Mechanism of efflux of noradrenaline from adrenergic nerves in rabbit atria

Dopamine release mediated by the dopamine transporter, facts and consequences

Spontaneous and/or stimulated neural activity of the nigrostriatal dopamine (DA) pathway makes amines run out from the neurons. This DA dynamic follows a rather complex path, running in or out the terminals, and flushing or diffusing into the extracellular space. The location of this leakage is not limited to the axon terminals; it also occurs from the cell bodies and dendrites. This molecular release mechanism was, for a long time, considered as being produced, in part, by the exocytosis of previously stored vesicles. The DA carrier protein (DAT, DA transporter) embedded in the DA cell membrane is known to clear previously released amines through an inward DA influx. The DAT also appears to be an active vector of amine release. Particular local conditions and the presence of numerous psychostimulant substances are able to trigger an outward efflux of DA through the DAT. This process, delivering slowly large amounts of amine could play a major regulatory role in extracellular DA homeostasis.

How addictive drugs disrupt presynaptic dopamine neurotransmission

The fundamental principle that unites addictive drugs appears to be that each enhances synaptic dopamine by means that dissociate it from normal behavioral control, so that they act to reinforce their own acquisition. This occurs via the modulation of synaptic mechanisms that can be involved in learning, including enhanced excitation or disinhibition of dopamine neuron activity, blockade of dopamine reuptake, and altering the state of the presynaptic terminal to enhance evoked over basal transmission. Amphetamines offer an exception to such modulation in that they combine multiple effects to produce nonexocytic stimulation-independent release of neurotransmitter via reverse transport independent from normal presynaptic function. Questions about the molecular actions of addictive drugs, prominently including the actions of alcohol and solvents, remain unresolved, but their ability to co-opt normal presynaptic functions helps to explain why treatment for addiction has been challenging.

Mechanisms of neurotransmitter release by amphetamines: a review

Amphetamine and substituted amphetamines, including methamphetamine, methylphenidate (Ritalin), methylenedioxymethamphetamine (ecstasy), and the herbs khat and ephedra, encompass the only widely administered class of drugs that predominantly release neurotransmitter, in this case principally catecholamines, by a non-exocytic mechanism. These drugs play important medicinal and social roles in many cultures, exert profound effects on mental function and behavior, and can produce neurodegeneration and addiction. Numerous questions remain regarding the unusual molecular mechanisms by which these compounds induce catecholamine release. We review current issues on the two apparent primary mechanisms--the redistribution of catecholamines from synaptic vesicles to the cytosol, and induction of reverse transport of transmitter through plasma membrane uptake carriers--and on additional drug effects that affect extracellular catecholamine levels, including uptake inhibition, effects on exocytosis, neurotransmitter synthesis, and metabolism.

Cardiac neural crest stem cells

Whereas the heart itself is of mesodermal origin, components of the cardiac outflow tract are formed by the neural crest, an ectodermal derivative that gives rise to the peripheral nervous system, endocrine cells, melanocytes of the skin and internal organs, and connective tissue, bone, and cartilage of the face and ventral neck, among other tissues. Cardiac neural crest cells participate in the septation of the cardiac outflow tract into aorta and pulmonary artery. The migratory cardiac neural crest consists of stem cells, fate-restricted cells, and cells that are committed to the smooth muscle cell lineage. During their migration within the posterior branchial arches, the developmental potentials of pluripotent neural crest cells become restricted. Conversely, neural crest stem cells persist at many locations, including in the cardiac outflow tract. Many aspects of neural crest cell differentiation are driven by growth factor action. Neurotrophin-3 (NT-3) and its preferred receptor, TrkC, play important roles not only in nervous system development and function, but also in cardiac development as deletion of these genes causes outflow tract malformations. In vitro clonal analysis has shown a premature commitment of cardiac neural crest stem cells in TrkC null mice and a perturbed morphology of the endothelial tube. Norepinephrine transporter (NET) function promotes the differentiation of neural crest stem cells into noradrenergic neurons. Surprisingly, many diverse nonneuronal embryonic tissues, in particular in the cardiovascular system, express NET also. It will be of interest to determine whether norepinephrine transport plays a role also in cardiovascular development.

Analysis of high intracellular [Na+]-induced release of [3H]noradrenaline in rat hippocampal slices

Our aim was to investigate the mechanisms involved in the high intracellular sodium-induced transmitter release in the CNS through the characterisation of the veratridine-evoked (40 microM) noradrenaline release from rat hippocampal slices. The response to veratridine was completely inhibited by tetrodotoxin (1 microM), indicating that the effect is due to the activation of sodium channels. Omission of Ca2+ from the superfusion fluid inhibited the veratridine-evoked release by 72%, showing that the majority of release results from external Ca2+-dependent exocytosis. The residual Ca2+-independent release was not blocked by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester (100 microM) suggesting that intracellular Ca2+ stores are not involved in this component of veratridine effect. The noradrenaline uptake blockers, desipramine (10 microM) and nisoxetine (10 microM), inhibited the external Ca2+-independent release by 50 and 46%, respectively, indicating that the release partly originates from the reversal of transporters (carrier-mediated release). In contrast to uptake blockers, lowering the temperature, another possibility to inhibit transporter function, completely inhibited the effect of veratridine in the absence of Ca2+. Further experiments revealed that low temperature (20 and 12 degrees C) reduces the veratridine-induced increase of intracellular sodium concentration ([Na+]i) in rat cortical synaptosomes (68 and 78% inhibition, respectively). The clinical relevance of our data is that during ischemia a massive release of transmitters occurs mainly due to the elevation of [Na+]i, which contributes to the development of ischemic brain injury. Our results show that low temperature may be a better therapeutic approach to the treatment of ischemia because it has a dual action on this process. Firstly, it inhibits the function of uptake transporters and hence reduces the carrier-mediated outflow of transmitters. Secondly, it inhibits the sodium influx and therefore prevents the unwanted elevation of [Na+]i. Our data also suggest that veratridine stimulation can be a suitable model for ischemic conditions.

Neuropharmacological characterization of local ibogaine effects on dopamine release

Local perfusion with ibogaine (10(-6) M-10(-3) M) via microdialysis probes in the nucleus accumbens or striatum of rats produced a biphasic dose-response effect on extracellular dopamine levels. Lower doses (10(-6) M-10(-4) M) produced a decrease while higher doses (5 x 10(-4) M-10(-3) M) produced an increase in dopamine levels. Dihydroxyphenylacetic acid (DOPAC) levels were not effected. Naloxone (10(-6) M) and norbinaltorphimine (10(-6) M-10(-5) M) did not affect dopamine levels, but when co-administered with ibogaine (10(-4) M) blocked the decrease in dopamine levels produced by ibogaine. Ibogaine (10(-3) M) stimulation of dopamine levels in the striatum was calcium independent and not blocked by tetrodotoxin (10(-5) M). Pretreatment with cocaine (15 mg/kg), reserpine (5 mg/kg) or alpha-methyl-para-tyrosine (250 mg/kg) given intraperitoneally significantly reduced ibogaine (10(-3)M) stimulation of striatal dopamine levels. In striatal synaptosomes, both ibogaine and harmaline (10(-7)-10(-4) M) produced dose-dependent inhibition of [3H]-dopamine uptake. These findings suggest that ibogaine has both inhibitory and stimulatory effects on dopamine release at the level of the nerve terminal. It is suggested that the inhibitory effect is mediated by kappa opiate receptors while the stimulatory effect is mediated by interaction with the dopamine uptake transporter.

The reverse transport of DA, what physiological significance?

It is well established that midbrain dopamine neurons innervating the striatum, release their neurotransmitter through an exocytotic process triggered by the neural firing and involving a transient calcium entry in the terminals. Long ago, it had been proposed, however, that another mechanism of release could co-exist with classical exocytosis, involving the reverse-transport of the cytosolic amine by the carrier, ordinarily responsible for uptake function. This atypical mode of release could be evoked directly at the preterminal level by multiple environmental endogenous factors involving transient alterations of the sodium gradient. It cannot be excluded that this mode of release participates in the firing-induced release. In contrast with the classical exocytosis of a preformed DA pool, the reverse-transport of DA requires simultaneous alterations of intraterminal amine metabolism including synthesis and displacement from storage compartment. The concept of a reverse-transport of dopamine is coming from the observations that releasing substances, such as amphetamine-related molecules, actually induce this type of transport. A large set of arguments advocates that reverse-transport plays a role in the maintenance of basal extracellular DA concentration in striatum. It was also often evoked in physiopathological situations including ischemia, neurodegenerative processes, etc. The most recent studies suggest that this release could occur mainly outside the synapses, and thus could constitute a major feature in the paracrine transmission, sometimes evoked for DA.

