Gonadotropin‐Releasing Hormone (GnRH) Neurons of the Developing Tectum of the Mouse

Using a modified immunocytochemistry protocol with enhanced sensitivity, we were able to visualize a population of neurons in the tectum of the developing mouse that contained GnRH‐like immunoreactivity. In microscopic studies conducted using 100 pm sections cut in sagittal or horizontal planes, 10–20 lightly‐stained neurons were first detected in the tectum at E 13.75 (morning of plug = E0.5). The number of immunostained cells increased exponentially reaching a peak at E 15.75 before decreasing in number. No positive neurons were seen in the tectum at PN20 or later. The GnRH cells were located medially along the dorsoventral axis of the tectum in a region of the brain distinct from that containing GnRH neurons that migrate into the CNS from the olfactory placode. To determine the nature of the immunoreactivity, two approaches were used. Analysis of tissue from an hpg mutant strongly supports the hypothesis that these cells make mammalian GnRH. lmmunocytochemical data suggest that although the precursor protein is synthesized, the cleaved and amidated decapeptide may be absent or be present at an undetectable level. Our results demonstrate that in addition to the GnRH neurons from the placade, a population of GnRH neurons exists in the mouse tectum. This population is developmentally regulated, appearing only during embryonic and early postnatal ages but not in the adult.