High-affinity uptake of noradrenaline in postsynaptic neurones

The cytotoxicity of the α1-adrenoceptor antagonist prazosin is linked to an endocytotic mechanism equivalent to transport-P

Since the α1-adrenergic antagonist prazosin (PRZ) was introduced into medicine as a treatment for hypertension and benign prostate hyperplasia, several studies have shown that PRZ induces apoptosis in various cell types and interferes with endocytotic trafficking. Because PRZ is also able to induce apoptosis in malignant cells, its cytotoxicity is a focus of interest in cancer research. Besides inducing apoptosis, PRZ was shown to serve as a substrate for an amine uptake mechanism originally discovered in neurones called transport-P. In line with our hypothesis that transport-P is an endocytotic mechanism also present in non-neuronal tissue and linked to the cytotoxicity of PRZ, we tested the uptake of QAPB, a fluorescent derivative of PRZ, in cancer cell lines in the presence of inhibitors of transport-P and endocytosis. Early endosomes and lysosomes were visualised by expression of RAB5-RFP and LAMP1-RFP, respectively; growth and viability of cells in the presence of PRZ and uptake inhibitors were also tested. Cancer cells showed co-localisation of QAPB with RAB5 and LAMP1 positive vesicles as well as tubulation of lysosomes. The uptake of QAPB was sensitive to transport-P inhibitors bafilomycin A1 (inhibits v-ATPase) and the antidepressant desipramine. Endocytosis inhibitors pitstop(®) 2 (general inhibitor of endocytosis), dynasore (dynamin inhibitor) and methyl-β-cyclodextrin (cholesterol chelator) inhibited the uptake of QAPB. Bafilomycin A1 and methyl-β-cyclodextrin but not desipramine were able to preserve growth and viability of cells in the presence of PRZ. In summary, we confirmed the hypothesis that the cellular uptake of QAPB/PRZ represents an endocytotic mechanism equivalent to transport-P. Endocytosis of QAPB/PRZ depends on a proton gradient, dynamin and cholesterol, and results in reorganisation of the LAMP1 positive endolysosomal system. Finally, the link seen between the cellular uptake of PRZ and cell death implies a still unknown pro-apoptotic membrane protein with affinity towards PRZ.

Radionuclide imaging of neurohormonal system of the heart

Heart failure is one of the growing causes of death especially in developed countries due to longer life expectancy. Although many pharmacological and instrumental therapeutic approaches have been introduced for prevention and treatment of heart failure, there are still limitations and challenges. Nuclear cardiology has experienced rapid growth in the last few decades, in particular the application of single photon emission computed tomography (SPECT) and positron emission tomography (PET), which allow non-invasive functional assessment of cardiac condition including neurohormonal systems involved in heart failure; its application has dramatically improved the capacity for fundamental research and clinical diagnosis. In this article, we review the current status of applying radionuclide technology in non-invasive imaging of neurohormonal system in the heart, especially focusing on the tracers that are currently available. A short discussion about disadvantages and perspectives is also included.

Molecular features of the prazosin molecule required for activation of Transport-P

Closely related structural analogues of prazosin have been synthesised and tested for inhibition and activation of Transport-P in order to identify the structural features of the prazosin molecule that appear to be necessary for activation of Transport-P. So far, all the compounds tested are less active than prazosin. It is shown that the structure of prazosin appears to be very specific for the activation. Only quinazolines have been found to activate, and the presence of the 6,7-dimethoxy and 4-amino groups appears to be critically important.

Search for the pharmacophore in prazosin for Transport-P

Partial structures of prazosin have been synthesised and tested for inhibition of Transport-P in order to identify the structural features of prazosin, which appear to be involved in binding to the putative transporter. It is shown that the pyrimidinyl 4-amino group is critically important for binding but that the 6,7-dimethoxy and 2-furoyl groups are not essential.

