Ultrastructural identification of noradrenergic axons and their distribution within the enteric plexuses of the guinea-pig small intestine

Individual sympathetic postganglionic neurons coinnervate myenteric ganglia and smooth muscle layers in the gastrointestinal tract of the rat

A full description of the terminal architecture of sympathetic axons innervating the gastrointestinal (GI) tract has not been available. To label sympathetic fibers projecting to the gut muscle wall, dextran biotin was injected into the celiac and superior mesenteric ganglia (CSMG) of rats. Nine days postinjection, animals were euthanized and stomachs and small intestines were processed as whole mounts (submucosa and mucosa removed) to examine CSMG efferent terminals. Myenteric neurons were counterstained with Cuprolinic Blue; catecholaminergic axons were stained immunohistochemically for tyrosine hydroxylase. Essentially all dextran-labeled axons (135 of 136 sampled) were tyrosine hydroxylase-positive. Complete postganglionic arbors (n = 154) in the muscle wall were digitized and analyzed morphometrically. Individual sympathetic axons formed complex arbors of varicose neurites within myenteric ganglia/primary plexus and, concomitantly, long rectilinear arrays of neurites within circular muscle/secondary plexus or longitudinal muscle/tertiary plexus. Very few CSMG neurons projected exclusively (i.e., ∼100% of an arbor's varicose branches) to myenteric plexus (∼2%) or smooth muscle (∼14%). With less stringent inclusion criteria (i.e., ≥85% of an axon's varicose branches), larger minorities of neurons projected predominantly to either myenteric plexus (∼13%) or smooth muscle (∼27%). The majority (i.e., ∼60%) of all individual CSMG postganglionics formed mixed, heterotypic arbors that coinnervated extensively (>15% of their varicose branches per target) both myenteric ganglia and smooth muscle. The fact that ∼87% of all sympathetics projected either extensively or even predominantly to smooth muscle, while simultaneously contacting myenteric plexus, is consistent with the view that these neurons control GI muscle directly, if not exclusively. J. Comp. Neurol. 524:2577-2603, 2016. © 2016 Wiley Periodicals, Inc.

Alpha2-adrenoceptors couple to inhibition of R-type calcium currents in myenteric neurons

Alpha2-adrenoceptors inhibit Ca2+ influx through voltage-gated Ca2+ channels throughout the nervous system and Ca2+ channel function is modulated following activation of some G-protein coupled receptors. We studied the specific Ca2+ channel inhibited following alpha2-adrenoceptor activation in guinea-pig small intestinal myenteric neurons. Ca2+ currents (I(Ca2+)) were studied using whole-cell patch-clamp techniques. Changes in intracellular Ca2+ (delta[Ca2+]i) in nerve cell bodies and varicosities were studied using digital imaging where Ca2+ influx was evoked by KCl (60 mmol L(-1)) depolarization. The alpha2-adrenoceptor agonist, UK 14 304 (0.01-1 micromol L(-1)) inhibited I(Ca2+) and delta[Ca2+]i; maximum inhibition of I(Ca2+) was 40%. UK 14 304 did not affect I(Ca2+) in the presence of SNX-482 or NiCl2 (R-type Ca2+ channel antagonists). UK 14 304 inhibited I(Ca2+) in the presence of nifedipine, omega-agatoxin IVA or omega-conotoxin, inhibitors of L-, P/Q- and N-type Ca2+ channels. UK 14 304 induced inhibition of I(Ca2+) was blocked by pertussis toxin pretreatment (1 microg mL(-1) for 2 h). Alpha2-adrenoceptors couple to inhibition of R-type Ca2+ channels via a pertussis toxin-sensitive pathway in myenteric neurons. R-type channels may be a target for the inhibitory actions of noradrenaline released from sympathetic nerves on to myenteric neurons.

Structure of peripheral synapses: autonomic ganglia

Final motor neurons in sympathetic and parasympathetic ganglia receive synaptic inputs from preganglionic neurons. Quantitative ultrastructural analyses have shown that the spatial distribution of these synapses is mostly sparse and random. Typically, only about 1%-2% of the neuronal surface is covered with synapses, with the rest of the neuronal surface being closely enclosed by Schwann cell processes. The number of synaptic inputs is correlated with the dendritic complexity of the target neuron, and the total number of synaptic contacts is related to the surface area of the post-synaptic neuron. Overall, most neurons receive fewer than 150 synaptic contacts, with individual preganglionic inputs providing between 10 and 50 synaptic contacts. This variation is probably one determinant of synaptic strength in autonomic ganglia. Many neurons in prevertebral sympathetic ganglia receive additional convergent synaptic inputs from intestinofugal neurons located in the enteric plexuses. The neurons support these additional inputs via larger dendritic arborisations together with a higher overall synaptic density. There is considerable neurochemical heterogeneity in presynaptic boutons. Some synapses apparently lack most of the proteins normally required for fast transmitter release and probably do not take part in conventional ganglionic transmission. Furthermore, most preganglionic boutons in the ganglionic neuropil do not form direct synaptic contacts with any neurons. Nevertheless, these boutons may well contribute to slow transmission processes that need not require conventional synaptic structures.

