Midline thalamic lesion and feline amygdaloid kindling. II. Effect of lesion placement upon completion of primary site kindling.

Lesion placement in the massa intermedia (MI) prior to amygdaloid (AM) kindling distinctively modifies the pattern of transhemispheric ictal transmission to one similar to that of forebrain-bisected cats, and eliminates the positive transfer effect (Hiyoshi and Wada 1988). Since the mechanism of AM seizure development is not the same as that for maintenance of AM-kindled seizure susceptibility, the effect of MI lesioning on established primary site AM-kindled seizures and secondary site AM kindling was studied in 8 cats. Upon the first primary site AM restimulation following MI lesioning, all animals responded with established bisymmetrical generalized tonic-clonic convulsions. Subsequent secondary site AM kindling showed (1) a distinctively modified seizure pattern identical to that observed during primary site kindling in cats with 'prior-to-kindling' MI lesions, and (2) complete absence of positive transfer effects. At primary site retest, the animals again responded with an established bisymmetrical convulsion and a marked interference effect (negative aftereffect) was noted. It is concluded that (1) the positive transfer effect at the contralateral homotopic AM is primary site hemisphere dependent, (2) the MI is critically involved in this effect, (3) the MI is not critical for maintaining the bisymmetrical pattern of AM-kindled convulsions, and (4) the mechanisms underlying the transhemispheric positive and negative transfer effects are different.