The too few mutant selectively affects subgroups of monoaminergic neurons in the zebrafish forebrain.

Monoaminergic neurons are present in small numbers and in multiple distinct locations of the vertebrate CNS. They are involved in important functions such as movement coordination, motivation, and the response to environmental stress. However, the mechanisms involved in their subtype specification are not well understood. In this study, we examined the states of forebrain dopaminergic (DA) and serotonergic (5HT) neurons in larval and adult zebrafish of wild type and the too few mutant. The majority of DA and 5HT neuronal subgroups that were found in adults were established in the 6-day old larval zebrafish. Rather than affecting all monoaminergic neurons in the forebrain, selective subgroups of these neurons are reduced in the too few mutant, starting from the larval stage. Taken together, our study establishes that similar to DA neurons, distinct subtypes of 5HT neurons exist in larval as well as adult zebrafish. The development of a subset of these monoaminergic neurons is dependent on the too few gene product. Thus, this mutant is potentially important for understanding the development as well as the function of forebrain DA and 5HT neurons.