A number of neurotransmitters and chemical substrates are involved in pain transmission; several are listed in Table 2-2. For example, in the periphery, tissue injury results in the activation of a number of cellular processes that release chemical compounds that can activate free nerve endings for pain transmission (Snyder 1980), such as acetylcholine, bradykinin, histamine, potassium ion, and serotonin (Levine et al. 1993). Additional agents that are active within the CNS are also listed in Table 2-2. Some of these substances have a pain-promoting role, whereas others have a pain inhibitory role. Many of these substances are the targets of influence when analgesics are employed (e.g., antiinflammatory agents and antidepressants), as is described further in Chapter 5, "Pharmacology of Pain," of this book.
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