Evoked potentials from the visual system in hypothermic hibernators and nonhibernators

EXPERIMEXTAL

14, 134-143

NEUROLOGY

Evoked

Potentials

(1966)

from

Hypothermic

the

Visual

Hibernators

System

in

and

Nonhibernators L. Laboratory

c.

JK.,

&lASSOPUST,

of Newophysiology,

Clevelalzd Received

.4ugusf

AND

L.

Psychiatric

R.

ifrOLIN’ Institute,

Cleveland,

Ohio

4, 1965

Evoked potentials from the visual system in hibernators and nonhibernators were examined under induced hypothermia. Responses were recorded from the eye (ERG), optic chiasm, superior colliculus, and visual cerebral cortex. The a-wave of the ERG disappeared early during hypothermia in both classes of animals at 24C esophageal or buccal temperature. The b-wave component of the ERG was lost at 24C in the nonhibernator but persisted to 17C in the hibernator. In the nonhibernators the evoked responses recorded from the optic chiasm dropped in amplitude slightly, then leveled off and remained at this level to 25C where a precipitous drop to zero amplitude took place at 24C. In hibernators this drop in amplitude was progressive and more linear. Progressive and linear drops in amplitudes of the evoked responses in the superior colliculus and visual cortex occurred in all animals. The latencies of these responses were most remarkable, increasing about 285% in the nonhibernators but increasing to over 500~ in the hibernators. The longest measurable latency was found in the optic chiasm of the prairie dog where the latency increased to about 90 msec at 17C. or a 600% increase over the IS-msec latencies measured at normothermic temperatures. Some protective mechanisms considered playing a role in these results were the resistance of the anterior reticular formation to cold which maintains minimal conditions within the central nervous system for transmission of impulses, the preferential shunting of blood to certain brain-stem areas, and the natural resistance to cold of certain heavily myelinated tracts of the brain stem. Introduction Previous investigations into the effects of induced hypothermia on the spontaneous and evoked electrical activity of the cortex, reticular formation and visual pathways have demonstrated the resistance of the mesencephalon to cold (12, 13, 17). 1 This investigation was supported in part by the National Institutes of Health grant NB-04393-03. The authors wish to acknowledge the assistance of Mrs. Judith Meder, R.N. in preparing the animals and in collecting the data. 134

EVOKED

POTENTIALS

AND

HYPOTHERMIA

135

Further, it has been shown that the anterior reticular formation retains excellent electrical activity below 16C buccal temperature in hibernators (14). Since this sustained electrical activity is present at low body temperature, a special protective mechanism probably operates in hibernators maintaining minimal metabolic conditions and circulatory integrity to the brain. Therefore, it should be of interest to compare the evoked potentials elicited along a brain-stem pathway of hibernating mammals with that of nonhibernators. Because of its long course within the brain stem, the visual pathway was selected for investigation. Materials

and

Methoas

Five black-tailed prairie dogs (Cynomys ludovkianus) and six thirteenstriped ground squirrels (Citellus tridecemlineatus) of both sexes were used in this investigation. These were compared with six common Egyptian cats (FeZis catus) and six laboratory guinea pigs (Cavia porcellus) . Animals were placed under anesthesia using either pentobarbital sodium (Nembutal), 30 mg/kg iv or ip, or thiamylal sodium (Suritol), 2% iv or ip. Beginning with the eye, a needle eIectrode about about 0.025 inch in diameter was inserted into the posterior chamber as close to the retina as could be determined with an ophthalmoscope. A second electrode was inserted into the optic chiasm through a trephine hole in the skull close to midline by means of a Baltimore stereotaxic instrument. Using this same instrument, another needle electrode was implanted into the stratum intermedium of the superior colliculus on the same side as ‘the eye being stimulated. Finally, a disc-type silver electrode was placed on the primary visual cortex on the contralateral side to the eye being stimulated. All electrodes were permanently fastened to the skull by dental cement. Location of the electrodes in the cat was determined from Jasper and AjmoneMarsan’s stereotaxic atlas (9). For the prairie dog, ground squirrel, and guinea pigs, model formalized heads were used to determine the position of each implanted electrode along the central visual pathway. The left eye of each animal was stimulated by a Grass P4 photostimulator set at its maximum intensity. The light pulse was brought to the eye by means of a flexible fiber optic (16-mm diameter) centered 13 mm in front of the cornea in the center of the dilated pupil. Five superimposed evoked responses from the visual pathway were recorded on a four-channel RM561A Tektronix oscilloscope via MS122 preamplifiers and photographed using the photographic elimination of artifactual transients (S), by a Polaroid camera. All recordings were of

