The temporomandibular joint (TMJ) formalin test was used to evaluate the effects of acute restraint stress on the nociceptive behavioral responses of female rats during proestrus and estrus phases of the estrous cycle. Rats were subjected to one session of restraint stress (15, 30 min or 1 h). They were then either immediately killed to allow the collection of blood for hormonal radioimmunoassay determinations or subjected to TMJ formalin test to evaluate nociception. All stress protocols significantly raised the plasma concentrations of corticosterone. The performance of rats subjected to 15 and 30 min of restraint stress was similar to that of control rats, whereas rats that were stressed for 1 h showed a decrease in nociceptive responses, during both proestrus and estrus phases. The stress-induced analgesia (SIA) was greater in the proestrus phase. To evaluate the role of kappa-opioid receptors, the selective receptor kappa-opioid antagonist nor-binaltorphimine (nor-BNI; 200 microg or saline) was injected into the TMJ 24 h prior to the 1 h stress period and the TMJ formalin test. The local administration of nor-BNI partially reversed the SIA during the proestrus phase. These findings suggest that (1) acute stress for 1 h can produce analgesia both during proestrus and estrus phases; this effect is greater during the proestrus phase and (2) kappa-opioid receptor activation is involved in the SIA observed in the proestrus phase.

Background: Skeletal muscle injuries may be caused by contraction of the muscle concerned. Objective: To analyze the tibialis anterior muscle at different times following injury induced by electrical stimulation. Method: Male Wistar rats (298.2 ± 16.0g) were divided into two electrically stimulated groups evaluated after three and five days (n= 20) and two control groups, also evaluated after three and five days (n= 14). While stretched, the tibialis anterior muscle was injured by neuromuscular electrical stimulation (90 minutes, 30 Hz, 1 m/s, Ton/Toff 4 s and 4 mA). Three and five days afterwards, the animals were sacrificed and the muscles were removed. Histological sections were cut (10 μm) using a cryostat and were stained with toluidine blue. The body and muscle weights were statistically analyzed using Student's t test (p≤ 0.05). Results: The final body weight was higher than the initial weight for the 3-day control group (288.5 ± 18.3g vs. 308.5 ± 24.3g) and 5-day control group (288.4 ± 15.0g vs. 305.5 ± 20.7g) and lower for the 3-day stimulated group (305.0 ± 13.0g vs. 285.6 ± 13.2g) and 5-day stimulated group (306.1 ± 12.4g vs. 278.4 ± 20.9g). The relative muscle weight in the 5-day stimulated group was lower than in the 5-day control group (0.20 ± 0.001% vs. 0.22 ± 0.01%, respectively). The histological analysis showed variance between the animals regarding the extent and signs of fiber damage and/or regeneration, and the distal region was the most injured. The 3-day stimulated group presented predominance of cell infiltrate and myofilament hypercontraction, while the 5-day stimulated group presented predominance of cell infiltrate, basophils and fibrosis. Conclusion: A period of two days following electrical stimulation was sufficient for showing a difference in the regeneration process. The distal region of the tibialis anterior muscle was more susceptible to injury.