title: Ultrastructural evidence that substance P neurons form synapses with noradrenergic neurons in the A7 catecholamine cell group that modulate nociception.
abstract: Potent antinociception can be produced by electrical stimulation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group and this effect is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. Microinjection of substance P near A7 neurons also produces antinociception that is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. These observations suggest that substance P produces antinociception by activating noradrenergic A7 neurons. However, it is not known whether this effect of substance P is produced by a direct or an indirect action on A7 neurons. Although light microscopic studies have demonstrated the existence of both substance P-containing axon terminals and neurokinin-1 receptors in the region of the A7 cell group, it is not known whether substance P terminals form synapses with noradrenergic A7 neurons. These experiments used double-labeling immunocytochemical methods and electron microscopic analysis to determine whether substance P-containing axons form synapses with noradrenergic neurons in the A7 cell group. Pre-embedding immunocytochemistry, combined with light and electron microscopic analysis, was used to provide ultrastructural evidence for synaptic connections between substance P-immunoreactive terminals labeled with immunoperoxidase and tyrosine hydroxylase-immunoreactive A7 neurons labeled with silver-enhanced immunogold. Tyrosine hydroxylase labeling was found in perikarya and dendrites in the A7 region, and substance P labeling was found in axons and synaptic terminals. Substance P-labeled terminals formed asymmetric synapses with tyrosine hydroxylase-labeled dendrites, but only a few of these were present on tyrosine hydroxylase-labeled somata. Substance P-labeled terminals also formed asymmetric synapses with unlabeled dendrites, and many unlabeled terminals formed both symmetric and asymmetric synapses with tyrosine hydroxylase-labeled dendrites. These results demonstrate that substance P neurons form a significant number of synapses with the dendrites of noradrenergic A7 neurons and support the conclusion that microinjection of substance P in the A7 cell group produces antinociception by direct activation of spinally projecting noradrenergic neurons.
authors.affiliation: Department of Pharmacology, University of Illinois at Chicago, 60612, USA.
authors.forename: H K
authors.initials: HK
authors.lastname: Proudfit
authors.forename: M
authors.initials: M
authors.lastname: Monsen
mesh_terms: D000818:Animals; D001369:Axons / Q000378:metabolism / Q000648:ultrastructure; D003712:Dendrites / Q000378:metabolism / Q000648:ultrastructure; D005260:Female; D007150:Immunohistochemistry; D008854:Microscopy, Electron; D009474:Neurons / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D009619:Nociceptors / Q000502:physiology*; D009638:Norepinephrine / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000378:metabolism / Q000648:ultrastructure; D051381:Rats; D017207:Rats, Sprague-Dawley; D013373:Substance P / Q000378:metabolism*; D013569:Synapses / Q000502:physiology* / Q000648:ultrastructure; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism
chemical_list: D013373:Substance P; D014446:Tyrosine 3-Monooxygenase; D009638:Norepinephrine