The organization of intrinsic hippocampal connections in explants of rat hippocampus studied by topical application of HRP crystals

A simple method to microinject solid neural tracers into deep structures of the brain

We have developed an instrument to perform microinjections of solid neural tracers into deep structures of the brain. The instrument consists of a thin hypodermic needle equipped with a movable internal rod, which is connected to a pressure chamber. When a pressure pulse is applied to the chamber, the rod moves forward and back inside the needle, pushing out a solid load previously packed inside the needle tip. By attaching a microelectrode to the instrument, it is also possible to have electrophysiological control of the injection placement. To test the instrument, we microinjected DiI and rhodamine crystals into selected structures of the visual system of pigeons. The results show small, well-defined injection sites, accurately located in the desired targets, together with well-developed anterogade and retrograde transport, selectively originated from the injection sites. This method extends the usage of solid tracers to most structures in the brain and may, in certain cases, be more advantageous than the conventional method of injecting tracer solutions.

Sprouting of CA3 pyramidal neurons to the dentate gyrus in rat hippocampal organotypic cultures

The understanding of the mechanisms of a functional synaptic plasticity in the hippocampus has been expanded greatly by the use of in vitro slice preparations. The question addressed in the present study was whether morphological plasticity observed in vivo can also be reproduced in hippocampal slices. In vivo, hippocampal commissural and association fibers are known to sprout and occupy synaptic sites vacated by deafferentation of the dentate gyrus (DG). In hippocampal slice preparations, the major input to the DG is eliminated, so that the DG is deafferented. Might intrinsic neurons sprout to the DG if the slice preparation is maintained for weeks? In this study hippocampal slices obtained from 6-day-old rats were cultured. Stimulation of the dentate stratum moleculare produced antidromic field potentials in the CA3 of the slices cultivated for more than 1 week. The antidromic response was not observed in CA1 pyramidal neurons. The CA3 to DG projection response was also observed in a CA3 mini-slice placed near a co-cultured whole hippocampal slice, when the DG in the latter was stimulated. Moreover, stimulation of the CA3 mini-slice induced synaptic responses in the DG of the whole-slice. The conclusion drawn is that deafferentation could induce axonal sprouting in a neuron-specific manner in hippocampal organotypic culture. This preparation would be potentially useful for the screening of chemical factors that influence sprouting of central neurons.

A developmental study of lactate dehydrogenase isozyme and aspartate aminotransferase activity in organotypic rat hippocampal slice cultures and primary cultures of mouse neocortical and cerebellar neurons

The development of enzyme activity and isozyme distribution of lactate dehydrogenase (LDH) was studied in murine organotypic hippocampal slice cultures and dissociated cultures of neocortical neurons and cerebellar granule cells and compared with that of the respective brain regions in vivo. In the hippocampal slice cultures and the hippocampus in vivo, the activity of aspartate aminotransferase (AAT) was also measured. During development in culture the specific activity of LDH increased in all types of cultures reaching values similar to that found in the corresponding brain areas in vivo. However, significant differences in the isozyme distribution were observed between the preparations in vitro and in vivo. During development in vivo, the LDH isozyme pattern changed from a preferential M-subunit composition to a preferential H-subunit composition regardless of the brain area. This shift was not observed in the respective cultures where the M4-isozyme prevailed at all culture periods examined accounting for 30-45% of the total LDH activity. The cultured cerebellar granule cells did not express the H4-isozyme at all, while in the hippocampal slice cultures and the cultured neo-cortical neurons this isozyme accounted for about 5% of the total LDH activity. The activity of AAT in the hippocampal organotypic slice cultures increased considerably during the culture period in parallel with the increase in AAT activity during postnatal development of hippocampus in vivo. The activity of AAT in the slice cultures was, however, consistently lower than the corresponding activity in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Biocytin pellets: an alternative technique for massive anterograde labeling of neuronal pathways in vivo and in vitro

The present study demonstrates that biocytin suspended in pellets of either coagulated chicken plasma (plasmaclot) or gelatine, produces intense anterograde axonal and terminal labeling and dense retrograde Golgi like labeling of neurons when injected into the brain parenchyma of young adult rats. The technique worked perfectly on hippocampal pathways like the mossy fiber system, the hilodentate associational and commissural fiber systems, CA3 Schaffer collaterals, the entorhinal perforant path to fascia dentata and hippocampus, as well as frontal motor cortical efferent and afferent fiber tracts. This pellet tracer delivery technique also proved very useful when applied on hippocampal slice cultures, where small pellets of plasmaclot embedded biocytin resulted in very discrete uptake sites with dense labeling of small groups of neurons and their projections.

