Characterization of dopaminergic innervation and receptor expression in mouse models and patients with Focal Cortical Dysplasia

Abstract

Focal Cortical Dysplasia (FCD) type 2 is the most prevalent malformation of cortical development (MCD) associated with pharmacoresistant focal epilepsy, frequently located in the frontal cortex. Neuropathological features include abnormal cortical layering and enlarged dysmorphic neurons and balloon cells. Profoundly altered local neuronal activity has been observed in epilepsy-surgical biopsies from patients with FCD type 2. Notably, FCD type 2 emerges during brain development and integrates into complex connectivity architectures with surrounding neuronal networks. Local cortical microcircuits, especially in the frontal cortex, are extensively modulated by monoaminergic axonal projections from the brainstem. Prior studies of human FCD type 2 biopsies suggested altered density and distribution of monoaminergic axons; however, a systematic investigation remains lacking. <br/> In this study, we provide a comprehensive analysis of dopaminergic (DA) innervation in human FCD type 2b biopsies and in the medial prefrontal cortex (mPFC) of a mouse model with mTOR hyperactivation (FCD type 2 model) during adolescence and adulthood. We also investigate DA receptor transcript expression using multiplex fluorescent RNA <em>in situ</em> hybridization in human specimens and the mPFC of this mouse model. Our findings in the mouse model reveal transient alterations in DA innervation density during adolescence and a trend toward decreased innervation in adulthood. Similarly, in human FCD type 2b biopsies, DA innervation density is reduced in the affected cortical areas compared to control regions from the same patients, with an altered lamination pattern of DA axons in the FCD type 2b regions. Additionally, DA receptor 1 (<em>DRD1/Drd1</em>) and receptor 2 (<em>DRD2/Drd2</em>) mRNA transcripts are upregulated in dysmorphic neurons in human samples and mTOR-mutated neurons in mice, compared to normally-developed neurons from control tissue or surrounding ones. <br/> These findings suggest complex molecular and structural cortical changes impacting DA neurotransmission within FCD type 2 and putatively affecting local cortical activity, manifestation of seizures and psychiatric comorbidities. This impaired DA signaling may have significant implications for understanding the pathology of FCD type 2 and developing targeted therapeutic strategies.