Transcallosal response (TCR) in the chronic photosensitive baboon preparation, Papio papio. II. Effect of premotor cortical kindling

The chronological pattern of transcallosal response (TCR) and its recovery cycle were studied by single or paired stimulation of the homotopic primary and the secondary (contralateral) cortical site before, during and after primary site premotor cortical kindling in Papio papio. Prior to kindling, the sequential pattern of TCR was early positive (P1), negative (N1), late small positive (P2) and large negative (N2) components. The recovery cycle of both P1 and N1 showed a marked supernormal phase at an inter-stimulus interval of 15-200 msec with a peak at about 30 msec, showing maximal facilitation up to 300%. Kindling, with or without prior low frequency cortical stimulation, resulted in a long-lasting amplitude potentiation of both P1 and N1 and a significant modification of the late components. However, a transient amplitude reduction was noted immediately following completion of stage 5 kindling, presumably due to repeated generalized convulsions. These changes occurred only during primary (kindling), but not secondary, homotopic site stimulation. These findings suggest that a persistently increased callosal transmission is probably due to primary site kindling-induced functional alteration of the synaptic site at the homotopic secondary site cortex. In contrast, the supernormal phase of the TCR recovery cycle was suppressed significantly at the secondary site cortex. Since a similar suppressive effect was observed with a small dose of pentobarbital in naive baboons, kindling is considered to produce enhancement of the inhibitory mechanism. However, a diminished suppressive effect on the supernormality 20 days after the termination of kindling suggests that this suppressive effect is transient. It is probably related to the repeated kindled convulsive seizures rather than to the kindling process itself.