Angiotensin receptors and actions in guinea pig enteric nervous system

Actions of ANG II on electrical and synaptic behavior of enteric neurons in the guinea pig small intestine were studied. Exposure to ANG II depolarized the membrane potential and elevated neuronal excitability. The number of responding neurons was small, with responses to ANG II in 32% of submucosal neurons and 25% of myenteric neurons. Hyperpolarizing responses were evoked by ANG II in 45% of the neurons. The hyperpolarizing responses were suppressed by alpha2-noradrenergic receptor antagonists, which suggested that the hyperpolarizing responses reflected stimulation of norepinephrine release from sympathetic neurons. Exposure to ANG II enhanced the amplitude and prolonged the duration of noradrenergic inhibitory postsynaptic potentials and suppressed the amplitude of both fast and slow excitatory postsynaptic potentials. The selective ANG II(1) receptor (AT1R) antagonists, ZD-7115 and losartan, but not a selective AT2R antagonist (PD-123319), suppressed the actions of ANG II. Western blot analysis and RT-PCR confirmed expression of AT1R protein and the mRNA transcript for the AT1R in the enteric nervous system. No expression of AT2R protein or mRNA was found. Immunoreactivity for AT1R was expressed by the majority of neurons in the gastric antrum and small and large intestine. AT1R immunoreactivity was coexpressed with calbindin, choline acetyltransferase, calretinin, neuropeptide Y, and nitric oxide synthase in subpopulations of neurons. The results suggest that formation of ANG II might have paracrine-like actions in the enteric nervous system, which include alterations in neuronal excitability and facilitated release of norepinephrine from sympathetic postganglionic axons. The enhanced presence of norepinephrine is expected to suppress fast and slow excitatory neurotransmission in the enteric microcircuits and to suppress neurogenic mucosal secretion.

Involvement of a purinergic pathway in the sympathetic regulation of motility in rat ileum

We investigated extrinsic neuronal regulation of intestinal motility. The mesenteric nerve stimulation (MNS; duration 0.5 ms, 10 Hz for 30 s) evoked relaxation in the longitudinal muscle direction of the isolated rat ileum. The MNS-induced relaxation was abolished by guanethidine (2 microM) or propranolol (10 microM), but was not affected by prazosin (10 microM), rauwolscine (10 microM), hexamethonium (100 microM) or capsaicin (1 microM). Exposure to a high concentration (100 microM) of ATP (ATP-desensitization) or ADP (ADP-desensitization) reduced the MNS-induced relaxation to 44.7% or 32.5% of the control (P<0.01), respectively. P2 purinoceptor antagonists [suramin (100 microM) and reactive blue-2 (RB-2, 50 microM)] or small conductance Ca(2+)-activated K(+) channel blocker, apamin (0.5 microM), significantly decreased the relaxation to 54.4% and 25.6% or 19.4% of the control (P<0.01), respectively, whereas selective P2Y(1) purinoceptor antagonist MRS2179 (10 microM) failed to affect the relaxation. Furthermore, exogenous ATP (1 microM) or ADP (1 microM) elicited relaxation in the rat ileum, which was almost abolished by reactive blue-2 (50 microM, 9.1% of control remained, P<0.05). In contrast, relaxation induced by noradrenalin (10 microM) was not antagonized by ATP-desensitization, apamin (0.5 microM) or reactive blue-2 (50 microM). From the present results, we conclude that noradrenergic sympathetic nerves might regulate intestinal motility mediated through a purinergic inhibitory neuronal pathway in the rat small intestine.

GI distension in severe ulcerative colitis

OBJECTIVES

In previous retrospective studies in patients with severe ulcerative colitis (UC), small bowel distension was found to characterize a subgroup of patients at higher risk for both toxic megacolon (TMC) and multiple organ dysfunction syndrome (MODS). In this study we prospectively evaluated the prevalence of GI distension and its relationship to clinical outcome in patients with severe UC.

METHODS

Of 109 consecutive inpatients with acute UC (admitted to the GI Unit of the University of Rome during the period 1995-2000), 45 had severe colitis. Routine blood tests and acid-base balance and plain abdominal film evaluations were performed upon admission and repeated every 1-3 days. The gas content of the stomach and small and large intestines was evaluated on plain abdominal films. All patients were submitted to the standard Oxford intensive medical regimen; clinical improvement, occurrence of major complications, need for surgery, and mortality were evaluated. Statistical analysis was carried out using Student's t, chi2, Fisher's exact, Mann-Whitney, and Wilcoxon rank sum tests, when appropriate.

RESULTS

Of 45 patients with severe UC, 24 (53%) had GI distension. Three of these 24 patients had TMC on admission (all underwent surgery and survived), 21 showed increased GI gas content (four developed TMC 1-4 days after the detection of GI distension and were operated on, two developed MODS and died, and eight did not improve but were submitted to surgery and survived). None of the 21 patients with normal GI gas content had complications; all survived (five did not improve and required surgery).

CONCLUSIONS

In severe UC, persistent GI distension characterized a subgroup of patients with poor response to medical therapy and at higher risk for TMC and of need for surgery. The development of MODS was the most important predicting factor for fatal outcome.

