The development of type I and type II benzodiazepine receptors in the mouse cortex and cerebellum

The postnatal development of benzodiazepine (BDZ) receptors was monitored in Heterogeneous Stock (HS) mice, and the BDZ receptors were characterized and categorized into Type I and Type II receptors. When the number of 3H-Flu binding sites (Bmax) was assessed at weekly intervals after the birth of the animal, the number of sites in both the cortex and cerebellum increased significantly if the data was expressed as fmol/mg tissue. On the other hand, no significant change in 3H-Flu binding sites was evidenced in the cortex, and the number of 3H-Flu binding sites in the cerebellum decreased during postnatal development if Bmax values were expressed as fmole/mg protein. When receptor binding data was analyzed for the presence of Type I and Type II BDZ receptors, the changes in KD values for 3H-Flu binding development could be accounted for by changes in relative proportions of Type I and Type II receptors present in the cortex and cerebellum during the maturation process. Type II receptors predominated in both cortex and cerebellum at birth, and Type I receptors proliferated primarily during the first two weeks of postnatal life. In the cortex of adult mice there were approximately equal numbers of Type I and Type II BDZ receptors. In the cerebellum of adult mice, computer assisted analysis of binding data could not distinguish the presence of two distinct BDZ binding sites. However, Hill coefficients and overall binding constants determined from data on CL-218,872 displacement of 3H-Flu binding to cerebellar membranes indicated that cerebellar tissue from adult mice did contain a heterogeneous array of BDZ receptors.