Contrasting patterns in the localization of glutamic acid decarboxylase and Ca2+/calmodulin protein kinase gene expression in the rat central nervous system

Comparative review of the brain development in Acomys cahirinus

Acomys cahirinus (referred to as "acomys" in this article) is a precocial rodent, born well-developed and mobile, capable of feeding independently and escaping predators shortly after birth. Notable for its advanced regenerative abilities and menstrual cycle, acomys serves as a unique model for studying diverse aspects of physiology and neuroscience, including developmental and regenerative neuroscience. Despite its significance, only sporadic and unsystematic data on the structure and development of the acomys brain are available. Therefore, the aim of this study was to systematically organize the existing information on the structure and development of the acomys brain and to compare it with that of commonly studied altricial rodent species (rats, mice, hamsters, and gerbils). This review is organized into several sections, focusing on general aspects of brain development, such as myelination and brain growth. It also discusses the development of brain structures involved in sensory processing (olfactory, visual, and auditory), motor control, learning and memory, and social behavior.

Serotonergic neuromodulation of synaptic plasticity

Synaptic plasticity constitutes a fundamental process in the reorganization of neural networks that underlie memory, cognition, emotional responses, and behavioral planning. At the core of this phenomenon lie Hebbian mechanisms, wherein frequent synaptic stimulation induces long-term potentiation (LTP), while less activation leads to long-term depression (LTD). The synaptic reorganization of neuronal networks is regulated by serotonin (5-HT), a neuromodulator capable of modify synaptic plasticity to appropriately respond to mental and behavioral states, such as alertness, attention, concentration, motivation, and mood. Lately, understanding the serotonergic Neuromodulation of synaptic plasticity has become imperative for unraveling its impact on cognitive, emotional, and behavioral functions. Through a comparative analysis across three main forebrain structures-the hippocampus, amygdala, and prefrontal cortex, this review discusses the actions of 5-HT on synaptic plasticity, offering insights into its role as a neuromodulator involved in emotional and cognitive functions. By distinguishing between plastic and metaplastic effects, we provide a comprehensive overview about the mechanisms of 5-HT neuromodulation of synaptic plasticity and associated functions across different brain regions.

Altered development and network connectivity in a human neuronal model of 15q11.2 deletion-related neurodevelopmental disorders

The chromosome 15q11.2 locus is deleted in 1.5% of patients with genetic epilepsy and confers a risk for intellectual disability and schizophrenia. Individuals with this deletion demonstrate increased cortical thickness, decreased cortical surface area and white matter abnormalities. Human induced pluripotent stem cell (iPSC)-derived neural progenitor cells (NPC) from 15q11.2 deletion individuals exhibit early adhesion junction and migration abnormalities, but later neuronal development and function have not been fully assessed. Imaging studies indicating altered structure and network connectivity in the anterior brain regions and the cingulum suggest that in addition to alterations in progenitor dynamics, there may also be structural and functional changes within discrete networks of mature neurons. To explore this, we generated human forebrain cortical neurons from iPSCs derived from individuals with or without 15q11.2 deletion and used longitudinal imaging and multielectrode array analysis to evaluate neuronal development over time. 15q11.2 deleted neurons exhibited fewer connections and an increase in inhibitory neurons. Individual neurons had decreased neurite complexity and overall decreased neurite length. These structural changes were associated with a reduction in multiunit action potential generation, bursting and synchronization. The 15q11.2 deleted neurons also demonstrated specific functional deficits in glutamate and GABA mediated network activity and synchronization with a delay in the maturation of the inhibitory response to GABA. These data indicate that deletion of the 15q11.2 region is sufficient to impair the structural and functional maturation of cortical neuron networks which likely underlies the pathologic changes in humans with the 15q11.2 deletion.

Distribution of vasoactive intestinal peptide (VIP) immunoreactivity in the rat pallial and subpallial amygdala and colocalization with γ-aminobutyric acid (GABA)

The amygdaloid complex, also known as the amygdala, is a heterogeneous group of distinct nuclear and cortical pallial and subpallial structures. The amygdala plays an important role in several complex functions including emotional behavior and learning. The expression of calcium-binding proteins and peptides in GABAergic neurons located in the pallial and subpallial amygdala is not uniform and is sometimes restricted to specific groups of cells. Vasoactive intestinal polypeptide (VIP) is present in specific subpopulations of GABAergic cells in the amygdala. VIP immunoreactivity has been observed in somatodendritic and axonal profiles of the rat basolateral and central amygdala. However, a comprehensive analysis of the distribution of VIP immunoreactivity in the various pallial and subpallial structures is currently lacking. The present study used immunohistochemical and morphometric techniques to analyze the distribution and the neuronal localization of VIP immunoreactivity in the rat pallial and subpallial amygdala. In the pallial amygdala, VIP-IR neurons are local inhibitory interneurons that presumably directly and indirectly regulate the activity of excitatory pyramidal neurons. In the subpallial amygdala, VIP immunoreactivity is expressed in several inhibitory cell types, presumably acting as projection or local interneurons. The distribution of VIP immunoreactivity is non-homogeneous throughout the different areas of the amygdaloid complex, suggesting a distinct influence of this neuropeptide on local neuronal circuits and, consequently, on the cognitive, emotional, behavioral and endocrine activities mediated by the amygdala.

Inner Structure of the Lateral Geniculate Complex of Adult and Newborn Acomys cahirinus

Acomys cahirinus is a unique Rodentia species with several distinctive physiological traits, such as precocial development and remarkable regenerative abilities. These characteristics render A. cahirinus increasingly valuable for regenerative and developmental physiology studies. Despite this, the structure and postnatal development of the central nervous system in A. cahirinus have been inadequately explored, with only sporadic data available. This study is the first in a series of papers addressing these gaps. Our first objective was to characterize the structure of the main visual thalamic region, the lateral geniculate complex, using several neuronal markers (including Ca2+-binding proteins, glutamic acid decarboxylase enzyme, and non-phosphorylated domains of heavy-chain neurofilaments) to label populations of principal neurons and interneurons in adult and newborn A. cahirinus. As typically found in other rodents, we identified three subdivisions in the geniculate complex: the dorsal and ventral lateral geniculate nuclei (LGNd and LGNv) and the intergeniculate leaflet (IGL). Additionally, we characterized internal diversity in the LGN nuclei. The "shell" and "core" regions of the LGNd were identified using calretinin in adults and newborns. In adults, the inner and outer parts of the LGNv were identified using calbindin, calretinin, parvalbumin, GAD67, and SMI-32, whereas in newborns, calretinin and SMI-32 were employed for this purpose. Our findings revealed more pronounced developmental changes in LGNd compared to LGNv and IGL, suggesting that LGNd is less mature at birth and more influenced by visual experience.

Effects of Interleukin-19 overexpression in the medial prefrontal cortex on anxiety-related behaviors, BDNF expression and p38/JNK/ERK pathways

Anxiety is a prevalent mental illness known for its high incidence, comorbidity, and tendency to recur, posing significant societal and individual burdens. Studies have highlighted Interleukin-19 (IL-19) as having potential relevance in neuropsychiatric disorders. Our previous research revealed that IL-19 overexpression in colonies exacerbated anxiety-related behaviors induced by dextran sodium sulfate/stress. However, the precise role and molecular mechanisms of IL-19 in anxiety regulation remain uncertain. In this study, we initiated an acute restraint stress (ARS)-induced anxious mouse model and identified heightened expression of IL-19 and IL-20Rα in the medial prefrontal cortex (mPFC) of ARS mice. Notably, IL-19 and IL-20Rα were predominantly present in the excitatory pyramidal neurons of the mPFC under both basal and ARS conditions. Utilizing the adeno-associated virus (AAV) strategy, we demonstrated that IL-19 overexpression in the mPFC induced anxiety-related behaviors and elevated stress susceptibility. Additionally, we observed decreased protein levels of brain-derived neurotrophic factor (BDNF) and postsynaptic density protein 95 (PSD95) in the mPFC of IL-19 overexpression mice, accompanied by reduced phosphorylation of in the p38, JNK, and Erk signaling pathways. These findings emphasize the role of IL-19 in modulating anxiety-related behaviors within the mPFC and suggest its potential as a pathological gene and therapeutic target for anxiety.

