Uptake of norepinephrine and related catecholamines by cultured chromaffin cells: characterization of cocaine-sensitive and -insensitive plasma membrane transport sites

Cell-Based Radiotracer Binding and Uptake Inhibition Assays: A Comparison of In Vitro Methods to Assess the Potency of Drugs That Target Monoamine Transporters

High-affinity monoamine transporters are targets for prescribed medications and stimulant drugs of abuse. Therefore, assessing monoamine transporter activity for candidate medications and newly-emerging drugs of abuse provides essential information for industry, academia, and public health. Radiotracer binding and uptake inhibition are the gold standard assays for determining drug–transporter interaction profiles. The combined results from such assays yield a unique biochemical fingerprint for each compound. Over time, different assay methods have been developed to assess transporter activity, and the comparability of data across various assay platforms remains largely unclear. Here, we compare the effects of six well-established stimulants in two different cell-based uptake inhibition assays, one method using adherent cells and the other using suspended cells. Furthermore, we compare the data from transfected cell lines derived from different laboratories and data reported from rat synaptosomes. For transporter inhibitors, IC₅₀ values obtained by the two experimental methods were comparable, but using different transfected cell lines yielded disparate results. For transporter substrates, differences between the two cell lines were less pronounced but the drugs displayed different inhibition potencies when evaluated by the two methods. Our study illustrates the inherent limitations when comparing transporter inhibition data from different laboratories and stresses the importance of including appropriate control experiments with reference compounds when investigating new drugs of interest.

Vesicular Transmitter Content in Chromaffin Cells Can Be Regulated via Extracellular ATP

The energy carrying molecule adenosine triphosphate (ATP) has been implicated for its role in modulation of chemical signaling for some time. Despite this, the precise effects and mechanisms of action of ATP on secretory cells are not well-known. Here, bovine chromaffin cells have been used as a model system to study the effects of extracellular ATP in combination with the catecholamine transmitter norepinephrine (NE). Both transmitter storage and exocytotic release were quantified using complementary amperometric techniques. Although incubation with NE alone did not cause any changes to either transmitter storage or release, coincubation with NE and ATP resulted in a significant increase that was concentration dependent. To probe the potential mechanisms of action, a slowly hydrolyzable version of ATP, ATP-γ-S, was used either alone or together with NE. The result implicates two different behaviors of ATP acting on both the purinergic autoreceptors and as a source of the energy needed to load chromaffin cell vesicles.

Amine weak bases disrupt vesicular storage and promote exocytosis in chromaffin cells

The vesicular contents in bovine chromaffin cells are maintained at high levels owing to the strong association of its contents, which is promoted by the low vesicular pH. The association is among the catecholamines, Ca2+, ATP, and vesicular proteins. It was found that transient application of a weak base, methylamine (30 mM), amphetamine (10 microM), or tyramine (10 microM), induced exocytotic release. Exposure to these agents was also found to increase both cytosolic catecholamine and intracellular Ca2+ concentration, as measured by amperometry and fura-2 fluorescence. Amphetamine, the most potent amine with respect to evoking exocytosis, was found to be effective even in buffer without external Ca2+; however, the occurrence of spikes was suppressed when BAPTA-acetoxymethyl ester was used to complex intracellular Ca2+. Amphetamine-induced spikes in Ca2+-free medium were not suppressed by thapsigargin or ruthenium red, inhibitors of the sarco(endo)plasmic reticulum Ca2+-ATPase and mitochondrial Ca2+ stores. Atomic absorption measurements of amphetamine- and methylamine-treated vesicles reveal that intravesicular Ca2+ stores are decreased after a 15-min incubation. Taken together, these data indicate that amphetamine and methylamine can disrupt vesicular stores to a sufficient degree that Ca2+ can escape and trigger exocytosis.

Uptake of [3H]dopamine in isolated chromaffin cells of the mouse: modulation by intra- and extra-adrenal peptides and other secretagogues

The effects of intra- and extra-adrenal peptides on [3H]dopamine uptake in adrenal chromaffin cells of the mouse were examined in vitro. Dopamine uptake was inhibited by acetylcholine, high potassium, reserpine, imipramine and desmethylimipramine as was in noradrenaline uptake. Among the intra-adrenal peptides, vasoactive intestinal peptide (VIP, 100 pmol/l) and neurotensin inhibited [3H]dopamine uptake by approximately 25%. Somatostatin, enkephalin, and neuropeptide Y did not cause any significant inhibition. An extra-adrenal peptide, bradykinin, inhibited the uptake while angiotensin II showed no significant effect. Intra-adrenal peptides which cause catecholamine secretion inhibit catecholamine uptake probably to extend its effect. Extra-adrenal peptide which causes catecholamine secretion also inhibits catecholamine uptake.

