Cerebrospinal fluid beta-endorphin in models of hyperalgesia in the rat

Cerebrospinal fluid (CSF) obtained by acute percutaneous puncture of the cisternal membrane of the halothane anesthetized rat has low but measurable concentrations of beta-endorphin-like immunoreactivity (beta-EPir: 32.8 +/- 3.0 pmol/l). Chromatographic separation of beta-EPir showed that authentic beta-endorphin1-31 was the main component of beta-EPir in cisternal CSF. Subcutaneous injection of 5% formalin in the hind paws did not increase beta-EPir in cisternal CSF. Rats with tactile paw hyperalgesia evoked by unilateral ligation of the L5/6 nerve roots 2 weeks earlier had beta-EPir concentrations that did not differ from sham operated or unoperated control animals. In contrast, capsaicin injected in the hindpaws increased the mean beta-EPir concentration compared to saline injections (P = 0.006) 45 min after emerging from anesthesia following injection. These results show that acute activation of C fibers (by capsaicin) will evoke the release of beta-endorphin into the CSF, suggesting activation of the beta-endorphin terminal systems in the brain/midbrain. The failure of formalin injections to release beta-EPir to CSF may be due to specificity of the afferent stimulus evoking beta-EPir release, a lower stimulus intensity, and/or the duration of the stimulus generated by formalin. The normal concentrations of beta-EPir found in the hyperalgesic state following nerve injury suggest that the supraspinal beta-endorphin system does not display tonic changes under such conditions.     

Hypoalgesia and hyperalgesia with inherited hypertension in the rat

Many studies indicate that blood pressure control systems can attenuate pain (hypoalgesia) of short duration; however, we recently found exaggerated nociceptive responses (hyperalgesia) of persistent duration in the spontaneously hypertensive rat (SHR). Here, we used SHR, Dahl Salt-Sensitive (SS), and normotensive control rats to evaluate the contribution of sustained elevations in arterial pressure to nociceptive responses. Compared with Sprague-Dawley and/or Wistar-Kyoto controls, SHR were 1) hypoalgesic in the hot plate test and 2) hyperalgesic in longer latency tail and paw-withdrawal tests and in two models of inflammatory nociception. These differences were not observed between SS and salt-resistant controls fed a high-salt diet. Inflammatory hyperalgesia in SHR was correlated with neither paw edema nor the number of Fos-positive spinal cord neurons. Our results indicate that "pain" phenotype of the SHR is not restricted to hypoalgesia. This phenotype is related to genetic factors or to the autonomic systems that control blood pressure and not to sustained elevations in blood pressure, differences in spinal neuron activity, or inflammatory edema.     

Lycopene ameliorates thermal hyperalgesia and cold allodynia in STZ-induced diabetic rat

Peripheral neuropathy is one of the common complications of diabetes mellitus. It is frequently associated with debilitating pain. The present study was designed to investigate effect of Lycopene, a carotenoid found in tomatoes, on hyperalgesia and cold allodynia in streptozotocin (STZ) induced diabetic rats. After 4-weeks of STZ injection, diabetic mice exhibited a significant thermal hyperalgesia cold allodynia, hyperglycemia and loss of body weights as compared with control rats. Chronic treatment of lycopene for 4 weeks significantly attenuated the cold allodynia and thermal hyperalgesia. The results emphasize the role of antioxidant such as lycopene as an adjuvant therapy in the treatment of diabetic neuropathy.     

