DEVELOPMENT AND REGULATION OF THE LUTEINIZING HORMONE RELEASING HORMONE SYSTEM

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LHRH neurons, critical for reproduction, are derived from the olfactory placode and migrate into the brain, where they become integral members of the hypothalamic-pituitary-gonadal axis. We study mechanism(s) underlying LHRH neuronal migration in normal/transgenic animals, and nasal explants. Intrinsic and trans-synaptic regulation of LHRH gene expression, peptide synthesis and secretion in embryonic LHRH neurons (outside the CNS) versus postnatal LHRH neurons (in the CNS) is studied using nasal explants and organotypic cultures, respectively. LHRH neurons express GABA receptors and GABAergic neurons are present in nasal regions. In vitro, migration of LHRH neurons is: a) inhibited by a GABA agonist, b) increased by GABA antagonists and treatment with tetrodotoxin. We hypothesize that GABAergic signals regulate the timing of entrance of LHRH neurons into the CNS. Examining the role of N- glycosylation on LHRH development, we found that tunicamycin treatment dramatically altered olfactory axon outgrowth - with fibers traversing between olfactory pits (OP), instead of exiting the explant upon reaching midline tissue. Although outgrowth of olfactory axons changed, treatment did not disrupt association of LHRH neurons with their axonal pathway, nor movement of LHRH neurons from the OP, indicating that the molecular mechanism(s) underlying these events is independent of N- gycosylation. Manipulations of midline/OP relation confirmed that a navigational signal(s) for appropriate positioning of olfactory axons is expressed by cartilagenous midline tissue. To investigate the relationship between gene expression, biosynthesis and secretion, we examined mRNA turnover rates in dopaminergic (DA) neurons and LHRH neurons. Differences in tyrosine hydroxlase (TH; marker of DA neurons) mRNA turnover rates were found between hypothalamic DA populations, with arcuate neurons exhibiting a TH mRNA turnover rate (6-7 hr) which corresponded to the known rhythmic output displayed by these neurons. LHRH neurosecretion is pulsatile (about 1 hr ). To maintain this pulsatile profile, we hypothesized that LHRH mRNA rapidly decays. We determined that there is a fast decay of LHRH mRNA (5-13 min), followed by a much slower decay rate (329-344 min). Notably, the rapid decay rate of LHRH mRNA corresponds to the rapid decay rate of LHRH pulses. By comparing the 3'UTR of mammalian LHRH mRNAs, we identified two conserved regions which may be involved in the observed turnover kinetics. Currently, we are determining: 1) the midline cues which influence olfactory axon outgrowth; 2) the molecular nature of the interaction between LHRH neurons and their axonal pathway; 3) genes which are selectively expressed in migrating LHRH neurons; 4) the relation of galanin expressing cells to the differentiation and/or migration of LHRH neurons and; 5) factors related to rapid LHRH mRNA turnover.