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Hypothermia-induced changes in rat short latency somatosensory evoked potentials
Electroencephalography and Clinical Neurophysiology , 1981, 51:19--31 © Elsevier/North-Holland Scientific Publishers, Ltd.
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HYPOTHERMIA-INDUCED CHANGES IN RAT SHORT LATENCY SOMATOSENSORY EVOKED POTENTIALS 1 B. BUDNICK, K.L. McKEOWN and W.C. WIEDERHOLT
Veteran's Administration Hospital, and Department of Neurosciences, University of California at San Diego, La Jolla, Calif. 92093 (U.S.A.) (Accepted for publication: August 14, 1980)
Short latency somatosensory evoked potentials (SEPs) preceding the cortical evoked potential have been recorded in animals (IraguiMadoz and Wiederholt 1977; Wiederholt and Iragui-Madoz 1977; Wiederholt 1978; Arezzo et al. 1979) and in man (Desmedt 1971; Cracco and Cracco 1976; Goff et al. 1977; Kritchevsky and Wiederholt, 1978). Clinical usefulness of evoked potentials has been reported for the somatosensory system (Giblin 1964; Larson and Sances 1968), and other sensory modalities (Starr and Achor 1975; Robinson and Rudge 1977; Stockard and Rossiter 1977). The short latency SEP of the rat consists of 3 well defined waves preceding the cortical potential occurring within 5 msec of forepaw stimulation and within 8 msec of hindpaw stimulation. On the basis of latency, surface distribution, depth-recording and lesioning, the anatomical generators of these potentials are thought to be in subcortical structures, brain stem, cord and peripheral nerve (Cracco and Cracco 1976; Iragui-Madoz and Wiederholt 1977; Wiederholt and Iragui-Madoz 1977). Jones et al. (1976) reported that localized cooling of auditory pathway fibers in the inferior colliculus changes the latency of some, but not all, waves of the brain stem auditory evoked response (BAER). Schorn et al.
I Supported by the Medical Research Service of the Veteran's Administration.
{1977) used the BAER to evaluate rats with hyperacute experimental autoimmune encephalomyelitis (HEAE). They reported that systemic cooling in control rats prolonged latencies and change wave forms of all BAER potentials. The changes were even more pronounced than those due to HEAE. Since local or systemic cooling alters BAERs, it can be assumed that hypothermia will also affect SEPs. To obtain data useful as reference in human SEPs under hypothermia, we studied the effect of systemic cooling upon surface and depth recorded SEPs and on frequency following of surface SEPs in rats. Methods Forty-six mature male albino rats, each weighing about 300 g, were anesthetized with intraperitoneal injections of sodium pentobarbital (40 mg/kg). Light anesthesia was maintained by additional injections at intervals and amounts established in control rats. Animals were tracheotomized, immobilized with curare, and artificially ventilated. The rat was placed in a stereotaxic frame, the skull was exposed, and small areas of bone and dura removed for depth and intracranial temperature recording. In most experiments, the spinal cord was exposed from medulla to C5 level. End-respiratory CO2 was monitored, and CO2 levels were kept in the normal range (2.5--4.5%) by adjusting respiratory rate and volume. Electroencephalogram, electrocardio-
20 gram, and rectal, peripheral (subcutaneous), spinal, and intracranial temperatures were monitored. The intracranial (i.c.) temperature probe, a 22-gauge hypodermic thermistor, was placed just above the superior colliculus. Spinal cord surface temperature was monitored by a similar probe. Control temperature was maintained at 37°C (i.c.) with a circulatingwater heating pad augmented by a feedback controlled heatlamp. The animals were systemically cooled by circulating ice water through the pad and placing crushed ice, sealed in a plastic bag, on the animal. Temperature was lowered to 32, 27 and 24°C (i.c.) and maintained at those levels. After cooling the animals, they were carefully warmed to establish the reversibility of effects of cooling on the SEP. Surface recording electrodes (00 by 1/8 in. gold-plated screws) were placed in rows on both sides of the skull, 3.5 mm lateral to the midline (Wiederholt and Iragui-Madoz 1977 ). A screw was placed as far anterior as possible, midline in the nasal bone for referential recording. Electrode impedances measured 15--20 k~t, Peripheral nerve activity was recorded by two closely spaced platinum needle electrodes (length 1 cm) placed subcutaneously in the rat's forelimb, the distal electrode about 4 cm from the site of stimulation. In 13 rats, bipolar depth electrodes were used for recording directly from structures in the somatosensory pathway. Co-axial, bipolar stainless steel electrodes (recording contacts 100 pm in diameter, 0.25 mm length, with 0.5 mm separation) were stereotaxically placed in distal and proximal dorsal column, cuneate nucleus, internal arcuate tract, medial lemniscus, inferior cerebellar peduncle, ventral posterior thalamus, cortex of the cerebellar vermis, and cerebral cortex in the primary forepaw somatosensory receiving area. Position was substantiated by comparing recordings to those obtained from animals in which appropriate structures were lesioned and histologically verified. Recording was done simultaneously from 3 or 4 of these structures plus several surface sites. In 3 of these rats, a
B. BUDNICK ET AL. bipolar depth electrode was placed in the dorsal column fibers at the caudal border of the cuneate nucleus, and another electrode placed 3 cm caudal to the first. Placement was verified by steep rise-time and by the potential's ability to follow 150/sec stimulation without significant amplitude decrement. A third electrode was placed at an angle into the internal arcuate fibers just ventral and rostra] to the cuneate nucleus. Position was verified by risetime of potential, progressive decrement of potential amplitude to high stimulation rates. and an interval of 0.9 msec between onsets of activity in the rostral dorsal column and internal arcuate electrodes. The separation between these electrodes was maximally 3 mm. Potentials from all electrodes could be recorded with ipsilateral, but not contralateral stimulation. Rats were stimulated unilaterally and bilaterally with needles inserted under the skin of the forepaw, 5 mm apart, with the cathode proximal. Square pulses (duration 0.1 msec at 2.5 mA) were delivered through stimulus isolation and constant current units. Stimulus rate for surface recording was 4/sec at i.c. temperatures of 37°C, 32°C, 27°C and 24°C. For the depth-recording experiments, stimuli were given at 1, 4 and 10/sec at 37°C and 27°C. For frequency-following experiments, stimuli were given at 1, 2, 4, 10, 12, 14, 16, 20, 40 and 60/sec at 37°C and 27°C. Activity was led through a high-impedance electrode board to a polygraph. System bandpass was limited at the polygraph with half amplitude at 10 Hz and 3 kHz. Amplified activity was simultaneously led to a 16-channel FM tape recorder, a storage oscilloscope, and a 12-bit laboratory computer triggered by the stimulator. Potentials were averaged both on-line and off-line. Two thousand responses were averaged for each stimulation run with a sweep length of 25 msec and a sampling rate of 127 psec. Averaged SEPs were displayed on the computer oscilloscope. Hard copies of potentials were obtained via a digital xerographic prin-
EFFECT OF HYPOTHERMIA ON SEPs
21
ter-plotter. For all potentials in this study, we measured onset and peak latency, and amplitude (onset to peak). Length of forelimb from stimulus site to recording locations at cord and peripheral nerve was measured and conduction velocities were calculated. Rise-time of potentials was defined as the interval between onset and peak latencies. In the frequency-following experiments, the amplitude of SEP potentials recorded at 1/sec stimulation was defined as baseline. Amplitude of potentials at faster rates of stimulation were divided by the baseline amplitude and multiplied by 100. Mean, standard deviation and standard error of the mean, and linear regression coefficients, were calculated.
