Effects of vagotomy on neurotransmitter receptors in the rat dorsal vagal complex

Gut-brain axis and the spread of α-synuclein pathology: Vagal highway or dead end?

Spread of α-synuclein pathology from the peripheral to central nervous system may be an important etiological factor in Parkinson's disease, although there are some unanswered questions about its correlation with neuronal loss. Experimental evidence has highlighted the gastrointestinal tract as a potential starting point for aggregated α-synuclein, with the vagus nerve acting as a "highway" by which pathology may be transmitted to the lower brain stem. This review begins by highlighting the key studies demonstrating that α-synuclein pathology has the ability to spread from certain sites in the gastrointestinal tract to the brain (and vice versa). We go on to assess the recent epidemiological studies that have shown that vagotomy and appendectomy may have the potential to reduce the risk of developing Parkinson's disease. Finally, we discuss the factors in the gastrointestinal tract (such as dysbiosis of the gut microbiota, infection, and inflammation) that may trigger α-synuclein aggregation in the first place, as well as other potential mechanisms underlying the distribution of α-synuclein pathology in the brain. © 2019 International Parkinson and Movement Disorder Society.

Effects of substance P on identified neurons of the rat dorsal motor nucleus of the vagus

Previous evidence suggests that substance P (SP) activates subpopulations of neurons within the dorsal motor nucleus of the vagus (DMV). In this study we aimed at identifying these subpopulations in relation to their gastrointestinal projection organs or vagal branches and characterizing pharmacologically the SP response. Using whole cell patch-clamp recordings from identified gastrointestinal-projecting vagal motoneurons, we found that SP induced an inward current in all neuronal groups except for cecum-projecting cells. The lowest percentage of SP-responding neurons was found in fundus-projecting cells, where SP also had a concentration-response curve that was shifted to the left (P < 0.05). Independently from the projections, the SP response was reduced by sendide and MEN 10,376 and mimicked by a combination of [Sar(9)-Met(O(2))(11)]SP and alpha-neurokinin. SP and alpha-neurokinin also increased the frequency, but not the amplitude, of postsynaptic currents. In conclusion, we demonstrated that SP induces both pre- and postsynaptic effects on DMV neurons via activation of neurokinin NK(1) and NK(2) receptors. The magnitude of the SP response was correlated to the peripheral target organ.

Activation of central opioid receptors may induce gastric mucosal defence in the rat

The effect of different opioid peptides on acidified ethanol- and indomethacin-induced gastric mucosal lesions was studied following intracerebroventricular (i.c.v.) administration. It was found that both the selective delta opioid receptor agonists--deltorphin II, [D-Ala(2), D-Leu(5)]-enkephalin (DADLE), [D-Pen(2), D-Pen(5)]-enkephalin (DPDPE)-, mu-opioid receptor agonist--[D-Ala(2), Phe(4), GlyT-ol]-enkephalin (DAGO)--as well as beta-endorphin inhibited the mucosal damage induced by both ethanol and indomethacin in pmolar dose range. In contrast, the gastric acid secretion was not influenced by DADLE in the dose of 16 nmol/rat and only a slight reduction (40%) was induced by DAGO in the dose of 1.9 nmol/rat. The protective effect was abolished in both ulcer models by bilateral cervical vagotomy. N(G)-nitro-L-arginine, an inhibitor of NO synthase, reduced the protective action in ethanol-induced, but not in indomethacin-induced gastric damage. The results suggest that activation of supraspinal delta and mu-opioid receptors resulted in inhibition of gastric mucosal lesions elicited by ethanol or indomethacin. The gastroprotective action is independent from the effect of opioids on acid secretion. Vagal nerve is involved in conveying the central action to the periphery. The mechanism of the gastroprotective effect of opioids is different in ethanol- and indomethacin-ulcer models: prostaglandins and nitric oxide are likely to be involved in the protective action of opioid peptides in ethanol-, but not in the indomethacin-ulcer model.

Vagal circuitry mediating cephalic-phase responses to food

The dorsal vagal complex in the medulla oblongata is the hub of the central nervous system network that produces vagal cephalic-phase reflexes. The preganglionic motor neurons controlling these cephalic responses of digestion and metabolism are organized topographically in longitudinal columnar subnuclei in the dorsal motor nucleus of the vagus. Gustatory and other visceral afferent inputs project into different subnuclei of the nucleus of the solitary tract capping the dorsal motor nucleus. Descending projections from more rostral stations of the neuroaxis project to the nuclei of the dorsal vagal complex, providing input both from exteroceptive senses, such as olfaction and vision, and from forebrain areas that modulate reflex strength. Recent structural analyses of the dorsal vagal complex, as well as characterizations of the region's inputs and neurochemistry, have provided a more complete understanding of the neural basis of cephalic-phase responses.

