Immunohistochemical evidence for the presence of calbindin containing neurones in the myenteric plexus of the guinea-pig stomach

Nitrergic and Purinergic Nerves in the Small Intestinal Myenteric Plexus and Circular Muscle of Mice and Guinea Pigs

ATP is an excitatory and inhibitory neurotransmitter, while nitric oxide (NO) is an inhibitory neurotransmitter in the enteric nervous system (ENS). We used a vesicular nucleotide transporter (SLC17A9, VNUT) antibody and a nitric oxide synthase (NOS) antibody to identify purinergic and nitrergic nerves in mouse and guinea ileum. Mouse: VNUT-immunoreactivity (ir) was detected in nerve fibers in myenteric ganglia and circular muscle. VNUT-ir fibers surrounded choline acetyltransferase (ChAT), nitric oxide synthase (nNOS), and calretinin-ir neurons. VNUT-ir nerve cell bodies were not detected. Tyrosine hydroxylase (TH)-ir nerves were detected in myenteric ganglia and the tertiary plexus. Guinea pig: VNUT-ir was detected in neurons and nerves fibers and did not overlap with NOS-ir nerve fibers. VNUT-ir was detected in nerve fibers in ganglia but not nerve cell bodies. VNUT-ir nerve fibers surrounded NOS-ir and NOS^- neurons. NOS-ir and VNUT-ir nerve fibers did not overlap in myenteric ganglia or circular muscle. VNUT-ir nerves surrounded some ChAT-ir neurons. VNUT-ir and ChAT-ir were detected in separate nerves in the CM. VNUT-ir nerve fibers surrounded calretinin-ir neurons.Conclusions: VNUT-ir neurons likely mediate purinergic signaling in small intestinal myenteric ganglia and circular muscle. ATP and NO are likely released from different inhibitory motorneurons. VNUT-ir and ChAT-ir interneurons mediate cholinergic and purinergic synaptic transmission in the myenteric plexus.

Characterization of projections of longitudinal muscle motor neurons in human colon

BACKGROUND

The enteric nervous system contains inhibitory and excitatory motor neurons which modulate smooth muscle contractility. Cell bodies of longitudinal muscle motor neurons have not been identified in human intestine.

METHODS

We used retrograde tracing ex vivo with DiI, with multiple labeling immunohistochemistry, to characterize motor neurons innervating tenial and inter-tenial longitudinal muscle of human colon.

KEY RESULTS

The most abundant immunohistochemical markers in the tertiary plexus were vesicular acetylcholine transporter, nitric oxide synthase (NOS), and vasoactive intestinal polypeptide (VIP). Of retrogradely traced motor neurons innervating inter-tenial longitudinal muscle, 95% were located within 6mm oral or anal to the DiI application site. Excitatory motor neuron cell bodies, immunoreactive for choline acetyltransferase (ChAT), were clustered aborally, whereas NOS-immunoreactive cell bodies were distributed either side of the DiI application site. Motor neurons had small cell bodies, averaging 438 + 18µm² in cross-sectional area, similar for ChAT- and NOS-immunoreactive subtypes. Motor neurons innervating the tenia had slightly longer axial projections, with 95% located within 9mm. ChAT-immunoreactive excitatory motor neurons to tenia were clustered aborally, whereas NOS-immunoreactive inhibitory motor neurons had both ascending and descending projections. VIP immunoreactivity was rarely present without NOS immunoreactivity in motor neurons.

CONCLUSIONS AND INFERENCES

Tenial and inter-tenial motor neurons innervating the longitudinal muscle have short projections. Inhibitory motor neurons have less polarized projections than cholinergic excitatory motor neurons. Longitudinal and circular muscle layers are innervated by distinct local populations of excitatory and inhibitory motor neurons. A population of human enteric neurons that contribute significantly to colonic motility has been characterized.

Distribution, projections, and association with calbindin baskets of motor neurons, interneurons, and sensory neurons in guinea-pig distal colon

Normal gut function relies on the activity of the enteric nervous system (ENS) found within the wall of the gastrointestinal tract. The structural and functional organization of the ENS has been extensively studied in the guinea pig small intestine, but less is known about colonic circuitry. Given that there are significant differences between these regions in function, observed motor patterns and pathology, it would be valuable to have a better understanding of the colonic ENS. Furthermore, disorders of colonic motor function, such as irritable bowel syndrome, are much more common. We have recently reported specialized basket-like structures, immunoreactive for calbindin, that likely underlie synaptic inputs to specific types of calretinin-immunoreactive neurons in the guinea-pig colon. Based on detailed immunohistochemical analysis, we postulated the recipient neurons may be excitatory motor neurons and ascending interneurons. In the present study, we combined retrograde tracing and immunohistochemistry to examine the projections of circular muscle motor neurons, myenteric interneurons, and putative sensory neurons. We focused on neurons with immunoreactivity for calbindin, calretinin and nitric oxide synthase and their relationship with calbindin baskets. Retrograde tracing using indocarbocyanine dye (DiI) revealed that many of the nerve cell bodies surrounded by calbindin baskets belong to motor neurons and ascending interneurons. Unique functional classes of myenteric neurons were identified based on morphology, neuronal markers and polarity of projection. We provide evidence for three groups of ascending motor neurons based on immunoreactivity and association with calbindin baskets, a finding that may have significant functional implications.

