Sympathetic control of substance P releasing enteric neurones in the guinea pig ileum

Effects of intestinal ischemia on in vitro activity of adjacent jejunum in samples obtained from ponies

OBJECTIVE

To determine whether intestinal ischemia would alter activity of the jejunum in vitro or alter staining characteristics for certain types of enteric neurotransmitters.

SAMPLE POPULATION

Jejunal samples obtained from 10 ponies.

PROCEDURE

Jejunal samples were obtained from locations proximal and distal to an area of small intestine made ischemic for 60 minutes. A portion of each sample was stained to detect substance P-like immunoreactivity, cholinergic and adrenergic neurons, and nitric oxide synthase. Portions of the remaining samples were suspended in muscle baths. General activity patterns (frequency and amplitude of contraction), responses to neuronal depolarization induced by electrical field stimulation (EFS), and responses to 1 microM norepinephrine (NE) were compared with responses of a normal section of small intestine obtained prior to ischemic insult.

RESULTS

Staining patterns were not altered. Proximal and distal sections had evidence of decreased contractility, compared with the normal section. Contraction frequency also was decreased, and distal sections had lower contraction frequency than proximal sections. Relaxation responses were decreased in distal sections. Responses to NE differed significantly for distal and proximal sections, compared with normal sections.

CONCLUSIONS AND CLINICAL RELEVANCE

Short-term ischemia can significantly affect adjacent bowel. Contractile and relaxation responses are impaired. Discrepancies in intestinal motility patterns and alterations in response to NE for sections proximal and distal to ischemic intestine could lead to clinical ileus or slowed transit of ingesta.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

Ileal distention relaxes the canine colon: a model of megacolon?

BACKGROUND/AIMS

Reflex relaxation is a mechanism whereby the colon can show short-term dilatation in the absence of mechanical obstruction. This study investigated the tonic response of the canine colon to ileal distention and its pharmacological control.

METHODS

In four dogs, the tone of the proximal colon was recorded by a barostat during distention of the terminal ileum.

RESULTS

Ileal distention inhibited ileal motility and relaxed the colon. Adrenergic blockade by propranolol plus phentolamine, nicotinic blockade by hexamethonium, and the nitric oxide synthase inhibitor N omega-nitro-L-arginine methylester significantly increased resting tone of the colon but did not inhibit relaxation induced by ileal distention. Muscarinic blockade by atropine completely relaxed the colon, and no further decrease in tone was observed after ileal distention. The neurokinin 2 antagonist SR48968 did not alter colonic tone.

CONCLUSIONS

The barostat was able to monitor resting tone of the canine colon, which was shown to be under inhibitory control by adrenergic, cholinergic-nicotinic, and nitric oxide-like transmitters. Inhibition of colonic tone by ileal distention was not mediated solely by adrenergic, cholinergic-nicotinic, or nitric oxide mechanisms. Reflex relaxation, possibly predisposing to acute colonic dilatation, may be activated by multiple mechanisms that may differ from those controlling resting tone.

Nerve-pulp interactions

Pulpal haemodynamics are naturally intermeshed with inflammatory responses. Cellular and humoral factors may be the vehicles that aid in physiological regulation, but when these systems are overly activated, they may lead to pathological changes. Sensory nerves may initiate inflammatory reactions when activated, and interestingly, recent findings show that vasoconstrictor nerves in the pulp can inhibit the release of neurally stored vasoactive and inflammatory mediators. Thus, there are options for endogenous control of inflammation. Perhaps a variation in the effectiveness of such control can explain why symptoms of hypersensitivity and pain are so unpredictable and individual. What naturally occurring agents are involved in early tissue changes and how do they act? Some agents exert their effects both on vessels and nerves. Thus, there is an intriguing mutual interplay between nerves and tissue reactions. A prolonged, painful stimulation may generate increased blood flow and inflammation, and vice versa, inflammation may lead to pain. This complexity of mechanisms generates many questions that need answers.

