Drug-induced changes in brain acetylcholine

Drug induces depression-like phenotypes and alters gene expression profiles in Drosophila

BACKGROUND

Major depressive disorder (MDD) is a severe mental illness that affects more than 350 million people worldwide. However, the molecular mechanisms of depression are currently unclear. Studies suggest that Drosophila and humans have similar depression-like symptoms under pressure. In this research, we choose Drosophila melanogaster as the animal model to explore the molecular mechanisms that trigger depression.

RESULTS

We found that feeding D. melanogaster with the medium containing Levodopa or Chlorpromazine could induce depression-like phenotypes in both behavioral and biochemical biomarkers, including significantly decreased food intake, mating frequency, serotonin (5-HT) concentration, and increased malondialdehyde (MDA) concentration as well as reduced activity of superoxide dismutase (SOD). Moreover, the progeny of Chlorpromazine-treated flies also showed these depression-like features. By RNA-seq technology, we identified 467 genes that were differentially expressed between Chlorpromazine treated (CPZ) and control male flies [fold-change of ≥2 (q-value<5%)]. When comparing CPZ with control flies, 312 genes were upregulated and 155 genes downregulated. Differential expression of genes related to metabolic pathway, Parkinson's disease, Huntington's disease, Alzheimer's disease and lysozyme pathways were observed. Quantitative reverse transcriptase PCR (qRT-PCR) confirmed that 19 genes are differentially expressed in CPZ and control male flies.

CONCLUSIONS

Levodopa, or Chlorpromazine can induce depression-like phenotypes in D. melanogaster regarding changes of appetite and sexual activity, and some key biochemical markers. A total of 467 genes were identified by RNA-seq analysis to have at least a 2-fold-change in expression between CPZ and control flies, including genes involved in metabolism, neurological diseases and lysozyme pathways. Our data provide additional insight into molecular mechanisms underlying depressive disorders in humans and may also contribute to clinical treatment.

Some neurochemical aspects of the depressant action of gamma-butyrolactone on the central nervous system

gamma-Butyrolactone, a depressant drug of the central nervous system, has been investigated for its ability to alter brain levels of 5-hydroxytryptamine, gamma-aminobutyric acid and acetylcholine in mice and rats; of these three compounds, only acetylcholine was changed in amount. Levels of acetylcholine in the cerebral cortex were increased by gamma-butyrolactone with a time-course which closely followed the depressed state of the animal. Indirect evidence has been presented to show that in the mid-brain and brain stem the change in acetylcholine level induced by gamma-butyrolactone is sharply localized in an area of the mesencephalon that contains the corpora quadrigemina.

Reserpine effects on neurotransmitters in chick heart during growth

Effects of tranquilizing agents on neurotransmitters in the heart have not been widely studied. Thus, the effect of intraperitoneal injection of reserpine, (2.5 mg/kg bw) on the concentrations of excitatory (glutamic acid, glutamine, aspartic acid, asparagine), inhibitory (GABA, glycine, alanine, taurine), neurotransmitters as well as the enzymes (GOT and GPT) and total protein were measured in both heart and serum chicks at different ages (1, 7, 30, 90 and 180 days). Reserpine induced a decrease in the excitatory amino acids and an increase in GABA in both heart and serum in most ages. Glycine and alanine increased in the heart and decreased in serum. Taurine increased in the heart of young ages (1 and 7 days) and decreased in older ones (90 and 180 days), however, it decreased in serum of most ages. Both GOT and GPT increased in heart but, in serum, GOT increased and GPT decreased in most ages. Total protein increased in the heart of young chicks and decreased in the 90- and 180-day-old chicks. In conclusion, reserpine induced a parallel decrease in the ratio glutamate, glutamine, aspartate/GABA in both myocardial tissue and serum of the different age groups. Changes observed in neurotransmitters of the heart suggest that these amino acids may play a similar role in the myocardial tissue, as is described in the central nervous system.

