The GABA receptor-chloride ion channel protein complex

Preparation of antibodies to beta subunits of gamma-aminobutyric acidA receptors

Antisera were produced in rabbits against synthetic peptides based on two regions of the cDNA sequence of the beta 1 subunit of bovine gamma-aminobutyric acidA (GABAA) receptors. The deduced amino acid sequences were similar in other beta subunits of bovine, rat, and chick receptors, predicting cross-reactability with all beta subunits. One antiserum (anti-beta e) was raised against an extracellular moiety near the invariant disulfide loop thought to be located near the neurotransmitter binding domain; the other (anti-beta c) was raised against an intracellular moiety containing a consensus sequence for cyclic AMP-dependent protein kinase phosphorylation of a serine residue. Predicted secondary structures suggested high potential immunogenicity for the chosen antigen peptides. Both antisera at high dilutions recognized the same polypeptide bands on western blots of GABAA receptors purified from three regions of bovine brain (four bands at 57, 54, 53, and 52 kDa in cerebral cortex) but fewer bands (57, 54, and 52 kDa) in hippocampus and cerebellum (one major band at 54 kDa, traces at 57 and 53 kDa). This is consistent with the presence of multiple beta subunits whose expression varies with brain region, as shown by molecular cloning. The anti-beta c antibody was able to immunoprecipitate purified GABAA receptor [3H]-muscimol binding, 87% in bovine cortex and 75% in total rat brain; the anti-beta e was unable to immunoprecipitate any antigen. These antibodies indicate a region-dependent heterogeneity of beta subunits and should be useful for analyzing structure, function, and localization of GABAA receptor subtypes in brain.

Brain region-dependent sensitivity of GABAA receptor-mediated responses to modulation by ethanol

Simultaneous extracellular and intracellular electrophysiological recordings were made from the CA1 region of rat hippocampal brain slices during superfusion with ethanol. Ethanol (80 mM) had a biphasic effect on the extracellularly recorded population spike, with an initial increase followed by a significant reduction (38%) in this response, which was maximal 10 to 15 min after the start of ethanol application. Concurrent intracellular recordings in the CA1 showed a small (0.7 mV) hyperpolarization of the resting membrane potential, with no significant change in the input impedance, EPSP, GABAA and GABAB IPSPs, or after hyperpolarization (AHP) following depolarizing current injection. Ethanol reduced the amplitude and duration of depolarizing responses to brief, localized pressure-ejection of N-methyl-D-aspartate (NMDA) onto pyramidal neuron dendrites, but did not affect the GABAA receptor-mediated depolarizing responses to the dendritic application of GABA. In parallel studies, the effect of ethanol on GABA-stimulated 36Cl- flux was measured in microsac preparations from rat hippocampus, cerebellum, and cerebral cortex. Ethanol application caused substantial enhancement of the chloride uptake from cerebellar and cerebral cortical microsacs, but had no effect on 36Cl- influx in hippocampal microsacs. These results suggest that there are important brain region-dependent differences in the sensitivity of the GABAA receptor/chloride channel to modulation by ethanol.

Immunological identification of multiple alpha-like subunits of the gamma-aminobutyric acidA receptor complex purified from neonatal rat cortex

Antibodies were prepared against a synthetic peptide corresponding to amino acid sequences 174-203 of the bovine gamma-aminobutyric acidA (GABAA) receptor alpha 1-subunit. The antibodies recognized this synthetic alpha 1-peptide, but failed to react with the homologous peptide sequence, 170-199, of the bovine beta 1-subunit. On Western blots, anti-alpha 1-subunit antibody recognized a 50-kilodalton (kDa) protein in affinity-purified receptor preparations from adult rat cortex and cerebellum. In receptor purified from neonatal cortex, the anti-alpha 1-antibody reacted with 50-kDa, 53-54-kDa, and 59-kDa proteins. After digestion with endoglycosidase F, these three protein bands retained differing electrophoretic mobilities. The 50-kDa and 59-kDa subunits of affinity-purified neonatal receptor, which were photoaffinity-labeled with [3H]flunitrazepam, were immunoprecipitated to different extents by alpha-subunit antibody. These data suggest the existence in GABAA receptor from neonatal cortex of three proteins (50 kDa, 53 kDa, and 59 kDa) which have immunological homology to alpha 1-subunit of bovine GABAA receptor. The presence of an alpha- and a beta-like subunit with similar mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis may account for the relatively high concentration of protein in the 53-54-kDa band which has been observed in receptor purified from neonatal cortex. The presence of multiple alpha-like subunits may be related to the presence of a relatively high concentration of type II GABA receptor in this tissue.