Inhibitory influence of sympathetic nerves on afferent nerve-induced extravasation in the rat incisor pulp upon direct electrical stimulation of the tooth

Previous studies have shown that sympathetic nerve stimulation reduces afferent nerve-induced vasodilation by mechanisms unrelated to vasoconstriction in the rat incisor pulp. The present investigation concerned whether similar modulatory mechanisms might also influence neurogenic plasma extravasation in dental pulp. Rat mandibular incisors were electrically stimulated and blood flow reactions in the pulp were recorded by laser Doppler flowmetry. Plasma extravasation in the incisor pulp, gingiva and lip were indirectly assessed by the Evans-blue method. Stimulation of teeth with 50 microA (5 min) did not cause increased dye accumulation in the stimulated pulps whereas stimulation with 100 microA significantly increased the dye content in ipsilateral pulps by 32% as compared to controls; 100 microA stimulation was without effect in unilaterally denervated animals. Tooth stimulation with 50 microA (5 min), in the presence of either the alpha-adrenergic blocker phenoxybenzamine (3 mg/kg), or the alpha 1-adrenergic antagonist prazosin (50 micrograms/kg), as well as in chronically sympathectomized animals, significantly increased the Evans-blue content in the stimulated pulps by 47, 83 and 86%, respectively. Application of short trains of impulses (same number of impulses as for the continuous stimulation but producing minimal vasoconstriction) resulted in some dye accumulation, which was enhanced in the ipsilateral pulps in the presence of prazosin (100 micrograms/kg) or after acute resection of the superior cervical sympathetic ganglion by 70 and 64%, respectively. The Evans-blue content in the lip and gingiva was uninfluenced by the tooth stimulation. The results indicate that activation of sympathetic nerves inhibits the afferent nerve-induced plasma extravasation in rat incisor pulp and this effect is mediated by alpha-adrenoceptors not associated with vasoconstriction.

Activation of sympathetic nerves exerts an inhibitory influence on afferent nerve-induced vasodilation unrelated to vasoconstriction in rat dental pulp

In order to elucidate a possible influence of the sympathetic nervous system on afferent nerve function, rat mandibular incisors were electrically stimulated and blood flow changes monitored in the incisor pulp of untreated and sympathectomized animals by a laser Doppler flowmeter. Monopolar electrical stimulation of the tooth (200 microA, 5 ms, 40 Hz, 1 s) in normal animals resulted in a transient reduction in pulpal blood flow (PBF) (16% reduction, n = 10) followed by a small but long-lasting increase (11% increase). After administration of phenoxybenzamine or phentolamine (3 mg kg-1, i.v.) the initial dip in PBF was reduced by 59% (P < 0.001) while the subsequent increase was enhanced by 185% (P < 0.001). Similarly, infusion of prazosin (50 micrograms kg-1, i.v.) and idazoxan (0.5 mg kg-1, i.v.) significantly enhanced the increase in PBF by 118 and by 79%, respectively. In chronically sympathectomized animals the increase in PBF was 250% larger than that seen in untreated animals (P < 0.001). This increase in PBF was not further enhanced after alpha-adrenergic blockade. Acute resection of the superior cervical sympathetic ganglion, also resulted in some enhancement (by 56%) of the stimulation-induced increase in PBF (P < 0.01, n = 6). The increase in PBF was unaffected by infusion of timolol (150 micrograms kg-1) and atropine (1 mg kg-1) but was totally abolished by intravenous pre-treatment with capsaicin (1-3 mg kg-1). The present results suggest that activation of sympathetic nerves exerts inhibitory effects on the afferent nerve-induced vasodilation in the rat incisor pulp unrelated to sympathetic vasoconstriction.

Tachykininergic transmission to the circular muscle of the guinea-pig ileum: evidence for the involvement of NK2 receptors

