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: suppression
of centrally stimulated gastric motility
Departments of 1 Physiology and 2 Cell Biology and Neuroanatomy, College of Medicine, Ohio State University, Columbus, Ohio 43210
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ABSTRACT |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Gastric stasis is frequently seen in
conjunction with critical infectious illness, chronic inflammatory
disorders, radiation sickness, and carcinogenesis. These conditions are
associated with elevated circulating levels of the cytokine tumor
necrosis factor-
(TNF-). The present studies examined the
relationship between endogenously produced TNF- and the central
neural mechanisms that augment gastric motility. Systemic
lipopolysaccharide (LPS) was employed to induce TNF- production in
thiobutabarbital-anesthetized rats. Sixty minutes after intravenous LPS
injection, gastric motility could not be stimulated by a potent
centrally acting gastrokinetic stimulant, thyrotropin-releasing hormone
(TRH). This failure to elicit gastric motility via central mechanisms
coincided with high circulating levels of TNF-. However, intravenous
injections of bethanecol, a peripherally acting cholinergic agonist
with direct gastrokinetic effects, were still able to elicit normal increases in gastric motility in the presence of TNF- and LPS. Therefore, the inability to stimulate gastric motility via central TRH
could not be attributed to the direct inhibitory effects of either LPS
or TNF- on the stomach. If the production of endogenous TNF- was
suppressed via the use of urethan as the anesthetic agent, then
intravenous injections of LPS were no longer effective in suppressing
gastric motility. Thus these effects on gastric motility are not
directly attributable to LPS nor are they due to direct effects on the
gastric smooth muscle. Our previous study demonstrated that
microinjection of femtomole quantities of TNF- in the brain stem
dorsal vagal complex (DVC) can modulate gastric motility. This central
TNF- effect on gastric motility was dose dependent and required an
intact vagal efferent pathway. The results from these two studies
suggest that systemically produced TNF- may gain access to the DVC
to modulate gastric function.
brain stem; gastric stasis; anesthesia; urethan; Inactin
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INTRODUCTION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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THE PRODUCTION OF cytokines by the immune system in response to infection or injury plays a critical role in coordinating the host's defense against pathogenic challenges or wounding. The acute physiological reactions by the host include local and systemic immunological, neuroendocrine, metabolic, and behavioral reactions that attempt to eliminate the challenge and restore homeostasis. These reactions are referred to collectively as the acute phase response (8) and include fever, increased sleep, decreased plasma iron levels, changes in lipid metabolism, and loss of appetite, gastrointestinal stasis, nausea, and vomiting (26).
Bacterial lipopolysaccharide (LPS; also referred to as endotoxin) is able to induce these acute phase responses and therefore has been used as a tool to mimic sickness without the use of a replicating pathogen. Loss of appetite and reductions in food intake are acute phase responses that are readily triggered by LPS (26). Conditioned taste aversion can be learned in response to exposure to LPS, thus the animal is able to associate the perception of sickness after the administration of endotoxin with a novel tasting substance (44).
LPS induces a delay in gastric emptying (i.e., gastric stasis) as early as 30 min and up to 8 h after endotoxin administration (6). Gastric stasis is one of the prodromal characteristics preceding vomiting and is perceived as nausea (23). Thus it would appear that the stasis induced by endotoxin may account for the hypophagia and nausea seen during sickness.
Although it is possible that LPS may have a direct effect on either the
stomach or the brain to affect gastric motility, this hypomotility
effect typically requires at least 30 min after endotoxin exposure to
occur. Therefore, it is difficult to argue that LPS is having a direct
effect. It is more plausible to suggest that these hypomotility effects
are attributable to the elaboration of one of the early cytokines that
are produced and released in response to endotoxin. Candidate
proinflammatory cytokines include tumor necrosis factor (TNF)- and
interleukins (IL)-1 and -6. These proinflammatory agents participate in
the cascading cytokine network critical for the development of the
immune response and elicit a variety of other physiological changes
associated with illness and development of immunity (8). The kinetics
of induced release of these cytokines from peripheral macrophages in
response to antigenic challenge is in the range of 30 min to 24 h (46). Microglia of the central nervous system (CNS) also respond to LPS
challenge by induction of proinflammatory cytokine production, although
the kinetics of induction are somewhat slower (24).
Feeding behavior studies have shown that both systemic and centrally
administered LPS induce anorexia within 30 min to 8 h after exposure
(6, 31). Similar studies have shown that central administration of
several cytokines, including TNF-, can also produce anorexia (20).
