Interaction of anesthetic barbiturates with the phosphoinositide-dependent pathway of signal transduction

An autoradiographic study of [3H]flunitrazepam binding sites in the brain of rat made tolerant to and dependent on pentobarbital

The effects of continuous administration of pentobarbital on the benzodiazepine receptor labeled by [3H]flunitrazepam were investigated. Animals were made tolerant to pentobarbital by intracerebroventricular (i.c.v.) infusion with pentobarbital (300 micrograms/10 microliters/h) for 6 days through pre-implanted canulae connected to osmotic mini-pumps. The dependent rats were assessed 24 h after cessation of pentobarbital infusion. Changes in [3H]flunitrazepam binding were examined in 37 brain regions at a concentration of [3H]flunitrazepam of 1 nM. In subsequent saturation studies, the binding parameters Bmax and KD were also investigated in 17 brain regions, most of which showed significant changes in [3H]flunitrazepam binding in experiments using a fixed concentration of radioligand. The pentobarbital-tolerant rats showed a significant increase in Bmax with an increase in KD for [3H]flunitrazepam in the ventroposterior nucleus of thalamus. In the dependent rats, a significant increase in Bmax for [3H]flunitrazepam binding, without a change in KD, was observed in all layers of the frontal cortex, the caudate-putamen, olfactory tubercle, and some nuclei in thalamus, compared to those in the control. Increased [3H]flunitrazepam binding in the molecular layer of the olfactory bulb, the ventral pallidum, and the cerebellum of the pentobarbital dependent rats at a fixed concentration of [3H]flunitrazepam was also observed. There was no significant change in [3H]flunitrazepam binding in the hippocampus and several nuclei of the brain stem. These findings suggest that benzodiazepine receptors are closely involved in the development of tolerance to and dependence on pentobarbital. Further studies on changes in gamma-aminobutyric acid (GABA)A receptor subunit mRNA or the effects of pentobarbital on GABAA receptor phosphorylation would be necessary for an explanation of the precise mechanisms underlying the development of tolerance to and dependence on pentobarbital.

Nitric oxide modulates the CBF response to increased extracellular potassium

The response of the regional cerebral blood flow (rCBF) to brain topical superfusion of 20 mM K+ was characterized in a closed cranial window preparation in barbiturate anesthetized and ventilated rats: Increasing K+ in the artificial cerebrospinal fluid (ACSF) induced a rCBF elevation (measured by laser-Doppler flowmetry) of +85 +/- 37% above baseline (n = 19). This elevation was stable for > 3 h with continuous superfusion of increased K+ (n = 5) and partially reversible to a level of +18 +/- 19% above baseline when returning to a physiological K+ concentration. Nitric oxide synthase (NOS) inhibition by brain topical superfusion with N omega-nitro-L-arginine (L-NA) revealed (a) Addition of L-NA to high-potassium ACSF reduced the rCBF increase from +94 +/- 36% to +21 +/- 18% (p < or = 0.01, n = 7). (b) When L-NA was superfused for 60 min before increasing K+, rCBF decreased to -17 +/- 7% below baseline. Subsequent coapplication of L-NA and increased K+ induced only an elevation of +7 +/- 4% above baseline (n = 4). (c) When the NO donor S-nitroso-N-acetylpenicillamine (SNAP) was added during NOS inhibition to restore basal tissue NO levels, the resultant level of rCBF was +28 +/- 54% above baseline. Subsequent increase of K+ in the presence of NOS inhibition and SNAP elevated rCBF to +137 +/- 89% above baseline (n = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of protein kinase C does not participate in disruption of the blood-brain barrier to albumin during acute hypertension

