Inhibition of nigral dopamine neurons by systemic and local apomorphine: possible contribution of dendritic autoreceptors.

Peripheral administration of low doses of dopamine agonist apomorphine induces a strong and short-latency inhibition of dopamine neurons in the substantia nigra, presumably via the activation of somatodendritic autoreceptors. We studied the site of action of apomorphine in anesthetized rats using volume-controlled pressure microejection combined with single unit recordings. Microapplication of apomorphine in the immediate vicinity of nigral dopamine neurons did not mimic the effect of intravenous administration of apomorphine (50 micrograms/kg), regardless of the concentration or volume used (10(-10)-10(-2) M, 10-100 nl). In contrast, the inhibition produced by systemic apomorphine was mimicked by drug application at a site 300 microns lateral and 600 microns ventral from the recording site in the zona reticulata of the substantia nigra, a region rich in dendrites of dopamine neurons. The inhibition induced by such a distant application of apomorphine could be reversed by systemic injection of D2, but not D1, receptor antagonists. Non-dopaminergic substances such as GABA, bicuculline or lidocaine were more effective when ejected close to rather than distant from the recording site, in a manner opposite to that of apomorphine. Similar to apomorphine, dopamine and D2 receptor agonists were more potent when intranigral applications were made at sites distant from, rather than close to, the recorded dopamine cells. Ejection of D2 antagonists in the substantia nigra zona reticulata attenuated the inhibitory effect of subsequent systemic apomorphine. Our results, together with other previous studies on the location of D2 receptors on dopamine neurons, suggest that peripheral administration of low doses of apomorphine inhibits nigral dopamine neurons by acting at D2 receptors located on the dendrites of these neurons.

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.

Combining in vivo volume-controlled pressure microejection with extracellular unit recording.

We report a method for combining extracellular single-unit recording with pressure ejection, permitting microvolume quantification through the measurement of meniscus movement. Good optimization of both high quality recording and precise determination (in the nanoliter range) of the pressure-ejected volume can be obtained by using a recording electrode affixed to a calibrated, narrow inner diameter ejection pipette.

Continuous volume infusion improves circulatory stability in anesthesized rats.

'Optimized' management (OM) was provided to chloralose-anesthetized rats for 12 h by combining continuous infusion (7 ml.kg-1). mechanical ventilation and strict control of acid-base equilibrium (n = 7). The chloralose-anesthetized rats managed conventionally (conventional management: CM, n = 9) received neither volume infusion, nor mechanical ventilation, nor correction of acid-base disturbances. All the OM rats completed the study while 6 out of 9 CM rats died before the end of the study period. Mean arterial pressure (MAP) remained at 100 mmHg for 12 h in the OM group. MAP stayed close to 70 mmHg in the CM group for 6 h and declined to very low levels thereafter (mean +/- S.E.M.: 46.0 +/- 3.9 mmHg at 12 h, P less than 10(-4) when compared to the other group). Central venous pressure and cardiac output remained close to baseline values for 12 h in the OM group. Acid-base equilibrium was preserved in the OM group in contrast to a severe metabolic acidosis in the CM group (pH = 7.14 +/- 0.03 at 12 h; P less than 10(-4). Such as 'optimized' management involving mechanical ventilation with oxygen, continuous infusion and acid-base monitoring may be of value to maintain circulatory stability in anesthetized rodent preparations during long periods of time, as in neurophysiological experiments.

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).

Apomorphine-induced inhibition of substantia nigra dopamine neurons: effects of unilateral injection through the internal carotid artery.

Possible indirect components in the inhibition of firing of A9 dopamine neurons induced by systemic apomorphine were studied using unilateral drug administration through the internal carotid artery, known to irrigate only the ipsilateral mid- and forebrain. When compared to intravenous injection, unilateral intracarotid administration inhibited ipsilateral neurons with a marked decrease of both the latency (less than 1 s) and the dose required for complete inhibition, whereas contralateral neurons were not affected. This suggests a first-pass central effect of apomorphine, presumably associated with brain extraction. Thus, peripheral and hindbrain targets do not seem to contribute to the inhibitory effect of low doses of systemic apomorphine. An intranigral possible mode of action is discussed in view of the particular arrangement of dopaminergic dendrites within the zona reticulata.

[Thoracoabdominal CT scan: a useful tool for the diagnosis of air embolism during an endoscopic retrograde cholangiopancreatography]. / Intérêt d'un scanner corps entier pour le diagnostic d'embolie gazeuse au cours d'une cholangiopancréatographie rétrograde endoscopique.

We report the case of an 82-year-old woman treated with biliary stents for an ampulloma of Vater's papilla, with recurrent stenosis of the common bile duct. She was hospitalized with a cholestasis. An endoscopic retrograde cholangiopancreatography (ERCP) was scheduled to change the biliary stent for a metallic one, under general anaesthesia, with oral intubation. The ERCP was performed initially without any complication, but as the metallic biliary stent was placed, an air embolism occurred and a cardiac arrest happened immediately. The etiologic diagnosis was quickly confirmed by an injected multislice body-scan, which showed liver, right heart and brain gas embolism. Cardiopulmonary resuscitation allowed a complete haemodynamic recovery but a poor neurological recovery. The patient was transferred in intensive care unit, were she died 12 days after, despite hyperbaric oxygen therapy and the disappearance of the air embolism on the following computed tomography scan. This case may be useful to recall the utility of a body-scan for the diagnosis, treatment and follow-up of an air embolism during ERCP.

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.

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.

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.

[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.

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.