Perineural Treatment Research Articles

Stimulation of pulmonary vagal C-fibres by anandamide in anaesthetized rats: role of vanilloid type 1 receptors.

This study was carried out to determine the effect of intravenous injection of anandamide on pulmonary C-fibre afferents and the cardiorespiratory reflexes. In anaesthetized, spontaneously breathing rats, intravenous bolus injection of anandamide near the right atrium immediately elicited the pulmonary chemoreflex responses, characterized by apnoea, bradycardia and hypotension. After perineural treatment of both cervical vagi with capsaicin to block the conduction of C-fibres, anandamide no longer evoked these reflex responses. In open-chest, and artificially ventilated rats, anandamide injection evoked an abrupt and intense discharge in vagal pulmonary C-fibres in a dose-dependent manner. After injection of the high dose, the fibre discharge generally started within 1 s, reached a peak in approximately 2 s, and returned to baseline within 7 s. The stimulation of C-fibres by anandamide was completely and reversibly blocked by pretreatment with capsazepine, a competitive antagonist of the vanilloid type 1 receptor. Anandamide (0.4 mg kg(-1)) stimulated approximately 93 % of pulmonary C-fibres that were activated by capsaicin at a much lower dose (0.6 microg kg(-1)); the response to anandamide showed similar intensity, but had slightly longer latency and duration than that to capsaicin. In conclusion, intravenous bolus injection of anandamide evokes a consistent and distinct stimulatory effect on pulmonary C-fibre terminals, and this effect appears to be mediated through an activation of the vanilloid type 1 receptor.

Effects of Capsaicin Application to Peripheral Nerves in a Rat Model for Human Postoperative Pain

Background: The interest in management of postoperative pain has increased with the development of medicine. The mechanism of postoperative pain should be understood first. Capsaicin has been proven to be a powerful agent acting selectively on nociceptive afferent fibers. This study was focused on how the perineural treatment of capsaicin affects pain after an incision in the rat model. Methods: Capsaicin was applied directly to sciatic nerve branches, on the are tibial, sural, and saphenous nerves. Pain behaviors to von Frey filaments, blunt probe stimulation, resting pain, and heat were evaluated on the incised feet. The grade of wound healing was assessed according to clinical findings of the wound 3 days postoperatively. Results: Capsaicin applied directly onto nerves elevated withdrawal latency to heat during the study. There were statistically significant increases of withdrawal latencies in the capsaicin group compared with the vehicle group. Capsaicin produced agreater increase of the withdrawal threshold to von Frey than the vehicle 2 days after the incision. Resting pain scores were lower in capsaicin - treated rats than in the vehicle group for 2 days. The grade of wound healing was poor in capsaicin - treated rats. Conclusions: The perineural application of capsaicin appears to affect the pathway of transmission of postoperative pain. Current results suggest that capsaicin has a strong analgesic effect on thermal hyperalgesia and attenuates mechanical hyperalgesia by an incision. Capsaicin sensitive nerves could be involved in wound healing.

Role of capsaicin-sensitive nerves in gastric and hepatic injury induced by cold-restraint stress.

The role of capsaicin-sensitive afferent fibers on cold-restraint stress-induced gastric and hepatic injury was examined at the macroscopic and ultrastructural levels. Wistar albino rats were treated with capsaicin either locally (intragastric, perivagal, and periceliac) or systemically (neonatal, intraperitoneal). Perineural and neonatal treatment with capsaicin was used to denervate afferent fibers, while intragastric capsaicin treatment would have activated mucosal afferent fibers just before the stress exposure. Capsaicin decreased significantly the formation of macroscopic gastric lesions caused by stress in all treatment groups. At the electron microscopic level, however, denervation of vagal afferent fibers with capsaicin was most effective in prevention of cellular injury in gastric mucosa. In the liver, systemic denervation of afferent fibers completely inhibited stress-induced cellular damage, while denervation of afferent fibers in vagus and splanchnic nerve was partially effective. Central neural pathways sensitive to capsaicin may mediate formation of both gastric and hepatic injury resulting from stress.

Blockade of capsaicin-induced reduction of GABA-immunoreactivity by spantide in cat spinal superficial dorsal horn.

