The effect of late intrauterine 5-bromo-2'-deoxyuridine administration on the development of GnRH neurons in the mouse

We have studied the effect of prenatal (late intrauterine) 5-bromo-2’-deoxyuridine (BrdU) administration on the development of the GnRH pathway. BrdU is widely accepted as a tool for studying neurogenesis (pre and postnatal). On the other hand, this thymidine analogue proved to be effective influencing different developmental events too. Dams of C57Bl6 mice were treated on the last days of pregnancy (between E16, E17, E18, E19 and the time of birth, E20/21 days, respectively). The dams received daily injections of 15 ug/g body weight BrdU subcutaneously, dissolved in physiological saline, between 11 and 12 am. BrdU treatment during the last 3-4 days of pregnancy resulted in a significant decrease of the number of GnRH immunoreactive (ir.) neuronal cell bodies as well as ir. fibers. This decrease could be detected at all postnatal ages (P0/P1, P3/4, P6/8 and P21/23) studied. The higher cumulative doses of BrdU produced a more significant decrease of the GnRH ir. structures in comparison with the lower ones. The morphological appearance of GnRH ir. neuronal cell bodies proved to be different from that of the controls too: bipolar cells were few in number, and most of the cells were round shaped, smaller in size, and in many cases they seemingly did not exhibit arborisation. As far as the distribution of GnRH ir. cell bodies is concerned, it was similar to that of the control animals (most of the cells were found in the preoptic area), however, immunopositive cells were detected in the nasal septum and along their migratory root at later postnatal ages (up to P6/P8 postnatal days) in comparison with the control (up to P3 postnatal day). However, the amount of the “missing” GnRH positive neurons present in the from hypothalamic areas was less than that we have found along the migratory path: near to the vomeronasal organ, within the nasal septum, passing through the cribriform plate and around the olfactory and accessory olfactory bulb. Significant differences were seen in the amount of GnRH ir. fibers in BrdU treated animals at all postnatal ages studied, especially at the lateral septal area, in the habenula, the preoptic and more caudal hypothalamic areas next to the arcuate nucleus and also to the mammilary bodies. As a consequence, late intrauterine BrdU administration influences the development of the GnRH system in a dose-(and time) dependent manner. One of the effects of BrdU is the retardation and diminishment of the migration of GnRH neurons, however, the marked decrease in their number (at hypothalamic territories areas) could be the reason either of apoptotic cell death or of the decrease of the GnRH synthesis below the level detectable with immunocytochemical method (or both). Application of the BrdU at low doses during the proper time-window of the pregnancy could be a useful tool in further studies on disturbed neuronal migration and development of the GnRH axis. Supported by a bilateral Cultural and Scientific Project between Italy and Hungary.