Synaptic connections of neuropeptide Y (NPY) immunoreactive neurons in the hilar area of the rat hippocampus

Synaptic connections and fine structural characteristics of neuropeptide Y-immunoreactive (NPY-i) neurons in the fascia dentata were studied using an antiserum against NPY. Normal and colchicine pretreated rats were examined to study the synaptic connections of NPY-i neurons in the normal fascia dentata. The perforant pathway and fimbria fornix were transected to label afferent fibers to NPY-positive cells. Horseradish peroxidase conjugated with wheat germ agglutinin (HRP-WGA) was injected into the contralateral hippocampus to study commissural projections of hippocampal NPY-i neurons, and to search for NPY-i synaptic contacts on immunonegative commissural cells. Since earlier reports have shown that at least half of the NPY-i neurons also contain somatostatin (SS), the distribution of NPY-i neurons in the hilar area was determined and compared with that of SS-i neurons. Four types of dentate NPY-i neurons were distinguished: Type 1: large multipolar cells in the deep hilus (9%). Type 2: medium-sized multipolar and fusiform hilar neurons with dendrites occasionally reaching the outer molecular layer (64%). Type 3: pyramidal shaped cells in the granule cell layer with long apical dendrites reaching the outer molecular layer (20%). Type 4: small multipolar NPY-i cells located in the molecular layer (7%). Our results indicate two overlapping but not identical cell populations of NPY-i and SS-i neurons. Light and electron microscopic analysis of the normal fascia dentata demonstrated that the majority of NPY-i terminals are located in the outer molecular layer of the dentate gyrus, where they establish symmetric synaptic contacts on dendritic shafts and occasionally on spines of granule cells. A moderate number of NPY-i synapses were also found on dendrites in the inner molecular layer and on the cell body of granule cells. Numerous symmetric NPY-i synapses were found on dendrites and somata of neurons in the hilar area. Some NPY-i dendrites in the hilar area received mossy axon collateral input. After transection of the perforant pathway degenerated axon terminals could be found in synaptic contact with NPY-i dendrites in the outer molecular layer. Commissurotomy revealed direct commissural input to NPY-i dendrites in the inner molecular layer and in the hilus. After injection of HRP-WGA into the contralateral hippocampus 2% of hilar NPY-i neurons were retrogradely labeled and symmetric NPY-i synapses were found on the cell bodies and dendrites of unstained HRP-WGA labeled neurons.(ABSTRACT TRUNCATED AT 400 WORDS)