In 2009, it was hypothesized that DMT may be an endogenous ligand for the σ1 receptor. The concentration of DMT needed for σ1 activation "in vitro" (50–100 μM) is similar to the behaviorally active concentration measured in mouse brain of approximately 106 μM This is minimally 4 orders of magnitude higher than the average concentrations measured in rat brain tissue or human plasma under basal conditions (see Endogenous DMT), so σ1 receptors are likely to be activated only under conditions of high local DMT concentrations. If DMT is stored in synaptic vesicles, such concentrations might occur during vesicular release. To illustrate, while the "average" concentration of serotonin in brain tissue is in the 1.5–4 μM range, the concentration of serotonin in synaptic vesicles was measured at 270 mM. Following vesicular release, the resulting concentration of serotonin in the synaptic cleft, to which serotonin receptors are exposed, is estimated to be about 300 μM. Thus, while "in vitro" receptor binding affinities, efficacies, and average concentrations in tissue or plasma are useful, they are not likely to predict DMT concentrations in the vesicles or at synaptic or intracellular receptors. Under these conditions, notions of receptor selectivity are moot, and it seems probable that most of the receptors identified as targets for DMT (see above) participate in producing its psychedelic effects.