Role of the RVM in Descending Pain Regulation Originating from the Cerebrospinal Fluid-Contacting Nucleus.

Evidence has suggested that cerebrospinal fluid-contacting nucleus (CSF-contacting nucleus) is correlated with the development and recurrence of pain. A recent research showed that the CSF-contacting nucleus acts as a component of the descending 5-hydroxytryptamine (5-HT) system and plays a role in descending pain inhibition. However, limited studies are conducted to investigate the relationship between the CSF-contacting nucleus and pain. In present study, we explored the effect of CSF-contacting nucleus on nociceptive behaviors in both normal and neuropathic rats via targeted ablation of the CSF-contacting nucleus in the brainstem, using cholera toxin subunit B-saporin (CB-SAP), a cytotoxin coupled to cholera toxin subunit B. The CB-SAP-treated rats showed aggravated thermal hyperalgesia and mechanical allodynia. Also, results from immunohistochemical experiments showed that rostral ventromedial medulla (RVM) received fiber projection from the CSF-contacting nucleus, which disappeared after ablation of the CSF-contacting nucleus, and the CB-SAP treated rats showed downregulation of c-Fos expression in the RVM as compared with the rats receiving i.c.v. injection of phosphate buffer saline (PBS). A significant downregulation of 5-HT-labeled neurons and tryptophan hydroxylase 2 (TPH2) as the marker of 5-HT cells in the RVM, and 5-HT expression in spinal dorsal horn in both normal and chronic constriction injury (CCI) rats after i.c.v. injection of CB-SAP was observed. These results suggested that RVM may be involved in descending pain modulation originating from the CSF-contacting nucleus.

Pain regulation of endokinin A/B or endokinin C/D on chimeric peptide MCRT in mice.

The present study focused on the interactive pain regulation of endokinin A/B (EKA/B, the common C-terminal decapeptide in EKA and EKB) or endokinin C/D (EKC/D, the common C-terminal duodecapeptide in EKC and EKD) on chimeric peptide MCRT (YPFPFRTic-NH2, based on YPFP-NH2 and PFRTic-NH2) at the supraspinal level in mice. Results demonstrated that the co-injection of nanomolar EKA/B and MCRT showed moderate regulation, whereas 30 pmol EKA/B had no effect on MCRT. The combination of EKC/D and MCRT produced enhanced antinociception, which was nearly equal to the sum of the mathematical values of single EKC/D and MCRT. Mechanism studies revealed that pre-injected naloxone attenuated the combination significantly compared with the equivalent analgesic effects of EKC/D alone, suggesting that EKC/D and MCRT might act on two totally independent pathways. Moreover, based on the above results and previous reports, we made two reasonable hypotheses to explain the cocktail-induced analgesia, which may potentially pave the way to explore the respective regulatory mechanisms of EKA/B, EKC/D, and MCRT and to better understand the complicated pain regulation of NK1 and µ opioid receptors, as follows: (1) MCRT and endomorphin-1 possibly activated different µ subtypes; and (2) picomolar EKA/B might motivate the endogenous NPFF system after NK1 activation.

Coordination between midcingulate cortex and retrosplenial cortex in pain regulation.

Introduction: The cingulate cortex, with its subregions ACC, MCC, and RSC, is key in pain processing. However, the detailed interactions among these regions in modulating pain sensation have remained unclear. Methods: In this study, chemogenetic tools were employed to selectively activate or inhibit neuronal activity in the MCC and RSC of rodents to elucidate their roles in pain regulation.Results: Our results showed that chemogenetic activation in both the RSC and MCC heightened pain sensitivity. Suppression of MCC activity disrupted the RSC's regulation of both mechanical and thermal pain, while RSC inhibition specifically affected the MCC's regulation of thermal pain. Discussion: The findings indicate a complex interplay between the MCC and RSC, with the MCC potentially governing the RSC's pain regulatory mechanisms. The RSC, in turn, is crucial for the MCC's control over thermal sensation, revealing a collaborative mechanism in pain processing. Conclusion: This study provides evidence for the MCC and RSC's collaborative roles in pain regulation, highlighting the importance of their interactions for thermal and mechanical pain sensitivity. Understanding these mechanisms could aid in developing targeted therapies for pain disorders.

