5-Hydroxytryptamine depolarizes neurons of cat pancreatic ganglia

Unravelling innervation of pancreatic islets

The central and peripheral nervous systems play critical roles in regulating pancreatic islet function and glucose metabolism. Over the last century, in vitro and in vivo studies along with examination of human pancreas samples have revealed the structure of islet innervation, investigated the contribution of sympathetic, parasympathetic and sensory neural pathways to glucose control, and begun to determine how the structure and function of pancreatic nerves are disrupted in metabolic disease. Now, state-of-the art techniques such as 3D imaging of pancreatic innervation and targeted in vivo neuromodulation provide further insights into the anatomy and physiological roles of islet innervation. Here, we provide a summary of the published work on the anatomy of pancreatic islet innervation, its roles, and evidence for disordered islet innervation in metabolic disease. Finally, we discuss the possibilities offered by new technologies to increase our knowledge of islet innervation and its contributions to metabolic regulation.

Optical Imaging of Pancreatic Innervation

At the time of Ivan Pavlov, pancreatic innervation was studied by looking at pancreas secretions in response to electrical stimulation of nerves. Nowadays we have ways to visualize neuronal activity in real time thanks to advances in fluorescent reporters and imaging techniques. We also have very precise optogenetic and pharmacogenetic approaches that allow neuronal manipulations in a very specific manner. These technological advances have been extensively employed for studying the central nervous system and are just beginning to be incorporated for studying visceral innervation. Pancreatic innervation is complex, and the role it plays in physiology and pathophysiology of the organ is still not fully understood. In this review we highlight anatomical aspects of pancreatic innervation, techniques for pancreatic neuronal labeling, and approaches for imaging pancreatic innervation in vitro and in vivo.

Intrapancreatic Ganglia and Neural Regulation of Pancreatic Endocrine Secretion

Extrapancreatic nerves project to pancreatic islets directly or converge onto intrapancreatic ganglia. Intrapancreatic ganglia constitute a complex information-processing center that contains various neurotransmitters and forms an endogenous neural network. Both intrapancreatic ganglia and extrapancreatic nerves have an important influence on pancreatic endocrine function. This review introduces the histomorphology, innervation, neurochemistry, and electrophysiological properties of intrapancreatic ganglia/neurons, and summarizes the modulatory effects of intrapancreatic ganglia and extrapancreatic nerves on endocrine function.

Impact of the 5-HT3 receptor channel system for insulin secretion and interaction of ginger extracts

The relevance of serotonin and in particular that of 5-HT(3) receptors is unequivocal with respect to emetic/antiemetic effects, but it is controversial with respect to antidiabetic effects. The effects of tropisetron (5-HT(3) receptor antagonist) and various ginger (Zingiber officinale) extracts (known to interact with the 5-HT(3) receptor channel system) were investigated. Serotonin (32 to 500 microM) inhibits insulin release (RIA) from INS-1 cells which is reversed by tropisetron (10 to 100 microM) and two different ginger extracts (spissum and an oily extract). Their effects are obvious even in the absence of serotonin but are more pronounced in its presence (doubled to tripled). Specific 5-HT(3) binding sites are present in INS-1 cells using 0.4 nM [3H] GR65630 in displacement experiments. The in vitro data with respect to ginger are corroborated by in vivo data on glucose-loaded rats showing that blood glucose (Glucoquant) is decreased by approximately 35% and plasma insulin (RIA) is increased by approximately 10%. Both the spissum extract and the oily ginger extract are effective in two other models: they inhibit [(14)C] guanidinium uptake into N1E-115 cells (model of 5-HT(3) effects) and relax rat ileum both directly and as a serotonin antagonistic effect. Other receptors addressed by ginger are 5-HT(2) receptors as demonstrated by using methysergide and ketanserin. They weakly antagonize the serotonin effect as well. It may be concluded that serotonin and in particular the 5-HT(3) receptor channel system are involved in modulating insulin release and that tropisetron and various ginger extracts can be used to improve a diabetic situation.

Serotonin (5-hydroxytryptamine, 5-HT) immunoreactive endocrine and neural elements in the chromaffin enteropancreatic system of amphibians and reptiles

The diffuse chromaffin enteropancreatic system of nine species of amphibians (newts, frogs) and reptiles (turtles, lizards, snakes) was investigated immunohistochemically for the presence and topographic distribution of serotonin (5-hydroxytryptamine, 5-HT). The study revealed various numbers of serotonin-producing cells in the pancreas and intestinal epithelium and also immunolabelled nerve profiles in the villi of all species studied. In addition, two different morphological populations of serotonin cells ("open" and "closed") were localized in the functional segments of the intestines in the representative species of all the taxa investigated. Semi-quantitative evaluation of the immunolabelled pancreatic and enteric cells revealed significantly different mean numbers of labelled cells in different amphibian and reptilian taxa, and also between the various successive gut segments of each taxon. The ratio between "open" and "closed" varieties of serotonin cells recorded along the intestines followed a decreasing trend, progressive in lizards and snakes and more abrupt in newts, frogs and turtles. The above findings may help resolve several key stages of the phylogenetic evolution of poikilothermic vertebrates.

