Temporal alterations of pituitary adenylate cyclase activating polypeptide and its receptors in a rat model induced by recurrent chemical stimulations: Relevant to chronic migraine

Background: Migraine is a common type of primary headache with disabling brain dysfunction. It has been found that pituitary adenylate cyclase activating polypeptide (PACAP) is involved in the pathogenesis of migraine, however, the role of PACAP and its receptors in chronic migraine remains unclear. Therefore, the present study aimed to explore the changes of PACAP and its receptors in different duration after recurrent dural inflammation soup stimulations and to investigate the co-expression between PACAP and calcitonin gene-related peptide (CGRP). Methods: Adult male rats were implanted with cannula surrounding superior sagittal sinus, which was followed by dural infusion of inflammatory soup (IS) or normal saline (NS). The rats were randomly divided into 4 groups (n = 8 for each group): IS stimulation for seven days (IS-7 group), IS stimulation for 14 days (IS-14 group), IS stimulation for 21 days (IS-21 group), and NS control for 21 days (CON group). The facial mechanical withdrawal threshold was daily measured during the whole experiment. The behavioral changes (ipsilateral and bilateral face grooming behavior) in a plastic cage of rats were observed and recorded. The expression of PACAP, its receptors (PAC1, VPAC1, VPAC2), and CGRP in the trigeminal ganglia (TG) and the trigeminal nucleus caudalis (TNC) was examined by immunohistochemistry. Immunofluorescence was used to explore the co-expression of PACAP, PAC1 receptor, and CGRP after repeated IS administration in the TG. Results: The ipsilateral facial grooming time of IS-21 group displayed an apparent increase than CON group after repeated stimulation on day 2, while significant differences were observed on day 14. No differences were found between the IS-21 and CON group in bilateral facial grooming. Dural IS stimulation induced a significantly decrease in facial mechanical withdrawal thresholds. PACAP positive cells in the regions of TNC were gradually decreased with the IS days increasing. PACAP and PAC1 receptor expression in the TG had a trend of increasing first and then decreasing. There was no significant difference in expression of VPAC1 and VPAC2 in the TG and the TNC. Immunofluorescence showed that PACAP was mainly expressed in TG neurons. PACAP and PAC1 receptor co-expression decreased gradually after repetitive IS stimulation. While the co-expression between PACAP and CGRP reached the peak in IS-7 group after repetitive IS stimulation, and then decreased. Conclusions: This study demonstrated that repetitive chemical stimulations induced a gradual decrease of PACAP in the TNC, while the PACAP and PAC1 receptor expression in TG showed dynamical changes of increasing first and then decreasing after repeated IS administration. These results suggested exhaustion of PACAP could be involved in the duration of chronic migraine and implied PACAP may contribute to the pathology of migraine through the PAC1 receptor, which was associated with CGRP.