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

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.

Non-synaptic transmission at autonomic neuroeffector junctions

Non-synaptic transmission is characteristic of autonomic neuroeffector junctions. The structure of the autonomic neuromuscular junction is described. The essential features are that: the terminal portions of autonomic nerve fibers are varicose and mobile, transmitters being released 'en passage' from varying distances from the effector cells; while there is no structural post-junctional specialization on effector cells, receptors for neurotransmitters accumulate on cell membranes at close junctions; muscle effectors are bundles rather than single smooth muscle cells, that are connected by gap junctions which allow electrotonic spread of activity between cells. A multiplicity of transmitters are utilized by autonomic nerves, and cotransmission occurs often involving synergistic actions of the cotransmitters, although pre- and post-junctional neuromodulation of neurotransmitter release also take place. It is suggested that autonomic neural control of immune, epithelial and endothelial cells also involves non-synaptic transmission.

Barium ions inhibit the dynamic response of guinea-pig corneal cold receptors to heating but not to cooling

An in vitro preparation of the guinea-pig cornea was used to study the effects of the K+ channel blockers 4-aminopyridine (4-AP), tetraethylammonium (TEA) and Ba2+ on nerve terminal impulses (NTIs) recorded extracellularly from cold sensory receptors. These receptors have an ongoing discharge of NTIs that is increased by cooling and decreased by heating. The K+ channel blocker 4-AP reduced the negative amplitude of the diphasic (positive-negative) NTIs, whereas TEA and Ba2+ prolonged the duration of the negative component. As the shape of the NTI is determined by the first derivative (dV/dt) of the membrane voltage change, these changes in the negative component are consistent with the blockade of K+ channels that contribute to action potential repolarization. Only TEA changed the basal activity of the receptors, increasing the likelihood of burst discharges. Ba2+ selectively reduced the response of the receptors to heating, whereas neither 4-AP nor TEA modified the response to heating or to cooling. The findings indicate that K+ channels blocked by 4-AP, TEA and Ba2+ contribute to action potential repolarization in corneal cold receptors, and that ionic mechanisms that underlie the reduction in NTI frequency in response to heating differ from those that increase activity in response to cooling.

Electrophysiology of corneal cold receptor nerve terminals

The mechanisms of sensory transduction in the fine nerve terminals of free nerve endings supplied by Adelta and C sensory axons are largely a matter of speculation. This is because the nerve terminals are small and inaccessible, particularly in intact tissues like skin. However, some of the difficulties associated with investigating the physiology of fine nerve terminals have recently been overcome using an in vitro preparation of the guinea-pig cornea that allows nerve terminal impulses (NTIs) to be recorded extracellularly from single polymodal and cold receptor nerve terminals. For cold receptors, the rate of spontaneously occurring NTIs is increased during cooling and decreased during heating. In addition, heating and cooling differentially modulate the shape of the recorded NTI. At the same temperature, NTIs are larger in amplitude and faster in time course during heating than those during cooling. The differential effect of heating and cooling on NTI shape is not considered to result simply from the temperature dependence of voltage-activated conductance kinetics or activity dependent changes in membrane excitability. Instead, changes in NTI shape may reflect changes in nerve terminal membrane potential that underlie the process of thermal transduction.

The effects of polarizing current on nerve terminal impulses recorded from polymodal and cold receptors in the guinea-pig cornea

