Neurotoxicity induced by continuous infusion of quinolinic acid into the lateral ventricle in rats

The rat as an animal model of Alzheimer's disease

As a disease model, the laboratory rat has contributed enormously to neuroscience research over the years. It has also been a popular animal model for Alzheimer's disease but its popularity has diminished during the last decade, as techniques for genetic manipulation in rats have lagged behind that of mice. In recent years, the rat has been making a comeback as an Alzheimer's disease model and the appearance of increasing numbers of transgenic rats will be a welcome and valuable complement to the existing mouse models. This review summarizes the contributions and current status of the rat as an animal model of Alzheimer's disease.

Corroborative cobalt and zinc model compounds of alpha-amino-beta-carboxymuconic-epsilon-semialdehyde decarboxylase (ACMSD)

We have synthesised and characterised a series of new Co(II) complexes (1-4, 6, 7) and one new Zn(II) complex (5) employing N(3)- and N(3)O-donor ligands [biap: N,N-bis(2-ethyl-5-methyl-imidazol-4-ylmethyl)amino-propane, KBPZG: potassium N,N-bis(3,5-dimethylpyrazolylmethyl) glycinate, KBPZA: potassium N,N-bis(3,5-dimethylpyrazolylmethyl) alaninate, KB(i)PrPZG: potassium N,N-bis(3,5-di-iso-propylpyrazolylmethyl) glycinate, and KB((t)BuM)PZG: potassium N,N-bis(3-methyl-5-tert-butyl-pyrazolylmethyl)glycinate] as structural models of the metalloenzyme alpha-amino-beta-carboxymuconic-epsilon-semialdehyde decarboxylase (ACMSD). These complexes were characterised by several techniques including X-ray crystallographic analysis, X-band EPR, and mass spectrometry (ESI-MS). The crystal structures of 1, 2, 6,7 revealed that they exist as mononuclear Co(II) complexes with trigonal-bipyramidal geometry in the solid state. Compounds 3 and 5 form infinite polymeric chains of Co(II) or Zn(II) complexes, respectively, linked by the pendant carboxylate arms of the BPZG(-) ligand. By comparing the degree of distortion in the penta-coordinate complexes, defined by the Addison-parameter tau, with the value determined for the five-coordinate centres found in the active site of ACMSD, it could be seen that complexes 5 and 7 are very good matches for the geometry of the zinc(II) centre in monomer A of the native enzyme. All complexes could be seen as model compounds for the active site of the enzyme ACMSD, where the Co(II) complexes reflected the structural flexibility found in case of two histidine (His177 and His228) residues found in the active site of the enzyme.

Animal models of cognitive dysfunction

The increased life expectancy in industrialised countries in the last half century has also brought to a greater incidence of neurological disorders, including neurodegenerative diseases and developing in a rather long time. In this respect, Alzheimer's disease (AD), for the large incidence, and the dramatic loss of autonomy caused by its cognitive and behavioural symptoms represents one of the main challenges of modern medicine. Although AD is a typical human disease and probably includes several nosographic entities, the use of animal models may contribute to understand specific aspects of pathophysiology of the disease. The most widely used animal models are rodents and non-human primates. In this review different animal models characterised by impaired cognitive functions are analysed. None of the models available mimics exactly cognitive, behavioural, biochemical and histopathological abnormalities observed in neurological disorders characterised by cognitive impairment. However, partial reproduction of neuropathology and/or cognitive deficits of Alzheimer's disease (AD), vascular dementia and dementia occurring in Huntington's and Parkinson's diseases, or in other neurodegenerative disorders may represent a basis for understanding pathophysiological traits of these diseases and for contributing to their treatments.

