Pharmacological characterisation of spontaneous or drug-associated purposeless chewing movements in rats

A Subset of Purposeless Oral Movements Triggered by Dopaminergic Agonists Is Modulated by 5-HT2C Receptors in Rats: Implication of the Subthalamic Nucleus

Dopaminergic medication for Parkinson’s disease is associated with troubling dystonia and dyskinesia and, in rodents, dopaminergic agonists likewise induce a variety of orofacial motor responses, certain of which are mimicked by serotonin2C (5-HT_2C) receptor agonists. However, the neural substrates underlying these communalities and their interrelationship remain unclear. In Sprague-Dawley rats, the dopaminergic agonist, apomorphine (0.03–0.3 mg/kg) and the preferential D2/3 receptor agonist quinpirole (0.2–0.5 mg/kg), induced purposeless oral movements (chewing, jaw tremor, tongue darting). The 5-HT_2C receptor antagonist 5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxyl]-5-pyridyl]carbamoyl]-6-trifluoromethylindone (SB 243213) (1 mg/kg) reduced the oral responses elicited by specific doses of both agonists (0.1 mg/kg apomorphine; 0.5 mg/kg quinpirole). After having confirmed that the oral bouts induced by quinpirole 0.5 mg/kg were blocked by another 5-HT_2C antagonist (6-chloro-5-methyl-1-[6-(2-methylpiridin-3-yloxy)pyridine-3-yl carbamoyl] indoline (SB 242084), 1 mg/kg), we mapped the changes in neuronal activity in numerous sub-territories of the basal ganglia using c-Fos expression. We found a marked increase of c-Fos expression in the subthalamic nucleus (STN) in combining quinpirole (0.5 mg/kg) with either SB 243213 or SB 242084. In a parallel set of electrophysiological experiments, the same combination of SB 243213/quinpirole produced an irregular pattern of discharge and an increase in the firing rate of STN neurons. Finally, it was shown that upon the electrical stimulation of the anterior cingulate cortex, quinpirole (0.5 mg/kg) increased the response of substantia nigra pars reticulata neurons corresponding to activation of the “hyperdirect” (cortico-subthalamonigral) pathway. This effect of quinpirole was abolished by the two 5-HT_2C antagonists. Collectively, these results suggest that induction of orofacial motor responses by D2/3 receptor stimulation involves 5-HT_2C receptor-mediated activation of the STN by recruitment of the hyperdirect (cortico-subthalamonigral) pathway.

The role of glutamate receptors and their interactions with dopamine and other neurotransmitters in the development of tardive dyskinesia: preclinical and clinical results

Tardive dyskinesia is a serious, disabling, movement disorder associated with the ongoing use of antipsychotic medication. Current evidence regarding the pathophysiology of tardive dyskinesia is mainly based on preclinical animal models and is still not completely understood. The leading preclinical hypothesis of tardive dyskinesia development includes dopaminergic imbalance in the direct and indirect pathways of the basal ganglia, cholinergic deficiency, serotonin receptor disturbances, neurotoxicity, oxidative stress, and changes in synaptic plasticity. Although, the role of the glutamatergic system has been confirmed in preclinical tardive dyskinesia models it seems to have been neglected in recent reviews. This review focuses on the role and interactions of glutamate receptors with dopamine, acetylcholine, and serotonin in the neuropathology of tardive dyskinesia development. Moreover, preclinical and clinical results of the differentiated effectiveness of N-methyl-D-aspartate (NMDA) receptor antagonists are discussed with a special focus on antagonists that bind with the GluN2B subunit of NMDA receptors. This review also presents new combinations of drugs that are worth considering in the treatment of tardive dyskinesia.

Haloperidol-Induced Preclinical Tardive Dyskinesia Model in Rats

Haloperidol is a first-generation antipsychotic used in the treatment of psychoses, especially schizophrenia. This drug acts by blocking dopamine D2 receptors, reducing psychotic symptoms. Notwithstanding its benefits, haloperidol also produces undesirable impacts, in particular extrapyramidal effects such as tardive dyskinesia (TD), which limit the use of this and related drugs. TD is characterized by repetitive involuntary movements occurring after chronic exposure therapy with haloperidol. Symptoms most commonly manifest in the orofacial area and include involuntary movements, tongue protrusion, pouting lips, chewing in the absence of any object to chew, and facial grimacing. The most serious aspect of TD is that it may persist for months or years after drug withdrawal and is irreversible in some patients. This unit, aimed at facilitating the study of TD, describes methods to induce TD in rats using haloperidol, as well as procedures for evaluating the animals's TD-related symptoms. © 2019 by John Wiley & Sons, Inc.

Scopolamine alleviates involuntary lingual movements: tardive dyskinesia or dystonia?

Cholinergic hypofunction was believed to be associated with the pathogenesis of tardive dyskinesia, and therefore, anticholinergic treatment might exacerbate the condition. We describe herein a middle-aged male with feeble chewing movements, involuntary rolling motions of the tongue, and abnormally tightened cheeks which developed after consuming different psychotropic medications. These symptoms did not improve after routine treatment for tardive dyskinesia, but responded well to anticholinergic agents, such as scopolamine and benzhexol hydrochloride. This case extended our understanding of the complexity of extrapyramidal effects and their pharmacologic management.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Lifelong Rodent Model of Tardive Dyskinesia-Persistence After Antipsychotic Drug Withdrawal

Tardive dyskinesia (TD), first appearing in humans after introduction of the phenothiazine class of antipsychotics in the 1950s, is now recognized as an abnormality resulting predominately by long-term block of dopamine (DA) D₂ receptors (R). TD is thus reproduced in primates and rodents by chronic administration of D₂-R antagonists. Through a series of studies predominately since the 1980s, it has been shown in rodent modeling of TD that when haloperidol or other D₂-R antagonist is added to drinking water, rats develop spontaneous oral dyskinesias, vacuous chewing movements (VCMs), after ~3 months, and this TD is associated with an increase in the number of striatal D₂-R. This TD persists for the duration of haloperidol administration and another ~2 months after haloperidol withdrawal. By neonatally lesioning dopaminergic nerves in brain in neonatal rats with 6-hydroxydopamine (6-OHDA), it has been found that TD develops sooner, at ~2 months, and also is accompanied by a much higher number of VCMs in these haloperidol-treated lesioned rats, and the TD persists lifelong after haloperidol withdrawal, but is not associated with an increased D₂-R number in the haloperidol-withdrawn phase. TD apparently is related in part to supersensitization of both D₁-R and serotoninergic 5-HT₂-R, which is also a typical outcome of neonatal 6-OHDA (n6-OHDA) lesioning. Testing during the haloperidol-withdrawn phase in n6-OHDA rats displaying TD reveals that receptor agonists and antagonists of a host of neuronal phenotypic classes have virtually no effect on spontaneous VCM number, except for 5-HT₂-R antagonists which acutely abate the incidence of VCMs in part. Extrapolating to human TD, it appears that (1) 5-HT₂-R supersensitization is the crucial alteration accounting for persistence of TD, (2) dopaminergic-perhaps age-related partial denervation-is a risk factor for the development of TD, and (3) 5-HT₂-R antagonists have the therapeutic potential to alleviate TD, particularly if/when an antipsychotic D₂-R blocker is withdrawn.

