Alteration of sensory and motor evoked responses by dieldrin

ALTERATION

OF SENSORY AND MOTOR RESPONSES BY DIELDRIN” R. M.

Department

EVOKED

JOY

of Physiological Sciences. School of Veterinary Medicine, University of California. Davis. California 95616

Summay

Dieldrin has specific effects on responses evoked to peripheral by central stimuli. Postsynaptic components of cortical responses to any modality of sensory stimulation are facilitated by dieldrin while subcortical potentials are moderately depressed. Similar postsynaptic facilitation occurs with direct cortical and transcallosal stimulation. In contrast to these findings, cortical and peduncular responses to stimulating fibres in the brachium conjunctivum or at ventralis lateralis were moderately depressed. With dieldrin a late positive-negative potential developed over frontal cortical areas which was indistinguishable from the irradiation potential produced by metrazol. The late wave was the source of the spike discharges that developed in the EEG records, The close similarities observed between dicldrin and metrarol strengthen the hypothesis that they share either a common mechanism or final common pathways responsible for electrophysiological events. Differences in reports of these agents seem more readily explained in terms of their diflercnt pharmacokinetic properties.

The convulsive properties of the chlorinated hydrocarbons, dieldrin and DDT, have been shown to possess many features in common with metrazol. In a previous report (JOY, 1973) it was demonstrated that dieldrin and DDT are characterized by the following EEG changes: (1) preseizure episodes of hypersynchrony, sporadic rhythmic discharges or spike discharges synchronous in cortex and deep structures, (2) bilaterally symmetrical seizure development, (3) cortical predominance. (4) independent seizure genesis in hippocampus and superior colliculus, and (5) induction of seizures by sensory stimulation. Also the protection against the convulsant properties of dieldrin afforded by various anticonvulsant drugs parallels the protection the same drugs provide against metrazol. Many other similarities have been observed such as veratrinic actions (EYZAGUIRRE and LILIENTHAL, 1949; NARAHASHI and HAAS, 1967,1968; HILLE, 1968) and alteration of evoked electrical activity (WOOLLEY,

1968

; REVZIN,

1966).

Our purpose in this study was two fold. First, we felt it important to extend the comparison of dieldrin and metrazol to include a systematic study of responses evoked by stimulating peripheral and central pathways. Second, we hoped to develop a clearer picture ofthe actions ofdieldrin in the CNS towards the end ofestablishing its mechanism of action. We purposefully chose to examine characteristics of the preconvulsive state since this is the time during which changes leading to epileptogenesis must develop. To the extent that dieldrin and metrazol share common effects the results may be of more general interest as an experimental study of epileptogenic processes. * This

work was supported

by t.JSPHS Grant

Number 93

ES-0620.

94

K. MM.JOY METIiODS

Sirt+cts

Fifty-three use.

malt cats. 2.5-3.5 kg, were used as subjects.

Cats \vcre fasted

12 hr prior to

Cats were anacsthetizcd with ether. trachcotomiled. placed in a stercotaxic frame and maintained with halothanc. Cannulac were placed in a brachioccphalic vein for infusion of gallamine. a femoral vein for drug injection and a femoral artcry for monitoring blood pressure. After reflecting the skin and muscles overlying the skull. holes were drilled through the bone to place subcortical clectrodcs. Various sections of bone wcrc rcmovcd. as ncccssarc. to expose the cortex. and wells of dental wax wcrc built up around the exposed areas. Subsequently, the dura was rcmovcd. and the wells wcrc filled with mineral oil at 37 C to prevent the cortex from drying. In some expcrimcnts the sciatic nerve \vas isolated and sectioned. and an electrode was placed around the cephalic end for stimulation. In experiments whcrc flashes of light wcrc to be employed as stimuli. the pupils were dilated with atropine and phcnylephrinc. At the end of surgery. prcssurc points and wound edges wcrc infiltrated with small amounts of dibucainc. and the wound edges were covcrcd with mineral oil and wrapped in saline-soaked Gaul. The animal was then paralyzed and artificial respiration begun. A Beckman LBI CO2 analyzer was used to insure an end-cxpiratory (‘O? bctwcen 3.X and 3,2”,,. Temperature was maintained between 37.5 and 3X C. Vital signs. c.g. blood prcssure. heart rate. CO? and LEG wcrc assessed continually to insure a viable and stable prcparation. At the end of the cxperimcnt the cats wcrc sucrificcd \vith pcntobarbital and 35 /tm froycn sections were stained \vith hcmatoxylin- eosin or Luxol fast blue. Stcrcotaxic placcmcntsand verifications were based on the atlas of JASPI:K and AJZKN.-MAKSAS (1953). Electrode locations were transferred to cross-sectional figures taken from this source and used in the figures.

