Pia Weikop

The Role of α1- and α2-Adrenoreceptors on Venlafaxineinduced Elevation of Extracellular Serotonin, Noradrenaline and Dopamine Levels in the Rat Prefrontal Cortex and Hippocampus:

The role of adrenergic α1- and α2-adrenoreceptors in augmentation of venlafaxine-induced elevation of extracellular serotonin (5-HT),noradrenaline (NA) and dopamine (DA) levels in the rat prefrontal cortex (PFC) and hippocampus (HIPP) was studied by in vivo microdialysis in anaesthetized rats. The α1-adrenoreceptor antagonist prazosin given alone (0.3 mg/kg, s.c.) induced only a moderate reduction of hippocampal 5-HT and NA levels. The α2-adrenoreceptor antagonistidazoxan (1.5 mg/kg, s.c.) causes moderate increases in the levels of 5-HT and DA in the PFC. The mixed 5-HT and NA reuptake inhibitor venlafaxine (10 mg/kg, i.p.) increased the efflux of 5-HT, NA and DA almost equally, to approximately 200% of the control levels in the PFC. The levels of 5-HT increased to 310%, an effect approximately twice the effect on NA in the HIPP. Venlafaxine also produced a moderate increase in DA levels in the PFC but had no effect in the HIPP. Pre-treatment with prazosin caused a significant attenuation of the venlafaxine induced 5-HT effect in the PFC, and a moderate increase in DA levels in the HIPP. Prazosin had no significant effect on the venlafaxine-induced increase of the NA levels in PFC or HIPP. A combined treatment of venlafaxine with idazoxan increased the venlafaxine NA and DA effects in PFC by a factor of two and resulted in a very robust five-foldaugmentation of NA and DA concentrations in the HIPP. In summary, idazoxan was found to produce a potent enhancement of the venlafaxine effect to increase extracellular NA and DA levels in the PFC and, inparticular, in the HIPP. Idazoxan had no effect on venlafaxine-induced elevation of extracellular 5-HT levels in either PFC or HIPP and prazosin induced a decrease of 5-HT in the PFC. The present data suggest that blockade of α2-adrenoreceptors may play an important role inaugmentation of the effects of mixed monoamine reuptake inhibitors.

The KCNQ Channel Opener Retigabine Inhibits the Activity of Mesencephalic Dopaminergic Systems of the Rat

Homo- and heteromeric complexes of KCNQ channel subunits are the molecular correlate of the M-current, a neuron-specific voltage-dependent K+ current with a well established role in control of neural excitability. We investigated the effect of KCNQ channel modulators on the activity of dopaminergic neurons in vitro and in vivo in the rat ventral mesencephalon. The firing of dopaminergic neurons recorded in mesencephalic slices was robustly inhibited in a concentration-dependent manner by the KCNQ channel opener N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester (retigabine). The effect of retigabine persisted in the presence of tetrodotoxin and simultaneous blockade of GABAA receptors, small-conductance calcium-activated K+ (SK) channels, and hyperpolarization-activated (Ih) channels, and it was potently reversed by the KCNQ channel blocker 4-pyridinylmethyl-9(10H)-anthracenone (XE991), indicating a direct effect on KCNQ channels. Likewise, in vivo single unit recordings from dopaminergic neurons revealed a prominent reduction in spike activity after systemic administration of retigabine. Furthermore, retigabine inhibited dopamine synthesis and c-Fos expression in the striatum under basal conditions. Retigabine completely blocked the excitatory effect of dopamine D2 autoreceptor antagonists. Again, the in vitro and in vivo effects of retigabine were completely reversed by preadministration of XE991. Dual immunocytochemistry revealed that KCNQ4 is the major KCNQ channel subunit expressed in all dopaminergic neurons in the mesolimbic and nigrostriatal pathways. Collectively, these observations indicate that retigabine negatively modulates dopaminergic neurotransmission, likely originating from stimulation of mesencephalic KCNQ4 channels.

