Multimodal neuroimaging of basal forebrain cholinergic systems in human focal epilepsy

Multimodal neuroimaging of basal forebrain cholinergic systems in human focal epilepsy

Project Summary

Septal nuclei, a primarily cholinergic cell group in basal forebrain, provide critical modulation to the hippocampus, facilitating synaptic plasticity necessary for encoding memories (1-4) and preventing pathologic hyperexcitability (5). In animal models of epilepsy, stimulation of septal nuclei is antiepileptic,(6-9) while septal lesions(8, 10-14) or septo-hippocampal disconnection (15) promote seizures and epileptogenesis. The role of septal nuclei in human epilepsy is unknown. Using MRI, we have demonstrated that average septal volume is significantly increased in patients with pharmacoresistant temporal lobe epilepsy (TLE). Here, we provide preliminary evidence that septal enlargement may predict seizure freedom following temporal lobectomy. Integrating our findings with evidence that basal forebrain cholinergic neurons exhibit remarkable neurotrophin-mediated neuroplasticity in response to brain insult or injury (16-24) including seizures (25) we propose a model in which septal enlargement in TLE represents MR-detectable evidence of plasticity/augmentation of the normally-antiepileptic cholinergic septo-hippocampal system. We will use functional and structural MRI, Positron Emission Tomography (PET) with a cholinergic radiotracer, and corroborative histological and molecular analysis of epilepsy surgical specimens to further characterize septo-hippocampal cholinergic neurocircuitry in human TLE. Our specific aims are to:

Aim 1: Determine septal volume contribution to surgical outcome in TLE patients. Building upon our preliminary results, we will test the hypothesis that septal enlargement in patients with pharmacoresistant TLE is an independent predictor of good outcome from temporal lobectomy. These results can validate septal enlargement as a novel MR biomarker to sub-type TLE patients and guide individualized therapy.

Aim 2: Use MRI to measure structural and functional septo-hippocampal connectivity in patients with TLE as compared to healthy controls and patients with extratemporal epilepsy. We will test the hypotheses that 1) Diffusion Tensor Imaging (DTI) measures of the integrity of septo-hippocampal structural connections (the fimbria/fornix) will correlate with the volume of structures whose axons comprise this tract (septal nuclei and hippocampus) and will be preserved in TLE; 2) septo-hippocampal functional connectivity measured using resting state fMRI will be diminished ipsilateral to the seizure focus in TLE, reflecting septohippocampal network dysfunction; and 3) normal high correlation between functional and structural connectivity (26, 27) will be disrupted ipsilateral to the seizure focus in TLE. Defining septo-hippocampal functional and structural connectivity in patients with TLE can inform use of novel therapies to modulate hippocampal excitability such as septal stimulation, which is antiepileptic in animals (8) and safe in humans (28, 29).

Aim 3: Use PET to determine the extent to which TLE-related alterations in septal volume and septohippocampal connectivity reflect alterations in cholinergic neurocircuitry. 18F-fluorocholine (18FCH) is a cholinergic PET tracer which in our preliminary results shows maximal brain uptake in hippocampus. We will test the hypotheses that 1) 18FCH PET signal will be abnormally increased in patients with TLE in the epileptic hippocampus, in accord with evidence of increased cholinergic hippocampal innervation in animal and human TLE;(25, 30-34) and 2) hippocampal 18FCH uptake will correlate with MR-measured septal volume, since we posit that these two measures reflect cell bodies and axon terminals of the same septo-hippocampal cholinergic neurons.

Aim 4: Verify and extend MRI and PET results via histological and molecular examination of surgicallyremoved TLE specimens. We will test the hypotheses that 1) MR-measured septal volume correlates with expression of markers of septo-hippocampal cholinergic terminals including the high-affinity choline transporter HACT (which our preliminary data shows to be overexpressed in epileptic hippocampus), reflecting augmentation of the septo-hippocampal cholinergic pathway, and 2) precise PET-MRI-surgical field-tissue coregistration will show that 18FCH PET signal correlates with density of cholinergic nerve terminals, thereby validating 18FCH PET for this novel brain use. In addition, to begin to elucidate the biological basis of septal enlargement in TLE, we will assess markers of growth factor signaling in epileptic hippocampus. Of particular interest is Nerve Growth Factor (NGF), which through interaction with its receptor TrkA, is essential for septal cholinergic neuron growth and morphology.(16, 20, 35) NGF is supplied almost exclusively by hippocampus(16, 20, 35-39) and is upregulated by seizures.(37) Excess NGF/TrkA activity in the septohippocampal system therefore represents a plausible mechanism underlying septal enlargement in TLE. We will test the hypothesis that 3) MR-measured septal volume correlates with increased hippocampal NGF/TrkA signaling.(40)

Significance

This project will determine if cholinergic septo-hippocampal neurocircuitry - little studied in humans but critically involved in epilepsy and neuroplasticity in animal models - shows structural, functional and neurochemical abnormalities in human epilepsy consistent with a cholinergically-mediated neuroplastic process potentially amenable to novel anticonvulsant and antiepileptogenic treatments.

Innovation

Our hypotheses regarding septal enlargement in TLE are novel. Development of multimodality biomarkers connecting septal volume and connectivity with cholinergic neurocircuitry would also be novel.

Approach

So far, patients for this study have undergone separated MR and PET examinations. Connection with the BTRC will allow us to perform the scans in a single session.

As of 5/10/2012, 14 patients with focal epilepsy and 8 controls have undergone PKPET for this study. We originally proposed to study 15 patients, and so have almost completed planned enrollment. However, we have expanded the number of subjects allowed by the IRB protocol, and will continue enrolling patients beyond the originally proposed 15 because we have been obtaining only 1 rather than 2 scans per patient. The 2 scans aimed to assess both a postictal (within 2 days post-seizure) and baseline (>2 weeks seizure-free) state, however, coordinating the PET schedule with patients’ seizures (which are of course unpredictable) has proven challenging. We did scan 1 patient in both states, and found little difference between the 2 scans, suggesting that epilepsy-related neuroinflammation, at least in patients withrefractory seizures, may be a tonic rather than phasic phenomenon, in accord with study hypotheses. Relatively few PET scans performed over the past year are due to 2 issues at Weill-Cornell (with whom we have collaborated prior to the arrival of the NYUSOM MR-PET scanner): the PET-CT scanner was replaced/upgraded, necessitating a period when scanning could not be performed, and there was an IRB lapse at Cornell (due to an IRB policy change concerning investigators whose primary academic appointment is not at Cornell.) IRB issues are now resolved, and we have a number of upcoming scans scheduled. Our aim is to use PKPET to detect the region of maximal inflammation to identify the seizure focus. This has been successful in several patients (see publications). However, in other patients, this tracer’s low specific to nonspecific binding has made interpretation challenging. We therefore continue to optimize analysis methods. We have devised a novel analysis method that uses a non-brain reference region: the clivus bone, which is included in brain PET field of view. As illustrated in Figure 1, images generated using a clivus reference were neurobiologically plausible, corresponding to patients’ clinical scenarios. Non-brain reference modeling of PKPET may be useful in diseases like epilepsy potentially affecting most or all of the brain.

The MRI portion of this project has been progressing well: using fMRI seed-based functional connectivity analyses we have identified abnormal, epilepsy-related networks that differ depending on patient’s seizure foci.

Key Personnel: 
Tracy Butler, MD

Sponsors

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Philanthropic Support

We gratefully acknowledge generous support for radiology research at NYU Langone Medical Center from:
 
• The Big George Foundation
• Raymond and Beverly Sackler
• Bernard and Irene Schwartz

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