Morphological evidence for novel enteric neuronal circuitry in guinea pig distal colon

The gastrointestinal (GI) tract is unique compared to all other internal organs; it is the only organ with its own nervous system and its own population of intrinsic sensory neurons, known as intrinsic primary afferent neurons (IPANs). How these IPANs form neuronal circuits with other functional classes of neurons in the enteric nervous system (ENS) is incompletely understood. We used a combination of light microscopy, immunohistochemistry and confocal microscopy to examine the topographical distribution of specific classes of neurons in the myenteric plexus of guinea-pig colon, including putative IPANs, with other classes of enteric neurons. These findings were based on immunoreactivity to the neuronal markers, calbindin, calretinin and nitric oxide synthase. We then correlated the varicose outputs formed by putative IPANs with subclasses of excitatory interneurons and motor neurons. We revealed that calbindin-immunoreactive varicosities form specialized structures resembling 'baskets' within the majority of myenteric ganglia, which were arranged in clusters around calretinin-immunoreactive neurons. These calbindin baskets directly arose from projections of putative IPANs and represent morphological evidence of preferential input from sensory neurons directly to a select group of calretinin neurons. Our findings uncovered that these neurons are likely to be ascending excitatory interneurons and excitatory motor neurons. Our study reveals for the first time in the colon, a novel enteric neural circuit, whereby calbindin-immunoreactive putative sensory neurons form specialized varicose structures that likely direct synaptic outputs to excitatory interneurons and motor neurons. This circuit likely forms the basis of polarized neuronal pathways underlying motility.

Downregulation of calbindin 1, a calcium-binding protein, reduces the proliferation of osteosarcoma cells

Osteosarcoma is the most common type of primary malignant bone tumor and has a high propensity to metastasize to the lungs and bones. Calbindin 1 (CALB1) is a constituent Ca2+ binding protein, which can prevent apoptotic death in several cell types induced through various pro-apoptotic signaling pathways. To investigate whether CALB1 is implicated in the tumor growth of human osteosarcoma, two different short hairpin RNAs (shRNAs) against CALB1 were used for CALB1-knockdown in osteosarcoma U2OS cells. The U2OS cells were divided into three groups: Two groups with CALB1 knockdown (CALB1-shRNA 1 and CALB1-shRNA 2) and one control group (Con-shRNA). Reverse transcription-quantitative polymerase chain reaction and western blot analysis confirmed that the CALB1-shRNA 1- and 2-infected cells exhibited significantly lower levels of CALB1 gene and protein expression compared with the Con-shRNA group. The proliferation and colony formation abilities were significantly inhibited in CALB1-deficient U2OS cells compared with the control, as measured using an MTT assay and crystal violet staining. Flow cytometry revealed that the number of CALB1-shRNA 2-injected cells was increased in the G0/G1 and G2/M phases, but decreased in the S phase, compared with the control group. The assessment of apoptosis and necrosis using Annexin V/7-aminoactinomycin D demonstrated that there was a significantly higher percentage of necrotic, early apoptotic, and late apoptotic cells, but a significantly lower percentage of viable cells in U2OS cells with CALB1-knockdown compared with the control group. In conclusion, CALB1 contributes to protecting osteosarcoma cells from apoptosis and provides a potential novel target for gene therapy to treat patients with osteosarcoma.

Mechanosensitive enteric neurons in the guinea pig gastric corpus

For long it was believed that a particular population of enteric neurons, referred to as intrinsic primary afferent neuron (IPAN)s, encodes mechanical stimulation. We recently proposed a new concept suggesting that there are in addition mechanosensitive enteric neurons (MEN) that are multifunctional. Based on firing pattern MEN behaved as rapidly, slowly, or ultra-slowly adapting RAMEN, SAMEN, or USAMEN, respectively. We aimed to validate this concept in the myenteric plexus of the gastric corpus, a region where IPANs were not identified and existence of enteric sensory neurons was even questioned. The gastric corpus is characterized by a particularly dense extrinsic sensory innervation. Neuronal activity was recorded with voltage sensitive dye imaging after deformation of ganglia by compression (intraganglionic volume injection or von Fry hair) or tension (ganglionic stretch). We demonstrated that 27% of the gastric neurons were MEN and responded to intraganglionic volume injection. Of these 73% were RAMEN, 25% SAMEN, and 2% USAMEN with a firing frequency of 1.7 (1.1/2.2), 5.1 (2.2/7.7), and of 5.4 (5.0/15.5) Hz, respectively. The responses were reproducible and stronger with increased stimulus strength. Even after adaptation another deformation evoked spike discharge again suggesting a resetting mode of the mechanoreceptors. All MEN received fast synaptic input. Fifty five percent of all MEN were cholinergic and 45% nitrergic. Responses in some MEN significantly decreased after perfusion of TTX, low Ca(++)/high Mg(++) Krebs solution, capsaicin induced nerve defunctionalization and capsazepine indicating the involvement of TRPV1 expressing extrinsic mechanosensitive nerves. Half of gastric MEN responded to intraganglionic volume injection as well as to ganglionic stretch and 23% responded to stretch only. Tension-sensitive MEN were to a large proportion USAMEN (44%). In summary, we demonstrated for the first time compression and tension-sensitive MEN in the stomach; many of them responded to one stimulus modality only. Their proportions and the basic properties were similar to MEN previously identified by us in other intestinal region and species. Unlike in the intestine, the responsiveness of some gastric MEN is enhanced by extrinsic TRPV1 expressing visceral afferents.

