Developmental Changes of Cerebral Cortical [3H]Clonidine Binding in Rats: Influences of Guanine Nucleotide and Cations

Abstract

Cerebral cortical [3H]clonidine binding and influences of GTP and cations were investigated in developing rats. Scatchard plots indicated the presence of 2 populations of binding sites [high-affinity binding (Kd = 0.59 nM) and low-affinity binding (Kd = 7.12 nM)] in 70 day old rats but only high-affinity binding (Kd = 0.27 nM) on day 1. Low-affinity binding was detectable on day 7. Kd values in high- and low-affinity binding were not significantly changed during development after 7 days. Bmax of high-affinity binding reached a peak on day 15 and the value of low-affinity binding gradually increased with age. The addition of 10 .mu.M GTP caused a significant reduction in Bmax of high-affinity binding after day 7. Neither Kd nor Bmax of low-affinity binding was affected by 10 .mu.M GTP during development. NaCl (10 and 100 mM) diminished the binding on days 7 and 70. MnCl2 (0.1 and 1.0 mM) markedly increased the binding on days 15 and 70, but not on day 7. Single binding sites of .alpha.2-adrenoceptors with higher affinity seem to be present on day 1. Low-affinity binding appears on day 7 and the number of high-affinity binding sites reaches a peak on day 15, followed by changes in populations of high-affinity as well as low-affinity sites without changing affinity. The regulatory mechanism in .alpha.2-receptors by guanine nucleotide reaches functional maturity between days 1 and 7, and the involvement of Na+ and Mn2+ in .alpha.2-receptor binding becomes functional by days 7 and 15, respectively.