K+ and Ca2+ channel blockers may enhance or depress sympathetic transmitter release via a Ca(2+)-dependent mechanism "upstream" of the release site

Sympathetic and Sensory-Motor Nerves in Peripheral Small Arteries

Small arteries, which play important roles in controlling blood flow, blood pressure, and capillary pressure, are under nervous influence. Their innervation is predominantly sympathetic and sensory motor in nature, and while some arteries are densely innervated, others are only sparsely so. Innervation of small arteries is a key mechanism in regulating vascular resistance. In the second half of the previous century, the physiology and pharmacology of this innervation were very actively investigated. In the past 10-20 yr, the activity in this field was more limited. With this review we highlight what has been learned during recent years with respect to development of small arteries and their innervation, some aspects of excitation-release coupling, interaction between sympathetic and sensory-motor nerves, cross talk between endothelium and vascular nerves, and some aspects of their role in vascular inflammation and hypertension. We also highlight what remains to be investigated to further increase our understanding of this fundamental aspect of vascular physiology.

Ion channel modulators that enhance acetylcholine release: potential therapies for Alzheimer’s disease

Enhancing the release of acetylcholine (ACh) in the brain is one approach to increasing neuronal activity, restoring central cholinergic tone and improving attention and cognition. ACh release is modulated by both ligand-gated (gamma-amino butyric acid A receptors/benzodiazepine [GABA(A)/BDZ], nicotinic-acetylcholine and serotonin, 5-HT3) and voltage-gated (calcium and potassium) ion channels. Of the ligand-gated channel modulators, the BDZ receptor (BDZR) inverse agonists (beta-CCM, ZK 93426) enhance activity-dependent release in animals, whereas S-8510, a partial inverse agonist, and the BDZR antagonist, flumazenil, show enhancement regardless of the behavioural state of the animal. Some of these agents have undergone limited clinical evaluation for Alzheimer's disease (AD) (ZK 93426, flumazenil, S-8510), but their potential anxiogenic liability makes their therapeutic use uncertain until more clinical data are available. Within the group of nicotinic agonists, ABT-418, though less potent than nicotine and epibatidine in promoting ACh release in vitro, was clinically evaluated based on its in vivo profile. Its lack of oral bioavailability has limited its acceptability, though transdermal administration has been used to circumvent this deficiency. Serotonin 5-HT3 receptor modulators have not been advanced for clinical evaluation for the treatment of AD. Among the voltage-gated ion channel modulators affecting L- or N-type calcium channels, nefiracetam, a nootropic agent, also increased ACh release in animal studies. It is currently undergoing clinical evaluation for AD, though a need for more potent and brain selective calcium channel blockers exists. Potassium channel modulators have been the most studied ACh release enhancing agents and several of these compounds (4-AP, 3,4-DAP, linopirdine) have been clinically evaluated. In AD patients, 4-AP, an A-type K+ channel blocker, elicited inconsistent and unremarkable effects. Linopirdine, whose enhancement of ACh release correlates with its ability to block M-type K+ channels, also produced disappointing clinical results, which may have been related to its suboptimal pharmacokinetic profile. Further work in this series has provided a compound (DMP 543) that should be a more reliable indicator of whether a blocker of this ion channel can activate the cholinergic system in man. This agent is currently undergoing clinical evaluation for AD.

Intermittent ATP release from nerve terminals elicits focal smooth muscle Ca2+ transients in mouse vas deferens

A confocal Ca2+ imaging technique has been used to detect ATP release from individual sympathetic varicosities on the same nerve terminal branch. Varicose nerve terminals and smooth muscle cells in mouse vas deferens were loaded with the Ca2+ indicator Oregon Green 488 BAPTA-1. Field (nerve) stimulation evoked discrete, focal increases in [Ca2+] in smooth muscle cells adjacent to identified varicosities. These focal increases in [Ca2+] have been termed 'neuroeffector Ca2+ transients' (NCTs). NCTs were abolished by alpha,beta-methylene ATP (1 microM), but not by nifedipine (1 microM) or prazosin (100 nM), suggesting that NCTs are generated by Ca2+ influx through P2X receptors without a detectable contribution from L-type Ca2+ channels or alpha(1)-adrenoceptor-mediated pathways. Action potential-evoked ATP release was highly intermittent (mean probability 0.019 +/- 0.002; range 0.001-0.10) at 1 Hz stimulation, even though there was no failure of action potential propagation in the nerve terminals. Twenty-eight per cent of varicosities failed to release transmitter following more than 500 stimuli. Spontaneous ATP release was very infrequent (0.0014 Hz). No Ca2+ transient attributable to noradrenaline release was detected even in response to 5 Hz stimulation. There was evidence of local noradrenaline release as the alpha(2)-adrenoceptor antagonist yohimbine increased the probability of occurrence of NCTs by 55 +/- 21 % during trains of stimuli at 1 Hz. Frequency-dependent facilitation preferentially occurred at low probability release sites. The monitoring of NCTs now allows transmitter release to be detected simultaneously from each functional varicosity on an identified nerve terminal branch on an impulse-to-impulse basis.

