Muscarinic receptors couple to modulation of nicotinic ACh receptor desensitization in myenteric neurons

The recruitment of mechanosensitive enteric neurons in the guinea pig gastric fundus is dependent on ganglionic stretch level

BACKGROUND

Serving as a reservoir, the gastric fundus can expand significantly, with an initial receptive and a following adaptive relaxation, controlled by extrinsic and intrinsic reflex circuits, respectively. We hypothesize that mechanosensitive enteric neurons (MEN) are involved in the adaptive relaxation, which is initiated when a particular gastric volume and a certain stretch of the stomach wall is reached. To investigate whether the responsiveness of MEN in the gastric fundus is dependent on tissue stretch, we performed mechanical stimulations in stretched versus ganglia "at rest".

METHODS

Responses of myenteric neurons in the guinea pig gastric fundus were recorded with membrane potential imaging using Di-8-ANEPPS. MEN were identified by small-volume intraganglionic injection in ganglia stretched to different degrees using a self-constructed stretching tool. Immunohistochemical staining identified the neurochemical phenotype of MEN. Hexamethonium and capsaicin were added to test their effect on recruited MEN.

KEY RESULTS

In stretched compared to "at rest" ganglia, significantly more MEN were activated. The change in the ganglionic area correlated significantly with the number of additional recruited MEN. The additional recruitment of MEN was independent from nicotinic transmission and the ratio of active MEN in stretched ganglia shifted towards a nitrergic phenotype.

CONCLUSION AND INFERENCES

The higher number of active MEN with increasing stretch of the ganglia and their greater share of nitrergic phenotype might indicate their contribution to the adaptive relaxation. Further experiments are necessary to address the receptors involved in mechanotransduction.

Mechanosensitive enteric neurons in the guinea pig gastric fundus and antrum

BACKGROUND

Coping with the ingested food, the gastric regions of fundus, corpus, and antrum display different motility patterns. Intrinsic components of such patterns involving mechanosensitive enteric neurons (MEN) have been described in the guinea pig gastric corpus but are poorly understood in the fundus and antrum.

METHODS

To elucidate mechanosensitive properties of myenteric neurons in the gastric fundus and antrum, membrane potential imaging using Di-8-ANEPPS was applied. A small-volume injection led to neuronal compression. We analyzed the number of MEN and their firing frequency in addition to the involvement of selected mechanoreceptors. To characterize the neurochemical phenotype of MEN, we performed immunohistochemistry.

KEY RESULTS

In the gastric fundus, 16% of the neurons reproducibly responded to mechanical stimulation and thus were MEN. Of those, 83% were cholinergic and 19% nitrergic. In the antrum, 6% of the neurons responded to the compression stimulus, equally distributed among cholinergic and nitrergic MEN. Defunctionalizing the sensory extrinsic afferents led to a significant drop in the number of MEN in both regions.

CONCLUSION

We provided evidence for MEN in the gastric fundus and antrum and further investigated mechanoreceptors. However, the proportions of the chemical phenotypes of the MEN differed significantly between both regions. Further investigations of synaptic connections of MEN are crucial to understand the hardwired neuronal circuits in the stomach.

Methyl palmitate modulates the nicotine-induced increase in basilar arterial blood flow

Methyl palmitate (MP) is a fatty acid methyl ester. Our recent study indicated that adrenergic nerve-dependent functional sympathetic-sensory nerve interactions were abolished by MP in mesenteric arteries. However, the effect of MP on perivascular nerves and cerebral blood flow remains unclear. In this study, the increase in basilar arterial blood flow (BABF) after the topical application of nicotinic acetylcholine receptor agonists was measured using laser Doppler flowmetry in anesthetized rats. The choline (a selective α7-nicotinic acetylcholine receptor agonist)-induced increase in BABF was abolished by tetrodotoxin (a neurotoxin), N^G -nitro-L-arginine (a nonselective NO synthase inhibitor), α-bungarotoxin (a selective α7-nicotinic acetylcholine receptor inhibitor), and chronic sympathetic denervation. In addition, the nicotine (a nicotinic acetylcholine receptor agonist)-induced increase in BABF was inhibited by MP in a concentration-dependent manner. The acetylcholine-induced increase in BABF was not affected by MP. The myography results revealed that nicotine-induced vasorelaxation was significantly inhibited by MP, but was reversed by chelerythrine (a protein kinase C inhibitor). MP-induced vasodilation was significantly greater in BA rings without endothelium compared to those with endothelium. Meanwhile, MP did not affect baseline BABF. Our results indicate that MP acts as a neuromodulator in the cerebral circulation where it activates the PKC pathway and causes a diminished nicotine-induced increase in blood flow in the brainstem, and that the vasorelaxation effect of MP may play a minor role.