CaMKIIa+ neurons in the bed nucleus of the stria terminalis modulate pace of natural reward seeking depending on internal state

This study aims to investigate the underlying neurobiological mechanisms that regulate natural reward seeking behaviors, specifically in the context of sexual behavior and sucrose self-administration. The role of CaMKIIa+ neurons in the bed nucleus of the stria terminalis (BNST) was explored using chemogenetic silencing and -stimulation. Additionally, the study examined how these effects interacted with the internal state of the animals. Through detailed behavioral analysis, it was demonstrated that CaMKIIa+ neurons in the BNST play a significant role in the regulation of both sexual behavior and sucrose self-administration. Although the behavioral outcome measures differed between the two behaviors, the regulatory role of the CaMKIIa+ neurons in the BNST was found to converge on the modulation of the pacing of engagement in these behaviors in male rats. Moreover, our study confirmed that the internal physiological state of the animal affects how the BNST modulates these behaviors. These findings suggest that different types of natural rewards may recruit a similar brain circuitry to regulate the display of motivated behaviors. Overall, this research provides valuable insights into the neural mechanisms underlying natural reward seeking and sheds light on the interconnected nature of reward-related behaviors in male rats.

SIRT1 mediates the excitability of spinal CaMKIIα-positive neurons and participates in neuropathic pain by controlling Nav1.3

AIMS

Neuropathic pain is a common chronic pain disorder, which is largely attributed to spinal central sensitization. Calcium/calmodulin-dependent protein kinase II alpha (CaMKIIα) activation in the spinal dorsal horn (SDH) is a major contributor to spinal sensitization. However, the exact way that CaMKIIα-positive (CaMKIIα+) neurons in the SDH induce neuropathic pain is still unclear. This study aimed to explore the role of spinal CaMKIIα+ neurons in neuropathic pain caused by chronic constriction injury (CCI) and investigate the potential epigenetic mechanisms involved in CaMKIIα+ neuron activation.

METHODS

CCI-induced neuropathic pain mice model, Sirt1loxP/loxP mice, and chemogenetic virus were used to investigate whether the activation of spinal CaMKIIα+ neurons is involved in neuropathic pain and its involved mechanism. Transcriptome sequence, western blotting, qRT-PCR, and immunofluorescence analysis were performed to assay the expression of related molecules and activation of neurons. Co-immunoprecipitation was used to observe the binding relationship of protein. Chromatin immunoprecipitation (ChIP)-PCR was applied to analyze the acetylation of histone H3 in the Scn3a promoter region.

RESULTS

The expression of sodium channel Nav1.3 was increased and the expression of SIRT1 was decreased in the spinal CaMKIIα+ neurons of CCI mice. CaMKIIα neurons became overactive after CCI, and inhibiting their activation relieved CCI-induced pain. Overexpression of SIRT1 reversed the increase of Nav1.3 and alleviated pain, while knockdown of SIRT1 or overexpression of Nav1.3 promoted CaMKIIα+ neuron activation and induced pain. By knocking down spinal SIRT1, the acetylation of histone H3 in the Scn3a (encoding Nav1.3) promoter region was increased, leading to an increased expression of Nav1.3.

CONCLUSION

The findings suggest that an aberrant reduction of spinal SIRT1 after nerve injury epigenetically increases Nav1.3, subsequently activating CaMKIIα+ neurons and causing neuropathic pain.

Chemogenetic Modulation of Posterior Insula CaMKIIa Neurons Alters Pain and Thermoregulation

The posterior insular cortex (PIC) is well positioned to perform somatosensory-limbic integration; yet, the function of neuronal subsets within the PIC in processing the sensory and affective dimensions of pain remains unclear. Here, we employ bidirectional chemogenetic modulation to characterize the function of PIC CaMKIIa-expressing excitatory neurons in a comprehensive array of sensory, affective, and thermoregulatory behaviors. Excitatory pyramidal neurons in the PIC were found to be sensitized under inflammatory pain conditions. Chemogenetic activation of excitatory CaMKIIa-expressing PIC neurons in non-injured conditions produced an increase in reflexive and affective pain- and anxiety-like behaviors. Moreover, activation of PIC CaMKIIa-expressing neurons during inflammatory pain conditions exacerbated hyperalgesia and decreased pain tolerance. However, Chemogenetic activation did not alter heat nociception via hot plate latency or body temperature. Conversely, inhibiting CaMKIIa-expressing neurons did not alter either sensory or affective pain-like behaviors in non-injured or under inflammatory pain conditions, but it did decrease body temperature and decreased hot plate latency. Our findings reveal that PIC CaMKIIa-expressing neurons are a critical hub for producing both sensory and affective pain-like behaviors and important for thermoregulatory processing. PERSPECTIVE: The present study reveals that activation of the posterior insula produces hyperalgesia and negative affect, and has a role in thermal tolerance and thermoregulation. These findings highlight the insula as a key player in contributing to the multidimensionality of pain.

Neurochemistry of the mammillary body

The mammillary body (MB) is a component of the extended hippocampal system and many studies have shown that its functions are vital for mnemonic processes. Together with other subcortical structures, such as the anterior thalamic nuclei and tegmental nuclei of Gudden, the MB plays a crucial role in the processing of spatial and working memory, as well as navigation in rats. The aim of this paper is to review the distribution of various substances in the MB of the rat, with a description of their possible physiological roles. The following groups of substances are reviewed: (1) classical neurotransmitters (glutamate and other excitatory transmitters, gamma-aminobutyric acid, acetylcholine, serotonin, and dopamine), (2) neuropeptides (enkephalins, substance P, cocaine- and amphetamine-regulated transcript, neurotensin, neuropeptide Y, somatostatin, orexins, and galanin), and (3) other substances (calcium-binding proteins and calcium sensor proteins). This detailed description of the chemical parcellation may facilitate a better understanding of the MB functions and its complex relations with other structures of the extended hippocampal system.

Effects of motor cortex neuromodulation on the specificity of corticospinal tract spinal axon outgrowth and targeting in rats

BACKGROUND

Neural activity helps construct neural circuits during development and this function is leveraged by neuromodulation protocols to promote connectivity and repair in maturity. Neuromodulation targeting the motor cortex (MCX) strengthens connections for evoking muscle contraction (MEPs). Mechanisms include promoting local MCX and corticospinal tract (CST) synaptic efficacy and also axon terminal structural changes.

OBJECTIVE

In this study, we address the question of potential causality between neuronal activation and the neuronal structural response.

METHODS

We used patterned optogenetic activation (ChR2-EYFP), daily for 10-days, to deliver intermittent theta burst stimulation (iTBS) to activate MCX neurons within the forelimb representation in healthy rats, while differentiating them from neurons in the same population that were not activated. We used chemogenetic DREADD activation to produce a daily period of non-patterned neuronal activation.

RESULTS

We found a significant increase in CST axon length, axon branching, contacts targeted to a class of premotor interneuron (Chx10), as well as projections into the motor pools in the ventral horn in optically activated but not neighboring non-activated neurons. A period of 2-h of continuous activation daily for 10 days using DREADD chemogenetic activation with systemic clozapine N-oxide (CNO) administration also increased CST axon length and branching, but not the ventral horn and Chx10 targeting effects. Both patterned optical and chemogenetic activation reduced MCX MEP thresholds.

CONCLUSION

Our findings show that targeting of CST axon sprouting is dependent on patterned activation, but that CST spinal axon outgrowth and branching are not. Our optogenetic findings, by distinguishing optically activated and non-activated CST axons, suggests that the switch for activity-dependent axonal outgrowth is neuron-intrinsic.

The circadian clock in the piriform cortex intrinsically tunes daily changes of odor-evoked neural activity

The daily activity in the brain is typically fine-tuned by the circadian clock in the local neurons as well as by the master circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus. In the olfactory response, odor-evoked activity in the piriform cortex (PC) and olfactory behavior retain circadian rhythmicity in the absence of the SCN, yet how the circadian rhythm in the PC is achieved independently of the SCN remains elusive. Here, to define neurons regulating the circadian rhythm of the odor-evoked activity in the PC, we knocked out the clock gene Bmal1 in a host of specific neurons along the olfactory circuit. We discovered that Bmal1 knockout in the PC largely abolishes the circadian rhythm of the odor-evoked activity. We further showed that isolated PC exhibits sustained circadian rhythms of the clock gene Per2 expression. Quantitative PCR analysis revealed that expression patterns of multiple genes involved in neural activity and synaptic transmission exhibit circadian rhythm in the PC in a BMAL1-dependent manner. Our findings indicate that BMAL1 acts intrinsically in the PC to control the circadian rhythm of the odor-evoked activity in the PC, possibly through regulating expression patterns of multiple genes involved in neural activity and transmission.

