Presence and differential distribution of distinct forms of immunoreactive gonadotropin-releasing hormone in the musk shrew brain.

Gonadotropin-releasing hormone (GnRH) immunoreactive cells and fibers were revealed in olfactory regions, the ventral forebrain, and in the midbrain of the musk shrew (Suncus murinus). Immunoreactive neurons in olfactory and telencephalic areas were specific for the mammalian form of GnRH. Cell bodies in the midbrain, however, cross-reacted with an antibody specific for chicken-II GnRH. High-performance liquid chromatography and radioimmunoassay analyses confirmed these results; high levels of chicken II GnRH were present in the midbrain, and mammalian GnRH was detected in both forebrain and midbrain. In addition, a third, late-eluting form of GnRH was revealed using high-performance liquid chromatography in both forebrain and midbrain of the musk shrew. Midbrain neurons containing GnRH have not been reported previously in a mammal, although mesencephalic GnRH immunoreactivity within cell bodies is common among nonmammalian vertebrates. Likewise, while multiple forms of GnRH have been reported in nonmammalian vertebrates and several metatherian species of mammals, this is the first report on multiple forms of GnRH in the brain of a placental mammal. Taken together, the findings suggest that this primitive eutherian mammal has retained the ability to produce GnRH protein in the midbrain. This feature of the GnRH system has been conserved among nonmammalian vertebrates, but appears to have been lost in modern placental mammal species. The functional significance of this group of neurons has yet to be determined.