Through hyperpolarization of post-synaptic neurons, action potential frequency is decreased.
Serotonin is recycled via a re-uptake mechanism, where it is taken back into the pre-synaptic terminal and readied for reuse. There is also a small amount of diffusion into the area surrounding the synapse, the neurotransmitter being ultimately taken into glial cells.
Desensitization can occur with chronic stimulation. Two methods are known. Decreased receptor density, as well as changes in the cell physiology such that the messenger complex is more efficient.