Thalamic midline cell populations projecting to the nucleus accumbens, amygdala, and hippocampus in the rat

Neural circuits for the adaptive regulation of fear and extinction memory

The regulation of fear memories is critical for adaptive behaviors and dysregulation of these processes is implicated in trauma- and stress-related disorders. Treatments for these disorders include pharmacological interventions as well as exposure-based therapies, which rely upon extinction learning. Considerable attention has been directed toward elucidating the neural mechanisms underlying fear and extinction learning. In this review, we will discuss historic discoveries and emerging evidence on the neural mechanisms of the adaptive regulation of fear and extinction memories. We will focus on neural circuits regulating the acquisition and extinction of Pavlovian fear conditioning in rodent models, particularly the role of the medial prefrontal cortex and hippocampus in the contextual control of extinguished fear memories. We will also consider new work revealing an important role for the thalamic nucleus reuniens in the modulation of prefrontal-hippocampal interactions in extinction learning and memory. Finally, we will explore the effects of stress on this circuit and the clinical implications of these findings.

The Basolateral Amygdala Sends a Mixed (GABAergic and Glutamatergic) Projection to the Mediodorsal Thalamic Nucleus

The medial prefrontal cortex receives converging inputs from the mediodorsal thalamic nucleus (MD) and basolateral amygdala (BLA). Although many studies reported that the BLA also projects to MD, there is conflicting evidence regarding this projection, with some data suggesting that it originates from GABAergic or glutamatergic neurons. Therefore, the present study aimed to determine the neurotransmitter used by MD-projecting BLA cells in male and female rats. We first examined whether BLA cells retrogradely labeled by Fast Blue infusions in MD are immunopositive for multiple established markers of BLA interneurons. A minority of MD-projecting BLA cells expressed somatostatin (∼22%) or calretinin (∼11%) but not other interneuronal markers, suggesting that BLA neurons projecting to MD not only include glutamatergic cells, but also long-range GABAergic neurons. Second, we examined the responses of MD cells to optogenetic activation of BLA axons using whole-cell recordings in vitro Consistent with our immunohistochemical findings, among responsive MD cells, light stimuli typically elicited isolated EPSPs (73%) or IPSPs (27%) as well as coincident EPSPs and IPSPs (11%). Indicating that these IPSPs were monosynaptic, light-evoked EPSPs and IPSPs had the same latency and the IPSPs persisted in the presence of ionotropic glutamate receptor antagonists. Overall, our results indicate that the BLA sends a mixed, glutamatergic-GABAergic projection to MD, which likely influences coordination of activity between BLA, MD, and medial prefrontal cortex. An important challenge for future studies will be to examine the connections formed by MD-projecting glutamatergic and GABAergic BLA cells with each other and other populations of BLA cells.SIGNIFICANCE STATEMENT The mediodorsal thalamic nucleus (MD) and basolateral amygdala (BLA) send convergent projections to the medial prefrontal cortex. Although many studies reported that the BLA also projects to MD, there is conflicting evidence as to whether this projection is glutamatergic or GABAergic. By combining tract tracing, immunohistochemistry, optogenetics, and patch clamp recordings in vitro, we found that BLA neurons projecting to MD not only include glutamatergic cells, but also long-range GABAergic neurons. Differential recruitment of these two contingents of cells likely influences coordination of activity between the BLA, MD, and medial prefrontal cortex.

Molecular and spatial profiling of the paraventricular nucleus of the thalamus

The paraventricular nucleus of the thalamus (PVT) is known to regulate various cognitive and behavioral processes. However, while functional diversity among PVT circuits has often been linked to cellular differences, the molecular identity and spatial distribution of PVT cell types remain unclear. To address this gap, here we used single nucleus RNA sequencing (snRNA-seq) and identified five molecularly distinct PVT neuronal subtypes in the mouse brain. Additionally, multiplex fluorescent in situ hybridization of top marker genes revealed that PVT subtypes are organized by a combination of previously unidentified molecular gradients. Lastly, comparing our dataset with a recently published single-cell sequencing atlas of the thalamus yielded novel insight into the PVT's connectivity with the cortex, including unexpected innervation of auditory and visual areas. This comparison also revealed that our data contains a largely non-overlapping transcriptomic map of multiple midline thalamic nuclei. Collectively, our findings uncover previously unknown features of the molecular diversity and anatomical organization of the PVT and provide a valuable resource for future investigations.

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.

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.

Dual medial prefrontal cortex and hippocampus projecting neurons in the paraventricular nucleus of the thalamus

The paraventricular nucleus (PVT) of the midline thalamus is a critical higher-order cortico-thalamo-cortical integration site that plays a critical role in various behaviors including reward seeking, cue saliency, and emotional memory. Anatomical studies have shown that PVT projects to both medial prefrontal cortex (mPFC) and hippocampus (HC). However, dual mPFC-HC projecting neurons which could serve a role in synchronizing mPFC and HC activity during PVT-dependent behaviors, have not been explored. Here we used a dual retrograde adenoassociated virus (AAV) tracing approach to characterize the location and proportion of different projection populations that send collaterals to mPFC and/or ventral hippocampus (vHC) in rats. Additionally, we examined the distribution of calcium binding proteins calretinin (CR) and calbindin (CB) with respect to these projection populations in PVT. We found that PVT contains separate populations of cells that project to mPFC, vHC, and those that innervate both regions. Interestingly, dual mPFC-HC projecting cells expressed neither CR nor CB. Topographically, CB+ and CR+ containing cells clustered around dual projecting neurons in PVT. These results are consistent with the features of dual mPFC-vHC projecting cells in the nucleus reuniens (RE) and suggestive of a functional mPFC-PVT-vHC system that may support mPFC-vHC interactions in PVT-dependent behaviors.

Amygdalar Functional Connectivity Differences Associated With Reduced Pain Intensity in Pediatric Peripheral Neuropathic Pain

Background

There is evidence of altered corticolimbic circuitry in adults with chronic pain, but relatively little is known of functional brain mechanisms in adolescents with neuropathic pain (NeuP). Pediatric NeuP is etiologically and phenotypically different from NeuP in adults, highlighting the need for pediatric-focused research. The amygdala is a key limbic region with important roles in the emotional-affective dimension of pain and in pain modulation.

Objective

To investigate amygdalar resting state functional connectivity (rsFC) in adolescents with NeuP.

Methods

This cross-sectional observational cohort study compared resting state functional MRI scans in adolescents aged 11–18 years with clinical features of chronic peripheral NeuP (n = 17), recruited from a tertiary clinic, relative to healthy adolescents (n = 17). We performed seed-to-voxel whole-brain rsFC analysis of the bilateral amygdalae. Next, we performed post hoc exploratory correlations with clinical variables to further explain rsFC differences.

