Subthalamic nucleus modulates burst firing of nigral dopamine neurones via NMDA receptors.

The role of the subthalamic nucleus in the burst firing of dopamine neurones of the substantia nigra was investigated using extracellular single unit recordings combined with pressure or iontophoretic micro-injections in anaesthetized rats. Inhibition of subthalamic neurones by pressure injection of gamma-aminobutyric acid (GABA) regularized the burst firing pattern in eight out of 17 dopamine neurones. Bicuculline injection near subthalamic neurones increased their firing rate and increased burst discharge in a subpopulation of dopamine neurones tested (34 out of 102). The increase was depressed by iontophoresis of the N-methyl-D-aspartate (NMDA) antagonist (+-)2-amino,5-phosphonopentanoic acid (AP-5), but not of the non-NMDA antagonist, 6-cyano,7-nitroquinoxaline-2,3-dione (CNQX). These data suggest that the subthalamic nucleus promotes burst discharge in a subpopulation of substantia nigra dopamine neurones via NMDA receptors.

Activation of brain noradrenergic neurons during recovery from halothane anesthesia. Persistence of phasic activation after clonidine.

BACKGROUND: alpha 2-Adrenoceptor agonists, known as antihypertensive agents, may be used during general anesthesia for their anesthetic sparing action and to reduce the occurrence of side effects. Previous studies have shown that the brain's noradrenergic nucleus, locus coeruleus, is an important target in mediating the hypnotic action of alpha 2 agonists. The authors studied the effects of recovery from halothane anesthesia on the electrical activity of locus coeruleus neurons to examine cellular substrates underlying the clinical effectiveness of alpha 2 agonists. METHODS: Experiments were performed in locally anesthetized rats, whose circulatory and acid-base stabilities were ensured by mechanical ventilation and volume infusion. Locus coeruleus neurons were recorded continuously while the rats were anesthetized with halothane (1%) and/or after the halothane was discontinued. RESULTS: Under the influence of halothane, locus coeruleus cells exhibited a slow, regular spontaneous discharge (1.95 +/- 0.23 Hz), and contralateral foot or tail pinch elicited a prominent, phasic activation in locus coeruleus neurons. Such phasic activation was blocked by local ejection of kynurenic acid, an excitatory amino acid antagonist, close to recorded neurons, but not by clonidine (up to 64 micrograms.kg-1). Thirty minutes after the halothane was discontinued, the mean firing rate of locus coeruleus neurons was increased by 87 +/- 20%. This excitation resulted from a prominent increase in bursting activity (21 +/- 5% of spikes in bursts vs. 4 +/- 1%) and was reversed by halothane readministration. This activation also was reduced by local ejection of kynurenic acid. Halothane discontinuance revealed the reactivity of locus coeruleus neurons to nonnoxious, sensory stimuli, and considerably reduced the apparent potency of intravenous administration of clonidine to inhibit locus coeruleus activity (effective dose for 50% of maximal effect (ED50), 25.48 +/- 8.26 micrograms.kg-1 vs. 4.81 +/- 0.80 micrograms.kg-1 under halothane). This decrease was caused by the persistence of bursting activity after the administration of clonidine, which was completely suppressed by readministration of halothane or local application of kynurenic acid. CONCLUSION: The data demonstrate: (1) that halothane withdrawal increases locus coeruleus neuronal activity via excitatory amino acid input, and this withdrawal-induced activity is characterized by a prominent burst (phasic) discharge; (2) that sedative doses of clonidine inhibit the tonic component of locus coeruleus activity but not the phasic activation of locus coeruleus neurons; and (3) that readministration of halothane or local ejection of an excitatory amino acid antagonist fully suppresses the bursting activity unaffected by clonidine.

Serotonin differentially modulates responses mediated by specific excitatory amino acid receptors in the rat locus coeruleus.

Microiontophoretic application of selective agonists for the three major excitatory amino acid receptors, N-methyl-D-aspartate (NMDA), quisqualate and kainate, increased the discharge rate of noradrenergic locus coeruleus (LC) neurons in vivo. NMDA activation was selectively attenuated by iontophoretic application of 2-amino-5-phosphonopentanoate (AP5), an antagonist at NMDA receptors, whereas kainate- and quisqualate-evoked responses were attenuated by both NMDA and non-NMDA antagonists iontophoresis. NMDA- and quisqualate-evoked responses were significantly decreased by co-iontophoresis of serotonin (5-HT). When the NMDA receptor-mediated component of the response to kainate was blocked with AP5 iontophoresis, 5-HT increased the response of LC neurons to kainate. These results revealed that 5-HT differentially modulates the responsiveness of LC neurons to excitatory amino acids, depending on the receptor subtypes responsible for the neuronal activation.

Tonic activation of NMDA receptors causes spontaneous burst discharge of rat midbrain dopamine neurons in vivo.

Midbrain dopamine neurons in vivo discharge in a single-spike firing pattern or in a burst-firing pattern. Such activity in vivo strikingly contrasts with the pacemaker activity of the same dopamine neurons recorded in vitro. We have recently shown that burst activity in vivo of midbrain dopamine neurons is due to the local activation of excitatory amino acid receptors, as microapplication of the broad-spectrum antagonist of excitatory amino acids, kynurenic acid, strongly regularized the spontaneous firing pattern of these dopamine neurons. In the present study, we investigated which subtypes of excitatory amino acid receptors are involved in the burst-firing of midbrain dopamine neurons in chloral hydrate-anaesthetized rats, iontophoretic or pressure microejections of 6-cyano, 7-nitroquinoxaline-2,3-dione (CNQX), a non-N-methyl-D-aspartate (NMDA) receptor antagonist, did not alter the spontaneous burst firing of dopamine neurons (n = 36). In contrast, similar ejections of (+-)2-amino,5-phosphonopentanoic acid (AP-5), a specific antagonist at NMDA receptors, markedly regularized the firing pattern by reducing the occurrence of bursts (n = 52). In addition, iontophoretic ejections of NMDA, but not kainate or quisqualate, elicited a discharge of these dopamine neurons in bursts (n = 20, 12 and 14, respectively). These data suggest that burst-firing of midbrain dopamine neurons in vivo results from the tonic activation of NMDA receptors by endogenous excitatory amino acids. In view of the critical dependency of catecholamine release on the discharge pattern of source neurons, excitatory amino acid inputs to midbrain dopamine neurons may constitute a major physiological substrate in the control of the dopamine level in target areas.

