Reversible control of synaptic transmission in a single gene mutant of Drosophila melanogaster

Synaptic transmission of the single gene mutant, shibirets1 (shi), of Drosophila melanogaster is reversibly blocked by elevated temperature. The presynaptic mechanism of transmission was studied in the neuromuscular junction of the dorsal longitudinal flight muscle of this mutant. It was observed that when the temperature was raised to 29 degrees C in shi flies, the amplitude of the excitatory junction potential (EJP) greatly diminished, the frequency of spontaneously released miniature excitatory junction potentials (MEJP's) was greatly reduced, and almost complete vesicle depletion was observed. These conditions were reversible if the temperature was lowered to 19 degrees C. These data suggest that the block in transmission is a result of vesicle depletion. It is suggested that depletion occurs not as a result of excessive release of transmitter but rather as a result of a block in the recycling of vesicles, which causes depletion as exocytosis (transmitter release) proceeds normally.     

Giant fiber activation of flight muscles in Drosophila: asynchrony in latency of wing depressor fibers

In Drosophila, brain stimulation of the giant fiber pathway brings about highly stereotyped electrical responses in target muscles involved in the escape response. Both the order of muscle response and the latency of that response are predictable in wild-type flies. The neuronal circuit to the targets is well defined and has been used in the analysis of a number of mutant phenotypes, including induced anomalies in temperature-sensitive (ts) mutations such as shibire (shi). It has been assumed that the stereotyped response includes simultaneous activation of all six fibers of the wing depressor muscle, DLM, resulting in equal latencies for all fibers. We report here a small, but distinct, inherent difference in latency between two sets of DLM fibers in a proportion of two wild-type strains as well as in a strain carrying the ts mutation shi. This difference may occur on one or both sides of an individual, is stable over time, and persists when the motor axon is stimulated peripherally. These results, due to the circuit leading to the target, suggest that the difference in latency arises peripherally. In flies reared at the shi permissive temperature (22 degrees C), the difference is more common in shi than in wild-type flies; however, in shi flies reared at 18 degrees C, the prevalence resembles that of wild-type flies. This indicates a subtle expression of the shi defect even at the presumed permissive temperature of 22 degrees C. The difference in latency is similar to that induced in shi flies whose development is affected by pupal heat pulse. Thus, correct interpretation of differences in latency, e.g., in shi/wild-type mosaic flies or in flies with mutations affecting the GF pathway, requires recognition of the inherent asynchrony that can occur between DLM fibers.     

Possible temperature-dependent blockage of synaptic vesicle recycling induced by a single gene mutation in Drosophila

The temperature-sensitive Drosophila mutant, shibirets1 (shi), has been shown to exhibit a reversible block in synaptic transmission at 30 degrees C. Various synaptic terminals (neuromuscular, sensory, central) of this mutant were observed by electron microscopy. At 19 degrees C, all terminals of shi showed essentially the same structural features as those of wild-type (Oregon-R) flies, while at 30 degrees C (5 or 10 min of exposure) shi terminals exhibited various structural changes not seen in the wild type. The major structural change observed in all of the various types of terminals was the accumulation of many pitlike structures on the plasma membrane near presynaptic sites. These structures consisted of a spherical head portion, about 50-100 nm in diameter, and a cylindrical neck portion, about 20 nm long and 20-25 nm in diameter. The neck portion was surrounded by a kind of cytoplasmic dense material, about 10 nm thick, reminiscent of a "collar." Thus, these pits are referred to as "collared pits." Similar kinds of pits were observed, although very rarely, in wild-type flies at 19 and 30 degrees C and in shi flies at 19 degrees C. In addition, various degrees of vesicle depletion, and an increase in membranous structures (infoldings and cisternalike or tubulelike structures) often accompanied pit formation. The possibility that these pits are the result of a blocked step in the endocytotic process, which in turn causes vesicle depletion as exocytosis proceeds, is discussed.     