Nicotine activates YAP1 through nAChRs mediated signaling in esophageal squamous cell cancer (ESCC)

Cigarette smoking is an established risk factor for esophageal cancers. Yes-associated protein 1 (YAP1), the key transcription factor of the mammalian Hippo pathway, has been reported to be an oncogenic factor for many cancers. In this study, we find nicotine administration can induce nuclear translocation and activation of YAP1 in ESCC. Consistently, we observed nuclear translocation and activation of YAP1 by knockdown of CHRNA3, which is a negative regulator of nicotine signaling in bronchial and esophageal cancer cells. Nicotine administration or CHRNA3 depletion substantially increased proliferation and migration in esophageal cancer cells. Interestingly, we find that YAP1 physically interacts with nAChRs, and nAChRs-signaling dissociates YAP1 from its negative regulatory complex composed with α-catenin, β-catenin and 14-3-3 in the cytoplasm, leading to upregulation and nuclear translocation of YAP1. This process likely requires PKC activation, as PKC specific inhibitor Enzastaurin can block nicotine induced YAP1 activation. In addition, we find nicotine signaling also inhibits the interaction of YAP1 with P63, which contributes to the inhibitory effect of nicotine on apoptosis. Using immunohistochemistry analysis we observed upregulation of YAP1 in a significant portion of esophageal cancer samples. Consistently, we have found a significant association between YAP1 upregulation and cigarette smoking in the clinical esophageal cancer samples. Together, these findings suggest that the nicotine activated nAChRs signaling pathway which further activates YAP1 plays an important role in the development of esophageal cancer, and this mechanism may be of a general significance for the carcinogenesis of smoking related cancers.

Mobility of acetylcholine receptors in command Helix lucorum neurons in a cellular analog of habituation

We investigated the role of the mobility of acetylcholine receptors in the depression of an acetylcholine-induced inward current (ACh-current) of Helix lucorum (a land snail) command neurons of defensive behavior in a cellular analog of habituation. The inhibitors of endocytosis and exocytosis, actin microfilaments and cytoskeleton microtubules, serine/threonine protein kinases (PKA, PKG, calcium calmodulin-dependent PK II, p38 mitogen-activated PK), tyrosine kinases (including Src-family kinases), serine/threonine phosphatases (PP1, PP2A, PP2B, PPM1D), and tyrosine protein phosphatases altered the depression of the ACh-current. A comparison of experimentally calculated curves of the ACh-current of these neurons and those obtained by mathematical modeling revealed the following: (a) ACh-current depression is caused by the reduction in the number of membranous ACh-receptors, which results from the shift in the balance of multidirectional transport processes of receptors toward the predominance of ACh-receptor internalization over their recycling; (b) depression of ACh-current depends on the activity of serine/threonine and tyrosine protein kinases and protein phosphatases, whose one of the main targets is the neuron transport system-actin microfilaments and microtubules of cytoskeleton, as well as motor proteins.

