Gonadotropin-releasing hormone-containing neurons change size with reproductive state in female Haplochromis burtoni.

In the preoptico-hypothalamic area (POA) of teleost fish, neurons containing gonadotropin-releasing hormone (GnRH) regulate reproduction through direct projections to pituitary gonadotropes. Here we show that these GnRH-containing cells change size depending on the reproductive and maturational state in female Haplochromis burtoni. We selected animals prior to, during, and after the reproductive portion of their life history, in both brooding and spawning states. Immunocytochemical staining of GnRH-containing neurons in the POA revealed that these cells are up to twice as large in females that have never spawned or are in the act of spawning than they are in females that are carrying broods. Older, postreproductive females have the largest cell sizes. Previous work on male H. burtoni has shown that soma sizes of the homologous neurons change according to social status, with dominant fish having larger cells than subordinates. Since reproductively active females have no apparent social hierarchy and are all exposed to approximately the same external stimuli, the primary factor(s) controlling GnRH-immunoreactive (irGnRH) neuron size appears to be internal reproductive state. Thus, while irGnRH neurons are pleiomorphic in both males and females, cell size change is differently regulated in each.