The role of endocannabinoids in mglur mediated long term depression in the rat hippocampus

Deirdre M O’Leary, Gemma Z Grennan, John J O’Connor and Caroline E Herron. School of Biomolecular and Biomedical Science, Conway Institute, University College Dublin, Belfield, Dublin 4.

We have investigated the mechanisms involved in metabotropic glutamate receptor (mGluR) mediated long-term depression (LTD) in the rat hippocampus in vitro. It has been suggested that DHPG-LTD may be induced postsynaptically, and expressed presynaptically, thereby requiring a retrograde messenger to be released from the postsynaptic neurone that can alter transmitter release from the presynaptic neurone. Arachidonic acid and its metabolites have been suggested as potential retrograde messengers, while endocannabinoids can also act as retrograde messengers; therefore here we investigated the role of arachidonic acid metabolites and the endocannabinoids in DHPG-LTD.

Field excitatory postsynaptic potentials (EPSPs) were recorded in area CA1 from hippocampal slices (350 μm) from male rats (3-4 weeks old). Data are expressed as mean ± SEM. The mGluR group I agonist, RS-DHPG (100 μM), was bath applied for 20 min, during which time, the EPSP slope was reduced to 33.1 ± 2.9% of pre-drug baseline. DHPG was then washed out, and 60 min after washout the EPSP slope was still significantly depressed at 68.5 ± 4.1% of baseline (n=11). We have shown previously that the 12-lipoxygenase inhibitor cinnamyl-3,4-dihydroxy-α -cyanocinnamate (CDC) could significantly attenuate LTD induced by DHPG (O’Leary et al., 2005). Bath application of the 5-lipoxygenase inhibitor, AA-861 (10 μM and 50 μM) for 30 min prior to DHPG significantly inhibited the DHPG-LTD (10 μM: 83.3 ± 3.7%, n=7 and 50 μM: 86.1± 4.5%, n=7, P<0.05, ANOVA). The cannabinoid (CB1) receptor antagonist, AM-251 (N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1Hpyrazole-3-carboxamide), was bath applied at a concentration of 1 μM or 5 μM. In the presence of AM-251, ESPS slope was not significantly different from control (1 μM: 97.7 ± 2.8%, n=6 and 5 μM: 101.6 ± 4.7%, n=7, P<0.05, ANOVA). Twenty minutes after AM-251 application, DHPG was added for 20 min and subsequently washed out. AM-251 significantly inhibited DHPG-LTD at both concentrations tested (1 μM: 89.6 ± 6.0%, n=6 and 5 μM: 97.5 ± 4.3%, n=7, P<0.05, ANOVA). Our results indicate that activation of the arachidonic acid and the endocannabinoid signalling pathways are likely to be involved in DHPG-LTD.

O’Leary DM, Grennan GZ, O’Connor JJ, Herron CE (2005). Acta Neurobiol Exp 65: 73.