Multiple controls exerted by 5-HT2C receptors upon basal ganglia function: from physiology to pathophysiology

Serotonin2C (5-HT2C) receptors are expressed in the basal ganglia, a group of subcortical structures involved in the control of motor behaviour, mood and cognition. These receptors are mediating the effects of 5-HT throughout different brain areas via projections originating from midbrain raphe nuclei. A growing interest has been focusing on the function of 5-HT2C receptors in the basal ganglia because they may be involved in various diseases of basal ganglia function notably those associated with chronic impairment of dopaminergic transmission. 5-HT2C receptors act on numerous types of neurons in the basal ganglia, including dopaminergic, GABAergic, glutamatergic or cholinergic cells. Perhaps inherent to their peculiar molecular properties, the modality of controls exerted by 5-HT2C receptors over these cell populations can be phasic, tonic (dependent on the 5-HT tone) or constitutive (a spontaneous activity without the presence of the ligand). These controls are functionally organized in the basal ganglia: they are mainly localized in the input structures and preferentially distributed in the limbic/associative territories of the basal ganglia. The nature of these controls is modified in neuropsychiatric conditions such as Parkinson's disease, tardive dyskinesia or addiction. Most of the available data indicate that the function of 5-HT2C receptor is enhanced in cases of chronic alterations of dopamine neurotransmission. The review illustrates that 5-HT2C receptors play a role in maintaining continuous controls over the basal ganglia via multiple diverse actions. We will discuss their interest for treatments aimed at ameliorating current pharmacotherapies in schizophrenia, Parkinson's disease or drugs abuse.

Pharmacological and physiological characterization of the tremulous jaw movement model of parkinsonian tremor: potential insights into the pathophysiology of tremor

Tremor is a cardinal symptom of parkinsonism, occurring early on in the disease course and affecting more than 70% of patients. Parkinsonian resting tremor occurs in a frequency range of 3-7 Hz and can be resistant to available pharmacotherapy. Despite its prevalence, and the significant decrease in quality of life associated with it, the pathophysiology of parkinsonian tremor is poorly understood. The tremulous jaw movement (TJM) model is an extensively validated rodent model of tremor. TJMs are induced by conditions that also lead to parkinsonism in humans (i.e., striatal DA depletion, DA antagonism, and cholinomimetic activity) and reversed by several antiparkinsonian drugs (i.e., DA precursors, DA agonists, anticholinergics, and adenosine A(2A) antagonists). TJMs occur in the same 3-7 Hz frequency range seen in parkinsonian resting tremor, a range distinct from that of dyskinesia (1-2 Hz), and postural tremor (8-14 Hz). Overall, these drug-induced TJMs share many characteristics with human parkinsonian tremor, but do not closely resemble tardive dyskinesia. The current review discusses recent advances in the validation of the TJM model, and illustrates how this model is being used to develop novel therapeutic strategies, both surgical and pharmacological, for the treatment of parkinsonian resting tremor.

Oral tremor induced by galantamine in rats: a model of the parkinsonian side effects of cholinomimetics used to treat Alzheimer's disease

Anticholinesterases are the most common treatment for Alzheimer's disease, and, in recent years, a new group of cholinesterase inhibitors (i.e. rivastigmine, galantamine, and donepezil) has become available. Although these drugs improve cognitive symptoms, they also can induce or exacerbate parkinsonian symptoms, including tremor. The present studies were conducted to determine if galantamine induces tremulous jaw movements, a rodent model of parkinsonian tremor, and to investigate whether these oral motor impairments can be reversed by co-administration of adenosine A(2A) antagonists. The first experiment demonstrated that systemic injections of galantamine (0.75-6.0 mg/kg I.P.) induced a dose-related increase in tremulous jaw movements in rats. In a second study, co-administration of the muscarinic antagonist scopolamine (0.0156-0.25 mg/kg I.P.) produced a dose dependent suppression of tremulous jaw movements induced by a 3.0 mg/kg dose of galantamine, indicating that galantamine induces these tremulous oral movements through actions on muscarinic acetylcholine receptors. In two additional studies, analyses of freeze-frame video and electromyographic activity recorded from the lateral temporalis muscle indicated that the local frequency of these galantamine-induced jaw movements occurs in the 3-7 Hz frequency range that is characteristic of parkinsonian tremor. In the final experiment, the adenosine A(2A) antagonist MSX-3 significantly attenuated the tremulous jaw movements induced by the 3.0mg/kg dose of galantamine, which is consistent with the hypothesis that co-administration of adenosine A(2A) antagonists may be beneficial in reducing parkinsonian motor impairments induced by anticholinesterase treatment.

Behavioral pharmacology of orofacial movement disorders

Dysfunction in orofacial movement is evident in patients with schizophrenia, Parkinson's disease and Huntington's disease. In animal studies on orofacial dyskinesia, these neurological disorders have been considered as a starting point to examine the pathophysiology and mechanisms underlying the symptoms. There is circumstantial evidence that orofacial dyskinesia in humans might be the consequence of hyperfunctioning mesolimbic-pallidal circuitry, in which the mesolimbic region occupies a central role, in contrast to typical Parkinson-like symptoms which involve hypofunction in the nigrostriato-nigral circuity. Studies in animals suffer from technical difficulties concerning the assessment of orofacial behaviors. There are some experimental designs that provide detailed information on the amplitude and the frequency of the jaw movements. By using such methods, the involvement of neurotransmitter systems and functional neural connections within the basal ganglia has been studied in rat rhythmical jaw movements. Regarding neurotransmitter systems, dopaminergic, cholinergic, γ-aminobutyric acid (GABA)ergic and glutamaterigic systems have been shown to be involved in rat rhythmical jaw movements. The involved neural connections have also been investigated, focusing on the differential role between the dorsal and ventral part of the striatum, the shell and core of the nucleus accumbens and the output pathways from the striatum and the nucleus accumbens. Taking available clinical and experimental evidence, the orofacial dyskinesias are thought to arise when hierarchically lower order output stations of the mesolimbic region start to dysfunction as a consequence of the arrival of distorted information sent by the mesolimbic region. This review seeks to provide an overview of prior and recent findings across several orofacial movement disorders and interpret new insights in the context of the limitations of behavioral pharmacology and prior knowledge of the regulation of behavior by dopamine receptors and other related neuronal systems.

The muscarinic receptor antagonist tropicamide suppresses tremulous jaw movements in a rodent model of parkinsonian tremor: possible role of M4 receptors

RATIONALE

Nonselective muscarinic acetylcholine antagonists have been used for several years as antiparkinsonian drugs. However, there are at least five subtypes of muscarinic receptor (M1-5). Neostriatal M4 receptors have been implicated in aspects of motor function, and it has been suggested that M4 antagonists could be used as treatments for parkinsonism.

OBJECTIVE

Currently, there is a lack of highly selective M4 antagonists that readily penetrate the blood brain barrier. Thus, the present studies focused upon the effects of tropicamide, a muscarinic acetylcholine receptor antagonist with moderate binding selectivity for the M4 receptor subtype.

MATERIALS AND METHODS

Tremulous jaw movements were used as a model of parkinsonian tremor in these studies, and the effects of tropicamide were compared with those of the nonselective muscarinic antagonist atropine.

RESULTS

Tropicamide suppressed the tremulous jaw movements induced by the muscarinic agonist pilocarpine and the dopamine antagonist pimozide. Analysis of the dose-response curves indicated that tropicamide showed approximately the same potency as atropine for suppression of pilocarpine-induced jaw movements but was more potent than atropine on the suppression of pimozide-induced jaw movements. In contrast, atropine was more potent than tropicamide in terms of impairing performance on visual stimulus detection and delayed nonmatch-to-position tasks.

CONCLUSIONS

These studies demonstrate that tropicamide, which currently is used clinically for ophthalmic purposes, can exert actions that are consistent with antiparkinsonian effects. Moreover, the different pattern of effects shown by tropicamide compared to those of atropine on motor vs cognitive tasks could be due to the modest M4 selectivity shown by tropicamide.

Modeling tardive dyskinesia: Predictive 5-HT2C receptor antagonist treatment

Tardive dyskinesia (TD), a movement disorder produced by long-term treatment with a classical antipsychotic drug, is generally considered to be a disorder of dopamine (DA) systems, since classical antipsychotics are potent DA D(2) receptor blockers. Also, acute DA D(1) agonist treatment of rats is known to produce vacuous chewing movements (VCMs), a behavioral feature resembling the oral dyskinesia that is so prominent in most instances of TD. In this paper we outline a series of studies in a new animal model of TD in which DA D(1) receptor supersensitivity was produced by neonatal 6-hydroxydopamine (6-OHDA) -induced destruction of nigrostriatal DA fibers. In rats so-lesioned 5-HT receptor supersensitivity is additionally produced, and in fact 5-HT receptor antagonists attenuate enhanced DA D(1) induction of VCMs. Moreover, in 6-OHDA-lesioned rats treated with haloperidol for one year, there a 2-fold increase in numbers of VCMs (vs intact rats treated with haloperidol); and this high frequency of VCMs persists for more than 6 months after discontinuing haloperidol treatment. During this stage, 5-HT(2) receptor antagonists, but not DA D(1) receptor antagonists, attenuate the incidence of VCMs. This series of findings implicates the 5-HT neuronal phenotype in TD, and promotes 5-HT(2) receptor antagonists, more specifically 5-HT(2C) receptor antagonists, as a rational treatment approach for TD in humans.

