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A key consideration in the clinical translation of nanomedicines is determining their in vivo biodistribution in preclinical studies, which is important for predicting and correlating therapeutic efficacy and safety. There are a number of techniques available for analyzing the in vivo biodistribution of nanoparticles, with each having its own advantages and limitations. However, conventional techniques are limited by their inability to image the three-dimensional (3D) association of nanoparticles with cells, vasculature and other biological structures in whole organs at a subcellular level. Recently, optical clearing techniques have been used to evaluate the biodistribution of nanoparticles by 3D organ imaging. Optical clearing is a procedure that is increasingly being used to improve the imaging of biological tissues, whereby light scattering substances are removed to better match the refractive indices of different tissue layers. The use of optical clearing techniques has the potential to transform the way we evaluate the biodistribution of new and existing nanomedicines, as it allows the visualization of the interaction of nanoparticles with the biological environment in intact tissues. This review will compare the main optical clearing techniques and will address the considerations for the use of these techniques to evaluate nanoparticle biodistribution.

In the weeks and months following an incomplete spinal cord injury (SCI) significant spontaneous recovery of function occurs in the absence of any applied therapeutic intervention. The anatomical correlates of this spontaneous plasticity are well characterized, however, the functional changes that occur in spinal cord interneurons after injury are poorly understood. Here we use a T10 hemisection model of SCI in adult mice (9-10 wks old) combined with whole-cell patch clamp electrophysiology and a horizontal spinal cord slice preparation to examine changes in intrinsic membrane and synaptic properties of deep dorsal horn (DDH) interneurons. We made these measurements during short-term (4 wks) and long-term (10 wks) spontaneous recovery after SCI. Several important intrinsic membrane properties are altered in the short-term, but recover to values resembling those of uninjured controls in the longer term. AP discharge patterns are reorganized at both short-term and long-term recovery time points. This is matched by reorganization in the expression of voltage-activated potassium and calcium subthreshold-currents that shape AP discharge. Excitatory synaptic inputs onto DDH interneurons are significantly restructured in long-term SCI mice. Plots of sEPSC peak amplitude vs. rise times suggest considerable dendritic expansion or synaptic reorganization occurs especially during long-term recovery from SCI. Connectivity between descending dorsal column pathways and DDH interneurons is reduced in the short-term, but amplified in long-term recovery. Our results suggest considerable plasticity in both intrinsic and synaptic mechanisms occurs spontaneously in DDH interneurons following SCI and takes a minimum of 10 wks after the initial injury to stabilize.

In spinal cord injury (SCI) research, axon regeneration across spinal lesions is most often assessed using anatomical methods. It would be extremely advantageous, however, to examine the functional synaptic connectivity of regenerating fibres, using high-resolution electrophysiological methods. We have therefore developed a mouse horizontal spinal cord slice preparation that permits detailed analysis of evoked dorsal column (DCol) synaptic inputs on spinal neurons, using whole-cell patch clamp electrophysiology. This preparation allows us to characterise postsynaptic currents and potentials in response to electrical stimulation of DCol fibres, along with the intrinsic properties of spinal neurons. In addition, we demonstrate that low magnification calcium imaging can be used effectively to survey the spread of excitation from DCol stimulation in horizontal slices. This preparation is a potentially valuable tool for SCI research where confirmation of regenerated, functional synapses across a spinal lesion is critical.