Abstract

Adenosine-induced modulation of neuromuscular transmission in young (3–4-week-old) rats was evaluated. Inhibition of adenosine kinase with iodotubercidin (ITU; 10 μM), which is known to induce adenosine release, enhanced the amplitude of evoked end-plate potentials (EPPs) recorded from innervated diaphragm muscle fibers. This facilitatory effect was transformed into an inhibitory one upon blockade of adenosine A_2A receptors with 4-(2-[7-amino-2-(2-furly)[1,2,4]triazolo[2,3-a][1,3,5]triazin5ylamino] ethyl) phenol (ZM 241385) (50 nM); further blockade of adenosine A₁ receptors with the selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 10 nM) abolished that inhibition. Adenosine or 2-chloroadenosine (CADO), at submicromolar concentrations, increased the amplitude and the quantal content of EPPs, whereas at low micromolar concentrations they decreased EPP amplitude. Blockade of A₁ receptors with DPCPX (10 nM) prevented both excitatory and inhibitory effects, whereas blockade of A_2A receptors with ZM241385 (50 nM) prevented only the excitatory effects. DPCPX and ZM241385 also prevented the excitatory effect of the selective A_2A receptor agonist 2-[p-(2-carboxyethyl) phenethylamino]-5′-N-ethylcarboxamido adenosine hydrochloride (CGS 21680; 10 nM). CADO (30 nM) also increased neuromuscular transmission in adult (12–16-week-old) rats. It is suggested that at the motor nerve endings, low extracellular concentrations of adenosine activate both A_2A and A₁ receptors, but activation of A_2A receptors predominates over A₁ receptors; the activity of A_2A receptors might, however, require coactivation of A₁ receptors. This facilitatory action of low concentrations of extracellular adenosine upon acetylcholine release may be particularly relevant at developing neuromuscular junctions, where subtle changes in synaptic levels of acetylcholine might influence synaptic stabilization.