Changes in brain content of nociceptin/orphanin FQ and endomorphin 2 in a rat model of neuropathic pain

Orphanin FQ (OFQ) and endomorphins (EM) are newly characterized members of opioid peptide family. OFQ has been shown to antagonize morphine analgesia at supraspinal level, whereas endomorphins are highly selective endogenous ligands for mu receptor, showing analgesic effect at both spinal and supras pinal level. OFQ and EM-2 (EM2) immunoreactivity (ir) was measured by radio immunoassay in nociception-related brain areas of rats subjected to L5/L6 s pinal nerve ligation, using sham operated rats as control. It was found tha t: (1) the content of EM2-ir of spinal nerve ligated rats showed a signific ant increase (778%) in periaqueductal gray (PAG), and a significant decreas e (43%) in striatum, compared with the control group. (2) a significant inc rease of the content of OFQ-ir was found in amygdala (+841%) and PAG (+459% ), respectively in spinal nerve ligated rats. High pressure liquid chromato graphy showed that the EM2-ir and OFQ-ir were both heterogeneous with the m ajor part eluting at the position of EM2 and OFQ standard, respectively. Th ese results suggest that spinal nerve ligation induces significant changes in the content of EM2-ir and OFQ-ir in some discrete brain areas, which may play a role in nociceptive modulation. (C) 2001 Elsevier Science Ireland L td. All rights reserved.