Respiratory actions of tachykinins in the nucleus of the solitary tract: effect of neonatal capsaicin pretreatment

1. The respiratory response to microinjection of capsaicin and tachykinin receptor agonists into the commissural nucleus of the solitary tract (cNTS) was investigated in adult, urethane-anaesthetized rats which had been pretreated with capsaicin (50 mg kg(-1) s.c.) or vehicle (10% Tween 80, 10% ethanol in saline) as day 2 neonates. 2. Microinjection of capsaicin (1 nmol) into the cNTS of vehicle-pretreated rats, significantly reduced respiratory frequency (59 breaths min(-1), preinjection control, 106 breaths min(-1)) without affecting tidal volume (VT). In capsaicin-pretreated rats, the capsaicin-induced bradypnoea was markedly attenuated (minimum frequency, 88 breaths min(-1); control, 106 breaths min(-1)). 3. In vehicle-pretreated rats, microinjection of substance P (SP, 33 pmol), neurokinin A (NKA, 33 pmol) and NKB (330 pmol), and the selective NK(1) tachykinin receptor agonists, [Sar(9), Met(O(2))(11)]-SP (33 pmol) and septide (10 pmol), increased VT (maxima, 3.60 - 3.93 ml kg(-1)) compared with preinjection control (2.82 ml kg(-1)), without affecting frequency. The selective NK(3) agonist senktide (10 pmol) also increased VT (3.93 ml kg(-1)) which was accompanied by a bradypnoea (-25 breaths min(-1)). The selective NK(2) agonist, [Nle(10)]-NKA(4-10) (330 pmol) increased VT slightly but significantly decreased frequency (-12 breaths min(-1)). In capsaicin-pretreated rats, VT responses to SP and [Sar(9), Met(O(2))(11)]-SP were increased whereas the response to septide was abolished. Both the VT and bradypnoeic responses to senktide and [Nle(10)]-NKA(4-10) were significantly enhanced. 4. These results show that neonatal capsaicin administration markedly reduces the respiratory response to microinjection of capsaicin into the cNTS. The destruction of capsaicin-sensitive afferents appears to sensitize the NTS to SP, NKB, [Sar(9),Met(O(2))(11)]-SP, senktide and [Nle(10)]-NKA(4-10). Moreover, the loss of septide responsiveness in capsaicin-pretreated rats, suggests that 'septide-sensitive' NK(1) receptors may be located on the central terminals of afferent neurons.

Intracisternal TRH and RX 77368 potently activate gastric vagal efferent discharge in rats

The influence of intracisternal (ic) TRH and the stable TRH analog, RX 77368, on gastric vagal efferent discharge (GVED) was investigated in urethane-anesthetized rats. Consecutive IC injections of TRH (3, 30, and 300 ng) at 60 min intervals stimulated dose dependently multi-unit GVED with a peak increase of 90 +/- 21%, 127 +/- 18% and 145 +/- 16% respectively. In two separate studies, IC injection of RX 77368 at 1.5 or 15 ng stimulated multi-unit GVED by 142 +/- 24% and 244 +/- 95% respectively. Saline injection IC had no effect on GVED. RX 77368 (1.5 ng, ic) action was long lasting (84 +/- 13 min) compared with TRH (3 ng: 44 +/- 7 min). Single-unit analysis also showed that 13 of 13 units responded to ic RX 77368 (1.5 ng) by an increase in activity. These data indicate that low doses of TRH injected ic stimulate vagal efferent outflow to the rat stomach and that RX 77368 action is more potent than TRH.

