Effects of Ketamine administration in rats: mitochondrial and functional changes in brain and liver
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Data
2012
Autores
Venâncio, Carlos Alberto e Silva
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Resumo
A cetamina é um anestésico dissociativo usado em anestesiologia humana e
veterinária. A sua utilização terapêutica, em doses baixas, para tratar dor crónica e
depressão tem vindo a ganhar uma importância crescente. Têm sido relatadas propriedades
controversas da cetamina, tanto a nível neuroprotetor/neurotóxico como hepatoprotetor/
hepatotóxico, existindo um interesse renovado no esclarecimento dos seus
mecanismos de ação e efeitos adversos. Foi recentemente sugerido que a cetamina pode
modificar a funcionalidade mitocondrial promovendo um aumento na produção de radicais
livres a nível do encéfalo e fígado.
Doses subanestésicas de cetamina induzem efeitos heterogéneos em diversas
regiões do encéfalo os quais têm sido associados a défices de memória e a sintomas
psicopatológicos. Contudo, existe pouca informação quanto à utilização da cetamina,
em doses baixas, na função cerebral e equilíbrio neuroquímico.
O objetivo deste estudo é contribuir para o esclarecimento sobre os efeitos da
exposição da cetamina no principal órgão alvo - encéfalo, e no órgão de metabolização -
fígado. Dois regimes de administração e várias doses de cetamina foram testados no rato adulto.
No primeiro estudo, avaliou-se o efeito agudo de uma dose anestésica intraperitoneal
de cetamina (50, 100 ou 150 mg/kg) na função mitocondrial e no estado redox
cerebral, seis horas após a sua administração. Observou-se que a cetamina aumentou o
consumo de oxigénio na respiração mitocondrial no estado 4 e diminuiu a eficiência da
fosforilação oxidativa quando se utilizou glutamato-malato. Esta interferência com o
complexo I foi confirmada pela diminuição da atividade neste complexo. Simultaneamente,
as doses 50 e 100 induziram um aumento na atividade da sintetase óxido nítrico
mitocondrial (mtNOS), assim como um aumento global dos níveis de óxido nítrico
(NO) plasmático. Além disso, a cetamina induziu um aumento na atividade da superóxido
dismutase (SOD). Estes resultados revelam que a administração aguda de cetamina
afeta negativamente a função mitocondrial e aumenta a atividade da mtNOS e da SOD,
preservando o estado redox geral.
No segundo estudo, avaliaram-se os efeitos da administração crónica de cetamina
no encéfalo e no fígado. Administrou-se subcutaneamente soro fisiológico, 5 ou 10
mg/kg de cetamina, duas vezes por dia, durante 14 dias consecutivos. A avaliação comportamental
iniciou-se 24 horas após a última administração, tendo-se verificado que a
cetamina aumentou a ansiedade no teste de exploração do objeto novo. A dose de 5
mg/kg apresentou também uma diminuição no índice de discriminação no teste de reconhecimento
do objeto, revelando uma incapacidade para atualizar representações espaciais
e uma deficiência no processo de habituação.
Os parâmetros neuroquímicos e neuroplásticos foram avaliados, dez dias após a
última administração, em regiões do encéfalo sensíveis aos efeitos da cetamina e envolvidas
nas alterações comportamentais. Na dose mais baixa, consistentemente com as
alterações comportamentais, verificou-se um aumento nos níveis de dopamina na amígdala
acompanhados por uma diminuição da funcionalidade na região frontal. Na dose
mais elevada verificou-se um aumento dos níveis de serotonina. Os níveis de glutamato
diminuíram no córtex pré-frontal (CPF) para a dose mais elevada, enquanto no hipocampo,
os níveis de glutamato aumentaram para as duas doses. Os neurónios imunorreativos
à parvalbumina diminuíram nas regiões pré-límbica e orbitofrontal do CPF,
cíngulo e CA1 do hipocampo. No seu conjunto, estes resultados sugerem um efeito
menos deletério da dose mais elevada, consequência duma diminuição da inibição do
sistema serotoninérgico (o que pode estar relacionado com o efeito antidepressivo),
enquanto a dose mais baixa parece ser prejudicial devido à desregulação dopaminérgica
e à diminuição dose-dependente do fluxo glutamatérgico aos neurónios GABAérgicos.
Foi também avaliado o efeito da exposição crónica na função mitocondrial encéfálica
e hepática. Nas mitocôndrias do encéfalo não foram observadas alterações significativas.
