Pecifically in the CFS brain without altering AChE activity. Five subtypes of mAChR, M1?, have been identified by molecular cloning [19]. M1, M2 and M4 receptors are predominant subtypes expressed in different percentages among brain regions. Quantitative immunoprecipitation study indicates that the distribution percentages of M1, M2 and M4 receptors are 60 , 20 and 20 in the cortex, respectively. In the striatum, their distribution percentages are 30 , 20 
and 50 , respectively [63]. We had expected a greater reduction of [11C](+)3-MPB BPND in the cortex than in the striatum because serum autoantibody detected in the present study was specific for the M1 receptor. However, similar reductions in the rate of [11C](+)3MPB BPND were observed between the cortex and striatum (Table 3). One possible explanation for this is the low selectivity of [11C](+)3-MPB to the subtype of mAChR. The Ki values of (+)3MPB for the human receptors from M1 to M5 were 1.34, 1.17, 2.82, 1.76, and 5.91 nM, respectively, as assessed with five clonedhuman mAChR subtypes expressed in CHO-K1 cells (unpublished data). These data indicate that the M1 Docosahexaenoyl ethanolamide receptor and the other subtype of mAChR contribute to the reduction in the rate of [11C](+)3-MPB BPND in CFS(+) patients. Because the M1 receptor has a significant role in cognitive function [3,20], we predicted cognitive impairment in CFS(+) patients. However, cognitive function in CFS patients was not associated with changes in [11C](+)3-MPB BPND. One plausible explanation is that reduction in the level of [11C](+)3-MPB BPND occurs within a range of preserved cognitive function. Indeed, we recently reported the relationship between [11C](+)3-MPB BPND and cognitive function in conscious monkeys, showing that there were thresholds (ca. 30?0 in cortex and ca. 20?0 in brainstem) of activity of the brain mAChR to induce cognitive impairment [64,65].LimitationsWe cannot exclude the possibility that the autoimmune reaction occurred as a secondary process to the reduction of the mAChR. In addition, our findings relate to a small subset of CFS patients. This was chiefly due to the difficulty in obtaining CFS patients’ consent to participate in the present study because it entailed a series of PET and MRI measurements, requiring a significant commitment of time from each subject. Additional 23727046 experiments will be necessary to fully validate the present findings. Increases in the serum autoantibody against the mAChR have also been reported in Sjogren syndrome [66] 
and other psychiatric disorders ?including schizophrenia [61,62,67]. Therefore, our results cannot be generalized to the entire CFS population.SummaryOur results demonstrate the usefulness of PET as a tool for detecting a reduction of neurotransmitter receptor binding in the brains of patients with high levels of serum autoantibody. Further follow up studies on a number of CFS patients are required in order to more thoroughly investigate alterations in cholinergic and neuronal functions with regard to levels of mAChR autoantibody and clinical symptoms.AcknowledgmentsThe authors thank the participants and the technical support team in charge of blood sampling.Author ContributionsConceived and designed the experiments: YW. Performed the experiments: SY YO DN TT ST EY HT MI KY HK. Analyzed the data: SY TT KM ST EY. Wrote the paper: SY YO KM HO YW.
Prions are the etiological agents responsible for a diverse set of transmissible fatal neurodegerative diseases of humans and an.
