Somatosensory evoked potentials in rat cerebral cortex before and after middle cerebral artery occlusion

The effect of cadmium (Cd) treatment on somatosensory evoked potentials (SEPs) and conduction velocity in alloxane-induced diabetic rats: relation to lipid peroxidation

Fifty-two healthy Swiss male albino rats, aged three months, were divided into four groups: Control (C), diabetic (D), cadmium (Cd), and diabetic + Cd (D+Cd). Diabetes was induced in D and D+Cd groups by administration of alloxane (5 mg/100 g). Cd and D+Cd groups were injected with CdCl2 i.p. (2 mg/kg/week) for 2 months. Somatosensory evoked potentials (SEPs) of the four groups were recorded following left posterior tibial nerve (PTN) stimulation. The mean latencies of P1, N1, P2 and N2 components were prolonged in all experimental groups compared with the control group. P1N1 and N1P2 amplitudes were significantly decreased in Cd and D+Cd groups in comparison with the control group. P2N2 amplitude was significantly decreased in the Cd group compared with the control group. In addition, conduction velocities and action potential amplitudes were determined from the sciatic nerves. The means of peripheral conduction velocities and action potential amplitudes were decreased significantly in all the experimental groups in comparison with the control group. Thiobarbituric acid reactive substances (TBARS), an indicator of lipid peroxidation, were increased in the kidneys and sciatic nerves of all experimental groups compared with the control group. A significant increase in the TBARS level of the brain was found in the Cd and D+Cd groups.

Analysis of optical signals evoked by peripheral nerve stimulation in rat somatosensory cortex: dynamic changes in hemoglobin concentration and oxygenation

The origins of reflected light changes associated with neuronal activity (optical signals) were investigated in rat somatosensory cortex with optical imaging, microspectrophotometry, and laser-Doppler flowmetry, and dynamic changes in local hemoglobin concentration and oxygenation were focused on. Functional activation was carried out by 2-second, 5-Hz electrical stimulation of the hind limb under chloralose anesthesia. These measurements were performed at the contralateral parietal cortex through a thinned skull. Regional cortical blood flow (rCBF) started to rise 1.5 seconds after the stimulus onset, peaked at 3.5 seconds (26.7% +/- 9.7% increase over baseline), and returned to near baseline by 10 seconds. Optical signal responses at 577, 586, and 805 nm showed a monophasic increase in absorbance coincident with the increase in rCBF; however, the signal responses at 605 and 760 nm were biphasic (an early increase and late decrease in absorbance) and microanatomically heterogeneous. The spectral changes of absorbance indicated that the concentrations of both total hemoglobin and oxyhemoglobin increased together with rCBF; deoxyhemoglobin, increased slightly but distinctly (P = 0.016 at 1.0 seconds, P = 0.00038 at 1.5 seconds) just before rCBF increases, then decreased. The authors conclude that activity-related optical signals are greatly associated with a moment-to-moment adjustment of rCBF and metabolism to neuronal activity.

Cholinergic modulation of cerebral cortical blood flow changes induced by trauma

These experiments tested the role of cholinergic mechanisms in the changes of cerebral cortical blood flow (CBF) induced by brain trauma. CBF was measured with Iodo-14C-antipyrine autoradiography, in 128 cerebral cortex regions of both hemispheres, distributed in eight coronal slices. The effects of a 6.3-mm diameter craniotomy over the left motor-sensory cortex with no weight drop, and of trauma (drop weight of 20 g from 30 cm height on left motor-sensory cortex through a 6.3 mm circular craniotomy) on CBF were studied at 2 and 24 h after the interventions. A group of control animals that received no intervention was also set up. Animals were treated with the cholinesterase inhibitor physostigmine salicylate (3.3 microg/kg/min i.v. infusion started 60 min before CBF measurements), the cholinergic blocker scopolamine hydrobromide (1 mg/kg i.v. pulse, 18 min before CBF measurements), or with the drugs vehicle (saline). A focus of decreased CBF at the site of impact was observed 2 h after trauma, extending caudally as far as the occipital cortex. CBF on the contralateral cerebral cortex was also decreased. Both phenomena reversed partially at 24 h. This spontaneous recovery of CBF was blocked by scopolamine. Physostigmine reversed the decrease in CBF of the traumatized cortex, partially around the contused area and completely in more distant regions. The cerebral cortex contralateral to the trauma showed significantly higher CBF 24 h after trauma when compared to intact controls or craniotomy that peaked at the area symmetrical to the center of trauma. This phenomenon was also enhanced by physostigmine and completely blocked by scopolamine. These results suggest a prominent role of cholinergic mechanisms in the vascular adjustments that accompany cerebral trauma.

