The benzodiazepine receptor: Its development and its modulation by γ-aminobutyric acid

Evidence for the presence of benzodiazepine receptors in brain tissue recently has been described. To assess the physiological significance of these receptors, we have examined the ontogeny of [³H]flunitrazepam binding in homogenates of mouse brain. Specific [³H]flunitrazepam binding appears early in fetal development (11.0 fmol/mg of tissue at 17 days gestation). There is a rapid increase in the neonate: 23% of adult levels in the 1-day neonate, 47% in the 7-day neonate, 58% in the 15-day neonate and adult levels in the 21-day neonate. This development profile for [³H]flunitrazepam binding is similar to the development of the γ-aminobutyric acid (GABA) receptor. In washed membrane preparations, the dissociation constant (KD) of [³H]flunitrazepam binding can be increased (whole homogenate 0.69 nM; twice washed membranes, 1.17 nM). This effect may be due to the removal of endogenous GABA. The direct effects of exogenously applied GABA have been studied at different ages. From fetal to adult ages, GABA increased the affinity of [³H]flunitrazepam binding approximately 2-fold. At the same ages, the GABA antagonist, (+)-bicuculline, decreased the affinity of [³H]flunitrazepam binding. Kinetic analysis of [³H]flunitrazepam binding indicated that the changes in KD could be accounted for by changes in the rate constant of dissociation. We conclude that the benzodiazepine receptor in the mouse brain: develops rapidly in the neonatal period, can be modulated by GABA at all ages, and shows changes in affinity that can be explained by changes in the rate constant of dissociation.