Donald S. Faber

Axotomy-induced alterations in the electrophysiological characteristics of neurons

The electrophysiological alterations provoked by axotomy have now been studied for almost half a century, in a number of different cell types. Consequently, it is now possible to detail some common mechanisms underlying these changes and to sort out certain trends in the data. The major phenomena reviewed in this section and some possible future directions are summarized below. (1) It is now possible to advance a unified hypothesis for the effects of axotomy on the conduction velocity of myelinated fibers. The key is that axon diameter, which is directly correlated with conduction velocity, is regulated, at least in part, by neurofilament protein gene expression and transport into the axon. Thus, the largest myelinated axons, with the fastest conduction velocities, have the highest neurofilament contents, and in turn, experience greater or faster declines in neurofilament content, axon caliber, and conduction speed following nerve injury. This regulation of neurofilament gene expression also appears to be target- and/or accessory cell-dependent. In fact, Hoffman and colleagues (1988) have hypothesized that neuron interactions with specific targets (via as yet unknown target-induced signals) may either specify or permit specification of the level of neurofilament gene expression in neurons. Imposed on this primary size determinant is an influence of activity, which also underlies the differential atrophy and decrement in conduction velocity exhibited by motor and sensory fibers of comparable diameters in the same lesioned nerve. Unmyelinated axons, whose structures are not dominated by neurofilament content and metabolism, react very differently to axotomy. The structural and metabolic basis of their reaction is not known. (2) Passive membrane properties, in particular neuronal input resistance, remain relatively stable in the majority of neurons after axotomy. The major exceptions, vertebrate spinal motoneurons, lamprey dorsal interneurons, and mammalian vagal motoneurons, all show an increase in input resistance after axotomy. This change in input resistance appears to be correlated with structural or geometric simplification of dendritic trees and real or apparent changes in specific membrane resistance in one case and with a reduction in cell body size in the other two; however, changes in specific membrane resistance cannot be excluded even in the latter two cases. In the spinal motoneurons, input resistance changes may be more pronounced in those neurons with the most extensive or complex dendritic geometries (i.e. F-type motoneurons). More combined electrophysiological (ideally under voltage or patch clamp conditions) and morphological investigations of single neurons need be done to resolve these questions.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of optic tectal input to the ventral dendrite of the goldfish Mauthner cell

Although visually evoked Mauthner cell (M-cell) startle responses occur in the goldfish, the afferent projections underlying these reactions have not been previously studied. We have recorded from the M-cell while stimulating the left optic nerve and/or right optic tectum and have traced projections of the optic nerve and restricted areas of the optic tectum using HRP histochemistry and autoradiography. Tectal stimulation elicits similar postsynaptic potentials (PSPs) in both M-cells. The responses recorded in the right (ipsilateral) cell were localized to its ventral dendrite. The existence of uncrossed tectal projections to the ventral dendrite was confirmed morphologically following application of horseradish peroxidase (HRP) to the optic tectum. The PSPs contained both inhibitory and excitatory components, but with adequate stimulus strength, excitation of either M-cell dominated. Thus, this pathway is probably sufficient to trigger visually evoked startle responses mediated by the M-cell. Stimulation of the left optic nerve also evoked PSPs capable of bringing both M-cells to threshold. The blockage of this response by conditioning stimulation of the right tectum suggests that the visual information is relayed to the M-cells through this structure. In support of these findings, no label was found near any portion of the M-cell after either intraocular injection of tritiated proline or application of HRP to the cut end of the optic nerve. In summary, visual input to the M-cell is mediated via projections from the tectum, is segregated onto the ventral dendrite, and is capable of bringing this neuron to threshold. This pathway presumably accounts for the demonstrated behavioral efficacy of visual stimuli in evoking a startle response.

Glutamate stimulates release of Ca2+ from internal stores in astroglia ☆

We report here that, in the absence of external Ca2+, glutamate produces a transient increase in intracellular Ca2+ [( Ca2+]i) in cultured spinal cord astrocytes, by mobilizing internal Ca2+ stores. [Ca2+]i was measured with Fura-2 using the ratiometric method. Astrocytes were identified by immunostaining for glial fibrillary acidic protein. Glutamate and quisqualate consistently caused a transient increase in [Ca2+]i, with both the response amplitude and latency being graded functions of agonist concentration. Some cells produced multiple transients. N-Methyl-D-aspartate, kainate and aspartate were ineffective. We conclude that the quisqualate subtype of glutamate receptors mediates release of Ca2+ from internal stores in mammalian astrocytes.

Spinal inputs to the ventral dendrite of the teleost Mauthner cell

Ascending excitatory inputs from the periphery to the ventral dendrite of the goldfish Mauthner (M)-cell are characterized in this report. Direct stimulation of the spinal cord, at strengths suprathreshold for antidromic activation of the M-axon, evoked a graded excitatory postsynaptic potential (EPSP) in the distal ventral dendrite of the cell. This localization was demonstrated by multiple intracellular recordings from the soma and dendritic loci. The EPSP had a relatively long latency (mean = 3.6 ms) and contained multiple components. Furthermore, the EPSP amplitudes were extremely sensitive to frequency, being reduced by more than 50% at frequencies of 1-2 Hz and maximal with interstimulus intervals of 30-60 s. The spinal input is, therefore, likely to be mediated by a polysynaptic pathway. Direct stimulation of the skin surface evoked similar EPSPs, in terms of latency, wave form, graded nature, frequency dependence and spatial distribution on the M-cell ventral dendrite. Thus, the spinal cord and skin inputs probably relay somatosensory information from the trunk to the M-cell ventral dendrite. This notion was further confirmed by an interaction study of the EPSPs evoked from the two sites. We also report that the ventral dendrite does not support active spike electrogenesis, as indicated by the spatial profile of the M-cell antidromic impulse amplitude.

Characteristics of glycine-activated conductances in cultured medullary neurons from embryonic rat.

Characteristics of glycine-activated currents in 10- to 20-day-old neurons were studied using the gigaseal whole-cell technique. Glycine activated a Cl- conductance that was blocked by strychnine acting as a mixed inhibitor, effecting both Imax and Kd. Glycine receptors with at least two different sensitivities to strychnine were found, based on the apparent inhibition constants (Ki). These two Ki values were associated with different Hill coefficients for the glycine responses. Cs+ activated the same receptor and also activated a relatively nonselective cation conductance.

Developmental changes in the regulation of glycine-activated Cl− channels of cultured rat medullary neurons ☆

Glycine-activated currents in 1- to 11-day-old rat medullary neurons were studied using patch clamp techniques. Glycine produced neither repeatable whole-cell current responses nor single-channel activity in the cell-attached mode until cells were in culture for a week or more. However, Cl- channels were present at the early stages because glycine-activated channels were seen in excised, inside-out patches. Furthermore, for cells less than a week in culture, 10 patches which did not exhibit glycine-activated Cl- channels in the cell-attached mode did upon excision. Consequently, the activation properties of these Cl- channels undergo a developmental change in that some cellular factor(s) presumably prevents the Cl- channels from opening in the intact cell during the initial stages in culture.