THE INFLUENCE OF CENTRALLY ACTING CHOLINOLYTIC DRUGS ON BRAIN ACETYLCHOLINE LEVELS

A number of centrally acting cholinolytic drugs reduced levels of cerebral acetylcholine in the rat. Among its naturally occurring analogues, hyoscine had the greatest potency, producing a decrease of 31% at a dose of 0.63 mg/kg. Atropine methyl nitrate, which acts as a cholinolytic drug in the periphery, had no effect on brain acetylcholine levels. The fall in acetylcholine produced by hyoscine was greatest after 60 min and disappeared at about 120 min. The animals tended to show a partial tolerance to this effect of hyoscine when the drug was administered repeatedly. The reduction in acetylcholine after hyoscine was restricted to the cerebral hemispheres, and did not appear in subcortical regions of the brain. Hyoscine had no influence on the net synthesis of acetylcholine by acetone-extracted powder of rat brain. In a series of four synthetic cholinolytic drugs, only the two with conspicuous psychotomimetic actions in man produced a decrease in brain acetylcholine comparable to that seen with hyoscine and related alkaloids.

THE EFFECT OF INTRA(CEREBRO)VENTRICULAR RESERPINE ON THE ACETYLCHOLINE CONTENT OF THE HEART, ILEUM AND HYPOTHALAMUS OF THE DOG

The effect of injection of reserpine into the cerebral ventricles on the acetylcholine contents of the sino-atrial node, ileum and hypothalamus of the dog was studied in ten dogs. Another group of five dogs served as a control. The effect of intravenous administration of reserpine, in the same dose as given intracerebroventricularly, was also studied on the acetylcholine content of these tissues in five dogs. General sedation, bradycardia, miosis, salivation, emesis and purgation were looked for. Tissues were removed 1 hr after administration of reserpine for estimation of acetylcholine content, which was increased in all the tissues studied. The increase in the peripheral tissues was greater than in the hypothalamus. The increase in the acetylcholine content was not quantitatively related to the other effects of reserpine. The increase in the acetylcholine content of the sino-atrial node and the ileum and also the peripheral effects observed on intracerebroventricular administration of reserpine can be attributed to its central action. With the same dose of reserpine given intravenously the acetylcholine content of the sino-atrial node was significantly increased, while that of the hypothalamus and ileum was not.

Effects of anticholinergic-antiparkinsonian drugs on striatal neurotransmitter levels of rats intoxicated with soman

Antimuscarinic drugs possessing antiparkinson activity that were effective in preventing convulsions induced by the organophosphorus cholinesterase (ChE) inhibitor soman were studied for their effects on spinal cord ChE activity and striatal levels of acetylcholine (ACh) and catecholamines in soman-intoxicated rats. Either biperiden (BPR) or trihexyphenidyl (THP) was administered to rats at an anticonvulsant dose (0.125 mg/kg, IM) in the presence or absence of soman (100 micrograms/kg, SC). The time course (up to 2 h) for ChE activity and levels of ACh and catecholamines were measured after soman, BPR, THP, soman and BPR, or soman and THP treatment. Soman rapidly inhibited ChE activity (65-75%; 15-120 min) and increased ACh levels (35%; at 30 min). It did not affect norepinephrine or dopamine (DA), but elevated at later time points (60-120 min) levels of the DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), thus indicating increased DA turnover. BPR and THP alone reduced striatal ACh level from control, but did not affect any other neurochemical parameters studied. THP and BPR each reversed the effects of soman on DOPAC and HVA levels, but neither affected ChE activity nor ACh level induced by soman. Thus, our findings suggest that the anticonvulsant effects of BPR and THP in soman poisoning may be attributed to their earlier reported muscarinic receptor blocking properties.

Anticonvulsants for poisoning by the organophosphorus compound soman: pharmacological mechanisms

