長期重度使用大麻與結構性腦異常有關


  June 6, 2008 — 一項新研究顯示,長期重度使用大麻與結構性腦異常有關。
  
  澳洲墨爾本大學的研究者發現,重度使用大麻的使用者,相較於從未使用過的人們,其海馬迴與杏仁體較小,海馬迴的體積平均下降了12%、杏仁體的體積平均下降了7.1%,使用大麻與精神異常的低閥值症狀有關。
  
  大麻的使用一直是個有爭議的議題;作者表示有許多人相信大麻是無害的,應該合法取得;然而,根據目前的研究結果,研究者並不這麼認為。
  
  這些發現挑戰了大麻被廣為認知的僅對神經有部分的或沒有影響,他們寫道,雖然輕度使用可能不會造成顯著的神經毒性作用,但這些結果顯示,重度每日使用可能確實對人類腦組織造成毒性。
  
  據估計,每個月有1,500萬美國人使用大麻,其中340萬人每日使用,且使用超過12個月以上,換算每年約有210萬人開始使用這個藥物。
  
  這項由Murat Yucel博士領導的研究發表於6月號的一般精神學誌。
  
  【機轉未知】
  根據作者表示,這是第一個長期重度使用大麻顯示海馬迴體積下降的人類研究,他們附帶表示,這項研究發現與使用大麻增加精神分裂症狀風險的看法有關。
  
  為了進行這項研究,研究者對15位每天使用五根大麻煙超過10年的男性,進行腦部結構高解析度核磁共振(MRI),這些男性平均年齡為39.8歲,規律使用大麻菸平均19.7年,這些受試者沒有濫用多種不同藥物或神經/精神疾病病史。
  
  這些受試者的MRI影像接著與16位相對應未使用大麻的控制組比較。
  
  儘管效應很顯著,但容積的下降是否是因為神經或膠質細胞缺損、細胞體積變化或是突觸密度下降仍然未知;研究者表示,需要進行更多的研究來解釋這些可能的機轉。
  
  這項研究也顯示大麻使用者的左半邊海馬迴體積與這個藥物的累積使用量,而不是右半邊大腦有關,這項發現顯示左海馬迴可能特別容易受到暴露在大麻之下影響,且可能與精神分裂症狀的產生更加有關。
  
  根據研究者表示,需要未來的前瞻性、縱向研究來確認長期使用大麻壞處的程度,以及戒除後神經恢復的時程。

Long-Term, Heavy Cannabis Use Linked to Structural Brain Changes

By Caroline Cassels
Medscape Medical News


June 6, 2008 — Long-term, heavy cannabis use has been linked with structural brain abnormalities, a new study shows.

Investigators at the University of Melbourne, Australia, found the hippocampus and the amygdala tend to be smaller in heavy cannabis users compared with nonusers, with average volume reductions of 12% in the hippocampus and 7.1% in the amygdala. Cannabis use was also associated with subthreshold symptoms of psychotic disorders.

Cannibis use is a controversial topic; the authors note there are many who believe cannabis is relatively harmless and should be legally available. However, the current findings suggest otherwise.

"These findings challenge the widespread perception of cannabis as having limited or no neuroanatomical sequelae. Although modest use may not lead to significant neurotoxic effects, these results suggest that heavy daily use might indeed be toxic to human brain tissue," they write.

It is estimated that 15 million Americans use cannabis in a given month, that 3.4 million are daily users with a duration of 12 months or more, and that every year 2.1 million start using the drug.

Led by Murat Yucel, PhD, the study is published in the June issue of Archives of General Psychiatry.

Mechanism Unclear

According to the authors, this is the first human study of long-term heavy cannabis use to show marked hippocampal volume reductions. The findings, they add, are consistent with the view that cannabis use increases the risk of psychotic symptoms.

For the study, researchers performed high-resolution structural magnetic resonance imaging (MRI) on 15 men who smoked more than 5 joints daily for more than 10 years. With an average age of 39.8 years and a mean duration of regular use of 19.7 years, the subjects had no history of polydrug abuse or neurologic/mental disorder.

MRI results from these subjects were then compared with those of 16 matched controls who did not use cannabis.

Despite the large magnitude of effect, it is unclear whether volumetric reductions are due to neuronal or glial loss, a change in cell size, or a reduction in synaptic density. The investigators say more research is needed to explain the underlying mechanisms.

The study also showed hippocampal volume in cannabis users was inversely correlated with cumulative exposure to the drug in the left, but not right, hemisphere — a finding that suggests that "the left hippocampus may be particularly vulnerable to the effects of cannabis exposure and may be more closely related to the emergence of psychotic symptoms."

According to the investigators, "further prospective, longitudinal research is required to determine the degree and mechanisms of long term cannabis-related harm and the time course of neuronal recovery after abstinence."

Arch Gen Psychiatry. 2008;65:694-701. Abstract

    
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