OOZS-3908192 $5.00+0.00 Copyright 0 1992 Pergamon Press plc

Neuropharmacology Vol.31,No. 2,pp.149-155, 1992 Printed in Great Britain. All rights reserved

EFFECTS OF PROLONGED ADMINISTRATION MILNACIPRAN, A NEW ANTIDEPRESSANT, RECEPTORS AND MONOAMINE UPTAKE THE BRAIN OF THE RAT M. B. ASSIE, M. CHARVERON,* C. PALMIER, C. Puozzo, Centre

de Recherche

Pierre Fabre,

17 avenue

OF ON IN

C. MORET and M. BRILEY

Jean Moulin,

81100 Castres,

France

(Accepted 16 September 1991) Summary-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 /?-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 30mg/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 /I-adrenoceptors. In addition, there were no alterations of a,- or qadrenoceptors, SHT,, SHT, 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 /?-adrenoceptors or the other receptors studied, suggesting that, at least for this antidepressant, these modifications are not essential for clinical activity. Key words-milnacipran,

prolonged

administration,

receptor

Recent research on the biological mechanisms of antidepressant treatments has led to major revisions in the earlier concepts of the pathogenesis of depression. The hypothesis that depression resulted from a deficiency of monoamines was, in part, based on the assumption that the therapeutic action of antidepressants was a consequence of an increased availability of synaptic noradrenaline (NA) and/or serotonin (5-HT), secondary to inhibition of highaffinity reuptake or decreased catabolism. This hypothesis, however, does not account for the temporal discrepancy between the rapidly induced biochemical effects on amine uptake and catabolism, which occur within hours and the response to the antidepressant in man, which appears only after several weeks of treatment. The observation of a down-regulation of fl-adrenoceptors, shown by a decreased density of binding sites (Banerjee, Kung, Riggh and Chanda, 1977) and/or a decreased response of adenylatecyclase to noradrenaline (Vetulani, Stawarz, Dingell and Sulser, 1976) following chronic treatment with various antidepressant drugs, has led to much specu-

*Present

address:

Dermo-Cosmetology

Centre de Recherche Dermatologie Vigoulet, 31322 Castanet, France.

down-regulation,

antidepressants.

lation concerning the importance of down-regulation of receptors in the action of antidepressants. The significance of down-regulation of b-adrenoceptors is that most clinically effective antidepressants and electroconvulsive-shock (ECS) (see Gleiter and Nutt, 1989) produce this effect in normal rats over a time-course similar to that required for antidepressant action to develop in man. However, there are a number of reports that various antidepressant drugs, including maprotiline, a specific inhibitor of the uptake of NA (Barbaccia, Ravizza and Costa, 1986), citalopram and paroxetine, specific inhibitors of the uptake of 5-HT (Hyttel, Overo and Arnt, 1984; Nelson, Palmer and Johnson, 1990) and atypical antidepressants, such as bupropion (Ferris and Beaman, 1983) do not down-regulate /I-adrenergic receptors. Milnacipran (previously named midalcipran, F 2207) (Fig. 1, Bonnaud, Cousse, Mouzin, Briley, Stenger, Fauran and Couzinier, 1987), is a new antidepressant drug which inhibits with equal potency the uptake of both NA and S-HT but, in contrast to the tricyclic antidepressants, has no affinity for monoamine or cholinergic receptors (Moret, Charveron, Finberg, Couzinier and Briley, 1985; Stenger, Couzinier and Briley, 1987). In doubleblind trials, milnacipran has been shown to be equi-

Cell Cultures, et Cosmetologie,

149

150

M. B. AWE et al.

\

N/%--H, ‘CH,--CCH,

Fig. 1. Structure of milnacipran (F 2207, midalcipran), I-phenyl-l-diethyl-amino-carbonyl-2-aminomethyl-cyc~opropane (Z) hydrochloride.