Results With cooling, there was little difference (maximal 1.3°C) between temperatures recorded at the surface of the brain, the superior colliculus, and the spinal cord. The forelimb temperature was not more than 2.5°C lower than the intracranial reading. Rapid cooling techniques caused differences as great as 9°C. To avoid this large temperature gradient, we used a slow cooling method. Even with slow cooling, the rectal temperature was often 6--12°C lower than the intracranial temperature. From 37°C to 27°C, 6 min of cooling were required to lower the i.c. temperature I°C. From 27°C to 24°C, 20 min
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Fig. 1. The reversible effects of cooling on evoked potentials and EEG activity. Bilateral forepaw stimulation at 4/sec, average of 2000 sweeps..SEP recording electrode at primary receiving area with nose reference; spinal cord bipolar electrode in left dorsal column at C3 level; peripheral nerve potential recorded from left median nerve. Temperature in °C.
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were required per degree of cooling. The mortality rate of animals cooled below 24°C (i.c.) was unacceptably high. Ongoing EEG activity was reduced by cooling (Fig. 1). At 37°C, the EEG showed highvoltage synchronized waves with frequent bursts of spikes and/or spindles. As the rat was cooled, the voltage steadily decreased until at 24°C the EEG showed only rare synchronized spikes at all surface recording sites. In spinal cord and peripheral nerve, conduction velocity decreased with cooling from 77 m/sec at 37°C (i.c.) to 43 m/sec at 24°C (Table I). Small differences in conduction velocity between peripheral nerve and spinal cord are probably due to difficulty in measuring path length in the intact animal. Cooling progressively increased the onset and peak latencies of peripheral, depth, and surface recorded potentials (Table I). For the surface SEP, the effect of cooling on latency was linear; regression coefficients were P = 0.89 (component I), 0.95 (component IlL 0.88 (component III), and 0.94 (cortical component) (Fig. 2). The absolute increase in latency and rise-time was smallest for component I and greatest for the cortical component as shown by the increasing slopes of the regression lines. This indicates that hypothermia affects earlier potentials less than later ones. Onset latencies and latency differences of depth activity recorded in internal arcuate fibers and dorsal columns are shown in Table II. The interval between onset latencies in the two dorsal column electrodes was 0.47 msec at 37°C and increased by 0.12 msec at 24°C. The interval between onset latencies of the proximal dorsal column electrode and the postsynaptic electrode in internal arcuate fibers was 0.98 msec at 37°C and increased by 1.04 msec at 24°C. Onset latencies, peak latencies, and risetimes for depth and surface potentials are shown in Table III and Fig. 3. At 37°C, cervical dorsal column activity and cuneate nucleus activity began slightly later than onset of component I. Rise-times of dorsal column
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Fig. 2. Relationship of S E P onset latency to intracranial temperature. 'r2' is Pearson's correlation coefficient for
each component's line. Regression lines are described by slope equation given beneath each line. and cuneate nucleus potentials substantially overlapped that of c o m p o n e n t I. Following cooling to 27°C, onset latencies of dorsal column, cuneate nucleus, and surface c o m p o n e n t I each increased by nearly 1.0 msec and their activities continued to overlap. At 37°C, onset latencies o f the cuneate nucleus and inferior cerebellar peduncle (ICP)
were 0.7 and 0.2 msec earlier, and medial lemniscus onset 0.2 msec later, than onset of component II. The rise-times of potentials in cuneate, medial lemniscus, and ICP overlapped t h a t of component II. After cooling to 27°C, the onset latencies of activity in cuneate, medial lemniscus, ICP, and c o m p o n e n t II increased by 0.8. 1.6, 1.4, and 2.3 msec
24
B. BUDNICK ET AL.
TABLE II Onset latencies (msec) and latency intervals (msec) of depth recorded SEPs at different temperatures and different rates of stimulation. Latency
1/sec
37°C 27°C
4/sec
37°C 27°C
10/sec
37°C 27°C
Latency interval
Distal cord
Proximal cord
Internal arcuate
Distalproximal cord
Proximal cord-internal arcuate
1.27 * (0.1) ** 1.93 (0.1)
1.73 (0.1) 2.52 (0)
2.7 (0.1) 4.4 (0.1)
0.46
0.97
0.59
1.88
1.24 (0.1) 1.82 (0.2)
1.7 (0) 2.37 (0.2)
2.67 (0.1) 4.43 (0.1)
0.46
0.97
0.55
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1.21 (0.1) 1.79 (0.2)
1.7 (0) 2.43 (0.1)
2.7 (0.2) 4.55 (0.2)
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* All values are the mean of 3 animals. ** Numbers in parentheses indicate standard deviation.
respectively. The rise-times of recorded potentials continued to rise-time of component II. Before cooling, onset latencies posterior thalamus and cerebellar
tentials were both 3,2 msec, which was 0.8 msec earlier than the onset of c o m p o n e n t III recorded anteriorly. There was extensive overlap of rise-time of potentials recorded in ventral-posterior thalamus and cerebellar surface
the depthoverlap the for ventralsurface po-
TABLE III Onset and peak latencies and rise-times (msec) of surface and depth recorded SEPs at different temperatures. Onset latency
Component I Cervical dorsal column Cuneate nucleus Inferior cerebellar peduncle Component II Medial lemniscus Ventral posterior thalamus C o m p o n e n t III, anterior CerebeUar cortex Sensory cortex Cortical c o m p o n e n t (surface)
Peak latency
37°C
27°C
1.1 * (0.1) ** 1.3 (0.1) 1.6 (0.1) 2.1 (0.4) 2.3 (0.1) 2.5 (0.3) 3.2 (0.2) 4.0 ( 0 . 3 ) 3.2 (0.4) 4.6 (0.6) 6.0 ( 0 . 3 )
2.1 2.2 2.4 3.5 4.6 4.1 5.3 7.0 6.0 11.6 10.4
S t i m u l a t i o n rate 4/sec. * All values are the mean o f 13 animals. ** Numbers in p a r e n t h e s e s indicate standard deviation.
(0.2) (0.2) (0.2) (0.8) (0.3) (0.3) (0.3) (0.7) (0.5) (2.5) (0.5)
Rise-time
37°C
27°C
1.8 (0.2) 3.2 (0.3) 3:4 (0.2) 5.0 (1.0) 3.3 (0.2) 3.9 (0.5) 4.9 (0.7) 5.1 (0.5) 6.4 (1.3) 13.2 (1.5) 6.4 (0.5)
2.9 4.1 5A 7.8 5.7 6.9 8.5 8.5 13.6 21.4 12.2
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37°C
27°C
0.7 1.9 1.8 2.9 1.0 1.4 1.7 1.1 3.2 8.6 0.4
0.8 1.9 3.0 4.3 1.1 2.8 2.7 1:5 7.6 9.8 1.8
EFFECT OF HYPOTHERMIA ON SEPs
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Fig. 3. Changes in depth recorded potentials with cooling. Bilateral forepaw stimulation at 4/sec, average of 500 sweeps. Recording sites: left posterior column at C3 level; left cuneate nucleus; right medial lemniscus; left restiform body; right ventral posterior nucleus of thalamus; left cerebellar vermis; right primary sensory cortical receiving area. Temperature in °C.
with that of component III. Following cooling, onset latencies of activity in ventral posterior thalamus, cerebellum, and component III increased by 2.0, 2.8, and 3.0 msec, respectively. Rise-times of potentials from the
depth structures continued to overlap the risetime of component III. At 37°C, the onset of directly recorded cortical activity was 1.4 msec earlier than onset of the cortical component of the SEP. The
TABLE IV Amplitudes (pV) of surface recorded SEPs at different temperatures.
37°C 32°C 27°C 24°C 37°C
Peripheral nerve
Component I
Component II
Component III
Cortical component
1.7 * 1.8 2.1 2.2 1.7
1.0 1.1 1.4 1.6 0.8
3.3 2.6 2.4 2.4 3.0
12.6 12.2 9.9 5.5 12.8
12.8 6.9 7.1 7.3 11.2
(0.5) ** (0.5) (0.6) (0.6) (0.6)
(0.8) (0.6) (0.7) (0.8) (0.7)
* All values are the mean of 28 animals. ** Numbers in parentheses indicate standard deviation.
