Releasing factors in the circumventricular organs in the rat brain.

With a recently developed microdissection technique, four circumventricular organs were removed from the rat brain, and their contents of LHRH and TRH were measured. The subfornical organ, the organon vasculosum lamia terminalis, the subcommissural organ, the area postrema all contained significant quantities of both releasing factors. The concentration of LHRH in the organon vasculosum (OVLT) was 14 ng/mg protein, 58% of that found in the median eminence. The concentration of LHRH in the remaining circumventricular organs ranged from 4.2 to 10.2 ng/mg protein. The concentration of TRH in these structures, however, was considerably lower, ranging from 0.7 to 1.8 ng/mg protein. The concentrations of LHRH and TRH in the tissue immediately adjacent to the organon vasculosum were nearly 50 times and 4 times less, respectively, than the concentration of these two releasing factors within the OVLT itself.