Cortical records wcrc obtained from 1.:2 mm chlorided silver ball clcctrodcs. Records were monopolar. the stereotaxic frame serving as rcferencc. Subcortical records were obtained from concentric stainless steel electrodes. Records were bipolar. ncgativit! at the central electrode being recorded as an upward dellcction. The data was recorded in parallel on a Tektronix 564 oscilloscope and a Hewlett Packard 3955C tape recorder. Stimuli were rectangular pulses gcncrated by a Grass S88 stimulator. isolated. and Icd to bipolar electrodes similar to those used for recording. Voltages used for stimulation wcrc kept constant between 2-3 times threshold unless otherwise indicated.

Animals were given 2 hr following completion of surgery to recover from effects of anaesthcsia. During this time clcctrodes wcrc located in the desired areas under stcrcotaxic and physiological control. After 1: hr. stimulation began and wascontinued without cessation at a rate of 0.5 per see throughout the remainder of the cxperimcnt. This procedure was selected to reduce the effects of habituation or changes in responses due to turning stimuli on and off. At 2 hr control recording began and this period was cxtcndcd for as long as necessary to assure stable baselines. This usually occurred within IO 20 min. Once

Dieldrin

and evoked

responses

95

controls were obtained, dieldrin was ad~nistered in incremen~l doses, and data was colfected continually on tape until the experiment was terminated. Dieldrin was solubilized in ethanol immediately prior to use in concentrations that allowed administration of less than 0.5 ml ethanol per experiment. Many control injections demonstrated that ethanol never produced any of the effects described in this study. Later the data was played back from the tapes and analyzed with a Speer p-Lint computer or a Mnemetron Computer of Average Transients. Groups of 20 responses were averaged sequentially to follow drug effects with time. These averages were used to generate both the amplitudes of responses and to provide examples for the figures. Selected single responses were photographed as well to provide examples of the potentials actually available for analysis. Because of the nature of the data all amplitudes were normalized and only changes in response amplitude or waveforms were considered for interpretation. RESULTS

Dieldrin changed the spontaneous EEG in a consistent dose-related fashion. Within l-2 min of injection of 224 mg/kg, a period of hypersynchrony ensued (Fig. 1B). This was usually intermittent in nature and, at least initially, it could be blocked by stimuli producing arousal. This period was followed by the development of spontaneous rhythmic discharges which were bilateral and appeared at highest amplitude over the anterior part of

Fig. 1. EEG effects of dieldrin. A: control record. B: period of hypersynchrony developing minutes after administering 2 mg/kg dieldrin. C: spontaneous rhythmic discharge with superimposed spikes. The development of such discharges was maximal from frontal cortical areas in monopolar records. D: spontaneous, grouped spikes occurring independently of rhythmic discharges. E: spontaneous seizure developing 25 mm after adminis~ation of 2 mg/kg dieldrin. Seizure episodes were usually followed by post-ictai depression. Amplitude calibrations, 200 @V. Time marks represent 1 and 5 sec.

96

R. M. JOY

the cortex (Fig. IC). Stimuli such as taps. clicks or flashes of light readily elicited such discharges, somatosensory and auditory stimuli being most consistently effective. In most subjects the discharges developed high amplitude spikes which occurred singly or multiply during the discharge. Often a seizure developed spontaneously from this type of background activity. It was not uncommon, however, for spikes to begin to appear independently (Fig. lD), increase in amplitude and frequency, and finally culminate in a seizure. Spikes, like the discharges, appeared widely distributed in the cortex and in many extracortical areas. Seizures, when they occurred, were always generalized and could last from 10 to 300 set (Fig. 1E). There was usually a period of post-ictal depression whose length was related to the duration of the seizure. At high doses of dieldrin recurrent seizures were the rule, and these could continue for hours.

