Serotonin is a key modulator of neuronal activity both at the system level and at the level of local (short-range) interactions. However, in contrast to the synaptically connected neuron ensembles, there are much less qualitative models that describe the serotonin-controlled neural circuits. In this paper, we propose a relatively simple model of serotonergic (serotonin-releasing and serotonin-sensitive) neuron.