168 Brain Research, 475 (1988) 168- 172 Elsevier BRE 23234 Demonstration of calcitonin gene-related peptide immunoreactive axons contacting dynorphin A(1-8) immunoreactive spinal neurons in a rat model of peripheral inflammation and hyperalgesia Osamu Takahashi, Richard J. Traub and M.A. Ruda Neurobiology and Anesthesiology Branch, National Institute of Dental Research. National Institutes of Health, Bethesda, MD 20892 (U.S.A.) (Accepted 23 August 1988) Key words: Dynorphin; Calcitonin gene-related peptide; Spinal cord; Rat; Immunocytochemistry; Nociception In a rat model of peripheral inflammation and hyperalgesia, dynorphin A(1-8)-like immunoreactive (DYN-LIr) spinal neurons were examined for contacts from calcitonin gene-related peptide-like immunoreactive (CGRP-LIr) varicosities using a double-label PAP method. Ipsilateral to the inflammation, CGRP-LIr varicosities contacted both dendrites and somata of DYN-LIr neurons in lumbar laminae I, II and V. Few such contacts were found on the contralateral side. The results suggest that opioid neurons which ex- hibit a dynamic change in dynorphin associated with inflammation, represent a subpopulation of neurons that receive contacts from presumptive nociceptive primary afferents. Recent immunocytochemical studies revealed that neurons containing dynorphin-like immunoreactivity are located in laminae 1-VII of the rat and cat spinal cord 2,11,16,tT. Although most dynorphin-like immuno- reactive (DYN-LIr) dorsal horn neurons appear to be local circuit neurons, a small number have been shown to be supraspinal projection neurons 12'13'19. It has also been shown that spinal dynorphin is involved in the reaction to peripheral nociceptive stimuli. An increase in preprodynorphin mRNA and dynorphin peptide has been demonstrated in rat lumbar spinal cord following unilateral experimental acute inflam- mation 6'7't6 and experimentally induced polyarthri- tisS,S-10. Calcitonin gene-related peptide (CGRP) is a 37 amino acid peptide 14 which in the dorsal horn may originate exclusively from small diameter primary af- ferents 3'15'21. A subpopulation of CGRP-Iike immu- noreactive (CGRP-LIr) dorsal root ganglion neurons co-localize substance P in their cell bodies and ax- ons 4"1s'22. These findings suggest that CGRP-Iike im- munoreactivity (CGRP-LI) is found in primary affer- ent axons that play a role in the processing of noci- ceptive information 1'15'2~. The present study was undertaken to examine the potential interaction between DYN-LIr neurons that respond to acute inflammation and hyperalgesia by increasing their content of dynorphin peptide and small diameter CGRP-LIr primary afferent axons that may carry nociceptive information to the spinal cord. Experiments were carried out on 11 male Spra- gue-Dawley rats (225-250 g). Under sodium pento- barbital anesthesia, inflammation of one hindpaw was induced by injection of 150 ~1 of a saline-com- plete Freund's adjuvant emulsion (75 ktg Mycobacte- rium butyricum, Sigma). The animals exhibited ede- ma and hyperalgesia to heat stimulation of the in- jected hindpaw that peaked at 2-4 days 6&16. The an- imals guarded the affected limb, but their general be- havior indicated that they experienced minimal dis- comfort 16. These experiments were reviewed and ap- Correspondence." O. Takahashi, NAB, NIDR, NIH, Building 30, Room B-20, 9000 Rockville Pike, Bethesda. MD 20892, U.S.A. 0006-8993/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)