Restricted cortical termination fields of the midline and intralaminar thalamic nuclei in the rat

Repeated Administrations of Polyphenolic Extracts Prevent Chronic Reflexive and Non-Reflexive Neuropathic Pain Responses by Modulating Gliosis and CCL2-CCR2/CX3CL1-CX3CR1 Signaling in Spinal Cord-Injured Female Mice

Neuropathic pain after spinal cord injury lacks any effective treatments, often leading to chronic pain. This study tested whether the daily administration of fully characterized polyphenolic extracts from grape stalks and coffee could prevent both reflexive and non-reflexive chronic neuropathic pain in spinal cord-injured mice by modulating the neuroimmune axis. Female CD1 mice underwent mild spinal cord contusion and received intraperitoneal extracts in weeks one, three, and six post-surgery. Reflexive pain responses were assessed weekly for up to 10 weeks, and non-reflexive pain was evaluated at the study's end. Neuroimmune crosstalk was investigated, focusing on glial activation and the expression of CCL2/CCR2 and CX3CL1/CX3CR1 in supraspinal pain-related areas, including the periaqueductal gray, rostral ventromedial medulla, anterior cingulate cortex, and amygdala. Repeated treatments prevented mechanical allodynia and thermal hyperalgesia, and also modulated non-reflexive pain. Moreover, they reduced supraspinal gliosis and regulated CCL2/CCR2 and CX3CL1/CX3CR1 signaling. Overall, the combination of polyphenols in these extracts may offer a promising pharmacological strategy to prevent chronic reflexive and non-reflexive pain responses by modifying central sensitization markers, not only at the contusion site but also in key supraspinal regions implicated in neuropathic pain. Overall, these data highlight the potential of polyphenolic extracts for spinal cord injury-induced chronic neuropathic pain.

Update on the connectivity of the paraventricular nucleus of the thalamus and its position within limbic corticostriatal circuits

The paraventricular nucleus of the thalamus (PVT) is generating interest because evidence establishes a role for this midline thalamic nucleus in behavior. Early tracing studies demonstrated that afferent fibers from the PVT and limbic cortex converge with dopamine fibers within subcompartments of the ventral striatum. Subsequent tracing studies expanded on these observations by establishing that the PVT provides a dense projection to a continuum of striatal-like regions that include the nucleus accumbens and the extended amygdala. These findings have been complemented by recent tracing evidence examining the organization of the PVT's efferent and afferent connections. An updated view of the organization of projection neurons in PVT is provided with a focus on the input-output relationship of these neurons. The review emphasizes recent findings demonstrating that the PVT is composed of intermixed populations of neurons with axons that collateralize to densely innervate limbic striatal regions while being reciprocally connected with limbic cortical areas that innervate the same regions of the striatum. An updated perspective of the PVT's anatomical relationship with limbic corticostriatal circuits is presented to stimulate research on how the PVT regulates behavioral responses associated with emotion and motivation.

A neural circuit from paratenial thalamic nucleus to anterior cingulate cortex for the regulation of opioid-induced hyperalgesia in male rats

Prolonged use of opioids can lead to increased sensitivity to painful stimuli, a condition referred to as opioid-induced hyperalgesia (OIH). However, the mechanisms underlying this contradictory situation remain unclear. This study elucidates the pivotal role of the paratenial thalamic nucleus (PT)-anterior cingulate cortex (ACC) neuronal circuit in the development of OIH in male rats. Immunofluorescence and electrophysiology experiments demonstrated aberrant activation of PT glutamatergic neurons (PTGlu) in rats with OIH. Optogenetic or chemogenetic activation of the PTGlu-ACC circuit aggravates mechanical and thermal hyperalgesia. Conversely, the inhibition of neuronal circuits showed analgesic effects. Additionally, PTGlu neurons project to both ACC pyramidal neurons and interneurons. Moreover, OIH affects the function of the ACC microcircuit, leading to decreased feedforward inhibition and an inhibitory/excitatory (I/E) imbalance in ACC pyramidal neurons. In conclusion, our findings highlighted the role of the PTGlu-ACC neuronal circuit in the development of opioid-induced hyperalgesia, suggesting that this circuit is a promising therapeutic target for addressing the side effects of opioids.

Topographic organization of bidirectional connections between the cingulate region (infralimbic area and anterior cingulate area, dorsal part) and the interbrain (diencephalon) of the adult male rat

The medial prefrontal cortex [cingulate region (Brodmann, 1909) (CNG)] in the rat is a connectionally and functionally diverse structure. It harbors cerebral nuclei that use long-range connections to promote adaptive changes to ongoing behaviors. The CNG is often described across functional and anatomical gradients, a dorsalventral gradient being the most prominent. Topographic organization is a general feature of the nervous system, and it is becoming clear that such spatial arrangements can reflect connectional, functional, and cellular differences. Portions of the CNG are known to form reciprocal connections with cortical areas and thalamus; however, these connectional features have not been described in detail or mapped to standardized rat brain atlases. Here, we used co-injected anterograde (Phaseolus vulgaris leucoagglutinin) and retrograde (cholera toxin B subunit) tracers throughout the CNG to identify zones of reciprocal connectivity in the diencephalon [or interbrain (Baer, 1837) (IB)]. Tracer distributions were observed using a Nissl-based atlas-mapping approach that facilitates description of topographic organization. This draft report describes CNG connections of the infralimbic area (Rose & Woolsey, 1948) (ILA) and the anterior cingulate area, dorsal part (Krettek & Price, 1977) (ACAd) throughout the IB. We found that corticothalamic connections are predominantly reciprocal, and that ILA and ACAd connections tended to be spatially segregated with minimal overlap. In the hypothalamus (Kuhlenbeck, 1927), we found dense and specific ILA-originating terminals in the following Brain Maps 4.0 atlas territories: dorsal region (Swanson, 2004) (LHAd) and suprafornical region (Swanson, 2004) (LHAs) of the lateral hypothalamic area (Nissl, 1913), parasubthalamic nucleus (Wang & Zhang, 1995) (PSTN), tuberal nucleus, terete part (Petrovich et al., 2001) (TUte), and an ill-defined dorsal cap of the medial mammillary nucleus (Gudden, 1881) (MM). We discuss these findings in the context of feeding behaviors.

Functional brain networks associated with the urge for action: Implications for pathological urge

Tics in Tourette syndrome (TS) are often preceded by sensory urges that drive the motor and vocal symptoms. Many everyday physiological behaviors are associated with sensory phenomena experienced as an urge for action, which may provide insight into the neural correlates of this pathological urge to tic that remains elusive. This study aimed to identify a brain network common to distinct physiological behaviors in healthy individuals, and in turn, examine whether this network converges with a network we previously localized in TS, using novel 'coordinate network mapping' methods. Systematic searches were conducted to identify functional neuroimaging studies reporting correlates of the urge to micturate, swallow, blink, or cough. Using activation likelihood estimation meta-analysis, we identified an 'urge network' common to these physiological behaviors, involving the bilateral insula/claustrum/inferior frontal gyrus/supplementary motor area, mid-/anterior- cingulate cortex (ACC), right postcentral gyrus, and left thalamus/precentral gyrus. Similarity between the urge and TS networks was identified in the bilateral insula, ACC, and left thalamus/claustrum. The potential role of the insula/ACC as nodes in the network for bodily representations of the urge to tic are discussed.

Local orchestration of distributed functional patterns supporting loss and restoration of consciousness in the primate brain

A central challenge of neuroscience is to elucidate how brain function supports consciousness. Here, we combine the specificity of focal deep brain stimulation with fMRI coverage of the entire cortex, in awake and anaesthetised non-human primates. During propofol, sevoflurane, or ketamine anaesthesia, and subsequent restoration of responsiveness by electrical stimulation of the central thalamus, we investigate how loss of consciousness impacts distributed patterns of structure-function organisation across scales. We report that distributed brain activity under anaesthesia is increasingly constrained by brain structure across scales, coinciding with anaesthetic-induced collapse of multiple dimensions of hierarchical cortical organisation. These distributed signatures are observed across different anaesthetics, and they are reversed by electrical stimulation of the central thalamus, coinciding with recovery of behavioural markers of arousal. No such effects were observed upon stimulating the ventral lateral thalamus, demonstrating specificity. Overall, we identify consistent distributed signatures of consciousness that are orchestrated by specific thalamic nuclei.

