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

Abstract

1. In order to investigate the modulatory effects of adenosine on excitatory amino acid projections onto striatal medium spiny neurones, 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 A_2A receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5′-N-ethylcarboxamidoadenosine (CGS 21680; 0.1 μM), the A₁ receptor agonist 2-chloro-N⁶-cyclopentyladenosine (CCPA; 10 μM) and the non-selective P1 purinoceptor antagonist 8-(p-sulphophenyl)-theophylline (8-SPT; 100 μM) 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 μM) 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 μM) and the accompanying increase in conductance were both inhibited by CGS 21680 (0.01–1 μM). The effect of CGS 21680 (0.1 μM) persisted in the presence of tetrodotoxin (0.5 μM) or in a Ca²⁺-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 μM) also diminished the effect of NMDA (10 μM), while the A₁ receptor agonists CCPA (0.01–10 μM) and (2S)-N⁶-[2-endo-norbornyl]adenosine [S(−)-ENBA; 10 μM] as well as the endogenous, non-selective P1 purinoceptor agonist adenosine (100 μM) were inactive. The endogenous non-selective P2 purinoceptor agonist ATP (1000 μM) also failed to alter the current response to NMDA (10 μM). Adenosine (100 μM), but not ATP (1000 μM) became inhibitory after blockade of nucleoside uptake by S(4-nitrobenzyl)-6-thioguanosine (NBTG; 30 μM).

4. 8-(p-Sulphophenyl)-theophylline (8-SPT; 100 μM), as well as the A_2A receptor antagonist 8-(3-chlorostyryl)caffeine (CSC; 1 μM) and the A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) at 0.03, but not 0.003 μM abolished the inhibitory action of CGS 21680 (0.1 μM). None of these compounds altered the effect of NMDA (10 μM) by itself. DPCPX (0.03 μM) prevented the inhibition by APNEA (10 μM).

5. There was no effect of CGS 21680 (0.1 μM), when guanosine 5′-O-(3-thiodiphosphate (GDP-β-S; 300 μM) was included in the pipette solution in order to block G protein-mediated reactions.

6. In conclusion, adenosine receptors, probably of the A_2A-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.

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