ULTRASTRUCTURAL DISTRIBUTION OF NADPH-DIAPHORASE IN THE NORMAL HIPPOCAMPUS AND AFTER LONG-TERM POTENTIATION

The distribution of the enzyme nitric oxide synthase (NOS) was investi gated at the ultrastructural level in synaptic structures of the hippo campal formation in relation to long-term potentiation (LTP), based on the histochemical NADPH-diaphorase (NADPH-d) staining with the tetraz olium salt BSPT. BSPT-formazan, the osmiophilic reaction product, was found to be selectively distributed and predominantly attached to memb ranes of the endoplasmic reticulum. In synaptic regions mainly the pre synaptic sides showed labeling. Although several groups have demonstra ted a principal involvement of NO in the LTP-mechanism, we found only a low, statistically insignificant increase in NADPH-d stained presyna ptic areas of the dentate gyrus, where LTP was evoked. Postsynaptic el ements also did not show any noticeable differences. Based on the pres ent results, the predominantly presynaptic localization of NOS should be preferably considered in models describing a functional role of NO in LTP formation, despite the fact that we failed to reveal any indica tions for an LTP-related change in synaptically located NADPH-d.