Two Visual Processing Pathways Are Targeted by Gonadotropin-Releasing Hormone in the Retina

In fish the terminal nerve is comprised of a group of cells with somata adjacent to the olfactory bulb and processes that extend both anteriorly to the olfactory mucosa and posteriorly to the telencephalon. In teleost fish an additional group of axons extends along the optic tract and delivers putative neuromodulators to the retina. One peptide - gonadotropin-releasing hormone (GnRH) - has been implicated as a prime candidate neuromodulator based on electrophysiological evidence that exogenous application influences neural activity. Here we describe the expression patterns of two GnRH receptor subtypes in the retina of a teleost fish, Astatotilapia (Haplochromis) burtoni. The type 1 GnRH receptor (GnRH-R1) was expressed in cells of the amacrine cell layer - where lateral inputs affect the flow of visual information from photoreceptors to the brain - and in a distribution and location pattern similar to dopaminergic interplexiform cells. Immunohistochemical labeling of GnRH fibers revealed varicosities along terminal nerve axons near the amacrine cell layer and near cells immunoreactive for tyrosine hydroxylase, a dopaminergic cell marker. This finding supports an existing model that the terminal nerve forms synapses with dopaminergic interplexiform cells. Surprisingly, the type 2 GnRH receptor (GnRH-R2) was abundantly expressed in ganglion cells, which lie along the direct pathway of visual information to the brain. These data suggest that GnRH from the TN could broadly influence processing of retinal signals both in lateral processing circuits through GnRH-R1 and in the vertical throughput pathway through GnRH-R2.