K-Cl Cotransporter 2-mediated Cl- Extrusion Determines Developmental Stage-dependent Impact of Propofol Anesthesia on Dendritic Spines

Delaying the GABA Shift Indirectly Affects Membrane Properties in the Developing Hippocampus

During the first two postnatal weeks, intraneuronal chloride concentrations in rodents gradually decrease, causing a shift from depolarizing to hyperpolarizing GABA responses. The postnatal GABA shift is delayed in rodent models for neurodevelopmental disorders and in human patients, but the impact of a delayed GABA shift on the developing brain remains obscure. Here we examine the direct and indirect consequences of a delayed postnatal GABA shift on network development in organotypic hippocampal cultures made from 6- to 7-d-old mice by treating the cultures for 1 week with VU0463271, a specific inhibitor of the chloride exporter KCC2. We verified that VU treatment delayed the GABA shift and kept GABA signaling depolarizing until DIV9. We found that the structural and functional development of excitatory and inhibitory synapses at DIV9 was not affected after VU treatment. In line with previous studies, we observed that GABA signaling was already inhibitory in control and VU-treated postnatal slices. Surprisingly, 14 d after the VU treatment had ended (DIV21), we observed an increased frequency of spontaneous inhibitory postsynaptic currents in CA1 pyramidal cells, while excitatory currents were not changed. Synapse numbers and release probability were unaffected. We found that dendrite-targeting interneurons in the stratum radiatum had an elevated resting membrane potential, while pyramidal cells were less excitable compared with control slices. Our results show that depolarizing GABA signaling does not promote synapse formation after P7, and suggest that postnatal intracellular chloride levels indirectly affect membrane properties in a cell-specific manner.SIGNIFICANCE STATEMENT During brain development, the action of neurotransmitter GABA shifts from depolarizing to hyperpolarizing. This shift is a thought to play a critical role in synapse formation. A delayed shift is common in rodent models for neurodevelopmental disorders and in human patients, but its consequences for synaptic development remain obscure. Here, we delayed the GABA shift by 1 week in organotypic hippocampal cultures and carefully examined the consequences for circuit development. We find that delaying the shift has no direct effects on synaptic development, but instead leads to indirect, cell type-specific changes in membrane properties. Our data call for careful assessment of alterations in cellular excitability in neurodevelopmental disorders.

General anesthesia in children and long-term neurodevelopmental deficits: A systematic review

Background

Millions of children experienced surgery procedures requiring general anesthesia (GA). Any potential neurodevelopmental risks of pediatric anesthesia can be a serious public health issue. Various animal studies have provided evidence that commonly used GA induced a variety of morphofunctional alterations in the developing brain of juvenile animals.

Methods

We conducted a systematic review to provide a brief overview of preclinical studies and summarize the existing clinical studies. Comprehensive literature searches of PubMed, EMBASE, CINAHL, OVID Medline, Web of Science, and the Cochrane Library were conducted using the relevant search terms “general anesthesia,” “neurocognitive outcome,” and “children.” We included studies investigating children who were exposed to single or multiple GA before 18, with long-term neurodevelopment outcomes evaluated after the exposure(s).

Results

Seventy-two clinical studies originating from 18 different countries published from 2000 to 2022 are included in this review, most of which are retrospective studies (n = 58). Two-thirds of studies (n = 48) provide evidence of negative neurocognitive effects after GA exposure in children. Neurodevelopmental outcomes are categorized into six domains: academics/achievement, cognition, development/behavior, diagnosis, brain studies, and others. Most studies focusing on children <7 years detected adverse neurocognitive effects following GA exposure, but not all studies consistently supported the prevailing view that younger children were at greater risk than senior ones. More times and longer duration of exposures to GA, and major surgeries may indicate a higher risk of negative outcomes.

Conclusion

Based on current studies, it is necessary to endeavor to limit the duration and numbers of anesthesia and the dose of anesthetic agents. For future studies, we require cohort studies with rich sources of data and appropriate outcome measures, and carefully designed and adequately powered clinical trials testing plausible interventions in relevant patient populations.

