Release of calcitonin gene-related peptide in guinea pig inferior mesenteric ganglion

Postoperative ileus-An ongoing conundrum

BACKGROUND

Postoperative ileus is common and is a major clinical problem. It has been widely studied in patients and in experimental models in laboratory animals. A wide variety of treatments have been tested to prevent or modify the course of this disorder.

PURPOSE

This review draws together information on animal studies of ileus with studies on human patients. It summarizes some of the conceptual advances made in understanding the mechanisms that underlie paralytic ileus. The treatments that have been tested in human subjects (both pharmacological and non-pharmacological) and their efficacy are summarized and graded consistent with current clinical guidelines. The review is not intended to provide a comprehensive overview of ileus, but rather a general understanding of the major clinical problems associated with it, how animal models have been useful to elucidate key mechanisms and, finally, some perspectives from both scientists and clinicians as to how we may move forward with this debilitating yet common condition.

A Novel Mode of Sympathetic Reflex Activation Mediated by the Enteric Nervous System

Enteric viscerofugal neurons provide a pathway by which the enteric nervous system (ENS), otherwise confined to the gut wall, can activate sympathetic neurons in prevertebral ganglia. Firing transmitted through these pathways is currently considered fundamentally mechanosensory. The mouse colon generates a cyclical pattern of neurogenic contractile activity, called the colonic motor complex (CMC). Motor complexes involve a highly coordinated firing pattern in myenteric neurons with a frequency of ∼2 Hz. However, it remains unknown how viscerofugal neurons are activated and communicate with the sympathetic nervous system during this naturally-occurring motor pattern. Here, viscerofugal neurons were recorded extracellularly from rectal nerve trunks in isolated tube and flat-sheet preparations of mouse colon held at fixed circumferential length. In freshly dissected preparations, motor complexes were associated with bursts of viscerofugal firing at 2 Hz that aligned with 2-Hz smooth muscle voltage oscillations. This behavior persisted during muscle paralysis with nicardipine. Identical recordings were made after a 4- to 5-d organotypic culture during which extrinsic nerves degenerated, confirming that recordings were from viscerofugal neurons. Single unit analysis revealed the burst firing pattern emerging from assemblies of viscerofugal neurons differed from individual neurons, which typically made partial contributions, highlighting the importance and extent of ENS-mediated synchronization. Finally, sympathetic neuron firing was recorded from the central nerve trunks emerging from the inferior mesenteric ganglion. Increased sympathetic neuron firing accompanied all motor complexes with a 2-Hz burst pattern similar to viscerofugal neurons. These data provide evidence for a novel mechanism of sympathetic reflex activation derived from synchronized firing output generated by the ENS.

Effects of calcitonin gene-related peptide on rat soleus muscle excitability: mechanisms and physiological significance

Intense exercise causes a large loss of K+ from contracting muscles. The ensuing elevation of extracellular K+ ([K+]o) has been suggested to cause fatigue by depressing muscle fiber excitability. In isolated muscles, however, repeated contractions confer some protection against this effect of elevated K+. We hypothesize that this excitation-induced force-recovery is related to the release of the neuropeptide calcitonin gene-related peptide (CGRP), which stimulates the muscular Na+-K+ pumps. Using the specific CGRP antagonist CGRP-(8-37), we evaluated the role of CGRP in the excitation-induced force recovery and examined possible mechanisms. Intact rat soleus muscles were stimulated to evoke short tetani at regular intervals. Increasing extracellular K+ ([K+]o) from 4 to 11 mM decreased force to ∼20% of initial force ( P < 0.001). Addition of exogenous CGRP (10−9 M), release of endogenous CGRP with capsaicin, or repeated electrical stimulation recovered force to 50–70% of initial force ( P < 0.001). In all cases, force recovery could be almost completely suppressed by CGRP-(8-37). At 11 mM [K+]o, CGRP (10−8 M) did not alter resting membrane potential or conductance but significantly improved action potentials ( P < 0.001) and increased the proportion of excitable fibers from 32 to 70% ( P < 0.001). CGRP was shown to induce substantial force recovery with only modest Na+-K+ pump stimulation. We conclude that the excitation-induced force recovery is caused by a recovery of excitability, induced by local release of CGRP. The data suggest that the recovery of excitability partly was induced by Na+-K+ pump stimulation and partly by altering Na+ channel function.

Influence of sex and experimental protocol on the effect of maternal deprivation on rectal sensitivity to distension in the adult rat

Neonatal maternal separation induces visceral hyperalgesia before and after stress in male rats. This study compares the effects on sensitivity to rectal distension in adult male and female rats, using two protocols of deprivation. Between postnatal days 1 and 14, maternal deprivation was performed for 2 h per day according to a protocol of type M (removal of all pups from home cage) or type P (separation of half of littermates). Visceral sensitivity was assessed at 12 weeks of age by the number of abdominal contractions induced by rectal distension before and after restraint stress. Calcitonin gene-related peptide (CGRP) was identified in the rectal wall by immunohistochemistry. In basal conditions, both separation protocols induced hyperalgesia, that was greater after type M than type P, and in females than in males for type P separation. Acute restraint stress induced hyperalgesia in control females only, and this effect was similarly enhanced by both type P and M separation. No difference was found between controls and deprived rats in rectal CGRP immunoreactivity which was greater in females and increased after rectal distension. These results indicate that long-term visceral hyperalgesia depends upon the type of maternal deprivation and that females are more sensitive than males.

Do sensory calcitonin gene-related peptide nerve fibres in the rat pelvic plexus supply autonomic neurons projecting to the uterus and cervix?