136

MASSOPUST

AND

WOLIN

the monopolar type with indifferent electrodes attached to the skull or skin of the head. Esophageal and buccal temperatures were recorded on a Yellow Springs Instrument Co. telethermometer by means of thermisters. The EKG records were made on a Grass electroencephalograph by means of needle leads inserted into the left fore and hind legs. Hypothermia was induced by placing cubed ice in plastic bags supported by a wire mesh over two-thirds of the animal’s body. Rewarming was accomplished by placing a special chest warmer under the animal and an infrared lamp over the exposed surface of the body. As mentioned previously ( 14)) resuscitation equipment was necessary during rewarming of the cat and guinea pig. The position of all electrodes was checked histologically (13). Results

In the cat the b-wave amplitude of the ERG averaged about 150 uv at normothermic temperatures, dropping precipitously to zero amplitude at 24C esophageal temperature. The wavelets superimposed on the main bwave were distinctly visible at temperatures of 26C. During rewarming the reverse of these results occurred. These same observations were also true in the guinea pig (Fig. 1). In the ground squirrel and prairie dog the amplitudes of the b-wave fell less precipitously and retained an amplitude of 20-30 yv at 19C buccal temperature, well below the isoelectric point of the cat and guinea pig. In fact, ERG potentials were still readable at 17C in the hibernators, allowing accurate evaluation of latencies of the responses (Fig. 1). It was noted that br and b, wavelets of the b-wave complex usually disappeared at around 2.X buccal temperature but the ba component was detectable to about 18C. During rewarming the responses mirrored the cooling phase, reaching maximum at 34-3X. The a-wave component of the ERG was always present at normothermic temperatures but would disappear early during induced hypothermia. Frequently, the a-wave would not reappear during rewarming. Therefore, only the b-wave amplitudes and latencies were determined. The optic chiasm responses to photic stimulation reveal an interesting phenomenon in the nonhibernating group of animals. In cats and guinea pigs, the amplitudes of the optic chiasm potentials dropped slightly, then leveled off and remained at this amplitude to 25C esophageal or buccal temperature. Between 25 and 24C there occurred a precipitous drop in

EVOKED

POTENTIALS

AND

137

HYPOTHERMIA

amplitudes, from an average of 200 uv in the cat and 50 p.v in the guinea pig to zero amplitudes for both species at 24C. In the case of the hibernators the potentials dropped in amplitude more gradually but were still readable (about 6 uv) at the 16C buccal temperature point. Likewise, the ERG

ox

CAT SC

oc

35

FIG. 1. Evoked potentials from the visual system of the cat (CAT) and ground squirrel (GS) during hypothermia. Each response is the average of five superimposed oscillographic traces. ERG = electroretinogram ; OX = optic chiasm ; SC = superior in microvolts and horizontal colliculus; OC = optic cortex. Vertical calibration calibration in milliseconds.

return of the potential was gradual during the rewarming phase, coming back to the normothermic amplitude at 35C. However, during rewarming the optic chiasm of the hibernators remained silent to about 19C where