KCC2 receptor upregulation potentiates antinociceptive effect of GABAAR agonist on remifentanil-induced hyperalgesia

GABAergic system disinhibition played an important role in the pathogenesis of remifentanil-induced hyperalgesia (RIH). K⁺-Cl^--cotransporter-2 (KCC2) has the potential to enhance the strength of GABAergic signaling function. However, few reports have focused on the additive analgesic effect of KCC2 enhancer and GABAA receptor agonist on the spinal dorsal horn. Therefore, we evaluated the role of GABA type A receptor (GABAAR) agonist (muscimol), KCC2 enhancer (CLP257) in remifentanil-induced hyperalgesia, as well as GABA and KCC2 receptors responses in the dorsal spinal horn. Remifentanil started to reduce paw withdrawal mechanical thresholds at postoperative 4 h and lasted to 72 h. The RIH associated decreases in spinal GABA release was transient. The amount of spinal GABA transmitter by microdialysis was observed to be decreased at the beginning and reached bottom at 150 min, then returned to the baseline level at 330 min. The synthesis and transportation of GABA transmitter were inhibited, characterized as spinal GAD67 and GAT1 downregulation after the establishment of RIH model. The effect of RIH on GABA receptor downregulation was linked to the reduced expression of spinal KCC2 receptor. This decrease in KCC2 expression has coincided with an early loss of GABA inhibition. KCC2 enhancer, which is reported to lead to a reduction in intracellular Cl⁻, can enhance GABA-mediated inhibitory function. Both muscimol and CLP257 could dose-dependently inhibit mechanical hypersensitivity caused by remifentanil-induced downregulation of GABAAα2R and KCC2, respectively. Compared with muscimol acting alone, the joint action of CLP257 and muscimol showed a higher pain threshold and less c-fos expression via upregulation of KCC2 and GABAAα2R. Taken together, these findings suggested that the RIH was initiated by decreased GABA release. Downregulation of GABAAα2R and KCC2 receptor contributed to spinally mediated hyperalgesia in RIH. KCC2 enhancer was proved to potentiate antinociceptive effect of GABAAR agonist in RIH.

HDAC6 is critical for ketamine-induced impairment of dendritic and spine growth in GABAergic projection neurons

Ketamine is widely used in infants and children for anesthesia; both anesthetic and sub-anesthetic doses of ketamine have been reported to preferentially inhibit the GABAergic neurons. Medium spiny neurons (MSNs), the GABAergic projection neurons in the striatum, are vulnerable to anesthetic exposure in the newborn brain. Growth of dendrites requires a deacetylase to remove acetyl from tubulin in the growth cone to destabilize the tubulin. Histone deacetylase 6 (HDAC6) affects microtubule dynamics, which are involved in neurite elongation. In this study we used a human induced pluripotent stem cells (iPSCs)-derived striatal GABA neuron system to investigate the effects of ketamine on HDAC6 and the morphological development of MSNs. We showed that exposure to ketamine (1-500 μM) decreased dendritic growth, dendrite branches, and dendritic spine density in MSNs in a time- and concentration-dependent manner. We revealed that ketamine treatment concentration-dependently inhibited the expression of HDAC6 or aberrantly translocated HDAC6 into the nucleus. Ketamine inhibition on HDAC6 resulted in α-tubulin hyperacetylation, consequently increasing the stability of microtubules and delaying the dendritic growth of MSNs. Finally, we showed that the effects of a single-dose exposure on MSNs were reversible and lasted for at least 10 days. This study reveals a novel role of HDAC6 as a regulator for ketamine-induced deficits in the morphological development of MSNs and provides an innovative method for prevention and treatment with respect to ketamine clinical applications.

Dendritic spine remodeling and plasticity under general anesthesia

Ever since its first use in surgery, general anesthesia has been regarded as a medical miracle enabling countless life-saving diagnostic and therapeutic interventions without pain sensation and traumatic memories. Despite several decades of research, there is a lack of understanding of how general anesthetics induce a reversible coma-like state. Emerging evidence suggests that even brief exposure to general anesthesia may have a lasting impact on mature and especially developing brains. Commonly used anesthetics have been shown to destabilize dendritic spines and induce an enhanced plasticity state, with effects on cognition, motor functions, mood, and social behavior. Herein, we review the effects of the most widely used general anesthetics on dendritic spine dynamics and discuss functional and molecular correlates with action mechanisms. We consider the impact of neurodevelopment, anatomical location of neurons, and their neurochemical profile on neuroplasticity induction, and review the putative signaling pathways. It emerges that in addition to possible adverse effects, the stimulation of synaptic remodeling with the formation of new connections by general anesthetics may present tremendous opportunities for translational research and neurorehabilitation.

