Involvement of protein kinase C in the mechanism of in vitro effects of imipramine on generation of second messengers by noradrenaline in cerebral cortical slices of the rat

Antidepressants Differentially Regulate Intracellular Signaling from α1-Adrenergic Receptor Subtypes In Vitro

Currently utilized antidepressants have limited effectiveness and frequently incur undesired effects. Most antidepressants are thought to act via the inhibition of monoamine reuptake; however, direct binding to monoaminergic receptors has been proposed to contribute to both their clinical effectiveness and their side effects, or lack thereof. Among the target receptors of antidepressants, α1‑adrenergic receptors (ARs) have been implicated in depression etiology, antidepressant action, and side effects. However, differences in the direct effects of antidepressants on signaling from the three subtypes of α1-ARs, namely, α1A-, α1B- and α1D‑ARs, have been little explored. We utilized cell lines overexpressing α1A-, α1B- or α1D-ARs to investigate the effects of the antidepressants imipramine (IMI), desipramine (DMI), mianserin (MIA), reboxetine (REB), citalopram (CIT) and fluoxetine (FLU) on noradrenaline-induced second messenger generation by those receptors. We found similar orders of inhibition at α1A-AR (IMI < DMI < CIT < MIA < REB) and α1D‑AR (IMI = DMI < CIT < MIA), while the α1B-AR subtype was the least engaged subtype and was inhibited with low potency by three drugs (MIA < IMI = DMI). In contrast to their direct antagonistic effects, prolonged incubation with IMI and DMI increased the maximal response of the α1B-AR subtype, and the CIT of both the α1A- and the α1B-ARs. Our data demonstrate a complex, subtype-specific modulation of α1-ARs by antidepressants of different groups.

Effects of the noradrenergic neurotoxin DSP-4 on the expression of α1-adrenoceptor subtypes after antidepressant treatment

We have previously reported that chronic imipramine and electroconvulsive treatments increase the α(1A)-adrenoceptor (but not the α(1B) subtype) mRNA level and the receptor density in the rat cerebral cortex. Furthermore, we have also shown that chronic treatment with citalopram does not affect the expression of either the α(1A)- or the α(1B)-adrenoceptor, indicating that the previously observed up-regulation of α(1A)-adrenoceptor may depend on the noradrenergic component of the pharmacological mechanism of action of these antidepressants. Here, we report that previous noradrenergic depletion with DSP-4 (50 mg/kg) (a neurotoxin selective for the noradrenergic nerve terminals) significantly attenuated the increase of α(1A)-adrenoceptor mRNA induced by a 14-day treatment with imipramine (IMI, 20 mg/kg, ip) and abolished the effect of electroconvulsive shock (ECS, 150 mA, 0.5 s) in the prefrontal cortex of the rat brain. The changes in the receptor protein expression (as reflected by its density) that were induced by IMI and ECS treatments were differently modulated by DSP-4 lesioning, and only the ECS-induced increase in α(1A)-adrenoceptor level was abolished. This study provides further evidence corroborating our initial hypothesis that the noradrenergic component of the action of antidepressant agents plays an essential role in the modulation of α(1A)-adrenoceptor in the rat cerebral cortex.

The glucocorticoid receptor: pivot of depression and of antidepressant treatment?

Hyperactivity of the hypothalamus-pituitary-adrenal (HPA) axis and increased levels of glucocorticoid hormones in patients with depression have mostly been ascribed to impaired feedback regulation of the HPA axis, possibly caused by altered function of the receptor for glucocorticoid hormones, the glucocorticoid receptor (GR). Antidepressants, in turn, ameliorate many of the neurobiological disturbances in depression, including HPA axis hyperactivity, and thereby alleviate depressive symptoms. There is strong evidence for the notion that antidepressants exert these effects by modulating the GR. Such modulations, however, can be manifold and range from regulation of receptor expression to post-translational modifications, which may result in differences in GR nuclear translocation and GR-dependent gene transcription. The idea that the therapeutic action of antidepressants is mediated, at least in part, by restoring GR function, is consistent with studies showing that decreased GR function contributes to HPA axis hyperactivity and to the development of depressive symptoms. Conversely, excessive glucocorticoid signalling, which requires an active GR, is associated with functional impairments in the depressed brain, especially in the hippocampus, where it results in reduced neurogenesis and impaired neuroplasticity. In this review, we will focus on the GR as a key player in the precipitation, development and resolution of depression. We will discuss potential explanations for the apparent controversy between glucocorticoid resistance and the detrimental effects of excessive glucocorticoid signalling. We will review some of the evidence for modulation of the GR by antidepressants and we will provide further insight into how antidepressants may regulate the GR to overcome depressive symptoms.

