Tachykinin neuropeptide-evoked intracellular calcium transients in cultured guinea pig myenteric neurons

Inhibitory effects of galanin on evoked [Ca2+]i responses in cultured myenteric neurons

Galanin modulates gastrointestinal motility by inhibiting the release of ACh from enteric neurons. It is, however, not known whether galanin also inhibits neuronal cholinergic transmission postsynaptically and whether galanin also reduces the action of other excitatory neurotransmitters. The aim of the present study was thus to investigate the effect of galanin on the evoked intracellular Ca(2+) concentration ([Ca(2+)](i)) responses in myenteric neurons. Cultured myenteric neurons from small intestine of adult guinea pigs were loaded with the Ca(2+) indicator fluo-3 AM, and the [Ca(2+)](i) responses following the application of different stimuli were quantified by confocal microscopy and expressed as a percentage of the response to high-K(+) solution (75 mM). Trains of electrical pulses (2 s, 10 Hz) were applied to stimulate the neuronal fibers before and after a 30-s superfusion with galanin (10(-6) M). Substance P (SP), 5-HT, 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP), and carbachol were used as direct postsynaptic stimuli (10(-5) M, 30 s) and were applied alone or after galanin perfusion. Galanin significantly reduced the responses induced by electrical fiber stimulation (43 +/- 2 to 35 +/- 3%, P = 0.01), SP (15.4 +/- 1 to 8.0 +/- 0.3%, P < 0.01), and 5-HT (26 +/- 2 to 21.4 +/- 1.5%, P < 0.05). On the contrary, galanin did not affect the responses induced by local application of DMPP and carbachol. We conclude that in cultured myenteric neurons, galanin inhibits the excitatory responses induced by electrical stimulation, SP, and 5-HT. Finally, the inhibitory effect of galanin on electrical stimulation, but not on DMPP- and carbachol-induced responses, suggests that, at least for the cholinergic component, galanin acts at the presynaptic level.

Calcium imaging of gut activity

The major cell types regulating gut motility include enteric neurones, interstitial cells of Cajal (ICC) and their effector smooth muscle cells. These cells are arranged conveniently in nested layers through the gut wall. Our knowledge of how many of these cells in each layer are integrated to produce the various patterns of motility is largely unknown. So far, much of our knowledge of gut motility has usually been obtained by examining point sources of activity (e.g. intracellular recordings from enteric neurones, ICC and smooth muscle cells), rather than the spread of activity through these spatially distributed nerve and ICC networks, or smooth muscle syncitia. Our understanding of how these cells are integrated to produce gut movements would be greatly enhanced if we could image the activity in many of these cells in each layer, or many cells in several layers, simultaneously. Calcium (Ca2+) is a major signalling and regulatory molecule in most cells. In fact, electrical excitability in enteric neurones, ICC and smooth muscle is associated with robust rises in intracellular Ca2+ that long outlast the electrical events (e.g. action potentials in neurones and smooth muscle) that gave rise to them. These prolonged Ca2+ responses, together with the development of several high quality Ca2+ indicators, has provided a unique opportunity to image many cells in intact tissues simultaneously using ICCD video-rate cameras along with conventional microscopy. However, confocal microscopy has also been used, and has several advantages over the above systems. These include reduced photo-toxicity and bleaching and the elimination of out of focus light from different layers within the tissue. So far, despite some limitations with the calcium imaging techniques, the spread of activity through the two layers of smooth muscle, ICC networks and myenteric neurones in intact preparations, or cultured myenteric neuronal networks, is beginning to yield exciting new data about how these different cells interact and process information.

Calcium- and superoxide anion-mediated mitogenic action of substance P on cardiac fibroblasts

Substance P is released from nerve endings in the heart under pathological conditions like ischemia, but its action on cardiac cells has not been investigated. This study tested the hypothesis that substance P is mitogenic to adult cardiac fibroblasts and delineated the underlying mechanism(s). Substance P, acting via neurokinin-1 (NK-1) receptors, stimulated cellular hyperplasia over a range of 1-10 micromol/l. It elicited no change in net collagen production, total protein synthesis, or cell protein content but increased (45)Ca uptake and superoxide generation. EGTA, N-acetyl-cysteine, and superoxide dismutase attenuated the hyperplastic response to substance P. A combination of substance P and EGTA enhanced superoxide generation without an increase in DNA synthesis, showing that an increase in superoxide production does not result in hyperplasia when extracellular Ca(2+) is chelated. Together, the data suggest that substance P may activate, via NK-1 receptors, a hyperplastic but not hypertrophic response in adult cardiac fibroblasts and that alterations in redox state and Ca(2+) homeostasis may act in concert to mediate its mitogenic action.

Immunohistochemical localization of voltage-activated calcium channels in the rat oesophagus

Voltage-activated calcium channels play an important role in the physiology of the enteric nervous system. To determine which types of voltage-activated calcium channels are present in the rat oesophagus, an immunohistochemical study was performed using specific antibodies for the alpha1 subunits of Cav2.1 (P/Q-type), Cav2.2 (N-type), Cav1.2 and Cav1.3 (L-type) calcium channels. All myenteric cell bodies showed Cav2.2 immunoreactivity, whereas labelling for this N-type channel was absent in nerve fibres. Cav1.2 immunoreactivity was found on nerve fibres in the myenteric plexus and on fibres innervating the striated muscle of the rat oesophagus, whereas no labelling was detected on neuronal somata. Immunoreactivity against Cav1.3 was not detected in the myenteric plexus or at the level of the striated muscle. Labelling for Cav2.1 was absent at the level of the myenteric plexus, but present in the striated muscle layer at the level of the motor endplates. Comparison with recent literature data from rat small intestine reveals region-specific distribution patterns of the various subtypes of voltage-activated calcium channels within the enteric nervous system. In addition, the present immunohistochemical data corroborate our physiological data (see accompanying paper), which indicate that the Cav2.2 (N-type) channel is the predominant channel involved in the generation of the calcium-dependent action potential evoked by intrasomatic depolarizing current pulses in all rat oesophageal myenteric neurones.

Differential Ca(2+) signaling characteristics of inhibitory and excitatory myenteric motor neurons in culture

Physiological studies on functionally identified myenteric neurons are scarce because of technical limitations. We combined retrograde labeling, cell culturing, and fluorescent intracellular Ca(2+) concentration ([Ca(2+)](i)) signaling to study excitatory neurotransmitter responsiveness of myenteric motor neurons. 1, 1-Didodecyl-3,3,3',3'-tetramethyl indocarbocyanine (DiI) was used to label circular muscle motor neurons of the guinea pig ileum. DiI-labeled neurons were easily detectable in cultures prepared from these segments. The excitatory neurotransmitters (10(-5) M) acetylcholine, substance P, and serotonin induced a transient rise in [Ca(2+)](i) in subsets of DiI-labeled neurons (66.7, 56.5, and 84. 3%, respectively). DiI-labeled motor neurons were either inhibitory (23.8%) or excitatory (76.2%) as assessed by staining for nitric oxide synthase or choline acetyltransferase. Compared with excitatory motor neurons, significantly fewer inhibitory neurons in culture responded to acetylcholine (0 vs. 69%) and substance P (12.5 vs. 69.2%). We conclude that combining retrograde labeling and Ca(2+) imaging allows identification of differential receptor expression in functionally identified neurons in culture.

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.