Flexible inhibitory control of defensive behaviour by the ventral lateral geniculate nucleus

Animals can react differently to similar sensory information depending on behavioural circumstances and previous experience. This flexibility is thought to depend on neural inhibition, through suppression of inappropriate and disinhibition of appropriate actions. In this thesis, I identified the ventral lateral geniculate nucleus (vLGN), an inhibitory prethalamic area, as a critical node for the control of visually evoked defensive responses in mice. First, I characterised the structural and functional organisation of the vLGN. Then, taking advantage of a well-characterised model for instinctive behavioural decisions – escape from imminent threat – I showed that GABAergic projections from vLGN to the medial superior colliculus (mSC), a known hub for threat-evoked defensive behaviours, convey information about previous experience of threat and assessment of risk in the environment. Activity in these projections was reduced when mice had experienced a threatening stimulus, while it was elevated after mice learned that stimuli did not pose any danger. Consistently, the chemogenetic suppression of vLGN activity increased risk-avoidance behaviour. The optogenetic activation of vLGN abolished escape responses to imminent visual threats, while suppressing vLGN activity increased the escape probability, demonstrating that vLGN exerts strong, bidirectional control over escape responses. Moreover, electrophysiological mSC recordings in vivo during optogenetic stimulation of vLGN, revealed that vLGN specifically suppresses the activity of visually responsive but not auditory-responsive neurons in the mSC. The optogenetic manipulation of GABAergic projections from vLGN to mSC more strongly influenced escape responses to visual than to auditory threats, suggesting a specificity of this pathway for visually guided behaviours. Together, these results indicate that vLGN flexibly controls the threshold for instinctive responses to imminent visual threat, depending on the animal’s prior experience and its anticipation of danger in the environment. These findings significantly strengthen the circuit-level understanding of flexible behaviour, and how different types of information are integrated to inform decisions

Interplay between Calcite, Amorphous Calcium Carbonate, and Intracrystalline Organics in Sea Urchin Skeletal Elements

Biomineralization processes in living organisms result in the formation of skeletal elements with complex ultrastructures. Although the formation pathways in sea urchin larvae are relatively well known, the interrelation between calcite, amorphous calcium carbonate (ACC), and intracrystalline organics in adult sea urchin biominerals is less clear. Here, we study this interplay in the spines and test plates of the Paracentrotus lividus sea urchins. Thermogravimetric analysis coupled with differential scanning calorimetry or mass spectrometry measurements, nuclear magnetic resonance technique, and high-resolution powder X-ray diffraction show that pristine spines and test plates are composed of Mg-rich calcite and comprise about 1.2 to 1.6 wt % organics, 10 wt % of anhydrous ACC and less than 0.2 wt % of water. Anhydrous ACC originates from incomplete crystallization of a precursor ACC phase during biomineralization and is associated with intracrystalline organics at the molecular level. Molecular interactions at organic/inorganic interfaces cause calcite lattice distortions of the tensile type. The latter are amplified during ACC crystallization and finally disappear after heat-assisted destruction of organic molecules. Converting the measured lattice distortions (strains) into internal stress components, we follow stress evolution upon annealing and find that complete crystallization of ACC leads to the isotropy of residual stresses in all investigated skeletal parts. These results allow us to speculate that organic macromolecules are preferentially attached to different crystallographic planes in the pristine test and spine samples

Interplay between Calcite, Amorphous Calcium Carbonate, and Intracrystalline Organics in Sea Urchin Skeletal Elements

Biomineralization processes in living organisms result in the formation of skeletal elements with complex ultrastructures. Although the formation pathways in sea urchin larvae are relatively well known, the interrelation between calcite, amorphous calcium carbonate (ACC), and intracrystalline organics in adult sea urchin biominerals is less clear. Here, we study this interplay in the spines and test plates of the Paracentrotus lividus sea urchins. Thermogravimetric analysis coupled with differential scanning calorimetry or mass spectrometry measurements, nuclear magnetic resonance technique, and high-resolution powder X-ray diffraction show that pristine spines and test plates are composed of Mg-rich calcite and comprise about 1.2 to 1.6 wt % organics, 10 wt % of anhydrous ACC and less than 0.2 wt % of water. Anhydrous ACC originates from incomplete crystallization of a precursor ACC phase during biomineralization and is associated with intracrystalline organics at the molecular level. Molecular interactions at organic/inorganic interfaces cause calcite lattice distortions of the tensile type. The latter are amplified during ACC crystallization and finally disappear after heat-assisted destruction of organic molecules. Converting the measured lattice distortions (strains) into internal stress components, we follow stress evolution upon annealing and find that complete crystallization of ACC leads to the isotropy of residual stresses in all investigated skeletal parts. These results allow us to speculate that organic macromolecules are preferentially attached to different crystallographic planes in the pristine test and spine samples

The impact of metastasis on the mineral phase of vertebral bone tissue

The negative impact of metastases on the mechanical performance of vertebral bone is often attributed to reduced bone density and/or compromised architecture. However limited characterization has been done on the impact of metastasis on the mineralization of bone tissue and resulting changes in material behaviour. This study aimed to evaluate the impact of metastasis on micro and nano scale characteristics of the mineral phase of bone, specifically mineral crystal growth, homogeneity of mineralization and changes in intrinsic material properties. Female athymic rats were inoculated with HeLa or Ace-1 cancer cells lines producing osteolytic or mixed (osteolytic & osteoblastic) metastases respectively (N=17 per group). A maximum of 21 days was allowed between inoculation and sacrifice of inoculated rats and healthy age-matched uninoculated controls (N=11). X-ray diffraction was used to assess average crystal size in crushed L1-L3 vertebrae; backscatter electron microscopy and nanoindentation were utilized to evaluate modifications in bone mineral density distribution and material behaviour (tissue hardness and modulus) in sagittal-sectioned, embedded and polished L5 vertebrae. HeLa inoculated samples showed reduced mineral crystal width compared to healthy controls. While both types of metastatic involvement reduced tissue mineral content, pathological osteoblastic bone, specific to Ace-1 inoculated samples, significantly decreased tissue mineral homogeneity, whereas osteolytic bone from HeLa samples saw a slight increase in homogeneity. The modulus and hardness of pathological osteoblastic bone was diminished compared to all other bone. Elucidating changes in material behaviour and mineralization of bone tissue is key to defining bone quality in the presence of metastatic involvement.Grant funding for this study was provided by the Canadian Institutes of Health Researc