It was reported recently that action potentials actively invade the sensory nerve terminals of corneal polymodal receptors, whereas corneal cold receptor nerve terminals are passively invaded (Brock, J.A., S. Pianova, and C. Belmonte. 2001. J. Physiol. 533:493-501). The present study investigated whether this functional difference between these two types of receptor was due to an absence of voltage-activated Na(+) conductances in cold receptor nerve terminals. To address this question, the study examined the effects of polarizing current on the configuration of nerve terminal impulses recorded extracellularly from single polymodal and cold receptors in guinea-pig cornea isolated in vitro. Polarizing currents were applied through the recording electrode. In both receptor types, hyperpolarizing current (+ve) increased the negative amplitude of nerve terminal impulses. In contrast, depolarizing current (-ve) was without effect on polymodal receptor nerve terminal impulses but increased the positive amplitude of cold receptor nerve terminal impulses. The hyperpolarization-induced increase in the negative amplitude of nerve terminal impulses represents a net increase in inward current. In both types of receptor, this increase in inward current was reduced by local application of low Na(+) solution and blocked by lidocaine (10 mM). In addition, tetrodotoxin (1 microM) slowed but did not reduce the hyperpolarization-induced increase in the negative amplitude of polymodal and cold nerve terminal impulses. The depolarization-induced increase in the positive amplitude of cold receptor nerve terminal impulses represents a net increase in outward current. This change was reduced both by lidocaine (10 mM) and the combined application of tetraethylammomium (20 mM) and 4-aminopyridine (1 mM). The interpretation is that both polymodal and cold receptor nerve terminals possess high densities of tetrodotoxin-resistant Na(+) channels. This finding suggests that in cold receptors, under normal conditions, the Na(+) conductances are rendered inactive because the nerve terminal region is relatively depolarized.

Neuropeptide Y inhibits double peaked vasoconstrictor responses to periarterial nerve stimulation primarily through prejunctional Y2 receptor subtype in canine splenic arteries

1 The effects of BIIE 0246, a novel and non-peptide neuropeptide Y (NPY) Y2 receptor antagonist on sympathetic vasoconstriction of the canine splenic artery were investigated. 2 The vasoconstrictor response to periarterial electrical nerve stimulation was described to be a double peaked vasoconstriction consisting of an initial transient, dominantly P2X purinoceptor-mediated constriction followed by a prolonged, mainly alpha1 adrenoceptor-induced response. 3 BIIE 0246 at a concentration of 0.1-1 microM dose-dependently potentiated double peaked constrictions at low frequencies (1 and 4 Hz), whereas at high frequency (10 Hz), it failed to affect these responses. BIIE 0246 (1 microM) also enhanced double peaked responses even in the presence of rauwolscine (0.1 microM). NPY (13-36) (1-100 nM), a selective Y2 receptor agonist reduced these two peaked responses in a dose-related manner. The vasoconstriction to noradrenaline (0.1-10 nmol) or adenosine triphosphate (0.01-1 micromol) was not significantly influenced by either 1 microM BIIE 0246 or 100 nM NPY (13-36). Exposure of tissues to 1 microM BIIE 0246 almost completely prevented the suppression of double peaked constrictions by NPY (13-36) (10 nM) or by NPY (10 nM). 4 We conclude that NPY inhibits sympathetic purinergic and adrenergic vasoconstrictions through an activation of prejunctional Y2 receptor subtype in the neurovascular junction of the canine splenic artery.

Effect of chronic clonidine treatment on transmitter release from sympathetic varicosities of the guinea-pig vas deferens

1. Previous studies have demonstrated that chronic pre-synaptic inhibition of transmitter release by morphine evokes a counter-adaptive response in the sympathetic nerve terminals that manifests itself as an increase in transmitter release during acute withdrawal. In the present study we examined the possibility that other pre-synaptically acting drugs such as clonidine also evoke a counter-adaptive response in the sympathetic nerve terminals. 2. In chronically saline treated (CST) preparations, clonidine (0.5 microM) completely abolished evoked transmitter release from sympathetic varicosities bathed in an extracellular calcium concentration ([Ca(2+)](o)) of 2 mM. The inhibitory effect of clonidine was reduced by increasing [Ca(2+)](o) from 2 to 4 mM and the stimulation frequency from 0.1 to 1 Hz. 3. The nerve terminal impulse (NTI) was not affected by concentrations of clonidine that completely abolished evoked transmitter release. 4. Sympathetic varicosities developed a tolerance to clonidine (0.5 microM) following 7-9 days of chronic exposure to clonidine. 5. Acute withdrawal of preparations following chronic clonidine treatment (CCT) resulted in a significant (P < 0.005) enhancement of neurotransmitter release (3.75 times) above control levels observed in CST preparations. 6. The present findings demonstrate an enhancement of neurotransmitter release from sympathetic varicosities following acute withdrawal from chronic clonidine treatment.

