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

Sphingosine-1-phosphate receptor 3 activation promotes sociability and regulates transcripts important for anxiolytic-like behavior

We previously demonstrated that sphingosine-1-phosphate receptor 3 (S1PR3) in the medial prefrontal cortex (mPFC) prevents reductions in sociability normally caused by stress. S1PR3 is a ubiquitously expressed G-protein coupled receptor that regulates immune system function, although its regulation of other biological processes is not well understood. Pharmacological activators of S1PR3 might provide important insights for understanding the neural substrates underlying sociability. Here we show that in mice, systemic injections of an S1PR3-specific agonist, CYM5541, promotes sociability in males and females whereas an S1PR3-specific antagonist, CAY10444, increases amygdala activation and increases social avoidance, particularly in females. S1PR3 expression is increased in the mPFC and dentate gyrus of females compared to males. RNA sequencing in the mPFC reveals that S1PR3 activation alters the expression of transcripts related to immune function, neurotransmission, transmembrane ion transport, and intracellular signaling. This work provides evidence that S1PR3 agonists, which have classically been used as immune modulators, might also be used to promote social behavior and, potentially, relieve symptoms of social anxiety. S1PR3 might be an important hub gene for mitigating maladaptive effects of stress as it reduces inflammatory processes, increases transcripts linked to anxiolytic neurotransmission, and promotes social behavior.

Persistently increased post-stress activity of paraventricular thalamic neurons is essential for the emergence of stress-induced alterations in behaviour

A single exposure to a stressful event can result in enduring changes in behaviour. Long-term modifications in neuronal networks induced by stress are well explored but the initial steps leading to these alterations remain incompletely understood. In this study, we found that acute stress exposure triggers an immediate increase in the firing activity of calretinin-positive neurons in the paraventricular thalamic nucleus (PVT/CR+) that persists for several days in mice. This increase in activity had a causal role in stress-induced changes in spontaneous behaviour. Attenuating PVT/CR+ neuronal activity for only 1 h after the stress event rescued both the protracted increase in PVT/CR+ firing rate and the stress-induced behavioural alterations. Activation of the key forebrain targets (basolateral amygdala, prelimbic cortex, and nucleus accumbens) that mediate defensive behaviour has also been reduced by this post-stress inhibition. Reduction of PVT/CR+ cell activity 5 days later remained still effective in ameliorating stress-induced changes in spontaneous behaviour. The results demonstrate a critical role of the prolonged, post-stress changes in firing activity of PVT/CR+ neurons in shaping the behavioural changes associated with stress. Our data proposes a therapeutic window for intervention in acute stress-related disorders, offering potential avenues for targeted treatment strategies.

The Paraventricular Thalamic Nucleus and Its Projections in Regulating Reward and Context Associations

The paraventricular thalamic nucleus (PVT) is a brain region that mediates aversive and reward-related behaviors as shown in animals exposed to fear conditioning, natural rewards, or drugs of abuse. However, it is unknown whether manipulations of the PVT, in the absence of external factors or stimuli (e.g., fear, natural rewards, or drugs of abuse), are sufficient to drive reward-related behaviors. Additionally, it is unknown whether drugs of abuse administered directly into the PVT are sufficient to drive reward-related behaviors. Here, using behavioral as well as pathway and cell-type specific approaches, we manipulate PVT activity as well as the PVT-to-nucleus accumbens shell (NAcSh) neurocircuit to explore reward phenotypes. First, we show that bath perfusion of morphine (10 µM) caused hyperpolarization of the resting membrane potential, increased rheobase, and decreased intrinsic membrane excitability in PVT neurons that project to the NAcSh. Additionally, we found that direct injections of morphine (50 ng) in the PVT of mice were sufficient to generate conditioned place preference (CPP) for the morphine-paired chamber. Mimicking the inhibitory effect of morphine, we employed a chemogenetic approach to inhibit PVT neurons that projected to the NAcSh and found that pairing the inhibition of these PVT neurons with a specific context evoked the acquisition of CPP. Lastly, using brain slice electrophysiology, we found that bath-perfused morphine (10 µM) significantly reduced PVT excitatory synaptic transmission on both dopamine D1 and D2 receptor-expressing medium spiny neurons in the NAcSh, but that inhibiting PVT afferents in the NAcSh was not sufficient to evoke CPP.

