Alleviation of the Temperature-Sensitive Paralytic Phenotype ofShibireTSMutants inDrosophilaby Sub-Anesthetic Concentrations of Carbon Dioxide

Abstract

Cellular mechanisms involved in general anesthesia are unknown. We report here that sub-anesthetic concentrations of carbon dioxide specifically suppress the temperature-sensitive paralytic phenotype of Drosophila shibire^ts mutants that have a conditional block in synaptic vesicle recycling. Carbon dioxide not only suppresses the onset of temperature-sensitive paralysis, but also rapidly reverses paralysis induced at the restrictive temperature. This effect of CO₂ is most pronounced at about 35% in air, and depends on the absolute concentration of available carbon dioxide rather than on the ratio of oxygen to CO₂. Other general anesthetics, halothane, N₂ or argon do not suppress the paralytic phenotype of shibire significantly at concentrations we tested. Paralysis of the other temperature sensitive paralytic mutants in our collection is not suppressed by carbon dioxide. These behavioral observations are discussed in the light of possible mechanisms underlying paralysis of shi^ts flies. We suggest that spontaneous seizures induced in shi^ts flies held at their restrictive temperatures cause vesicle depletion at critical synapses and consequent behavioral paralysis. The effect of subanesthetic concentrations of CO₂ may be to depress spontaneous CNS activity, thus raising the threshold temperature at which synaptic vesicle depletion occurs. In support of this model, we show that the threshold temperature for paralysis is reduced in shi^ts flies when CNS activity is increased by pharmacological or genetic manipulations, and that subanesthetic concentrations of CO₂ aggravate, rather than alleviate, the t.s. paralytic phenotype of hypoactive para^ts flies defective in axonal voltage-gated sodium channels.