Effect of skin temperature on vibrotactile sensitivity

The vibration problems relating to living bodies have so far been studied from the perspectives of engineering physiology and psychology. This study shows the relationship between vibratory sensibility and temperature in the living body. Psychological experiments were carried out by using the vibrometer of an acoustic calibration apparatus in sine, triangular and square waves. The sensibility-threshold measurements were made using 30–700 Hz sine waves, 30–300 Hz triangular and sawtooth waves, or 30–250 Hz square waves. Each of ten subjects was kept seated. The average value of the vibratory levels, varied by ascending and descending steps, was taken as that of the threshold. As the vibrometer in the apparatus used makes a noise at frequencies greater than 250 Hz it was masked from the subject by presenting him with a different noise. The threshold curve for square waves is lower by 12·3 dB than that for sine waves at about 30Hz. The threshold curve of the 26°C sine wave was lower by 10 dB than that of the 58°C sine wave vibration near 200 Hz. For example with a sine wave, at 58°C the amplitude threshold was lowest at about 270 Hz, but at −11°C at about 200 Hz. At frequency stimulation higher than 120 Hz, as the temperature of the contact point was lowered, the amplitude threshold increased and the frequency at which the threshold curve was at a minimum shifted to a lower frequency.     

Tuning of the ocular vestibular evoked myogenic potential (oVEMP) to air- and bone-conducted sound stimulation in superior canal dehiscence

Recent studies have demonstrated the frequency selectivity of air-conducted (AC) and bone-conducted (BC) stimuli in eliciting ocular vestibular evoked myogenic potentials (oVEMPs). In this study, frequency tuning of the oVEMP was assessed in patients with superior canal dehiscence (SCD) and compared with responses previously reported for healthy subjects. Six (five unilateral) SCD patients were stimulated using AC sound (50–1,200 Hz) and BC transmastoid vibration (50–1,000 Hz). Stimuli were delivered at two standardized intensities: one the same as previously used for healthy controls and the other at 10 dB above vestibular threshold (a similar relative intensity to that used in controls). For AC stimulation, SCD patients had larger oVEMP amplitudes across all frequencies tested for both stimulus intensities. Normalized tuning curves demonstrated greater high-frequency responses with the stronger stimulus. For BC stimulation, larger oVEMP amplitudes were produced at frequencies at and above 100 Hz using standard intensity stimuli. For the matched intensity above vestibular threshold, enhancement of the oVEMP response was present in SCD patients for 500–800 Hz only. We conclude that SCD causes greater facilitation for AC than BC stimuli. The high-frequency response is likely to originate from the superior (anterior) canal and is consistent with models of inner ear changes occurring in SCD.     

The effect of the subcutaneous fat on the transfer of current through skin and into muscle

The present investigation was conducted to see the effect of subcutaneous fat on the transmission characteristics of an electrical stimulus applied to the skin and conducted to skeletal muscle. Two groups of subjects participated. In one, the subjects were three males and three females whose average age was 24.6+/-1.5 years, average weight was 74.8+/-18.2kg, and average height was 176.4+/-10.3cm. The other was a group of 30 subjects who average age was 26.2+/-1.9 years, average height 177.3+/-11.5cm, average weight 92.4+/-19.8kg. Electrical stimulation was applied above the quadriceps muscle at a current of 5mA and with sine and square wave stimulation at a frequency of 30Hz and a pulse width of 250micros. Current movement was measured on the skin and into muscle with surface and needle electrodes. The results showed that the thickness of the subcutaneous fat layer was directly related to signal loss from the skin (correlation between subcutaneous fat thickness and RC time constant was 0.96, p<0.001). Because of the subcutaneous fat layer and the resulting capacitance, an RC low pass filter is created such that square wave stimuli are not transmitted well into muscle whereas sine wave stimuli pass easily. Thus, when considering surface stimulation of nerve or muscle, any volume conductor model must take subcutaneous fat into consideration since the RC low pass filter created by fat will filter surface signals or, conversely, signals such as EMG which are generated in muscle but measured on the surface of the skin.     

