Rat brain synaptic vesicles: uptake specificities of [3H]norepinephrine and [3H]serotonin in preparations from whole brain and brain regions

Autoreceptor control of serotonin dynamics

BACKGROUND

Serotonin is a neurotransmitter that has been linked to a wide variety of behaviors including feeding and body-weight regulation, social hierarchies, aggression and suicidality, obsessive compulsive disorder, alcoholism, anxiety, and affective disorders. Full understanding involves genomics, neurochemistry, electrophysiology, and behavior. The scientific issues are daunting but important for human health because of the use of selective serotonin reuptake inhibitors and other pharmacological agents to treat disorders. This paper presents a new deterministic model of serotonin metabolism and a new systems population model that takes into account the large variation in enzyme and transporter expression levels, tryptophan input, and autoreceptor function.

RESULTS

We discuss the steady state of the model and the steady state distribution of extracellular serotonin under different hypotheses on the autoreceptors and we show the effect of tryptophan input on the steady state and the effect of meals. We use the deterministic model to interpret experimental data on the responses in the hippocampus of male and female mice, and to illustrate the short-time dynamics of the autoreceptors. We show there are likely two reuptake mechanisms for serotonin and that the autoreceptors have long-lasting influence and compare our results to measurements of serotonin dynamics in the substantia nigra pars reticulata. We also show how histamine affects serotonin dynamics. We examine experimental data that show very variable response curves in populations of mice and ask how much variation in parameters in the model is necessary to produce the observed variation in the data. Finally, we show how the systems population model can potentially be used to investigate specific biological and clinical questions.

CONCLUSIONS

We have shown that our new models can be used to investigate the effects of tryptophan input and meals and the behavior of experimental response curves in different brain nuclei. The systems population model incorporates individual variation and can be used to investigate clinical questions and the variation in drug efficacy. The codes for both the deterministic model and the systems population model are available from the authors and can be used by other researchers to investigate the serotonergic system.

Mathematical insights into the effects of levodopa

Parkinson’s disease has been traditionally thought of as a dopaminergic disease in which cells of the substantia nigra pars compacta (SNc) die. However, accumulating evidence implies an important role for the serotonergic system in Parkinson’s disease in general and in physiological responses to levodopa therapy, the first line of treatment. We use a mathematical model to investigate the consequences of levodopa therapy on the serotonergic system and on the pulsatile release of dopamine (DA) from dopaminergic and serotonergic terminals in the striatum. Levodopa competes with tyrosine and tryptophan at the blood-brain barrier and is taken up by serotonin neurons in which it competes for aromatic amino acid decarboxylase. The DA produced competes with serotonin (5HT) for packaging into vesicles. We predict the time courses of LD, cytosolic DA, and vesicular DA in 5HT neurons during an LD dose. We predict the time courses of DA and 5HT release from 5HT cell bodies and 5HT terminals as well as the changes in 5HT firing rate due to lower 5HT release. We compute the time course of DA release in the striatum from both 5HT and DA neurons and show how the time course changes as more and more SNc cells die. This enables us to explain the shortening of the therapeutic time window for the efficacy of levodopa as Parkinson’s disease progresses. Finally, we study the effects 5HT1a and 5HT1b autoreceptor agonists and explain why they have a synergistic effect and why they lengthen the therapeutic time window for LD therapy. Our results are consistent with and help explain results in the experimental literature and provide new predictions that can be tested experimentally.

Vesicular monoamine transporters: structure-function, pharmacology, and medicinal chemistry

Vesicular monoamine transporters (VMAT) are responsible for the uptake of cytosolic monoamines into synaptic vesicles in monoaminergic neurons. Two closely related VMATs with distinct pharmacological properties and tissue distributions have been characterized. VMAT1 is preferentially expressed in neuroendocrine cells and VMAT2 is primarily expressed in the CNS. The neurotoxicity and addictive properties of various psychostimulants have been attributed, at least partly, to their interference with VMAT2 functions. The quantitative assessment of the VMAT2 density by PET scanning has been clinically useful for early diagnosis and monitoring of the progression of Parkinson's and Alzheimer's diseases and drug addiction. The classical VMAT2 inhibitor, tetrabenazine, has long been used for the treatment of chorea associated with Huntington's disease in the United Kingdom, Canada, and Australia, and recently approved in the United States. The VMAT2 imaging may also be useful for exploiting the onset of diabetes mellitus, as VMAT2 is also expressed in the β-cells of the pancreas. VMAT1 gene SLC18A1 is a locus with strong evidence of linkage with schizophrenia and, thus, the polymorphic forms of the VMAT1 gene may confer susceptibility to schizophrenia. This review summarizes the current understanding of the structure-function relationships of VMAT2, and the role of VMAT2 on addiction and psychostimulant-induced neurotoxicity, and the therapeutic and diagnostic applications of specific VMAT2 ligands. The evidence for the linkage of VMAT1 gene with schizophrenia and bipolar disorder I is also discussed.

