Phylogenetic Distribution and Function of the Hypophysiotropic Hormones of the Hypothalamus

SYNOPSIS. Following the isolation, synthesis and subsequentdevelopment of specific and sensitive radioimmunoassays forthe hypothalamic hormones thyrotropin-releasing hormone (TRH),luteinizing hormone-releasing hormone (LH-RH) and growth hormonerelease-inhibiting hormone (somatostatin), it was recognizedthat these peptides were not localized solely in the hypothalamus,but were widely distributed throughout the mammalian nervoussystem. Somatostatin occurs outside the nervous system altogether,being located in the gastrointestinal tract of vertebrates whereit may have a physiologic role in the secretion of gastrointestinalhormones. TRH, also, has been located outside the nervous system,occurring in large quantities in the skin of Rana species whereit may be of physiologic importance in skin function. This tripeptideis found throughout the nervous system of vertebrate and invertebratespecies in situations where it has no pituitary-thyroid function.Thesepeptides are present in brain synaptosomes and enzymatic degradingsystems have been recognized for each in brain tissue. For TRH,specific receptors and synthesizing activity have been detectedoutside the hypothalamic-pituitary system. The anatomic location,phylogenetic distribution, neurophysiologic and behavioral effectsstrongly support a role for these substances in neuronal regulation,apart from control of pituitary secretion. Evolutionary studies,especially of TRH, suggest that their primary function may beas neurotransmitters.