Two types of electrocortical paroxysms in an inbred strain of rats

Neuroscience Letters, 70 (1986) 393-397 Elsevier ScientificPublishers Ireland Ltd.

393

NSL 04198

Two types of electrocortical paroxysms in an inbred strain of rats E.L.J.M. van Luijtelaar and A.M.L. Coenen

Department of Psychology. University of Nijmegen, Nijrnegen (The Netherlands) (Received 9 September 1985; Revised version received and accepted 7 July 1986)

Key words'." EEG paroxysm -- Spike wave complex - - Petit mal epilepsy-- Rat - - Sleep - Wakefulness In 10 subjects of a particular inbred strain of rats (WAG/Rij), two types of spike-wave complexes were detected in the cortical electroencephalogram (EEG) during the two recording hours. The first type, already well-known, occurred in 8 of the I0 rats. This type was seen both during sleep and wakefulness and was accompanied by behavioural concomitants, reminiscent of human petit-mal epilepsy. A mean of 18 discharges/h was found. The second type, a novel one with a differential shape, occurred in 7 rats with a mean number of 14 discharges/h. In contrast to the first one, this spike-wave complex was not accompanied by behavioural correlates and was almost exclusivelypresent during wakefulness. Four reasons make this strain of rats attractive as an additional animal model for epileptogenic activity: the occurrence of two types of spike wave complexes, the high percentage of animals showing these phenomena, the genetic constitution of these rats and the fact that they are commerciallyavailable.

While s t u d y i n g the sleep wake characteristics of a particular inbred strain of rats ( W A G / R i j ) [2], we discovered a b e r r a n t discharges s p o n t a n e o u s l y occurring in the cortical e l e c t r o e n c e p h a l o g r a m (EEG). It soon became evident that these discharges appeared in a high percentage o f rats. Several, but n o t all, of these paroxysms strongly resembled the often described s p i k e - w a v e complexes which are regarded as epileptogenic [I, 3, 5, 6, 8]. The purpose of the present paper is to describe the aberrant E E G p h e n o m e n a , to estimate their incidence, to establish an eventual relationship with the states of vigilance a n d to investigate whether the electrophysiological p h e n o m e n a are a c c o m p a n i e d by changes in o n g o i n g behaviour. In general, it was d e t e r m i n e d whether or not this strain of rats could serve as an a d d i t i o n a l a n i m a l model for epileptogenic activity. Ten male rats (age 4-28 m o n t h s ; weight 260-360 g) o f the W A G / R i j strain [2], b o r n and raised in o u r laboratory, served as subjects. Ancestors were purchased from the R E P G O - l n s t i t u t e o f T N O at Rijswijk, The Netherlands. Rats were m a i n t a i n e d on a 12-12 h l i g h t - d a r k regime with lights on at 02.30 h. A n i m a l s were chronically provided with s t a n d a r d E E G (Plastic P r o d u c t s C o m p a n y , MS 333/2-A) a n d electrom y o g r a m ( E M G ) electrodes (MS 303/71) u n d e r complete N e m b u t a l anesthesia [7].

('orrespondence." E.L.J.M. van Luijtelaar, Department of Psychology, University of Nijmegen, P.O. Box 9104, 6500 H E Nijmegen. The Netherlands. 0304-3940;86iS 03.50 O 1986 ElsevierScientificPublishers Ireland Ltd.

394

EEG electrodes were placed in the cortex: one in the frontal region (coordinates with skull surface flat and bregma zero-zero: A 2.0, L 3.5) and a second one in the parietal region (A 6.0, L 4.0). The earth electrode was placed in the cerebellum. The E M G electrodes were subcutaneously placed on the dorsal neck muscles. Following surgery. a 10-day recovery and experimental habituation, EEG (registered between I and, to include also high frequencies, 700 Hz) and E M G (registered between 27 and 700 Hz) recordings were made during I h in the light (13.00 14.00 h) and I h in the dark ( 15.00-16.00 h). At the same time, rats were closely observed through a window. t-:or detailed behavioural observations, videorecordings were made. To allow simultaneous analysis of EEG and behaviour, the frequency transformed EEG was also recorded on the videorecorder. Two different categorics of aberrant discharges could bc distinguished in thc cortical EEG. The first category consists of spike- wave complexes, also dcscribed by othi

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Fig. I. E x a m p l e s o f type 1 spike wave complexes of 5 rats at two different recording speeds. The n u m b e r s on the left arc the c o d e - n u m b e r s of the rats. Time and voltage are calibrated.

