In vivo imaging of the 5-hydroxytryptamine reuptake site in primate brain using single photon emission computed tomography and [123I]5-iodo-6-nitroquipazine.

Previous experiments have demonstrated that 5-iodo-6-nitro-2-piperazinylquinoline (5-I-6-NQP) is a potent and selective ligand for studying brain 5-hydroxytryptamine (5-HT) reuptake sites. We performed in vivo imaging in non-human primates using single photon emission computed tomography (SPECT) and the 123I-labeled compound [123I]5-I-6-NQP. These studies showed rapid brain uptake, with slow egress of the tracer from the brainstem, a region rich in 5-HT reuptake sites. Loss of the tracer from regions with a lower density of these sites, such as cerebellum, was relatively more rapid. Pretreatment of animals with paroxetine increased the washout of tracer from the brainstem to rates similar to that seen in cerebellum. Brainstem to cerebellar ratios of tracer accumulation were > 2 by 8 h after injection, and in paroxetine pretreated animals remained close to 1. These results indicate that the radiotracer has characteristics suitable for use as a SPECT imaging agent of serotonin reuptake sites.