(R,S)-trihexyphenidyl, acting via a muscarinic receptor-independent mechanism, inhibits hippocampal glutamatergic and GABAergic synaptic transmissions: Potential relevance for treatment of organophosphorus intoxication

Preclinical studies have reported that, compared to the muscarinic receptor (mAChR) antagonist atropine, (R,S)-trihexyphenidyl (THP) more effectively counters the cholinergic crisis, seizures, and neuropathology triggered by organophosphorus (OP)-induced acetylcholinesterase (AChE) inhibition. The greater effectiveness of THP was attributed to its ability to block mAChRs and N-methyl-d-aspartate-type glutamatergic receptors (NMDARs) in the brain. However, THP also inhibits α7 nicotinic receptors (nAChRs). The present study examined whether THP-induced inhibition of mAChRs, α7 nAChRs, and NMDARs is required to suppress glutamatergic synaptic transmission, whose overstimulation sustains OP-induced seizures. In primary hippocampal cultures, THP (1-30 μM) suppressed the frequency of excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs, respectively) recorded from neurons in nominally Mg2+-free solution. A single sigmoidal function adequately fit the overlapping concentration-response relationships for THP-induced suppression of IPSC and EPSC frequencies yielding an IC50 of 6.3 ± 1.3 μM. Atropine (1 μM), the NMDAR antagonist d,l-2-amino-5-phosphonopentanoic acid (D,L-AP5, 50 μM), and the α7 nAChR antagonist methyllycaconitine (MLA, 10 nM) did not prevent THP-induced inhibition of synaptic transmission. THP (10 μM) did not affect the probability of transmitter release because it had no effect on the frequency of miniature IPSCs and EPSCs recorded in the presence of tetrodotoxin. Additionally, THP had no effect on the amplitudes and decay-time constants of miniature IPSCs and EPSCs; therefore, it did not affect the activity of postsynaptic GABAA and glutamate receptors. This study provides the first demonstration that THP can suppress action potential-dependent synaptic transmission via a mechanism independent of NMDAR, mAChR, and α7 nAChR inhibition.

Central inhibition prevents the in vivo acute toxicity of harmine in mice

BACKGROUND

Harmine is a β-carboline alkaloid that displays antidepressant, antitumor and other pharmacological effects. However, the strong toxic effects limit its clinical application, and should be first considered.

PURPOSE

To evaluate the in vivo toxicity of harmine and explore intervention strategies against its toxicity.

METHODS

The in vivo toxicity of harmine was assessed from the symptoms, biochemical indices, and cardiovascular effects in mice. The intervention experiments were performed by using anesthetics, central drugs, and peripheral anticholinergics.

RESULTS

The acute toxicity of harmine is significantly dose-dependent and the median lethal dose is 26.9 mg/kg in vivo. The typical symptoms include convulsion, tremor, jumping, restlessness, ataxia, opisthotonos, and death; it also changes cardiovascular function. The anesthetics improved the survival rate and abolished the symptoms after harmine poisoning. Two central inhibitors, benzhexol and phenytoin sodium, uniformly improved the survival rates of mice poisoned with harmine. The peripheral anticholinergics didn't show any effects.

CONCLUSION

Harmine exposure leads to central neurological symptoms, cardiovascular effects and even death through direct inhibition of the central AChE activity, where the death primarily comes from central neurological symptoms and is cooperated by the secondary cardiovascular collapse. Central inhibition prevents the acute toxicity of harmine, and especially rapid gaseous anesthetics such as isoflurane, might have potential application in the treatment of harmine poisoning.

Changes in sensitivity of reward and motor behavior to dopaminergic, glutamatergic, and cholinergic drugs in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is a leading cause of intellectual disability. FXS is caused by loss of function of the FMR1 gene, and mice in which Fmr1 has been inactivated have been used extensively as a preclinical model for FXS. We investigated the behavioral pharmacology of drugs acting through dopaminergic, glutamatergic, and cholinergic systems in fragile X (Fmr1 (-/Y)) mice with intracranial self-stimulation (ICSS) and locomotor activity measurements. We also measured brain expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis. Fmr1 (-/Y) mice were more sensitive than wild type mice to the rewarding effects of cocaine, but less sensitive to its locomotor stimulating effects. Anhedonic but not motor depressant effects of the atypical neuroleptic, aripiprazole, were reduced in Fmr1 (-/Y) mice. The mGluR5-selective antagonist, 6-methyl-2-(phenylethynyl)pyridine (MPEP), was more rewarding and the preferential M1 antagonist, trihexyphenidyl, was less rewarding in Fmr1 (-/Y) than wild type mice. Motor stimulation by MPEP was unchanged, but stimulation by trihexyphenidyl was markedly increased, in Fmr1 (-/Y) mice. Numbers of midbrain TH+ neurons in the ventral tegmental area were unchanged, but were lower in the substantia nigra of Fmr1 (-/Y) mice, although no changes in TH levels were found in their forebrain targets. The data are discussed in the context of known changes in the synaptic physiology and pharmacology of limbic motor systems in the Fmr1 (-/Y) mouse model. Preclinical findings suggest that drugs acting through multiple neurotransmitter systems may be necessary to fully address abnormal behaviors in individuals with FXS.

Colonic architectural change on colonoscopy in patients taking psychotropic medications

BACKGROUND

Patients on psychotropic medications have been clinically observed to have higher rates of abnormal colonic architecture resulting in difficult colonoscopies. This study aims to determine if a correlation between use of psychotropic medications and colonic architectural change seen on colonoscopy exists.

METHODS

A retrospective case-control study was undertaken with 252 adults selected from the hospital endoscopy database between January 2006 and July 2008. Cases were selected if they had 'capacious', 'megacolon', 'redundant' and/or 'featureless' colonic architecture reported in their first completed colonoscopy (n = 63). Demographic information and medication records were collected for both cases and controls. Logistic regression analysis was performed for each of the medication groups.

RESULTS

Medication groups associated with increased incidence for colonic architectural changes observed during colonoscopy include: antipsychotic medications [odds ratio (OR) 7.79, confidence interval (CI) 2.59-23.41], benzhexol (OR 23.50, CI 2.83-195.08) and iron tablets (OR 2.97, CI 1.39-6.33). Antidepressants, laxatives, benzodiazepines, gastroprotective medications and antihypertensive medications were not found to have any significant effect on changes to colonic architecture.

CONCLUSIONS

Use of antipsychotic medications is associated with changes to colonic architecture. This could predispose such a patient to difficult colonoscopy and therefore increase colonoscopy-associated risks. Medication history should be elicited prior to colonoscopy.

An anticholinergic reverses motor control and corticostriatal LTD deficits in Dyt1 ΔGAG knock-in mice

DYT1 early-onset generalized torsion dystonia is an inherited movement disorder associated with mutations in DYT1 that codes for torsinA protein. The most common mutation seen in this gene is a trinucleotide deletion of GAG. We previously reported a motor control deficit on a beam-walking task in our Dyt1 ΔGAG knock-in heterozygous mice. In this report we show the reversal of this motor deficit with the anticholinergic trihexyphenidyl (THP), a drug commonly used to treat movement problems in dystonia patients. THP also restored the reduced corticostriatal long-term depression (LTD) observed in these mice. Corticostriatal LTD has long been known to be dependent on D2 receptor activation. In this mouse model, striatal D2 receptors were expressed at lower quantities in comparison to wild-type mice. Furthermore, the mice were also partially resistant to FPL64176, an agonist of L-type calcium channels that have been previously reported to cause severe dystonic-like symptoms in wild-type mice. Our findings collectively suggest that altered communication between cholinergic interneurons and medium spiny neurons is responsible for the LTD deficit and that this synaptic plasticity modification may be involved in the striatal motor control abnormalities in our mouse model of DYT1 dystonia.