Effects of unilateral vagotomy on nitric oxide synthase and histamine H3 receptors in the rat dorsal vagal complex

Nitric oxide synthase (NOS) and histamine H3 receptors are both markedly increased by neuronal injuries. To examine whether peripheral axotomy produced differential changes in NOS and H3 receptors, both NOS and H3 receptors were measured in the dorsal vagal complex after unilateral vagotomy. The presence of NOS-positive neurons was examined using both NADPH-diaphorase histochemistry and neuronal NOS-immunohistochemistry in rats vagotomized at the mid-cervical level. NADPH-diaphorase activity and NOS-immunoreactivity were markedly enhanced on the dorsal motor nucleus of the vagus (DMX) and in the ambiguous nucleus at the denervated side. Intraperitoneal injection of NOS inhibitors, N omega-nitro-L-arginine (10 mg/kg) or dexamethasone (0.5 mg/kg) attenuated the increase in NADPH-diaphorase activity. Glial fibrillary acidic protein (GFAP) was similarly induced 2 weeks after vagotomy in the vagal complex and surrounding area. Histamine H3 receptors in the vagal complex were visualized with [3H]N alpha-methylhistamine. The ligand-labeled H3 receptors were mainly located at the nucleus of the solitary tract (NST). The densities of H3 receptors did not change in the NST after unilateral vagotomy. These results suggest that peripheral axotomy such as mid-cervical vagotomy preferentially induces NOS in damaged neurons without affecting the level of H3 receptors.

Association of substance P and its receptor with efferent neurons projecting to the greater curvature of the rat stomach

Retrograde tracing and immunocytochemistry were used to identify and map the distribution of substance P (SP) and its receptor (NK-1r) associated with gastric motor neurons in the dorsal motor nucleus of the vagus (DMV) in the rat brain stem. The presence of peptide and receptor in surrounding regions within the dorsal vagal complex were also observed. Injection of the retrograde tracer Fluorogold (FG) into the greater curvature of the stomach produced bilateral labelling of neurons within the DMV. The majority of the NK-1r immunoreactivity appeared as an intricate lattice of fibres with a small number of immunoreactive cell bodies. The NK-1r-labelled fibres were detected within the DMV in close association with FG-labelled neurons and in the region between the DMV and nucleus of the solitary tract (NTS). A proportion of FG-labelled neuronal cell bodies were also labelled with NK-1r (7% of total). The greatest density of NK-1r-labelled fibres was observed at the rostral end of the FG-labelled neuron columns in the DMV (close to the IV ventricle) in the region where gastric vagal afferents terminate. Little NK-1r labelling was observed at the caudal end of the FG-labelled neuron tracts adjacent to the central canal. In the coronal plane, the NK-1r-labelled fibres were seen at the edges of the DMV extending into overlying NTS. Substance P was visualized as a dense network of fibres spanning the entire length of the DMV and in close association with FG-labelled neurons. Substance P staining was also detected in the NTS and in the ventral AP. Most of the association between SP/NK-1r immunoreactive fibres was observed within the DMV and at the border between the DMV and NTS. These findings suggest that SP directly regulates a subpopulation of efferent neurons in the DMV which project to the greater curvature of stomach.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Effects of vagotomy on cholecystokinin- and dexfenfluramine-induced Fos-like immunoreactivity in the rat brain

This study compared the effects of bilateral subdiaphragmatic vagotomy on the Fos-like immunoreactivity (FLI), a marker of neuronal activation, in rat brain induced by two anorectic agents, cholecystokinin (CCK) and the serotonin agonist, dexfenfluramine (DFEN). In the nonvagotomized rats, both CCK (5 micrograms/kg, IP) and DFEN (2 mg/kg, IP) induced FLI in the nucleus of the solitary tract (NST), the external subdivision of the lateral parabrachial nuclei (LPBE), the lateral subdivision of the central amygdaloid nucleus (CeL), and the bed nucleus of the stria terminalis (BST). However, subregional distribution of the FLI induced by the two agents was different in most of these regions. Additionally, the area postrema and the medial subdivision of the hypothalamic paraventricular nucleus were preferentially activated by CCK but not DFEN, while the caudate-putamen was activated by DFEN but not CCK. Bilateral subdiaphragmatic vagotomy completely abolished CCK-induced FLI in all the brain regions but did not attenuate DFEN-induced FLI in any of these regions, including the NST. The results of the present study suggest that DFEN-activation of the NST-LPBE-CeL/BST neuraxis is not mediated by the vagus nerve. On the other hand, and consistent with a variety of other data, activation of various parts of the brain by peripherally administered CCK depends on a vagal pathway. These data are discussed in relation to a previously proposed interaction between CCK and serotonin in mediating satiety.

