CHRM4 and schizophrenia: Given the lack of agonist SPECT or PET tracers, current studies provide no direct evidence as to whether there is a loss of receptors in the high affinity (g-coupled) state in people with schizophrenia, but this limitation may be overcome with the development of 11C-MK6884 (CHRM4 PAM) and 11C -LSN3172176 (bitopic CHRM1 agonist) PET tracers.