In vitro loading of human synovial membrane with 5-hydroxydopamine: evidence for dense core secretory granules in type B cells

Ultrastructural studies of the synovial membrane were performed on tissue samples obtained from the human lumbar facet joint. Ultrastructural changes in synoviocytes were studied after loading synovial samples with 5-hydroxydopamine (5-OHDA) in an oxygenated Krebs' solution, prior to fixation. Synoviocytes were set loosely in the intimal matrix and classified into type A (phagocytic) and type B (secretory) cells. In general, type A cells populated the surface of the synovial lining, whereas type B cells were located deeper in the tissue, extending a process into the synovial fluid. Type B cells in control samples contained sparse secretory granules. Free nerve endings were not found in the synovial intima. In response to incubation in 5-OHDA, a precursor of biogenic monoamines, synoviocytes clustered and established contact. The ultrastructure of type B cells in the loaded group clearly differed from controls. They possessed typical membrane-bound vesicles, containing an electron dense interior surrounded by a lucent space. The size of these dense core vesicles ranged from 100 to 260 nm (on average 180 nm). They were in relation to microtubules and located preferentially in the marginal area of the cytoplasm, close to the Golgi complex. The ultrastructure of type A cells was not significantly altered. The present observations provide morphological evidence for the amine-handling properties of type B cells, indicating that they might be added to the list of 'APUD' cells of the diffuse neuroendocrine system. A recepto-secretory function for type B cells is discussed.

Specialised sympathetic neuroeffector associations in immature rat iris arterioles

Sympathetic nerve-mediated vasoconstriction in iris arterioles of mature rats occurs via the activation of alpha(1B)-adrenoceptors alone, while in immature rat iris arterioles, vasoconstriction occurs via activation of both alpha1- and alpha2-adrenoceptors. In mature rats the vast majority of sympathetic varicosities form close neuroeffector junctions. Serial section electron microscopy of 14 d iris arterioles has been used to determine whether restriction in physiological receptor types with age may result from the establishment of these close neuroeffector junctions. Ninety varicosities which lay within 4 microm of arteriolar smooth muscle were followed for their entire length. Varicosities rarely contained dense cored vesicles even after treatment with 5-hydroxydopamine. 47 % of varicosities formed close associations with muscle cells and 88 % formed close associations with muscle cells or melanocytes. Varicosities in bundles were as likely as single varicosities to form close associations with vascular smooth muscle cells, although the distribution of synaptic vesicles in single varicosities did not show the asymmetric accumulation towards the smooth muscle cells seen in the varicosities in bundles which were frequently clustered together. We conclude that restriction of physiological receptor types during development does not appear to correlate with the establishment of close neuroeffector junctions, although changes in presynaptic structures may contribute to the refinement of postsynaptic responses.

Duodenal neurons provide nicotinic fast synaptic input to sphincter of Oddi neurons in guinea pig

We have investigated the existence of neural connections between the duodenum and the sphincter of Oddi (SO). Stimulation of duodenal myenteric fiber bundles elicited synaptic responses in SO neurons, which included nicotinic fast excitatory postsynaptic potentials (EPSPs), slow EPSPs, and alpha(2)-adrenoreceptor-mediated inhibitory postsynaptic potentials. After 48 h in organ culture, when extrinsic fibers had diminished, only the fast EPSPs persisted. Duodenal mucosal stimulation also elicited nicotinic fast EPSPs in SO neurons. There was no association between the SO neurons that received duodenal input and their chemical coding. A reciprocal projection also exists from the SO to the duodenum. In acute and cultured preparations, duodenal myenteric stimulation caused antidromic responses in 20% of SO neurons. Furthermore, 45.6 +/- 10.5 neurons in SO ganglia were retrogradely labeled from dye application sites in the duodenum. It is proposed that bidirectional neural communication occurs between the duodenum and the SO and that duodenal neurons provide excitatory fast synaptic input to SO neurons through a reflex that can be activated at the duodenal mucosa.

Ultrastructure of intramural ganglia in the striated muscle portions of the guinea pig oesophagus

The ultrastructure of the myenteric plexus located in the striated muscle portion of the guinea pig oesophagus was examined and compared with that of the plexus associated with the smooth muscle portion of the rest of the digestive tract. The oesophageal ganglia had essentially the same architecture as those of the smooth muscle portion, such as a compact neuropil without the intervention of connective tissue and blood vessels. Some features, however, were particular to the striated muscle part of the oesophagus. It was clearly demonstrated that myelinated fibres, probably sensory terminals of vagal origin, join the myenteric ganglia. Synapses and terminal varicosities are sparsely distributed within the ganglia and fewer morphological types of axon varicosities could be distinguished compared with other regions. Glial cells are well developed in the oesophageal myenteric ganglia. These cells outnumber the ganglion cells, having a higher ratio than in the lower digestive tract, and form numerous cytoplasmic lamellar processes. The lamellar processes, located at the surface of the ganglia, considerably reduce the area of neuronal membrane which directly contacts the basal lamina. The role of these lamellar processes in the oesophageal ganglia is discussed.