Movement disorders: neurodevelopment and neurobehavioural expression

Braak and co-workers have recently shown that movement disorders such as Parkinson's disease develop progressively over years with early neuronal losses in brainstem regions caudal to the substantia nigra. The relevance of this finding to notions of comorbidity between movement disorders and psychiatric symptoms was recognised at the recent meeting concerning, "Implications of Comorbidity for the Etiology and Treatment of Neuropsychiatric Disorders" held in Oct. 2005 in Mazagon, Spain. The identification of stages in the early development of neurodegenerative disorders appeared to unify multiple, diverse findings. These included: novel therapeutic innovations for Parkinson's disease, Alzheimer's disease and depression in the aged; the neurochemical ontogeny of drug-induced oral dyskinesias; the types of chemical agents abused in neuropsychiatric states; postnatal iron overload effects upon the functional and interactive role of dopaminergic and noradrenergic pathways that contribute to the expression of movement disorders; and the spectrum of motor symptoms expressed in schizophrenia and attention deficit hyperactivity disorder and the eventual treatment of these disorders. A continued focus on a number of neuropsychiatric diseases as progressive disorders may lead to further advances in understanding their etiology and in developing better therapeutics.

Effects of baclofen on reserpine-induced vacuous chewing movements in mice

We have described that GABA mimetic drugs present the ability to inhibit the expression of reserpine-induced oral movements. In this respect, oral movements is associated with important neuropathologies. This study investigates the effects of an acute or a repeated treatment of different doses of the GABA(B) agonist baclofen, as well as withdrawal from these treatments, on the development and/or expression of reserpine-induced vacuous chewing movements (VCM). Male mice received two injections of vehicle or of 1mg/kg reserpine separated by 48 h. In the first experiment, 24h later, animals were acutely treated with vehicle or baclofen (1, 2 or 4 mg/kg). In the second experiment, animals were treated with vehicle or baclofen (1 or 4 mg/kg) for four consecutive days receiving a concomitant injection of 1mg/kg reserpine (or vehicle) on Days 2 and 4. Twenty-four hours later, animals received vehicle or baclofen. Thirty minutes after the last injection, they were observed for quantification of VCM and open-field general activity. The acute administration of all the doses of baclofen abolished the manifestation of reserpine-induced VCM. Repeated treatment with 1mg/kg baclofen induced tolerance to the ability of an acute injection of this dose to reduce VCM. Treatment with baclofen (4 mg/kg) did not modify spontaneous VCM. Acute administration of the highest dose induced a decrease in general motor activity and a potentiation of the reserpine-induced decrease in general activity. These results reinforce the involvement of GABAergic hypofunction in the expression of oral movements and suggest that a repeated treatment with baclofen induces compensatory changes in GABAergic transmission that can attenuate its acute property to decrease VCM.

Enhanced 5-HT2C receptor signaling is associated with haloperidol-induced "early onset" vacuous chewing in rats: implications for antipsychotic drug therapy

RATIONALE

Haloperidol is a representative of typical antipsychotics that are still in clinical use and which can lead to abnormal motor activity following repeated administration. The mechanisms underlying antipsychotic-induced dyskinesias are not well understood but are widely held to be related to excessive loss of dopamine function. In several models of dopamine hypofunction, serotonin 5-HT2C receptors have been shown to mediate vacuous chewing movements (VCM), a form of abnormal motor activity. It is well established that repeated haloperidol administration also elicits VCM, but there is no information on how repeated haloperidol administration affects 5-HT2C receptor signaling.

OBJECTIVES

In the present study, we tested the hypothesis that repeated daily administration of haloperidol leads to enhanced serotonin 5-HT2C receptor signaling that is associated with increased 5-HT2C-mediated VCM.

METHODS

Rats were treated by subcutaneous injection once daily for 21 days with either vehicle, a low dose of haloperidol (0.1 mg kg(-1) day(-1)), or a high dose of haloperidol (1.0 mg kg(-1) day(-1)). Following 1-day withdrawal, rats were either used for behavioral scoring of VCM or sacrificed for biochemical assessment of 5-HT2 receptor-mediated phospholipase C activity and radioligand binding. VCM were scored following two successive "drug" challenges. The first challenge was an injection of vehicle (0.9% saline), and the second challenge was an injection of the 5-HT2C agonist meta-chlorophenylpiperazine (1.0 mg/kg). In this manner, a measure of "spontaneous" and "5-HT2C-elicited" orofacial activity could be made while minimizing animal use.

RESULTS

Following 21-day haloperidol treatment at either dose, there was an increase in expression of meta-chlorophenylpiperazine-induced VCM. In a separate experiment, meta-chlorophenylpiperazine-induced VCM were shown to be mediated through 5-HT2C receptors. Striatal 5-HT2C receptor-mediated phospholipase C (PLC) activity and high-affinity agonist-labeled 5-HT2C receptors were also increased following either dose of haloperidol as compared to vehicle treatment. GTP-stimulated PLC activity and striatal Gq proteins were unchanged by haloperidol suggesting that enhanced signaling could be accounted for by alterations at the level of the receptor and not at downstream mechanisms.

CONCLUSIONS

Repeated daily administration of haloperidol leads to an adaptive increase in 5-HT2C signaling which may contribute to abnormal motor function associated with antipsychotic use.

Quetiapine (Seroquel) shows a pattern of behavioral effects similar to the atypical antipsychotics clozapine and olanzapine: studies with tremulous jaw movements in rats

RATIONALE

Previous studies demonstrated that clozapine and olanzapine suppressed tacrine-induced jaw movements at lower doses than those required for suppression of lever pressing.

OBJECTIVE

The present studies were undertaken to evaluate the novel atypical antipsychotic quetiapine using the jaw movement model.

METHODS

The effect of acute quetiapine on the suppression of tacrine-induced tremulous jaw movements was examined. To determine the relative potency of this effect compared with other behavioral effects of quetiapine, suppression of lever pressing also was studied. In other studies, rats received quetiapine for 14 consecutive days to study the effects of repeated injections of this drug.

RESULTS

Acute quetiapine injections decreased tacrine-induced jaw movements and lever pressing. The ratio of the ED50 for suppression of jaw movements divided by the ED50 for suppression of lever pressing was used as an index of liability to produce motor side effects, and the present results demonstrate that quetiapine has a ratio similar to that previously shown for clozapine and olanzapine. In the repeated-administration studies, quetiapine failed to induce jaw movements. On day 14, quetiapine reduced tacrine-induced tremulous jaw movements, and in a parallel experiment quetiapine significantly suppressed lever pressing on days 1-14. Repeated injections of quetiapine reduced tacrine-induced jaw movements over a dose range lower than that required for suppression of lever pressing.

CONCLUSIONS

On tests of jaw movement activity and lever pressing after both acute and repeated drug administration, quetiapine showed a profile somewhat similar to clozapine and olanzapine. A theoretical model is offered suggesting that atypical antipsychotics that act on 5-HT or muscarinic receptors have intrinsic antiparkinsonian actions that work in opposition to the motor effects produced by dopamine antagonism.

