Subunit Composition and Pharmacology of Two Classes of Striatal Presynaptic Nicotinic Acetylcholine Receptors Mediating Dopamine Release in Mice

Abstract

Pharmacological evaluation of nicotine-stimulated dopamine release from striatum has yielded data consistent with activation of a single population of nicotinic acetylcholine receptors (nAChR). However, discovery that ␣-conotoxin MII (␣-CtxMII) partially inhibits the response indicates that two classes of presynaptic nAChRs mediate dopamine release. We have investigated the pharmacology and subunit composition of these two classes of nAChR. Inhibition of nicotine-stimulated dopamine release from mouse striatal synaptosomes by ␣-CtxMII occurs within minutes; recovery is slow. The IC 50 is 1 to 3 nM. ␣-CtxMII-sensitive and -resistant components have significant differences in pharmacology. The five agonists tested were more potent at activating the ␣-CtxMII-sensitive nAChRs; indeed, this receptor is the highest affinity functional nAChR found, so far, in mouse brain. In addition, cytisine was more efficacious at the ␣-CtxMII-sensitive sites. Methyllycaconitine was 9-fold more potent at inhibiting the ␣-CtxMII-sensitive sites, whereas dihydro-␤-erythroidine was a 7-fold more potent inhibitor of the ␣-CtxMII-resistant response. Both the transient and persistent phases of nicotine-stimulated dopamine release were partially inhibited by ␣-CtxMII with equal potency. The subunit composition of functional nAChRs, was assessed in mice with null mutations for individual nAChR subunits. The ␤2 subunit is an absolute requirement for both classes. In contrast, deletion of ␤4 or ␣7 subunits had no effect. The ␣-CtxMIIsensitive response requires ␤3 and is partially dependent upon ␣4 subunits, probably ␣6␤3␤2 and ␣4␣6␤3␤2, whereas the ␣-CtxMII-resistant release requires ␣4 and is partially dependent upon ␣5 subunits, probably ␣4␤2 and ␣4␣5␤2.Nicotinic cholinergic receptors (nAChRs) are expressed both peripherally and centrally. Many, if not most, brain nAChRs are located presynaptically, where they serve to modulate neurotransmitter release (Wonnacott, 1997;Nishi et al., 2000). Activation of presynaptic nAChRs may elicit neurotransmitter release in the absence of a propagated signal (Vizi and Lendvai, 1999), and may increase action-potential induced transmitter release and, consequently, enhance synaptic fidelity (McGehee et al., 1995;Gray et al., 1996).Neuronal nAChR subunits form pentameric complexes, closely resembling the nAChR found at the neuromuscular junction (Leonard and Bertrand, 2001). Some of these receptors are homomeric (␣7), but most are heteromeric. Subunit composition of heteromeric nAChRs expressed in Xenopus laevis oocytes or transfected cell lines has profound effects on agonist potency and efficacy as well as sensitivity to antagonists (Luetje and Patrick, 1991;Harvey and Luetje, 1996;

The subtypes of nicotinic acetylcholine receptors on dopaminergic terminals of mouse striatum

“…Those receptors that are expressed on or near nerve terminals modulate calcium-dependent release of neurotransmitters [35,36,37,38] including dopamine [39,40,41,42], norepinephrine [43,44], glutamate [45,46], GABA [47], and acetylcholine [48,49]. Nicotinic agonist-stimulated release of dopamine has been studied extensively in the nucleus accumbens because these nAChRs may play a major role in regulating the reinforcing effects of nicotine [50,51,52].…”

“…Panel B: ACh-stimulated dopamine release measured from C57Bl striatal synaptosomes with (MII resistant activity) and without (total activity) prior exposure to α-CtxMII (50nM for 5 min). The MII-sensitive activity is determined by difference (from [42]). Units are calculated as for Panel A.…”

“…Units are calculated as for Panel A. Panel C: α-CtxMII-resistant dopamine release stimulated by various agonists as % of maximal α-CtxMII-resistant release by ACh (replotted from [42]). Panel D: α-CtxMII-sensitive dopamine release stimulated by various agonists as Effect of nAChR subunit null mutations on agonist-stimulated dopamine release from striatal synaptosomes.…”

“…Panel D: α-CtxMII-sensitive dopamine release stimulated by various agonists as Effect of nAChR subunit null mutations on agonist-stimulated dopamine release from striatal synaptosomes. Panel A: Effects on α-CtxMII-resistant dopamine release by ACh (10μM) with prior exposure to α-CtxMII (50nM for 5 min) as % of wildtype response (replotted from [42] and unpublished data from SR Grady). Panel B: Effects on α-CtxMII-sensitive dopamine release stimulated by ACh (1μM) by difference between samples of synaptosomes with and without prior exposure to α-CtxMII (50nM for 5 min) as % of wildtype response (replotted from [42] and unpublished data from SR Grady).…”

Discovery of a novel nicotinic receptor antagonist for the treatment of nicotine addiction: 1-(3-Picolinium)-12-triethylammonium-dodecane dibromide (TMPD)

“…α4β2*, α6β2*, and α4α6β2* subtypes are suggested to mediate the DA response to nicotine [38]. Results from a comprehensive molecular genetics study in which an individual subunit gene (i.e., α4, α5, α7, β2, β3, and β4) was deleted suggested that at least 6 different subtypes mediate nicotine-evoked DA release from mouse striatal synaptosomes, including 2 classes of nAChRs: α-CtxMII-sensitive nAChRs (i.e., α6β2β3*, α4α6β2β3* and possibly a small amount of α6β2* or α4α6β2* subtypes) and α-CtxMIIresistant nAChRs (i.e., α4β2* and α4α5β2* subtypes), whereas deletion of β4 and α7 subunits had no effect [39]. Also, α6-and β3-containing nAChRs have been implicated in nicotineevoked DA release [24,40,41].…”