1. The effect of newly developed, receptor-selective tachykinin antagonists (GR 71,251 for NK1 receptors, MEN 10,376 and L 659,877 for NK2 receptors) on noncholinergic transmission to the circular muscle of the guinea-pig ileum has been investigated. 2. In circular muscle strips of the ileum, electrical field stimulation in the presence of atropine (2 microM) and apamin (0.1 microM) evoked a complex motor response. The tonic primary contraction in this response was reduced by GR 71,251 (10 microM) and MEN 10,376 (3-10 microM) but not by L 659,877 (up to 10 microM). The presence of apamin was necessary in this experimental arrangement to unmask an atropine-resistant primary contraction, sensitive to tachykinin antagonists. The motor response was abolished by tetrodotoxin. 3. In circular strips of the ileum GR 71,251 (10 microM) inhibited the tonic contraction produced by [Sar9] substance P sulphone, a selective NK1 receptor agonist but not that produced by [beta Ala8] neurokinin A (4-10), a selective NK2 receptor agonist. By contrast, MEN 10,376 antagonized the effect of the NK2 agonist while leaving the response to the NK1 agonist unaffected. 4. In whole segments of the ileum, distension of the gut wall by an intraluminal balloon placed at about 1 cm from the point of recording of mechanical activity of the circular muscle produced atropine-sensitive phasic contractions (ascending enteric reflex). In the presence of atropine (2 microM), a noncholinergic response was elicited, which required larger volumes of distension that the cholinergic one. The atropine-resistant ascending enteric reflex was enhanced by apamin (0.1 microM) and abolished by tetrodotoxin, either in the presence or absence of apamin.5. MEN 10,376 (3-lOmicroM) inhibited the atropine-resistant ascending enteric reflex in the presence of apamin while GR 71,251 or L 659,877 (10 microM each) were ineffective. MEN 10,376 inhibited the atropine-resistant ascending enteric reflex to a larger extent in the absence than in the presence of apamin and also slightly inhibited the ascending enteric reflex in the absence of atropine.6. These findings provide evidence for an involvement of NK2 tachykinin receptors in excitatory transmission to the circular muscle of the guinea-pig ileum. NK2 receptors are also involved in the physiological-like circular muscle activation produced by stimulation of intramural neuronal pathways which subserve the atropine-resistant ascending enteric reflex.

[Role of substance P in the nervous system control of digestive motility]

Substance P is a 11 amino-acids peptide which belongs to the tachykinins, a family of peptide which induces a rapid contraction of the smooth muscle of the digestive tract. The occurrence of substance P has been demonstrated by immunohistochemical and radioimmunological techniques in most parts of the central and peripheral nervous system. Substance P exerts on the smooth muscle of all the areas of the digestive tract a strong excitatory effect which is either direct or relayed by the cholinergic intramural neurones. Numerous electrophysiological, pharmacological and immunohistochemical data lead to the conclusion that substance P is released by intrinsic neurones of the digestive tract or by extrinsic nerves (vagus and splanchnic nerves, etc...). This release is enhanced by acetylcholine, cholecystokinin, serotonin and neurotensin, it is reduced by opioid peptides and noradrenaline. Substance P participates in the intestinal peristaltic reflex by the activation of the smooth muscle cells of the intestine, either directly or through the activation of the cholinergic intrinsic neurones. Substance P is also involved in the genesis of a non-cholinergic ascending excitatory activity likely occurring during vomiting. Lastly, substance P participates in the reflex contraction of the lower oesophageal sphincter following acidification of the distal part of the oesophagus.

Neuromodulation of guinea pig intestinal electrolyte transport by cholecystokinin octapeptide

We examined the effect of cholecystokinin octapeptide on electrolyte transport across isolated guinea pig mucosa. Segments of distal ileum stripped of longitudinal muscle and bathed on both sides with a Krebs'-bicarbonate buffer responded to cholecystokinin octapeptide when studied under short-circuited conditions. Cholecystokinin octapeptide (0.5-50 nmol/L) evoked a transient (4-10-minute) increase in transepithelial potential difference and short-circuit current upon application to the serosal side. Maximal increases in short-circuit current, achieved at 50-500 nmol/L, were 67 +/- 11 microA/cm2, whereas half-maximal effects occurred at a concentration of 0.7 +/- 0.2 nmol/L. Pretreatment of the tissues with 0.5 mumol/L atropine reduced the maximal short-circuit response to cholecystokinin octapeptide by 53%. The change in short-circuit current due to cholecystokinin octapeptide was nearly abolished by pretreatment with 0.5 mumol/L tetrodotoxin, suggesting neuronal involvement. Cholecystokinin octapeptide-induced increases in short-circuit current were halved by removal of serosal buffer Ca2+ and were abolished in Cl(-)- and HCO3(-)-free buffer. The cholecystokin-receptor antagonists proglumide and lorglumide shifted the concentration-response curve for cholecystokinin octapeptide competitively to the right, having antagonists potencies of 130 and 0.03 mumol/L, respectively. Cerulein (0.1-500 nmol/L) also increased short-circuit current, whereas nonsulfated cholecystokinin octapeptide was ineffective. In conclusion, cholecystokinin octapeptide seems to act at neuronal cholecystokinin receptors to stimulate mucosal anion secretion, in part, by releasing acetylcholine.

