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Chronic imipramine reduces [3H]SCH 23390 binding and DA-sensitive adenylate cyclase in the limbic system
European Journal of Pharmacology, 167 (1989) 299-303
299
Elsevier EJP 20445 Short communication
Chronic imipramine reduces [3HISCH 23390 binding and DA-sensitive adenylate cyclase in the limbic system Graziella M. D e M o n t i s *, P a o l a D e v o t o , G i a n Luigi G e s s a a, D o m e n i c o Meloni, A n n a Porcella, Pierluigi Saba, G i n o Serra ~ a n d A l e s s a n d r o T a g l i a m o n t e Institute of Pharmacology and Biochemical Pathology, J Department of Neuroscience, University of Cagliari, Via Porcell 4, 09100 Cagliari, Italy
Received 8 June 1989, accepted 20 June 1989
[3H]SCH 23390 binding and dopamine (DA)-stimulated adenylate cyclase activity were measured in brain membrane preparations from rats chronically treated with imipramine (10 mg/kg twice daily for 14 days). [3H]SCH 23390 binding sites were decreased by 27% in the limbic system but by only 14% in the striatum. The responsiveness of adenylate cyclase to DA was reduced by 38% in the limbic system but was not modified in the striatum. Concomitant treatment with a-methyltyrosine (a-MPT) (50 mg/kg daily for 14 days) prevented the imipramine-induced reduction in both [3H]SCH23390 binding sites and the responsiveness of adenylate cyclase to DA. Imipramine; Dopamine D-1 receptors; Adenylate cyclase; a-Methyltyrosine
1. Introduction
The failure of acutely administered tricyclic antidepressants to modify dopaminergic transmission has been considered a strong argument against the possible involvement of brain dopamine (DA) in the mechanism of antidepressant action of these compounds. However, recent data have shown that chronic treatment with different antidepressants results in a number of persistent changes not only in noradrenergic and serotoninergic transmission (Zis and Goodwin, 1979) but in dopaminergic transmission as well (Maj, 1986). Indeed, chronic administration of different antidepressants results in potentiation of the behavioral responses to apomorphine, amphetamine and other DA agonists, which are considered to be
* To whom all correspondenceshould be addressed: Institute of Pharmacologyand BiochemicalPathology, University of Cagliari, Via Porcell 4, 09100 Cagliari, Italy.
mediated by activation of the D-2 receptors in the limbic system (Spyraki and Fibiger, 1981). In contrast to the behavioral evidence, binding studies have failed to detect changes in D-2 receptors, labelled with [3H]spiperone, in the limbic areas or in the striatum after long-term treatment with antidepressants (Klimek and Nielsen, 1987). More recently, however, it has been reported that chronic treatment with different antidepressants decreases the number of D-1 receptors, labelled with [3H]SCH 23390, both in the striatum and to a greater extent in the limbic areas (Klimek and Nielsen, 1987), raising the problem of whether D-1 receptor subsensitivity is correlated to the behavioral and therapeutic effect of these compounds. Since D-1 receptors are positively coupled to adenylate cyclase, we have studied whether the imipramine-induced decrease in D-1 receptor density is associated with a parallel decrease in the responsiveness of adenylate cyclase to DA. Moreover, we have studied whether the imipramine-in-
0014-2999/89/$03.50 © 1989 ElsevierSciencePublishers B.V. (BiomedicalDivision)
300 duces changes are prevented by a-methyltyrosine (a-MPT), an inhibitor of catecholamine biosynthesis.
2. Materials and methods
2.1. Animals Experiments were carried out with male Sprague Dawley rats (Charles River, Como, Italy, 175-200 g). The animals were kept in a controlled environment with a temperature of 22 ° C, a 12 h light-dark cycle, and water and standard laboratory food available ad libitum. A group of rats was treated 2 times daily for 2 weeks with imipramine, 10 m g / k g i.p.; a second group received a combination of imipramine and ct-MPT, the latter was given at a dose of 50 m g / k g i.p. once a day; the controls received saline 1 ml/kg. The rats were decapitated 24 h after the last injection and the striata and the limbic system (containing the olfactory tubercle, the nucleus accumbens, and the septum) were dissected out and used immediately in the adenylate cyclase assay or frozen at - 8 0 ° C for the evaluation of [3H]SCH 23390 binding and DA and DOPAC concentrations.
