Neurons in the barosensory area of the caudal ventrolateral medulla project monosynaptically on to sympathoexcitatory bulbospinal neurons in the rostral ventrolateral medulla.

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.