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Effects of chronic antidepressant treatment on nigrostriatal and mesolimbic dopamine autoreceptors in the rat
Neurochem. Int. Vol. 9, No. 1, pp. 61-67, 1986
0197-0186/86 $3.00 +0.00 Pergamon Journals Ltd
Printed in Great Britain
EFFECTS OF CHRONIC ANTIDEPRESSANT TREATMENT ON NIGROSTRIATAL A N D MESOLIMBIC DOPAMINE AUTORECEPTORS IN THE RAT J. A. NIELSEN* Department of Pharmacology, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio, U.S.A. (Received 12 September 1985; accepted 25 November 1985) Abstract--Dopamine autoreceptors were studied by determining the effects of chronic antidepressant treatment on the ability of several doses of apomorphine to decrease 3,4-dihydroxyphenylalanine accumulation (an index of dopamine synthesis in vivo) after saline or ).-hydroxybutyric acid lactone (~-butyrolactone). 3,4-Dihydroxyphenylalanine accumulation was measured in nigrostriatal [nucleus caudatus putamen] and mesolimbic [nucleus accumbens and tuberculum olfactorium] nerve terminals. Apomorphine decreased 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen, tuberculum olfactorium and nucleus accumbens in a dose-related manner. Chronic imipramine (10 days) treatment attenuated the low and high dose apomorphine-induced decrease in 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen to a greater extent than the tuberculum olfactorium or nucleus accumbens. In 7-butyrolactone-treated animals chronic treatment with imipramine, amitriptyline or bupropion (10 days) attenuated the low dose apomorphine effect in the nucleus caudatus putamen, but not the tuberculum olfactorium or nucleus accumbens. Only 2 days of imipramine treatment had no effect on the apomorphine-induced decrease in 3,4-dihydroxyphenylalanineaccumulation in the nucleus caudatus putamen with or without y-butyrolactone treatment. These data suggest that chronic treatment with three antidepressants produces dopamine autoreceptor subsensitivity in nigrostriatal neurons more than mesolimbic neurons and that this effect is not seen with short-term imipramine treatment.
The involvement of dopamine (DA) in the state of depression and the action of antidepressants was first suggested by Randrup and coworkers (1975). Several recent reports have investigated the possibility that chronic antidepressant treatment alters DA autoreceptors (for reviews see Willner, 1983; Waldmeier, 1984). Subsensitivity of DA autoreceptors after repeated antidepressant treatment has been suggested by the following findings. Serra and coworkers (1979, 1980) reported that chronic administration of several antidepressants were found to attenuate the reduction of motor activity and of striatal 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations induced by small doses of the DA agonist apomorphine which are thought to act preferentially on DA autoreceptors. This biochemical finding was replicated by Holcomb and coworkers (1982) although they suggest that this does not reflect subsensitivity of nerve terminal autoreceptors. Arnt and colleagues
(1984) showed that chronic treatment with citalopram attenuated the reduction of motor activity induced by low doses of apomorphine. Antelman and coworkers (Chiodo and Antelman, 1980, 1982; Antelman et al., 1982) found that long-term administration of several antidepressants attenuated the apomorphine-induced decrease in firing of DA neurons located within the zona compacta of the substantia nigra. This effect was found to depend on passage of time rather than daily drug administration (Chiodo and Antelman, 1982). Finally, Fibiger and Phillips (1981) found that chronic desipramine treatment enhanced self-stimulation obtained from electrodes in the ventromedial tegmentum. Other findings do not suggest that chronic antidepressant treatment produces DA autoreceptor subsensitivity. Spyraki and Fibiger (1981) showed that chronic desmethylimipramine did not alter the low dose apomorphine-induced decrease in motor activity of rats. Two studies found no change in apomorphine-induced inhibition of DA cell firing following chronic antidepressant treatment (Welch et al., 1982; MacNeil and Gower, 1982). Waldmeier
*Address correspondence to: Dr. J. A. Nielsen, Pfizer Central Research, Eastern Point Road, Groton, CT 06340, U.S.A. 61
62
,1. A. Naitst:,,
(1984) was unable to reproduce the biochemical findings of Serra a n d coworkers (1979, 1980) with long-term a d m i n i s t r a t i o n of imipramine, maprotiline and two e n a n t i o m e r s of oxaprotiline. Diggory and Buckett (1984) have shown that chronic treatment with four different antidepressants did not alter the low dose a p o m o r p h i n e - i n d u c e d decrease in the D A metabolites D O P A C or 3-methoxy-4-hydroxyphenylacetic acid (homovanillic acid) in rat striatum. Finally, H o l c o m b and coworkers (1982) employed a specific biochemical model of D A autoreceptor stimulation to determine the effects of chronic antidepressant treatment on these receptors. Following blockade of D A impulse flow with 7-hydroxybutyric acid lactone (7-butyrolactone, G B L ) (Waiters et al., 1973; Waiters and Roth, 1976), a p o m o r p h i n e stimulates D A autoreceptors, thus decreasing D A synthesis as evidenced by a decrease in 3,4-dihydroxyphenylalanine ( D O P A ) accumulation ( R o t h and Nowycky, 1977). H o l c o m b and coworkers (1982) found that long-term antidepressant t r e a t m e n t did not alter the effect of high doses of a p o m o r p h i n e in this model. The present study extends these findings by using lower doses of a p o m o r p h i n e . Most of the reports cited above investigated the effects of chronic antidepressants on nigrostriatal D A neurons. Maj (1984) has s h o w n that mesolimbic D A neurons may be altered by chronic antidepressant treatment. Chronic treatment with imipramine increased the l o c o m o t o r stimulation induced by da m p h e t a m i n e injected into the nucleus accumbens. However, the concentration of D A and its metabolites in the nucleus a c c u m b e n s were not significantly affected by chronic imipramine treatment. This suggests that perhaps the imipramine-induced increase in responsiveness to d - a m p h e t a m i n e may be due to postsynaptic changes in mesolimbic D A neurons. The aim of the present study was to determine the effects of chronic antidepressant treatment on a neurochemical index of nigrostriatal and mesolimbic D A autoreceptors in the rat. l m i p r a m i n e , amitriptyline and b u p r o p i o n were the antidepressants used because the first two are typical antidepressants while b u p r o p i o n is newer, "atypical," a n d affects DA neurons (Nielsen et al., in press). Two methods were used to specifically identify D A autoreceptors neurochemically: low dose a p o m o r p h i n e - i n d u c e d decrease in D O P A a c c u m u l a t i o n with and without G B L treatment. The major difference between the two models is that impulse flow is blocked when G B L is present, implying that any drug-induced changes in D O P A accumulation results from effects on nerve terminal
