Lack of desensitization of muscarinic receptor-mediated second messenger signals in rat brain upon acute and chronic inhibition of acetylcholinesterase.

We studied the effects of acute and chronic in vivo inhibition of acetylcholinesterase on both the density and function of brain muscarinic cholinergic receptors. Adult male rats were treated either once or multiple times over a period of 10 days with the irreversible acetylcholinesterase inhibitor diisopropylfluorophosphate (DFP). The concentration and affinity of muscarinic receptors in various brain regions were determined using radioligand binding techniques. Acute DFP treatment resulted in a significant reduction in receptor number only in the brain stem, while chronic treatment caused receptor down-regulation in the brain stem, cerebral cortex, and striatum. There was no change in ligand affinity in any of the brain regions. In sharp contrast, muscarinic receptor function was fully preserved, in terms of coupling of the receptors to increased phosphoinositide hydrolysis in the cerebral cortex, hippocampus, and striatum, or inhibition of cyclic AMP formation in the cerebral cortex or striatum. Therefore, there is a marked lack or correlation between DFP-induced muscarinic receptor down-regulation and receptor desensitization.     

Lack of desensitization of muscarinic receptor–mediated second messenger signals in rat brain upon acute and chronic inhibition of acetylcholinesterase

We studied the effects of acute and chronic in vivo inhibition of acetylcholinesterase on both the density and function of brain muscarinic cholinergic receptors. Adult male rats were treated either once or multiple times over a period of 10 days with the irreversible acetylcholinesterase inhibitor diisopropylfluorophosphate (DFP). The concentration and affinity of muscarinic receptors in various brain regions were determined using radioligand binding techniques. Acute DFP treatment resulted in a significant reduction in receptor number only in the brain stem, while chronic treatment caused receptor downregulation in the brain stem, cerebral cortex, and striatum. There was no change in ligand affinity in any of the brain regions. In sharp contrast, muscarinic receptor function was fully preserved, in terms of coupling of the receptors to increased phosphoinositide hydrolysis in the cerebral cortex, hippocampus, and striatum, or inhibition of cyclic AMP formation in the cerebral cortex or striatum. Therefore, there is a marked lack or correlation between DFP-induced muscarinic receptor down-regulation and receptor desensitization.     

Changes in striatal specific 3-H-atropine binding after unilateral 6-hydroxydopamine lesions of nigrostriatal dopaminergic neurones.

Specific binding of ³H-atropine to crude synaptosomal membrane fractions of the rat striatum was measured at different times after unilateral 6-hydroxydopamine lesions of the nigrostriatal dopaminergic neurones. In a group of rats killed between 4 to 15 days after lesioning the right side, specific ³H-atropine binding was reduced by 20 percent compared to the right side of unlesioned rats. There was a concomitant increase (20 percent) of specific ³H-atropine binding in the contralateral side compared to control animals. These changes in muscarinic receptor binding depended on the time after which the lesions were made : maximum effects occured about 8 days after lesioning but almost completely disappeared 13 days later. Dissociation constants for ³H-atropine in the right and left striata of control and lesioned rats were not significantly different. The decrease in muscarinic receptor binding in the ipsilateral striatum of lesioned animals may result from an activation of cholinergic neurones produced by removal of the inhibitory dopaminergic terminals.     

Differential modulation of organophosphate-sensitive muscarinic receptors in rat brain by parathion and chlorpyrifos

We previously reported similar levels of brain cholinesterase inhibition but marked differences in toxicity following acute maximum tolerated doses of the organophosphate pesticides parathion and chlorpyrifos. Because extensive acetylcholinesterase inhibition often induces compensatory changes in cholinergic receptor populations, we compared the effects of parathion and chlorpyrifos on brain muscarinic receptors. Adult male rats were treated with vehicle or the maximum tolerated dose of parathion (18 mg/kg, sc) or chlorpyrifos (279 mg/kg, sc) and observed for signs of acute toxicity. Similarly treated animals were sacrificed at 2, 7, or 14 days after treatment for measurement of cholinesterase activity and binding to the nonselective muscarinic antagonist [³H]quinuclidinyl benzilate, the M2-preferential antagonist [³H]AFDX-384, and the high-affinity agonist [³H]cis-methyldioxolane. More acute toxicity was noted after parathion treatment. Both insecticides caused similar levels (> 85%) of maximal cholinesterase inhibition and reductions (up to 55%) in atropine-sensitive quinuclidinyl benzilate binding (i.e., total muscarinic receptors) and [³H]AFDX-384 binding in cortex and striatum. Parathion also reduced, whereas chlorpyrifos increased, total muscarinic receptor binding and [³H]AFDX-384 binding in the cerebellum. When tissues were preincubated with paraoxon (10 μM), radiolabeling of a subset of quinuclidinyl benzilate binding sites was blocked and the apparent densities of these organophosphate-sensitive receptors in all three tissues were decreased (16% maximal) by parathion but increased (up to 37%) by chlorpyrifos. Similarly, parathion decreased whereas chlorpyrifos increased [³H]cis-methyldioxolane binding sites in all three brain regions. We propose that differential modulation of these organophosphate-sensitive muscarinic receptors contributes to differences in acute toxicity following exposure to these pesticides.     

