Nicotine trapping causes the persistent desensitization of alpha 4 beta 2 nicotinic receptors expressed in oocytes

To determine whether prolonged nicotine exposure persistently inactivates rat alpha 4 beta 2 nicotinic receptors expressed in Xenopus oocytes, we measured the voltage-clamped alpha 4 beta 2 response to acetylcholine (ACh) before and 24 h after, 1-h or 12-h incubations in 10 mu m nicotine. A 12-h incubation in 10 mu m nicotine depressed the alpha 4 beta 2 ACh response for 24 h without affecting total or surface alpha 4 beta 2 expression. To determine whether oocyte-mediated nicotine release caused this depression, we co-incubated an alpha 4 beta 2-expressing oocyte with an un-injected one (pre-incubated in 10 mu m nicotine for 12 h) for 24 h and measured the change in the alpha 4 beta 2 ACh response. The response decreased by the same factor after the co-incubation as it did after a 12-h incubation in 10 mu m nicotine and a 24-h incubation in nicotine-free media. Thus, oocyte-mediated nicotine release caused the persistent desensitization we observed after a 12-h incubation in 10 mu m nicotine. Consistent with this result, measurements of [ super(3)H]nicotine release show that oocytes release enough nicotine into the wash media to desensitize alpha 4 beta 2 receptors and that prolonged incubation in 300 mu m ACh (which cannot readily cross the membrane or accumulate in acidic vesicles) did not persistently depress the alpha 4 beta 2 response.