By definition, feeding studies examine a behavior that may be modified
for a variety of reasons and may require a time frame of minutes to
hours to be evaluated. However, one possible explanation for the
suppression of feeding observed in these studies is impairment of
normal gastric function (e.g., gastric stasis) and the attendant
perception of nausea.
Multiple clinical trials investigating the possible therapeutic value
of recombinant human TNF- in the treatment of advanced cancers
repeatedly cite nausea, vomiting, and malaise as symptomatic toxicities
(e.g., see Ref. 18). In the rodent, intravenous recombinant human TNF
has been shown to inhibit gastric emptying (30).
The dorsal vagal complex (DVC), located in the medulla of the brain stem, constitutes the basic neural circuitry of vago-vagal reflex control of gastrointestinal function (e.g., motility, tone, and acid secretion; see Ref. 34). This complex (DVC) is composed of a mix of sensory (nucleus of the solitary tract) and motor (dorsal motor nucleus of the vagus) components. Similar to the overlying area postrema, subregions of the nucleus of the solitary tract have fenestrated capillaries and enlarged perivascular spaces (12). Therefore, this region possesses the vascular characteristics of a circumventricular organ and is able to monitor the presence or concentrations of blood-borne chemicals, peptides, and toxins. Broadwell and Sofroniew (3) have demonstrated that large serum proteins readily bypass the blood-brain barriers in this area within minutes of intravenous injection. Additionally, dendrites from neurons of the DVC have been seen to penetrate the ependymal layer and enter the floor of the fourth ventricle (33, 40). These anatomical characteristics place the DVC in a position to potentially monitor either the blood or ventricular fluids. Thus it may be that circulating TNF may access the CNS at this locus and interact with neurons of the DVC to provoke gastric hypomotility.
Our previous experiments (17) demonstrated that microinjection of
femtomole quantities of TNF- directly in the DVC of the anesthetized
rat immediately (i.e., within 30 s) suppressed thyrotropin-releasing hormone (TRH)-stimulated gastric motility for prolonged periods of
time. This suppression demonstrated a dose-dependent effect of TNF-
and required an intact efferent vagal pathway.
Urethan (ethyl carbamate) anesthesia is commonly used for nonrecovery
laboratory surgery due to its long-lasting effects. It has recently
been observed that urethan anesthesia protects rats against lethal
doses of endotoxin by reducing TNF- release (22). Kotanidou et
al.'s (22) study demonstrated that TNF- mRNA expression was
suppressed by serum from urethan-treated rats. Urethan anesthesia
reduced the peak increase in circulating TNF- (at 90 min after LPS
administration) by 88% compared with levels of TNF production after
LPS exposure during thiobutabarbital anesthesia. Urethan reduction of
TNF- release is attributed to its ability to block the
2-adrenoreceptors
(10) on macrophages (41); these immune effector cells are the primary
sources of TNF- after acute LPS exposure.
Therefore, the purpose of the present experiments was to determine
whether endogenously produced TNF- was associated with the
suppression of centrally mediated increases in gastric motility. Based
on the results of our previous studies of centrally administered TNF- effects on gastric motility, our hypothesis was that TNF-, and not LPS, is responsible for these gastric responses and that the
effects were not attributable to a peripheral mechanism (i.e., direct
effects on the stomach). In these experiments, gastric motility was
continuously monitored in anesthetized rats exposed to either PBS or
LPS (intravenously). One-half of the rats was anesthetized with urethan
to specifically suppress the production of endogenous TNF- in
response to LPS, whereas the other one-half was anesthetized with
Inactin, which does not interfere with TNF- production (22). Blood
samples were taken to determine plasma TNF- levels, and gastric
motility was stimulated centrally and peripherally. These studies
demonstrated that gastric motility could not be stimulated centrally in
rats with elevated TNF- levels despite the observation that the
stomach could be readily stimulated by peripheral cholinergic agonists.
These results are consistent with our overall hypothesis that
endogenous TNF- production may provoke gastric stasis by acting
directly on neurons in the brain stem DVC that control gastric functions.