Characterization of action potential-evoked calcium transients in mouse postganglionic sympathetic axon bundles

1. Action potential-evoked Ca(2+) transients in postganglionic sympathetic axon bundles in mouse vas deferens have been characterized using confocal microscopy and Ca(2+) imaging. 2. Axonal Ca(2+) transients were tetrodotoxin sensitive. The amplitude depended on both the frequency of stimulation and the number of stimuli in a train. 3. Removal of extracellular Ca(2+) abolished the Ca(2+) transient. Cd(2+)(100 microM) inhibited the Ca(2+) transient by 78 +/- 10 %. The N-type Ca(2+) channel blocker omega-conotoxin GVIA (0.1 microM) reduced the amplitude by -35 +/-4 %, whereas nifedipine (10 microM; L-type) and omega-conotoxin MVIIC (0.1 microM; P/Q type) were ineffective. 4. Caffeine (10 mM), ryanodine (10 microM), cyclopiazonic acid (30 microM) or CCCP (10 microM) had no detectable effects. 5. Blockade of large and small conductance Ca(2+)-dependent K+ channels with iberiotoxin (0.1 microM) and apamin (1 microM), respectively, or Ca(2+)-dependent Cl(-) channels by niflumic acid (100 microM) did not alter Ca(2+) transients. 6. In contrast, the non-specific K+ channel blockers tetraethylammonium (10 mM) and 4-aminopyridine (10 mM) markedly increased the amplitude of the Ca(2+) transient. Blockade of delayed rectifiers and A-like K+ channels, by tityustoxin-K (alpha) (0.1 microM) and pandinustoxin-K (alpha) (10 nM), respectively, also increased the Ca(2+) transient amplitude. 7. Thus, Ca(2+) transients are evoked by Na(+)-dependent action potentials in axons. These transients originate mainly from Ca(2+) entry through voltage-dependent Ca(2+) channels (80 % Cd(2+) sensitive of which 40 % was attributable to N-type). Twenty per cent of the Ca(2+) transient was not due to Ca(2+) entry through voltage-gated Ca(2+) channels. Intracellular stores and mitochondria were not involved in the generation of the transient. Ca(2+) transients are modulated by A-like K+ channels and delayed rectifiers (possibly K(V)1.2) but not by Ca(2+)-activated ion channels.

Paired pulse analysis of ATP and noradrenaline release from sympathetic nerves of rat tail artery and mouse vas deferens: effects of K+ channel blockers

1. The paired pulse stimulus paradigm - two pulses of equal strength delivered at variable interpulse intervals was used to study the release of ATP and noradrenaline (NA) from post ganglionic sympathetic nerves of rat tail artery and mouse vas deferens. 2. Excitatory junction currents (EJCs) were used to measure the release of ATP, and differential pulse amperometry to measure that of NA. 3. At interpulse intervals of 0.1 - 1 s paired pulse stimulation caused an increase in the size of the second EJC, both in rat tail artery and mouse vas deferens. As the interpulse interval was increased to 10 s or more, the two EJCs became of equal size. 4. In both preparations the K+ channel blockers tetraethylammonium (TEA, 20 mM) and 4-aminopyridine (4-AP, 1 mM) prolonged the duration of the nerve terminal spike and greatly amplified the first EJC of the pair. 5. In the presence of TEA and 4-AP in rat tail artery paired pulse stimulation caused a dramatic depression of the second EJC without markedly affecting the nerve terminal spike. The depression of the second EJC decreased with increasing interpulse intervals, and also when external Ca2+ was reduced to 0.2 mM. In mouse vas deferens, TEA and 4-AP caused only a modest depression of the second EJC. 6. In rat tail artery in the presence of TEA and 4-AP paired pulse stimulation caused a depression of the NA oxidation current evoked by the second pulse, which was similar in magnitude and time course to that of the EJC. Similar TEA and 4-AP induced depression of the second pulse response was also observed when the purinergic and noradrenergic components of the contractile response were investigated. 7. The results show that in rat tail artery K+ channel blockers cause a dramatic paired pulse depression of the release of ATP and NA. The similarity in the depression of the EJC, the NA oxidation current, and the purinergic and noradrenergic components of the contractile response is compatible with the hypothesis that ATP and NA are released in parallel from the same neuronal sources.

