Post-receptor-mediated increases in adenylate cyclase activity after chronic antidepressant treatment: relationship to receptor desensitization

Functional interaction between α2-adrenoceptors, μ- and κ-opioid receptors in the guinea pig myenteric plexus: Effect of chronic desipramine treatment

The existence of a functional interplay between alpha(2)-adrenoceptor and opioid receptor inhibitory pathways modulating neurotransmitter release has been demonstrated in the enteric nervous system by development of sensitivity changes to alpha(2)-adrenoceptor, mu- and kappa-opioid receptor agents on enteric cholinergic neurons after chronic sympathetic denervation. In the present study, to further examine this hypothesis we evaluated whether manipulation of alpha(2)-adrenoceptor pathways by chronic treatment with the antidepressant drug, desipramine (10 mg/kg i.p. daily, for 21 days), could entail changes in enteric mu- and kappa-opioid receptor pathways in the myenteric plexus of the guinea pig distal colon. In this region, subsensitivity to the inhibitory effect of both UK14,304 and U69,593, respectively alpha(2A)-adrenoceptor and kappa-opioid receptor agonist, on the peristaltic reflex developed after chronic desipramine treatment. On opposite, in these experimental conditions, supersensitivity developed to the inhibitory effect of [D-Ala, N-Me-Phe4-Gly-ol5]-enkephalin (DAMGO), mu-opioid receptor agonist, on propulsion velocity. Immunoreactive expression levels of alpha(2A)-adrenoceptors, mu- and kappa-opioid receptors significantly decreased in the myenteric plexus of the guinea pig colon after chronic desipramine treatment. In these experimental conditions, mRNA levels of alpha(2A)-adrenoceptors, mu- and kappa-opioid receptors significantly increased, excluding a direct involvement of transcription mechanisms in the regulation of receptor expression. Levels of G protein-coupled receptor kinase 2/3 and of inhibitory G(i/o) proteins were significantly reduced in the myenteric plexus after chronic treatment with desipramine. Such changes might represent possible molecular mechanisms involved in the development of subsensitivity to UK14,304 and U69,593 on the efficiency of peristalsis. Alternative molecular mechanisms, including a higher efficiency in the coupling between receptor activation and downstream intracellular effector systems, possibly independent from inhibitory G(i/o) proteins, may be accounted for the development of supersensitivity to DAMGO. Increased sensitivity to the mu-opioid agonist might compensate for the development of alpha(2A)-adrenoceptor and kappa-opioid receptor subsensitivity. On the whole, the present data further strengthen the concept that, manipulation of alpha(2)-adrenergic inhibitory receptor pathways in the enteric nervous system entails changes in opioid inhibitory receptor pathways, which might be involved in maintaining homeostasis as suggested for mu-opioid, but not for kappa-opioid receptors.

Signaling networks in the pathophysiology and treatment of mood disorders

Over the past decade, the focus of research into the pathophysiology of mood disorders (bipolar disorder and unipolar depression in particular) has shifted from an interest in the biogenic amines to an emphasis on second messenger systems within cells. Second messenger systems rely on cell membrane receptors to relay information from the extracellular environment to the interior of the cell. Within the cell, this information is processed and altered, eventually to the point where gene and protein expression patterns are changed. There is a preponderance of evidence implicating second messenger systems and their primary contact with the extracellular environment, G proteins, in the pathophysiology of mood disorders. After an introduction to G proteins and second messenger pathways, this review focuses on the evidence implicating G proteins and two second messenger systems-the adenylate cyclase (cyclic adenosine monophosphate, cAMP) and phosphoinositide (protein kinase C, PKC) intracellular signaling cascades-in the pathophysiology and treatment of bipolar disorder and unipolar depression. Emerging evidence implicates changes in cellular resiliency, neuroplasticity and additional cellular pathways in the pathophysiology of mood disorders. The systems discussed within this review have been implicated in neuroplastic processes and in modulation of many other cellular pathways, making them likely candidates for mediators of these findings.