Serotonin synthesis, release and reuptake in terminals: a mathematical model

BACKGROUND

Serotonin is a neurotransmitter that has been linked to a wide variety of behaviors including feeding and body-weight regulation, social hierarchies, aggression and suicidality, obsessive compulsive disorder, alcoholism, anxiety, and affective disorders. Full understanding of serotonergic systems in the central nervous system involves genomics, neurochemistry, electrophysiology, and behavior. Though associations have been found between functions at these different levels, in most cases the causal mechanisms are unknown. The scientific issues are daunting but important for human health because of the use of selective serotonin reuptake inhibitors and other pharmacological agents to treat disorders in the serotonergic signaling system.

METHODS

We construct a mathematical model of serotonin synthesis, release, and reuptake in a single serotonergic neuron terminal. The model includes the effects of autoreceptors, the transport of tryptophan into the terminal, and the metabolism of serotonin, as well as the dependence of release on the firing rate. The model is based on real physiology determined experimentally and is compared to experimental data.

RESULTS

We compare the variations in serotonin and dopamine synthesis due to meals and find that dopamine synthesis is insensitive to the availability of tyrosine but serotonin synthesis is sensitive to the availability of tryptophan. We conduct in silico experiments on the clearance of extracellular serotonin, normally and in the presence of fluoxetine, and compare to experimental data. We study the effects of various polymorphisms in the genes for the serotonin transporter and for tryptophan hydroxylase on synthesis, release, and reuptake. We find that, because of the homeostatic feedback mechanisms of the autoreceptors, the polymorphisms have smaller effects than one expects. We compute the expected steady concentrations of serotonin transporter knockout mice and compare to experimental data. Finally, we study how the properties of the the serotonin transporter and the autoreceptors give rise to the time courses of extracellular serotonin in various projection regions after a dose of fluoxetine.

CONCLUSIONS

Serotonergic systems must respond robustly to important biological signals, while at the same time maintaining homeostasis in the face of normal biological fluctuations in inputs, expression levels, and firing rates. This is accomplished through the cooperative effect of many different homeostatic mechanisms including special properties of the serotonin transporters and the serotonin autoreceptors. Many difficult questions remain in order to fully understand how serotonin biochemistry affects serotonin electrophysiology and vice versa, and how both are changed in the presence of selective serotonin reuptake inhibitors. Mathematical models are useful tools for investigating some of these questions.

Influence of age and dietary restriction on norepinephrine uptake into cardiac synaptosomes

The purpose of this study was to determine whether aging alters neuronal uptake of norepinephrine (NE) in the rat heart and if dietary restriction influenced the effect of age on this system. Cardiac synaptosomes were prepared from 6-, 12- and 24-month-old male F344 rats fed ad libitum (AL) or a diet restricted (DR) to 60% of AL intake. Cardiac synaptosomes were incubated with 50, 100, 200, or 400 nM [3H]NE for 10 min at 37 degrees C with and without desmethylimipramine (DMI), a selective neuronal-uptake blocker. DMI-sensitive [3H]NE uptake was calculated as the difference between samples with and without DMI. NE uptake was adjusted for the number of cardiac synaptosomes in each sample by dividing by the endogenous NE content in each sample. The Vmax for uptake ([3H]NE/min/ng NE) declined significantly between 6 and 12 months in AL rats and between 12 and 24 months in DR rats. Km was not significantly different between age or diet groups. The change in Vmax with age suggests that the number of NE transporters per synaptosome may decline with age and that DR delays this effect of age. There were no differences in the sensitivity to DMI between age or diet groups.

Uptake of glycine, GABA and glutamate by synaptic vesicles isolated from different regions of rat CNS

Synaptic vesicle fractions have been isolated from cerebral cortex, subcortical telencephalon, whole brain and spinal cord by density gradient centrifugation. The Mg2+ ATP-dependent vesicular uptake and the Na(+)-dependent synaptosomal uptake of glycine, GABA and L-glutamate has been compared in the different regions. All these regions contain GABA as inhibitory neurotransmitter, whereas glycine only plays a dominant role as such in the spinal cord. The ratio between GABA and glycine uptake in the different vesicle fractions was similar, and the ratios differed greatly from the ratios in the synaptosomal uptake. In contrast, the ratio between glutamate and GABA uptake in vesicles from different regions differed, and these ratios corresponded to the ratios in the synaptosomal uptake. These results indicate that glycine is taken up into synaptic vesicles from non-glycine terminals, and we suggest that GABA and glycine can be taken up into the same vesicle population.

