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Potassium Activity Measurements in the Microenvironment of the Central Nervous System of an Insect
1 Department of Pure and Applied Biology, Neuroscience Group, Imperial College, London SW7 2BB
The activity of K+ and the control of influx of + into the extracellular space (micro-environment) of the central nervous system of the cockroach, Periplaneta americana, were measured directly with K+-sensitive microelectrodes. Using an in vivo preparation, it was possible to follow the effects of changes in K+ concentration in the medium bathing the nervous system on extracellular K+ and spontaneous and evoked neuronal activity. For bath K+ levels less than 31 mmoll-1, roughly corresponding to maximal haemolymph level in natural physiological conditions, the blood-brain barrier was found to be suitably efficient in restricting the influx of K+ and thereby allowing normal neural activity. At an external K+ concentration of 100 mmoll-1, however, the system was unable to maintain a sufficiently low extracellular K+ concentration and neuronal activity was suppressed. Influx of K+ from the external medium into the micro-environment occurred mainly in two phases. The early phase had a fast time course and probably reflects the physical aspects of the blood-brain barrier. The later, second phase was a slower process possibly corresponding to activation of metabolic ion pumps. The time courses of the functioning of these two systems and their control of the extraaxonal K+ activity are also discussed.
Key words: blood-brain barrier, extracellular potassium, ion-sensitive microelectrodes, Periplaneta americana
Accepted on August 27, 1986
