Enhanced afferent synaptic functions in fetal mouse spinal cord-sensory ganglion explants following NGF-induced ganglion hypertrophy

Organotypic brain slice cultures: A review

In vitro cell cultures are an important tool for obtaining insights into cellular processes in an isolated system and a supplement to in vivo animal experiments. While primary dissociated cultures permit a single homogeneous cell population to be studied, there is a clear need to explore the function of brain cells in a three-dimensional system where the main architecture of the cells is preserved. Thus, organotypic brain slice cultures have proven to be very useful in investigating cellular and molecular processes of the brain in vitro. This review summarizes (1) the historical development of organotypic brain slices focusing on the membrane technology, (2) methodological aspects regarding culturing procedures, age of donors or media, (3) whether the cholinergic neurons serve as a model of neurodegeneration in Alzheimer’s disease, (4) or the nigrostriatal dopaminergic neurons as a model of Parkinson’s disease and (5) how the vascular network can be studied, especially with regard to a synthetic blood–brain barrier. This review will also highlight some limits of the model and give an outlook on future applications.

Development of specific synaptic network functions in organotypic central nervous system (CNS) cultures: implications for transplantation of CNS neural cells in vivo

This article provides a broad overview of the significant roles that morphophysiologic analyses of organotypic cultures of neural tissues explanted in vitro-initiated during the 1950s-have played in stimulating the more recent development of techniques for transplantation of neural cells and tissues into specific regions of the central nervous system (CNS) in vivo. The demonstrations by Crain and co-workers in the 1950s and 1960s that fetal rodent and human CNS neurons can continue to develop a remarkable degree of mature structure and function during many months of complete isolation in culture provided crucial evidence that development of many organotypic properties of nerve cells is regulated by epigenetic factors that ensure rather stereotyped expression despite wide variations in environmental conditions. These in vitro studies strongly suggested that fetal neural cells should, indeed, be capable of even more highly organotypic development after transplantation in vivo, as has been elegantly demonstrated by many of the successful CNS transplantation studies reviewed here.

Modulatory effects of Gs-coupled excitatory opioid receptor functions on opioid analgesia, tolerance, and dependence

Electrophysiologic studies of opioid effects on nociceptive types of dorsal root ganglion (DRG) neurons in organotypic cultures have shown that morphine and most mu, delta, and kappa opioid agonists can elicit bimodal excitatory as well as inhibitory modulation of the action potential duration (APD) of these cells. Excitatory opioid effects have been shown to be mediated by opioid receptors that are coupled via Gs to cyclic AMP-dependent ionic conductances that prolong the APD, whereas inhibitory opioid effects are mediated by opioid receptors coupled via Gi/Go to ionic conductances that shorten the APD. Selective blockade of excitatory opioid receptor functions by low (ca. pM) concentrations of naloxone, naltrexone, etorphine and other specific agents markedly increases the inhibitory potency of morphine or other bimodally acting agonists and attenuates development of tolerance/dependence. These in vitro studies have been confirmed by tail-flick assays showing that acute co-treatment of mice with morphine plus ultra-low-dose naltrexone or etorphine remarkably enhances the antinociceptive potency of morphine whereas chronic co-treatment attenuates development of tolerance and naloxone-precipitated withdrawal-jumping symptoms.

Biphalin, an enkephalin analog with unexpectedly high antinociceptive potency and low dependence liability in vivo, selectively antagonizes excitatory opioid receptor functions of sensory neurons in culture

The mechanism of action of the dimeric enkephalin peptide, biphalin (Tyr-D-Ala-Gly-Phe-NH2)2, which was previously shown to have remarkable high antinociceptive potency and low dependence liability in vivo, has now been studied by electrophysiologic analyses of its effects on the action potential duration (APD) of nociceptive types of sensory dorsal root ganglion (DRG) neurons in culture. Acute application of biphalin (pM-microM) elicited only dose-dependent, naloxone-reversible inhibitory (APD-shortening) effects on DRG neurons. Furthermore, at pM concentrations that evoked little or no alteration of the APD of DRG neurons biphalin selectively antagonized excitatory (APD-prolonging) effects of low (fM-nM) concentrations of bimodally-acting mu and delta opioid agonists and unmasked potent inhibitory effects of these opioids. This dual opioid inhibitory-agonist/excitatory-antagonist property of biphalin is remarkably similar to that previously observed in studies of the ultra-potent opioid analgesic, etorphine on DRG neurons and in sharp contrast to the excitatory agonist action of most mu, delta and kappa opioid alkaloids and peptides when tested at low (pM-nM) concentrations. Chronic treatment of DRG neurons with high (microM) concentrations of biphalin did not result in supersensitivity to the excitatory effects of naloxone nor in tolerance to opioid inhibition effects, in contrast to the excitatory opioid supersensitivity and tolerance that develop in chronic morphine- or DADLE-treated, but not chronic etorphine-treated, neurons. These studies on DRG neurons in vitro may help to account for the unexpectedly high antinociceptive potency and low dependence liability of biphalin as well as etorphine in vivo.

Chronic morphine-treated sensory ganglion neurons remain supersensitive to the excitatory effects of naloxone for months after return to normal culture medium: an in vitro model of 'protracted opioid dependence'

Chronic morphine-treated dorsal-root ganglion (DRG) neurons in DRG/spinal cord explant cultures were previously shown to become supersensitive to the excitatory effects of remarkably low concentrations of the opioid agonists, morphine and dynorphin, and the opioid antagonist, naloxone. The present study demonstrates that this opioid excitatory supersensitivity of chronic morphine-treated DRG neurons (1 microM for > 1 week) is retained for periods > 3 months after return to control culture medium. Acute application of femtomolar dynorphin, as well as nanomolar naloxone, to the treated neurons after months in control medium evoked characteristic prolongation of the action potential duration (APD), as occurs in cells tested during or shortly after chronic opioid exposure. The threshold concentrations for eliciting these excitatory effects in naive DRG neurons are > 1000-fold higher. Furthermore, treatment of micromolar morphine-sensitized neurons with 1 nM etorphine (which is a potent excitatory opioid receptor antagonist) for I week prior to return to control medium blocked further expression of opioid excitatory supersensitivity when tested after an additional 1-7 weeks in culture. These results provide a unique in vitro model system for analyses of some of the cellular mechanisms underlying protracted opioid dependence in vivo.