Paroxetine pretreatment does not change the effects induced in the rat cortical beta-adrenergic receptor system by repetitive transcranial magnetic stimulation and electroconvulsive shock

Repetitive transcranial magnetic stimulation (rTMS) has been proposed as a clinically effective antidepressant treatment, but meta-analysis suggests that its efficacy is marginal. We investigated whether the administration of rTMS together with paroxetine would enhance its effects on the beta-adrenergic system of the rat. We compared our results with the effects of electroconvulsive shock therapy (ECS). The experiment was performed for 12 d on male Wistar rats that received a physical treatment of either rTMS (B=1.4 T, f=10 Hz, 300 s) or ECS (I=130 mA, f=50 Hz, t=500 ms), preceded by sterile water or paroxetine (10 mg/kg i.p. 30 min earlier). All rats were decapitated 24 h after the final treatment. Cyclic AMP (cAMP) was measured in cortical slices prelabelled with [3H]adenine and stimulated with noradrenaline. beta-adrenoceptor parameters (Bmax and KD) were assessed in the P2 fraction of cortical homogenates using [3H]CGP 12177 as a ligand. ECS resulted in down-regulation of both the cAMP response and beta-adrenoceptor density, while rTMS depressed only the responsiveness of the cAMP-generating system. Paroxetine, which was only effective in dampening the cAMP response, did not change the effects of either physical treatment. The data suggest that any possible interaction between paroxetine and rTMS or ECS does not involve the beta-adrenergic mechanisms.

In vitro modulation of the glucocorticoid receptor by antidepressants

Clinical studies have demonstrated an impairment of glucocorticoid receptor (GR)-mediated negative feedback on the hypothalamus-pituitary-adrenal (HPA) axis in patients with major depression (GR resistance), and its resolution by antidepressant treatment. Accordingly, reduced GR function has also been demonstrated in vitro, in peripheral tissues of depressed patients, as shown by reduced sensitivity to the effects of glucocorticoids on immune and metabolic functions. We and others have shown that antidepressants in vitro are able to modulate GR mRNA expression, GR protein level and GR function. This paper reviews the in vitro studies that have examined the effect of antidepressants on GR expression, number and function in human and animal cell lines, and the possible molecular mechanisms underlying these effects. Antidepressants are shown to both increase and decrease GR function in vitro, based on different experimental conditions. Specifically, increased GR function is likely to be mediated by an increased intracellular concentration of glucocorticoids, while decreased GR function seems to be the consequence of GR downregulation. We suggest that the study of the effects of antidepressants on glucocorticoid function might help clarify the therapeutic action of these drugs.

Amitriptyline preserves morphine’s antinociceptive effect by regulating the glutamate transporter GLAST and GLT-1 trafficking and excitatory amino acids concentration in morphine-tolerant rats

The present study was undertaken to examine the effect of amitriptyline on the antinociceptive effect of morphine and its underlying mechanisms in regulating glutamate transporters trafficking in morphine-tolerant rats. Long-term morphine infusion induced antinociceptive tolerance and down-regulation of glutamate transporters (GTs), GLAST, GLT-1, and EAAC1, expression in the rat spinal cord dorsal horn. Acute amitriptyline treatment potentiated morphine's antinociceptive effect, with a 5.3-fold leftward shift of morphine's dose-response curve in morphine-tolerant rats, and this was associated with GLAST and GLT-1 trafficking onto the cell surface. Similar to our previous studies, morphine challenge (10 microg/10 microl, i.t.) significant by increased the excitatory amino acids (EAAs) aspartate and glutamate level in the CSF dialysates of morphine-tolerant rats. Acute amitriptyline treatment not only suppressed this morphine-evoked EAA release, but further reduced the EAA concentration than baseline level. Furthermore, long-term morphine infusion up-regulated PKA and PKC protein expression in the spinal cord dorsal horn, while amitriptyline inhibited the increase in expression of phospho-PKA, PKCalpha, PKCbetaII, and PKCgamma. In morphine-tolerant rats, acute treatment with PKA inhibitor H89 and PKC inhibitor Gö6805 attenuated morphine tolerance and the morphine-induced CSF glutamate and aspartate elevation, and induced trafficking of GLAST and GLT-1 from cytosol onto the cell surface. These results show that acute amitriptyline treatment preserved morphine's antinociceptive effect in morphine-tolerant rats; the mechanisms may be involved in inhibition of phospho-PKA and PKC expression, and thus inducing the GLAST and GLT-1 trafficking onto glial cell surface which enhances the EAA uptake from the synaptic cleft and reduces EAA concentration in the spinal CSF.