2.2. Adenylate cyclase Adenylate cyclase activity was assayed in membranes prepared from the striata and limbic system according to Olianas et al. (1983). The results are expressed as pmol of cAMP formed per mg of protein per rain or as a percentage of the basal values. The concentrations of DA and forskolin required to induce half-maximal activation of adenylate cyclase activity (km) and maximal activation (Vm~x) were calculated by linear regression analysis of Eadie-Hofstee plots of the concentration-response curves. Protein concentrations were determined with the method of Lowry et al. (1951).
2.3. [3H]SCH 23390 binding The tissues were homogenized with a Polytron PT 10 for 20 s in 100 volumes (w/v) of ice-cold 50
mM Tris-HC1 buffer, p H 7.4, and centrifuged for 10 rain at 48 000 × g. The pellet was washed once by resuspension and recentrifugation in 100 volumes of the same buffer. The final pellet was resuspended in 200 volumes of 50 mM Tris-HC1 buffer, p H 7.4, containing 120 mM NaC1, 5 mM KC1, 2 mM CaC12 and 1 mM MgC12. Binding assays were performed according to the method of Billard et al. (1985), with minor modifications. Briefly, 400 /~1 of membrane suspension was incubated with various concentrations of [3H]SCH 23390 (spec. act. 85 C i / m m o l , Amersham), ranging from 0.098 to 3 nM, in a final volume of 0.5 ml. Non-specific binding was determined in the presence of 10 /~M cisflupentixol. The reaction was stopped after 20 min incubation at 37 o C, by the addition of 4 ml of ice-cold Tris HC1 buffer, p H 7.4, and the mixture was filtered under vacuum through Whatman G F / B filters.
2.4. Determination of DA and DOPAC Dopamine and DOPAC were measured by H P L C (Waters) with electrochemical detection (Waters 460) according to Giorgi et al (1987). The concentrations of DA and DOPAC were calculated from the ratio of their detector response relative to the response of the internal standard introduced into the calibration curve equation obtained by linear regression analysis with a Water 745 data module integrator.
3. Results
In agreement with previous observations we found that chronic treatment with imipramine decreased the number of [3H]SCH 23390 binding sites by 27% in the limbic system and by 14% in the striatum, there was no change in the affinity of [3H]SCH 23390 for the D-receptors in either area (table 1). Figure 1 shows that chronic imipramine markedly reduced the stimulatory effect of DA on adenylate cyclase in the limbic system, but failed to modify the DA response in the striatum. As shown in table 1, the concomitant administration of a-MPT at the daily dose of 50 m g / k g pre-
301 TABLE 1 [3H]SCH 23390 binding and DA-stimulated adenylate cyclase activity in the striatum and limbic system oI rats chronically treated with imipramine or imipramine + a-MPT. Data are the means_+ S.E. of at least four experiments performed in triplicate. Bmax is expressed as f m o l / m g protein, K a as nM, Vmax as p m o l / m g protein per min, K m as /~M. Statistical analysis was done with a two-tailed Student's t-test. Striatum
[3H]SCH 23390 binding D A stimulated A.C.
Limbic system
Saline
Imipramine
Imipramine + a-MPT
Saline
Imipramine
Imipramine + a-MPT
Bm~ Kd
780.4 -t-45.1 0.41+ 0.03
672.5 +33.6 a 0.35-l- 0.03
802.5 +59.0 0.38+ 0.04
220.4 +18.8 0.45+ 0.04
160.9 +20.3 0.40+ 0.03
251.0 +21.9 0.43+ 0.04
Vmax
96.8 + 5.5 4.71-1- 0.41
94.1 + 6.2 4.60+ 0.55
100.4 + 7.5 4.23+ 0.33
33.3 + 4.6 6.78 + 0.91
20.9 + 3.8 a 6.19+ 0.71
37.8 + 4.1 6.49+ 0.58
Km
a p < 0.05 with respect to control values.
vented both the imipramine-induced reduction in [3H]SCH 23390 binding sites and in the responsiveness of adenylate cyclase to DA. Treatment with a-MPT reduced the DA and DOPAC content by 16 and 24%, respectively, in the limbic system, but failed to modify DA metabolism in the striatum.
