Acidic sphingomyelinase: relationship with antidepressant-induced desensitization of beta-adrenoceptors

Interactions of acid sphingomyelinase and lipid bilayers in the presence of the tricyclic antidepressant desipramine

The tricyclic antidepressant desipramine causes a decrease in cellular acid sphingomyelinase (A-SMase, EC 3.1.4.12) activity when added to culture medium of human fibroblasts. This effect can be prevented by incubation of the cells with the protease inhibitor leupeptin, which suggests that desipramine induces proteolytic degradation of the lysosomal enzyme. By using surface plasmon resonance (SPR, Biacore) we were able to monitor the interactions of A-SMase and substrate-containing lipid bilayers immobilized on the surface of a Pioneer trade mark L1 sensor chip. SPR binding curves show that the enzyme hardly dissociates from the lipid surface at acidic pH values. On the other hand, a drop in binding signals (resonance units, RU) of approximately 50% occurred after injection of 20 mM desipramine. Our findings indicate that desipramine interferes with the binding of A-SMase to the lipid bilayers and thereby displaces the enzyme from its membrane-bound substrate. The application of control substances suggests a key role for the cationic moiety of desipramine. We hypothesize that the displacement of the glycoprotein A-SMase from the inner membranes of late endosomes and lysosomes by desipramine renders it susceptible to proteolytic cleavage by lysosomal proteases.

Lack of effect of repeated administration of milnacipran, a double noradrenaline and serotonin reuptake inhibitor, on the beta-adrenoceptor-linked adenylate cyclase system in the rat cerebral cortex

The effects of subacute administration of the double noradrenaline and serotonin uptake inhibitor antidepressant, milnacipran, and the tricyclic antidepressant, imipramine, on radioligand binding to beta-adrenergic receptors and on beta-adrenergic agonist-stimulated adenylate cyclase activity, in the rat cerebral cortex, have been determined. Rats were injected intraperitoneally for 21 days with milnacipran (3, 10 or 30 mg/kg/day) or imipramine (10 mg/kg/day). The treatment with milnacipran up to 30 mg/kg/day did not modify either the maximum number of [3H]CGP-12177 binding sites (Bmax) or the equilibrium dissociation constant (Kd). On the other hand, treatment of the rats with 10 mg/kg/day imipramine induced a decrease (27%) in Bmax [3H]CGP-12177 binding sites without affecting the Kd value. Furthermore, milnacipran did not affect the stimulation of cAMP production induced by either 30 microM isoprenaline, 10 microM GTP gamma S or 10 microM forskolin. Under similar conditions, treatment with imipramine reduced by 70% the isoprenaline-induced stimulation of cAMP production without affecting that induced by either GTP gamma S or forskolin. These results demonstrate that, unlike imipramine, subacute administration of milnacipran does not produce any change in beta-adrenoceptor sensitivity in the rat brain cortex.

Effect of milnacipran and desipramine on noradrenergic alpha 2-autoreceptor sensitivity

1. The effects of chronic administration with milnacipran and desipramine on the noradrenergic alpha 2-autoreceptor sensitivity in the rat hypothalamus were compared. 2. Rats were administered, in their diet, milnacipran (50 mg/kg/day for 21 days, 24 h wash-out), desipramine (35 mg/kg/day for 21 days, 24 h wash out) or desipramine (14 mg/kg/day for 21 days, 41 h wash-out). Hypothalamic slices were incubated with [3H]noradrenaline, superfused and stimulated electrically. 3. Chronic administration with milnacipran did not modify basal or electrically induced release of [3H]noradrenaline, tissue incorporation of [3H]noradrenaline or the sensitivity of the alpha 2-autoreceptor assessed by the inhibition of the stimulation-evoked release of [3H]noradrenaline by the alpha 2-adrenoceptor agonist, guanabenz, in comparison to controls. After chronic desipramine (35 mg/kg), basal and evoked release of [3H]noradrenaline were increased, tissue incorporation of [3H]noradrenaline decreased and the inhibitory effect of guanabenz was diminished. At the lower dose (14 mg/kg), chronic desipramine increased only the evoked release of noradrenaline but did not modify the sensitivity of the alpha 2-autoreceptor. 4. Desipramine at 35 mg/kg remains in the tissue after 24 h wash out, causing a reduction of uptake and complicating the interpretation of the data.

