Brain Research, 525 (1990) 215-224
Elsevier BRES 15774
The ontogeny of feline temporal lobe epilepsy: kindling a spontaneous seizure disorder in kittens Margaret N. Shouse 1'2, Alison King 1'2, James Langer 1'2, Tanya Vreeken 1'2, Ken King 1'2 and Melvyn Richkind 3 1Department of Anatomy and Cell Biology, UCLA School of Medicine, Los Angeles, CA 90024 (U.S.A.), 2Sleep Disturbance Research (151A3), VA Medical Center, Sepulveda, CA 91343 (U.S.A.) and 3Animal Research Facility, VA Medical Center, Sepulveda, CA 91343 (U.S.A.) (Accepted 20 February 1990)
Key words: Developmental epilepsy; Amygdala kindling; Kitten; Spontaneous seizure
We describe the ontogeny of feline temporal lobe epilepsy after amygdala kindling in 24 cats, aged 2.5 months to over 1 year. In so doing, we report the first experimental model of spontaneous epilepsy in immature animals. Preadolescent kittens (n = 12 ~ 6.5 months) are far more likely to develop epilepsy, indexed by spontaneous seizures, than are adult cats (n = 12 > 1 year). Moreover, youth accelerated the development of epilepsy. The younger the kitten at the beginning of kindling, the more probable and rapid the onset of spontaneous seizures. Failed postictal depression was the most reliable precursor of spontaneous seizures in immature cats. However, spontaneous epilepsy continued after postictal refractory periods stabilized and was still present when kittens matured to adulthood. Collectively, the results suggest that failed inhibition contributes to the onset of spontaneous epilepsy in immature animals but that other morphologic, physiological and/or chemical changes might sustain epilepsy afterwards.
Studies of developmental epilepsy in immature animals are rare in spite of the fact that the majority of human seizure disorders have their onset from infancy through adolescence11'14'45. Even human temporal lobe epilepsy is prone to early onset, although it can develop at any age for a variety of traumatic, symptomatic and hereditary reasons 14. This report demonstrates that experimental temporal lobe epilepsy, induced by amygdala kindling in cats, can also occur at any age but that formative changes induced by repetitive temporal lobe stimulation are most pronounced in immature cats. Specifically, preadolescent kittens are far more likely to develop epilepsy, indexed by spontaneous seizures, than are adult cats. Moreover, the younger the kitten at the beginning of kindling, the more probable and rapid the onset of spontaneous seizures. MATERIALS AND METHODS
Subjects Fifty-seven cats were procured for this study. The age distribution at procurement was: n = 12 neonates, n = 23 prepubertal kittens (2-6.5 months) and n = 12 adult cats (1-3 years) ~3. All unweaned kittens (~6 weeks) died before surgery due to an epidemic of upper respiratory disease for which specific-pathogen-free (SPF) mothers had not been inoculated. Accordingly, aseptic neurosurgery was
performed only on weaned kittens. Of the 36 operated kittens, 11 aged 5 months or less died within a week after surgery, 8 from thermoregulatory instability associated with anesthesia and 3 from upper respiratory disease.
Stereotaxic surgery Twenty-four cats (n = 12 kittens, 11 female and 1 male at 0.8-2.2 kg and n = 12 adults, 8 female and 3 male at 3.32-5.22 kg) survived stereotaxic surgery. Sodium pentobarbital anesthesia at 35 mg/kg, i.p. was used to implant electrodes for basolateral amygdala kindling and sleep-waking state evaluation 35. Brain atlases are available for cats 5.5 months of age at the beginning of kindling, as reported elsewhere 35. Threshold testing during sleep had to be discontinued in kittens younger than 5.5 months, as spontaneous seizures disrupted threshold procedures. Spontaneous seizures were documented during behavioral observation, chronic polygraphic recordings and/or split-screen video monitoring. Histology Amygdala electrode placements were verified either visually or histologically in 6 animals kindled as kittens and in all adults. The remaining kittens are still under observation. Data analysis Cats were divided into 3 groups according to age at initial AD to assess age-related differences in: (1) seizure induction during kindling; and (2) spontaneous seizure activity after kindling. For most parameters, age groups were: 2.5-5 months (young preadolescent n = 8), 5.5-6.5 months (older preadolescent, n = 4) and >1 year (adult cats, n = 12) 13. Simple analysis of variance compared indices of evoked or spontaneous kindled seizure activity in the 3 groups. Post-hoc tests were independent Student t-tests; statistical significance is denoted by asterisks on tables. Finally, Pearson product moment correlations were computed between indices to determine interactions between variables.
