Mechanistic pathways of fibromyalgia induced by intermittent cold stress in mice is sex-dependently

Fibromyalgia results from a complex interplay of biochemical and neurobiological elements mediated sensitization of nociceptive pathways. Despite the symptoms of fibromyalgia negatively affect the quality of life of patients, the pathophysiology of this disease remains inconclusive, which difficult the development of an appropriate treatment. The present study investigated the involvement of the serotonergic receptors, the N-methyl-D-aspartate (NMDA)/ nitric oxide (NO)/ cyclic guanosine monophosphate (cGMP) pathway and the oxidative stress in an animal model of fibromyalgia induced by intermittent cold stress (ICS), considering the specificities of male and female Swiss mice. The ICS exposure increased mechanical and thermal sensitivities, and decreased muscle strength in mice of both sexes. Female mice exhibited a longer-lasting mechanical sensitivity than male mice exposed to ICS along with an enhancement of the Na⁺, K⁺-ATPase activity in the spinal cord and cerebral cortex. Conversely, an inhibition in the Na⁺, K⁺-ATPase and glutathione peroxidase activities accompanied by an increase in the reactive species levels in the cerebral cortex of male mice were observed. The treatment with different serotonergic antagonists (pindolol, ketanserin and ondasetron) reversed the mechanical sensitivity in mice of both sexes, after the ICS exposure. The administration of MK-801, L-arginine and methylene blue also blocked the mechanical sensitivity in female mice exposed to ICS. Except L-arginine, MK-801 and methylene blue also attenuated this nociceptive signal in male mice, after ICS exposure. In conclusion, the modulation of serotonergic receptors, the NMDA/NO/cGMP pathway, and the oxidative stress seems contribute to nociceptive behaviors induced by ICS exposure sex-dependent.

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.