PhDs in Press: TRAPing Neurons and Tracking Temporal Lobe Epilepsy

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Part 7 in an occasional feature, highlighting recently published articles featuring an author (or authors) affiliated with the Stanford Neuroscience Ph.D program. This round, we've got two lovely first author papers, by Casey Guenthner and Izumi Toyoda.

Let's begin with my year-mate, Casey Guenthner (Luo Lab), who published his development of a technique, TRAP (Targeted Recombination in Active Populations), that allows genetic targeting of populations of neurons that are defined by whether they were activated in vivo by a set stimulus. For details, check out Casey's abstract, below.

Targeting genetically encoded tools for neural circuit dissection to relevant cellular populations is a major challenge in neurobiology. We developed an approach, targeted recombination in active populations (TRAP), to obtain genetic access to neurons that were activated by defined stimuli. This method utilizes mice in which the tamoxifen-dependent recombinase CreER(T2) is expressed in an activity-dependent manner from the loci of the immediate early genes Arc and Fos. Active cells that express CreER(T2) can only undergo recombination when tamoxifen is present, allowing genetic access to neurons that are active during a time window of less than 12 hr. We show that TRAP can provide selective access to neurons activated by specific somatosensory, visual, and auditory stimuli and by experience in a novel environment. When combined with tools for labeling, tracing, recording, and manipulating neurons, TRAP offers a powerful approach for understanding how the brain processes information and generates behavior.

Guenthner, Miyamichi, Yang, Heller and Luo. Permanent Genetic Access to Transiently Active Neurons via TRAP: Targeted Recombination in Active Populations. Neuron. 2013: 78(5):773-84. doi:10.1016/j.neuron.2013.03.025

I will leave you all with the comment that Casey's cartoon of the mouse whisker pad is glorious in its anatomical accuracy. And it's pretty cute, too. Figure 3b. Check it out below, along with data demonstrating targeting of active neurons in the whisker barrel system.

Figure 3. FosTRAP in the Barrel Cortex of Whisker-Plucked Mice.(A) Experimental scheme: FosTRAP mice had either all whiskers except C2 plucked unilaterally or had only the C2 whisker plucked. After a 2 day recovery period, mice were injected with 150 mg/kg TM, and recombination was examined 7 days later.(B) Tangential views of flattened layer 4 of the primary somatosensory barrel cortex (top) or coronal views through the C2 barrel (bottom). White dots indicate the corners of the C2 barrel on the basis of dense DAPI staining of the barrel walls. Compared with controls (left), removal of only the C2 whisker results in elimination of TRAP signal from the C2 barrel (middle), whereas removal of all whiskers except C2 results in absence of most TRAPed cells in all barrels except C2 (right). The left and middle images are from the same mouse. Images are representative of at least 3–4 mice for each condition. The scale bar represents 250 μm. Guenthner et al 2013.

Just last week, Izumi Toyoda (Buckmaster lab) published her work recording spontaneous seizures in rats with temporal lobe epilepsy. Using 32 recording electrodes per rat, Izumi records from a massive number of brain structures, tracking the spread of seizures within the rodent brain. Her paper compares the propagation of the spontaneous seizures in the rats to known seizure activity in human patients with temporal lobe epilepsy. The results validate the pilocarpine model of temporal lobe epilepsy, showing seizures begin in similar brain locations in human patients and rodent subjects.

Temporal lobe epilepsy is the most common form of epilepsy in adults. The pilocarpine-treated rat model is used frequently to investigate temporal lobe epilepsy. The validity of the pilocarpine model has been challenged based largely on concerns that seizures might initiate in different brain regions in rats than in patients. The present study used 32 recording electrodes per rat to evaluate spontaneous seizures in various brain regions including the septum, dorsomedial thalamus, amygdala, olfactory cortex, dorsal and ventral hippocampus, substantia nigra, entorhinal cortex, and ventral subiculum. Compared with published results from patients, seizures in rats tended to be shorter, spread faster and more extensively, generate behavioral manifestations more quickly, and produce generalized convulsions more frequently. Similarities to patients included electrographic waveform patterns at seizure onset, variability in sites of earliest seizure activity within individuals, and variability in patterns of seizure spread. Like patients, the earliest seizure activity in rats was recorded most frequently within the hippocampal formation. The ventral hippocampus and ventral subiculum displayed the earliest seizure activity. Amygdala, olfactory cortex, and septum occasionally displayed early seizure latencies, but not above chance levels. Substantia nigra and dorsomedial thalamus demonstrated consistently late seizure onsets, suggesting their unlikely involvement in seizure initiation. The results of the present study reveal similarities in onset sites of spontaneous seizures in patients with temporal lobe epilepsy and pilocarpine-treated rats that support the model's validity.

Toyoda, Bower, Leyva, Buckmaster. Early Activation of Ventral Hippocampus and Subiculum during Spontaneous Seizures in a Rat Model of Temporal Lobe Epilepsy. J Neurosci, 3 July 2013, 33(27):11100-11115. doi:10.1523/JNEUROSCI.0472-13.2013

And because I showed a figure from Casey's paper, here is one from Izumi's, highlighting a subset of 16 recording electrodes on which a spontaneous seizure is recorded in an epileptic, pilocarpine-treated rat. Let there be no doubt in your mind - the skill required to implant 32 recording electrodes into a rat is large. I suggest being very impressed with Izumi's surgical skills.

Figure 2. Spontaneous seizures in epileptic pilocarpine-treated rats. A, Seizures spread quickly and extensively. Recordings from a subset of 16/32 electrodes implanted in various brain regions. The approximate seizure offset (arrow) and onset window (bar), bracketed by the latest normal activity and earliest clear seizure activity, are indicated. L indicates left; M, medial; EC, entorhinal cortex; sub, subiculum; R, right; amyg, amygdala; SNpr/c, substantia nigra pars reticulata/compacta; sept, septum; D, dorsal; hipp, hippocampus; olf ctx, olfactory cortex; vdb, ventral diagonal band; dm thal, dorsomedial thalamus; endo n, endopiriform nucleus. B, Precise seizure onsets (arrows) in three regions. C, Seizure durations. Bars represent averages; symbols indicate durations of individual seizures. D, Behavioral seizure onset latencies. Electrographic seizure onset is at 0. Positive (negative) values indicate that behavioral onset followed (preceded) electrographic onset. Bars represent averages; symbols indicate individual seizures. From Toyoda et al, 2013.

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Astra Bryant

Astra Bryant is a graduate of the Stanford Neuroscience PhD program in the labs of Drs. Eric Knudsen and John Huguenard. She used in vitro slice electrophysiology to study the cellular and synaptic mechanisms linking cholinergic signaling and gamma oscillations – two processes critical for the control of gaze and attention, which are disrupted in many psychiatric disorders. She is a senior editor and the webmaster of the NeuWrite West Neuroblog