Oxidative stress induced by cumene hydroperoxide evokes changes in neuronal excitability of rat motor cortex neurons. Neuroscience. 2015;289:85-98. [PMID: 25592424 DOI: 10.1016/j.neuroscience.2014.12.055]

Migratory Response of Cells in Neurogenic Niches to Neuronal Death: The Onset of Harmonic Repair?

Harmonic mechanisms orchestrate neurogenesis in the healthy brain within specific neurogenic niches, which generate neurons from neural stem cells as a homeostatic mechanism. These newly generated neurons integrate into existing neuronal circuits to participate in different brain tasks. Despite the mechanisms that protect the mammalian brain, this organ is susceptible to many different types of damage that result in the loss of neuronal tissue and therefore in alterations in the functionality of the affected regions. Nevertheless, the mammalian brain has developed mechanisms to respond to these injuries, potentiating its capacity to generate new neurons from neural stem cells and altering the homeostatic processes that occur in neurogenic niches. These alterations may lead to the generation of new neurons within the damaged brain regions. Notwithstanding, the activation of these repair mechanisms, regeneration of neuronal tissue within brain injuries does not naturally occur. In this review, we discuss how the different neurogenic niches respond to different types of brain injuries, focusing on the capacity of the progenitors generated in these niches to migrate to the injured regions and activate repair mechanisms. We conclude that the search for pharmacological drugs that stimulate the migration of newly generated neurons to brain injuries may result in the development of therapies to repair the damaged brain tissue.

Glia-neuron coupling via a bipartite sialylation pathway promotes neural transmission and stress tolerance in Drosophila

Modification by sialylated glycans can affect protein functions, underlying mechanisms that control animal development and physiology. Sialylation relies on a dedicated pathway involving evolutionarily conserved enzymes, including CMP-sialic acid synthetase (CSAS) and sialyltransferase (SiaT) that mediate the activation of sialic acid and its transfer onto glycan termini, respectively. In Drosophila, CSAS and DSiaT genes function in the nervous system, affecting neural transmission and excitability. We found that these genes function in different cells: the function of CSAS is restricted to glia, while DSiaT functions in neurons. This partition of the sialylation pathway allows for regulation of neural functions via a glia-mediated control of neural sialylation. The sialylation genes were shown to be required for tolerance to heat and oxidative stress and for maintenance of the normal level of voltage-gated sodium channels. Our results uncovered a unique bipartite sialylation pathway that mediates glia-neuron coupling and regulates neural excitability and stress tolerance.

Equine Muscle Derived Mesenchymal Stem Cells Loaded with Water-Soluble Curcumin: Modulation of Neutrophil Activation and Enhanced Protection against Intracellular Oxidative Attack

We investigated the antioxidant potential of equine mesenchymal stem cells derived from muscle microbiopsies (mdMSCs), loaded by a water-soluble curcumin lysinate incorporated into hydroxypropyl-β-cyclodextrin (NDS27). The cell loading was rapid and dependent on NDS27 dosage (14, 7, 3.5 and 1 µM). The immunomodulatory capacity of loaded mdMSCs was evaluated by ROS production, on active and total myeloperoxidase (MPO) degranulation and neutrophil extracellular trap (NET) formation after neutrophil stimulation. The intracellular protection of loaded cells was tested by an oxidative stress induced by cumene hydroperoxide. Results showed that 10 min of mdMSC loading with NDS27 did not affect their viability while reducing their metabolism. NDS27 loaded cells in presence of 14, 7 µM NDS27 inhibited more intensively the ROS production, the activity of the MPO released and bound to the NET after neutrophil stimulation. Furthermore, loaded cells powerfully inhibited intracellular ROS production induced by cumene as compared to control cells or cyclodextrin-loaded cells. Our results showed that the loading of mdMSCs with NDS27 significantly improved their antioxidant potential against the oxidative burst of neutrophil and protected them against intracellular ROS production. The improved antioxidant protective capacity of loaded mdMSCs could be applied to target inflammatory foci involving neutrophils.

