Chemogenetic inactivation of the nucleus reuniens impairs object placement memory in female mice.

Episodic memory is a complex process requiring input from several regions of the brain. Emerging evidence suggests that coordinated activity between the dorsal hippocampus (DH) and medial prefrontal cortex (mPFC) is required for episodic memory consolidation. However, the mechanisms through which the DH and mPFC interact to promote memory consolidation remain poorly understood. A growing body of research suggests that the nucleus reuniens of the thalamus (RE) is one of several structures that facilitate communication between the DH and mPFC during memory and may do so through bidirectional excitatory projections to both regions. Furthermore, recent work from other labs indicates that the RE is necessary for spatial working memory. However, it is not clear to what extent the RE is necessary for memory of object locations. The goal of this study was to determine whether activity in the RE is necessary for spatial memory as measured by the object placement (OP) task in female mice. A kappa-opioid receptor DREADD (KORD) virus was used to inactivate excitatory neurons in the RE pre- or post-training to establish a role for the RE in spatial memory acquisition and consolidation, respectively. RE inactivation prior to, or immediately after, object training blocked OP memory formation relative to chance and to control mice. Moreover, expression of the immediate early gene EGR-1 was reduced in the RE 1 hour after an object training trial, supporting the conclusion that reduced neuronal activity in the RE impairs the formation of object location memories. In summary, the findings of this study support a key role for the RE in spatial memory acquisition and consolidation.

Thalamic Massa Intermedia in Children with and without Midline Brain Malformations.

BACKGROUND AND PURPOSE: The massa intermedia is a normal midline transventricular thalamic connection. Massa intermedia aberrations are common in schizophrenia, Chiari II malformation, X-linked hydrocephalus, Cornelia de Lange syndrome, and diencephalic-mesencephalic junction dysplasia, among others. We have noticed that massa intermedia abnormalities often accompany other midline malformations. The massa intermedia has never been formally evaluated in a group of exclusively pediatric patients, to our knowledge. We sought to compare and contrast the prevalence, size, and location of the massa intermedia in pediatric patients with and without congenital midline brain abnormalities. MATERIALS AND METHODS: Successive 3T brain MR imaging examinations from pediatric patients with and without midline malformations were procured from the imaging data base at a pediatric hospital. Massa intermedia presence, size, morphology, and position were determined using 3D-TIWI with 1-mm isotropic resolution. The brain commissures, septum pellucidum, hypothalamus, hippocampus, vermis, and brain stem were evaluated to determine whether alterations were related to or predictive of massa intermedia abnormalities. RESULTS: The massa intermedia was more frequently absent, dysmorphic, and/or displaced in patients with additional midline abnormalities than in those without. The massa intermedia was absent in 40% of patients with midline malformations versus 12% of patients with normal findings (P < .001). Massa intermedia absence, surface area, and morphology were predictable by various attributes and alterations of the commissures, hippocampus, hypothalamus, vermis, brain stem, and third ventricle. CONCLUSIONS: Most pediatric patients have a thalamic massa intermedia centered in the anterior/superior third ventricle. Massa intermedia abnormalities are commonly associated with other midline malformations. Normal-variant massa intermedia absence is a diagnosis of exclusion.

Afferent connections of the thalamic nucleus reuniens in the mouse.

The nucleus reuniens (RE) is part of the midline thalamus and one of the major sources of thalamic inputs to the hippocampal formation and the medial prefrontal cortex. However, it not only sends strong efferents to these areas but is also heavily innervated by both brain regions. Based on its connectivity and supported by functional studies the RE has been suggested to represent a major hub in reciprocal hippocampal-prefrontal communication. Indeed, inactivation studies have demonstrated that this nucleus is particularly important for cognitive behaviors which depend on prefrontal-hippocampal communication, such as working memory or memory consolidation. However, besides its central role in mediating hippocampal-prefrontal communication, the RE is target of a multitude of other cortical and subcortical afferents, which likely modulate its function. So far, however, studies that have systematically investigated the afferents of the RE have only been performed in rats. Because of the unique role of the mouse as a genetically accessible model system for mammalian brain circuit analysis we have mapped the afferent connectivity of the mouse RE using retrograde Fluoro-Gold tracing. Comparison with similar data from rats indicated a very high level of similarity in prefrontal and hippocampal afferents but some differences in afferent connectivity with other brain regions. In particular, our results suggest interspecies differences regarding the integration of the RE in circuits of fear, aversion, and defense.