Specific N- or C-terminus modified dynorphin and beta-endorphin peptides can selectively block excitatory opioid receptor functions in sensory neurons and unmask potent inhibitory effects of opioid agonists

We recently showed that the opioid alkaloids, etorphine, dihydroetorphine and diprenorphine, have remarkably potent antagonist actions on excitatory opioid receptor functions in mouse sensory dorsal root ganglion (DRG) neurons. Pretreatment of naive nociceptive types of neurons with pM concentrations of these antagonists blocks excitatory prolongation of the calcium-dependent component of the action potential duration (APD) elicited by pM-nM morphine or other bimodally acting mu, delta and kappa opioid agonists and unmasks inhibitory APD shortening which usually requires much higher (ca. microM) concentrations. The present study demonstrates that pM concentrations of [des-Tyr1] fragments of dynorphin and beta-endorphin, as well as beta-endorphin-(1-27), can also selectively block excitatory opioid receptor functions in DRG neurons and unmask potent inhibitory effects of low concentrations of bimodally acting mu, delta and kappa opioid peptides and alkaloid agonists. These N- or C-terminus modified dynorphin or beta-endorphin peptides can be readily formed in neurons by specific peptidase activities. Since sustained activation of excitatory opioid receptor functions is essential for the development of tolerance/dependence in chronic morphine-treated DRG neurons in culture, the present in vitro study may help to account for the unexplained efficacy of [des-Tyr1] dynorphin fragments, as well as the endogenous opioids dynorphin A and beta-endorphin, in suppressing development and expression of naloxone-precipitated withdrawal and morphine tolerance in vivo.

Effects of nerve growth factor on whole-cell currents and other electrical membrane properties in cultured dorsal root ganglion neurons from normal and trisomy 16 mice

The trisomy 16 mouse is considered to be a model of human trisomy 21 (Down syndrome). Dorsal root ganglia (DRG) from trisomy 16 and diploid control fetuses were cultured in the presence of nerve growth factor (NGF) for 10 days, after which NGF was withdrawn from 50% of the dishes. Withdrawing NGF at 10 days did not affect the survival rate of either trisomy 16 or control neurons. With or without NGF, trisomy 16 neurons had a significantly larger inward current (171%, 163%) and larger inward conductance (156%, 166%), a faster rate of depolarization (219%, 149%), and a shorter duration of the action potential (83%, 81%) than control neurons, indicating that these parameters are determined solely by the trisomic state. In the absence of NGF, the outward conductance was significantly larger (143%), and the rate of repolarization was faster (131%), in trisomy cells compared to controls. Withdrawing NGF resulted in a smaller outward conductance (86%) in control neurons and a larger outward conductance (132%) and faster rate of repolarization (118%) in trisomy neurons, indicating that these parameters are NGF-dependent, and that trisomy and control neurons exhibit a differential sensitivity to NGF. This is the first report of a differential sensitivity of trisomic and control neurons to NGF, and demonstrates significant abnormalities in active electrical membrane properties of trisomic DRG neurons.

Networks formed by dorsal root ganglion neurites within spinal cord explants: a computer-aided analysis of HRP intracellularly labeled neurons

Dorsal root ganglion (DRG) neurons from rat embryos were explanted either alone or with the attached spinal cord (SC). Neuritic processes were mapped out histologically using an intracellular iontophoretic HRP method. Computer reconstructions and morphometric parameters allowed a comparative quantitative analysis of the DRG neurons and of their neuritic processes in both models. The first model, strongly dependent on nerve growth factor (NGF) for its survival, developed large multidirectional processes. The second model showed a bipolar distribution of its neuritic processes, the central one entering predominantly the dorsolateral part of the cord explant and ramifying both homo- and heterolaterally. The quantitative data revealed a significant decrease in the overall size of the neuritic networks of the second model (with attached SC). A discriminant analysis permitted the recognition of these two populations of DRG neurons. The role of the spinal cord explant, and more precisely the target cells of the dorsal horn, was considered to be a prominent factor in the development of the DRG neuritic networks.

Human beta nerve growth factor obtained from a baculovirus expression system has potent in vitro and in vivo neurotrophic activity

A baculovirus expression vector, which contains the coding sequences for human prepro (beta) nerve growth factor under control of the viral polyhedrin promoter, was constructed. Upon infection of insect cells with the recombinant virus, mature human beta nerve growth factor (rhNGF) was released into the culture fluid. The mature rhNGF was biologically active since rat pheochromocytoma (PC12) and human neuroblastoma (SH-SY5Y) cells were induced to extend neurites upon treatment with this material. This activity was abolished by treating with antiserum prepared against mature mouse beta NGF (mNGF). When compared with mNGF, rhNGF more rapidly elicited the differentiation response in both PC12 and SH-SY5Y cells. In an in vivo assay of cholinergic cell survival, rhNGF was nearly as potent as mNGF in protecting cholinergic neurons from degeneration following a fimbria-fornix lesion. These results show that the baculovirus expression system provides quantities of biologically potent human beta NGF suitable for a comprehensive program of research to ascertain beta NGF's potential as a therapeutic agent for the treatment of Alzheimer's disease.

