Inactivity produces increases in neurotransmitter release and synapse size

Synapse composition and organization following chronic activity blockade in cultured hippocampal neurons

Activity plays multiple roles in the expression of synaptic plasticity, and has been shown to regulate the localization of both neurotransmitter receptors and downstream signaling machinery. However, the role of activity in central synapse formation and organization is incompletely understood. Some studies indicate that synapse formation can occur in the absence of synaptic activity, while others indicate that activity is required for synapse maintenance and receptor recruitment. In addition, the effects of long-term blockade of transmission generally, rather than blockade of specific receptors, on postsynaptic protein complement has been poorly characterized. In order to address the role of activity in synapse formation and postsynaptic specialization, we used tetanus toxin to chronically cleave VAMP2 and inhibit SNARE-mediated neurotransmitter release in cultured hippocampal neurons. Although these neurons are deficient in synaptic release, they are of normal size and morphology. In addition, both excitatory and inhibitory synapses form along their processes with normal density. These synapses have a remarkably similar cellular and molecular organization compared to controls, and are capable of recruiting postsynaptic scaffolding proteins, GABA, and glutamate receptors. Subcellular enrichment of synaptic proteins into specialized domains also appears intact. These data indicate that global activity inhibition is insufficient to disrupt central synapse formation or organization.

Age-related changes in the number and structure of synapses in the lip region of the mushroom bodies in the ant Pheidole dentata

Behavioral development in the worker caste of many adult ants follows a pattern of task transitions that contribute to the division of labor within colonies. In the ant Pheidole dentata, the number of tasks that minor workers attend to increases as they progress from brood-care activities within the nest to acts outside the nest such as foraging and defense. In this study we investigated synapse maturation in the lip region of mushroom bodies in young and old minor workers because of its potentially crucial role in behavioral development, task performance, and repertoire expansion. As minor workers aged, individual presynaptic boutons enlarged and acquired more synapses and vesicles, but the total number of synapses in the lip region did not change significantly. Glial cell processes occupied less of the synaptic neuropil as ants matured. These findings indicate an expansion and enhancement of efficacy at specific sets of synaptic connections between the projection interneurons and Kenyon cell dendrites and a commensurate loss of other connections as minor workers age and expand their behavioral repertoire.

Adaptation to synaptic inactivity in hippocampal neurons

In response to activity deprivation, CNS neurons undergo slow adaptive modification of unitary synaptic transmission. The changes are comparable in degree to those induced by brief intense stimulation, but their molecular basis is largely unknown. Our data indicate that prolonged AMPAR blockade acts through loss of Ca2+ entry through L-type Ca2+ channels to bring about an increase in both vesicle pool size and turnover rate, as well as a postsynaptic enhancement of the contribution of GluR1 homomers, concentrated at the largest synapses. The changes were consistent with a morphological scaling of overall synapse size, but also featured a dramatic shift toward synaptic drive contributed by the Ca2+-permeable homomeric GluR1 receptors. These results extend beyond "synaptic homeostasis" to involve more profound changes that can be better described as "metaplasticity".

Enhanced neurally evoked responses and inhibition of norepinephrine reuptake in rat mesenteric arteries after spinal transection

In patients with high thoracic spinal lesions that remove most of the central drive to splanchnic preganglionic neurons, visceral or nociceptive stimuli below the lesion can provoke large increases in blood pressure (autonomic dysreflexia). We have examined the effects of T4 spinal transection on isometric contractions of mesenteric arteries isolated from spinalized rats. Nerve-evoked contractions involved synergistic roles for norepinephrine and ATP. At 7 wk after spinal transection, responses to perivascular stimulation at 1-5 Hz were enhanced fivefold, whereas the alpha1-adrenoceptor antagonist prazosin (10 nM) produced a twofold larger reduction in contraction (to 20 pulses at 10 Hz) than in unoperated controls. In contrast, the reduction in nerve-evoked contractions by the P2-purinoceptor antagonist suramin (0.1 mM) and the responses to the P2-purinoceptor agonist alpha,beta-methylene ATP or to high K+ concentration did not greatly differ between groups, indicating that arteries from spinalized rats were not generally hyperreactive. Sensitivity to the alpha1-adrenoceptor agonist phenylephrine was enhanced in arteries from spinalized rats, and the difference from controls was abolished by the norepinephrine uptake blocker desmethylimipramine. Sensitivity to the alpha1-adrenoceptor agonist methoxamine, which is not a substrate for the neuronal norepinephrine transporter, was similar among the groups. Thus the increased neurally evoked response after spinal transection appeared to be due to a reduction in neuronal uptake of released norepinephrine, a mechanism that did not explain the enhanced response of tail arteries after spinal transection that we previously reported. The findings provide further support for potentiated neurovascular responses contributing to the genesis of autonomic dysreflexia.

Evidence that disinhibition is associated with a decrease in number of vesicles available for release at inhibitory synapses

We used three-dimensional reconstruction from serial electron micrographs to investigate two structural changes that could underlie estrogen-induced disinhibition of hippocampal CA1 pyramidal cells: a decrease in the number of inhibitory inputs per neuron and/or a change in inhibitory boutons that could limit GABA release. We analyzed 373 boutons forming 510 inhibitory synapses in estrogen-treated and control animals. Our results show that estrogen specifically decreases the number of synaptic vesicles adjacent to the presynaptic membrane of inhibitory synapses without affecting the overall number of vesicles. We detected no difference in the density of inhibitory inputs. These findings provide a novel mechanism for the functional effects of estrogen on synaptic inhibition and represent the first in vivo evidence that the number of presynaptic vesicles available for release is a regulated property of synapses that affects synaptic physiology.

Efficient quantification of afferent cochlear ultrastructure using design-based stereology

The afferent synapse between the auditory nerve fiber and the inner hair cell (IHC) represents a critical junction for hearing. Elucidation of the structure at this site will help establish the substrate for normal sound encoding as well as pathologic processes associated with hearing dysfunction. Previous applications of unbiased (design-based) stereological principles have expanded our knowledge of neuro-morphological changes evident with the light microscope. Applying these principles at the level of the synapse is a promising morphometric approach for the efficient sampling of large reference spaces with electron microscopy. This study tests the accuracy of using ultra-thin sections at a fixed interval, known as disector pairs, to quantify afferent innervation density. We analyzed the total numbers of afferent terminals, synaptic thickenings, and synaptic bodies associated with each IHC in the C57BL/6J mouse cochlea, and confirmed the accuracy of the stereological approach in comparison to three-dimensional reconstructions of serial alternate sections. The higher sampling efficiency of the disector pair method rapidly increases precision while also reducing the largest source of variability, inter-animal differences. We conclude that ultrastructural quantification of afferent innervation can be accomplished in the cochlea using efficient design-based stereology.

