Nicotinic receptor-mediated regulation of the dopamine transporter in rat prefrontocortical slices following chronic in vivo administration of nicotine
Introduction
We have shown previously that nicotine, acting through receptors likely to be composed of α4 and β2 subunits based upon pharmacological profile, enhance amphetamine-stimulated (transporter-mediated) [3H]DA release from slices of prefrontal cortex, but not from nucleus accumbens or striatum Drew et al., 2000, Drew and Werling, 2001 prepared from drug-naı̈ve rats. This suggests a regional selectivity of nicotinic effects, which could be exploited in treatment of certain disorders where activity in a particular area of brain might be desired while activity in other areas was not. One such application might be enhancement of prefrontal dopamine release in schizophrenia. In schizophrenia, there is dopaminergic hypofunction in prefrontal areas, and dopaminergic hyperfunction in accumbens (reviewed by Davis et al., 1991). One reason that the smoking rate in schizophrenics is so high (88% vs. 25% in non-schizophrenics; Hughes et al., 1986; reviewed by Dalack et al., 1998 and by Glassman, 1993) might relate to the ability of nicotine to elevate selectively prefrontal dopamine levels. However, for nicotinic agonists to be medically relevant through the mechanism of dopamine elevation, it would be necessary that the response to nicotine or a congener not desensitize during continuous exposure.
It has been argued that chronic nicotine increases nicotinic receptor number, but whether the upregulated receptors are desensitized, or capable of acting identically to normal nicotinic receptors, has been debated. Most studies of dopamine release have addressed only exocytotic release (e.g. Grady et al., 1992, Benwell et al., 1995, Soliakov and Wonnacott, 2001). In most, though not all, studies of exocytotic release, chronic nicotine exposure results in a desensitization of response to nicotinic agonist. We have concentrated on release of [3H]DA via reversal of the dopamine transporter (DAT) Drew et al., 2000, Drew and Werling, 2001. We now report that, even after chronic nicotine treatment (2 mg/kg nicotine tartrate, injected subcutaneously twice a day for 10 days), nicotinic receptors of the α4β2 type are still capable of regulating amphetamine-stimulated [3H]DA release through the DAT. After chronic treatment, nicotine, epibatidine, and A85380 all enhanced amphetamine-stimulated release to the same extent as these drugs enhanced release in prefrontocortical tissue from saline-treated or untreated animals. We also tested whether receptors of the α4β2 subunit composition involved in regulation of the DAT-mediated [3H]DA release could be localized to nerve terminals by examining regulation of amphetamine-stimulated release from synaptosomal preparations of prefrontal cortex. Lastly, we tested whether chronic nicotine altered the ability of the DAT to take up dopamine, and whether it affected potential regulation of uptake by several nicotinic drugs. Our findings suggest that nicotine or other nicotinic agonists might be used therapeutically without a loss of effectiveness over time for at least a 10-day interval, that at least a portion of α4β2 subunit containing nicotinic receptors is on nerve terminals, and that DAT uptake properties are not changed by chronic nicotine treatment.
Section snippets
Chemicals and reagents
The following drugs and reagents were kindly provided or obtained through the following sources, 3-(2(S)-azentidinylmethoxy)pyridine (A85380), amphetamine, dihydro-β-erythroidine hydrochloride (DHβE), domperidone, (±)-epibatidine 2HCl, GBR12909, (−)-nicotine ditartrate (nicotine), nisoxetine hydrochloride, (Research Biochemicals International, Natick, MA); [3H]DA (specific activity 46 to 51 Ci/mmol) (Amersham, Arlington Heights, IL).
Measurement of amphetamine-stimulated release of [3H]DA from brain slices
All experiments were carried out in accordance with the
Results
We have previously constructed concentration–response curves for production of an enhancement of amphetamine-stimulated [3H]DA release via the DAT from slices of prefrontal cortex by all nicotinic agonists used in the current study. We chose concentrations of agonists for testing in the current study that produced the maximum enhancement of stimulated release in slices of prefrontal cortical slices. Rats were injected for 10 days twice a day with either 2 mg/kg nicotine, or an equivalent volume
Discussion
Schizophrenia is associated with a high rate of smoking. Patients report improved cognition while smoking, which has been confirmed in clinical tests Dalack and Meador-Woodruff, 1996, Levin et al., 1996, Dalack et al., 1998. Since the patients' cognitive function is also improved by application of nicotine via a patch (Levin et al., 1996), it is most likely that the benefit is due largely if not wholly to the nicotine ingested in tobacco smoke. These findings have led many investigators to
Acknowledgments
This work was supported by an Independent Investigator Award to LLW from the National Alliance for Research on Schizophrenia and Depression. We thank Alicia Derbez, Tammy Awtry, Samer Nuwayhid, and Caroline Sanchez for performing some of the experiments. We also thank Dr. Pamela Puttfarcken for her critical reading of the manuscript.
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