Modulating Stress Susceptibility and Resilience: Insights from miRNA Manipulation and Neural Mechanisms in Mice

This study explored the impact of microRNAs, specifically mmu-miR-1a-3p and mmu-miR-155-5p, on stress susceptibility and resilience in mice of different strains. Previous research had established that C57BL/6J mice were stress-susceptible, while NET-KO and SWR/J mice displayed stress resilience. These strains also exhibited variations in the serum levels of mmu-miR-1a-3p and mmu-miR-155-5p. To investigate this further, we administered antagonistic sequences (Antagomirs) targeting these microRNAs to C57/BL/6J mice and their analogs (Agomirs) to NET-KO and SWR/J mice via intracerebroventricular (i.c.v) injection. The impact of this treatment was assessed using the forced swim test. The results showed that the stress-susceptible C57/BL/6J mice could be transformed into a stress-resilient phenotype through infusion of Antagomirs. Conversely, stress-resilient mice displayed altered behavior when treated with Ago-mmu-miR-1a-3p. The study also examined the expression of mmu-miR-1a-3p in various brain regions, revealing that changes in its expression in the cerebellum (CER) were associated with the stress response. In vitro experiments with the Neuro2a cell line indicated that the Antago/Ago-miR-1a-3p and Antago/Ago-miR-155-5p treatments affected mRNAs encoding genes related to cAMP and Ca2+ signaling, diacylglycerol kinases, and phosphodiesterases. The expression changes of genes such as Dgkq, Bdnf, Ntrk2, and Pde4b in the mouse cerebellum suggested a link between cerebellar function, synaptic plasticity, and the differential stress responses observed in susceptible and resilient mice. In summary, this research highlights the role of mmu-miR-1a-3p and mmu-miR-155-5p in regulating stress susceptibility and resilience in mice and suggests a connection between these microRNAs, cerebellar function, and synaptic plasticity in the context of stress response.

Effect of clozapine on ketamine-induced deficits in attentional set shift task in mice

RATIONALE

Clozapine (CLZ) is an effective treatment for schizophrenia, producing improvements in both negative symptoms and cognitive impairments. Cognitive impairments can be modelled in animals by ketamine (KET) and assessed using the attentional set-shift task (ASST).

OBJECTIVE

Our first aim was to determine whether CLZ improves cognitive function and reverses KET-induced cognitive impairments using the ASST. Our second aim was to assess dose dependency of these effects.

RESULTS

Our findings demonstrate that acute as well as sub-chronic administration of KET cause cognitive deficits observed as increase in number of trails and errors to reach the criterion in the EDS phase. CLZ 0.3 mg/kg reversed the effects of both acute and sub-chronic KET, with no effects on locomotor activity. However, clozapine's effect after sub-chronic administration of dose 0.3 mg/kg was not as explicit as in the case of acute treatment. Moreover, administration of 1 mg/kg CLZ to KET-treated mice induced or enhanced deficits in the extra-dimensional shift phase compared to 1 mg/kg CLZ administration to mice not receiving KET. Locomotor activity test showed sedation effects of CLZ 1 mg/kg after acute treatment; therefore, effect of CLZ 1 mg/kg on KET-induced cognitive deficits was not evaluated in the attentional set-shift task (ASST) test.

CONCLUSIONS

The present findings support dose-dependent effects of CLZ to reverse KET-induced cognitive deficits. The observed dose dependency may be mediated by activation of different receptors, including monomers and/or heterodimers.

Basal prolactin levels in rat plasma correlates with response to antidepressant treatment in animal model of depression

Prolactin (PRL) has been shown to be altered by psychotropic drugs, including antidepressant drugs (ADs). Many studies have focused on the response to antidepressant treatment (especially related to the serotonergic system) using the fenfluramine test (PRF), however some data suggest lack of correlation between PRF and prediction of clinical response to ADs. In our study we have investigated the hypothesis that basal plasma level of prolactin is a better predictor of antidepressant treatment. We have used Chronic Mild Stress (CMS) - the animal model of depression. Rats are exposed to CMS in combination with imipramine (IMI) treatment for 5 consecutive weeks. Blood samples were collected from the rat tail vein three times: before the CMS procedure, after 2 weeks of stress and after the complete CMS procedure (after 5 weeks of stress and IMI treatment). The PRL level in plasma was determined using the commercially available ELISA kit. In CMS, anhedonia in rats is manifested by reduced consumption of sucrose solution while administration of antidepressant drugs reverses anhedonia. Some animals (ca.30%) did not respond to antidepressant therapy and were considered treatment-resistant. There was no correlation between basal PRL levels and stress response, however, from the results obtained by Spearman Rank Correlation analysis we have observed a significant negative correlation between basal PRL levels before the CMS procedure and behavioral response to IMI administration. The obtained results indicate that the basal PRL level in rat plasma correlates with a good response to treatment in the animal model of depression.

Chronic mild stress alters the somatostatin receptors in the rat brain

RATIONALE

The involvement of somatostatin (SST) and its receptors in the pathophysiology of depression and stress has been evidenced by numerous studies.

OBJECTIVES

The purpose of the present study was to find whether chronic mild stress (CMS), an animal model of depression, affects the SST receptors in the rat brain and pituitary, as well as the level of SST in plasma.

METHODS

In CMS model, rats were subjected to 2 weeks of stress and behaviorally characterized using the sucrose consumption test into differently reacting groups based on their response to stress, i.e., stress-reactive (anhedonic), stress-non-reactive (resilient), and invert-reactive rats (characterized by excessive sucrose intake). We measured specific binding of [125I]Tyr3-Octreotide, expression of mRNA encoding sst2R receptors in the rat brains, expression of SST and its receptors in rat pituitary, and the level of SST in the plasma.

RESULTS

The obtained results show decreases in binding of [125I]Tyr3-Octreotide in most of rat brain regions upon CMS and no significant differences between three stressed groups of animals, except for significant up-regulation of sst2 receptor in medial habenula (MHb) in the stress-reactive group. In the same group of animals, significant increase in plasma SST level was observed.

CONCLUSIONS

There are two particularly sensitive sites distinguishing the response to stress in CMS model. In the brain, it is MHb, while on the periphery this predictor is SST level in plasma. These changes may broaden an understanding of the mechanisms involved in the stress response and point to the intriguing role of MHb.