Conventional anticonvulsant drugs in the guinea pig kindling model of partial seizures: effects of acute phenobarbital,valproate, and ethosuximide

This study addressed the anticonvulsant effects of phenobarbital, valproate, and ethosuximide in the amygdala of kindled guinea pigs to further validate this model for the screening of anticonvulsant drugs. Behavioral toxic effects were assessed at 30 min following drug administration using quantitative locomotor tests, as well as scores on a sedation and muscle relaxation rating index. The anticonvulsant efficacy of the drugs were evaluated from measurements of afterdischarge threshold (ADT), afterdischarge duration (ADD), and behavioral seizure severity (SS) during early and late phases of kindling acquisition, and in kindled guinea pigs. ADD and SS were also measured in response to both threshold and suprathreshold kindling stimulation. All drugs exerted slight to moderate sedative effects in guinea pigs on both the behavioral tests and rating index. We found that phenobarbital exhibited effective anticonvulsant properties in guinea pigs by consistently reducing ADD and SS in response to both threshold and suprathreshold kindling stimulation. Valproate exhibited effective anticonvulsant properties at threshold stimulation and less effective properties at suprathreshold stimulation. Lastly, we found that ethosuximide lacked effective anticonvulsant action at either threshold or suprathreshold kindling stimulation. Our results indicate that the guinea pig kindling model correctly predicted the actions of phenobarbital, valproate, and ethosuximide in the treatment of partial seizures. Guinea pig amygdala kindling appears to serve as a useful and valid model for partial epilepsy.     

Estimation of the distribution of intramuscular current during electrical stimulation of the quadriceps muscle

Electrical stimulation is commonly used for strengthening muscle but little evidence exists as to the optimal electrode size, waveform, or frequency to apply. Three male and three female subjects (22–40 years old) were examined during electrical stimulation of the quadriceps muscle. Two self adhesive electrode sizes were examined, 2 cm × 2 cm and 2 cm × 4 cm. Electrical stimulation was applied with square and sine waveforms, currents of 5, 10 and 15 mA, and pulse widths of 100–500 μs above the quadriceps muscle. Frequencies of stimulation were 20, 30, and 50 Hz. Current on the skin above the quadriceps muscle was measured with surface electrodes at five positions and at three positions with needle electrodes in the same muscle. Altering pulse width in the range of 100–500 μs, the frequency over a range of 20–50 Hz, or current from 5 to 15 mA had no effect on current dispersion either in the skin or within muscle. In contrast, the distance separating the electrodes caused large changes in current dispersion on the skin or into muscle. The most significant finding in the present investigation was that, while on the surface of the skin current dispersion was not different between sine and square wave stimulation, significantly more current was transferred deep in the muscle with sine versus square wave stimulation. The use of sine wave stimulation with electrode separation distances of less then 15 cm is recommended for electrical stimulation with a sine wave to achieve deep muscle stimulation.     

Loud sound-induced changes in cochlear mechanics

To investigate the inner ear response to intense sound and the mechanisms behind temporary threshold shifts, anesthetized guinea pigs were exposed to tones at 100-112 dB SPL. Basilar membrane vibration was measured using laser velocimetry, and the cochlear microphonic potential, compound action potential of the auditory nerve, and local electric AC potentials in the organ of Corti were used as additional indicators of cochlear function. After exposure to a 12-kHz intense tone, basilar membrane vibrations in response to probe tones at the characteristic frequency of the recording location (17 kHz) were transiently reduced. This reduction recovered over the course of 50 ms in most cases. Organ of Corti AC potentials were also reduced and recovered with a time course similar to the basilar membrane. When using a probe tone at either 1 or 4 kHz, organ of Corti AC potentials were unaffected by loud sound, indicating that transducer channels remained intact. In most experiments, both the basilar membrane and the cochlear microphonic response to the 12-kHz overstimulation was constant throughout the duration of the intense stimulus, despite a large loss of cochlear sensitivity. It is concluded that the reduction of basilar membrane velocity that followed loud sound was caused by changes in cochlear amplification and that the cochlear response to intense stimulation is determined by the passive mechanical properties of the inner ear structures.     

