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 intermediate ganglion cells. To investigate whether acetylcholine plays a role in the transduction of pineal photoreceptor signals, extracellular recordings were performed from ganglion cells of intact superfused pineal organs of the rainbow trout. Bath-applied acetylcholine increased the spike discharge rate of 96% of achromatic ganglion cells in a dose-dependent 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 also active if applied during synaptic blockade with low Ca2+/high Mg(2+)-medium, demonstrating the presence of cholinergic receptors at the ganglion cell level. These data represent the first demonstration of acetylcholine constituting a postsynaptic modulation of photoreceptor signals in the trout pineal organ.