An Su Bin, Cho Yi Sul, Park Sook Kyung, Kim Yun Sook, Bae Yong Chul
Department of Anatomy and Neurobiology, School of Dentistry, Kyungpook National University, Daegu, Republic of Korea.
Front Cell Neurosci. 2023 Mar 30;17:1162874. doi: 10.3389/fncel.2023.1162874. eCollection 2023.
Recent studies have shown a direct projection of nociceptive trigeminal afferents into the lateral parabrachial nucleus (LPBN). Information about the synaptic connectivity of these afferents may help understand how orofacial nociception is processed in the LPBN, which is known to be involved primarily in the affective aspect of pain. To address this issue, we investigated the synapses of the transient receptor potential vanilloid 1-positive (TRPV1+) trigeminal afferent terminals in the LPBN by immunostaining and serial section electron microscopy. TRPV1 + afferents arising from the ascending trigeminal tract issued axons and terminals (boutons) in the LPBN. TRPV1+ boutons formed synapses of asymmetric type with dendritic shafts and spines. Almost all (98.3%) TRPV1+ boutons formed synapses with one (82.6%) or two postsynaptic dendrites, suggesting that, at a single bouton level, the orofacial nociceptive information is predominantly transmitted to a single postsynaptic neuron with a small degree of synaptic divergence. A small fraction (14.9%) of the TRPV1+ boutons formed synapses with dendritic spines. None of the TRPV1+ boutons were involved in axoaxonic synapses. Conversely, in the trigeminal caudal nucleus (Vc), TRPV1+ boutons often formed synapses with multiple postsynaptic dendrites and were involved in axoaxonic synapses. Number of dendritic spine and total number of postsynaptic dendrites per TRPV1+ bouton were significantly fewer in the LPBN than Vc. Thus, the synaptic connectivity of the TRPV1+ boutons in the LPBN differed significantly from that in the Vc, suggesting that the TRPV1-mediated orofacial nociception is relayed to the LPBN in a distinctively different manner than in the Vc.
近期研究表明，伤害性三叉神经传入纤维可直接投射至外侧臂旁核（LPBN）。了解这些传入纤维的突触连接信息，可能有助于理解LPBN如何处理口面部伤害感受，已知该核主要参与疼痛的情感方面。为解决这一问题，我们通过免疫染色和连续切片电子显微镜研究了LPBN中瞬时受体电位香草酸受体1阳性（TRPV1+）三叉神经传入终末的突触。来自三叉神经升支的TRPV1+传入纤维在LPBN发出轴突和终末（终扣）。TRPV1+终扣与树突干和树突棘形成不对称型突触。几乎所有（98.3%）的TRPV1+终扣与一个（82.6%）或两个突触后树突形成突触，这表明在单个终扣水平，口面部伤害性信息主要以较小程度的突触发散传递至单个突触后神经元。一小部分（14.9%）的TRPV1+终扣与树突棘形成突触。没有TRPV1+终扣参与轴-轴突触。相反，在三叉神经尾侧核（Vc）中，TRPV1+终扣常与多个突触后树突形成突触，并参与轴-轴突触。LPBN中每个TRPV1+终扣的树突棘数量和突触后树突总数显著少于Vc。因此，LPBN中TRPV1+终扣的突触连接与Vc中的显著不同，表明TRPV1介导的口面部伤害感受以与Vc明显不同的方式传递至LPBN。