Dopamine agonists suppress cholinomimetic-induced tremulous jaw movements in an animal model of Parkinsonism: tremorolytic effects of pergolide, ropinirole and CY 208-243

Considerable evidence indicates that cholinomimetic-induced tremulous jaw movements in rats share many characteristics with human Parkinsonian tremor, and several antiparkinsonian drugs suppress cholinomimetic-induced tremulous jaw movements. The present study investigated three different types of dopamine agonists, which have known antiparkinsonian characteristics, for their ability to suppress the tremulous jaw movements induced by tacrine (5.0 mg/kg). The non-selective dopamine agonist pergolide, a widely used antiparkinsonian drug, was highly potent at suppressing tacrine-induced jaw movements (e.g. 0.125-1.0 mg/kg). The selective D2 agonist ropinirole, which also is used clinically as an antiparkinsonian drug, suppressed jaw movements in the dose range of 2.5-20.0 mg/kg. The D1 agonist CY 208-243, which has been reported to suppress tremor, also reduced jaw movement activity (4.0 mg/kg). Across several studies, the rank order of potency for suppressing cholinomimetic-induced jaw movements in rats is related to the potency for producing antiparkinsonian effects in humans. Together with previous studies, the present results suggest that cholinomimetic-induced jaw movements in rats can be used to characterize dopaminergic antiparkinsonian agents and to investigate the basal ganglia circuits involved in the generation of tremulous movements.

Continuous but not intermittent olanzapine infusion induces vacuous chewing movements in rats

BACKGROUND

Continuous, but not intermittent, infusion with a conventional antipsychotic (haloperidol, HAL) can induce the vacuous chewing movement (VCM) syndrome in rats. The objective of this study was to determine whether continuous, versus intermittent, olanzapine (OLZ) infusion differently affects the development of VCMs.

METHODS

Experiment 1: Animals were treated with 7.5 mg/kg/day of OLZ or vehicle (VEH) via either minipump (MP) or daily subcutaneous (SC) injections for 8 weeks. Experiment 2: A separate group of rats were treated with 15 mg/kg/day of OLZ, or 1 mg/kg/day of HAL or VEH via MP for 8 weeks. Dopamine D2 receptor occupancy levels were measured, ex vivo, with [3H]-raclopride.

RESULTS

Experiment 1: Rats receiving 7.5 mg/kg/day of OLZ via MP (51% D2 occupancy), but not those receiving the same dose via daily SC injections (94% peak D2 occupancy), showed significant VCM levels compared with control animals (p = .02). Experiment 2: Both OLZ (67% D2 occupancy) and HAL (79% D2 occupancy) led to similar increases in VCMs compared with VEH (p = .005).

CONCLUSIONS

This study provides strong evidence that even an atypical antipsychotic like OLZ, which rarely gives rise to tardive dyskinesia in the clinic, can lead to the VCM syndrome in rats if the antipsychotic is administered in a method (via MP) that leads to continuous presence of the drug in the brain.

Validation of the tremulous jaw movement model for assessment of the motor effects of typical and atypical antipychotics: effects of pimozide (Orap) in rats

Drug-induced tremulous jaw movements (TJMs) in rats have been used as a model of parkinsonian tremor. Previous studies demonstrated that the typical antipsychotic haloperidol induced TJMs after acute or subchronic administration, while atypical antipsychotics did not. Moreover, it has been suggested that the relative potency for suppression of tacrine-induced TJMs relative to the suppression of lever pressing can be used to discriminate between typical and atypical antipsychotics. In order to validate this model with additional drugs, the present studies assessed the effects of the typical antipsychotic pimozide. In the first series of experiments, the effects of acute pimozide on tacrine-induced TJMs and lever pressing were examined. As with haloperidol, pimozide failed to suppress tacrine-induced TJMs, even at doses considerably higher than those that suppressed lever pressing. In the second group of experiments, rats were given single daily injections of pimozide (0.125-1.0 mg/kg) or tartaric acid vehicle for 13 days, and were observed for TJMs on days 1, 7, and 13. Pimozide induced TJMs in a dose-related manner on all days. The jaw movements occurred largely in the 3-7 Hz frequency range characteristic of parkinsonian tremor. These data support the hypothesis that typical antipsychotics can induce TJMs in rats, and demonstrate that chronic administration of typical antipsychotics is not necessary for induction of TJMs. TJMs induced by acute or subchronic pimozide may be related to early-onset motor syndromes such as drug-induced parkinsonism.

Motor effects of GABA(A) antagonism in globus pallidus: studies of locomotion and tremulous jaw movements in rats

RATIONALE

Although most rodent studies related to parkinsonian symptoms have focused on locomotion, tremulous jaw movements also have been used as a rodent model of tremor for investigating the circuitry of the basal ganglia.

OBJECTIVE

There are multiple pathways involved in the generation of parkinsonian symptoms. The globus pallidus is a basal ganglia relay nucleus, and the present study was conducted to investigate the effect of pallidal GABA antagonism on locomotion and tremulous jaw movements.

METHODS

Suppression of locomotion and induction of tremulous jaw movements were produced by repeated (i.e., 14 day) systemic administration of the dopamine D2 antagonist haloperidol, and by acute systemic injection of the muscarinic agonist pilocarpine. The GABA(A) antagonist bicuculline was injected into the globus pallidus, and its effects on locomotion in haloperidol- and pilocarpine-treated rats were assessed in the first group of experiments. In the second group of experiments, the effects of intrapallidal infusions of bicuculline on haloperidol- and pilocarpine-induced jaw movements were observed.

RESULTS

Pallidal GABA antagonism stimulated locomotion when no other treatment was present, and also when animals were coadministered haloperidol or pilocarpine. Bicuculline suppressed haloperidol-induced jaw movements in a dose-related manner, and had no effect on pilocarpine-induced jaw movements.

CONCLUSIONS

These results support the notion that there are distinct pathways conveying basal ganglia outflow and demonstrate that the striatopallidal pathway is involved in the generation of the haloperidol-induced tremulous jaw movements. These findings are consistent with some features of current models of basal ganglia function and may lead to an understanding of the specific mechanisms that generate parkinsonian symptoms.

Relationship of orofacial movements to behavioural repertoire as assessed topographically over the course of 6-month haloperidol treatment followed by 4-month withdrawal

RATIONALE

Late-onset vacuous chewing movements (VCMs) arise in a significant proportion of rats treated chronically with conventional antipsychotic drugs. Given their common action to block dopamine D2-like receptors, VCMs may be related to changes in dopaminergic function; if so, other typical dopamine-mediated behaviours might be altered also.

OBJECTIVE

To examine this hypothesis, behavioural repertoire was studied topographically over the course of chronic treatment and withdrawal.

METHODS

Animals were injected with haloperidol decanoate 28 mg/kg IM, or vehicle, every 3 weeks for 27 weeks, and then maintained without treatment for a further 18 weeks. Immediately before each injection and during withdrawal, VCMs and other topographies of behaviour were assessed.

RESULTS

In both control and haloperidol-treated rats, exploratory behaviours declined over the study, indicating habituation effects. Conversely, VCMs emerged after 6 weeks of treatment with haloperidol and persisted after withdrawal; VCM and locomotion were not related, indicating that in treated rats, increased VCMs are not an artifact of reduced locomotion. Treated animals with VCMs evidenced increases in buccal tremor and grooming behaviour relative to those without VCMs, although no clear relationship to the emergence of VCMs was established; there were no material differences in any other topographies of behaviour.

CONCLUSION

The effect of long-term treatment with haloperidol to induce VCMs is not reflected in fundamental changes in dopamine-mediated behavioural topography but, rather, appears to affect neural mechanisms involved in orofacial movement preferentially.