Differential effects of dietary fatty acids on rat liver alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase activity and gene expression

Hepatic alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (ACMSD; formerly termed picolinic carboxylase) [EC4.1.1.45] plays a key role in regulating NAD biosynthesis and the generation of quinolinate (quinolinic acid) from tryptophan. Quinolinate is a potent endogenous excitotoxin of neuronal cells. We previously reported that ingestion of fatty acids by rats leads to a decrease in their hepatic ACMSD activity. However, the mechanism of this phenomenon is not clarified. We previously purified ACMSD and cloned cDNA encoding rat ACMSD. Therefore, in this study, we examined the differential effect of fatty acids on ACMSD mRNA expression by Northern blot. Moreover, we measured quinolinic acid concentration in rats fed on fatty acid. When diets containing 2% level of fatty acid were given to male Sprague-Dawley rats (4 weeks old) for 8 days, long-chain saturated fatty acids and oleic acid did not affect ACMSD mRNA expression in the liver. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) strongly suppressed the liver ACMSD mRNA expression. In rats fed with high linoleic acid diet for 8 days, serum quinolinic acid was significantly increased as compared with the rats fed on a fatty acid-free diet under the condition of the approximately same calorie ingestion. These results suggest that the transcription level of ACMSD is modulated by polyunsaturated fatty acids, and suppressive potency of ACMSD mRNA is n-3 fatty acid family>linoleic acid (n-6 fatty acid)>saturated fatty acid. Moreover, this study provides the information that a high polyunsaturated fatty acid diet affects the production of quinolinic acid in serum by suppressing the ACMSD activity.

The neuropharmacological basis for the use of memantine in the treatment of Alzheimer's disease

Memantine has been demonstrated to be safe and effective in the symptomatic treatment of Alzheimer's disease (AD). While the neurobiological basis for the therapeutic activity of memantine is not fully understood, the drug is not a cholinesterase inhibitor and, therefore, acts differently from current AD therapies. Memantine can interact with a variety of ligand-gated ion channels. However, NMDA receptors appear to be a key target of memantine at therapeutic concentrations. Memantine is an uncompetitive (channel blocking) NMDA receptor antagonist. Like other NMDA receptor antagonists, memantine at high concentrations can inhibit mechanisms of synaptic plasticity that are believed to underlie learning and memory. However, at lower, clinically relevant concentrations memantine can under some circumstances promote synaptic plasticity and preserve or enhance memory in animal models of AD. In addition, memantine can protect against the excitotoxic destruction of cholinergic neurons. Blockade of NMDA receptors by memantine could theoretically confer disease-modifying activity in AD by inhibiting the "weak" NMDA receptor-dependent excitotoxicity that has been hypothesized to play a role in the progressive neuronal loss that underlies the evolving dementia. Moreover, recent in vitro studies suggest that memantine abrogates beta-amyloid (Abeta) toxicity and possibly inhibits Abeta production. Considerable attention has focused on the investigation of theories to explain the better tolerability of memantine over other NMDA receptor antagonists, particularly those that act by a similar channel blocking mechanism such as dissociative anesthetic-like agents (phencyclidine, ketamine, MK-801). A variety of channel-level factors could be relevant, including fast channel-blocking kinetics and strong voltage-dependence (allowing rapid relief of block during synaptic activity), as well as reduced trapping (permitting egress from closed channels). These factors may allow memantine to block channel activity induced by low, tonic levels of glutamate--an action that might contribute to symptomatic improvement and could theoretically protect against weak excitotoxicity--while sparing synaptic responses required for normal behavioral functioning, cognition and memory.

Expression of rat hepatic 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase is affected by a high protein diet and by streptozotocin-induced diabetes

