Abstracts

Rationale: The paired-pulse technique has been widely used to study synaptic inhibition in the hippocampus, particularly in epilepsy research. Nearly all of the physiological evidence in support of increased inhibition (i.e. the “hyperinhibition” hypothesis) following epilepsy-associated synaptic reorganization in the hippocampus (e.g., mossy fiber sprouting) derives from this technique. We previously showed that measures of “inhibition” using paired-pulse suppression, particularly with repetitive paired-pulse stimulation, greatly depend on the stimulus parameters and can provide erroneous data on GABA-mediated inhibition (Waldbaum and Dudek, 2005, Soc. Neurosci. Abstr. 276.12). The present study further investigated the influence of stimulus parameters and the effects of relatively small changes in membrane excitability and of GABAA-receptor mediated inhibition on paired-pulse suppression.Methods: Paired-pulse suppression was analyzed as a function of perforant-path stimulus intensity and interpulse interval with field-potential recordings from the granule cell layer in hippocampal slices.Results: Increasing [Ca++]o from 1.3 to 2.5 mM, which is known to raise action potential threshold, reduced paired-pulse suppression. Raising [K+]o from 3.0 to 6.0 mM and lowering [Ca++]o from 1.3 to 1.0 mM, which elevated membrane excitability, significantly increased paired-pulse suppression. Under physiological ionic conditions, increased levels of paired-pulse suppression were also measured during bath application of the GABAA-receptor antagonist SR-95531 (Gabazine, 0.3 µM), which reduced GABAA-receptor mediated inhibition.Conclusions: Modest increases in membrane excitability and small reductions in GABAA-receptor mediated inhibition lead to an increase in paired-pulse suppression. One likely mechanism for these effects is that the treatments enhance the amplitude of the response to the conditioning stimulus, and thus are presumably similar to an increase in stimulus intensity of the conditioning pulse. These data indicate that the paired-pulse technique erroneously measures increased levels of “inhibition” when excitability is modestly increased or when GABAA-receptor mediated inhibition is slightly decreased. Thus, measurements of increased suppression using the paired-pulse technique have high potential to be an artifact, because this can occur when excitability is increased and/or when GABA-mediated inhibition is decreased. These results challenge the physiological evidence supporting the “hyperinhibition” hypothesis after epilepsy-associated synaptic reorganization in the hippocampus. (Supported by NS45465 (SW) and NS16883 (FED)).