PHYSIOLOGY AND PHARMACOLOGY OF CORTICOTHALAMIC STIMULATION-EVOKED RESPONSES
IN RAT SOMATOSENSORY THALAMIC NEURONS IN VITRO.
Chang-Qing Kao and Douglas A. Coulter.
Department of Neurology, Medical College of Virginia, and the MCV
Comprehensive Epilepsy Institute of Virginia Commonwealth University,
Richmond, VA. 23298-0599.
APStracts 4:0034N, 1997.
ABSTRACT
Whole-cell current- and voltage-clamp recording techniques were employed in a
rat thalamocortical slice preparation to characterize corticothalamic
stimulation-evoked responses in thalamic neurons. Three types of
corticothalamic stimulation-evoked responses were observed in thalamic
neurons. Fifty seven percent of thalamic neurons responded to corticothalamic
stimulation with purely excitatory synaptic responses, while 27% had
inhibitory synaptic responses, and 16% had mixed excitatory/inhibitory
responses. This suggested corticothalamic activation of multiple distinct
synaptic circuits, presumably involving both nucleus reticularis thalami (NRT)
and thalamus, since the rat ventrobasal (VB) complex is virtually devoid of
GABAergic interneurons. Corticothalamic stimulation-evoked excitatory
postsynaptic currents (EPSCs) were predominantly slow-rising currents which
showed non-linear voltage-dependence, characteristics of an NMDA receptor-
mediated synaptic current. These slow rising EPSCs were blocked by the NMDA
antagonist 2-amino-5-phosphonovaleric acid (APV). A minority of
corticothalamic EPSCs had faster kinetics, and were blocked by 6-cyano-7
nitroquinoxaline-2,3-dione (CNQX). Varying frequency corticothalamic
stimulation optimally activated burst responses in thalamic neurons at low
frequencies (3-6 Hz). The optimal 3-6 Hz response was reduced by ethosuximide,
APV, and by detaching the neocortex from the thalamocortical slice, suggesting
that T current, NMDA receptors, and neocortical properties all contributed to
generation of this 3-6 Hz frequency preference. In contrast to
corticothalamic EPSCs, medial thalamic stimulation-evoked responses consisted
of fast CNQX-sensitive EPSCs that were predominantly voltage-insensitive, with
no 3-6 Hz frequency preference. In thalamic neurons in which corticothalamic
stimulation evoked predominantly inhibitory synaptic responses, this IPSP had
early and late phases, often followed by a rebound burst. The early IPSP
reversed at -95 mV and was bicuculline sensitive, while the late IPSP reversed
at -113 mV, and was blocked by the GABAB antagonist CGP-55845A. In thalamic
neurons in which corticothalamic stimulation evoked a mixed EPSP/IPSP
response, repetitive corticothalamic stimulation rapidly reduced IPSPs, and
enhanced EPSPs at higher frequencies. This resulted in burst firing being
triggered in these mixed response neurons at frequencies >6 Hz.
Received 8 November 1996 accepted in final form 9 Jannuary 1997.
APS Manuscript Number J889-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997