In the tryptophan-niacin conversion, 2-amino-3-carboxymuconate-6-semiardehyde decarboxylase (ACMSD; formerly termed picolinic carboxylase) is an important enzyme regulating the generation of quinolinate. In a series of experiments, we investigated alterations of ACMSD expression in rats by feeding a high protein diet and by inducing diabetes with streptozotocin (STZ). Male Sprague-Dawley rats (5-wk-old) were fed a diet containing 40% casein for 11 d, and hepatic ACMSD activity and mRNA expression were determined at intervals. The enzyme activity had increased at d 2, and it continued to increase through d 11. ACMSD mRNA expression had increased at d 1 and the elevated levels were maintained through d 11. Shifting from the 40% casein diet to a 20% casein diet restored hepatic ACMSD activity and mRNA expression to normal levels within 5 d and 2 d, respectively. In another series of experiments, male Wistar rats were injected with STZ (50 mg/kg) and the time-course (d 0, 1, 2, 4, 8 and 14) of the change in hepatic ACMSD activity and mRNA expression were examined. The activity increased dramatically after d 4, while mRNA expression was significantly elevated at d 2, followed by slight increases through d 14. Insulin administration (2 U/12 h) reduced the elevated ACMSD activity and fully suppressed the elevated ACMSD mRNA expression due to STZ injection. These results indicated that the fluctuation of hepatic ACMSD mRNA expression was followed by that of ACMSD activity.

Purification and molecular cloning of rat 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase

2-Amino-3-carboxymuconate-6-semialdehyde decarboxylase (ACMSD; EC 4.1.1.45) is one of the important enzymes regulating tryptophan-niacin metabolism. In the present study, we purified the enzyme from rat liver and kidney, and cloned the cDNA encoding rat ACMSD. The molecular masses of rat ACMSDs purified from the liver and kidney were both estimated to be 39 kDa by SDS/PAGE. Analysis of N-terminal amino acid sequences showed that these two ACMSDs share the same sequence. An expressed sequence tag (EST) of the mouse cited from the DNA database was found to be identical with this N-terminal sequence. Reverse transcription-PCR (RT-PCR) was performed using synthetic oligonucleotide primers having the partial sequences of the EST, and then cDNAs encoding rat ACMSDs were isolated by using subsequent 3'-rapid amplification of cDNA ends and RT-PCR methods. ACMSD cDNAs isolated from liver and kidney were shown to be identical, consisting of a 1008 bp open reading frame (ORF) encoding 336 amino acid residues with a molecular mass of 38091 Da. The rat ACMSD ORF was inserted into a mammalian expression vector, before transfection into human hepatoma HepG2 cells. The transfected cells expressed ACMSD activity, whereas the enzyme activity was not detected in uninfected parental HepG2 cells. The distribution of ACMSD mRNA expression in various tissues was investigated in the rat by RT-PCR. ACMSD was expressed in the liver and kidney, but not in the other principal organs examined.

Kynurenines in the CNS: from endogenous obscurity to therapeutic importance

In just under 20 years the kynurenine family of compounds has developed from a group of obscure metabolites of the essential amino acid tryptophan into a source of intensive research, with postulated roles for quinolinic acid in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease. One of the kynurenines, kynurenic acid, has become a standard tool for use in the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke. The kynurenines represent a major success in translating a basic discovery into a source of clinical understanding and therapeutic application, with around 3000 papers published on quinolinic acid or kynurenic acid since the discovery of their effects in 1981 and 1982. This review concentrates on some of the recent work most directly relevant to the understanding and applications of kynurenines in medicine.

Animal models of Alzheimer's disease and evaluation of anti-dementia drugs

Alzheimer's disease (AD) is the most common cause of progressive decline of cognitive function in aged humans, and is characterized by the presence of numerous senile plaques and neurofibrillary tangles accompanied by neuronal loss. Some, but not all, of the neuropathological alterations and cognitive impairment in AD can be reproduced genetically and pharmacologically in animals. It should be possible to discover novel drugs that slow the progress or alleviate the clinical symptoms of AD by using these animal models. We review the recent progress in the development of animal models of AD and discuss how to use these model animals to evaluate novel anti-dementia drugs.

