Ultrastructural localization of neuropeptide Y-like immunoreactivity in the rat hippocampal formation.

Neuropeptide Y (NPY) has been implicated in the modulation of hippocampal neuronal activity and in the pathophysiology of several neurological disorders involving the hippocampal formation. Thus, this study examines the light and electron microscopic immunoperoxidase labeling of a rabbit polyclonal antibody against porcine NPY in single sections through each lamina of the CA1 and CA3 regions of the hippocampus and the dentate gyrus (DG) of normal adult rats. By light microscopy, the majority of perikarya with intense NPY-like immunoreactivity (NPYLI) were located in stratum oriens of CA1 and CA3 of the hippocampus and in the hilus of the DG. Fine varicose processes with NPY-LI were found in all layers of the hippocampal formation, but were densest in the outer third of the molecular layer of the DG. The density of NPY-labeling was greater in the ventral portion of the hippocampal formation. By electron microscopy, most NPY-containing perikarya in all three hippocampal regions were: small (8–12 μm) or medium-sized (12–18 μm) and elongated; or medium-sized and round. A dense accumulation of NPY-LI was commonly observed within the individual saccules of Golgi complexes and some rough endoplasmic reticulum in the cytoplasm. Perikarya and dendrites with NPY-LI usually were directly apposed to other neuronal processes (mostly terminals) and lacked astrocytic appositions. The majority of terminals in contact with NPY immunoreactive neurons were unlabeled and synapsed with the shafts of large and small dendrites. In CA1 and CA3 of the hippocampus, the types of synapses formed by the unlabeled terminals were not significantly different; however, more asymmetric synapses than symmetric synapses were formed by the unlabeled terminals on the shafts of small NPY-labeled dendrites in the DG. The terminals with NPY-LI (0.25–1.2 μm) contained many small, clear vesicles and 0–2 large, dense-core vesicles. The types of synapses (i.e., asymmetric and symmetric) and distribution of NPY-labeled terminals on the targets were remarkably similar in each lamina of the hippocampal subregions. The NPY-labeled terminals usually synapsed with one unlabeled perikaryon or dendrite. However, others synapsed either (1) with two unlabeled perikarya or dendrites simultaneously or (2) with one NPY-containing perikaryon or dendrite. Most of the terminals with NPY-LI formed symmetric junctions with the shafts of small (distal) dendrites. The remaining NPY-labeled terminals either (1) formed appositions that lacked a membrane specialization, but were without apparent glial intervention in the plane of section analyzed, with NPY-labeled and unlabeled perikarya and dendrites; or (2) were closely apposed without glial intervention to unlabeled and NPY-labeled terminals. The dense input from terminals forming asymmetric (excitatory) junctions and the limited glial coverage may contribute to the vulnerability of NPY-immunoreactive neurons in disorders such as Alzheimer's disease or epilepsy. Additionally, these results provide cellular substrates in the rat hippocampal formation for (1) the direct synaptic input of NPY-labeled terminals to non-NPY-containing hippocampal neurons; and (2) presynaptic interactions between NPY-containing terminals and terminals and terminals containing other transmitters.