Prenatal minocycline treatment alters synaptic protein expression, and rescues reduced mother call rate in oxytocin receptor-knockout mice
Introduction
Microglia are the resident immunocompetent cells of the central nervous system (CNS), involved in phagocytic events in the CNS under inflammatory conditions, such as strokes, ischemia and infectious disease [1]. Although the role of microglia in the immune system in the CNS has been well characterized, researchers currently believe that in addition to the immunological function, microglia also contribute to brain physiology and the pathology of many mental disorders. Recent studies have shown the associations between the disruptions of microglia-expressed genes and mental disorders, including autism spectrum disorders (ASD) [2], [3], [4].
ASD is one of the most common mental disorders. ASD comprises a group of developmental disabilities that can cause significant defect in social communication and behavioral challenges as well as morbid repetitive behaviors and restricted interests [5], [6]. ASD is common in all racial, ethnic, and socioeconomic groups; it is, however, almost five times more common among boys than girls. It is thought that about 1% of children are affected with ASD [7], but no effective cures has been identified to date. Because of this situation, ASD is the subject of public concern, and ASD research has grown a great deal in recent years. Recently, several reports demonstrated a notable amelioration in ASD episodes by intranasal administration of oxytocin (Oxt) [8], [9], [10]. Moreover, clinical application of oxytocin to ASD patients has begun.
Oxt is a nonapeptide neurohormone synthetized in the supraoptic nucleus and paraventricular nucleus and secreted into the blood by the pituitary gland. The most noted function of Oxt in its history is in peripheral. Peripherally secreted Oxt has long been known as a reproductive hormone during pregnancy and parturition [11], and recent studies demonstrated a function of central Oxt in prosocial behaviors, such as trust, social memory, maternal behavior, and pair bonding [12], [13], [14], [15]. Interestingly, these are some of the behaviors that are often impaired in patients with ASD. Mice deficient in the Oxt gene, generated by our group, showed social amnesia [16]. In Oxt receptor (Oxtr) knockout mice, we also reported notable abnormal phenotypes of aggressive behavior, impaired maternal behavior, and suppressed ultrasonic vocalization by infant mice [17]. Oxt and Oxtr- deficient mice can therefore serve as useful animal models of human ASD.
Interestingly, a recent study suggested that deficient pruning of excess synapses by microglial cells in the brain can be one potential mechanism of ASD development [18]. Here, we analyzed the pathological phenotype of microglial cells in sociality-related brain regions of Oxtr deficient mice. Inhibition of abnormal microglial activation by minocycline can ameliorates PSD95 expression and mother–infant communication in the Oxtr deficient mice. Our results suggest that microglial abnormality is a potential mechanism of the development of Oxt/Oxtr mediated ASD-like phenotypes.
Section snippets
Mice
Oxtr-deficient mice used in this work have been described previously [16]. Only male mice were used for behavioral analyses. Mice were fed a standard chow diet and water ad libitum, and kept at 25 °C in a room with a 12-h light/dark cycle. All animal experiments were performed according to the Tohoku University guidelines for animal experimentation.
Drug administration
Minocycline (Wako; 100 mg/kg/day) was dissolved into drinking water at the required concentration, which was determined based on each dam's body
Results
Synaptic scaffolding protein expression is decreased in brains of adult Oxtr-deficient mice.
Recent studies reported that the number of synapses is altered in ASD patients and several ASD model animals. Therefore, we first assessed the expression of PSD95, a major synaptic scaffolding protein, in Oxtr-deficient mouse brains. Surprisingly, the expression level of PSD95 was markedly reduced in the Oxtr null brain (Fig. 1 A, B).
Excessive microglial activation was observed in several
Discussion
Oxt administration has recently been reported to ameliorate ASD episodes [9]; however, the molecular, biochemical, and physiological mechanisms that underlie this improvement are still unknown. Oxt is thought to function through the activation of neurons via the release of Ca2+ ions from intracellular stores, which, in turn, activates protein kinase C. Interestingly, functional magnetic resonance imaging analyses of patients with ASD indicate that nasal administration of Oxt activates nuclei
Competing interests
The authors declare that they have no competing interests.
Authors' contribution
SM, YH and KN performed this research. YH designed this research. SM and YH performed the experiments. SH contributed essential reagents of this research. SM and YH analyzed the data. YH and KN wrote the paper. All authors have read and approved the manuscript for publication.
Acknowledgments
This work was supported by a Grant-in Aid from the Japan Society for the Promotion of science of Japan (23380055, 15H02442), and part of this study is the result of “Integrated Research on Neuropsychiatric Disorders” carried out under the Strategic Research Program for Brain Sciences by the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
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- 1
Contributed equally to this work.