Algogenic mediator-induced nociceptive response in diabetic mice

Mechanical hyperalgesia is attenuated by local administration of octreotide in pristane-induced arthritis in Dark-Agouti rats

AIMS

The Dark-Agouti (DA) rat is very susceptible to pristane-induced arthritis (PIA) and represents a suitable model for rheumatoid arthritis. In the present study, we examined the pain sensitivity and the effect of local administration of octreotide (OCT) on mechanical hyperalgesia in PIA DA rats.

MAIN METHODS

Arthritis was induced by intradermal injection of pristane (300 microl). The mechanical withdrawal threshold (MWT) and heat withdrawal latency (HWL) were used to evaluate the pain sensitivity. In addition, we recorded the discharge firings in the tibial nerve sensory C-fibers innervating the inflamed toe joints of arthritic DA rats.

KEY FINDINGS

Two weeks after injection of pristane, all DA rats developed severe arthritis. This symptom was associated with a decreased MWT (78.50+/-5.68 mN before pristane injection, 19.50+/-6.27 mN on day 14 after pristane injection), indicating a mechanical hyperalgesia in PIA. In contrast, HWL was comparable before and after pristane injection (10.25+/-0.70 s before injection; 9.45+/-1.23 s on day 14 after injection). Local injection of OCT markedly increased MWT and relieved the hyperalgesia in PIA. In addition, OCT significantly decreased the discharge rate of afferent C units evoked by both non-noxious and noxious joint movements.

SIGNIFICANCE

Taken together, the results demonstrate that mechanical hyperalgesia, but not thermal hyperalgesia is associated with PIA and that the mechanical hyperalgesia and the discharge of afferent C units are attenuated by local administration of OCT. These observations provide evidence for a novel therapeutic strategy for pain control in rheumatoid arthritis.

Spinal mechanisms of antinociceptive action caused by diphenyl diselenide

The present study was designed to investigate further the mechanisms involved in the antinociception caused by diphenyl diselenide in behavioral model of pain in mice. Diphenyl diselenide (1-100 mg/kg), given orally, produced significant inhibition of the biting behavior induced by intrathecal (i.t.) injection of glutamate (175 nmol/site) and N-methyl-d-aspartate (NMDA; 450 pmol/site), with mean ID(50) values of 45.92 (39.74-60.4) and 55.77 (36.52-77.5) mg/kg respectively. However, diphenyl diselenide completely failed to affect the nociception induced by alpha-amino-3-hydroxy-5-mehtyl-4-isoxazolepropionic acid (AMPA; 135 pmol/site), (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 50 nmol/site) and kainate (110 pmol/site). This compound also reduced the nociceptive response induced by substance P (SP) (135 ng/site, i.t.), interleukin 1beta (IL-1beta; 1 pg/site), tumor necrosis factor-alpha (TNF-alpha; 0.1 pg/site), bradykinin (BK; 0.1 microg/site) and capsaicin (30 ng/site) with mean ID(50) values of 16.22, 7.06, 6.06, 4.18 and 7.90 mg/kg, respectively. Together, these results indicate that diphenyl diselenide produced antinociception at spinal sites, with a possible interaction with glutamatergic pathways, more specifically via interaction with NMDA receptors, peptidergic or vanilloid systems.

Peripheral participation of cholecystokinin in the morphine-induced peripheral antinociceptive effect in non-diabetic and diabetic rats

The effects of cholecystokinin (CCK-8) and the CCK receptor antagonist proglumide, on antinociception induced by local peripheral (subcutaneous) injected morphine in non-diabetic (ND) and streptozotocin-induced diabetic (D) rats, were examined by means of the formalin test. Morphine induced dose-dependent antinociception both in ND and D rats. However, in D rats, antinociceptive morphine potency was about twofold less than in ND rats. Pre-treatment with CCK-8 abolished the antinociceptive effect of morphine in a dose-dependent manner in both groups of rats. Additionally, proglumide enhanced the antinociceptive effect induced by all doses of morphine tested. Both CCK-8 and proglumide had no effect on flinching behaviour when given alone to ND rats. Unlike ND rats, in D rats proglumide produced dose-dependent antinociception and CCK-8 enhanced formalin-evoked flinches, as observed during the second phase of the test. In conclusion, our data show a decrease in peripheral antinociceptive potency of morphine when diabetes was present. Additionally, peripheral CCK plays an antagonic role to the peripheral antinociceptive effect of morphine, additional to the well known CCK/morphine interaction at spinal and supraspinal level.

