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Journal of Neuroscience, Vol 6, 2691-2706, Copyright © 1986 by Society for Neuroscience
Inhibitory circuitry in the ventral cochlear nucleus is probably mediated by glycine
SH Wu and D Oertel
Intracellular recordings from brain slice preparations of the ventral
cochlear nuclei (VCN) of mice have shown that both the major cell types,
stellate and bushy cells, distinguishable by their responses to
intracellularly injected current (Oertel, 1983; Wu and Oertel, 1984),
receive late inhibitory as well as early excitatory inputs when the
auditory nerve is stimulated electrically. When the extracellular Cl-
concentration was lowered or when the intracellular Cl- was raised, the
reversal potential of IPSPs became more positive; the reversal potentials
were independent of the extracellular K+ concentration. Therefore, IPSPs
result from increases in Cl- permeability. To determine whether either or
both GABA or glycine might mediate the inhibition, the sensitivity of cells
to bath-applied putative neurotransmitters was tested. All cells responded
to applications of 0.1-10 mM GABA and glycine with large drops in input
resistance; these drops were Cl- dependent. To determine which of these 2
substances was more likely to mediate the IPSPs, antagonists specific to
GABA and glycine were tested for their ability to block the IPSPs. All
IPSPs were eliminated by 1 microM strychnine, a blocker of glycine-mediated
inhibition; they were not consistently blocked by 100 microM bicuculline or
by 100 microM picrotoxin, blockers of GABA-mediated inhibition. These
results indicate that the inhibition is likely to be mediated by glycine. A
simple interpretation of the finding that IPSPs have latencies (1.2-4 msec)
at least 2X as long as EPSPs (0.6-0.9 msec) is that cells in the VCN are
excited monosynaptically by auditory nerve fibers, and that they are
inhibited disynaptically through interneurons within the VCN. To test
physiologically whether EPSPs and IPSPs are, respectively, monosynaptic and
polysynaptic, 500-700 microM sodium pentobarbital was applied to the
preparation. Pentobarbital raised the thresholds of all impaled cells and
their synaptic inputs. EPSPs could be evoked in the presence of
pentobarbital by raising the stimulus strength, as expected when thresholds
are raised in a monosynaptic circuit; even if the thresholds of IPSPs were
lower than those of EPSPs in normal saline, they were raised above those of
EPSPs in the presence of pentobarbital. The finding that the thresholds of
IPSPs are raised more than those of EPSPs supports the interpretation that
IPSPs are mediated through a polysynaptic pathway, and this may explain why
inhibition has been detected inconsistently in vivo.
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