Cortical independence of type I and II barbiturate spindles

502

Electroencephalogruphy and Clinical Neurophysiology, 1980, 5 0 : 5 0 2 - - 5 0 8 © Elsevier/North-Holland Scientit'ic Publishers, Ltd.

CORTICAL INDEPENDENCE OF TYPE I AND II BARBITURATE SPINDLES J. E R I C J O R D A N 1 and S. TOM G R I C E

EEG Laboratory, VA Hospital, Jackson, Miss. 39216 (~LS.A.) (Accepted for publication : J u n e 17, 1980)

In 1961 Spencer and Brookhart defined two varieties of cortical spindles in the cerveau isol6 cat which were arbitrarily labeled type I and type II (Spencer and Brookhart 1961). They further showed by similarities of laminar profiles, unit-EEG correlations and surface properties that type I spindle waves were analogous to the augmenting response and type II to the recruiting response. This clearly indicated utilization of thalamo-cortical mechanisms for each type. Earlier work appears to support this view in part. Ralston and Ajmone Marsan (1956) produced several changes in cortical barbiturate spindles following thalamic penicillin injection. Several of their illustrations show what appear to be type II spindle waves following penicillin, whereas, control recordings show only type I waves. However, these changes were observed with both non-specific and specific nuclear application. Although cortical spindles induced by mesencephalic lesions may not be the same as barbiturate spindles (Ball et al. 1977; Spencer and Brookhart 1961), Ganes, Andersson and co-workers classified barbiturate spindles as type I or II in a series of more recent papers on barbiturate spindles (Andersson and Manson 1971; Andersson et al. 1971; Ganes and

' S u p p o r t e d by the Medical Research Service of the Veterans A d m i n i s t r a t i o n and BRSG Grant No. 5 S07 R R 0 5 3 8 6 awarded by the Biomedical S u p p o r t Grant Program Division of Re~earch Services, NIH. Presented in part at the 33rd annual meeting, American EEG Society, Atlanta, Ga., S e p t e m b e r 21, 1979.

Andersen 1975; Ganes 1976a, b). Their results confirmed a subcortical origin of type I spindles. However, type II spindles were felt to be entirely cortical in nature and triggered locally by type I spindles (Ganes m~d Andersen 1975). This latter conclusion was based on temporal relationships between the two spindle types plus the lack of correlation between thMamic ventroposterolateral (VPL) nucleus activity and type II spindles. Resolution of these different views is important since fundamental questions of basic cerebral electrophysiology are involved. For example, evidence exists that the isolated cortex is electrically silent (Burns 1950) and yet this is the generally accepted mechanism of suppression burst activity (Brenner et al. 1975). The more recent view (Ganes and Andersen 1975) can be challenged on theoretical grounds. The observed temporal relationship can be explained in several ways. For example, subcortical sites other than the VPL may provide such a link (Spencer and Brookhart 1961; Andersson et al. 1971; Skinner and Yingling 1977). On the other hand, no current evidence exists to directly refute Ganes and Andersen's (1975) view. During the course of other work, glaring exceptions to the recent data (Ganes and Andersen 1975) were repeatedly observed (Jordan et al. 1978). In the present paper our observations are documented using a modification of Ganes and Andersen's analysis methods. The results show a random relationship between type I and II spindles and suggest

BARBITURATE SPINDLES

t h a t t h e r e is no p a r t i c u l a r r e a s o n t o s u p p o s e t h a t t y p e II spindles are g e n e r a t e d b y the cort e x exclusively.

