Pswhiatry Research, 7, I I I - 1 I9 ( 1982) Elsevier Biomedical Press

111

Reduced Presynaptic Dopamine Receptor Density After Chronic Antidepressant Treatment in Rats

Tyrone Lee and Siu Wa Tang

Received December 23. 1981; revised version reeeived March 10. 1982; accepted March 22, 1982.

Abstract. Chronic administration of desipramine and nomifensine to rats reduced the number of presynaptic dopamine receptors as labeled by 3H-dopamine in rat striatal tissues, with no change in receptor affinity. This change can only be brought about by continuous daily treatment with antidepressants for a duration of at least 21 days or longer. All antidepressants did not displace JH-spiperone or 3H- dopamine at low concentrations in vitro, suggesting that the agents may not act on the dopamine receptors directly. A possible interaction between neurotransmitters in the central nervous system for the overall therapeutic effect of antidepressant clrugs was proposed.

Key Words. Antidepressants, dopamine, presynaptic receptors, subsensitivity.

According to the biogenic amine hypothesis of affective disorders proposed by Schildkraut (1965) depression is thought to be due to a functional deficit and mania to an excess of activity in the catecholaminergic and indoleaminergic neurotransmitter systems. Tricyclic antidepressants have been thought to benefit depressed patients because they are potent uptake inhibitors of norepinephrine, serotonin, or both at synapses, thus creating an abundance of the transmitter at synaptic cleft to overcome the deficit. However, such action cannot explain the delayed onset of action of these tricyclics commonly observed by psychiatrists. Also, other atypical antidepressants, e.g., iprindole and mianserin, may not act via blockade of transmitter uptake systems. They have been shown to affect monoamine synthesis (Kafoe et al., 1976; Carlsson and Lindqvist, 1978) and the adenylate cyclase enzyme system (Vetulani et al., 1976).

There have been suggestions that other neuronal systems, such as dopamine, could be involved in the antidepressant drug action. Van Praag et al. (1973) reported a decrease of the dopamine metabolite homovanillic acid in depressed patients. Post et al. (1978) and Shopsin and Gershon (1978) found that piribedil (ET-495), a dopamine- mimetic drug, had a mild to moderate antidepressant effect in patients with affective disorders. The potent antidepressant, nomifensine, is itself a powerful inhibitor of dopamine uptake (Hunt et al., 1974).

Recent investigations by Serra et al. (1979, 1980), as well as by Chiodo and Antelman (1980a, 1980b), demonstrated that chronic administration of trycyclic antidepressant to rats led to subsensitivity of dopamine autoreceptors in the brain.

Tyrone Lee, Ph.D., is Assistant Director, Psychopharmacology Unit, Clarke Institute of Psychiatry, and Assistant Professor of Pharmacology and Psychiatry, University of Toronto; Siu Wa Tang, M.B., B.S., Ph.D., is Director, Psychopharmacology Unit, Clarke Institute of Psychiatry, and Assistant Professor of Psychiatry and Pharmacology, University of Toronto. (Reprint requests to Dr. T. Lee, Psychopharmacol- ogy Unit. Clarke Institute of Psychiatry, 250 College St., Toronto, Ontario, Canada M5T I R8.)

0165-1781:82:0000-0000/$02.75 0 Elsevier Biomedical Press

112

These findings suggest that dopamine might play an important role in the mechanism of action of antidepressant drugs.

To investigate further the possibility that antidepressants may act through the dopamine neurotransmission system, we used radioreceptor binding techniques to measure the number of presynaptic and postsynaptic dopamine receptors of rats chronically treated with either desipramine or nomifensine. 3H-Dopamine and 3H- spiperone were used to label the presynaptic (DJ and postsynaptic (DJ dopamine receptors, respectively.

Methods

Chronic Drug Treatment. Adult male Wistar rats (I 50- 175 g) were allowed to adapt in animal housings for 1 week before drug administration began. Desipramine (DMI) or nomifensine (NMF) was injected intraperitoneally once daily at 20 mg/ kg to groups of rats according to the schedule shown in Table 1.

