Modification of the apparent molecular weight of different benzodiazepine binding proteins from rat brain membranes by various endoglycosidases

Deduction of amino acid residues in the GABA(A) receptor alpha subunits photoaffinity labeled with the benzodiazepine flunitrazepam

Peptide mapping and microsequencing were used to infer the site of photoaffinity labeling by the gamma-aminobutyric acidA receptor modulator [3H]flunitrazepam. Peptide mapping with and without N-deglycosylation was used to restrict the domain for photoaffinity labeling to residues 74-123 of the bovine alpha1 subunit, in agreement with a previously predicted labeling domain between residues 59-148 based on cyanogen bromide fragmentation. Edman degradation of partially purified photolabeled peptides gave release of 3H counts in the ninth cycle of a tryptic peptide sequence. A second V8/chymotryptic peptide produced an impure sequence with release of 3H counts in the seventh through ninth cycle of sequence. The combined data support those previously reported, i.e., that the primary site for photoaffinity labeling by [3H]flunitrazepam is His102 of the bovine alpha1 subunit. In addition we also detected possible secondary labeling of Pro97.

Differential ontogenesis of type I and II benzodiazepine receptors in mouse cerebellum

The ontogeny of type I and type II benzodiazepine binding sites was studied in mouse cerebellum by displacement of [3H]flunitrazepam binding by zolpidem, a ligand specific for the type I sites. Type I binding sites predominate throughout development and in the adult while type II sites account for 25% of total cerebellar benzodiazepine binding sites at birth and, during development, decrease to 10% or less in the adult. On a per cerebellum basis type II sites increase during the first postnatal week and then remain at a steady level while type I sites increase until adulthood. These results may indicate a specific localization of the type II sites (and of the corresponding alpha-protein subunits in the GABA/benzodiazepine receptor complex) in structures already present at birth and developing during a short early postnatal period. The affinity of zolpidem for its high affinity (type I) binding sites increases during cerebellar ontogeny, this increase possibly indicates an epigenetic (post-translational) 'maturation' process of the corresponding receptor molecule. Hill numbers indicate the existence of an additional binding site heterogeneity greater during development but still present in the adult; probably this is to be related to the simultaneous presence of different 'maturation' stages during development and with a certain variety of the final products.

Distinct GABAA receptor alpha subunit mRNAs show differential patterns of expression in bovine brain

Specific oligonucleotide probes have been used to visualize the regional and cellular distribution of the mRNAs encoding three structurally distinct GABAA receptor alpha subunits in bovine brain. In situ hybridization analysis showed that these transcripts differ in distribution and in relative abundance. In frontal cortex the alpha 1 and alpha 2 transcripts are most abundant in layers II-IV, whereas the alpha 3 mRNA is most abundant in layers V and VI. In the hippocampal complex, the alpha transcripts are differentially distributed in the entorhinal cortex and subiculum. The alpha 2 transcript is enriched in the dentate gyrus and CA4/CA3 regions of the hippocampus. In the cerebellum, essentially only the alpha 1 transcript is detectable in granule cells, Purkinje cells, and stellate/basket cells. These results suggest that the different alpha subunits represent components of distinct GABAA receptor subtypes.

Persistence of species variation and regional heterogeneity of the apparent molecular masses of benzodiazepine-binding proteins after deglycosylation

Brain membrane preparations of different vertebrates were photoaffinity labeled with [3H]flunitrazepam and subsequently deglycosylated with endoglycosidase F and peptide N-glycopeptidase. SDS-polyacrylamide gel electrophoresis followed by fluorography revealed that each benzodiazepine-binding protein is deglycosylated in two steps, indicating that each protein has two glycosylation sites. Species variation of the apparent molecular masses of the benzodiazepine-binding proteins and regional heterogeneity in avians persist after deglycosylation. These results indicate that the alpha-subunit(s) of the GABA/benzodiazepine receptor has undergone electrophoretically detectable changes in its amino acid composition during vertebrate evolution. The existence of at least two different alpha-subunits in avians is further substantiated.