Department of Neurology, University of Iowa, Iowa, USA.
J Comp Neurol. 2021 Aug 1;529(11):2911-2957. doi: 10.1002/cne.25136. Epub 2021 Mar 25.
The parabrachial nucleus (PB) is composed of glutamatergic neurons at the midbrain-hindbrain junction. These neurons form many subpopulations, one of which expresses Calca, which encodes the neuropeptide calcitonin gene-related peptide (CGRP). This Calca-expressing subpopulation has been implicated in a variety of homeostatic functions, but the overall distribution of Calca-expressing neurons in this region remains unclear. Also, while previous studies in rats and mice have identified output projections from CGRP-immunoreactive or Calca-expressing neurons, we lack a comprehensive understanding of their efferent projections. We began by identifying neurons with Calca mRNA and CGRP immunoreactivity in and around the PB, including populations in the locus coeruleus and motor trigeminal nucleus. Calca-expressing neurons in the PB prominently express the mu opioid receptor (Oprm1) and are distinct from neighboring neurons that express Foxp2 and Pdyn. Next, we used Cre-dependent anterograde tracing with synaptophysin-mCherry to map the efferent projections of these neurons. Calca-expressing PB neurons heavily target subregions of the amygdala, bed nucleus of the stria terminalis, basal forebrain, thalamic intralaminar and ventral posterior parvicellular nuclei, and hindbrain, in different patterns depending on the injection site location within the PB region. Retrograde axonal tracing revealed that the previously unreported hindbrain projections arise from a rostral-ventral subset of CGRP/Calca neurons. Finally, we show that these efferent projections of Calca-expressing neurons are distinct from those of neighboring PB neurons that express Pdyn. This information provides a detailed neuroanatomical framework for interpreting experimental work involving CGRP/Calca-expressing neurons and opioid action in the PB region.
脑桥臂旁核（PB）由中脑-后脑交界处的谷氨酸能神经元组成。这些神经元形成许多亚群，其中一个亚群表达 Calca，它编码降钙素基因相关肽（CGRP）的神经肽。这个表达 Calca 的亚群已被牵涉到多种体内平衡功能中，但该区域中表达 Calca 的神经元的总体分布仍不清楚。此外，尽管以前在大鼠和小鼠中的研究已经确定了来自 CGRP-免疫反应性或 Calca 表达神经元的传出投射，但我们缺乏对其传出投射的全面了解。我们首先在 PB 及其周围区域中鉴定出具有 Calca mRNA 和 CGRP 免疫反应性的神经元，包括蓝斑核和运动三叉神经核中的神经元。PB 中的表达 Calca 的神经元显著表达μ阿片受体（Oprm1），并且与表达 Foxp2 和 Pdyn 的邻近神经元不同。接下来，我们使用 Cre 依赖性顺行示踪剂 synaptophysin-mCherry 来绘制这些神经元的传出投射。表达 Calca 的 PB 神经元强烈靶向杏仁核的亚区、终纹床核、基底前脑、丘脑板内核和腹后小细胞核以及后脑，其投射模式取决于 PB 区域内注射部位的位置。逆行轴突追踪显示，以前未报道的后脑投射来自 CGRP/Calca 神经元的头腹亚群。最后，我们表明，这些表达 Calca 的神经元的传出投射与表达 Pdyn 的邻近 PB 神经元的传出投射不同。这些信息为解释涉及 PB 区域中 CGRP/Calca 表达神经元和阿片作用的实验工作提供了详细的神经解剖学框架。
