Abstracts

In tangential slices that include the distinctive barrel structures in layer 4, application of the GABA[sub]A[/sub] receptor blocker, bicuculline, leads to a slow barrel-to-barrel propagation of paroxysmal discharges (Fleidervisch et al., 1998). The unmasking of this horizontal spread of activity in layer 4 may indicate that GABA[sub]A[/sub] receptor mediated inhibition between horizontally connected septa neurons (located in-between the barrels) is powerful. The present study seeks to identify separate inhibitory pathways in barrel cortex through examination of the biophysical properties of GABA[sub]A[/sub] receptor mediated IPSCs in septa neurons.
Whole cell voltage clamp recordings were made from neurons in the barrel cortex of mice in live slices cut tangential to the pial surface. GABA[sub]A[/sub] receptor-mediated IPSCs were chemically isolated and examined in septa neurons and compared to IPSCs recorded in barrel neurons. In other experiments, sections were sliced tangentially on a freezing microtome, postfixed in 4% paraformaldehyde and processed for in situ hybridization histochemistry of complementary RNA (cRNA) probes specific to the [alpha]1 and [alpha]5 subunits. The probes were transcribed in vitro from sub-cloned portions of the [alpha]1 and [alpha]5 subunit cDNAs in the pBluescript II KS(+) transcription vector.
IPSCs recorded from septa neurons decayed at much slower rates than barrel neurons. When measured at 90% of total decay from the peak current amplitude, IPSCs in septa neurons were almost twice as slow as IPSCs in the barrels. IPSCs in neurons located in the [italic]wall[/italic] of the barrel exhibited extremely fast decay times [ndash] at one-third the rate of septa neurons. In separate experiments, in situ hybridization of GABA[sub]A[/sub] receptor subunit cRNAs confirmed that [alpha]1 subunit expression is very high in the walls of barrels. Conversely, adjacent sections showed that [alpha]5 cRNAs were concentrated in septa neurons.
Differential subunit expression is considered the foundation for functional heterogeneity of GABA[sub]A[/sub] receptor-mediated IPSCs. The [alpha] subunit class of receptor subunits determines the kinetic properties of the GABA[sub]A[/sub] ion channel. In particular, GABA-gated currents composed of [alpha]1 subunits produce briefer currents that those containing [alpha]2, [alpha]3 or [alpha]5 subunits. These data show that IPSCs in the septa have much slower decay kinetics than neurons in the barrels. Slower decay times may indicate a need for a more powerful inhibition for neurons responsible for controlling the spread of epileptiform activity. The basis for this slower decay may result from the expression of the [alpha]5 subunit in septa neurons. It is hoped that this study will lead to an improved understanding of GABA[sub]A[/sub] receptor subunit contributions in intracortical circuits, in addition to, improved therapies in the treatment of certain types of epilepsies.
[Supported by: a grant from the Epilepsy Foundation. ]