Exposure to high doses of organophosphorus nerve agents such as soman, even with carbamate pretreatment, produces a variety of toxic cholinergic signs, including secretions, convulsions and death. Evidence suggests that soman-induced convulsions may be associated with postexposure brain neuropathology. The purpose of this study was to investigate the pharmacologic mechanism of action of soman-induced convulsions and of anticonvulsant drugs. Various classes of compounds were evaluated for their efficacy in preventing soman-induced convulsions in rats pretreated with the oxime HI-6 to increase survival time, along with various doses of the test compounds (IM) either in the absence or presence of atropine sulfate (16 mg/kg, IM) 30 minutes prior to a soman challenge dose (180 micrograms/kg, SC; equivalent to 1.6 x LD50) that produced 100% convulsions. Without atropine sulfate, only tertiary anticholinergics (scopolamine, trihexyphenidyl, biperiden, benactyzine, benztropine, azaprophen and aprophen), caramiphen, carbetapentane and MK-801 were effective anticonvulsants. In the presence of atropine sulfate, the benzodiazepines (diazepam, midazolam, clonazepam, loprazolam and alprazolam), mecamylamine, flunarizine, diphenylhydantoin, clonidine, CGS 19755 and Organon 6370 studied were effective. We have examined the possibility that diazepam may exert some of its anticonvulsant effects through cholinergic mechanisms and found that a reduced release of ACh into synapses after diazepam and atropine treatment may account for diazepam's anticonvulsant activity against soman. We also found that at anticonvulsant doses biperiden and trihexyphenidyl each significantly reversed the effects of soman on striatal levels of DOPAC and HVA, the metabolites of dopamine, and have concluded that in addition to actions on muscarinic receptors, the anticonvulsant effects of these anticholinergics in soman poisoning may be partially related to their actions on the striatal dopaminergic system. These findings allow us to postulate that central muscarinic cholinergic mechanisms are primarily involved in eliciting the convulsions following exposure to soman and that subsequent recruitment of other excitatory neurotransmitter systems and loss of inhibitory control may be responsible for sustaining the convulsions and for producing the subsequent brain damage. Future studies to confirm these neuropharmacological mechanisms are proposed.

Cholinergic actions of diazepam and atropine sulfate in soman poisoning

The effectiveness of diazepam alone or in the presence of atropine sulfate in reversing soman-induced convulsions, inhibition of blood and brain cholinesterase (ChE) activity, and elevation of brain acetylcholine (ACh) and choline (Ch) concentrations in rats was studied. Diazepam (5 mg/kg, IM) blocked the convulsive activity of soman (100 micrograms/kg, SC) whereas atropine sulfate (12 mg/kg, IM) did not. Inclusion of atropine sulfate enhanced the anticonvulsant effects of diazepam. Neither diazepam nor atropine sulfate alone affected ChE activity in the blood and brain of rats, nor did they alone, or in combination, reverse the ChE inhibition induced by soman. Diazepam by itself caused an increase in ACh concentrations in the striatum and a decrease in Ch concentrations in the cortex and striatum. On the other hand, atropine sulfate produced a decrease in ACh and an increase in Ch concentrations in these two brain regions. With combined treatment, diazepam reversed the effect of atropine sulfate on brain ACh and Ch concentrations. Diazepam attenuated the soman-induced elevation of ACh and Ch concentrations in most of the brain regions studied, while atropine sulfate did not. Only when diazepam was given concurrently with atropine sulfate did the elevated brain ACh or Ch concentrations induced by soman return to normal. These results suggest that the anticonvulsant activity of diazepam in soman poisoning may be partially related to its action on presynaptic cholinergic mechanism.

Minaprine improves impairment of working memory induced by scopolamine and cerebral ischemia in rats

Using a repeated acquisition procedure in a three-panel runway apparatus, the effects of minaprine on the impairment of working memory produced by scopolamine, ethylcholine aziridinium ion (AF64A) or cerebral ischemia were investigated in rats. Minaprine (3.2-32 mg/kg IP) as well as idebenone (10-100 mg/kg IP) and physostigmine (0.1-0.32 mg/kg IP) dose-dependently reduced the increase of errors (pushes made on the two incorrect panels located at each choice point) induced by 0.56 mg/kg IP scopolamine. Cerebral ischemia for 5 min caused a significant increase of errors in the runway task. Minaprine at 3.2 and 10 mg/kg administered IP immediately after blood recirculation and again 30 min before the runway test conducted 24 h after ischemia, significantly reduced increases in errors expected to occur after 5 min of ischemia. Physostigmine 0.1 mg/kg similarly attenuated the increase in errors in ischemic rats. However, minaprine at doses up to 32 mg/kg IP failed to reduce the increase of errors induced by AF64A 2.5 nmol injected into the dorsal hippocampus. These findings suggest that minaprine exerts an ameliorating effect on amnesia produced by scopolamine and cerebral ischemia, probably through mediation of its stimulant action on central cholinergic systems.

