Differential effects of voltage-dependent Ca2+ channels on low and high frequency mediated neurotransmission in guinea-pig ileum and rat vas deferens

Alpha2-adrenoceptors couple to inhibition of R-type calcium currents in myenteric neurons

Alpha2-adrenoceptors inhibit Ca2+ influx through voltage-gated Ca2+ channels throughout the nervous system and Ca2+ channel function is modulated following activation of some G-protein coupled receptors. We studied the specific Ca2+ channel inhibited following alpha2-adrenoceptor activation in guinea-pig small intestinal myenteric neurons. Ca2+ currents (I(Ca2+)) were studied using whole-cell patch-clamp techniques. Changes in intracellular Ca2+ (delta[Ca2+]i) in nerve cell bodies and varicosities were studied using digital imaging where Ca2+ influx was evoked by KCl (60 mmol L(-1)) depolarization. The alpha2-adrenoceptor agonist, UK 14 304 (0.01-1 micromol L(-1)) inhibited I(Ca2+) and delta[Ca2+]i; maximum inhibition of I(Ca2+) was 40%. UK 14 304 did not affect I(Ca2+) in the presence of SNX-482 or NiCl2 (R-type Ca2+ channel antagonists). UK 14 304 inhibited I(Ca2+) in the presence of nifedipine, omega-agatoxin IVA or omega-conotoxin, inhibitors of L-, P/Q- and N-type Ca2+ channels. UK 14 304 induced inhibition of I(Ca2+) was blocked by pertussis toxin pretreatment (1 microg mL(-1) for 2 h). Alpha2-adrenoceptors couple to inhibition of R-type Ca2+ channels via a pertussis toxin-sensitive pathway in myenteric neurons. R-type channels may be a target for the inhibitory actions of noradrenaline released from sympathetic nerves on to myenteric neurons.

Expression Profiles of High Voltage-Activated Calcium Channels in Sympathetic and Parasympathetic Pelvic Ganglion Neurons Innervating the Urogenital System

Among the autonomic ganglia, major pelvic ganglia (MPG) innervating the urogenital system are unique because both sympathetic and parasympathetic neurons are colocalized within one ganglion capsule. Sympathetic MPG neurons are discriminated from parasympathetic ones by expression of low voltage-activated Ca2+ channels that primarily arise from T-type alpha1H isoform and contribute to the generation of low-threshold spikes. Until now, however, expression profiles of high voltage-activated (HVA) Ca2+ channels in these two populations of MPG neurons remain unknown. Thus, in the present study, we dissected out HVA Ca2+ channels using pharmacological and molecular biological tools. Reverse transcription-polymerase chain reaction analysis showed that MPG neurons contained transcripts encoding all of the known HVA Ca2+ channel isoforms (alpha1B, alpha1C, alpha1D and alpha1E), with the exception of alpha1A. Western blot analysis and pharmacology with omega-agatoxin IVA (1 microM) confirmed that MPG neurons lack the alpha1A Ca2+ channels. Unexpectedly, the expression profile of HVA Ca2+ channel isoforms was identical in the sympathetic and parasympathetic neurons of the MPG. Of the total Ca2+ currents, omega-conotoxin GVIA-sensitive N-type (alpha1B) currents constituted 57 +/- 5% (n = 9) and 60 +/- 3% (n = 6), respectively; nimodipine-sensitive L-type (alpha1C and alpha1D) currents made up 17 +/- 4% and 14 +/- 2%, respectively; and nimodipine-resistant and omega-conotoxin GVIA-resistant R-type currents were 25 +/- 3% and 22 +/- 2%, respectively. The R-type Ca2+ currents were sensitive to NiCl2 (IC50 = 22 +/- 0.1 microM) but not to SNX-482, which was able to potently (IC50 = 76 +/- 0.4 nM) block the recombinant alpha1E/beta2a/alpha2delta Ca2+ currents expressed in human embryonic kidney 293 cells. Taken together, our data suggest that sympathetic and parasympathetic MPG neurons share a similar but unique profile of HVA Ca2+ channel isoforms.