(6.0) (7.4) (8.6) (5.0) (6.8)
(7.8) (3.3) (5.0) (1.7) (6.2)
26
rise-time of directly recorded cortical activity entirely overlapped that of the surface cortical component. After cooling, the onset latency of the directly recorded cortex activity increased by 7.0 msec compared to a 4.4 msec increase in onset latency of the cortical component of the SEP, The rise-time of the directly recorded cortical activity was overlapped by a substantial portion of the longlasting cortical component. The effect of hypothermia on SEP amplitudes was complex. At 37°C, the amplitude distribution of potentials was the same as reported by Wiederholt and Iragui-Madoz in 1977. Components I and II increased in amplitude from anterior to posterior, while components II and primary cortical were maximal over contralateral central areas. Component I amplitude progressively increased with cooling. At 24°C, the increase was 60% recorded anteriorly. All other components decreased in amplitude with cooling. At 24°C, component II decreased by 19%, component II by 56%, and the cortical component by 43% (Table IV). We observed that during prolonged direct recording from dorsal column and cuneate nucleus, the amplitude changed even in control conditions. This was probably due to small, gradual changes in position of the bipolar electrodes caused by movements with artificial ventilation. Therefore, we did not assess the effect of hypothermia on amplitude of potentials recorded directly from depth structures. Peripheral nerve potential amplitude was variable from animal to animal, but in each experiment the amplitude increased an average of 28% at 24°C. In this respect, the peripheral nerve potential amplitude behaved similarly to component I. In the frequency,following experiments, SEP amplitudes attenuated more, and at slower rates of stimulation, in the cooled state than at 37°C (Table V). At 27°C, component III and cortical component attenuated markedly at even 2/sec stimulation, while at 37°C their attenuation was negligible at any rate
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EFFECT OF HYPOTHERMIAON SEPs less than 10/sec. Component II was also attenuated more at 27°C than at 37°C, with the increased attenuation becoming apparent at rates of 16/sec or greater. Component I, in contrast, showed little attenuation at any stimulation rate whether at 27°C or 37°C. The amplitude of component I was actually larger at 27°C than 37°C for stimulus rates of 2, 4, 10 and 12/sec. In addition, at 27°C the amplitude of component I at these rates of stimulation was larger than its amplitude at 1/sec stimulation.
Discussion
Animals were cooled slowly to achieve uniform systemic temperature. Large, rapid temperature changes elicit neural activity (Bernhard and Granit 1946; Brooks et al. 1955; Barker and Carpenter 1970; Pierau et al. 1971) as do temperature gradients between cooled and non-cooled areas (Von Euler 1947; Brooks et al. 1955; Granit 1955; Suda et al. 1957). At a locally cooled segment of nerve, the compound action potential consistently increases in amplitude (Brooks et al. 1955; Bindman et al. 1963). Therefore, in our study we maintained a minimal temperature difference (1.3°C) between brain surface, subcortical areas, and spinal cord; there was a difference of only 2.5°C between forelimb and intracranial temperatures. A 6--12°C gradient between rectal and intracranial temperatures was unavoidable. The onset latency of each potential increased proportionately to the degree of cooling. Conduction velocity in peripheral nerve decreased by 2.7 -+ 0.1 m/sec/°C, which agrees with the 2.3 m/sec/°C obtained by Kraft (1972). Therefore, prolonged latencies of potentials from presynaptic structures in the somatosensory path are due to slowed conduction velocity, potentials from structures with at least one interposed synapse show a more pronounced delay of onset latency with cooling. This suggests that cooling affects synaptic transmission more than
27 activity in fiber tracts. The increased rise-time of potentials is attributed to increased risetimes of intracellularly recorded action potentials (Suda et al. 1957; Klee et al. 1974), and to increased separation of unit potentials with different conduction velocities that form the compound action potential, the separation being proportional to the amount of cooling (Brooks et al. 1955). To gain information on the relative contribution of fiber and synapse to latency increase, we tested the differential effect of hypothermia on conduction and transmission. In frequency-following experiments, we found that at both 37°C and 27°C the amplitude of SEP components attenuated with increasing rates of stimulation. As shown in Table V, the attenuation was greatest for the cortical component and least for component I. For components III and cortical, stimulation at 2/sec caused negligible attenuation at 37°C, but at 27°C caused significant attenuation. Likewise, component II showed more attenuation at 27°C than 37°C, but the attenuation became markedly greater only when stimulation rates of 16/sec or higher were used. In contrast, component I, considered to arise from presynaptic structures, did not show a decrease in frequency following between 27°C and 37°C. These observations support the view that synaptic transmission is more affected by hypothermia than conduction. The more interposed synapses, the more apparent is the attenuation of potential amplitudes, and the more striking is the difference between cooled and warm animals. The facilitory effect of cooling (Koizumi et al. 1954) can be seen in component I. The amplitude of potentials increased with stimulation rates of 2, 4, 10 and 12/sec at 27°C. This increase, relative to stimulation at 1/sec, was n o t seen at 37°C. For more precise definition of effects of cold on synapse and fiber, we recorded directly from fibers pre- and postsynaptic to the cuneate nucleus. As shown in Table II, cooling by 10°C increased the conduction time between proximal and distal dorsal col-
28 umn electrodes by 26%. The time required to cross the cuneate synapse between proximal dorsal column and internal arcuate electrodes increased by 106%. This 4-fold difference demonstrates hypothermia has a greater effect on synaptic transmission. Benita and Cond~ (1972) showed that synaptic transmission in cat red nucleus was blocked completely at 18 ° C. Our data show that as systemic temperature is lowered, there is a progressive increase in synaptic delay before transmission blockage occurs. A temperature drop of 10°C more than doubles the time required for synaptic transmission. Therefore, prolonged onset latencies of potentials arising from postsynaptic structures are primarily due to increased synaptic delay. This is consistent with observations by Dondey et al. (1962), Benita and Cond~ {1972), and Katz and Miledi, who in 1965 established that synaptic delay in frogs can increase by over 1000% when cooled by 17.5°C. Our observations and those mentioned above disagree with the conclusion of Schorn et al. (1977) that the latency increase of evoked potentials must be due to slowed conduction velocity. Because synaptic delay increases greatly with cooling, the observed latency increase for SEP components II, III and cortical (each considered to be generated by postsynaptic structures) must b e primarily due to synaptic delay. The prolonged rise-times of these components are probably due to several factors including: temporal dispersion and increased duration of unit action potentials, and slower rise-times of EPSPs and lower EPSP amplitudes (Brooks et al. 1955; Suda et al. 1957; Lester 1970; Weight and Erulkar 1976). The considerations elaborated above can also explain the increased latencies and risetimes of potentials directly recorded from depth structures (Table III). There are onset latency similarities between component I, the dorsal column, and the cuneate nucleus, also between component II, the medial lemniscus and the inferior cerebellar peduncle. These latency similarities are preserved at 27°C.
