DENSITY OF ALPHA-2A ADRENOCEPTORS AND G(I) PROTEINS IN THE HUMAN BRAIN - RATIO OF HIGH-AFFINITY AGONIST SITES TO ANTAGONIST SITES AND EFFECT OF AGE

In the postmortem human brain, the specific binding of the agonists [H -3]clonidine and [H-3]bromoxidine ([H-3]UK14304) and of the antagonist [H-3]2-methoxyidazoxan (RX821002) and inhibition by l-epinephrine of [H-3]RX821002 binding were determined in parallel to quantify the frac tions of high (R(H))- and low-affinity (R(L)) agonist sites of alpha-2 A adrenoceptors and to investigate the association of these inhibitory receptors with various regulatory Gi proteins and the effect of aging . In the frontal cortex, the density of R(H) with [H-3]clonidine was n ot different from that with [H-3]UK14304 and the ratio of B-max values in the same subjects was 0.88 +/- 0.05 (n = 39). The inhibition of [H -3] RX821002 binding by l-epinephrine allowed the resolution and quant itation of the R(H) and R(L) with similar values to those obtained in saturation experiments. The simultaneous analysis and computer modelin g of competition and saturation experiments performed in the same subj ects indicated that a high proportion of alpha-2A adrenoceptors exist in the R(H) conformation (70-80%). The density of alpha-2A adrenocepto rs (B-max for [H-3]agonist and [H-3]antagonist binding) and age (range , 4-89 years) were negatively correlated (r = -.60, n = 39, P <.001). However, only the density of R(H) declined with aging (r = -.68, n = 3 8, P <.001); that of R(L) did not appear to be age related (r = -.26, n = 37, P >.05). Similarly, there was a negative correlation between t he immunoreactivity of G(i) proteins and the age at death for each of the various G(alpha l1/2) (r = -.84, P <.005), G(alpha i2) (r = -.67, P <.05) and G(alpha i3) (r = - 80, P <.005) subunits. In the same subj ects, the density of alpha-2A adrenoceptors and the abundance of the v arious Gi proteins showed positive correlations (e.g., B-max for [H-3] UK14304 versus G(alpha l1/2), r =.88, P <.001). However, the immunorea ctivities of the various G(alpha l1/2/3) subunits not only correlated with the density of R(H) but also with that of R(L). The results indic ate that, in the human brain, a high proportion (70-80%) of alpha-2A a drenoceptors exist in the R(H) conformation and that only this functio nally relevant fraction of receptors declines with age in parallel wit h the abundance of regulatory G(i) proteins.