Distributions of two gonadotropin-releasing hormone receptor types in a cichlid fish suggest functional specialization

Gonadotropin-releasing hormone 1 (GnRH1) from the brain controls reproduction in vertebrates via a GnRH-specific receptor in the pituitary; however, other forms of GnRH are found in all species, suggesting additional roles for this family of peptides. GnRH action depends critically on the location of its cognate receptors in the brain. To understand the potential roles of additional GnRH forms, we localized two known GnRH receptor types in a cichlid fish, Astatotilapia burtoni, in which GnRH1 is socially regulated. Using in situ hybridization, we describe the mRNA expression pattern of these GnRH receptor (GnRH-R) subtypes in the brain, specifically with respect to GnRH-producing neurons. Our data suggest that following a gene duplication, the two GnRH receptors have evolved to serve different functions. The type 1 receptor (GnRH-R1) is expressed less widely than the type 2 receptor (GnRH-R2). Specifically, GnRH-R1 is expressed in groups of neurons in the telencephalon, preoptic area, ventral hypothalamus, thalamus, and pituitary. In contrast, GnRH-R2 is expressed in many more brain areas, including the olfactory bulb, telencephalon, preoptic area, hypothalamus, thalamus, midbrain, optic tectum, cerebellum, hindbrain, and pituitary. The specific distribution of GnRH-R2 suggests that the GnRH ligands may act via this receptor to influence behavior in A. burtoni. Moreover, only GnRH-R2 mRNA is colocalized in the three known groups of GnRH-containing neurons, suggesting that any direct feedback regulation of GnRH by itself must act through this receptor type. Taken together, these data suggest that the two GnRH receptor types serve different functional roles in A. burtoni.