Involvement of Spinal Somatostatin Receptor SST2A in Inflammation-Induced Thermal Hyperalgesia: Ultrastructural and Behavioral Studies in Rats

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

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.

Somatostatin Type 2 Receptor Antibody Enhances Mechanical Hyperalgesia in the Dorsal Root Ganglion Neurons after Sciatic Nerve-pinch Injury: Evidence of Behavioral Studies and Bax Protein Expression

BACKGROUND

Our previous study has indicated that somatostatin potently inhibits neuropathic pain through the activation of its type 2 receptor (SSTR2) in mouse dorsal root ganglion and spinal cord. However, the underlying mechanism of this activation has not been elucidated clearly.

OBJECTIVE

The aim of this study is to perform the pharmacological studies on the basis of sciatic nerve-pinch mice model and explore the underlying mechanism involving SSTR2.

METHODS

On the basis of a sciatic nerve-pinch injury model, we aimed at comparing the painful behavior and dorsal root ganglion neurons neurochemical changes after the SSTR2 antibody (anti- SSTR2;5μl,1μg/ml) administration in the mouse.

RESULTS

After pinch nerve injury, we found that the mechanical hyperalgesia and severely painful behavior (autotomy) were detected after the application of SSTR2 antibody (anti-SSTR2; 5μl, 1μg/ml) on the pinch-injured nerve. The up-regulated phosphorylated ERK (p-ERK) expression and the apoptotic marker (i.e., Bax) were significantly decreased in DRGs after anti-SSTR2 treatment.

CONCLUSION

The current data suggested that inhibitory changes in proteins from the apoptotic pathway in anti-SSTR2-treated groups might be taking place to overcome the protein deficits caused by SSTR2 antibody and supported the new therapeutic intervention with SSTR2 antagonist for neuronal degeneration following nerve injury.

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.

Somatostatin and its 2A receptor in dorsal root ganglia and dorsal horn of mouse and human: expression, trafficking and possible role in pain

Background

Somatostatin (SST) and some of its receptor subtypes have been implicated in pain signaling at the spinal level. In this study we have investigated the role of SST and its sst2A receptor (sst2A) in dorsal root ganglia (DRGs) and spinal cord.

Results

SST and sst2A protein and sst2 transcript were found in both mouse and human DRGs, sst2A-immunoreactive (IR) cell bodies and processes in lamina II in mouse and human spinal dorsal horn, and sst2A-IR nerve terminals in mouse skin. The receptor protein was associated with the cell membrane. Following peripheral nerve injury sst2A-like immunoreactivity (LI) was decreased, and SST-LI increased in DRGs. sst2A-LI accumulated on the proximal and, more strongly, on the distal side of a sciatic nerve ligation. Fluorescence-labeled SST administered to a hind paw was internalized and retrogradely transported, indicating that a SST-sst2A complex may represent a retrograde signal. Internalization of sst2A was seen in DRG neurons after systemic treatment with the sst2 agonist octreotide (Oct), and in dorsal horn and DRG neurons after intrathecal administration. Some DRG neurons co-expressed sst2A and the neuropeptide Y Y1 receptor on the cell membrane, and systemic Oct caused co-internalization, hypothetically a sign of receptor heterodimerization. Oct treatment attenuated the reduction of pain threshold in a neuropathic pain model, in parallel suppressing the activation of p38 MAPK in the DRGs

Conclusions

The findings highlight a significant and complex role of the SST system in pain signaling. The fact that the sst2A system is found also in human DRGs and spinal cord, suggests that sst2A may represent a potential pharmacologic target for treatment of neuropathic pain.

The somatostatin analogue octreotide inhibits capsaicin-mediated activation of nociceptive primary afferent fibres in spinal cord lamina II (substantia gelatinosa)

Somatostatin (SST) in spinal cord has been linked with the inhibition of nociceptive neurotransmission in several experimental paradigms. The SST2 receptor (SSTR2) is the main SST receptor subtype in the superficial dorsal horn (DH) and is activated, besides to the naïve peptide, by the SST synthetic analogue octreotide (OCT). In the present work, we have studied the central effects of SSTR2 activation on capsaicin (CAP)-induced glutamate release in mouse DH. In neurons of the lamina II of DH, CAP (2 μM) induced a strong increase of mEPSC frequency that was significantly reduced (70%) by OCT. SSTR2 involvement was assessed by using the specific antagonist CYN 154806. No differences were observed between frequency increase in CAP alone vs. CAP in the presence of CYN 154806+OCT. The effect of OCT was further investigated by studying c-fos expression in spinal cord slices. The CAP-induced increase in density of Fos immunoreactive nuclei in the superficial DH was strongly prevented by OCT. SSTR2a (a splicing variant of SSTR2) immunoreactivity was found in both pre- and post-synaptic compartments of laminae I-II synapses. By light and electron microscopy, SSTR2a was mainly localized onto non-peptidergic isolectin B4 (IB4)-positive primary afferent fibres (PAFs). A subset of them was also found to express the CAP receptor TRPV1. These data show that the SST analogue OCT inhibits CAP-mediated activation of non-peptidergic nociceptive PAFs in lamina II. Our data indicate that SSTR2a plays an important role in the pre-synaptic modulation of central excitatory nociceptive transmission in mouse.