Interplay between expressed non-NMDA receptors and endogenous calcium-activated chloride currents in Xenopus laevis oocytes

The use of Xenopus oocytes in drug screening

INTRODUCTION

The physiological roles of ion channels are receiving increased interest in both basic research and drug discovery, and a demand for pharmacological approaches that can characterize or screen ion channels and their ligands with higher throughput has emerged. Traditionally, screening of compound libraries at ion channel targets has been performed using assays such as binding assays, fluorescence-based assays and flux assays that allow high-throughput, but sacrifice high data quality. The use of these assays with ion channel targets can also be problematic, emphasizing the usefulness of automated Xenopus oocyte electrophysiological assays in drug screening.

AREAS COVERED

This review summarizes the use of Xenopus oocytes in drug screening, presents the advantages and disadvantages of the use of Xenopus oocytes as expression system, and addresses the options available for automated two-electrode voltage-clamp recordings from Xenopus oocytes.

EXPERT OPINION

Automated and manual Xenopus oocyte two-electrode voltage-clamp recordings are useful and important techniques in drug screening. Although they are not compatible with high-throughput experimentation, these techniques are excellent in combination or as alternatives to fluorescence-based assays for hit validation, screening of focused compound libraries and safety screening on ion channels with their high flexibility for the choice of molecular targets, quality of data and reproducibility.

Species-dependent functional properties of non-NMDA receptors expressed in Xenopus laevis oocytes injected with mammalian and avian brain mRNA

1. Species-dependent variation in the functional properties of non-NMDA receptors was investigated by intracellular recording in Xenopus laevis oocytes injected with rat, chick and calf brain mRNA. 2. In all mRNA-injected oocytes, kainic acid (KA), domoic acid (Dom) and 5-bromowillardiine (BrW) evoked large, maintained membrane currents, in contrast to the smaller, desensitizing responses elicited by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), quisqualic acid (QA) and L-glutamic acid (L-Glu). Dose-response curves for KA in oocytes injected with calf (EC50 = 96.4 +/- 12.3 microM; mean +/- s.e. mean), chick (87.0 +/- 8.9 microM) or rat (88.7 +/- 4.3 microM) brain mRNA were similar. 3. Current-voltage (I-V) relationships determined with KA inwardly rectified in oocytes injected with calf or chick mRNA; whereas, outward rectification was observed in oocytes injected with rat brain mRNA. 4. In oocytes injected with rat brain mRNA, AMPA antagonized responses evoked by KA in a competitive manner. The absolute amplitudes of KA and AMPA responses in the same oocytes were significantly correlated, which is consistent with both agonists acting on the same receptor-ionophore complex. 5. In contrast, in oocytes injected with calf or chick brain mRNA, AMPA (QA and L-Glu) antagonized the response evoked by KA in a non-competitive manner. The response amplitudes of KA compared to AMPA, QA or L-Glu in the same oocytes were not correlated suggesting discrete receptor-ionophores. 6. This study favours the existence of distinct non-NMDA receptor subtypes that are equi-sensitive to KA. The expressed receptors from different species of mRNA may be distinguished by their voltage sensitivities and the type of antagonism exerted by AMPA on KA-activated responses. Our observations may reflect further heterogeneity of non-NMDA receptors in the central nervous system of different vertebrate species.