Effect of sodium, hemicholinium-3 and antiparkinson drugs on (14C)acetylcholine synthesis and (3H)choline uptake in rat striatal synaptosomes

Molecular properties of the high-affinity choline transporter CHT1

This article summarizes molecular properties of the high-affinity choline transporter (CHT1) with reference to the historical background focusing studies performed in laboratories of the author. CHT1 is present on the presynaptic terminal of cholinergic neurons, and takes up choline which is the precursor of acetylcholine. The Na(+)-dependent uptake of choline by CHT1 is the rate-limiting step for synthesis of acetylcholine. CHT1 is the integral membrane protein with 13 transmembrane segments, belongs to the Na(+)/glucose co-transporter family (SLC5), and has 20-25% homology with members of this family. A single nucleotide polymorphism (SNP) for human CHT1 has been identified, which has a replacement from isoleucine to valine in the third transmembrane segment and shows the choline uptake activity of 50-60% as much as that of wild-type CHT1. The proportion of this SNP is high among Asians. Possible importance of choline diet for those with this SNP was discussed.

Transgenic overexpression of the presynaptic choline transporter elevates acetylcholine levels and augments motor endurance

The hemicholinium-3 (HC-3) sensitive, high-affinity choline transporter (CHT) sustains cholinergic signaling via the presynaptic uptake of choline derived from dietary sources or from acetylcholinesterase (AChE)-mediated hydrolysis of acetylcholine (ACh). Loss of cholinergic signaling capacity is associated with cognitive and motor deficits in humans and in animal models. Whereas genetic elimination of CHT has revealed the critical nature of CHT in maintaining ACh stores and sustaining cholinergic signaling, the consequences of elevating CHT expression have yet to be studied. Using bacterial artificial chromosome (BAC)-mediated transgenic methods, we generated mice with integrated additional copies of the mouse Slc5a7 gene. BAC-CHT mice are viable, appear to develop normally, and breed at wild-type (WT) rates. Biochemical studies revealed a 2 to 3-fold elevation in CHT protein levels in the CNS and periphery, paralleled by significant increases in [(3)H]HC-3 binding and synaptosomal choline transport activity. Elevations of ACh in the BAC-CHT mice occurred without compensatory changes in the activity of either choline acetyltransferase (ChAT) or AChE. Immunohistochemistry for CHT in BAC-CHT brain sections revealed markedly elevated CHT expression in the cell bodies of cholinergic neurons and in axons projecting to regions known to receive cholinergic innervation. Behaviorally, BAC-CHT mice exhibited diminished fatigue and increased speeds on the treadmill test without evidence of increased strength. Finally, BAC-CHT mice displayed elevated horizontal activity in the open field test, diminished spontaneous alteration in the Y-maze, and reduced time in the open arms of the elevated plus maze. Together, these studies provide biochemical, pharmacological and behavioral evidence that CHT protein expression and activity can be elevated beyond that seen in wild-type animals. BAC-CHT mice thus represent a novel tool to examine both the positive and negative impact of constitutively elevated cholinergic signaling capacity.

Ultrastructural localization of high-affinity choline transporter in the rat anteroventral thalamus and ventral tegmental area: differences in axon morphology and transporter distribution

The high-affinity choline transporter (CHT) is a protein integral to the function of cholinergic neurons in the central nervous system (CNS). We examined the ultrastructural distribution of CHT in axonal arborizations of the mesopontine tegmental cholinergic neurons, a cell group in which CHT expression has yet to be characterized at the electron microscopic level. By using silver-enhanced immunogold detection, we compared the morphological characteristics of CHT-immunoreactive axon varicosities specifically within the anteroventral thalamus (AVN) and the ventral tegmental area (VTA). We found that CHT-immunoreactive axon varicosities in the AVN displayed a smaller cross-sectional area and a lower frequency of synapse formation and dense-cored vesicle content than CHT-labeled profiles in the VTA. We further examined the subcellular distribution of CHT and observed that immunoreactivity for this protein was predominantly localized to synaptic vesicles and minimally to the plasma membrane of axons in both regions. This pattern is consistent with the subcellular distribution of CHT displayed in other cholinergic systems. Axons in the AVN showed significantly higher levels of CHT immunoreactivity than those in the VTA and correspondingly displayed a higher level of membrane CHT labeling. These novel findings have important implications for elucidating regional differences in cholinergic signaling within the thalamic and brainstem targets of the mesopontine cholinergic system.

Reduced expression and capacity of the striatal high-affinity choline transporter in hyperdopaminergic mice

Behavioral and neuronal abnormalities observed in mice exhibiting a reduced expression of the dopamine transporter model important aspects of schizophrenia, addiction, and attentional disorders. As the consequences of a chronic hyperdopaminergic tone for striatal output regulation have remained poorly understood, the present experiments were designed to determine the status of striatal interneuronal cholinergic neurotransmission in dopamine transporter knockdown animals. The high-affinity choline transporter represents the rate-limiting step of acetylcholine synthesis and release. Compared with wild type mice, striatal high-affinity choline transporter expression in dopamine transporter knockdown mice was significantly decreased. As in vivo basal striatal acetylcholine release did not differ between the strains, reduced high-affinity choline transporter expression in dopamine transporter knockdown mice was not due to reduced basal cholinergic activity. Furthermore, the proportion of high-affinity choline transporters expressed in plasma membrane-enriched versus vesicular membrane-enriched fractions did not differ from wild type animals, suggesting that changes in intracellular high-affinity choline transporter trafficking were not associated with lower overall levels of striatal high-affinity choline transporters. Synaptosomal choline uptake assays indicated a reduced capacity of striatal high-affinity choline transporters in dopamine transporter knockdown mice, and thus the functional significance of the reduced level of high-affinity choline transporter expression. Likewise, in vivo measures of the capacity of striatal high-affinity choline transporters to clear increases in extracellular choline concentrations, using choline-sensitive microelectrodes, revealed a 37-41% reduction in hemicholinium-sensitive clearance of exogenous choline in dopamine transporter knockdown mice. Furthermore, clearance of potassium-evoked choline signals was reduced in dopamine transporter knockdown mice (1.63+/-0.15 microM/s) compared with wild type animals (2.29+/-0.21 microM/s). Dysregulated striatal cholinergic neurotransmission is hypothesized to disrupt the integration of thalamic and cortical information at spiny projection neurons and thus to contribute to abnormal striatal information processing in dopamine transporter knockdown mice.