Restriction of the activity of the recombination site dif to a small zone of the Escherichia coli chromosome

The recombination site dif is the target on the Escherichia coli chromosome of the site-specific recombinases XerC and XerD. The dif/XerC-D system plays a role during the cell cycle, probably by favoring sister chromosome monomerization or separation. A phenomenon of regional control over dif activity, also analyzed in this issue, is demonstrated here by translocation of dif to a series of loci close to the normal locus. We found that the site is physiologically active only within a narrow zone around its natural position. Competence for dif activity does not depend on the sequence of the normal dif activity zone (DAZ), because delta(dif) deletions larger than the DAZ result in Dif+ bacteria when dif is reinserted at the junction point. Although dif maps where replication normally terminates, termination of replication is not the elicitor. A strain with a large inversion that places dif and its surrounding region close to oriC remains Dif+, even when a Tus- mutation allows replication to terminate far away from it. Preliminary data suggest the possibility that specialized sequences separate the competent zone from the rest of the chromosome. We suspect that these sequences are members of a set of sequences involved in a polarized process of postreplicative reconstruction of the nucleoid structure. We propose that this reconstruction forces catenation links between sister chromosomes to accumulate within the DAZ, where they eventually favor recombination at dif.

Cocaine as a naturally occurring insecticide

Although cocaine has a fascinating and complex medicinal history in man, its natural function in plants is unknown. The present studies demonstrate that cocaine exerts insecticidal effects at concentrations which occur naturally in coca leaves. Unlike its known action on dopamine reuptake in mammals, cocaine's pesticidal effects are shown to result from a potentiation of insect octopaminergic neurotransmission. Amine-reuptake blockers of other structural classes also exert pesticidal activity with a rank order of potency distinct from that known to affect vertebrate amine transporters. These findings suggest that cocaine functions in plants as a natural insecticide and that octopamine transporters may be useful sites for targeting pesticides with selectivity toward invertebrates.

Morphological aspects of chromaffin tissue: the differential fixation of adrenaline and noradrenaline

The morphological aspects of chromaffin tissue are reviewed, based mainly on our studies on the mouse adrenal gland. Particular attention was focused on the differential fixation of adrenaline and noradrenaline, and on the uptake and storage of [3H]dopa, [3H]dopamine and related substances in the adrenaline-storing (A) and noradrenaline-storing (NA) cells. Scanning electron microscopy combined with the NaOH-maceration method was useful for demonstrating the 3-dimensional organisation of nerve terminals, chromaffin cells, glial elements and vascular elements. In transmission electron microscopy, 3 types of chromaffin cell were distinguished. They were A, NA and SGC (small granule chromaffin) cells. After glutaraldehyde fixation followed by postosmication, A cell granules showed lower electron density, whereas NA cell granules were solid and dark. This difference in appearance between A and NA cells was first explained by the hypothesis that, after glutaraldehyde fixation, most of the adrenaline dissolved, whereas noradrenaline was precipitated in situ. Later, this hypothesis was supported by a series of autoradiographic and radioisotopic assay studies using [3H]dopa, [3H]dopamine and related substances; when [3H]adrenaline occurred, radioactivity in A cells mostly disappeared in the specimen, whereas that in the NA cells remained. At 15-60 min after an i.p. injection of [3H]dopa or [3H]dopamine, the concentration of radioactivity in A cells was higher than that in NA cells. However, in hypophysectomised mice, the radioactivity was low and evenly distributed in these 2 types of chromaffin cell. It was deduced that the carrier activities for extracellular dopa and dopamine were made greater in the A cells than in the NA cells by the pituitary gland. The hypophysectomy effects were restored by i.p. administration of ACTH.

Hypoxia induces different responses of striatal high- and low- affinity dopamine uptake sites

The dopamine (DA) uptake over a concentration range from 0.03 to 100 microM was studied in S1 fractions of the rat striatum prepared from control rats and those exposed for 14 h to hypobaric hypoxia. The uptake exhibited non-Michaelis-Menten kinetics, which were evaluated by applying an equation assuming two transport sites. The high-affinity uptake site was characterized by an apparent Michaelis-Menten constant of 0.47 microM and an apparent maximal transport rate of 113 pmol/mg protein/30 s. The respective constants of the low-affinity uptake site were 52.8 microM and 1490 pmol/mg protein/30 s. One hour after hypoxia kinetic constants of the high-affinity uptake were unchanged but the maximal transport rate of the low-affinity uptake was increased by 50%. The elevated low-affinity uptake capacity may represent a means of adaptation to hypoxia allowing a faster removal of high extracellular concentrations of DA.

