Comparison of the peripheral antinociceptive effect of somatostatin with bupivacaine and morphine in the rodent postoperative pain model

Concomitant use of pre-emptive analgesia with local and general anesthesia in rat uterine pain surgical model

Preemptive analgesia is used for postoperative pain management, providing pain relief with few adverse effects. In this study, the effect of a preemptive regime on rat behavior and c-fos expression in the spinal cord of the uterine surgical pain model was evaluated. It was a lab-based experimental study in which 60 female Sprague-Dawley rats; eight to 10 weeks old, weighing 150-300 gm were used. The rats were divided into two main groups: (i) superficial pain group (SG) (with skin incision only), (ii) deep pain group (with skin and uterine incisions). Each group was further divided into three subgroups based on the type of preemptive analgesia administered i.e., "tramadol, buprenorphine, and saline subgroups." Pain behavior was evaluated using the "Rat Grimace Scale" (RGS) at 2, 4, 6, 9 and 24 h post-surgery. Additionally, c-fos immunohistochemistry was performed on sections from spinal dorsal horn (T12-L2), and its expression was evaluated using optical density and mean cell count 2 hours postoperatively. Significant reduction in the RGS was noted in both the superficial and deep pain groups within the tramadol and buprenorphine subgroups when compared to the saline subgroup (p ≤ .05). There was a significant decrease in c-fos expression both in terms of number of c-fos positive cells and the optical density across the superficial laminae and lamina X of the spinal dorsal horn in both SD and DG (p ≤ .05). In contrast, the saline group exhibited c-fos expression primarily in laminae I-II and III-IV for both superficial and deep pain groups and lamina X in the deep pain group only (p ≤ .05). Hence, a preemptive regimen results in significant suppression of both superficial and deep components of pain transmission. These findings provide compelling evidence of the analgesic efficacy of preemptive treatment in alleviating pain response associated with uterine surgery.

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.