Regional Alterations in Rat Brain Neurotransmitter Systems Following Chronic Lithium Treatment

Abstract: Chronic, but not acute, consumption of lithium leads to a significant decrease in serotonin and GABA receptor binding in selected regions of the rat brain, with no changes noted in P-adrenergic or cholinergic muscarinic receptor binding. In addition, the concentration of β-methoxytyramine, a dopamine metabolite, in the corpus striatum was increased in the animals treated chronically with lithium, suggesting a possible enhancement in dopamine release, or inhibition of uptake, in this brain area. In contrast, chronic consumption of rubidium had no effect on any of the parameters studied. The results suggest that lithium administration causes selective changes in brain neurotransmitter receptor systems and that the net result of these changes may be a decrease in GABAergic and serotoninergic activity. The fact that these alterktions are noted only after chronic administration suggests that they may be related to the therapeutic action of lithium in the prophylactic treatment of recurrent manic- depressive psychosis.     

A comparison of the muscarinic cholinoceptors and benzodiazepine receptors of guinea-pig brain and rat brain

Some properties of muscarinic cholinoceptors and benzodiazepine receptors in selected brain regions of guinea-pigs and rats were compared under identical experimental conditions. The regions investigated were striatum, hippocampus and pons-medulla, and the properties examined were the concentrations of receptors; apparent dissociation constants of the ligands [³H]quinuclidinyl benzilate (for muscarinic receptors) and [³H]flunitrazepam (for benzodiazepine receptors); Hill coefficients for the interactions of the antagonist atropine and the agonist acetylcholine with the muscarinic receptors; the affinities of these compounds for the muscarinic receptors; and the effects of chronic administration of an organophosphate cholinesterase inhibitor (di-isopropylfluorophosphate) on the concentrations of receptors. Rat striatal and hippocampal muscarinic receptors were found to have a slightly higher affinity for acetylcholine than the corresponding guinea-pig receptors. Administration of di-isopropylfluorophosphate reduced the concentration of muscarinic receptors in rat brain by 30%, but had no significant effect on the concentration of receptors in guinea-pig brain. In all other aspects, the properties of the brain receptors of the two species were very similar. For both species, the affinities of the muscarinic receptors for acetylcholine were higher in the pons-medulla than in the striatum and hippocampus. This was found to be the result of differences in the values of the association constants of the high- and low-affinity states of the receptors, rather than because of varying proportions of two states which have the same association constant in all regions.The insensitivity of guinea-pig brain muscarinic receptors to chronic administration of an organophosphate confirms the results of a previous study on the guinea-pig alone, and makes this system unique. Many other studies on various species have all indicated that prolonged activation of a receptor by an agonist (caused in the present work by inactivation of acetyl-cholinesterase) leads to a decrease in the concentration of the receptor.     