Effects of prolonged administration of milnacipran, a new antidepressant, on receptors and monoamine uptake in the brain of the rat

Most antidepressants produce changes in monoamine receptors in brain after chronic administration in animals. The most commonly described alterations are a decreased density and function of beta-adrenergic receptors and have been postulated to be the mechanisms by which antidepressants exert their therapeutic effect. Milnacipran (previous name midalcipran) is a new, clinically-effective antidepressant, which inhibits the uptake of both serotonin and noradrenaline but has no affinity for any pre- or postsynaptic receptor studied. When given either orally at 7.5 mg/kg twice daily for 3 days, at 30 mg/kg once daily for 3 weeks, by osmotic mini-pump at 30 mg/kg/day for 27 days, or in drinking water at approximately 15 mg/kg/day for 6 weeks and after a washout period of 24 hr, milnacipran produced no down-regulation of beta-adrenoceptors. In addition, there were no alterations of alpha 1- or alpha 2-adrenoceptors, 5-HT1, 5-HT2 receptors or benzodiazepine binding sites. Moreover, uptake and accumulation of serotonin and noradrenaline were unmodified. In addition, the potency for milnacipran to inhibit monoamine uptake in vitro in the cortex was not altered in treated rats, compared to control animals. Thus, in spite of its action on both the uptake of serotonin and noradrenaline, milnacipran appears to be without long-term action on beta-adrenoceptors or the other receptors studied, suggesting that, at least for this antidepressant, these modifications are not essential for clinical activity.

Rapid desensitization of central beta-adrenoceptors in rat after subacute treatment with imipramine and calcium entry blockers

The subcutaneous (s.c.) administration of isoprenaline to rats produced a dose-dependent increase in water drinking which was effectively antagonized by propranolol. This dipsogenic response was significantly inhibited after the intraperitoneal (i.p.) administration of imipramine (15 mg/kg/day), together with either of the following calcium entry blockers, for four days: diltiazem (15 mg/kg/day), verapamil (10 mg/kg/day), nifedipine (10 mg/kg/day) or nicardipine (15 mg/kg/day). Simultaneous injection of the inhibitor of the synthesis of serotonin, p-chlorophenylalanine (200 mg/kg/day, i.p.), did not affect this attenuation of the isoprenaline-induced response. Similarly, the selective 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) or the 5-HT2 receptor antagonist, ketanserin, had no significant effect on the attenuation of isoprenaline-induced drinking behaviour. The inhibition of isoprenaline-induced drinking, was, however, effectively attenuated after treatment of the animals with 6-hydroxydopamine (2.5 micrograms) or clonidine (30 micrograms), injected intracerebroventricularly (i.c.v.). These results indicate that the calcium entry blockers accelerate the desensitization of central beta-adrenoceptors possibly by an action on central adrenoceptors of the rat.

Inhibition of the adrenergic phenotype in cultured neural crest cells by norepinephrine uptake inhibitors

Tricyclic antidepressants in combination with in vitro clonal analysis of quail neural crest cells were used to examine the role the norepinephrine uptake mechanism might play in the development of adrenergic neural crest derivatives. Norepinephrine (NE) uptake inhibitors blocked expression of the adrenergic phenotype by neural crest cells. The degree of inhibition of phenotypic expression correlated with the potency and specificity of the uptake inhibitors. The drugs acted during the early phase of in vitro development, i.e., several days before overt expression of the adrenergic phenotype in clonal culture. They were nontoxic, and a chronic exposure of the cells to NE uptake inhibitors was necessary to cause an effect. These observations suggest that norepinephrine and possibly related neurotransmitters play a direct or indirect role in the expression of the adrenergic phenotype by neural crest cells and that tricyclic antidepressants may affect neurogenesis during sensitive stages of embryonic development. The data may reflect in vivo mechanisms, since there are neurotransmitters present in the migratory pathway of presumptive sympathetic neurons and the norepinephrine uptake system is expressed in the embryo by these cells before they synthesize and accumulate catecholamines.