Causality methods to study the functional connectivity in brain networks: the basal ganglia - thalamus causal interactions

This study uses methods recently developed to study the complex evolution of atmospheric phenomena which have some similarities with the dynamics of the human brain. In both cases, it is possible to record the activity of particular centers (geographic regions or brain nuclei) but not to make an experimental modification of their state. The study of "causality", which is necessary to understand the dynamics of these complex systems and to develop robust models that can predict their evolution, is hampered by the experimental restrictions imposed by the nature of both systems. The study was performed with data obtained in the thalamus and basal ganglia of awake humans executing different tasks. This work studies the linear, non-linear and more complex relationships of these thalamic centers with the cortex and main BG nuclei, using three complementary techniques: the partial correlation regression method, the Gaussian process regression/distance correlation and a model-free method based on nearest-neighbor that computes the conditional mutual information. These causality methods indicated that the basal ganglia present a different functional relationship with the anterior-ventral (motor), intralaminar and medio-dorsal thalamic centers, and that more than 60% of these thalamus-basal ganglia relationships present a non-linear dynamic (35 of the 57 relationships found). These functional interactions were observed for basal ganglia nuclei with direct structural connections with the thalamus (primary somatosensory and motor cortex, striatum, internal globus pallidum and substantia nigra pars reticulata), but also for basal ganglia without structural connections with the thalamus (external globus pallidum and subthalamic nucleus). The motor tasks induced rapid modifications of the thalamus-basal ganglia interactions. These findings provide new perspectives of the thalamus - BG interactions, many of which may be supported by indirect functional relationships and not by direct excitatory/inhibitory interactions.

Two Distinct Neuronal Populations in the Rat Parafascicular Nucleus Oppositely Encode the Engagement in Stimulus-driven Reward-seeking

BACKGROUND

The thalamus is a phylogenetically well-preserved structure. Known to densely contact cortical regions, its role in the transmission of sensory information to the striatal complex has been widely reconsidered in recent years.

METHODS

The parafascicular nucleus of the thalamus (Pf) has been implicated in the orientation of attention toward salient sensory stimuli. In a stimulus-driven reward-seeking task, we sought to characterize the electrophysiological activity of Pf neurons in rats.

RESULTS

We observed a predominance of excitatory over inhibitory responses for all events in the task. Neurons responded more strongly to the stimulus compared to lever-pressing and reward collecting, confirming the strong involvement of the Pf in sensory information processing. The use of long sessions allowed us to compare neuronal responses to stimuli between trials when animals were engaged in action and those when they were not. We distinguished two populations of neurons with opposite responses: MOTIV+ neurons responded more intensely to stimuli followed by a behavioral response than those that were not. Conversely, MOTIV- neurons responded more strongly when the animal did not respond to the stimulus. In addition, the latency of excitation of MOTIV- neurons was shorter than that of MOTIV+ neurons.

CONCLUSION

Through this encoding, the Pf could perform an early selection of environmental stimuli transmitted to the striatum according to motivational level.

The Arousal-Related "Central Thalamus" Stimulation Site Simultaneously Innervates Multiple High-Level Frontal and Parietal Areas

In human and nonhuman primates, deep brain stimulation applied at or near the internal medullary lamina of the thalamus [a region referred to as "central thalamus," (CT)], but not at nearby thalamic sites, elicits major changes in the level of consciousness, even in some minimally conscious brain-damaged patients. The mechanisms behind these effects remain mysterious, as the connections of CT had not been specifically mapped in primates. In marmoset monkeys (Callithrix jacchus) of both sexes, we labeled the axons originating from each of the various CT neuronal populations and analyzed their arborization patterns in the cerebral cortex and striatum. We report that, together, these CT populations innervate an array of high-level frontal, posterior parietal, and cingulate cortical areas. Some populations simultaneously target the frontal, parietal, and cingulate cortices, while others predominantly target the dorsal striatum. Our data indicate that CT stimulation can simultaneously engage a heterogeneous set of projection systems that, together, target the key nodes of the attention, executive control, and working-memory networks of the brain. Increased functional connectivity in these networks has been previously described as a signature of consciousness.SIGNIFICANCE STATEMENT In human and nonhuman primates, deep brain stimulation at a specific site near the internal medullary lamina of the thalamus ["central thalamus," (CT)] had been shown to restore arousal and awareness in anesthetized animals, as well as in some brain-damaged patients. The mechanisms behind these effects remain mysterious, as CT connections remain poorly defined in primates. In marmoset monkeys, we mapped with sensitive axon-labeling methods the pathways originated from CT. Our data indicate that stimulation applied in CT can simultaneously engage a heterogeneous set of projection systems that, together, target several key nodes of the attention, executive control, and working-memory networks of the brain. Increased functional connectivity in these networks has been previously described as a signature of consciousness.

The structural connectivity mapping of the intralaminar thalamic nuclei

The intralaminar nuclei of the thalamus play a pivotal role in awareness, conscious experience, arousal, sleep, vigilance, as well as in cognitive, sensory, and sexual processing. Nonetheless, in humans, little is known about the direct involvement of these nuclei in such multifaceted functions and their structural connections in the brain. Thus, examining the versatility of structural connectivity of the intralaminar nuclei with the rest of the brain seems reasonable. Herein, we attempt to show the direct structural connectivity of the intralaminar nuclei to diencephalic, mesencephalic, and cortical areas using probabilistic tracking of the diffusion data from the human connectome project. The intralaminar nuclei fiber distributions span a wide range of subcortical and cortical areas. Moreover, the central medial and parafascicular nucleus reveal similar connectivity to the temporal, visual, and frontal cortices with only slight variability. The central lateral nucleus displays a refined projection to the superior colliculus and fornix. The centromedian nucleus seems to be an essential component of the subcortical somatosensory system, as it mainly displays connectivity via the medial and superior cerebellar peduncle to the brainstem and the cerebellar lobules. The subparafascicular nucleus projects to the somatosensory processing areas. It is interesting to note that all intralaminar nuclei have connections to the brainstem. In brief, the structural connectivity of the intralaminar nuclei aligns with the structural core of various functional demands for arousal, emotion, cognition, sensory, vision, and motor processing. This study sheds light on our understanding of the structural connectivity of the intralaminar nuclei with cortical and subcortical structures, which is of great interest to a broader audience in clinical and neuroscience research.

Projections from the five divisions of the orbital cortex to the thalamus in the rat

The orbital cortex (ORB) of the rat consists of five divisions: the medial (MO), ventral (VO), ventrolateral (VLO), lateral (LO), and dorsolateral (DLO) orbital cortices. No previous report has comprehensively examined and compared projections from each division of the ORB to the thalamus. Using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin, we describe the efferent projections from the five divisions of the ORB to the thalamus in the rat. We demonstrated that, with some overlap, each division of the ORB distributed in a distinct (and unique) manner to nuclei of the thalamus. Overall, ORB projected to a relatively restricted number of sites in the thalamus, and strikingly distributed entirely to structures of the medial/midline thalamus, while completely avoiding lateral regions or principal nuclei of the thalamus. The main termination sites in the thalamus were the paratenial nucleus (PT) and nucleus reuniens (RE) of the midline thalamus, the medial (MDm) and central (MDc) divisions of the mediodorsal nucleus, the intermediodorsal nucleus, the central lateral, paracentral, and central medial nuclei of the rostral intralaminar complex and the submedial nucleus (SM). With some exceptions, medial divisions of the ORB (MO, VO) mainly targeted "limbic-associated" nuclei such as PT, RE, and MDm, whereas lateral division (VLO, LO, DLO) primarily distributed to "sensorimotor-associated" nuclei including MDc, SM, and the rostral intralaminar complex. As discussed herein, the medial/midline thalamus may represent an important link (or bridge) between the orbital cortex and the hippocampus and between the ORB and medial prefrontal cortex. In summary, the present results demonstrate that each division of the orbital cortex projects in a distinct manner to nuclei of the thalamus which suggests unique functions for each division of the orbital cortex.

Parallel Pathways Provide Hippocampal Spatial Information to Prefrontal Cortex

Long-range synaptic connections define how information flows through neuronal networks. Here, we combined retrograde and anterograde trans-synaptic viruses to delineate areas that exert direct and indirect influence over the dorsal and ventral prefrontal cortex (PFC) of the rat (both sexes). Notably, retrograde tracing using pseudorabies virus (PRV) revealed that both dorsal and ventral areas of the PFC receive prominent disynaptic input from the dorsal CA3 (dCA3) region of the hippocampus. The PRV experiments also identified candidate anatomical relays for this disynaptic pathway, namely, the ventral hippocampus, lateral septum, thalamus, amygdala, and basal forebrain. To determine the viability of each of these relays, we performed three additional experiments. In the first, we injected the retrograde monosynaptic tracer Fluoro-Gold into the PFC and the anterograde monosynaptic tracer Fluoro-Ruby into the dCA3 to confirm the first-order connecting areas and revealed several potential relay regions between the PFC and dCA3. In the second, we combined PRV injection in the PFC with polysynaptic anterograde viral tracer (HSV-1) in the dCA3 to reveal colabeled connecting neurons, which were evident only in the ventral hippocampus. In the third, we combined retrograde adeno-associated virus (AAV) injections in the PFC with an anterograde AAV in the dCA3 to reveal anatomical relay neurons in the ventral hippocampus and dorsal lateral septum. Together, these findings reveal parallel disynaptic pathways from the dCA3 to the PFC, illuminating a new anatomical framework for understanding hippocampal-prefrontal interactions. We suggest that the representation of context and space may be a universal feature of prefrontal function.SIGNIFICANCE STATEMENT The known functions of the prefrontal cortex are shaped by input from multiple brain areas. We used transneuronal viral tracing to discover multiple prominent disynaptic pathways through which the dorsal hippocampus (specifically, the dorsal CA3) has the potential to shape the actions of the prefrontal cortex. The demonstration of neuronal relays in the ventral hippocampus and lateral septum presents a new foundation for understanding long-range influences over prefrontal interactions, including the specific contribution of the dorsal CA3 to prefrontal function.

