Intracellular calcium changes associated with cholinergic nicotinic receptor activation in cultured myenteric plexus neurones

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

Intracellular Ca2+ dynamics of hippocampal interneurons following nicotinic acetylcholine receptor activation

Ca2+ permeability of central nicotinic acetylcholine receptors (nAChRs), especially the alpha7 subunits, are exceptionally high and this important feature provide a special functional importance for these receptors at the system level. Although studies at the cellular level extensively characterized the molecular properties of Ca2+ influx following nAChR activation, much less is known about the time-related Ca2+ dynamics during nicotine administration in integration units of neurons. Such studies are of particular relevance to understanding in situ nonsynaptic actions of nicotine. Puff ejection of drugs produce a rapid drug delivery and elimination from the cell surface allowing the activation of extrasynaptic receptors within desensitization time-frame. In this report we provide evidence that rapid nicotine application is able to produce irregular Ca2+ transients in the dendrites of stratum radiatum interneurons in the hippocampal CA1 region. Potential components and mechanisms of nAChR-mediated Ca2+ influx are discussed in details to demonstrate the unique feature of activation of nAChRs involved in nonsynaptic function in interneurons as compared to other types of nicotinic activity.

Electrical stimulation reveals complex neuronal input and activation patterns in single myenteric guinea pig ganglia

The myenteric plexus plays a key role in the control of gastrointestinal motility. We used confocal calcium imaging to study responses to electrical train stimulation (ETS) of interganglionic fiber tracts in entire myenteric ganglia of the guinea pig small intestine. ETS induced calcium transients in a subset of neurons: 52.2% responded to oral ETS, 65.4% to aboral ETS, and 71.7% to simultaneous oral and aboral ETS. A total of 41.3% of the neurons displayed convergence of oral and aboral ETS-induced responses. Responses could be reversibly blocked with TTX (10(-)6 M), demonstrating involvement of neuronal conduction, and by removal of extracellular calcium. omega-Conotoxin (5 x 10(-7) M) blocked the majority of responses and reduced the amplitude of residual responses by 45%, indicating the involvement of N-type calcium channels. Staining for calbindin and calretinin did not reveal different response patterns in these immunohistochemically identified neurons. We conclude that, at least for ETS close to a ganglion, confocal calcium imaging reveals complex oral and aboral input to individual myenteric neurons rather than a polarization in spread of activity.

Modulation of inhibitory synaptic activity by a non-alpha4beta2, non-alpha7 subtype of nicotinic receptors in the substantia gelatinosa of adult rat spinal cord

The GABA/glycine-mediated inhibitory activity in the substantia gelatinosa (SG) of the spinal cord is critical in the control of nociceptive transmission. We examined whether and how SG inhibitory activity might be regulated by neuronal nicotinic receptors (nAChRs). Patch-clamp recordings were performed in SG neurons of spinal slice preparations from adult rats. We provided electrophysiological evidence that inhibitory presynaptic terminals in the SG expressed nAChRs and their activation resulted in large increases in the frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) in over 90% SG neurons tested. The enhancement of inhibitory activity was mediated by increases in the release of GABA/glycine, and direct Ca(2+) entry through SG presynaptic nAChRs appeared to be involved. Miniature IPSC frequency could be enhanced by the nAChR agonists nicotine or cytisine. Nicotine could still elicit large increases in mIPSC frequency in the presence of the alpha4beta2 nAChR antagonist dihydro-beta-erythroidine (5 microM) and the alpha7 nAChR-selective antagonist methyllycaconitine (40 nM). However, nicotine did not produce a significant enhancement of mIPSC frequency in the presence of the broad spectrum nAChR antagonist mecamylamine (5 microM). Nicotinic agonist-evoked whole-cell currents from SG neurons and the antagonist profiles also indicated the presence of a subtype of nAChRs, which were different from the major central nervous system nAChR subtypes, i.e. alpha4beta2* or alpha7 nAChRs. Together, our results suggest that a subtype of nAChR, possibly alpha3beta4* nAChR or a new nAChR type, is highly expressed at the inhibitory presynaptic terminals in SG of adult rats and play a role in the control of inhibitory activity in SG.

