Comparison of Antinociceptive Effect of Octreotide With Morphine in a Rat Model of Acute Inflammatory Pain

Abstract

Background: Opioids such as morphine are used for treating moderate to severe pain. However, they also produce adverse effects such as nausea, constipation, addiction, and respiratory depression. Thus, other suitable analgesics need to be identified. Somatostatin is an inhibitory neuropeptide that modulates the transmission of pain. However, the half-life of somatostatin is short. In the present study, the antinociceptive effect of octreotide (a stable long-acting analog of somatostatin) was evaluated in rats with acute inflammatory pain. Methods: Sprague Dawley rats ( n = 42) were divided into control ( n = 6) and carrageenan injected groups ( n = 36). The carrageena group was divided into three equal subgroups and treated with saline, morphine (10 mg/kg), and octreotide (3 µg). Rats belonging to each subgroup ( n = 12) were again randomly divided into two equal sets. They were subjected to (a) behavioral evaluation of pain (allodynia) and estimation of paw edema, followed by immunohistochemical analysis of the expression of somatostatin type 2 receptor (sst2r) in the spinal cord and (b) estimation of open-field activity. Allodynia and paw edema were measured by von Frey filaments and plethysmometer, respectively, at 3 and 4 h after carrageenan injection. Expression of sst2r was examined after 24 hours, whereas open-field activity was evaluated after 3 hours. Results: In comparison to the saline-treated group, allodynia was partially attenuated by octreotide, though this was almost completely reversed by morphine. Paw edema was unaffected by octreotide, though it was marginally increased by morphine. This was not related to increased activity of rats, following relief from pain. Immunohistochemistry revealed a significant increase in the expression of sst2r in saline-treated rats, but a decrease in other groups. Conclusion: Octreotide has an antinociceptive effect, which was less than morphine. Increased edema following morphine could result from venodilation. Variations in the sst2r expression suggest its involvement in pain modulation at the spinal level. This information may have clinical relevance.