Stathmin reduction and cytoskeleton rearrangement in rat nucleus accumbens in response to clozapine and risperidone treatment - Comparative proteomic study

The complex network of anatomical connections of the nucleus accumbens (NAc) makes it an interface responsible for the selection and integration of cognitive and affective information to modulate appetitive or aversively motivated behaviour. There is evidence for NAc dysfunction in schizophrenia. NAc also seems to be important for antipsychotic drug action, but the biochemical characteristics of drug-induced alterations within NAc remain incompletely characterized. In this study, a comprehensive proteomic analysis was performed to describe the differences in the mechanisms of action of clozapine (CLO) and risperidone (RIS) in the rat NAc. Both antipsychotics influenced the level of microtubule-regulating proteins, i.e., stathmin, and proteins of the collapsin response mediator protein family (CRMPs), and only CLO affected NAD-dependent protein deacetylase sirtuin-2 and septin 6. Both antipsychotics induced changes in levels of other cytoskeleton-related proteins. CLO exclusively up-regulated proteins involved in neuroprotection, such as glutathione synthetase, heat-shock 70-kDa protein 8 and mitochondrial heat-shock protein 75. RIS tuned cell function by changing the pattern of post-translational modifications of some proteins: it down-regulated the phosphorylated forms of stathmin and dopamine and the cyclic AMP-regulated phosphoprotein (DARPP-32) isoform but up-regulated cyclin-dependent kinase 5 (Cdk5). RIS modulated the level and phosphorylation state of synaptic proteins: synapsin-2, synaptotagmin-1 and adaptor-related protein-2 (AP-2) complex.

cAMP receptor protein from escherichia coli as a model of signal transduction in proteins--a review

In Escherichia coli, cyclic AMP receptor protein (CRP) is known to regulate the transcription of about 100 genes. The signal to activate CRP is the binding of cyclic AMP. It has been suggested that binding of cAMP to CRP leads to a long-distance signal transduction from the N-terminal cAMP-binding domain to the C-terminal domain of the protein, which is responsible for interaction with specific sequences of DNA. The signal transduction plays a crucial role in the activation of the protein. The most sophisticated spectroscopic techniques, other techniques frequently used in structural biochemistry, and site-directed mutagenesis have been used to investigate the details of cAMP-mediated allosteric control over CRP conformation and activity as a transcription factor. The aim of this review is to summarize recent works and developments pertaining to cAMP-dependent CRP signal transduction in E. coli.

Norepinephrine transporter knockout-induced up-regulation of brain alpha2A/C-adrenergic receptors

The norepinephrine transporter (NET) is responsible for the rapid removal of norepinephrine released from sympathetic neurons; this release is controlled by inhibitory alpha(2)-adrenergic receptors (alpha(2)ARs). Long-term inhibition of the NET by antidepressants has been reported to change the density and function of pre- and postsynaptic ARs, which may contribute to the antidepressant effects of NET inhibitors such as desipramine. NET-deficient (NET-KO) mice have been described to behave like antidepressant-treated mice. By means of quantitative real-time PCR we show that mRNAs encoding the alpha(2A)-adrenergic receptor (alpha(2A)AR) and the alpha(2C)-adrenergic receptor (alpha(2C)AR) are up-regulated in the brainstem, and that alpha(2C)AR mRNA is also elevated in the hippocampus and striatum of NET-KO mice. These results were confirmed at the protein level by quantitative autoradiography. The NET-KO mice showed enhanced binding of the selective alpha(2)AR antagonist [(3)H]RX821002 in several brain regions. Most robust increases (20-25%) in alpha(2)AR expression were observed in the hippocampus and in the striatum. Significant increases (16%) were also seen in the extended amygdala and thalamic structures. In an 'in vivo' test, the alpha(2)AR agonist clonidine (0.1 mg/kg) caused a significantly greater reduction of locomotor activity in NET-KO mice than in wild-type mice, showing the relevance of our findings at the functional level.

The effect of combined treatment with imipramine and amantadine on the behavioral reactivity of central ??1-adrenergic system in rats

The problem of drug-resistant depression implies a strong need for alternative antidepressant therapies. Recently, it has been shown that joint administration of a tricyclic antidepressant, imipramine (IMI), with amantadine (AMA), a drug already approved for clinical use in the treatment of other diseases, induces a stronger 'antidepressant' effect in the forced swimming test in rats than treatment with either drug given separately. Combined treatment with IMI and AMA also induces up-regulation of dopamine D2 and D3 receptors in the rat brain, and appears to be effective in the treatment of patients with drug-resistant unipolar depression. In the present study, we examined the effect of IMI (5 or 10 mg/kg p.o.) and AMA (10 mg/kg p.o.) given separately or jointly, either as a single dose or repeatedly (twice daily for 14 days) on the development of adaptive changes in the behavioral reactivity of the central alpha1-adrenergic system. Following repeated administration of the higher dose of IMI together with AMA, we observed an increase in clonidine-induced aggression in mice, and significant enhancement of D-amphetamine-induced locomotor hyperactivity, as well as phenylephrine-induced exploratory behavior, in rats. In binding studies using [3H]prazosin, no changes in the density (Bmax) or affinity (Kd) of alpha1-adrenergic receptors were observed in rat brain cortex. However, competition analysis allowed us to observe an increase in the affinity of alpha1-adrenergic receptors (Ki) for an agonist (phenylephrine) upon repeated treatment with IMI, given alone or in combination with AMA. AMA appears to act through several pharmacological mechanisms, none of which has been identified as the chief mode of action. In the light of data obtained in the present study, one can supplement the postulated mechanisms of antidepressant action of AMA by adaptive changes in the reactivity of alpha1-adrenergic receptors, which develop upon repeated combined treatment with IMI.

Effect of combined treatment with imipramine and amantadine on the central dopamine D2 and D3 receptors in rats

In spite of intensive research, the problem of treating antidepressant-resistant depressive patients has not yet been solved. Our previous studies demonstrated that joint administration of a tricyclic antidepressant drug, imipramine (IMI) with the uncompetitive antagonist of NMDA receptors, amantadine (AMA), produced stronger "antidepressant" effect in the forced swimming test (Porsolt's test) than the treatment with either drug alone given. Since it has been suggested that dopamine receptors, among others, may play a role in anti-immobility effect of IMI, in the present study we examined the effect of AMA (10 mg/kg) and IMI (5 and 10 mg/kg) given separately or jointly, as a single dose or repeatedly (twice daily for 14 days) on the dopamine D2 and D3 receptors in the rat brain, using receptor autoradiography. Following repeated administration of AMA alone or given in combination with IMI (5 mg/kg), the binding of [3H]quinpirole (dopamine D2/D3 receptors agonist) was increased, and similar changes were observed at the level of mRNA encoding dopamine D2 receptors. We used [3H]7-OH-DPAT to selectively label the dopamine D3 receptors. This experiment has shown that AMA given repeatedly did not induce statistically significant changes in the D3 receptor binding, while IMI at both used doses, increased the [3H]7-OH-DPAT binding, and this effect was still observed after repeated joint administration of AMA with both doses of IMI. However, using both radioligands, we did not observe any synergistic or even additive effects in the binding studies after joint administration of AMA and IMI. Nevertheless, we can conclude that repeated administration of AMA, given together with IMI, induces the up-regulation of dopamine D2 and D3 receptors in the rat brain, and this effect may explain their synergistic action observed in the behavioral studies involving dopaminergic transmission.

