Chemical messengers in development: a hypothesis

Differential expression of acetylcholinesterase molecular forms in neural retina and retinal pigmented epithelium during chick development

Neural retina (NR) and retinal pigmented epithelium (RPE) were used as a model for studying acetylcholinesterase (AChE) expression in neuronal and non-neuronal tissues during development. In chick embryo retina, increasing AChE activity appeared from day 7 to hatching, first in NR and then in RPE. NR contained 3 main AChE forms, 11.3S, 6.5S and 4.5S, resolved by sucrose density gradient centrifugation. An additional 19S form was exclusively detected in RPE whatever the extraction procedure followed. During differentiation, the proportions of AChE molecular form changed until they reached a steady state characteristic of mature neural retina, whereas in RPE, patterns of AChE molecular forms did not change significantly during development. Thus, 19S AChE appeared to be characteristic of non-neuronal retinal tissue. The expression of the AChE molecular forms in NR and RPE, and particularly the 19S form, was independent of the presence of the lens during retinal differentiation.

Recovery of acetylcholinesterase activity after irreversible inhibition by organophosphorous compounds in embryonic development

1. Recovery of acetylcholinesterase (AChE) activity was studied using the embryos of sea urchins Strongylocentrotus intermedius and S. nudus, embryos of axolotl Ambystoma mexicanum and in the chick embryo muscle culture treated by "irreversible" organophosphorous inhibitors (OPI). 2. AChE activity was assayed by a modified Ellman's procedure. 3. It follows from the data obtained that, unlike the plutei of sea urchins and the monolayer culture of chick embryo muscle cells, the embryos of axolotl show a compensatory increase in AChE biosynthesis after inhibition by OPI. 4. This mechanism is assumed to be related to the presence of a well developed neuromuscular system in the A. mexicanum embryos. 5. It is possible that acetylcholine accumulated as a result of partial AChE inhibition is responsible for the compensatory increase in AChE biosynthesis.

Asymmetry in the hippocampal region specific for one of two closely related species of wild mice

The volumes of the components of the left and right hippocampal regions were compared in male, adult specimens of the two closely related species of wild mice, Apodemus flavicollis (yellow-necked wood mice) and Apodemus sylvaticus (long-tailed wood mice). In one of the species, Apodemus flavicollis, the component containing the deep layers of the subiculum was found to be significantly larger in the hippocampal region of the left side than it was in that of the right side. No other significant asymmetries were found in the hippocampal components of the two species. These findings indicate that significant asymmetries in localized brain regions can arise during a single speciation event.

Noradrenergic stimulation of mitochondriogenesis in brown adipocytes differentiating in culture

The ability of adrenergic agents to promote the differentiation and especially the mitochondriogenesis of brown fat precursor cells, grown in culture, was investigated. These cells begin to differentiate during the days preceding confluence. We found here that, already during the early growth phase, the cultures (essentially precursor cells and preadipocytes at this stage) show increased cyclic AMP (cAMP) levels when acutely stimulated with norepinephrine (NE). The cultured cells were therefore chronically treated with NE up to the time of confluence, and their cytochrome-c oxidase activity was measured as an index of mitochondriogenesis. Chronic NE treatment resulted in an increased cytochrome-c oxidase activity of the cells at confluence. This effect was reproduced by selective activation of adenylate cyclase with cholera toxin, suggesting that the NE effect was exerted through an increase in cAMP. Ascorbate (added with NE as an antioxidant) had in itself a positive effect, both on final cell number and on cytochrome-c oxidase activity. It is concluded that NE, working through beta-adrenergic receptors, can stimulate mitochondriogenesis in brown fat cells through a direct effect on the cells, in accordance with the suggestion (based on in vivo experiments) that NE accelerates, rather than initiates, the differentiation process.

Collagen and fibronectin synthesis by trisomic and triploid fibroblasts from human spontaneous abortuses

Collagen and fibronectin synthesis by trisomic and triploid fibroblasts derived from human spontaneous abortuses was studied. It was demonstrated that the level of fibronectin and collagen production in fibroblasts with trisomy 7, trisomy 9, and triploidy was reduced as compared with diploid cells. A correlation between this observation and an increased rate of intracellular 14C-procollagen degradation was also established for the anomalous strains. No difference in hydroxylation of 14C-proline residues in alpha 1(I) and alpha 2(I) collagen chains and no fluctuation in the collagen type (I): type III ratio was found in the strains with the abnormal karyotypes. It was concluded that differentiation of the abnormal fibroblasts was impaired. The data also favour the hypothesis that the deficiency of the fibroblasts in producing proteins may account for a variety of anatomic abnormalities of embryos.

