"Resting" CBF in the epileptic baboon: correlation with ketamine dose and interictal epileptic discharges

Neuroimaging in the Epileptic Baboon

Characterization of baboon model of genetic generalized epilepsy (GGE) is driven both electroclinically and by successful adoption of neuroimaging platforms, such as magnetic resonance imaging (MRI) and positron emission tomography (PET). Based upon its phylogenetic proximity and similar brain anatomy to humans, the epileptic baboon provides an excellent translational model. Its relatively large brain size compared to smaller nonhuman primates or rodents, a gyrencephalic structure compared to lissencephalic organization of rodent brains, and the availability of a large pedigreed colony allows exploration of neuroimaging markers of diseases. Similar to human idiopathic generalized epilepsy (IGE), structural imaging in the baboon is usually normal in individual subjects, but gray matter volume/concentration (GMV/GMC) changes are reported by statistical parametric mapping (SPM) analyses. Functional neuroimaging has been effective for mapping the photoepileptic responses, the epileptic network, altered functional connectivity of physiological networks, and the effects of anti-seizure therapies. This review will provide insights into our current understanding the baboon model of GGE through functional and structural imaging.

Cerebral blood flow differences between high- vs low-frequency VNS therapy in the epileptic baboon

PURPOSE

Cerebral blood flow (CBF) tracks physiological effects of ictal or interictal epileptic discharges (IEDs) and neurostimulation. This study compared CBF changes between high-frequency (HF; 300 Hz) microburst, and standard, low-frequency (LF; 30 Hz) vagal nerve stimulation (VNS) Therapy in 2 baboons with genetic generalized epilepsy (GGE), including one with photosensitivity.

METHODS

The baboons were selected based on video recordings and scalp EEG studies. They were both implanted with Sentiva™ 1000 devices capable of stimulating at standard and microburst frequencies. Nine H₂¹⁵O (10-20 mCi) positron emission tomographic (PET) scans were performed each session (two PET sessions acquired for each animal). The baboons were sedated with ketamine, paralyzed, and monitored with scalp EEG. CBF changes were compared between the two modes of stimulation and resting scans in the first study, while in the second, VNS Therapy trials were combined with intermittent light stimulation (ILS) at 25 Hz and compared to CBF changes induced by ILS alone.

RESULTS

ILS-associated IED rates were slightly reduced by HF- and LF-VNS Therapies in B1, while spontaneous IEDs were completely suppressed by HF-VNS Therapy in B2. Regional CBF changes were consistent between the two modes of therapy in each baboon, in particular with respect to the activation of the superior colliculus and cerebellum. Neither VNS mode suppressed the photoepileptic response in B1. In B2, IED suppression was associated with bilateral deactivations of the frontal and temporal cortices, cingulate and anterior striatum, as well as bilateral cerebellar activations.

CONCLUSIONS

This pilot study reveals similar activation/deactivation patterns between LF- and HF-VNS Therapies, but the most pronounced CBF differences between the two baboons and the two modes of stimulation may have been driven by the suppression of the epileptic network by HF-VNS Therapy in B2. Some therapeutic targets appear to be subcortical, including the putamen, superior colliculus, brainstem nuclei, as well as the cerebellum, all of which modulate corticothalamic networks, which is particularly reflected by CBF changes associated with HF-VNS Therapy. These findings need to be replicated in larger samples and correlated with long-term clinical outcomes.

The baboon in epilepsy research: Revelations and challenges

The baboon offers a natural model for genetic generalized epilepsy with photosensitivity. In this review, we will summarize some of the more important clinical, neuroimaging, and elctrophysiological findings form recent work performed at the Southwest National Primate Research Center (SNPRC, Texas Biomedical Research Institute, San Antonio, Texas), which houses the world's largest captive baboon pedigree. Due to the phylogenetic proximity of the baboon to humans, many of the findings are readily translatable, but there may be some important differences, such as the mutlifocality of the ictal and interictal epileptic discharges (IEDs) on intracranial electroencephalography (EEG) and greater parieto-occipital connectivity of baboon brain networks compared to juvenile myoclonic epilepsy in humans. Furthermore, there is still limited knowledge of the natural history of the epilepsy, which could be transformative for research into epileptogenesis in genetic generalized epilepsy (GGE) and sudden unexpected death in epilepsy (SUDEP).