Increased cocaine self-administration in M4 muscarinic acetylcholine receptor knockout mice

RationaleThe reinforcing effects of cocaine are mediated by the mesolimbic dopamine system. Behavioral and neurochemical studies have shown that the cholinergic muscarinic M4 receptor subtype plays an important role in regulation of dopaminergic neurotransmission.ObjectivesHere we investigated for the first time the involvement of M4 receptors in the reinforcing effects of cocaine using chronic intravenous cocaine self-administration in extensively backcrossed M4 receptor knockout (M4−/−) mice.MethodsWe evaluated acquisition of cocaine self-administration in experimentally naïve mice. Both cocaine self-administration and food-maintained operant behavior were evaluated under fixed ratio 1 (FR 1) and progressive ratio (PR) schedules of reinforcement. In addition, cocaine-induced dopamine release and cocaine-induced hyperactivity were evaluated.ResultsM4−/− mice earned significantly more cocaine reinforcers and reached higher breaking points than their wild-type littermates (M4+/+) at intermediate doses of cocaine under both FR 1 and PR schedules of reinforcement. Under the PR schedule, M4−/− mice exhibited significantly higher response rates at the lowest liquid food concentration. In accordance with these results, cocaine-induced dopamine efflux in the nucleus accumbens and hyperlocomotion were increased in M4−/− mice compared to M4+/+ mice.ConclusionsOur data suggest that M4 receptors play an important role in regulation of the reward circuitry and may serve as a new target in the medical treatment of drug addiction.

Differential effects of adjunctive methylphenidate and citalopram on extracellular levels of serotonin, noradrenaline and dopamine in the rat brain

Several clinical studies have suggested that the combined treatment with methylphenidate and citalopram may accelerate the onset of antidepressant action and induce an improvement even in treatment-refractory patients. In the present study, in vivo microdialysis was used to monitor the extracellular levels of serotonin, noradrenaline and dopamine in the prefrontal cortex, hippocampus, nucleus accumbens and striatum of the rat. Administration of methylphenidate (2.5 mg/kg s.c.) with citalopram (5 mg/kg i.p.) compared to methylphenidate alone caused a marked enhancement of dopamine levels in the prefrontal cortex, n. accumbens and hippocampus, but not in the striatum. Citalopram-induced increase in serotonin levels was strongly enhanced by adjunctive methylphenidate in the hippocampus, but attenuated in the cortex. These findings suggest that the proposed augmentation effects of adjuvant methylphenidate to citalopram are most likely associated with enhanced dopamine transmission in the corticolimbic areas, whereas serotonin and noradrenaline levels show differential and region specific responses.

Hypericum perforatum L (St John's wort) preferentially increases extracellular dopamine levels in the rat prefrontal cortex

The effects of hydro‐alcoholic extracts of Hypericum perforatum L on extracellular serotonin (5‐HT), noradrenaline (NA) and dopamine (DA) levels and the acidic metabolites (3,4‐dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5‐hydroxy‐3‐indoleacetic acid (5‐HIAA)) were examined by in vivo microdialysis in the prefrontal cortex of awake rats. Thus, a single dose (60 mg kg−1 i.p. or 300 mg kg−1 p.o.) of H. perforatum increased DA concentrations to 165 and 140% of control values, respectively, and increased locomotor activity in nonhabituated rats. DOPAC and HVA levels were markedly reduced. 5‐HT concentrations were elevated only moderately, while the NA levels were not affected by any treatment. The whole‐tissue analysis revealed that hypericum increased, whereas the monoamine oxidase (MAO) A/B inhibitor phenelzine decreased DA and 5‐HT turnover. The present data indicate that the mechanism of action of hypericum extract in vivo is more complex than the inhibition of monoamine reuptake or metabolism observed in vitro. The finding of preferential enhancement of DA transmission is in agreement with human studies measuring DA‐mediated neuroendocrine responses.