Ligand-gated ion channels in the enteric nervous system

There are many cell surface receptors expressed by neurones in the enteric nervous system (ENS). These receptors respond to synaptically released neurotransmitters, circulating hormones and locally released substances. Cell surface receptors are also targets for many therapeutically used drugs. This review will focus on ligand-gated ion channels, i.e. receptors in which the ligand binding site and the ion channel are parts of a single multimeric receptor. Ligand-gated ion channels expressed by enteric nerves are: nicotinic acetylcholine receptors (nAChRs), P2X receptors, 5-hydroxytryptamine3 (5-HT3) receptors, gamma-aminobutyric acid (GABAA) receptors, N-methyl-d-aspartate (NMDA) receptors,alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and glycine receptors. P2X, 5-HT3 and nAChRs participate in fast synaptic transmission in S-type neurones in the ENS. Fast synaptic transmission occurs in some AH-type neurones, and AH neurones express all the ligand-gated ion channels listed above. Ligand-gated ion channels may be localized at extra-synaptic sites in some AH neurones and these extra-synaptic receptors may be useful targets for drugs that can be used to treat disorders of gastrointestinal function.

Multiple mechanisms of fast excitatory synaptic transmission in the enteric nervous system

The enteric nervous system (ENS) can control gastrointestinal function independent of direct connections with the central nervous system. Enteric nerves perform this important function using multiple mechanisms of excitatory neurotransmission in enteric ganglia. Fast excitatory synaptic transmission in the autonomic nervous system (ANS) is largely mediated by acetylcholine (ACh) acting at nicotinic cholinergic receptors but in the ENS there are noncholinergic fast excitatory neurotransmitters. There are two broad types of neurons in the ENS: S neurons and AH neurons. S neurons are interneurons and motoneurons while AH neurons are sensory neurons. Three subsets of S neurons in the myenteric plexus can be distinguished on the basis of the neurotransmitters producing fast excitatory postsynaptic potentials (fEPSPs) in each subset. In one subset, fEPSPs are mediated solely by ACh acting at nicotinic cholinergic receptors. In a second subset of S neurons, ATP acting at P2X purine receptors and ACh contribute to the fEPSP while in a third subset, fEPSPs are mediated by 5-hydroxytryptamine (5-HT) acting at 5-HT(3) receptors and ACh. Some AH neurons also receive fast excitatory synaptic input. The fEPSPs recorded from AH neurons are mediated ACh and also by glutamate acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. Multiple mechanisms of fast excitatory synaptic transmission in the ENS are likely to contribute to its capacity to regulate complex gastrointestinal functions.

Receptor-induced Ca(2+) signaling in cultured myenteric neurons

We studied the effect of excitatory neurotransmitters (10(-5) M) on the intracellular Ca(2+) concentration ([Ca(2+)](i)) of cultured myenteric neurons. ACh evoked a response in 48.6% of the neurons. This response consisted of a fast and a slow component, respectively mediated by nicotinic and muscarinic receptors, as revealed by specific agonists and antagonists. Substance P evoked a [Ca(2+)](i) rise in 68.2% of the neurons, which was highly dependent on Ca(2+) release from intracellular stores, since after thapsigargin (5 microM) pretreatment only 8% responded. The responses to serotonin, present in 90.7%, were completely blocked by ondansetron (10(-5) M), a 5-HT(3) receptor antagonist. Specific agonists of other serotonin receptors were not able to induce a [Ca(2+)](i) rise. Removing extracellular Ca(2+) abolished all serotonin and fast ACh responses, whereas substance P and slow ACh responses were more persistent. We conclude that ACh-induced signaling involves both nicotinic and muscarinic receptors responsible for a fast and a more delayed component, respectively. Substance P-induced signaling requires functional intracellular Ca(2+) stores, and the 5-HT(3) receptor mediates the serotonin-induced Ca(2+) signaling in cultured myenteric neurons.

