Cytological evidence for activation of neuroendocrine cells in the parvocellular preoptic nucleus of the goldfish hypothalamus following pharmacological adrenalectomy.

The nucleus preopticus (NPO) of the goldfish hypothalamus is composed of parvocellular (NPOpc) and magnocellular (NPOmc) neurosecretory neutrons. The cytology of NPOpc and NPOmc neurons was examined with light and electron microscopy following pharmacological adrenalectomy with the adrenocortical inhibitor, metopirone. After five days of metopirone administration, light microscopy revealed a significant increase in nuclear area of NPOpc, but not of NPOmc, neurons. Ultrastructural examination of NPOpc neurons revealed two cell types, PC1 and PC2 neurons, which could be distinguished by the relative abundance and the size of the neurosecretory granules in the cytoplasm. The ultrastructural appearance of the NPOmc neurons revealed a single cell type containing abundant neurosecretory granules. Following five days of metopirone administration, the ultrastructural appearance of the PC1 neurons indicated a state of enhanced secretory activity. Metopirone had no observable effect on the appearance of the PC2 or NPOmc neurons. These observations demonstrate that PC1 neurons are activated under the conditions of pharmacological adrenalectomy and suggest that the secretory activity of these neurons is inhibited by adrenocorticosteroids.