Specialised sympathetic neuroeffector associations in rat iris arterioles

Vascular sympathetic neuroeffector associations have been examined in rat iris arterioles using serial section electron microscopy and reconstruction techniques. Examination of random sections showed that, of all profiles of varicosities (199) seen to lie closer than 4 microns to vascular smooth muscle cells, only a small proportion (29/199) were found in close association with vascular smooth muscle cells, where adjacent membranes were separated by less than 100 nm. However, serial section examination, from intervaricose region to intervaricose region, of 79 varicosities similarly observed lying within 4 microns of vascular smooth muscle cells showed that 54 formed close associations with vascular smooth muscle cells. In serial sections, all these varicosities were also closely associated with melanocytes and of the 25 remaining varicosities, 22 formed close associations with melanocytes alone, whilst 3 did not come into close association with any effector cell. The increased observation of close associations with vascular smooth muscle cells in serial sections, compared with random sections, is consistent with the demonstration that the area of contact only occupies, on average, a small percentage (5%) of the total surface area of the varicosity as seen in the 3-dimensional reconstructions. In both random and serial sections, close associations were observed between varicosities and vascular smooth muscle cells or melanocytes irrespective of whether fibres were present singly or in small nerve bundles. Three-dimensional reconstruction of associations of varicosities and vascular smooth muscle cells demonstrated several common features, such as accumulations of synaptic vesicles and loss of Schwann cell covering at the region of membrane facing the effector cell. The similarity in the appearance of the neuroeffector association seen in this study and those described in previous studies provides evidence for the existence of a common sympathetic neuroeffector association, irrespective of the receptor subtype involved in neurotransmission.

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.

Ultrastructural examination of the targets of serotonin-immunoreactive descending interneurons in the guinea pig small intestine

Serotonin neurons are descending interneurons in the myenteric plexus of the guinea pig small intestine. Preembedding single- and double-label immunocytochemistries at the ultrastructural level were used to identify the targets of these serotonin interneurons. Serial ultrathin sections were taken through a myenteric ganglion that had been processed for serotonin immunocytochemistry. The ganglion contained the cell bodies of 69 neurons, including 2 serotonin neurons and 6 neurons with the ultrastructural features of Dogiel type II cells. For each cell body in the ganglion, the number of serotonin inputs (synapses and close contacts) was determined. About 59% of the cell bodies did not receive any serotonin inputs. The most abundant serotonin terminals were related to two targets: other serotonin descending interneurons and a population of neurons with Dogiel type I morphology, but whose neurochemistry and function is unknown. The serotonin inputs to the serotonin cell bodies were located predominantly on the lamellar dendrites. Each of the Dogiel type II neurons received 3 or fewer serotonin inputs, and none of the serotonin inputs to Dogiel type II neurons formed a synapse. Overall, about 40% of the serotonin inputs formed synapses. The serotonin inputs to neurons that received many serotonin inputs were more likely to show synaptic specializations than serotonin inputs to neurons that received few serotonin inputs. Inhibitory motor neurons contain nitric oxide synthase (NOS). At the light microscope level, serotonin nerve fibers do not form dense pericellular baskets around NOS cell bodies. To determine whether there are serotonin inputs to NOS neurons, serial ultrathin sections were taken through a myenteric ganglion that had been processed for preembedding double-label immunocytochemistry, in which the NOS neurons were labeled with peroxidase-diaminobenzidine and the serotonin neurons with silver-intensified 1 nm gold. Only 1 out of 9 NOS cells examined in serial section received more than 5 serotonin inputs. The results suggest that, in the guinea pig small intestine, the serotonin descending interneurons are not an essential element of the descending inhibitory reflex.

Sensory ganglia as a target of autonomic and sensory nerve fibres in the guinea-pig

The distribution of neuropeptide- (neuropeptide Y, substance P, vasoactive intestinal peptide) and catecholamine-synthesizing enzyme-immunoreactive axons in guinea-pig trigeminal, nodose, and cervical dorsal root ganglia was studied by double-labelling immunofluorescence in controls and after extirpation of either the cervical sympathetic trunk or the stellate ganglion; tyrosine hydroxylase- and dopamine-beta-hydroxylase-immunoreactive terminals in dorsal root ganglia were ultrastructurally investigated. Six neurochemically identifiable axons innervated the trigeminal ganglion, five kinds were found in the nodose and dorsal root ganglia. Two of them (catecholaminergic with and without neuropeptide Y) were of sympathetic origin and, besides their termination at arteries, provided a direct innervation of capsule cells of the trigeminal and cervical dorsal root ganglia facing the subarachnoid space. Varicosities which were interpreted as being of sensory origin were equally numerous in all ganglia, whereas those being likely of parasympathetic origin decreased in numbers from the trigeminal to the dorsal root and nodose ganglia. It is concluded that the sensory ganglia are the target of postganglionic sympathetic, parasympathetic and primary afferent neurons, each of which are specifically organized with respect to the neurochemical phenotype and inter- and intraganglionic distribution. Among other targets, these "nervi gangliorum" appear to be intimately linked to the ganglionic capsular cells and meningeal sheaths facing the liquor spaces.

Sympathetic innervation of the surface of the mouse vas deferens

The distribution of axons and axon varicosities on the surface of the mouse vas deferens has been determined following fluorescence of these structures with 3,3-diethyloxardicarbocyanide (DiOC2 (5)) to locate varicosities and FAGLU to detect catecholamine containing nerves. Small bundles of fluorescent axons treated with DiOC2(5), were shown to give rise to single or very small bundles of 2 or 3 varicose axons that passed over the surface of the muscle bundles. Varicosities had the same average diameter of 0.9 microns, length of 1.1 micron and spacing apart of 4.6 microns whether identified following DiOC2(5) fluorescence or the FAGLU method for catecholamines and this was shown statistically to imply that they came from the same population of varicosities. Serial thin sections through small axon bundles and single axons, viewed with the electron microscope, confirmed the dimensions of varicosities along axons observed following DiOC2(5) staining or use of the FAGLU method.

