Elevated Neurosteroids in the Lateral Thalamus Relieve Neuropathic Pain in Rats with Spared Nerve Injury

Impaired neuronal macroautophagy in the prelimbic cortex contributes to comorbid anxiety-like behaviors in rats with chronic neuropathic pain

A large proportion of patients with chronic pain experience co-morbid anxiety. The medial prefrontal cortex (mPFC) is proposed to underlie this comorbidity, but the molecular and neuronal mechanisms are not fully understood. Here, we reported that impaired neuronal macroautophagy in the prelimbic cortical (PrL) subregion of the mPFC paralleled the occurrence of anxiety-like behaviors in rats with chronic spared nerve injury (SNI). Intriguingly, such macroautophagy impairment was mainly observed in a FOS/c-Fos+ neuronal subpopulation in the PrL. Chemogenetic inactivation of this comorbid anxiety-related neuronal ensemble relieved pain-induced anxiety-like behaviors. Rescuing macroautophagy impairment in this neuronal ensemble relieved chronic pain-associated anxiety and mechanical allodynia and restored synaptic homeostasis at the molecular level. By contrast, artificial disruption of macroautophagy induced early-onset co-morbid anxiety in neuropathic rats, but not general anxiety in normal rats. Taken together, our work identifies causal linkage between PrL neuronal macroautophagy dysfunction and comorbid anxiety in neuropathic pain and provides novel insights into the role of PrL by differentiating its contribution in pain-induced comorbid anxiety from its modulation over general anxiety-like behaviors.Abbreviation: AAV: adeno-associated viruses; ACC: anterior cingulate cortex; ATG5: autophagy related 5; ATG7: autophagy related 7; ATG12: autophagy related 12; CAMK2/CaMKII: calcium/calmodulin-dependent protein kinase II; CNO: clozapine-N-oxide; CQ: chloroquine; DIA: data independent acquisition; DIO: double floxed inverse orf; DLG4/PSD-95: discs large MAGUK scaffold protein 4; Dox: doxycycline; GABA: γ-aminobutyric acid; GFP: green fluorescent protein; GO: gene ontology; Gi: inhibitory guanine nucleotide-binding proteins; HsCHRM4/M4D: human cholinergic receptor muscarinic 4; HsSYN: human synapsin; KEGG: Kyoto encyclopedia of genes and genomes; LAMP1: lysosomal-associated membrane protein 1; LC3-II: PE conjugated microtubule-associated protein 1 light chain3; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; mPFC: medial prefrontal cortex; P2A: 2A self-cleaving peptide; PPI: protein-protein interaction networks; PrL: prelimbic cortex; RBFOX3/NeuN: RNA binding protein, fox-1 homolog (C. elegans) 3; rtTA: reverse tetracycline-transactivator; SDS-PAGE: sodium dodecylsulfate-polyacrylamide gel electrophoresis; SHANK3: SH3 and multiple ankyrin repeat domains 3; SLC1A1/EAAC1: solute carrier family 1 (neuronal/epithelial high affinity glutamate transporter, systemXag), member 1; SNAP23: synaptosomal-associated protein 23; SNI:spared nerve injury; SQSTM1/p62: sequestosome 1; SYT3: synaptotagmin 3; TRE: tetracycline-responsive element; TRE3G: third-generation tetracycline-responsive element.

Research Progress on the Mechanisms of Central Post-Stroke Pain: A Review

Central Post-Stroke Pain (CPSP) is a primary sequelae of stroke that can develop in the body part corresponding to the cerebrovascular lesion after stroke, most typically after ischemic stroke but also after hemorrhagic stroke. The pathogenesis of CPSP is currently unknown, and research into its mechanism is ongoing. To summarize current research on the CPSP mechanism and provide guidance for future studies. Use "central post-stroke pain," "stroke AND thalamic pain," "stroke AND neuropathic pain," "post-stroke thalamic pain" as the search term. The search was conducted in the PubMed and China National Knowledge Infrastructure databases, summarizing and classifying the retrieved mechanism studies. The mechanistic studies on CPSP are extensive, and we categorized the included mechanistic studies and summarized them in terms of relevant pathway studies, relevant signals and receptors, relevant neural tissues, and described endoplasmic reticulum stress and other relevant studies, as well as summarized the mechanisms of acupuncture treatment. Studies have shown that the pathogenesis of CPSP involves the entire spinal-thalamo-cortical pathway and that multiple substances in the nervous system are involved in the formation and development of CPSP. Among them, the relevant receptors and signals are the hotspot of research, and the discovery and exploration of different receptors and signals have provided a wide range of therapeutic ideas for CPSP. As a very effective treatment, acupuncture is less studied regarding the analgesic mechanism of CPSP, and further experimental studies are still needed.

