Distribution and chemical coding of orphanin FQ/nociceptin-immunoreactive neurons in the myenteric plexus of guinea pig intestines and sphincter of Oddi

Longitudinal muscle-myenteric plexus preparations of guinea pig intestines and sphincter of Oddi (SO) were immunostained for orphanin FQ/nociceptin. Orphanin FQ-immunoreactive (OFQ-IR) neurons and nerve fibers were relatively abundant in the SO, duodenum, ileum, cecum, and distal colon, with fewer neurons and nerve fibers observed in the proximal colon. Double staining with antibodies directed against the neuron-specific RNA binding protein Hu revealed that while the numbers of OFQ-IR neurons per ganglion decreased along the gut tube, similar proportions (7-9%) of neurons in these regions were OFQ-IR, whereas <1% of the neurons in the proximal colon were OFQ positive. In the ileum, where 8% of the myenteric neurons were OFQ-IR, all OFQ-IR neurons expressed choline acetyltransferase. In addition, multiple-label immunohistochemistry demonstrated that 58% of the OFQ-IR neurons were calretinin-IR, 52% were substance P-IR, and 28% were enkephalin-IR. Nitric oxide synthase immunoreactivity was observed in about 5% of OFQ-IR neurons, or 0.4% of the total population, and a similar proportion of the OFQ-IR neurons was positive for vasoactive intestinal peptide. No OFQ-IR neurons were immunoreactive for calbindin, somatostatin, or serotonin. These results, combined with previous studies of chemical coding and projection patterns in the guinea pig myenteric plexus, indicate that OFQ-IR is expressed preferentially in excitatory motor neurons projecting to the longitudinal and circular muscle layers, as well as a small subgroup of descending interneurons. Because OFQ is expressed by excitatory motor neurons, and because this peptide inhibits excitatory neurotransmission in the guinea pig ileum, it is likely that OFQ acts through a feedback autoinhibitory mechanism.

Chemical coding of intrinsic and extrinsic nerves in the guinea pig gallbladder: distributions of PACAP and orphanin FQ

The complexity of the neural regulation of the gallbladder is reflected by the variety of neuroactive compounds that are found in the intrinsic and extrinsic nerves of the guinea pig gallbladder. The studies reported here used antisera to test for the presence of gallbladder nerves that are immunoreactive for the neuroactive peptides, pituitary adenylyl activating polypeptide (PACAP), and/or orphanin FQ (OFQ, also known as nociceptin). PACAP immunoreactivity was observed in nerve fibers of the paravascular plexus that were also immunoreactive for calcitonin gene-related peptide. These nerve fibers, which are also immunoreactive for substance P, could be followed into the ganglionated plexus. Within the ganglia, a small proportion of neurons was found to be immunoreactive for PACAP; these neurons were also immunoreactive for vasoactive intestinal peptide and nitric oxide synthase. Immunoreactivity for OFQ was observed in the perivascular plexus in nerve fibers that were also immunoreactive for tyrosine hydroxylase. These nerves were previously shown to be immunoreactive for neuropeptide Y. In the ganglionated plexus, immunoreactivity was observed in all gallbladder neurons, as demonstrated by double staining with antiserum directed against the neuron-specific RNA binding protein, Hu. OFQ immunoreactivity was also present in the small catecholaminergic neurons that are observed in a subset of the ganglia. These results further demonstrate the neurotransmitter diversity of the nerves of the gallbladder, and they provide an incentive for studies of the actions of these compounds in the gallbladder wall.

Neural control of the gallbladder: an intracellular study of human gallbladder neurons

BACKGROUND/AIMS

Gallbladder neurons are important governors of gallbladder function. In animal models, gallbladder ganglia can be regulated both by neural and hormonal inputs. The purpose of this study was to demonstrate the feasibility of obtaining recordings from human gallbladder neurons.

METHODS

Human gallbladders (n = 33) were bathed in oxygenated Krebs solution (37 degrees C) containing the vital fluorescent stain 4-Di-2-ASP to localize the ganglia. Cells were characterized using conventional intracellular recording techniques.

RESULTS

The mean resting membrane potential of human gallbladder neurons was -51.2 +/- 1.8 mV (n = 11). Depolarizing current pulses elicited only 1-4 spikes regardless of the amplitude or duration of the stimulus. Afterspike hyperpolarizations had a mean duration of 144.5 +/- 19.2 ms (n = 10). Anodal break excitation was not recorded with hyperpolarizing current pulses. Fiber tract stimulation elicited fast excitatory postsynaptic potentials in all neurons tested.