Oxidative Stress as a Potential Mechanism Underlying Membrane Hyperexcitability in Neurodegenerative Diseases

Neurodegenerative diseases are characterized by gradually progressive, selective loss of anatomically or physiologically related neuronal systems that produce brain damage from which there is no recovery. Despite the differences in clinical manifestations and neuronal vulnerability, the pathological processes appear to be similar, suggesting common neurodegenerative pathways. It is well known that oxidative stress and the production of reactive oxygen radicals plays a key role in neuronal cell damage. It has been proposed that this stress, among other mechanisms, could contribute to neuronal degeneration and might be one of the factors triggering the development of these pathologies. Another common feature in most neurodegenerative diseases is neuron hyperexcitability, an aberrant electrical activity. This review, focusing mainly on primary motor cortex pyramidal neurons, critically evaluates the idea that oxidative stress and inflammation may be involved in neurodegeneration via their capacity to increase membrane excitability.

Acute Colon Inflammation Triggers Primary Motor Cortex Glial Activation, Neuroinflammation, Neuronal Hyperexcitability, and Motor Coordination Deficits

Neuroinflammation underlies neurodegenerative diseases. Herein, we test whether acute colon inflammation activates microglia and astrocytes, induces neuroinflammation, disturbs neuron intrinsic electrical properties in the primary motor cortex, and alters motor behaviors. We used a rat model of acute colon inflammation induced by dextran sulfate sodium. Inflammatory mediators and microglial activation were assessed in the primary motor cortex by PCR and immunofluorescence assays. Electrophysiological properties of the motor cortex neurons were determined by whole-cell patch-clamp recordings. Motor behaviors were examined using open-field and rotarod tests. We show that the primary motor cortex of rats with acute colon inflammation exhibited microglial and astrocyte activation and increased mRNA abundance of interleukin-6, tumor necrosis factor-alpha, and both inducible and neuronal nitric oxide synthases. These changes were accompanied by a reduction in resting membrane potential and rheobase and increased input resistance and action potential frequency, indicating motor neuron hyperexcitability. In addition, locomotion and motor coordination were impaired. In conclusion, acute colon inflammation induces motor cortex microglial and astrocyte activation and inflammation, which led to neurons’ hyperexcitability and reduced motor coordination performance. The described disturbances resembled some of the early features found in amyotrophic lateral sclerosis patients and animal models, suggesting that colon inflammation might be a risk factor for developing this disease.

Hydrogen peroxide suppresses excitability of gonadotropin-releasing hormone neurons in adult mouse

It has been reported that reactive oxygen species (ROS) derived from oxygen molecule reduction can interfere with the cross-talk between the hypothalamic-pituitary-gonadal (HPG) axis and other endocrine axes, thus affecting fertility. Furthermore, ROS have been linked to GnRH receptor signaling in gonadotropes involved in gonadotropin release. There has been evidence that ROS can interfere with the HPG axis and gonadotropin release at various levels. However, the direct effect of ROS on gonadotropin-releasing hormone (GnRH) neuron remains unclear. Thus, the objective of this study was to determine the effect of hydrogen peroxide (H₂O₂), an ROS source, on GnRH neuronal excitabilities in transgenic GnRH-green fluorescent protein-tagged mice using the whole-cell patch-clamp electrophysiology. In adults, H₂O₂ at high concentrations (mM level) hyperpolarized most GnRH neurons tested, whereas low concentrations (pM to μM) caused slight depolarization. In immature GnRH neurons, H₂O₂ exposure induced excitation. The sensitivity of GnRH neurons to H₂O₂ was increased with postnatal development. The effect of H₂O₂ on adult female GnRH neurons was found to be estrous cycle-dependent. Hyperpolarization mediated by H₂O₂ persisted in the presence of tetrodotoxin, a voltage-gated Na⁺ channel blocker, and amino-acids receptor blocking cocktail containing blockers for the ionotropic glutamate receptors, glycine receptors, and GABA_A receptors, indicating that H₂O₂ could act on GnRH neurons directly. Furthermore, glibenclamide, an ATP-sensitive K⁺ (K_ATP) channel blocker, completely blocked H₂O₂-mediated hyperpolarization. Increasing endogenous H₂O₂ by inhibiting glutathione peroxidase decreased spontaneous activities of most GnRH neurons. We conclude that ROS can act as signaling molecules for regulating GnRH neuron's excitability and that adult GnRH neurons are sensitive to increased ROS concentration. Results of this study demonstrate that ROS have direct modulatory effects on the HPG axis at the hypothalamic level to regulate GnRH neuron's excitabilities.