Cortical choline transporter function measured in vivo using choline-sensitive microelectrodes: clearance of endogenous and exogenous choline and effects of removal of cholinergic terminals

The capacity of the high-affinity choline transporter (CHT) to import choline into presynaptic terminals is essential for acetylcholine synthesis. Ceramic-based microelectrodes, coated at recording sites with choline oxidase to detect extracellular choline concentration changes, were attached to multibarrel glass micropipettes and implanted into the rat frontoparietal cortex. Pressure ejections of hemicholinium-3 (HC-3), a selective CHT blocker, dose-dependently reduced the uptake rate of exogenous choline as well as that of choline generated in response to terminal depolarization. Following the removal of CHTs, choline signal recordings confirmed that the demonstration of potassium-induced choline signals and HC-3-induced decreases in choline clearance require the presence of cholinergic terminals. The results obtained from lesioned animals also confirmed the selectivity of the effects of HC-3 on choline clearance in intact animals. Residual cortical choline clearance correlated significantly with CHT-immunoreactivity in lesioned and intact animals. Finally, synaptosomal choline uptake assays were conducted under conditions reflecting in vivo basal extracellular choline concentrations. Results from these assays confirmed the capacity of CHTs measured in vivo and indicated that diffusion of substrate away from the electrode did not confound the in vivo findings. Collectively, these results indicate that increases in extracellular choline concentrations, irrespective of source, are rapidly cleared by CHTs.

Effects of AF64A on gene expression of choline acetyltransferase (ChAT) in the septo-hippocampal pathway and striatum in vivo

AF64A (ethylcholine mustard aziridinium ion) was stereotaxically administered bilaterally (1 nmol/side) into rat lateral cerebral ventricles. Choline acetyltransferase (ChAT) activity and ChAT mRNA levels were measured at predetermined time points in the septo-hippocampal pathway and striatum, both well identified as rich in cholinergic neurons. AF64A caused a rapid but transient increase in ChAT mRNA (167%, P < 0.05) and ChAT activity (164%, P < 0.01) in the septum. By day 7 post treatment, there was a significant decrease in ChAT mRNA (42.5% of control, P < 0.05) in the septum although the ChAT activity still stayed high. This decreased ChAT mRNA level in the septum lasted for at least four weeks, and was paralleled by a long-lasting decrease in ChAT activity in the hippocampus. In the striatum, on the other hand, there were no observed changes in either ChAT activity or ChAT mRNA. These data suggest that the long term effect of AF64A on the septo-hippocampal cholinergic pathway may, at least in part, be due to an action of AF64A on gene expression in the cholinergic neuron. The difference in the response to AF64A between the septo-hippocampal and striatal cholinergic systems might be due to their difference in neuron types.

Intra-retrosplenial cortical grafts of cholinergic neurons: functional incorporation and restoration of high affinity choline uptake

Fetal septal neurons transplanted into the deafferented retrosplenial cortex (RSC) of rats have been shown to reinnervate the host brain and ameliorate spatial memory deficits. In the present study we examined the effects of implanting cholinergic neurons on high affinity choline uptake (HACU) in the denervated RSC and the correlational relationship between this cholinergic parameter and the level of behavioral recovery. Three groups of animals were used: 1) normal control rats (NC), 2) rats with lesions of the fornix and cingulate pathways (FX), and 3) lesioned rats with fetal septal grafts in the RSC (RSCsep-TPL). We found that intra-RSC septal grafts produced significant increases in HACU, and that recovery of HACU was significantly correlated with the improvements in the performance of spatial reference memory, spatial navigation, and spatial working memory tasks. We have also investigated the ability of the host brain to modulate the activity of the implanted neurons. In particular we evaluated the effect of the animals' performance in a 6-arm radial maze task on high affinity choline uptake (HACU). Animals in each of the NC, FX, and RSCsep-TPL groups were randomly assigned one of the following subgroups: 1) rats that performed the maze task before the determination of HACU (BEH), or 2) rats that did not perform the maze task before the determination of HACU (NON-BEH). Significant increases were observed in the NC and RSCsep-TPL groups, but not in the FX animals, indicating that fetal septal grafts in the RSC can become functionally incorporated with the host neural circuitry, and that the activity of the implanted cholinergic neurons can be modulated by the host brain.

Neural grafting of cholinergic neurons in the hippocampal formation

The cholinergic septohippocampal system plays an important role in spatial learning and memory functions. Transections of the septohippocampal pathway have been shown to result in a near complete loss of cholinergic innervation in the hippocampus and induce severe spatial memory impairments. In this article, we have reviewed the studies which demonstrate the ability of intrahippocampal septal grafts to reinnervate the hippocampal formation and ameliorate spatial learning and memory deficits. Neuroanatomical studies suggest that grafts of cholinergic tissue can innervate the host hippocampal formation in a pattern that mimics that of the normal septohippocampal pathway. This innervation, in turn, is associated with the formation of graft-to-host synaptic connections. Neurochemical studies reveal that intrahippocampal grafts of septal cells can restore choline acetyltransferase activity, acetylcholine synthesis, and high affinity choline uptake in presynaptic terminals of grafted neurons. In addition, these grafts can normalize the upregulation of cholinergic muscarinic receptors seen postsynaptically in the hippocampus following lesions of the septohippocampal pathway. The functional nature of these grafts is also substantiated by electrophysiological recordings which demonstrate stimulus-evoked graft-to-host synaptic transmission as well as the reinstatement of EEG activity typical of septohippocampal connectivity. In addition to graft-to-host connections, behavioral and neurochemical studies also provide evidence for host-to-graft connections that can regulate the activity of grafted cholinergic neurons during the performance of specific behavioral tasks requiring spatial memory function. Together, these studies suggest that grafts of cholinergic neurons from the medial septal nucleus can become anatomically and functionally incorporated into the circuitry of the host hippocampal formation.

