Hypothalamic lesions stimulating growth hormone cell activity in the goldfish

The cytology of the growth-hormone (GH) cells of the goldfish pituitary were examined following electrothermic lesions of the anterior preoptic hypothalamus and telencephalon. Following lesions of the nucleus preopticus (NPO) light microscopy of the pituitary revealed a significant increase in the nuclear diameter and a degranulation of the GH cells. Lesions of the telencephalon anterior or dorsal to the NPO had no cytological effect on the GH cells. The ultrastructural appearance of the GH cells of NPO-lesioned fish was characterized by a marked degranulation of the cytoplasm and a proliferation of the rough endoplasmic reticulum indicative of enhanced secretory activity. The GH cells of the proximal pars distalis (PPD) are directly innervated by peptidergic (type A) and aminergic-like (type B) neurosecretory axons. Following lesions of the NPO, there was a marked reduction in the number of type A fibers in the PPD. These results suggest that the type A fibers innervating the GH cells originate in the NPO and act to inhibit the secretory activity of the GH cells.