Quantitative autoradiography of 3H-paroxetine binding sites in rat brain

The use of 3H-paroxetine as a ligand for quantitative autoradiography of serotonin (5-HT) transport sites was optimized, and the distribution of 3H-paroxetine binding sites in rat brain was studied. Under the conditions described, 3H-paroxetine binding in forebrain sections was of high affinity and saturable, with a Kd of 0.18 +/- 0.02 nM (mean +/- SEM) and Bmax of 268 +/- 12 fmol/mg of protein (n = 3). Nonspecific binding was 10.7% +/- 1.0 of the total binding (n = 8). The distribution of 3H-paroxetine binding sites closely matched the regional distribution of 5-HT nerve terminals and cell bodies. The highest concentrations of 3H-paroxetine binding sites were found in the dorsal raphe nucleus (563 +/- 55 fmol/mg tissue, n = 4), and high densities of binding were also found in the locus coeruleus, medial forebrain bundle, substantia nigra, several limbic structures (amygdala, hippocampus, hypothalamus, olfactory tubercle, septum, and thalamus), and components of the visual relay system (superior colliculus and the lateral geniculate body). Although lesioning of 5-HT neurons with p-chloroamphetamine (PCA) drastically eliminated 3H-paroxetine binding in most regions of the rat brain, significant binding remained in the raphe nuclei and medial forebrain bundle suggesting that 3H-paroxetine binding in these regions was to presynaptic sites on cell bodies or axons relatively resistant to PCA.