Isoform-specific changes of adenylate cyclase mRNA expression in rat brains following chronic electroconvulsive shock

1. Electroconvulsive shock (ECS) has been reported to regulate the cAMP signaling system at various levels, suggesting that the cAMP system is involved in the therapeutic mechanism. 2. Chronic ECS has been suggested to change the expressions of adenylate cyclase (AC) genes, which constitute at least 9 families. However, little is known about its effect on the expression of AC. Therefore, to understand how chronic ECS alters the expression of AC genes in the brain, the authors analyzed the expression of 9 AC isoforms at the transcriptional level in rat hippocampus and cerebellum by quantitative RT-PCR following chronic ECS treatment. 3. Chronic ECS treatment was found to induce differential changes in the expression of AC isoforms in an isoform- and brain region-specific manner in the rat hippocampus and cerebellum. 4. Thus, it is concluded that chronic ECS induces differential changes in the expression of AC isoform mRNA in an isoform- and brain region-specific manner in the rat hippocampus and cerebellum. This suggests that the differential expression of AC isoforms might be an important mechanism by which chronic ECS treatment regulates the cAMP signaling system in rat brains.

Chronic clomipramine alters presynaptic 5-HT(1B) and postsynaptic 5-HT(1A) receptor sensitivity in rat hypothalamus and hippocampus, respectively

Clomipramine is a tricyclic antidepressant drug with a high affinity for the serotonin (5-HT) uptake site or transporter. Electrophysiological experiments have provided evidence that repeated administration of clomipramine induces an increase in the sensitivity of postsynaptic 5-HT(1A) receptors in the hippocampus. We have studied the effects of clomipramine, administered to rats at a dose of 10mg/kg/day for 28 days by osmotic minipumps, on presynaptic 5-HT(1A) and 5-HT(1B) autoreceptors in the hypothalamus, and on postsynaptic 5-HT(1A) receptors in the hippocampus, by using in vivo microdialysis to measure 5-HT and cyclic adenosine monophosphate (cAMP) levels. Postsynaptic 5-HT(1A) receptor sensitivity in the hypothalamus was determined by means of a neuroendocrine challenge procedure. Although the sensitivity of presynaptic 5-HT(1A) autoreceptors, as measured by the effect of a subcutaneous (s.c.) injection of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.2mg/kg or 50 microg/kg) to reduce 5-HT levels, did not change, there was a reduction in sensitivity of presynaptic 5-HT(1B) receptors, as measured by the effect of an injection of the 5-HT(1B/1D) antagonist GR 127935 (5mg/kg, s.c.) to increase 5-HT levels. This effect probably accounted for the increase in basal 5-HT levels observed in the hypothalamus after chronic clomipramine administration. Postsynaptic 5-HT(1A) receptor sensitivity in the hippocampus, measured by the effect of 8-OH-DPAT to increase cAMP levels in the dialysate, was increased after chronic clomipramine. Animals that had received daily intraperitoneal injections of 10mg/kg clomipramine for 28 days did not show a change in postsynaptic 5-HT(1A) receptor sensitivity in the hypothalamus as measured by the ability of 8-OH-DPAT (50 microg/kg, s.c.) to stimulate secretion of corticosterone. Taken together with the results of previous experiments involving the cerebral cortex, these in vivo results show that chronic clomipramine exerts effects on both pre- and postsynaptic serotonin receptors, but that these effects are highly region-specific.

Increased adenylyl cyclase type 1 mRNA, but not adenylyl cyclase type 2 in the rat hippocampus following antidepressant treatment

The adenylyl cyclase (AC) system is affected by several types of antidepressant treatments, and increased activity in this system is linked to the therapeutic action of antidepressants. The present study was undertaken to compare the effects of single-dose and long-term treatment with the selective serotonin reuptake inhibitor, citalopram (10 mg/kg, i.p.), on the AC system in the male rat brain of Wistar rats. Furthermore, we compared the effects of long-term citalopram and lithium treatments on the AC system. Long-term citalopram, but not single-dose administration, increased the AC type 1 mRNA in the hippocampus, whereas type 2 mRNA was unaffected. Long-term lithium treatment also increased AC1 in the hippocampus. However, long-term citalopram treatment did not increase AC type 1 protein, basal or forskolin-stimulated AC activity, but GTP increased AC activity in the hippocampus. This may indicate enhanced AC/G protein coupling. Thus, citalopram may increase cAMP signalling by acting on components of the AC system without affecting the protein level of the AC type 1.

Regulation of signal transduction pathways and gene expression by mood stabilizers and antidepressants

OBJECTIVE

To determine whether the currently available evidence supports the hypothesis that antidepressants and mood stabilizers may bring about some of their long-term therapeutic effects by regulating signal transduction pathways and gene expression in the central nervous system.

