Therapeutic effect of opioids (heroin)
Opioids are effective as analgesics when given in minute doses. They excite neurones in the periaqueductal grey matter and nucleus reticularis paragigantocellularis, which project to the nucleus raphe magnus. Descending pathways from the midbrain exert a strong inhibitory effect on pain transmission in the dorsal horn (mediated by 5-HT, enkephalins and noradrenaline). Opioids also inhibit pain transmission by acting directly on the dorsal horn, and by inhibiting excitation of peripheral nociceptive afferent neurones.
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References
Analgesic drugs. In: Pharmacology, 4th edition. Rang HP, Dale MM and Ritter JM. Edinburgh, UK: Harcourt Publishers Ltd, 2001:579–603.