Fast-axon synapses of a crab leg muscle

Neuromuscular synapses of the "fast" excitatory axon supplying the main extensor muscle in the leg of the shore crab Pachygrapsus crassipes were studied with electrophysiological and electron-microscopic techniques. Electrical recording showed that many muscle fibers of the central region of the extensor muscle responded only to stimulation of the fast axon, and electron microscopy revealed many unitary subterminal axon branches. Maintained stimulation, even at a low frequency, resulted in depression of the excitatory junctional potentials (EJPs) set up by the fast axon but EJPs of different muscle fibers depressed at different rates, indicating some physiological heterogeneity among the fast-axon synapses. Focal recording at individual synaptic sites on the surfaces of the muscle fibers showed quantal contents ranging from 1.4 to 5.5 at different synapses; these values are relatively high in comparison with similar determinations made in the crayfish opener muscle. Synapse-bearing nerve terminals were generally relatively small in diameter and filiform, with many individual synaptic contact areas of uniform size averaging 0.6 micron2. All of the individual synapses had a presynaptic "dense body" at which synaptic vesicles clustered. If these structures represent release points for transmitter quanta, the initial high quantal content would have an ultrastructural basis. The mitochondial content of the nerve terminals, the synaptic vesicle population, and the specialized subsynaptic sarcoplasm were all much reduced in comparison with tonic axon synaptic regions in this and other crustaceans. The latter features may be correlated with the relatively infrequent use of this axon by the animal, and with rapid fatigue.     

AMPA receptor trafficking at excitatory synapses

Excitatory synapses in the CNS release glutamate, which acts primarily on two sides of ionotropic receptors: AMPA receptors and NMDA receptors. AMPA receptors mediate the postsynaptic depolarization that initiates neuronal firing, whereas NMDA receptors initiate synaptic plasticity. Recent studies have emphasized that distinct mechanisms control synaptic expression of these two receptor classes. Whereas NMDA receptor proteins are relatively fixed, AMPA receptors cycle synaptic membranes on and off. A large family of interacting proteins regulates AMPA receptor turnover at synapses and thereby influences synaptic strength. Furthermore, neuronal activity controls synaptic AMPA receptor trafficking, and this dynamic process plays a key role in the synaptic plasticity that is thought to underlie aspects of learning and memory.     

Postsynaptic organization and regulation of excitatory synapses

Dynamic regulation of synaptic efficacy is one of the mechanisms thought to underlie learning and memory. Many of the observed changes in efficacy, such as long-term potentiation and long-term depression, result from the functional alteration of excitatory neurotransmission mediated by postsynaptic glutamate receptors. These changes may result from the modulation of the receptors themselves and from regulation of protein networks associated with glutamate receptors. Understanding the interactions in this synaptic complex will yield invaluable insight into the molecular basis of synaptic function. This review focuses on the molecular organization of excitatory synapses and the processes involved in the dynamic regulation of glutamate receptors.     

Optimal learning with excitatory and inhibitory synapses

Characterizing the relation between weight structure and input/output statistics is fundamental for understanding the computational capabilities of neural circuits. In this work, I study the problem of storing associations between analog signals in the presence of correlations, using methods from statistical mechanics. I characterize the typical learning performance in terms of the power spectrum of random input and output processes. I show that optimal synaptic weight configurations reach a capacity of 0.5 for any fraction of excitatory to inhibitory weights and have a peculiar synaptic distribution with a finite fraction of silent synapses. I further provide a link between typical learning performance and principal components analysis in single cases. These results may shed light on the synaptic profile of brain circuits, such as cerebellar structures, that are thought to engage in processing time-dependent signals and performing on-line prediction.     

Heterogeneity of excitatory synapses at the ends of single muscle fibers in lobster,Homarus americanus