autoreceptors. Without GBL, drug-induced changes in impulse flow can also produce indirect effects on D O P A synthesis. By using both methods we hope to investigate the effects of chronic antidepressant treatmenl on D A autoreceptor and n o n - a u t o r e c e p t o r influence on DA synthesis. This study expands on the work of H o l c o m b and coworkers (1982) in that different antidepressants were used, mesolimbic neurons were analyzed, and the effects of lower doses of a p o m o r p h i n e were investigated. EXPERIMENT&I. PROCEDURES Male Sprague Dawle~ rals (liarlan Spraguc Dawley, Inc., Haslett, MI: or Zivic-Miller Labs, Allison, PA) weighmg 175 200 g were maintained under 12 13 periods of light and dark. Rats were housed 3 4cage and had access to standard rat chow (Wayne t.ab Blox: Allied Mills. Chicago, 1I,) and tap water ad lihitum. The rats were treated with apomorphine HC1 (Eli Lilly Co., Indianapolis, IN: 30, 100 or 300 pg/kg, s.c.) or saline 35 rain before and an inhibitor of aromatic I-amino-acid decarboxylasc, 3-hydroxybenzylhydrazmc HC1 (NSD 1015: Sigma Chemical Co., Saint Louis, MO: 100mg/kg, i.p.) 30min before decapitation. In a second experiment, imipramine HCI (Geigy, Ardsley, NY: 10mg/kg, s.c.) or saline was administered twice a day (BID) (about 1000 and 2200 h) for 10 days. Three days after the last treatment the animals were injected with apomorphine HC1 (30, 100 or 300 pg/kg, s.c.) or saline and NSD 1015 as described above. A third experiment was performed as described above, except that GBL (750 mg/kg, i.p.) was administered 1o some animals 35 rain bel\~re decapitation. The fourlh cxpermlent inw)Ived chronic administration of imipramine HC1 (10 mgkg, s.c.), amitriptylinc HCI (Merck Sharp and Dohme Research Lab, Rahway, N J: I0 mg/kg, s.c.), bupropion HCI (Burroughs Wellcome Co., Research Triangle Park, NC; 30 mg/kg, s.c.) or saline twice a (lay (at about 1000 anti 2200 h) for l0 days. Three days after the last treatment the rats received apomorphine, GBL and,'or saline plus NSD 1015 as described above. The fifth and final experiment involved treatment with imipraminc HCI (10mg'kg, s.c.) for 10 days, imipramine tICI (10mg/kg, s.c.) for 2 days and saline l\)r 8 days, or saline twice a day for 10 days. Three days after the last treatmenl {when essentially all of the antidepressant had left the body) the rats received apomorphine, GBL a n d o r saline plus NSD 1015 as described above. The m rit'o synthesis of DA was estimated in the DA nerve terminals of the nigroslriatal [nucleus caudatus putamen (CP)] and mesolimbic [nucleus accumbens (NA) and tuberculum olfaclorium (-I"O)l neurons by measurirtg the accumulation of DOPA after NSD 1015 treatment. After decapitation the brain was quickly removed from the skull. lhe tissues mentioned above dissected as described previously (Roth and Nowycky, 1977L and the tissues homogenized such that approximately lllmg tissue was homogenized in 1001~1 cold 0.2N perchloric acid containing 10rag% ethyleneglycol-hiv-(/~-amino-ethyl ether) N,N'tetraacetic acid or high pressure liquid chromotography (LC) buffer (see below). Following 30 s centrifugafion in it microfuge (Beckman, Fullerton, CAI, DOPA concentrations in the samples was analyzed by radioenzymatic
Chronic antidepressant treatment on dopamine autoreceptors assay or LC with electrochemical detection (EC). The radioenzymatic assay standards (125 2000 ng DOPA) and supernatants of perchloric acid extracts of tissue were incubated in the presence of catechol-o-methyltransferase (EC 2.1.1.6) and [3H]S-adenosyl-L-methionine. The resuiting tritiated o-methylated product of DOPA, 3-methoxytyrosine, was separated from interfering omethylated catecholamines and unreacted [3H]S-adenosyl-Lmethionine by cation exchange chromatography, absorption onto activated charcoal followed by anion exchange chromatography (Demarest and Moore, 1980). The DOPA content of each sample was calculated directly from standards after subtracting blank values. Blank values were obtained by assaying duplicate 10/11 aliquots of perchloric acid solution. The sensitivity of the assay (the amount of DOPA yielding counts at least twice the blank) was about 125 pg. Individual brain regions from each experiment were analyzed for DOPA in the same assay and treatment effects were compared only to control values determined in that assay. For the LCEC assay, the supernatant from each sample was injected via a fixed loop (Rheodyne, Cotati, CA) onto a 30cm × 3.9mm i.d. /~Bondapak C m column (Bioanalytical Systems, West Lafayette, IN). A model LC 9533 (IBM Instruments, Wallingford, CT) liquid chromatographic system was used with an electrochemical detector (IBM Instruments). Detector potential was set at +0.75 V and the reference electrode was Ag+/AgC1. The mobile phase contained 0.1 M citrate-phosphate, 0.1 mM ethylene-diamine-tetraacetic acid, and methanol. The methanol (0-7%) and pH (2 3.6) were altered in order to maintain separation of peaks as the column aged. Details of the chromatographic analysis have been published previously (Nielsen et al., 1982). The concentrations of DOPA in each sample was determined by comparing areas under the curve (CS 9000 computer, IBM Instruments) for samples and standard analyzed the same day. The sensitivity of the assay varied inversely with the age of the #Bondapak column. The sensitivity was typically about 50 pg, and never less than 200 pg per sample. In all samples, the pellet was analyzed for protein (Lowry et al., 1951). There was no significant difference between controls using the radioenzymatic or LCEC assays. The effects of treatments were determined using a oneway analysis of variance followed by StudentNewman-Keuls' test (Steel and Torrie, 1980). A critical value of P < 0.05 was set as that required to indicate a statistically reliable effect of experimental manipulation.
6O 4O 2O _
0
I
§ g 6o ~ 2o u '~
NCI. 9/I--E
I
To
0
13_
I
I
I
I
L NA
4O 2O 0
I
I
I
1
0
50
100
500
Apomorphine
(/J.glkg )
Fig. 1. Effect of chronic treatment with imipramine on apomorphine-induced decrease in DOPA accumulation. Three days after a 10-day treatment with imipramine (10mg/kg, s.c., BID) (A) or saline (O), apomorphine (s.c.) or sodium metabisulfite vehicle was administered at 35min and NSD 1015 (100mg/kg, i.p.) at 30min before sacrifce. Each data point represents the mean (M) and the vertical line one standard error (SE) of t~8 determinations. Where no vertical line is depicted, the SE is less than the radius of the symbol. Variability around the zero apomorphine points is not shown for clarity, but it was always less than 12%. The only significant differences (P < 0.05) between animals receiving chronic saline and chronic imipramine were at doses of 30 and 300/lg/kg apomorphine in the CP. The following are baseline (saline and NSD 1015 treatment) values (ng/mg protein, M + I S E ) for DOPA accumulation: Chronic
RESULTS
There was no significant difference in the baseline c o n c e n t r a t i o n s of D O P A a c c u m u l a t i o n in any brain region between experiments (legends to Figs 1 a n d 2, Tables 1 a n d 2). A p o m o r p h i n e (30, 100 a n d 300/lg/kg) p r o d u c e d a dose-related decrease in D O P A a c c u m u l a t i o n in the CP, T O a n d N A (experim e n t 1, data not shown). C h r o n i c i m i p r a m i n e - t r e a t m e n t a t t e n u a t e d the apom o r p h i n e (30 a n d 3 0 0 / t g / k g ) - i n d u c e d decrease in D O P A a c c u m u l a t i o n in the C P (experiment 2, Fig. 1). C h r o n i c imipramine t r e a t m e n t had no effect o n the a p o m o r p h i n e - i n d u c e d reduction in D O P A accumulation in the T O or N A (Fig. 1).