CONCLUSION

Intracellular recordings of human gallbladder neurons utilizing 4-Di-2-ASP are thus feasible, but are very problematic due to the density of connective tissue overlying the ganglia. As human and guinea pig gallbladder neurons have similar basic electrical properties, the guinea pig may be an appropriate model for further electrophysiological studies into gallbladder disease.

Actions of histamine on muscle and ganglia of the guinea pig gallbladder

Histamine is an inflammatory mediator present in mast cells, which are abundant in the wall of the gallbladder. We examined the electrical properties of gallbladder smooth muscle and nerve associated with histamine-induced changes in gallbladder tone. Recordings were made from gallbladder smooth muscle and neurons, and responses to histamine and receptor subtype-specific compounds were tested. Histamine application to intact smooth muscle produced a concentration-dependent membrane depolarization and increased excitability. In the presence of the H(2) antagonist ranitidine, the response to histamine was potentiated. Activation of H(2) receptors caused membrane hyperpolarization and elimination of spontaneous action potentials. The H(2) response was attenuated by the ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide in intact and isolated smooth muscle. Histamine had no effect on the resting membrane potential or excitability of gallbladder neurons. Furthermore, neither histamine nor the H(3) agonist R-alpha-methylhistamine altered the amplitude of the fast excitatory postsynaptic potential in gallbladder ganglia. The mast cell degranulator compound 48/80 caused a smooth muscle depolarization that was inhibited by the H(1) antagonist mepyramine, indicating that histamine released from mast cells can activate gallbladder smooth muscle. In conclusion, histamine released from mast cells can act on gallbladder smooth muscle, but not in ganglia. The depolarization and associated contraction of gallbladder smooth muscle represent the net effect of activation of both H(1) (excitatory) and H(2) (inhibitory) receptors, with the H(2) receptor-mediated response involving the activation of K(ATP) channels.

Pharmacology and modulation of K(ATP) channels by protein kinase C and phosphatases in gallbladder smooth muscle

ATP-sensitive K(+) (K(ATP)) channels exhibit pharmacological diversity, which is critical for the development of novel therapeutic agents. We have characterized K(ATP) channels in gallbladder smooth muscle to determine how their pharmacological properties compare to K(ATP) channels in other types of smooth muscle. K(ATP) currents were measured in myocytes isolated from gallbladder and mesenteric artery. The potencies of pinacidil, diazoxide, and glibenclamide were similar in gallbladder and vascular smooth muscle, suggesting that the regions of the channel conferring sensitivity to these agents are conserved among smooth muscle types. Activators of protein kinase C (PKC), however, were less effective at inhibiting K(ATP) currents in myocytes from gallbladder than mesenteric artery. The phosphatase inhibitor okadaic acid increased the efficacy of PKC activators and revealed ongoing basal activation of K(ATP) channels by protein kinase A in gallbladder. These results suggest that phosphatases and basal kinase activity play an important role in controlling K(ATP) channel activity.

Direct neuronal interactions between the duodenum and the sphincter of Oddi

The sphincter of Oddi (SO) is a complex structure that must function in coordination with the motor activities of the gallbladder and the duodenum. It is now clear that a neural circuit exists between the duodenum and the SO, and it is likely that this network is largely responsible for the regulation of SO motility. Recent studies have demonstrated that this circuit provides excitatory cholinergic input to SO ganglia that can be activated by electrical stimulation of the duodenal mucosa, distention of the duodenum, and increased motor activity of the duodenum.

Cholesterol inhibits spontaneous action potentials and calcium currents in guinea pig gallbladder smooth muscle

Elevated cholesterol decreases agonist-induced contractility and enhances stone formation in the gallbladder. The current study was conducted to determine if and how the electrical properties and ionic conductances of gallbladder smooth muscle are altered by elevated cholesterol. Cholesterol was delivered as a complex with cyclodextrin, and effects were evaluated with intracellular recordings from intact gallbladder and whole cell patch-clamp recordings from isolated cells. Cholesterol significantly attenuated the spontaneous action potentials of intact tissue. Furthermore, calcium-dependent action potentials and calcium currents were reduced in the intact tissue and in isolated cells, respectively. However, neither membrane potential hyperpolarizations induced by the ATP-sensitive potassium channel opener, pinacidil, nor voltage-activated outward potassium currents were affected by cholesterol. Hyperpolarizations elicited by calcitonin gene-related peptide were reduced by cholesterol enrichment, indicating potential changes in receptor ligand binding and/or second messenger interactions. These data indicate that excess cholesterol can contribute to gallbladder stasis by affecting calcium channel activity, whereas potassium channels remained unaffected. In addition, cholesterol enrichment may also modulate receptor ligand behavior and/or second messenger interactions.