Contribution of baroreceptor reflexes to blood pressure and sympathetic responses to cholecystokinin and vasoactive intestinal peptide in anesthetized dogs

The effects of synthetic vasoactive intestinal peptide (VIP) and cholecystokinin (CCK) on systemic blood pressure and renal nerve activity were studied before and after cervical vagotomy, and sino-aortic denervation with vagotomy in anesthetized dogs. Intravenous injection of VIP (5 micrograms/kg) in animals with an intact neuraxis produced a significant decrease in systemic blood pressure and a significant increase in renal nerve activity. These responses to VIP did not change after vagotomy and after complete denervation, VIP did not cause any change in renal nerve activity, even during hypotension. The level of hypotension after complete denervation was greater than that under other conditions. These results indicate that the cardiovascular effects of VIP are reduced by activation of the systemic baroreceptors. Intravenous injection of CCK (10 micrograms/kg) in animals with an intact neuraxis produced significant decreases in blood pressure and renal nerve activity. These responses to CCK were abolished in animals with cervical vagotomy only. However, following complete denervation of the carotid sinus and total section of the vagal nerves, CCK caused a significant increase in blood pressure and renal sympathetic nerve activity. These results indicate that the sympathetic depressor effect of CCK may be mediated by activation of the vagal afferents, and that the sympathetic pressor effect may be due to a direct action of CCK on the central nervous system. Thus, each gastrointestinal peptide may regulate the cardiovascular system through a different mechanism.

Immunocytochemical analysis of potential neurotransmitters present in the myenteric plexus and muscular layers of the corpus of the guinea pig stomach

Recent electrophysiological studies of neurons of the myenteric plexus of the corpus of the guinea pig stomach have revealed that slow synaptic events are extremely rare. In contrast, they are commonly encountered in similar investigations of myenteric ganglia of the guinea pig small intestine. The current immunocytochemical analysis of the myenteric plexus and innervation of the muscularis externa of the corpus of the guinea pig stomach was undertaken in order to determine whether putative neurotransmitters capable of mediating slow synaptic events are present in gastric ganglia. A major difference between the small intestine and the stomach was found in the innervation of the musculature. Whereas the longitudinal muscle layer of the small intestine contains very few nerve fibers and is innervated mainly at its interface with the myenteric plexus, the longitudinal muscle of the corpus of the stomach contained as many varicose substance P (SP)-, vasocative intestinal polypeptide (VIP)-, and neuropeptide Y (NPY)-immunoreactive axons as the circular muscle layer. These putative neurotransmitters were also present in the ganglia of the myenteric plexus, where varicose SP-, VIP-, and NPY-immunoreactive fibers encircled nonimmunoreactive neurons. Varicose 5-hydroxytryptamine (5-HT)-immunoreactive terminal axons were essentially limited to the myenteric plexus and were found both in ganglia and in interganglionic connectives, where they were particularly numerous; 5-HT-immunoreactive neurons appeared to be more abundant in the stomach than in the small intestine. Tyrosine hydroxylase (TH)- and calcitonin-gene-related-peptide (CGRP)-immunoreactive axons were also more common in the myenteric plexus than in the musculature, but of these, only the TH-immunoreactive neurites tended, like those of the other putative transmitters, to encircle neurons in myenteric ganglia. Evidence was obtained that, as in the small intestine, at least some of the SP-, VIP-, NPY-, and 5-HT-immunoreactive fibers in the stomach are derived from intrinsic gastric myenteric neurons. In contrast, unlike the small intestine, gastric myenteric ganglia appeared to lack intrinsic CGRP-immunoreactive neurons; therefore, the CGRP-immunoreactive gastric axons are probably of extrinsic origin.(ABSTRACT TRUNCATED AT 400 WORDS)

Propagated motor activity in the small intestine of urethane-anaesthetized rats: inhibitory action of sympathetic and capsaicin-sensitive nerves