Results

Adolescents with NeuP had stronger negative rsFC between right amygdala and right dorsolateral prefrontal cortex (dlPFC) and stronger positive rsFC between right amygdala and left angular gyrus (AG), compared to controls (P_FDR<0.025). Furthermore, lower pain intensity correlated with stronger negative amygdala-dlPFC rsFC in males (r = 0.67, P = 0.034, n = 10), and with stronger positive amygdala-AG rsFC in females (r = −0.90, P = 0.006, n = 7). These amygdalar rsFC differences may thus be pain inhibitory.

Conclusions

Consistent with the considerable affective and cognitive factors reported in a larger cohort, there are rsFC differences in limbic pain modulatory circuits in adolescents with NeuP. Findings also highlight the need for assessing sex-dependent brain mechanisms in future studies, where possible.

Circuit and neuropeptide mechanisms of the paraventricular thalamus across stages of alcohol and drug use

The paraventricular nucleus of the thalamus (PVT) is a midline thalamic brain region that has emerged as a critical circuit node in the regulation of behaviors across domains of affect and motivation, stress responses, and alcohol- and drug-related behaviors. The influence of the PVT in this diverse array of behaviors is a function of its ability to integrate and convey information about salience and valence through its connections with cortical, hypothalamic, hindbrain, and limbic brain regions. While understudied to date, recent studies suggest that several PVT efferents play critical and complex roles in drug and alcohol-related phenotypes. The PVT is also the site of signaling for many neuropeptides released from the synaptic terminals of distal inputs and local neuropeptidergic neurons within. While there is some evidence that neuropeptides including orexin, neurotensin, substance P, and cocaine and amphetamine-related transcript (CART) signal in the PVT to regulate alcohol/drug intake and reinstatement, there remains an overall lack of understanding of the roles of neuropeptides in the PVT in addiction-related behaviors, especially in a circuit-specific context. In this review, we present the current status of preclinical research regarding PVT circuits and neuropeptide modulation of the PVT in three aspects of the addiction cycle: reward/acquisition, withdrawal, and relapse, with a focus on alcohol, opioids (particularly morphine), and psychostimulants (particularly cocaine). Given the PVT's unique position within the broader neural landscape, we further discuss the potential ways in which neuropeptides may regulate these behaviors through their actions upon PVT circuits.

Brain-wide mapping of presynaptic inputs to basolateral amygdala neurons

The basolateral amygdala (BLA), a region critical for emotional processing, is the limbic hub that is connected with various brain regions. BLA neurons are classified into different subtypes that exhibit differential projection patterns and mediate distinct emotional behaviors; however, little is known about their presynaptic input patterns. In this study, we employed projection-specific monosynaptic rabies virus tracing to identify the direct monosynaptic inputs to BLA subtypes. We found that each neuronal subtype receives long-range projection input from specific brain regions. In contrast to their specific axonal projection patterns, all BLA neuronal subtypes exhibited relatively similar input patterns. This anatomical organization supports the idea that the BLA is a central integrator that associates sensory information in different modalities with valence and sends associative information to behaviorally relevant brain regions.

Thalamo-Nucleus Accumbens Projections in Motivated Behaviors and Addiction

The ventral striatum, also called nucleus accumbens (NAc), has long been known to integrate information from cortical, thalamic, midbrain and limbic nuclei to mediate goal-directed behaviors. Until recently thalamic afferents have been overlooked when studying the functions and connectivity of the NAc. However, findings from recent studies have shed light on the importance and roles of precise Thalamus to NAc connections in motivated behaviors and in addiction. In this review, we summarize studies using techniques such as chemo- and optogenetics, electrophysiology and in vivo calcium imaging to elucidate the complex functioning of the thalamo-NAc afferents, with a particular highlight on the projections from the Paraventricular Thalamus (PVT) to the NAc. We will focus on the recent advances in the understanding of the roles of these neuronal connections in motivated behaviors, with a special emphasis on their implications in addiction, from cue-reward association to the mechanisms driving relapse.

The Paraventricular Thalamus as a Critical Node of Motivated Behavior via the Hypothalamic-Thalamic-Striatal Circuit

In this review, we highlight evidence that supports a role for the paraventricular nucleus of the thalamus (PVT) in motivated behavior. We include a neuroanatomical and neurochemical overview, outlining what is known of the cellular makeup of the region and its most prominent afferent and efferent connections. We discuss how these connections and distinctions across the anterior-posterior axis correspond to the perceived function of the PVT. We then focus on the hypothalamic-thalamic-striatal circuit and the neuroanatomical and functional placement of the PVT within this circuit. In this regard, the PVT is ideally positioned to integrate information regarding internal states and the external environment and translate it into motivated actions. Based on data that has emerged in recent years, including that from our laboratory, we posit that orexinergic (OX) innervation from the lateral hypothalamus (LH) to the PVT encodes the incentive motivational value of reward cues and thereby alters the signaling of the glutamatergic neurons projecting from the PVT to the shell of the nucleus accumbens (NAcSh). The PVT-NAcSh pathway then modulates dopamine activity and resultant cue-motivated behaviors. As we and others apply novel tools and approaches to studying the PVT we will continue to refine the anatomical, cellular, and functional definitions currently ascribed to this nucleus and further elucidate its role in motivated behaviors.

Comprehensive topographical map of the serotonergic fibers in the male mouse brain

It is well established that serotonergic fibers distribute throughout the brain. Abnormal densities or patterns of serotonergic fibers have been implicated in neuropsychiatric disorders. Although many classical studies have examined the distribution pattern of serotonergic fibers, most of them were either limited to specific brain areas or had limitations in demonstrating the fine axonal morphology. In this study, we utilize male mice expressing green fluorescence protein under the serotonin transporter (SERT) promoter to map the topography of serotonergic fibers across the rostro-caudal extent of each brain area. We demonstrate previously unreported regional density and fine-grained anatomy of serotonergic fibers. Our findings include: (a) SERT fibers distribute abundantly in the thalamic nuclei close to the midline and dorsolateral areas, in most of the hypothalamic nuclei with few exceptions such as the median eminence and arcuate nuclei, and within the basal amygdaloid complex and lateral septal nuclei, (b) the source fibers of innervation of the hippocampus traverse through the septal nuclei before reaching its destination, (c) unique, filamentous type of straight terminal fibers within the nucleus accumbens, (d) laminar pattern of innervation in the hippocampus, olfactory bulb and cortex with heterogenicity in innervation density among the layers, (e) cortical labeling density gradually decreases rostro-caudally, (f) fibers traverse and distribute mostly within the gray matter, leaving the white fiber bundles uninnervated, and (g) most of the highly labeled nuclei and cortical areas have predominant anatomical connection to limbic structures. In conclusion, we provide novel, regionally specific insights on the distribution map of serotonergic fibers using transgenic mouse.