4. Discussion
The present study confirms a previous observation that chronic imipramine treatment results in a decrease in the number of D-1 receptors labelled with [3H]SCH 23390, an effect that is more pro-
STRIATUM
OF
BASAL
LIMBIC
SYSTEM
ACTIVITY
180 140 160
t! 140 120 12o
1oo
I
i
--6
--5 LOG
[DA]
J
--4
i
--3
100
--7
i
--6
i
--5 LOG
--4
i
--3
[DA]
Fig. 1. Adenylate cyclase activity in rats chronically treated with imipramine. Values are expressed as percentages of the basal activity (126.66 p m o l / m g protein per rain ( o ) controls and 112.12 p m o l / m g protein per min (O) imipramine in the striatum; 60.66 p m o l / m g protein per rain ( o ) controls and 62.78 p m o l / m g protein per min (e) imipramine in the limbic system). Each value is the mean + S.E. of six separate experiments. Statistical analysis of the response to individual agonist concentrations was done with a two-tailed Student's test; * P < 0.01 versus the corresponding value for the control.
302 nounced in the limbic system than in the striatum. Moreover, we found that the decrease in D-1 receptor density was associated with a reduced response of adenylate cyclase to the stimulatory effect of DA. The latter effect was restricted to the limbic system and was absent in the striatum. The question arises how imipramine reduced both the number of D-1 receptors and the sensitivity of adenylate cyclase to DA, and whether these effects are correlated to the imipramine-induced potentiation of those behavioral effects of D A agonists that are considered to be mediated by D-2 receptors in the limbic areas (Spyraki and Fibiger, 1981). While no changes in D-2 receptors have been observed after chronic antidepressant treatment (Klimek and Nielsen, 1987), recent studies indicate that the concomitant activation of D-1 receptors is needed for the motor response to D-2 receptors stimulation to occur (Longoni et al., 1987). In order to reconcile the apparent paradox of behavioral facilitation associated with the reduction in D-1 receptor number and in the sensitivity of adenylate cyclase to DA, it might be suggested that, despite this down-regulation, the formation of cyclic-AMP in the limbic structures is sufficiently high to facilitate D-2 r~ceptor activation. This hypothesis implies that adenylate cyclase activation in the limbic system takes place at a step distal to the D-1 receptor itself. Such a possibility is supported by the recent finding that chronic electroconvulsive shock and chronic desipramine treatment increase adenylate cyclase activity in cortical, cerebellar and hippocampal membranes in responses to forskolin, an agent acting distal to the receptor at either the nucleotide binding or the catalytic subunit of adenylate cyclase (Newman and Lererl 1989). We have extended this finding recently by showing that chronic imip'ramine treatment increases the adenylate cyclase response to forskolin in limbic areas (De Montis et al., in preparation). According to our hypothesis, a reduction in D-1 receptor number and in the sensitivity of adenylate cyclase to D A is an adaptive mechanism to compensate for the post-receptor-mediated increase in adenylate cyclase activity. In agreement with such an interpretation, we can explain
the finding that a - M P T treatment prevented the occurrence of down-regulation of D-1 receptors by suggesting that the compensatory mechanism does not need to operate when dopaminergic neurotransmission is reduced. Irrespective of the mechanism involved, the question arises as to whether down-regulation of the D-1 receptor complex in the limbic areas might be correlated with the antidepressant action of imipramine, and whether the observed changes are specific for imipramine or are a general characteristic of antidepressant treatment. Research in our laboratory has shown that an increase in forskolin-stimulated adenylate cyclase activity is produced in the limbic area not only by imipramine but also by repeated ECS and that this increase is associated with downregulation of D-1 receptors.
Acknowledgements The authors wish to thank Mrs. Graziella Satta for technical assistance. This work was supported by a grant from Ministero della Pubblica Istruzione.