Expression Mapping and Functional Analysis of Orphan G-Protein-Coupled Receptor GPR158 in the Adult Mouse Brain Using a GPR158 Transgenic Mouse

Aberrant expression of G-protein-coupled receptor 158 (GPR158) has been reported to be inextricably linked to a variety of diseases affecting the central nervous system, including Alzheimer’s disease (AD), depression, intraocular pressure, and glioma, but the underlying mechanism remains elusive due to a lack of biological and pharmacological tools to elaborate its preferential cellular distribution and molecular interaction network. To assess the cellular localization, expression, and function of GPR158, we generated an epitope-tagged GPR158 mouse model (GPR158Tag) that exhibited normal motor, cognitive, and social behavior, no deficiencies in social memory, and no anxiety-like behavior compared to C57BL/6J control mice at P60. Using immunofluorescence, we found that GPR158+ cells were distributed in several brain regions including the cerebral cortex, hippocampus, cerebellum, and caudate putamen. Next, using the cerebral cortex of the adult GPR158Tag mice as a representative region, we found that GPR158 was only expressed in neurons, and not in microglia, oligodendrocytes, or astrocytes. Remarkably, the majority of GPR158 was enriched in Camk2a+ neurons whilst limited expression was found in PV+ interneurons. Concomitant 3D co-localization analysis revealed that GPR158 was mainly distributed in the postsynaptic membrane, but with a small portion in the presynaptic membrane. Lastly, via mass spectrometry analysis, we identified proteins that may interact with GPR158, and the relevant enrichment pathways were consistent with the immunofluorescence findings. RNA-seq analysis of the cerebral cortex of the GPR158−/− mice showed that GPR158 and its putative interacting proteins are involved in the chloride channel complex and synaptic vesicle membrane composition. Using these GPR158Tag mice, we were able to accurately label GPR158 and uncover its fundamental function in synaptic vesicle function and memory. Thus, this model will be a useful tool for subsequent biological, pharmacological, and electrophysiological studies related to GPR158.

Activity in a prefrontal-periaqueductal gray circuit overcomes behavioral and endocrine features of the passive coping stress response

The question of how the brain links behavioral and biological features of defensive responses has remained elusive. The importance of this problem is underscored by the observation that behavioral passivity in stress coping is associated with elevations in glucocorticoid hormones, and each may carry risks for susceptibility to a host of stress-related diseases. Past work implicates the medial prefrontal cortex (mPFC) in the top-down regulation of stress-related behaviors; however, it is unknown whether such changes have the capacity to buffer against the longer-lasting biological consequences associated with aversive experiences. Using the shock probe defensive burying test in rats to naturalistically measure behavioral and endocrine features of coping, we observed that the active behavioral component of stress coping is associated with increases in activity along a circuit involving the caudal mPFC and midbrain dorsolateral periaqueductal gray (PAG). Optogenetic manipulations of the caudal mPFC-to-dorsolateral PAG pathway bidirectionally modulated active (escape and defensive burying) behaviors, distinct from a rostral mPFC-ventrolateral PAG circuit that instead limited passive (immobility) behavior. Strikingly, under conditions that biased rats toward a passive coping response set, including exaggerated stress hormonal output and increased immobility, excitation of the caudal mPFC-dorsolateral PAG projection significantly attenuated each of these features. These results lend insight into how the brain coordinates response features to overcome passive coping and may be of importance for understanding how activated neural systems promote stress resilience.

Anti-Nociceptive and Anti-Aversive Drugs Differentially Modulate Distinct Inputs to the Rat Lateral Parabrachial Nucleus

The lateral parabrachial nucleus (LPBN) plays an important role in the processing and establishment of pain aversion. It receives direct input from the superficial dorsal horn and forms reciprocal connections with the periaqueductal grey matter (PAG), which is critical for adaptive behaviour and the modulation of pain processing. Here, using in situ hybridization and optogenetics combined with in vitro electrophysiology, we characterized the spinal- and PAG-LPBN circuits of rats. We found spinoparabrachial projections to be strictly glutamatergic, while PAG neurons send glutamatergic and GABAergic projections to the LPBN. We next investigated the effects of drugs with anti-aversive and/or anti-nociceptive properties on these synapses: The µ-opioid receptor agonist DAMGO (10 µM) reduced spinal and PAG synaptic inputs onto LPBN neurons, and the excitability of LPBN neurons receiving these inputs. The benzodiazepine receptor agonist diazepam (5 µM) strongly enhanced GABAergic action at inhibitory PAG-LPBN synapses. The cannabinoid receptor agonist WIN 55,212-2 (5 µM) led to a reduction in inhibitory and excitatory PAG-LPBN synaptic transmission, without affecting excitatory spinoparabrachial synaptic transmission. Our study reveals that opioid, cannabinoid and benzodiazepine receptor agonists differentially affect distinct LPBN synapses. These findings may support the efforts to develop pinpointed therapies for pain patients. PERSPECTIVE: The LPBN is an important brain region for the control of pain aversion versus recuperation, and as such constitutes a promising target for developing new strategies for pain management. We show that clinically-relevant drugs have complex and pathway-specific effects on LPBN processing of putative nociceptive and aversive inputs.

Repeated motor cortex theta-burst stimulation produces persistent strengthening of corticospinal motor output and durable spinal cord structural changes in the rat

BACKGROUND

The strength of connections between motor cortex (MCX) and muscle can be augmented with a variety of stimulation protocols. Augmenting MCX-to-muscle connection strength by neuromodulation may be a way to enhance the intact motor system's capacity for acquiring motor skills and promote function after injury to strengthen spared connections. But this enhancement must be maintained for functional improvements.

OBJECTIVE

We determined if brief MCX muscle evoked potential (MEP) enhancement produced by intermittent theta burst stimulation (iTBS) can be converted into a longer and structurally durable form of response enhancement with repeated daily and longer-term application.

METHODS

Electrical iTBS was delivered through an implanted MCX epidural electrode and MEPs were recorded using implanted EMG electrodes in awake naïve rats. MCX activity was modulated further using chemogenetic (DREADDs) excitation and inhibition. Corticospinal tract (CST) axons were traced and immunochemistry used to measure CST synapses.

RESULTS

A single MCX iTBS block (600 pulses) produced MEP LTP lasting ∼30-45 min. Concatenating five iTBS blocks within a 30-min session produced MEP LTP lasting 24-48 h, which could be strengthened or weakened by bidirectional MCX activity modulation. Effect duration was not changed. Finally, daily induction of this persistent MEP LTP with daily iTBS for 10-days produced MEP enhancement outlasting the stimulation period by at least 10 days, and accompanied by CST axonal outgrowth and structural changes at the CST-spinal interneuron synapse.

CONCLUSION

Our findings inform the mechanisms of iTBS and provide a framework for designing neuromodulatory strategies to promote durable enhancement of cortical motor actions.

Bilateral Interactions in the Mouse Dorsal Inferior Colliculus Enhance the Ipsilateral Neuronal Responses and Binaural Hearing

The inferior colliculus (IC) is a critical centre for the binaural processing of auditory information. However, previous studies have mainly focused on the central nucleus of the inferior colliculus (ICC), and less is known about the dorsal nucleus of the inferior colliculus (ICD). Here, we first examined the characteristics of the neuronal responses in the mouse ICD and compared them with those in the inferior colliculus under binaural and monaural conditions using in vivo loose-patch recordings. ICD neurons exhibited stronger responses to ipsilateral sound stimulation and better binaural summation than those of ICC neurons, which indicated a role for the ICD in binaural hearing integration. According to the abundant interactions between bilateral ICDs detected using retrograde virus tracing, we further studied the effect of unilateral ICD silencing on the contralateral ICD. After lidocaine was applied, the responses of some ICD neurons (13/26), especially those to ipsilateral auditory stimuli, decreased. Using whole-cell recording and optogenetic methods, we investigated the underlying neuronal circuits and synaptic mechanisms of binaural auditory information processing in the ICD. The unilateral ICD provides both excitatory and inhibitory projections to the opposite ICD, and the advantaged excitatory inputs may be responsible for the enhanced ipsilateral responses and binaural summation of ICD neurons. Based on these results, the contralateral ICD might modulate the ipsilateral responses of the neurons and binaural hearing.