Repetitive extrasystole as an index of vulnerability to ventricular fibrillation.

The assessment of ventricular vulnerability by inducing ventricular fibrillation (VF) presents limitations when neural activity is being investigated, especially in the unanesthetized animal. As repetitive extrasystoles (RE) have been observed to precede the occurrence of VF, it was relevant to determine whether the RE threshold provides a reliable index of cardiac susceptivility to fibrillation. The RE and VF threshold relationships were studied in 32 chloralose-anesthetized dogs during left stellate ganglion stimulation, vagus nerve stimulation, and beta-adrenergic blockage with practolol. The vulnerable period was scanned at 1-ms intervals and at 2-mA increments with a single, 2-ms, constant-current cathodal stimulus; RE and multiple RE were induced reproducibly when 66% and 82%, respectively, of the fibrillatory current was administered. The nadirs for RE and multiple RE were coincident in the cardiac cycle with the vulnerable-period threshold for VF. Stellate and vagal stimulation and beta-adrenergic blockade resulted in comparable changes in RE and VF thresholds and produced equivalent shifts in the cardiac cycle of the RE and VF vulnerable-period nadirs. These observations suggest that RE and VF phenomena share a common electrophysiologic basis and that the RE threshold can be used as an end point for measuring ventricular vulnerability to VF.     

心室颤动的时-频分析和胺碘酮的作用

研究心室颤动时主导频率的动态性空间时间变化。在19只犬中建立正交心电图和心脏电除颤系统;诱发心室颤动持续10~30 s;使用时-频分析法分析心室颤动时频率的时频变化。另有4只犬在诱发心室颤动后静注胺碘酮100 mg以观察主导频率的改变。结果显示:在427个10 s VF和335个30 s VF的试验中,主导频率的变化与平均频率相差12%~18%;79个使用胺碘酮的试验中,主导频率的均值和变异性均降低。表明:在10~30 s心室颤动时心电图主导频率有明显和持续性的变异,胺碘酮可减小心室颤动时的频率和变异。     

Surprising similarity between mechanisms mediating low (1 Hz)-and high (100 Hz)-induced long-lasting synaptic potentiation in CA1 of the intact hippocampus

It is generally assumed that long lasting synaptic potentiation (long-term potentiation, LTP) and depression (long-term depression, LTD) result from distinct patterns of afferent activity, with high and low frequency activity favouring LTP and LTD, respectively. However, a novel form of N-methyl-d-aspartate (NMDA) receptor-dependent synaptic potentiation in the hippocampal CA1 area in vivo induced by low frequency afferent stimulation has recently been demonstrated. Here, we further characterize the mechanisms mediating this low frequency stimulation (LFS)-induced LTP in area CA1 of intact, urethane-anesthetized preparations. Consistent with previous reports, alternating, low frequency (1 Hz) stimulation of CA1 afferents originating in the contralateral CA3 area and the medial septum resulted in gradually developing, long lasting (>2 h) LTP of field excitatory postsynaptic potentials (fEPSPs) recorded in CA1. Local application of the protein synthesis inhibitor anisomycin in CA1 blocked LFS-induced LTP, as did application of H89, an inhibitor of protein kinase A. Given the apparent overlap in molecular mechanisms mediating LFS-LTP and “classic” high-frequency stimulation (HFS)-induced LTP in CA1, we examined the relation between these forms of LTP by means of occlusion experiments. LFS, delivered to synapses saturated by initial HFS, resulted in a gradually developing LTD, rather than the normally seen LTP. Conversely, initial induction of LFS-LTP reduced the amount of subsequent HFS-LTP. Together, these experiments reveal a surprising similarity in the molecular mechanisms (dependence on NMDA receptors, protein kinase A, protein synthesis) mediating LTP induced by highly distinct (1 vs. 100 Hz) induction protocols. Importantly, these findings further challenge the “high-frequency-LTP, low-frequency LTD” dogma by demonstrating that this dichotomy does not account for all types of plasticity phenomena at central synapses.     