Crustacean neuromuscular synapses arising from a single excitor axon are known to be well differentiated among different muscle fibers but little is known about their condition along single fibers. Focal recording techniques were used to examine the quantal transmitter release and facilitation properties of synapses in the single excitatory innervated distal accessory flexor muscle of the lobster, Homarus americanus. Synapses were reliably differentiated with respect to quantal output so that those located near the tendon end were 1.15–4.12 times greater than those at the opposite, exoskeletal end (p < 0.01, paired t-test). Regional differences were also seen in the amount of facilitation determined from twin pulse experiments. The fine structural basis for these differences was determined by serial section electron microscopy of 10-μm segments at each end to ensure that the area of focal recording was sampled. No quantitative differences were found in the terminals or synapses in the two regions. Instead, the physiological diversity was correlated with number and size of presynaptic dense bars. Thus, the tendon end had a greater number and larger mean surface area of dense bars compared to the exoskeletal end. This heterogeneity of excitatory multiterminal innervation is correlated with the axonal branching pattern. Thus, the main axon and the larger primary axon branches lie in close proximity to the tendon end of the muscle fibers, whereas the exoskeletal end is innervated by smaller secondary and tertiary axonal branches. This proximity to the large axonal branches of the higher quantal output synapses at the tendon end may be regulated by some neural influence including a timing of innervation and/or access to greater amounts of metabolites in the larger branches which may be conducive to forming high-output synapses.     

Receptor trafficking and the plasticity of excitatory synapses

Newly discovered features of the trafficking of AMPA receptors to and from the postsynaptic membrane of excitatory synapses are now bringing the mechanisms of synaptic plasticity into focus. Recent advances, including the existence of slots, anchors, transport factors and pathways for activity-dependent control, have elucidated the role of the individual AMPA receptor subunits and their binding partners. The latest views describe how subunit type dictates the assembly of heteromeric receptors, and how these heteromers interact with the receptor trafficking machinery and synaptic anchorage factors. Moreover, phosphorylation may play an important role in receptor transport and synaptic turnover.     

Neuroenergetics and the kinetic design of excitatory synapses

Why is the characteristic timescale of neural information processing in the millisecond range, corresponding to a 'clock speed' of about 1 kHz, whereas the clock speed of modern computers is about 3 GHz? Here we investigate how the brain's energy supply limits the maximum rate at which the brain can compute, and how the molecular components of excitatory synapses have evolved properties that are matched to the information processing they perform.     

Modulation of smooth muscle activity by excitatory and inhibitory nerves in the guinea-pig stomach

• 1.1. In smooth muscle of the guinea-pig stomach, intramural nerve stimulation evoked cholinergic excitatory junction potential in the fundus and non-adrenergic non-cholinergic inhibitory junction potential in the antrum, yet cholinergic contractions in both regions.
• 2.2. This dissociation between electrical and mechanical responses was mainly due to different sensitivity of the membrane for depolarization to acetylcholine.

     

Integrins modulate fast excitatory transmission at hippocampal synapses

The present study provides the first evidence that adhesion receptors belonging to the integrin family modulate excitatory transmission in the adult rat brain. Infusion of an integrin ligand (the peptide GRGDSP) into rat hippocampal slices reversibly increased the slope and amplitude of excitatory postsynaptic potentials. This effect was not accompanied by changes in paired pulse facilitation, a test for perturbations to transmitter release, or affected by suppression of inhibitory responses, suggesting by exclusion that alterations to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate receptors cause the enhanced responses. A mixture of function-blocking antibodies to integrin subunits alpha(3), alpha(5), and alpha(v) blocked ligand effects on synaptic responses. The ligand-induced increases were (i) blocked by inhibitors of Src tyrosine kinase, antagonists of N-methyl-d-aspartate receptors, and inhibitors of calcium calmodulin-dependent protein kinase II and (ii) accompanied by phosphorylation of both the Thr(286) site on calmodulin-dependent protein kinase II and the Ser(831) site on the GluR1 subunit of the AMPA receptor. N-Methyl-d-aspartate receptor antagonists blocked the latter two phosphorylation events, but Src kinase inhibitors did not. These results point to the conclusion that synaptic integrins regulate glutamatergic transmission and suggest that they do this by activating two signaling pathways directed at AMPA receptors.     

Excitatory synapses of blue crab gastric mill muscles

Summary Physiological and ultrastructural studies were made of neuromuscular synapses in stomach muscles, especially two gastric mill muscles of the blue crab innervated by neurons of the stomatogastric ganglion. These muscles depolarized and contracted with application of glutamate, but not acetylcholine, whereas the dorsal dilator muscles of the pyloric region depolarized and contracted in acetylcholine, but not in glutamate. Large excitatory postsynaptic potentials (EPSP's) of 5–20 mV were recorded in the gastric mill muscles. At low frequencies of activation, individual synapses released on average about 2 quanta of transmitter for each nerve impulse. Facilitation of EPSP's after a single nerve impulse could be detected for at least 10 s. Synapses were found on enlarged terminals of the motor axon; their contact areas ranged from 0.2 m² up to 3 m². Both electron-lucent, round synaptic vesicles and dense-cored vesicles occurred near these synapses. A possible correlation between contact area of a synapse and output of transmitter, is discussed.Supported by grants from the National Research Council of Canada and the Muscular Dystrophy Association of Canada to H.L. Atwood and C.K. Govind. We thank Kazuko Hay, Eva Yap-Chung and Irene Kwan for technical assistance with electron microscopy and reconstruction of nerve terminals from micrographs     