A transient embryonic dopamine receptor inhibits growth cone motility and neurite outgrowth in a subset of avian retina neurons

To investigate the possible developmental significance of a transient dopamine receptor in the avian central nervous system, we examined the effects of dopamine on the morphology and motility of cultured retina neurons. Neurite arborization was significantly reduced by chronic dopamine stimulation. Using continuous video microscope monitoring, we observed that a subset of neurons responded to short-term dopamine with decreased filopodial activity and retracted neurites. The effects of dopamine could be blocked or reversed by haloperidol or SCH23390 and forskolin produced a response similar to dopamine, indicating the morphological changes were mediated by D1-receptor stimulation of adenylate cyclase.

Transient expression of 5-HT1A receptor binding sites in some areas of the rat CNS during postnatal development

The developmental evolution of 5-HT1A receptor binding sites was examined in the rat CNS during the early postnatal period using quantitative autoradiography and binding assays with 3H-8-OH-DPAT as the selective ligand. A progressive increase in the density of 5-HT1A sites was observed in the hippocampus, septum and cerebral cortex, up to adult levels which were reached around the third postnatal week. In contrast, complex biphasic (increase then decrease) changes were noted in other structures (for instance the nucleus of the lateral lemniscus), and even a progressive decrease in the density of 5-HT1A sites took place in the cerebellum during the first two postnatal weeks. The transient expression of 5-HT1A receptor binding sites in a structure such as the cerebellum which develops exclusively for the postnatal period further supports that 5-HT might play a trophic role during maturation of the CNS.

The muscarinic receptor-mediated action of acetylcholine in the gastrulating chick embryo

Some of the muscarinic receptor-mediated effects of acetylcholine in early chick embryo cells at stages three-four by Hamburger and Hamilton were studied. Acetylcholine increased the intracellular level of cGMP about two-fold. Acetylcholine raised the intracellular level of free calcium from the basal level of 120 nM to 140 nM. Atropine, a muscarinic antagonist, blocked both the above-mentioned responses.

Preliminary evidence for a cholinergic-like system in lichen morphogenesis

Membrane acetylcholinesterase activity is considered to be a marker for a cholinergic system. When temporarily expressed in differentiating cells other than the nervous or muscular ones, it may play a role in morphogenesis. In the lichen Parmelia caperata (L.) Ach., acetylcholinesterase is histochemically localized mainly in the cell walls and/or membranes of both symbionts just where they proliferate and form well-organized propagation structures, the soredia. The enzyme activity is first detected in a few algae undergoing aplanosporogenesis and later in medullary hyphae that reach the dividing algae by elongating perpendicularly to the thallus surface. This histochemical pattern that is associated with algal proliferation and oriented hyphal growth is characteristic of early morphogenesis of the soredia; when fully differentiated, they consist of an inner dividing alga and an outer hyphal envelope, both showing cholinesterase activity. Substrate specificity and inhibitor sensitivity of the histochemical staining indicate an acetylcholinesterase-like activity. However, extracts of the thallus areas where soredia develop give four bands of cholinesterase activity on disc electrophoresis: the two cathodal bands have the characteristics of acetylcholinesterase, the others of pseudocholinesterase. One of the latter hydrolyses propionylthiocholine very rapidly. The findings suggest that in lichen symbiosis, a cholinergic-like system participates in regulating morphogenetic processes such as cell division, oriented tip growth and alga-fungus membrane interactions. Environmental stimuli, particularly light, might trigger the development of soredia by modulating the activity of the cholinergic mechanism.

Neural tube defects: a review of human and animal studies on the etiology of neural tube defects

Although neural tube defects are a common congenital anomaly, their etiology is not known. Human studies have emphasized the pathology and epidemiology of the defects and suggest that in the majority of cases the etiology is multifactorial. Factors which appear possibly to be important are genetic predisposition, maternal illness, and fetal drug exposure. Animal studies have utilized naturally occurring neural tube defects and teratologically induced lesions. No animal model has been convincingly established as the equivalent of human neural tube defects. However, animal models have allowed investigation of the mechanisms of suggested human teratogens and determination of the pathogenesis of naturally occurring animal defects. Their most important contribution has been in furthering the understanding of the normal mechanisms of neural tube closure. It may be through this understanding that the etiology of human neural tube defects will be determined.

The effects of serotonin on the morpho-functional development of rat cerebral neocortex in tissue culture

The morpho-functional development of the visual area of the cerebral cortex of newborn rats and effects of serotonin (5-HT) on these processes were studied in long-term (up to 40 days) organotypic cultures. It was shown that systematic addition of 5-HT in physiological concentrations to the nutrient medium during cultivation of the explants stimulates glia proliferation, neuron differentiation, neuropil formation, axon myelination and synaptogenesis. Electrophysiological studies of cortical cells in 5-HT-treated cultures showed earlier exhibition of spontaneous activity, increased number (by 33%) of spontaneously firing neurons and prevalence of periodic (bursting) type of discharges. Cultivation of neurons in a medium with 5-HT approximates their sensitivity to 5-HT to that observed in situ. The data obtained are indicative of the stimulating effects of 5-HT on the morpho-functional development of the rat cerebral cortex in tissue culture.

