A-current expression is regulated by activity but not by target tissues in developing lumbar motoneurons of the chick embryo

Spatiotemporal integration of developmental cues in neural development

Nervous system development relies on the generation of neurons, their differentiation and establishment of synaptic connections. These events exhibit remarkable plasticity and are regulated by many developmental cues. Here, we review the mechanisms of three classes of these cues: morphogenetic proteins, electrical activity, and the environment. We focus on second messenger dynamics and their role as integrators of the action of diverse cues, enabling plasticity in the process of neural development.

Homeostatic synaptic plasticity in developing spinal networks driven by excitatory GABAergic currents

Homeostatic plasticity refers to mechanisms that the cell or network engage in order to homeostatically maintain a preset level of activity. These mechanisms include compensatory changes in cellular excitability, excitatory and inhibitory synaptic strength and are typically studied at a developmental stage when GABA or glycine is inhibitory. Here we focus on the expression of homeostatic plasticity in the chick embryo spinal cord at a stage when GABA is excitatory. When spinal activity is perturbed in the living embryo there are compensatory changes in postsynaptic AMPA receptors and in the driving force for GABAergic currents. These changes are triggered by reduced GABAA receptor signaling, which appears to be part of the sensing machinery for triggering homeostatic plasticity. We compare and contrast these findings to homeostatic plasticity expressed in spinal systems at different stages of development, and to the developing retina at a stage when GABA is depolarizing. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Downregulation of GluA2 AMPA receptor subunits reduces the dendritic arborization of developing spinal motoneurons

AMPA receptors lacking the GluA2 subunit allow a significant influx of Ca(2+) ions. Although Ca(2+)-permeable AMPA receptors are a familiar feature at early stages of development, the functional significance of these receptors during the maturation of the nervous system remains to be established. Chicken lumbar motoneurons express Ca(2+)-permeable AMPA receptors at E6 but the Ca(2+) permeability of AMPA receptors decreases ∼3-fold by E11. Considering that activity-dependent changes in intracellular Ca(2+) regulates dendritic outgrowth, in this study we investigated whether downregulation of GluA2 expression during a critical period of development alters the dendritic arborization of spinal motoneurons in ovo. We use an avian replication-competent retroviral vector RCASBP (B) carrying the marker red fluorescent protein (RFP) and a GluA2 RNAi construct to downregulate GluA2 expression. Chicken embryos were infected at E2 with one of the following constructs: RCASBP(B)-RFP, RCASBP(B)-RFP-scrambled RNAi, or RCASBP(B)-RFP-GluA2 RNAi. Infection of chicken embryos at E2 resulted in widespread expression of RFP throughout the spinal cord with ≥60% of Islet1/2-positive motoneurons infected, resulting in a significant reduction in GluA2 protein expression. Downregulation of GluA2 expression had no effect on the dendritic arborization of E6 motoneurons. However, downregulation of GluA2 expression caused a significant reduction in the dendritic arborization of E11 motoneurons. Neither motoneuron survival nor maturation of network activity was affected by changes in GluA2 expression. These findings demonstrate that increased GluA2 expression and changes in the Ca(2+) permeability of AMPA receptors regulate the dendritic arborization of spinal cord motoneurons during a critical period of development.

Pharmacological manipulation of GABA-driven activity in ovo disrupts the development of dendritic morphology but not the maturation of spinal cord network activity

Background

In the adult nervous system, GABA acts as a major inhibitory neurotransmitter; however, at early stages of neurodevelopment, GABA receptor activation leads to membrane depolarization and accumulation of [Ca²⁺]_i. The role of excitatory GABAergic neurotransmission in the development of the nervous system is not fully understood. In this study, we investigated the role of excitatory GABA-driven activity in regulating the dendritic morphology and network function in the developing chicken spinal cord.

Results

Both bicuculline, a GABA receptor antagonist, and muscimol, a GABA agonist, inhibit the generation of spontaneous network activity in the isolated spinal cord at E8 or E10, indicating that altering GABA receptor activation disrupts the generation of spontaneous network activity in the chicken spinal cord. Treatment of chicken embryos with bicuculline or muscimol between E5 and E8 (or between E8 and E10), inhibits the dendritic outgrowth of motoneurons when compared to vehicle-treated embryos. The inhibitory effect of bicuculline or muscimol on the dendritic morphology of motoneurons was likely due to inhibition of GABA-driven network activity since a similar effect was also observed following reduction of network activity by Kir2.1 overexpression in the spinal cord. The inhibitory effect of bicuculline or muscimol was not caused by an adverse effect on cell survival. Surprisingly, chronic treatment of chicken embryos with bicuculline or muscimol has no effect on the shape and duration of the episodes of spontaneous activity, suggesting that maturation of network activity is not altered by disruption of the dendritic outgrowth of motoneurons.