Enhanced muscarinic M1 receptor gene expression in the corpus striatum of streptozotocin-induced diabetic rats

Acetylcholine (ACh), the first neurotransmitter to be identified, regulate the activities of central and peripheral functions through interactions with muscarinic receptors. Changes in muscarinic acetylcholine receptor (mAChR) have been implicated in the pathophysiology of many major diseases of the central nervous system (CNS). Previous reports from our laboratory on streptozotocin (STZ) induced diabetic rats showed down regulation of muscarinic M1 receptors in the brainstem, hypothalamus, cerebral cortex and pancreatic islets. In this study, we have investigated the changes of acetylcholine esterase (AChE) enzyme activity, total muscarinic and muscarinic M1 receptor binding and gene expression in the corpus striatum of STZ – diabetic rats and the insulin treated diabetic rats. The striatum, a neuronal nucleus intimately involved in motor behaviour, is one of the brain regions with the highest acetylcholine content. ACh has complex and clinically important actions in the striatum that are mediated predominantly by muscarinic receptors. We observed that insulin treatment brought back the decreased maximal velocity (V_max) of acetylcholine esterase in the corpus striatum during diabetes to near control state. In diabetic rats there was a decrease in maximal number (B_max) and affinity (K_d) of total muscarinic receptors whereas muscarinic M1 receptors were increased with decrease in affinity in diabetic rats. We observed that, in all cases, the binding parameters were reversed to near control by the treatment of diabetic rats with insulin. Real-time PCR experiment confirmed the increase in muscarinic M1 receptor gene expression and a similar reversal with insulin treatment. These results suggest the diabetes-induced changes of the cholinergic activity in the corpus striatum and the regulatory role of insulin on binding parameters and gene expression of total and muscarinic M1 receptors.     

Quantitative in vivo receptor binding IV: Detection of muscarinic receptor down-regulation by equilibrium and by tracer kinetic methods

Newly-developed methods for estimation of in vivo binding to neurotransmitter receptors should enable the detection and quantification of physiologic or pathologic changes in receptor numbers. In the present study, both equilibrium and kinetic experimental strategies for in vivo muscarinic receptor determination were applied to the detection of receptor changes induced by chronic inhibition of acetylcholinesterase in the rat. Following one week of treatment, in vitro receptor autoradiography utilizing [³H]scopolamine revealed significant losses of muscarinic binding in the cerebral cortex, hippocampus, striatum and in cranial nerve motor nuclei. The in vivo distribution of [³H]scopolamine, following infusion to approach equilibrium binding in the brain, revealed reductions in binding which paralleled the pattern and magnitude of changes detected in vitro. A simplified tracer kinetic estimation following bolus injection of the ligand also detected substantial reductions in forebrain muscarinic receptor binding. These results indicate the feasibility of detecting receptor changes underlying neuropathologic conditions in vivo, and suggest that either equilibrium or kinetic experimental approaches may be extended to clinical research applications with the use of positron or single-photon emission tomography.Special issue dedicated to Dr. Louis Sokoloff.     

Regional distribution of the muscarinic cholinoceptor and acetylcholinesterase in guinea pig brain

The muscarinic receptors in membranes prepared from guinea pig brain were studied using a radiolabeled antagonist, [³H]quinuclidinyl benzilate (QNB). The apparent dissociation constant of the QNB-receptor complex (K _d ) was similar in all regions, but the concentration of receptors was highest in the striatum, cerebral cortex, and hippocampus and lowest in the cerebellum. Similar distributions have been reported for other species, although the concentration of receptors in guinea pig brain is higher than in other species. Acetylcholine inhibited QNB binding with a Hill coefficient of 0.4–0.6. The concentration of acetylcholine required to inhibit binding by 50% (I₅₀) was lowest in the brain stem and more than 10 times higher in the hippocampus. Similar results have been reported for mouse brain. The activity of acetylcholinesterase was highest in the striatum, where the concentration of muscarinic receptors is highest, but did not vary greatly in other brain regions.RMD was seconded to the University of Melbourne to undertake this study.     

Effect of Chronic Nicotine Treatment on Nicotinic Autoreceptor Function and N-[3H]Methylcarbamylcholine Binding Sites in the Rat Brain

It has been reported that N-methylcarbamylcholine (MCC), a nicotinic agonist, binds to central nicotinic receptors and causes an increase of acetylcholine (ACh) release from certain central cholinergic nerve terminals. The present experiments determine whether these two phenomena change in response to the chronic administration of nicotine, a procedure known to result in an increase in nicotinic binding sites. Chronic nicotine caused a brain region-specific up-regulation of [3H]MCC sites; binding increased in the frontal cortex, parietal cortex, striatum, and hippocampus, but not in the occipital cortex or cerebellum. The effect of nicotine was selective to nicotinic binding sites, because muscarinic sites, both M1 ([ 3H]pirenzepine) and M2 ([3H]ACh), were unaffected by chronic nicotine treatment. MCC increased the release of ACh from the frontal cortex and hippocampus by a calcium-dependent mechanism; MCC did not alter ACh release from striatum or occipital cortex of control animals. The MCC-induced increase in ACh release was not apparent in those animals which had been treated with nicotine. There was a partial recovery of nicotinic autoreceptor function when animals were allowed to recover (4 days) following chronic nicotine treatment, but the density of binding sites remained increased compared to control. Chronic nicotine did not change the potassium-evoked release of ACh from the frontal cortex or hippocampus, but decreased this measure from striatum. It also decreased the ACh content of the striatum, but not that of the cortex or the hippocampus; the activity of choline acetyltransferase was not altered in any of the regions tested.(ABSTRACT TRUNCATED AT 250 WORDS)     

