A PET imaging study of 5-HT1A receptors in cat brain after acute and chronic fluoxetine treatment

Immuno-electron microscopic and β-microprobe studies have demonstrated that the internalization of serotonin 5-HT1A autoreceptors, after acute treatment with the selective 5-HT1A receptor agonist 8-OH-DPAT or with the specific serotonin reuptake inhibitor (SSRI) fluoxetine, is associated with a marked decrease in the in vivo binding of [18F]MPPF in the nucleus raphe dorsalis (NRD) of rat. To determine whether this event might be amenable to brain imaging, the present [18F]MPPF positron emission tomographic (PET) study was carried out in anesthetized cats given or not a single dose (5 mg/kg, i.v.) or chronically treated with fluoxetine (5 mg/kg, s.c. for 21 days). Compared to control, [18F]MPPF binding potential was considerably (and visibly) decreased in the cat NRD after acute fluoxetine treatment, while it remained unchanged in other brain regions. Unexpectedly, after chronic fluoxetine treatment, [18F]MPPF binding potential was not affected in any brain region. In parallel immuno-electron microscopic experiments carried out in rat, the density of 5-HT1A autoreceptors on the plasma membrane of NRD dendrites was comparable to control after chronic fluoxetine treatment. If the decrease in [18F]MPPF binding at the onset of SSRI treatment was detectable by PET imaging, it could potentially serve as a biological index of efficacy.