Conclusions

Taken together, these findings indicate that excitatory GABA receptor activation regulates the maturation of dendritic morphology in the developing spinal cord by an activity-dependent mechanism. However, inhibition of dendritic outgrowth caused by disruption of GABA-driven activity does not alter the maturation of spontaneous electrical activity generated by spinal cord networks, suggesting that compensatory mechanisms can reverse any adverse effect of dendritic morphology on network function.

Compensatory changes in cellular excitability, not synaptic scaling, contribute to homeostatic recovery of embryonic network activity

When neuronal activity is reduced over a period of days, compensatory changes in synaptic strength and/or cellular excitability are triggered, which are thought to act in a manner to homeostatically recover normal activity levels. The time course over which changes in homeostatic synaptic strength and cellular excitability occur are not clear. Although many studies show that 1-2 days of activity block are necessary to trigger increases in excitatory quantal strength, few studies have been able to examine whether these mechanisms actually underlie recovery of network activity. Here, we examine the mechanisms underlying recovery of embryonic motor activity following block of either excitatory GABAergic or glutamatergic inputs in vivo. We find that GABA(A) receptor blockade triggers fast changes in cellular excitability that occur during the recovery of activity but before changes in synaptic scaling. This increase in cellular excitability is mediated in part by an increase in sodium currents and a reduction in the fast-inactivating and calcium-activated potassium currents. These findings suggest that compensatory changes in cellular excitability, rather than synaptic scaling, contribute to activity recovery. Further, we find a special role for the GABA(A) receptor in triggering several homeostatic mechanisms after activity perturbations, including changes in cellular excitability and GABAergic and AMPAergic synaptic strength. The temporal difference in expression of homeostatic changes in cellular excitability and synaptic strength suggests that there are multiple mechanisms and pathways engaged to regulate network activity, and that each may have temporally distinct functions.

NKCC1 and AE3 appear to accumulate chloride in embryonic motoneurons

During early development, gamma-aminobutyric acid (GABA) depolarizes and excites neurons, contrary to its typical function in the mature nervous system. As a result, developing networks are hyperexcitable and experience a spontaneous network activity that is important for several aspects of development. GABA is depolarizing because chloride is accumulated beyond its passive distribution in these developing cells. Identifying all of the transporters that accumulate chloride in immature neurons has been elusive and it is unknown whether chloride levels are different at synaptic and extrasynaptic locations. We have therefore assessed intracellular chloride levels specifically at synaptic locations in embryonic motoneurons by measuring the GABAergic reversal potential (EGABA) for GABAA miniature postsynaptic currents. When whole cell patch solutions contained 17-52 mM chloride, we found that synaptic EGABA was around -30 mV. Because of the low HCO3- permeability of the GABAA receptor, this value of EGABA corresponds to approximately 50 mM intracellular chloride. It is likely that synaptic chloride is maintained at levels higher than the patch solution by chloride accumulators. We show that the Na+-K+-2Cl- cotransporter, NKCC1, is clearly involved in the accumulation of chloride in motoneurons because blocking this transporter hyperpolarized EGABA and reduced nerve potentials evoked by local application of a GABAA agonist. However, chloride accumulation following NKCC1 block was still clearly present. We find physiological evidence of chloride accumulation that is dependent on HCO3- and sensitive to an anion exchanger blocker. These results suggest that the anion exchanger, AE3, is also likely to contribute to chloride accumulation in embryonic motoneurons.