Nicotine produces antidepressant-like actions: Behavioral and neurochemical evidence

Converging lines of evidence indicate the involvement of nicotinic acetylcholine receptors in depressive illness and antidepressant drug action. We investigated the effects of sub-chronic and chronic treatment with imipramine, nicotine and their combination on: (a) the ability of a dopamine-mimetic challenge to produce locomotor stimulation and (b) cortical density of beta-adrenoceptors. One week of treatment with imipramine (10 mg/kg, twice daily) did not result in an altered response to the apomorphine (0.15 mg/kg) challenge, but after 2 weeks, the imipramine-treated rats demonstrated hyperactivity. Conversely, such increased locomotor response was observed in rats treated with nicotine (0.4 mg/kg, twice daily) for 1 but not for 2 weeks. Groups treated with nicotine+imipramine for 1 and 2 weeks demonstrated equally high hyperactivity in response to the apomorphine challenge. This effect was not different from the effects of 1-week treatment with nicotine or 2-week treatment with imipramine. The density of beta-adrenoceptors was equally decreased by 2 (but not 1) weeks of the treatment with imipramine, nicotine and their combination. The present behavioral and neurochemical data suggest the antidepressant-like effect of the chronic treatment with nicotine. It appears that the potentiation of the dopamine-mimetic-induced hyperactivity cannot be explained by beta-adrenoceptor down-regulation.

Carane derivative stereoisomers of different local anaesthetic and antiplatelet activity similarly potentiate forskolin-stimulated cyclic AMP response and bind to beta-adrenoceptors in the rat brain cortex

A carane derivative, KP-23 [RS](-)-4-(2-hydroxy-3)N-isopropylamino)-propoxyimino)-cis-carane, was earlier described as a potential local anaesthetic and antiplatelet agent, and the following studies revealed that its R and S stereoisomers, KP-23R and KP-23S, have different potencies in the infiltration anaesthesia and platelet aggregation tests. The effects of these stereoisomers on the cyclic AMP (cAMP) generating system and the displacement of [(3)H]CGP 12177 (a beta-adrenoceptor ligand) from its binding sites in the rat cerebral cortical tissue were investigated. The stereoisomers did not affect the basal cAMP level, but, at concentrations between 10(-4) and 10(-3) M, they elevated the forskolin-induced accumulation of cAMP with similar potency. The compounds displaced [(3)H]CGP 12177, however the stereoisomer R was less potent than the racemic KP-23 and the S form (K(i) = 64.1 +/- 5.9 nM, 161.1 +/- 10 nM and 62.1 +/- 5.6 nM for KP-23, KP-23R and KP-23S, respectively). The fact that the stereoisomers differed in both tests only slightly, if at all, suggests that their pharmacological effects are not related to the action on the beta-adrenoceptor/adenylate cyclase system.

Repeated imipramine and electroconvulsive shock increase alpha 1A-adrenoceptor mRNA level in rat prefrontal cortex

alpha(1)-Adrenoceptors have been implicated in the mechanism of action of antidepressants, but their action on specific receptor subtypes was rarely reported. We compared now the action of two prototypic antidepressant treatments: repeated imipramine and electroconvulsive shock, on the expression of the alpha(1A)- and alpha(1B)-adrenoceptor mRNAs and on the receptor density in rats. The mRNA expression was assessed by Northern blot in the prefrontal cortex and the hippocampus, the receptor density was measured by [3H]prazosin binding in the total cerebral cortex and hippocampus. In the cortex, both treatments elevated the alpha(1A)-adrenoceptor mRNA and the expression of receptor protein. The expression of alpha(1B)-adrenoceptor mRNA remained unaffected. In contrast, in the hippocampus, the antidepressant treatments augmented the density of alpha(1A)-adrenoceptor protein without changing the level of its mRNA expression there. The results suggest that the alpha(1A)-adrenoceptor subtype is specifically involved in the mechanism of action of classical antidepressant treatments.

