Abstract
Serotonin and octopamine have opposite effects on a simple behavioral response, the crayfish's lateral giant escape reaction. Specifically, serotonin depresses the lateral giants' responsiveness, whereas octopamine enhances it. Both effects are largely confined to the disynaptic pathway from the sensory afferents to the lateral giants, although, occasionally, small effects are also seen in the monosynaptic pathway to the lateral giants. One specific locus of the octopaminergic effect is the synapse between the afferents and the largest of the sensory interneurons in the disynaptic pathway, interneuron A. Serotonin, however, does not have a consistent effect at this synapse. Although serotonin and octopamine modulate the excitability of the escape response, neither monoamine appears to significantly alter the rate at which this response habituates.