Selective cortical decrease of high-affinity choline uptake carrier in Alzheimer's disease: an autoradiographic study using 3H-hemicholinium-3

3H-hemicholinium-3 (3H-HC-3) binding, a marker of the presynaptic high-affinity choline uptake carrier (HACU), was measured by autoradiography in several brain regions of 17 Alzheimer's disease (AD) patients and of 11 matched controls. A significant decrease in the density of 3H-HC-3 binding sites was found in entorhinal cortex, hippocampus and layers I-III of the frontal cortex. By contrast, in the caudate-putamen the number of 3H-HC-3 binding sites in AD cases was comparable to that of control striata. These data concur with previous results using classical presynaptic markers and reflect the loss in the activity of HACU, and, hence, in the synthesis of acetylcholine, that selectively occurs in cortical areas of AD brains due to the degeneration of presynaptic cholinergic terminals arising from the basal forebrain. However, the relatively low mean reduction in HACU in cortical areas (-40%), together with the apparent indemnity of this marker in certain severely demented AD cases, suggest that AD dementia cannot be explained simply by the loss of presynaptic terminals originating in the basal forebrain. These data seem to be a good explanation for the poor response to cholinergic replacement in AD.

A selective lesion of striatonigral neurons decreases presynaptic binding of [3H]hemicholinium-3 to striatal interneurons

We have used the suicide transport agent, volkensin, to produce selective lesions of striatal efferent neurons projecting to the substantia nigra in the rat. In order to evaluate potential trans-synaptic effects, we examined cholinergic interneurons intrinsic to the striatum following destruction of striatonigral projection neurons by nigral injection of volkensin. There was no change in the number of large interneurons identified either by Nissl stain or by immunocytochemistry for choline acetyltransferase, indicating that volkensin was not directly toxic to this group of neurons. However, [3H]hemicholinium-3 binding to the choline re-uptake site on the presynaptic cholinergic terminals decreased. No change in [3H]hemicholinium-3 binding was seen after destruction of dopaminergic afferents with 6-hydroxydopamine. Striatonigral afferents to the cholinergic interneurons contain substance P which has been shown to stimulate acetylcholine release. The decrease in [3H]hemicholinium-3 binding may reflect loss of this afferent input. However, striatonigral neurons are an efferent target of the cholinergic interneuron as well, and a presynaptic effect due to loss of target neurons also may contribute.

Further studies on the biochemical characterization and autoradiographic distribution of [3H]hemicholinium-3 binding sites in rat brain: a presynaptic cholinergic marker

Hemicholinium-3 (HC-3) is a potent inhibitor of the high-affinity choline uptake system (HACU). Here we report on the biochemical characterization and autoradiographic distribution of [3H]hemicholinium-3 binding sites in rat brain, confirming and expanding results from previous studies. The binding of [3H]HC-3 to striatal membranes was specific, to a single site, sodium-dependent, saturable, and of high-affinity, Kd values being about 3 nM for striatum, 5 nM for the hippocampus and 12 nM for neocortex. [3H]HC-3 specific binding exhibited a pharmacological profile suggestive of physiologically relevant interactions and fully comparable to that reported for HACU. The uneven distribution of [3H]HC-3 binding sites exhibited a high degree of correspondence with the reported distribution of HACU and other enzymatic presynpatic cholinergic markers. The punctual differences between our study and previous works on [3H]HC-3 binding are analysed. We conclude that [3H]HC-3 labelling may be used as a selective and quantifiable marker of the cholinergic presynaptic terminals in close relationship with HACU.

High-affinity choline uptake carrier in Alzheimer's disease: implications for the cholinergic hypothesis of dementia

We examined the density and the state of affinity of [3H]hemicholinium-3 ([3H]HC-3) binding sites, a marker of the presynaptic high-affinity choline uptake (HACU) carrier, in 4 representative regions of 13 postmortem Alzheimer's disease (AD) brains, as well as in 12 matched control brains. Significant reductions in the densities of [3H]HC-3 binding sites were found both in frontal cortex (-44.7%) and hippocampus (-36.5%) of AD brains in comparison to controls. On the other hand the densities of [3H]HC-3 binding sites in AD brains in caudate-putamen and cerebellar cortex showed no significant differences when compared to controls. No significant change in the state of affinity of these sites could be observed in the saturation assays carried out in hippocampus and frontal cortex. Our findings concur with the reported data by using other presynaptic cholinergic markers in AD and confirm that some degree of cholinergic degeneration, highly specific for the basal forebrain neurons, occurs in AD. However, these results, obtained in a group of AD brains belonging to severely demented patients, do not show a dramatic loss of the HACU in many AD brains. Although this fact could be due to the existence of a compensatory mechanism, our results probably suggest that dementia in AD cannot be explained only by the loss of neocortical cholinergic presynaptic terminals arising from the basal forebrain and also may clarify as to why the acetylcholine precursors or the muscarinic agonists are not effective in AD dementia.

Quantitative immunohistochemical distribution of choline acetyltransferase in the rostral forebrain of the rat

The immunohistochemical distribution of choline acetyltransferase (CAT) in the rat rostral forebrain was analyzed quantitatively and minutely by means of a microphotometry system. The CAT concentrations varied greatly depending on the brain region. Within the neostriatum, CAT tended to be distributed with a lateral (high) to medial (low) gradient of approximately 1.2:1 and a caudal (high) to rostral (low) gradient of approximately 1.4:1, with the highest level in the medius lateralis. In the cortex cerebri, the CAT concentration in the area cinguli was high, while those in the area frontalis, area parietalis and area pyriformis were relatively low. High levels of CAT were also localized in other regions: e.g., hippocampus pars posterior, nucleus preopticus, nucleus anterior hypothalami, nucleus interstitialis striae terminalis, nucleus suprachiasmaticus and nucleus accumbens septi. The quantitative data obtained from the present microphotometric examination can be useful for analysis of a dynamic aspect of neurochemical substances under physiological as well as pathological conditions of the brain.