Methods D a t a w e r e c o l l e c t e d on 22 adult cats o f 3--6 kg. A n i m a l s were a n e s t h e t i z e d with 22 m g / k g o f k e t a m i n e . S u b d e r m a l (8 animals) or stainless steel screw (14 a n i m a l s ) e l e c t r o d e s were p l a c e d o v e r t h e lateral and cruciate gyri f o r E E G r e c o r d i n g . E E G r e c o r d i n g s were m a d e using an e l e c t r o d e p l a c e d in or o v e r the right f r o n t a l b o n e ( r e p r e s e n t i n g the s u p e r i o r a s p e c t o f t h e f r o n t a l sinus} as a r e f e r e n c e elect r o d e at a b a n d p a s s o f 1 - - 7 0 c/see. B a r b i t u r a t e spindles were i n d u c e d b y intrav e n o u s infusion o f e i t h e r p e n t o b a r b i t a l or t h i o p e n t a l . T h e doses ranged f r o m 18.3 to 1 4 2 . 8 m g / k g and were a d e q u a t e to m a i n t a i n anesthesia t h r o u g h o u t the e x p e r i m e n t . Correct e l e c t r o d e p l a c e m e n t a n d the lack o f cortical d a m a g e was verified at the t e r m i n a t i o n of eacb e x p e r i m e n t . Data amdysis was p e r f o r m e d on all t y p e II b a r b i t u r a t e spindles. T h e defining criteria o f each spindle t y p e as well as o t h e r d e f i n i t i o n s and scoring criteria used in t h e p r e s e n t s t u d y are given in T a b l e I. T h e s e were derived f r o m Ganes and Andersen (1975) with some modif i c a t i o n and e x p a n s i o n . T h e r a t i o n a l e f o r the choice o f m e a s u r e s is given in the Discussion section. G a n e s a n d A n d e r s e n ( 1 9 7 5 ) used the t e r m ' s t a r t interval' to m e a n the t i m e f r o m the e n d o f o n e spindle to the b e g i n n i n g o f t h e n e x t and studied o n l y t h e intervals f o r m e d w h e n a type I spindle was f o l l o w e d b y a type II. In the p r e s e n t s t u d y this interval is arbitrarily d e s i g n a t e d as a t y p e X start interval to distinguish it f r o m the o p p o s i t e s t a r t interval, the t i m e b e t w e e n the end o f a type H spindle and the start o f a type I, a r b i t r a r i l y labeled as a t y p e Y s t a r t interval. All s t a r t intervals involving a t y p e II spindle were c o n s i d e r e d in this study. A n u m e r i c a l score based o n t h e length o f

503 'FABLE I D e f i n i t i o n a n d s c o r i n g criteria u s e d in this s t u d y .

Abbreviations as follows: n = number of scores with common score value; nt = total number of scores. A score of zero was also given if the preceding or subsequent spindle was also a type II spindle. No score was given for type II waves imbedded in a type I spindle. S p i n d l e - - A b u r s t o f h i g h voltage a c t i v i t y c o n t a i n i n g m o r e t h a n 3 r h y t h m i c waves w h o s e a m p l i t u d e is m o r e t h a n twice t h e a d j a c e n t b a c k g r o u n d . T y p e I - - s p i n d l e s c o n s i s t i n g o f b i p h a s i c or t r i p h a s i c waves. T y p e II -- s p i n d l e s c o n s i s t i n g o f m ( m o p h a s i c waves. S p i n d l e i n d e x - total sec o f s p i n d l e a c t i v i t y / 6 0 sec of recording. Start interval -- t i m e b e t w e e n a t y p e II s p i n d l e a n d a n y p r e c e d i n g or s u b s e q u e n t spindle. T y p e X s t a r t interval - t h e t i m e b e t w e e n t h e start o f a t y p e II s p i n d l e a n d t h e e n d of a n y preceding spindle. T y p e Y s t a r t interval - - t h e t i m e b e t w e e n t h e e n d o f a t y p e II s p i n d l e a n d t h e start of a n y s u b s e q u e n t spindle. Start interval secure criteria: Slart-inlerval tim(" < 100 m s e e 101--400 msee 401--2000 msec 2 s e e - - 2 rain > 2 min

Score I,ah.' 4 3; 2 1 0

4 M e a n w e i g h t e d score .

.

0

.