The blood-brain barrier minimizes the entry of macromolecules into brain tissue. During acute increases in arterial blood pressure, disruption of the blood-brain barrier occurs primarily in cerebral venules and veins. Mechanisms by which increases in cerebral venous pressure produce disruption of the blood-brain barrier during acute hypertension are not clear. The goal of this study was to determine the role of activation of protein kinase C in disruption of the blood-brain barrier during acute hypertension. We examined the microcirculation of the cerebrum in vivo. Permeability of the blood-brain barrier was quantitated by the formation of venular leaky sites and clearance of fluorescent-labeled albumin (FITC-albumin) before and during phenylephrine-induced acute hypertension. In addition, we examined changes in pial arteriolar and pial venular pressure before and during phenylephrine-induced acute hypertension. We compared responses of the blood-brain barrier to acute hypertension in control (untreated) rats and in rats treated with inhibitors of protein kinase C; calphostin C (0.1 microM) or sphingosine (1.0 microM). Under control conditions, no venular leaky sites were visible and clearance of FITC-albumin was minimal in all groups. Phenylephrine infusion increased systemic arterial, pial arteriolar and pial venular pressures, and increased the formation of venular leaky sites and clearance of FITC-albumin by a similar magnitude in all groups. The findings of the present study suggest that inhibition of protein kinase C does not significantly alter the formation of venular leaky sites and/or clearance of FITC-albumin during acute hypertension. Thus, disruption of the blood-brain barrier during acute hypertension does not appear to be influenced by activation of protein kinase C.

Effets des anesthésiques intraveineux sur les neurones du système nerveux central : mécanismes d'action cellulaires et moléculaires

The mechanisms of action of intravenous anaesthetics are not yet completely elucidated. Until recently, most of the studies had focused on the interactions between anaesthetics and lipid bilayers. It has been proposed that loss of consciousness is produced by disorganization of the lipid phase of nerve membranes, which impairs the action potential propagation. However, new data obtained with sophisticated neuropharmacological tools such as the patch clamp technique have recently contributed to challenge this hypothesis. Indeed, several lines of evidence suggest that intravenous anaesthetics are thought to induce loss of consciousness by blocking the excitatory synaptic transmission. This can be achieved presynaptically, by inhibiting glutamate release from nerve endings via alterations in the gating properties of voltage-dependent calcium channels. Blockade of excitatory synaptic transmission can also occur at the postsynaptic level by antagonizing the glutamate receptors of the N-methyl D-aspartate subtype. Some anaesthetic agents including ketamine also block the nicotinic receptors, however the relevance of this finding with respect to clinical anaesthesia requires further investigation. Preliminary data also suggest that propofol and etomidate elicit uncoupling of gap junctions between astrocytes, which represent a major nonneuronal cell population in the central nervous system. This phenomenon might indirectly contribute to the hypnotic action of these compounds. Whether loss of consciousness involves preferential target structures within the brain remains to be delineated. A better understanding of the mechanisms of action of general anaesthetics might contribute to generate new agents with more pharmacological selectivity and less undesirable side-effects.

Barbiturates inhibit intracellular Ca2+ rise induced by thrombin in rat platelets

The effects of a number of barbiturates (anesthetic as well as anticonvulsant) on thrombin-induced calcium mobilization were tested in rat platelets using the fluorescent Ca2+ probe Fura-2. All drugs, except barbituric acid and Na-barbital, inhibited the thrombin-induced intracellular Ca2+ rise. Both the uptake of extracellular Ca2+ and the release of calcium from intracellular organelles were affected but influx was inhibited more strongly and at lower concentrations of the drugs (e.g. IC50 of thiopental was 0.83 mM for influx and 1.2 mM for intracellular release). Inhibitory potencies of the various barbiturates were markedly different. Thiopental was the most and barbital the least potent inhibitor. The order of inhibitory potency of the drugs appeared generally to follow their lipid solubility and the order of their hypnotic efficiency, with hexobarbital as the most conspicuous exception. Therefore, barbiturate treatment of cells perturbs agonist-induced calcium mobilization. This effect may be partially linked to their previously reported inhibitory action on two kinases, protein kinase C and phosphatidylinositol 4-phosphate kinase [1, 2].