Chapter 16 Physiological functions of pontomedullary raphe and medial reticular neurons

Abstract

This chapter focuses on the nociceptive modulatory role of neurons in the pontomedullary raphe magnus (RM) and the adjacent nucleus reticularis paragigantocellularis pars α (NRPGα) in the rat. This region is part of a system originally described by Oliveral and classically reported as the “endogenous analgesia system” by Fields and Basbaum. In the classic formulation, neurons in the midbrain periaqueductal gray (PAG) were thought to suppress pain transmission via a monosynaptic relay with RM neurons, including serotonergic cells, that project to the spinal dorsal horn. Since the original description of descending pain modulation, the understanding of these pathways has grown in several ways. First, it is know that the PAG-RM-dorsal horn pathway is only one of several pathways that can affect spinal and trigeminal nociceptive transmission. Other brainstem areas, including the parabrachial nuclei the lateral reticular nucleus, and the A5 and A7 catecholaminergic nuclei, as well as forebrain areas, such as the amygdala and medial preoptic nucleus are now thought to play important roles in nociceptive modulation. Second, the pharmacology of pain modulation, once thought to center on endogenous opioids, serotonin and norepinephrine, has been complicated by the discovery of a multitude of peptides and amino acid transmitters contained in both pain modulatory and pain transmission neurons.