Abstracts

RATIONALE: Interactions between thalamocortical (TC) and thalamic reticular (RE) neurons generate spindle oscillations and may also contribute to spike-wave seizures in pathological states such as generalized absence epilepsy. Although the anti-absence drug, clonazepam (CZP), enhances inhibitory connections from RE cells onto both other RE cells and TC cells, the former action is thought to explain its therapeutic effect. To test this hypothesis, we studied mice with mutations in the benzodiazepine binding site on particular GABA[sub]A[/sub] receptor subunits. In [alpha]1H101R [dsquote]knock-in[dsquote] mice, CZP selectively enhances spontaneous IPSCs (sIPSCs) in RE cells, whereas in [alpha]3H101R [dsquote]knock-in[dsquote] mice, CZP selectively enhances sIPSCs in TC cells. Thus, the effects of CZP on intrathalamic oscillations in these mutant mice reveal how modulation of specific inhibitory loci might differentially regulate these oscillations.
METHODS: We evoked oscillations in acute thalamic slices by stimulating internal capsule, and recorded the resulting TC cell activity using arrays of multiunit electrodes in ventrobasal (VB) thalamus.
RESULTS: Stimulation evoked 5-8 Hz oscillations in VB. Each cycle consisted of a burst of spikes followed by quiescence, and this pattern continued for approximately one second after the stimulus. CZP did not abolish oscillations, as do blockers of excitatory synaptic transmission. Rather, in wild-type and [alpha]1H101R knock-in mice, CZP (100 nM) reversibly reduced the number of spikes across multiple electrodes during evoked oscillations by 7.6 [plusminus] 1.5% (p [lt] .001; n = 15) and 9.7 [plusminus] 1.3% (p [lt] .001; n = 12), respectively. This decreased spike count corresponded to a suppression of activity throughout the oscillation, not simply a shortening of its duration. In contrast, in [alpha]3H101R [dsquote]knock-in[dsquote] mice, CZP produced a statistically insignificant enhancement of the spike count (2.3 [plusminus] 3.6%; n = 5).
CONCLUSIONS: The CZP-dependent suppression of thalamic oscillations is preserved in [alpha]1H101R mutants, in which CZP selectively modulates intra-RE inhibition, but abolished in [alpha]3H101R mutants, in which CZP selectively modulates inhibition from RE cells onto TC cells. This suggests that CZP suppresses thalamic oscillations by enhancing inhibitory synapses between RE cells. Furthermore, the CZP-dependent suppression was actually greater in [alpha]1H101R mutants than in wild-type mice, and in [alpha]3H101R mutants, CZP produced a slight enhancement of spike number. Thus, by enhancing inhibitory synapses from RE cells to TC cells, CZP in wild-type slices might compromise the anti-oscillatory effects of enhanced intra-RE inhibition. This suggests that benzodiazepine-like drugs which specifically target the [alpha]3 subunit of the GABA[sub]A[/sub] receptor, which is found on RE cells but not on TC cells, should be powerful anti-absence agents.
Support: NIGMS, NINDS, and SNSF.