potent with amitriptyline (Ansseau, Von Frenckell, Mertens, De Wilde, Botte, Devoitille, Evrard, De Nayer, Darimont, Dejaiffe, Mirel, Meurice, Parent, Couzinier, Demarez and Serre, 1989a; Ansseau, Von Frenckell, Papart, Mertens, De Wilde, Botte, Devoitille, Evrard, De Nayer, Koch-Bourdouxhe, Darimont, Lecoq, Mire], Couzinier, Demarez and Serre, 1989b) and clomipramine (Clerc, Pagot, Bouchard, Oules, Guibert, Assicot, Guillard, Cottin, Dachary, Bezaury, Parmentier, Gresle, Von Frenckell and Serre, 1990). In an initial study, repeated administration of lO-16mg/kg/day of milnacipran caused no modification of the binding of /J-adrenoceptors (Moret et al., 1985). Here the effect of various repeated or prolonged administration paradigms in rats, on the binding of the cortical /I-adrenoceptors and various other receptors, often reported to be modified by antidepressants is reported. In addition, the effects of chronic administration of milnacipran on its own potency as an inhibitor of uptake are reported. METHODS

Animals Male Sprague-Dawley rats (Charles River, France), housed at 21-23°C in cages of 6 or 7 with food and water ad libitum, were used for all the experiments. Administration of drugs (1) Oral administration. The rats weighed 170-190 g at the beginning of the experiment. Desipramine 30 mg/kg, citalopram 30 mg/kg and milnacipran 30 mg/kg were administered by oral gavage once daily for 21 days. The animals were sacrificed 24 hr after the last administration. For the 3-day treatment, animals (220-240 g) were given milnacipran 7.5 mg/kg, twice daily. The animals were sacrificed 24 hr after the last administration. Control animals received vehicle. (2) Administration by osmotic mini-pumps. Animals (365-375 g at the beginning of the experiment) were implanted with mini-pumps (Alzet, model 2 ML 4, Alza, Palo Alto) under light ether anesthesia. Milnacipran or desipramine were dissolved in 25% PEG 300 in distilled water and the solutions were

filtered through Millex Millipore filters (0.22 pm pore size). Control animals were implanted with minipumps, fihed with the vehicle. Animals implanted with mini-pumps filled with desipramine, developed necrosis of the skin and only 2 out of 6 implanted rats completed the 27 days. Thus, no meaningful results could be obtained with desipramine. The mini-pumps released 12 mg of compound/ rat/day. Blood samples (2 ml) were collected under light ether anesthesia from the caudal artery into heparinized tubes 7, 14 and 27 days after the implantation of the mini-pump. After centrifugation, the plasma was stored at -20°C until analysis of the drug. The mini-pumps were removed on day 27 under light ether anesthesia and the animals were sacrificed 24 hr later. (3) Administration in drinking water. The rats weighed 220-240 g at the beginning of the experiment, Desipramine, imipramine or milnacipran were given in the drinking water for 6 weeks. The compounds were dissolved in tap water, the bottles were changed every 2 or 3 days and the concentration of drug (150 mg/l at the beginning of the experiment), was adjusted according to the volume drunk so that an approximately constant dose of 15mg/kg/day was absorbed. The animals were sacrificed 24 hr after replacing the solution of drug by drinking water, without the drug. Control animals were given normal drinking water, without drug. Binding experiments Preparation of membranes. The rats were killed by decapitation and the brains quickly removed and dissected; each region (see Table 1) was weighed and homogenized, using an Ultra-Turrax in 20 volumes of cold buffer:Tris 50 mM, HCl pH 7.4 (containing NaCl 120mM, KC1 5 mM and MgCl, 25 mM for p-adrenergic receptors). The homogenate was centrifuged at 48,000g for 15 min, the pellet was resuspended in the same volume of buffer and centrifuged under the same conditions. The final membrane pellet was resuspended in an appropriate volume (see Table 1) of cold buffer and frozen at -2O”C, until the day of the experiment (maximum storage 15 days). Incubation. Membranes were incubated as described in Table 1. At the end of the incubation period, the mixtures were filtered through Whatman GFjC filters for P-adrenergic receptors and GF/B for the other binding assays. The filters were washed twice with 5 ml of cold buffer and the retained tritium was counted by liquid scintillation spectrometry. using Instagel (Packard) in a Packard Tri-carb 4640 counter. The dissociation constant (&) and maximum number of binding sites (B,,,,,) were determined from saturation curves of free against bound ligand by non-linear regression analysis (Fit P, Biosoft), assuming a single site. A minimum of 6 concentrations was used, each determined in duplicate.

