• chloride
• cortex
• oscillation

Intracellular chloride concentration ([Cl]i) influences the action of gamma-aminobutyric acid (GABA) on neurons. Usually, this neurotransmitter elicits inhibition by binding to GABAA receptors thus allowing chloride ions to flow inside the postsynaptic cell. However, GABA was reported to exert an excitatory action early during development and in pathology. In these cases, GABA acts as a depolarizing neurotransmitter because of a higher [Cl]i allowing an outflow of chloride ions. We reported for the first time diurnal oscillations in intracellular chloride levels in vivo in pyramidal neurons of the primary visual cortex of rodents; [Cl]i was low at midday and increased at midnight. The aim of this thesis is to demonstrate that diurnal chloride oscillations drive physiological changes in the response and elaboration of external stimuli. Electrophysiological recordings were acquired to evaluate the visually-driven response at day and night, the time at which [Cl]i differs the most, resulting in a different response at different times. To further analyse the cause of this difference, parvalbumin interneurons were optogenetically activated at different times; we found that a diverse inhibitory strength is exerted at day and at night, with the minimal inhibitory activity reported at night. These results matched with our previous ones reporting a change in brain excitability that follows the chloride rhythm and shed light on a new scenario in which GABA changes its effect according to [Cl]i also in the cortex of adult mammals following diurnal oscillations. As GABA exerts plenty of physiological action, further analysis will be carried out to assess new biological questions.