The nucleus reuniens of the thalamus sits at the nexus of a hippocampus and medial prefrontal cortex circuit enabling memory and behavior.

The nucleus reuniens of the thalamus (RE) is a key component of an extensive network of hippocampal and cortical structures and is a fundamental substrate for cognition. A common misconception is that RE is a simple relay structure. Instead, a better conceptualization is that RE is a critical component of a canonical higher-order cortico-thalamo-cortical circuit that supports communication between the medial prefrontal cortex (mPFC) and the hippocampus (HC). RE dysfunction is implicated in several clinical disorders including, but not limited to Alzheimer's disease, schizophrenia, and epilepsy. Here, we review key anatomical and physiological features of the RE based primarily on studies in rodents. We present a conceptual model of RE circuitry within the mPFC-RE-HC system and speculate on the computations RE enables. We review the rapidly growing literature demonstrating that RE is critical to, and its neurons represent, aspects of behavioral tasks that place demands on memory focusing on its role in navigation, spatial working memory, the temporal organization of memory, and executive functions.

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.

Molecular neuroanatomy of the mouse medial preoptic area with reference to parental behavior.

The medial preoptic area (MPOA), an anterior part of the hypothalamus, is one of the most important areas for the regulation of instinctively motivated behaviors, such as parental behavior, mating behavior and aggression. Consistent with its role in reproductive behaviors, the MPOA abundantly expresses gonadal steroid hormone receptors and shows distinct sexual dimorphism in its morphology. Despite the functional importance of the MPOA, the anatomical demarcations of the mouse MPOA subregions have been confusing and remained undefined because of their heterogeneity and complexity. In this review, I first introduce our histological examination showing differential expression of various molecules among the MPOA subregions. I also provide useful molecular markers to delineate the mouse MPOA subregions showing sexual dimorphism. Based on this anatomical study at the subregion level, I also summarize the current understanding of the role of the mouse MPOA and adjacent bed nucleus of the stria terminalis in parental motivation: the central part of the MPOA is essential for parental motivation, and this area exerts an inhibitory effect on the neural activity in the BNST rhomboid nucleus resulting in suppressed infanticide.

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.

MIDA: A Multimodal Imaging-Based Detailed Anatomical Model of the Human Head and Neck.

Computational modeling and simulations are increasingly being used to complement experimental testing for analysis of safety and efficacy of medical devices. Multiple voxel- and surface-based whole- and partial-body models have been proposed in the literature, typically with spatial resolution in the range of 1-2 mm and with 10-50 different tissue types resolved. We have developed a multimodal imaging-based detailed anatomical model of the human head and neck, named "MIDA". The model was obtained by integrating three different magnetic resonance imaging (MRI) modalities, the parameters of which were tailored to enhance the signals of specific tissues: i) structural T1- and T2-weighted MRIs; a specific heavily T2-weighted MRI slab with high nerve contrast optimized to enhance the structures of the ear and eye; ii) magnetic resonance angiography (MRA) data to image the vasculature, and iii) diffusion tensor imaging (DTI) to obtain information on anisotropy and fiber orientation. The unique multimodal high-resolution approach allowed resolving 153 structures, including several distinct muscles, bones and skull layers, arteries and veins, nerves, as well as salivary glands. The model offers also a detailed characterization of eyes, ears, and deep brain structures. A special automatic atlas-based segmentation procedure was adopted to include a detailed map of the nuclei of the thalamus and midbrain into the head model. The suitability of the model to simulations involving different numerical methods, discretization approaches, as well as DTI-based tensorial electrical conductivity, was examined in a case-study, in which the electric field was generated by transcranial alternating current stimulation. The voxel- and the surface-based versions of the models are freely available to the scientific community.

