Abstract
Nicotine can enhance working memory and attention. Activation of both α7 and β2* nicotinic acetylcholine receptors (nAChRs) in the prefrontal cortex (PFC) has been implicated in these processes. The ability of presynaptic nAChRs to modulate neurotransmitter release, notably glutamate release, is postulated to contribute to nicotine's effects. We have examined the cellular mechanisms underlying α7 and β2* nAChR-mediated [3H]d-aspartate release from the PFC in vitro. Using the α7 and β2* nAChR-selective agonists (R)-N-(1-azabicyclo[2.2.2]-oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide) (compound A) and 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-iodo-A-85380), respectively, in conjunction with inhibitors of voltage-operated Ca2+ channels (VOCCs) and intracellular Ca2+ stores, we show that [3H]d-aspartate release evoked by activation of β2* nAChRs occurs via VOCCs. In contrast, α7 nAChR-evoked release was unaffected by VOCC blockers but was abolished by modulators of Ca2+ stores, including ryanodine. The α7 nAChR ligand α-bungarotoxin and ryanodine receptors were colocalized to a subpopulation of PFC synaptosomes. Compound A-evoked [3H]d-aspartate release was also blocked by mitogen-activated protein kinase kinase 1 inhibitors, implicating extracellular signal-regulated kinase (ERK)1/2 in α7 nAChR-evoked exocytosis. Western blotting confirmed that compound A, but not 5-iodo-A-85380, application increased ERK2 phosphorylation in PFC synaptosomes, and this was dependent on ryanodine-sensitive stores. Compound A also promoted synapsin-1 phosphorylation at ERK1/2-dependent sites, in a ryanodine-sensitive manner. Thus, β2* and α7 nAChR subtypes in the PFC mediate [3H]d-aspartate release via distinct mechanisms as a result of their differential coupling to VOCCs and Ca2+-induced Ca2+ release (CICR), respectively. The ability of α7 nAChRs to promote the phosphorylation of presynaptic ERK2 and synapsin-1, downstream of CICR, provides a potential mechanism for presynaptic facilitation in the PFC.
Footnotes
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This work was supported by Biological and Biotechnological Research Council (BBSRC) grant BBS/B/15600 (to S.W.) and a Ph.D. studentship from the BBSRC in conjunction with GlaxoSmithKline (to J.A.D.).
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ABBREVIATIONS: nAChR, nicotinic acetylcholine receptor; PFC, prefrontal cortex; VOCC, voltage-operated calcium channel; CICR, calcium-induced calcium release; ERK, extracellular signal-regulated kinase; MAPK, mitogen-activated protein kinase; αBgt, α-bungarotoxin; compound A, (R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide); TBOA, dl-threo-β-benzoyloxyaspartic acid; U0126, 1,4-diamino-2,3-dicyano-1,4-bis(methylthio)butadiene; PD 98059 (PD), 2′-amino-3′-methoxyflavone; 5-iodo-A-85380, 5-iodo-3-(2(S)-azetidinylmethoxy)-pyridine; DHβE, dihydro-β-erythroidine; BSA, bovine serum albumin; PBS, phosphate-buffered saline; KB, Krebs-bicarbonate buffer; p-, phospho-; MEK, mitogen-activated protein kinase kinase; Rya, ryanodine; Mec, mecamylamine.
- Received February 25, 2008.
- Accepted April 28, 2008.
- The American Society for Pharmacology and Experimental Therapeutics
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