RESULTS E l e v e n of 12 k i t t e n s in this study were females, w h o reach p u b e r t y from 7 to 9 m o n t h s of age 13. A l l were p r e p u b e r t a l at initial A D ( 2 . 5 - 6 . 5 m o n t h s ) , a n d n o n e had r e a c h e d p u b e r t y by t h e e n d of k i n d l i n g , i n c l u d i n g the
Kindling development in kittens and adult cats: focal AD thresholds during and after kindling AD, afterdischarge. Values are means ± S.E.M. Range is given in parentheses.
Age at initial AD (months)
Initital A D AD threshold (mA )
Immediate post-kindling AD threshold (mA)
2-4 months post-kindling AD threshold (mA)
2.5-5.0 (n=8) 5.5-6.5 (n =4) >12 (n=12)
8.9±8.0* (0.5-20) 1.0±0.6 (0.8-1.1) 0.9±0.4 (0.2-1.5)
4.0±6.0* (0.4-18) 1.0±0.4 (0.3-1.25) 0.6±0.4 (0.2-1.4)
0.7±0.5** (0.4-1.5) 0.8±0.4 (0.3-1.3) 0.5±0.4 (0.2-1.0)
*P < 0.05 from older kittens and adults over I year. **n = 4 surviving kittens.
6 . 5 - m o n t h - o l d female. T h e o n l y m a l e in the study was 3.5 m o n t h s of age at initial A D a n d was d e a r l y p r e a d o l e s c e n t at the b e g i n n i n g a n d e n d of k i n d l i n g 13.
Kindling development (Table I) Table I has s o m e typical indices o f k i n d l i n g d e v e l o p m e n t p r e s e n t e d as a f u n c t i o n of age at initial A D . T h e y o u n g e s t k i t t e n s differed f r o m o l d e r k i t t e n s a n d adults b y h a v i n g e n o r m o u s variability o n n e a r l y all p a r a m e t e r s . In spite of sizable r a n g e s a n d s t a n d a r d d e v i a t i o n s , there were statistically significant differences o n a n u m b e r of variables. A D thresholds (Table Ia). Initial a n d i m m e d i a t e p o s t k i n d l i n g t h r e s h o l d s d i m i n i s h e s with age in that y o u n g e s t kittens h a d highest t h r e s h o l d s , o l d e r kittens were i n t e r m e d i a t e a n d adults lowest. O l d e r k i t t e n s were n o t , h o w e v e r , o u t s i d e t h e n o r m a t i v e r a n g e for adults (~12(n= 12)
Facility personnel during this interval. It should be noted that most of the kittens who survived long enough for reasonable follow-up had routine threshold tests (see Table IV). Consequently, it is not certain whether the maintenance of spontaneous seizures is dependent upon recently elicited convulsions. Table IV shows the number of evoked convulsions after kindling as a function of age and the development of spontaneous epilepsy. The number of evoked convulsions after kindling did not seem to figure prominently in the development of spontaneous epilepsy in young kittens or adults (Table IVa). Youngest kittens (~5 months; Table IVa,b). These kittens, especially the oldest one (5.5 months), had the longest postkindling delay before onset of spontaneous epilepsy (Table III) and significantly more evoked seizures before onset of spontaneous epilepsy than did the younger kittens (Table IVb, n = 7, r = 0.82, P < 0.01).
