Reserpine causes biphasic nociceptive sensitivity alteration in conjunction with brain biogenic amine tones in rats

Diosmetin attenuates fibromyalgia-like symptoms in a reserpine-induced model in mice

Fibromyalgia is a potentially disabling idiopathic disease characterized by widespread chronic pain associated with comorbidities such as fatigue, anxiety, and depression. Current therapeutic approaches present adverse effects that limit adherence to therapy. Diosmetin, an aglycone of the flavonoid glycoside diosmin found in citrus fruits and the leaves of Olea europaea L., has antinociceptive, anti-inflammatory, and antioxidant properties. Here, we investigated the effect of diosmetin on nociceptive behaviors and comorbidities in an experimental fibromyalgia model induced by reserpine in mice. To induce the experimental fibromyalgia model, a protocol of subcutaneous injections of reserpine (1 mg/kg) was used once a day for three consecutive days in adult male Swiss mice. Mice received oral diosmetin on the fourth day after the first reserpine injection. Nociceptive (mechanical allodynia, muscle strength, and thermal hyperalgesia) and comorbid (depressive-like and anxiety behavior) parameters were evaluated. Potential adverse effects associated with diosmetin plus reserpine (locomotor alteration, cataleptic behavior, and body weight and temperature changes) were also evaluated. Oral diosmetin (0.015-1.5 mg/kg) reduced the mechanical allodynia, thermal hyperalgesia, and loss of muscle strength induced by reserpine. Diosmetin (0.15 mg/kg) also attenuated depressive-like and anxiety behaviors without causing locomotor alteration, cataleptic behavior, and alteration in weight and body temperature of mice. Overall, diosmetin can be an effective and safe therapeutic alternative to treat fibromyalgia symptoms, such as pain, depression and anxiety.

Spinal dopaminergic D1 and D5 receptors contribute to reserpine-induced fibromyalgia-like pain in rats

Fibromyalgia is a complex pain syndrome without a precise etiology. Reduced monoamines levels in serum and cerebrospinal fluid in fibromyalgia patients has been reported and could lead to a dysfunction of descending pain modulatory system producing the painful syndrome. This study evaluated the role of D₁-like dopamine receptors in the reserpine-induced fibromyalgia-like pain model in female Wistar rats. Reserpine-treated animals were intrathecally injected with different dopamine receptors agonists and antagonists, and small interfering RNAs (siRNAs) against D₁ and D₅ receptor subtypes. Withdrawal and muscle pressure thresholds were assessed with von Frey filaments and the Randall-Selitto test, respectively. Expression of D₁-like receptors in lumbar spinal cord and dorsal root ganglion was determined using real time polymerase chain reaction (qPCR). Reserpine induced tactile allodynia and muscle hyperalgesia. Intrathecal dopamine and D₁-like receptor agonist SKF-38393 induced nociceptive hypersensitivity in naïve rats, whilst this effect was prevented by the D₁-like receptor antagonist SCH-23390. Moreover, SCH-23390 induced a sex-dependent antiallodynic effect in reserpine-treated rats. Furthermore, transient silencing of D₁ and D₅ receptors significantly reduced reserpine-induced hypersensitivity in female rats. Reserpine slightly increased mRNA D₅ receptor expression in dorsal spinal cord, but not in DRG. This work provides new insights about the involvement of the spinal dopaminergic D₁/D₅ receptors in reserpine-induced hypersensitivity in rats.

Antihyperalgesic and Antiallodynic Effects of Amarisolide A and Salvia amarissima Ortega in Experimental Fibromyalgia-Type Pain

Salvia amarissima Ortega is an endemic species of Mexico used in folk medicine to alleviate pain and as a nervous tranquilizer. The S. amarissima extract and one of its abundant metabolites, identified and isolated through chromatographic techniques, were investigated to obtain scientific evidence of its potential effects to relieve nociplastic pain such as fibromyalgia. Then, the extract and amarisolide A (3-300 mg/kg, i.p.) were pharmacologically evaluated in reserpine-induced fibromyalgia-type chronic pain and in depressive-like behavior (as a common comorbidity) by using the forced swimming test in rats. The 5-HT_1A serotonin receptor (selective antagonist WAY100635, 1 mg/kg, i.p.) was explored after the prediction of a chemical interaction using in silico analysis to look for a possible mechanism of action of amarisolide A. Both the extract and amarisolide A produced significant and dose-dependent antihyperalgesic and antiallodynic effects in rats, as well as significant antidepressive behavior without sedative effects when the antinociceptive dosages were used. The 5-HT_1A serotonin receptor participation was predicted by the in silico descriptors and was corroborated in the presence of WAY100635. In conclusion, S. amarissima possesses antihyperalgesic, antiallodynic, and anti-depressive activities, partially due to the presence of amarisolide A, which involves the 5-HT_1A serotonin receptor. This pharmacological evidence suggests that S. amarissima and amarisolide A are both potential alternatives to relieve pain-like fibromyalgia.