Cholinesterase inhibition improves blood flow in the ischemic cerebral cortex

The ability of central cholinesterase inhibition to improve cerebral blood flow in the ischemic brain was tested in Sprague-Dawley rats with tandem occlusion of left middle cerebral and common carotid arteries. Cerebral blood flow was measured with lodo- 14C-antipyrine autoradiography in 170 regions of cerebral cortex. The regional distribution of blood flow was characterized in normal animals by cerebral blood flow maxima in the temporal regions. After 2 h ischemia, minimum cerebral blood flow values were found in the lateral frontal and parietal areas on the left hemisphere, and a new maximum was found in the right hemisphere in an area approximately symmetrical to the ischemic focus. Heptyl-physostigmine (eptastigmine), a carbamate cholinesterase inhibitor with prolonged time of action improved cerebral blood flow in most regions, with the exception of the ischemic core. The drug also enhanced the ischemia-induced rostral shift of cerebral blood flow maxima in the right hemisphere. The effects of eptastigmine were more marked 24 h after ischemia. Discriminant analysis showed that data from only 22 regions was sufficient to achieve 100% accuracy in classifying all cases into the various experimental conditions. The redistribution of cerebral blood flow to the sensorimotor area of the right hemisphere of animals with cerebral ischemia, a phenomenon possibly related to recovery of function, was also enhanced by eptastigmine.

Ligation of lateral carotid artery attenuates disturbance of brain function caused by subsequent cerebral ischemia in rabbits

The effects of right carotid artery ligation on the subsequent cerebral ischemia, induced by iron particle injection, in rabbits, were evaluated by recording somatosensory evoked potentials (SEPs) and cerebral blood flow (CBF) using laser Doppler flowmetry. Iron particle injection decreased CBF over 120 min and delayed SEP onset latency in rabbits with no previous carotid artery ligation (control group). In rabbits with a carotid ligation 3 days before, iron particle injection induced the decrease of CBF, as in the control group, but did not prolong the latency of SEP. Injection of iron particles induced only a transient decrease of CBF (less than 10 min) followed by an abrupt recovery, and no prolongation of SEP latency was observed in rabbits with a carotid ligation 6 days before. These results suggest that carotid artery ligation induces a beneficial effect on cerebral function during the subsequent ischemia, which is independent on the CBF changes.

A comparison of forepaw and vibrissae somatosensory cortical evoked potentials in the rat

Somatosensory evoked potentials were elicited in anesthetized rats by electrical stimulation of the forepaw (F-SEP) or the vibrissae (V-SEP) and were compared in order to study which of these is a more valid animal model for studying the physiology and pathophysiology of somatosensory evoked potentials (SEPs) that are often recorded in man in a clinical setting. Intensity and rate functions were measured for the two potentials. The V-SEPs had larger amplitudes than the F-SEPs at high stimulus intensity and low stimulus rate. Furthermore, the ratios of the maximal amplitude of the F-SEP to that of the V-SEP (0.66) and of the areas under the curves of the two responses (0.75) reflected the smaller representation of the forepaw in the primary somatosensory cortex of the rat, compared to the vibrissae (ratio of cortical areas about 0.79). The differences should be taken into account when using median nerve SEP in the rat as a model of the human SEP. Study of V-SEPs in rat may provide insight into trigeminal nerve SEPs in man, which are also occasionally used for neurological evaluation.