Pharmacological comparison between the actions of methamphetamine and 1-aminoindan stereoisomers on sympathetic nervous function in rat vas deferens

The selective monoamine oxidase-B inhibitor selegiline (deprenyl) causes sympathomimetic effects and is metabolised to R(-)-methamphetamine and R(-)-amphetamine. The new monoamine oxidase-B inhibitor rasagiline is devoid of sympathomimetic effects and is metabolised to R(+)-1-aminoindan. Sympathomimetic effects of methamphetamine and 1-aminoindan enantiomers were compared in the rat vas deferens. R(-)-methamphetamine and S(+)-methamphetamine caused initial potentiation and subsequent inhibition of the field stimulation-induced twitch response of isolated rat vas deferens (0.1 Hz). EC(50) values for inhibition of twitch in prazosin-treated vas deferens were 0.36+/-0.13 and 1.64+/-0.10 microM (mean+/-S.E.M.) for S(+)- and R(-)-methamphetamine, respectively. There was no difference between S(+)-methamphetamine and R(-)-methamphetamine in potentiation of postsynaptic contractile response to noradrenaline. R(+)- and S(-)-1-aminoindan increased twitch response only at concentrations above 30 microM. R(-)-methamphetamine has similar potency to S(+)-methamphetamine in potentiation of noradrenaline-mediated responses and can therefore play a role in the sympathomimetic effects of selegiline.

Atrial supersensitivity to noradrenaline in stressed female rats

Stress can change the responses to catecholamines in many tissues. The aim of this study was to investigate the influence of the estrous cycle on the sensitivity of right atria to noradrenaline in female rats subjected to acute swimming stress. Female Wistar rats in proestrus, estrus, metestrus or diestrus were submitted to a 50 min-swimming session. Immediately after the exercise, the rats were killed and their right atria were mounted for isometric recording of the spontaneous beating rate. Concentration-effect curves to noradrenaline were obtained before and after the inhibition of neuronal uptake with phenoxybenzamine (10 microM) and of extraneuronal uptake with estradiol (5 microM). Acute swimming stress did not change the right atrial sensitivity to noradrenaline in rats in estrus, metestrus and diestrus. However, swimming stress produced supersensitivity to noradrenaline in proestrus (pD(2) control: 7.14 +/- 0.03 vs. pD(2) swimming: 7.55 +/- 0.04; p<0.05). This supersensitivity was still observed after uptake inhibition. When catecholamine uptake was inhibited, the concentration-effect curve to noradrenaline was shifted to the left 2.5-fold in the proestrus control group and 1.7-fold in the proestrus stress group (p<0.05). In conclusion, the estrous cycle influenced the acute stress-induced atrial supersensitivity to noradrenaline.

Modeling of membrane excitability in gonadotropin-releasing hormone-secreting hypothalamic neurons regulated by Ca2+-mobilizing and adenylyl cyclase-coupled receptors

Gonadotropin-releasing hormone (GnRH) secretion from native and immortalized hypothalamic neurons is regulated by endogenous Ca(2+)-mobilizing and adenylyl cyclase (AC)-coupled receptors. Activation of both receptor types leads to an increase in action potential firing frequency and a rise in the intracellular Ca(2+) concentration ([Ca(2+)](i)) and neuropeptide secretion. The stimulatory action of Ca(2+)-mobilizing agonists on voltage-gated Ca(2+) influx is determined by depletion of the intracellular Ca(2+) pool, whereas AC agonist-stimulated Ca(2+) influx occurs independently of stored Ca(2+) and is controlled by cAMP, possibly through cyclic nucleotide-gated channels. Here, experimental records from immortalized GnRH-secreting neurons are simulated with a mathematical model to determine the requirements for generating complex membrane potential (V(m)) and [Ca(2+)](i) responses to Ca(2+)-mobilizing and AC agonists. Included in the model are three pacemaker currents: a store-operated Ca(2+) current (I(SOC)), an SK-type Ca(2+)-activated K(+) current (I(SK)), and an inward current that is modulated by cAMP and [Ca(2+)](i) (I(d)). Spontaneous electrical activity and Ca(2+) signaling in the model are predominantly controlled by I(d), which is activated by cAMP and inhibited by high [Ca(2+)](i). Depletion of the intracellular Ca(2+) pool mimics the receptor-induced activation of I(SOC) and I(SK), leading to an increase in the firing frequency and Ca(2+) influx after a transient cessation of electrical activity. However, increasing the activity of I(d) simulates the experimental response to forskolin-induced activation of AC. Analysis of the behaviors of I(SOC), I(d), and I(SK) in the model reveals the complexity in the interplay of these currents that is necessary to fully account for the experimental results.