Release of amines from acidified stores following accumulation by Transport-P

1. Transport-P is an uptake process for amines in peptidergic neurones of the hypothalamus. It differs from other uptake processes by its anatomical location in post-synaptic neurones, its functional properties and by the structure of its ligands. Transport-P accumulates amines in intracellular vesicles, derives its energy from the electrochemical proton gradient and is linked to vacuolar-type ATPase (V-ATPase). Transport-P is blocked by antidepressants. We have now studied the release of amines following uptake by Transport-P in a cell line of hypothalamic peptidergic neurones. 2. Release of prazosin was not inhibited by the antidepressant desipramine; as Transport-P is blocked by desipramine, this indicated that amines are released by a mechanism which is independent of Transport-P. 3. Release of prazosin was sensitive to temperature and conformed to the Arrhenius equation. Release was minimal in the range 0-25 degrees C but accelerated exponentially at higher temperatures up to 33 degrees C. The activation energy for the release of prazosin is 83.1 kJ x mol(-1), corresponding to a temperature quotient (Q10) value of 3. 4. Release was accelerated by the organic base chloroquine, the ionophore monensin, bafilomycinA1 which inhibits V-ATPase and by increasing extracellular pH. Thus, retention of prazosin requires an intracellular proton gradient which is generated by V-ATPase. 5. Fluorescence microscopy demonstrated that release of BODIPY FL prazosin was temperature dependent and was accelerated by chloroquine and monensin. 6. Thus, following uptake by Transport-P, amines are accumulated in acidified intracellular stores. Their retention in peptidergic neurones requires intracellular acidity. The amines are released by a temperature-dependent process which is resistant to antidepressants.

In vivo histamine release from brain cortex: the effects of modulating cellular and extracellular sodium and calcium channels

The in vivo mechanisms underlying the actions of modulating Na(+)- and Ca(2+)-sensitive channels and its effect on basal histamine release in the cerebral cortex of freely-moving unanesthetized rats was investigated. Basal histamine release in the cerebral cortex was determined by in vivo microdialysis coupled with high-performance liquid chromatography (HPLC) fluorometry detection. Basal levels of histamine were 0.67+/-0.02 pmol/10 microl of dialysate. Diltiazem, a Ca(2+) channel antagonist, produced a dose-dependent decrease in dialysate basal histamine concentration. Elevated K(+) (100 mM) in the perfusion medium increased basal histamine to a maximum of 223% of the baseline value. Similarly, diltiazem (60 mM) reduced the K(+), veratridine (100 microg/ml) and ouabain (100 microM)-evoked increase in dialysate histamine. Basal histamine decreased by 48% when the perfusate contained 3 microM of voltage dependent Na(+) antagonist tetrodotoxin. The results of these studies indicate that the release of histamine in rat cerebral cortex can be induced by modulating Na(+) and Ca(2+) channels and that the L-type voltage-dependent sensitive Ca(2+) channels are involved in this release process.

Ion dependence of carrier-mediated release in dopamine or norepinephrine transporter-transfected cells questions the hypothesis of facilitated exchange diffusion

The mechanism of release mediated by the human dopamine and norepinephrine transporter (DAT and NET, respectively) was studied by a superfusion technique in human embryonic kidney 293 cells stably transfected with the respective transporter cDNA and loaded with the metabolically inert substrate [(3)H]1-methyl-4-phenylpyridinium. Release was induced by amphetamine, dopamine, and norepinephrine or by lowering the sodium or chloride concentration in the superfusion buffer (iso-osmotic replacement by lithium and isethionate, respectively). Efflux of [(3)H]1-methyl-4-phenylpyridinium was analyzed at 30-s time resolution. In both transporters, release induced by the substrates amphetamine, dopamine, and norepinephrine followed the same time course as release induced by the removal of chloride and was faster than that caused by the removal of sodium. In the presence of low sodium (DAT: 10 mM; NET: 5 mM) none of the substrates was able to induce release from either type of cell, but adding back sodium to control conditions promptly restored the releasing action. In the presence of low chloride (DAT: 3 mM; NET: 2 mM), however, amphetamine as well as the catecholamines stimulated release from both types of cell. In contrast with the ion dependence of release observed in superfusion experiments, uptake initial rates of substrates at concentrations used in release experiments were the same or even higher at low sodium than at low chloride. The results indicate a decisive role of extracellular sodium for carrier-mediated release unrelated to the sodium-dependent uptake of the releasing substrate, and suggest a release mechanism different from simple exchange diffusion considering only the amines as substrates.

Inhibition of plasma membrane monoamine transporters by beta-ketoamphetamines

Methcathinone and methylone, the beta-ketone analogues of methamphetamine and 3,4-methylenedioxymethamphetamine (MDMA), respectively, were tested for neurotransmitter uptake inhibition in vitro. The beta-ketones were threefold less potent than the nonketo drugs at inhibiting platelet serotonin accumulation, with IC(50)'s of 34.6+/-4.8 microM and 5.8+/-0.7 microM, respectively. Methcathinone and methylone were similar in potency to methamphetamine and MDMA at catecholamine transporters individually expressed in transfected glial cells. For dopamine uptake, IC(50)'s were 0.36+/-0.06 microM and 0.82+/-0.17 microM, respectively; for noradrenaline uptake, IC(50) values were 0.51+/-0.10 microM and 1. 2+/-0.1 microM, respectively. In chromaffin granules, IC(50)'s for serotonin accumulation were 112+/-8.0 microM for methcathinone and 166+/-12 microM for methylone, 10-fold higher than the respective values for methamphetamine and MDMA. Our results indicate that methcathinone and methylone potently inhibit plasma membrane catecholamine transporters but only weakly inhibit the vesicle transporter.

Co-release of endogenous ATP and [3H]noradrenaline from rat hypothalamic slices: origin and modulation by alpha2-adrenoceptors

The release of endogenous ATP, measured by the luciferin-luciferase assay, and of [3H]noradrenaline from the in vitro superfused rat hypothalamic slices were studied. ATP and [3H]noradrenaline were released simultaneously during resting conditions and in response to low and high frequency field electrical stimulation; the release of both substances were frequency dependent between 2 Hz and 16 Hz. The stimulation-induced release of ATP and [3H]noradrenaline was diminished by more than 80% under Ca2+-free conditions. Tetrodotoxin inhibited the majority of the evoked release of both ATP and [3H]noradrenaline, however, it was less effective in reducing the release of [3H]noradrenaline, than that of ATP. Bilateral stereotaxic injection of 6-hydroxydopamine (4 microg/side) to the ventral part of the ventral noradrenergic bundle, originating from the A1 cell group in the brainstem, resulted in a 55% reduction of endogenous noradrenaline content of the hypothalamic slices, and the tritium uptake and the stimulation-evoked release of [3H]noradrenaline was also markedly reduced. While the basal release of ATP was not affected, the evoked release was diminished by 72% by this treatment. Perfusion of the slices with noradrenaline (100 microM) initiated rapid and continuous tritium release; on the other hand, it did not release any ATP. In contrast, 6 min perfusion of (-)nicotine and 1,1-dimethyl-4-phenyl-piperazinium iodide evoked parallel release of ATP and [3H]noradrenaline which was inhibited by the nicotinic receptor antagonist mecamylamine; 6-hydroxydopamine lesion of the ventral part of the ventral noradrenergic bundle did not affect the nicotine-evoked ATP and [3H]noradrenaline release. While CH 38083, a non subtype-selective alpha2-antagonist and BRL44408, the subtype-selective alpha2AD antagonist augmented the evoked release of [3H]noradrenaline, ARC239, a selective alpha2BC antagonist was without effect. In contrast, neither of the alpha2-antagonists significantly affected the evoked-release of ATP. In summary, we report here that endogenous ATP and [3H]noradrenaline are co-released stimulation-dependently from superfused rat hypothalamic slices. A significant part of the release of both compounds is derived from the nerve terminals, originating from the A1 catecholaminergic cell group of brainstem nuclei. Unlike that from the peripheral sympathetic transmission, noradrenaline and alpha1-adrenoceptor agonists were unable to promote the release of ATP. Conversely, parallel ATP and noradrenaline release could be induced by nicotine receptor activation, but this release does not originate from the same nerve endings. The evoked-release of [3H]noradrenaline is inhibited by endogenous noradrenaline via alpha2AD subtype of adrenoreceptors, while the release of ATP is not subject to this autoinhibitory modulation. In conclusion, our results support the view that ATP is involved in the neurotransmission in the hypothalamus, but the sources of the released ATP and noradrenaline seem to be not identical under different stimulatory and modulatory conditions.

