Proteinase K These anatomical studies were followed by the f
These anatomical studies were followed by the first functional MRI studies of auditory processing in songbirds that were performed in starlings (Van Meir et al., 2005; De Groof et al., 2017; De Groof et al., 2013b) and in zebra finches (Boumans et al., 2008a; Boumans et al., 2007; Boumans et al., 2008b; Poirier et al., 2011; Poirier et al., 2010; van der Kant et al., 2013; Van Ruijssevelt et al., 2018; Van Ruijssevelt et al., 2017a; Van Ruijssevelt et al., 2017b; Vignal et al., 2008)(For reviews see: (Hamaide et al., 2016; Van der Linden et al., 2009; Van Ruijssevelt et al., 2013b)). Auditory processing in zebra finches was further investigated using functional MRI by Voss et al. (Voss et al., 2007), focusing on modulations in auditory responses in a model for stuttering (Voss et al., 2010) and in song-deprived males and females (Maul et al., 2010).
In principle, MRI is a non-invasive imaging technique used both in clinical and research settings to investigate all types of soft tissue including the brain. MRI allows the acquisition of virtual slices in any direction throughout the entire organism or any part of it such as the brain. Its success mainly resides in its high spatial resolution (e.g., 150 μm or even less in small animals), superior tissue contrast and non-invasive nature, which allows repeated imaging of the same subject. This permits longitudinal studies where for each subject the outcome can be compared to its own baseline measurement along the course of a pathology, over different training sessions or before and after neuromodulatory interventions of any kind (For more details on the MRI method, we refer the reader to the following books: (McRobbie et al., 2006; Westbrook et al., 2005)).
This latter approach is called functional MRI (fMRI) and uses neuronal activity-induced changes in the local blood perfusion (the so-called hemodynamic response) as readout for Proteinase K activity. The responsible contrast mechanism is the Blood-Oxygenation Level-Dependent (BOLD) contrast that relies on subtle differences in the magnetic properties of oxygenated (diamagnetic) versus deoxygenated (paramagnetic, disturbing the local magnetic field) hemoglobin. When challenged by a specific task or stimulation, specific neuronal populations will metabolize more glucose and thus use more oxygen. This will induce a hemodynamic response resulting in an increase of the blood flow and blood volume in that area within a few seconds. This will also result in a local change of the ratio of oxygenated versus deoxygenated hemoglobin evoking a local BOLD contrast (Logothetis, 2002, Logothetis, 2008; Logothetis et al., 2001). Subjects must remain immobilized throughout the scanning procedure for fMRI to be successful. For animals, this usually requires the use of anesthesia, which limits the type of experiments or stimulation paradigms that can be applied (see also (Van der Linden et al., 2007)). However, a number of studies have explored possible ways of performing fMRI studies by implementing training protocols to teach animals to remain immobile in combination with restraining. Examples of awake fMRI are found in rodents (Ferris et al., 2011; Jonckers et al., 2014; King et al., 2005) but also in pigeons (De Groof et al., 2013a) and zebra finches (Van Ruijssevelt et al., 2017a).
Exact details on how fMRI can be performed in songbirds are provided in a review paper (Poirier et al., 2010) but also in a video available in JoVe (Van Ruijssevelt et al., 2013a). In the next chapter, we now summarize a study that we recently published (De Groof et al., 2017) to illustrate the use of fMRI in studying the spatial extend of rapid effects of aromatase inhibition on auditory processing in the telencephalon/brain of male starlings over different seasons.
Functional MRI visualizes fast effects of brain estrogens depletion following acute aromatase inhibition on auditory processing in a seasonal songbird