The influence of second messengers and related substances on the sensitivity of early embryos to cytostatic neurochemicals

"Prenervous" neurotransmitters interact with second messengers in the regulation of early embryogenesis of sea urchins and the clawed frog Xenopus laevis. Propranolol and atropine inhibit cleavage divisions of X. laevis only after their intracellular administration. The level of the membrane potential (including a zero level) of the embryos studied is not essential for the cleavage divisions. Possible mechanisms of action of the "prenervous" neurotransmitters at the intracellular level and their coupling with second messengers are discussed.

Growth patterns of primary cultures dissociated from 3-day-old chick embryos: morphological and biochemical comparisons

Cultures were prepared by dissociating 3-day-old whole chick embryos and plating the dispersed cells on poly-L-lysine-coated dishes in Dulbecco's Modified Eagle's Medium with 10% fetal calf serum. By 48 hr in culture, aggregates and neuritic sprouting were observed. Long neuritic bundles connecting cell aggregates were evident by 4 days in culture. Consistent patterns throughout the lifespan of the cultures were contacts between neurites, and flat isolated cells, presumptively glial, emerged. Throughout the lifespan of the cultures, the cholinergic cell population was characterized histochemically by the method of Karnovsky and Roots and biochemically by assaying choline acetyltransferase. By 4 days in culture, all aggregates showed light cholinesterase-positive staining; however, with days in culture, several aggregates had no staining, and some positive-stained aggregates were interconnected with other aggregates showing only spotted positive staining. Choline acetyltransferase activity showed a developmental profile in agreement with the histological findings. The early presence of choline acetyltransferase activity is taken as indication of the early commitment of cholinergic neurons.

An immunohistochemical study of serotonin development in the opossum cerebellum

In the present study we have used the indirect antibody peroxidase anti-peroxidase technique to analyze the development of serotonin in the cerebellum of pouch young opossums ranging in age from birth (postnatal day (PD) 1) - to PD 47. The pathways by which serotoninergic axons enter the cerebellum appear to change during development. Between PD 1 and PD 11 varicose serotoninergic fibers course though the continuity between the tectum and the dorsal medial aspect of the cerebellar plate and distribute primarily to the intermediate zone. By PD 11 serotoninergic fibers enter the cerebellum via a connecting band between the cerebellum and the dorsal lateral aspect of the rostral medulla. Fibers entering the cerebellum via this later route course around the lateral recess of the fourth ventricle and initially distribute to ventral and lateral areas of the cerebellum. At later developmental ages (PD 14-PD 47) serotoninergic fibers are present in: 1) the cellular zone of migration between the Purkinje cell layer and deep nuclei; 2) the Purkinje cell layer; 3) the internal granule cell layer; and 4) the deep cerebellar nuclei. The external granule cell and the molecular layers rarely contain serotoninergic fibers. The present study has shown that serotonin is present in the cerebellar anlage on PD 1 (within 13 days of conception). This is prior to the arrival of other major afferent systems (King et al. 1982; Bishop et al. 1983; Martin et al. 1983; Morgan et al. 1983). Further, serotoninergic axons reach the cerebellum via different routes at different stages of development. The presence of serotonin in the intermediate zone early in development supports the hypothesis that it may influence neuronal migration and differentiation (Lauder and Krebs 1976). Axons containing serotonin and arriving later in development may function in synaptic transmission, a role proposed for this indoleamine in the adult (Strahlendorf et al. 1979).

Nerve growth factor induces neural differentiation in undifferentiated cell of early chick embryos

Nerve growth factor (NGF) induced differentiation in postnodal pieces (PNPs) of stage 4 chick embryos. This induction was highly selective for neural tissue; no other structures developed in the NGF-treated PNPs. Furthermore, the number of PNPs showing neural differentiation was dependent on the concentration of NGF, but there was no correlation between the concentration of NGF (5-100 ng/ml) and extent of neuralization. The neural inducing capacity of NGF could be abolished by anti-NGF antibody. NGF-induced neural differentiation was accompanied by elevated intracellular levels of cyclic AMP. Exogenous cyclic AMP (175 micrograms/ml) was able to stimulate neural differentiation but, unlike NGF, induced other structures (e.g., notochord and pulsatile tissue). Overall results suggest that cells from chick embryos at developmental stages much earlier than previously thought are responsive to NGF and NGF or a a closely related substance may serve as a neural inducer in the chick embryo.