Optogenetic study of central medial and paraventricular thalamic projections to the basolateral amygdala

The central medial (CMT) and paraventricular (PVT) thalamic nuclei project strongly to the basolateral amygdala (BL). Similarities between the responsiveness of CMT, PVT, and BL neurons suggest that these nuclei strongly influence BL activity. Supporting this possibility, an electron microscopic study reported that, in contrast with other extrinsic afferents, CMT and PVT axon terminals form very few synapses with BL interneurons. However, since limited sampling is a concern in electron microscopic studies, the present investigation was undertaken to compare the impact of CMT and PVT thalamic inputs on principal and local-circuit BL neurons with optogenetic methods and whole cell recordings in vitro. Optogenetic stimulation of CMT and PVT axons elicited glutamatergic excitatory postsynaptic potentials (EPSPs) or excitatory postsynaptic currents (EPSCs) in principal cells and interneurons, but they generally had a longer latency in interneurons. Moreover, after blockade of polysynaptic interactions with tetrodotoxin (TTX), a lower proportion of interneurons (50%) than principal cells (90%) remained responsive to CMT and PVT inputs. Although the presence of TTX-resistant responses in some interneurons indicates that CMT and PVT inputs directly contact some local-circuit cells, their lower incidence and amplitude after TTX suggest that CMT and PVT inputs form fewer synapses with them than with principal BL cells. Together, these results indicate that CMT and PVT inputs mainly contact principal BL neurons such that when CMT or PVT neurons fire, limited feedforward inhibition counters their excitatory influence over principal BL cells. However, CMT and PVT axons can also recruit interneurons indirectly, via the activation of principal cells, thereby generating feedback inhibition.NEW & NOTEWORTHY Midline thalamic (MTh) nuclei contribute major projections to the basolateral amygdala (BL). Similarities between the responsiveness of MTh and BL neurons suggest that MTh neurons exert a significant influence over BL activity. Using optogenetic techniques, we show that MTh inputs mainly contact principal BL neurons such that when MTh neurons fire, little feedforward inhibition counters their excitatory influence over principal cells. Thus, MTh inputs may be major determinants of BL activity.

The paraventricular thalamus input to central amygdala controls depression-related behaviors

The dysregulation of neuronal networks may contribute to the etiology of major depressive disorder (MDD). However, the neural connections underlying the symptoms of MDD have yet to be elucidated. Here, we observed that glutamatergic neurons in the paraventricular thalamus (PVT) were activated by chronic unpredictable stress (CUS) with higher expression numbers of ΔFosB-labeled neurons and protein expression levels, activation of PVT neurons caused depressive-like phenotypes, whereas suppression of PVT neuronal activity induced an antidepressant effect in male, but not female mice, which were achieved by using a chemogenetic approach. Moreover, we found that PVT glutamatergic neurons showed strong neuronal projections to the central amygdala (CeA), activation of the CeA-projecting neurons in PVT or the neuronal terminals of PVT-CeA projection neurons induced depression-related behaviors or showed enhanced stress-induced susceptibility. These results suggest that PVT is a key depression-controlling nucleus, and PVT-CeA projection regulates depression-related behaviors in a sex-dependent manner, which could be served as an essential pathway for morbidity and treatment of depression.

The role of the anterior pretectal nucleus in pain modulation: A comprehensive review

Descending pain modulation involves multiple encephalic sites and pathways that range from the cerebral cortex to the spinal cord. Behavioral studies conducted in the 1980s revealed that electrical stimulation of the pretectal area causes antinociception dissociation from aversive responses. Anatomical and physiological studies identified the anterior pretectal nucleus and its descending projections to several midbrain, pontine, and medullary structures. The anterior pretectal nucleus is morphologically divided into a dorsal part that contains a dense neuron population (pars compacta) and a ventral part that contains a dense fiber band network (pars reticulata). Connections of the two anterior pretectal nucleus parts are broad and include prominent projections to and from major encephalic systems associated with somatosensory processes. Since the first observation that acute or chronic noxious stimuli activate the anterior pretectal nucleus, it has been established that numerous mediators participate in this response through distinct pathways. Recent studies have confirmed that at least two pain inhibitory pathways are activated from the anterior pretectal nucleus. This review focuses on rodent anatomical, behavioral, molecular, and neurochemical data that have helped to identify mediators of the anterior pretectal nucleus and pathways related to its role in pain modulation.

CaMKIIα Expressing Neurons to Report Activity-Related Endogenous Hypoxia upon Motor-Cognitive Challenge

We previously introduced the brain erythropoietin (EPO) circle as a model to explain the adaptive 'brain hardware upgrade' and enhanced performance. In this fundamental circle, brain cells, challenged by motor-cognitive tasks, experience functional hypoxia, triggering the expression of EPO among other genes. We attested hypoxic cells by a transgenic reporter approach under the ubiquitous CAG promoter, with Hif-1α oxygen-dependent degradation-domain (ODD) fused to CreERT2-recombinase. To specifically focus on the functional hypoxia of excitatory pyramidal neurons, here, we generated CaMKIIα-CreERT2-ODD::R26R-tdTomato mice. Behavioral challenges, light-sheet microscopy, immunohistochemistry, single-cell mRNA-seq, and neuronal cultures under normoxia or hypoxia served to portray these mice. Upon complex running wheel performance as the motor-cognitive task, a distinct increase in functional hypoxic neurons was assessed immunohistochemically and confirmed three-dimensionally. In contrast, fear conditioning as hippocampal stimulus was likely too short-lived to provoke neuronal hypoxia. Transcriptome data of hippocampus under normoxia versus inspiratory hypoxia revealed increases in CA1 CaMKIIα-neurons with an immature signature, characterized by the expression of Dcx, Tbr1, CaMKIIα, Tle4, and Zbtb20, and consistent with accelerated differentiation. The hypoxia reporter response was reproduced in vitro upon neuronal maturation. To conclude, task-associated activity triggers neuronal functional hypoxia as a local and brain-wide reaction mediating adaptive neuroplasticity. Hypoxia-induced genes such as EPO drive neuronal differentiation, brain maturation, and improved performance.

Silencing and stimulating the medial amygdala impairs ejaculation but not sexual incentive motivation in male rats

The medial amygdala (MeA) is a sexually dimorphic brain region that integrates sensory information and hormonal signaling, and is involved in the regulation of social behaviors. Lesion studies have shown a role for the MeA in copulation, most prominently in the promotion of ejaculation. The role of the MeA in sexual motivation, but also in temporal patterning of copulation, has not been extensively studied in rats. Here, we investigated the effect of chemogenetic inhibition and stimulation of the MeA on sexual incentive motivation and copulation in sexually experienced male rats. AAV5-CaMKIIa viral vectors coding for Gi, Gq, or no DREADDs (sham) were bilaterally infused into the MeA. Rats were assessed in the sexual incentive motivation test and copulation test upon systemic clozapine N-oxide (CNO) or vehicle administration. We report that MeA stimulation and inhibition did not affect sexual incentive motivation. Moreover, both stimulation and inhibition of the MeA decreased the number of ejaculations in a 30 min copulation test and increased ejaculation latency and the number of mounts and intromissions preceding ejaculation, while leaving the temporal pattern of copulation intact. These results indicate that the MeA may be involved in the processing of sensory feedback required to reach ejaculation threshold. The convergence of the behavioral effects of stimulating as well as inhibiting the MeA may reflect opposing behavioral control of specific neuronal populations within the MeA.

Protocol for BATTLE-1EX: A High-Resolution Imaging Method to Visualize Whole Synaptic Structures and their Components in the Nervous System

This protocol describes BATTLE-1EX, which is a combined method of BATTLE-1 and expansion microscopy to obtain high-resolution imaging of whole synaptic structures and their components of hippocampal neural circuits. BATTLE-1 uses two genetically engineered recombinase proteins and competition between two recombinases that can be independently titrated, resulting in a tunable proportion of mCherry+/YFP− and YFP+/mCherry− cells. As a combinational method, BATTLE-1EX has the potential to visualize and dissect whole synaptic structures in numerous regions in the brain.

