EXCITATORY EFFECT OF THE A2A ADENOSINE RECEPTOR AGONIST CGS-21680 ON SPONTANEOUS AND K+-EVOKED ACH RELEASE AT THE MOUSE NEUROMUSCULAR JUNCTION

NEUROSCIENCE

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2010

0306-4522

The mechanism of action of the A2A adenosine receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5¡ä-N-ethylcarboxamidoadenosine hydrochloride (CGS-21680) in the facilitation of spontaneous (isotonic and hypertonic condition) and K+-evoked ACh release was investigated in the mouse diaphragm muscles. At isotonic condition, the CGS-21680-induced excitatory effect on MEPP frequency was not modified in the presence of CdCl2 and in a medium free of Ca2+ (0Ca2+-EGTA), but it was abolished after buffering the rise of intracellular Ca2+ with BAPTA-AM and when the Ca2+-ATPase inhibitor thapsigargin was used to deplete intracellular Ca2+ stores. CGS-21680 did not have a direct effect on the Ca2+-independent neurotransmitter-releasing machinery, since the modulatory effect on the hypertonic response was also occluded by BAPTA-AM and thapsigargin. CGS-21680 facilitation on K+-evoked ACh release was not altered by the P/Q-type voltage-dependent calcium channel (VDCC) blocker ¦Ø-Agatoxin IVA, but it was completely prevented by both, the L-type VDCC blocker nitrendipine (which is known to immobilize their gating charges), or thapsigargin, suggesting that the effects of CGS-21680 on L-type VDCC and thapsigargin-sensitive internal stores are associated. We found that the VDCC pore blocker Cd2+ (2 mM Ca2+ or 0Ca2+ - EGTA) failed to affect the CGS-21680 effect in high K+ whereas nitrendipine in 0Ca2+-EGTA + Cd2+ occluded its action. The blockade of Ca2+ release from endoplasmic reticulum with ryanodine antagonized the facilitating effect of CGS-21680 in control and high K+ concentration. It is concluded that, at the mouse neuromuscular junction, activation of A2A receptors facilitates spontaneous and K+-evoked ACh release by an external Ca2+¨Cindependent mechanism but that involves mobilization of Ca2+ from internal stores: during spontaneous ACh release stimulating directly the ryanodine-sensitive stores and, at high K+, probably modulating the L-type VDCCs which may cause the opening of the ryanodine receptors that would be directly coupled to the channels. In both cases, Ca2+ released from the endoplasmic reticulum would be capable of activating the exocytotic machinery, thus producing facilitation of ACh release.