Serotonin receptors in parasitic worms

It is evident from the above review that during the last two decades a great deal of interest in investigating the action of serotonin in parasitic worms has been shown by parasitologists as well as by scientists from several other disciplines. What we have initially reported concerning the effect of serotonin on motility and carbohydrate metabolism of F. hepatica has been pursued on several other parasitic worms. The studies so far indicate that serotonin stimulates motility of every species tested among the phylum Platyhelminthes. The indoleamine also stimulates glycogenolysis in the few flatworm parasites that have been investigated. The information in nematodes is scanty and the role of serotonin in these parasites is still open for experimentation. Recent biochemical investigations on F. hepatica and S. mansoni demonstrated that serotonin and related compounds utilize a common class of receptors in plasma membrane particles which I designate as 'serotonin receptors'. These receptors are linked to an adenylate cyclase that catalyses the synthesis of the second messenger, cyclic 3',5'-AMP. Serotonin and its congeners increase the concentration of cyclic AMP in intact parasites whereas antagonists inhibit such an effect. Cyclic AMP stimulates glycogenolysis, glycolysis and some rate-limiting glycolytic enzymes. It activates a protein kinase that may be involved in activation of glycogen phosphorylase and phosphofructokinase. Serotonin-activated adenylate cyclase in S. mansoni is activated early in the life of the schistosomule. The possibility is discussed that the availability of cyclic AMP through serotonin activation in these parasites may be a prelude to the development processes that take place in the parasite. The different components of the serotonin-activated adenylate cyclase in the parasite are the same as those that have been previously described for the host. Binding characteristics of the receptors indicate that the receptors in F. hepatica appear to be different from those that have been described in the host. The discovery of these receptors and their differences from those in the host offer a new site which is amenable to pharmacological manipulation. The search for new agents that influence serotonin receptors in these parasites could be included in a strategy for the development of new chemotherapeutic agents against these parasites.

Molecular regulators of brain function: a new view

A working hypothesis is proposed based on two mutually dependent concepts: neurons may be functionally regulated not only by presently known neurotransmitters but by many kinds of informational substances. Known informational substances are considered in categories corresponding to major regulators of the central nervous system, including transmitters, peptides, hormones, "factors" and various proteins. Many new informational substances are being discovered by the application of DNA technology. Alongside neuronal circuitry that forms the basis for conventional neuroanatomy and neurophysiology, and that operates through conventional synaptic junctions, is a system here called parasynaptic, i.e. in "parallel with" synapse-linked circuitry. In the parasynaptic system, informational substances reach specific target cell receptors by diffusion from release points through extracellular fluids. The parasynaptic system has the same degree of selectivity as synaptic circuitry but possesses, in addition, a domain of versatility and plasticity lacking in "hardwired" circuitry; the latter is, however, also influenced by highly potent informational substances in the ambient extracellular fluid.

The cholinergic system of the primitive streak chick embryo

The presence of neurotransmitters at stages of embryonic development prior to neurulation has been demonstrated in several systems. Although the functions of these molecules at early stages of embryogenesis have not been ascertained, it is possible that they are involved in aspects of cell migration, regulation of the synthesis of macromolecules, intercellular communication, and in the transmission of positional information during gastrulation. As an initial approach to the resolution of questions concerning the function of transmitters during early development, we have begun a study of the cholinergic system in the primitive streak chick embryo (Hamburger-Hamilton stages 3 + to 5). We have found that the chick embryo: (1) can use exogenously applied choline for the synthesis of acetylcholine; (2) possesses a true acetylcholinesterase, which is predominantly in the form of the 4-6s monomer; and (3) can take up exogenous choline through a sodium-dependent, high-affinity choline transport system. To date we do not have any evidence for the presence of nicotinic or muscarinic receptors at the primitive streak stage.

Perinatal dopamine-related drugs demasculinize rats

Administration of haloperidol, a common neuroleptic, to pregnant or lactating rats impaired the masculine sex behavior of their male offspring. Prenatal haloperidol did not affect testosterone concentrations in fetuses. Maternal administration of apomorphine, a dopamine agonist, and of alpha-methyl-p-tyrosine, an inhibitor of dopamine synthesis, also demasculinized male offspring. In both experiments other behaviors and developmental milestones were unaffected. Perinatal haloperidol, apomorphine, and alpha-methyl-p-tyrosine did not lower testosterone in adulthood. These drugs may act directly on neurons that control masculine behavior without lowering testosterone prenatally or in adulthood.