Selective coexpression of synaptic proteins, α-synuclein, cysteine string protein-α, synaptophysin, synaptotagmin-1, and synaptobrevin-2 in vesicular acetylcholine transporter-immunoreactive axons in the guinea pig ileum

Parkinson's disease is a neurodegenerative disorder characterized by Lewy bodies and neurites composed mainly of the presynaptic protein α-synuclein. Frequently, Lewy bodies and neurites are identified in the gut of Parkinson's disease patients and may underlie associated gastrointestinal dysfunctions. We recently reported selective expression of α-synuclein in the axons of cholinergic neurons in the guinea pig and human distal gut; however, it is not clear whether α-synuclein expression varies along the gut, nor how closely expression is associated with other synaptic proteins. We used multiple-labeling immunohistochemistry to quantify which neurons in the guinea pig ileum expressed α-synuclein, cysteine string protein-α (CSPα), synaptophysin, synaptotagmin-1, or synaptobrevin-2 in their axons. Among the 10 neurochemically defined axonal populations, a significantly greater proportion of vesicular acetylcholine transporter-immunoreactive (VAChT-IR) varicosities (80% ± 1.7%, n = 4, P < 0.001) contained α-synuclein immunoreactivity, and a significantly greater proportion of α-synuclein-IR axons also contained VAChT immunoreactivity (78% ± 1.3%, n = 4) compared with any of the other nine populations (P < 0.001). Among synaptophysin-, synaptotagmin-1-, synaptobrevin-2-, and CSPα-IR varicosities, 98% ± 0.7%, 96% ± 0.7%, 88% ± 1.6%, and 85% ± 2.9% (n = 4) contained α-synuclein immunoreactivity, respectively. Among α-synuclein-IR varicosities, 96% ± 0.9%, 99% ± 0.6%, 83% ± 1.9%, and 87% ± 2.3% (n = 4) contained synaptophysin-, synaptotagmin-1-, synaptobrevin-2-, and CSPα immunoreactivity, respectively. We report a close association between the expression of α-synuclein and the expression of other synaptic proteins in cholinergic axons in the guinea pig ileum. Selective expression of α-synuclein may relate to the neurotransmitter system utilized and predispose cholinergic enteric neurons to degeneration in Parkinson's disease.

An immunohistochemical study of the distribution of nitric oxide synthase-immunoreactive neurons and fibers in the reticular groove of suckling lambs

The reticular groove (RG) is a specialized region of ruminant forestomach which, in suckling animals, via a vagovagal reflex, transforms itself into a tube to ensure the direct transport of milk from the esophagus to the abomasum. The nervous mechanism controlling the RG movement is not fully understood; however, at this level, the enteric nervous system (ENS) shows the highest neuronal density when compared with other forestomach compartments. Because nitric oxide is considered the putative major mediator of non-adrenergic non-cholinergic smooth muscle relaxation, the aim of the present study was to investigate the ENS of the RG of suckling lambs, both in the floor and in the lip, with particular regard to nitric oxide synthase (NOS)-immunoreactivity (-IR), by means of double immunohistochemical staining. NOS antiserum was used in association with some neurochemical markers which have been utilized by many authors in ENS. A rich innervation of fibers extended along the entire length of the RG. Proceeding distally, the number of neurons stained with a pan-neuronal marker increased; they were more numerous in the lips and lip-floor junction than in the floor itself. However, the percentage of NOS-IR neurons was the same in the proximal and distal parts. Many NOS-IR neurons often co-expressed galanin and dopamine β-hydroxylase. Neurochemical markers, such as calbindin, calcitonin gene-related peptide, IB4 and neurofilament 200 kDa, usually used to identify primary sensory neurons were not expressed in RG neurons, and the co-localization of NOS with tyrosine hydroxylase and substance P was rarely found. When compared with other districts, the RG showed some peculiar aspects, such as the lack of both neurons in the submucosal plexus and the lack of typical sensory neurons.

