Anti-seizure effect and neuronal activity change in the genetic-epileptic model rat with acute and chronic vagus nerve stimulation.

BACKGROUND: VNS showed time-dependent anti-seizure effect. However, the precise mechanism of VNS in acute and chronic anti-seizure effect has not been fully elucidated. Noda epileptic rat (NER) is genetic epilepsy model rat which exhibits spontaneous generalized tonic-clonic seizure (GTC) approximately once per 30 h and frequent dialeptic seizure (DS). We performed acute and chronic VNS on NER to focus on the acute and chronic anti-epileptic effect and neuronal activity change by VNS. METHODS: We performed acute VNS (2 h) on 22 NERs (VNS, n = 11, control, n = 11), then subsequently administered chronic (4 weeks) VNS on 10 of 22 NERs (VNS n = 5, control n = 5). We evaluated the acute and chronic anti-seizure effects of VNS on GTC and DS by behavioral and electroencephalographical observation (2 h every week). We carried out double immunofluorescence for biomarkers of short-term (c-Fos) and long-term (ΔFosB) neuronal activation to map regions in the brain that were activated by acute (VNS n = 6, control n = 6) or chronic VNS (VNS n = 5, control n = 5). Furthermore, we performed chronic VNS (4 w) on 12 NERs (VNS n = 6, control n = 6) with long-term observation (8 h a day, 5d per week) to obtain an adequate number of GTCs to elucidate the time dependent anti-epileptic effect on GTC. RESULTS: Acute VNS treatment reduced GTC seizure frequency and total duration of the DS. Chronic VNS resulted in a time-dependent reduction of DS frequency and duration. However, chronic VNS did not show time-dependent reduction of GTC frequency. There were significant c-Fos expressions in the central medial nucleus (CM), mediodorsal thalamic nucleus (MDM), locus coeruleus (LC), and nucleus of solitary tract (NTS) after acute VNS. And there were significant ΔFosB expressions in the lateral septal nucleus (LSV), medial septal nucleus (MSV), MDM, and pontine reticular nucleus caudal (PnC) after chronic VNS. Any decrease in frequency of GTCs by chronic VNS could not be confirmed even with long-term observation. CONCLUSION: We confirmed acute VNS significantly reduced the frequency of GTC and duration of DS. Chronic VNS decreased the frequency and duration of DS in a time-dependent manner. The brainstem and midline thalamus were activated after acute and chronic VNS. The forebrain was activated only after chronic VNS.

Effects of heat stress on Young's modulus of outer hair cells in mice.

Intense sound exposure causes permanent hearing loss due to hair cell and cochlear damage. Prior conditioning with sublethal stressors, such as nontraumatic sound, heat stress and restraint protects the ear from acoustic injury. However, the mechanisms underlying conditioning-related cochlear protection remain unknown. In this paper, Young's modulus and the amount of filamentous actin (F-actin) of outer hair cells (OHCs) with/without heat stress were investigated by atomic force microscopy and confocal laser scanning microscopy, respectively. Conditioning with heat stress resulted in a statistically significant increase in Young's modulus of OHCs at 3-6 h after application, and such modulus then began to decrease by 12 h and returned to pre-conditioning level at 48 h after heat stress. The amount of F-actin began to increase by 3 h after heat stress and peaked at 12 h. It then began to decrease by 24 h and returned to the pre-conditioning level by 48-96 h after heat stress. These time courses are consistent with a previous report in which heat stress was shown to suppress permanent threshold shift (PTS). In addition, distortion product otoacoustic emissions (DPOAEs) were confirmed to be enhanced by heat stress. These results suggest that conditioning with heat stress structurally modifies OHCs so that they become stiffer due to an increase in the amount of F-actin. As a consequence, OHCs possibly experience less strain when they are exposed to loud noise, resulting in protection of mammalian hearing from traumatic noise exposure.