Model organism: We use the mouse (Mus musculus) as our model organism. Using mice, we can more easily link cellular/synaptic-level phenomena with systems- and behavior-level questions. Using mice also offers us the opportunity to use state-of-the-art genetic tools in our investigation of cortical and thalamic circuitry.
Transsynaptic anatomical tracing: Many of the neuronal circuits we study are defined by directionally specific pre- and postsynaptic regions. For instance, sensory cortex can be connected disynaptically via the higher-order thalamus to the prefrontal cortex in the feedforward direction, or vice versa in the feedback direction. We need methods to genetically label these specific pathways. By combining anterograde transsynaptic viral tracing with intersectional genetic techniques (e.g. Cre-Lox and Flp-Frt), we can label neurons based both on where they project to and where they receive input from (yellow, right). Additionally, by combining this approach with retrograde transsynaptic viral tracing (i.e. G-deleted rabies), we can also reveal which other brain areas (both cortical and subcortical) are presynaptic to these specific neurons. Together, these cutting-edge anatomical techniques allow us to uncover the input-output organization of the higher-order thalamus and how its functional integration with the cortex is modulated.
Ev vivo electrophysiology and optogenetics: To explore the detailed synaptic physiology of thalamocortical (TC) and corticothalamic (CT) interactions, we use ex vivo slice preparations of the mouse brain in which excitatory opsins (e.g. ChR2) are expressed in TC or CT axon terminals. By pulsing light onto these terminals while recording the postsynaptic responses in cortical or thalamic neurons using the whole-cell patch clamp technique, we can uncover multiple features of synaptic signaling (e.g. short-term dynamics, postsynaptic receptor composition). This technique provides us with a foundational view of how the cortex and thalamus bidirectionally communicate.
In vivo electrophysiology: We also monitor the neuronal activity of cortical and thalamic neurons in awake, head-fixed mice. Head-fixation allows for stable neuronal recordings, yet also allows mice to engage in various spontaneous behaviors, such as locomotion, whisking, and grooming. With this preparation, we can relate awake behavioral state with cortical and thalamic spiking activity, or even membrane potential with whole-cell patch clamp recordings (right).
In vivo imaging: By expressing genetically-encoded indicators that change their fluorescence in response to activity (such as changes in intracellular calcium levels), we can monitor the activity of specific cortical and thalamic cell bodies or axons in awake, behaving animals using either fiber photometry or 2-photon microscopy (right). We can also combine fluorescent indicators of different colors to simultaneously monitor the activity of multiple cell types to study their functional interactions as animals behave.
Behavior: We can combine many of the above in vivo techniques with behavioral tasks in which mice need to make a response to specific sensory events in order to receive a reward, such as licking for a target visual stimulus while ignoring distractors (right). These experiments provide us with information about how cortical and thalamic circuits transform sensory inputs into goal-directed behavior.