Local translational control in dendrites and its role in long-term synaptic plasticity

Local protein synthesis in dendrites has emerged as a key mechanism contributing to enduring forms of synaptic plasticity. Although the translational capability of dendrites has been appreciated for over 20 years, it is only recently that significant progress has been made in elucidating mechanisms that contribute to its regulation. It is clear from work over the last few years that the control of translation in dendrites is complex, involving a host of unique (and often surprising) mechanisms that can operate together or in parallel to tightly control gene expression in time and space. Here, we discuss the strategies used by neurons to regulate translation in dendrites and how these are implemented in the service of long-term information storage.

Probing vesicle dynamics in single hippocampal synapses

We use fluorescence correlation spectroscopy and fluorescence recovery after photobleaching to study vesicle dynamics inside the synapses of cultured hippocampal neurons labeled with the fluorescent vesicle marker FM 1-43. These studies show that when the cell is electrically at rest, only a small population of vesicles is mobile, taking seconds to traverse the synapse. Applying the phosphatase inhibitor okadaic acid causes vesicles to diffuse freely, moving 30 times faster than vesicles in control synapses. These results suggest that vesicles move sluggishly due to binding to elements of the synaptic cytomatrix and that this binding is altered by phosphorylation. Motivated by these results, a model is constructed consisting of diffusing vesicles that bind reversibly to the cytomatrix. This stick-and-diffuse model accounts for the fluorescence correlation spectroscopy and fluorescence recovery after photobleaching data, and also predicts the well-known exponential refilling of the readily releasable pool. Our measurements suggest that the movement of vesicles to the active zone is the rate-limiting step in this process.

Mechanisms of target-cell specific short-term plasticity at Schaffer collateral synapses onto interneurones versus pyramidal cells in juvenile rats

Although it is presynaptic, short-term plasticity has been shown at some synapses to depend upon the postsynaptic cell type. Previous studies have reported conflicting results as to whether Schaffer collateral axons have target-cell specific short-term plasticity. Here we investigate in detail the short-term dynamics of Schaffer collateral excitatory synapses onto CA1 stratum radiatum interneurones versus pyramidal cells in acute hippocampal slices from juvenile rats. In response to three stimulus protocols that invoke different forms of short-term plasticity, we find differences in some but not all forms of presynaptic short-term plasticity, and heterogeneity in the short term plasticity of synapses onto interneurones. Excitatory synapses onto the majority of interneurones had less paired-pulse facilitation than synapses onto pyramidal cells across a range of interpulse intervals (20-200 ms). Unlike synapses onto pyramidal cells, synapses onto most interneurones had very little facilitation in response to short high-frequency trains of five pulses at 5, 10 and 20 Hz, and depressed during trains at 50 Hz. However, the amount of high-frequency depression was not different between synapses onto pyramidal cells versus the majority of interneurones at steady state during 2-10 Hz trains. In addition, a small subset of interneurones (approximately 15%) had paired-pulse depression rather than paired-pulse facilitation, showed only depression in response to the high-frequency five pulse trains, and had more steady-state high-frequency depression than synapses onto pyramidal cells or the majority of interneurones. To investigate possible mechanisms for these differences in short-term plasticity, we developed a mechanistic mathematical model of neurotransmitter release that explicitly explores the contributions to different forms of short-term plasticity of the readily releasable vesicle pool size, release probability per vesicle, calcium-dependent facilitation, synapse inactivation following release, and calcium-dependent recovery from inactivation. Our model fits the responses of each of the three cell groups to the three different stimulus protocols with only two parameters that differ with cell group. The model predicts that the differences in short-term plasticity between synapses onto CA1 pyramidal cells and stratum radiatum interneurones are due to a higher initial release probability per vesicle and larger readily releasable vesicle pool size at synapses onto interneurones, resulting in a higher initial release probability. By measuring the rate of block of NMDA receptors by the open channel blocker MK-801, we confirmed that the initial release probability is greater at synapses onto interneurones versus pyramidal cells. This provides a mechanism by which both the initial strength and the short-term dynamics of Schaffer collateral excitatory synapses are regulated by their postsynaptic target cell.

Homeostatic plasticity and NMDA receptor trafficking

Learning, memory and brain development are associated with long-lasting modifications of synapses that are guided by specific patterns of neuronal activity. Such modifications include classical Hebbian plasticities (such as long-term potentiation and long-term depression), which are rapid and synapse-specific, and others, such as synaptic scaling and metaplasticity, that work over longer timescales and are crucial for maintaining and orchestrating neuronal network function. The cellular mechanisms underlying Hebbian plasticity have been well studied and involve rapid changes in the trafficking of highly mobile AMPA receptors. An emerging concept is that activity-dependent alterations in NMDA receptor trafficking contribute to homeostatic plasticity at central glutamatergic synapses.

Decreased synaptic activity shifts the calcium dependence of release at the mammalian neuromuscular junction in vivo

We examined the mechanism underlying increased quantal content after block of activity at the mouse neuromuscular junction in vivo. We found that, when quantal content was measured in solution containing normal extracellular calcium, block of activity had no effect on either quantal content or the response to repetitive stimulation. However, when quantal content was measured in low extracellular calcium, there was a large increase in quantal content after block of activity. The increase in quantal content was accompanied by increased depression during repetitive stimulation. The explanation for these findings was that there was a shift in the calcium dependence of release after block of activity that manifested as an increase in probability of release in low extracellular calcium. Block of presynaptic P/Q channels eliminated the increase in probability of release. We propose that activity-dependent regulation of presynaptic calcium entry may contribute to homeostatic regulation of quantal content.

