Effect of butorphanol on equine antroduodenal motility.

Six healthy six to eight-month-old horses were surgically prepared with Ag bipolar electrodes sutured to the gastric antrum and duodenum. Leads from the electrodes were exteriorised through a stab incision in the flank. During experimental sessions the horses were lightly restrained in stocks and electrode leads were connected to a physiograph to record antroduodenal myoelectrical activity. Intravenous (i.v.) injection of 0.05 mg/kg bodyweight (bwt) of the opioid agonist/antagonist, butorphanol was followed within 2 to 3 mins by a normal appearing period of repetitive spike activity, or phase III, of the migrating motor complex (MMC) on the duodenum. This was followed by a period of no spike activity, or phase I, of the MMC and then resumption of intermittent spike activity, or phase II, of normal duration. Pre-treatment with 15 micrograms/kg bwt of the non-selective opioid antagonist, naloxone, or with 1 mg/kg bwt of the alpha 2-adrenergic antagonist, tolazoline, did not block the myoelectrical response to butorphanol. It was concluded that a dose of butorphanol that has effective analgesic effects in a colicky horse resets the antroduodenal MMC without causing undesirable effects on antroduodenal motility. This particular effect of butorphanol might not be mediated by either a2-adrenergic or opioid receptors, although the latter question needs further investigation.