Jelena Mijatovic

Pharmacologically Diverse Antidepressants Rapidly Activate Brain-Derived Neurotrophic Factor Receptor TrkB and Induce Phospholipase-Cγ Signaling Pathways in Mouse Brain

Previous studies suggest that brain-derived neurotrophic factor and its receptor TrkB are critically involved in the therapeutic actions of antidepressant drugs. We have previously shown that the antidepressants imipramine and fluoxetine produce a rapid autophosphorylation of TrkB in the rodent brain. In the present study, we have further examined the biochemical and functional characteristics of antidepressant-induced TrkB activation in vivo. We show that all the antidepressants examined, including inhibitors of monoamine transporters and metabolism, activate TrkB rapidly in the rodent anterior cingulate cortex and hippocampus. Furthermore, the results indicate that acute and long-term antidepressant treatments induce TrkB-mediated activation of phospholipase-Cγ1 (PLCγ1) and increase the phosphorylation of cAMP-related element binding protein, a major transcription factor mediating neuronal plasticity. In contrast, we have not observed any modulation of the phosphorylation of TrkB Shc binding site, phosphorylation of mitogen-activated protein kinase or AKT by antidepressants. We also show that in the forced swim test, the behavioral effects of specific serotonergic antidepressant citalopram, but not those of the specific noradrenergic antidepressant reboxetine, are crucially dependent on TrkB signaling. Finally, brain monoamines seem to be critical mediators of antidepressant-induced TrkB activation, as antidepressants reboxetine and citalopram do not produce TrkB activation in the brains of serotonin- or norepinephrine-depleted mice. In conclusion, our data suggest that rapid activation of the TrkB neurotrophin receptor and PLCγ1 signaling is a common mechanism for all antidepressant drugs.

Constitutive Ret activity in knock-in multiple endocrine neoplasia type B mice induces profound elevation of brain dopamine concentration via enhanced synthesis and increases the number of TH-positive cells in the substantia nigra.

Ret is the common signaling receptor for glial cell line-derived neurotrophic factor (GDNF) and other ligands of the GDNF family that have potent effects on brain dopaminergic neurons. The Met918Thr mutation leads to constitutive activity of Ret receptor tyrosine kinase, causing the cancer syndrome called multiple endocrine neoplasia type B (MEN2B). We used knock-in MEN2B mice with the Ret-MEN2B mutation to study the effects of constitutive Ret activity on the brain dopaminergic system and found robustly increased concentrations of dopamine (DA) and its metabolites in the striatum, cortex, and hypothalamus. The concentrations of brain serotonin were not affected and those of noradrenaline were slightly increased only in the lower brainstem. Tyrosine hydroxylase (TH) protein levels were increased in the striatum and substantia nigra/ventral tegmental area (SN/VTA), and TH mRNA levels were increased in SN/VTA of MEN2B mice, suggesting that constitutive Ret activity increases DA levels by increasing its synthesis. Also, the striatal DA transporter protein levels in the MEN2B mice were increased, which agrees with increased sensitivity of these mice to the stimulatory effects of cocaine. In the SN pars compacta of homozygous MEN2B mice, we found a 26% increase in the number of TH-positive cells, but no differences were found in the VTA. Thus, we show here that the constitutive Ret activity in mice is sufficient to increase the number of dopaminergic neurons and leads to profound elevation of brain DA concentration. These data clearly suggest that Ret activity per se can have a direct biological function that actively changes and shapes the brain dopaminergic system.

GDNF Overexpression from the Native Locus Reveals its Role in the Nigrostriatal Dopaminergic System Function

