Relationship between serum growth hormone levels and the brain and pituitary content of immunoreactive somatostatin in the goldfish, Carassius auratus L

Abstract

In this study, the relationships between endogenous brain and pituitary immunoreactive somatostatin (irSRIF) and circulating growth hormone (GH) levels in the goldfish were examined using two approaches. First, the amount of irSRIF in extracts of the pituitary gland and various brain regions was measured by radioimmunoassay several times throughout the year and was compared to serum GH levels at each time. The amounts of irSRIF in extracts of the pituitary gland, hypothalamus, and telencephalon were found to be inversely related to seasonal changes in serum GH levels, such that irSRIF was highest in these regions when serum GH levels were lowest (November and February). Conversely, irSRIF in these regions was lower in May, June, and July when serum GH levels were highest. These results suggest that endogenous irSRIF in the pituitary and forebrain may participate in the regulation of seasonal changes in serum GH levels in the goldfish. In extracts from other brain regions (thalamus + midbrain and cerebellum + medulla), some changes in the amount of irSRIF were observed among the various sample times, but these variations were not related to changes in serum GH levels. In a second set of experiments, the origin of irSRIF fibers innervating the goldfish pituitary gland was examined by using brain lesioning techniques to destroy regions of the forebrain known to contain irSRIF perikarya and fibers, and subsequently measuring the amount of irSRIF in the pituitary gland. Lesions in the preoptic area of the forebrain resulted in increased serum GH levels concomitant with a decrease in pituitary irSRIF content. This provides direct evidence that the preoptic area is the origin of a somatostatinergic projection inhibiting GH secretion from the goldfish pituitary. Lesions centered in the nucleus lateral tuberis (NLT) pars anterioris did not influence serum GH levels or the pituitary content of irSRIF. In contrast, more posterior lesions centered in the NLT pars posterioris (NLTp) resulted in a dramatic reduction in the amount of irSRIF in the pituitary. This suggests that the majority of irSRIF projections to the goldfish pituitary pass through the area destroyed by the lesion centered in the NLTp; it is also possible that perikarya within this area may be the origin of at least some of the irSRIF-containing fibers in the goldfish pituitary. Together, results from the present study provide evidence of a functional relationship between circulating levels of GH and endogenous brain and pituitary irSRIF in the goldfish.