[3H]paroxetine binding and serotonin content of rat and rabbit cortical areas, hippocampus, neostriatum, ventral mesencephalic tegmentum, and midbrain raphe nuclei region.

The high-affinity binding of [3H]paroxetine to membranes was measured in different regions of the rat and rabbit brain: cingulate, frontal, parietal, piriform, entorhinal, and visual cortical areas; dorsal and ventral hippocampus; rostral and caudal halves of neostriatum (rat) or caudate nucleus and putamen (rabbit); ventral mesencephalic tegmentum; and midbrain raphe nuclei region. The tissue concentrations of serotonin (5-HT), 5-hydroxyindole-3-acetic acid (5-HIAA) and 5-hydroxy-l-tryptophan (5-HTP) were also determined by high-performance liquid chromatography (HPLC) in the same brain samples. The regional density of [3H]paroxetine binding varied in both species; the highest values (Bmax) were found in the midbrain raphe region and ventral mesencephalic tegmentum. The cortical values ranged from moderate to low, with a significantly higher density in the cingulate cortex of the rat compared with rabbit. In the rat, there was also a higher density in the ventral than dorsal hippocampus, and the caudal than rostral neostriatum. In the rabbit, the hippocampal and neostriatal values were generally lower and more uniform. In both species, there was an excellent correlation between regional 5-HT levels and specific [3H]paroxetine binding (r = 0.87 in the rat and 0.96 in the rabbit). Considering the available quantitative data on the number of 5-HT nerve cell bodies and axon terminals in different regions of the rat brain, it appears likely that the high amount of [3H]paroxetine binding in the midbrain raphe region and ventral mesencephalic tegmentum reflects the presence of 5-HT uptake sites on 5-HT nerve cell bodies and dendrites as well as axon terminals. In other brain regions, the heterogeneous distribution of [3H]paroxetine binding parallels that of the number of 5-HT axon terminals, emphasizing the potential usefulness of this radioligand as a marker of 5-HT innervation density.