We systematically characterized shape alterations of 14 subcortical structures in GGE using a surface-based morphological analysis pipeline. We found that GGE was associated with extensive morphological changes in the shape of all subcortical structures. Using a collective toolbox that aggregated neurotransmitter profiles derived from PET scans, we found that shape area deviations were associated with multiple neurotransmitter expression profiles across different systems, including acetylcholine and norepinephrine. These results increased our understanding of the relationship between macroscale morphological information and microscale molecular pathways in GGE.
Subcortical structures play a critical role in behavioral symptoms, propagation, and even initiation of epileptic seizures33. However, the pattern and extent of subcortical structural disruptions due to epilepsy development is still unclear, given the absence of systematic examination of fine-grained morphological features. Our study utilized a surface-based pipeline to provide detailed morphometric information on the subcortical nuclei in GGE patients. The morphological profiles examined in this study included distances between each vertex to the medial curve of the specific structure, namely the shape thickness; and the surface dilation ratio of mapping the corresponding vertex on individual surfaces to the standard template, namely the shape area. These two metrics described the fundamental structural property of the subcortical nuclei. Compared to volumetric analysis, the surface-based pipeline provided a more precise representation of the shape of brain structures, and was better able to distinguish morphological abnormalities. While inconsistent results have been reported in previous volumetric analyses, and a systematic examination of subcortical structures remains lacking, our study indicated that GGE patients had widespread morphological alterations in both shape thicknesses and areas. The deviation patterns of shape areas and thicknesses were coarsely symmetric across hemispheres, with some apparent asymmetries in the thalamus and ventral hippocampus. Bilateral ventral, medial, and right dorsal thalamus, and bilateral lateral putamen exhibited notable shape surface dilation, whereas other structures had spatially scattered patterns with mixed dilation and contraction distributions. Compared to previous studies that used volumetric pipelines, we obtained a more fine-grained analysis of the morphological alterations of subcortical nuclei in GGE. These results provided systematic knowledge about the disruption of these structures and potential targets for new hypotheses of pathological circuits, as well as detailed locations for precision intervention and associations to other modalities.
Neurotransmitters play an increasing role in the theory of pathogenesis of epilepsy, including GGE. The development of new generation anti-epileptic drugs that target the restoration of the balance between neurotransmitters is a core element of this theory, as opposed to mainstream anti-seizure medications, which only provide symptomatic relief29. Our results provided a comprehensive characterization of the relationship between subcortical structural vulnerabilities and neurotransmitter profiles. We examined the associations of the disruption levels of both shape areas and thicknesses using PET-derived neurotransmitter expression maps across nine different systems, and found two kinds of neurotransmitter profiles, which provided a close relationship with the shape area deviation pattern of GGE: Ach and NE. Ach is a neurotransmitter that consists of acetic acid and choline ester34, and is involved in multiple cognitive functions, including memory and learning35. Animal and preclinical studies have suggested dysfunction of the cholinergic system, mediated by release of ACh, which may lead to epilepsy36. Several studies have reported that ACh and related receptors and precursors were involved in the pathogenesis of epilepsy37–39. The profiles of two ACh subtype receptors, muscarinic and nicotinic Ach, exhibited distinct relationships to shape area deviations. The muscarinic ACh (M1) expression profile of subcortical structures was negatively correlated with shape area alteration levels, which suggested that the effect of GGE exerted in subcortex may involve muscarinic Ach, because low concentration areas were more vulnerable to structural damage. Muscarinic ACh (M1) has been shown to be involved in the initiation of generalized epileptic seizures in the subcortex of rat models40, and is also associated with hippocampal dysfunction41. In contrast, the nicotinic ACh (α4β2) profile exhibited positive correlations with shape area deviations, suggesting a possible molecular mechanism involving nicotinic Ach, leading to subcortex structural disruptions in GGE. Previous studies have reported that nicotinic ACh receptors, which are extensively expressed in hippocampal neurons, may contribute to epilepsy, with decreased expression being observed in patients42. In the present study, we found a significant correlation between NET expressions and shape area deviations in two PET maps from different sources. NE is produced from dopamine and released from noradrenergic neurons as a neurotransmitter43. NE is deemed as a seizure suppressor44 and may also play a part in seizure modulation by slowing the stimulation of limbic areas29.
The present study showed widespread structural alterations of subcortical nuclei in GGE. The surface-based subcortical morphological pipeline was more effective at detecting fine-grained defects than its volumetric counterparts. The results indicated a general abnormality of shape morphology in all subcortical structures included in the analysis, with the most significant disruptions found in the thalamus, putamen, and hippocampus. Volumetric PET data, which analyzed neurotransmitter profiles, was transformed to shape surfaces and associated with morphological deviation patterns. Two types of neurotransmitters, ACh and NE, were shown to be closely related to morphological disruptions. Taken together, these results provided a comprehensive characterization of subcortical morphological disruptions, and suggested corresponding molecular pathways that may aid the development of new types of anti-epileptic drugs and precision interventions, as well as expand our understanding of the pathogenesis of GGE.