Synaptotagmin mutants Y311N and K326/327A alter the calcium dependence of neurotransmission

Synaptotagmin I, a calcium-binding synaptic vesicle protein, is thought to act as the calcium sensor for fast neurotransmission, but what synaptotagmin I does, upon binding calcium, to trigger exocytosis is still unknown. To begin to examine the role of synaptotagmin I's interactions with calcium-dependent binding partners, three mutant versions of synaptotagmin I reported to affect calcium-dependent self-oligomerization (Y311N, K327A, and K326/327A) were expressed in cultured mouse hippocampal neurons lacking endogenous synaptotagmin I, and effects on neurotransmission were evaluated by comparison with transmission rescued by wild-type synaptotagmin I. All three mutants reduced transmitter release. To separate effects on calcium binding from effects on calcium-dependent oligomerization, we measured the calcium dependence of exocytosis for two of the mutants. Both showed apparent calcium affinity much lower than wild-type, a reduction sufficient to account for the neurotransmission defects. We conclude that self-oligomerization is unlikely to play any significant role in triggering synaptic vesicle exocytosis.

AP180 maintains the distribution of synaptic and vesicle proteins in the nerve terminal and indirectly regulates the efficacy of Ca2+-triggered exocytosis

AP180 plays an important role in clathrin-mediated endocytosis of synaptic vesicles (SVs) and has also been implicated in retrieving SV proteins. In Drosophila, deletion of its homologue, Like-AP180 (LAP), has been shown to increase the size of SVs but decrease the number of SVs and transmitter release. However, it remains elusive whether a reduction in the total vesicle pool directly affects transmitter release. Further, it is unknown whether the lap mutation also affects vesicle protein retrieval and synaptic protein localization and, if so, how it might affect exocytosis. Using a combination of electrophysiology, optical imaging, electron microscopy, and immunocytochemistry, we have further characterized the lap mutant and hereby show that LAP plays additional roles in maintaining both normal synaptic transmission and protein distribution at synapses. While increasing the rate of spontaneous vesicle fusion, the lap mutation dramatically reduces impulse-evoked transmitter release at steps downstream of calcium entry and vesicle docking. Notably, lap mutations disrupt calcium coupling to exocytosis and reduce calcium cooperativity. These results suggest a primary defect in calcium sensors on the vesicles or on the release machinery. Consistent with this hypothesis, three vesicle proteins critical for calcium-mediated exocytosis, synaptotagmin I, cysteine-string protein, and neuronal synaptobrevin, are all mislocalized to the extrasynaptic axonal regions along with Dap160, an active zone marker (nc82), and glutamate receptors in the mutant. These results suggest that AP180 is required for either recycling vesicle proteins and/or maintaining the distribution of both vesicle and synaptic proteins in the nerve terminal.

Glutamate induces the rapid formation of spine head protrusions in hippocampal slice cultures

Synaptic plasticity at neuronal connections has been well characterized functionally by using electrophysiological approaches, but the structural basis for this phenomenon remains controversial. We have studied the dynamic interactions between presynaptic and postsynaptic structures labeled with FM 4-64 and a membrane-targeted GFP, respectively, in hippocampal slices. Under conditions of reduced neuronal activity (1 muM tetrodotoxin), we observed extension of glutamate receptor-dependent processes from dendritic spines of CA1 pyramidal cells to presynaptic boutons. The formation of these spine head protrusions is blocked by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonists and by agents that reduce the release of glutamate from presynaptic terminals. Moreover, spine head protrusions form in response to exogenously applied glutamate, with clear directionality toward the glutamate electrode. Our results suggest that spontaneously released glutamate is sufficient to activate nearby spines, which can then lead to the growth of new postsynaptic processes connecting to a presynaptic site. Spines thus can compare their recent history with that of neighboring synapses and modify local connectivity accordingly.

A morphological correlate of synaptic scaling in visual cortex

We studied the response of dendritic spines in the thalamic-recipient zone of rat visual cortex to simple manipulations of the visual environment. We measured the morphologies of a total of 3824 spines located on the basal dendrites of 60 layer 3 pyramidal cells. As expected from previous studies, we found a significantly lower spine density in dark-reared animals at postnatal day 30 (P30) compared with light-reared controls. Additional analysis revealed that the spines in dark-reared animals were significantly shorter and more bulbous than in light-reared animals. When these two results were combined, we found that the total synaptic area per unit length of dendrite was conserved, compatible with the phenomenon of "synaptic scaling." We also found that the increase in average spine head diameter is reversed by 10 d of light exposure (starting at P20), but surprisingly, the decrease in spine density is not. Thus, not all effects of dark rearing can be reversed by subsequent visual experience, even when the experience occurs during the third postnatal week.

Synaptic vesicle pools

Communication between cells reaches its highest degree of specialization at chemical synapses. Some synapses talk in a 'whisper'; others 'shout'. The 'louder' the synapse, the more synaptic vesicles are needed to maintain effective transmission, ranging from a few hundred (whisperers) to nearly a million (shouters). These vesicles reside in different 'pools', which have been given a bewildering array of names. In this review, we focus on five tissue preparations in which synaptic vesicle pools have been identified and thoroughly characterized. We argue that, in each preparation, each vesicle can be assigned to one of three distinct pools.

Regulation of a DLK-1 and p38 MAP Kinase Pathway by the Ubiquitin Ligase RPM-1 Is Required for Presynaptic Development

Synapses display a stereotyped ultrastructural organization, commonly containing a single electron-dense presynaptic density surrounded by a cluster of synaptic vesicles. The mechanism controlling subsynaptic proportion is not understood. Loss of function in the C. elegans rpm-1 gene, a putative RING finger/E3 ubiquitin ligase, causes disorganized presynaptic cytoarchitecture. RPM-1 is localized to the presynaptic periactive zone. We report that RPM-1 negatively regulates a p38 MAP kinase pathway composed of the dual leucine zipper-bearing MAPKKK DLK-1, the MAPKK MKK-4, and the p38 MAP kinase PMK-3. Inactivation of this pathway suppresses rpm-1 loss of function phenotypes, whereas overexpression or constitutive activation of this pathway causes synaptic defects resembling rpm-1(lf) mutants. DLK-1, like RPM-1, is localized to the periactive zone. DLK-1 protein levels are elevated in rpm-1 mutants. The RPM-1 RING finger can stimulate ubiquitination of DLK-1. Our data reveal a presynaptic role of a previously unknown p38 MAP kinase cascade.