Sex differences in electrophysiological properties and voltage-gated ion channel expression in the paraventricular thalamic nucleus following repeated stress

Background

Habituation to repeated stress refers to a progressive reduction in the stress response following multiple exposures to the same, predictable stressor. We previously demonstrated that the posterior division of the paraventricular thalamic nucleus (pPVT) nucleus regulates habituation to 5 days of repeated restraint stress in male rats. Compared to males, female rats display impaired habituation to 5 days of restraint. To better understand how activity of pPVT neurons is differentially impacted in stressed males and females, we examined the electrophysiological properties of pPVT neurons under baseline conditions or following restraint.

Methods

Adult male and female rats were exposed to no stress (handling only), a single period of 30 min restraint or 5 daily exposures to 30 min restraint. 24 h later, pPVT tissue was prepared for recordings.

Results

We report here that spontaneous excitatory post-synaptic current (sEPSC) amplitude was increased in males, but not females, following restraint. Furthermore, resting membrane potential of pPVT neurons was more depolarized in males. This may be partially due to reduced potassium leakage in restrained males as input resistance was increased in male, but not female, rats 24 h following 1 or 5 days of 30-min restraint. Reduced potassium efflux during action potential firing also occurred in males following a single restraint as action potential half-width was increased following a single restraint. Restraint had limited effects on electrophysiological properties in females, although the mRNA for 10 voltage-gated ion channel subunits was altered in the pPVT of female rats.

Conclusions

The results suggest that restraint-induced changes in pPVT activation promote habituation in males. These findings are the first to describe a sexual dimorphism in stress-induced electrophysiological properties and voltage-gated ion channel expression in the pPVT. These results may explain, at least in part, why habituation to 5 days of restraint is disrupted in female rats.

Male, but not female, pPVT neurons display increases in EPSC amplitude and decay time 24 h following one and five restraints.

Input resistance is increased 24 h following one and five restraints in male, but not female, pPVT neurons.

Afterhyperpolarization potential is greater in pPVT neurons of females compared to males, regardless of restraint.

Restraint alters the expression of 10 voltage-gated ion channel transcripts in the pPVT of females, but only 3 in males.

Structural and functional organization of the midline and intralaminar nuclei of the thalamus

The midline and intralaminar nuclei of the thalamus form a major part of the "limbic thalamus;" that is, thalamic structures anatomically and functionally linked with the limbic forebrain. The midline nuclei consist of the paraventricular (PV) and paratenial nuclei, dorsally and the rhomboid and nucleus reuniens (RE), ventrally. The rostral intralaminar nuclei (ILt) consist of the central medial (CM), paracentral (PC) and central lateral (CL) nuclei. We presently concentrate on RE, PV, CM and CL nuclei of the thalamus. The nucleus reuniens receives a diverse array of input from limbic-related sites, and predominantly projects to the hippocampus and to "limbic" cortices. The RE participates in various cognitive functions including spatial working memory, executive functions (attention, behavioral flexibility) and affect/fear behavior. The PV receives significant limbic-related afferents, particularly the hypothalamus, and mainly distributes to "affective" structures of the forebrain including the bed nucleus of stria terminalis, nucleus accumbens and the amygdala. Accordingly, PV serves a critical role in "motivated behaviors" such as arousal, feeding/consummatory behavior and drug addiction. The rostral ILt receives both limbic and sensorimotor-related input and distributes widely over limbic and motor regions of the frontal cortex-and throughout the dorsal striatum. The intralaminar thalamus is critical for maintaining consciousness and directly participates in various sensorimotor functions (visuospatial or reaction time tasks) and cognitive tasks involving striatal-cortical interactions. As discussed herein, while each of the midline and intralaminar nuclei are anatomically and functionally distinct, they collectively serve a vital role in several affective, cognitive and executive behaviors - as major components of a brainstem-diencephalic-thalamocortical circuitry.

The central medial thalamic nucleus facilitates bilateral movement execution in rats

Intralaminar thalamic nuclei, including the central medial nucleus (CMT), have been classically implicated in the control of attentional functional states such as sleep-wake transitions. In rodents, the CMT innervates large cortical and subcortical areas bilaterally, including sensorimotor regions of the cortex and striatum, but its contribution to motor function, which regularly develops in faster temporal scales than attentional states, is still far from being completely understood. Here, by using a novel behavioral protocol to evaluate bilateral coordination in rats, combined with electrophysiological recordings and optogenetic manipulations, we studied the contribution of the CMT to motor control and coordination. We found that optogenetic stimulation of the central region of the CMT produced bilateral recruitment of neural activity in the sensorimotor cortex and striatum. The same type of stimulations produced a significant increase in bilateral movement coordination of the forelimbs accompanied by a decrease in movement trajectory variability. Optogenetic inactivation of the CMT did not affect motor execution but significantly increased execution times, suggesting less interest in the task. Altogether, our results indicate that brief CMT activations create windows of synchronized bilateral cortico-striatal activity, suitable to facilitate motor coordination in temporal scales relevant for motor execution. Significance Statement The central medial thalamic nucleus (CMT) has been classically implicated in attentional processes. However, it also innervates large motor cortico-striatal regions, but its participation in motor control and coordination is still not well understood. Here, by combining a novel behavioral protocol with optogenetic manipulations, we have found that brief CMT activations create windows of synchronized bilateral cortico-striatal activity, suitable to facilitate motor coordination in temporal scales relevant for motor execution.

Arc-Mediated Plasticity in the Paraventricular Thalamic Nucleus Promotes Habituation to Stress

BACKGROUND

Habituation is defined as a progressive decline in response to repeated exposure to a familiar and predictable stimulus and is highly conserved across species. Disrupted habituation is a signature of posttraumatic stress disorder. In rodents, habituation is observed in neural, neuroendocrine, and behavioral responses to repeated exposure to predictable and moderately intense stress or restraint. We previously demonstrated that lesioning the posterior paraventricular thalamic nucleus (pPVT) impairs habituation. However, the underlying molecular mechanisms and specific neural connections among the pPVT and other brain regions that underlie habituation are unknown.

METHODS

Behavioral and neuroendocrine habituation was assessed in adult male Sprague Dawley rats using the repeated restraint paradigm. Pan-neuronal and Cre-dependent DREADDs (designer receptors exclusively activated by designer drugs) were used to chemogenetically inhibit the pPVT and the subpopulation of pPVT neurons that project to the medial prefrontal cortex (mPFC), respectively. Activity-regulated cytoskeleton-associated protein (Arc) expression was knocked down in the pPVT using small interfering RNA. Structural plasticity of pPVT neurons was assessed using Golgi staining. Local field potential recordings were used to assess coherent neural activity between the pPVT and mPFC. The attentional set shifting task was used to assess mPFC-dependent behavior.

RESULTS

Here, we show that Arc promotes habituation by increasing stress-induced spinogenesis in the pPVT, increasing coherent neural activity with the mPFC, and improving mPFC-mediated cognitive flexibility.

CONCLUSIONS

Our results demonstrate that Arc induction in the pPVT regulates habituation and mPFC function. Therapies that improve synaptic plasticity during posttraumatic stress disorder therapy may enhance habituation and the efficacy of posttraumatic stress disorder treatment.

Eating driven by the gustatory insula: contrasting regulation by infralimbic vs. prelimbic cortices

Subregions within insular cortex and medial prefrontal cortex (mPFC) have been implicated in eating disorders; however, the way these brain regions interact to produce dysfunctional eating is poorly understood. The present study explored how two mPFC subregions, the infralimbic (IL) and prelimbic (PRL) cortices, regulate sucrose hyperphagia elicited specifically by a neurochemical manipulation of the agranular/dysgranular region of gustatory insula (AI/DI). Using intra-AI/DI infusion of the mu-opioid receptor (µ-OR) agonist, DAMGO (1 µg), sucrose hyperphagia was generated in ad-libitum-maintained rats, while in the same rat, either the IL or prelimbic (PRL) subregion of mPFC was inactivated bilaterally with muscimol (30 ng). Intra-IL muscimol markedly potentiated AI/DI DAMGO-induced sucrose hyperphagia by increasing eating bout duration and food consumption per bout. In contrast, PRL attenuated intra-AI/DI DAMGO-driven sucrose intake and feeding duration and eliminated the small DAMGO-induced increase in feeding bout initiation. Intra-IL or -PRL muscimol alone (i.e., without intra-AI/DI DAMGO) did not alter feeding behavior, but slightly reduced exploratory-like rearing in both mPFC subregions. These results reveal anatomical heterogeneity in mPFC regulation of the intense feeding-motivational state engendered by µ-OR signaling in the gustatory insula: IL significantly curtails consummatory activity, while PRL modestly contributes to feeding initiation. Results are discussed with regard to potential circuit-based mechanisms that may underlie the observed results.

