The agonist and antagonist properties of bremazocine, ethylketazocine, Mr 2034 and Mr 2092, benzomorphans with the same pharmacological actions in vitro, were studied in vivo in rats. Overnight fasted animals were tested for nociceptive reaction on a 55°C hot-plate and for the intestinal transit of a charcoal meal fed 5 min earlier. The antagonist activity was assessed by the drug's ability to prevent morphine-induced constipation. Bremazocine was an agonist on the nociceptive reaction and a pure antagonist on intestinal propulsion; ethylketazocine and Mr 2034 were pure agonists in both tests and Mr 2092 was a mixed agonist-antagonist in the intestinal transit test. Thus the in vivo agonist-antagonist properties of opiates cannot always be predicted from their in vitro characteristics.
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