Effect of adenosine and some of its structural analogues on the conductance of NMDA receptor channels in a subset of rat neostriatal neurones.

1. In order to investigate the modulatory effects of adenosine on excitatory amino acid projections onto striatal medium spiny neurons, whole-cell patch clamp experiments were carried out in rat brain slices. The effects of various agonists for P1 (adenosine) and P2 (ATP) purinoceptors and their antagonists were investigated. The A2A receptor agonist 2-p-(2-carboxyethyl)phenythylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 0.1 microM), the A1 receptor agonist 2-chloro-N6-cyclcopentyladenosine (CCPA; 10 microM) and the non-selective P1 purinoceptor antagonist 8-(p sulphophenyl)-theophylline (8-SPT; 100 microM) did not alter the resting membrane potential, the threshold current necessary to elicit an action potential, the amplitude of spikes, their rise time, the amplitude of the afterhyperpolarization (AHP) and the time to peak of the AHP. 2. N-methyl-D-aspartate (NMDA; 1-1000 microM) caused a concentration-dependent inward current which was larger in the absence than in the presence of Mg2+ (1.3 mM). In a subset of striatal neurones, the current response to NMDA (10 microM) and the accompanying increase in conductance were both inhibited by CGS 21680 (0.01-1 microM). The effect of CGS 21680 (0.1 microM) persisted in the presence of tetrodotoxin (0.5 microM) or in a Ca(2+)-free medium, under conditions when synaptically mediated influences may be negligible. 3. The A3 receptor agonist N6-2-(4-aminophenyl)ethyladenosine (APNEA; 0.1-10 microM) also diminished the effect of NMDA (10 microM), while the A1 receptor agonists CCPA (0.1-10 microM) and (2S)-N6-[2-endonorbornyl] adenosine [S(-)-ENBA; 10 microM] as well as the endogenous, non-selective P1 purinoceptor agonist adenosine (100 microM) were inactive. The endogenous non-selective P2 purinoceptor agonist ATP (1000 microM) also failed to alter the current response to NMDA (10 microM). Adenosine (100 microM), but not ATP (1000 microM) became inhibitory after blockade of nucleoside uptake by S[4-nitrobenzyl)-6-thioguanosine (NBTG; 30 microM). 4. 8-(p-Sulphophenyl)-theophylline (8-SPT; 100 microM), as well as the A2A receptor antagonist 8-(3-chlorostyryl) caffeine (CSC; 1 microM) and the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) at 0.03, but not 0.003 microM abolished the inhibitory action of CGS 21,680 (0.1 microM). None of these compounds altered the effect of NMDA (10 microM) by itself. DPCPX (0.03 microM) prevented the inhibition of APNEA (10 microM). 5. There was no effect of CGS 21,680 (0.1 microM), when guanosine 5'-O-(3-thiodiphosphate (GDP-beta-S; 300 microM) was included in the pipette solution in order to block G protein-mediated reactions. 6. In conclusion, adenosine receptors, probably of the A2A-subtype, inhibit the conductance of NMDA receptor channels in a subset of medium spiny neurones of the rat striatum by a transduction mechanism which involves a G protein.