Mapping thalamic-anterior cingulate monosynaptic inputs in adult mice

The anterior cingulate cortex (ACC) is located in the frontal part of the cingulate cortex, and plays important roles in pain perception and emotion. The thalamocortical pathway is the major sensory input to the ACC. Previous studies have show that several different thalamic nuclei receive projection fibers from spinothalamic tract, that in turn send efferents to the ACC by using neural tracers and optical imaging methods. Most of these studies were performed in monkeys, cats, and rats, few studies were reported systematically in adult mice. Adult mice, especially genetically modified mice, have provided molecular and synaptic mechanisms for cortical plasticity and modulation in the ACC. In the present study, we utilized rabies virus-based retrograde tracing system to map thalamic-anterior cingulate monosynaptic inputs in adult mice. We also combined with a new high-throughput VISoR imaging technique to generate a three-dimensional whole-brain reconstruction, especially the thalamus. We found that cortical neurons in the ACC received direct projections from different sub-nuclei in the thalamus, including the anterior, ventral, medial, lateral, midline, and intralaminar thalamic nuclei. These findings provide key anatomic evidences for the connection between the thalamus and ACC.

Dual projecting cells linking thalamic and cortical communication routes between the medial prefrontal cortex and hippocampus

The interactions between the medial prefrontal cortex (mPFC) and the hippocampus (HC) are critical for memory and decision making and have been specifically implicated in several neurological disorders including schizophrenia, epilepsy, frontotemporal dementia, and Alzheimer's disease. The ventral midline thalamus (vmThal), and lateral entorhinal cortex and perirhinal cortex (LEC/PER) constitute major communication pathways that facilitate mPFC-HC interactions in memory. Although vmThal and LEC/PER circuits have been delineated separately we sought to determine whether these two regions share cell-specific inputs that could influence both routes simultaneously. To do this we used a dual fluorescent retrograde tracing approach using cholera toxin subunit-B (CTB-488 and CTB-594) with injections targeting vmThal and the LEC/PER in rats. Retrograde cell body labeling was examined in key regions of interest within the mPFC-HC system including: (1) mPFC, specifically anterior cingulate cortex (ACC), dorsal and ventral prelimbic cortex (dPL, vPL), and infralimbic cortex (IL); (2) medial and lateral septum (MS, LS); (3) subiculum (Sub) along the dorsal-ventral and proximal-distal axes; and (4) LEC and medial entorhinal cortex (MEC). Results showed that dual vmThal-LEC/PER-projecting cell populations are found in MS, vSub, and the shallow layers II/III of LEC and MEC. We did not find any dual projecting cells in mPFC or in the cornu ammonis (CA) subfields of the HC. Thus, mPFC and HC activity is sent to vmThal and LEC/PER via non-overlapping projection cell populations. Importantly, the dual projecting cell populations in MS, vSub, and EC are in a unique position to simultaneously influence both cortical and thalamic mPFC-HC pathways critical to memory. SIGNIFICANCE STATEMENT: The interactions between mPFC and HC are critical for learning and memory, and dysfunction within this circuit is implicated in various neurodegenerative and psychiatric diseases. mPFC-HC interactions are mediated through multiple communication pathways including a thalamic hub through the vmThal and a cortical hub through lateral entorhinal cortex and perirhinal cortex. Our data highlight newly identified dual projecting cell populations in the septum, Sub, and EC of the rat brain. These dual projecting cells may have the ability to modify the information flow within the mPFC-HC circuit through synchronous activity, and thus offer new cell-specific circuit targets for basic and translational studies in memory.

Evolution of prefrontal cortex

Subdivisions of the prefrontal cortex (PFC) evolved at different times. Agranular parts of the PFC emerged in early mammals, and rodents, primates, and other modern mammals share them by inheritance. These are limbic areas and include the agranular orbital cortex and agranular medial frontal cortex (areas 24, 32, and 25). Rodent research provides valuable insights into the structure, functions, and development of these shared areas, but it contributes less to parts of the PFC that are specific to primates, namely, the granular, isocortical PFC that dominates the frontal lobe in humans. The first granular PFC areas evolved either in early primates or in the last common ancestor of primates and tree shrews. Additional granular PFC areas emerged in the primate stem lineage, as represented by modern strepsirrhines. Other granular PFC areas evolved in simians, the group that includes apes, humans, and monkeys. In general, PFC accreted new areas along a roughly posterior to anterior trajectory during primate evolution. A major expansion of the granular PFC occurred in humans in concert with other association areas, with modifications of corticocortical connectivity and gene expression, although current evidence does not support the addition of a large number of new, human-specific PFC areas.

Neurophysiological characteristics in the periventricular/periaqueductal gray correlate with pain perception, sensation, and affect in neuropathic pain patients

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The PAG/PVG carries out its biology function by oscillatory network.

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Three distinct local networks of oscillations involved in pain perception, sensory and affective.

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The delta oscillation is a key hub for coding pain perception.

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The high-gamma oscillation is a key hub for coding sensory pain.

The periventricular/periaqueductal gray (PAG/PVG) is critical for pain perception and is associated with the emotional feelings caused by pain. However, the electrophysiological characteristics of the PAG/PVG have been little investigated in humans with chronic pain. The present study analyzed the oscillatory characteristics of local field potentials (LFPs) in the PAG/PVG of eighteen neuropathic pain patients. Power spectrum analysis and neural state analysis were applied to the PAG/PVG LFPs. Neural state analysis is based on a dynamic neural state identification approach and discriminates the LFPs into different neural states, including a single neural state based on one oscillation and a combinational neural state based on two paired oscillations. The durations and occurrence rates were used to quantify the dynamic features of the neural state. The results show that the combined neural state forms three local networks based on neural oscillations that are responsible for the perceptive, sensory, and affective components of pain. The first network is formed by the interaction of the delta oscillation with other oscillations and is responsible for the coding of pain perception. The second network is responsible for the coding of sensory pain information, uses high gamma as the main node, and is widely connected with other neural oscillations. The third network is responsible for the coding of affective pain information, and beta oscillations play an important role in it. This study suggested that the combination of two neural oscillations in the PAG/PVG is essential for encoding perceptive, sensory, and affective measures of pain.

Centromedian Nucleus and Epilepsy

SUMMARY

Centromedian thalamic nucleus is an intralaminar nucleus with vast connectivity to cerebral cortex and basal ganglia. It receives afferents from the brain stem through the central tegmental tract and is part of the diffuse thalamic projection system. Because the reticulothalamic system has been related to initiation and propagation of epileptic activity (centroencephalic theory of epilepsy), deep brain stimulation has been proposed to interfere with seizure genesis or propagation. Centromedian thalamic nucleus is a large nucleus laying nearby the anatomical references for stereotaxis and therefore a convenient surgical target to approach. Electrodes are implanted in the anterior ventral lateral part of the nucleus (parvocellular area), guided by intraoperative recruiting responses elicited by unilateral 6 to 8 Hz electrical stimulation delivered through the deep brain stimulation electrode. Therapeutic stimulation is delivered with the following parameters: 60 Hz, 450 μs, 3.0 V. Seizure control runs between 69% and 83% in different reports, decreasing mainly generalized seizures from the start, with significant improvement in neuropsychological performance. Significant decrease in seizure occurs from hours to days after the onset of deep brain stimulation. Some reports refer that seizure improvement may occur by the simple insertion of the deep brain stimulation electrodes, and therefore, it was used to treat refractory epileptic status.

Hypoactive Thalamic Crh+ Cells in a Female Mouse Model of Alcohol Drinking After Social Trauma

BACKGROUND

Comorbid stress-induced mood and alcohol use disorders are increasingly prevalent among female patients. Stress exposure can disrupt salience processing and goal-directed decision making, contributing to persistent maladaptive behavioral patterns; these and other stress-sensitive cognitive and behavioral processes rely on dynamic and coordinated signaling by midline and intralaminar thalamic nuclei. Considering the role of social trauma in the trajectory of these debilitating psychopathologies, identifying vulnerable thalamic cells may provide guidance for targeting persistent stress-induced symptoms.

