Diurnal modulation of optogenetically evoked neural signals

Neural signal processing in the cerebral cortex is often regarded as robust and stereotyped; however, the brain's internal environment undergoes dynamic fluctuations across the day. Whether these diurnal rhythms modulate cortical responsiveness and plasticity remains unclear. Here, we examined diurnal modulation of neural responsiveness and plasticity in the primary visual cortex (V1). Using transgenic rats expressing channelrhodopsin-2 (ChR2), we optically stimulated V1 neurons with brief light pulses and recorded local field potentials (LFPs) over several days. V1 responses to single-pulse stimulation showed clear diurnal variation, with delta- and gamma-band activity modulated in a time-of-day-dependent manner. Administration of the adenosine A1 receptor antagonist DPCPX enhanced neural responses at Zeitgeber time (ZT) 0 (Sunrise) but not at ZT 12 (Sunset). LTP-like potentiation was observed only when train stimulation was applied at Sunrise, indicating that plasticity is also gated by diurnal phase. These findings demonstrate that both excitability and plasticity of V1 circuits are regulated by diurnal factors. Although it remains unclear whether these effects are driven by intrinsic circadian rhythms or light/dark-triggered mechanisms, our results highlight that cortical processing is dynamically modulated across the day, with implications for sensory function, learning, and neuromodulatory regulation.