Corticotropin-releasing activity in the rat gastric antrum

Rat gastric antrum, duodenum, pancreas, and spleen were extracted in acetic acid, treated with acetone, and purified on a C-18 cartridge. These extracts, in a dose equivalent to one respective organ, were examined for CRF bioactivity in vitro using rat half pituitaries, with gastric antrum extract showing a significant CRF activity. The antrum extract showed a dose-related CRF activity in vitro using rat pituitary cell culture, and the dose-response curve appeared to be parallel with that of synthetic rat hypothalamic CRF. Subsequent ion-exchange chromatography on a SP-Sephadex column showed that antrum CRF coeluted with basic materials (SP-III fraction), while rat hypothalamic CRF coeluted with weakly basic materials (SP-II fraction). The SP-III fraction was further purified by gel filtration on Sephadex G-50. CRF activity was eluted in two areas: large mol wt fraction (10,000-15,000) and small mol wt fraction (1500-2000). Hypothalamic CRF was eluted between them. The CRF activities of the two fractions were completely abolished by trypsin digestion, suggesting a peptide nature. The large molecular weight fraction exhibited a steeper dose-response curve than the hypothalamic CRF in vitro using cell culture, and the response to a dose equivalent to two antra exceeded the maximum response exhibited by the hypothalamic CRF. However, the fraction failed to increase serum corticosterone when injected in pharmacologically blocked rats. On the other hand, the small molecular weight fraction showed a lesser CRF activity and a similar dose-response curve to that of the hypothalamic CRF as tested in vitro. This fraction significantly stimulated corticosterone secretion in vivo as well. The small molecular weight activity did not appear to be due to other peptides or amines which have been reported as causing ACTH release. Although the physiological roles of the small molecular weight antrum CRF are unknown, it is possible that this CRF plays a role during stress as a tissue CRF.     

Peptic erosion of gastric mucus in the rat

1. The effect of pepsin on the loss of mucus glycoprotein from the gastric epithelial mucus layer was studied in the rat. 2. Pepsin was instilled into the gastric lumen, and luminal contents were subsequently assayed. 3. Glycoprotein loss increased with luminal pepsin, up to a concentration of 1 mg pepsin/ml. 4. Luminal glycoprotein had a molecular size distribution intermediate between subunit, and native mucus glycoprotein of the epithelial mucus layer. 5. Incubation of gastric epithelial scrapings with pepsin demonstrated that insoluble, native mucus glycoprotein was rapidly degraded to soluble glycoprotein of similar molecular size distribution to that found in vivo in the lumen.     

Inhibitory effect of anandamide on resiniferatoxin-induced sensory neuropeptide release in vivo and neuropathic hyperalgesia in the rat

Anandamide (AEA) is an endogenous cannabinoid ligand acting predominantly on the cannabinoid 1 (CB(1)) receptor, but it is also an agonist on the capsaicin VR(1)/TRPV(1) receptor. In the present study we examined the effects of AEA and the naturally occurring cannabinoid 2 (CB(2)) receptor agonist palmitylethanolamide (PEA) on basal and resiniferatoxin (RTX)-induced release of calcitonin gene-related peptide (CGRP) and somatostatin in vivo. Since these sensory neuropeptides play important role in the development of neuropathic hyperalgesia, the effect of AEA and PEA was also examined on mechanonociceptive threshold changes after partial ligation of the sciatic nerve. Neither AEA nor PEA affected basal plasma peptide concentrations, but both of them inhibited RTX-induced release. The inhibitory effect of AEA was prevented by the CB(1) receptor antagonist SR141716A. AEA abolished and PEA significantly decreased neuropathic mechanical hyperalgesia 7 days after unilateral sciatic nerve ligation, which was antagonized by SR141716A and the CB(2) receptor antagonist SR144528, respectively. Both SR141716A and SR144528 increased hyperalgesia, indicating that endogenous cannabinoids acting on CB(1) and peripheral CB(2)-like receptors play substantial role in neuropathic conditions to diminish hyperalgesia. AEA and PEA exert inhibitory effect on mechanonociceptive hyperalgesia and sensory neuropeptide release in vivo suggesting their potential therapeutical use to treat chronic neuropathic pain.     

An explanation of gastric hypersecretion in the pylorus-ligated rat

The hypersecretion of gastric acid in the pylorus-ligated rat has been shown to be of vagal origin. The present series of experiments were performed to identify the stimulus. The pyloric sphincter was ligated in a series of Sprague Dawley rats. Along with pylorus ligation, various other surgical manipulations were performed. Intestinal obstruction by ligation approximately 20 cm aboral to the cecum reduced unstimulated gastric secretion in the pylorus-ligated rat. However, perfusion of the lower small intestine with bicarbonate (143 mEq/L) stimulated secretion. Perfusion with either saline or deoxycholic acid (20 mEq/L) did not alter secretion. This supports a role for bicarbonate in the hypersecretion of gastric acid in the pylorus-ligated rat. The reflex appears to involve the myenteric plexus, since section of the pylorus seemed to attenuate gastric secretion. Plasma from animals with pylorus ligation, either alone or with intestinal ligation, equally inhibited gastric secretion. This suggests that while some factor inhibiting gastric secretion may be present, it appears to be unrelated to pylorus ligation.     

