Harmonic discharge pattern reveals fundamental frequency (ca. 40 Hz) synchronising elements of the rat subcortical visual system

The lateral geniculate nucleus (LGN) is a retinorecipient thalamic structure serving both vision and non-vision forming functions. Subpopulation of LGN neurons is characterised by isoperiodic, infra slow oscillation (ISO; <0.01 Hz) in the rate of action potential firing. This ISO is common in the subcortical visual system and is synchronised among nuclei innervated by the same eye. Recently a new feature of light responsive neurons in the suprachiasmatic nuclei (SCN) has been described – harmonic distribution pattern (HDP) of interspike intervals (ISI), revealing fundamental frequency of ca. 30 Hz. The aim of this study was to determine the existence of HDP in the firing of the rat LGN. Single- or 32-channel extracellular recordings of neuronal firing were performed on in vivo (urethane anaesthesia) and in vitro preparations of Wistar rat brain. In vivo recordings were performed in experimentally controlled lighting conditions and during alternating brain states determined by EEG monitoring. We have discovered harmonic distribution pattern (HDP) of interspike intervals (fundamental ISI ~25–30 ms) in half of the in vivo recorded LGN and OPN neurons that were characterised by ISOs, but not in vitro. HPD was resistant to sustained light changes, although altered by transient (5 s) light pulses. Inactivation of contralateral retina (TTX injection) abolished HDP observed in LGN. HDP was correlated within each LGN and OPN and also between ipsilateral nuclei. For the first time we show that in LGN and OPN there is a subpopulation of neurons that fires in synchrony, governed by common HDP. Our results, combined with observation in SCN, allow as to propose the retina as a common source of HDP in light-responsive neurons and bring us closer to understanding the function of widespread retinal innervation of the mammalian brain. FINANCIAL SUPPORT: Supported by MSHE grant: 0001/ DIA/2014/43