Paraventricular Thalamus Activity during Motivational Conflict Highlights the Nucleus as a Potential Constituent in the Neurocircuitry of Addiction

Common neural deficits across reward functions in major depression: a meta-analysis of fMRI studies

Major depressive disorder (MDD) is characterized by deficient reward functions in the brain. However, existing findings on functional alterations during reward anticipation, reward processing, and learning among MDD patients are inconsistent, and it was unclear whether a common reward system implicated in multiple reward functions is altered in MDD. Here we meta-analyzed 18 past studies that compared brain reward functions between adult MDD patients (N = 477, mean age = 26.50 years, female = 59.40%) and healthy controls (N = 506, mean age = 28.11 years, females = 55.58%), and particularly examined group differences across multiple reward functions. Jack-knife sensitivity and subgroup meta-analyses were conducted to test robustness of findings across patient comorbidity, task paradigm, and reward nature. Meta-regression analyses assessed the moderating effect of patient symptom severity and anhedonia scores. We found during reward anticipation, MDD patients showed lower activities in the lateral prefrontal-thalamus circuitry. During reward processing, patients displayed reduced activities in the right striatum and prefrontal cortex, but increased activities in the left temporal cortex. During reward learning, patients showed reduced activity in the lateral prefrontal-thalamic-striatal circuitry and the right parahippocampal-occipital circuitry but higher activities in bilateral cerebellum and the left visual cortex. MDD patients showed decreased activity in the right thalamus during both reward anticipation and learning, and in the right caudate during both reward processing and learning. Larger functional changes in MDD were observed among patients with more severe symptoms and higher anhedonia levels. The thalamic-striatal circuitry functional alterations could be the key neural mechanism underlying MDD patients overarching reward function deficiencies.

Paraventricular Thalamic MC3R Circuits Link Energy Homeostasis with Anxiety-Related Behavior

The hypothalamic melanocortin system is critically involved in sensing stored energy and communicating this information throughout the brain, including to brain regions controlling motivation and emotion. This system consists of first-order agouti-related peptide (AgRP) and pro-opiomelanocortin (POMC) neurons located in the hypothalamic arcuate nucleus and downstream neurons containing the melanocortin-3 (MC3R) and melanocortin-4 receptor (MC4R). Although extensive work has characterized the function of downstream MC4R neurons, the identity and function of MC3R-containing neurons are poorly understood. Here, we used neuroanatomical and circuit manipulation approaches in mice to identify a novel pathway linking hypothalamic melanocortin neurons to melanocortin-3 receptor neurons located in the paraventricular thalamus (PVT) in male and female mice. MC3R neurons in PVT are innervated by hypothalamic AgRP and POMC neurons and are activated by anorexigenic and aversive stimuli. Consistently, chemogenetic activation of PVT MC3R neurons increases anxiety-related behavior and reduces feeding in hungry mice, whereas inhibition of PVT MC3R neurons reduces anxiety-related behavior. These studies position PVT MC3R neurons as important cellular substrates linking energy status with neural circuitry regulating anxiety-related behavior and represent a promising potential target for diseases at the intersection of metabolism and anxiety-related behavior such as anorexia nervosa.SIGNIFICANCE STATEMENT Animals must constantly adapt their behavior to changing internal and external challenges, and impairments in appropriately responding to these challenges are a hallmark of many neuropsychiatric disorders. Here, we demonstrate that paraventricular thalamic neurons containing the melanocortin-3 receptor respond to energy-state-related information and external challenges to regulate anxiety-related behavior in mice. Thus, these neurons represent a potential target for understanding the neurobiology of disorders at the intersection of metabolism and psychiatry such as anorexia nervosa.

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.

A thalamic bridge from sensory perception to cognition

The ability to adapt to dynamic environments requires tracking multiple signals with variable sensory salience and fluctuating behavioral relevance. This complex process requires integrative crosstalk between sensory and cognitive brain circuits. Functional interactions between cortical and thalamic regions are now considered essential for both sensory perception and cognition but a clear account of the functional link between sensory and cognitive circuits is currently lacking. This review aims to document how thalamic nuclei may effectively act as a bridge allowing to fuse perceptual and cognitive events into meaningful experiences. After highlighting key aspects of thalamocortical circuits such as the classic first-order/higher-order dichotomy, we consider the role of the thalamic reticular nucleus from directed attention to cognition. We next summarize research relying on Pavlovian learning paradigms, showing that both first-order and higher-order thalamic nuclei contribute to associative learning. Finally, we propose that modulator inputs reaching all thalamic nuclei may be critical for integrative purposes when environmental signals are computed. Altogether, the thalamus appears as the bridge linking perception, cognition and possibly affect.