A technique for the primary dissociation of neurons from restricted regions of the vertebrate CNS

Releasing the peri-neuronal net to patch-clamp neurons in adult CNS

The extracellular matrix of adult neural tissue contains chondroitin sulphated proteogylcans that form a dense peri-neuronal net surrounding the cell body and proximal dendrites of many neuronal classes. Development of the peri-neuronal net beyond approximately postnatal day 17 obscures visualization and often access by patch electrodes to neuronal membranes with the result that patch clamp recordings are most readily obtained from early postnatal animals. We describe a technique in which the surface tension of a sucrose-based medium promotes partial dissociation of thin tissue slices from adult tissue. Surface tension spreads the tissue and loosens the peri-neuronal net from neuronal membranes within minutes and in the absence of proteolytic enzymes. Furthermore, the extent of dissociation can be controlled so as to maintain the overall slice structure and allow identification of specific cell classes. Excellent structural preservation of neurons and dendrites can be obtained and full access by patch electrodes made possible for current- or voltage-clamp recordings in tissue well beyond the development of peri-neuronal nets. We demonstrate the feasibility of using this approach through patch recordings from neurons in the brainstem and cerebellum of adult gymnotiform fish and in deep cerebellar nuclei of rats as old as 6 months.

Precision of the pacemaker nucleus in a weakly electric fish: network versus cellular influences

We investigated the relative influence of cellular and network properties on the extreme spike timing precision observed in the medullary pacemaker nucleus (Pn) of the weakly electric fish Apteronotus leptorhynchus. Of all known biological rhythms, the electric organ discharge of this and related species is the most temporally precise, with a coefficient of variation (CV = standard deviation/mean period) of 2 x 10(-4) and standard deviation (SD) of 0.12-1.0 micros. The timing of the electric organ discharge is commanded by neurons of the Pn, individual cells of which we show in an in vitro preparation to have only a slightly lesser degree of precision. Among the 100-150 Pn neurons, dye injection into a pacemaker cell resulted in dye coupling in one to five other pacemaker cells and one to three relay cells, consistent with previous results. Relay cell fills, however, showed profuse dendrites and contacts never seen before: relay cell dendrites dye-coupled to one to seven pacemaker and one to seven relay cells. Moderate (0.1-10 nA) intracellular current injection had no effect on a neuron's spiking period, and only slightly modulated its spike amplitude, but could reset the spike phase. In contrast, massive hyperpolarizing current injections (15-25 nA) could force the cell to skip spikes. The relative timing of subthreshold and full spikes suggested that at least some pacemaker cells are likely to be intrinsic oscillators. The relative amplitudes of the subthreshold and full spikes gave a lower bound to the gap junctional coupling coefficient of 0.01-0.08. Three drugs, called gap junction blockers for their mode of action in other preparations, caused immediate and substantial reduction in frequency, altered the phase lag between pairs of neurons, and later caused the spike amplitude to drop, without altering the spike timing precision. Thus we conclude that the high precision of the normal Pn rhythm does not require maximal gap junction conductances between neurons that have ordinary cellular precision. Rather, the spiking precision can be explained as an intrinsic cellular property while the gap junctions act to frequency- and phase-lock the network oscillations.

N-methyl-D-aspartate receptor 1 mRNA distribution in the central nervous system of the weakly electric fish Apteronotus leptorhynchus

We have isolated a partial cDNA for the N-methyl-D-aspartate (NMDA) receptor 1 (NMDAR1) subunit from an Apteronotus leptorhynchus brain cDNA library. The A. leptorhynchus cDNA fragment, which corresponds to nucleotides 135-903 within the 5' region of the rat NR1 mRNA, encodes 252 amino acids that are >80% identical to the homologous segments of the rat, human, and duck NR1 proteins. RNAse protection assays revealed that the A. leptorhynchus NR1 mRNA was highly enriched in the forebrain and hypothalamus, with lesser amounts in the brainstem, and very low levels in the cerebellum. In situ hybridization also demonstrated that neurons in the pallial forebrain were highly enriched in NR1 transcripts. High levels of NR1 mRNA were found in pyramidal cells within the optic tectum and octavolateral regions. Pyramidal cells of the electrosensory lateral line lobe had the highest levels of expression, and the NR1 mRNA was found to be selectively enriched in their apical dendrites.

A Streptomyces fradiae protease dissociates structurally preserved neurons and glial cells from the embryonic and adult central nervous system of vertebrates

Nerve cell dissociation has become a key procedural tool in the implementation of a number of techniques in cellular and molecular neurobiology. We report that a protease preparation from Streptomyces fradiae (henceforth SF-protease) dissociates viable and morphologically identifiable embryonic and mature neurons and glial cells from the central nervous system of chick and rat, when used under strictly controlled conditions. Typical dendritic and axonal growth cones, with their lamellipodia and filopodia, are seen in many neuroblast types - growth cones in the case of embryonic glial cells and even the thinnest processes of some cells, such as the microvilli of adult chick retinal Müller (glial) cells, or the cilia of photoreceptors appear intact. Our results suggest that the SF-protease releases cells from tissue in a way that ensures the continuity of the plasma membrane and cuts through the transmembrane attachment systems (either cell-cell or cell-extracellular matrix) without compromising the cytoskeletal integrity underlying native cell shape.

Ligand-gated ion channels in acutely dissociated rat hippocampal neurons with long dendrites

A technique for dissociation of hippocampi of 3-25-day-old rats is described by which pyramidal and bipolar neurons with many long (up to 200 microns) dendrites can be obtained. Dissociation of CA1 neurons was achieved by mechanical means, in the absence of Ca2+, and without the use of proteolytic enzymes. The functional properties of the dissociated neurons were assessed using the whole-cell patch-clamp technique. Whole-cell currents were elicited by U-tube application of the agonists N-methyl-D-aspartate (NMDA), gamma-aminobutyric acid (GABA), and acetylcholine (ACh), and spontaneous miniature currents were also observed in these neurons. ACh-elicited currents were blocked by methyllycaconitine (MLA, 1 nM) and Pb2+ (0.1-10 microM). These results establish acutely dissociated neurons as a simple and reliable preparation for the study of the pharmacology, kinetics and subcellular distribution of ligand-gated ion channels.

Acutely isolated and cultured cells from the electrosensory lateral line lobe of a gymnotiform teleost

The present study established the morphological and immunocytochemical criteria necessary to identify neuronal and nonneuronal cells after dissociating select regions of the medullary electrosensory lateral line lobe of adult weakly electric fish (Apteronotus leptorhynchus). Cells dissociated from the pyramidal cell body layers of the centromedial and lateral segments exhibited similar characteristics in the acutely dissociated preparation and up to 14 days in culture. Basilar and nonbasilar pyramidal cells were tentatively identified according to a bipolar or monopolar process extension, and polymorphic cells by the extension of three or more processes and positive immunoreactivity for gamma-aminobutyric acid. Nonneuronal cells were identified by the pattern of process arborization and positive immunolabel for gamma-aminobutyric acid or glial fibrillary acidic protein. Neuronal cells increased in total number over the first 4 days and could appear for the first time on any day in culture. Individual pyramidal cells could maintain their morphology from the time of dissociation and over several days in culture. Pyramidal cell processes were phenotypically similar to apical and basal dendrites found in situ but were reduced in size and in the degree of process branching. These results indicate that dissociated adult apteronotid neurons can maintain a morphology sufficiently similar to that found in situ as to allow tentative identification, opening up a wide range of possibilities for studying the electrophysiological and regenerative properties of electrosensory neurons.