Jeske I, Reis D J, Milner T A
Department of Neurology and Neuroscience, Cornell University Medical College, New York, NY 10021, USA.
Neuroscience. 1995 Mar;65(2):343-53. doi: 10.1016/0306-4522(94)00470-p.
Neurons in the caudal ventrolateral medulla may function as interneurons in the baroreceptor reflex are by inhibiting sympathoexcitatory bulbospinal neurons in rostral ventrolateral medulla. While some caudal ventrolateral medullary neurons are excited orthodromically by baroreceptors and antidromically from the rostral ventrolateral medulla, there is no anatomical evidence to prove that these barosensory neurons of the caudal ventrolateral medulla monosynaptically innervate the bulbospinal neurons in the rostral ventrolateral medulla. To establish the presence of such a direct projection, barosensory neurons were identified in the rostral caudal ventrolateral medulla of anesthetized rats by criteria that they spontaneously discharged with a cardiac rhythm and were excited by baroreceptor stimulation. The anterograde tracer biocytin was iontophoresed onto these neurons and, in the same animal, the retrograde tracer wheatgerm-agglutinated apo-horseradish peroxidase conjugated to gold particles was injected by micropressure into the ipsilateral spinal (thoracic level 3) intermediolateral cell column to label bulbospinal neurons. After 18-24 h, rats were killed and sections through the rostral ventrolateral medulla were processed for both markers. By light microscopy, numerous biocytin-labeled varicose processes overlapped rostral ventrolateral medullary neurons containing wheatgerm-agglutinated apo-horseradish peroxidase conjugated to gold particles. By electron microscopy, biocytin was found in axons and terminals. The terminals (n = 76) were large (0.6-1.2 microns in diameter), contained numerous small, clear vesicles and formed primarily symmetric synapses on perikarya and large (1.5-4.5 microns) dendrites within the rostral ventrolateral medulla. Some of these target neurons contained wheatgerm-agglutinated apo-horseradish peroxidase conjugated to gold particles associated with lysosomes and multivesicular bodies in the cytoplasm. The results indicate that: (i) neurons in the barosensory sympathoinhibitory region of the caudal ventrolateral medulla directly synapse on bulbospinal neurons in the rostral ventrolateral medulla; and (ii) the synaptic profile (symmetric synapse) and location (perikarya and large dendrites) is consistent with the conclusion that baroreceptor neurons of the caudal ventrolateral medulla potently and monosynaptically inhibit sympathoexcitatory neurons of the rostral ventrolateral medulla. The findings support the hypothesis that the barosensory region of the rostral caudal ventrolateral medulla is an intermediate relay in the baroreceptor reflex are.
延髓尾端腹外侧的神经元可能通过抑制延髓头端腹外侧的交感兴奋型延髓脊髓神经元，而在压力感受器反射弧中充当中间神经元。虽然一些延髓尾端腹外侧神经元可被压力感受器顺向兴奋，并被来自延髓头端腹外侧的逆向兴奋，但尚无解剖学证据证明延髓尾端腹外侧的这些压力感受神经元能单突触地支配延髓头端腹外侧的延髓脊髓神经元。为确定是否存在这种直接投射，在麻醉大鼠的延髓头端尾端腹外侧，根据能随心脏节律自发放电且能被压力感受器刺激兴奋的标准来识别压力感受神经元。将顺行示踪剂生物胞素离子导入这些神经元，在同一只动物中，通过微压将与金颗粒偶联的逆行示踪剂麦胚凝集素-脱辅基辣根过氧化物酶注入同侧脊髓（胸段3水平）的中间外侧细胞柱，以标记延髓脊髓神经元。18 - 24小时后，处死大鼠，对延髓头端腹外侧的切片进行两种标记物的处理。通过光学显微镜观察，大量生物胞素标记的曲张突起与含有与金颗粒偶联的麦胚凝集素-脱辅基辣根过氧化物酶的延髓头端腹外侧神经元重叠。通过电子显微镜观察，在轴突和终末中发现了生物胞素。这些终末（n = 76）较大（直径0.6 - 1.2微米），含有大量小而清亮的囊泡，主要在延髓头端腹外侧的胞体和大（1.5 - 4.5微米）树突上形成对称性突触。这些靶神经元中的一些含有与细胞质中的溶酶体和多泡体相关的、与金颗粒偶联的麦胚凝集素-脱辅基辣根过氧化物酶。结果表明：（i）延髓尾端腹外侧压力感受性交感抑制区的神经元直接与延髓头端腹外侧的延髓脊髓神经元形成突触；（ii）突触形态（对称性突触）和位置（胞体和大树突）与以下结论一致，即延髓尾端腹外侧的压力感受神经元能有效且单突触地抑制延髓头端腹外侧的交感兴奋神经元。这些发现支持了这样一种假说，即延髓头端尾端腹外侧的压力感受区是压力感受器反射弧中的一个中间中继站。