Each group was assigned 20 rats, 10 of which were used as controls,receiving equivalent volume of saline for the various injection periods.

Table 1. Schedule of drua administration

Drugs

Groups

Drug treatment i days I

Drug free idavsi

DMI NMF

1 2 3 4 5 6 7 a

1 2 2 10 14 21 28 28

2 2 10 2 2 2 2 2

Tissue Preparations. At the end of the drug withdrawal period, rats were sacrificed by decapitation (usually around 9 a.m.). Brains were quickly removed and the corpus striata were dissected out on ice. The corpus striata from each rat were separately homogenized with a Teflon-glass homogenizer (clearance: 0.13-o. I8 mm) in 20 volumes of ice cold TEAN buffer which contained 15 mM Tris-HCl, 5 mMNa,EDTA, I. I mMascorbate and 12.5 PMnialamide withpH adjusted to 7.4 using 0. I N HCl. The crude homogenates, obtained after homogenizing with 20 up-and-down strokes, were centrifuged at 39,000g for 15 minutes at 4°C. The supernat- ant was then discarded and the pellet was resuspended in fresh TEAN buffer. The tissue suspension was again centrifuged and resuspended in buffer. The process was repeated one more time (total three times). At the end of the third centrifugation, the pellet was resuspended in a known volume of buffer such that tissue concentration was approximately 20 mg wet tissue per ml of buffer. The washed crude homogenates were then frozen at -20°C for radioreceptor assays at a later date.

Radioreceptor Binding Assay. The frozen crude homogenates were thawed and centrifuged at 39,000g for 15 minutes at 4°C. They were then resuspended in TEAN buffer such that final membrane protein concentration was about 0.2 mg/ ml for 3H-spiperone binding and 0.4 mg/ ml for 3H-dopamine binding. The tissue suspension was further homogenized with a Brinkmann Polytron (PT- 10, full scale of IO) for 20 seconds at a setting of 6. Then, 0.2 ml aliquots of such membrane suspension were added to test tubes already containing 0.2ml of3H-spiperone (New England Nuclear Corp., specific activity = 31-34 Ci/mmole) or 3H-dopamine (Amersham Corp., specific activity = 43-48 Ci/mmole) at various concentrations and 0.2 ml of either buffer or IO-5 M sulpiride for JH-spiperone binding or IO-6 Mapomorphine for 3H-dopamine binding.

113

The ingredients were thoroughly mixed with a vortex and allowed to incubate at room temperature for 30 minutes. At the end of the incubation period, 0.5 ml of the incubate was rapidly filtered through Whatman GF/ B glass fiber filters and immediately followed by a wash with IO ml of buffer. The filters were then transferred to scintillation vials and 9 ml of Aquasol was added to each vial. The radioactivity was measured by a liquid scintillation counter after the filters were equilibrated in Aquasol overnight.

The radioreceptor binding assays were carried out using different concentrations of radioli- gands, thus allowing determination of receptor densities (Bmax) and receptor affinities (Kd) by Scatchard analysis. Statistical significance was tested using two-tailed Student’s f test.

In the determination of relative potencies of various antidepressants in displacing 3H- spiperone or 3H-dopamine binding, different concentrations of the drugs were incubated with the respective radioligand. The relative potency (IC,,) was then expressed as concentration of the drug required to displace 50% of the radioligand that was bound to the receptor sites.

Results

The effects of chronic administration of DMI on rat dopamine receptors is illustrated in Fig. 1. DMI did not appear to affect the density of postsynaptic dopamine receptors (D, type) at all different durations of drug treatment tested. Similarly, the density of the presynaptic dopamine receptors (D, type) was not affected until the animals were exposed to DMI for 21 days or more. Theeffect shown at 28 days was a 30Yoreduction in the D, receptor density (p < 0.01) with no change in receptor affinity.

