Effects of the Nucleus Raphe Magnus Stimulation on Nociceptive Neurons of the Rat Caudal Ventrolateral Medulla in Normal Conditions and after Intestinal Inflammation

The nucleus raphe magnus (RMg) is a key structure of the endogenous antinociceptive system, the activity of which is regulated by serotonin 5-HT1A receptors. A recipient of the RMg descending projections is the caudal ventrolateral medulla (cVLM) - the first supraspinal center for processing visceral and somatic pain signals. Intestinal pathology is known to cause persistent functional alterations in the RMg, which are associated with the development of visceral and somatic hyperalgesia. Presumably, a consequence of the alterations may be changes in the RMg modulating effects on cVLM nociceptive activity. However, the specific neuronal and molecular mechanisms underlying such influence in normal conditions, as well as their changes in pathology remain unexplored. The aim of our neurophysiological experiments performed in anesthetized adult male Wistar rats was to compare the effects of RMg electrical stimulation on the activity of cVLM neurons evoked by visceral (colorectal distension, CRD) and somatic (tail squeezing) pain stimulations that occur in normal conditions and after intestinal inflammation (colitis), with an assessment of the contribution to these processes of the supraspinal 5-HT1A receptor activation with intracerebroventricular buspirone. It has been shown that RMg can exert an inhibitory effect on both non-selective and differential responses of the cVLM neurons to diverse pain stimuli, causing a weakening of excitatory neuronal ractions and an increase in inhibitory responses to CRD while inhibiting both types of reactions to tail squeezing. The RMg-evoked suppression of nociceptive excitation in the caudal medullary neurons is enhanced under activation of supraspinal 5-HT1A receptors by buspirone. It has been established that in postcolitis period the RMg inhibitory action on different populations of cVLM neurons are significantly diminished, indicating an impairment of the nucleus’ antinociceptive function. In these conditions, the RMg descending influence loses its 5-HT1A receptor-dependent component. The changes described may contribute to the supraspinal mechanisms underlying pathogenesis of post-inflammatory abdominal pain and comorbid somatic hyperalgesia.