[The antinociceptive effect of intrathecal somatostatin in monkeys]

The distribution of somatostatin (SST) throughout the nervous system suggests that this tetradecapeptide may play a physiological role in CNS in the mediation of analgesia. The present study was undertaken to evaluate the antinociceptive properties of intrathecal (IT) injection of SST in the comparison of morphine sulfate (MS) in a primate model. The study was conducted after institutional approval and adhered to the regulations of the animal research committee. Seven adult monkeys (Maccaca cyclopis Swinhoe) weighing 4-6 kg were used. In each animal a L5 laminectomy window was created to facilitate IT injection. No neurological damage from surgery was noted. With the monkey standing in a specially constructed cage, all animals randomly received the following agents at one-week interval: (1) MS 1 mg, IT; (2) SST 50 micrograms, IT; (3) SST 250 micrograms, IT; and (4) SST 250 micrograms, IT + intramuscular (IM) naloxone 400 micrograms. The measured withdrawal latency (HPWL) was converted to the maximal percentage effect (MPE %) for comparison. The HPWL was measured at predrug and 5, 15, 30, 45, 60, 90 and 120 min after injection. Venous blood sample was obtained every 15 min to determine the plasma SST level by radioimmunoassay (RIA) technique in group 3 only. The results showed that MS (1 mg, IT) produced potent antinociception (MPE 100%) for more than 2 h. Intrathecal SST 50 micrograms, however, induced mild antinociception (MPE 43%) for only a short period and a 5-fold larger dose (250 micrograms) did not significantly change the nociceptive threshold with MPE only up to 47%.(ABSTRACT TRUNCATED AT 250 WORDS)