A comparative study of the respiratory depressant and analgesic effects of bremazocine, a kappa-agonist.

The respiratory depression and analgesia induced by a kappa-agonist, bremazocine, were studied by the ED50 ratios and the apparent pA2 values of the interaction with naloxone. The ratio between the ED50 in respiratory depression and analgesia (i.c.v.) was very high compared to that of other mu- and delta-agonists, indicating its small activity on the respiratory center. pA2 values were 7.33 and 7.20 for analgesia and respiratory depression, respectively, after i.c.v. administration. After s.c. injection, the pA2 value for analgesia was 6.16. The results confirm the involvement of different opiate receptors in the mediation of supraspinal and spinal analgesia. The results suggest that bremazocine may induce analgesia by interacting with different types of receptors located at different levels of the CNS. This property and the low activity on the respiratory center make drugs like bremazocine good candidates in the search for safer opiate analgesics.