The postnatal GABA shift: A developmental perspective

GABA is the major inhibitory neurotransmitter that counterbalances excitation in the mature brain. The inhibitory action of GABA relies on the inflow of chloride ions (Cl^-), which hyperpolarizes the neuron. In early development, GABA signaling induces outward Cl^- currents and is depolarizing. The postnatal shift from depolarizing to hyperpolarizing GABA is a pivotal event in brain development and its timing affects brain function throughout life. Altered timing of the postnatal GABA shift is associated with several neurodevelopmental disorders. Here, we argue that the postnatal shift from depolarizing to hyperpolarizing GABA represents the final shift in a sequence of GABA shifts, regulating proliferation, migration, differentiation, and finally plasticity of developing neurons. Each developmental GABA shift ensures that the instructive role of GABA matches the circumstances of the developing network. Sensory input may be a crucial factor in determining proper timing of the postnatal GABA shift. A developmental perspective is necessary to interpret the full consequences of a mismatch between connectivity, activity and GABA signaling during brain development.

Is Anesthesia Bad for the Brain? Current Knowledge on the Impact of Anesthetics on the Developing Brain

There are compelling preclinical data that common general anesthetics cause increased neuroapoptosis in juvenile animals. Retrospective studies demonstrate that young children exposed to anesthesia have school difficulties, which could be caused by anesthetic neurotoxicity, perioperative hemodynamic and homeostatic instability, underlying morbidity, or the neuroinflammatory effects of surgical trauma. Unnecessary procedures should be avoided. Baseline measures of blood pressure are important in determining perioperative blood pressure goals. Inadvertent hypocapnia or moderate hypercapnia and hyperoxia or hypoxia should be avoided. Pediatric patients should be maintained in a normothermic, euglycemic state with neutral positioning. Improving outcomes of infants and children requires the collaboration of anesthesiologists, surgeons, pediatricians and neonatologists.

Nest Carbon Dioxide Masks GABA-Dependent Seizure Susceptibility in the Naked Mole-Rat

African naked mole-rats were likely the first mammals to evolve eusociality, and thus required adaptations to conserve energy and tolerate the low oxygen (O₂) and high carbon dioxide (CO₂) of a densely populated fossorial nest. As hypercapnia is known to suppress neuronal activity, we studied whether naked mole-rats might demonstrate energy savings in GABAergic inhibition. Using whole-colony behavioral monitoring of captive naked mole-rats, we found a durable nest, characterized by high CO₂ levels, where all colony members spent the majority of their time. Analysis of the naked mole-rat genome revealed, uniquely among mammals, a histidine point variation in the neuronal potassium-chloride cotransporter 2 (KCC2). A histidine missense substitution mutation at this locus in the human ortholog of KCC2, found previously in patients with febrile seizures and epilepsy, has been demonstrated to diminish neuronal Cl^- extrusion capacity, and thus impairs GABAergic inhibition. Seizures were observed, without pharmacological intervention, in adult naked mole-rats exposed to a simulated hyperthermic surface environment, causing systemic hypocapnic alkalosis. Consistent with the diminished function of KCC2, adult naked mole-rats demonstrate a reduced efficacy of inhibition that manifests as triggering of seizures at room temperature by the GABA_A receptor (GABA_AR) positive allosteric modulator diazepam. These seizures are blocked in the presence of nest-like levels of CO₂ and likely to be mediated through GABA_AR activity, based on in vitro recordings. Thus, altered GABAergic inhibition adds to a growing list of adaptations in the naked mole-rat and provides a plausible proximate mechanism for nesting behavior, where a return to the colony nest restores GABA-mediated inhibition.

