Original article 8
Electrocorticogram and Sensory Evoked Potentials in the Young Hydrocephalic H-Tx Rat R. M. Bucknall l , H. C.]ones l I Physiology
Group, Biomedical Sciences Division, King's College London, Campden Hill Road, London, England W8 7AH
Summary The ECoG and the sensory evoked potential in response to stimulation of the contralateral forelimb were studied in control and hydrocephalic H -Tx rats between 21 and 35 days of age by recording in several positions from the dura overlying the somatosensory cortex. All hydrocephalics showed Ionger periods of low amplitude, high frequency ECoG activity than the controls. For the evoked potentials the hydrocephalic rats were divided into two groups: those that gave an evoked potential in at least one electrode position and those that did not give any evoked potentials. In the hydrocephalic rats that gave an evoked response the latency of the fourth phase of the evoked potential (the N 2 component) was significantly longer than in the contro) rats suggesting that the processing of the signal within the cortex was affected by the hydrocephalus. The absence of an evoked potential in some hydrocephalic rats suggests there may be a disruption of the thalamo-cortical pathway preventing the response from reaching the cortex.
Key words Hydrocephalus - Evoked potential- Rat
The H-Tx rat, first described by Kohn et al (4), has severe inherited congenital hydrocephalus caused by obstruction of the aqueduct in late gestation (3). The hydrocephalus is detectable outwardly at birth by doming of the skull and tautness of the skin overlying the cranium. The condition progresses rapidly with symptoms such as ataxia appearing soon after weaning and death occurs at approximately 7 weeks of age. The lateral ventricles are dilated from the onset of the condition and continue to enlarge during the progression of the hydrocephalus with consequent distortion of the overlying cerebral cortex. The present study was undertaken to ascertain how these events affect the electrical activity within the cortex. Two types of activity were measured: the electrocorticogram (ECoG) which represents the continuous background activity within the cortex, and the sensory evoked potential which represents synchronous activity in groups of cells in response to a contralateral peripheral stimulus. The sensory pathway involved is the dorsal column mediallemniscal system (5) with synapses in the medulla and in the ventral posterior nucleus of the thalamus. Received July 14, 1990 Z Kinderchir 45, Suppl. I (1990) 8-10 (0 Hippokrates Verlag Stuttgart
Hydrocephalic (n = 20) H -Tx rats and normallittermates or rats from another non-hydrocephalic strain (CFY) between 21 and 35 days after birth were anaesthetised with 20 % urethane (0.65 mll 100 g i. p.) together with a supplementary dose (0.1 mlll00 g) after 15 min if required. The rats were placed in a stereotaxic frame and the surface ofthe skull exposed. The frame was adjusted so that lambda and bregma were at the same level. Stimulating electrodes, connected to a Harvard stimulator via a stimulus isolator, were placed close to the nerve trunk in the right forelimb. A burr hole was drilled in the left frontal bone dose to the midline for the placement of an Ag/AgCI indifferent electrode. From 2 to 6 further holes were drilled in the left frontal and parietal bones dose to the temporal ridge which corresponds to the main sensory receiving area for the forelimb (1). A platinum wire recording electrode was placed in contact with the dura through each of these holes and the position in relation to bregma noted for mapping the receiving area. The recording and indifferent electrodes were connected to a microelectrode preamplifier, through a preamplifier, to an oscilloscope and two-channel pen recorder. The signal was also sent through a signal averager to the second channel of the pen recorder. For each electrode position on the surface of the dura a recording of the ECoG was made followed by 4 or more responses to stimulation of the contralateral forelimb (0.3 Hz, 7.5 or 10 V). These were separated by at least 5 minutes to prevent fatigue of the pathway and each response was the average of 16 evoked potentials. For each electrode position the amplitude and waveform of the ECoG was analysed and the mean latencies of the components of the averaged evoked potentials were calculated. The latencies for each of the components were compared for control and hydrocephalic rats using analysis of variance.
Results 1. The Electrocorticogram (ECoG): An ECoG was recorded in all electrode positions in all animals. The predominant activity in the ECoG in both the control and hydrocephalic rats was a slow wave of 0.9-1.3 Hz with a maximum amplitude of 800 JlV. This slow wave was interspersed with short periods of low amplitude, high frequency (30-40 Hz) activity lasting 0.2-0.4 s in the control rats and 0.2-1.2 s in the hydrocephalic rats. 2. Sensory Evoked Potentials: All control rats gave evoked potentials in at least one of the electrode positions tested. The hydrocephalic rats consisted of two groups: one where an evoked potential was recorded in at least one of the electrode positions tested (group 1, n = 10) and a second which did not show an evoked response in any electrode position tested (group 2, n = 10). There was no significant difference between the mean age of the two hydrocephalic groups (group 1: 27.8 ± 1.64 days and group 2: 30.9 ± 1.08 days), nor between their weights (group 1: 52.3 ± 4.6 g and group 2: 57.5 ± 7.2 g). For each group the electrode positions tested were plotted in relation to bregma (Fig. 1A, Band C). In control rats the majority of positions tested gave a response within an area between 2 and 5 mm lateral to
Downloaded by: NYU. Copyrighted material.
