Abstract

Bifenthrin, a relatively stable type I pyrethroid that causes tremors and impairs motor activity in rodents, is broadly used. Whether nanomolar bifenthrin alters synchronous Ca²⁺ oscillations (SCO) necessary for activity dependent dendritic development was investigated. Primary mouse cortical neurons were cultured 8-9 days in vitro (DIV), loaded with the Ca²⁺ indicator Fluo-4, and imaged using FLIPR Tetra. Acute exposure to bifenthrin rapidly increased the frequency of SCO 2.7-fold (EC₅₀=58nM) and decreased SCO amplitude by 36%. Changes in SCO properties were independent of modifications in voltage-gated sodium channels since 100nM bifenthrin had no effect on the whole-cell Na⁺ current, nor influenced neuronal E_rest. The L-type Ca²⁺ channel blocker nifedipine failed to ameliorate bifenthrin-triggered SCO activity. By contrast, the mGluR 5 antagonist MPEP normalized bifenthrin-triggered increase in SCO frequency without altering baseline SCO activity, indicating that bifenthrin amplifies mGluR 5 signaling independent of Na⁺ channel modification. Competitive (AP-5) and non-competitive (MK801) NMDAR antagonists partially decreased both basal and bifenthrin-triggered SCO frequency increase. Bifenthrin-modified SCO rapidly enhanced the phosphorylation of cAMP response element-binding protein (CREB). Sub-acute (48h) exposure to bifenthrin commencing 2 DIV enhanced neurite outgrowth and persistently increased SCO frequency and reduced SCO amplitude. Bifenthrin-stimulated neurite outgrowth and CREB phosphorylation were dependent on mGluR 5 activity since MPEP normalized both responses. Collectively these data identify a new mechanism by which bifenthrin potently alters Ca²⁺ dynamics and Ca²⁺-dependent signaling in cortical neurons that have long term impacts on activity driven neuronal plasticity.