Identification of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian palate

A direct radioligand binding technique utilizing a beta-adrenergic antagonist [3H]Dihydroalprenolol [( 3H]DHA) was employed in the identification and characterization of fetal palatal beta-adrenergic receptors. [3H]DHA binding was saturable (Bmax 16 fmol/mg protein) with high affinity and an apparent equilibrium dissociation constant (KD) of 1.5 nM. Binding of [3H]DHA was displaced by the competitive beta-adrenergic antagonist propranolol in a concentration-dependent manner. Dissociation kinetic studies demonstrated almost complete reversibility of radioligand binding within 60 min. The functionality of these beta-adrenergic receptors was demonstrated by showing that fetal palatal mesenchymal cells responded to catecholamine agonists with dose-dependent accumulations of intracellular cAMP. This effect could be entirely blocked by the beta-antagonist, propranolol. The relative potency order of catecholamines in eliciting an elevation of cellular cAMP was characteristic of a beta 2-adrenergic receptor-mediated response: (-) isoproterenol greater than (-) epinephrine greater than (-) norepinephrine. In addition, this response was found to be stereospecific with (-) isoproterenol being significantly more potent than (+) isoproterenol. Both the [3H]DHA binding characteristics and the catecholamine sensitivity of fetal palatal tissue support the presence of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian secondary palate.

Sodium, phosphorus, sulphur, chlorine and potassium shifts in rat brain during embryonic development

Concentrations of sodium (Na), phosphorus (P), sulphur (S), chlorine (Cl) and potassium (K) and their variations during brain development were measured in freeze-dried thick sections from rat brain (16-20 micron). Sprague-Dawley rats were bred and the day of finding vaginal spermatozoa was considered as day zero of pregnancy. On days 12E, 13E, 14E, 16E, 19E, 21E (embryonic) and postnatal day one whole embryos or fetal heads were rapidly frozen in liquid Freon 22 cooled with liquid nitrogen (-180 degrees C), sectioned in a cryostat (-20 to -40 degrees C), and processed for X-ray microanalysis on pure carbon plates. Concentrations of Na and Cl differed in the cells of the cerebral cortex, ependyma, choroid plexus and cerebral spinal fluid (CSF). During cerebral development, Na and Cl concentrations appeared to be correlated, while K was more related to P. S was low and unchanged in all compartments during development and was thus considered as an internal control. K was inversely related to Na and Cl fluctuations within the choroid plexus epithelia. Sharp phase changes of elemental composition appeared in all tissues at specific growth stages, e.g. days 14E and 19E. These results demonstrate rhythmic changes in the inorganic components of developing rat brain cells and fluid environment presumably reflecting physiological fluctuations and cell cycle phenomena. Such changes may also be related directly or indirectly to known 'growth phase changes' in the developing rat.

Disturbances in collagen synthesis in trisomic cells from spontaneously aborted embryos

Collagen synthesis in cells with trisomy 7 and 9 derived from human spontaneous abortuses was found to be lower (5.06% and 5.53% respectively) than in the control diploid cells (8.80%). The ratio of collagen types (I/III) in trisomic strains did not differ from the control data while the amount of the degraded procollagen in trisomic cells was increased.

Aminergic neurotransmitters and adenylate cyclase in hydra

Serotonin, dopamine and, in lesser amounts, norepinephrine were detected in Chlorohydra viridissima with electrochemical detection coupled to liquid chromatography (LCED). Treatment with reserpine induces a significant decrease in amine levels. Adenylate cyclase was found in Hydra tissue; the enzyme is stimulated by Mg, Mn and F and sensitive to guanine nucleotide activation. Dopamine, serotonin, GSH and glutamate do not affect cyclase activity.

Occurrence of type A and type B monoamine oxidase activities in toad egg mitochondria

Monoamine oxidase activity has been assayed radiochemically in mitochondria from Bufo bufo eggs. Time courses of MAO activity towards PEA and 5-HT indicate that when PEA is used as a substrate higher specific activities are obtained. The inhibition patterns by clorgyline and deprenyl demonstrate that both type A and type B MAO are present. Type A activity is more sensitive than type B to the inhibitory effect of Triton X-100. Low concentrations of 2-mercaptoethanol cause a slight stimulation of type A and B activities. Increasing concentrations result in a decrease of activity.

Acetylcholinesterase activity of Xenopus laevis oocytes

The cholinesterase activity of Xenopus laevis oocytes was assessed using [3H]acetylcholine in a simple radiometric procedure. The cholinesterase activity of mature (stage V-Vl) oocytes was very sensitive to inhibition by the specific acetylcholinesterase inhibitor, BW284-C5l, and relatively insensitive to an inhibitor of non-specific, or butyrylcholinesterase. The Km and Vmax of the acetylcholinesterase measured in homogenates of oocytes were 312 microM and 4.6 nmol-oocyte 1-h 1, respectively. Triton X-100 increased the enzyme activity of homogenates four- to five-fold while collagenase treatment displaced into the medium none of the acetylcholinesterase activity from either homogenates or intact oocytes. Cations were found generally to diminish the acetylcholinesterase activity of oocyte homogenates, and lanthanum ions inhibited acetylcholine hydrolysis with an IC50 of 0.63 mM. Subcellular fractionation of oocytes revealed that the bulk of enzyme activity was associated with particulate fractions. Acetylcholinesterase activity was also detected on the surface, and in homogenates, of immature oocytes. Peak enzyme activity resided in stage IV oocytes. Eggs obtained from females induced to spawn were found to have acetylcholinesterase activity in homogenates but little or no hydrolytic activity was detected on the egg surface. These results provide a point of departure for further investigations of the functional significance of this enzyme in Xenopus oocytes.