References Billard, W., V. Ruperto, G. Crosby, L.C. Iorio and A. Barnett, 1985, Characterization of the binding of [3H]SCH 23390, a selective D-1 receptor antagonist ligand, in rat striatum, Life Sci 5, 1885. Giorgi, O., G. De Montis, M.L. Porceddu, S. Mele, G. Calderini, G. Toffano and G. Biggio, 1987, Developmental and agerelated changes in D-1 dopamine receptores and dopamine content in the rat striatum, Develop. Brain Res. 35, 283. Klimek, V. and M. Nielsen, 1987, Chronic treatment with antidepressants decreases the number of [3H]SCH 23390 binding sites in rat striatum and the limbic system, European J. Pharmacol. 139, 163. Longoni, R., L. Spina and G. Di Chiara, 1987, Permissive role of D-1 receptor stimulation by endogenous dopamine for the expression of postsynaptic D-2 mediated behavioural responses, European J. Pharmacol. 134, 163. Lowry, O.H., N.J. Rosenbrough, A.L. Farr and R.J. Randall, 1951, Protein measurement with the Folin phenol reagent, J. Biol. Chem. 193, 265. Maj, J., 1986, Repeated treatment with antidepressant drugs: responses mediated by brain dopamine receptors, in: New Results in Depression Research, eds. H. Hippius, G.L. Klerman and N. Matussek (Springer-Verlag, Berlin, Heidelberg) p. 90.
303 Newman, M.E. and B. Lerer, 1989, Post-receptor-mediated increases in adenylate cyclase activity after chronic antidepressant treatment: relationship to receptor desensitization, European J. Pharmacol. 162, 345. Olianas, M.C., P. Onali, N.H. Neff and E. Costa, 1983, Adenylate cyclase activity of synaptic membranes from rat striatum, Mol. Pharmacol. 23, 393.
Spyraki, C. and H.C. Fibiger, 1981, Behavioural evidence for supersensitivity of postsynaptic dopamine receptors in the mesolimbic system after chronic administration of desipramine, European J. Pharmacol. 74, 195. Zis, A.P. and F.K. Goodwin, 1979, Novel antidepressants and the biogenic amine hypothesis of depression, Arch. Gen. Psychiatr. 36, 1097.
299
Elsevier EJP 20445 Short communication
Chronic imipramine reduces [3HISCH 23390 binding and DA-sensitive adenylate cyclase in the limbic system Graziella M. D e M o n t i s *, P a o l a D e v o t o , G i a n Luigi G e s s a a, D o m e n i c o Meloni, A n n a Porcella, Pierluigi Saba, G i n o Serra ~ a n d A l e s s a n d r o T a g l i a m o n t e Institute of Pharmacology and Biochemical Pathology, J Department of Neuroscience, University of Cagliari, Via Porcell 4, 09100 Cagliari, Italy
Received 8 June 1989, accepted 20 June 1989
[3H]SCH 23390 binding and dopamine (DA)-stimulated adenylate cyclase activity were measured in brain membrane preparations from rats chronically treated with imipramine (10 mg/kg twice daily for 14 days). [3H]SCH 23390 binding sites were decreased by 27% in the limbic system but by only 14% in the striatum. The responsiveness of adenylate cyclase to DA was reduced by 38% in the limbic system but was not modified in the striatum. Concomitant treatment with a-methyltyrosine (a-MPT) (50 mg/kg daily for 14 days) prevented the imipramine-induced reduction in both [3H]SCH23390 binding sites and the responsiveness of adenylate cyclase to DA. Imipramine; Dopamine D-1 receptors; Adenylate cyclase; a-Methyltyrosine
1. Introduction
The failure of acutely administered tricyclic antidepressants to modify dopaminergic transmission has been considered a strong argument against the possible involvement of brain dopamine (DA) in the mechanism of antidepressant action of these compounds. However, recent data have shown that chronic treatment with different antidepressants results in a number of persistent changes not only in noradrenergic and serotoninergic transmission (Zis and Goodwin, 1979) but in dopaminergic transmission as well (Maj, 1986). Indeed, chronic administration of different antidepressants results in potentiation of the behavioral responses to apomorphine, amphetamine and other DA agonists, which are considered to be
* To whom all correspondenceshould be addressed: Institute of Pharmacologyand BiochemicalPathology, University of Cagliari, Via Porcell 4, 09100 Cagliari, Italy.