63
CP TO NA
Saline
Imipramine
26.8 4- 1.4 11.9 + 1.1 28.0 4- 3.4
16.4 + 1.4 9.3 + 1.1 20.9 4- 3.5
In G B L - t r e a t e d animals, chronic imipraminet r e a t m e n t a t t e n u a t e d the low dose a p o m o r p h i n e effect in the CP, b u t not the effects o f higher doses in the CP, or any dose of a p o m o r p h i n e in the T O a n d N A (experiment 3, Fig. 2). In experiment 4, these effects of chronic i m i p r a m i n e - t r e a t m e n t were replicated a n d ami-
64
J.A. NIELSEN
'°°F 6O 4O 2O 0
I
J
J
I
I TO
~_
60
2o o '~ n 0
0
1
[
i
J
I NA
100
6O 4O
morphine [Tables I and 2, ( - ) G B L ( )APO] and with GBL, but without apomorphine [Tables I and 2, ( + )GBL( - )APO]. The effects of the lowest dose of apomorphine (30 l~g/kg) in animals chronically treated with imipramine for 10 days in experiment 5 was similar to that mentioned above for experiments 2~4. Ten days" treatment with imipramine produced the following effects in the CP: (1) no effect on animals treated with neither GBL nor apomorphine [Table 2, (-)GBL(-)APO]; (2) an attenuation of the apomorphine-induced decrease in D O P A accumulation [Table 2, ( - )GBL( + )APO]; and (3) a similar attenuation in animals treated with GBL and apomorphine [Table 2, ( + ) G B L ( + )APO]. Two days" treatment with imipramine produced none of the effects described above (Table 2). Neither 2 nor 10 days' treatment with imipramine had any effect on apomorphine- and/or GBL-induced changes in D O P A accumulation in the TO or N A (Table 2).
2O 0
J 0 nB°k
I 0
I 30
Apomorphine
I 100
J 300
Fig. 2. Effect of chronic imipramine treatment on apomorphine-induced decline of DOPA accumulation in terminals of nigrostriatal and mesolimbic DA neurons in GBL-treated rats. Three days after a 10-day treatment with imipramine (10 mg/kg, s.c., BID) (A) or saline (©), GBL (750mg/kg, i.p.) or saline was administered as 35min, apomorphine (s.c.) or sodium metabisulfite vehicle at 35 min and NSD 1015 (100 mg/kg, i.p.) at 30 min before sacrifice. Each data point represents the mean (M) and the vertical line one standard error (SE) of 6 8 determinations. Variability around the zero apomorphine points is not shown for clarity, but it was always less than 12%. The only significant difference (P <0.05) between animals receiving chronic saline and chronic imipramine was at a dose of 30 #g/kg apomorphine in the CP. The following are baseline (saline, GBL and NSD 1015 treatment) values (ng/mg protein, M + I SE) for DOPA accumulation. Chronic CP TO NA
DISCUSSION
(b~g/kg)
Saline
lmipramine
30.1 + 2.4 13.1 + 1.4 35.8 + 4.4
40.0 _+2.6 12.3 + t.0 29.7 + 2.1
triptyline and bupropion produced similar effects (Table 1). Imipramine, amitriptyline and bupropion attenuated the apomorphine-induced decrease in D O P A accumulation in the CP but not the TO or NA. Chronic imipramine-, amitriptyline-, or bupropion-treatment had no effect on D O P A accumulation in animals treated without GBL and apo-
Chronic treatment with imipramine, amitriptyline and bupropion attenuates the inhibition of D O P A accumulation in the CP by apomorphine. This may be due to a change in DA autoreceptors. This effect occurs at a low dose of apomorphine, which may be preferentially activating DA autoreceptors, and at a low dose of apomorphine in GBL-treated animals which probably represents DA autoreceptor stimulation. Chronic antidepressant treatment did not alter the decrease in D O P A accumulation induced by high doses of apomorphine in animals treated with GBL, but it did in rats not treated with GBL. This probably suggests that antidepressants have several effects on DA neurons. First of all, when GBL is present they produce autoreceptor subsensitivity to very low doses of apomorphine. The response to higher doses is not altered, presumably since higher doses of apomorphine can overcome the subsensitivity. However, when impulse flow is not blocked, antidepressants can exert a second kind of effect on DA neurons which attenuates the response to high doses of apomorphine. The ability of antidepressants to exert this second effect may depend on impulse flow through postsynaptic feedback mechanisms. Chronic imipramine treatment did attenuate the D O P A accumulation lowering effect of a high dose of apomorphine in animals not treated with GBL. This effect was not seen in animals treated with GBL. This suggests that chronic imipramine may produce changes on postsynaptic DA receptors and/or indi-
Chronic antidepressant treatment on dopamine autoreceptors
65
Table 1. Effects of chronic antidepressant treatment on a test for DA autoreceptor sensitivity Pretreatment
( - ) GBL ( - ) APO
Saline Imipramine Amitriptyline Bupropion
48 ± 53 ± 50 ± 45 ±
8 7 6 9
Saline Imipramine Amitriptyline Bupropion
63 ± 54 ± 67 ± 59 ±
7 9 11 6
Saline lmipramine Amitriptyline Bupropion
58 ± 58 ± 67 ± 61 ±
6 9 4 7
( + ) GBL ( - ) APO
( + ) GBL ( + ) APO
Caudatus Putamen (CP) 100 ± 8 100 ± 7 100 ± 11 100 ± 10 Tuberculum Olfactorium (TO) 100 ± 5 100 _+ 11 100 ± 9 100 ± 7 Nucleus Accumbens (NA) 100 + 11 100 ± 9 100 ± 8 100 ± 9
77 ± 95 ± 111 ± 93 ±
8 7* 10" 5*
82 ± 77 ± 80 ± 95 ±
10 6 9 6
71 ± 77 ± 75 ± 79 ±
8 10 4 9
Three days after a 10 day treatment with saline, imipramine (10 mg/kg, s.c., BID), amitriptyline (10mg/kg, s.c., BID) or bupropion (30mg/kg, s.c., BID), GBL (750mg/kg, i.p.) or saline was administered at 35 minutes, apomorphine (30 pg/kg, s.c.) or saline at 35 min and NSD 1015 (100 mg/kg, i.p.) at 30 rain before sacrifice. Values (percentage of the baseline values listed below) represent the mean (M) ± one standard error (SE) of 6~7 determinations. *Significantly (P < 0.05) different from saline pretreatment. The following are baseline (saline, GBL and NSD 1015 treatment) values (ng/mg protein, M ± 1 SE) for D O P A accumulation. Saline CP TO NA
rectly on DA because
neurons.