Neurochemical effects of minaprine, a novel psychotropic drug, on the central cholinergic system of the rat

Minaprine, a novel psychotropic drug with antidepressant, anticataleptic and antiaggressive properties, produced an increase in rat brain regional acetylcholine content at a subconvulsant dose of 30 mg/kg IP. The greatest increase (60%) was produced in the striatum, whereas an increase of about 35% was obtained in the hippocampus and the rest of the cortex. A small but significant increase of 14% was also found in the midbrain-hindbrain region. Minaprine decreased choline content only in the striatum. No tolerance to acute challenge was observed after 10-day chronic treatment. In vitro, the drug had no effect on striatal choline acetyltransferase activity up to a concentration of 160 microM and only weakly displaced (3H) dexetimide from its specific muscarinic receptor binding sites in striatum (IC50, 2 X 10(-4) M). After in vivo administration the drug did not affect sodium-dependent high affinity choline uptake by a hippocampal homogenate. On the other hand, the drug inhibited both striatal and hippocampal acetylcholinesterase activity at high (40-160 microM) concentrations in vitro. In vivo the drug produced a brief (5 min), small (18%) decrease in the enzymic activity which corresponded in time to the peak drug level attained in the brain, but was not concomitant with a change in striatal acetylcholine content. By contrast, the increase in striatal acetylcholine appeared after 30 min when there was no longer inhibition of acetylcholinesterase activity and when the level of minaprine in brain was reduced by 78%. Blockade of dopamine receptors by pimozide pretreatment partially prevented the increase in striatal acetylcholine produced by minaprine, whereas interference with cholinergic or serontonergic neurotransmission was without effect.(ABSTRACT TRUNCATED AT 250 WORDS)

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 4-aminopyridine on acetylcholine output from the cerebral cortex of the rat in vivo

1 The effects of 4-aminopyridine (4AP) on the output of acetylcholine (ACh) from the cerebral cortex were investigated in unanaesthetized freely moving rats and in anaesthetized rats by means of the ;cup technique'. ACh was determined by bioassay on the dorsal muscle of the leech.2 In unanaesthetized rats intraperitoneal injection of 4AP (3 mg/kg) had no effect on the cortical output of ACh.3 After injection of morphine (10 mg/kg s.c.), which depressed the spontaneous output of ACh, 4AP increased the cortical output to a level significantly higher than that determined before morphine injection.4 In rats anaesthetized with either urethane or pentobarbitone, drugs known to decrease cortical output of ACh, 4AP (i.v. or i.p.) elicited a significant increase in the output of ACh. The time-courses of the 4AP-induced effects were different depending on the anaesthetic drug used: an immediate increase slowly fading in urethane anaesthesia and a gradual increase after delayed onset in pentobarbitone-anaesthetized rats.5 In some urethane-anaesthetized rats, respiratory frequency was kept constant (tracheotomy, connection to respirator, bilateral vagotomy) and prazosin (1 mg/kg i.v.) was administered to reduce the 4AP-induced increase of blood pressure. Cortical output of ACh was not related to changes in blood pressure. Moreover, the 4AP-induced increase in cortical ACh output was not related to changes in respiratory frequency.6 In summary systemic administration of 4AP in subconvulsive doses (1 and 3 mg/kg) increased cortical output of ACh in rats anaesthetized with urethane or pentobarbitone or after injection of morphine, but not in untreated freely moving rats. It is suggested that the anaesthetic agents and morphine may cause an imbalance between excitatory and inhibitory central pathways, and that this imbalance may play a role in their depressant effect on cortical output of ACh and/or in the 4AP-induced facilitation described in this paper.

The effect of naloxone on opioid-induced inhibition and facilitation of acetylcholine release in brain slices

1 The effect of morphine, methionine-enkephalin (Met-enkephalin) and D-Ala2-D-Leu5-enkephalin (DADLE) were tested on the spontaneous and electrically-evoked release of acetylcholine (ACh) from superfused slices of guinea-pig thalamus, caudate nucleus and cerebral cortex. 2 At no concentration did morphine, Met-enkephalin or DADLE modify the outflow of ACh at rest but Met-enkephalin in the presence of naloxone, reduced the resting ACh release. 3 Morphine, at a low dose (3 microM) had no effect in slices of cerebral cortex, but it enhanced the evoked release of ACh in thalamic and caudate, slices. At higher doses of morphine (10-30 microM), the ACh release evoked by electrical pulses was significantly inhibited in every area. 4 Met-enkephalin behaved like morphine in thalamic slices, whereas DADLE, a specific delta agonist, produced a slight inhibition of ACh outflow only at 10 microM. 5 Naloxone antagonized the inhibitory effect of morphine in the cerebral cortex and caudate nucleus slices. Naloxone and also spiroperidol blocked the releasing effect of morphine in caudate slices. In contrast naloxone did not affect the increase of ACh release caused by morphine and Met-enkephalin in thalamic slices. The inhibitory effect of both opioids at high doses was reversed by naloxone so that they then enhanced ACh release. 6 A two fold increase of calcium concentration in the Krebs solution prevented the inhibitory effects of morphine 10 microM. 7 It is suggested that two receptors are present in thalamic slices, one of which inhibits and the other facilitates ACh release.

