Maintenance of amphetamine-induced place preference does not correlate with astrocytosis

Connecting Metainflammation and Neuroinflammation Through the PTN-MK-RPTPβ/ζ Axis: Relevance in Therapeutic Development

Inflammation is a common factor of pathologies such as obesity, type 2 diabetes or neurodegenerative diseases. Chronic inflammation is considered part of the pathogenic mechanisms of different disorders associated with aging. Interestingly, peripheral inflammation and the associated metabolic alterations not only facilitate insulin resistance and diabetes but also neurodegenerative disorders. Therefore, the identification of novel pathways, common to the development of these diseases, which modulate the immune response and signaling is key. It will provide highly relevant information to advance our knowledge of the multifactorial process of aging, and to establish new biomarkers and/or therapeutic targets to counteract the underlying chronic inflammatory processes. One novel pathway that regulates peripheral and central immune responses is triggered by the cytokines pleiotrophin (PTN) and midkine (MK), which bind its receptor, Receptor Protein Tyrosine Phosphatase (RPTP) β/ζ, and inactivate its phosphatase activity. In this review, we compile a growing body of knowledge suggesting that PTN and MK modulate the immune response and/or inflammation in different pathologies characterized by peripheral inflammation associated with insulin resistance, such as aging, and in central disorders characterized by overt neuroinflammation, such as neurodegenerative diseases and endotoxemia. Evidence strongly suggests that regulation of the PTN and MK signaling pathways may provide new therapeutic opportunities particularly in those neurological disorders characterized by increased PTN and/or MK cerebral levels and neuroinflammation. Importantly, we discuss existing therapeutics, and others being developed, that modulate these signaling pathways, and their potential use in pathologies characterized by overt neuroinflammation.

Mechanisms of Action and Persistent Neuroplasticity by Drugs of Abuse

Adaptation of the nervous system to different chemical and physiologic conditions is important for the homeostasis of brain processes and for learning and remembering appropriate responses to challenges. Although processes such as tolerance and dependence to various drugs of abuse have been known for a long time, it was recently discovered that even a single pharmacologically relevant dose of various drugs of abuse induces neuroplasticity in selected neuronal populations, such as the dopamine neurons of the ventral tegmental area, which persist long after the drug has been excreted. Prolonged (self-) administration of drugs induces gene expression, neurochemical, neurophysiological, and structural changes in many brain cell populations. These region-specific changes correlate with addiction, drug intake, and conditioned drugs effects, such as cue- or stress-induced reinstatement of drug seeking. In rodents, adolescent drug exposure often causes significantly more behavioral changes later in adulthood than a corresponding exposure in adults. Clinically the most impairing and devastating effects on the brain are produced by alcohol during fetal development. In adult recreational drug users or in medicated patients, it has been difficult to find persistent functional or behavioral changes, suggesting that heavy exposure to drugs of abuse is needed for neurotoxicity and for persistent emotional and cognitive alterations. This review describes recent advances in this important area of research, which harbors the aim of translating this knowledge to better treatments for addictions and related neuropsychiatric illnesses.

An animal model of differential genetic risk for methamphetamine intake

The question of whether genetic factors contribute to risk for methamphetamine (MA) use and dependence has not been intensively investigated. Compared to human populations, genetic animal models offer the advantages of control over genetic family history and drug exposure. Using selective breeding, we created lines of mice that differ in genetic risk for voluntary MA intake and identified the chromosomal addresses of contributory genes. A quantitative trait locus was identified on chromosome 10 that accounts for more than 50% of the genetic variance in MA intake in the selected mouse lines. In addition, behavioral and physiological screening identified differences corresponding with risk for MA intake that have generated hypotheses that are testable in humans. Heightened sensitivity to aversive and certain physiological effects of MA, such as MA-induced reduction in body temperature, are hallmarks of mice bred for low MA intake. Furthermore, unlike MA-avoiding mice, MA-preferring mice are sensitive to rewarding and reinforcing MA effects, and to MA-induced increases in brain extracellular dopamine levels. Gene expression analyses implicate the importance of a network enriched in transcription factor genes, some of which regulate the mu opioid receptor gene, Oprm1, in risk for MA use. Neuroimmune factors appear to play a role in differential response to MA between the mice bred for high and low intake. In addition, chromosome 10 candidate gene studies provide strong support for a trace amine-associated receptor 1 gene, Taar1, polymorphism in risk for MA intake. MA is a trace amine-associated receptor 1 (TAAR1) agonist, and a non-functional Taar1 allele segregates with high MA consumption. Thus, reduced TAAR1 function has the potential to increase risk for MA use. Overall, existing findings support the MA drinking lines as a powerful model for identifying genetic factors involved in determining risk for harmful MA use. Future directions include the development of a binge model of MA intake, examining the effect of withdrawal from chronic MA on MA intake, and studying potential Taar1 gene × gene and gene × environment interactions. These and other studies are intended to improve our genetic model with regard to its translational value to human addiction.