Pharmacological Interaction of Quercetin Derivatives of Tilia americana and Clinical Drugs in Experimental Fibromyalgia

Fibromyalgia (FM) is a pain syndrome characterized by chronic widespread pain and CNS comorbidities. Tilia americana var. mexicana is a medicinal species used to treat anxiety, insomnia, and acute or chronic pain. However, its spectrum of analgesic efficacy for dysfunctional pain is unknown. To investigate a possible therapeutic alternative for FM-type pain, an aqueous Tilia extract (TE) and its flavonoid fraction (FF) containing rutin and isoquercitrin were evaluated alone and/or combined with clinical drugs (tramadol—TRA and pramipexol—PRA) using the reserpine-induced FM model in rats. Chromatographic analysis allowed the characterization of flavonoids, while a histological analysis confirmed their presence in the brain. TE (10–100 mg/kg, i.p.) and FF (10–300 mg/kg, i.p.) produced significant and dose-dependent antihyperalgesic and antiallodynic effects equivalent to TRA (3–10 mg/kg, i.p.) or PRA (0.01–1 mg/kg, s.c.). Nevertheless, the combination of FF + TRA or FF + PRA resulted in an antagonistic interaction by possible competitive action on the serotonin transporter or µ-opioid and D₂ receptors, respectively, according to the in silico analysis. Flavonoids were identified in cerebral regions because of their self-epifluorescence. In conclusion, Tilia possesses potential properties to relieve FM-type pain. However, the consumption of this plant or flavonoids such as quercetin derivatives in combination with analgesic drugs might reduce their individual benefits.

Physical exercise prevents behavioral alterations in a reserpine-treated zebrafish: A putative depression model

Major depressive disorder (MDD) has increasingly reached the world population with an expressive increase in recent years due to the COVID-19 pandemic. Here we used adult zebrafish (Danio rerio) as a model to verify the effects of reserpine on behavior and neurotransmitter levels. We observed an increase in the immobile time and time spent in the bottom zone of the tank in reserpine-exposed animals. The results demonstrated a decrease in distance traveled and velocity. Reserpine exposure did not induce changes in memory and social interaction compared to the control group. We also evaluated the influence of exposure to fluoxetine, a well-known antidepressant, on the behavior of reserpine-exposed animals. We observed a reversal of behavioral alterations caused by reserpine. To verify whether behavioral alterations in the putative depression model induced by reserpine could be prevented, the animals were subjected to physical exercise for 6 weeks. The results showed a protective effect of the physical exercise against the behavioral changes caused by reserpine in zebrafish. In addition, we observed a reduction in dopamine and serotonin levels and an increase in the 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the brain. Physical exercise was able to prevent the changes in dopamine and serotonin levels, reinforcing that the preventive effect promoted by physical exercise is related to the modulation of neurotransmitter levels. Our findings showed that reserpine was effective in the induction of a putative depression model in zebrafish and that physical exercise may be an alternative to prevent the effects induced by reserpine.

Involvement of Parvalbumin-Positive Neurons in the Development of Hyperalgesia in a Mouse Model of Fibromyalgia

Fibromyalgia (FM) presents as chronic systemic pain, which might be ascribed to central sensitization, in which pain information processing is amplified in the central nervous system. Since patients with FM display elevated gamma oscillations in the pain matrix and parvalbumin (PV)-positive neurons play a critical role in induction of gamma oscillations, we hypothesized that changes in PV-positive neurons are involved in hyperalgesia in fibromyalgia. In the present study, to investigate a role of PV-positive neurons in neuropathic pain, mice received reserpine administration for 3 consecutive days as an animal model of FM (RES group), while control mice received vehicle injections in the same way (VEH group). The mice were subjected to hot-plate and forced swim tests, and immuno-stained PV-positive neurons were counted in the pain matrix. We investigated relationships between PV-positive neuron density in the pain matrix and pain avoidance behaviors. The results indicated that the mice in the RES group showed transient bodyweight loss and longer immobility time in the forced swim test than the mice in the VEH group. In the hot-plate test, the RES group showed shorter response latencies and a larger number of jumps in response to nociceptive thermal stimulus than the VEH group. Histological examination indicated an increase in the density of PV-positive neurons in the primary somatosensory cortex (S1) in the RES group. Furthermore, response latencies to the hot-plate were significantly and negatively correlated with the density of PV-positive neurons in the S1. These results suggest a critical role for PV-positive neurons in the S1 to develop hyperalgesia in FM.

Nociceptive chemical hypersensitivity in the spinal cord of a rat reserpine-induced fibromyalgia model

The pathological mechanisms of fibromyalgia (FM) are largely unknown. Recently, a rat reserpine-induced pain model showing exaggerated pain-related behaviors to mechanical and thermal stimuli has been used in FM research. However, the model has not been fully characterized. Here, we investigated nociceptive hypersensitivity to chemical stimuli and its spinal mechanisms to further characterize the model. The rat model was induced by administering reserpine to the nervous system. Nociceptive behaviors to chemical stimuli were quantified using the formalin pain test, and neuronal activation of the stimuli was examined using spinal c-Fos immunohistochemistry and electrophysiological recordings of superficial dorsal horn (SDH) neurons. The duration of pain-related behaviors was prolonged in both phases I (0-5min) and II (10-60min) and the interphase; and the number of c-Fos-immunoreactive nuclei increased in laminae I-II, III-IV, and V-VI at the spinal segments L3-L5 on the side ipsilateral to the formalin injection, and these factors were significantly and positively correlated. The action potentials of SDH neurons induced by formalin injection were markedly increased in rats treated with reserpine. These results demonstrate that pain-related behaviors are facilitated by noxious chemical stimuli in a rat reserpine-induced FM model, and that the behavioral hypersensitivity is associated with hyperactivation of SDH neurons.

