Adenosine A1 and A2 receptors of the substantia gelatinosa are located predominantly on intrinsic neurons: an autoradiography study.

Tissue autoradiography was used to examine the distribution of A1 and A2 adenosine receptors in the rat spinal cord. The distribution of binding sites for the A1 selective agonist, N6-[3H]-[(R)-1-methyl-2-phenylethyl]adenosine (R-[3H]PIA), was similar to the distribution of binding sites for [3H]-5'-N-ethylcarboxamide adenosine [( 3H]NECA), an agonist that binds to A1 and A2 receptors with nearly equal affinities. Binding site densities were highest in the substantia gelatinosa, followed by lamina 10; the remainder of the spinal cord displayed uniformly low levels of binding sites. In each of these three areas, [3H]NECA bound to more sites than R-[3H]PIA. When the A1-selective agonist, N6-(cyclopentyl)adenosine, was used to block [3H]NECA binding to A1 receptors, the autoradiograms demonstrated a band of A2 receptor density which tended to be localized in the ventral portion of the substantia gelatinosa. Unilateral dorsal rhizotomy, hemitransection and complete transection of the spinal cord failed to alter the [3H]NECA or R-[3H]PIA binding in the substantia gelatinosa. In contrast, unilateral microinjections of kainic acid into the dorsal horn decreased [3H]NECA and R-[3H]PIA binding at the level of the injections by 42 and 37%, respectively. In some areas, kainic acid injection eliminated all R-[3H]PIA and [3H]NECA binding in the substantia gelatinosa. It is therefore concluded that spinal cord A1 and A2 adenosine receptors in the substantia gelatinosa are located predominantly on intrinsic neurons.