138

MASSOPUST

AND

WOLIN

barely readable responses were found, then increased in amplitude rapidly reaching the normothermic amplitude levels at 25C. Responses elicited from the superior colliculi and optic cortices of both hibernators and nonhibernators followed the usual course of events. No significant differences were noted in responses from the above two brain areas during the cooling or rewarming phases except that the detectable potentials were found at much lower temperatures in the ground squirrel and prairie dog. However, in the guinea pig a considerable increase in amplitude of evoked cortical responses occurred at 29C buccal temperature during the cooling phase and also recurred at 29C during the rewarming phase. This phenomenon was not observed in the cat, ground squirrel, or prairie dog. Latencies. Figure 2 illustrates the percentage change of amplitudes and latencies in the four speciesduring cooling and rewarming. As was observed previously (17), the stability of the latencies is much greater than the stability of the amplitudes of evoked responsesfrom the visual pathway. Latencies of the ERG were measuredfrom the stimulus artifact to the first deflection of the waveform from the baseline with the exception of the cat, which was measured to the second deflection or the b-wave. Since the a-wave dropped out early during cooling in the rodents, it could not be usedfor comparative purposes.The a-wave persisted throughout cooling and rewarming phases in the cat, leaving a normothermic latency of 4-5 msec and increasing to 12-14 msec at 24C esophageal temperature. It returned to the normothermic level of 4-5 msec during rewarming. The bl component of the b-wave complex had a latency of 14-18 msec at normothermic temperatures. It increased to 35-55 msec at 24C esophagealtemperature and returned to normothermic level during rewarming. This represents an average hypothermic change of 1705% from the normothermic level (Fig. 2), In the guinea pig, the bi component of the b-wave had about a 10 mseclonger latency than the cat at the normothermic level and increased an average of 180% at 24C buccal temperature. Below 24C there occurred a rapid loss of all potentials in these two nonhibernators, associated with cardiovascular difficulties (14), whereas in the hibernators all potentials persisted to just below 16C buccal temperatures where the isoelectric point was reached. Potentials from the optic chiasm were unusual during cooling. These

EVOKED

14c- .

POTENTIALS

AND

HYPOTHERMIA

139

..

a q--s w....

PD GS CAT GP

FIG. 2. Multiple graph showing average response evoked potentials in visual system during hypothermia. percentage change from baseline response. PD = prairie CAT = Cat; GP = guinea pig. ERG = electroretinogram; = superior colliculus; OC = optic cortex. Percentage degrees centigrade on the abscissas,

amplitudes and latencies of All responses are given as dog; GS = ground squirrel; OX = optic chiasm ; SC changes on the ordinates;

140

MASSOPUST

AND

WOLIN

potentials not only retained an excellent amplitude at hypothermic temperatures but had remarkab!y extended latencies. For example, in the cat these latencies at 25C increased 280% over their precooling level. In the guinea pig, this increase was on the order of 290%. Responses from the superior colliculus followed the usual pattern in the nonhibernators, having an almost linear increase in latency until the precipitous dropoff point was reached below 24C. The optic cortex potentials also showed this linear increase in latency to the isoelectric point at 24C. However, the afterpotentials usually observed following the initial evoked response disappeared at about 28 to 27C esophageal temperature, and in several nonhibernators did not return during rewarming. In the hibernators the most striking finding was the extreme length of latency still measurable at the low buccal temperature of 16C. The ERG b wave showed a 60-90 msec latency at 16C, approximately 700% average increase over the normothermic latency of 12-14 msec in the prairie dog, while in the ground squirrel the hypothermic latency extended to 30-60 msec (Fig. 2). The longest latency of all evoked responses was found in the optic chiasm of the prairie dog where the average latency was 90 msec or an increase of 60070 over the normothermic latency of 15 msec at 35C buccal temperature. The latency of responses from the superior colliculus increased about 75070 over the normothermic average in the ground squirrel and about 450% in the prairie dog. Similar latencies of around 450 to 600% increase during hypothermia were found in the visual cortex. Heart Rate. In the nonhibernators the heart rates fell linearly with the body temperature to about 50% of its normothermic level at 24C, where little electrical activity remained in the central nervous system. However, in the hibernators the heart rates fell more precipitously than the body temperature and decreased to about 15% of their normothermic level at 2OC buccal temperature, while the brain still retained good electrical activity. For example, the precooling average heart rate of the cat was about 200 beats/min. It decreased to about 100 beats/min at 24C and increased to 230 beats/min at 35C during rewarming. In the prairie dog the normothermic average heart rate was 370 beats/min, dropping to 120 beats/min at 24C and 45 beats/min at 16C, and returning to 330 beats/min as 35C during rewarming.