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.

Orexin Modulation of VTA Dopamine Neuron Activity: Relevance to Schizophrenia

BACKGROUND

The hippocampus is a region consistently implicated in schizophrenia and has been advanced as a therapeutic target for positive, negative, and cognitive deficits associated with the disease. Recently, we reported that the paraventricular nucleus of the thalamus (PVT) works in concert with the ventral hippocampus to regulate dopamine system function; however, the PVT has yet to be investigated as target for the treatment of the disease. Given the dense expression of orexin receptors in the thalamus, we believe these to be a possible target for pharmacological regulation of PVT activity.

METHODS

Here we used the methylazoxymethanol acetate (MAM) rodent model, which displays pathological alterations consistent with schizophrenia to determine whether orexin receptor blockade can restore ventral tegmental area dopamine system function. We measured dopamine neuron population activity, using in vivo electrophysiology, following administration of the dual orexin antagonist, TCS 1102 (both intraperitoneal and intracranial into the PVT in MAM- and saline-treated rats), and orexin A and B peptides (intracranial into the PVT in naïve rats).

RESULTS

Aberrant dopamine system function in MAM-treated rats was normalized by the systemic administration of TCS 1102. To investigate the potential site of action, the orexin peptides A and B were administered directly into the PVT, where they significantly increased ventral tegmental area dopamine neuron population activity in control rats. In addition, the direct administration of TCS 1102 into the PVT reproduced the beneficial effects seen with the systemic administration in MAM-treated rats.

CONCLUSION

Taken together, these data suggest the orexin system may represent a novel site of therapeutic intervention for psychosis via an action in the PVT.

Structural changes in brains of patients with disorders of consciousness treated with deep brain stimulation

Disorders of consciousness (DOC) are one of the major consequences after anoxic or traumatic brain injury. So far, several studies have described the regaining of consciousness in DOC patients using deep brain stimulation (DBS). However, these studies often lack detailed data on the structural and functional cerebral changes after such treatment. The aim of this study was to conduct a volumetric analysis of specific cortical and subcortical structures to determine the impact of DBS after functional recovery of DOC patients. Five DOC patients underwent unilateral DBS electrode implantation into the centromedian parafascicular complex of the thalamic intralaminar nuclei. Consciousness recovery was confirmed using the Rappaport Disability Rating and the Coma/Near Coma scale. Brain MRI volumetric measurements were done prior to the procedure, then approximately a year after, and finally 7 years after the implementation of the electrode. The volumetric analysis included changes in regional cortical volumes and thickness, as well as in subcortical structures. Limbic cortices (parahippocampal and cingulate gyrus) and paralimbic cortices (insula) regions showed a significant volume increase and presented a trend of regional cortical thickness increase 1 and 7 years after DBS. The volumes of related subcortical structures, namely the caudate, the hippocampus as well as the amygdala, were significantly increased 1 and 7 years after DBS, while the putamen and nucleus accumbens presented with volume increase. Volume increase after DBS could be a result of direct DBS effects, or a result of functional recovery. Our findings are in accordance with the results of very few human studies connecting DBS and brain volume increase. Which mechanisms are behind the observed brain changes and whether structural changes are caused by consciousness recovery or DBS in patients with DOC is still a matter of debate.

The lateral hypothalamus and orexinergic transmission in the paraventricular thalamus promote the attribution of incentive salience to reward-associated cues

RATIONALE

Prior research suggests that the neural pathway from the lateral hypothalamic area (LHA) to the paraventricular nucleus of the thalamus (PVT) mediates the attribution of incentive salience to Pavlovian reward cues. However, a causal role for the LHA and the neurotransmitters involved have not been demonstrated in this regard.

OBJECTIVES

To examine (1) the role of LHA in the acquisition of Pavlovian conditioned approach (PavCA) behaviors, and (2) the role of PVT orexin 1 receptors (OX1r) and orexin 2 receptors (OX2r) in the expression of PavCA behaviors and conditioned reinforcement.

METHODS

Rats received excitotoxic lesions of the LHA prior to Pavlovian training. A separate cohort of rats characterized as sign-trackers (STs) or goal-trackers (GTs) received the OX1r antagonist SB-334867, or the OX2r antagonist TCS-OX2-29, into the PVT, to assess their effects on the expression of PavCA behavior and on the conditioned reinforcing properties of a Pavlovian reward cue.

RESULTS

LHA lesions attenuated the development of sign-tracking behavior. Administration of either the OX1r or OX2r antagonist into the PVT reduced sign-tracking behavior in STs. Further, OX2r antagonism reduced the conditioned reinforcing properties of a Pavlovian reward cue in STs.

CONCLUSIONS

The LHA is necessary for the development of sign-tracking behavior; and blockade of orexin signaling in the PVT attenuates the expression of sign-tracking behavior and the conditioned reinforcing properties of a Pavlovian reward cue. Together, these data suggest that LHA orexin inputs to the PVT are a key component of the circuitry that encodes the incentive motivational value of reward cues.

Nucleus reuniens of the thalamus controls fear memory reconsolidation

The nucleus reuniens has been shown to support the acquisition, consolidation, maintenance, destabilization upon retrieval, and extinction of aversive memories. However, the direct participation of this thalamic subregion in memory reconsolidation is yet to be examined. The present study addressed this question in contextually fear-conditioned rats. Post-reactivation infusion of the GABA_A receptor agonist muscimol, the glutamate N2A-containing NMDA receptor antagonist TCN-201, or the protein synthesis inhibitor anisomycin into the NR induced significant impairments in memory reconsolidation. Administering muscimol or TCN-201 and anisomycin locally, or associating locally infused muscimol or TCN-201 with systemically administered clonidine, an α₂-receptor adrenergic agonist that attenuates the noradrenergic tonus associated with memory reconsolidation, produced no further reduction in freezing times when compared with the muscimol-vehicle, TCN-201-vehicle, vehicle-anisomycin, and vehicle-clonidine groups. This pattern of results indicates that such treatment combinations produced no additive/synergistic effects on reconsolidation. It is plausible that NR inactivation and antagonism of glutamate N2A-containing NMDA receptors weakened/prevented the subsequent action of anisomycin and clonidine because they disrupted the early stages of signal transduction pathways involved in memory reconsolidation. It is noteworthy that these pharmacological interventions, either alone or combined, induced no contextual memory specificity changes, as assessed in a later test in a novel and unpaired context. Besides, omitting memory reactivation precluded the impairing effects of muscimol, TCN-201, anisomycin, and clonidine on reconsolidation. Together, the present findings demonstrate interacting mechanisms through which the NR can regulate contextual fear memory restabilization.