Behavioural and biochemical studies of citalopram and WAY 100635 in rat chronic mild stress model

Reversal of chronic mild stress (CMS)-induced decrease of sucrose consumption has been studied in rats after 2, 7, 14, and 35 days treatment with imipramine, citalopram (both 10 mg/kg per day, i.p.), WAY 100635 (0.2 mg/kg sc, b.i.d.), and citalopram plus WAY 100635. Bmax, Kd, and functional status [cyclic AMP (cAMP) generation] of beta1-adrenoceptors were assessed in cortical tissue at the same time points. Citalopram reversed CMS-induced reduction of sucrose intake at an earlier time point than imipramine. WAY 100635 was not effective and did not potentiate the effect of citalopram. CMS produced increase of Bmax. Imipramine decreased Bmax in controls (Days 2, 7, 14, and 35) and normalised Bmax in stressed animals (Day 35). Citalopram, WAY 100635, and the combination increased Bmax in stressed animals and controls (Days 14 and 35). Inconsistent changes of Kd values and of cAMP responses to noradrenaline (NA) stimulation were observed. Thus stress- and drug-induced effects on beta1-adrenoceptors do not appear to be a common biochemical marker of antidepressant-like activity in the CMS model.

Synthesis, structure, and binding of some 2-imidazolines to rat brain alfa-1 and alfa-2-adrenergic receptors

A series of novel 2-[(2-aminophenyl)imino]imidazolinium salts 3a-d and N-benzyl-N-(4,5-dihydro-imidazol-2-yl)-O-methylhydroxylamine hydrochloride 7a-c were prepared and their structure was determined by IR and NMR spectroscopic data as well as X-ray analysis of the imidazolinium azide salt 3e. Binding evaluation for both alpha 1- and alpha 2-adrenergic receptors in rat brain preparations of these compounds and the previously described alpha-hydroxy-2-aryliminoimidazolines 11a-d, N-(4,5-dihydroimidazol-2-yl)-1,3-2-oxodihydrobenzimidazoles 12a-b, 2-amino-N-(4,5-dihydroimidazol-2-yl)-benzimidazoles 13a-b, and N-(4,5-dihydroimidazol-2-yl)-indoles 14a-b was performed. Among the compounds tested, 2-[(2-amino-4,5-dichlorophenyl)imino]imidazolinium chloride 3c showed highest binding affinity to alpha 2-adrenoreceptors (Ki = 30 nM).

Antidepressants: past, present and future

Since the discovery of first antidepressants in mid-1950's, the field has been intensively studied. Several new classes of compounds emerged and several hypotheses on the mechanism of their action were proposed. The novel antidepressants are either selective and reversible monoamine oxidase inhibitors, (e.g., moclobemide), or selective serotonin reuptake inhibitors (e.g., citalopram or paroxetine), or serotonin and noradrenaline reuptake inhibitors (e.g. , venlafaxine). Recently neuropeptides (e.g., thyrotropin-releasing hormone,TRH) or antagonists of neuropeptide receptors (e.g., tachykinin NK(1) receptor) undergo clinical tests. Several hypotheses proposed the predominant involvement of one or few neurotransmitter receptors in the mechanism of antidepressant action, but it is now assumed that several distinct receptor mechanisms' trigger different but converging intracellular signal cascades that activate transcription factors, which, in turn, promote the expression of genes encoding for proteins, that play a crucial role in restoring of neuronal functions involved in mood regulation.