Materials and methods
Z Kinderchir 45, Suppl. I (1990)
Electrocorticogram and Sensory EL'oked Potentials in the Young Hydrocephalic H- Tx Rat
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Electrocorticogram and Sensory Evoked Potentials in the Young Hydrocephalic H-Tx Rat R. M. Bucknall l , H. C.]ones l I Physiology
Group, Biomedical Sciences Division, King's College London, Campden Hill Road, London, England W8 7AH
Summary The ECoG and the sensory evoked potential in response to stimulation of the contralateral forelimb were studied in control and hydrocephalic H -Tx rats between 21 and 35 days of age by recording in several positions from the dura overlying the somatosensory cortex. All hydrocephalics showed Ionger periods of low amplitude, high frequency ECoG activity than the controls. For the evoked potentials the hydrocephalic rats were divided into two groups: those that gave an evoked potential in at least one electrode position and those that did not give any evoked potentials. In the hydrocephalic rats that gave an evoked response the latency of the fourth phase of the evoked potential (the N 2 component) was significantly longer than in the contro) rats suggesting that the processing of the signal within the cortex was affected by the hydrocephalus. The absence of an evoked potential in some hydrocephalic rats suggests there may be a disruption of the thalamo-cortical pathway preventing the response from reaching the cortex.
Key words Hydrocephalus - Evoked potential- Rat
The H-Tx rat, first described by Kohn et al (4), has severe inherited congenital hydrocephalus caused by obstruction of the aqueduct in late gestation (3). The hydrocephalus is detectable outwardly at birth by doming of the skull and tautness of the skin overlying the cranium. The condition progresses rapidly with symptoms such as ataxia appearing soon after weaning and death occurs at approximately 7 weeks of age. The lateral ventricles are dilated from the onset of the condition and continue to enlarge during the progression of the hydrocephalus with consequent distortion of the overlying cerebral cortex. The present study was undertaken to ascertain how these events affect the electrical activity within the cortex. Two types of activity were measured: the electrocorticogram (ECoG) which represents the continuous background activity within the cortex, and the sensory evoked potential which represents synchronous activity in groups of cells in response to a contralateral peripheral stimulus. The sensory pathway involved is the dorsal column mediallemniscal system (5) with synapses in the medulla and in the ventral posterior nucleus of the thalamus. Received July 14, 1990 Z Kinderchir 45, Suppl. I (1990) 8-10 (0 Hippokrates Verlag Stuttgart
Hydrocephalic (n = 20) H -Tx rats and normallittermates or rats from another non-hydrocephalic strain (CFY) between 21 and 35 days after birth were anaesthetised with 20 % urethane (0.65 mll 100 g i. p.) together with a supplementary dose (0.1 mlll00 g) after 15 min if required. The rats were placed in a stereotaxic frame and the surface ofthe skull exposed. The frame was adjusted so that lambda and bregma were at the same level. Stimulating electrodes, connected to a Harvard stimulator via a stimulus isolator, were placed close to the nerve trunk in the right forelimb. A burr hole was drilled in the left frontal bone dose to the midline for the placement of an Ag/AgCI indifferent electrode. From 2 to 6 further holes were drilled in the left frontal and parietal bones dose to the temporal ridge which corresponds to the main sensory receiving area for the forelimb (1). A platinum wire recording electrode was placed in contact with the dura through each of these holes and the position in relation to bregma noted for mapping the receiving area. The recording and indifferent electrodes were connected to a microelectrode preamplifier, through a preamplifier, to an oscilloscope and two-channel pen recorder. The signal was also sent through a signal averager to the second channel of the pen recorder. For each electrode position on the surface of the dura a recording of the ECoG was made followed by 4 or more responses to stimulation of the contralateral forelimb (0.3 Hz, 7.5 or 10 V). These were separated by at least 5 minutes to prevent fatigue of the pathway and each response was the average of 16 evoked potentials. For each electrode position the amplitude and waveform of the ECoG was analysed and the mean latencies of the components of the averaged evoked potentials were calculated. The latencies for each of the components were compared for control and hydrocephalic rats using analysis of variance.
Results 1. The Electrocorticogram (ECoG): An ECoG was recorded in all electrode positions in all animals. The predominant activity in the ECoG in both the control and hydrocephalic rats was a slow wave of 0.9-1.3 Hz with a maximum amplitude of 800 JlV. This slow wave was interspersed with short periods of low amplitude, high frequency (30-40 Hz) activity lasting 0.2-0.4 s in the control rats and 0.2-1.2 s in the hydrocephalic rats. 2. Sensory Evoked Potentials: All control rats gave evoked potentials in at least one of the electrode positions tested. The hydrocephalic rats consisted of two groups: one where an evoked potential was recorded in at least one of the electrode positions tested (group 1, n = 10) and a second which did not show an evoked response in any electrode position tested (group 2, n = 10). There was no significant difference between the mean age of the two hydrocephalic groups (group 1: 27.8 ± 1.64 days and group 2: 30.9 ± 1.08 days), nor between their weights (group 1: 52.3 ± 4.6 g and group 2: 57.5 ± 7.2 g). For each group the electrode positions tested were plotted in relation to bregma (Fig. 1A, Band C). In control rats the majority of positions tested gave a response within an area between 2 and 5 mm lateral to
Downloaded by: NYU. Copyrighted material.
Materials and methods
Z Kinderchir 45, Suppl. I (1990)
Electrocorticogram and Sensory EL'oked Potentials in the Young Hydrocephalic H- Tx Rat
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Fig. 1 Cortical maps of sites for surface recording electrodes. A: Control rats, B: Hydrocephalic rats with evoked potentials, C: Hydrocephalic rats without evoked potentials. Closed circles: Electrode positions where an evoked potential was recorded, Open circles: Electrode positions where an
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