Effect of tianeptine and fluoxetine on the levels of Met-enkephalin and mRNA encoding proenkephalin in the rat

The purpose of the present study was to evaluate the effects of acute and repeated treatment with two antidepressant drugs (ADs) of opposite pharmacological profile, i.e. tianpetine (TIA, serotonin reuptake enhancer) and fluoxetine (FLU, serotonin reuptake inhibitor) on the levels of Met-Enkephalin, (Met-Enk, a member ofopioid peptide family, which has been suggested to play a role in the mechanism of action ADs) as well as on mRNA coding for proenkephalin (mRNA PENK) in various regions of the rat brain, pituitary, adrenal glands and plasma. Male Wistar rats were treated acutely or repeatedly (10 mg/kg p.o., twice daily for 14 days) with TIA or FLU. Tissue for biochemical experiments was taken 2 h after last dose of appropriate drug. The levels of Met-Enk were estimated by radioimmunoassay, mRNA PENK was measured using in situ hybridization. From the results obtained in the present study it may be concluded that repeated administration of TIA or FLU induced similar changes in the levels of Met-Enk in the rat hippocampus, striatum, hypothalamus and neurointermediate lobe of pituitary. Such an effect is interesting, especially if one takes into account the differences in pharmacological profile between these two antidepressant drugs. It may be suggested that serotonin level might not be crucial for inducing the alterations in the content of Met-Enk. Since we did not observe any changes in the levels of PENK mRNA in the studied rat brain regions after repeated administration of TIA or FLU, it seems that the observed changes in the levels of Met-Enk do not result from effects of these antidepressants on biosynthesis of PENK, but rather from alterations in the peptide release. Another interesting finding of the present study was that in the anterior lobe of pituitary, adrenal glands and plasma, repeated administration of TIA induced alterations in the contents of Met-Enk, while repeated administration of FLU remained without any effect. It is tempting to speculate that such a differentiation between the effects of these two antidepressants might be linked to the well known feature of TIA (but not FLU) which has been shown to reduce both basal and stress-evoked activity of the hypothalamic-pituitary-adrenal (HPA) axis.

Effect of repeated treatment with tianeptine and fluoxetine on central dopamine D(2) /D(3) receptors

Tianeptine (TIA) is an antidepressant drug that has been shown to decrease extracellular serotonin level and reveals no affinity for neurotransmitter receptors. The present study was aimed at determining whether repeated TIA treatment induced any adaptive changes in the central dopamine D(2)/D(3) system (behavioural and biochemical) similar to those reported earlier for tricyclic antidepressants. Experiments were carried out on male Wistar rats. TIA was administered at a dose of 5 and 10 mg/kg once or repeatedly (twice daily for 14 days). Fluoxetine (FLU), used as a reference compound, was also administered at a dose of 10 mg/kg. The results obtained showed that TIA or FLU administered repeatedly increased the hyperlocomotion induced by D-amphetamine and 7-hydroxy-dipropylaminotetralin (7-OH-DPAT). Biochemical study revealed a decrease in the [(3)H]7-OH-DPAT binding sites after acute and repeated treatment with TIA or FLU in the islands of Calleja minor, as well as in the shell part of nucleus accumbens septi. On the other hand, both TIA and FLU administered repeatedly increased the binding of [(3)H]quinpirole (a D(2)/D(3) receptor agonist) in the nucleus caudatus as well as in the core part of the nucleus accumbens septi. Similar effects have been observed when dopamine D(2)/D(3) receptors were visualized with the use of [3H]raclopride, a dopamine D(2)/D(3) receptor antagonist. However, TIA and FLU induced a decrease in the level of mRNA encoding for dopamine D(2) receptors, not only after repeated but also after acute treatment. These results indicate that repeated TIA and FLU administration induces adaptive changes in the dopaminergic D(2)/D(3) system and especially enhances the functional responsiveness of dopamine D(2) and D(3) receptors. However, the question of whether this increased responsiveness is important for clinical antidepressant efficacy remains open.

Effect of repeated treatment with mirtazapine on the central alpha1-adrenergic receptors

Mirtazapine (MIR) is an antidepressant which enhances noradrenergic and serotonergic 5-HT1A neurotransmission via antagomism of central alpha2-adrenergic autoreceptors and heteroreceptors. The drugs does not inhibit noradrenaline and serotonin reuptake but blocks the 5-HT, and 5-HT3 receptors and has high affinity only for central and peripheral histamine H1 receptors. The present study was aimed at determining whether repeated MIR treatment induced adaptive changes in the alpha1-adrenergic receptors, similar to those reported by us early for tricyclic antidepressants, The experiments were carried out on male mice and rats. MIR was administered at a dose of 10 mg/kg once or repeatedly (twice daily for 14 days). The obtained results showed that MIR administrated repeatedly potentiated the methoxamine- induced exploratory hyperactivity in rats and clonidine-induced aggressiveness in mice, those effects being mediated by alpha1-adrenergic receptors. MIR given repeatedly (but not acutely) increased the binding (Bmax ) of [3H]prazosin to alpha1-adrenergic receptors in cerebral cortex, however, the ability of the alpha1-adrenoceptor agonist phenylephrine to compete for the these sites was not significantly changed. The above results indicate that repeated MIR administration increases the responsiveness of alpha1-adrenergic system (behavioural and biochemical changes), as tricyclics do. However, the question whether the increased functional responsiveness found in the present study is important for the clinical antidepressant efficacy, remains open.

Effect of repeated treatment with tianeptine and fluoxetine on the central alpha(1)-adrenergic system

Tianeptine (TIA) is an antidepressant drug which enhances the reuptake of serotonin but, in contrast to tricyclics, shows no affinity for neurotransmitter receptors. The present study was aimed at determining whether repeated TIA treatment induced adaptive changes in the alpha(1)-adrenergic system, similar to those reported by us earlier for tricyclic antidepressants. The experiments were carried out on male mice and rats. TIA was administered at a dose of 5 or 10mg/kg once or repeatedly (twice daily for 14 days) and fluoxetine (FLU), used as a reference compound, at a dose of 10mg/kg. The obtained results showed that TIA administered repeatedly potentiated the methoxamine- and phenylephrine (PHEN)-induced exploratory hyperactivity in rats and clonidine-induced aggressiveness in mice, the effects mediated by alpha(1)-adrenoceptors. TIA given repeatedly (but not acutely) increased the binding (B(max)) of alpha(1)-adrenergic receptors in cerebral cortex for [(3)H]prazosin. However, the ability of the alpha(1)-adrenoceptor agonist PHEN to compete for these sites was not significantly changed. The above results indicate that repeated TIA administration increases the responsiveness of the alpha(1)-adrenergic system (behavioural and biochemical changes). On the other hand, FLU did not affect any behavioural and biochemical changes in this system.

