Protein kinase C inhibition rescues manic-like behaviors and hippocampal cell proliferation deficits in the sleep deprivation model of mania.

BACKGROUND: Recent studies revealed that bipolar disorder may be associated with deficits of neuroplasticity. Additionally, accumulating evidence has implicated alterations of the intracellular signaling molecule protein kinase C (PKC) in mania. METHODS: Using sleep deprivation (SD) as an animal model of mania, this study aimed to examine the possible relationship between PKC and neuroplasticity in mania. Rats were subjected to SD for 72 h and tested behaviorally. In parallel, SD-induced changes in hippocampal cell proliferation were evaluated with bromodeoxyuridine (BrdU) labeling. We then examined the effects of the mood stabilizer lithium, the antipsychotic agent aripiprazole, and the PKC inhibitors chelerythrine and tamoxifen on both behavioral and cell proliferation impairments induced by SD. The antidepressant fluoxetine was used as a negative control. RESULTS: We found that SD triggered the manic-like behaviors such as hyperlocomotion and increased sleep latency, and reduced hippocampal cell proliferation. These alterations were counteracted by an acute administration of lithium and aripiprazole but not of fluoxetine, and only a single administration of aripiprazole increased cell proliferation on its own. Importantly, SD rats exhibited increased levels of phosphorylated synaptosomal-associated protein 25 (SNAP-25) in the hippocampus and prefrontal cortex, suggesting PKC overactivity. Moreover, PKC inhibitors attenuated manic-like behaviors and rescued cell proliferation deficits induced by SD. CONCLUSIONS: Our findings confirm the relevance of SD as a model of mania, and provide evidence that antimanic agents are also able to prevent SD-induced decrease of hippocampal cell proliferation. Furthermore, they emphasize the therapeutic potential of PKC inhibitors, as revealed by their antimanic-like and pro-proliferative properties.

Astrocytes and gliotransmitters: new players in the treatment of major depression?

With a lifetime prevalence of more than 16% worldwide, major depressive disorder is one of the most common psychiatric disorders. Only one third of patients experience a complete therapeutic improvement with the use of current antidepressant drugs, with a therapeutic effect appearing only after several weeks of treatment. Hence, a better understanding of the mechanisms of action of current antidepressant treatments is needed to ultimately identify new targets and enhance beneficial effects. Given the intimate relationships between astrocytes and neurons at synapses and the ability of astrocytes to "sense" neuronal communication and release gliotransmitters, an attractive hypothesis is emerging stating that the effects of antidepressants on brain function could be, at least in part, mediated by direct influences of astrocytes on neuronal networks. This review aims at highlighting the involvement of astrocytes and gliotransmission in the antidepressant effects of both non- and pharmacological therapies.

Protein kinase C regulates mood-related behaviors and adult hippocampal cell proliferation in rats.

The neurobiological mechanisms underlying the pathophysiology and therapeutics of bipolar disorder are still unknown. In recent years, protein kinase C (PKC) has emerged as a potential key player in mania. To further investigate the role of this signaling system in mood regulation, we examined the effects of PKC modulators in behavioral tests modeling several facets of bipolar disorder and in adult hippocampal cell proliferation in rats. Our results showed that a single injection of the PKC inhibitors tamoxifen (80 mg/kg, i.p.) and chelerythrine (3 mg/kg, s.c.) attenuated amphetamine-induced hyperlocomotion and decreased risk-taking behavior, supporting the efficacy of PKC blockade in acute mania. Moreover, chronic exposure to tamoxifen (10 mg/kg/day, i.p., for 14 days) or chelerythrine (0.3 mg/kg/day, s.c., for 14 days) caused depressive-like behavior in the forced swim test, and resulted in a reduction of cell proliferation in the dentate gyrus of the hippocampus. Finally, we showed that, contrary to the PKC inhibitors, the PKC activator phorbol 12-myristate 13-acetate (PMA) enhanced risk-taking behavior and induced an antidepressant-like effect. Taken together, these findings support the involvement of PKC in regulating opposite facets of bipolar disorder, and emphasize a major role for PKC in this disease.

A role for the PKC signaling system in the pathophysiology and treatment of mood disorders: involvement of a functional imbalance?

