Alpha 2-adrenoceptor mediated inhibition of exocytotic noradrenaline release in the absence of extracellular Ca2+

Expression pattern of voltage-dependent calcium channel subunits in hippocampal inhibitory neurons in mice

Different subtypes of voltage-dependent calcium channels (VDCCs) generate various types of calcium currents that play important role in neurotransmitter release, membrane excitability, calcium transients and gene expression. Well-established differences in the physiological properties and variable sensitivity of hippocampal GABAergic inhibitory neurons to excitotoxic insults suggest that the calcium homeostasis, thus VDCC subunits expression pattern is likely different in subclasses of inhibitory cells. Using double-immunohistochemistry, here we report that in mice: 1) Cav2.1 and Cav3.1 subunits are expressed in almost all inhibitory neurons; 2) subunits responsible for the L-type calcium current (Cav1.2 and Cav1.3) are infrequently co-localized with calretinin inhibitory cell marker while Cav1.3 subunit, at least in part, tends to compensate for the low expression of Cav1.2 subunit in parvalbumin-, metabotropic glutamate receptor 1alpha- and somatostatin-immunopositive inhibitory neurons; 3) Cav2.2 subunit is expressed in the majority of inhibitory neurons except in calbindin-reactive inhibitory cells; 4) Cav2.3 subunit is expressed in the vast majority of the inhibitory cells except in parvalbumin- and calretinin-immunoreactive neurons where the proportion of expression of this subunit is considerably lower. These data indicate that VDCC subunits are differentially expressed in hippocampal GABAergic interneurons, which could explain the diversity in their electrophysiological properties, the existence of synaptic plasticity in certain inhibitory neurons and their vulnerability to stressful stimuli.

Cognitive deficits in aged rats correlate with levels of L-arginine, not with nNOS expression or 3,4-DAP-evoked transmitter release in the frontoparietal cortex

Aging is associated with altered neurotransmitter function in the brain. In this study, we measured release parameters for acetylcholine (ACh), norepinephrine and serotonin in the frontoparietal cortex of young and aged rats. We also determined cortical amino acid concentrations and nitric oxide (NO) synthase function. Prior to sacrifice, the rats had been tested for Morris water-maze performance. In aged, compared with young rats, we observed a reduction in both uptake of choline and acetylcholine release. Serotonin release and L-arginine concentrations (a precursor of NO) showed an aging-related increase; however, L-citrulline/L-arginine ratios were decreased in aged rats. Moreover, while most age-related changes in transmitter release or neurochemical markers were not related to the learning performance, L-arginine concentrations were positively correlated to cognitive deficits. NO synthase concentrations were not affected by aging. It is suggested that events related to L-arginine-to-L-citrulline/NO metabolism in the frontoparietal cortex may take part in age-related cognitive deficits.

Raphé grafts and 3,4-diaminopyridine-evoked overflow of serotonin in the rat hippocampus after 5,7-dihydroxytryptamine lesions: evidence for 5-HT1A autoreceptors

A first experiment verified that the overflow of 5-HT evoked by 75 microM 3,4-diaminopyridine in superfused hippocampal slices was calcium-dependent, tetrodotoxin-sensitive and modulable by drugs acting on 5-HT autoreceptors. Subsequently, the technique was used in rats to investigate the effects of 5,7-dihydroxytryptamine lesions and intrahippocampal serotonergic grafts. The lesions reduced the accumulation (-81%) and relative evoked overflow (-23%; absolute evoked overflow -86%) of [ H]5-HT, but increased the relative baseline overflow (+23%; absolute baseline overflow -78%). Grafts partially compensated for these effects. In slices from grafted rats, the evoked overflow was reduced by application of a 5-HT receptor agonist (8-OH-DPAT), a response not found in sham-operated and lesion-only rats. Although the graft-induced effects were less marked than in previous studies, they were beneficial and modulated by a mechanism that normally does not operate in the intact hippocampus.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

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.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Properties of 3,4-diaminopyridine-evoked dopamine and acetylcholine release in rabbit caudate nucleus slices: involvement of facilitatory adenosine A2 receptors or nitric oxide?

