Localization and functional characterization of the nitric oxide and carbon monoxide signaling pathways in the postnatal rat hippocampal formation
Sanchez, Eva Shirley
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In the rat hippocampus, nitric oxide (NO) and carbon monoxide (CO) both function as signaling molecules involved in long-term plasticity. In neurons, NO generated by the enzyme neuronal nitric oxide synthase (nNOS) and CO generated by hemeoxygenase-2 (HO-2), can activate soluble guanylyl cyclase (sGC) to produce cyclicguanosine-monophosphate (cGMP). There has not been a complete functional localization of these signaling pathways in the sub-regions and laminae of the hippocampal formation. In this work, transverse slices of ventral hippocampus (200-250 um thick) from P17-P21 Long Evans rats were used to characterize and localize the NO and CO signaling pathways in the hippocampus. Immunohistochemistry was used to characterize and localize cells containing nNOS, H0-2 and cGMP. Additionally, the NO sensitive fluorescent dye, DAF-FM, was used to localize the production of NO in response to stimulation with NMDA or AMPA. nNOS was immunolocalized in twenty-four distinct neuron types, in varicose and smooth processes, and in axons of the alveus fiber pathway. H0-2 was found in all granule cells, hilar cells and pyramidal cells. Sparse principal cells had both nNOS and H0-2, and sparse non-principal cells had H0-2 and cGMP, but no cells had both nNOS and cGMP. In response to stimulation with a NO-donor, cGMP was found in the following: dendrites and somata of three distinct basket cell types in the dentate gyrus; three distinct interneurons; axonal fibers in the perforant pathway and alveus; and in astrocytes in CA1. Simultaneous stimulation with both NO and CO produced a region specific synergistic increase in cGMP production. NMDA or AMPA stimulated cGMP was localized in the following: varicose processes in the neuropil; in varicose processes surrounding· pyramidal cells in CAl and CA3; in astrocytes in CAl; and in smooth long range processes in CA1 and the alveus. In response to stimulation with NMDA or AMPA NO-induced fluorescence was found in the principal cell layer, alveus, apical dendrites of pyramidal cells in CA1, and in occasional granule-cell somata. The results presented here indicate that NO-cGMP functions in a wide variety of hippocampal synaptic circuits and that CO interacts with NO in the modulation of cGMP levels.
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