[3H]Desipramine Binding to Rat Brain Tissue: Binding to Both Noradrenaline Uptake Sites and Sites Not Related to Noradrenaline Neurons

Abstract The pharmacological and biochemical characteristics of [³H]desipramine binding to rat brain tissue were investigated. Competition studies with noradrenaline, nisoxetine, nortriptyline, and desipramine suggested the presence of more than one [³H]desipramine binding site. Most of the noradrenaline-sensitive binding represented a high-affinity site, and this site appeared to be the same as the high-affinity site of nisoxetine-sensitive binding. The [³H]desipramine binding sites were abolished by protease treatment, a result suggesting that the binding sites are protein in nature. When specific binding was defined by 0.1 μM nisoxetine, the binding was saturable and fitted a single-site binding model with a binding affinity of ～1 nM. This binding fraction was abolished by lesioning of the noradrenaline neurons with the noradrenaline neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromo-benzylamine (DSP4). In contrast, when 10 μM nisoxetine was used to define the specific binding, the binding was not saturable over the nanomolar range, but the binding fitted a two-site binding model with K_D values of 0.5 and >100 nM for the high- and low-affinity components, respectively. The high-affinity site was abolished after DSP4 lesioning, whereas the low-affinity site remained. The binding capacity (B_max) for binding defined by 0.1 μM nisoxetine varied between brain regions, with very low density in the striatum (B_max not possible to determine), 60-90 fmol/mg of protein in cortical areas and cerebellum, and 120 fmol/mg of protein in the hypothalamus. The binding capacities of these high-affinity sites correlated significantly with the regional distribution of [³H]noradrenaline uptake but not with 5-[³H]hydroxytryptamine uptake. The low-affinity sites did not correlate with the regional distribution of [³H]noradrenaline uptake. Drug inhibition studies showed that noradrenaline inhibits the binding defined by 0.1 μM nisoxetine in a competitive manner. Together, these findings suggest that only a small fraction of the [³H]desipramine binding can be regarded as “specific” binding, and this binding fraction may represent the substrate recognition site for noradrenaline uptake. Assuming that one molecule of desipramine binds to each carrier molecule, the turnover number for the noradrenaline carrier was calculated to be 20/min, i.e., the duration of one transport cycle was 3 s.     

High-Affinity Uptake of [3H]Norepinephrine by Primary Astrocyte Cultures and Its Inhibition by Tricyclic Antidepressants

Abstract: Primary astrocyte cultures from neonatal rat brains show uptake of [³H]norepinephrine ([³H]NE). This uptake has a high-affinity component with an apparent K_m of approximately 3 × 10^?7 M. At 10^?7 M [³H]NE both the initial rate of uptake and steady-state content of [³H]NE is inhibited by up to 95% by omission of external Na⁺. The Na⁺-dependent component of this uptake is totally inhibited by the tricyclic antidepressants desipramine (DMI) and amitryptyline with IC₅₀ values of 2 × 10^?9 and 4 × 10^?8 M, respectively. Inhibition of [³H]NE uptake by DMI shows competitive kinetics. These characteristics are essentially identical to those found for high-affinity uptake of NE in total membrane or synaptosome fractions from rodent brains and suggests that such uptake in neural tissue is not exclusively neuronal.     

High Serotonin and 5-Hydroxyindoleacetic Acid Levels in Limbic Brain Regions in a Rat Model of Depression; Normalization by Chronic Antidepressant Treatment

Abstract: Although alterations in serotonin levels and neurotransmission are associated with depressive disorders and effective antidepressant therapy, the exact cause of these disorders and the mode of action of anti-depressant drugs are poorly understood. In a genetic rat model of depression [Flinders sensitive line (FSL) rats], deviations from normal serotonin (5-HT) levels and metabolism in specific brain regions were determined. The levels of 5-HT and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in tissue punches of various brain regions were quantitated simultaneously with an HPLC apparatus coupled to an electrochemical detector. In the nucleus accumbens, prefrontal cortex, hippocampus, and hypothalamus of FSL rats, the levels of 5-HT and 5-HIAA were three- to eightfold higher than in control Sprague-Dawley rats. Significant differences in the levels of 5-HT and 5-HIAA in the striatum and raphe nucleus of the "depressed" and normal rats were not observed. After chronic treatment with the antidepressant desipramine (5 mg/kg/day for 18 days), the immobility score in a swim test, as a measure of a behavioral deficit, and 5-HT levels of the FSL rats became normalized, but these parameters in the control rats did not change. The [5-HIAA]/[5-HT] ratio was lower in the nucleus accumbens and hypothalamus of the FSL than in the control rats, and increased after desipramine treatment only in the nucleus accumbens of the FSL rats. These results indicate that the behavioral deficits expressed in the FSL model for depression correlate with increased 5-HT levels in specific limbic sites and suggest the FSL rats as a novel model for clarification of the molecular mechanism of clinically used antidepressant drugs.     