A synthetic pregnenolone analog promotes microtubule dynamics and neural development

BACKGROUND

Pregnenolone (P5) is a neurosteroid that promotes microtubule polymerization. It also reduces stress and negative symptoms of schizophrenia, promotes memory, as well as recovery from spinal cord injury. P5 is the first substance in the steroid-synthetic pathway; it can be further metabolized into other steroids. Therefore, it is difficult to differentiate the roles of P5 versus its metabolites in the brain. To alleviate this problem, we synthesized and screened a series of non-metabolizable P5 derivatives for their ability to polymerize microtubules similar to P5.

RESULTS

We identified compound #43 (3-beta-pregnenolone acetate), which increased microtubule polymerization. We showed that compound #43 modified microtubule dynamics in live cells, increased neurite outgrowth and changed growth cone morphology in mouse cerebellar granule neuronal culture. Furthermore, compound #43 promoted the formation of stable microtubule tracks in zebrafish developing cerebellar axons.

CONCLUSIONS

We have developed compound #43, a nonmetabolized P5 analog, that recapitulates P5 functions in vivo and can be a new therapeutic candidate for the treatment of neurodevelopmental diseases.

Translocator Protein 18 kDa (TSPO) as a Novel Therapeutic Target for Chronic Pain

Chronic pain is an enormous modern public health problem, with significant numbers of people debilitated by chronic pain from a variety of etiologies. Translocator protein 18 kDa (TSPO) was discovered in 1977 as a peripheral benzodiazepine receptor. It is a five transmembrane domain protein, mainly localized in the outer mitochondrial membrane. Recent and increasing studies have found changes in TSPO and its ligands in various chronic pain models. Reversing their expressions has been shown to alleviate chronic pain in these models, illustrating the effects of TSPO and its ligands. Herein, we review recent evidence and the mechanisms of TSPO in the development of chronic pain associated with peripheral nerve injury, spinal cord injury, cancer, and inflammatory responses. The cumulative evidence indicates that TSPO-based therapy may become an alternative strategy for treating chronic pain.

Allopregnanolone: An overview on its synthesis and effects

Allopregnanolone, a 3α,5α-progesterone metabolite, acts as a potent allosteric modulator of the γ-aminobutyric acid type A receptor. In the present review, the synthesis of this neuroactive steroid occurring in the nervous system is discussed with respect to physiological and pathological conditions. In addition, its physiological and neuroprotective effects are also reported. Interestingly, the levels of this neuroactive steroid, as well as its effects, are sex-dimorphic, suggesting a possible gender medicine based on this neuroactive steroid for neurological disorders. However, allopregnanolone presents low bioavailability and extensive hepatic metabolism, limiting its use as a drug. Therefore, synthetic analogues or a different therapeutic strategy able to increase allopregnanolone levels have been proposed to overcome any pharmacokinetic issues.

Neurosteroids: A novel promise for the treatment of stroke and post-stroke complications

Stroke is the primary reason for death and disability worldwide, with few treatment strategies to date. Neurosteroids, which are natural molecules in the brain, have aroused great interest in the field of stroke. Neurosteroids are a kind of steroid that acts on the nervous system, and are synthesized in the mitochondria of neurons or glial cells using cholesterol or other steroidal precursors. Neurosteroids mainly include estrogen, progesterone (PROG), allopregnanolone, dehydroepiandrosterone (DHEA), and vitamin D (VD). Most of the preclinical studies have confirmed that neurosteroids can decrease the risk of stroke, and improve stroke outcomes. In the meantime, neurosteroids have been shown to have a positive therapeutic significance in some post-stroke complications, such as epilepsy, depression, anxiety, cardiac complications, movement disorders, and post-stroke pain. In this review, we report the historical background, modulatory mechanisms of neurosteroids in stroke and post-stroke complications, and emphasize on the application prospect of neurosteroids in stroke therapy.

Physiopathological role of the enzymatic complex 5α-reductase and 3α/β-hydroxysteroid oxidoreductase in the generation of progesterone and testosterone neuroactive metabolites

The enzymatic complex 5α-reductase (5α-R) and 3α/3β-hydroxysteroid oxidoreductase (HSOR) is expressed in the nervous system, where it transforms progesterone (PROG) and testosterone (T) into neuroactive metabolites. These metabolites regulate myelination, brain maturation, neurotransmission, reproductive behavior and the stress response. The expression of 5α-R and 3α-HSOR and the levels of PROG and T reduced metabolites show regional and sex differences in the nervous system and are affected by changing physiological conditions as well as by neurodegenerative and psychiatric disorders. A decrease in their nervous tissue levels may negatively impact the course and outcome of some pathological events. However, in other pathological conditions their increased levels may have a negative impact. Thus, the use of synthetic analogues of these steroids or 5α-R modulation have been proposed as therapeutic approaches for several nervous system pathologies. However, further research is needed to fully understand the consequences of these manipulations, in particular with 5α-R inhibitors.

