Axonal growth from insect neurons in glia-free cultures

Molluscan neurons in culture: shedding light on synapse formation and plasticity

From genes to behaviour, the simple model system approach has played many pivotal roles in deciphering nervous system function in both invertebrates and vertebrates. However, with the advent of sophisticated imaging and recording techniques enabling the direct investigation of single vertebrate neurons, the utility of simple invertebrate organisms as model systems has been put to question. To address this subject meaningfully and comprehensively, we first review the contributions made by invertebrates in the field of neuroscience over the years, paving the way for similar breakthroughs in higher animals. In particular, we focus on molluscan (Lymnaea, Aplysia, and Helisoma) and leech (Hirudo) models and the pivotal roles they have played in elucidating mechanisms of synapse formation and plasticity. While the ultimate goal in neuroscience is to understand the workings of the human brain in both its normal and diseased states, the sheer complexity of most vertebrate models still makes it difficult to define the underlying principles of nervous system function. Investigators have thus turned to invertebrate models, which are unique with respect to their simple nervous systems that are endowed with a finite number of large, individually identifiable neurons of known function. We start off by discussing in vivo and semi-intact preparations, regarding their amenability to simple circuit analysis. Despite the 'simplicity' of invertebrate nervous systems however, it is still difficult to study individual synaptic connections in detail. We therefore emphasize in the next section, the utility of studying identified invertebrate neurons in vitro, to directly examine the development, specificity, and plasticity of synaptic connections in a well-defined environment, at a resolution that it is still unapproachable in the intact brain. We conclude with a discussion of the future of invertebrates in neuroscience in elucidating mechanisms of neurological disease and developing neuron-silicon interfaces.

Effects of nicotinic and muscarinic ligands on embryonic neurones of Periplaneta americana in primary culture: a whole cell clamp study

The pharmacological properties of acetylcholine (ACh) receptors of cultured neurones from embryonic cockroach brains were studied using the whole-cell configuration of the patch-clamp technique. More than 90% of the studied neurones responded to ACh by a monophasic inward current, the intensity of which varied from cell to cell. The sequence of potency of the five tested agonists was ACh > nicotine=carbamylcholine > suberyldicholine=oxotremorine. The dose-response relationship was complex, suggesting the existence of two populations of receptors: high-affinity receptors (extrapolated K(d) around 10(-7) M) and low-affinity receptors (extrapolated K(d) around 5x10(-5) M). The current-voltage relationship of the induced current was linear between -80 and -40 mV and the extrapolated reversal potential was not significantly different from 0 mV. The sequence of decreasing potency of the antagonists of the ACh response was: methyllycaconitine > alpha-bungarotoxin > mecamylamine > curare > strychnine > bicuculline > atropine > picrotoxin. These results show: (1) that, in embryonic brain neurones, the response to ACh corresponds to the opening of non-selective cationic channels; and (2) that the pharmacology of the ACh receptors is mainly but not solely nicotinic. The nature of the single events which underlie this response, as well as the structure of the channels (homo or hetero-oligomeric) remain to be investigated.

Growth properties of larval and adult locust neurons in primary cell culture

We developed a cell culture system for thoracic neurons of fifth instar or adult locusts (Locusta migratoria) in order to obtain maximum visualization of cellular morphology and direct access to the neurons for electrophysiological analysis. The dissociated neurons regenerated new neurites in a serum-free defined culture medium, in which they remained viable for up to 3 weeks. Viability of the cells was confirmed by intracellular recordings demonstrating active membrane properties and action potentials. While the morphology of the cultured neurons is distinct from their in vivo counterparts, they retained some cellular surface properties and markers related to transmitter metabolism. Two factors influencing cellular morphology in vitro were identified in Locusta: 1) the presence of a primary neurite stump, and 2) membrane contacts between cells. Dissociated neurons of the locust species Schistocerca gregaria grown in a hemolymph-enriched medium showed a marked reduction in branching patterns and a tenfold increase in neurite length compared to neurons growing in a medium without hemolymph. This culture system could prove useful for identifying the action of hemolymph-derived growth factors.

Dissociated neurons of the pupal honeybee brain in cell culture

Primary cell cultures were prepared from specific regions of the pupal honeybee brain which are involved in proboscis extension learning. Defined areas could be dissociated purely by mechanical treatment. We show that cultured neurons regenerate new neurites and remain viable for up to three weeks in a serum-free, chemically-defined medium. Several labelling techniques were employed to identify subpopulations of cultured neurons. For example, acetylcholinesterase staining; fluorescent beads to distinguish identified cell populations of co-cultured brain areas; various markers for surface antigens such as a monoclonal antibody to olfactory projection neurons of the antennoglomerular tracts and monopolar cells of the optic lobes, as well as anti-HRP immunoreactivity and alpha-bungarotoxin binding; and various antisera for detecting transmitter phenotype. The appearance of transmitter-immunoreactive cells agreed closely with that expected from their known distribution in situ. Our results suggest that cultured cells retain surface properties and transmitter phenotype of their in vivo counterparts, despite differences in basic morphology. Thus our culture system provides the important initial step for future in vitro investigations of the cellular and electrophysiological properties of neurons mediating proboscis extension learning.