Refinement of Active and Passive Membrane Properties of Layer V Pyramidal Neurons in Rat Primary Motor Cortex During Postnatal Development

Achieving the distinctive complex behaviors of adult mammals requires the development of a great variety of specialized neural circuits. Although the development of these circuits begins during the embryonic stage, they remain immature at birth, requiring a postnatal maturation process to achieve these complex tasks. Understanding how the neuronal membrane properties and circuits change during development is the first step to understand their transition into efficient ones. Thus, using whole cell patch clamp recordings, we have studied the changes in the electrophysiological properties of layer V pyramidal neurons of the rat primary motor cortex during postnatal development. Among all the parameters studied, only the voltage threshold was established at birth and, although some of the changes occurred mainly during the second postnatal week, other properties such as membrane potential, capacitance, duration of the post-hyperpolarization phase or the maximum firing rate were not defined until the beginning of adulthood. Those modifications lead to a decrease in neuronal excitability and to an increase in the working range in young adult neurons, allowing more sensitive and accurate responses. This maturation process, that involves an increase in neuronal size and changes in ionic conductances, seems to be influenced by the neuronal type and by the task that neurons perform as inferred from the comparison with other pyramidal and motor neuron populations.

Age-Dependent Vulnerability to Oxidative Stress of Postnatal Rat Pyramidal Motor Cortex Neurons

Oxidative stress is one of the main proposed mechanisms involved in neuronal degeneration. To evaluate the consequences of oxidative stress on motor cortex pyramidal neurons during postnatal development, rats were classified into three groups: Newborn (P2-P7); infantile (P11-P15); and young adult (P20-P40). Oxidative stress was induced by 10 µM of cumene hydroperoxide (CH) application. In newborn rats, using the whole cell patch-clamp technique in brain slices, no significant modifications in membrane excitability were found. In infantile rats, the input resistance increased and rheobase decreased due to the blockage of GABAergic tonic conductance. Lipid peroxidation induced by CH resulted in a noticeable increase in protein-bound 4-hidroxynonenal in homogenates in only infantile and young adult rat slices. Interestingly, homogenates of newborn rat brain slices showed the highest capacity to respond to oxidative stress by dramatically increasing their glutathione and free thiol content. This increase correlated with a time-dependent increase in the glutathione reductase activity, suggesting a greater buffering capacity of newborn rats to resist oxidative stress. Furthermore, pre-treatment of the slices with glutathione monoethyl ester acted as a neuroprotector in pyramidal neurons of infantile rats. We conclude that during maturation, the vulnerability to oxidative stress in rat motor neurons increases with age.

Deficiency of Mitochondrial Functions and Peroxidation of Frontoparietal Cortex Enhance Isoflurane Sensitivity in Aging Mice

Background: Hypersensitivity to general anesthetics may predict poor postoperative outcomes, especially among the older subjects. Therefore, it is essential to elucidate the mechanism underlying hypersensitivity to volatile anesthetics in the aging population. Given the fact that isoflurane sensitivity increases with aging, we hypothesized that deficiencies of mitochondrial function and elevated oxidative levels in the frontoparietal cortex may contribute to the enhanced sensitivity to isoflurane in aging mice. Methods: Isoflurane sensitivity in aging mice was determined by the concentration of isoflurane that is required for loss of righting reflex (LORR). Mitochondrial bioenergetics of the frontoparietal cortex was measured using a Seahorse XFp analyzer. Protein oxidation and lipid oxidation in the frontoparietal cortex were assessed using the Oxyblot protein oxidation detection kit and thiobarbituric acid reactive substance (TBARS) assay, respectively. Contributions of mitochondrial complex II inhibition by malonate and peroxidation by ozone to isoflurane sensitivity were tested in vivo. Besides, effects of antioxidative therapy on mitochondrial function and isoflurane sensitivity in mice were also measured. Results: The mean concentration of isoflurane that is required for LORR in aging mice (14-16 months old) was 0.83% ± 0.13% (mean ± SD, n = 80). Then, the mice were divided into three groups as sensitive group (S group, mean - SD), medium group (M group), and resistant group (R group, mean + SD) based on individual concentrations of isoflurane required for LORR. Activities of mitochondrial complex II and complex IV in mice of the S group were significantly lower than those of the R group, while frontoparietal cortical malondialdehyde (MDA) levels were higher in the mice of S group. Both inhibition of mitochondrial complexes and peroxidation significantly decreased the concentration of isoflurane that is required for LORR in vivo. After treatment with idebenone, the levels of lipid oxidation were alleviated and mitochondrial function was restored in aging mice. The concentration of isoflurane that required for LORR was also elevated after idebenone treatment. Conclusions: Decreased mitochondrial functions and higher oxidative stress levels in the frontoparietal cortex may contribute to the hypersensitivity to isoflurane in aging mice.