1. In the duodenum/small intestine of urethane-anesthetized rats, a series of atropine- and hexamethonium-sensitive phasic contractions activity were recorded by using a balloon. With two balloons, one in the proximal duodenum (B1) and the other at a distal site (B2) a co-ordinated motor activity was observed which migrated from B1 to B2. 2. Spontaneous activity of the duodenum was greater in either adrenalectomized, guanethidine- or capsaicin-pretreated than in control rats. In guanethidine-pretreated animals a series of atropine-resistant but hexamethonium-resistant rhythmic contractions could be demonstrated. 3. In control rats, distension of the balloon produced an inhibitory effect on rhythmic contractions which were ultimately suppressed. Distension had little inhibitory effect on duodenal motor activity of adrenalectomized, guanethidine- or capsaicin-pretreated rats. 4. These findings indicate that the propagated motor activity in the duodenum/small intestine of urethane anesthetized rats can be modulated by the adrenals as well as sympathetic and capsaicin-sensitive nerves.

Cholecystokinin octapeptide activates an opioid mechanism in the guinea-pig ileum: a possible role for substance P

To test the hypothesis that excitatory peptides release endogenous opioids from the myenteric plexus longitudinal muscle (MPLM) preparation of the guinea-pig ileum (GPI), the effect of cholecystokinin (CCK8) was studied in the absence and presence of the opioid antagonist naloxone. The maximum height of the contracture induced by CCK8 was not altered by the presence of naloxone in the incubation medium, however, the subsequent sustained excitation was clearly increased. This effect is interpreted as being a result of the release of endogenous opioids during the first moments of the CCK8-evoked excitation of the plexus. CCK8 still induced neurogenic contractures in the presence of atropine; these contractures were probably mediated by the release of substance P. Naloxone was used to evidence the opioid control of the CCK8-induced release of substance P. Desensitization to the effect of substance P reduced the action of CCK8 and also abolished the non-cholinergic contractures evoked by CCK8 and the subsequent effect of naloxone. These facts suggest the release of endogenous opioids within the plexus in response to the neurally mediated excitatory action of CCK8.

Evaluation of a new and potent cholecystokinin antagonist on motor responses of the guinea-pig intestine

The potency and selectivity of D,L-4-(3,4-dichloro-benzoyl-amino)-5-(dipentyl-amino)-5-oxo-pen tan oic acid (CR 1409) as a cholecystokinin (CCK) antagonist was investigated on motor responses of the longitudinal and circular muscles of the guinea-pig isolated ileum. CR 1409 was further used to examine whether nerve-mediated motor responses to electrical field stimulation or distension of the gut wall may involve the release of CCK-like peptides. CR 1409 (0.06-2.1 microM) antagonized longitudinal muscle responses to ceruletide (caerulein, a CCK-related decapeptide) in a concentration dependent and competitive manner (pA2 7.77); responses to CCK-octapeptide (CCK-8) were antagonized with a similar potency. Contractions of the circular muscle evoked by ceruletide were also blocked by CR 1409 (0.2-0.4 microM). Longitudinal muscle contractions in response to dimethylphenylpiperazinium, bethanechol, histamine, substance P, or 5-hydroxytryptamine (5-HT), and circular muscle contractions evoked by acetylcholine, 5-HT, substance P, or substance K were not altered by CR 1409 (0.4 microM). Longitudinal muscle contractions induced by electrical field stimulation (with pulses delivered at 0.05 and 1 Hz in the absence, and at 5 Hz in the presence of atropine) were not or only slightly reduced by CR 1409 (0.4 microM). Longitudinal contractions due to activation of extrinsic nerves by capsaicin remained unaltered in the presence of CR 1409 (0.4 microM). Reflex contractions of the circular muscle, induced by balloon distension and recorded orally to the site of distension, and peristaltic activity elicited by intraluminal infusion of Tyrode solution remained unaffected by CR 1409 (0.4 microM). 5 These findings indicate that CR 1409 is a potent and selective antagonist of CCK-like peptides in the guinea-pig ileum. The results do not provide any evidence that CCK-like peptides, released from extrinsic or intrinsic neurones, are involved in nerve-mediated contractions of intestinal muscle and in the peristaltic reflex.

