Responses of neurons in thalamic ventrobasal complex of rats to graded distension of uterus and vagina and to uterine suprafusion with bradykinin and prostaglandin F2 alpha

The Effects of Platelet-Activating Factor on Uterine Contractility, Perfusion, Hypoxia, and Pain in Mice

It is widely hypothesized that menstrual pain is triggered by prostaglandin synthesis that evokes high-pressure uterine contractions and ischemia. However, the effects of molecules implicated in menstrual pain on uterine contractility, perfusion, and oxygenation in vivo have been rarely demonstrated. Studies in women that do not respond to nonsteroidal anti-inflammatory drugs (NSAIDs) have reported elevated levels of platelet-activating factor (PAF). To establish in vivo evidence of PAF's capability to impair uterine homeostasis and to elicit visceral pain, we examined the effects of the PAF receptor agonist (carbamyl PAF [CPAF]) in comparison to other molecules hypothesized to play a role in uterine pain in mice. Uterine pressure was increased by oxytocin, prostaglandin F2α (PGF2α), and CPAF. Even in the absence of inflammatory molecules, uterine contractions reduced uterine oxygenation by 38%. CPAF reduced uterine perfusion by 40% ± 8% and elicited further oxygen desaturation approaching hypoxia (9.4 ± 3.4 mm Hg Pao₂). Intraperitoneal injections of CPAF and PGF2α evoked visceral pain and pelvic hyperalgesia in awake wild-type mice. However, pain was not observed in identically injected PAF-receptor knockout mice. Thus, our model provides a demonstration that a molecule implicated in NSAID-resistant dysmenorrhea has a detrimental effect on uterine homeostasis and is capable of causing visceral pain. Our results support the general hypothesis that menstrual cramps are caused by uterine contractions, impaired perfusion, and reduced oxygenation. Since this study was limited to mice, confirmation of these results in humans would be valuable for development of novel therapeutics targeted at inflammatory precursors, contractility, perfusion, and tissue oxygenation.

Proprioceptive and Cutaneous Representations in the Rat Ventral Posterolateral Thalamus

Determining how and where proprioceptive information is represented in the rat ventral posterolateral (VPL) is important in allowing us to further investigate how this sense is utilized during motor control and learning. Here we demonstrate using electrophysiological techniques that the rostral portion of the rat VPL nucleus (rVPL, −2 to −2.5 mm bregma) carries a large amount of proprioceptive information. Caudal to this region is a zone where the cutaneous receptive fields are focal (mVPL for middle VPL, −2.5 to −3.2 mm bregma) with a fine topographic map of the fore- and hindlimbs. The forepaw is represented with digit 1 medial and each subsequent digit increasingly lateral, all of which are dorsal to the pads. The caudal VPL (cVPL, −3.2 to −4.0 mm bregma) has broad receptive fields and is the target of lamina 1 and lamina 2, as well as the dorsal column nuclei, and may represent the flow of nociceptive information through the VPL. Thus we propose that the VPL may be thought of as three subnuclei—the rostral, middle, and caudal VPL—each carrying preferentially a different modality of information. This pattern of information flow through the rat VPL is similar, although apparently rotated, to that of many primates, indicating that these regions in the rat (rVPL, mVPL, and cVPL) have become further differentiated in primates where they are seen as separate nuclei (VPS, VPL, and VPI/VMpo).

Characterization of thalamic neuronal responses to urinary bladder distention, including the effect of acute spinal lesions in the rat

Chronic visceral pain has proved to be difficult to treat. This study characterized urinary bladder distention (UBD)-evoked responses of neurons located within the ventrobasal group of the thalamus. Units were also characterized for responses to cutaneous stimuli and colorectal distention (CRD). In addition, the effects of spinal lesions on UBD-evoked responses were examined in a subset of neurons. After a stable response to UBD was established, 3 sequential lesions of the spinal cord at the mid-cervical level were performed, and responses to UBD were determined 1 and 5 minutes later. A majority of the neurons in the ventrobasal group of the thalamus were excited by UBD, demonstrated graded responses to graded distention pressures, and responded to cutaneous stimulation. No correlation between the magnitude of the responses of thalamic neurons to UBD and CRD was found. UBD-evoked thalamic neuronal activity was significantly attenuated after dorsal midline lesions of the spinal cord. The present study is a quantitative description of ventrobasal thalamic neuronal responses to UBD in the rat and provides direct neurophysiologic evidence that nociceptive information from the urinary bladder to the ventrobasal group of the thalamus ascends via a dorsal midline pathway.

