Effects of LTP-inducing tetanic stimulations of the perforant path on the commissural inhibition and facilitation of dentate granule cell discharge

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Effects of diazepam (1 mg/kg, i.p.) on the commissural influences on granule cells were first examined to further assess its GABAergic inhibitory mechanism. Whereas the commissural inhibition at an interval of 5–8 ms of the contralateral dentate-perforant path (CP) combined stimulus was not enhanced by diazepam, the commissural facilitation at a CP interval of 11–20 ms was reduced, suggesting that the GABAergic inhibition is involved in rather a part of the commissural facilitation at a CP interval of 11–20 msec than a part of the ‘commissural’ inhibition at a shorter CP interval. Based on the results of diazepam, effects of high-frequency stimulations of the perforant path on the commissural inhibition of dentate granule cells were then examined, in relation to their effects on the dentate paired-pulse depression. These stimulations produced the long-term potentiation of the perforant path-dentate excitatory synapse and significant reduction of the paired-pulse depression. The commissural inhibition at a CP interval of 5–8 ms remained unchanged following tetanization. The commissural facilitation at a CP interval of 11–20 ms was, however, slightly enhanced by tetanic stimulations and a statistical significance was obtained at a CP interval of 19 ms. These results imply that tetanic stimulations of the perforant path selectively reduce the GABAergic component of the commissural inhibition, as well as that of the paired-pulse depression.