Twenty-eight days of chronic treatment with NMF produced a slight reduction (11%) of D, receptor density and significant reduction of the D, density (21%, p < 0.05) but had no influence on the affinities of the two receptor populations (Table 2).

Of all the antidepressants tested for relative potencies in displacing 3H-spiperone and 3H-dopamine binding, only clomipramine and amitriptyline showed some action in displacing 3H-spiperone at concentrations around 300 nM. All others required micromolar or submillimolar concentrations to displace either radioligand (Table 3).

Discussion

The use of 3H-dopamine or 3H-apomorphine to label the presynaptic dopamine receptors can be justified by the finding that 3H-apomorphine binding was signifi- cantly reduced in rats in which the nigrostriatal dopamine pathway had been des- troyed by 6-OH-dopamine lesions (Nagy et al., 1978). The dopamine receptors in post-mortem Parkinsonian brains were also greatly reduced as measured by 3H- apomorphine (Lee et al., 1978) and by 3H-dopamine (Lee et al., 1981).In Parkinson’s disease patients, a progressive degeneration of dopamine cell bodies in the substantia nigra of the brain was demonstrated (Ehringer and Hornykiewicz, 1960). Also, 3H-apomorphine binding to striatal tissues from rats after kainic acid lesions was unaffected (Bannon et al., 1980).

When 10-5 M sulpiride was used to define specific 3H-spiperone binding in the corpus striatum, it was shown that the postsynaptic dopamine receptors were selec- tively labeled (Lee and Seeman, 1980; Alander et al., 1980). 3H-Spiperone also was found to label, presumably, the postsynaptic dopamine receptors in the anterior pituitary gland of the human (Cronin et al., 1980), sheep and steer (Cronin and Weiner, 1979), calf (Creese et al., 1977), and rat (Stefanini et al., 1980). Further evidence that 3H-spiperone binding was significantly reduced in rats lesioned by kainic

I14

acid suggested that postsynaptic receptor binding sites were involved (Garau et al. 1978; Govoni et al., 1978; Leff et al., 198 I).

Fig. 1. Effect of chronic administration of desipramine (DMI) to rats on striatal dopamine receptor D2 density (upper panel) and D3 density (lower panel)

DYI TREATMENT ON DOPAYINE RECEPTOR (De) DENSlTY

Rx I 2 2 IO I4 21 20

Wlthdraral 2 2 IO 2 2 2 2

:: 40

d

0 L

DAYS

DYI TREATMENT ON DOPAYINE RECEPTOR (01) DENSITY

IL 3 3

Rx I

Withdrawal 2

IO

2

DAYS

2 2

P<O.Ol H h 8 0

2E

2

The D3 and D3 dopamine receptor densities (Bmax in femtomoles per mg membrane protein I were determined by Scatchard analyses of 3H-spiperone and 3H-dopamine binding, respectively. The hatched columns repre- sent experimental animals treated with the drug, and open columns representcontrol animals receivingequiva- lent volume of saline. Each column represents the average of replicate receptor determinations (indicated by numbers inside) with standard errors of the mean indicated by the vertical bars. Rx stands for duration of drug

treatment in days.

In the present study, 3H-dopamine binding in striatal tissues from rats chronically treated with DMI of NMF was significantly reduced after 4 weeks of continuous drug treatment. Shorter duration of exposure to DMI, e.g., 3 weeks, showed a tendency for the receptors to be down-regulated, but this effect was not significant (JJ < 0.1). The drugs appeared to affect the density of the receptors without causing any significant change in the affinity of the receptors.

115

Table 2. Effect of 28 days’ continuous treatment with antidepressants on 3H-spiperone and 3H-dopamine binding to rat striatal tissue (mean +

SE) Saline Desirxamine Nomifensine

sH-Spiperone Bmax Kd n

3H-Dopamine Bmax Kd n

241.3 k 14.8 232.7 k 8.4 215.7 zk 4.1 0.159 IL 0.07 0.144 2 0.05 0.157 i 0.07

3 3 3

113.2 + 9.9 78.7 !I 4.1++ 89.3 IL 5.7’ 3.73 ? 0.30 2.41 f 0.32 2.99 + 0.24

8 8 8

‘p c: 0.05. “p < 0.01.