The 3H-overflow from slices of the rabbit caudate nucleus preincubated with tritiated dopamine (DA), or choline, and then superfused and stimulated twice with 3,4-diaminopyridine (3,4-DAP; 25 microM, 1 min), was explored as an in vitro model for evoked release of DA, or acetylcholine (ACh), respectively. In both cases the 3,4-DAP-evoked 3H-overflow was tetrodotoxin-sensitive and Ca(2+)-dependent and hence most probably represents action potential-induced exocytotic release of DA or ACh, respectively. Using pairs of preferential agonists/antagonists it was shown, that evoked DA release was inhibited via presynaptic D2 autoreceptors (quinpirole/domperidone) and kappa-opioid receptors (U-50488H/norbinaltorphimine). No evidence was found for the presence of presynaptic adenosine A1 or A2 receptors on dopaminergic terminals. Moreover, 3,4-DAP-evoked DA release was unaffected by increased intracellular cyclic AMP levels or by drugs affecting the NO/guanylate cyclase pathway. In a similar manner it was shown that 3,4-DAP-evoked ACh release was inhibited via presynaptic muscarine autoreceptors (oxotremorine/atropine) and dopamine D2 heteroreceptors (quinpirole/domperidone). Again, no evidence for the involvement of the NO/guanylate cyclase system in the modulation of ACh release was found, whereas the presence of inhibitory adenosine A1 receptors, but not of facilitatory A2 receptors, could be clearly established. It is concluded, that 3,4-DAP-evoked 3H-overflow from rabbit caudate nucleus slices preincubated with [3H]DA or [3H]choline, represents a simple and useful in vitro model for action potential-induced DA or ACh release, respectively. Moreover, at least in this model or rabbit brain region, facilitatory adenosine A2 receptors and the NO/guanylate cyclase system seem not to be involved in the release of these transmitters.

Inhibition of N-type calcium channels: the only mechanism by which presynaptic alpha 2-autoreceptors control sympathetic transmitter release

Alpha 2-Adrenoceptors are known to inhibit voltage-dependent Ca2+ channels located at neuronal cell bodies; the present study investigated whether this or alternative mechanisms, possibly downstream of Ca2+ entry, underlie the presynaptic alpha 2-adrenergic modulation of transmitter release from chick sympathetic neurons. Using chick sympathetic neurons, overflow of previously incorporated [3H]noradrenaline was elicited in the presence of extracellular Ca2+ by electrical pulses, 25 mM K+ or 10 microM nicotine, or by adding Ca2+ to otherwise Ca(2+)-free medium when cells had been made permeable by the calcium ionophore A23187 or by alpha-latrotoxin. Pretreatment of neurons with the N-type Ca2+ channel blocker omega-conotoxin GVIA and application of the alpha 2-adrenergic agonist UK 14304 reduced the overflow elicited by electrical pulses, K+ or nicotine, but not the overflow caused by Ca2+ after permeabilization with alpha-latrotoxin or A23187. In contrast, the L-type Ca2+ channel blocker nitrendipine reduced the overflow due to K+ and nicotine, but not the overflow following electrical stimulation or alpha-latrotoxin- and A23187-permeabilization. The inhibition of electrically evoked overflow by UK 14304 persisted in the presence of nitrendipine and the L-type Ca2+ channel agonist BayK 8644, which per se enhanced overflow. In omega-conotoxin GVIA-treated cultures, electrically evoked overflow was also enhanced by BayK 8644 and almost reached the value obtained in untreated neurons. However, UK 14304 lost its effect under these conditions. Whole-cell recordings of voltage-activated Ca2+ currents corroborated these results: UK 14304 inhibited Ca2+ currents by 33%, nitrendipine caused a 7% reduction, and BayK 8644 increased the currents by 30%. Moreover, the dihydropyridines failed to abolish the inhibition by UK 14304, but pretreatment with omega-conotoxin GVIA, which reduced mean amplitude from 0.95 to 0.23 nA, entirely prevented alpha 2-adrenergic effects. Our results indicate that the alpha 2-autoreceptor-mediated modulation of noradrenaline release from chick sympathetic neurons relies exclusively on the inhibition of omega-conotoxin GVIA-sensitive N-type Ca2+ channels. Mechanisms downstream of these channels and voltage-sensitive Ca2+ channels other than N-type appear not to be important.