Involvement of GABA in palate morphogenesis and its relation to diazepam teratogenesis in two mouse strains

Previous studies have indicated that serotonin and acetylcholine stimulate palate shelf reorientation. The present studies were undertaken to determine whether gamma-aminobutyric acid (GABA) functions as an inhibitory neurotransmitter in the palate and whether diazepam mimics GABA to inhibit shelf reorientation and cause cleft palate. First, it was shown that 10(-4) M GABA inhibits palate shelf reorientation in day 14.5 AJ embryos cultured for 2 hours. Anterior palate reorientation stimulated by 10(-5) M serotonin was decreased by GABA; 10(-5) M picrotoxin (GABA antagonist) stimulated anterior shelf reorientation and reversed the effect of GABA. Diazepam (10(-4) M) partially inhibited palate shelf reorientation and that stimulated by 10(-5) M serotonin. Diazepam (400 mg/kg) was administered to AJ mice at day 13.5 of gestation and embryos were cultured at day 14.5. The inhibition produced by diazepam was significantly reduced by 10(-5) M picrotoxin. The teratogenic effect of diazepam was compared with AJ and Swiss-Webster Vancouver (SWV) inbred strains. Diazepam produced greater clefting in SWV mice (57% net) than in the AJ (18% net) when compared to their water- and food-starved controls. The greater sensitivity of the SWV strain than the AJ strain to diazepam, as well as to GABA, was also observed in embryo culture. GABA (10(-5) M) markedly inhibited posterior palate reorientation and reversed the stimulation produced by bethanechol in SWV mice. The inhibitory effects of GABA on the posterior palate were partially reversed by picrotoxin. Furthermore, diazepam inhibited palate reorientation either when administered to the pregnant dam or added in embryo culture.(ABSTRACT TRUNCATED AT 250 WORDS)

On the role of serotonin and 5-methoxy-tryptamine in the regulation of cell division in sea urchin eggs

The presence of serotonin in sea urchin eggs has been ascertained by high-performance liquid chromatography and thin-layer chromatography analysis of tissue-free bioamines. The results show the presence of both serotonin and 5-methoxytryptamine. The role of these substances in the cell division process has been studied by using the serotonin antagonists, gramine and metergoline. Both antagonists cause a significant delay of the cell division which, however, can be prevented by the addition of either 5-hydroxytryptophane, serotonin, or 5-methoxytryptamine. The effect of gramine on the different stages of the cell division process has also been investigated. Neither S phase nor mitosis are affected by the serotonin antagonist, while cleavage is delayed. The effect of serotonin seems mediated by calcium ions and cAMP. Gramine causes a marked increase in radio-calcium efflux from the fertilized egg, and at the same time lowers the cAMP level.

Cholinergic transmission by embryonic retinal neurons in culture: inhibition by dopamine

The function of neurotransmitters in ontogeny remains unclear, although it is well known that both pre- and postsynaptic components of certain neurotransmitter systems are present from early in morphogenesis. The objective of this study was to establish a culture system that would permit an analysis of the physiological effects of dopamine on immature neurons. Specifically, dopamine-mediated effects on synaptic transmission by cholinergic neurons of the embryonic chick retina were explored. To do this, a retina-muscle culture system was used. In previous physiological studies, striated muscle cells in culture have proved useful as postsynaptic targets for cholinergic neurons of the immature retina. It is reported here that dopamine can inhibit synaptic responses of cultured muscle cells that are innervated by neurons of the embryonic chick retina. This inhibitory effect is blocked reversibly by the dopamine antagonists, haloperidol and fluphenazine. With the culture system used in this developmental study, dopamine-mediated inhibition can be examined with either explants of retina or with dissociated retinal neurons. When a low density of dissociated cells is plated, it is possible to examine relatively isolated, visually identified, presynaptic, cholinergic neurons. The results show that an inhibitory response to dopamine is expressed by neurons derived from retinas which are at an early stage of ontogeny. The finding that inhibition by dopamine could be demonstrated to develop at 90% of the retina-muscle synapses indicates that the cholinergic neurons studied in this experimental system are a relatively homogenous population with respect to their responsiveness to dopamine.