Serotonergic fibers distribution in the midline and intralaminar thalamic nuclei in the rock cavy (Kerodon rupestris).

The thalamic midline/intralaminar complex is part of the higher-order thalamus, which receives little sensory input, and instead forms extensive cortico-thalamo-cortical pathways. The midline thalamic nuclei connect with the medial prefrontal cortex and the medial temporal lobe. On the other hand, the intralaminar nuclei connect with the fronto-parietal cortex. Taking into account this connectivity pattern, it is not surprising that the midline/intralaminar complex has been implicated in a broad variety of cognitive functions, including memory process, attention and orientation, and also reward-based behavior. Serotonin (5-HT) is a neurotransmitter that exerts different post-synaptic roles. Serotonergic neurons are almost entirely restricted to the raphe nuclei and the 5-HT fibers are distributed widely throughout the brain, including the midline/intralaminar complex. The present study comprises a detailed description of the morphologic features and semiquantitative analysis of 5-HT fibers distribution in the midline/intralaminar complex in the rock cavy, a typical rodent of the Northeast region of Brazil, which has been used by our group as an anatomical model to expand the comprehension about phylogeny on the nervous system. The 5-HT fibers in the midline/intralaminar nuclei of the rock cavy were classified into three distinct categories: (1) beaded fibers, which are relatively fine and endowed with large varicosities; (2) fine fibers, with thin axons and small varicosities uniformly distributed in whole axon; and (3) stem axons, showing thick non-varicose axons. Moreover, the density of 5-HT fibers is variable among the analyzed nuclei. On the basis of this diversity of the morphological fibers and the differential profile of optical density among the midline/intralaminar nuclei of the rock cavy, we conclude that the serotonergic system uses a diverse morphologic apparatus to exert a large functional repertory in the midline/intralaminar thalamic nuclei.

Nucleus reuniens of the thalamus contains head direction cells.

Discrete populations of brain cells signal heading direction, rather like a compass. These 'head direction' cells are largely confined to a closely-connected network of sites. We describe, for the first time, a population of head direction cells in nucleus reuniens of the thalamus in the freely-moving rat. This novel subcortical head direction signal potentially modulates the hippocampal CA fields directly and, thus, informs spatial processing and memory.

The reuniens and rhomboid nuclei: neuroanatomy, electrophysiological characteristics and behavioral implications.

The reuniens and rhomboid nuclei, located in the ventral midline of the thalamus, have long been regarded as having non-specific effects on the cortex, while other evidence suggests that they influence behavior related to the photoperiod, hunger, stress or anxiety. We summarise the recent anatomical, electrophysiological and behavioral evidence that these nuclei also influence cognitive processes. The first part of this review describes the reciprocal connections of the reuniens and rhomboid nuclei with the medial prefrontal cortex and the hippocampus. The connectivity pattern among these structures is consistent with the idea that these ventral midline nuclei represent a nodal hub to influence prefrontal-hippocampal interactions. The second part describes the effects of a stimulation or blockade of the ventral midline thalamus on cortical and hippocampal electrophysiological activity. The final part summarizes recent literature supporting the emerging view that the reuniens and rhomboid nuclei may contribute to learning, memory consolidation and behavioral flexibility, in addition to general behavior and aspects of metabolism.

Validation of connectivity-based thalamic segmentation with direct electrophysiologic recordings from human sensory thalamus.