Effects of trihexyphenydil, the structural analog of phencyclidine, on neocortical and hippocampal electrical activity in sleep-waking cycle

Finding about structural and functional relation between NMDA receptors specific binding and phencyclidine sites was very important for a possible modulation of NMDA receptors' function. We have therefore got interested what would happen with EEG and vegetative patterns of PS in the case when NMDA receptors function is modulated by blocking of phencyclidines' site. Consequently, we studied the effects of Trihexyphenydil, the structural analog of phencyclidine, on neocortical and hippocampal electrical activity in SWC. On cats (n=5) metallic electrodes were implanted under Nembutal anesthesia. EEG registration lasting 12 hr daily started after animals' recovery. Trihexyphenydil was administered intraperitoneally (0.5 mg/kg - 1 mg/kg). Statistical processing was made by Students' t-test. Trihexyphenydil resulted in dissociated triggering of PS. Rapid eye movements and PGO waves appeared on the face of active waking state. Therefore on the background of behavioral active waking according to electrical activity of the visual cortex and rapid eye movements, electrographic patterns of paradoxical sleep were recorded. Thus in our experiments it was shown firstly that the mechanism of hallucinogenic action of Trihexyphenydil is closely related to the disturbance of paradoxical sleep integrity. Blocking of NMDA receptors phencyclidines site and therefore functional modulation of these receptors produce the splitting of PS patterns and their intrusion in waking state. Such an effect never takes place in normal conditions since the waking system has the powerful inhibitory influence on the PS triggering system. Suggestion is make that NMDA glutamate receptors must be involved in mechanisms providing structural and functional integrity of PS and that fulfillment of such function is possible in the case when the NMDA receptors phencyclidine site isn't in blocked state. Normal functioning of NMDA receptors phencyclidine site represents the mechanism which inhibits and/or hampers appearance of hallucination. NMDA glutamate receptors, possessing phencyclidine site, are implicated in the mechanisms providing structural and functional integrity of PS.

Muscarinic preferential M(1) receptor antagonists enhance the discriminative-stimulus effects of cocaine in rats

Previous studies of benztropine analogues have found them to inhibit dopamine uptake like cocaine, but with less effectiveness than cocaine in producing behavioral effects related to drug abuse. Studies have assessed whether nonselective muscarinic antagonists decrease the effects of cocaine because many of the benztropine analogues are also muscarinic antagonists. As previous studies were conducted with nonselective muscarinic antagonists and the benztropine analogues show preferential affinity for the M(1) muscarinic receptor subtype, the present study examined interactions of cocaine and the preferential M(1) antagonists, telenzepine (TZP) and trihexyphenidyl (TXP) on subjective effects in rats trained to discriminate cocaine (10 mg/kg, i.p.) from saline injections. Cocaine dose-dependently increased the percentage of responses on the cocaine-appropriate lever, with full substitution at the training dose. In contrast neither TZP nor TXP produced more than 25% cocaine-appropriate responding at any dose. Both M(1) antagonists produced significant leftward shifts in the cocaine dose-effect curve, TZP at 3.0 and TXP at 0.3 and 1.0 mg/kg. The present results indicate that preferential antagonist actions at muscarinic M(1) receptors enhance rather than attenuate the discriminative-stimulus effects of cocaine, and thus those actions unlikely contribute to the reduced cocaine-like effects of BZT analogues.

Anti-fibrillogenic and fibril-destabilizing activities of anti-Parkinsonian agents for α-synuclein fibrils in vitro

The aggregation of alpha-synuclein (alphaS) in the brain has been implicated as a critical step in the development of Lewy body diseases (LBD) and multiple system atrophy (MSA). Among the antioxidant strategies proposed, increasing evidence points to the possibility of achieving neuroprotection by dopamine agonists, as well as monoamine oxidase B inhibitors. We showed previously that the anti-Parkinsonian agents dose-dependently inhibited beta-amyloid fibrils (fAbeta)(1-40) and fAbeta(1-42) formation as well as destabilized preformed fAbetas. Using fluorescence spectroscopy with thioflavin S, electron microscopy, and atomic force microscopy, we examined the effects of anti-Parkinsonian agents, selegiline, dopamine, pergolide, bromocriptine, and trihexyphenidyl on the formation of alphaS fibrils (falphaS) and on preformed falphaS. All molecules except for trihexyphenidyl, dose-dependently inhibited the formation of falphaS. Moreover, these molecules dose-dependently destabilized preformed falphaS. The overall activity of the molecules examined was in the order of: selegiline = dopamine > pergolide > bromocriptine. These agents and other compounds related structurally could be key molecules for the development of therapeutics for LBD and MSA.

Role of Cholinergic Structures in Individual Resistance of Rat Circulatory System to Posthemorrhagic Hypoxia

Experiments employing ultrasound technique showed that nonselective blockade of central muscarinic cholinoceptors with amizyl significantly increases the number and lifespan of rats highly resistant to acute massive blood loss. This pretreatment increased individual resistance of the circulatory system to posthemorrhagic hypoxia (blood pressure and portal blood flow rate). Preliminary blockade of central nicotinic cholinoceptors and peripheral muscarinic cholinoceptors with cyclodol and methacin, respectively, had no effect on the percentage of rats highly and low resistant to acute blood loss. Preliminary blockade of peripheral muscarinic cholinoceptors with methacin prevented the decrease in the cardiac output in low resistant animals during the posthemorrhagic period.

Structure-activity relationship of trihexyphenidyl analogs with respect to the dopamine transporter in the on going search for a cocaine inhibitor

A series of trihexyphenidyl (THP) analogs were used to search for a derivative that could serve as a cocaine inhibitor. A compound that blocks binding of the cocaine analog carboxyfluorotropane (CFT), allows dopamine uptake and exhibits low side effects could serve as a good candidate for that purpose. All analogs were tested for the extent to which they inhibit CFT binding, dopamine uptake and n-methyl scopolamine (NMS) binding. Several structure-function relationships emerged. Methylation/halogenation of THP's benzene ring enhanced the compound's ability to block CFT binding in comparison to its ability to block dopamine uptake (5a-e). Replacement of the cyclohexyl ring with a benzene ring tended to create compounds that had lower affinities to the dopamine transporter (7b compared to THP, 7d compared to 5h, 7c compared to 8c) and modification of THP's piperidine ring tended to enhance affinity to the dopamine transporter (5f-h, 8a, 8c). One analog (5f) that showed little muscarinic activity indicating that it would probably have few side effects was investigated for its effects as an in vivo cocaine inhibitor. However, it showed few antagonistic effects in vivo. Nevertheless, this work greatly elucidates the structure-function relationships required for potential cocaine inhibitors and so lays out promising directions for future research.

Treatment with subthreshold doses of caffeine plus trihexyphenidyl fully restores locomotion and exploratory activity in reserpinized rats

Trihexyphenidyl (THP) is a drug commonly used to reduce parkinsonian symptoms. An important side effect of this agent is memory impairment. Since caffeine enhances the potency of THP to inhibit haloperidol-induced catalepsy, caffeine may be used as an adjuvant of lower doses of THP, in order to improve its antiparkinsonian effects without causing memory disruption. To further assess the synergism between caffeine and THP, both drugs were tested in reserpinized rats, another preclinical model of Parkinson's disease. Four groups of rats (n = 7) were treated with reserpine (5 mg/kg, i.p.). A control group (n = 7) was treated only with the vehicle for reserpine (dimethylsulphoxide). The spontaneous locomotor behavior was tested 24 h later in a box with infrared sensors, 30 min after receiving one of the following treatments: distilled water (1 ml/kg), caffeine (1 mg/kg), THP (0.1 mg/kg) or caffeine plus THP. The levels of horizontal locomotion (14 +/- 5%) and vertical exploration (15 +/- 10%) were significantly lower in reserpinized rats treated with distilled water, compared with the mean activity values (100%) recorded in animals pretreated only with the vehicle for reserpine. The reserpine-induced hypokinesia was neither reversed by caffeine alone nor by THP alone. However, the combination of caffeine plus THP restored locomotion (141 +/- 19%) and vertical exploration (82 +/- 17%) to levels not significantly different to those of non-reserpinized rats. Moreover, the time-course of locomotion and exploration displayed the characteristic habituation over time, in which short-term memory processes are involved. Also, the thigmotaxis index indicated that the combined treatment did not induce anxiety-like behavior. Hence, these results support the proposal that low, subthreshold doses of caffeine plus THP have the potential to alleviate the motor disabilities in parkinsonian patients, with a low risk of causing anxiety or memory impairment.

Effect of M2Muscarinic Receptor Blockade in Rats with Haloperidol-Produced Catatonic Syndrome

We studied the functional role of individual subtypes of muscarinic cholinoceptors in the pathogenesis of neuroleptic parkinsonism in rats. Blockade of M4 receptors prevented the development of extrapyramidal disorders, which was abolished by simultaneous blockade of M2 receptors. The data suggest that various subtypes of muscarinic receptors are involved in the regulation of dopamine concentration.