A possible role of the cholinergic and purinergic receptor interaction in the regulation of the rat urinary bladder function

The contractile activation of the upper (dome) and lower (base) parts of the urinary bladder show some differences. Cellular mechanisms that might be responsible for cholinergic effects blocking non-adrenergic non-cholinergic contractions in the base of the rat urinary bladder were investigated. Smooth muscle cells were thus freshly isolated or cultured both from the dome and the base of the rat urinary bladder and the contribution from cholinergic and purinergic pathways to their Ca(2+) homeostasis was examined. The expression of nicotinic acetylcholine (nAChR) and P2X2 purinergic receptors on the cultured cells and on tissue sections was investigated. The ATP-evoked Ca(2+) transients in rat smooth muscle cells did not show any desensitization. However, when ATP was administered together with carbamylcholine (CCh), the latter essentially prevented ATP from evoking Ca(2+) transients in smooth muscle cells from the base (suppression to 12 ± 2.5% of control, n = 57; p < 0.01), but not from the dome (99 ± 5% of control, n = 52; p > 0.05) of the rat urinary bladder. While atropine was unable to modify (6 ± 3% of control, n = 14; p < 0.05), α-bungarotoxin (118 ± 12% of control, n = 20; p > 0.05) blocked the inhibitory effects of CCh. Additionally, α7 subunits of nAChR and P2X2 purinergic receptors were identified using immunocytochemistry, immunohistochemistry, and Western blot in cultured urinary bladder smooth muscle cells, in urinary bladder sections, and in urinary bladder muscle strips, respectively, suggesting that the activation of nAChR modifies the action of ATP.

Plasticity of non-adrenergic non-cholinergic bladder contractions in rats after chronic spinal cord injury

The purpose of this study was to examine the pharmacologic plasticity of cholinergic, non-adrenergic non-cholinergic (NANC), and purinergic contractions in neurogenic bladder strips from spinal cord injured (SCI) rats. Bladder strips were harvested from female rats three to four weeks after T(9)-T(10) spinal cord transection. The strips were electrically stimulated using two experimental protocols to compare the contribution of muscarinic and NANC/purinergic contractions in the presence and the absence of carbachol or muscarine. The endpoints of the study were: (1) percent NANC contraction that was unmasked by the muscarinic antagonist 4-DAMP, and (2) P2X purinergic contraction that was evoked by α,β-methylene ATP. NANC contraction accounted for 78.5% of the neurally evoked contraction in SCI bladders. When SCI bladder strips were treated with carbachol (10 μM) prior to 4-DAMP (500 nM), the percent NANC contraction decreased dramatically to only 13.1% of the neurally evoked contraction (P=0.041). This was accompanied by a substantial decrease in α,β-methylene ATP evoked P2X contraction, and desensitization of purinergic receptors (the ratio of subsequent over initial P2X contraction decreased from 97.2% to 42.1%, P=0.0017). Sequential activation of the cholinergic receptors with carbachol (or with muscarine in neurally intact bladders) and unmasking of the NANC response with 4-DAMP switched the neurally evoked bladder contraction from predominantly NANC to predominantly cholinergic. We conclude that activation of muscarinic receptors (with carbachol or muscarine) blocks NANC and purinergic contractions in neurally intact or in SCI rat bladders. The carbachol-induced inhibition of the NANC contraction is expressed more in SCI bladders compared to neurally intact bladders. Along with receptor plasticity, this change in bladder function may involve P2X-independent mechanisms.

Cholinergic Currents in Leg Motoneurons of Carausius morosus

We used patch-clamp recordings and fast optical Ca2+ imaging to characterize an acetylcholine-induced current ( IACh) in leg motoneurons of the stick insect Carausius morosus. Our long-term goal is to better understand the synaptic and integrative properties of the leg sensory-motor system, which has served extremely successfully as a model to study basic principles of walking and locomotion on the network level. The experiments were performed under biophysically controlled conditions on freshly dissociated leg motoneurons to avoid secondary effects from the network. To allow for unequivocal identification, the leg motoneurons were backfilled with a fluorescent label through the main leg nerve prior to cell dissociation. In 87% of the motoneurons, IACh consisted of a fast-desensitizing ( IACh1) and a slow-desensitizing component ( IACh2), both of which were concentration dependent, with EC50 values of 3.7 × 10−5 and 2.0 × 10−5 M, respectively. Ca2+ imaging revealed that a considerable portion of IACh (∼18%) is carried by Ca2+, suggesting that IACh, besides mediating fast synaptic transmission, could also induce Ca2+-dependent processes. Using specific nicotinic and muscarinic acetylcholine receptor ligands, we showed that IACh was exclusively mediated by nicotinic acetylcholine receptors. Distinct concentration–response relations of IACh1 and IACh2 for these ligands indicated that they are mediated by different types of nicotinic acetylcholine receptors.