Quinolinic acid is increased in CSF and associated with mortality after traumatic brain injury in humans

We tested the hypothesis that quinolinic acid, a tryptophan-derived N-methyl-D-aspartate agonist produced by macrophages and microglia, would be increased in CSF after severe traumatic brain injury (TBI) in humans, and that this increase would be associated with outcome. We also sought to determine whether therapeutic hypothermia reduced CSF quinolinic acid after injury. Samples of CSF (n = 230) were collected from ventricular catheters in 39 patients (16 to 73 years old) during the first week after TBI, (Glasgow Coma Scale [GCS] < 8). As part of an ongoing study, patients were randomized within 6 hours after injury to either hypothermia (32 degrees C) or normothermia (37 degrees C) treatments for 24 hours. Otherwise, patients received standard neurointensive care. Quinolinic acid was measured by mass spectrometry. Univariate and multivariate analyses were used to compare CSF quinolinic acid concentrations with age, gender, GCS, time after injury, mortality, and treatment (hypothermia versus normothermia). Quinolinic acid concentration in CSF increased maximally to 463 +/- 128 nmol/L (mean +/- SEM) at 72 to 83 hours after TBI. Normal values for quinolinic acid concentration in CSF are less than 50 nmol/L. Quinolinic acid concentration was increased 5- to 50-fold in many patients. There was a powerful association between time after TBI and increased quinolinic acid (P < 0.00001), and quinolinic acid was higher in patients who died than in survivors (P = 0.003). Age, gender, GCS, and treatment (32 degrees C versus 37 degrees C) did not correlate with CSF quinolinic acid. These data reveal a large increase in quinolinic acid concentration in CSF after TBI in humans and raise the possibility that this macrophage-derived excitotoxin may contribute to secondary damage.

Neurochemical and metabolic consequences of elevated cerebrospinal fluid quinolinic acid concentrations in rat brain

Quinolinic acid (QUIN) is an endogenous excitatory amino acid, which is elevated in brain tissues or cerebrospinal fluid (CSF) in several acute and chronic inflammatory central nervous system (CNS) diseases. The functional significance of this elevation is unknown but speculations of excitotoxicity have been raised. We have begun to address the pathologic consequences of elevated CSF QUIN by studying the effects of intracerebroventricular (i.cv) administration of QUIN on regional choline acetyltransferase (ChAT) activity, somatostatin content and glucose metabolism in the rat brain. QUIN (12 and 60 nmol) i.cv administration once a day for 7 days (total dose; 84 and 420 nmol, respectively) had minimal effect on somatostatin content and no effect on ChAT activity. In contrast, following continuous i.cv infusion of QUIN for 14 days using an osmotic minipump (480 nmol), ChAT activity dropped in the hippocampus and the striatum and somatostatin content was reduced in the frontal cortex, hippocampus, striatum and amygdala. Moreover, following the QUIN infusion, glucose utilization decreased in the basal nucleus of Meynert, frontal cortex, and portions of the basal ganglia and the limbic system. These results indicate that subchronic i.cv infusion of QUIN to rats results in selective regional neurochemical and metabolic changes distributed throughout the CNS. These results suggest target brain areas and transmitter systems which may be associated with neurologic syndromes characterized by elevated CSF QUIN levels.

Biochemical characteristics of gamma-glutamyl transpeptidase in capillaries from entorhinohippocampal complex of quinolinate-lesioned rat brain

Quinolinic acid (QUIN) is an endogenous excitotoxic agonist of the N-methyl-D-aspartate (NMDA) type of glutamate receptor, which causes slowly progressing degeneration of vulnerable neurons in some brain regions. Using changes in the activity of membrane-bound gamma-glutamyl transpeptidase (GGT) as a marker of cell damage, we found a significant decrease of this enzyme activity, which was preferentially located in the ipsilateral hippocampal formation and entorhinal cortex, 4 d after the unilateral intracerebroventricular (icv) injection of 0.5 mumol QUIN. The dose of QUIN divided into two half-doses injected bilaterally led to a symmetrical decline of GGT activity in hippocampal areas. The lesion was characterized by a suppression of GGT activity in hippocampal and entorhinal capillaries, corresponding to 60 and 81% of their initial value, respectively, but no significant changes were ascertained in synaptosomal membranes. The changes in the activity of capillary GGT were associated with the decrease of apparent maximal velocity Vmaxapp, whereas apparent Michaelis constant K(m)app (0.69-0.79 mM) remained unaffected. In the nonlesioned brain, concanavalin A (Con A) affinity chromatography revealed five glycoforms of synaptosomal GGT in contrast to only one found in hippocampal and entorhinal capillaries. The results document that neither the saccharide moiety of GGT nor the value of enzyme K(m)app is significantly affected by the QUIN-induced lesion of the rat brain. However, the suppression of GGT activity, which is accompanied by a decrease in the value of Vmaxapp in brain microvessels, may suggest dysfunction of the blood-brain barrier (BBB) in the QUIN-injured rat brain.