The effect of repeated treatment with pramipexole on the central dopamine D3 system

The study examined the effect of pramipexole (2-amino-4,5,6,7-tetrahydro-6-propyl-aminobenzthiazole dihydrochloride; PRA), a new potent dopamine receptor agonist with the high preference for D3 receptors, as compared to D2 or D4, on the central dopamine D3 system. Experiments were conducted on male Wistar rats. PRA was injected subcutaneously. PRA given repeatedly (14 days, twice a day, in doses of 0.3 and 1 mg/kg), but not acutely, potentiated the locomotor hyperactivity induced by (+/-)-7-OH-DPAT (3mg/kg s.c.), when given 24h after the single or the last dose of PRA. Administration of PRA, 1 mg/kg, for 3 or 7 days produced an effect similar to that described above, whereas a dose of 0.3 mg/kg produced such an effect only after 7, but not 3, days. Repeated treatment with PRA (0.3 and 1 mg/kg, 14 days, twice daily) also enhanced the D3 receptor binding in the islands of Calleja and nucleus accumbens (shell)--the brain region known to be rich in D3 receptors--when [3H]7-OH-DPAT was used as a ligand. Repeated PRA administration did not change the concentration of mRNA coding for D3 receptors in the islands of Calleja. The obtained results indicate that-- like the previously studied typical antidepressants given repeatedly--PRA increases the functional responsiveness and the binding to the brain dopamine D3 receptors. Hence PRA may be considered as a potential antidepressant drug.

Pharmacological effects of milnacipran, a new antidepressant, given repeatedly on the alpha1-adrenergic and serotonergic 5-HT2A systems

Milanacipran (MIL) is a representative of a new class of antidepressants (SNRIs) which inhibit selectively the reuptake of serotonin and noradrenaline but, in contrast to tricyclics, show no affinity for neurotransmitter receptors. The present study was aimed at determining whether repeated MIL treatment induced adaptive changes in the alpha1-adrenergic and serotonergic 5-HT2A systems, similar to those reported by us earlier for tricyclic antidepressants. The experiments were carried out on male mice and rats. MIL was administered at a dose of 10 or 30 mg/kg p.o. once or repeatedly (twice daily for 14 days). The obtained results showed that MIL administered repeatedly potentiated the clonidine-induced aggressiveness and the methoxamine-induced exploratory hyperactivity, the effects mediated by alpha1-adrenoceptors. MIL did not change the number or affinity (Bmax and K(D)) of alpha1-adrenergic receptors in the cerebral cortex for [3H]prazonsin, however, the ability of the alpha1-adrenoceptor agonist phenylephrine to compete for these sites was significantly enhanced. MIL given repeatedly (but not acutely) inhibited both the head twitch reaction induced by L-5-HTP or (+/-)DOI, the effects mediated by serotonergic 5-HT2A receptors. MIL also decreased the binding (Bmax) or [3H]-ketanserin to 5-HT2A receptors in the cerebral cortex. The above results indicate that repeated MIL administration increases the responsiveness of alpha1-adrenergic system (behavioural and biochemical changes) and decreases the responsiveness of the serotonergic 5-HT2A receptors (especially behavioural changes) as tricyclics do. It may be concluded that the lack of MIL affinity for neurotransmitter receptors is of no importance to the development of adaptive changes in the studied systems, observed after repeated treatment with antidepressants.

Some behavioural effects of antidepressant drugs are time-dependent

1. The effects of repeated administration of antidepressant drugs (imipramine, IMI and citalopram, CIT) on the beta- and alpha2-adrenergic as well as dopaminergic D3 receptors were compared with time-dependent changes in the receptor responsiveness after acute treatment. 2. Repeated treatment with IMI or CIT (administered at a dose of 10 mg/kg p.o. twice a day for 14 days) induced down-regulation of beta-adrenergic receptors, demonstrated by behavioural experiment using salbutamol-induced hypoactivity and by binding studies using [3H]CGP12177. The changes in alpha2-adrenergic receptors were studied using clonidine-induced hypoactivity, which was attenuated by repeated treatment with IMI or CIT. Behavioural responsiveness of dopamine D3 receptors was investigated using two doses of 7-OH-DPAT. This drug at a dose of 0.05 mg/kg s.c. induced locomotor hypoactivity (interpreted as a result of stimulation of presynaptic dopamine D3 receptors), which was reversed by repeated administration of IMI or CIT, while 7-OH-DPAT at a dose of 3 mg/kg s.c. (which stimulated postsynaptic dopamine D3 receptors) induced significant hyperactivity, which was markedly enhanced by repeated administration of antidepressant drugs. 3. The effect of acute administration of IMI or CIT measured 14 days after drug treatment were similar to the described above alterations at the level of alpha2 adrenoreceptors and presynaptic dopamine D3 receptors, i.e. the drugs attenuated clonidine-induced hypoactivity and reversed locomotor hypoactivity evoked by low dose of 7-OH-DPAT. To induce the down-regulation of beta-adrenergic receptors or up-regulation of the behavioural responsiveness of dopaminergic D3 postsynaptic receptors, the repeated administration of IMI or CIT was necessary. 4. Therefore it has been concluded that presynaptic dopaminergic D3 and alpha2-adrenergic receptors are more sensitive to the acute treatment with antidepressant drugs than postsynaptic D3 and beta-adrenergic receptors.

Antidepressant drugs attenuate 7-OH-DPAT-induced hypoactivity in rats

Various antidepressant drugs given repeatedly induce the supersensitivity of postsynaptic dopamine D2 and D3 receptors. Several reports have also suggested the subsensitivity of presynaptic dopamine D2 receptors. The aim of the present study was to investigate the effect of two antidepressant drugs with different pharmacological profile, i.e. imipramine and citalopram, administered repeatedly, on the hypoactivity induced by low dose (0.05 mg/kg sc) of (+/-)7-hydroxy-dipropylaminotetralin (7-OH-DPAT), a dopamine D3 receptor preferring agonist. Male Wistar rats were treated with antidepressant drugs (10 mg/kg po) either acutely (single dose) or repeatedly (twice daily for 14 days). Two or 24 h after the last dose of antidepressant drug, the locomotor activity induced by (+/-)7-OH-DPAT was measured in photoresistor actometers. Additionally, the influence of nafadotride (0.2 or 1 mg/kg ip), a dopamine D3 preferring antagonist, on the (+/-)7-OH-DPAT-induced changes in locomotor activity was studied. Low dose of (+/-)7-OH-DPAT induced the locomotor hypoactivity, however, this effect was not modulated by nafadotride. Antidepressant drugs given repeatedly, but not acutely, reversed the effect of (+/-)7-OH-DPAT, and this effect of antidepressants was antagonized by nafadotride. The obtained results indicate that the sensitivity of dopamine D3 receptors might be altered by the repeated treatment with antidepressant drugs.

The role of dopamine D2 receptor in the behavioral effects of imipramine--study with the use of antisense oligonucleotides

Antisense strategies have a potential to specifically block the production of a given protein, e.g. receptor subtype, thus may help to uncover its behavioral and/or biochemical function. In the present study we demonstrated the utility of this approach for studying the role of dopamine D2 receptors in the anti-immobility effect of imipramine in the forced swimming test. Following intracerebroventricular (i.c.v.) administration of phosphorothioate oligonucleotide complementary to mRNA encoding for dopamine D2 receptors (D2 antisense ODN; 1 nmol/1 microl H2O, twice a day for 5 days) to the rats, the decrease in the locomotor activity (shortened total distance travelled and decrease in vertical activity, without differences in the stereotypic movements of animals), as well as the decrease of specific binding of [3H]raclopride in the striatum and limbic forebrain were observed. At the same time, i.c.v. administration of D2 antisense ODN reversed the effect of imipramine in the forced swimming test, what may indicate that the dopamine D2 receptors play a significant role in the behavioral anti-immobility effects of imipramine.

