Abstracts

Rationale: The paired-pulse technique has been widely used to assess GABAergic inhibition. This technique measures the amplitude of the population spike in response to the second stimulus relative to the first one, with the rationale being that the degree of suppression (i.e., smaller amplitude) to the second stimulus is a measure of GABAergic inhibition. Thus, in epilepsy research, a change in paired-pulse suppression has been used as evidence for changes in GABAergic inhibition. The present study challenges the paired-pulse technique, and examines more closely the relationship between paired-pulse suppression and GABAergic inhibition. Methods: Field-potential and whole-cell recordings were obtained from dentate granule cells in hippocampal slices. Paired-pulse stimulation was delivered to the perforant path (20-ms interval). Stimulus intensity was varied to examine the intensity-response relationship. Paired stimuli were given every 20 sec. In some experiments, a low dose of GABAzine (0.1-0.3 μM) was applied to partially block GABAA receptor-mediated inhibition. Results: The preliminary data show that: (1) The amount of paired-pulse suppression largely depended on stimulus intensity. At low intensity, paired-pulse stimulation generally yielded facilitation (2nd population spike larger than the 1st), and high-intensity paired-pulses often led to suppression (2nd pulse smaller than 1st), (2) Some slices with intact GABAergic inhibition only exhibited paired-pulse facilitation, regardless of stimulus intensity (i.e., would have been interpreted as no paired-pulse “inhibition”), (3) In slices with paired-pulse suppression, partial pharmacological reduction in GABAA receptor-mediated inhibition (i.e., by 0.3 μM GABAzine) actually resulted in increased paired-pulse suppression, and (4) In slices with paired-pulse facilitation, partial blockade of GABAA receptors (in 0.1 μM GABAzine) converted paired-pulse facilitation into paired-pulse suppression (i.e., reduced GABAergic inhibition led to increased paired-pulse suppression). Conclusions: These preliminary results suggest that several technical and physiological factors affect the results of experiments using the paired-pulse suppression technique. Paired-pulse suppression is often increased after GABAA receptors are partially blocked; therefore, the data with this technique often has no relation to GABAergic inhibition.