PCP and ketamine inhibit non-NMDA glutamate receptor mediated hsp70 induction

The physiological model for glutamate receptor mediated excitotoxicity entails elevation of intraneuronal calcium levels. Excessive activation of the NMDA receptor leads to excitotoxicity by prolonged calcium influx via its calcium channel. The purpose of this research was to examine the mechanism of non-NMDA glutamate receptor mediated excitotoxicity. Mammalian AMPA receptors do not show significant calcium conductance. However, some kainate receptors show significant calcium conductance. The hypothesis of this research states that non-NMDA glutamate agonists (quisqualate (5 microliters of 2 mg/ml i.c.v.), AMPA (4 microliters of 1 mg/ml i.c.v.), and kainate (15 mg/kg i.p.)) produce significant heat shock gene, hsp70, induction via glutamate release with subsequent opening of the NMDA receptor calcium channel. PCP (phencyclidine) and ketamine are noncompetitive blockers of the NMDA calcium channel. They act to prevent significant NMDA receptor excitotoxicity. PCP (20 mg/kg i.p.) and ketamine (60 mg/kg i.p.) both diminished quisqualate and AMPA hsp70 induction in the CA1, CA2, CA3 areas of the hippocampus, in the polymorph area of the dentate gyrus, and in the parietal neocortex. PCP significantly (P < 0.05) diminished kainate hsp70 induction only in the CA1 area and the neocortex. Ketamine failed to reduce kainate hsp70 induction. AMPA receptors appear to result in excitotoxic damage via glutamate release. Glutamate opens NMDA receptor calcium channels which increases intraneuronal calcium levels. Kainate receptors probably mediate excitotoxicity via direct calcium conductance with glutamate release being important in the CA1 area and neocortex.

Subchronic intraventricular infusion of quinolinic acid produces working memory impairment--a model of progressive excitotoxicity

It has been proposed by Yamada et al. [Neurosci. Lett. 118: 128-131 (1990); J. Pharmacobiodyn. 14: 351-355 (1991)] that subchronic i.c.v. infusion of the NMDA receptor agonist quinolinic acid may serve as a model for some aspects of neurodegenerative dementia. In the present study, quinolinic acid (9 mM) was infused i.c.v. by ALZET osmotic minipumps for 2 weeks. This treatment produced a short-term working memory deficit in the T-maze (alternation) but no change in reversal learning in the same test. The working memory deficit in the T-maze was progressive i.e. seen after 14, but not 3 days of infusion and persisted for at least for 3 weeks after the termination of the infusion. Histological examination revealed a modest decrease in the number of cells in the nucleus basalis magnocellularis but not in the striatum, entorhinal cortex, or hippocampus. However, in most of the structures studied, morphological changes such as swollen somata and irregular shape were observed indicative of alterations in neuronal function. Autoradiography in the hippocampus revealed a decrease in [3H]hemicholinium and [3H]quinuclidinyl benzilate (QNB) binding to choline uptake sites and muscarinic receptors respectively. Surprisingly no change was observed in [3H]MK-801 binding to NMDA receptor channels in the hippocampus and cortex. The subchronic infusion of quinolinic acid may serve as a model of progressive deterioration of cognitive functions.