these
studies
31.4±1.9 16.7±1.3 28.6±2.3
Chronic Imipramine Amitriptyline 33.0±2.9 15.8±2.0 26.4±2.3
This finding was not pursued
However,
concentrated
cussing
on
DA
auto-
receptors. Chronic
Bupropion
39.3±5.4 13.9±1.1 31.1±3.7
29.8±3.3 16.8±1.7 29.8+3.4
it is n e c e s s a r y regional
effects of antidepressants antidepressant
striatal more
than
treatment
mesolimbic
DA
affected
nigro-
nerve terminals.
are much limbic
to exercise caution
differences,
needs
further
findings
in dis-
since
in the mesolimbic
less clear cut. Our
regions
especially
in the meso-
verification,
since, for
Table 2. Effect of 2 and 10 day treatment with imipramine on a test for DA autoreceptor sensitivity Pretreatment
( - ) GBL ( - ) APO
Saline Imipramine x 2 Imipramine × 10
43 ± 6 48 _+ 5 45 ± 7
Saline Imipramine x 2 Imipramine × 10
60 _+ 8 59 ± 6 69 ± 7
Saline Imipramine x 2 lmipramine x 10
68 ± 7 61 ± 6 60 ± 9
( - ) GBL ( + ) APO
( + ) GBL ( - ) APO
Caudatus Putamen (CP) 29 ± 4 100 _+ 2 30 ± 5 100 ± 9 50 ± 6* 100 ± 11 Tuberculum Olfactorium (TO) 36 ± 5 100 + 7 39 ± 8 100 + 8 43 ± 5 100 + 6 Nucleus Accumbens (NA) 42 ± 8 100 ± 12 45 ± 4 100 ± 6 39 ± 7 100 ± 8
( + ) GBL ( + ) APO 71 ± 6 79 + 9 102 ± 11" 85 ± 11 79 ± 6 91 ± 8 82 ± 7 82 ± 7 91 ± 6
Three days after treatment with saline for l0 days, imipramine for 2 days then saline for 8 days, or imipramine for 10 days, GBL (750 mg/kg, i.p.) or saline was administered at 35 min, apomorphine (30 #g/kg, s.c.) or saline at 35 min and NSD 1015 ( 100 mg/kg, i.p.) at 30 rain before sacrifice. Values (percentage of the baseline values listed below) represent the mean (M) _+ one standard error (SE) of 5-7 determinations. *Significantly (P < 0.05) different from saline pretreatment. The following are baseline (saline, GBL and NSD 1015 treatment) values (ng/mg protein, M + I S E ) for DOPA accumulation.
CP TO NA
Saline
Chronic Imipramine x 2
Imipramine x 10
25.7 ± 2.6 12.8 ± 1.6 23.5 ± 2.5
23.9 ± 1.1 14.1 ± 1.1 24.8 + 2.1
28.0 ± 3.0 13.8 ± 2.4 26.9 ± 1.9
the
regions
66
J.A. Nlt!I.SEN
example, we were unable to c o r r o b o r a t e previous studies which reported that mesolimbic synthesism o d u l a t i n g autoreceptors are more sensitive to apom o r p h i n e than nigrostriatal autoreceptors in the G B L model (Demarest et al., 1983: Roth and Nowycky, 1977). However, it is not surprising that long-term antidepressant a d m i n i s t r a t i o n had greater effects in one brain area t h a n another, since K a r o u m and coworkers (1984) reported that D A neurons in the striatum are more sensitive to chronic antidepressant t r e a t m e n t than D A neurons in the hypothalamus. A n t e l m a n and coworkers showed that 2 days treatment with imipramine followed by 10 days administration of saline attenuated autoreceptor stimulation by low doses of a p o m o r p h i n e inhibiting the firing o f DA cells in the substantia nigra (Chiodo and Antelman, 1982). We were unable to find any biochemical correlation to this effect. It may not be possible to correlate electrophysiological findings concerning the sensitivity of impulse-regulating autoreceptors in the substantia nigra with the biochemical methods described herein. The importance of timing in chronic antidepressant treatment effects on D A neurons requires more research. in summary, chronic antidepressant treatment produces D A autoreceptor subsensitivity in nigrostriatal but not mesolimbic neurons. This effect is not seen with short-term imipramine treatment. The author thanks Dr Kenneth Moore for his assistance with the work described herein, and Susan Stahl for valuable technical assistance. This work was supported in part by USPHS grant NS 15911. Acknowledgenwnts
REFERENCES
Antelman S. M., Chiodo L. A. and DeGiovani L. A. (1982) Antidepressants and dopamine autoreceptors: Implications for both a novel means of treating depression and understanding bipolar illness. In Typieal and AO'pieal Antidepressants, (Costa E. and Racagni G.. eds), pp. 121-146. Raven Press, New York. Arnt J., Overe~ K. F., Hyttel J. and Olsen R. (1984) Changes in rat dopamine- and serotonin function in rivo after prolonged administration of the specific 5-HT uptake inhibitor citalopram. Psyehopharmacology 84, 457 465. Chiodo L. A. and Antelman S. M. (1980) Repeated tricyclics induce a progressive dopamine autoreceptor subsensitivity independent of daily drug treatment. Nature 287, 451 454. Chiodo L. A. and Antelman S. M. (1982) Do antidepressants induce dopamine autoreceptor subsensitivity? Nature 298, 302303. Demarest K. T., Lawson-Wcndling K. L. and Moore K. E. (1983) d-Amphetamine and butyrolactone alteration of dopamine synthesis in the terminals of nigrostriatal and mesolimbic neurons. Bioehem. Pharmaeol. 32, 691 697. Demarest K. T. and Moore K. E. (1980) Accumulation of
L-DOPA in the median eminence: An index of tuberoinfundibular dopaminergic nerve activity. Endocrinology 106, 463 468. Diggory G. L. and Buckett W. R. (1984) Chronic antidepressant administration fails to attenuate apomorphine-induced decreases in rat striatal dopamine metabolites. Eur. J. Pharmaeol. 105, 257 263. Fibiger H. C. and Phillips A. G. (1981) Increased intracranial sell-stimulation in rats after long-term administration of desipramine. &'ience 214, 683 685. Holcomb H. H., Bannon M. J. and Roth R. H. (1982) Striatal dopamine autoreceptors uninfluenced by chronic administration of antidepressants. Eur. J. Pharmaeol. 82, 173 178. Karoum F., Korpi E. R., Linnoila M., Chuang L.-W. and Wyatt R. J. (1984) Reduced metabolism and turnover rates of rat brain dopamine, norepinephrine and serotonin by chronic desipraminc and zimelidine treatments. Eur. J. Pharmaeol. 100, 137 144. Lowry O., Rosebrough N., Farr A. and Randall R. (1951) Protein measurement with the Folin phenol reagent../. biol. Chem. 193, 265 275. MacNeil D. A. and Gower M. (1982) Do antidepressants induce dopamine autoreceptor subsensitivity? Nature 298, 302. Maj J. (1984) Central effects following repeated treatment with antidepressant drugs. Pol. J. Pharmacol. Pharm. 36, 87 99. Nielsen J. A.. Duda N. J. and Moore K. E. (1982)Tolerance develops to the haloperidol-induced increase in the efflux of dopamine metabolites from the brains of unanesthetized, freely-moving rats. L(I~' Sei. 31, 1495 1500. Nielsen J. A., Shannon N. J.. Bero L. and Moore K. E. Effects of acute and chronic bupropion on locomotor activity and dopaminergic neurons. Pharmaeol. Bioehem. Behae. In press. Randrup A., Munkvad I., Fog. R.. Gerlach J.. Molander L., Kjellberg B. and Scheel-Kruger J. (1975) Mania, depression and brain dopamine. In Current Developments m Psychopharmacology, (Essman W. B. and Valzelli V., eds.), pp. 206-248. Spectrum Publication, New York. Roth R. H. and Nowycky M. C. (1977) Nonstriatal dopammergic neurons: Role of presynaptic receptors in the modulation of transmitter synthesis. In Advances in Biochemical Psyehopharmacoh~g:v, (Costa E. and Gessa G. L.. eds.), pp. 465 470. Raven Press, New York. Serra G., Argiolas A., Fadda F. and Gessa G. L. (1980) Hyposensitivity ofdopamine "'autoreceptors'" reduced by chronic administration of tricyclic antidepressants. PharmacoL Res. Commun. 12, 619 624. Serra G., Argiolas A.. Klimek V.. Fadda 1:. and Gessa G. L. (1979) Chronic treatment with antidepressants prevents the inhibitory effect of small doses of apomorphine on dopamine synthesis and motor activity, l,(/b Sei. 25, 415 424. Spyraki C. and Fibiger H. C. (1981) Behavioral evidence for supersensitivity of postsynaptic dopamine receptors in the mesolimbic system after chronic administration of desipramine. Eur. J. PharmacoL 74, 195 206. Steel R. G. l). and Torrie J. U. (1980) Prhwiples and Procedures
O/ Statistics:
4
Biomelrieal
Approach.