NGF-dependent sprouting and regeneration in the hippocampus

While a variety of sprouting and regenerative responses have been investigated in the hippocampus, the cellular and molecular events responsible for these plastic responses have not been determined. One transmitter system, the cholinergic system, shows several distinct responses to damage in the septohippocampal circuit. Present evidence strongly supports a role for nerve growth factor (NGF) in these responses. NGF is not only important for the survival of the adult cholinergic neurons, but can also induce regrowth of the damaged fibers given an appropriate substratum for growth. These reparative effects of NGF can manifest themselves in functional recovery in the aged rat and the young rat with fimbria-fornix lesions. Finally, a role for glia cells is proposed to clarify how NGF availability may be regulated during the degenerative and regenerative events. While all plasticity events certainly cannot be explained by the coincidence of NGF and the cholinergic system, their interaction may provide a template for other transmitter/trophic factor interactions.

Biological activity of a new neuronal growth factor from injured peripheral nerve

In response to transection injury, the distal nerve segment produces a soluble neurite promoting factor (SN). In this study, the ability to support neuronal survival and differentiation have been studied. Embryonic rat dorsal root ganglion (DRG) neurons were plated out on collagen substrates and incubated in medium containing either SN or nerve growth factor (NGF). The number of surviving neurons was counted after 1, 2, 4, 7, and 15 days in vitro. After fixation and staining, the diameter of the surviving neurons was measured. During the period of observation, 60.8 +/- 5.8% of plated neurons survived in the presence of NGF and 90.5 +/- 12.9% survived with SN (P less than 0.05). The mean of median neuronal cell diameter was 28 +/- 2.7 microns with NGF and 34.2 +/- 3.7 microns with SN, (P less than 0.01). This increased diameter was due to enhanced survival of 30-50 microns diameter neurons. In parallel experiments, the degree of myelination of DRG neurons by Schwann cells was assessed morphometrically. In the presence of SN there was an 86% increased in myelination compared with NGF which indicates that not only is the survival of neurons increased but they are able to become fully differentiated in the presence of SN.

Androgen increases the number of cells in fetal mouse spinal cord cultures: implications for motoneuron survival

Androgen effects were studied in organotypic cultures of the E12 fetal mouse lumbosacral spinal cord labeled in utero with [3H]thymidine on E10. Following continuous exposure to androgens for one month in vitro, the number of labeled cells was significantly increased in whole explants, and in hemisected segments in the presence or absence of co-cultured fetal thigh muscle. Because lumbosacral motoneurons undergo their final mitosis predominantly on E10 and thus remain permanently labeled, the results suggest that androgens increase neuronal numbers by directly modulating motoneuron survival rather than stimulating mitosis. These findings demonstrate for the first time that in addition to the well documented role of the muscle target in motoneuron survival, the direct neuronotrophic effects of androgen at the level of the spinal cord must also be considered.

Death of sensory ganglion neurons after acute withdrawal of nerve growth factor in dissociated cell cultures

The time course of dependence on nerve growth factor (NGF) for survival in sensory neurons in vitro was examined with microscopic and biochemical methods. Primary dorsal root ganglion (DRG) cultures from embryonic-day-15 (E-15) and day-19 (E-19) rats were maintained with standard dissociated cell culture techniques in the absence of most non-neuronal cells. After various times in culture, neurons were acutely deprived of neurotrophic support by changing to NGF-free medium and adding NGF antiserum to eliminate any residual NGF. Neuronal cultures were examined with phase microscopy; and, their metabolic activity was measured with a protein assay at various time points after NGF deprivation. E-15 neurons grown in culture for 5 days were exquisitely sensitive to acute NGF deprivation. By 12 h after NGF deprivation, neuronal morphology was severely disrupted and the majority of neurons appeared dead. E-15 neurons grown in culture for 8 or 11 days showed progressively less dependence on NGF for survival. These older neurons did not die until 24 and 48 h, respectively, following NGF withdrawal. Neurons grown in culture for 20 days did not show any morphologic changes by phase microscopy up to 4 days after NGF deprivation. Protein incorporation progressively decreased between 12 and 48 h after NGF withdrawal in E-15 neurons grown in culture for 5, 8, or 11 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Antisera to the ganglioside GM1 do not have anti-myelin or anti-axon activities in vitro

Four antisera to the ganglioside GM1 were tested for effects on myelin and axons when applied to mouse spinal cord-dorsal root ganglia explant cultures. None of the antisera to GM1 caused myelination inhibition or demyelination, while an antiserum to galactocerebroside caused both. Antisera to GM1 did not inhibit axonal outgrowth or destroy mature outgrowth zone axons, while an antiserum to a rat brain axolemma-enriched fraction did both. These results suggest that antibodies to GM1 do not have significant anti-myelin or anti-axon activity.

Opioids excite rather than inhibit sensory neurons after chronic opioid exposure of spinal cord-ganglion cultures

