Abstracts

Mossy cells are unique glutamatergic local circuit neurons located in the hilar region of the dentate gyrus. The activity of mossy cells is thought to contribute to the regulation of granule cell activity along the septo-temporal axis of the hippocampus, while the loss of mossy cells is strongly implicated in the etiology of temporal lobe epilepsy. Nevertheless, relatively little is known about the neurophysiological mechanisms responsible for regulating mossy cell activity. This study was designed to test the hypothesis that excitation of mossy cells is capable of modulating transmitter release from inhibitory afferents via a mechanism that involves endocannabinoid dependent retrograde signalling. Hilar mossy cells were voltage clamped at -70 mV in horizontal slices of rat brain made from p18-28 Sprague Dawley rats. Glutamatergic transmission was blocked by bath application of NBQX and APV, and a bipolar stimulator placed in the hilus was used to evoke inhibitory post synaptic currents (eIPSCs). Apparent retrograde transmission was initiated by transiently depolarizing mossy cells from -70 mV to 0 mV. We determined that a 5 second depolarization of hilar mossy cells from -70 mV to 0 mV is capable of transiently reducing the amplitude of eIPSCs by 19 [plusmn] 4.4% (n=10). Bath application of 3 [mu]M carbachol, a muscarinic acetylcholine receptor agonist, enhances this effect (to 31 [plusmn] 3.8%, n=13). Carbachol also increases spontaneous IPSC frequency, often to the point that depolarization induced suppression of inhibition (DSI) can also be detected as a transient reduction in that frequency (by 35 [plusmn] 7.3%, n=11). We further demonstrated that the DSI of eIPSCs in hilar mossy cells is blocked by bath application of 5 [mu]M AM-251, an endocannabinoid receptor (CB1) antagonist, and occluded by bath application of 5 [mu]M WIN55,212-2, a CB1 agonist. Consistent with results in other systems, DSI of eIPSCs in hilar mossy cells also varies with duration of postsynaptic depolarization (between .1 and 5 seconds) and is blocked by chelation of postsynaptic calcium with 10 mM internal BAPTA. Current studies are examining the synapse specificity of this phenomenon and determining the extent to which CB1 mediated inhibition of transmitter release depends on presynaptic calcium influx. We conclude that depolarization of hilar mossy cells leads to calcium dependent release of endogenous cannabinoids which can be enhanced by activation of somatic mAChRs, and further, that this retrograde transmission modulates transmitter release from GABAergic afferents, at least in part, through activation of presynaptic CB1 receptors. Further investigation into the extent of endocannabinoid mediated retrograde signalling in the dentate gyrus is likely to significantly enhance our understanding of how excitability is regulated in this area. (Supported by The University of Florida College of Pharmacy and the Evelyn F. McKnight Brain Research Grant Program.)