The Journal of Neurophysiology Vol. 79 No. 6 June 1998, pp. 2999-3011
Copyright ©1998 The American Physiological Society

Changes in Quantal Size Distributions Upon Experimental Variations in the Probability of Release at Striatal Inhibitory Synapses

Jan C. Behrends and Gerrit ten Bruggencate

Department of Physiology, Universität München, 80336 Munich, Germany

Behrends, Jan C. and Gerrit ten Bruggencate. Changes in quantal size distributions upon experimental variations in the probability of release at striatal inhibitory synapses. J. Neurophysiol. 79: 2999-3011, 1998. Postsynaptic inhibitory -aminobutyric acid-A (GABA_A)-receptor-mediated current responses were measured using simultaneous pre- and postsynaptic whole cell recordings in primary cell cultures of rat striatum. Substitution of Sr²⁺ for extracellular Ca²⁺ strongly desynchronized the inhibitory postsynaptic currents (IPSCs), resulting in a succession of asynchronous IPSCs (asIPSCs). The rise times and decay time constants of individual evoked asIPSCs were not significantly different from those of miniature IPSCs that are the result of spontaneous vesicular release of GABA. Thus asIPSCs reflect quantal transmission at the individual contacts made by one presynaptic neuron on the recorded postsynaptic 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 probability of release (P_r). At the same time the amplitude distribution of asIPSCs was shifted toward larger values, whereas responses to exogenously applied GABA on average were slightly decreased in amplitude. Application of the GABA_B-receptor agonist baclofen (3-10 µM) strongly reduced the frequency of asIPSC, consistent with a decrease in P_r, and led to a shift of the amplitude distribution toward smaller values. 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 variation in the amount of transmitter released.

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