Pharmacological sensitivity of reflexive and nonreflexive outcomes as a correlate of the sensory and affective responses to visceral pain in mice

Resilience to Pain-Related Depression in σ1 Receptor Knockout Mice Is Associated with the Reversal of Pain-Induced Brain Changes in Affect-Related Genes

Mice lacking the σ1 receptor chaperone (σ1R-/-) are resilient to depressive-like behaviors secondary to neuropathic pain. Examining the resilience's brain mechanisms could help develop conceptually novel therapeutic strategies. We explored the diminished motivation for a natural reinforcer (white chocolate) in the partial sciatic nerve ligation (PSNL) model in wild-type (WT) and σ1R-/- mice. In the same mice, we performed a comprehensive reverse transcription quantitative PCR (qPCR) analysis across ten brain regions of seven genes implicated in pain regulation and associated affective disorders, such as anxiety and depression. PSNL induced anhedonic-like behavior in WT but not in σ1R-/- mice. In WT mice, PSNL up-regulated dopamine transporter (DAT) and its rate-limiting enzyme, tyrosine hydroxylase (Th), in the ventral tegmental area (VTA) and periaqueductal gray (PAG) as well as the serotonin transporters (SERT) and its rate-limiting enzyme tryptophan hydroxylase 2 (Tph2) in VTA. In addition, μ-opioid receptor (MOR) and σ1R were up-regulated in PAG, and MOR was also elevated in the somatosensory cortex (SS) but down-regulated in the striatum (STR). Finally, increased BDNF was found in the medial prefrontal cortex (mPFC) and hypothalamus (HPT). Sham surgery also produced PSNL-like expression changes in VTA, HPT, and STR. Genetic deletion of the σ1R in mice submitted to PSNL or sham surgery prevented changes in the expression of most of these genes. σ1R is critically involved in the supraspinal gene expression changes produced by PSNL and sham surgery. The changes in gene expression observed in WT mice may be related to pain-related depression, and the absence of these changes observed in σ1R-/- mice may be related to resilience.

Piezo2 channels expressed by colon-innervating TRPV1-lineage neurons mediate visceral mechanical hypersensitivity

Inflammatory and functional gastrointestinal disorders such as irritable bowel syndrome (IBS) and obstructive bowel disorder (OBD) underlie the most prevalent forms of visceral pain. Although visceral pain can be generally provoked by mechanical distension/stretch, the mechanisms that underlie visceral mechanosensitivity in colon-innervating visceral afferents remain elusive. Here, we show that virally mediated ablation of colon-innervating TRPV1-expressing nociceptors markedly reduces colorectal distention (CRD)-evoked visceromotor response (VMR) in mice. Selective ablation of the stretch-activated Piezo2 channels from TRPV1 lineage neurons substantially reduces mechanically evoked visceral afferent action potential firing and CRD-induced VMR under physiological conditions, as well as in mouse models of zymosan-induced IBS and partial colon obstruction (PCO). Collectively, our results demonstrate that mechanosensitive Piezo2 channels expressed by TRPV1-lineage nociceptors powerfully contribute to visceral mechanosensitivity and nociception under physiological conditions and visceral hypersensitivity under pathological conditions in mice, uncovering potential therapeutic targets for the treatment of visceral pain.

Transient Reflexive Pain Responses and Chronic Affective Nonreflexive Pain Responses Associated with Neuroinflammation Processes in Both Spinal and Supraspinal Structures in Spinal Cord-Injured Female Mice

Central neuropathic pain is not only characterized by reflexive pain responses, but also emotional or affective nonreflexive pain responses, especially in women. Some pieces of evidence suggest that the activation of the neuroimmune system may be contributing to the manifestation of mood disorders in patients with chronic pain conditions, but the mechanisms that contribute to the development and chronicity of CNP and its associated disorders remain poorly understood. This study aimed to determine whether neuroinflammatory factor over-expression in the spinal cord and supraspinal structures may be associated with reflexive and nonreflexive pain response development from acute SCI phase to 12 weeks post-injury in female mice. The results show that transient reflexive responses were observed during the SCI acute phase associated with transient cytokine overexpression in the spinal cord. In contrast, increased nonreflexive pain responses were observed in the chronic phase associated with cytokine overexpression in supraspinal structures, especially in mPFC. In addition, results revealed that besides cytokines, the mPFC showed an increased glial activation as well as CX3CL1/CX3CR1 upregulation in the neurons, suggesting the contribution of neuron-glia crosstalk in the development of nonreflexive pain responses in the chronic spinal cord injury phase.