Choline uptake in cholinergic nodose cell bodies

Isolated living cell bodies were obtained by mechanical and enzymatic dissociation from adult rabbit nodose ganglion followed by separation of fibres and cells using a Percoll gradient. A purification yield of 45% was measured. Based on previous results obtained in whole ganglion and showing the presence of cholinergic cell bodies among the afferent fibres of the vagus nerve, this preparation was used to study choline uptake by neuron cell somata. Cholinergic cells counted after choline acetyltransferase immunohistological staining showed a stained population of 2.9% among the isolated population. Two [3H]choline uptake mechanisms were detected at the cell body level. The first, with Km1 = 7 microM and Vm1 = 200 pmol/h per ganglion is sodium dependent, related to acetylcholine synthesis (43%) and has an IC50 with hemicholinium-3 equal to 50 microM. The second, with Km2 = 54 microM and Vm2 = 2235 pmol/h per ganglion is sodium independent, poorly associated to acetylcholine synthesis (12%) and exhibits an IC50 of 2 microM with hemicholinium-3. Except for their sensitivity to hemicholinium-3, the high and low affinity choline uptake mechanisms observed at the somatic level have, respectively, the same characteristics as the high and low affinity mechanisms described at the synaptic level. Their physiological role, their opposed sensitivity to hemicholinium-3 compared to the synaptic uptake systems and the relation between the somatic high affinity choline transport and an acetylcholine somatic release are discussed.

Characterization of [3H]hemicholinium-3 binding sites in human brain membranes: a marker for presynaptic cholinergic nerve terminals

We report here on the binding properties of [3H]hemicholinium-3, a selective inhibitor of the high-affinity choline uptake process, to human brain membranes. Under the assay conditions described, the binding of [3H]hemicholinium-3 exhibited a dependency of physiological conditions on pH, temperature, and NaCl concentrations. Striatal binding proved to be specific, to a single site, saturable, and reversible, with an apparent KD of 10 nM and a Bmax of 82 fmol/mg of protein. [3H]Hemicholinium-3 specific binding exhibited a pharmacological profile and an ionic dependency suggestive of physiologically relevant interactions and comparable with those reported for the high-affinity choline uptake. Moreover, specific [3H]hemicholinium-3 binding exhibited an uneven regional distribution: striatum much greater than nucleus basalis greater than spinal cord much greater than midbrain = cerebellum greater than or equal to hippocampus greater than neocortex = anterior thalamus greater than posterior thalamus much much greater than white matter. This distribution closely corresponds to the reported activity of both enzymatic cholinergic presynaptic markers and high-affinity choline uptake in mammalian brain. There are no significant differences between these results and those previously found in the rat brain using this radioligand. Our results demonstrate, for the first time, the presence of [3H]hemicholinium-3 binding sites in human brain and strongly support the proposal that this radioligand binds to the carrier site mediating the high-affinity choline uptake process on cholinergic neurons. Thus, [3H]hemicholinium-3 binding may be used in postmortem human brain as a selective and quantifiable marker of the presynaptic cholinergic terminals.

Autoradiographic distribution of [3H]hemicholinium-3 binding sites in human brain

Since previous radioligand binding studies support the evidence that [3H]hemicholinium-3 ([3H]HC-3) selectively labels the high-affinity choline uptake (HACU) process, we have studied the autoradiographic characteristics and regional distribution of [3H]HC-3 binding to post mortem human brain tissue. [3H]HC-3 specific binding was saturable, of high affinity and exhibited an uneven distribution. High densities were observed in caudate-putamen, nucleus basalis accesorius of the amygdala, hippocampal gyrus dentatus and CA3 field, locus niger, nucleus interpeduncularis and motor trigeminal and facial nuclei. Low densities were found in areas such as neocortex, thalamus, hypothalamus or cerebellum. Our results agree with those obtained in human brain membranes and are comparable to previous autoradiographic data from rat brain. Remarkably, the distribution of [3H]HC-3 binding sites closely corresponds with that of cholinergic enzymatic presynaptic markers and HACU. These findings, together with previous data from membrane studies, allow the use of [3H]HC-3 as a selective anatomical marker of cholinergic presynaptic terminals.

Effects of soman and sarin on high affinity choline uptake by rat brain synaptosomes

Synaptosomes were incubated at various time intervals following injection of 120 micrograms/kg SC of soman or sarin or with various concentrations (10(-8) to 10(-2) M) of soman or sarin in vitro. Total cholinesterase (ChE) activities in each brain region were also measured. Following soman injection, sodium-dependent, high affinity choline uptake (SDHACU) was decreased from 1 to 4 hr in the cortex and from 1 to 2 hr in the hippocampus, but increased from 2 to 24 hr in the striatum. Similarly, following sarin injection SDHACU was decreased at 0.5 hr in the cortex and from 1 to 4 hr in the hippocampus, but increased at 1 hr in the striatum. Injection of soman severely inhibited (83-99%) total ChE activity in the cortex, hippocampus and striatum from 1 to 24 hr. In contrast, sarin did not severely inhibit ChE activity in these regions and maximal inhibition (40-60%) did not occur until 24 hr after injection. With both compounds, by 168 hr ChE activity in all regions had partially recovered. Incubation of synaptosomes with soman or sarin in vitro at concentrations below 10(-4) M did not affect SDHACU in any of the brain regions. These data demonstrated that acute soman and sarin injection produced similar effects upon SDHACU in different brain regions, although the time-course of these effects was different for the two compounds. These effects were probably neither due to a direct action of these compounds on the uptake process nor dependent on ChE inhibition.