Anti-androgenic, anti-oxidant and anti-apoptotic effects of the aqueous and methanol extracts of Pterorhachis zenkeri (Meliaceae): Evidence from in vivo and in vitro studies

The aim of this study was to evaluate the effects of Pterorhachis zenkeri (Meliaceae) on sex organ growth in immature male rats and, oxidative stress and apoptosis markers in CCL-97 (R2C) Leydig cells. For the in vivo studies, 70 immature male Wistar rats (n = 10/group) were treated for 2 or 4 weeks with: distilled water (10 ml/kg, per os) plus soya oil (1 ml/kg, sc), bicalutamide (10 mg/kg, per os), aqueous or methanol extract of P. zenkeri (10 mg/kg or 62 mg/kg, per os) or testosterone propionate (3 mg/kg, sc). After each treatment period, body and sexual organ weights, plasmatic testosterone, total proteins and total cholesterol levels were measured. In the in vitro test, the effects of the methanol extract of P. zenkeri on cell viability, apoptosis, reactive oxygen species (ROS) production, intracellular calcium release and caspases 3/9 were assessed using CCL-97 Leydig cells. Pterorhachis zenkeri extracts decreased sex organ weights, plasmatic testosterone and protein levels in rats. In the in vitro studies, P. zenkeri inhibited apoptosis, ROS production, calcium release and caspase 3/9 activities. These results suggest that P. zenkeri has anti-androgenic, anti-oxidant and anti-apoptotic activities with methanol extract being the most active and could be an effective alternative for the management of androgen-related diseases.

Hydrogen peroxide inhibits neurons in the paraventricular nucleus of the hypothalamus via potassium channel activation

The paraventricular nucleus (PVN) of the hypothalamus is an important homeostatic and reflex center for neuroendocrine, respiratory, and autonomic regulation, including during hypoxic stressor challenges. Such challenges increase reactive oxygen species (ROS) to modulate synaptic, neuronal, and ion channel activity. Previously, in the nucleus tractus solitarius, another cardiorespiratory nucleus, we showed that the ROS H₂O₂ induced membrane hyperpolarization and reduced action potential discharge via increased K⁺ conductance at the resting potential. Here, we sought to determine the homogeneity of influence and mechanism of action of H₂O₂ on PVN neurons. We recorded PVN neurons in isolation and in an acute slice preparation, which leaves neurons in their semi-intact network. Regardless of preparation, H₂O₂ hyperpolarized PVN neurons and decreased action potential discharge. In the slice preparation, H₂O₂ also decreased spontaneous excitatory postsynaptic current frequency, but not amplitude. To examine potential mechanisms, we investigated the influence of the K⁺ channel blockers Ba²⁺, Cs⁺, and glibenclamide on membrane potential, as well as the ionic currents active at resting potential and outward K⁺ currents (I_K) upon depolarization. The H₂O₂ hyperpolarization was blocked by K⁺ channel blockers. H₂O₂ did not alter currents between -50 and -110 mV. However, H₂O₂ induced an outward I_K at -50 mV yet, at potentials more positive to 0 mV H₂O₂, decreased I_K. Elevated intracellular antioxidant catalase eliminated H₂O₂ effects. These data indicate that H₂O₂ alters synaptic and neuronal properties of PVN neurons likely via membrane hyperpolarization and alteration of I_K, which may ultimately alter cardiorespiratory reflexes.

Current perspectives on the development and treatment of chronic daily headache in children and adolescents

Headache disorders subsumed under the term chronic daily headache (CDH), including chronic migraine, chronic tension-type headache and new daily persistent headache, affect up to 4% of the pediatric population and can be highly disabling and challenging to effectively treat. Although historically the knowledge base about this group of headache disorders in children primarily was derived from clinical observation and extrapolation from adult studies, over the past several years there have been important research findings relevant to the development and management of pediatric CDH that can help inform clinical practice. The intent of this paper is to provide a focused review on recent empirical work done on pediatric CDH and suggest avenues for future work.