Discriminative stimulus properties of 1.25 and 5.0 mg/kg doses of clozapine in rats: examination of the role of dopamine, serotonin, and muscarinic receptor mechanisms

Clozapine (CLZ), an atypical antipsychotic drug (APD), produces minimal extrapyramidal side effects (EPS) and has significant advantages for treating both positive and negative symptoms in schizophrenic patients. CLZ has been established as a discriminative cue in the drug discrimination paradigm and in generalization tests the CLZ cue is more selective for atypical, rather than typical, APDs. However, greater selectivity for atypical antipsychotics has been demonstrated with a lower (1.25 mg/kg) CLZ training dose in rats [Psychopharmacology, 149 (2000) 189], rather than the traditional, higher training dose (5.0 mg/kg). It is therefore of interest to evaluate the properties mediating the 1.25 mg/kg CLZ discriminative cue. In the present study, rats were trained to discriminate either 1.25 mg/kg (N=7) or 5.0 mg/kg (N=7) CLZ from vehicle in a two-lever drug discrimination task. The typical antipsychotic haloperidol (0.1-0.4 mg/kg) did not substitute for either CLZ cue, whereas the atypical antipsychotic melperone (0.37-3.0 mg/kg) provided full substitution in both groups (>80% CLZ-appropriate responding). The 5-HT(1A) receptor agonist (+)-8-OH-DPAT (0.04-0.16 mg/kg), and the selective 5-HT(2A) receptor antagonist M100907 (0.03-1.0 mg/kg) did not produce substitution in either group. (+)-8-OH-DPAT combined with haloperidol (0.05 mg/kg) engendered only partial substitution (>60% CLZ-appropriate responding) for both CLZ cues, and M100907 combined with haloperidol (0.05 and 0.1 mg/kg doses) failed to provide substitution in either group. Trihexyphenidyl (0.18-6.0 mg/kg), a muscarinic M(1)-preferring receptor antagonist, engendered full substitution for the 1.25 mg/kg CLZ cue, but only partial substitution for the 5.0 mg/kg CLZ cue. These results provide evidence that antagonism at the muscarinic M(1) receptor is sufficient to provide 1.25 mg/kg CLZ-like discriminative stimulus effects.

Selective A2A, but not A1 adenosine antagonists enhance the anticataleptic action of trihexyphenidyl in rats

In rats made cataleptic with haloperidol (5.32 micromol/kg), the bar test was used to assess the possible synergism between the muscarinic antagonist trihexyphenidyl (THP) and selective adenosine A(1) and A(2A) receptor antagonists. Neither catalepsy intensity nor latency were affected by a subthreshold dose of THP (0.33 micromol/kg). The selective adenosine A(1) antagonist 8-cyclopentyl-1,3-dipropyl-xanthine (DPCPX) (5.15 micromol/kg) caused a small, but significant reduction of catalepsy intensity that remained unchanged when combined with THP. DPCPX had no effect on catalepsy latency, either alone or combined with THP. In contrast, an equimolar dose of the selective adenosine A(2A) antagonist 4-(2-[7-amino-2-(2-furyl)1,2,4-triazolo[2,3-a]-[1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) (5.15 micromol/kg) produced a significant reduction of catalepsy intensity and increased catalepsy latency. Both effects were potentiated when ZM 241385 was combined with THP. The synergism was more evident when rats were pretreated with a subthreshold dose of ZM 241385 (1.55 micromol/kg) that was unable to modify catalepsy parameters when applied alone, but produced a significant reduction in catalepsy intensity and an increase in catalepsy latency when administered with THP. Catalepsy was unaffected by a combination of equimolar, subthreshold doses of DPCPX (1.55 micromol/kg) and ZM 241385 (1.55 micromol/kg). These findings indicate that the anticataleptic effect of anticholinergics is enhanced only by the selective blockade of adenosine A(2A) receptors.

Suppressive effects of trihexyphenidyl on methamphetamine-induced dopamine release as measured by in vivo microdialysis

Abuse of methamphetamine (MAP) and cocaine causes severe medical and social problems throughout the world. Our previous study found that trihexyphenidyl (THP), a muscarinic cholinergic receptor antagonist, specifically suppressed the rewarding properties of MAP but not of cocaine, as measured by conditioned place preference in mice. The present study examined using in vivo microdialysis whether THP differentially affects the extracellular dopamine (DA) levels in the nucleus accumbens and striatum of mice injected with MAP and cocaine in comparison with another antimuscarinic agent, scopolamine (SCP). In addition, locomotor activity was simultaneously measured during microdialysis. In vivo microdialysis experiments revealed that during the initial hour after injection of MAP (1 mg/kg) DA levels increased up to 698% in the nucleus accumbens and 367% in the striatum as compared to the basal level. These increases were reduced to 293% in the nucleus accumbens and 207% in the striatum by treatment with 5 mg/kg THP. However, SCP (3 mg/kg) had no effect on the increases in extracellular DA levels in both regions after MAP injection. Cocaine (10 mg/kg) increased DA levels during the initial hour to 254% in the nucleus accumbens and 220% in the striatum as compared to the basal level. These increases were unaffected by treatment with either THP or SCP. On the contrary, both THP and SCP enhanced the locomotor-stimulant action of MAP and cocaine. These results, together with our previous finding, suggest that THP may specifically antagonize the rewarding properties of MAP through suppression of DA release in the mesolimbic area without retarding locomotor activity.

Qualitative assessment of selective blockade of M(4)-cholinoreceptors in the whole organism

Quantitative assessment of selective blockade of M4-subtype muscarinic receptors was performed by the number of pilocarpine-induced movements of lower jaw in rats. Three antagonists (atropine, cyclodol, and glipin) were used in the experiments. Glipin produced the most potent blockade of M4 receptors in the whole organism compared to other test antagonist.

Network activity in neurons of the motor and prefrontal areas of the cortex in trained cats in conditions of systemic administration of m-cholinoreceptor blockers

Experiments on five cats already trained to an operant conditioned food-procuring reflex to light were used to study the network activity of cells in the frontal and motor areas of the cortex accompanying disruption of conditioned reflex behavior in conditions of systemic administration of m-cholinoreceptor blockers. The activity of cortical neurons and their network properties were assessed using auto- and cross-correlation histograms. Doses of central m-cholinoreceptor blockers (the non-selective blocker scopolamine and the relatively selective m1-cholinoreceptor blocker trihexyphenidyl) disrupted performance of the operant motor reflex but had no effect on the appearance of contextual behavior and responses to switching on of the conditioned signal (standing up, elevating the paw). This was accompanied by 1) changes in the patterns of neuron activity in the moor and frontal areas of the cortex, with increases in train, rhythmic, and rhythmic train activity in cortical cells; 2) appearance of synchronicity in the operation of cortical neurons; 3) decreases in the numbers of direct interneuronal connections in the motor and frontal areas of the cortex and in the numbers of connections between these structures.

[Cooperative activity of motor and frontal cortex neurons in trained cats systemically administered M-cholinoreceptor blockers]

It was shown previously that peripherally administered antagonists of the central 1 M-cholinoreceptors led to a selective impairment of bar-pressing response in a food-reinforced operant conditioned task but did not alter contextual behavior and functions such as motivation, perception, and locomotion. To obtain information about the central mechanisms of the conditioning impairment, we recorded simultaneously the extracellular multiunit activity from the frontal and motor neocortical areas of five cats trained to acquisition criteria in a food-reinforced operant conditioning task. Multiunit recordings were performed drur 1) normal conditioning; 2) conditioning during subcutaneous administration of muscarinic antagonists scopolamine (0.03 mg/kg), trihexyphenidyl (1 mg/kg), and methylscopolamine (0.03 mg/kg). Autocorrelation analysis showed that scopolamine and trihexyphenidyl but not methylscopolamine led to a significant increase in the tendency of cortical cells to fire in a cyclic way (i.e., the shift of the firing pattern from a single-spike discharge to burst, rhythmic, or rhythmic-burst discharge) both in the motor and frontal areas. Cross-correlation analysis showed that the bursting and rhythmic-bursting cells synchronized their activity within and (in a number of cases) between the cortical areas. These changes in the neuronal activity within the motor cortex and frontal cortex were accompanied by a significant decrease in the functional connectivity both inside and between the cortical areas in parallel with selective impairment of the conditioned response.

