Analgesic effects of subarachnoidally administered hyperbaric opioids in horses.

The research studies the pain-relieving effects of certain hyperbaric opioids, specifically morphine, buprenorphine, and methadone, when administered through the subarachnoid space in horses. The results indicate that these substances can significantly increase the threshold for pain experienced from electrical stimulation, without causing harmful side effects such as cardiorespiratory depression, ataxia, or central nervous system excitement.

Methodology

• The study involved six healthy adult horses that were administered these substances subarachnoidally – that is, into the space surrounding the spinal cord.
• Each horse received a 5 mL volume of either morphine, buprenorphine, methadone, or dextrose solution for control.
• After administration, each horse was exposed to electrical stimulation in different dermatome regions, which refer to specific areas of the skin that are supplied by single spinal nerves.
• Every ten minutes for the first 60 minutes, and then every 30 minutes for up to 120 minutes post-injection, the researchers measured the avoidance threshold voltage – the voltage at which the horse would exhibit behaviors to avoid the stimulation.

Results

• The control group, which received a dextrose solution, showed no change in avoidance threshold.
• Morphine and methadone significantly increased the avoidance threshold within ten minutes of injection in all tested areas, lasting up to 120 minutes.
• All areas treated with morphine or methadone achieved profound analgesia, a term meaning a high level of pain relief, indicated by an avoidance threshold over 40 volts.
• Buprenorphine also increased the avoidance threshold, albeit it only lasted for 60 minutes and was deemed as providing moderate analgesia.
• Importantly, none of the horses displayed any sign of ataxia (loss of full control of bodily movements), cardiorespiratory depression, or central nervous system excitement, indicating that these treatments have a high safety profile.

Conclusion

• The study concludes that the administration of hyperbaric morphine or methadone into the subarachnoid space can offer significant analgesic effects in horses, across various dermatome regions for up to two hours post-application.
• Furthermore, this analgesic effect was achieved without inducing any harmful side effects commonly associated with opioid use, suggesting these treatments could be safe and effective for managing pain in horses.