Ultrastructural evidence that substance P neurons form synapses with noradrenergic neurons in the A7 catecholamine cell group that modulate nociception

Potent antinociception can be produced by electrical stimulation of spinall y projecting noradrenergic neurons in the A7 catecholamine cell group and t his effect is blocked by intrathecal injection of alpha(2)-adrenoceptor ant agonists. Microinjection of substance P near A7 neurons also produces antin ociception that is blocked by intrathecal injection of alpha(2)-adrenocepto r antagonists. These observations suggest that substance P produces antinoc iception by activating noradrenergic A7 neurons. However, it is not known w hether this effect of substance P is produced by a direct or an indirect ac tion on A7 neurons. Although light microscopic studies have demonstrated th e 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 subst ance P terminals form synapses with noradrenergic A7 neurons. These experim ents used double-labeling immunocytochemical methods and electron microscop ic analysis to determine whether substance P-containing axons form synapses with noradrenergic neurons in the A7 cell group. Pre-embedding immunocytoc hemistry, combined with light and electron microscopic analysis, was used t o provide ultrastructural evidence for synaptic connections between substan ce P-immunoreactive terminals labeled with immunoperoxidase and tyrosine hy droxylase-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 termin als. Substance P-labeled terminals formed asymmetric synapses with tyrosine hydroxylase-labeled dendrites, but only a few of these were present on tyr osine 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-la beled dendrites. These results demonstrate that substance P neurons form a significant numbe r of synapses with the dendrites of noradrenergic A7 neurons and support th e conclusion that microinjection of substance P in the A7 cell group produc es antinociception by direct activation of spinally projecting noradrenergi c neurons. (C) 1999 IBRO. Published by Elsevier Science Ltd.