Neurochemical coding of myenteric neurons in the guinea-pig antrum

Electrophysiological studies of myenteric neurons in the guinea-pig antrum suggest that different neuroactive compounds are involved in synaptic transmission. It is not known what neurotransmitters and neuropeptides are present and to what extent they colocalize. Immunohistochemical stainings were performed on whole-mount preparations of the guinea-pig antrum. Immunoreactivity for neuron-specific enolase was used as a general marker and was set at 100%. There was no overlap between cholinergic and nitrergic neurons, resulting in two separate subpopulations. The presence of choline acetyltransferase immunoreactivity was used to identify the cholinergic subset, which accounted for 56% of the cells. Immunoreactivity for nitric oxide synthase, on the other hand, was displayed in 40.7% of the neurons. Substance-P immunoreactivity was present in 37.4% of the cells and vasoactive intestinal peptide and neuropeptide Y in 21.7% and 28.6%, respectively. Small subsets of neurons had immunoreactivity for serotonin (3.9%), calretinin (6.8%) and calbindin (0.5%). Colocalization studies revealed several subgroups of neurons, containing one or more of the screened markers. Though some similarity is found in the chemical coding of the antrum compared to that of the small intestine and the corpus, remarkable differences can be seen in the occurrence of some subpopulations. Cholinergic neurons are not as predominant as in other parts of the gut, serotonin presence is doubled and some vasointestinal-peptide-positive neurons express substance P. These differences might reflect the highly specialized function of the antrum; however, the exact role of these classes remains to be established.

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.

Neuropeptide Y (NPY) expression is increased in explanted guinea pig parasympathetic cardiac ganglia neurons

While expression of neuropeptides by sympathetic neurons is altered by decentralization and axotomy, it is not known whether similar experimental paradigms also modulate the chemical phenotype of parasympathetic cardiac ganglia neurons. The present study tested whether guinea pig parasympathetic neuron neuropeptide Y (NPY) expression was altered when cardiac ganglia preparations were maintained as organ explants in the presence or absence of colchicine. Two experimental approaches were used to examine NPY expression. First, immunocytochemical techniques were used to quantitate numbers of neurons within the cardiac ganglia exhibiting NPY-immunoreactivity; second, reverse transcription PCR was used to examine proNPY mRNA expression. In control cardiac ganglia preparations, approximately 4% of ganglia neurons exhibited NPY-immunoreactivity. The percentage of NPY-immunopositive neurons in 30- and 72-h explanted cardiac ganglia preparations, maintained in the absence of colchicine, increased to 11 and 16%, respectively. Colchicine treatment of explanted preparations further increased the percentage of NPY-positive ganglia cells 24% (30 h) and 32% (72 h). All NPY-immunoreactive neurons from control ganglia and explanted ganglia were choline acetyltransferase(ChAT)-immunoreactive, indicating retention of the cholinergic phenotype. ProNPY mRNA also was increased following ganglia explantation, consistent with the increase in the numbers of NPY-immunoreactive neurons. NPY transcripts were further increased after 30 h, but not after 72 h in colchicine-treated, explanted cardiac ganglia preparations. These results demonstrate that NPY expression is altered in explanted cardiac ganglia preparations, providing evidence that the chemical phenotype of parasympathetic cardiac neurons can be modulated.

5-HT is present in nerves of guinea pig sphincter of Oddi and depolarizes sphincter of Oddi neurons