B. BUDNICK ET AL. Although the shorter latency of component I compared to dorsal column indicates that activity distal to the C3 recording site may contribute to the onset of component I (Wiederholt and Iragui-Madoz 1977), it is reasonable to propose that combined activities of peripheral structures, dorsal columns and, to a lesser degree, cuneate nucleus are mainly responsible for component I because their activities overlap one another substantially. For the same considerations, it is probable that activity in the medial lemniscus and inferior cerebellar peduncle generate component II, with late activity in the cuneate nucleus possibly contributing to the onset of component II. While their rise-times greatly overlap, the onset latency of component III is clearly later than the onset of activity in the ventral posterior thalamus and the cerebellar vermis, and clearly precedes activity recorded from cortex. These relationships do not change with cooling. Thus, thalamocortical fibers are suggested as the major source of component III with a smaller contribution from ventral posterior nucleus and cerebellum. Nuclear structures are less likely to contribute to surface SEPs not only because of latency differences, but because their geometric orientation is less likely to produce a strong electromagnetic field compared with that of a uniformly aligned fiber tract (Nufiez 1980). The amplitude of component I was the only part of the SEP to increase with cooling (Table IV}. Other studies on far-field potentials have sh own both increased and decreased amplitudes under hypothermia (Jones et al. 1976; Schorn et al. 1977; Stockard et al. 1978). Earlier studies reporting amplitude augmentation by cooling (Granit and Skoglund 1945; Bernhard and Granit 1946; Bindman et al. 1963; Pasztor and Kukorelli 1967) mentioned that rapid and transitory temperature changes or s~eep local temperature gradients were required. These conditions were n o t present in this study. Suda et al. (1957) reported in their work on cat lumbar motor neurons that both mono- and polysynaptic reflexes in the spinal cord were greatly
EFFECT OF HYPOTHERMIA ON SEPs increased by cooling that did not involve rapid temperature changes or local gradients. Barton and Mathews (1938) reported that cooling by 5°C causes an antidromic dorsal r o o t volley in response to afferent activity in the dorsal roots. Previous work done on normothermic rats in our laboratory indicated that an early latency potential was still recorded from the skull after dorsal rhizoto m y from C4 to T2 which shows that activity can be recorded from as far as brachial plexus with our technique. Therefore, the increase in c o m p o n e n t I amplitude can be attributed to several factors, including farfield recording of the augmented spinal reflex and of the antidromic dorsal r o o t volley, plus the increased amplitude of the peripheral c o m p o u n d action potential. The remaining components of the SEP all decreased in amplitude with cooling. Because hypothermia depresses metabolism, blocks synaptic transmission (Dondey et al. 1962; Benita and Condd 1972) and raises threshold (Brooks et al. 1955), a decrease in amplitude with cooling is reasonably accounted for.
Summary Under general anesthesia, rats were gradually cooled from 37°C to 24°C. Slowly cooling avoided large temperature gradients between central and peripheral nervous systems. Short latency somatosensory potentials were evoked by forepaw stimulation and recorded from skull and depth structures. Cooling progressively increased onset and peak latencies and duration of all potentials. Amplitude of surface and depth recorded potentials decreased with decreasing temperatures except amplitude of surface c o m p o n e n t I increased. The response of surface and depth components to different rates of stimulation and cooling clearly indicates that cooling slows synaptic transmission much more than fiber conduction. The response of surface and depth recorded potentials to hypothermia suggests that evoked activity in cervical dorsal
29 column and cuneate nucleus contributes to surface c o m p o n e n t I, that activity in cuneate nucleus, medial lemniscus, and inferior cerebellar peduncle contributes to surface component II, and that activity in thalamocortical fibers and probably cerebellum contributes t o surface c o m p o n e n t III. These conclusions agree with our previous thoughts a b o u t the origin of short latency, surface recorded somatosensory evoked potentials.
Rdsum6 Modifications des potentiels dvoqugs somatosensoriels de courte latence induites par hypothermie chez le rat Sous anesth6sie g6ndrale, des rats ont 6td refroidis progressivement de 37°C ~ 24°C. Le refroidissement lent dvite de grands ~carts de tempdrature entre les syst6mes nerveux central et p6riph6rique. Les potentiels somato-sensoriels de courte latence ont dt6 dvoqu6s par stimulation du membre ant6rieur et enregistrds au niveau d u scalp et des structures profondes. Le refroidissement augmente progressivement les latences de ddbut et de pic et la durde de tous les potentiels. L'amplitude des potentiels enregistrds en surface et en profondeur diminue avec la diminution des temp6ratures ~ l'exception de l'amplitude de la composante I de surface qui augmente. La rdponse des composantes en surface et en profondeur diffdrentes vitesses de stimulation lors du refroidissement indique clairement que le refroidissement ralentit la transmission synaptique beaucoup plus que la conduction par fibres. La rdponse des potentiels enregistr~s en surface et en profondeur ~ l'hypothermie sugg6re que l'activitd dvoqu6e dans la colonne cervico-dorsale et le n o y a u cund'/forme contribue ~ la composante de surface I, que l'activitd du n o y a u cund'/forme, du lemniscus m6dian et du pddoncule cdrdbelleux infdrieur contribue ~ la composante de surface II et que l'activitd des fibres thalamo-corticales et probablement celle du cervelet contribuent ~ la
30 c o m p o s a n t e d e s u r f a c e III. Ces c o n c l u s i o n s s o n t e n a c c o r d avec n o s h y p o t h e s e s a n t d r i e u r e s s u r l ' o r i g i n e des p o t e n t i e l s ~ v o q u d s somato-sensoriels de c o u r t e latence enregistrds en surface. The authors gratefully acknowledge the expert technical assistance of Mr. Jeff Borchardt.
References Arezzo, J., Leggatt, A.D. and Vaughan, Jr., H.G. Topography and intracranial sources of somatosensory evoked potentials in the monkey. I. Early components. Electroenceph. clin. Neurophysiol., 1979, 46: 155--172. Barker, J.L. and Carpenter, D.O. Thermosensitivity of neurons in the sensorimotor cortex of the cat. Science, 1970, 169: 597--598. Barron, D.H. and Matthews, B.H.C. Dorsal root reflexes. J. Physiol. (Lond.), 1938, 94: 26P--27P. Benita, M. and CondO, H. Effects of local cooling upon conduction and synaptic transmission. Brain Res., 1972, 36: 133--151. Bernhard, C.G. and Granit, R. Nerve as a model temperature end organ. J. gen. Physiol., 1946, 29: 257--265. Bindman, L.J., Lippold, O.C.J. and Readfern, J.W.T. Comparison of the effects on electrocortieal activity of general body cooling and local cooling of the surface of the brain. Electroenceph. clin. Neurophysiol., 1963, 15: 238--245. Brooks, C. McC., Koizumi, K. and Malcolm, J.L. Effects of changes in temperature on reactions of spinal cord: J. Neurophysiol., 1955, 18: 205--216. Cracco, R.Q. and Cracco, J.B. Somatosensory evoked potential in man: far-field potentials. Electroenceph. clin. Neurophysiol., 1976, 41: 460--466. Desmedt, J.E. Somatosensory cerebral evoked potentials in man. In: A. Rt!mond (Ed.), Handbook of Electroencephalography and Clinical Neurophysiology, Vol. 8. Elsevier, Amsterdam, 1971: 55--82. Dondey, M., Albe-Fessard, D. et LeBeau, J. Premieres applications neurophysiologigues d'une m~thode permettant le blocage ~lectif et r~versible de structures centrates par r~frig~ration localis~e. Electroenceph, clin. Neurophysiol., 1962, 14: 758--763. Giblin, D.R. Somatosensory evoked potentials in healthy subjects and in patients with lesions of the nervous system. Ann. N.Y. Acad.Sci., 1964, 112: 93--142. Goff, G.D., Mitsumiya, Y., Allison, T. and Goff, W.R. The scalp topography of human somatosensory and auditory evoked potentials. Electroenceph. clin. Neurophysiol., 1977, 42 : 57--76.