知母皂苷元对痴呆模型大鼠脑内M受体密度分布的影响

目的：观察知母皂苷元对痴呆大鼠模型脑内M受体密度分布的影响。方法：单侧基底核内联合注射β-淀粉样肽25-35片段(Aβ25-35)和兴奋性氨基酸建立大鼠痴呆模型．然后将模型动物分为假手术组、模型组和ZMS组,采用放射配基结合分析法测定皮层、海马和纹状体中的M受体密度。结果：脑内联合注射Aβ和Ibotenicacid(IBO)后,模型大鼠脑内有明显的Aβ斑块沉积．同时上述的三个区域中的M受体密度明显比假手术组减少,而模型大鼠喂服知母皂苷元60天后,能有效地增加模型大鼠脑内不同区域中的M受体密度。结论：知母皂苷元能使痴呆动物脑内M受体密度增加。说明它对老年性痴呆的胆碱系统功能渐进性退化有一定的预防和治疗作用。     

Effect of long-term L-dopa administration on the dopaminergic and cholinergic (muscarinic) receptors of striatum in 6-hydroxydopamine lesioned rats

L-Dihydroxyphenylalanine (L-Dopa) (200 mg/kg/day) was administered for 30 days to the rats whose nigrostriatal dopamine pathway was lesioned unilaterally with 6-hydroxydopamine and the receptor binding of ³H-spiperone and ³H-quinuclidinyl benzilate (³HQNB) was measured in the dopaminergic and muscarinic cholinergic receptors of the striatum. ³H-spiperone binding increased by 73% and ³HQNB binding decreased by 14% in the lesioned side when compared to the control side of L-Dopa-non-treated rats. ³H-spiperone binding was measured in the lesioned sides of L-Dopa-treated and L-Dopa-non-treated rats and was found to have decreased by 21% in the former. In the control side of the L-Dopa-treated lesioned rats, however, ³H-spiperone binding increased by 27% when compared to the opposite striatum of the same rats. ³HQNB binding in the lesioned side of L-Dopa-treated rats was not significantly different from that of the control side statistically. These results suggest that changes in functional equilibrium between the dopaminergic and cholinergic mechanisms influence the muscarinic cholinergic receptors and that supersensitivity of dopamine receptors after lesion of the nigrostriatal pathway also remains after long-term L-Dopa treatment.     

The effect of chronic ethanol consumption on muscarinic receptors in rat brain

Ethanol (15% v/v) was administered in the drinking water to male Wistar rats over period of 3 months. Binding properties of muscarinic receptors were studied in synaptosomes from selected brain areas using [³H]quinuclidinyl benzilate and its displacement by the selective antagonist, pirenzepine and the agonist, carbachol. Dissociation constants (K_d) of all three ligands in the cerebral cortex, hippocampus and striatum of ethanol-treated groups did not differ from those in controls. Density of [³H]quinuclidinyl benzilate binding sites in the cortex of ethanol-treated animals was approx. 50% higher than in controls (2.06 ± 0.2 and 1.32 ± 0.2 pmol/mg of protein respectively, mean ± SD, n = 6, P < 0.001). This was largely attributable to an increase in M₁ binding sites as shown by pirenzepine displacement studies. In the hippocampus and striatum binding capacity of muscarinic receptors was not affected by ethanol treatment. Synthesis of acetylcholine in cerebral cortex prisms from ethanol-treated animals was not inhibited under resting conditions, but stimulation of synthesis by high K⁺ concentration was significantly altenuated by comparison with controls. These results suggest that chronic ethanol consumption induces changes in cholinergic neurotransmission in selected brain areas.     