Mood disorders, such as bipolar and major depressive disorders, are frequent, severe, and often disabling neuropsychiatric diseases affecting millions of individuals worldwide. Available mood stabilizers and antidepressants remain unsatisfactory because of their delayed and partial therapeutic efficacy. Therefore, the development of targeted therapies, working more rapidly and being fully effective, is urgently needed. In this context, the protein kinase C (PKC) signaling system, which regulates multiple neuronal processes implicated in mood regulation, can constitute a novel therapeutic target. This paper reviews the currently available knowledge regarding the role of the PKC signaling pathway in the pathophysiology of mood disorders and the therapeutic potential of PKC modulators. Current antidepressants and mood stabilizers have been shown to modulate the PKC pathway, and the inhibition of this intracellular signaling cascade results in antimanic-like properties in animal models. Disrupted PKC activity has been found both in postmortem brains and platelet from patients with mood disorders. Finally, the PKC inhibitor tamoxifen has recently demonstrated potent antimanic properties in several clinical trials. Overall, emerging data from preclinical and clinical research suggest an imbalance of the PKC signaling system in mood disorders. Thus, PKC may be a critical molecular target for the development of innovative therapeutics.

In vivo effects of pardoprunox (SLV308), a partial D2/D3 receptor and 5-HT1A receptor agonist, on rat dopamine and serotonin neuronal activity.

The nonergot ligand pardoprunox (SLV308) is a dopamine (DA) D2/D3 and serotonin (5-HT)(1A) receptor agonist and a new candidate for the treatment of Parkinson's disease. We used in vivo electrophysiological paradigm in the rat to assess the effects of pardoprunox on DA neuronal activity in ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) as well as on 5-HT neuronal activity in dorsal raphe nucleus (DRN). In the VTA, pardoprunox (2-20 µg kg⁻¹, i.v.) decreased partially the firing activity of DA neurons. Interestingly, the bursting activity of VTA DA neurons was completely suppressed. This compound both reversed and prevented the inhibition of firing rate induced by the full D2-like receptor agonist apomorphine, confirming its partial D2-like receptor agonistic property. Surprisingly in the SNc, pardoprunox (10 µg kg⁻¹, i.v.) either partially or fully suppressed the firing activity in two separate populations of DA neurons. Finally, in the DRN, pardoprunox (5-40 µg kg⁻¹, i.v.) completely suppressed the firing activity of 5-HT neurons. Moreover, the selective 5-HT(1A) receptor antagonist WAY-100,635 prevented and reversed the effects of pardoprunox. The present study shows that pardoprunox acts in the VTA as a potent partial D2-like receptor agonist reducing preferentially the burst activity linked to the phasic activity of DA neurons. Unexpectedly in the SNc, pardoprunox behaves either as apartial or a full D2-like receptor agonist. Finally in the DRN, pardoprunox is a potent full 5-HT(1A) receptor agonist. Hence, this in vivo study suggests that pardoprunox represents a promising approach for the treatment of Parkinson's disease.

Pharmacological blockade of 5-HT7 receptors as a putative fast acting antidepressant strategy.

Current antidepressants still display unsatisfactory efficacy and a delayed onset of therapeutic action. Here we show that the pharmacological blockade of serotonin 7 (5-HT(7)) receptors produced a faster antidepressant-like response than the commonly prescribed antidepressant fluoxetine. In the rat, the selective 5-HT(7) receptor antagonist SB-269970 counteracted the anxiogenic-like effect of fluoxetine in the open field and exerted an antidepressant-like effect in the forced swim test. In vivo, 5-HT(7) receptors negatively regulate the firing activity of dorsal raphe 5-HT neurons and become desensitized after long-term administration of fluoxetine. In contrast with fluoxetine, a 1-week treatment with SB-269970 did not alter 5-HT firing activity but desensitized cell body 5-HT autoreceptors, enhanced the hippocampal cell proliferation, and counteracted the depressive-like behavior in olfactory bulbectomized rats. Finally, unlike fluoxetine, early-life administration of SB-269970, did not induce anxious/depressive-like behaviors in adulthood. Together, these findings indicate that the 5-HT(7) receptor antagonists may represent a new class of antidepressants with faster therapeutic action.