METHODS

To address this question, we reviewed the evidence showing that chronic administration of antidepressants and mood stabilizers involves alterations in signaling pathways and gene expression in the central nervous system.

RESULTS

A large body of data has shown that lithium and valproate exert effects on the protein kinase C signaling pathway and the activator protein 1 family of transcription factors; in contrast, antidepressants affect the cyclic adenosine monophosphate pathway and may bring about their therapeutic effects by modulating cyclic adenosine monophosphate-regulated gene expression in the central nervous system.

CONCLUSIONS

Given the key roles of these signaling cascades in the amplification and integration of signals in the central nervous system, the findings have clear implications not only for research into the etiology and pathophysiology of the severe mood disorders but also for the development of novel and innovative treatment strategies.

Adrenergic receptors in premenstrual dysphoric disorder. II. Neutrophil beta2-adrenergic receptors: Gs protein coupling, phase of menstrual cycle and prediction of luteal phase symptom severity

Abnormal beta2-adrenergic receptor coupling to Gs protein is implicated in depressive disorders. Steroid hormones and antidepressants modulate beta-adrenergic receptor coupling, which may relate to the therapeutic efficacy of antidepressants. We examined beta2-adrenergic receptors in 18 patients with premenstrual dysphoric disorder (PMDD), in 15 control subjects during the follicular phase and in 12 patients during late luteal phase. Antagonist-measured receptor density, agonist-measured receptor density in the high- and low-conformational states and agonist affinity to both states were measured. Coupling indices to Gs protein were determined from agonist-displacement experiments. Follicular beta2-adrenergic receptor density was higher in patients than in control subjects, with a trend for higher receptor density in the high-conformational state. The phase of menstrual cycle had no effect on beta2-adrenergic receptor regulation in PMDD. Exploratory correlations showed that the K(L)/K(H) ratio was related to anxiety ratings in control subjects and %R(H) was correlated with symptom severity in patients. In patients, follicular beta2-adrenergic receptor binding measures were correlated with luteal symptom severity. These findings suggest abnormal beta2-adrenergic receptor regulation in PMDD. Further exploration of the role of beta-adrenergic receptor kinase, sex steroid hormones and antidepressants on beta-adrenergic receptor regulation in PMDD is warranted.

Effects of subchronic administration of antidepressants and anxiolytics on levels of the alpha subunits of G proteins in the rat brain

The aim of this study was to examine the effects of subchronic administration of psychoactive drugs on the alpha subunits of G proteins in the rat brain, and also to determine if different classes of psychoactive drugs share a common property, i.e., of altering levels of these proteins. For this purpose, we selected the psychoactive drugs desipramine and phenelzine (antidepressants), lithium (antimanic), alprazolam and buspirone (anxiolytics), and metachlorophenylpiperazine (anxiogenic). The levels of alpha subunits of G proteins (Gs, Gi 1/2, Gq/11) expressed in cortical, hippocampal, and cerebellar brain regions were studied by the Western blot technique. We observed that subchronic treatment with lithium significantly decreased, and with phenelzine significantly increased levels of Gi 1/2 alpha protein in the cortex and the hippocampus. On the other hand, buspirone significantly decreased levels of Gi 1/2 alpha protein only in the cerebellum. Other psychoactive drugs, however, namely desipramine, meta-chlorophenylpiperazine, and alprazolam, did not alter levels of Gs, Gi 1/2, or Gq/11 alpha proteins in any of the brain regions studied. Since other studies have shown the involvement of G proteins in the mechanism of action of psychoactive drugs, our results demonstrate that expressed protein levels of the alpha subunit of G proteins are not altered by all the psychoactive drugs.