Synaptic structural abnormalities in the Ts65Dn mouse model of Down Syndrome

The Ts65Dn mouse is a genetic model for Down syndrome. Although this mouse shows abnormalities in cognitive function that implicate hippocampus as well as marked deficits in hippocampal long-term potentiation, the structure of the hippocampus has been little studied. We characterized synaptic structure in Ts65Dn and control (2N) mice, studying the hippocampus (fascia dentata, CA1) as well as the motor and somatosensory cortex, entorhinal cortex, and medial septum. Confocal microscopy was used to examine immunostained presynaptic boutons and to detail the structure of dendrites after Lucifer yellow microinjection. Both presynaptic and postsynaptic elements were significantly enlarged in Ts65Dn in all regions examined. The changes were detected at the youngest age examined (postnatal day 21) and in adults. In studies detailing the changes in fascia dentata and motor cortex, the enlargement of spines affected the entire population, resulting in the presence of spines whose volume was greatly increased. Electron microscopy confirmed that boutons and spines were enlarged and demonstrated abnormalities in the internal membranes of both. In addition, spine density was decreased on the dendrites of dentate granule cells, and there was reorganization of inhibitory inputs, with a relative decrease in inputs to dendrite shafts and an increase in inputs to the necks of spines. Taken together, the findings document widespread abnormalities of synaptic structure that recapitulate important features seen in Down syndrome. They establish the Ts65Dn mouse as a model for abnormal synapse structure and function in Down syndrome and point to the importance of studies to elucidate the mechanisms responsible for synapse enlargement.

Enhancement of synaptic plasticity through chronically reduced Ca2+ flux during uncorrelated activity

The plasticity of synapses within neural circuits is regulated by activity, but the underlying mechanisms remain elusive. Using the dye FM1-43 to directly image presynaptic function, we found that large numbers of presynaptic terminals in hippocampal cultures have a low release probability. While these terminals were not readily modifiable, a transient but not permanent long-term reduction of network activity or Ca2+ influx could increase their modifiability. This modulation of plasticity was mediated by Ca2+ flux through NMDA and voltage-gated calcium channels and was lost within 48 hr. A more permanent enhancement of synaptic plasticity was achieved by selectively reducing the Ca2+ flux associated with uncorrelated activity via adjustment of the voltage-dependent Mg2+ block of the NMDAR. Upregulation of NR2B-containing NMDARs induced by this treatment is an important but not sole contributor to the enhancement of plasticity. Thus, quantity and quality of activity have differential effects on the intrinsic plasticity of neurons.

Selective reduction of weak synaptic activity awakens dormant synapses

Slutsky et al. (this issue of Neuron) report that by selectively filtering out low-level uncorrelated synaptic activity at NMDA receptors in hippocampal cultures they can unlock a large reserve of quiescent synapses and make them available for potentiation with theta burst stimulation. These findings differ from previously reported activity-dependent mechanisms in that inactivity does not necessarily increase synaptic activity globally.

Age-dependence in the homeostatic upregulation of hippocampal dendritic spine number during blocked synaptic transmission

Homeostatic regulation of spine number in mature hippocampal neurons results in more dendritic spines when synaptic transmission is blocked, providing a mechanism to compensate for diminished synaptic input. It is unsettled whether blockade of synaptic transmission also elevates spine number during development. To address this question, synaptic transmission was blocked in rat hippocampal slices during critical developmental stages of spine formation at postnatal days (P) 6-P22 and compared to adults. CA1 pyramidal cells were labeled with DiI and maintained for 5 h in one of three conditions, control artificial cerebrospinal fluid (ACSF), block media containing synaptic transmission antagonists in ACSF, or block media containing synaptic transmission antagonists in a nominally calcium-free ACSF with high magnesium. Slices were fixed in mixed aldehydes, sectioned, and the lateral dendrites were imaged in stratum radiatum with confocal microscopy. Dendritic spine density was quantified per unit length of dendrite. At P6-7 there were only a few protrusions emerging from the dendrites, which were predominantly filopodia-like in appearance. At both P11-12 and P15-16 there was a mixture of dendritic spines and filopodia-like structures. By P20-22 dendritic spines predominated and spine density was about 82% of the adult level. Dendritic spine density increased during blocked synaptic transmission at P20-22 as in adults, but was unchanged during blockade at younger ages. When extracellular calcium was nominally zero, dendritic spine density further increased on P20-22 dendrites as in adults. In contrast, spine density decreased along P11-12 dendrites under the nominally zero calcium condition. Under control conditions, dendritic protrusions were longer at P6-7 than at all other ages, which did not differ from one another. When synaptic transmission was blocked, dendritic protrusions further elongated at P6-7 only. Under the nominally zero calcium condition with blocked synaptic transmission, dendritic protrusions shortened at P11-12 only. These findings reveal age-dependent changes in the manifestation of homeostatic control of dendritic spines that could be mediated by maturational changes in mechanisms regulating postsynaptic calcium.

Synaptic vesicle cycling at type-identified diaphragm neuromuscular junctions

Differences in neuromuscular transmission and neuromuscular junction morphology exist across muscle fiber types. We hypothesized that these fiber-type differences are reflected in the size of the cycling synaptic vesicle pool. Synaptic vesicle cycling at type-identified rat diaphragm neuromuscular junctions was examined by fluorescently labeling presynaptic vesicles with FM4-64. We found that FM4-64 fluorescence uptake was higher at presynaptic terminals of type I/IIa fibers than type IIx/IIb fibers. However, no fiber-type differences in the rate of FM4-64 destaining were found with repetitive nerve stimulation. Synaptic vesicle density at active zones was examined by transmission electron microscopy. In accordance with FM4-64 uptake, synaptic vesicle density was greater at type I/IIa than IIx/IIb fibers. These results demonstrate differences in synaptic vesicle cycling across diaphragm muscle fiber types, which may underlie previously observed differences in neuromuscular transmission across diaphragm muscle fiber types. In the diaphragm, motor units comprising type I and type IIa fibers are most frequently recruited with a duty cycle of approximately 40%. Motor units comprising IIx/IIb fibers are infrequently recruited and only for short durations. The capacity for synaptic vesicle release and cycling at different muscle fiber types matches the functional requirements of these motor units. If the demand for recruitment of motor units comprising IIx/IIb fibers increases, for example, with mechanical loading, there is an increased risk for neuromuscular transmission failure that my relate to the capacity for synaptic vesicle release and cycling. Muscle fiber type-specific adaptations should be considered when examining neuromuscular disorders.

