Excitability changes in thalamocortical relay neurons during synchronous discharges in cat neocortex.

Abstract

Intracellular events in thalamocortical relay (TCR) cells in VPL nucleus of cat thalamus were analyzed during and after focal electrical stimulation of the homotopic somatosensory cortex. During a 5-s, 50-HZ tetanic stimulation, the cortically evoked IPSPs [inhibitory post synaptic potential] in TCR [thalamocortical relay] cells were replaced by rapidly facilitating depolarizations, accompanied by the appearance of repetitive depolarizing afterpotentials and facilitation of sensory transmission. During the ensuing cortical afterdischarges, selective posttetanic potentiation of the corticothalamic pathway was accompanied by prolonged (about 200 ms), low-amplitude (10 mV) rhythmic depolarizations of TCR cells. During the early stage of the afterdischarges, a 2nd population of action potentials differing in amplitude and waveform appeared in brief bursts with regular interspike intervals of 3-4 ms. Axonal bursting did not occur following repetitive stimulation subthreshold for seizures, and disappeared within 10 s of the onset of continuous synchronous activity in cortex. Analysis of axonal burst patterns demonstrated nonlinear repetitive firing properties in CNS axons. The initiation of threshold alterations and repetitive firing in intracortical axons, as a result of cortical discharges produced in the absence of convulsant agents, indicates that axonal burst generation can be a general mechanism within the CNS for increasing excitatory synaptic drive and synchrony among neuronal aggregates under abnormal conditions of hyperexcitability, such as epilepsy.