Biomedical subjects

G E Spencer

Publications and source records attributed to G E Spencer.

At least 19 recordsLinked to original sources

PubMed2001

Functional implications of neurotransmitter expression during axonal regeneration: serotonin, but not peptides, auto-regulate axon growth of an identified central neuron.

We studied the regenerative properties of one of two electrically coupled molluscan neurons, the serotonergic cerebral giant cells (CGCs) of Lymnaea stagnalis, after axotomy. The CGCs play a crucial role in feeding behavior, and when both cells are disconnected from their target neurons, animals no longer feed. When one CGC was permanently disconnected from its targets and the other was reversibly damaged by a nerve crush, the latter one regenerated over a period of 2 weeks to reform functional synapses with specific target neurons. At the same time, recovery of the feeding behavior was observed. After the crush, neuropeptide gene expression in the CGC was downregulated to approximately 50%. Serotonin synthesis, on the other hand, remained unaffected, suggesting that serotonin might have an active role in regeneration. In primary neuron culture, CGCs failed to extend neurites in the presence of serotonin; in cells that extended neurites in the absence of serotonin, focally applied serotonin, but not neuropeptides, induced growth cone collapse. Using serotonin-sensitive sniffer cells, we show that CGC neurites and growth cones release serotonin in culture. Finally, both the spontaneous and stimulation-induced release of serotonin from CGCs in culture resulted in growth cone collapse responses that could be blocked by the serotonin receptor antagonist methysergide. Our data suggest that auto-released serotonin is inhibitory to CGC neurite outgrowth in vitro. During regeneration in vivo, serotonin release might fine-tune axon guidance and branching by inducing local collapse responses in extending neurites.

Animals↗

PubMed2000

Transmitter-receptor interactions between growth cones of identified Lymnaea neurons determine target cell selection in vitro.

In addition to their involvement in transsynaptic communication in the adult nervous system, neurotransmitters also participate in many developmental events, such as neurite initiation and outgrowth. Although growth cones can release transmitters and are themselves sensitive to exogenously applied neurotransmitters, a direct causal relationship between the release of transmitter from one growth cone and its effect on another has not yet been demonstrated. In this study, we provide evidence that dopamine release from the growth cones of an identified Lymnaea neuron, right pedal dorsal 1 (RPeD1), differentially regulates the growth cone behavior of its in vivo target and nontarget neurons in vitro. In coculture, RPeD1 growth cones enhanced the rate of growth cone advance from target cells and synaptic connections developed immediately after contact. In contrast, RPeD1 growth cones not only inhibited the rate of growth cone advance from nontarget cells but they also induced growth cone collapse. Using a "sniffer cell" approach, we demonstrated that both RPeD1 growth cones and somata released dopamine, which can be detected at a distance of several hundred micrometers. RPeD1 somata were used to demonstrate that spontaneous release of dopamine also acted as a chemoattractant for target growth cones but as a chemorepellent for nontarget growth cones. These effects were mimicked by exogenous dopamine application, and both RPeD1 growth cone and soma-induced effects were also blocked in the presence of dopamine receptor antagonists. This study emphasizes the importance of transmitter-receptor interactions between growth cones in target cell selection.

Animals↗

PubMed2000

Synthesis and functional integration of a neurotransmitter receptor in isolated invertebrate axons.

Neurotransmitter receptors are considered an important class of membrane proteins that are involved in plasticity-induced changes underlying learning and memory. Recent studies, which demonstrated that the mRNAs encoding for various receptor proteins are localized to specific dendritic domains, allude toward the possibility that these membrane bound molecules may be synthesized locally. However, direct evidence for the local axonal or dendritic synthesis and functional integration of receptor proteins in either vertebrates or invertebrates is still lacking. In this study, using an invertebrate model system we provide the first direct evidence that isolated axons (in the absence of the soma) can intrinsically synthesize and functionally integrate a membrane-bound receptor protein from an axonally injected mRNA. Surgically isolated axons from identified neurons were injected with mRNA encoding a G-protein-coupled conopressin receptor. Immunocytochemical and electrophysiological techniques were used to demonstrate functional integration of the receptor protein into the membrane of the isolated axon. Ultrastructural analysis of axonal compartments revealed polyribosomes, suggesting that some components of the protein synthesizing machinery are indeed present in these extrasomal compartments. Such axonal propensity to locally synthesize and functionally insert transmitter receptors may be instrumental in plasticity induced changes, for instance those that underlie learning and memory.