Morphology of synapses in the autonomic nervous system

The ultrastructure of synapses in the autonomic nervous system is reviewed. The synaptic organization of the parasympathetic ganglia is relatively simple. Preganglionic axons form synapses either on the soma or on short perikaryal processes of the ganglionic neurons. The presynaptic terminals have a cholinergic morphology and contain mainly small clear vesicles with a few large dense cored vesicles. A few neuropeptides have been localized to the large dense cored vesicles of these terminals. The postganglionic parasympathetic axons ramify within their target tissues where they form close associations, but not true synaptic contacts. Sites of release of transmitter are recognized morphologically as varicosities along the length of the axon that contain clusters of small clear vesicles with a few large dense cored vesicles. The organization of the sympathetic nervous system is somewhat more complex. In addition to acetylcholine, enkephalin also exists in these terminals, probably in the large dense cored vesicles. There are at least three types of ganglion cell neurons in the paravertebral portion of the sympathetic nervous system: those that contain norepinephrine alone, those that contain norepinephrine along with neuropeptide Y, and those that contain acetylcholine and vasoactive intestinal polypeptide. The first type provides innervation to the parenchyma of the target tissues, while the second mainly innervates blood vessels. The third type innervates the sweat glands. In the prevertebral ganglia, a fourth type of neuron exists that contains norepinephrine and somatostatin. This neuron probably innervates the gut. Preganglionic terminals of the cholinergic type form synaptic connections mainly with the dendrites of the sympathetic ganglion neurons. In addition to the types of synapses described for the paravertebral ganglia, neurons in the prevertebral ganglia receive synaptic connections from dorsal root ganglia and from the enteric nervous system. The sympathetic ganglia also contain interneurons that receive preganglionic synapses and form efferent synapses with some of the principal ganglion cells. The interneurons have been shown to contain a variety of transmitters, including norepinephrine, epinephrine, dopamine, serotonin, and a number of neuropeptides. The postganglionic sympathetic axons have a similar morphology to the parasympathetic axons. They form networks in their targets, and the axons display varicosities with concentrations of both small and large vesicles. After appropriate fixation, these vesicles are seen to possess dense cores.(ABSTRACT TRUNCATED AT 400 WORDS)

Innervation and gap junction formation in the myometrium of pregnant little brown bats, Myotis lucifugus

Pregnant Myotis lucifugus were captured in mist nets set outside a large maternity colony and, in most cases, were examined 12-15 hours later. Anterior and posterior halves of uteri were pinned to dental wax and either incubated in glyoxylic acid to produce adrenergic nerve fluorescence, or fixed in buffered glutaraldehyde for electron microscopy. Blood was collected for radioimmunoassay of plasma progesterone. We found no evidence of decreased nerve fluorescence even in late pregnancy when plasma progesterone levels were 10-20 times preovulatory values. Ultrastructural examination also showed no evidence of damage to, or destruction of, nerves in the myometrium. However, we did find gap junctions between myometrial muscle cells during the periparturient period in both normal and aborting bats. Gap junctions began to form several days before term, increased in number until parturition, then decreased dramatically within a day or two thereafter. The percentage of muscle cell plasma membrane involved in gap junction formation was closely correlated with plasma progesterone levels, although whether this is causal or coincidental is not certain. These data do not agree with the conclusions drawn from observations in other mammals that a disappearance of adrenergic nerves from the myometrium is associated with the initiation of parturition, or that gap junction formation is associated with changes in nerve function. They do, however, lend further support to the hypothesis that there is neurogenic control of myometrial contractility in M. lucifugus uteri at term.

Noradrenergic innervation of serotoninergic neurons in the myenteric plexus

The monoaminergic innervation of the guinea pig small intestine was investigated to determine if there is an anatomical basis for the hypothesis that serotoninergic and noradrenergic neurons physiologically interact in the enteric nervous system. Initial rates of uptake of tritiated 5-hydroxytryptamine (3H-5-HT) or norepinephrine (3H-NE) by segments of guinea pig small intestine were measured in order to estimate the regional density of the serotoninergic and noradrenergic innervation. No change was found in the uptake of 3H-5-HT as a function of distance between duodenum and ileum, whereas the relative uptake of 3H-NE declined. The pattern of serotoninergic elements demonstrated radioautographically was compared with that obtained by visualizing 5-HT immunoreactivity. Both methods revealed that a small number of serotoninergic neurons, located in 35.3% +/- 1.5% of myenteric ganglia, give rise to many fibers that form thick bundles in interganglionic connectives. Moreover, there was a pronounced heterogeneity in the serotoninergic innervation of individual myenteric neurons and ganglia. In material fixed with aldehydes and postfixed with NaMnO4, noradrenergic axon terminals were identified by their characteristic small dense-cored vesicles. Following incubation with 3H-NE only terminals with small dense-cored vesicles were radioautographically labeled, confirming that these terminals are noradrenergic. When 3H-5-HT was substituted for 3H-NE, noradrenergic terminals were not labeled, showing that nonspecific uptake of 3H-5-HT into noradrenergic axons did not occur in the presence of 5-hydroxydopamine. The combination of aldehyde-NaMnO4 fixation with the radioautographic localization of 3H-5-HT thus permitted the simultaneous identification of serotoninergic and noradrenergic neural elements. Serotoninergic varicosities were found to differ from noradrenergic varicosities in the size, appearance, and packing density of their synaptic vesicles. In addition, recognizable but rudimentary pre- and postsynaptic membrane specializations were associated with serotoninergic but not noradrenergic varicosities. Most serotoninergic neuronal cell bodies were contacted both by serotoninergic synapses and noradrenergic varicosities. Similar appositions of noradrenergic varicosities with nonserotoninergic neurons appeared to be rare. In view of earlier observations that sympathetic nerves affect the release of 5-HT from stimulated enteric serotoninergic neurons, it seems likely that the noradrenergic appositions with serotoninergic neurons are the anatomical substrate for this effect.(ABSTRACT TRUNCATED AT 400 WORDS)