Synaptic transmission induces transient Ca2+ concentration changes in cultured myenteric neurones

The enteric nervous system controls most of the gastrointestinal functions. We applied confocal microscopy and the Ca2+ indicator Fluo-3 as an optical approach to study synaptic activation in cultures of myenteric neurones. The optical recording of [Ca2+]i (the intracellular Ca2+ concentration) was used to monitor activation, since [Ca2+]i is crucial in the coupling between neuronal excitation and the activation of several intracellular events. Extracellular fibre tract stimulation (2 s, 30 Hz) caused a transient [Ca2+]i rise in a subset of neurones (50%). These transients lasted for 5.2 s (n=36), with an average amplitude of 3.4 +/- 1.3 times the basal concentration. The removal of extracellular Ca2+ (n=15) or the application of 10-6 M tetrodotoxin (n=16) blocked this response. The N-type Ca2+-channel blocker omega-conotoxin (5 x 10 -7M) abolished the [Ca2+]i increase, while blockade of L-type and P/Q type Ca2+ channels had no effect. Single stimuli evoked a [Ca2+]i rise in the processes. omega-conotoxin-sensitive postsynaptic events required repetitive stimulation. Cholinergic blockade did not inhibit the [Ca2+]i rise in all neurones, suggesting that, besides acetylcholine, other neurotransmitters are involved. Optical imaging of [Ca2+]i can be used to study synaptic spread of activation in enteric neuronal circuits expressed in culture.

Action potential-dependent calcium transients in myenteric S neurons of the guinea-pig ileum

Simultaneous intracellular microelectrode recording and Fura-2 imaging was used to investigate the relationship between intracellular calcium ion concentration ([Ca2+]i) and excitability of tonic S neurons in intact myenteric plexus of the guinea-pig ileum. S neurons were impaled in myenteric ganglia, at locations near connections with internodal strands. The calcium indicator Fura-2 was loaded via the recording microelectrode. The estimated [Ca2+]i of these neurons was approximately 95 nM (n = 25). Intracellular current injection (200 ms pulses, 0.2 nA, delivered at 0.05 Hz) resulted in action potential firing throughout the stimulus pulse, accompanied by transient increases in [Ca2+]i (to approximately 240 nM, n = 12). Increasing the number of evoked action potentials by increasing stimulus duration (100-500 ms) or intensity (0.05-0.3 nA) produced correspondingly larger [Ca2+]i transients. Single action potentials rarely produced resolvable [Ca2+]i events, while short bursts of action potentials (three to five events) invariably produced resolvable [Ca2+]i increases. Some neurons demonstrated spontaneous action potential firing, which was accompanied by sustained [Ca2+]i increases. Action potential firing and [Ca2+]i increases were also observed by activation of slow synaptic input to these neurons, in cases where the slow depolarization initiated action potential firing. Action potentials (evoked or spontaneous) and associated [Ca2+]i transients were abolished by tetrodotoxin (1 microM). Omega-conotoxin GVIA (100 nM) reduced [Ca2+]i transients by approximately 67%, suggesting that calcium influx through N-type calcium channels contributes to evoked [Ca2+]i increases. The S neurons in this study showed prominent afterhyperpolarizations following bursts of action potential firing. The time-course of afterhyperpolarizations was correlated with the time-course of evoked [Ca2+]i transients. Afterhyperpolarizations were blocked by tetrodotoxin and reduced by omega-conotoxin GVIA, suggesting that calcium influx through N-type channels contributes to these events. The electrical properties of Fura-2-loaded neurons were not significantly different from properties of neurons recorded without Fura-2 injection, suggesting that Fura-2 injection alone does not significantly influence the electrical properties of these cells. These data indicate that myenteric S neurons in situ show prominent, activity-dependent increases in [Ca2+]i. These events can be generated spontaneously, or be evoked by intracellular current injection or synaptic activation. [Ca2+]i transients in these neurons appear to involve action potential-dependent opening of N-type calcium channels, and the elevation in [Ca2+]i increase may underlie afterhyperpolarizations and regulate excitability of these enteric neurons.

