, 2005). We provide evidence that eCBs, through actions at CB1Rs, gate LTP at GABA synapses. In addition, our study also reveals two interesting interactions between the NO and eCB systems in regulating GABA transmission in the DMH. First, eCB signaling impairs NO-mediated potentiation of GABA synapses. This is evident following a prolonged burst of afferent activity

where eCB-mediated LTDGABA is favored over NO-mediated potentiation. With shorter durations of stimulation, however, we observed a shift from LTDGABA to LTPGABA. It is likely that shorter ABT-199 cost bursts of afferent activity favor the production of NO over eCBs. Although both retrograde signals are produced following a rise in intracellular Ca2+, it is possible that NO may be synthesized at a faster rate because of coupling of NO synthase to the NMDA receptor (Bredt and Snyder, 1989 and Garthwaite et al., 1989). With longer stimulation,

both NO and eCBs are present and eCB signaling impairs NO-mediated LTPGABA. Figure 7 summarizes our current hypothesis regarding the activity-dependent this website production and action of NO and eCBs in regulating GABA transmission in satiated and food-deprived conditions. The mechanism of the eCB-mediated blockade of NO action is not known, but our observation that the NO donor SNAP fails to potentiate GABA synapses in the presence of WIN 55,212-2 suggests that CB1R activation impedes NO signaling in the DMH. eCB-mediated LTD requires inhibition of protein kinase A (PKA). Thus, one possibility is that there may be an interaction between the cAMP-PKA and cGMP-PKG signaling pathways (Barman et al., 2003 and Nugent et al., 2009) such that inhibition of PKA interferes with PKG.

We also show that NO signaling is necessary for eCB-mediated LTD of GABA synapses. When NO production is blocked, found GABA synapses do not depress in response to HFS-induced eCB production or application of a CB1R agonist. Conversely, when NO signaling is augmented, CB1R-induced depression of GABA synapses is even more effective. These findings are consistent with evidence indicating that the induction of eCB-mediated plasticity in other brain areas is blocked by disrupting NO signaling (Daniel et al., 1993, Kyriakatos and El Manira, 2007, Makara et al., 2007, Safo and Regehr, 2005 and Shibuki and Okada, 1991). The exact mechanism of this blockade is not known (Alger, 2005), but several potential mechanisms have been proposed. NO appears to be acting downstream of CB1R activation to mediate LTD in the cerebellum (Safo and Regehr, 2005) and striatum (Chepkova et al., 2009), whereas in the hippocampus, under certain conditions, eCB-mediated plasticity requires NO actions upstream of the CB1R (Makara et al., 2007). Alternatively, NO may act directly at the CB1R to enhance eCB signaling.