395 er researchers [1, 3, 5, 6, 8]. A discharge of this type lasts at least 1 s and is characterized by a train of sharp spikes and slow waves. The spikes are directed upwards with an amplitude (mean 300/tV; range IIXr-450 ,uV) of at least twice the background EEG activity. The spike-wave complexes are asymmetric, the repetition of spikes within a burst varies from 7.5 to 9.5 Hz with a mean frequency o f 8.7 Hz. The mean duration of the complexes is about 5 s (range 1-30 s) whereas the mean number of discharges per hour is 18 (range 4-33). From the 10 rats investigated, 8 showed these complexes (an example of 5 of these rats is presented in Fig. 1). This high incidence was confirmed in a follow up study in which I I out of 12 individuals had spike-wave complexes; the mean number of discharges per hour was 21, while the mean duration was 3.2 s. The second category of deviating E E G patterns is a type of spike-wave complex which, to our knowledge, has not been described so far. They were found in 7 of the 10 rats and examples are presented in Fig. 2. In comparison to the above-

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396 mentioned discharges, the polarity o f the spikes is opposite and they appear to havc another position to the waves. Furthermore, the amplitude o f the spikes is smaller (mean 150/aV; range 80- 215/tV). Discharges o f this type are classified as such when they last at least 0.5 s and have a spike amplitude o f minimally twice the background activity. In general, the duration o f the bursts is 0.8 s (range 0.5-2.0 s); the spikefrcquency is 8.0 Hz (range 7.6-8.3 Hz) and the mean n u m b e r o f discharges is 14/h (range 4--17). Aberrant E E G p h e n o m e n a not meeting the requirements o f both types of spike--wave complexes and sometimes difficult to distinguish from sleep spindles, were not considered at further length in this study. In order to establish the behavioural states in which the aberrant paroxysms took place, wakefulness, slow wave sleep and paradoxical sleep were discriminated by means o f electrophysiological registrations [7]. This was supported by close behavioural observations. Moreover, it was assessed whether the behavioural state changed directly following E E G paroxysms. Whereas spike wave complexes o f the first type occurred in 60% o f all cases during slow wave sleep (here used to signify all non-rapid eye movement sleep), nearly 80% o f them were directly followed by a transition to wakefulness. The remaining 40% o f these bursts took place when the animals were awakc. The occurrence ot" the second type o f spike wave complex is in contrast to this. A b o u t 90% o f these complexes appeared during wakefulness and only 10% during slow wave sleep. Dcviating E E G phenomena were found in all investigated rats. Five animals showed both types o f spike-wave discharges, three only the first type and two only the second type. No aberrant p h e n o m e n a were found during rapid eye movement sleep. In addition to electrophysiological parameters, behavioural indices were also investigated. In 5 subjects, several characteristics were studied during 15 spike-wave complexes o f the first type, occurring during all states o f vigilance. Results arc presented in Table !. In all cases, rats were lying or sitting immobile at the onset o f a discharge. After a latency o f 0.5-1.0 s, behavioural p h e n o m e n a became visible, usually starting with a small muscle shock, followed by head tilting, acceleration o f breathing, and sometimes an incidental eye twitch. When sleep preceded the spike rAB[.I! I StJRVEY OF BEHAVI()I,JRAL CORRELATES OC(.'URRIN() I)URING 15 SPIKE WAVE COMPI.EXES OF TYPE I OF 5 INDIVIDUAL RATS rhe latency (s" mean _+_S.I).) is taken from the onset of the spike wave complex, v,'hile the other numbers represent the number of times (max. 15) that the behavioural correlate was observed l.atency to behavioural changes