Effect of repeated treatment with milnacipran on the central dopaminergic system

Milnacipran (MIL) is a representative of a new class of antidepressants (SNRIs) which inhibit the reuptake of serotonin and noradrenaline, but, in contrast to tricyclics, show no affinity for neurotransmitters receptors. The present study was aimed at determining whether repeated MIL administration (given at doses of 10 or 30 mg/kg, twice daily for 14 days) induced the adaptive changes in the dopaminergic system similar to those reported by us earlier for tricyclic antidepressants. The obtained results showed that MIL administered repeatedly did not change the responsiveness of dopamine D1 receptors since it did not change the SKF 38393-induced grooming. Repeated MIL treatment increased the hyperlocomotion induced by D-amphetamine and 7-OH-DPAT, but did not affect the D-amphetamine and apomorphine stereotypies. The binding parameters (Bmax and Kd) to dopamine D1 and D2 receptors in the limbic forebrain were not affected by repeated MIL treatment when [3H]SCH 23390 and [3H]spiperone, respectively, were used as ligands. On the other hand, the increased density of dopamine D2 receptors (Bmax) was observed in the striatum after repeated treatment with MIL. MIL administered acutely or repeatedly did not change the binding of [3H]7-OH-DPAT to dopamine D3 receptors in the islands of Calleja and the shell region of the nucleus accumbens septi. The above results indicate that repeated MIL administration induces the adaptive changes in the dopaminergic system, especially it enhances the functional responsiveness of dopamine D2 and D3 receptors. However, the question whether this increased functional responsiveness is important for the clinical antidepressant efficacy, remains open.

Effects of antidepressant drugs on the dopamine D2/D3 receptors in the rat brain differentiated by agonist and antagonist binding--an autoradiographic analysis

In the present study the effect of antidepressant drugs on the density of dopamine D2/D3 receptors in the rat forebrain was examined using autoradiography, since this technique provides the appropriate anatomical resolution. Male Wistar rats were treated with three various antidepressant drugs: imipramine, citalopram and mianserin in a dose of 10 mg/kg p.o., acutely (single dose) or repeatedly (twice a day for 14 days). To estimate the distribution of D2/D3 receptors, we chose following radioligands: [3H]raclopride, a non-selective antagonist of D2/D3 receptors, and [3H]quinpirole, a non-selective D2/D3 agonist. When [3H]raclopride was used as a radioligand, no significant differences in the density of D2/D3 receptors were observed after administration of the investigated drugs. However, following repeated administration of imipramine, citalopram and mianserin, a significant increase in the binding of [3H]quinpirole was observed, both in the nucleus caudatus and nucleus accumbens septi. In some cases the increase of [3H]quinpirole binding was also observed after acute treatment with antidepressant drugs. Thus, using an agonist as a radioligand, we were able to see upregulation of dopamine D2/D3 receptors in the rat forebrain following administration of antidepressant drugs, which might be interpreted as the biochemical correlative for the postsynaptic dopamine D2/D3 receptor supersensitivity observed in previous behavioral studies.

Effect of local intracerebral administration of EMD 57445, a selective sigma receptor ligand, on the locomotor activity of the rat

EMD 57445 is a new compound which has been characterized by high affinity for sigma receptor sites. It has not been shown to bind to any other receptors, including dopamine ones. However, hitherto existing data (behavioral as well as biochemical) have suggested that this drug exhibits functional antidopaminergic activity. Therefore, in the present study, the local cerebral administration of EMD 57445 has been used in order to better elucidate the actual site of action of this compound in the central nervous system. EMD 57445 given unilaterally into the nucleus accumbens, striatum and lateral ventricle decreases the locomotor activity of the rats, the effect being the most pronounced in the case of administration into the nucleus accumbens. Moreover, unilateral intraaccumbal injection of EMD 57445 significantly diminishes apomorphine (given peripherally)-induced hyperactivity. Local administration of EMD 57445 into the prefrontal cortex of the rats also attenuated the locomotor activity but the effect was statistically significant only after bilateral administration of the compound. Additionally, the lack of influence of EMD 57445, administered po, on the dopamine D1 and D2 receptor binding in the striatal membranes was observed in these studies. However, in limbic forebrain the density of D2 receptors decreased after the higher dose of EMD 57445. In conclusion, the results obtained in this paper support the hypothesis that EMD 57445 exerts antidopaminergic activity through indirect inhibition of dopamine system.

A search for new 5-HT1A/5-HT2A receptor ligands. In vitro and in vivo studies of 1-[omega-(4-aryl-1-piperazinyl)alkyl]indolin-2(1H)-ones

A series of 1-¿omega-(4-aryl-1-piperazinyl)alkyl]indolin-2(1H)-one derivatives 2-14 was synthesized in order to obtain ligands with a dual 5-HT1A/5-HT2A activity. The majority of those compounds (2-5, 7, 10-13) exhibited a high 5-HT1A (Ki = 2-44 nM) and/or 5-HT2A affinity (Ki = 51 and 39 for 5 and 7, respectively). Induction of lower lip retraction (LLR) and behavioral syndrome and inhibition of these effects evoked by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) were used for determination the agonistic and antagonistic activity, respectively, at 5-HT1A receptors. The 5-HT2A antagonistic activity was assessed by the blocking effect on the head twitches induced by (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) in mice. Two of the tested compounds, 1-¿3-[4-(3-chlorophenyl)-1-piperazinyl]propyl¿-6-fluoroindolin-2(1 H)-one (5) and 1-¿3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl¿indolin-2(1H)-one (7), demonstrated a high 5-HT1A/5-HT2A affinity and an in vivo antagonistic activity towards both receptor subtypes.

Effect of antidepressant drugs administered repeatedly on the dopamine D3 receptors in the rat brain

Previous studies have indicated that antidepressant drugs displaying different pharmacological profiles, administered repeatedly, increase the locomotor hyperactivity induced by various dopaminomimetics, among others by quinpirole. As this drug, according to a recent study, shows high affinity not only for dopamine D2 but also for dopamine D3 receptors, the question arises if dopamine D3 receptors are involved in the increase in quinpirole-elicited locomotor hyperactivity induced by repeated treatment with antidepressant drugs. In the present study we administered imipramine, amitriptyline, citalopram and mianserin (in a dose of 10 mg/kg p.o., twice a day, 14 days) to male Wistar rats and then (+/-)-7-OH-DPAT (7-hydroxy-dipropylaminotetralin), a dopamine D3 receptor agonist, was given (3 mg/kg s.c.). Hyperlocomotion induced by (+/-)-7-OH-DPAT was significantly increased by repeated administration of antidepressant drugs. The receptor autoradiography technique with [3H]7-OH-DPAT as a radioligand was applied to measure the effects of antidepressant drugs treatment on the dopamine D3 receptors in the islands of Calleja and in the shell of the nucleus accumbens septi, which are brain regions with highly selective expression of dopamine D3 receptors. The biochemical studies indicated that in both examined brain regions there was an increase in the binding of [3H]7-OH-DPAT following the repeated administration of antidepressant drugs. In some cases this increase was also observed after the acute administration of antidepressants. The results obtained in the present study indicate that antidepressant drugs administered repeatedly enhance the responsiveness of dopamine D3 receptors, probably via an increase in the density of these receptors. This mechanism is probably similar to that observed already in the case of dopamine D2 receptors. Therefore it is hypothesized that dopamine D3 receptors are also involved in the increased responsiveness to dopamine D3 receptor agonists observed after antidepressants.