Development of gastric sensitivity to histamine in the rat

Sensitivity of the developing rat stomach to histamine (HA) was examined on isolated gastric mucosae of rats of various ages from the fetal to adult periods. Spontaneous acid secretion in mu eq/h.cm2 occurred at all the ages studied, at a basal rate of 0.45 +/- 0.07 in fetuses to 0.22 +/- 0.03 (day 5), 0.11 +/- 0.04 (day 10), 0.12 +/- 0.04 (day 12), 0.22 +/- 0.08 (day 16) and 0.33 +/- 0.04 (adults). In the fetal rats as in the adults, marked responses to respectively 10(-5) and 10(-4) M HA were demonstrated. The H2-receptor antagonist cimetidine diminished HA-induced secretion by 66 and 57% in fetuses and adults respectively. Between these two stages (from days 5 to 12), basal secretion and the response to HA dropped significantly. On day 21 of gestation, as well as on the critical days 5 and 12 after parturition, db-cAMP (10(-4) M) caused maximal stimulation of acid secretion. These results indicate that the development of responsiveness to HA in the rat is biphasic. They suggest that after birth, the H2-receptor adenylate cyclase system undergoes major modifications which might lead to the complete lack of responsiveness to HA by day 12.     

Effect of mepacrine on gastric motility in the rat

Mepacrine given orally to rats inhibits gastric motility; its blocking effect is comparable to that of chloroquine, papaverine, and drotaverine, but less expressed than that of bencyclane. Similarly as the delayed gastric emptying induced by chloroquine, the effect of mepacrine is antagonized by acetyl-beta-methylcholine in a dose-related fashion, while that of papaverine, drotaverine, and bencyclane remains unchanged after treatment with the cholinomimetic drug.     

Sprouted innervation into uterine transplants contributes to the development of hyperalgesia in a rat model of endometriosis

Endometriosis is an enigmatic painful disorder whose pain symptoms remain difficult to alleviate in large part because the disorder is defined by extrauteral endometrial growths whose contribution to pain is poorly understood. A rat model (ENDO) involves autotransplanting on abdominal arteries uterine segments that grow into vascularized cysts that become innervated with sensory and sympathetic fibers. ENDO rats exhibit vaginal hyperalgesia. We used behavioral, physiological, and immunohistochemical methods to test the hypothesis that cyst innervation contributes to the development of this hyperalgesia after transplant. Rudimentary sensory and sympathetic innervation appeared in the cysts at two weeks, sprouted further and more densely into the cyst wall by four weeks, and matured by six weeks post-transplant. Sensory fibers became abnormally functionally active between two and three weeks post-transplant, remaining active thereafter. Vaginal hyperalgesia became significant between four and five weeks post-transplant, and stabilized after six to eight weeks. Removing cysts before they acquired functional innervation prevented vaginal hyperalgesia from developing, whereas sham cyst removal did not. Thus, abnormally-active innervation of ectopic growths occurs before hyperalgesia develops, supporting the hypothesis. These findings suggest that painful endometriosis can be classified as a mixed inflammatory/neuropathic pain condition, which opens new avenues for pain relief. The findings also have implications beyond endometriosis by suggesting that functionality of any transplanted tissue can be influenced by the innervation it acquires.     

Intrathecal adenosine A1 receptor agonist attenuates hyperalgesia without inhibiting spinal glutamate release in the rat

The analgesia effects of intrathecal adenosine A₁ receptor agonist, R-PIA, on the hyperalgesia and CSF-glutamate release after formalin injection into the rat paw were evaluated. R-PIA significantly and dose-dependently attenuated increases in flinching behavior, and this attenuating effect was reversed by the adenosine A₁ receptor antagonist, aminophylline. Morphine blocked flinchs, however MK-801 partially abolished. The increase in CSF-glutamate release evoked by formalin stimulation was inhibited by morphine but not by either R-PIA or MK-801. These findings suggest that the intrathecal adenosine A₁ receptor agonist provokes analgesic effect via the postsynaptic action independent of an effect upon spinal glutamate release.