Zinc Nanoparticles Enhance Brain Connectivity in the Canine Olfactory Network: Evidence From an fMRI Study in Unrestrained Awake Dogs

Prior functional Magnetic Resonance Imaging (fMRI) studies have indicated increased neural activation when zinc nanoparticles are added to odorants in canines. Here we demonstrate that zinc nanoparticles up-regulate directional brain connectivity in parts of the canine olfactory network. This provides an explanation for previously reported enhancement in the odor detection capability of the dogs in the presence of zinc nanoparticles. In this study, we obtained fMRI data from awake and unrestrained dogs while they were being exposed to odorants with and without zinc nanoparticles, zinc nanoparticles suspended in water vapor, as well as just water vapor alone. We obtained directional connectivity between the brain regions of the olfactory network that were significantly stronger for the condition of odorant + zinc nanoparticles compared to just odorants, water vapor + zinc nanoparticles and water vapor alone. We observed significant strengthening of the paths of the canine olfactory network in the presence of zinc nanoparticles. This result indicates that zinc nanoparticles could potentially be used to increase canine detection capabilities in the environments of very low concentrations of the odorants, which would have otherwise been undetected

Functional MRI of the olfactory system in conscious dogs.

We depend upon the olfactory abilities of dogs for critical tasks such as detecting bombs, landmines, other hazardous chemicals and illicit substances. Hence, a mechanistic understanding of the olfactory system in dogs is of great scientific interest. Previous studies explored this aspect at the cellular and behavior levels; however, the cognitive-level neural substrates linking them have never been explored. This is critical given the fact that behavior is driven by filtered sensory representations in higher order cognitive areas rather than the raw odor maps of the olfactory bulb. Since sedated dogs cannot sniff, we investigated this using functional magnetic resonance imaging of conscious dogs. We addressed the technical challenges of head motion using a two pronged strategy of behavioral training to keep dogs' head as still as possible and a single camera optical head motion tracking system to account for residual jerky movements. We built a custom computer-controlled odorant delivery system which was synchronized with image acquisition, allowing the investigation of brain regions activated by odors. The olfactory bulb and piriform lobes were commonly activated in both awake and anesthetized dogs, while the frontal cortex was activated mainly in conscious dogs. Comparison of responses to low and high odor intensity showed differences in either the strength or spatial extent of activation in the olfactory bulb, piriform lobes, cerebellum, and frontal cortex. Our results demonstrate the viability of the proposed method for functional imaging of the olfactory system in conscious dogs. This could potentially open up a new field of research in detector dog technology

Comparisons of fitted time series obtained from the GLM for ROIs in awake dogs.

<p>The ROIs are in brain regions that were activated by low and high odor concentration, as well as parametric modulation by odor intensity in awake dogs. These regions are olfactory bulb and cerebellum, which are shown in bold face in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t003" target="_blank">Tables 3</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t004" target="_blank">4</a>, and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t006" target="_blank">6</a>. In each of these regions, the ROI was determined by a sphere which centers at the peak activation of parametric modulation and has a radius of 2 mm. Fitted time series for low concentration are shown in blue, and high concentration in red.</p

Cluster-level statistics of activations for anesthetized dogs, low concentration of odorant.^*

<p>*: ROIs shown in bold face were commonly activated for low and high (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t002" target="_blank">Table 2</a>) odor concentration, as well as parametric modulation by odor concentration (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t005" target="_blank">Table 5</a>).</p

Group activation maps for awake dogs.

<p>(Overall FDR = 0.05, cluster threshold  = 15 voxels using AlphaSim, t-contrast) Three orthogonal views are shown for each subfigure. Hot colormap is used for activation intensity, and important areas are indicated by arrows with labels. Subfigure (A) corresponds to low concentration odorant (0.016 mM), subfigure (B) corresponds to high concentration odorant (0.16 mM). The activation in olfactory bulb for low concentration is not visible in this view, please refer to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t003" target="_blank">Table 3</a> for activation statistics with regard to this region. (A: Anterior, P: posterior, S: superior, I: inferior, L: left, R: right)</p

Group activation maps for parametric modulation in anesthetized dogs (A) and awake dogs (B).

<p>(Overall FDR = 0.05, cluster threshold  = 15 voxels using AlphaSim, t-contrast). Three orthogonal views are shown for each subfigure. Hot colormap is used for activation intensity, and important areas are indicated by arrows with labels. (A: Anterior, P: posterior, S: superior, I: inferior, L: left, R: right)</p

Cluster-level statistics of activations for parametric modulation of awake dogs.^*

<p>*: ROIs shown in bold face were commonly activated for low (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t003" target="_blank">Table 3</a>) and high (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t004" target="_blank">Table 4</a>) odor concentration, as well as parametric modulation by odor concentration.</p

The interlinked trigger system.

<p>Arrows denote the triggering direction. A laptop with VT-8 software <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone.0086362-Warner1" target="_blank">[35]</a> provided the interface to trigger the odorant applicator. The VT-8 software is a platform that can be used to design and display sequence of odorant flow and clearance, and provides communication and control to odorant applicator to generate the expected experimental sequence. Once the odorant applicator started to give odorant stimulus, it sent a signal to the trigger synchronizer, which then triggered the scanner and sent a signal to the manual trigger. The manual trigger, as the name suggests, was manually set for switching between two states. One was waiting for signals from trigger synchronizer, and the other was waiting for signal from a hand-pressed button. In our experiment, the first state was used for data collection and the second was used only for testing. Upon receiving the signal, it triggered the infrared radiation transmitter to give off infrared rays, and the infrared camera to start recording infrared reflections from the dog's head, and the motion parameter recording palmtop to start calculating displacement parameters. When the camera was triggered, it sent the signal to the monitor for display.</p

Cluster-level statistics of activations for awake dogs, high concentration of odorant.^*

<p>*: ROIs shown in bold face were commonly activated for low (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t003" target="_blank">Table 3</a>) and high odor concentration, as well as parametric modulation by odor concentration (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086362#pone-0086362-t006" target="_blank">Table 6</a>).</p