Behavioural and functional neurotoxicological changes caused by cadmium in a three-generational study in rats

1. Three consecutive generations of Wistar rats were orally treated by gavage with 3.5, 7.0 or 14.0 mg/kg cadmium (in form of cadmium chloride diluted in distilled water) over the period of pregnancy, lactation and 8 weeks after weaning. 2. Behavioural (open field behaviour) and electrophysiological (spontaneous and evoked cortical activity, etc.) parameters of male rats from each generation were investigated at the age of 12 weeks. 3. The main behavioural outcomes were change in vertical exploration activity (rearing) and increased exploration of an open field centre. The spontaneous and evoked electrophysiological variables showed dose- and generation-dependent changes (increased frequencies in the electrocorticogram, lengthened latency and duration of evoked potentials, etc.) signalling a change in neural functions. 4. The data show that low-level, multigeneration exposure to inorganic cadmium can affect functions of the nervous system. This suggests that cadmium exposed human populations may be at risk of developing nervous system disorders.

Calcium in sympathetic varicosities of mouse vas deferens during facilitation, augmentation and autoinhibition

1. The sympathetic nerve terminals of the mouse vas deferens were loaded with the calcium indicator Oregon Green 488 BAPTA-1 by orthograde transport along the postganglionic nerves. Changes in the calcium concentration in the varicosity (delta [Ca2+]v) were determined following single impulses, and short (5-impulse) and long (200-impulse) trains at 5 Hz. 2. All varicosities showed a significant delta [Ca2+]v in response to every single impulse. The elevated delta [Ca2+]v declined in two phases with similar kinetics for all varicosities: a fast phase (time constant, 0.42 +/- 0.05 s) and a moderate phase (3.6 +/- 0.4 s). 3. Line scanning confocal microscopy revealed that the delta [Ca2+] of a single terminal following single impulses was smaller for the intervaricose regions than for the varicosities. 4. Blockade of the voltage-sensitive calcium channels with Cd2+ (in calcium-free solution) completely blocked the delta [Ca2+]v on stimulation. The addition of either nifedipine (10 microM), omega-conotoxin GVIA (100 nM) or omega-agatoxin TK (100 nM) showed that 47 +/- 6% of the evoked response was mediated by N-type calcium channels. 5. Ryanodine (10 microM) did not significantly change the amplitude of delta [Ca2+]v in response to short trains. 6. Spontaneous increases in delta [Ca2+]v were observed in individual varicosities, with coupling in the increase of delta [Ca2+]v between varicosities. 7. The presynaptic alpha 2-receptor antagonist yohimbine (10 microM) increased the amplitude of delta [Ca2+]v in response to five impulses (5 Hz) by 54 +/- 14%, while the alpha 2-receptor agonist clonidine (1 microM) decreased the delta [Ca2+]v by 55 +/- 4%. 8. These results are discussed in terms of the hypotheses that the increased probability for secretion at sympathetic nerve terminals which accompanies facilitation and augmentation is due to the residual delta [Ca2+]v remaining after the calcium influx following impulses and that noradrenaline acts presynaptically to decrease the probability of secretion by modifying calcium influx.