McGraw-Hill. New York. Waldmeier P. C. (1984) Effects of chronic antidepressant treatment on pre- and post-synaptic dopaminergic mechanisms. A short review of the literature and some corn-
Chronic antidepressant treatment on dopamine autoreceptors plementary experiments. Pol. J. Pharmacol. Pharm. 36, 201-216. Walters J. R. and Roth R. H. 0976) Dopaminergic neurons: An in viro system for measuring drug interactions with presynaptic receptors. Naunyn Schmied. Arch. Pharmacol. 296, 5-14. Waiters J. R., Roth R. H. and Aghajanian G. K. (1973) Dopaminergic neurons: Similar biochemical and histochemical effects of 7-hydroxybutyrate and acute lesions of
67
the nigroneostriatal pathway. J. Pharmacol. Exp. Ther. 186, 630q539. Welch J., Kim H., Fallon S. and Liebman J. (1982) Do antidepressants induce dopamine autoreceptor subsensitivity? Nature 298, 301 302. Willner P. (1983) Dopamine and depression: A review of recent evidence. III. The effects of antidepressant treatments. Brain Res. Rev. 6, 237-246.
0197-0186/86 $3.00 +0.00 Pergamon Journals Ltd
Printed in Great Britain
EFFECTS OF CHRONIC ANTIDEPRESSANT TREATMENT ON NIGROSTRIATAL A N D MESOLIMBIC DOPAMINE AUTORECEPTORS IN THE RAT J. A. NIELSEN* Department of Pharmacology, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio, U.S.A. (Received 12 September 1985; accepted 25 November 1985) Abstract--Dopamine autoreceptors were studied by determining the effects of chronic antidepressant treatment on the ability of several doses of apomorphine to decrease 3,4-dihydroxyphenylalanine accumulation (an index of dopamine synthesis in vivo) after saline or ).-hydroxybutyric acid lactone (~-butyrolactone). 3,4-Dihydroxyphenylalanine accumulation was measured in nigrostriatal [nucleus caudatus putamen] and mesolimbic [nucleus accumbens and tuberculum olfactorium] nerve terminals. Apomorphine decreased 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen, tuberculum olfactorium and nucleus accumbens in a dose-related manner. Chronic imipramine (10 days) treatment attenuated the low and high dose apomorphine-induced decrease in 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen to a greater extent than the tuberculum olfactorium or nucleus accumbens. In 7-butyrolactone-treated animals chronic treatment with imipramine, amitriptyline or bupropion (10 days) attenuated the low dose apomorphine effect in the nucleus caudatus putamen, but not the tuberculum olfactorium or nucleus accumbens. Only 2 days of imipramine treatment had no effect on the apomorphine-induced decrease in 3,4-dihydroxyphenylalanineaccumulation in the nucleus caudatus putamen with or without y-butyrolactone treatment. These data suggest that chronic treatment with three antidepressants produces dopamine autoreceptor subsensitivity in nigrostriatal neurons more than mesolimbic neurons and that this effect is not seen with short-term imipramine treatment.
The involvement of dopamine (DA) in the state of depression and the action of antidepressants was first suggested by Randrup and coworkers (1975). Several recent reports have investigated the possibility that chronic antidepressant treatment alters DA autoreceptors (for reviews see Willner, 1983; Waldmeier, 1984). Subsensitivity of DA autoreceptors after repeated antidepressant treatment has been suggested by the following findings. Serra and coworkers (1979, 1980) reported that chronic administration of several antidepressants were found to attenuate the reduction of motor activity and of striatal 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations induced by small doses of the DA agonist apomorphine which are thought to act preferentially on DA autoreceptors. This biochemical finding was replicated by Holcomb and coworkers (1982) although they suggest that this does not reflect subsensitivity of nerve terminal autoreceptors. Arnt and colleagues
(1984) showed that chronic treatment with citalopram attenuated the reduction of motor activity induced by low doses of apomorphine. Antelman and coworkers (Chiodo and Antelman, 1980, 1982; Antelman et al., 1982) found that long-term administration of several antidepressants attenuated the apomorphine-induced decrease in firing of DA neurons located within the zona compacta of the substantia nigra. This effect was found to depend on passage of time rather than daily drug administration (Chiodo and Antelman, 1982). Finally, Fibiger and Phillips (1981) found that chronic desipramine treatment enhanced self-stimulation obtained from electrodes in the ventromedial tegmentum. Other findings do not suggest that chronic antidepressant treatment produces DA autoreceptor subsensitivity. Spyraki and Fibiger (1981) showed that chronic desmethylimipramine did not alter the low dose apomorphine-induced decrease in motor activity of rats. Two studies found no change in apomorphine-induced inhibition of DA cell firing following chronic antidepressant treatment (Welch et al., 1982; MacNeil and Gower, 1982). Waldmeier
*Address correspondence to: Dr. J. A. Nielsen, Pfizer Central Research, Eastern Point Road, Groton, CT 06340, U.S.A. 61
62
,1. A. Naitst:,,
(1984) was unable to reproduce the biochemical findings of Serra a n d coworkers (1979, 1980) with long-term a d m i n i s t r a t i o n of imipramine, maprotiline and two e n a n t i o m e r s of oxaprotiline. Diggory and Buckett (1984) have shown that chronic treatment with four different antidepressants did not alter the low dose a p o m o r p h i n e - i n d u c e d decrease in the D A metabolites D O P A C or 3-methoxy-4-hydroxyphenylacetic acid (homovanillic acid) in rat striatum. Finally, H o l c o m b and coworkers (1982) employed a specific biochemical model of D A autoreceptor stimulation to determine the effects of chronic antidepressant treatment on these receptors. Following blockade of D A impulse flow with 7-hydroxybutyric acid lactone (7-butyrolactone, G B L ) (Waiters et al., 1973; Waiters and Roth, 1976), a p o m o r p h i n e stimulates D A autoreceptors, thus decreasing D A synthesis as evidenced by a decrease in 3,4-dihydroxyphenylalanine ( D O P A ) accumulation ( R o t h and Nowycky, 1977). H o l c o m b and coworkers (1982) found that long-term antidepressant t r e a t m e n t did not alter the effect of high doses of a p o m o r p h i n e in this model. The present study extends these findings by using lower doses of a p o m o r p h i n e . Most of the reports cited above investigated the effects of chronic antidepressants on nigrostriatal D A neurons. Maj (1984) has s h o w n that mesolimbic D A neurons may be altered by chronic antidepressant treatment. Chronic treatment with imipramine increased the l o c o m o t o r stimulation induced by da m p h e t a m i n e injected into the nucleus accumbens. However, the concentration of D A and its metabolites in the nucleus a c c u m b e n s were not significantly affected by chronic imipramine treatment. This suggests that perhaps the imipramine-induced increase in responsiveness to d - a m p h e t a m i n e may be due to postsynaptic changes in mesolimbic D A neurons. The aim of the present study was to determine the effects of chronic antidepressant treatment on a neurochemical index of nigrostriatal and mesolimbic D A autoreceptors in the rat. l m i p r a m i n e , amitriptyline and b u p r o p i o n were the antidepressants used because the first two are typical antidepressants while b u p r o p i o n is newer, "atypical," a n d affects DA neurons (Nielsen et al., in press). Two methods were used to specifically identify D A autoreceptors neurochemically: low dose a p o m o r p h i n e - i n d u c e d decrease in D O P A a c c u m u l a t i o n with and without G B L treatment. The major difference between the two models is that impulse flow is blocked when G B L is present, implying that any drug-induced changes in D O P A accumulation results from effects on nerve terminal
