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

Localisation of pre- and postsynaptic cholinergic markers in the human brain

The cholinergic neurotransmission in the central nervous system plays an important role in modulating cognitive processes such as learning, memory, arousal and sleep as well as in modulating locomotor activity. Dysfunction of the central cholinergic system is involved in numerous neuropsychiatric diseases. This review will provide a synopsis on the regional localisation of cholinergic and cholinoceptive structures within the adult human brain. On the cholinergic site data based on the distribution of choline acetyltransferase-immunoreactive structures are in the focus, complemented by data from acetylcholinesterase and vesicular acetylcholine transporter studies. On the cholinoceptive site, the distribution and localisation of receptors that transduce the acetylcholine message, i.e. the muscarinic and the nicotinic acetylcholine receptors is summarized. In addition to these data obtained on post mortem brain an overview of markers which allow for the in vivo monitoring of the cholinergic system in the brain is given. The detailed knowledge on the distribution and localisation of cholinergic markers in human brain will provide further information on the cholinergic circuits of neurotransmission - a prerequisite for the interpretation of in vivo imaging data and the development of selective diagnostic and therapeutic compounds.

Effect of postmortem storage on the [3H]hemicholinium-3 binding site in the rat brain. Preliminary study for investigations of human patients with Alzheimer's disease

The effect of postmortem storage at room temperature (24-26 degrees C, 0-4 h) and cold-room temperature (4 degrees C, (0-24 h) on the [(3)H]hemicholinium-3 binding sites in the brain hippocampus, cortex and cerebellum of 3-month-old Wistar rats was studied. A slow decrease in the density of the binding sites was observed at both temperatures, which was best fit by a linear model common for all three brain regions. No systematic alterations of the affinity of the binding sites for hemicholinium-3 were found. The values obtained from experiments with animals were compared with the values measured in the frontal cortex of old men. Approaches to the evaluation of data obtained from postmortem samples of human brain tissue of patients with Alzheimer's disease are proposed.

Assessment of choline uptake for the synthesis and release of acetylcholine in brain slices by a dynamic autoradiographic technique using [11C]choline

The uptake of choline for the synthesis and release of acetylcholine was investigated in brain slices by dynamic positron autoradiography using [11C]choline. Brain slices (330 microm) were incubated with [11C]choline in oxygenated Krebs-Ringer medium at 34 degrees C and serial two-dimensional time-resolved images of the uptake and release of radioactivity were recorded on Storage Phosphor screens. [11C]choline uptake increased with the period of incubation and was 1.9 times higher in the striatum than cerebral cortex. The uptake in the striatum was significantly diminished by hemicholinium-3 (HC-3), an inhibitor of high-affinity choline uptake. Pretreatment of brain slices with 50 mM K(+) for 20 min enhanced the uptake in striatum. The uptake of [11C]choline in brain slices was saturable using nonlabeled choline. Two uptake systems, a high-affinity and a low-affinity system, were confirmed to exist by kinetic analysis using Lineweaver-Burk plots. The 11C radioactivity that had accumulated in the striatum disappeared on treatment with veratridine, a depolarization agent, in the presence of HC-3. This pattern of disappearance was consistent with that of the appearance of unlabeled and labeled acetylcholine in the medium. These results indicate that this method is useful for obtaining information regarding the uptake of choline for the synthesis and release of acetylcholine in live brain tissues.

Morphine-induced spinal cholinergic activation: in vivo imaging with positron emission tomography

