Differential synaptic inhibition and serotonin 5-HT₇ receptor-mediated modulation in identified dorsal horn neurons

Serotonergic modulation of pain transmission in the spinal cord involves the activation of multiple receptor types, including 5-HT₇ receptors. Activation of spinal 5-HT₇ receptors appears to have a predominant antinociceptive effect in various animal models. Although the serotonergic modulation of dorsal horn circuits has been extensively investigated, information about the specific effects of serotonergic receptors on identified neuron types remains limited. To address this, we have employed transgenic mice expressing channelrhodopsin-2 (ChR2) in inhibitory neurons, under the control of the vesicular GABA transporter (VGAT) promoter. Postsynaptic inhibitory responses (oIPSCs) induced by optogenetic stimulation of spinal cord slices displayed distinct properties in superficial dorsal horn VGAT+ and VGAT- neurons (inhibitory and putative excitatory neurons, respectively). While oIPSCs recorded from VGAT + neurons showed GABA- and glycine-mediated components of similar amplitudes, oIPSCs from VGAT- neurons were predominantly mediated by glycine. Consistently, immunofluorescence staining for the glycine transporter GlyT2 in mice expressing dTomato in GAD2 neurons revealed that GlyT2+ boutons primarily contact putative excitatory interneurons, which are negative for GAD2. Activation of 5-HT₇ receptors by the agonist LP-211 significantly enhanced both the frequency of spontaneous inhibitory currents and the amplitude of oIPSCs in VGAT- neurons. In minimal optical stimulation experiments, application of LP-211 reduced the number of synaptic failures and increased the quantal content of oIPSCs, indicating presynaptic modulation mediated by 5-HT₇ receptors. Our results suggest that enhanced synaptic inhibition of dorsal horn excitatory interneurons may contribute to the role of 5-HT₇ receptors in suppressing pain transmission at the spinal cord level.