Radioligand binding and autoradiographic visualization of adenosine transport sites in human inferior vagal ganglia and their axonal transport along rat vagal afferent neurons

The present study has employed membrane-binding studies and in vitro autoradiography to demonstrate the presence of adenosine transport sites in human inferior vagal ganglia using [³H]nitrobenzylthioinosine ([³H]NBMPR), a potent inhibitor of adenosine transport. In addition, [³H]NBMPR was used to determine whether adenosine transport sites are subject to axonal transport along the rat vagus nerve. Binding of [³H]NBMPR to human inferior vagal ganglia membranes was saturable and reversible. Saturation experiments revealed a single class of high affinity-binding sites with a K(d) of 93.73 ± 23.13 pM and B(max) of 413.50 ± 50.40 fmol/mg protein. In displacement experiments, the adenosine transport inhibitor dipyridamole was the most potent displacer of [³H]NBMPR binding (K(i) = 42.7 ± 28.0 nM). Adenosine itself was able to fully displace [³H]NBMPR binding with a K(i) of 115.0 ± 34.0 μM. The A(1)/A(2a) adenosine receptor agonist 5'-(N-ethylcarboxamido)-adenosine (NECA) was able to fully displace [³H]NBMPR binding in only one experiment at a concentration of 100 μM, yielding an affinity 1000-fold higher than its affinity for adenosine receptors. All competition curves obtained from displacement experiments displayed monophasic profiles, indicating the presence of a single class of [³H]NBMPR binding sites. Incubation of human inferior vagal ganglia sections with [³H]NBMPR (0.7 nM) revealed dense binding which appeared to be consistent with the distribution of neuronal cell bodies in this tissue. Following unilateral ligation of the vagus nerve in the rat, acccumulation of [³H]NBMPR binding sites occurred both proximal and distal to the vagal ligatures. These results suggest that [³H]NBMPR binds with high affinity to a single class of adenosine transport sites, and that these sites are present on vagal afferent neurons in the human and undergo bidirectional axonal transport along the rat vagus nerve.