Differential effects of within-day continuous vs. transient dopamine D2 receptor occupancy in the development of vacuous chewing movements (VCMs) in rats

Accumulating evidence suggests that antipsychotics (APs) that lead to sustained blockade of dopamine D(2) receptors are more likely to induce acute extrapyramidal side effects (EPS) compared to APs that only occupy D(2) receptors transiently. It is unclear, however, whether a similar relationship exists for long-term AP-induced motoric side effects like tardive dyskinesia (TD). The objective of this study was to ascertain whether transient (via daily subcutaneous (s.c.) injections) vs continuous (via osmotic minipump) AP-induced D(2) receptor occupancy differentially affects the development of haloperidol-induced vacuous chewing movements (VCMs), an animal model of TD. Six groups of 12 rats received 0.1, 0.25, or 1 mg/kg of haloperidol or vehicle (n=36) via osmotic minipump (to provide within-day sustained) or daily s.c. injection (within-day transient) for 8 weeks. VCMs were measured on a weekly basis and D(2) occupancy levels were measured in vivo using [(3)H]-raclopride at the end of the experiment. Minipump-treated rats developed HAL dose-dependent D(2) occupancies of 0.1 mg/kg/day (57%), 0.25 mg/kg/day (70%), and 1 mg/kg/day (88%). S.C.-treated rats also developed HAL dose-dependent D(2) occupancies of 0.1 mg/kg/day (83% peak, 3% trough), 0.25 mg/kg/day (89% peak, 0% trough), and 1 mg/kg/day (94% peak, 17% trough). A total of 43% of rats given 0.25 and 1 mg/kg/day of HAL via minipump developed high VCMs compared to only 8% of the rats given the same doses via daily s.c. injections. The 0.1 mg/kg dose did not give rise to VCMs beyond vehicle levels regardless of the route of administration. These findings support the contention that D(2) occupancy levels induced by chronic HAL must be high and sustained through the day before significant risk of VCMs, and perhaps also TD, emerges.

Pro-Leu-glycinamide and its peptidomimetic, PAOPA, attenuate haloperidol induced vacuous chewing movements in rat: A model of human tardive dyskinesia

In the present experimental paradigm, we examine the effect of L-prolyl-L-leucyl-glycinamide (PLG) co-administration with haloperidol on vacuous chewing movements (VCM) in rats-a model of tardive dyskinesia (TD) in humans. We examined the dose dependent induction of VCM through both injected and orally administered PLG (MIF-1). Our results show significant levels of VCM attenuation (P<0.05) in rats treated with 10mg/kg of PLG. Doses of 1 and 100mg/kg were ineffective. Reductions were present in both orally treated and injected rats. We also examined the therapeutic effect of a peptidomimetic of PLG-PAOPA. PAOPA was able to produce similar behavioral effects to PLG at a dose, which was 100-fold lower than the effective dose of PLG. These results suggest that PLG may play a role in D2 receptor expression and function, as well as providing a therapy for neuroleptic induced TD.

Blockade of nigral and pallidal opioid receptors suppresses vacuous chewing movements in a rodent model of tardive dyskinesia

Chronic neuroleptic treatment leads to the development of tardive dyskinesia in 20-30% of patients. While the pathogenesis of tardive dyskinesia remains elusive, altered opioid peptide function in striatal projection pathways of the basal ganglia has been implicated. Using a rodent model of vacuous chewing movements induced by chronic neuroleptic administration, we investigated regional involvement of opioid transmission in tardive dyskinesia. We examined the role of dynorphin in the direct striatonigral pathway by infusing nor-binaltorphimine, a selective kappa opioid receptor antagonist, into the substantia nigra pars reticulata. As well, infusions of naloxone (a non-specific opioid receptor antagonist), D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr amide (CTOP; a mu opioid receptor antagonist) or naltrindole (a delta opioid receptor antagonist) into the globus pallidus were used to establish the contribution of the striatopallidal pathway. Chronic fluphenazine treatment (25 mg/kg i.m. every 3 weeks for 18 weeks) resulted in a robust increase in vacuous chewing movements. Infusion of nor-binaltorphimine (5.0 nmol) into the substantia nigra pars reticulata significantly attenuated vacuous chewing movements. Infusion of naloxone (0.5 and 2.0 nmol) into the globus pallidus also significantly attenuated vacuous chewing. Infusion of naltrindole into the globus pallidus blocked vacuous chewing at all doses administered (0.5, 1.0, 2.0 nmol) while CTOP was only effective at the two higher doses. From these results we suggest that increases in dynorphin in the direct striatonigral pathway and enkephalin in the indirect striatopallidal pathway following chronic neuroleptic administration are both likely to contribute to tardive dyskinesia.

The vacuous chewing movement (VCM) model of tardive dyskinesia revisited: is there a relationship to dopamine D(2) receptor occupancy?

Tardive dyskinesia (TD) is a late side effect of long-term antipsychotic use in humans, and the vacuous chewing movement (VCM) model has been used routinely to study this movement disorder in rats. Recent receptor occupancy studies in humans and rats have found that antipsychotics given in doses which lead to moderate levels of D(2) receptor blockade can achieve optimal clinical response while minimizing the emergence of acute motor side effects. This suggests that clinicians may have been using inappropriately high doses of antipsychotics. A review of the existing VCM literature indicates that most animal studies have similarly employed antipsychotic doses that are high, i.e. doses that lead to near complete D(2) receptor saturation. To verify whether the incidence or severity of VCMs would decrease with lower antipsychotic doses, we conducted initial experiments with different doses of haloperidol (HAL) given either as repeated daily injections or as depot injections over the course of several weeks. Our results demonstrate that (1) the incidence of VCMs is significantly related to HAL dose, and (2) significant levels of VCMs only emerge when haloperidol is continually present. These findings are consistent with the possibility that total D(2) occupancy, as well as 'transience' of receptor occupation, may be important in the development of late-onset antipsychotic-induced dyskinetic syndromes.

Neurotoxicity associated with neuroleptic-induced oral dyskinesias in rats. Implications for tardive dyskinesia?

Tardive dyskinesia is a serious motor side effect of long-term treatment with neuroleptics, with an unknown pathophysiologic basis. Brain damage and aging are prominent risk-factors, and together with the persistent character of the disorder, it is likely that long-lasting neuronal changes are involved in the pathogenesis. It has been hypothesized that striatal neurodegeneration caused by excitotoxic mechanisms and oxidative stress may play an important role in the development of the disorder, and the scope of the present work is to review the evidence supporting this hypothesis. The rat model of tardive dyskinesia has been used extensively in the field, and the usefulness of this model will be discussed. Neuroleptics are able to induce oxidative stress in vitro and increase striatal glutamatergic activity in rats, which may lead to toxic effects in the striatum. Drugs that block excitotoxicity inhibit the development of persistent oral dyskinesia in the rat model, and impaired energy metabolism leads to increased frequency of oral dyskinesia. There are also signs of altered striatal histology in rats with high frequency of oral dyskinesia. Furthermore, markers of increased oxidative stress and glutamatergic neurotransmission have been found in the cerebrospinal fluid of patients with tardive dyskinesia. In conclusion, several lines of evidence implicate neurotoxic events in the development of neuroleptic induced tardive dyskinesia.

Effects of H1 antagonists on cholinomimetic-induced tremulous jaw movements: studies of diphenhydramine, doxepin, and mepyramine

In several previous studies, tremulous jaw movements in rats have been used to assess the effects of antiparkinsonian drugs and atypical antipsychotics. Because antihistamines such as diphenhydramine are used as antiparkinsonian agents, and atypical antipsychotic drugs such as clozapine and olanzapine have high affinity for histamine H1 receptors, the present study investigated the effects of H1 antagonists on cholinomimetic-induced jaw movements. Diphenhydramine, doxepin, and mepyramine (all injected IP 2.5-20.0 mg/kg) were assessed for their ability to block the jaw movements induced by 5.0 mg/kg of the anticholinesterase tacrine. Within this dose range, only diphenhydramine produced a robust and significant reduction in jaw movement activity. Thus, diphenhydramine was subjected to further testing, which employed procedures previously used to assess the effects of other antitremorogenic drugs, such as clozapine. Diphenhydramine did not induce jaw movement activity. In addition to suppressing jaw movement activity after acute injections, diphenhydramine also suppressed tacrine-induced jaw movements after repeated (14-day) administration. In summary, the present results show that diphenhydramine suppresses cholinomimetic-induced jaw movements, an effect that is similar to other antiparkinsonian or antitremor drugs such as anticholinergics, L-DOPA, DA antagonists, and clozapine. Nevertheless, doxepin produced only mild effects, and mepyramine, which has a higher affinity and selectivity than diphenhydramine for H1 receptors, failed to suppress cholinomimetic-induced jaw movements. These results suggest that diphenhydramine suppresses tremulous movements through a mechanism that does not depend upon antagonism of histamine H1 receptors.

