Striatal NMDA receptors gate cortico-pallidal synchronization in a rat model of Parkinson's disease.

Anomalous patterns of synchronization between basal ganglia and cortex underlie the symptoms of Parkinson's disease. Computational modeling studies suggest that changes in cortical feedback loops involving trans-striatal and trans-subthalamic circuits bring up this anomalous synchronization. We asked whether striatal outflow synchronizes globus pallidus neurons with cortical activity in a rat model of Parkinson's disease. We found that striatal firing is highly increased in rats with chronic nigrostriatal lesion and that this hyperactivity can be reduced by locally infusing a competitive NMDA receptor antagonist. Moreover, NMDA receptor-dependent striatal output had frequency dependent effects on distinct pathological patterns of cortico-pallidal coupling. Blockade of striatal NMDA receptors almost completely abolished an anomalous ~1Hz cortico-pallidal anti-phase synchronization induced by nigrostriatal degeneration. Moreover, under striatal NMDA receptor blockade, synchronization with 2.5-5Hz cortical oscillations falls to negligible levels and oscillations at 10-20Hz are markedly attenuated, whereas beta synchronization (with a peak at ~26Hz) is marginally reduced. Thus, tonic activation of striatal NMDA receptors allows different forms of anomalous oscillations along the cortico-striato-pallidal axis. Moreover, the frequency dependent effects of NMDA receptors suggest that low and high frequency parkinsonian oscillations stem from partially different mechanisms. Finally, our results may help to reconcile views about the contributions of changes in firing rate and oscillatory synchronization to Parkinson's disease symptoms by showing that they are related to each other.

NMDA receptor gating of information flow through the striatum in vivo.

A role of NMDA receptors in corticostriatal synaptic plasticity is widely acknowledged. However, the conditions that allow NMDA receptor activation in the striatum in vivo remain obscure. Here we show that NMDA receptors contribute to sustain the membrane potential of striatal medium spiny projection neurons close to threshold during spontaneous UP states in vivo. Moreover, we found that the blockade of striatal NMDA receptors reduces markedly the spontaneous firing of ensembles of medium spiny neurons during slow waves in urethane-anesthetized rats. We speculate that recurrent activation of NMDA receptors during UP states allows off-line information flow through the striatum and system level consolidation during habit formation.

Flavonoid modulation of ionic currents mediated by GABA(A) and GABA(C) receptors.

The modulation of ionotropic gamma-aminobutyric acid (GABA) receptors (GABA-gated Cl(-) channels) by a group of natural and synthetic flavonoids was studied in electrophysiological experiments. Quercetin, apigenin, morine, chrysin and flavone inhibited ionic currents mediated by alpha(1)beta(1)gamma(2s) GABA(A) and rho(1) GABA(C) receptors expressed in Xenopus laevis oocytes in the micromolar range. alpha(1)beta(1)gamma(2s) GABA(A) and rho(1) GABA(C) receptors differ largely in their sensitivity to benzodiazepines, but they were similarly modulated by different flavonoids. Quercetin produced comparable actions on currents mediated by alpha(4)beta(2) neuronal nicotinic acetylcholine, serotonin 5-HT(3A) and glutamate AMPA/kainate receptors. Sedative and anxiolytic flavonoids, like chrysin or apigenin, failed to potentiate but antagonized alpha(1)beta(1)gamma(2s) GABA(A) receptors. Effects of apigenin and quercetin on alpha(1)beta(1)gamma(2s) GABA(A) receptors were insensitive to the benzodiazepine antagonist flumazenil. Results indicate that mechanism/s underlying the modulation of ionotropic GABA receptors by some flavonoids differs from that described for classic benzodiazepine modulation.

Benzodiazepine modulation of homomeric GABAAρ1 receptors: differential effects of diazepam and 4'-chlorodiazepam.

GABA(A) receptors (GABA(A)Rs) are ligand-gated ion channels that mediate inhibitory neurotransmission in the central nervous system (CNS). They are members of the Cys-loop receptor family and display marked structural and functional heterogeneity. Many GABA(A)Rs receptor subtypes are allosterically modulated by benzodiazepines (BDZs), which are drugs extensively used as anxiolytics, sedative-hypnotics and anticonvulsants. One high-affinity site and at least three additional low-affinity sites for BDZ recognition have been identified in several heteromeric and homomeric variants of the GABA(A)Rs (e.g.: α1ß2γ2, α1ß2/3, ß3, etc.). However, the modulation of homomeric GABA(A)ρRs by BDZs was not previously revealed, and these receptors, for a long a time, were assumed to be fully insensitive to the actions of these drugs. In the present study, human homomeric GABA(A)ρ1 receptors were expressed in Xenopus oocytes and GABA-evoked responses electrophysiologically recorded in the presence or absence of BDZs. GABA(A)ρ1 receptor-mediated responses were modulated by diazepam and 4'-chlorodiazepam in the micromolar range, in a concentration-dependent, voltage-independent and reversible manner. Diazepam produced potentiating effects on GABA-evoked Cl(-) currents and 4'-Cl diazepam induced biphasic effects depending on the GABA concentration, whereas Ro15-4513 and alprazolam were negative modulators. BDZ actions were insensitive to flumazenil. Other BDZs showed negligible activity at equivalent experimental conditions. Our results suggest that GABA(A)ρ1 receptor function can be selectively and differentially modulated by BDZs.

Striatal gating through up states and oscillations in the basal ganglia: Implications for Parkinson's disease.

Up states are a hallmark of striatal physiology. Spontaneous activity in the thalamo-cortical network drives robust plateau depolarizations in the medium spiny projection neurons of the striatum. Medium spiny neuron firing is only possible during up states and is very tightly regulated by dopamine and NMDA receptors. In a rat model of Parkinson's disease the medium spiny neurons projecting to the globus pallidus (indirect pathway) show more depolarized up states and increased firing. This is translated into abnormal patterns of synchronization between the globus pallidus and frontal cortex, which are believed to underlie the symptoms of Parkinson's disease. Here we review our work in the field and propose a mechanism through which the lack of D2 receptor stimulation in the striatum allows the establishment of fixed routes of information flow in the cortico-striato-pallidal network.