METHODS

A novel behavioral protocol traced the progression from social trauma to the development of social defensiveness and chronically escalated alcohol consumption in female mice. Recent cell activation-measured as cFos-was quantified in thalamic cells after safe social interactions, revealing stress-sensitive corticotropin-releasing hormone-expressing (Crh+) anterior central medial thalamic (aCMT) cells. These cells were optogenetically stimulated during stress-induced social defensiveness and abstinence-escalated binge drinking.

RESULTS

Crh+ aCMT neurons exhibited substantial activation after social interactions in stress-naïve but not in stressed female mice. Photoactivating Crh+ aCMT cells dampened stress-induced social deficits, whereas inhibiting these cells increased social defensiveness in stress-naïve mice. Optogenetically activating Crh+ aCMT cells diminished abstinence-escalated binge alcohol drinking in female mice, regardless of stress history.

CONCLUSIONS

This work uncovers a role for Crh+ aCMT neurons in maladaptive stress-induced social interactions and in binge drinking after forced abstinence in female mice. This molecularly defined thalamic cell population may serve as a critical stress-sensitive hub for social deficits caused by exposure to social trauma and for patterns of excessive alcohol drinking in female populations.

Efferent and afferent connections of supratrigeminal neurons conveying orofacial muscle proprioception in rats

The supratrigeminal nucleus (Su5) is a key structure for controlling jaw movements; it receives proprioceptive sensation from jaw-closing muscle spindles (JCMSs) and sends projections to the trigeminal motor nucleus (Mo5). However, the central projections and regulation of JCMS proprioceptive sensation are not yet fully understood. Therefore, we aimed to reveal the efferent and afferent connections of the Su5 using neuronal tract tracings. Anterograde tracer injections into the Su5 revealed that the Su5 sends contralateral projections (or bilateral projections with a contralateral predominance) to the Su5, basilar pontine nuclei, pontine reticular nucleus, deep mesencephalic nucleus, superior colliculus, caudo-ventromedial edge of the ventral posteromedial thalamic nucleus, parafascicular thalamic nucleus, zona incerta, and lateral hypothalamus, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) to the intertrigeminal region, trigeminal oral subnucleus, dorsal medullary reticular formation, and hypoglossal nucleus as well as the Mo5. Retrograde tracer injections into the Su5 demonstrated that the Su5 receives bilateral projections with a contralateral predominance (or contralateral projections) from the primary and secondary somatosensory cortices, granular insular cortex, and Su5, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) from the dorsal peduncular cortex, bed nuclei of stria terminalis, central amygdaloid nucleus, lateral hypothalamus, parasubthalamic nucleus, trigeminal mesencephalic nucleus, parabrachial nucleus, juxtatrigeminal region, trigeminal oral and caudal subnuclei, and dorsal medullary reticular formation. These findings suggest that the Su5, which receives JCMS proprioception, has efferent and afferent connections with multiple brain regions that are involved in emotional and autonomic functions as well as orofacial motor functions.

Midline Thalamic Damage Associated with Alcohol-Use Disorders: Disruption of Distinct Thalamocortical Pathways and Function

The thalamus, a significant part of the diencephalon, is a symmetrical and bilateral central brain structure. The thalamus is subdivided into three major groups of nuclei based on their function: sensorimotor nuclei (or principal/relay nuclei), limbic nuclei and nuclei bridging these two domains. Anatomically, nuclei within the thalamus are described by their location, such as anterior, medial, lateral, ventral, and posterior. In this review, we summarize the role of medial and midline thalamus in cognition, ranging from learning and memory to flexible adaptation. We focus on the discoveries in animal models of alcohol-related brain damage, which identify the loss of neurons in the medial and midline thalamus as drivers of cognitive dysfunction associated with alcohol use disorders. Models of developmental ethanol exposure and models of adult alcohol-related brain damage and are compared and contrasted, and it was revealed that there are similar (anterior thalamus) and different (intralaminar [adult exposure] versus ventral midline [developmental exposure]) thalamic pathology, as well as disruptions of thalamo-hippocampal and thalamo-cortical circuits. The final part of the review summarizes approaches to recover alcohol-related brain damage and cognitive and behavioral outcomes. These approaches include pharmacological, nutritional and behavioral interventions that demonstrated the potential to mitigate alcohol-related damage. In summary, the medial/midline thalamus is a significant contributor to cognition function, which is also sensitive to alcohol-related brain damage across the life span, and plays a role in alcohol-related cognitive dysfunction.

Functional Connectivity of the Human Paraventricular Thalamic Nucleus: Insights From High Field Functional MRI

The paraventricular thalamic nucleus (PVT) is a small but highly connected nucleus of the dorsal midline thalamus. The PVT has garnered recent attention as a context-sensitive node within the thalamocortical arousal system that modulates state-dependent motivated behaviors. Once considered related to generalized arousal responses with non-specific impacts on behavior, accumulating evidence bolsters the contemporary view that discrete midline thalamic subnuclei belong to specialized corticolimbic and corticostriatal circuits related to attention, emotions, and cognition. However, the functional connectivity patterns of the human PVT have yet to be mapped. Here, we combined high-quality, high-resolution 7T and 3T resting state MRI data from 121 young adult participants from the Human Connectome Project (HCP) and thalamic subnuclei atlas masks to investigate resting state functional connectivity of the human PVT. The 7T results demonstrated extensive positive functional connectivity with the brainstem, midbrain, ventral and dorsal medial prefrontal cortex (mPFC), anterior and posterior cingulate, ventral striatum, hippocampus, and amygdala. These connections persist upon controlling for functional connectivity of the rest of the thalamus. Whole-brain contrasts provided further evidence that, compared to three nearby midline thalamic subnuclei, functional connectivity of the PVT is strong with the hippocampus, amygdala, ventral and dorsal mPFC, and middle temporal gyrus. These findings suggest that, even during rest, the human PVT is functionally coupled with many regions known to be structurally connected to rodent and non-human primate PVT. Further, cosine similarity analysis results suggested the PVT is integrated into the default mode network (DMN), an intrinsic connectivity network associated with episodic memory and self-referential thought. The current work provides a much-needed foundation for ongoing and future work examining the functional roles of the PVT in humans.

Rostral Intralaminar Thalamus Engagement in Cognition and Behavior

The thalamic rostral intralaminar nuclei (rILN) are a contiguous band of neurons that include the central medial, paracentral, and central lateral nuclei. The rILN differ from both thalamic relay nuclei, such as the lateral geniculate nucleus, and caudal intralaminar nuclei, such as the parafascicular nucleus, in afferent and efferent connectivity as well as physiological and synaptic properties. rILN activity is associated with a range of neural functions and behaviors, including arousal, pain, executive function, and action control. Here, we review this evidence supporting a role for the rILN in integrating arousal, executive and motor feedback information. In light of rILN projections out to the striatum, amygdala, and sensory as well as executive cortices, we propose that such a function enables the rILN to modulate cognitive and motor resources to meet task-dependent behavioral engagement demands.

Central Thalamic-Medial Prefrontal Control of Adaptive Responding in the Rat: Many Players in the Chamber

The medial prefrontal cortex (mPFC) has robust afferent and efferent connections with multiple nuclei clustered in the central thalamus. These nuclei are elements in large-scale networks linking mPFC with the hippocampus, basal ganglia, amygdala, other cortical areas, and visceral and arousal systems in the brainstem that give rise to adaptive goal-directed behavior. Lesions of the mediodorsal nucleus (MD), the main source of thalamic input to middle layers of PFC, have limited effects on delayed conditional discriminations, like DMTP and DNMTP, that depend on mPFC. Recent evidence suggests that MD sustains and amplifies neuronal responses in mPFC that represent salient task-related information and is important for detecting and encoding contingencies between actions and their consequences. Lesions of rostral intralaminar (rIL) and ventromedial (VM) nuclei produce delay-independent impairments of egocentric DMTP and DNMTP that resemble effects of mPFC lesions on response speed and accuracy: results consistent with projections of rIL to striatum and VM to motor cortices. The ventral midline and anterior thalamic nuclei affect allocentric spatial cognition and memory consistent with their connections to mPFC and hippocampus. The dorsal midline nuclei spare DMTP and DNMTP. They have been implicated in behavioral-state control and response to salient stimuli in associative learning. mPFC functions are served during DNMTP by discrete populations of neurons with responses related to motor preparation, movements, lever press responses, reinforcement anticipation, reinforcement delivery, and memory delay. Population analyses show that different responses are timed so that they effectively tile the temporal interval from when DNMTP trials are initiated until the end. Event-related responses of MD neurons during DNMTP are predominantly related to movement and reinforcement, information important for DNMTP choice. These responses closely mirror the activity of mPFC neurons with similar responses. Pharmacological inactivation of MD and adjacent rIL affects the expression of diverse action- and outcome-related responses of mPFC neurons. Lesions of MD before training are associated with a shift away from movement-related responses in mPFC important for DNMTP choice. These results suggest that MD has short-term effects on the expression of event-related activity in mPFC and long-term effects that tune mPFC neurons to respond to task-specific information.

