oa Editorial [Hot topic:Pharmacological Brain Imaging in the Healthy and Sick Brain (Executive Editor: F.M. Van Der Veen)]

With the development of neuroimaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), it has become possible to directly monitor the effects of pharmacological agents on brain functioning. This is especially important for the research in the area of psychiatry and psychology. With fMRI it has become possible to see which brain functions are affected by the agent and how this change in functioning is related to symptoms of the disease. With PET it has become possible to monitor more basic processes like receptor binding and metabolic functions of different types of brain cells. Both techniques have become increasingly important in establishing the role of neurotransmitter systems in psychiatric diseases and monitoring the effects of different pharmacological treatments. One of the dominant hypotheses with respect to major depression is that disrupted serotonin function plays a key role. Various studies have found evidence for lowered serotonin function and increasing central serotonin levels by means of administration of selective serotonin reuptake inhibitors (SSRI) is the dominant treatment. However, very few studies have established a direct link. With recent developments in PET it has become possible to directly map serotonin metabolism in vivo and establish such a direct link. The paper of Veltman et al. [1] nicely reviews the recent PET findings with respect to the role of serotonin in major depression and other psychiatric disorders. By using fMRI is has become possible to directly look at effects of treatment with an SSRI on brain function. The paper of Murphy [2] reviews the literature in this area and focuses on emotion-related processing. Finally, fMRI has also made it possible to examine the effects of changes in central serotonin levels in healthy controls, which provides the opportunity to acquire more basic knowledge regarding the role of serotonin in various aspects of normal information processing. The paper Evers et al. [3] reviews the literature in this area and focuses on the effect of acute tryptophan depletion, a well-established method to transiently lower central serotonin levels. The dominant hypothesis with respect to schizophrenia is that dopamine function is disrupted. Various forms of the dopamine hypothesis have been put forward and a recent version of the hypothesis states that in schizophrenic patients dopamine activity is lowered in prefrontal brain areas leading to negative symptoms and to stronger dopamine activity in mesolimbic areas which might be the cause of the positive symptoms. First generation antipsychotics have explicitly targeted the dopamine system but second generation antipsychotics also aim at influencing other neurotransmitter systems. The paper of Roder et al. [4] reviews the literature with respect to the effects of both first and second generation antipsychotics on brain function of schizophrenic patients, as can be measured with fMRI. The fMRI technique has also been used to further elucidate the role of dopamine in information processing in healthy controls. Literature in this area is reviewed by Schouwenburg et al. [5] who especially focus on the role of dopamine in cognitive control. This special issue reviews the role of dopamine and serotonin in information processing in healthy and sick brains as can be established by the new and exciting neuroimaging techniques of PET and fMRI. The included reviews will present a broad and informative perspective of this new and rapidly developing field of research.