The postnatal development of intrinsic properties and spike encoding at cortical GABAergic neurons

GABAergic neurons play a critical role in maintaining the homeostasis of brain functions for well-organized behaviors. It is not known about the dynamical change in signal encoding at these neurons during postnatal development. We investigated this issue at GFP-labeled GABAergic neurons by whole-cell recording in cortical slices of mice. Our results show that the ability of spike encoding at GABAergic neurons is improved during postnatal development. This change is associated with the reduction of refractory periods and threshold potentials of sequential spikes, as well as the improvement of linear correlations between intrinsic properties and spike capacity. Therefore, the postnatal maturation of the spike encoding capacity at GABAergic neurons will stabilize the excitatory state of cerebral cortex.     

Neural modeling of intrinsic and spike-discharge properties of cochlear nucleus neurons

The purpose of this study was to develop neurobiologically plausible models to account for the response properties of several types of cochlear nucleus neurons. Three cell types--the bushy cells, stellate cells, and fusiform cells--were selected because useful data from intracellular recordings were available for these cell types, and because these three cell types exhibit distinct contrasts in their neuronal signal coding strategies. Stellate cells have primarily linear current-voltage (I-V) characteristics, but both bushy and fusiform cells have highly non-linear I-V characteristics. In light of this, we hypothesize that some of these cells have non-linear voltage-dependent conductances which alter their response properties. We modeled the bushy cell membrane conductance as an exponentially increasing function of membrane voltage, that of the fusiform cell as an exponentially decreasing function of the voltage, and that of the stellate cell as being voltage-independent. We have combined the voltage-dependent non-linear conductances of the cell membrane with a simple R-C circuit type of neuron model. These models reproduced the salient features of the experimentally observed I-V characteristics of the cells. In addition, we found that the models reproduced the spike discharge behavior to intracellularly injected current steps. Moreover, a more detailed study of stellate cell 'chopper'-type response patterns yielded hypotheses regarding the nature of the current that must exist at the soma during a pure-tone stimulus in order for the cells to exhibit various chopper subtype patterns, such as chop-S, chop-T, and Oc. The chop-S pattern requires a steady average current level with a relatively small variability during the tone-burst stimulus. The chop-T pattern, in contrast, requires that the current become more irregular during the tone-burst stimulus. The Oc pattern arises, however, when the input is similar to the chop-T case but the intrinsic properties of the cell model have been changed to increase the accommodation of the threshold. The implications of these findings for circuitry in the cochlear nucleus are discussed. Our analysis of these models revealed that this approach can be used to simulate neuronal cell types where I-V characteristics are known but more detailed ion channel data are not known.     

Phenotypic properties of adult mouse intrinsic cardiac neurons maintained in culture

Intrinsic cardiac neurons are core elements of a complex neural network that serves as an important integrative center for regulation of cardiac function. Although mouse models are used frequently in cardiovascular research, very little is known about mouse intrinsic cardiac neurons. Accordingly, we have dissociated neurons from adult mouse heart, maintained these cells in culture, and defined their basic phenotypic properties. Neurons in culture were primarily unipolar, and 89% had prominent neurite outgrowth after 3 days (longest neurite length of 258 +/- 20 microm, n = 140). Many neurites formed close appositions with other neurons and nonneuronal cells. Neurite outgrowth was drastically reduced when neurons were kept in culture with a majority of nonneural cells eliminated. This finding suggests that nonneuronal cells release molecules that support neurite outgrowth. All neurons in coculture showed immunoreactivity for a full complement of cholinergic markers, but about 21% also stained for tyrosine hydroxylase, as observed previously in sections of intrinsic cardiac ganglia from mice and humans. Whole cell patch-clamp recordings demonstrated that these neurons have voltage-activated sodium current that is blocked by tetrodotoxin and that neurons exhibit phasic or accommodating patterns of action potential firing during a depolarizing current pulse. Several neurons exhibited a fast inward current mediated by nicotinic ACh receptors. Collectively, this work shows that neurons from adult mouse heart can be maintained in culture and exhibit appropriate phenotypic properties. Accordingly, these cultures provide a viable model for evaluating the physiology, pharmacology, and trophic factor sensitivity of adult mouse cardiac parasympathetic neurons.     

