Potent analgesic and anti-inflammatory actions of a novel thymulin-related peptide in the rat

Abstract

1. The present study examines the effect of PAT (peptide analogue of thymulin) in two rat models of inflammatory hyperalgesia induced by either i.pl. (1.25 microg in 50 microl saline) or i.p. (50 microg in 100 microl) injections of endotoxin ET. 2. Pretreatment with PAT (1, 5 or 25 microg in 100 microl saline, i.p.) decreased, in a dose dependent manner, both mechanical hyperalgesia, determined by the paw pressure (PP) test and thermal hyperalgesia determined by the hot plate (HP), the paw immersion (PI) and the tail flick (TF) tests. 3. Compared to the tripeptides K(D)PT and K(D)PV, known to antagonize interleukin (IL)-1beta or IL-1beta and PGE(2) mechanisms, PAT, at lower dosages, exerted stronger anti-hyperalgesic effects. 4. When compared with the effect of a steroidal (dexamethasone) and a non-steroidal (indomethacin) anti-inflammatory drugs (NSAID), PAT demonstrated equal analgesic actions. 5. Pretreatment with PAT, reduced significantly the increased concentration of IL-1beta, IL-6, TNF-alpha and NGF due to i.pl. injection of ET. 6. Injection of i.p. ET produced sickness behaviour characterized by hyperalgesia and fever. Pretreatment with PAT prevented the hyperalgesia and maintained the body temperature within the normal range and was accompanied by a down-regulation of the levels of pro-inflammatory cytokines and PGE(2) in the liver. 7. PAT, in all doses used, did not result in any evident changes in the physiological parameters or in the normal behaviour of the rats. 8. The anti-hyperalgesic and anti-inflammatory effects of PAT can be attributed, at least partially, to the down-regulation of pro-inflammatory mediators.

Figure 1

Dose-dependent reduction of the endotoxin-induced hyperalgesia by PAT. Each bar represents the mean±s.e.mean of measurements made on a separate group of rats (n=5) for each treatment. All measurements were performed at 9 h following the injections, the time that corresponds to the peak of hyperalgesia induced by ET. The significance of differences was measured for each experimental group in reference to the control group (injected with saline).

Figure 2

The effect of PAT pretreatment on ET-induced inflammatory hyperalgesia is compared with pretreatment with steroidal, NSAID and other analgesic tripeptides. Each bar represents the mean±s.e.mean of measurements performed on a separate group of rats (n=5) for each treatment. All measurements were performed at 9 h following the injections. The significance of differences was measured in reference to the value of saline control for each test. Abbreviations: K(D)PV, Lys. D. Pro. Val, (10 mg kg⁻¹; 1.1 μmol); K(D)PT, Lys. D. Pro. Thr, (10 mg kg⁻¹; 1.1 μmol); Indo; Indomethacin (4 mg kg⁻¹; 11.2 μmol); Dex., Dexamethasone (200 μg kg⁻¹; 1.01 μmol); PAT, peptide analogue of thymulin (114 pmol).

Figure 3

Pretreatment with PAT prevented the increase in cytokines and NGF concentration due to ET injection. Each bar represents the determination of the level of each cytokine and NGF in the skin tissues of the injected paws of a separate group of rats (n=5) for the indicated treatment. The levels of cytokines and NGF were measured at the time of peak of their upregulation by ET, which corresponds to 1 h for TNF-α and 3 h for the remaining factors. The significance of differences was measured by comparing their levels with saline injected controls.

Figure 4

Pretreatment with PAT prevented the hyperalgesia and fever induced by systemic injection of ET. Two groups (n=5) of rats were used, one injected with ET the other received ET preceded by PAT injection. The pairs of bars in each panel represent the measurements made before (control) or at the indicated time interval following treatments. The significance of difference was established by the two-tailed student t-test with reference to the control (^*) before injection or between the two groups of each time interval (+).

Figure 5

Dose-dependent effect of PAT pretreatment on the hyperalgesia induced by systemic injection of ET. Each bar, in each panel, represents the mean±s.e.mean of measurements made on one group of rats for the indicated treatment. The significance of difference was established with reference either to control (^*) measured before any treatment or to the peak of hyperalgesia induced by ET (+).

Figure 6

Pretreatment with PAT reduced the up-regulation of cytokine and PGE₂ levels by systemic injection of ET. Each bar represents the mean±s.e.mean of measurements performed on one group of rats for the indicated treatment, 1 h following the injection.