3,4-Diaminopyridine masks the inhibition of noradrenaline release from chick sympathetic neurons via presynaptic alpha 2-adrenoceptors: insights into the role of N- and L-type calcium channels

We have investigated the participation of the N-type (omega-conotoxin GVIA-sensitive) and L-type (nifedipine-sensitive) calcium channels in the alpha 2-adrenoceptor mediated autoinhibition of the release of [3H]noradrenaline from chick sympathetic neurons in culture. Blockade of 3,4-diaminopyridine-sensitive potassium channels resulted in tetrodotoxin-sensitive and calcium-dependent increase of the release of [3H]noradrenaline evoked by electrical stimulation. Nifedipine attenuated the evoked release under control conditions by 20%, but in the presence of 3,4-diaminopyridine by 51%, while omega-conotoxin decreased the release under control conditions by 87% and in the presence of 3,4-diaminopyridine by only 43%. The L-type calcium channel activator Bay k 8644 increased the evoked release of the transmitter both in the absence and in the presence of 3,4-diaminopyridine. Under control conditions, the alpha 2-adrenoceptor agonist UK 14304 decreased the evoked release by 57% and the alpha 2-adrenoceptor antagonist rauwolscine increased it by 14%. Nifedipine did not prevent this modulation. In the presence of 3,4-diaminopyridine, UK 14304 lost its effect on the release of noradrenaline, but its inhibitory action was restored when nifedipine, but not omega-conotoxin, was added. Changes in the increase of intracellular calcium concentration ([Ca2+]i) evoked by electrical stimulation, measured in the cell processes by microfluorimetry, paralleled the changes in the release of [3H]noradrenaline. Under control conditions, nifedipine attenuated the rise of intracellular calcium by only 16%, while omega-conotoxin did so by 66%. 3,4-Diaminopyridine enhanced the evoked rise of [Ca2+]i; in its presence the rise of intracellular calcium was about equally reduced by nifedipine and omega-conotoxin (by 46 and 36%, respectively). These effects were additive. UK 14304 diminished the peak concentration of [Ca2+]i elicited by the standard electrical stimulation by 31% and rauwolscine antagonised this effect. UK 14304 did not measurably inhibit the stimulation-evoked rise of intraterminal [Ca2+]i in the presence of 3,4-diaminopyridine but it produced an inhibition by 26% if nifedipine had been applied together with 3,4-diaminopyridine. Our observations show that, under control conditions, the stimulated release of [3H]noradrenaline is mainly associated with the opening of N-type channels, while in the presence of 3,4-diaminopyridine the contribution of L-type channels becomes more important. The alpha 2-adrenoceptor stimulation by UK 14304 inhibits the release of [3H]noradrenaline but, in the presence of 3,4-diaminopyridine, the inhibition of release can only be observed if the massive influx through L-type calcium channels is prevented. These data suggest that presynaptic alpha 2-adrenoceptors of chick sympathetic neurons preferentially influence the N-type calcium channels.

Toosendanin facilitates [3H]noradrenaline release from rat hippocampal slices

Slices of hippocampus of the rat, preincubated with [3H]noradrenaline ([3H]NA), were used to investigate the effects of toosendanin on the release of [3H]NA. Toosendanin potently enhanced spontaneous 3H outflow. Seventy-four percent of the enhancement was inhibited by reserpine pretreatment. The toosendanin-induced 3H overflow was in a concentration-dependent manner (5-60 microM) both in the presence and absence of extracellular calcium. Under Ca(2+)-free conditions, the effect of toosendanin on 3H outflow was unchanged by TTX, but inhibited by Ca(2+)-chelator BAPTA-AM; dantrolene sodium failed to affect the toosendanin-induced 3H outflow, while 3,4-diaminopyridine showed an additive effect on the outflow with this substance. The findings suggest that in the absence of extracellular Ca2+, toosendanin enhances [3H]NA release through the liberation of intracellular Ca2+ stores.

