Altered neuronal excitability accompanying experimental prevention of supersensitivity in undercut cortex.

Cerebral cortex, partially neuronally isolated by undercutting, undergoes changes resulting in decreased threshold for, and increased duration of, afterdlscharge ehmted by surface electrical stimulation3, a4 This phenomenon has been called supersensmvlty and can be prevented in most cases by daily sessions of brain stmlulatlon begun shortly after the undercutting operation a' The absence of supersensmwty to test snmuh persists after chronic stimulation has been d~scontlnued p Neuronal excitability in undercut cortex can be evaluated by determining the effect of surface electrmal stimulation upon postsynaptlc or neuronal spike potentmls 7 9 W e report here a study of neuronal response patterns to surface stlmulatmn in intact, undercut supersensitive and undercut non-supersensmve cortex and discuss possible mechamsms for the exmtabdlty changes observed The methods are described in detail elsewhere 1~' Briefly, the marginal gyrus of adult cat cortex was undercut on one s~de, 3-4 mm below the surface Ammals which were to receive long-term electrical brain stlmulatmn also had platinum w~re electrodes ~mplanted shghtly wxthm the undercut cortex and a pa~r of electrocomcogram electrodes placed on the dura Daily brain stmaulatmn of twenty 2-sec trams was g~ven for an average of 9 weeks The intensity of stimulation was below the threshold for ehcltatlon of an afterdlscharge At least one week following cessation of long-term stimulation, a terminal acute experiment was performed For this, most of the ammals were anesthetized w~th chloralose, others were prepared under a short lasting barbiturate Cut edges and pressure points were locally anesthetized and lmmobdlzatmn was achieved with gallamme trmthlodlde (Flaxedll) Using bipolar surface elecmcal stlmulatmn, each cat was tested for supersensmwty of the undercut cortex in comparison with contralateral cortex Extracellular umt studies were then made m undercut and contralateral cortex, comparing histograms generated by at least 50 surface stimulations at a frequency of less than l/sec Cells without injury discharges were studied for 10-45 mm and at least two postst~mulus h~stograms and two spontaneous d~scharge