Tests were carried out to determine if the tolerance that develops in dorsal-horn network responses of mouse dorsal root ganglion (DRG)-spinal cord explants after chronic exposure to opioids could be accounted for by alterations in the excitability and pharmacologic properties of the afferent DRG cells. Intracellular recordings were made from DRG neurons in organotypic DRG-cord explants after chronic treatment with 1 microM D-Ala2-D-Leu5-enkephalin (DADLE) for greater than 4 days in vitro. Acute application of 10 microM DADLE shortened the duration of the Ca2+ component of the somatic action potential (APD) in only 5% of the treated neurons (4 out of 79 cells), in contrast to about 50% of the cells in naive explants (36 out of 74). Thus many DRG neuron perikarya became tolerant to the APD-shortening effects of DADLE. Furthermore, 77% of the treated DRG cells (61 out of 79) showed prolongation of the APD in response to an acute increase in DADLE concentration vs 34% in naive explants (25 out of 74). However, when the DADLE responsivity tests were carried out in the presence of multiple K+ channel blockers, only 20% of the treated DRG neurons showed APD prolongation (3 out of 15 cells), whereas 73% showed APD-shortening responses (11 out of 15 cells). The results suggest that: (1) DADLE-induced APD prolongation of the treated DRG neurons is mediated by opioid receptor subtypes that decrease a voltage-sensitive K+ conductance; (2) the DADLE-induced APD-shortening effects which are unmasked during more complete K+ channel blockade are mediated by opioid-receptor subtypes in the same neuron that reduce a voltage-sensitive Ca2+ conductance (resembling kappa receptors). DRG neurons did not become tolerant to either of these two opioid effects after chronic exposure to DADLE. Opioid shortening of the APD of DRG neuron perikarya has been generally accepted to be a model of opioid inhibition of calcium influx and transmitter release at presynaptic DRG terminals6,52,53,65,75,76. It is postulated that the opioid-induced APD prolongation observed in the present study provides evidence that opioids can also evoke direct excitatory effects on neurons. The enhancement of DADLE-induced excitatory responses and attenuation of DADLE-induced inhibitory responses of DRG neurons after chronic exposure to this opioid show striking similarities to the effects of forskolin or pertussis toxin treatment. These in vitro studies may provide clues to compensatory mechanisms underlying physiologic expression of tolerance to opioid analgesic effects in primary afferent synaptic networks.

Effects of opioid peptides and naloxone on nervous tissue in culture

It was shown that opioid peptides stimulate nervous tissue growth in culture in the rat, which manifests itself in augmented outgrowth of neurites from explants and in an increase in the number of glial and fibroblast-like cells in the growth zone. The effects of opioid peptides ([Leu]- and [Met]-enkephalins, beta- and gamma-endorphins and some synthetic analogues of [Leu]-enkephalin) on the growth of organotypic cultures of rat sympathetic and dorsal root ganglia and spinal cord were investigated. Neurite outgrowth, cell composition, and size of the growth zone as well as the dynamics of its formation were estimated. Changes in the survival of neurons in dorsal root ganglion cultures were determined. The experiments were performed with living cultures as well as with fixed preparations. In experiments with sympathetic ganglia, it was demonstrated that a significant growth-promoting effect is exerted by peptides taken at concentrations of 10(-8) M to 10(-14) M. Naloxone does not eliminate the effects of peptides, but stimulates the growth at 10(-5) M to 10(-7) M. Studies with spinal cord revealed that naloxone (10(-6) M) enhances the response to [Leu]-enkephalin (10(-9) M). The survival of dorsal root ganglion neurons under the influence of a [leu]-enkephalin analog (10(-9) M) exceeds control values by approximately two to four times. Thus, opioid peptides were shown to exert a strong growth-promoting effect on nervous tissue in culture. This effect is dual: in neurons the peptides stimulate the outgrowth of neurites and their survival, while in glial cells they change the rate of their migration and, probably, their proliferation. It is suggested that opioid peptides, besides their already established functions, may play a role in the development and regeneration of nervous tissue.

Nerve growth factor regulates the action potential duration of mature sensory neurons

The effect of nerve growth factor (NGF) on the action potential of sensory ganglion neurons was investigated in long-term organotypic cultures of embryonic mouse dorsal root ganglia grown isolated or attached to spinal cord explants. The present study demonstrates that NGF regulates a specific bioelectric property of these neurons--the duration of the Ca2+ component of the somatic action potential--at mature stages when they no longer require NGF for survival. Prolonged culture of fetal mouse dorsal root ganglion neurons with relatively low levels of NGF shortens the duration of the action potential. Furthermore, addition or withdrawal of NGF in mature cultures results, within several days, in shorter or longer action potential durations, respectively. Exposure to anti-NGF antiserum accelerates the onset of the longer-lasting action potentials elicited by simple withdrawal of NGF. This plastic response of sensory neurons to NGF may be important in regulating their physiological properties and/or their response to injury.

Chemical neurotoxicity: detection and analysis in organotypic cultures of sensory and motor systems

Screening of chemical substances for human neurotoxic (and therapeutic) properties may be carried out with the aid of organotypic tissue cultures composed of foetal explants of mouse sensory and neuromuscular tissues that develop in vitro their characteristic cytoarchitectural and functional organization. Supporting this statement is a wealth of studies describing a range of specific, chemically-induced responses in organotypic neural cultures that parallel changes induced in the nervous system of humans and animals.

Cyclic AMP or forskolin rapidly attenuates the depressant effects of opioids on sensory-evoked dorsal-horn responses in mouse spinal cord-ganglion explants

Exposure of fetal mouse spinal cord-ganglion explants to morphine (greater than 0.1 microM) results in naloxone-reversible, dose-dependent depression of sensory-evoked dorsal-horn synaptic-network responses within a few minutes. After chronic opiate exposure (1 microM) for 2-3 days, these dorsal cord responses recover and can then occur even in greater than 10 microM morphine. In the present study, when naive explants were treated with forskolin (10-50 microM)--a selective activate activator of cyclase (AC)--for 10-30 min prior to and during exposure to morphine (0.1-0.3 microM) or D-Ala2-D-Leu5-enkephalin (0.03-0.1 microM), the usual opioid depressant effects on dorsal-horn responses generally failed to occur (10-30 min tests). Dibutyryl cyclic AMP (10 microM) or the more lipid-soluble analog, dioctanoyl cyclic AMP (0.1 mM), produced a similar degree of subsensitivity to opiates as 10 microM forskolin. With high levels of forskolin (50 microM), even concentrations of morphine up to 1-10 microM were far less effective in depressing cord responses. These effects of exogenous cAMP analogs and forskolin on cord-ganglion explants are probably both mediated by increases in intracellular cAMP. The marked decrease in opioid sensitivity of cAMP or forskolin-treated cord-ganglion explants provides significant electrophysiologic data compatible with the hypothesis that neurons may develop tolerance and/or dependence during chronic opioid exposure by a compensatory enhancement of their AC/cAMP system following initial opioid depression of AC activity. Previous evidence relied primarily on behavioral tests and biochemical analyses of cell cultures. It will be of interest to determine if dorsal-horn tissues of cord-ganglion explants do, in fact, develop increased AC/cAMP levels as they express physiologic signs of tolerance during chronic exposure to opioids.