The effectiveness of sensory stimuli in precipitating rhythmic discharges and seizures in cats administered dieldrin is one of the most significant actions that can be observed with EEG recording, but such records cannot define these changes in detail. To examine this more specifically. responses along the somatosensory pathway to sciatic stimulation were studied. Figure 2 illustrates a typical result. Responses at the level of the medial lemniscus (ML) and ventralis posterolateralis (VPL) were both moderately depressed by dieldrin, the depression of the VPL response being proportional to that occurring in ML. In contrast, the primary response at the somatosensory cortex was greatly enhanced. Changes in the primary evoked response at the cortex developed progressively at about the same time that hypersynchrony appeared in the EEG. Cortical responses generally proceeded to grow in a constant manner until a spontaneous seizure developed. Although the earliest changes were restricted to the amplitude of the primary response, in the later stages of intoxication the waveform was also altered. The most obvious change was the development of a late positive-negative deflection which encroached upon the negative portion of the primary response (Fig. 2C). Once it appeared. it. too. grew in amplitude in a constant fashion. This late deflection was synonymous with, and its appearance coincided exactly with, the presence of spikes in the EEG records. The late potential could be recorded over wide areas of the cortex, but it appeared most developed in the anterior sigmoid and surround. To determine whether these alterations were specific to the somatosensory system or whether they were generalizable to other sensory systems, responses to optic tract stimulation were also examined. As Figure 3 indicates, similar changes occurred to visual as to somatosensory stimuli. The tract response recorded at the lateral geniculate (LG) was not affected whereas the geniculate response was slightly reduced in amplitude. The cortical response demonstrated three significant changes. First, the initial positive deflection (Lat. Gyrus-Pre) which has previously been attributed to the arrival of impulses of the radiation fibres at the cortex (BISHOPand O’LEARY, 1938; CHANG and KAADA, 1950; BISHOPand CLARE,1951) was reduced proportionately to the geniculate response. Second, the portions of the response attributable to cortical events (Lat. Gyrus-Post) were markedly enhanced. As shown in Figure 3A, immediately following a seizure, the postsynaptic components wcrc reduced in amplitude. They subsequently began to increase again and continued to grow until another seizure began the process over again. Third, a late wave developed in response to stimulation. This wave was visible in records from the lateral gyrus as a positive deflection encroaching upon the negative part of the primary evoked response (Fig.

Dieldrin

and evoked responses

97

-PSG ---ML

- - - -vPL

VPL

ML

ASG

Fig, 2, Alterations of responses evoked by sciatic stimulation. A: graph correlating changes in evoked response amplitudes with EEG characteristics. ‘The ordinate represents time, each successive point on the ordinate being the averaged response amplitude for a 4 min time period. Abscissa represents amplitudes referred to mean control values. Small numbers along the graph indicate the times at which the records in C were obtained. B: cross-sectional diagrams indicating the determined location of each recording electrode. marked by filled circles. Subcortical diagrams are taken from the atlas of JASPERand AJMONE-MARSAN (1954). C: averages of 20 responses taken at the times indicated in A. In this and subsequent figures. vertical lines on potentials in top row represent points used to determine amplitudes of responses. D: single response taken during the control period to indicate waveform and latency correlates along the somatosensory pathway. Top: PSG, middle: VPL, bottom: ML. Time calibrations for C and D. 10 msec. Amplitude calibrations for D, 100 1tV. Photographs in this and subsequent figures have been retouched for clarity. In this and subsequent figures abbreviations used are: ASG, anterior igmoid gyrus: LG. lateral geniculate nucleus; Lat. gyrus. Intcrai gyrus: MG, medial geniculate nucleus; ML. medial lemniscus: Pcd.. peduncle; PSG, posterior sigmoid gyrus; VL, ventralis lateralis; VPL, nucleus ventralis posterolateralis.

3C), but it was most well developed at the level of the anterior sigmoid and surround. Its distribution was similar to the late wave produced by sciatic stimulation, but it differed in its latency. The latency of the potential was 5.5 msec to stimulation of the optic tract whereas it was 17.1 msec to sciatic stimulation measured from equivalent points on the anterior sigmoid gyrus. Because the stimuli in the above situations were applied beyond the receptor, we also examined responses evoked by clicks, light flashes or mechanical pressure to see whether the presence of a receptor would affect the results. Responses recorded along the auditory pathway to click stimuli are shown in Figure 3D. Transduction via a receptor did not alter

98

R. M. JOY

-LAT GYRUSfpcst) - -‘- J_ATGYRUS(pre) ---LG

LAT GYRUS

Fig. 3. Alteration of responses evoked by optic tract stimulation or by clicks. A: similar to Figure 1 but representing responses along the visual pathway elicited by optic tract stimulation. Black square indicates occurrence of a seizure. Inserts at right show location of recording electrodes. “Post” and “pre” after Lat. gyrus in legend signify postsynaptic and presynaptic components as explained in text. B: averages of 20 responses at times indicated in A. Columns 3 and 4 are the same data with time bases of 32.5 and 65 msec respectively. C: single responses indicating latencies of various potentials to optic tract stimulation. Top record taken during control and bottom record after 3 mg/kg dieldrin. In each record top trace is LG; middle is Lat. gyrus; bottom is ASG. D: single responses to click. Top record taken during control period and bottom record after 2 mg/kg dieldrin. In each record top trace is MG; middle is auditory cortex; bottom is ASG. Time calibrations for B, C and D, 10 msec. Amplitude calibrations for C and D, 100 /IV.

