CHANGES IN QUANTAL SIZE DISTRIBUTIONS UPON EXPERIMENTAL VARIATIONS INTHE PROBABILITY OF RELEASE AT STRIATAL INHIBITORY SYNAPSES

Postsynaptic inhibitory gamma-aminobutyric acid-A (GABA(A))-receptor-m ediated current responses were measured using simultaneous pre- and po stsynaptic whole cell recordings in primary cell cultures of rat stria tum. Substitution of Sr2+ for extracellular Ca2+ strongly desynchroniz ed the inhibitory postsynaptic currents (IPSCs), resulting in a succes sion of asynchronous IPSCs (asIPSCs). The rise times and decay time co nstants of individual evoked asIPSCs were not significantly different from those of miniature IPSCs that are the result of spontaneous vesic ular release of GABA. Thus asIPSCs reflect quantal transmission at the individual contacts made by one presynaptic neuron on the recorded po stsynaptic cell. Increasing the concentration of Sr2+ from 2 to 10 mM and decreasing that of Mg2+ from 5 to 1 mM produced an increase in the frequency of asIPSCs consistent with an enhancement of the mean proba bility of release (P-r). At the same time the amplitude distribution o f asIPSCs was shifted toward larger values, whereas responses to exoge nously applied GABA on average were slightly decreased in amplitude. A pplication of the GABA(B)-receptor agonist baclofen (3-10 mu M) strong ly reduced the frequency of asIPSC, consistent with a decrease in P-r, and led to a shift of the amplitude distribution toward smaller value s. Baclofen had no effect on responses to exogenously applied GABA. In summary, our data suggest that at striatal inhibitory connections the weight of single contacts may be controlled presynaptically by variat ion in the amount of transmitter released.