(score value x n) . . . . . nt

s t a r t interval was assigned to each interval (Table I). T h e s e n u m b e r s were used to c o m p u t e a ' m e a n w e i g h t e d s c o r e ' (Table I) expressing the overall score d i s t r i b u t i o n f o r each cat, c o m p a r e t y p e X to t y p e Y s t a r t interval d a t a and c o n t r a s t the p r e s e n t results with G a n e s and A n d e r s e n ' s (1975} results.

Results E x a m p l e s o f t y p e s I and II spindles are s h o w n in Fig. 1. As c o m p a r e d with t y p e II, t y p e I waves were g r e a t e r in a m p l i t u d e , less

J.E. JORDAN, S.T. GRICE

504 TYPE I SPINDLES

TYPE lI SPINDLES

Fig. 1. Examples of the two types of barbiturate spindle activity from several different experiments. Spindles are underlined. Calibration marks refer to 100 pV and 1 sec.

well organized (in terms of a regular repetition rate and amplitude envelope) and biphasic. Type II waves were usually surface negative and appeared when other activity tended to be relatively more suppressed. In the lighter stages of anesthesia type II spindles were extremely rare. As anesthesia progressively deepened, they became progressively more frequent while type I spindles became more infrequent. The doses of barbiturate used were adequate to produce EEG suppression burst activity in 20 of the 22 cats. Type I spindles were frequently absent at this stage, whereas, type II were often abundant. Therefore, isolated type II spindles were noted in all cats. A brief instance of this is illustrated in Fig. 2. Another differential dose-related effect on spindles is shown in Fig. 2. Smaller fluctuations in spindle index (Table I) were superimposed on the more general trends described above. In lighter anesthesia, these fluctuations tended to mirror each other (i.e. an increase in type I spindle index was associated with an

increase in type II spindle index). In deep anesthesia, however, an increase in one index was associated with a decrease in the other. This effect was observed in 88% of the plots. Start-interval analysis was performed on 3325 intervals related to a type II spindle. The results are given in Table II. Here it can be seen that the vast majority of intervals received a zero score value. The data were recalculated in terms of percentage incidence (Table I) to eliminate variation related to the large differences in number of type II spindles between animals (range = 10--640). These data show the same result. Individual cat data closely paralleled grouped data. In 17 cats (77%) the incidence of zero scores was greater than any other score. In 13 (59%) zero scores represented an absolute majority of all scores. Conversely combined percentages of scores of 3--4 never represented the majority and exceeded the total of scores of 0, 1 and 2 only once. A mean weighted score representing the combined influence of score incidence and value was computed for each cat (Table I). The results are shown in Fig. 3. Mean weighted scores were less than 2 in all cats except one whose score was 2.03. Chi square comparison of type X and Y start-intervals revealed no significant differences. This was true for grouped as well as individual data. On the other hand, type X start interval scores were significantly (P < 0.05, linear regression analysis) correlated with the corresponding type I spindle index (representing type I spindle density) in 77% of cats. In another 10% the correlation just barely missed significance. Thus, the postulated (Ganes and Andersen 1975) strict temporal relationship between types I and II spindles could not be demonstrated in this study and, moreover, any temporal relationship that occurred was possibly a spurious consequence of a larger number of type I spindles. Ganes and Andersen's (1975) data were recalculated according to Table I and compared with the present results. Significant differences (P < 0.001, X2 with Yates and per-

BARBITURATE SPINDLES

505

CHANGESIN SPINDLE INDEX PENTOBARBITAL 16.7mg/kg 4;.