Reversible blockage of membrane retrieval and endocytosis in the garland cell of the temperature-sensitive mutant of Drosophila melanogaster, shibirets1

Temperature-induced structural changes in the cortical region of the garland cell, which is considered to be active in endocytosis, were investigated in a temperature-sensitive, single gene mutant of Drosophila melanogaster, shibirets1 (shi) and wild-type (Oregon-R). At 19 degrees C, both shi and wild type showed similar structural features: an irregularly extended network of labyrinthine channels, coated pits and vesicles, tubular elements and alpha vacuoles. Tannic acid (TA) impregnation showed that coated pits comprised approximately 20-25% of the total coated profiles at 19 degrees C in both shi and wild-type. When flies were incubated in a horseradish peroxidase (HRP) solution for 5 min, organelles such as coated profiles, tubular elements, and alpha vacuoles were labeled. In wild-type at 30 degrees C, minor changes were observed--mainly a decrease in the distribution of the labyrinthine channels and an increase in HRP uptake. On the other hand, in shi at 30 degrees C, the labyrinthine channels were much elongated and their network became far more complex, indicating the expansion of the surface area of the cell. Also, the coated profiles were increased in number while the number of tubular elements was decreased considerably. The TA method showed that almost all of the coated profiles were coated pits, coated vesicles being almost completely absent at 30 degrees C in shi. Furthermore, HRP uptake activity was considerably decreased at 30 degrees C. These structural changes, as well as the reduced HRP uptake activity, were reversible when the temperature was lowered to 19 degrees C. The observations suggest that in the garland cell of shi the conversion of coated pits to coated vesicles, that is, membrane pinch-off, is blocked at high temperature.     

The cdc 22 mutation by Schizosaccharomyces pombe is a temperature-sensitive defect in nucleoside diphosphokinase

A number of temperature-sensitive cdc- mutants of Schizosaccharomyces pombe that are affected in DNA replication, were screened for the absence of deoxynucleoside triphosphate(s) when blocked at their restrictive temperature. The preliminary screening simply involved analysis of perchloric acid-soluble cell extracts by two-dimensional thin-layer chromatography on poly(ethyleneimine)-impregnated cellulose. One mutant strain, cdc 22-M45, was found which apparently lacked dTTP. Pulse-labelling of intracellular nucleotides revealed that not only did dTTP become depleted, but that dTDP accumulated when this mutant was blocked by a temperature shift-up, indicating a defective nucleoside diphosphokinase. Nucleoside diphosphokinase from cdc 22-M45 was less active than that from wild-type strain 972 when assayed at high temperatures. The nucleoside diphosphokinase of the mutant also has an altered Km for dTDP at both permissive (25 degrees C), and at the restrictive (36.8 degrees C) temperatures. At the restrictive temperature the Km for dTDP of the mutant enzyme is more than 11-times greater than that of the wild type. Characterisation of the biochemical basis of the defect in this cdc- mutant has shown that in S. pombe, despite its having an apparently complex system of genetic control over progression through S-phase, one factor at least is merely availability of a nucleoside triphosphate precursor to DNA synthesis.     

Increased neuromuscular activity causes axonal defects and muscular degeneration

Before establishing terminal synapses with their final muscle targets, migrating motor axons form en passant synaptic contacts with myotomal muscle. Whereas signaling through terminal synapses has been shown to play important roles in pre- and postsynaptic development, little is known about the function of these early en passant synaptic contacts. Here, we show that increased neuromuscular activity through en passant synaptic contacts affects pre- and postsynaptic development. We demonstrate that in zebrafish twister mutants, prolonged neuromuscular transmission causes motor axonal extension and muscular degeneration in a dose-dependent manner. Cloning of twister reveals a novel, dominant gain-of-function mutation in the muscle-specific nicotinic acetylcholine receptor alpha-subunit, CHRNA1. Moreover, electrophysiological analysis demonstrates that the mutant subunit increases synaptic decay times, thereby prolonging postsynaptic activity. We show that as the first en passant synaptic contacts form, excessive postsynaptic activity in homozygous embryos severely impedes pre- and postsynaptic development, leading to degenerative defects characteristic of the human slow-channel congenital myasthenic syndrome. By contrast, in heterozygous embryos, transient and mild increase in postsynaptic activity does not overtly affect postsynaptic morphology but causes transient axonal defects, suggesting bi-directional communication between motor axons and myotomal muscle. Together, our results provide compelling evidence that during pathfinding, myotomal muscle cells communicate extensively with extending motor axons through en passant synaptic contacts.     