Involvement of a Subpopulation of Neuronal M4 Muscarinic Acetylcholine Receptors in the Antipsychotic-like Effects of the M1/M4 Preferring Muscarinic Receptor Agonist Xanomeline

Disturbances in central dopaminergic neurotransmission are believed to be centrally involved in the pathogenesis of schizophrenia. Central dopaminergic and cholinergic systems interact and the cholinergic muscarinic agonist xanomeline has shown antipsychotic effects in clinical studies. Preclinical studies indicate that the M4 muscarinic cholinergic receptor subtype (mAChR) modulates the activity of the dopaminergic system and that this specific mAChR subtype is involved in mediating the antipsychotic-like effects of xanomeline. A specific neuronal subpopulation that expresses M4 mAChRs together with D1 dopamine receptors seems to be especially important in modulating dopamine-dependent behaviors. Using mutant mice that lack the M4 mAChR only in D1 dopamine receptor-expressing cells (D1-M4-KO), we investigated the role of this neuronal population in the antipsychotic-like effects of xanomeline in amphetamine-induced hyperactivity and apomorphine-induced climbing. Interestingly, the antipsychotic-like effects of xanomeline in the two models were almost completely abolished in D1-M4-KO mice, suggesting that M4 mAChRs colocalized with D1 dopamine receptors are centrally involved in mediating the antipsychotic-like effects of xanomeline. This is consistent with the hypothesis that activation of the M4 mAChR represents a potential target for the future medical treatment of psychosis.

Central serotonin depletion affects rat brain areas differently : A qualitative and quantitative comparison between different treatment schemes

Depletion of rat brain serotonin (5-hydroxytryptamin, 5-HT) has been widely used to study effects of serotonin and its interaction with other transmitter systems. Various treatment regimes for serotonin depletion have been applied, but the efficacy of these seems to vary considerably. So far, no studies have systematically examined and compared different approaches. The present work combines quantitative and qualitative measurements and compares six different treatment schemes for 5-HT depletion. Treatment outcome was evaluated by HPLC measurements of 5-HT and 5-HIAA concentrations, and by 5-HT and tyrosine hydroxylase immunocytochemistry. The schemes included repeated administration of fenfluramine (FEN) and/or p-chlorophenylalanine (pCPA). The most efficient treatment for rat brain 5-HT depletion was the combined treatment with one daily pCPA (200 mg/kg) injection for 3 days followed by one injection of d,l-FEN (20 mg/kg) on the fourth day, causing a 94.9% brain 5-HT depletion. Immunostaining revealed a distinct brain distribution of the remaining 5-HT, with an almost complete depletion of 5-HT in the cerebral cortex, hippocampus and thalamus, while a substantial amount of 5-HT still was left in the raphe nuclei, the medial forebrain bundle, and the medial eminence. FEN or pCPA treatment alone caused from 68.2 to 94.0% decrease in 5-HT levels. While the pattern of 5-HT depletion using pCPA alone was comparable to the one seen with the combined treatment, the 5-HT depletion using FEN alone showed a different pattern with 5-HT distributed in several brain regions.

Elevated levels of IL-6 and IL-18 in manic and hypomanic states in rapid cycling bipolar disorder patients

Inflammatory system dysregulation may be involved in the pathophysiology of bipolar disorder with peripheral cytokine levels varying between affective states; however, the evidence is based primarily on case-control studies and limited by methodological issues. The objectives of the present study were to assess alterations of peripheral cytokine levels between affective states in rapid cycling bipolar disorder patients and to compare these with levels in healthy control subjects. In a longitudinal design, repeated measurements of plasma levels of IL-6, IL-10, IL-18, IL-1β and TNF-α were obtained in affective states of varying polarity during 6-12 months in 37 rapid cycling bipolar disorder patients and compared with repeated measurements in 40 age- and gender matched healthy control subjects, using rigorous laboratory-, clinical- and statistical methodology. Adjusting for demographical, clinical- and lifestyle factors, levels of IL-6 (p<0.05) and IL-18 (p<0.005) were significantly elevated in rapid cycling bipolar disorder patients in a manic/hypomanic state, compared with a depressed and a euthymic state. Compared with healthy control subjects, unadjusted levels of IL-6 (p<0.05) and IL-18 (p<0.05) were elevated in manic/hypomanic bipolar disorder patients. Levels of IL-10 and IL-1β were undetectable in the majority of samples; high TNF-α assay variability was found. The results support a role for altered peripheral immune response signaling in rapid cycling bipolar disorder and suggest that IL-6 and IL-18 could be markers of manic episodes.

