Role of dopamine and serotonin in modulation of snail defensive behavior

The application of 10(-5)-10(-6) M dopamine to physiologic saline surrounding snail CNS leads to decreased excitability of the LPa7 neuron (presynaptic in relation to the defensive behavior command neurons) and decreased amplitude of the monosynaptic excitatory postsynaptic potential (EPSP) in the command neurons brought about by intracellular stimulation of the LPa7 neuron. In addition, dopamine causes an average 70% decrease in the amplitude of the summation EPSP in command neurons in response to intestinal nerve stimulation, a 6-8 mV change in the resting potential towards hyperpolarization, and an average 20% decrease in the command neurons' input resistance. These actions can lead to an overall increase in the threshold of the defensive system's reaction to stimulation. The effect of dopamine on command neurons is significantly reduced in the presence of serotonin. In the presence of dopamine, the efficacy of serotonin action on the size of the response elicited in command neurons is reduced. Based on the data obtained, it was concluded that the interrelation of dopamine and serotonin concentrations could be the basis for the formation of behavioral choice in snails.