IN-SITU DETECTION OF H1-RECEPTOR MESSENGER-RNA AND ABSENCE OF APOPTOSIS IN THE TRANSIENT HISTAMINE SYSTEM OF THE EMBRYONIC RAT-BRAIN

In the developing brain, histamine is one of the first neurotransmitte rs to appear. The concentration of histamine in the prenatal brain is fivefold that of adult levels. During the prenatal development a large transiently histamine-immunoreactive cell population distinct from th e adult histaminergic system can be found within a subpopulation of th e developing serotonergic raphe nuclei neurons. Also histamine-immunor eactive nerve fibers are widely distributed already during the prenata l development extending to the diencephalon, the thalamus, the cortex, and the spinal cord. Large numbers of histamine-containing mast cells also migrate into the brain during the late prenatal life. The wide d istribution and high prenatal concentrations imply important functions for the histaminergic system during intrauterine development. However , little is known about the actual functions of histamine during devel opment, and which of the histamine receptors are present in the prenat al rat brain is currently unknown. In the present study, we used in si tu hybridization to study the distribution of H1-receptor (H1R) mRNA i n the embryonic rat brain and spinal cord. H1R mRNA could be detected in rat brain and in spinal cord on embryonic day (E) 14, and the expre ssion pattern seemed to partially localize in areas containing histami ne-immunoreactive nerve fibers through E14-E20. H1R mRNA was also dete cted by reverse transcriptase polymerase chain reaction from embryonic brain samples and by Northern hybridization. The possible involvement of apoptosis in the disappearance of the developing transiently hista minergic system was studied by using apoptosis detection based on the terminal dUTP nick end labeling (TUNEL) technique and with c-Fos immun ostaining. Although histamine immunoreactivity disappears dramatically from the developing raphe nuclei after E18, only occasional apoptotic nuclei could be seen in the histamine-immunoreactive cell bodies. The presence of H1R mRNA during the embryonic development renders it poss ible that histamine could exert an H1R-specific function at the time o f the embryonic histamine peak. (C) 1998 Wiley-Liss, Inc.