Therapeutic Approaches to Nociplastic Pain Based on Findings in the Reserpine-Induced Fibromyalgia-Like Animal Model

Nociplastic pain, the third category of chronic pain, has emerged as a serious medical issue. Due to its significant negative influences on patients and society, high prevalence, and lack of sufficiently effective treatments, more efficacious therapies are required. This review highlights the potential therapeutic approaches identified in studies that used reserpine-induced myalgia (RIM) animal model that exhibits nociplastic pain-associated phenotypes. These studies have revealed that biological processes including the chronic reduction of monoamines, increase of oxidative/nitrosative stresses and inflammatory mediators, upregulation of pronociceptive neurotransmitters and their receptors, increase of trophic factors, enhancement of the apoptotic pathway, sensory nerve sensitization, and activation of immune cells in central and/or peripheral regions, underly the nociplastic pain-associated phenotypes in RIM animal model. Potential therapeutic approaches to nociplastic pain, i.e., 1) functional modification of specific molecules which expression is distinctly altered following monoamine reduction, 2) targeting the molecules which are responsible for other major categories of chronic pain (i.e., chronic inflammatory pain and neuropathic pain), 3) supplementation of nutrition to correct the disrupted nutritional balance, 4) improvement of physical constitution by natural substances, and 5) nonpharmacological interventions, have been identified. Significance Statement Studies in RIM animal model have revealed the pathologies that occur after the chronic reduction of monoamines and identified potential therapeutic approaches to nociplastic pain. Translation of their analgesic efficacy from RIM animal model to patients remains an issue to be addressed. Successful translation would lead to better therapies for nociplastic pain.

Neuronal Sensitization and Synaptic Facilitation in the Superficial Dorsal Horn of a Rat Reserpine-induced Pain Model

Chronic widespread pain is one of the important issues to be solved in medical practice. Impaired spinal descending pain inhibitory system due to decreased monoamine neurotransmitters is assumed to cause nociceptive hypersensitivities in chronic painful conditions like that described in patients with fibromyalgia (FM). However, response behaviors and synaptic transmission of the spinal dorsal horn neurons in response to reserpine remain to be clarified. Here we examined the activities of superficial dorsal horn (SDH) neurons, as well as excitatory and inhibitory postsynaptic inputs to SDH neurons, using a putative rat model of FM that was established by injecting reserpine. Extracellular recordings in vivo revealed that SDH neurons were sensitized to mechanical stimulation applied to the neurons' receptive fields, and the mechanically sensitized neurons were spontaneously more active. The sensitizing effect was evident 1 day and 3 days after the reserpine treatment, but subsided 5 days after the treatment or later. Using patch-clamp recordings in vivo, spontaneous excitatory postsynaptic currents (sEPSCs) to SDH neurons were found to increase in the pain model, while spontaneous inhibitory postsynaptic currents (sIPSCs) to SDH neurons decreased. These results demonstrate that the SDH neurons were strongly sensitized in response to the reserpine treatment, and that increased excitatory and decreased inhibitory postsynaptic inputs could be responsible for the spinal nociceptive hypersensitivity in the putative FM model.

Animal models of fibromyalgia: What is the best choice?

Fibromyalgia (FM) is a complex syndrome, with an indefinite aetiology and intricate pathophysiology that affects 2 - 3% of the world population. From the beginning of the 2000s, experimental animal models have been developed to mimic clinical FM and help obtain a better understanding of the relevant neurobiology. These animal models have enabled a broad study of FM symptoms and mechanisms, as well as new treatment strategies. Current experimental FM models include the reserpine-induced systemic depletion of biogenic amines, muscle application of acid saline, and stress-based (cold, sound, or swim) approaches, among other emerging models. FM models should: (i) mimic the cardinal symptoms and complaints reported by FM patients (e.g., spontaneous nociception, muscle pain, hypersensitivity); (ii) mimic primary comorbidities that can aggravate quality of life and lead to worse outcomes (e.g., fatigue, sleep disturbance, depression, anxiety); (iii) mimic the prevalent pathological mechanisms (e.g., peripheral and central sensitization, inflammation/neuroinflammation, change in the levels of the excitatory and inhibitory neurotransmitters); and (iv) demonstrate a pharmacological profile similar to the clinical treatment of FM. However, it is difficult for any one of these models to include the entire spectrum of clinical FM features once even FM patients are highly heterogeneous. In the past six years (2015 - 2020), a wide range of experimental FM studies has amounted to the literature reinforcing the need for an updated review. Here we have described, in detail, several approaches used to experimentally study FM, with a focus on recent studies in the field and in previously less discussed mechanisms. We highlight each model's challenges, limitations, and future directions, intending to help preclinical researchers establish the correct experimental FM model to use depending on their goals.

