The Journal of Neuroscience, 0000, 20:RC100:1-5
RAPID COMMUNICATION
Inhibition Suppresses Transmission of Tonic Vibrissa-Evoked
Activity in the Rat Ventrobasal Thalamus
Jed A.
Hartings and
Daniel J.
Simons
Department of Neurobiology, University of Pittsburgh, Pittsburgh,
Pennsylvania 15261
Previous studies have demonstrated that tonic responses of
trigeminal ganglion neurons to maintained whisker deflections are transformed to mainly phasic responses in thalamocortical neurons. The
high tonic responsiveness of thalamic reticular neurons suggests that
thalamic inhibition may contribute to this suppression of tonic
activity. To test this hypothesis we recorded responses of
thalamocortical neurons in the ventroposterior medial (VPm) nucleus to
200 and 400 msec sustained whisker deflections during simultaneous
microiontophoresis of the GABA receptor antagonists bicuculline
and phaclofen. Under control conditions, VPm units responded to
deflection plateaus with mean activities of only 18 spikes/sec,
compared with 16 spikes/sec spontaneous firing. A minority of cells
(5/19) had significantly greater plateau than spontaneous activity, and
these cells were classified as tonic; the other 14/19 were considered
phasic. Under GABA receptor antagonism, however, mean plateau activity
increased to 53 spikes/sec compared with 30 spikes/sec spontaneous
activity, and 7 of the 14 phasic units became tonically responsive.
Increases in plateau activity were significantly greater, by both
absolute and relative measures, than increases in spontaneous activity.
Transient responses to stimulus onsets and offsets also increased in
magnitude 4.0- and 2.9-fold, attributable mainly to their increased
duration. These data indicate that VPm neurons receive tonic excitatory
inputs that under normal conditions are masked by inhibition.
Suppression of tonic activity in VPm by inhibitory thalamic reticular
neurons may reduce tonic inhibition in cortical layer IV circuits,
preserving their responsiveness to transient signals.
Key words:
whisker; thalamocortical; reticular nucleus; inhibition; microiontophoresis; bicuculline
Copyright © 0000 Society for Neuroscience 0270-6474/00/$05.00/0
Previous Article | Next Article
