Differential effects of iontophoretic in vivo application of the GABA_A-antagonists bicuculline and gabazine in sensory cortex

Abstract

We have compared the effects of microiontophoretic application of the GABA_A-receptor antagonists bicuculline (BIC) and gabazine (SR95531) on responses to pure tones and to sinusoidally amplitude-modulated (AM) tones in cells recorded extracellularly from primary auditory cortex (AI) of Mongolian gerbils. Besides similar effects in increasing spontaneous and stimulus-evoked activity and their duration, both drugs elicited differential effects on spectral tuning and synchronized responses to AM tones. In contrast to gabazine, iontophoresis of the less potent GABA_A-antagonist BIC often resulted in substantial broadening of frequency tuning for pure tones and an elimination of synchronized responses to AM tones, particularly with high ejecting currents. BIC-induced effects which could not be replicated by application of gabazine were presumably due to the well-documented, non-GABAergic side-effects of BIC on calcium-dependent potassium channels. Our results thus provide strong evidence that GABA_A-mediated inhibition in AI does not sharpen frequency tuning for pure tones, but rather contributes to the processing of fast temporal modulations of sound envelopes. They also demonstrate that BIC can have effects on neuronal response selectivity which are not due to blockade of GABAergic inhibition. The results have profound implications for microiontophoretic studies of the role of intracortical inhibition in sensory cortex.

Introduction

γ-Aminobutric acid (GABA) is the major inhibitory neurotransmitter in cerebral cortex (Curtis and Johnston, 1974, Krnjević, 1984, Winer, 1992) where it acts mainly on GABA_A- and GABA_B-receptors (Bormann, 1988, Bormann, 2000, Chebib and Johnston, 1999). The structure, distribution and functional role of GABA_A-receptors has been the subject of intense investigation (e.g., Bowery, 1983, Hevers and Luddens, 1998; for review see Egebjerg et al., 2002). These receptors mediate fast inhibitory postsynaptic potentials (Metherate, 1998, Li et al., 1996) which are particularly well-suited to generate or sharpen receptive field properties of cortical neurons. Following the pioneering studies of Sillito, 1975a, Sillito, 1975b, the functional role of GABA_A-receptors in sensory cortex has been investigated extensively by iontophoretically applying the competitive GABA_A-receptor antagonist bicuculline (BIC) whilst monitoring the responses of extracellularly recorded cells. However, it has been known for many years that, in addition to blocking GABA_A-receptors, BIC has several non-GABAergic side-effects which may affect neuronal discharges in a dose-dependent manner. These include inhibition of GABA uptake, reduction of resting membrane conduction resulting in membrane depolarization, prolongation of calcium-dependent action potentials, paroxysmal depolarization shifts and apamin-like potentiation of burst firing (Olsen et al., 1976, Heyer et al., 1981, Johnson and Seutin, 1997). At least some of these secondary effects are thought to result from actions of BIC on calcium-dependent potassium channels (Johansson et al., 2001). In contrast, the pyridazinyl-GABA derivative gabazine has a higher affinity for the GABA_A-receptor than BIC (Michaud et al., 1986) and does not seem to induce the non-GABAergic effects associated with the application of BIC (Chambon et al., 1985, Heaulme et al., 1986, Hamann et al., 1988). Despite this, gabazine has not been widely used as a GABA_A-antagonist in studies designed to assess the functional role of intracortical inhibition.

Here, we have compared for the first time the effects of microiontophoretic application of BIC and gabazine on the selectivity of cortical cells to the same stimuli. We studied drug-induced effects on frequency tuning for pure tones and temporal processing of AM tones in AI. We focused on these properties, because (i) studies which have employed iontophoretic application of BIC have yielded conflicting results on the contribution of GABAergic inhibition to frequency tuning for pure tones (Schulze and Langner, 1999, Wang et al., 2000, Wang et al., 2002, Foeller et al., 2001); and (ii) there is substantial evidence that GABAergic inhibition plays an important role in the temporal processing of sounds, and (iii) GABAergic inhibition has been implicated in phase-locking of responses to AM tones at the level of AI (Grothe, 2000, Grothe and Klump, 2000, Schulze et al., 2002).

The results clarify the contribution of GABA_A-mediated inhibition to the spectral and temporal processing of sounds in AI and demonstrate that iontophoretic application of BIC can have deleterious effect on neuronal response selectivity which do not reflect the loss of GABAergic inhibition. They cast doubt on the conclusions drawn from previous studies in which iontophoretic application of BIC has been used to elucidate the functional role of GABAergic inhibition in sensory cortex. Parts of this study have been published in abstract form (Kurt et al., 2004, Kurt et al., 2005).