Differential recruitment of brain circuits during fear extinction in non-stressed compared to stress resilient animals

Dysfunctional fear responses in post-traumatic stress disorder (PTSD) may be partly explained by an inability to effectively extinguish fear responses elicited by trauma-related cues. However, only a subset of individuals exposed to traumatic stress develop PTSD. Therefore, studying fear extinction deficits in animal models of individual differences could help identify neural substrates underlying vulnerability or resilience to the effects of stress. We used a rat model of social defeat in which rats segregate into passively and actively coping rats. In previous work, we showed that passively coping rats exhibit disruptions in social interaction whereas actively coping rats do not display behaviors differently from controls, indicating their resilience. Here, adult male rats exposed to 7 days of social defeat were tested for fear extinction, retention of extinction, and persistence of retention using contextual fear and ethologically-relevant fear tests. Passively coping rats exhibited elevated freezing in response to the previously extinguished context. Analyses of cFos expressing cells across select brain regions showed high correlations within dorsal hippocampal subregions, while passively coping rats had high correlations between the dorsal hippocampus CA1 and the central and basolateral subregions of the amygdala. Importantly, although control and actively coping rats showed similar levels of behavioral extinction, there was little similarity between activated structures, suggesting stress resilience in response to chronic social defeat involves an adaptive differential recruitment of brain circuits to successfully extinguish fear memories.

Stress circuitry: mechanisms behind nervous and immune system communication that influence behavior

Inflammatory processes are increased by stress and contribute to the pathology of mood disorders. Stress is thought to primarily induce inflammation through peripheral and central noradrenergic neurotransmission. In healthy individuals, these pro-inflammatory effects are countered by glucocorticoid signaling, which is also activated by stress. In chronically stressed individuals, the anti-inflammatory effects of glucocorticoids are impaired, allowing pro-inflammatory effects to go unchecked. Mechanisms underlying this glucocorticoid resistance are well understood, but the precise circuits and molecular mechanisms by which stress increases inflammation are not as well known. In this narrative review, we summarize the mechanisms by which chronic stress increases inflammation and contributes to the onset and development of stress-related mood disorders. We focus on the neural substrates and molecular mechanisms, especially those regulated by noradrenergic signaling, that increase inflammatory processes in stressed individuals. We also discuss key knowledge gaps in our understanding of the communication between nervous and immune systems during stress and considerations for future therapeutic strategies. Here we highlight the mechanisms by which noradrenergic signaling contributes to inflammatory processes during stress and how this inflammation can contribute to the pathology of stress-related mood disorders. Understanding the mechanisms underlying crosstalk between the nervous and immune systems may lead to novel therapeutic strategies for mood disorders and/or provide important considerations for treating immune-related diseases in individuals suffering from stress-related disorders.

Time-restricted feeding is an intervention against excessive dark-phase sleepiness induced by obesogenic diet

High-fat diet (HFD)-induced obesity is a growing epidemic and major health concern. While excessive daytime sleepiness (EDS) is a common symptom of HFD-induced obesity, preliminary findings suggest that reduced wakefulness could be improved with time-restricted feeding (TRF). At present, however, the underlying neural mechanisms remain largely unknown. The paraventricular thalamic nucleus (PVT) plays a role in maintaining wakefulness. We found that chronic HFD impaired the activity of PVT neurons. Notably, inactivation of the PVT was sufficient to reduce and fragment wakefulness during the active phase in lean mice, similar to the sleep-wake alterations observed in obese mice with HFD-induced obesity. On the other hand, enhancing PVT neuronal activity consolidated wakefulness in mice with HFD-induced obesity. We observed that the fragmented wakefulness could be eliminated and reversed by TRF. Furthermore, TRF prevented the HFD-induced disruptions on synaptic transmission in the PVT, in a feeding duration-dependent manner. Collectively, our findings demonstrate that ad libitum access to a HFD results in inactivation of the PVT, which is critical to impaired nocturnal wakefulness and increased sleep, while TRF can prevent and reverse diet-induced PVT dysfunction and excessive sleepiness. We establish a link between TRF and neural activity, through which TRF can potentially serve as a lifestyle intervention against diet/obesity-related EDS.

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.