Effect of kainic acid and pentetrazole administration on the activity of nitric oxide synthase in the mouse brain

The present study examined ex vivo effect of kainic acid and pentetrazole administration on the activity of nitric oxide synthase (NOS) in the mouse brain. NOS activity was assayed by measuring the formation of [3H] citrulline from [3H]arginine in the homogenates of mouse hippocampus, neocortex and cerebellum. The highest basal activity of the enzyme was found in this latter brain region. Administration of kainic acid (30 mg/kg) increased the NOS activity in all brain regions examined. On the other hand, pentetrazole (60 mg/kg) did not evoke any significant changes in the NOS activity at 5 min after the administration. Only in cerebellum, at 10 min after administration of pentetrazole, the increase in the activity of the enzyme was observed. The obtained results indicate that the two particular convulsants used in this study differ not only in respect of behavioral signs of seizures which they evoke, but also in respect of the effect on mouse brain NOS activity.

Pharmacological profile of venlafaxine, a new antidepressant, given acutely

Pharmacological effects of acute treatment with venlafaxine (VEN), a clinically active antidepressant [a noradrenaline (NA) and 5-hydroxytryptamine (5-HT) reuptake inhibitor without any affinity for neurotransmitter receptors] were studied in mice and rats. VEN inhibited the reserpine- or apomorphine-induced hypothermia and enhanced the L-5-HTP-induced head twitches in mice. It reduced the immobility time in Porsolt's test in mice and rats, but either did not change the locomotor activity (mice) or decreased it (rats). VEN reduced the locomotor hyperactivity induced by amphetamine (AMP), apomorphine (APO) and quinpirole (QUI), as well as the APO-induced stereotypy; the stereotypy induced by AMP in rats was prolonged. VEN neither changed the clonidine-induced aggressiveness in mice nor the behavioral syndrome induced by oxotremorine in rats. The obtained results indicate that VEN, given acutely, shows a pharmacological profile similar to that of tricyclic NA and 5-HT reuptake inhibitors. In contrast to the antidepressants mentioned above, VEN does not exhibit an alpha 1-adrenolytic or a cholinolytic activity (in vivo tests).

The effect of prolonged treatment with imipramine on the biosynthesis and functional characteristics of D2 dopamine receptors in the rat caudate putamen

1. The present study shows the effects of imipramine in a single dose (10 mg kg(-1), p.o.) or following repeated (14 days, twice a day) treatment on the level of mRNA coding for D2 dopamine receptors in the rat caudate putamen (CP). Repeated administration of imipramine resulted in the increase of the level of mRNA coding for D2 dopamine receptors. 2. Radioligand binding studies with the D2 receptor agonist, [3H]-N-0437, indicated, that following imipramine administration, the affinity of the agonist for the D2 dopamine receptor significantly increased, though without any alterations in the Bmax. 3. Pharmacological manipulations (by use of forskolin, GppNHp and quinpirole) of the cyclic AMP generating system, ex vivo following administration of imipramine indicated that an up-regulation of factors inhibiting cyclic GMP formation takes place. 4. Most probably it is the D2 dopamine receptor which undergoes functional up-regulation, resulting from the enhancement of its biosynthesis.

Changes in dopamine receptor mRNA expression following chronic mild stress and chronic antidepressant treatment

This study used in-situ hybridization to examine D1 and D2 receptor mRNA expression in the brains of rats subjected to chronic mild stress and/or chronic treatment (5 weeks) with the antidepressant drugs imipramine and fluoxetine. As in previous studies using the chronic mild stress procedure, stress decreased the consumption of a palatable sucrose solution, and both antidepressants reversed this effect. In-situ hybridization to D1 and D2 receptor probes was studied in midbrain and forebrain sections. In the midbrain sections, stress decreased the D2 receptor message in the substantia nigra and in the lateral part of the ventral tegmental area (VTA), but not in the medial part of the VTA. Imipramine, but not fluoxetine, increased the D2 receptor message in the VTA, but only in the non-stressed group. In the basal ganglia, stress decreased the D2 receptor message in the nucleus accumbens (core and shell) and in the lateral, but not the medial, part of the caudate nucleus, in both control and stressed groups. Both imipramine and fluoxetine increased the D2 receptor message in the lateral, but not the medial, part of the caudate nucleus and in the shell, but not the core, of the nucleus accumbens. There were no significant changes in D1 receptor message. The results, which are consistent in some respects, and inconsistent in others, with predictions from earlier work, support the hypothesis that forebrain dopamine systems are involved in mediating the behavioural effects of chronic mild stress and their reversal by antidepressants.

The effect of imipramine on the amount of mRNA coding for rat dopamine D2 autoreceptors

Several reports have investigated the possibility that chronic antidepressant treatment alters dopamine autoreceptors. Since radioligand binding studies do not differentiate between presynaptic and postsynaptic dopamine D2 receptors in the rat forebrain, we used the in situ hybridization technique to measure the amount of mRNA coding for dopamine D2 autoreceptors in the dopaminergic cell bodies. The amount of mRNA coding for dopamine D2 autoreceptors in the rat mesencephalon was analyzed following acute and repeated treatment with imipramine, the most widely used antidepressant drug. No significant changes in the amount of mRNA were observed in the substantia nigra of the rat, after acute or repeated treatment with imipramine. In the ventral tegmental area repeated treatment with imipramine (14 days, twice a day) increased the amount of dopamine D2 autoreceptor mRNA in the lateral part of this brain region (containing nucleus paranigralis and n. parabrachialis pigmentosus), without there being any significant changes in the more medial part (n. interfascicularis and n. linearis). The increase in the amount of dopamine D2 autoreceptor mRNA in the ventral tegmental area started to be significant 72 h after acute imipramine. Moreover, this increase was also observed after 14 drug-free days following the acute administration of the drug. The results indicate the different sensitivity of neurons synthesizing dopamine autoreceptors for imipramine. Another interesting finding is the observation that acute treatment with imipramine seems to be sufficient to trigger changes as a function of time regardless of whether imipramine is again administered, providing a possible explanation for the delayed therapeutic effect of the drug.

