Received for publication February 25, 2008.
Revised April 28, 2008.
Accepted for publication April 28, 2008.

Presynaptic 7 and 2-containing nicotinic acetylcholine receptors modulate excitatory amino acid release from rat prefrontal cortex nerve terminals via distinct cellular mechanisms

Abstract

Nicotine can enhance working memory and attention. Activation of both 7 and 2* 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 7 and 2* nAChR-mediated [³H]D-aspartate release from the PFC in vitro. Using the 7 and 2* nAChR selective agonists compound A and 5-iodo-A-85380, respectively, in conjunction with inhibitors of voltage-operated Ca²⁺ channels (VOCC) and intracellular Ca²⁺ stores, we show that [³H]D-aspartate release evoked by activation of 2* nAChRs occurs via VOCCs. In contrast, 7 nAChR-evoked release was unaffected by VOCC blockers but was abolished by modulators of Ca²⁺ stores, including ryanodine. The 7 nAChR ligand -bungarotoxin and ryanodine receptors were colocalised to a subpopulation of PFC synaptosomes. Compound A-evoked [³H]D-aspartate release was also blocked by MEK1 inhibitors, implicating ERK1/2 in 7 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 2* and 7 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 7 nAChRs to promote the phosphorylation of presynaptic ERK2 and synapsin-1, downstream of CICR, provides a potential mechanism for presynaptic facilitation in the PFC.

Key words: Nicotinic cholinergic, MAP Kinase, Immunocytochemistry, Exocytosis