Bisphenol A Impairs Synaptic Plasticity by Both Pre- and Postsynaptic Mechanisms

Abstract

Bisphenol A (BPA), an environmental xenoestrogen, has been reported to induce learning and memory impairments in rodent animals. However, effects of BPA exposure on synaptic plasticity and the underlying physiological mechanisms remain elusive. Our behavioral and electrophysiological analyses show that BPA obviously perturbs hippocampal spatial memory of juvenile Sprague-Dawley rats after four weeks exposure, with significantly impaired long-term potentiation (LTP) in the hippocampus. These effects involve decreased spine density of pyramidal neurons, especially the apical dendritic spine. Further presynaptic findings show an overt inhibition of pulse-paired facilitation during electrophysiological recording, which suggest the decrease of presynaptic transmitter release and is consistent with reduced production of presynaptic glutamate after BPA exposure. Meanwhile, LTP-related glutamate receptors, NMDA receptor 2A (NR2A) and AMPA receptor 1 (GluR1), are significantly downregulated in BPA-exposed rats. Excitatory postsynaptic currents (EPSCs) results also show that EPSC_NMDA, but not EPSC_AMPA, is declined by 40% compared to the baseline in BPA-perfused brain slices. Taken together, these findings reveal that juvenile BPA exposure has negative effects on synaptic plasticity, which result from decreases in dendritic spine density and excitatory synaptic transmission. Importantly, this study also provides new insights into the dynamics of BPA-induced memory deterioration during the whole life of rats.

Keywords: bisphenol A; hippocampus; spatial memory; spine; synaptic plasticity.

Figure 1

BPA crosses the BBB in CNS. Cerebrospinal fluid (CSF) samples were collected from the third ventricle of rats with or without juvenile BPA exposure at P49. BPA levels in CSF were determined by UPLC. Histograms were plotted by the mean of eight rat hippocampus per group. (*p < 0.05, n = 8 rats per group).

Figure 2

Deterioration of spatial memory by juvenile BPA exposure. A) Rats after chronic oral treatment of BPA (n = 8 rats, red) or DMSO (n = 8 rats, black) were assessed for spatial memory by the Marris‐water maze test, as shown by B) escape latency to reach the platform after 5 d of training (p > 0.05), C) platform duration (*p < 0.05), D) escape latency (p > 0.05), and E,F) swimming speed and distance (p > 0.05). G) Rats (n = 8 rats per group) were assessed for spatial memory in the Y maze test (without stress punishment or reward), as shown by H) the percentage of duration (*p < 0.05) and I) entry number in the novel arm (**p < 0.01), and I) percentage of entry number in the home arm (**p < 0.01).

Figure 3

Impairment of basic synaptic transmission and long‐term plasticity (LTP) in the hippocampal CA1 areas after juvenile BPA exposure. A) Input/output (I/O) curves showed the fEPSP slopes as a function of stimulus current intensities in control and BPA‐exposed groups. I/O was significantly depressed in BPA‐exposed rats compared with that of the control value (n = 10 rats per group, **p < 0.01). B) The magnitude of LTP was assessed by the fEPSP slope (percentage of baseline) in every 5 min within 60 min after high frequency tetanic stimulation (HFS) (n = 10 rats per group, ***p < 0.001). C) Histogram showing a significant decrease of LTP magnitude in 60 min of juvenile BPA‐exposed rats (n = 10 rats per group, **p < 0.01). Arrow indicates of HFS application.

Figure 4

Alteration of dendritic morphology of pyramidal neurons in the hippocampal CA1 areas after juvenile BPA exposure. A) A representative Golgi‐Cox impregnated pyramidal neuron by Sholl analysis for measuring dendritic length. Sholl analysis and histograms plot showing no difference of dendritic length between the control and BPA exposed groups (n = 10 per group, p > 0.05); B) Representative sections (50 µm) of Golgi‐Cox stained apical and basal dendrites of pyramidal neurons in hippocampus. Histograms showing decreased dendritic spine density (spines per 10 µm), especially apical spine density, after juvenile BPA exposure. (n = 10 rats per group, ***p < 0.001).

Figure 5

Downregulation of excitatory receptor and postsynaptic transmission after BPA exposure. A,B) Representative immunoblot and corresponding densitometric analysis showing the ratios of expression amounts of NMDA receptor subtypes (NR1, NR2A, and NR2B to GAPDH (#p > 0.05, *p < 0.05, and #p > 0.05, respectively) and AMPA receptor subtypes (GluR1 and GluR2) to GAPDH in control and juvenile BPA treated groups (n = 8 rats per group, *p < 0.05 and p > 0.05, respectively); C,D) Excitatory synaptic transmission in BPA‐treated brain slices (10 × 10⁻⁶ m) was assessed by recording NMDA and AMPA receptor mediated current in the hippocampal CA1 areas, as shown by decreased EPSC_NMDA, not EPSC_AMPA, after acute BPA treatment (n = 8 rats per group, *p < 0.05 and p > 0.05, respectively).

Figure 6

Decrease of pre‐synaptic transmission, and transmitter synthesis and release after juvenile BPA exposure. A,B) Paired‐pulse facilitation in the hippocampal CA1 areas was obviously inhibited in BPA exposed rats, as shown by reduced paired‐pulse ratio (fEPSP2/fEPSP1) (n = 10 rats per group, **p < 0.01); C,D) Glutamate synthesis was decreased by juvenile BPA exposure, as shown by downregulation of glutaminase activity and glutamate level (n = 8 rats per group, **p < 0.01, and *p < 0.05, respectively).