Competitive inhibition of sodium-dependent high affinity choline uptake by harmala alkaloids

Harmaline tremor: underlying mechanisms in a potential animal model of essential tremor

BACKGROUND

Harmaline and harmine are tremorigenic β-carbolines that, on administration to experimental animals, induce an acute postural and kinetic tremor of axial and truncal musculature. This drug-induced action tremor has been proposed as a model of essential tremor. Here we review what is known about harmaline tremor.

METHODS

Using the terms harmaline and harmine on PubMed, we searched for papers describing the effects of these β-carbolines on mammalian tissue, animals, or humans.

RESULTS

Investigations over four decades have shown that harmaline induces rhythmic burst-firing activity in the medial and dorsal accessory inferior olivary nuclei that is transmitted via climbing fibers to Purkinje cells and to the deep cerebellar nuclei, then to brainstem and spinal cord motoneurons. The critical structures required for tremor expression are the inferior olive, climbing fibers, and the deep cerebellar nuclei; Purkinje cells are not required. Enhanced synaptic norepinephrine or blockade of ionic glutamate receptors suppresses tremor, whereas enhanced synaptic serotonin exacerbates tremor. Benzodiazepines and muscimol suppress tremor. Alcohol suppresses harmaline tremor but exacerbates harmaline-associated neural damage. Recent investigations on the mechanism of harmaline tremor have focused on the T-type calcium channel.

DISCUSSION

Like essential tremor, harmaline tremor involves the cerebellum, and classic medications for essential tremor have been found to suppress harmaline tremor, leading to utilization of the harmaline model for preclinical testing of antitremor drugs. Limitations are that the model is acute, unlike essential tremor, and only approximately half of the drugs reported to suppress harmaline tremor are subsequently found to suppress tremor in clinical trials.

Antidepressant binding site in a bacterial homologue of neurotransmitter transporters

Sodium-coupled transporters are ubiquitous pumps that harness pre-existing sodium gradients to catalyse the thermodynamically unfavourable uptake of essential nutrients, neurotransmitters and inorganic ions across the lipid bilayer. Dysfunction of these integral membrane proteins has been implicated in glucose/galactose malabsorption, congenital hypothyroidism, Bartter's syndrome, epilepsy, depression, autism and obsessive-compulsive disorder. Sodium-coupled transporters are blocked by a number of therapeutically important compounds, including diuretics, anticonvulsants and antidepressants, many of which have also become indispensable tools in biochemical experiments designed to probe antagonist binding sites and to elucidate transport mechanisms. Steady-state kinetic data have revealed that both competitive and noncompetitive modes of inhibition exist. Antagonist dissociation experiments on the serotonin transporter (SERT) have also unveiled the existence of a low-affinity allosteric site that slows the dissociation of inhibitors from a separate high-affinity site. Despite these strides, atomic-level insights into inhibitor action have remained elusive. Here we screen a panel of molecules for their ability to inhibit LeuT, a prokaryotic homologue of mammalian neurotransmitter sodium symporters, and show that the tricyclic antidepressant (TCA) clomipramine noncompetitively inhibits substrate uptake. Cocrystal structures show that clomipramine, along with two other TCAs, binds in an extracellular-facing vestibule about 11 A above the substrate and two sodium ions, apparently stabilizing the extracellular gate in a closed conformation. Off-rate assays establish that clomipramine reduces the rate at which leucine dissociates from LeuT and reinforce our contention that this TCA inhibits LeuT by slowing substrate release. Our results represent a molecular view into noncompetitive inhibition of a sodium-coupled transporter and define principles for the rational design of new inhibitors.

The transport of choline

Choline has many physiological functions throughout the body that are dependent on its available local supply. However, since choline is a charged hydrophilic cation, transport mechanisms are required for it to cross biological membranes. Choline transport is required for cellular membrane construction and is the rate-limiting step for acetylcholine production. Transport mechanisms include: (1) sodium-dependent high-affinity uptake mechanism in synaptosomes, (2) sodium-independent low-affinity mechanism on cellular membranes, and (3) unique choline uptake mechanisms (e.g., blood-brain barrier choline transport). A comprehensive overview of choline transport studies is provided. This review article examines landmark and current choline transport studies, molecular mapping, and molecular identification of these carriers. Information regarding the choline-binding site is presented by reviewing choline structural analog (hemicholinium-3 and 15, and other nitrogen/methyl-hydroxyl compounds) inhibition studies. Choline transport in Alzheimer's disease, brain ischemic events, and aging is also discussed. Emphasis throughout the article is placed on targeting the choline transporter in disease and use of this carrier as a drug delivery vector.

