium Signaling in Schizophrenia. Focus on GPCRs Activated by Neurotransmitters and Chemokines

Schizophrenia is a common debilitating disease characterized by continuous or relapsing episodes of psychosis. Although the molecular mechanisms underlying this psychiatric illness remain incompletely understood, a growing body of clinical, pharmacological, and genetic evidence suggests that G protein-coupled receptors (GPCRs) play a critical role in disease development, progression, and treatment. This pivotal role is further highlighted by the fact that GPCRs are the most common targets for antipsychotic drugs. The GPCRs activation evokes slow synaptic transmission through several downstream pathways, many of them engaging intracellular Ca²⁺ mobilization. Dysfunctions of the neurotransmitter systems involving the action of GPCRs in the frontal and limbic-related regions are likely to underly the complex picture that includes the whole spectrum of positive and negative schizophrenia symptoms. Therefore, the progress in our understanding of GPCRs function in the control of brain cognitive functions is expected to open new avenues for selective drug development. In this paper, we review and synthesize the recent data regarding the contribution of neurotransmitter-GPCRs signaling to schizophrenia symptomology.

Phosphodiesterase type 1 inhibition alters medial prefrontal cortical activity during goal-driven behaviour and partially reverses neurophysiological deficits in the rat phencyclidine model of schizophrenia

Positive modulation of cAMP signalling by phosphodiesterase (PDE) inhibitors has recently been explored as a potential target for the reversal of cognitive and behavioural deficits implicating the corticoaccumbal circuit. Previous studies show that PDE type 1 isoform B (PDE1B) inhibition may improve memory function in rodent models; however, the contribution of PDE1B inhibition to impulsivity, attentional and motivational functions as well as its neurophysiological effects have not been investigated. To address this, we recorded single unit activity in medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) in Lister Hooded rats treated with the PDE1B inhibitor Lu AF64386 and tested in the 5-choice serial reaction time task (5-CSRTT). We also asked whether PDE1B inhibition modulates neurophysiological deficits produced by subchronic phencyclidine (PCP) treatment, a rat pharmacological model of schizophrenia. Lu AF64386 significantly affected behavioural parameters consistent with a reduction in goal-directed behaviour, however without affecting accuracy. Additionally, it reduced mPFC neuronal activity. Pre-treatment with PCP did not affect behavioural parameters, however it significantly disrupted overall neuronal firing while increasing phasic responses to reward-predicting cues and disrupting mPFC-NAc cross-talk. The latter two effects were reversed by Lu AF64386. These findings suggest PDE1B inhibition may be beneficial in disorders implicating a dysfunction of the mPFC-NAc network.

Guanfacine's mechanism of action in treating prefrontal cortical disorders: Successful translation across species

The selective norepinephrine (NE) α2A-adrenoceptor (α2A-AR) agonist, guanfacine (Intuniv™), is FDA-approved for treating Attention Deficit Hyperactivity Disorder (ADHD) based on research in animals, a translational success story. Guanfacine is also widely used off-label in additional mental disorders that involve impaired functioning of the prefrontal cortex (PFC), including stress-related disorders such as substance abuse, schizotypic cognitive deficits, and traumatic brain injury. The PFC subserves high order cognitive and executive functions including working memory, abstract reasoning, insight and judgment, and top-down control of attention, action and emotion. These abilities arise from PFC microcircuits with extensive recurrent excitation through NMDAR synapses. There is powerful modulation of these synapses, where cAMP-PKA opening of nearby potassium (K+) channels can rapidly and dynamically alter synaptic strength to coordinate arousal state with cognitive state, e.g. to take PFC "offline" during uncontrollable stress. A variety of evidence shows that guanfacine acts within the PFC via post-synaptic α2A-AR on dendritic spines to inhibit cAMP-PKA-K+ channel signaling, thus strengthening network connectivity, enhancing PFC neuronal firing, and improving PFC cognitive functions. Although guanfacine's beneficial effects are present in rodent, they are especially evident in primates, where the PFC greatly expands and differentiates. In addition to therapeutic actions in PFC, stress-related disorders may also benefit from additional α2-AR actions, such as weakening plasticity in the amygdala, reducing NE release, and anti-inflammatory actions by deactivating microglia. Altogether, these NE α2-AR actions optimize top-down control by PFC networks, which may explain guanfacine's benefits in a variety of mental disorders.