Levels of cyclic GMP and cyclic AMP in regenerating forelimbs of adult newts following denervation

The effect of short-term denervation (0, 12, 24, and 72 hours) on the levels of cyclic 3'5'-guanosine monophosphate (cGMP) and cyclic 3'5'-adenosine monophosphate (cAMP) in adult newt (Notophthalmus viridescens) forelimbs at 15, 22, and 35 days of regeneration was investigated. Regenerate blastema and stump cyclic nucleotide levels were compared with those of the contralateral intact forelimb and hindlimb, with levels in the normally regenerating blastema, and with levels measured in the forelimbs of intact, nonoperated animals. Variations in cyclic nucleotide levels occurred according to regeneration stage and tissue type. Changes in level were noted immediately upon denervation and subsequently at other sample times in all regenerate and control series. Parallel fluctuations occurred in regenerate stump and contralateral intact forelimbs. Our results from nonamputated denervated and sham-denervated animals indicate that short-term, denervation-associated cyclic nucleotide fluctuations cannot be attributed solely to the loss of innervation.

Cyclic nucleotides during chondrogenesis: concentration and distribution in vivo and in vitro

This study correlates endogenous levels of cAMP and cGMP with their immunohistochemical localization during chondrogenic differentiation of C57B1/6J mouse limb mesenchyme in vivo and in vitro. A transient decrease in cGMP but not cAMP was found from days 12 to 13 in vivo correlating with early stages of chondrogenesis in the developing limb. Intracellular levels of both cAMP and cGMP in high density limb mesenchyme cultures increased 25% after 24 hr in culture when aggregate and nodule formation was detectable. When cells were seeded at different initial plating densities to delay the onset of aggregate and nodule formation, increased levels of intracellular cAMP correlated temporally with the appearance of nodules. Both cyclic AMP and cGMP were immunohistochemically localized in perichondrial cells and chondrocytes in vivo and in vitro. Therefore, (1) cAMP levels correlated temporally with the appearance of chondrogenic cells and (2) cAMP and cGMP were immunohistochemically localized to chondrogenic cells. These data indicate that fluctuations of both cAMP and cGMP levels may be involved in limb cartilage differentiation. Although increases in both nucleotides were found to correlate with the onset of chondrogenesis in vitro, in vivo data suggest that the amount of cAMP relative to cGMP rather than the absolute amount of an individual cyclic nucleotide may be more significant in modulating differentiation.

Development of apparent presynaptic elements formed in response to polylysine coated surfaces

Formation of apparent presynaptic elements on polylysine-coated surfaces was examined with both scanning and transmission electron microscopy. Neurons from 2- or 8-day-old rat cerebellums were used in dissociated cell cultures as a source of growing neurites. An apparent presynaptic element is a defined swelling on a neurite with 40 nm diameter vesicles accumulated at the membrane which has a slight thickening. Apparent presynaptic elements do not have a normal postsynaptic element. In place of the postsynaptic element was the polylysine-coated surface of a large diameter Sepharose bead. The first apparent presynaptic elements were seen at 2 h of incubation, suggesting that morphologically identifiable synapses may form in this short a time. The number of apparent presynaptic elements on beads increased from 2 h to 24 h incubation and decreased from 5 to 9 days incubation. At all times non-neuronal cells grew up on to the beads and often covered both neurites and apparent presynaptic elements. In the longer incubations degenerating apparent presynaptic elements were seen engulfed by non-neuronal cells, suggesting that non-neuronal cells may have the ability to remove presynaptic elements that are not functioning. The number of synaptic vesicles per apparent presynaptic elements increased continuously between 2 h and 9 day incubation, eventually surpassing the number of synaptic vesicles seen in other presynaptic elements in cultures. This result suggests that an interaction between presynaptic and postsynaptic elements may be necessary to limit the number of synaptic vesicles found in presynaptic elements. Cultures grown for only 5 days in vitro (DIV) and incubated for 1 day with coated beads had the most apparent presynaptic elements, while those at 28 DIV and incubated for 1 day had the least. In the cultures at 5 DIV, neurons formed many apparent presynaptic elements, but these neurons could form only a very few normal synapses between themselves. Thus the beginning of the formation of synaptic contacts may depend on the availability of future postsynaptic elements and not on the lack of future presynaptic elements.

Perturbation of the switch-on of transcriptase activity in intermediate subviral particles from reovirus

Intermediate subviral particles (ISVP) derived from reovirus represent a simple model system for the switch-on of transcriptase function. In such particles the endogenous transcriptase is present in a switched-off form, one step removed from the switched-on state. Switch-on of transcriptase function is an active process in this system and can be triggered by K+ions. A variety of agents which affect gene expression in cells were tested for an effect on switch-on in ISVP. Marked effects on switch-on in ISVP were observed with a diverse group of test agents, including DMSO and other solvents, BUdR, TdR, caffeine, theophylline, and temperature. The correlation in response between ISVP and cells suggests that the ISVP system may be useful as a model for studying the biochemical mechanisms underlying the perturbative effects of such agents on gene expression in cells.