Release mechanisms of [125I]meta-iodobenzylguanidine in neuroblastoma cells: evidence of a carrier-mediated efflux

[131I]metaiodobenzylguanidine ([131I]MIBG) is selectively taken up and stored by tumours derived from the neural crest, and is used for diagnosis and treatment of neuroblastoma (NB). The antitumoral effect of [131I]MIBG is closely related to the intracellular level of the radiopharmaceutical compound, which is dependent on uptake and storage/release mechanisms. While MIBG uptake is well characterised, storage and release mechanisms are still controversial. In order to better characterise [125I]MIBG release mechanisms, we studied the basal and stimulated efflux of [125I]MIBG in the human NB cell line, SH-SY5Y, preloaded with 0.1 microM [125I]MIBG for 1 h. We found that [125I]MIBG basal efflux is highly temperature-dependent, that [125I]MIBG release, induced by cell depolarisation with high potassium, is mainly calcium-independent, and induced by exchange with cold MIBG or noradrenaline, inversion of the sodium gradient across the cell membrane by veratridine by substitution of sodium chloride with equimolar concentration of lithium chloride. The exposure of NB cells to imipramine, an Uptake-1 inhibitor, also produces a net stimulatory effect on [125I]MIBG release. However, when used in association with other releasing stimuli, such as higher levels of intracellular sodium or external agonists, imipramine abolishes the consequent increase of [125I]MIBG release. Our findings suggest that stimulated [125I]MIBG release is mediated by a carrier, most probably the uptake carrier working in a reverse mode, while a minimal fraction of [125I]MIBG is released by an exocytotic mechanism.

Contribution of intra- and extracellular Ca2+ to noradrenaline exocytosis induced by ouabain and monensin from guinea-pig vas deferens

1. Contributions of intra- and extracellular Ca2+ to noradrenaline (NA) release evoked by increasing intracellular Na+ concentrations (ouabain plus monensin) from adrenergic nerves of guinea-pig vas deferens were evaluated under conditions eliminating carrier-mediated NA release (with 100 microM cocaine). 2. Ouabain (100 microM) plus monensin (10 microM), unlike 100 mM KCl, produced a marked NA release which was unchanged by Ca(2+)-removal. 3. In normal solution but not in Ca(2+)-free solution, the release of NA evoked by ouabain plus monensin was reduced by adenosine, clonidine and neuropeptide Y, and by Ca(2+)-channel blockers such as omega-conotoxin GVIA and nifedipine. The release of NA was also decreased by cromakalim in a glibenclamide-sensitive fashion. 4. In contrast, in the absence but not in the presence of Ca2+, the drug-evoked NA release was inhibited by mitochondrial inhibitors (carbonylcyanide-m-chlorophenylhydrazone and oligomycin) and further by immobilizers of intracellular Ca2+ (TMB-8 and BAPTA-AM) and calmodulin antagonists (W-7 and trifluoperazine). 5. These findings suggest that the release of NA evoked by elevation of [Na+]i from adrenergic nerves in the presence and absence of Ca2+ involves, in part, exocytotic processes which are triggered by depolarization-induced Ca2+ influx and by utilization of Ca2+ from intracellular Ca2+ store sites such as mitochondria, respectively.

Dual effect of nicotine on cardiac noradrenaline release during metabolic blockade

Nicotine-induced noradrenaline was investigated in perfused guinea pig hearts subjected to metabolic blockade that was caused either by anoxia or by cyanide intoxication. Noradrenaline, neuropeptide Y, and dihydroxyphenylethyleneglycol (DOPEG) were determined in the coronary venous overflow Neuropeptide Y is a sympathetic cotransmitter of noradrenaline, and concomitant release of both transmitters indicates an exocytotic, calcium-dependent release mechanism, whereas neuropeptide Y overflow does not occur during nonexocytotic noradrenaline release. Nonexocytotic, calcium-independent noradrenaline release, however, is associated with an increase of DOPEG overflow, which is the main intraneuronal metabolite of noradrenaline formed by monoamine oxidase if oxygen is present. Anoxia per se caused a nonexocytotic release of noradrenaline starting after 10 min of anoxia and reaching peak levels at 30 min. During anoxia, nicotine (3 and 10 mumol/l) accelerated and enhanced noradrenaline overflow, i.e., the period between the onset of anoxia and the begin of noradrenaline release was shortened and peak levels were increased. Nicotine-induced noradrenaline release was accompanied by neuropeptide Y overflow. The action of nicotine was further evaluated during energy depletion caused by cyanide. As anoxia did, cyanide administration alone resulted in noradrenaline release. In accordance with a nonexocytotic mechanism and due to the presence of oxygen, this release of noradrenaline was accompanied by an increase of DOPEG. When added 10 min after the onset of energy depletion, nicotine (10 mumol/l) caused a brief but marked enhancement of exocytotic noradrenaline release, since this release was calcium-dependent and was accompanied by a significant rise of neuropeptide Y overflow. In absence of extracellular calcium to avoid exocytosis, concomitant administration of nicotine (3-100 mumol/l) and cyanide caused a concentration-dependent acceleration of both the overflow of noradrenaline and DOPEG, whereas overflow of neuropeptide Y was not increased, thus indicating a nonexocytotic release mechanism. In conclusion, the application of nicotine during myocardial energy depletion increases overflow of noradrenaline by both calcium-dependent exocytotic release and calcium-independent nonexocytotic release mechanisms.

Prejunctional effects of cromakalim, nicorandil and pinacidil on noradrenergic transmission in rat isolated mesenteric artery

The present study investigated the effects of cromakalim, nicorandil and pinacidil on resting and stimulation-induced (S-I) effluxes of radioactivity from rat isolated mesenteric artery preparations in which the noradrenergic transmitter stores had been radiolabelled with [3H]-noradrenaline. The efflux of radioactivity evoked by field stimulation of peri-arterial sympathetic nerves (pulses at 2 Hz frequency in trains of 60 s duration) was taken as an index of transmitter noradrenaline release. Cromakalim (1-100 microM) and nicorandil (1-1000 microM) produced minor effects on resting and S-I effluxes of radioactivity, but these did not exhibit concentration-dependency. Pinacidil (1-1000 microM) produced concentration-dependent increases, in both resting and S-I effluxes of radioactivity. With 1000 microM pinacidil, resting and S-I effluxes were increased to approximately 348% and 358% of their respective control values. The effects of pinacidil on resting and S-I effluxes were unaltered when the neuronal amine transport system was inhibited by desipramine (1 microM). Inhibition of monoamine oxidase with pargyline (100 microM) treatment markedly reduced the enhancement of resting efflux by 1000 microM pinacidil but did not alter its effect on S-I efflux. It is proposed that the enhanced resting efflux produced by pinacidil without pargyline treatment consists of deaminated [3H]-noradrenaline metabolites formed from [3H]-noradrenaline displaced from transmitter storage vesicles by pinacidil. The enhancement of S-I efflux by pinacidil does not appear to involve disruption of alpha 2-adrenoceptor auto-inhibition of transmitter release since equi-effective concentrations of phentolamine (1 microM) and pinacidil (1000 microM) produced additive effects on S-I efflux, whereas increasing the concentration of phentolamine from 1 to 2M produced no further increases in S-I efflux. In conclusion, this study has provided no evidence of a prejunctional inhibitory effect of the potassium channel openers cromakalim, nicorandil and pinacidil on transmitter noradrenaline release. However, the findings with pinacidil suggest that, in high concentrations, pinacidil displaces noradrenaline from transmitter stores, such that deaminated noradrenaline metabolites are released from the nerve terminals. Furthermore, pinacidil enhances S-I transmitter noradrenaline release, possibly by blocking neuronal potassium channels.