Differential effects of trihexyphenidyl on place preference conditioning and locomotor stimulant activity of cocaine and methamphetamine

Cocaine produces rewarding and locomotor stimulant effects by increasing extracellular dopamine (DA) levels in the terminal areas of the mesolimbic DA system. Our recent in vitro studies have shown that a muscarinic receptor antagonist, trihexyphenidyl (THP) inhibits the binding of a cocaine analogue to the DA transporter at concentrations that are ineffective in inhibiting 3H-DA uptake, suggesting that THP may attenuate the actions of cocaine selectively. The present study examined whether THP could affect conditioned place preference (CPP) for and locomotor stimulant activity of cocaine and methamphetamine (MAP) in mice. Mice were injected with cocaine (10 mg/kg) or MAP (1 mg/kg) in one compartment of the CPP chamber 4 times every second day. On alternate days the animals received saline in the other compartment of the CPP chamber. Pretreatment with THP was made 10 min before cocaine or MAP injection. The CPP score and locomotor activity were assessed using a novel activity monitor, SCANET. Cocaine and MAP produced CPP for the drug-paired compartment. Pretreatment with THP (0.05-5 mg/kg) had no influence on cocaine-induced CPP at any dose tested. In contrast, MAP-induced CPP was completely antagonized by THP at 5 mg/kg, which produced no CPP by itself. Another muscarinic receptor antagonist, scopolamine (SCP, 3 mg/kg) neither caused CPP by itself nor affected the development of cocaine- or MAP-induced CPP. Both THP and SCP enhanced spontaneous, cocaine- or MAP-induced locomotor activity. Though the present conditioning treatments failed to develop locomotor sensitization to cocaine, THP, but not SCP, acted cooperatively with cocaine to develop locomotor sensitization. The development of locomotor sensitization to MAP was retarded by SCP but was not affected by THP. These results suggest that, contrary to our anticipation, THP has a unique characteristic of specifically counter-acting the rewarding properties of MAP via a non-cholinergic (muscarinic) mechanism.

The role of muscarinic cholinoceptors in the retrieval of an operant food-related conditioned reflex in cats

Experiments on cats showed that lesions affecting retrieval of an operant food-procuring reflex. occurring on a background of systemic administration of the centrally-acting muscarinic cholinoceptor blocker scopolamine (a non-selective M-cholinoceptor blocker) and trihexyphenidyl (a relatively selective M1-cholinoceptor blocker), might be associated with the central and peripheral side effects of these blockers, preventing performance of the conditioned reflex. It was established that when no side effects were present (low doses of trihexyphenidyl, 1 mg/kg), blockade of M1-cholinoceptors led to selective loss of the motor operant reflex while contextual behavior and other conditioned responses were retained or led to errors in performance of the reflex: this appears to be evidence that derangement of launching and performing the motor program is the most important component of the conditioned reflex. Systemic administration of trihexyphenidyl at a dose of 10 mg/kg, scopolamine at doses of 0.03 and 0.06 mg/kg, and the peripherally-acting non-selective blocker methylscopolamine at a dose of 0.03 mg/kg led to changes in the general functional state (disturbances in the emotional-motivational sphere), the extent of which depended on the individual sensitivity of the animal to the anticholinergic agents. The presence of side effects led to complete cessation of conditioned reflex activity, though this appeared not to be associated with memory impairment.

Muscarinic antagonists in substantia nigra influence the decarboxylation of L-dopa in striatum

This study was designed to investigate whether anticholinergic drugs acting at the level of the substantia nigra can affect basal extracellular dopamine concentrations and the levodopa (L-dopa)-induced increases in dopamine levels in the striatum. Dual probe in vivo microdialysis in freely moving rats was used. One microdialysis probe was implanted in the substantia nigra and the other in the ipsilateral striatum. Muscarinic receptor antagonists were perfused into the substantia nigra and changes in neurotransmitter levels in the substantia nigra and at the axon terminals in the striatum were monitored simultaneously. Nigral perfusion of the non-selective muscarinic receptor antagonist trihexyphenidyl (1 mM) produced an increase in extracellular dopamine and gamma-aminobutyric acid (GABA) levels in the substantia nigra. Perfusion with the muscarinic M(1) receptor antagonist telenzepine (0.1 microM) produced a significant decrease in nigral dopamine and GABA levels in the substantia nigra. The muscarinic M(2) receptor antagonist methoctramine (75 microM) produced an increase in dopamine levels in the substantia nigra. No significant changes in nigral extracellular GABA levels were observed. The L-dopa-induced increases in extracellular dopamine levels in the striatum were clearly attenuated under nigral perfusion of these drugs. This in vivo study demonstrates that anticholinergic drugs perfused at the level of the substantia nigra can modulate dopamine and GABA levels and attenuate the L-dopa decarboxylation in the striatum, possibly via modulation of the nigrostriatal dopaminergic system. We add further evidence that the substantia nigra is an important site of action of antimuscarinic drugs. The attenuation of L-dopa-induced dopamine release in the striatum exerted by nigral perfusion of these antimuscarinic drugs is probably mediated via different mechanisms. This attenuation is regarded as a beneficial effect of the muscarinic antagonists as adjuncts to L-dopa in Parkinson's disease treatment. We postulate that drugs that enhance dopamine release, after L-dopa administration, in a less extreme way than L-dopa administered on its own could prevent further neurodegeneration and dyskinesias thought to result from extremely high extracellular dopamine levels following L-dopa treatment.

[Effect of biotransformation on the receptor selectivity of muscarinic antagonists in vivo]

In the experiments in vivo it is found that the receptor selectivity of muscarine antagonist glypine is time-dependent unlike atropine, amedine, benzhexol, benactyzine, and thropacin. Using modulation of the metabolic system activity it is shown that upon biotransformation glypine forms active metabolites that differs in receptor selectivity of the action.

[The role of muscarinic cholinoreceptors in the retrieval of the instrumental food conditioned reflex in cats]

It was demonstrated in cat experiments that impairment of the retrieval of appetitive instrumental conditioning observed after systemic administration of antagonists of muscarinic central cholinoreceptors scopolamine (a nonselective M1 antagonist) and trihexyphenidyl (relatively selective M1 antagonist) could be connected with central and peripheral side effects of these antagonists. It was established that in case of the absence of side effects (low doses of trihexyphenidyl, 1 mg/kg) the blockade of M1-cholinoreceptors led either to selective fall-off of the motor instrumental reaction with in the presence of contextual behavior and other conditioned reactions or the appearance of errors that seemingly was indicative of the disturbance of triggering and realization of the motor program as the most important component of conditioning performance. The systemic injection of trihexyphenidyl (10 mg/kg), scopolamine (0.03 and 0.06 mg/kg) and nonselective peripheral antagonist methylscopolamine (0.03 mg/kg) led to changes in the general functional state (disorders in the emotional and motivational sphere), the expression of which depended on the individual sensitivity to anticholinergic drugs. The disturbance of CR retrieval observed in parallel with side reactions was characterized by a complete cessation of conditioning and apparently was not associated with memory deterioration.

Comparison of central and peripheral pharmacologic effects of biperiden and trihexyphenidyl in human volunteers

In this double-blind, randomized study, indices of central (memory, sedation) and peripheral (salivation, ratio of R-R interval on electrocardiogram) muscarinic function were evaluated in 14 healthy volunteers who received trihexyphenidyl, biperiden, and placebo. Additionally, serum drug levels were obtained 2 hours after oral administration. All subjects participated in three study sessions. During each session, subjects received two doses of biperiden (4 mg), trihexyphenidyl (5 mg), or placebo, and four series of tests were administered. The tests included the determination of cardiac response to standing (R-R ratio), mouth salivation, finger-tapping speed, digit span (forward and backward), a selective reminding task, and visual analog scales (VAS). On the VAS, subjects rated biperiden as significantly more sedating than either trihexyphenidyl or placebo, and both biperiden and trihexyphenidyl were associated with more dizziness than was placebo. Saliva production was significantly reduced by both trihexyphenidyl and biperiden compared with placebo. Digit span performance was significantly decreased in only the backward direction. The selective reminding task revealed highly significant decrements in the number of words recalled and consistent long-term retrieval after both biperiden and trihexyphenidyl. Delayed recall was significantly decreased by both active drugs. Both trihexyphenidyl and biperiden caused a significant increase in the R-R ratio comparison with placebo. With the exception of the VAS measurement of sedation, the effects caused by biperiden and trihexyphenidyl did not differ. The results of this study do not support the hypothesis that the side effect profile of biperiden is significantly different from that of trihexyphenidyl.

[The effect of panpal prophylaxis on acetylcholinesterase activity in the blood, diaphragm and various parts of the brain in rats during treated and untreated poisoning with the organophosphorus insecticide phosdrine]]

In experiments on male rats the paper investigated the effect of pharmacological prophylaxis with Panpal (pyridostigmine in combination with benactyzine and trihexyphenidyl) on the activity of acetylcholinesterase in the whole blood, diaphragm, and selected parts of the brain (frontal cortex, pontomedular region, hippocampus, cerebellum) at hour 1 and 3 of untreated and treated (oxime HI-6 with atropine) intoxication with the organophosphorous insecticide phosdrine. Whereas in the CNS Panpal did not produce statistically significant changes in the activity of acetylcholinesterase in the course of untreated and treated phosdrine intoxication, in the blood and diaphragm Panpal markedly intensified phosdrine-induced inhibition of the acetylcholinesterase activity and, in addition, decreased the reactivating effect of the oxime HI-6. The data give evidence of the importance of the combination of the prophylactically used reversible acetylcholinesterase inhibitor pyridostigmine with anticholinergics, which could eliminate the consequences of a pyridostigmine-induced decrease in the activity of the enzyme in the periphery.

