ACETYLCHOLINE MODULATES GANGLION-CELL ACTIVITY IN THE TROUT PINEAL ORGAN

IN the teleost pineal organ light activates functional photoreceptors, which transmit electrical activity to the brain via axons of intermed iate ganglion cells. To investigate whether acetylcholine plays a role in the transduction of pineal photoreceptor signals, extracellular re cordings were performed from ganglion cells of intact superfused pinea l organs of the rainbow trout. Bath-applied acetylcholine increased th e spike discharge rate of 96% of achromatic ganglion cells in a dose-d ependent manner. The light response curve of ganglion cells, which was obtained by plotting spike rate vs light intensity, was significantly shifted by acetylcholine to higher frequencies. Acetylcholine was als o active if applied during synaptic blockade with low Ca2+/high Mg2+-m edium, demonstrating the presence of cholinergic receptors at the gang lion cell level. These data represent the first demonstration of acety lcholine constituting a postsynaptic modulation of photoreceptor signa ls in the trout pineal organ.