Acetylcholinesterase function is dispensable for sensory neurite growth but is critical for neuromuscular synapse stability

The enzyme acetylcholinesterase (AChE) terminates synaptic transmission at cholinergic synapses by hydrolyzing the neurotransmitter acetylcholine. In addition, AChE is thought to play several 'non-classical' roles that do not require catalytic function. Most prominent among these is facilitation of neurite growth. Here, we report that the zebrafish zieharmonika (zim) locus encodes AChE. We show that one mutant zim allele is caused by a pre-mature stop codon, resulting in a truncated protein that lacks both the catalytic site and the carboxy-terminal neuritogenic domain. To explore the 'non-classical' role of AChE, we examined embryos mutant for this allele. In contrast to previous results using a catalytic-inactive allele, our analysis demonstrates that AChE is dispensable for muscle fiber development and Rohon-Beard sensory neuron growth and survival. Moreover, we show that in the absence of AChE, acetylcholine receptor clusters at neuromuscular junctions initially assemble, but that these clusters are not maintained. Taken together, our results demonstrate that AChE is dispensable for its proposed non-classical roles in muscle fiber formation and sensory neuron development, but is crucial for regulating the stability of neuromuscular synapses.     

Neuromuscular synapses mediate motor axon branching and motoneuron survival during the embryonic period of programmed cell death

The embryonic period of motoneuron programmed cell death (PCD) is marked by transient motor axon branching, but the role of neuromuscular synapses in regulating motoneuron number and axonal branching is not known. Here, we test whether neuromuscular synapses are required for the quantitative association between reduced skeletal muscle contraction, increased motor neurite branching, and increased motoneuron survival. We achieved this by comparing agrin and rapsyn mutant mice that lack acetylcholine receptor (AChR) clusters. There were significant reductions in nerve-evoked skeletal muscle contraction, increases in intramuscular axonal branching, and increases in spinal motoneuron survival in agrin and rapsyn mutant mice compared with their wild-type littermates at embryonic day 18.5 (E18.5). The maximum nerve-evoked skeletal muscle contraction was reduced a further 17% in agrin mutants than in rapsyn mutants. This correlated to an increase in motor axon branch extension and number that was 38% more in agrin mutants than in rapsyn mutants. This suggests that specializations of the neuromuscular synapse that ensure efficient synaptic transmission and muscle contraction are also vital mediators of motor axon branching. However, these increases in motor axon branching did not correlate with increases in motoneuron survival when comparing agrin and rapsyn mutants. Thus, agrin-induced synaptic specializations are required for skeletal muscle to effectively control motoneuron numbers during embryonic development.     

Kismet Positively Regulates Glutamate Receptor Localization and Synaptic Transmission at the Drosophila Neuromuscular Junction

The Drosophila neuromuscular junction (NMJ) is a glutamatergic synapse that is structurally and functionally similar to mammalian glutamatergic synapses. These synapses can, as a result of changes in activity, alter the strength of their connections via processes that require chromatin remodeling and changes in gene expression. The chromodomain helicase DNA binding (CHD) protein, Kismet (Kis), is expressed in both motor neuron nuclei and postsynaptic muscle nuclei of the Drosophila larvae. Here, we show that Kis is important for motor neuron synaptic morphology, the localization and clustering of postsynaptic glutamate receptors, larval motor behavior, and synaptic transmission. Our data suggest that Kis is part of the machinery that modulates the development and function of the NMJ. Kis is the homolog to human CHD7, which is mutated in CHARGE syndrome. Thus, our data suggest novel avenues of investigation for synaptic defects associated with CHARGE syndrome.     