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.

alpha(1B) adrenergic receptors in gonadotrophin-releasing hormone neurones: relation to Transport-P

1. Peptidergic neurones accumulate amines via an unusual uptake process, designated Transport-P. [(3)H]-prazosin binds to alpha(1) adrenoceptors on these cells and is displaceable by unlabelled prazosin in concentrations up to 10(-7) M. However, at greater concentrations of prazosin, there is a paradoxical accumulation of [(3)H]-prazosin which we have attributed to Transport-P. Uptake of prazosin via Transport-P is detectable at 10(-10) M prazosin concentration, is linear up to 10(-7) M and at greater concentrations becomes non-linear. In contrast, in noradrenergic neurones, noradrenaline uptake is linear and saturates above 10(-7) M. In noradrenergic neurones and in non-neuronal cells, there is no uptake of prazosin in concentrations up to 10(-6) M, suggesting that Transport-P is a specialised function of peptidergic neurones. 2. Using a mouse peptidergic (gonadotrophin-releasing hormone, GnRH) neuronal cell line which possesses Transport-P, we have studied the interaction of alpha(1) adrenoceptors with Transport-P. Polymerase chain reactions and DNA sequencing of the products demonstrated that only the alpha(1B) sub-type of adrenoceptors is present in GnRH cells. 3. In COS cells transfected with alpha(1b) adrenoceptor cDNA and in DDT(1) MF-2 cells which express native alpha(1B) adrenoceptors, [(3)H]-prazosin was displaced by unlabelled prazosin in a normal equilibrium process, with no prazosin paradox in concentrations up to 10(-6) M. In DDT(1) MF-2 cells, [(3)H]-prazosin was displaced likewise by a series of alpha(1) adrenergic agonists, none of which increased the binding of [(3)H]-prazosin. Hence, the prazosin paradox is not due to some function of alpha(1) adrenoceptors, such as internalization of ligand-receptor complexes. 4. In neurones which possess Transport-P, transfection with alpha(1b) adrenoceptor cDNA resulted in over-expression of alpha(1B) adrenoceptors, but the prazosin paradox was unaltered. Thus, alpha(1) adrenoceptors and Transport-P mediate distinct functions in peptidergic neurones.

Modeling of membrane excitability in gonadotropin-releasing hormone-secreting hypothalamic neurons regulated by Ca2+-mobilizing and adenylyl cyclase-coupled receptors

Gonadotropin-releasing hormone (GnRH) secretion from native and immortalized hypothalamic neurons is regulated by endogenous Ca(2+)-mobilizing and adenylyl cyclase (AC)-coupled receptors. Activation of both receptor types leads to an increase in action potential firing frequency and a rise in the intracellular Ca(2+) concentration ([Ca(2+)](i)) and neuropeptide secretion. The stimulatory action of Ca(2+)-mobilizing agonists on voltage-gated Ca(2+) influx is determined by depletion of the intracellular Ca(2+) pool, whereas AC agonist-stimulated Ca(2+) influx occurs independently of stored Ca(2+) and is controlled by cAMP, possibly through cyclic nucleotide-gated channels. Here, experimental records from immortalized GnRH-secreting neurons are simulated with a mathematical model to determine the requirements for generating complex membrane potential (V(m)) and [Ca(2+)](i) responses to Ca(2+)-mobilizing and AC agonists. Included in the model are three pacemaker currents: a store-operated Ca(2+) current (I(SOC)), an SK-type Ca(2+)-activated K(+) current (I(SK)), and an inward current that is modulated by cAMP and [Ca(2+)](i) (I(d)). Spontaneous electrical activity and Ca(2+) signaling in the model are predominantly controlled by I(d), which is activated by cAMP and inhibited by high [Ca(2+)](i). Depletion of the intracellular Ca(2+) pool mimics the receptor-induced activation of I(SOC) and I(SK), leading to an increase in the firing frequency and Ca(2+) influx after a transient cessation of electrical activity. However, increasing the activity of I(d) simulates the experimental response to forskolin-induced activation of AC. Analysis of the behaviors of I(SOC), I(d), and I(SK) in the model reveals the complexity in the interplay of these currents that is necessary to fully account for the experimental results.

Structural properties of phenylethylamine derivatives which inhibit transport-P in peptidergic neurones

1. Transport-P is an antidepressant-sensitive, proton-dependent, V-ATPase-linked uptake process for amines in peptidergic neurones of the hypothalamus. It is unusual in its anatomical location in postsynaptic neurones and in that it is activated by its substrate (prazosin). This study examined the structural properties of phenylethylamine derivatives which are substrates for transport-P, as judged by competitive inhibition of the uptake of prazosin 10(-6) M in immortalized hypothalamic peptidergic neurones. 2. A basic amine was essential for activity; absence of the amine or neutralization with a carboxyl group abolished activity. Primary, secondary and tertiary amines were active but quaternary and guanyl amines were inactive. 3. A phenyl group was essential for activity at transport-P. Potency at transport-P was reduced by phenolic hydroxyl groups and enhanced by phenolic halogens. Thus, for maximal potency, the phenyl group should be hydrophobic. Phenolic methoxyl groups had no effect on potency at transport-P. 4. A side chain was necessary for activity at transport-P. Potency at transport-P was reduced by beta-hydroxyl and enhanced by alpha-methyl groups. 5. These findings further distinguish transport-P from other amine uptake processes in the brain.