Animals↗

PubMed1999

Neural changes after operant conditioning of the aerial respiratory behavior in Lymnaea stagnalis.

In this study, we demonstrate neural changes that occurred during operant conditioning of the aerial respiratory behavior of Lymnaea stagnalis. Aerial respiration in Lymnaea occurs at the water interface and is achieved by opening and closing movements of its respiratory orifice, the pneumostome. This behavior is controlled by a central pattern generator (CPG), the neurons of which, as well as the motoneurons innervating the pneumostome, have previously been identified and their synaptic connections well characterized. The respiratory behavior was operantly conditioned by applying a mechanical stimulus to the open pneumostome whenever the animal attempted to breathe. This negative reinforcement to the open pneumostome resulted in its immediate closure and a significant reduction in the overall respiratory activity. Electrophysiological recordings from the isolated CNSs after operant conditioning showed that the spontaneous patterned respiratory activity of the CPG neurons was significantly reduced. This included reduced spontaneous activity of the CPG interneuron involved in pneumostome opening (input 3 interneuron) and a reduced frequency of spontaneous tonic activity of the CPG interneuron [right pedal dorsal 1 (RPeD1)]. The ability to trigger the patterned respiratory activity by electrical stimulation of RPeD1 was also significantly reduced after operant conditioning. This study therefore demonstrates significant changes within a CPG that are associated with changes in a rhythmic homeostatic behavior after operant conditioning.

Activity Cycles↗

PubMed1998

Modulation of reconstructed peptidergic synapses and electrical synapses by general anaesthetics.

1. The actions of clinically relevant concentrations of general anaesthetics on reconstructed peptidergic synapses and electrical synapses in the intact brain of the mollusc Lymnaea stagnalis (L.) are described. 2. At identified, reconstructed, FMRFamidergic synapses, chemical synaptic transmission is completely blocked in 2% halothane. 3. Inhibitory postsynaptic responses to directly applied FMRFamide are maintained in 2% halothane and are enhanced in 1% halothane, unlike excitatory responses which are abolished at this concentration. 4. Met-enkephalin normally produces inhibitory responses on postsynaptic PeA neurones, but these are non-reversibly abolished by halothane, whose presence induces novel, dose-dependent, enkephalinergic depolarising responses. 5. The biophysical effects of volatile anaesthetics and sodium pentobarbital on neuronal membranes have been described and they are shown to have opposite dose-dependent effects on input resistance, input conductance and time constant of the electrically coupled neurones VD1 and RPD2. 6. Volatile anaesthetics decouple the neurones VD1 and RPD2 in a dose dependent manner, whilst sodium pentobarbital either enhances coupling or has no effect, depending on the concentration used.

Anesthetics, General↗

PubMed1998

Neurotransmitters and neurodevelopment. Role of dopamine in neurite outgrowth, target selection and specific synapse formation.

Neurotransmitters and their receptors appear early during nervous system development and are thought to play important roles in neurite outgrowth, growth cone motility, target cell selection and synaptogenesis. In vivo studies in both vertebrates and invertebrates have shown that the perturbations of embryonic transmitter expression result in abnormal morphological and synaptic development. In vitro studies have further revealed that transmitters are capable of affecting neurite outgrowth and growth cone behaviour. The precise cellular mechanisms by which neurotransmitters affect these developmental steps are, however, poorly defined. In vitro, a presynaptic neuron from the mollusc Lymnaea stagnalis releases dopamine, which induces both growth cone attraction and growth cone collapse of target and non-target cell growth cones, respectively. We propose that the ability of dopamine to differentially affect growth cone motility of two cell types results from a divergence of the dopamine receptor-activated second messenger pathways at the G-protein level. Such transmitter-receptor interactions between growth cones of specific neurons may not only induce changes in the growth cone motility, but may subsequently play an important role in target cell selection and specificity of synaptogenesis.