Comparison of the effect of reversible and irreversible MAO inhibitors on renal nerve activity in the anesthetized rat

Cardiovascular effects of the irreversible MAO-A inhibitor clorgyline and reversible MAO-inhibitors, moclobemide and brofaromine, were compared in the anesthetized rat. Electrical activity of the sympathetic renal nerve was monitored as an index of central sympathetic output. A long lasting decrease in the recorded parameters: blood pressure (BP), renal nerve activity (RNA) and heart rate (HR) was produced by acute administration of clorgyline (2 mg/Kg, IP). Acute treatment with moclobemide (10 mg/Kg, IP) or brofaromine (10 mg/Kg, IP) caused only a transient decrease in RNA. Pretreatment with the alpha 2 antagonist yohimbine, decreased significantly the inhibitory effect of clorgyline on all three parameters. The selective alpha 2 antagonist CH-38083 blocked the sympathoinhibitory effect of brofaromine. These results indicate an alpha 2 adrenoceptor involvement in the central sympathoinhibitory effect of MAO inhibitors, which may be manifested as a hypotensive effect, including orthostatic hypotension, in patients treated with irreversible selective MAO inhibitors.

Effects of the venom of the theridiid spider, Steatoda capensis Hann, on autonomic transmission in rat isolated atria and caudal artery

The possibility of alpha-latrotoxin-like activity in the crude venom gland extract (VGE) of a related Theridiid spider, Steatoda capensis Hann, was investigated. The VGE from female S. capensis Hann spiders produced vasoconstriction in isolated segments of rat caudal artery but was without effect in artery segments obtained from rats that had been pretreated with reserpine (2.5 mg/kg) 24 hr prior to experimentation, indicating that the vasoconstriction was due to the release of noradrenaline from periarterial sympathetic nerves. Steatoda capensis Hann VGE also increased the rate of beating of rat isolated atrial preparations. The positive chronotropic action of the VGE was partly due to the release of noradrenaline from atrial sympathetic nerves since it was reduced by the beta-adrenoceptor antagonist propranolol, and smaller increases in rate were observed in atria taken from rats pretreated with reserpine. The positive chronotropic effect of the VGE was enhanced by atropine, suggesting that the VGE also releases acetylcholine from atrial parasympathetic nerves. The VGE evoked release of radioactivity from rat atria in which the transmitter stores of the atrial intramural noradrenergic nerves had been labelled with [3H]noradrenaline. There appeared to be two components of the release, one involving omega-conotoxin GVIA-sensitive Ca2+ channels, and the other independent of extracellular Ca2+.

Prejunctional actions of tacrine on autonomic neuroeffector transmission in rabbit isolated pulmonary artery and rat isolated atria

1. This study investigated the effects of tacrine (1,2,3,4-tetrahydro-9-aminoacridine) on the resting and stimulation-induced (SI) release of radioactive substances from isolated preparations of rat atria and rabbit pulmonary artery in which the noradrenergic transmitter stores had been labelled with [3H]-noradrenaline, and from rat atrial preparations in which cholinergic transmitter stores had been labelled with [3H]-acetylcholine. In addition, the effect of tacrine on the uptake of [3H]-noradrenaline by noradrenergic nerves in rat atria was determined. 2. Tacrine produced concentration-dependent increases in the resting efflux of radioactivity from both the [3H]-noradrenaline-loaded artery and atrial preparations. Blockade of neuronal amine transport with desipramine reduced the release of radioactivity evoked by tacrine from atria but not that evoked from artery preparations. Inhibition of monoamine oxidase by pargyline pretreatment markedly reduced the tacrine-evoked release of radioactivity in both atrial and artery preparations. 3. The radioactivity released from [3H]-noradrenaline-labelled rat atrial preparations by 30 mumol/L tacrine consisted entirely of the deaminated metabolite [3H]-DOPEG. The evoked release of [3H]-DOPEG from atria was reduced by approximately 50% by desipramine (1 mumol/L). When atrial monoamine oxidase had been inhibited by pargyline treatment in vivo and in vitro, 30 mumol/L tacrine evoked the release of [3H]-noradrenaline instead of [3H]-DOPEG. However, the amounts of [3H]-noradrenaline released by tacrine when monoamine oxidase was inhibited were only about 25% of the amounts of [3H]-DOPEG released in untreated atria. 4. Tacrine, in concentrations of 1 and 10 mumol/L, enhanced the release of radioactivity evoked by field stimulation of [3H]-noradrenaline-loaded rabbit pulmonary artery preparations. This effect was unaltered by desipramine or pretreatment with pargyline. However, in artery preparations pretreated with pargyline, a high concentration of tacrine (100 mumol/L) markedly reduced SI efflux. In contrast to the findings with artery preparations, tacrine (1-30 mumol/L) did not alter SI efflux in rat atrial preparations. 5. It is concluded that tacrine displaces noradrenaline from intraneuronal transmitter stores of sympathetically-innervated tissues, and that the displaced amine is totally metabolized by monoamine oxidase before leaving the nerve terminals. When deamination of neuronal cytoplasmic noradrenaline is prevented, only a portion of the noradrenaline displaced from storage vesicles passes to the extracellular space. It is likely that the transfer of cytoplasmic noradrenaline out of the terminals is limited by the activity of the amine transport mechanism.

Cathinone, a natural amphetamine

Cathinone is an alkaloid that has been discovered some fifteen years ago in the leaves of the khat bush. This plant grows in East Africa and in southern Arabia, and the inhabitants of these regions frequently chew khat because of its stimulating properties. Cathinone, which is S(-)-alpha-aminopropiophenone, was soon found to have a pharmacological profile closely resembling that of amphetamine; indeed, in a wide variety of in vitro and in vivo experiments it was demonstrated that cathinone shares the action of amphetamine on CNS as well as its sympathomimetic effects; thus, for example, drug-conditioned animals will not distinguish between cathinone and amphetamine. These various observations were confirmed by a clinical experiment showing that cathinone also in humans produces amphetamine-like objective and subjective effects. Finally, it was demonstrated that cathinone operates through the same mechanism as amphetamine, i.e. it acts by releasing catecholamines from presynaptic storage sites. Thus, much experimental evidence indicates that cathinone is the main psychoactive constituent of the khat leaf and that, in fact, this alkaloid is a natural amphetamine.

Carrier-mediated outward transport of dopamine from adrenergic varicosities of the vas deferens of reserpine-pretreated rats

In vasa deferentia of reserpine-pretreated rats a carrier-mediated (i.e., desipramine-sensitive) outward transport of endogenous dopamine was induced by either tyramine or ouabain. The dopamine taking part in the efflux induced by tyramine (and the concomitant efflux of DOPAC) was derived from ongoing synthesis of dopamine. Inhibition of MAO trebled the rate of spontaneous efflux of dopamine and reduced the spontaneous efflux of DOPAC by 90%. After inhibition of MAO, desipramine caused a further five-fold increase in the basal efflux of dopamine with no change in the basal efflux of DOPAC. Inhibition of COMT failed to affect the spontaneous efflux of dopamine but increased that of DOPAC. It is concluded that, after depletion of the noradrenaline stores by pretreatment with reserpine, an outward transport of axoplasmic dopamine is induced by the same mechanisms that (without any pretreatment with reserpine) are known to initiate an outward transport of noradrenaline.

