Almost 25 years ago, it was already postulated that controlling GABA delivery to specific brain areas should benefit each of these diseases [9, 10]. Cell transplantation is a powerful tool to introduce a new source of GABA and may allow reconstitution of neural circuits in the diseased brain [11, 12]. To be successful, grafted cells should possess the ability to disperse through affected areas and differentiate into fully mature neurons expressing appropriate neurotransmitters, in this case GABA. Ideally, these cells should also functionally integrate and modulate circuitry activity in the damaged host brain; for instance, affecting its plasticity. Since the pioneer works from Lindvall and Björklund [9] and Isacson et al. [10], several transplantation assays with different GABA-producing cell types have been performed with disparate success in animal models of diseases. Many cell types were partially successful in reverting some of the pathological anomalies observed in the grafted models. However, some of them presented important drawbacks, such as their poor tissue distribution, transient effect, maybe due to decreased GABA release over time [13–15], or in the case of ES cells, the lack of safety due to potential generation of teratocarcinomas [16, 17].

In the last decade, a better comprehension of how and where the cortical and hippocampal interneurons originate has led to use their neuronal precursors in transplantation [18, 19]. We currently know that most of the GABAergic interneurons in the cortex and hippocampus are mainly generated in two regions of the subcortical telencephalon, known as the caudal and medial ganglionic eminence (CGE and MGE), from where they migrate tangentially to their final destination in the cerebral cortex [19–22]. In the last years, several groups have reported regenerative works using these MGE-derived GABAergic precursors, with striking results [23–28]. At present, they represent the most promising cell-based therapeutic alternative for GABA-related diseases.

According to these strategies, several GABAergic cell types, with different origins and characteristics, have been assayed in animal models of epilepsy to evaluate their therapeutic potential. In the following sections, we describe in detail the most representative cell types and assays.

In addition, many attempts of gene and cell-based therapies are in progress to establish treatments that can be complementary and additive to the standard pharmacological approach. In particular, a gene therapy approach has been developed to deliver the glutamic acid decarboxylase (GAD) gene, catalysing the synthesis of GABA, directly into neurons of the subthalamic nucleus [76]. In PD, activity of the subthalamic nucleus (STN) is increased mainly because of reduced GABAergic input from the globus pallidus. Interestingly, the focal delivery of GAD in the STN contributed to reducing its overactivity leading to an amelioration of the clinical neurological symptoms. Safety and tolerability of this gene therapy protocol has been demonstrated in a phase I trial although with a reduced number of patients and over a limited period of time (1 year) [77]. Thus, promoting GABAergic neuronal activity in specific nuclei can contribute in restoring a balance in the basal ganglia neuronal outflow controlling the extrapyramidal motor system.

In addition to these approaches, the clinical impact of cell replacement has been evaluated in animals and humans over the last two decades. These approaches are aimed to reconstitute a local dopaminergic network capable of a feedback controlled dopamine release upon delivering of dopaminergic neurons in the affected striatal compartment. A similar procedure has been pioneered in humans using cellular grafts of fetal nigral tissues [78–80]. This approach led to some important symptomatic improvements, however, often associated with the development of extremely severe dyskinesia [81]. These side effects are probably due to the high heterogeneity of the grafted tissue containing only a minimal fraction of dopaminergic precursors (5%) in an overall population of different cell types including serotonergic and GABAergic neurons [82, 83]. Recently, an alternative strategy of cell transplantation has been validated in a PD animal model. This is based on transplanting GABAergic precursors isolated from the embryonic MGE/CGE into the adult striatum [84]. At first glance, this approach introducing inhibitory neurons in a tissue already deprived of dopamine might seem counterintuitive. However, thinking of PD as an activity outflow unbalance among different striatal neuronal networks, this methodology finds a strong rationale [85]. Noteworthy, E14.5 MGE/CGE cells injected into a single site were able to migrate throughout the striatal tissue, but not beyond it. The extent of migration is similar to that described for these cells upon transplantation into the adult cortex (see above). Therefore, MGE/CGE GABAergic neuronal precursors have a tendency to actively disperse within different adult brain tissues, and this represents a strong attractive feature for an efficient cell-based therapy. However, this should not be considered of general rule, since the same cells grafted in the subthalamic nucleus are unable to migrate from the injection site [84]. Possibly, this is the case since cortex and striatum, contrary to thalamic areas, are the forebrain regions which are normally colonized by these cells during embryogenesis and might retain some molecular or structural cues that allow this behaviour even in adulthood. Are these cells able subsequently to survive, mature, and integrate in the host striatal tissue? Martínez-Cerdeño and colleagues found that despite the great majority of the MGE/CGE-transplanted cells which were lost after 1 year from transplantation, 1% of them survived and presented morphological and functional features of mature interneurons of the three major subtypes, namely, calretinin+, parvalbumin+, and somatostatin+ cells [84]. Therefore, only a minimal fraction of MGE-transplanted cells are able to survive for long time in the striatal tissues and this probably reflects the need for establishing stable and functional connections with the host neuronal network for promoting their survival. Remarkably, even though the transplanted interneurons accounted for only about 5% of the total endogenous GABAergic neuronal population, they were sufficient to elicit a significant motor and behavioural recovery in the 6-hydroxydopamine-lesioned rats. How might this occur? The authors revealed the integration of the grafted interneurons by showing the formation of de novo synapses with the host neurons, and hence, they suggest that it is the graft-mediated reorganization of the basal ganglia network that fosters the functional recovery observed [84]. In fact, the striatum is the key centre of the extrapyramidal tract which controls thalamic efferents to the motor cortical regions. This circuitry is organized in two main neuronal assemblies known as the direct and the indirect pathways. The first connects striatum-internal globus pallidus and thalamus and activates thalamic activity. The second restrains thalamic activity and is connecting striatum-internal and -external globus pallidus and thalamus. Hence, these two pathways converge to the thalamus as their final target centre and regulate its activity by playing reciprocal opposing functions. In PD, dopamine depletion in the striatum produces two concomitant effects. First, it reduces the activity of the direct pathway while promoting the indirect pathway creating an upraised inhibitory outflow to the thalamus.