PERSPECTIVE

The effect of dorsal midline lesions is of profound clinical interest because it points to a potential treatment for urinary bladder pain, such as that which is characteristic of interstitial cystitis. Further research might reveal pharmacologic approaches to modulate this pain pathway and result in novel treatments for interstitial cystitis.

Effect of kappa opioid agonists on visceral nociception induced by uterine cervical distension in rats

Although uterine distension in rats results in an escape reflex, there exists no model of uterine cervical distension (UCD), the pain stimulus during the first stage of labor. The aims of this study were to develop such a model in virgin rats and to test whether peripherally restricted kappa opioid receptor (KOR) agonists (ADL 10-0101, ADL 10-0102, ADL 10-0116) inhibit responses to UCD. Under intravenous (i.v.) pentobarbital and alpha-chloralose anesthesia, fine metal rods were inserted in both uterine cervical osses through a small midline laparotomy. UCD was performed by manual separation of the rods (25-100 g). Single-unit afferent responses in hypogastric nerve or reflex rectus abdominis electromyographic (EMG) activity were determined before and after i.v. KOR agonists. UCD resulted in a stimulus-dependent increase in single-unit afferent activity. Units could be characterized as low threshold (mean threshold 6.6+/-2.7 g), or high threshold (mean threshold 55+/-8.8 g); all were C fibers, all responded to topical bradykinin. ADL 10-0116 (10 mg/kg) reduced the afferent response to UCD. Reflex EMG response occurred over a distension force range of 25-100 g, unaffected by i.v. saline. All three KOR agonists produced a dose-dependent, naloxone-reversible inhibition of the EMG response with a potency relationship of ADL 10-0102 (ED50 0.04 mg/kg)>ADL 10-0101 (ED50 0.65 mg/kg)=ADL 10-0116 (ED50 0.60 mg/kg). These data support the use of acute UCD as a noxious stimulus, inducing afferent and reflex activity. Like other visceral stimuli, UCD is sensitive to inhibition by KOR agonists.

Mechanisms of referred visceral pain: uterine inflammation in the adult virgin rat results in neurogenic plasma extravasation in the skin

The purpose of this study was to investigate the mechanisms of referred pain observed in female patients with pain from the reproductive organs. We developed a model of inflammatory uterine pain in the rat. Inflammation of the uterus in rats pretreated with Evans Blue Dye resulted in dye extravasation in the skin over the abdomen, groin, lower back, thighs, perineal area and proximal tail, thus providing for the first time evidence for the trophic changes observed in the area of referred visceral pain in an animal model of uterine pain. The neuronal pathways mediating the observed dye extravasation in the skin after uterine inflammation may include dichotomizing afferent fibers, afferent-afferent interactions via a spinal cord pathway or a sympathetic reflex. This model will allow to gain further insight into the mechanisms of referred pain and the trophic changes observed in the area of referred pain in visceral disease.

Antinociceptive effects of morphine and U-50,488H on vaginal distension in the anesthetized rat

The antinociceptive activity of the kappa- and mu-opioid receptor agonists, (+/-)-U-50,488H and morphine, was examined in a vaginal distension model in anaesthetized female rats. Vaginal distension induced a reproducible cardiovascular response (CVR) which was inhibited in a dose related manner by morphine (0.03-1.0 mg/kg i.v., ED50 = 0.16 mg/kg) and (+/-)-U-50,488H (0.08-1.6 mg/kg i.v., ED50 = 0.49 mg/kg). Morphine (0.3 microg/rat) administered i.c.v. inhibited the CVR by 81.6 +/- 7.9% whereas (+/-)-U-50,488H (30-300 microg/rat) was inactive by this route. A low dose of naloxone (30 microg/kg i.v.) blocked the effect of morphine but not that of (+/-)-U-50,488H. The kappa-opioid antagonist, nor-binaltorphimine (10 mg/kg s.c.) abolished the response to (+/-)-U-50,488H but not that of morphine. This demonstrates that both central and peripheral mu-opioid receptors may be involved in morphine-induced antinociception whereas the kappa-opioid agonist, (+/-)-U-50,488H, blocks vaginal nociception by acting on peripheral kappa-opioid receptors.

