Abstracts

A significant proportion of antiepileptic drugs function to selectively enhance post-synaptic currents by targeting certain GABA[sub]A[/sub] receptor subtypes. GABA[sub]A[/sub]-mediated inhibition can manifest in a synaptically discrete [ldquo]phasic[rdquo] form, or as an extrasynaptically localized, more persistent, [ldquo]tonic[rdquo] form. The presence of tonic inhibition within a neuron can powerfully mediate the excitability of that cell. Here we employ the drug THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol), a superagonist for GABA[sub]A[/sub] receptors containing [alpha]4 and [delta] subunits (Brown et al. 2002), to reveal the presence of a GABA[sub]A[/sub]-mediated tonic conductance within the inhibitory interneurons of the mouse somatosensory [quot]barrel[quot] cortex. Whole-cell voltage-clamp recordings were made from neurons in live slices from mouse barrel cortex. Interneurons were identified by inducing stereotypic firing patterns in current clamp mode. THIP (20uM) was bath applied in the presence of APV and DNQX. In separate experiments, GABA[sub]A[/sub] antagonists bicuculline, picrotoxin, and gabazine (20-100uM) were bath and focally applied. Bath application of THIP caused a pronounced ([gt]100pA) inward shift in baseline holding current and a two to three-fold increase in baseline noise. This effect was not attributable to an increase in amplitude or frequency of spontaneous phasic events as no significant difference was observed in sIPSC amplitude (Control: 29.3 [plusmn] 6.6 pA, THIP: 34.3 [plusmn] 7.2 pA, [italic]p[/italic] = 0.63, n=3), half-width (Control: 6.7 [plusmn] 0.2 ms, THIP: 6.5 [plusmn] 0.6 ms, [italic]p[/italic] = 1, n=3), or frequency (Control: 6.9 [plusmn] 1.5 ms, THIP: 6.1 [plusmn] 0.4 ms, [italic]p[/italic] = 0.61, n=3) in the presence of THIP as compared to control. Application of GABA[sub]A[/sub] antagonists resulted in an outward shift in baseline holding current confirming the presence of a GABA[sub]A[/sub]-mediated tonic conductance in these interneurons. Within the CNS, excitation and inhibition must be precisely coordinated in order to maintain proper function. Here we illustrate the presence of a powerful inhibitory mechanism within interneurons of the barrel cortex. Furthermore, these data suggest that THIP is a powerful and selective activator of tonic inhibition. As THIP preferentially activates [delta] subunit containing receptors, these data offer insights into the GABA receptor subunit composition of cortical interneurons and potential direction for new pharmacological agents that seek to target these neurons. (Supported by a grant from the Epilepsy Foundation.)