Statistical learning shapes pain perception and prediction independently of external cues.

The placebo and nocebo effects highlight the importance of expectations in modulating pain perception, but in everyday life we don't need an external source of information to form expectations about pain. The brain can learn to predict pain in a more fundamental way, simply by experiencing fluctuating, non-random streams of noxious inputs, and extracting their temporal regularities. This process is called statistical learning. Here, we address a key open question: does statistical learning modulate pain perception? We asked 27 participants to both rate and predict pain intensity levels in sequences of fluctuating heat pain. Using a computational approach, we show that probabilistic expectations and confidence were used to weigh pain perception and prediction. As such, this study goes beyond well-established conditioning paradigms associating non-pain cues with pain outcomes, and shows that statistical learning itself shapes pain experience. This finding opens a new path of research into the brain mechanisms of pain regulation, with relevance to chronic pain where it may be dysfunctional.

Activating PV-positive neurons in ventral thalamic reticular nucleus reduces pain sensitivity in mice.

Previous studies have demonstrated that thalamic reticular nucleus (TRN) and the sub-nuclei play important roles in pain sensation. Our previous findings showed that activating parvalbumin-positive (PV+) neurons in dorsal sector of TRN (dTRN) could reduce the pain threshold and consequently increase the pain sensitivity of mice. Recent studies have shown that activation of GABAergic projection of TRN to ventrobasal thalamus (VB) alleviated pathological pain. GABAergic neurons in TRN are mainly PV+ neurons. However, the exact roles of ventral TRN (vTRN) PV+ neurons in pain sensation remain unclear. In this study, the designer receptors exclusively activated by designer drugs (DREADD) method was used to activate the PV+ neurons in vTRN of PV-Cre transgenic mice, and the mechanical threshold and thermal latency were measured to investigate the regulatory effects of vTRN on pain sensitivity in mice. Thereafter, PV-Cre transgenic mice, conditional anterograde axonal tract tracing, and immunohistochemistry were used to investigate the distribution of PV+ neurons fibers in vTRN. The results showed that the activation of PV+ neurons in vTRN increased the mechanical threshold and thermal latency, which indicated reduction of pain sensitivity. The fibers of these neurons mainly projected to ventral posterolateral thalamic nucleus (VPL), ventral posteromedial thalamic nucleus (VPM), ventrolateral thalamic nucleus (VL), centrolateral thalamic nucleus (CL) and various other brain regions. These findings indicated that activation of PV+ neurons in the vTRN decreased pain sensitivity in mice, which provided additional evidence on the mechanisms of PV+ neurons of TRN in regulating neuralgia.

Developmental-Based Classification of Enkephalin and Somatostatin Containing Neurons of the Chicken Central Extended Amygdala.

The central extended amygdala, including the lateral bed nucleus of the stria terminalis and the central amygdala, plays a key role in stress response. To understand how the central extended amygdala regulates stress it is essential to dissect this structure at molecular, cellular and circuit levels. In mammals, the central amygdala contains two distinct cell populations that become active (on cells) or inactive (off cells) during the conditioned fear response. These two cell types inhibit each other and project mainly unidirectionally to output cells, thus providing a sophisticated regulation of stress. These two cell types express either protein kinase C-delta/enkephalin or somatostatin, and were suggested to originate in different embryonic domains of the subpallium that respectively express the transcription factors Pax6 or Nkx2.1 during development. The regulation of the stress response by the central extended amygdala is poorly studied in non-mammals. Using an evolutionary developmental neurobiology approach, we previously identified several subdivisions in the central extended amygdala of chicken. These contain Pax6, Islet1 and Nkx2.1 cells that originate in dorsal striatal, ventral striatal or pallidopreoptic embryonic divisions, and also contain neurons expressing enkephalin and somatostatin. To know the origin of these cells, in this study we carried out multiple fluorescent labeling to analyze coexpression of different transcription factors with enkephalin or somatostatin. We found that many enkephalin cells coexpress Pax6 and likely derive from the dorsal striatal division, resembling the off cells of the mouse central amygdala. In contrast, most somatostatin cells coexpress Nkx2.1 and derive from the pallidal division, resembling the on cells. We also found coexpression of enkephalin and somatostatin with other transcription factors. Our results show the existence of multiple cell types in the central extended amygdala of chicken, perhaps including on/off cell systems, and set the basis for studying the role of these cells in stress regulation.

