Adenosine A(2A) Receptors Activation Facilitates Neuromuscular Transmission in the Pre-Symptomatic Phase of the SOD1(G93A) ALS Mice, but Not in the Symptomatic Phase

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease leading to motor neuron dysfunction resulting in impairment of neuromuscular transmission. A(2A) adenosine receptors have already been considered as a potential therapeutical target for ALS but their neuromodulatory role at the neuromuscular junction in ALS remains to be clarified. In the present work, we evaluated the effects of A(2A) receptors on neuromuscular transmission of an animal model of ALS: SOD1(G93A) mice either in the pre-symptomatic (4–6 weeks old) or in the symptomatic (12–14 weeks old) stage. Electrophysiological experiments were performed obtaining intracellular recordings in Mg(2+) paralyzed phrenic nerve-hemidiaphragm preparations. Endplate potentials (EPPs), quantal content (q. c.) of EPPs, miniature endplate potentials (MEPPs) and giant miniature endplate potential (GMEPPs) were recorded. In the pre-symptomatic phase of the disease (4–6 weeks old mice), the selective A(2A) receptor agonist, CGS 21680, significantly enhanced (p<0.05 Unpaired t-test) the mean amplitude and q.c. of EPPs, and the frequency of MEPPs and GMEPPs at SOD1(G93A) neuromuscular junctions, the effect being of higher magnitude (p<0.05, Unpaired t-test) than age-matched control littermates. On the contrary, in symptomatic mice (12–14 weeks old), CGS 21680 was devoid of effect on both the amplitude and q.c. of EPPs and the frequency of MEPPs and GMEPPs (p<0.05 Paired t-test). The results herein reported clearly document that at the neuromuscular junction of SOD1(G93A) mice there is an exacerbation of A(2A) receptor-mediated excitatory effects at the pre-symptomatic phase, whereas in the symptomatic phase A(2A) receptor activation is absent. The results thus suggest that A(2A) receptors function changes with ALS progression.