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

Sammanfattning: 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.