Journal of Neuroscience, Vol 8, 160-175, Copyright © 1988 by Society for Neuroscience
The pharmacology of synapses formed by identified corticocollicular neurons in primary cultures of rat visual cortex
JE Huettner and RW Baughman
Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115.
Primary cultures of neurons from the visual cortex of 7-10-d-old Long Evans
rats were used to study the pharmacology of synaptic transmission.
Dissociated cells were grown either in mass cultures, which contained
8000-10,000 neurons, or in miniature island cultures of 50-100 cells. Prior
to dissociation, cells in layer V of visual cortex that project to the
superior colliculus were labeled in vivo by retrograde transport of
fluorescent latex microspheres-a permanent fluorescent marker. After 2 d to
8 weeks in culture, labeled neurons were identified by epifluorescent
illumination, and electrophysiological recordings were obtained from a
labeled cell and, simultaneously, from a nearby unlabeled neuron in the
same field of view. The 2 neurons were stimulated sequentially by current
injection and the pharmacology of evoked postsynaptic potentials (PSPs) was
investigated. In mass cultures, relatively few pairs of neurons from which
we recorded were synaptically connected, although nearly every cell
exhibited abundant spontaneous EPSPs and IPSPs. Neurons grown on island
cultures generally did not exhibit spontaneous synaptic activity; however,
stimulation of one of the cells in a pair frequently elicited a
short-latency PSP in the follower neuron. Retrogradely labeled
corticocollicular neurons produced only excitatory PSPs in follower cells,
while unlabeled neurons were either excitatory or inhibitory. Three
antagonists of excitatory amino acid receptors, kynurenic acid, piperidine
dicarboxylic acid, and gamma-D- glutamylglycine, completely blocked EPSPs
produced by labeled corticocollicular neurons, as well as EPSPs produced by
nearly all of the unlabeled excitatory cells. We have previously shown that
these compounds block both N-methyl-D-aspartate (NMDA)-type and non-NMDA
receptors on cultured cortical neurons (Huettner and Baughman, 1986). The
specific NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV) did
not alter short-latency EPSPs recorded in 1 mM Mg2+, but did reduce
longer-latency EPSPs polysynaptic activity. Since responses mediated by the
NMDA receptor are known to be antagonized by Mg2+ (Mayer and Westbrook,
1985), we perfused cultures with Mg2+-free medium and found that the
falling phase of some monosynaptic EPSPs was prolonged. Addition of APV to
Mg2+-free medium reduced the duration of the falling phase of EPSPs such
that they returned to the time course obtained in 1 mM Mg2+.(ABSTRACT
TRUNCATED AT 400 WORDS)
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