Estradiol and testosterone differently affect visceral pain-related behavioural responses in male and female rats.

In the study of pain, the presence of sex differences is well known, with female subjects being more affected in a number of chronic painful conditions; however, the underlying mechanisms and the involvement of gonadal hormones, are still controversial. This study evaluated visceral pain in a validated rat model of artificial calculosis and the effects of estradiol and testosterone administration. Adult male and female rats were divided into groups and treated with one of the substances or Oil (vehicle) for 5 days, starting 2 days before surgery, with half receiving an artificial calculosis (Stone) and half only a sham (Sham) procedure. The animals' behaviour (ureteral crises, frequency and duration) were recorded for 72 h; estradiol and testosterone plasma levels were determined in all groups at the end of the observation period. After surgery, only Stone rats showed ureteral pain crises, with a significant sex difference in the Oil-treated groups in which the number and duration of crises were higher in females than in males. This difference was not present in the estradiol-treated groups in which ureteral crises were decreased only in females while testosterone treatment had no effect in either sex. Estradiol and testosterone plasma levels were affected by treatments in both sexes. These results confirm that, also in this model of visceral pain, females experience more pain than males; moreover, they show that supraphysiological levels of estradiol, but not of testosterone, are analgesic only in females. A dose and sex-dependent efficacy of gonadal hormones is suggested and discussed.

Trigeminal pain transmission requires reactive oxygen species production.

Three experiments were conducted in order to investigate the possible involvement of the reactive oxygen species in the nociception within the subnucleus caudalis of the spinal trigeminal nucleus (Vc). In the first experiment the extracellular level of hydrogen peroxide was evaluated by microdialysis in the Vc of two groups of six rats before and after a formalin (group 1) or saline solution (group 2) injection into the upper lip. In the second experiment the formalin test was conducted in three groups of 6 rats after a microinjection of 2-methoxyestradiol (2-ME, a superoxide-dismutase inhibitor; group 1) or N-acetylcysteine (NAC, an oxygen intermediate scavenger; group 2) or saline solution (group 3) into the Vc. In the third experiment an histochemical assay for superoxide dismutase activity was performed on two groups of 4 rats each 2 h after a formalin (group 1) or saline solution (group 2) injection into the upper lip. The results showed that (1) the level of hydrogen peroxide increases into the Vc during facial pain (134% of baseline); (2) the inhibition of superoxide dismutase or the removal of oxygen intermediate within the Vc decreases the sensibility to facial pain stimuli; and (3) persistent facial pain stimuli decrease the superoxide activity into the Vc (90% of counter-lateral). These data indicate that reactive oxygen species are produced in the Vc during persistent facial pain and are necessary for the transmission of pain.

Evidence that GABAergic neurons in the spinal trigeminal nucleus are involved in the transmission of inflammatory pain in the rat: a microdialysis and pharmacological study.

The aim of this experiment was to investigate the role of the gamma-aminobutyric acid (GABA)-ergic transmission in the nociception within the spinal trigeminal nucleus. The formalin test was used as an animal model of inflammatory pain. Two groups of six rats were used. The behavioural response to the labial injection of formaldehyde (50 microl of a 5% solution) (group 1) or saline (group 2) was evaluated by recording the time spent in facial grooming during a period of 8 min (one period before and seven consecutive periods after the injection). The extracellular concentration of GABA in the trigeminal caudalis nucleus was evaluated, during the formalin test, on samples of 30 microl each (one sample before and three samples after the labial injection) obtained by microdialysis and analysed by HPLC with electrochemical detection of the o-phtalaldeyde pre-column derivate. Subsequently, three more groups of six rats each were injected with saline, muscimol (GABAa receptor agonist), or bicuculline (GABAa receptor antagonist) in the trigeminal caudalis nucleus, before performing the formalin test. The injection of formaldehyde induced a biphasic behavioural response and an increase of the GABA levels at 15-45 min. The injection of bicuculline, but not muscimol or saline, strongly decreased the behavioural response of the formalin test. These findings suggest that GABAergic neurons in the trigeminal caudalis nucleus are involved in the transmission of nociceptive information.