the observations, and the same relationships were determined for auditory, visual or somatosensory stimuli whether applied at the receptor or directly to an afferent nerve. The magnitude of the effect of dieldrin was always greatest, however, when afferent pathways were directly stimulated, perhaps because of the greater synchrony possible when this mode of stimulation is used. These studies indicate that dieldrin alters the responses evoked in sensory systems in a specific manner. At all levels below the cortex there is either no change or some depression in the amplitude of responses. At the cortex there is a specific enhancement of the response attributable to cortical elements. After an initial phase of growth of the primary response, a late discharge develops. This late discharge is not restricted to the specified sensory cortex but is widely distributed, appearing maximally at or around the motor area, the anterior sigmoid gyrus

Dieldrin

Eflects of dieldrin on responses

99

and evoked responses

evoked

by stimulating j2hre.s of the brachium

conjunctivum

The brachium conjunctivum contains fibres conveying proprioceptive and other inforMatson from the cerebellum which synapse in the thala~~ nucleus, ventralis lateralis (VL). The ventralis lateralis then projects to the anterior sigmoid gyrus. As such this represents a relay for information from the cerebellum through the thalamus to the motor cortex. Responses evoked by stimulation of the fibres of the brachium conjunctivum were examined to observe whether this direct motor projection pathway would respond similarly to dieldrin as did the sensory projection pathways. Results are shown in Figure 4. The effects of dieldrin in this situation were essentially opposite to that seen for the sensory systems. The postsynaptic response in VL was often slightly enhanced while the cortical response was depressed, Whereas the direct effect of such stimulation was to produce an afterdischarge in the anterior sigmoid gyrus following each stimulus, dieldrin depressed these discharges. Late potentials like those seen with sensory stimulation never developed. In fact, this stimulation appeared to depress cortical responses to applied sensory stimuli. In a number of cases this depression was made graphically evident in that unilateral seizures developing on the non-stimulated side occurred (Fig. 5). This was remarkable in that a unilateral seizure has never been observed under any other conditions.

._-.VL(

-.- - .VL&

I

Control

3-

HwGrs”“chmnv

Dlsldrin,m

~Afterdmhame

Dieldrin

t)

-ASG

nmefko

-J--ye Fig. 4. Alteration of responses evoked by stimulating fibres of the brachium conjunctivum. A: similar to Figures 1 and 2. B: averages of 20 responses taken at times indicated in A. C: location of stimulating electrode (top) and recording electrodes (bottom). D: single responses to demonstrate latencies of various components. Left record is control; right is after 2 mg/kg dieldrin. Top trace is VL, and bottom is ASG response. Time calibrations for B and D, 10 msec. Amplitude calibrations for D, 100 PV.

R. M. JOY

100

L. PEDUNCLE -

Fig. 5. Unilateral seizure occurring during continual stimulation of left VL. Seizure develops on the right hemisphere from a series of slow spikes. There is minimal involvement of the stimulated hemisphere and a corresponding lack of discharge at the peduncle. Responses to VL stimulation are not obvious before the seizure but appear clearly during the period of post-ictal depression. Time marks. I and 5 sec. Amphtude calibrations. 200 pV.

E&G&tof dieldrin on direct and callosal

mediated responses

In addition to observing changes in cortical responses resulting from thalamocortical pathways, we also examined the effects of dieldrin on cortical responses evoked by direct cortical stimulation or via callosal fibres. In one set of experiments the cortex of the left suprasylvian gyrus was directly stimulated through a pair of silver ball electrodes placed 2 mm apart, and responses were recorded 3 mm from the stimulating electrodes (Fig. 6A) and at the homologous point in the contralateral cortex (Fig. 6B). Dieldrin enhanced both responses. The direct cortical response was enhanced regardless of stimulus intensity applied. The postsynaptic component of the direct cortical response (ECCLES, 1951; PURPURAand GRUNDFEST, 1956) was clearly evident when strong stimuli were used, and it was also evident that this component was most sensitive to the presence of dieldrin. Concurrent with the changes in the direct cortical response the transcallosal response was also enhanced (Fig. 6B). During the control period only the stronger stimuli produced a clear response. After dieldrin both stimulus intensities were effective. In this situation, however, it was not possible to determine whether the effects of dieldrin were due to actions on cortical cells at the stimulated site or due to an increased response of cells in the contralateral cortex. To resolve this the callosum was stimulated directly (Fig. 6C). In this circumstance dieldrin still potentiated the callosal response. Since the positive and