Potential antidepressant properties of forskolin and a novel water-soluble forskolin (NKH477) in the forced swimming test

The mechanisms of the antidepressant activity of forskolin and a novel water soluble forskolin analog (NKH477) were studied using the forced swimming method in rats. Forskolin (0.01-0.1 mg/kg) and NKH477 (0.01-0.1 mg/kg) dose-dependently decreased ratings of immobility, with effects similar to those of amitriptyline treatment. The maximum effects of forskolin and NKH477 were observed at 0.01 mg/kg dose which is 150 more times potent than that (15 mg/kg) of amitriptyline. At a high dose (1.0 mg/kg) of forskolin and NKH477, the duration of immobility was returned to control levels. Forskolin and NKH477 did not influence the spontaneous locomotor activity at intraperitoneal injection doses from 0.01 to 1 mg/kg. Furthermore chronic administration with NKH477 at oral dose from 0.5 to 1.5 mg/kg significantly decreases the duration of immobility. These data indicate that both forskolin and NKH477 have strong antidepressive potency, consistent with the hypothesis that elevation of the cAMP cascade system may have an important role in antidepressive effects.

In vivo cyclic AMP responses in rat brain are not modified by chronic electroconvulsive shock

In vivo microdialysis was used to determine the effects of chronic electroconvulsive shock (ECS), given daily for 10 days, on the cyclic AMP (cAMP) responses to noradrenaline (NA) and to forskolin in the cortex of freely-moving conscious rats. Microdialysis probes were inserted on the final day of ECS administration, and cAMP responses measured the following day. Chronic ECS did not modify the responses to either NA or forskolin. These results differ markedly from those previously obtained with incubated slices of rat brain. It is concluded that care is needed in interpreting results obtained by either ex vivo or in vitro methods.

Dimeric tubulin-stimulated adenylyl cyclase activity is augmented after long-term amitriptyline treatment

We have investigated altered association of tubulin dimers interacting with G proteins and modulating adenylyl cyclase (AC) as a result of long-term amitriptyline (AMT) treatment. Gpp(NH)p-stimulated, but not basal or manganese-stimulated, AC activity was significantly augmented in cortex membranes prepared from rats chronically treated with AMT. The enhancement of AC activity by Gpp(NH)p-liganded tubulin (tubulin-Gpp(NH)p) was significantly higher in chronically AMT-treated rats than in control rats. Moreover, in cortex membranes from controls, tubulin-Gpp(NH)p prepared from chronically AMT-treated rats was more effective to activate AC activity than tubulin-Gpp(NH)p from controls. Immunoblotting and photoaffinity guanine nucleotide binding procedures showed no significant differences in the amount and the function of G proteins between controls and AMT-treated groups. It is suggested that long-term AMT treatment causes alteration in the functional interaction between tubulin and G protein, and this modification may participate in enhanced coupling of Gs to the catalytic subunit of AC induced by the chronic antidepressant treatment.

Adenylyl cyclase activity and G-protein subunit levels in postmortem frontal cortex of suicide victims

Basal and stimulated adenylyl cyclase activities and Gs and Gi protein alpha-subunit levels (Gs alpha and Gi alpha) were compared in postmortem frontal cortex from 18 suicide cases and 22 matched controls. Basal, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) stimulated and forskolin stimulated enzyme activities were significantly lower in the suicide cases, compared to controls. These effects were most apparent in those suicides that had died from violent means or that had had a history of depression and appeared to reflect the lowered basal activity rather than a reduced ability of either GTP gamma S or forskolin to activate the enzyme. No significant correlations were found between adenylyl cyclase activity and either subject age or postmortem delay. Western blotting revealed no significant differences in Gs alpha and Gi alpha levels between control and suicide cases. However, levels of the smaller Gs alpha isoform (Gs alpha-S) showed a tendency to be increased in the violent death suicide and depressed suicide subgroups, compared to controls. Levels of the larger Gs alpha isoform (Gs alpha-L) showed a significant positive correlation with subject age. Gi alpha levels showed a significant negative correlation with subject age and a positive correlation with postmortem delay. These results support the hypothesis that suicidal behaviour and depressive illness may be associated with an altered regulation of adenylyl cyclase.

Post-train administration of 9-amino-1,2,3,4-tetrahydroacridine enhances passive avoidance retention and decreases beta-adrenoceptor-linked cyclic AMP formation in middle-aged rats

The possible involvement of beta-adrenoceptor system in the effectiveness of 9-amino-1,2,3,4-tetrahydroacridine (THA) to attenuate retention deficits exhibited by middle-aged rats in a one-trial passive avoidance task has been investigated. THA (2.5 mg.kg-1), injected i.p. after training, induced a significant increase in test step-through latency (STL) in middle-aged rats. Post-training injection of THA reduced basal and isoprenaline stimulated cyclic AMP accumulation in cortex and hippocampus of every group of rats. It is suggested that the effect of THA on memory processes may involve an action on beta-adrenoceptor-linked cyclic AMP accumulation.