Antidepressant drugs inhibit glucocorticoid receptor-mediated gene transcription - a possible mechanism

1. Antidepressant drugs are known to inhibit some changes evoked by glucocorticoids, as well as a hyperactivity of hypothalamic-pituitary-adrenal (HPA) axis, often observed in depression. 2. The aim of present study was to investigate effects of various antidepressant drugs on the glucocorticoid-mediated gene transcription in fibroblast cells, stably transfected with an MMTV promoter (LMCAT cells). 3. The present study have shown that antidepressants (imipramine, amitriptyline, desipramine, fluoxetine, tianeptine, mianserin and moclobemide), but not cocaine, inhibit the corticosterone-induced gene transcription in a concentration- and a time-dependent manner. 4. Drugs which are known to augment clinical effects of medication in depressed patients (lithium chloride, amantadine, memantine), do not affect the inhibitory effects of imipramine on the glucocorticoid receptor (GR)-mediated gene transcription. 5. Inhibitors of phospholipase C (PLC), protein kinase C (PKC), Ca(2+)/calmodulin-dependent protein kinase (CaMK) and antagonists of the L-type Ca(2+) channel also inhibit the corticosterone-induced gene transcription. 6. Inhibitors of protein kinase A (PKA) and protein kinase G (PKG) are without effect on the GR-induced gene transcription. 7. Phorbol ester (an activator of PKC) attenuates the inhibitory effect of imipramine on the GR-induced gene transcription. 8. Imipramine decreases binding of corticosterone-receptor complex to DNA. 9. It is concluded that antidepressant drugs inhibit the corticosterone-induced gene transcription, and that the inhibitory effect of imipramine depends partly on the PLC/PKC pathway.

Different regulation of phospholipase D activity in glioma C6 cells by sphingosine, propranolol, imipramine and phorbol ester

In has been found that sphingosine, propranolol, imipramine and phorbol ester (12-O-tetradecanoylphorbol-13-acetate, TPA) have a stimulatory effect on phospholipase D activity in glioma C6 cells. The cells were prelabelled with [1-(14)C]palmitic acid and phospholipase D-mediated synthesis of [(14)C]phosphatidylethanol was measured. The enhancing effect of TPA was almost completely blocked by a specific protein kinase C inhibitor, GF 109203X. In contrast, GF 109203X failed to inhibit the sphingosine, imipramine and propranolol stimulatory effects, indicating that their stimulation was independent of protein kinase C. The effect of TPA on phospholipase D was also blocked by imipramine and propranolol, whereas sphingosine additively potentiated TPA-mediated phospholipase D activity, both at shorter and longer (2-60 min) times of incubation. These results suggest that in glioma C6 cells, sphingosine is not only involved in a different phospholipase D activation than the TPA regulatory system, but also that it operates in a different compartment of the cell.

Does Ca2+ channel blockade modulate the antidepressant-induced changes in mechanisms of adrenergic transduction?

We investigated how the L-type calcium channel blockade (CCB) with nifedipine affects the cyclic AMP responses to noradrenaline or isoproterenol in cerebral cortical slices from rats receiving antidepressant treatments that induce (electroconvulsive shock, imipramine) or do not induce (amitriptyline) beta-downregulation. To assess the role of protein kinase C (PKC) in receptor crosstalk under CCB conditions, the cyclic AMP responses were tested also in the presence of a PKC activator, TPA. CCB alone induced no changes, but modulated the action of those antidepressants that down regulate the beta-adrenergic system. Chronic ECS and imipramine treatments were differently affected. ECS, under conditions of CCB, down regulated the response to isoproterenol in the presence of TPA, while imipramine ceased to block the TPA-potentiation of cyclic AMP responses. Thus, CCB affects the processes related to the antidepressant-induced changes on the crosstalk between alpha1- and beta-adrenergic receptors, depending on the specific properties of the antidepressant.

Thyroid hormones and the treatment of depression: an examination of basic hormonal actions in the mature mammalian brain

Numerous clinical reports indicate that thyroid hormones can influence mood, and a change in thyroid status is an important correlate of depression. Moreover, thyroid hormones have been shown to be effective as adjuncts for traditional antidepressant medications in treatment-resistant patients. In spite of a large clinical literature, little is known about the mechanism by which thyroid hormones elevate mood. The lack of mechanistic insight reflects, in large part, a longstanding bias that the mature mammalian central nervous system is not an important target site for thyroid hormones. Biochemical, physiological, and behavioral evidence is reviewed that provides a clear picture of their importance for neuronal function. This paper offers the hypothesis that the thyroid hormones influence affective state via postreceptor mechanisms that facilitate signal transduction pathways in the adult mammalian brain. This influence is generalizable to widely recognized targets of antidepressant therapies such as noradrenergic and serotonergic neurotransmission.