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MATERIALS AND METHODS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Chemicals
Rats were anesthetized with either thiobutabarbital (Inactin; Research Biochemicals International, Natick, MA) dissolved in saline (100 mg/ml; 100 mg/kg ip) or urethan (ethyl carbamate; Sigma, St. Louis, MO) dissolved in water (33% wt/vol; 1.5 g/kg ip). Endogenous production of TNF- was induced by intravenous injections of 1 mg/kg body wt LPS
(43) derived from Escherichia coli
serotype 0111:B4 (Sigma) dissolved in saline at 2.5 mg/ml concentration. One hundred micromolar concentration of TRH (Bachem, Torrance, CA) was dissolved in PBS (124 mM NaCl, 26 mM
NaHCO3, 2 mM
KH2PO4;
304 mosmol; pH 7.4) and placed directly on the area postrema to
maximally stimulate gastric activity via a vagally dependent
cholinergic mechanism (16). Bethanecol chloride
(carbamyl-
-methylcholine chloride; Sigma), a cholinergic agonist
with direct, muscarinic, gastrokinetic effects, was dissolved in saline
at a concentration of 200 µg/ml (100 µg/kg iv; see Ref. 5).
Surgical Preparations
Male Long-Evans rats (300-500 g body wt, n = 31) were food deprived 12 h before either thiobutabarbital (Inactin, 100 mg/kg ip) or urethan anesthesia (ethyl carbamate, 1.5 g/kg ip). All other preparations were the same in all groups of animals. Loss of the withdrawal reflex in response to pinching the footpad marked the necessary plane of anesthesia before any surgeries were initiated. The trachea was cannulated to maintain an open airway. An abdominal laparotomy was performed, and a miniature strain gauge (RB Products, Madison, WI) was sewn on the ventral surface of the stomach in parallel with the circular smooth muscle to measure corpus gastric motility and tone, as previously described (17). Blood sample collections were obtained from an arterial catheter installed in the middle caudal (i.e., tail) artery. All intravenous injections were made through a jugular cannula. After initial surgical preparations were complete, the animal was mounted in a stereotaxic frame in a 10° nose-down orientation. The dorsal spinomedullary junction was exposed by resecting the dorsal cervical musculature, removing the occipital skull plate, and resecting both the dura mater and arachnoid meninges. Anesthetic and surgical procedures were according to National Institutes of Health guidelines and protocols approved by the Ohio State University Institutional Laboratory Animal Care and Use Committee.Experimental Design
A time line of the experimental design (Fig. 1) is provided to facilitate visualizing the sequence and timing of the experimental manipulations and data gathered.
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The first blood sample ("pre"; 0.4 ml) was obtained immediately
after the tail arterial cannula was in place. Withdrawal of all blood
samples from the animal was followed by an equal volume replacement
with PBS. Blood samples were transferred to heparinized microcentrifuge
tubes and briefly stored in the refrigerator. Plasma samples for
quantitating TNF production were stored in a 
70°C freezer.
Gastric motility was continuously monitored once the animal was secured
in the stereotaxic frame. Upon completion of the brain stem exposure,
the second blood sample ("T = 0 h") was obtained. Immediately after the second blood sample, the
animal received an intravenous injection of either 1 mg/kg body wt LPS
(43) or an equal volume of PBS via the jugular cannula. Sixty minutes after the intravenous administration of either LPS or PBS, the final
blood sample (T = 1 h) was
obtained. Gastric motility was centrally stimulated by 2 µl of
10
4 M TRH (i.e., 0.2 nmol
TRH) placed directly on the surface of the fourth ventricle. To test
the responsiveness of peripheral cholinergic mechanisms that activate
gastric motility, the cholinergic agonist bethanecol (100 µg/kg iv)
was administered at least 1 h after central stimulation with TRH. At
this point in time, any residual TRH effects on gastric motility have
been resolved.
Although small amounts of TNF- can be detected in the plasma within
30 min of systemic LPS exposure, maximum plasma concentrations occur
within 1-2 h, and significant TNF- levels are maintained at
least 6 h after LPS treatment (43). Therefore, the timing of the
central stimulation of gastric motility with TRH (i.e., 1 h post-LPS)
and peripheral stimulation with bethanecol (i.e., 2 h post-LPS)
were designed to coincide with the peak period of circulating TNF- levels.