One plausible mechanism by which MGE-transplanted cells promote a symptomatic relief in PD animals is to restore a balance in the total output of these two pathways over the thalamus by preferentially inhibiting the indirect pathway [85]. Although this explanation needs more experimental evidences, nonetheless these studies reveal how a small transplanted population of interneurons has the capability to modulate the plasticity of long-ranging and complex neuronal circuitry and restore a functional unbalance between related neuronal systems.

Recently, similar cell transplantations of embryonic MGE GABAergic precursor cells have been carried out in a mouse model of stroke [86]. Focal ischemia in cortical and nearby striatal areas was produced by middle cerebral artery occlusion, and embryonic MGE cells were transplanted in multiple sites in adjoining regions. Noteworthy, MGE-transplanted animals improved in their locomotion and motor coordination with a significant improvement in both tests respect to sham-injected controls [86]. Similar to previous studies, embryonic MGE cells developed in fully mature neurons featuring spontaneous action potentials and connecting to host neurons. However, the amount of MGE grafted cells that differentiated into mature neurons after 4 weeks from transplantation were only a limited fraction accounting for 20% of the total. Surprisingly, the rest of the cells resulted negatively for astrocyte or oligodendrocyte markers indicating that the transplanted cells remain blocked to a progenitor state unable to complete the differentiation in any cell lineage [86]. This is in striking contrast with the differentiation behavior of grafted MGE cells in the other disease murine models previously described [82].

Nonetheless, the authors noted that the MGE cell grafting stimulates axonal reorganization of the host tissue [86]. In fact, the axonal sprouting and neurite reorganization in the injured site was strongly increased after cell transplantation. These results suggest that grafted MGE neurons might stimulate endogenous repairing mechanisms or formation of alternative neuronal assemblies to support the functions of the lost tissue. To which extent exogenous MGE cells can trigger neuronal rewiring and plasticity of the host tissue remains to be better exemplified. To be noted, also MGE cell graftings in PD animal models induced some changes in the host tissue as for instance the re-expression of the calcium-binding proteins calretinin and calbindin by host striatal cells nearby the transplantation site [84]. The changes might also be promoted by all sorts of trophic factors released by the grafted GABA neuronal precursors that can stimulate neuritogenesis or synaptic connections.

Accumulating evidence supports a pivotal role for late-developing excitation versus inhibition circuit balance in the initiation of sensitive periods. For example, the onset of visual cortical plasticity is delayed by genetic disruption of GABA synthesis [91, 92]. Conversely, the application of benzodiazepines or other treatments that accelerate GABA circuit function triggers premature plasticity [93]. Therefore, the onset of OD plasticity is triggered by the establishment of a functional network of inhibitory synaptic transmission. Southwell and colleagues asked whether transplantation of MGE GABAergic precursors could be sufficient to trigger a plasticity respond in the host cortex [94]. In mice, OD plasticity reaches a peak in the fourth postnatal week, when cortical inhibitory neurons are 33–35 days old. Thus, the authors transplanted E13.5 mouse MGE GABAergic precursors in perinatal or early-postnatal brains and ascertained the induction of visual plasticity in the host brains [94]. Grafted MGE cells differentiated into GABAergic interneurons with a great efficiency and only 0.2% of them developed morphology of astrocytes. Remarkably, the cellular graftings were able to successfully trigger visual plasticity in the host [94]. However, this induction was achieved only for a short period of time which correlated with the age of the transplanted cells. In fact, transplantation was effective when the cells were 33–35 days while soon later at 43–46 days old the same cells failed to trigger the same effect. Therefore, the cellular age of the transplanted population strictly determines the effects on cortical plasticity.

The introduction of a supplemental amount of inhibitory interneurons would suggest that an increased inhibitory tone is the trigger for such neural rejuvenation. However, this is not the case since pharmacological enhancement of inhibition does not induce similar effects [95]. The answer is rooted probably in the nature of the synaptic contacts established by grafted interneurons with the host neuronal circuitry. Indeed, transplanted inhibitory neurons form weak but numerous synaptic connections with neighbouring excitatory neurons in the host brain. Thus, these new connections are believed not to simply raise the general inhibitory tone, but rather to promote an overall reorganization of the cortical circuitry by introducing a new set of weak inhibitory synapses. This pattern of newly established synaptic contacts represent an ideal biological substrate capable of enhancing the Hebbian plasticity mechanisms during the critical period [96].

It is noteworthy that the grafted interneurons promote plasticity only when they reach a cellular age comparable to that of the endogenous counterpart during the critical period. This data strongly suggest that plasticity is successfully initiated by a cell-autonomous program endowed in interneuron progenitors which is minimally influenced by the age of the host tissue. These findings open a new scenario where cell transplantation might be effective in reprogramming neural activity up to triggering plasticity processes. Nonetheless, a number of questions need to be answered to understand the safety and efficacy of this procedure. For instance, (i) whether transplantations of interneurons are able to induce visual plasticity even in the adulthood, (ii) if the grafted animals display any neuronal misbehaviour at later stages triggered by the action of the transplanted cells, (iii) if plasticity is promoted by a specific class of interneurons, and (iv) the assessment of the minimal number of cells to be grafted for inducing brain plasticity.