Diurnal inhibition of NMDA‐EPSCs at rat hippocampal mossy fibre synapses through orexin‐2 receptors
Authors: Perin Martina , Longordo Fabio , Massonnet Christine , Welker Egbert , Lüthi Anita 
Published: August 2014

Key points

Orexins are well described for their excitatory actions on feeding‐ and arousal‐promoting brain centres; we describe here an inhibitory action of orexin‐A on synaptic NMDA receptors in hippocampus, a key area for synaptic plasticity and memory formation.

Orexin‐A inhibited NMDA receptor responses at mossy fibre–CA3 connections through postsynaptically expressed orexin‐2 receptors, whereas a minor inhibition was observed at Schaffer collateral–CA1 connections, and no effect occurred at non‐mossy‐fibre excitatory synapses in CA3.

Exogenously applied orexin‐A inhibited NMDA receptors in slices prepared during the rats’ resting phase, when endogenous orexin levels are low, but not in slices prepared in the active phase, when endogenous orexins peak.

Through intraperitoneal administration of an orexin receptor antagonist during the active period, exogenous orexin‐A‐mediated inhibition was restored in the slice.

Endogenous orexins suppress hippocampal synaptic NMDA receptor function in a diurnally cyclic manner, probably restraining synaptic plasticity and learning during certain periods of waking.

Diurnal release of the orexin neuropeptides orexin‐A (Ox‐A, hypocretin‐1) and orexin‐B (Ox‐B, hypocretin‐2) stabilises arousal, regulates energy homeostasis and contributes to cognition and learning. However, whether cellular correlates of brain plasticity are regulated through orexins, and whether they do so in a time‐of‐day‐dependent manner, has never been assessed. Immunohistochemically we found sparse but widespread innervation of hippocampal subfields through Ox‐A‐ and Ox‐B‐containing fibres in young adult rats. The actions of Ox‐A were studied on NMDA receptor (NMDAR)‐mediated excitatory synaptic transmission in acute hippocampal slices prepared around the trough (Zeitgeber time (ZT) 4–8, corresponding to 4–8 h into the resting phase) and peak (ZT 23) of intracerebroventricular orexin levels. At ZT 4–8, exogenous Ox‐A (100 nm in bath) inhibited NMDA receptor‐mediated excitatory postsynaptic currents (NMDA‐EPSCs) at mossy fibre (MF)–CA3 (to 55.6 ± 6.8% of control, P = 0.0003) and at Schaffer collateral–CA1 synapses (70.8 ± 6.3%, P = 0.013), whereas it remained ineffective at non‐MF excitatory synapses in CA3. Ox‐A actions were mediated postsynaptically and blocked by the orexin‐2 receptor (OX2R) antagonist JNJ10397049 (1 μm), but not by orexin‐1 receptor inhibition (SB334867, 1 μm) or by adrenergic and cholinergic antagonists. At ZT 23, inhibitory effects of exogenous Ox‐A were absent (97.6 ± 2.9%, P = 0.42), but reinstated (87.2 ± 3.3%, P = 0.002) when endogenous orexin signalling was attenuated for 5 h through i.p. injections of almorexant (100 mg kg−1), a dual orexin receptor antagonist. In conclusion, endogenous orexins modulate hippocampal NMDAR function in a time‐of‐day‐dependent manner, suggesting that they may influence cellular plasticity and consequent variations in memory performance across the sleep–wake cycle.