Repeated administration of antidepressant drugs affects the levels of mRNA coding for D1 and D2 dopamine receptors in the rat brain

The present study examined the effects of acute and repeated administration of three antidepressant drugs (imipramine, citalopram and (+)-oxaprotiline) on the levels of mRNA coding for dopamine D1 and D2 receptors in the rat brain. Quantitive in situ hybridization with 35S-labelled oligonucleotide probes has been utilised. The level of mRNA coding for dopamine D1 receptor (D1 mRNA) is decreased following repeated administration of imipramine, both in the nucleus accumbens and in the striatum. On the other hand, the repeated administration of citalopram, the selective inhibitor of serotonin reuptake, resulted in an increase in the level of D1 mRNA in the striatum and in the core region of nucleus accumbens. A similar tendency, i.e.: an increase in the level of D1 mRNA was observed after repeated administration of (+)-oxaprotiline, a selective inhibitor of noradrenaline reuptake. The level of mRNA coding for dopamine D2 receptors (D2 mRNA) was increased in all the brain regions studied, both after administration of imipramine and citalopram. (+)-Oxaprotiline did not produce any statistically significant changes in the level of D2 mRNA. The results obtained in this study indicate that the levels of mRNA coding for dopamine D1 and D2 receptors are regulated by the antidepressant drugs. The changes concerning the dopamine D2 receptors are more consistent and fit in with the previously described binding and behavioral effects and seem to be important for the mechanism of action of antidepressant drugs.

Time-dependent effects of antidepressant drugs on the low dose of apomorphine-induced locomotor hypoactivity in rats

In the present study the hypothesis of antidepressant-induced subsensitivity of presynaptic dopamine receptors was readdressed. We used the behavioral model of low dose of apomorphine-induced locomotor hypoactivity in rats. Indeed, repeated administration of imipramine and citalopram attenuated the effect of low dose of apomorphine. However, the most interesting finding of the present study was the observation that similar and even stronger attenuation of low dose of apomorphine-induced hypoactivity was obtained by the acute treatment with imipramine, citalopram and amitriptyline, followed by 14 drug free days. That means that the acute antidepressant drug treatment seems to be sufficient to trigger the changes at the level of dopamine D2 presynaptic receptors and/or signal transduction mechanisms as a function of time, regardless of whether the drug is again administered. On the other hand, such the effect of time was not observed in the experiments using D-amphetamine as a motor stimulant. Only the repeated administration of antidepressant drugs enhanced the locomotor hyperactivity induced by D-amphetamine. Acute treatment, neither 2 h nor 14 days after the last dose, changed the D-amphetamine-induced hyperactivity. Such a difference in the time-dependent effects of antidepressant drugs on the locomotor activity induced by either the low dose of apomorphine or D-amphetamine may possibly result from the different mechanisms involved in the regulation of the biosynthesis of dopamine presynaptic and postsynaptic receptors in the rat brain.

The corticosterone synthesis inhibitor metyrapone decreases dopamine D1 receptors in the rat brain

Experiments were performed to examine the effect of metyrapone, an inhibitor of corticosterone synthesis, on the level of dopamine D1 receptors and their transcripts in the caudate-putamen, nucleus accumbens and olfactory tubercle of the rat brain. The binding to dopamine D1 receptors was measured by receptor autoradiography using the specific D1 receptor antagonist [3H]SCH 23390. The level of dopamine D1 receptor messenger RNA was determined by in situ hybridization histochemistry. The results obtained have shown that metyrapone (two injections of 150 and 50 mg/kg, i.p., given 20 and 3 h before killing, respectively) induced a decrease in the D1 receptor-specific binding in the studied areas of the rat brain. In the caudate putamen, the decrease in [3H]SCH 23390 binding was stronger in the medial (31-39%) than in the lateral part (24-27%). Decreases similar to those in the caudate-putamen were observed in the nucleus accumbens (21%) and olfactory tubercle (32%). Furthermore, metyrapone decreased the level of dopamine D1 receptor messenger RNA in the caudate putamen (17-28%), nucleus accumbens (20%) and olfactory tubercle (18%). In conclusion, our study indicates that glucocorticoids might be involved in the regulation of dopamine D1 receptor level in the rat brain. since metyrapone (which inhibits the synthesis of these hormones) decreases the messenger RNA encoding D1 receptor synthesis, as well as the specific binding to this receptor.

The effects of paroxetine given repeatedly on the 5-HT receptor subpopulations in the rat brain

Effects of paroxetine (10 mg/kg PO, twice daily, 14 days) on 5-HT receptor subpopulations in the brain were evaluated pharmacologically, electrophysiologically and biochemically in male Wistar rats. Imipramine was used for comparison. Repeated paroxetine antagonized the 8-OH-DPAT-induced behavioural syndrome (a 5-HT1A effect); imipramine showed similar, yet weaker, activity. The 5-HT-or 8-OH-DPAT-induced inhibition of population spikes in hippocampal slices was increased by both those repeated antidepressants. Repeated (or acute) paroxetine decreased the density of and increased the affinity for 5-HT1A receptors ([3H]-8-OH-DPAT used as ligand) in the hippocampus, while imipramine induced opposite effects. m-Chlorophenyl piperazine (m-CPP)-evoked exploratory hypoactivity, a 5-HT2C effect, was reduced by repeated paroxetine, but not by imipramine. Either of the antidepressants given repeatedly antagonized TFMPP-induced hyperthermia (another putative 5-HT2C effect). 5-HTP-induced head twitches (a 5-HT2A effect) were inhibited by repeated paroxetine or imipramine. Either antidepressant given repeatedly decreased the density of 5-HT2A receptors ([3H]-ketanserin as a ligand) in the brain cortex, but did not change their affinity. The present results indicate that paroxetine given repeatedly induces secondary changes in 5-HT2 receptors, which lead to reduction of the 5-HT2 neurotransmission (reduced responsiveness of 5-HT2 postsynaptic receptors). The consequences of the secondary changes in 5-HT1A receptors, found here still await clarification.

Comparison of the effects of antidepressant drugs on the level of cAMP in the rat striatum and nucleus accumbens septi

The effect of acute and repeated administration of three antidepressant drugs with various pharmacological profile: imipramine (IMI), (+)-oxaprotiline (OXA) and citalopram (CIT) on the level of cAMP in the rat striatum (STR) and nucleus accumbens septi (NAS) was studied. Acute and chronic IMI treatment reduced the basal level and the stimulatory effect of forskolin, quinpirole and Gpp(NHp) (guanosine-5'-imidotriphosphate). Forskolin-stimulated level of cAMP was increased in both of examined structures not only following IMI administration but also after acute and repeated administration of CIT and OXA. It is noteworthy that the increase in the sensitivity of adenylate cyclase to forskolin was significantly attenuated by the blockade of D1 receptor with SCH 23390 in the STR, except for CIT. Similar results were obtained in NAS after administration of OXA and CIT. The obtained results indicate that the effect of the antidepressant drugs used on the level of cAMP in the examined brain structures of the rat strongly depends on the pharmacological profile of the given drug.

