Stereoselective modulatory actions of oleamide on GABAA receptors and voltage-gated Na+ channels in vitro:: a putative endogenous ligand for depressant drug sites in CNS

1 cis-9,10-octadecenoamide ('oleamide') accumulates in CSF on sleep deprivation. It induces sleep in animals (the trans form is inactive) but its cellular actions are poorly characterized. We have used electrophysiology in cultures from embryonic rat cortex and biochemical studies in mouse nerve preparations to address these issues. 2 Twenty mu M cis-oleamide (but not trans) reversibly enhanced GABA(A) currents and depressed the frequency of spontaneous excitatory and inhibitory synaptic activity in cultured networks. 3 cis-oleamide stereoselectively blocked veratridine-induced (but not K+-induced) depolarisation of mouse synaptoneurosomes (IC50, 13.9 mu M). 4 The cis isomer stereoselectively blocked veratridine-induced (but not K+-induced) [H-3]-GABA release from mouse synaptosomes (IC50, 4.6 mu M). 5 At 20 mu M cis-oleamide, but not trans, produced a marked inhibition of Na+ channel-dependent rises in intrasynaptosomal Ca2+. 6 The physiological significance of these observations was examined by isolating Na+ spikes in cultured pyramidal neurones. Sixty-four mu M cis-oleamide did not significantly alter the amplitude, rate of rise or duration of unitary action potentials (1 Hz). 7 cis-Oleamide stereoselectively suppressed sustained repetitive firing (SRF) in these cells with an EC50 of 4.1 mu M suggesting a frequency- or state-dependent block of voltage-gated Na+ channels. 8 Oleamide is a stereoselective modulator of both postsynaptic GABA(A) receptors and presynaptic or somatic voltage-gated Na+ channels which are crucial for synaptic inhibition and conduction. The modulatory actions are strikingly similar to those displayed by sedative or anticonvulsant barbiturates and a variety of general anaesthetics. 9 Oleamide may represent an endogenous modulator for drug receptors and an important regulator of arousal.