Reciprocal effects of combined administration of serotonin, noradrenaline and dopamine reuptake inhibitors on serotonin and dopamine levels in the rat prefrontal cortex: the role of 5-HT1A receptors

The purpose of the present study was to examine, by in vivo microdialysis technique, the effects of triple acting monoamine reuptake inhibitors, constructed by combinations of a selective serotonin reuptake inhibitor citalopram with a noradrenaline/dopamine reuptake inhibitor methylphenidate and a serotonin/noradrenaline reuptake inhibitor venlafaxine with a dopamine reuptake inhibitor GBR12909, on extracellular levels of serotonin (5-HT), noradrenaline (NA) and dopamine (DA) in the prefrontal cortex (PFC) of anaesthetized rats. At the highest dose tested, adjunctive methylphenidate (10 mg/kg s.c.) to citalopram markedly attenuated by 63% the extracellular levels of 5-HT as compared to the levels induced by citalopram (5 mg/kg i.p.) alone, whereas the overall DA concentrations significantly increased to about 149% of those induced by methylphenidate alone. Similarly, the combination of venlafaxine with GBR12909 (10 mg/kg s.c.) caused a reduction of 5-HT levels to 66% of the levels induced by venlafaxine (10 mg/kg i.p.) alone, whereas the overall DA levels increased to 151% of the venlafaxine-treated group. The extracellular levels of NA were only marginally affected by the treatments with combined reuptake inhibitors compared to the effects induced by methylphenidate or venlafaxine alone. The modulatory effects of combined administration of the DA/NA reuptake inhibitors with the 5-HT reuptake inhibitors (citalopram and venlafaxine) on potentiation of DA and attenuation of 5-HT efflux were completely reversed by a pre-treatment with the 5-HT1A receptor antagonist WAY-100635. These findings suggest a crucial role played by the 5-HT1A receptors in balancing the reuptake inhibitory efficacy for the enhancement of 5-HT and DA transmission in the PFC by the drugs combining the reuptake inhibition of all three monoamines.

An allosteric enhancer of M4 muscarinic acetylcholine receptor function inhibits behavioral and neurochemical effects of cocaine

RationaleThe mesostriatal dopamine system plays a key role in mediating the reinforcing effects of psychostimulant drugs like cocaine. The muscarinic M4 acetylcholine receptor subtype is centrally involved in the regulation of dopamine release in striatal areas. Consequently, striatal M4 receptors could be a novel target for modulating psychostimulant effects of cocaine.ObjectivesFor the first time, we here addressed this issue by investigating the effects of a novel selective positive allosteric modulator of M4 receptors, VU0152100, on cocaine-induced behavioral and neurochemical effects in mice.MethodsTo investigate the effect of VU0152100 on the acute reinforcing effects of cocaine, we use an acute cocaine self-administration model. We used in vivo microdialysis to investigate whether the effects of VU0152100 in the behavioral studies were mediated via effects on dopaminergic neurotransmission. In addition, the effect of VU0152100 on cocaine-induced hyperactivity and rotarod performance was evaluated.ResultsWe found that VU0152100 caused a prominent reduction in cocaine self-administration, cocaine-induced hyperlocomotion, and cocaine-induced striatal dopamine increase, without affecting motor performance. Consistent with these effects of VU0152100 being mediated via M4 receptors, its inhibitory effects on cocaine-induced increases in striatal dopamine were abolished in M4 receptor knockout mice. Furthermore, selective deletion of the M4 receptor gene in dopamine D1 receptor-expressing neurons resulted in a partial reduction of the VU0152100 effect, indicating that VU0152100 partly regulates dopaminergic neurotransmission via M4 receptors co-localized with D1 receptors.ConclusionsThese results show that positive allosteric modulators of the M4 receptor deserve attention as agents in the future treatment of cocaine abuse.