EVOKED

POTENTIALS

AND

HYPOTHERMIA

141

Discussion

There remains considerable controversy over the origin of the various ,components of the ERG. Evidence continues to accumulate which assigns the a-wave to the photoreceptors and the b-wave to the bipolar and ganglion cell layers of the retina. Most investigators now agree that the b-wave arises from the bipolar layer or structures proximal to the receptors, or both (Z-4). The profound effects of hypothermia on amplitudes and latencies of neural transmission within the central visual pathway provide an excellent tool for dealing with the above issues. If the a-wave is the receptor potential of the ERG, then what activates the central visual pathway when the a-wave disappears at hypothermic temperatures of 2X? The early loss of the a-wave during hypothermia in hibernators indicates that the electrophysiological changes are not merely due to metabolic deficiencies since experiments which directly deprive the retina of essential metabolites such as oxygen and glucose immediately decrease the amplitude of the b-wave ,while leaving the awave unchanged or even enhanced (6-8, 10). Earlier data from nonhibernators strengthen the premise that the reduction of amplitudes and remarkable increases in latency were due to the direct effect of cold in slowing the chemical reactions involved in phototransduction and nervous transmission ( 13). Furthermore, the b-wave, optic chiasm potentials, superior colliculus potentials and visual cortex potentials were still measurable at temperatures below 20C in the hibernators. If the sequence, a-wave, b-wave, optic chiasm response, and so on through the visual pathway, actuahy represents the course of transmission of the neural impulse in response to visual stimulation, then it follows that a photoreceptor potential must occur in order for the neuroretina and the central visual pathway to respond. With the slowing .of photochemical processes during hypothermia, it is conceivable that the time period during which temporal summation is effective may be extended. This would permit the transmission of an impulse across a synapse and the recording of the postsynaptic potential even under conditions where the presynaptic potentials do not achieve sufficient amplitude to be detected by these methods. Further evidence for the “occurrence” of the ‘La-wave,” even at temperatures where it is no longer measurable, is clearly provided by the fact that optic chiasm potentials, and sometimes the cortical potentials, precede the appearance of the b-wave. Ponte and

142

M.4SSOPUST

AND

WOLIN

Monaco (16) have reported that in the normothermic rabbit the b-wave latency is characteristically longer than that of the optic nerve potential and, while at high stimulus intensities the two potentials may be simuitaneous, the appearance of the b-wave never precedes the optic nerve POtentiai. Previous investigations (13, 14) have demonstrated that the anterior reticular formation resists the depressant effects of cold and helps maintain minimal metabolic and possibly circulatory conditions to protect the vital processes of nonhibernating species. However, in the hibernators a special mechanism must be present which protects all regions of the central nervous system against cold. The anterior reticular formation could act as the control center for this mechanism since it retains some spontaneous electrical activity at buccai temperatures of 16C. Further, the preferential shunting of blood away from the cerebral cortex to certain brain-stem areas could function to protect the nonhibernators only at lower body temperatures (15). In the hibernators, the low heart rate observed at temperatures below 2OC indicates a relatively poor circulation to the brain. However, the electrical activity is still present at fairly good amplitudes. Therefore, the blood-shunting mechanism mentioned above probably would not play as great a role in the hibernators as it would in the nonhibernators. It would appear that whatever protective mechanism exists lies within neural tissue, probably inherent within the cellular matrix of the brain itself. The early loss of the afterpotentials observed in the primary visual areas of the cerebral cortex demonstrates a high sensitivity of the neuropil of the cortex to changes in intracerebral temperature. A few degree drop in intracorticai temperature must have a blocking effect on the reverberating association cortical circuits. thus causing the loss of the afterpotentials. The fact that the amplitudes of the evoked cortical response either maintain their normothermic level or increase sharply from 30 to 27C esophageal or buccai temperatures indicates an increase in cellular and muscular metabolism to compensate for the heat loss during the early phases of hypothermia (1). Shivering occurred in most of the animals after a l- to 3-degree drop in body temperature. Similarly, during rewarming the amplitudes of the evoked cortical responses returned to the normothermic level at about 26C and leveled off (Fig. 2). The esception to this was the extreme increases in cortical potentials observed in the guinea pig at 29C buccai temperature. Since the iatencies can be measured at various stations along the