Dieldrin

and

evoked responses

Control

I

Dieldrin

Control

2

Dieldrin

Fig. 6. Alterations of responses evoked by direct cortical or callosal stimulation. Column A: direct cortical responses. Column B: simultaneously relayed transcallosal responses. Column C: responses to callosal stimulation. Row 1: stimulus intensity of I.5 times threshold for the direct cortical response. Row 2: stimulus intensity of three times threshold. Time calibration in C for all responses. IO msec. Amplitude calibrations for A. B and C, 200 pV.

negative portions of the transcallosal response have been shown to represent postsynaptic activity to different sets of afferent fibres (GRAFSTEIN, 1959) it is evident that dieldrin has increased the responsivity of cortical neurones to both direct and transcallosal stimuli secondary to synaptic excitation. The direct cortical response and transcallosal response, therefore, share in common with the primary sensory evoked response the property of increasing in amplitude in the presence of dieldrin. The most sensitive element is the cortical neurone since postsynaptic portions of these responses appear to be selectively affected. This contrasts with responses to VL stimulation where the cortical response, and presumably the activation of cortical neurones, was depressed. Comparison

qf VL and VPL stimulation on eferent activity from the motor cortex

The differential was paralleled by motor cortex was major part of this

effect of dieldrin on the cortical response to VPL and VL stimulation a differential effect on corticofugal discharge when the outflow from the examined at the level of the cerebral peduncle. The identification of the peduncular response as originating from the motor cortex was based

IO2

R. M. Jo\

on the following: latency. correlation of amplitude with surface rcsponsc in the anterior sigmoid gyrus. failure to follow stimulus frequcncics above Xk30 Hz, production of an antidromic response in the anterior sigmoid when stimulating via the peduncular clectrodc. and the disappearance of most or all of the pcduncular response following removal of the anterior sigmoid gyrus by suction.

b‘ig. 7. Kclatlonahips bcrwc~n stimulus inwblt) and rcsponsc amplitude 10 \:L or VPL stimulation. Each column rqresents rchponscs rccordcd from the indicated location 10 VL (top) or VPL (bottom) stimulation. The ordinarc reprcscnts stimulu\ intensities in multiple\ of threshold value while the abscissa reprcscnt\ normahA amplirudes. Fach point on the graph represents the average of 2X0 powntial\. In wch situation rhc relationship for one cupcrimcnt (black lines) i\ graphed :ltonp with Ihc 5tand:ird dcu;~t~on kG Itic ohscrr:bll,,n\. Addltlonal c\pcrImsnlaI rcsulb arc lndl-

Our first concern was to establish what relationship existed under control conditions between stimulus intensity and cfferent responses. To determine this WC stimulated VPL or VL at stimulus strengths between I.5 and 3 times threshold for the appearance of the cortical response. and then measured the response amplitudes at the cortex and in the pcduncular outflow. The responses in both arcas increased with increasing stimulus intcnsity rcgardlcss of whcthcr VL or VPL was stimulated (Fig. 7). Although it cannot bc assumed that the same population of cells are involved in the responses. it is cvidcnt that both VL and VPL stimulation product a net excitatory action on cortical ncuroncs whose axons discharge through the pedunclc. Subsequent to dicldrin administration the two modes of stimulation wcrc difkrentially affected. Thcrc ivas a tendency for both the cortical and pcduncular responses to decrease when VL was stimulated (Fig. 8). If a scizurc occurred. the cortical rcspnnsc was enhanced

Dieldrin

and evoked responses

103

A200

ASG

PED

Fig. 8. Alterations of responses in motor system to stimulation of VL. A: similar to previous figures. B: location of stimulating (top) and recording (bottom) electrodes. ASG location the same as indicated in Figure 4. C: averages of 20 responses taken at times indicated in A. Row 3 are averages takon at the end of the experiment following injection of 5 mg/kg pcntobarbital. D: single responses taken during control period (top) and after 2 m&kg dieldrin (bottom). Top trace is from ASG and bottom trace from peduncle. Time for C and D, 10 msec. Amplitude calibrations in D, 200 pv. during the period of post-ictal depression. This was accomplished without a marked change in the peduncular response. When VPL was stimulated the cortical response was enhanced as indicated pre~ously. This was associated with a tremendous increase in the firing of cells whose axons were contained in the peduncular outflow (Fig. 9). The peduncle response increased in amplitude and was spread out tremendously in time suggesting that a prolonged state of excitability had resulted from the procedures. When a seizure occurred, both the cortical response and peduncle response were obliterated and only recovered when spontaneous activity returned to the EEG.