The transfer of current through skin and muscle during electrical stimulation with sine,square, Russian and interferential waveforms

Electrical stimulation is a commonly used modality for both athletic training and physical therapy. However, there are limited objective data available to determine the waveform which provides the maximum muscle strength as well as minimizing pain. In the present investigation, two groups of subjects were examined. Group 1 was composed of six males and four females and group 2 was composed of three male and three female subjects. The first series of experiments investigated muscle strength with stimulation at currents of 20, 40 and 60 milliamps using sine, square, Russian and interferential waveforms evaluating strength production and pain as outcomes. The second phase of experiments compared the effect of the different waveforms on current dispersion in surface versus deep muscle electrodes with these same waveforms. The results of the experiments showed that sine wave stimulation produced significantly greater muscle strength and significantly less pain than square wave, Russian or interferential stimulation at that same current. The most painful stimulation was square wave. Strength production was greatest with sine wave and least with Russian and interferential. An explanation of these findings may be the filtering effect of the fat layer separating skin from muscle. The highly conductive muscle and skin dermal layers would form the plates of a capacitor separated by the subcutaneous fat layer providing an RC filter. This filtering effect, while allowing sine wave stimulation to pass to the muscle, reduced power transfer in square wave, Russian and interferential stimulation is observed.     

体表脉冲电刺激大动物的安全性评估

背景：目前电刺激技术已成为研究热点,但其对机体心电活动的影响尚不明确,且既往此方面研究主要针对小型动物。目的：在不影响心电活动的前提下,寻找刺激猪肝区所能使用最大的脉冲电压及频率。方法：取健康雄性荣昌猪3头,将脉冲电刺激仪输出电极固定于猪肝右叶体表投影前后对应部位,设置固定电刺激参数为波宽10 ms、方波,刺激时间为1 h,分别进行5-100 V的电压耐受实验和1-5 Hz的频率耐受实验,观察猪的动态心电图变化、其不适反应和生活行为的改变。结果与结论：电压耐受实验中脉冲电刺激电压高低与心率增量呈正相关;当电压> 35 V时,可发生窦性心律不齐、室性早搏等轻度心律失常,无恶性心律失常、死亡;当电压> 60 V时,可见电刺激诱发形成的qRs波。频率耐受实验中当频率> 2 Hz时可见心律失常发生,无恶性心律失常,脉冲电刺激可诱发形成qRs波。提示选择参数为电压35 V、频率2 Hz的脉冲电刺激猪肝区是安全的,电压大于35 V或频率大于2 Hz的脉冲电刺激猪肝区均可引发心律失常,多为良性心律失常。中国组织工程研究杂志出版内容重点：肾移植;肝移植;移植;心脏移植;组织移植;皮肤移植;皮瓣移植;血管移植;器官移植;组织工程     

Changes in BOLD transients with visual stimuli across 1-44 Hz

The dependency of positive BOLD (PBOLD) and post-stimulus undershoot (PSU) on the temporal frequency of visual stimulation was investigated using stimulation frequencies between 1 and 44 Hz. The PBOLD peak at 8 Hz in primary visual cortex was in line with previous neuroimaging studies. In addition to the 8 Hz peak, secondary peaks were observed for stimulation frequencies at 16 and 24 Hz. These additional local peaks were contrary to earlier fMRI studies which reported either a decrease or a plateau for frequencies above 8 Hz but in line with electrophysiological results obtained in animal local field potential (LFP) measurements and human steady-state visual evoked potential (SSVEP) recordings. Our results also indicate that the dependency of PSU amplitude on stimulus frequency deviates from that of PBOLD. Although their amplitudes were correlated within the 1-13 Hz range, they changed independently at stimulation frequencies between 13 and 44 Hz. The different dependency profiles of PBOLD and PSU to stimulation frequency points to different underlying neurovascular mechanisms responsible for the generation of these BOLD transients with regard to their relation to inhibitory and excitatory neuronal activity.     