Degeneration of nigrostriatal dopaminergic system is the principal lesion in Parkinson’s disease. Because glial cell line-derived neurotrophic factor (GDNF) promotes survival of dopamine neurons in vitro and in vivo, intracranial delivery of GDNF has been attempted for Parkinson’s disease treatment but with variable success. For improving GDNF-based therapies, knowledge on physiological role of endogenous GDNF at the sites of its expression is important. However, due to limitations of existing genetic model systems, such knowledge is scarce. Here, we report that prevention of transcription of Gdnf 3’UTR in Gdnf endogenous locus yields GDNF hypermorphic mice with increased, but spatially unchanged GDNF expression, enabling analysis of postnatal GDNF function. We found that increased level of GDNF in the central nervous system increases the number of adult dopamine neurons in the substantia nigra pars compacta and the number of dopaminergic terminals in the dorsal striatum. At the functional level, GDNF levels increased striatal tissue dopamine levels and augmented striatal dopamine release and re-uptake. In a proteasome inhibitor lactacystin-induced model of Parkinson’s disease GDNF hypermorphic mice were protected from the reduction in striatal dopamine and failure of dopaminergic system function. Importantly, adverse phenotypic effects associated with spatially unregulated GDNF applications were not observed. Enhanced GDNF levels up-regulated striatal dopamine transporter activity by at least five fold resulting in enhanced susceptibility to 6-OHDA, a toxin transported into dopamine neurons by DAT. Further, we report how GDNF levels regulate kidney development and identify microRNAs miR-9, miR-96, miR-133, and miR-146a as negative regulators of GDNF expression via interaction with Gdnf 3’UTR in vitro. Our results reveal the role of GDNF in nigrostriatal dopamine system postnatal development and adult function, and highlight the importance of correct spatial expression of GDNF. Furthermore, our results suggest that 3’UTR targeting may constitute a useful tool in analyzing gene function.

Pharmacological Characterisation of a Structurally Novel α2C ‐Adrenoceptor Antagonist ORM‐10921 and its Effects in Neuropsychiatric Models

The α2‐adrenoceptors (ARs) are important modulators of a wide array of physiological responses. As only a few selective compounds for the three α2‐AR subtypes (α2A, α2B and α2C) have been available, the pharmacological profile of a new α2C‐selective AR antagonist ORM‐10921 is reported. Standard in vitro receptor assays and antagonism of α2, and α1‐AR agonist ‐evoked responses in vivo were used to demonstrate the α2C‐AR selectivity for ORM‐10921 which was tested in established behavioural models related to schizophrenia and cognitive dysfunction with an emphasis on pharmacologically induced hypoglutamatergic state by phencyclidine or MK‐801. The Kb values of in vitro α2C‐ AR antagonism for ORM‐10921 varied between 0.078–1.2 nM depending on the applied method. The selectivity ratios compared to α2A‐AR subtype and other relevant receptors were 10‐100 times in vitro. The in vivo experiments supported its potent α2C‐antagonism combined with only a weak α2A‐antagonism. In the pharmacodynamic microdialysis study, ORM‐10921 was found to increase extracellular dopamine levels in prefrontal cortex in the baseline conditions. In the behavioural tests, ORM‐10921 displayed potent antidepressant and antipsychotic‐like effects in the forced swimming test and prepulse‐inhibition models analogously with the previously reported results with structurally different α2C‐selective AR antagonist JP‐1302. Our new results also indicate that ORM‐10921 alleviated the NMDA‐antagonist‐induced impairments in social behaviour and watermaze navigation. This study extends and further validates the concept that α2C‐AR is a potential therapeutic target in CNS disorders such as schizophrenia or Alzheimers disease and suggests the potential of α2C‐antagonism to treat such disorders.

Effect of morphine on locomotor activity and striatal monoamine metabolism in nicotine-withdrawn mice.

Evidence for an interaction between nicotine and morphine has been found in several studies. In the present study mice withdrawn from a 7-week oral nicotine treatment were administered morphine, following which their locomotor activities were recorded and the concentrations of dopamine, 5-hydroxytryptamine (5-HT), noradrenaline and their metabolites were measured in the caudate putamen (CPu) and nucleus accumbens (NAc). For comparison, the effect of cocaine on locomotor activity was studied in mice withdrawn from nicotine. Morphine (15 mg/kg s.c.) enhanced locomotor activity significantly more in the nicotine-withdrawn mice than in the controls, whereas cocaine (20 mg/kg i.p.) stimulated the locomotor activity similarly in the nicotine-withdrawn and in the control mice. Morphine (10 mg/kg s.c.) elevated dopamine and 5-HT metabolites to the same degree in the NAc of the nicotine-withdrawn and the control mice. However, in the CPu morphine enhanced the metabolism of dopamine and also that of 5-HT in the nicotine-withdrawn mice but not in the controls. In addition, the basal concentrations of dopamine metabolites were reduced in the CPu of the nicotine-withdrawn mice. Thus, the enhancement of morphine-induced locomotor activation in the nicotine-withdrawn mice could be related to nicotine-induced changes in the regulation of the nigrostriatal dopaminergic and serotonergic systems.

