In vivo synaptic potency, short-term and long-term plasticity at the hippocampal Schaffer collateral-CA1 synapses: Role of different light-dark cycles in male rats

Abstract

The changes in the light-dark (L/D) cycle could modify cellular mechanisms in some brain regions. The present study compared the effects of various L/D cycles on in vivo synaptic potency, short-term and long-term plasticity in the hippocampal CA1 area, adrenal glands weight (AGWs), corticosterone (CORT) levels, and body weight differences (BWD) in male rats. Male rats were assigned into different L/D cycle groups: L4/D20, L8/D16, L12/D12 (control), L16/D8, and L20/D4. The slope, amplitude, and the area under curve (AUC) related to the field excitatory postsynaptic potentials (fEPSPs) were assessed, using the input-output (I/O) functions, paired-pulse (PP) responses at different interpulse intervals, and after the induction of long-term potentiation (LTP) in the hippocampal CA1 area. Also, the CORT levels, AGWs, and BWDs were measured in all groups. The slope, amplitude, and AUC of fEPSP in the I/O functions, all three phases of PP, before and after the LTP induction, were significantly decreased in all experimental groups, especially in the L20/D4 and L4/D20 groups. As such, the CORT levels and AGWs were significantly increased in all experimental groups, especially in the L20/D4 group. Overall, the uncommon L/D cycles (minimum and particularly maximum durations of light) significantly reduced the cellular mechanism of learning and memory. Also, downtrends were observed in synaptic potency, as well as short-term and long-term plasticity. The changes in PP with high interpulse intervals, or activity of GABAB receptors, were more significant than the changes in other PP phases with different L/D durations. Additionally, the CORT levels, adrenal glands, and body weight gain occurred time-independently concerning different L/D lengths.