Role of sigma-1 receptors in carrageenan-induced mechanical and thermal hypersensitivity in mice

MA Tejada-Giraldez1,2, C Sánchez-Fernández1,2, JM Entrena1,2, E Portillo-Salido3, JM Baeyens1,2. 1Granada University, Department of Pharmacology and Institute of Neuroscience, 18012, Spain, 2Granada University, Biomedical Research Centre, 18100, Spain, 3Laboratorios Dr. Esteve, Drug Discovery and Preclinical Development,08041, Spain

Sigma-1 (σ1) receptors play a significant role in the hypersensitivity evoked by nociceptive stimuli in models of neuropathic pain. However, their role in inflammation-induced pain hypersensitivity is almost unexplored. Therefore, we compared carrageenan-induced mechanical and thermal hypersensitivity (as well as paw inflammation) in wild-type and σ1-receptor knockout (σ1-KO) mice. Moreover, we evaluated the effect of a selective σ1-receptor antagonist (BD-1063) on carrageenan effects in both types of animals.

Experiments were performed in CD-1 wild-type (WT) and σ1-KO mice weighing 25-30g. The animals received an intraplantar (i.pl.) injection of carrageenan (1% in saline, 50μl), or its solvent, in the right hind paw. Paw volume, mechanical and thermal sensitivity were measured 1.5, 3, 4.5, 6 and 7.5h after carrageenan injection. To evaluate mechanical hypersensitivity, pressure (150g) was applied alternatively to both hind paws with a rounded tip cone-shaped paw-presser analgesimeter, until the mouse showed a struggling behaviour or 50s had passed (cut-off time). Thermal hypersensitivity was evaluated with a plantar test analgesimeter alternatively in both hind paws. The intensity of the radiant heat beam was adjusted to have control paw withdrawal latency values of 10s and the cut-off time was 20s. Hind paw volume was measured with an electronic plethysmometer. The selective σ1-receptor antagonist, BD-1063, or solvent, was administered subcutaneously (s.c) 150 minutes after the i.pl. injection of carrageenan, and the behavioural tests were performed 30 min later, since the maximum hypersensitivity occurred 3 hours after carrageenan injection. BD-1063 was also administered 5 minutes before carrageenan to evaluate whether the drug could prevent inflammation.

The differences between values obtained in the time-courses were analyzed with two-way repeated-measures analysis of variance, and the effect of drugs was compared to their controls by a one-way analysis of variance; a Bonferroni post hoc test was performed in both cases.

The i.pl. injection of carrageenan produced mechanical hypersensitivity of the injected paw in WT mice but not in σ1-KO mice (p < 0.01 between WT and KO animals at all the times evaluated). Ipsilateral thermal hypersensitivity appeared in both carrageenan-injected WT and σ1-KO mice reached the maximum at 3h and was non-significantly different (p > 0.05) between both types of animals at any time-points evaluated. Moreover, carrageenan produced inflammation of the injected paw (31-65µl depending on the time considered) in WT and also in σ1-KO mice. No effect of carrageenan on the paw volume, mechanical and thermal sensitivity of the contralateral paw was observed in either type of mice at any time.

BD-1063 (1-32mg/kg, s.c.) administration dose-dependently inhibited the carrageenan-induced mechanical and thermal hypersensitivity in WT mice (ED50 = 4.23±0.22 and 1.47±0.46mg/kg, respectively), whereas it was devoid of effect in σ1-KO mice. The increase of paw volume induced by carrageenan was not reduced, either in WT or σ1-KO mice, when BD-1063 (16mg/kg) was injected before or after carrageenan.

These data suggest that σ1-receptors are involved in the mechanisms underlying the mechanical and thermal hypersensitivity induced by carrageenan and that σ1 receptor antagonists may have therapeutic interest for treatment of pain hypersensitivity induced by an inflammatory process.