Reduced TrkB expression results in precocious age-like changes in neuromuscular structure, neurotransmission, and muscle function

Acute blockade of signaling through the tyrosine kinase receptor B (TrkB) attenuates neuromuscular transmission and fragments postsynaptic acetylcholine receptors (AChRs) in adult mice, suggesting that TrkB signaling is a key regulator of neuromuscular function. Using immunohistochemical, histological, and in vitro muscle contractile techniques, we tested the hypothesis that constitutively reduced TrkB expression would disrupt neuromuscular pre- and postsynaptic structure, neurotransmission, muscle fiber size, and muscle function in the soleus muscle of 6- to 8-mo-old TrkB?/? mice compared with age-matched littermates. Age-like expansion of postsynaptic AChR area, AChR fragmentation, and denervation was observed in TrkB?/? mice similar to that found in 24-mo-old wild-type mice. Neurotransmission failure was increased in TrkB?/? mice, suggesting that these morphologic changes were sufficient to alter synaptic function. Reduced TrkB expression resulted in decreased muscle strength and fiber cross-sectional area. Immunohistochemical and muscle retrograde labeling experiments show that motor neuron number and size are unaffected in TrkB?/? mice. These results suggest that TrkB- signaling at the neuromuscular junction plays a role in synaptic stabilization, neurotransmission, and muscle function and may impact the aging process of sarcopenia.     

The oculomotor system of Daphnia magna

Summary The highly mobile cyclopic compound eye of Daphnia magna is rotated by six muscles arranged as three bilateral pairs. The three muscles on each side of the head share a common origin on the carapace and insert dorsally, laterally and ventrally on the eye. The dorsal and ventral muscles are each composed of two muscle fibers and the lateral muscle is composed of from two to five fibers, with three the most common number. Individual muscle fibers are spindle-shaped mononucleated cells with organized bundles of myofilaments. Lateral eye-muscle fibers are thinner than those of the other muscles but are otherwise similar in ultrastructure. Two motor neurons innervate each dorsal and each ventral muscle and one motor neuron innervates each lateral muscle. The cell bodies of the motor neurons are situated dorsally in the supraesophageal ganglion (SEG) and are ipsilateral to the muscles they innervate. The dendritic fields of the dorsal-muscle motor neurons are ipsilateral to their cell bodies; those of the ventral-muscle motor neurons are bilateral though predominantly contralateral. The central projections of the lateral-muscle motor neurons are unknown. In the dorsal and ventral muscles one motor axon synapses principally with one muscle fiber; in each lateral muscle the single motor axon branches to, and forms synapses with, all the fibers. The neuromuscular junctions, characterized by pre- and postsynaptic densities and clear vesicles, are similar in all the eye muscles.     

A CHO-cell mutant with a defect in cytokinesis.

In a selection procedure designed to enrich for temperature-sensitive mutant cells blocked in mitosis a CHO-cell mutant was isolated which has a defect in cytokinesis as the basis of its temperature-sensitive phenotype. Cultures of the mutant had an abnormally high percentage (ie. 34%) of polyploid cells at the permissive temperature of 34 degress C and showed further increased frequencies of polyploidy as well as many multinucleated cells at 38.5 degrees 39.5 degrees. When the mutant cells were synchronized in metaphase by Colcemid arrest and then placed into fresh medium at nonpermissive temperature, they did not divide although the completion of mitosis appeared cytologically normal. Ultrastructural examination by electron microscopy of such synchronized cells at telophase revealed no specific defects in cellular components other than failure of development of a normal midbody. The sensitivity of the mutant to cytochalasin B and to Colcemid was the same as for wild-type cells. This mutation behaved as recessive in tetraploid cell hybrids constructed by fusing the mutant with a CHO strain which was wild-type with respect to temperature sensitivty.     