Expression of tryptophan 2,3-dioxygenase in mature granule cells of the adult mouse dentate gyrus

New granule cells are continuously generated in the dentate gyrus of the adult hippocampus. During granule cell maturation, the mechanisms that differentiate new cells not only describe the degree of cell differentiation, but also crucially regulate the progression of cell differentiation. Here, we describe a gene, tryptophan 2,3-dioxygenase (TDO), whose expression distinguishes stem cells from more differentiated cells among the granule cells of the adult mouse dentate gyrus. The use of markers for proliferation, neural progenitors, and immature and mature granule cells indicated that TDO was expressed in mature cells and in some immature cells. In mice heterozygous for the alpha-isoform of calcium/calmodulin-dependent protein kinase II, in which dentate gyrus granule cells fail to mature normally, TDO immunoreactivity was substantially downregulated in the dentate gyrus granule cells. Moreover, a 5-bromo-2'-deoxyuridine labeling experiment revealed that new neurons began to express TDO between 2 and 4 wk after the neurons were generated, when the axons and dendrites of the granule cells developed and synaptogenesis occurred. These findings indicate that TDO might be required at a late-stage of granule cell development, such as during axonal and dendritic growth, synaptogenesis and its maturation.

Multifunctional rapidly adapting mechanosensitive enteric neurons (RAMEN) in the myenteric plexus of the guinea pig ileum

An important feature of the enteric nervous system (ENS) is its capability to respond to mechanical stimulation which, as currently suggested for the guinea-pig ileum, is encoded by specialized intrinsic primary afferent neurons (IPANs). We used von Frey hairs or intraganglionic volume injections to mimic ganglion deformation as observed in freely contracting preparations. Using fast voltage-sensitive dye imaging we identified rapidly adapting mechanosensitive enteric neurons (RAMEN, 25% of all neurons) in the myenteric plexus of the guinea pig ileum. RAMEN responded with phasic spike discharge to dynamic changes during ganglion deformation. This response was reproducible and increased with increasing forces. Deformation-evoked spike discharge was not changed by synaptic blockade with hexamethonium, omega-conotoxin or low Ca(2+)/high Mg(2+), defunctionalization of extrinsic afferents with capsaicin or muscle paralysis with nifedipine, suggesting direct activation of RAMEN. All RAMEN received hexamethonium-sensitive fast EPSPs, which were blocked by omega-conotoxin and low Ca(2+)/high Mg(2+). Seventy-two per cent of RAMEN were cholinergic, 22% nitrergic, and 44% were calbindin and NeuN negative, markers used to identify IPANs. Mechanosensitivity was observed in 31% and 47% of retrogradely traced interneurons and motor neurons, respectively. RAMEN belong to a new population of mechanosensitive neurons which differ from IPANs. We provided evidence for multifunctionality of RAMEN which may fulfil sensory, integrative and motor functions. In light of previously identified mechanosensitive neuron populations, mechanosensitivity appears to be a property of many more enteric neurons than generally assumed. The findings call for a revision of current concepts on sensory transmission within the ENS.

Differential expression of canonical (classical) transient receptor potential channels in guinea pig enteric nervous system

The canonical transient receptor potential (TRPC) family of ion channels is implicated in many neuronal processes including calcium homeostasis, membrane excitability, synaptic transmission, and axon guidance. TRPC channels are postulated to be important in the functional neurobiology of the enteric nervous system (ENS); nevertheless, details for expression in the ENS are lacking. Reverse transcriptase-polymerase chain reaction, Western blotting, and immunohistochemistry were used to study the expression and localization of TRPC channels. We found mRNA transcripts, protein on Western blots, and immunoreactivity (IR) for TRPC1/3/4/6 expressed in the small intestinal ENS of adult guinea pigs. TRPC1/3/4/6-IR was localized to distinct subpopulations of enteric neurons and was differentially distributed between the myenteric and submucosal divisions of the ENS. TRPC1-IR was widely distributed and localized to neurons with cholinergic, calretinin, and nitrergic neuronal immunochemical codes in the myenteric plexus. It was localized to both cholinergic and noncholinergic secretomotor neurons in the submucosal plexus. TRPC3-IR was found only in the submucosal plexus and was expressed exclusively by neuropeptide Y-IR neurons. TRPC4/6-IR was expressed in only a small population of myenteric neurons, but was abundantly expressed in the submucosal plexus. TRPC4/6-IR was coexpressed with both cholinergic and nitrergic neurochemical codes in the myenteric plexus. In the submucosal plexus, TRPC4/6-IR was expressed exclusively in noncholinergic secretomotor neurons. No TRPC1/3/4/6-IR was found in calbindin-IR neurons. TRPC3/4/6-IR was widely expressed along varicose nerve fibers and colocalized with synaptophysin-IR at putative neurotransmitter release sites. Our results suggest important roles for TRPC channels in ENS physiology and neuronal regulation of gut function.

