Expression Of BAM22 Peptide In The Spinal Dorsal Horn And Dorsal Root Ganglion In Naive And CFA-treated Rats

Bovine adrenal medulla 22 (BAM22) is one of the proteolytic cleavage products of proenkephalin A and displays high affinity with opioid receptors and sensory neuron-specific receptor (SNSR) that is exclusively distributed in dorsal root ganglion (DRG) cells. The present study was designed to examine expression of BAM22 in the spinal dorsal horn and DRG of the rat with or without unilateral paw inflammation produced by complete Freund's adjuvant (CFA). BAM22 immunoreactivity (BAM22-IR) was expressed in fibers and neurons in the spinal dorsal horn with high density in laminae I in naive rat. The expression of BAM22-IR in the superficial laminae was greatly reduced following dorsal rhizotomy. BAM22-IR was also located in 19% of DRG cells, mainly in small- and medium-size subpopulations. Following intraplantar injection of CFA into one hindpaw, the expression of BAM22-IR in the superficial laminar of the spinal cord and small-size DRG neurons on ipsilateral side was increased by approximately 2 folds.Double-labeling study showed that Fos-positive nucleus was surrounded by BAM22-IR cytoplasm in the spinal dorsal horn neurons or associated closely with BAM22-IR fibers in the superficial laminae. Furthermore, CFA-induced mechanical allodynia in inflamed paw was reinforced by intrathecal administration of BAM22 antibody. The results suggest that the BAM22 peptide is largely synthesized in primary sensory neurons in ganglia, transported and stored in the central terminal of primary afferents in the superficial dorsal horn, and that there may be close relationship between BAM22 and SNSR. The results indicate that BAM22 is involved in modulation of nociceptive processing at the spinal level under normal and inflammatory status.