The main excitatory pathway of the hippocampal formation is controlled by a network of morphologically distinct populations of GABAergic interneurons. Here we describe a novel type of GABAergic interneuron located in the outer molecular layer (OML) of the rat dentate gyrus with a long-range forward projection from the dentate gyrus to the subiculum across the hippocampal fissure. OML interneurons were recorded in hippocampal slices by using the whole-cell patch-clamp configuration. During recording, cells were filled with biocytin for subsequent light and electron microscopic analysis. Neurons projecting to the subiculum were distributed throughout the entire OML. They had round or ovoid somata and a multipolar dendritic morphology. Two axonal domains could be distinguished: an extensive, tangential distribution within the OML and a long-range vertical and tangential projection to layer 1 and stratum pyramidale of the subiculum. Symmetric synaptic contacts were established by these interneurons on dendritic shafts in the OML and subiculum. OML interneurons were characterized physiologically by short action potential duration and marked afterhyperpolarization that followed the spike. On sustained current injection, they generated high-frequency (up to 130 Hz, 34°C) trains of action potentials with only little adaptation. In situ hybridization and single-cell RT-PCR analysis for GAD67 mRNA confirmed the GABAergic nature of OML interneurons. GABAergic interneurons in the OML projecting to the subiculum connect the input and output regions of the hippocampus. Hence, they could mediate long-range feedforward inhibition and may participate in an oscillating crossregional interneuron network that may synchronize the activity of spatially distributed principal neurons in the dentate gyrus and the subiculum. Key words: GABAergic interneurons; dentate-subicular projection; glutamate decarboxylase; single-cell RT-PCR; feedforward inhibition; dentate gyrus; ratThe neuronal network of the hippocampus consists of glutamatergic principal neurons (granule cells and pyramidal neurons) and GABAergic interneurons (Amaral, 1978;Buhl et al., 1994; Schwartzkroin, 1995a,b) (for review, see Freund and. Although interneurons numerically represent only ϳ10% of the neuronal population, they regulate the activity of the entire network. GABAergic interneurons mediate feedback or feed-forward inhibition by local synaptic interactions with principal neurons and thereby control their activity (Andersen et al., 1963;Buzsáki, 1984). In addition, GABAergic interneurons form a network by mutual synaptic interactions. This interneuron network is thought to be involved in the generation of oscillatory activity and may provide the clock signal for temporal encoding of information in principal neurons (Soltész and Deschênes, 1993;Bragin et al., 1995;Buzsáki and Chrobak, 1995;Cobb et al., 1995;Whittington et al., 1995;Jefferys et al., 1996).The axons of GABAergic interneurons innervate specific regions of their postsynaptic target cells (Somogyi, 1977...

Cajal-Retzius (CR) cells are characteristic horizontally orientated, early-generated transient neurons in the marginal zones of the neocortex and hippocampus that synthesize the extracellular matrix protein reelin. They have been implicated in the pathfinding of entorhino-hippocampal axons, but their role in this process remained unclear. Here we have studied the axonal projection of hippocampal CR cells. Following injection of the carbocyanine dye DiI into the entorhinal cortex of aldehyde-fixed rat embryos and young postnatal rats, neurons in the outer molecular layer of the dentate gyrus and stratum lacunosum-moleculare of the hippocampus proper with morphological characteristics of CR cells were retrogradely labelled. In a time course analysis, the first retrogradely labelled CR cells were observed on embryonic day 17. This projection of hippocampal CR cells to the entorhinal cortex was confirmed by retrograde tracing with Fast Blue in new-born rats and by intracellular biocytin filling of CR cells in acute slices from young postnatal rat hippocampus/entorhinal cortex and in entorhino-hippocampal slice cocultures using infrared videomicroscopy in combination with the patch-clamp technique. In double-labelling experiments CR cells were identified by their immunocytochemical staining for reelin or calretinin, and their interaction with entorhino-hippocampal axons labelled by anterograde tracers was analysed. Future studies need to investigate whether this early transient projection of hippocampal CR cells to the entorhinal cortex is used as a template by the entorhinal axons growing to their target layers in the hippocampus.

The entorhinal cortex gives rise to a massive projection to the hippocampus and fascia dentata. In the rat, this projection forms early in development with first entorhinal axons reaching the hippocampus around embryonic day (E) 17. From the very beginning, the entorhinal axons recognize their appropriate termination zones in the hippocampus proper and fascia dentata, i.e., stratum lacunosum-moleculare and the outer molecular layer of the dentate. This is remarkable, because at the time of entorhinal fiber ingrowth, the definitive target cells of entorhinal axons, pyramidal cells and granule cells, are not yet fully developed, and the majority of their distal dendritic tips have not yet reached these layers. This raises the question as to the cellular and molecular signals guiding the entorhinal axons to and keeping them in their target layers. Here we hypothesize that early generated Cajal-Retzius (CR) cells located in stratum lacunosum-moleculare and the outer molecular layer of the dentate, and in particular their axons projecting to the entorhinal cortex, provide a template that is used by the entorhinal axons to find their target layers in the hippocampus.