Data Collection and Analysis
Gastric motility. Gastric motility was continuously monitored via the miniature strain gauge connected to a Wheatstone bridge-based amplifier (17). Each strain gauge was calibrated with specific weights; thus, calibration curves could convert voltage data from the amplifier into approximate grams of force. Output from this amplifier was directed to a Grass polygraph and to the analog-to-digital convertor inputs of a waveform storage/analysis system (RC Electronics, Santa Barbara, CA). Gastric motility data could be displayed in real time and digitized for subsequent analysis or graphic display.Motility records were analyzed on the video monitor in 300-s (i.e., 5-min) epochs; data were converted to motility indexes (MIs) for statistical and graphic purposes. MIs were calculated as the area under the curve (i.e., volt-seconds) during the 300-s time interval. The baseline was adjusted such that the area under the curve was the most conservative calculation of changes in motility, exclusive of changes in gastric tone (see Fig. 2). Although motility was monitored continuously throughout the duration of the experiment, MI were calculated and subjected to statistical analysis for specific periods in the experiment (refer to time line of experimental design, Fig. 1). These periods included 1) basal motility levels (defined as those after all surgical preparations were completed and before the intravenous LPS or PBS injections were made), 2) pre-TRH motility levels (defined as the motility activity level during the 5 min preceding the TRH stimulation), 3) post-TRH motility levels (defined as the activity level during the 5 min after TRH stimulation), 4) prebethanecol (defined as the activity level 5 min preceding bethanecol injection), and 5) postbethanecol (defined as the motility activity during the 5 min after bethanecol stimulation).
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TNF- assay. Plasma TNF- was
determined by an enzyme-linked immunosorbent assay (ELISA) for rat
TNF- (Genzyme Diagnostics, Cambridge, MA). Fifty-microliter plasma
samples, in duplicate, were incubated (at 37°C) in microwells
coated with monoclonal anti-rat TNF- antibody. After a 2-h
incubation, the wells were aspirated and washed with buffer; next, 100 µl of horseradish peroxidase-conjugated anti-TNF- were added.
After an additional 1-h incubation period, the wells were aspirated
again and washed with buffer. One hundred microliters of a
tetramethylbenzidine peroxidase substrate solution were added to all
wells. After a 10-min incubation at room temperature, the reaction was
quenched by the addition of a stop solution. The optical absorbance of each well was read in a spectrophotometer (Bio-Rad model 2550 EIA
reader; Bio-Rad, Hercules, CA). Absorbance values were converted to
TNF- concentrations by comparison with a simultaneously generated standard curve. The limits of detection per well of this assay kit were
10-2,240 pg/ml; the inter- and intra-assay variabilites were 7.4 and 5.5%, respectively (manufacturer's data).
Statistical Analysis
MIs from each of the four treatment groups (e.g., urethan/PBS, urethan/LPS, Inactin/PBS, or Inactin/LPS) were analyzed across the five time points described in Data Collection and Analysis. Basal motility levels (i.e., before any intravenous treatments) under both anesthesias were compared using unpaired Student's t-test.Basal MIs before (i.e., "basal" as defined in Data Collection and Analysis) any intravenous treatments and 60 min after (i.e., "pre-TRH") were compared across the four treatment groups using repeated measures analysis of variance (ANOVA). An overall P < 0.05 was considered significant and suggested posttests.
Each animal's stimulated MI (i.e., after either TRH or bethanecol stimulation) was compared with its own basal gastric activity level immediately before stimulation. The use of each animal as its own control minimized the variability of gastric motility levels between animals in each group. Thus changes in gastric activity levels were normalized and reported as ratios (e.g., post-TRH/basal MIs). These normalized indexes of change for each animal were compared across treatment groups. ANOVA tests were performed on the motility ratio changes 1) before and after TRH stimulation and 2) before and after bethanecol stimulation. An overall P < 0.05 was considered significant and warranted Bonferroni posttests on selected pairs of treatment groups.
Plasma TNF- levels in each of the four treatment groups were
determined across the three time points described in Experimental Design (see Fig. 2). TNF- levels were analyzed by a one-way
ANOVA. Significance was defined as an overall
P < 0.05; Bonferroni posttests were
applied to selected pairs of treatment groups.
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RESULTS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Basal Gastric Motility
Basal gastric motility tends to be minimal in anesthetized animals that have been fasted overnight. Although basal gastric motility was slightly higher in Inactin versus urethan-anesthetized animals, this difference was not statistically significant (Fig. 3; P > 0.05). Additionally, comparison of the basal versus pre-TRH MIs across the four treatment groups indicated that intravenous treatment with either PBS or LPS did not alter unstimulated gastric motility 60 min later under either anesthesia (ANOVA; P > 0.05; data not shown). Thus neither LPS nor presumptive increased TNF- levels affected basal
motility levels.
