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

Relevance of animal models to human tardive dyskinesia

Tardive dyskinesia remains an elusive and significant clinical entity that can possibly be understood via experimentation with animal models. We conducted a literature review on tardive dyskinesia modeling. Subchronic antipsychotic drug exposure is a standard approach to model tardive dyskinesia in rodents. Vacuous chewing movements constitute the most common pattern of expression of purposeless oral movements and represent an impermanent response, with individual and strain susceptibility differences. Transgenic mice are also used to address the contribution of adaptive and maladaptive signals induced during antipsychotic drug exposure. An emphasis on non-human primate modeling is proposed, and past experimental observations reviewed in various monkey species. Rodent and primate models are complementary, but the non-human primate model appears more convincingly similar to the human condition and better suited to address therapeutic issues against tardive dyskinesia.

The styrene metabolite, phenylglyoxylic acid, induces striatal-motor toxicity in the rat: influence of dose escalation/reduction over time

Exposure to the industrial solvent, styrene, induces locomotor and cognitive dysfunction in rats, and parkinsonian-like manifestations in man. The antipsychotic, haloperidol (HP), well known to induce striatal toxicity in man and animals, and styrene share a common metabolic pathway yielding p-fluoro phenylglyoxylic acid and phenylglyoxylic acid (PGA), respectively. Using an exposure period of 30 days and the vacous chewing movement (VCM) model as an expression of striatal-motor toxicity, we found that incremental PGA dosing (220–400 mg/kg) significantly increased VCMs up to day 25, but decreased to control levels shortly after reaching maximum dose. However, a diminishing dose of PGA (400–200 mg/kg) did not evoke an immediate worsening of VCMs but precipitated a significant increase in VCMs following dosage reduction to 200 mg/kg on day 22. PGA exposure, therefore, compromises striatal-motor function that is especially sensitive to changes in exposure dose. Longer alternating dose exposure studies are needed to establish whether motor dysfunction is progressive in severity or longevity. These findings are of significance for the environmental toxicology of styrene in the chemical industry.

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.

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.

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.

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.

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.