| 15066904 |
De Lorenzo S, Veggetti M, Muchnik S, Losavio A: Presynaptic inhibition of spontaneous acetylcholine release induced by adenosine at the mouse neuromuscular junction. Br J Pharmacol. 2004 May;142(1):113-24. Epub 2004 Apr 5.
1. At the mouse neuromuscular junction, adenosine (AD) and the A (1) agonist 2-chloro-N (6)-cyclopentyl-adenosine (CCPA) induce presynaptic inhibition of spontaneous acetylcholine (ACh) release by activation of A (1) AD receptors through a mechanism that is still unknown. To evaluate whether the inhibition is mediated by modulation of the voltage-dependent calcium channels (VDCCs) associated with tonic secretion (L- and N-type VDCCs), we measured the miniature end-plate potential (mepp) frequency in mouse diaphragm muscles. 2. Blockade of VDCCs by Cd (2+) prevented the effect of the CCPA. Nitrendipine (an L-type VDCC antagonist) but not omega-conotoxin GVIA (an N-type VDCC antagonist) blocked the action of CCPA, suggesting that the decrease in spontaneous mepp frequency by CCPA is associated with an action on L-type VDCCs only. 3. As A (1) receptors are coupled to a G (i/o) protein, we investigated whether the inhibition of PKA or the activation of PKC is involved in the presynaptic inhibition mechanism. Neither N-(2 [p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide (H-89, a PKA inhibitor), nor 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine (H-7, a PKC antagonist), nor phorbol 12-myristate 13-acetate (PHA, a PKC activator) modified CCPA-induced presynaptic inhibition, suggesting that these second messenger pathways are not involved. 4. The effect of CCPA was eliminated by the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) and by ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid-acetoxymethyl ester epsilon6TDelta-BM, which suggests that the action of CCPA to modulate L-type VDCCs may involve Ca (2+)-calmodulin. 5. To investigate the action of CCPA on diverse degrees of nerve terminal depolarization, we studied its effect at different external K (+) concentrations. The effect of CCPA on ACh secretion evoked by 10 mm K (+) was prevented by the P/Q-type VDCC antagonist omega-agatoxin IVA. 6. CCPA failed to inhibit the increases in mepp frequency evoked by 15 and 20 mm K (+). We demonstrated that, at high K (+) concentrations, endogenous AD occupies A1 receptors, impairing the action of CCPA, since incubation with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, an A (1) receptor antagonist) and adenosine deaminase (ADA), which degrades AD into the inactive metabolite inosine, increased mepp frequency compared with that obtained in 15 and 20 mm K (+) in the absence of the drugs. Moreover, CCPA was able to induce presynaptic inhibition in the presence of ADA. It is concluded that, at high K (+) concentrations, the activation of A (1) receptors by endogenous AD prevents excessive neurotransmitter release. |
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