The effect of catecholamines and sympathetic stimulation on the release of acetylcholine from the guinea-pig colon

Circadian rhythms in colonic function

A rhythmic expression of clock genes occurs within the cells of multiple organs and tissues throughout the body, termed "peripheral clocks." Peripheral clocks are subject to entrainment by a multitude of factors, many of which are directly or indirectly controlled by the light-entrainable clock located in the suprachiasmatic nucleus of the hypothalamus. Peripheral clocks occur in the gastrointestinal tract, notably the epithelia whose functions include regulation of absorption, permeability, and secretion of hormones; and in the myenteric plexus, which is the intrinsic neural network principally responsible for the coordination of muscular activity in the gut. This review focuses on the physiological circadian variation of major colonic functions and their entraining mechanisms, including colonic motility, absorption, hormone secretion, permeability, and pain signalling. Pathophysiological states such as irritable bowel syndrome and ulcerative colitis and their interactions with circadian rhythmicity are also described. Finally, the classic circadian hormone melatonin is discussed, which is expressed in the gut in greater quantities than the pineal gland, and whose exogenous use has been of therapeutic interest in treating colonic pathophysiological states, including those exacerbated by chronic circadian disruption.

Stimulation of extrinsic sympathetic nerves differentially affects neurogenic motor activity in guinea pig distal colon

The speed of pellet propulsion through the isolated guinea pig distal colon in vitro significantly exceeds in vivo measurements, suggesting a role for inhibitory mechanisms from sources outside the gut. The aim of this study was to investigate the effects of sympathetic nerve stimulation on three different neurogenic motor behaviors of the distal colon: transient neural events (TNEs), colonic motor complexes (CMCs), and pellet propulsion. To do this, segments of guinea pig distal colon with intact connections to the inferior mesenteric ganglion (IMG) were set up in organ baths allowing for simultaneous extracellular suction electrode recordings from smooth muscle, video recordings for diameter mapping, and intraluminal manometry. Electrical stimulation (1-20 Hz) of colonic nerves surrounding the inferior mesenteric artery caused a statistically significant, frequency-dependent inhibition of TNEs, as well as single pellet propulsion, from frequencies of 5 Hz and greater. Significant inhibition of CMCs required stimulation frequencies of 10 Hz and greater. Phentolamine (3.6 μM) abolished effects of colonic nerve stimulation, consistent with a sympathetic noradrenergic mechanism. Sympathetic inhibition was constrained to regions with intact extrinsic nerve pathways, allowing normal motor behaviors to continue without modulation in adjacent extrinsically denervated regions of the same colonic segments. The results demonstrate differential sensitivities to sympathetic input among distinct neurogenic motor behaviors of the colon. Together with findings indicating CMCs activate colo-colonic sympathetic reflexes through the IMG, these results raise the possibility that CMCs may paradoxically facilitate suppression of pellet movement in vivo, through peripheral sympathetic reflex circuits.

Sympathetic Pathways Target Cholinergic Neurons in the Human Colonic Myenteric Plexus

Background

The sympathetic nervous system inhibits human colonic motility largely by effects on enteric neurons. Noradrenergic axons, which branch extensively in the myenteric plexus, are integral to this modulatory role, but whether they contact specific types of enteric neurons is unknown. The purpose of this study was to determine the association of noradrenergic varicosities with types of enteric neurons.

Methods

Human colonic tissue from seven patients was fixed and dissected prior to multi-layer immunohistochemistry for human RNA binding proteins C and D (HuC/D) (pan-neuronal cell body labelling), tyrosine hydroxylase (TH, catecholaminergic labelling), Enkephalin (ENK), choline acetyltransferase (ChAT, cholinergic labelling) and/or nitric oxide synthase (NOS, nitrergic labelling) and imaged using confocal microscopy. TH-immunoreactive varicose nerve endings and myenteric cell bodies were reconstructed as three dimensional digital images. Data was exported to a purpose-built software package which quantified the density of varicosities close to the surface of each myenteric cell body.

Results

TH-immunoreactive varicosities had a greater mean density within 1 μm of the surface of ChAT +/NOS− nerve cell bodies compared with ChAT−/NOS + cell bodies. Similarly, ENK-immunoreactive varicosities also had a greater mean density close to ChAT +/NOS− cell bodies compared with ChAT−/NOS + cells.

Conclusion

A method for quantifying close associations between varicosities and nerve cell bodies was developed. Sympathetic axons in the myenteric plexus preferentially target cholinergic excitatory cells compared to nitrergic neurons (which are largely inhibitory). This connectivity is likely to be involved in inhibitory modulation of human colonic motility by the sympathetic nervous system.

Characterization of Cecal Smooth Muscle Contraction in Laying Hens

The ceca play an important role in the physiology of the gastrointestinal tract in chickens. Nevertheless, there is a gap of knowledge regarding the functionality of the ceca in poultry, especially with respect to physiological cecal smooth muscle contraction. The aim of the current study is the ex vivo characterization of cecal smooth muscle contraction in laying hens. Muscle strips of circular cecal smooth muscle from eleven hens are prepared to investigate their contraction ex vivo. Contraction is detected using an isometric force transducer, determining its frequency, height and intensity. Spontaneous contraction of the chicken cecal smooth muscle and the influence of buffers (calcium-free buffer and potassium-enriched buffer) and drugs (carbachol, nitroprusside, isoprenaline and Verapamil) affecting smooth muscle contraction at different levels are characterized. A decrease in smooth muscle contraction is observed when a calcium-free buffer is used. Carbachol causes an increase in smooth muscle contraction, whereas atropine inhibits contraction. Nitroprusside, isoprenaline and Verapamil result in a depression of smooth muscle contraction. In conclusion, the present results confirm a similar contraction behavior of cecal smooth muscles in laying hens as shown previously in other species.

Autonomic nerve regulation of colonic peristalsis in Guinea pigs

Background/Aims

Colonic peristalsis is mainly regulated via intrinsic neurons in guinea pigs. However, autonomic regulation of colonic motility is poorly understood. We explored a guinea pig model for the study of extrinsic nerve effects on the distal colon.