Synthesis and Biological Evaluation of Scutellarein Alkyl Derivatives as Preventing Neurodegenerative Agents with Improved Lipid Soluble Properties

BACKGROUND

Exogenous antioxidants are considered as a promising therapeutic approach to treat neurodegenerative diseases since they could prevent and/or minimize the neuronal damage by oxidation.

OBJECTIVE

Three series of lipophilic compounds structurally based on scutellarein (2), which is one metabolite of scutellarin (1) in vivo, have been designed and synthesized.

METHODS

Their antioxidant activity was evaluated by detecting the 2-thiobarbituric acid reactive substance (TBARS) produced in the ferrous salt/ascorbate-induced autoxidation of lipids, which were present in microsomal membranes of rat hepatocytes. The lipophilicity of these compounds indicated as partition coefficient between n-octanol and buffer was investigated by ultraviolet (UV) spectrophotometer.

RESULTS

This study indicated that compound 5e which had a benzyl group substituted at the C4'- OH position showed a potent antioxidant activity and good lipophilicity.

CONCLUSION

5e could be an effective candidate for preventing or reducing the oxidative status associated with the neurodegenerative processes.

Astrocytic CCAAT/Enhancer-binding protein delta contributes to reactive oxygen species formation in neuroinflammation

Excessive reactive oxygen species (ROS) can form an oxidative stress and an associated neuroinflammation. However, the contribution of astrocytes to ROS formation, the cause of the resistance of astrocytes to oxidative stress, and the consequences on neurons remain largely uninvestigated. The transcription factor CCAAT/enhancer-binding protein delta (CEBPD) is highly expressed in astrocytes and has been suggested to contribute to the progress of Alzheimer's disease (AD). In this study, we found that ROS formation and expression of p47^phox and p67^phox, subunits of NADPH oxidase, were increased in AppTg mice but attenuated in AppTg/Cebpd^-/- mice. Cebpd can up-regulate p47^phox and p67^phox transcription via a direct binding on their promoters, which results in an increase in intracellular oxidative stress. In addition, Cebpd also up-regulated Cu/Zn superoxide dismutase (Sod1) in astrocytes. Inactivation of Sod1 increased the sensitization to oxidative stress, which provides a reason for the resistance of astrocytes in an oxidative stress environment. Taken together, the study first revealed and dissected the involvement of astrocytic Cebpd in the promotion of oxidative stress and the contribution of CEBPD to the resistance of astrocytes in an oxidative stress environment.

Neurodegenerative and neurological disorders by small inhaled particles

The world's population is steadily ageing and as a result, health conditions related to ageing, such as dementia, have become a major public health concern. In 2001, it was estimated that there were almost 5 million Europeans suffering from Alzheimer's disease (AD) and this figure has been projected to almost double by 2040. About 40% of people over 85 suffer from AD, and another 10% from Parkinson's disease (PD). The majority of AD and PD cases are of sporadic origin and environmental factors play an important role in the aetiology. Epidemiological research identified airborne particulate matter (PM) as one of the environmental factors potentially involved in AD and PD pathogenesis. Also, cumulating evidence demonstrates that the smallest sizes of the inhalable fraction of ambient particulate matter, also referred to as ultrafine particulate matter or nano-sized particles, are capable of inducing effects beyond the respiratory system. Translocation of very small particles via the olfactory epithelium in the nose or via uptake into the circulation has been demonstrated through experimental rodent studies with engineered nanoparticles. Outdoor air pollution has been linked to several health effects including oxidative stress and neuroinflammation that may ultimately result in neurodegeneration and cognitive impairment. This review aims to evaluate the relationship between exposure to inhaled ambient particles and neurodegeneration.

The neurotoxic effects of hydrogen peroxide and copper in Retzius nerve cells of the leech Haemopis sanguisuga

