Photoacoustic imaging of fear conditioning neuronal ensembles using a Fos-LacZ gene reporter system in the rat brain in vivo after intrathecal X-gal injection

Current functional imaging techniques, such as functional magnetic resonance imaging, rely upon activity-induced blood flood changes to neurons. This indirect measurement of neuronal activity inherently limits image resolution and specificity, however, advances in transgenic technology and Photoacoustic (PA) methodology have offered new solutions. We previously demonstrated using PA imaging in conjunction with a Fos-LacZ rat model to map activated neurons through selective formation of PA-active X-gal product within Fos-expressing neurons. While we have previously used intracranial injections for successful delivery of X-gal to detect neurons that are highly-active after classical fear conditioning using PA imaging, now, we have further advanced this technique by using an intrathecal route of administration of X-gal combined with in vivo imaging to improve the translatability of this approach and whole-brain imaging capabilities. We subjected Fos/LacZ rats to fear conditioning, or control conditions. Ninety minutes post stimulus presentation, X-gal was delivered via intrathecal or intravenous injection. After 24 hours, a cranial window was made in animals and they were PA imaged in vivo. PA intensity within the medial prefrontal cortex (mPFC) of acquired images was quantified using ImageJ software. We presently report quantification of PA images of rat brains stained with X-gal product generated in vivo within activated neurons. Detailed PA signal differences are reported between mPFC subregions. We discuss the feasibility and translatability of this reporter method for neuronal activity based on our acquired images. With this technique, we propose a method of longitudinally monitoring activated neurons in vivo with high resolution and specificity.

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