Methods

Guinea pigs were sacrificed, their distal colons isolated, preserving pelvic nerves (PN) and inferior mesenteric ganglia (IMG), and placed in a tissue bath. Fecal pellet propagation was conducted during PN and IMG stimulation at 10 Hz, 0.5 ms and 5 V. Distal colon was connected to a closed circuit system, and colonic motor responses were measured during PN and IMG stimulation.

Results

PN stimulation increased pellet velocity to 24.6 ± 0.7 mm/sec (n = 20), while IMG stimulation decreased it to 2.0 ± 0.2 mm/sec (n = 12), compared to controls (13.0 ± 0.7 mm/sec, P < 0.01). In closed circuit experiments, PN stimulation increased the intraluminal pressure, which was abolished by atropine (10⁻⁶ M) and hexamethonium (10⁻⁴ M). PN stimulation reduced the incidence of non-coordinated contractions induced by NG-nitro-L-arginine methyl ester (L-NAME; 10⁻⁴ M). IMG stimulation attenuated intraluminal pressure increase, which was partially reversed by alpha-2 adrenoceptor antagonist (yohimbine; 10⁻⁶ M).

Conclusions

PN and IMG input determine speed of pellet progression and peristaltic reflex of the guinea pig distal colon. The stimulatory effects of PN involve nicotinic, muscarinic and nitrergic pathways. The inhibitory effects of IMG stimulation involve alpha-2 adrenoceptors.

Use of inotropes and vasopressor agents in critically ill patients

Inotropes and vasopressors are biologically and clinically important compounds that originate from different pharmacological groups and act at some of the most fundamental receptor and signal transduction systems in the body. More than 20 such agents are in common clinical use, yet few reviews of their pharmacology exist outside of physiology and pharmacology textbooks. Despite widespread use in critically ill patients, understanding of the clinical effects of these drugs in pathological states is poor. The purpose of this article is to describe the pharmacology and clinical applications of inotropic and vasopressor agents in critically ill patients.

Effects of extrinsic autonomic inputs on expression of c-Fos immunoreactivity in myenteric neurons of the guinea pig distal colon

c-Fos protein is a nuclear protein coded by c-fos proto-oncogene subsequent to synaptic activation of the neurons. We used immunohistochemical methods to visualize the expression of c-Fos protein in myenteric neurons of the guinea pig distal colon and examined the effects of the extrinsic autonomic inputs on the enteric circuits. No c-Fos immunoreactivity was observed in the colonic segments fixed immediately after removal from the animal body. A number of c-Fos-immunoreactive nuclei of myenteric neurons, however, appeared in all preparations that were incubated in Krebs solution in vitro (n=10). Application of tetrodotoxin (0.2 microM) abolished the expression of c-Fos-immunoreactivity (n=6), but hexamethonium (100 microM) failed to decrease the number of c-Fos-positive neurons despite a complete suppression of spontaneous peristaltic movements (n=5). Neither the electrical stimulation (n=8) nor the severing of the pelvic nerves (n=5) changed the number of c-Fos-positive neurons. Application of clonidine, an alpha(2)-agonist, (0.1 microM) abolished the expression of c-Fos protein in all preparations (n=5), while denervation of the sympathetic fibers in the lumbar colonic and hypogastric nerves in vivo increased the number of c-Fos-positive neurons (n=5). The results indicate that the enteric circuit in the distal part of the gastrointestinal tract is under tonic inhibition by the sympathetic nervous system from the lumbar spinal cord. c-Fos immunoreactivity expressed in the colonic preparations in vivo might be the results of enhanced activation of non-nicotinic receptors after removal of the sympathetic inhibition.

In vivo assessment of the regulatory mechanism of cholinergic neuronal activity associated with motility in dog small intestine

Intestinal motor activity associated with acetylcholine (ACh) release was assessed in the small intestine of anesthetized dogs by simultaneous measurement of motor activity and local ACh concentrations within the intestinal wall with in vivo microdialysis. Basal concentration of ACh measured in the dialysate was 1.12 +/- 0.08 pmol/15 min (n = 10), a value that remained constant until 3 h after perfusion. Intra-arterial infusion of tetrodotoxin reduced dialysate ACh concentration, while the motor activity accelerated at the early phase after infusion of tetrodotoxin and then decreased, thereby suggesting that the motor activity is regulated by not only excitatory cholinergic neurons, but also inhibitory neurons. Intraarterial infusion of atropine increased dialysate ACh concentration but reduced motor activity, thereby indicating that the cholinergic neurons are tonically active and the muscarinic autoreceptors operate to inhibit the ACh release. Intraarterial infusion of norepinephrine reduced, but yohimbine increased both motor activity and dialysate ACh concentration, thereby indicating that the adrenergic neurons regulate the motor activity due to control of cholinergic neuronal activity. This in vivo microdialysis method demonstrated in the whole body of animals that the activity of cholinergic neurons was physiologically regulated by itself and adrenergic neurons.

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

Impulse interval-dependent effect of sympathetic nerve stimulation on evoked acetylcholine release from the rabbit perfused atria preparation

The aim of the present study was to explore possible prejunctional effects mediated by impulse activity of sympathetic terminals on evoked acetylcholine release in an organ innervated by the autonomic ground plexus. Rabbit atria were isolated with the extrinsic right vagus and sympathetic nerves intact and perfused with Tyrode solution. Acetylcholine overflow was determined after labelling of the transmitter stores with [14C]choline and fractionation of the radioactivity on cation exchange columns. The overflow of endogenous noradrenaline was measured by HPLC and electrochemical detection. The vagus nerve was stimulated at 2 Hz for 3 min four times at intervals of 10 min. During the second stimulation the postganglionic sympathetic nerves were stimulated (2 Hz, 3 min) in such a way that the impulses preceded the vagus stimuli by a fixed time interval which was varied in different experiments (0, 7, 19, 50, 132, and 350 ms). Evoked acetylcholine release was significantly enhanced when the vagus was excited 7, 19 and 50 ms after the sympathetic nerves but it was unaltered at the 132 or 350 ms intervals, and when both nerves were stimulated simultaneously. Noradrenaline release was similar (about 6 ng per stimulation period) in all experimental groups. When sympathetic nerve stimulation had little effect in releasing noradrenaline (< 2.0 ng per stimulation period), facilitation of acetylcholine release at the 19 ms pulse interval was absent. The resting outflow of acetylcholine was unaffected by sympathetic nerve stimulation. The experiments show a facilitation of evoked acetylcholine release by sympathetic activity. As revealed by the pulse-to-pulse method this effect is confined to a relatively brief interval immediately following the excitation of the noradrenergic terminal, and is unlikely to be mimicked by exogenous drug application.

