Role of connexin36 proteins in peripheral nerves during the triple response and pain
Introduction: Gap junctions allow direct cell-to-cell communication and form electrical synapses between neurons primarily via the gap junction protein connexin36 (Cx36). We examined the potential contribution of Cx36 in spinal dorsal root ganglia and peripheral nerves to the classic triple response (flare, wheal, and edema) following stimulation of nociceptors. The flare reflects increased blood flow beyond the injured area and the wheal is due to changes in the vascular permeability. Studies conducted in the 1980s suggested that electrical coupling between unmyelinated C-fibers in the periphery may contribute to these responses. Cx36 is a protein allowing gap junction formation between neurons. Objectives: We aimed to quantify the contribution of Cx36-containing gap junctions to plasma extravasation responses following stimulation of nociceptors. Methods: Wild-type (WT) and Cx36 knockout (KO) mice lacking this protein in all neurons were used for studies in which application of noxious stimuli (xylene, formalin, electrical stimulation of the sciatic nerve) was performed. Extravasation of Evans Blue dye from the circulation in the limbs was monitored via spectrophotometry, laser Doppler flux and in vivo fluorescence imaging after noxious stimulation in anesthetized mice. Results: A small but non-significant difference was found between KO and WT animals in the extravasated EB dye after front-paw’s chemical stimulation with xylene. A trend for greater responses as well as a faster rate of change in the Evans Blue fluorescence was found within the first 2-5 minutes after xylene application in WT vs. KO mice. Conclusions: The enhanced early vascular response observed in this study after nociceptor stimulation in the WT animals as compared to the KO animals is consistent with earlier reports of direct electrical coupling between C-fibers and with evidence of Cx36 expression in small sensory neurons. These results suggest that Cx36 transiently contributes to the triple response.