An Integrative Model of Effortful Control

This article presents an integrative model of effortful control, a resource-limited top-down control mechanism involved in mental tasks and physical exercises. Based on recent findings in the fields of neuroscience, social psychology and cognitive psychology, this model posits the intrinsic costs related to a weakening of the connectivity of neural networks underpinning effortful control as the main cause of mental fatigue in long and high-demanding tasks. In this framework, effort reflects three different inter-related aspects of the same construct. First, effort is a mechanism comprising a limited number of interconnected processing units that integrate information regarding the task constraints and subject’s state. Second, effort is the main output of this mechanism, namely, the effort signal that modulates neuronal activity in brain regions involved in the current task to select pertinent information. Third, effort is a feeling that emerges in awareness during effortful tasks and reflects the costs associated with goal-directed behavior. Finally, the model opens new avenues for research investigating effortful control at the behavioral and neurophysiological levels.

Role of KCNQ potassium channels in stress-induced deficit of working memory

The prefrontal cortex (PFC) mediates higher cognition but is impaired by stress exposure when high levels of catecholamines activate calcium-cAMP-protein kinase A (PKA) signaling. The current study examined whether stress and increased cAMP-PKA signaling in rat medial PFC (mPFC) reduce pyramidal cell firing and impair working memory by activating KCNQ potassium channels. KCNQ2 channels were found in mPFC layers II/III and V pyramidal cells, and patch-clamp recordings demonstrated KCNQ currents that were increased by forskolin or by chronic stress exposure, and which were associated with reduced neuronal firing. Low dose of KCNQ blockers infused into rat mPFC improved cognitive performance and prevented acute pharmacological stress-induced deficits. Systemic administration of low doses of KCNQ blocker also improved performance in young and aged rats, but higher doses impaired performance and occasionally induced seizures. Taken together, these data demonstrate that KCNQ channels have powerful influences on mPFC neuronal firing and cognitive function, contributing to stress-induced PFC dysfunction.

Dopamine effects on stress-induced working memory deficits

The prefrontal cortex (PFC) plays a critical role in mediating executive functions and orchestrating the way in which we think, decide, and behave. Many studies have shown that PFC neurons not only play a major role in mediating behavioral responses to stress but are also sensitive to stress and undergo remodeling following stress exposure. Activation of the hypothalamic-pituitary-adrenal axis as a result of stress initiates a flood of alterations in prefrontal neurotransmitter release. Dopamine (DA) neurotransmission in the PFC is involved in the modulation of stress responsiveness. Compelling results show that stressful events are associated with increased DA concentrations in the medial PFC. Excessive DA-ergic activity in the medial prefrontal cortex following stress has a negative impact on working memory and executive functions in rodents, monkeys, and humans, making them unable to processing information selectively and impairing cognitive function. Therefore, an exact understanding of these mechanisms may provide important insights into the pathophysiology of executive dysfunction and novel treatment avenues. The present review provides a summary of the neuronal circuitry involved in alterations of PFC dopaminergic neurons under conditions of stress, and then addresses the interaction of PFC DA with glucocorticoids leading to impairment of working memory under conditions of stress.

Core Differences in Synaptic Signaling Between Primary Visual and Dorsolateral Prefrontal Cortex

Neurons in primary visual cortex (V1) are more resilient than those in dorsolateral prefrontal cortex (dlPFC) in aging, schizophrenia and Alzheimer’s disease. The current study compared glutamate and neuromodulatory actions in macaque V1 to those in dlPFC, and found striking regional differences. V1 neuronal firing to visual stimuli depended on AMPA receptors, with subtle NMDA receptor contributions, while dlPFC depends primarily on NMDA receptors. Neuromodulatory actions also differed between regions. In V1, cAMP signaling increased neuronal firing, and the phosphodiesterase PDE4A was positioned to regulate cAMP effects on glutamate release from axons. HCN channels in V1 were classically located on distal dendrites, and enhanced cell firing. These data contrast with dlPFC, where PDE4A and HCN channels are concentrated in thin spines, and cAMP-HCN signaling gates inputs and weakens firing. These regional differences may explain why V1 neurons are more resilient than dlPFC neurons to the challenges of age and disease.