The sensitivity of adrenergic varicosities to the 3H-noradrenaline-releasing effect of potassium

After the loading of incubated, homogeneously innervated tissues with 3H-noradrenaline (monoamine oxidase and catechol-O-methyl transferase inhibited, calcium-containing solution) high K+ released the 3H-amine from adrenergic varicosities. In paired experiments the sensitivity of rat atria to high K+ exceeded that of vasa deferentia. In the rat vas deferens the releasing effect of high K+ was enhanced by drugs or procedures which induce a carrier-mediated outward transport of 3H-noradrenaline, i.e., by ouabain, by glucose deprivation and by hypoxia. In the presence of extracellular calcium desipramine failed to affect the releasing effect of high K+ (except in the absence of glucose or during hypoxia), but in the absence of calcium desipramine reduced it. Apparently, whenever the axoplasmic levels of 3H-noradrenaline are increased, high K+ is able to induce some carrier-mediated outward transport of the 3H-amine. It is suggested that "organ differences" with respect to the sensitivity to high K+ may well be due to hypoxia (plus some lack of glucose) of those varicosities that had been loaded with 3H-noradrenaline. The risk of storage of 3H-noradrenaline in hypoxic varicosities appears to be greater in incubated than in perfused organs, and in the former it is greater in sparsely than in densely innervated tissues.

Mechanism of neurotransmitter release elicited by the preferential alpha 1-adrenergic receptor agonist phenylephrine in superfused superior cervical ganglion cells in culture

Phenylephrine increased [3H]norepinephrine efflux and accumulation of cyclic AMP in cultured rat superior cervical ganglion cells superfused with Tyrode's solution. The purpose of this study was to determine the mechanism and relationship between these two events. Electrical stimulation (1-2 Hz), potassium chloride (50 mM), and the preferential alpha 1-adrenergic receptor agonist phenylephrine (1-100 microM) increased fractional tritium efflux, whereas methoxamine, cirazoline, and amidephrine were relatively ineffective. Phenylephrine, but not methoxamine and cirazoline, also increased cyclic AMP accumulation. Phenylephrine-induced tritium efflux was not altered by alpha- and beta-adrenergic receptor antagonists or by removal of extracellular calcium. Phenylephrine-induced cyclic AMP accumulation was blocked by the beta-adrenergic receptor antagonists propranolol and atenolol. Forskolin (10 microM) and the nonhydrolyzable cyclic AMP analogue 8-(4-chlorophenylthio)cyclic AMP (100 microM) had minimal effect on tritium efflux. However, phenylephrine-evoked increase in tritium efflux was dose dependently attenuated by the neuronal uptake blocker cocaine, and phenylephrine dose-dependently inhibited the incorporation of [3H]norepinephrine into neuronal stores. We conclude that the increase in tritium efflux induced by phenylephrine is independent of cyclic AMP accumulation and appears to be mediated by uptake of phenylephrine via the neuronal carrier-mediated amine transport process, which in turn promotes efflux of the adrenergic transmitter from its storage sites.

The energy requirements for the basal efflux of 3H-noradrenaline from sympathetically innervated organs

In the rat vas deferens (preloaded with 3H-noradrenaline, catechol-O-methyl transferase inhibited, calcium-free solution) ouabain, glucose deprivation or the combination of hypoxia plus presence of lactate were found to induce a carrier-mediated (desipramine-sensitive) outward transport of the 3H-amine. Glucose deprivation additionally increased the efflux of deaminated 3H-metabolites, as a consequence of an increased net leakage of vesicular 3H-noradrenaline; moreover, 3H-dihydroxymandelic acid then became the predominant neuronal metabolite. The simultaneous lack of oxygen and glucose resulted in a very pronounced release of the 3H-amine. Moreover, during spontaneous efflux more outward transport of 3H-noradrenaline was observed in the absence than in the presence of extracellular calcium. In rat atria (under the same experimental conditions) the contribution by carrier-mediated outward transport to the spontaneous efflux of tritium exceeded that in vasa deferentia. Moreover, the efflux of lactate (as an index of hypoxia of the tissue) exceeded that observed in vasa deferentia, under aerobic and anaerobic conditions. It is proposed that the greater contribution by outward transport of 3H-noradrenaline to spontaneous efflux in atria than in vasa deferentia does not reflect any basic difference between the varicosities in two different organs. It is likely that the less heterogeneous distribution of the 3H-amine in atria than in vasa deferentia is responsible for storage of the exogenous amine in atrial varicosities that are subject to some hypoxia, to an increased extracellular lactate level and to perhaps a minor degree of glucose deficiency; these factors may well be responsible for the difference with regard to outward transport of 3H-noradrenaline during spontaneous efflux. Thus, in addition to the heterogeneity of the distribution of 3H-noradrenaline, an additional heterogeneity with regard to the energy supply must be expected for incubated organs.

Effect of digitalis glycosides on norepinephrine release in the heart. Dual mechanism of action

The effect of ouabain on exocytotic and nonexocytotic norepinephrine release was investigated in perfused rat and guinea pig hearts. The overflow of endogenous norepinephrine and its neuronal metabolite 3,4-dihydroxyphenylethyleneglycol (DOPEG) was determined by high-pressure liquid chromatography. DOPEG served as the indicator of free axoplasmic norepinephrine concentrations. The overflow of the norepinephrine cotransmitter neuropeptide Y (NPY) was determined by radioimmunoassay and NPY was used as marker for exocytotic release. Electrical stimulation of the left stellate ganglion resulted in exocytotic norepinephrine release in rat and guinea pig hearts. Ouabain caused an increase in stimulation-induced norepinephrine overflow from rat and guinea pig hearts by 40%. However, overflow of NPY was decreased by 40%, indicating a reduced exocytosis rate. Ouabain increased both norepinephrine and NPY overflow, suggesting enhancement of exocytosis, when neuronal catecholamine uptake (uptake1) was blocked by desipramine or when presynaptic alpha 2-adrenoceptors were inhibited by yohimbine. The results demonstrate an interaction of ouabain with both calcium-dependent exocytosis and uptake1 of norepinephrine. Under calcium-free conditions, ouabain or potassium-free perfusate resulted in norepinephrine release from hearts when the axoplasmic norepinephrine concentration was elevated by the reserpinelike agent Ro 4-1284. This release was independent from neural activity, not accompanied by NPY overflow, and suppressed by the uptake1 blocker desipramine. These findings are in keeping with carrier-mediated nonexocytotic norepinephrine release that is caused by reversal of the transport direction of the uptake1 carrier. During myocardial ischemia nonexocytotic norepinephrine release was accelerated and enhanced by inhibition of Na+,K(+)-ATPase before ischemia. This study demonstrates the potential of digitalis glycosides to interact both with transmitter exocytosis and with the neuronal catecholamine transport system by Na+,K(+)-ATPase inhibition. Interaction with the catecholamine transport system involves both inhibition of norepinephrine inward transport and induction of norepinephrine outward transport, resulting in nonexocytotic norepinephrine release.

Bay K 8644 enhances the outflow of [3H]-noradrenaline and [3H]-DOPEG from isolated rat atria

The effect of Bay K 8644 (a dihydropyridine Ca(2+)-channel activator), was examined on spontaneous and stimulus-evoked release of tritium from isolated rat atria prelabelled with [3H]-noradrenaline. Bay K 8644 (3 mumol/l) significantly increased atrial rate from 206 +/- 7 to 259 +/- 9 beats.min-1 (P less than 0.05) and also tritium outflow (expressed as fractional rate of loss in min-1 x 10(3)) from 6.49 +/- 0.35 to 8.61 +/- 0.74 (P less than 0.05). Neither the maximal rate nor the overflow of tritium induced by stimulation of sympathetic nerve terminals was changed by the compound. The increase in basal tritium outflow produced by Bay K 8644 was calcium-dependent. However, it could not be antagonized by nitrendipine. The overflow of tritium induced by Bay K 8644 consisted mainly of 3,4-dihydroxyphenylglycol ([3H]-DOPEG), indicating that the compound produces a leakage from the storage vesicles of sympathetic nerve terminals of the isolated rat atria.

