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Nicotine trapping causes the persistent desensitization of alpha4beta2 nicotinic receptors expressed in oocytes
Li Jia1, Karen Flotildes, Maureen Li
1Division of Biomedical Sciences, University of California, Riverside, CA, USA.
Abstract:
To determine whether prolonged nicotine exposure persistently inactivates rat alpha4beta2 nicotinic receptors expressed in Xenopus oocytes, we measured the voltage-clamped alpha4beta2 response to acetylcholine (ACh) before and 24 h after, 1-h or 12-h incubations in 10 microm nicotine. A 12-h incubation in 10 microm nicotine depressed the alpha4beta2 ACh response for 24 h without affecting total or surface alpha4beta2 expression. To determine whether oocyte-mediated nicotine release caused this depression, we co-incubated an alpha4beta2-expressing oocyte with an un-injected one (pre-incubated in 10 microm nicotine for 12 h) for 24 h and measured the change in the alpha4beta2 ACh response. The response decreased by the same factor after the co-incubation as it did after a 12-h incubation in 10 microm 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 microm nicotine. Consistent with this result, measurements of [3H]nicotine release show that oocytes release enough nicotine into the wash media to desensitize alpha4beta2 receptors and that prolonged incubation in 300 microm ACh (which cannot readily cross the membrane or accumulate in acidic vesicles) did not persistently depress the alpha4beta2 response.
Insights
Prolonged nicotine exposure persistently desensitizes rat alpha4beta2 nicotinic receptors. Oocytes release nicotine, causing this persistent depression, not direct receptor inactivation.
Area of Science:
- Neuroscience
- Pharmacology
- Molecular Biology
Background:
- Nicotinic acetylcholine receptors (nAChRs) are crucial for neurotransmission.
- Rat alpha4beta2 nAChRs are implicated in nicotine addiction and withdrawal.
Purpose of the Study:
- To investigate persistent inactivation of rat alpha4beta2 nAChRs.
- To determine if nicotine exposure or oocyte-mediated release causes receptor desensitization.
Main Methods:
- Xenopus oocytes expressing alpha4beta2 nAChRs were exposed to nicotine.
- Voltage-clamp electrophysiology measured acetylcholine (ACh) response.
- Co-incubation experiments assessed oocyte-mediated nicotine release.
Main Results:
- 12-hour nicotine incubation caused 24-hour depression of alpha4beta2 ACh response.
- This depression persisted without changes in receptor expression.
- Co-incubation confirmed oocyte-mediated nicotine release as the cause of desensitization.
Conclusions:
- Oocyte-mediated nicotine release persistently desensitizes alpha4beta2 nAChRs.
- This finding clarifies the mechanism of nicotine-induced receptor downregulation.