The regenerative response of single mature muscle fibers isolated in vitro.

Segments of individual differentiated muscle fibers, ranging from 1–10 mm in length, were dissected from the pectoral muscle of juvenile Japanese quail and cultured for periods ranging up to 2–3 weeks under conditions known to promote the proliferation and eventual differentiation of embryonic myoblasts. Approximately 22% of such fibers give rise to a colony of bipolar cells which expands in area and cell number. Sometime during the second or third week the process of myoblast fusion is initiated and a network of long cross-striated multinuclear cells is formed.In the vast majority of fibers the colony arises from only one highly localized site along the fiber suggesting that proliferative competence is restricted to extremely few fiber nuclei. This same conclusion is suggested, as well, by the observation that fiber degeneration, in terms of the loss of intact nuclei, occurs rapidly and is completed within 24 hr after explantation (and perhaps as soon as 4–8 hr). Those nuclei which survive (usually one per fiber) are found to be contained within separate bipolar cells closely applied to the fiber.An examination of the fine structure of these surviving cells shows them to be separate, mononucleated cells contained within the basement lamina of the degenerating fiber. These cells are identical, on the basis of their ultrastructure as well as their location, to satellite cells associated with muscle fibers in the source tissue.     

Brimonidine promotes axon growth after optic nerve injury through Erk phosphorylation

It is well known that axons of the adult mammalian central nervous system have a very limited ability to regenerate after injury. Therefore, the neurodegenerative process of glaucoma results in irreversible functional deficits, such as blindness. Brimonidine (BMD) is an alpha2-adrenergic receptor agonist that is used commonly to lower intraocular pressure in glaucoma. Although it has been suggested that BMD has neuroprotective effects, the underlying mechanism remains unknown. In this study, we explored the molecular mechanism underlying the neuroprotective effect of BMD in an optic nerve injury (ONI) model. BMD treatment promoted optic nerve regeneration by inducing Erk1/2 phosphorylation after ONI. In addition, an Erk1/2 antagonist suppressed BMD-mediated axonal regeneration. A gene expression analysis revealed that the expression of the neurotrophin receptor gene p75 was increased and that the expression of the tropomyosin receptor kinase B (TrkB) gene was decreased after ONI. BMD treatment abrogated the changes in the expression of these genes. These results indicate that BMD promotes optic nerve regeneration via the activation of Erk1/2.     

Long-term transplantation of mature sensory neurons

The spinal ganglia were transplanted into the mesocolon of adult cats for periods of from 1 day to 9 months. About 30% of differentiated sensory neurons survived during the longterm transplantation. The intensive regeneration of the sensory neurons processes was characteristic of the transplanted neurons. Total myelinization of the regenerating nerves occurred during the 3rd--5th month. Potential regeneration capacity of the differentiated neurons and possibly of their prolonged transplantation were revealed.     

From snails to sciatic nerve: Retrograde injury signaling from axon to soma in lesioned neurons

The cell body of a lesioned neuron must receive accurate and timely information on the site and extent of axonal damage, in order to mount an appropriate response. Specific mechanisms must therefore exist to transmit such information along the length of the axon from the lesion site to the cell body. Three distinct types of signals have been postulated to underlie this process, starting with injury-induced discharge of axon potentials, and continuing with two distinct types of retrogradely transported macromolecular signals. The latter include, on the one hand, an interruption of the normal supply of retrogradely transported trophic factors from the target; and on the other hand activated proteins emanating from the injury site. These activated proteins are termed "positive injury signals", and are thought to be endogenous axoplasmic proteins that undergo post-translational modifications at the lesion site upon axotomy, which then target them to the retrograde transport system for trafficking to the cell body. Here, we summarize the work to date supporting the positive retrograde injury signal hypothesis, and provide some new and emerging proteomic data on the system. We propose that the retrograde positive injury signals form part of a complex that is assembled by a combination of different processes, including post-translational modifications such as phosphorylation, regulated and transient proteolysis, and local axonal protein synthesis.     