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Central Stimulation of Gastric Motility
Direct application of 2 µl of 104 M TRH (i.e., 0.2 nmol
TRH) on the area postrema maximally stimulates gastric activity via a
vagally dependent cholinergic mechanism (17). Both groups that had
received intravenous PBS (i.e., urethan/PBS and Inactin/PBS) 60 min
before TRH stimulation demonstrated the classic increase in gastric
motility. TRH produced a fivefold increase in the gastric motility
ratio (i.e., ratio of motility post-TRH/pre-TRH) under either Inactin
or urethan anesthesia (Figs.
4-6).
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The urethan/LPS group also showed a similar increase in gastric motility after central stimulation with TRH (Figs. 4 and 6). In contrast, when the Inactin/LPS group was stimulated with TRH, gastric motility could not be activated centrally (mean motility ratio post-TRH/pre-TRH = 1.1 ± 0.1, mean ± SE; Figs. 5 and 6). This inability to activate motility is significantly different from the responses of the other groups after TRH stimulation [ANOVA, F(3,28) = 35.6, P < 0.0001; Bonferroni posttest; P < 0.001].
Peripheral Stimulation of Gastric Motility
Peripheral stimulation of the stomach by systemic intravenous injection of the cholinergic agonist bethanecol occurred ~2 h after either PBS or LPS administration. [Note, the timing of either central (i.e., TRH) or peripheral (i.e., bethanecol) stimulation of gastric motility was specifically designed to bracket peak circulating levels of endogenous TNF- production after LPS exposure (43).]
Comparisons of motility ratios (i.e., postbethanecol/prebethanecol) across the four experimental groups demonstrated comparable levels of
peripherally stimulated gastric activity [~3-fold increase in
motility; ANOVA F(3,17) = 0.89;
P = 0.47; Figs. 4, 5, and
7]. Thus the stomach was still
intrinsically capable of increased motility in all treatment groups
despite the observation that central stimulation of the Inactin/LPS
group could not increase motility.
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Plasma TNF- Levels
levels in the four experimental groups
across the three time points were subjected to one-way ANOVA
[F(11,75) = 25.39;
P < 0.0001]. Pre and
T = 0 levels were not significantly
different in all four experimental groups; therefore, only pre data are
shown in Fig. 8. Although the urethan/LPS
group demonstrated an elevation in circulating TNF- levels
(1,250 ± 206 pg/ml, mean ± SE) relative to either the
pre or T = 0 time periods,
this difference was not statistically significant (Bonferroni
P > 0.05; Fig. 8).
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In contrast, the elevation in circulating endogenous TNF- levels in
the Inactin/LPS group 1 h after LPS administration (5,695 ± 1,046 pg/ml, mean ± SE) was significantly greater than either the pre or
T = 0 time points of the Inactin/LPS
group. This level of plasma TNF- was also significantly greater than
that seen at the T = 1 h time point of
the urethan/LPS group (1,250 ± 206, mean ± SE;
Bonferroni posttests, P < 0.001;
Fig. 8).
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DISCUSSION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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These studies demonstrated that the endogenous production of TNF-
was essential for the suppression of centrally mediated increases in
gastric motility. Systemic LPS only suppressed centrally commanded
increases in motility when endogenous TNF- production was not
impaired (i.e., Inactin anesthesia group). The TNF-
production-related suppression of gastric responsiveness to central
stimulation was not due to any intrinsic inability of the gastric
smooth muscle to augment motility, since intravenous bethanecol
produced equivalent gastrokinetic effects under all experimental conditions.
Direct Action of LPS or Cytokines on Gastric Functions
It is possible that LPS (or the cytokines induced by LPS administration) acts peripherally to yield the observed gastric hypomotility in response to central stimulation. For example, Hellstrom and colleagues (15) have suggested that LPS acts peripherally to change the rate of migrating myoelectric complexes (MMC) of the small intestine of the rat. Intravenous administration of endotoxin to conscious, awake rats was shown to disrupt the MMC of the small intestine within 30 min of administration and, ultimately, result in diarrhea within 1-2 h. Note, however, that pretreatment with dexamethasone prevented the rapid intestinal transit induced by LPS. Given that dexamethasone inhibits the production of cytokines (14), which normally can be detected ~30 min after LPS exposure, this inhibition of changes in transit suggests that cytokine production is implicated in mediating this effect. Second, pretreatment with N
-nitro-L-arginine
methyl ester, an inhibitor of nitric oxide synthase, also suppressed
the increased transit induced by LPS exposure, suggesting that nitric
oxide is involved in the transduction of these effects (15).