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.

Neurotransmitter release mechanisms in sympathetic neurons: past, present, and future perspectives

In 1969, Paton and Vizi described the inhibitory actions of noradrenaline on acetylcholine release from the innervation of the guinea-pig ileum longitudinal muscle. They concluded "that acetylcholine output by the nervous networks of the longitudinal strip is under the normal control of the sympathetic by a species of presynaptic inhibition mediated by <==> receptors". This work was carried out in the Pharmacology Department at Oxford University. Clearly, a period in the 'Dreaming Spires' of Oxford sufficiently inspired Sylvester to take up a life long career in scientific research. He has published more than 300 papers on a wide range of topics but clearly has a strong interest in neurotransmitter release mechanisms and recently, non-synaptic interactions between neurons. It seems fitting therefore to write a brief review on the continuing studies on neurotransmitter release mechanisms in sympathetic neurons in a volume honoring the now distinguished Professor Vizi.

Differences between nerve terminal impulses of polymodal nociceptors and cold sensory receptors of the guinea-pig cornea

1. Extracellular recording techniques were used to study nerve terminal impulses (NTIs) recorded from single polymodal nociceptors and cold-sensitive receptors in guinea-pig cornea isolated in vitro. 2. The amplitude and time course of NTIs recorded from polymodal nociceptors was different from those of cold-sensitive receptors. 3. Bath application of tetrodotoxin (1 microM) changed the time course of spontaneous NTIs recorded from both polymodal and cold-sensitive receptors. 4. Bath application of lignocaine (lidocaine; 1-5 mM) abolished all electrical activity. 5. Local application of lignocaine (2.5 and 20 mM) through the recording electrode changed the time course of the NTIs recorded from polymodal nociceptors but not that of NTIs recorded from cold-sensitive nerve endings. 6. It is concluded that action potentials propagate actively in the sensory nerve endings of polymodal nociceptors. In contrast, cold-sensitive receptor nerve endings appear to be passively invaded from a point more proximal in the axon where the action potential can fail or be initiated.

Calcium-activated potassium currents in mammalian neurons

1. Influx of calcium via voltage-dependent calcium channels during the action potential leads to increases in cytosolic calcium that can initiate a number of physiological processes. One of these is the activation of potassium currents on the plasmalemma. These calcium-activated potassium currents contribute to action potential repolarization and are largely responsible for the phenomenon of spike frequency adaptation. This refers to the progressive slowing of the frequency of discharge of action potentials during sustained injection of depolarizing current. In some cell types, this adaptation is so marked that despite the presence of depolarizing current, only a single spike (or a few spikes) is initiated. Following cessation of current injection, slow deactivation of calcium-activated potassium currents is also responsible for the prolonged hyperpolarization that often follows. 2. A number of macroscopic calcium-activated potassium currents that can be separated on the basis of kinetic and pharmacological criteria have been described in mammalian neurons. At the single channel level, several types of calcium-activated potassium channels also have been characterized. While for some macroscopic currents the underlying single channels have been unambiguously defined, for other currents the identity of the underlying channels is not clear. 3. In the present review we describe the properties of the known types of calcium-activated potassium currents in mammalian neurons and indicate the relationship between macroscopic currents and particular single channels.

