Estrous cycle phase variations in visceromotor and cardiovascular responses to colonic distension in the anesthetized rat

Stress-induced visceral hypersensitivity in female rats is estrogen-dependent and involves tachykinin NK1 receptors

Hormonal cycling may be related to a higher incidence of pain syndrome in female. As tachykinins are pivotal in stress-induced colonic dysfunction, we investigated whether ovarian steroids influence stress-induced visceral hypersensitivity to rectal distension (RD) in female rats and further, whether this influence involves NK1 receptors. Female Wistar rats, either intact or ovariectomized (OVX), were equipped for abdominal muscle electromyography and submitted to 2-h partial restraint stress (PRS) or sham-PRS. First, the effect of PRS was evaluated in intact rats. Second, abdominal response to RD was recorded in OVX rats treated with either, progesterone, 17beta-estradiol, 17beta-estradiol-plus-progesterone, or vehicle, in both basal and PRS conditions. Third, the NK1 receptor-antagonist, SR140333, was tested in PRS-intact and PRS-OVX rats under 17beta-estradiol or 17beta-estradiol-plus-progesterone treatment. PRS induced visceral hypersensitivity to RD and this effect was prevented by ovariectomy. OVX rats treated with 17beta-estradiol or 17beta-estradiol-plus-progesterone, but not progesterone alone, exhibited visceral hypersensitivity after PRS similar to that of intact rats. Both stress-induced visceral hypersensitivity in intact rats and the hormonally-restored visceral hyper-responsiveness of OVX rats were antagonized by SR140333. It is concluded, therefore, that stress-induced visceral hypersensitivity in female rats is estrogens-dependent and mediated through NK1 receptor activation.

Long-lasting changes in central nervous system responsivity to colonic distention after stress in rats

BACKGROUND & AIMS

The highly prevalent functional gastrointestinal disorders involve visceral pain and disturbed bowel habit and are associated with preceding stressful experiences, although causality and biological mechanisms remain unclear. The aim of the present study was to establish whether stress can directly and lastingly alter central nervous system responsivity to colonic distention in the rat as well as which neural pathways are likely to be involved.

METHODS

Rats were treated with a brief session of stressful foot shocks known to induce long-term behavioral and autonomic sensitization. Two weeks later, after induction of inhalation anesthesia, a balloon catheter was inserted in the distal colon and repeatedly inflated with brief, constant-pressure air pulses.

RESULTS

Reflex decreases in blood pressure and heart rate indicative of visceral afferent activation were greater in previously shocked rats than in controls. Colonic distention increased the expression of Fos, a marker of neuronal activation, in the sacral spinal cord and caudal brain stem. In the central amygdala and several cortical areas (prelimbic, infralimbic, agranular insular, cingulate), previously shocked rats showed reduced Fos expression following colonic distention compared with relevant controls.

CONCLUSIONS

The results indicate that a brief but intense stressful experience causes long-lasting alterations in higher-order central nervous system responsivity to colonic distention even in the absence of conscious affective responses, pointing to basic alterations in the neural pathways involved.

Gender differences and hormonal modulation in visceral pain

Women seek healthcare and are diagnosed more frequently with chronic somatic and visceral pain conditions relative to men. These conditions tend not to be life-threatening disorders, but rather ones that decrease people's quality of life, impinge on work and recreational activities, and increase healthcare resource utilization. With increased awareness of basic gender differences in biology and responsiveness to therapies, there has been renewed interest in factors which may account for the gender disparity in chronic visceral pain conditions. Basic and clinical evidence primarily from patients with irritable bowel syndrome has provided initial insights into visceral pain sensitivity, perception, and responsitivity.

Sex and oestrous cycle differences in visceromotor responses and vasopressin release in response to colonic distension in male and female rats anaesthetized with halothane

Visceromotor responses and vasopressin release before and after colonic visceral distension were compared between male (n=5 (n=4 for vasopressin)) and female rats and between females during the oestrous cycle (proestrus n=6, oestrus n=5, metestrus n=5, diestrus n=6) at a controlled depth of anaesthesia. Pre-stimulation vasopressin and blood pressures demonstrated oestrous cycle variability. The mean (SEM) colonic balloon pressure triggering visceromotor responses was significantly higher in males (64 (4) mm Hg) than females (41 (1) mm Hg), P=0.002 and within females, proestrus rats had the lowest thresholds, (29 (1) mm Hg, P<0.01). Post-stimulation, vasopressin concentrations increased significantly in all groups (males 1.34 (0.39) to 2.24 (0.74) pmol litre(-1); females 1.54 (0.24) to 2.88 (0.58) pmol litre(-1); P=0.002). Within groups statistically significant differences were measured in proestrus 2.06 (0.56) to 3.42 (1.12) and oestrus 1.16 (0.38) to 2.76 (0.60) pmol litre(-1) (P<0.05). High vasopressin concentrations coupled with low-pressure stimulation during proestrus shows sex-hormone dependent integration of the neuroendocrine response to noxious visceral stimulation.