Systemic harmaline blocks associative and motor learning by the actions of the inferior olive

Is there a role for the inferior olive in learning? Novel paradigms of conditioning involving tongue protrusion were developed using the rat to test whether: (a) the indole alkaloid harmaline blocks associative learning via actions within the inferior olive, and (b) the inferior olive is required for associative and motor learning. Harmaline blocked associative learning as measured by the absence of conditioned responses to a tone over six daily sessions of conditioning and the absence of retention without harmaline. Harmaline's effect on associative learning was completely blocked by prior removal of the inferior olive with 3-acetylpyridine. Rats whose inferior olives were chronically lesioned showed normal associative learning, normal associative memory, and could learn to modify tongue protrusion via a motor learning paradigm involving response shaping. Removal of the inferior olive degraded the performance of the licking motor system by increasing the latency of conditioned tongue protrusions and by increasing the temporal variability of rhythmic licking elicited by intraoral water. The experiments raise doubt as to whether the inferior olive encodes memory in the cerebellum but demonstrate that the inferior olive is essential for the temporal precision of movement. The results indicate that harmaline's antilearning action is produced by its ability to exaggerate the normal propensity of olivary neurons to fire rhythmically, a process that must be constrained under physiological conditions for normal learning to occur. It is concluded that there may be an important role for the rhythmic activity of inferior olivary neurons in the temporal processes that underlie both motor control and learning.

The cation receptor subsite of the choline transporter in preimplantation mouse conceptuses resembles a cation receptor subsite of several amino acid transporters

Mediated choline transport in preimplantation mouse conceptuses was inhibited competitively by Na+ and other cationic osmolites. Uptake of choline by conceptuses was also inhibited relatively strongly by ethanolamine, hemicholinium-3, harmaline, harmalol and harmine. The Ki values for inhibition of choline transport by most of the latter inhibitors were of the same order of magnitude as the Km value for choline transport (approximately 100 microM). To our knowledge, we are the first to show that mediated 'Na(+)-independent' choline transport is, nevertheless, inhibited strongly by the Na(+)-site inhibitor, harmaline. Inhibitions by harmaline, Na+ and other cations have been used to draw a parallel between the substrate receptor sites of amino acid transport systems y+ and bo.+. We suggest that the latter parallel should be extended to include the Na(+)-independent mammalian choline transporter. In addition, the choline transport activity in conceptuses increased by more than 100-fold between the 2-cell and blastocyst stages of development. Mouse blastocysts probably utilize choline for the synthesis of membrane phospholipids during cellular differentiation and when they begin to grow about ten hours prior to implantation. Since we show here that mouse conceptuses develop the capacity to transport choline prior to the onset of growth, some of the choline utilized for growth could come from an exogenous source.

Dopamine uptake inhibitory capacities of beta-carboline and 3,4-dihydro-beta-carboline analogs of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) oxidation products

Potentially endogenous beta-carboline and 3,4-dihydro-beta-carboline alkaloidal compounds were compared, generally as 2-methylated (quaternary) and normethylated pairs, to the neurotoxin, 1-methyl-4-phenyl-dihydropyridinium ion (MPP+), with respect to inhibition of [3H]dopamine uptake into rat striatal synaptosomal preparations. Although less potent than MPP+, several compounds displayed IC50 values for inhibition in the moderate range (12-24 microM). Notably, quaternization generally did not improve inhibitory potency, and the 3,4-dihydro-compounds often were more effective inhibitors than their heteroaromatic analogs. The partially competitive nature of inhibition by one of the more effective pairs, 2-methyl-harmine and harmine, was consistent with uptake of the beta-carbolines by the synaptosomal dopamine uptake system, as was the fact that the accumulation of 2-[14C]methyl-harmine was significantly reduced by low Na+ media and by nomifensine, a potent inhibitor of the dopamine transporter. When viewed with reports that certain 2-methyl-beta-carbolines show MPP+-like toxicity in vitro and in vivo, these studies support the proposal that a mammalian beta-carbolinium compound may be taken up by nigrostriatal neurons and provoke the neuronal degeneration underlying Parkinson's disease.

Choline transport specificity in animal cells and tissues

1. Beta carbolines inhibit choline transport in rat brain. 2. The aziridinium ring on the nitrogen of mustard analogs of choline causes irreversible binding to the carrier in rat brain. 3. The uptake system in rat brain is stereoselective, requires a quaternary nitrogen, and prefers analogs with a nitrogen-oxygen distance of about 3.26 A. 4. In mouse brain troxonium derivatives inhibit choline transport. 5. In cuttlefish optic lobes and torpedo electric organ pyrene derivatives potently inhibit choline transport. 6. In guinea pig placenta, the affinity of the choline carrier remains high even when this molecule lacks one or two methyl groups.