Estrogen therapy and brain muscarinic receptor density in healthy females: a SPET study

Estrogen Therapy (ET) may protect against age-related cognitive decline and neuropsychiatric disorders (e.g. Alzheimer's disease). The biological basis for this putative neuroprotective effect is not fully understood, but may include modulation of cholinergic systems. Cholinergic dysfunction has been implicated in age-related memory impairment and Alzheimer's disease. However, to date no one has investigated the effect of long-term ET on brain cholinergic muscarinic receptor aging, and related this to cognitive function. We used Single Photon Emission Tomography (SPET) and (R,R)[(123)I]-I-QNB, a novel ligand with high affinity for m(1)/m(4) muscarinic receptors, to examine the effect of long-term ET and age on brain m(1)/m(4) receptors in healthy females. We included 10 younger premenopausal subjects and 22 postmenopausal women; 11 long-term ET users (all treated following surgical menopause) and 11 ET never-users (surgical menopause, n=2). Also, verbal memory and executive function was assessed in all postmenopausal subjects. Compared to young women, postmenopausal women (ET users and never-users combined) had significantly lower muscarinic receptor density in all brain regions examined. ET users also had higher muscarinic receptor density than ET never-users in all the brain regions, and this reached statistical significance in left striatum and hippocampus, lateral frontal cortex and thalamus. Moreover, in ET users, (R,R)[(123)I]-I-QNB binding in left hippocampus and temporal cortex was significantly positively correlated with plasma estradiol levels. We also found evidence for improved executive function in ET users as compared to ET never-users. However, there was no significant relationship between receptor binding and cognitive function within any of the groups. In healthy postmenopausal women use of long-term ET is associated with reduced age-related differences in muscarinic receptor binding, and this may be related to serum estradiol levels.     

Huntington's disease and its animal model: alterations in kainic acid binding.

The density of ³H-kainic acid (KA) binding was determined in several regions of Huntington's Diseased (HD) and control human brains. ³H-Kainic acid binding was significantly reduced by 55% in the caudate nucleus and by 53% in the putamen of HD brains. In addition, ³H-KA binding was determined in rat striatum at various intervals following lesion with KA, a procedure which produces an animal model of HD. After KA lesion, ³H-KA binding in the rat striatum underwent a slow reduction, reaching 25% of control after 6 weeks. Several properties of ³H-KA binding to rat brain membranes were also investigated, including inhibition by ions, regional distribution and displacement by various compounds. The findings confirm the validity of the KA-lesioned model for HD and suggest a post-synaptic location for kainic acid receptors in the striatum.     

Effect of aging on the interaction of quinuclidinyl benzilate,N-methylscopolamine,pirenzepine, and gallamine with brain muscarinic receptors

The objective of the present study was to investigate the effects of senescence on the binding characteristics of muscarinic receptors by using [³H]quinuclidinyl benzilate ([³H]QNB) and [³H]N-methylscopolamine ([³H]NMS) as ligands in young (3months), middle-age (10months) and old (24 months) male Fischer 344 rats. Muscarinic receptor density was found to decrease significantly with aging in certain brain regions, depending on the ligand employed. Moreover, the relative proportions of M₁ and M₂ muscarinic receptor subtypes was not significantly altered by aging, except in the aged striatum. Furthermore, the dissociation kinetics of [³H]NMS in the cerebral cortex and their allosteric modulation by gallamine were only slightly influenced by age.     

Alterations in the γ-Aminobutyric Acid-Gated Chloride Channel Following Transient Forebrain Ischemia in the Gerbil

The role of inhibitory neurotransmission in selective neuronal degeneration after transient forebrain ischemia was studied by binding of t-[35S]butylbicyclophosphorothionate ([35S]TBPS) to the gamma-aminobutyric acid (GABA)-gated chloride channel and measurement of GABAA receptor function in Mongolian gerbil brain. [35S]TBPS binding to the hippocampus, striatum, and cortex quantified by autoradiography and muscimol-stimulated 36Cl- uptake in synaptoneurosomes of the same regions were examined 1, 4, and 29 days after a 5-min bilateral carotid occlusion. [35S]TBPS binding was decreased in the pyramidal cell dendritic layers, stratum oriens, and stratum lacunosum-moleculare of the CA1 hippocampus, 4 and 29 days after occlusion, and in the stratum radiatum 29 days after occlusion. [35S]TBPS binding sites in the lateral striatum decreased 47% 4 days after occlusion. At the same time, there was a corresponding decrease in muscimol-stimulated 36Cl- uptake in the striatal synaptoneurosomes. Muscimol-stimulated 36Cl- uptake in the hippocampus decreased slightly 4 days after occlusion and more so after 29 days, although these decreases were not significant. No changes were observed in somatosensory cortex at any time point. These data suggest that a portion of GABAA receptors in areas sensitive to ischemic insult are associated with degenerating neurons, whereas other GABAA) receptors are spared.     

