Biomedical subjects

N I Syed

Publications and source records attributed to N I Syed.

49 records · Page 3Linked to original sources

PubMed1993

In vitro evidence for multiple neuritogenic factors in the central nervous system of pulmonate molluscs.

Previous work has shown that neurons of the fresh water pond snails, Lymnaea and Helisoma, require soluble factors produced by neural tissues for neurite outgrowth to occur in vitro. In the present study, we show that mammalian nerve growth factor (NGF) stimulates neurite outgrowth of specific Lymnaea neurons. In contrast to motoneurons and interneurons, which show a robust dose response to NGF, no response was observed in neurosecretory cells. In an attempt to localize neuritogenic activity to specific ganglia or organs, we show that the dorsal bodies, endocrine structures of the cerebral ganglia, promote neurite outgrowth of specific neurons. In general, however, the spectrum of neurons that respond to dorsal body cell conditioned medium differs from that which respond to NGF. We conclude that Lymnaea neurons respond both to NGF and also to a separate factor derived from the dorsal body cells.

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PubMed1993

In vitro connections between Lymnaea and Helisoma identified interneurons and their follower cells.

The mechanisms that underly the specificity of synaptic connections are poorly understood. In this study we used two homologous interneurons, the giant dopamine cell (GDC), in two species of pond snails, Lymnaea and Helisoma. We examined the ability of the Lymnaea GDC to form specific synapses with known follower or non-follower cells in vitro. Similar tests were performed for the Helisoma GDC. Both of these interneurons form appropriate connections not only with homologous follower neurons, but also with follower neurons from the alternative species. These results suggest that common mechanisms of cell recognition and synapse formation exist in the nervous systems of these two different families of molluscs.

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PubMed1992

Reconstruction of neuronal networks in culture.

Since the 1960s, the large neurones of some invertebrates have been exploited in attempts to define the neural circuits that underlie simple behaviours. Even in the relatively 'simple' nervous systems of these animals, it is often difficult to study individual synaptic connections in detail and to rule out involvement of unidentified neurones. These limitations have been overcome by reconstruction of partial circuits of identified neurones in cell culture. This approach has provided opportunities to examine the function of small neuronal circuits in a manner that is unapproachable in the intact nervous system.

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PubMed1992

Transplantation and functional integration of an identified respiratory interneuron in Lymnaea stagnalis.

The possibility that damaged neural circuitries can be repaired through grafting has raised questions regarding the cellular mechanisms required for functional integration of transplanted neurons. Invertebrate models offer the potential to examine such mechanisms at the resolution of single identified neurons within well-characterized neural networks. Here it is reported that a specific deficit in the respiratory behavior of a pulmonate mollusc, caused by the ablation of a solitary interneuron, can be restored by grafting an identical donor interneuron. The transplanted interneuron not only survives and extends neurites within the host nervous system, but under specific conditions forms synapses with appropriate target neurons and is physiologically integrated into the host's circuitry, thereby restoring normal behavior.

Action Potentials↗

PubMed1992

Specific in vitro synaptogenesis between identified Lymnaea and Helisoma neurons.

We tested the ability of identified neurons from two different families of pulmonate molluscs to form specific connections in vitro. The presynaptic neuron chosen for this study was the giant dopamine cell of Lymnaea stagnalis and Helisoma trivolvis which is known to synapse upon specific visceral and parietal ganglion neurons in both species. Here we show that the giant dopamine cells can reform specific connections in vitro on follower neurons from both species. Thus the mechanisms that determine synapse specificity are conserved between two different families of molluscs.

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PubMed1992

The respiratory central pattern generator of Lymnaea.

We have recently described the respiratory behavior of a pulmonate mollusc, Lymnaea stagnalis, and identified relevant motor neurons and interneurons involved in this behavior. Three interneurons, namely right pedal dorsal 1 (R.Pe.D1), visceral dorsal 4 (V.D4) and Input 3 interneuron (Ip.3.I) comprise the central pattern generator (CPG). We demonstrate that appropriate connections exist between these interneurons and that they are sufficient to form the basis for the CPG.

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PubMed1992

The respiratory central pattern generator (CPG) of Lymnaea reconstructed in vitro.