Responses of cutaneous C-fiber afferents and spinal microglia after hindlimb cast immobilization in rats

Previous studies have shown that persistent limb immobilization using a cast increases nociceptive behavior to somatic stimuli in rats. However, the peripheral neural mechanisms of nociception remain unclear. Using single-fiber electrophysiological recordings in vitro, we examined the general characteristics of cutaneous C-fiber afferents in the saphenous nerve and their responsiveness to mechanical and heat stimuli in a rat model of immobilization-induced pain by subjecting the rats to hindlimb cast immobilization for 4 weeks. The mechanical response of C-fibers appeared to increase in the model; however, statistical analysis revealed that neither the response threshold nor the response magnitude was altered. The general characteristics and heat responses of the C-fibers were not altered. The number of microglia and cell diameters significantly increased in the superficial dorsal horn of the lumbar spinal cord. Thus, activated microglia-mediated spinal mechanisms are associated with the induction of nociceptive hypersensitivity in rats after persistent cast immobilization.

Serotonergic Neurotransmission System Modulator, Vortioxetine, and Dopaminergic D2/D3 Receptor Agonist, Ropinirole, Attenuate Fibromyalgia-Like Symptoms in Mice

Fibromyalgia is a disease characterized by lowered pain threshold, mood disorders, and decreased muscular strength. It results from a complex dysfunction of the nervous system and due to unknown etiology, its diagnosis, treatment, and prevention are a serious challenge for contemporary medicine. Impaired serotonergic and dopaminergic neurotransmission are regarded as key factors contributing to fibromyalgia. The present research assessed the effect of serotonergic and dopaminergic system modulators (vortioxetine and ropinirole, respectively) on the pain threshold, depressive-like behavior, anxiety, and motor functions of mice with fibromyalgia-like symptoms induced by subcutaneous reserpine (0.25 mg/kg). By depleting serotonin and dopamine in the mouse brain, reserpine induced symptoms of human fibromyalgia. Intraperitoneal administration of vortioxetine and ropinirole at the dose of 10 mg/kg alleviated tactile allodynia. At 5 and 10 mg/kg ropinirole showed antidepressant-like properties, while vortioxetine had anxiolytic-like properties. None of these drugs influenced muscle strength but reserpine reduced locomotor activity of mice. Concluding, in the mouse model of fibromyalgia vortioxetine and ropinirole markedly reduced pain. These drugs affected emotional processes of mice in a distinct manner. Hence, these two repurposed drugs should be considered as potential drug candidates for fibromyalgia. The selection of a specific drug should depend on patient’s key symptoms.

A KCa3.1 Channel Opener, ASP0819, Modulates Nociceptive Signal Processing from Peripheral Nerves in Fibromyalgia-Like Pain in Rats

Purpose

Although abnormal peripheral and central pain processing has been observed in fibromyalgia (FM) patients, the biomechanics and pathophysiology, surrounding the peripheral mechanism are not well understood. An intermediate conductance channel, K_Ca3.1, is expressed in peripheral sensory nerve fibers where it maintains the resting membrane potential and controls nerve firing, making it a plausible target for peripheral therapeutic interventions. ASP0819, a K_Ca3.1 channel opener, is an orally available molecular entity and is used in this investigation to elucidate the role of K_Ca3.1 in signal processing of pain in FM.

Methods

Human or rat K_Ca3.1 channel-expressing cells were used for evaluating the main action of the compound. Effects of the compound on withdrawal behavior by mechanical stimulation were examined in reserpine-induced myalgia (RIM) and vagotomy-induced myalgia (VIM) models of rats. In addition, in vivo electrophysiological analysis was performed to examine the peripheral mechanisms of action of the compound. Other pain models were also examined.

Results

ASP0819 increased the negative membrane potential in a concentration-dependent manner. Oral administration of ASP0819 significantly recovered the decrease in muscle pressure threshold in rat FM models of RIM and VIM. The in vivo electrophysiological experiments showed that Aδ- and C-fibers innervating the leg muscles in the RIM model demonstrated increased spontaneous and mechanically evoked firing compared with normal rats. Intravenous infusion of ASP0819 significantly reduced both the spontaneous activity and mechanically evoked responses in Aδ-fibers in the rat RIM model. ASP0819 significantly reduced the number of abdominal contractions as an indicator of abdominal pain behaviors in the rat visceral extension model and withdrawal responses in the osteoarthritis model, respectively.

Conclusion

These findings suggest that ASP0819 may be a promising analgesic agent with the ability to modulate peripheral pain signal transmission. Its use in the treatment of several pain conditions should be explored, chief amongst these being FM pain.

