MODULATION OF [S-35] TBPS BINDING BY LIGANDS WITH PREFERENTIAL AFFINITY FOR BENZODIAZEPINE BZ(1) SITES IN THE CEREBRAL-CORTEX OF NEWBORN AND ADULT-RATS

The present study was designed to compare the allosteric coupling betw een the Cl- channel of the GABA, receptor and the different benzodiaze pine recognition site subtypes (BZ sites) in the cerebral cortex of ne wborn (5-day-old) and adult rats (90-day-old). To this aim, we reexami ned the heterogeneity of cortical GABA(A) receptors in self- and cross -competition binding experiments using [H-3]flunitrazepam and two liga nds with higher affinity for benzodiazepine BZ(1) sites relative to be nzodiazepine BZ(2) sites, the triazolopyridazine ethyl-6-[3-(trifluoro methyl)phenyl]-1,2,4-triazolo [4,3-b] pyridazine (CL 218,872) and the imidazopyridine -methylphenyl)-imidazo[1,2-a]-pyridine-3-acetamide hem itartrate (zolpidem). Benzodiazepine BZ(1) sites accounted for 52% of the total number of binding sites in adult rats, but were not detected in newborn rats. On the other hand, two classes of benzodiazepine BZ( 2) sites with high and low affinity for zolpidem were present in newbo rn and adult rats. These sites were designated as benzodiazepine BZ(2H ) (high affinity for zolpidem, K-d similar to 150 nM) and benzodiazepi ne BZ(2L) (low affinity for zolpidem, K-d similar to 3000 nM). High de nsities of benzodiazepine BZ(2H) sites were measured in both newborn a nd adult rats (75% and 41% of the total number of [H-3]flunitrazepam b inding sites, respectively), whereas benzodiazepine BZ(2L) sites accou nted for 25% and 7% of the total number of cortical sites in neonates and adults, respectively. Flunitrazepam, CL 218,872 and zolpidem inhib ited in a concentration-dependent manner the binding of [S-35]t-butylb icyclophosphorothionate ([S-35]TBPS) to the convulsant site of cortica l GABA, receptors in newborn and adult rats. The IC50 for flunitrazepa m was about 3-fold greater in adults than in neonates. This rightward shift in the concentration-response curve may be due to a decrease wit h age in the intrinsic efficacy of flunitrazepam. In contrast, CL 218, 872 and zolpidem were 4-fold more potent at inhibiting [S-35]TBPS bind ing in adult rats relative to neonates. The different affinities of CL 218,872 and zolpidem for benzodiazepine BZ(1) and BZ(2) receptors may account, at least in part, for the age-related changes in their inhib itory potencies. These results demonstrate that benzodiazepine BZ(2) s ites mediate the modulation of [S-35]TBPS binding by benzodiazepine re cognition site ligands in the cerebral cortex of newborn rats. Further , benzodiazepine BZ(2) sites may be involved in the inhibition of [S-3 5]TBPS binding by flunitrazepam, CL 218,872 and zolpidem in the cerebr al cortex of adult rats.