Differences in cerebral blood vasculature and flow in awake and anesthetized mouse cortex revealed by quantitative optical coherence tomography angiography

BACKGROUND

Most of the in vivo neurovascular imaging studies are performed in anesthetized animals. However, anesthesia significantly affects cerebral hemodynamics.

NEW METHOD

We applied optical coherence tomography (OCT) methods such as optical microangiography (OMAG) and Doppler optical microangiography (DOMAG) to quantitatively evaluate the effect of anesthesia in cerebral vasculature and blood flow in mouse brain.

RESULTS

The OMAG results indicated the increase of large vessel diameter and capillary density induced by ketamine-xylazine and isoflurane, meaning that both anesthetics caused vasodilation. In addition, the preliminary results from DOMAG showed that isoflurane increased the baseline cerebral blood flow.

COMPARISON WITH EXISTING METHODS

In comparison with other in vivo imaging modalities, OCT can provide label-free assessment of cortical tissue including tissue morphology, cerebral blood vessel network and flow information down to capillary level, with a large field of view and high imaging speed.

CONCLUSIONS

OCT angiography methods demonstrated the ability to measure the differences in the baseline morphological and flow parameters of both large and capillary cerebrovascular networks between awake and anesthetized mice.

Changes in resting-state cerebral blood flow and its connectivity in patients with focal to bilateral tonic-clonic seizures

Arterial spin labeling (ASL) is an important tool for understanding cerebral perfusion in epilepsy patients. The aim of this study was to explore patterns of change in cerebral blood flow (CBF) and CBF connectivity in patients with focal to bilateral tonic-clonic seizures (FBTCS). High-resolution three-dimensional (3-D) T1-weighted and 3-D pseudo-continuous ASL magnetic resonance imaging (MRI) was collected from 32 patients with FBTCS and 16 healthy volunteers using a 3.0 T MRI scanner. Cerebral blood flow and its connectivity were compared between the FBTCS and control group. Correlation analysis was used to explore relationships of CBF and its connectivity changes with clinical parameters. Cerebral blood flow data of spatial standardization and normalization were used to improve statistical power. Patients with FBTCS exhibited increased CBF in the bilateral thalamus, caudate nucleus, olfactory cortex, and gyrus rectus, but decreased CBF in the bilateral supplementary motor areas (SMA) and middle cingulate cortex (MCC). Patients with FBTCS showed significant positive correlation between CBF and gray matter volume (GMV) in bilateral SMA and MCC. No significant correlations between CBF and clinical parameters were found among FBTCS patients. The anterior cingulate cortex (ACC) showed positive CBF connectivity with the bilateral SMA and MCC, and these CBF connectivity measures differed significantly between groups (cluster-level, FWE-corrected, P < 0.001). These findings suggest that patients with FBTCS have changes in cerebral CBF and CBF connectivity, which may relate to the underlying neuropathology of FBTCS.