Effect of desipramine-induced blockade of neuronal uptake mechanisms on adrenoceptor-mediated responses in the guinea-pig colon

In order to clarify whether adrenoceptors in the guinea-pig distal colon are under sympathetic control, we assessed possible variations in the sensitivity to adrenoceptor agonists after blockade of neuronal catecholamine uptake mechanisms by desipramine (DMI). First, experiments were carried out to investigate the effects of DMI added in the organ bath on propulsion velocity, endogenous and [3H] prelabelled acetylcholine overflow, electrically evoked noradrenaline overflow and longitudinal smooth muscle tone. Secondly, we studied the effects of adrenoceptor agonists on the above parameters in untreated animals and in animals chronically treated with DMI. DMI added in the organ bath at concentrations equal to or higher than 30 nM inhibited all the parameters under study. Thus, when evaluating the effect of DMI on concentration-response curves to adrenoceptor agonists, concentrations which were per se inactive were used. DMI added in the organ bath at concentrations up to 30 nM potentiated the inhibitory effects of exogenous noradrenaline on propulsion velocity and acetylcholine overflow, but it did not affect the concentration-response curve to exogenous noradrenaline on longitudinal smooth muscle tone. Furthermore, 30 nM DMI inhibited propulsion velocity during sympathetic nerve stimulation. In preparations obtained from animals chronically treated with DMI, no significant change of propulsion velocity, endogenous and [3H] prelabelled acetylcholine overflow was found with respect to untreated animals. Nevertheless, in such preparations subsensitivity to isoprenaline (acting mainly on muscular beta-adrenoceptors) and clonidine (acting on neuronal alpha 2-adrenoceptors) and super-sensitivity to phenylephrine were observed. Electrically evoked noradrenaline overflow was enhanced, in a frequency-dependent way, by yohimbine and inhibited by clonidine.(ABSTRACT TRUNCATED AT 250 WORDS)

Mathematical modelling of the enteric nervous network. II: Facilitation and inhibition of the cholinergic transmission

The pharmacokinetic responses of the cholinergic enteric neurone to treatment with acetylcholinesterases, tetrodotoxin, some chloride salts of divalent cations, botulinum toxin, beta-bungarotoxin and changes in the concentration of calcium ions in the external medium and repetitive stimulation are presented. The numerical results obtained reproduce quantitatively the effects of toxins and salts of divalent cations acting at different levels of acetylcholine release from the nerve-terminal. The addition of cholinergic agonists potentiates the action of acetylcholine and increases the amplitude of the generated excitatory postsynaptic potential. A decrease in the concentration of extracellular Ca2+ ions reduces the amplitude of the excitatory postsynaptic potential and significantly increases synaptic transmission time. The effect of tetrodotoxin is the blockade propagation of the action potential along the nerve axon and, as a consequence, acetylcholine release from the vesicular store. All these effects have been shown to be dose-dependent. The repetitive stimulation of the neurone reproduces the effects of accumulation and potentiation. The possible applications of the model for the analysis of the enteric nervous system function are discussed.

Characteristics of cholinergic neuroeffector transmission of ganglionic and aganglionic colon in Hirschsprung's disease

Differences in the release and content of acetylcholine and the alpha 2 adrenoceptor mediated interaction between noradrenergic and cholinergic neurons were investigated by neurochemical and pharmacological methods in aganglionic and ganglionic segments of isolated human colon taken from children suffering from Hirschsprung's disease. Both at rest and during transmural stimulation the release of acetylcholine was significantly higher in the spastic (aganglionic) segment than in the proximal dilated bowel. Significant differences were found in the tissue concentration of acetylcholine between ganglionic and aganglionic specimens. The pattern of response to transmural stimulation was also different in the spastic and dilated bowel. Transmural stimulation induced relaxation and contraction in ganglionic specimens but only contractions in aganglionic specimens. The sensitivity of the smooth muscle in the aganglionic portion to exogenous acetylcholine and to field stimulation was found to be higher than in the ganglionic portion. While noradrenaline added to the organ bath reduced the stimulation-evoked release of acetylcholine from spastic segments, via an alpha 2 adrenoceptor mediated process, yohimbine did not enhance the release. It is suggested that in Hirschsprung's disease the increased acetylcholine release, the enhanced sensitivity of smooth muscle cells to acetylcholine, and the lack of alpha 2 adrenoceptor mediated noradrenergic modulation of acetylcholine release from cholinergic interneurons might be responsible for the spasm of aganglionic segments.

The effect of a new selective alpha 2-adrenoreceptor antagonist, idazoxan, and the agonist, clonidine, on fasting antroduodenal motility in healthy volunteers

Studies were carried out on 16 healthy male volunteers to investigate whether intravenous administration of the alpha 2-adrenoreceptor antagonist, idazoxan, could affect fasting antroduodenal motility with and without administration of the agonist, clonidine. Contractile activity was recorded using an oral tube with perfused side holes positioned in the stomach and duodenum. Clonidine decreased antral contractile activity, an effect that idazoxan did not restore. Idazoxan alone did not affect antral motility. In the duodenum, clonidine decreased the number of contractions significantly and idazoxan restored them. Idazoxan alone did not increase duodenal motility but clonidine induced phase-III activity within the first 15 min after administration. The observations indicate that regulation of antroduodenal motility is influenced by alpha 2-adrenoreceptor drugs. Idazoxan may have potential as a motility restoring drug, for example, in postoperative ileus.