The preventative role of antioxidants (selegiline and vitamin E) in a rat model of tardive dyskinesia

BACKGROUND

We examined the potential protective effects of two potent antioxidants, selegiline and vitamin E, in a rodent model of tardive dyskinesia (TD), viz. neuroleptic-induced spontaneous orofacial movements.

METHODS

Rats were treated with fortnightly injections of fluphenazine decanoate for 12 weeks, and examined at baseline and at fortnightly intervals for vacuous chewing movements, mouth tremors and tongue protrusions.

RESULTS

The administration of fluphenazine led to a progressive increase of all three types of orofacial movements. In the first study, the impact of the concomitant administration of selegiline on orofacial movements was examined. Selegiline led to a reduction in orofacial movements in neuroleptic-treated rats to the level of control rats not being administered a neuroleptic drug. In the second study, rats were fed diets either high or low in their vitamin E content. High and low vitamin E diets did not significantly affect neuroleptic-induced orofacial movements.

CONCLUSIONS

Our studies provide some support for the hypothesis that oxidative injury may play a role in the genesis of neuroleptic-induced movement disorder, and prompt further examination of this hypothesis in both animals and humans.

The effect of novel antipsychotics in rat oral dyskinesia

1. The effect of the D1 agonist SKF38393 and the 5HT2C agonist m-CPP on repetitive jaw movements (RJM) was studied in rats. Acute administration of SKF38393 and/or m-CPP induced RJM in a dose dependent manner. In rats treated with both drugs, RJM responses were about equal to the sum of those obtained with each drug alone. 2. The induction of RJM by SKF38393 was somewhat lower in rats pretreated with 5HT2C receptor antagonist, mianserin, whereas mianserin severely reduced RJM induced by m-CPP alone. 3. D1 antagonist SCH23390 inhibited SKF38393 induced RJM but had no effect on m-CPP induced chewing behavior. 4. The present study confirms earlier evidence that D1 agonists used at optimal doses for the induction of RJM do not involve the serotonergic system in a significant way. It does, however, implicate the system in the emergence of drug induced oral behavior in rats. 5. The effect of the atypical antipsychotics, clozapine, olanzapine and risperidone was studied on SKF38393 and m-CPP induced RJM. Pretreatment with the atypical antipsychotics clozapine and olanzapine inhibit SKF38393 and m-CPP induced RJM. Pretreatment with risperidone inhibits m-CPP induced oral behavior in rats while increases dose dependently SKF38393 induced RJM.

A phosphoinositide-linked dopamine D1 receptor mediates repetitive jaw movements in rats

BACKGROUND

We have demonstrated that rats injected with D1 agonists SKF 38393 or A68930 demonstrate repetitive jaw movements (RJM). These agonist-induced movements in rats are similar in their appearance to those induced in rats by long-term treatment with antipsychotic drugs. Over recent years D-1 receptors were discovered which showed linkage not only to c-AMP but also to PI hydrolysis. We examined the effect of EEDQ inactivation of D1 receptors on D-1 mediated PI hydrolysis and RJM.

METHODS

Twenty four hours following EEDQ or vehicle administration D-1 agonists or vehicle were administered. The number of RJM episodes was assessed in EEDQ and vehicle treated rats. D-1 receptor density and inositol phosphate formation were determined in the striata.

RESULTS

EEDQ administration resulted, 24 hours later, in 70-80% selective depletion of D-1 receptors in the striata but did not modify the rate of RJM induced by D-1 agonists. There was no significant difference in D-1 mediated PI hydrolysis in EEDQ treated rats when compared to vehicle treated group.

CONCLUSIONS

The present data support the earlier demonstration of D-1 agonist induced RJM, an effect mediated by a subpopulation of a D-1 receptor subtype and constitute the first behavioral evidence for the existence of a behavioral response mediated by D-1 like dopamine receptors linked to an alternate second messenger system-PI hydrolysis.

Tremulous jaw movements in rats: a model of parkinsonian tremor

Several pharmacological and neurochemical conditions in rats induce 'vacuous' or 'tremulous' jaw movements. Although the clinical significance of these movements has been a subject of some debate, considerable evidence indicates that the non-directed, chewing-like movements induced by cholinomimetics, dopamine antagonists and dopamine depletions have many of the characteristics of parkinsonian tremor. These movements occur within the 3-7 Hz peak frequency range that is characteristic of parkinsonian tremor. Tremulous jaw movements are induced by many of the conditions that are associated with parkinsonism, and suppressed by several different antiparkinsonian drugs, including scopolamine, benztropine, L-DOPA, apomorphine, bromocriptine, amantadine and clozapine. Striatal cholinergic and dopaminergic mechanisms are involved in the generation of tremulous jaw movements, and substantia nigra pars reticulata appears to be a major basal ganglia output region through which the jaw movements are regulated. Future research on the neurochemical and anatomical characteristics of tremulous jaw movements could yield important insights into the brain mechanisms that generate tremulous movements.

Behavioral and electromyographic characterization of the local frequency of tacrine-induced tremulous jaw movements

Rats were implanted with fine-wire electromyograph (EMG) electrodes and were videotaped to identify the local frequency characteristics and muscle activity associated with tacrine-induced tremulous jaw movements. All rats received intraperitoneal injections of 2.5 mg/kg tacrine. The videotape sessions were played back in slow motion (i.e., one-sixth normal speed), and an observer entered each jaw movement into a computer program that recalculated the interresponse time and the local frequency (in hertz) for each movement within a burst. Analyses of the distribution of frequencies showed that the peak frequency of jaw movements was in the 3- to 5-Hz frequency range, with an average frequency of 4.0 Hz. EMG electrodes were implanted into three jaw muscles: temporalis, anterior belly of digastricus, and masseter. Tremulous jaw movements were not accompanied by consistent changes in masseter activity. The anterior belly of digastricus showed bursts of EMG activity during some jaw movements, although the temporal relation between jaw movements and EMG activity was somewhat inconsistent. The muscle that showed activity most closely related to tremulous jaw movements was the temporalis. During bursts of jaw movements, temporalis muscles across several different rats showed bursts of EMG activity. Sections of videotape corresponding to bursts of EMG activity were reanalyzed by freeze-frame examination of the tape; typically, the temporalis showed a burst for each jaw movement, with the burst of activity occurring during the jaw-closing phase and the transition between jaw closing and opening. These results indicate that the local frequency of tremulous jaw movements is within the 3- to 7-Hz frequency that is typically associated with parkinsonian tremor. Moreover, the EMG data suggest that temporalis is a major contributor to the muscle activity that underlies tremulous jaw movements.

Motor dysfunction produced by tacrine administration in rats

In the present study, three experiments were conducted to provide a characterization of some of the motor effects of the anticholinesterase tacrine (1.25-5.0 mg/kg I.P.) in rats. In the first experiment, tacrine was found to produce tremulous jaw movements in the dose range of 1.25-5.0 mg/kg. The second experiment examined the effects of tacrine on locomotion, and it was demonstrated that tacrine produced a dose-related suppression of open-field motor activity. In the third experiment, the effects of tacrine were assessed using operant conditioning procedures. Behavioral output during lever pressing on a fixed ratio 5 schedule was recorded by a computerized system that measured response initiation time (time from offset of one response to onset of the next) and duration for each lever press. Tacrine administration substantially depressed lever pressing response rate. This deficit was largely due to a substantial increase in the average response initiation time. Analysis of the distribution of response initiation times indicated that tacrine-treated rats made relatively few responses with fast initiation times (e.g., 0-125 ms), and also that tacrine led to a dramatic increase in the number of pauses in responding (i.e., response initiation times greater than 2.5 s). Tacrine-treated rats showed a slight increase in the average initiation time for fast responses (i.e., a slight decrease in the local rate of responding), and also showed a substantial increase in the average length of pauses greater than 2.5 s. Analysis of response durations indicated that there was an overall increase in average response duration among animals that received the higher doses of tacrine. Although tacrine-induced decreases in the local rate of responding and increases in response duration contribute to the overall deficit, the major reason why tacrine-treated animals responded less was because they took much longer breaks in responding. It is possible that the tacrine-induced increases in pausing reflect a drug-induced akinesia. Thus, the present experiments indicate that tacrine impairs several aspects of motor function in the dose range tested. In view of the fact that tremor and motor slowing are classic symptoms of Parkinsonism, the present results in rats are consistent with the human literature indicating that tacrine (Cognex) can produce Parkinsonian side effects. Studies of the motor dysfunctions produced by tacrine in rats could be useful for investigating the motor side effects of tacrine in humans.

