The Journal of Neurophysiology Vol. 86 No. 1 July 2001, pp. 75-85
Copyright ©2001 by the American Physiological Society

Activation of GABA_A Receptors in Subthalamic Neurons In Vitro: Properties of Native Receptors and Inhibition Mechanisms

Laboratoire de Neurophysiologie Centre National de la Recherche Scientifique Unité Mixte de Recherche 5543, Université de Bordeaux 2, 33076 Bordeaux cedex, France

Baufreton, Jérôme, Maurice Garret, Sandra Dovero, Bernard Dufy, Bernard Bioulac, and Anne Taupignon. Activation of GABA_A Receptors in Subthalamic Neurons In Vitro: Properties of Native Receptors and Inhibition Mechanisms. J. Neurophysiol. 86: 75-85, 2001. The subthalamic nucleus (STN) influences the output of the basal ganglia, thereby interfering with motor behavior. The main inputs to the STN are GABAergic. We characterized the GABA_A receptors expressed in the STN and investigated the response of subthalamic neurons to the activation of GABA_A receptors. Cell-attached and whole cell recordings were made from rat brain slices using the patch-clamp technique. The newly identified  subunit confers atypical pharmacological properties on recombinant receptors, which are insensitive to barbiturates and benzodiazepines. We tested the hypothesis that native subthalamic GABA_A receptors contain  proteins. Applications of increasing concentrations of muscimol, a selective GABA_A agonist, induced Cl and HCO currents with an EC₅₀ of 5 µM. Currents induced by muscimol were fully blocked by the GABA_A receptor antagonists, bicuculline and picrotoxin. They were strongly potentiated by the barbiturate, pentobarbital (+190%), and by the benzodiazepines, diazepam (+197%) and flunitrazepam (+199%). Spontaneous inhibitory postsynaptic currents were also significantly enhanced by flunitrazepam. Furthermore, immunohistological experiments with an  subunit-specific antibody showed that the  protein was not expressed within the STN. Native subthalamic GABA_A receptors did not, therefore, display pharmacological or structural properties consistent with receptors comprising . Burst firing is a hallmark of Parkinson's disease. Half of the subthalamic neurons have the intrinsic capacity of switching from regular-firing to burst-firing mode when hyperpolarized by current injection. This raises the possibility that activation of GABA_A receptors might trigger the switch. Statistical analysis of spiking activity established that 90% of intact neurons in vitro were in single-spike firing mode, whereas 10% were in burst-firing mode. Muscimol reversibly stopped recurrent electrical activity in all intact neurons. In neurons held in whole cell configuration, membrane potential hyperpolarized by 10 mV whilst input resistance decreased by 50%, indicating powerful membrane shunting. Muscimol never induced burst firing, even in neurons that exhibited the capacity of switching from regular- to burst-firing mode. These molecular and functional data indicate that native subthalamic GABA_A receptors do not contain the  protein and activation of GABA_A receptors induces membrane shunting, which is essential for firing inhibition but prevents switching to burst-firing. They suggest that the STN, like many other parts of the brain, has the physiological and structural features of the widely expressed GABA_A receptors consisting of subunits.