Serotonin Reuptake Sites Research Articles

Radiosynthesis of [F-18]fluoxetine as a potential radiotracer for serotonin reuptake sites

Synthesis of 4-nitro-α-bromo-α,α-difluorotoluene was accomplished in two steps starting from 4-nitrobenzaldehyde, with a 30% overall yield. Radiolabeling of 4-nitro-α-bromo-α,α-difluorotoluene with no-carrier-added [ 18F]fluoride provided 4-nitro-α,α-difluoro-α-[ 18F]fluorotoluene in 2–4% yields with a specific activity of 2590 GBq/mmol (70 Ci/mmol). The effect of the reaction temperature on the radiochemical yield and specific activity of the radiolabeling reaction was studied. Radiochemical yields increased, whereas specific activity decreased, with increasing temperature. Radiosynthesis of [ 18F]fluoxetine involved coupling of 4-nitro-α,α-difluoro-α-[ 18F]fluorotoluene with the sodium alkoxide of ( S)-3-(methylamino)-1-phenyl-1-propanol. The overall yield of HPLC purified [ 18F]fluoxetine was 1–2% (decay-corrected; total radiosynthesis time, 150–180 min). The specific activity of the product was 1480 GBq/mmol (40 Ci/mmol).

4-Iodotomoxetine: A novel ligand for serotonin uptake sites

The tomoxetine analog, R-4-iodotomoxetine, binds in vitro to a single site of rat cortical membranes with high affinity (Kd=0.03±0.01 nM, n=4) and can be blocked by a selective serotonin reuptake site inhibitor, paroxetine. The [ 125l]R-4-iodotomoxetine binding at equilibrium is saturable and is temperature- and Na +-dependent. The number of specific [ 125l]R-4- iodotomoxetine binding sites (Bmax=356±20 fmol/mg protein) is similar to that of [ 3H]citalopram (329±30 fmol/mg protein), a known serotonin uptake inhibitor. The binding of [ 125l]R-4-iodotomoxetine is selectively inhibited by several serotonin uptake blockers, and a good correlation is demonstrated between the potency of various drugs to inhibit in vitro binding of [ 125l]R-4- iodotomoxetine and [ 3H]citalopram. In addition, lesions performed with the neurotoxin p-chloroamphetamine, which destroys monoamine neurons, including serotonergic neuronal system, result in a 90% reduction of [ 125l]R-4- iodotomoxetine binding when compared to sham controls. These results indicate that the binding sites labeled by [ 125l]R-4-iodotomoxetine are associated with the neuronal serotonin uptake sites. However, the in vivo and ex vivo results do not show regional localization corresponding to the distribution of serotonin uptake sites. The nonspecific uptake may be related to this compound's high lipophilicity (octanol-buffer partition coeffient = 1100−1400 at pH 7). Although the in vivo properties of [ 125l]R-4- iodotomoxetine make it an unlikely candidate for mapping serotonin uptake sites with SPECT, the high affinity and selectivity should make it a useful tool for in vitro studies of the serotonin uptake sites.

Serotonergic denervation does not reduce brain uptake of n-Isopropyl- p-[ 123I]iodoamphetamine in vivo

The relation between brain uptake of radiolabeled n-isopropyl- p-iodoamphetamine ([ 123I]IMP) and serotonin re-uptake sites was studied in vivo by lesioning serotonergic nerve endings. The reduction of brain serotonin demonstrated the effectiveness of the procedure. Accumulation of radioactivity/g brain tissue was higher in the lesioned than in the sham-operated rats. This was explained by the loss of body weight of the lesioned animals, allowing an increased distribution to the brain. These results demonstrate that the in vivo distribution and binding of [ 123I]IMP in the brain is not limited to the specific serotonin re-uptake sites on nerve endings.

Differential 3H-imipramine platelet binding in patients with panic disorder and depression

3H-Imipramine ( 3H-IMI) binding to platelet membranes was measured in 19 patients with agoraphobia with panic attacks, 9 patients with major depression, and 22 healthy subjects. In comparison to healthy subjects, the maximal number of binding sites ( B max) was significantly decreased in depressed patients but not in panic disorder patients, and the apparent affinity of binding was slightly decreased in depressed patients but not in panic disorder patients. The B max and K d of 3H-IMI platelet binding did not differ between panic disorder patients with and without a history of a major depressive episode. Thus, 3H-IMI platelet binding is clearly different in patients with panic disorder compared to those with an active depression. Because 3H-IMI binding is associated with the serotonin reuptake site in platelet and brain membranes, these findings give further support to abnormalities in serotonergic function in patients with major depression.

Does high affinity [ 3H] imipramine binding label serotonin reuptake sites in brain and platelet?

Recent studies have demonstrated the presence of high affinity binding sites for [ 3H] imipramine in membrane preparations derived from rat brain, human platelet, and human brain. Although initial reports concluded that there was no relationship between these binding sites and the reuptake sites for biogenic amines, subsequent studies in our laboratory suggested a close relationship between the high affinity imipramine binding site and the serotonin uptake or transport site (cf. ref. 9). To further establish whether these binding sites are associated with either platelet or neuronal uptake of serotonin, the relative potencies of a series of tricyclic antidepressants in inhibiting [ 3H] imipramine binding and serotonin uptake were determined under identical assay conditions. A close correlation between inhibition of serotonin uptake and [ 3H] imipramine binding was observed (r = 0.99, p < 0.001). In addition, electrolytic lesions of the midbrain raphe produced a decrease in [ 3H] imipramine binding in hypothalamic synaptosomes that paralleled the decrease in [ 3H] serotonin uptake. Finally incubation of synaptosomal membranes with 2,8-dinitroimipramine, an irreversible inhibitor of [ 3H] imipramine binding, produced a dose-dependent decrease in serotonin uptake, without altering the uptake of nonrepinephrine or dopamine. Taken together our results strongly suggest that high affinity binding of [ 3]] imipramine selectively labels serotonin uptake sites in brain and platelet.