Factorial analysis of the cardiovascular responses to carotid sinus nerve stimulation

The changes in arterial blood pressure, heart period, and respiration evoked by carotid sinus nerve (CSN) stimulation were studied in closed-chest, anesthetized dogs. Multifactorial regression equations were derived to express the various steady-state responses as functions of the parameters of stimulation. With the CSN and vagi intact, the changes in arterial pressure produced by CSN stimulation were much less pronounced than after the CSN and vagi were interrupted. When the CSN and vagi were intact, changes in stimulus duration had relatively little influence on heart period at the lower voltage levels. However, at higher voltages, increases in pulse duration produced substantial increments in heart period. TheRR interval increased progressively as the frequency of CSN stimulation was varied from 15 to 45 Hz; the magnitude of this effect varied directly with the stimulus voltage. This work was supported by U.S. Public Health Service Grants HL 15758 and HL 10951 from the National Heart and Lung Institute.     

Differential effects of duration for ocular and cervical vestibular evoked myogenic potentials evoked by air- and bone-conducted stimuli

We investigated the changes in cervical (cVEMP) and ocular (oVEMP) vestibular evoked myogenic potentials in response to differing stimulus durations. cVEMPs (n = 12 subjects) and oVEMPs (n = 13 subjects) were recorded using air-conducted (AC: 500 Hz) and bone-conducted (BC: 500 Hz) tone burst stimuli with durations varying from 2 to 10 ms. BC stimulation was applied both frontally and to the mastoid. AC cVEMPs showed an increase in amplitude with stimuli up to 6-ms duration associated with a prolonged latency, as previously reported. In contrast, AC oVEMP amplitude decreased with increasing stimulus duration. BC stimuli showed no significant increase in amplitude with increasing stimulus duration for either reflex using either location of stimulation. BC cVEMPS following forehead stimulation showed a significant decrease as duration increased, and BC oVEMPs to mastoid stimulation were largest at 2 ms and decreased thereafter. We conclude that an increase in amplitude with increasing stimulus duration, using 500 Hz stimuli, only occurs for AC cVEMPs. There is no definite benefit in using longer stimuli than 2 ms for BC or oVEMP studies. Shorter stimuli also minimise subject exposure to sound and vibration.     

Tetanic depression and catch-like effect in fast motor units of the rat medial gastrocnemius at linearly increasing and decreasing stimulation frequencies

The forces developed by fast resistant (FR) and fast fatigable (FF) motor units of the rat medial gastrocnemius during trains of electrical stimuli at linearly increasing or decreasing frequency were measured at the instantaneous frequency of 60 Hz and compared with the force evoked at a constant 60 Hz. In both motor unit types, the mean forces during stimulation at increasing frequency were depressed by 17%, while those recorded during stimulation at decreasing frequency were increased by 15% (FR) or 10% (FF) compared to values observed during constant-rate stimulation. During trains of stimuli at an increasing rate, the instantaneous frequency necessary to induce a force comparable to that produced at constant 60 Hz stimulation in FR and FF units was 84 and 88 Hz, respectively; whereas for the same units during stimulation at a decreasing rate these values were 45 and 47 Hz, respectively. When the stimulation frequency was increased up to 60 Hz and then held at this level, the force of both motor unit types was depressed by approximately 6% compared to 60 Hz constant-rate stimulation. From the available data it may be concluded that the phenomenon of tetanic depression is able to limit the development of force when the motoneuronal firing rate increases.     

