The distribution of adenosine A1 receptors and 5′‐nucleotidase in the hippocampal formation of several mammalian species

Abstract

The distributions of adenosine Al receptors, as demonstrated by ³H‐cyclohexyladenosine (³H‐CHA) binding, and the adenosine‐producing enzyme 5′‐nucleotidase were examined in the hippocampal formation of the rat, mouse, gerbil, cat, hamster, rabbit, and guinea pig. The enzyme and binding sites were restricted to subregions and often individual layers of this structure. The distribution of ³H‐CHA binding was consistent among the species with the strata radiatum and oriens of fields CA1 and CA3 exhibiting the highest levels of binding. A distinct band of ³H‐CHA binding was observed in the stratum moleculare of the dentate gyrus; and in most species, this band was restricted to the inner one‐third of the stratum moleculare (i.e., proximal to the stratum granulosum). The strata pyramidale, granulosum, and lucidum were in general only weakly positive for ³H CHA binding. The binding to the stratum lacunosum/moleculare (or the distinct strata lacunosum and moleculare in the rabbit and cat) was moderate. In contrast to the relative consistency of the patterns of ³H‐CHA binding in these species, 5′‐nucleotidase exhibited wide variations in both the absolute amount of activity and its localization. In all species, the strata granulosum and pyramidale appeared devoid of 5′‐nucleotidase activity. The only clear exception to this rule was the CA3 region of the cat where activity was seen between the cell bodies of stratum pyramidale. The strata radiatum and oriens of CAI were strongly positive in the rat and hamster but only low to moderately stained in the other species examined. The stratum lucidum of CA3 showed high amounts of staining in the hamster and moderate to high in the rat and rabbit. The cat, mouse, gerbil, and guinea pig exhibited only low to moderate 5′‐nucleotidase activity in the stratum lucidum of CA3. The stratum oriens of CA3 was low to moderate in all species except the cat, in which a dense band was observed near the stratum pyramidale. The only evidence of distinct binding in field CA2 was observed in the mouse and gerbil, in which all strata were moderately labeled except the stratum pyramidale. The dentate gyrus presented a variety of patterns with the stratum moleculare being low to moderate and relatively homogeneously stained in the mouse, rat, guinea pig, and rabbit. In the cat and gerbil, a dense band of enzymatic activity was seen in the outer one‐third (i.e., distal to the stratum granulosum) of the stratum moleculare near the obliterated hippocampal fissure. The gerbil exhibited an additional dense band in the inner third of the stratum moleculare of the dentate gyrus. The hamster, in constrast to the other species, showed dense 5′‐nucleotidase activity throughout the stratum moleculare. It thus is apparent that no simple correlation between the distributions of 5′‐nucleotidase and the ³H‐CHA binding site exists. These findings are discussed in terms qf the roles of adenosine receptors and related enzymes in controlling purine modulation of central nervous system activity.