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BACKGROUND: The pyrethroid deltamethrin (DM) is broadly used for insect control. Although DM hyperexcites neuronal networks by delaying inactivation of axonal voltage-dependent Na+ channels, this mechanism is unlikely to mediate neurotoxicity at lower exposure levels during critical perinatal periods in mammals.

OBJECTIVES: We aimed to identify mechanisms by which acute and subchronic DM altered axonal and dendritic growth, patterns of synchronous Ca2+ oscillations (SCOs), and electrical spike activity (ESA) functions critical to neuronal network formation.

METHODS: Measurements of SCOs using Ca2+ imaging, ESA using microelectrode array (MEA) technology, and dendritic complexity using Sholl analysis were performed in primary murine cortical neurons from wild-type (WT) and/or ryanodine receptor 1 (RyR1(T4826I/T4826I)) mice between 5 and 14 d in vitro (DIV). [H-3]ryanodine binding analysis and a single-channel voltage clamp were utilized to measure engagement of RyRs as a direct target of DM.

RESULTS: Neuronal networks responded to DM (30-70 nM) as early as 5 DIV, reducing SCO amplitude and depressing ESA and burst frequencies by 60-70%. DM (10-300 nM) enhanced axonal growth in a nonmonotonic manner. DM >= 100 nM enhanced dendritic complexity. DM stabilized channel open states of RyR1, RyR2, and cortical preparations expressing all three isoforms. DM (30 nM) altered gating kinetics of RyR1 channels, increasing mean open time, decreasing mean closed time, and thereby enhancing overall open probability. SCO patterns from cortical networks expressing RyR1(T4826I/T4826I) were more responsive to DM than WT. RyR1(T4826I/T4826I) neurons showed inherently longer axonal lengths than WT neurons and maintained less length-promoting responses to nanomolar DM.

CONCLUSIONS: Our findings suggested that RyRs were sensitive molecular targets of DM with functional consequences likely relevant for mediating abnormal neuronal network connectivity in vitro.