Crayfish brain-protocerebrum and retina show serotonergic functional relationship

The results from various studies have indicated possible functional relationships between crayfish electroretinogram (ERG) rhythmic amplitude changes and the serotonergic pathways projecting from the central brain through the optic neuropils to the eye, but to date, this functional interaction has not been proven. Here, in a set of experiments using an isolated eyestalk–brain preparation, we investigated whether there is a circadian input from the brain to retina that regulates this rhythm. We sought to determine whether the protocerebral bridge (PB) stimulation affects the ERG amplitude in accordance with the zeitgeber time (ZT) and whether 5-HT modulates the associate input. Our results showed that photic stimulation of retina produced changes in both the amplitude and the frequency of spontaneous electrical activity in the protocerebral neuropils. In addition, electrical stimulation of the medial protocerebrum, particularly the PB, produced statistically significant changes in the ERG that depended on both the time of day and the level of serotonin. This suggests that pathways between retina and PB seem to be serotonergic.

► Stimulation of the protocerebral bridge in vitro increments the amplitude of the ERG in crayfish, particularly at night.
► The effect of protocerebral bridge is enhanced by exogenous 5HT and antagonized by 5HT1 and 5HT2 antagonists.
► Retinal stimulation of the central brain increases electrical activity of the protocerebral brain and the anterior and median neuropils.
► Light stimulus elicites a sustained high amplitude unitary activity in the protocerebral bridge but not in the anterior and median neuropils.