GABA RELEASE IN RAT CORTICAL SLICES IS UNABLE TO COPE WITH DEMAND IF THE AUTORECEPTOR IS BLOCKED

Electrically stimulated release of neurotransmitters in brain slices n ormally displays frequency dependence because of progressive activatio n of autoreceptors by endogenously released transmitter, which is abol ished by blockade of autoreceptors. In consequence, the maximal increa se caused by an autoreceptor antagonist in percent of the correspondin g controls should be greater at higher than at lower frequencies. In t he case of gamma-aminobutyric acid (GABA), we have previously found a marked deviation from this expectation. Among several explanations env isaged, computer simulation suggested only one to be compatible with t he experimental data: the release mechanism may not be able to cope wi th high demand. This hypothesis was tested by investigating the freque ncy dependence of the release of H-3-GABA in the presence and absence of a high concentration of the potent GABA(B) antagonist, CGP 55845, u sing extremely short stimulation periods. To this end, slices were sti mulated with groups of 4 POPs (a POP - pseudo-one-pulse - consists of 4 pulses delivered at 100 Hz). The intervals between the POPs within a group were varied from 60-0.5 s, corresponding to frequencies within the POP group of 0.0167-2 Hz. Under such circumstances, the theoretica lly expected pattern was indeed observed: the GABA(B) antagonist aboli shed the frequency dependence. In a second series of experiments, frac tional release per POP was determined when 4-32 POPs were delivered at 2 Hz, with and without CGP 55845. The increase of GABA release elicit ed by the GABA(B) antagonist gradually subsided with increasing number of POPs. It was about 50% of that observed during the first 4 POPs in the 4 subsequent ones, and almost nil in the last 16 of a total of 32 POPs. The results of this study support the hypothesis generated with the help of computer simulation, that release may not be able to keep up with high demand. They further suggest that exhaustion of the rele asable transmitter pool occurs surprisingly fast. It is not known whet her this phenomenon is physiologically relevant or a consequence of me tabolic stress to which the slices are subjected during preparation an d superfusion, or of their exposure to transaminase and uptake inhibit ors, but it provides an explanation for the anomalies observed in the studies of the effects of GABA(B) antagonists on H-3-GABA release.