Interaction between serotonin and nitric oxide (NO) in the activation of the serotoninergic system in the common snail.

The effects of serotonin and NO donors on serotoninergic neurons (more than 60) in the brain of the common snail Helix lucorum were studied. Serotonin and NO donors induced depolarization in all neurons. and increased spike activity and activated the synchronous synaptic input, including train-like input. resulting in the onset of synchronous train activity in all these neurons. The excitatory effect of serotonin was significantly decreased by 5,7-dihydroxytryptamine (5.7-DOT) and monomethylarginine--a blocker of endogenous NO synthesis. Both these substances blocked the serotonin activation of the synchronous train input. 5.7-DOT also blocked the activation of this input by NO donors. but had no effect on their excitatory actions. The effects of 5,7-DOT developed quickly, were reversible, and were comparable to the effects of serotonin receptor antagonists. The data obtained here provide evidence that serotonin and NO have similar regulatory effects on the serotoninergic system in the snail brain. Not only do they excite serotoninergic neurons, but they also coordinate their functioning by activating common synaptic inputs. which are apparently also serotoninergic. It is suggested that NO has the role of a second messenger during serotoninergic excitation and functions as a co-transmitter for the presynaptic input.