Constitutive Ret signaling is protective for dopaminergic cell bodies but not for axonal terminals

Ret is the canonical signaling receptor for glial cell line-derived neurotrophic factor (GDNF), which has been shown to have neuroprotective effects when administered prior to neurotoxic challenge. A missense Meth918Thr mutation causes the constitutive activation of Ret, resulting in multiple endocrine neoplasia type 2 B (MEN2B). To clarify the role of Ret signaling in neuroprotection, we studied the effects of the neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) on the dopaminergic system of mice carrying the MEN2B mutation. We found that MEN2B mice were significantly more resistant to nigral tyrosine hydroxylase (TH)-positive cell loss induced by unilateral striatal 6-OHDA than Wt mice. However, 6-OHDA caused profound dopamine (DA) depletion in the striatum of both MEN2B and Wt mice. Systemic MPTP caused similar DA depletion and a decrease in TH-immunostaining in the striatum of MEN2B and Wt mice. Neither neurotoxin induced a compensatory increase in striatal metabolite/DA ratios in the MEN2B mice, possibly contributing to an increased amphetamine-induced turning behavior observed in behavioral assessments of these mice. Thus, our data suggest that activated Ret protects DA cell bodies in the substantia nigra pars compacta, but does not protect DA axons in the striatum.

Characterization of the striatal dopaminergic neurotransmission in MEN2B mice with elevated cerebral tissue dopamine

The Ret receptor tyrosine kinase is the common signaling receptor for the glial cell line‐derived neurotrophic factor (GDNF) family ligands. The Met918Thr mutation leads to constitutive activation of Ret and is responsible for dominantly inherited cancer syndrome MEN2B. Previously, we found that the mice carrying the mutation (MEN2B mice) have profoundly increased tissue dopamine (DA) concentrations in the striatum as well as increased striatal levels of tyrosine hydroxylase (TH) and dopamine transporter. The aim of this study was to characterize the striatal dopaminergic neurotransmission in MEN2B mice and to clarify the mechanisms by which they compensate their over‐production of DA. We found that tyrosine hydroxylase activity and DA synthesis are increased in MEN2B mice. Augmented effects of α‐methyl‐para‐tyrosine (αMT, an inhibitor of TH) and tetrabenazine (VMAT2 blocker) on DA levels suggest that also storage of DA is increased in MEN2B mice. There was no difference in the basal extracellular DA concentrations or potassium‐evoked DA release between the genotypes. The effects of cocaine and haloperidol were also similar between the genotypes as assessed by in vivo microdialysis. However, with in vivo voltammetry we found increase in stimulated DA release in MEN2B mice and detailed analysis of DA overflow showed that uptake of DA was also enhanced in MEN2B mice. Thus, our data show that enhanced synthesis of DA leading to increased storage and releasable pools in pre‐synaptic terminals in MEN2B mice apparently also leads to increased DA release, which in turn is compensated by higher dopamine transporter activity.

Mice deficient in transmembrane prostatic acid phosphatase display increased GABAergic transmission and neurological alterations.

Prostatic acid phosphatase (PAP), the first diagnostic marker and present therapeutic target for prostate cancer, modulates nociception at the dorsal root ganglia (DRG), but its function in the central nervous system has remained unknown. We studied expression and function of TMPAP (the transmembrane isoform of PAP) in the brain by utilizing mice deficient in TMPAP (PAP−/− mice). Here we report that TMPAP is expressed in a subpopulation of cerebral GABAergic neurons, and mice deficient in TMPAP show multiple behavioral and neurochemical features linked to hyperdopaminergic dysregulation and altered GABAergic transmission. In addition to increased anxiety, disturbed prepulse inhibition, increased synthesis of striatal dopamine, and augmented response to amphetamine, PAP-deficient mice have enlarged lateral ventricles, reduced diazepam-induced loss of righting reflex, and increased GABAergic tone in the hippocampus. TMPAP in the mouse brain is localized presynaptically, and colocalized with SNARE-associated protein snapin, a protein involved in synaptic vesicle docking and fusion, and PAP-deficient mice display altered subcellular distribution of snapin. We have previously shown TMPAP to reside in prostatic exosomes and we propose that TMPAP is involved in the control of GABAergic tone in the brain also through exocytosis, and that PAP deficiency produces a distinct neurological phenotype.