mediated by activation of the D-2 receptors in the limbic system (Spyraki and Fibiger, 1981). In contrast to the behavioral evidence, binding studies have failed to detect changes in D-2 receptors, labelled with [3H]spiperone, in the limbic areas or in the striatum after long-term treatment with antidepressants (Klimek and Nielsen, 1987). More recently, however, it has been reported that chronic treatment with different antidepressants decreases the number of D-1 receptors, labelled with [3H]SCH 23390, both in the striatum and to a greater extent in the limbic areas (Klimek and Nielsen, 1987), raising the problem of whether D-1 receptor subsensitivity is correlated to the behavioral and therapeutic effect of these compounds. Since D-1 receptors are positively coupled to adenylate cyclase, we have studied whether the imipramine-induced decrease in D-1 receptor density is associated with a parallel decrease in the responsiveness of adenylate cyclase to DA. Moreover, we have studied whether the imipramine-in-
0014-2999/89/$03.50 © 1989 ElsevierSciencePublishers B.V. (BiomedicalDivision)
300 duces changes are prevented by a-methyltyrosine (a-MPT), an inhibitor of catecholamine biosynthesis.
2. Materials and methods
2.1. Animals Experiments were carried out with male Sprague Dawley rats (Charles River, Como, Italy, 175-200 g). The animals were kept in a controlled environment with a temperature of 22 ° C, a 12 h light-dark cycle, and water and standard laboratory food available ad libitum. A group of rats was treated 2 times daily for 2 weeks with imipramine, 10 m g / k g i.p.; a second group received a combination of imipramine and ct-MPT, the latter was given at a dose of 50 m g / k g i.p. once a day; the controls received saline 1 ml/kg. The rats were decapitated 24 h after the last injection and the striata and the limbic system (containing the olfactory tubercle, the nucleus accumbens, and the septum) were dissected out and used immediately in the adenylate cyclase assay or frozen at - 8 0 ° C for the evaluation of [3H]SCH 23390 binding and DA and DOPAC concentrations.
2.2. Adenylate cyclase Adenylate cyclase activity was assayed in membranes prepared from the striata and limbic system according to Olianas et al. (1983). The results are expressed as pmol of cAMP formed per mg of protein per rain or as a percentage of the basal values. The concentrations of DA and forskolin required to induce half-maximal activation of adenylate cyclase activity (km) and maximal activation (Vm~x) were calculated by linear regression analysis of Eadie-Hofstee plots of the concentration-response curves. Protein concentrations were determined with the method of Lowry et al. (1951).
2.3. [3H]SCH 23390 binding The tissues were homogenized with a Polytron PT 10 for 20 s in 100 volumes (w/v) of ice-cold 50
mM Tris-HC1 buffer, p H 7.4, and centrifuged for 10 rain at 48 000 × g. The pellet was washed once by resuspension and recentrifugation in 100 volumes of the same buffer. The final pellet was resuspended in 200 volumes of 50 mM Tris-HC1 buffer, p H 7.4, containing 120 mM NaC1, 5 mM KC1, 2 mM CaC12 and 1 mM MgC12. Binding assays were performed according to the method of Billard et al. (1985), with minor modifications. Briefly, 400 /~1 of membrane suspension was incubated with various concentrations of [3H]SCH 23390 (spec. act. 85 C i / m m o l , Amersham), ranging from 0.098 to 3 nM, in a final volume of 0.5 ml. Non-specific binding was determined in the presence of 10 /~M cisflupentixol. The reaction was stopped after 20 min incubation at 37 o C, by the addition of 4 ml of ice-cold Tris HC1 buffer, p H 7.4, and the mixture was filtered under vacuum through Whatman G F / B filters.
2.4. Determination of DA and DOPAC Dopamine and DOPAC were measured by H P L C (Waters) with electrochemical detection (Waters 460) according to Giorgi et al (1987). The concentrations of DA and DOPAC were calculated from the ratio of their detector response relative to the response of the internal standard introduced into the calibration curve equation obtained by linear regression analysis with a Water 745 data module integrator.