Animals↗

PubMed1996

Halothane affects both inhibitory and excitatory synaptic transmission at a single identified molluscan synapse, in vivo and in vitro.

In the isolated CNS of Lymnaea, a peptidergic neuron termed VD4 makes monosynaptic connections with identified pedal A cluster neurons. In this study, the pedal A (PeA) neurons were further divided into two subgroups depending upon whether they received an inhibitory or excitatory input from VD4. PeA cells inhibited by VD4 were designated PeA(I), whereas those excited by VD4 were termed PeA(E). Both inhibitory and excitatory effects of VD4 stimulation on the PeA(I) and PeA(E) cells, respectively, were mimicked by exogenous FMRFamide in culture (in vitro), implicating this or a related peptide as the transmitter utilized at the VD4-to-PeA synapses. We tested the ability of the general anesthetic, halothane, to affect either the inhibitory or the excitatory peptidergic synapses between VD4 and the PeA neurons, both in the isolated CNS (in vivo) and at the in vitro reconstructed synapses. In the presence of 1% halothane, the excitatory synaptic potential between VD4 and the PeA(E) cells was either depressed or completely abolished, whereas the inhibitory synaptic potential between VD4 and the PeA(I) cells was unaffected in the presence of 1% halothane. The inhibitory potential between VD4 and the PeA(I) cells was, however, blocked in 2% halothane. In order to determine halothane' 5 site of action, exogenous FMRFamide was applied to both PeA(E) and PeA(I) cells in the presence of 1 or 2% halothane. In 1% halothane, the excitatory responses produced by FMRFamide were substantially reduced or abolished, whereas the inhibitory responses to FMRFamide were maintained and enhanced in duration in 1% halothane. In 2% halothane, the inhibitory responses to exogenous FMRFamide remained unchanged. It, therefore, appears that halothane exerts effects at both the pre- and postsynaptic level of the synapse, although presynaptic transmitter release is probably not substantially affected until a concentration of 2% halothane is reached. Our data provide the first evidence that clinically relevant concentrations of halothane (1-2%) affect both excitatory and inhibitory peptidergic synaptic transmission between identified neurons in the nervous system. Furthermore, excitatory transmission is abolished at lower anesthetic concentrations than inhibitory transmission.

Animals↗

PubMed1996

Dopamine regulation of neurite outgrowth from identified Lymnaea neurons in culture.

1. An identified dopaminergic interneuron (RPeD1) of the snail Lymnaea stagnalis, makes specific synaptic connections with a number of target (VI and VJ) but not non-target (VF and RPB) neurons in vivo. When cultured in vitro with both target and non-target cells, RPeD1 re-establishes synapses with target cells only. 2. To test whether exogenous dopamine exerts effects on the neurite outgrowth of both target and non-target neurons respectively, these cells were cultured in conditioned media (CM) in the presence of dopamine (10(-5) M). The growth of the non-target cells was severely restricted and retarded in the presence of dopamine. These data suggest that dopamine may regulate neurite outgrowth of non-target cells in culture. 3. The growth regulatory effects of dopamine on the non-target cells were blocked in the presence of a dopamine receptor antagonist (R(+) SCH-23390, 10(-4) M). These results indicate that dopamine-induced growth regulation of the non-target cells is mediated via dopamine receptors on these cells. 4. In the absence of conditioned media, dopamine was not sufficient to exert growth promoting effects on either target or non-target cells. 5. Taken together, our data show that dopamine differentially regulates growth of identified Lymnaea neurons in culture. Dopamine alone, however, is not sufficient to initiate and support neurite outgrowth from these cells. Rather, it functions to suppress the neurite outgrowth of the non-target cells, initiated by the conditioned media.

