Neuronal location of N⁶-cyclohexyl[³H]adenosine binding sites in rat and guinea-pig brain

N⁶-cyclohexyl[³H]adenosine, ([³H]CHA), was used to label adenosine a₁ receptors in crude synaptic membranes prepared from rat and guinea pig brain. The density of [³H]CHA binding sites was highest in the rat hippocampus and cerebellum and in the guinea-pig hippocampus. A microdissection of coronal sections of guinea-pig hippocampus revealed that the specific binding capacity for [³H]CHA in area CA-1 was 20-30% higher than in area CA-3, dentate gyrus and subiculum. Selective neuronal lesions of serotonergic, noradrenergic and cholinergic afferents to the hippocampus failed to alter [³H]CHA binding to hippocampal membranes. These results suggest that [³H]CHA binding sites are not associated with axons or terminals of these neurones in the hippocampus. Intrahippocampal injection of kainic acid reduced the number of [³H]CHA recognition sites by 30% with no alteration in the affinity of [³H]CHA for these receptors. Thus, a significant portion of A₁ receptors may be associated with intrinsic neurones of the hippocampus which do not appear to be innervated by noradrenergic, cholinergic or serotonergic axons.