Individual sympathetic varicosities possess different sensitivities to alpha 2 and P2 receptor agonists and antagonists in mouse vas deferens

1. The diversity of alpha(2) and purinergic autoreceptor actions on action potential evoked calcium transients in single varicosities has been investigated using the calcium indicator Oregon Green 488 BAPTA-1. 2. During long trains of impulses (10 Hz for 30 s), the change in calcium concentration in varicosities from its resting level (Delta[Ca(2+)](v)) increased in many varicosities during the first 3 s of stimulation before reaching a plateau. 3. The alpha(2) adrenoceptor agonist clonidine (1 microM) decreased Delta[Ca(2+)](v) by over 40% during short trains (five impulses at 5 Hz) in most varicosities, although some were unaffected. The alpha(2) adrenoceptor antagonist idazoxan (2 microM) increased the Delta[Ca(2+)](v) plateau following long trains in most varicosities. Hence, most varicosities possess alpha(2) adrenoceptors which are activated when noradrenaline accumulates extracellularly. 4. During long trains of impulses, the P(2y)-purinergic receptor agonist 2-methyl-thio-ATP (100 microM) decreased Delta[Ca(2+)](v) plateau by about 50% in most varicosities; alpha,beta-methylene ATP (100 microM) decreased it by about 50% in a minority of varicosities; adenosine (200 microM) had no significant effect. Suramin (100 microM) increased the Delta[Ca(2+)](v) during all stimulus protocols in most varicosities, suggesting that ambient ATP modulates Delta[Ca(2+)](v) responses. The P(2y) receptor antagonist reactive blue (100 microM) affected a minority of varicosities. Given that most varicosities respond to suramin, other P(2) receptor subtypes are probably present. 5. The ATP ectoenzyme antagonist ARL67157 (50 microM) decreased the plateau Delta[Ca(2+)](v) during long trains in complete strings of varicosities but not in others. 6. The present technique indicates that varicosities have diverse autoreceptor utilization.

Electrophysiological and morphological diversity of mouse sympathetic neurons

We have used multiple-labeling immunohistochemistry, intracellular dye-filling, and intracellular microelectrode recordings to characterize the morphological and electrical properties of sympathetic neurons in the superior cervical, thoracic, and celiac ganglia of mice. Neurochemical and morphological characteristics of neurons varied between ganglia. Thoracic sympathetic ganglia contained three main populations of neurons based on differential patterns of expression of immunoreactivity to tyrosine hydroxylase, neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP). In the celiac ganglion, nearly all neurons contained immunoreactivity to both tyrosine hydroxylase and NPY. Both the overall size of the dendritic tree and the number of primary dendrites were greater in neurons from the thoracic and celiac ganglia compared with those from the superior cervical ganglion. The electrophysiological properties of sympathetic neurons depended more on their ganglion of origin rather than their probable targets. All neurons in the superior cervical ganglion had phasic firing properties and large afterhyperpolarizations (AHPs). In addition, 34% of these neurons displayed an afterdepolarization preceding the AHP. Superior cervical ganglion neurons had prominent I(M), I(A), and I(H) currents and a linear current-voltage relationship between -60 and -110 mV. Neurons from the thoracic ganglia had significantly smaller action potentials, AHPs, and apparent cell capacitance compared with superior cervical ganglion neurons, and only 18% showed an afterdepolarization. All neurons in superior cervical ganglia and most neurons in celiac ganglia received at least one strong preganglionic input. Nearly one-half the neurons in the celiac ganglion had tonic firing properties, and another 15% had firing properties intermediate between those of tonic and phasic neurons. Most celiac neurons showed significant inward rectification below -90 mV. They also expressed I(A), but with slower inactivation kinetics than that of superior cervical or thoracic neurons. Both phasic and tonic celiac ganglion neurons received synaptic inputs via the celiac nerves in addition to strong inputs via the splanchnic nerves. Multivariate statistical analysis revealed that the properties of the action potential, the AHP, and the apparent cell capacitance together were sufficient to correctly classify 80% of neurons according to their ganglion of origin. These results indicate that there is considerable heterogeneity in the morphological, neurochemical, and electrical properties of sympathetic neurons in mice. Although the morphological and neurochemical characteristics of the neurons are likely to be related to their peripheral projections, the expression of particular electrophysiological traits seems to be more closely related to the ganglia within which the neurons occur.