Intrinsic ruminal innervation in ruminants of different feeding types

According to their feeding habits, ruminants can be classified as grazers, concentrate selectors and those of intermediate type. The different feeding types are reflected in distinct anatomical properties of the forestomachs. The present study was designed to investigate whether the intrinsic innervation patterns of the rumen (the main part of the forestomach) differ between intermediate types and grazers. Myenteric plexus preparations from the rumen of goats (intermediate type), fallow deer (intermediate type), cattle (grazer) and sheep (grazer) were analysed by immunohistochemical detection of the following antigens: Hu-protein (HuC/D), choline acetyltransferase (ChAT), nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), substance P (SP), calbindin (CALB) and somatostatin (SOM). Myenteric ganglia of cattle contained 73 +/- 6 neurons per ganglion, whereas the ganglia of sheep were significantly smaller (45 +/- 18 neurons per ganglion). The ganglion density of the myenteric plexus was highest in fallow deer (15 +/- 3 ganglia per cm(2)) and lowest in cattle (6 +/- 1 ganglia per cm(2)). All myenteric neurons were either ChAT or NOS positive. The proportion of NOS-positive neurons was significantly lower in sheep (29.5 +/- 8.2% of all neurons) than in goats (44.2 +/- 9.8%). In all species, additional analysis of the different neuropeptides revealed the following subpopulations in descending order of percentile appearance: ChAT/SP > NOS/VIP/NPY > ChAT/- > NOS/NPY. Expression of CALB was detected in a minority of the ChAT-positive neurons in all species. Somatostatin immunoreactive somata were found only in preparations obtained from fallow deer and sheep. These data suggest that the rumen of grazers is under stronger cholinergic control than the rumen of species belonging to the intermediate type, although most subpopulations of neurons are present in all species. However, whether the strong mixing patterns of low quality roughage during digestion are enabled by the prominent excitatory input of the rumen of grazers requires elucidation in further studies.

Targets of myenteric interneurons in the guinea-pig small intestine

Polarized outputs of myenteric interneurons in guinea-pig small intestine have been well studied. However, the variety of motility patterns exhibited suggests that some interneuron targets remain unknown. We used antisera selected to distinguish interneuron varicosities and known myenteric neuron types to investigate outputs of three interneuron classes in guinea-pig jejunum; two classes of descending interneurons immunoreactive (IR) for somatostatin (SOM) or nitric oxide synthase (NOS)/vasoactive intestinal peptide (VIP), and one class of ascending interneurons [calretinin/enkephalin (ENK)-IR]. Varicosities apposed to immunohistochemically identified cell bodies were quantified by confocal microscopy. Intrinsic sensory neurons (calbindin-IR) were apposed by few varicosities. Cholinergic secretomotor neurons (neuropeptide Y-IR) were apposed by many SOM-IR varicosities. Longitudinal muscle excitatory motor neurons (calretinin-IR) were apposed by some VIP- and ENK-IR varicosities, but few SOM-IR varicosities. Ascending interneurons (calretinin-IR) were apposed by many varicosities of all types. NOS-IR interneurons and inhibitory motor neurons were apposed by numerous VIP-IR and SOM-IR varicosities. NOS-IR short inhibitory motor neurons were apposed by significantly fewer ENK-IR varicosities than other NOS-IR neurons. Based on the specific chemical coding of ascending (ENK) and descending (SOM) interneurons, we conclude that cholinergic secretomotor neurons and short inhibitory neurons are located in descending reflex pathways, while ascending interneurons and NOS-IR descending interneurons are focal points at which ascending and descending pathways converge.

Fate mapping Nkx2.1-lineage cells in the mouse telencephalon

The homeodomain transcription factor Nkx2.1 is expressed in the pallidal (subcortical) telencephalon, including the medial ganglionic eminence (MGE) and preoptic area. Studies have shown that Nkx2.1 is required for normal patterning of the MGE and for the specification of the parvalbumin (PV)- and somatostatin (SST)-expressing cortical interneurons. To define the contribution of Nkx2.1 lineages to neurons in the mature telencephalon, we have generated transgenic mice carrying the genomic integration of a modified bacterial artificial chromosome (BAC) in which the second exon of Nkx2.1 is replaced by the Cre recombinase. Analysis of these mice has found that they express the Cre recombinase and Cre reporters within Nkx2.1-expressing domains of the brain, thyroid, pituitary, and lung. Telencephalic expression of reporters begins at about embryonic day 10.5. Expression both of Cre and of recombination-based Cre reporters is weaker within the dorsalmost region of the MGE than in other Nkx2.1-expressing regions. In this paper, we present fate-mapping data on Nkx2.1-lineage neurons throughout the telencephalon, including the cerebral cortex, amygdala, olfactory bulb, striatum, globus pallidus, septum, and nucleus basalis.