PENTOOARBITAL

PEIITOIIAROITAL

|. 3mg/kg

E.3q/Iql

4P"

4;I-

PREVIOUS PENTOBARBITAL

SUlqqlESSlOll BURST

IIL3q/Iql

....__,,TYPE X SPININ.E ~ TYPE ][ SIqlIOLE

.7

m.ll

11

w

I

It

I

.i 40

60

60

70

88 TIME IN MINUTES

II0

IN

110

120

Fig. 2. Changes in spindle o c c u r r e n c e over the course o f o n e e x p e r i m e n t . In lighter anesthesia (left) b o t h spindle indices increase in parallel and t y p e I p r e d o m i n a t e s . A f t e r s u b s e q u e n t injections t y p e II p r e d o m i n a t e s . In d e e p anesthesia (right) an increase in the t y p e II spindle i n d e x is associated with a decrease in the t y p e I index. A b r i e f p e r i o d o f isolated t y p e II spindles is s h o w n (right). Longer periods o c c u r r e d in o t h e r e x p e r i m e n t s .

T A B L E II S u m m a r y o f 3325 start-interval scores in 22 cats Interval

Score 0

1

2

3

4

2119 96.3 -+ 137.2 * 56.7 -+ 23.2

633 28.8 -+ 36.0 25.8 _+16.2

249 11.3 + 21.2 7.3 -+ 5.6

82 3.7 -+ 8.6 2.3 + 2.9

242 11.0 + 19.6 8.0 -+ 8.5

2141 97.3 + 138.7 57.0 + 23.7

576 26.2 _+ 34.4 23.2 + 12.4

275 12.5 -+ 23.4 8.2 -+ 6.8

72 3.3 _+6.3 2.6 -+ 3.0

262 11.9 -+ 18.6 9.2 -+ 9.5

Type X Total no. Mean no. per cat Mean % incidence ** per cat Type Y Total no. Mean no. per cat Mean % incidence per cat * Mean

+ 1 S.D.

** I n c i d e n c e = n u m b e r o f scores o f given value/total n u m b e r o f scores for each individual cat.

506

J.E. J O R D A N ,

DISTRIBUTION OF MEAN WEIGHTED SCORES

• :

TYPE X I N T E R V A L S

[:3 :

TYPE Y INTERVALS

: Z0(so°RE VALUE..)

(>025

MEAN

0 2 6 (15 c)51075 076 1 WEIGHTED

SCORE

I 125

12615

151 175

176 2 2 2 2 5

RANGE

Fig. 3. Distribution o f mean weighted scores calculated as shown for each type o f i n t e r v a l The score for the one cat shown at the extreme right was 2.03.

centage corrections) in the score distributions {percentage of intervals with a score of 3 or 4 vs. a score o f 0--2) and incidence of 3 or 4 scores were found. The lowest possible mean weighted score in Ganes and Andersen's (1975) results was 2.6, whereas, the highest found in the present results was 2.03.

Discussion Although evidence for a subcortical source of t y p e II spindles exists in the earlier literature (Spencer and B r ookhar t 1961; Ralston and Ajmone Marsan 1956), Ganes and Andersen (1975} postulated a purely cortical mechanism triggered by t ype I spindle activity. This view was based on the lack o f correlation between cortical t y p e II spindle waves and thalamic VPL activity and the fact that 80% o f the type II spindles occurred within 400 msec of a ty p e I spindle. This time between spindles th ey referred to as a 'start interval'. This study is an a t t e m p t to replicate Ganes and Andersen's {1975) findings. It was undertaken for two reasons: (1) repeated observations during o th er work {Jordan et al. 1978) suggested that type II spindles may be indep e n d e n t of ty p e I; (2) the findings have n o t been previously replicated. In order to accomplish this, analysis methods were chosen that closely paralleled Ganes and Andersen (1975). Ganes and Andersen