3. Results
In agreement with previous observations we found that chronic treatment with imipramine decreased the number of [3H]SCH 23390 binding sites by 27% in the limbic system and by 14% in the striatum, there was no change in the affinity of [3H]SCH 23390 for the D-receptors in either area (table 1). Figure 1 shows that chronic imipramine markedly reduced the stimulatory effect of DA on adenylate cyclase in the limbic system, but failed to modify the DA response in the striatum. As shown in table 1, the concomitant administration of a-MPT at the daily dose of 50 m g / k g pre-
301 TABLE 1 [3H]SCH 23390 binding and DA-stimulated adenylate cyclase activity in the striatum and limbic system oI rats chronically treated with imipramine or imipramine + a-MPT. Data are the means_+ S.E. of at least four experiments performed in triplicate. Bmax is expressed as f m o l / m g protein, K a as nM, Vmax as p m o l / m g protein per min, K m as /~M. Statistical analysis was done with a two-tailed Student's t-test. Striatum
[3H]SCH 23390 binding D A stimulated A.C.
Limbic system
Saline
Imipramine
Imipramine + a-MPT
Saline
Imipramine
Imipramine + a-MPT
Bm~ Kd
780.4 -t-45.1 0.41+ 0.03
672.5 +33.6 a 0.35-l- 0.03
802.5 +59.0 0.38+ 0.04
220.4 +18.8 0.45+ 0.04
160.9 +20.3 0.40+ 0.03
251.0 +21.9 0.43+ 0.04
Vmax
96.8 + 5.5 4.71-1- 0.41
94.1 + 6.2 4.60+ 0.55
100.4 + 7.5 4.23+ 0.33
33.3 + 4.6 6.78 + 0.91
20.9 + 3.8 a 6.19+ 0.71
37.8 + 4.1 6.49+ 0.58
Km
a p < 0.05 with respect to control values.
vented both the imipramine-induced reduction in [3H]SCH 23390 binding sites and in the responsiveness of adenylate cyclase to DA. Treatment with a-MPT reduced the DA and DOPAC content by 16 and 24%, respectively, in the limbic system, but failed to modify DA metabolism in the striatum.
4. Discussion
The present study confirms a previous observation that chronic imipramine treatment results in a decrease in the number of D-1 receptors labelled with [3H]SCH 23390, an effect that is more pro-
STRIATUM
OF
BASAL
LIMBIC
SYSTEM
ACTIVITY
180 140 160
t! 140 120 12o
1oo
I
i
--6
--5 LOG
[DA]
J
--4
i
--3
100
--7
i
--6
i
--5 LOG
--4
i
--3
[DA]
Fig. 1. Adenylate cyclase activity in rats chronically treated with imipramine. Values are expressed as percentages of the basal activity (126.66 p m o l / m g protein per rain ( o ) controls and 112.12 p m o l / m g protein per min (O) imipramine in the striatum; 60.66 p m o l / m g protein per rain ( o ) controls and 62.78 p m o l / m g protein per min (e) imipramine in the limbic system). Each value is the mean + S.E. of six separate experiments. Statistical analysis of the response to individual agonist concentrations was done with a two-tailed Student's test; * P < 0.01 versus the corresponding value for the control.