Inhibition of gamma-aminobutyrate and glycine uptake into synaptic vesicles

The substrate specificity of vesicular GABA and glycine uptake was studied in vesicle fractions from brain and spinal cord, respectively. Glycine, beta-alanine and gamma-vinyl-GABA were competitive inhibitors of the GABA uptake were competitive inhibitors of the GABA uptake by synaptic vesicles in brain. Likewise GABA and beta-alanine turned out to be competitive inhibitors of vesicular uptake of glycine in spinal cord. The apparent K1 values were in the same range as the respective Km values for the transport systems. Accumulation of different amino acids were examined, and the structurally related amino acids GABA, beta-alanine and glycine were all taken up by both vesicle fractions. In the present study, we suggest that there are similarities in the vesicular transporters for GABA and glycine, and the two amino acids are probably taken up into the same vesicle population. The key factor in differentiating between GABA and glycine as transmitters in the terminals could be the synthesis and the high-affinity synaptosomal uptake.

Serotonin-storing secretory vesicles

Advances have been made in the characterization of 5-HT-storing organelles of neurectodermal cells. The parafollicular cell of the thyroid has been used as a model. This cell stores 5-HT, shares many properties with neurons, and can be induced to change its phenotype from endocrine to neuronal by exposure in vitro to NGF. The membranes of isolated parafollicular 5-HT storage vesicles appear to contain a chloride channel that is gated in response to stimulation of the cells by secretogogues. Opening of this channel permits the interior of the vesicle to acidify in response to the action of a H+ ATPase in the vesicular membrane. Development of a delta psi appears to limit acidification of the vesicular interior when the chloride conductance is low. Transmembrane transport of 3H-5-HT into parafollicular vesicle is inhibited by dissipating the delta pH across the granular membranes. The physiological significance of the ability of parafollicular vesicles to modify the internal pH of their 5-HT-storing organelles remains to be determined. Like the synaptic vesicles of central and peripheral serotonergic neurons parafollicular vesicles contain a specific 5-HT binding protein, SBP. 5-HT storage organelles and SBP have been found in medullary thyroid carcinoma (MTC) cells, a tumor line derived from parafollicular cells. The cell biology of SBP is now under study utilizing the MTC cells.

D-tartrate alters uptake of [3H]dopamine into brain synaptic vesicles

The use of D-tartrate containing media for measuring uptake of catecholamines into brain synaptic vesicles alters the properties of transport. Absolute concentrations of inhibitors determined in competition studies should be viewed with caution.

Kinetic studies on uptake of serotonin and noradrenaline into pial arteries of rats

A population of cerebrovascular nerve fibers have recently been found to store serotonin (5-hydroxytryptamine; 5-HT). There is reason to assume that these 5-HT-containing fibers have a sympathetic rather than an intracerebral origin. This was further elucidated in the present study in which the uptake mechanisms of 5-HT and noradrenaline (NA) were characterized and compared in rat pial arteries by measuring the accumulation of [3H]5-HT and [14C]NA under various experimental conditions in vitro. Sympathectomized vessels served as blanks. The uptake into the perivascular sympathetic nerves was dependent on time as well as concentration and was saturable. The Km values were similar, 0.17 microM for 5-HT and 0.15 microM for NA, but the Vmax value was 10 times higher for NA (2.38 and 25 pmol/mg/15 min, respectively). The two amines competed with each other in the sympathetic uptake, as studied by inhibition of the accumulation of one labeled amine by the other nonlabeled amine. Corticosterone, acting on the extraneuronal process, significantly inhibited the 5-HT uptake but had no substantial effect on NA. Reserpine, blocking the intraaxonal vesicular stores, markedly attenuated the accumulation of NA, but not of 5-HT. The selective uptake blocker paroxetine reduced the 5-HT uptake with much higher potency than the NA uptake, whereas desipramine predominantly inhibited NA uptake. The pial 5-HT uptake was not significantly affected by lesion of the raphe complex, whereas it was reduced to half following superior cervical ganglionectomy. The results suggest that the 5-HT present in nerves associated with pial vessels at the base of the brain is taken up through an efficient axonal mechanism, functionally related but not identical to the uptake process for NA.

