Dependency on sex and stimulus quality of nociceptive behavior in a conscious visceral pain rat model

Background Determination of the phases of the estrous cycle and induction of estrus (heat) in experimental animals remains useful, especially in reproductive function research. Main body of the abstract This review provides a detailed description and discusses extensively the variations observed in different phases of the estrous cycle in laboratory animals using rats and mice as examples. It also illustrates how these phases can be determined and how to induce estrus ‘heat’ when required. The phases of the estrous cycle can be determined using various methods such as visual assessment, vaginal smear/cytology, histology of female reproductive organs (vagina, uterus and ovaries), vaginal wall impedance assessment and determination of urine biochemical parameters. Female animals can be artificially brought to estrus phase ‘heat’ to make them receptive to male counterparts. Conclusion Determination of the length and phases of the estrous cycle and induction of estrus are useful in teaching and research and evaluating the effects of drugs/chemicals on the reproductive functions.

This study investigated the combined effect of neonatal maternal separation and acute psychological stress on pain responses in adult rats. Long-Evans dams and their male pups were reared under two conditions: 1) 180 min daily maternal separation (MS180) on postnatal days 2-14 or 2) no handling or separation (NH). At 2 mo of age, visceromotor responses to graded intensities of phasic colorectal distension (10-80 mmHg) at baseline as well as following acute 60 min water avoidance stress (WA) were significantly higher in MS180 rats. Both groups showed similar stress-induced visceral hyperalgesia in the presence of naloxone (20 mg/kg ip). MS180 rats had smaller stress-induced cutaneous analgesia in the tail-flick test compared with NH rats, with a residual naloxone-resistant component. MS180 rats showed an enhanced fecal pellet output following WA or exposure to a novel environment. These data suggest that early life events predispose adult Long-Evans rats to develop visceral hyperalgesia, reduced somatic analgesia, and increased colonic motility in response to an acute psychological stressor, mimicking the cardinal features of irritable bowel syndrome.

We have previously shown that the menstrual cycle has no effect on rectal sensitivity of normal healthy women, despite them having looser stools at the time of menses. Patients with irritable bowel syndrome (IBS) often report significant exacerbation of their IBS symptoms with menses, raising the possibility that IBS patients may respond differently to the menstrual cycle. Rectal responses to balloon distension during days 1-4 (menses), 8-10 (follicular phase), 18-20 (luteal phase), and 24-28 (premenstrual phase) of the menstrual cycle were assessed in 29 female IBS patients (aged 21-44 years), diagnosed by the Rome I criteria. During the course of the study patients completed symptom diaries to assess abdominal pain and bloating (visual analogue scale), and frequency and consistency of bowel habits. In addition, levels of anxiety and depression were assessed using the hospital anxiety and depression questionnaire. Menses was associated with a worsening of abdominal pain and bloating compared with most other phases of the menstrual cycle (p<0.05). Bowel habits also became more frequent (p<0.05) and patients tended to have a lower general well being. Rectal sensitivity increased at menses compared with all other phases of the cycle (p<0.05). There was no associated change in rectal compliance, wall tension, or motility index. Neither was there any difference in resting anal pressure or the distension volumes required to relax the internal anal sphincter during the menstrual cycle. These data (1) confirm that IBS symptomatology is exacerbated at menses and (2) show for the first time that in contrast with healthy women, rectal sensitivity changes with the menstrual cycle. These cyclical changes in sensitivity suggest that women with IBS respond differently to fluctuations in their sex hormonal environment or its consequences compared with healthy females.