In Vitro Brain Slice Studies of the Rat's Dorsal Nucleus of the Lateral
Lemniscus: III. Synaptic Pharmacology.
Wu, Shu Hui and Jack B. Kelly.
Laboratory of Sensory Neuroscience, Department of Psychology, Carleton
University, Ottawa, Canada K1S 5B6.
APStracts 2:0305N, 1995.
SUMMARY AND CONCLUSIONS
1) The synaptic pharmacology of the dorsal nucleus of the lateral lemniscus
(DNLL) of the rat was investigated in a brain slice preparation of the
auditory midbrain. The brain slice was cut in the coronal plane and placed in
a small recording chamber where warm, oxygenated saline was continuously
perfused over and underneath the tissue. Intracellular recordings were made
with glass microelectrodes filled with 4M potassium acetate. Synaptic
potentials were elicited by electrical stimulation of the lateral lemniscus or
commissure of Probst and pharmacological effects were tested by bath
application of amino acid agonists and antagonists. 2) The cells in DNLL were
challenged with the excitatory amino acid (EAA) agonists, {SYMBOL 97 \f
"Symbol"}-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-
D-aspartic acid (NMDA) in 0 Mg2+ and L-glutamate. Each of these caused a
depolarization of the cell membrane, a reduction in cell membrane resistance
and the onset of spontaneous firing. 3) Short latency excitatory postsynaptic
potentials (EPSPs) were evoked by stimulation of the lateral lemniscus in 77%
of the neurons tested. The mean latency to initial depolarization was 0.9 ms.
A single spike with relatively constant latency (mean 1.5 ms) was typically
elicited when the strength of lemniscal stimulation was increased. A longer
latency EPSP (mean 2.9 ms) was seen in 34 % of the neurons tested either with
the slice in normal saline or after pharmacological block of the earlier,
short latency EPSP. The long latency EPSP was followed by a single spike or
multiple spikes with highly variable latencies (range: 3.2-24 ms). In 28 % of
the neurons tested both early and late EPSPs were observed in response to
stimulation of a single location on the lateral lemniscus. 4) Stimulation of
the commissure of Probst elicited short latency EPSPs (mean 0.9 ms) in 37% of
the neurons tested. Longer latency EPSPs (mean 3.0 ms) were found in only 3 %
of the neurons in response to commissural stimulation. 5) The non-specific EAA
antagonist, kynurenic acid, blocked both short and long latency EPSPs evoked
by either lemniscal or commissural stimulation. The non-NMDA antagonist, 6-
cyano-7-nitroquinoxaline-2,3-dione (CNQX), at very low concentrations blocked
the short latency EPSPs but had no effect on the longer latency EPSPs. The
short latency EPSPs were unaffected by the NMDA antagonist, D,L-2-amino-5-
phosphonovaleric acid (APV). In contrast, the longer latency EPSPs were
blocked by APV, but never by CNQX. 6) DNLL neurons were affected by the
inhibitory amino acid agonists, {SYMBOL 103 \f "Symbol"}-aminobutyric acid
(GABA) and glycine. The membrane resistance of the neurons was decreased by
GABA and glycine in a solution of either normal or calcium free saline in a
concentration dependent manner. 7) Inhibitory postsynaptic potentials (IPSPs)
were elicited by stimulation of the lateral lemniscus in 53 % of the neurons
and the commissure of the Probst in 18 % of the neurons tested. The mean
latencies were 1.0 and 0.9 ms respectively. The reversal potentials of the
IPSPs were around -70 mV. 8) The IPSPs evoked by stimulation of the lateral
lemniscus were blocked by the glycine receptor antagonist, strychnine, but not
by the GABA receptor antagonist, bicuculline, whereas the IPSPs elicited by
stimulation of the commissure of Probst were blocked by bicuculline, but not
strychnine.
Received 30 May 1995; accepted in final form 17 October 1995.
APS Manuscript Number J353-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95