Inhibitory effects of ventral tegmental area stimulation on the activity of prefrontal cortical neurons: evidence for the involvement of both dopaminergic and GABAergic components.

The medial prefrontal cortex of the rat receives dopamine and non-dopaminergic projections from the ventral tegmental area. Both electrical stimulation of the ventral tegmental area and local application of dopamine induce an inhibition of the spontaneous activity of most prefrontal cortical neurons, including efferent neurons. In the present study, the techniques of extracellular recording and microiontophoresis were used in anesthetized rats in order to determine whether these dopamine- and ventral tegmental area-induced inhibitory responses involve GABAergic components. Prefrontal cortex output neurons were identified by antidromic activation from subcortical structures. The inhibitory responses evoked by the local application of dopamine were blocked by the iontophoretic application of the D2 antagonist sulpiride, and the GABAA antagonist bicuculline in 89 and 57% of the cases, respectively. In addition, sulpiride and bicuculline abolished the inhibition induced by ventral tegmental area stimulation in 54 and 51% of the prefrontal cortical cells tested, respectively. The implication of a non-dopaminergic mesocortical system in the ventral tegmental area-induced inhibition was further analysed using rats pre-treated with alpha-methylparatyrosine to deplete dopamine stores. The proportion of prefrontal cortical cells inhibited by ventral tegmental area stimulation was markedly reduced (39%) in alpha-methylparatyrosine-treated rats, when compared to controls (86%). Remaining ventral tegmental area-induced inhibition was no longer affected by sulpiride, but in all cases blocked by the local microiontophoretic application of bicuculline. The present results suggest that: (1) the dopamine-induced inhibition of prefrontal cortex neurons could involve cortical GABAergic interneurones; (2) the non-dopaminergic mesocortical system exerts also an inhibitory influence on prefrontal cortical cells and appears to be GABAergic.

Acute mercury chloride intoxication. Effects of hemodialysis and plasma exchange on mercury kinetic.

A 27 year-old man developed after ingestion of mercury chloride, 6 g, a hypovolemic shock, an acute renal failure and a necrosis of the stomach which required a total gastrectomy. The anuria did not improve and required 42 hemodialyses. Subsequent evolution showed numerous complications and the patient died on the 91st day. On admission mercury plasma concentration was 5 mg/L and decreased slowly with an apparent half-life of 226 hours. Hemodialyses were ineffective for mercury elimination: mercury clearances varied between -10 and + 1.5 ml/min. Seventeen mg of mercury were removed by six plasma exchanges: the mercury clearance was mean 17.3 ml/min. Among the extracorporeal elimination methods, plasma exchange appears to be the most efficient for inorganic mercury and it could be usefull in association with chelation therapy at the early phase of the intoxication.

Effects of a low dose of pancuronium on oxygen consumption during sustained muscular exercise.

The effects of a low dose of pancuronium on muscular performance were studied in six healthy volunteers. Three dynamic exercise tests at increasing levels of power were performed by each subject on two consecutive days using an ergometric bicycle. To assess muscular function, oxygen consumption was measured in a steady state at each level of power output. Pancuronium 6-8 micrograms kg-1 or placebo was administered i.v. in a double-blind fashion before the second test each day. Plasma pancuronium levels were measured 8 min after administration but even with pancuronium clearly detectable in the plasma no significant differences were found either in oxygen consumption or in the haemodynamic measurements made. The authors conclude that such doses of non-depolarizing neuromuscular blocking agents do not disturb oxygen consumption during dynamic muscular exercise and are therefore unlikely to have a significant effect on muscular function.