Expression of the Human R163C-RYR1 Gain-of-Function Mutation Modified 2,2',3,5',6-Pentachlorobiphenyl (PCB 95)...

Epidemiological studies have identified the developing brain as a target of concern for polychlorinated biphenyls (PCBs). In animal models, behavioral deficits caused by developmental exposure to PCBs have been associated with altered patterns of dendritic arborization in functionally relevant brain regions. In vitro studies revealed that PCB 95 promoted dendritic growth in primary rat hippocampal neuron-glia co-cultures via ryanodine receptor 1 (RYR1)-dependent Ca²⁺ signaling. However, it is not yet known whether RYR1 dysregulation contributes to disruption of dendritic morphogenesis in the intact developing brain or whether PCB 95 affects translationally relevant behavioral endpoints in juvenile animals. To address these data gaps, we assessed Morris water maze (MWM) performance and dendritic arborization of hippocampal CA1 pyramidal neurons in C57BL/6 mice heterozygous for the human R163C-RYR1 gain-of-function mutation (HET) and congenic wildtype (WT) littermates exposed to vehicle or PCB 95 at 0.1, 1.0, or 6.0 mg/kg/d in the dam's diet from conception through weaning. MWM performance was not altered in HET vehicle controls relative to WT vehicle controls; however, compared to genotype-matched vehicle controls, spatial learning was impaired in WT and HET male and female weanlings in the 1.0 mg/kg/d PCB 95 dose group and WT males in the 6.0 mg/kg/d PCB 95 dose group. Spatial memory was altered only in WT females exposed to 1.0 or 6.0 mg/kg/d PCB 95. Sholl analyses of Golgi-stained hippocampal neurons in male and female WT and HET weanlings from the 1.0 mg/kg/d PCB 95 and vehicle groups indicated that relative to WT vehicle controls, basal dendritic arborization was significantly increased in HET vehicle controls and in PCB 95-exposed WT weanlings; however, PCB 95 did not alter basal dendritic growth in HET weanlings relative to genotype-matched vehicle controls. PCB 95 significantly reduced training-induced dendritic arborization in WT but not HET weanlings. Measurement of tritiated ryanodine ([³H]Ry) binding in cortical tissue from these same animals revealed interactions between genotype, PCB 95 exposure, and dose that altered [³H]Ry binding relative to WT vehicle controls. Quantitative analyses confirmed a dose-dependent increase in PCB tissue burden that was not significantly altered by genotype, MWM training, or sex. Serum levels of progesterone, estradiol, cortisol, and thyroid hormone (TH) and brain transcript levels of TH-responsive genes were not significantly altered by genotype or PCB 95 dose. These data demonstrate that PCB 95 caused behavioral deficits coincident with altered patterns of basal and training-induced dendritic arborization. Furthermore, behavioral and dendritic responses to PCB 95 were altered by expression of the human R163C-RYR1 gain-of-function mutation, supporting the involvement of RYR1-dependent mechanisms in PCB 95 DNT in vivo.