Default Mode Network as a Neural Substrate of Acupuncture: Evidence, Challenges and Strategy

Acupuncture is widely applied all over the world. Although the neurobiological underpinnings of acupuncture still remain unclear, accumulating evidence indicates significant alteration of brain activities in response to acupuncture. In particular, activities of brain regions in the default mode network (DMN) are modulated by acupuncture. DMN is crucial for maintaining physiological homeostasis and its functional architecture becomes disrupted in various disorders. But how acupuncture modulates brain functions and whether such modulation constitutes core mechanisms of acupuncture treatment are far from clear. This Perspective integrates recent literature on interactions between acupuncture and functional networks including the DMN, and proposes a back-translational research strategy to elucidate brain mechanisms of acupuncture treatment.

Increased expression of CaV3.2 T-type calcium channels in damaged DRG neurons contributes to neuropathic pain in rats with spared nerve injury

Ion channels are very important in the peripheral sensitization in neuropathic pain. Our present study aims to investigate the possible contribution of Ca_V3.2 T-type calcium channels in damaged dorsal root ganglion neurons in neuropathic pain. We established a neuropathic pain model of rats with spared nerve injury. In these model rats, it was easy to distinguish damaged dorsal root ganglion neurons (of tibial nerve and common peroneal nerve) from intact dorsal root ganglion neurons (of sural nerves). Our results showed that Ca_V3.2 protein expression increased in medium-sized neurons from the damaged dorsal root ganglions but not in the intact ones. With whole cell patch clamp recording technique, it was found that after-depolarizing amplitudes of the damaged medium-sized dorsal root ganglion neurons increased significantly at membrane potentials of −85 mV and −95 mV. These results indicate a functional up-regulation of Ca_V3.2 T-type calcium channels in the damaged medium-sized neurons after spared nerve injury. Behaviorally, blockade of Ca_V3.2 with antisense oligodeoxynucleotides could significantly reverse mechanical allodynia. These results suggest that Ca_V3.2 T-type calcium channels in damaged medium-sized dorsal root ganglion neurons might contribute to neuropathic pain after peripheral nerve injury.

Positron Emission Tomography Studies of the Glial Cell Marker Translocator Protein in Patients With Psychosis: A Meta-analysis Using Individual Participant Data

BACKGROUND

Accumulating evidence suggests that the immune system may be an important target for new treatment approaches in schizophrenia. Positron emission tomography and radioligands binding to the translocator protein (TSPO), which is expressed in glial cells in the brain including immune cells, represents a potential method for patient stratification and treatment monitoring. This study examined whether patients with first-episode psychosis and schizophrenia had altered TSPO levels compared with healthy control subjects.

METHODS

PubMed was searched for studies comparing patients with psychosis with healthy control subjects using second-generation TSPO radioligands. The outcome measure was total distribution volume (VT), an index of TSPO levels, in frontal cortex, temporal cortex, and hippocampus. Bayes factors (BFs) were applied to examine the relative support for higher, lower, or no difference in patients' TSPO levels compared with healthy control subjects.

RESULTS

Five studies, with 75 participants with first-episode psychosis or schizophrenia and 77 healthy control subjects, were included. BFs showed strong support for lower VT in patients relative to no difference (all BFs > 32), or relative to higher VT (all BFs > 422), in all brain regions. From the posterior distributions, mean patient-control differences in standardized VT values were -0.48 for frontal cortex (95% credible interval [CredInt] = -0.88 to 0.09), -0.47 for temporal cortex (CredInt = -0.87 to -0.07), and -0.63 for hippocampus (CredInt = -1.00 to -0.25).

CONCLUSIONS

The lower levels of TSPO observed in patients may correspond to altered function or lower density of brain immune cells. Future studies should focus on investigating the underlying biological mechanisms and their relevance for treatment.