Sensory fibers of the pelvic nerve innervating the Rat's urinary bladder

Much attention has been given to the pelvic nerve afferent innervation of the urinary bladder; however, reports differ considerably in descriptions of afferent receptor types, their conduction velocities, and their potential roles in bladder reflexes and sensation. The present study was undertaken to do a relatively unbiased sampling of bladder afferent fibers of the pelvic nerve in adult female rats. The search stimulus for units to be studied was electrical stimulation of both the bladder nerves and the pelvic nerve. Single-unit activity of 100 L(6) dorsal root fibers, activated by both pelvic and bladder nerve stimulation, was analyzed. Sixty-five units had C-fiber and 35 units had Adelta-fiber conduction velocities. Receptive characteristics were established by direct mechanical stimulation, filling of the bladder with 0.9% NaCl at a physiological speed and by filling the bladder with solutions containing capsaicin, potassium, or turpentine oil. The majority (61) of these fibers were unambiguously excited by bladder filling with 0.9% NaCl and were classified as mechanoreceptors. All mechanoreceptors with receptive fields on the body of the bladder had low pressure thresholds (</=10 mmHg). Receptive fields of units with higher thresholds were near the ureterovesical junction, on the base of the bladder or could not be found. Neither thresholds nor suprathreshold responses could be related to conduction velocity. Bladder compliance and mechanoreceptor thresholds were influenced by the stage of the estrous cycle: both were lowest in proestrous rats and highest in metaestrous rats. Mechanoreceptors innervating the body of the bladder and the region near the ureterovesical junction showed two patterns of responsiveness to slow bladder filling. One group of units exhibited increasing activity with increasing pressure up to 40 mmHg, while the other group showed a peak in activity at pressures below 40 mmHg followed by a plateau or decrease in activity with increasing pressure. It is proposed that differences in stimulus transduction relate to the different response patterns. Thirty-nine units failed to respond to bladder filling. Eight of these were excited by intravesical potassium or capsaicin and were classified as chemoreceptors. The remaining 31 units were not excited by any stimulus tested. Chemoreceptors and unexcited units had both Adelta and C afferent fibers. We conclude that the pelvic nerve sensory innervation of the rat bladder is complex, may be sensitive to hormonal status, and that the properties of individual sensory receptors are not related in an obvious manner to the conduction velocity of their fibers.

Sex-related hormonal influences on pain and analgesic responses

Considerable evidence indicates sex-related differences in pain responses and in the effectiveness of various analgesic agents. Specifically, females are at greater risk for experiencing many forms of clinical pain and are more sensitive to experimentally induced pain relative to males. Regarding analgesic responses, nonhuman animal studies indicate greater opioid analgesia for males, while a limited human literature suggests the opposite. Though the mechanisms underlying these effects remain unclear, the influence of gonadal hormones on nociceptive processing represents one plausible pathway whereby such sex differences could emerge. The present article reviews the complex literature concerning sex steroid effects on pain responses and analgesia. First, nonhuman animal research related to hormonal effects on nociceptive sensitivity and analgesic responses is presented. Next, human studies regarding gonadal hormonal influences on experimental pain responses are reviewed. Several potential mechanisms underlying hormonal effects on nociceptive processing are discussed, including hormonal effects to both peripheral and central nervous system pathways involved in pain transmission. Finally, based on these findings we draw several conclusions and make specific recommendations that will guide future research as it attempts to elucidate the magnitude and importance of sex-related hormonal effects on the experience of pain.

Kappa opioid receptors in rat spinal cord vary across the estrous cycle

Kappa opioid receptors (KORs) were immunocytochemically localized in the lumbosacral spinal cord of female rats in different stages of the estrous cycle to examine the influence of hormonal status on receptor density. KOR labeling was primarily in fine processes and a few neuronal cell bodies in the superficial dorsal horn and the dorsolateral funiculus. Quantitative light microscopic densitometry of the superficial dorsal horn revealed that rats in diestrus had significantly lower KOR densities than those in proestrus or estrus. This suggests that female reproductive hormones regulate spinal KOR levels, which may contribute to variations in analgesic effectiveness of KOR agonists across the estrous cycle.