Decreased neurogenesis in aged rats results from loss of granule cell precursors without lengthening of the cell cycle

It is well established that neurogenesis in the dentate gyrus slows with aging, but it is unclear whether this change is due to slowing of the cell cycle, as occurs during development, or to loss of precursor cells. In the current study, we find that the cell cycle time of granule cell precursors in middle-aged male rats is not significantly different from that in young adults. The size of the precursor pool, however, was 3-4 times smaller in the middle-aged rats, as determined using both cumulative bromodeoxyuridine (BrdU) labeling as well as labeling with the endogenous marker of cell proliferation, proliferating cell nuclear antigen (PCNA). Loss of precursor cells was much greater in the granule cell layer than in the hilus, suggesting that dividing cells in the hilus belong to a distinct population, most likely glial progenitors, that are less affected by aging than neuronal precursors. BrdU-labeled precursor cells and young neurons, labeled with doublecortin, appeared to be lost equally from rostral and caudal, as well as suprapyramidal and infrapyramidal, subregions of the granule cell layer. However, doublecortin staining did show large parts of the caudal granule cell layer with few if any young neurons at both ages. Taken together, these findings indicate that precursor cells are not distributed evenly within the dentate gyrus in adulthood but that precursors are lost from throughout the dentate gyrus in old age with no concomitant change in the cell cycle time.

Mapping 5-HT inputs to enteric neurons of the guinea-pig small intestine

5-HT released by gastrointestinal mucosa and enteric interneurons has powerful effects on gut behavior. However, the targets of 5-HT-containing neurons within enteric circuits are not well characterized. We used antisera against 5-HT and selected markers of known enteric neuron types to investigate the connections made by 5-HT-containing neurons in the guinea-pig jejunum. Confocal microscopy was used to quantify the number of 5-HT-immunoreactive varicosities apposed to immunohistochemically identified cell bodies. Large numbers of varicosities were identified apposing cholinergic secretomotor neurons, immunoreactive for neuropeptide Y, in both myenteric and submucous plexuses. Subgroups of neurons identified by calretinin (ascending interneurons) and nitric oxide synthase (descending interneurons and inhibitory motor neurons) immunoreactivity were also apposed by many varicosities. Longitudinal muscle motor neurons (calretinin immunoreactive) and AH/Dogiel type II (sensory) neurons (calbindin immunoreactive) were apposed by small numbers of varicosities. Combined retrograde tracing and immunohistochemistry were used to identify excitatory circular muscle motor neurons; these were encircled by 5-HT-immunoreactive varicosities, but the appositions could not be quantified. We suggest that 5-HT-containing interneurons are involved in secretomotor pathways and pathways to subgroups of other interneurons, but not longitudinal muscle motor neurons. There also appear to be connections between 5-HT-containing interneurons and excitatory circular muscle motor neurons. Physiological evidence demonstrates a functional connection between 5-HT-containing interneurons and AH/Dogiel type II neurons, but few 5-HT-immunoreactive varicosities were observed apposing calbindin-immunoreactive cell bodies. Taken together these results suggest that neural 5-HT may have significant roles in excitatory pathways regulating both motility and secretion.

Binding of isolectin IB4 to neurons of the mouse enteric nervous system

The plant lectin, IB4, binds to primary afferent neurons of dorsal root and trigeminal ganglia, where it is selective for nociceptive neurons. In the enteric nervous system of the guinea-pig IB4 labels intrinsic primary afferent neurons, which are believed to have roles as nociceptors. Here we investigate whether IB4 binding is also a marker of intrinsic primary afferent neurons in the mouse. Neurons that bound IB4 were common in the enteric plexuses of the small intestine and colon. Labeled neurons were rare in the stomach, and absent from the esophagus and gallbladder. Binding was to the cell surface, initial parts of axons and to clumps in the cytoplasm. Similar binding occurred on small and medium sized neurons of dorsal root, nodose and trigeminal ganglia. In the enteric nervous system, IB4 revealed large round or oval (type II) neurons, type I neurons with prominent laminar dendrites and small neurons of myenteric ganglia. The type II neurons were immunoreactive for calretinin, and some type I neurons were immunoreactive for nitric oxide synthase. Most neurons in the submucosal ganglia bound IB4, and some of these were vasoactive intestinal peptide immunoreactive. Thus IB4 binds to specific subgroups of enteric neurons in the mouse. These include intrinsic primary afferent neurons, but other neurons, including secretomotor neurons, are labeled. The results suggest that IB4 is not a specific label for enteric nociceptive neurons.

