Abstract
It is well known that the intermediate layer (SGI) of the mammalian superior colliculus
(SC) receives cholinergic inputs originating from the pedunculopontine tegmental nucleus
(PPTN). The action of the cholinergic input on the SGI neurons was investigated using
whole-cell patch-clamp recording technique in slice preparations obtained from rats.
Application of acetylcholine (ACh) induced fast inward currents mediated by nicotinic
ACh receptors in the SGI neurons. Depolarization induced by nicotine enhanced the
N-methyl-d-aspartate receptor-mediated excitatory postsynaptic potential component and lowered
the threshold of bursting response in the SGI neurons to stimulation of the superficial
layer. Thus, the cholinergic input to the SGI facilitates the signal transmission
through the direct visuomotor pathway in the SC. The behavioral correlate of this
observation was explored by microinjection of nicotine into the SC of awake monkeys
during visually guided saccade task; injection of nicotine increased frequency of
express saccades, the saccades with extremely short reaction times (<120 ms). Analysis
of single unit activity of the PPTN neurons revealed that a population of the PPTN
neurons increased firing preceding saccades in a particular direction and also during
the GAP period between the offset of fixation point and onset of the saccade target.
Thus, PPTN neurons may be involved in execution and preparation of saccades. All these
results explain the mechanisms of how the brainstem cholinergic system facilitates
initiation of saccades presumably depending on attention or vigilance level of the
animal.
Keywords
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© 2001 Elsevier Science B.V. Published by Elsevier Inc. All rights reserved.