A continuous striatal infusion of 6-hydroxydopamine produces a terminal axotomy and delayed behavioral effects

Rat models of Parkinson's disease typically employ a rapid nigral injection of 6-hydroxydopamine (6-OHDA) to produce a near-complete loss of nigrostriatal dopamine neurons, and thus, model end stage disease. The present report describes the use of a continuous, low dose infusion of 6-OHDA into the striatum which produces a terminal axotomy of nigrostriatal dopamine neurons and protracted behavioral response. A solution of 6-OHDA in 0.4% ascorbate, delivered at 37 degrees C from osmotic minipumps, was stable for 8 days as determined by its retained toxicity to a dopaminergic neuroblastoma cell line. The continuous infusion of 0.2 mu g 6-OHDA per h did not affect the striatal uptake of [3H]%GABA, [3H]choline, or [3H]glutamate but reduced [3H]dopamine uptake by 55% within 1.5 days after the start of the infusion. The striatal infusion of 6-OHDA produced a dose-dependent reduction of striatal dopamine and DOPAC levels but did not alter HVA, 5-HT, or 5-HIAA. An increase in amphetamine-induced ipsiversive rotations occurred within 1.5 days after the acute striatal injection of 20 mu g or 30 mu g of 6-OHDA but required 4 days to develop with the continuous 6-OHDA infusion. The topography of the lesion mapped by [3H]mazindol binding showed that, beginning by 1.5 days, a diffuse depletion of terminals encompassed much of the striatum in the 30 mu g acute injection group, whereas in the continuously infused rats, the lesion was apparent only by 4 days and was restricted to a smaller and more completely lesioned area. Unlike acutely lesioned animals, continuously infused rats revealed no obvious loss of dopamine neurons in the pars compacta by 5 weeks after 6-OHDA. The continuous striatal infusion of 6-OHDA can produce a topographically limited terminal axotomy of dopamine neurons and a protracted behavioral impairment.

Learning deficits induced by chronic intraventricular infusion of quinolinic acid--protection by MK-801 and memantine

The NMDA receptor agonist quinolinic acid (9 mM) was infused i.c.v. via ALZET osmotic minipumps for 2 weeks. This treatment produced a persistent, short-term memory deficit in the T-maze. Autoradiography revealed a decrease in the density of choline uptake sites in the hippocampus. Parallel s.c. infusion by another minipump of the uncompetitive NMDA receptor antagonist memantine (1-amino-3,5-dimethyladamantane, 20 mg/kg per day) or (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate ((+)-MK-801, 0.31 mg/kg day) prevented the learning deterioration induced by quinolinic acid. The treatment with memantine resulted in steady-state serum levels of 1.2 mu M which, based on in vitro data, should assure inhibition of NMDA receptors and are similar to levels seen in the serum of demented patients treated with this agent. In naive animals this treatment had no effect on either learning or on ex vivo induction of long-term potentiation, indicating that under chronic conditions it is possible to obtain neuroprotective effects with NMDA receptor antagonists without negative effects on memory processes. This contrasts to some acute insults (e.g. ischaemia) where high doses of NMDA receptor antagonists that produce side effects are required.

Possible involvement of nitric oxide in quinolinic acid-induced convulsion in mice

Quinolinic acid (QA) induced clonic and tonic convulsions in mice when it was injected into the cerebral ventricle. Pretreatment with L-arginine (L-Arg), a substrate of nitric oxide (NO) synthase (NOS), and/or 5,6,7,8-tetrahydrobiopterin (THB), cofactor of NOS, tended to potentiate QA-induced convulsion. NG-monomethyl-L-arginine (NMMA), a competitive NOS inhibitor, diminished QA-induced convulsion. This effect of NMMA was attenuated by coadministration of L-Arg or THB. Sodium nitroprusside (SNP), which spontaneously releases NO, did not potentiate, but diminished QA-induced convulsion. These findings suggest that an endogenous NO may be involved, at least in part, in QA-induced convulsion in mice, and that an exogenous NO released from SNP may cause downregulation of N-methyl-D-aspartate (NMDA) receptor activity, and thereby prevent the excessive excitation of NMDA receptors and subsequent convulsion caused by QA.