ATP-regulated neuronal catecholamine uptake: a new mechanism

Uptake of the catecholamines (CA), dopamine (DA) and norepinephrine (NE) into synaptosomes prepared from rat and bovine brains was potentiated by ATP (from 0.1 to 5.0 mM) in a dose-dependent manner. Other nucleotides, particularly the nonhydrolyzable ATP analogs beta,gamma-imidoadenosine-5'-triphosphate (AMP-PNP) and beta,gamma-methyladenosine-5'-triphosphate (AMP-PCP) also potentiated [3H]DA and [3H]NE uptake. Several endogenous 5'-nucleotide triphosphates (e.g. GTP, UTP and CTP) potentiated [3H]CA uptake, but were less effective than ATP. Among the ATP metabolites, only ADP potentiated uptake whereas AMP and adenosine did not. [3H]Dopamine uptake measured in Krebs bicarbonate buffer had a Km of 2.1 microM and a Vmax of 163.9 pmol/mg prot./min. In presence of ATP, [3H]DA uptake had much higher affinity (Km = 0.56 microM) and larger capacity (Vmax = 333 pmol/mg prot./min) than uptake in absence of added ATP. Furthermore, [3H]DA uptake in presence of ATP had faster rate of uptake, and was independent of temperature while in absence of added ATP it was temperature-dependent. This ATP-dependent [3H]DA uptake was retained by synaptosomal ghosts that were obtained after lysing the striatal synaptosomes and removing their contents of synaptic vesicles and mitochondria. It is proposed that, in addition to the carrier-mediated (neuronal) uptake of CA, there is neuronal uptake that is regulated by ATP and inhibited by cocaine, which may be more relevant for terminating the synaptic action of CA because of its faster rate of uptake and larger capacity.

The effect of nicotine on catecholaminergic storage vesicles

The present study examined the action of nicotine on the accumulation of [3H]dopamine into synaptic vesicles prepared from mouse cerebral cortex or bovine striatum. Nicotine was shown to be a weak inhibitor of [3H]dopamine accumulation, with an IC50 of approximately 0.2-0.4 mM. In addition, repeated nicotine administration (1.2 mg (-)-nicotine di-(+)tartrate/kg s.c., twice daily for 10 days) in vivo in BALB/cBy male mice did not alter the potency of reserpine in inhibiting [3H]dopamine accumulation into synaptic vesicles, nor did it change the slight shift induced by nicotine in the potency of reserpine in inhibiting [3H]dopamine accumulation. The present results show that nicotine is an inhibitor of vesicular dopamine accumulation at high concentrations.

Anesthetic effects on 5-hydroxytryptamine uptake by rat brain synaptosomes

As a neurotransmitter involved in modulating central nervous system nociception and awareness, 5-hydroxytryptamine (5-HT) may play an important role in the clinical sequelae of certain anesthetic compounds. Anesthetic agents are known to affect peripheral, non-neuronal 5-HT uptake but little is known about their effects on 5-HT metabolism in the central nervous system. The effects of several anesthetic compounds on 5-HT uptake were examined in synaptosomes isolated from rat brain cortex. Inhibition of this uptake process was observed by exposure to clinically relevant concentrations of the volatile anesthetics halothane, isoflurane, and enflurane. The non-volatile agent, ketamine also inhibited uptake while the narcotic fentanyl had an effect only at the highest concentrations tested. Non-volatile agents which had neither a consistent nor significant effect on synaptosomal 5-HT uptake included pentobarbital, sufentanil, and etomidate. These alterations of 5-HT metabolism could represent a mechanism that contributes to anesthetic action.

Synaptic vesicles from mammalian brain: large-scale purification and physical and immunochemical characterization

Purification of synaptic vesicles directly from homogenates of mammalian brain is compared with a classical method based on osmotic lysis of brain synaptosomes. The direct method affords increased yield and purity of synaptic vesicles prepared under isoosmotic conditions. Antigen SV2 and the antigens (primarily synaptophysin) recognized by rabbit antiserum R10, raised to purified rat brain synaptic vesicles, are localized specifically on approximately 40-nm-diameter microsomal vesicles from rat brain. Rat brain synaptic vesicles have equilibrium densities of approximately 1.11 g/ml on Nycodenz density gradients, 1.12 g/ml on glycerol/Nycodenz, and 1.07 g/ml on Ficoll gradients. Both SV2 and the R10 antigens are enriched approximately 50-fold in purified rat brain synaptic vesicles. Synaptic vesicles purified from rat or cow brain show active uptake of [3H]norepinephrine that is reserpine sensitive and dependent on ATP and Mg2+. Synaptic vesicles exhibiting [3H]norepinephrine uptake comigrate with approximately 40-nm-diameter synaptic vesicles carrying SV2 or R10 antigens during permeation chromatography. After the Sephacryl S-1000 chromatography step, [3H]-norepinephrine uptake activity is purified approximately 90-fold. Highly purified brain synaptic vesicles should facilitate studies at the molecular level of the roles of these organelles in neurotransmission at mammalian synapses.