Influence of inspiratory threshold load on cardiovascular responses to controlled breathing at 0.1 Hz

Slow, deep breathing is being used as a self‐management intervention for various health conditions including pain and hypertension. Stimulation of the arterial baroreceptors and increased vagal modulation are among the proposed mechanisms for the therapeutic effects of slow, deep breathing. We investigated whether adding inspiratory threshold load can enhance the cardiovascular responses to controlled breathing at the frequency of 0.1 Hz, a common form of slow, deep breathing. Healthy volunteers (N = 29) performed controlled breathing at 0.1 Hz (6 breaths/minute) without load and with inspiratory threshold loads of 5 cmH₂O and 10 cmH₂O. Respiratory airflow, heart rate, and blood pressure were continuously recorded. The amplitude of the systolic blood pressure variation during respiratory cycles increased with increasing loads. Respiratory sinus arrhythmia was higher during controlled breathing at 0.1 Hz with the load of 10 cmH₂O compared to without load. Baroreflex sensitivity was not affected by loads. The effect of loads on respiratory sinus arrhythmia was mediated by increasing the amplitude of systolic blood pressure variation during respiratory cycles. These results suggest that applying small inspiratory threshold loads during controlled breathing at 0.1 Hz increases cardiac vagal modulation by this breathing exercise. This effect seems to be mediated by stronger stimulation of the arterial baroreceptors because of larger systolic blood pressure swings along the respiratory cycle. The potential benefit of long‐term practice of controlled breathing at 0.1 Hz with inspiratory threshold loads on baroreflex function and cardiac vagal control needs to be investigated, particularly in pain and hypertension patients.     

Modulation of visual cortical excitability in migraine with aura: effects of 1 Hz repetitive transcranial magnetic stimulation

Recent studies showed hyperexcitability of the occipital cortex in subjects affected by migraine with aura. It has been shown that 1 Hz repetitive transcranial magnetic stimulation (rTMS) reduces excitability of visual cortex in normal subjects. The aim of the study was to investigate the effects of low frequency (1 Hz) rTMS on visual cortical excitability by measuring changes in phosphene threshold (PT) in subjects with migraine with aura. Thirteen patients with migraine with aura and 15 healthy controls were examined. Using a standardized transcranial magnetic stimulation protocol of the occipital cortex, we assessed the PT (the lowest magnetic stimulation intensity at which subjects just perceived phosphenes) before and after a 1-Hz rTMS train delivered at PT intensity for 15 min. The difference in the proportion of subjects reporting phosphenes in migrainer and control groups was significant (migrainers: 100% vs controls 47%; P<0.05), and 1 Hz rTMS over the occipital cortex led to a significantly increased visual cortex excitability expressed as a decrease in PT in subjects affected by migraine with aura. Conversely, after a 1-Hz TMS train normal subjects showed increased PT values, which suggests a decreased visual cortex excitability. Our findings confirm that the visual cortex is hyperexcitable in migrainers and suggest a failure of inhibitory circuits, which are unable to be upregulated by low frequency rTMS.     

The regulation of ciliary beat frequency by ovarian steroids in the guinea pig Fallopian tube: interactions between oestradiol and progesterone