Inter- and intraindividual variability of posterior tibial nerve somatosensory evoked potentials in comatose patients

Thirty-two sequential posterior tibial nerve somatosensory evoked potentials (PTN-SEP) were recorded in 30 neurologically impaired, ventilated, comatose patients. To establish the time invariance of PTN-SEP in this population, Spearman rank correlations of latencies, interpeak latency, amplitudes, and mean absolute amplitude with time were computed. The results revealed no significant time dependency. The mean, standard deviation, and 5th and 95th percentiles for the inter- and intraindividual distribution of PTN-SEP parameters, pairwise PTN-SEP parameter differences, and direct cross-correlation of PTN-SEP were estimated using bootstrap procedures. The standard deviations of the interindividual distribution of PTN-SEP parameters are two to three times higher than the standard deviations of the intraindividual distributions. The coefficients of variation, that is, standard deviations divided by means, for the intraindividual distribution of latencies ranged from 0.012 to 0.042, of amplitudes from 0.146 to 0.230. The mean maximal cross-correlation coefficient of two randomly chosen PTN-SEP across patients equaled 0.65, and within patients 0.91. These data demonstrate the interindividual variability and intraindividual stability of PTN-SEP. The normal limits of intraindividual variability (1.96 x standard deviation) are 3.76, 1.33, 2.92, 6.00, and 3.04 ms for latencies N1, P1, N2, P2, and interpeak latency P1-N2, respectively. The intraindividual differences of amplitudes N1/P1, P1/N2, N2/P2, and the mean absolute amplitude should not exceed 0.67, 0.67, 0.90, and 0.27 microV or, expressed as quotients, 61, 52, 41, and 61%, respectively. The intraindividual maximal cross-correlation coefficient should not be lower than 0.74 with a lag of < 2.00 ms. The results of this study are of use for discrete and continuous PTN-SEP monitoring on intensive care units and during neuroradiological interventions and neurosurgery.

Rapid measurement of somatosensory evoked potential response to cerebral artery occlusion

The aim of the paper is to determine the speed of the neurological response to cerebral artery occlusion by monitoring transient changes in somatosensory evoked potentials (SEPs). SEPs, continuously monitored during temporary clipping of the middle cerebral artery (MCA) in anaesthetised cats, are analysed. The SEP signals are modelled by a quasi-periodic Fourier series, the coefficients of which are estimated with the aid of two adaptive least squares estimation algorithms. The energy levels at various harmonics throughout the protocol are obtained directly from the filter weights. Noise covariance is estimated from pre-stimulus recording, and the adaptation rate of the algorithm is adjusted sweep-by-sweep to accommodate transient changes in the pre-stimulus noise level. After the occlusion, a significant decrease (p < 0.05) in SEP amplitude is observed. The change in latency is not statistically significant (p approximately equal to 0.5). The spectral trends show a sudden decline in energy at all harmonics immediately following occlusion, although when the amplifier bandwidth is changed to 5-1500 Hz (from an initial setting of 30-1500 Hz), the fundamental frequency component of the SEP signal shows the greatest responsiveness to injury. The average time constant of the decline in amplitude resulting from MCA occlusion is only 10.6 +/- 4.0 s. It is concluded that rapid detection of cerebral artery occlusion and ischaemia may be feasible by continuously monitoring SEP signals and analysing transient changes in time and frequency domains.

Effect of thyroxine on the development of somatosensory and visual evoked potentials in the rat

The thyroid hormone thyroxine (T4), administered post-natally to neonatal rats, has been shown to accelerate development of auditory function, as expressed by auditory nerve-brainstem evoked responses. This study investigated whether this earlier development was also reflected in other sensory modalities. Rat pups were injected with T4 from the day of birth for 10 consecutive days. Somatosensory evoked potentials, both from the cortex and from sub-cortical structures, and flash-elicited visual evoked potentials (VEP), were recorded at various ages up to 3 months. The recordings were compared with those from control rats from the same litters. Only a minimal difference was found between the experimental and control groups, the most significant being in the VEP at age 12 days, by which time the eyes of most of the experimental rats had opened, which was not the case for the majority of control rats. This difference disappeared with eye-opening in the control rats. Although T4 is known to affect myelinization and synaptic transmission in developing rat brain, this apparently only minimally affects the functioning of the brain as expressed by evoked potentials, both in the short and long term. The main effect of neonatal hyperthyroidism in these rats appeared to be accelerated development of the end organ (the eye and the ear).

