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The Journal of Neurophysiology Vol. 81 No. 5 May 1999, pp. 2066-2074
Copyright ©1999 by the American Physiological Society
1Department of Neurology, Medical College of
Virginia, Richmond, Virginia 23298; 2Department
of Medicine,
Caddick, Sarah J.,
Chunsheng Wang,
Colin F. Fletcher,
Nancy A. Jenkins,
Neal G. Copeland, and
David A. Hosford.
Excitatory But Not Inhibitory Synaptic Transmission Is Reduced in
Lethargic (Cacnb4lh) and Tottering
(Cacna1atg) Mouse Thalami. J. Neurophysiol. 81: 2066-2074, 1999.
Excitatory but not inhibitory synaptic transmission is reduced in
lethargic (Cacnb4lh) and tottering
(Cacna1atg) mouse thalami. Recent
studies of the homozygous tottering (Cacna1atg)
and lethargic mouse (Cacnb4lh) models of absence
seizures have identified mutations in the genes encoding the
1A and
4 subunits, respectively, of voltage-gated Ca2+ channels
(VGCCs).
subunits normally regulate Ca2+ currents via a
direct interaction with
1 (pore-forming) subunits of VGCCs, and
VGCCs are known to play a significant role in controlling the release
of transmitter from presynaptic nerve terminals in the CNS. Because the
gene mutation in Cacnb4lh homozygotes results in
loss of the
4 subunit's binding site for
1 subunits, we
hypothesized that synaptic transmission would be altered in the CNS of
Cacnb4lh homozygotes. We tested this hypothesis
by using whole cell recordings of single cells in an in vitro slice
preparation to investigate synaptic transmission in one of the critical
neuronal populations that generate seizure activity in this strain, the
somatosensory thalamus. The primary finding reported here is the
observation of a significant decrease in glutamatergic synaptic
transmission mediated by both
N-methyl-D-aspartate (NMDA) and non-NMDA
receptors in somatosensory thalamic neurons of
Cacnb4lh homozygotes compared with matched,
nonepileptic mice. In contrast, there was no significant decrease in
GABAergic transmission in Cacnb4lh homozygotes
nor was there any difference in effects mediated by presynaptic
GABAB receptors. We found a similar decrease in glutamatergic but not GABAergic responses in
Cacna1atg homozygotes, suggesting that the
independent mutations in the two strains each affected P/Q channel
function by causing defective neurotransmitter release specific to
glutamatergic synapses in the somatosensory thalamus. This may be an
important factor underlying the generation of seizures in these models.
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