autoreceptors. Without GBL, drug-induced changes in impulse flow can also produce indirect effects on D O P A synthesis. By using both methods we hope to investigate the effects of chronic antidepressant treatmenl on D A autoreceptor and n o n - a u t o r e c e p t o r influence on DA synthesis. This study expands on the work of H o l c o m b and coworkers (1982) in that different antidepressants were used, mesolimbic neurons were analyzed, and the effects of lower doses of a p o m o r p h i n e were investigated. EXPERIMENT&I. PROCEDURES Male Sprague Dawle~ rals (liarlan Spraguc Dawley, Inc., Haslett, MI: or Zivic-Miller Labs, Allison, PA) weighmg 175 200 g were maintained under 12 13 periods of light and dark. Rats were housed 3 4cage and had access to standard rat chow (Wayne t.ab Blox: Allied Mills. Chicago, 1I,) and tap water ad lihitum. The rats were treated with apomorphine HC1 (Eli Lilly Co., Indianapolis, IN: 30, 100 or 300 pg/kg, s.c.) or saline 35 rain before and an inhibitor of aromatic I-amino-acid decarboxylasc, 3-hydroxybenzylhydrazmc HC1 (NSD 1015: Sigma Chemical Co., Saint Louis, MO: 100mg/kg, i.p.) 30min before decapitation. In a second experiment, imipramine HCI (Geigy, Ardsley, NY: 10mg/kg, s.c.) or saline was administered twice a day (BID) (about 1000 and 2200 h) for 10 days. Three days after the last treatment the animals were injected with apomorphine HC1 (30, 100 or 300 pg/kg, s.c.) or saline and NSD 1015 as described above. A third experiment was performed as described above, except that GBL (750 mg/kg, i.p.) was administered 1o some animals 35 rain bel\~re decapitation. The fourlh cxpermlent inw)Ived chronic administration of imipramine HC1 (10 mgkg, s.c.), amitriptylinc HCI (Merck Sharp and Dohme Research Lab, Rahway, N J: I0 mg/kg, s.c.), bupropion HCI (Burroughs Wellcome Co., Research Triangle Park, NC; 30 mg/kg, s.c.) or saline twice a (lay (at about 1000 anti 2200 h) for l0 days. Three days after the last treatment the rats received apomorphine, GBL and,'or saline plus NSD 1015 as described above. The fifth and final experiment involved treatment with imipraminc HCI (10mg'kg, s.c.) for 10 days, imipramine tICI (10mg/kg, s.c.) for 2 days and saline l\)r 8 days, or saline twice a day for 10 days. Three days after the last treatmenl {when essentially all of the antidepressant had left the body) the rats received apomorphine, GBL a n d o r saline plus NSD 1015 as described above. The m rit'o synthesis of DA was estimated in the DA nerve terminals of the nigroslriatal [nucleus caudatus putamen (CP)] and mesolimbic [nucleus accumbens (NA) and tuberculum olfaclorium (-I"O)l neurons by measurirtg the accumulation of DOPA after NSD 1015 treatment. After decapitation the brain was quickly removed from the skull. lhe tissues mentioned above dissected as described previously (Roth and Nowycky, 1977L and the tissues homogenized such that approximately lllmg tissue was homogenized in 1001~1 cold 0.2N perchloric acid containing 10rag% ethyleneglycol-hiv-(/~-amino-ethyl ether) N,N'tetraacetic acid or high pressure liquid chromotography (LC) buffer (see below). Following 30 s centrifugafion in it microfuge (Beckman, Fullerton, CAI, DOPA concentrations in the samples was analyzed by radioenzymatic
Chronic antidepressant treatment on dopamine autoreceptors assay or LC with electrochemical detection (EC). The radioenzymatic assay standards (125 2000 ng DOPA) and supernatants of perchloric acid extracts of tissue were incubated in the presence of catechol-o-methyltransferase (EC 2.1.1.6) and [3H]S-adenosyl-L-methionine. The resuiting tritiated o-methylated product of DOPA, 3-methoxytyrosine, was separated from interfering omethylated catecholamines and unreacted [3H]S-adenosyl-Lmethionine by cation exchange chromatography, absorption onto activated charcoal followed by anion exchange chromatography (Demarest and Moore, 1980). The DOPA content of each sample was calculated directly from standards after subtracting blank values. Blank values were obtained by assaying duplicate 10/11 aliquots of perchloric acid solution. The sensitivity of the assay (the amount of DOPA yielding counts at least twice the blank) was about 125 pg. Individual brain regions from each experiment were analyzed for DOPA in the same assay and treatment effects were compared only to control values determined in that assay. For the LCEC assay, the supernatant from each sample was injected via a fixed loop (Rheodyne, Cotati, CA) onto a 30cm × 3.9mm i.d. /~Bondapak C m column (Bioanalytical Systems, West Lafayette, IN). A model LC 9533 (IBM Instruments, Wallingford, CT) liquid chromatographic system was used with an electrochemical detector (IBM Instruments). Detector potential was set at +0.75 V and the reference electrode was Ag+/AgC1. The mobile phase contained 0.1 M citrate-phosphate, 0.1 mM ethylene-diamine-tetraacetic acid, and methanol. The methanol (0-7%) and pH (2 3.6) were altered in order to maintain separation of peaks as the column aged. Details of the chromatographic analysis have been published previously (Nielsen et al., 1982). The concentrations of DOPA in each sample was determined by comparing areas under the curve (CS 9000 computer, IBM Instruments) for samples and standard analyzed the same day. The sensitivity of the assay varied inversely with the age of the #Bondapak column. The sensitivity was typically about 50 pg, and never less than 200 pg per sample. In all samples, the pellet was analyzed for protein (Lowry et al., 1951). There was no significant difference between controls using the radioenzymatic or LCEC assays. The effects of treatments were determined using a oneway analysis of variance followed by StudentNewman-Keuls' test (Steel and Torrie, 1980). A critical value of P < 0.05 was set as that required to indicate a statistically reliable effect of experimental manipulation.