Amnesic effects of the anticholinergic drugs, trihexyphenidyl and biperiden: differences in binding properties to the brain muscarinic receptor

An amnesic effect of anticholinergic drugs was previously described from several behavioral studies. We examined this effect induced by trihexyphenidyl and biperiden, clinically used in the parkinsonism and schizophrenic patients, by using passive avoidance tasks. Both of these drugs (0.1-10 mg/kg, s.c.) showed dose-dependent amnesic effects in the acquisition and retrieval phases. However, the effect induced by trihexyphenidyl was transient, whereas that of biperiden was long-lasting. To clarify the reason for the different duration of the amnesic activity, binding to the muscarinic receptor was examined. In the Scatchard analysis, trihexyphenidyl competed with [(3)H]quinuclidinyl benzilate ([(3)H]QNB) on the muscarinic receptor (showed increased K(d) and unchanged B(max) value), while biperiden decreased [(3)H]QNB binding (B(max) value) significantly. Furthermore, in an exchange assay for receptor inactivation, trihexyphenidyl binding to muscarinic receptors was exchanged by [(3)H]QNB completely, but biperiden decreased the exchangeable binding of [(3)H]QNB in a dose dependent manner (0.1-100 nM). These results suggested that the binding of trihexyphenidyl and biperiden to muscarinic receptor might be completely reversible and partially irreversible, respectively, whereas the K(i) values of these two drugs were similar. In conclusion, this difference in binding property may explain the difference in the time-course of the amnesic effect induced by trihexyphenidyl and biperiden.

Effect of trihexyphenidyl, a non-selective antimuscarinic drug, on decarboxylation of L-dopa in hemi-Parkinson rats

In vivo microdialysis was used to study the effect of the non-selective muscarinic antagonist, trihexyphenidyl, on the decarboxylation of levodopa (L-dopa) in the striatum of hemi-Parkinson rats. In normal rats, continuous perfusion of trihexyphenidyl (1 mM) via the microdialysis probe induced a significant increase in striatal dopamine release, followed by a decrease to below baseline values. A similar effect was observed, though less pronounced, in denervated striatum of rats with a unilateral 6-hydroxydopamine lesion of the nigrostriatal pathway. In these hemi-Parkinson rats, continuous striatal perfusion of trihexyphenidyl had no effect on the biotransformation of locally applied L-dopa (2 microM for 20 min) to dopamine in either intact or denervated striatum. However, systemic administration of trihexyphenidyl (1.5 mg/kg i.p.) produced an attenuation of the L- dopa-induced dopamine release in the intact striatum (contralateral to the lesion) of hemi-Parkinson rats. This effect was absent in the denervated striatum of these animals. We confirmed that L-dopa induces an increase in striatal dopamine output which is influenced by the severity of the dopaminergic denervation. The absence of an effect of trihexyphenidyl locally applied in the striatum, on biotransformation of L-dopa suggests that the site of action of antimuscarinic drugs may not be in the striatum and, therefore, remains unclear. The mechanism of action of these drugs is not well understood but appears more complicated than previously thought.

[The effect of pharmacologic prophylaxis with Panpal on acetylcholinesterase activity in the diaphragm and various parts of the brain in rats during treated and untreated Soman poisoning]

BACKGROUND

The pharmacological prophylaxis protecting the organism against organophosphorus compounds could increase the effect of antidotal treatment of poisoning with organophosphates.

METHODS AND RESULTS

The influence of the pharmacological prophylaxis with Panpal (pyridostigmine in combination with benaetyzine and trihexyphenidyle) on acetylcholinesterase activity in diaphragm and various parts of brain at 1 and 3 h following non-treated and treated (the oxime HI-6 in combination with atropine) soman poisoning was tested on male rats. While Panpal did not significantly influence the acetylcholinesterase activity in brain following non-treated as well as treated soman poisoning. Panpal increased so many-induced acetylcholinesterase inhibition following non-treated poisoning and decreased the reactivating effect of the oxime HI-6 following treated soman poisoning in diaphragm.

CONCLUSIONS

Our data confirm the importance of the combination of reversible acetylcholinesterase inhibitor pyridostigmine with anticholinergic drugs in the pharmacological prophylaxis of soman poisoning because of the elimination of consequences of pyridostigmine-induced increasing in acetylcholinesterase inhibition in the peripheral compartment.

Chronic haloperidol produces a time- and dose-related slowing of lick rhythm in rats: implications for rodent models of tardive dyskinesia and neuroleptic-induced parkinsonism

In order to characterize the development of orolingual motor effects of chronic haloperidol treatment in rats, this typical neuroleptic was administered for 102 days while daily measurements of tongue movement dynamics (peak force, lick rhythm, number of licks) during water licking were recorded. After chronic haloperidol dosing (vehicle, 0.06. 0.12, 0.24 mg/kg for four separate groups) for 32 days and continuing every second or third day of the chronic dosing period, the effects of cholinergic (scopolamine: 0.05-0.20 mg/kg; trihexyphenidyl: 0.15-1.0 mg/kg) or serotonergic (ritanserin: 0.5-4.0 mg/kg; quipazine: 0.5-4.0 mg/kg) probe drugs were examined for their capacity to antagonize the alterations in licking behavior induced by haloperidol. Haloperidol dose-dependently reduced peak force and number of licks, effects which were apparent within 2 or 3 days of the start of treatment. Significant effects of haloperidol on lick rhythm first emerged on day 13 and gradually increased in magnitude through the remaining treatment period. Scopolamine, trihexyphenidyl, and quipazine reduced haloperidol's effects on at least one measure of licking behavior. During a 7-day haloperidol withdrawal period, the four dosage groups were similar on all measures of tongue dynamics. Overall, the results exhibited features suggesting the co-occurrence of Parkinson-like and tardive dyskinesia-like effects.

Characteristic subcellular distribution, in brain, heart and lung, of biperiden, trihexyphenidyl, and (-)-quinuclidinyl benzylate in rats

The subcellular distribution of biperiden (BP), trihexyphenidyl (TP) and (-)-quinuclidinyl benzylate (QNB) in brain, heart and lung following high dose (3.2 mg/kg) i.v. administration was investigated in rats. The subcellular distribution of BP or TP used clinically conformed with that of QNB, a typical potent central muscarinic antagonist. The concentration-time courses of the brain subcellular fractions for these drugs were of two types which decreased slowly and in parallel to the plasma concentration. The subcellular distribution in the brain and heart was dependent on the protein amount of each fraction. The percent post-nuclear fraction (P2) of the total concentration in the lung was characteristically about 3-5 times larger than that in the heart. It was elucidated that the distribution in the lung differs from that in the brain and heart, with high affinity which is not dependent on the protein amount in the P2 fraction containing lysosomes. On the other hand, at a low dose (650 ng/kg) of 3H-QNB, each fraction as a percentage of the total concentration in the brain increased in synaptic membrane and synaptic vesicles and decreased in nuclei and cytosol as compared with the high dose. These results show that although the tissue concentration-time courses of anticholinergic drugs appear to decrease simply in parallel to plasma concentration, the subcellular distribution exhibits a variety of patterns among various tissues.

Similarities and differences between the subchronic and withdrawal effects of clozapine and olanzapine on forelimb force steadiness

The purpose of this study was to compare the subchronic, low-dose effects of clozapine with those of olanzapine in a learned behavioral task previously shown to distinguish between clozapine and haloperidol with acute and subchronic treatment regimes. Rats were trained to use a single forelimb to press a force-recording operandum and simultaneously to lick water from a dipper that remained available while forelimb force exceeded a modest lower limit. Analysis of the resulting forcetime recordings provided measures of task engagement (time on task-analogous to response rate), lick rhythm, tremor, ballistic (maximum force) and tonic (hold force) forelimb force measures, as well as the durations of the individual responses. In a between-groups dosing design, five separate groups of rats received vehicle, clozapine 1.0 or 5.0 mg/kg, olanzapine 0.5 or 1.0 mg/kg daily for 27 days. A 7-day withdrawal period followed. On days 22 and 26 of antipsychotic drug treatment, all rats additionally received 0.3 mg/kg trihexyphenidyl or 1.0 mg/kg quipazine, respectively. The effects of olanzapine and clozapine were similar in that both drugs reduced time on task, increased response duration, and slowed lick rhythm. The two drugs differed in that clozapine reduced the force and tremor measures but olanzapine did not. Both tolerance and withdrawal effects, as reflected by the tremor measure, were observed for clozapine but not for olanzapine. Trihexyphenidyl further increased the duration of responses already lengthened by clozapine; in contrast, trihexyphenidyl decreased the duration lengthening effect of olanzapine. Taken together, the results indicated that olanzapine did not have the antitremor and hypotonic effects displayed by clozapine, and olanzapine did not induce tolerance and withdrawal phenomena as clozapine did.