The Paraventricular Nucleus of the Thalamus Is an Important Node in the Emotional Processing Network

The paraventricular nucleus of the thalamus (PVT) has for decades been acknowledged to be an important node in the limbic system, but studies of emotional processing generally fail to incorporate it into their investigational framework. Here, we propose that the PVT should be considered as an integral part of the emotional processing network. Through its distinct subregions, cell populations, and connections with other limbic nuclei, the PVT participates in both major features of emotion: arousal and valence. The PVT, particularly the anterior PVT, can through its neuronal activity promote arousal, both as part of the sleep-wake cycle and in response to novel stimuli. It is also involved in reward, being both responsive to rewarding stimuli and itself affecting behavior reflecting reward, likely via specific populations of cells distributed throughout its subregions. Similarly, neuronal activity in the PVT contributes to depression-like behavior, through yet undefined subregions. The posterior PVT in particular demonstrates a role in anxiety-like behavior, generally promoting but also inhibiting this behavior. This subregion is also especially responsive to stressors, and it functions to suppress the stress response following chronic stress exposure. In addition to participating in unconditioned or primary emotional responses, the PVT also makes major contributions to conditioned emotional behavior. Neuronal activity in response to a reward-predictive cue can be detected throughout the PVT, and endogenous activity in the posterior PVT strongly predicts approach or seeking behavior. Similarly, neuronal activity during conditioned fear retrieval is detected in the posterior PVT and its activation facilitates the expression of conditioned fear. Much of this involvement of the PVT in arousal and valence has been shown to occur through the same general afferents and efferents, including connections with the hypothalamus, prelimbic and infralimbic cortices, nucleus accumbens, and amygdala, although a detailed functional map of the PVT circuits that control emotional responses remains to be delineated. Thus, while caveats exist and more work is required, the PVT, through its extensive connections with other prominent nuclei in the limbic system, appears to be an integral part of the emotional processing network.

Dysregulation of Midbrain Dopamine System and the Pathophysiology of Schizophrenia

Dysregulation of the dopamine system is central to many models of the pathophysiology of psychosis in schizophrenia. However, emerging evidence suggests that this dysregulation is driven by the disruption of upstream circuits that provide afferent control of midbrain dopamine neurons. Furthermore, stress can profoundly disrupt this regulatory circuit, particularly when it is presented at critical vulnerable prepubertal time points. This review will discuss the dopamine system and the circuits that regulate it, focusing on the hippocampus, medial prefrontal cortex, thalamic nuclei, and medial septum, and the impact of stress. A greater understanding of the regulation of the dopamine system and its disruption in schizophrenia may provide a more complete neurobiological framework to interpret clinical findings and develop novel treatments.

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.

The lateral neocortex is critical for contextual fear memory reconsolidation

Memories are a product of the concerted activity of many brain areas. Deregulation of consolidation and reprocessing of mnemonic traces that encode fearful experiences might result in fear-related psychopathologies. Here, we assessed how pre-established memories change with experience, particularly the labilization/reconsolidation of memory, using the whole-brain analysis technique of positron emission tomography in male mice. We found differences in glucose consumption in the lateral neocortex, hippocampus and amygdala in mice that underwent labilization/reconsolidation processes compared to animals that did not reactivate a fear memory. We used chemogenetics to obtain insight into the role of cortical areas in these phases of memory and found that the lateral neocortex is necessary for fear memory reconsolidation. Inhibition of lateral neocortex during reconsolidation altered glucose consumption levels in the amygdala. Using an optogenetic/neuronal recording-based strategy we observed that the lateral neocortex is functionally connected with the amygdala, which, along with retrograde labeling using fluorophore-conjugated cholera toxin subunit B, support a monosynaptic connection between these areas and poses this connection as a hot-spot in the circuits involved in reactivation of fear memories.

Distributed Encoding of Reinforcement in Rat Cortico-Striatal-Limbic Networks

Decision-making in the mammalian brain typically involves multiple brain structures within the midbrain, thalamus, striatum, limbic system, and cortex. Although task specific contributions of each brain region have been identified, neurons responding to reinforcement have been found throughout these structures. We sought to determine if any brain area, or cluster of areas, are the source of information, and if the fidelity of information varies among the areas. We recorded simultaneous field potentials (FPs) in rats from seven brain regions as they completed a binary choice task. The FPs of a 0.5 s window following reinforcement were given as input to a classifier that attempted to predict whether or not the rat received reward on each trial. The classifier correctly categorized reward on 77% of trials. Any region-specific signal could be omitted without lowering accuracy. Frequencies above 40 Hz and signals recorded later than 0.25 s following reinforcement were necessary to achieve this accuracy. Further, the classifier was able to predict reinforcement outcome above chance levels when using FPs from any single recorded brain region. Some combinations of structures, however, were more predictive than others. Analysis of FPs prior to reward revealed most regions reflected the prior probability of reward. Lastly, analyses of information flow suggested reinforcement information does not originate within a single structure of the network, within the resolution afforded by FP recordings. These data suggest reward delivery information is rapidly distributed non-uniformly across the network, and there is no canonical flow of information about reward events in the recorded structures.

A Resting-State Functional MR Imaging and Spectroscopy Study of the Dorsal Hippocampus in the Chronic Unpredictable Stress Rat Model

Exposure to chronic stress leads to an array of anatomical, functional, and metabolic changes in the brain that play a key role in triggering psychiatric disorders such as depression. The hippocampus is particularly well known as a target of maladaptive responses to stress. To capture stress-induced changes in metabolic and functional connectivity in the hippocampus, stress-resistant (low-responders) or -susceptible (high-responders) rats exposed to a chronic unpredictable stress paradigm (categorized according to their hormonal and behavioral responses) were assessed by multimodal neuroimaging; the latter was achieved by using localized ¹H MR spectroscopy and resting-state functional MRI (fMRI) at 11,7T data from stressed (n = 25) but also control (n = 15) male Wistar rats.Susceptible animals displayed increased GABA-glutamine (+19%) and glutamate-glutamine (+17%) ratios and decreased levels of macromolecules (-11%); these changes were positively correlated with plasma corticosterone levels. In addition, the neurotransmitter levels showed differential associations with functional connectivity between the hippocampus and the amygdala, the piriform cortex and thalamus between stress-resistant and -susceptible animals. Our observations are consistent with previously reported stress-induced metabolomic changes that suggest overall neurotransmitter dysfunction in the hippocampus. Their association with the fMRI data in this study reveals how local adjustments in neurochemistry relate to changes in the neurocircuitry of the hippocampus, with implications for its stress-associated dysfunctions.SIGNIFICANCE STATEMENT Chronic stress disrupts brain homeostasis, which may increase the vulnerability of susceptible individuals to neuropsychiatric disorders such as depression. Characterization of the differences between stress-resistant and -susceptible individuals on the basis of noninvasive imaging tools, such as magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI), contributes to improved understanding of the mechanisms underpinning individual differences in vulnerability and can facilitate the design of new diagnostic and intervention strategies. Using a combined functional MRI/MRS approach, our results demonstrate that susceptible- and non-susceptible subjects show differential alterations in hippocampal GABA and glutamate metabolism that, in turn, associate with changes in functional connectivity.