Sensory nerve fibres containing calcitonin gene-related peptide (CGRP) innervate neurons of the paracervical ganglion (PCG) in the female rat pelvic plexus. We have combined retrograde tracing with immunocytochemistry to investigate whether CGRP-immunoreactive (-IR) fibres supply neurons targeting the genital tract. Of the total neurons projecting to either the uterine horns or the cervix, 38 and 41% received CGRP-IR innervation, respectively. All these neurons displayed choline acetyltransferase-IR, thus are cholinergic. They were found throughout the PCG and other pelvic plexus ganglia, namely accessory ganglia (AG) and hypogastric plexus (HP). Pelvic nerve section showed that afferent fibres in these nerves provided most of the CGRP-IR fibres supplying uterine- or cervical-related neurons in the PCG/AG, none in HP. It is suggested that such sensory-motor network may provide a local pathway for reflex control of genital tract activity, acting through cholinergic nerve projections.

Substance P, calcitonin gene related peptide and PGE2 co-released from the mouse colon: a new model to study nociceptive and inflammatory responses in viscera, in vitro

Visceral inflammation is thought to play an important role in the sensitization of low and high threshold mechanosensory and polymodal afferents and to recruit silent nociceptors. Yet, little is known about the potential role of the mediators involved in nociceptor sensitization to mechanical stimulation as compared to heat sensitization in the skin. In the present study we developed a new isolated preparation of the mouse colon which allowed to apply controlled mechanical distensions. Excised segments of colon from CD mice were immersed in synthetic interstitial fluid (SIF) exposing the serosal surface during 5 min to different types of noxious stimuli; the increase in neuropeptide and PGE(2) release were analyzed (by EIA technique). Capsaicin, heat and pH 5.2 were able to induce significant increases in calcitonin gene related peptide (CGRP) release (14.6-, 5.1-, and 2.3-fold over baseline), however, only capsaicin induced a significant increase in substance P (SP) levels (1.8-fold over baseline). When pH 3.4 was used, a massive liberation of both CGRP and SP was obtained (14- and 15-fold from baseline) which was Ca(2+)-independent and not recovering, suggesting unphysiological release. Mechanical distensions in the noxious range (45, 60 and 90 mmHg) evoked a long-linear graded release of CGRP (1.3-, 1.6- and 2.6-fold over baseline) and of PGE(2) (1.9- 3.8-, 12.3-fold over baseline). Only the 90 mmHg distension evoked a significant increase of SP (1.9-fold over baseline). We conclude that the mouse colon preparation is a suitable model to study inflammatory and nociceptive mechanisms in viscera. Furthermore, a potentially important and yet unexplored role of PGE(2) in noxious visceral distension has been revealed.

Patterns of innervation of sympathetic vascular neurons by peptide-containing primary sensory fibers

The purpose of this study was to determine whether there is a specific organization of the primary sensory innervation on to identified vascular neurons in the inferior mesenteric ganglion (IMG) in guinea-pig. Retrograde tracers were placed intraluminally in inferior mesenteric artery (IMA) or inferior mesenteric vein (IMV) in vitro to identify ganglionic neurons as arterial, venous or unlabeled neurons. The distribution of primary sensory nerve fibers containing calcitonin gene-related peptide (CGRP), neuronal nitric oxide synthase (NOS) and substance P immunoreactivity (SP-IR) was compared before and after treatment with capsaicin. In control animals the density of immunoreactivity varied both with the transmitter and the type of neuron innervated. The density of immunoreactivity for all the three substances was reduced by capsaicin treatment. The degree of reduction of immunoreactivity in the fibers varied with the transmitter and the type of neuron. The density of CGRP and SP immunoreactive fibers was greatest around unlabeled neurons; 78% of the CGRP fibers were of primary sensory origin and all of the SP fibers were primary sensory. Around arterial neurons 44% of the CGRP fibers were of primary sensory origin and around venous 68% were primary sensory. NOS positive innervation around venous neurons was denser than around arterial neurons and all of it was completely (97%) eliminated by capsaicin, indicating that it was solely of primary sensory origin. We conclude that the primary sensory fibers innervating the IMG are differentially distributed to arterial and venous neurons and that the pattern of distribution is characteristic for each sensory neurotransmitter.

Involvement of prostaglandins and CGRP-dependent sensory afferents in peritoneal irritation-induced visceral pain

This study investigates the contribution of prostaglandins (PG) and calcitonin gene-related peptide (CGRP) pathways in visceral pain induced by peritoneal irritation in rats. Peritoneal irritation was produced by i.p. administration of acetic acid (AA: 0.06-1.0%, 10 ml/kg). Visceral pain was scored by counting abdominal contractions. The effect of CGRP (3-100 microg/kg, i.p.) was also evaluated. Like AA, CGRP induced abdominal pain. Neonatal pretreatment with capsaicin reduced abdominal contractions produced by AA (0.6%) and CGRP (20 microg/kg) with 64.6% and 45.6%, respectively. Abdominal contractions induced by AA and CGRP were blocked by two antinociceptive drugs, mu-and kappa-opioid agonists, morphine and (+/-)-U-50,488H, respectively. Indomethacin (3 mg/kg, s.c.) reduced the number of abdominal contractions produced by AA by 78.1%+/-6.4% but did not inhibit abdominal contractions produced by CGRP. The CGRP, receptor antagonist, hCGRP(8-37) (300 microg/kg, i.v.) inhibited AA- and CGRP-induced abdominal contractions with 57.5%+/-12.4% and 51.6%+/-11.3%, respectively. Concomitant i.p. administration of PGE1 and PGE2 (0.3 mg/kg of each) produced abdominal contractions which were inhibited 45.6%+/-9.3% by hCGRP(8-37) (300 microg/kg i.v.). Taken together, these results suggest that peritoneal irritation is likely to trigger the release of prostaglandins, which in turn produces a release of CGRP from primary sensory afferents.