Positron emission tomography (PET) imaging of spinal cord in monkeys with a cholinergic tracer demonstrates increased spinal cholinergic activity in response to an analgesic dose of morphine, and this PET result correlates with measurement of acetylcholine spillover into spinal cord extracellular space induced by morphine, as measured by microdialysis. Previous studies in rats, mice, and sheep demonstrate activation of spinal cholinergic neurons by systemic opioid administration, and participation of this cholinergic activity in opioid-induced analgesia. Testing the relevance of this observation in humans has been limited to measurement of acetylcholine spillover into lumbar cerebrospinal fluid. The purpose of this study was to apply a recently developed method to image spinal cholinergic terminals non-invasively via PET and to test the hypothesis that the tracer utilized would reflect changes in local cholinergic activity. Following Animal Care and Use Committee approval, seven adult male rhesus monkeys were anesthetized on three separate occasions. On two of the occasions PET scans were performed using [(18)F] (+)-4-fluorobenzyltrozamicol ([(18)F]FBT), which selectively binds to the vesicular acetylcholine (ACh) transporter in the presynaptic cholinergic terminals. PET scans were preceded by injection of either saline or an analgesic dose of IV morphine (10 mg/kg). On the third occasion, microdialysis catheters were inserted in the spinal cord dorsal horn and acetylcholine concentrations in dialysates determined before and after IV morphine injection. Morphine increased cholinergic activity in the spinal cord, as determined by blood flow corrected distribution volume of [(18)F]FBT in the cervical cord compared to the cerebellum. Morphine also increased acetylcholine concentrations in microdialysates from the cervical cord dorsal horn. The one animal which did not show increased spinal cholinergic activity by PET from this dose of morphine also did not show increased acetylcholine from this morphine dose in the microdialysis experiment. These data confirm the ability to use PET to image spinal cholinergic terminals in the monkey spinal cord and suggest that acute changes in cholinergic activity can be imaged with this non-invasive technique. Following preclinical screening, PET scanning with [(18)F]FBT may be useful to investigate mechanisms of analgesic action in normal humans and in those with pain.

Functional characterization of the human high-affinity choline transporter

Na(+)-dependent, high-affinity choline uptake in cholinergic neurons is the rate-limiting step in acetylcholine synthesis. Here we report the molecular cloning and functional characterization of the human high-affinity choline transporter (hCHT1). The hCHT1 exhibits significant homology with known members of the Na(+)-dependent glucose transporter family, but not with members of the neurotransmitter transporter family. The human CHT1 gene is 25 kb in length with 9 exons and was assigned to chromosome II at position IIq11-12. Northern blot analysis showed that a 5.4 kb hCHT1 transcript was expressed exclusively in tissues containing cholinergic neurons. When expressed in Xenopus oocytes, the human clone induced Na(+)- and Cl(-)-dependent, high-affinity choline uptake, which was sensitive to the specific inhibitor hemicholinium-3, with a K(i) of 1.3 nM. The hCHT1-mediated choline uptake increased with increasing concentrations of choline, Na(+) and Cl(-), with EC(50) values of 2.0 microM, 76 mM, and 48 mM, and with apparent Hill coefficients of 1, 2.5 and 2.3, respectively.

In vivo imaging of the spinal cord cholinergic system with PET

PURPOSE

Our goal was to demonstrate the feasibility of an in vivo noninvasive method for imaging spinal cord cholinergic terminals using (+)-4-[18F]fluorobenzyltrozamicol ([18F]FBT) and PET.

METHOD

In vitro and in vivo experiments in rats were conducted to demonstrate the specific binding characteristics, localization, and time course of [3H]FBT binding in the spinal cord. PET imaging was then performed on seven rhesus monkeys.

RESULTS

The rat studies demonstrate high specific binding in the spinal cord with a distribution coinciding with the known distribution of cholinergic terminals. In vivo tracer concentrations in the spinal cord and basal ganglia were of the same magnitude. With use of [18F]FBT and PET in the rhesus monkey, the spinal cord was clearly visualized, with tracer concentration in the spinal cord being approximately one-fourth of that seen in the basal ganglia.

CONCLUSION

This work demonstrates the feasibility of imaging cholinergic terminals in vivo in the spinal cord using [18F]FBT and PET.