Evidence that Extracellular Concentrations of Dopamine Are Regulated by Noradrenergic Neurons in the Frontal Cortex of Rats

Abstract: Experiments were performed to confirm that noradrenergic terminals regulate extracellular concentrations of dopamine (DA) in the frontal cortex of rats. The effects of 20 mg/kg 1-[2-[bis(4-fluorphenyl)methoxy]-ethyl]-4-(3-phenylpropyl)piperazine (GBR 12909), a selective inhibitor of DA uptake, and 2.5 mg/kg desipramine (DMI) on the extracellular concentrations of DA in the frontal cortex and striatum were studied in rats given 6-hydroxydopamine (6 µg/µl) bilaterally into the locus coeruleus to destroy noradrenergic terminals. GBR 12909 increased dialysate DA similarly in the striatum of vehicle and 6-hydroxydopamine-treated rats, whereas in the frontal cortex it raised DA concentrations only in lesioned animals. DMI raised extracellular DA concentrations in the frontal cortex but not in the striatum of controls. The effect of DMI on cortical DA was abolished by the 6-hydroxydopamine lesion. GBR 12909, at a subcutaneous dose of 20 mg/kg, further increased cortical dialysate DA in rats given DMI intraperitoneally at 20 mg/kg or through the probe at 10⁻⁵ mol/L. The data support the hypothesis of an important regulation of the extracellular concentrations of DA in the frontal cortex by noradrenergic terminals.     

NMDA Receptor Subtype Selectivity: Eliprodil, Polyamine Spider Toxins, Dextromethorphan, and Desipramine Selectively Block NMDA-Evoked Striatal Acetylcholine but Not Spermidine Release

Abstract: NMDA receptor stimulation concomitantly increases the release of [¹⁴C]acetylcholine and [³H]spermidine from rat striatal slices in vitro. The NMDA-induced release of both acetylcholine and spermidine was blocked with equal potency by the NMDA channel blocker phencyclidine (0.1–10 µ M ). However, certain other channel blockers, including dextromethorphan (1–100 µ M ), which antagonized NMDA-evoked acetylcholine release without affecting NMDA-evoked spermidine release, and dextrorphan (1–100 µ M ) and memantine (1–100 µ M ), which block NMDA-evoked acetylcholine release more potently than NMDA-evoked spermidine release, showed greater selectivity of action. As previously shown for ifenprodil, eliprodil (SL82.0715; 1–100 µ M ) blocked NMDA-evoked acetylcholine but not spermidine release. This selectivity is also observed for other agents interacting with the polyamine site(s) on the NMDA receptor, including arcaine (1–1,000 µ M ), philanthotoxin₃₄₃, and argiotoxin₆₃₆ (10 µ M ) and was also noted for desipramine (1–100 µ M ). The NMDA-induced release of acetylcholine and spermidine is likely to be mediated by different native NMDA receptor subtypes, and several NMDA antagonists may be candidates for a selective action at a particular NMDA receptor subtype.     

Noradrenergic Activity Differentially Regulates the Expression of Rolipram-Sensitive, High-Affinity Cyclic AMP Phosphodiesterase (PDE4) in Rat Brain

Abstract: In a previous study, it was observed that the activity of rolipram-sensitive, low- K _m, cyclic AMP phosphodiesterase (PDE4) was decreased in vivo with diminished noradrenergic stimulation. The results of the present experiments indicated that the reduction in the activity may be associated with down-regulation of PDE4 protein. Immunoblot analysis using PDE4-specific, subfamily-nonspecific antibody (K116) revealed four major bands of PDE4 in rat cerebral cortex; those with apparent molecular masses of 109 and 102 kDa are variants of PDE4A. Diminished noradrenergic activity, produced by intracerebroventricular infusion of 6-hydroxydopamine (6-OHDA) or chronic subcutaneous infusion of propranolol, decreased the intensities of the protein bands for the 109- and 102-kDa PDE4A variants in rat cerebral cortex but not of the 98- or 91-kDa PDE4 forms. 6-OHDA-induced noradrenergic lesioning also decreased the content of 102-kDa PDE4A in hippocampus as labeled by PDE4A-specific antibody (C-PDE4A). Enhanced noradrenergic stimulation up-regulated PDE4 in cerebral cortex. This was indicated by the finding that repeated treatment with desipramine increased the intensity of the protein band for the 102-kDa PDE4 but not for the other variants of PDE4. These results suggest that PDE4 subtypes are differentially regulated at the level of expression, as evidenced by an apparent change in the amount of PDE4 protein, following changes in noradrenergic activity. These observations are consistent with the notion that PDE4s, especially the PDE4A variants with molecular masses of 109 and 102 kDa, play an important role in maintaining the homeostasis of the noradrenergic signal transduction system in the brain and may be involved in the mediation of antidepressant activity.     

