Because chronic haloperidol-treated rats demonstrate an increased incidence of spontaneous oral activity, while neonatal 6-hydroxydopamine-lesioned rats demonstrate an increased incidence of dopamine agonist-induced oral activity, we studied the influence of haloperidol in 6-hydroxydopamine-lesioned rats. At 3 days after birth rats received 6-hydroxydopamine hydrobromide (200 μg intracerebroventricularly; desipramine pretreatment, 20 mg/kg i.p., 1 h) or vehicle. Two months later haloperidol (1.5 mg/kg per day × 2 days per week, for 4 weeks; then 1.5 mg/kg per day, every day for 10 months) was added to the drinking water. After 15 weeks the level of spontaneous oral activity was stable. At 11 months there were 35.8 ± 4.9 vs. 18.4 ± 2.1 oral movements in 6-hydroxydopamine-lesioned vs. intact rats receiving haloperidol. This effect persisted unabated in lesioned rats for 4 months after haloperidol withdrawal. This stable high frequency of oral dyskinesias is an advantage for studying putative therapeutic drugs for tardive dyskinesia.

Neonatal 6-hydroxydopamine (6-OHDA) treatment of rats is associated with supersensitization of the dopamine D1 agonist induction of oral activity. The present study was conducted to determine whether induced oral responses to serotonin (5-HT) agonists would be similarly altered in this rat model. At 3 days after birth, rats received desipramine HCl (20 mg/kg, IP) 1 h before 6-OHDA HBr (100 μg in each lateral ventricle) or saline-ascorbic acid (0.1%) vehicle. At approximately 9 mo, rats were challenged with the mixed 5-HT1C and 5-HT2 receptor agonist, m-chlorophenylpiperazine diHCl (m-CPP 2HCl; 0.30-6.0 mg/kg, IP) and were then observed for 1 min every 10 min over a 60-min period. m-CPP induced oral activity in both the vehicle and 6-OHDA groups, with the responses of the 6-OHDA group being much greater. An m-CPP dose of 3.0 mg/kg produced a maximal response of 63.6 ± 3.2 oral movements in the 6-OHDA group. A bell-shaped response curve was obtained, with lower and higher doses of m-CPP producing less of an effect. Attenuation of the m-CPP-induced response by the 5-HT receptor antagonist, mianserin HCl (1.0 mg/kg, IP, 30 min before m-CPP), indicates that the m-CPP effect is receptor mediated. These findings demonstrate that neonatal 6-OHDA treatment produces ontogenic long-lived supersensitization of a 5-HT receptor system in rats.

Enhanced oral responses have been observed in rats that are treated shortly after birth with 6-hydroxydopamine (6-OHDA). A series of studies was conducted to characterize this effect. A dose-response curve demonstrated that the dopamine D1 receptor agonist, SKF 38393, produced a maximal response in 6-OHDA-treated rats at a dose of 0.10 mg/kg (IP). With the D2 receptor antagonist, spiperone, a bell-shaped dose-response curve was seen, with a maximal effect in the 6-OHDA group occurring at 80 μg/kg. There were only slight increases in oral activity with different SKF 38393 or spiperone doses in the saline group, indicating that there was an overt supersensitization of D1 receptors in the 6-OHDA-treated rats. The D1 antagonist SCH 23390 (0.30 mg/kg, IP) attenuated the response to both SKF 38393 and spiperone. The oral response to the D2 agonist, quinpirole (0.10 mg/kg, IP) was not preferentially increased in the 6-OHDA group of rats. These findings indicate that the enhanced oral response in neonatal 6-OHDA-treated rats is mediated by supersensitive dopamine D1 receptors. The persistence of the enhanced oral ersponse in 6-OHDA-treated rats at 8 months demonstrates that this sensitization of D1 receptors is a long-lived phenomenon.