Animals↗

PubMed1995

Halothane-induced synaptic depression at both in vivo and in vitro reconstructed synapses between identified Lymnaea neurons.

1. In the present study we tested the ability of the general anesthetic, halothane, to affect synaptic transmission at in vivo and in vitro reconstructed peptidergic synapses between identified neurons of Lymnaea stagnalis. 2. An identified respiratory interneuron, visceral dorsal 4 (VD4), innervates a number of postsynaptic cells in the central ring ganglia of Lymnaea. Because VD4 has previously been shown to exhibit immunoreactivity for FMRFamide-related peptides, it was hypothesized that these peptides may be utilized by VD4 during synaptic transmission. In the intact, isolated CNS of Lymnaea, we have identified novel connections between VD4 and the pedal A (PeA) cells. We demonstrate that VD4 makes inhibitory connections with the PeA neurons, in particular PeA4, and that these synaptic responses are mimicked by exogenous application of FMRFamide. 3. The synaptic transmission between VD4 and the PeA cells in an intact, isolated CNS preparation was completely blocked in 2%, but not 1% halothanc. Interestingly, the postsynaptic responses (PeA) to exogenous FMRFamide were maintained in the presence of both 1 and 2% halothane. 4. To determine the specificity of the observed responses and to determine the precise synaptic site of anesthetic action, we reconstructed the VD4/PeA synapses in vitro. After isolation from their respective ganglia, both cell types extended processes and established neuritic contact. We demonstrated that not only did the presynaptic neuron reestablish the appropriate inhibitory synapses with the PeA neurons, but that the PeA cells also maintained their responsiveness to exogenous FMRFamide. 5. Superfusion of the in vitro synaptically connected VD4 and PeA cells with 2% halothane completely abolished the synaptic transmission between these cells. However, even higher concentrations of 4% halothane failed to block the responsiveness of the PeA neurons to exogenous FMRFamide. Moreover, both 1 and 2% halothane enhanced the duration of the postsynaptic response to exogenously applied FMRFamide. These data suggest that the halothane-induced depression of synaptic transmission most likely occurred at the presynaptic level. 6. This study provides the first direct evidence that peptidergic transmission in the nervous system may also be susceptible to the actions of general anesthetics. In addition, we utilized a novel approach of in vitro reconstructed synapses for studying the effects of general anesthetics on monosynaptic transmission in the absence of other synaptic influences.

Anesthetics, Inhalation↗

PubMed1992

Transmucosal electrical resistance in rabbit isolated gastric mucosa during exposure to acid.