Comparison

of dirldrin to rnetrazol

In 15 experiments metrazol was administered rather than dieldrin. Metrazol was infused at a rate of 1 mg/kg per min to provide similar time-response conditions as used for dieldrin. In all respects the two drugs were comparable. Two primary changes developed to sensory stimuli : the enhancement of postsynaptic cortical responses and the development of a late wave distributed broadly but preferentially in frontal cortical regions. Stimulation

104

R. M. JOY

300

200

200

_PED -._ASG -.- _ PSG controt

C

PSG

Dieldrin

Im ,,, 4

Dieidrin

ASG

2mg,kg

h

PED

10

D

I&------

Fig. 9. Alterations of responses in motor system to stimulation of VPL. A: similar to previous Figures. B: location of stimulating (top) and recording electrodes (bottom). C: average of 20 responses taken at the times indicated in A. Third pedunclc response recorded at 2.‘5 amplitude of first and second. D: single rcsponscs from a dilfercnt sub.jcct to dcmonstratc latencies of the var-ions potentials. top record from control period and bottom word after 2 mg’kg dieldrin. In each record top trace is PSG, middle from ASG. and bottom trace from peduncle. Time marks. IO msec. Amplitude calibration in D. 200 !tV.

of VL was not changed by metrazol, whereas the cortical and peduncular responses to VPL stimulation were increased. Because most of these effects have been previously reported for metrazol (see Discussion) specific data will not be presented here. DISCUSSION

Our primary concern in this study has been to investigate certain features of the preconvulsive period following administration of dieldrin. This period represents a time of transition for the nervous system from its normal state to a state in which it is capable of spontaneous epileptiform activity. It is during this transitional period that changes leading to epileptogenesis must develop, and it would appear to be the most productive period in which to determine what factors change and how they must change in order for the convulsive process to develop. One point of interest was to determine how changes in the EEG correlated with changes in evoked response subsequent to administering dieldrin. Two relevant facts emerged. First, the evoked response began to change at about the same time EEG changes became obvious. The primary components of evoked responses changed progressively over time

Dieldrinand evoked responses

105

without any abrupt steps until a seizure occurred. The responses were usually normalized immediately after a seizure, but then they began to change as before until the next seizure occurred. This process could continue over and over again. The only clear demarcation of events that developed during this period occurred when a late response appeared suddenly at some point in time, then grew to become a dominating activity. It was clearly correlated with the development of evoked rhythmic discharges in the EEG. As the amplitude of this late response increased, the EEG records developed into classical spike responses. It was evident from the data that the spikes of the EEG records were not correlated with the primary evoked response complex, but instead were a manifestation of the late response. The primary responses evoked by sensory stimuli were consistently changed by dieldrin in a specific manner. Similar changes were observed whether stimuli were presented to receptors or applied along the sensory pathways of somatosensory. auditory or visual systems. Below the level of the cortex changes were comparatively small, but usually responses were decreased in amplitude without much modification in waveform. Primary components were never enhanced, and consequently the observation made by JOY (1973) that responses from such areas are increased in EEG records must be related to later components of the response. The frequency response of the electroencephalograph used was y 60 Hz which is too low to record primary events faithfully. The most predominant alterations take place at the cortical level. Responses to all sensory stimuli or to direct stimulation along a sensory pathway were enhanced 2-5-fold. Where pre- and postsynaptic components could be separated, it was the postsynaptic part that was affected. Presynaptic components, as expected, followed changes observed at the thalamic level. Our observations that events subsequent to dieldrin are equivalent to those produced by metrazol are abundantly supported by previous studies of metrazol. Geniculate responses to photic stimuli are unchanged by metrazol (HUNTERand INGVAR,1955). In the somatosensory system metrazol has been reported to depress the lemniscal response (BANNAand HAZBUN, 1969) as well as to depress responses in VPL (OKUMA, 1960). Metrazol shares with dieldrin a specificity for enhancing cortical responses to sensory stimulation. The postsynaptic component of the response in striate cortex to photic stimuli is enhanced (GASTAI’Tand HIIYTER.1950; DELL, BONVALLET and DELL, 1951; HUNTERand INGVAR,1955). Such observations led BISHOP and CLARE(1952) to postulate that metrazol increased cortical interneuronal activity while other authors (BREMER,1952) have emphasized the effect on firing of corticofugal elements. Similar observations and interpretations have been made for somatosensory (OKUMA. 1960) and auditory (LORENTZDE HAAS, LOMBROSO and MERLIS,1953) systems. The late positive--negative wave that develops to dieldrin is of considerable interest. It is an extremely rare event in the unanaesthetized cat during control periods, and when something like it does occur, it is of low amplitude. Its development during dieldrin administration and its appearance were common to all sensory modalities. Although its latency depended upon the type of stimuli used, its waveform and preferential distribution around the anterior sigmoid gyrus were independent of modality. The late potential could become very large, over a millivolt in amplitude. In the absence of stimulation similar potentials developed simultaneously over the same areas. The late response is not unique to dieldrin. It is well known for metrazol where it has been termed the irradiation response (GASTAUT, 1950; GASTAUT and HUNTER, 1950; NPVol.