The effect of chronic administration of amitriptyline on the effects of subsequent electroconvulsive treatment on responsiveness of alpha 1-and beta-adrenoceptors in the rat cortical slices

Both antidepressant drugs and repeated electroconvulsive shock (ECS) produce adaptive changes in cerebral neurotransmitter systems. As in the clinical practice ECS is used almost always after therapeutical failure of pharmacotherapy, we investigated presently how chronic administration of an antidepressant amitriptyline affects the action of subsequent multiple ECS in rats. Amitriptyline differed from ECS and from other classical antidepressant in producing no beta-downregulation and potentiating the inhibitory effect of protein kinase C activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), on responses of alpha 1-adrenoceptor system to noradrenaline. The action of ECS on alpha 1-adrenoceptor system remained essentially unaffected by previous amitriptyline administration. Its downregulatory effect on responses of beta-adrenoceptor system to noradrenaline, and particularly to isoproterenol, were attenuated by previous drug treatment. The present results suggest that previous chronic administration of antidepressant drugs may alter the effect of subsequent ECS.

Novel benzodioxan derivative, 5-(3-[((2S)-1,4-benzodioxan-2- ylmethyl)amino]propoxy)-1,3-benzodioxole HCl (MKC-242), with a highly potent and selective agonist activity at rat central serotonin1A receptors

The present study characterizes the neurochemical profile of the newly synthesized compound 5-(3-[((2S)-1,4-benzodioxan-2-ylmethyl)amino]propoxy)-1,3-be nzodioxole HCl (MKC-242). In in vitro experiments, MKC-242 had high affinity for serotonin1A (5-HT1A) receptors (Ki: 0.35 nM) and moderate affinity for alpha 1-adrenoceptors (Ki: 21 nM), whereas it had no appreciable affinity for any other neurotransmitter recognition sites studied and 5-HT transporter. MKC-242 (0.3-3.0 mg/kg, s.c.; 1-10 mg/kg, p.o.) caused presynaptic 5-HT1A-receptor-mediated responses (decreases in 5-HT turnover and 5-HT release) and postsynaptic 5-HT1A-receptor-mediated responses (hypothermia, an increase in serum corticosterone level and 5-HT1A behavioral syndrome). The effects of MKC-242 on decarboxylase inhibitor-induced 5-hydroxytryptophan accumulation and rectal temperature were blocked by the 5-HT1A-receptor antagonist N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylpropana mide. The comparative studies on the in vivo responses induced by MKC-242 and the 5-HT1A-receptor full agonist 8-hydroxy-2-(di-n-propyl-amino)tetralin (8-OH-DPAT) showed that MKC-242 and 8-OH-DPAT had similar efficacy at presynaptic 5-HT1A receptors, whereas the former had less efficacy than the latter at postsynaptic 5-HT1A receptors. Furthermore, MKC-242 partially inhibited forskolin-stimulated adenylate cyclase activity in hippocampal membranes. These findings suggest that MKC-242 acts as a full and partial agonist at pre- and postsynaptic 5-HT1A receptors, respectively, in the central nervous system.

Protein kinase C in rat brain cortex and hippocampus: effect of repeated administration of fluoxetine and desipramine