No fígado, as duas doses usadas inibiram o complexo I assim como o consumo
de oxigénio quando foi utilizado o glutamato-malato como substrato respiratório. A
cetamina também diminuiu o conteúdo de glicogénio hepático, o que pode contribuir
para a diminuição da resistência hepática às perturbações tóxicas. O tratamento com
cetamina induziu uma diminuição no aumento de peso corporal o que sugere alguma
precaução na sua utilização.
Em geral, este trabalho permite uma melhor compreensão da interferência da
cetamina na função mitocondrial, ajudando ao esclarecimento do mecanismo subjacente
aos seus efeitos. No uso crónico da cetamina em doses baixas, destaca-se a existência de
efeitos dependentes da dose com alterações heterogéneas em diferentes regiões do encéfalo,
sendo estas alterações tanto ao nível dos parâmetros neuroquímicos como de neuroplasticidade
com consequentes reflexos a nível comportamental.
Ketamine is a dissociative anaesthetic used in Human and Veterinary anaesthesiology. In low doses, ketamine gained an increasing relevance in the therapy of chronic pain and depression. Since ketamine has been reported to have controversial properties such as neuroprotective/hepatoprotective and neurotoxic/hepatotoxic, there is now a renewed interest in clarifying its mechanisms of action and side effects. Recently, it was suggested that ketamine can modify the mitochondrial bioenergetic function, as well as the rate of free radical production, affecting, in particular, the neural and liver tissues. Subanaesthetic ketamine doses induce heterogeneous region effects that were associated with impaired memory function and increased psychopathological symptoms. However, there is a lack of information regarding the safety of chronic low antidepressant or analgesic ketamine doses in the brain function and neurochemical homeostasis. The objective of the present study is to contribute to further characterize the effects of ketamine exposure in its main targets - the brain, and in its metabolisation organ – the liver. Two regimes of ketamine administration and a range of doses were evaluated in an adult rat model. In the first study, the acute effect of a single intraperitoneal anaesthetic ketamine dose (50, 100 or 150 mg/kg) was evaluated six hours later in the brain mitochondrial function and redox state. The ketamine was shown to increase mitochondrial oxygen consumption in the state 4 of respiration and to impair the oxidative phosphorylation efficiency of complex I. This interference with the complex I was confirmed by the observation of its decreased activity. Simultaneously, ketamine lead to increased activity of the mitochondrial nitric oxide synthase (mtNOS) in the lowest doses, and to a global increase of the plasmatic levels of nitric oxide (NO). In addition, ketamine increased the activity of superoxide dismutase (SOD) in all tested doses. These results reveal that acute ketamine administration impairs mitochondrial function and increases the activity of mtNOS. However, ketamine also triggered SOD activity, potentially to preserve the overall redox status. In the second study, the effects of chronic ketamine were evaluated in brain and liver tissues. Rats were administered with saline solution, 5 or 10 mg/kg of ketamine, twice a day, subcutaneously, for 14 consecutive days. Behavioural evaluation started 24 hours after the last administration. Ketamine increased anxiety-like behaviour in the novel object exploration test. The 5 mg/kg group presented also decreased discrimination index in the object recognition task, inability to update spatial representation, and deficient habituation processes. Ten days after the last administration, neurochemical and neuroplastic parameters were evaluated in specific brain regions known to be susceptible to the effects of ketamine, and involved in the reported behavioural changes. Consistently with the behavioural observations for the lower dose, there was an increase in the amygdalar dopamine levels accompanied by frontal hipofunctionality; while a higher dose resulted in increased 5-HT levels. Glutamate levels were decreased in the prefrontal cortex (PFC) for the highest dose; while in the hippocampus, glutamate levels were increased for both doses. Moreover, parvalbumin immunoreactive neurons were reduced in prelimbic and orbitofrontal PFC regions, cingulate cortex and hippocampus CA1 region. Taking all these together, it is propose that a high dose of ketamine may be less deleterious through decreased inhibition of the serotonergic system (a potential antidepressant-like effect), while a lower dose may be detrimental due to altered dopaminergic regulation caused by dose-dependent reduction of the glutamatergic flow to GABAergic neurons. Moreover, the effect of chronic exposure was also assessed in the brain and liver mitochondrial function. While no effects could be observed in brain mitochondria, in the liver both doses inhibited mitochondrial complex I and oxygen consumption when glutamate-malate substrate was used. Ketamine also decreased the hepatic glycogen content, which may contribute to decrease hepatic resistance to toxic insults. Chronic ketamine groups also showed a decreased evolution of body weight gain through the treatment period, clearly suggesting a morbidity induced effect. In general, this work allows a better comprehension of ketamine interference with the mitochondrial function that helps to clarify the mechanism underlying its effects. Moreover, regarding the use of ketamine in chronic low doses, it highlights the existence of dose-dependent effects that lead to heterogeneous changes in different brain regions, either in neurochemical or neuroplastic parameters, with relevant consequences at the behavioural level.