Role of adrenergic alpha-receptors in regulation of acetylcholine release evoked by distension of guinea pig ileum

The significance of adrenergic nerves in the regulation of acetylcholine (ACh) release evoked by distension of the guinea pig ileum was evaluated by pharmacological manipulations. ACh release was measured by bioassay. Release in response to distension was completely abolished by epinephrine and norepinephrine. Tyramine also suppressed the distension-evoked ACh release, while dopamine was ineffective. The release of ACh in the anal segment, adjacent to the distended part, was abolished by epinephrine and norepinephrine. Dibenamine and phentolamine abolished ACh release anal to the distension, but augmented release orally, while dichloroisoproterenol and propranolol were ineffective. The present results give direct evidence that adrenergic nerves modulate cholinergic transmission in the myenteric plexus through alpha-receptors.

Phenylethylamine-induced release of noradrenaline fails to stimulate alpha 1-adrenoceptors modulating [3H]-acetylcholine release in rat atria, but activates alpha 2-adrenoceptors modulating [3H]-serotonin release in the hippocampus

The modulation of the depolarization induced release of [3H]-acetylcholine by agonists acting on alpha-adrenoceptors was studied in superfused rat atrial slices. In this model, noradrenaline and methoxamine, but not UK 14304 reduced the potassium evoked release of [3H]-acetylcholine. The inhibitory action of these drugs was antagonized by the alpha 1 selective adrenoceptor antagonist prazosin. Propranolol, idazoxan and sulpiride did not antagonize the inhibition by noradrenaline of the potassium-evoked release of [3H]-acetylcholine. Exposure to amphetamine, beta-phenylethylamine, m- or p-tyramine, increased in a concentration-dependent manner the spontaneous outflow of [3H]-noradrenaline from atrial slices. Yet, these concentrations of the indirectly acting sympathomimetic amines, tested in the presence of an inhibitor of monoamine oxidase (MAO), failed to modify the potassium evoked release of [3H]-acetylcholine. Desipramine 3 mumol/l or cocaine 10 mumol/l did not affect the release of [3H]-acetylcholine evoked by potassium stimulation. Under similar experimental conditions, beta-phenylethylamine facilitated the spontaneous outflow of [3H]-noradrenaline, and inhibited the electrically-evoked release of [3H]-serotonin from the hippocampus by activation of alpha 2-adrenoceptors. It is concluded that the release of acetylcholine from atrial cholinergic neurons can be modulated through inhibitory alpha 1-adrenoceptors, which are not activated when the release of noradrenaline is induced by indirectly acting sympathomimetic amines. In addition, amphetamine or structurally related amines do not activate directly recognition sites in the cholinergic postganglionic parasympathetic neuron to modify the release of [3H]-acetylcholine.

Effect of alpha-adrenoceptor agonists and antagonists on cholinergic transmission in guinea-pig isolated atria

Evidence was sought for the existence on cholinergic nerve terminals in guinea-pig atria of alpha-adrenoceptors subserving inhibition of acetylcholine release. The experiments were performed with atria which had been incubated with 3H-choline and transmitter release was deduced from the efflux of radioactivity elicited by field stimulation. In preparations which had been incubated with 3H-choline, field stimulation (60 pulses, 2 Hz) evoked release and radioactivity which was inhibited by 1.0 mumol/l noradrenaline, in the presence of propranolol (1.0 mumol/l), but was unaltered by clonidine (1.0 and 10.0 mumol/l). The inhibitory effect noradrenaline on the stimulation-induced efflux of radioactivity was blocked by idazoxan (0.3 mumol/l), and phentolamine (1.0 mumol/l) but not by prazosin (0.3 mumol/l). In the presence of propranolol (1.0 mumol/l), neither phentolamine (1.0 mumol/l), idazoxan (0.3 mumol/l) nor prazosin (0.3 mumol/l) had any effect on stimulation-induced efflux of radioactivity. Stimulation of the extrinsic vagus nerve of atrial preparations with trains of pulses at frequencies of 2, 4, 8, and 16 Hz produced graded decreases in the rate of atrial beating. The negative chronotropic responses to vagus stimulation were unaffected by noradrenaline (1.0 mumol/l) in the presence of propranolol (1.0 mumol/l). These findings indicate that the release of acetylcholine from the cholinergic terminals in guinea-pig atria can be inhibited by a mechanism apparently involving prejunctional alpha 2-adrenoceptors. However, under the experimental conditions used here the chronotropic responses of atria to stimulation of the extrinsic vagus nerve was not affected by activation of the prejunctional alpha 2-adrenoceptors associated with the cholinergic terminals.

Hirschsprung's disease: catecholamine content, alpha-adrenoceptors, and the effect of electrical stimulation in aganglionic colon

In order to assess abnormalities in the adrenergic mechanism in the intestine of Hirschsprung's disease, catecholamine concentrations, alpha-adrenoceptors, and the effect of electrical field stimulation were examined in aganglionic segments of colon or rectum. The aganglionic segment had a higher concentration of norepinephrine, assayed with high performance liquid chromatography with an electrochemical detector, whereas concentrations of epinephrine or dopamine were similar in normal and pathological segments. In four patients with extensive aganglionosis, the norepinephrine concentration in aganglionic colon segments decreased progressively in descending, transverse, and ascending colon. The tissue content of alpha-adrenoceptors and their affinity assayed from the specific binding of [3H]dihydro-alpha-ergocryptine appeared similar in normal and aganglionic segments of the rectosigmoidal colon. Electrical field stimulation of normal rectosigmoidal colon segments caused relaxation at low frequencies and contraction at a very high frequency. Relaxation was not abolished by blocking concentrations of propranolol or phentolamine. In aganglionic segments, the predominant response to electrical field stimulation was contraction, which was inhibited by either atropine or tetrodotoxin. These results indicate that an alpha-adrenergic system and cholinergic innervation apparently exist in aganglionic colon segments and that dysfunction of the colon appears to result from lack of a nonadrenergic inhibitory system.