Inhibition of alpha 7-containing nicotinic ACh receptors by muscarinic M1 ACh receptors in rat hippocampal CA1 interneurones in slices

Cys-loop ligand-gated nicotinic ACh receptors (nAChRs) and G protein-coupled muscarinic ACh receptors (mAChRs) are expressed on rat hippocampal interneurones where they can regulate excitability, synaptic communication and cognitive function. Even though both nAChRs and mAChRs appear to co-localize to the same interneurones, it is not clear whether there is crosstalk between them. We utilized patch-clamp techniques to investigate this issue in rat hippocampal CA1 interneurones in slices under conditions where synaptic transmission was blocked. The alpha7 nAChR-mediated currents were activated by choline, and when the activation of this receptor was preceded by the activation of the M(1) mAChR subtype, the amplitude of alpha7 responses was significantly reduced in a rapidly reversible and voltage-independent manner, without any change in the kinetics of responses. This M(1) mAChR-mediated inhibition of alpha7 nAChRs was through a PLC-, calcium- and PKC-dependent signal transduction cascade. These data show that M(1) mAChRs and alpha7 nAChRs are functionally co-localized on individual rat hippocampal interneurones where the activation of these particular mAChRs inhibits alpha7 nAChR function. This information will help to understand how these cholinergic receptor systems might be regulating neuronal excitability in the hippocampus in a manner that has relevance for synaptic plasticity and cognition.

Insights into mechanisms of intestinal segmentation in guinea pigs: a combined computational modeling and in vitro study

Segmentation in the guinea pig small intestine consists of a number of discrete motor patterns including rhythmic stationary contractions that occur episodically at specific locations along the intestine. The enteric nervous system regulates segmentation, but the exact circuit is unknown. Using simple computer models, we investigated possible circuits. Our computational model simulated the mean neuron firing rate in the feedforward ascending and descending reflex pathways. A stimulus-evoked pacemaker was located in the afferent pathway or in a feedforward pathway. Output of the feedforward pathways was fed into a simple model to determine the response of the muscle. Predictions were verified in vitro by using guinea pig jejunum, in which segmentation was induced with luminal fatty acid. In the computational model, local stimuli produced an oral contraction and anal dilation, similar to in vitro responses to local distension, but did not produce segmentation. When the stimulus was distributed, representing a nutrient load, the result was either a tonic response or globally synchronized oscillations. However, when we introduced local variations in synaptic coupling, stationary contractions occurred around these locations. This predicts that severing the ascending and descending pathways will induce stationary contractions. An acute lesion in our in vitro model significantly increased the number of stationary contractions immediately oral and anal to the lesion. Our results suggest that spatially localized rhythmic contractions arise from a local imbalance between ascending excitatory and descending inhibitory muscle inputs and require a distributed stimulus and a rhythm generator in the afferent pathway.

Activation of cholinergic receptors blocks non-adrenergic non-cholinergic contractions in the rat urinary bladder

