Heterogeneous distribution of adenosine transport sites labelled by [³H]nitrobenzylthioinosine in rat brain: An autoradiographic and membrane binding study

A highly heterogeneous distribution of [³H]nitrobenzylthioinosine ([³H]NBI) binding sites was observed using both autoradiographic and membrane binding methodology. Of the 24 brain regions examined in the radio-ligand binding studies, the highest levels of [³H]NBI sites were found in the thalamus, followed by midbrain, superior colliculus, olfactory cortex and hypothalamus. The thalamus contained over 5 times more sites than cerebellum which exhibited the lowest [³H]NBI binding levels. The results obtained from autoradiographic analysis agreed well with quantitative measurements and revealed that subnuclei of thalamus and hypothalamus as well as specific layers of the superior colliculus contained particularly high concentrations of [³H]NBI sites. When the [³H]NBI autoradiograms were compared with the distribution of adenosine deaminase in brain it was found that brain regions richest in neural elements immunoreactive for adenosine deaminase contained the greatest numbers of [³H]NBI sites. In contrast, a poor correlation was found between the distribution of [³H]NBI binding and adenosine receptors labelled with [³H]cyclohexyladenosine. The co-localization of [³H]NBI binding and adenosine deaminase in brain indicates the existence of neural systems having a high capacity to take up and metabolize adenosine.