Activation of GABA(A) receptors in subthalamic neurons in vitro: Properties of native receptors and inhibition mechanisms

The subthalamic nucleus (STN) influences the output of the basal ganglia, t hereby interfering with motor behavior. The main inputs to the STN are GABA ergic. We characterized the GABA(A) receptors expressed in the STN and inve stigated the response of subthalamic neurons to the activation of GABA(A) r eceptors. Cell-attached and whole cell recordings were made from rat brain slices using the patch-clamp technique. The newly identified epsilon subuni t confers atypical pharmacological properties on recombinant receptors, whi ch are insensitive to barbiturates and benzodiazepines. We tested the hypot hesis that native subthalamic GABA(A) receptors contain epsilon proteins. A pplications of increasing concentrations of muscimol, a selective GABA(A) a gonist, induced Cl- and HCO3- currents with an EC50 of 5 muM. Currents indu ced by muscimol were fully blocked by the GABA(A) receptor antagonists, bic uculline and picrotoxin. They were strongly potentiated by the barbiturate, pentobarbital (+190%), and by the benzodiazepines, diazepam (+197%) and fl unitrazepam (+199%). Spontaneous inhibitory postsynaptic currents were also significantly enhanced by flunitrazepam. Furthermore, immunohistological e xperiments with an epsilon subunit-specific antibody showed that the epsilo n protein was not expressed within the STN. Native subthalamic GABA(A) rece ptors did not, therefore, display pharmacological or structural properties consistent with receptors comprising epsilon. Burst firing is a hallmark of Parkinson's disease. Half of the subthalamic neurons have the intrinsic ca pacity of switching from regular-firing to burst-firing mode when hyperpola rized 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-sp ike firing mode, whereas 10% were in burst-firing mode. Muscimol reversibly stopped recurrent electrical activity in all intact neurons. In neurons he ld in whole cell configuration, membrane potential hyperpolarized by -10 mV whilst input resistance decreased by 50%, indicating powerful membrane shu nting. Muscimol never induced burst firing, even in neurons that exhibited the capacity of switching from regular- to burst-firing mode. These molecul ar and functional data indicate that native subthalamic GABA(A) receptors d o not contain the epsilon protein and activation of GABA(A) receptors induc es membrane shunting, which is essential for firing inhibition but prevents switching to burst-firing. They suggest that the STN, like many other part s of the brain, has the physiological and structural features of the widely expressed GABAA receptors consisting of alpha beta gamma subunits.