S.T. GRICE

{1975) considered a 'coupling relationship' to be present if the start interval was less than 400 msec and absent if it was more than 2 sec while a start interval o f less than 100 msec implied a 'simultaneous onset'. These intervals were used in the present study and assigned numerical values in order to quantitate the temporal relationship between spindles. For example, this allows the c o m p u t a t i o n of a mean weighted score (Table I) expressing the overall start interval data. Ganes and Andersen (1975) did not consider start-intervals of more than 4 sec, permitting a statistical test of the null hypot hesis. We chose instead to consider all start intervals, assigning a zero score to intervals greater than 2 min in duration. This extends the scoring scheme and permits a further expression of no temporal relationship. Zero scores were assigned to start intervals between two t ype II spindles since they imply no relationship between types I and II spindles. In order to test the null hypothesis we analyzed t w o start intervals -- the interval considered by Ganes and Andersen (1975) between the end of a t y p e I spindle and the start of a t y p e II spindle as well as the interval between the end of a type II and the start of a t ype I spindle. These we arbitrarily labeled as types X and Y start intervals, respectively. In terms of the measures of the present study, the Ganes and Andersen (1975) idea predicts several findings: (1) type II spindles c a n n o t occur in isolation; (2) type Y start intervals should be significantly longer than type X; (3) type II spindle index plots should mirror type I plots at all times; (4) 80% of all t ype X start intervals should receive a score of 3 or 4; and {5) the lowest possible mean weighted score for any cat can be 2.6. None o f these predictions was confirmed and when the present data were directly com pared with Ganes and Andersen's (1975) data the differences were significant. The reason for the discrepancy of results is u n k n o w n but there are several possibilities. A sampling error probably played a role since 3325 t ype II spindles were studied here while

BARBITURATE SPINDLES only 40 were studied previously (Ganes and Andersen 1975). Moreover, these 40 were collected in only one cat. Perhaps of greater importance is the depth of anesthesia. At lighter levels type II spindles were rare, did not occur in isolation and indeed did appear related to type I spindles on our index plots. Thus, had we restricted the analysis to these levels our results might have been in accord with previous results. However, the anesthetic level in the previous work was not specified (Ganes and Andersen 1975) and, therefore, this possibly cannot be pursued further. Finally, Ganes and Andersen (1975) studied intervals of 4 sec or less. This induces an automatic bias against zero scores and may help to explain their higher scores. The present results, especially the similarity of types X and Y intervals, suggest that any temporal relationship between types I and II spindles is randomly determined by the mere occurrence of each spindle type; in other words there is no real 'connection' between spindles. This postulate is based on the simple fact that the more type I spindles that occur in any time period containing a type II spindle the more likely it is that short start intervals will occur. This consideration applies equally to type X and Y start intervals but, since the interest of this study was in type X intervals, the postulate was tested by correlating the average of all type X interval scores against the a m o u n t of type I spindle (spindle index) in all 1 min periods. The significant correlations found in most cats support the postulate. This does not rule out the possibility of any connection between types I and II spindles. In fact our spindle index pl6ts (Fig. 2) suggest some relationship in light anesthesia which appears to be lost as anesthesia deepens. Perhaps both types of spindles may be independently but differentially related to the level of barbiturate. This in turn might be related to the biphasic effects of barbiturates on sensory transmission (King et al. 1957). Until now, the only reason to suspect a cortical source of type I! spindles was Ganes and

507 Andersen's (1975) start-interval data. The present results fail to confirm any definite time relationship between the two spindle types. Thus, we must conclude that no evidence for a cortical type II spindle source exists. Further work using the same approach as Ganes and Andersen (1975) but studying other deep areas such as the thalamic non-specific nuclei (Spencer and Brookhart 1961) may be helpful in addressing this issue more directly. The importance of the present results is in the contribution they offer toward the eventual understanding of the mechanisms of spindle production. This, in turn, is important because fundamental questions regarding EEG generation are involved. Moreover, spindle waves may play a role in seizure discharge production (Gloor et al. 1979). We hope further work will enable us to understand these spindles more fully.

Summary Two varieties of barbiturate-induced EEG spindles are recognized. Type I originates subcortically but recent data suggest that type II spindles are triggered by type I at the cortical level. This is based on a temporal relationship between the two spindle types in the gross electrocorticogram. In the present study no such relationship could be found except possibly in light anesthesia. These results fail to confirm the only data supporting a cortical mechanism underlying type II spindles.

