Modulation of GABA_A receptors and inhibitory synaptic currents by the endogenous CNS sleep regulator cis-9,10-octadecenoamide (cOA)

Abstract

1. Cis-9,10-octadecenoamide (cOA) accumulates in the CSF of sleep-deprived cats and may represent a novel signalling molecule. Synthetic cOA has been shown to induce physiological sleep when injected into laboratory rats. Here we assess the cellular mode of action of cOA in vitro.

2. In all rat cultured cortical neurones (pyramidal cells) examined, the synthetic brain lipid (3.2–64 μM) enhanced the responses to subsaturating GABA concentrations (up to circa 2×) in a concentration-dependent manner (EC₅₀, circa 15 μM).

3. (20 μM) cOA significantly enhanced the affinity of exogenous GABA for its receptor without changing the Hill slope or the maximal response. These effects were not voltage-dependent or secondary to shifts in E_Cl.

4. In the absence of GABA, cOA directly evoked small inhibitory currents in a subpopulation (<7%) of sensitive cells.

5. 20 μM cOA reversibly enhanced the duration of spontaneous inhibitory post synaptic currents (circa 2 fold) without significantly altering their amplitude.

6. At 32–64 μM, cOA reversibly reduced the incidence and amplitude of both inhibitory post synaptic currents (i.p.s.cs) and excitatory post synaptic currents (e.p.s.cs) in the cultured neuronal circuits in common with other depressant drugs acting at the GABA_A receptor.

7. 32 μM Oleic acid did not modulate exogenous GABA currents or synaptic activity suggesting that cOAs actions are mediated through a specific receptor.

8. A specific, protein-dependent interaction with GABA_A receptors was confirmed in Xenopus oocytes. Recombinant human receptors were modulated by 10 μM cOA (and diazepam) only when a γ₂ subunit was co-expressed with α₁β₂: the cOA response was not sensitive to the specific benzodiazepine antagonist flumazenil (1 μM).

9. cOA may represent an endogenous ligand for allosteric modulatory sites on isoforms of GABA_A receptors which are crucial for the regulation of arousal and have recently been implicated in the circadian control of physiological sleep.

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