TABLE IVb Total number of elicited convulsions before and after the first spontaneous seizure in cats with spontaneous epilepsy
Values are means + S.E.M. Range is given in parentheses. Age at initial A D (months)
No. before 1st spontaneous seizure
No. after 1st spontaneous seizure
2.5-4.0 (n = 4)
5.7 ± 6 (1-13) 29.7 ± 10 (21-41)
4.3 ± 3 (2-9) 29.0 ± 16 (11-43)
5.0-6.5 (n = 3)
With the exception of a brief report of our preliminary findings 35, subcortical kindling in kittens has never been described before. There are two reports of cortical kindling in kittens. Visual cortex kindling evoked AD without clinical accompaniment in l-month-old kittens and adult cats; AD threshold were higher in kittens than adults 19, consistent with our finding in amygdala-kindled kittens. Motor cortex kindling was successful at 1 month, but kindling parameters were indistinguishable from 3-month-old kittens 7. We cannot comment on differences in subcortical kindling between weaned and unweaned kittens. However, there were clear age-related differences in amygdala kindling phenomena between prepubertal kittens and adults in our study, the most notable of which is the development of spontaneous seizures. This novel observation could stem from the well known susceptibility of limbic system to epilepsy. An experimental model of spontaneous seizures has not been described in any other immature species, including kindled rat pups 24. Detection of spontaneous seizures in rat pups may be complicated by group housing and the comparatively minor nature of kindled seizures when compared to kittens. High intensity stimulation during kindling in 4 kittens (/>15 mA) may have contributed to the development of feline epilepsy via local lesions, and subsequent status epilepticus could have figured in the maintenance of spontaneous seizures and/or the development of multifocal epilepsy via local and global cell loss ~7. These factors do not seem essential to onset or maintenance, however, as histology in one kitten with a history of non-terminal status and high intensity stimulation (40 mA) revealed no more gliosis around electrode tips than occurs in adult cats. More work has been published on developmental epilepsy in rodents than cats, using either kindling or convulsant drug models 3'2°-24'41. There are many similarities between our findings and the kindling literature on rodents 2° 24. In some ways, immature animals are less vulnerable to seizures than mature animals. For example, kindled kittens, like kindled rat pups 23, are resistant to focal seizures, as both have high focal A D thresholds which subside to adult levels with maturity. Moreover, young kittens, also like young rat pups 23, can manifest low voltage focal AD and slow propagation. On the other hand, several findings indicate that young animals are more vulnerable to seizures than older animals. Young kittens and rat pups 24 require fewer focal ADs to reach generalized seizure stages, suggesting rapid progression. Young animals also have more difficulty inhibiting seizures, evidenced by the absence of postictal refractory periods and increased incidence of status epilepticus 16'
22,35. Finally, kindled kittens and rat pups appear more 'epileptic' than adults 12'21, exemplified by more severe evoked seizures in rat pups 24 and by frequent, spontaneous seizures in kindled kittens. Several features of immature neurons might underlie these paradoxical, age-related differences in kindling phenomena, as recently reviewed by Moshe et al. 24. Immature neurons are thought to posses higher input resistance and fewer voltage-dependent channels 15'25 as well as incomplete dendritic branching and myelination 1' 5. These properties could explain the resistance to A D at the focus as well as the slow propagation of AD to other structures. On the other hand, there is evidence that brains of young animals contain a higher ratio of excitatory to inhibitory synapses 15'26'3°'31. Furthermore, neurotransmitters thought to inhibit seizure activity, notably norepinephrine (NE) and ),-amino-butyric acid ( G A B A ) , are less abundant in immature than mature brains 23'32'4°. These factors might contribute to the rapid transition from focal to generalized seizure stages during kindling, the absence of postictal depression and the propensity to status in kindled rat pups and kittens as well as to the development of multifocal interictal discharges and spontaneous seizures in kittens. The role of specific maturational changes in developmental kindling phenomena remains highly speculative. None of the above neuronal properties has been documented in amygdala neurons of kittens at the ages we studied, and few have been linked to limbic seizure susceptibility in immature organisms of any species 16. Even so, changes in cell physiology, synaptic morphology and neurotransmitter concentrations have been reported after kindling in adult rats and/or cats. Longterm potentiation 36,37 as well as increased mossy fiber
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