Synopsis. Following the isolation, synthesis and subsequent development of specific and sensitive radioimmunoassays for the hypothalamic hormones thyrotropin-releasing hormone (TRH), luteinizing hormone-releasing hormone (LH-RH) and growth hormone release-inhibiting hormone (somatostatin), it was recognized that these peptides were not localized solely in the hypothalamus, but were widely distributed throughout the mammalian nervous system. Somatostatin occurs outside the nervous system altogether, being located in the gastrointestinal tract of vertebrates where it may have a physiologic role in the secretion of gastrointestinal hormones. TRH, also, has been located outside the nervous system, occurring in large quantities in the skin of Rana species where it may be of physiologic importance in skin function. This tripeptide is found throughout the nervous system of vertebrate and invertebrate species in situations where it has no pituitary-thyroid function. These peptides are present in brain synaptosomes and enzymatic degrading systems have been recognized for each in brain tissue. For TRH, specific receptors and synthesizing activity have been detected outside the hypothalamic-pituitary system. The anatomic location, phylogenetic distribution, neurophysiologic and behavioral effects strongly 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 be as neurotransmitters.
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IVOR M. D. ACKSON (1978) studied this question.