Effects of neurochemicals upon a dinoflagellate photoresponse

Photoresponsiveness by Gymnodinium splendens Lebour was monitored quantitatively by a microscope-television system. Exposure to the catecholamines DOPA and Dopamine caused a decrease in light sensitivity, while 0.01 mM norepinephine, or isoproterenol did not affect photoresponsiveness. Classical catecholamine blocking agents, dichloroisoproterenol, propranolol, and dibenzyline, and an inhibitor of DOPA synthesis, alpha-methyl-rho-tyrosine, caused an increase in sensitivity. In addition, acetylcholine and an inhibitor of acetylcholinesterase activity, eserine, caused an increase in sensitivity, while an inhibitor of acetylcholine action atropine, had the opposite effect. These experiments suggest that G. splendens may have an antagonistic catecholamine-cholinergic system which participates in regulating photosensitivity.