Table 3. Relative potencies of antidepressants in displacing 3H-spiperone or 3H-dopamine on normal rat striatal tissue

Drugs

Apomorphine

Dopamine

Serotonin

Spiperone

Haloperidol

Clomipramine

lmipramine

Desipramine

Amitriptyline

Nortriptyline

Protriptyline

Mianserin

lprindol

Nomifensine

3H-Spiperone 3H-Dopamine (02 sites) (D3 sites) IC50 @A4 lC50 WA)

150

5,000

> 100,000

0.5

2.6

280

1,700

2,000

300

2,100

n

-

3,500

> 100,000

> 100.000

L

2 -

’ 1,000

> 1,000

> 100,000

’ 100,000

> 10,000

’ 1,000

’ 100,000

’ 100,000

> 100,000

> 100,000

> 100.000

The reduction of presynaptic dopamine receptor numbers by the antidepressants demonstrated in this study is in accord with the in vivo behavioral studies of Serra et al. (1979, 1980) and the in vivo electrophysiological studies of Chiodo and Antelman (1980h) in which antidepressant treatment in rats induced subsensitivity of dopamine autoreceptors. Our results are also in agreement with those of Koide and Matsushita (198 1), who reported a reduction of dopamine receptor density in rats chronically treated with antidepressants. However, these investigators did not specify which population of dopamine receptors was affected by the drugs.

116

The postsynaptic dopamine receptors (D, sites labeled by 3H-spiperone) did not seem to be affected by DMI in this study. This finding is supported by reports of Rosenblatt et al. (1979), Rehavi et al. (1980) Peroutka and Snyder ( 1980), and Tang et al. (1981). The lack of effect of chronic DMI treatment on Dz type of dopamine receptors indicates that the presynaptically located receptor sites (D3) are more sensitive to the drug. NMF-induced decrease in D, dopamine receptor density may possibly be due to dopamine reuptake blockade (Hunt et al., 1974).

Chiodo and Antelman (19806) suggested that long-term repeated drug treatment may not be necessary in order to induce dopaminergic subsensitivity. They demon- strated that the same effect could be achieved by treating the animals for just 2 days and allowing a IO-day drug-free period. Under similar conditions employed in our present study, we did not observe any changes for periods shorter than 3 weeks, however. Perhaps the discrepancy lies in the difference in methodologies employed in the two studies.

Since the acute pharmacological action of typical antidepressants such as DMI and amitriptyline cannot explain the delayed onset of therapeutic action of the drugs, it has been proposed that the latency may be directly related to the time it takes to induce progressive changes in the sensitivity of receptors. Indeed, numerous recent investiga- tions have been carried out to study the effect of chronic antidepressant administra- tion on a,-adrenergic, /3-adrenergic, and serotonin (S,) receptors (Banerjee et al., 1977; Clements-Jewery, 1978; Crews and Smith, 1978; Saraiet al., 1978; Kinnier et al., 1980; Peroutka and Snyder, 1980; Rehavi et al., 1980; Spyraki and Fibiger, 1980; Crews et al., 1981; Fuxe et al., 198 1; Siever et al., 1981; Smith et al., 198 1; Tang et al., 198 1; Wong and Bymaster, 198 1). All the studies cited here reported that a minimum of 14 treatment days were required to bring about subsensitivity or desensitization of the three types of receptors. This interval appears to match the clinical latency of the drugs used in treating depressed patients.

