Narcotics and local anesthetics

The recognition and alleviation of animal pain is a growing veterinary and public concern. Pain can be of an acute or chronic nature with different behavioral manifestations. Physiologically, pain is a dynamic and complex phenomenon that produces changes in the central and autonomic nervous systems as well as in the endocrine system. Horses and other animals appear to possess an endogenous pain-suppressing system involving the brainstem and spinal cord. This system can modulate pain perception and the responses to it. The recently discovered endogenous opioid peptides (endorphins and enkephalins) appear to play a role in this system, which is activated by stress. Opioids (narcotic analgesics) act to selectively depress pain-sensitive cells. Opioid analgesics may act via multiple opioid receptors. Each subclass of opioid receptor has a different pharmacologic profile. Classical opioids that act at mu (morphine) receptors typically produce analgesia, increased locomotor activity, cardiorespiratory stimulation, and a decrease in intestinal peristalsis in the horse. Opioids that act at kappa receptors produce analgesia, sedation, ataxia, and minimal autonomic effects in the horse. Owing to their lack of excitatory actions, the kappa opioids represent a potentially useful class of analgesics for use in equine species. Local anesthetics depress all excitable cells and can diminish sensory, motor, and muscular function. They do not act selectively on pain fibers, although pain is among the first sensations lost following a nerve block. Local anesthetic activity is enhanced by increased extraneuronal pH, nerve cooling, increased nervous activity, coadministration of a vasoconstrictor or hyaluronidase, delayed systemic absorption, prolonged drug metabolism, and by using agents with high lipid solubility. Procaine, lidocaine, and mepivacaine are among the most widely used and studied agents in horses. These agents and/or their metabolites can be readily detected in urine; in some cases, for prolonged periods.