Journal of Neuroscience, Vol 8, 2948-2960, Copyright © 1988 by Society for Neuroscience
Glutamate-positive corticocortical neurons in the somatic sensory areas I and II of cats
F Conti, M Fabri and T Manzoni
Institute of Human Physiology, University of Ancona, Italy.
Combined retrograde transport-immunocytochemical experiments were carried
out on cats to study the morphology, laminar distribution, and percentages
of corticocortical projecting neurons of somatosensory area I (SI) and II
(SII) showing immunoreactivity to an antiserum raised against the amino
acid glutamate (Glu). A previously characterized anti- Glu serum (Conti et
al., 1987a, b; Hepler et al., 1987) was used in conjunction with HRP. This
tracer was injected either in SI to label retrogradely neurons in
ipsilateral SII (SII-SI association neurons) and contralateral SI (SI-SI
callosal neurons) or in SII to label retrogradely neurons in ipsilateral SI
(SI-SII association neurons) and contralateral SII (SII-SII callosal
neurons). In sections from SI and SII processed for simultaneous
visualization of Glu and HRP (Bowker et al., 1982), and containing the
cells from which every one of the 4 corticocortical projections arise, 3
types of labeled neurons were observed: (1) single-labeled neurons showing
the homogeneous brown immunoreaction product of Glu (Glu-positive neurons);
(2) single- labeled neurons containing the granular black reaction product
of retrogradely transported HRP (Glu-negative, association or callosal
neurons); and (3) double-labeled neurons in which both the black HRP
granules and the brown immunostaining were present (Glu-positive,
association or callosal neurons). Double-labeled neurons were all pyramidal
in shape and were distributed intermingled with Glu-negative
corticocortical neurons in all layers of SI and SII known to give rise to
association and callosal projections. Counts from 25-micron-thick sections
showed that of 432 association and callosal neurons sampled from SI and
SII, 214 (49.5%) were Glu-negative and 218 (50.5%) Glu- positive. In counts
carried out on 5-micron-thick sections, the percentage of Glu-positive
corticocortical neurons raised to about 70%. The 2 populations of single-
and double-labeled corticocortical neurons showed no difference in their
perikaryal cross-sectional areas. The present results show that a large
fraction of association and callosal neurons of SI and SII are
immunoreactive for Glu, and, therefore, these neurons probably use this
excitatory amino acid, or a closely related compound, as neurotransmitter.