The optic tectum of the salmon:site of interaction of neurohormonal photoperiodic and neural visual signals. The GABAergic neuronal system and melatonin receptors.

Melatonin is a neurohormone which mediates photoperiodic signals from the pineal organ to the brain. GABA (g-aminobutyric acid) is the major inhibitory transmitter in the central nervous system. It has previously been suggested that functional interaction exists between central binding sites for GABA/benzodiazepines (GABAA/bzd receptors) and melatonin in the central nervous system, and that GABA and benzodiazepines may be involved in circadian responses to light. To investigate if there is a structural basis for interactions between the neurohormone melatonin and GABA in the primary visual center, the optic tectum of the brain of the Atlantic salmon (Salmo salar) I compared the distribution of the GABAA/benzodiazepine receptor complex(GABAA/bzd receptor) to that of melatonin receptors, using immunohistochemistry (for GABAA/bzd receptors) and quantitative autoradiography (for melatonin receptors). I also investigated if there is any difference in the binding of 2-[125I]iodomelatonin at day compared to at night. I was also interested to know which transmitter substance that mediates the light-/dark message from the retina to the optic tectum via the optic tract. By using immunohistochemistry I studied the distribution of GABA in the retina and in the optic tectum. The ganglion cells in the retina showed no GABA immunoreactivity indicating that GABA is not the transmitter mediating the light-/dark message to the optic tectum. An overlap in the distribution of GABAA/bzd receptors, melatonin receptors, and GABAergic neurons and fibers in the retinorecipient layers of the optic tectum was observed. Thus, there is a structural basis for interactions between GABA and melatonin in the optic tectum of the Atlantic salmon. It is likely that GABA is involved in the intrinsic processing of the light-/dark message in the optic tectum. The 2-[125I]iodomelatonin was found to be specific, saturable, displaceable and of high affinity, with a dissociation constant (Kd) 85±14 pM and a maximum number of binding sites (Bmax) 58±7 fmol/mg protein. The Hill plot coefficient (nH) was 2.0 which indicates the presence of two types of binding sites. The presence of two binding sites could, however, not be statistically verified using MACLIGAND. No significant differences were found when comparing the 2-[125I]iodomelatonin at night to that at day

GABA and GABA-transporter (GAT-1) immunoreactivities in the retina of the salmon (Salmo salar L.)

Putative GABAergic elements in the retina of the Atlantic salmon have been identified by immunohistochemistry, utilising polyclonal antisera against γ-aminobutyric acid (GABA) and the GABA transporter GAT-1. Cell types immunoreactive (ir) for GABA comprise horizontal cells, amacrine cells, displaced amacrine cells in the ganglion cell layer, displaced amacrine cells in the inner plexiform layer (interstitial cells), and Muller cells. In addition, a GABA-immunonegative type of interstitial cell was also identified. In the inner plexiform layer, GABAir fibres were organised in sublayers that were strikingly similar to the sublayering of GAT-1ir fibres. GAT-lir cell bodies comprise amacrine cells and displaced amacrine cells that may represent a subpopulation of the GABAir ones. In view of the very similar sublayering of GABAir and GAT-ir fibres in the IPL we suggest that a similar type of GABA transporter, that can be recognised with antibodies against rat GAT-1, is present at least in the dendrites of all GABAir amacrine cells but is not expressed in the cell bodies of all GABAir cells