Unilateral cochlear ablation produces greater loss of inhibition in the contralateral inferior colliculus

Bilateral cochlear ablation leads to a profound weakening of synaptic inhibition within the inferior colliculus (IC) of gerbils [Vale & Sanes (2000) J. Neurosci., 20, 1912-1921]. To examine whether unilateral deafening leads to similar functional alterations, we studied the effect of unilateral cochlear ablation on inhibitory synaptic properties both ipsilateral and contralateral to the deafened ear. Lateral lemniscal and commissure of the IC-evoked inhibitory postsynaptic currents (IPSCs) were recorded in an IC brain slice preparation using whole-cell and gramicidin perforated-patch electrodes in the presence of kynurenic acid. Unilateral cochlear ablation led to a 23 mV depolarizing shift in the IPSC equilibrium potential for IC neurons contralateral to the deafened ear, but only a 10 mV depolarization in the ipsilateral IC. Lateral lemniscal-evoked inhibitory synaptic conductance declined significantly in the ipsilateral and contralateral IC, whereas commissural-evoked inhibitory synaptic conductance declined only contralateral to the ablated cochlea. An analysis of paired-pulse facilitation showed that inhibitory transmitter release was more affected ipsilateral to the ablated cochlea. Thus, unilateral cochlear ablation modifies inhibitory synapses in the inferior colliculus, but these changes appear to be dominated by postsynaptic alterations in the contralateral IC, and by presynaptic changes in the ipsilateral IC.

Chronic decentralization potentiates neurovascular transmission in the isolated rat tail artery, mimicking the effects of spinal transection

Spinal cord transection produces a marked increase in the response of the isolated rat tail artery to sympathetic nerve stimulation, possibly as a result of a decrease in ongoing sympathetic activity. We have tested the effects of removing ongoing nerve activity on neurovascular transmission by cutting the preganglionic input to postganglionic neurones supplying the tail artery (decentralization). Isometric contractions to nerve stimulation were compared between decentralized arteries and those from age-matched and sham-operated controls. Nerve-evoked responses of decentralized arteries were much larger than those of control arteries at 2 and 7 weeks post operatively. The extent of blockade of nerve-evoked contraction by alpha-adrenoceptor antagonists prazosin (10 nM) or idazoxan (0.1 microM) was reduced. Decentralized arteries were transiently supersensitive to the alpha1-adrenoceptor agonist phenylephrine and the alpha2-adrenoceptor agonist clonidine; the unchanged sensitivity to methoxamine and phenylephrine after 2 weeks indicated no effect on the neuronal noradrenaline uptake transporter. Decentralized arteries were hypersensitive to alpha,beta methylene-ATP, but the P2-purinoceptor antagonist suramin (0.1 mM) did not reduce nerve-evoked contractions. Enlarged responses to 60 mM K+ after both 2 and 7 weeks were correlated with the response of the arteries to nerve stimulation, suggesting that increased postjunctional reactivity contributes to the enhanced contraction. Comparison between data from decentralized arteries and our previous data from spinalized animals showed that the two lesions similarly potentiate nerve-evoked contractions and have similar but not identical postjunctional effects. The enhanced vascular responses following a reduction in tonic nerve activity may contribute to the hypertensive episodes of autonomic dysreflexia in spinally injured patients.

Homeostatic synaptic plasticity can explain post-traumatic epileptogenesis in chronically isolated neocortex

Chronically isolated neocortex develops chronic hyperexcitability and focal epileptogenesis in a period of days to weeks. The mechanisms operating in this model of post-traumatic epileptogenesis are not well understood. We hypothesized that the spontaneous burst discharges recorded in chronically isolated neocortex result from homeostatic plasticity (a mechanism generally assumed to stabilize neuronal activity) induced by low neuronal activity after deafferentation. To test this hypothesis we constructed computer models of neocortex incorporating a biologically based homeostatic plasticity rule that operates to maintain firing rates. After deafferentation, homeostatic upregulation of excitatory synapses on pyramidal cells, either with or without concurrent downregulation of inhibitory synapses or upregulation of intrinsic excitability, initiated slowly repeating burst discharges that closely resembled the epileptiform burst discharges recorded in chronically isolated neocortex. These burst discharges lasted a few hundred ms, propagated at 1-3 cm/s and consisted of large (10-15 mV) intracellular depolarizations topped by a small number of action potentials. Our results support a role for homeostatic synaptic plasticity as a novel mechanism of post-traumatic epileptogenesis.

Receptor clustering: nothing succeeds like success

By experimentally limiting the amount of receptor molecules available in a postsynaptic neuron in Drosophila, it has been shown that receptors can preferentially cluster opposite active zones with a high release probability. This asymmetry in receptor distribution can alter the apparent probability and calcium-dependence of neurotransmitter release.

Preferential localization of glutamate receptors opposite sites of high presynaptic release

BACKGROUND

The localization of glutamate receptors is essential for the formation and plasticity of excitatory synapses. These receptors cluster opposite neurotransmitter release sites of glutamatergic neurons, but these release sites have heterogeneous structural and functional properties. At the Drosophila neuromuscular junction, receptors expressed in a single postsynaptic cell are confronted with an array of hundreds of apposed active zones. Hence, this is an ideal preparation for the investigation of whether receptor clustering is sensitive to the morphological and physiological properties of the apposed active zones.

RESULTS

To investigate the relationship between the localization of glutamate receptors and the properties of the apposed active zones, we investigated receptor localization in mutants in which receptors are limited. We find that receptors are not uniformly distributed opposite the full array of active zones but that some active zones have a disproportionately large share of receptors as assayed by receptor levels and response to transmitter. The active zones at which receptors preferentially cluster are larger and have a higher neurotransmitter release probability than the average active zone. We find a similar relationship between glutamate receptor clusters and active-zone size at wild-type synapses.

CONCLUSIONS

When confronted with an array of active zones, glutamate receptors preferentially cluster opposite the largest and most physiologically active sites. These results suggest an activity-dependent matching of pre- and postsynaptic function at the level of a single active zone.

