Gonadotropin-releasing hormone and thyrotropin-releasing hormone: distribution and effects in the central nervous system.

TLDR: Evidence is provided for the ubiquitous distribution of GnRH and TRH in extrahypothalamic regions of the rat nervous system, including the pineal gland, anterior pituitary, midbrain, cerebral and cerebellar cortices, and brain stem, and for the possibility that these peptidergic neurons can be a potential source for hypothalamic peptides identified in the outer regions of median eminence.

Abstract: Publisher Summary This chapter discusses the distribution and effects of gonadotropin-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH) in the central nervous system (CNS). The studies presented in the chapter provide evidence for the ubiquitous distribution of GnRH and TRH in extrahypothalamic regions of the rat nervous system, including the pineal gland, anterior pituitary, midbrain, cerebral and cerebellar cortices, and brain stem. TRH has also been demonstrated by radioimmunoassay in the rat spinal cord and in human cerebral spinal fluid. The heterogeneous distribution pattern of these peptides, found in greatest concentrations in hypothalamus and midbrain, corresponds with regions endowed with biogenic amines, inducing dopamine and norepinephrine. Hypothalamic peptides, such as melanocyte-inhibiting factor (MIF)-1, ProLeuGlyAmide, TRH, GnRH, and growth hormone-inhibiting hormone (GHIH), exert behavioral influences in animal bioassay models that do not depend on the integrity of the endocrine system. These have been exemplified by L-dopa potentiation in MIF-1, TRH, GnRH, and GHIH, serotonin potentiation in TRH and GnRH, reversal of barbiturate hypnosis in TRH, and protection against audiogenic seizures in GnRH. The direct application of three of these peptides, TRH, GnRH, and GHIH, on individual neurons at many levels of the rat CNS results in rapid and reversible suppression of action potentials, similar to the inhibitory effects of histamine and dopamine on identical neurons. Electrical stimulation of 134 tuberoinfundibular neuron axon terminals reveals efferent projections to unexpected areas of the CNS, including the anterior hypothalamic area, medial preoptic area, and the nucleus dorsalis medialis of the thalamus. These latter observations raise the possibility that these peptidergic neurons can be a potential source for hypothalamic peptides identified in the outer regions of median eminence.