Autonomic neuromuscular transmission at a varicosity

This review attempts to clarify the definition of what constitutes an autonomic neuromuscular function formed by a varicosity. Ultrastructural studies of serial sections through varicosities, partly or wholly bare of Schwann cell covering, show that areas of close apposition occur between varicosities and muscle cell membrane that vary between 20 and 150 nm, depending on the muscle considered. Consideration of the diffusion of purine transmitters and their receptor kinetics after secretion in a packet show that the number of purinergic receptor channels opened at a site of 150 nm apposition by a varicosity is about 15% of that at a site of 50 nm apposition. These results, together with the analysis of the stochastic fast component and the deterministic slow components of the rising phase of the EJP suggest that the stochastic fast component is due to varicosities that form especially close appositions (20-50 nm), whereas the deterministic slow component is due to the large number of varicosities at distances up to about 150 nm. Varicosities forming appositions of 20-150 nm with muscle cells several hundred micrometers long possess junctional receptor types distinct from extrajunctional receptors. According to this argument, then, there are two different classes of varicosities: one that gives rise to a relatively large junctional current and another that is responsible for a very small junctional current. Present evidence suggests that two subclasses of varicosities can be discerned amongst the varicosities that generate large junctional currents. One of these subclasses of varicosity possesses relatively few post-junctional receptors compared with the amount of transmitter reaching the receptors from the varicosity, so that the junctional current generated is determined by the size of the receptor population; in this case, the size of the transmitter packages released from these varicosities is unknown and the size of the junctional current is relatively constant. The other subclass of varicosity possesses large receptor patches, sufficient to accommodate the largest amounts of transmitter released from the varicosities: in this case, the size of the transmitter packages is shown to be highly non-uniform. These speculations await confirmation by direct labelling of the receptor patches beneath varicosities, a possibility that is likely to be realized in the near future.

Functional reconstitution of KCl-evoked, Ca(2+)-dependent acetylcholine release system in Xenopus oocytes microinjected with presynaptic plasma membranes and synaptic vesicles

We have developed a new method for the generation of functionally active presynaptic chimeras in Xenopus laevis oocytes. Frog oocytes injected with presynaptic subcellular fractions extracted from the electric organ of Torpedo marmorata release acetylcholine in a calcium-dependent manner upon chemical stimulation. Neither oocytes injected without presynaptic plasma membranes nor oocytes injected with ghost erythrocyte plasma membrane instead of presynaptic plasma membrane release acetylcholine. This suggests that specific presynaptic components necessary for KCl-evoked, Ca(2+)-dependent acetylcholine release become functionally integrated in the Xenopus laevis oocytes. Moreover, rhodaminated presynaptic plasma membranes and the synaptic vesicle protein synaptophysin are detected on the oocyte surface by fluorescence or immunofluorescence, respectively, showing that the injected presynaptic components are incorporated into the membrane of the frog oocyte. Furthermore, Botulinum neurotoxin type A, a specific blocker of acetylcholine release in the neuromuscular junction, inhibits the neurotransmitter release from the chimerical oocytes. This suggests that targets for toxin action are also functionally incorporated in the oocyte upon injection of membranous presynaptic components. Our results show that oocytes injected with presynaptic components behave as cholinergic nerve ending chimeras, at least in terms of neurotransmitter release and toxin targets. The system bypasses some problems associated with messenger RNA expression because not only proteins, but native presynaptic components are incorporated. This new technique may provide a useful approach for electrophysiological and pharmacological studies in order to characterize the synaptic transmission.

Effects of Ca2+ and K+ channel blockers on nerve impulses recorded from guinea-pig postganglionic sympathetic nerve terminals

1. A focal extracellular suction electrode was used to investigate the contributions of K+ and Ca2+ currents to the nerve impulse recorded from sympathetic nerve terminals innervating the guinea-pig vas deferens in vitro. 2. Perfusing the electrode with Cd2+ (0.1-0.5 mM) had little effect on the configuration of the nerve impulse. 3. Perfusing the electrode with Ba2+ (1-3 mM) caused the appearance of a second negative-going component of the nerve impulse. Local application of Cd2+ (0.1 mM) had little affect on this component of the nerve impulse. 4. Perfusing the electrode with 4-aminopyridine (4-AP) and/or tetraethylammonium (TEA) caused the appearance of a second negative-going component of the nerve impulse. This component has been termed the late negative-going component (LNC). 5. The LNC produced by local application of 1 mM 4-AP and 10 mM TEA was not changed when the solution perfusing the electrode contained no added Ca2+, 10 mM Ca2+ or omega-conotoxin GVIA (0.1 microM). Perfusion of the electrode with Cd2+ (0.1 mM) reduced the amplitude and slowed the time course of the LNC. 6. The LNC was markedly inhibited when the organ bath was perfused with TEA (10 mM) or 4-AP and TEA (1 and 10 mM, respectively). In some experiments the LNC was completely abolished. 7. The LNC was reduced in amplitude and slowed in time course when the solution perfusing the organ bath contained no added Ca2+. A similar effect on the LNC was observed when the solution perfusing the organ bath contained omega-conotoxin GVIA (0.1 microM), charybdotoxin (0.05 microM) or low concentrations of TEA (0.3-1 mM) or Ba2+ (10-500 microM). 8. Bath application of the alpha 2-adrenoceptor agonist clonidine (0.1-0.3 microM) did not detectably change the LNC. 9. The results demonstrate that the LNC produced by the local application of K+ blockers is due primarily to K+ efflux from sites outside the recording electrode and that a part of the change in conductance that underlies this component is due to opening of Ca(2+)-activated K+ channels. The failure to detect an effect of clonidine on the LNC suggests that activation of presynaptic alpha 2-adrenoceptors does not change either the K+ or the Ca2+ conductance of the nerve terminals.