Somatostatin and neuropeptide Y in organotypic slice cultures of the rat hippocampus: An immunocytochemical and in situ hybridization study

The neuronal distributions of somatostatin and neuropeptide Y and their respective mRNAs in hippocampal slice cultures were examined by immunohistochemical staining and in situ hybridization. For the in situ hybridization we used an alkaline phosphatase-labelled oligodeoxynucleotide probe for somatostatin mRNA and an 35S-labelled oligodeoxynucleotide probe for neuropeptide Y mRNA. For both neuropeptides the immunostained and hybridized neurons displayed a comparable, organotypic distribution. Most labelled neurons were located in the dentate hilus and stratum oriens of CA3 and CA1. Additional neurons were found in stratum radiatum and pyramidale of CA3, but very few in the corresponding layers of CA1. In all locations the density of somatostatin- and neuropeptide Y-reactive cells exceeded that observed in vivo. Also, the hybridization signal of the individual neurons appeared enhanced in the slice cultures. Methodologically it was noted that the non-radioactive alkaline phosphatase-labelled oligodeoxynucleotide probe gave excellent in situ hybridization results with detailed cellular resolution and no apparent problems of tissue penetration, even when used on whole-mount explants. These results demonstrate that somatostatin and neuropeptide Y-immunoreactive and mRNA containing neurons retain their organotypic distribution and basic morphological characteristics in the slice cultures. The supernormal density of these neurons and their hybridization signals indicate that a transient developmental increase in neuropeptide expression may persist in vitro.

Neuroactive amino acids in organotypic slice cultures of the rat hippocampus: An immunocytochemical study of the distribution of GABA, glutamate, glutamine and taurine

Antisera raised against protein-glutaraldehyde-amino acid conjugates were used to study the light and electron microscopic distribution of GABA, glutamate, glutamine and taurine in organotypic slice cultures of rat hippocampi. In the stratum oriens and radiatum, glutamate-like immunoreactivity was particularly concentrated in nerve endings establishing asymmetric junctions with dendritic spines. Mossy fiber terminals in CA3 and the dentate hilus were also strongly labeled. A quantitative immunogold analysis of the glutamate-immunolabelled profiles showed a pattern that was highly reminiscent of that previously observed in perfusion-fixed hippocampi, including a correspondingly sparse labeling of glial processes and of presynaptic elements in symmetric synapses. GABA-like immunoreactivity was localized predominantly in interneurons and in presynaptic terminals contacting dendritic shafts and neuronal cell bodies, while immunoreactivities for glutamine and taurine were found mainly in astroglial cells and pyramidal cells, respectively. Our data indicate that the major intrinsic fiber systems of the cultured hippocampi have retained their normal transmitter phenotypes.

An Organotypic Spinal Cord - Dorsal Root Ganglion - Skeletal Muscle Coculture of Embryonic Rat. I. The Morphological Correlates of the Spinal Reflex Arc