The regenerative effects of electromagnetic field on spinal cord injury

Traumatic spinal cord injury (SCI) is typically the result of direct mechanical impact to the spine, leading to fracture and/or dislocation of the vertebrae along with damage to the surrounding soft tissues. Injury to the spinal cord results in disruption of axonal transmission of signals. This primary trauma causes secondary injuries that produce immunological responses such as neuroinflammation, which perpetuates neurodegeneration and cytotoxicity within the injured spinal cord. To date there is no FDA-approved pharmacological agent to prevent the development of secondary SCI and induce regenerative processes aimed at healing the spinal cord and restoring neurological function. An alternative method to electrically activate spinal circuits is the application of a noninvasive electromagnetic field (EMF) over intact vertebrae. The EMF method of modulating molecular signaling of inflammatory cells emitted in the extra-low frequency range of <100 Hz, and field strengths of <5 mT, has been reported to decrease inflammatory markers in macrophages, and increase endogenous mesenchymal stem cell (MSC) proliferation and differentiation rates. EMF has been reported to promote osteogenesis by improving the effects of osteogenic media, and increasing the proliferation of osteoblasts, while inhibiting osteoclast formation and increasing bone matrix in vitro. EMF has also been shown to increase chondrogenic markers and collagen and induce neural differentiation, while increasing cell viability by over 50%. As advances are made in stem cell technologies, stabilizing the cell line after differentiation is crucial to SCI repair. Once cell-seeded scaffolds are implanted, EMF may be applied outside the wound for potential continued adjunct treatment during recovery.     

Recurrent axon collaterals of lumbar motor neurons in turtles

A combined morphophysiological study was made of connections between motoneurons on the superfused isolated lumbar spinal cord of Testudo horsfieldi. Postsynaptic potentials of motoneurons, followed by antidromic stimulations of ventral root filaments (VR-PSPs), were recorded intracellularly. Depolarizing VP-PSPs had short latencies (1.0-1.5 mc) and amplitudes in the range of 0.3-3.0 mV. At the constant stimulus intensity, the fluctuations of amplitudes were recorded. In some motoneurons, hyperpolarizing VP-PSRs with the latencies 2.5-3.0 mc were observed. A possible structural basis of VR-PSPs was studied by the horseradish peroxidase (HRP) method. After HRP application on thin ventral root filaments the retrograde staining of motoneurons revealed recurrent axon collaterals of labeled motoneurons. Three-dimensional computer reconstructions showed one to three collaterals given off by motoneuron axons. There were up to 19 points of branching in a single collateral. In some cases the full length of collateral trees reached 4.0 mm. The collateral branches had up to 72 "en passant" and terminal axon swellings. The swellings (presumed contacting boutons) were distributed in the ventral and intermedial gray matter and in the ventromedial while matter and revealed on motoneurons and inerneurons. These data suggest the participation of the motor axon collaterals in the motoneuron--motoneuron communication in the turtle spinal cord whereas only dendro-dendritic contacts had been discussed earlier.     

RhoA drives actin compaction to restrict axon regeneration and astrocyte reactivity after CNS injury

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Identification of intrinsic determinants of midbrain dopamine neurons

The prospect of using cell replacement therapies has raised the key issue of whether elucidation of developmental pathways can facilitate the generation of therapeutically important cell types from stem cells. Here we show that the homeodomain proteins Lmx1a and Msx1 function as determinants of midbrain dopamine neurons, cells that degenerate in patients with Parkinson's disease. Lmx1a is sufficient and required to trigger dopamine cell differentiation. An early activity of Lmx1a is to induce the expression of Msx1, which complements Lmx1a by inducing the proneural protein Ngn2 and neuronal differentiation. Importantly, expression of Lmx1a in embryonic stem cells results in a robust generation of dopamine neurons with a "correct" midbrain identity. These data establish that Lmx1a and Msx1 are critical intrinsic dopamine-neuron determinants in vivo and suggest that they may be essential tools in cell replacement strategies in Parkinson's disease.