In the present study, the plasticity of the non-adrenergic non-cholinergic (NANC) response was investigated. Isolated rat bladder strips were electrically stimulated and the evoked contractions were isometrically recorded. The NANC part of the contractions were unmasked by applying 500 nM 4-DAMP, a potent muscarinic antagonist. Treatment of the bladder strips with 10 microM carbachol (a cholinergic agonist) increased the muscle tone but did not alter the neurally evoked contractions. However, carbachol decreased: (1) the NANC response from 74.6% to 33.3% of control and (2) the purinergic contractile response to alpha,beta-methylene ATP (alpha,beta-mATP) (10 microM) from 97.0% to 43.4% (p<0.05). Treatment with the cholinesterase inhibitor eserine (10 microM) also significantly decreased the NANC response to 21.1% (p<0.0001). The purinergic receptor antagonist suramin (100 microM) did not affect the neurally evoked contractions, however; subsequent addition of 4-DAMP decreased the contractions to 31%. Activation of the smooth muscle cholinergic receptors (with carbachol or eserine) and purinergic receptors (with alpha,beta-mATP) decreased the NANC contractions and the direct contractile response to alpha,beta-mATP. When the electrically evoked contractions were facilitated by the L-type Ca2+ channel activator, Bay-K 8644 the subsequent application of 4-DAMP did not unmask inhibited NANC contractions. We conclude that activation of muscarinic receptors by cholinergic agonist, carbachol or by endogenous acetylcholine (ACh) induce a cascade of events that leads to diminished purinergic response and consequently an inhibition of the bladder NANC response.

Fine-tuning modulation of myenteric motoneurons by endogenous adenosine: on the role of secreted adenosine deaminase

Besides the well-characterized inhibitory effect of adenosine in the gastrointestinal tract mediated by A1 receptors, we recently demonstrated that endogenously generated adenosine facilitates [3H]acetylcholine release from myenteric neurons through preferential activation of prejunctional A2A receptors. The co-existence of both receptor subtypes on cholinergic neurons prompted the question of how does adenosine discriminate between these receptors to regulate synaptic transmission in the longitudinal muscle-myenteric plexus (LM-MP) of the rat ileum. Electrical stimulation of the LM-MP increased the outflow of adenosine, inosine and hypoxanthine. Myenteric neurons seem to be the main source of endogenous adenosine, since blockade of action potentials with tetrodotoxin (1 microM) or omission of Ca2+ (plus EGTA, 1 mM) in the buffer essentially abolished nucleosides release, while adenosine outflow remained unchanged when smooth muscle contractions were prevented by nifedipine (1 microM). Inhibition of ecto-5'-nucleotidase by concanavalin A (0.1 mg ml-1) produced only a moderate decrease (approximately 25%) on adenosine accumulation in the LM-MP, indicating that the extracellular catabolism of released ATP might not be a major source of the nucleoside. Data using the acetylcholinesterase inhibitor, physiostigmine (10 microM), and several subtype-specific muscarinic receptor antagonists, 4-DAMP (100 nM), AF-DX 116 (10 microM) and muscarinic toxin-7 (1 nM), suggest that cholinergic motoneurons are endowed with muscarinic M3 autoreceptors facilitating the outflow of adenosine. Surprisingly, bath samples collected after stimulating the LM-MP exhibited a relatively high adenosine deaminase (ADA) activity (0.60+/-0.07 U ml-1), which increased in parallel with the accumulation of adenosine and its deamination products. Our findings are in keeping with the hypothesis that ADA secretion, along with a less-efficient dipyridamole-sensitive nucleoside transport system, may restrict endogenous adenosine actions to the synaptic region channelling to facilitatory A2A receptors activation. Such a local environment may also limit diffusion of exogenously added adenosine towards the active zones, as we showed that this constrain may be overcome by inhibiting ADA activity with erythro-9(2-hydroxy-3-nonyl) adenine (50 microM).

Dynamics of fast synaptic excitation during trains of stimulation in myenteric neurons of guinea-pig ileum

