Release of Purines, Noradrenaline, and GABA from Rat Hippocampal Slices by Field Stimulation

Abstract

Labelled adenine, noradrenaline (NA), and γ-aminobutyric acid (GABA) were taken up by the transversely cut hippocampal slice. [³H]NA and [¹⁴C]GABA were retained as such, [³H]- (or [¹⁴C]-) adenine mainly as adenine nucleotides. There was a spontaneous overflow of all three types of compounds ranging from 0.1 (GABA) to 0.21 (NA) %/min. The rate of [³H]NA overflow increased rapidly during electrical field stimulation. The release rate was well maintained over a 15-min period. The rate of [¹⁴C]GABA release also increased rapidly but it was not maintained over a 15-min period even if uptake and/or metabolism was inhibited by nipecotic acid (1 mM) and aminooxyacetic acid (AOAA, 0.1 mM). The bulk of the purines was released after the stimulation period. For all compounds the amounts released were frequency- and calcium-dependent. At a frequency of 3 Hz a 10 V stimulation was sufficient to cause a maximal [³H]NA release and 20 V to cause maximal [¹⁴C]GABA release, but ¹⁴C-purine release was increased further by increasing the voltage to 40 V. The evoked purine release was inhibited by a nucleoside uptake inhibitor (dipyridamole). On stimulation of [³H]NA-labelled slices the released radioactivity was composed of >95% unchanged NA. The specific activities of NA in the slice and in the superfusate were practically identical. In [³H]adenine-labelled slices the released radioactivity was composed of adenosine, inosine, and hypoxanthine, but the activity in the slice of ATP, ADP, and AMP. The specific activity of the released purines was about four times higher than that of the purines retained in the slice but similar to that of the hypoxanthine in the slice. It is suggested that the purine release induced by electrical field stimulation differs in important respects from the release of a traditional neurotransmitter such as NA and that it depends on the intracellular formation of adenosine from a labile fraction of adenine nucleotides that is affected by neurotransmission.