Oleamide (cOA) is a sleep-inducing brain lipid, which has been reported to be a potent modulator (1-100 nM) of serotonergic receptors in oocytes and cell lines (_2A/C and _1A subtypes) (e.g. Boger et al. 1998). In vivo studies provide evidence for, and against, a serotonergic mechanism (Cheer et al. 1999; Federova et al. 2001). The CA1 region of the hippocampus bears somatic 5HT_1A receptors, which mediate hyperpolarisation and a concentration-dependent depression of the population spike (Pugliese et al. 1998). We have assessed the ability of cOA to modulate serotonergic transmission in the in vitro hippocampal slice exposed briefly to exogenous 5HT.

Male Wistar rats (ca 150 g) were humanely killed and sagittal brain slices (400 µm) were prepared for superfusion at 35 °C with ACSF (mM): NaCl 119, KCl 3, MgCl₂ 1, CaCl₂ 2, NaHCO₃ 26.2, NaH₂PO₄ 1.25, D-glucose 10, equilibrated with 95 % O₂ and 5 % CO₂). Field population spikes (fPS) from CA1 stratum pyramidale were recorded following stimulation of the Schaffer collaterals using a bipolar stimulus (0.1 ms pulses at 0.05 Hz) at a stimulus intensity giving ca 40 % of the maximal fPS. ACSF was supplemented with 0.1 % DMSO plus 0.1 % BSA to aid the dissolution of cOA. Results are normalised as % of pretreatment fPS amplitude ± S.E.M. Paired t tests were used to compare averaged responses before and after oleamide.

5HT (0.1-100 mM) caused a concentration-dependent depression of the population spike (EC₅₀: ca 10 mM), which was maximal between 64 and 100 mM (n = 5). We selected a concentration of 3.2 mM (ca EC₂₅) to measure the effects of repeated 5HT application. Three 10 min applications of 5HT alone (with 15 min wash-out to avoid desensitisation) were applied followed by a further three applications in the presence of 1 mM cOA (30 min pretreatment). cOA did not attenuate the amplitude of population spikes when applied alone at this concentration, nor did it potentiate the depressant effects of 5HT after 30-60 min of superfusion. The responses obtained with repeated application of 5HT in control saline were not significantly different from those obtained with cOA (5HT against 5HT and cOA, 30.7 ± 2.7 % against 28.5 ± 3.7 %, P > 0.05, n = 5)

We conclude that cOA, at a concentration that would saturate recombinant receptors in expression systems, does not modulate serotonergic transmission in the CA1 neurons. 5HT _2A/C receptors also mediate inhibitory responses on the CA1 soma. We are currently examining the possibility that such low-affinity metabotropic receptors are differentially sensitive to oleamide in these functional CNS circuits.

Thanks to The Wellcome Trust, College of Pharmacy Practice and The RPharmS for financial support.

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