High-Affinity Binding of [3H]Desipramine to Rat Brain: A Presynaptic Marker for Noradrenergic Uptake Sites

Abstract: High-affinity binding sites (apparent K _D= 1.5 nM) for [³H]desipramine have been demonstrated and characterized in membranes prepared from rat brain. The binding of [3H]desipramine was found to be saturable, reversible, heat-sensitive, sodium-dependent, and regionally distributed among various regions of the brain. High concentrations of [³H]desipramine binding sites were found in the septum, cerebral cortex, and hypothalamus, whereas lower concentrations were found in the medulla, cerebellum, and corpus striatum. A very good correlation ( r = 0.81, P < 0.001) was observed between the potencies of a series of drugs in inhibiting high-affinity [³H]desipramine binding and their capacity to block norepinephrine uptake into synaptosomes. In 6-hydroxydopamine-lesioned rats there was a marked decrease in [³H]norepinephrine uptake and [3H]desipramine binding with no significant alterations in either [³H]serotonin uptake or [³H]imipramine binding. These results suggest that the high-affinity binding of [³HJdesipramine to rat brain membranes is pharmacologically and biochemically distinct from the high-affinity binding of [3H]imipramine, and that there is a close relationship between the high-affinity binding site for [³H]desipramine and the uptake site for norepinephrine.     

Evidence for κ- and μ-Opioid Receptor Expression in C6 Glioma Cells

Abstract: The astrocytoma cell line rat C6 glioma has been used as a model system to study the mechanism of various opioid actions. Nevertheless, the type of opioid receptor(s) involved has not been established. Here we demonstrate the presence of high-affinity U69,593, endomorphin-1, morphine, and β-endorphin binding in desipramine (DMI)-treated C6 cell membranes by performing homologous and heterologous binding assays with [³H]U69,593, [³H]morphine, or ¹²⁵I-β-endorphin. Naive C6 cell membranes displayed U69,593 but neither endomorphin-1, morphine, nor β-endorphin binding. Cross-linking of ¹²⁵I-β-endorphin to C6 membranes gave labeled bands characteristic of opioid receptors. Moreover, RT-PCR analysis of opioid receptor expression in control and DMI-treated C6 cells indicate that both κ- and μ-opioid receptors are expressed. There does not appear to be a significant difference in the level of μ nor κ receptor expression in naive versus C6 cells treated with DMI over a 20-h period. Collectively, the data indicate that κ- and μ-opioid receptors are present in C6 glioma cells.     

Desipramine administered chronically inhibits lipopolysaccharide-stimulated production of IL-1β in the brain and plasma of rats

Nowadays, it is assumed that therapeutic efficacy of antidepressants depends, at least partly, on their anti-inflammatory properties. The present study investigated for the first time the effect of 21-day oral administration of desipramine on the lipopolysaccharide (LPS)-stimulated IL-1β concentration in the olfactory bulb, hypothalamus, frontal cortex, hippocampus and plasma of rats, and on the LPS-induced IL-1β mRNA level in the olfactory bulb. Desipramine (15 mg/kg/day) reduced significantly the LPS (250 μg/kg i.p.)-induced IL-1β concentration in the olfactory bulb, hypothalamus and in plasma, and diminished the LPS effect on IL-1β mRNA in the olfactory bulb. Plasma concentration of desipramine was comparable to its therapeutic range. By using the α1/α2-adrenoceptor antagonist prazosin and the unspecific β-adrenoceptor antagonist propranolol given prior to LPS, we found that the effect of desipramine on LPS-induced IL-1β production was partially mediated by both adrenoceptors in the olfactory bulb and plasma, and that β-adrenoceptors contributed also to its effect on the stimulated IL-1β concentration in the hypothalamus. The effect of LPS on the cerebral IL-1β levels was, in part, mediated by β-adrenoceptors and, in a region-specific manner, by α1/α2-adrenoceptors. The findings provide evidence for central and peripheral anti-inflammatory activity of desipramine and confirm the impact of the noradrenergic system on IL-1β production induced by an immunostimulatory challenge.     

Cytoprotection of rat hepatocytes by desipramine in a model of simulated ischemia/reperfusion

We investigated the cytoprotective effect of desipramine (DMI) during in vitro simulated ischemia/reperfusion (I/R) of rat hepatocytes. Primary hepatocytes isolated from male Sprague-Dawley rats were subjected to 4 h of anoxia at pH 6.2 followed by normoxia at pH 7.4 for 2 h to simulate ischemia and reperfusion, respectively. During simulated reperfusion, some hepatocytes were reoxygenated using media containing 5 μM DMI. Necrotic cell death and the onset of mitochondrial permeability transition (MPT) were assessed using fluorometry and confocal microscopy. Changes in autophagic flux and autophagy-related proteins (ATGs) were analyzed by immunoblotting. DMI was shown to substantially delay MPT onset and suppress I/R related cell damage. Mechanistically, DMI treatment during reperfusion increased the expression level of the microtubule-associated protein 1A/1B-light chain 3 (LC3) processing enzymes, ATG4B and ATG7. Genetic knockdown of ATG4B abolished the cytoprotective effect of DMI. Together, these results indicate that DMI is a unique agent which enhances LC3 processing in an ATG4B-dependent way.