Journal of Neurophysiology, Vol 38, Issue 2 250-256, Copyright © 1975 by APS
Abnormal neuronal responses during evolution of a penicillin epileptic focus in cat visual cortex
J. S. Ebersole and R. A. Levine
After defining the receptive fields of single units in cortical area 17 of
anesthetized cats, recurrent on-off stimulation with bars of light of
optimal configuration win from a second micropipette; Progressively, three
distinct alterations of neuronal activity developed. The most longlasting
and usually the earliest abnormality was an increase in the number and
frequency of spikes comprising a neuron's response to stimuli that were
effective prior to iontophoresis. This enhanced physiologic response (EPR)
could be elicited from a cell independently of the discharge activity of an
induced focus, but only with stimuli appropriate for the cell's receptive
field. With additional iontophoresis an entirely new response developed,
which was consistent with an extracellular paroxysmal depolarization shift
(PDS). This high-frequency burst of spikes appeared only in association
with an ECoG interictal potential. It could be triggered, however, by
stimuli which were previously effective or ineffective, as well as occur
spontaneously. Characteristics which further distinguished the PDS from EPR
included a longer and more-variable latency, a longer recovery period, and
a different sensitivity to changes of stimulus intensity. A period of
response inhibition also accompanied each interictal potential and
persisted with a variable duration afterward. It was most noticeable as an
interruption in the activity of tonically responding neurons and was often
present before the cell began to generate PDSs. It was concluded that the
EPR represents a direct effect of penicillin on the cell or its immediate
synaptic connections, while the PDS appears dependent on the altered
interactions within a population of such affected cells. The inhibitory
phenomenon, in addition, seems a result of projected influences from cells
more fully involved with the developing focus. A dynamic model of the
EPR-PDS relationship is proposed.
