Intrinsic Circuitry of the Superior Colliculus: Pharmacophysiological
Identification of Horizontally-oriented Inhibitory Interneurons.
M. Alex Meredith, and Ary S. Ramoa.
Department of Anatomy, Visual/Motor Neuroscience Division, Medical College
of Virginia, Virginia Commonwealth University, PO Box 980709, Richmond, VA
23298-0709 USA.
APStracts 4:356N, 1997.
ABSTRACT
Much of what is known about the organization of the superior colliculus is
based on the arrangement of its external connections. Consequently, there is
little information regarding pathways that remain intrinsic to it, even though
recent data suggest that a horizontally-oriented local circuit may mediate the
functional reciprocity among fixation and saccade-related neurons (Munoz and
Wurtz, 1993a, 1995). Therefore, the present experiments sought physiological
evidence for neurons intrinsic to the superior colliculus which might
participate in a horizontally-oriented local circuit.
Parasagittal slices of the ferret superior colliculus were prepared for in
vitro recording and 125 intermediate/deep layer neurons were examined in
response to electrical stimulation rostral or caudal to the recording site. A
substantial proportion (37%) of neurons responded with a prolonged period (_ =
59.3 ñ 30 ms) of post-stimulus suppression of spontaneous action potential
activity. Of the suppressed neurons, most (53%) were disinhibited when the
excitatory amino acid receptor antagonists d-APV and NBQX were administered,
indicating that excitatory input to inhibitory interneurons was blocked. Of
the neurons that received inputs from inhibitory interneurons, all had their
suppressive responses decreased or eliminated by the ë-aminobutyric acid
(GABA) antagonist, bicuculline. Finally, severing the superficial layers from
the slice had no effect on intermediate layer responses to intrinsic
stimulation.
These data provide physiological evidence for the presence of horizontally-
oriented inhibitory interneurons in the superior colliculus. Furthermore,
these findings are consistent with the hypothesis that an intrinsic circuit,
routed through interneurons, might account for the reciprocal inhibition
observed among fixation and saccade-related neurons.
Received 31 March 1997; accepted in final form 4 December 1997.
APS Manuscript Number J263-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997