Long-lasting reflexive and nonreflexive pain responses in two mouse models of fibromyalgia-like condition

Nociplastic pain arises from altered nociception despite no clear evidence of tissue or somatosensory system damage, and fibromyalgia syndrome can be highlighted as a prototype of this chronic pain subtype. Currently, there is a lack of effective treatments to alleviate both reflexive and nonreflexive pain responses associated with fibromyalgia condition, and suitable preclinical models are needed to assess new pharmacological strategies. In this context, although in recent years some remarkable animal models have been developed to mimic the main characteristics of human fibromyalgia, most of them show pain responses in the short term. Considering the chronicity of this condition, the present work aimed to develop two mouse models showing long-lasting reflexive and nonreflexive pain responses after several reserpine (RIM) or intramuscular acid saline solution (ASI) injections. To our knowledge, this is the first study showing that RIM6 and ASI mouse models show reflexive and nonreflexive responses up to 5-6 weeks, accompanied by either astro- or microgliosis in the spinal cord as pivotal physiopathology processes related to such condition development. In addition, acute treatment with pregabalin resulted in reflexive pain response alleviation in both the RIM6 and ASI models. Consequently, both may be considered suitable experimental models of fibromyalgia-like condition, especially RIM6.

Comprehensive Preclinical Assessment of Sensory, Functional, Motivational-Affective, and Neurochemical Outcomes in Neuropathic Pain: The Case of the Sigma-1 Receptor

Chronic pain remains a major health problem and is currently facing slow drug innovation. New drug treatments should address not only the sensory-discriminative but also functional and motivational-affective components of chronic pain. In a mouse model of neuropathic pain induced by partial sciatic nerve ligation (PSNL), we analyzed sensory and functional-like outcomes by hindpaw mechanical stimulation and automated gait analysis (CatWalk). We characterized over time a reward-seeking task based on diminished motivation for natural reinforcers (anhedonic-like behavior). To differentiate the appetitive ("wanting") and consummatory ("liking") aspects of motivational behavior, we quantified the latency and number of approaches to eat white chocolate, as well as the eating duration and amount consumed. We explored a putative chronic pain-induced dysregulation of monoamine function by measuring monoamine levels in the nucleus accumbens (NAc), a well-known brain reward area. Finally, we investigated the role of sigma-1 receptor (σ₁R) modulation, a nonopioid target, in these multiple dimensions by genetic deletion and pharmacological dose-response studies. After 6 weeks, PSNL increased the approach latency and reduced the consumption of white chocolate in 20-25% of the mice, while around 50-60% had one or the other parameter affected independently. After 10 weeks, sham-operated mice also displayed anhedonic-like behavior. PSNL was associated with reduced extracellular baseline dopamine and increased norepinephrine in the NAc and with a suppression of increased dopamine and serotonin efflux in response to the rewarding stimulus. Genetic and pharmacological blockade of σ₁R relieved these multiple alterations in nerve-injured mice. We comprehensively describe sensory, functional, and depression-like impairment of key components of motivated behavior associated with nerve injury. We provide a neurochemical substrate for the depressed mesocorticolimbic reward processing in chronic pain, with a potentially increased translational value. Our results also highlight σ₁R for the therapeutic intervention of neuropathic pain.

The search for translational pain outcomes to refine analgesic development: Where did we come from and where are we going?

Pain measures traditionally used in rodents record mere reflexes evoked by sensory stimuli; the results thus may not fully reflect the human pain phenotype. Alterations in physical and emotional functioning, pain-depressed behaviors and facial pain expressions were recently proposed as additional pain outcomes to provide a more accurate measure of clinical pain in rodents, and hence to potentially enhance analgesic drug development. We aimed to review how preclinical pain assessment has evolved since the development of the tail flick test in 1941, with a particular focus on a critical analysis of some nonstandard pain outcomes, and a consideration of how sex differences may affect the performance of these pain surrogates. We tracked original research articles in Medline for the following periods: 1973-1977, 1983-1987, 1993-1997, 2003-2007, and 2014-2018. We identified 606 research articles about alternative surrogate pain measures, 473 of which were published between 2014 and 2018. This indicates that preclinical pain assessment is moving toward the use of these measures, which may soon become standard procedures in preclinical pain laboratories.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).