Volume 17, Number 14,
Issue of July 15, 1997
pp. 5509-5527
Copyright ©1997 Society for Neuroscience
GABAergic Neurons in Barrel Cortex Show Strong, Whisker-Dependent
Metabolic Activation during Normal Behavior
Received Dec. 11, 1996; revised April 28, 1997; accepted April 30, 1997.
James S. McCasland1 and
Lyndon S. Hibbard2
1 Department of Anatomy and Cell Biology, State
University of New York Health Science Center at Syracuse, Syracuse, New
York 13210, and 2 Division of Experimental Neurology and
Neurological Surgery and McDonnell Center for Studies of Higher Brain
Function, Washington University School of Medicine, St. Louis, Missouri
63110
Electrophysiological data from the rodent whisker/barrel cortex
indicate that GABAergic, presumed inhibitory, neurons respond more
vigorously to stimulation than glutamatergic, presumed excitatory, cells. However, these data represent very small neuronal samples in
restrained, anesthetized, or narcotized animals or in cortical slices.
Histochemical data from primate visual cortex, stained for the
mitochondrial enzyme cytochrome oxidase (CO) and for GABA, show that
GABAergic neurons are more highly reactive for CO than glutamatergic
cells, indicating that inhibitory neurons are chronically more active
than excitatory neurons but leaving doubt about the short-term stimulus
dependence of this activation. Taken together, these results suggest
that highly active inhibitory neurons powerfully influence relatively
inactive excitatory cells but do not demonstrate directly the relative
activities of excitatory and inhibitory neurons in the cortex
during normal behavior.
We used a novel double-labeling technique to approach the issue
of excitatory and inhibitory neuronal activation during behavior. Our
technique combines high-resolution 2-deoxyglucose (2DG),
immunohistochemical staining for neurotransmitter-specific antibodies,
and automated image analysis to collect the data. We find that putative
inhibitory neurons in barrel cortex of behaving animals are, on
average, much more heavily 2DG-labeled than presumed excitatory cells, a pattern not seen in animals anesthetized at the time of 2DG injection. This metabolic activation is dependent specifically on
sensory inputs from the whiskers, because acute trimming of most
whiskers greatly reduces 2DG labeling in both cell classes in columns
corresponding to trimmed whiskers. Our results provide confirmation of
the active GABAergic cell hypothesis suggested by CO and single-unit
data. We conclude that strong activation of inhibitory cortical neurons
must confer selective advantages that compensate for its inherent
energy inefficiency.
Key words:
barrels;
hamster;
neural inhibition;
somatosensory
cortex;
deoxyglucose;
cortical circuits