Time course effects of soman on acetylcholine and choline levels in six discrete areas of the rat brain

The time course of changes in rat brain levels of acetylcholine (ACh) and choline (Ch) was investigated following a single SC injection of soman (0.9 LD50, 120 micrograms/kg) to understand the relationship between central neurotransmitter alteration and soman toxicity. Of the animals exposed to the dose of soman, 46% died within 24 h, with maximum mortality occurring during the first 40 min following soman administration. In a second group, surviving rats were killed at various times after treatment by a beam of focused microwave radiation to the head, and ACh and Ch levels were determined by gas chromatography-mass spectrometry. Soman produced a maximal ACh elevation in the brain stem at 20 min (34.4%), in cerebellum at 40 min (51.9%), in cortex and striatum at 2 h (320.3% and 35.2%, respectively), and in hippocampus and midbrain at 3 h (94.5% and 56.8%, respectively). ACh levels remained above normal approximately 30 min in the brain stem; 2 h in the midbrain, cerebellum, and striatum; 8 h in the cortex; and 16 h in the hippocampus. Ch levels were elevated in all areas except the striatum. Ch maxima occurred at 10-40 min and returned to control levels approximately 3 h after injection. Results suggest that perturbation of ACh levels due to soman was not uniform throughout the brain and that soman toxicity may reflect ACh changes in multiple areas, rather than changes in any given area. These data further suggest a possible relationship between elevated Ch levels and soman toxicity.

Modification of the 5-hydroxytryptophan-induced head-twitch response by exogenous endocrine agents

Estrogens both alone and in combination with progestagens, gonadotrophins, gonadotrophin-releasing factor, corticosteroids, and corticotrophin antagonise the head-twitch response produced by 5-hydroxytryptophan in mice. In contrast, a potentiation was seen following thyroid hormones, thyrotrophin, and thyrotrophin-releasing hormone. Pre-treatment with androgens had no significant effect. Possible mechanisms for these interactions are discussed.

Regulation of acetylcholine synthesis: does cytoplasmic acetylcholine control high affinity choline uptake?

When brain synaptosomes are obtained from animals that have been injected intravenously with [2H4]choline 1 minute before being killed, their high affinity [3H] choline uptake is correlated inversely with their acetylcholine content and directly with the rate at which they synthesize [2H4]acetylcholine. The control of such choline uptake by the cytoplasmic acetylcholine concentration is proposed as a mechanism regulating acetylcholine synthesis in cholinergic nerve terminals.

Effects of drugs and axotomy on acetylcholine levels in central cholinergic neurons

Administration of cholinergic nuscarinic agonists either did not alter, or increased hippocampal Ach levels, while antagonists lowered the levels. As observed previously, placement of lesions in the medial septal area resulted in a rise in hippocampal Ach levels in the first thirty minutes. Agonist drugs administered immediately before lesion, further increased the post-lesion rise in Ach levels. With antagonists, inconsistent results were obtained in that atropine blocked the post-lesion rise, while scopolamine and quinuclidinyl benzilate (QNB) potentiated the post-lesion rise. These results suggest that there may be receptor-mediated effects on Ach synthesis or release after axotomy.

Effects of some centrally acting drugs on acetylcholine synthesis by rat cerebral cortex slices