Upregulation of Cav3.2 T-type calcium channels in adjacent intact L4 dorsal root ganglion neurons in neuropathic pain rats with L5 spinal nerve ligation

Besides the injured peripheral dorsal root ganglion (DRG) neurons, the adjacent intact DRG neurons also have important roles in neuropathic pain. Ion channels including Ca_v3.2 T-type calcium channel in the DRG neurons are important in the development of neuropathic pain. In the present study, we aimed to examine the expression of Ca_v3.2 T-type calcium channels in the intact DRG neurons in neuropathic pain. A neuropathic pain model of rat with lumbar 5 (L5) spinal nerve ligation (SNL) was established, in which the L4 DRG was separated from the axotomized L5 DRG, and the molecular, morphological and electrophysiological changes of Ca_v3.2 T-type calcium channels in L4 DRG neurons were investigated. Western blotting showed that total and membrane protein levels of Ca_v3.2 in L4 DRG neurons increased, and voltage-dependent patch clamp recordings revealed an increased T-type current density with a curve shift to the left in steady-state activation in the acutely isolated L4 DRG neurons in neuropathic pain rats. Immunofluorescent staining further showed that the membrane expression of Ca_v3.2 increased in CGRP-, IB4-positive small neurons and NF200-positive large ones. In conclusion, the membrane expression and the function of Ca_v3.2 T-type calcium channels are increased in the intact L4 DRG neurons in neuropathic pain rats with peripheral nerve injury like SNL.

Hypersensitivity of Prelimbic Cortex Neurons Contributes to Aggravated Nociceptive Responses in Rats With Experience of Chronic Inflammatory Pain

Previous experience of chronic pain causes enhanced responses to upcoming noxious events in both humans and animals, but the underlying mechanisms remain unclear. In the present study, we found that rats with complete Freund's adjuvant (CFA)-induced chronic inflammatory pain experience exhibited aggravated pain responses to later formalin test. Enhanced neuronal activation upon formalin assaults and increased phosphorylated cAMP-response element binding protein (CREB) were observed in the prelimbic cortex (PL) of rats with chronic inflammatory pain experience, and inhibiting PL neuronal activities reversed the aggravated pain. Inflammatory pain experience induced persistent p38 mitogen-activated protein kinase (MAPK; p38) but not extracellular regulated protein kinase (ERK) or c-Jun N-terminal kinase (JNK) hyperphosphorylation in the PL. Inhibiting the p38 phosphorylation in PL reversed the aggravated nociceptive responses to formalin test and down-regulated enhanced phosphorylated CREB in the PL. Chemogenetics identified PL-periaqueductal gray (PAG) but not PL-nucleus accumbens (NAc) as a key pathway in inducing the aggravated formalin pain. Our results demonstrate that persistent hyperphosphorylation of p38 in the PL underlies aggravated nociceptive responses in rats with chronic inflammatory pain experience.

Accumulation of Cav3.2 T-type Calcium Channels in the Uninjured Sural Nerve Contributes to Neuropathic Pain in Rats with Spared Nerve Injury

Injuries to peripheral nerve fibers induce neuropathic pain. But the involvement of adjacent uninjured fibers to pain is not fully understood. The present study aims to investigate the possible contribution of Ca_v3.2 T-type calcium channels in uninjured afferent nerve fibers to neuropathic pain in rats with spared nerve injury (SNI). Aβ-, Aδ- and C-fibers of the uninjured sural nerve were sensitized revealed by in vivo single-unit recording, which were accompanied by accumulation of Ca_v3.2 T-type calcium channel proteins shown by Western blotting. Application of mibefradil, a T-type calcium channel blocker, to sural nerve receptive fields increased mechanical thresholds of Aβ-, Aδ- and C-fibers, confirming the functional involvement of accumulated channels in the sural nerve in SNI rats. Finally, perineural application of mibefradil or TTA-P2 to the uninjured sural nerve alleviated mechanical allodynia in SNI rats. These results suggest that axonal accumulation of Ca_v3.2 T-type calcium channels plays an important role in the uninjured sural nerve sensitization and contributes to neuropathic pain.

Brain-derived neurotrophic factor in the infralimbic cortex alleviates inflammatory pain

In chronic pain, it has been reported that the medial prefrontal cortex (mPFC) takes important regulatory roles, and may change functionally and morphologically in result of chronic pain. Brain-derived neurotrophic factor (BDNF) is well known as a critical modulator of neuronal excitability and synaptic transmission in the central nervous system. The aim of the present study is to investigate the role of BDNF in the infralimbic cortex and the prelimbic cortex of the mPFC in complete Freund's adjuvant (CFA)-induced inflammatory pain. We found that the BDNF level decreased in the infralimbic cortex, but not in the prelimbic cortex, 3days after the CFA induction of the inflammatory pain. BDNF infusion into bilateral infralimbic cortices to activate neuronal activities could alleviate inflammatory pain and accelerate long-term recovery from pain. In conclusion, BDNF in the infralimbic cortex of the mPFC could accelerate recovery from inflammatory pain.