LP-211

Neuropsychiatric disorders, such as depression and anxiety, are frequently associated with neuropathic pain. Despite the availability of various analgesics, their efficacy in treating neuropathic pain comorbidities has been limited. The aim of this study was to evaluate the impact of a 5-hydroxytryptamine 7 agonist (LP-211) in combination with gabapentin on two distinct models of neuropathic pain in rats, namely streptozotocin-induced diabetic neuropathic pain and partial sciatic nerve ligation. The sensory-discriminative parameter of mechanical allodynia was assessed using Von Frey monofilaments. We evaluated the affective components of neuropathic mechanical allodynia, such as depression and anxiety, using a forced swim test, sucrose preference test, elevated plus maze, and novelty-induced hypophagia, respectively. We measured the levels of monoamines in the hippocampus using HPLC. The electrical activity of neurons was estimated through in-vivo electrophysiology. LP-211 alone did not result in a significant increase in paw withdrawal thresholds, but when combined with gabapentin, it showed a significant increase. Furthermore, the combination treatment reduced the neuronal response of wide dynamic range neurons because of mechanical stimulation, and a significant modulation of monoamines in the hippocampus was observed. Importantly, the combination treatment exhibited antidepressant-like activity, by a significant decrease in immobility time and an increase in percentage sucrose preference. It also demonstrated anxiolytic-like activity, as indicated by an increase in time spent in open arms and an increase in food intake in a novel environment. Overall, the results of this study provide evidence that multiple therapies with different mechanisms may alleviate mechanical allodynia and its comorbidities.

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.