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.

Convergent Inputs from the Hippocampus and Thalamus to the Nucleus Accumbens Regulate Dopamine Neuron Activity

Aberrant hippocampal activity is observed in individuals with schizophrenia and is thought to underlie the augmented dopamine system function associated with psychosis. The pathway by which the ventral hippocampus (vHipp) regulates dopamine neuron activity has been demonstrated previously and involves a glutamatergic projection to the nucleus accumbens (NAc). Recent postmortem studies have confirmed glutamatergic abnormalities in the NAc of individuals with schizophrenia. Specifically, an increase in vesicular glutamate transporter 2 (vGlut2) expression was reported. Although projections from the hippocampus do express vGlut2, inputs from the thalamus are more likely to account for this alteration; however, the role of thalamic inputs to the NAc in the regulation of dopamine neuron activity has not been elucidated. Here, using male Sprague Dawley rats, we demonstrate that a subset of NAc medium spiny neurons receive convergent inputs from the vHipp and paraventricular nucleus of the thalamus (PVT), with both regions working synergistically to regulate dopamine neuron activity. Activation of either the vHipp or PVT increases the number of spontaneously active dopamine neurons in the ventral tegmental area. Moreover, this regulation requires simultaneous activity in both regions because PVT inactivation can reverse vHipp-induced increases in dopamine neuron population activity and vHipp inactivation can reverse PVT-induced increases. This is relevant to schizophrenia because inactivation of either the vHipp or PVT is sufficient to reverse aberrant dopamine system function in two distinct rodent models. These data suggest that thalamic abnormalities may contribute to the aberrant dopamine system function observed in schizophrenia and that the PVT represents a novel site of intervention for psychosis.SIGNIFICANCE STATEMENT Current treatments for schizophrenia are far from adequate and a more complete understanding of the pathophysiology underlying this disease is warranted if we are to discover novel therapeutic targets. We have previously demonstrated that the aberrant dopamine system function observed in individuals with schizophrenia and rodent models is driven by increases in hippocampal activity. We now demonstrate that thalamic (paraventricular nucleus, PVT) and ventral hippocampal afferents converge in the nucleus accumbens to regulate dopamine system function. Such information provides a potential site for therapeutic intervention for schizophrenia. Indeed, inactivation of the PVT can effectively reverse aberrant dopamine system function in two distinct rodent models displaying circuit level alterations and corresponding behavioral deficits relevant to schizophrenia.

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.

Quantitative Analyses of the Projection of Individual Neurons from the Midline Thalamic Nuclei to the Striosome and Matrix Compartments of the Rat Striatum

A fundamental organizing principle of the striatum is the striosome/matrix system that is defined by inputs/outputs and neurochemical markers. The thalamostriatal projection is highly heterogeneous originating in many subnuclei of the thalamus including the midline (ML) and intralaminar (IL) nuclei. We examined the dendritic morphology and axonal trajectory of 15 ML and 11 IL neurons by single-neuron labeling with viral vectors in combination with mu-opioid receptor immunostaining in rat brains. Dendritic and axonal morphology defined ML neurons as type II cells consisting of at least two subclasses according to the presence or absence of striatal axon collaterals. In the striatum, ML neurons preferentially innervated striosomes, whereas parafascicular neurons preferentially innervated the matrix. Almost all single thalamostriatal neurons favoring striosome or matrix compartments also innervated the cerebral cortical areas that supplied cortical input to the same striatal compartment. We thus revealed that thalamostriatal projections are highly organized 1) by the similarity in morphological characteristics and 2) their preference for the striatal compartments and cortical areas. These findings demonstrate that the functional properties of striatal compartments are influenced by both their cortical and thalamic afferents presumably with a different time latency and support selective dynamics for the striosome and matrix compartments.

Chemogenetic inhibition reveals midline thalamic nuclei and thalamo-accumbens projections mediate cocaine-seeking in rats

Drug addiction is a chronic disease that is shaped by alterations in neuronal function within the cortical-basal ganglia-thalamic circuit. However, our understanding of how this circuit regulates drug-seeking remains incomplete, and relapse rates remain high. The midline thalamic nuclei are an integral component of the cortical-basal ganglia-thalamic circuit and are poised to mediate addiction behaviors, including relapse. It is surprising that little research has examined the contribution of midline thalamic nuclei and their efferent projections in relapse. To address this, we expressed inhibitory, G_i/o -coupled DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) in a subset of the midline thalamic nuclei or in midline thalamic nuclei neurons projecting to either the nucleus accumbens or the amygdala. We examined the effect of transiently decreasing activity of these neuronal populations on cue-induced and cocaine-primed reinstatement of cocaine-seeking. Reducing activity of midline thalamic nuclei neurons attenuated both cue-induced and cocaine-primed reinstatement, but had no effect on cue-induced reinstatement of sucrose-seeking or locomotor activity. Interestingly, attenuating activity of efferent projections from the anterior portion of midline thalamic nuclei to the nucleus accumbens blocked cocaine-primed reinstatement but enhanced cue-induced reinstatement. Decreasing activity of efferent projections from either the posterior midline thalamic nuclei to the nucleus accumbens or the midline thalamic nuclei to amygdala had no effect. These results reveal a novel contribution of subsets of midline thalamic nuclei neurons in drug-seeking behaviors and suggest that modulation of midline thalamic nuclei activity may be a promising therapeutic target for preventing relapse.