Dual action of cholecystokinin-octapeptide on the guinea pig antrum

We examined the mechanism of action of cholecystokinin-octapeptide (CCK-OP) on longitudinal strips of the lesser and greater curvatures of the guinea pig antrum. In the strips of the lesser curvature, CCK-OP produced a concentration-dependent tonic contraction, which was significantly reduced by atropine, but not by tetrodotoxin. In contrast, CCK-OP caused a relaxation of the preparation of the greater curvature in a concentration-dependent manner. The relaxation induced by the peptide was enhanced by atropine, whereas it was blocked by tetrodotoxin. Propranolol, phentolamine, desensitization to adenosine-5'-triphosphate, and desensitization to vasoactive intestinal polypeptide had no effect on CCK-OP-induced relaxation. Cholecystokinin-octapeptide evoked the release of acetylcholine from strips of both sides, and it was not blocked by tetrodotoxin. These findings indicate that the mechanism of action of CCK-OP on the lesser curvature differs from that on the greater curvature. The peptide seems to act directly on smooth muscle cells and to stimulate cholinergic neural activity by sodium channel-independent mechanisms. Additionally, nonadrenergic inhibitory neurons appear to be activated by CCK-OP in the greater curvature.

Motor activity of the rat duodenum in vivo: evidence for the existence of an atropine-resistant peristalsis

The motor activity of the proximal duodenum has been investigated by means of a balloon method in urethane-anaesthetized rats. A series of atropine- (1 mg/kg i.v.) or hexamethonium- (20 mg/kg i.v.) sensitive rhythmic contractions can be demonstrated in the rat duodenum which represents a peristaltic reflex mediated by intramural cholinergic neurons. The atropine-sensitive peristaltic reflex is transiently suppressed by intravenous DMPP (0.1 mg/kg) GABA (3 mg/kg) or noradrenaline (10 micrograms/kg). In rats pretreated with guanethidine (20 mg/kg s.c.) plus naloxone (2 mg/kg i.v.) and theophylline (2 mg/kg i.v.) a second type of peristaltic reflex which is atropine-resistant but hexamethonium-sensitive can be demonstrated. These findings indicate that in the rat small intestine a peristaltic activity can be sustained by both atropine-sensitive and atropine-resistant mechanism, which may explain failure of atropine to affect intestinal motility in conscious rats.

Innervation of airway smooth muscle. Efferent mechanisms

The classical view, with one excitatory (cholinergic) and one inhibitory (noradrenergic) component, of the innervation of airway smooth muscle is incomplete and at least two other, possibly peptidergic, types of innervation must be included when the innervation of airways is considered. A summary of these neuronal components is given in Fig. 1 and their possible origin is outlined. Besides the inhibitory noradrenergic innervation of the airways observed in some species, an inhibitory NANC (i-NANC) innervation has been demonstrated. The polypeptide, VIP, seems to be the most likely candidate for the neurotransmitter in the i-NANC innervation of the airways. The excitatory cholinergic innervation is present in the airways from the trachea down to the peripheral bronchi. In the guinea-pig bronchi an excitatory NANC (e-NANC) innervation has been demonstrated as well. The e-NANC nerves may correspond to chemosensitive primary afferent nerves with substance P or a related tachykinin as transmitter. When the innervation of airway smooth muscle of different mammalian species is compared it is evident that all nerve components except the cholinergic, show a considerable variability among species. The cholinergic innervation seems to be present in all mammalian species whereas the other components may be completely absent from some species. Distinct regional variations in the innervation of the airways may occur, which is exemplified by the distribution of the autonomic innervation in the guinea-pig tracheo-bronchial tree. Cholinergic neurotransmission in for example the guinea-pig and human airways can be modulated by NA via prejunctional inhibitory alpha 2-adrenoceptors. Furthermore, the e-NANC neurotransmission in the guinea-pig airways may be modulated by NA or by selective alpha 2-adrenoceptor agonists, acting via prejunctional inhibitory alpha 2-adrenoceptors. The clinical importance of the NANC innervation in relation to asthma is discussed. The i-NANC nerves may exert a modulating effect on bronchoconstriction, and a functional defect would presumably lead to an exaggerated response to constrictor stimuli. The e-NANC nerves in the airways may also be clinically relevant since the transmitter (tachykinins) from these nerves can produce bronchoconstriction and promote inflammation of the airway epithelium, either by direct mechanisms or indirectly by activation of mast cells, and thus contribute to the features of asthma.(ABSTRACT TRUNCATED AT 400 WORDS)