Disease-linked mutations alter the stoichiometries of HCN-KCNE2 complexes

The four hyperpolarization-activated cylic-nucleotide gated (HCN) channel isoforms and their auxiliary subunit KCNE2 are important in the regulation of peripheral and central neuronal firing and the heartbeat. Disruption of their normal function has been implicated in cardiac arrhythmias, peripheral pain, and epilepsy. However, molecular details of the HCN-KCNE2 complexes are unknown. Using single-molecule subunit counting, we determined that the number of KCNE2 subunits in complex with the pore-forming subunits of human HCN channels differs with each HCN isoform and is dynamic with respect to concentration. These interactions can be altered by KCNE2 gene-variants with functional implications. The results provide an additional consideration necessary to understand heart rhythm, pain, and epileptic disorders.

Loss of Prefrontal Cortical Higher Cognition with Uncontrollable Stress: Molecular Mechanisms, Changes with Age, and Relevance to Treatment

The newly evolved prefrontal cortex (PFC) generates goals for "top-down" control of behavior, thought, and emotion. However, these circuits are especially vulnerable to uncontrollable stress, with powerful, intracellular mechanisms that rapidly take the PFC "off-line." High levels of norepinephrine and dopamine released during stress engage α1-AR and D1R, which activate feedforward calcium-cAMP signaling pathways that open nearby potassium channels to weaken connectivity and reduce PFC cell firing. Sustained weakening with chronic stress leads to atrophy of dendrites and spines. Understanding these signaling events helps to explain the increased susceptibility of the PFC to stress pathology during adolescence, when dopamine expression is increased in the PFC, and with advanced age, when the molecular "brakes" on stress signaling are diminished by loss of phosphodiesterases. These mechanisms have also led to pharmacological treatments for stress-related disorders, including guanfacine treatment of childhood trauma, and prazosin treatment of veterans and civilians with post-traumatic stress disorder.

A novel dopamine D1 receptor agonist excites delay-dependent working memory-related neuronal firing in primate dorsolateral prefrontal cortex

Decades of research have emphasized the importance of dopamine (DA) D1 receptor (D1R) mechanisms to dorsolateral prefrontal cortex (dlPFC) working memory function, and the hope that D1R agonists could be used to treat cognitive disorders. However, existing D1R agonists all have had high affinity for D1R, and engage β-arrestin signaling, and these agonists have suppressed task-related neuronal firing. The current study provides the first physiological characterization of a novel D1R agonist, PF-3628, with low affinity for D1R -more similar to endogenous DA actions- as well as little engagement of β-arrestin signaling. PF-3628 was applied by iontophoresis directly onto dlPFC neurons in aged rhesus monkeys performing a delay-dependent working memory task. Aged monkeys have naturally-occurring loss of DA, and naturally-occurring reductions in dlPFC neuronal firing and working memory performance. We found the first evidence of excitatory actions of a D1R agonist on dlPFC task-related firing, and this PF-3628 beneficial response was blocked by co-application of a D1R antagonist. These D1R actions likely occur on pyramidal cells, based on previous immunoelectron microscopic studies showing expression of D1R on layer III spines, and current microarray experiments showing that D1R are four times more prevalent in pyramidal cells than in parvalbumin-containing interneurons laser-captured from layer III of the human dlPFC. These results encourage the translation of D1R mechanisms from monkey to human, with the hope PF-3628 and related, novel D1R agonists will be more appropriate for enhancing dlPFC cognitive functions in patients with mental disorders.

Characterization of drug binding within the HCN1 channel pore

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels mediate rhythmic electrical activity of cardiac pacemaker cells, and in neurons play important roles in setting resting membrane potentials, dendritic integration, neuronal pacemaking, and establishing action potential threshold. Block of HCN channels slows the heart rate and is currently used to treat angina. However, HCN block also provides a promising approach to the treatment of neuronal disorders including epilepsy and neuropathic pain. While several molecules that block HCN channels have been identified, including clonidine and its derivative alinidine, lidocaine, mepivacaine, bupivacaine, ZD7288, ivabradine, zatebradine, and cilobradine, their low affinity and lack of specificity prevents wide-spread use. Different studies suggest that the binding sites of these inhibitors are located in the inner vestibule of HCN channels, but the molecular details of their binding remain unknown. We used computational docking experiments to assess the binding sites and mode of binding of these inhibitors against the recently solved atomic structure of human HCN1 channels, and a homology model of the open pore derived from a closely related CNG channel. We identify a possible hydrophobic groove in the pore cavity that plays an important role in conformationally restricting the location and orientation of drugs bound to the inner vestibule. Our results also help explain the molecular basis of the low-affinity binding of these inhibitors, paving the way for the development of higher affinity molecules.