Maturation of opioid sensitivity of fetal mouse dorsal root ganglion neuron perikarya in organotypic cultures: regulation by spinal cord

Opioid agonists selectively decrease the duration of the Ca2+ component of the action potential recorded from embryonic dorsal root ganglion neurons in dissociated cell cultures. In contrast, no significant alterations in the action potentials generated by adult dorsal root ganglion neurons in vivo were detected during opioid exposure. In the present study, the perikaryal opioid sensitivity of fetal mouse dorsal root ganglion neurons was analyzed during maturation in organotypic explant cultures. To determine whether spinal cord might influence this sensitivity, neuron perikarya were tested in ganglia grown: (a) in isolation; (b) attached to spinal cord explants; and (c) attached to spinal cord, but decentralized by a dorsal root transection in mature explants 1-2 weeks before the tests. After 2-8 weeks in culture, the duration of the Ba2+-enhanced Ca2+ component of intracellularly recorded action potentials was measured prior to and during bath exposure to the opioid, [D-Ala2, D-Leu5]enkephalin. Sensitive neurons were characterized by a marked, reversible reduction (averaging about 50%) in the duration of the Ca2+ component (which was antagonized by naloxone). The fraction of opioid-sensitive neuron perikarya in dorsal root ganglia grown attached to cord explants was significantly lower (48%) than in ganglia grown isolated (78%) or decentralized in vitro (79%). The mean duration of the Ca2+ component was significantly shorter in ganglion cells which had been grown attached to cord, or subsequently decentralized, compared to cells grown in isolated ganglia (by 24 and 38%, respectively). This difference was even larger in the opioid-insensitive groups. Although opioid-sensitive perikarya in ganglia grown attached to cord had a significantly longer Ba2+-enhanced Ca2+ component than that of insensitive neurons, some of the insensitive perikarya in all 3 types of explant paradigms displayed Ca2+ components which were as prolonged as those of sensitive cells. The results obtained in this study support the hypothesis that the observed decrease in the fraction of opioid-sensitive perikarya during development of fetal mouse dorsal root ganglia is due to regulation by interactions with their central target tissue, the spinal cord. The developmental decrease in the duration of the Ca2+ component of the action potential of these ganglion cells is also enhanced by the presence of the spinal cord. However, regulation of functional opiate receptors and Ca2+ component duration of the ganglion cell perikarya appear to be independent processes.

Antisera to an axolemma-enriched fraction inhibit neurite outgrowth and destroy axons in vitro

Antisera prepared to an axolemma-enriched fraction derived from rat brain inhibited neurite outgrowth and destroyed mature axons in spinal cord-dorsal root ganglia cultures. Similar antibody-mediated anti-axon effects may be important in some diseases of the human nervous system.

Development of Met-enkephalin immunoreactivity in organotypic explants of fetal mouse spinal cord and attached dorsal root ganglia

Radioimmunoassays of methionine-enkephalin (Met-Enk) in organotypic cultures of 13-day fetal mouse spinal cord explants with attached dorsal root ganglia (DRG) demonstrate a progressive development of immunoreactivity (IR) during 5 weeks in vitro. Met-Enk IR in these cultures increased to levels observed in adult rodent spinal cord. most of the Met-Enk IR assays were made on cord explants excised from cord-DRG cultures. In smaller numbers of assays performed on entire DRG-cord cultures or on cord cultured in the absence of DRGs, similar levels of Met-Enk IR were obtained. Thus most of the Met-Enk IR appeared to be located within the cord tissue. No Met-Enk IR was detected in DRGs cultured in the absence of cord. In contrast, low levels of Met-Enk IR were present in about 50% of the assays of DRGs cultured attached to the cord. Since these assays included the neuritic outgrowths of the cultures, our data do not preclude possible contamination by Met-Enk immunoreactive cord neurites that may have aberrantly projected into the outgrowth zones. Nevertheless, the data raise the possibility of a trophic influence of cord tissue on the development of Met-Enk IR in DRG neurons. The development of Met-Enk IR in cord regions of cord-DRG explants extends previous binding assays demonstrating development of opiate receptors in these cultures and provides further support to electrophysiological analyses suggesting tonic opioid inhibitory networks in these explants.

Morphological alterations in dorsal root ganglion neurons and supporting cells of organotypic mouse spinal cord-ganglion cultures exposed to taxol