Autonomic pathways regulating pancreatic exocrine secretion

The parasympathetic (PNS) and sympathetic (SNS) and nervous systems densely innervate the exocrine pancreas. Efferent PNS pathways, consisting of central dorsal motor nucleus of the vagus (DMV) and peripheral pancreatic neurons, stimulate exocrine secretion. The DMV integrates cortical (olfactory, gustatory) and gastric, and intestinal vagal afferent input to determine central PNS outflow during cephalic, gastric and intestinal phases of exocrine secretion. Pancreatic neurons integrate DMV input with peripheral enteric, sympathetic, and, possibly, afferent axon reflexes to determine final PNS input to all exocrine effectors. Gut and islet hormones appear to modulate both central and peripheral PNS pathways. Preganglionic sympathetic neurons in the intermediolateral (IML) column of the spinal cord receive inputs from brain centers, some shared with the PNS, and innervate postganglionic neurons, mainly in prevertebral ganglia. Sympathetic innervation of the exocrine pancreas is primarily indirect, and inhibits secretion by decreasing blood flow and inhibiting transmission in pancreatic ganglia. Interactions between SNS and PNS pathways appear to occur in brain, spinal cord, pancreatic and prevertebral ganglia, and at neuroeffector synapses. Thus, the PNS and SNS pathways regulating the exocrine pancreas are directly or indirectly antagonistic at multiple sites: the state of exocrine secretion reflects the balance of these influences. Despite over a century of study, much remains to be understood about the connections of specific neurons forming pancreatic pathways, their processes of neurotransmission, and how disruption of these pathways contributes to pancreatic disease.

Catecholamines and 5-hydroxytryptamine in tissues of the rabbit exocrine pancreas

OBJECTIVES

Norepinephrine (NE), dopamine (DA), epinephrine (Epi), and 5-hydroxytryptamine (5-HT) all modulate pancreatic exocrine secretion, yet their concentrations in specific tissues of the exocrine pancreas are unknown.

METHODS

Concentrations of catecholamines and 5-HT in rabbit pancreatic ganglia, acini, ducts and ampullae, and arteries and veins were measured using HPLC.

RESULTS

Concentrations of NE in ganglia from the head/neck region were significantly higher than those from the body (1620 +/- 220 vs. 778 +/- 179 pmol/mg protein). Acini contained little NE, DA, or 5-HT (9 +/- 2, 0.9 +/- 0.2, 13 +/- 5 pmol/mg protein). Ducts and ampullae contained NE (314 +/- 74 and 156 +/- 24 pmol/mg protein), DA (43 +/- 14 and 13 +/- 4 pmol/mg protein), Epi (63 +/- 29 and 39 +/- 6 pmol/mg protein), and 5-HT (696 +/- 151 and 3563 +/- 288 pmol/mg protein). Arteries and veins contained the highest concentrations of NE (1962 +/- 463 and 736 +/- 80 pmol/mg protein, respectively).

CONCLUSIONS

Pancreatic ganglia and blood vessels, rather than acini, are the main sites of noradrenergic sympathetic innervation of the rabbit exocrine pancreas. These nerves preferentially target ganglionic transmission in the head/neck versus the body. Serotonergic nerves provide little or no innervation of rabbit pancreatic ganglia or acini.

Electrophysiological effects of GABA on cat pancreatic neurons

In mammalian peripheral sympathetic ganglia GABA acts presynaptically to facilitate cholinergic transmission and postsynaptically to depolarize membrane potential. The GABA effect on parasympathetic pancreatic ganglia is unknown. We aimed to determine the effect of locally applied GABA on cat pancreatic ganglion neurons. Ganglia with attached nerve trunks were isolated from cat pancreata. Conventional intracellular recording techniques were used to record electrical responses from ganglion neurons. GABA pressure microejection depolarized membrane potential with an amplitude of 17.4 +/- 0.7 mV. Electrically evoked fast excitatory postsynaptic potentials were significantly inhibited (5.4 +/- 0.3 to 2.9 +/- 0.2 mV) after GABA application. GABA-evoked depolarizations were mimicked by the GABA(A) receptor agonist muscimol and abolished by the GABA(A) receptor antagonist bicuculline and the Cl(-) channel blocker picrotoxin. GABA was taken up and stored in ganglia during preincubation with 1 mM GABA; beta-aminobutyric acid application after GABA loading significantly (P < 0.05) increased depolarizing response to GABA (15.6 +/- 1.0 vs. 7.8 +/- 0.8 mV without GABA preincubation). Immunolabeling with antibodies to GABA, glial cell fibrillary acidic protein, protein gene product 9.5, and glutamic acid decarboxylase (GAD) immunoreactivity showed that GABA was present in glial cells, but not in neurons, and that glial cells did not contain GAD, whereas islet cells did. The data suggest that endogenous GABA released from ganglionic glial cells acts on pancreatic ganglion neurons through GABA(A) receptors.