B. BUDN[CK ET AL. Granit, R. Receptors and Sensory Perception. Yale Univ. Press, New Haven, Conn., 1955. Granit, R. and Skoglund, L.R. The effect of temperature on the artificial synapse formed by cut end of mammalian nerve. J. Neurophysiol., 1945, 8: 211--217. Iragui-Madoz, V.J. and Wiederholt, W.C. Far-field somatosensory evoked potentials in the cat: correlation with depth recording. Ann. Neurol., 1977, 1 : 569--574 Jones. T., Stockard, J.J., Rossiter. V. and Bickford, R. Application of cryogenic techniques in the evaluation of afferent pathway and coma mechanism. Proc. San Diego biomed. Syrup., 1976, 15: 249 255. Katz, B. and Miledi, R. The effect of temperature on the synaptic delay at the neuromuscular junction. J Physiol. (Lond.), 1965, 181 : 656--670. Klee, M.R., Pierau, F.K. and Faber, D.S. Temperature effects on resting potential and spike parameters of cat motoneuron. Exp. Brain Res., 1974, 19: 478--492. Koizumi, K., Malcolm, J.L. and Brooks, C. McC. Effect of temperature on facilitation and inhibition of reflex activity. Amer. J. Physiol., 1954, 179: 507--512. Kraft, G.H. Effects of temperature and age on nerve conduction velocity in the guinea pig. Arch. phys. Med. Rehab., 1972, 53: 328--332. Kritchevsky, M. and Wiederholt, W.C. Short latency somatosensory evoked potentials. Arch. Neurol. (Chic.), 1978, 35: 706--711. Larson, S.J. and Sances, A. Averaged evoked potentials in stereotaxic surgery J. Neurosurg., 1968, 28: 227--232. Lester, H.A. Transmitter release by presynaptic impulses in squid stellate ganglion. Nature (Lond.), 1970, 227: 493--496. Nufiez, P. Electric Fields in the Brain. Oxford Univ. Press, New York, 1980. Pasztor, E. and Kukorelli. T. Effect on cortical evoked potentials of local cooling of the cerebral surface. Acta physiol. Acad. Sci. hung., 1967, 31 : 41--50. Pierau, F.K., Klee, M.R.. Faber, D.S. and Klussman, F.W. Mechanism of cellular thermoreception in mammals. Int. J. Biometeor., 1971, 15: 134--140. Robinson, K. and Rudge, P. Abnormalities in the auditory evoked potentials in patients with multiple sclerosis. Brain, 1 9 7 7 , 1 0 0 : 19--40. Schorn, V., Lennon, V. and Bickford, R. Temperature effects on the brainstem auditory evoked responses (BAERs) of the rat. Proc. San Diego bioreed. Syrup., 1977, 16: 313--318. Starr, A. and Achor, L.J. Auditory brainstem responses in neurological disease. Arch. Neurol. (Chic.), 1975, 32: 761--768.
EFFECT OF HYPOTHERMIA
O N SEPs
Stockard, J.J. and Rossiter, V.S. Clinical and pathological correlates of brainstem auditory response abnormalities. Neurology (Minneap.), 1977, 27: 316--325. Stockard, J.J., Sharbrough, F.W. and Tinker, J.A. Effects of hypothermia on the human brainstem auditory response. Ann. Neurol., 1978, 3: 368--370. Suda, I., Koizumi, K. and Brooks, C. McC. Analysis of effects of hypothermia on central nervous system responses. Amer. J. Physiol., 1957, 189: 373--380.
31
Von Euler, C. Selective responses to thermal stimulation o f mammalian nerves. Acta physiol, scand., 1947, 14 (Suppl. 45): 1--75. Weight, F.F. and Erulkar, S.D. Synaptic transmission and effects of temperature at the squid giant synapse. Nature (Lond.), 1976, 261: 720--722. Wiederholt, W.C. Recovery function of short latency components of surface and depth recorded somatosensory evoked potentials in the cat. Electroe~ceph. clin. Neurophysiol., 1978, 45: 259--267. Wiederholt, W.C. and Iragui-Madoz, V.J. Far-field somatosensory potentials in the rat. Electroenceph. clin. Neurophysiol., 1977, 42: 456--465.
19
HYPOTHERMIA-INDUCED CHANGES IN RAT SHORT LATENCY SOMATOSENSORY EVOKED POTENTIALS 1 B. BUDNICK, K.L. McKEOWN and W.C. WIEDERHOLT
Veteran's Administration Hospital, and Department of Neurosciences, University of California at San Diego, La Jolla, Calif. 92093 (U.S.A.) (Accepted for publication: August 14, 1980)
Short latency somatosensory evoked potentials (SEPs) preceding the cortical evoked potential have been recorded in animals (IraguiMadoz and Wiederholt 1977; Wiederholt and Iragui-Madoz 1977; Wiederholt 1978; Arezzo et al. 1979) and in man (Desmedt 1971; Cracco and Cracco 1976; Goff et al. 1977; Kritchevsky and Wiederholt, 1978). Clinical usefulness of evoked potentials has been reported for the somatosensory system (Giblin 1964; Larson and Sances 1968), and other sensory modalities (Starr and Achor 1975; Robinson and Rudge 1977; Stockard and Rossiter 1977). The short latency SEP of the rat consists of 3 well defined waves preceding the cortical potential occurring within 5 msec of forepaw stimulation and within 8 msec of hindpaw stimulation. On the basis of latency, surface distribution, depth-recording and lesioning, the anatomical generators of these potentials are thought to be in subcortical structures, brain stem, cord and peripheral nerve (Cracco and Cracco 1976; Iragui-Madoz and Wiederholt 1977; Wiederholt and Iragui-Madoz 1977). Jones et al. (1976) reported that localized cooling of auditory pathway fibers in the inferior colliculus changes the latency of some, but not all, waves of the brain stem auditory evoked response (BAER). Schorn et al.
I Supported by the Medical Research Service of the Veteran's Administration.
{1977) used the BAER to evaluate rats with hyperacute experimental autoimmune encephalomyelitis (HEAE). They reported that systemic cooling in control rats prolonged latencies and change wave forms of all BAER potentials. The changes were even more pronounced than those due to HEAE. Since local or systemic cooling alters BAERs, it can be assumed that hypothermia will also affect SEPs. To obtain data useful as reference in human SEPs under hypothermia, we studied the effect of systemic cooling upon surface and depth recorded SEPs and on frequency following of surface SEPs in rats. Methods Forty-six mature male albino rats, each weighing about 300 g, were anesthetized with intraperitoneal injections of sodium pentobarbital (40 mg/kg). Light anesthesia was maintained by additional injections at intervals and amounts established in control rats. Animals were tracheotomized, immobilized with curare, and artificially ventilated. The rat was placed in a stereotaxic frame, the skull was exposed, and small areas of bone and dura removed for depth and intracranial temperature recording. In most experiments, the spinal cord was exposed from medulla to C5 level. End-respiratory CO2 was monitored, and CO2 levels were kept in the normal range (2.5--4.5%) by adjusting respiratory rate and volume. Electroencephalogram, electrocardio-
20 gram, and rectal, peripheral (subcutaneous), spinal, and intracranial temperatures were monitored. The intracranial (i.c.) temperature probe, a 22-gauge hypodermic thermistor, was placed just above the superior colliculus. Spinal cord surface temperature was monitored by a similar probe. Control temperature was maintained at 37°C (i.c.) with a circulatingwater heating pad augmented by a feedback controlled heatlamp. The animals were systemically cooled by circulating ice water through the pad and placing crushed ice, sealed in a plastic bag, on the animal. Temperature was lowered to 32, 27 and 24°C (i.c.) and maintained at those levels. After cooling the animals, they were carefully warmed to establish the reversibility of effects of cooling on the SEP. Surface recording electrodes (00 by 1/8 in. gold-plated screws) were placed in rows on both sides of the skull, 3.5 mm lateral to the midline (Wiederholt and Iragui-Madoz 1977 ). A screw was placed as far anterior as possible, midline in the nasal bone for referential recording. Electrode impedances measured 15--20 k~t, Peripheral nerve activity was recorded by two closely spaced platinum needle electrodes (length 1 cm) placed subcutaneously in the rat's forelimb, the distal electrode about 4 cm from the site of stimulation. In 13 rats, bipolar depth electrodes were used for recording directly from structures in the somatosensory pathway. Co-axial, bipolar stainless steel electrodes (recording contacts 100 pm in diameter, 0.25 mm length, with 0.5 mm separation) were stereotaxically placed in distal and proximal dorsal column, cuneate nucleus, internal arcuate tract, medial lemniscus, inferior cerebellar peduncle, ventral posterior thalamus, cortex of the cerebellar vermis, and cerebral cortex in the primary forepaw somatosensory receiving area. Position was substantiated by comparing recordings to those obtained from animals in which appropriate structures were lesioned and histologically verified. Recording was done simultaneously from 3 or 4 of these structures plus several surface sites. In 3 of these rats, a