All antidepressants were shown to be weak in displacing 3H-dopamine or 3H- spiperone at respective receptor sites (see Table 3). This observation seemed to indicate that the drugs may not be acting directly at dopamine receptors causing sensitivity changes. Thus, the dopaminergic subsensitivity we have detected in rats after long-term administration of DMI or NMF could be due to initial interaction of other neuronal systems than dopamine which lead to the final outcome of reduction in presynaptic dopamine receptor density. Such interactions (e.g., noradrenergic- dopaminergic) have been hypothesized and reported by Antelman and Caggiula (1977a, 19776). Another possible mechanism is that the long-term treatment of rats with DMI may cause reuptake blockade at dopaminergic synapses (Halaris et al., 1975) which then lead to desensitization of the dopamine autoreceptors located at the nerve terminals.

In summary, long-term administration of DMI or NMF to rats resulted in subsensi- tivity of presynaptic dopamine receptors. This effect was only seen after repeated injections for a duration of at least 3 weeks. The drugs do not seem to act directly on dopamine receptors but rather through other neuronal systems, e.g., noradrenergic, serotonergic, or both. The exact nature of interactions remains to be elucidated.

Acknowledgments. The authors would like to thank Mrs. Lydia Burkitt for her excellent technical assistance. Wealso thank CIBA-GEIGY Canada Ltd. and Hoechst

117

Pharmaceuticals Canada Ltd. for their gifts of desipramine HCl and nomifensine. This study is supported by the Research Fund of the Clarke Institute of Psychiatry. Dr. Tang is an Ontario Mental Health Foundation Scholar.

References

Alander, T. Andtn, N.E., and Cirabowska-And&, M. Metoclopramine and sulpiride as selective blocking agents of pre- and postsynaptic dopamine receptors. Naunyn-Schmiede- berg’s Archives of Pharmacology, 312, 145 ( 1980).

Antelman, S.M., and Caggiula, A.R. Norepinephrine-dopamine interactions and behavior. Science, 195, 646 (1977a).

Antelman, S.M., and Caggiula, A.R. Tails of stress-related behavior: Aneuropharmacological model. In: Hanin, I., and Usdin, E., eds. Animal Models in Psychiatry and Neurology. Per- gamon Press, New York (19776).

Banerjee, S.P., Kung, L.S., Riggi, S.J., and Chanda, S.K. Development of P-adrenergic receptor subsensitivity by antidepressants. Nature, 268, 455 (1977).

Bannon, M.J., Bunney, E.D., Zigun, J.R., Skirboll, L.R., and Roth, R.H. Presynaptic dopa- mine receptors: Insensitivity to kainic acid and the development of supersensitivity following chronic haloperidol. Naunyn-Schmiedeberg’s Archives of Pharmacology, 312, 161 (1980).

Carlsson, A., and Lindqvist, M. Effects of antidepressant agents on the synthesis of brain monoamines. Journal of Neural Transmission, 43, 73 (1978).

Chiodo, L.A., and Antelman, S.M. Electroconvulsive shock: Progressive dopamine auto- receptor subsensitivity independent of repeated treatment. Science, 210, 799 (1980a).

Chiodo, L.A., and Antelman, S.M. Repeated tricyclics induce a progressive dopamine auto- receptor subsensitivity independent of daily drug treatment. Nature, 287, 45 1 (198Ob).

Clements-Jewery, S. The development of cortical P-adrenoceptor subsensitivity in the rat by chronic treatment with trazodone, doxepin, and mianserin. Neuropharmacology, 17, 779 (1978).

Creese, I., Schneider, R., and Snyder, S.H. 3H-Spiroperidol labels dopamine receptors in pituitary and brain. European Journal of Pharmacology, 46, 377 (1977).

Crews, F.T., Paul, S.M., and Goodwin, F.K. Acceleration of P-receptor desensitization in combined administration of antidepressants and phenoxybenzamine. Nature, 290, 787 (1981).

Crews, F.T., and Smith, C. 9. Presynaptic a-receptor subsensitivity after long-term antidepres- sant treatment. Science, 202, 322 (1978).