1. Transmucosal electrical resistance (Rt) and short-circuit current (Isc) were determined in rabbit isolated fundic mucosa. Under basal conditions, with a HCO(3-)-free HEPES-buffered solution (pH 7.4) bathing both sides of the mucosae, Rt was 161.5 + 5.0 omega cm2 and Isc 41.8 +/- 1.8 microA cm-2, and these values were not significantly different to values observed in HCO(3-)-buffered Krebs-Hensleit solution. 2. The basal Isc was inhibited by the Cl- channel blocker diphenylamine-2-carboxylate, and ouabain, but unaffected by the Na+ channel blocker amiloride (10(-5) M), consistent with electrogenic chloride secretion dependent upon a sodium gradient. Prostaglandin E2 (10(-7) M) stimulated an increase in Isc which was susceptible to inhibition by diphenylamine-2-carboxylate, but not amiloride, again consistent with Cl- secretion. 3. Stepwise acidification of the mucosal solution to pH 2.8 resulted in an increase in Rt of 43%, as compared with that measured with mucosal pH 7.4. Isc did not change during the acidification to pH 2.8, indicating retention of tissue viability. Increased Rt while Isc remained constant is consistent with an acid-induced decrease in the shunt (paracellular) conductance in this Cl(-)-secreting tissue. At pH less than 2.8, Rt declined rapidly and Isc declined and reversed, consistent with H+ back-diffusion. Scanning electron microscopic investigation of tissue exposed to mucosal pH 2.8 revealed little difference from control (pH 7.4) tissue, but there was considerable evidence of cellular damage and membrane disruption in tissue exposed to pH 1.8. 4. Acidification of the serosal solution did not increase Rt, which was maintained until pH 3.7, and then rapidly declined at pH less than 3.7. Bilateral acidification produced a mixed response; Rt increased, as for mucosal acidification, down to pH 2.8, after which there was a rapid decline in Rt following the pattern observed for serosal acidification. 5. Compared at a mucosal pH approximately 2.8, DIDS (4 x 10(-4) M) and amiloride (10(-3) M) inhibited the acid-induced increase in Rt, suggesting a role for both Cl(-)-HCO3- and Na(+)-H+ exchange in the response. In contrast, the acid-induced increase in Rt was unaffected by a lower concentration of amiloride (10(-5) M), acetazolamide and ouabain. Therefore, neither Na+ channels nor a Na+ gradient appear to be involved in the acid-induced increase in Rt.(ABSTRACT TRUNCATED AT 400 WORDS)

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗

PubMed1987

Destructive monarticular arthritis secondary to anticoagulant therapy.

Hemarthrosis secondary to anticoagulant therapy is a well-known clinical problem. The pathologic process usually occurs in large joints and is reversible with the discontinuation of the anticoagulant medication. The condition presented for consideration here is unusual for two reasons. First, it is relatively uncommon for the ankle joint to be involved. Second, the destructive arthritis progressed after the medication was discontinued. Resting the involved joint until symptoms subside is often adequate treatment for anticoagulant-induced hemarthrosis. In an 84-year-old man, an arthrodesis was necessary to achieve a symptom-free ankle joint. Hemarthrosis secondary to anticoagulant medication may not be a benign disease process.

Aged↗

PubMed1983

Predicting the success of reambulation in patients with Duchenne muscular dystrophy.

We used biochemical and clinical variables to develop a method to predict the expected duration of independent walking following surgery and bracing in patients with Duchenne muscular dystrophy. Data from the records of fifty patients were analyzed by linear and multiple regression. The most useful factors, applied in combination, in predicting the duration of walking ability after bracing were: percentage of residual muscle strength, vital capacity, creatinine coefficient, motivation of the patient at the time of bracing, and decrease in creatinine coefficient in the two years prior to bracing. This system uses readily available variables to predict the response to bracing in patients with Duchenne muscular dystrophy. Improvement in the criteria for the selection of patients for surgery and bracing is important in view of the economic cost as well as the demands on the time and energy of these children and their parents.

Braces↗

PubMed1977

A non-clogging suction system.

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Humans↗

PubMed1977

Fibrous contracture of muscles following intramuscular injections in adults.

Periarticular fibrous muscle contractures in adults from repeated injections in the same site is predictable. The causes of joint contracture in children are many and complex, but in adults it seems certain that this phenomenon is the result of repeated injections of analgesics or other agents into 1 muscle area. Any drug if repeatedly injected locally may cause fibrosis of the muscle and subsequent joint contracture. Five cases of bilateral abduction contracture of the shoulder in adults including the first case of bilateral abduction contractures of shoulder and hip plus bilateral flexion contracture of elbow and extension contracture of a knee are reported. No underlying disease which might predispose to this fibrosis of muscles was noted. The frequency and period of injections were variable over several years. In all patients the interference in activtities of daily living were serious, but the deformities were corrected by release of the fibrous band with relief of discomfort and restoration of joint motion without recurrence. Noting the potential complication of repeated intramuscular injections in one area, this practice should be avoided whenever possible in adults, as well as in children.

Adult↗

PubMed1975