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

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Abstract
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. This study suggests that NPY‐i neurons receive entorhinal and commissural afferents and in turn form contacts with granule cells and commissural neurons in the fascia dentata. Some NPY‐i neurons may be the targets of perforant pathway afferents as well as mossy axon collaterals which may explain their vulnerability to hippocampal seizure activity.

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