R-type calcium channels in myenteric neurons of guinea pig small intestine

Currents carried by L-, N-, and P/Q-type calcium channels do not account for the total calcium current in myenteric neurons. This study identified all calcium channels expressed by guinea pig small intestinal myenteric neurons maintained in primary culture. Calcium currents were recorded using whole cell techniques. Depolarizations (holding potential = -70 mV) elicited inward currents that were blocked by CdCl(2) (100 microM). Combined application of nifedipine (blocks L-type channels), Omega-conotoxin GVIA (blocks N-type channels), and Omega-agatoxin IVA (blocks P/Q-type channels) inhibited calcium currents by 56%. Subsequent addition of the R-type calcium channel antagonists, NiCl(2) (50 microM) or SNX-482 (0.1 microM), abolished the residual calcium current. NiCl(2) or SNX-482 alone inhibited calcium currents by 46%. The activation threshold for R-type calcium currents was -30 mV, the half-activation voltage was -5.2 +/- 5 mV, and the voltage sensitivity was 17 +/- 3 mV. R-type currents activated fully in 10 ms at 10 mV. R-type calcium currents inactivated in 1 s at 10 mV, and they inactivated (voltage sensitivity of 16 +/- 1 mV) with a half-inactivation voltage of -76 +/- 5 mV. These studies have accounted for all of the calcium channels in myenteric neurons. The data indicate that R-type calcium channels make the largest contribution to the total calcium current in myenteric neurons. The relatively positive half-activation voltage and rapid activation kinetics suggest that R-type channels could contribute to calcium entry during somal action potentials or during action potential-induced neurotransmitter release.

Neuronal Ca2+ Channel Blocking Action of an Antihypertensive Drug, Cilnidipine, in IMR-32 Human Neuroblastoma Cells

Although the anti-sympathetic mechanisms of the antihypertensive drug cilnidipine have been analyzed in neuronal cells derived from rodents, there is little information regarding the effects of cilnidipine in human neuronal cells. We investigated the effects of cilnidipine on N-type Ca2+ channels in IMR-32 human neuroblastoma cells using fura-2-based microfluorimetry. The ratio of the intensities of the emitted fluorescence at an excitation wavelength of 340 nm to that at 380 nm was calibrated to estimate the intracellular concentration of Ca2+. Stimulation of IMR-32 cells by 40 mmol/l KCl immediately increased the intensities ratio. In the presence of 10 micromol/l of nifedipine to block L-type Ca2+ channels, omega-conotoxin GVIA, a selective N-type Ca2+ channel blocker, in a concentration of 1 micromol/l suppressed the elevation of the intensities ratio induced by 40 mmol/l KCl. Similarly, cilnidipine in a concentration of 10 micromo/l suppressed the elevation of the ratio induced by 40 mmol/l KCl, and this suppression was effectively inhibited after the treatment with omega-conotoxin GVIA. These results suggest that cilnidipine potentially inhibits N-type Ca2+ channels in human neuronal cells and might be applied as a prospective therapeutic tool to provide neuronal protection as well as its antihypertensive effects and anti-sympathetic actions.

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 involvement of conotoxin-sensitive mechanisms in neurogenic vasodilatation responses: effects of age

During aging there is a decline in sensory nerve function that is associated with reduced neurogenic inflammation and poor wound repair. The cellular mechanism(s) responsible for this decline in function with age is not well understood. We previously reported that sensory nerves in aged rats release sensory neuropeptides preferentially in response to low-frequency (5 Hz) as compared with higher-frequency (15 Hz) antidromic electrical stimulation, and that low-frequency transcutaneous electrical nerve stimulation accelerates wound healing. The present study investigates possible mechanisms for this preferential response. Using laser Doppler techniques, we have measured changes in blood flow in the base of vacuum-induced blisters induced in the rat hind footpad of young and old animals in response to low-frequency (5 Hz) or high-frequency (15 Hz) electrical stimulation (20 V, 2 ms for 1 minute) of the sciatic nerve. The relative contributions of the sensory neuropeptides, substance P and calcitonin gene-related peptide (CGRP), and of N-type voltage-gated calcium channels to the vascular responses were assessed by using the specific receptor antagonists RP67580, which is 2-(1-imino-2-(2 methoxy phyenyl) ethyl)-7,7 diphenyl-4 perhydroisoindolone-(3aR, 7aR); CGRP(8-37); and omega-conotoxin GVIA (Conus geographus), respectively. The results showed a greater involvement of substance P at high-frequency electrical stimulation and of CGRP at low-frequency stimulation. Our finding that omega-conotoxin-sensitive N-type calcium channel function was preserved with age and was only involved in the vascular response to low-frequency electrical stimulation could explain our previous report demonstrating beneficial effects of low-frequency transcutaneous electrical nerve stimulation to wound repair in aged animals. The current results have important practical implications for improving tissue repair in the aged.