B. BUDNICK ET AL. bipolar depth electrode was placed in the dorsal column fibers at the caudal border of the cuneate nucleus, and another electrode placed 3 cm caudal to the first. Placement was verified by steep rise-time and by the potential's ability to follow 150/sec stimulation without significant amplitude decrement. A third electrode was placed at an angle into the internal arcuate fibers just ventral and rostra] to the cuneate nucleus. Position was verified by risetime of potential, progressive decrement of potential amplitude to high stimulation rates. and an interval of 0.9 msec between onsets of activity in the rostral dorsal column and internal arcuate electrodes. The separation between these electrodes was maximally 3 mm. Potentials from all electrodes could be recorded with ipsilateral, but not contralateral stimulation. Rats were stimulated unilaterally and bilaterally with needles inserted under the skin of the forepaw, 5 mm apart, with the cathode proximal. Square pulses (duration 0.1 msec at 2.5 mA) were delivered through stimulus isolation and constant current units. Stimulus rate for surface recording was 4/sec at i.c. temperatures of 37°C, 32°C, 27°C and 24°C. For the depth-recording experiments, stimuli were given at 1, 4 and 10/sec at 37°C and 27°C. For frequency-following experiments, stimuli were given at 1, 2, 4, 10, 12, 14, 16, 20, 40 and 60/sec at 37°C and 27°C. Activity was led through a high-impedance electrode board to a polygraph. System bandpass was limited at the polygraph with half amplitude at 10 Hz and 3 kHz. Amplified activity was simultaneously led to a 16-channel FM tape recorder, a storage oscilloscope, and a 12-bit laboratory computer triggered by the stimulator. Potentials were averaged both on-line and off-line. Two thousand responses were averaged for each stimulation run with a sweep length of 25 msec and a sampling rate of 127 psec. Averaged SEPs were displayed on the computer oscilloscope. Hard copies of potentials were obtained via a digital xerographic prin-
EFFECT OF HYPOTHERMIA ON SEPs
21
ter-plotter. For all potentials in this study, we measured onset and peak latency, and amplitude (onset to peak). Length of forelimb from stimulus site to recording locations at cord and peripheral nerve was measured and conduction velocities were calculated. Rise-time of potentials was defined as the interval between onset and peak latencies. In the frequency-following experiments, the amplitude of SEP potentials recorded at 1/sec stimulation was defined as baseline. Amplitude of potentials at faster rates of stimulation were divided by the baseline amplitude and multiplied by 100. Mean, standard deviation and standard error of the mean, and linear regression coefficients, were calculated.
Results With cooling, there was little difference (maximal 1.3°C) between temperatures recorded at the surface of the brain, the superior colliculus, and the spinal cord. The forelimb temperature was not more than 2.5°C lower than the intracranial reading. Rapid cooling techniques caused differences as great as 9°C. To avoid this large temperature gradient, we used a slow cooling method. Even with slow cooling, the rectal temperature was often 6--12°C lower than the intracranial temperature. From 37°C to 27°C, 6 min of cooling were required to lower the i.c. temperature I°C. From 27°C to 24°C, 20 min
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22
B. BUDNICK ET AL.
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were required per degree of cooling. The mortality rate of animals cooled below 24°C (i.c.) was unacceptably high. Ongoing EEG activity was reduced by cooling (Fig. 1). At 37°C, the EEG showed highvoltage synchronized waves with frequent bursts of spikes and/or spindles. As the rat was cooled, the voltage steadily decreased until at 24°C the EEG showed only rare synchronized spikes at all surface recording sites. In spinal cord and peripheral nerve, conduction velocity decreased with cooling from 77 m/sec at 37°C (i.c.) to 43 m/sec at 24°C (Table I). Small differences in conduction velocity between peripheral nerve and spinal cord are probably due to difficulty in measuring path length in the intact animal. Cooling progressively increased the onset and peak latencies of peripheral, depth, and surface recorded potentials (Table I). For the surface SEP, the effect of cooling on latency was linear; regression coefficients were P = 0.89 (component I), 0.95 (component IlL 0.88 (component III), and 0.94 (cortical component) (Fig. 2). The absolute increase in latency and rise-time was smallest for component I and greatest for the cortical component as shown by the increasing slopes of the regression lines. This indicates that hypothermia affects earlier potentials less than later ones. Onset latencies and latency differences of depth activity recorded in internal arcuate fibers and dorsal columns are shown in Table II. The interval between onset latencies in the two dorsal column electrodes was 0.47 msec at 37°C and increased by 0.12 msec at 24°C. The interval between onset latencies of the proximal dorsal column electrode and the postsynaptic electrode in internal arcuate fibers was 0.98 msec at 37°C and increased by 1.04 msec at 24°C. Onset latencies, peak latencies, and risetimes for depth and surface potentials are shown in Table III and Fig. 3. At 37°C, cervical dorsal column activity and cuneate nucleus activity began slightly later than onset of component I. Rise-times of dorsal column
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Fig. 2. Relationship of S E P onset latency to intracranial temperature. 'r2' is Pearson's correlation coefficient for
each component's line. Regression lines are described by slope equation given beneath each line. and cuneate nucleus potentials substantially overlapped that of c o m p o n e n t I. Following cooling to 27°C, onset latencies of dorsal column, cuneate nucleus, and surface c o m p o n e n t I each increased by nearly 1.0 msec and their activities continued to overlap. At 37°C, onset latencies o f the cuneate nucleus and inferior cerebellar peduncle (ICP)
were 0.7 and 0.2 msec earlier, and medial lemniscus onset 0.2 msec later, than onset of component II. The rise-times of potentials in cuneate, medial lemniscus, and ICP overlapped t h a t of component II. After cooling to 27°C, the onset latencies of activity in cuneate, medial lemniscus, ICP, and c o m p o n e n t II increased by 0.8. 1.6, 1.4, and 2.3 msec
24
B. BUDNICK ET AL.
TABLE II Onset latencies (msec) and latency intervals (msec) of depth recorded SEPs at different temperatures and different rates of stimulation. Latency
1/sec
37°C 27°C
4/sec
37°C 27°C
10/sec
37°C 27°C
Latency interval
Distal cord
Proximal cord
Internal arcuate
Distalproximal cord
Proximal cord-internal arcuate
1.27 * (0.1) ** 1.93 (0.1)
1.73 (0.1) 2.52 (0)
2.7 (0.1) 4.4 (0.1)
0.46
0.97
0.59
1.88
1.24 (0.1) 1.82 (0.2)
1.7 (0) 2.37 (0.2)
2.67 (0.1) 4.43 (0.1)
0.46
0.97
0.55
2.06
1.21 (0.1) 1.79 (0.2)
1.7 (0) 2.43 (0.1)
2.7 (0.2) 4.55 (0.2)
0149
1.0
0.64
2.12
* All values are the mean of 3 animals. ** Numbers in parentheses indicate standard deviation.
respectively. The rise-times of recorded potentials continued to rise-time of component II. Before cooling, onset latencies posterior thalamus and cerebellar
tentials were both 3,2 msec, which was 0.8 msec earlier than the onset of c o m p o n e n t III recorded anteriorly. There was extensive overlap of rise-time of potentials recorded in ventral-posterior thalamus and cerebellar surface
the depthoverlap the for ventralsurface po-
TABLE III Onset and peak latencies and rise-times (msec) of surface and depth recorded SEPs at different temperatures. Onset latency
Component I Cervical dorsal column Cuneate nucleus Inferior cerebellar peduncle Component II Medial lemniscus Ventral posterior thalamus C o m p o n e n t III, anterior CerebeUar cortex Sensory cortex Cortical c o m p o n e n t (surface)
Peak latency
37°C
27°C
1.1 * (0.1) ** 1.3 (0.1) 1.6 (0.1) 2.1 (0.4) 2.3 (0.1) 2.5 (0.3) 3.2 (0.2) 4.0 ( 0 . 3 ) 3.2 (0.4) 4.6 (0.6) 6.0 ( 0 . 3 )
2.1 2.2 2.4 3.5 4.6 4.1 5.3 7.0 6.0 11.6 10.4
S t i m u l a t i o n rate 4/sec. * All values are the mean o f 13 animals. ** Numbers in p a r e n t h e s e s indicate standard deviation.