Somatosensory evoked potentials in cerebral ischemia of rabbits

Twenty-one rabbits were used in the ischemic group and six in the control group. Cerebral ischemia of variable degree was induced by Fe particle injection method. Somatosensory evoked potentials (SEPs) and cerebral blood flow (CBF) were compared when the CBF levels decreased to their minimum. The latency of the SEPs increased along with the decrease of the CBF when it was lower than 20 ml/100 g/min (68% of the pre-ischemic control level). This may be related to the ischemic change of the white matter. The amplitude showed diphasic changes. When the CBF decreased below 20 ml/100 g/min, the amplitude increased; when the CBF was lower than 11 ml/100 g/min (38% of the pre-ischemic level), it decreased. These results indicate that the functions of the cerebral cortex might be excited in mild ischemia, and be suppressed in severe ischemia.

Sensory evoked potentials for selective monitoring of the rat spinal cord: a cerebellar evoked potential to assess ventral cord integrity

The purpose of this study was to evaluate two types of ascending sensory evoked potentials (SEPs) in the rat and their capacity for selective monitoring of dorsal versus ventral spinal cord integrity. SEPs were elicited by direct sciatic nerve stimulation. A cerebellar evoked response was recorded over the paramedian lobule of the cerebellar hemisphere (CEPpml) while somatosensory evoked potentials (SSEPs) were simultaneously recorded over the sensorimotor cortex. All components of the CEPpml and SSEP except the longest latency positive waves were present in each animal. At stimulus intensities of 3 to 10 mA, no significant changes in latency of the peaks were observed, but amplitudes of the longer latency responses tended to increase throughout this stimulation range. Unilateral sciatic stimulation resulted in bilateral cortical responses, larger ipsilaterally for the CEPpml, and contralaterally for the SSEP. Selective spinal cord lesions demonstrated N9 and P14 of the CEPpml to be mediated primarily through the ventral spinal cord, while P14 and N19 of the SSEP were conducted primarily through the dorsal columns. Sectioning of the cerebellar peduncles abolished N9 and P14 of the CEPpml despite persistence of the SSEP. This study demonstrates that selective assessment of the ventral and dorsal spinal cord is possible in the rat by monitoring SEPs.

Development of epileptic activity induced by iron injection into rat cerebral cortex: electrographic and behavioral characteristics

Unilateral injection of ferrous chloride solution into the rat sensorimotor cortex produced epileptic discharges in the electrocorticograms (ECoGs). The discharges were isolated spikes and spike and wave complexes, and the epileptic activity lasted for more than 12 months after the injection. Isolated spike activity often appeared on the left or right side of the cortex, whereas spike and wave complex activity appeared bilaterally. In rats showing dominant isolated spike activity in the secondary epileptic cortex, there was a deviation in somatosensory evoked potentials (SEPs). Rats showing isolated spikes and spike and wave complexes exhibited vibrissa tremors and head nodding. Rats showing only isolated spikes exhibited no abnormal behavior, but their convulsion thresholds to pentylenetetrazol were lowered. The results including ECoGs, SEPs, behavior and convulsion threshold were characterized with reference to the development of iron-induced epilepsy. The profiles of ECoG discharge activity and SEP configuration suggest that the process of iron-induced epilepsy consists of 3 stages.