This study involved immunohistochemistry and intracellular electrophysiology to investigate serotonergic neurotransmission in the sphincter of Oddi (SO). 5-Hydroxytryptamine (HT)-positive neurons (14 cells/preparation) and nerve fibers were observed in the ganglionated plexus. Serotonergic nerve fibers, which persisted under 2- to 6-day organ culture, were densely distributed, with varicose endings encircling some SO neurons. When 5-HT was applied to SO neurons, it elicited three different responses: 1) a fast depolarization to 5-HT in 31 of 62 cells was mimicked by 2-methyl-5-HT and blocked by LY-278584 (1 microM); 2) a prolonged depolarization to 5-HT in 21 of 62 cells evoked an increase in input resistance and was attenuated by the 5-HT1P antagonist renzapride (1 microM) but not by the 5-HT4 antagonist SDZ-205557 (0.1-10 microM); and 3) an indirect depolarization blocked by TTX or atropine was observed in 32 of 62 cells. 5-HT superfusion elicited a dose-dependent monophasic depolarization (EC50 = 2 microM, n=14). In conclusion, 5-HT is present in nerves of the SO and elicits both 5-HT3 and 5-HT1P receptor-mediated depolarizations, supporting the concept that 5-HT plays a role in SO regulation.

Duodenal sensory neurons project to sphincter of Oddi ganglia in guinea pig

Retrograde labeling of duodenum-sphincter of Oddi (SO) preparations in vitro with the carbocyanine dye DiI revealed that duodenal neurons project to the SO. The duodenum-SO-projecting neurons were immunoreactive (IR) for choline acetyltransferase but not nitric oxide synthase or calretinin, indicating that this is a cholinergic projection and that this pathway is distinct from the circuitry involved in the ascending limb of the peristaltic reflex. Approximately 20% of the duodenum-SO projection neurons were IR for calbindin. Calbindin-IR nerves within SO ganglia degenerated when the SO was maintained in organ culture alone, but persisted when the SO was cultured with the duodenum intact. Therefore, SO ganglia are a target of the calbindin-positive duodenum-SO projection. Because calbindin is a marker of intrinsic sensory neurons that have processes that pass to the mucosa, these neurons are in position to detect the release of a compound from the mucosa and signal its release to SO ganglia. When applied to retrogradely labeled neurons, cholecystokinin (CCK) elicited a prolonged depolarization, indicating that duodenum-SO-projecting neurons could be capable of detecting CCK released from the mucosa. It is proposed that the role of the intrinsic sensory neurons that project to the SO may be to signal the postprandial release of CCK, thus providing an instruction to decrease SO resistance and facilitate the flow of bile into the duodenum.

Expression and physiological actions of neuropeptide Y in guinea pig parasympathetic cardiac ganglia

Guinea pig atrial whole mount preparations containing the parasympathetic cardiac ganglia were used to establish the expression, distribution and actions of neuropeptide Y (NPY) in atrial tissues. NPY-immunoreactive fibers densely innervated the atrial myocardium and blood vessels. Fibers containing NPY also innervated intrinsic parasympathetic cardiac neurons. Four percent of the cardiac neurons, identified using microtubule associated protein-2 antiserum, were NPY-positive. An endogenous source of NPY was confirmed with reverse transcription PCR which demonstrated the presence of proNPY mRNA. Sixty percent of the parasympathetic cardiac neurons were hyperpolarized by local application of NPY. NPY also decreased the amplitude and duration of the action potential after hyperpolarization in 60% of the neurons and decreased the fast excitatory postsynaptic potential in about 50% of the cells. These observations indicate that NPY is anatomically positioned to directly alter the output of the parasympathetic cardiac ganglia either by hyperpolarizing the cardiac neurons or by decreasing the fast synaptic input which drives individual neurons.

Correlation of electrophysiology, neurochemistry and axonal projections of guinea-pig sphincter of Oddi neurones

Sphincter of Oddi (SO) ganglia are comprised of two main types of neurones based either on their electrical or neurochemical properties. This study investigated whether any correlation exists between the electrical and neurochemical properties of these cells. SO neurones were characterized electrically as either Tonic or Phasic cells, labelled with neurobiotin, fixed, and processed for beta-nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-DA) staining and choline acetyltransferase immuno-reactivity to identify whether electrically characterized neurones were nitrergic or cholinergic. A total of 119 cells were analysed in this manner; 45% of cells were Tonic and 37% were Phasic. An equivalent number of Tonic (58.1%, 18/31) and Phasic cells (60%, 21/35) were choline acetyltransferase (ChAT) positive. Three of 34 Phasic cells were NADPH-DA positive, whereas 11/33 Tonic cells were NADPH-DA positive. In none of the preparations was ChAT immunoreactivity and NADPH-DA reactivity ever observed in the same neurone. Calretinin immunoreactivity was present in a subpopulation of both Tonic and Phasic neurones. No correlation was observed between the direction of axon projections and the electrophysiological or neurochemical properties of the cell. These results suggest that there is a lack of correlation between the electrical properties and the neurochemical content of SO neurones. Various explanations for these findings are discussed.