Effects of ketamine on EEG in baboons with genetic generalized epilepsy

Ketamine, a noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist, used as an anesthetic has been reported to induce seizures both in humans and baboons predisposed to epilepsy. In this study, we aimed to characterize the acute effects of ketamine on scalp (sc-EEG) and intracranial EEG (ic-EEG) in the baboon, which offers a natural model of genetic generalized epilepsy (GGE). We evaluated the electroclinical response to ketamine in three epileptic baboons. The raw EEG data were analyzed within 10 min of intramuscular ketamine (5-6 mg/kg) administration. Earliest EEG changes occurred after 30 s in sc-EEG and after 15 s in ic-EEG of ketamine administration. These initial changes involved increased paroxysmal fast activity (PFA) followed by slowing, the latter emerging first occipitally, and then spreading more anteriorly. Generalized spike-and-wave discharges (GSWDs) were evident on both sc-EEG and ic-EEG within two minutes, but focal occipital discharges were already increased on ic-EEG after 15 s. Occipital slowing emerged on ic-EEG after 30 s, before spreading fronto-centrally and orbito-frontally. By 60-120 seconds post-injection, ic-EEG demonstrated a parieto-occipital burst suppression (BS), which was not noted on sc-EEG. Ketamine waves and seizures, especially if the latter were subclinical, also appeared earlier on ic-EEG. This study highlights the anesthetic and proconvulsant effects of ketamine originate in the occipital lobes before fronto-central regions. We speculate that NMDAR concentration difference in cortical regions, such as the occipital and frontal cortices, are mainly involved in the expression of ketamine's EEG effects, both physiological and epileptic.

Resting-state functional connectivity changes due to acute and short-term valproic acid administration in the baboon model of GGE

Resting-state functional connectivity (FC) is altered in baboons with genetic generalized epilepsy (GGE) compared to healthy controls (CTL). We compared FC changes between GGE and CTL groups after intravenous injection of valproic acid (VPA) and following one-week of orally administered VPA. Seven epileptic (2 females) and six CTL (3 females) baboons underwent resting-state fMRI (rs-fMRI) at 1) baseline, 2) after intravenous acute VPA administration (20 mg/kg), and 3) following seven-day oral, subacute VPA therapy (20–80 mg/kg/day). FC was evaluated using a data-driven approach, while regressing out the group-wise effects of age, gender and VPA levels. Sixteen networks were identified by independent component analysis (ICA). Each network mask was thresholded (z > 4.00; p < 0.001), and used to compare group-wise FC differences between baseline, intravenous and oral VPA treatment states between GGE and CTL groups. At baseline, FC was increased in most cortical networks of the GGE group but decreased in the thalamic network. After intravenous acute VPA, FC increased in the basal ganglia network and decreased in the parietal network of epileptic baboons to presumed nodes associated with the epileptic network. After oral VPA therapy, FC was decreased in GGE baboons only the orbitofrontal networks connections to the primary somatosensory cortices, reflecting a reversal from baseline comparisons. VPA therapy affects FC in the baboon model of GGE after a single intravenous dose—possibly by facilitating subcortical modulation of the epileptic network and suppressing seizure generation—and after short-term oral VPA treatment, reversing the abnormal baseline increases in FC in the orbitofrontal network. While there is a need to correlate these FC changes with simultaneous EEG recording and seizure outcomes, this study demonstrates the feasibility of evaluating rs-fMRI effects of antiepileptic medications even after short-term exposure.

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This resting-state fMRI study evaluates treatment-related functional connectivity (FC) changes in the baboon model of GGE.

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Pre-treatment FC is mostly increased in cortical networks, but decreased for the thalamic network in epileptic baboons.

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Treatment-related FC changes were noted both after single intravenous dose of VPA and short-term oral VPA treatment.

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FC studies may provide a novel approach to evaluate antiepileptic medication effects.

Changes in Cerebral Blood Flow during an Alteration in Glycemic State in a Large Non-human Primate (Papio hamadryas sp.)

Changes in cerebral blood flow (CBF) during a hyperglycemic challenge were mapped, using perfusion-weighted MRI, in a group of non-human primates. Seven female baboons were fasted for 16 h prior to 1-h imaging experiment, performed under general anesthesia, that consisted of a 20-min baseline, followed by a bolus infusion of glucose (500 mg/kg). CBF maps were collected every 7 s and blood glucose and insulin levels were sampled at regular intervals. Blood glucose levels rose from 51.3 ± 10.9 to 203.9 ± 38.9 mg/dL and declined to 133.4 ± 22.0 mg/dL, at the end of the experiment. Regional CBF changes consisted of four clusters: cerebral cortex, thalamus, hypothalamus, and mesencephalon. Increases in the hypothalamic blood flow occurred concurrently with the regulatory response to systemic glucose change, whereas CBF declined for other clusters. The return to baseline of hypothalamic blood flow was observed while CBF was still increasing in other brain regions. The spatial pattern of extra-hypothalamic CBF changes was correlated with the patterns of several cerebral networks including the default mode network. These findings suggest that hypothalamic blood flow response to systemic glucose levels can potentially be explained by regulatory activity. The response of extra-hypothalamic clusters followed a different time course and its spatial pattern resembled that of the default-mode network.

