ABSTRACT
Opioids act primarily at pre- and postsynaptic receptors present in the peripheral and central nervous systems. All opioids have a similar mode of action, though activity at various receptor subtypes varies. Presently three opioid receptors have been cloned, OP1, OP2, OP3 that are known to mediate analgesia. Activation of opioid receptors by either an exogenous opioid agonist or endogenous ligand inhibits the presynaptic release of excitatory neurotransmitters from nerve terminals in the dorsal horn of the spinal cord. Opioid agonists typically increase vagal tone resulting in a reduced heart rate and in cats may increase body temperature transiently. Some opioids may cause vomiting and may be contraindicated if an increase in intra-ocular, intra-cranial, intra-abdominal, or esophageal pressure will be detrimental. Opioids have additive or synergistic analgesic effects with alpha2 agonists, NSAIDs, phenothiazines, benzodiazepines, and local anesthetics. Opioids may enhance the respiratory and cardiac depression associated with most anesthetic drugs.