Hemicholinium-3 binding sites in rat brain: a quantitative autoradiographic study

Quantitative in vitro autoradiography was used to study the distribution of [3H]hemicholinium-3 ([3H]HC-3) binding sites in the rat brain. Regional concentrations of HC-3 binding sites were corrected for regional tissue quenching of tritium in a number of brain structures. Specific binding of 10 nM [3H]HC-3 was highest in the interpeduncular nucleus, followed by the caudate-putamen, olfactory tubercle, amygdala, and the medial and lateral habenulae. There was a high positive correlation between regional HC-3 binding and choline acetyltransferase activity in rat brain; however, a novel pattern of the distribution of cholinergic terminals in the subnuclei of the interpeduncular nucleus was discovered. The apparent Kd in the 1-5 nM range and the pharmacological specificity of the HC-3 binding site agreed with data for choline uptake and for the HC-3 binding site as determined in membrane preparations. HC-3 autoradiography appears to be a useful anatomical marker for cholinergic terminals.

Acetylcholine in the posterior hypothalamic nucleus is involved in the elevated blood pressure in the spontaneously hypertensive rat

Intravenous injection of physostigmine, 40 and 80 ug/kg, in unanesthetized normotensive rats increased systolic blood pressure (SBP) by 21 +/- 3 and 42 +/- 7 mmHg. This pressor response was 80% inhibited by intracerebroventricular (icv) injection of hemicholinium-3 (HC-3), 20 ug. Simultaneous icv injection of HC-3 and choline (365 ug) prevented the inhibition of the pressor response by HC-3. In spontaneously hypertensive rats, injection of HC-3 either icv (20 ug) or bilaterally into the posterior hypothalamic nuclei (1 ug) decreased SBP by about 40 mmHg. The effect of intrahypothalamic HC-3 was completely blocked by simultaneous injection of choline (24.3 ug) into the same site. The hypotensive effect of icv HC-3 was completely blocked by icv choline (243 ug) and was inhibited up to 60% by injections of choline (24.3 ug) into the posterior hypothalamic nuclei.

Hemipalmitoylcarnitinium, a strong competitive inhibitor of purified hepatic carnitine palmitoyltransferase

We have synthesized (2S,6R:2R,6S)-6-carboxymethyl-2-hydroxy-2-pentadecyl-4,4-dimethylmorp holinium bromide (hemipalmitoylcarnitinium, HPC) which is a conformationally restricted analog inhibitor of carnitine palmitoyltransferase (CPT; EC 2.3.1.21). rac-HPC inhibits catalytic activity in purified rat liver CPT. In the forward reaction, HPC competes with both (R)-carnitine (Ki(app) = 5.1 +/- 0.7 microM) and palmitoyl-CoA (Ki(app) = 21.5 +/- 4.9 microM). In the reverse reaction, inhibition by HPC is competitive with palmitoyl-(R)-carnitine (Ki(app) = 1.6 +/- 0.6 microM), but inhibition is uncompetitive with CoA. The forward reaction is also competitively inhibited by its product, palmitoyl-(R)-carnitine, Ki(app)'s 14.2 +/- 2.1 microM relative to (R)-carnitine and 8.7 +/- 2.6 microM relative to palmitoyl-CoA. rac-HPC is the most potent synthetic reversible inhibitor of purified CPT. HPC fails to inhibit carnitine acetyltransferase (CAT; EC 2.3.1.7). Palmitoylcholine also inhibits CPT in the forward reaction, competing with (R)-carnitine (Ki(app) = 18.6 +/- 4.5 microM) and with palmitoyl CoA (Ki(app) = 10.4 +/- 2.5 microM). Choline is not an effective CPT inhibitor. We have shown [R.D. Gandour et al. (1986) Biochem. Biophys. Res. Commun. 138, 735-741] that hemiacetylcarnitinium inhibits CAT but not CPT. The combined data demonstrate further differences between the carnitine recognition sites in CPT and CAT.

Cholinergic neurons in the rat nodose ganglia

Presence of acetylcholine (ACh) in the vagal afferent fibres of the rat was investigated. In the nodose ganglion, which contains the cell bodies of this sensitive contingent, a choline acetyltransferase (ChAT) activity, a choline (Ch) uptake and an endogenous content of acetylcholine were detected. These data were confirmed by ChAT immunohistological visualization.

Active-site probes of carnitine acyltransferases. Inhibition of carnitine acetyltransferase by hemiacetylcarnitinium, a reaction intermediate analogue

Hemiacetylcarnitinium (2S,6R:2R,65)-6-carboxymethyl-2-hydroxy-2,4,4- trimethylmorpholinium) chloride is a relatively potent competitive inhibitor (Ki = 0.89 mM) of pigeon breast carnitine acetyltransferase (CAT) and of the crude rat liver CAT (Ki = 4.72 mM) but is neither an inhibitor nor an effective substrate for purified rat liver carnitine palmitoyltransferase (CPT). It does not inhibit state 3 oxygen consumption in isolated hepatic mitochondria using palmitoyl-CoA or palmitoylcarnitine as substrates. This compound is a reaction intermediate analogue of the proposed tetrahedral intermediate for acetyl transfer between acetylcarnitine and CoASH. Because the hemiketal carbon is chiral, a suggestion is made that one of the enantiomers has the same relative configuration as the proposed tetrahedral intermediate.

Characterization of lithium potentiation of pilocarpine-induced status epilepticus in rats

Subcutaneous administration of pilocarpine to rats that were pretreated with a small dose of lithium chloride results in the evolution of generalized convulsive status epilepticus. The production of status epilepticus is absolutely reproducible, has a very consistent time to onset (22 min), has a duration of several hours, and is extremely severe with a high mortality rate. Experimental results show that this animal model of status epilepticus: (i) requires activation of muscarinic receptors because the initiation of seizures is blocked by atropine; (ii) requires presynaptic cholinergic activity because it is attenuated by hemicholinium-3; (iii) recruits noncholinergic cells because when status epilepticus is established it is not altered by atropine administration; and (iv) is blocked by pretreatment with diazepam and ongoing seizures are terminated by administration of diazepam, similar to certain forms of status epilepticus in humans. The reproducibility, prolonged nature, and involvement of a clearly defined neurochemical system as the triggering mechanism, i.e., cholinergic activation, makes this a potentially valuable animal model of generalized convulsive status epilepticus.

