A wide range of physiological and behavioral processes, such as social, sexual, 
and maternal behaviors, learning and memory, and osmotic homeostasis are 
influenced by the neurohypophysial peptides oxytocin and vasopressin. 
Disruptions of these hormone systems have been linked to several neurobehavioral 
disorders, including autism, Prader-Willi syndrome, affective disorders, and 
obsessive-compulsive disorder. Studies in zebrafish promise to reveal the 
complex network of regulatory genes and signaling pathways that direct the 
development of oxytocin- and vasopressin-like neurons, and provide insight into 
factors involved in brain disorders associated with disruption of these systems. 
Isotocin, which is homologous to oxytocin, is expressed early, in a simple 
pattern in the developing zebrafish brain. Single-minded 1 (sim1), a member of 
the bHLH-PAS family of transcriptional regulatory genes, is required for 
terminal differentiation of mammalian oxytocin cells and is a master regulator 
of neurogenesis in Drosophila. Here we show that sim1 is expressed in the 
zebrafish forebrain and is required for isotocin cell development. The 
expression pattern of sim1 mRNA in the embryonic forebrain is dynamic and 
complex, and overlaps with isotocin expression in the preoptic area. We provide 
evidence that the role of sim1 in zebrafish neuroendocrine cell development is 
evolutionarily conserved with that of mammals.