For complete details on the use and execution of this protocol, please refer to Kohara et al. (2020).

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BATTLE-1EX enables 3D high-resolution imaging of whole synapses in the hippocampus.

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Split-tunable allocation of transgenes by competition between two recombinases

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Entire synaptic morphologies can be expanded without changing protein placement

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Localizations of synaptic proteins can be visualized in whole synaptic structures

CaMKIIα-Positive Interneurons Identified via a microRNA-Based Viral Gene Targeting Strategy

Single-cell analysis is revealing increasing diversity in gene expression profiles among brain cells. Traditional promotor-based viral gene expression techniques, however, cannot capture the growing variety among single cells. We demonstrate a novel viral gene expression strategy to target cells with specific miRNA expression using miRNA-guided neuron tags (mAGNET). We designed mAGNET viral vectors containing a CaMKIIα promoter and microRNA-128 (miR-128) binding sites, and labeled CaMKIIα⁺ cells with naturally low expression of miR-128 (Lm128C cells) in male and female mice. Although CaMKIIα has traditionally been considered as an excitatory neuron marker, our single-cell sequencing results reveal that Lm128C cells are CaMKIIα⁺ inhibitory neurons of parvalbumin or somatostatin subtypes. Further evaluation of the physiological properties of Lm128C cell in brain slices showed that Lm128C cells exhibit elevated membrane excitability, with biophysical properties closely resembling those of fast-spiking interneurons, consistent with previous transcriptomic findings of miR-128 in regulating gene networks that govern membrane excitability. To further demonstrate the utility of this new viral expression strategy, we expressed GCaMP6f in Lm128C cells in the superficial layers of the motor cortex and performed in vivo calcium imaging in mice during locomotion. We found that Lm128C cells exhibit elevated calcium event rates and greater intrapopulation correlation than the overall CaMKIIα⁺ cells during movement. In summary, the miRNA-based viral gene targeting strategy described here allows us to label a sparse population of CaMKIIα⁺ interneurons for functional studies, providing new capabilities to investigate the relationship between gene expression and physiological properties in the brain.SIGNIFICANCE STATEMENT We report the discovery of a class of CaMKIIα⁺ cortical interneurons, labeled via a novel miRNA-based viral gene targeting strategy, combinatorial to traditional promoter-based strategies. The fact that we found a small, yet distinct, population of cortical inhibitory neurons that express CaMKIIα demonstrates that CaMKIIα is not as specific for excitatory neurons as commonly believed. As single-cell sequencing tools are providing increasing insights into the gene expression diversity of neurons, including miRNA profile data, we expect that the miRNA-based gene targeting strategy presented here can help delineate many neuron populations whose physiological properties can be readily related to the miRNA gene regulatory networks.

The Dorsal Raphe Regulates the Duration of Attack through the Medial Orbitofrontal Cortex and Medial Amygdala

The dorsal raphe (DR) is an evolutionarily conserved brain structure that is involved in aggressive behavior. It projects onto numerous cortical and limbic areas underlying attack behavior. The specific neurocircuit through which the DR regulates aggression, however, is largely unclear. In this study we show that DR neurons expressing CaMKIIα are activated by attack behavior in mice. These neurons project to the medial aspect of the orbitofrontal cortex (OFC; MeOC) and the medial amygdala (MeA), two key regions within the neural circuit known to control aggressive behavior. Using an in vivo optogenetic approach, we show that attack bouts are shortened by inhibiting CaMKIIα+ neurons in the DR and their axons at the MeOC and prolonged by stimulating the DR-MeOC axons during an attack. By contrast, stimulating the axons of CaMKIIα+ DR neurons at the MeA shortens attack. Notably, neither the DR-MeOC or DR-MeA pathway initiates attack when stimulated. These results indicate that the DR-MeOC and DR-MeA pathways regulate the duration of attack behavior in opposite directions, revealing a circuit mechanism for the control of attack by the DR.

Potentiation of Divergent Medial Amygdala Pathways Drives Experience-Dependent Aggression Escalation

Heightened aggression can be serious concerns for the individual and society at large and are symptoms of many psychiatric illnesses, such as post-traumatic stress disorder. The circuit and synaptic mechanisms underlying experience-induced aggression increase, however, are poorly understood. Here we find that prior attack experience leading to an increase in aggressive behavior, known as aggression priming, activates neurons within the posterior ventral segment of the medial amygdala (MeApv). Optogenetic stimulation of MeApv using a synaptic depression protocol suppresses aggression priming, whereas high-frequency stimulation enhances aggression, mimicking attack experience. Interrogation of the underlying neural circuitry revealed that the MeApv mediates aggression priming via synaptic connections with the ventromedial hypothalamus (VmH) and bed nucleus of the stria terminalis (BNST). These pathways undergo NMDAR-dependent synaptic potentiation after attack. Furthermore, we find that the MeApv-VmH synapses selectively control attack duration, whereas the MeApv-BNST synapses modulate attack frequency, both with no effect on social behavior. Synaptic potentiation of the MeApv-VmH and MeApv-BNST pathways contributes to increased aggression induced by traumatic stress, and weakening synaptic transmission at these synapses blocks the effect of traumatic stress on aggression. These results reveal a circuit and synaptic basis for aggression modulation by experience that can be potentially leveraged toward clinical interventions.SIGNIFICANCE STATEMENT Heightened aggression can have devastating social consequences and may be associated with psychiatric disorders, such as post-traumatic stress disorder. The circuit and synaptic mechanisms underlying experience-induced aggression escalation, however, are poorly understood. Here we identify two aggression pathways between the posterior ventral segment of the medial amygdala and its downstream synaptic partners, the ventromedial hypothalamus and bed nucleus of the stria terminalis that undergo synaptic potentiation after attack and traumatic stress to enhance aggression. Notably, weakening synaptic transmission in these circuits blocks aggression priming, naturally occurring aggression, and traumatic stress-induced aggression increase. These results illustrate a circuit and synaptic basis of aggression modulation by experience, which can be potentially targeted for clinical interventions.

BATTLE: Genetically Engineered Strategies for Split-Tunable Allocation of Multiple Transgenes in the Nervous System

Elucidating fine architectures and functions of cellular and synaptic connections requires development of new flexible methods. Here, we created a concept called the “battle of transgenes,” based on which we generated strategies using genetically engineered battles of multiple recombinases. The strategies enabled split-tunable allocation of multiple transgenes. We demonstrated the versatility of these strategies and technologies in inducing strong and multi-sparse allocations of multiple transgenes. Furthermore, the combination of our transgenic strategy and expansion microscopy enabled three-dimensional high-resolution imaging of whole synaptic structures in the hippocampus with simultaneous visualizations of endogenous synaptic proteins. These strategies and technologies based on the battle of genes may accelerate the analysis of whole synaptic and cellular connections in diverse life science fields.

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Generation of BATTLE-recombinase systems for allocation of multiple transgenes

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Split-tunable allocation in BATTLE-1 and multi-sparse allocation in BATTLE-2

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Clear and strong labeling of dendrites and axons using BATTLE-2

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3D high-resolution imaging of whole synapses in hippocampus in BATTLE-1EX

Functional Access to Neuron Subclasses in Rodent and Primate Forebrain

Viral vectors enable foreign proteins to be expressed in brains of non-genetic species, including non-human primates. However, viruses targeting specific neuron classes have proved elusive. Here we describe viral promoters and strategies for accessing GABAergic interneurons and their molecularly defined subsets in the rodent and primate. Using a set intersection approach, which relies on two co-active promoters, we can restrict heterologous protein expression to cortical and hippocampal somatostatin-positive and parvalbumin-positive interneurons. With an orthogonal set difference method, we can enrich for subclasses of neuropeptide-Y-positive GABAergic interneurons by effectively subtracting the expression pattern of one promoter from that of another. These methods harness the complexity of gene expression patterns in the brain and significantly expand the number of genetically tractable neuron classes across mammals.