Omega-conotoxin GVIA inhibits the methylphenidate-induced but not methamphetamine-induced behavior

We investigated the effects of antagonists for omega-conotoxin GVIA (omega-CTX)-sensitive N-type voltage-sensitive calcium channels (N-channels) on methylphenidate- and methamphetamine-induced behavior. I.c.v. injection of omega-CTX or neomycin, both N-channel antagonists, caused a dose-dependent inhibition of methylphenidate-induced hypermotility in mice but failed to inhibit methamphetamine-induced hyperactivity. Further, omega-CTX inhibited the circling behavior induced by methylphenidate in rats that had kainic acid-induced unilateral striatal lesions. These results suggest that calcium influx through omega-CTX-sensitive N-channels plays an important role in methylphenidate-induced behavior.

Neuropharmacological characterization of voltage-sensitive calcium channels: possible existence of neomycin-sensitive, omega-conotoxin GVIA- and dihydropyridines-resistant calcium channels in the rat brain

We attempted to characterize the functional roles of subtypes of voltage-sensitive calcium channels in the brain. The maximal number of [125I]omega-conotoxin GVIA (omega-CTX) binding sites in rat brain associated with N-type calcium channels (N-channels) was approximately 10 times more than that of [3H]-PN200-110 associated with L-type calcium channels (L-channels). [125I]omega-CTX binding was inhibited by aminoglycoside antibiotics, neomycin and dynorphin A(1-13), but not by various classes of L-channel antagonists. A 6-hydroxydopamine-induced lesion of the striatum resulted in a marked reduction of both [125I]-omega-CTX and [3H]PN200-110 binding. Kainic acid-induced lesion of the striatum reduced [3H]PN200-110 binding by 57%, but did not reduce [125I]omega-CTX binding. Omega-CTX produced a small (18%) but significant reduction of potassium-stimulated Ca2+ influx into rat brain synaptosomes, although it produced a concentration-dependent inhibition in chick brain synaptosomes. Neomycin inhibited Ca2+ influx in both preparations in a concentration-dependent manner. Both omega-CTX and neomycin inhibited potassium-stimulated [3H]dopamine (DA) release from rat striatal slices. The L-channel antagonists had no effect on either Ca2+ influx or [3H]DA release. These results suggest that DA release in the striatum is regulated by Ca2+ influx through N-channels located in presynaptic nerve terminals, and that the most of the Ca2+ influx in rat brain appears to be governed by neomycin-sensitive, omega-CTX- and DHP-resistant calcium channels.

High-affinity choline transport sites: use of [3H]hemicholinium-3 as a quantitative marker

High-affinity choline transport (HAChT), the rate-limiting and regulatory step in acetylcholine (ACh) synthesis, is selectively localized to cholinergic neurons. Hemicholinium-3 (HC3), a potent and selective inhibitor of HAChT, has been used as a specific radioligand to quantify HAChT sites in membrane binding and autoradiographic studies. Because both HAChT velocity and [3H]HC3 binding change as in vivo activity of cholinergic neurons is altered, these markers are also useful measures of cholinergic neuronal activity. Evidence that [3H]HC3 is a specific ligand for HAChT sites on cholinergic terminals is reviewed. The ion requirements of HAChT and [3H]HC3 binding indicate that sodium and chloride are required for recognition of both choline and [3H]HC3. A common recognition site is also indicated by the close correspondence of the potency of HC3 and choline analogues for inhibiting both HAChT and [3H]HC3 binding. The parallel regional distributions of both markers in adult brain, during development and after specific lesions, all indicate specific cholinergic localization. The close association of HAChT and [3H]HC3 binding sites is also supported by parallel regulatory changes occurring after in vivo drug treatments and in vitro depolarization. Overall, the data indicate a close association between HAChT and [3H]HC3 binding and are consistent with the sites being identical. Methodologic considerations in using [3H]HC3 as a ligand and considerations in interpretation of results are also discussed.