Modeling the effective connectivity of the visual network in healthy and photosensitive, epileptic baboons

The baboon provides a model of photosensitive, generalized epilepsy. This study compares cerebral blood flow responses during intermittent light stimulation (ILS) between photosensitive (PS) and healthy control (CTL) baboons using H 2 (15) O-PET. We examined effective connectivity associated with visual stimulation in both groups using structural equation modeling (SEM). Eight PS and six CTL baboons, matched for age, gender and weight, were classified on the basis of scalp EEG findings performed during the neuroimaging studies. Five H 2 (15) O-PET studies were acquired alternating between resting and activation (ILS at 25 Hz) scans. PET images were acquired in 3D mode and co-registered with MRI. SEM demonstrated differences in neural connectivity between PS and CTL groups during ILS that were not previously identified using traditional activation analyses. First-level pathways consisted of similar posterior-to-anterior projections in both groups. While second-level pathways were mainly lateralized to the left hemisphere in the CTL group, they consisted of bilateral anterior-to-posterior projections in the PS baboons. Third- and fourth-level pathways were only evident in PS baboons. This is the first functional neuroimaging study used to model the photoparoxysmal response (PPR) using a primate model of photosensitive, generalized epilepsy. Evidence of increased interhemispheric connectivity and bidirectional feedback loops in the PS baboons represents electrophysiological synchronization associated with the generation of epileptic discharges. PS baboons demonstrated decreased model stability compared to controls, which may be attributed to greater variability in the driving response or PPRs, or to the influence of regions not included in the model.

Repetitive Transcranial Magnetic Stimulation Educes Frequency-Specific Causal Relationships in the Motor Network

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) has the potential to treat brain disorders by modulating the activity of disease-specific brain networks, yet the rTMS frequencies used are delivered in a binary fashion - excitatory (>1 Hz) and inhibitory (≤1 Hz).

OBJECTIVE

To assess the effective connectivity of the motor network at different rTMS stimulation rates during positron-emission tomography (PET) and confirm that not all excitatory rTMS frequencies act on the motor network in the same manner.

METHODS

We delivered image-guided, supra-threshold rTMS at 3 Hz, 5 Hz, 10 Hz, 15 Hz and rest (in separate randomized sessions) to the primary motor cortex (M1) of the lightly anesthetized baboon during PET imaging. Each rTMS/PET session was analyzed using normalized cerebral blood flow (CBF) measurements. Path analysis - using structural equation modeling (SEM) - was employed to determine the effective connectivity of the motor network at all rTMS frequencies. Once determined, the final model of the motor network was used to assess any differences in effective connectivity at each rTMS frequency.

RESULTS

The exploratory SEM produced a very well fitting final network model (χ(2) = 18.04, df = 21, RMSEA = 0.000, p = 0.647, TLI = 1.12) using seven nodes of the motor network. 5 Hz rTMS produced the strongest path coefficients in four of the seven connections, suggesting that this frequency is the optimal rTMS frequency for stimulation the motor network (as a whole); however, the premotor cerebellum connection was optimally stimulated at 10 Hz rTMS and the supplementary motor area caudate connection was optimally driven at 15 Hz rTMS.

CONCLUSION(S)

We have demonstrated that 1) 5 Hz rTMS revealed the strongest path coefficients (i.e. causal influence) on the nodes of the motor network, 2) stimulation at "excitatory" rTMS frequencies did not produce increased CBF in all nodes of the motor network, 3) specific rTMS frequencies may be used to target specific none-to-node interactions in the stimulated brain network, and 4) more research needs to be performed to determine the optimum frequency for each brain circuit and/or node.