3H-5-hydroxytryptamine accumulation by rat brain synaptic vesicles in a membrane-impermeant medium, and selective reduction by 5,7-dihydroxytryptamine

In order to examine possible selectivity of amine uptake by synaptic vesicles, the ATP-stimulated accumulation of 3H-5-hydroxytryptamine (5HT) by synaptic vesicles from rat whole brain was examined in a medium comprised largely of membrane-impermeant anions (d-tartrate). Such media have previously been shown to stabilize vesicular accumulation of several neurotransmitters. Accumulation of 3H-5HT did not occur in tartrate medium alone, but was increased biphasically with increasing concentrations of both potassium phosphate and potassium bicarbonate. At optimal concentrations of each anion (10 mM), stable accumulation of 3H-5HT was observed at 37 degrees (26.1 +/- 1.2 pmol/mg protein; Km 6 X 10(-7) M), which was reduced by greater than 95% in the absence of K2ATP, at 4 degrees C, in the presence of 10(-6) M reserpine, or in the presence of the proton ionophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). Uptake was significantly antagonized by millimolar concentrations of Na+, Mg++ or Cl-, but was unaffected by ouabain (10(-5) M). Pretreatment of animals with 5,7-dihydroxytryptamine (5,7-DHT) (200 micrograms, intraventricular) 10 days prior to sacrifice reduced endogenous 5HT levels by 70%, while levels of endogenous norepinephrine (NE) and dopamine (DA) were unaffected. Accumulation of 3H-5HT, examined in the presence of 10(-6) M NE to block 3H-5HT accumulation by vesicles from noradrenergic nerve endings, was reduced by 40% in vesicles from treated animals. Vesicular accumulation of 3H-(-)-NE and 3H-DA was unaffected by 5,7-DHT treatment. The data suggest the possibility of preferential accumulation of 3H-5HT by vesicles arising from serotonergic nerve endings.

Influence of hypoxia on release and uptake of neurotransmitters in guinea pig striatal slices: dopamine and acetylcholine

We studied the influence of hypoxia on the release of [3H]dopamine ([3H]DA) and [3H]acetylcholine ([3H]ACh), uptake of [3H]DA and [3H]choline and Ca2+-influx in guinea pig striatal slices. Tetrodotoxin (TTX)-sensitive and Ca2+-dependent electrically evoked release of [3H]DA was not affected by hypoxia, while spontaneous release of [3H]DA was rapidly increased. On the other hand, by hypoxia, the evoked [3H]ACh release gradually decreased and was diminished to about 45% 40 min later. Hypoxia suppressed the Vmax of [3H]DA uptake to one third and that of [3H]choline to half of the control values, but with no change in either of the Km values. Hypoxia reduced both the acetylation and the uptake of [3H]choline in slices preliminarily incubated with 3 mM or 25 mM K+ medium. Stimulation-induced Ca2+-influx was slightly suppressed and was 78.1% of the control values even after 40 min exposure to hypoxia. The Ca2+-dependent neurotransmitter release process itself appears to be well preserved against hypoxia as compared with the uptake process. Our findings imply that hypoxia could result in differential alterations of neural activity depending on the specific sensitivity of the presynaptic process of neurotransmission.

Binding of [3H]serotonin to skeletal muscle actin

We previously observed that the neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) binds with high- and low-affinity interactions to an actin-like protein prepared from rat brain synaptosomes. In this study, we examined its binding to highly purified actin obtained from rabbit skeletal muscle. Monomeric G-actin bound serotonin with high and low affinities, exhibiting equilibrium dissociation constants (KD values) of 5 X 10(-5) M and 4 X 10(-3) M, respectively. The serotonin binding site on actin was distinct from those sites previously characterized for divalent cations, nucleotides, and cytochalasin alkaloids. The binding of serotonin (1 microM) to G-actin was increased as much as 26-fold by divalent cations. Potassium iodine (KI) increased the affinity of G-actin for serotonin, KD values for this binding being 3 X 10(-7) M and X 10(-5) M. Serotonin bound with even higher affinity to polymerized F-actin, with KD values of 2 X 10(-8) M and 2 X 10(-5) M. However, the total number of binding sites on F-actin was only about 4% of the number of G-actin. The binding of serotonin (0.1 microM) to G-actin could be inhibited by phenothiazines (1 microM) or reserpine (10 microM), but not by classical antagonists of serotonin receptors or by drugs that release serotonin or inhibit its uptake. The binding of serotonin to actin in vivo may participate in a contractile process related to neurotransmitter release.

