A unique inbred rat strain with sustained cephalic hypersensitivity as a model of chronic migraine-like pain

Abstract Animal models of migraine-like pain enabling ongoing study of behaviour typically involve the systemic administration of chemical vasodilators or dural administration of inflammatory algogens. However, neither method mediates prolonged effects on behavior indicative of enduring pathophysiological changes occurring within dural or trigeminal pain circuits. We generated successive generations of a unique inbred rat strain, spontaneous trigeminal allodynia (STA) rats, previously reported to exhibit an episodic migraine-like behavioural phenotype. We show that both male and female STA rats display robust and sustained reductions in periorbital thresholds to cutaneous mechanical stimulation. Otherwise, the general behavior (e.g. locomotor, grooming) of these rats appeared normal. In female STA rats, the mechanical hypersensitivity was confined to the cephalic region, manifested after puberty through adolescence, and was sustained into adulthood recapitulating the clinical manifestation of migraine. We exploited this hitherto unidentified chronic phenotype to show that the migraine-specific drugs sumatriptan (5-HT1B/1D receptor agonist) and olcegepant (CGRP receptor antagonist) could completely reverse cephalic hypersensitivity using a within subject cross-over paradigm. Our findings indicate that STA rats actually possess a phenotype indicative of migraine chronicity which is exquisitely sensitive to migraine therapeutics. This unique strain could prove to be an invaluable resource in preclinical migraine drug discovery

PACAP-38 and PACAP(6–38) Degranulate Rat Meningeal Mast Cells via the Orphan MrgB3-Receptor

Infusion of pituitary adenylate cyclase activating peptide-38 (PACAP-38) provokes migraine attacks in migraineurs and headache in non-migraineurs. Adverse events like long-lasting flushing and heat sensation can be terminated with oral antihistamine treatment, indicating the involvement of mast cell activation after PACAP-infusion. Degranulation of rat peritoneal mast cells was provoked by several isoforms of PACAP via previously unknown receptor pharmacology. The effect might thus be mediated either via specific splice variants of the PAC1-receptor or via an unknown receptor for PACAP-38. In the present study, we characterize degranulation of rat meningeal mast cells in response to PACAP-receptor ligands. Furthermore, we investigate if PACAP-38-induced mast cell degranulation is mediated via PAC1-receptor splice variants and/or via the orphan Mas-related G-protein coupled member B3 (MrgB3)-receptor. To address this, the pharmacological effect of different PACAP isoforms on meningeal mast cell degranulation was investigated in the hemisected skull model after toluidine blue staining followed by microscopic quantification. Presence of mRNA encoding PAC1-receptor splice variants and the MrgB3-receptor in rat mast cells was investigated by Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) analysis. The effect of PACAP isoforms on PAC1- and MrgB3-receptor-expressing Xenopus laevis oocytes were performed by two-electrode voltage-clamp (TEVC) electrophysiology. PACAP-38 is a more potent mast cell degranulating agent than Pituitary Adenylate Cyclase Activating Peptide-27 (PACAP-27) in the meninges. Presence of mRNA encoding the PAC1-receptor and its different splice variants could not be detected in peritoneal mast cells by RT-PCR, whereas the orphan MrgB3-receptor, recently suggested to be a mediator of basic secretagogues-induced mast cell degranulation, was widely present. In PAC1-receptor-expressing Xenopus laevis oocytes both PACAP-38, PACAP-27 and the specific PAC1-receptor agonist maxadilan were equipotent, however, only PACAP-38 showed a significant degranulatory effect on mast cells. We confirmed Pituitary Adenylate Cyclase Activating Peptide(6–38) [PACAP(6–38)] to be a PAC1-receptor antagonist, and we demonstrated that it is a potent mast cell degranulator and have an agonistic effect on MrgB3-receptors expressed in oocytes. The present study provides evidence that PACAP-induced mast cell degranulation in rat is mediated through a putative new PACAP-receptor with the order of potency being: PACAP-38 = PACAP(6–38) > > PACAP-27 = maxadilan. The results suggest that the observed responses are mediated via the orphan MrgB3-receptor

Effect of two novel CGRP-binding compounds in a closed cranial window rat model

We investigated the in vivo effects of two novel calcitonin gene-related peptide (CGRP) binding molecules in the genuine closed cranial window model in the rat. The RNA-Spiegelmer (NOX-C89) and the monoclonal CGRP antibody are CGRP scavengers and might be used as an alternative to CGRP-receptor antagonists in the treatment of migraine. Rats were anaesthetized and a closed cranial window established. Changes in dural and pial artery diameter and mean arterial blood pressure were measured simultaneously. Infusion of the RNA-Spiegelmer or the CGRP antibody alone had no effect on the arteries or the mean arterial blood pressure. We then used a bolus of 0.3 mu g/kg CGRP (n=6) or electrical stimulation (25 V, 5 Hz, 1 ms pulse width and of 10 s of duration) (n=6) to induce dilatation of dural and pial arteries (mediated via CGRP-receptors). Pre-treatment with the RNA-Spiegelmer inhibited CGRP-induced vasodilatation of the dural artery (from 38 +/- 17% to 7 +/- 3%) and the pial artery (from 14 +/- 1% to 3 +/- 2%) (P 0.05). The CGRP antibody caused a significant reduction of the dural artery diameter caused by intravenous CGRP-in fusion (from 23 +/- 5% to 12 +/- 3%) (P 0.05). In conclusion, the CGRP scavengers effectively inhibited the effect of circulating CGRP but do not modify the effect of electdcal stimulation and the consequent liberation of CGRP from perivascular sensory nerve fibres. (c) 2007 Elsevier B.V. All rights reserved