[Wakefulness and central basal nuclei: experimental observations and possible implications in Parkinson's disease]. / Vigilance et noyaux gris centraux: observations expérimentales et possibles implications dans la maladie de Parkinson.

Movement disorders in Parkinson disease, notably dampened during sleep, are associated with hyperactivity of the subthalamic nucleus (STN), whose origin is controversial. We have studied, on non-anaesthetized head-restrained rats, the STN spontaneous unit activity and the one of its principal GABAergic afferents, the globus pallidus (GP). In normal rats, STN neurons shifted from a random discharge in wakefulness (W) to a bursting pattern in slow wave sleep (SWS), without any change in their mean firing rate. In contrast GP neurons, with a mean firing rate higher in W than in SWS, exhibited a relatively regular discharge rate whatever the vigilance state. During paradoxical sleep, both STN and GP neurons increased markedly their firing rate. When applied during W, GABA-A antagonists increased the STN firing rate but did not change the typical W random pattern. When applied during SWS, they strongly reinforced the spontaneous burst pattern into a particularly marked one with instantaneous frequencies reaching 500Hz. SWS-W transitions occurring during ongoing antagonist iontophoresis invariably disrupted this burst pattern into a random one. On 6-OHDA unilaterally treated rats, the ipsilateral STN was hyperactive whatever the vigilance state (with an abnormal burst pattern during W on some neurons), but this hyperactivity did not seem to be associated with a GP hypoactivity. These results show that STN activity is not inversely correlated with GP activity, that its discharge pattern is strongly dependent on vigilance states, that GABA receptors do not play an exclusive role in regulating its firing pattern, and question the depolarization block hypothesis during STN high frequency stimulation.

Effects of high frequency stimulation of subthalamic nucleus on extracellular glutamate and GABA in substantia nigra and globus pallidus in the normal rat.

High frequency stimulation (130 Hz) of the subthalamic nucleus has dramatic beneficial motor effects in severe parkinsonian patients. However, the mechanisms underlying these clinical results remain obscure. The objective of the present work was to study the neurochemical changes induced in rats by high frequency stimulation of the subthalamic nucleus by using intracerebral microdialysis within its target structures. Our results show that high frequency stimulation of the subthalamic nucleus induces a significant increase of extracellular glutamate levels in the ipsilateral globus pallidus and substantia nigra while GABA was augmented only in the substantia nigra. These data suggest that functional effects induced by high frequency stimulation of the subthalamic nucleus might imply distal mechanisms involving the synaptic relationships with the subthalamic efferences. They question the current view that the direct inhibition of the subthalamic neurons is induced by high frequency stimulation.

Unrelated course of subthalamic nucleus and globus pallidus neuronal activities across vigilance states in the rat.

The pallido-subthalamic pathway powerfully controls the output of the basal ganglia circuitry and has been implicated in movement disorders observed in Parkinson's disease (PD). To investigate the normal functioning of this pathway across the sleep-wake cycle, single-unit activities of subthalamic nucleus (STN) and globus pallidus (GP) neurons were examined, together with cortical electroencephalogram and nuchal muscular activity, in non-anaesthetized head-restrained rats. STN neurons shifted from a random discharge in wakefulness (W) to a bursting pattern in slow wave sleep (SWS), without any change in their mean firing rate. This burst discharge occurred in the 1-2 Hz range, but was not correlated with cortical slow wave activity. In contrast, GP neurons, with a mean firing rate higher in W than in SWS, exhibited a relatively regular discharge whatever the state of vigilance. During paradoxical sleep, both STN and GP neurons increased markedly their mean firing rate relative to W and SWS. Our results are not in agreement with the classical 'direct/indirect' model of the basal ganglia organization, as an inverse relationship between STN and GP activities is not observed under normal physiological conditions. Actually, because the STN discharge pattern appears dependent on coincident cortical activity, this nucleus can hardly be viewed as a relay along the indirect pathway, but might rather be considered as an input stage conveying corticothalamic information to the basal ganglia.

Single-unit and polygraphic recordings associated with systemic or local pharmacology: a multi-purpose stereotaxic approach for the awake, anaesthetic-free, and head-restrained rat.

In order to avoid any artifactual pharmacological interferences with anaesthetic agents, a procedure has been developed for working on the awake, anaesthetic-free rat in a head-restrained condition. It allows, on the same animal and over several consecutive days, single-unit recordings in combination with systemic or local pharmacology (microiontophoresis or micropressure ejections), as well as monitoring vigilance states via the electroencephalogram and the electromyogram. After the cementing of a special "U"-shaped device on its skull under general anaesthesia, the animal is progressively habituated to stay daily, for several hours, under a painless corresponding stereotaxic restraint. This system can be easily adapted to different stereotaxic frames and, because of its spatial flexibility for targetting the desired rostrocaudal or lateral positions, allows access to a large number of cerebral structures. Experiments performed on Globus Pallidus, Substantia Nigra, and Locus Coeruleus neurons, combining the different possibilities of this system, are reported. They demonstrate, on the awake anaesthetic-free head-restrained rat, and under suitable ethical conditions, the feasibility of single-unit recordings of identified neurons associated with the study of their pharmacological reactivity after systemic or local drug administrations without any other drug interferences, and in physiologically relevant conditions such as the spontaneous alternance of vigilance states.

Role and origin of the GABAergic innervation of dorsal raphe serotonergic neurons.

Extracellular electrophysiological recordings in freely moving cats have shown that serotonergic neurons from the dorsal raphe nucleus (DRN) fire tonically during wakefulness, decrease their activity during slow wave sleep (SWS), and are nearly quiescent during paradoxical sleep (PS). The mechanisms at the origin of the modulation of activity of these neurons are still unknown. Here, we show in the unanesthetized rat that the iontophoretic application of the GABA(A) antagonist bicuculline on dorsal raphe serotonergic neurons induces a tonic discharge during SWS and PS and an increase of discharge rate during quiet waking. These data strongly suggest that an increase of a GABAergic inhibitory tone present during wakefulness is responsible for the decrease of activity of the dorsal raphe serotonergic cells during slow wave and paradoxical sleep. In addition, by combining retrograde tracing with cholera toxin B subunit and glutamic acid decarboxylase immunohistochemistry, we demonstrate that the GABAergic innervation of the dorsal raphe nucleus arises from multiple distant sources and not only from interneurons as classically accepted. Among these afferents, GABAergic neurons located in the lateral preoptic area and the pontine ventral periaqueductal gray including the DRN itself could be responsible for the reduction of activity of the serotonergic neurons of the dorsal raphe nucleus during slow wave and paradoxical sleep, respectively.