Receptor-Receptor Interactions as a Widespread Phenomenon: Novel Targets for Drug Development?

The discovery of receptor-receptor interactions (RRI) has expanded our understanding of the role that G protein-coupled receptors (GPCRs) play in intercellular communication. The finding that GPCRs can operate as receptor complexes, and not only as monomers, suggests that several different incoming signals could already be integrated at the plasma membrane level via direct allosteric interactions between the protomers that form the complex. Most research in this field has focused on neuronal populations and has led to the identification of a large number of RRI. However, RRI have been seen to occur not only in neurons but also in astrocytes and, outside the central nervous system, in cells of the cardiovascular and endocrine systems and in cancer cells. Furthermore, RRI involving the formation of macromolecular complexes are not limited to GPCRs, being also observed in other families of receptors. Thus, RRI appear as a widespread phenomenon and oligomerization as a common mechanism for receptor function and regulation. The discovery of these macromolecular assemblies may well have a major impact on pharmacology. Indeed, the formation of receptor complexes significantly broadens the spectrum of mechanisms available to receptors for recognition and signaling, which may be implemented through modulation of the binding sites of the adjacent protomers and of their signal transduction features. In this context, the possible appearance of novel allosteric sites in the receptor complex structure may be of particular relevance. Thus, the existence of RRI offers the possibility of new therapeutic approaches, and novel pharmacological strategies for disease treatment have already been proposed. Several challenges, however, remain. These include the accurate characterization of the role that the receptor complexes identified so far play in pathological conditions and the development of ligands specific to given receptor complexes, in order to efficiently exploit the pharmacological properties of these complexes.

Unique Molecular Regulation of Higher-Order Prefrontal Cortical Circuits: Insights into the Neurobiology of Schizophrenia

Schizophrenia is associated with core deficits in cognitive abilities and impaired functioning of the newly evolved prefrontal association cortex (PFC). In particular, neuropathological studies of schizophrenia have found selective atrophy of the pyramidal cell microcircuits in deep layer III of the dorsolateral PFC (dlPFC) and compensatory weakening of related GABAergic interneurons. Studies in monkeys have shown that recurrent excitation in these layer III microcircuits generates the precisely patterned, persistent firing needed for working memory and abstract thought. Importantly, excitatory synapses on layer III spines are uniquely regulated at the molecular level in ways that may render them particularly vulnerable to genetic and/or environmental insults. Glutamate actions are remarkably dependent on cholinergic stimulation, and there are inherent mechanisms to rapidly weaken connectivity, e.g. during stress. In particular, feedforward cyclic adenosine monophosphate (cAMP)-calcium signaling rapidly weakens network connectivity and neuronal firing by opening nearby potassium channels. Many mechanisms that regulate this process are altered in schizophrenia and/or associated with genetic insults. Current data suggest that there are "dual hits" to layer III dlPFC circuits: initial insults to connectivity during the perinatal period due to genetic errors and/or inflammatory insults that predispose the cortex to atrophy, followed by a second wave of cortical loss during adolescence, e.g. driven by stress, at the descent into illness. The unique molecular regulation of layer III circuits may provide a nexus where inflammation disinhibits the neuronal response to stress. Understanding these mechanisms may help to illuminate dlPFC susceptibility in schizophrenia and provide insights for novel therapeutic targets.

Elevated dopamine D1 receptor availability in striatum of Göttingen minipigs after electroconvulsive therapy

Electroconvulsive therapy (ECT), a direct form of brain stimulation, is an effective antidepressant. We hypothesized that the beneficial effects of ECT are mediated by increased dopaminergic neurotransmission, in which the baseline activity of D1 receptors may predict the response to ECT. We established a novel model of brain stimulation in Göttingen minipigs based on the protocol of ECT applied in humans. With positron emission tomography (PET), we determined a measure of dopaminergic neurotransmission with the dopamine D1 receptor antagonist [¹¹C]SCH23390. Seven minipigs were anesthetized and completed PET at baseline, prior to the onset of ECT treatment, and at 24-48 h and 8-10 days after the end of a clinical course of ECT, consisting of 10 ECT sessions over a 3.5-week period. In all pigs, the binding of [¹¹C]SCH23390 to striatal D1 receptors had increased by 24-48 h after ECT, and in most, binding returned towards baseline at 8-10 days. Increased binding was observed in inverse proportion to baseline binding rates. Increased binding to dopamine D1 receptors suggests facilitation of dopaminergic neurotransmission, which may contribute to the therapeutic effects of ECT. Importantly, the baseline binding capacity of D1 receptors predicts the magnitude of increased binding, up to a maximum binding capacity.