Cronin, W.J., Cheung, C.Y., Wilson, C.B., Jaffe, R.B., and Weiner, R.I. 3H-Spiperone binding to human anterior pituitaries and pituitary adenomas secreting prolactin, growth hormone, and adrenocorticotropic hormone. Journal of Clinical Endocrinology and Metabolism, 50, 387 (1980).

Cronin, M.J., and Weiner, R.I. 3H-Spiroperidol (Spiperone) binding to a putative dopamine receptor in sheep and steer pituitary and stalk median eminence. Endocrinolog.v, 104, 307 (1979).

Ehringer, H., and Hornykiewicz, 0. Verteilung von Noradrenalin und dopamin (3-hydroxy- tyramin) Im Gehirn des Menschen und ihr Verhalten beim Erkrankungen des extrapyramid- alen systems. Klinische Wochenschrifr, 38, 1236 (1960).

Fuxe, K., Ogren, S.O., Agnati, L.F., Eneroth, P., Holm, A.C., and Andersson, K. Long-term treatment with zimelidine leads to a reduction in 5-hydroxytryptamine neurotransmission within the central nervous system of the mouse and rat. Neuroscience Letters, 21,57 (198 1).

Garau, L., Govoni, S., Stefanini, E., Trabucchi, M., and Spano, P.F. Dopamine receptors: Pharmacological and anatomical evidences indicate that two distinct dopamine receptor populations are present in rat striatum. Life Sciences, 23, 1745 (1978).

Govoni, S., Olgiati, V.R., Trabucchi, M., Garau, L., Stefanini, E., and Spano, P.F. 3H-Halo- peridol and 3H-spiroperidol receptor binding after striatal injection of kainic acid. Neuro- science Letters, 8, 207 (1978).

118

Halaris, A.E., Belendiuk, K.T., and Freedman, D.X. Antidepressant drugs affect dopamine uptake. Biochemical Pharmacology, 24, I896 ( 1975).

Hunt, P., Kannengiesser, M.-H., and Raynaud, J.-P. Nomifensine: A new potent inhibitor of dopamine uptake into synaptosomes from rat brain corpus striatum. Journalof Pharmaq~

and Pharmacologic, 26, 370 ( 1974). Kafoe, W.F., DeRidder, J.J., and Leonard, B.E. The effect of a tetracyclic antidepressant

compound, ORG GB94, on the turnover of biogenic amines in rat brain. Biochemical Pharmacolog,~, 25, 2455 ( 1976).

Kinnier, W.J., Chuang, D.-M., and Costa, E. Down regulation of dihydroalprenolol and imipramine binding sites in brain of rats repeatedly treated with imipramine. European Journal of Pharmacolog),, 67, 289 ( 1980).

Koide, T., and Matsushita, H. An enhanced sensitivity of muscarinic cholinergic receptor associated with dopaminergic receptor subsensitivity after chronic antidepressant treat- ment. L@ Sciences, 28, I I39 ( I98 I).

Lee, T., and Seeman, P. Dopamine receptor binding in schizophrenia and Parkinson’s disease, Progress in Neurops,~chopharmaco/og.v, Suppl. 4, Abstract 391 (1980).

Lee, T.. Seeman, P., Hornykiewicz, O., Bilbao, J., Deck, J., and Tourtellotte, W.W. Parkin- son’s disease: Low density and presynaptic location of D, dopamine receptors. Brain Research, 212, 494 ( 198 I).

Lee, T., Seeman, P., Rajput, A.H., Farley, I.J., and Hornykiewicz, 0. Receptor basis for dopaminergic supersensitivity in Parkinson’s disease. Nature, 273, 59 (1978).

Leff, S., Adams, L., Hyttel, J., and Creese, I. Kainate lesion dissociates striatal dopamine receptor radioligand binding sites. European Journal of Pharmncolog_v, 70, 7 I ( I98 I).

Nagy, J.I., Lee, T., Seeman, P., and Fibiger, H.C. Dopamine receptor binding in brain: Direct evidence for presynaptic and postsynaptic receptors. Nature. 274, 278 (1978).