Further characterization of the phenotype of ts20, a DNAts mutant of BALB/3T3 cells

Ts20 is a temperature-sensitive mutant cell line derived from BALB/3T3 cells that is blocked at a step in DNA synthesis involving chain elongation. Following a shift from 33 degrees to 39 degrees C, mutant cells lost ability to grow or form colonies. When mutant cells were infected with polyomavirus, both cell and virus DNA synthesis were inhibited at the restrictive temperature of 39 degrees C. When cell extracts from wild-type cells were added in vitro to lysed infected mutant cells that had been incubated in vivo at 39 degrees C for expression of the mutation, cell DNA synthesis was increased 3-fold (similar to the effect in uninfected mutant cells), whereas virus DNA synthesis was increased only 60%. With harsher lysis conditions, the effect of added extract on virus DNA synthesis was greater, although baseline DNA synthesis (prior to addition of extracts) was much lower. Analysis by alkaline sucrose gradients showed that the addition of cell extract converted small cellular DNA molecules into larger ones, while it increased the synthesis of small virus DNA molecules rather than completed genomes. Analysis of cytosol extracts (in which the activity stimulating DNA synthesis resides) showed that DNA topo-isomerase I activity was more heat-labile when assayed in mutant extracts compared to wild-type extracts. In contrast, cytosol DNA polymerase activity was equally heat-labile in mutant and wild-type extract. This suggested the factor in extract was likely associated with the activity of DNA topo-isomerase I. Analysis of virus DNA synthesized in vitro in restricted mutant cells by gel electrophoresis and fluorography showed an accumulation of topo-isomers migrating between form I and II. These topo-isomers, thought to be a manifestation of the ts defect, did not disappear when extract from wild-type cells was added back in vitro or when mutant cells were shifted back to permissive temperature prior to lysis for in vitro synthesis. The results indicate that polyoma DNA synthesis and cell DNA synthesis differ in their response to the mutant gene product in ts20, although both are inhibited at a step early in DNA chain elongation that may involve DNA topo-isomerase I.     

A quantum of neurotransmitter causes minis in multiple postsynaptic cells at the Caenorhabditis elegans neuromuscular junction

The polyadic synapse, where a single presynaptic active zone associates with two or more postsynaptic cells, exists in both mammals and invertebrates. An important but unresolved question is whether synaptic transmission occurs between the presynaptic site and its various postsynaptic partners. Using the dual whole-cell voltage clamp technique, we analyzed miniature postsynaptic currents (mPSCs or minis) at the C. elegans neuromuscular junction (NMJ), which is a polyadic synapse. We found that neighboring muscle cells at the same position along the body axis had high frequencies of concurrent mPSCs, which could not be explained by pure chance. Although body-wall muscle cells are electrically coupled, the high frequency of concurrent mPSCs was not due to electrical coupling because there was no correlation between the frequency of concurrent mPSCs and the degree of electrical coupling; the rise time of concurrent mPSCs was identical to that of nonconcurrent mPSCs but distinct from that of junctional currents (I(j)); and a mutant defective in electrical coupling showed normal frequency of concurrent mPSCs. Our analyses suggest that a single quantum of neurotransmitter may cause mPSCs in multiple postsynaptic cells at polyadic synapses, and that high-fidelity synaptic transmission occurs between the presynaptic site and its various postsynaptic partners. Thus, polyadic synapses could be a distinct mechanism for synaptic divergence and for synchronizing activities of postsynaptic cells.     

Introduction of wild-type p53 in a human ovarian cancer cell line not expressing endogenous p53.

