Developmental prefrontal mRNA expression of D2 dopamine receptor splice variants and working memory impairments in rats after early life Interleukin-1β elevation

Highlights

• D2S mRNA level decreases in intact rat mPFC from juvenile to adolescent period.

• Prefrontal D2S/D2L ratio is decreased during adolescence in intact rats.

• Early life IL-1β treatment induces long-lasting working memory (WM) deficit in rats.

• IL-1β-induced WM deficit appears in juvenile and persists in both adolescent & adult rats.

• IL-1β treatment alters developmental D2 expression: possible mechanism of WM decline.

Abstract

Long (D2L) and Short (D2S) isoforms of D2 dopamine receptor differ in their biochemical and physiological properties, which could affect functioning of prefrontal cortex. Contribution of distinct D2 dopamine receptor isoforms to cognitive dysfunctions and its developmental regulation are currently not fully elucidated. In the present study, we evaluated developmental mRNA expression of D2S/D2L dopamine receptor isoforms within the rat medial prefrontal cortex (mPFC) in the model of neurodevelopmental cognitive dysfunction. Working memory performance (Y-maze spontaneous alternations) and D2S/D2L mRNA expression in the mPFC (by qRT-PCR) were evaluated in juvenile (P27), adolescent (P42–47) and adult (P75–90) rats after chronic early life treatment with proinflammatory cytokine interleukin (IL)-1β (1 µg/kg i.p. daily P15–21). It was shown that IL-1β elevation during the 3rd week of life leads to working memory deficit originating in juvenile animals and persisting into adulthood. D2S mRNA expression was strongly downregulated during adolescence, and such downregulation was exaggerated in animals injected with IL-1β during P15–21. Early life IL-1β administrations influenced developmental changes in the D2S/D2L mRNA ratio. This measure was found to be decreased in adolescent and adult control (intact and vehicle-treated) rats compared to juvenile control, while in the case of IL-1β-treated animals, the decrease in D2S/D2L ratio was observed only in adulthood but not in adolescence compared to juvenile rats. During the adolescence, D2S mRNA expression was downregulated and D2S/D2L ratio was upregulated in the mPFC of rats treated with IL-1β during the 3rd week of life compared to controls. Based on these data we conclude that changes in the developmental expression of D2 dopamine receptor splice variants within mPFC may underlie long-lasting cognitive deficit associated with neonatal pathology.

Introduction

D2 dopamine receptors are known to be perspective target for developing new drugs to treat schizophrenia, affective disorders and other neuropsychiatric pathologies (Beaulieu and Gainetdinov, 2011, Beaulieu et al., 2015, Seeman, 2011). D2 receptors have two isoforms, short (D2S) and long (D2L) one, produced by alternative mRNA splicing (Giros et al., 1989, Monsma et al., 1989). These isoforms differ by their signal transduction pathways (Gantz et al., 2015, Lindgren et al., 2003, Takeuchi and Fukunaga, 2004, Van-Ham et al., 2007) as well as cellular localization: the D2L receptors are localized postsynaptically, while D2S are predominantly presynaptic autoreceptors (Khan, Mrzljak, Gutierrez, de la Calle, & Goldman-Rakic, 1998). Based on human genetic data and D2L knockout animal studies these two isoforms of D2 dopamine receptor are believed to play different roles in the regulation of working memory processes, prefrontal and motor cortical activity, striatal functions and mood regulation, behavioral flexibility, response to novelty (Bertolino et al., 2009, Bertolino et al., 2010, Fazio et al., 2011, Hranilovic et al., 2008, Macpherson et al., 2016, Morita et al., 2016, Quarto et al., 2017, Usiello et al., 2000, Wang et al., 2000). Postmortem and genetic studies indicate that D2 dopamine receptor splice variants may also be differentially implicated in the pathophysiology of schizophrenia and affective disorders (Bertolino et al., 2009, Kaalund et al., 2014).

Relatively high D2S/D2L ratio within dopaminoceptive brain areas is observed in the prefrontal cortex (Khan et al., 1998), which is well known to regulate working memory processing (Tzschentke, 2001). Decreased D2S/D2L ratio is believed to be implicated in working memory deficit in healthy subjects as well as schizophrenic patients (Zhang et al., 2007). Cognitive impairments may originate from neurodevelopmental alterations associated with negative genetic and environmental factors during pre- and early postnatal development (Bilbo and Schwarz, 2012, Zubareva and Klimenko, 2013). Brain dopaminergic system is considered to be implicated in the pathogenesis of various neurodevelopmental disorders (Money & Stanwood, 2013).

Little is known about regulation of D2S/D2L dopamine receptor mRNA expression during normal brain maturation and development of cognitive deficit in neurodevelopmental disorders associated with action of negative environmental factors during early development (Bilbo and Schwarz, 2012, Zubareva and Klimenko, 2013). One approach to model such disorders is the induction of early postnatal inflammation by proinflammatory cytokine interleukin (IL)-1β treatment. Elevated expression of this cytokine plays crucial role in the development of neuropsychiatric pathology after various neonatal brain injuries including hypoxic-ischemic encephalopathy, infectious diseases, perinatal traumas (Zubareva & Klimenko, 2013). Adult rats treated with IL-1β during the 3rd week of life demonstrate learning deficit in active and passive avoidance (Zubareva, Shcherbakova, Kalemenev, Simbirtsev, & Klimenko, 2013) as well as in Morris water maze paradigms (Trofimov, Zubareva, Simbirtsev, & Klimenko, 2014).

In the present study we investigated the effect of early life IL-1β treatment on the working memory performance and D2S/D2L dopamine receptor mRNA expression within mPFC during rat postnatal (from juvenile period to adulthood) development.