Local Opioids in the Inflamed Dental Pulp

We have looked for the presence of beta-endorphin and somatostatin in normal and inflamed pulps. In 25 adult rats, under anesthesia, small openings were made into the pulp on the mesial surface of both first molars on one side. One week later all four first molars were removed, half were processed for immunohistochemistry using specific antibodies to beta-endorphin, somatostatin, and CD3, a marker for T lymphocytes. The pulps of the remainder were removed, solubilized, and subjected to enzyme-linked immunosorbent assay for somatostatin and beta-endorphin. Many cells that labeled for beta-endorphin and somatostatin in the injured pulps also stained for CD3. The levels of both beta-endorphin and somatostatin, were higher in the exposed than in the uninjured pulps (t test, p < 0.05). beta-endorphin and somatostatin are both produced in increased amounts in the dental pulp during inflammation attributable, at least in part, to the presence of T lymphocytes producing these substances.

Tonic control of peripheral cutaneous nociceptors by somatostatin receptors

The peptide somatostatin [somatotropin release-inhibiting factor (SRIF)] is widely distributed in the body and exerts a variety of hormonal and neural actions. Several lines of evidence indicate that SRIF is important in nociceptive processing: (1) it is localized in a subset of small-diameter dorsal root ganglion cells; (2) activation of SRIF receptors results in inhibition of both nociceptive behaviors in animals and acute and chronic pain in humans; (3) SRIF inhibits dorsal horn neuronal activity; and (4) SRIF reduces responses of joint mechanoreceptors to noxious rotation of the knee joint. The goal of the present study is to show that cutaneous nociceptors are under the tonic inhibitory control of SRIF. This is accomplished using behavioral and electrophysiological paradigms. In a dose-dependent manner, intraplantar injection of the SRIF receptor antagonist cyclo-somatostatin (c-SOM) results in nociceptive behaviors in normal animals and enhancement of nociceptive behaviors in formalin-injected animals, and these actions can be blocked when c-SOM is coapplied with three different SRIF agonists. Furthermore, intraplantar injection of SRIF antiserum also results in nociceptive behaviors. Electrophysiological recordings using an in vitro glabrous skin-nerve preparation show increased nociceptor activity in response to c-SOM, and this increase is blocked by the same three SRIF agonists. Parallel behavioral and electrophysiological studies using the opioid antagonist naloxone demonstrate that endogenous opioids do not maintain a tonic inhibitory control over peripheral nociceptors, nor does opioid receptor antagonism influence peripheral SRIF effects on nociceptors. These findings demonstrate that SRIF receptors maintain a tonic inhibitory control over peripheral nociceptors, and this may contribute to mechanisms that control the excitability of these terminals.

Effect of diabetes on bradykinin-induced thermal hyperalgesia in mice

To investigate the role of protein kinase C in the attenuation of bradykinin-induced thermal hyperalgesia in diabetic mice, we examined the effects of a protein kinase C activator or inhibitor on the i.t. bradykinin-induced hyperalgesia in diabetic and non-diabetic mice. Intrathecal injection of bradykinin caused a transient antinociceptive effect, which diminished within 30 min, and then produced a thermal hyperalgesia, which lasted about 120 min, in non-diabetic mice. Although the duration of the antinociceptive phase was longer in diabetic mice than in non-diabetic mice, the hyperalgesic response was not observed in diabetic mice. The bradykinin-induced hyperalgesia was dose-dependently and significantly enhanced by pretreatment with calphostin C (0.3 to 3 pmol, i.t.), a specific protein kinase C inhibitor, in diabetic mice. However, calphostin C (3 pmol, i.t.) had no significant effect on bradykinin-induced hyperalgesia in non-diabetic mice. On the other hand, pretreatment with phorbol-12, 13-dibutyrate (12.5 to 50 pmol, i.t.), a protein kinase C activator, significantly and dose-dependently reduced bradykinin-induced hyperalgesia in non-diabetic mice. However, phorbol-12, 13-dibutyrate (50 pmol, i.t. ) had no significant effect on bradykinin-induced hyperalgesia in diabetic mice. These results suggest that the change in bradykinin-induced thermal hyperalgesia in diabetic mice may be due, at least in part, to the modification of nociceptive transmission in the spinal cord by the activation of protein kinase C.