Prolonged Table

Receptor binding

site

a,-adrenergic (Ref. I) r,-adrenergic (Ref. I) fl-adrenergic (Ref. 2) 5-HT, (Ref. 3) 5-HT, (Ref. i) Benzodiazepine (Ref. 4) The methods Peroutka

[‘H]Ligand (specific activitv) [‘H]WB4101 (27 Ci/mmol) [‘Hlclonidine (27 Ci/mmol) 13H]dihydroalprenolol (7;;w~n;l)

I. Exoerimental

assay Incubation

Concentration (nM)

Area of’ brain

0.125-4

Forebrain

7

I5

25

Forebrain

7

30

25

Cerebral cortex

I2

30

25

Propranolol IO

Frontal cortex Frontal cortex Forebrain

8

I5

37

4

I5

37

Serotonin IO Methysergide IO

3.5

20

0

l-32 0.2s-8

0.125-4 0.25-a

Time (min)

Temperature (“C)

Drug used for nonspecific binding (PM) Prazosin IO Clonidine

I

were taken from the literature: Ref. I: U’Prichard, Greenberg and Snyder (1977); Ref. 2: Bylund and Snyder (1976); Ref. 3: and Snyder (1979); Ref. 4: Cha&ron, Ass%, Stenger and Briley (1985).

Monoamine uptake

Rats were killed by decapitation and the cortex was rapidly dissected, weighed and homogenized in 10 volumes of sucrose 0.32 M with a Potter S teflomglass homogenizer at 700 rpm. The homogenate was centrifuged 10 min at 1000 g and the supernatant was subsequently centrifuged 10 min at 10,000 g, to give a pellet which was resuspended in 5 volumes of sucrose 0.32 M and homogenized with a manual glass/glass homogenizer. This tissue homogenate was immediately used for uptake measurements. The uptake of 5-HT and NA were measured in parallel, in phosphate buffered saline, pH 7.2 (NaCl 137 mM, K,HPO, 7 mM, KH*PO, 2.5 mM containing pargyline 25 PM), saturated with 0z/C02 (95: 5) using 100 nM of either [3H]5-HT (21.8 Ci/mmol) or [‘H]NA (33 Ci/mmol). Initial uptake and accumulation were determined by incubation at 37°C for 4min and 20 min, respectively. Uptake was stopped by addition of 2.5 ml ice-cold buffer and rapid filtration through Whatman GF/F filters. The filters were rinsed twice with 2.5 ml buffer and the retained radioactivity was counted by liquid scintillation using Instagel (Packard) and a Packard Tri-carb 4640. Analysis of the concentration of milnacipran in plasma

Concentrations of milnacipran in plasma were determined by high performance liquid chromatography with fluorescence detection (Puozzo, Rostin, Montastruc and Houin, 1987). After thawing, the samples were centrifuged at 15OOg for 10 min. One ml of plasma was mixed in a stoppered glass vessel with 100 ~1 of 1 N sodium hydroxide and 4 ml of diethyl ether. The mixture was shaken for 10 min, centrifuged (1000 g, 15 min) and the organic layer removed and evaporated to dryness. The residue was dissolved with 50 ~1 of phosphate buffer (66 mM, pH 8) and 25 ~1 of acetonic solution of fluorescamine (0.3 mg/ml) was added. The mixture was vortexmixed for 1 min, then 200 ~1 of the mobile phase (methanol/phosphate buffer 6.6mM, pH 7, 63/37 v/v) were added and 50 ~1 injected into a reversed NP31,2--1)

details for binding

151

Final tissue concentration in incubation (mgiml)

0.5-10 (21.8 Ci/mmol) [‘Hlspiroperidol (23.8 Ci/mmol) [‘Hlflunitrazepam (74 Ci/mmol)

of milnacipran

administration

phase column (Ultrasphere, Cl8, 5 pm x 15 cm) with a flow rate of 1 ml/min. The fluorescence detector wavelength was set at 280 nm excitation and 480 nm emission. Statistics

One-way analysis of variance, followed by Dunnett’s test, was used throughout. Drugs

Imipramine HCl and desipramine HCl were kindly provided by Ciba-Geigy, citalopram HBr by Lundbeck. Milnacipran was prepared by the Chemistry Department of the Pierre Fabre Research Centre. Tritiated chemicals were purchased from Amersham, with the exception of spiroperidol, which was purchased from New England Nuclear. Doses of drugs are expressed as the salt. RESULTS