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.

Tetrodotoxin-resistant impulses in single nociceptor nerve terminals in guinea-pig cornea

1. Extracellular recording techniques have been used to study nerve impulses in single sensory nerve terminals in guinea-pig cornea isolated in vitro. 2. Nerve impulses occurred spontaneously and were evoked by electrical stimulation of the ciliary nerves. 3. The nerve impulses were identified as originating in polymodal receptors, mechano-receptors or 'cold' receptors. All three types are believed to be nociceptors. 4. Tetrodotoxin (TTX, 1 microM) blocked nerve impulses evoked by electrical stimulation of the ciliary nerves. However, ongoing and/or naturally evoked nerve impulses persisted in the presence of TTX in all three types of receptors. Lignocaine (lidocaine; 1 mM) blocked all electrical activity. 5. TTX-resistant sodium channels therefore play a major role in generating the action potentials that signal pain to the brain.

Omega-conotoxin GVIA-resistant neurotransmitter release from postganglionic sympathetic nerves in the guinea-pig vas deferens and its modulation by presynaptic receptors

1 Intracellular recording techniques were used to study neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals in the guinea-pig isolated vas deferens. 2 Recently, a component of action potential-evoked release which is insensitive to high concentrations of the selective N-type calcium channel blocker omega-conotoxin GVIA termed 'residual release' has been described. Under these conditions, release of the neurotransmitter ATP evoked by trains of low frequency stimuli is abolished, but at higher frequencies a substantial component of release is revealed. 3 'Residual release' was studied with trains of 5 or 10 stimuli at stimulation frequencies of 10, 20 and 50 Hz. The alpha2-adrenoceptor agonist clonidine (30-100 nM) inhibited 'residual release', the degree of inhibition being most marked at the beginning of a train. 4 The alpha2-adrenoceptor antagonist yohimbine (1 microM) induced a marked increase in 'residual release' which was dependent on both the frequency of stimulation and the number of stimuli in a train. 5 Prostaglandin E2 (30 nM) and neuropeptide Y (100 nM) caused a rapid inhibition of 'residual release' at all stimulation frequencies examined. 6 4-Aminopyridine (100 microM) induced a powerful potential of 'residual release' and could reverse the inhibition of omega-conotoxin GVIA. 7 'Residual release' was modulated through presynaptic alpha2-adrenoceptors suggesting that (i) residual release of ATP is subject to alpha-autoinhibition through the co-release of noradrenaline, (ii) noradrenaline release can be triggered by calcium channels other than the N-type and (iii) when presynaptic receptors are activated, inhibition of transmitter release can occur by mechanisms other than modulation of calcium-entry through N-type calcium channels in postganglionic sympathetic nerves. Prostaglandin E2 and neuropeptide Y also modulated neurotransmitter release.

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.

Sources of Ca2+ for different Ca(2+)-activated K+ conductances in neurones of the rat superior cervical ganglion

1. The role of various Ca(2+)-activated K+ conductances were investigated using intracellular recording and single-electrode voltage clamp in neurones of superior cervical ganglia isolated in vitro from young adult rats. 2. Following replacement of Ca2+ with Co2+ (2 mM) or the addition of Cd2+ (100 microM), action potential amplitude and half-width either increased or decreased (in different cells), but both the after-hyperpolarization (AHP) and the outward tail current following a suprathreshold voltage step were markedly attenuated (by about 75%). 3. Addition of charybdotoxin (60 nM) or nifedipine (10 microM) increased action potential half-width (by about 25%) but had no significant effect on the AHP or tail current. 4. Addition of apamin (100 nM) or omega-conotoxin GVIA (100 nM) reduced the AHP and tail current (by about 60%) but did not significantly affect the action potential. A prolonged apamin-resistant component of the AHP present in 50% of neurones was blocked by ryanodine (20 microM). 5. Omega-Conotoxin MVIIC (150 nM) and omega agatoxin IVA (200 nM) had no significant effects on the action potential half-width or the AHP. 6. None of the Ca2+ channel blockers affected the prolonged ryanodine-sensitive component of the AHP and tail current. 7. We conclude that, in rat sympathetic neurones, Ca2+ entry via L-type channels selectively activates large conductance Ca(2+)-activated K+ channels (BK type) contributing to action potential repolarization, whereas Ca2+ entry via N-type channels selectively activates small conductance Ca(2+)-activated K+ channels (SK type) contributing to the AHP. Ca2+ entry via R-type Ca2+ channels prolongs the AHP by activating Ca2+ release from intracellular stores.