Chronic electroconvulsive treatment augments coupling of the GTP-binding protein Gs to the catalytic moiety of adenylyl cyclase in a manner similar to that seen with chronic antidepressant drugs

A significant increase of guanylylimidodiphosphate (GppNHp)-, fluoride-, and forskolin-stimulated adenylyl cyclase was observed in synaptic membrane preparations from rat cerebral cortex subsequent to chronic electroconvulsive shock (ECS) treatment. This effect required at least five treatments over a course of 10 days. The inhibition of adenylyl cyclase induced by GppNHp was not affected by these treatments. The dissociation constant (KD) and maximal binding for the photoaffinity GTP analog, [32P]P3-(4-azidoanilido)-P1-5'-GTP [( 32P]AAGTP), to each of the synaptic membrane G proteins also were unchanged after ECS treatment. Nonetheless, the transfer of [32P]AAGTP from Gi to Gs, which we suggest is indicative of the coupling between Gs and the adenylyl cyclase catalytic moiety, was accelerated by chronic ECS treatment but not by acute or sham treatment. Furthermore, chemical uncoupling of Gs from adenylyl cyclase rendered membranes from treated animals indistinguishable from controls. Finally, in all cases tested, membranes prepared from animals subjected to chronic treatment with amitriptyline or iprindole showed similar changes in the Gs-mediated activation of adenylyl cyclase. Acute treatments produced effects similar to controls, and liver and kidney membranes from animals receiving chronic treatment showed no changes in adenylyl cyclase despite the marked changes seen in brain. These results suggest that chronic administration of ECS enhances coupling between Gs and adenylyl cyclase enzyme and modifies interactions between Gs and Gi.

Possible role of dopamine D1 receptor in the behavioural supersensitivity to dopamine agonists induced by chronic treatment with antidepressants

The effect of chronic treatment with antidepressants (ADs) on the behavioral responses to LY 171555, a selective D2 receptor agonist, SKF 38393, a selective D1 receptor agonist, and B-HT 920, a selective DA autoreceptor agonist, was studied in rats. In normal rats small, intermediate and high doses of LY 171555 produced hypomotility, hyperactivity and stereotypies, respectively. Chronic but not acute pretreatment with imipramine (IMI) greatly potentiated the motor stimulant effect of LY 171555, but failed to modify its stereotypic and sedative effect. The potentiation of the motor stimulant effect of LY 171555 was observed also after chronic, but not acute, treatment with desmethylimipramine (DMI), mianserin (MIA) or repeated electroconvulsive shock (ECS). Chronic treatment with IMI failed to modify the effect of SKF 38393 (motor stimulation, grooming and penile erection), but reversed the sedative effect of B-HT 920 into a motor stimulant response. The motor stimulant response to LY 171555 in IMI-pretreated animals was suppressed by L-sulpiride, a D2 antagonist, and by a combination of reserpine with alpha-methyltyrosine (alpha-MT), but it was only partially antagonized by high doses of SCH 23390, a selective D1 antagonist. The results indicate that chronic treatment with ADs potentiates the behavioural responses mediated by the stimulation of postsynaptic D2 receptors in the mesolimbic system and suggest that this behavioural supersensitivity is due to enhanced neurotransmission at the D1 receptor level.

Beta-adrenoceptor and post-receptor components show different rates of desensitization to desipramine

Both 3-day and 15-day desipramine treatments (10 mg/kg, once daily) significantly reduced beta 1- but not beta 2-adrenoceptor and isoproterenol-stimulated adenylyl cyclase activities in rat cortical, hippocampal and amygdaloid membrane preparations. In contrast, 15-day but not 3-day desipramine treatment resulted in a significant reduction in NaF-, GppNHp- and forskolin-stimulated adenylyl cyclase activity. The results suggest that post-beta-adrenoceptor component desensitization occurs more slowly than receptor downregulation in response to desipramine.

