Parvalbumin-deficiency facilitates repetitive IPSCs and gamma oscillations in the hippocampus

In the hippocampus, the calcium-binding protein parvalbumin (PV) is expressed in interneurons that innervate perisomatic regions. PV in GABAergic synaptic terminals was proposed to limit repetitive GABA release by buffering of "residual calcium." We assessed the role of presynaptic PV in Ca2+-dependent GABA release in the hippocampus of PV-deficient (PV-/-) mice and wild-type (PV+/+) littermates. Pharmacologically isolated inhibitory postsynaptic currents (IPSCs) were evoked by low-intensity stimulation of the stratum pyramidale and recorded from voltage-clamped CA1 pyramidal neurons. The amplitude and decay time constant of single IPSCs were similar for both genotypes. Under our experimental conditions of reduced release probability and minimal presynaptic suppression, paired-pulse facilitation of IPSCs occurred at intervals from 2 to 50 ms, irrespective of the presence of PV. The facilitation of IPSCs induced by trains of 10 stimuli at frequencies >20 Hz was enhanced in cells from PV-/- mice, the largest difference between PV-/- and PV+/+ animals (220%) being observed at 33 Hz. The effect of IPSC facilitation at sustained gamma frequencies was assessed on kainate-induced rhythmic IPSC-paced neuronal oscillations at gamma frequencies, recorded with dual field potential recordings in area CA3. The maximum power of the oscillation was 138 muV(2) at 36 Hz in slices from PV+/+ mice and was trebled in slices from PV-/- mice. PV deficiency caused a similar increase in gamma power under conditions used to study IPSC facilitation and can be explained by an increased facilitation of GABA release at sustained high frequencies. The dominant frequency and coherence were not affected by PV deficiency. These observations suggest that PV deficiency, due to an increased short-term facilitation of GABA release, enhances inhibition by high-frequency burst-firing PV-expressing interneurons and may affect the higher cognitive functions associated with gamma oscillations.