R6sum6 Indgpendance corticale des fuseaux de type I et H induits par les barbituriques Deux sortes de fuseaux EEG induits par les barbituriques sont reconnus. Le type I prend son origine dans les structures sous-corticales mais des donndes rdcentes sugg~rent que les fuseaux de type II sont d6clench6s par ceux

508 d u t y p e I a u n i v e a u c o r t i c a l . C e c i e s t bas~ s u r u n e r e l a t i o n t e m p o r e l l e e n t r e les d e u x t y p e s de fuseaux dans l'~lectrocorticogramme. Dans l'~tude actuelle, aucune relation de cette sorte n'a pu ~tre trouv~e except~ peut ~tre au cours d e l ' a n e s t h ~ s i e l~g~re. Ces r ~ s u l t a t s n e c o n f i r m e n t p a s les d o n n ~ e s q u i s o n t e n f a v e u r d ' u n e o r i g i n e c o r t i c a l e d e s f u s e a u x d e t y p e II.

References Andersson, S.A. and Manson, J.R. Rhythmic activity in the thalamus of the unanesthetized decorticate cat. Electroenceph. clin. Neurophysioi., 1971, 31: 21--34. Andersson, S.A., Holmgren, E. and Manson, J.R. Synchronization and desynchronization in the thalamus of the unanesthetized decorticate cat. Electroenceph, clin. Neurophysiol., 1971, 31: 335--345. Ball, G.J., Gloor, P. and Thompson, C.J. Computed unit-EEG correlations and laminar profiles of spindle waves in the electroencephalogram of cats. Electroenceph. clin. Neurophysiol., 1977, 43: 330--345. Brenner, R.P., Schwartzman, R.J. and Richey, E.T. Prognostic significance of episodic low amplitude or relatively isoelectric EEG patterns. Dis. herr. Syst., 1975, 36: 582--587. Burns, B.D. Some properties of the cat's isolated cerebral cortex. J. Physiol. (Lond.), 1950, 11: 5 0 - 6 8 . Ganes, T. Barbiturate spindle activity in the thalamic lateral ventroposterior nucleus and the second

J.E. JORDAN, S.T. GRICE somatosensory area of the cortex. Brain Res., 1975; 98: 473--485. Ganes, T. Spindle wave synchrony in the somatosensory cortex of the cat. Brain Res., 1976a, 101: 11--22. Ganes, T. Barbiturate spindle activity in the association cortex of the cat and its relation to spindle activity in the somatosensory system. Brain Res., 1976b, 101: 23--35. Ganes, T. and Andersen, P. Barbiturate spindle activity in functionally corresponding thalamic and cortical somatosensory areas in the cat. Brain Res., 1975, 98: 457.--472. Gloor, P., Pellegrini, A. and Kostopoulos, G.K. Effects of changes in cortical excitability upon the epileptic bursts in generalized penicillin epilepsy of the cat. Electroenceph. clin. Neurophysiol., 1979, 46: 274--289. Jordan, J.E., Mikiten, T.M. and Grice, S.T. Varieties of barbiturate-induced suppression burst activity in the cat. Exp. Neurol., 1978, 61: 121--135. King, E.E., Naquet, R. and Magoun, H.W. Alterations in somatic afferent transmission through the thalamus by central mechanisms and barbiturates. J. Pharmacol. exp. Ther., 1957, 119: 48--63. Ralston, R. and Ajmone Marsan, C. Thalamic control of certain normal and abnormal cortical rhythms. Electroenceph. clin. Neurophysiol., 1956, 8: 559--582. Skinner, J.E. and Yingling, C.D. Central gating mechanisms that regulate event-related potentials and behavior. Prog. clin. Neurophysiol., 1977, 1: 30--69. Spencer, W.A. and Brookhart, J.M. A study of spontaneous spindle waves in sensorimotor cortex of cat. J. Neurophysiol., 1961, 24: 50--65.