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Mechanism Underlying the Analgesic Effect Exerted by Endomorphin-1 in the rat Ventrolateral Periaqueductal Gray

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Abstract

The ventrolateral periaqueductal gray (vlPAG) is an important brain area, in which 5-HTergic neurons play key roles in descending pain modulation. It has been proposed that opioid peptides within the vlPAG can excite the 5-HTergic neurons by alleviating tonic inhibition from GABAergic neurons, the so-called disinhibitory effect. However, no direct morphological evidence has been observed for the micro-circuitry among the opioid peptide-, GABA-, and 5-HT-immunoreactive (ir) profiles nor for the functional involvement of the opioid peptides in the intrinsic properties of GABAergic and 5-HTergic neurons. In the present study, through microscopic observation of triple-immunofluorescence, we firstly identified the circuitry among the endomorphin-1 (EM1, an endogenous ligand for the μ-opioid receptor)-ir terminals and GABA-ir and 5-HT-ir neurons within the rat vlPAG. The synaptic connections of these neurons were further confirmed by electron microscopy. Through the in vitro whole-cell patch-clamp method, we showed that EM1 has strong inhibitory effects on the spiking of GABAergic neurons. However, although the resting membrane potential was hyperpolarized, EM1 actually increased the firing of 5-HTergic neurons. More interestingly, EM1 strongly inhibited the excitatory input to GABAergic neurons, as well as the inhibitory input to 5-HTergic neurons. Finally, behavioral results showed that pretreatment with a GABAA receptor antagonist potentiated the analgesic effect of EM1, while treatment with a GABAA receptor agonist blocked its analgesic effect. In summary, by utilizing morphological and functional methods, we found that the analgesic effect of EM1 is largely dependent on its potent inhibition on the inhibitory inputs to 5-HTergic neurons, which overwhelms EM1’s direct inhibitory effect on 5-HTergic neurons.

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Acknowledgments

The authors wish to thank Dr. Giannina Descalzi (Department of Neuroscience, Icahn School of Medicine, USA) for carefully reading and constructive suggestions on the article. This work was supported by National Natural Science Foundation of China (31010103909, 81371239 to Y.-Q. Li, 31171068 to J.-Q. Du and 31371126 to T. Chen).

Conflict of Interest

The authors declare that they have no conflicts of interest with any of the work presented in this manuscript.

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    Authors and Affiliations

    1. Department of Anatomy, Histology and Embryology, K. K. Leung Brain Research Centre, The Fourth Military Medical University, Xi’an, 710032, China

      Tao Chen, Ban Feng, Yu-Lin Dong, Ting Zhang, Jun-Bin Yin & Yun-Qing Li

    2. Department of Physiology and Pathophysiology, College of Medicine, Xi’an Jiaotong University, Xi’an, 710061, China

      Jing Li, Fu-Quan Huo & Jian-Qing Du

    3. Department of Neurosurgery, Navy General Hospital, Beijing, 100048, China

      Rui Hui

    4. Collaborative Innovation Center for Brain Science, Fudan University, Shanghai, 200032, China

      Yun-Qing Li

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    1. Tao Chen
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    Tao Chen, Jing Li, Ban Feng, Rui Hui and Yu-Lin Dong contributed equally to this work.

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    Chen, T., Li, J., Feng, B. et al. Mechanism Underlying the Analgesic Effect Exerted by Endomorphin-1 in the rat Ventrolateral Periaqueductal Gray. Mol Neurobiol 53, 2036–2053 (2016). https://doi.org/10.1007/s12035-015-9159-5

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