Eflect of repeated or prolonged administration on various receptor binding sites /I-Adrenergic receptors. Administration of mil-

nacipran for 3 days, at 7.5 mg/kg (p.0.) twice daily, did not change the number of /I-adrenergic receptors (B,,,) or affinity (&) (Table 2). Milnacipran, 30 mg/kg (p.0.) or citalopram 30mg/kg (p.0.) administered for 21 days, did not modify the Kd or B,,,,, of fi-adrenergic receptors, while desipramine 30mg/kg (p.0.) caused a significant (43%, P < 0.01) decrease in the number of /I-adrenoceptors (Table 2). After 27 days of delivery of drug by mini-pump at 30 mg/kg/day, the Kd and B,,,,, for animals given milnacipran were not significantly different from control (Table 2). During the experiment, blood samples were taken on days 7, 14 and 27 after the implantation of the mini-pumps. The concentrations of milnacipran in plasma were, respectively, 221.9 f 15.3, 239.9 f 22.5 and 308.9 +_26.4 ng/ml (n = 6). Administration of milnacipran, 15 mg/kg/day in the drinking water for 6 weeks, had no effect on the

M. B. AWE et al.

152

Table 2. Binding of [3H]dihydroalprenolo1 to cortical membranes rat. Effects of prolonged administration of the compounds 4 3 Days, 2 x ZSmglkg Control(n = 4) Milnacipran (n = 4)

B,,,

(“M)

(pmol/g

of

tissue)

(po.) 1.20+0.17 I .64 f 0.37

5.56 f 0.36 6.54* 1.18

I .46 k

21 Days, 3Omglkg (pa.) Control (n = 5) Milnacipran (n = 5) Citalopram (n = 5) Desipramine(n = 5)

0.25 I.38 + 0.18 I .07 f 0.05 I .27 f 0.22

7.38 7.90 7.51 4.05

27 Days, jlOmg/kglda~ imp/unfed mini-pumps Control (n = 6) Milnacipran (n = 6)

2.1 I & 0.37 I .94 * 0.50

6.92 k 0.54 6.54 + 0.68

6 Weeks, 15mglkglday drinking warn Control (n = 5) Milhacipran (n = 5) Desipramine (n = 6)

2.00 + 0.51 2.12io.49 I .75 f 0.20

9.29 f 0.39 9.46 f 0.59 6.53 ? 0.42’

+ + f f

& (“M) B,_ (pmol/g

Control

tissue)

a,-adrenergic 4 (“M) E,,, (pmol/g

tissue)

5-HT, serotonergic 4 (“M) B,,, (pmol/g tissue) 5-HT, serotonergic 4 (“M) E,., (pmol/g tissue) Results are expressed

I .88 f 0.07 90.2 * 3.1

DISCUSSION

uptake

Receptor

Imipramine

1.83 kO.13 92.1 f 3.5

each

Initial uptake (4 min) or accumulation (20 min) of 5-HT and NA by well washed homogenates of the cerebral cortex from animals that had received administration imipramine or of milnacipran, vehicle, for 6 weeks, are shown in Table 5. Neither milnacipran nor imipramine, at the dose of 15 mg/ kg/day, had any effect on either initial uptake or when compared to control rats.

a,-adrenergic K+ (“M) i&, (pmol/g

Milnacipran

Table 6 shows the inhibition of the uptake of 5-HT and NA by milnacipran 100 nM and imipramine 100 nM on homogenates of cortex from rats, treated with milnacipran or imipramine 15 mg/kg/day, compared to control animals. The ability of milnacipran and imipramine to inhibit the uptake was not significantly altered in rats given milnacipran and imipramine, as compared to control animals.

parameters of fl-adrenergic receptors whereas there was a significant (31%, P < 0.01) decrease in the number of B-adrenoceptors with desipramine 15 mg/kg/day (Table 2). Other monoamine receptors. After 21 days of oral administration, there were no changes in the affinity of LX,-or a,-adrenergic, SHT, or 5-HT, receptors or number of binding sites with milnacipran, citalopram or desipramine (Table 3). Benzodiazepine binding sites. After 6 weeks of treatment, neither milnacipran nor imipramine had any effect on benzodiazepine receptors (Table 4).

Table 3. Effect of the oral administration

tissue)

Control I .98 + 0.07 96.3 k 2.8

Results are expressed as mean + SEM of 6 separate determinations, each performed in duplicate.

0.59 0.52 0.41 0.9 I *

Results are expressed as mean f SEM of n determinations, performed in duplicate. *P < 0.01 Dunnett’s test.