Therapeutic potential of 5-HT7 receptors in mood disorders.

Serotonin (5-HT) exerts its diverse physiological and pharmacological effects through the activation of multiple receptor subtypes. One of the newest members of this family is the 5-HT(7) receptor. Increasing investigations on this receptor are currently undertaken to highlight its physiological and physiopathological significance. With the development of selective 5-HT(7) receptor ligands, preclinical studies have started to elucidate the functions of this receptor subtype in more details. Hence, growing body of evidence suggests that the 5-HT(7) receptor is involved in biological processes such as circadian rhythm and thermoregulation, in addition to disorders in which disturbances of the latter are considered to be an important contributing factor. Moreover, preclinical data support the use of 5-HT(7) receptor antagonism as a promising mechanism for the treatment of several dysfunctions such as cognitive deficits and, importantly, have also unveiled anxiolytic and antidepressant-like properties. In this review, we will report major advances in the discovery of 5-HT(7) receptor roles, with special emphasis on the potential application of their antagonists as novel anxiolytic and antidepressant drugs.

Functional correlates for 5-HT(1A) receptors in maternally deprived rats displaying anxiety and depression-like behaviors.

Maternal separation is known to induce long-term changes in neuroendocrine and emotional responsiveness to stress in a large variety of models. We examined an animal model of early deprivation in Sprague-Dawley rats consisting of separating litters from their mothers and littermates 3 h daily during postnatal days 2 to 15. In adulthood, maternally deprived rats in comparison with non-deprived controls exhibited an increase in anxiety and depression-related behaviors in the open-field and forced swim tests. Because serotonin (5-HT) 5-HT(1A) receptors seem to play an important role in the pathophysiology of major depression and in the mechanism of action of antidepressants, we investigated if 5-HT(1A) receptor function is altered in deprived rats. Although the hypothermic response to the 5-HT(1A) receptor agonist 8-OH-DPAT was increased in adult deprived rats compared to non-deprived control group, no differences between groups were found in the effect of the systemic 8-OH-DPAT administration on serotoninergic cell firing in dorsal raphe nucleus and in the 5-HT release at the ventral hippocampus levels. These results suggest that 5-HT(1A) receptors are not substantially affected in adult Sprague-Dawley rats that were subjected to a maternal deprivation 3 h daily during the neonatal period.

Serotonin(4) (5-HT(4)) receptor agonists are putative antidepressants with a rapid onset of action.

Current antidepressants are clinically effective only after several weeks of administration. Here, we show that serotonin(4) (5-HT(4)) agonists reduce immobility in the forced swimming test, displaying an antidepressant potential. Moreover, a 3 day regimen with such compounds modifies rat brain parameters considered to be key markers of antidepressant action, but that are observed only after 2-3 week treatments with classical molecules: desensitization of 5-HT(1A) autoreceptors, increased tonus on hippocampal postsynaptic 5-HT(1A) receptors, and enhanced phosphorylation of the CREB protein and neurogenesis in the hippocampus. In contrast, a 3 day treatment with the SSRI citalopram remains devoid of any effect on these parameters. Finally, a 3 day regimen with the 5-HT(4) agonist RS 67333 was sufficient to reduce both the hyperlocomotion induced by olfactory bulbectomy and the diminution of sucrose intake consecutive to a chronic mild stress. These findings point out 5-HT(4) receptor agonists as a putative class of antidepressants with a rapid onset of action.

R-citalopram prevents the neuronal adaptive changes induced by escitalopram.

This study examined the long-term effects of the antidepressant escitalopram on rat serotonin (5-HT) neuronal activity and hippocampal neuroplasticity. In the dorsal raphe nucleus, a 2-week treatment with escitalopram (10 mg/kg/day, subcutaneous) did not modify the firing activity of 5-HT neurons, whereas a cotreatment with R-citalopram (20 mg/kg/day, subcutaneous) decreased it. In the dentate gyrus of dorsal hippocampus, escitalopram increased significantly (57%) the number of de novo cells and this was prevented by a cotreatment with R-citalopram. The present results support the role of the allosteric modulation of the 5-HT transporter in the regulation of the recovery of 5-HT neuronal activity and long-lasting hippocampal cellular plasticity induced by escitalopram, two adaptive changes presumably associated with the antidepressant response.