Cholinergic differentiation of clonal rat pheochromocytoma cells (PC12) induced by factors contained in glioma-conditioned medium: enhancement of high-affinity choline uptake system and reduction of norepinephrine uptake system

The effects of glioma-conditioned medium (GCM) and factors contained in GCM on the neurochemical differentiation of the PC12 clone of rat pheochromocytoma cells were investigated. The results obtained are as follows. The accumulation of choline into PC12 cells proceeded through two uptake systems with high (Km = 3.20 microM) and low (Km = 65.2 muM) affinities as revealed by least-squares iterative fitting of a substrate-velocity curve to the data. Culturing of PC12 cells in the presence of GCM led to a 5-fold increase in the Vmax value of the high-affinity uptake system without affecting the Km of the high-affinity uptake system. Both Km and Vmax of the low-affinity uptake system were unaffected by the GCM treatment. The high-affinity choline uptake system in both GCM-treated and untreated PC12 cells was devoid of Na+ dependency and showed low sensitivity to hemicholinium-3. The ratio of [3H]acetylcholine converted from [3H]choline taken up by PC12 cells at 1 muM choline for 1 h was two-fold higher than that by untreated cells. PC12 possess a high-affinity norepinephrine uptake system. Culturing of PC12 cells in the presence of GCM led to a decrease in the rate of uptake of 3 muM norepinephrine to 43% of that in control cells. The 40-K and 10-K fractions isolated by gel filtration of GCM had both abilities to enhance the high-affinity choline uptake system and to suppress the high-affinity norepinephrine uptake system. From these observations it was concluded that GCM contains factors which induce the cholinergic neuronal differentiation of PC12 cells.

Activation of acetylcholine synthesis in cat sympathetic ganglia: dependence on external choline and sodium-pump rate

Acetylcholine synthesis in the perfused cat superior cervical ganglion is maximally activated without activation of release during a 10 min recovery in Locke solution following a 15 min period of Na-pump inhibition by perfusion with K-free Locke solution; choline (5 X 10(-5) M) being present throughout. This procedure combined with the use of very high rates of perfusion flow has now permitted an examination of the roles of choline uptake and Na in the activation of synthesis. The data were analysed by analysis of variance as a basis for assessing experimental error and by Bartlett's test to assess equality of variance. Significance of differences between groups was estimated from this analysis (see Appendix). By selective omission of choline, either with or without addition of hemicholinium-3 (HC-3), in the K-free or in the recovery period it was found that choline is only taken up for formation of acetylcholine in the recovery period. With the use of different concentrations of choline in the recovery period, and omission of choline in the K-free period, it was found that the rate of acetylcholine synthesis increased with increasing choline concentration in conformity with Michaelis-Menten kinetics. The choline concentration giving half-maximal synthesis rate was 3.6 microM. Addition of 10(-6) M-HC-3 during recovery completely abolished synthesis in the presence of 5 X 10(-6) M-choline, and 2.5 X 10(-7) M-HC-3 reduced it by 68%. These values for choline dependence and inhibitory potency of HC-3 are similar to those found for high affinity choline transport in brain synaptosomes, indicating that the same system operates in brain and in ganglia. In additional experiments in which choline was omitted in the K-free period and with 5 X 10(-5) M-choline in the recovery fluid a reduction of external Na to 50 mM during recovery did not reduce significantly the maximal rate of acetylcholine synthesis. Further reduction to 25 nM, which would be expected to abolish the Na gradient, reduced the rate of synthesis by only 18%. The presence of 2 X 10(-5) M-ouabain during recovery in normal Locke solution containing 5 X 10(-5) M-choline abolished synthesis. It is concluded that choline uptake for acetylcholine synthesis in ganglia is via the high affinity transporter; that the transport is rate limiting for acetylcholine synthesis and; that the transport process is intimately linked to Na-pump rate.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium-dependent high-affinity binding of [3H]hemicholinium-3 in the rat brain: a potentially selective marker for presynaptic cholinergic sites

This report describes the membrane binding properties of [3H]hemicholinium-3 ([3H]HC-3), a selective inhibitor of sodium-dependent high-affinity choline uptake (SDHACU) in cholinergic nerve terminals. Under the described assay conditions, [3H]HC-3 bind with a saturable population of high-affinity (apparent Kd = 1.9 nM) CNS membrane sites having the regional distribution: striatum much greater than hippocampus greater than cerebral cortex greater than cerebellum. High-affinity [3H]HC-3 binding is entirely dependent upon the presence of sodium chloride (EC50 = 35-50 mM) and is markedly reduced when other salts of sodium or monovalent ions are substituted. [3H]HC-3 binding is inhibited by choline (Ki = 6 microM) and acetylcholine (Ki = 35 microM) but markedly less sensitive to other cholinergic agents and metabolic inhibitors. In light of the similar ionic dependencies, regional distributions and pharmacological specificities of [3H]HC-3 binding and SDHACU, closely associated sites may be involved in both processes.

