Selective GABAergic input to SATB1-immunonegative axo-axonic cells and inhibition during sharp waves in vivo. GABAergic interneurons immunoreactive for the calcium-binding protein parvalbumin (magenta) are found in the CA3 area of the hippocampus. Some express the transcription factor SATB1 (yellow; top two cells), whereas others lack detectable SATB1 immunoreactivity (bottom cell). Axo-axonic cells, which are SATB1 immunonegative interneurons, are selectively innervated by GABAergic neurons of the medial septum (cyan, anterogradely-labelled medial septal axons and boutons). In CA3 during sleep and awake immobility, sharp waves (top trace) are initiated and occur simultaneously with high-frequency 'ripples' (middle trace; 130-230 Hz). The firing of axo-axonic cells (bottom trace; action potentials from a single cell) is silenced during sharp waves.