Transplantation of inhibitory precursor cells from medial ganglionic eminence produces distinct responses in two different models of acute seizure induction

GABAergic interneurons in epilepsy: More than a simple change in inhibition

The pathophysiology of epilepsy has been historically grounded on hyperexcitability attributed to the oversimplified imbalance between excitation (E) and inhibition (I) in the brain. The decreased inhibition is mostly attributed to deficits in gamma-aminobutyric acid-containing (GABAergic) interneurons, the main source of inhibition in the central nervous system. However, the cell diversity, the wide range of spatiotemporal connectivity, and the distinct effects of the neurotransmitter GABA especially during development, must be considered to critically revisit the concept of hyperexcitability caused by decreased inhibition as a key characteristic in the development of epilepsy. Here, we will discuss that behind this known mechanism, there is a heterogeneity of GABAergic interneurons with distinct functions and sources, which have specific roles in controlling the neural network activity within the recruited microcircuit and altered network during the epileptogenic process. This article is part of the Special Issue "NEWroscience 2018.

Anticonvulsant effects after grafting of rat, porcine, and human mesencephalic neural progenitor cells into the rat subthalamic nucleus

Cell transplantation based therapy is a promising strategy for treating intractable epilepsies. Inhibition of the subthalamic nucleus (STN) or substantia nigra pars reticulata (SNr) is a powerful experimental approach for remote control of different partial seizure types, when targeting the seizure focus is not amenable. Here, we tested the hypothesis that grafting of embryonic/fetal neural precursor cells (NPCs) from various species (rat, human, pig) into STN or SNr of adult rats induces anticonvulsant effects. To rationally refine this approach, we included NPCs derived from the medial ganglionic eminence (MGE) and ventral mesencephalon (VM), both of which are able to develop a GABAergic phenotype. All VM- and MGE-derived cells showed intense migration behavior after grafting into adult rats, developed characteristics of inhibitory interneurons, and survived at least up to 4 months after transplantation. By using the intravenous pentylenetetrazole (PTZ) seizure threshold test in adult rats, transient anticonvulsant effects were observed after bilateral grafting of NPCs derived from human and porcine VM into STN, but not after SNr injection (site-specificity). In contrast, MGE-derived NPCs did not cause anticonvulsant effects after grafting into STN or SNr (cell-specificity). Neither induction of status epilepticus by lithium-pilocarpine to induce neuronal damage prior to the PTZ test nor pretreatment of MGE cells with retinoic acid and potassium chloride to increase differentiation into GABAergic neurons could enhance anticonvulsant effectiveness of MGE cells. This is the first proof-of-principle study showing anticonvulsant effects by bilateral xenotransplantation of NPCs into the STN. Our study highlights the value of VM-derived NPCs for interneuron-based cell grafting targeting the STN.

MGE-derived nNOS+ interneurons promote fear acquisition in nNOS-/- mice

Neuronal nitric oxide synthase (nNOS) ¹, mainly responsible for NO release in central nervous system (CNS) ², plays a significant role in multiple physiological functions. However, the function of nNOS⁺ interneurons in fear learning has not been much explored. Here we focused on the medial ganglionic eminences (MGE) ³-derived nNOS⁺ interneurons in fear learning. To determine the origin of nNOS⁺ interneurons, we cultured neurons in vitro from MGE, cortex, lateral ganglionic eminence (LGE) ⁴, caudal ganglionic eminences (CGE) ⁵ and preoptic area (POA) ⁶. The results showed that MGE contained the most abundant precursors of nNOS⁺ interneurons. Moreover, donor cells from E12.5 embryos demonstrated the highest positive rate of nNOS⁺ interneurons compared with other embryonic periods (E11.5, E12, E13, E13.5 and E14). Additionally, these cells from E12.5 embryos showed long axonal and abundant dendritic arbors after 10 days culture, indicating the capability to disperse and integrate in host neural circuits after transplantation. To investigate the role of MGE-derived nNOS⁺ interneurons in fear learning, donor MGE cells were transplanted into dentate gyrus (DG) ⁷ of nNOS knock-out (nNOS^-/-) or wild-type mice. Results showed that the transplantation of MGE cells promoted the acquisition of nNOS^-/- but not the wild-type mice, suggesting the importance of nNOS⁺ neurons in fear acquisition. Moreover, we transplanted MGE cells from nNOS^-/- mice or wild-type mice into DG of the nNOS^-/- mice and found that only MGE cells from wild-type mice but not the nNOS^-/- mice rescued the deficit in acquisition of the nNOS^-/- mice, further confirming the positive role of nNOS⁺ neurons in fear learning.