Depolarizing GABA Transmission Restrains Activity-Dependent Glutamatergic Synapse Formation in the Developing Hippocampal Circuit

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mature brain but has the paradoxical property of depolarizing neurons during early development. Depolarization provided by GABA_A transmission during this early phase regulates neural stem cell proliferation, neural migration, neurite outgrowth, synapse formation, and circuit refinement, making GABA a key factor in neural circuit development. Importantly, depending on the context, depolarizing GABA_A transmission can either drive neural activity or inhibit it through shunting inhibition. The varying roles of depolarizing GABA_A transmission during development, and its ability to both drive and inhibit neural activity, makes it a difficult developmental cue to study. This is particularly true in the later stages of development when the majority of synapses form and GABA_A transmission switches from depolarizing to hyperpolarizing. Here, we addressed the importance of depolarizing but inhibitory (or shunting) GABA_A transmission in glutamatergic synapse formation in hippocampal CA1 pyramidal neurons. We first showed that the developmental depolarizing-to-hyperpolarizing switch in GABA_A transmission is recapitulated in organotypic hippocampal slice cultures. Based on the expression profile of K⁺−Cl⁻ co-transporter 2 (KCC2) and changes in the GABA reversal potential, we pinpointed the timing of the switch from depolarizing to hyperpolarizing GABA_A transmission in CA1 neurons. We found that blocking depolarizing but shunting GABA_A transmission increased excitatory synapse number and strength, indicating that depolarizing GABA_A transmission can restrain glutamatergic synapse formation. The increase in glutamatergic synapses was activity-dependent but independent of BDNF signaling. Importantly, the elevated number of synapses was stable for more than a week after GABA_A inhibitors were washed out. Together these findings point to the ability of immature GABAergic transmission to restrain glutamatergic synapse formation and suggest an unexpected role for depolarizing GABA_A transmission in shaping excitatory connectivity during neural circuit development.

Loss of non-canonical KCC2 functions promotes developmental apoptosis of cortical projection neurons

KCC2, encoded in humans by the SLC12A5 gene, is a multifunctional neuron-specific protein initially identified as the chloride (Cl^- ) extruder critical for hyperpolarizing GABA_A receptor currents. Independently of its canonical function as a K-Cl cotransporter, KCC2 regulates the actin cytoskeleton via molecular interactions mediated through its large intracellular C-terminal domain (CTD). Contrary to the common assumption that embryonic neocortical projection neurons express KCC2 at non-significant levels, here we show that loss of KCC2 enhances apoptosis of late-born upper-layer cortical projection neurons in the embryonic brain. In utero electroporation of plasmids encoding truncated, transport-dead KCC2 constructs retaining the CTD was as efficient as of that encoding full-length KCC2 in preventing elimination of migrating projection neurons upon conditional deletion of KCC2. This was in contrast to the effect of a full-length KCC2 construct bearing a CTD missense mutation (KCC2^R952H ), which disrupts cytoskeletal interactions and has been found in patients with neurological and psychiatric disorders, notably seizures and epilepsy. Together, our findings indicate ion transport-independent, CTD-mediated regulation of developmental apoptosis by KCC2 in migrating cortical projection neurons.

Sex differences in neurodevelopmental abnormalities caused by early-life anaesthesia exposure: a narrative review

Exposure to anaesthetic drugs during the fetal or neonatal period induces widespread neuronal apoptosis in the brains of rodents and non-human primates. Hundreds of published preclinical studies and nearly 20 clinical studies have documented cognitive and behavioural deficits many months or years later, raising the spectre that early life anaesthesia exposure is a long-term, perhaps permanent, insult that might affect the quality of life of millions of humans. Although the phenomenon of anaesthesia-induced developmental neurotoxicity is well characterised, there are important and lingering questions pertaining to sex differences and neurodevelopmental sequelae that might occur differentially in females and males. We review the relevant literature on sex differences in the field of anaesthesia-induced developmental neurotoxicity, and present an emerging pattern of potential sex-dependent neurodevelopmental abnormalities in rodent models of human infant anaesthesia exposure.

General Anesthetic-Induced Neurotoxicity in the Immature Brain: Reevaluating the Confounding Factors in the Preclinical Studies

General anesthetic (GA) is used clinically to millions of young children each year to facilitate surgical procedures, relieve perioperative stress, and provide analgesia and amnesia. During recent years, there is a growing concern regarding a causal association between early life GA exposure and subsequently long-term neurocognitive abnormalities. To address the increasing concern, mounting preclinical studies and clinical trials have been undergoing. Until now, nearly all of the preclinical findings show that neonatal exposure to GA causally leads to acute neural cell injury and delayed cognitive impairment. Unexpectedly, several influential clinical findings suggest that early life GA exposure, especially brief and single exposure, does not cause adverse neurodevelopmental outcome, which is not fully in line with the experimental findings and data from several previous cohort trials. As the clinical data have been critically discussed in previous reviews, in the present review, we try to analyze the potential factors of the experimental studies that may overestimate the adverse effect of GA on the developing brain. Meanwhile, we briefly summarized the advance in experimental research. Generally, our purpose is to provide some useful suggestions for forthcoming preclinical studies and strengthen the powerfulness of preclinical data.

