Defining the disturbance in cortical glutamate and GABA function in psychosis, its origins and consequences

点击翻译按钮获取中文摘要

Schizophrenia is a common disorder which typically begins in the late teens and twenties. Often there is a period (the prodrome) of gradual decline in motivation, interest and sociability before the acute onset of psychotic symptoms such as hearing voices and having threatening paranoid beliefs. The psychotic symptoms usually respond quite well to antipspychotic drugs but there are often residual symptoms when the psychosis has died down and patients are left with a degree of apathy that leads to a poor quality of life. We do not have medications that reverse or prevent these residual symptoms. Finding better drugs is difficult because we do not know what the underlying brain changes are; if we did we could develop drugs targeted on the process and reverse the illness or prevent the prodrome progressing to psychosis. Increasing evidence from brain imaging studies suggest that subtle changes to the grey matter of the brain are occurring in the prodrome that continue into the acute phase. There is much enthusiasm for the idea that chemical messengers in the grey matter (neurotransmitters) called glutamate and GABA are somehow bound up with the process of becoming psychotic and with the residual state. Much of the interest comes from the effect of drugs like phencyclidine that can induce a state like a psychosis. However, to really know whether there is something wrong with glutamate and GABA we need to measure its release and activity in living people. It is possible to measure these chemicals using a technique called magnetic resonance spectroscopy (MRS). At high magnetic field strengths, the different compounds can be clearly separated and measured. A related technique allows us to go a step further and measure how much glutamate neurones are actually releasing. This method has never been applied to a brain condition. We will use these spectroscopic methods to give a decisive yes or no to the question of whether glutamate and GABA are abnormal, either early on the illnesses or in those with more than 10 years of illness. We also want to know what might cause glutamate/GABA abnormalities. There is a good case that some form of inflammatory response may be involved in acute psychosis that dies down having left some mild damage that accounts for the residual symptoms. We will check this using the most sensitive Positron Emission Tomography (PET) camera in the country. It detects tracers that bind to inflammatory cells in the brain and this is clearly seen in diseases such as Parkinson's disease. We might find that glutamate problems were present in those with PET evidence of inflammation. Or it might be that they are independent risk factors. Finally we want to know whether the glutamate/GABA changes actually produce symptoms and how they might do this. We can use magneto-encephalography (MEG) to detect tiny magnetic fields that brain cells induce outside the head when they fire. We are beginning to understand that different parts of the grey matter communicate with one another by firing in step to produce waves of activity. This has revealed that different networks do different jobs in the brain such as focussing attention or remembering things. Glutamate and GABA keep cells firing in step with each other and so abnormalities in these neurotransmitters may produce symptoms by affecting how networks operate. Measuring MRS, PET and MEG together in the same people would be ideal but very demanding. We have devised a series of overlapping pairs of tests that will enable us to finally settle whether glutamate and GABA are functioning abnormally in schizophrenia, whether inflammation is anything to do with the process and how symptoms might result. The results are potentially game-changing and could point the way to new drug treatments and re-invigorate the interest of industry in developing new treatments for schizophrenia.