Homeostatic plasticity in the CNS: synaptic and intrinsic forms

The study of experience-dependent plasticity has been dominated by questions of how Hebbian plasticity mechanisms act during learning and development. This is unsurprising as Hebbian plasticity constitutes the most fully developed and influential model of how information is stored in neural circuits and how neural circuitry can develop without extensive genetic instructions. Yet Hebbian plasticity may not be sufficient for understanding either learning or development: the dramatic changes in synapse number and strength that can be produced by this kind of plasticity tend to threaten the stability of neural circuits. Recent work has suggested that, in addition to Hebbian plasticity, homeostatic regulatory mechanisms are active in a variety of preparations. These mechanisms alter both the synaptic connections between neurons and the intrinsic electrical properties of individual neurons, in such a way as to maintain some constancy in neuronal properties despite the changes wrought by Hebbian mechanisms. Here we review the evidence for homeostatic plasticity in the central nervous system, with special emphasis on results from cortical preparations.

Developmental changes in release properties of the CA3-CA1 glutamate synapse in rat hippocampus

Developmental changes in release probability (Pr) and paired-pulse plasticity at CA3-CA1 glutamate synapses in hippocampal slices of neonatal rats were examined using field excitatory postsynaptic potential (EPSP) recordings. Paired-pulse facilitation (PPF) at these synapses was, on average, absent in the first postnatal week but emerged and became successively larger during the second postnatal week. This developmental increase in PPF was associated with a reduction in Pr, as indicated by the slower progressive block of the N-methyl-D-aspartate (NMDA) EPSP by the noncompetitive NMDA receptor antagonist MK-801. This developmental reduction in Pr was not homogenous among the synapses. As shown by the MK-801 analysis, the Pr heterogeneity observed among adult CA3-CA1 synapses is present already during the first postnatal week, and the developmental Pr reduction was found to be largely selective for synapses with higher Pr values, leaving Pr of the vast majority of the synapses essentially unaffected. A reduction in Pves, the release probability of the individual vesicle, possibly caused by reduction in Ca2+ influx, seems to explain the reduction in Pr. In vivo injection of tetanus toxin at the end of the first postnatal week did not prevent the increase in PPF, indicating that this developmental change in release is not critically dependent on normal neural activity during the second postnatal week.

Altered cortical synaptic morphology and impaired memory consolidation in forebrain- specific dominant-negative PAK transgenic mice

Molecular and cellular mechanisms for memory consolidation in the cortex are poorly known. To study the relationships between synaptic structure and function in the cortex and consolidation of long-term memory, we have generated transgenic mice in which catalytic activity of PAK, a critical regulator of actin remodeling, is inhibited in the postnatal forebrain. Cortical neurons in these mice displayed fewer dendritic spines and an increased proportion of larger synapses compared to wild-type controls. These alterations in basal synaptic morphology correlated with enhanced mean synaptic strength and impaired bidirectional synaptic modifiability (enhanced LTP and reduced LTD) in the cortex. By contrast, spine morphology and synaptic plasticity were normal in the hippocampus of these mice. Importantly, these mice exhibited specific deficits in the consolidation phase of hippocampus-dependent memory. Thus, our results provide evidence for critical relationships between synaptic morphology and bidirectional modifiability of synaptic strength in the cortex and consolidation of long-term memory.

Plastic elimination of functional glutamate release sites by depolarization

To examine persisting effects of depolarizing rises in extracellular potassium concentration ([K+](o)) on synapses, we depolarized cells to simulate ischemia-like rises in [K+](o). Elevated [K+](o) for 1-16 hr severely depressed glutamate signaling, while mildly depressing GABA transmission. The glutamate-specific changes were plastic over several hours and involved a decrease in the size of the pool of releasable vesicles. Rather than a reduction of the number of vesicles per release site, the change involved functional elimination of release sites. This change was clearly dissociable from a second effect, depressed probability of transmitter release, which was common to both glutamate and GABA transmission. Thus, while other recent evidence links alteration of the releasable pool size with changes in p(r), our results suggest the two can be independently manipulated. Selective depression of glutamate release may provide an adaptive mechanism by which neurons limit excitotoxicity.

Regulation of dendritic protein synthesis by miniature synaptic events

We examined dendritic protein synthesis after a prolonged blockade of action potentials alone and after a blockade of both action potentials and miniature excitatory synaptic events (minis). Relative to controls, dendrites exposed to a prolonged blockade of action potentials showed diminished protein synthesis. Dendrites in which both action potentials and minis were blocked showed enhanced protein synthesis, suggesting that minis inhibit dendritic translation. When minis were acutely blocked or stimulated, an immediate increase or decrease, respectively, in dendritic translation was observed. Taken together, these results reveal a role for miniature synaptic events in the acute regulation of dendritic protein synthesis in neurons.

Spontaneous synaptic activity is required for the formation of functional GABAergic synapses in the developing rat hippocampus

Here we examine the role of the spontaneous synaptic activity generated by the developing rat hippocampus in the formation of functional gamma-aminobutyric acid (GABA) synapses. Intact hippocampal formations (IHFs) were dissected at birth and incubated for 1 day in control or tetrodotoxin (TTX)-supplemented medium at 25 degrees C. After the incubation, miniature GABA(A)-mediated postsynaptic currents (mGABA(A)-PSCs) were recorded in whole-cell voltage-clamped CA3 pyramidal neurones from IHF-derived slices. After 1 day in vitro in control medium, the frequency of mGABA(A)-PSCs was similar to that recorded in acute slices obtained 1 day after birth, but significantly higher than the frequency recorded from acute slices just after birth. These results suggest that the factors required in vivo for the formation of functional GABAergic synapses are preserved in the IHFs in vitro. The frequency increase was prevented when IHFs were incubated for 1 day with TTX. TTX treatment affected neither the morphology of CA3 pyramidal neurones nor cell viability. The TTX effects were reproduced when IHFs were incubated in the presence of glutamatergic or GABAergic ionotropic receptor antagonists or in high divalent cationic medium. The present results indicate that the spontaneous synaptic activity generated by the developing hippocampus is a key player in the formation of functional GABAergic synapses, possibly via network events requiring both glutamatergic and GABAergic receptors.

