Distribution and amino acid content of enkephalin-immunoreactive inputs onto juxtacellularly labelled bulbospinal barosensitive neurons in rat rostral ventrolateral medulla

• Published in: Neuroscience Vol. 108; no. 2; pp. 307 - 322
• Main Authors: Llewellyn-Smith, I.J, Schreihofer, A.M, Guyenet, P.G
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.01.2001; Elsevier
• Subjects: Action Potentials - physiology; Animals; Axons - metabolism; Axons - ultrastructure; baroreflex; Biological and medical sciences; Biotin - analogs & derivatives; Dendrites - metabolism; Dendrites - ultrastructure; Efferent Pathways - metabolism; Efferent Pathways - ultrastructure; Enkephalins - metabolism; Fundamental and applied biological sciences. Psychology; GABA; gamma-Aminobutyric Acid - metabolism; glutamate; Glutamic Acid - metabolism; glycine; Glycine - metabolism; Immunohistochemistry; Male; Medulla Oblongata - metabolism; Medulla Oblongata - ultrastructure; Microscopy, Electron; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Neural Conduction - physiology; Neural Inhibition - physiology; Neurotransmitter Agents - metabolism; opiate; Pressoreceptors - metabolism; Pressoreceptors - ultrastructure; Presynaptic Terminals - metabolism; Presynaptic Terminals - ultrastructure; Rats; Rats, Sprague-Dawley; Reticular Formation - metabolism; Reticular Formation - ultrastructure; Spinal Cord - metabolism; Spinal Cord - ultrastructure; Sympathetic Nervous System - metabolism; Sympathetic Nervous System - ultrastructure; sympathetic tone; Synaptic Transmission - physiology; Vertebrates: nervous system and sense organs; GLU, glutamate; PPD, preprodynorphin; baroreflex; DAB, 3,3′-diaminobenzidine; NHS, normal horse serum; TBS, Tris-buffered saline; opiate; LM, light microscopy; TPBS, Tris phosphate-buffered saline; EM, electron microscopy; IR, immunoreactive; ENK, enkephalin; HRP, horseradish peroxidase; glycine; GLY, glycine; TH, tyrosine hydroxylase; BSBS, bulbospinal barosensitive; GAD, glutamic acid decarboxylase; GABA; PPE, preproenkephalin; AP, arterial pressure; RVLM, rostral ventrolateral medulla; glutamate; sympathetic tone; Rat; Rodentia; Central nervous system; Enkephalin; Glutamate; Glycine; Sympathetic nervous system; Neuropeptide; Opioid peptide; Vertebrata; Mammalia; Neuron; Aminoacid; Baroreflex; Distribution; Medulla oblongata; Brain (vertebrata)
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/S0306-4522(01)00415-8

The activity of bulbospinal (presympathetic) vasomotor neurons of the rostral ventrolateral medulla is modulated pre- and postsynaptically by exogenously applied opioid agonists. To determine whether these neurons receive direct opioid inputs, we examined the relationship between bulbospinal barosensitive neurons and nerve terminals immunoreactive for enkephalin in the rostral ventrolateral medulla of rats. By light microscopy, we mapped the distribution of close appositions by enkephalin-immunoreactive varicosities on 10 bulbospinal barosensitive neurons labelled in vivo with biotinamide. We also examined four labelled neurons ultrastructurally for synapses by enkephalin-immunoreactive terminals and determined with post-embedding immunogold labelling whether these enkephalin-positive terminals contained amino acids. Enkephalin-immunoreactive varicosities closely apposed all bulbospinal barosensitive neurons. Maps of the dendritic distribution of appositions indicated that fast-conducting bulbospinal barosensitive neurons with myelinated axons (conduction velocity >3 m/s; n=3) received many appositions (up to 470/neuron); and slowly conducting neurons with unmyelinated axons (conduction velocity <0.90 m/s; n=3), substantially fewer. Ultrastructural analysis of three fast- and one slowly conducting bulbospinal barosensitive neurons revealed numerous synapses from enkephalin-immunoreactive terminals on cell bodies and dendrites. Enkephalin-positive terminals synapsing on bulbospinal barosensitive neurons contained one or more amino acid: GABA+glycine, glutamate alone or GABA+glutamate. Enkephalin-immunoreactive terminals located near biotinamide-labelled cells contained a similar variety of amino acids. In summary, enkephalin-immunoreactive terminals in the rostral ventrolateral medulla densely innervate lightly myelinated presympathetic neurons and more sparsely those with unmyelinated axons. Enkephalin is present in both excitatory (glutamate-immunoreactive) and inhibitory (GABA- and/or glycine-immunoreactive) terminals. The data suggest that endogenous enkephalin inhibits amino acid release from terminals that innervate bulbospinal barosensitive neurons of the rostral ventrolateral medulla.