1. Recent evidence indicates that changes in the activity of cyclic AMP-dependent protein kinase may be involved in neuroadaptive mechanisms after chronic treatment with antidepressants. The aim of this study was to investigate the effect of repeated administration of fluoxetine (FL) and desipramine (DMI) on the distribution and activity of protein kinase C (PKC) in subcellular fractions of rat cortex (Cx) and hippocampus (Hc) under basal conditions and in response to a single in vivo administration of 5-HT2A/2C agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). 2. Rats were treated for 21 days with FL (5 mg kg-1 day-1, i.p.) or DMI (10 mg kg-1 day-1, i.p.). DOI was injected to groups of rats receiving repeated doses of antidepressants or to control rats 1 h before ex vivo PKC assay. Distribution of PKC was determined by [3H]-phorbol-12,13-dibutyrate ([3H]-PDBu) binding and PKC activity by the Amersham enzyme assay system. 3. Autoradiography of tissue sections revealed decreased [3H]-PDBu binding in CA1 region of hippocampus (by 18%) and paraventricular thalamic nucleus (by 28%) of rats after repeated administration of FL. 4. In vitro exposure of brain sections to 50 microM FL resulted in significant decreases (by 23-32%) of [3H]-PDBu binding in six out of seven regions examined; exposure to 100 microM FL reduced [3H]-PDBu binding (by 36-52%) in all regions. In contrast, exposure of brain sections to 100 microM DMI failed to alter specific [3H]-PDBu binding in brain sections. 5. The activity of PKC in subcellular fractions of Cx and Hc was significantly (by 40-50%) decreased in rats given repeated doses of FL or DMI. A single administration of either drug was without effect.6. A single in vivo administration of DOI to control rats resulted in reduced PKC activity (by 30-40%)in the particulate fraction of both Cx and Hc. This response to DOI was similar in DMI-treated rats but was not seen in rats given repeated doses of FL. A single administration of DOI to animals given repeated doses of FL resulted in PKC activities higher than those seen in rats treated with FL alone.7. The results indicate that repeated administration of FL and DMI produced similar changes in basal PKC activity but differentially affected the PKC response to the 5-HT2A/2c receptor agonist, DOI. The effect on basal PKC activity may result from a post-receptor action of antidepressants; the alteration of PKC response to DOI after fluoxetine could be due to receptor-mediated desensitization of the signalling system.

Reversal by imipramine of beta-adrenoceptor up-regulation induced in a chronic mild stress model of depression

Male Wistar rats were subjected to a chronic mild stress procedure involving different stress stimuli applied for 8 weeks. During this time the consumption of 1% sucrose solution was monitored at weekly intervals. After the first 3 weeks, when stressed animals displayed a reduction of sucrose consumption, the control and stressed groups were divided into subgroups receiving daily placebo or imipramine (10 mg/kg/day) treatment. After 5 weeks of treatment, 24 h after the last injection, the rats were killed and beta-adrenoceptor density and affinity in cortical membrane preparations and the accumulation of cyclic AMP in cortical slices stimulated with noradrenaline were assessed. While in stressed placebo-treated rats the sucrose consumption remained reduced, in the imipramine-treated group the level of consumption gradually returned to control values. The stressed placebo-treated rats also displayed an increase in cortical beta-adrenoceptor density (by 34%) with no changes in affinity, and an increase (22%) in the cyclic AMP response to noradrenaline in cortical slices. Imipramine, which in non-stressed rats did not affect sucrose intake but depressed the beta-adrenoceptor density and the cyclic AMP response, reversed the stress-induced decrease in sucrose consumption and the increase in the beta-adrenoceptor density; at physiological noradrenaline concentrations it also reduced the enhanced cyclic AMP response. The results suggest that the chronic mild stress procedure produces behavioral and biochemical changes consistent with a realistic model of depression in animals.

The effect of (-)-4-(2-hydroxy-3(N-isopropylamino)-propoxyimino)-cis-carane on basal and forskolin-stimulated accumulation of cyclic AMP in the cerebral cortical slices of the rat

(-)-4-(2-Hydroxy-3(N-isopropylamino)-propoxyimino)-cis-carane++ +, a local anaesthetic and platelet aggregation inhibitor which is much more potent than lignocaine, facilitated forskolin-induced cyclic (c) AMP accumulation in cerebral cortical slices of the rat. Lignocaine was ineffective in this respect. It is hypothesized that a cAMP-related mechanism may be involved in increased efficacy of the compound.

Retrieval associated cholinergic activity and its inhibition by memory updating

Both hippocampal cholinergic and glutamatergic systems are believed to be engaged in learning and memory. By measuring behavior and ex vivo second messenger inositol phosphate (IP) accumulation, we investigated biochemical responses of cholinergic receptors to retrieval and acquisition processes in rats trained in a spatial task. We report that in rats retrieving spatial information, carbachol--induced IP accumulation strongly and transiently increased above values observed in handled controls and rats acquiring new information, and that this increase was profoundly inhibited by N-methyl-D-aspartate (NMDA). These results suggest that memory retrieval, rather than formation of a memory trace, is related to increased responsiveness of the hippocampal cholinergic system, and that formation of a new memory trace, which updates long-term memory, inhibits this cholinergic activation, possibly by a learning-associated increase in NMDA receptor activation. Moreover, the present study shows that the distinction between acquisition and retrieval processes can be demonstrated on both a behavioral and biochemical level.