Autoradiographic distribution of M1, M2, M3, and M4 muscarinic receptor subtypes in Alzheimer's disease

We studied the autoradiographic densities of all pharmacologically characterised muscarinic receptors (MR) in frontal, temporal, and visual cortex, hippocampal formation, and striatum in autopsied brains from 19 histopathologically verified patients of Alzheimer's disease (AD) and in matched controls. Almost all (16 of 19) of the AD cases were severe. In AD brains, total MR, M1, and M3 MR subtypes were found to be significantly decreased in entorhinal cortex and in most hippocampal strata. Total MR and M1 receptors were also significantly reduced in visual area and in frontal cortex of AD brains, respectively. M2 receptors were significantly reduced over hippocampal formation but increased significantly in striatum of AD brains as compared with controls. M3 receptors in AD were in the range of controls in neocortex and striatum, whereas the M4 receptor subtype was also preserved in all brain regions in AD brains when compared with controls. This is the first autoradiographic study analysing the distribution of all MR subtypes in AD brains. These changes in MR densities concur with the general pattern of neuronal degeneration occurring in AD brains and partly explain the poor response of AD cognitive decline to present cholinergic supplementation therapies. Although M3 and M4 MR were labelled with nonselective approaches, the preservation of M4 and to a lesser degree M3 MR subtypes in AD brains could open an alternative way for the symptomatic therapy of AD dementia.

Imaging of cholinergic terminals using the radiotracer [18F](+)-4-fluorobenzyltrozamicol: in vitro binding studies and positron emission tomography studies in nonhuman primates

The goal of the present set of studies was to characterize the in vitro binding properties and in vivo tissue kinetics for the vesicular acetylcholine transporter (VAcChT) radiotracer, [18F](+)-4-fluorobenzyltrozamicol ([18F](+)-FBT). In vitro binding studies were conducted in order to determine the affinity of the (+)- and (-)-stereoisomers of FBT for the VAcChT as well as sigma (sigma 1 and sigma 2) receptors. (+)-FBT was found to have a high affinity (Ki = 0.22 nM) for the VAcChT and lower affinities for sigma 1 (21.6 nM) and sigma 2 (35.9 nM) receptors, whereas (-)-FBT had similar affinities for the VAcChT and sigma 1 receptors (approximately 20 nM) and a lower affinity for sigma 2 (110 nM) receptors. PET imaging studies were conducted in rhesus monkeys (n = 3) with [18F](+)-FBT. [18F](+)-FBT was found to have a high accumulation and slow rate of washout from the basal ganglia, which is consistent with the labeling of cholinergic interneurons in this brain region. [18F](+)-FBT also displayed reversible binding kinetics during the 3 h time course of PET and produced radiolabeled metabolites that did not cross the blood-brain barrier. The results from the current in vitro and in vivo studies indicate that [18F](+)-FBT is a promising ligand for studying cholinergic terminal density, with PET, via the VAcChT.

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.

'Choline/orphan V8-2-1/creatine transporter' mRNA is expressed in nervous, renal and gastrointestinal systems

Several cDNAs with substantial sequence homologies to members of the neurotransmitter transporter gene family currently remain 'orphan' transporters, without clearly-identified substrates. We were concerned that a cDNA 'V8-2-1' isolated from a ventral midbrain cDNA library in this laboratory and a virtually-identical cDNA 'CHOT1' reported by Mayser et al. [J. Neurochem., 20 (1973) 581-593] might represent such an orphan. Despite initial reports that it could mediate some choline uptake; neither CHOT1 nor V8-2-1 was demonstrated to confer pharmacologically appropriate choline uptake not already present in either Xenopus oocytes or COS cells. Determination of the regional and tissue-specific distribution of mRNA hybridizing with V8-2-1 cDNA was undertaken to aid in identifying its function. Examination of the distribution of V8-2-1 expression reveals several novel features of this transporter gene family member's distribution, including several features that add to current evidence suggesting that the clone may not encode the classical pharmacologically-defined, hemicholinium-3 sensitive high affinity transporter of cholinergic neurons. These data fit with and extend recent data that suggest that this cDNA represents creatine transporter, and provide initial documentation of its regional distribution in brain.

