High level of adenosine A1 receptor-like immunoreactivity in the CA2/CA3a region of the adult rat hippocampus

We describe the immunocytochemical distribution of adenosine Al receptors i n the rat hippocampus. Adenosine Al receptor-like immunoreactivity was seen on the cell soma and dendrites of pyramidal cells and the cell soma and pr oximal part of dendrites of granule cells, but not on glial cells. Developm entally, adenosine Al receptor-like immunoreactivity was diffuse on postnat al day 7 and increased in intensity in individual cells by day 21. In the C A2/CA3a region, the adult pattern of Al receptor distribution was establish ed by day 28. In the adult rat hippocampus, rostrocaudal inspection reveale d that immunoreactivity in CA2/CA3a was greatest. Confocal microscopy revea led differences in the staining patterns for the adenosine Al receptor and synaptophysin, a marker of presynaptic terminals. This result suggests that the adenosine Al receptor might have postsynaptic physiological functions. Double-labeling of adenosine Al receptors and antero,anteradely-labeled fi bers from the supramammillary nucleus showed that the fibers from the supra mammillary nucleus terminate directly on the cell soma of the Al receptor-i mmunopositive neurons in CA2/CA3a and the dentate gyrus. These results indicate that the adenosine Al receptor in CA2/CA3a and the d entate gyrus are in a position to regulate hippocampal theta activity and t hat resultant strong synaptic depression in CA2/CA3a could play a role in r egulating the intrinsic signal how between CA3 and CA1. (C) 1999 IBRO. Publ ished by Elsevier Science Ltd.