Research progress on the mechanism of orexin in pain regulation in different brain regions.

Orexin is a neuropeptide that is primarily synthesized and secreted by the lateral hypothalamus (LH) and includes two substances derived from the same precursor (orexin A [OXA] and orexin B [OXB]). Studies have shown that orexin is not only involved in the regulation of eating, the sleep-wake cycle, and energy metabolism, but also closely associated with various physiological functions, such as cardiovascular control, reproduction, stress, reward, addiction, and the modulation of pain transmission. At present, studies that have been performed both domestically and abroad have confirmed that orexin and its receptors are closely associated with pain regulation. In this article, the research progress on acute pain regulation involving orexin is reviewed.

Acceptance-Based Emotion Regulation Reduces Subjective and Physiological Pain Responses.

Acceptance-based regulation of pain, which focuses on the allowing of pain and pain related thoughts and emotions, was found to modulate pain. However, results so far are inconsistent regarding different pain modalities and indices. Moreover, studies so far often lack a suitable control condition, focus on behavioral pain measures rather than physiological correlates, and often use between-subject designs, which potentially impede the evaluation of the effectiveness of the strategies. Therefore, we investigated whether acceptance-based strategies can reduce subjective and physiological markers of acute pain in comparison to a control condition in a within-subject design. To this end, participants (N = 30) completed 24 trials comprising 10 s of heat pain stimulation. Each trial started with a cue instructing participants to welcome and experience pain (acceptance trials) or to react to the pain as it is without employing any regulation strategies (control trials). In addition to pain intensity and unpleasantness ratings, heart rate (HR) and skin conductance (SC) were recorded. Results showed significantly decreased pain intensity and unpleasantness ratings for acceptance compared to control trials. Additionally, HR was significantly lower during acceptance compared to control trials, whereas SC revealed no significant differences. These results demonstrate the effectiveness of acceptance-based strategies in reducing subjective and physiological pain responses relative to a control condition, even after short training. Therefore, the systematic investigation of acceptance in different pain modalities in healthy and chronic pain patients is warranted.

Activation of Parvalbumin Neurons in the Rostro-Dorsal Sector of the Thalamic Reticular Nucleus Promotes Sensitivity to Pain in Mice.

The calcium-binding protein, parvalbumin (PV), is highly expressed in thalamic reticular nucleus (TRN) GABAergic neurons, which receive input from the cerebral cortex and thalamus and send inhibitory output to the thalamic relay nucleus. Previous studies suggest that the TRN is involved in pain regulation as an important relay nucleus of the ascending pain pathway. However, little is known about its functional role in pain regulation and interconnectivity. In our study, the role of rostro-dorsal sector of TRN (TRNrd) PV-positive neurons in pain regulation was studied using chemogenetics based on designer receptors exclusively activated by designer drugs (DREADD). Then, projections from the TRNrd PV-positive neurons were explored using PV-Cre transgenic mice, conditional anterograde axonal tract tracing, and optogenetics, combined with immunohistochemistry and electrophysiology. The results showed that activation of PV-positive neurons in the TRNrd decreased the mechanical threshold and thermal latency of behaving mice during the light period when neuronal activity was low. Furthermore, the anterodorsal and paratenial thalamic nucleus received innervation from PV-positive neurons in the TRNrd. They were specifically inhibited by GABA, which is released from local axonal endings of PV neurons. These findings indicate that activation of PV neurons in the TRNrd increases pain sensitivity in PV-Cre transgenic mice.

Interaction of endokinin A/B and (Mpa(6))-γ2-MSH-6-12 in pain regulation in mice.

