Serotonergic modulation of inhibitory synaptic transmission in mouse inferior colliculus

The inferior colliculus (IC) transmits the ascending auditory signal to the thalamic medial geniculate nucleus. Previous studies have reported that serotonergic input originating from the raphe nuclei has a strong influence on signal processing within the central nucleus of the IC. To identify the cellular target for the serotonergic modulation in the IC, we examined the effect of serotonin as well as selective serotonin reuptake inhibitor (SSRI) fluvoxamine on spontaneous GABAergic and glycinergic inhibitory postsynaptic currents (sIPSCs) recorded with whole-cell recordings.Consistent with earlier studies, we confirmed that serotonin robustly enhanced the frequency, but not amplitude, of GABAergic sIPSCs. It should be noted that the application of fluvoxamine alone marginally increased the frequency of GABAergic sIPSCs. These findings suggest that serotonin is endogenously released even in slice preparations, and it negatively modulates the tone of activity of inhibitory neurons within IC. We also examined the effect of serotonin and fluvoxamine on glycinergic sIPSCs and found that serotonin has a significantly weaker effect on glycinergic sIPSCs than on GABAergic sIPSCs. The differential sensitivity of the GABAergic and glycinergic sIPSCs to serotonin implies that serotonergic input plays a specific role in auditory information processing.Moreover, it suggests that the serotonergic input may contribute to pathological conditions such as tinnitus.