Free gait in a shallow pool accelerates recovery after exercise in model mice with fibromyalgia

This study aimed to determine the effect of pool gait exercise using fibromyalgia-induced model mice. The sensory threshold, locomotive behavior, electrocardiogram, and onset time after the gait test in shallow water using male C57BL/6J mice (weight, 30–35 g; n=21) were investigated. To induce fibromyalgia in model mice, reserpine was injected intraperitoneally into wild-type mice once a day for 3 days. Subsequently, the fibromyalgia-induced model mice were randomly classified into two groups as follows: the control group (n=11) and the pool gait group (n=10). The mice in the pool gait group walked in the same cage containing shallow warm water 5 times per week. Both groups underwent sensory thresholds and video recordings to determine locomotive behaviors weekly. Further, both heart rate and video recordings for observation of a recovery after the gait test in shallow water were undertaken (control group; n=5, pool gait group; n=5). The pool gait did not affect sensory thresholds and locomotive behavior; however, in the pool gait group, both the recovery after the test, such as onset time and gait distance, were considerably better than those of the control group. Furthermore, changes in heart rate and heart rate irregularity after the test were more apparent in the control group than in the pool gait group. The free gait in a shallow pool accelerated recovery after exercise, unlike the sensory threshold.

Excessive exercise induces cardiac arrhythmia in a young fibromyalgia mouse model

Background

Fibromyalgia patients experience cardiovascular complications in addition to musculoskeletal pain. This study aimed to investigate the cardiac effects of a prolonged shallow water gait in a fibromyalgia-induced young mouse model.

Methods

To produce a fibromyalgia mouse model, wild-type mice were administered an intraperitoneal injection of reserpine once a day for three days, and two primary experiments were performed. First, three types of gait tests were performed before and after the reserpine injections as follows: (i) 5 minutes of free gait outside the water, (ii) 1 minute of free gait in shallow warm water, and (iii) 5 minutes of free gait in shallow warm water. Second, electrocardiogram recordings were taken before and after the three gait tests. The average heart rate and heart rate irregularity scores were analyzed.

Results

Exercise-induced cardiac arrhythmia was observed at 1-minute gait in shallow water during the acute stage of induced FM in young mice. Further, both cardiac arrhythmia and a decrease in HR have occurred at 5-minute gait in shallow water at the same mice. However, this phenomenon was not observed in the wild-type mice under any test conditions.

Conclusion

Although a short-term free gait in shallow warm water may be advantageous for increasing the motor activity of FM-model mice, we should be aware of the risk of prolonged and excessive exercise-induced cardiac arrhythmia. For gait exercises in shallow water as a treatment in FM patients. We suggest a gradual increase in exercise duration may be warranted.

Effects of NBI-98782, a selective vesicular monoamine transporter 2 (VMAT2) inhibitor, on neurotransmitter efflux and phencyclidine-induced locomotor activity: Relevance to tardive dyskinesia and antipsychotic action

Valbenazine, a vesicular monoamine transporter 2 (VMAT2, SLC18A2) inhibitor, is a newly approved treatment for tardive dyskinesia. VMAT2 is present in the membrane of secretory vesicles and transports dopamine (DA), norepinephrine (NE), serotonin (5-HT), histamine, glutamate (Glu), and GABA into vesicles for presynaptic release. We utilized microdialysis in awake, freely moving mice to determine the effect of NBI-98782, the active metabolite of valbenazine, alone, or in combination with several antipsychotic drugs (APDs), to influence neurotransmitter efflux in the medial prefrontal cortex (mPFC), dorsal striatum (dSTR), hippocampus and nucleus accumbens (NAC); we also compared it with tetrabenazine, the prototypical VMAT2 inhibitor. Acute NBI-98782 and tetrabenazine decreased mPFC, dSTR, hippocampus, and NAC DA, 5-HT, and NE efflux, while increasing that of DOPAC, HVA, and 5-HIAA. Sub-chronic NBI-98782 (7 days) decreased baseline DA and 5-HT efflux in both mPFC and dSTR. NBI-98782 elicited similar effects on neurotransmitter efflux in sub-chronic NBI-98782-treated mice but also enhanced ACh and GABA; the decrease in DA efflux in mPFC and dSTR was not significant in the sc-treated animals. NBI-98782 suppressed clozapine-, olanzapine- and risperidone-induced DA efflux in both mPFC and dSTR, and ACh efflux in mPFC. NBI-98782 suppressed the haloperidol-induced DA efflux in dSTR, with minimal effect on GABA efflux. NBI-98782 attenuated PCP-induced DA, 5-HT, NE and Glu efflux, and AMPH-induced DA and NE efflux, in both mPFC and dSTR, as well as PCP- and AMPH-induced hyperlocomotion, suggesting possible beneficial antipsychotic effects.

