Induction of multiple photophobic behaviors in a transgenic mouse sensitized to CGRP

Abstract

Migraine is a complex neurological disorder with a significant impact on patients and society. Clinical and preclinical studies have established the neuropeptide calcitonin gene-related peptide (CGRP) as a key player in migraine and other neurovascular headaches. To study the role of CGRP in these disorders, we have characterized the photophobic phenotype of nestin/hRAMP1 mice, a transgenic model with genetically engineered increased sensitivity to CGRP. These mice have increased nervous system expression of a regulatory subunit of the CGRP receptor, human receptor activity-modifying receptor (hRAMP1). We have previously demonstrated that nestin/hRAMP1 mice display a light-aversive behavior that is greatly enhanced by CGRP and blocked by a CGRP receptor antagonist used to treat migraine. Here we have compared their behavior in two different experimental setups with testing chambers of different sizes and light intensities as well as in complete darkness. We demonstrated similar degrees of light aversion in nestin/hRAMP1 mice with 1000 and 50 lux. To control for other possible factors driving nestin/hRAMP1 mice to the dark zone, we tested them in the absence of any light, and they showed identical behavior as littermates. Furthermore, both nestin/hRAMP1 and control mice have decreased motility in response to CGRP in the dark, but not the light side of the chamber. Our findings confirm the robust CGRP-induced light-aversive phenotype of nestin/hRAMP1 mice, which can be a surrogate of photophobia, and validates its usefulness as a model of migraine and other disorders associated with photophobia.

Introduction

Migraine is a chronic neurovascular disorder characterized by recurrent episodes of severe unilateral throbbing head pain and associated symptoms, such as photophobia (Goadsby et al., 2002). Mechanistic studies in the field of migraine pathophysiology are hampered by the lack of appropriate animal models. We have developed a transgenic mouse that displays a migraine-like behavioral phenotype, the nestin/hRAMP1 mouse (Zhang et al., 2007, Marquez de Prado et al., in press, Recober et al., 2009). This mouse is sensitized to the calcitonin gene-related peptide (CGRP), a key player in migraine pathophysiology (Arulmani et al., 2004), by overexpression of the human receptor activity-modifying protein 1 (hRAMP1) subunit of the CGRP receptor in the nervous system.

Nestin/hRAMP1 mice exhibit light-aversive behavior that is greatly enhanced by exogenous CGRP (Recober et al., 2009). This behavior cannot be attributed to differences in anxiety or motor activity (Recober et al., 2009). The specificity of CGRP actions was confirmed by the use of a CGRP receptor antagonist, olcegepant, which prevented the CGRP-induced light-aversive behavior (Recober et al., 2009). A potential mechanism of the light-aversion observed at baseline may be the higher levels of CGRP present in cerebrospinal fluid of nestin/hRAMP1 mice (Recober et al., 2009).

We studied light-aversive behavior as a surrogate of photophobia, a common symptom reported by patients both during migraine attack and to a lesser degree between attacks (Mulleners et al., 2001). While the mechanisms underlying photophobia remain unknown, the trigeminal system appears to be involved (Drummond and Woodhouse, 1993, Kowacs et al., 2001, Okamoto et al., 2009). As a major neuropeptide in the trigeminal system, CGRP is likely to be implicated in the development of photophobia. Our previous studies suggest that nestin/hRAMP1 mice are a useful model for mechanistic and therapeutic studies of migraine and other trigeminal mediated disorders and conditions associated with photophobia (Zhang et al., 2007, Marquez de Prado et al., in press, Recober et al., 2009).

The aim of the current study is to fully characterize the photophobic phenotype of nestin/hRAMP1 mice and confirm the robustness of this behavior by replicating the findings in a different experimental setup that allows measurement of additional parameters. The times spent in light in chamber # 1 (Table 1) are taken from a previous publication for comparison (Recober et al., 2009). Transitions and latencies in chamber # 1 and all the parameters measured in chamber # 2 are original data for this study.