Routes of the thalamus through the history of neuroanatomy

The most distant roots of neuroanatomy trace back to antiquity, with the first human dissections, but no document which would identify the thalamus as a brain structure has reached us. Claudius Galenus (Galen) gave to the thalamus the name 'thalamus nervorum opticorum', but later on, other names were used (e.g., anchae, or buttocks-like). In 1543, Andreas Vesalius provided the first quality illustrations of the thalamus. During the 19th century, tissue staining techniques and ablative studies contributed to the breakdown of the thalamus into subregions and nuclei. The next step was taken using radiomarkers to identify connections in the absence of lesions. Anterograde and retrograde tracing methods arose in the late 1960s, supporting extension, revision, or confirmation of previously established knowledge. The use of the first viral tracers introduced a new methodological breakthrough in the mid-1970s. Another important step was supported by advances in neuroimaging of the thalamus in the 21th century. The current review follows the history of the thalamus through these technical revolutions from Antiquity to the present day.

The Paraventricular Nucleus of the Thalamus as an Integrating and Relay Node in the Brain Anxiety Network

The brain anxiety network is composed of a number of interconnected cortical regions that detect threats and execute appropriate defensive responses via projections to the shell of the nucleus accumbens (NAcSh), dorsolateral region of the bed nucleus of the stria terminalis (BSTDL) and lateral region of the central nucleus of the amygdala (CeL). The paraventricular nucleus of the thalamus (PVT) is anatomically positioned to integrate threat- and arousal-related signals from cortex and hypothalamus and then relay these signals to neural circuits in the NAcSh, BSTDL, and CeL that mediate defensive responses. This review describes the anatomical connections of the PVT that support the view that the PVT may be a critical node in the brain anxiety network. Experimental findings are reviewed showing that the arousal peptides orexins (hypocretins) act at the PVT to promote avoidance of potential threats especially following exposure of rats to a single episode of footshocks. Recent anatomical and experimental findings are discussed which show that neurons in the PVT provide divergent projections to subcortical regions that mediate defensive behaviors and that the projection to the NAcSh is critical for the enhanced social avoidance displayed in rats exposed to footshocks. A theoretical model is proposed for how the PVT integrates cortical and hypothalamic signals to modulate the behavioral responses associated with anxiety and other challenging situations.

Widespread corticopetal projections from the oval paracentral nucleus of the intralaminar thalamic nuclei conveying orofacial proprioception in rats

The oval paracentral nucleus (OPC) was initially isolated from the paracentral nucleus (PC) within the intralaminar thalamic nuclei in rats. We have recently shown that the rat OPC receives proprioceptive inputs from jaw-closing muscle spindles (JCMSs). However, it remains unknown which cortical areas receive thalamic inputs from the OPC, and whether the cortical areas receiving the OPC inputs are distinct from those receiving inputs from the other intralaminar nuclei and sensory thalamic nuclei. To address this issue, we injected an anterograde tracer, biotinylated dextranamine (BDA), into the OPC, which was electrophysiologically identified by recording of proprioceptive inputs from the JCMSs. Many BDA-labeled axonal fibers and terminals from the OPC were ipsilaterally observed in the rostral and rostroventral regions of the primary somatosensory cortex (S1), the rostral region of the secondary somatosensory cortex (S2), and the most rostrocaudal levels of the granular insular cortex (GI). In contrast, a BDA injection into the caudal PC, which was located slightly rostral to the OPC, resulted in ipsilateral labeling of axonal fibers and terminals in the rostrolateral region of the medial agranular cortex and the rostromedial region of the lateral agranular cortex. Furthermore, injections of a retrograde tracer, Fluorogold, into these S1, S2, and GI regions, resulted in preferential labeling of neurons in the ipsilateral OPC among the intralaminar and sensory thalamic nuclei. These findings reveal that the rat OPC has widespread, but strong corticopetal projections, indicating that there exist divergent corticopetal pathways from the intralaminar thalamic nucleus, which process JCMS proprioceptive sensation.

Cortical influences of serotonin and glutamate on layer V pyramidal neurons

Layer V pyramidal neurons constitute principle output neurons of the medial prefrontal cortex (mPFC)/neocortex to subcortical regions including the intralaminar/midline thalamic nuclei, amygdala, basal ganglia, brainstem nuclei and the spinal cord. The effects of 5-hydroxytryptamine (5-HT) on layer V pyramidal cells primarily reflect a range of excitatory influences through 5-HT_2A receptors and inhibitory influences through non-5-HT_2A receptors, including 5-HT_1A receptors. While the 5-HT_2A receptor is primarily a postsynaptic receptor on throughout the apical dendritic field of 5-HT_2A receptors, activation of a minority of 5-HT_2A receptors also appears to increase spontaneous excitatory postsynaptic currents/potentials (EPSCs/EPSPs) via a presynaptic effect on thalamocortical terminals arising from the midline and intralaminar thalamic nuclei. Activation of 5-HT_2A receptors by the phenethylamine hallucinogen also appears to increase asynchronous release of glutamate upon the layer V pyramidal dendritic field, an effect that is suppressed by 5-HT itself through non-5-HT_2A receptors. Serotonergic hallucinogens acting on 5-HT_2A receptors also appears to increase gene expression of immediate early genes (iEG) and other receptors appearing to induce an iEG-like response like BDNF. Psychedelic hallucinogens acting on 5-HT_2A receptors also induce head twitches in rodents that appear related to induction of glutamate release. These electrophysiological, biochemical and behavioral effects of serotonergic hallucinogens appear to be related to modulating glutamatergic thalamocortical neurotransmission and/or shifting the balance toward 5-HT_2A receptor activation and away from non-5-HT_2A receptor activation. These 5-HT_2A receptor induced responses are modulated by feedback homeostatic mechanisms through mGlu₂, mGlu₄, and mGlu₈ presynaptic receptors on thalamocortical terminals. These 5-HT_2A receptor and glutamatergic interactions also appear to play a role on higher cortical functions of the mPFC such as motoric impulsivity and antidepressant-like behavioral responses on the differential-reinforcement-of low rate 72-s (DRL 72-s schedule). These mutually opposing effects between 5-HT_2A receptor and mGlu autoreceptor activation (e.g., blocking 5-HT_2A receptors and enhancing activity at mGlu₂ receptors) may play a clinical role with respect to currently prescribed or novel antidepressant drugs. Thus, there is an important balance between 5-HT_2A receptor activation and activation of mGlu autoreceptors on prefrontal cortical layer V pyramidal cells with respect to the electrophysiological, biochemical and behavioral effects serotonergic hallucinogenic drugs.

Cell Assemblies in the Cortico-Hippocampal-Reuniens Network during Slow Oscillations

The nucleus reuniens (NR) is an important anatomic and functional relay between the medial prefrontal cortex (mPFC) and the hippocampus (HPC). Whether the NR controls neuronal assemblies, a hallmark of information exchange between the HPC and mPFC for memory transfer/consolidation, is not known. Using simultaneous local field potential and unit recordings in NR, HPC, and mPFC in male rats during slow oscillations under anesthesia, we identified a reliable sequential activation of NR neurons at the beginning of UP states, which preceded mPFC ones. NR sequences were spatially organized, from dorsal to ventral NR. Chemical inactivation of the NR disrupted mPFC sequences at the onset of UP states as well as HPC sequences present during sharp-wave ripples. We conclude that the NR contributes to the coordination and stabilization of mPFC and HPC neuronal sequences during slow oscillations, possibly via the early activation of its own sequences.SIGNIFICANCE STATEMENT Neuronal assemblies are believed to be instrumental to code/encode/store information. They can be recorded in different brain regions, suggesting that widely distributed networks of networks are involved in such information processing. The medial prefrontal cortex, the hippocampus, and the thalamic nucleus reuniens constitute a typical example of a complex network involved in memory consolidation. In this study, we show that spatially organized cells assemblies are recruited in the nucleus reuniens at the UP state onset during slow oscillations. Nucleus reuniens activity appears to be necessary to the stability of medial prefrontal cortex and hippocampal cell assembly formation during slow oscillations. This result further highlights the role of the nucleus reuniens as a functional hub for exchanging and processing memories.

