This chapter discusses the pharmacologic evidence for the modulation of nociception by noradrenergic (NA) neurons. Four major systems of NA neurons that modulate nociception have been described. Three of these systems are involved in producing antinociception, while the fourth enhances nociception. System of bulbospinal NA neurons that modulates nociception is located in the locus coeruleus (LC) and subcoeruleus regions of the dorsolateral medulla. Stimulation of these neurons produces antinociception and inhibition of spinal cord dorsal horn neurons activated by noxious cutaneous stimulation. The NA neurons located in the A1 nucleus, part of the lateral reticular nucleus of the ventrolateral medulla, constitute a second NA pain modulation system. Electrical or chemical stimulation of the neurons in the A1 region produces antinociception and inhibition of spinal cord dorsal horn neurons. A third bulbospinal NA system originates in the rostral part of the A5 nucleus and includes the A7 nucleus. Electrical stimulation of neurons in these regions produces antinociception that can be blocked by intrathecal injection of NA antagonists. The fourth system of NA neurons, located in the caudal A5 nucleus, projects to, and tonically inhibits spinally projecting neurons located in the ventromedial medulla (VMM). © 1988, Elsevier Science Publishers, B.V. All rights reserved.
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