The influence of central dopaminergic fibers on drug-induced oral activity in rats has not been well studied. Rats were treated 3 days after birth with bilateral intracerebroventricular 6-hydroxydopamine (6-OHDA; 134 ωg total, base form) to destroy dopaminergic fibers in the brain. Control rats received vehicle by the same route. At about 10 weeks of age, a challenge dose of the dopamine D2 receptor antagonist, spiperone (40 ωg/kg, IP), produced an 8-fold increase in the number of oral movements during a 60-minute observation period, vs. the control group. SKF 38393 (3.0 mg/kg, IP), a D1 agonist, produced the same number of oral movements as spiperone in the 6-OHDA group, representing a 2.4-fold increase over the controls. The Bmax and Kd for both D1 and D2 receptors was not changed in rat striatum by neonatal 6-OHDA treatment, even though dopamine content was reduced by 96%. These findings demonstrate that oral activity in rats can be greatly altered, even when there is no change in absolute numbers of D1 and D2 receptors and no change in the ratio of D1:D2 receptors.

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.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Considering the hypothesis that GABA and oxidative stress are involved in the development of oral movements associated with important neuropathologies, the present study investigated the possible ability of V. officinalis in the prevention of vacuous chewing movements (VCMs) induced by reserpine in rats. Adult male rats were treated with reserpine (1 mg/kg, s.c.) and/or with V. officinalis (in the drinking water). VCMs, locomotor activity and oxidative stress measurements were evaluated. The neuroprotective effect of V. officinalis against iron-induced cell toxicity was investigated in brain cortical slices. Furthermore, we carried out the identification of valeric acid and gallic acid by HPLC in the V. officinalis tincture. Our findings demonstrate that reserpine caused a marked increase on VCMs and the co-treatment with V. officinalis was able to reduce the intensity of VCM. Reserpine did not induce oxidative stress in cerebral structures (cortex, hippocampus, striatum and substantia nigra). However, a significant positive correlation between DCF-oxidation (an estimation of oxidative stress) in the cortex and VCMs (p<0.05) was observed. Moreover, a tendency for a negative correlation between Na+K+- ATPase activity in substantia nigra and the number of VCMs was observed (p= 0.06). In vitro, V. officinalis protected brain cortical slices viability against Fe(II)-induced neurotoxicity. In conclusion, V. officinalis had in vitro and in vivo neuroprotective effects in rats, i.e., reduced Fe(II) neurotoxicity and reserpine-induced VCMs, probably via modulation of oxidative stress in specific brain nucleus and its GABA-mimetic action. However, the mechanisms involved in this protective activity needs to further investigated to better understand the action of V. officinalis. / Considerando as hipóteses do papel da neurotransmissão gabaérgica e do estresse oxidativo no desenvolvimento de movimentos orais associados a neuropatologias importantes, o presente estudo investigou a possível habilidade da tintura de V. officinalis na prevenção dos movimentos de mascar no vazio (MMV) induzidos por reserpina em ratos. Os animais foram tratados com reserpina (1 mg/Kg, s.c.) e/ou com V. officinalis (na água de beber). MMV, atividade locomotora e medidas de estresse oxidativo foram avaliadas. O efeito neuroprotetor da V. officinalis contra a toxicidade celular induzida por ferro foi investigada em fatias de córtex cerebral. Além disso, fez-se a identificação do ácido valérico e do ácido gálico por HPLC na tintura de V. officinalis. Os resultados demonstram que a reserpina causou um aumento nos MMV quando comparado com o seu veículo e o co-tratamento com V. officinalis foi capaz de reduzir a intensidade dos MMV. A reserpina não alterou de forma significativa alguns parâmetros de estresse oxidativo analisados nas estruturas do cérebro (córtex, hipocampo, estriado e substantia nigra). Porém, uma correlação positiva entre os níveis de oxidação da DCF (uma estimativa do estresse oxidativo) no cortex e o número de MMV (p<0.05) foi observada. Além disso, foi observada uma tendência a haver uma correlação negativa entre a atividade da Na+/K+- ATPase na substantia nigra e o número de MMV (p= 0.055). In vitro, V. officinalis protegeu as fatias de córtex cerebral contra a neurotoxicidade induzida por ferro. Desta forma, pode-se concluir que a V. officinalis apresentou efeitos neuroprotetores em ratos tanto in vitro quanto in vivo, ou seja, reduziu a neurotoxicidade induzida por ferro e os MMV induzidos por reserpina, provavelmente via modulação do estresse oxidativo em núcleos específicos do cérebro e sua ação gabamimética. Porém, os mecanismos envolvidos nesta atividade protetora necessitam de mais investigações para melhor entender a ação da V. officinalis.