Involvement of TRPV1 and the efficacy of α-spinasterol on experimental fibromyalgia symptoms in mice

Fibromyalgia is characterised mainly by symptoms of chronic widespread pain and comorbidities like depression. Although these symptoms cause a notable impact on the patient's quality of life, the underlying aetiology and pathophysiology of this disease remain incompletely elucidated. The transient receptor potential vanilloid type 1 (TRPV1) is a polymodal receptor that is involved in the development of nociceptive and depressive behaviours, while α-spinasterol, a multitarget TRPV1 antagonist and cyclooxygenase inhibitor, presents antinociceptive and antidepressant effects. The present study investigated the involvement of the TRPV1 channel and the possible effects of α-spinasterol on nociceptive and depressive-like behaviours in an experimental fibromyalgia model. The fibromyalgia model was induced with a subcutaneous (s.c.) injection of reserpine (1 mg/kg) once daily for 3 consecutive days in male Swiss mice. Reserpine administration depleted monoamines and caused mechanical allodynia. This dysfunction was inhibited by SB-366791 (1 mg/kg, oral route [p.o.]), a selective TRPV1 antagonist, with a maximum inhibition (I_max) of 73.4 ± 15.5%, or by the single or 3-day-repeated administration of α-spinasterol (0.3 mg/kg, p.o.), with an I_max of 72.8 ± 17.8% and 78.9 ± 32.9%, respectively. SB-366791 also inhibited the increase of the reserpine-induced immobility time, with an I_max of 100%, while α-spinasterol inhibited this parameter with an I_max of 98.2 ± 21.5% and 100%, by single or repeated administration, respectively. The reserpine-induced mechanical allodynia and the thermal hyperalgesia were abolished by TRPV1-positive fibers desensitization induced by previous resiniferatoxin (RTX) administration. In summary, the TRPV1 channel is involved in the development and maintenance of nociception and depressive-like behaviours in a fibromyalgia model, while the α-spinasterol has therapeutic potential to treat the pain and depression symptoms in fibromyalgia patients.

Sex differences and estradiol involvement in hyperalgesia and allodynia in an experimental model of fibromyalgia

Fibromyalgia (FM) is a musculoskeletal chronic pain syndrome. Its prevalence in women is higher than in men possibly by hormonal factors given that symptoms are aggravated during sex hormone-related events, such as the premenstrual period, pregnancy, postpartum or menopause. The aim of the present study was to investigate whether hyperalgesia and allodynia, in reserpine-induced experimental FM, depend on sex, estrous cycle, ovariectomy and replacement with 17β-estradiol. To fulfill this objective, we compared males, intact females with known estrous cycle phases and ovariectomized (OVX) rats treated with 17β-estradiol. Data demonstrated that reserpine administration disrupted the normal estrous cycle and produced that all females entered metestrus/diestrus. In addition, this treatment leads to muscle hyperalgesia and tactile allodynia in a similar manner in male and intact female rats. However, the absence of ovarian hormones (in OVX rats) increased muscle nociception. 17β-estradiol (2.5-10μg/rat) produced antihyperalgesic and antiallodynic effects 24h, but not 8h, after its administration, suggesting a genomic mechanism. The present results support the validity of the reserpine-induced FM model for searching alternatives of treatment, particularly during endocrine phases when pain is exacerbated such as menopause, and that 17β-estradiol replacement might be useful.

Short- and long-term dopamine depletion causes enhanced beta oscillations in the cortico-basal ganglia loop of parkinsonian rats

Abnormally enhanced beta oscillations have been found in deep brain recordings from human Parkinson's disease (PD) patients and in animal models of PD. Recent correlative evidence suggests that beta oscillations are related to disease-specific symptoms such as akinesia and rigidity. However, this hypothesis has also been repeatedly questioned by studies showing no changes in beta power in animal models using an acute pharmacologic dopamine blockade. To further investigate the temporal dynamics of exaggerated beta synchrony in PD, we investigated the reserpine model, which is characterized by an acute and stable disruption of dopamine transmission, and compared it to the chronic progressive 6-hydroxydopamine (6-OHDA) model. Using simultaneous electrophysiological recordings in urethane anesthetized rats from the primary motor cortex, the subthalamic nucleus and the reticulate part of the substantia, we found evidence for enhanced beta oscillations in the basal ganglia of both animal models during the activated network state. In contrast to 6-OHDA, reserpine treated animals showed no involvement of primary motor cortex. Notably, beta coherence levels between primary motor cortex and basal ganglia nuclei were elevated in both models. Although both models exhibited elevated beta power and coherence levels, they differed substantially in respect to their mean peak frequency: while the 6-OHDA peak was located in the low beta range (17Hz), the reserpine peak was centered at higher beta frequencies (27Hz). Our results further support the hypothesis of an important pathophysiological relation between enhanced beta activity and akinesia in parkinsonism.

