SYNAPTIC TRANSMISSION IN THE PINEAL EYE OF YOUNG XENOPUS-LAEVIS TADPOLES - A ROLE FOR NMDA AND NON-NMDA GLUTAMATE AND NON-GLUTAMINERGIC RECEPTORS

The pineal eye of Xenopus laevis tadpoles is directly photosensitive. A sudden reduction in light intensity produces a burst of activity in the pineal ganglion cells, which is closely followed by the onset of s wimming. In this paper I present the results of experiments on the eff ects of agonists and antagonists of candidate pineal transmitters on g anglion cell activity. I found that NMDA and non-NMDA excitatory amino acid (EAA) agonists increased pineal activity, indicating the presenc e of both types of receptor. Kynurenic acid reduced activity, thus con firming that the photoreceptor transmitter is an EAA. Under physiologi cal conditions, CNQX blocked activity almost completely whilst APS had little effect. In Mg2+-free saline CNQX had a considerably smaller ef fect, but joint application of CNQX and AP5 blocked almost all activit y; therefore, the NMDA receptors are subject to blockage by Mg2+. Alth ough GABA(A) and ACh receptors appear to be present, no evidence was f ound for GABA or ACh as pineal transmitters. In addition, 5-HT had no effect on pineal activity. The main pineal transmitter is an EAA actin g on ganglion cells through both NMDA and non-NMDA receptors. Other re ceptors are present but appear to have no role in controlling pineal a ctivity at this stage.