Participation of presynaptic noradrenergic fibers in the suppression of alpha 2-adrenoceptor activity by substance P at the nucleus reticularis gigantocellularis of the rat

We applied reverse microdialysis and high performance liquid chromatography (HPLC) analysis to evaluate the participation of presynaptic noradrenergic neurotransmission in the suppression by substance P (SP) of alpha 2-adrenoceptor activity at the nucleus reticularis gigantocellularis (NRGC) in Sprague-Dawley rats anesthetized with pentobarbital sodium. Microinfusion of SP (600 microM/min) into the NRGC through a stereotaxically positioned microdialysis probe attenuated the hypotensive and bradycardiac actions of the alpha 2-adrenoceptor agonist, guanabenz (100 micrograms/kg, i.v.). This inhibitory effect correlated positively with the time course of elevation in the estimated extracellular concentrations of SP and norepinephrine (NE) in the NRGC. Direct microinfusion of NE (50 nM/min) into the NRGC also lessened the cardiosuppressant effects of guanabenz. These circulatory and NE responses to SP were, however, significantly blunted in rats in which the noradrenergic innervation in the NRGC was depleted with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) pretreatment. Microinfusion of NE into the NRGC of DSP4-pretreated animals restored the attenuation of guanabenz-induced cardiovascular suppression. These results suggest that SP may depress the activity of alpha 2-adrenoceptor at the NRGC that are involved in circulatory regulation by increasing the extracellular concentration of NE via a presynaptic modulation of noradrenergic neurotransmission.

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.

alpha 2-Adrenoreceptor-mediated inhibition of acetylcholine-induced noradrenaline release from rat sympathetic neurons: an action at voltage-gated Ca2+ channels

[3H]Noradrenaline release was studied in cultured sympathetic neurons derived from superior cervical ganglia of neonatal rats. Acetylcholine elicited a concentration- and time-dependent increase in 3H outflow which was half-maximal at about 300 microM and within 5 s. The overflow induced by 10 s exposure to 300 micro A acetylcholine was reduced by the nicotinic antagonist hexamethonium, but increased by the muscarinic antagonist atropine. Cd2+ (300 microM) prevented the overflow evoked by electrical field stimulation, but reduced acetylcholine-induced overflow by less than 50%. Removal of extracellular Ca2+ abolished stimulation-evoked tritium overflow irrespective of the stimulus. The selective alpha2-adrenoceptor agonist UK 14,304 inhibited acetylcholine-evoked overflow to a significantly smaller extent (approximately 25% maximal inhibition) than electrically induced overflow ( > or = 45% maximal inhibition). These inhibitory effects were antagonized by the alpha2-adrenoceptor antagonist yohimbine. Noradrenaline (0.1 microM) reduced acetylcholine-evoked overflow to the same extent as did UK 14,304 (0.1 microM). UK 14,304 had no effect when 3H overflow was evoked by acetylcholine in the presence of 300 microM Cd2+. Currents through nicotinic acetylcholine receptors and voltage-activated Ca2+ currents were studied with the whole-cell variant of teh patch-clamp technique. UK 14,304 reduced nicotinic acetylcholine receptor currents and voltage-activated Ca2+ currents with similar potency and efficacy. Yohimbine, however, antagonized only the inhibition of voltage-activated Ca2+ currents, but not the effects of UK 14,304 on nicotinic receptor currents. Furthermore, yohimbine per se reduced currents through nicotinic receptors. Noradrenaline (10 microM) inhibited voltage-dependent Ca2+ currents just as did UK 14,304 (10 microM), but failed to reduce currents through nicotinic acetylcholine receptor channels. Cd2+ (300 microM) abolished voltage-activated Ca2+ currents and reduced nicotinic acetylcholine receptor currents by 65%. These results indicate that acetylcholine evokes noradrenaline release from rat sympathetic neurons by activation of nicotinic receptors and restricts this release via muscarinic receptors. The acetylcholine-induced transmitter release is based on two mechanisms, one involving and the other one bypassing voltage-dependent Ca2+ channels. alpha2-Adrenoceptor activation reduces voltage-activated Ca2+ currents and effects exclusively the component of acetylcholine-induced release which involves voltage-dependent Ca2+ channels. These results support the hypothesis that voltage-activated Ca2+ channels are the sole site of autoinhibitory alpha2-adrenergic effects on transmitter release from rat sympathetic neurons. The inhibitory effects of alpha2-adrenoceptor agonists and antagonists on currents through nicotinic acetylcholine receptors are not mediated by an alpha2-adrenoceptor.

