Electrophysiological studies on neuronal transmission in the frog's photosensory pineal organ. The effect of amino acids and biogenic amines.

A variety of putative neurotransmitter substances and their analogues were used to characterize the synaptic connections from photoreceptors to ganglion cells in the pineal organ of the frog. The effects of all agents were tested on electrophysiologically identified luminance cells of the pineal organ. L-Aspartate and L-glutamate caused a significant increase of neuronal cell firing, the effects were dose-related, reversible and reproducible. The aspartate and glutamate agonists N-methyl-D-aspartate and kainate possess a similar ability to induce neuronal excitations in the pineal organ, N-methyl-D-aspartate being more potent than kainic acid. The excitatory action of all agents persisted if the ganglion cells were isolated from other synaptic inputs by Co2+ treatment. The excitatory action of the amino acids was antagonized by D-alpha-aminoadipate (D-alpha-AA); the order of sensitivity to antagonism by D-alpha-AA was: N-methyl-D-aspartate greater than L-aspartate greater than L-glutamate approximately equal to kainate. Taurine, by far the most abundant amino acid in pineal tissues, markedly decreased the spontaneous activity in half of the neurons tested, the remaining cells being unresponsive. The indoleamine serotonin effectively depressed the maintained activity also in half of the cells tested. Acetylcholine had only small effects on pineal luminance cells. It is concluded that L-aspartate (and/or glutamate) might interact with postsynaptic receptors in the ganglion cell membrane and mimics the action of the natural photoreceptor transmitter.