Consistency in the number of dopaminergic paraventricular organ-accompanying neurons in the posterior tuberculum of the zebrafish brain.

The teleostean diencephalon contains a relatively large number of dopaminergic neurons compared to other vertebrates. In the zebrafish, 17 groups of such neurons have been distinguished. One of the most unusual among these is the group of paraventricular organ-accompanying cells, which are easily distinguished by their large somal size, high tyrosine hydroxylase content, and characteristic dendritic architecture. This cell group is also heterogeneous-subsets of neurons can be identified on the bases of dendritic orientation. In this study, the number of paraventricular organ-accompanying neurons is counted in adult brain sections stained with anti-tyrosine hydroxylase antibodies. There is an average of 7.2+/-1.0 neurons on each side of the brain, and an average sum of 14+/-1.1 neurons on both sides. The average difference between the left and the right sides is 0.8+/-0.5 neurons. Neuron numbers between the two sides of the same brain are highly correlated. These results suggest that there is a relatively stringent regulation of paraventricular organ-accompanying neuron number in the zebrafish brain. The correlated left-right numbers suggests that genetic factors may play a major role in this regulation. The consistent and low cell number should be helpful in elucidating the number of subsets of these neurons, the anatomical and functional organization of some of the dopaminergic neurons along the paraventricular organ, as well as factors that play a role in regulating neuron numbers.