Utilizing a temperature sensitive p53 mutant (pLTRp53cGval135) which expresses mutant p53 at 37 degrees C and a wild-type like p53 at 32 degrees C, we transfected a human ovarian cancer cell line (SKOV3) which does not express endogenous p53. Among the different clones obtained, we selected three clones. Two were obtained from simultaneous transfection of p53 and neomycin resistance expression plasmids (SK23a and SK9), the other was obtained from transfection experiments utilizing the neomycin resistance gene only (SKN). Introduction of mutant p53 did not alter the morphology or growth characteristics of this ovarian cancer cell line. Upon shifting to the permissive temperature, a dramatic change in morphology and growth rate was observed in SK23a and SK9 cells that is associated with the presence of a wild-type like p53. SKN and SKOV3 cells maintained at 32 degrees C did not change morphology and only slightly reduced proliferation. Both SK23a and SK9 cells did not show evidence of apoptosis when measured up to 72 hours of maintenance at 32 degrees C. In contrast to what observed in other cell lines, SK23a and SK9 cells maintained at 32 degrees C were not blocked in G1, but they were accumulated in G2-M. This accumulation was transient and could be due either to a blockade or to a delay in the G2 progression. No down-regulation of c-myc was observed in p53 expressing clones when shifted to the permissive temperature. In these conditions gadd45 mRNA expression was highly stimulated in SK9 and SK23a cells but not in SKN cells. In both clones Gas1 mRNA was not detected either at 37 degrees C or 32 degrees C. This system represents a new and useful model for studying the effect of the absence of p53 (SKOV3 or SKN), presence of mutated p53 (SK23a and SK9 kept at 37 degrees C) or wild type p53 (SK23a and SK9 kept at 32 degrees C) on the mechanism of response of cancer cells to DNA damaging agents.     

Synaptic dendritic activity modulates the single synaptic event

Synaptic transmission is the key system for the information transfer and elaboration among neurons. Nevertheless, a synapse is not a standing alone structure but it is a part of a population of synapses inputting the information from several neurons on a specific area of the dendritic tree of a single neuron. This population consists of excitatory and inhibitory synapses the inputs of which drive the postsynaptic membrane potential in the depolarizing (excitatory synapses) or depolarizing (inhibitory synapses) direction modulating in such a way the postsynaptic membrane potential. The postsynaptic response of a single synapse depends on several biophysical factors the most important of which is the value of the membrane potential at which the response occurs. The concurrence in a specific time window of inputs by several synapses located in a specific area of the dendritic tree can, consequently, modulate the membrane potential such to severely influence the single postsynaptic response. The degree of modulation operated by the synaptic population depends on the number of synapses active, on the relative proportion between excitatory and inbibitory synapses belonging to the population and on their specific mean firing frequencies. In the present paper we show results obtained by the simulation of the activity of a single Glutamatergic excitatory synapse under the influence of two different populations composed of the same proportion of excitatory and inhibitory synapses but having two different sizes (total number of synapses). The most relevant conclusion of the present simulations is that the information transferred by the single synapse is not and independent simple transition between a pre- and a postsynaptic neuron but is the result of the cooperation of all the synapses which concurrently try to transfer the information to the postsynaptic neuron in a given time window. This cooperativeness is mainly operated by a simple mechanism of modulation of the postsynaptic membrane potential which influences the amplitude of the different components forming the postsynaptic excitatory response.     

Characterization of Temperature-Sensitive, Fertilization-Defective Mutants of the Nematode CAENORHABDITIS ELEGANS

The isolation and characterization of three Caenorhabditis elegans temperature-sensitive mutants that are defective at fertilization are described. All three are alleles of the gene fer-1. At the restrictive temperature of 25 degrees, mutant hermaphrodites make sperm and oocytes in normal numbers. No oocytes are fertilized, although they pass through the spermatheca and uterus normally. The oocytes can be fertilized by sperm transferred by wild-type males, indicating that the mutant defect is in the sperm. The temperature-sensitive period for the mutants coincides with spermatogenesis. Sperm made by mutants at 25 degrees cannot be distinguished from wild-type sperm by light microscopy. The sperm do contact oocytes in mutant hermaphrodites, but do not fertilize. Mutant sperm appear to be nonmotile. Mutant males are also steril when grown at 25 degrees. They trnasfer normal numbers of sperm to hermaphrodites at mating, but these sperm fail to migrate to the spermatheca and are infertile. The phenotype of these mutants is consistent with a primary defect in sperm motility, but the cause of this defect is not known.     