Cytoplasmic, but not nuclear, expression of the neuronal nuclei (NeuN) antibody is an exclusive feature of Dogiel type II neurons in the guinea-pig gastrointestinal tract

This study aimed to reveal if NeuN, a neuronal nuclei (NeuN) antibody, is a selective marker of intrinsic primary afferent neurons (IPANs) in the guinea-pig gastrointestinal tract as previously hypothesised. The NeuN immunoreactivity was found in the enteric nervous system with exception of the esophagus. Two groups of NeuN-expressing neurons were observed: neurons with immunostained nuclei and cytoplasm (NeuN(NC)) and neurons only expressing immunoreactivity in their nuclei (NeuN(N)). The NeuN(N)-immunoreactive neurons were found in the myenteric plexus of the stomach and the colon. In the stomach, none of the NeuN(N)-expressing neurons, of which 55+/-3% co-expressed calbindin, had a Dogiel type I or II morphology. The NeuN(N)-positive neurons of the colon, which did not express calbindin, did not resemble a Dogiel type II morphology either, but were small-sized neurons. The NeuN(NC)-immunoreactive neurons were observed in both the small and large intestine. These neurons were smooth-contoured and bigger-sized, resembling a Dogiel type II morphology. Some of these neurons co-expressed calbindin. The present data reveal the existence of two populations of Dogiel type II neurons, exhibiting NeuN(NC)+/calbindin+ or NeuN(NC)+/calbindin- immunoreactivity, in the intestine. Assuming that all IPANs exhibit a Dogiel type II morphology, we conclude that the cytoplasmic expression of NeuN is an exclusive feature of IPANs.

Distribution of P2Y6 and P2Y12 receptor: their colocalization with calbindin, calretinin and nitric oxide synthase in the guinea pig enteric nervous system

The distribution of P2Y(6) and P2Y(12) receptor-immunoreactive (ir) neurons and fibers and their coexistence with calbindin, calretinin and nitric oxide synthase (NOS) has been investigated with single and double labeling immunostaining methods. The results showed that 30-36% of the ganglion cells in the myenteric plexus are strongly P2Y(6) receptor-ir neurons; they are distributed widely in the myenteric plexus of stomach, jejunum, ileum and colon, but not in the submucosal plexus, with a typical morphology of multipolar neurons with a long axon-like process. About 42-46% of ganglion cells in both the myenteric and submucosal plexuses show P2Y(12) receptor-ir. About 28-35% of P2Y(6) receptor-ir neurons were found to coexist with NOS and 41-47% of them coexist with calretinin, but there was no coexistence of P2Y(6) receptor-ir with calbindin. In contrast, all P2Y(12) receptor-ir neurons were immunopositive for calbindin, although occasionally P2Y(12) receptor-ir neurons without calbindin immunoreactivity were found, while none of the P2Y(12) receptor-ir neurons were found to coexist with calretinin or NOS in the gastrointestinal system of guinea pig. The P2Y(12) receptor-ir neurons coexpressing calbindin-ir in the small intestine are Dogiel type II/AH, intrinsic primary afferent neurons.

Calbindin-immunopositive cells are cholinergic interneurons in the myenteric plexus of rabbit ileum

The 28-kDa calcium-binding protein (calbindin) is a widely studied neuronal marker in the enteric nervous system of numerous species. Calbindin has previously been detected in myenteric neurons of rabbit ileum in which 3% of all myenteric neurons are calbindin-immunopositive. We have studied the detailed morphology and chemical coding of calbindin-immunopositive neurons in this segment of the gut. We have found calbindin immunoreactivity in both strongly and weakly stained neurons. Of these, the strongly immunoreactive neurons belong to the Dogiel type I category. These neurons project only to other ganglia and primary strands of the plexus and their processes never run to the muscle or mucosal layers. The neurons within this group are 29.5+/-6.6 microm in length and 14.7+/-3.8 microm in width. The second smaller group of immunoreactive cells (27%) label faintly and have different morphological properties. They are characterized by their round medium-sized cell bodies (long axis: 24.4+/-5.2 microm; short axis: 15.5+/-2.9 microm) and do not exhibit immunoreactivity either in their dendrites or in their axonal processes. Double-label studies show that all calbindin-immunopositive neurons lack immunoreactivity for nitric oxide synthase, vasoactive intestinal peptide and substance P but all are immunoreactive for the synthesizing enzyme of acetylcholine, choline acetyltransferase. Thus, populations of neurons containing calbindin are cholinergic interneurons in the myenteric plexus of rabbit ileum.

