Role of glutamate transporters in corticostriatal synaptic transmission.

High-affinity glutamate transporters (GTs) play a major role in controlling the extracellular level of this excitatory neurotransmitter in the CNS. Here we have characterized, by means of in vitro patch-clamp recordings from medium spiny neurons (MSNs), the role of GTs in regulating corticostriatal glutamatergic synaptic transmission in the adult rat. Charge transfer and decay-time, but not amplitude, of excitatory postsynaptic currents (EPSCs) were enhanced by dl-threo-beta-benzyloxyaspartate (TBOA), a broad inhibitor of GTs. Moreover, TBOA also potentiated currents induced by high-frequency stimulation (HFS) protocols. Interestingly, the effect of TBOA on EPSCs was lost when MSNs were clamped at +40 mV, a condition in which neuronal GTs, that are voltage-dependent, are blocked. However, in this condition TBOA was still able to enhance HFS-induced currents, suggesting that glial GT's role is to regulate synaptic transmission when glutamate release is massive. These data suggest that neuronal GTs, rather than glial, shape EPSCs' kinetics and modulate glutamate transmission at corticostriatal synapse. Moreover, the control of glutamate concentration in the synaptic cleft by GTs may play a role in a number of degenerative disorders characterized by the hyperactivity of corticostriatal pathway, as well as in synaptic plasticity.

Preservation of the direct and indirect pathways in an in vitro preparation of the mouse basal ganglia.

We have developed a slice preparation of the mouse basal ganglia which contains portions of the striatum, external pallidum, subthalamic nucleus and substantia nigra and the neocortex. This basal ganglia slice is unique in preserving functional direct and indirect connections between the striatum and the substantia nigra as well as interconnectivity between the globus pallidus and the subthalamic nucleus. We used fiber tract tracing studies and electrophysiological recordings to demonstrate the full functionality of these pathways. Deposits of 1,1'-dioctadecyl-3,3,3',3'-tetra-methylindocarbocyamine perchlorate in the different basal ganglia resulted in labeled fibers in each of their target nuclei. Confirming these results, electrical stimulation of the different nuclei elicited whole-cell recorded postsynaptic potentials in their target neurons with an appropriate pharmacological profile. Electrical and glutamate activation of the striatum evoked bursts of glutamatergic and GABAergic activities in whole-cell recorded nigral neurons indicating that the direct and indirect pathways are operative in this slice. It also showed that the responses evoked are not due to fibers en passant but to the activation of striatal cell bodies. These findings provide the first direct evidence for a preserved basal ganglia circuitry in vitro and make the basal ganglia slice a suitable preparation for analyzing the activity of the direct and indirect pathways in physiological and pathological conditions.

Long-term depression in the nucleus accumbens: a neural correlate of behavioral sensitization to cocaine.

A compelling model of experience-dependent plasticity is the long-lasting sensitization to the locomotor stimulatory effects of drugs of abuse. Adaptations in the nucleus accumbens (NAc), a component of the mesolimbic dopamine system, are thought to contribute to this behavioral change. Here we examine excitatory synaptic transmission in NAc slices prepared from animals displaying sensitization 10-14 days after repeated in vivo cocaine exposure. The ratio of AMPA (alpha-amino-3-hydroxy-5-methyl-4- isoxazole propionic acid) receptor- to NMDA (N-methyl-d-aspartate) receptor-mediated excitatory postsynaptic currents (EPSCs) was decreased at synapses made by prefrontal cortical afferents onto medium spiny neurons in the shell of the NAc. The amplitude of miniature EPSCs at these synapses also was decreased, as was the magnitude of long-term depression. These data suggest that chronic in vivo administration of cocaine elicits a long-lasting depression of excitatory synaptic transmission in the NAc, a change that may contribute to behavioral sensitization and addiction.

High-frequency stimulation produces a transient blockade of voltage-gated currents in subthalamic neurons.

The effect of high-frequency stimulation (HFS) of the subthalamic nucleus (STN) was analyzed with patch-clamp techniques (whole cell configuration, current- and voltage-clamp modes) in rat STN slices in vitro. A brief tetanus, consisting of 100-micros bipolar stimuli at a frequency of 100--250 Hz during 1 min, produced a full blockade of ongoing STN activity whether it was in the tonic or bursting mode. This HFS-induced silence lasted around 6 min after the end of stimulation, was frequency dependent, could be repeated without alteration, and was not synaptically induced as it was still observed in the presence of blockers of ionotropic GABA and glutamate receptors or in the presence of cobalt at a concentration (2 mM) that blocks voltage-gated Ca(2+) channels and synaptic transmission. During HFS-induced silence, the following alterations were observed: the persistent Na(+) current (I(NaP)) was totally blocked (by 99%), the Ca(2+)-mediated responses were strongly reduced including the posthyperpolarization rebound (-62% in amplitude) and the plateau potential (-76% in duration), suggesting that T- and L-type Ca(2+) currents are transiently depressed by HFS, whereas the Cs(+)-sensitive, hyperpolarization-activated cationic current (I(h)) was little affected. Thus a high-frequency tetanus produces a blockade of the spontaneous activities of STN neurons as a result of a strong depression of intrinsic voltage-gated currents underlying single-spike and bursting modes of discharge. These effects of HFS, which are completely independent of synaptic transmission, provide a mechanism for interrupting ongoing activities of STN neurons.