Presynaptic adenosine A2a receptors on soma and central terminals of rat vagal afferent neurons

The dorsal vagal complex of the medulla oblongata is a key centre involved in the regulation of numerous autonomic functions, including cardiovascular control. Adenosine has been implicated as a potential neuromodulator of the baroreceptor reflex, and therefore the current study has investigated the presence and characteristics of adenosine receptors on rat vagal afferent neurons. In the nodose-vagal grease gap preparation, the adenosine A2a agonist CGS-21680 evoked a depolarisation only in the presence of the selective adenosine A1 antagonist PACPX. Autoradiography using [3H]NECA (4 nM) with suppression of A1 binding enabled the first visualisation of high affinity adenosine A2 receptors in the nucleus tractus solitarius (NTS). Unilateral nodose ganglionectomy resulted in over 90% reduction in binding in the lesioned (ipsilateral) NTS compared to a sham control. Furthermore, local administration of CGS-21680 increased evoked glutamate release in the NTS, as measured by in vivo microdialysis. These data suggest the presence of presynaptic adenosine A2a receptors on both the soma and central terminals of rat vagal afferent neurons, and thereby support the hypothesis that adenosine may have a modulatory role in the baroreceptor reflex.

Electrophysiological effects of serotonin in the solitary tract nucleus of the rat

The nucleus tractus solitarius (nTS) is an important site for the integration of visceral information and its modification by afferent neural systems. One such afferent system arises from the raphe nuclei. This study investigated the electrophysiological effects of the primary transmitter of the raphe nuclei, serotonin (5-HT), on neurones in the nTS of the rat. Extracellular single unit recordings were made of the spontaneous activity of nTS neurones in isolated, superfused brainstem slices during bath-application of 5-HT (50, 100, 250, 500, 1000 nM). Twenty-seven of 46 neurones studied (approximately 59%) showed concentration-dependent decreases of firing rate, with a calculated EC50 of 261 nM. An additional 3 neurones displayed excitatory responses, while the remaining 16 were unaffected. The broad-spectrum 5-HT1 antagonist methysergide (200 nM) was highly effective in producing blockade of 5-HT-evoked depressions of neuronal activity (4 of 4), whereas the 5-HT1A receptor-selective antagonist spiroxatrine (5 nM) and the 5-HT2-selective antagonist mianserin (200 nM) were considerably less effective (1 of 5 and 2 of 5, respectively). Seven additional neurones were examined during exposure to the 5-HT1-selective agonist 5-carboxamidotryptamine (5-CT) or the 5-HT2-selective agonist DOI. 5-CT depressed the activity of three of the four neurones tested, the remaining neurone being unresponsive at concentrations up to 50 nM. DOI at concentrations up to 100 nM failed to affect the activity of two of the three neurones tested, and depressed the activity of the third at a concentration of 50 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

CNS monoamine cell groups projecting to pancreatic vagal motor neurons: a transneuronal labeling study using pseudorabies virus

The CNS monoamine cell groups that project to the pancreatic parasympathetic preganglionic neurons were identified with the use of the viral retrograde transneuronal labeling method. Pseudorabies virus (PRV) was injected into the pancreas of C8 spinal rats and subsequently, transneuronally-labelled central monoamine neurons were mapped in brain tissue sections that had been stained by an immunohistochemical procedure that allowed for the visualization of PRV products and biogenic amine neurotransmitter enzymes or serotonin (5-HT) in the same neuron. The enzymes studied were tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), phenylethanolamine-N-methyltransferase (PNMT), and histidine decarboxylase. Pancreatic vagal motor neurons originate exclusively from the dorsal vagal motor nucleus and some of these may be dopamine neurons because they were TH immunopositive, but DBH and PNMT immunonegative. Transneuronally labeled aminergic neurons were found throughout the medulla oblongata. The adrenergic inputs arose from the C1, C2, and C3 cell groups. Noradrenergic inputs originated predominantly from the A5 cell group, with lesser contributions from the A1 and A2 cell groups as well as from the area postrema. None of the other CNS catecholamine cells were labeled, except for some weakly staining TH-immunoreactive neurons, presumably dopaminergic, in the paraventricular hypothalamic nucleus (PVN). The greatest number of 5-HT neurons that innervate the pancreatic vagal motor neurons come from the gigantocellular reticular nucleus, pars alpha with lesser inputs from the raphe magnus, obscurus, and pallidus nuclei. None of the CNS histaminergic cell groups were labeled.(ABSTRACT TRUNCATED AT 250 WORDS)