A circadian rhythm in neural activity can be recorded from the central nervous system of the cockroach

Evidence presented in this paper indicates that a robust circadian rhythm in the frequency of neural activity can be recorded from the central nervous system of intact cockroaches, Leucophaea maderae. This rhythmicity was abolished by optic lobe removal. Spontaneous neural activity was then used as an assay to demonstrate that the optic lobe is able to generate circadian oscillations in vitro. These results provide direct evidence that the cockroach optic lobe is a self-sustained circadian oscillator capable of generating daily rhythms in the absence of neural or hormonal communications with the rest of the organism.

Sensitivity of Aplysia neurons in primary culture to putative neurotransmitters

Aplysia neurons grown in primary cell culture (Dagan and Levitan, 1981) were exposed to the putative neurotransmitters acetylcholine and serotonin by local iontophoretic application, and changes in membrane potential or voltage clamp currents were examined. It was found that 47% of the neurons were sensitive to cholinergic agonists, 14% to serotonin, and 9% responded to both. Responses could be recorded upon application of the transmitters to the cell bodies as well as along the regenerated neurites. An identified group of neurons, the neurosecretory bag cells, exhibited similar responses to cholinergic agonists in culture and in situ. Pleural medial neurons exhibited cholinergic responses in culture similar to those previously reported in situ. Thus neurotransmitter receptor/ion channel complexes characteristic for a specific cell type in the intact ganglion are also present on this cell type in culture.

Fine structure of Periplaneta americana neurons in long-term culture

A new culture technique is described for dissociated ganglia of embryos of Periplaneta americana. Neurons obtained by the mechanical dissociation of ganglia from 23-26 day old embryos were grown for seven days in a combination of Schneider's Drosophila Medium and Eagle's Basal Medium and then transferred to a combination of Leibovitz's L-15 Medium and Yunker's Modified Grace's Medium supplemented with ecdysone. Neurons cultured in this way survived for periods in excess of one month. The cells had a typically neuronal ultrastructure and produced axonal processes that associated to form fibre bundles. Within these processes neurosecretory granules, dense-core vesicles and clear vesicles were found. In some instances these vesicles aggregated at points of contact between fibres giving the appearance of presynaptic endings. These cultures would appear to be suitable for pharmacological studies of the insect nervous system.

Regeneration of spinal electrocyte fibers in Sternarchus albifrons: development of axon-Schwann cell relationships and nodes of Ranvier

The electrocyte fibers in the gymnotid Sternarchus albifrons are highly differentiated myelinated axons which exhibit several types of nodes of Ranvier and characteristically short internode lengths. In the present study, regeneration of the electrocyte fibers following removal of the tail was examined by electron microscopy. By 36 days following extirpation, the regenerating electrocyte axons exhibit Type I nodes of Ranvier, with a normal morphology, and Type II nodes of Ranvier with a large nonmyelinated gap and a polypoid elaboration of the axon surface. Moreover, in the regenerating axons the internode length: diameter ratios are quite small. Thus, relatively normal axons-Schwann cell relations and a relatively normal differentiation of the axon surface are achieved during regeneration of the Sternarchus electrocyte fibers.

Neuronal properties of nymphal and adult insect neurosecretory cells in vitro

Neurosecretory brain cells from late nymphal and adult cockroaches were grown for 1 to 3 months in a chemically defined medium in combined cultures with embryonic organs or oocytes from nymphal and adult specimens. Under these conditions, neurosecretory cells show vigorous axonal growth. Electron dense granules in cell perikarya and axons of 3-month-old cultures are suggestive of neurosecretory activity in vitro.

Branching patterns of individual sympathetic neurons in culture

The growth of single sympathetic neurons in tissue culture was examined with particular regard to the way in which the patterns of axonal or dendritic processes (here called nerve fibers), were formed. The tips of the fibers were seen to advance in straight lines and to grow at rates that did not vary appreciably with time, with their position in the cell outgrowth, or with the fiber diameter. Most of the branch points were formed by the bifurcation of a fiber tip (growth cone), apparently at random, and thereafter remained at about the same distance from the cell body. It seemed that the final shape of a neuron was the result of the reiterated and largely autonomous activities of the growth cones. The other parts of the cell played a supportive role but, apart from this, had no obvious influence on the final pattern of branches formed.