Melatonin synthesis and circadian tryptophan hydroxylase activity in chicken retina following destruction of serotonin immunoreactive amacrine and bipolar cells by kainic acid.

The neurotoxic excitatory amino acid analog, kainic acid, was used to destroy serotonin-immunoreactive inner retinal neurons, bipolar cells and amacrine cells, in retinas of chickens. Tryptophan hydroxylase activity and melatonin content were examined in the kainic acid-lesioned retinas. Tryptophan hydroxylase activity was present in kainic acid-lesioned retinas and displayed a circadian rhythm. Nocturnal levels of activity in lesioned and control retinas were similar. Melatonin synthesis occurred in kainic acid-lesioned retinas in a diurnal cycle as in normal retinas. Dark-phase melatonin content of kainic acid-lesioned retinas exceeded that of controls. We conclude that most, if not all, circadian tryptophan hydroxylase activity and melatonin synthesis occurs in cells other than the cells that contain most of the serotonin in retina, serotonin-immunoreactive bipolar and amacrine cells.