Stress increases descending inhibition in mouse and human colon

BACKGROUND

A relationship between stress and the symptoms of irritable bowel syndrome (IBS) has been well established but the cellular mechanisms are poorly understood. Therefore, we investigated effects of stress and stress hormones on colonic descending inhibition and transit in mouse models and human tissues.

METHODS

Stress was applied using water avoidance stress (WAS) in the animal model or mimicked using stress hormones, adrenaline (5 nM), and corticosterone (1 μM). Intracellular recordings were obtained from colonic circular smooth muscle cells in isolated smooth muscle/myenteric plexus preparations and the inhibitory junction potential (IJP) was elicited by nerve stimulation or balloon distension oral to the site of recording.

KEY RESULTS

Water avoidance stress increased the number of fecal pellets compared to control (p < 0.05). WAS also caused a significant increase in IJP amplitude following balloon distension. Stress hormones also increased the IJP amplitude following nerve stimulation and balloon distension (p < 0.05) in control mice but had no effect in colons from stressed mice. No differences were observed with application of ATP between stress and control tissues, suggesting the actions of stress hormones were presynaptic. Stress hormones had a large effect in the nerve stimulated IJP in human colon (increased >50%). Immunohistochemical studies identified alpha and beta adrenergic receptor immunoreactivity on myenteric neurons in human colon.

CONCLUSIONS & INFERENCES

These studies suggest that WAS and stress hormones can signal via myenteric neurons to increase inhibitory neuromuscular transmission. This could lead to greater descending relaxation, decreased transit time, and subsequent diarrhea.

Differential functional role of purinergic and nitrergic inhibitory cotransmitters in human colonic relaxation

AIM

ATP and nitric oxide (NO) are released from enteric inhibitory motor neurones and are responsible for colonic smooth muscle relaxation. However, how frequency of neural stimulation affects this cotransmission process and the post-junctional responses has not been systematically characterized in the human colon.

METHODS

The dynamics of inhibitory cotransmission were studied using different protocols of electrical field stimulation (EFS) to characterize the inhibitory junction potentials (IJP) and the corresponding relaxation in colonic strips obtained from 36 patients.

RESULTS

Single pulses elicited a fast IJP (IJPf(MAX) = -27.6 ± 1.6 mV), sensitive to the P2Y1 antagonist MRS2500 1 μm, that ran down with frequency increase leaving a residual hyperpolarization at high frequencies (IJPf∞ = -3.7 ± 0.6 mV). Accordingly, low frequencies of EFS caused purinergic transient relaxations that cannot be maintained at high frequencies. Addition of the P2Y1 agonist MRS2365 10 μm during the purinergic rundown did not cause any hyperpolarization. Protein kinase C (PKC), a putative P2Y1 desensitizator, was able to reduce the amplitude of the IJPf when activated, but the rundown was not modified by PKC inhibitors. Frequencies higher than 0.60 ± 0.15 Hz were needed to evoke a sustained nitrergic hyperpolarization that progressively increased reaching IJPs∞ = -13 ± 0.4 mV at high frequencies and leading to a sustained inhibition of spontaneous motility.

CONCLUSION

Changes in frequency of stimulation possibly mimicking neuronal firing will post-junctionally determine purinergic vs. nitrergic responses underlying different functional roles. NO will be responsible for sustained relaxations needed in physiological processes such as storage, while purinergic neurotransmission evoking sharp transient relaxations will be dominant in processes such as propulsion.

Interstitial cells of Cajal mediate nitrergic inhibitory neurotransmission in the murine gastrointestinal tract

Nitric oxide (NO) is a major inhibitory neurotransmitter in the gastrointestinal (GI) tract. Its main effector, NO-sensitive guanylyl cyclase (NO-GC), is expressed in several GI cell types, including smooth muscle cells (SMC), interstitial cells of Cajal (ICC), and fibroblast-like cells. Up to date, the interplay between neurons and these cells to initiate a nitrergic inhibitory junction potential (IJP) is unclear. Here, we investigate the origin of the nitrergic IJP in murine fundus and colon. IJPs were determined in fundus and colon SMC of mice lacking NO-GC globally (GCKO) and specifically in SMC (SM-GCKO), ICC (ICC-GCKO), and both SMC/ICC (SM/ICC-GCKO). Nitrergic IJP was abolished in ICC-GCKO fundus and reduced in SM-GCKO fundus. In the colon, the amplitude of nitrergic IJP was reduced in ICC-GCKO, whereas nitrergic IJP in SM-GCKO was reduced in duration. These results were corroborated by loss of the nitrergic IJP in global GCKO. In conclusion, our results prove the obligatory role of NO-GC in ICC for the initiation of an IJP. NO-GC in SMC appears to enhance the nitrergic IJP, resulting in a stronger and prolonged hyperpolarization in fundus and colon SMC, respectively. Thus NO-GC in both cell types is mandatory to induce a full nitrergic IJP. Our data from the colon clearly reveal the nitrergic IJP to be biphasic, resulting from individual inputs of ICC and SMC.

