Protective sensitivity changes of the motor cortex due to epileptiform experience of the visual cortex

In awake rats, experiments were performed to study early epileptiform events (interictal spike, myoclonic jerk) in relation to a) the strength of the convulsive stimulus, b) the site of the focus, and c) epileptiform experience. For this reason, Na-penicillin G (PCN) was injected either into the motor or visual cortex and, in a second test 2 weeks thereafter, into the motor cortex in all these rats. The median latencies of both the first interictal potential and jerk were independent of the applied PCN concentration in the range between 16-1,000 IU/0.5 microliter (90.5-113 s, and from 106-196 s, respectively), as well as from the injection site in the visual or motor cortex (potential: 80 vs. 69 s; jerk: 124 vs. 129 s, respectively). After epileptiform experience in the visual cortex, the latencies of the first potential and jerk were significantly (p < 0.05) increased compared to animals with an experience in the motor cortex (first potential: 100 vs. 66 s; first jerk: 159 vs. 116 s, respectively). The results show that a PCN focus in the visual cortex decreases the susceptibility of the motor cortex for the convulsant action of PCN. This means that an autoprotective mechanism is activated whose efficiency depends upon a close linkage between the visual and motor cortex.