Postembryonic development of rectifying electrical synapses in crayfish: physiology

Autoinhibition of serotonin cells: an intrinsic regulatory mechanism sensitive to the pattern of usage of the cells

After periods of high-frequency firing, the normal rhythmically active serotonin (5HT)-containing neurosecretory neurons of the lobster ventral nerve cord display a period of suppressed spike generation and reduced synaptic input that we refer to as "autoinhibition." The duration of this autoinhibition is directly related to the magnitude and duration of the current injection triggering the high-frequency firing. More interesting, however, is that the autoinhibition is inversely related to the initial firing frequency of these cells within their normal range of firing (0.5-3 Hz). This allows more active 5HT neurons to resume firing after shorter durations of inhibition than cells that initially fired at slower rates. Although superfused 5HT inhibits the spontaneous firing of these cells, the persistence of autoinhibition in saline with no added calcium, in cadmium-containing saline, and in lobsters depleted of serotonin suggests that intrinsic membrane properties account for the autoinhibition. A similar autoinhibition is seen in spontaneously active octopamine neurons but is absent from spontaneously active gamma-aminobutyric acid cells. Thus, this might be a characteristic feature of amine-containing neurosecretory neurons. The 5HT cells of vertebrate brain nuclei share similarities in firing frequencies, spike shapes, and inhibition by 5HT with the lobster cells that were the focus of this study. However, the mechanism suggested to underlie autoinhibition in vertebrate neurons is that 5HT released from activated or neighboring cells acts back on inhibitory autoreceptors that are found on the dendrites and cell bodies of these neurons.

Biotin staining in the giant fiber systems of the lobster

The avidin-biotin-complex method is a popular immunocytochemical technique. This method labels consistently a group of neurons in the lobster ventral nerve cord in the absence of primary antibodies. The specific staining is due to a relatively high level of endogenous biotin (or biocytin) in these neurons. These biotin-positive neurons are located in the supraesophageal, thoracic, and abdominal ganglia. Intraaxonal injection of Lucifer yellow followed by Texas red-conjugated streptavidin staining reveals that the neurons are members of the medial giant (MG) and lateral giant (LG) systems, which are important in mediating rapid tail flipping during escape maneuvers. In neuronal somata, staining is restricted to the cytoplasm. Within MG axons, staining appears as punctate, subaxolemmal structures. Preincubating nerve cords in biocytin or direct intraaxonal injection of biocytin enhances staining of these punctate organelles. In LG axons, staining is localized to fragments of braided filamentous structures that also appear to be associated with the axolemma. Preincubation of ventral nerve cords in various concentrations of biocytin results in the appearance of additional groups of stained neurons, suggesting that there are subsets of neurons with specific biocytin-uptake or -retention mechanisms. In the crayfish, biotin-positive staining is confined to the MG neurons; the LG neurons are not stained. In the earthworm, no staining is observed in the MG and LG axon escape systems. In the goldfish, no biotin-staining is seen in the Mauthner neurons and their axons. The significance of specific localization of biotin or biocytin to subsets of neurons is unclear. It may reflect the presence of high levels of biocytin moieties on biotin-dependent enzymes. Biotin is an important cofactor in the catalytic functions of several decarboxylases crucial in energy production and lipogenesis. Axons of the giant fiber systems in lobsters and crayfish may have high energy and fatty acid synthesis requirements. Increased levels of biotin accumulation may also be related to other functions of the giant axon systems, such as the formation of electrical synapses among themselves and with phasic motoneurons.

Two classes of vesicles are present and change in relative proportion during post-embryonic development of rectifying electrical synapses in the crayfish

The size and shape of vesicles at junctional appositions of the rectifying electrical synapses between the medial giant fibre and motor giant neurone of the crayfish were measured during the first 2 months after hatching. Summed data over this period reveal a bimodal distribution in vesicle diameter. From the day of hatching until about 7 days of age, small vesicles (circa 25 nm diameter) predominate. From day 7 onwards, larger vesicles (circa 55 nm diameter) occur in increasing numbers, until at day 56 they constitute about 85% of the population at any one junctional apposition. At intermediate ages (day 7-28) individual junctional appositions may show the same bimodal distribution in size as does the age group as a whole, indicating that large and small vesicles occur together at the same junction. The larger vesicles are mainly circular, while the small vesicles are pleomorphic, with shapes ranging from almost circular down to a shape factor of about 0.6.

Structural and functional post-embryonic development of a non-rectifying electrical synapse in the crayfish

The post-embryonic development of the non-rectifying septate synapse between homologous lateral giant (LG) fibre segments has been investigated using electron microscopy and electrophysiology. In adults, the LG-LG synapse is characterized by closely apposed membranes (approximately 4 nm separation) traversed by regularly spaced particles, and large (60-80 nm) spherical vesicles on both sides of the junction. In newly hatched crayfish the junction between lateral giant fibre segments comprises regions of close membrane apposition as seen in the adult along with non-specialized areas of wide (10-15 nm) membrane separation. Vesicles associated with these junctions are small (25-40 nm) and pleomorphic. The number of vesicles is low by comparison with adult junctions; in most sections of hatchling junctions there are normally fewer than five vesicles, although as many as 30 have occasionally been seen. During development the non-specialized areas of wide membrane separation become rare and the vesicle population changes to a mixture of small pleomorphic forms and larger (60-80 nm) spherical ones. However even at two months the number of large spherical vesicles is markedly less than that at the adult synapse, while small pleomorphic vesicles are still abundant. Despite the difference between the adult and hatchling vesicle populations, intracellular recordings have shown that the synapse is fully functional as a non-rectifying electrical junction on hatching and that the intracellular marker Lucifer Yellow can pass between adjacent lateral giant fibre neurons.

Ultrastructure of electrical synapses: review

This article reviews studies providing information on the ultrastructure of electrical synapses. Although the review focuses on electron-microscopic investigations, its aim is to examine how the structure of an electrical synapse relates to its function. It begins by presenting a historical overview of the early studies which were responsible for the recognition of electrical synapses. The structure of gap junctions which are the morphological correlates of electrical synapses is illustrated and the ultrastructure and function of the two types of electrical synapse, rectifying and non-rectifying, described. Recent papers investigating the ultrastructure of electrical and mixed electrical-chemical synapses in invertebrates and vertebrates are reviewed. For earlier references, the reader is directed to previous reviews on the subject. Much new information, however, on the structure and formation of electrical synapses has been obtained from work on cultured neurons and from electron-microscopic, immunocytochemical, conformational and molecular studies. This article reviews those studies and in light of their findings, re-examines the relationships of the structure of electrical synapses with their function.