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 neurotransmission over prolonged periods of high-frequency stimulation. Previously, we showed that synaptojanin 1 (SJ1), a presynaptic polyphosphoinositide phosphatase, is required for normal synaptic vesicle recycling (Cremona et al., 1999). We asked whether the stability of inhibitory synaptic responses 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 responses that decreased in size with a bi-exponential time course. A fast component developed within a few stimuli and was quantified with paired-pulse protocols. Paired-pulse depression (PPD) appeared to be independent of previous GABA release at intervals of greater than or equal to 100 msec. The characteristics of PPD, and synaptic depression induced within the first similar to 80 stimuli in the trains, were unaltered in SJ1-deficient inhibitory synapses. A slow component of depression developed within hundreds of stimuli, and steady-state depression showed a sigmoidal dependence on stimulation frequency, with half-maximal depression at 6.0 +/-0.5 Hz. Slow depression was increased when release probability was augmented, and there was a small negative correlation between consecutive synaptic amplitudes during steady-state depression, 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 monitored synaptic depression and a molecular modification known to affect synaptic vesicle reformation.