Ketamine is a dissociative anaesthetic used in Human and Veterinary anaesthesiology. In low doses, ketamine gained an increasing relevance in the therapy of chronic pain and depression. Since ketamine has been reported to have controversial properties such as neuroprotective/hepatoprotective and neurotoxic/hepatotoxic, there is now a renewed interest in clarifying its mechanisms of action and side effects. Recently, it was suggested that ketamine can modify the mitochondrial bioenergetic function, as well as the rate of free radical production, affecting, in particular, the neural and liver tissues. Subanaesthetic ketamine doses induce heterogeneous region effects that were associated with impaired memory function and increased psychopathological symptoms. However, there is a lack of information regarding the safety of chronic low antidepressant or analgesic ketamine doses in the brain function and neurochemical homeostasis. The objective of the present study is to contribute to further characterize the effects of ketamine exposure in its main targets - the brain, and in its metabolisation organ – the liver. Two regimes of ketamine administration and a range of doses were evaluated in an adult rat model. In the first study, the acute effect of a single intraperitoneal anaesthetic ketamine dose (50, 100 or 150 mg/kg) was evaluated six hours later in the brain mitochondrial function and redox state. The ketamine was shown to increase mitochondrial oxygen consumption in the state 4 of respiration and to impair the oxidative phosphorylation efficiency of complex I. This interference with the complex I was confirmed by the observation of its decreased activity. Simultaneously, ketamine lead to increased activity of the mitochondrial nitric oxide synthase (mtNOS) in the lowest doses, and to a global increase of the plasmatic levels of nitric oxide (NO). In addition, ketamine increased the activity of superoxide dismutase (SOD) in all tested doses. These results reveal that acute ketamine administration impairs mitochondrial function and increases the activity of mtNOS. However, ketamine also triggered SOD activity, potentially to preserve the overall redox status. In the second study, the effects of chronic ketamine were evaluated in brain and liver tissues. Rats were administered with saline solution, 5 or 10 mg/kg of ketamine, twice a day, subcutaneously, for 14 consecutive days. Behavioural evaluation started 24 hours after the last administration. Ketamine increased anxiety-like behaviour in the novel object exploration test. The 5 mg/kg group presented also decreased discrimination index in the object recognition task, inability to update spatial representation, and deficient habituation processes. Ten days after the last administration, neurochemical and neuroplastic parameters were evaluated in specific brain regions known to be susceptible to the effects of ketamine, and involved in the reported behavioural changes. Consistently with the behavioural observations for the lower dose, there was an increase in the amygdalar dopamine levels accompanied by frontal hipofunctionality; while a higher dose resulted in increased 5-HT levels. Glutamate levels were decreased in the prefrontal cortex (PFC) for the highest dose; while in the hippocampus, glutamate levels were increased for both doses. Moreover, parvalbumin immunoreactive neurons were reduced in prelimbic and orbitofrontal PFC regions, cingulate cortex and hippocampus CA1 region. Taking all these together, it is propose that a high dose of ketamine may be less deleterious through decreased inhibition of the serotonergic system (a potential antidepressant-like effect), while a lower dose may be detrimental due to altered dopaminergic regulation caused by dose-dependent reduction of the glutamatergic flow to GABAergic neurons. Moreover, the effect of chronic exposure was also assessed in the brain and liver mitochondrial function. While no effects could be observed in brain mitochondria, in the liver both doses inhibited mitochondrial complex I and oxygen consumption when glutamate-malate substrate was used. Ketamine also decreased the hepatic glycogen content, which may contribute to decrease hepatic resistance to toxic insults. Chronic ketamine groups also showed a decreased evolution of body weight gain through the treatment period, clearly suggesting a morbidity induced effect. In general, this work allows a better comprehension of ketamine interference with the mitochondrial function that helps to clarify the mechanism underlying its effects. Moreover, regarding the use of ketamine in chronic low doses, it highlights the existence of dose-dependent effects that lead to heterogeneous changes in different brain regions, either in neurochemical or neuroplastic parameters, with relevant consequences at the behavioural level.
Descrição
Tese de Doutoramento em Ciências Veterinárias
Palavras-chave
Cérebro , Cetamina , Fígado , Mitocondria , Comportamento , Neurotransmissor , Stress oxidativo , Neurociências