Arruda-Carvalho, M. | Search bioRxiv

bioRxiv2023

Maturation of Nucleus Accumbens Synaptic Transmission Signals a Critical Period for the Rescue of Social Deficits in a Mouse Model of Autism Spectrum Disorder

Social behavior emerges early in development, a time marked by the onset of neurodevelopmental disorders featuring social deficits, including autism spectrum disorder (ASD). Although deficits in social interaction and communication are at the core of the clinical diagnosis of ASD, very little is known about their neural correlates at the time of clinical onset of the disorder. The nucleus accumbens (NAc), a brain region extensively implicated in social behavior, undergoes synaptic, cellular and molecular alterations in early life, and is particularly affected in ASD mouse models. To explore a link between the maturation of the NAc and neurodevelopmental deficits in social behavior, we compared age-dependent changes in spontaneous synaptic transmission in NAc shell medium spiny neurons (MSNs) between the highly social C57BL/6J mouse strain and the idiopathic ASD mouse model BTBR T+ Itpr3tf/J at postnatal day (P) 4, P6, P8, P12, P15, P21 and P30. We found that MSNs from both C57BL/6J and BTBR mice display age-dependent increases in spontaneous excitatory and inhibitory synaptic currents between P4 and P30. Comparison of NAc spontaneous transmission between strains showed that BTBR MSNs display increased excitatory transmission during the first postnatal week, and increased inhibition across the first, second and fourth postnatal weeks, suggesting accelerated maturation of excitatory and inhibitory synaptic inputs onto BTBR MSNs compared to C57BL/6J mice. These early life changes in synaptic transmission are consistent with a potential critical period in the maturation of the NAc, which could maximize the efficacy of interventions affecting social behavior. To test this possibility, we treated BTBR mice in either early life (P4-P8) or adulthood (P60-P64) with the mTORC1 antagonist rapamycin, a well-established rescue intervention for ASD-like behavior. We found that rapamycin treatment rescued social interaction deficits in BTBR mice when injected in infancy, but not in adulthood. These data emphasize the importance of studying brain regions involved in the pathophysiology of neurodevelopmental disorders at clinically-relevant time points, which may offer novel insight into the timing and targets of therapeutic interventions to maximize positive outcomes.

neuroscience↗

bioRxiv2021

Off-target expression of Cre-dependent adeno-associated viruses in wild type C57BL/6J mice

Adeno-associated viruses (AAVs) are a commonly used tool in neuroscience to efficiently label, trace, and/or manipulate neuronal populations. Highly specific targeting can be achieved through recombinase-dependent AAVs in combination with transgenic rodent lines that express Cre-recombinase in specific cell types. Visualization of viral expression is typically achieved through fluorescent reporter proteins (e.g., GFP or mCherry) packaged within the AAV genome. Although non-amplified fluorescence is usually sufficient to observe viral expression, immunohistochemical amplification of the fluorescent reporter is routinely used to improve viral visualization. In the present study, Cre-dependent AAVs were injected into the hippocampus and cortex of wild-type C57BL/6J mice. While we observed weak but consistent non-amplified off-target DIO expression in C57BL/6J mice, antibody amplification of the GFP or mCherry reporter revealed extensive Cre-independent viral expression. Off-target expression of DIO constructs in wild-type C57BL/6J mice occurred independent of vendor, AAV serotype or promoter. We also evaluated whether Cre-independent expression had functional effects via Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). The DREADD agonist C21 had no effect on contextual fear conditioning or cFos expression in DIO-hM3Dq-mCherry+ cells of C57BL/6J mice. Taken together, our results indicate that DIO constructs have considerable off-target expression in wild type subjects. Our findings are particularly important for the design of experiments featuring sensitive systems and/or quantitative measurements that could be negatively impacted by off-target expression. Significance StatementAdeno-associated viruses (AAV) are widely used in neuroscience because of their safety and ease of use. Combined with specific promoters, Cre/loxP, and stereotaxic injections, highly specific targeting of cells and circuits within the brain can be achieved. In the present study we injected Cre-dependent AAVs into wild-type C57BL/6J mice and found considerable Cre-independent viral expression of AAVs encoding mCherry, GFP, or hM3Dq following immunohistochemical amplification of the fluorescent reporter protein. Importantly, we observed no functional effects of the Cre-independent expression in the hippocampus, as C21 had no detectable effect on DIO-hM3Dq-mCherry infected neurons in C57BL/6J mice. Given the widespread use of DIO rAAVs by the neuroscience community, our data supports careful consideration when using DIO constructs in control animals.

neuroscience↗