Pathophysiology of equine postoperative ileus: effect of adrenergic blockade, parasympathetic stimulation and metoclopramide in an experimental model

An experimental model of postoperative ileus was developed in ponies using trauma to, and exposure of, a length of small intestine which gave rise to a reproducible and reversible set of changes in gut activity. This was assessed by recordings of electrical and mechanical activity and by propulsion of spheres from stomach to anus. Activity was depressed, especially in the stomach and colon, and transit was slowed. All drugs given increased electromechanical activity but propranolol was the least effective and did not alter the delayed transit of spheres. Yohimbine was more effective and the addition of bethanechol produced a little extra propulsive action. Metoclopramide had the best effect, virtually returning transit to normal and was the only drug fully restoring coordination of gastric and small intestinal activity which was disrupted by the ileus procedure. Loss of gastroduodenal coordination is probably the central lesion in equine ileus and may be mediated by dopamine.

Pharmacological characterization of the receptor mediating the adrenergic inhibition of responses to substance P in the cingulate cortex

The excitatory responses of neurones in the anterior cingulate cortex of the rat to iontophoretically applied substance P (SP) are reduced by noradrenaline (NA) applied iontophoretically or released from noradrenergic pathways. In order to determine the receptor involved in this inhibitory effect we have studied the effects of a number of receptor-specific adrenergic agonists and antagonists on responses of cingulate neurones to SP in rats anaesthetized with chloral hydrate. Low iontophoretic currents (0-15 nA) of NA, adrenaline and the beta-agonist, clenbuterol, all strongly reduced responses to SP. Isoprenaline was also effective but less consistently so, although problems were experienced with its iontophoretic release from micropipettes. The alpha 1-agonists, phenylephrine and methoxamine were also able to reduce responses to SP. However, this reduction required higher iontophoretic currents (15-60 nA) and was associated with depressant effects on baseline firing rate. The alpha 2-agonist clonidine was only weakly active at high currents and this too was associated with depression of baseline firing. Similar weak effects were noted with dopamine. The inhibitory effects of NA on SP responses were convincingly blocked or reversed by the beta-antagonist, practolol, but not by the alpha 1-antagonist, prazosin. The reduction of SP responses by phenylephrine was also blocked by practolol but unaffected by prazosin. Finally, reduction of SP excitations by activation of the coeruleocortical pathway was also blocked by practolol applied iontophoretically to the cortical cells. These results are consistent with the hypothesis that the effect of NA on SP responsiveness in the cingulate cortex is mediated by beta-adrenoreceptors.

The properties of the clonidine withdrawal response of guinea-pig isolated ileum

The dependence-inducing effects of clonidine were investigated on the guinea-pig isolated ileum. Clonidine produced relaxation of the ileum with a threshold concentration between 0.01 and 0.1 mumol 1(-1). Washout of clonidine did not induce a withdrawal contraction. Following 2 min contact of the ileum with clonidine, 1 mumol 1(-1), addition of phentolamine, 5 mumol 1(-1), induced a contracture. The phentolamine-precipitated withdrawal contracture did not increase in height with a longer period of contact (32 min) of the ileum with clonidine. The phentolamine-precipitated withdrawal contracture following 2 min contact of ileum with clonidine was abolished by atropine, 5 mumol 1(-1), and substance P (SP) antagonists, (D-Pro2,D-Phe7, D-Trp9)-SP and spantide, 10 mumol 1)-1). [Met5]enkephalin, 1 mumol 1(-1), abolished the withdrawal response to clonidine and clonidine, 1 mumol 1(-1), abolished the withdrawal response to [Met5]enkephalin. Following 2 min contact of the ileum with noradrenaline, 5 mumol 1(-1), washout or addition of phentolamine or yohimbine, 5 mumol 1(-1), also induced a withdrawal response. The noradrenaline washout withdrawal response was abolished by atropine, 5 mumol 1(-1), and spantide, 10 mumol 1(-1). Since clonidine dependence may be induced as rapidly as opiate dependence in the ileum and the pharmacology of the withdrawal responses is similar, it is suggested that they both induce the same post-receptor neuronal feedback disturbance in which substance P neurones play a major role.

