Development of NADPH-diaphorase activity in the central nervous system of the cichlid fish, Tilapia mariae

The distribution of nicotinamide adenine dinucleotide phosphate (NADPH)-dia phorase activity was studied in the cichlid fish Tilapia mariae during onto genesis by the histochemical reaction of NADPH-diaphorase that indicates, i n aldehyde-fixed tissue, the presence of nitric oxide synthase, which is th e enzyme responsible for nitric oxide production. The first appearance of N ADPH-diaphorase-positive neurons has a striking bilateral symmetry and occu rs 20 h after fertilization (stage 8) in the olfactory placodes and in the neural tube where two clusters of positive neurons were seen in the diencep halon and in the rhombomere r4 of the hindbrain. Two days after fertilizati on (stage 10), the clusters of positive neurons showed labeled axons. The t wo longitudinal fiber bundles that arose from the diencephalic positive neu rons ran caudally in the tract of the postoptic commissure. At stage 12 (3. 5 days after fertilization), new populations of NADPH-diaphorase-positive n eurons appeared in the telencephalon, in some diencephalic nuclei, and in t he hypothalamus. Several trigeminal motor neurons showed strong NADPH-diaph orase activity, whereas the optic tectum and cerebellum were completely fre e of enzymatic activity. In the hindbrain, clusters of positive neurons wer e seen in the octavolateral region and in the region defined by the exit of the vagus nerve. in the cervical spinal cord, some ventral putative motor neurons were labeled. At stage 14(5.5 days after fertilization), several pe riventricular neurons of the optic tectum and some neurons of the cerebella r lamina were labeled. Dorsal neurons, including a few large superficial ne urons were also labeled in the cervical spinal cord. NADPH-diaphorase activ ity was seen in the neuropil area of the telencephalon, the target of olfac tory inputs, and in the sensory dorso-lateral area of the spinal cord.