13 No 2-B

106

R. M, JOY

HUNTER and INGVAR,1955). The irradiation

response has not yielded to any simple analysis of origin, and its mechanism remains largely unknown. Both subcortical and transcortical pathways have been advocated (GASTAUTand HUNTER, 1950; STARZL,NIEMER,DELL and FORGRAVE,1963; LORENTZ:DE HAAS, LOMBRO~Oand MERLE, 1953; HUNTER and INGVAR,1955). Similar phenomena have been reported for megimide (ROIXN, RUTLEI~GE and CALHOUN,1958), and they are well known to occur with chloralose (PURPURA,1959; BUSER, 1966). All of these late potentials bear a certain similarity to the secondary discharges that have been observed in unanaesthetized subjects (BIJSER,1957, 1966; BUSER and IMBERT,1961; THOMPSON,1970; FESSARD,1961) especially to novel or sudden excessive stimulation. The metrazol spike, or the irradiation potential, has been analyzed by CRWUTZFELL>T, WATANABEand Lux (1966b). They have shown that cell depolarization and discharge accompany the positive portion of the response, while the negative portion is correlated with repolarization and often with a late inhibitory postsynaptic potential (IPSP). Cell discharge is con~rmed by the simultaneous pyramidal discharge that accompanies the response (BUSERand ASC~IER?1960; PATTO~\~ and A~SSIAN, 1960). By extension the spike that develops with dieldrin most likely represents the synchronous firing of massive numbers of corticofugal neurones. The cortical events that develop with sensory stimulation are not restricted solely to these afferent systems. Similar effects develop to either direct or transcallosal stimuli as well. On the basis of the analyses of others (CL~RTIS, 1940; PEACOCK,1957; ~RAFSTE~~, 1959) and on the interpretation of waveforms produced in this study, the elements involved are postsynaptic in nature. The experiments on direct and callosal stimulation were done in a cortical association area which precludes any possibility that the architecture of sensory cortex per se or of its aflerents are essential for the changes developing to dieldrin. The data discussed so far suggest that dieldrin or metrazol may produce a generalized increase in the responsiveness ofthe cortex to affercnt input. If this were true, then it would be expected that responses to all afferent input of an excitatory nature would be enhanced. The projection from VL to the motor cortex appears to be of this type on the basis of intracellular studies of pyramidal cells in the motor cortex (CKEUTZFELLX, WATANABE and Lux, 1966a) and on the basis of the observed correlation between stimulus intensities and peduncular response. Responses in this system should also be enhanced by dieldrin if the assumption were true. However, the administration of dieldrin resulted in a reduction of the cortical responses and simultaneously reduced the peduncular outflow. Recordings from the same location were markedly enhanced when VPL rather than VL was stimulated. Although it is not possible to determine whether or not the same cells are involved when VL or VPL is stimulated, it is clear that dieldrin has selectively altered responses to one type of stimuli but not the other. This rules out any simple hypothesis that the excitability of cortical neurones is generally increased by dieldrin. Some other explanation must be sought to explain the observed results. Similar experiments with metrazol show that the same specificity is present. Neither the cortical response nor the peduncle response was greatly affected by slow infusion of metrazol. This confirms an earlier report (ZANCXETI‘Iand BROOKHART,1955) that metrazol does not alter pyramidal discharge to direct stimulation of the motor cortex. Although it is unlikely that the mechanism of action of dieldrin is based solely upon a generalized increase of excitability of cortical cells per se, there are at least two other possibilities to consider. It is possible that the amount of afferent drive impinging on cer-