The effect of choline mustard on the rat superior cervical ganglia

The effect of choline mustard aziridinium ion (ChM) on the isolated superior cervical ganglion of the rat was investigated. In the presence of ChM (22.5 microM), stimulation at 1 Hz resulted in a slowly developing blockade of transmission, which did not occur at a stimulus frequency of 0.1 Hz. Choline (0.1 mM) slowed the onset of the blockade produced by stimulation at 1 Hz in the presence of ChM. The presence of excess thiosulphate ions prevented the action of ChM on the transmission in the superior cervical ganglion. Treatment of the ganglion with ChM (22.5 microM) only slightly inhibited the depolarization produced by carbachol (dose ratio 1.3), suggesting the drug produced a small degree of receptor blockade. [3H]-choline accumulation in the rat superior cervical ganglia displays several components: (a) sodium-dependent high affinity uptake (SDHAU) that can be activated further by preincubation in a high concentration of K+ ions; (b) sodium-dependent low affinity uptake (SILAU); (c) linear diffusional accumulation which does not saturate. Hemicholinium-3 selectively inhibits the activated sodium-dependent high affinity uptake, but is a weak inhibitor of resting sodium-dependent high affinity uptake and sodium-independent low affinity uptake. ChM inhibits both activated and resting sodium-dependent high affinity uptake, but is a very weak inhibitor of sodium-independent low affinity uptake. Homocholine shows similar selectivity. ChM inhibition of activated sodium-dependent high affinity uptake is very much more persistent than that of hemicholinium-3. Hemicholinium-3 and ChM both inhibit [3H]-acetylcholine synthesis.

Peripheral toxicity of hemicholinium-3 in mice

1 The site (i.e. peripheral or central) of the toxicity produced by hemicholinium-3 in mice was investigated. 2 Hemicholinium-3 was measured fluorometrically and acetylcholine was determined by gas chromatography after intraventricular or intraperitoneal administration of hemicholinium-3. 3 Hemicholinium-3 was not detected in the brain nor were acetylcholine levels decreased in the brain after systemic administration. 4 The dose-response curve following intraventricular administration demonstrated that hemicholinium-3 was not as lethal after central administration as it was after peripheral administration. 5 Approximately 24% of a 75 microgram intraventricular dose of hemicholinium-3 was found in the periphery at death. 6 These results suggest that hemicholinium-3 manifests its toxicity primarily in the periphery.

Effects of hemicholinium-3 and choline on hippocampal electrical activity during immobility vs. movement

Earlier studies have shown that the hippocampal rhythmical slow activity (RSA or theta) which may occur during behavioral immobility (IRSA) is abolished by systemically administered atropine (is atropine-sensitive), although the RSA which accompanies movements, such as walking, running or swimming (MRSA) is atropine-resistant. This study was designed to manipulate brain cholinergic activity in ways other than through the use of postsynaptic receptor antagonists, and to determine the effects of such manipulations on IRSA and MRSA. The IRSA elicited by electrical stimulation of the reticular formation in urethanized rats was severely attenuated by intraventricular injections of hemicholinium-3 (HC-3), a drug which depletes brain acetylcholine. A subsequent systemic injection of choline chloride restored IRSA elicited by electrical stimulation. In contrast, HC-3 had no deleterious effects on the MRSA recorded from freely moving rats. Therefore, atropine-sensitive IRSA is also HC-3 sensitive, and atropine-resistant MRSA is also HC-3 resistant. These results support the hypothesis that there are two pharmacologically distinct neurochemical systems which may produce hippocampal RSA. It is suggested that acetylcholine is necessary for the production of IRSA, but is not necessary for the production of MRSA.

Choline uptake by the neuroblastoma x glioma hybrid, NG108-15

The neuroblastoma x glioma hybrid clone NG108-15 is able to release acetylcholine upon depolarization and form cholinergic neuromuscular synapses in culture. Normal functioning of cholinergic synapses is thought to be dependent on the ability of a neuron to take up extracellular choline, since neurons are unable to synthesize choline de novo. For these two reasons it became important to characterize the choline uptake system of NG108-15 cells. The uptake system appears to bear little if any resemblance to the Na+-dependent high-affinity choline uptake system normally associated with cholinergic neurons. Although the cells appear to possess both high- and low-affinity choline uptake systems, neither system is dependent on Na+ and uptake actually is increased about 60% by the substitution of sucrose for NaCl. Acetylcholine synthesis also is not dependent on Na+, since sucrose, substituted for NaCl, also stimulates acetylcholine synthesis. Changes in the concentrations of the other ions in the uptake medium have little effect on uptake, with the exception that elevated Ca2+ or Mg2+ reverses the stimulation of choline uptake produced by substitution of sucrose for NaCl. Choline uptake is inhibited by hemicholinium-3, but only at high concentrations of the drug (IC50 = 30-80 microM). The metabolic poisons cyanide and iodoacetate inhibit uptake by only 30-40%. Growth of the cells in N6,O2' dibutyryladenosine-3',5'-cyclic monophosphate, which promotes functional and morphological differentiation of the cells, decreased slightly the total amount of choline taken up but had no additional effect on the uptake system. Thus, it appears that NG108-15 cells are capable of forming functional cholinergic synapses with muscle cells even though the neuroblastoma does not possess the high-affinity choline uptake system normally associated with cholinergic neurons.

Choline uptake and permanent memory storage

The uptake of 3H-choline and its incorporation into 3H-acetylcholine was studied in vitro on hippocampus slices obtained from animals showing a good or poor long-term memory. The animals were selected on the basis of their retention performance when tested by a brightness discrimination model. The 3H-choline uptake and the incorporation of 3H-choline into 3H-acetylcholine was higher in hippocampus slices from animals showing good retention compared to those from animals with poor retention. The level of high affinity uptake of choline into hippocampus slices may serve as an indicator of the cholinergic activity in this structure under in vitro conditions. The present findings suggest that individual differences in the activity level of the hippocampal cholinergic system do exist and are capable of influencing the retention of the individual animals to a variable degree.