Functional interactions between nicotinic and P2X channels in short-term cultures of guinea-pig submucosal neurons

1. Functional interactions between nicotinic and P2X receptors in submucosal neurons were investigated. Whole-cell currents induced by ACh (IACh) and ATP (IATP) were blocked by hexamethonium and PPADS), respectively. Currents induced by simultaneous application of the two transmitters (IACh+ATP) were only as large as the current induced by the most effective of these substances. This current occlusion indicates that activation of nicotinic and P2X channels is not independent. 2. Kinetic parameters of IACh+ATP indicate that they are carried through channels activated by either substance. In agreement with this interpretation, both IACh and IATP amplitudes were decreased when ATP and ACh were applied simultaneously, whereas no cross-desensitization was observed when nicotinic and P2X receptors were desensitized individually. 3. Current occlusion was observed at membrane potentials of -60 and +10 mV, when IACh and IATP were inward. However, when these currents were outward (at +40 mV), current occlusion was not observed. Current occlusion was still observed at +40 mV in experiments in which the reversal potential of these currents had been adjusted to more positive values. 4. Current occlusion occurred as soon as currents were detected (< 5 ms), was still present in the absence of Ca2+, Na+ or Mg2+, and after adding staurosporine, genistein, K-252a, or N-ethylmaleimide to the pipette solution. Similar observations were noted after substituting alpha, beta-methylene ATP for ATP, or GTP for GTP-gamma-S in the pipette and in experiments carried out at 36, 23 and 9 C. 5. We propose that nicotinic and P2X channels are in functional clusters of at least two, and that the influx of ions through one activates (through allosteric interactions) a mechanism that inhibits the other channel.

Nitric oxide modulates cholinergic reflex pathways to the longitudinal and circular muscle in the isolated guinea-pig distal colon

1. The involvement of nitric oxide (NO) in enteric neural pathways underlying reflex responses of the longitudinal muscle (LM) and circular muscle (CM) layers activated by mucosal stimulation was examined in the isolated guinea-pig distal colon. 2. A segment of colon spanned two partitions (10 mm apart), which divided the organ bath into three chambers: a recording chamber where LM and CM tension was measured; a stimulation chamber where mucosal stimulation was applied; and a middle chamber separating them. 3. Brushing the mucosa anal and oral to the recording site evoked simultaneous oral contraction and anal relaxation of both the LM and CM. 4. N omega-nitro-L-argininel-NA; 100 microM) or N omega-nitro-L-arginine methyl ester (L-NAME; 100 microM) applied to the middle chamber or stimulation chamber decreased the oral contractile response of the LM and CM (by about 30-40 %), but increased the anal relaxation (> 600 %) and exposed an anal contraction (> 1000 % increase) of both muscles. The addition of L-NA to the recording chamber reduced the anal relaxation of the LM and CM and the anal contraction of the LM, but slightly increased the anal contraction of the CM. 5. S-Nitroso-N-acetylpenicillamine (SNAP; 10 microM), an NO donor, reversed the effects of L-NA in the middle or stimulation chambers. 6. 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM), a soluble guanylate cyclase inhibitor, mimicked the effects of L-NAin the middle chamber or stimulation chamber, but these effects were not reversed by SNAP. 7. The oral contractile responses, and the anal relaxation and contractile responses of the LM and CM produced by L-NA in the stimulation or middle chambers, were blocked by hexamethonium (300 microM) in any chamber. Atropine (1 microM) in the recording chamber reduced the contractile responses of the LM and CM. 8. In conclusion, endogenous NO facilitates and depresses release of acetylcholine from interneurons in ascending and descending nervous pathways, respectively. These NO effects are mediated through soluble guanylate cyclase in cholinergic interneurons