EVOKED

POTENTI.4LS

AND

HYPOTHERMIA

143

visual pathway in the brain stem at the lower hypothermic limits, a mechanism such as that described by Lundberg (11) may explain the presence of responses at low body temperatures along myelinated brain-stem pathways such as the visual pathway. References 1. 2. 3. 4. 5.

7. 8. 9. 10. 11.

12.

13.

14.

15.

16. 17.

1963. Body temperature regulation in the normal and cold acclimatized cat. J. Appl. Physiol. 18: 772-777. ARMINGTON, J. C. 1965. Vision. Ann. Rev. Physiol. 27: 163-182. BROWX, K. T., and M. MURAXAMI. 1964. A new receptor potential of the monkey retina with no detectable latency, Natzlre 201: 626.627. BROWN, K. T., and K. WATANABE. 1962. Rod receptor potential from the retina of the night monkey. Natzrre 196: j47-550. COLLINS, R. L. 1964. Photographic elimination of transients (PET): A simple technique for retrieving bioelectrical signals from noise. PYOC. Sot. Exptl. Biol. Med. 117: 724-726. GRANiT, R. 1959. Neural activity in the retina, pp. 623-712. In “Handbook of Physiology, Section 1, Neurophysiology,” Vol. 1. American Physiological Society, Washington, D. C. HENKES, H. E. 1953. Electroretinography in circulatory disturbances of the retina. Arch. Ophthalnzol. 49: 190-201. HENKES, H. E. 1957. Electroretinography. Am. J. Ophthabwol. 43: 67-86. JASPER, H. H., and C. AJMONE-MARSAN. 1952. “A Stereotaxic Atlas of the Diencephalon of the Cat.” National Research Council of Canada, Ottawa. KRILL, A. E., M. DIABZOND, and G. ISER. 1962. The electroretinogram in carotid artery disease. Arch. Ophthalnzol. 68: 42-51. LUNDBERG, A. 1948. Potassium and the differential sensitivity of membrane potential, spike and negative after-potential in mammalian A and C fibres. Acta Physiol. Stand. .%ppl. 50, 15: l-67. MASSOPUST, L. C., JR., M. S. ALBIN, H. W. BARNES, R. MEDER, and H. W. KRETCHMER. 1964. Cortical and subcortical responses to hypothermia. Exptl. Neural. 9: 249-261. MASSOPUST, L. C., JR., L. R. WOLIN, M. S. ALBIN, and J. MEDER. 1964. Evoked responses from the eye and visual pathways in the hypothermic cat. Ezptl. Neurol. 10: 383-392. MASSOPUST, L. C., JR., L. R. WOLIN, and J. MEDER. 1963. Spontaneous electrical activity of the brain in hibernators and nonhibernators during hypothermia. Exptl. Newrol. 12: 25-32. METER, J, S., and J, HUNTER. 1957. Effects of hypothermia on local blood flow and metabolism during cerebral ischemia and hypoxia. J. Neurosurg. 14: 210-227. PONTE, F., and P. MONACO. 1964. A study on the relationship between electroretinogram and optic nerve discharge. OphthaZnzoZogica 147: 57-66. WOLIN, L. R., L. C. MASSOPUST, JR., and J. MEDER. 1964. Electroretinogram and cortical evoked potentials under hypothermia. Arch. Ophthalmol. 72: SZI524. ADAMS,

T.