6O 4O 2O _
0
I
§ g 6o ~ 2o u '~
NCI. 9/I--E
I
To
0
13_
I
I
I
I
L NA
4O 2O 0
I
I
I
1
0
50
100
500
Apomorphine
(/J.glkg )
Fig. 1. Effect of chronic treatment with imipramine on apomorphine-induced decrease in DOPA accumulation. Three days after a 10-day treatment with imipramine (10mg/kg, s.c., BID) (A) or saline (O), apomorphine (s.c.) or sodium metabisulfite vehicle was administered at 35min and NSD 1015 (100mg/kg, i.p.) at 30min before sacrifce. Each data point represents the mean (M) and the vertical line one standard error (SE) of t~8 determinations. Where no vertical line is depicted, the SE is less than the radius of the symbol. Variability around the zero apomorphine points is not shown for clarity, but it was always less than 12%. The only significant differences (P < 0.05) between animals receiving chronic saline and chronic imipramine were at doses of 30 and 300/lg/kg apomorphine in the CP. The following are baseline (saline and NSD 1015 treatment) values (ng/mg protein, M + I S E ) for DOPA accumulation: Chronic
RESULTS
There was no significant difference in the baseline c o n c e n t r a t i o n s of D O P A a c c u m u l a t i o n in any brain region between experiments (legends to Figs 1 a n d 2, Tables 1 a n d 2). A p o m o r p h i n e (30, 100 a n d 300/lg/kg) p r o d u c e d a dose-related decrease in D O P A a c c u m u l a t i o n in the CP, T O a n d N A (experim e n t 1, data not shown). C h r o n i c i m i p r a m i n e - t r e a t m e n t a t t e n u a t e d the apom o r p h i n e (30 a n d 3 0 0 / t g / k g ) - i n d u c e d decrease in D O P A a c c u m u l a t i o n in the C P (experiment 2, Fig. 1). C h r o n i c imipramine t r e a t m e n t had no effect o n the a p o m o r p h i n e - i n d u c e d reduction in D O P A accumulation in the T O or N A (Fig. 1).
63
CP TO NA
Saline
Imipramine
26.8 4- 1.4 11.9 + 1.1 28.0 4- 3.4
16.4 + 1.4 9.3 + 1.1 20.9 4- 3.5
In G B L - t r e a t e d animals, chronic imipraminet r e a t m e n t a t t e n u a t e d the low dose a p o m o r p h i n e effect in the CP, b u t not the effects o f higher doses in the CP, or any dose of a p o m o r p h i n e in the T O a n d N A (experiment 3, Fig. 2). In experiment 4, these effects of chronic i m i p r a m i n e - t r e a t m e n t were replicated a n d ami-
64
J.A. NIELSEN
'°°F 6O 4O 2O 0
I
J
J
I
I TO
~_
60
2o o '~ n 0
0
1
[
i
J
I NA
100
6O 4O
morphine [Tables I and 2, ( - ) G B L ( )APO] and with GBL, but without apomorphine [Tables I and 2, ( + )GBL( - )APO]. The effects of the lowest dose of apomorphine (30 l~g/kg) in animals chronically treated with imipramine for 10 days in experiment 5 was similar to that mentioned above for experiments 2~4. Ten days" treatment with imipramine produced the following effects in the CP: (1) no effect on animals treated with neither GBL nor apomorphine [Table 2, (-)GBL(-)APO]; (2) an attenuation of the apomorphine-induced decrease in D O P A accumulation [Table 2, ( - )GBL( + )APO]; and (3) a similar attenuation in animals treated with GBL and apomorphine [Table 2, ( + ) G B L ( + )APO]. Two days" treatment with imipramine produced none of the effects described above (Table 2). Neither 2 nor 10 days' treatment with imipramine had any effect on apomorphine- and/or GBL-induced changes in D O P A accumulation in the TO or N A (Table 2).
