Changes in neuronal excitability in the rat hippocampus in a prolonged febrile seizures model

Abstract

Febrile seizures (FSs) are a prevalent neurological disorder among children aged 3 months to 5 years, with the highest incidence observed in the second year of life [1]. Considering that neuronal and glial cell development and synaptic contact formation are ongoing during this period [2, 3], FSs can potentially influence these processes. However, the present data on the effect of FSs on brain development remain inconsistent. This study aims to examine the impact of extended seizures on the attributes of hippocampal pyramidal neurons in rats of varying ages. Wistar rats were used in this study. FSs were induced on postnatal day (P)10 through placing pups onto the bottom of a glass chamber for 30 minutes and exposing them to a controlled stream of heated air, causing their body temperature to rise to 39 °C and trigger the occurrence of FSs. Only animals that underwent FSs that persisted for at least 15 minutes were included in the study. Littermates were employed as controls and were placed away from the nest for the same duration but kept at room temperature. At postnatal days 12, 21–23, and 51–55, the rats were decapitated, and their brains were extracted. Horizontal brain slices (400 micrometers) were sliced. The study examined the biophysical properties of CA1 pyramidal neurons using the whole-cell patch-clamp method. 1.5-second current pulses were injected, and subthreshold membrane properties such as resting membrane potential, input resistance, membrane time constant, and intrinsic firing properties, were assessed. Extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded from the CA1 stratum radiatum of the hippocampus to assess the efficacy of synaptic neurotransmission at CA3-CA1 pyramidal neuron synapses. The amplitude and fiber volley (FV) amplitude of each fEPSP were measured by applying high amplitude currents to each slice. A sigmoidal Gompertz function was used to evaluate the efficacy of neurotransmission. Paired-pulse stimulation was used to examine potential alterations in short-term synaptic plasticity. Paired pulses were administered at intervals spanning from 10 to 500 ms, and the paired-pulse ratio (PPR) was assessed as the proportion of the amplitude of the second to the first fEPSP for each interval. Maximum electroshock seizure threshold (MEST) was determined two months after FS to assess the animals’ susceptibility to seizures. The lowest current at which tonic hind limb extension was exhibited was determined for every animal. No significant changes in subthreshold firing properties were observed in P12 or P21 rats following FSs. However, we did observe several changes in intrinsic firing properties. For example, the maximum firing frequency decreased by 23% in P12 rats exposed to FSs, relative to age-matched control rats. Additionally, we noticed that firing frequency adaptation was significantly less pronounced in P12 rats that underwent FSs, compared to control rats of the same age. Seizure-induced alterations in firing characteristics were absent in P21 rats. No significant variations in fEPSP or FV amplitudes were found among the different groups of P12 and P55 rats at different current intensities. However, a significant increase in FV amplitudes and decrease in neuronal input-output (I/O) relationships between fEPSP and FV amplitudes were observed in P21 rats following FSs. At P12, short-term synaptic plasticity was disrupted, evidenced by a significant increase in PPR in rats two days post-FS. However, at P21 and P55, experimental and control groups were not significantly different. MEST test displayed a significant rise in the hind limb extension threshold in rats two months following FS as compared to control animals. Overall, these findings suggest alterations in neuronal excitability after prolonged FS. Two days after FS, neuronal excitability transiently decreased while PPR increased, indicating a decrease in the probability of presynaptic glutamate release in hippocampal neurons. The efficiency of synaptic neurotransmission in CA3-CA1 was reduced in three-week-old animals. Additionally, the MEST test demonstrated that rats, two months following FS, exhibited a higher hind limb extension threshold in comparison to control animals.