Selective M1 muscarinic receptor antagonists disrupt memory consolidation of inhibitory avoidance in rats

The effect of three different M1 muscarinic antagonists, pirenzepine, biperiden, and trihexyphenidyl on memory consolidation was investigated. Rats were trained in a one-trial step-through inhibitory avoidance task and injected intraperitoneally immediately afterwards, either with pirenzepine, biperiden, or trihexyphenidyl (dose range from 0 to 16 mg/kg). The non-selective antimuscarinic compound scopolamine, was also administered for comparison. One day later, rats were tested for retention. Results show that biperiden, trihexyphenidyl and scopolamine produced a dose-dependent impairment of inhibitory avoidance consolidation, while pirenzepine had no effect. The amnestic state produced by biperiden and trihexyphenidyl was comparable to that observed after the administration of scopolamine. These results indicate that the selective blockade of the central M1 muscarinic receptors interfere with memory consolidation of inhibitory avoidance and suggest that this receptor subtype is critically involved in mnemonic functions.

No effect of the anticholinergic drugs trihexyphenidyl and biperiden on the plasma concentrations of bromperidol and its reduced metabolite

Effects of the anticholinergic drugs trihexyphenidyl and biperiden on plasma concentrations of bromperidol and its reduced metabolite were studied. Subjects comprised 20 schizophrenic inpatients taking bromperidol, 6-18 mg/ day for 1-9 weeks. Patients were randomly allocated to one of two treatment sequences: trihexyphenidyl-biperiden (n = 12) or biperiden-trihexyphenidyl (n = 8). Each sequence consisted of two 2-week phases, with no washout period between the two phases. The daily dose of trihexyphenidyl was 8 mg and that of biperiden 6 mg. Plasma concentrations of bromperidol and reduced bromperidol were measured using high-performance liquid chromatography (HPLC). There was no significant difference in plasma bromperidol or reduced bromperidol concentrations among baseline, trihexyphenidyl and biperiden phases: 7.3 +/- 3.7 versus 7.2 +/- 4.1 versus 7.0 +/- 4.3 ng/ml and 2.0 +/- 2.1 versus 2.2 +/- 2.1 versus 1.9 +/- 2.0 ng/ml, respectively. The present study thus suggests that neither trihexyphenidyl nor biperiden affects plasma concentrations of bromperidol and its reduced metabolite.

Effects of antimuscarinic antiparkinsonian drugs on brightness discrimination performance in rats

Biperiden (BPR) and trihexyphenidyl (THP), the current antimuscarinic drugs of choice in the management of parkinsonism, have been shown to exert anticonvulsant effects induced by poisoning by the organophosphorus compound soman. The present study was undertaken to evaluate the effects of these drugs on performance of a simple light-intensity discrimination task in rats under a tandem schedule of fixed-ratio (FR) reward/ differential-reinforcement-of-low-rate (DRL) nonreward contingencies, for water reinforcement in 2-h experimental sessions. Both BPR (0.125-2.0 mg/kg, SC) and THP (0.25-8.0 mg/kg, SC) in general decreased overall reinforcement rates in a similar dose dependent and parallel manner, concurrent with increased overall nonreinforced responses in an inverted U-shaped dose-response relationship. Lower doses of BPR (0.125-0.5 mg/kg) and and THP (0.25-2.0 mg/kg) produced a moderate reduction in reinforcement (> or = 50% of baseline controls), which was correlated well with increases in nonreinforced responses emitted, whereas, higher doses of BPR (> 0.5 mg/kg) and TPH (> or = 2.0 mg/kg) markedly decreased reinforcements, which mainly resulted from the pausing of responding in the presence of stereotyped behavior. The behavioral disruption induced by BPR was much more rapid than that induced by THP. The ED50 values (0.6 mg/kg vs. 1.3 mg/kg, respectively) and parallel dose-effect curves suggest that these drugs have similar efficacy, and that BPR is about twice as potent as THP, a ranking that corresponds with their binding affinity at M-1 muscarinic acetylcholine receptors in rat cerebral cortex. Based on the similarity between the anticonvulsant doses of these drugs and the maximal doses that in this study did not disrupt operant responses (0.125 mg/kg vs. 0.25 mg/kg, respectively), it is suggested that both drugs may be useful in protection against seizures produced by the cholinesterase inhibitor soman. Overall, these results suggest that this multiple schedule operant contingency may have promise as a behavioral model to identify the therapeutic or toxic potentials of centrally acting antimuscarinic antiparkinsonian drugs based on their congnitive side effects.

Objective measurement of activation of rigidity: diagnostic, pathogenetic and therapeutic implications in parkinsonism

1. Quantification of the effect on rigidity of its 'activation', by isometric grip, of standardized pressure, of the contralateral hand, was explored. Torque required to move the forearm through a fixed angle of 40 degrees, at a controlled rate of 0.5 Hz, in a horizontal plane about a pivotal axis aligned to the elbow joint, was recorded before (12 'baseline' recordings), during (10), and after (> or = 8) activation. Work required per unit displacement was calculated. 2. Specificity: Pilot serial daytime measurements gave an overall mean ratio, work required on activation over baseline, of 2.94 (95% CI 2.53, 3.42) in two elderly untreated parkinsonians, and 3.19 (2.75, 3.71) in two elderly subjects with isolated, clinically activation phenomenon, compared with 1.90 (1.64, 2.21) in two elderly without (P < 0.001), whilst two young adults did not activate, 0.98 (0.85, 1.14). In elderly subjects, work required under activation decreased during the day in health (-10 (-5, -14)% h-1, P = 0.0002), showed no significant change in those with clinical activation (4 (-1, 9)% h-1), and increased in parkinsonians (6 (0, 12)% h-1, P = 0.05): there appeared to be a transitionary state. 3. Validation of methodology: Quantifying the same work ratio on a single occasion in 20 aged parkinsonians (P), their spouses (Ps), 20 index controls (C) without parkinsonism, matched to (P), and their spouses (Cs) gave corroborative evidence of a pre-clinical state, defined by other measurements, in the spouses of sufferers. Values for C, Cs and Ps, 1.89 (1.42, 2.52), 2.38 (1.79, 3.17) and 2.93 (2.20, 3.90) respectively, were in consecutive positions, from health to (P, 2.96 (2.22, 3.95)) disease (P = 0.001 for Ps c.f. C; P = 0.1 for Ps c.f. Cs). Data on change over the day may enhance discrimination. 4. Sensitivity to medicines was illustrated, in two parkinsonians, by randomised, placebo balanced and controlled challenges: 1 and 2 tablets, Sinemet CR (Du Pont Pharmaceuticals, each levodopa 200 mg/carbidopa 50 mg) and 1 tablet, Sinemet-Plus (levodopa 100 mg/carbidopa 25 mg), then two 2 mg tablets, benzhexol. The dopaminergic effect (P < 0.001) was selective for activation (treatment.test-condition interaction, P = 0.004), and showed the expected time profiles. The effect of benzhexol (P = 0.008) lacked such selectivity. Its onset (> 4, < or = 6 h) was delayed, compatible with a gastrointestinal anti-muscarinic action and the subjects' ages. 5. Reliability (Fleiss's criterion) was shown to be good in 30 untreated parkinsonians.