Genetic resistance to kindling associated with alterations in circuit function

How a seizure spreads from a focal onset zone to other regions of the brain is not well understood, and animal studies suggest that there is a genetic influence. To understand how genetic factors may influence seizure spread, we examined whether the kindling resistance of WAG/Rij rats, which are slow to develop kindled motor seizures, is independent of the site of seizure induction and thus a global phenomenon, or whether it is circuit specific. We compared the kindling rates (number of stimulations to induce kindled motor seizures) of WAG/Rij rats to the rates of kindling in Sprague Dawley rats. Both groups underwent a standard hippocampal kindling protocol and a separate group was kindled from the medial dorsal nucleus of the thalamus, a site that has been previously demonstrated to result in the very rapid development of motor seizures. To examine whether there were differences in the interaction in a circuit involved with the motor seizures, evoked responses were obtained from the prefrontal cortex following stimulation of the subiculum or medial dorsal thalamic nucleus. The WAG/Rij rats once again demonstrated resistance to kindling in the hippocampus, but both strains kindled rapidly from the medial dorsal nucleus. In the WAG/Rij rats there was also a reduction in the duration of the afterdischarge in the frontal cortex during hippocampal stimulation, but there was no reduction during thalamic kindling. The prefrontal cortex evoked responses were reduced following stimulation of the subiculum in the WAG/Rij rats, but the evoked responses to thalamic stimulation were the same in both strains. These findings suggest that there are genetic influences in the strength of the input from the subiculum to the prefrontal cortex in WAG/Rij rats that could explain the resistance to limbic kindling because of reduced excitatory drive onto a key target region.

Collateralization of projections from the paraventricular nucleus of the thalamus to the nucleus accumbens, bed nucleus of the stria terminalis, and central nucleus of the amygdala

The paraventricular nucleus of the thalamus (PVT) is a midline thalamic nucleus with dense projections to the nucleus accumbens (NAc), dorsolateral region of the bed nucleus of the stria terminalis (BSTDL) and the lateral/capsular region of the central nucleus of the amygdala (CeL/CeC). Recent experimental evidence indicates that the PVT is involved in both appetitive and aversive behaviors. However, it is unknown if subgroups of neurons in the PVT innervate different subcortical targets or if the same neurons issue collaterals to multiple areas. To address this issue, we injected two different fluorescent retrograde tracers, cholera toxin subunit B conjugated to Alexa Fluor-488 or Alexa Fluor-594, into different pairs of the subcortical targets including different parts of the NAc (shell, core, dorsomedial shell, and ventromedial shell), BSTDL, and amygdala (basolateral amygdala and CeL/CeC). The results indicate a moderate to high level of collateralization of projections from neurons in the PVT to NAc, BSTDL, and CeL/CeC suggesting a potential importance of the PVT in simultaneously coordinating the activity of key regions of the brain involved in mediating emotional and motivational behaviors. We also observed a difference in the subcortical targets innervated by the anterior PVT (aPVT) and posterior PVT (pPVT) showing that more neurons in the aPVT innervate the dorsomedial part of the NAc shell, while more neurons in the pPVT innervate the ventromedial NAc shell, BSTDL, and CeL/CeC. This observation is suggestive of a potential functional difference between the aPVT and pPVT.

Limbic grey matter changes in early Parkinson's disease

The purpose of this study was to investigate local and network‐related changes of limbic grey matter in early Parkinson's disease (PD) and their inter‐relation with non‐motor symptom severity. We applied voxel‐based morphometric methods in 538 T1 MRI images retrieved from the Parkinson's Progression Markers Initiative website. Grey matter densities and cross‐sectional estimates of age‐related grey matter change were compared between subjects with early PD (n = 366) and age‐matched healthy controls (n = 172) within a regression model, and associations of grey matter density with symptoms were investigated. Structural brain networks were obtained using covariance analysis seeded in regions showing grey matter abnormalities in PD subject group. Patients displayed focally reduced grey matter density in the right amygdala, which was present from the earliest stages of the disease without further advance in mild‐moderate disease stages. Right amygdala grey matter density showed negative correlation with autonomic dysfunction and positive with cognitive performance in patients, but no significant interrelations were found with anxiety scores. Patients with PD also demonstrated right amygdala structural disconnection with less structural connectivity of the right amygdala with the cerebellum and thalamus but increased covariance with bilateral temporal cortices compared with controls. Age‐related grey matter change was also increased in PD preferentially in the limbic system. In conclusion, detailed brain morphometry in a large group of early PD highlights predominant limbic grey matter deficits with stronger age associations compared with controls and associated altered structural connectivity pattern. This provides in vivo evidence for early limbic grey matter pathology and structural network changes that may reflect extranigral disease spread in PD. Hum Brain Mapp 38:3566–3578, 2017. © 2017 The Authors Human Brain Mapping Published by Wiley Periodicals, Inc.

Paraventricular Thalamus Balances Danger and Reward

Foraging animals balance the need to seek food and energy against the accompanying dangers of injury and predation. To do so, they rely on learning systems encoding reward and danger. Whereas much is known about these separate learning systems, little is known about how they interact to shape and guide behavior. Here we show a key role for the rat paraventricular nucleus of the thalamus (PVT), a nucleus of the dorsal midline thalamus, in this interaction. First, we show behavioral competition between reward and danger: the opportunity to seek food reward negatively modulates expression of species-typical defensive behavior. Then, using a chemogenetic approach expressing the inhibitory hM4Di designer receptor exclusively activated by a designer drug in PVT neurons, we show that the PVT is central to this behavioral competition. Chemogenetic PVT silencing biases behavior toward either defense or reward depending on the experimental conditions, but does not consistently favor expression of one over the other. This bias could not be attributed to changes in fear memory retrieval, learned safety, or memory interference. Rather, our results demonstrate that the PVT is essential for balancing conflicting behavioral tendencies toward danger and reward, enabling adaptive responding under this basic selection pressure.SIGNIFICANCE STATEMENT Among the most basic survival problems faced by animals is balancing the need to seek food and energy against the accompanying dangers of injury and predation. Although much is known about the brain mechanisms that underpin learning about reward and danger, little is known about how these interact to solve basic survival problems. Here we show competition between defensive (to avoid predatory detection) and approach (to obtain food) behavior. We show that the paraventricular thalamus, a nucleus of the dorsal midline thalamus, is integral to this behavioral competition. The paraventricular thalamus balances the competing behavioral demands of danger and reward, enabling adaptive responding under this selection pressure.