Explants of 14-day fetal mouse spinal cord with attached dorsal root ganglia, which had become differentiated over 2-3 weeks in culture, were exposed to 1-2 microM taxol for up to 6 days. The culture medium was supplemented with nerve growth factor (300 units/ml) during exposure to the drug. By 3-6 days in taxol, unusually numerous microtubules were seen in peripheral perikaryal and proximal neuritic regions of ganglion neurons. Microtubules also engirdled massive aggregations of pleomorphic vesicular/cisternal elements in many neurons. These aggregates were visible as unusual 'clear' spheroidal regions in the living cells, and were often as large as the nuclei. Some of the elements comprising these striking vesicular/cisternal accumulations appeared to be portions of disrupted Golgi complexes normally polarized around the cytocentrum, as well as hypertrophied smooth endoplasmic reticulum formations. In other neuronal areas, Golgi complexes and other organelles were altered or disrupted to lesser degrees. Ordered microtubular arrays occurred along endoplasmic reticulum cisternae both in neuron somata and neurites. Over time, a plethora of microtubules assembled throughout the perikarya in various orientations apparently unrelated to microtubule organizing centers. Unlike the effects of other plant alkaloids that interact with tubulin, there was no discernible increase in filaments, although their distribution appeared altered. Concentric ordered microtubular-macromolecular lamellated complexes were seen only in neurites. Neuronal nuclei were misshapen, often displaced, and displayed fine structure reminiscent of chromatolysis. Satellite and Schwann cells contained atypically abundant microtubules, abnormal cisternae, disrupted Golgi complexes, and increased lysosomes. Some nuclei displayed abnormal chromatin, and in rare cases even microtubules. We suggest that taxol alters the distribution, integrity, and/or organization of organelle systems in dorsal root ganglion cells by engendering unusually abundant microtubules in abnormal groupings and aberrant locations in these cells.

Nerve growth factor attenuates neurotoxic effects of taxol on spinal cord-ganglion explants from fetal mice

Most neurons in organotypic cultures of dorsal root ganglia from 13-day-old fetal mice require high concentrations of nerve growth factor for survival during the first week after explanation. These nerve growth factor-enhanced sensory neurons mature and innervate the dorsal regions of attached spinal cord tissue even after the removal of exogenous growth factor after 4 days. In cultures exposed for 4 days to nerve growth factor and taxol (a plant alkaloid that promotes the assembly of microtubules) and returned to medium without growth factor, greater than 95 percent of the ganglionic neurons degenerated and the spinal cord tissues were reduced almost to monolayers. In contrast, when the recovery medium was supplemented with nerve growth factor, the ganglionic neurons and dorsal (but not ventral) cord tissue survived remarkably well. Dorsal cord neurons do not normally require an input from dorsal root ganglia for long-term maintenance in vitro, but during and after taxol exposure they become dependent for survival and recovery on the presence of neurite projections from nerve growth-factor-enhanced dorsal root ganglia.

Histocompatibility and isoenzyme differences in commercially supplied "BALB/c" mice

BALB/c mice obtained commercially were found to differ significantly from the standard phenotype of BALB/c strain mice. Isoenzyme tests and H-2 haplotype analyses indicated that the majority of mice from two of the three sources tested appeared mixed, frequently heterozygous, and did not consistently express either the expected H-2 or glucose phosphate isomerase type.

Development of cross-tolerance to 5-hydroxytryptamine in organotypic cultures of mouse spinal cord-ganglia during chronic exposure to morphine

Exposure of organotypic explants of mouse spinal cord with attached dorsal root ganglia (DRGs) to low concentrations (approximately 10nM) of 5-hydroxytryptamine (5-HT) markedly depressed sensory-evoked dorsal-horn network responses, resembling the acute effects of opioids in these cultures. Attenuation of cord responses by 5-HT was not prevented by exposure to the 5-HT antagonists, methysergide and cyproheptadiene, nor to the opiate antagonist, naloxone. Explants that had become tolerant to morphine after chronic exposure (1 microM) for greater than 2 days often developed cross-tolerance to 5-HT. Acute exposure of morphine-tolerant explants to naloxone (1 microM) further attenuated the effects of 5-HT so that the minimum depressant levels of 5-HT were often increased up to 30-fold. Increasing the extra-cellular Ca++ concentration (to 5 mM) and/or introduction of 4-aminopyridine markedly antagonized the depressant effects of 5-HT on DRG-evoked cord responses, so that 5-HT levels comparable to those used on morphine-tolerant explants were required to depress naive explants. These depressant effects of 5-HT on cord-DRG explants are consonant with antinociceptive actions of 5-HT administered to dorsal cord in situ. Our data suggest that 5-HT may block neuronal components of dorsal horn networks at similar regions to those that are depressed by opiates, i.g. presynaptic DRG nerve terminals where abundant opiate receptors are located. The marked attenuation of the depressant effects of both 5-HT and opiates on cord-DRG explants by high Ca++ raises the possibility that cross-tolerance to 5-HT in morphine-tolerant explants may result from the same neuronal alterations that render dorsal-horn networks tolerant to opiates. Furthermore, the increased degree of cross-tolerant cultures may be an expression of opiate dependence.

Exposure to 4-aminopyridine prevents depressant effects of opiates on sensory-evoked dorsal-horn network responses in spinal cord cultures

The depressant effects of morphine (0.1-1 microM) on sensory-evoked dorsal-horn network responses in explants of mouse spinal cord with attached dorsal root ganglia (DRGs) were rapidly restored after addition of 4-aminopyridine (4-AP; 0.1 mM) and major components of these cord responses were stably maintained in the presence of the opiate. Moreover, prior exposure of cord-DRG explants to 0.1 mM 4-AP prevented the depressant effects of 0.1 microM morphine on DRG-evoked dorsal-horn responses, and the effects of 1-10 microM morphine were at least partly antagonized. Increased Ca++ levels (5 microM) attenuated the depression of dorsal horn responses by 1-10 micro M morphine and these effects of Ca++ were greatly enhanced in the presence of 4-AP--in some cultures, concentrations of morphine as high as 100 micro M were strongly antagonized during test periods up to 2 hours. Receptor assays showed that 0.1 mM 4-AP +/- 5 mM Ca++ had no effect on stereospecific opiate binding, indicating that the antagonist actions of these agents in our cultures do not occur at the level of the opiate receptor. The relevance of our in vitro studies of 4-AP antagonism of opiate-depressant effects on sensory-evoked dorsal-horn network responses for analyses of problems in opiate analgesia has been strengthened by a recent report demonstrating that 4-AP does, in fact, reverse morphine analgesia in rats, as determined by tail flick tests.

