Immunohistochemical localization of the carboxy terminus of the novel mu opioid receptor splice variant MOR-IC within the human spinal cord

The present study examined the distribution in the human spinal cord of a unique carboxy terminus sequence contained within MOR-IC, one of the recently described splice variants of the cloned mu opioid receptor gene MOR-I. Although MOR-I-like immunoreactivity (LI) and delta opioid receptor-like immunoreactivity were observed only in the superficial laminae, MOR-IC-LI was abundant in the superficial laminae of the dorsal horn and around the central canal. In the substantia gelatinosa, MOR-IC-LI was found in small diameter axonal elements, the cytoplasm and the plasmalemma of small spinal neurons and dendrites in inner lamina II and in some fibers within Lissauerʼs tract. These studies imply the presence of MOR-IC in human spinal cord and its distribution suggests that it plays a role in the control of pain processing.