(0.2) (0.2) (0.2) (0.8) (0.3) (0.3) (0.3) (0.7) (0.5) (2.5) (0.5)
Rise-time
37°C
27°C
1.8 (0.2) 3.2 (0.3) 3:4 (0.2) 5.0 (1.0) 3.3 (0.2) 3.9 (0.5) 4.9 (0.7) 5.1 (0.5) 6.4 (1.3) 13.2 (1.5) 6.4 (0.5)
2.9 4.1 5A 7.8 5.7 6.9 8.5 8.5 13.6 21.4 12.2
(0.3) (0.9) (0.6) (1.9) (0.4) (1.2) (1.5) (0.5) (2.5) (3.7) (1.2)
37°C
27°C
0.7 1.9 1.8 2.9 1.0 1.4 1.7 1.1 3.2 8.6 0.4
0.8 1.9 3.0 4.3 1.1 2.8 2.7 1:5 7.6 9.8 1.8
EFFECT OF HYPOTHERMIA ON SEPs
25
DEPTH-ItlCORNDPOTINTIALS
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with that of component III. Following cooling, onset latencies of activity in ventral posterior thalamus, cerebellum, and component III increased by 2.0, 2.8, and 3.0 msec, respectively. Rise-times of potentials from the
depth structures continued to overlap the risetime of component III. At 37°C, the onset of directly recorded cortical activity was 1.4 msec earlier than onset of the cortical component of the SEP. The
TABLE IV Amplitudes (pV) of surface recorded SEPs at different temperatures.
37°C 32°C 27°C 24°C 37°C
Peripheral nerve
Component I
Component II
Component III
Cortical component
1.7 * 1.8 2.1 2.2 1.7
1.0 1.1 1.4 1.6 0.8
3.3 2.6 2.4 2.4 3.0
12.6 12.2 9.9 5.5 12.8
12.8 6.9 7.1 7.3 11.2
(0.5) ** (0.5) (0.6) (0.6) (0.6)
(0.8) (0.6) (0.7) (0.8) (0.7)
* All values are the mean of 28 animals. ** Numbers in parentheses indicate standard deviation.
(6.0) (7.4) (8.6) (5.0) (6.8)
(7.8) (3.3) (5.0) (1.7) (6.2)
26
rise-time of directly recorded cortical activity entirely overlapped that of the surface cortical component. After cooling, the onset latency of the directly recorded cortex activity increased by 7.0 msec compared to a 4.4 msec increase in onset latency of the cortical component of the SEP, The rise-time of the directly recorded cortical activity was overlapped by a substantial portion of the longlasting cortical component. The effect of hypothermia on SEP amplitudes was complex. At 37°C, the amplitude distribution of potentials was the same as reported by Wiederholt and Iragui-Madoz in 1977. Components I and II increased in amplitude from anterior to posterior, while components II and primary cortical were maximal over contralateral central areas. Component I amplitude progressively increased with cooling. At 24°C, the increase was 60% recorded anteriorly. All other components decreased in amplitude with cooling. At 24°C, component II decreased by 19%, component II by 56%, and the cortical component by 43% (Table IV). We observed that during prolonged direct recording from dorsal column and cuneate nucleus, the amplitude changed even in control conditions. This was probably due to small, gradual changes in position of the bipolar electrodes caused by movements with artificial ventilation. Therefore, we did not assess the effect of hypothermia on amplitude of potentials recorded directly from depth structures. Peripheral nerve potential amplitude was variable from animal to animal, but in each experiment the amplitude increased an average of 28% at 24°C. In this respect, the peripheral nerve potential amplitude behaved similarly to component I. In the frequency,following experiments, SEP amplitudes attenuated more, and at slower rates of stimulation, in the cooled state than at 37°C (Table V). At 27°C, component III and cortical component attenuated markedly at even 2/sec stimulation, while at 37°C their attenuation was negligible at any rate
B. BUDNICK ET AL.
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EFFECT OF HYPOTHERMIAON SEPs less than 10/sec. Component II was also attenuated more at 27°C than at 37°C, with the increased attenuation becoming apparent at rates of 16/sec or greater. Component I, in contrast, showed little attenuation at any stimulation rate whether at 27°C or 37°C. The amplitude of component I was actually larger at 27°C than 37°C for stimulus rates of 2, 4, 10 and 12/sec. In addition, at 27°C the amplitude of component I at these rates of stimulation was larger than its amplitude at 1/sec stimulation.
Discussion
Animals were cooled slowly to achieve uniform systemic temperature. Large, rapid temperature changes elicit neural activity (Bernhard and Granit 1946; Brooks et al. 1955; Barker and Carpenter 1970; Pierau et al. 1971) as do temperature gradients between cooled and non-cooled areas (Von Euler 1947; Brooks et al. 1955; Granit 1955; Suda et al. 1957). At a locally cooled segment of nerve, the compound action potential consistently increases in amplitude (Brooks et al. 1955; Bindman et al. 1963). Therefore, in our study we maintained a minimal temperature difference (1.3°C) between brain surface, subcortical areas, and spinal cord; there was a difference of only 2.5°C between forelimb and intracranial temperatures. A 6--12°C gradient between rectal and intracranial temperatures was unavoidable. The onset latency of each potential increased proportionately to the degree of cooling. Conduction velocity in peripheral nerve decreased by 2.7 -+ 0.1 m/sec/°C, which agrees with the 2.3 m/sec/°C obtained by Kraft (1972). Therefore, prolonged latencies of potentials from presynaptic structures in the somatosensory path are due to slowed conduction velocity, potentials from structures with at least one interposed synapse show a more pronounced delay of onset latency with cooling. This suggests that cooling affects synaptic transmission more than
27 activity in fiber tracts. The increased rise-time of potentials is attributed to increased risetimes of intracellularly recorded action potentials (Suda et al. 1957; Klee et al. 1974), and to increased separation of unit potentials with different conduction velocities that form the compound action potential, the separation being proportional to the amount of cooling (Brooks et al. 1955). To gain information on the relative contribution of fiber and synapse to latency increase, we tested the differential effect of hypothermia on conduction and transmission. In frequency-following experiments, we found that at both 37°C and 27°C the amplitude of SEP components attenuated with increasing rates of stimulation. As shown in Table V, the attenuation was greatest for the cortical component and least for component I. For components III and cortical, stimulation at 2/sec caused negligible attenuation at 37°C, but at 27°C caused significant attenuation. Likewise, component II showed more attenuation at 27°C than 37°C, but the attenuation became markedly greater only when stimulation rates of 16/sec or higher were used. In contrast, component I, considered to arise from presynaptic structures, did not show a decrease in frequency following between 27°C and 37°C. These observations support the view that synaptic transmission is more affected by hypothermia than conduction. The more interposed synapses, the more apparent is the attenuation of potential amplitudes, and the more striking is the difference between cooled and warm animals. The facilitory effect of cooling (Koizumi et al. 1954) can be seen in component I. The amplitude of potentials increased with stimulation rates of 2, 4, 10 and 12/sec at 27°C. This increase, relative to stimulation at 1/sec, was n o t seen at 37°C. For more precise definition of effects of cold on synapse and fiber, we recorded directly from fibers pre- and postsynaptic to the cuneate nucleus. As shown in Table II, cooling by 10°C increased the conduction time between proximal and distal dorsal col-
28 umn electrodes by 26%. The time required to cross the cuneate synapse between proximal dorsal column and internal arcuate electrodes increased by 106%. This 4-fold difference demonstrates hypothermia has a greater effect on synaptic transmission. Benita and Cond~ (1972) showed that synaptic transmission in cat red nucleus was blocked completely at 18 ° C. Our data show that as systemic temperature is lowered, there is a progressive increase in synaptic delay before transmission blockage occurs. A temperature drop of 10°C more than doubles the time required for synaptic transmission. Therefore, prolonged onset latencies of potentials arising from postsynaptic structures are primarily due to increased synaptic delay. This is consistent with observations by Dondey et al. (1962), Benita and Cond~ {1972), and Katz and Miledi, who in 1965 established that synaptic delay in frogs can increase by over 1000% when cooled by 17.5°C. Our observations and those mentioned above disagree with the conclusion of Schorn et al. (1977) that the latency increase of evoked potentials must be due to slowed conduction velocity. Because synaptic delay increases greatly with cooling, the observed latency increase for SEP components II, III and cortical (each considered to be generated by postsynaptic structures) must b e primarily due to synaptic delay. The prolonged rise-times of these components are probably due to several factors including: temporal dispersion and increased duration of unit action potentials, and slower rise-times of EPSPs and lower EPSP amplitudes (Brooks et al. 1955; Suda et al. 1957; Lester 1970; Weight and Erulkar 1976). The considerations elaborated above can also explain the increased latencies and risetimes of potentials directly recorded from depth structures (Table III). There are onset latency similarities between component I, the dorsal column, and the cuneate nucleus, also between component II, the medial lemniscus and the inferior cerebellar peduncle. These latency similarities are preserved at 27°C.