Activity-dependent changes in intracellular calcium in myenteric neurons

The spatial distribution and changes in intracellular calcium concentration ([Ca2+]i) in myenteric neurons were measured using fura 2 in the longitudinal muscle-myenteric plexus preparation from the guinea pig duodenum. These measurements were made simultaneously with intracellular voltage recordings. The generation of action potentials in the cell bodies of both S- and AH-type neurons increased [Ca2+]i in the processes and cell bodies. There was no measurable delay between the [Ca2+]i changes in the somata and the processes, indicating that these changes were caused by the spread of electrical signals and not by diffusion. The rate of Ca2+ removal was faster in the processes than in the somata, apparently due to the large surface-to-volume ratio in the former. In AH neurons, the [Ca2+]i transient was shorter than the duration of the after-spike hyperpolarization. It is concluded that the two main types of myenteric neurons possess voltage-gated Ca2+ channels in both somata and processes.

Comparison of the binding of nicotinic agonists to receptors from human and rat cerebral cortex and from chick brain (alpha 4 beta 2) transfected into mouse fibroblasts with ion channel activity

(-)-Nicotine, cytisine and carbachol evoked 86Rb efflux from mouse fibroblasts stably transfected with alpha 4 beta 2 chick brain nicotinic subunits. This response to (-)-nicotine was inhibited by mecamylamine and dihydro-beta-erythroidine and was mirrored by a rise in intracellular Ca2+ measured by microspectrofluorimetry. Lobeline and isoarecolone methiodide evoked no significant 86Rb from cells and unlike the above agonists displayed significantly different IC50 values for the displacement of [3H]nicotine from mammalian (rat and human cerebral cortex) and transfected fibroblast membranes.

Augmenting action of nicotine on population spikes in the dentate gyrus of the guinea pig

Effects of nicotinic cholinergic agents on field potentials recorded from the dentate gyrus were studied in thin transverse sections of the hippocampus of the guinea pig. Nicotine augmented the population spike elicited by the second stimulus of a paired stimulation to the molecular layer. The threshold concentration of nicotine to cause this effect was 5-10 microM. The augmentation of the spike was not accompanied by an increase in the rising slope of population excitatory postsynaptic potentials, and was not observed in the presence of bicuculline. Carbamylcholine had a weak and inconsistent effect. D-tubocurarine and mecamylamine also augmented the population spike. The action of nicotine was blocked by hexamethonium. These results suggest that nicotine facilitates the generation of action potentials in granule cells by depressing inhibitory processes, and that properties of nicotinic cholinergic receptors are different in different subfields of the hippocampal formation, presumably reflecting the diversity of the receptors.

Long-term potentiation and depression in the dentate gyrus, and effects of nicotine

Effects of a brief high-frequency stimulation or a prolonged low-frequency stimulation in the presence and absence of nicotine were studied in thin transverse slices of the dentate gyrus prepared from the guinea pig. Test and conditioning stimulations were delivered to the middle one-third of the molecular layer, and the slope of the population excitatory postsynaptic potentials (EPSPs) elicited by the test stimulation was taken as an indicator of potentiation or depression. Nicotine was without effects on the N-methyl-D-aspartate (NMDA) component of EPSPs at 50 or 100 microM. A brief high-frequency stimulation induced long-term potentiation (LTP) in the presence of bicuculline. Nicotine (50 microM) almost doubled the magnitude of LTP. In the absence of bicuculline, an identical high-frequency stimulation induced a brief depression, the duration and magnitude of which were increased by nicotine. The increase was not statistically significant, however. In contrast to observations in the region CA1, 720 pulses at 1 Hz delivered after induction of LTP failed to induce long-term depression. Nicotine did not modify the after-effect of the low-frequency stimulation. It was discussed that the facilitation of LTP by nicotine probably resulted from suppression of inhibitory processes.