Release of cholecystokinin-immunoreactivity into the vascular bed of the guinea-pig small intestine during peristalsis

The release of cholecystokinin-immunoreactivity (CCK-IR) into the venous effluate of the isolated and vascularly perfused guinea-pig small intestine was measured. Raising the intraluminal pressure by 5 mbar for 8 min initiated peristalsis and was associated with an increased release of CCK-IR. The ganglion stimulant dimethylphenyl-piperazinium failed to alter the release of CCK-IR. The results are discussed in the light of a possible involvement of CCK-containing neurones in intestinal peristalsis.

In vivo demonstration of alpha-2-adrenoceptor-mediated inhibition of the excitatory non-cholinergic neurotransmission in guinea pig airways

In anaesthetized guinea pigs, vagal nerve stimulation caused an atropine- and hexamethonium-resistant increase in the respiratory insufflation pressure. Clonidine (0.05 mg/kg i.v.) inhibited the increase in insufflation pressure caused by vagal stimulation. This effect of clonidine was antagonized by the alpha-2-antagonist yohimbine (0.5 mg/kg i.v.), but not by the alpha-1-antagonist prazosin (0.03 mg/kg i.v.). In conclusion, the increase in the respiratory insufflation pressure, caused by stimulation of noncholinergic nerves, seemed to be controlled by inhibitory alpha-2-adrenoceptors in guinea pig airways.

Release of dynorphin, somatostatin and substance P from the vascularly perfused small intestine of the guinea-pig during peristalsis

The release of dynorphin-(1-17), somatostatin and substance P into the venous effluate of the isolated and vascularly perfused guinea-pig small intestine was measured during rest and peristaltic activity. The peptides were determined by specific radioimmunoassays. Increasing the intraluminal pressure by 5 mbar increased the release of dynorphin-(1-17), somatostatin and substance P. A substantial increase in the release of substance P was only seen in the presence of naloxone (1.5 microM) indicating an inhibitory influence of opioid peptide-containing neurones on the release of substance P. The pressure-induced release of substance P and dynorphin-(1-17) was completely prevented by tetrodotoxin (1.3 microM), which suggests a neural origin of these two peptides. The pressure-induced release of somatostatin was only partially inhibited by tetrodotoxin (1.3 microM) suggesting that somatostatin may also be released from non-neuronal sources, i.e. endocrine mucosal cells. Dimethylphenylpiperazinium (32 microM) increased the release of somatostatin and substance P and this effect was inhibited by tetrodotoxin (1.3 microM). Cholecystokinin-octapeptide (38 nM) induced a large increase in the release of somatostatin but only a minute increase in the release of substance P; these effects of cholecystokinin-octapeptide were not blocked by tetrodotoxin (1.3 microM). Noradrenaline (59 microM) inhibited the pressure-induced release of substance P but not that induced by dimethylphenylpiperazinium (32 microM). Neither the pressure-induced nor the dimethylphenylpiperazinium-evoked release of somatostatin was significantly diminished by noradrenaline. These results indicate that dynorphin-(1-17), somatostatin and substance P may be transmitters involved in the coordination of the peristaltic reflex. Part of the inhibitory effects of opioid peptides and noradrenaline on intestinal motility may be brought about by inhibition of the release of substance P.

Behavioral evidence in rats for a peptidergic-noradrenergic interaction in cutaneous sensory and vascular function

Cutaneous sensory and vascular function was examined following application of capsaicin to the sciatic nerve and systemic injection of guanethidine. Together the two drugs produced a reduction in sensitivity to heat-pain, inflammatory pain (formalin test), tactile stimulation and skin temperature of the foot that exceeded the effects of either drug alone. The inflammation produced by an injection of formalin to the plantar surface of the hind paw was reduced equally by capsaicin or capsaicin + guanethidine. Cold sensitivity and inflammation produced by yeast injection were unaffected by all treatments. The data imply a peripheral interaction between peptidergic and noradrenergic systems with significant functional implications that may be important in the pathology of familial dysautonomia.