Peroutka, S.J., and Snyder, S.H. Long-term antidepressant treatment decreases spiroperidol- labeled serotonin receptor binding. Science. 210, 88 (1980).

Post, R.M., Gerner, R.H., Carman, J.S., Gillin, J.C., Jimerson. D.C., Goodwin, F.K., and Bunney, W.E., Jr. Effects of a dopamine agonist piribedil in depressed patients. Archives of General fs~~chiatry, 35, 609 ( 1978).

Rehavi, M., Ramot, O., Yavetz, B., and Sokolovsky, M. Amitriptyline: Long term treatment elevates a-adrenergic and muscarinic receptor binding in mouse brain. Brain Research, 194,

443 ( 1980). Rosenblatt, J.E., Pert, C.B., Tallman, J.F., Pert, A., and Bunney, W.E., Jr. The effect of

imipramine and lithium on o- and P-receptor binding in rat brain. Brain Research, 160,

I86 (1979). Sarai, K., Frazer, A., Brunswick, D., and Mendels, J. Desmethylimipramine-induced decrease

in P-adrenergic receptor binding in rat cerebral cortex. Biochemical Pharmacologic. 27,

2179 (1978). Schildkraut, J.J. The catecholamine hypothesis of affective disorders: A review of supporting

evidence. American Journal of fs.r’chiatv,~, 122, 509 ( 1965). Serra, G., Argiolas, A., Fadda, F., and Gessa, G.L. Hyposensitivity of dopamine “autorecep-

tors” induced by chronic administration of tricyclic antidepressants. Pharmacological Research Communicarions. 12, 6 I9 ( 1980).

Serra, G., Argiolas. A., Klimek, V., Fadda, F., and Gessa, G.L. Chronic treatment with antidepressants prevents the inhibitory effect of small doses of apomorphine on dopamine synthesis of motor activity. Life Sciences, 25, 415 (1979).

Shopsin, B., and Gershon, S. Dopamine receptor stimulation in the treatment of depression: Piribedil (ET-495). Neuropsyhohiolog~~, 4, I (1978).

Siever, L.J., Cohen, R.M., and Murphy, D.L. Antidepressants and a-adrenergic autoreceptor desensitization. Atnerkan Journal of Ps.whiarry, 138, 68 I (I 98 I).

Smith, C.B., Garcia-Sevilla, J.A.. and Hollingsworth, P.J. nz-Adrenoceptors in rat brain are decreased after long-term tricyclic antidepressant drug treatment. Brain Reseawh. 210, 413 (1981).

Spyraki, C., and Fibiger, H.C. Functional evidence for subsensitivity of noradrenergic 01~ receptors after chronic desipramine treatment. r.@ Sciences. 27, I863 ( 1980).

119

Stefanini, E., Devoto, P., Marchisio, A.M., Vernaleone, F., and Collu, R. 3H-Spiroperidol binding to a putative dopaminergic receptor in rat pituitary gland. Life Sciences. 26, 583 ( 1980).

Tang, SW., Seeman, P., and Kwan, S. Differential effect of chronic desipramine and amitrip- tyline treatment on rat brain adrenergic and serotonergic receptors. fs.vchiarry Research, 4, 129 (1981).

Van Praag, H.M., Korf, J., and Schut, D. Cerebral monoamines and depression: An investiga- tion with the probenecid technique. Archives of General Psychiatry, 28, 827 (1973).

Vetulani, J., Stawarz, R.J., Dingell, J.V., and Sulser, F. A possible common mechanism of action of antidepressant treatments. Naun_vn-Schmiedeberg’s Archives of Pharmacology, 293, 109 ( 1976).

Wong, D.T., and Bymaster, F.P. Subsensitivity of serotonin receptors after long-term treat- ment of rats with fluoxetine. Research Communications in Chemical Patho1og.v and Phar- macology. 32, 41 ( I98 I).