Pleiotrophin differentially regulates the rewarding and sedative effects of ethanol

Pleiotrophin (PTN) is a cytokine with important roles in dopaminergic neurons. We found that an acute ethanol (2.0 g/kg, i.p.) administration causes a significant up-regulation of PTN mRNA and protein levels in the mouse prefrontal cortex, suggesting that endogenous PTN could modulate behavioural responses to ethanol. To test this hypothesis, we studied the behavioural effects of ethanol in PTN knockout (PTN(-/-) ) mice and in mice with cortex- and hippocampus-specific transgenic PTN over-expression (PTN-Tg). Ethanol (1.0 and 2.0 g/kg) induced an enhanced conditioned place preference in PTN(-/-) compared to wild type mice, suggesting that PTN prevents ethanol rewarding effects. Accordingly, the conditioning effects of ethanol were completely abolished in PTN-Tg mice. The ataxic effects induced by ethanol (2.0 g/kg) were not affected by the genotype. However, the sedative effects of ethanol (3.6 g/kg) tested in a loss of righting reflex paradigm were significantly reduced in PTN-Tg mice, suggesting that up-regulation of PTN levels prevents the sedative effects of ethanol. These results indicate that PTN may be a novel genetic factor of importance in alcohol use disorders, and that potentiation of the PTN signalling pathway may be a promising therapeutic strategy in the treatment of these disorders.

Genetic inactivation of midkine modulates behavioural responses to ethanol possibly by enhancing GABA(A) receptor sensitivity to GABA(A) acting drugs

Midkine (MK) is a cytokine with important functions in dopaminergic neurons that is found upregulated in the prefrontal cortex of alcoholics. We have studied the behavioural effects of ethanol in MK genetically deficient (MK-/-) and wild type (MK+/+) mice. A low dose of ethanol (1.0g/kg), unable to cause conditioned place preference (CPP) in MK+/+ mice, induced a significant CPP in MK-/- mice, suggesting that MK prevents the rewarding effects of low doses of ethanol. However, this difference between genotypes is lost when a higher, rewarding, dose of ethanol (2.0g/kg) is used. Accordingly, the anxiolytic effects of 1.0mg/kg diazepam, other GABA(A) acting drug, were significantly enhanced in MK-/- mice compared to MK+/+ mice; however, 2.0mg/kg diazepam caused increased anxiolytic effects in MK+/+ mice. In addition, MK-/- mice showed a significant delayed recovery from ethanol (2.0g/kg)-induced ataxia whereas the sedative effects induced by ethanol (3.6g/kg), tested in a loss of righting reflex paradigm, were found to be similar in MK-/- and MK+/+ mice. The data indicate that MK differentially regulates the behavioural responses to ethanol. The results suggest that differences in the sensitivity of GABA(A) receptors to GABA(A) acting drugs caused by genetic inactivation of MK could underlie the different behavioural responses to ethanol in MK-/- mice. Overall, these results suggest that MK may be a novel genetic factor of importance in alcohol use disorders, and that potentiation of MK signalling pathway may be a promising therapeutic strategy in the treatment of these disorders.