The effects of co-administration of benzhexol on the peripheral pharmacokinetics of oral levodopa in young volunteers

1. The effects of benzhexol on the absorption and pharmacokinetics of an oral dose of levodopa have been studied in 10 young healthy volunteers. Subjects were given a suspension of levodopa (250 mg) 90 min after either benzhexol (5 mg) or placebo in a randomized cross over design with doses separated by at least 1 week; on each occasion carbidopa was given 1 h before and 5 h after the dose of levodopa. Soluble paracetamol and radiolabelled DTPA were given with the levodopa as markers of gastric emptying. 2. Most subjects showed two peaks in the levodopa plasma concentration-time curve on the placebo day, with the second minor peak occurring 1-2 h after the dose. After benzhexol administration all subjects showed two or more peak levodopa concentrations in plasma. Benzhexol administration caused a significant decrease in the maximum concentration (43%; P < 0.05) of the initial peak and an increase (22%; P < 0.1) in the maximum concentration of the second peak. This change in absorption profile caused by benzhexol significantly altered the ratios of the second peak compared with the initial peak for both the maximum concentrations (P < 0.02) and for the AUC values (P < 0.05). Benzhexol administration did not affect the total AUC of levodopa (7.30 +/- 1.09 vs 7.19 +/- 1.26 micrograms ml-1 h; means +/- s.d.). 3. The plasma concentration-time curves for paracetamol showed similar profiles to those for levodopa and the ratios of the peak concentrations and AUC values for the second peak compared with the initial peak were increased significantly by benzhexol administration (P < 0.05). The total AUC of paracetamol was not affected by benzhexol administration (39.4 +/- 8.2 vs 40.0 +/- 8.9 micrograms ml-1 h; mean +/- s.d.) 4. Benzhexol altered the gastric emptying profile, shown by gamma-scintigraphy, with a reduced extent of initial emptying prior to the establishment of the plateau which is characteristic of levodopa administration in the fasting state. In consequence the ratio of the second to the initial phase of emptying was significantly higher (P < 0.01) following benzhexol treatment. 5. Benzhexol reduces the initial phase of gastric emptying after a dose of levodopa so that there is a decrease in the initial peak and a greater proportion of the dose is absorbed subsequently following the second phase of gastric emptying which occurs approximately 1 h later. Theoretically, this altered concentration-time profile could be an advantage for some patients with Parkinson's disease.

Differential effects of anticholinergic drugs on paired discrimination performance

Working and reference memory processes were simultaneously evaluated during the performance of a paired discrimination (PD) task in which visual and spatial discrimination trials were combined within the same session. Atropine (1 and 5 mg/kg), scopolamine (0.02-0.20 mg/kg), benactyzine (1-4 mg/kg), trihexyphenidyl (1-10 mg/kg), and aprophen (5-20 mg/kg) were all found to increase the number of errors performed by overtrained rats during the spatial but not during the visual trials. Although all the anticholinergic drugs tested induced specific working memory impairment at low doses, they differentially affected other, simultaneously recorded, behavioral parameters. Thus, while atropine affected most of the recorded parameters, aprophen induced only a mild effect. Benactyzine was found to have the most specific effect on working memory, with only minimal side effects, a combination that supports its use as the preferred psychopharmacological model of working memory impairment.

Chronic blockade of muscarinic cholinergic receptors by systemic trihexyphenidyl (Artane) administration modulates but does not mediate the dopaminergic regulation of striatal prepropeptide messenger RNA expression

A striatal dopaminergic denervation leads to changes in the expression of messenger RNA encoding prepropeptides contained in striatal efferent neurons. Such a dopaminergic lesion also abolishes a functional equilibrium between dopaminergic and cholinergic transmissions, generally believed to operate within the neostriatum, which constitutes the theoretical basis for the clinical use of antimuscarinic drugs in extrapyramidal diseases. It is possible, therefore, that changes in prepropeptide messenger RNA expression are mediated by an alteration in cholinergic transmission. To test this hypothesis, we have examined in rats whether trihexyphenidyl, an antimuscarinic drug of wide clinical use, can counteract the changes in preproenkephalin, preprotachykinin and preprodynorphin messenger RNA expression produced by a unilateral 6-hydroxydopamine lesion of substantia nigra dopaminergic neurons. Two weeks after the lesion, trihexyphenidyl was continuously administered through an osmotic minipump (5 mg/day for 15 days) to half of the lesioned and sham-operated rats, the other half receiving the vehicle. Using quantitative in situ hybridization histochemistry, messenger RNAs were analysed at two rostrocaudal levels (anterior and central) of the neostriatum. In parallel, M1 muscarinic receptors were measured by autoradiography of [3H]pirenzepine binding sites. In sham-operated rats, trihexyphenidyl administration produced a significant increase (17-27%) in M1 binding sites. In addition, preproenkephalin messenger RNA levels were decreased (-38%) in the central part, while preprodynorphin messenger RNA levels were significantly increased (+22%) at both striatal levels. In 6-hydroxydopamine-lesioned rats, the expected changes in messenger RNAs were observed when ipsi- versus contralateral side values were compared, but changes were not always detected when comparison was established between values from the dopamine-denervated neostriatum and those from sham-operated rats. The trihexyphenidyl administration in 6-hydroxydopamine-lesioned animals was unable to reproduce the up-regulation of M1 receptors, even in the intact neostriatum. This antimuscarinic treatment further increased preproenkephalin messenger RNA levels in the denervated anterior neostriatum, amplifying the ipsi- versus contralateral difference. It also potentiated the imbalance in preprotachykinin messenger RNA expression, mainly as a result of an increase of preprotachykinin messenger RNA levels in the intact neostriatum. In contrast, trihexyphenidyl treatment by increasing preprodynorphin messenger RNA in both neostriata abolished the ipsi- versus contralateral difference observed in lesioned rats. In conclusion, with the exception of preprodynorphin messenger RNA, trihexyphenidyl treatment was unable to counteract the imbalance in prepropeptide messenger RNA expression produced by a unilateral striatal dopaminergic denervation and even amplified this effect. These results question the neostriatum as the site of action of antimuscarinic drugs in producing their therapeutic effect in extrapyramidal syndromes.

Antagonism by (R)- and (S)-trihexyphenidyl of muscarinic stimulation of adenylyl cyclase in rat olfactory bulb and inhibition in striatum and heart

1. Activation of muscarinic receptors in rat olfactory bulb stimulates adenylyl cyclase activity. This response was competitively antagonized by the (R)- and (S)-enantiomers of trihexyphenidyl with pA2 values of 8.84 and 6.09, respectively. 2. Similarly, in rat striatal homogenates, muscarinic inhibition of adenylyl cyclase activity was antagonized by the (R)- and (S)-enantiomers with pA2 values of 8.75 and 6.12, respectively. 3. In contrast, in rat myocardium the muscarinic inhibition of the adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation was more weakly antagonized by trihexyphenidyl, with a particularly marked loss (15 fold) in activity of the (R)-enantiomer. The (R)- and (S)-enantiomers had pA2 values of 7.64 and 5.72, respectively. 4. Each muscarinic response was completely antagonized by increasing concentrations of (R)-trihexyphenidyl with a Hill coefficient not significantly different from unity. 5. The present study shows that the muscarinic receptors coupled to stimulation of adenylyl cyclase in the olfactory bulb display high stereoselectivity for the enantiomers of trihexyphenidyl. The affinities of these receptors for the antagonists are similar to those shown by the striatal receptors. This finding supports the hypothesis that both the muscarinic stimulation of adenylyl cyclase in the olfactory bulb and the muscarinic inhibition of the enzyme in striatum are mediated by activation of a receptor subtype pharmacologically equivalent to the m4 gene product. On the other hand, the weaker affinities and the lower stereoselectivity for the trihexyphenidyl enantiomers exhibited by the muscarinic inhibition of adenylyl cyclase in the heart are consistent with the involvement of M2 receptors in this response.

Effects of enantiomers of M3 antagonists on muscarinic receptors in rabbit trachea

The effects of the enantiomers of structurally related chiral M3 antagonists (trihexyphenidyl, p-fluorohexahydrodifenidol, hexahydrodifenidol and p-fluorohexbutinol) were studied at the presynaptic M2 and postsynaptic M3 receptor level in the rabbit trachea. All isomers were M3- over M2-selective as they did not increase the release of acetylcholine (an M2 effect) at concentrations that significantly inhibited smooth muscle contraction (an M3 effect). At the smooth muscle receptor, the R-enantiomers were consistently more potent than the S-enantiomers. The potency ratios (IC50(S)/IC50(R)) varied from 6 for p-fluorohexbutinol to 288 for trihexyphenidyl, and increased with higher eutomer potencies, in accordance to Pfeiffer's rule. Furthermore, we found that the potency of the racemic mixture of hexahydrodifenidol was significantly lower than that of the eutomer R-hexahydrodifenidol. To exclude that this difference was due to the lower concentration of the more active isomer, present in a racemic mixture, we calculated the potencies (-log IC50 values) of mixtures of the isomers of hexahydrodifenidol with varying amounts of S-hexahydrodifenidol and a constant amount of R-hexahydrodifenidol. We found that the presence of the distomer altered the potency of the eutomer in a dose-related manner. In conclusion, we have shown that the muscarinic smooth muscle receptor can be blocked differentially by the isomers of muscarinic antagonists and that the presence of the less active compound alters the potency of the most active isomer. We, therefore, suggest that, in bronchodilating therapy, the use of the pure eutomer might have advantages.