Characteristics of [3H]GBR 12935 binding in the human and rat frontal cortex

Binding characteristics of the selective dopamine uptake inhibitor [3H]GBR 12935 have been described for the striatum but not for the frontal cortex. We have developed assay conditions for quantifying [3H]GBR 12935 binding in the frontal cortex. In both the rat and human frontal cortex, the assay required four times more tissue (8 mg/ml) than in the striatum (2 mg/ml). [3H]GBR 12935 binding in the frontal is complex, as it involves multiple binding sites. The high-affinity binding site is sodium dependent and is inhibited by sodium. In human but not in rat frontal cortex, addition of K+ reversed the sodium inhibition. The pharmacological profile of the high-affinity [3H]GBR 12935 binding site is consistent with that of the dopamine transporter, because drugs with the most selective dopamine reuptake blocking activities are the most potent displacers of [3H]GBR 12935 binding. There is a positive correlation between the rat and human inhibitory constants, a finding indicating that there are similar pharmacological profiles across at least these two species. Rats with a 6-hydroxydopamine lesion had a 47% decrease in number of [3H]GBR 12935 binding sites, a result indicating that at least a portion of these sites had been on presynaptic dopamine terminals.

Carrier-mediated efflux of [3H]dopamine and [3H]1-methyl-4-phenylpyridine: effect of ascorbic acid

The carrier-mediated efflux of [3H]1-methyl-4-phenylpyridine (MPP+) and [3H]dopamine was examined in mouse striatal synaptosomal P2 fractions. Although the two compounds are transported by the same carrier, the translocation of the carrier-ligand complex is more rapid with MPP+ than with dopamine. With dopamine-stimulated efflux of preloaded [3H]dopamine, externally present dopamine at a concentration of 1.3 microM reduced the intrasynaptosomal concentration of [3H]dopamine by 50% (the ECR value) with 8 min of incubation. The ECR value of dopamine in promoting the efflux of [3H]MPP+, however, was only 0.15 microM. Similarly, ascorbic acid was weaker in enhancing the efflux of [3H]dopamine (ECR greater than 2000 microM) than that of [3H]MPP+ (ECR = 567 microM). This effect of ascorbic acid on the efflux of [3H]MPP+ was attenuated by mazindol, a blocker of dopamine uptake. It is proposed that ascorbic acid has a neuromodulatory role involving changes at the level of carrier-membrane translocation and/or orientation.

Effect of dose on cocaine self-administration behavior and dopamine levels in the nucleus accumbens

The reinforcing properties of cocaine are thought to be primarily mediated by the release of dopamine (DA) in the nucleus accumbens (N ACC). The extracellular concentration of DA in the N ACC was monitored with in vivo microdialysis procedures during ongoing cocaine self-administration to achieve a more detailed understanding of how DA mediates the reinforcing effects of cocaine. A dose-dependent decrease in lever pressing behavior occurred as the dose of cocaine was increased. The mean number of lever presses (in 20 min intervals) for 0.25, 0.50 and 0.75 mg/infusion doses was 5.6 +/- 0.7, 3.3 +/- 0.3 and 2.4 +/- 0.3, respectively. However, a simple inverse relationship did not occur between lever pressing behavior and the total amount of cocaine injected. Lever pressing behavior significantly increased cocaine intake as the dose of cocaine was increased. The total amount of cocaine intake that occurred during the 3 h self-administration period of the 0.25, 0.50 and 0.75 mg/infusion doses was 12.0 +/- 1.8 mg, 14.6 +/- 0.37 mg and 16.6 +/- 1.2 mg. Correspondingly, the extracellular concentration of DA in the N ACC was increased and maintained at significantly higher levels as the dose of cocaine was increased. The average concentration of DA that occurred during the self-administration of 0.25, 0.50 and 0.75 mg/infusion doses of cocaine was 269 +/- 26%, 381 +/- 21% and 464 +/- 49% of the basal DA concentration. As dose is increased, a corresponding increase occurs in both cocaine intake and in the extracellular concentration of DA in the N ACC.(ABSTRACT TRUNCATED AT 250 WORDS)

Reserpine induced intraneuronal dopamine oxidation: reversal by MPP+ action

1-methyl-4-phenylpyridinium ion (MPP+) was tested for its effects upon dopamine level after incubating striatal synaptosomes in medium with and without reserpine. In the absence of reserpine, MPP+ enhanced the total incubation mixture dopamine level when tyrosine was present in the medium but that enhancing effect was considerably weaker when tyrosine was replaced by alpha methyl p-tyrosine. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) also had effects upon dopamine but likely due to MPP+, which was formed from MPTP by free mitochondrial MAO present in the tissue preparation. The incubation mixture dopamine level was drastically reduced by the addition of only reserpine and its presence in the medium markedly raised the ability of MPP+ to increase dopamine; the effects of MPTP in this medium were weaker than those of MPP+. Pargyline also raised dopamine levels under these conditions but only at concentrations much higher than those of MPP+. The particulate uptake of MPP+, at several medium concentrations, and the corresponding value of dopamine increase above the basal level were determined; the dopamine increase in p-moles was much greater than the p-moles of MPP+ uptake. These results indicate that, in the presence of reserpine, MPP+ has a potent action and that may lead to a release of intraneuronal free dopamine; this action is also likely to be independent of the countertransport from MPP+ uptake. The possibility of MPP+ being a potent inhibitor of intraneuronal MAO may have to be considered.

Cardiac sympathetic activity in myocardial ischemia: release and effects of noradrenaline

Sympathetic overactivity in myocardial ischemia is closely associated with the progression of myocyte injury and the incidence of malignant arrhythmias. Adrenergic stimulation of the ischemic myocardium is predominantly due to increased local noradrenaline concentrations in the heart, whereas plasma catecholamine levels are of minor relevance. During the first few minutes of ischemia, efferent sympathetic nerves are activated. Excessive accumulation of noradrenaline, however, is prevented since adenosine, formed in the ischemic myocardium, suppresses exocytotic noradrenaline release, and released noradrenaline is rapidly removed as long as catecholamine reuptake is functional. With progression of ischemia to more than 10 min, the myocardium is no longer protected against excess catecholamine accumulation in the interstitial space, since local metabolic release mechanisms become increasingly important. This release, which is independent of central sympathetic activity and from extracellular calcium, occurs in two steps: First, noradrenaline escapes from its intracellular storage vesicles and accumulates in the cytoplasm of the neuron. In a second, rate-limiting step, noradrenaline is transported across the plasma membrane into the interstitial space, using the neuronal uptake carrier in reverse of its normal transport direction. As a consequence of local metabolic catecholamine release, extracellular noradrenaline reaches 1000 times the normal plasma concentration within 20 min of ischemia. Studies using acute and chronic sympathetic denervation and antiadrenergic agents demonstrate that local metabolic, rather than centrally induced noradrenaline release is critically involved in the progression of ischemic cell damage within the occurrence of ventricular fibrillation in early ischemia. Myocardial ischemia results in a temporary supersensitivity of the myocytes to catecholamines. This is due to a twofold increase of alpha 1- and a 30% increase of beta-adrenergic receptor number at the cell surface. The sensitization of adenylate cyclase during the first 20 min of total ischemia is followed by a rapid inactivation of the enzyme. The beta-adrenergic hyperresponsiveness to catecholamines is therefore limited to the first few minutes of ischemia. The deleterious combination of extremely high noradrenaline concentrations with a temporarily enhanced responsiveness to catecholamines of the tissue is thought to accelerate the propagation of the wavefront of irreversible cell damage within the ischemic myocardium. Moreover, the inhomogenous distribution of catecholamine excess within the heart is considered to promote malignant arrhythmias by unmasking and enhancing electrophysiological disturbances in early ischemia.

Noradrenaline release from the rat heart during anoxia: effects of changes in extracellular sodium concentration and inhibition of sodium uptake mechanisms

1. Anoxic perfusion of the isolated rat heart releases noradrenaline in the absence of sympathetic nerve fibre stimulation. 2. Anoxic noradrenaline release is enhanced by reducing the extracellular Na+ concentration, consistent with the proposal that such release occurs by carrier-mediated efflux. 3. Release is also enhanced by lignocaine but inhibited by amiloride and ethylisopropylamiloride, suggesting that sodium entry into adrenergic nerve terminals during anoxia occurs by Na+/H+ (and possibly Na+/Ca2+) exchange.

Energy requirements for the basal efflux of noradrenaline and its metabolites from adrenergic varicosities

The combination of hypoxia plus glucose deprivation or of hypoxia plus lactate induces carrier-mediated outward transport of 3H-noradrenaline in the rat vas deferens. Lactate efflux is higher from atria than from vas deferens. Hence, the much lower contribution by outward transport to the spontaneous efflux of 3H-noradrenaline in vas deferens than atria is likely to be due to a better supply of oxygen (and perhaps also glucose) to the 3H-noradrenaline-storing varicosities in vas deferens than in atria.