Connectivity-based segmentation has been used to identify functional gray matter subregions that are not discernable on conventional magnetic resonance imaging. However, the accuracy and reliability of this technique has only been validated using indirect means. In order to provide direct electrophysiologic validation of connectivity-based thalamic segmentations within human subjects, we assess the correlation of atlas-based thalamic anatomy, connectivity-based thalamic maps, and somatosensory evoked thalamic potentials in two adults with medication-refractory epilepsy who were undergoing intracranial EEG monitoring with intrathalamic depth and subdural cortical strip electrodes. MRI with atlas-derived localization was used to delineate the anatomic boundaries of the ventral posterolateral (VPL) nucleus of the thalamus. Somatosensory evoked potentials with intrathalamic electrodes physiologically identified a discrete region of phase reversal in the ventrolateral thalamus. Finally, DTI was obtained so that probabilistic tractography and connectivity-based segmentation could be performed to correlate the region of thalamus linked to sensory areas of the cortex, namely the postcentral gyrus. We independently utilized these three different methods in a blinded fashion to localize the "sensory" thalamus, demonstrating a high-degree of reproducible correlation between electrophysiologic and connectivity-based maps of the thalamus. This study provides direct electrophysiologic validation of probabilistic tractography-based thalamic segmentation. Importantly, this study provides an electrophysiological basis for using connectivity-based segmentation to further study subcortical anatomy and physiology while also providing the clinical basis for targeting deep brain nuclei with therapeutic stimulation. Finally, these direct recordings from human thalamus confirm early inferences of a sensory thalamic component of the N18 waveform in somatosensory evoked potentials.

Orexins in the midline thalamus are involved in the expression of conditioned place aversion to morphine withdrawal.

Previous studies have implicated the bed nucleus of the stria terminalis, central nucleus of the amygdala and the shell of the nucleus accumbens (collectively called the extended amygdala) as playing an important role in mediating the aversive emotion associated with opioid withdrawal. The paraventricular nucleus of the thalamus (PVT) provides a very dense input to the extended amygdala, and the PVT is densely innervated by orexin neurons, which appear to be involved in producing some of the physical and emotional effects associated with morphine withdrawal. In the present study, we confirm that the PVT is densely innervated by orexin fibers, whereas the regions of the extended amygdala associated with the effects of morphine withdrawal are poorly innervated. Microinjections of the orexin-1 receptor (OX1R) antagonist SB334867 or the orexin-2 receptor (OX2R) antagonist TCSOX229 at doses of 5.0 or 15.0 microg into the PVT region did not affect the acquisition of the conditioned place aversion (CPA) nor the physical effects produced by naloxone-precipitated morphine withdrawal. In contrast, microinjections of TCSOX229 (15.0 microg) in the PVT region significantly attenuated the expression of naloxone-induced CPA while microinjections of SB334867 at the same dose had no effect. The results from these experiments indicate a role for OX2R in the PVT on the expression of CPA associated with morphine withdrawal. Orexins may mediate the aversive effects of morphine withdrawal by engaging the extended amygdala indirectly through the action of orexins on the PVT.

Negative BOLD-fMRI signals in large cerebral veins.

Reductions in blood oxygenation level dependent (BOLD)-functional magnetic resonance imaging (fMRI) signals below baseline levels have been observed under several conditions as negative activation in task-activation studies or anticorrelation in resting-state experiments. Converging evidence suggests that negative BOLD signals (NBSs) can generally be explained by local reductions in neural activity. Here, we report on NBSs that accompany hemodynamic changes in regions devoid of neural tissue. The NBSs were investigated with high-resolution studies of the visual cortex (VC) at 7 T. Task-activation studies were performed to localize a task-positive area in the VC. During rest, robust negative correlation with the task-positive region was observed in focal regions near the ventricles and dispersed throughout the VC. Both positive and NBSs were dependent on behavioral condition. Comparison with high-resolution structural images showed that negatively correlated regions overlapped with larger pial and ependymal veins near sulcal and ventricular cerebrospinal fluid (CSF). Results from multiecho fMRI showed that NBSs were consistent with increases in local blood volume. These findings confirm theoretical predictions that tie neural activity to blood volume increases, which tend to counteract positive fMRI signal changes associated with increased blood oxygenation. This effect may be more salient in high-resolution studies, in which positive and NBS may be more often spatially distinct.

Circuit-based localization of ferret prefrontal cortex.