The present study focused on the interactive effects of (Mpa(6))-γ2-MSH-6-12 (Mpa, spinal level) and endokinin A/B (EKA/B, supraspinal level) on pain regulation in mice. EKA/B (30 pmol) only weakened 100 pmol Mpa-induced hyperalgesia at 5 min, but could enhance it during 20-30 min. However, EKA/B (100 pmol) antagonized all dose levels of Mpa significantly at 5 min and blocked them completely at 10 min. EKA/B (3 nmol) co-injected with Mpa presented marked analgesia at 5 min and enduring hyperalgesia within 20-60 min. To investigate the underlying mechanisms between Mpa and EKA/B, SR140333B and SR142801 (NK1 and NK3 receptor antagonists, respectively) were utilized. SR140333B had no influence on Mpa, while SR142801 potentiated it during 20-30 min. Whereas, SR140333B and SR142801 could block the co-administration of Mpa and EKA/B (30 pmol) separately at 5 min and 30 min. These phenomena might attribute to that these two antagonists promoted the antagonism of EKA/B (30 pmol) at the early stage, while antagonized EKA/B preferentially in the latter period. SR140333B weakened the analgesia of EKA/B (3 nmol), but produced no effect on Mpa. However, SR140333B failed to affect the co-injection of Mpa and EKA/B, which implied that EKA/B cooperated with Mpa prior to SR140333B. These results could potentially help to better understand the interaction of NK and MrgC receptors in pain regulation in mice.

The noradrenergic pain regulation system: a potential target for pain therapy.

Noradrenaline, through action on α1- and α2-adrenoceptors, is involved in intrinsic control of pain. Peripheral noradrenaline that is mainly released by the sympathetic nervous system has little influence on healthy tissues, whereas in injured or inflamed tissues it has varying effects, including aggravation of pain in neuropathy. The peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ion channel properties on primary afferent nociceptors, whereas an interaction with the immune system may contribute to peripheral antinociceptive effect of noradrenaline. In the spinal dorsal horn, noradrenaline released from descending pathways originating in the pontine A5-A7 cell groups attenuates pain by inhibitory action on α(2A)-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct α2-adrenergic action on spinal pain-relay neurons (postsynaptic inhibition), and by α1-adrenergic activation of inhibitory interneurons. Moreover, α(2C)-adrenoceptors on axon terminals of excitatory interneurons might contribute to spinal control of pain. At supraspinal levels, the effect of noradrenergic system on pain has varied depending on many factors such as the type of the adrenoceptor, pathophysiological condition, and the brain area. In general, the baseline pain sensitivity is only little influenced by the noradrenergic system, whereas in injured conditions the noradrenergic system contributes to feedback inhibition of pain. The central as well as the peripheral noradrenergic system is subject to various plastic changes following injury or inflammation that influence its antinociceptive efficiency. α2-Adrenoceptor agonists have proven effective in treating various pain conditions.

Astrocytic expression of GFAP and serum levels of IL-1ß and TNF-α in rats treated with different pain relievers

ABSTRACT Pro-inflammatory cytokines and glial cells, especially microglial cells, have been implicated in persistent pain sensitization. Less is known about the role of astrocytes in pain regulation. This study aimed to observe the expression of the astrocytic biomarker glial fibrillary acidic protein (GFAP) and the serum levels of interleukin 1 beta (IL-1ß) and tumor necrosis factor alpha (TNF-α) after short-term administration of central pain relievers in rats not submitted to noxious stimuli. Male Wistar rats were divided into five groups, receiving for nine days- (1) amitriptyline (Amt-10 mg/kg/day, by gavage); (2) gabapentin (Gb-60 mg/kg/day, by gavage; (3) methadone (Me-4.5 mg/kg/day, intraperitoneal route [IP]); (4) morphine (Mo-10 mg/kg/day, IP); or (5) 0.9% saline solution, IP. Brain samples were collected for immunohistochemical study of GFAP expression in the mesencephalon and nucleus accumbens (NAc). The area of GFAP-positive cells was calculated using MetaMorph software and serum levels of IL-1ß and TNF-α were measured by enzyme-linked immunosorbent assay. Serum TNF-α levels were decreased in the groups treated with Mo, Me and Gb, but not in the Amt-treated group. IL-1ß decreased only in rats treated with Me. The astrocytic expression of GFAP was decreased in the brainstem with all drugs, while it was increased in the NAc with Amt, Me and Mo