Meso-diencephalic dopaminergic neurons are known to modulate locomotor behaviors through their ascending projections to the basal ganglia, which in turn project to the mesencephalic locomotor region, known to control locomotion in vertebrates. In addition to their ascending projections, dopaminergic neurons were found to increase locomotor movements through direct descending projections to the mesencephalic locomotor region and spinal cord. Intriguingly, fibers expressing tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis, were also observed around reticulospinal neurons of lampreys. We now examined the origin and the role of this innervation. Using immunofluorescence and tracing experiments, we found that fibers positive for dopamine innervate reticulospinal neurons in the four reticular nuclei of lampreys. We identified the dopaminergic source using tracer injections in reticular nuclei, which retrogradely labeled dopaminergic neurons in a caudal diencephalic nucleus (posterior tuberculum [PT]). Using voltammetry in brain preparations isolated in vitro, we found that PT stimulation evoked dopamine release in all four reticular nuclei, but not in the spinal cord. In semi-intact preparations where the brain is accessible and the body moves, PT stimulation evoked swimming, and injection of a D₁ receptor antagonist within the middle rhombencephalic reticular nucleus was sufficient to decrease reticulospinal activity and PT-evoked swimming. Our study reveals that dopaminergic neurons have access to command neurons that integrate sensory and descending inputs to activate spinal locomotor neurons. As such, our findings strengthen the idea that dopamine can modulate locomotor behavior both via ascending projections to the basal ganglia and through descending projections to brainstem motor circuits.

SIGNIFICANCE STATEMENT Meso-diencephalic dopaminergic neurons play a key role in modulating locomotion by releasing dopamine in the basal ganglia, spinal networks, and the mesencephalic locomotor region, a brainstem region that controls locomotion in a graded fashion. Here, we report in lampreys that dopaminergic neurons release dopamine in the four reticular nuclei where reticulospinal neurons are located. Reticulospinal neurons integrate sensory and descending suprareticular inputs to control spinal interneurons and motoneurons. By directly modulating the activity of reticulospinal neurons, meso-diencephalic dopaminergic neurons control the very last instructions sent by the brain to spinal locomotor circuits. Our study reports on a new direct descending dopaminergic projection to reticulospinal neurons that modulates locomotor behavior.

已知中脑多巴胺能神经元通过向上投射到基底神经节来调节运动行为，基底神经节又投射到中脑运动区域，已知控制脊椎动物的运动。除了它们的上升投射之外，还发现多巴胺能神经元通过对中脑运动区域和脊髓的直接下降投射来增加运动。有趣的是，在七鳃鳗的网状脊髓神经元周围也观察到表达酪氨酸羟化酶（TH）的纤维，酪氨酸羟化酶是多巴胺合成的限速酶。现在我们研究了这种神经支配的起源和作用。通过免疫荧光和示踪实验，我们发现多巴胺阳性纤维支配七鳃鳗四个网状核中的网状脊髓神经元。我们通过在网状核中注射示踪剂来识别多巴胺能来源，该示踪剂逆行标记尾侧间脑核（后结节[PT]）中的多巴胺能神经元。在体外分离的大脑制剂中使用伏安法，我们发现 PT 刺激在所有四个网状核中引起多巴胺释放，但不在脊髓中。在大脑可接近且身体移动的半完整制剂中，PT刺激诱发游泳，并且在中菱脑网状核内注射D ₁受体拮抗剂足以减少网状脊髓活动和PT诱发的游泳。我们的研究表明，多巴胺能神经元可以访问指挥神经元，这些神经元整合感觉和下行输入以激活脊髓运动神经元。 因此，我们的研究结果强化了这样的观点：多巴胺可以通过对基底神经节的上行投射和对脑干运动回路的下行投射来调节运动行为。

意义声明中脑多巴胺能神经元通过在基底神经节、脊髓网络和中脑运动区域（以分级方式控制运动的脑干区域）释放多巴胺，在调节运动中发挥关键作用。在这里，我们在七鳃鳗中报告，多巴胺能神经元在网状脊髓神经元所在的四个网状核中释放多巴胺。网状脊髓神经元整合感觉和下行网状上输入来控制脊髓中间神经元和运动神经元。通过直接调节网状脊髓神经元的活动，中脑多巴胺能神经元控制大脑发送到脊髓运动回路的最后指令。我们的研究报告了一种新的直接下降的多巴胺能投射到网状脊髓神经元，调节运动行为。