Selective adenylate cyclase increase in the limbic area of long-term imipramine-treated rats

Long-term administration of imipramine to rats produced an increase in the Vmax of forskolin- or guanylylimidodiphosphate (Gpp(NH))p-activated adenylate cyclase only in the limbic area. This effect was prevented by the daily administration of alpha-methyl-p-tyrosine (alpha-MPT), given together with imipramine, at a dose (50 mg/kg) which had no effect on adenylate cyclase activity per se. The time course of the effects of chronic imipramine on dopaminergic transmission in the limbic area showed that the decrease in both D-1 receptor number and adenylate cyclase stimulation by dopamine (DA) reached significance on day 8 of treatment and were maximal on day 15. The Vmax of the enzyme started to increase on day 15 and was further increased on day 21. Possible mechanisms underlying these effects are discussed.

Effects of treatment with a lithium-imipramine combination on components of adenylate cyclase in the cerebral cortex of the rat

This study was aimed at investigating the effects of treatment with a lithium-imipramine combination on the activity of adenylate cyclase in membranes from the cerebral cortex of the rat. Treatment with (1) lithium for 2 weeks, yielding a level of lithium in serum of 0.54 +/- 0.12 mmol/l, (2) imipramine for 4 weeks (10 mg/kg i.p. twice per day) and (3) a combination of the two drugs reduced isoprenaline-induced stimulation of adenylate cyclase by GTP, with a greater decrement with the combined treatment. None of the treatments exerted any effect on the activity of the enzyme stimulated by GTP alone. Lithium ex vivo inhibited the calcium (Ca2+)- and Gpp(NH)p-stimulated activity of adenylate cyclase, but imipramine ex vivo did not affect the activity of adenylate cyclase, stimulated by these activators. The lithium-imipramine treatment reduced Ca2(+)- and Gpp(NH)p-stimulated activity of adenylate cyclase, but this was not different from that observed in the lithium-treated group. In conclusion, the beta-adrenoceptor-stimulated adenylate cyclase was affected markedly by administration of lithium and imipramine together. In contrast to lithium ex vivo, imipramine ex vivo did not impair the activity of either the guanine nucleotide regulatory protein or the catalytic subunit, since no change in activity was observed in the presence of beta,gamma-imidoguanosine-5' triphosphate (Gpp(NH)p) or Ca2+. Furthermore, lithium ex vivo exerted its post-receptor effects on the adenylate cyclase, independent of imipramine. The decrement in activity of beta-adrenergic adenylate cyclase, induced by administration of the two drugs together may partly be involved in the therapeutic action of the augmentation of antidepressants by lithium in refractory depression.

Effects of chronic exposure to ethanol alone and in combination with desipramine on beta-adrenoceptors of rat brain

The effect of chronic ethanol exposure alone or in combination with desipramine on agonist and antagonist binding to beta-adrenoceptors was studied in membrane preparations from rat frontal cortex and hippocampus. Ten day exposure of animals to ethanol vapor (25 mg/l) in inhalation chambers had no effect on binding properties of antagonist iodocyanopindolol (ICYP) in either brain region. However, ethanol in combination with chronic desipramine treatment prevented the reduction of beta-adrenoceptor density in frontal cortex produced by desipramine administration. Similar to its effects on antagonist binding, chronic ethanol exposure did not change the agonist isoproterenol binding characteristics measured in membranes from either rat frontal cortex or hippocampus. However, the combination of ethanol plus desipramine reduced the dissociation constant of the low affinity state of the receptor (KL) in frontal cortex from 23.1 +/- 3.7 microM in controls to 11.2 +/- 1.7 microM. Moreover, ethanol plus desipramine produced a greater decrease in the percentage of cortical receptors in the high affinity state for agonist (%RH) than did desipramine alone. This suggests that ethanol enhances desipramine-induced desensitization of beta-adrenoceptors in frontal cortex in spite of the prevention of reduction in density of the receptors. In hippocampal membranes, ethanol together with desipramine prevented desipramine-induced changes in agonist binding characteristics, i.e. the decrease in KH (dissociation constant from high affinity state of the receptor) and the consequent enhancement in KL/KH ratio. Thus, chronic exposure to relatively low concentrations of ethanol partially prevents effects of desipramine on beta-adrenoceptors.

Chronic imipramine reduces [3H]SCH 23390 binding and DA-sensitive adenylate cyclase in the limbic system

[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 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 alpha-methyltyrosine (alpha-MPT) (50 mg/kg daily for 14 days) prevented the imipramine-induced reduction in both [3H]SCH 23390 binding sites and the responsiveness of adenylate cyclase to DA.