Early Sharp Wave Synchronization along the Septo-Temporal Axis of the Neonatal Rat Hippocampus

In the neonatal rat hippocampus, the first and predominant pattern of synchronized neuronal network activity is early sharp waves (eSPWs) occurring at a frequency of ~2–4 events per minute. However, how eSPWs are organized longitudinally along the septo-temporal hippocampal axis remains unknown. Using silicone probe recordings from the septal and intermediate segments of the CA1 hippocampus in neonatal rats in vivo we found that eSPWs are highly synchronized longitudinally. The amplitudes of eSPWs in the septal and intermediate segments of the hippocampus were also highly correlated. eSPWs also supported longitudinal synchronization of CA1 multiple unit activity. Spatial-temporal analysis revealed a septal-temporal gradient with more frequent initiation of eSPWs in the septal regions. The speed of eSPW longitudinal propagation attained ~ 250 mm/s. We suggest that longitudinal correlated activity supported by synchronized eSPWs emerges early during postnatal development and may participate in the formation of intrahippocampal connections in the developing hippocampus.