Microanatomy of dendritic spines: emerging principles of synaptic pathology in psychiatric and neurological disease

Psychiatric and neurologic disorders ranging from mental retardation to addiction are accompanied by structural and functional alterations of synaptic connections in the brain. Such alterations include abnormal density and morphology of dendritic spines, synapse loss, and aberrant synaptic signaling and plasticity. Recent work is revealing an unexpectedly complex biochemical and subcellular organization of dendritic spines. In this review, we highlight the molecular interplay between functional domains of the spine, including the postsynaptic density, the actin cytoskeleton, and membrane trafficking domains. This research points to an emerging level of analysis--a microanatomical understanding of synaptic physiology--that will be critical for discerning how synapses operate in normal physiologic states and for identifying and reversing microscopic changes in psychiatric and neurologic disease.

Neocortical seizures: initiation, development and cessation

Different forms of electrical paroxysms in experimental animals mimic the patterns of absence seizures associated with spike-wave complexes at approximately 3 Hz and of Lennox-Gastaut seizures with spike-wave or polyspike-wave complexes at approximately 1.5-2.5 Hz, intermingled with fast runs at 10-20 Hz. Both these types of electrical seizures are preferentially generated during slow-wave sleep. Here, we challenge the hypothesis of a subcortical pacemaker that would account for suddenly generalized spike-wave seizures as well as the idea of an exclusive role of synaptic excitation in the generation of paroxysmal depolarizing components, and we focus on three points, based on multiple intracellular and field potential recordings in vivo that are corroborated by some clinical studies: (a) the role of neocortical bursting neurons, especially fast-rhythmic-bursting neurons, and of very fast oscillations (ripples, 80-200 Hz) in seizure initiation; (b) the cortical origin of both these types of electrical paroxysms, the synaptic propagation of seizures from one to other, local and distant, cortical sites, finally reaching the thalamus, where the synchronous cortical firing excites thalamic reticular inhibitory neurons and thus leads to steady hyperpolarization and phasic inhibitory postsynaptic potentials in a majority of thalamocortical neurons, which might explain the obliteration of signals from the external world and the unconsciousness during absence seizures; and (c) the cessation of seizures, whose cellular mechanisms have only begun to be investigated and remain an open avenue for research.

Mossy fibre contact triggers the targeting of Kv4.2 potassium channels to dendrites and synapses in developing cerebellar granule neurons

Compartmentalization of neuronal function is achieved by highly localized clustering of ion channels in discrete subcellular membrane domains. Voltage-gated potassium (Kv) channels exhibit highly variable cellular and subcellular patterns of expression. Here, we describe novel activity-dependent synaptic targeting of Kv4.2, a dendritic Kv channel, in cerebellar granule cells (GCs). In vivo, Kv4.2 channels are highly expressed in cerebellar glomeruli, specializations of GC dendrites that form synapses with mossy fibres. In contrast, in cultured GCs, Kv4.2 was found localized, not to dendrites but to cell bodies. To investigate the role of synaptic contacts, we developed a co-culture system with cells from pontine grey nucleus, the origin of mossy fibres. In these co-cultures, synaptic structures formed, and Kv4.2 was now targeted to these synaptic sites in a manner dependent on synaptic activity. Activation of NMDA- and/or AMPA-type glutamate receptors was necessary for the targeting of Kv4.2 in co-cultures, and activation of these receptor systems in GC monocultures induced dendritic targeting of Kv4.2 in the absence of synapse formation. These results indicate that the proper targeting of Kv4.2 channels is dynamically regulated by synaptic activity. This activity-dependent regulation of Kv4.2 localization provides a crucial yet dynamic link between synaptic activity and dendritic excitability.

The number of glutamate receptors opened by synaptic stimulation in single hippocampal spines

The number of receptors opening after glutamate release is critical for understanding the sources of noise and the dynamic range of synaptic transmission. We imaged [Ca2+] transients mediated by synaptically activated NMDA receptors (NMDA-Rs) in individual spines in rat brain slices. We show that Ca2+ influx through single NMDA-Rs can be reliably detected, allowing us to estimate the number of receptors opening after synaptic transmission. This number is small: at the peak of the synaptic response, less than one NMDA-R is open, on average. Therefore, stochastic interactions between transmitter and receptor contribute substantially to synaptic noise, and glutamate occupies a small fraction of receptors. The number of receptors opening did not scale with spine volume, and smaller spines experience larger [Ca2+] transients during synaptic transmission. Our measurements further demonstrate that optical recordings can be used to study single receptors in intact systems.

Local structural balance and functional interaction of excitatory and inhibitory synapses in hippocampal dendrites

Theoretical and experimental studies on the computation of neural networks suggest that neural computation results from a dynamic interplay of excitatory and inhibitory (E/I) synaptic inputs. Precisely how E/I synapses are organized structurally and functionally to facilitate meaningful interaction remains elusive. Here we show that E/I synapses are regulated across dendritic trees to maintain a constant ratio of inputs in cultured rat hippocampal neurons. This structural arrangement is accompanied by an E/I functional balance maintained by a 'push-pull' feedback regulatory mechanism that is capable of adjusting E/I efficacies in a coordinated fashion. We also found that during activity, inhibitory synapses can determine the impact of adjacent excitatory synapses only if they are colocalized on the same dendritic branch and are activated simultaneously. These fundamental relationships among E/I synapses provide organizational principles relevant to deciphering the structural and functional basis for neural computation within dendritic branches.

