Modulation by adenosine of A delta and C primary-afferent glutamatergic transmission in adult rat substantia gelatinosa neurons

The present study examined the actions of adenosine on monosynaptic Adelta and C primary-afferent excitatory postsynaptic currents (EPSCs) recorded from substantia gelatinosa (SG) neurons of an adult rat spinal cord slice. In 67% of the neurons examined, adenosine reversibly decreased the amplitude of the Adelta-fiber EPSC, while in 13% of the neurons the amplitude was reduced or unaffected, which was followed by its increase persisting for several minutes after adenosine washout. The remaining neurons did not exhibit a change in the amplitude. The reduction in Adelta-fiber EPSC amplitude by adenosine was dose-dependent with an effective concentration for half-inhibition (EC50) value of 217 muM. When examined by using a paired-pulse stimulus, a ratio of the second to first Adelta-fiber EPSC amplitude under the reduction was larger than that of EPSC amplitude in the control, suggesting a presynaptic action of adenosine. In 69% of the neurons tested, the C-fiber EPSC was reversibly decreased in amplitude by adenosine (100 muM) by an extent comparable to that of Adelta-fiber EPSC; the remaining neurons were without adenosine actions. Similar inhibitory actions of adenosine were also seen in neurons where both Adelta-fiber and C-fiber EPSCs were elicited. Similar reduction in the Adelta-fiber or C-fiber EPSC amplitude was induced by an A(1) adenosine-receptor agonist, N-6-cyclopentyladenosine (1 muM), and the adenosine-induced reduction was not observed in the presence of an A(1) antagonist, 8-cyclopentyl-1,3-dipropylxanthine (1 muM). An A(2a) agonist, CGS 21680 (1 muM), did not significantly affect the AB-fiber EPSC amplitude. It is concluded that adenosine presynaptically inhibits monosynaptic Adelta-fiber and C-fiber transmission by a similar extent through the activation of the A(1) receptor in many but not all SG neurons; this could contribute to at least a part of antinociception by intrathecally administered adenosine analogues and probably by endogenous adenosine. (C) 2004 IBRO. Published by Elsevier Ltd. All rights reserved.