Neuroscience Letters, 66 (1986) 263-268 263 Elsevier Scientific Publishers Ireland Ltd. NSL 03930 VARIATION OF POTASSIUM ION CONCENTRATIONS IN THE RAT HIPPOCAMPUS SPECIFICALLY AFFECTS EXTRACELLULAR TAUR1NE LEVELS JOS]~ M. SOLiS*, ANTONIO S. HERRANZ, OSCAR HERRERAS, MARIA D. MU~,IOZ, RAFAEL MARTIN DEL RIO and JUAN LERMA Departamento de Investigaci6n, Centro 'Rambn y C~jal', E-28034 Madrid (Spain) (Received January 21st, 1986; Revised version received February 17th, 1986; Accepted February 19th, 1986) Key wor~,(sv low potassium taurine hippocampus extracellular fluid rat The effects of different K- concentrations (3-100 mM) on both the extracellular amino acid levels and field potentials, evoked by perforant pathway stimulation, were studied 'in vivo" in the rat dentate gyrus by means of a brain dialysis device, formed by a hollow fiber plus a stainless-steel electrode. Perfusion with low K concentrations (3-12 raM; Krebs Ringer bicarbonate) specifically enhanced the dialysate lev- els of taurine and concomitantly increased the population spike amplitude. High K concentrations in pert'usate ( > 25 raM) did not further increase the levels of taurine but enhanced both glutamate and 7- aminobutyric acid levels, whereas the population spike diminished drastically. The absence of calcium ions in the perfusion liquid increased both basal and K+-enhanced taurine levels. The specific enhancement of cxtracellular taurine by physiological K- concentrations may represent an autoregulative mechanism of nervous tissue excitability. While exposure of the central nervous system to solutions with high concentrations of K (40 100 raM) evokes the efflux of putative amino acid neurotransmitters, it is improbable that these extracellular K ~ concentrations ([K~]o) occur in nature. It is well-known, however, that the [K *]o increases up to 12 mM as a consequence of neuronal activity [9, 19] and that these elevated concentrations (6-12 raM) enhance the amplitude [6] and duration [20] of hippocampal evoked field potentials, playing an important role in regulating neuronal excitability. The purpose of the present study was to determine the possible influence of physiological [K+]o on the extracellu- lar amino acid levels in the 'in vivo' dentate gyrus of the rat hippocampal formation. Adult Sprague-Dawley rats were studied while maintained under urethane (1.2 g/ kg, i.p.) anaesthesia. A modified dialytrode [2], formed by a 200 ~m thick hollow fiber plus a 60 ktm stainless-steel recording electrode, was implanted stereotaxically in the dorsal dentate gyrus of the hippocampus. Field potentials, evoked by perforant path- way stimuli, served to test the response of dentate gyrus cells to monosynaptic activa- *Author for reprint requests at: Depto. Investigaci6n, Ctro, 'Ram6n y Cajal', Ctra. Colmenar Km. 9, E- 28034 Madrid, Spain. 0304-394(I/86/$ 03.50 © 1986 Elsevier Scientific Publishers Ireland Ltd.