Visual detection of transport-P in peptidergic neurones

1. Hypothalamic peptidergic neurones possess an uptake process for amines (transport-P), for which prazosin is a substrate. It is characterized by a paradoxical increase in the accumulation of [3H]-prazosin when the concentration of unlabelled prazosin is increased above 10(-7) M. This increase is due to activation of a proton-dependent, vacuolar-type ATPase-linked pump that is blocked by tricyclic antidepressants. This study utilized a fluorescence method to detect amine uptake in individual cells. 2. Prazosin is fluorescent but most of its emission spectrum is in the ultraviolet range. We therefore used an analogue of prazosin in which the furan ring had been substituted with a fluorescent group, BODIPY FL. This compound's emission maximum is in the green part of the visible spectrum. 3. BODIPY FL prazosin accumulated in immortalised peptidergic neurones and the characteristic emission spectrum of the compound was evident in these cells. Accumulation of BODIPY FL prazosin was saturable and was inhibited by the tricyclic antidepressant desipramine and by unlabelled prazosin. As previously described for prazosin, uptake of BODIPY FL prazosin was blocked by cold temperature and by the organic base chloroquine. Thus, prazosin and BODIPY FL prazosin were accumulated by the same uptake process. 4. BODIPY FL prazosin accumulated in a granular distribution, which is compatible with storage in intracellular vesicles. 5. Hypothalamic cells from foetal rats in primary culture also accumulated BODIPY FL prazosin by a desipramine-sensitive process. Uptake was predominantly in neurones and glial cells did not accumulate the amine. 6. Fluorescent detection provides visual evidence for amine uptake in peptidergic neurones and should enable detailed study of the distribution of this process in the brain.

Binding and competitive inhibition of amine uptake at postsynaptic neurones (transport-P) by tricyclic antidepressants

1. We have provided evidence for a novel amine uptake process for which prazosin is a substrate in postsynaptic neurones, characterized by a paradoxical increase in accumulation of the radioligand when the concentration of the unlabelled drug is increased above 10(-7) M. This increase is due to activation of a proton-dependent, vacuolar type-ATPase-linked uptake process which is blocked by desipramine but is resistant to reserpine. We have now examined the effects of tricyclic antidepressants on this uptake system in a cell line derived from hypothalamic peptidergic neurones, known to be innervated by noradrenergic nerve terminals in vivo. 2. [3H]-imipramine bound to the cells and was displaced by unlabelled imipramine, desipramine, amitriptyline and nortriptyline. The data fitted a single binding site model. This is the first demonstration of antidepressant binding sites in postsynaptic neurones. 3. There was no increase in the binding of [3H]-imipramine at high concentrations of unlabelled imipramine, suggesting that antidepressants inhibit uptake but are not themselves accumulated by peptidergic gonadotrophin releasing hormone neurones. 4. Accumulation of prazosin was competitively inhibited by antidepressants. Tertiary amines were slightly more potent than secondary amines and the presence of a nitrogen atom in the heterocyclic ring enhanced blocking activity. 5. The affinities of the antidepressants for the uptake process are within the range of plasma concentrations that are observed during therapeutic use of these compounds. Since it is likely that this uptake process has a physiological function, its inhibition by antidepressants may provide a new avenue for investigating the mechanism of action of these compounds.

Functional properties of the uptake of amines in immortalised peptidergic neurones (transport-P)

