Diurnal rhythm regulates the frequency of carbachol-induced beta oscillation via inhibitory neural system in rat hippocampus

Animals have a diurnal rhythm with a cycle of approximately 1 day modulated by light information. This rhythm modulates memory performance. Relatedly, the hippocampal neural circuit has a dynamic property that can induce theta, beta, and gamma brain waves. However, the associated between the diurnal rhythm and these waves has not yet been elucidated. Carbachol, a cholinergic agent, can induce oscillations (e.g., beta waves) in rat hippocampal slices. In this study, we investigate the diurnal changes in the dynamic properties of hippocampal neuronal circuits using carbachol-induced beta oscillations (CIBOs). The hippocampal slices were made from rats adapted to 12-h-each light and dark conditions. The frequency of CIBO was significantly decreased more at midnight than at midday. There was no significant difference both under 12-h-each dark/dark condition and in the shuffled data of diurnal condition. The frequency at midday was significantly decreased by the application of SR95531 (gabazine) and bicuculline gamma-aminobutyric acid (GABA)-A receptor antagonists. In paired-pulse inhibition (PPI) experiments, the PPI ratio at midnight was larger than that at midday. The PPI ratio reflects the degree of recurrent inhibition. The expression level of Glutamate decarboxylase 65, an enzyme that synthetizes GABA, was significantly higher at midnight than at midday. These results suggest that the CIBO frequency can be modulated by diurnal changes of hippocampal inhibitory neurons, and the modulation may lead to a diurnal change in memory performance.