ABSTRACT
The opioid analgesics appear to produce analgesia by inhibiting the ascending pain pathways and activate the descending pain control pathways, which go from the CNS down the ventro-medial medulla and down to the spinal cord dorsal horn. Opioids act in the periaqueductal gray to decrease GABAergic inhibition of the desceding pathways. The basic mode of opioid action is to inhibit the release of excitatory amino acids such as glutamate from peripheral nociceptors and postsynaptic neurons in the spinal cord dorsal horn. The nitric oxide leaves the cell and reacts with guanyl synthase, which closes the sodium channel. This enables the development of pain that will not respond to opioids, as opioids can only work on the terminal if the sodium channel is open. Opioid medications have multiple routes of administration, including oral, IV, IM, SQ, sublingual, intranasal, inhaled, transdermal, vaginal, rectal, intrathecal, and epidural.