The cytoarchitecture of a spinal cord - dorsal root ganglion - skeletal muscle tissue coculture system was investigated at the level of the light microscope using a number of different staining techniques. In these cultures central synapses between dorsal root ganglion (DRG) cells and interneurons in the ventral spinal cord and between DRG cells and motoneurons were visualized by parvalbumin immunostaining and by intracellular horseradish peroxidase (HRP) filling of DRG cells. Skeletal muscle fibres regenerated in vitro first into multinucleated myotubes, and around day 8 in vitro into well differentiated muscle fibres with regular cross-striation. At the same time newly formed motor endplates could be visualized using acetylcholinesterase staining. The axons of motoneurons could be traced retrogradely by local application of HRP to the regenerated muscle fibres. The motor axons sometimes gave off collaterals reminiscent of Renshaw collaterals at about 300 microm from the axon hillock. Intracellular filling to motoneurons with HRP revealed that only a minority of the motoneurons within a culture had reached their appropriate target. Comparing the dendrograms of the motoneurons which had innervated muscles to those which had not suggested that motoneurons innervating muscle tissue had more complex dendritic trees and larger somata than those which did not innervate muscle tissue. Peripheral neurites of parvalbumin-immunoreactive DRG cells coiling around regenerated muscle fibres could be demonstrated in these cultures. These probably correspond to that part of the sensory muscle spindle apparatus which developed in vivo. However, only a few of the several hundred DRG cells found in every culture were parvalbumin-immunoreactive, suggesting that the actual number of Ia and II afferents within the population of DRG cells in culture is very small. This study demonstrates that all the neural elements necessary for the segmental spinal reflexes develop and can be maintained for several weeks in vitro.

Cajal-Retzius cell ontogenesis and death in mouse brain visualized with horseradish peroxidase and electron microscopy

The ontogenetic development of Cajal-Retzius cells was studied in mouse by local application of horseradish peroxidase over the developing neocortex, revelation with 3,3'-diaminobenzidene and examination from horizontal thick sections. Cajal-Retzius cells were completely stained in Golgi-like fashion. The Cajal-Retzius cells were seen to be elongated spindle-shaped bipolar neurons with their main processes horizontally oriented. They were exclusively located in the first cortical layer and were connected to the cortex surface by the numerous vertical appendages. Except for these appendages, the Cajal-Retzius cells were two-dimensional, with an immature structure at their tips resembling a growth cone. Cajal-Retzius cell dendrites were up to 400-microns-long and reached their maximal length prenatally. Their axon and its collaterals were very fine and sometimes measured several millimetres. It followed a random but planar trajectory confined to the first layer. Healthy Cajal-Retzius bearing growth cones were seen until one week after birth when signs of Cajal-Retzius cell degeneration began to occur and intensified in the days that followed. Rough endoplasmic reticulum and Golgi complex swelling along with a progressive darkening of the Cajal-Retzius cells were revealed by electron microscopy, strongly suggesting that most Cajal-Retzius cells disappear from the first cortical layer. Usually neuronal death is the result of cell deafferentation following synapse retraction; however, this effect does not seem to apply to Cajal-Retzius cells engaged in the process of death since normal synaptic junctions were seen on them. No signs of the morphological transformation of Cajal-Retzius cells into persisting horizontal first layer cells were observed. The concept of dual origin of neocortex is discussed in light of the similar fate of Cajal-Retzius cells and subplate neurons which both are transient neurons.

A membrane glycoprotein associated with the limbic system mediates the formation of the septo-hippocampal pathway in vitro

The ability of a neuronal surface glycoprotein to mediate the formation of neuronal connections was tested in an explant culture system. A monoclonal antibody against the limbic system-associated membrane protein (LAMP) was used in co-cultures containing cholinergic neurons of the septum and their hippocampal target neurons. Antibody treatment had no effect on general axon outgrowth, but significantly diminished the ability of septal cholinergic axons to invade and collateralize in the hippocampus. The results suggest that factors regulating general axon outgrowth may be distinct from those regulating the patterns of outgrowth that define the formation of neural circuits.

A modified roller tube technique for organotypic cocultures of embryonic rat spinal cord, sensory ganglia and skeletal muscle

The roller tube technique as initially described in the literature in 1981, was modified in several aspects for the coexplantation of embryonic rat spinal cord with attached dorsal root ganglia and skeletal muscle from newborn rats. The high metabolic activity of this coculture system required a particular culturing protocol to stabilize pH and osmotic pressure. The appropriate adjustment of the partial pressure of carbon dioxide gas in the incubator proved to be essential for the control of the pH within narrow limits (7.3 +/- 0.1). The adjustment of the osmotic pressure of the medium (290-300 mOsm) improved the growth of the cultures considerably. Roller drum speed was set to 120 revolutions per hour for enhanced flattening of the culture. A simple rating system was used to evaluate neuronal and non-neuronal outgrowth under different modifications of the culture system. Furthermore, morphological and electrophysiological criteria were defined for evaluating individual neurons. The technique described insures the growth of long-term organotypic cocultures of spinal cord, sensory ganglia and skeletal muscle.