The distribution of P2X3 purine receptor subunits in the guinea pig enteric nervous system

Adenosine 5'-triphosphate (ATP) excites 70-90% of enteric neurons through P2X type purine receptors, and is likely to be an enteric neurotransmitter. Recent studies indicate that the P2X2 subunit is expressed by specific subgroups of enteric neurons, and that there are enteric neurons that are responsive to ATP but lack this subunit. In the present work, we have investigated whether the P2X3 subunit is similarly localised to specific subgroups of neurons, and whether these are different from the P2X2 subunit-expressing neurons. The P2X3 subunit was localised by immunohistochemistry to nerve cells of the myenteric ganglia of the stomach, small and large intestines, and nerve cells of the submucosal ganglia in the small and large intestines of the guinea pig. All immunoreactivity was absorbed with the P2X3 receptor peptide against which the antiserum was raised. In myenteric ganglia of the ileum, P2X3 receptor immunoreactivity was in calretinin, enkephalin and nitric oxide synthase (NOS)-immunoreactive neurons. In submucosal ganglia, all calretinin-immunoreactive nerve cells were P2X3 receptor immunoreactive. In the submucosal ganglia of the ileum, 13 +/- 3% of neuropeptide Y (NPY)-immunoreactive neurons were also P2X3 receptor immunoreactive, whereas in the distal colon, almost all NPY-expressing nerve cells were P2X3 receptor immunoreactive. The localisation of the P2X3 subunit was largely distinct from that of the P2X2 subunit, although both subunits occur in some NOS neurons, where P2X2 and P2X3 subunits may form heteromeric receptors. Unlike the P2X2 subunit, the P2X3 subunit is not expressed in intrinsic sensory neurons in the ileum. It is concluded that the P2X3 receptor subunit is expressed in specific functional groups of neurons; the major types are excitatory and inhibitory muscle motor neurons, ascending interneurons and cholinergic secretomotor neurons.

Retrograde tracing of enteric neuronal pathways

Neuroanatomical tracing techniques, and retrograde labelling in particular, are widely used tools for the analysis of neuronal pathways in the central and peripheral nervous system. Over the last 10 years, these techniques have been used extensively to identify enteric neuronal pathways. In combination with multiple-labelling immunohistochemistry, quantitative data about the projections and neurochemical profile of many functional classes of cells have been acquired. These data have revealed a high degree of organization of the neuronal plexuses, even though the different classes of nerve cell bodies appear to be randomly assorted in ganglia. Each class of neurone has a predictable target, length and polarity of axonal projection, a particular combination of neurochemicals in its cell body and distinctive morphological characteristics. The combination of retrograde labelling with targeted intracellular recording has made it possible to target small populations of cells that would rarely be sampled during random impalements. These neuroanatomical techniques have also been applied successfully to human tissue and are gradually unravelling the complexity of the human enteric nervous system.

Enteric pathways in the stomach

This report summarises the characteristics of target specific projection and neurochemical coding patterns of motor and interneuronal pathways in the gastric enteric nervous system (ENS) which are involved in the innervation of the mucosa, the circular and the longitudinal muscle. The pathways were identified by retrograde tracing and further characterised by optical and intracellular recordings of the synaptic activation of muscle motor neurones, and by recordings of pathway-specific muscle responses. All motor pathways had polarised projections consisting of ascending cholinergic and descending nitrergic populations. Thus, both muscle layers were innervated by excitatory and inhibitory motor neurones. Their projections indicated the presence of intrinsic circuits that mediate excitatory and inhibitory components of a peristaltic reflex and/or are involved in reflex mediated changes in gastric tone. Although polarised projections were also identified for interneuronal pathways, a substantial proportion of descending interneurones was cholinergic. Interneurones and longitudinal muscle motor pathways had longitudinal projection preferences whereas circular muscle motor pathways had circumferential projection preferences. Target-specific coding was primarily revealed for cholinergic populations; ChAT/ENK/+/-SP neurones projected to the muscle layers, ChAT/NPY/+/-VIP projected to the mucosa and ChAT/+/-SP/+/-5-HT/+/-Calret/+/-Calb were interneurones. Muscle strip recordings revealed the functional significance of ascending excitatory and descending inhibitory pathways to the circular muscle and the prominent influence of ascending and descending cholinergic interneurones which activated excitatory and inhibitory circular muscle motor neurones through nicotinic synapses. It is concluded that enteric pathways in the stomach have region specific features which reflect structural and functional adaptation of the gastric ENS.

Projections and neurochemistry of interneurones in the myenteric plexus of the guinea-pig gastric corpus

Recently, motor neurones of the myenteric plexus innervating the muscle layers or the mucosa have been identified in the guinea-pig stomach. We applied the neuronal tracer DiI (1,1'-didodecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorat) onto myenteric ganglia in order to identify populations of interneurones in the myenteric plexus of the guinea-pig stomach. The tracing was combined with the immunohistochemical detection of calbindinD28k (CALB), choline acetyltransferase (ChAT), neuropeptide Y (NPY) and 5-hydroxytryptamine (serotonin) (5-HT) and the results were compared to the neurochemical coding of target specific motor neurones. Long projecting ( approximately 5.4 mm) ChAT/CALB/+/-5-HT-, nitric oxide synthase (NOS)/CALB- and short projecting ( approximately 1.1 mm) ChAT/NPY-neurones were identified as descending interneurones. CALB positive ascending interneurones contained ChAT but rarely 5-HT (code: ChAT/CALB). This study identified ascending and descending interneurones in the gastric myenteric plexus and revealed the neurochemical coding of some of the interneurone populations.