IMMUNOCYTOCHEMICAL LOCALIZATION OF THE GABA(A) BENZODIAZEPINE RECEPTOR BETA(2) BETA(3) 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 GABA(A)/benzodiazepin e receptor beta(2)/beta(3) subunits with a monoclonal antibody (BD-17) in the optic tectum of the salmon brain. A weak immunoreactivity is f ound in the rostral stratum marginale (SM), strong labelling of the ne uropil is shown in a thin band in stratum opticum (SO), two bands in s tratum fibrosum et griseum superficiale (SFGS) and two bands in stratu m griseum centrale (SGC). Immunoreactive perikarya with neurites that extend radially through the stratum album centrale (SAC) are located i n the stratum periventriculare. BD-17 immunoreactivity is to a great e xtent located in tectal layers that receive direct retinal input, i.e the SO, SFGS and SGC. These layers are known to receive input also fro m other visual centers, such as the pretectum (SO, SFGS), the nucleus isthmi (SO, SFGS, SGC), as well as non-visual regions as the telenceph alon (SGC). High levels of 2-[I-125]-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 benzodi azepines and melatonin play a role in visual processing in the optic t ectum of teleost fish.