Quantitative study of synapse formation in the duck olfactory bulb

Synaptic emergence and development in the duck olfactory bulb was quantitatively studied by electron microscopy from the 14th day of incubation (E 14) to the adult stage. Overall synaptic density in this bulb grew considerably during the last weeks of embryonic life and the first postnatal week. The pattern of synaptic density development was similar in the four main architectonic layers of the bulb. However, lower density values were observed in the mitral and inner granule cell layers. In the glomerular layer (GL), axodendritic synapse density was always higher than dendrodentritic synapse density. In the external plexiform layer, most synapses were dendrodendritic and were established between the gemmules of the granule cells (GC) and the dendrites of the mitral cells (MC) or tufted cells (TC). Synapses established by MC and TC on GC gemmules, or by GC on MC and TC dendrites had densities very similar to each other at all the stages studied. Reciprocal synapses already appeared at E 14; their density grew until a week after birth (P7) and thereafter remained stable. In the internal granular layer, the density of asymmetrical synapses was always higher than that of symmetrical synapses. Excitatory synapses formed earlier on MC and TC than inhibitory synapses. The ratio of inhibitory-to-excitatory synapses rose rapidly after birth, reaching 2.5 in the adult duck. The density of excitatory synapses received by granule cells was as high in the external plexiform layer as in the inner granule layer, at all stages of GC development. However, the ratio of received-to-formed synapses fell in these cells from 8.42 at E 14 to 2 after birth. These results are discussed as a function of the evolution of the different synaptic balances during olfactory bulb development. Synaptic development in the duck olfactory bulb at birth is relatively close to the adult state. It appears sufficiently advanced to enable the olfactory system to function in a way compatible with the relatively independent behavior displayed by the duckling.

Preferential growth of neurites from isolated fetal mouse dorsal root ganglia in relation to specific regions of co-cultured spinal cord explants

Clusters of dorsal root ganglia (DRGs) from 13- to 14-day fetal mice were co-cultured with specific fragments of deafferented spinal cord (0.5-1 mm apart) on collagen-coated coverslips in Maximow slide chambers. Nerve growth factor (NGF) was added to the culture medium (1000 biological units/ml, at explantation) to ensure optimal survival and growth of a large fraction of the fetal DRG neurons. Sequential microscopic observations of the living cultures and cytologic studies after silver impregnation demonstrate that many neurites from isolated DRGs can invade dorsal (DC) regions of co-cultured spinal cord explants, whereas they are deflected from ventral cord (VC) tissue and its neuritic-glial outgrowth. Furthermore, some DRG neurites may become redirected towards distant DC target explants even after long circuitous detours around proximally arrayed VC explants. DRG neurites also show remarkably sharp projections to DC tissue and more complete avoidance of adjacent VC tissue when the DRG neurites approach a suitably arranged DC-VC-DC interface, e.g. forming de novo 'dorsal roots' at the end of a longitudinal strip of whole spinal cord. These experiments suggest that DC-VC boundaries may be particularly effective in guiding DRG neurites to specific regions of the CNS. The present studies of co-cultured fetal mouse DRG and spinal cord explants provide the first demonstration of preferential neuritic growth in vitro in relation to specific CNS target tissues.

Specific neuritic pathways and arborizations formed by fetal mouse dorsal root ganglion cells within organized spinal cord explants in culture: a peroxidase-labeling study

Extracellular microiontophoretic injections of horseradish peroxidase (HRP) into NGF-enhanced fetal mouse dorsal root ganglia (DRGs) produced an anterograde solid Golgi-like labeling of DRG neurites and their terminal arborizations within co-cultured spinal cord explants. In cultures of spinal cord transverse cross-sections with attached DRGs, the large NGF-enhanced DRGs remained in close proximity to the cord, often adjacent to both dorsal and ventral cord regions. Despite this, nearly all DRG neurites that entered the cord did so via dorsal root fascicles. They branched and ramified extensively within the dorsal region, taking on a wavy or kinky course and showed various types of arborizations. The density of cord innervation was much lower when isolated DRGs and cord explants were co-cultured 0.5-1 mm apart. Although fewer entering DRG fibers were labeled by our HRP injections the same qualitative growth and arborization patterns were seen within dorsal and ventral cord regions as in explants of cord with attached DRGs. When the facing edge contained both dorsal and ventral tissues, HRP-labeled DRG fibers entered dorsal regions selectively. DRG fibers readily entered, ramified and arborized within isolated strips of dorsal cord, whereas they sharply avoided isolated ventral cord explants. The avoidance of ventral cord cannot simply be due to the paucity of specific synaptic targets within the tissue, for larger numbers of DRG fibers entered completely inappropriate CNS target tissues, e.g. superior colliculus explants--though they did not ramify or arborize to any degree comparable to that seen within dorsal cord regions.

Selective innervation of target regions within fetal mouse spinal cord and medulla explants by isolated dorsal root ganglia in organotypic co-cultures

Correlative electrophysiologic and cytologic analyses demonstrate that a significant group of neurons in isolated (NGF-enhanced) fetal mouse dorsal root ganglia (DRGs) can grow across a collagen substrate and selectively innervate specific dorsal horn and dorsal column nuclei regions in co-cultured explants of deafferented spinal cord and medulla. Neurites from this group of DRG cells from connections in CNS target zones that generate characteristic primary afferent network responses to sensory stimuli, as observed in cultures of spinal cord with attached DRGs. Systematic microelectrode stimulus-mapping tests revealed that many DRG neurites were preferentially distributed in sensory target zones of co-cultured cord and medulla explants and that few collaterals of these DRG neurons were present in neighboring inappropriate regions, especially in the ventral cord. Another group of DRG neurons appears to be responsible for the less prominent, but clear-cut, innervation that developed in some of the co-cultured ventral cord explants. Fetal DRGs were also able to establish characteristic primary afferent dorsal horn or dorsal column nuclei networks when introduced into cultures of deafferented spinal cord and medulla that had been explanted alone for 1-3 weeks prior to introduction of the DRGs. These experiments demonstrate that CNS target neurons remain receptive to DRG innervation even after 1-3 weeks of maturation in vitro. Our electrophysiologic and cytologic analyses of DRG and CNS explants in organotypic co-cultures provide the first systematic attempt to establish conditions under which preferential neuritic growth to and functional innervation of specific CNS target tissues can occur in vitro. This model system should facilitate analyses of mechanisms underlying development, as well as regeneration, of specific synaptic connections in the CNS.