In our previous study, perineural application of capsaicin not only produced release and depletion of substance P from primary nociceptive afferent terminals, but also reduced GABA immunoreactivity in the superficial dorsal horn. The aim of the present study was to determine whether the release of GABA is triggered by substance P released from primary nociceptive afferent terminals by capsaican. GABA and substance P immunoreactivity in the lumbar dorsal horn was examined in two groups: in the first group the tibial nerve was treated with 3% capsaicin, and in the second group the dorsal surface of the lumbar cord was infused with spantide (50 nM), a substance P receptor antagonist, before application of capsaicin to the tibial nerve. Following perineural treatment of capsaicin for 30 min, both the GABA-immunostaining density and the number of GABA immunoreactive neurons were reduced significantly in the ipsilateral laminae I-II at L5 through L7. GABA immunoreactivity was reduced by 54.12%, 44.46% and 31.0% in the medial, central and lateral parts of the ipsilateral laminae I-II at L7, respectively. With pre-application of spantide to the spinal cord, GABA immunoreactivity was reduced only to 14.4%, 16.4% and 10.16%, respectively, in the medial, central and lateral parts of laminae I-II at L7 and no reduction of GABA immunoreactive neurons was observed. Additionally, capsaicin-induced reduction of substance P immunoreactivity was partially blocked by spantide. These results suggest that capsaicin produces substance P release from primary nociceptive afferent terminals, and that substance P, in turn, activates the second-order GABAergic interneurons in the dorsal horn. The functional significance of capsaicin-induced activation of GABAergic neurons in modulation of spinal nociception is discussed.

Reduction of substance P binding sites in the spinal dorsal horn after perineural capsaicin treatment in the rat

The long-term effect of perineural capsaicin treatment on the distribution of substance P (SP) binding sites was studied in the rat spinal dorsal horn using 125I-labelled Bolton-Hunter-SP. Three months after local application of capsaicin onto the sciatic nerve quantitative evaluation of the autoradiograms revealed a significant decrease in the densiy of SP binding sites of up to 48% in regions of laminae I and II of the spinal dorsal horn somatotopically related to the capsaicin treated sciatic nerve. It is suggested that reduction in SP binding may result from transganglionic and transsynaptic degenerative changes affecting postsynaptic structures. Changes in the distribution of SP binding sites may significantly contribute to functional alterations observed after perineural treatment with capsaicin.

Pathobiological reactions of C-fibre primary sensory neurones to peripheral nerve injury.

Mammalian primary sensory neurones display profound anatomical and chemical changes in response to injury of their peripheral processes. Peripheral nerve damage results in transganglionic degeneration of central terminals of A-fibre primary afferent neurones terminating in the deeper layers of the medullary or spinal dorsal horn. The depletion of neuropeptides and other neurone-specific macromolecules from neurones in sensory ganglia and from the superficial dorsal horn, the known central projection area of C-fibre primary afferents, is a salient phenomenon commencing after peripheral nerve lesions. A recently devised new experimental procedure, termed the capsaicin-gap method, permitted evaluation of the transganglionic degenerative phenomena which develop in C-fibre primary sensory neurones after a lesion is inflicted upon their peripheral branches. The experimental findings indicated that C-fibre primary afferent terminals underwent transganglionic degeneration following a perineural treatment with capsaicin, and that this was associated with, and probably resulted from, ganglionic cell degeneration. Studies on the effects of peripheral nerve section yielded similar results. It has therefore been suggested that the observed depletion of specific macromolecules, including sensory peptides, specific glycoconjugates and sensory neurone specific acid phosphatase, may be accounted for, at least in part, by an irreversible loss of sensory ganglion neurones. In contrast, many injured neurones express peptides, including vasoactive intestinal polypeptide, peptide histidine-isoleucine and galanin, which can be demonstrated in only a few neurones under normal conditions. These seem to be involved in dorsal horn regenerative and/or compensatory processes following peripheral nerve damage. There is suggestive evidence that the partial deafferentation caused by the transganglionic degeneration of C-fibre primary afferents creates favourable circumstances for an anatomical rearrangement of neuronal connections within the spinal cord dorsal horn. These changes may provide a morphological substrate of some of the functional alterations demonstrated after peripheral nerve lesions.

Capsaicin-sensitive vagal afferents and CCK in inhibition of gastric motor function induced by intestinal nutrients

The role of vagal afferent pathways and cholecystokinin (CCK) in mediating changes in gastric motor function after a meal was investigated in urethane-anesthetized rats. Proximal gastric motor function was measured manometrically, and nutrients were infused into an isolated segment of duodenum. Inhibition of gastric motility in response to duodenal infusion of protein (peptone or casein), but not carbohydrate (glucose), was significantly attenuated by administration of the CCK antagonist, L364,718. Selective ablation of vagal afferents by perineural treatment with the sensory neurotoxin, capsaicin, significantly reduced responses to both duodenal protein and glucose. These results suggest that protein in the duodenum decreases proximal gastric motor function via release of CCK and a vagal capsaicin-sensitive afferent pathway. In contrast, glucose acts via a capsaicin-sensitive vagal pathway not involving CCK. Thus separate neural and hormonal mechanisms mediate the effects of different nutrients in the duodenal feedback regulation of gastric motor function.

