Subcellular site of action of imipramine in rodent brain

New insights into the intracellular distribution pattern of cationic amphiphilic drugs

Cationic amphiphilic drugs (CADs) comprise a wide variety of different substance classes such as antidepressants, antipsychotics, and antiarrhythmics. It is well recognized that CADs accumulate in certain intracellular compartments leading to specific morphological changes of cells. So far, no adequate technique exists allowing for ultrastructural analysis of CAD in intact cells. Azidobupramine, a recently described multifunctional antidepressant analogue, allows for the first time to perform high-resolution studies of CADs on distribution pattern and morphological changes in intact cells. We showed here that the intracellular distribution pattern of azidobupramine strongly depends on drug concentration and exposure time. The mitochondrial compartment (mDsRed) and the late endo-lysosomal compartment (CD63-GFP) were the preferred localization sites at low to intermediate concentrations (i.e. 1 μM, 5 μM). In contrast, the autophagosomal compartment (LC3-GFP) can only be reached at high concentrations (10 μM) and long exposure times (72 hrs). At the morphological level, LC3-clustering became only prominent at high concentrations (10 μM), while changes in CD63 pattern already occurred at intermediate concentrations (5 μM). To our knowledge, this is the first study that establishes a link between intracellular CAD distribution pattern and morphological changes. Therewith, our results allow for gaining deeper understanding of intracellular effects of CADs.

Desipramine modulates 3H-ouabain binding in rat hypothalamus

We have previously shown that Na+, K(+)-ATPase activity in hypothalamus is increased after administration of an acute dose of desipramine, a noradrenaline uptake inhibitor (Viola et al., Cell Molec Neurobiol 9:263-271, 1989). In this report the same treatment (10 mg per kg) was applied to evaluate 3H-ouabain binding in rat brain sections by quantitative autoradiography. Results disclosed an increase in the number of ouabain binding sites in hypothalamus but not in cerebral cortex. Concomitantly, such acute DMI treatment enhanced K(+)-stimulated-p-nitrophenylphosphatase activity in hypothalamus membranes whereas it failed to modify cerebral cortex membranes. A direct interaction of DMI with the enzyme was ruled out since in vitro DMI is known to inhibit the enzyme. It may be speculated that DMI indirectly stimulates Na+, K(+)-ATPase through the increase in noradrenaline which acts in turn on the external phosphorylated site of the enzyme.

[3H]imipramine binding in the retinas of chickens and pigs

High-affinity [3H]imipramine binding sites detected in the brain are associated with the neural uptake mechanism for serotonin. In this study [3H]imipramine binding in the retinal synaptosomal P2-fraction, which contains the serotonin-accumulating nerve terminals, was characterized using two different animal species, chicken and pig, and with a wide range of imipramine concentrations. The specific binding of imipramine was saturable. When homogenates were prepared from fresh retinas, a high-affinity binding site in the nanomolar range and a low-affinity binding site in the micromolar range were always found. In the pig retina only a high-affinity binding site was found after freezing. The Bmax-value of the high-affinity binding was about 10 times greater in the retinas of pigs than in those of chickens; Bmax-values for fresh homogenates were 1711- and 235 fmol mg-1 protein and those for frozen homogenates were 2066- and 146 fmol mg-1 protein in pig and chicken retinas, respectively. Our results confirm the complexity of imipramine binding described for different regions of the brain. The great difference in the Bmax-values of the high-affinity binding in chicken and pig retinas might indicate differences between species in the serotonergic system.