Isoflurane Modulates Hippocampal Cornu Ammonis Pyramidal Neuron Excitability by Inhibition of Both Transient and Persistent Sodium Currents in Mice

BACKGROUND

Volatile anesthetics inhibit presynaptic voltage-gated sodium channels to reduce neurotransmitter release, but their effects on excitatory neuron excitability by sodium current inhibition are unclear. The authors hypothesized that inhibition of transient and persistent neuronal sodium currents by the volatile anesthetic isoflurane contributes to reduced hippocampal pyramidal neuron excitability.

METHODS

Whole-cell patch-clamp recordings of sodium currents of hippocampal cornu ammonis pyramidal neurons were performed in acute mouse brain slices. The actions of isoflurane on both transient and persistent sodium currents were analyzed at clinically relevant concentrations of isoflurane.

RESULTS

The median inhibitory concentration of isoflurane for inhibition of transient sodium currents was 1.0 ± 0.3 mM (~3.7 minimum alveolar concentration [MAC]) from a physiologic holding potential of -70 mV. Currents from a hyperpolarized holding potential of -120 mV were minimally inhibited (median inhibitory concentration = 3.6 ± 0.7 mM, ~13.3 MAC). Isoflurane (0.55 mM; ~2 MAC) shifted the voltage-dependence of steady-state inactivation by -6.5 ± 1.0 mV (n = 11, P < 0.0001), but did not affect the voltage-dependence of activation. Isoflurane increased the time constant for sodium channel recovery from 7.5 ± 0.6 to 12.7 ± 1.3 ms (n = 13, P < 0.001). Isoflurane also reduced persistent sodium current density (median inhibitory concentration = 0.4 ± 0.1 mM, ~1.5 MAC) and resurgent currents. Isoflurane (0.55 mM; ~2 MAC) reduced action potential amplitude, and hyperpolarized resting membrane potential from -54.6 ± 2.3 to -58.7 ± 2.1 mV (n = 16, P = 0.001).

CONCLUSIONS

Isoflurane at clinically relevant concentrations inhibits both transient and persistent sodium currents in hippocampal cornu ammonis pyramidal neurons. These mechanisms may contribute to reductions in both hippocampal neuron excitability and synaptic neurotransmission.

Hypothermia: Impact on plasticity following brain injury

Therapeutic hypothermia (TH) is a potent neuroprotectant against multiple forms of brain injury, but in some cases, prolonged cooling is needed. Such cooling protocols raise the risk that TH will directly or indirectly impact neuroplasticity, such as after global and focal cerebral ischemia or traumatic brain injury. TH, depending on the depth and duration, has the potential to broadly affect brain plasticity, especially given the spatial, temporal, and mechanistic overlap with the injury processes that cooling is used to treat. Here, we review the current experimental and clinical evidence to evaluate whether application of TH has any adverse or positive effects on postinjury plasticity. The limited available data suggest that mild TH does not appear to have any deleterious effect on neuroplasticity; however, we emphasize the need for additional high-quality preclinical and clinical work in this area.

KCC2-Mediated Cl- Extrusion Modulates Spontaneous Hippocampal Network Events in Perinatal Rats and Mice

It is generally thought that hippocampal neurons of perinatal rats and mice lack transport-functional K-Cl cotransporter KCC2, and that Cl- regulation is dominated by Cl- uptake via the Na-K-2Cl cotransporter NKCC1. Here, we demonstrate a robust enhancement of spontaneous hippocampal network events (giant depolarizing potentials [GDPs]) by the KCC2 inhibitor VU0463271 in neonatal rats and late-gestation, wild-type mouse embryos, but not in their KCC2-null littermates. VU0463271 increased the depolarizing GABAergic synaptic drive onto neonatal CA3 pyramidal neurons, increasing their spiking probability and synchrony during the rising phase of a GDP. Our data indicate that Cl- extrusion by KCC2 is involved in modulation of GDPs already at their developmental onset during the perinatal period in mice and rats.