Serotonin receptor subtypes that depolarize guinea pig inferior mesenteric ganglion neurons

Our previous studies indicated that serotonin (5-HT) depolarized a majority of guinea pig inferior mesenteric ganglion (IMG) neurons and may be another transmitter for the noncholinergic late slow excitatory postsynaptic potential (ls-EPSP) in the IMG. However, the subtypes of 5-HT receptor mediating these responses have not yet been identified. Using intracellular recording, we examined the effect of 5-HT receptor antagonists with specificity to various 5-HT receptor subtypes on the 5-HT-mediated depolarization and ls-EPSP in IMG neurons in vitro. Cyproheptadine, a 5-HT(1/2) receptor antagonist, reversibly inhibited the slow, but not the fast, depolarization and ls-EPSP in the 5-HT-sensitive neurons. Both mianserin and spiperone, 5-HT(2) and 5-HT(1A) receptor antagonists, did not significantly alter either the fast or slow depolarizing responses or the ls-EPSP. The 5-HT(3) receptor antagonist MDL 72222 (Bemesetron) completely inhibited the fast depolarization with little diminution of the slow depolarization and ls-EPSP. Superfusion of putative 5-HT(1P) receptor antagonist, BRL 24924 (Renzapride), reversibly attenuated both the depolarization and ls-EPSP. However, 5-HT-insensitive neurons with ls-EPSP were found to be insensitive to both cyproheptadine and BRL 24924. In most 5-HT-sensitive neurons, the 5-HT(3) receptor agonist, 2-methyl-5-HT, and the selective 5-HT(1P) agonist, MCPP or 5-OHIP, evoked a fast and a slow depolarization in 55.6 and 71.4% of the neurons, respectively, without a significant effect on the membrane potential in 85.7 and 100% of the 5-HT-insensitive neurons. In 5-HT-sensitive neurons, MDL 72222 reversibly abolished the fast depolarization induced by 2-methyl-5-HT; BRL 24924 significantly inhibited the slow depolarization induced by MCPP or 5-OHIP, but not by SP. Prolonged superfusion of 5-HT-sensitive neurons with MCPP abolished the evoked ls-EPSP without inhibition of action potential. These results suggest that the fast and slow depolarizations in these neurons are mediated by 5-HT(3) and 5-HT(1P) receptor subtypes, respectively. The latter may also mediate the ls-EPSP in 5-HT-sensitive neurons.

Guinea pig pancreatic neurons: morphology, neurochemistry, electrical properties, and response to 5-HT

The morphology, neurochemistry, and electrical properties of guinea pig pancreatic neurons were determined. The majority of neurons expressed choline acetyltransferase (ChAT) immunoreactivity; however, ChAT-negative neurons were also found. Both cholinergic and noncholinergic neurons expressed nitric oxide synthase (NOS) immunoreactivity. Three types of pancreatic neurons were distinguished. Phasic neurons fired action potentials (APs) at the onset of depolarizing current pulse, tonic neurons spiked throughout the duration of a suprathreshold depolarizing pulse, and APs could not be generated in nonspiking neurons, even though they did receive synaptic input. APs were tetrodotoxin sensitive, and all types of neurons received fast and slow excitatory postsynaptic potentials (EPSPs). Fast EPSPs had cholinergic and noncholinergic components. The majority of pancreatic neurons appeared to innervate the acini. NOS- and/or neuropeptide Y-immunoreactive phasic and tonic neurons were found. Microejection of 5-hydroxytryptamine (5-HT) caused a slow depolarization that was inhibited by the 5-HT1P antagonist N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide and mimicked by the 5-HT1P agonist 6-hydroxyindalpine. A pancreatic 5-HT transporter was located, and inhibition of 5-HT uptake by fluoxetine blocked slow EPSPs in 5-HT-responsive neurons by receptor desensitization.

Involvement of 5-hydroxytryptamine (5-HT)3 receptor mechanisms in regulation of basal pancreatic secretion in conscious rats

The effects of 5-hydroxytryptamine (5-HT)3 receptor antagonists (azasetron and granisetron) on basal pancreatic exocrine secretion were examined in conscious rats. Rats were prepared with cannulae draining bile and pancreatic juice separately. Intravenous injection of azasetron significantly increased pancreatic fluid and protein outputs in a dose-dependent manner. These increases were completely abolished by treatment with atropine, but not affected by bilateral truncal vagotomy. Intravenous injection of granisetron also increased pancreatic secretion, significantly. Intragastric injection of azasetron increased pancreatic secretion, although a double dose was required to elicit the stimulatory effect compared with intravenous injection. It is concluded that 5-HT3 receptor activity is involved in regulation of basal pancreatic secretion in conscious rats.