Changes in Proenkephalin mRNA expression in forebrain areas after amphetamine-induced behavioural sensitization.

Acute and repeated psychostimulant administration induces a long-lasting enhanced behavioural response to a subsequent drug challenge, known as behavioural sensitization. This phenomenon involves persistent neurophysiological adaptations, which may lead to drug addiction. Brain dopaminergic pathways have been implicated as the main neurobiological substrates of behavioural sensitization, although other neurotransmitters and neuromodulators may also participate. In order to investigate a possible involvement of opioid systems in amphetamine (AMPH) behavioural sensitization, we studied the AMPH-induced changes in Proenkephalin (Pro-Enk) mRNA expression in forebrain areas in both drug-naïve and AMPH-sensitized rats. Male Sprague-Dawley rats were sensitized to AMPH by means of a single AMPH (1 mg/kg s.c.) injection and the same dose was injected 7 days later to assess the expression of sensitization. Pro-Enk mRNA levels were evaluated by in situ hybridization in coronal brain sections. AMPH injection induced an increase in Pro-Enk mRNA expression in the nucleus accumbens and the medial-posterior caudate-putamen in drug-naïve rats. Challenge with AMPH to rats injected 1 week earlier with AMPH induced motor sensitization and increased and decreased Pro-Enk mRNA expression in the prefrontal cortex and the anterior medial caudate-putamen, respectively. Our results suggest that alterations in cortical and striatal enkephalinergic systems could contribute to the expression of AMPH behavioural sensitization.

Highly sensitive assay for the measurement of serotonin in microdialysates using capillary high-performance liquid chromatography with electrochemical detection.

A highly sensitive isocratic capillary high-performance liquid chromatographic (HPLC) method with electrochemical detection (ED) for the simultaneous measurement of serotonin (5-hydroxytryptamine, 5-HT) and its metabolite 5-hydroxyindole-3-acetic acid (5-HIAA) in microdialysates has been developed using a 0.5 mm i.d. capillary column and a 11-nL detection cell. This method, validated on both pharmacological and analytical bases, can be performed using injection volumes as low as 1 microL. The limits of detection were 5.6 x 10(-11)mol/L and 3.0 x 10(-9)mol/L for 5-HT and 5-HIAA. Several applications of the present method are given on microdialysates from rodent brain and human spinal cord.

5-HT7 receptor antagonists as a new class of antidepressants.

It is now admitted that major depression is associated with monoaminergic dysfunctions as well as with functional brain plasticity impairments. Despite the wide variety of medications available to treat such a syndrome, two foremost problems still remain unresolved: one-third of patients do not respond to any treatment and there is an unwanted 2-4 week delay in the onset of therapeutic action of all available antidepressant drugs. These issues draw attention to the need and urgency to develop more efficacious treatments and to accelerate the antidepressant response. The combination of an atypical antipsychotic, known to be a potent 5-HT(7) receptor antagonist, with an antidepressant has been recently proposed as an alternative therapy. Hence, blockade of 5-HT(7) receptors might represent a key determinant for this hastening strategy. This review summarizes recent data that put emphasis on the putative antidepressant properties of selective 5-HT(7) receptor antagonists. The use of such ligands seems very promising to elaborate novel generations of antidepressants that surpass the efficacy and onset of action limitations of existing antidepressants.

Blockade of beta-adrenergic receptors prevents amphetamine-induced behavioural sensitization in rats: a putative role of the bed nucleus of the stria terminalis.