Exposure to methylmercury in utero: effects on biochemical development of catecholamine neurotransmitter systems

Administration of methylmercury to pregnant rats resulted in major alterations in synaptic dynamics of brain dopamine systems in the offspring which were prominent even at doses of the organomercurial which did not produce acute toxicity, fetal or neonatal death, low birth weight or reduced litter sizes. The abnormalities were typified by shortfalls in both the levels and turnover rate of the transmitter in vivo, accompanied by elevations in synaptic uptake as assessed in synaptosomal preparations in vitro. These effects were not apparent in the immediate postnatal period but instead showed a delayed onset beginning at about the time of weaning. Methylmercury exposure displayed selectivity in that central noradrenergic systems showed only the synaptic uptake alterations without changes in transmitter levels or turnover; targeted interactions were also apparent in peripheral sympathetic pathways to the heart and kidney. The threshold dose required to elicit damage to biochemical development of neurotransmitter systems was the same as that to alter more generalized cellular development, as assessed through measurements of brain ornithine decarboxylase activity. These studies indicate that neurochemical damage produced by prenatal exposure of the developing organism to methylmercury involves transmitter-selective alterations in synaptic dynamics and function which may contribute to adverse behavioral outcomes; the underlying mechanisms, however, do not necessarily reflect actions of the organomercurial which are primary or specific to these particular neuronal tissues.

Effects of neonatal mercuric chloride administration on growth and biochemical development of neuronal and non-neuronal tissues in the rat: comparison with methylmercury

Mercuric chloride (HgCl2) was administered at 3 dosage levels (0.5, 1, and 2.5 mg/kg s.c.) daily to newborn rats beginning at 1 day of age and continuing through weaning. HgCl2 produced a dose-dependent inhibition of body growth which was apparent only after weaning and which worsened as the animals approached adulthood; brain growth was also adversely affected, but less so than was body weight. Growth of other tissues (heart, kidney, liver) was influenced in a different manner, with initial increases over control organ weights and a subsequent decline toward normal (kidney) or to subnormal levels (heart, liver). Examination of ornithine decarboxylase (ODC) activity, an index of cellular maturation, confirmed that HgCl2 produced separable types of effects in different organ systems. Although the patterns of growth alterations caused by HgCl2 resembled those seen with methylmercury, the effects on ODC were dissimilar, suggesting that there were some differences in the underlying biochemical mechanisms. In addition to causing generalized alterations of cellular development, HgCl2 produced specific effects on catecholamine neurotransmitter systems, with increases in brain norepinephrine levels and turnover as well as elevations in synaptosomal uptake capabilities for norepinephrine. Dopamine levels and turnover were slightly reduced or unchanged, but synaptosomal uptake was still elevated. Target-specific effects were also apparent in the peripheral sympathetic nervous system, where renal but not cardiac norepinephrine was elevated in the HgCl2 group. Again, some but not all of these changes can be produced by comparable exposure to methylmercury; many of the neuronal effects in animals exposed to HgCl2 were in the opposite direction from those seen with the organomercurial.

Lability in storage of 3H-dopamine and 3H-norepinephrine in crude synaptosome (P2) and vesicle-associated fractions of rat brain

Crude synaptosome (P2) fractions prepared from rat striatum and hypothalamus, preloaded with 3H-dopamine (DA) or 3H-norepinephrine (NE), were incubated at 37 degrees C for 5 min. The addition of reserpine at a concentration of 0.1 microM to the striatal synaptosomes substantially depleted 3H-DA to about 45% of control values, but had no effect on 3H-NE. An analogous difference in sensitivity to reserpine, though less pronounced, was observed between 3H-DA and 3H-NE loaded into hypothalamic synaptosomes. Preloaded synaptosome fractions prepared from striatum and hypothalamus were also lysed under hypoosmotic conditions, filtered, and then washed with 130 mM KH2PO4 buffer, pH 7.4, maintained at 0 degrees or 37 degrees C. Washing with 0 degrees C buffer produced no appreciable change in the amount of 3H-DA or 3H-NE retained by the hypoosmotic-resistant subsynaptosomal fractions. Increasing the temperature of the wash buffer to 37 degrees C, however, elicited a volume-dependent depletion of 3H-DA about 2.5-fold higher than that obtained for 3H-NE. Consistent with this finding, the retention of 3H-DA by a crude vesicle fraction prepared from striatum was found to be significantly less than the retention of 3H-NE following 4.5 and 6 min of incubation at 20 degrees C. Thus, in intact synaptosomes, 3H-DA appears to be stored in a form that is more susceptible than 3H-NE to depletion by reserpine, and this effect may be related to differences between the intravesicular storage stability of DA and NE.

