South African Acute Pain Guidelines | 2016

S6 SASA Acute Pain Guidelines act at this level, but as Na channels are present in all nerve fibres, blocking of the autonomic motor and sensory fibres can occur. Agents which block subtypes of Na channels (specific to sensory fibres) are not yet available. Pain modulation can be achieved by decreasing excitation (opioid receptor, Na channel blockers and ketamine) and/or increasing inhibition [increased alpha-2 agonist (clonidine) and glycine (GABA agonists) at the level of the spinal cord]. The most common receptors and ligands are outlined in Table 1. 3.4.1 Ascending pathways The spinothalamic tract originates in laminae I, II andV, ascends to the thalamus and then the somatosensory cortex, providing information on the type and the site of the painful stimulus. Table 1: Receptors and ligands Receptor Subtypes Ligands Transient receptor potential receptors (TRPs) TRPV1 Heat ≥ 42 °C, H+ and capsaicin TRPV2 Heat ≥ 54 °C TRPA Cold ≤ 17 °C Acid sensing ASIC Protons DRASIC Purine P2X3 ATP Serotonin 5-HT 3 5-HT NMDA receptor NRI Glutamate AMPA iGlutR1 Glutamate Kainate iGlutR5 Glutamate Prostanoids EP1-4 PGE 2 IP PGI 2 Histamine HI HA Serotonine 5-HT 1A , 5-HT 2A and 5-HT 4 5-HT Bradykinine BK1 and BK2 BK Cannabinoids CB1-2 Anandamide Opioids Mu, delta and kappa Enkepalin, dynorphin and beta-endorphin Thacykinine NK-1 Substance P and neurokinine A 5-HT: 5-hydroxytryptamine, AMPA: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, ASIC: acid-sensing ion channel, ATP: adenosine triphosphate, DRASIC: dorsal root acid sensing ion channel, NK-1: neurokinin 1, NMDA: N-methyl-D-aspartic acid, PGE 2 : prostaglandin 2, PG1 2 : prostacyclin, TRP: transient receptor potential