Peripheral and spinal mechanisms of nociception in a rat reserpine-induced pain model

Chronic widespread pain is a serious medical problem, yet the mechanisms of nociception and pain are poorly understood. Using a reserpine-induced pain model originally reported as a putative animal model for fibromyalgia, this study was undertaken to examine the following: (1) expression of several ion channels responsible for pain, mechanotransduction, and generation/propagation of action potentials in the dorsal root ganglion (DRG), (2) activities of peripheral nociceptive afferents, and (3) alterations in spinal microglial cells. A significant increase in mRNA expression of the acid-sensing ion channel (ASIC)-3 was detected in the DRG, and the behavioral mechanical hyperalgesia was significantly reversed by subcutaneous injection of APETx2, a selective blocker of ASIC3. Single-fiber recordings in vitro revealed facilitated mechanical responses of mechanoresponsive C-fibers both in the skin and muscle although the proportion of mechanoresponsive C-nociceptors was paradoxically decreased. In the spinal dorsal horn, microglial cells labeled with Iba1 immunoreactivity was activated, especially in laminae I-II where the nociceptive input is mainly processed compared with the other laminae. The activated microglia and behavioral hyperalgesia were significantly tranquilized by intraperitoneal injection of minocycline. These results suggest that the increase in ASIC3 in the DRG facilitated mechanical response of the remaining C-nociceptors and that activated spinal microglia may direct to intensify pain in this model. Pain may be further amplified by reserpine-induced dysfunction of the descending pain inhibitory system and by the decrease in peripheral drive to this system resulting from a reduced proportion of mechanoresponsive C-nociceptors.

Depressive-like symptoms in a reserpine-induced model of fibromyalgia in rats

Since the pathogenesis of fibromyalgia is unknown, treatment options are limited, ineffective and in fact based on symptom relief. A recently proposed rat model of fibromyalgia is based on central depletion of monamines caused by reserpine administration. This model showed widespread musculoskeletal pain and depressive-like symptoms, but the methodology used to measure such symptoms has been criticized. Evidence relates the high prevalence of pain and depression in fibromyalgia to common pathogenic pathways, most probably focused on the monoaminergic system. The present study aims at a validation of the reserpine model of fibromyalgia. For this purpose, rats undergoing this model have been tested for depressive-like symptoms with a Novelty-Suppressed Feeding Test adaptation. Animals administered with reserpine and subjected to forced food deprivation performed a smaller number of incursions to the center of the open field, evidenced by a decrease in the per-minute rate of the rats' approaching, smelling or touching the food. They also took more time to eat from the central food than control rats. These NSFT findings suggest the presence of depressive-like disorders in this animal model of fibromyalgia.

Molecular, Neurochemical, and Behavioral Hallmarks of Reserpine as a Model for Parkinson's Disease: New Perspectives to a Long-Standing Model

The administration of reserpine to rodents was one of the first models used to investigate the pathophysiology and screening for potential treatments of Parkinson's disease (PD). The reserpine model was critical to the understanding of the role of monoamine system in the regulation of motor and affective disorders, as well as the efficacy of current PD treatments, such as L-DOPA and dopamine agonists. Nevertheless, with the introduction of toxin-induced and genetic models of PD, reserpine became underused. The main rationale to this drawback was the supposed absence of reserpine construct validity with PD. Here, we highlight classical and recent experimental findings that support the face, pharmacological, and construct validity of reserpine PD model and reason against the current rationale for its underuse. We also aim to shed a new perspective upon the model by discussing the main challenges and potentials for the reserpine model of PD.

Potential benefits of berberine in the management of perimenopausal syndrome

Cardiovascular diseases are one of the leading causes of morbidity and mortality in women after menopause and 56% of all causes of death in Western European countries. Nowadays, with increasing life span, women spend approximately one-third of their life-time in postmenopausal state; therefore, the development of new strategies to improve the prevention and treatment of menopause-associated pathologies is important topic in clinical practice. The studies to assess the safety of hormone replacement therapy in women with estrogen deficiency have not been conclusive due to the relative contraindications; therefore, hormone replacement therapy is prescribed only in selected cases and for a limited time. For this reason, today women are encouraged to use naturally available compounds to prevent or to attenuate menopausal symptoms and correlated pathologies, with fewer side effects. Among these compounds, berberine, an isoquinoline alkaloid derived from plants of the generis Berberis, has been recognized as being capable of decreasing oxidative stress, LDL, triglycerides, and insulin resistance and of improving the mood. This review describes the cellular and clinical effects associated with the use of berberine, which suggest that this molecule could be an effective natural supplement to ensure a smooth peri- and postmenopausal transition.

Correlation between ventral striatal catecholamine content and nociceptive thresholds in neuropathic mice

Neuropathic pain is characterized by persistent, intractable pain following damage or dysfunction of the nervous system. Analgesics that include central, rather than purely peripheral, targets are more effective when treating neuropathic pain, highlighting the spinal and/or supraspinal mechanisms that contribute to this aberrant pain condition. The striatum represents one of the brain regions that have been implicated in pain processing. Release of dopamine in the ventral striatum is normally associated with analgesia. Clinical and human imaging studies suggest that dopamine is disrupted in neuropathic pain patients, although the conclusions drawn from these studies are limited by their noninvasive imaging or pharmacologic approaches. In this study, we used a C57Bl/6 mouse model of neuropathic pain to describe the changes in neurotransmitter content in the striatum and their relationship to evoked pain thresholds. Striatal dopamine content negatively correlated with mechanical thresholds in sham animals. Neuropathic pain animals had reduced dopamine content that was not correlated with mechanical thresholds. In contrast, norepinephrine content was significantly increased and correlated with mechanical thresholds in neuropathic, but not sham, animals. These results describe changes in striatal signaling in neuropathic pain animals and contribute to the literature defining the role of dopamine and norepinephrine in mediating sensory thresholds in healthy and neuropathic pain states.