Voltage-dependent K+ currents in smooth muscle cells from mouse gallbladder

The ionic mechanisms associated with the control of gallbladder contractility are incompletely understood. One type of K+ current, the voltage-dependent K+ (KV) current, is relatively uncharacterized in gallbladder cells and may contribute to muscular excitability. The main focus of this study was therefore to determine the voltage dependence and pharmacological nature of this K+ current in isolated myocytes from mouse gallbladder, using the patch-clamp technique. Currents through Ca(2+)-activated K+ channels were minimized by buffering of intracellular Ca2+ (20 nM free Ca2+) and by inclusion of 1 mM tetraethylammonium (TEA+) in the bathing solution. With 140 mM symmetrical K+, membrane depolarization increased K+ currents, independent of driving force, as assessed by tail current analysis. Half-maximal activation of K+ currents occurred at approximately 1 mV and increased e-fold per 9 mV. Inactivation also increased on depolarization, with a midpoint of -24 mV. Single KV channels were recorded in the cell-attached configuration, exhibiting a single-channel conductance of 4.9 pS. TEA+ at 10 mM reduced KV currents by 36%. At +50 mV, 1 mM and 10 mM 4-aminopyridine inhibited currents by 18% and 35%, respectively, whereas 1 and 10 mM 3,4-diaminopyridine inhibited currents by 11% and 21%, respectively. Quinine inhibited KV currents (at +50 mV, 100 microM and 1 mM quinine inhibited current by 24% and 70%, respectively). In summary, we describe voltage-activated K+ currents from the mouse gallbladder that are likely to contribute to the control of muscular excitability.

PGE2 hyperpolarizes gallbladder neurons and inhibits synaptic potentials in gallbladder ganglia

Gallbladder prostaglandin E2 (PGE2) levels are significantly elevated in pathophysiological conditions, resulting in changes in gallbladder motility or secretion that may involve actions of the prostanoid in intramural ganglia. This study was undertaken to examine the effects of PGE2 on neurons of the intramural ganglia of the guinea pig gallbladder. Application of PGE2 by microejection or superfusion elicited a complex triphasic change in the resting membrane potential (RMP). For example, application of PGE2 by microejection (100 microM) resulted in a brief hyperpolarization (mean duration 11.1 +/- 1.3 s), followed by a mid-phase repolarization toward or above RMP (mean duration 50.7 +/- 8.1 s), and finally a long-lasting hyperpolarization (mean duration 157.3 +/- 36.7 s). Associated with these PGE2-evoked alterations in RMP were changes in input resistance measured via injection of hyperpolarizing current pulses. An examination of the action potential afterhyperpolarization (AHP) during the PGE2-evoked response revealed an attenuation of both the amplitude and duration of the AHP. However, only a slight increase in excitability of gallbladder neurons in the presence of PGE2 was evident in response to depolarizing current pulses, and PGE2 did not cause the cells to fire spontaneous action potentials. Application of PGE2 reduced the amplitudes of both fast and slow excitatory synaptic potentials. These results suggest that increased prostaglandin production may decrease ganglionic output and therefore contribute to gallbladder stasis.

Innervation of the gallbladder: structure, neurochemical coding, and physiological properties of guinea pig gallbladder ganglia