Calbindin-D 28K in hippocampal organotypic cultures

Slices of hippocampus from 6-day-old rats were cultured for 2-4 weeks using the roller-tube technique. The organization of these explants was studied by immunocytochemical labeling of calbindin-D 28K (CaBP 28K). The development of the CaBP 28K staining was very close to that of the rat hippocampus in vivo with only 3 subpopulations of labeled cells: granule cells and their mossy fibers, pyramidal cells in the subiculum-CA1 zone and interneurons scattered in strata oriens and radiatum.

Developmental and regeneration-associated regulation of the limbic system associated membrane protein in explant cultures of the rat brain

In the present study we have examined the topographic and temporal patterns of expression of the limbic system associated membrane protein by light and electron microscopic immunocytochemistry in organotypic cultures of the rat brain. The regional, cellular and subcellular distribution of staining in young cultures was similar to that in the intact brain of corresponding age. Since the tissue in vitro is isolated both from afferents and targets, short-term protein expression appears to be regulated by factors intrinsic to the neuron. In culture, the protein was present on the surface of neurons which are physiologically interconnected, such as neurons belonging to the septohippocampal system (cholinergic neurons in the septum and pyramidal and granule cells in the hippocampus). It was also present on the surface of axons and growth cones during process outgrowth. Thus, the limbic system associated membrane protein is expressed in an appropriate spatial and temporal pattern for mediating interactions between growing axons and their targets. The expression of the protein in culture showed some important differences as compared to the intact brain. With increasing age, there was an increasing scattering and disappearance of immunoreactivity in cultures fixed with paraformaldehyde/glutaraldehyde. The decreased immunoreactivity in aged cultures does not appear to reflect decreased protein synthesis, because unfixed and acetone-fixed explants continued to show immunostaining. Furthermore, dot-blot assays showed similar amounts of immunoreactivity in culture as in the intact brain of corresponding age. Thus, the age-dependent decrease of immunoreactivity may reflect altered insertion of the protein into the membrane or a modification of the epitope recognized by the antibody. There was a rapid increase (within 1 hour) of immunostaining on the surface of sprouting processes following mechanical lesion of mature, unstained axons. The altered distribution after tissue injury could be a means of ensuring specificity of connectivity during nerve fiber regeneration. On the basis of the reported findings, we suggest that system-specific membrane proteins, including the limbic system associated membrane protein, may mediate the formation of specific connections in the brain. Furthermore, we suggest that the reinnervation processes taking place after central nervous system injury may exhibit a similar molecular basis to the development of neural pathways.

Selective kainic acid lesions in cultured explants of rat hippocampus

The influence of the excitotoxin kainic acid (KA) on cultivated explants of rat hippocampus was investigated. Addition of 3 microM KA to the culture medium over 24-48 h induced a destruction of the pyramidal cells in the CA3 region, whereas the CA1 pyramidal cells and the granule cells were left undamaged. Higher concentrations (10-100 microM) of KA destroyed also the latter cell groups. The selectivity of the KA lesion at 3 microM was further indicated by the fact that the acetylcholinesterase-positive neurons in the hippocampus were not destroyed through KA administration and that the stereoisomer dihydrokainic acid was ineffective in inducing lesions. Application of tetrodotoxin did not protect the CA3 pyramidal cells from KA lesion, whereas gamma-glutamylaminomethylsulphonic acid (GAMS) only offered a very small, statistically not significant, protection. Baclofen protected the cultures slightly from KA lesions but not when added together with GAMS. Possible mechanisms responsible for the KA lesions in these cultures are discussed.