Subsensitivity of enteric cholinergic neurones to alpha 2-adrenoceptor agonists after chronic sympathetic denervation

The concentration-effect relationships of noradrenaline, dopamine and clonidine in inhibiting resting and stimulated acetylcholine output have been studied in intact and in sympathetically denervated preparations of guinea pig isolated distal colon. The order of potencies for the inhibition of resting acetylcholine release in intact preparations was clonidine greater than dopamine greater than noradrenaline while the order of intrinsic activities was noradrenaline greater than dopamine greater than clonidine. Sympathetic denervation was able to modify the potency of either clonidine, dopamine and noradrenaline. Noradrenaline was 6 times more potent in inhibiting resting acetylcholine release in denervated than in intact preparations, while clonidine and dopamine underwent a 18-fold and a 11-fold decrease in potency after denervation. The potency of clonidine relative to noradrenaline was 110 in intact preparations and only 1.2 in denervated organs. The intrinsic activities of noradrenaline, dopamine and clonidine were almost unchanged in denervated organs. A dose-dependent facilitatory effect of yohimbine on both the resting acetylcholine output and the peristaltic reflex could be observed in intact but not in sympathetically denervated preparations at concentrations ranging from 2.5 X 10(-8) M to 2.5 X 10(-7) M. Yohimbine was able to counteract the inhibitory effect of dopamine and to remove the inhibitory effect of periarterial nerve stimulation on both acetylcholine release and the peristaltic reflex. Our results are consistent with the existence of a tonic physiological modulation of enteric cholinergic neurones by postganglionic sympathetic fibres. The order of potencies of adrenoceptor agonists and the antagonism by yohimbine is consistent with such a modulation being entirely carried out through alpha 2-heteroceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the integrated extrinsic nervous control of rectal motility in the cat

The effect of sympathetic nervous activity on rectal motility induced by pelvic nerve stimulation (PNS) was studied in anaesthetized cats. Division of the sympathetic lumbar colonic and hypogastric nerves or alpha-adrenoreceptor blockade, both of which reduced rectal tone, also reversed a predominantly relaxatory pelvic nerve response into a pure contraction. Contractions to pelvic nerve stimulation were reduced by simultaneous lumbar colonic nerve stimulation. This lumbar colonic nerve-induced inhibition was augmented by alpha-adrenoceptor blockade and abolished by beta-blockade. Close intra-arterial injection of a beta-adrenergic agonist reduced contractions to PNS, while an alpha-adrenergic agonist had no effect. Stimulation of the hypogastric nerves enhanced rectal contractions to simultaneous PNS. The apparent similarity with the arrangement of extrinsic nervous control of the internal anal sphincter suggests that the rectum is functionally involved in continence mechanisms.

Pelvic afferent reflex control of rectal motility and lumbar colonic efferent discharge mediated by the pontine sympatho-inhibitory region in guinea pigs

Rectal motility and the efferent discharge of lumbar colonic nerves (LCED) have previously been shown to be affected by reflex activity activated by rectal stimulation. The sensory limb of this reflex is represented by afferent fibers in pelvic nerves. The present study revealed that this reflex is modulated by supraspinal sympatho-inhibitory regions. Pelvic afferent stimulation led to rectal contraction through the withdrawal of a tonic inhibitory influence of lumbar colonic nerves. The supraspinal region responsible for this antagonism of the rectal-inhibitory colonic nerve activity was localized to the pons. Neither the intravenous administration of atropine nor that of guanethidine (and Eisai compound 865-123, another adrenergic neuron blocking agent) effected the ability of pelvic afferent stimulation to inhibit tonic discharge of lumbar colonic efferent nerves; nevertheless, both agents eliminated the mechanical response of the rectum to stimulation of pelvic afferents. These observations suggest that lumbar sympathetic nerves may tonically inhibit the release of acetylcholine from excitatory neurons in the rectal myenteric plexus. We conclude that descending fibers from the pons are activated as a result of pelvic afferent nerve stimulation. These descending pontine fibers in turn inhibit the firing of sympathetic lumbar colonic nerves. Removal of this tonic restraint leads to rectal contraction.

Beta adrenergic influence on esophageal and colonic motility in man

Gastrointestinal (GI) motility is centrally controlled through the sympathetic and parasympathetic nerves, sympathetic effects being partly mediated by beta adrenoceptors. Although beta adrenoceptor agonists and antagonists are widely used for different disorders, little is known about the influence of these agents on GI motility. The present study was initiated to investigate whether there is a physiological, beta adrenergic influence on human GI motility and to describe the effects of selective beta adrenoceptor stimulation on motility in the proximal and distal parts of the GI tract. Esophageal peristalsis was measured in healthy subjects using electronic catheters. Distal colonic motility was measured with an open-tipped, water-perfused catheter in the sigmoid colon and from an air-filled balloon in the rectum in healthy subjects and in patients with the irritable bowel syndrome (IBS). In one study, colonic motility was stimulated with continuous infusion of the octapeptide of cholecystokinin (CCK-OP). Esophagus: Peristaltic amplitude was increased in the distal smooth muscle part of the esophageal body after infusion of both the nonselective beta blocker propranolol and the beta-1 selective blocker metoprolol. After infusion of the beta-1 agonist prenalterol and the beta-2 selective agonist terbutaline, a profound decrease in esophageal peristaltic amplitude was seen. Pretreatment with metoprolol selectively blocked the response to a moderate dose of prenalterol but did not block the response to terbutaline. The latter response was blocked by propranolol. Peristaltic velocity in the proximal part of the esophagus was decreased by beta-1 stimulation and in the distal part by beta-2 stimulation. Distal colon: In healthy subjects the sigmoid motility index showed a dose-dependent increase after metoprolol and propranolol, respectively. The increase was more marked after propranolol infusion. Terbutaline decreased the sigmoid motility index both in healthy subjects and in patients with the IBS. Furthermore, the rectal motility index was decreased in the group of healthy subjects. The effects of prenalterol on rectal and sigmoid motility did not differ from those of placebo. The IBS patient group showed larger intraindividual variations in sigmoid motility from day to day and also lower rectal motility indices than the healthy subjects. Infusion of CCK-OP increased the sigmoid motility index compared to non-stimulated conditions. No effects on CCK-OP stimulated motility were seen after either terbutaline, prenalterol or placebo.(ABSTRACT TRUNCATED AT 400 WORDS)