Effects of clozapine and haloperidol on baseline levels of vacuous jaw movements in aged rats

Some studies have indicated that aged rats have elevated basal levels of vacuous jaw movements and these vacuous jaw movements are exacerbated by classic neuroleptic drugs like haloperidol, but the effects of the atypical antipsychotic clozapine on vacuous jaw movements in aged rats has not previously been studied. Aged rats were administered daily intraperitoneal injections of either haloperidol (0.04, 0.1 or 0.4 mg/kg), clozapine (0.4, 1.0, 4.0 mg/kg) or 0.3% tartaric acid vehicle for 22 days. On days 1, 8, 15 and 22 these rats were placed in an observation tube and vacuous jaw movements were recorded by two trained observers. Vacuous jaw movements were present in the aged rats receiving vehicle. Haloperidol produced a dose-dependent increase in vacuous jaw movements while clozapine produced a dose-dependent attenuation of vacuous jaw movements, relative to the vehicle-treated rats. These results indicate that screening for vacuous jaw movements may provide a useful behavioral assay for atypical antipsychotic drugs which do not produce extrapyramidal side effects and that clozapine's resistance to these side effects may extend to populations of elderly human patients.

Tremulous characteristics of the vacuous jaw movements induced by pilocarpine and ventrolateral striatal dopamine depletions

Vacuous jaw movements induced by the muscarinic agonist pilocarpine and striatal dopamine depletions were examined using a slow motion videotape system. With this procedure, rats were videotaped in a Plexiglas tube so that the profile of the head region could be seen. Vacuous jaw movements were analyzed by examining the tape at 1/6 normal speed. An observer recorded each jaw movement using a computer, and the computer program re-calculated the temporal characteristics of jaw movement responses back to normal speed. The interresponse time was recorded for each jaw movement, and each jaw movement interresponse time was assigned to a 50 ms wide time bin. Thus, the distribution of interresponse times could be used to analyze the temporal characteristics of jaw movement responses. In the first experiment, rats were administered saline vehicle, 1.0 mg/kg and 2.0 mg/kg pilocarpine. The rats were videotaped 10-15 min after injection, and the data were analyzed as described above. Pilocarpine induced very high levels of vacuous jaw movements, and the vast majority of all movements occurred in "bursts" with interresponse times of 1.0 s or less. Analysis of the interresponse time distributions showed that most of the jaw movements were within the 150-350 ms range. The modal jaw movement interresponse time was in the 150-200 ms range, which corresponds to a local frequency of 5-6.66 Hz. In the second experiment, the neurotoxic agent 6-hydroxydopamine was injected directly into the ventrolateral striatum in order to produce a local dopamine depletion. The dopamine-depleted rats were observed for jaw movements 7 days after surgery. The overall level of jaw movement activity resulting from dopamine-depletion was much lower than that produced by pilocarpine. There was a significant inverse correlation between ventrolateral striatal dopamine levels and total number of vacuous jaw movements. Videotape analysis indicated that the temporal characteristics of jaw movements induced by dopamine depletions were similar to those shown with pilocarpine. These experiments indicate that vacuous jaw movements induced by pilocarpine and striatal dopamine depletion occur in a frequency range similar to that shown in parkinsonian tremor.

The neurotensin antagonist SR 48692 fails to modify the behavioural responses to a dopamine D1 receptor agonist in the rat

The effects of the neurotensin antagonist SR 48692 on the behavioural responses to the dopamine D1 receptor agonist SKF 38393 were investigated in the rat. SKF 38393 (5 mg/kg s.c.) elicited vacuous chewing movements (VCMs) and grooming, which were unaffected by SR 48692 (50 micrograms/kg i.p.). The dopamine D2 receptor antagonist raclopride (0.5 mg/kg s.c.) elicited a small increase in VCMs in animals treated with SR 48692 and attenuated grooming induced by SKF 38393. These effects were not otherwise modified by SR 48692. We conclude that VCMs induced by acute administration of a dopamine D1 receptor agonist are unlikely to be dependent upon enhanced release of neurotensin in the striatum or its projections. This is contrast to the vacuous chewing response which emerges following chronic administration of neuroleptics, which is attenuated by neurotensin receptor antagonist. Thus, inasmuch as chronic neuroleptic-induced VCMs in the rat may be analogous to tardive dyskinesia in humans, the responses induced by acute administration of a D1 agonist to the rat cannot be used as a model of this disorder. Furthermore, the behavioural effects of chronic neuroleptic administration reflect more than a simple shift in the balance of D1 versus D2 receptor stimulation.

The effects of raclopride on vacuous jaw movements in rats following acute administration

Classic neuroleptics produce a syndrome of vacuous jaw movements in rats, whereas atypical neuroleptics like clozapine do not. The present study compared the effects of repeated administration of raclopride, clozapine, haloperidol, or vehicle on vacuous jaw movements in rats over a 4-week period. Rats received an IP injection of drug once a day. On days 1, 8, 15, and 29 the rats were observed for a 5-min period by two trained observers who recorded their vacuous jaw movements. The dose-response curves at which each drug produced vacuous jaw movements are presented and discussed in terms of their predictive capabilities of early onset extrapyramidal side effects.

Enhanced oral activity responses to intrastriatal SKF 38393 and m-CPP are attenuated by intrastriatal mianserin in neonatal 6-OHDA-lesioned rats

Enhanced oral activity is induced in neonatal 6-hydroxydopamine- (6-OHDA-) lesioned rats by systemic administration of the dopamine (DA) D1 receptor agonist SKF 38393 and serotonin (5-HT) 5-HT2A,2C agonist m-chlorophenylpiperazine (m-CPP). The DA D1 receptor antagonist SCH 23390 effectively attenuates the effect of SKF 38393 but not m-CPP. The 5-HT2 antagonist mianserin attenuates the effects of both m-CPP and SKF 38393, suggesting that DA agonist effects are mediated by 5-HT neurochemical systems. To test whether DA and 5-HT agonist effects and interactions might occur within the neostriatum, rats were implanted with permanent injection cannulae, with tips in the ventral striatum. One group of rats was lesioned at 3 days after birth with 6-OHDA HBr (100 micrograms salt form, in each lateral ventricle; desipramine HCl pretreatment, 20 mg/kg IP, base form, 1 h), while controls received the vehicle in place of 6-OHDA. Cannulae were implanted when rats weighed 200-250 g. During a 1-h observation session SKF 38393 (5 nmol per side) produced 74.3 +/- 19.2 oral movements in intact rats and 310.7 +/- 97.0 oral movements in 6-OHDA-lesioned rats. m-CPP (10 nmol per side) produced 72.6 +/- 15.1 and 274.5 +/- 65.0 oral movements in these respective groups. These responses were several-fold greater than the 25.3 +/- 7.3 and 41.8 +/- 9.5 oral movements in the same groups after saline (0.5 microliter per side) (P < 0.05). Mianserin (6 nmol per side) alone had no effect on oral activity but attenuated responses to both SKF 38393 and m-CPP in intact and 6-OHDA-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of neurotensin in a rodent model of tardive dyskinesia

The role of neurotensin (NT) in a putative model of tardive dyskinesia (TD) was examined in the rat. When administered directly into the ventrolateral striatum of neuroleptic-naive animals, NT (2.5 micrograms/side) elicited vacuous chewing movements. This response was not seen following administration of NT into other striatal regions or the substantia nigra and was suppressed by the NT antagonist SR 48692 (100 micrograms/kg i.p.). Vacuous chewing movements were also seen following chronic administration of fluphenazine decanoate. These movements were likewise suppressed by SR 48692 (10-100 micrograms/kg i.p.), which failed to affect other behavioural responses and was without effect in neuroleptic-naive animals. Our data suggest that increased levels of endogenous NT within the ventrolateral striatum may play a critical role in the development of TD following chronic neuroleptic administration and that NT antagonists may be beneficial for the treatment of this disorder.