We examined the extent of the ferret prefrontal cortex (PFC) and its reciprocal connections with the mediodorsal nucleus of the thalamus (MD) by anterograde and retrograde labeling in 6- to 14-week-old male ferrets. Our results indicate that in the ferret, as in other species, MD projects heavily to the PFC although it also projects to other cortical and subcortical structures. The MD projection to PFC terminates largely in layer IV with lighter innervation of layers II, III, V, and VI. The cells projecting back to MD are mostly in layer VI. The parvocellular component of MD projects to and receives projections from the more caudal and dorsomedial component of the PFC, whereas the magnocellular portion of MD projects to and receives projections from the more rostral and lateral component of the PFC. With these results we have localized the ferret PFC, defined as a frontal cortical region with heavy reciprocal connections with the MD.

Computational macroscopical patterning of the medullary striae of fourth ventricle

The aim of this study was to pattern macroscopically, by use of computational tools, the number and distributionof the medullary striae (MS) of fourth ventricle. After removing 71 fresh human brain stems, each respectiverhomboid fossa was photographed. The MS were carefully identified to be shaped and fulfilled by means ofa digital pen, using the Adobe Photoshop CS3® program. For absolute and relative analyses of number anddistribution, it was considered the maximum and minimum numbers of striae; striae that reached the ipsilaterallateral recess; presence of horizontal or oblique striae, with or without parallelism; and striae located at pontineor bulbar part of the rhomboid fossa. At least two MS per side were macroscopically detectable in 90.6% ofcases; they were bilaterally absent in 5.3% of pieces; and at least one medullary stria was present in both sidesof the rhomboid fossa in 92% of cases. As on the right side (36% of cases) as on the left (26.6%), two MS werefrequently more present. In 60% of cases, striae reached ipsilateral lateral recess on the left, and in 40% of caseson the right. It was detected horizontal, (non-parallel) oblique and parallel striae in 50.7, 86.7 and 26.7%of cases, respectively. Medial medullary striae were observed in the bulbar part of rhomboid fossa in 80% ofpieces, and in 36% of cases in the pontine part. The MS of fourth ventricle show high morphological variabilitydegree in relation to number and distribution.

Preliminary study in patients with obsessive-compulsive disorder treated with electrical stimulation in the inferior thalamic peduncle.

OBJECTIVE: Deep brain stimulation has been used in the treatment of refractory obsessive-compulsive disorder (OCD). Our principal objective was to determine the safety and effectiveness of deep brain stimulation of the inferior thalamic peduncle in the treatment of refractory OCD. METHODS: An open protocol was performed from March 2003 to April 2007 in 5 patients with OCD refractory to conventional treatments. Bilateral stereotactic implantation of tetrapolar electrodes was aimed at the inferior thalamic peduncle and corroborated by electrophysiological responses and magnetic resonance imaging. All patients were off stimulation for 1 month after implantation. In the on-stimulation period, parameters were set at 5 V, 450 microseconds, 130 Hz in bipolar and continuous mode. Clinical changes were evaluated every 3 months for 12 months by means of the Yale-Brown Obsessive Compulsive Scale and the Global Assessment of Functioning scale. Statistical significance was assessed by the Friedman and Wilcoxon tests. RESULTS: The mean Yale-Brown Obsessive Compulsive Scale score decreased from 35 to 17.8 (P < 0.001), and the mean Global Assessment of Functioning scale score improved from 20% to 70% (P < 0.0001). The neuropsychological battery did not show significant changes, and there were no side effects related to electrical stimulation in the chronic period. CONCLUSION: We conclude that inferior thalamic peduncle stimulation is a safe procedure and may be an effective alternative in the treatment of those OCD cases refractory to conventional treatments.

An immunohistochemical investigation of the relationship between neuronal nitric oxide synthase, GABA and presympathetic paraventricular neurons in the hypothalamus.

