Kindling of the midbrain periaqueductal gray in rats

We investigated the role of midbrain periaqueductal gray matter (PAG) in the manifestation of generalized seizures by administering electrical stimulation to this area in rats. Electrical stimulation of 60 Hz biphasic square pulses of a 1-s duration administered to the PAG-induced convulsive responses in the following order: (1) Type I, running (stimulus intensity; range 50--200 microA, mean 73.1 microA) without afterdischarge (AD), (2) Type IIa, running (stimulus intensity; range 50--300 microA, mean 111.8 microA) with AD at the PAG and the amygdala (AMY), and (3) Type IIb, generalized tonic--clonic seizures (GTCS) (stimulus intensity; range 50--250 microA, mean 182.1 microA) with AD at the PAG and AMY. Twenty daily PAG stimulations at the non-GTCS inducing threshold failed to produce kindling. However, 20 daily PAG stimulations at the GTCS threshold produced progressive AD spread involving the motor cortex, and progressive changes in the behavioral seizure pattern. These findings indicate that the PAG can be effectively kindled. However, PAG kindling has no apparent influence on subsequent AMY kindling.