The muscle and epithelial tissues of the gallbladder are regulated by a ganglionated plexus that lies within the wall of the organ. Although these ganglia are derived from the same set of precursor neural crest cells that colonize the gut, they exhibit structural, neurochemical and physiological characteristics that are distinct from the myenteric and submucous plexuses of the enteric nervous system. Structurally, the ganglionated plexus of the guinea pig gallbladder is comprised of small clusters of neurons that are located in the outer wall of the organ, between the serosa and underlying smooth muscle. The ganglia are encapsulated by a shell of fibroblasts and a basal lamina, and are devoid of collagen. Gallbladder neurons are rather simple in structure, consisting of a soma, a few short dendritic processes and one or two long axons. Results reported here indicate that all gallbladder neurons are probably cholinergic since they all express immunoreactivity for choline acetyltransferase. The majority of these neurons also express substance P, neuropeptide Y, and somatostatin, and a small remaining population of neurons express vasoactive intestinal peptide (VIP) immunoreactivity and NADPH-diaphorase enzymatic activity. We report here that NADPH-diaphorase activity, nitric oxide synthase immunoreactivity, and VIP immunoreactivity are expressed by the same neurons in the gallbladder. Physiological studies indicate that the ganglia of the gallbladder are the site of action of the following neurohumoral inputs: 1) all neurons receive nicotinic input from vagal preganglionic fibers; 2) norepinephrine released from sympathetic postganglionic fibers acts presynaptically on vagal terminals within gallbladder ganglia to decrease the release of acetylcholine from vagal terminals; 3) substance P and calcitonin gene-related peptide, which are co-expressed in sensory fibers, cause prolonged depolarizations of gallbladder neurons that resemble slow EPSPs; and 4) cholecystokinin (CCK) acts presynaptically within gallbladder ganglia to increase the release of acetylcholine from vagal terminals. Results reported here indicate that hormonal CCK can readily access gallbladder ganglia, since there is no evidence for a blood-ganglionic barrier in the gallbladder. Taken together, these results indicate that gallbladder ganglia are not simple relay stations, but rather sites of complex modulatory interactions that ultimately influence the functions of muscle and epithelial cells in the organ.

Innervation of the gallbladder: structure, neurochemical coding, and physiological properties of guinea pig gallbladder ganglia

The muscle and epithelial tissues of the gallbladder are regulated by a ganglionated plexus that lies within the wall of the organ. Although these ganglia are derived from the same set of precursor neural crest cells that colonize the gut, they exhibit structural, neurochemical and physiological characteristics that are distinct from the myenteric and submucous plexuses of the enteric nervous system. Structurally, the ganglionated plexus of the guinea pig gallbladder is comprised of small clusters of neurons that are located in the outer wall of the organ, between the serosa and underlying smooth muscle. The ganglia are encapsulated by a shell of fibroblasts and a basal lamina, and are devoid of collagen. Gallbladder neurons are rather simple in structure, consisting of a soma, a few short dendritic processes and one or two long axons. Results reported here indicate that all gallbladder neurons are probably cholinergic since they all express immunoreactivity for choline acetyltransferase. The majority of these neurons also express substance P, neuropeptide Y, and somatostatin, and a small remaining population of neurons express vasoactive intestinal peptide (VIP) immunoreactivity and NADPH-diaphorase enzymatic activity. We report here that NADPH-diaphorase activity, nitric oxide synthase immunoreactivity, and VIP immunoreactivity are expressed by the same neurons in the gallbladder. Physiological studies indicate that the ganglia of the gallbladder are the site of action of the following neurohumoral inputs: 1) all neurons receive nicotinic input from vagal preganglionic fibers; 2) norepinephrine released from sympathetic postganglionic fibers acts presynaptically on vagal terminals within gallbladder ganglia to decrease the release of acetylcholine from vagal terminals; 3) substance P and calcitonin gene-related peptide, which are co-expressed in sensory fibers, cause prolonged depolarizations of gallbladder neurons that resemble slow EPSPs; and 4) cholecystokinin (CCK) acts presynaptically within gallbladder ganglia to increase the release of acetylcholine from vagal terminals. Results reported here indicate that hormonal CCK can readily access gallbladder ganglia, since there is no evidence for a blood-ganglionic barrier in the gallbladder. Taken together, these results indicate that gallbladder ganglia are not simple relay stations, but rather sites of complex modulatory interactions that ultimately influence the functions of muscle and epithelial cells in the organ.

Tachykinin-induced activation of non-specific cation conductance via NK3 neurokinin receptors in guinea-pig intracardiac neurones

1. Whole mount preparations from guinea-pig hearts were used to characterize the receptors and ionic mechanisms mediating the substance P (SP)-induced depolarization of parasympathetic postganglionic neurones of the cardiac ganglion. 2. Measurement of the amplitude of depolarization in response to superfusion of different tachykinin agonists (neurokinins A (NKA) and B (NKB), SP, and senktide) gave a rank-order potency of NKB = senktide > NKA > SP, indicating involvement of an NK3 receptor. The use of the selective tachykinin receptor antagonists SR 140333, SR 48986, and SR 142801 demonstrated that only the NK3 receptor antagonist SR 142801 inhibited the SP-induced depolarization. 3. The SP-induced depolarization was not inhibited by Ba2+, TEA, or niflumic acid, or altered by reduced Cl- solutions, but was attenuated in reduced Na+ solutions. Single electrode voltage clamp studies demonstrated that the SP-induced inward current increased in amplitude at more negative potentials, had a reversal potential of approximately 0 mV, and was reduced in amplitude in reduced Na+ solutions. 4. We conclude that the SP-induced depolarization in guinea-pig postganglionic parasympathetic neurones of the cardiac ganglion is due to NK3-mediated activation of a non-selective cation conductance.