1. Brain cortex slices from rats injected i.p. with urethane (1 g/kg), chloral hydrate (350 mg/kg) or physostigmine (0.75 mg/kg) were examined for acetylcholine (ACh) content, cholinesterase (total enzyme) activity and formation of (14)C-ACh from carbon (14)-uniformly labelled glucose (U-(14)C-D-glucose) in the presence of 0.01 mM physostigmine.2. Slices from rats treated with urethane, chloral hydrate, or physostigmine contained significantly higher concentrations of ACh than slices from untreated animals.3. Only slices from physostigmine-treated rats had a significantly lower cholinesterase activity.4. Slices from urethane- or chloral hydrate-treated animals formed significantly less (14)C-ACh than slices from untreated or physostigmine-treated rats when incubated in 4 mM K(+) medium. In an ACh-releasing medium (31 mM K(+)) slices from rats treated with urethane or chloral hydrate and slices from untreated rats formed similar amounts of (14)C-ACh.5. Slices from rats treated with atropine (25 mg/kg) or pentylenetetrazol (75 mg/kg) had a similar ability to form (14)C-ACh as slices from untreated animals when incubated in either 4 or 31 mM K(+) medium.6. These findings suggest that the intraneuronal ACh concentration is a limiting factor in the regulation of ACh synthesis.

Morphine-induced kinetic alterations of choline acetyltransferase of the rat caudate nucleus

1. In order to explain the decrease of choline acetyltransferase (2.3.1.6.) activity observed in the caudate nucleus of morphine-treated rats, partially purified preparations of the enzyme were used in kinetic studies, with choline as substrate.2. The apparent Michaelis constant for the enzyme obtained from normal rats was found to be 0.9 mM choline; this value doubled when the animals were killed one hour after a single injection of morphine (30 mg/kg). When the rats were injected daily for 4 or 15 days, and killed one hour after the last injection, the apparent Km value was 2.1 mM in each case. Prolonged daily treatment with morphine, followed by 48 h withdrawal, or by administration of 4 mg/kg of naloxone (given half an hour after the last injection of morphine) resulted in apparent Km values of 1.3-1.5 mM of choline, suggesting a gradual return to the lower, normal substrate requirement. Vmax changes were insignificant.3. The effect of morphine added in vitro to different enzyme preparations was also studied. The Km values of 0.9 mM, in the enzyme isolated from normal rats, increased to 2.0 after incubation in vitro with 12.5 mM morphine. Similar increases were found in enzymes obtained from rats 48 h after the withdrawal of morphine or from rats injected with naloxone after prolonged morphine treatment. The high apparent Km values, found in enzyme obtained from animals killed one hour after the last dose of morphine, did not change upon incubation with 12.5 mM morphine. A similar pattern of Km changes was noticed after incubation with 25 mM acetylcholine.4. An increase of 32% in acetylcholine (ACh) level was found in the caudate nucleus one hour after subcutaneous injection of 30 mg/kg of morphine. Return to normal values was observed when morphine was administered daily. After two to three weeks of daily treatment and subsequent withdrawal from morphine for 48 h, the levels of ACh were normal. If the daily treated rats were given naloxone within half an hour of the last injection of morphine, and killed 30 min later, the levels of ACh remained normal.5. Fifty per cent inhibition of enzyme activity was observed upon in vitro incubation with 75 mM acetylcholine, or with 25 mM morphine. The same degree of inhibition was noticed when the enzyme was obtained from normal or from morphine-treated rats.

Effects of morphone on choline acetyltransferase levels in the caudate nucleus of the rat

1. Choline acetyltransferase (choline-o-acetyltransferase 2.3.1.6.) concentrations were determined in the caudate nucleus, thalamus, and cortex of control and morphine treated rats. The enzyme was assayed using a modified radiochemical method on a number of selected days, one hour after the last injection of 30 mg/kg of morphine and also during the subsequent phase of abstinence from morphine.2. Significant lowering of choline acetyltransferase activity in the caudate nucleus area was found in two cases, one hour after the first dose of morphine and upon subsequent abstinence from morphine.3. The enzyme activity in the two other parts of the brain remained at the normal levels.4. The presence of endogenous inhibitors formed during morphine administration was excluded.5. The relationship of a possible effect of morphine on the tissue binding of the enzyme and the subsequent lowering of its activity was tested by homogenization of the caudate nucleus area in different media. The decrease in enzyme activity occurred in all extraction media one hour after morphine administration.6. Inhibitory effects of in vitro addition of morphine to caudate nucleus homogenate, obtained from normal and morphine treated rats, were found to occur only at very high concentrations of the drug, negating the possibility of direct inhibitory effects of morphine.7. These experiments suggest two possible causes of the observed effects, which can be responsible for the lowering of enzyme activity, and can be operative simultaneously: (1) a negative feedback mechanism of accumulated acetylcholine, occurring after the first dose of morphine, and (2) the possible changes in enzyme configuration produced by morphine treatment.