ZINC-POSITIVE AFFERENTS TO THE RAT SEPTUM ORIGINATE FROM DISTINCT SUBPOPULATIONS OF ZINC-CONTAINING NEURONS IN THE HIPPOCAMPAL AREAS AND LAYERS - A COMBINED FLUOROGOLD TRACING AND HISTOCHEMICAL-STUDY

The purpose of the present study was to examine whether zinc-positive and zinc-negative hippocampal neurons in rats differed with respect to their projections to the septum. By combining retrograde axonal trans port of the fluorescent tracer Fluoro-Gold with histochemical demonstr ation of zinc selenide complexes in zinc-containing neurons after intr aperitoneal injection of sodium selenite, we were able to visualize th e distribution of retrogradely Fluoro-Gold labeled neurons and zinc-co ntaining neurons in the same sections. After unilateral injection of F luoro-Gold into the rat septum a few retrogradely labeled cells were o bserved in layer IV of the ipsilateral medial entorhinal area, and num erouS labeled cells were observed mainly in the superficial layers of the ipsilateral subicular areas and throughout the CA1 and CA3 pyramid al cell layers, as well as in the contralateral CA3 pyramidal cell lay er. Zinc-containing neurons were observed in layers IV-VI of the media l entorhinal area, layers II and III of the parasubiculum, layers II, III and V of presubiculum, and in the superficial CA1 and deep CA3 pyr amidal cell layers. Cells double-labeled with Fluoro-Gold and zinc sel enide complexes were primarily located in distal (relative to the area dentata) parts of the superficial CA1 pyramidal cell layer and distal parts of the deep CA3 pyramidal cell layer and in layers II and III o f presubiculum. Only a very few double-labeled cells were seen in the contralateral CA3. The result demonstrates that the hippocampo-septal projection of rats is a mixture of zinc-positive and zinc-negative fib ers. Whereas zinc-negative fibers originate from neurons throughout th e hippocampal and retrohippocampal areas, zinc-positive fibers origina te from distinct subgroups of zinc-containing cells in different areas and layers.