1. Most neurotransmitters are inactivated by uptake into presynaptic nerve terminals and into glial cells. We recently provided evidence for uptake of amines in postsynaptic neurones. Uptake was evident at nanomolar concentrations of prazosin, but at concentrations of unlabelled prazosin greater than 10(-7) M, there was a further activation of uptake, manifested by a paradoxical increase in accumulation of the radioligand. We have now studied further characteristics of amine uptake in immortalised gonadotrophin-releasing hormone (GnRH) neurones. Control cells included SK-N-SH neuroblastoma cells (which possess presynaptic type amine transporters) and non-neuronal (COS-7) cells. 2. [3H]-prazosin bound to intact GnRH cells and was displaced by unlabelled prazosin in concentrations of 10(-9) to 10(-7) M. However, at higher concentrations of unlabelled prazosin, there was an increase in apparent [3H]-prazosin binding, as we had previously described. This paradoxical increase in accumulation of the radioligand was abolished by desipramine. 3. Desipramine had no effect on the association of prazosin with COS-7 cells. There was no paradoxical increase in accumulation of [3H]-prazosin in COS-7 cells, indicating that this effect requires the presence of a desipramine-blockable uptake process. 4. The increase in binding of the radioligand that was observed in the GnRH cells is not a general property of neuronal transporters; in SK-N-SH cells, there was no increase in accumulation of (-)-[3H]-noradrenaline in the presence of concentrations of unlabelled (-)-noradrenaline greater than 10(-7) M. 5. The uptake of prazosin and the increase in accumulation of [3H]-prazosin were abolished in the cold, indicating that this is an active, energy-requiring process. 6. Desipramine-sensitive uptake of prazosin was demonstrable in the GnRH cells in the absence of sodium. Further, the Na+/K(+)-ATPase inhibitor, vanadate, abolished noradrenaline uptake in SK-N-SH cells but had no effect on prazosin uptake in GnRH cells. Thus, the uptake of prazosin does not derive its energy from the sodium pump. 7. Prazosin uptake was inhibited by the V-ATPase inhibitor bafilomycin A1, the H+/Na+ ionophore, monensin and the organic base, chloroquine, indicating that uptake derives its energy from a proton pump. In contrast to other proton-dependent amine transporters, the uptake of prazosin was unaffected by reserpine. 8. Increasing extracellular pH did not increase the uptake of prazosin into GnRH cells, indicating that it is unlikely to be due to non-specific diffusion and concentration of a lysosomotropic drug into intracellular acidic particles. 9. The uptake of prazosin was unaffected by steroid hormones. 10. In COS-7 cells transfected with alpha 1-adrenoceptor cDNA, [3H]-prazosin was displaced by unlabelled prazosin without causing an increase in binding of the radioligand. This indicated that the increase in accumulation of the radioligand is unlikely to be due simply to some function of alpha 1-adrenoceptors. 11. Thus, peptidergic neurones possess an uptake process with properties that are distinguishable from known amine transporters.

Glial alpha 2-receptors probably inhibit the high-affinity uptake of noradrenaline into astrocytes in the rat brain in vivo

The effect of alpha 2-receptor blockage on the extraneuronal turnover of noradrenaline (NA) has been studied in the intact rat brain. Tropolone and yohimbine, along with reserpine or desmethylimipramine, were given 30 min after intracerebroventricular injection of [7-3H]NA, i.e. after the tracer had been stored or inactivated. Tropolone given alone did not change the fractions of 3H-activity recovered as [3H]NA from hypothalamus, septum, striatum and pons-medulla, but in the presence of yohimbine improved the [3H]NA recovery in all areas except pons-medulla. The maximum effect was seen in the hypothalamus of reserpine-treated rats. Since the alpha 2-autoreceptors were blocked, the increased [3H]NA recovery does not reflect a down-regulated neuronal NA turnover. Instead it seems to show that a fraction greater than normal of neuronally released NA had been taken up into astrocytes and remained unmetabolized if catechol-O-methyltransferase was inactive. It is assumed that yohimbine enabled the protective tropolone effect by blocking astrocytic alpha 2-receptors that otherwise, either by itself or by antagonizing beta-receptor-induced hyperpolarization or cAMP formation, had impaired parameters that stimulate the high-affinity NA Uptake 1 of astrocytes (e.g. membrane potential, Na+,K(+)-ATPase) or control the gap junction permeability in the glial syncytium.

Gonadotropin-releasing hormone neurons: intrinsic pulsatility and receptor-mediated regulation

The pulsatile pattern of gonadotropin-releasing hormone (GnRH) release from the hypothalamus is driven by a functionally interconnected and synchronized network of GnRH neurons termed the GnRH pulse generator. Several recent observations have revealed that immortalized GnRH neurons can generate an episodic pattern of GnRH release when cultured in the absence of other cell types. The in vitro operation of the pulse generator depends on the development of synaptic contacts among GnRH neurons, the electrical properties of individual GnRH neurons, and the GnRH-induced modulation of its secretory mechanism. The expression o f several other receptors by GnRH neurons provides the means for integrated regulation of pulse generator activity from without the network by agonists including glutamate, GABA, endothelin, and catecholamines.