Cholinergic markers in degenerative parkinsonism: autoradiographic demonstration of high-affinity choline uptake carrier hyperactivity

[3H]Hemicholinium-3 ([3H]HC-3) binding, as a marker of the presynaptic high-affinity choline uptake carrier (HACU), and cholinergic muscarinic receptors were measured by autoradiography in several brain regions of levodopa-responsive parkinsonism and matched cases. A significant increase in the density of [3H]HC-3 binding sites was found in the striatum of parkinsonian brains, while there was a slight decrease in the parkinsonian hippocampus. Total, M1 and non-M1 muscarinic receptors remained unchanged in frontal cortex and striatum of parkinsonian brains as compared to controls. Total and non-M1 muscarinic receptors were significantly reduced in the parkinsonian hippocampus, whereas hippocampal M1 receptors were preserved. These data demonstrate a hyperactivity of the HACU, and thus of the acetylcholine synthesis, in parkinsonian brains probably compensatory of the loss of both dopaminergic terminals in the striatum and of basal forebrain neurons in the hippocampus. Our results emphasize the value of [3H]HC-3 binding in the study of the functional status of the cholinergic synapse in neurodegenerative disorders.

Comparison of the effects of aging in vivo and of oxygen free radicals in vitro on high-affinity choline uptake and hemicholinium-3 binding in the rat brain

The effects of aging in vivo (Wistar rats aged 3-26 months) and of an oxygen free-radical generating system in vitro (Fe(2+)/ascorbic acid) on high-affinity choline uptake in the hippocampus and on (3H)hemicholinium-3 binding sites in the cortex and hippocampus are compared. The high-affinity choline transport system was found to be more damaged than the low-affinity system during aging (Na(+)-dependent part of the uptake drops to 76%: Na(+)-independent part increases to 120%). The decrease in high-affinity choline uptake values is probably more influenced by the impairment of correct function of carriers (the fall in the turnover rate of each carrier) than by a decrease in the number of transport sites (no change of the density of the carriers in the hippocampus and cortex). The causes of the defect in high-affinity choline transport during aging are discussed.

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.

Development of high-affinity choline transport sites in rat forebrain: a quantitative autoradiography study with [3H]hemicholinium-3

The development of cholinergic terminals in rat brain has been quantitatively analyzed by [3H]hemicholinium-3 autoradiography. [3H]Hemicholinium-3 binds to high affinity choline transport sites, a specific marker for cholinergic neurons. In neonatal animals, kinetic and pharmacologic binding characteristics and regional distribution of [3H]hemicholinium-3 sites are consistent with specific cholinergic localization, as in the adult. The distribution of cholinergic terminals is described in the adult rat brain and during development, including heterogeneity of binding within several regions such as the striatum, nucleus accumbens, olfactory tubercle, cortex, and hippocampus. Early development and maturation vary greatly between brain regions. At embryonic day E18 and day 0, specific binding density is high only in the medial habenula. Development occurs primarily during the postnatal period in most brain regions examined. Many brain regions exhibit a lull in development between days 5 and 10, although the rate of development is highly region specific. Specific binding increases 2-12-fold between day 5 and adult animals, with adult density being achieved anywhere from day 15 to after day 21. The ontogeny of [3H]hemicholinium-3 binding sites generally occurs in a rostral to caudal direction. In the striatal body the characteristic lateral to medial gradient of binding site density is apparent by day 5, and development is more rapid in the lateral striatum. Patches of dense [3H]hemicholinium-3 binding coincident with acetylcholinesterase are observed on day 5 in the caudal striatum. The various patterns of cholinergic terminal development suggest that factors regulating cholinergic development are regional and complex.

Quantitative autoradiography of hemicholinium-3 binding sites in human amygdala

We report the binding properties and subnuclear localization of [3H]hemicholinium-3 binding sites in human amygdala using quantitative autoradiography. Specific binding was saturable and high affinity (apparent Kd 2-11 nM). Binding was highest in the basolateral nucleus which receives dense cholinergic innervation from the basal forebrain. Binding closely approximated acetylcholinesterase reactivity. These data support [3H]hemicholinium-3 as a quantitative marker for cholinergic terminals in human brain.

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.