Neuroleptic-induced vacuous chewing movements in rodents: incidence and effects of long-term increases in haloperidol dose

Rats treated chronically with neuroleptics develop vacuous chewing movements (VCMs), similar in some respects to tardive dyskinesia (TD) in man. The VCM syndrome was used as a model of TD to examine the ability of increased neuroleptic doses to produce long-term suppression of dyskinetic movements. The incidence and persistence of the VCM syndrome in individual rats were also assessed to look for affected and unaffected subgroups. Rats were initially treated for 15 weeks and haloperidol decanoate. For the next 21 weeks, half the group received a 50-150% increase in dose while the other half continued to receive the same dose. Animals were also followed during a 28-week withdrawal period. Total VCM ratings showed a skewed distribution, with some rats exhibiting few movements while others developed marked and persistent movements. Increasing doses did not suppress VCMs, nor did they exacerbate movements during the withdrawal period. To the extent that the VCM syndrome models TD, the absence of long-term suppression of the VCM syndrome suggests that, at this dosage range, increasing depot neuroleptic doses may not be a useful long-term strategy for TD suppression.

Repeated scopolamine injections sensitize rats to pilocarpine-induced vacuous jaw movements and enhance striatal muscarinic receptor binding

This experiment was conducted to determine if repeated administration of the muscarinic antagonist scopolamine could increase pilocarpine-induced vacuous jaw movements and also enhance muscarinic receptor binding. Rats received daily injections of either scopolamine (0.5 mg/kg IP) or saline for 14 days. On day 15 rats received no injections of scopolamine, but did receive injections of pilocarpine (1.0, 2.0 or 4.0 mg/kg IP) or saline. After administration of pilocarpine or saline, all rats were observed for vacuous jaw movements and rearing behavior. The day after pilocarpine injections, rats were sacrificed and samples of tissue from the lateral neostriatum were removed to assess muscarinic receptor binding using 3H-QNB as the ligand. Analyses of the vacuous jaw movement data indicated that there was a significant dose-related increase in vacuous jaw movements induced by pilocarpine, and also that there was a significant enhancement of pilocarpine-induced vacuous jaw movements in rats pretreated with repeated scopolamine injections. There was not a significant scopolamine x pilocarpine interaction, suggesting that pretreatment with scopolamine produced an apparent parallel shift in the pilocarpine dose-response curve. Pilocarpine significantly suppressed rearing behavior, and scopolamine pretreatment significantly enhanced the suppression of rearing produced by pilocarpine. Analysis of the receptor binding data indicated that there was a significant increase in the number of muscarinic receptor sites (Bmax) in rats that received repeated scopolamine injections as compared to saline-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Vacuous jaw movements in rats induced by acute reserpine administration: interactions with different doses of apomorphine

Two experiments were conducted to study the vacuous jaw movements induced in rats by acute administration of the monoamine-depleting agent reserpine. In the first experiment, different doses of reserpine (1.25, 2.5, and 5.0 mg/kg) were assessed for their ability to induce vacuous jaw movements. Acute administration of reserpine induced a dose-related increase in vacuous jaw movements, with the two highest doses being significantly different from the vehicle control. In the second experiment, interactions between 5.0 mg/kg reserpine and the dopamine agonist apomorphine were investigated. Coadministration of reserpine with the lowest dose of apomorphine (0.1 mg/kg) significantly increased vacuous jaw movements relative to reserpine alone. The two higher doses of apomorphine (0.5 and 1.0 mg/kg) significantly decreased vacuous jaw movements in reserpine-treated rats. These results demonstrate that vacuous jaw movements are induced by acute reserpine treatment in a dose-related manner. In addition, the interactions with apomorphine suggest that vacuous jaw movements are stimulated by decreases in dopamine release produced by low doses of apomorphine that are thought to have mainly presynaptic actions, but that these movements are decreased by higher doses of apomorphine that are known to act postsynaptically.

The opiate antagonist naloxone suppresses a rodent model of tardive dyskinesia

The effects of both opiate agonists and the opiate antagonist naloxone were examined in a rodent model of tardive dyskinesia (TD). Chronic (approximately 20 weeks) administration of fluphenazine resulted in the emergence of vacuous chewing mouth movements (VCMs), a response which may be a useful model for this disorder. Fluphenazine-induced VCMs were not affected by a variety of selective opiate agonists administered intracerebroventricularly, but were potently suppressed by subcutaneous administration of the opiate antagonist naloxone. These findings suggest that increased opiate transmission may contribute to the pathogenesis of TD. Further investigation of the role of opiate antagonists in treating this disorder are warranted.

Effects of acute haloperidol and reserpine administration on vacuous jaw movements in three different age groups of rats

In these experiments three different age groups of rats were tested for vacuous jaw movements. The age groups included rats aged 3 months, rats aged 6-9 months, and rats aged 12-15 months. In the first experiment, rats were given a single IP injection of either 0.3% tartaric acid vehicle or 0.4 mg/kg haloperidol. In the second experiment, rats were given injections of vehicle or 5.0 mg/kg reserpine. Haloperidol and reserpine significantly increased vacuous jaw movements. There were significant effects of age on vacuous jaw movements, with rats aged 6-9 months and rats aged 12-15 months showing more jaw movements than 3-month-old rats. In both experiments, the drug x age interaction was not significant, indicating that the combined effects of age and neuroleptic treatment were additive and not synergistic. Haloperidol and reserpine also reduced rearing behavior in all age groups. It is concluded that age is an important variable in studies of vacuous jaw movements.

Electrical kindling of rats treated discontinuously or continuously with haloperidol

Intermittent as opposed to continuous treatment of rats with haloperidol resulted in a long-lasting potentiation of oral activity. To examine if this behavioural sensitization to discontinuous neuroleptic treatment facilitates seizure development in electrical kindling, rats treated either intermittently or continuously with haloperidol for 15 weeks were kindled in the nucleus amygdala. Development of kindled seizures was significantly faster in the intermittently treated group (P < 0.01) than in controls or continuously treated rats. Furthermore, discontinuously treated animals displayed electroencephalographic afterdischarges in the substantia nigra from the beginning of treatment. The findings of cross-sensitivity between electrical amygdala kindling and pharmacological sensitization and of early appearance of epileptiform nigral activity have implications for the pathogenesis of both conditions. We suggest that depressed gamma-aminobutyric acid activity in substantia nigra could be a common mechanism.

Increases in rat striatal extracellular dopamine and vacuous chewing produced by two sigma receptor ligands

The present studies were carried out to analyze the neurochemical and behavioral effects of peripheral sigma ligand administration in the rat. Based upon previous studies which showed an increase in turning behavior following unilateral intranigral administration of sigma ligands, we determined the effects of two sigma ligands, 1,3-di-o-tolylguanidine (DTG) and (+)-pentazocine, on extracellular dopamine levels in the rat striatum. Dopamine levels were monitored via microdialysis in awake freely moving animals following i.p. injection of the ligands. Both DTG (1 and 3 mg/kg) and (+)-pentazocine (10 mg/kg) produced a significant (30-50%) increase in extracellular dopamine. Given the relatively high concentration of sigma receptors in brain nuclei involved in facial and mouth movements, we have also determined the effects of the two sigma ligands on facial movements. Both ligands produced a significant increase in vacuous chewing movements, suggesting that studies on the consequences of sigma receptor activation may have relevance to animal models of human dystonia and/or dyskinesia.