Abstract
Long-term potentiation (LTP) in the dentate gyrus was previously shown to be enhanced by nicotine, an effect dependent on both homomeric α7 and heteromeric α2β2 nicotinic acetylcholine receptors (nAChR). In our experiments, bath-applied nicotine produced no significant enhancement of LTP. The α7 nAChR silent agonist NS6740, a weak activator of α7 nAChR ion channels but an effective modulator of the cholinergic anti-inflammatory pathway, decreased LTP and, additionally, produced a substantial reduction in the baseline synaptic function prior to the high frequency stimulation used to induce LTP. The effects of NS6740 on the various ligand-gated ion channels associated with the generation and modulation of dentate LTP were evaluated with receptors expressed in Xenopus oocytes. A 60 s pre-application of 5 μM NS6740 to α7 receptors blocked the response to subsequent applications of acetylcholine (ACh). In contrast, the responses of α2β2 nAChR to control applications of ACh were not significantly affected by NS6740. Likewise, responses of cells expressing GluR1 + GluR2 AMPA-type glutamate receptor subunits or GABA(A) α1, β2, and γ2L subunits to control agonist applications (100 μM kainic acid or 10 μM GABA, respectively), were unaffected by NS6740. The effects of NS6740 on α7 were inconsistent with simple antagonism since, while unresponsive to ACh, the receptors exposed to NS6740 were effectively activated by the positive allosteric modulator PNU-120596. The results support the hypothesis that NS6740 switches the mode of α7 signaling in a channel-independent manner that can reduce synaptic function.