Pain matrix shift in the rat brain following persistent colonic inflammation revealed by voxel-based statistical analysis

Inflammatory bowel disease (IBD), mainly comprising Crohn’s disease and ulcerative colitis, is characterized by chronic inflammation in the digestive tract. Approximately 60% of the patients experience abdominal pain during acute IBD episodes, which severely impairs their quality of life. Both peripheral and central mechanisms are thought to be involved in such abdominal pain in IBD. Although much attention has been paid to peripheral mechanisms of abdominal pain in IBD pathophysiology, the involvement of supraspinal mechanisms remains poorly understood. To address this issue, we investigated regional brain activity in response to colorectal distension in normal and IBD model rats using voxel-based statistical analysis of 2-deoxy-2-[18F]fluoro-_D-glucose positron emission tomography imaging. The rat IBD model was generated by colorectal administration of 2,4,6-trinitrobenzene sulfonic acid, a chemical compound widely used to generate colitis. Tissue damage and inflammation were induced and dynamically changed with time after 2,4,6-trinitrobenzene sulfonic acid injection, while colorectal distension-induced visceromotor response showed corresponding temporal changes. We found that characteristic brain activations were observed in response to visceral innocuous and noxious colorectal distension and supraspinal nociception shared some physiological sensory pathway. Moreover, widespread brain regions were activated, and the functional coupling between the central medial thalamic nucleus and anterior cingulate cortex was enhanced after noxious colorectal distension in IBD model of rats. Increased brain activity in the anterior insular cortex and anterior cingulate cortex was positively correlated with noxious colorectal distension-induced pain severity in normal and IBD rats, respectively. These findings suggest that the pain matrix was shifted following persistent colonic inflammation, and thalamocortical sensitization in the pathway from the central medial thalamic nucleus to anterior cingulate cortex might be a central mechanism of the visceral hyperalgesia in IBD pathophysiology.

A disinhibitory nigra-parafascicular pathway amplifies seizure in temporal lobe epilepsy

The precise circuit of the substantia nigra pars reticulata (SNr) involved in temporal lobe epilepsy (TLE) is still unclear. Here we found that optogenetic or chemogenetic activation of SNr parvalbumin⁺ (PV) GABAergic neurons amplifies seizure activities in kindling- and kainic acid-induced TLE models, whereas selective inhibition of these neurons alleviates seizure activities. The severity of seizures is bidirectionally regulated by optogenetic manipulation of SNr PV fibers projecting to the parafascicular nucleus (PF). Electrophysiology combined with rabies virus-assisted circuit mapping shows that SNr PV neurons directly project to and functionally inhibit posterior PF GABAergic neurons. Activity of these neurons also regulates seizure activity. Collectively, our results reveal that a long-range SNr-PF disinhibitory circuit participates in regulating seizure in TLE and inactivation of this circuit can alleviate severity of epileptic seizures. These findings provide a better understanding of pathological changes from a circuit perspective and suggest a possibility to precisely control epilepsy.

The neural circuits through which the substantia nigra pars reticulata (SNr) exerts its role in epilepsy control are not known. Here the authors reveal that a long-range SNr-parafascicular nucleus disinhibitory circuit participates in regulating seizures in temporal lobe epilepsy and inhibition of this circuit can alleviate severity of epileptic seizures.

Electroacupuncture Alleviates Pain-Related Emotion by Upregulating the Expression of NPS and Its Receptor NPSR in the Anterior Cingulate Cortex and Hypothalamus

Objective

Electroacupuncture (EA) is reported effective in alleviating pain-related emotion; however, the underlying mechanism of its effects still needs to be elucidated. The NPS-NPSR system has been validated for the involvement in the modulation of analgesia and emotional behavior. Here, we aimed to investigate the role of the NPS-NPSR system in the anterior cingulate cortex (ACC), hypothalamus, and central amygdala (CeA) in the use of EA to relieve affective pain modeled by complete Freund's adjuvant- (CFA-) evoked conditioned place aversion (C-CPA). Materials and Methods. CFA injection combined with a CPA paradigm was introduced to establish the C-CPA model, and the elevated O-maze (EOM) was used to test the behavioral changes after model establishment. We further explored the expression of NPS and NPSR at the protein and gene levels in the brain regions of interest by immunofluorescence staining and quantitative real-time PCR.

Results

We observed that EA stimulation delivered to the bilateral Zusanli (ST36) and Kunlun (BL60) acupoints remarkably inhibited sensory pain, pain-evoked place aversion, and anxiety-like behavior. The current study showed that EA significantly enhanced the protein expression of this peptide system in the ACC and hypothalamus, while the elevated expression of NPSR protein alone was just confined to the affected side in the CeA. Moreover, EA remarkably upregulated the mRNA expression of NPS in CeA, ACC, and hypothalamus and NPSR mRNA in the hypothalamus and CeA.

Conclusions

These data suggest the effectiveness of EA in alleviating affective pain, and these benefits may at least partially be attributable to the upregulation of the NPS-NPSR system in the ACC and hypothalamus.

Two genetically, anatomically and functionally distinct cell types segregate across anteroposterior axis of paraventricular thalamus

Unlike the sensory thalamus, studies on the functional organization of the midline and intralaminar nuclei are scarce, and this has hindered the establishment of conceptual models of the function of this brain region. We investigated the functional organization of the paraventricular nucleus of the thalamus (PVT), a midline thalamic structure that is increasingly being recognized as a critical node in the control of diverse processes such as arousal, stress, emotional memory and motivation, in mice. We identify two major classes of PVT neurons-termed type I and type II-that differ in terms of gene expression, anatomy and function. In addition, we demonstrate that type II neurons belong to a previously neglected class of PVT neurons that convey arousal-related information to corticothalamic neurons of the infralimbic cortex. Our results uncover the existence of an arousal-modulated thalamo-corticothalamic loop that links the PVT and the ventromedial prefrontal cortex.

Distribution of Midbrain Cholinergic Axons in the Thalamus

Cholinergic transmission is essential for adaptive behavior and has been suggested to play a central role in the modulation of brain states by means of the modulation of thalamic neurons. Midbrain cholinergic neurons from the pedunculopontine nucleus (PPN) and the laterodorsal tegmental nucleus (LDT) provide dense innervation of the thalamus, but a detailed connectivity mapping is missing. Using conditional tracing of midbrain cholinergic axons in the rat, together with a detailed segmentation of thalamic structures, we show that projections arising in PPN and LDT are topographically organized along the entire extent of the thalamus. PPN cholinergic neurons preferentially innervate thalamic relay structures, whereas LDT cholinergic neurons preferentially target thalamic limbic nuclei. Moreover, both PPN and LDT provide a dense innervation of the intralaminar thalamic nuclei. Notably, we observe a differential synaptic density that functionally dissociates between PPN and LDT innervation. Our results show that midbrain cholinergic neurons innervate virtually all thalamic structures and this innervation is functionally segregated.

Differentiating Cerebellar Impact on Thalamic Nuclei

The cerebellum plays a role in coordination of movements and non-motor functions. Cerebellar nuclei (CN) axons connect to various parts of the thalamo-cortical network, but detailed information on the characteristics of cerebello-thalamic connections is lacking. Here, we assessed the cerebellar input to the ventrolateral (VL), ventromedial (VM), and centrolateral (CL) thalamus. Confocal and electron microscopy showed an increased density and size of CN axon terminals in VL compared to VM or CL. Electrophysiological recordings in vitro revealed that optogenetic CN stimulation resulted in enhanced charge transfer and action potential firing in VL neurons compared to VM or CL neurons, despite that the paired-pulse ratio was not significantly different. Together, these findings indicate that the impact of CN input onto neurons of different thalamic nuclei varies substantially, which highlights the possibility that cerebellar output differentially controls various parts of the thalamo-cortical network.

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Cerebello-thalamic axons form terminals of varying size in distinct thalamic nuclei

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Cerebello-thalamic responses vary in amplitude in distinct thalamic nuclei

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Repetitive stimuli depress cerebello-thalamic responses in all thalamic nuclei

Microcircuit Mechanisms through which Mediodorsal Thalamic Input to Anterior Cingulate Cortex Exacerbates Pain-Related Aversion

Hyperexcitability of the anterior cingulate cortex (ACC) is thought to drive aversion associated with chronic neuropathic pain. Here, we studied the contribution of input from the mediodorsal thalamus (MD) to ACC, using sciatic nerve injury and chemotherapy-induced mouse models of neuropathic pain. Activating MD inputs elicited pain-related aversion in both models. Unexpectedly, excitatory responses of layer V ACC neurons to MD inputs were significantly weaker in pain models compared to controls. This caused the ratio between excitation and feedforward inhibition elicited by MD input to shift toward inhibition, specifically for subcortically projecting (SC) layer V neurons. Furthermore, direct inhibition of SC neurons reproduced the pain-related aversion elicited by activating MD inputs. Finally, both the ability to elicit pain-related aversion and the decrease in excitation were specific to MD inputs; activating basolateral amygdala inputs produced opposite effects. Thus, chronic pain-related aversion may reflect activity changes in specific pathways, rather than generalized ACC hyperactivity.