Adaptive changes in the rat dopaminergic transmission following repeated lithium administration

In the present study the alterations in the contents of dopamine (DA) and metabolites, as well as in the levels of mRNA coding for DA receptor D2, were determined in the rat striatum (STR) and nucleus accumbens septi (NAS), in correlation with the duration of lithium administration. Single or subchronic (3 days) administration of lithium produced less consistent effects as far as the levels of DA and metabolites are concerned; however, following 7 or 14 days of lithium administration, the DA release from terminals was significantly attenuated and the effect was more pronounced in NAS. After the same time of treatment, the increase in the levels of mRNA coding for the D2 receptor was increased; this might be interpreted as an adaptive change to the decreased dopaminergic transmission following the prolonged administration of lithium.

Antidepressant drugs given repeatedly change the binding of the dopamine D2 receptor agonist, [3H]N-0437, to dopamine D2 receptors in the rat brain

The effects of antidepressants given in a single dose or repeatedly (10 mg/kg p.o., twice daily, 14 days) on binding to dopamine D2 receptors in the striatum and limbic forebrain of Wistar male rats were studied. [3H]N-0437, (2-(N[2,3(n)-3H]propyl-N-(2-thiofuranyl)-2'-ethylamino) -5-hydroxy-1,2,3,4-tetrahydronaphthalene), a dopamine D2 receptor agonist, was used as a ligand. Already a single dose of imipramine and fluoxetine caused a statistically significant decrease in the affinity of the ligand for dopamine D2 receptors in the striatum, but only at 72 h after drug administration. Also at 72 h after the single dose of mianserin a significant increase in the density of dopamine D2 receptors was observed. Repeated imipramine, amitriptyline and mianserin increased the affinity for dopamine D2 receptors in the striatum and in the limbic forebrain. Repeated fluoxetine increased that affinity in the striatum, but decreased it in the limbic forebrain. The density of dopamine D2 receptors was increased by the repeated administration of the antidepressants studied in the limbic forebrain, but was not changed in the striatum. The results obtained in the present study are in good agreement with the previously reported enhancement of behavioural responsiveness to dopamine and dopamine stimulants (dopamine D2 up-regulation) evoked by repeated treatment with antidepressants.

Effect of chronic mild stress and prolonged treatment with imipramine on the levels of endogenous Met-enkephalin in the rat dopaminergic mesolimbic system

Chronic mild stress (CMS)-an established behavioral model of anhedonia, which is one of the core symptoms of major depression-significantly decreases the level of endogenous Met-enkephalin in the rat nucleus accumbens (NAS). Prolonged administration of imipramine (IMI)-the most often used antidepressant drug-increases the level of this peptide in the NAS and reverses the effect of CMS, when administered simultaneously to the animals subjected to CMS. Prolonged administration of IMI decreases the level of Met-enkephalin in the ventral tegmental area (VTA) both in control rats as well as in rats subjected to CMS. Since the endogenous enkephalins have been demonstrated to influence the activity of dopaminergic projection from the VTA to the NAS, the obtained results are discussed in the light of the data pointing to the involvement of mesolimbic dopamine system both in the CMS-induced anhedonia and in the mechanism of therapeutic effect of IMI.

The effect of prolonged administration of lithium on the level of dopamine D2 receptor mRNA in the rat striatum and nucleus accumbens

In the present study the alterations in the levels of mRNA coding for dopamine receptor D2, were determined in the rat striatum (STR) and the nucleus accumbens septi (NAS), in dependence on duration of lithium administration. The levels of mRNA coding for D2 receptor (determined using an in situ hybridization technique) were determined after 1, 7 and 14 days of lithium administration (LiCl, 6 mEq/kg, p.o.), at 3 and 24 h after the last dose of the drug. This treatment resulted in the increase in the levels of mRNA coding for dopamine receptor D2 in all brain regions examined and the effect depended on the time after lithium administration. However, the effect was most pronounced in the shell region of NAS, 24 h after the 14-day treatment.

The effect of prolonged treatment with imipramine and electroconvulsive shock on the levels of endogenous enkephalins in the nucleus accumbens and the ventral tegmentum of the rat

The present study was designed to find out whether the prolonged administration of imipramine (IMI) or electroconvulsive shock (ECS) influences levels of endogenous enkephalins in the nucleus accumbens (NAS) and the ventral tegmentum (VTA) of the rat. Ressults indicate that treatment with IMI as well as with ECS has a profound effect on the levels of enkephalins in both structures. In the NAS both treatments lead to an increase in the levels of endogenous enkephalins and this effect is accompanied by an increase in mRNA coding for proenkephalin (measured by in situ hybridization) in this structure, indicating the enhancement of biosynthesis of endogenous enkephalinergic peptides following antidepressant treatment. The results are discussed in the light of the hypothesis concerning the influence of endogenous enkephalins on mesolimbic dopamine neurons, the activity of which plays a crucial role in the etiology of depression.

The effect of mu and kappa opioid receptor agonists on cAMP level in hippocampus of kainic acid-treated rats

The effect of opioid receptor activation in vitro on the cAMP level was studied in the slices of hippocampus obtained from kainic acid (KA)-treated rats. In the tissue obtained from control rats both U50,488H (kappa opioid receptor agonist) and morphine (mu opioid receptor agonist) produced an increase in the cAMP level, reversed by nor-binaltorphimine (nor-BNI) and naloxone, respectively. When the level of cAMP was stimulated by forskolin (5 microM), the effects of U50,488H and morphine differentiated. Namely, the activation of kappa opioid receptor led to the decrease in this stimulation, but activation of mu opioid receptor was without any effect. The basal level of cAMP in the hippocampal tissue obtained from rats treated with KA, 24 h after the single dose (12 mg/kg ip), was significantly increased, as compared to the control value. Activation of kappa opioid receptors by U50,488H, ex vivo, in such tissue produced a marked decrease in the level of cAMP, while morphine was without any effect. Our results indicate, that in the excited hippocampal tissue, i.e. following KA administration, activation of kappa opioid receptors lowers also the increased level of cAMP. Since the modulation of the intracellular cAMP level may be an important mechanism in regulation of the excitatory transmission in the hippocampus, the above result might point to an additional mechanism, by which U50,488H participates in the lowering the susceptibility of the limbic system to the excitatory effects of KA.

The effect of prolonged lithium administration on the cAMP level in the rat striatum

In the present paper the effect of lithium on the level of cAMP in the rat striatum was studied. Incubation of the striatal slices in the presence of LiCl (1 mM) resulted in the significant decrease in the cAMP level stimulated by dopamine (via dopaminergic receptor), guanylyl-5-imidodiphosphate (GppNHp, via G protein) and forskolin (a direct activator of adenylate cyclase), without any effect on the basal level of cAMP. On the other hand, the effect of prolonged administration of LiCl (5 mEq/kg po) on the cAMP level in the striatal tissue was much more complicated and strongly depended not only on the stimulating factor used ex vivo (i.e. dopamine, GppNHp or forskolin) but also on the period of lithium administration (i.e. 1, 3, 7 or 21 days) and on the time after each of the above mentioned doses (i.e. 3 or 24 h). From the obtained results it can be concluded that initial inhibitory effect of LiCl on the cAMP level can be overcome upon the prolonged administration of the drug, by the adaptive changes which take place apparently at the level of catalytic unit of adenylate cyclase.