A rat model of somatosensory-evoked reflex seizures induced by peripheral stimulation

Abstract

Objective: We introduce a novel animal model of somatosensory stimulation-induced reflex seizures which generates focal seizures without causing damage to the brain.Methods: Specifically, we electrically stimulated digits or forepaws of adult rats sedated with dexmedetomidine while imaging cerebral blood volume and recording neurophysiological activity in cortical area S1FL. For the recordings, we either inserted a linear probe into the D3 digit representation or we performed surface electrocorticography (ECoG) recordings.Results: Peripheral stimulation of a digit or the forepaw elicited seizures that were followed by a refractory period with decreased neuronal activity, or another seizure or a normal response. LFP amplitudes in response to electrical pulses during the seizures (0.28 ± 0.03 mV) were higher than during normal evoked responses (0.25 ± 0.05 mV) and refractory periods (0.2 ± 0.08 mV). Seizures generated during the stimulation period showed prolonged after-discharges that were sustained for 20.9±1.9 s following the cessation of the stimulus. High-frequency oscillations were observed prior to and during the seizures, with amplitudes higher than those associated with normal evoked responses. The seizures were initially focal. Optical imaging of the cerebral blood volume response showed that they propagated from the onset zone to adjacent cortical areas, beyond the S1FL representation of the stimulated digit or forepaw. The spatial extent during seizures was on average 1.74 times larger during the stimulation and 4.1 times following its cessation relative to normal evoked responses. Seizures were recorded not only by probes inserted into cortex but also with ECoG arrays (24.1±5.8 seizures per rat) placed over the dura matter, indicating that the seizures were not induced by damage caused by inserting the probes to cortex. Stimulation of the forepaw elicited more seizures (18.8±8.5 seizures per rat) than stimulation of a digit (1.7±0.7). Unlike rats sedated with dexmedetomidine, rats anesthetized with urethane showed no seizures, indicating that the seizures may depend on the use of the mild sedative dexmedetomidine.Significance: Our proposed animal model generates seizures induced by electrical sensory stimulation free of artifacts and brain damage. It can be used for studying the mechanisms underlying the generation and propagation of reflex seizures and for evaluating antiepileptic drugs..

Dexmedetomidine - commonly used in functional imaging studies - increases susceptibility to seizures in rats but not in wild type mice

“…In line with the findings by Whittington et al (2002) and Airaksinen et al (2012), we did not observe any seizure-like responses to short and weak forelimb stimuli (1s-long, 0.6-0.8 mA current pulses delivered at 8 Hz) under dexmedetomidine sedation (Sotero et al, 2015). Nevertheless, under the same anesthesia regime, long (10 s), frequent (8Hz) and potent (2 mA) peripheral electrical stimuli evokes seizures (Bortel et al, 2019). Similar potent peripheral stimulation in the range of 8-12 Hz and 1.5-2 mA is commonly used to elicit BOLD signal in dexmedetomidine sedated rodents.…”

“…Accordingly, forelimb stimulation elicited seizure-like responses accompanied by changes in cerebral blood flow (CBF) in Sprague-Dawley (SD) rats (Fukuda et al, 2013). These results have been corroborated by Bortel et al (2019), who characterized the pattern of seizures and demonstrated that the determining factors for inducing the seizures are the administration of dexmedetomidine combined with potent sensory electrical stimulation. Indeed, dexmedetomidine sedation alone, even if combined with short and weak peripheral stimulation is not sufficient for inducing epileptiform activity (Sotero et al, 2015).…”

“…Therefore, urethane is suitable for maintaining anesthesia during electrophysiological recording (Hara and Harris, 2002). We previously applied the exact same experimental and forepaw stimulation procedures under urethane anesthesia (Bortel et al, 2019). Under urethane anesthesia, we did not evoke any seizures.…”

“…All procedures applied for optical imaging of intrinsic signals (OIS) are described in detail by Bortel et al (2019). The ROI we imaged was centered on the atlas coordinates of area S1FL in the left hemisphere in rats (AP 0.00 mm, ML ±4.50 mm, DV -2.5 mm; Paxinos and Watson (2005)) and mice (AP -0.30 mm, ML ±2.20 mm, DV -1.3 mm; Franklin and Paxinos (2007)).…”

“…However, at higher doses (≥1.6%) isoflurane increases the baseline cerebral blood flow (CBF) (Eger, 1984;Franceschini et al, 2010) and for this reason, it is no longer the anesthetics of choice for hemodynamics-based functional studies. Therefore, to evaluate the dependence of the seizure generation on the anesthesia regime other than dexmedetomidine sedation, we performed six experiments using urethane (Bortel et al, 2019). Urethane is an anesthetic that has modest effects on both the inhibitory and excitatory systems and does not affect the noradrenergic system (Hara and Harris, 2002).…”