An ultrastructural analysis of the sympathetic neuromuscular junctions on arterioles of the submucosa of the guinea pig ileum

The relationship of the varicosities of sympathetic postganglionic nerve terminals to the smooth muscle cells of arterioles in the submucosa of the guinea pig ileum has been investigated quantitatively by electron microscopy. Longitudinal sections were cut through arterioles about 50 micron in diameter after fixation in vitro or in situ under pressure. About 13% of the varicosities in individual ultrathin sections made contact with the outer surface of the smooth muscle cells. The neuromuscular junctions resembled those in skeletal muscle: the basal laminae of the axon bundle and of the smooth muscle were fused, and synaptic vesicles were accumulated close to the region of fusion. When individual varicosities were examined in serial sections, 92% and 83% in two preparations were found to form junctions of this kind. Most of the noncontacting varicosities were bare of Schwann cell toward the arteriolar surface and separated from it by less than 200 nm. Almost all axon profiles contained synaptic vesicles with electron dense cores after exposure to 5-hydroxydopamine. In electrophysiological experiments, ionophoretic application of noradrenaline to the arteriolar surface along the nerve bundles (demonstrated subsequently by fluorescence histochemistry) produced responses resembling those evoked by nerve stimulation. These anatomical and physiological data, taken together with the evidence for quantal release in this preparation (see Hirst et al., '85), suggest that neuromuscular transmission involves the rare release of a quantum of noradrenaline at discrete points on the smooth muscle membrane.

Physiological responses of guinea-pig myenteric neurons secondary to the release of endogenous serotonin by tryptamine

Intracellular recordings showed that administration of pulses of tryptamine mimicked one of the actions of serotonin (a slow depolarization associated with an increased input resistance) on type II/AH neurons of the myenteric plexus. After superfusion at high concentration tryptamine initially acted like serotonin, but then blocked the action of serotonin on these cells. Measurements of the release of preloaded [3H]serotonin or [3H]norepinephrine revealed that tryptamine is a potent releaser of these labeled amines; this release is Ca2+ independent but temperature dependent. Moreover, incubation with tryptamine depleted the myenteric plexus of endogenous serotonin. Since tryptamine has previously been demonstrated not to inhibit the binding of [3H]serotonin to its enteric neural receptor we framed the hypothesis that the serotonin-releasing action of tryptamine is responsible for its ability to mimic serotonin when given in pulses or to desensitize serotonin receptors through the prolonged release of serotonin when it is superfused. This hypothesis was tested by examining the action of tryptamine on the serotonin-mediated slow excitatory postsynaptic potentials evoked in type II/AH neurons by fiber tract stimulation. Tryptamine superfusion antagonized these slow potentials as predicted. Moreover, after a long time when endogenous serotonin was depleted, the response of type II/AH neurons to exogenous serotonin recovered but the slow synaptic potential did not. The action of tryptamine on this neuron was relatively specific. When the slow synaptic potential and serotonin responses were blocked by tryptamine the type II/AH neurons still responded to acetylcholine. Fast excitatory postsynaptic potentials were not affected by tryptamine. Furthermore, other types of neurons (I/S) and other neuronal responses to serotonin (such as a fast depolarization with decreased input resistance or presynaptic inhibition of acetylcholine release) were not blocked by tryptamine. Finally, radioautographic studies revealed a neural uptake of tryptamine in the chemically sympathectomized myenteric plexus; however, the distribution of tryptamine in the plexus was different from that of serotonin and was not blocked by excess non-radioactive serotonin. Therefore tryptamine does not enter myenteric neurons via the specific serotonin uptake mechanism; however, zimelidine, found to be a selective inhibitor of the enteric uptake of serotonin, antagonized the release of serotonin by tryptamine and attenuated the effect of tryptamine on responses to serotonin.(ABSTRACT TRUNCATED AT 400 WORDS)

Somatostatin is present in a subpopulation of noradrenergic nerve fibres supplying the intestine

Somatostatin and dopamine beta-hydroxylase have been localized in the coeliaco-mesenteric ganglia, in mesenteric nerves and in the wall of the guinea-pig small intestine. Nerve lesions were used to determine the sources of the nerves. Nerve cell bodies in the coeliaco-mesenteric ganglia with immunoreactivity for both somatostatin and dopamine beta-hydroxylase project to the intestine via the mesenteric nerves. Most of their terminals are in the submucous ganglia, where they make up the full complement of noradrenergic terminals, and in the mucosa where other noradrenergic terminals, not containing somatostatin immunoreactivity, are also present. The small number of noradrenergic fibres present in the tertiary component of the myenteric plexus and in the circular muscle all show immunoreactivity for somatostatin. The noradrenergic fibres supplying the mesenteric and intestinal blood vessels and those ramifying in the myenteric ganglia do not contain somatostatin. The numerous somatostatin-immunoreactive nerves in the enteric plexuses that do not contain dopamine beta-hydroxylase come from enteric nerve cell bodies. These results, considered in the context of other published work, indicate that post-ganglionic sympathetic noradrenergic neurons are chemically coded according to the target tissue they supply and suggest that neurons that were hitherto thought to be neurochemically equivalent, but which serve different functions, are in fact chemically distinct.

