Dopamine Modulates the Lateral Giant Neuron and Serotonergic Facilitation in Crayfish

The lateral giant neuron (LG) and neurons in many other animals are modulated by dopamine and serotonin. Though individually the amines have opposite effects on the LG, the previous chapter showed that serotonergic facilitation is “chosen” by the cell when both modulators are present at the same time. Preliminary data showed that serotonin conceals dopaminergic depression when serotonin is applied prior to dopamine. However, when the dopamine system is activated first, the effect of dopamine on serotonergic facilitation in the LG seemed to depend on how long the dopamine system was activated prior to an application of serotonin. Using the same crayfish nerve cord preparation and electrophysiological methods, this chapter aimed to understand these temporal differences in the interaction between the two modulatory effects. It was found that long doses of dopamine blocked serotonergic facilitation. Short doses of dopamine given prior to serotonin application produced the strong facilitatory effect observed when the amine systems were activated simultaneously, as indicated by the occurrence of superthreshold EPSPs that caused the LG to fire action potentials. These results show that dopamine has two distinct effects on serotonergic facilitation that depends on how long dopaminergic pathways have been activated.