Inhibition of Gastric Motor Function by Circulating Corticotropin‐Releasing Factor in Anesthetized Rats

We have investigated the influence of intravenous corticotropin‐releasing factor (CRF) on intraluminal pressure of the body of the stomach and the neurohumoral pathways through which CRF inhibits gastric motor function in rats. CRF (0.21–210 pmol) injected intravenously produced a dose‐dependent, long‐lasting decrease in baseline tone and inhibited phasic gastric contractions measured manometrically in urethane‐anesthetized rats. The inhibitory effect of CRF was abolished by administration of hexamethonium or by subdiaphragmatic vagotomy but was unaffected by perineural treatment of the vagus nerves with capsaicin, adrenalectomy, or celiac/superior mesenteric ganglionectomy. Intravenous administration of CRF (126 pmol) inhibited by 69% the emptying of a noncaloric liquid meal as measured by the phenol red technique in conscious rats. Hexamethonium pretreatment significantly attenuated the CRF ‐induced delay in gastric emptying; bretylium, naloxone, adrenalectomy, and celiac I superior mesenteric ganglionectomy had no such effect. In an isolated tissue preparation, CRF (10−11 to 10−6 M) inhibited spontaneous, but not carbachol‐induced, contractions of rat antral longitudinal muscle. The effect of CRF was dose‐dependent and abolished in the presence of tetrodotoxin. These results suggest that peripheral CRF‐induced inhibition of corpus and antral motor contractility may mediate delayed gastric emptying via an action involving a nicotinic synapse, presumably in the enteric nervous system, that requires a vagal input. The in vitro studies support the concept that the action of CRF is not direct on smooth muscle but involves neural transmission within the enteric nervous system.

Transganglionic degeneration of capsaicin-sensitive C-fiber primary afferent terminals

Local treatment of rat peripheral nerves with capsaicin induces permanent impairment of afferent C-fiber functions and changes in the response properties of spinal dorsal horn neurons. In this study a new experimental approach, the “capsaicin gap” technique, has been utilized in an attempt to unravel pathomorphological alterations which commence in the domain of primary sensory neurons as a consequence of perineural treatment with capsaicin. The technique relies on the facts that peripheral nerves in the spinal dorsal horn are represented in a strict somatotopic manner, and on the observation that in the adult rat systemic injection of appropriate doses of capsaicin results in a selective degeneration of primary afferent fibers within Rexed's laminae I and II of the spinal cord. We have assumed that, if treatment of a peripheral nerve with capsaicin results in an irreversible destruction of primary afferent terminals related to that nerve, then the corresponding spinal projection areas will be free of degeneration following a subsequent systemic injection of capsaicin. The topographical distribution of degenerating axon terminals, induced by a systemic injection of capsaicin, was studied within the lumbar spinal cord of rats whose sciatic nerves had been treated with capsaicin or its vehicle one to three months previously. In the dorsal horn ipsilateral to the vehicle-treated nerve, axon terminal degeneration was found in the entire mediolateral extent of Rexed's laminae I and II. In contrast, in the dorsal horn ipsilateral to the capsaicin-treated nerve, virtually no degeneration was observed in areas which correspond to the projection territories of sciatic afferents. It is concluded that the central terminals of capsaicin-sensitive sciatic afferents underwent transganglionic degeneration as a result of prior perineural treatment with capsaicin, and a subsequent systemic injection of this neurotoxin therefore failed to cause axon terminal degeneration in somatotopic areas served by the treated nerve. Comparative quantitative morphometric analysis of cell populations of dorsal root ganglia related to capsaicin- or vehicle-treated nerves disclosed (1) a marked reduction in the proportion of small-sized neurons, (2) a fall of about 80% in the percentage of neurons which undergo degeneration after the systemic injection of capsaicin, (3) a marked decrease in the total number of neurons in ganglia ipsilateral to the capsaicin-treated nerves. Quantitative electron microscopic studies on saphenous nerves treated perineurally with capsaicin revealed a 32% reduction in the number of unmyelinated axons as compared with the controls, whereas the number of myelinated fibers was unchanged. The results demonstrate that unmyelinated primary afferent terminals may undergo transganglionic degeneration associated with, and probably resulting from, ganglionic cell degeneration. The “capsaicin gap” technique may be of considerable potential for evaluating transganglionic degenerative changes of unmyelinated primary afferents following various types of peripheral nerve injuries.