The inhibitory effect of somatostatin receptor activation on bee venom-evoked nociceptive behavior and pCREB expression in rats

Inhibition of ASIC-Mediated Currents by Activation of Somatostatin 2 Receptors in Rat Dorsal Root Ganglion Neurons

Somatostatin (SST) and its analogues like octreotide (OCT) have analgesic effect on a variety of pain through peripheral SST receptors (SSTRs). However, the precise molecular mechanisms have not yet been fully elucidated. This research aimed to identify possible antinociceptive mechanisms, showing functional links of the SSTR2 and acid-sensing ion channels (ASICs). Herein, we reported that OCT inhibited the electrophysiological activity of ASICs in rat dorsal root ganglia (DRG) neurons. OCT concentration-dependently decreased the peak amplitude of acid-evoked inward currents, which were mediated by ASICs. OCT shifted concentration-response curve to protons downwards, with a decrease of 36.53 ± 5.28% in the maximal current response to pH 4.5 in the presence of OCT. OCT inhibited ASIC-mediated currents through SSTR2, since the inhibition was blocked by Cyn 154806, a specific SSTR2 antagonist. The OCT inhibition of ASIC-mediated currents was mimicked by H-89, a membrane-permeable inhibitor of PKA, and reversed by internal treatment of an adenylyl cyclase activator forskolin or 8-Br-cAMP. OCT also decreased the number of action potentials induced by acid stimuli through SSTR2. Finally, peripheral administration of 20 μM OCT, but not 2 μM OCT, significantly relieved nociceptive responses to intraplantar injection of acetic acid in rats. This occurred through local activation of SSTR2 in the injected hindpaw and was reversed following co-application of Cyn 154806. Our results indicate that activation SSTR2 by OCT can inhibit the activity of ASICs via an intracellular cAMP and PKA signaling pathway in rat DRG neurons. These observations demonstrate a cross-talk between ASICs and SSTR2 in peripheral sensory neurons, which was a novel peripheral analgesic mechanism of SST and its analogues.

Role of somatostatin and somatostatin receptor type 2 in postincisional nociception in rats

Somatostatin (SST) and the somatostatin receptor type 2 (sstr2) are expressed in the superficial part (Laminae I-III) of the dorsal horn of the spinal cord. Since the neurons in these laminae also receive nociceptive sensation from the periphery, it was hypothesized that both SST and sstr2 could be involved in the modulation of nociceptive transmission. To the best of knowledge, there are no studies on the involvement of SST and sstr2 in hind paw incision model in rats, which mimics postoperative pain in humans. Sprague-Dawley rats were subjected to hind paw incision under isoflurane anaesthesia and the resulting mechanical allodynia and thermal hyperalgesia were evaluated for 5 days. In another set of animals, the spinal cord was isolated at specified time intervals after incision and examined for SST and sstr2 expression using immunohistochemistry and immunoblotting procedures. Finally, nociceptive parameters were again evaluated in incised rats, which had received SST (400 µg/kg i.p. three times per day). Blood glucose level and locomotor activity were determined after SST treatment. Both allodynia and hyperalgesia were highest immediately after incision. Spinal SST expression increased at 2 h. A further increase was noted on day 3. Expression of sstr2 increased initially but decreased at day 1. These changes could be due to exocytosis of SST and internalization of the ligand-receptor complex. SST injection significantly attenuated mechanical allodynia but not thermal hyperalgesia. Significant change in blood glucose level or locomotor activity was absent. SST appears to contribute to postincisional pain. This finding could be of clinical relevance.