An ultrastructural comparison of neuromuscular relationships in blood vessels with functional and 'non-functional' neuromuscular transmission

The neuromuscular relationships of the guinea-pig renal artery, which has previously been shown to be noradrenergically innervated but 'non-functional', i.e. not responsive to nerve stimulation in vitro, is compared with those of two arteries where functional neuromuscular transmission occurs: the carotid artery, where responses to nerve stimulation are slow and of medium amplitude, and the mesenteric artery, where the responses to nerve stimulation are relatively fast and of large amplitude. Substantial differences of ultrastructure were demonstrated. In the renal artery, there were no nerve varicosities closer than 400 nm to the smooth muscle layer, all varicosities had connective tissue interposed between them and the muscle wall, and there were significantly fewer varicosities (6 +/- 3.1 mm-1) compared with the carotid (33 +/- 8.3 mm-1; P less than 0.05) or mesenteric (105 +/- 15.8 mm-1; P less than 0.001) arteries. In the carotid artery, a small group of varicosities (17%), with little interposed connective tissue, had an average neuromuscular gap of 1.5 micron, whilst the remainder were separated by an average of 4.3 micron and by cellular and other connective tissue elements from the nearest smooth muscle cell. In the mesenteric artery, about half of the nerve varicosities were closer than 400 nm to the muscle layer and 25% of the varicosities had no connective tissue except basal lamina interposed in the neuromuscular space. Differences were also shown in the ultrastructural appearance of the nerves: analysis of the neuronal vesicles showed that the majority of varicosities were noradrenergic in all three vessels, with non-adrenergic varicosities forming 6% of the total in the carotid artery and 32% of the total in the mesenteric artery. Noradrenaline content was greater in the mesenteric artery (0.37 +/- 0.052 ng mm-2) compared with the carotid (0.17 +/- 0.017 ng mm-2; P less than 0.01) and renal (0.11 +/- 0.024 ng mm-2; P less than 0.01) arteries. These differences appear to correlate with the differences of neuromuscular activity in the three arteries.

Ultrastructure of catecholamine-containing axons in the intestine of the domestic fowl

Axons in the duodenum, ileum and rectum of the domestic fowl were identified as catecholamine-containing (CA) on the basis of positive reactivity following chromaffin fixation for electron microscopy. CA-axons in association with blood vessels in all regions of the intestine and in non-vascular sites in the small intestine had a 'typical' adrenergic appearance, in that they contained many small granular vesicles (SGV) and variable numbers of large granular vesicles (LGV). In the rectum the non-vascular CA-axon profiles were atypical, in that there were many elongated LGV and few SGV, and the chromaffin reactivity was weak. The nerve profiles in the rectum were dramatically reduced following 6-hydroxydopamine and reserpine treatment and were absent in rectum cultured in the absence of extrinsic ganglia. It was concluded that the profiles, in spite of their low chromaffin reactivity, truely represent CA-axons. The possibility was raised that the atypical morphology and reduced chromaffin reactivity is due to the presence of adrenaline.

Morphological studies of synapses and varicosities in dissociated cell cultures of sympathetic neurons

Neurons dissociated from the superior cervical ganglia of newborn rats can be grown under conditions which support either adrenergic or cholinergic differentiation. In both cases, the neurons form numerous morphologically specialized synaptic terminals or synapses as well as relatively unspecialized varicosities. The ultrastructure of both types of terminal was compared in mature neuronal cultures and the effects of growth conditions on terminal morphology examined. After aldehyde-osmium fixation, synapses in cultures grown under adrenergic or cholinergic conditions were characterized by asymmetrical membrane specializations comparable to type I or asymmetric synapses; bismuth iodide and ethanolic phosphotungstic acid impregnation of neuronal cultures revealed the presence of characteristic synaptic membrane specializations: a presynaptic grid of dense projections and a wide postsynaptic dense band of uniform thickness. No membrane specializations were apparent in varicosities after aldehyde-osmium fixations or with these stains. Intramembranous particle distributions were examined in freeze-fracture replicas of neurons. Aggregates of large, 10-12 nm particles were found on P-face membrane leaflets of cell bodies and large diameter processes; this distribution is the same as that of synapses in thin-sectioned preparations. These particle aggregates may represent postsynaptic membrane specializations or acetylcholine receptors. The cytoplasmic leaflet of boutons contained large, 12-14 nm particles, which appeared to be concentrated at the region of synaptic contact at putative synapses, but were diffusely distributed in varicosity membranes. Similar large particles were also seen at a much lower density in the membrane E-face. None of these ultrastructural characteristics appeared to vary with transmitter identity or growth conditions. Synaptic vesicle shape, however, did vary in glutaraldehyde-fixed cultures. At all ages examined, neurons grown on monolayers of heart cells contained predominantly round vesicles, whereas neurons grown in the virtual absence of non-neuronal cells possessed pleiomorphic synaptic vesicles. This difference in vesicle shape appeared to be correlated more closely with growth in the presence of non-neuronal cells than with the transmitter present at the time of fixation.