Cholecystokinin depolarizes guinea pig sphincter of Oddi neurons by activating CCK-A receptors

Motility studies indicate that cholecystokinin (CCK) acts through a neural mechanism in the sphincter of Oddi (SO) after meals. To evaluate its actions in SO ganglia, CCK was applied by microejection (0.1 mM) or superfusion (0.1 to 300 nM) while recording was carried out intracellularly from intact SO neurons. In tonic cells, microejection and superfusion of CCK caused a prolonged depolarization accompanied by action potentials. In phasic cells, microejection of CCK caused brief and/or prolonged depolarizations, but superfusion caused only prolonged depolarizations. In afterhyperpolarized cells, CCK did not cause a detectable change in the resting membrane potential. In low-Na+ Krebs solution, the prolonged depolarizations in both tonic and phasic cells were significantly reduced. Unsulfated CCK (100 nM) had no effect. CCK-induced depolarization was significantly reduced by a CCK-A, but not a CCK-B, receptor antagonist. It is concluded that CCK can act on CCK-A receptors to depolarize SO neurons. However, it is unlikely that hormonal CCK could mediate such an action because of the discrepancy between the sensitivity of SO neurons for CCK and the peak concentrations of CCK in the serum after a meal.

Identification of the cholinergic neurons in guinea-pig sphincter of Oddi ganglia

The muscular tone of the sphincter of Oddi (SO) can be up- or down-regulated by neurons that lie within ganglia in the wall of the tissue. Previous studies have demonstrated that neurons in the ganglia of the guinea-pig SO can be classified into two major populations, one of which expresses tachykinins and enkephalin and another which expresses nitric oxide synthase. Although results of previous pharmacological studies indicate that acetylcholine is released in the SO, the neurons that express this neurotransmitter have not previously been identified. This study was conducted to establish which neurons in the ganglia of the guinea-pig SO are cholinergic by examining the distribution of choline acetyltransferase (ChAT) immunoreactivity, since the enzyme, ChAT is necessary for acetylcholine synthesis. Choline acetyltransferase immunoreactivity was intense and widespread in the ganglionated plexus of the SO. ChAT-immunoreactive nerve fibers were present in ganglia, interganglionic fiber bundles and in the circular muscle layer. Neurons that were immunoreactive for ChAT comprised about 69% of the population and most of these neurons were also tachykinin-immunoreactive. Co-expression of ChAT and nitric oxide synthase was not observed in nerve cell bodies or nerve fibers. Data from this study support the concept that SO ganglia are largely made up of two populations of neurons, one excitatory and the other inhibitory, on the basis of their chemical coding. The excitatory neurons are cholinergic and co-express tachykinin and opiate peptides and the inhibitory neurons are ChAT-negative and express nitric oxide synthase.

Actions of calcitonin gene-related peptide in guinea pig gallbladder ganglia

The actions of calcitonin gene-related peptide (CGRP) have been determined from intracellular recordings obtained from gallbladder neurons in intact whole mount preparations. In most cells, pressure microejection of CGRP elicited a slow, monophasic depolarization, 4 mV in amplitude, that was associated with a decrease in input resistance and increased excitability. The CGRP-induced depolarization was attenuated in a low-Na+ solution and had a reversal potential of -8 mV. In 10% of the cells, microejection of CGRP elicited a biphasic response that was composed of a rapid transient depolarization followed by a slow depolarization that was similar to the monophasic response. Addition of CGRP (1-10 nM) to the bathing solution elicited a monophasic depolarization and desensitized the cells to applications of CGRP by microejection. Forskolin, applied either by microejection or bath application, also depolarized gallbladder neurons and produced cross-desensitization to CGRP. Responses to substance P were not enhanced by CGRP, and CGRP did not affect fast synaptic responses. It is concluded that CGRP may contribute to a local axon reflex response in gallbladder ganglia.

