Ontogenetic appearance of the adenosine receptor precedes N-protein coupling in rat forebrain

The ontogenetic development of rat forebrain adenosine receptors labelled by [3H]cyclohexyladenosine [( 3H]CHA) and to the corresponding susceptibility of such [3H]CHA binding to the non-hydrolysable GTP analogue, GppNHp is reported. The present studies reveal that: (1) in neonatal forebrain, [3H]CHA binding can be detected but there is little or no GppNHp-induced reduction in such binding; (2) susceptibility of [3H]CHA binding to GppNHp develops slowly with maximum (adult) levels of inhibition not being observed until approximately 16 days postpartum; (3) changing susceptibility of adenosine receptors to GppNHp is due to the maximal effect of GppNHp increasing with time. The potency of GppNHp remains constant at around 6.3 X 10(-7)M over this period of change; (4) Scatchard analysis of [3H]CHA binding to 30-day forebrain membranes reveals the presence of two binding sites--a high-affinity, low-capacity site and a low-affinity, high-capacity site. In the presence of 10(-4)M GppNHp, only a low affinity, high capacity site is detected; (5) Scatchard analysis of [3H]CHA binding to 6-day forebrain membranes (where GppNHp has no effect) reveals the presence of only a single low-affinity, high-capacity binding site. The data strongly suggests that in very young neonates adenosine receptors can be detected but that many detected binding sites are not functionally linked to associated N-proteins.