Postsynaptic regulation of the development and long-term plasticity of Aplysia sensorimotor synapses in cell culture

The monosynaptic component of the neuronal circuit that mediates the withdrawal reflex of Aplysia californica can be reconstituted in dissociated cell culture. Study of these in vitro monosynaptic connections has yielded insights into the basic cellular mechanisms of synaptogenesis and long-term synaptic plasticity. One such insight has been that the development of the presynaptic sensory neurons is strongly regulated by the postsynaptic motor neuron. Sensory neurons which have been cocultured with a target motor neuron have more elaborate structures—characterized by neurites with more branches and varicosities—than do sensory neurons grown alone in culture or sensory neurons that have been cocultured with an inappropriate target cell. Another way in which the motor neuron regulates the development of sensory neurons is apparent when sensorimotor cocultures with two presynaptic cells are examined. In such cocultures the outgrowth from the different presynaptic cells is obviously segregated on the processes of the postsynaptic cell. By contrast, when two sensory neurons are placed into cell culture without a motor neuron, thier processes readily grow together. In addition to regulating the in vitro development of sensory neurons, the motor neuron also regulates learning-related changes in the structure of sensory neurons. Application of the endogenous facilitatory trasmitter serotonin (5-HT) causes long-term facilitation of in vitro sensorimotor synapses due in part to growth of new presynatpic varicosities. But 5-HT applied to sensory neurons alone in cultuer does not produce structural changes in these cells. More recently it has been found that sensorimotor synapses in cell culture can exhibit long-term potentiation (LTP). Like LTP of some hippocampal synapses, LTP of in vitro Aplysia syanpses is regulated by the voltage of the postsynaptic cell. Pairing high-frequency stimulation of sensory neurons with strong hyperpolarization of the motor neuron blocks the induction of LTP. Moreover, LTP of sensorimotor synapses can be induced in Hebbian fashion by pairing weak presynaptic stimulation with strong postsynaptic depolarization. These findings implicate a Habbian mechanism in classical conditioning in Aplysia. They also indicate that Hebbian LTP is a phylogenetically ancient form of synaptic plasticity. 1994 John Wiley & Sons, Inc.     

Inhibitory innervation of a lobster muscle

Summary Inhibitory neuromuscular synapses formed by the common inhibitor (CI) neuron on the distal accessory flexor muscle (DAFM) in the lobster, Homarus americanus, were studied with electrophysiological and electron-microscopic (thin-section and freeze-fracture) techniques. Postsynaptic inhibition as indicated by inhibitory junctional potentials was several-fold stronger on distal compared to proximal muscle fibers. This difference correlated with the results of serial thin-section studies, which showed more inhibitory synapses on distal fibers than on their proximal counterparts. Effects of postsynaptic inhibition on excitatory junctional potentials via current shunting had a morphological correlate in the spatial relationship between inhibitory and excitatory synapses on the distal fibers. Inhibitory synapses were larger than their excitatory counterparts and had fewer glial processes. In freeze-fracture views, inhibitory synapses did not appear as raised plateaus in the P-face as do excitatory synapses, and their active zones were more widely scattered. The intramembrane particles in the inhibitory postsynaptic membrane-representing neurotransmitter receptors-are arranged in parallel rows in the sarcolemmal P-face and have complementary furrows in the sarcolemmal E-face. Altogether, our findings help to describe a population of inhibitory neuromuscular synapses formed by the CI neuron in lobster muscle.