Microtubule arrays in taxol-treated mouse dorsal root ganglion-spinal cord cultures

Exposure of organotypic dorsal root ganglion-spinal cord cultures to taxol, a potent microtubule promoting and stabilizing agent, results in an unusual abundance of microtubules in neurons, and the presence of microtubule-endoplasmic reticulum arrays in their perikarya and processes. Ordered concentric arrays of microtubules alternating with marcromolecular material are observed in dorsal root ganglion neurites. Short linear structures are discernible between some microtubules and the macromolecular material, as well as between microtubules and endoplasmic reticulum cisternae. Analyses of such unusual microtubule arrays in taxol-treated cultures may provide valuable insights into tubulin-related systems in neurons, as well as in other cells.

Neurites from mouse retina and dorsal root ganglion explants show specific behavior within co-cultured tectum or spinal cord

We have utilized extracellular microiontophoretic injections of horseradish peroxidase into fetal mouse retinal explants to label retinal ganglion cell axons innervating co-cultured tectal explants in a solid Golgi-like manner. Using dorsal root ganglia-tectum and retina-spinal cord co-cultures as controls, our results indicate that retinal neurites show selective growth and arborizations within their appropriate tectal, target tissue. Retinotectal explant co-cultures may be a useful model system for studying aspects of neuronal specificity.

Chemically defined medium enhances bioelectric activity in mouse spinal cord-dorsal root ganglion cultures

Co-cultures of mouse spinal cord with dorsal root ganglion (DRG) cultures were grown either in horse serum (HS)-supplemented medium or in a serum-free, chemically defined medium (CDM). The cytoarchitecture of cord--DRG explants was fully retained in CDM, with little or no distortion due to flattening of the explant, as is invariably observed in HS-supplemented cultures. Functional properties such as bioelectric activity and DRG--spinal cord interconnectivity were well sustained in CDM.

Presence of leucine-enkephalin in organotypic explants of fetal mouse spinal cord

Spinal cord explants with attached dorsal root ganglia (DRGs), from 14-day fetal mice were fixed at 1-3 weeks in vitro and incubated for leucine-enkephalin (LE) immunoreactivity by the peroxidase-anti-peroxidase (PAP) immunohistochemical method. Results show long processes with labeled varicosities seen more often in dorsal regions of the cord explants. Stained punctate bodies and varicosities were often seen close to large cells in these cultures, whereas no label was detected in neuronal perikarya. A prominent laminar array of stained punctate bodies was noted in one cord explant, concentric with the perimeter of the explant. No LE label was detected in the neuritic outgrowths from the cord-DRG explants, whereas high levels of opiate receptors develop in these outgrowths, primarily on the DRG neurites, by 1-2 weeks in culture. The results indicate the presence of LE in explants of fetal mouse spinal cord with attached DRGs and offer an in vitro model system in which the onset and development of peptidergic neurons can be studied as they form functional cellular interrelationships with neurons bearing opioid and monoaminergic receptors in these organotypic cultures.

Dorsal root ganglion neurons are destroyed by exposure in utero to maternal antibody to nerve growth factor

Rats and guinea pigs, when immunized with mouse nerve growth factor, produce antibodies that cross-react with their own nerve growth factor. The antibodies reach developing offspring of these animals both prenatally (rats and guinea pigs) and postnatally (rats). Depriving the fetus of nerve growth factor in this way results in the destruction of up to 85 percent of dorsal root ganglion neurons as well as destruction of sympathetic neurons. Sensory neurons of placodal origin in the nodose ganglion were not affected. These data demonstrate that dorsal root ganglion neurons go through a phase of nerve growth factor dependence in vivo.

Patterns of reaggregation and formation of linear aggregate chains in collagen-well cultures of dissociated mouse brain and spinal cord cells

With the use of the newly developed collagen-well culture technique spontaneous reaggregation of cells from dissociated embryonic mouse brain and spinal cord were studied. Within 20 h in culture, aggregates are formed and settled onto collagen substrate. Two patterns of aggregate arrangement were observed; random and linear. Linear chains of aggregates appeared to be more characteristic of dissociated spinal cord cells, although the linear patterns were not uncommon in cultures of dissociated cortex. Formation of aggregate chains appeared to be dependent on the stage of neuronal and glial differentiation. After attachment to the collagen substrate, the general pattern of aggregate organization was not greatly altered except for changes which resulted from cellular migration and proliferation, the formation of connections between aggregates, or incorporation of small aggregates into larger ones. The number of non-aggregated cells in collagen-well cultures was small. Single, non-aggregated neurons seldom survived individually. Fiber connections between aggregates began to form after the first day in vitro, and by 2 or 3 days, the growing fibers formed neuritic bridges connecting aggregates. By the end of the first week growing fibers often organized compact bundles, but part connections between aggregates were presented by separate fibers in a diffused manner. Silver impregnation revealed that these connections were formed by the axons of neurons located in the aggregates. Thus, progression of the above described processes resulted in the de' novo' formation of linear organized or random systems of interconnected neuronal centers.