Labeled norepinephrine uptake and release in rat globus pallidus

The uptake and release of [3H]norepinephrine [( 3H]NE) were investigated using isolated rat globus pallidus slices to determine the possible neurotransmitter role of this catecholamine. The uptake into these slices was linear for the first 10 min. Kinetic analysis indicated two components of NE accumulation, one representing a high (Km1 2.9 X 10(-7) M and Vmax1 1.4 pmol/mg/10 min) and other a low (Km2 1.6 X 10(-6) M and Vmax2 5 pmol/mg/10 min) affinity uptake system. Desmethylimipramine at a concentration of 10(-5) M reduced the high affinity uptake of [3H]NE by 25% of the control values. Electrical stimulation of the slices increased the efflux of [3H]NE and its metabolites from tissues preloaded with [3H]NE, in a current- and frequency-dependent fashion. The release of [3H]NE and its metabolites induced by electrical stimulation (1 mA, 20 Hz, 1 ms for 2 min) was inhibited by tetrodotoxin (10(-6) M), and by a calcium-free medium containing EGTA (10(-4) M) or medium with a high magnesium-concentration (2 X 10(-2) M). These findings provide strong evidence for the neurotransmitter role of NE, in these tissues.

Characterization of high affinity dopamine uptake into the dopamine neurons of the hypothalamus

The study of hypothalamic dopamine (DA) neurons is complicated by the difficulty in distinguishing DA neurons from norepinephrine (NE) neurons and by the fact that they comprise only a small proportion of the catecholamine neuron population of the hypothalamus. We have studied DA uptake into nerve terminals of hypothalamic DA neurons using a synaptosomal preparation. Desmethylimipramine (DMI) was used to prevent uptake into synaptosomes from NE neurons, thus pharmacologically isolating dopaminergic from noradrenergic nerve terminals. This DMI-insensitive DA uptake in hypothalamus had all the properties of a high affinity uptake process; it was saturable, dependent on incubation time and incubation temperature, increased linearly with increasing amounts of tissue and was completely abolished by excess unlabeled DA. Also, it was completely abolished by benztropine, an inhibitor of amine uptake into DA neurons. We believe that DMI-insensitive DA uptake into hypothalamic synaptosomes represents uptake into DA nerve terminals. The DMI-insensitive DA accumulation in discrete areas of hypothalamus correlated well with the known prevalence of DA neurons relative to NE neurons in these areas: median eminence greater than median eminence-arcuate nucleus greater than mediobasal hypothalamus greater than whole hypothalamus. Comparison of the affinity constants for DA uptake into synaptosomes incubated without DMI revealed a 2-fold higher affinity constant for DA uptake in median eminence compared with striatum, but affinity constants in all the other hypothalamic regions examined (mediobasal hypothalamus, hypothalamus minus median eminence, whole hypothalamus) were identical to that of striatum. In contrast, comparison on the affinity constants for DA uptake in the presence of DMI revealed a 2-3-fold higher affinity constant for DA uptake in all these hypothalamic regions compared with striatum. It appears the tuberoinfundibular DA neurons and the other DA neurons of the hypothalamus have a high affinity uptake system for DA, although affinity for DA in all of these hypothalamic DA neurons appears to be 2-3-fold lower than that in striatal DA neurons. The data also suggest that the much larger mediobasal hypothalamus may serve as a model for studies of DA uptake into tuberoinfundibular DA neurons of the median eminence.

Participation of a transmembrane proton gradient in 5-hydroxytryptamine transport by platelet dense granules and dense-granule ghosts