Central thalamic inactivation impairs the expression of action- and outcome-related responses of medial prefrontal cortex neurons in the rat

The mediodorsal (MD) and adjacent intralaminar (IL) and midline nuclei provide the main thalamic input to the medial prefrontal cortex (mPFC) and are critical for associative learning and decision-making. MD neurons exhibit activity related to actions and outcomes that mirror responses of mPFC neurons in rats during dynamic delayed non-match to position (dDNMTP), a variation of DNMTP where start location is varied randomly within an open octagonal arena to avoid confounding behavioral events with spatial location. To test whether the thalamus affects the expression of these responses in mPFC, we inhibited the central thalamus unilaterally by microinjecting muscimol at doses and sites found to affect decision-making when applied bilaterally. Unilateral inactivation reduced normalized task-related responses in the ipsilateral mPFC without disrupting behavior needed to characterize event-related neuronal activity. Our results extend earlier findings that focused on delay-related activity by showing that central thalamic inactivation interferes with responses related to actions and outcomes that occur outside the period of memory delay. These findings are consistent with the broad effects of central thalamic lesions on behavioral measures of reinforcement-guided responding. Most (7/8) of the prefrontal response types affected by thalamic inactivation have also been observed in MD during dDNMTP. These results support the hypothesis that MD and IL act as transthalamic gates: monitoring prefrontal activity through corticothalamic inputs; integrating this information with signals from motivational and sensorimotor systems that converge in thalamus; and acting through thalamocortical projections to enhance expression of neuronal responses in the PFC that support adaptive goal-directed behavior.

The paraventricular thalamic nucleus: A key hub of neural circuits underlying drug addiction

Drug addiction is a chronic relapsing brain disease characterized by compulsive, out-of-control drug use and the appearance of negative somatic and emotional consequences when drug access is prevented. The limited efficacy of treatment urges researchers toward a deeper understanding of the neural mechanism of drug addiction. Brain circuits that regulate reward and motivation are considered to be the neural substrate of drug addiction. An increasing body of literature indicates that the paraventricular thalamic nucleus (PVT) could serve as a key node in the neurocircuits that control goal-directed behaviors. In this review, we summarize the anatomical and functional evidence that the PVT regulates drug-related behaviors. The PVT receives extensive inputs from the brainstem and hypothalamus, and is reciprocally connected with the limbic system. Neurons in the PVT are recruited by drug exposure as well as cues and context associated with drug taking. Pathway-specific perturbation studies have begun to decipher the precise role of PVT circuits in drug-related behaviors. We also highlight recent findings about the involvement of neural plasticity of the PVT pathways in drug addiction and provide perspectives on future studies.

Biphasic effect of abstinence duration following cocaine self-administration on spine morphology and plasticity-related proteins in prelimbic cortical neurons projecting to the nucleus accumbens core

Cocaine self-administration (SA) in rats dysregulates glutamatergic signaling in the prelimbic (PrL) cortex and glutamate release in the nucleus accumbens (NA) core, promoting cocaine seeking. PrL adaptations that affect relapse to drug seeking emerge during the first week of abstinence, switching from an early (2 h) hypoglutamatergic state to a later (7 days) hyperglutamatergic state. Different interventions that normalize glutamatergic signaling in PrL cortex at each timepoint are necessary to suppress relapse. We hypothesized that plasticity-related proteins that regulate glutamatergic neurotransmission as well as dendritic spine morphology would be biphasically regulated during these two phases of abstinence in PrL cortical neurons projecting to the NA core (PrL-NA core). A combinatorial viral approach was used to selectively label PrL-NA core neurons with an mCherry fluorescent reporter. Male rats underwent 2 weeks of cocaine SA or received yoked-saline infusions and were perfused either 2 h or 7 days after the final SA session. Confocal microscopy and 3D reconstruction analyses were performed for Fos and pCREB immunoreactivity (IR) in the nucleus of layer V PrL-NA core neurons and GluA1-IR and GluA2-IR in apical dendritic spines of the same neurons. Here, we show that cocaine SA decreased PrL-NA core spine head diameter, nuclear Fos-IR and pCREB-IR, and GluA1-IR and GluA2-IR in putative mushroom-type spines 2 h after the end of cocaine SA, whereas the opposite occurred following 1 week of abstinence. Our findings reveal biphasic, abstinence duration-dependent alterations in structural plasticity and relapse-related proteins in the PrL-NA core pathway after cocaine SA.

Drug Seeking and Relapse: New Evidence of a Role for Orexin and Dynorphin Co-transmission in the Paraventricular Nucleus of the Thalamus

The long-lasting vulnerability to relapse remains the main challenge for the successful treatment of drug addiction. Neural systems that are involved in processing natural rewards and drugs of abuse overlap. However, neuroplasticity that is caused by drug exposure may be responsible for maladaptive, compulsive, and addictive behavior. The orexin (Orx) system participates in regulating numerous physiological processes, including energy metabolism, arousal, and feeding, and is recruited by drugs of abuse. The Orx system is differentially recruited by drugs and natural rewards. Specifically, we found that the Orx system is more engaged by drugs than by non-drugs, such as sweetened condensed milk (SCM) or a glucose saccharin solution (GSS), in an operant model of reward seeking. Although stimuli (S+) that are conditioned to cocaine (COC), ethanol, and SCM/GSS equally elicited reinstatement, Orx receptor blockade reversed conditioned reinstatement for drugs vs. non-drugs. Moreover, the hypothalamic recruitment of Orx cells was greater in rats that were tested with the COC S+ vs. SCM S+, indicating of a preferential role for the Orx system in perseverative, compulsive-like COC seeking and not behavior that is motivated by palatable food. Accumulating evidence indicates that the paraventricular nucleus of the thalamus (PVT), which receives major Orx projections, mediates drug-seeking behavior. All Orx neurons contain dynorphin (Dyn), and Orx and Dyn are co-released. In the VTA, they play opposing roles in reward and motivation. To fully understand the physiological and behavioral roles of Orx transmission in the PVT, one important consideration is that Orx neurons that project to the PVT may co-release Orx with another peptide, such as Dyn. The PVT expresses both Orx receptors and κ opioid receptors, suggesting that Orx and Dyn act in tandem when released in the PVT, in addition to the VTA. The present review discusses recent findings that suggest the maladaptive recruitment of Orx/Dyn-PVT neurotransmission by drugs of abuse vs. a highly palatable food reward.

Target Interneuron Preference in Thalamocortical Pathways Determines the Temporal Structure of Cortical Responses

Sensory processing relies on fast detection of changes in environment, as well as integration of contextual cues over time. The mechanisms by which local circuits of the cerebral cortex simultaneously perform these opposite processes remain obscure. Thalamic “specific” nuclei relay sensory information, whereas “nonspecific” nuclei convey information on the environmental and behavioral contexts. We expressed channelrhodopsin in the ventrobasal specific (sensory) or the rhomboid nonspecific (contextual) thalamic nuclei. By selectively activating each thalamic pathway, we found that nonspecific inputs powerfully activate adapting (slow-responding) interneurons but weakly connect fast-spiking interneurons, whereas specific inputs exhibit opposite interneuron preference. Specific inputs thereby induce rapid feedforward inhibition that limits response duration, whereas, in the same cortical area, nonspecific inputs elicit delayed feedforward inhibition that enables lasting recurrent excitation. Using a mean field model, we confirm that cortical response dynamics depends on the type of interneuron targeted by thalamocortical inputs and show that efficient recruitment of adapting interneurons prolongs the cortical response and allows the summation of sensory and contextual inputs. Hence, target choice between slow- and fast-responding inhibitory neurons endows cortical networks with a simple computational solution to perform both sensory detection and integration.

Effect of electrical stimulation of the infralimbic and prelimbic cortices on anxiolytic-like behavior of rats during the elevated plus-maze test, with particular reference to multiunit recording of the behavior-associated neural activity

Fear and anxiety affect the activities of daily living and require concerted management, such as coping strategies, to preserve quality of life. The infralimbic (IL) and prelimbic (PL) medial prefrontal cortices have been implicated in the regulation of fear- and anxiety-like behavior, but their roles in overcoming fear- and anxiety-like behavior remain unknown. We investigated the anxiolytic-like effects of electrical stimulation of the IL and PL cortices in rats during the elevated plus-maze test. IL stimulation led to a significantly higher percentage of time spent and entries in the open arms, whereas PL stimulation did not have any significant behavioral effects. Subsequently, we recorded multiunit activity from the IL and PL cortices in rats using a wireless telemetry device, to determine whether activation of the IL occurs when rats enter the open arms in the elevated plus-maze test. The firing rate of IL neurons increased 1-3 s prior to entry from the closed arm to the open arm, whereas there were no corresponding changes in the firing rate of PL neurons. Taken together, the present findings suggest that the IL plays a key role in exerting active action to overcome anxiety-like behavior.

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

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