Pertussis toxin inhibits noradrenaline accumulation by bovine adrenal medullary chromaffin cells

Bovine adrenal medullary chromaffin cells maintained in tissue culture accumulated [3H]-noradrenaline by a high affinity, Na(+)-dependent, desipramine-sensitive process. The accumulation was linear with time (1-90 min) and had an apparent Km of 0.52 +/- 0.24 mumol/l and Vmax of 1.70 +/- 0.48 pmol/(10(5) cells.15 min). Pretreatment of the cells with the ADP-ribosylating agent pertussis toxin resulted in a reduction in the Vmax [0.81 +/- 0.39 pmol/(10(5)cells.15 min)] but no significant change in the apparent affinity (Km = 0.42 +/- 0.07 mumol/l). This inhibition of [3H]noradrenaline accumulation was distinct from that produced by the vesicular transport inhibitor reserpine. Pertussis toxin inhibition probably did not arise through an indirect action on the Na(+)-gradient because while, as expected, Na+,K(+)-ATPase inhibition reduced [3H]noradrenaline accumulation, pertussis toxin pretreatment always caused a further significant reduction even in the presence of maximally effective concentrations of ouabain. Stimulation of the cAMP-protein kinase A system by forskolin or 8-bromocyclic AMP also caused a reduction in [3H] noradrenaline accumulation but again pertussis toxin pretreatment always resulted in a further reduction. Thus, the data provide evidence for a pertussis toxin-sensitive element in the catecholamine accumulation process and are consistent with an action at a site directly associated with the transporter itself rather than with an indirect action via secondary processes.

Quantitative autoradiography of cocaine binding sites in human brain postmortem

Quantitative autoradiography was used to study cocaine binding sites in the human brain postmortem. Tritiated cocaine was applied to brain sections from three drug- and disease-free subjects at a low (10 nM) concentration and at a high (1 microM) concentration, the latter being in the range of brain concentrations of cocaine found in users of the drug. Nonspecific binding was assessed in the presence of 100 microM unlabeled cocaine. At low (10 nM) concentrations of labeled cocaine, the basal ganglia exhibit the highest density of binding sites, with considerably lower densities in thalamus, cortex, and hippocampus. Cocaine binding at high (1 microM) concentrations displayed a different distribution pattern, more homogeneous with some cortical regions exhibiting binding site densities close to those seen in the basal ganglia. Preliminary competition experiments with several drugs indicate that dopamine uptake inhibitors completely block cocaine binding to the basal ganglia, while serotonin uptake inhibitors were more effective in the hippocampus. These findings suggest that cocaine binds to dopamine uptake sites in the human basal ganglia postmortem but that it also interacts with other classes of binding sites, depending on the concentration and brain region examined.

Recent Advances in Pharmacological Research on Alcohol

Alcohol dependence is a major public health problem. Studies have shown that a person dependent on alcohol often coabuses other substances, such as cocaine. Cocaine is a powerful stimulant whereas ethanol is generally considered to be a depressant, with some stimulating properties. The subjective effects of these two substances in a dependent individual may often appear to be more similar than they are different. Animals also self-administer both substances. Basically, although both substances have anesthetic properties and both act to functionally increase catecholaminergic function, especially that of dopamine, there are some differences in their actions. Both alcohol and cocaine have various effects on several neurotransmitters and systems, which ultimately interact to produce the feeling of well-being avidly sought by many individuals today. This drive often eventually produces a dependence which has associated social and medical consequences. It seems likely that the neurochemical changes that ensue following abuse of these substances underlie the phenomena of dependence, tolerance, and subsequent withdrawal. The apparent similarities and differences between these two substances will be reviewed in this chapter.

Role of the sigma receptor in the inhibition of [3H]-noradrenaline uptake in brain synaptosomes and adrenal chromaffin cells

1. Rat brain synaptosomes and cultured bovine adrenal chromaffin cells were used to monitor the inhibitory effects of phencyclidine (PCP) and sigma (sigma)-receptor ligands on the uptake of [3H]-noradrenaline ([3H]-NA). 2. A Na(+)-dependent high affinity uptake was observed in synaptosomes (30 degrees C) and chromaffin cells (37 degrees C) with Km of 0.22 and 0.56 microM and Vmax of 2.5 pmol min-1 mg-1 protein and 0.7 pmol min-1 per 10(6) cells, respectively. 3. PCP and haloperidol inhibited the high affinity uptake with IC50 of 0.17 and 0.42 microM, respectively in synaptosomes and 0.24 and 0.47 microM, respectively in adrenal chromaffin cells. 4. A close correlation (r = 0.96) was established between the ability of various PCP and sigma-receptor ligands to inhibit [3H]-NA uptake in both systems: PCP greater than TCP greater than haloperidol greater than 3-(+)-PPP greater than MK-801 greater than or equal to (-)-butaclamol greater than (+)-SKF-10047 greater than DTG. Spiperone and opioid receptor ligands were ineffective at 20 microM. 5. These results indicate that the central and peripheral inhibitory effects of PCP and sigma-receptor ligands on [3H]-NA uptake involves a receptor (sigma 1-like) which is distinct from that (PCP2) recognized for the inhibition of [3H]-dopamine uptake by PCP.