Another proinflammatory cytokine, IL-1, also appears to modulate gastric functions directly. Intraperitoneal injection of IL-1 has been reported to stimulate prostaglandin synthesis by the stomach, which was correlated with an inhibition of gastric secretion and retardation of gastric emptying in rats (32).
In vitro, both IL-1 and TNF- can inhibit motility of the gastric
fundus (28). These results suggest that neither cyclooxygenase nor
nitric oxide synthase activities are involved in this inhibition of
motility but rather production of leukotriene
B4 is responsible for the
gastric-strip relaxation seen in these studies.
The possibility exists that the vagal postganglionic, i.e., enteric, neurons may be one of the sites of action of LPS or the induced cytokines. However, capillaries do not enter the enteric ganglia, and a blood-ganglion barrier, similar to the blood-brain barrier, has been demonstrated in the myenteric plexus (11). The capillaries that supply the myenteric plexus layer differ from those of other layers of the gut and have impermeable junctions that prevent passage of tracer (and macromolecules) between endothelial cells and the enteric nervous system. Recent work by Schmitz et al. (36) demonstrated that the contribution of TNF to the pathogenesis of diarrhea in inflammatory bowel disease was not mediated by the enteric nervous system. Rather, the secretory changes in human colon are mediated by the TNF-induced prostaglandin E2 production by subepithelial cells.
Although multiple pathways exist by which gastric function may be influenced by either endotoxin or the cytokines it induces, it is important to note that, in our studies, basal gastric motility was not influenced by either intravenous PBS or LPS regardless of the anesthetic used. Second, the responsiveness to peripheral cholinergic stimulation with bethanecol produced equivalent gastrokinetic effects under all experimental conditions. Thus our primary observations were not due to changes in the intrinsic ability of the stomach to contract.
TNF- (or LPS) and the Vagus
. These authors conclude that the
effects of these agents on feeding behavior rely on humoral
and/or splanchnic visceral afferent pathways.
Possible CNS Sites Activated by Endotoxin or TNF-
showed complete inhibition to centrally mediated gastric stimulation.
TNF- and the DVC
within the CNS is not complete, the highest concentration of specific
and saturable TNF- binding sites is located within the brain stem
(21). In addition to demonstrations that the DVC brain stem region is
functionally devoid of a blood-brain barrier (3, 12), a specific,
saturable transport system for TNF- to enter the CNS has been
described (13). The possibility exists that TNF- may exert direct
action on the brain stem circuitry DVC that influences descending
control of gastric function. Our own studies have shown that
subpicomole doses of TNF- injected directly in the DVC produce an
immediate reduction in gastric motility; this effect is vagally
mediated (17).
A similar relationship between a peripherally produced, large peptide hormone, peptide YY (PYY), and its ability to suppress gastric motility via direct action on the DVC has already been demonstrated. For example, the "ileal brake" effect is a long-lasting suppression of gastric motility response that is triggered by the appearance of fatty acids in the ileum. PYY is a 36-amino acid peptide that is released in the bloodstream from endocrine secreting cells of the ileum after the appearance of fatty acids in the distal small bowel. PYY circulates, enters the dorsal medulla through fenestrated capillaries, and binds with Y2 receptors in the DVC (i.e., the nucleus of the solitary tract and the dorsal motor nucleus of the vagus). PYY acts directly on vago-vagal reflex circuitry to cause a withdrawal of cholinergic excitation of the stomach, suppressing transit and the further delivery of ingested fats to the intestine. These centrally mediated gastroinhibitory effects were lost with vagotomy (4).
Thus there are several similarities between the proposed routes of
action and the neural circuitry involved in modulating gastric motility
of the peptides PYY and TNF-. However, both in vivo and in vitro
neurophysiological studies will be required to verify the specific site
of endogenous TNF- action responsible for these changes in gastric function.
TNFergic Pathways in the CNS
TNF--ir has been observed in cell bodies, fibers, and terminals
within the CNS, which suggests that this central cytokine may function
as an intrinsic neuromodulator. Breder et al. (1) have shown that
TNF--ir cell bodies are located within the dorsomedial nucleus of
the hypothalamus, the lateral hypothalamic area, caudal raphe nuclei,
ventral medullary surface, and possibly in the nodose ganglion.