Functional changes in thalamic relay neurons after focal cerebral infarct: a study of unit recordings from VPL neurons after MCA occlusion in rats

We evaluated neuronal and histological changes of thalamic neurons 1, 4, 7, and 14 days after middle cerebral artery (MCA) occlusion in rats. After the somatosensory evoked potentials (SEPs) were measured from the cerebral cortex, the thalamic relay neuronal activities were recorded with a glass microelectrode following repetitive electrical stimulation of the contralateral forepaw at frequencies ranging from 1 to 50 Hz. In approximately 95% of the occluded rats, the ipsilateral somatosensory cortex and/or the subcortical somatosensory pathway developed infarct, resulting in SEP loss. We evaluated unit data from rats with abolished SEPs. The average firing rate of the nucleus ventralis posterolateralis (VPL) neurons in response to 25 stimulations at 30 Hz was significantly reduced to 0.1 spike/stimulus 1 day after MCA occlusion. In sham-operated rats, the same stimulation produced 0.7 spike/stimulus. The firing rate recovered to 0.4 spike/stimulus at 30-Hz stimulation 4 and 7 days after occlusion. This was followed by resuppression (0.1 spike/stimulus) 14 days after occlusion. Histological study revealed some abnormal neurons in the ipsilateral thalamus 7 days after occlusion. We were unable to find normal-shaped neurons in the VPL 14 days after occlusion. The present study demonstrates that cortical infarct produces functional and morphologic changes that gradually and progressively affect the ipsilateral thalamus, although incomplete transient recovery of somatosensory transmission may occur.

Pure motor hemiparesis with stable somatosensory evoked potential monitoring during aneurysm surgery: case report

We report a patient who sought treatment for an acute subarachnoid hemorrhage as a result of an intracranial aneurysm. Management included early surgical repair and intraoperative monitoring of evoked potentials. Pan-angiography revealed berry aneurysms of the communicating anterior artery and right middle cerebral artery. Surgery was uneventful, and the somatosensory evoked potential monitoring did not show any abnormalities. Nevertheless, the patient showed a neurological deficit due to a clip-related infarct in the right middle cerebral artery territory characterized by a right hemiparesis with no sensory deficit. This case report supports the possibility of false-negative results in single-mode intraoperative monitoring during aneurysm surgery.

Transhemispheric diaschisis. A review and comment

We review here the literature in both animal models and humans concerning electrical activity, blood flow, and metabolism in the hemisphere contralateral to unilateral cerebral ischemia. We analyze the data by periods based on the time from initial injury to emphasize the time course of transhemispheric diaschisis. Contralateral electrical activity, such as evoked potential amplitude, is increased in the late stages after unilateral infarction, with the data from the more acute periods being inconclusive. Contralateral blood flow changes probably depend on the magnitude of the ischemic injury, with a larger insult resulting in a decrease not seen with smaller insults. Some studies have shown a decrease in contralateral blood flow over the first week followed by a gradual return toward baseline. Most measures of contralateral metabolism show a time course similar to blood flow, that is, a decrease followed by gradual recovery. The effects of corpus callosum section on transhemispheric diaschisis are not yet established. We provide examples to show that under certain conditions, diaschisis may represent a loss of remote inhibition rather than a loss of remote facilitation, as von Monakow originally suggested. By following the contralateral changes over time, particularly during the first minutes and hours of ischemia, insight will be gained into the brain's responses remote from the focus of ischemic injury. These responses should bear a relation to the brain's defense mechanisms ipsilaterally to the region of ischemia.

PET measured evoked cerebral blood flow responses in an awake monkey

We have developed a method to measure task-related regional cerebral blood flow (BF) responses in an awake, trained monkey using positron emission tomography (PET) and H215O. We trained an animal with operant conditioning using only positive reinforcement to climb unassisted into a modified primate chair that was then positioned in the PET scanner. A special headholder and acrylic skull cap permitted precise placement and accurate repositioning. We measured BF qualitatively with bolus injection of H215O and 40-s scan. Each session included scans at rest interposed with scans during vibration of a forepaw. Regional responses were identified using subtraction image analysis. After global normalization, a resting image was subtracted on a pixel-by-pixel basis from a comparable image collected during vibration. The region of peak response occurred in contralateral sensorimotor cortex with a mean magnitude of 11.6% (+/- 3.2%) of the global mean value for 10 separate experiments, significantly greater than the mean qualitative BF change (0.4 +/- 3.6%; p less than 0.00001) in the same region for seven rest-rest pairs. This newly developed technique forms the basis for a wide variety of experiments.