Febrile seizures cause a decrease in calcium-permeable AMPA receptors at synapses of rat cortical and hippocampal pyramidal neurons

Abstract

Febrile seizures (FS) are a prevalent childhood neurological disorder that can result in lasting functional alterations in neural networks, contributing to the onset of epilepsy and cognitive impairment [1]. Higher levels of calcium-permeable AMPA receptors (CP-AMPARs), which lack the GluA2 subunit, are detected in the hippocampus and cortex at an early age. CP-AMPARs are involved in various plastic changes within the central nervous system (CNS), including regular physiological processes (synaptic plasticity) and various pathological conditions. Such changes occur when these receptors are included in the neurons’ membrane, where they are not typically expressed. CP-AMPARs were demonstrated to incorporate into synapses during seizures [2]. The effect of FSs on CP-AMPAR expression is uncertain, and the resultant alterations in neuronal network function are unclear. The aim of this study was to determine whether the proportion of CP-AMPARs at synapses of pyramidal neurons in the rat entorhinal cortex and hippocampus alters immediately (at 15 min) and 48 h post FS. Ten-day-old rats were exposed to a stream of warm air (46 °C) for 30 minutes to induce hyperthermia, resulting in the development of FS. Only animals with FS that lasted for a minimum of 15 minutes were included in the study. The control group was comprised of littermates removed from the dam for an equivalent period but kept at room temperature. Isolated pyramidal neurons were used to determine the proportion of CP-AMPARs in the hippocampus. AMPAR-mediated currents were induced by application of 100 μM kainate. Excitatory postsynaptic currents (EPSCs) were evoked by extracellular stimulation in the entorhinal cortex. The antagonist IEM-1460 was used to selectively block CP-AMPARs. The rectification index of AMPA-mediated EPSCs was calculated to better assess the contribution of CP-AMPARs. Neurons expressing CP-AMPARs were visualized using the kainate-induced cobalt uptake method. Brain slices were stimulated with kainate while AR-5 and TTX were present. The DNQX blocker was used for the determination that the influx of Co2+ was mediated by AMPARs. Basal synaptic transmission was assessed by recording field postsynaptic responses in the hippocampus stimulated by Shaffer collaterals at different current strengths. FS induced a rapid decrease in the levels of CP-AMPARs on the membranes of pyramidal neurons in the hippocampus. As a result, 15 min after FS, IEM-1460 caused significantly less blockade of kainate-evoked current in neurons isolated from rat hippocampus compared to control (22% vs. 14%, p 0.05). A similar finding was observed for EPSCs evoked extracellularly in the entorhinal cortex, with a frequency of 10% in the FS group compared to 3% in the control group (p 0.05). Furthermore, the FS group’s neurons exhibited a significantly greater rectification index of EPSCs compared to the control group’s neurons. However, two days post-FS, no significant differences existed between the two groups. This observation may be attributed to the rapid decrease in the proportion of CP-AMPARs in pyramidal neurons at this stage. The cobalt uptake method supported electrophysiological findings, revealing higher staining levels in the CA1 field of the hippocampus and entorhinal cortex of control rats compared to FS rats. The observed effect surfaced 15 minutes after FS, and no divergences emerged between the groups after two days. Although the proportion of CP-AMPARs was reduced, basal neurotransmission levels in brain slices obtained from rats that underwent FS did not differ from control values. In summary, the expression of CP-AMPARs in entorhinal cortex and hippocampal pyramidal neurons in young rats decreases significantly with FS. These alterations do not impact the characteristics of basal synaptic transmission in the hippocampus.