Inhibition of purinergic transmission by prostaglandin E1 and E2 in the guinea-pig vas deferens: an electrophysiological study

1. The effects of prostaglandin E1 (PGE1) and E2 (PGE2) on postjunctional electrical activity in the guinea-pig vas deferens evoked by sympathetic nerve stimulation were investigated using both intracellular and focal extracellular recording techniques in vitro. 2. Bath application of PGE1 (1-100 nM) or PGE2 (0.1-100 nM) concentration-dependently inhibited the amplitudes of all excitatory junction potentials (e.j.ps) evoked during short trains of stimuli (10 stimuli at 1 Hz). Increasing the duration of nerve stimulation (100 stimuli at 1 Hz) did not overcome this inhibitory effect. At these concentrations PGE1 and PGE2 were without any apparent inhibitory effect on the amplitudes of spontaneous e.j.ps. 3. Local application of PGE1 (10-100 nM) or PGE2 (10-30 nM) markedly reduced the frequency of occurrence of excitatory junction currents (e.j.cs) evoked by trains of 20-100 stimuli at 1 to 4 Hz without changing the amplitudes of spontaneous e.j.cs or the configuration of the nerve terminal impulse. 4. In the presence of PGE1 or PGE2, raising the frequency of stimulation (from 1 to 4 Hz), increased the likelihood of e.j.c. occurrence. 5. The postjunctional electrical activity recorded in the guinea-pig vas deferens is believed to be due to ATP released from the sympathetic nerve endings. Thus the present study demonstrates that both PGE1 and PGE2 powerfully inhibit quantal ATP release in the guinea-pig vas deferens.

Omega-conotoxin GVIA-resistant neurotransmitter release in postganglionic sympathetic nerve terminals

Intracellular recording techniques were used to study neurotransmitter release in the guinea-pig isolated vas deferens. Low concentrations of the irreversible and selective N-type calcium channel blocker omega-conotoxin GVIA have previously been shown to block excitatory junction potentials evoked by low frequencies (< or = 1 Hz) of nerve stimulation. Here we report a component of action potential-evoked release which is insensitive to high concentrations of omega-conotoxin GVIA. We have termed this component "residual release" and show (i) it is positively frequency-dependent, (ii) its magnitude is dependent on both the train length and interval between trains, (iii) "residual release" can be modulated through prejunctional alpha 2-adrenoceptors and (iv) "residual release" is insensitive to many calcium entry blockers but abolished by omega-grammotoxin SIA and cadmium ions. Although noradrenaline is released by nerve action potentials, residual excitatory junction potentials were abolished by alpha,beta-methylene-ATP and therefore resulted entirely from the actions of neuronally released ATP acting through postjunctional P2x-purinoceptors. The results suggest that calcium entry through a novel, pharmacologically uncharacterized voltage-dependent calcium channel is responsible for "residual release" in sympathetic nerve terminals. It seems that in response to single or short trains of nerve action potentials, N-type calcium channels dominate the release process. However, at higher frequencies other voltage-dependent calcium channels are recruited and these may have an important role to play in triggering the mechanisms underlying frequency-dependent facilitation.