ABSTRACT
The aim of this study was to explore the modulation by alpha7 nicotinic receptors (nAChRs) of dopamine and glutamate release in the rat prefrontal cortex where these receptors are implicated in attentional processes and are therapeutic targets for cognitive deficits. The presence of presynaptic alpha7 nAChRs on glutamate terminals is supported by the ability of the subtype-selective agonist Compound A to evoke [(3)H]D-aspartate release from synaptosomes: This response was potentiated by the selective allosteric potentiator PNU-120596 and blocked by alphabungarotoxin. Compound A also evoked dopamine overflow in the prefrontal cortex in vivo, and this was potentiated by PNU-120596. alpha7 nAChR-evoked [(3)H]dopamine release from tissue prisms in vitro was blocked by antagonists of NMDA and AMPA receptors. These data are consistent with a model in which alpha7 nAChRs present on glutamate terminals increase glutamate release that (1) contributes to presynaptic facilitation and synaptic plasticity and (2) co-ordinately enhances dopamine release from neighbouring boutons.
Subject(s)
Dopamine/metabolism , Glutamine/metabolism , Prefrontal Cortex/drug effects , Prefrontal Cortex/metabolism , Receptors, Nicotinic/drug effects , Receptors, Nicotinic/metabolism , Acetylcholine/metabolism , Animals , Aspartic Acid/metabolism , Excitatory Amino Acid Antagonists/pharmacology , Extracellular Fluid/drug effects , Extracellular Fluid/metabolism , Isoxazoles/pharmacology , Microdialysis , Neuronal Plasticity/drug effects , Neuronal Plasticity/physiology , Nicotinic Agonists/pharmacology , Nicotinic Antagonists/pharmacology , Phenylurea Compounds/pharmacology , Presynaptic Terminals/drug effects , Presynaptic Terminals/metabolism , Rats , Synaptic Transmission/drug effects , Synaptic Transmission/physiology , Synaptosomes , alpha7 Nicotinic Acetylcholine ReceptorABSTRACT
Nicotine can enhance working memory and attention. Activation of both alpha7 and beta2(*) 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 alpha7 and beta2(*) nAChR-mediated [(3)H]d-aspartate release from the PFC in vitro. Using the alpha7 and beta2(*) 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 Ca(2+) channels (VOCCs) and intracellular Ca(2+) stores, we show that [(3)H]d-aspartate release evoked by activation of beta2(*) nAChRs occurs via VOCCs. In contrast, alpha7 nAChR-evoked release was unaffected by VOCC blockers but was abolished by modulators of Ca(2+) stores, including ryanodine. The alpha7 nAChR ligand alpha-bungarotoxin and ryanodine receptors were colocalized to a subpopulation of PFC synaptosomes. Compound A-evoked [(3)H]d-aspartate release was also blocked by mitogen-activated protein kinase kinase 1 inhibitors, implicating extracellular signal-regulated kinase (ERK)1/2 in alpha7 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, beta2(*) and alpha7 nAChR subtypes in the PFC mediate [(3)H]d-aspartate release via distinct mechanisms as a result of their differential coupling to VOCCs and Ca(2+)-induced Ca(2+) release (CICR), respectively. The ability of alpha7 nAChRs to promote the phosphorylation of presynaptic ERK2 and synapsin-1, downstream of CICR, provides a potential mechanism for presynaptic facilitation in the PFC.
Subject(s)
Excitatory Amino Acids/metabolism , Prefrontal Cortex/metabolism , Presynaptic Terminals/metabolism , Receptors, Nicotinic/physiology , Animals , Extracellular Signal-Regulated MAP Kinases/metabolism , Male , Nicotinic Agonists/pharmacology , Prefrontal Cortex/drug effects , Prefrontal Cortex/enzymology , Presynaptic Terminals/enzymology , Rats , Rats, Sprague-Dawley , Receptors, Presynaptic/agonists , Receptors, Presynaptic/metabolism , alpha7 Nicotinic Acetylcholine ReceptorABSTRACT
Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation channels that can modulate various neuronal processes by altering intracellular Ca(2+) levels. Following nAChR stimulation Ca(2+) can enter cells either directly, through the intrinsic ion channel, or indirectly following voltage-operated Ca(2+) channel (VOCC) activation; Ca(2+) levels can subsequently be amplified via Ca(2+)-induced Ca(2+) release from intracellular stores. We have used subtype-selective nAChR agonists to investigate the Ca(2+) sources contributing to alpha7 and non-alpha7 nAChR-mediated increases in intracellular Ca(2+) in PC12 cells. Application of the alpha7 nAChR positive allosteric modulator PNU 120596 (10 mum), in conjunction with the alpha7 nAChR agonist, compound A [(R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide), 10 nm], produces a rapid increase in fluo-3 fluorescence that is prevented by the selective alpha7 nAChR antagonist alpha-bungarotoxin. The non-alpha7 nAChR agonist 5-Iodo-A-85380 produces alpha-bungarotoxin-insensitive increases in intracellular Ca(2+) (EC(50) = 11.2 mum). Using these selective agonists or KCl in conjunction with general and selective VOCC inhibitors, we demonstrate that the primary route of Ca(2+) entry following either non-alpha7 nAChR activation or KCl stimulation is via L-type VOCCs. In contrast, the alpha7 nAChR-mediated response is unaffected by VOCC blockers but is inhibited by modulators of intracellular Ca(2+) stores. These results indicate that alpha7 and non-alpha7 nAChRs are differentially coupled to Ca(2+)-induced Ca(2+) release and VOCCs, respectively.