Targeting midkine and pleiotrophin signalling pathways in addiction and neurodegenerative disorders: recent progress and perspectives

Midkine (MK) and pleiotrophin (PTN) are two neurotrophic factors that are highly up-regulated in different brain regions after the administration of various drugs of abuse and in degenerative areas of the brain. A deficiency in both MK and PTN has been suggested to be an important genetic factor, which confers vulnerability to the development of the neurodegenerative disorders associated with drugs of abuse in humans. In this review, evidence demonstrating that MK and PTN limit the rewarding effects of drugs of abuse and, potentially, prevent drug relapse is compiled. There is also convincing evidence that MK and PTN have neuroprotective effects against the neurotoxicity and development of neurodegenerative disorders induced by drugs of abuse. Exogenous administration of MK and/or PTN into the CNS by means of non-invasive methods is proposed as a novel therapeutic strategy for addictive and neurodegenerative diseases. Identification of new molecular targets downstream of the MK and PTN signalling pathways or pharmacological modulation of those already known may also provide a more traditional, but probably effective, therapeutic strategy for treating addictive and neurodegenerative disorders.

LINKED ARTICLES

This article is part of a themed section on Midkine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-4.

Phosphoproteomic analysis of the striatum from pleiotrophin knockout and midkine knockout mice treated with cocaine reveals regulation of oxidative stress-related proteins potentially underlying cocaine-induced neurotoxicity and neurodegeneration

The neurotrophic factors pleiotrophin (PTN) and midkine (MK) are highly upregulated in different brain areas relevant to drug addiction after administrations of different drugs of abuse, including psychostimulants. We have previously demonstrated that PTN and MK modulate amphetamine-induced neurotoxicity and that PTN prevents cocaine-induced cytotoxicity in NG108-15 and PC12 cells. In an effort to dissect the different mechanisms of action triggered by PTN and MK to exert their protective roles against psychostimulant neurotoxicity, we have now used a proteomic approach to study protein phosphorylation, in which we combined phosphoprotein enrichment, by immobilized metal affinity chromatography (IMAC), with two-dimensional gel electrophoresis and mass spectrometry, in order to identify the phosphoproteins regulated in the striatum of PTN knockout, MK knockout and wild type mice treated with a single dose of cocaine (15mg/kg, i.p.). We identified 7 differentially expressed phosphoproteins: 5'(3')-deoxyribonucleotidase, endoplasmic reticulum resident protein 60 (ERP60), peroxiredoxin-6 (PRDX6), glutamate dehydrogenase 1 (GLUD1), aconitase and two subunits of hemoglobin. Most of these proteins are related to neurodegeneration processes and oxidative stress and their variations specially affect the PTN knockout mice, suggesting a protective role of endogenous PTN against cocaine-induced neural alterations. Further studies are needed to validate these proteins as possible targets against neural alterations induced by cocaine.

Regulation of extinction of cocaine-induced place preference by midkine is related to a differential phosphorylation of peroxiredoxin 6 in dorsal striatum

The neurotrophic factors Midkine (MK) and Pleiotrophin (PTN) have been suggested to modulate drugs of abuse-induced effects. To test this hypothesis, cocaine (10 and 15mg/kg)-induced conditioned place preference (CPP) was rendered in PTN knockout (PTN-/-), MK knockout (MK-/-) and wild type (WT+/+) mice, and then extinguished after repeated saline injections (distributed in 4 extinction sessions). Cocaine induced a similar CPP in all the three genotypes. We found a significantly increased percentage of MK-/- mice that did not extinguish cocaine CPP at the end of the extinction sessions. Particularly, 40% of MK-/- mice did not extinguish cocaine (15mg/kg)-induced CPP compared to WT+/+ and PTN-/- mice (∼0-6%). Interestingly, we found that a greater magnitude of extinction of CPP after the first extinction session (5 days after last administration of cocaine) correlates with increased tyrosine phosphorylation of the enzyme peroxiredoxin 6 in the dorsal striatum of MK-/- mice. On the other hand, a greater magnitude of CPP extinction correlates with increased tyrosine phosphorylation of aconitase 2 in the prefrontal cortex of WT+/+ mice. In contrast, a lower magnitude of CPP extinction correlates with increased phosphorylation of aconitase 2 in the prefrontal cortex of PTN-/- mice, suggesting that the correlation between the tyrosine phosphorylation levels of aconitase 2 and magnitude of CPP extinction depends on the genotype considered. The data demonstrate that MK is a novel genetic factor that plays a role in the extinction of cocaine-induced CPP by mechanisms that may involve specific phosphorylation of striatal peroxiredoxin 6.