Influence of Catecholamines on Migration and Differentiation of GnRH Neurons in Rats

The GnRH producing neurons are the key link of neuroendocrine regulation of the adult reproductive system. Synthesis and secretion of GnRH are, in turn, under the afferent catecholaminergic control. Taking into account that catecholamines exert morphogenetic effects on target cells during ontogenesis, this study was aimed at investigation of the effects of catecholamines on development of GnRH neurons in rats during ontogenesis. We carried out comparative quantitative and semiquantitative analyses of differentiation and migration of GnRH neurons in fetuses of both sexes under the conditions of normal metabolism of catecholamines (administration of saline) or their pharmacologically induced deficiency (administration of -methyl-para-tyrosine). The inhibition of catecholamine synthesis from day 11 of embryogenesis led to an increasing number of GnRH neurons in rostral regions of the trajectory of their migration over the brain: in the area of olfactory tubercles on day 17 and in the area of olfactory bulb on days 18 and 21. In addition, the optical density of GnRH neurons located in the rostral regions of migration was higher in the fetuses after administration of -methyl-para-tyrosine during embryogenesis, as compared to the control. It has been concluded that catecholamines stimulate the migration of GnRH neurons and affect their differentiation.     

A Population of Neurons Expressing Tyrosine Hydroxylase and Migrating from Olfactory Placodes into Forebrain during Ontogenesis in Rats

Olfactory placodes, that give rise to the olfactory and respiratory epithelia during ontogenesis, are a source of many neurons migrating into forebrain in the direction of growth along the olfactory nerves. The neurons expressing gonadotropin releasing hormone (GnRH) are among the best studied in the population in question. This hormone is responsible for the central regulation of reproduction in adult animals. It was already shown that, in addition to the GnRH-immunoreactive neurons, a small amount of neurons expressing tyrosine hydroxylase (TH), the first enzyme of catecholamine synthesis, migrates into the forebrain. Such a transient population of TH-immunoreactive neurons was shown by means of single and double immmunohistochemical labeling. The TH neurons were first found on branches of the olfactory, terminal, and vomeronasal nerves, along the trajectory of migration of GnRH-immunoreactive neurons on day 15 of embryogenesis, which preceded the appearance of GnRH-immunoreactive neurons. On days 17–21 of embryogenesis, both populations of neurons were found in almost the same areas and on day 21 single neurons contained both GnRH and TH. There were no neurons expressing decarboxylase of aromatic amino acids (DAA), the second enzyme of catecholamine synthesis, among TH-immunoreactive neurons, thus suggesting noncatecholaminergic nature of these neurons. However, single nonenzymatic DAA-immunoreactive neurons were found in the area of anterior olfactory nuclei in the forebrain, which suggests their involvement in local cooperative synthesis of catecholamines in the area where GnRH-immunoreactive neurons penetrate in the forebrain. Thus, the neurons expressing TH, TH and GnRH, and DAA were found in rats during prenatal period in the nasal part of the head along the nerves projecting into the forebrain. The origin and functional significance of these neurons are discussed.