Absence seizures afflict a significant percentage of children with epilepsy. Attempts to understand the molecular mechanisms of absence seizures may be facilitated by studying animal models. Our earlier studies using the lethargic (lh/lh) mutant mouse model of absence seizures showed: i) an apparent requirement for GABAB receptors in absence seizures; ii) an increased number of GABAB receptors in neocortex and thalamic nuclei compared to control (+/+) mice; iii) no change in electrophysiologic responses mediated by postsynaptic GABAB receptors in thalamic neurons in lh/lh compared to +/+ mice; and iv) alterations in biochemical functions mediated by presynaptic GABAB receptors in neocortex and thalamus in lh/lh compared to +/+ mice. The short-term goals of this proposal are to seek physiologic correlates to our findings regarding GABAB responses in lh/lh and +/+ mice, and to determine if these responses cause absence seizures. We will perform 6 specific aims to answer the following questions.
First (aims 1 -3)), what is the physiologic correlate of our evidence that presynaptic GABAB receptor-mediated function is altered in thalamic and neocortical neurons from lh/lh mice? Second (aims 4 and 5), is the altered function mediated by presynaptic GABAB receptors in lh/lh mice caused by the increased numbers of GABAB receptors in these mice? Third (aim 6), are absence seizures in lh/lh mice caused by the increased numbers of GABAB receptors in these mice? In the long-term, the answers forthcoming will increase our understanding of the physiologic and pathophysiologic roles of GABAB receptor-mediated function. More importantly, this information may lead to better therapies for patients with absence seizures.
In specific aims 1 and 2 we will use Cs+-filled microelectrodes and whole- cell voltage-clamp techniques in ventrobasal (VB) thalamic neurons (aim 1) or neocortical pyramidal neurons (aim 2) of lh/lh and +/_ slices to measure the effect of presynaptic GABAB receptor activation on IPSCs (autoreceptor function) and EPSCs (heteroreceptor function) evoked by local stimulation. We will activate presynaptic GABAB receptors by: i) applying the GABAB receptor agonist (-) baclofen to the bath; and ii) measuring paired-pulse and homosynaptic depression.
In specific aim 3 we will use Cs+ filled microelectrodes and whole-cell voltage clamp techniques in VB thalamic neurons to measure the effect of presynaptic GABAB receptor activation on spindles evoked by stimulation of nucleus reticularis thalami (NRT).
In specific aim 4 we will down-regulate GABAB receptors in lh/lh mice by chronic administration of GABAB receptor agonists. We will then measure the effect of presynaptic GABAB receptors on [3H]-GABA release in thalamic and neocortical synaptosomes from these mice and from """"""""normal"""""""" lh/lh mice.
In specific aim 5 we will use whole-cell voltage-clamp techniques in VB thalamic neurons of down-regulated lh/lh mice to measure the effect of presynaptic GABAB receptors on spindles evoked by stimulation of NRT.
In specific aim 6 we will implant bipolar recording electrodes into neocortex of +/+ and lh/lh mice. After up-regulating GABAB receptors by chronic administration of GABAB antagonists to half of the +/+ mice, we will use EEG recordings to compare absence seizure frequency in the groups.

Agency
National Institute of Health (NIH)
Institute
National Institute of Neurological Disorders and Stroke (NINDS)
Type
Research Project (R01)
Project #
5R01NS030977-05
Application #
2379674
Study Section
Neurological Sciences Subcommittee 1 (NLS)
Program Officer
Jacobs, Margaret
Project Start
1993-05-01
Project End
1999-02-28
Budget Start
1997-03-01
Budget End
1998-02-28
Support Year
5
Fiscal Year
1997
Total Cost
Indirect Cost
Name
Duke University
Department
Internal Medicine/Medicine
Type
Schools of Medicine
DUNS #
071723621
City
Durham
State
NC
Country
United States
Zip Code
27705
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Hosford, D A; Lin, F H; Wang, Y et al. (1999) Studies of the lethargic (lh/lh) mouse model of absence seizures: regulatory mechanisms and identification of the lh gene. Adv Neurol 79:239-52
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