CEREBELLAR AND MESENCEPHALIC INFLUENCE ON BULBAR PENICILLIN-G EPILEPTOGENESIS IN RATS

The influence of the cerebellum and mesencephalon on epileptic bulbar discharge induced by topical application of penicillin-G on the floor of the IVth ventricle was analyzed in rats. Bulbar multiunit activity was recorded at different depths. The animals were divided into two ma in groups: totally cerebellectomized rats (Group I) and lobus anterior cerebellectomized rats (Group II). Each main group was further subdiv ided into two subgroups: animals with intact mesencephalon and animals with transected mesencephalon. In Group I: the total cerebellectomy, in intact mesencephalic rats (first subgroup) induced a sudden disappe arance of bulbar epileptic discharge. The mid-collicular transection ( second subgroup) produced the immediate disappearance of bulbar paroxy sms and the total cerebellectomy, subsequently performed, further decr eased the spontaneous firing rate. In Group II: (first subgroup) the l obus anterior ablation in rats with intact mesencephalon, significantl y enhanced me paroxysmal discharge. In the second subgroup, where the midcollicular transection had provoked the disappearance of bulbar par oxysms, the lobus anterior ablation induced the immediate reappearance of the paroxysmal activity. The penicillin-G epileptogenic activity s howed a different intensity at different depths in the bulb with a max imum intensity at the level of the vestibular nuclei. In conclusion, t he present study shows that both the mesencephalon and the cerebellum have a facilitating influence on bulbar epileptic discharge induced by the topical application of the GABA antagonist. However, not all the cerebellum has a facilitating effect, because the anterior lobus was f ound to have an inhibitory influence on bulbar discharge. The higher r esponsiveness to the paroxysmal action of GABA antagonists found at th e level of the vestibular nuclei may be due to an increased number of GABA receptors present in the vestibular neuron membrane, compared to those in the membrane of the remaining bulbar cells.