Functional studies suggest that nitric oxide (NO) modulates sympathetic outflow by enhancing synaptic GABAergic function. Furthermore, the paraventricular nucleus of the hypothalamus (PVN), an important site for autonomic and endocrine homeostasis constitutes an important center mediating NO actions on sympathetic outflow. However, the exact anatomical organization of GABA and NO releasing neurons with the PVN neurons that regulate autonomic activity is poorly understood. The present study addressed this by identifying PVN-presympathetic neurons in the rat with the retrograde tracer Fluorogold injected into T2 segment of the spinal cord or herpes simplex virus injected into the adrenal medulla (AM). GABAergic or nitric oxide cell bodies were identified by antibodies directed towards GABA or glutamate decarboxylase (GAD67) enzyme or neuronal nitric oxide synthase. This revealed a population of GABAergic neurons to be synaptically associated with a chain of pre-sympathetic neurons targeting the AM. Furthermore, this GABAergic population is not a cellular source of NO. Within the PVN, the majority of cellular nitric oxide was localized to non-spinally projecting neurons while for the PVN-spinally projecting neuronal pool only a minority of neuron were immunopositive for neuronal nitric oxide synthase. In summary, nitrergic and GABAergic neurons are associated with a hierarchical chain of neurons that regulate autonomic outflow. This anatomical arrangement supports the known function role of a NO-GABA modulation of sympathetic outflow.

Accelerated postnatal growth increases lipogenic gene expression and adipocyte size in low-birth weight mice.

OBJECTIVE: To characterize the hormonal milieu and adipose gene expression in response to catch-up growth (CUG), a growth pattern associated with obesity and diabetes risk, in a mouse model of low birth weight (LBW). RESEARCH DESIGN AND METHODS: ICR mice were food restricted by 50% from gestational days 12.5-18.5, reducing offspring birth weight by 25%. During the suckling period, dams were either fed ad libitum, permitting CUG in offspring, or food restricted, preventing CUG. Offspring were killed at age 3 weeks, and gonadal fat was removed for RNA extraction, array analysis, RT-PCR, and evaluation of cell size and number. Serum insulin, thyroxine (T4), corticosterone, and adipokines were measured. RESULTS: At age 3 weeks, LBW mice with CUG (designated U-C) had body weight comparable with controls (designated C-C); weight was reduced by 49% in LBW mice without CUG (designated U-U). Adiposity was altered by postnatal nutrition, with gonadal fat increased by 50% in U-C and decreased by 58% in U-U mice (P < 0.05 vs. C-C mice). Adipose expression of the lipogenic genes Fasn, AccI, Lpin1, and Srebf1 was significantly increased in U-C compared with both C-C and U-U mice (P < 0.05). Mitochondrial DNA copy number was reduced by >50% in U-C versus U-U mice (P = 0.014). Although cell numbers did not differ, mean adipocyte diameter was increased in U-C and reduced in U-U mice (P < 0.01). CONCLUSIONS: CUG results in increased adipose tissue lipogenic gene expression and adipocyte diameter but not increased cellularity, suggesting that catch-up fat is primarily associated with lipogenesis rather than adipogenesis in this murine model.

Paraventricular thalamic nucleus: subcortical connections and innervation by serotonin, orexin, and corticotropin-releasing hormone in macaque monkeys.

The present study examines subcortical connections of paraventricular thalamic nucleus (Pa) following small anterograde and retrograde tracer injections in cynomolgus monkeys (Macaca fascicularis). An anterograde tracer injection into the dorsal midline thalamus revealed strong projections to the accumbens nucleus, basal amygdala, lateral septum, and hypothalamus. Retrograde tracer injections into these areas labeled neurons specifically in Pa. Following a retrograde tracer injection into Pa, labeled neurons were found in the hypothalamus, dorsal raphe, and periaqueductal gray. Pa contained a remarkably high density of axons and axonal varicosities immunoreactive for serotonin (5-HT) and orexin/hypocretin (ORX), as well as a moderate density of fibers immunoreactive for corticotropin-releasing hormone (CRH). A retrograde tracer injection into Pa combined with immunohistochemistry demonstrated that ORX and 5-HT axons originate from neurons in the hypothalamus and midbrain. Pa-projecting neurons were localized in the same nuclei of the hypothalamus, amygdala, and midbrain as CRH neurons, although no double labeling was found. The connections of Pa and its innervation by 5-HT, ORX, and CRH suggest that it may relay stress signals between the midbrain and hypothalamus with the accumbens nucleus, basal amygdala, and subgenual cortex as part of a circuit that manages stress and possibly stress-related psychopathologies.