Interactions between splanchnic and vagus nerves in the control of mean intragastric pressure in the ferret

To determine whether splanchnic nerves relax the stomach by direct or indirect mechanisms, ramp inflations of the stomach, section and electrical stimulation of the vagus and greater splanchnic nerves, and step inflations of the duodenum were used. A high threshold, sustained inhibition of the gastric pressure response to ramp inflation was mediated by the vagus. Prior splanchnectomy increased vagal inhibition. The greater splanchnic nerves had no effect on gastric responses to inflation, although after vagotomy they were shown to be mediators of a low threshold, powerful but transient inhibition of the stomach. This was not dependent on intrinsic neurones with nicotinic receptors. Electrical stimulation of the greater splanchnic nerves produced a relaxation of the stomach, the magnitude of which was determined by resting pressure. Splanchnically mediated relaxation was not abolished by atropine, nor was it reduced by concurrent vagal stimulation. At submaximal levels of vagal stimulation the two nerves had a partially additive effective on relaxation. Duodenal inflation had an effect on intracorpus pressure similar to that of electrical stimulation of the greater splanchnic nerves on intragastric pressure. Reflex relaxation of the corpus evoked by duodenal distension was decreased by atropine but greatly increased by atropine coupled with vagotomy. These changes were caused by variations in resting pressure. It was concluded that the principal effect of splanchnic nerves on mean gastric pressure is direct and does not depend on inhibition of cholinergic neurones either centrally or peripherally. Evidence is presented for central interactions between the vagus and the greater splanchnic nerves in the anaesthetized ferret.

Problems and ambiguities in the identification of autonomic neurotransmitters

Although the traditional classification of autonomic neurons as either cholinergic or adrenergic is still subscribed to in most textbooks of physiology, it is apparent that many of the postganglionic neurons do not fall readily into either of these categories. Upwards of a dozen other transmitter candidates have been proposed, but in no instance has the identity of a non-cholinergic, non-adrenergic autonomic transmitter been established. In this paper some of the problems and ambiguities relating to satisfaction of the classical criteria for neurotransmitter identification are briefly surveyed, with special reference to the neuropeptides.

Pre- and postjunctional effects of clonidine- and oxymetazoline-like compounds in guinea-pig ileal preparations

1 Noradrenaline and 28 imidazolidine (clonidine-like) and imidazoline (oxymetazoline-like) compounds with various phenyl ring substituents have been examined for their ability to inhibit responses to transmural stimulation and exogenous acetylcholine in ileal preparations from reserpine-treated guinea-pigs.2 The bathing solution contained propranolol, mepyramine, cimetidine and desipramine to preclude interference with the responses by other than the alpha-receptor-mediated actions of the compounds.3 In transmurally stimulated preparations the inhibitory response to noradrenaline is due to a combination of prejunctional alpha-adrenoceptor stimulation and a postjunctional depressant effect that does not involve adrenoceptor activation.4 Of the 28 imidazolidines and imidazolines studied, 21 inhibited transmurally elicited responses. In the various compounds studied this effect involved actions at pre- or postjunctional sites as indicated by (a) the frequency-dependence of the inhibitory response, (b) its susceptibility to blockade by alpha-receptor antagonists and (c) the relative concentrations required to inhibit responses to transmural stimulation and exogenous acetylcholine.

Aminergic neurons: state control and plasticity in three model systems

Aminergic neurons have particular functions in many systems, and in this review their role is discussed and compared in three systems: those parts of the central nervous system controlling sleep and waking in the cut; the superior cervical ganglion: and the isolated nervous system of Aplysia. In the cat the aminergic neurons are most important in a waking state during which time external information is received, processed, and can be retrieved, and during which time habituation and sensitization occur. Aminergic neurons appear to have similar roles in state control in plasticity in both the Aplysia nervous system and the superior cervical ganglion. The striking similarities in the role of aminergic neurons in these three systems support the speculation that aminergic neurons have uniquely important roles in regulation of the plastic properties of neurons.

Pre- and postjunctional effects of prostaglandin E2, prostaglandin synthetase inhibitors and atropine on cholinergic neurotransmission in guinea pig ileum and bovine iris

The effects of prostaglandin E2, arachidonic acid, prostaglandin synthetase inhibitors and atropine on cholinergic neuromuscular transmission were examined in isolated guinea pig ileum longitudinal muscle and bovine iris sphincter muscle. Prostaglandin E2 and arachidonic acid markedly enhanced contraction responses induced by nerve stimulation. In addition, prostaglandin E2, enhanced contraction responses to acetylcholine and direct muscle stimulation, approximately to the same extent as those to nerve stimulation. Prostaglandin synthetase inhibitors (indomethacin, meclofenamic acid and eicosatetraynoic acid) very effectively reduced contraction responses to nerve stimulation or acetylcholine, and they annulled the stimulant effect of arachidonic acid. Basal and nerve-induced efflux of acetylcholine from the eserinized tissues, as measured by mass fragmentography, was unaltered by prostaglandin E2, but diminished slowly during indomethacin. Subsequent prostaglandin administration caused a slight increase of nerve-induced release of acetylcholine. Atropine markedly increased overflow of acetylcholine form stimulated preparations of both types. This indicates that muscarinic receptors, causing diminished acetylcholine release in eserinized tissue, may be present also at postganglionic terminals. During atropine, an effect of prostaglandin E2 on acetylcholine release could still not be seen. Thus, using physicochemical determination of acetylcholine, we could not verify earlier reports, employing bioassay, claiming enhanced release of transmitter during prostaglandin E2 treatment. However, it seems likely that prostaglandin E2 takes part in the regulation of contractility and tone of the smooth muscle cells.