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wave c o m p l e x , in 80% o f the cases it is followed by a w a k e n i n g a c c o m p a n i e d by an a b r u p t finishing o f the discharge. Hence, the question arises w h e t h e r the rats a w a k e themselves, e.g. by the i n v o l u n t a r y g e n e r a t e d m o v e m e n t s . N o t e w o r t h y is that behavioural p h e n o m e n a a c c o m p a n y i n g a s p i k e - w a v e c o m p l e x d u r i n g sleep c o u l d not always be observed. In these cases, sleep was then c o n t i n u e d a n d no a w a k e n i n g t o o k place. In c o n t r a s t to the b e h a v i o u r a l changes, o b s e r v a b l e d u r i n g a spike- wave c o m plex o f the first type, there was no i n d i c a t i o n o f such changes when a c o m p l e x o f the second type t o o k place. S o m e t i m e s active b e h a v i o u r , such as g r o o m i n g , continued. W h e t h e r a n d h o w the two types o f spike wave c o m p l e x e s are related to one a n o t h er is as yet unclear. A possibility is t h a t their origin is similar but that the second c o m p l e x is a ' w e a k e r ' form o f the m o r e d e v e l o p e d first type. The d u r a t i o n o f the second form might be too s h o r t to express the b e h a v i o u r a l correlates. G i v e n the differential characteristics o f the two E E G p a r o x y s m s , it is also possible that the disc h a r g i n g activity is m o r e restricted than the m o r e generalized discharges o f the first c o m p l e x a n d does not involve b r a i n a r e a s responsible for the a c c o m p a n y i n g m o t o r p h e n o m e n a . F u r t h e r n e u r o p h y s i o l o g i c a l research is necessary to clarify this point. A l t h o u g h therc is little d o u b t that the second type o f s p i k e - w a v e c o m p l e x is an aberrant E E G activity, the fact that the b e h a v i o u r a i correlates are not present, means that this p h e n o m e n o n c a n n o t bc c o n s i d e r e d as an epileptic a t t a c k [4]. In c o n t r a s t with this, b o t h the e l e c t r o p h y s i o l o g i c a l a n d the b e h a v i o u r a l characteristics o f the s p i k e wave c o m p l e x o f the first type are u n d o u b t e d l y related to p h e n o m e n a o f the petit-real type described by others [1, 3, 5, 6, 8]. Thc W A G / R i j strain has been i n b r e d for at least 100 g e n e r a t i o n s [2], which m a k e s rats o f this strain a l m o s t h o m o z y g o u s . This offers some a d v a n t a g e s for genetic a n a lyses. F u r t h e r m o r e , a n i m a l s o f this strain b e h a v e n a t u r a l l y , have regular s l e e p - w a k e patterns, are easy to handle a n d are c o m m e r c i a l l y available. In all, this m a k e s this p a r t i c u l a r strain highly a t t r a c t i v e as an a d d i t i o n a l a n i m a l m o d e l for petit-mal epilepsy. W e greatly a c k n o w l e d g e the creative a n d enthusiastic c o n t r i b u t i o n o f K a r i n K r o m m e n h o e k a n d the skilled biotechnical assistance o f Thijs Janssen. I Chocholovfi, L. and Kolinovfi, M., Rhythmic activity of the awake phase in the rat, Activ. Nerv. Sup. (Praha), 21 (1979) 36 38. 2 Fcsting, M.F.W., Inbred Strains in Biomedical Research, MacMillan Press, London, 1979, pp. 267-296. 3 Kleinlogcl, H., Spontaneous EEG paroxysms in the rat: effects of psychotropic and alpha-adrenergic agents. Neuropsychobiology, 13 (1985) 206-213. 4 L6schcr, W., Genetic animal models of epilepsy as unique resource for thc evaluation of anticonvulsant drugs. A revicw, Meth. Find. Exp. Clin. Pharmacol., 6 (1984) 531--547. 5 Marescaux, C., Vergnes, M., Micheletti, G., Depaulis, A, Reis, J., Rumbach, L., Warter, J.M. and Kurtz, D, Une forme g,~n&ique d'absences petit real chez le rat wistar. Rev. Neurol. (Paris), 140 (1984) 63 66. 6 Robinson, P.F. and Gilmore, S.A., Spontaneous generalized spike wave diseharges in the electrocorticograms of albino rats, Brain Res., 201 (1980) 452 458. 7 Van Luijtelaar, E.L.J.M. and Coenen, A.M.L., An EEG averaging technique for automated sleep-wake stage identification in the rat, Physiol. Behav., 33 (1984) 837 841. 8 Vcrgncs, M., Marescaux, C., Micheletti, G., Reis, J., Depaulis, A., Rumbach, I. and Warier, J.M., Spontaneous paroxysmal electroclinical patterns in the rat: a model of generalized non-convulsive epilepsy, Neurosci. Lctt., 33 (1982) 97 101.