Methodologic recommendations and possible interpretations of video-EEG recordings in immature rodents used as experimental controls: A TASK1-WG2 report of the ILAE/AES Joint Translational Task Force

The use of immature rodents to study physiologic aspects of cortical development requires high-quality recordings electroencephalography (EEG) with simultaneous video recording (vEEG) of behavior. Normative developmental vEEG data in control animals are fundamental for the study of abnormal background activity in animal models of seizures or other neurologic disorders. Electrical recordings from immature, freely behaving rodents can be particularly difficult because of the small size of immature rodents, their thin and soft skull, interference with the recording apparatus by the dam, and other technical challenges. In this report of the TASK1 Working Group 2 (WG2) of the International League Against Epilepsy/American Epilepsy Society (ILAE/AES) Joint Translational Task Force, we provide suggestions that aim to optimize future vEEG recordings from immature rodents, as well as their interpretation. We focus on recordings from immature rodents younger than 30 days old used as experimental controls, because the quality and correct interpretation of such recordings is important when interpreting the vEEG results of animals serving as models of neurologic disorders. We discuss the technical aspects of such recordings and compare tethered versus wireless approaches. We also summarize the appearance of common artifacts and various patterns of electrical activity seen in young rodents used as controls as a function of behavioral state, age, and (where known) sex and strain. The information herein will hopefully help improve the methodology of vEEG recordings from immature rodents and may lead to results and interpretations that are more consistent across studies from different laboratories.

Update on developmental anesthesia neurotoxicity

PURPOSE OF REVIEW

Adverse long-term impact of general anesthesia on the developing brain is a widely discussed and controversial issue with potential public health relevance. The goal of this article is to give insights into the most recent experimental and clinical observations aimed to advance our understanding in this field.

RECENT FINDINGS

Recent investigations demonstrate long-term behavioral consequences of early-life anesthesia exposure in nonhuman primates under experimental conditions that are translationally relevant to human clinical practice. Converging evidence from rodent experiments strongly suggest that anesthetics exert developmental stage-dependent and context-dependent impact on developing neuronal circuitry and, therefore, may induce lasting changes in neuronal plasticity. Although three recent population-based human studies found a strong evidence for small increase in risk, the two most robust studies (General Anaesthesia compared to Spinal anaesthesia trial and Pediatric Anesthesia Neurodevelopment Assessment) did not find an association between brief anesthesia exposure and poor neurodevelopmental outcome.

SUMMARY

Experimental data with reasonable translational relevance suggest that early-life exposure to general anesthetics can induce lasting behavioral and cognitive deficits. In contrast, human studies provide, at best, mixed evidence about developmental anesthesia neurotoxicity. Future research, both experimental and human, is needed to clarify this important issue.

Development of inhibitory synaptic inputs on layer 2/3 pyramidal neurons in the rat medial prefrontal cortex

Inhibitory control of pyramidal neurons plays a major role in governing the excitability in the brain. While spatial mapping of inhibitory inputs onto pyramidal neurons would provide important structural data on neuronal signaling, studying their distribution at the single cell level is difficult due to the lack of easily identifiable anatomical proxies. Here, we describe an approach where in utero electroporation of a plasmid encoding for fluorescently tagged gephyrin into the precursors of pyramidal cells along with ionotophoretic injection of Lucifer Yellow can reliably and specifically detect GABAergic synapses on the dendritic arbour of single pyramidal neurons. Using this technique and focusing on the basal dendritic arbour of layer 2/3 pyramidal cells of the medial prefrontal cortex, we demonstrate an intense development of GABAergic inputs onto these cells between postnatal days 10 and 20. While the spatial distribution of gephyrin clusters was not affected by the distance from the cell body at postnatal day 10, we found that distal dendritic segments appeared to have a higher gephyrin density at later developmental stages. We also show a transient increase around postnatal day 20 in the percentage of spines that are carrying a gephyrin cluster, indicative of innervation by a GABAergic terminal. Since the precise spatial arrangement of synaptic inputs is an important determinant of neuronal responses, we believe that the method described in this work may allow a better understanding of how inhibition settles together with excitation, and serve as basics for further modelling studies focusing on the geometry of dendritic inhibition during development.