TNF--ir innervation was observed in the dorsal motor vagal nucleus,
the compact portion of the nucleus ambiguus, as well as the medial and
dorsomedial nucleus of the solitary tract and area postrema. These
TNF- neural pathways may offer another pathway for modulating (via
descending inputs) gastrointestinal function similar to that seen with
oxytocin (25).
Perspectives
Teleological significance? One of the primary functions of the immune system is surveillance of foreign and/or harmful agents. The detection of foreign antigens by elements of the immune system is translated into the production of cytokines (e.g., TNF-, IL-1, or IL-6). These chemical messengers may
have localized (e.g., changes in regional blood flow and localized
edema) and more global effects (e.g., production of fever, fatigue,
loss of appetite, or nausea).
The results of the present studies taken together with our previous
work (17) suggest one pathway by which TNF- may modulate gastric
function. The effective gastric stasis associated with the presence of
TNF- would offer the host protection against further ingestion of
the pathogen or toxin (due to perception of nausea and/or
vomiting) as well as retard intestinal absorption by reducing gastric
emptying. Additionally, other studies (15) indicate that the intestinal
response to endotoxin exposure is an acceleration of intestinal
transport (i.e., production of diarrhea). These two mechanisms together
would effectively purge the offending agent from the alimentary tract
of the host.
Pregnancy and thalidomide: early evidence of
correlation between TNF-, fetal development, and
nausea? Many women experience nausea and vomiting (also
referred to as "morning sickness") during pregnancy, especially
during the first trimester. During the late 1950s, a new sedative was
found to also give relief to the nausea associated with pregnancy. Thus
began the tragedy of infant deformities associated with the use of
thalidomide (27). If the mother was given thalidomide during the first
trimester of pregnancy, often, the child was born with phocomelia (or
limb deficiencies). Other symptoms included hemangiomas, severe
malformation of the digestive tract, and absence of the external ear.
The common denominator between the relief of morning sickness and the
teratogenic effects of thalidomide may have been the suppression of
TNF- production by thalidomide.
It has recently been recognized that circulating levels of TNF- are
elevated in the mother during pregnancy. Apparently TNF- is produced
by both fetal and maternal tissue during gestation (19) and may play a
critical role during blastocyst implantation and fetal development
(45). TNF- is a pleiotrophic molecule with angiogenic properties
(8); thus, TNF- may be critical for maintenance of pregnancy and
fetal development. In particular, proper development of the limb bud
requires both vasculogenesis (i.e., formation of a capillary bed from
endothelial cells) and angiogenesis (i.e., the formation of new blood
vessels from sprouts of existing vessels; see Ref. 39), which may be
modified by the presence or absence of TNF-.
Recent work by Moreira et al. (29) has demonstrated that thalidomide
specifically enhances the degradation of the mRNA for TNF-
production. Thus inhibition of this cytokine by thalidomide is rather
selective. D'Amato and colleagues (7) have demonstrated that
thalidomide has anti-angiogenic properties that are attributed to its
ability to prevent TNF- production. These authors suggest that the
teratogenic effects seen with thalidomide were secondary to inhibition
of blood vessel growth of the developing limb bud.
Our studies have demonstrated an association between TNF- and
gastric stasis. The condition of gastric stasis is typically perceived
as a general feeling of malaise and nausea (23). Multiple clinical
trials repeatedly cite nausea, vomiting, and malaise as symptoms
associated with recombinant human TNF- treatment (e.g., see Ref.
18).
Bringing together the results from the current study and these
divergent databases, it appears that symptoms of nausea and morning
sickness associated with pregnancy may be attributable to elevated
circulating levels of TNF-. The use of thalidomide, which alleviated
these gastric symptoms, may have done so by suppressing TNF-
production. Tragically, the drug also interfered with normal limb bud
development by inhibiting the production of the TNF-, which was
necessary to maintain appropriate angiogenesis.
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ACKNOWLEDGEMENTS |
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This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-52142 to G. E. Hermann and R. C. Rogers.
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: G. E. Hermann, Dept. of Physiology, College of Medicine, Ohio State University, 302 Hamilton Hall, 1645 Neil Ave., Columbus, Ohio 43210.
Received 21 April 1998; accepted in final form 3 September 1998.
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REFERENCES |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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