Noradrenergic and non-noradrenergic nerves containing small granular vesicles in Auerbach's plexus of the guinea-pig: evidence against the presence of noradrenergic synapses

The appearance and distribution of varicosities containing small granular vesicles in Auerbach's plexus of the guinea-pig ileum, distal colon and rectum has been studied with the electron-microscope. Two types of varicosity were recognised. The first type was located predominantly at the surface of the plexus and did not form synapses on intrinsic neurons. This type became labelled with 5-hydroxydopamine, a specific marker for noradrenergic axons, and was destroyed by 6-hydroxydopamine and extrinsic denervation, procedures which lead to degeneration of noradrenergic nerves in the gut. The second type formed axodendritic and axosomatic synapses on intrinsic neurons and the morphology of its synaptic vesicles differed subtly from that of the first type. The second type was unaffected by 5-hydroxydopamine, 6-hydroxydopamine, or extrinsic denervation. It is concluded that the two types of small granular vesicle-containing varicosities belong to different neurons and that the first type is noradrenergic. Noradrenergic varicosities do not, therefore, form synapses in Auerbach's plexus. This conclusion is in accord with the electrophysiological findings. The second type of small granular vesicle-containing varicosity is not noradrenergic although it was formerly thought to be so. It is intrinsic to the gut and is resistant to the serotoninergic neurotoxin, 5,6-dihydroxytryptamine.

The fine structure of the submucous plexus of the guinea-pig ileum. I. The ganglia, neurons, Schwann cells and neuropil

A fine structural study was made of the ganglia, neurons, Schwann cells and neuropil of the submucous plexus of the guinea-pig ileum. The arrangement of the plexus as seen by light microscopy is briefly described. Submucous ganglia are small, containing an average of eight neurons per ganglion (compared with 43 in myenteric ganglia) and are connected with each other by fine nerve strands. The cell bodies of neurons and Schwann cells and a neuropil consisting of neuronal and Schwann cell processes from the ganglia. No other cell types or blood vessels are found within the ganglia. Ganglia are surrounded by a continuous basal lamina but lack a well-defined connective tissue investment. The glial investment of neurons is incomplete: many neurons lie directly beneath the basal lamina with no intervening Schwann cell processes, and the plasma membranes of adjacent neurons are often directly apposed over large areas. Other areas of apposition occur between the cell bodies and processes of neurons and Schwann cells. Desmosome-like membrane specializations may be seen between neurons and other neurons or Schwann cells. Submucous neurons could not be categorized according to size, shape, organelle content or types of processes. Processes emerging from nerve-cell bodies were placed into four broad categories on the basis of shape and microtubule content. Many bundles of closely apposed small nerve profiles lacking intervening Schwann processes are found in the neuropil in addition to a large number of vesiculated varicosities, some of which are directly apposed to the plasma membranes of nerve-cell bodies. A small proportion of vesiculated profiles from synapses with nerve cell bodies, their processes and profiles in the neuropil. From their structure, submucous neurons appear to form a more homogeneous population than myenteric neurons. Because of their incomplete investment they are more likely to be freely exposed to substances diffusing in the extraganglionic tissue than are neurons of sympathetic ganglia.

The fine structure of the submucous plexus of the guinea-pig ileum. II. Description and analysis of vesiculated nerve profiles

A fine structural study has been made of the vesiculated nerve profiles of the submucous plexus of both normally innervated and extrinsically denervated segments of guinea-pig ileum. Two types of nerve profiles could be readily distinguished by their vesicular content after conventional fixation. The first type, comprising 5% of all intrinsic profiles, consisted of predominantly small vesicles containing electron dense material which usually formed a ring around the inner face of the vesicular membrane but sometimes partially or completely filled the vesicle. These profiles, termed ring-vesicle-containing profiles, remained after extrinsic denervation, and their vesicular content did not change following injection of reserpine or 5-hydroxydopamine. Thus ring-vesicle-containing profiles are not adrenergic. Profiles which were positive for the uranaffin method were similar in morphology and frequency of occurrence to ring-vesicle-containing profiles, although it is not possible to say that they are the same. The second type of profile, comprising 95% of all intrinsic profiles, contained varying proportions of large granular and small clear vesicles. These heterogeneous profiles were present in both normally innervated and extrinsically denervated tissue. Their vesicular content did not change following injection of reserpine, however, some profiles of this type in normally innervated, but not in extrinsically denervated, intestine contained electron dense deposits after injection of 5-hydroxydopamine. This means that noradrenergic profiles are a subpopulation of the heterogeneous profiles in normally innervated tissue. Analysis of intrinsic heterogeneous profiles showed that the proportion and packing density of large granular vesicles formed continuous distributions which did not provide any basis for further subdivision of this type of profile. Ring-vesicle-containing and heterogeneous profiles often formed synapses with neuronal cell bodies and processes. Two rarer types of profiles were also seen. The first type contained mainly small flattened vesicles which took up 5-hydroxydopamine and was not present in extrinsically denervated tissue. This type, like the group described above, is considered to be noradrenergic. The second rare type contained large numbers of lysosome-like dense bodies and vesicles of different sizes and content and was seen in both normally innervated and denervated tissue. This type probably represents spontaneously degenerating nerve profiles.