Procedures for Culturing and Genetically Manipulating Murine Hippocampal Postnatal Neurons

Neuronal hippocampal cultures are simple and valuable models for studying neuronal function. While embryonic cultures are widely used for different applications, mouse postnatal cultures are still challenging, lack reproducibility and/or exhibit inappropriate neuronal activity. Yet, postnatal cultures have major advantages such as allowing genotyping of pups before culture and reducing the number of experimental animals. Herein we describe a simple and fast protocol for culturing and genetically manipulating hippocampal neurons from P0 to P3 mice. This protocol provides reproducible cultures exhibiting a consistent neuronal development, normal excitatory over inhibitory neuronal ratio and a physiological neuronal activity. We also describe simple and efficient procedures for genetic manipulation of neurons using transfection reagent or lentiviral particles. Overall, this method provides a detailed and validated protocol allowing to explore cellular mechanisms and neuronal activity in postnatal hippocampal neurons in culture.

Zona incerta as a therapeutic target in Parkinson's disease

The zona incerta has recently become an important target for deep-brain stimulation (DBS) in Parkinson's disease (PD). The present review summarizes clinical, animal and anatomical data which have indicated an important role of this structure in PD, and discusses potential mechanisms involved in therapeutic effects of DBS. Animal studies have suggested initially some role of neurons as well as GABAergic and glutamatergic receptors of the zona incerta in locomotion and generation of PD signs. Anatomical data have indicated that thanks to its multiple interconnections with the basal ganglia, thalamus, cerebral cortex, brainstem, spinal cord and cerebellum, the zona incerta is an important link in a neuronal chain transmitting impulses involved in PD pathology. Finally, clinical studies have shown that DBS of this structure alleviates parkinsonian bradykinesia, muscle rigidity and tremor. DBS of caudal zona incerta seemed to be the most effective therapeutic intervention, especially with regard to reduction of PD tremor as well as other forms of tremor.

Sociability development in mice with cell-specific deletion of the NMDA receptor NR1 subunit gene

Social affiliative behavior is an important component of everyday life in many species and is likely to be disrupted in disabling ways in various neurodevelopmental and neuropsychiatric disorders. Therefore, determining the mechanisms involved in these processes is crucial. A link between N-methyl-d-aspartate (NMDA) receptor function and social behaviors has been clearly established. The cell types in which NMDA receptors are critical for social affiliative behavior, however, remain unclear. Here, we use mice carrying a conditional allele of the NMDA R1 subunit to address this question. Mice bearing a floxed NMDAR1 (NR1) allele were crossed with transgenic calcium/calmodulin-dependent kinase IIα (CaMKIIα)-Cre mice or parvalbumin (PV)-Cre mice targeting postnatal excitatory forebrain or PV-expressing interneurons, respectively, and assessed using the three-chambered Social Approach Test. We found that deletion of NR1 in PV-positive interneurons had no effect on social sniffing, but deletion of NR1 in glutamatergic pyramidal cells resulted in a significant increase in social approach behavior, regardless of age or sex. Therefore, forebrain excitatory neurons expressing NR1 play an important role in regulating social affiliative behavior.

Kcnab1 Is Expressed in Subplate Neurons With Unilateral Long-Range Inter-Areal Projections

Subplate (SP) neurons are among the earliest-born neurons in the cerebral cortex and heterogeneous in terms of gene expression. SP neurons consist mainly of projection neurons, which begin to extend their axons to specific target areas very early during development. However, the relationships between axon projection and gene expression patterns of the SP neurons, and their remnant layer 6b (L6b) neurons, are largely unknown. In this study, we analyzed the corticocortical projections of L6b/SP neurons in the mouse cortex and searched for a marker gene expressed in L6b/SP neurons that have ipsilateral inter-areal projections. Retrograde tracing experiments demonstrated that L6b/SP neurons in the primary somatosensory cortex (S1) projected to the primary motor cortex (M1) within the same cortical hemisphere at postnatal day (PD) 2 but did not show any callosal projection. This unilateral projection pattern persisted into adulthood. Our microarray analysis identified the gene encoding a β subunit of voltage-gated potassium channel (Kcnab1) as being expressed in L6b/SP. Double labeling with retrograde tracing and in situ hybridization demonstrated that Kcnab1 was expressed in the unilaterally-projecting neurons in L6b/SP. Embryonic expression was specifically detected in the SP as early as embryonic day (E) 14.5, shortly after the emergence of SP. Double immunostaining experiments revealed different degrees of co-expression of the protein product Kvβ1 with L6b/SP markers Ctgf (88%), Cplx3 (79%), and Nurr1 (58%), suggesting molecular subdivision of unilaterally-projecting L6b/SP neurons. In addition to expression in L6b/SP, scattered expression of Kcnab1 was observed during postnatal stages without layer specificity. Among splicing variants with three alternative first exons, the variant 1.1 explained all the cortical expression mentioned in this study. Together, our data suggest that L6b/SP neurons have corticocortical projections and Kcnab1 expression defines a subpopulation of L6b/SP neurons with a unilateral inter-areal projection.

Thalamic inhibitory circuits and network activity development

Inhibitory circuits in thalamus and cortex shape the major activity patterns observed by electroencephalogram (EEG) in the adult brain. Their delayed maturation and circuit integration, relative to excitatory neurons, suggest inhibitory neuronal development could be responsible for the onset of mature thalamocortical activity. Indeed, the immature brain lacks many inhibition-dependent activity patterns, such as slow-waves, delta oscillations and sleep-spindles, and instead expresses other unique oscillatory activities in multiple species including humans. Thalamus contributes significantly to the generation of these early oscillations. Compared to the abundance of studies on the development of inhibition in cortex, however, the maturation of thalamic inhibition is poorly understood. Here we review developmental changes in the neuronal and circuit properties of the thalamic relay and its interconnected inhibitory thalamic reticular nucleus (TRN) both in vitro and in vivo, and discuss their potential contribution to early network activity and its maturation. While much is unknown, we argue that weak inhibitory function in the developing thalamus allows for amplification of thalamocortical activity that supports the generation of early oscillations. The available evidence suggests that the developmental acquisition of critical thalamic oscillations such as slow-waves and sleep-spindles is driven by maturation of the TRN. Further studies to elucidate thalamic GABAergic circuit formation in relation to thalamocortical network function would help us better understand normal as well as pathological brain development.

Midline thalamic inputs to the amygdala: Ultrastructure and synaptic targets

One of the main subcortical inputs to the basolateral nucleus of the amygdala (BL) originates from a group of dorsal thalamic nuclei located at or near the midline, mainly from the central medial (CMT), and paraventricular (PVT) nuclei. Although similarities among the responsiveness of BL, CMT, and PVT neurons to emotionally arousing stimuli suggest that these thalamic inputs exert a significant influence over BL activity, little is known about the synaptic relationships that mediate these effects. Thus, the present study used Phaseolus vulgaris-leucoagglutinin (PHAL) anterograde tracing and electron microscopy to shed light on the ultrastructural properties and synaptic targets of CMT and PVT axon terminals in the rat BL. Virtually all PHAL-positive CMT and PVT axon terminals formed asymmetric synapses. Although CMT and PVT axon terminals generally contacted dendritic spines, a substantial number ended on dendritic shafts. To determine whether these dendritic shafts belonged to principal or local-circuit cells, calcium/calmodulin-dependent protein kinase II (CAMKIIα) immunoreactivity was used as a selective marker of principal BL neurons. In most cases, dendritic shafts postsynaptic to PHAL-labeled CMT and PVT terminals were immunopositive for CaMKIIα. Overall, these results suggest that CMT and PVT inputs mostly target principal BL neurons such that when CMT or PVT neurons fire, little feed-forward inhibition counters their excitatory influence over principal cells. These results are consistent with the possibility that CMT and PVT inputs constitute major determinants of BL activity.

Pool size estimations for dense-core vesicles in mammalian CNS neurons

Neuropeptides are essential signaling molecules transported and secreted by dense-core vesicles (DCVs), but the number of DCVs available for secretion, their subcellular distribution, and release probability are unknown. Here, we quantified DCV pool sizes in three types of mammalian CNS neurons in vitro and in vivo Super-resolution and electron microscopy reveal a total pool of 1,400-18,000 DCVs, correlating with neurite length. Excitatory hippocampal and inhibitory striatal neurons in vitro have a similar DCV density, and thalamo-cortical axons in vivo have a slightly higher density. Synapses contain on average two to three DCVs, at the periphery of synaptic vesicle clusters. DCVs distribute equally in axons and dendrites, but the vast majority (80%) of DCV fusion events occur at axons. The release probability of DCVs is 1-6%, depending on the stimulation. Thus, mammalian CNS neurons contain a large pool of DCVs of which only a small fraction can fuse, preferentially at axons.

