Light and electron microscopic localization of glutamate immunoreactivity in the supraoptic nucleus of the rat hypothalamus

The distribution of glutamate immunoreactivity was mapped within the supraoptic nucleus of the rat hypothalamus utilizing a specific anti-glutamate antibody. Magnocellular neuroendocrine cells of the supraoptic nucleus showed intense immunoreactivity for glutamate which varied with the conditions of fixation. Within the perikarya, reaction product was found associated with the endoplasmic reticulum but not the mitochondria, Golgi, dense bodies or neurosecretory granules. A relatively high density of glutamate-immunoreactive terminals was found in the supraoptic nucleus. These terminals were less affected by fixation condition and were generally found contacting large, glutamate-immunoreactive processes within the ventral dendritic neuropil of the supraoptic nucleus. The pattern and characteristics of glutamate immunoreactivity in the supraoptic nucleus suggested the presence of two distinct glutamate pools. The magnocellular neuroendocrine cells may contain a large, labile metabolic pool of glutamate. These cells, in turn, appear to receive glutamate synaptic input from a more stable pool consistent with suggestions that glutamate may be used as a transmitter within this system.