3,4-Diaminopyridine-evoked noradrenaline release in rat hippocampal slices: facilitation by endogenous or exogenous nitric oxide

The involvement of nitric oxide (NO) in the evoked release of noradrenaline (NA) was studied in rat hippocampal slices preincubated with [3H]NA and stimulated with 3,4-diaminopyridine (3,4-DAP; 200 microM) for 2 min. The 3,4-DAP-evoked [3H]overflow was enhanced by the NO synthase substrate L-arginine, but not by D-arginine; it was reduced by the NO synthase inhibitor NG-nitro-L-arginine, which also antagonized the effects of L-arginine. The corresponding nitro derivative of D-arginine was inactive and unable to block the effects of L-arginine. Also drugs known to produce NO in-vitro, like sodium nitroprusside (SNP), 3-morpholino-sydnonimine (SIN-1) and S-nitroso-N-acetylpenicillamine (SNAP) enhanced the 3,4-DAP-evoked NA release. The NO scavenger hemoglobin showed no significant effects when given alone, but reduced or abolished, respectively, the facilitatory effects of SNP, or SNAP and L-arginine. The cyclic GMP derivatives 8-Br-cGMP and Sp-8-p-chlorophenylthioguanosine-3',5'-cyclic monophosphorothioate (Sp-8-pCPT-cGMPS) also acted facilitatory, whereas the corresponding Rp-enantiomer of the latter compound was inactive, but antagonized the effect of Sp-8-pCPT-cGMPS. NA release evoked by 3,4-DAP (10 microM) from rat hippocampus synaptosomes was not affected by L-arginine or NG-nitro-L-arginine but slightly increased by SNAP and Sp-8-pCPT-cGMPS. Antagonists at NMDA, non-NMDA and metabotropic glutamate receptors neither affected the 3,4-DAP-evoked NA release nor the facilitatory effect of L-arginine.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Aminopyridine differentially affects the spontaneous release of radiolabelled transmitters from rat brain slices in vitro

4-Aminopyridine increased the release of [3H]noradrenaline from dorsal hippocampus slices in vitro in a concentration-dependent manner. When the slices were exposed to 4-aminopyridine for 5 min, the overflow of radioactivity returned to pre-exposure values within 20-25 min. When the exposure of the slices was continued, a sustained enhancement of the release of [3H]noradrenaline was observed for the duration of the exposure. 4-Aminopyridine, 10(-4) M, had an effect of similar magnitude, or an even more pronounced effect, on the release of [3H]catecholamine from cortex, septum, periaqueductal gray and striatum slices. The effects of the compound on the release of [3H]5-hydroxytryptamine and [14C]acetylcholine were less pronounced. At this concentration 4-aminopyridine had no effect on the release of [3H]D-aspartate from hippocampus or septum slices, whereas the effect on the release of this transmitter in striatal slices was marginal. The effect of 4-aminopyridine on the release of [3H]noradrenaline in hippocampus slices was largely dependent on the presence of Ca2+ in the superfusion medium. This was also the case for the effect on the release of [3H]noradrenaline from preloaded dorsal hippocampus synaptosomes. In the presence of nitrendipine the effect of 4-aminopyridine was dose-dependently reduced, but the maximal reduction, at a nitrendipine concentration of 10(-4) M, was only 40%. Cd2+ completely abolished the effect of 4-aminopyridine on the release of [3H]noradrenaline. These results confirm that the enhancing effect of 4-aminopyridine on the release of [3H]noradrenaline depends on the entry of extracellular Ca2+ into the nerve terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic A2-adrenoceptors and neuropeptide Y Y2 receptors inhibit [3H]noradrenaline release from rat hypothalamic synaptosomes via different mechanisms