Geometry, kinetics and plasticity of release and clearance of ATP and noradrenaline as sympathetic cotransmitters: roles for the neurogenic contraction

The paper compares the microphysiology of sympathetic neuromuscular transmission in three model preparations: the guinea-pig and mouse vas deferens and rat tail artery. The first section describes the quantal release of ATP and noradrenaline from individual sites. The data are proposed to support a string model in which: (i) most sites (> or = 99%) ignore the nerve impulse and a few (< or = 1%) release a single quantum of ATP and noradrenaline; (ii) the probability of monoquantal release is extremely non-uniform; (iii) high probability varicosities form 'active' strings; and (iv) an impulse train causes repeated quantal release from these sites. Analogy with molecular mechanisms regulating transmitter exocytosis in other systems is proposed to imply that coincidence of at least two factors at the active zone, Ca2+ and specific cytosolic protein(s), may be required to remove a 'fusion clamp', form a 'fusion complex' and trigger exocytosis of a sympathetic transmitter quantum, and that the availability of these proteins may regulate the release probability. The second section shows that clearance of noradrenaline in rat tail artery is basically > or = 30-fold slower than of co-released ATP, and that saturation of local reuptake and binding to local buffering sites maintain the noradrenaline concentration at the receptors, in spite of a profound decline in per pulse release during high frequency trains. The third section describes differences in the strategies by which mouse vas deferens and rat tail artery use ATP and noradrenaline to trigger and maintain the neurogenic contraction.

Spatiotemporal pattern of quantal release of ATP and noradrenaline from sympathetic nerves: consequences for neuromuscular transmission

The recent explosive development in research concerning the fundamental mechanisms of synaptic transmission helps put the present paper in context. It is now evident that not all transmitter vesicles in a nerve terminal, not even all those docked at its active zones, are immediately available for release (36). We watch, fascinated, the unraveling of the amazingly complex cellular mechanisms and molecular machinery that determine whether or not a vesicle is "exocytosis-competent" (77,78,39,79). Studies on quantal release in different systems show that neurons are fundamentally similar in one respect: that transmitter release from individual active zones is monoquantal (2). But they also show that active zones in different neurons differ drastically in the probability of monoquantal release and in the number of quanta immediately available for release (3). This implies that one should not extrapolate directly from transmitter release in one set of presynaptic terminals (e.g., in neuromuscular endplate or squid giant synapse) to that in other nerve terminals, especially if they have a very different morphology. As shown here, one should not even extrapolate from transmitter release in sympathetic nerves in one tissue (e.g., rat tail artery) to that in other tissues or species (e.g., mouse vas deferens). It is noteworthy that most studies of quantal release are based on electrophysiological analysis and therefore deal with release of fast, ionotropic transmitters from small synaptic vesicles at the active zones, especially in neurons in which these events may be examined with high resolution (49,48,46,33,32). Such data are useful as general models of the release of both fast and slow transmitters from small synaptic vesicles at active zones in other systems, provided that these transmitters are released in parallel, as are apparently ATP and NA in sympathetic nerves. They tell us little or nothing, however, about the release of transmitters (e.g., neuropeptides) from the large vesicles, nor about the spatiotemporal pattern of monoquantal release from small synaptic vesicles in the many neurons that have boutons-en-passent terminals. They show that the time course of effector responses to fast, rapidly inactivated transmitters such as ACh or ATP is necessarily release related. But they do not even address the possibility that the effector responses to slow transmitters such as NA, co-released from the same terminals, may obey completely different rules and perhaps rather be clearance related (7).(ABSTRACT TRUNCATED AT 400 WORDS)