2O 0
J 0 nB°k
I 0
I 30
Apomorphine
I 100
J 300
Fig. 2. Effect of chronic imipramine treatment on apomorphine-induced decline of DOPA accumulation in terminals of nigrostriatal and mesolimbic DA neurons in GBL-treated rats. Three days after a 10-day treatment with imipramine (10 mg/kg, s.c., BID) (A) or saline (©), GBL (750mg/kg, i.p.) or saline was administered as 35min, apomorphine (s.c.) or sodium metabisulfite vehicle at 35 min and NSD 1015 (100 mg/kg, i.p.) at 30 min before sacrifice. Each data point represents the mean (M) and the vertical line one standard error (SE) of 6 8 determinations. Variability around the zero apomorphine points is not shown for clarity, but it was always less than 12%. The only significant difference (P <0.05) between animals receiving chronic saline and chronic imipramine was at a dose of 30 #g/kg apomorphine in the CP. The following are baseline (saline, GBL and NSD 1015 treatment) values (ng/mg protein, M + I SE) for DOPA accumulation. Chronic CP TO NA
DISCUSSION
(b~g/kg)
Saline
lmipramine
30.1 + 2.4 13.1 + 1.4 35.8 + 4.4
40.0 _+2.6 12.3 + t.0 29.7 + 2.1
triptyline and bupropion produced similar effects (Table 1). Imipramine, amitriptyline and bupropion attenuated the apomorphine-induced decrease in D O P A accumulation in the CP but not the TO or NA. Chronic imipramine-, amitriptyline-, or bupropion-treatment had no effect on D O P A accumulation in animals treated without GBL and apo-
Chronic treatment with imipramine, amitriptyline and bupropion attenuates the inhibition of D O P A accumulation in the CP by apomorphine. This may be due to a change in DA autoreceptors. This effect occurs at a low dose of apomorphine, which may be preferentially activating DA autoreceptors, and at a low dose of apomorphine in GBL-treated animals which probably represents DA autoreceptor stimulation. Chronic antidepressant treatment did not alter the decrease in D O P A accumulation induced by high doses of apomorphine in animals treated with GBL, but it did in rats not treated with GBL. This probably suggests that antidepressants have several effects on DA neurons. First of all, when GBL is present they produce autoreceptor subsensitivity to very low doses of apomorphine. The response to higher doses is not altered, presumably since higher doses of apomorphine can overcome the subsensitivity. However, when impulse flow is not blocked, antidepressants can exert a second kind of effect on DA neurons which attenuates the response to high doses of apomorphine. The ability of antidepressants to exert this second effect may depend on impulse flow through postsynaptic feedback mechanisms. Chronic imipramine treatment did attenuate the D O P A accumulation lowering effect of a high dose of apomorphine in animals not treated with GBL. This effect was not seen in animals treated with GBL. This suggests that chronic imipramine may produce changes on postsynaptic DA receptors and/or indi-
Chronic antidepressant treatment on dopamine autoreceptors
65
Table 1. Effects of chronic antidepressant treatment on a test for DA autoreceptor sensitivity Pretreatment
( - ) GBL ( - ) APO
Saline Imipramine Amitriptyline Bupropion
48 ± 53 ± 50 ± 45 ±
8 7 6 9
Saline Imipramine Amitriptyline Bupropion
63 ± 54 ± 67 ± 59 ±
7 9 11 6
Saline lmipramine Amitriptyline Bupropion
58 ± 58 ± 67 ± 61 ±
6 9 4 7
( + ) GBL ( - ) APO
( + ) GBL ( + ) APO
Caudatus Putamen (CP) 100 ± 8 100 ± 7 100 ± 11 100 ± 10 Tuberculum Olfactorium (TO) 100 ± 5 100 _+ 11 100 ± 9 100 ± 7 Nucleus Accumbens (NA) 100 + 11 100 ± 9 100 ± 8 100 ± 9
77 ± 95 ± 111 ± 93 ±
8 7* 10" 5*
82 ± 77 ± 80 ± 95 ±
10 6 9 6
71 ± 77 ± 75 ± 79 ±
8 10 4 9
Three days after a 10 day treatment with saline, imipramine (10 mg/kg, s.c., BID), amitriptyline (10mg/kg, s.c., BID) or bupropion (30mg/kg, s.c., BID), GBL (750mg/kg, i.p.) or saline was administered at 35 minutes, apomorphine (30 pg/kg, s.c.) or saline at 35 min and NSD 1015 (100 mg/kg, i.p.) at 30 rain before sacrifice. Values (percentage of the baseline values listed below) represent the mean (M) ± one standard error (SE) of 6~7 determinations. *Significantly (P < 0.05) different from saline pretreatment. The following are baseline (saline, GBL and NSD 1015 treatment) values (ng/mg protein, M ± 1 SE) for D O P A accumulation. Saline CP TO NA
rectly on DA because
neurons.
these
studies
31.4±1.9 16.7±1.3 28.6±2.3
Chronic Imipramine Amitriptyline 33.0±2.9 15.8±2.0 26.4±2.3
This finding was not pursued
However,
concentrated
cussing
on
DA
auto-
receptors. Chronic
Bupropion
39.3±5.4 13.9±1.1 31.1±3.7
29.8±3.3 16.8±1.7 29.8+3.4
it is n e c e s s a r y regional
effects of antidepressants antidepressant
striatal more
than
treatment
mesolimbic
DA
affected
nigro-
nerve terminals.
are much limbic
to exercise caution
differences,
needs
further
findings
in dis-
since
in the mesolimbic
less clear cut. Our
regions
especially
in the meso-
verification,
since, for
Table 2. Effect of 2 and 10 day treatment with imipramine on a test for DA autoreceptor sensitivity Pretreatment
( - ) GBL ( - ) APO
Saline Imipramine x 2 Imipramine × 10
43 ± 6 48 _+ 5 45 ± 7
Saline Imipramine x 2 Imipramine × 10
60 _+ 8 59 ± 6 69 ± 7
Saline Imipramine x 2 lmipramine x 10
68 ± 7 61 ± 6 60 ± 9
( - ) GBL ( + ) APO
( + ) GBL ( - ) APO
Caudatus Putamen (CP) 29 ± 4 100 _+ 2 30 ± 5 100 ± 9 50 ± 6* 100 ± 11 Tuberculum Olfactorium (TO) 36 ± 5 100 + 7 39 ± 8 100 + 8 43 ± 5 100 + 6 Nucleus Accumbens (NA) 42 ± 8 100 ± 12 45 ± 4 100 ± 6 39 ± 7 100 ± 8
( + ) GBL ( + ) APO 71 ± 6 79 + 9 102 ± 11" 85 ± 11 79 ± 6 91 ± 8 82 ± 7 82 ± 7 91 ± 6
Three days after treatment with saline for l0 days, imipramine for 2 days then saline for 8 days, or imipramine for 10 days, GBL (750 mg/kg, i.p.) or saline was administered at 35 min, apomorphine (30 #g/kg, s.c.) or saline at 35 min and NSD 1015 ( 100 mg/kg, i.p.) at 30 rain before sacrifice. Values (percentage of the baseline values listed below) represent the mean (M) _+ one standard error (SE) of 5-7 determinations. *Significantly (P < 0.05) different from saline pretreatment. The following are baseline (saline, GBL and NSD 1015 treatment) values (ng/mg protein, M + I S E ) for DOPA accumulation.
CP TO NA
Saline
Chronic Imipramine x 2
Imipramine x 10
25.7 ± 2.6 12.8 ± 1.6 23.5 ± 2.5
23.9 ± 1.1 14.1 ± 1.1 24.8 + 2.1
28.0 ± 3.0 13.8 ± 2.4 26.9 ± 1.9
the
regions
66
J.A. Nlt!I.SEN
example, we were unable to c o r r o b o r a t e previous studies which reported that mesolimbic synthesism o d u l a t i n g autoreceptors are more sensitive to apom o r p h i n e than nigrostriatal autoreceptors in the G B L model (Demarest et al., 1983: Roth and Nowycky, 1977). However, it is not surprising that long-term antidepressant a d m i n i s t r a t i o n had greater effects in one brain area t h a n another, since K a r o u m and coworkers (1984) reported that D A neurons in the striatum are more sensitive to chronic antidepressant t r e a t m e n t than D A neurons in the hypothalamus. A n t e l m a n and coworkers showed that 2 days treatment with imipramine followed by 10 days administration of saline attenuated autoreceptor stimulation by low doses of a p o m o r p h i n e inhibiting the firing o f DA cells in the substantia nigra (Chiodo and Antelman, 1982). We were unable to find any biochemical correlation to this effect. It may not be possible to correlate electrophysiological findings concerning the sensitivity of impulse-regulating autoreceptors in the substantia nigra with the biochemical methods described herein. The importance of timing in chronic antidepressant treatment effects on D A neurons requires more research. in summary, chronic antidepressant treatment produces D A autoreceptor subsensitivity in nigrostriatal but not mesolimbic neurons. This effect is not seen with short-term imipramine treatment. The author thanks Dr Kenneth Moore for his assistance with the work described herein, and Susan Stahl for valuable technical assistance. This work was supported in part by USPHS grant NS 15911. Acknowledgenwnts
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