Racing the clock: The role of circadian rhythmicity in addiction across the lifespan

Although potent effects of psychoactive drugs on circadian rhythms were first described over 30 years ago, research into the reciprocal relationship between the reward system and the circadian system - and the impact of this relationship on addiction - has only become a focus in the last decade. Nonetheless, great progress has been made in that short time toward understanding how drugs of abuse impact the molecular and physiological circadian clocks, as well as how disruption of normal circadian rhythm biology may contribute to addiction and ameliorate the efficacy of treatments for addiction. In particular, data have emerged demonstrating that disrupted circadian rhythms, such as those observed in shift workers and adolescents, increase susceptibility to addiction. Furthermore, circadian rhythms and addiction impact one another longitudinally - specifically from adolescence to the elderly. In this review, the current understanding of how the circadian clock interacts with substances of abuse within the context of age-dependent changes in rhythmicity, including the potential existence of a drug-sensitive clock, the correlation between chronotype and addiction vulnerability, and the importance of rhythmicity in the mesocorticolimbic dopamine system, is discussed. The primary focus is on alcohol addiction, as the preponderance of research is in this area, with references to other addictions as warranted. The implications of clock-drug interactions for the treatment of addiction will also be reviewed, and the potential of therapeutics that reset the circadian rhythm will be highlighted.

G protein-coupled receptor-receptor interactions give integrative dynamics to intercellular communication

The proposal of receptor-receptor interactions (RRIs) in the early 1980s broadened the view on the role of G protein-coupled receptors (GPCR) in the dynamics of the intercellular communication. RRIs, indeed, allow GPCR to operate not only as monomers but also as receptor complexes, in which the integration of the incoming signals depends on the number, spatial arrangement, and order of activation of the protomers forming the complex. The main biochemical mechanisms controlling the functional interplay of GPCR in the receptor complexes are direct allosteric interactions between protomer domains. The formation of these macromolecular assemblies has several physiologic implications in terms of the modulation of the signaling pathways and interaction with other membrane proteins. It also impacts on the emerging field of connectomics, as it contributes to set and tune the synaptic strength. Furthermore, recent evidence suggests that the transfer of GPCR and GPCR complexes between cells via the exosome pathway could enable the target cells to recognize/decode transmitters and/or modulators for which they did not express the pertinent receptors. Thus, this process may also open the possibility of a new type of redeployment of neural circuits. The fundamental aspects of GPCR complex formation and function are the focus of the present review article.

The impact of traumatic stress on Pavlovian biases

BACKGROUND

Disturbances in Pavlovian valuation systems are reported to follow traumatic stress exposure. However, motivated decisions are also guided by instrumental mechanisms, but to date the effect of traumatic stress on these instrumental systems remain poorly investigated. Here, we examine whether a single episode of severe traumatic stress influences flexible instrumental decisions through an impact on a Pavlovian system.

METHODS

Twenty-six survivors of the 2011 Norwegian terror attack and 30 matched control subjects performed an instrumental learning task in which Pavlovian and instrumental associations promoted congruent or conflicting responses. We used reinforcement learning models to infer how traumatic stress affected learning and decision-making. Based on the importance of dorsal anterior cingulate cortex (dACC) for cognitive control, we also investigated if individual concentrations of Glx (=glutamate + glutamine) in dACC predicted the Pavlovian bias of choice.

RESULTS

Survivors of traumatic stress expressed a greater Pavlovian interference with instrumental action selection and had significantly lower levels of Glx in the dACC. Across subjects, the degree of Pavlovian interference was negatively associated with dACC Glx concentrations.

CONCLUSIONS

Experiencing traumatic stress appears to render instrumental decisions less flexible by increasing the susceptibility to Pavlovian influences. An observed association between prefrontal glutamatergic levels and this Pavlovian bias provides novel insight into the neurochemical basis of decision-making, and suggests a mechanism by which traumatic stress can impair flexible instrumental behaviours.