Enkephalinergic Circuit Involved in Nociceptive Modulation in the Spinal Dorsal Horn

• Published in: Neuroscience Vol. 429; pp. 78 - 91
• Main Authors: Bai, Yang, Li, Meng-Ying, Ma, Jiang-Bo, Li, Jia-Ni, Teng, Xiao-Yu, Chen, Ying-Biao, Yin, Jun-Bin, Huang, Jing, Chen, Jing, Zhang, Ting, Qiu, Xin-Tong, Chen, Tao, Li, Hui, Wu, Sheng-Xi, Peng, Ya-Nan, Li, Xiang, Kou, Zhen-Zhen, Li, Yun-Qing
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 01.03.2020
• Subjects: Animals; Axons; enkephalin; Mice; Mice, Inbred C57BL; Neurons; Nociception; pain; Posterior Horn Cells; preproenkephalin-GFP transgenic mouse; Spinal Cord Dorsal Horn; spinal dorsal horn; synaptic transmission; PBS; pain; ENK; PPE; IHC; DOR; HRP; mEPSC; ir; mRNA; ACSF; DPDPE; GFP; enkephalin; preproenkephalin-GFP transgenic mouse; SDH; GABA; TMR; IB4; synaptic transmission; PBN; SP; spinal dorsal horn; ISH
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/j.neuroscience.2019.12.020

Local ENKergic circuits involved in nociceptive modulation within the SDH: ENKergic neurons receive excitatory inputs from primary afferents and send projections toward PBN-projecting neurons. In the condition of pain stimuli, primary afferents may directly activate PBN-projecting neurons within lamina I to transmit pain signals toward upper brain structures, as well as ENKergic interneurons within lamina II to modulate spinal pain processing. ENK might inhibit presynaptic glutamate release via presynaptic DORs through non-synaptic mechanisms and suppress the excitability of projection neurons via postsynaptic DORs. [Display omitted] •Non-peptidergic afferents were in close apposition to PBN-projecting neurons within lamina I.•Non-peptidergic afferents were in close apposition to ENKergic neurons within lamina II.•ENKergic terminals were in close apposition to PBN-projecting neurons within lamina I.•ENK might inhibit the glutamatergic transmission towards PBN-projecting neurons via presynaptic and postsynaptic DORs. Enkephalin (ENK) has been implicated in pain modulation within the spinal dorsal horn (SDH). Revealing the mechanisms underlying ENK analgesia entails the anatomical and functional knowledge of spinal ENK-ergic circuits. Herein, we combined morphological and electrophysiological studies to unravel local ENK-ergic circuitry within the SDH. First, the distribution pattern of spinal ENK-ergic neurons was observed in adult preproenkephalin (PPE)-GFP knock-in mice. Next, the retrograde tracer tetramethylrhodamine (TMR) or horseradish peroxidase (HRP) was injected into the parabrachial nucleus (PBN) in PPE-GFP mice. Immunofluorescent staining showed I-isolectin B4 (IB4) labeled non-peptidergic afferents were in close apposition to TMR-labeled PBN-projecting neurons within lamina I as well as PPE-immunoreactivity (-ir) neurons within lamina II. Some TMR-labeled neurons were simultaneously in close association with both IB4 and PPE-ir terminals. Synaptic connections of these components were further confirmed by electron microscopy. Finally, TMR was injected into the PBN in adult C57BL/6 mice. Whole-cell patch recordings showed that δ-opioid receptor (DOR) agonist, [D-Pen2,5]-enkephalin (DPDPE, 1 µM), significantly reduced the frequency of miniature excitatory postsynaptic current (mEPSC) and decreased the activity of TMR-labeled neurons. In conclusion, spinal ENKergic neurons receive direct excitatory inputs from primary afferents, which might be directly recruited to release ENK under the condition of noxious stimuli; ENK could inhibit the glutamatergic transmission towards projecting neurons via presynaptic and postsynaptic DORs. These morphological and functional evidence may explain the mechanisms underlying the analgesic effects exerted by ENK within the SDH.