We have recently developed a model system for testing the necessity, appropriateness and sufficiency of individual components of the respiratory central pattern generator (CPG) in Lymnaea stagnalis. In order to examine the intrinsic and network properties of the three CPG interneurons (R.Pe.D1, V.D4 and Ip.3.I), these cells were isolated and cultured in vitro. All of these cells exhibited extensive neurite outgrowth within 18-24 h of plating in conditioned medium. These isolated neurons maintained their intrinsic properties in culture and also formed specific synapses with each other similar to those observed in vivo. However, only when all three interneurons were plated together was it possible to initiate the alternating rhythm characteristic of the respiratory activity observed in semi-intact or isolated brain preparations.

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PubMed1992

Regeneration of an interneuronal network in Helisoma: re-establishment of synaptic contacts.

We have identified a network of three interneurons located in the central ring ganglia of Helisoma. Two of these interneurons, designated Left Pedal Dorsal 1 (LPeD1) and Right Pedal Dorsal 1 (RPeD1), are the largest neurons of the pedal ganglia and appear to contain dopamine and serotonin, respectively. A third interneuron, identified as Visceral Dorsal 4 (VD4), is a small FMRFamide immunoreactive cell located on the dorsal surface of the visceral ganglion. Monosynaptic chemical connections exist between all these interneurons. For instance, a reciprocal inhibitory connection exists between LPeD1 and RPeD1, whereas VD4 has inhibitory effects on both LPeD1 and RPeD1. Furthermore, LPeD1, but not RPeD1, has an excitatory connection with VD4. We demonstrate that following axotomy these interneurons not only regenerate their axons but re-establish their appropriate synaptic connections.

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PubMed1991

Nerve growth factor (NGF) induces sprouting of specific neurons of the snail, Lymnaea stagnalis.

Nerve growth factor (NGF) was examined for its ability to elicit sprouting by adult molluscan neurons. Motoneurons and interneurons (but not neurosecretory cells) from Lymnaea exhibited a sprouting response to murine 2.5S NGF in defined medium with a half-maximal response at about 150 ng/mL. Furthermore, an NGF antiserum blocked sprouting by all normally responsive neurons. We tested whether an NGF-like molecule is a component of conditioned medium (CM) by attempting to preabsorb its sprout-inducing activity with NGF antiserum. Treatment of CM with immune (but not nonimmune) serum largely blocked the response of motoneurons, but not that of neurosecretory cells, to CM. We conclude that NGF exerts neurotrophic activity on specific adult Lymnaea neurons, and suggest the possibility that an NGF-like molecule may exist in the molluscan nervous system.

Absorption↗

PubMed1991

Coordination of locomotor and cardiorespiratory networks of Lymnaea stagnalis by a pair of identified interneurones.

1. The morphology and electrophysiology of a newly identified bilateral pair of interneurones in the central nervous system of the pulmonate pond snail Lymnaea stagnalis is described. 2. These interneurones, identified as left and right pedal dorsal 11 (L/RPeD11), are electrically coupled to each other as well as to a large number of foot and body wall motoneurones, forming a fast-acting neural network which coordinates the activities of foot and body wall muscles. 3. The left and right sides of the body wall of Lymnaea are innervated by left and right cerebral A cluster neurones. Although these motoneurones have only ipsilateral projections, they are indirectly electrically coupled to their contralateral homologues via their connections with L/RPeD11. Similarly, the activities of left and right pedal G cluster neurones, which are known to be involved in locomotion, are also coordinated by L/RPeD11. 4. Selective ablation of both neurones PeD11 results in the loss of coordination between the bilateral cerebral A clusters. 5. Interneurones L/RPeD11 are multifunctional. In addition to coordinating motoneuronal activity, they make chemical excitatory connections with heart motoneurones. They also synapse upon respiratory motoneurones, hyperpolarizing those involved in pneumostome opening (expiration) and depolarizing those involved in pneumostome closure (inspiration). 6. An identified respiratory interneurone involved in pneumostome closure (visceral dorsal 4) inhibits L/RPeD11 together with all their electrically coupled follower cells. 7. Both L/RPeD11 have strong excitatory effects on another pair of electrically coupled neurones, visceral dorsal 1 and right parietal dorsal 2, which have previously been shown to be sensitive to changes in the partial pressure of environmental oxygen (PO2). 8. Although L/RPeD11 participate in whole-body withdrawal responses, electrical stimulation applied directly to these neurones was not sufficient to induce this behaviour.

