Evaluation of mRNA Expression of Calcitonin Gene-Dependent Peptide (CGRP) and Rat Calcitonin (rCT) in the Periaqueductal Gray Area (PAG) of Diabetic Rats in the Formalin Test

Introduction: CGRP and rCT are involved in descending pain control areas. This study aimed to evaluate the effect of intracerebroventricular administration (ICV) of CGRP and rCT on mRNA expression of CGRP and rCT peptides in the Periaqueductal Gray Area (PAG) of the diabetic rats in the formalin test. Material & Methods: This study investigated 24 male Sprague-Dawley rats in four groups (n=6). To induce diabetes, streptozotocin at a dose of 45 mg/kg was used intraperitoneally. CGRP or rCT peptides at a dose of 1.5 nmol with a volume of 5 μl were ICV injected daily for seven days. Pain-related behaviors were recorded in the formalin test for up to 60 min in the study groups. The PAG was then removed to assess the changes made in the mRNA expression of the CGRP and rCT. (Ethic code: INT2M129396) Findings: ICV injection of CGRP or rCT in diabetic rats reduced pain in the acute and middle phases of the formalin test. In addition, ICV administration of CGRP increased CGRP mRNA expression in the PAG. However, ICV administration of rCT increased the mRNA expression of both CGRP and rCT peptides after seven days in the PAG. Discussion & Conclusion: ICV injection of CGRP and rCT peptides reduced the pain of formalin injection in rats in the experimental model of streptozotocin-induced diabetes mellitus, possibly by altering the mRNA expression of both peptides.