RELEASE OF [3H]GABA FROM IN VITRO PREPARATIONS: COMPARISON OF THE EFFECT OF DABA AND β-ALANINE ON THE K+ AND PROTOVERATRINE STIMULATED RELEASE OF [3H]GABA FROM BRAIN SLICES AND SYNAPTOSOMES1

It has been proposed that the major portion of ³H]GABA released from rat cortical slices upon exposure to high K⁺ comes from a neuronal pool. Using carrier mediated exchange diffusion of DABA or β-alanine in the superfusion medium for GABA in the slice as a technique for manipulating neuronal and glial pools of GABA, it was found that DABA but not β-alanine substantially reduced the K⁺ stimulated release of ³H]GABA. The present study using synaptosomes as an in vitro model of the nerve ending was undertaken to ascertain whether this neuronal pool of releasable ³H]GABA was associated with a specific transmitter pool in nerve endings. A continuous superfusion system employing a Ca²⁺ pulse to produce a calcium coupled release (Levy et al, 1973) was used to study the effect of two concentrations (20 μm , 1 mm ) of DABA and β-alanine on the release of ³H]GABA from synaptosomes. In contrast to the results in slices, DABA at both concentrations had no effect on the release of ³H]GABA from synaptosomes in spite of evidence that exchange diffusion was occurring. With protoveratrine as the releasing agent there was no effect of DABA on the release of ³H]GABA from either slices or synaptosomes. The results suggest that the major portion of ³H]GABA released from cortical slices by high K⁺ comes from a non-transmitter pool in the neuron. Use of K⁺ stimulated release of amino acids from cortical slices as a criterion for neurotransmitter function must be viewed with caution.