Synaptojanin 1 contributes to maintaining the stability of GABAergic transmission in primary cultures of cortical neurons

Inhibitory synapses in the CNS can exhibit a considerable stability of neur otransmission over prolonged periods of high-frequency stimulation. Previou sly, we showed that synaptojanin 1 (SJ1), a presynaptic polyphosphoinositid e phosphatase, is required for normal synaptic vesicle recycling (Cremona e t al., 1999). We asked whether the stability of inhibitory synaptic respons es was dependent on SJ1. Whole-cell patch-clamp recordings of unitary IPSCs were obtained in primary cortical cultures between cell pairs containing a presynaptic, fast-spiking inhibitory neuron (33.5-35 degreesC). Prolonged presynaptic stimulation (1000 stimuli, 2-20 Hz) evoked postsynaptic respons es that decreased in size with a bi-exponential time course. A fast compone nt developed within a few stimuli and was quantified with paired-pulse prot ocols. Paired-pulse depression (PPD) appeared to be independent of previous GABA release at intervals of greater than or equal to 100 msec. The charac teristics of PPD, and synaptic depression induced within the first similar to 80 stimuli in the trains, were unaltered in SJ1-deficient inhibitory syn apses. A slow component of depression developed within hundreds of stimuli, and st eady-state depression showed a sigmoidal dependence on stimulation frequenc y, with half-maximal depression at 6.0 +/-0.5 Hz. Slow depression was incre ased when release probability was augmented, and there was a small negative correlation between consecutive synaptic amplitudes during steady-state de pression, consistent with a presynaptic depletion process. Slow depression was increased in SJ1-deficient synapses, with half-maximal depression at 3. 3 +/-0.9 Hz, and the recovery was retarded similar to3.6-fold. Our studies establish a link between a distinct kinetic component of physiologically mo nitored synaptic depression and a molecular modification known to affect sy naptic vesicle reformation.