Evolutionary significance of the phylogenetic distribution of the mammalian hypothalamic releasing hormones.

The hypophysiotrophic hormones isolated from the mammalian hypothalamus are distributed throughout the nervous system of vertebrate species. Although their role in regulating pituitary hormone secretion in mammals is clear, a similar function in lower species has not been established. Thyrotropin-releasing hormone is unable to stimulate thyroid function in amphibia and fish, despite being present in the hypothalamus and brain of these species of high concentration. The tripeptide is also found in high concentration in frog skin, a tissue derived from (or programed by) primitive neuroectoderm that is also a rich source of other peptides structurally related to neural peptides located in mammalian brain and gut. Luteinizing hormone-releasing hormone (LHRH) is able to activate gonadotropin secretion in submammalian species but there is evidence that the LHRH material present in avian, reptilian, and piscine brain is not identical to the mammalian decapeptide. An LHRH-like material present in frog sympathetic ganglia appears to function as a neurotransmitter in this location. Somatostatin is present in high concentrations in the hypothalamus, brain, pancreas, and gastrointestinal tract of all vertebrates and chromatographically is identical to the mammalian material, suggesting that this peptide is an "ancient" molecule with an important role in neuronal pancreatic and digestive function. The hypothalamic releasing hormones are part of a family of neural peptides that have a widespread anatomic and phylogenetic distribution and form a diffuse neuroendocrine system. It an material, suggesting that this peptide is an "ancient" molecule with an important role in neuronal pancreatic and digestive function. The hypothalamic releasing hormones are part of a family of neural peptides that have a widespread anatomic and phylogenetic distribution and form a diffuse neuroendocrine system. It an material, suggesting that this peptide is an "ancient" molecule with an important role in neuronal pancreatic and digestive function. The hypothalamic releasing hormones are part of a family of neural peptides that have a widespread anatomic and phylogenetic distribution and form a diffuse neuroendocrine system. It appears likely that the releasing hormones initially arose with a neurocrine or paracrine function, and that only later in evolution did they acquire the role of regulating adenohypophysial secretion.