| dc.description.abstracteng | Understanding the brain is aided by visualizing neural activity over time. The most popular
method for doing so in humans is functional magnetic resonance imaging (fMRI)—a
method that tracks blood oxygenation as a proxy for neural activity. fMRI relies on neurovascular
coupling, the brain’s capacity to increase its blood supply locally and on demand.
Apart from humans, fMRI can be also applied to experimental animals and thereby
plays an essential role in translating findings across species. Additionally, the combination
of animal fMRI with electrophysiological and optical methods is crucial for uncovering
the neural correlates of the observed blood-oxygen-level-dependent (BOLD) fMRI signal.
Since fMRI necessitates immobility, animals must be either restrained or anesthetized.
Most researchers take the latter approach, for both practical and ethical reasons. However,
anesthesia confounds the results of fMRI experiments by profoundly altering neural
activity and by interfering with neurovascular coupling. This conundrum, which can be
viewed both as a challenge and an opportunity, motivated the three studies presented in
this thesis.
The challenge lies in choosing the right anesthesia for animal fMRI experiments. The
ideal anesthetic protocol must provide sufficient sedation, guarantee immobility, and crucially,
preserve a degree of neural responsiveness and neurovascular coupling. Anesthetic
protocols based on the continuous infusion of the sedative medetomidine exhibit these
qualities and have thus become a popular choice for rats—the most widely used animal
fMRI model. Despite this, it has not yet been established how fMRI readouts evolve over
several hours of medetomidine anesthesia and how they are affected by variations in
timing, dose, and route of administration. In my first study (Chapter 2), I used four different
protocols of medetomidine administration to anesthetize rats for up to six hours and
repeatedly evaluated stimulus-evoked responses and fMRI measures of functional connectivity.
I found that the temporal evolution of fMRI readouts varied between administration
schemes. Based on the results, I made recommendations regarding the administration of
medetomidine and the timing of fMRI experiments. These factors are important for obtaining
reproducible results and should be considered for the design and interpretation
of future rat fMRI studies.
The opportunity lies in exploiting anesthesia’s effects on fMRI to better understand
large-scale phenomena in the anesthetized brain. The case in point is burst-suppression,
a poorly understood pattern of neural activity that appears in deep anesthesia and coma.
In animals anesthetized with isoflurane, burst-suppression has been associated with the
widespread synchronization of brain areas. In the second study (Chapter 3), I used fMRI
data from four species—humans, macaques, marmosets, and rats—to precisely describe
the fMRI signatures of anesthesia-induced burst-suppression and to map their distribution
across the brain. I discovered a marked difference between primates and rodents. In
rats the entire neocortex engaged in burst-suppression, while in the three primate species
certain cortical areas were excluded—most notably the visual cortex. Based on the fMRI
data alone, I could not determine the underlying cause of this exclusion. I concluded
that answering this question would necessitate direct recordings of neural activity in the
visual cortex of both primates and rodents.
In the third study (Chapter 4), I aimed to develop the methods required for such direct
neural recordings. Specifically, I conducted a series of pilot experiments in isoflurane-anesthetized
rats and demonstrated the feasibility of in vivo two-photon calcium imaging
through chronically implanted cranial windows. I was able to record the activity of
hundreds of layer 2/3 neurons in the rat somatosensory and visual cortex and confirm
my previous findings regarding the pancortical distribution of burst-suppression. I also
examined the effects of varying the isoflurane dose on spontaneous activity and stimulus-evoked
responses, thereby reproducing several known properties of burst-suppression
in rodents. The developed methods can be easily adapted to record from the marmoset
visual cortex, with the aim of understanding the primate-rodent difference described in
Chapter 3.
The above studies showcase that anesthetizing animals for functional neuroimaging experiments
should not be viewed as a necessary evil. Anesthetic protocols can be optimized
to allow for a host of neuroscientific questions to be asked. Moreover, such experiments
can shed light on the functional organization of the anesthetized brain and on elusive
anesthetic mechanisms of actions. | de |