Presynaptic receptors may reduce transmitter release with different mechanisms. Both the alpha 2-agonist, clonidine and the Y2-agonist, neuropeptide Y fragment 13-36 (NPY 13-36), induce a concentration-dependent inhibition of the 4-aminopyridine (4-AP)-evoked [3H]noradrenaline ([3H]NA) release from hypothalamic synaptosomes. Changes in alpha 2- and Y2-modulation of noradrenaline (NA) release were observed by lowering the calcium influx with the use of omega-conotoxin (omega-CgTx), a calcium-channel blocking agent. In these experimental conditions, clonidine was less active, whereas NPY 13-36 preserved its efficacy. It therefore seems possible that presynaptic alpha 2-adrenoceptors can primarily inhibit NA release by reducing calcium influx via voltage-sensitive calcium channels (VSCC), while Y2-receptors may inhibit the intracellular release process with a mechanism independent of the calcium entry.

Relation of [Ca2+]i to dopamine release in striatal synaptosomes: role of Ca2+ channels

We compared the effects of KCl and 4-aminopyridine (4-AP) stimulation on the coupling of Ca2+ channel activation to [3H]dopamine ([3H]DA) release in rat striatal synaptosomes and used specific Ca2+ channel blockers to discriminate between the different VSCC's activated by the two stimulatory agents. We found that whereas [3H]DA release is strictly Ca(2+)-dependent in the case of KCl depolarization, 4-AP, at concentrations above 100 microM, progressively causes a large Ca(2+)-independent release of [3H]DA. Thus, at 1 to 3 mM 4-AP, as much as 80-95% of the [3H]DA release is Ca(2+)-independent and can be partially blocked by nomifensine, indicating that some [3H]DA release is occurring through reversal of the DA carrier. Therefore, in the studies relating [Ca2+]i to [3H]DA release we selected 4-AP concentrations lower than 100 microM and corrected for the Ca(2+)-independent release. Under these conditions, we determined that: (1) Ca2+ entry through N-type VSCC's is involved in [3H]DA release both in the case of KCl depolarization (35% inhibition by omega-CgTx) and in 4-AP stimulation (23% inhibition by omega-CgTx); (2) Ca2+ entering through P-type and/or Q-type VSCC's is also involved in [3H]DA release due to 4-AP stimulation (26% inhibition by 200 nM omega-Aga IVA); (3) Neomycin (0.35 mM) inhibited the [3H]DA release due to 4-AP stimulation by about 20% and decreased the KCl induced [3H]DA release by 55%; the effects of neomycin (0.35 mM) and omega-CgTx were additive in both cases, indicating that, at this concentration, the antibiotic does not affect significantly N-type Ca2+ channels; (4) When applied together, omega-CgTx and omega-Aga IVA inhibited the 4-AP stimulated [3H]DA release by about 40-50%, suggesting that the remaining large fraction of the VSCC's activated by 4-AP stimulation are non-N, non-P VSCC's and are coupled to Ca(2+)-dependent [3H]DA release; (5) The contribution of L-type VSCC's is uncertain, since there seemed to be a small contribution in the case of KCl depolarization, but not in the case of 4-AP stimulation. On the whole, the results suggest that the release of [3H]DA in the rat striatal nerve terminals depends on Ca2+ entry through N-, P-, possibly Q-, and other non-N-, non-P-type VSCC's when either KCl or 4-AP stimulation is utilized.