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PubMed1990

In vitro reconstruction of the respiratory central pattern generator of the mollusk Lymnaea.

Most rhythmic behaviors such as respiration, locomotion, and feeding are under the control of networks of neurons in the central nervous system known as central pattern generators (CPGs). The respiratory rhythm of the pond snail Lymnaea stagnalis is a relatively simple, CPG-based behavior for which the underlying neural elements have been identified. A three-neuron network capable of generating the respiratory rhythm of this air-breathing mollusk has been reconstructed in culture. The intrinsic and network properties of this neural ensemble have been studied, and the mechanism of postinhibitory rebound excitation was found to be important for the rhythm generation. This in vitro model system enables a better understanding of the neural basis of rhythm generation.

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PubMed1989

The morphology and electrophysiology of the neurones of the paired pedal ganglia of Lymnaea stagnalis (L.).

1. A morphological and electrophysiological map of the identifiable neurones and neuronal clusters of the paired pedal ganglia has been prepared. 2. Neuronal morphology was investigated using the fluorescent dye, Lucifer Yellow CH, whilst electrophysiological properties were studied using conventional intracellular recording techniques and the phase plane technique. 3. The paired pedal ganglia are largely symmetrical and giant neurones usually have contralateral homologues. 4. Neuronal clusters are also paired, but minor asymmetries, both of identifiable neurones and neuronal clusters have been found to exist. 5. These asymmetries are thought to be related to asymmetries of body form. 6. Most of the individually identifiable neurones possess obligatory axon branches which are invariant from one preparation to the next, but variant branches also occur. 7. Within the neuronal clusters, morphology appears to be more variable. 8. Individually identifiable neurones and neuronal clusters were characterized electrophysiologically according to the criteria of action potential shape, spontaneous activity pattern, electrical coupling and common synaptic inputs. 9. Homologous pairs of neurones usually have similar electrophysiological properties, as do those within clusters. 10. A number of wide-acting synaptic inputs have been identified on neurones of the pedal, buccal, visceral and parietal ganglia.

Action Potentials↗

PubMedDate not supplied

Trophic factor-induced plasticity of synaptic connections between identified Lymnaea neurons.

Neurotrophic factors participate in both developmental and adult synaptic plasticity; however, the underlying mechanisms remain unknown. Using soma-soma synapses between the identified Lymnaea neurons, we demonstrate that the brain conditioned medium (CM)-derived trophic factors are required for the formation of excitatory but not the inhibitory synapse. Specifically, identified presynaptic [right pedal dorsal 1 (RPeD1) and visceral dorsal 4 (VD4)] and postsynaptic [visceral dorsal 2/3 (VD2/3) and left pedal dorsal 1 (LPeD1)] neurons were soma-soma paired either in the absence or presence of CM. We show that in defined medium (DM-does not contain extrinsic trophic factors), appropriate excitatory synapses failed to develop between RPeD1 and VD2/3. Instead, inappropriate inhibitory synapses formed between VD2/3 and RPeD1. Similarly, mutual inhibitory synapses developed between VD4 and LPeD1 in DM. These inhibitory synapses were termed novel because they do not exist in the intact brain. To test whether DM-induced, inappropriate inhibitory synapses could be corrected by the addition of CM, cells were first paired in DM for an initial period of 12 hr. DM was then replaced with CM, and simultaneous intracellular recordings were made from paired cells after 6-12 hr of CM substitution. Not only did CM induce the formation of appropriate excitatory synapses between both cell pairs, but it also reduced the incidence of inappropriate inhibitory synapse formation. The CM-induced plasticity of synaptic connections involved new protein synthesis and transcription and was mediated via receptor tyrosine kinases. Taken together, our data provide the first direct insight into the cellular mechanism underlying trophic factor-induced specificity and plasticity of synaptic connections between soma-soma paired Lymnaea neurons.

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