The endogenous pain control system: Facilitating and inhibitory pathways

• Type: Article in International Conference Proceedings Book
• Date: 1998

Authors

Deolinda Lima (Author) FMUP; Almeida, A (Author) Other; Isaura Tavares (Author) FMUP

Conference proceedings International

Title: PAIN MECHANISMS AND MANAGEMENT

Pages: 192-211

International Symposium on the Application of the Theory of Metabolic Regulation to Pain

EREVAN, ARMENIA, SEP 22-28, 1996

Indexing

ISI Web of Knowledge - 3 Citations

Other information

Authenticus ID: P-001-9KE

Abstract (EN): Since the first studies on the antinociceptive role of the periaqueductal gray matter and nucleus raphe magnus, several regions have been included in the endogenous pain control system based on the ability of local stimulation to depress nociceptive reflexes or inhibit nociresponsive dorsal horn neurons. There is increasing evidence, however, on the occurrence of excitatory mechanisms which are likely to act in parallel with the inhibitory ones in order to enhance pain reactions or promote a balanced response adequate to the specific nature of the painful event. Such a bidirectionally organized system may play a role in the genesis of sensitization of central neurons during repetitive or prolonged noxious stimulation by favoring descending facilitating actions in such nociceptive conditions. The present study congregates new data on the functional anatomy of caudal medullary nuclei, which point to both the specific organization of each component of the pain control system, and the common arrangement in recurrent loops capable of counteracting the modulatory effect triggered in each one. Reciprocal connections were shown to occur between subareas of the dorsal reticular nucleus (DRt), nucleus tractus solitarius (NTS) and caudal ventrolateral reticular formation (caudal VLM, and specific spinal dorsal horn regions, particularly lamina I. Asymmetrical, putative excitatory synaptic contacts were observed between DRt and lamina I neurons at both spinal and medullary levels, suggesting that in this case the reciprocal loop may function as a reverberating circuit aimed at facilitating nociceptive transmission. Accordingly, behavioral studies pointed to the involvement of the DRt in pain facilitation. Electrical or chemical lessening of the nucleus resulted in the increase of pain threshold in the tail-flick and hot plate tests, and decrease of pain behavior during the acute and tonic phases of the Formalin test. Conversely, stimulation of the nucleus by local instillation of glutamate produced a decrease in pain threshold during the tail-flick test. Data based on double retrograde tracing showed, however, that lamina I neurons activating the DRt are also connected with nuclei in the caudal mesencephalon, indicating that the DRt can be simultaneously engaged in promoting a potent antinociceptive effect. As to the reciprocal connections concerning the NTS and caudal VLM, studies on their anatomy and neurochemistry are still lacking. Nevertheless, the possibility that they mediate descending inhibition upon activation by nociceptive input arriving from the spinal cord is suggested by the fact that inhibitory actions can be induced by stimulating both areas. However, the antinociceptive action elicited from the caudal VLM, which is mediated by spinal a, adrenoreceptors and, in a smaller extent, 5-HT receptors, cannot be conveyed by the direct spinopetal descending pathway. Neurons containing 5-HT do not occur in the caudal VLM, whereas noradrenergic neurons of the Al group could not be labelled retrogradely from the spinal cord. Tracing studies revealed that dysynaptic pathways relayed in the A(5) and the rostral ventromedial medulla are probably conveying, respectively, the noradrenergic and 5-HT antinociceptive effects. Again, negative feedback circuits capable of controlling the ascending triggering effect appear to occur at both the caudal VLM and the spinal cord. Axonal collaterals of spinal-projecting A(5) neurons contact ventrolateral modullary neurons acting upon them. Spinal neurons connected with the caudal ventrolateral medulla receive appositions from descending noradrenergic and serotoninergic fibers.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 20