Neuromodulatory effects of gonadotropin-releasing hormone on retinotectal synaptic transmission in the optic tectum of rainbow trout

Gonadotropin-releasing hormone (GnRH) is a hypophysiotropic decapeptide that stimulates the release of gonadotropins from the pituitary. In addition, there are extra-hypothalamic GnRH neurons that project to all regions of the brain and whose function remains unknown. Here, we investigated the effects of GnRH on retinotectal synaptic transmission, the synapses of which are formed between retinal fibers and tectal periventricular neurons that express GnRH receptor mRNA. We used rainbow trout as our study model. The excitatory postsynaptic currents (EPSCs), which were evoked by electrical stimulation of the retinal fibers and recorded in periventricular neurons, were suppressed by antagonists of ionotropic glutamate receptors. EPSCs were increased by application of each of two types of GnRH (GnRH2 and GnRH3) in the trout tectum. Such facilitation lasted for at least 10 min after application of the GnRH. To our knowledge, this is the first report of GnRH modulating conventional synaptic transmission in the brain, suggesting that tectal GnRH enhances tectal sensitivity for retinal inputs. Furthermore, such long-lasting facilitation might occur across all the brain regions innervated by GnRH neurons, and GnRH might simultaneously switch neuronal activities in the brain regions relevant to reproductive behaviors.