Rank-order inhibition by omega-conotoxins in human and animal autonomic nerve preparations

The inhibitory effects of the omega-conotoxins GVIA, MVIIA and MVIIC on electrically-evoked, tetrodotoxin (10(-7) M)-sensitive, autonomic nerve activity were studied using human, rat or guinea-pig vas deferens and intestinal tissues. In each preparation from each species, nM concentrations of omega-conotoxins GVIA and MVIIA prevented the neuronally-mediated contractions, whereas omega-conotoxin MVIIC was either markedly less potent (IC(50)'s 1.4 or 2.9 log units more than for omega-conotoxin GVIA in guinea-pig ileum and rat vas deferens, respectively) or was without significant activity (human vas deferens, human Taenia coli) when tested at similar concentrations. In contrast the differences in potency between omega-conotoxins GVIA and MVIIC were considerably less when assayed directly on Ca(2+) channel currents evoked from rat superior cervical ganglion neurons in culture (approximately 0.1 log unit difference) and from a stable cell line expressing rat alpha(1B), alpha(2)delta, beta(1b) Ca(2+) channel subunits (approximately 0.9 log unit). These different rank-orders of inhibitory activity of the conotoxins support the suggestion that there are pharmacologically distinct N-type Ca(2+) channels in the peripheral nervous system, and that this tissue-dependent difference is seen in man.

Differential localization of Ca2+ channel alpha1 subunits in the enteric nervous system: presence of alpha1B channel-like immunoreactivity in intrinsic primary afferent neurons

Immunocytochemistry was employed to locate calcium (Ca2+) channel proteins in the enteric nervous system (ENS) of the rat and guinea pig. Anti-peptide antibodies that specifically recognize the alpha1 subunits of class A (P/Q-type), B (N-type), C and D (L-type) Ca2+ channels were utilized. Alpha1B channel-like immunoreactivity was abundant in both enteric plexuses, the mucosa, and circular and longitudinal muscle layers. Immunoreactivity was predominantly found in cholinergic varicosities, supporting a role for Ca2+ channels, which contain the alpha1B subunit, in acetylcholine release. Immunoreactivity was also associated with the cell soma of calbindin-immunoreactive submucosal and myenteric neurons, cells that have been proposed to be intrinsic primary afferent neurons. Alpha1C channel-like immunoreactivity was distributed diffusely in the cell membrane of a large subset of neuronal cell bodies and processes, whereas alpha1D was found mainly in the cell soma and proximal dendrites ofvasoactive intestinal polypeptide-immunoreactive neurons in the guinea pig gut. Alpha1A channel-like immunoreactivity was found in a small subset of cell bodies and processes in the rat ENS. The differential localization of the alpha1 subunits of Ca2+ channels in the ENS implies that they serve distinct roles in neuronal excitation and signaling within the bowel. The presence of alpha1B channel-like immunoreactivity in putative intrinsic primary afferent neurons suggested that class B Ca2+ channels play a role in enteric sensory neurotransmission; therefore, we determined the effects of the N-type Ca2+ channel blocker, omega-conotoxin GVIA (omega-CTx GVIA), on the reflex-evoked activity of enteric neurons. Demonstrating the phosphorylation of cyclic AMP (cAMP)-responsive element-binding protein (pCREB) identified neurons that became active in response to distension. Distension elicited hexamethonium-resistant pCREB immunoreactivity in calbindin-immunoreactive neurons in each plexus; however, in preparations stimulated in the presence of omega-CTx GVIA, pCREB immunoreactivity was found only in calbindin-immunoreactive neurons in the submucosal plexus and not in myenteric ganglia. These data confirm that intrinsic primary afferent neurons are located in the submucosal plexus and that N-type Ca2+ channels play a role in sensory neurotransmission.