Effects of prenatal methamphetamine exposure on learning and memory and synaptogenesis in mouse hippocampus

Abstract

The dynamic processes of brain development during perinatal period are vulnerable to neurotoxic insults leading to changes in brain structure and functions. Hippocampus plays an important role in learning and memory. Hippocampal formation can be influenced by substance used during brain developmental processes including neuron differentiation and synaptogenesis. Brain-derived neurotrophic factor (BDNF) regulates neuron development and neuroplasticity via its receptor TrkB. Methamphetamine (MA) abused and the risk of MA exposed during pregnancy have increased recently. The present study aimed to elucidate the effects of prenatal MA exposure on learning and memory. Prenatal effects of MA on neuronal morphology and synaptogenesis, and the underlying neurotoxic mechanism involving BDNF-TrkB signaling pathway were also determined. Pregnant C57BL/6JNc mice were treated with either MA (5 mg/kg, s.c.) or normal saline on gestation day (GD) 8-15. Primary hippocampal cultures were prepared from embryos at GD16.5. Hippocampal neurons were used to examine morphology of axons and dendrites, synaptogenesis and BDNF-TrkB signaling protein expression. For in vivo study, adolescent (6 weeks old) and adult mice (12 weeks old) were conducted behavioral studies in open-field test, novel object recognition (NOR), novel location test (NLT), elevated plus maze (EPM) and forced swimming test (FST). Brains from prenatal day 14 (PND14), adolescent and adult mice were collected to determine presynaptic (synapsin-I), post-synaptic (PSD-95), and BDNF-TrkB signaling protein expression. Prenatal MA exposure impaired NLT performance in female adolescent mice while MA-exposed male adolescent mice showed memory deficit in NOR test. Moreover, prenatal MA exposure caused memory impairments in NOR and NLT in female and male adult mice. However, there were no effects on locomotor activity, anxiety and depression levels. MA-exposed hippocampal primary culture demonstrated decreases in secondary dendritic branches, axonal length and diameter. The number of presynaptic terminals and co-localization between pre- and postsynapse in MA group were also declined. The expression levels of presynaptic protein were decreased in the hippocampus of MA-exposed PND14 mice, but an increase in postsynaptic protein expression levels were presented in adolescent and adult mice. Prenatal MA exposure reduced TrkB expressions in primary cultures, while the expression levels of BDNF-TrkB signaling proteins in hippocampal tissue were not changed. The increased BDNF expression was observed in the prefrontal cortex and striatum of MA-exposed PND14 mice. This study showed the long-lasting effects of prenatal MA exposure on learning and memory impairments in adolescent and adult mice. These effects resulted from the disruptions of hippocampal neuronal growth and synaptogenesis in the developing brain. Alterations of BDNF-TrkB signaling and presynaptic proteins were found in early age, which lead to an adaptation in postsynaptic proteins.