Fine structural studies of growth-hormone-releasing-factor (GRF)-immunoreactive neurons and their synaptic connections in the guinea pig arcuate nucleus

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Abstract
The fine structure of neurons containing human growth‐hormone‐releasing factor (hGRF) immunoreactivity located in the arcuate nucleus of the guinea pig was studied by means of the preembedding immunohistochemical technique. The perikaryon of labeled neurons was fusiform or ovoid; the nucleus was regular in shape and contained a prominent nucleolus. The main ultrastructural features of the hGRF‐immunoreactive neurons were (1) the presence of numerous labeled secretory granules (100–120 nm in diameter) and (2) the abundance and the enlargement, of the organelles involved in the synthesis of the peptides: a well‐developed rough endoplasmic reticulum and a conspicuous Golgi apparatus. Synaptic inputs were observed on immunoreactive perikarya but, above all, on the labeled dendrites. The unstained presynaptic nerve endings most often contained only small clear vesicles and formed symmetrical contacts. In rare cases, the presynaptic terminals exhibited both small clear and large dense vesicles and constituted asymmetrical contacts. Immunoreactive nerve endings were also observed in this area: the synaptic boutons contained large, stained vesicles and small, unlabeled, clear vesicles. These axon terminals made synaptic contacts with unstained dendritic processes; the contacts were symmetrical. The results indicate that hGRF‐immunoreactive neurons of the guinea pig arcuate nucleus present morphological features of neuroendocrine cells. Moreover, the presence of hGRF‐labeled nerve endings in the arcuate nucleus itself suggests that a substance related to hGRF might be a neuromodulator, at least in this area.

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