Characterization of the muscarine receptor subtype on sympathetic nerve endings in the rat caudal artery

The prejunctional muscarine receptor on sympathetic nerves in the rat caudal artery was characterized using several selective antagonists. The inhibitory response of carbachol on vasoconstriction elicited by sympathetic nerve stimulation was antagonized by benzhexol (trihexyphenidyl; pKB 7.1), heptane-1,7-bis(dimethyl-3'-phthalimidopropyl ammonium bromide) (C7/3-phth; pKB 6.5) and hexahydrosiladifenidol (HHSiD; apparent pKB 6.0). These pKB values suggest that the receptor most closely resembles the muscarine M2 receptor subtype rather than the muscarine M1, M3 or M4 receptor subtypes.

Effects of trihexyphenidyl and L-dopa on brain muscarinic cholinergic receptor binding measured by positron emission tomography

The effects of pharmacological intervention on brain muscarinic cholinergic receptor (mAChR) binding were assessed in seven patients with Parkinson's disease by positron emission tomography and carbon-11 labelled N-methyl-4-piperidyl benzilate ([11C]NMPB). [11C]NMPB was injected twice, approximately 2 hours apart, in each patient, to assess the effect of single doses of 4 mg of trihexyphenidyl (n = 5) or 400 mg of L-dopa with 57 mg of benserazide (n = 2) on the binding parameter of mAChRs (K3). There was a mean 28% inhibition of K3 values in the brain in the presence of trihexyphenidyl, which was assumed to reflect mAChR occupancy. No significant change in K3 was observed in the presence of L-dopa. This study demonstrates the feasibility of measuring mAChR occupancy by an anticholinergic medication with PET.

Evidence that the M1 muscarinic receptor subtype mediates the effects of oxotremorine on masculine sexual behavior

The cholinergic system participates in the regulation of masculine sexual behavior, mainly through the muscarinic system. Recently, muscarinic receptors have been subdivided into at least two subtypes, M1 and M2, according to their differential affinity for pirenzepine. In this study, we analyzed the possible participation of the M1 muscarinic receptor subtype on masculine sexual behavior regulation. In the first experiment, trihexyphenidyl, a specific M1 antagonist, was administered to experienced adult male rats in a wide range of doses (from 0.1 to 6.4 mg/kg). No modification was observed in any of the male sexual behavior parameters recorded, with the exception of the highest dose at which an increase of the intromission frequency and a decrease of the ejaculation frequency were observed. In the second experiment, trihexyphenidyl was administered in several doses (from 0.2 to 1.6 mg/kg), before the administration of oxotremorine, a muscarinic agonist, at a dose that readily facilitates masculine sexual behavior. Trihexyphenidyl completely prevented the facilitatory effects of oxotremorine even at the smallest dose used. These results strongly suggest that the M1 muscarinic receptor subtype participates in the cholinergic facilitation of masculine sexual behavior.

Anticholinergic agents. 4. Stereocontrolled synthesis of fluorinated acetylcholine antagonists; syntheses of the two 1-cyclohexyl-1-(4-fluorophenyl)-3-piperidyl-1-propanols and their methiodides

Four new putative muscarinic antagonists, (R)-(-)- and (S)-(+)-1-cyclohexyl-1-(4-fluorophenyl)-3-piperidyl-1-propanol and their methiodides, have been synthesised. Absolute configurations have been assigned on the basis of the anticipated chirality of the products of the Sharpless asymmetric epoxidation reaction.

Comparative behavioral and neurochemical activities of cholinergic antagonists in rats

The comparative behavioral and neurochemical activities of the muscarinic cholinergic antagonists scopolamine, trihexyphenidyl and pirenzepine, and the nicotinic cholinergic antagonist mecamylamine were evaluated in rats. The three muscarinic antagonists, but not the nicotinic antagonist, impaired memory performance in a spatial alternation task. The minimal effective doses required to disrupt behavior were 0.03, 1.0 and 10 mg/kg for scopolamine, trihexyphenidyl and pirenzepine, respectively. Scopolamine and trihexyphenidyl inhibited ex vivo binding of [3H] pirenzepine to M1 receptors in cerebral cortex, indicating ready penetration into the brain. In contrast, pirenzepine penetrated into the brain at relatively high doses, suggesting poor penetration into the brain. Scopolamine and trihexyphenidyl, but not pirenzepine, inhibited [3H]quinuclidinyl benzilate binding to brain stem M2 receptors ex vivo after subcutaneous administration. In addition, scopolamine and trihexyphenidyl, but not pirenzepine, decreased acetylcholine (ACh) levels in striatum and hippocampus, presumably by increasing ACh release by blocking ACh feedback inhibition at M2 receptors. Scopolamine and trihexyphenidyl also produced modest decreases in levels of the dopamine metabolite 3,4-dihydroxyphenylacetic acid in striatum, most likely due to blockade of M1 heteroreceptors on dopamine nerve terminals. The present results are consistent with the interpretation that muscarinic antagonists impair memory performance in rats, at least in part, by blocking M1 muscarinic receptors. The present results do not support a role for blockade of M2 receptors. Further research is needed to determine the extent to which blockade of other (M3, M4, M5) muscarinic receptor subtypes contributes to the memory-impairing effects of muscarinic cholinergic antagonists.

Rapid response of striatal muscarinic M1-receptor mRNA to muscarinic cholinergic agents in rat brain

The effects of a single administration of muscarinic cholinergic agents on the level of muscarinic M1-receptor messenger RNA (M1-R mRNA) in the rat striatum were studied. Carbachol increased the M1-R mRNA expression rapidly and transiently, while trihexyphenidyl decreased it. These results suggest that muscarinic cholinergic agents participate in the positive regulation of muscarinic receptor mRNA in the early stage after treatment, contrary to the negative regulation in the chronic stage.

Pharmacological modulation of soman-induced seizures

Anticholinergics, benzodiazepines and N-methyl-D-aspartate (NMDA) antagonists have been shown to modulate the expression of nerve agent-induced seizures. This study examined whether the anticonvulsant actions of these drugs varied depending on the duration of prior seizure activity. Rats implanted with electrodes to record electroencephalographic (EEG) activity were pretreated with the oxime HI-6 (125 mg/kg, IP) to prolong survival, and then challenged with a convulsant dose of the nerve agent soman (180 micrograms/kg, SC); treatment compounds (scopolamine, diazepam, MK-801, atropine, benactyzine, and trihexyphenidyl) were delivered IV at specific times after seizure onset. Both diazepam and MK-801 displayed a similar profile of activity: At both short or long times after seizure initiation the anticonvulsant efficacy of each drug remained the same. Diazepam, and especially MK-801, enhanced the lethal actions of soman by potentiating the respiratory depressant effects of the agent; scopolamine given prior to diazepam or MK-801 protected against the respiratory depression. Scopolamine and atropine showed a dose- and time-dependent effectiveness; the longer the seizure progressed the higher the dose of drug required to terminate the seizure, with eventual loss of anticonvulsant activity if the seizure had progressed for 40 min. In contrast, benactyzine and trihexyphenidyl showed a third profile of activity: There was a smaller increase in drug dosage required for anticonvulsant activity as seizure duration increased, and both drugs could terminate seizures that had progressed for 40 min. The early anticonvulsant action of anticholinergics is interpreted as a specific effect that blocks the primary cholinergic excitatory drive that initiates, and first maintains, nerve agent seizures. If allowed to progress, the seizure activity itself recruits excitatory neurotransmitter systems (i.e., NMDA) that eventually maintain the seizure independent of the initial cholinergic drive. This is indicated by the eventual ineffectiveness of scopolamine and atropine as the duration of the seizure progresses. Diazepam and MK-801 appear to act to moderate nerve agent seizures by enhancing inhibitory activity (diazepam) or dampening the secondarily activated noncholinergic excitatory system (MK-801). Benactyzine and trihexyphenidyl represent compounds that possibly have both anticholinergic and NMDA antagonistic properties.

The relative effects of selective M1 muscarinic antagonists on rapid eye movement sleep

Three muscarinic antagonists, scopolamine, trihexyphenidyl and biperiden were systemically administered (0, 0.5, 1, 2 and 4 mg/kg) in rats. Scopolamine increased wakefulness and deceased sleep, both slow wave and REM. Trihexyphenidyl increased wakefulness and decreased REM sleep while biperiden decreased REM sleep selectively. The rank order REM-suppressing effect was roughly scopolamine and trihexyphenidyl having a greater suppressing effect than biperiden. These results suggest that the regulation of the sleep-wake cycle is at least partially controlled by the M1 muscarinic receptor.