Expression of choline acetyltransferase immunoreactivity in guinea pig cardiac ganglia

Recent reports indicate that a considerable amount of heterogeneity exists amongst cardiac postganglionic neurons in their chemical coding patterns and electrical properties, and that some of these cells may serve in roles as sensory and interganglionic neurons as well as motor neurons. This study was undertaken to ascertain whether or not all of these neurons are cholinergic by immunostaining whole-mount preparations of the guinea pig heart for choline acetyltransferase (ChAT). Counts of neurons that were immunostained for microtubule-associated protein-2 revealed that about 1000 neurons exist in about 100 ganglia on the posterior atrial surface. ChAT immunoreactivity was expressed by all of the postganglionic neurons in the cardiac ganglia, including the 5% of neurons that also expressed immunoreactivity for nitric oxide synthase. Varicose nerve fibers that were immunoreactive for ChAT were abundant in ganglia, with every cardiac neuron lying in close apposition to one or more labelled varicosities. ChAT-immunoreactive nerve fibers were also observed in large vagosympathetic fiber bundles, in interganglionic fiber bundles, and passing individually within the myocardium. Immunoreactivity for ChAT was also observed in a large proportion of the small tyrosine hydroxylase-immunoreactive neurons that exist in guinea pig cardiac ganglia. These results indicate that all postganglionic neurons in guinea pig cardiac ganglia are likely to utilize acetylcholine as a neurotransmitter, regardless of their functional role in circuitry of cardiac innervation, and each of these neurons is likely to receive cholinergic input.

Structure and chemical coding of human, canine and opossum gallbladder ganglia

Immunohistochemistry and cholinesterase histochemistry were used to evaluate the structure and neurotransmitter content of the ganglionated plexuses of the human, canine, and opossum (Monodelphis domestica) gallbladders. In each species, the ganglionated plexus consisted of small (mean approximately 4 neurons/ganglion), irregularly dispersed ganglia that were interconnected by bundles of nerve fibers. The density of ganglia was about ten-fold higher in the opossum than in the human or the dog. Immunostaining for choline acetyltransferase (ChAT) was accomplished in the human, dog, opossum, and the guinea pig where all neurons were found to express ChAT-immunoreactivity. In the human, immunoreactivities for vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY) were the most abundant followed by substance P (SP). In the dog, immunoreactivity for galanin (GAL) was the strongest, followed closely by VIP and then by SP. NPY-immunoreactive neurons were not observed in the dog, but immunoreactive nerve fibers were seen in the perivascular plexus. In the opossum, immunoreactivity for GAL was the most intense and abundant followed by SP, which was followed by VIP. NPY-immunoreactivity in the opossum was limited to scarce perivascular nerve fibers. Immunoreactivity for calcitonin-gene-related peptide (CGRP) was not observed in neuronal somata, but CGRP/SP-immunoreactive nerve fibers were a feature of each species studied. These findings, along with previously published work on the guinea pig, indicate that it is likely that all gallbladder neurons are cholinergic, and that VIP, SP, and NPY and/or GAL are commonly expressed in gallbladder neurons.

Evidence for afferent fiber innervation of parasympathetic neurons of the guinea-pig cardiac ganglion

The present study was done to establish whether peptidergic afferent inputs can modulate parasympathetic neurons of the guinea-pig cardiac ganglion. Whole mount preparations from the guinea-pig heart were utilized to localize afferent terminals by immunohistochemistry and for intracellular recordings from individual neurons in situ. Action potentials could be elicited by both intracellular current injection and stimulation of interganglionic fiber bundles. Two types of neuron, phasic (95%) and tonic (5%) as defined by their firing properties, were observed. High frequency (5-10 Hz) interganglionic fiber stimulation produced a calcium-dependent, slow depolarization in many cells which was not blocked by 100 microM hexamethonium or 1 microM atropine. A prolonged depolarization was also produced by local application of capsaicin (1 mM), which releases substance P and CGRP from afferent nerve terminals. Microinjection of the mammalian tachykinins substance P, neurokinin A and neurokinin B (all at 100 microM), also produced a slow depolarization. Application of specific agonists for the tachykinin receptor subtypes indicated that these neurons express both NK2 and NK3 receptors. Individual cells were filled with neurobiotin to examine their morphology and the preparations were counter-stained for SP-like immunoreactivity. The results demonstrated that SP-positive fibers are found in close apposition to both phasic and tonic neurons. From these results, we suggest that the parasympathetic neurons of the guinea-pig cardiac ganglion receive inputs from peptidergic, afferent fibers and that this input provides a pathway for potential local reflex control of cardiac function.