Fast excitatory postsynaptic potentials (fEPSPs) occur in bursts in the myenteric plexus during evoked motor reflexes in the guinea-pig ileum in vitro. This study used electrophysiological methods to study fEPSPs during stimulus trains to mimic bursts of synaptic activity in vitro. The amplitude of fEPSPs or fast excitatory postsynaptic currents (EPSCs) declined (rundown) during stimulus trains at frequencies of 0.5, 5, 10 and 20 Hz. At 0.5 Hz, fEPSP or fEPSC amplitude declined by 50% after the first stimulus but remained constant for the remainder of the train. At 5, 10 and 20 Hz, synaptic responses ran down completely with time constants of 0.35, 0.21 and 0.11 s, respectively. Recovery from rundown occurred with a time constant of 7 s. Mecamylamine, a nicotinic cholinergic receptor antagonist, or PPADS, a P2X receptor antagonist, reduced fEPSP amplitude, but they had no effect on rundown. Responses caused by trains of ionophoretically applied ATP or ACh (to mimic fEPSPs) did not rundown. Blockade of presynaptic inhibitory muscarinic, adenosine A1, opioid, alpha2-adrenergic and 5-HT1A receptors or pertussis toxin (PTX) treatment did not alter rundown. Antidromic action potentials followed a 10-Hz stimulus train. Iberiotoxin (100 nM), a blocker of large conductance calcium activated K+ (BK) channels, did not alter rundown. These data suggest that synaptic rundown is not due to: (a) action potential failure; (b) nicotinic or P2X receptor desensitization; (c) presynaptic inhibition mediated by pertussis-toxin sensitive G-proteins, or (d) BK channel activation. Synaptic rundown is likely due to depletion of a readily releasable pool (RRP) of neurotransmitter.

Desensitization of α7 nicotinic receptors potentiated the inhibitory effect on M-current induced by stimulation of muscarinic receptors in rat superior cervical ganglion neurons

Whole-cell patch-clamp recording from rat superior cervical ganglion neurons in culture was used to investigate the modulatory effect of desensitized alpha7-nAChRs on mAChRs. An inward alpha-bungarotoxin and methyllycaconitine sensitive current was elicited by rapid application of choline, which consisted of a fast and a slow desensitizing component. The amplitude of choline-evoked currents recorded 0.5, 1, 2, and 3 min after the prolonged application of choline (10 mM, 30 s) decreased to 25.3 +/- 9.2%, 45.9 +/- 11.8%, 66.3 +/- 14.5%, and 73.9 +/- 13.3% of their baseline levels, respectively. The amplitudes of M-currents, recorded at the same time intervals after the similar prolonged stimulation with choline, were decreased to 52.7 +/- 17.4%, 63.9 +/- 4.2%, 70.9 +/- 2.8%, and 72.9 +/- 17.3% of initial values respectively by focal application of pilocarpine (1 mM, 5 s) onto the soma of neurons. By contrast, before the desensitization of alpha7-nAChRs, M-currents were only decreased to 79.8 +/- 13.7% of baseline levels by pilocarpine (1 mM, 5 s). Whereas the desensitization of alpha7-nAChRs had no direct effects on M-currents, and the facilitated effects on muscarinic agonists on the M-currents induced by desensitized alpha7-nAChRs, were removed in the presence of alpha-bungarotoxin and methyllycaconitine. These results indicated that desensitization of alpha7-nAChRs could potentiate the inhibitory effect on M-current by stimulation of mAChRs with their agonist.

Blockade of Gi/o proteins modifies electrical activity of S-myenteric neurons from guinea-pig ileum

We have investigated the effect of blockade of G(i/o) proteins on the electrical activity of S-myenteric neurons from guinea-pig ileum longitudinal muscle-myenteric plexus preparations. Intracellular recordings were made from either control or pertussis toxin (PTX) treated tissues. PTX is known to disrupt the function of G(i/o) proteins. Incubation with PTX (300 ng/ml) induced three main changes in the electrophysiological characteristics of S-neurons: (1) resting membrane potential was more depolarised; (2) fast excitatory postsynaptic potentials (EPSPs) were faster and narrower; and (3) spontaneous fast EPSPs and action potentials were more frequent. Amplitude of slow EPSPs was not modified. This general increase in excitability after blockade of G(i/o) proteins shares characteristics with the slow depolarisation induced by long-lasting trains of low-frequency electrical stimulation and could be mediated by similar mechanisms. Electrophysiological activity of S-myenteric neurons could be modulated by an inhibitory tone, which PTX could modify by either pre- or post-synaptic mechanisms, such as inhibition of the tonic release of inhibitory neurotransmitters and/or intracellular signal transduction involving PTX-sensitive G(i/o) proteins.