Alterations in central neurotransmitter receptor binding sites following estradiol implantation in female rats

The subcutaneous implantation of an estradiol pellet (10 mg) into female rats induced a hypophyseal hyperplasia with hyperprolactinaemia. Examination of neurotransmitter receptors in the hippocampus, striatum and cerebral cortex one month after the implantation revealed that estrogenization was associated with: an increased density of ³H-domperidone binding sites (D₂ receptors) in the striatum and reduced numbers of ³H-serotonin high affinity sites (5-HT₁ receptors) in the hippocampus and of ³H-muscimol binding sites (GABA receptors) in the hippocampus, striatum and cerebral cortex. In contrast, the characteristics of ³H-spiperone binding to 5-HT₂ receptors (in the cerebral cortex) and those of ³H-flunitrazepam binding to benzodiazepine sites (in the three brain regions examined) were not significantly different in estrogenized and in control female rats. However, the enhancing effect of GABA on ³H-flunitrazepam binding was markedly reduced in brain membranes from estrogenized animals. The respective roles of estradiol and prolactin in mediating these changes in neurotransmitter receptors are discussed notably with regard to the regional heterogeneity of estradiol binding capacity in the rat brain.     

The role of dopamine in behavioral supersensitivity to muscarinic antagonists following cholinesterase inhibition

The role of brain dopamine (DA) in the enhancement of muscarinic antagonist-induced hyperactivity was investigated. The effects of atropine and scopolamine on the concentrations of DA and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), following DFP administration were determined. In control animals, atropine and scopolamine decreased the concentration of DA and increased the ratios of DOPAC/DA and HVA/DA in the striatum, but not in the N. accumbens - T. olfactorium (mesolimbic) area. Following a single dose of DFP, the two antimuscarinic drugs caused decreases of DA and further increases of the above ratios in both brain regions. However, following repeated DFP treatment for 2 weeks, these antimuscarinic drug-induced changes were observed only in the mesolimbic area, but not in the striatum. It is suggested that an increased DA turnover, indicated by elevated DOPAC/DA and HVA/DA ratios, underlies the muscarinic antagonist-induced hyperactivity. The well-known occurrence of muscarinic receptor down-regulation after DFP administration, could be responsible for the enhancement of the actions of muscarinic antagonists in DFP-treated animals. The observed differential effect on DA turnover in the two broad areas may involve both muscarinic and DA receptors.     

Huntington's disease: regional alteration in muscarinic cholinergic receptor binding in human brain.

Huntington's Disease, an autosomal dominant neurological disorder, is characterized by diffuse neuronal degeneration particularly in the basal ganglia and cerebral cortex. The purpose of this study was to examine various discrete regions of choreic and control brains for alterations in muscarinic cholinergic receptor binding and choline acetyltransferase (ChAc) activity. Nine postmortem brains, three from patients with Huntington's Disease and six controls, were dissected into 17 discrete regions. Each regional homogenate was assayed for muscarinic receptor concentration by measuring specific membrane binding of [³H]-QNB, a potent muscarinic antagonist which selectively labels brain muscarinic receptors. Aliquots from each brain region were also assayed for ChAc activity. Of significance was the marked reduction in specific [³H]-QNB receptor binding in the caudate nucleus, putamen and globus pallidus of choreic brain while no significant alterations were detected in other brain regions. Significant decreases in ChAc activity were found in the caudate nucleus, putamen, and globus pallidus with no alterations in ChAc activity in the rest of the brain regions examined. The tissues were chosen such that protein levels were similar in both choreic and normal brain samples. The apparent reduction in the number of muscarinic cholinergic receptors in the choreic brains suggests that treatment with cholinomimetic drugs might be beneficial in Huntington's Disease.