Ciliary beat frequency (CBF) was measured in slice preparations of the Fallopian tube fimbria, using videomicroscopy with a high-speed (500 Hz) camera in guinea pigs that were treated with β-oestradiol benzoate (βE2B) and medroxy progesterone (mPRG). In non-ovulating guinea pigs at 4 weeks of age, the CBF of the fimbria was high (17.8 Hz). In sexually mature guinea pigs (12-16 weeks of age) with constant ovulation, the CBF varied from 12 Hz to 16 Hz. The in vivo administration of both ICI-182,780 (a blocker of βE2 receptors) and mifepristone (a blocker of PRG receptors) induced high CBF (17.4 Hz). The administration of βE2B at a low (3.2 mg/kg/day) or high (32 mg/kg/day) dose decreased the CBF to 14.5 Hz or 11 Hz, respectively. ICI-182,780 abolished the βE2B-induced changes in CBF and decreased CBF to 12 Hz. The administration of mPRG (6.4 mg/kg/day) decreased CBF to 12.5 Hz. Mifepristone abolished this mPRG-induced decrease in CBF and maintained the CBF at 15 Hz. However, administering both βE2B and mPRG increased CBF to 17.5 Hz, suggesting that βE2B inhibits mPRG actions and vice versa. To confirm the interactions between βE2B and mPRG, we administered both βE2B and mPRG to guinea pigs that were pretreated for 1.5 days with either mPRG (6.4 mg/kg/day) or βE2B (3.2 mg/kg/day). Prior treatment with βE2B or mPRG prevented the increase in CBF that was otherwise by βE2B plus mPRG, and maintained the CBF at 14.5 Hz or 13 Hz, respectively. The administration of βE2B plus mPRG still induced the expression of PRG receptors, indicating that the highest CBF is not the result of no expression of the receptors. In the beating cilia of the fimbria, the signals that are activated by the βE2 and PRG receptors are proposed to antagonize each other in regulating the frequency.     

Corticofugal projection inhibits the auditory thalamus through the thalamic reticular nucleus

Electrical stimulation of the auditory cortex (AC) causes both facilitatory and inhibitory effects on the medial geniculate body (MGB). The purpose of this study was to identify the corticofugal inhibitory pathway to the MGB. We assessed two potential circuits: 1) the cortico-colliculo-thalamic circuit and 2) cortico-reticulo-thalamic one. We compared intracellular responses of MGB neurons to electrical stimulation of the AC following bilateral ablation of the inferior colliculi (IC) or thalamic reticular nucleus (TRN) in anesthetized guinea pigs. Cortical stimulation with intact TRN could cause strong inhibitory effects on the MGB neurons. The corticofugal inhibition remained effective after bilateral IC ablation, but it was minimized after the TRN was lesioned with kainic acid. Synchronized TRN neuronal activity and MGB inhibitory postsynaptic potentials (IPSPs) were observed with multiple recordings. The results suggest that corticofugal inhibition traverses the corticoreticulothalamic pathway, indicating that the colliculi-geniculate inhibitory pathway is probably only for feedforward inhibition.     

Ultra-slow oscillation (0.025 Hz) triggers hippocampal afterdischarges in Wistar rats

Oscillations in neuronal networks are assumed to serve various physiological functions, from coordination of motor patterns to perceptual binding of sensory information. Here, we describe an ultra-slow oscillation (0.025 Hz) in the hippocampus. Extracellular and intracellular activity was recorded from the CA1 and subicular regions in rats of the Wistar and Sprague-Dawley strains, anesthetized with urethane. In a subgroup of Wistar rats (23%), spontaneous afterdischarges (4.7+/-1.6 s) occurred regularly at 40.8+/-15.7 s. The afterdischarge was initiated by a fast increase of population synchrony (100-250 Hz oscillation; "tonic" phase), followed by large-amplitude rhythmic waves and associated action potentials at gamma and beta frequency (15-50 Hz; "clonic" phase). The afterdischarges were bilaterally synchronous and terminated relatively abruptly without post-ictal depression. Single-pulse stimulation of the commissural input could trigger afterdischarges, but only at times when they were about to occur. Commissural stimulation evoked inhibitory postsynaptic potentials in pyramidal cells. However, when the stimulus triggered an afterdischarge, the inhibitory postsynaptic potential was absent and the cells remained depolarized during most of the afterdischarge. Afterdischarges were not observed in the Sprague-Dawley rats. Long-term analysis of interneuronal activity in intact, drug-free rats also revealed periodic excitability changes in the hippocampal network at 0.025 Hz. These findings indicate the presence of an ultra-slow oscillation in the hippocampal formation. The ultra-slow clock induced afterdischarges in susceptible animals. We hypothesize that a transient failure of GABAergic inhibition in a subset of Wistar rats is responsible for the emergence of epileptiform patterns.