Dieldrinand evokedresponses

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tain cortical systems via pathways directly stimulated is increased. Alternatively dieldrin may affect the activity of systems which act as modulators of cortical excitability and thus influence the cortex only indirectly. Indeed, all might occur simultaneously. The possibility that afferent processes may be affected is suggested by the observation that drugs similar to dieldrin such as DDT and metrazol display veratrinic activity in many peripheral systems (EYZAGLJIRRE and LILIENTHAL,1949; SHANKLAND,1964; NARAHASWand HAAS, 1967, 1968). If similar repetitive axonal discharges occur centrally, this process could serve to tremendously amplify an externally applied stimulus. The evaluation of macroelectrode responses as recorded in this study is not well suited for such an analysis, but it can be anticipated that veratrinic effects would increase the amplitude and particularly the duration of wave components involved. Close observation of the responses recorded from all regions examined in this study failed to demonstrate any changes in latency, amplitude or duration from aflerent systems that would fit such a hypothesis. The clearest example of this was obtained when responses to optic tract stimulation were considered. Tract responses and radiation responses are well separated from other components of the primary response in LG and cortex respectively. Neither were modified in latency or waveform at times when postsynaptic cortical components were 2--5 times greater than normal. Although more direct testing is now underway, these data do not provide evidence for a vcratrinic efkct within afferent systems. A similar exclusion of veratrinic action has been made for metrazol (BORYS and ESPLIN, 1969; DL‘NLAP,AUEY,KILLAMand BRAZIER, 1960). The great body of data dealing with the eflect of stimulating nonspecific and diffuse thalamic systems or of large areas of the reticular formation provide the basis for hypothesizing that many of the actions of dieldrin are due to effects at these sites. Conditioning stimuli applied to the ascending reticular activating system (ARAS) enhance cortical responses to sensory stimuli but depress cortical responses to VL stimulation. The ascending reticular activating system effects on evoked responses have been thoroughly reviewed by STERIADE(1970), and the reader is referred to this review for details. KL.E (1966) has demonstrated that motor cortex pyramidal cells are hyperpolarized by ARAS stimulation. Besides the facilitation of visual evoked responses with ARAS stimulation, stimulation of the pulvinar and lateral posterior nuclei of the thalamus have been shown to potentiate the postsynaptic components of the visual evoked response (MORILLO, 1961). The distribution of late responses and some of the characteristics of afterdischarges are similar to the recruiting responses obtained by stimulating the diffuse thalamic projection system (P~~R~~IRA,1959; JASPER, 1959). Processes other than stimulating subcortical regions may also potentiate cortical components of sensory evoked responses. Background retinal illumination and optic tract transection both result in enhancement (CHANG. 19.52;D~JMONTand DELL, 1960; STERIADE and DEMETRESCU, 1967). Transcallosal (AJMONL-MARSAN and MORILLO,1963) and direct stimulation (STERIADE,1964) of the striate cortex are also enhanced by background illumination. There are obvious difficulties in trying to relate the results of such experiments to the action of a convulsant drug, but the many common features mentioned raise the possibility that similar mechanisms may be at least partly operative for dieldrin. The similar effects of ARAS stimulation and of convulsant drugs on responses of sensory and motor cortical areas seem particularly significant. It is quite possible that the action of convulsive drugs such as dieldrin are related to changes in excitability of many various systems, and that

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the effects seen at the cortical level represent a complex action on many systems simultaneously. If this is true, both the interpretation of evoked waveforms and the elucidation of events leading up to the convulsive state are rendered quite difficult. One additional observation from our study was the unilateral depression of convulsive activity during continual stimulation of VL. This was the sole circumstance in our experiments in which unilateral seizures have been observed to occur with dieldrin or metrazol, and we have been unable to find previous reports of such a phenomenon. This finding may relate to reports of suppression of epileptiform discharge by cerebellar activity (Dow, 1965) since VL is a major relay station for cerebellocortical systems. Finally, the similarities between dieldrin and metrazol that we have previously reported (JOY, 1973) can be extended to include effects on evoked responses. The similarities are sufficient to propose that dieldrin and metrazol act by the same general mechanisms or at least that they share final common pathways responsible for a myriad of electrophysiological and pharmacological actions. Differences when encountered are most likely due to differences in mode of administration, onset and duration of action. or other pharmacokinetic properties. AcknoMllrdgrrlzmts-The author wishes to thank PATRICIA J. ANLXR~ONfor her technical, histological and artistic assistance. and G. PALJL MABRFY for his skillful technical assistance. The author appreciates the donation of dieldrin by Shell Development Company. REFERENCES AIMONE-MARSAN, C. and MORILLO, A. (1963). Callosal and specific response in the visual cortex of cat. Archs ital. Biol. 101: l-29. BANNA, N. R. and HAZBUN. J. (1969). Analysis of the convulsant action of pentylenetetrazol. Specialia 15: 382383.

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