Comparison of pre-junctional alpha-adrenoceptors at the neuromuscular junction with vascular post-junctional alpha-receptors in cat skeletal muscle

1 Activation of pre-junctional alpha-adrenoceptors at the skeletal neuromuscular junction enhances acetylcholine release whereas activation of such receptors at autonomic nerve endings inhibits transmitter output. In the present study the characteristics of pre-junctional alpha-adrenoceptors at motor nerve terminals have been compared with post-junctional (vascular) alpha-adrenoceptors in the cat hind limb.2 Reversal of partial (+)-tubocurarine blockade of contractions of the tibialis anterior muscle was used to monitor pre-junctional activity and increases in hindlimb vascular resistance to assess post-junctional actions at alpha-adrenoceptors.3 Responses to intra-arterial injections of noradrenaline, adrenaline, phenylephrine, oxymetazoline, methoxamine and clonidine were monitored. Dose-response lines for all the compounds except clonidine were parallel. The latter agent produced only weak and inconsistent effects.4 Ratios of the doses of the agents required to produce pre- and post-junctional effects indicated that oxymetazoline and adrenaline possessed some preferential activity at post-junctional sites, whereas the remaining agents were non-selective in their actions. If dose-ratios with respect to noradrenaline were compared at the two sites none of the compounds possessed a marked degree of selectivity.5 In the presence of phentolamine or tolazoline, dose-response curves to the pre- and post-junctional effects of phenylephrine were shifted to a similar extent. Thymoxamine showed preferential activity as a pre-junctional alpha-receptor antagonist.6 In comparing the results of this study with those of other authors, it is apparent that there are marked differences in the characteristics of pre-junctional alpha-receptors at the skeletal neuromuscular junction and at autonomic nerve endings. The pre- and post-junctional alpha-receptors in skeletal muscle show less divergence.

Presynaptic inhibitory effects of catecholamines on cholinergic transmission in the smooth muscle of the chick stomach

In the vagus nerve--smooth muscle preparation isolated from the chick proventriculus, adrenaline, clonidine (10(-8) - 2.5 x 10(-7) M), noradrenaline (10(-7) - 2.5 x 10(-6) M) and dopamine (10(-5) - 10(-4) M) inhibited the contraction induced by low frequency (0.5 Hz) stimulation of the vagus nerve, but they did not inhibit the contraction elicited by acetylcholine (5 x 10(-8) - 5 x 10(-7) M). The concentration producing 50% inhibition was 10(-7) M for adrenaline and clonidine, 10(-6) M for noradrenaline, and 5 x 10(-5) M for dopamine. Isoproterenol (5 x 10(-8) - 5 x 10(-7) M) inhibited the responses induced by both stimulation of the vagus nerve and acetylcholine. The inhibitory effects of the catecholamine and clonidine were blocked by phentolamine (2.7 x 10(-6) M) but not by 5-(3-tert-Butylamino-2-hydroxy)-propoxy-3, 4-dihydrocarbostyril hydrochloride (OPC 1085) which blocked the effect of isoproterenol. It is suggested that presynaptic alpha-receptors are present in the myenteric plexus of the chick proventriculus, and that the catecholamines and clonidine exert their inhibitory effects on cholinergic transmission via these receptors.

Noradrenaline inhibition of acetylcholine release from guinea-pig brain

The effect of noradrenaline (NA) on acetylcholine (ACh) release from guinea-pig brain was investigated in superfused cerebral cortex slices and in unrestrained unanaesthetized animals provided with epidural cups. The amine reduced the ACh release from electrically stimulated tissue and its effect was antagonized by phentolamine and phenoxybenzamine, but not by propranolol and spiroperidol. The injection of NA (150 microgram) into the cerebral ventricles caused sedation, E.Co.G. synchronization and reduced ACh outflow from the parietal cortex. This inhibition was counteracted by alpha-blocking agents. A lower dose of NA (50 microgram) did not change the behaviour, but produced a late increase in ACh outflow, prevented by spiroperidol. These results fit well with the hypothesis that NA restrains, via alpha-receptors, the ACh secretion from the nerve endings and indirectly support the view that the amine reduces the firing rate of the corticopetal cholinergic neurones. The late increase in ACh outflow, observed in vivo, may be referred to secondary activation of the dopaminergic neurones, known to enhance the cortical ACh release in this animal species.

The site and receptors responsible for the inhibition by sympathetic nerves of intestinal smooth muscle and its parasympathetic motor nerves

1. The effects of three inhibitory stimuli, sympathetic nerve stimulation, noradrenaline (NA) and isoprenaline have been examined on three forms of motor activity in the rabbit colon, the response to pelvic (parasympathetic) nerve stimulation, acetylcholine (ACh) and spontaneous tone.2. The response to pelvic nerve stimulation is most effectively inhibited by sympathetic nerve stimulation, much less effectively by NA and hardly at all by isoprenaline. The sympathetic nerves can inhibit the pelvic response at frequencies of stimulation which do not affect spontaneous tone. The inhibitory effect of sympathetic stimulation, and of NA, on the pelvic response is reduced by phentolamine 5 x 10(-6)M and unaffected by propranolol 5 x 10(-6)M suggesting the effect is mediated via alpha receptors.3. The response to ACh is inhibited by all three stimuli equally. The inhibitory effect of sympathetic nerve stimulation and of isoprenaline is reduced by propranolol 5 x 10(-6)M. The inhibitory effect of NA is also reduced by propranolol but to a lesser extent. Phentolamine 5 x 10(-6)M has a small effect in reducing the inhibitory effect of sympathetic nerve stimulation or of NA. This effect of phentolamine is lost if the participation of motor nerves in the response to ACh is excluded by either tetrodotoxin 10(-7) g/ml. or cold storage for 10-14 days. These results suggest that inhibition of the ACh response takes place mainly at the muscle by activation of beta receptors but that ACh may have a small indirect stimulant action through motor nerves and this is susceptible to inhibition through alpha receptors.4. All three stimuli are equally effective in lowering smooth muscle tone. This inhibitory effect of sympathetic nerve stimulation and of isoprenaline is reduced by propranolol 5 x 10(-6)M and unaffected by phentolamine 5 x 10(-6)M. The inhibitory effect of NA is reduced by propranolol but again is less sensitive to block than the other two inhibitory stimuli. Phentolamine is without effect on the inhibitory action of NA and the combination of phentolamine with propranolol is no more effective than propranolol alone. These results suggest that NA liberated by sympathetic nerves and isoprenaline inhibit myogenic tone in the smooth muscle by an action on beta receptors but the action of NA added to the bath cannot be fully explained in this way.