Monoamine

Table 4. Binding of [‘Hjflunitrazepam: effect of administration of the compounds I5 mg/kg/day for 6 weeks in the drinking water

Previous studies have shown that milnacipran, like tricyclic antidepressants, is an equipotent inhibitor of the reuptake of both NA and 5-HT but, unlike tricyclics, it has no direct effect on a variety of postsynaptic receptors (Moret et al., 1985). The present studies were undertaken to determine whether the antidepressant activity of milnacipran could be explained on the basis of long-term alterations of postsynaptic receptors. As widely described, desipramine was found to significantly decrease the density of jl-adrenoceptors after repeated administration to rats. In contrast, milnacipran, like citalopram (Hyttel et al., 1984), did not modify the density, affinity or function of /?adrenoceptors. In the first experiment, milnacipran was given once daily by oral gavage for 21 days. Autoradiographic data shows that milnacipran does not accumulate in the brain but is quite rapidly eliminated (Benard and Puozzo, 1986). This is probably related to the fact that milnacipran is less lipophilic than tricyclic compounds (Bauer, Megret, Lamure, Lacabanne and Fauran-Clavel, 1990). In an attempt to maintain stable levels of the compound,

(30mg/kg) of antidepressants bindinn sites

for 21 days on various

Milnacipran

Citalopram

0.64 f 0.22 5.07 f 1.13 n=4

0.73 f 0.23 6.64 f 1.17 n=5

0.76 + 0.33 7.47 f I .50 n=5

0.64+0.16 7.38 + 1.49 n=5

3.51 f 0.57 12.1 * 1.5 ?I=5

4.19+0.50 13.3 * I.3 n=5

6.36 f 1.24 16.3 f 2.3 n=5

4.03 + 0.63 13.0 + 0.4 n=5

5.38 + I .20 28.8 + 2.0 n=3

3.72 + 0.70 25.1 f 2.3 n=3

3.34 f 0.49 22.8 + 0.1 n=3

4.17 f 0.46 21.6 f 1.9 n=4

2.05 + 0.57 26.6 f 3.5 n=3

1.94 + 0.51 25.9 + 6.2 n=3

2.01 f 0.67 27.4 i 7.7 n=3

2.14 f 0.49 17.2 + 2.5 n=4

as mean + SEM of n determinations,

each performed

Desipramine

in duplicate.

Prolonged administration Table 5. Initial uptake and accumulation

of 5-HT and NA in cerebral cortex of rat: effect of administration for 6 weeks in the drinking water Initial

Monoamines

Control

5-H-I NA

uptake

in fmol monoamine

Imipramine

3005 f 263 1835 + 219 taken up/mg

2165 + 146 1276 + 109

protein

6. Percentage

inhibition of initial administration

3327 f 286 3140+411

in vitro

Milnacipran Imipramine

100 nM 100 nM

Data are presented VI = 2.

Control

in the form of percentage

Milnacipran

inhibition

of uptake

cortex of rat: effect of

NA uptake (% control) prolonged administration Imipramine

71 f 0.7 64.5 f 6.4

3235 f 90 2721 + 199

study, neither milnacipran, desipramine and citalopram altered a,-adrenoceptors. Although functional supersensitivity of a,-adrenoceptors has been reported after repeated administration of antidepressants (Mogilnicka, Zazula and Wedzony, 1987), the same authors and others (Peroutka and Snyder, 1980) have failed to find a change in the number of cc,-adrenoceptor binding sites, while Stockmeier, McLeskey, Blendy, Armstrong and Kellar (1987) found an increase in a,-adrenergic binding sites with electroconvulsive shock but not antidepressant drugs. In contrast, Heal (1984) showed no functional modification of a,-adrenoceptors, either with antidepressant compounds or with electroconvulsive shock. In the present, no alteration of the number of a,-adrenoceptors was found with milnacipran, citalopram or desipramine. The long-term effect of antidepressant drugs on 5-HT receptors has been investigated by several groups. Peroutka and Snyder (1980) reported that repeated administration to rats of a variety of antidepressant drugs decreased the number of 5-HT, receptors sites in the frontal cortex. In contrast electroconvulsive shock has been found to increase both the number and function of 5-HT, receptors (Goodwin, Green and Johnson, 1984). In the present study, desipramine, citalopram or milnacipran produced no significant changes in the number of 5-HT, receptors. Reports on changes in 5-HT, receptors after chronic treatment with antidepressants are contradictory (see Briley, 1981). The present study showed no alteration of 5-HT, receptor sites with milnacipran, citalopram or desipramine. Down-regulation of benzodiazepine receptors has been reported after repeated administration of certain antidepressants (Suranyi-Cadotte, Dam and Quirion, 1985; Barbaccia et al., 1986). The present study, however, showed no difference between animals given antidepressant and controls. The induction of up-regulation of y-aminobutyric acid (GABA,) receptors and their function by a number of