Pharmacological properties of non-adrenergic, non-cholinergic inhibitory transmission in chicken gizzard

1. Non-adrenergic, non-cholinergic (NANC) inhibitory transmission in chicken gizzard was studied by use of intracellular microelectrode techniques. Changes in membrane potential in response to NANC nerve stimulation were recorded in the gizzard smooth muscle pretreated with atropine (1 microM) and guanethidine (1 microM). 2. Field stimulation of the intramural nerves (FS) evoked inhibitory junction potentials (i.j.ps) which were abolished by tetrodotoxin (1 microM), but not inhibited at all by K+ channel blockers including apamin (0.5 microM), tetraethylammonium (TEA, 10 mM), charybdotoxin (0.2 microM) and glibenclamide (10 microM). 3. NG-nitro-L-arginine (3 mM), an inhibitor of nitric oxide (NO) synthase, inhibited i.j.ps. The effect was reversed by L-arginine (3 mM), but not by D-arginine (3 mM). 4. 8-Bromo cyclic GMP (100 microM), a membrane permeable analogue of cyclic GMP, produced a membrane hyperpolarization which was blocked by TEA (10 mM) or glibenclamide (10 microM). 5. NO at concentrations of up to 400 microM affected neither i.j.ps nor resting membrane potential. On the other hand, NO (80 microM) caused the membrane to hyperpolarize in the smooth muscle of guinea-pig ileum. 6. These results suggest that in the chicken gizzard, NANC i.j.ps may not arise from opening of conventional types of K+ channel and that NO seems unlikely to be involved in the generation of i.j.ps. A possible mechanism by which the inhibitory effect of NG-nitro-L-arginine on i.j.ps was brought about will be discussed.

Regulation of citrulline recycling in nitric oxide-dependent neurotransmission in the murine proximal colon

1. We investigated the contribution of nitric oxide (NO) to inhibitory neuromuscular transmission in murine proximal colon and the possibility that citrulline is recycled to arginine to maintain the supply of substrate for NO synthesis. 2. Intracellular microelectrode recordings were made from circular smooth muscle cells in the presence of nifedipine and atropine (both 1 microM). Electrical field stimulation (EFS, 0.3-20 Hz) produced inhibitory junction potentials (i.j.ps) composed of an initial transient hyperpolarization (fast component) followed by a slow recovery to resting potential (slow component). 3. L-Nitro-arginine-methyl ester (L-NAME, 100 microM) selectively abolished the slow component of i.j.ps. The effects of L-NAME were reversed by L-arginine (0.2-2 mM) but not by D-arginine (2 mM). Sodium nitroprusside (an NO donor, 1 microM) reversibly hyperpolarized muscle cells. This suggests that NO mediates the slow component of i.j.ps. 4. L-Citrulline (0.2 mM) also reversed the effects of L-NAME, and this action was maintained during sustained exposures to L-citrulline (0.2 mM). This may reflect intraneuronal recycling of L-citrulline to L-arginine. 5. Higher concentrations of L-citrulline (e.g. 2 mM) had time-dependent effects. Brief exposure (15 min) reversed the effects of L-NAME, but during longer exposures (30 min) the effects of L-NAME gradually returned. In the continued presence of L-citrulline, L-arginine (2 mM) readily restored nitrergic transmission, suggesting that during long exposures to high concentrations of L-citrulline, the ability to generate arginine from citrulline was reduced. 6. Aspartate (2 mM) had no effect on i.j.ps, the effects of L-NAME, or the actions of L-citrulline in the presence of L-NAME, L-Citrulline (0.2-2 mM) alone had no effect on i.j.ps under control conditions. 7. S-methyl-L-thiocitrulline (10 microM), a novel NOS inhibitor, blocked the slow component of i.j.ps. The effects of this inhibitor were reversed by L-arginine (2 mM), but not by L-citrulline (2 mM). 8. These results suggest that i.j.ps in the murine colon result from release of multiple inhibitory neurotransmitters. NO mediates a slow component of enteric inhibitory neurotransmission. Recycling of L-citrulline to L-arginine may sustain substrate concentrations in support of NO synthesis and this pathway may be inhibited when concentrations of L-citrulline are elevated.