Structure-activity studies on inhibition of choline uptake by a mouse brain synaptosomal preparation: basic data

Over 80 substances were studied for their inhibition of high-affinity uptake of [3H]choline into a mouse brain synaptosomal fraction. Kinetic experiments tested a number of them for competitive behavior. A minimal provisional model for the choline uptake process is envisioned that is consistent with current data and with relevant observations in the literature. There are two hydrophilic anionic sites on the choline transporter that are separated from each other by a cationic hydrophobic domain. Association of choline in a Na+-dependent manner with one or both of the sites is necessary for the transport of choline to take place. The choline binding anionic sites are sufficiently large and/or flexible to allow attachment of cationic moieties larger than choline. The cationic hydrophobic domain of the transporter is flexible, probably tending to planarity. The length of the hydrophobic region between the anionic sites is approximately that of 10 extended methylene groups, and the minimal width is approximated by the distance across the condensed ring system of chlorpromazine. The probability of attachment of the highly hydrophilic choline to its binding sites is increased both by hydrogen-bonding to a proton-acceptor within the anionic sites and by repulsion from the cationic hydrophobic region. A number of substances that potently and competitively inhibit high affinity choline uptake possess quaternary ammonium groups and neutral or negatively charged lipophilic groups. In general, substances in which two quaternary ammonium groups are separated by an appropriately configured hydrophobic group and which can combine with both anionic sites and the hydrophobic region between them are more potent inhibitors than monoquaternary substance with the same or similar groups. However, substances with a single high-affinity quaternary group and an appropriately structured hydrophobic group, e.g. the trimethoxy-3-butynyl quaternary ammonium compounds, possess inhibitory efficacies similar to those shown by the most potent bisquaternaries. The above suggests that further delineation of the characteristics of the structures of the above sites of the transporter could lead to devisal of more potent reversible inhibitors of choline uptake than now are available.

Amiloride and 5-N,N-dimethylamiloride inhibit the carrier mediated uptake of choline in Ehrlich ascites tumor cells

Amiloride and 5-N,N-dimethylamiloride (DMA) inhibit the choline uptake of Ehrlich ascites tumor cells. The inhibition by DMA is competitive with a KI value of 20 microM. The apparent KM value for choline was determined as 15 microM. Amiloride is approximately three times less potent. Amiloride uptake is not antagonized by choline or impaired in cells characterized by a deficient choline carrier. This indicates that amiloride is not transported into the cell by the choline carrier. The inhibition of the choline uptake by DMA cannot be attributed to a depression of choline kinase (EC 2.7.1.32) and is therefore considered to be due to a direct interaction between DMA and the choline carrier. DMA does not compete with sodium ions for its effect on the choline carrier. It is suggested that the choline carrier of Ehrlich ascites tumor cells exhibits a binding site for DMA similar to the one on the Na+/H+ antiporter.

Inhibition of Na+-Ca2+ exchange mechanism in cardiac sarcolemmal vesicles by harmaline

1. Harmaline was found to inhibit the Na+-Ca2+ exchange mechanism present in cardiac sarcolemmal vesicles. 2. The inhibition was dose-dependent and was observed in the range 10(-5) M-10(-2) M harmaline. 3. The effect was demonstrated on both 45Ca2+-uptake and 45Ca2+-efflux. 4. The observed Ki value for harmaline inhibition of 45Ca2+-uptake was found to be 2.5 X 10(-4) M.

Action of hemicholinium-3 on phospholipid metabolism in Krebs II ascites cells

Incorporation of [Me-14C]choline or/and [2-14C]ethanolamine into phospholipids of Krebs II ascites cells in toto have been tested in the presence of hemicholinium-3. With [Me-14C]choline, labelling of cell pellet, intracellular choline, phosphocholine and total lipid extract is inhibited by hemicholinium-3 in a dose-dependent way between 6.25 X 10(-6) M and 10(-3) M. These effects are caused by a diminution of the choline or/and ethanolamine transport across the cell membrane and by a choline-kinase inhibition. In Krebs cells, choline is taken up by a low affinity Na+ sensitive uptake system (KT = 46 X 10(-6) M) which is competitively inhibited by hemicholinium-3 (KTi = 161 X 10(-6) M). Krebs cells exert a counter-transport (i.e. an exchange of choline across the membrane) against a concentration gradient of 10 mM choline whereas 10 mM hemicholinium-3 has no effect. Choline-kinase is also inhibited (I50 = 57 X 10(-6) M) in Krebs cells in toto and time-course data suggest that choline transport and phosphorylation might be tightly coupled. Specific radioactivities of phosphocholine and choline-glycerophospholipids decrease owing to the effect of the drug on the uptake and phosphorylation system. With 4 X 10(-5) M hemicholinium-3 and [Me-14C]choline as a marker, labelled choline-glycerophospholipids are decreased by 22%. With [2-14C]ethanolamine, labelled ethanolamine-phospholipids are decreased by 26% and choline-glycerophospholipids remain unlabelled. With the two markers, the additional effect produces a 35% decrease. It is concluded that hemicholinium-3 might be able to induce a depression of the intracellular choline and phosphocholine pool which could provoke a serious quantitative deficiency of major phospholipids in Krebs cells.