Slowly inactivating sodium current (I(NaP)) underlies single-spike activity in rat subthalamic neurons.

One-half of the subthalamic nucleus (STN) neurons switch from single-spike activity to burst-firing mode according to membrane potential. In an earlier study, the ionic mechanisms of the bursting mode were studied but the ionic currents underlying single-spike activity were not determined. The single-spike mode of activity of STN neurons recorded from acute slices in the current clamp mode is TTX-sensitive but is not abolished by antagonists of ionotropic glutamatergic and GABAergic receptors, blockers of calcium currents (2 mM cobalt or 40 microM nickel), or intracellular Ca(2+) ions chelators. Tonic activity is characterized by a pacemaker depolarization that spontaneously brings the membrane from the peak of the afterspike hyperpolarization (AHP) to firing threshold (from -57.1 +/- 0.5 mV to -42.2 +/- 0.3 mV). Voltage-clamp recordings suggest that the Ni(2+)-sensitive, T-type Ca(2+) current does not play a significant role in single-spike activity because it is totally inactivated at potentials more depolarized than -60 mV. In contrast, the TTX-sensitive, I(NaP) that activated at -54.4 +/- 0.6 mV fulfills the conditions for underlying pacemaker depolarization because it is activated below spike threshold and is not fully inactivated in the pacemaker range. In some cases, the depolarization required to reach the threshold for I(NaP) activation is mediated by hyperpolarization-activated cation current (I(h)). This was directly confirmed by the cesium-induced shift from single-spike to burst-firing mode which was observed in some STN neurons. Therefore, a fraction of I(h) which is tonically activated at rest, exerts a depolarizing influence and enables membrane potential to reach the threshold for I(NaP) activation, thus favoring the single-spike mode. The combined action of I(NaP) and I(h) is responsible for the dual mode of discharge of STN neurons.

Subthalamic nucleus neurons switch from single-spike activity to burst-firing mode.

The modification of the discharge pattern of subthalamic nucleus (STN) neurons from single-spike activity to mixed burst-firing mode is one of the characteristics of parkinsonism in rat and primates. However, the mechanism of this process is not yet understood. Intrinsic firing patterns of STN neurons were examined in rat brain slices with intracellular and patch-clamp techniques. Almost half of the STN neurons that spontaneously discharged in the single-spike mode had the intrinsic property of switching to pure or mixed burst-firing mode when the membrane was hyperpolarized from -41.3 +/- 1.0 mV (range, -35 to -50 mV; n = 15) to -51.0 +/- 1.0 mV (range, -42 to -60 mV; n = 20). This switch was greatly facilitated by activation of metabotropic glutamate receptors with 1S,3R-ACPD. Recurrent membrane oscillations underlying burst-firing mode were endogenous and Ca2+-dependent because they were largely reduced by nifedipine (3 microM), Ni2+ (40 microM), and BAPTA-AM (10-50 microM) at any potential tested, whereas TTX (1 microM) had no effect. In contrast, simultaneous application of TEA (1 mM) and apamin (0.2 microM) prolonged burst duration. Moreover, in response to intracellular stimulation at hyperpolarized potentials, a plateau potential with a voltage and ionic basis similar to those of spontaneous bursts was recorded in 82% of the tested STN neurons, all of which displayed a low-threshold Ni2+-sensitive spike. We propose that recurrent membrane oscillations during bursts result from the sequential activation of T/R- and L-type Ca2+ currents, a Ca2+-activated inward current, and Ca2+-activated K+ currents.

Subthalamic stimulation elicits hemiballismus in normal monkey.

High frequency stimulation (HFS) of the subthalamic nucleus (STN) reduces parkinsonian symptoms in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkey and in human patients. The effects of stimulation on normal waking primates have never been evaluated. While low frequency stimulation has no effect, HFS induces dyskinesias contralateral to the stimulated STN resembling human hemiballismus and those obtained in primates after neurotoxic lesion or pharmacological blockade of the STN. In the normal monkey, HFS appears reversibly to incapacitate the STN and allow the emergence of involuntary proximal displacements, due to disinhibition of the thalamo-cortical pathway. In the MPTP-treated monkey HFS buffers STN overactivity and alleviates akinesia and rigidity by reducing inputs to the internal segment of the globus pallidus.