Recent findings have given evidence a role for noradrenergic transmission in the mechanisms underlying behavioural sensitization to psychostimulants. This work was undertaken to investigate the possible role of beta-adrenergic receptors in amphetamine-induced behavioural sensitization in rats. Rats were sensitized by a single administration of amphetamine (1 mg/kg s.c.) and challenged with the same dose 7 d later. The beta(1) /beta(2) -adrenergic receptor antagonists timolol (10 mg/kg i.p.) and nadolol (10 mg/kg i.p.), which respectively cross or do not readily cross the blood-brain barrier, were injected prior to the first or second amphetamine administration. Timolol, but not nadolol, prevented the initiation of behavioural sensitization without interfering with the expression of the sensitized response or the acute locomotor response to amphetamine. Since we found amphetamine-induced fos-activated cells closely associated with dopamine beta-hydroxylase immunoreactive varicosities in the bed nucleus of the stria terminalis (BNST), we investigated the effect of a bilateral micro-injection of timolol into this nucleus. Similarly to systemic administration, intra-BNST timolol (2.5 microg/side) prevented the development of behavioural sensitization. These results suggest that central beta-adrenergic receptors could specifically modulate early neuronal changes leading to the development of behavioural sensitization to psychostimulants, and that the BNST could be an important part of the brain circuitry involved in these long-term neuroadaptations.

Endogenous neurotensin in the ventral tegmental area contributes to amphetamine behavioral sensitization.

Studies showing psychostimulant-like effects of exogenous neurotensin (NT) infused into the ventral tegmental area (VTA) prompted us to examine the role in the VTA of the endogenous NT in behavioral sensitization to amphetamine. Rats were sensitized to amphetamine by means of a subcutaneous amphetamine (1 mg/kg) injection, and the same dose was injected 7 days later to evaluate the expression of sensitization. The highly selective NT-receptor antagonist SR 142948A was injected into the VTA prior to the first and/or second amphetamine administration. SR 142948A (5 pmol/side) given before the first amphetamine exposure prevented the induction of behavioral sensitization, but did not alter the acute response to amphetamine. SR 142948A given with the second amphetamine administration did not affect the expression of behavioral sensitization. In contrast to administration into the VTA, intraperitoneal administration of SR 142948A (0.03, 0.1, or 0.3 mg/kg) had no detectable effect on the induction of amphetamine sensitization. These results suggest that activation of VTA NT receptors by endogenous NT may contribute to the neuroadaptations underlying behavioral sensitization to amphetamine.

Enhanced tail pinch-induced activation of catecholamine metabolism in the pericerulean area of RU 24722-treated rats.

Our study was devoted to determine in freely moving rats whether the increase in tissue concentration of tyrosine hydroxylase (TH) elicited by a single administration of RU 24722 could modify the catecholaminergic reactivity of neuronal processes present in the rostrolateral part of the pericerulean area (r-lPCA) in response to tail pinch. Catecholaminergic activity was monitored by measuring in vivo the concentration of dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) using microdialysis coupled to HPLC detection. In this study, the microdialysis probe was implanted at a sufficient distance from the lateral border of rostral nucleus locus ceruleus (LC) to avoid a large contribution of the noradrenergic cell bodies in the measurements performed. We first evidenced that DOPAC measured in the r-lPCA indicated the functional state of catecholaminergic metabolism in neuronal processes (dendrites and fibers) laying in this region. We also showed that the enhancement of TH protein concentration in the r-lPCA following RU 24722 treatment supported an increased in vivo catecholaminergic metabolism in this region. Furthermore, catecholaminergic metabolism response to tail pinch was potentiated in animals with greater TH tissue concentration. Thus, our study reveals that the modulation of both TH concentration and catecholaminergic metabolism in the r-lPCA may be critical in the functioning of cells and neuronal elements present in this region, notably in adaptive responses to noxious stimuli.

Microdialysis monitoring of catecholamines and excitatory amino acids in the rat and mouse brain: recent developments based on capillary electrophoresis with laser-induced fluorescence detection--a mini-review.

1. Although microdialysis is a widely used approach for in vivo monitoring extracellular neurotransmitter concentrations, it has been previously limited in many cases by its poor temporal resolution. It is clear that when 10-30-min sampling is performed, short-lasting changes in extracellular neurotransmitter concentrations can be overlooked. Such a low sampling rate is necessary when combining microdialysis with the conventional analytical methods like high performance liquid chromatography. 2. Since capillary electrophoresis coupled to laser-induced fluorescence detection (CE-LIFD) allows the detection of attomoles of neurotransmitters, the temporal resolution of microdialysis may be significantly improved: high sampling rates, in the range of 5 s to 1 min, have been already reported by our group and others using CE-LIFD for simultaneously analyzing catecholamines and amino acids in microdialysates. 3. The power of combining microdialyis and CE-LIFD is shown, using examples of physiological and pharmacological studies dealing with the dynamics of in vivo efflux processes and/or interactions between neurotransmitters.