Responses of neurons in the caudal intralaminar thalamic complex of the rat to stimulation of the uterus, vagina, cervix, colon and skin

This study examined responses of 35 neurons in the caudal intralaminar (IL) thalamic nuclei in 12 adult female virgin rats to mechanical stimulation of the skin (brush, pressure, pinch) and to 4 different visceral stimuli (noxious distension of the uterine horns and vaginal canal; gentle distension of the colon and probing the cervix). As in male rats and other species, many IL neurons (24/35) responded to frankly noxious somatic stimuli applied to several bodily regions. Some of these (16/24) also responded to one or more of the visceral stimuli (mainly the noxious ones), while 4/35 responded only to a visceral stimulus. Thus, unlike neurons in lateral thalamus studied under identical conditions, IL neurons appear to be signalling information primarily when intense somatic and visceral stimuli are frankly above the noxious threshold.

Behavioral responses to uterine or vaginal distension in the rat

Afferent fibers in the rat hypogastric and pelvic nerves supply the uterus and vagina, respectively, the former being responsive mainly to intense uterine stimuli, the latter to gentle and intense vaginal stimuli (Berkley et al. 1993c). If such responses result in sensory experiences, those produced by uterine and vaginal stimulation should differ, uterine stimuli being experienced mainly as pain and vaginal stimuli experienced in various ways, including pain. To test this hypothesis, 48 young virgin rats were first trained to make an operant response to escape a noxious tail-pinch stimulus. Latex balloons inserted into the rat's uterine horn or vagina were then distended to various volumes and the metestrous rat's detection behaviors and operant escape response probabilities measured. Approximately 26% of the 23 rats tested failed to respond to uterine stimulation, even when it produced ischemia. For the rest, detection and escape responses occurred only to ischemic stimuli and never to all of them, even at the highest volumes. In contrast, all 25 rats tested responded readily to vaginal distension, often to all of them at high volumes. Detection behaviors occurred at distension magnitudes lower than those that evoked escape responses. These results support the hypothesis that sensory consequences of uterine and vaginal stimulation differ. Because effective uterine stimuli were larger than any that would occur in a normal physiological state in non-pregnant/parturient rats, normally occurring uterine states in such rats are probably insensible. In addition, while the behavioral responses did indeed reflect differences in hypogastric and pelvic nerve response properties, the results also indicated that activity produced in those fibers, even by abnormal stimuli, does not inevitably result in behavior.

Responses of lateral thalamic neurons to algesic chemical stimulation of the cat knee joint

In order to gain insight into the representation of articular pain of the knee at the supraspinal level, recordings were made from lateral thalamic neurons receiving input from afferent fibres of the knee joint in chloralose-anaesthetized cats. Dorsoventral penetrations were made through the ventral posterior lateral nucleus (VPL) using high intensity electrical stimulation of the medial articular nerve (MAN), which contains a high proportion (80%) of A delta and C afferent fibres. All recording sites were verified histologically. Close retrograde injections (300 microliters over 6 s) into geniculate artery of KCl (2 x isotonic), bradykinin (BK, 2.6 or 26 micrograms) and capsaicin (200 microM) were used to test the response properties of thalamic neurons. Of the 50 MAN-positive units tested, 20 showed a response to intra-arterial KCl; of these 20, 12 had a response to BK; 8 of these 12 units were additionally tested with capsaicin and all responded. KCl and capsaicin injections had similar mean response latencies (4.5 and 6.8 s), whereas BK had a longer mean latency (18.6 s). The mean peak response was greatest for capsaicin (168 impulses/s), then KCl (87.5 imp/s) and least with BK (36.4 imp/s). The mean response duration was longest with capsaicin (118 s), followed by BK (67.5 s) and least with KCl (27.9 s). Most of these were convergent wide dynamic range (WDR) neurons with a deep receptive field in the knee joint and hindlimb muscle and/or cutaneous distal hind limb digit, located to the dorsal or ventral periphery of the lateral division of the VPL, the VPL1. In addition, 8 neurons showed inhibitory responses to KCl and/or BK injections. The background activity of VPL1 neurons activated by saphenous nerve stimulation was inhibited by the nociceptive articular stimulus with a magnitude and time course which mirrored the excitatory responses in the periphery of VPL1. These results support the concept that the lateral thalamus plays an important role in mediating discriminative aspects of joint pain.