uptake of 5-HT and NA by milnacipran and imipramine in cerebral of the compounds 15 mg/kg/day for 6 weeks in the drinking water

74.4 f 2.4 70.3 k 2.3

Imipramine

3997 f 278 3075 + 524

determinations.

5-HT uptake (% control) prolonged administration Compounds

15 mg/kg/day

(20 min)

Milnacipran

Control

as mean f SEM of 6 separate

milnacipran was administered continuously for 27 days using a mini-pump. In this experiment, relatively stable pharmacologically active levels of milnacipran in plasma were achieved. These concentrations in plasma were similar to that obtained approximately 1 hr after acute administration of 5 mg/kg (p.0.) of milnacipran (Puozzo, Maynadier, Ramouneau-Pigot and Solles, 1988), a dose which produces a number of pharmacological effects (Stenger et al., 1987) and inhibits the uptake of NA and S-HT in vivo by at least 50% (Moret et al., 1985). Under these conditions, binding to /I-adrenoceptors remained unchanged. In the third experiment, a dose of 15 mg/kg/day was administered in the drinking water so that its absorption was distributed, albeit not equally, throughout the 24 hr. Under these conditions, after 6 weeks there were no alterations of /I-adrenoceptor binding. Recent reports, showing a down-regulation of padrenoceptors after only 3 days administration of various antidepressants and potential antidepressant compounds (Buckett, Luscombe and Thomas, 1987, 1988) led to a consideration of the possibility that down-regulation of /I-adrenoceptors could occur with milnacipran in a few days and return to normal values by the time measurements were made after 21 days or later. No down-regulation was seen, however, with milnacipran, given at 7.5 mg/kg, twice daily for 3 days. In addition, since the initial report of a lack of down-regulation of /I-adrenergic receptors with milnacipran (Moret et al., 1985), this observation has been confirmed by various authors, either by binding (Buckett, personal communication; Cart+, Boutry, Baumann and Maurin, 1988; Matsubara, Koyama, Odagaki, Nakayama, Inoue and Yamashita, 1988) or isoproterenol-stimulated activity of adenylate cyclase (Matsubara, Koyama, Muraki, Matsubara, Odagaki, Nakayama and Yamashita, 1990). Modifications of cc,-adrenoceptor binding, following repeated administration of antidepressants are equivocal (see Green and Nutt, 1985). In the present

Table

of the compounds

Accumulation

(4 min)

Milnacipran

2631 + 115 1503 + 231

Data are expressed

153

of milnacipran

77 * 4.4 71.5 + 5.2

Control 73’ 81.6 f 6.2

as mean f SEM of 3 separate

Milnacipran 68.2 + 4.0 88 + 2.8 determinations.

Imipramine 78.8 f 1.5 73.4 * 10.2

M. B. AWE el al.

154

antidepressants, has been suggested to be involved in their mechanism of action (Lloyd, Thuret and Pile, 1985; Gray, Goodwin, Heal and Green, 1987). Milnacipran however showed no alteration of these receptors (Cross and Horton, personal communication). In the present study, the uptake of 5-HT or NA were unchanged in the cerebral cortex after repeated administration of antidepressants. Moreover, no alteration of the effect of imipramine or milnacipran in vitro on the uptake of S-HT or NA, after repeated administration, was seen. These data suggest that no adaptative changes occur in the monoamine uptake

mechanism, following repeated administration of these compounds. Thus, milnacipran, administered repeatedly or continuously in pharmacologically-active doses, appears not to produce any adaptative changes in postsynaptic receptors or in the mechanisms of monoamine uptake. Acknowledgements-The authors would like to thank Isabelle Pastrie for skilful technical assistance, Bernadette Maynadier for the HPLC analysis of milnacipran in plasma and Martine Dehaye for the preparation of the manuscript. REFERENCES

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