Biphasic effect of tricyclic antidepressants on the release of norepinephrine from the adrenergic nerves of the rabbit heart

The release of norepinephrine (NE) from the right atrium of the rabbit heart was used as a model to investigate biphasic effects due to tricyclic antidepressants, similar to those clinically observed in the treatment of depression and known as "therapeutic window". Strips of the atrium were loaded with 3H-NE, and then superfused by Krebs solution. The basal release and the electrical stimulation evoked release of 3H-NE were measured in the presence and absence of four clinically used tricyclic antidepressants: imipramine, amitriptyline, desipramine and nortriptyline. In addition, guanethidine, an adrenergic neuron blocker, was also studied. At lower concentrations (0.5-10 microM) tricyclic antidepressants increased, whereas higher concentrations (50-100 microM), inhibited the evoked release of NE. This inhibition was not prevented by the alpha2 adrenoceptor antagonist yohimbine, excluding the possibility of alpha 2 adrenoceptor-mediated inhibition of NE release. In higher concentrations the tricyclic antidepressants increased the basal release of NE in a Ca-independent way. Secondary amine derivatives were more potent inhibitors of the evoked release, and enhance the resting basal release of NE to a greater extent than the tertiary ones. Similarly, guanethidine (1-50 microM) also decreased the evoked release and increased the basal release of NE in a concentration dependent manner. Yohimbine failed to counteract the inhibition caused by guanethidine and the increment of the basal release was Ca-independent. It is concluded that the effect of tricyclic antidepressants in potentiating the release of NE is masked by their adrenergic neuron blocking properties, i.e. they inhibit the release of NE.(ABSTRACT TRUNCATED AT 250 WORDS)

Depolarization promotes caffeine induced [3H]-noradrenaline release in calcium-free solution from peripheral sympathetic nerves

The transmitter releasing action of caffeine was studied in the absence of extracellular Ca2+ from the peripheral sympathetic nerves of the rabbit main pulmonary artery. Caffeine (10 mM) increased the release of [3H]-noradrenaline moderately, but not significantly in Ca2(+)-free (+1 mM EGTA) Krebs solution. When peripheral nerve endings/varicosities were depolarized by elevating extracellular K+ to 47.2 mM and 70.8 mM in Ca2(+)-free solution, the transmitter releasing effect of 10 mM caffeine became significant. Ca2+ removal itself transiently increased the [3H]-noradrenaline outflow. In the individual experiments the amount of the caffeine evoked transmitter release at 47.2 mM and 70.8 mM K(+)-depolarization was inversely correlated to the release evoked by Ca2(+)-removal. Our results suggest that caffeine-sensitive calcium stores are present in peripheral nerve terminals of rabbit pulmonary artery, and part of the caffeine sensitive calcium stores may discharge during Ca2(+)-removal from the extracellular solution.

Microdialysis in the posterior hypothalamus: sodium chloride affects norepinephrine release, mean arterial pressure, heart rate and behavior in awake rats

A microdialysis probe, implanted in the posterior hypothalamus (PH) was used to examine changes of extracellular norepinephrine (NE) in freely moving rats from which mean arterial pressure (mAP) and heart rate (HR) were continuously monitored. Artificial cerebrospinal fluid (aCSF) was pumped through the probe and 10-microliters dialysate samples were collected at 5-minute intervals and analyzed for NE by radioenzymatic assay. High sodium chloride (NaCl)-aCSF solution elicited pressor and tachycardiac responses and locomotor activity coupled with significant increases in levels of dialysate NE. The latter and the associated cardiovascular effects were significantly attenuated by perfusate lidocaine (0.5%). When alpha-adrenergic receptors in PH were blocked by phenoxybenzamine (0.165 M), high NaCl-aCSF released NE but the associated cardiovascular effects were attenuated. In addition, intravenous ganglionic blockade with hexamethonium (25 mg/kg) also attenuated cardiovascular responses during the high NaCl-aCSF perfusion of PH. These results indicate that PH is one of the important areas for central actions of NaCl and that the cardiovascular and locomotor responses produced by central NaCl, in part, depend on neuroadrenergic activity in PH.

In vivo mechanisms underlying dopamine release from rat nigrostriatal terminals: II. Studies using potassium and tyramine

The brain microdialysis technique has been used to examine the in vivo effects of potassium and tyramine on dopamine (DA) release and metabolism in the striatum of halothane-anaesthetised rats. Increasing the concentration of potassium perfusing the dialysis probe (30-120 mM) induced a dose-related efflux of DA. A dose-related release of DA was also observed following addition of tyramine (1-100 microM) to the perfusing buffer. High concentrations of potassium were found to reduce the dialysate content of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid and the serotonin metabolite 5-hydroxyindoleacetic acid. No such effect was observed even when using the highest concentration of tyramine tested. Potassium-evoked DA release was facilitated by pretreatment with the DA uptake inhibitor nomifensine, was inhibited by depletion of extracellular calcium, and was not significantly affected by tetrodotoxin (TTX). The effect of tyramine on DA efflux was inhibited by nomifensine and was insensitive to both TTX and calcium depletion. These data suggest that potassium and tyramine induce release of DA via different mechanisms. Potassium-induced DA release involves a carrier-independent process and may utilise an exocytotic release mechanism. On the other hand, tyramine-induced DA release would appear to involve a carrier-dependent process. Depletion of vesicular stores of DA by pretreatment with reserpine did not significantly affect potassium-induced DA release, whereas a marked inhibition of the effects of tyramine was noted. However, in reserpinised animals the potassium-induced release of DA was inhibited by nomifensine, a result suggesting that a carrier-dependent release mechanism operates in the absence of vesicular DA.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo mechanisms underlying dopamine release from rat nigrostriatal terminals: I. Studies using veratrine and ouabain

The in vivo mechanisms underlying the dopamine (DA)-releasing actions of veratrine and ouabain in the striatum of halothane-anaesthetised rats have been investigated using brain microdialysis. Relevant catecholamines and indoleamines were separated and quantified using HPLC combined with an electrochemical detection system. Veratrine (10 micrograms/ml-1 mg/ml) and ouabain (10 microM-1 mM) were added to the medium perfusing the dialysis probes. Both compounds increased dialysate DA content in a dose-related manner. Dialysate levels of the DA metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid and the serotonin metabolite 5-hydroxyindoleacetic acid were reduced by both veratrine and ouabain. Veratrine-induced DA efflux was maximal in the first 20-min sample collected after drug infusion began, whereas the maximal effect of ouabain was not observed until 20-40 min after administration began. Veratrine-induced DA efflux was unaffected by systemic injection of the DA uptake inhibitor nomifensine but was inhibited by either coperfusion of tetrodotoxin (TTX) or removal of calcium from the perfusing buffer. These data suggest that veratrine induces release of DA via a carrier-independent mechanism, perhaps involving an exocytotic release process. In contrast, ouabain-induced DA release was reduced by nomifensine but was inhibited to a lesser degree by calcium depletion and TTX. Detailed analyses of these data suggest that although ouabain initially induces release of DA via a carrier-dependent mechanism, an exocytotic process may also be involved. The finding that ouabain-induced DA efflux exhibits a degree of TTX and calcium sensitivity suggests that membrane depolarisation caused by Na+,K(+)-ATPase blockade opens voltage-gated sodium channels and initiates an exocytotic release of DA. The intracellular pools of DA involved in the release of DA induced by veratrine and ouabain were also examined. Depletion of vesicular pools of DA by pretreatment with reserpine reduced the amount of DA release induced by both agents, although this effect was only significant in the case of veratrine. However, in reserpinised animals the residual amount of DA release induced by veratrine was inhibited by nomifensine, a result suggesting that DA may be released via a carrier-dependent process in the absence of vesicular DA. Newly synthesised pools of DA were also depleted by pretreatment with the DA synthesis inhibitor alpha-methyl-p-tyrosine. Under these conditions, both veratrine- and ouabain-induced DA efflux was reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Apomorphine does not alter amphetamine-induced dopamine release measured in striatal dialysates

Amphetamine facilitates the release of dopamine from nerve terminals, but the mechanisms underlying this effect have not been fully delineated. The present experiments were designed to test the extent to which amphetamine-induced dopamine release is dependent on impulse flow and autoreceptor function in dopaminergic neurons. Rats were pretreated with a low dose of apomorphine (0.05 mg/kg) to inhibit dopamine neuronal activity, and the striatal dopaminergic response to amphetamine (0.5 mg/kg) was assessed by in vivo dialysis in freely moving animals. Consistent with previous results, apomorphine alone substantially decreased, whereas amphetamine increased, striatal dialysate dopamine concentrations. However, whereas apomorphine pretreatment decreased the locomotor response to amphetamine, the amphetamine-induced increase in dialysate dopamine was unaffected. These results indicate that amphetamine-facilitated dopamine release is independent of neuronal firing and autoreceptor regulation, consistent with the putative accelerative exchange-diffusion mechanism of amphetamine-induced dopamine release. Other possible mechanisms underlying the inhibitory effects of apomorphine on amphetamine locomotor activation are discussed.

