Differences in paired-pulse inhibition and facilitation in the dentate gyrus and CA3 field between dorsal and ventral rat hippocampus.

We studied the processes of inhibition and facilitation in the dentate gyrus (DG) and the CA3 field by examining the effects of paired-pulse stimulation on the evoked population spike (PS) in dorsal (DH) and ventral (VH) hippocampal slices from the adult rat. The antidromic-orthodromic (A-O) and the orthodromic-orthodromic (O-O) paired-pulse stimulation protocols were used at varying inter-pulse intervals (IPI). In the DG, the A-O stimulation produced an early depression of PS lasting 30-40ms which was significantly stronger in the VH compared with DH. The O-O stimulation produced a biphasic pattern of effects, in both dorsal and ventral DG, consisting of an early depression of PS followed by facilitation at relatively longer intervals. In the DH but not the VH the phase of facilitation was followed by a late depression of PS (>200ms). In the CA3 field both A-O and O-O stimulation had a biphasic effect consisting of an early phase of strong depression of similar strength in DH and VH. The depression was followed by a phase of facilitation which was more pronounced with O-O stimulation. The facilitation observed with the O-O stimulation was much stronger in DH than VH and in DH only it was significantly reduced by the antagonist of GABAB receptors CGP52432. Furthermore, the facilitation was insensitive to changes in [Ca(2+)]o in both hippocampal poles. These findings suggest that the dorsal compared with ventral DG is more amenable to fast-frequency input but filters out slow-frequency inputs more reliably while the gating and amplification of the excitatory input in the CA3 circuitry is more prominent in DH than in VH.