this may enhance their susceptibility to CB1-mediated inhibition of intrinsic excitability. In addition, while a small portion of calbindin-positive interneurons express CB1 receptors, the majority of CB1-positive GABA terminals are found on perisomatic synapses onto PNs formed by cholecystokinin-positive interneurons. The position of these synapses on PNs makes them particularly well-suited to regulate synchronized patterns of cortical network activity suggesting that disruption of GABAergic signaling Endocannabinoid Modulation of Up-States from CCK+ neurons may outweigh diminished glutamatergic input due to the sub-cellular compartmentalization of these inhibitory inputs. A comparison of IPSCs between cortical layers revealed that WIN produced greater inhibition of GABAA currents in superficial layer neurons. These data agree with published findings that CB1-mediated inhibition of GABA terminals occurs predominantly in layer II/III PNs. We note that although we monitored upstates in deep-layer neurons in this study, up-states are a network phenomena and essentially all areas of the slice culture show synchronous entry and exit from up-states. In addition, a recent study using an optogenetic approach showed that, in sensorimotor cortex, only a small number of layer V/VI pyramidal neurons need to be activated to initiate up-states and network oscillations in vivo Overall, these data suggest that a CB1-mediated disinhibition of layer II/III PN inputs results in a net excitation of deep layer PNs that is reflected in the alterations in up-state parameters observed in this study. PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19643932 Up-state Vorapaxar supplier amplitude and spiking were also enhanced when inhibitors of 2-AG or AEA metabolism were applied. AEA is also an agonist at TRPV1 receptors that are expressed in cortex, but neither the TRPV1 agonist, CAP, nor the TRPV1 antagonist, CPZ, significantly altered up-state parameters as compared to those obtained during control recordings. While both exogenous and endogenous activators of CB1 receptors enhanced up-state amplitude, application of the CB1 antagonists AM251 or AM281 significantly reduced up-state duration and amplitude. As 
these compounds are actually inverse agonists at the CB1 receptor, their effects may reflect a constitutive activity of CB1 receptors in the slice culture or actions at other G-protein coupled receptors. This seems unlikely, however, as NESS0327, a putative neutral antagonist of CB1, also reduced up-state duration and AM281 had no effect on up-state parameters in CB1 KO cultures. These results suggest that cortical slice cultures generate an EC tone that regulates synaptic activity similar to findings from studies of ECs in the central amygdala, hippocampus, and spinal cord. Although the mechanism underlying the reduction in up-state duration by CB1 antagonists is not fully known, AM281 also increased the frequency of layer II/III GABAA sIPSCs suggesting that an increased inhibitory drive onto layer II/III PNs may effectively shorten up-states. This proposal appears counter to findings in the literature showing that partial block of GABAergic transmission with bicucculine or SR95531 also reduces up-state duration while simultaneously enhancing spiking. While these differences could reflect methodological variations between these studies involving both species and brain areas, it should be noted that application of Endocannabinoid Modulation of Up-States GABAA antagonists will affect all inhibitory synapses while modulators of EC