A food-predictive cue attributed with incentive salience engages subcortical afferents and efferents of the paraventricular nucleus of the thalamus

The paraventricular nucleus of the thalamus (PVT) has been implicated in behavioral responses to reward-associated cues. However, the precise role of the PVT in these behaviors has been difficult to ascertain since Pavlovian-conditioned cues can act as both predictive and incentive stimuli. The "sign-tracker/goal-tracker" rat model has allowed us to further elucidate the role of the PVT in cue-motivated behaviors, identifying this structure as a critical component of the neural circuitry underlying individual variation in the propensity to attribute incentive salience to reward cues. The current study assessed differences in the engagement of specific PVT afferents and efferents in response to presentation of a food-cue that had been attributed with only predictive value or with both predictive and incentive value. The retrograde tracer fluorogold (FG) was injected into the PVT or the nucleus accumbens (NAc) of rats, and cue-induced c-Fos in FG-labeled cells was quantified. Presentation of a predictive stimulus that had been attributed with incentive value elicited c-Fos in PVT afferents from the lateral hypothalamus, medial amygdala (MeA), and the prelimbic cortex (PrL), as well as posterior PVT efferents to the NAc. PVT afferents from the PrL also showed elevated c-Fos levels following presentation of a predictive stimulus alone. Thus, presentation of an incentive stimulus results in engagement of subcortical brain regions; supporting a role for the hypothalamic-thalamic-striatal axis, as well as the MeA, in mediating responses to incentive stimuli; whereas activity in the PrL to PVT pathway appears to play a role in processing the predictive qualities of reward-paired stimuli.

Verbal Memory Decline following DBS for Parkinson's Disease: Structural Volumetric MRI Relationships

Background

Parkinson’s disease is a chronic degenerative movement disorder. The mainstay of treatment is medical. In certain patients Deep Brain Stimulation (DBS) may be offered. However, DBS has been associated with post-operative neuropsychology changes, especially in verbal memory.

Objectives

Firstly, to determine if pre-surgical thalamic and hippocampal volumes were related to verbal memory changes following DBS. Secondly, to determine if clinical factors such as age, duration of symptoms or motor severity (UPDRS Part III score) were related to verbal memory changes.

Methods

A consecutive group of 40 patients undergoing bilateral Subthalamic Nucleus (STN)-DBS for PD were selected. Brain MRI data was acquired, pre-processed and structural volumetric data was extracted using FSL. Verbal memory test scores for pre- and post-STN-DBS surgery were recorded. Linear regression was used to investigate the relationship between score change and structural volumetric data.

Results

A significant relationship was demonstrated between change in List Learning test score and thalamic (left, p = 0.02) and hippocampal (left, p = 0.02 and right p = 0.03) volumes. Duration of symptoms was also associated with List Learning score change (p = 0.02 to 0.03).

Conclusion

Verbal memory score changes appear to have a relationship to pre-surgical MRI structural volumetric data. The findings of this study provide a basis for further research into the use of pre-surgical MRI to counsel PD patients regarding post-surgical verbal memory changes.

Theta Rhythm of the Hippocampus: Subcortical Control and Functional Significance

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain and has been strongly implicated in mnemonic processes of the hippocampus. We describe (a) ascending brain stem–forebrain systems involved in controlling theta and nontheta (desynchronization) states of the hippocampal electroencephalogram; (b) theta rhythmically discharging cells in several structures of Papez's circuit and their possible functional significance, specifically with respect to head direction cells in this same circuit; and (c) the role of nucleus reuniens of the thalamus as a major interface between the medial prefrontal cortex and hippocampus and as a prominent source of afferent limbic information to the hippocampus. We suggest that the hippocampus receives two main types of input: theta rhythm from ascending brain stem– diencephaloseptal systems and information bearing mainly from thalamocortical/cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it from the entorhinal cortex and nucleus reuniens.

The Spatiotemporal Dynamics of Phase Synchronization during Epileptogenesis in Amygdala-Kindling Mice

The synchronization among the activities of neural populations in functional regions is one of the most important electrophysiological phenomena in epileptic brains. The spatiotemporal dynamics of phase synchronization was investigated to reveal the reciprocal interaction between different functional regions during epileptogenesis. Local field potentials (LFPs) were recorded simultaneously from the basolateral amygdala (BLA), the cornu ammonis 1 of hippocampus (CA1) and the mediodorsal nucleus of thalamus (MDT) in the mouse amygdala-kindling models during the development of epileptic seizures. The synchronization of LFPs was quantified between BLA, CA1 and MDT using phase-locking value (PLV). During amygdala kindling, behavioral changes (from stage 0 to stage 5) of mice were accompanied by after-discharges (ADs) of similar waveforms appearing almost simultaneously in CA1, MDT, as well as BLA. AD durations were positively related to the intensity of seizures. During seizures at stages 1~2, PLVs remained relatively low and increased dramatically shortly after the termination of the seizures; by contrast, for stages 3~5, PLVs remained a relatively low level during the initial period but increased dramatically before the seizure termination. And in the theta band, the degree of PLV enhancement was positively associated with seizure intensity. The results suggested that during epileptogenesis, the functional regions were kept desynchronized rather than hyper-synchronized during either the initial or the entire period of the seizures; so different dynamic patterns of phase synchronization may be involved in different periods of the epileptogenesis, and this might also reflect that during seizures at different stages, the mechanisms underlying the dynamics of phase synchronization were different.

Midline thalamic reuniens lesions improve executive behaviors

The role of the thalamus in complex cognitive behavior is a topic of increasing interest. Here we demonstrate that lesions of the nucleus reuniens (NRe), a midline thalamic nucleus interconnected with both hippocampal and prefrontal circuitry, lead to enhancement of executive behaviors typically associated with the prefrontal cortex. Rats were tested on four behavioral tasks: (1) the combined attention-memory (CAM) task, which simultaneously assessed attention to a visual target and memory for that target over a variable delay; (2) spatial memory using a radial arm maze, (3) discrimination and reversal learning using a touchscreen operant platform, and (4) decision-making with delayed outcomes. Following NRe lesions, the animals became more efficient in their performance, responding with shorter reaction times but also less impulsively than controls. This change, combined with a decrease in perseverative responses, led to focused attention in the CAM task and accelerated learning in the visual discrimination task. There were no observed changes in tasks involving either spatial memory or value-based decision making. These data complement ongoing efforts to understand the role of midline thalamic structures in human cognition, including the development of thalamic stimulation as a therapeutic strategy for acquired cognitive disabilities (Schiff, 2008; Mair et al., 2011), and point to the NRe as a potential target for clinical intervention.