Dense granules, the storage organelles for 5-hydroxytryptamine in blood platelets, have been isolated from porcine platelets and are shown to transport 5-hydroxytryptamine in response to a transmembrane proton gradient (delta pH). Transport in the absence of delta pH is minimal, and it is shown that a rapid increase in transport takes place as delta pH increases. Direct measurements with [14C]methylamine show a delta pH of 1.1 units (acid inside) for intact granules. Osmotically active ghosts of dense granules from which 95% of the endogenous 5-hydroxytryptamine content has been released have also been prepared. Ghosts swell in the presence of ATP and Mg2+, and this swelling is shown to be due to the entry of protons via a process linked to ATP hydrolysis. Proton entry is also apparently linked to anion penetration in ghosts. Steady-state 5-hydroxytryptamine transport in ghosts is stimulated approx. 3-fold on the addition of ATP to the incubation medium, and the stimulation of 5-hydroxytryptamine transport in ghosts correlates with the formation of a transmembrane delta pH. Ghosts generate a delta pH of 1.1-1.3 pH units (acid inside) in the presence of 5 mM-ATP/2.5 mM-MgSO4. delta pH is generated within 3 min at 37 degrees C and is dissipated by the ionophore nigericin and by NH4Cl. It is shown that an Mg2+-stimulated ATPase activity is present on the ghost membrane, and inhibition of the ATPase leads to a corresponding decrease in 5-hydroxytryptamine transport. The results presented support the idea that 5-hydroxytryptamine transport into platelet dense granules is dependent on the presence of a transmembrane delta pH and, together with previous findings by others, suggest a generalized mechanism for biogenic amine transport into subcellular storage organelles.

Further studies on 5-hydroxytryptamine transport in pancreatic islets and isolated beta-cells

1 The transport mechanism for (3)H-labelled 5-hydroxytryptamine (5-HT) in isolated pancreatic islets of non-inbred ob/ob mice was further characterized.2 Isolated beta-cells accumulated 5-HT to the same degree and with the same Na(+)-dependence as whole islets.3 Imipramine inhibited the uptake in a concentration-dependent way.4 Reserpine did not affect the uptake or efflux rates.5 Glucose stimulation of insulin secretion did not affect the uptake rate.6 It is concluded that the observed islet uptake of [(3)H]-5-HT represents an intracellular accumulation by the beta-cells. Mechanisms at the level of the plasma membrane may be rate-limiting for this process.

Transport of catecholamines by native and reconstituted rat heart synaptic vesicles

Highly purified "heavy" synaptic vesicles were isolated from rat heart by differential centrifugation. Because of the high intravesicular concentrations of proteins, catecholamine, and ATP, resealed vesicle ghosts were prepared and used to study the detailed kinetics of catecholamine transport. ATP stimulated the uptake of l-norepinephrine and was saturable with a Km for l-norepinephrine at 3.3 microM and 1.8 mM for ATP. The ghosts also accumulated 5-hydroxytryptamine and l-epinephrine via an ATP-dependent mechanism. Uptake was stereospecific for the l-form. A functional catecholamine transporter could be solubilized by the detergent octyl-glucoside and incorporated into phospholipid vesicles, which, after detergent removal, generated proteoliposomes that accumulated l-norepinephrine. Reserpine- and l-propranolol-sensitive accumulation against a concentration gradient is achieved by artificially creating a pH gradient across the membrane, and lends further support to the idea that at least the initial phase of catecholamine transport is driven by the trans-membrane pH gradient created by the proton-translocating ATPase.

Synaptic development in brains of rats exposed perinatally to ethanol

Development of brain synaptosomal uptakes of 3H-norepinephrine and 3H-dopamine in pups whose mothers received ethanol were nearly normal. However, development of synaptosomal uptake of 3H-serotonin was significantly lower than in controls, while uptake of 3H-norepinephrine into synaptic storage vesicles was increased.

Methadone inhibits serotonin and norephinephrine uptake into rat brain synaptosomes and synaptic vesicles in vitro but not in vivo

Methadone in vitro inhibited biogenic amine uptake into synaptosomes isolated from rat whole brain; the IC50 for inhibition of serotonin uptake was between 0.1 and 1 micron, and for norepinephrine uptake, 10 micron when the serotonin or norepinephrine concentration was 0.05 micron. Methadone in vitro also inhibited uptake of norepinephrine into rat brain synaptic vesicles to the same extent that it inhibited synaptosomal norepinephrine uptake. In all cases, all cases, morphine in vitro was much less effective in blocking uptake. However, in vivo acute or chronic administration of methadone to rats failed to cause inhibition of serotonin or norepinephrine uptakes in synaptosomes isolated from the brains of treated rats, nor was inhibition of vesicular norepinephrine uptake produced. Amine uptake in situ also was unaffected by methadone administration, as evidenced by normal incorporation of intracisternally administered 3H-serotonin into synaptic endings. These data show that inhibition of synaptosomal and vesicular biogenic amine uptake by methadone in vitro is probably not a major determinant of the alterations in biogenic amine turnover caused by methadone administration. Thus, results obtained with platelets and adrenomedullary chromaffin vesicles, in which methadone administration does cause effective inhibition of amine uptake in vivo, cannot be extended to the central nervous system.