BACKGROUND AND AIMS: The dorsal lateral geniculate nucleus (dLGN) is a relay station for the transmission of visual information to the cerebral cortex. It receives input from all three types of photoreceptors: rods, cones and melanopsin cells. Electrophysiological studies on mice have shown that eye illumination induces three types of neuronal responses within the dLGN: sustained, transient ON and OFF. The purpose of the present study was to verify, if similar types of light-induced responses occur in the dLGN of pigmented rat and whether they are associated with the specific firing pattern of neurons (bursting, tonic, oscillatory). The second goal was to determine if these neurons are modulated by sleep-like cycle (cortical activation/deactivation) visible in the EEG signal. METHODS: We performed in vivo extracellular single-unit neuronal recordings from the dLGN of urethane-anaesthetized Long Evans rats in combination with EEG recordings and light stimulations with different intensities. RESULTS: We observed excitation orsuppression of dLGN cells upon the eye illumination.The most common response (63%) was a transient ON, and only sustained cells support irradiance-dependent increases in the firing rate of dLGN neurons. Furthermore, the type of light-induced responses was independent of the neuronal firing pattern. The activity of 44% of recorded neurons was reflected by fluctuations of the EEG signal (reduced activity during NREM-like sleep). CONCLUSIONS: All three types of light-induced neuronal responses were present within the dLGN of pigmented rat. Only sustained cells were able to code light intensity, what may suggest, that they received input from melanopsin cells, like it was observed in mice. Moreover, the obtained results showed that rhythmic changes in the EEG signal reflects alterations in the mean firing rate of recorded cells, what indicating the influence of state-dependent changes in CNS activity on sensitive to light dLGN neurons. This study was supported by grant 2013/08/W/N23/00700.
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