PERSPECTIVE

Results show significant loss of ventral striatal dopamine in neuropathic pain conditions, and the relationship of ventral striatal catecholamines to pain thresholds is changed in neuropathic pain. These results complement human imaging studies and provide evidence that chronic pain alters the function of reward systems.

Possible involvement of oxido-nitrosative stress induced neuro-inflammatory cascade and monoaminergic pathway: underpinning the correlation between nociceptive and depressive behaviour in a rodent model

BACKGROUND

Pain and depression are frequent co-morbid disorders. The prevalence rate of depression is several times higher in patients with chronic pain than in the general population but the mechanism underlying this association is unknown. A combination of interactions between neurotransmitters, neuropeptides, oxidative and nitrosative stress and cytokines are thought to take part in pathogenesis of pain as well as depression. Thus, the aim of the present study was two-fold, first to investigate the interplay between nociception and associated depression and second to investigate the protective potential of berberine against the reserpine-induced nociceptive and depressive behaviour and further to explore the role of oxidative-nitrosative stress mediated inflammatory cascade and apoptotic signalling pathway in this dyad.

METHODS AND RESULTS

Nociception and associated depression were induced by administration of reserpine (1mg/kg subcutaneous daily) for three consecutive days. This behavioural deficit was integrated with decrease in the biogenic amine (dopamine, norepinephrine and serotonin) levels along with increased substance P concentration, oxidative-nitrosative stress, inflammatory cytokines, NF-κβ and caspase-3 levels in different brain regions (cortex and hippocampus) of the reserpinised rats.

LIMITATION

More studies are still warranted in similar rodent models of pain and depression, so, that the present findings can be further substantiated to establish the clinical effectiveness of berberine in a subset of patients suffering from pain as well as depression.

CONCLUSION

The findings from the current study suggested that reserpine-induced neurochemical alterations and dysregulation of oxidative-nitrosative stress induced inflammatory cascade underlies the co-morbidity of nociceptive behaviour and associated depression in rats.

Revealing interaction between sulfobutylether-β-cyclodextrin and reserpine by chemiluminescence and site-directed molecular docking

The host-guest interaction between sulfobutylether-β-cyclodextrin (SBE-β-CD) and reserpine (RSP) is described using flow injection-chemiluminescence (FI-CL) and site-directed molecular docking methods. It was found that RSP could inhibit the CL intensity produced by a luminol/SBE-β-CD system. The decrease in CL intensity was logarithmic over an RSP concentration range of 0.03 to 700.0 nM, giving a regression equation of ∆I = 107.1lgCRES  + 186.1 with a detection limit of 10 pM (3σ). The CL assay was successfully applied in the determination of RSP in injection, saliva and urine samples with recoveries in the range 93.5-106.1%. Using the proposed CL model, the binding constant (KCD-R ) and the stoichiometric ratio of SBE-β-CD/RSP were calculated to be 7.4 × 10(6)  M(-1) and 1 : 1, respectively. Using molecular docking, it was confirmed that luminol binds to the small cavity of SBE-β-CD with a nonpolar interaction, while RSP targeted the larger cavity of SBE-β-CD and formed a 1 : 1 complex with hydrogen bonds. The proposed new CL method has the potential to become a powerful tool for revealing the host-guest interaction between CDs and drugs, as well as monitoring drugs with high sensitivity.

Different pathophysiology underlying animal models of fibromyalgia and neuropathic pain: comparison of reserpine-induced myalgia and chronic constriction injury rats

The reserpine-induced myalgia (RIM) rat manifests fibromyalgia-like chronic pain symptoms. The present study explored the pathophysiology underlying the pain symptoms in the RIM rat and the chronic constriction injury (CCI) rat, an animal model of neuropathic pain as a reference. Nerve tissue samples were collected from the nociception-tested animals for pathological examinations. Additionally, the therapeutic efficacy of a sodium channel blocker mexiletine was assessed in both rats. A slight vacuolization in the substantia nigra (SN) occurred in some of the RIM rats without any other histopathological changes in the brain or peripheral neurons. All the RIM rats, with or without vacuolization, showed hypersensitivity to tactile, muscle pressure, and cold stimuli. In the CCI rat, neurodegenerative changes were apparent in the sciatic nerve and the spinal cord only. CCI rats displayed muscle hyperalgesia in addition to tactile and cold allodynia. Pharmacotherapy with mexiletine did not attenuate the pain in the RIM rat, although it was effective in the CCI rat. Taken together, it is not likely that pain symptoms in RIM rats are caused by degenerative changes at the level of primary afferents and spinal cord, as is the case for CCI rats. The significance of the vacuolization in the SN is less clear at present because of the minor extent of the change and the lack of correlation with nociceptive sensitivity. The pain symptoms in RIM rats could be associated with dysfunction of biogenic amines-mediated CNS pain control even without apparent pathologies in the nervous system.