Sex-related differences in the transcriptional effects of CGRP in an animal model of migraine-like photophobic behaviour

ABSTRACT Migraine is one of the most disabling neurological disorders, characterized by moderate, intense, and pulsating pain often localized on one side of the head, accompanied by associated symptoms like photophobia, nausea, vomiting, or sensitivity to sounds. Migraine is considered a global health issue with a significant impact on quality of life and healthcare costs. The literature estimates that migraine affects approximately 14% of the global population, its prevalence varies by age, sex, ethnicity, and income. In fact, women are three times more likely than men to suffer from migraine. Possible mechanisms underlying migraine pathophysiology include trigeminal vascular system activation and central sensitization sustained by the release of vasoactive peptides, such as calcitonin gene-related peptide (CGRP). CGRP is one of the main molecular mediators associated with the neurovascular theory of migraine. It is released from the terminal branches of the trigeminal nerve and causes vasodilatation and inflammation in the meninges. So far, the molecular mechanisms of this neurological disorder are still poorly understood, therefore preclinical studies could improve our understanding of the mechanisms underlying this disease and its sex-related prevalence. In this thesis, we employed a validated animal model of migraine-like photophobic behaviour to study the behavioural and molecular differences between male and female animals. In fact, it has been demonstrated that peripheral administration (intraperitoneal injection, i.p.) of CGRP in adult rodents induces migraine-like symptoms, including photophobia and reduced motility. Four different groups of adult male and female mice (CD1; n=13-15 per group) received one of the following treatments: saline (i.p.), CGRP (0.1 mg/kg; i.p.), sumatriptan (0.6 mg/kg; i.p.) or co-treatment with CGRP and sumatriptan. All animals underwent a 20 minutes-long behavioural test (light-dark box) 30 minutes after treatment and were sacrificed two hours after the injection. Consistently with the literature, we observed that the injection of CGRP resulted in light-aversive behaviour irrespective of the biological sex, and that this effect was attenuated by a clinically effective migraine drug, the 5-HT1B/D agonist sumatriptan. After the behavioural test, in a subgroup of animals (n=9 per group) we measured the expression levels of targets involved in neuronal activation and plasticity (BDNF, c-Fos, and Fos-B) and in the inflammatory response (TNFα, IL-1β, IL-6, and TGFβ1). Additionally, we evaluated the expression levels of the alpha oestrogen receptor (ESR1). Molecular analyses were performed on two different brain areas, namely the cerebellum and the hypothalamus, closely associated with migraine pathology. In fact, the cerebellum has recently been shown to be central in pain processing, while the hypothalamus is closely related to the onset of migraine. Furthermore, CGRP and its receptors are widely expressed in both areas. The results of this study demonstrate the complex interaction between biological sex and pharmacological treatment in migraine-like behaviours. This thesis also provide evidence on the intricate neural circuitry of migraine and the interplay between CGRP and sumatriptan in determining migraine symptomatology and treatment outcome in adult male and female mice.