Quantitative tracking of trans-synaptic nose-to-brain transport of nanoparticles and its modulation by odor,aging,and Parkinson’s disease

Nanoparticles(NPs)can be transported via the nose-to-brain(N_(2)B)route.Nonetheless,quantitative data on their spatiotemporal dynamics and regulation of the N_(2)B transport are largely lacking.We surveyed metal oxide/hydroxide NPs as magnetic resonance imaging(MRI)contrasts for quantitative N_(2)B tracking.NPs containing divalent transition metals were the only ones capable of N_(2)B transmission.Using T1-weighted(T1W)MRI,we showed that Mn_(3)O_(4)-NPs were readily engulfed by olfactory receptor neurons(ORNs)without disrupting olfactory sensing,we mapped their N_(2)B trajectory.Within neurons,the Mn_(3)O_(4)-NPs were localized to the cytosol,mitochondria,vesicles,moved at mixed fast and slow axonal transport velocities intraand extra-vesicularly through ORNs.The NPs’axonal transport is dependent on neuronal activity and microtubule integrity.The Mn_(3)O_(4)-NPs were trans-synaptically transmitted through at least four synapses across the olfactory tract.Trans-synaptic transmission of the NPs was dependent on N-type Ca^(2+)channels and NMDA receptors but blocked by GABAB receptor activation.A five-parameter Weibull signal increase/decrease model fitted to the T1W MRI data allowed for estimating kinetic parameters of Mn_(3)O_(4)-NP accumulation/elimination.Absolute and relative accumulation rates,but not elimination,correlated negatively with the number of synapses from ORNs,indicating a coupling of the NPs’N_(2)B transport with spontaneous neuronal activity.Accordingly,olfactory stimuli(2,5-dimethylpyrazine and acetophenone)significantly modulated and rerouted the Mn_(3)O_(4)-NP N_(2)B transport odor specifically.Finally,the NPs’trans-synaptic transmission was impaired by aging and the onset of Parkinson’s disease.These data suggest new approaches to diagnostics,functional neuroimaging,controlling N_(2)B drug delivery.

Trans-synaptic Neural Circuit-Tracing with Neurotropic Viruses

A central objective in deciphering the nervous system in health and disease is to define the connections of neurons. The propensity of neurotropic viruses to spread among synaptically-linked neurons makes them ideal for mapping neural circuits. So far, several classes of viral neuronal tracers have become available and provide a powerful toolbox for delineating neural networks. In this paper, we review the recent developments of neurotropic viral tracers and highlight their unique properties in revealing patterns of neuronal connections.

Cortical Organization of Centrifugal Afferents to the Olfactory Bulb: Mono-and Trans-synaptic Tracing with Recombinant Neurotropic Viral Tracers

Sensory processing is strongly modulated by different brain and behavioral states,and this is based on the top-down modulation.In the olfactory system,local neural circuits in the olfactory bulb(OB)are innervated by centrifugal afferents in order to regulate the processing of olfactory information in the OB under different behavioral states.The purpose of the present study was to explore the organization of neural networks in olfactory-related cortices and modulatory nuclei that give rise to direct and indirect innervations to the glomerular layer(GL)of the OB at the whole-brain scale.Injection of different recombinant attenuated neurotropic viruses into the GL showed that it received direct inputs from each layer in the OB,centrifugal inputs from the ipsilateralanterior olfactory nucleus(AON),anterior piriform cortex(Pir),and horizontal limb of diagonal band of Broca(HDB),and various indirect inputs from bilateral cortical neurons in the AON,Pir,amygdala,entorhinal cortex,hippocampus,HDB,dorsal raphe,median raphe and locus coeruleus.These results provide a circuitry basis that will help further understand the mechanism by which olfactory informationprocessing in the OB is regulated.

Whole-Brain Mapping of Direct Inputs to and Axonal Projections from GABAergic Neurons in the Parafacial Zone

The GABAergic neurons in the parafacial zone（PZ） play an important role in sleep-wake regulation and have been identified as part of a sleep-promoting center in the brainstem, but the long-range connections mediating this function remain poorly characterized. Here, we performed whole-brain mapping of both the inputs and outputs of the GABAergic neurons in the PZ of the mouse brain. We used the modified rabies virus Env A-DG-Ds Red combined with a Cre/lox P gene-expression strategy to map the direct monosynaptic inputs to the GABAergic neurons in the PZ, and found that they receive inputs mainly from the hypothalamic area, zona incerta, and parasubthalamic nucleus in the hypothalamus; the substantia nigra, pars reticulata and deep mesencephalic nucleus in the midbrain;and the intermediate reticular nucleus and medial vestibular nucleus（parvocellular part） in the pons and medulla.We also mapped the axonal projections of the PZ GABAergic neurons with adeno-associated virus, and defined the reciprocal connections of the PZ GABAergic neurons with their input and output nuclei. The newlyfound inputs and outputs of the PZ were also listed compared with the literature. This cell-type-specific neuronal whole-brain mapping of the PZ GABAergic neurons may reveal the circuits underlying various functions such as sleep-wake regulation.

Viral Tools for Neural Circuit Tracing

Neural circuits provide an anatomical basis for functional networks.Therefore,dissecting the structure of neural circuits is essential to understanding how the brain works.Recombinant neurotropic viruses are important tools for neural circuit tracing with many advantages over non-viral tracers:they allow for anterograde,retrograde,and trans-synaptic delivery of tracers in a cell type-specific,circuit-selective manner.In this review,we summarize the recent developments in the viral tools for neural circuit tracing,discuss the key principles of using viral tools in neuroscience research,and highlight innovations for developing and optimizing viral tools for neural circuit tracing across diverse animal species,including nonhuman primates.

Three-Dimensional Heterogeneity of Cerebellar Interposed Nucleus-Recipient Zones in the Thalamic Nuclei

The cerebellum is conceptualized as a processor of complex movements and is also endowed with roles in cognitive and emotional behaviors.Although the axons of deep cerebellar nuclei are known to project to primary thalamic nuclei,macroscopic investigation of the characteristics of these projections,such as the spatial distribution of recipient zones,is lacking.Here,we studied the output of the cerebellar interposed nucleus(IpN)to the ventrolateral(VL)and centrolateral(CL)thalamic nuclei using electrophysiological recording in vivo and trans-synaptic viral tracing.We found that IpN stimulation induced mono-synaptic evoked potentials(EPs)in the VL but not the CL region.Furthermore,both the EPs induced by the IpN and the innervation of IpN projections displayed substantial heterogeneity across the VL region in three-dimensional space.These findings indicate that the recipient zones of IpN inputs vary between and within thalamic nuclei and may differentially control thalamo-cortical networks.