Hippocampal rhythmic slow activity (RSA; theta): a critical analysis of selected studies and discussion of possible species-differences

The literature concerning the correlates of hippocampal RSA (theta) has seen a wealth of hypotheses generated from seemingly contradictory data. Two possible reasons for this are examined here. (1) An analysis of the maximum published RSA amplitudes in over 70 papers shows that there is enormous variation in how effective various research groups have been in tapping the hippocampal RSA generator zones. It is suggested that this variation is a major source of 'contradictory data'. The enormous variability is probably due to the fact that the laminar structure of the hippocampus, and the location of two seemingly independent 180 degrees out-of-phase RSA generators, results in very disparate signals being recorded by electrodes of different configurations. Electrodes which are not optimally placed result in records which may provide misinformation as to whether or not the hippocampus is in the RSA 'mode'. The results of studies with less than adequate records must therefore be viewed with great caution. (2) An explanation often evoked to account for much of the controversy is that of species differences. This idea is examined and it is suggested that there are probably not major species differences in that all of the species appropriately examined thus far have neural systems capable of producing both an atropine-sensitive and an atropine-resistant form of RSA. All species (with the exception of primates) also show relations of RSA to ongoing motor behavior. However, there are definitely species differences in the neural mechanisms underlying the production of atropine-sensitive, immobility-related RSA. Although all species appear to be capable of producing immobility-related RSA some do so only rarely (e.g. rats), while others do so frequently, particularly in response to sensory stimulation (e.g. rabbits, cats, guinea pigs). Therefore, the answer to the question as to whether there are species differences in the occurrence of RSA may be yes, or not, depending upon how specifically the question is posed.

Regulation of phospholipid metabolism in differentiating cells from rat brain cerebral hemispheres in culture: ontogenesis of carrier-specific transport of choline and N-methyl-substituted choline analogs

Dimethylaminoethanol was studied both as a substrate and as an inhibitor of choline uptake in long-term cultures of foetal rat cerebral hemispheres. A saturable component with an apparent Km of 28 microM and Vmax of 11 pmol/min/microgram DNA for dimethylaminoethanol, was observed. Like choline, dimethylaminoethanol was also taken up by a second, low-affinity component, the apparent Vmax of which was about 102 pmol/min/microgram DNA. Dimethylaminoethanol inhibited the high-affinity but not the low-affinity choline uptake in a competitive manner with an apparent inhibition constant of 6.0 microM. Monomethylaminoethanol (Ki approximately 60 microM) competitively inhibited high-affinity choline transport. At low concentrations hemicholinium-3, but not ethanolamine, effectively inhibited high-affinity uptake of choline and to a lesser degree the uptake of the dimethylaminoethanol. While the high-affinity uptake of both substrates was inhibited by high concentrations of hemicholinium-3 or ethanolamine, the low-affinity system was not affected by hemicholinium-3. From the kinetics of uptake and inhibition patterns of choline and its related analogs, the methyl group seems to play a major role in determining the affinity rate constants for these substrates. The maximum rate of choline uptake via the high-affinity component increases about sixfold during a period of 2 weeks. In the absence of serum the maximum velocity of the high-affinity component is greatly reduced. These observations suggest that the high-affinity choline uptake component is an integral property, and a useful marker, of the developing cerebral cells.

High affinity choline transport and acetylCoA production in brain and their roles in the regulation of acetylcholine synthesis

This review describes recent advances made in the understanding of the regulation of acetylcholine synthesis in brain with regard to the availability of its two precursors, choline and acetylCoA. Choline availability appears to be regulated by the high affinity choline transport system. Investigations of the localization and inhibition of this system are reviewed. Procedures for measuring high affinity choline transport and their shortcomings are described. The kinetics and effects of previous in vivo and in vitro treatments on high affinity choline transport are reviewed. Kinetic and direct coupling of the transport and acetylation of choline are discussed. Recent investigations of the source of acetylCoA used for the synthesis of acetylcholine are reviewed. Three sources of acetylCoA have recently received support: citrate conversion catalyzed by citrate lyase, direct release of acetylCoA from mitochondria following its synthesis from pyruvate catalyzed by pyruvate dehydrogenase, and production of acetylCoA by cytoplasmic pyruvate dehydrogenase. Investigations indicating that acetylCoA availability may limit acetylcholine synthesis are reviewed. A model for the regulation of acetylcholine synthesis which incorporates most of the reviewed material is presented.

Development of high affinity choline uptake and associated acetylcholine synthesis in the rat fascia dentata

The ontogenic development of hemicholinium-sensitive, high affinity choline uptake and the synthesis of acetylcholine from exogenous choline have been studied in particulate preparations of the rat fascia dentata. Between 6 days of age and adulthood the rate of high affinity choline uptake increases 3-fold, when expressed with respect to protein, and 125-fold, when expressed independently of protein. This process develops most rapidly during the period around 16-17 days of age, similar to the ontogenesis of choline acetyltransferase activity. This observation supports the idea that cholinergic septohippocampal boutons develop mainly at this time. Unlike choline acetyltransferase activity, the velocity of high affinity choline uptake increases to as much as 161% of the adult value at about 30 days of age. It is suggested that at 25-31 days of age a relatively high endogenous septohippocampal firing rate increases the rate of choline uptake. At 6 days of age we detected no synthesis of acetylcholine from the accumulated choline. Uptake-synthesis coupling develops mainly between 6 and 13 days of age, earlier than any other presynaptic cholinergic property. Acetylcholine synthesis from exogenous choline develops in paralled with high affinity choline uptake, but developmental increases in uptake velocity result in comparable increases in synthesis rate only after a delay of several days. Some limiting factor other than choline acetyltransferase activity appears to link the accumulation of exogenous choline to acetylcholine synthesis during development.

Isolation of pure cholinergic nerve endings from Torpedo electric organ. Evaluation of their metabolic properties

Pure cholinergic nerve endings (synaptosomes) were isolated from the electric organ of Torpedo by a rapid procedure. These synaptosomes are approximately 3 micron in diameter. They contain an occasional mitochondrion, numerous synaptic vesicles, and sometimes an active zone is observed. No postynaptic membrane attachment is found. This nerve ending fraction is extremely pure as shown by morphological controls and biochemical data. It is rich in choline acetyltransferase (450 nmol/h per mg protein) and acetylcholine (ACh) (130 nmol/mg protein). The isolated endings retain their cytoplasmic components and they synthesize ACh and are stable in vitro for several hours, as shown by biochemical measurements and morphological analysis.