302 nounced in the limbic system than in the striatum. Moreover, we found that the decrease in D-1 receptor density was associated with a reduced response of adenylate cyclase to the stimulatory effect of DA. The latter effect was restricted to the limbic system and was absent in the striatum. The question arises how imipramine reduced both the number of D-1 receptors and the sensitivity of adenylate cyclase to DA, and whether these effects are correlated to the imipramine-induced potentiation of those behavioral effects of D A agonists that are considered to be mediated by D-2 receptors in the limbic areas (Spyraki and Fibiger, 1981). While no changes in D-2 receptors have been observed after chronic antidepressant treatment (Klimek and Nielsen, 1987), recent studies indicate that the concomitant activation of D-1 receptors is needed for the motor response to D-2 receptors stimulation to occur (Longoni et al., 1987). In order to reconcile the apparent paradox of behavioral facilitation associated with the reduction in D-1 receptor number and in the sensitivity of adenylate cyclase to DA, it might be suggested that, despite this down-regulation, the formation of cyclic-AMP in the limbic structures is sufficiently high to facilitate D-2 r~ceptor activation. This hypothesis implies that adenylate cyclase activation in the limbic system takes place at a step distal to the D-1 receptor itself. Such a possibility is supported by the recent finding that chronic electroconvulsive shock and chronic desipramine treatment increase adenylate cyclase activity in cortical, cerebellar and hippocampal membranes in responses to forskolin, an agent acting distal to the receptor at either the nucleotide binding or the catalytic subunit of adenylate cyclase (Newman and Lererl 1989). We have extended this finding recently by showing that chronic imip'ramine treatment increases the adenylate cyclase response to forskolin in limbic areas (De Montis et al., in preparation). According to our hypothesis, a reduction in D-1 receptor number and in the sensitivity of adenylate cyclase to D A is an adaptive mechanism to compensate for the post-receptor-mediated increase in adenylate cyclase activity. In agreement with such an interpretation, we can explain
the finding that a - M P T treatment prevented the occurrence of down-regulation of D-1 receptors by suggesting that the compensatory mechanism does not need to operate when dopaminergic neurotransmission is reduced. Irrespective of the mechanism involved, the question arises as to whether down-regulation of the D-1 receptor complex in the limbic areas might be correlated with the antidepressant action of imipramine, and whether the observed changes are specific for imipramine or are a general characteristic of antidepressant treatment. Research in our laboratory has shown that an increase in forskolin-stimulated adenylate cyclase activity is produced in the limbic area not only by imipramine but also by repeated ECS and that this increase is associated with downregulation of D-1 receptors.
Acknowledgements The authors wish to thank Mrs. Graziella Satta for technical assistance. This work was supported by a grant from Ministero della Pubblica Istruzione.
References Billard, W., V. Ruperto, G. Crosby, L.C. Iorio and A. Barnett, 1985, Characterization of the binding of [3H]SCH 23390, a selective D-1 receptor antagonist ligand, in rat striatum, Life Sci 5, 1885. Giorgi, O., G. De Montis, M.L. Porceddu, S. Mele, G. Calderini, G. Toffano and G. Biggio, 1987, Developmental and agerelated changes in D-1 dopamine receptores and dopamine content in the rat striatum, Develop. Brain Res. 35, 283. Klimek, V. and M. Nielsen, 1987, Chronic treatment with antidepressants decreases the number of [3H]SCH 23390 binding sites in rat striatum and the limbic system, European J. Pharmacol. 139, 163. Longoni, R., L. Spina and G. Di Chiara, 1987, Permissive role of D-1 receptor stimulation by endogenous dopamine for the expression of postsynaptic D-2 mediated behavioural responses, European J. Pharmacol. 134, 163. Lowry, O.H., N.J. Rosenbrough, A.L. Farr and R.J. Randall, 1951, Protein measurement with the Folin phenol reagent, J. Biol. Chem. 193, 265. Maj, J., 1986, Repeated treatment with antidepressant drugs: responses mediated by brain dopamine receptors, in: New Results in Depression Research, eds. H. Hippius, G.L. Klerman and N. Matussek (Springer-Verlag, Berlin, Heidelberg) p. 90.
303 Newman, M.E. and B. Lerer, 1989, Post-receptor-mediated increases in adenylate cyclase activity after chronic antidepressant treatment: relationship to receptor desensitization, European J. Pharmacol. 162, 345. Olianas, M.C., P. Onali, N.H. Neff and E. Costa, 1983, Adenylate cyclase activity of synaptic membranes from rat striatum, Mol. Pharmacol. 23, 393.
Spyraki, C. and H.C. Fibiger, 1981, Behavioural evidence for supersensitivity of postsynaptic dopamine receptors in the mesolimbic system after chronic administration of desipramine, European J. Pharmacol. 74, 195. Zis, A.P. and F.K. Goodwin, 1979, Novel antidepressants and the biogenic amine hypothesis of depression, Arch. Gen. Psychiatr. 36, 1097.