The synaptic structure of catecholaminergic axon varicosities in the dorso-medial portion of the nucleus tractus solitarius of the cat: possible roles in the regulation of cardiovascular reflexes.
The study was performed on adult mongrel cats to investigate synaptic structures of nerve terminals in the dorso-medial portion of the nucleus tractus solitarius (NTS), where majorities of the cardiovascular afferent fibers terminate and a dense plexus of the catecholaminergic nerves exists. In normal control groups, axo-dendritic and axo-somatic contacts were commonly observed but no axo-axonic contacts were seen. Synaptic vesicles were almost exclusively spherical (97%) and less frequently flat (3%). Small cored vesicles were not observed in animals with formalin-glutaraldehyde and osmium tetroxide fixation. Two or three days after denervation of IXth and Xth cranial nerves, degenerated axon varicosities occupied about 15-20% of total population. Quantitative analysis after intracisternal injection of 5-hydroxydopamine revealed 3 different types of the varicosity: varicosities with spherical clear vesicles (85%), flat clear vesicles (3%) and small cored vesicles (12%). Axon varicosities with small cored vesicles predominantly exhibited axo-dendritic synapses and occasionally showed the closed apposition with those containing spherical clear vesicles. 6-Hydroxydopamine produced a minimal degree of degenerative changes in some axon varicosities (less than 5%). Treatment with 5-hydroxydopamine after IXth and Xth cranial nerve section gave intact axon varicosities with small cored vesicles. In conclusion, there is a relatively dense innervation of catecholaminergic nerves in the dorso-medial portion of the NTS, which apparently does not stem from the IXth and Xth cranial nerves. Possible roles of catecholaminergic nerves are discussed.
D000818:Animals; D001369:Axons / Q000502:physiology / Q000648:ultrastructure*; D002319:Cardiovascular System / Q000294:innervation*; D002415:Cats; D003391:Cranial Nerves / Q000502:physiology; D003714:Denervation; D005260:Female; D006892:Hydroxydopamines / Q000494:pharmacology; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects / Q000502:physiology / Q000648:ultrastructure*; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence; D009411:Nerve Endings / Q000648:ultrastructure; D009434:Neural Pathways; D009475:Neurons, Afferent / Q000502:physiology; D012018:Reflex; D013569:Synapses / Q000502:physiology / Q000648:ultrastructure*; D013572:Synaptic Vesicles / Q000648:ultrastructure
D006892:Hydroxydopamines