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.

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.

Two Na,K-ATPase isoenzymes in canine cardiac myocytes. Molecular basis of inotropic and toxic effects of digitalis.

Canine cardiac myocytes contain two distinct molecular forms of the Na,K-ATPase catalytic subunit. They are resolved by gel electrophoresis and identified using immunological techniques. The apparent molecular weights of the catalytic subunits are 95,000 (alpha) and 98,000 (alpha +). As judged by [3H]ouabain-binding measurements and Na,K-ATPase assays, the two forms are active and differ by a factor of 150 in their respective affinity for digitalis (ouabain and digitoxigenin). The dissociation constant of the high affinity form (alpha +) is KD, 2 nM, and that of the low affinity molecular form (alpha) is KD, 300 nM. According to both enzymatic and binding assays, up to 70% of maximum inhibition is caused by occupation of the high affinity sites (alpha +). Inasmuch as the pharmacological and toxic concentrations of digitalis in dog are 1 and 200 nM, respectively, and as maximum inhibition of Na+ pump in vivo should not exceed 80% to avoid toxicity (Akera, T. and Brody, T. (1982) Annu. Rev. Physiol. 44, 375-388), it appears that the high affinity molecular form (alpha +) is the pharmacological receptor exclusively related to positive inotropy, whereas the low affinity form (alpha) is mainly associated with toxicity.