Dopamine release and metabolism in the rat striatum: an analysis by 'in vivo' brain microdialysis

Brain microdialysis studies on the mechanisms underlying dopamine release in the rat striatum provide evidence that both exocytotic and carrier-dependent processes operate in vivo. While several releasers (potassium, veratrine, amphetamine, ouabain) utilize newly synthesized stores of dopamine, tyramine is uniquely sensitive to depletion of vesicular storage by reserpine. Extracellular DOPAC is closely associated with the newly synthesized pool of dopamine and experiments with selective monoamine oxidase inhibitors suggest that DOPAC is formed mainly by MAO-A. Recent work on the two dopamine receptors suggest that release by different mechanisms may selectively activate D1 or D2 receptor subtypes.

Extracellular sodium and chloride depletion enhances nonexocytotic noradrenaline release induced by energy deficiency in rat heart

The effect of either extracellular sodium or extracellular chloride reduction on the release of endogenous noradrenaline and its deaminated metabolite dihydroxyphenylglycol (DOPEG) has been studied in the isolated perfused rat heart under conditions of ischaemia and cyanide intoxication. The overflow of noradrenaline and DOPEG was determined by high pressure liquid chromatography. The efflux of DOPEG, the predominant neuronal noradrenaline adrenaline metabolite, served as indicator of the free axoplasmic plasmic amine concentration. A calcium-free perfusion buffer was used to avoid exocytotic noradrenaline release. Sodium and chloride in the perfusion buffer were replaced by lithium and isethionate, respectively. (1) Reduction of extracellular sodium or chloride increased noradrenaline overflow in ischaemia. The release was suppressed by the uptake1 blocker cocaine indicating carrier-mediated outward transport of noradrenaline. (2) In cyanide intoxication sodium or chloride reduction accelerated the onset of DOPEG efflux reflecting increased axoplasmic noradrenaline concentrations. This was accompanied by increased noradrenaline release. The ratio of noradrenaline/DOPEG overflow was increased by reduced sodium or chloride, indicating facilitation of carrier-mediated noradrenaline net outward transport. (3) In the presence of unaltered energy metabolism overflow of both, noradrenaline and DOPEG, was not enhanced by sodium or chloride reduction. The results demonstrate that reduction of extracellular sodium or chloride has two effects on noradrenaline release from the sympathetic neuron with reduced energy supply. First, reduced sodium or chloride induces increased axoplasmic noradrenaline concentrations by interference with vesicular storage function. Second, both interventions enhance carrier-mediated noradrenaline release.

Multiple effects of cocaine upon evoked secretion in bovine adrenal medullary chromaffin cells. Additional insight into the mechanism of action of cardiac glycosides

Experiments to determine the effects of the catecholamine neuronal uptake blockers cocaine and desipramine, and of the cardiac glycoside, ouabain, upon 3H-(noradrenaline) efflux have been performed with bovine adrenal medullary chromaffin cells in tissue culture. Both cocaine and desipramine reduced 3H-noradrenaline uptake into chromaffin cells. Inhibitable uptake was 80% of total accumulation over 60 min; this degree of inhibition was produced by cocaine (30 mumol/l) or desipramine (1 mumol/l). Cocaine (30 mumol/l) had no effect upon spontaneous 3H-efflux measured over 60 min, but reduced that evoked over the same period by carbachol (EC50), veratridine (EC50) and by ouabain (100 mumol/l). Cocaine did not reduce that efflux evoked by raised levels of K+ (28 mmol/l; EC50). Desipramine (1 mumol/l), like cocaine, had no effect upon spontaneous efflux of 3H, but reduced that efflux evoked by carbachol, veratridine and ouabain. Tetrodotoxin (TTX) inhibited veratridine-evoked 3H efflux (IC50 0.2 mumol/l). The degree of inhibition caused by TTX (0.2 mumol/l) was not increased by cocaine (30 mumol/l). TTX also inhibited ouabain-evoked 3H efflux: this was reduced by 55% by a concentration of TTX (1 mumol/l) sufficient to virtually abolish veratridine-evoked efflux. Cocaine (30 mumol/l) in the presence of TTX (1 mumol/l) did not further inhibit ouabain-evoked efflux. Cocaine (30 mumol/l) did not alter 86Rb+ uptake into chromaffin cells, nor did it alter that inhibition of 86Rb+ uptake produced by ouabain (100 mumol/l) indicating that cocaine has no effect upon Na,K-ATPase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The K+-induced increases in noradrenaline and dopamine release are accompanied by reductions in the release of their intraneuronal metabolites from the rat anterior hypothalamus. An in vivo brain microdialysis study

The novel technique of microdialysis has been used to examine the basal and K+-induced release of catecholamines and metabolites from the anterior hypothalamus of the urethane-anaesthetized rat in vivo. A high pressure liquid chromatographic assay was developed to simultaneously measure endogenous noradrenaline, dopamine and their intraneuronal metabolites 3,4-dihydroxyphenylglycol (DOPEG) and 3,4-dihydroxyphenylacetic acid (DOPAC) respectively, in each 60 microliters dialysate sample. The effect of replacing Ca2+ in the perfusion fluid with a low concentration of Cd2+, which blocks Ca2+ effects, was also studied. Increasing the K+ concentration in the perfusion fluid elicited a concentration-dependent increase in noradrenaline and dopamine release. In contrast, there were marked reductions in DOPEG and DOPAC which were not concentration-dependent. In the Ca2+-depleted conditions, the K+-induced increase in amine release was significantly attenuated, but the reductions in the metabolites were not affected. We suggest that the mechanisms contributing to the observed reductions in the metabolites may be inhibition of neuronal reuptake, an increase in neuronal efflux, an enhancement of vesicular uptake and a decrease in vesicular efflux.

Influence of a carrier transport process on in vivo release and metabolism of dopamine: dependence on extracellular Na+

In vivo microdialysis was utilized to evaluate the role of extracellular Na+ in regulating dopamine (DA) neurotransmission in the caudate-putamen of halothane-anaesthetized rats. Reduction of the extracellular Na+ concentration by introduction of a perfusion media containing 50mM Na+ (with choline replacement) produced an excessive release of DA that could be effectively blocked by nomifensine and Lu 19-005, potent inhibitors of an amine transport carrier. These results substantiate reports of a carrier-mediated efflux of DA from presynaptic terminals. Pretreatment with amphetamine, considered both a DA uptake inhibitor and releaser, did not, however, influence the efflux of DA induced by the low extracellular Na+ environment. Although the release of DA from an apparent non-granular cytosolic pool was greatly enhanced by the low extracellular Na+ environment, 3,4-dihydrophenylacetic acid (DOPAC) levels, which supposedly reflect metabolism of non-vesicular DA, were minimally effected. In contrast, homovanillic acid (HVA) was sensitive to extracellular Na+ and not directly related to extracellular levels of either DA or DOPAC, suggesting the possibility of a Na+-sensitive (carrier-mediated?) process involved in the formation of HVA. Overall, the results of this paper cannot be completely reconciled with the traditional concept of intracellular organization of DA pools and suggests the possibility of various non-granular pools being differentially sensitive to efflux and metabolism.