Estrous variations in behavioral responses to vaginal and uterine distention in the rat

Response properties of peripheral afferent fibers supplying the vagina and uterus of the rat vary with estrous stage (Robbins A, Berkley KJ, Sato Y. Estrous cycle variation of afferent fibers supplying reproductive organs in the female rat. Brain Res 1992,596:353-356), suggesting that behavioral sensitivity to vaginal and uterine stimulation might similarly vary. To test this hypothesis, detection and escape responses of 12 rats to different volumes of distention of the vaginal canal or uterine horn (six rats each) were compared during each of the four estrous stages, proestrus (P), estrus (E), metestrus (M), and diestrus (D), using previously-published behavioral techniques (Berkley KJ, Wood E, Scofield SL, Little M. Behavioral responses to uterine or vaginal distention in the rat. Pain 1995;61:121-131). Estrous variations in vaginal or uterine tone (pressure-volume functions) were also measured in the same rats. Vaginal canal: escape response percentages increased significantly as vaginal distention volume increased, but neither escape nor detection responses varied with estrous stage. Vaginal tone, however, was greater in P and E than in M and D, with the greatest tone in E and the least in D. When escape responses to different pressures were analyzed, it was found that escape response percentages to the same vaginal pressure were lower during P and E than during M and D. One outcome of these estrous influences would be that a vaginal stimulus of a given volume (such as an erect penis) would exert higher pressures during P and E (i.e. the penis would be held within the vaginal canal more firmly) than during M and D, but would be less likely during P and E to provoke escape responses. This modification of nociceptive sensitivity has obvious reproductive advantages, because P and E constitute the rat's fertile period. Uterine horn: escape response percentages increased significantly as uterine distention volume increased only during M and D, with no differences between them. Detection responses also occurred only during M and D. Similarly, uterine tone was significantly greater in M and D than in P and E, with the greatest tone occurring during M and the least during P. Although these changes in uterine tone were opposite to those of the vaginal canal, escape response percentages to the same uterine pressures were, in a manner similar to vaginal pressures, lower during P and E than during M and D. The functional significance of these variations for the uterus is unclear, but does suggest that, under pathophysiological conditions, stimulation of the uterine horn, as well as the vaginal canal, would be more likely to provoke escape behaviors in M and D than in P and E. A similar increased sensitivity in rats during M and D to noxious stimulation of other pelvic organs has been observed by others. This situation resembles that in women, for whom many visceral pain conditions are exacerbated during a comparable part of their ovarian cycle, i.e. perimenstrually.

Uterine inflammation as a noxious visceral stimulus: behavioral characterization in the rat

We have developed a model of uterine inflammation in the rat. The purpose of this study was to characterize the behavioral manifestations of uterine pain. Mustard oil was injected into one uterine horn to produce chemical inflammation. Control rats were sham-operated. Non-stop videotape recording was performed for 7 days to monitor rat behavior. Rats with uterine inflammation showed abnormal behavior during the first 4 days (hunching, hump-backed position, licking of the lower abdomen, repeated waves of contraction of the ipsilateral oblique musculature with inward turning of the ipsilateral hindlimb, stretching, squashing of the lower abdomen against the floor) suggestive of visceral pain and evidence of flank muscle hyperalgesia over 7 days indicative of referred visceral pain. This model resembles closely a state of inflammatory uterine pain and will allow to gain further insight into the neural processes which contribute to visceral nociception.

Sex, time-of-day and estrous variations in behavioral and bladder histological consequences of cyclophosphamide-induced cystitis in rats

This study examined how cyclophosphamide (CP)-induced cystitis related manifestations (bladder inflammation and behavioral impairment) differed in female and male Sprague-Dawley rats. Under transient halothane-O2-N2O gas anesthesia, a single dose of CP was injected (100 mg/kg i.p. in 1 ml saline) and the animal's behaviors analyzed for a period of 4 h using a protocol that permits quantitative analysis of behavioral impairment. The rats were then sacrificed and their bladders removed for histological quantification of inflammation. All CP-injected, but not control rats, exhibited a range of impairment behaviors that increased rapidly over a period of 2 h, gradually reaching plateau levels over the next 2 h. Female rats initially developed behavioral responses faster than male rats, but reached the same mean peak values overall as males. No sex differences were observed in CP-induced bladder inflammation. Influences of time-of-day and estrous stage were further examined in females. Time-of-day had no effect on the degree of bladder inflammation. Although there were also no significant time-of-day differences in behavioral impairments, impairment scores from 90 min after the injection consistently tended to be lower for rats injected 5 h versus 9 h after lights on. Overall, the effects of estrous stage were also insignificant. However, a subset of rats who were in the estrous stage of their cycle early in the morning of the experimental day developed the most severe degree of bladder inflammation, but failed to develop the severe behavioral impairments shown by all the other rats. These results show that there are seemingly only minor sex differences in the overall behavioral and inflammatory consequences of CP injections, as evidenced by similar final degrees of behavioral impairment and inflammation. These results also suggest, however, that there are sex differences in the etiology of the disease process. These differences are evidenced by the more rapid development of behavioral symptoms in females and the susceptibility of some of those having shown morning estrous smears to develop very severe bladder inflammation in absence of corresponding behavioral impairment. The multiple influences of sex and estrous condition on CP-induced cystitis related manifestations observed here underline the complexity of the etiological factors associated with the cystitis disease process.