Separately developing axonal uptake of 5-hydroxytryptamine and norepinephrine in the fetal ileum of the rabbit

Uptake of 5-hydroxytryptamine (5-HT) by adult and fetal rabbit's ileum was studied. The adult myenteric plexus accumulated tritium when incubated with tritiated 5-HT. However, in addition to labeled 5-HT, tritiated 5-hydroxyindole acetic acid and, when monoamine oxidase (MAO) was inhibited, 5-HT-o-glucuronide were found in the tissue. Two uptake processes differing in affinity could be defined. Only the high affinity process was saturable. Fetal ileum took up tritiated 5-HT but glucuronidation did not occur when MAO was inhibited. The uptake of tritiated 5-HT by the fetal ileum was due to a single, saturable, temperature sensitive (Q10 at 27-37 degress C = 2.4) process inhibited by ouabain. It was identical to the high affinity uptake found in adult tissue. This specific high affinity uptake could be found as early as the 16th day of gestation, 5-8 days before uptake of norepinephrine (NE) begins. Light and electron microscope radioautography revealed that the uptake of 5-HT was primarily into axons and a characteristic structure called the expanded process, both in the myenteric plexus. Both contained dense-cored vesicles. Axons were not labeled by tritiated NE until after 24 days and the expanded process was never labeled by tritiated NE. This study shows that uptake of 5-HT is a property of distinct system of axons in the mammalian myenteric plexus which develops prior to adrenergic axons during ontogeny.

Adrenoreceptors of the guinea-pig urinary bladder

1 Adrenaline, noradrenaline and isoprenaline (5 mug/ml) did not affect the resting tone of the isolated urinary bladder of the guinea-pig. 2 The catecholamines (1-2 mug/ml) inhibited neuronally evoked contractions at various stimulation frequencies; the inhibition was maximum at 2 Hz and minimum at 50 Hz. Isoprenaline produced maximum inhibition. 3 Propranolol (0.5 mug/ml) completely blocked the catecholamine-induced inhibition at all the frequencies employed. The concentration-response curves of isoprenaline at 2, 10 and 50 Hz were characteristically shifted by propranolol (50 ng/ml). Phenoxybenzamine (0.2 mug/ml) was totally ineffective. 4 In some experiments adrenaline significantly raised the tone of the bladder exposed to propranolol; this effect could be blocked by phenoxybenzamine. 5 Acetylcholine-induced bladder contractions were inhibited by adrenaline (2 mug/ml); the inhibition was completely blocked by propranolol (0.5 mug/ml). 6 The results indicate the presence of an inhibitory beta-adrenoceptor and suggest the possibility of an excitatory alpha-adrenoceptor in guinea-pig urinary bladder.

A study of peripheral input to and its control by post-ganglionic neurones of the inferior mesenteric ganglion

1. Intracellular recordings were made, in vitro, from neurones of guinea-pig inferior mesenteric ganglia (IMG) attached, via the lumbar colonic nerves, to segments of distal colon. 2. 'Spontaneous' synaptic input from colonic afferent fibres was observed in 79% of the neurones tested. In any given preparation, the level and pattern of this synaptic input to different neurones varied considerably. 3. Superfusion of colonic segments with drugs (papaverine, isoprenaline, and adenosine triphosphate) which reduce colonic motility decreased colonic afferent input to IMG neurones. 4. Superfusion of colonic segments with acetylcholine or stimulation of pelvic nerves, both of which increase colonic motility, increased colonic afferent input to IMG neurones. 5. Superfusion of colonic segments with either atropine or tubocurarine reduced the level of 'spontaneous', colonic afferent input. However, distension of these relaxed segments increased the colonic afferent input. 6. Repetitive stimulation of preganglionic inputs to the IMG inhibited afferent input from drug relaxed segments of colon that were moderately distended by the injection of air into the lumen. Superfusion of the colon with phentolamine blocked this inhibition. 7. The results of this study suggest that IMG neurones receive afferent input from mechanoreceptors located in the distal colon and that the mechanosensitivity of this afferent pathway is in part controlled by efferent noradrenergic neurones of the IMG. The IMG-colon neural circuitry can therefore be considered to form a feed-back control system which participates in the regulation of colonic motility.

Some observations on the intrinsic nervous mechanism in Hirschsprung's disease

Both at rest and during transmural stimulation acetylcholine output from isolated longitudinal and circular muscle strips is significantly higher in the spastic segment than in the proximal dilated bowel. No difference has been found in the tissue concentration of acetylcholine between ganglionic and aganglionic specimens. The pattern of response to transmural stimulation is also similar in the spastic and dilated bowel. However, after cholinergic and adrenergic blockade transmural stimulation fails to induce relaxation in aganglionic specimens, as it does in normal colon. The hypotheses are advanced that the increase in acetylcholine output may be partly dependent on a failure of the intrinsic modulating mechanisms and that an alteration of the non-adrenergic inhibitory neurons may be involved in the motor disturbances of the aganglionic tract.