Endogenous Opiates

The endogenous opiates consist of P-endorphin, enkephalins, and dynor-phins (Sewell and Lee 1980). These are abundantly distributed throughout the CNS, thereby modulating pain transmission. Enkephalins are endogenous opiates found in the interneurons of the substantia gelatinosa that mediate the effects of inhibitory interneurons within the dorsal horn. Binding to opioid receptors, enkephalins can inhibit the release of substance P from nociceptors. In fact, intraspinal application of opiates (e.g., morphine) is thought to influence the enkephalin receptors, thereby mitigating pain transmission from the spinal cord. Cells producing P-endorphin arise from the hypothalamus and are thought to exert their influence within the limbic system and midbrain.

Table 2-2. Mediators of pain processing and transmission

Pain promoting

Pain inhibiting

Peripheral nervous system

Central nervous system

Acetylcholine

Adenosine

Bradykinin

Cytokines

Glutamate

Histamine

Prostaglandins (E series)

Serotonin

Substance P

Cholecystokinin Glutamate Serotonin Norepinephrine Substance P

Endogenous opiates

Endogenous opiates ß-Endorphin Endorphins Dynorphins Serotonin Norepinephrine Neurotensin

Source. Adapted from Terman GW, Bonica JJ: "Spinal Mechanisms and Their Modulation," in Bonicas Management of Pain, 3rd Edition. Edited by Loeser JD, Butler SH, Chapman CR, et al. Philadelphia, PA, Lippincott Williams & Wilkins, 2001, pp. 73-152.

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