Actions of lithium on the cyclic AMP signalling system in various regions of the brain--possible relations to its psychotropic actions. A study on the adenylate cyclase in rat cerebral cortex, corpus striatum and hippocampus

It has been estimated that in most industrialized countries 1 person out of every 1000 in the population is undergoing lithium treatment to stabilize their episodic mood disturbances due to manic-depressive illness. Lithium may stabilize mood swings by altering the action of certain neurotransmitters at the synaptic level in the brain. Recent research suggests that lithium alters neurotransmission by affecting neurotransmitter-coupled second messenger systems. A major second messenger system is the adenylate cyclase, which generates intracellular cAMP from ATP. The adenylate cyclases (type I-IV) are regulated by stimulatory and inhibitory receptors, which either stimulate or inhibit the adenylate cyclase activity. The stimulatory and inhibitory neurotransmitter-receptor signals are transferred to the catalytic unit of the adenylate cyclase by Gs and Gi, respectively. The activated receptor induces GTP stimulation of the heterotrimeric G protein, leading to a dissociation of the protein into the active alpha*GTP and the beta gamma complex. The former stimulates the catalytic unit of adenylate cyclase. The stimulation is terminated by a GTPase located on the alpha subunit that converts GTP to inactive GDP. At present, G proteins are known to play a central role in coupling receptors to effector proteins. In addition to extracellular regulation due to neurotransmitters, some adenylate cyclases (type I, III) are regulated by CaM as a consequence of enhanced intracellular concentrations of free Ca2+. The Ca(2+)-dependent stimulation of adenylate cyclase by CaM is assumed to occur by a direct effect on the catalytic unit. The catalytic units sensitive to Ca(2+)-CaM are also subjected to regulation by stimulatory and inhibitory neurotransmitter stimuli. Magnesium is essential for adenylate cyclase activity, since MgATP2- is the enzyme substrate. Furthermore, one Mg2+ site located on the G protein regulates both the receptor agonist affinity and the dissociation of the G protein during the activation cycle. A second Mg2+ site on the catalytic unit is responsible for Mg2+ regulation of the catalytic activity. The present work aimed at investigating the mechanisms by which lithium in vitro and after chronic treatment (ex vivo) affects adenylate cyclase activities in various regions of the rat brain. Lithium in vitro and ex vivo inhibited the selective stimulation of adenylate cyclase by Ca(2+)-CaM in the cerebral cortex. Furthermore, lithium in vitro interacted directly with the catalytic unit of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhancement of the responsiveness of cortical adrenergic receptors by chronic administration of the 5-hydroxytryptamine uptake inhibitor citalopram

The aim of this study was to evaluate the effect of citalopram, a second generation antidepressant agent producing no beta-down-regulation, on the receptors and second messenger systems related to noradrenergic transmission in the cerebral cortex of the rat. We confirmed that citalopram does not bind to alpha 1-, alpha 2-, and beta 1-adrenoceptors, but we found that it attenuates the inhibitory action of the protein kinase C activator, 12-O-tetradecanoylphorbol 13-acetate, on the noradrenergic response from alpha 1-adrenoceptor. In contrast to most antidepressants, chronic treatment with citalopram does not produce beta-down-regulation, but increases the responses to noradrenaline from beta-adrenoceptors without increasing the beta 1-adrenoceptor density. Chronic treatment with citalopram also increases the maximal response from alpha 1-adrenoceptor. The results indicate that beta-down-regulation is not a necessary characteristic of an efficient antidepressant drug.

Centpropazine affinity to cortical noradrenergic receptors and effect on their responsiveness in the rat

We have studied the in-vitro effect of centpropazine on cerebral cortical noradrenergic receptors measured as the accumulation of second messengers, cyclic AMP and inositol phosphate, stimulated by noradrenaline, and the binding to alpha 1- and beta-adrenoceptors. Centpropazine inhibited inositol phosphate, but not the cyclic AMP accumulation in the cerebral cortical slices of the rat. It moderately antagonized the specific binding of [3H]prazosin, but did not affect the specific binding of the beta-adrenoceptor ligand, [3H]CGP 12177, to cerebral cortical membranes.