Effect of long-term administration of antidepressant drugs on the 5-HT3 receptors that enhance the electrically evoked release of [3H]noradrenaline in the rat hippocampus

The present study investigated the effects of various classes of antidepressant drugs (10 mg/kg per day, s.c. during 21 days) on the electrically evoked release of [3H]noradrenaline and on its modulation by the 5-HT3 receptor agonist 2-methyl-5-hydroxy-tryptamine (2-methyl-5-HT) using preloaded rat hippocampal slices. Treatments with either fluoxetine, a selective serotonin (5-HT) reuptake inhibitor, or moclobemide, a reversible type A monoamine oxidase inhibitor, increased the evoked release of [3H]noradrenaline. These two antidepressant treatments did not change, however, the magnitude of the enhancing effect of 2-methyl-5-HT on the electrically evoked release of [3H]noradrenaline. Desipramine produced a much larger increase of the electrically evoked release of [3H]noradrenaline than fluoxetine or moclobemide, and desensitized the 5-HT3 receptors that modulate this release. Trimipramine, which like desipramine has a tricyclic structure but does not block the reuptake of noradrenaline or that of 5-HT, did not increase the evoked release of [3H]noradrenaline and did not desensitize the 5-HT3 receptors that enhance the release of [3H]noradrenaline. Maprotiline, a selective noradrenaline reuptake inhibitor, did not produce the same changes as desipramine, but maprotiline inhibited noradrenaline reuptake to a lesser extent (50%) than desipramine (80%). These results suggest that the high potency noradrenaline reuptake blocker desipramine desensitizes 5-HT3 receptors modulating [3H]noradrenaline release, but that this effect is not common to all antidepressant drugs.

Involvement of nitric oxide synthase in 3,4-diaminopyridine-evoked noradrenaline release in rat hippocampus

The release of tritiated noradrenaline, evoked by stimulation of rat hippocampus slices with 3,4-diaminopyridine (200 microM, 2 min), was enhanced by the nitric oxide (NO) synthase substrate, L-arginine (but not by D-arginine), by NO donors (sodium nitroprusside, 3-morpholino-sydnonimine), and by 8-Br-cGMP. The effect of L-arginine was stereospecifically antagonized by NG-nitro-L-arginine, which given alone was inhibitory. From these findings we conclude that endogenously formed NO facilitates 3,4-diaminopyridine-evoked noradrenaline release in rat hippocampus.

Effect of an intracellular calcium chelator on the regulation of electrically evoked [3H]-noradrenaline release from rat hippocampal slices

1. The electrically (3 Hz, 5 min) evoked [3H]-noradrenaline ([3H]-NA) release from rat hippocampal slices was reduced by prior treatment of the slices with 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetraacetomethylester (BAPTA/AM) in a concentration-(10 to 500 microM) dependent manner (40% at 30 microM). Reduction of medium calcium from 1.3 to 0.5 mM caused a larger (70%) decrease. BAPTA free acid (100 mM), a non-permeable Ca(2+)-chelator had no significant effect. 2. Basal [3H]-noradrenaline release was reduced by BAPTA/AM in a concentration-dependent manner (50% at 30 microM), but reduction of external Ca2+ from 1.3 to 0.5 mM did not alter basal release. 3. About 10% of total [3H]-NA in the slices was released at 3 Hz stimulation in 1.3 mM Ca2+ buffer. Addition of the alpha 2-adrenoceptor antagonist, idazoxan (1 microM), increased electrically evoked [3H]-NA release to 26% but stimulated release was not altered by the adenosine A1-receptor antagonist, 8-cyclopentyl theophylline (8-CPT) (1 microM). 4. Evoked release was reduced by the alpha 2-receptor agonist, UK 14,304, in a concentration-dependent manner in the presence of 8-CPT (1 microM). The magnitude of this effect was not altered by the treatment of slices with 30 microM BAPTA/AM. 5. The adenosine A1 receptor agonist, N6-cyclohexyl adenosine (CHA) (1 microM) inhibited electrically evoked [3H]-NA release by about 40% in the presence of idazoxan (1 microM). The effect of CHA was not significantly altered by treatment of slices with BAPTA/AM. 7. The present results show that spontaneous [3H]-NA release is affected by reduction of intracellular Ca2+, but not by reduction of extracellular Ca2+ or by the presynaptic agonists or w-conotoxin. By contrast, electrically evoked release was affected more strongly by alterations of extracellular Ca2+ than by buffering intracellular Ca2+. The reduction of electrically evoked [3H]-NA release by agonists at the adenosine Al-receptor and a2-adrenoceptor is probably mediated through the control of Ca2+ entry via membrane ion channels or at a low affinity Ca2'-site governing evoked release.