A direct magnocellular-preopticospinal pathway in goldfish: implications for control of sex behavior.

Neurons in the pars magnocellularis and gigantocellularis of the goldfish magnocellular preoptic nucleus concentrate horseradish peroxidase by retrograde transport following its placement into sectioned spinal cord. Implants at rostral levels labeled the greatest number of preoptic cells. The pathway demonstrated is primarily ipsilateral and appears equivalent to the paraventricular-spinal system of tetrapods. In goldfish it may control sperm release and other stereotyped reproductive responses.