Oxidative stress and the generation of reactive oxygen species (ROS) play an important role in cellular damage. Electrophysiological analyses have shown that membrane transport proteins are susceptible to ROS. In the present study, oxidative stress was induced in Retzius nerve cells of the leechHaemopis sanguisugaby bath application of 1 mM of hydrogen peroxide (H2O2) and 0.02 mM of copper (Cu) for 20 min. The H2O2/Cu(II) produced considerable changes in the electrical properties of the Retzius nerve cells. Intracellular recording of the resting membrane potential revealed that the neuronal membrane was depolarized in the presence of H2O2/Cu(II). We found that the amplitude of action potentials decreased, while the duration augmented in a progressive way along the drug exposure time. The combined application of H2O2and Cu(II) caused an initial excitation followed by depression of the spontaneous electrical activity. Voltage-clamp recordings revealed a second effect of the oxidant, a powerful inhibition of the outward potassium channels responsible for the repolarization of action potentials. The neurotoxic effect of H2O2/Cu(II) on the spontaneous spike electrogenesis and outward K(+)current of Retzius nerve cells was reduced in the presence of hydroxyl radical scavengers, dimethylthiourea and dimethyl sulfoxide, but not mannitol. This study provides evidence for the oxidative modification of outward potassium channels in Retzius nerve cells. The oxidative mechanism of the H2O2/Cu(II) system action on the electrical properties of Retzius neurons proposed in this study might have a wider significance, referring not only to leeches but also to mammalian neurons.

Time and dose dependent effects of oxidative stress induced by cumene hydroperoxide in neuronal excitability of rat motor cortex neurons

It has been claimed that oxidative stress and the production of reactive oxygen radicals can contribute to neuron degeneration and might be one factor in the development of different neurological diseases. In our study, we have attempted to clarify how oxidative damage induces dose dependent changes in functional membrane properties of neurons by means of whole cell patch clamp techniques in brain slices from young adult rats. Our research demonstrates physiological changes in membrane properties of pyramidal motor cortex neurons exposed to 3 concentrations of cumene hydroperoxide (CH; 1, 10 and 100μM) during 30min. Results show that oxidative stress induced by CH evokes important changes, in a concentration and time dependent manner, in the neuronal excitability of motor cortex neurons of the rat: (i) Low concentration of the drug (1μM) already blocks inward rectifications (sag) and decreases action potential amplitude and gain, a drug concentration which has no effects on other neuronal populations, (ii) 10μM of CH depresses the excitability of pyramidal motor cortex neurons by decreasing input resistance, amplitude of the action potential, and gain and maximum frequency of the repetitive firing discharge, and (iii) 100μM completely blocks the capability to produce repetitive discharge of action potentials in all cells. Both larger drug concentrations and/or longer times of exposure to CH narrow the current working range. This happens because of the increase in the rheobase, and the reduction of the cancelation current. The effects caused by oxidative stress, including those produced by the level of lipid peroxidation, are practically irreversible and, this, therefore, indicates that neuroprotective agents should be administered at the first symptoms of alterations to membrane properties. In fact, the pre-treatment with melatonin, acting as an antioxidant, prevented the lipid peroxidation and the physiological changes induced by CH. Larger cells (as estimated by their cell capacitance) were also more susceptible to oxidative stress. Our results provide previously unavailable observations that large size and high sensitivity to oxidative stress (even at low concentrations) make pyramidal neurons of the motor cortex, in particular corticofugal neurons, more susceptible to cell death when compared with other neuronal populations. These results could also shed some light on explaining the causes behind diseases such as Amyotrophic Lateral Sclerosis.

Time windows for postnatal changes in morphology and membrane excitability of genioglossal and oculomotor motoneurons

Time windows for postnatal changes in morphology and membrane excitability of genioglossal (GG) and oculomotor (OCM) motoneurons (MNs) are yet to be fully described. Analysis of data on brain slices in vitro of the 2 populations of MNs point to a well-defined developmental program that progresses with common age-related changes characterized by: (1) increase of dendritic surface along with length and reshaping of dendritic tree complexity; (2) disappearance of gap junctions early in development; (3) decrease of membrane passive properties, such as input resistance and time constant, together with an increase in the number of cells displaying sag, and modifications in rheobase; (4) action potential shortening and afterhyperpolarization; and (5) an increase in gain and maximum firing frequency. These modifications take place at different time windows for each motoneuronal population. In GG MNs, active membrane properties change mainly during the first postnatal week, passive membrane properties in the second week, and dendritic increasing length and size in the third week of development. In OCM MNs, changes in passive membrane properties and growth of dendritic size take place during the first postnatal week, while active membrane properties and rheobase change during the second and third weeks of development. The sequential order of changes is inverted between active and passive membrane properties, and growth in size does not temporally coincide for both motoneuron populations. These findings are discussed on the basis of environmental cues related to maturation of the respiratory and OCM systems.