DISTRIBUTION OF NITRIC-OXIDE SYNTHASE-IMMUNOREACTIVE NEURONS IN FETAL-RAT BRAINS AT EMBRYONIC DAY-15 AND DAY-19

Although nitric oxide (NO) is hypothesized to play a role in the model ling of neuronal connections in the developing brain, no precise histo logical information has been reported on nitric oxide synthase immunor eactive (NOS-IR) structures in the embryonic brain. Thus, we examined the distribution of NOS-IR structures in the rat brain at embryonic da y 15 (E15) and day 19 (E19), as a stage of early embryonic neurogenesi s and active synaptogenesis, respectively. In the brain at E15, a few NOS-IR cell bodies were observed in the diencephalon near the 3rd vent ricle and in the pens. By E19, the number of NOS-IR cells was greatly increased. Many NOS-IR cells were distributed throughout the forebrain and the medulla oblongata. NOS-IR cells of the neostriatum were locat ed in its ventrolateral two-thirds. NOS-IR cells in the hypothalamus w ere distributed densely in the medial part while sparsely in the later o-ventral part, suggesting medio-lateroventral directions of NOS devel opment. In the lower brainstem, on the other hand, NOS-IR cells were d istributed in a similar pattern to that reported in adult rat, indicat ing earlier development of synaptic refinement in this area. The numbe r of NOS-IR cells in the cerebral cortex and the cerebellum was very f ew.