Endomorphin-2 is not released from rat spinal dorsal horn in response to intraplantar formalin

Abstract

Antibody coated microprobes, inserted into the spinal cord at the L4–5 level, were used to detect whether endomorphin-2 (Endo2) was released from spinal dorsal horns in anesthetized rats in response to formalin injected into the hindpaw footpads. Saline injections were used as a control and substance P (SP) was measured to verify activation of nociceptive afferent fibers. SP but not Endo2 was released during pre-stimulation periods. Saline injections did not cause the release of either Endo2 or SP from the spinal cord. Formalin injections caused an increase in Fos expression as well as a release of SP, but not Endo2 from the ipsilateral side dorsal horn in L4–5. We conclude that Endo2 does not play a role in mediating the in vivo responses to acute inflammatory nociceptive signals at the spinal level in the anesthetized rat model.