Traumatic Stress Produces Distinct Activations of GABAergic and Glutamatergic Neurons in Amygdala

Posttraumatic stress disorder (PTSD) is an anxiety disorder characterized by intrusive recollections of a severe traumatic event and hyperarousal following exposure to the event. Human and animal studies have shown that the change of amygdala activity after traumatic stress may contribute to occurrences of some symptoms or behaviors of the patients or animals with PTSD. However, it is still unknown how the neuronal activation of different sub-regions in amygdala changes during the development of PTSD. In the present study, we used single prolonged stress (SPS) procedure to obtain the animal model of PTSD, and found that 1 day after SPS, there were normal anxiety behavior and extinction of fear memory in rats which were accompanied by a reduced proportion of activated glutamatergic neurons and increased proportion of activated GABAergic neurons in basolateral amygdala (BLA). About 10 days after SPS, we observed enhanced anxiety and impaired extinction of fear memory with increased activated both glutamatergic and GABAergic neurons in BLA and increased activated GABAergic neurons in central amygdala (CeA). These results indicate that during early and late phase after traumatic stress, distinct patterns of activation of glutamatergic neurons and GABAergic neurons are displayed in amygdala, which may be implicated in the development of PTSD.

Targeted overexpression of CRH receptor subtype 1 in central amygdala neurons: effect on alcohol-seeking behavior

RATIONALE

The corticotropin-releasing hormone (CRH) system is a key mediator of stress-induced responses in alcohol-seeking behavior. Recent research has identified the central nucleus of the amygdala (CeA), a brain region involved in the regulation of fear and stress-induced responses that is especially rich in CRH-positive neurons, as a key player in mediating excessive alcohol seeking. However, detailed characterization of the specific influences that local neuronal populations exert in mediating alcohol responses is hampered by current limitations in pharmacological and immunohistochemical tools for targeting CRH receptor subtype 1 (CRHR1).

OBJECTIVE

In this study, we investigated the effect of cell- and region-specific overexpression of CRHR1 in the CeA using a novel transgenic tool.

METHODS

Co-expression of CRHR1 in calcium-calmodulin-dependent kinase II (αCaMKII) neurons of the amygdala was demonstrated by double immunohistochemistry using a Crhr1-GFP reporter mouse line. A Cre-inducible Crhr1-expressing adeno-associated virus (AAV) was site-specifically injected into the CeA of αCaMKII-Cre^ERT2 transgenic rats to analyze the role of CRHR1 in αCaMKII neurons on alcohol self-administration and reinstatement behavior.

RESULTS

Forty-eight percent of CRHR1-containing cells showed co-expression of αCaMKII in the CeA. AAV-mediated gene transfer in αCaMKII neurons induced a 24-fold increase of Crhr1 mRNA in the CeA which had no effect on locomotor activity, alcohol self-administration, or cue-induced reinstatement. However, rats overexpressing Crhr1 in the CeA increased responding in the stress-induced reinstatement task with yohimbine serving as a pharmacological stressor.

CONCLUSION

We demonstrate that CRHR1 overexpression in CeA-αCaMKII neurons is sufficient to mediate increased vulnerability to stress-triggered relapse into alcohol seeking.

Developmental changes of GABA immunoreactivity in cortico-thalamic networks of an absence seizure model

Absence seizures (ASs) are associated with abnormalities in gamma-aminobutyric acid (GABA) neurotransmission in the thalamus and the cortex. In the present study, we used light microscopy GABA immunocytochemistry to quantify the GABA-immunoreactive (GABA-IR) neurons and neuropil in the thalamic ventral basal (VB) nucleus, the nucleus reticularis thalami (NRT), the dorsal lateral geniculate (dLGN), the primary motor cortex (M1) and perioral region of the somatosensory cortex (S1po) of genetic absence epilepsy rats from Strasbourg (GAERS). We used both the relative non-epileptic control (NEC) and normal Wistar rats as control strains, and investigated GABA immunostaining at postnatal day 15 (P15), P25, and P90. The main findings were i) an increase in GABA-IR neuropil in the VB at P25 and P90 in GAERS but not in NEC and Wistar rats; ii) an increase in NRT GABA-IR neurons in GAERS and NEC, but not Wistar, rats at both P25 and P90; and iii) an increase in GABA-IR neuron density in S1po of GAERS at P25 and P90 and in Wistar at P90. These results indicate that the increased GABAergic innervation in the VB at P25 most likely contributes to the enhanced tonic inhibition observed in GAERS prior to AS onset, whereas the lack of any anatomo-morphological GABAergic differences in GAERS S1po suggests that functional more than structural abnormalities underlie the origin of cortical paroxysms in S1po of this AS model.

This article is part of the “Special Issue Dedicated to Norman G. Bowery”.

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GABA-IR profiles increase in P25 to P90 VB neuropil in GAERS but not in NEC and Wistar rats.

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NRT GABA-IR neurons increase in P25 and P90 GAERS and NEC, but not in Wistar rats.

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GABA-IR neuron density increases in S1po of GAERS at P25 and P90 and in Wistar at P90.

Non-canonical heterogeneous cellular distribution and co-localization of CaMKIIα and CaMKIIβ in the spinal superficial dorsal horn

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a key enzyme in long-term plasticity in many neurons, including in the nociceptive circuitry of the spinal dorsal horn. However, although the role of CaMKII heterooligomers in neuronal plasticity is isoform-dependent, the distribution and co-localization of CaMKII isoforms in the dorsal horn have not been comprehensively investigated. Here, quantitative immunofluorescence analysis was used to examine the distribution of the two major neuronal CaMKII isoforms, α and β, in laminae I–III of the rat dorsal horn, with reference to inhibitory interneurons and neuronal populations defined by expression of parvalbumin, calretinin, and calbindin D28k. Unexpectedly, all or nearly all inhibitory and excitatory neurons showed both CaMKIIα and CaMKIIβ immunoreactivity, although at highly variable levels. Lamina III neurons showed less CaMKIIα immunoreactivity than laminae I–II neurons. Whereas CaMKIIα immunoreactivity was found at nearly similar levels in inhibitory and excitatory neurons, CaMKIIβ generally showed considerably lower immunoreactivity in inhibitory neurons. Distinct populations of inhibitory calretinin neurons and excitatory parvalbumin neurons exhibited high CaMKIIα-to-CaMKIIβ immunoreactivity ratios. CaMKIIα and CaMKIIβ immunoreactivity showed positive correlation at GluA2⁺ puncta in pepsin-treated tissue. These results suggest that, unlike the forebrain, the dorsal horn is characterized by similar expression of CaMKIIα in excitatory and inhibitory neurons, whereas CaMKIIβ is less expressed in inhibitory neurons. Moreover, CaMKII isoform expression varies considerably within and between neuronal populations defined by laminar location, calcium-binding protein expression, and transmitter phenotype, suggesting differences in CaMKII function both between and within neuronal populations in the superficial dorsal horn.

CaMKIIα Expression Defines Two Functionally Distinct Populations of Granule Cells Involved in Different Types of Odor Behavior

Granule cells (GCs) in the olfactory bulb (OB) play an important role in odor information processing. Although they have been classified into various neurochemical subtypes, the functional roles of these subtypes remain unknown. We used in vivo two-photon Ca²⁺ imaging combined with cell-type-specific identification of GCs in the mouse OB to examine whether functionally distinct GC subtypes exist in the bulbar network. We showed that half of GCs express Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα⁺) and that these neurons are preferentially activated by olfactory stimulation. The higher activity of CaMKIIα⁺ neurons is due to the weaker inhibitory input that they receive compared to their CaMKIIα-immunonegative (CaMKIIα^-) counterparts. In line with these functional data, immunohistochemical analyses showed that 75%-90% of GCs expressing the immediate early gene cFos are CaMKIIα⁺ in naive animals and in mice that have been exposed to a novel odor and go/no-go operant conditioning, or that have been subjected to long-term associative memory and spontaneous habituation/dishabituation odor discrimination tasks. On the other hand, a perceptual learning task resulted in increased activation of CaMKIIα^- cells. Pharmacogenetic inhibition of CaMKIIα⁺ GCs revealed that this subtype is involved in habituation/dishabituation and go/no-go odor discrimination, but not in perceptual learning. In contrast, pharmacogenetic inhibition of GCs in a subtype-independent manner affected perceptual learning. Our results indicate that functionally distinct populations of GCs exist in the OB and that they play distinct roles during different odor tasks.