Limbic circuitry of the midline thalamus

The thalamus was subdivided into three major groups: sensorimotor nuclei (or principal/relay nuclei), limbic nuclei and nuclei bridging these two domains. Limbic nuclei of thalamus (or 'limbic thalamus') consist of the anterior nuclei, midline nuclei, medial division of the mediodorsal nucleus (MDm) and central medial nucleus (CM) of the intralaminar complex. The midline nuclei include the paraventricular (PV) and paratenial (PT) nuclei, dorsally, and the reuniens (RE) and rhomboid (RH) nuclei, ventrally. The 'limbic' thalamic nuclei predominantly connect with limbic-related structures and serve a direct role in limbic-associated functions. Regarding the midline nuclei, RE/RH mainly target limbic cortical structures, particularly the hippocampus and the medial prefrontal cortex. Accordingly, RE/RH participate in functions involving interactions of the HF and mPFC. By contrast, PV/PT mainly project to limbic subcortical structures, particularly the amygdala and nucleus accumbens, and hence are critically involved in affective behaviors such as stress/anxiety, feeding behavior, and drug seeking activities. The anatomical/functional characteristics of MDm and CM are very similar to those of the midline nuclei and hence the collection of nuclei extending dorsoventrally along the midline/paramidline of the thalamus constitute the core of the 'limbic thalamus'.

Dynorphin activation of kappa opioid receptor reduces neuronal excitability in the paraventricular nucleus of mouse thalamus

It has been reported that kappa opioid receptor (KOR) is expressed in the paraventricular nucleus of thalamus (PVT), a brain region associated with arousal, drug reward and stress. Although intra-PVT infusion of KOR agonist was found to inhibit drug-seeking behavior, it is still unclear whether endogenous KOR agonists directly regulate PVT neuron activity. Here, we investigated the effect of the endogenous KOR agonist dynorphin-A (Dyn-A) on the excitability of mouse PVT neurons at different developmental ages. We found Dyn-A strongly inhibited PVT neurons through a direct postsynaptic hyperpolarization. Under voltage-clamp configuration, Dyn-A evoked an obvious outward current in majority of neurons tested in anterior PVT (aPVT) but only in minority of neurons in posterior PVT (pPVT). The Dyn-A current was abolished by KOR antagonist nor-BNI, Ba(2+) and non-hydrolyzable GDP analogue GDP-β-s, indicating that Dyn-A activates KOR and opens G-protein-coupled inwardly rectifying potassium channels in PVT neurons. More interestingly, by comparing Dyn-A currents in aPVT neurons of mice at various ages, we found Dyn-A evoked significant larger current in aPVT neurons from mice around prepuberty and early puberty stage. In addition, KOR activation by Dyn-A didn't produce obvious desensitization, while mu opioid receptor (MOR) activation induced obvious desensitization of mu receptor itself and also heterologous desensitization of KOR in PVT neurons. Together, our findings indicate that Dyn-A activates KOR and inhibits aPVT neurons in mice at various ages especially around puberty, suggesting a possible role of KOR in regulating aPVT-related brain function including stress response and drug-seeking behavior during adolescence.

Gray matter atrophy in narcolepsy: An activation likelihood estimation meta-analysis

The authors reviewed the literature on the use of voxel-based morphometry (VBM) in narcolepsy magnetic resonance imaging (MRI) studies via the use of a meta-analysis of neuroimaging to identify concordant and specific structural deficits in patients with narcolepsy as compared with healthy subjects. We used PubMed to retrieve articles published between January 2000 and March 2014. The authors included all VBM research on narcolepsy and compared the findings of the studies by using gray matter volume (GMV) or gray matter concentration (GMC) to index differences in gray matter. Stereotactic data were extracted from 8 VBM studies of 149 narcoleptic patients and 162 control subjects. We applied activation likelihood estimation (ALE) technique and found significant regional gray matter reduction in the bilateral hypothalamus, thalamus, globus pallidus, extending to nucleus accumbens (NAcc) and anterior cingulate cortex (ACC), left mid orbital and rectal gyri (BAs 10 and 11), right inferior frontal gyrus (BA 47), and the right superior temporal gyrus (BA 41) in patients with narcolepsy. The significant gray matter deficits in narcoleptic patients occurred in the bilateral hypothalamus and frontotemporal regions, which may be related to the emotional processing abnormalities and orexin/hypocretin pathway common among populations of patients with narcolepsy.

Advances in understanding mechanisms of thalamic relays in cognition and behavior

The main impetus for a mini-symposium on corticothalamic interrelationships was the recent number of studies highlighting the role of the thalamus in aspects of cognition beyond sensory processing. The thalamus contributes to a range of basic cognitive behaviors that include learning and memory, inhibitory control, decision-making, and the control of visual orienting responses. Its functions are deeply intertwined with those of the better studied cortex, although the principles governing its coordination with the cortex remain opaque, particularly in higher-level aspects of cognition. How should the thalamus be viewed in the context of the rest of the brain? Although its role extends well beyond relaying of sensory information from the periphery, the main function of many of its subdivisions does appear to be that of a relay station, transmitting neural signals primarily to the cerebral cortex from a number of brain areas. In cognition, its main contribution may thus be to coordinate signals between diverse regions of the telencephalon, including the neocortex, hippocampus, amygdala, and striatum. This central coordination is further subject to considerable extrinsic control, for example, inhibition from the basal ganglia, zona incerta, and pretectal regions, and chemical modulation from ascending neurotransmitter systems. What follows is a brief review on the role of the thalamus in aspects of cognition and behavior, focusing on a summary of the topics covered in a mini-symposium held at the Society for Neuroscience meeting, 2014.

Activity-based anorexia has differential effects on apical dendritic branching in dorsal and ventral hippocampal CA1

Anorexia nervosa (AN) is an eating disorder to which adolescent females are particularly vulnerable. Like AN, activity-based anorexia (ABA), a rodent model of AN, results in elevation of stress hormones and has genetic links to anxiety disorders. The hippocampus plays a key role in the regulation of anxiety and responds with structural changes to hormones and stress, suggesting that it may play a role in AN. The hippocampus of ABA animals exhibits increased brain-derived neurotrophic factor and increased GABA receptor expression, but the structural effects of ABA have not been studied. We used Golgi staining of neurons to determine whether ABA in female rats during adolescence results in structural changes to the apical dendrites in hippocampal CA1 and contrasted to the effects of food restriction (FR) and exercise (EX), the environmental factors used to induce ABA. In the dorsal hippocampus, which preferentially mediates spatial learning and cognition, cells of ABA animals had less total dendritic length and fewer dendritic branches in stratum radiatum (SR) than in control (CON). In the ventral hippocampus, which preferentially mediates anxiety, ABA evoked more branching in SR than CON. In both dorsal and ventral regions, the main effect of exercise was localized to the SR while the main effect of food restriction occurred in the stratum lacunosum-moleculare. Taken together with data on spine density, these results indicate that ABA elicits pathway-specific changes in the hippocampus that may underlie the increased anxiety and reduced behavioral flexibility observed in ABA.