Photoperiod during development does not change GABA signalling of the murine central circadian clock in adulthood

Perinatal exposure to different photoperiods can lead to differences in behaviour and sleep during adulthood. Information about acute photoperiod is first processed in the brain through the suprachiasmatic nuclei (SCN), the primary brain nuclei responsible for circadian rhythms. In these nuclei, the photoperiodic information is coded and related to the excitatory/inhibitory (E/I) balance of GABA. We investigated in mice, whether behavioural effects of perinatal photoperiod (PNP) in adulthood are represented in a difference in the E/I balance in SCN cells. We exposed males and females to a short or long PNP from conception until weaning, followed by an intermediate photoperiod for 4 weeks. Subsequently, we measured neuronal calcium responses in response to GABA in SCN brain slices. We compared the E/I ratio of the neuronal calcium responses, response type distribution and the baseline intracellular calcium concentration ([Ca²⁺]_i) between the two groups. Mice from the long and short PNP had a similar E/I ratio and distribution of response types. There was no significant difference in [Ca²⁺]_ibetween the PNPgroups, but the cell response type was correlated with baseline [Ca²⁺]_i. On average, baseline [Ca²⁺]_iof inhibitory cells was higher than that of excitatory cells. PNP does not seem to affect GABAergic E/I balance of calcium responses in the SCN in the long-term. This means that previously found behavioural differences in adult mice caused by PNP are probably not mediated through SCN E/I balance. It is possible that the observed long-term differences stem from changes in other brain nuclei or systems.