Inhibition of locus coeruleus neurons by the phencyclidine analog, N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine: evidence for potent indirect adrenoceptor agonist properties.

The effects of the phencyclidine derivative, N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine (BTCP), on the electrical activity of noradrenaline (NA) neurons of the locus coeruleus (LC) were studied in halothane-anesthetized rats. Systemic administration of BTCP potently inhibited LC neurons (ID50 of 1.1 +/- 0.1 mg/kg i.v.). This effect was mimicked by local microejection of BTCP into the LC. Both the systemic and local effects of BTCP were blocked by alpha 2-adrenoceptor antagonists and prevented by prior depletion of catecholamines with reserpine. These and other data suggest that BTCP behaves as a potent indirect NA agonist (i.e. via NA re-uptake and/or release systems).

Anatomical and electrophysiological evidence for a glycinergic inhibitory innervation of the rat locus coeruleus.

Using a highly specific antiserum to glycine and a very sensitive immunohistochemical technique with streptavidin-horseradish peroxidase, we visualized for the first time a dense plexus of glycine varicose fibers in the locus ceruleus (LC) of the rat. We further demonstrated that iontophoretically applied glycine inhibits the spontaneous LC noradrenergic cell discharge and that this inhibition is blocked by co-iontophoresis of strychnine. These anatomical and electrophysiological results indicate that the rat locus ceruleus receives an inhibitory glycinergic input.

A method to maintain normal respiratory and metabolic state in artificially respired rats.

Analysis of arterial blood gases (ABG) in awake, paralyzed, locally anesthetized, and artificially respired rats revealed the development with time of severe hypoxemia associated with metabolic acidosis despite adequate ventilation as assessed by normal PaCO2. These respiratory and metabolic disturbances may underlie the progressive deterioration experienced with this preparation frequently used in neuropharmacological experiments. We report here that the intravascular infusion of bicarbonated artificial plasma, associated with continuous positive pressure ventilation, prevents the deterioration of the respiratory and metabolic state in this preparation, which can be maintained within the range of that of the freely moving animal. This stabilized preparation may thus be highly suitable for neuropharmacological experiments extending for several hours.

Burst firing of mesencephalic dopamine neurons is inhibited by somatodendritic application of kynurenate.

Midbrain dopamine neurons of the zona compacta substantia nigra (SN) and ventral tegmental area (VTA), giving rise to the nigrostriatal and mesolimbocortical midbrain dopamine pathways, respectively, typically display a spontaneous activity consisting of single spikes and bursts. Previously, intracerebroventricular administration of the excitatory amino acid (EAA) antagonist kynurenate has been shown to inhibit burst firing and induce a regular, pacemaker-like firing of ventral tegmental area midbrain dopamine neurons. In the present experiments, zona compacta substantia nigra and ventral tegmental area midbrain dopamine neurons were recorded in the chloral hydrate anaesthetized male rat. Kynurenate was administered locally, either by micro-iontophoresis or by pneumatic (micropressure) ejection. Both forms of local kynurenate application produced an immediate inhibition of burst firing and a slightly increased regularity of firing in both zona compacta substantia nigra and ventral tegmental area midbrain dopamine neurons. The present results indicate that excitatory amino acid nerves tonically modulate midbrain dopamine neuronal burst firing directly on the midbrain dopamine cell bodies, further stressing the importance of excitatory amino acid innervation in the physiological function of midbrain dopamine neurons, particularly in the dynamic aspects involved in the behavioural modulation and pharmacological responses of these psychopharmacologically important neurons.

[Participation of 5-HT 1A receptors in the decrease by serotonin of activation of locus coeruleus neurons by glutamate]. / Participation des récepteurs 5-HT 1A dans la diminution par la sérotonine de l'excitation des neurones du locus coeruleus évoquée par le glutamate.

The serotonin-induced decrease of glutamate-evoked activation of noradrenergic locus coeruleus neurons is mimicked by agonists of 5-HT 1 and 1A but not by 5-HT 1B or by 5-HT 2 agonists. Moreover, this effect is reversed by a broad-spectrum 5-HT antagonist but not by a 5-HT 2 antagonist, indicating that this effect is mediated primarily through 5-HT 1A receptors.

[Participation of NMDA receptors in spontaneous burst firing of dopaminergic mesencephalic neurons]. / Participation des récepteurs NMDA dans la genèse des bouffées spontanées des neurones dopaminergiques du mésencéphale.

In the rat, somatodendritic application of the NMDA antagonist AP-5, within the Substantia Nigra Zona Compacta and Ventral Tegmental Area, either by micro-iontophoresis or pressure ejection, reduces burst firing of dopamine neurons. Similar local application of the non-NMDA antagonist CNQX does not affect their firing pattern. These results indicate that, in vivo, excitatory amino acid afferents participate through NMDA receptors in the control of the spontaneous burst firing of midbrain dopamine neurons.