The dorsal raphe nucleus (DRN) is the origin of the central serotonin 5-hydroxytryptamine (5-HT) system and plays an important role in the regulation of many physiological functions such as sleep/arousal, food intake and mood. In order to understand the regulatory mechanisms of 5-HT system, characterization of the types of neurons is necessary. We performed electrophysiological recordings in acute slices of glutamate decarboxylase 67-green fluorescent protein knock-in mice. We utilized this mouse to identify visually GABAergic cells. Especially, we examined postsynaptic responses mediated by 5-HT receptors between GABAergic and serotonergic cells in the DRN. Various current responses were elicited by 5-HT and 5-HT(1A) or 5-HT(2A/2C) receptor agonists in GABAergic cells. These results suggested that multiple 5-HT receptor subtypes overlap on GABAergic cells, and their combination might control 5-HT cells. Understanding the postsynaptic 5-HT feedback mechanisms may help to elucidate the 5-HT neurotransmitter system and develop novel therapeutic approaches.