While searching for factors that could explain the vastly different results obtained in the different buffers, we noticed that the two buffers differed greatly in osmolarity: NGM is 160 mOsm, while Dent's buffer is 300 mOsm. We tested if C. elegans' sensitivity to intoxication while swimming depends on osmolarity, and found that worms became rapidly intoxicated when assayed in NGM buffer in which we had adjusted the osmolarity to match Dent's buffer by adding 130 mOsm sorbitol (Figure 5d). Assaying worms in only 300 mOsm sorbitol without any salts reproduced a dose response for intoxication that was characteristic of Dent's buffer (Figure 5d). Moreover, we found that pre-incubation for 20 minutes in Dent's buffer or 300 mOsm sorbitol conferred enhanced sensitivity to intoxication when the animals were assayed in 150 mOsm NGM buffer (data not shown). Together, these results suggest that acute sensitivity to exogenous ethanol in C. elegans depends on osmolarity, that this sensitivity can be dynamically adjusted, and that this is an explanation that can resolve the conflicting reports of dose response sensitivity to ethanol while swimming.