Rapid binge-like eating and body weight gain driven by zona incerta GABA neuron activation

The neuronal substrate for binge eating, which can at times lead to obesity, is not clear. We find that optogenetic stimulation of mouse zona incerta (ZI) γ-aminobutyric acid (GABA) neurons or their axonal projections to paraventricular thalamus (PVT) excitatory neurons immediately (in 2 to 3 seconds) evoked binge-like eating. Minimal intermittent stimulation led to body weight gain; ZI GABA neuron ablation reduced weight. ZI stimulation generated 35% of normal 24-hour food intake in just 10 minutes. The ZI cells were excited by food deprivation and the gut hunger signal ghrelin. In contrast, stimulation of excitatory axons from the parasubthalamic nucleus to PVT or direct stimulation of PVT glutamate neurons reduced food intake. These data suggest an unexpected robust orexigenic potential for the ZI GABA neurons.

Tectal-derived interneurons contribute to phasic and tonic inhibition in the visual thalamus

The release of GABA from local interneurons in the dorsal lateral geniculate nucleus (dLGN-INs) provides inhibitory control during visual processing within the thalamus. It is commonly assumed that this important class of interneurons originates from within the thalamic complex, but we now show that during early postnatal development Sox14/Otx2-expressing precursor cells migrate from the dorsal midbrain to generate dLGN-INs. The unexpected extra-diencephalic origin of dLGN-INs sets them apart from GABAergic neurons of the reticular thalamic nucleus. Using optogenetics we show that at increased firing rates tectal-derived dLGN-INs generate a powerful form of tonic inhibition that regulates the gain of thalamic relay neurons through recruitment of extrasynaptic high-affinity GABA_A receptors. Therefore, by revising the conventional view of thalamic interneuron ontogeny we demonstrate how a previously unappreciated mesencephalic population controls thalamic relay neuron excitability.

Selective Manipulation of Neural Circuits

Unraveling the complex network of neural circuits that form the nervous system demands tools that can manipulate specific circuits. The recent evolution of genetic tools to target neural circuits allows an unprecedented precision in elucidating their function. Here we describe two general approaches for achieving circuit specificity. The first uses the genetic identity of a cell, such as a transcription factor unique to a circuit, to drive expression of a molecule that can manipulate cell function. The second uses the spatial connectivity of a circuit to achieve specificity: one genetic element is introduced at the origin of a circuit and the other at its termination. When the two genetic elements combine within a neuron, they can alter its function. These two general approaches can be combined to allow manipulation of neurons with a specific genetic identity by introducing a regulatory gene into the origin or termination of the circuit. We consider the advantages and disadvantages of both these general approaches with regard to specificity and efficacy of the manipulations. We also review the genetic techniques that allow gain- and loss-of-function within specific neural circuits. These approaches introduce light-sensitive channels (optogenetic) or drug sensitive channels (chemogenetic) into neurons that form specific circuits. We compare these tools with others developed for circuit-specific manipulation and describe the advantages of each. Finally, we discuss how these tools might be applied for identification of the neural circuits that mediate behavior and for repair of neural connections.

Selective Control of Fear Expression by Optogenetic Manipulation of Infralimbic Cortex after Extinction

Evidence from rodent and human studies has identified the ventromedial prefrontal cortex, specifically the infralimbic cortex (IL), as a critical brain structure in the extinction of conditioned fear. However, how IL activity controls fear expression at the time of extinction memory retrieval is unclear and controversial. To address this issue, we used optogenetics to precisely manipulate the activity of genetically targeted cells and to examine the real-time contribution of IL activity to expression of auditory-conditioned fear extinction in mice. We found that inactivation of IL, but not prelimbic cortex, impaired extinction retrieval. Conversely, photostimulation of IL excitatory neurons robustly enhanced the inhibition of fear expression after extinction, but not before extinction. Moreover, this effect was specific to the conditioned stimulus (CS): IL activity had no effect on expression of fear in response to the conditioned context after auditory fear extinction. Thus, in contrast to the expectation from a generally held view, artificial activation of IL produced no significant effect on expression of non-extinguished conditioned fear. Therefore, our data provide compelling evidence that IL activity is critical for expression of fear extinction and establish a causal role for IL activity in controlling fear expression in a CS-specific manner after extinction.

GABAergic Neuron-Specific Loss of Ube3a Causes Angelman Syndrome-Like EEG Abnormalities and Enhances Seizure Susceptibility

Loss of maternal UBE3A causes Angelman syndrome (AS), a neurodevelopmental disorder associated with severe epilepsy. We previously implicated GABAergic deficits onto layer (L) 2/3 pyramidal neurons in the pathogenesis of neocortical hyperexcitability, and perhaps epilepsy, in AS model mice. Here we investigate consequences of selective Ube3a loss from either GABAergic or glutamatergic neurons, focusing on the development of hyperexcitability within L2/3 neocortex and in broader circuit and behavioral contexts. We find that GABAergic Ube3a loss causes AS-like increases in neocortical EEG delta power, enhances seizure susceptibility, and leads to presynaptic accumulation of clathrin-coated vesicles (CCVs)-all without decreasing GABAergic inhibition onto L2/3 pyramidal neurons. Conversely, glutamatergic Ube3a loss fails to yield EEG abnormalities, seizures, or associated CCV phenotypes, despite impairing tonic inhibition onto L2/3 pyramidal neurons. These results substantiate GABAergic Ube3a loss as the principal cause of circuit hyperexcitability in AS mice, lending insight into ictogenic mechanisms in AS.

Large-scale imaging of cortical dynamics during sensory perception and behavior

Sensory-driven behaviors engage a cascade of cortical regions to process sensory input and generate motor output. To investigate the temporal dynamics of neural activity at this global scale, we have improved and integrated tools to perform functional imaging across large areas of cortex using a transgenic mouse expressing the genetically encoded calcium sensor GCaMP6s, together with a head-fixed visual discrimination behavior. This technique allows imaging of activity across the dorsal surface of cortex, with spatial resolution adequate to detect differential activity in local regions at least as small as 100 μm. Imaging during an orientation discrimination task reveals a progression of activity in different cortical regions associated with different phases of the task. After cortex-wide patterns of activity are determined, we demonstrate the ability to select a region that displayed conspicuous responses for two-photon microscopy and find that activity in populations of individual neurons in that region correlates with locomotion in trained mice. We expect that this paradigm will be a useful probe of information flow and network processing in brain-wide circuits involved in many sensory and cognitive processes.

Pilocarpine-induced seizures trigger differential regulation of microRNA-stability related genes in rat hippocampal neurons

Epileptogenesis in the temporal lobe elicits regulation of gene expression and protein translation, leading to reorganization of neuronal networks. In this process, miRNAs were described as being regulated in a cell-specific manner, although mechanistics of miRNAs activity are poorly understood. The specificity of miRNAs on their target genes depends on their intracellular concentration, reflecting the balance of biosynthesis and degradation. Herein, we confirmed that pilocarpine application promptly (<30 min) induces status epilepticus (SE) as revealed by changes in rat electrocorticogram particularly in fast-beta range (21–30 Hz). SE simultaneously upregulated XRN2 and downregulated PAPD4 gene expression in the hippocampus, two genes related to miRNA degradation and stability, respectively. Moreover, SE decreased the number of XRN2-positive cells in the hilus, while reduced the number of PAPD4-positive cells in CA1. XRN2 and PAPD4 levels did not change in calretinin- and CamKII-positive cells, although it was possible to determine that PAPD4, but not XRN2, was upregulated in parvalbumin-positive cells, revealing that SE induction unbalances the accumulation of these functional-opposed proteins in inhibitory interneurons that directly innervate distinct domains of pyramidal cells. Therefore, we were able to disclose a possible mechanism underlying the differential regulation of miRNAs in specific neurons during epileptogenesis.