Resting-state functional connectivity in the baboon model of genetic generalized epilepsy

OBJECTIVE

The baboon provides a natural model of genetic generalized epilepsy (GGE). This study compares the intrinsic connectivity networks of epileptic and healthy control baboons using resting-state functional magnetic resonance imaging (rs-fMRI) and data-driven functional connectivity mapping.

METHODS

Twenty baboons, matched for gender, age, and weight, were classified into two groups (10 epileptic [EPI], 10 control [CTL]) on the basis of scalp electroencephalography (EEG) findings. Each animal underwent one MRI session that acquired one 5-min resting state fMRI scan and one anatomic MRI scan-used for registration and spatial normalization. Using independent component analysis, we identified 14 unique components/networks, which were then used to characterize each group's functional connectivity maps of each brain network.

RESULTS

The epileptic group demonstrated network-specific differences in functional connectivity when compared to the control animals. The sensitivity and specificity of the two groups' functional connectivity maps differed significantly in the visual, motor, amygdala, insular, and default mode networks. Significant increases were found in the occipital gyri of the epileptic group's functional connectivity map for the default mode, cingulate, intraparietal, motor, visual, amygdala, and thalamic regions.

SIGNIFICANCE

This is the first study using resting-state fMRI to demonstrate intrinsic functional connectivity differences between epileptic and control nonhuman primates. These results are consistent with seed-based GGE studies in humans; however, our use of a data-driven approach expands the scope of functional connectivity mapping to include brain regions/networks comprising the whole brain.

Functional PET Evaluation of the Photosensitive Baboon

The baboon provides a unique, natural model of epilepsy in nonhuman primates. Additionally, photosensitivity of the epileptic baboon provides an important window into the mechanism of human idiopathic generalized epilepsies. In order to better understand the networks underlying this model, our group utilized functional positron emission tomography (PET) to compare cerebral blood flow (CBF) changes occurring during intermittent light stimulation (ILS) and rest between baboons photosensitive, epileptic (PS) and asymptomatic, control (CTL) animals. Our studies utilized subtraction and covariance analyses to evaluate CBF changes occurring during ILS across activation and resting states, but also evaluated CBF correlations with ketamine doses and interictal epileptic discharge (IED) rate during the resting state. Furthermore, our group also assessed the CBF responses related to variation of ILS in PS and CTL animals. CBF changes in the subtraction and covariance analyses reveal the physiological response and visual connectivity in CTL animals and pathophysiological networks underlying responses associated with the activation of ictal and interictal epileptic discharges in PS animals. The correlation with ketamine dose is essential to understanding differences in CBF responses between both groups, and correlations with IED rate provides an insight into an epileptic network independent of visual activation. Finally, the ILS frequency dependent changes can help develop a framework to study not only spatial connectivity but also the temporal sequence of regional activations and deactivations related to ILS. The maps generated by the CBF analyses will be used to target specific nodes in the epileptic network for electrophysiological evaluation using intracranial electrodes.

Functional neuroimaging of the baboon during concurrent image-guided transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) has well-established applications in basic neuroscience and promising applications in neurological and psychiatric disorders. However the underlying mechanisms of TMS-induced alterations in brain function are not well understood. As a result, treatment design parameters are determined ad hoc and not informed by any coherent theory or model. Once the mechanisms underlying TMS's modulatory effects on brain systems are better understood and modeled, TMS's potential as a therapeutic and/or investigative tool will be more readily explored and exploited. An animal model is better suited to study different TMS variables, therefore we developed a baboon model to facilitate testing of some of the current theoretical models of TMS interactions with brain regions. We have demonstrated the feasibility of this approach by successfully imaging cerebral blood flow (CBF) changes with H(2)(15)O positron emission tomography imaging during high-frequency, suprathreshold repetitive TMS in the primary motor cortex of five healthy, adult baboons.