The Distribution of μ and δ Opioid Binding Sites Belonging to a Single Cervical Dorsal Root in the Superficial Dorsal Horn of the Rat Spinal Cord: A Quantitative Autoradiographic Study

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Abstract
Numerous studies have demonstrated a dense concentration of opioid receptors in the superficial layers (laminae I‐II) of the spinal cord. These receptors are located both pre‐ and postsynaptically at this level. The purpose of this study was to assess the distribution of opioid receptors belonging to a single (C7) dorsal root. Thus, quantitative autoradiography of γ ([3H]Tyr‐d‐Ala‐Gly‐NMe‐Phe‐Gly‐ol; [3H]DAMGO) and δ ([3H]Tyr‐d‐Thr‐Gly‐Phe‐Leu‐Thr; [3H]DTLET) opioid binding sites was performed for several experimental groups: control rats with intact dorsal roots and lesioned rats with a unilateral dorsal rhizotomy of (a) the C7 root alone, (b) the three successive roots rostral and caudal to the spared C7 root, and (c) the seven roots C4‐Th2. By subtracting results of the ‘C7 cut’ group from the ‘intact’ group or by subtracting results of the C4‐Th2 cut group from the C7 spaced group, it was possible to measure the distribution of γ. and δ opioid binding sites belonging to the C7 root. The combination of these two methods of calculation allowed us to demonstrate a significant distribution over two segments rostral and one segment caudal to the segment of entry. For [3H]DAMGO, the distribution was 10% (P < 0.05) in the C5, 27%, (P < 0.001) in the C6, 38% (P < 0.001) in the C7 and 14% (P < 0.05) in the C8 segment. For [3H]DTLET, the distribution was 11% (P= 0.05) in the C5, 27%, (P < 0.01) in the C6, 37% (P < 0.001) in the C7 and 18% (P < 0.05) in the C8 segment. It is also noted that rostral distributions spread more densely and further than the caudal ones.

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