In vivo comparison of two 5-HT1A receptors agonists alnespirone (S-20499) and buspirone on locus coeruleus neuronal activity.

The aim of the present study was to compare, in chloral-hydrate anaesthetized rats, the alpha(2)-adrenergic properties of the selective 5-HT(1A) receptor agonist, alnespirone (S-20499), with those of buspirone, a 5-HT(1A) receptor agonist exhibiting potent alpha(2)-adrenoceptor antagonist properties via its principal metabolite, 1-(2-pyrimidinyl)-piperazine. Both locus coeruleus spontaneous firing activity and noradrenaline release in the medial prefrontal cortex were potently inhibited by the alpha(2)-adrenoceptor agonist clonidine, at a dose of 40 microg/kg (i.p.). Such an inhibition was neither prevented nor reversed by alnespirone (10 mg/kg, i.p.), while buspirone, at the same dose, potently antagonized the locus coeruleus inhibitory effects of clonidine. These data demonstrate that, in contrast with some aryl-piperazine compounds (such as buspirone), alnespirone, either on its own or via a possible metabolite such as buspirone, is devoid in vivo of significant alpha(2)-adrenoceptor antagonist properties.

NO synthase inhibitors attenuate locus coeruleus catecholamine metabolism and behavior induced by morphine withdrawal.

The effects of nitric oxide synthase (NOS) inhibitors were examined simultaneously on the behavior and on the catecholaminergic metabolism in the locus coeruleus (LC) during morphine withdrawal using microdialysis in freely moving rats. Morphine withdrawal was precipitated by naltrexone administration to morphine-treated rats. Acute pretreatment of rats with NOmicron-nitro-L-arginine-p-nitroanilide (L-NAPNA) or 7-nitroindazole (7-NI) before naltrexone challenge attenuated the behavioral expression of morphine withdrawal and strongly reduced the withdrawal-induced increase in 3,4-dihydroxyphenylacetic acid (DOPAC) in the LC. The two NOS inhibitors also decreased DOPAC in absence of naltrexone challenge. These results suggest a role for NO in the expression of morphine withdrawal syndrome that may be mediated, at least in part, by LC noradrenergic neurons.

Chronic blockade of neurotensin receptors strongly reduces sensitized, but not acute, behavioral response to D-amphetamine.

This study investigated the effect of a chronic blockade of neurotensin (NT) receptors on the sensitized behavioral response to amphetamine using a nonpeptide NT receptor antagonist, SR 48692. Rats received four injections of D-amphetamine (0.5 or 1 mg/kg, IP) every other day (day 1, 3, 5 and 7) and were then challenged with the same dose of amphetamine after a 6-day withdrawal (day 14) to establish the presence of locomotor sensitization. Daily administration of SR 48692 (1 mg/kg, IP) throughout the amphetamine regimen (day 1 to day 14) almost completely blocked the sensitized locomotor response to amphetamine without affecting stereotyped behaviors (experiment 1). The decreased amphetamine-induced sensitization in chronically SR 48692-treated rats did not appear to result from an influence on basal locomotor activity, as chronic SR 48692 treatment did not modify the spontaneous locomotor activity developed in response to mild stresses (experiment 2). Moreover, we showed that chronic pretreatment with SR 48692 (1 mg/kg, 14 daily IP injections) had no effect on the locomotor activation induced by a single IP administration of amphetamine (experiment 3). These data suggest that a sustained blockade of NT receptors considerably reduces the sensitized behavioral response to amphetamine without altering the acute effect of this psychostimulant or the locomotor activation induced by a mild stress. This ability of SR 48692 to specifically reduce the behavioral sensitization to amphetamine suggests that NT receptor antagonists could have potential clinical utility in the treatment of some psychiatric disorders.