Inhibition of electrical activity by retroviral infection with Kir2.1 transgenes disrupts electrical differentiation of motoneurons

Network-driven spontaneous electrical activity in the chicken spinal cord regulates a variety of developmental processes including neuronal differentiation and formation of neuromuscular structures. In this study we have examined the effect of chronic inhibition of spinal cord activity on motoneuron survival and differentiation. Early spinal cord activity in chick embryos was blocked using an avian replication-competent retroviral vector RCASBP (B) carrying the inward rectifier potassium channel Kir2.1. Chicken embryos were infected with one of the following constructs: RCASBP(B), RCASBP(B)-Kir2.1, or RCASBP(B)-GFP. Infection of chicken embryos at E2 resulted in widespread expression of the viral protein marker p27 gag throughout the spinal cord. Electrophysiological recordings revealed the presence of functional Kir2.1 channels in RCASBP(B)-Kir2.1 but not in RCASBP(B)-infected embryos. Kir2.1 expression significantly reduced the generation of spontaneous motor movements in chicken embryos developing in ovo. Suppression of spontaneous electrical activity was not due to a reduction in the number of surviving motoneurons or the number of synapses in hindlimb muscle tissue. Disruption of the normal pattern of activity in chicken embryos resulted in a significant downregulation in the functional expression of large-conductance Ca²⁺-dependent K⁺ channels. Reduction of spinal cord activity also generates a significant acceleration in the inactivation rate of A-type K⁺ currents without any significant change in current density. Kir2.1 expression did not affect the expression of voltage-gated Na⁺ channels or cell capacitance. These experiments demonstrate that chronic inhibition of chicken spinal cord activity causes a significant change in the electrical properties of developing motoneurons.

GABAA transmission is a critical step in the process of triggering homeostatic increases in quantal amplitude

When activity levels are altered over days, a network of cells is capable of recognizing this perturbation and triggering several distinct compensatory changes that should help to recover and maintain the original activity levels homeostatically. One feature commonly observed after activity blockade has been a compensatory increase in excitatory quantal amplitude. The sensing machinery that detects altered activity levels is a central focus of the field currently, but thus far it has been elusive. The vast majority of studies that reduce network activity also reduce neurotransmission. We address the possibility that reduced neurotransmission can trigger increases in quantal amplitude. In this work, we blocked glutamatergic or GABA(A) transmission in ovo for 2 days while maintaining relatively normal network activity. We found that reducing GABA(A) transmission triggered compensatory increases in both GABA and AMPA quantal amplitude in embryonic spinal motoneurons. Glutamatergic blockade had no effect on quantal amplitude. Therefore, GABA binding to the GABA(A) receptor appears to be a critical step in the sensing machinery for homeostatic synaptic plasticity. The findings suggest that homeostatic increases in quantal amplitude may normally be triggered by reduced levels of activity, which are sensed in the developing spinal cord by GABA, via the GABA(A) receptor. Therefore, GABA appears to be serving as a proxy for activity levels.

Chloride-sensitive MEQ fluorescence in chick embryo motoneurons following manipulations of chloride and during spontaneous network activity

Intracellular Cl(-) ([Cl(-)](in)) homeostasis is thought to be an important regulator of spontaneous activity in the spinal cord of the chick embryo. We investigated this idea by visualizing the variations of [Cl(-)](in) in motoneurons retrogradely labeled with the Cl-sensitive dye 6-methoxy-N-ethylquinolinium iodide (MEQ) applied to cut muscle nerves in the isolated E10-E12 spinal cord. This labeling procedure obviated the need for synthesizing the reduced, cell-permeable dihydro-MEQ (DiH-MEQ). The specificity of motoneuron labeling was confirmed using retrograde co-labeling with Texas Red Dextran and immunocytochemistry for choline acetyltransferase (ChAT). In MEQ-labeled motoneurons, the GABA(A) receptor agonist isoguvacine (100 muM) increased somatic and dendritic fluorescence by 7.4 and 16.7%, respectively. The time course of this fluorescence change mirrored that of the depolarization recorded from the axons of the labeled motoneurons. Blockade of the inward Na(+)/K(-)/2Cl(-) co-transporter (NKCC1) with bumetanide (20 microM) or with a low-Na(+) bath solution (12 mM), increased MEQ fluorescence by 5.3 and 11.4%, respectively, consistent with a decrease of [Cl(-)](in). After spontaneous episodes of activity, MEQ fluorescence increased and then declined to the pre-episode level during the interepisode interval. The largest fluorescence changes occurred over motoneuron dendrites (19.7%) with significantly smaller changes (5.2%) over somata. Collectively, these results show that retrogradely loaded MEQ can be used to detect [Cl(-)](in) in motoneurons, that the bumetanide-sensitive NKCC1 co-transporter is at least partially responsible for the elevated [Cl(-)](in) of developing motoneurons, and that dendritic [Cl(-)](in) decreases during spontaneous episodes and recovers during the inter-episode interval, presumably due to the action of NKCC1.