B. BUDNICK ET AL. Although the shorter latency of component I compared to dorsal column indicates that activity distal to the C3 recording site may contribute to the onset of component I (Wiederholt and Iragui-Madoz 1977), it is reasonable to propose that combined activities of peripheral structures, dorsal columns and, to a lesser degree, cuneate nucleus are mainly responsible for component I because their activities overlap one another substantially. For the same considerations, it is probable that activity in the medial lemniscus and inferior cerebellar peduncle generate component II, with late activity in the cuneate nucleus possibly contributing to the onset of component II. While their rise-times greatly overlap, the onset latency of component III is clearly later than the onset of activity in the ventral posterior thalamus and the cerebellar vermis, and clearly precedes activity recorded from cortex. These relationships do not change with cooling. Thus, thalamocortical fibers are suggested as the major source of component III with a smaller contribution from ventral posterior nucleus and cerebellum. Nuclear structures are less likely to contribute to surface SEPs not only because of latency differences, but because their geometric orientation is less likely to produce a strong electromagnetic field compared with that of a uniformly aligned fiber tract (Nufiez 1980). The amplitude of component I was the only part of the SEP to increase with cooling (Table IV}. Other studies on far-field potentials have sh own both increased and decreased amplitudes under hypothermia (Jones et al. 1976; Schorn et al. 1977; Stockard et al. 1978). Earlier studies reporting amplitude augmentation by cooling (Granit and Skoglund 1945; Bernhard and Granit 1946; Bindman et al. 1963; Pasztor and Kukorelli 1967) mentioned that rapid and transitory temperature changes or s~eep local temperature gradients were required. These conditions were n o t present in this study. Suda et al. (1957) reported in their work on cat lumbar motor neurons that both mono- and polysynaptic reflexes in the spinal cord were greatly
EFFECT OF HYPOTHERMIA ON SEPs increased by cooling that did not involve rapid temperature changes or local gradients. Barton and Mathews (1938) reported that cooling by 5°C causes an antidromic dorsal r o o t volley in response to afferent activity in the dorsal roots. Previous work done on normothermic rats in our laboratory indicated that an early latency potential was still recorded from the skull after dorsal rhizoto m y from C4 to T2 which shows that activity can be recorded from as far as brachial plexus with our technique. Therefore, the increase in c o m p o n e n t I amplitude can be attributed to several factors, including farfield recording of the augmented spinal reflex and of the antidromic dorsal r o o t volley, plus the increased amplitude of the peripheral c o m p o u n d action potential. The remaining components of the SEP all decreased in amplitude with cooling. Because hypothermia depresses metabolism, blocks synaptic transmission (Dondey et al. 1962; Benita and Condd 1972) and raises threshold (Brooks et al. 1955), a decrease in amplitude with cooling is reasonably accounted for.
Summary Under general anesthesia, rats were gradually cooled from 37°C to 24°C. Slowly cooling avoided large temperature gradients between central and peripheral nervous systems. Short latency somatosensory potentials were evoked by forepaw stimulation and recorded from skull and depth structures. Cooling progressively increased onset and peak latencies and duration of all potentials. Amplitude of surface and depth recorded potentials decreased with decreasing temperatures except amplitude of surface c o m p o n e n t I increased. The response of surface and depth components to different rates of stimulation and cooling clearly indicates that cooling slows synaptic transmission much more than fiber conduction. The response of surface and depth recorded potentials to hypothermia suggests that evoked activity in cervical dorsal
29 column and cuneate nucleus contributes to surface c o m p o n e n t I, that activity in cuneate nucleus, medial lemniscus, and inferior cerebellar peduncle contributes to surface component II, and that activity in thalamocortical fibers and probably cerebellum contributes t o surface c o m p o n e n t III. These conclusions agree with our previous thoughts a b o u t the origin of short latency, surface recorded somatosensory evoked potentials.
Rdsum6 Modifications des potentiels dvoqugs somatosensoriels de courte latence induites par hypothermie chez le rat Sous anesth6sie g6ndrale, des rats ont 6td refroidis progressivement de 37°C ~ 24°C. Le refroidissement lent dvite de grands ~carts de tempdrature entre les syst6mes nerveux central et p6riph6rique. Les potentiels somato-sensoriels de courte latence ont dt6 dvoqu6s par stimulation du membre ant6rieur et enregistrds au niveau d u scalp et des structures profondes. Le refroidissement augmente progressivement les latences de ddbut et de pic et la durde de tous les potentiels. L'amplitude des potentiels enregistrds en surface et en profondeur diminue avec la diminution des temp6ratures ~ l'exception de l'amplitude de la composante I de surface qui augmente. La rdponse des composantes en surface et en profondeur diffdrentes vitesses de stimulation lors du refroidissement indique clairement que le refroidissement ralentit la transmission synaptique beaucoup plus que la conduction par fibres. La rdponse des potentiels enregistr~s en surface et en profondeur ~ l'hypothermie sugg6re que l'activitd dvoqu6e dans la colonne cervico-dorsale et le n o y a u cund'/forme contribue ~ la composante de surface I, que l'activitd du n o y a u cund'/forme, du lemniscus m6dian et du pddoncule cdrdbelleux infdrieur contribue ~ la composante de surface II et que l'activitd des fibres thalamo-corticales et probablement celle du cervelet contribuent ~ la
30 c o m p o s a n t e d e s u r f a c e III. Ces c o n c l u s i o n s s o n t e n a c c o r d avec n o s h y p o t h e s e s a n t d r i e u r e s s u r l ' o r i g i n e des p o t e n t i e l s ~ v o q u d s somato-sensoriels de c o u r t e latence enregistrds en surface. The authors gratefully acknowledge the expert technical assistance of Mr. Jeff Borchardt.
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