Novel protein kinase A-dependent long-term depression of excitatory synapses

Dopamine neurons of the ventral tegmental area (VTA) are critically involved in processing novel and rewarding information, and mediate the addictive properties of many drugs of abuse. Excitatory synapses on these neurons, like those in other brain regions, exhibit long-term depression (LTD). Amphetamine or dopamine block LTD at VTA synapses, indicating that both pathological and local physiological stimuli regulate LTD. Here we show that in common with other forms of LTD, VTA LTD results from a selective decrease in AMPA receptor function accompanied by a decrease in cell surface AMPA receptors. However, unlike the case for any previously described form of LTD, activation of cyclic AMP-dependent protein kinase (PKA) is necessary and sufficient to trigger LTD at synapses on VTA dopamine neurons.     

The self-tuning neuron: synaptic scaling of excitatory synapses

Homeostatic synaptic scaling is a form of synaptic plasticity that adjusts the strength of all of a neuron's excitatory synapses up or down to stabilize firing. Current evidence suggests that neurons detect changes in their own firing rates through a set of calcium-dependent sensors that then regulate receptor trafficking to increase or decrease the accumulation of glutamate receptors at synaptic sites. Additional mechanisms may allow local or network-wide changes in activity to be sensed through parallel pathways, generating a nested set of homeostatic mechanisms that operate over different temporal and spatial scales.     

Structural-functional differences of a crab motoneuron to four stomach muscles

A motor unit in the stomach of the blue crab, Callinectes sapidus, consists of four separate muscles involved in different aspects of the trituration and filtering of food. Motor nerve terminals to two of the muscles (CPV7a and GM5) release small amounts of transmitter (low-output) while those to the other two muscles (CV2 and CV3) release between three and five-fold greater amounts (high-output). Structural features underlying the disparity in synaptic strength were analysed with thin serial-section electron microscopy. Nerve terminals were similar in their volume percent of mitochondria, clear vesicles and dense core vesicles among the four muscles. This was also the case for the number and size of synaptic contacts. However, presynaptic dense bars representing active zones were longer and occurred more frequently at high-output synapses than at low-output ones. High-output synapses were also characterized by the close spacing of adjacent dense bars. The longer and more closely spaced dense bars at high-output synapses would be factors in the generation of larger synaptic potentials in these terminals compared to their low-output counterparts. Other factors, however, need to be considered to fully account for the physiological differences in synaptic strength among the four muscles.     

Building excitatory and inhibitory synapses: balancing neuroligin partnerships

Processing of neural information is thought to occur by integration of excitatory and inhibitory synaptic inputs. As such, precise control mechanisms must exist to maintain an appropriate balance between each synapse type. Recent findings indicate that neuroligins and their synaptic binding partners modulate the development of both excitatory and inhibitory synapses. Here we highlight these findings and discuss a mechanism potentially involved in controlling the balance between excitation and inhibition.     

Differences in the cytoskeleton in inhibitory and excitatory synapses

The cytoskeleton of afferent chemical synapses, with various ultrastructure of contact zones, was examined in the Mauthner cells of the goldfish. The synapses with combined active zones and desmosome-like specialized contacts possessed a well developed cytoskeleton consisting of filaments and microtubules oriented towards the synaptic apposition. Regular arrays of synaptic vesicles oriented in the same direction were observed beyond and near the active zones. The cytoskeleton of the synapses lacking desmosome-like formations was diffusely organized throughout the boutons. The distribution of vesicles in the vicinity of active zones was also not ordered. The role of cytoskeleton in organization of the two morphologically distinct synapses is discussed. A special function of cytoskeleton as an intermediary between synaptoplasm and membrane is regarded as a necessary basis for plasticity of excitatory rather than inhibitory synapses.