In vivo and in vitro development of serotonergic neurons

The monoamines are one of the earliest developing neurotransmitter systems in the mammalian brain. The first part of this paper describes the normal ontogeny of the serotonergic (5-HT) system in the rat brain as studied using long survival 3H-thymidine autoradiography (time of neuronal genesis, time of origin) and the Falck-Hillarp histofluorescence method, electron microscopy, and immunocytochemistry (anti-5-HT). Due to their early ontogeny relative to other brain regions, 5-HT neurons (as well as monoamine neurons in general) have been suggested to exert some type of "trophic" influence on brain development. Results of pharmacological experiments designed to inhibit 5-HT synthesis in the embryonic rat brain by maternal treatment with p-chlorophenylalanine (pCPA) at a time when this monoamine might exert such an influence are discussed with regard to effects on the time course of neuronal genesis (time of origin) of 5-HT neurons and their target cells. These results, which prompted us to propose that 5-HT might act as a "differentiation signal" for certain of its target cells, are now discussed in light of our more recent immunocytochemical-autoradiographic studies (anti-5-HT, 3H-thymidine) which morphologically demonstrate close associations between developing 5-HT neurons and proliferating neuroepithelial cells in the embryonic brain. Postnatal studies using this immunocytochemical-autoradiographic method also provide evidence for interactions of 5-HT axons with proliferating glioblasts in the developing cerebellum and with immature granule cells and their precursors in the hippocampus. These findings, in conjunction with the results of our pCPA experiments, further enhance the possibility that 5-HT neurons could exert an epigenetic influence on the development of less differentiated cells with which they come into contact. Finally, preliminary studies using dissociated cell cultures containing 5-HT neurons suggest that interactions between 5-HT neurons and glial elements may be important for the differentiation of these neurons in vitro. Whether 5-HT neurons in turn influence the development of glial or neuronal cells in these cultures remains to be determined. These studies are evaluated with regard to a possible pre-transmission role for 5-HT during key phases of neuronal and glial genesis.

Modifications of chromatin structure in control of gene expression

Repetitive sequences in intron and spacer DNA could be sites for binding of chromosomal proteins which maintain chromatin structure and control gene activity. Methylation of DNA guides the binding of acidic nonhistone proteins and maintains the differentiation state during DNA replication. Differentiation inducers modify repressor proteins permitting unfolding of chromatin. Histone H 1 must be removed for gene activity. Phosphorylation of nonhistone proteins probably induces allosteric modifications which permit unfolding of chromatin. Acetylation of nucleosomal histones is necessary to permit passage of RNA polymerase. Deacetylation quickly returns the gene to a normal histone repressed state. Chromosomal RNA attached to nonhistone proteins aids the binding of RNA polymerase to the DNA template. Carcinogens can disrupt normal gene control leading to circumvention of normal cell cycle controls.

Development of beta-adrenergic receptors and the in vitro accumulation of cyclic AMP in the chick spinal cord

To clarify the functional development of the descending monoaminergic input to the chick spinal cord we have studied the ontogeny of beta-adrenergic receptors by measuring the specific binding the tritiated dihydroalprenolol (DHA). In addition, we examined the ability of isoproterenol to stimulate the accumulation of cyclic AMP in slices of developing chick spinal cord. Results show that the chick spinal cord contains a high density of beta-adrenergic receptors that are apparently linked to adenylate cyclase. During development, both the density of beta-receptors, as determined by the specific binding of DHA, and the response of tissue slices to isoproterenol underwent marked changes. beta-Adrenergic receptors (approximately 4 fmol/mg tissue) were first detected on the fourteenth day in ovo. Receptor density increased to approximately 20 fmol/mg by day 20. Between day 20 and the time of hatching, a sharp increase in receptor density, to approximately 50 fmol/mg, was seen. The density of receptors remained high until the second day after hatching, fell off to approximately 30 fmol/mg by the fourth day, and remained relatively unchanged through day 30. The response of spinal cord slices to isoproterenol showed a similar pattern of development with the peak response (7-fold increase in levels of cyclic AMP) occurring at or near the time of hatching. During the period between day 18 in ovo and the time of hatching, when both the response of tissue slices to isoproterenol and the density of beta-receptors increased markedly, the activity of phosphodiesterase did not change. Therefore, the pronounced changes in adrenergic responsiveness that occurred near the time of hatching appear to be related primarily to changes in the density of beta-adrenergic receptors coupled to adenylate cyclase. Such developmental changes in the density of beta-adrenergic receptors and adrenergic responsiveness may play an important role in determining the functional state of the descending monoaminergic systems in the chick spinal cord.