Immunocytochemical localization of the GABAA/benzodiazepine receptor beta2/beta3 subunits in the optic tectum of the salmon

The optic tectum of the salmon is a primary visual center with direct input from the retina via the optic tract. The structure is homologous with the superior colliculus of the mammalian brain. We have studied the distribution of immunoreactivity against the GABAA/benzodiazepine receptor beta2/beta3 subunits with a monoclonal antibody (BD-17) in the optic tectum of the salmon brain. A weak immunoreactivity is found in the rostral stratum marginale (SM), strong labelling of the neuropil is shown in a thin band in stratum opticum (SO), two bands in stratum fibrosum et griseum superficiale (SFGS) and two bands in stratum griseum centrale (SGC). Immunoreactive perikarya with neurites that extend radially through the stratum album centrale (SAC) are located in the stratum periventriculare. BD-17 immunoreactivity is to a great extent located in tectal layers that receive direct retinal input, i.e. the SO, SFGS and SGC. These layers are known to receive input also from other visual centers, such as the pretectum (SO, SFGS), the nucleus isthmi (SO, SFGS, SGC), as well as non-visual regions as the telencephalon (SGC). High levels of 2-[125I]-iodomelatonin binding sites have previously been demonstrated in all layers of the salmon optic tectum except the SM and SPV. Thus it appears likely that GABA and/or benzodiazepines and melatonin play a role in visual processing in the optic tectum of teleost fish.