Competition between phasic and asynchronous release for recovered synaptic vesicles at developing hippocampal autaptic synapses

Developing hippocampal neurons in microisland culture undergo rapid and extensive transmitter release-dependent depression of evoked (phasic) excitatory synaptic activity in response to 1 sec trains of 20 Hz stimulation. Although evoked phasic release was attenuated by repeated stimuli, asynchronous (miniature like) release continued at a high rate equivalent to approximately 2.8 readily releasable pools (RRPs) of quanta/sec. Asynchronous release reflected the recovery and immediate release of quanta because it was resistant to sucrose-induced depletion of the RRP. Asynchronous and phasic release appeared to compete for a common limited supply of release-ready quanta because agents that block asynchronous release, such as EGTA-AM, led to enhanced steady-state phasic release, whereas prolongation of the asynchronous release time course by LiCl delayed recovery of phasic release from depression. Modeling suggested that the resistance of asynchronous release to depression was associated with its ability to out-compete phasic release for recovered quanta attributable to its relatively low release rate (up to 0.04/msec per vesicle) stimulated by bulk intracellular Ca2+ concentration ([Ca2+]i) that could function over prolonged intervals between successive stimuli. Although phasic release was associated with a considerably higher peak rate of release (0.4/msec per vesicle), the [Ca2+]i microdomains that trigger it are brief (1 msec), and with asynchronous release present, relatively few quanta can accumulate within the RRP to be available for phasic release. We conclude that despite depression of phasic release during train stimulation, transmission can be maintained at a near-maximal rate by switching to an asynchronous mode that takes advantage of a bulk presynaptic [Ca2+]i.

Tail arteries from chronically spinalized rats have potentiated responses to nerve stimulation in vitro

Patients with severe spinal cord lesions that damage descending autonomic pathways generally have low resting arterial pressure but bladder or colon distension or unheeded injuries may elicit a life-threatening hypertensive episode. Such episodes (known as autonomic dysreflexia) are thought to result from the loss of descending baroreflex inhibition and/or plasticity within the spinal cord. However, it is not clear whether changes in the periphery contribute to the exaggerated reflex vasoconstriction. The effects of spinal transection at T7-8 on nerve- and agonist-evoked contractions of the rat tail artery were investigated in vitro. Isometric contractions of arterial segments were recorded and responses of arteries from spinalized animals ('spinalized arteries') and age-matched and sham-operated controls were compared. Two and eight weeks after transection, nerve stimulation at 0.1-10 Hz produced contractions of greater force and duration in spinalized arteries. At both stages, the alpha-adrenoceptor antagonists prazosin (10 nm) and idazoxan (0.1 microm) produced less blockade of nerve-evoked contraction in spinalized arteries. Two weeks after transection, spinalized arteries were supersensitive to the alpha(1)-adrenoceptor agonist phenylephrine, and the alpha(2)-adrenoceptor agonist, clonidine, but 8 weeks after transection, spinalized arteries were supersensitive only to clonidine. Contractions of spinalized arteries elicited by 60 mm K(+) were larger and decayed more slowly at both stages. These findings demonstrate that spinal transection markedly increases nerve-evoked contractions and this can, in part, be accounted for by increased reactivity of the vascular smooth muscle to vasoconstrictor agents. This hyper-reactivity may contribute to the genesis of autonomic dysreflexia in patients.

Postsynaptic contributions to hippocampal network hyperexcitability induced by chronic activity blockade in vivo

Neuronal activity is thought to play an important role in refining patterns of synaptic connectivity during development and in the molecular maturation of synapses. In experiments reported here, a 2-week infusion of tetrodotoxin (TTX) into rat hippocampus beginning on postnatal day 12 produced abnormal synchronized network discharges in in vitro slices. Discharges recorded upon TTX washout were called 'minibursts', owing to their small amplitude. They were routinely recorded in area CA3 and abolished by CNQX, an AMPA receptor antagonist. Because recurrent excitatory axon collaterals remodel and glutamate receptor subunit composition changes after postnatal day 12, experiments examined possible TTX-induced alterations in recurrent excitation that could be responsible for network hyperexcitability. In biocytin-labelled pyramidal cells, recurrent axon arbors were neither longer nor more highly branched in the TTX infusion site compared with saline-infused controls. However, varicosity size and density were increased. Whereas most varicosities contained synaptophysin and synaptic vesicles, many were not adjacent to postsynaptic specializations, and thus failed to form anatomically identifiable synapses. An increased pattern of excitatory connectivity does not appear to explain network hyperexcitability. Quantitative immunoblots also indicated that presynaptic markers were unaltered in the TTX infusion site. However, the postsynaptic AMPA and NMDA receptor subunits, GluR1, NR1 and NR2B, were increased. In electrophysiological studies EPSPs recorded in slices from TTX-infused hippocampus had an enhanced sensitivity to the NR2B containing NMDA receptor antagonist, ifenprodil. Thus, increases in subunit protein result in alterations in the composition of synaptic NMDA receptors. Postsynaptic changes are likely to be the major contributors to the hippocampal network hyperexcitability and should enhance both excitatory synaptic efficacy and plasticity.

The biology of epilepsy genes

Mutations in over 70 genes now define biological pathways leading to epilepsy, an episodic dysrhythmia of the cerebral cortex marked by abnormal network synchronization. Some of the inherited errors destabilize neuronal signaling by inflicting primary disorders of membrane excitability and synaptic transmission, whereas others do so indirectly by perturbing critical control points that balance the developmental assembly of inhibitory and excitatory circuits. The genetic diversity is now sufficient to discern short- and long-range functional convergence of epileptogenic molecular pathways, reducing the broad spectrum of primary molecular defects to a few common processes regulating cortical synchronization. Synaptic inhibition appears to be the most frequent target; however, each gene mutation retains unique phenotypic features. This review selects exemplary members of several gene families to illustrate principal categories of the disease and trace the biological pathways to epileptogenesis in the developing brain.

Neurotransmitter Release at Central Synapses

Our understanding of synaptic transmission has grown dramatically during the 15 years since the first issue of Neuron was published, a growth rate expected from the rapid progress in modern biology. As in all of biology, new techniques have led to major advances in the cell and molecular biology of synapses, and the subject has evolved in ways (like the production of genetically engineered mice) that could not even be imagined 15 years ago. My plan for this review is to summarize what we knew about neurotransmitter release when Neuron first appeared and what we recognized we did not know, and then to describe how our views have changed in the intervening decade and a half. Some things we knew about synapses--"knew" in the sense that the field had reached a consensus--are no longer accepted, but for the most part, impressive advances have led to a new consensus on many issues. What I find fascinating is that in certain ways nothing has changed--many of the old arguments persist or recur in a different guise--but in other ways the field would be unrecognizable to a neurobiologist time-transported from 1988 to 2003.

Defects in synaptic vesicle docking in unc-18 mutants

Sec1-related proteins function in most, if not all, membrane trafficking pathways in eukaryotic cells. The Sec1-related protein required in neurons for synaptic vesicle exocytosis is UNC-18. Several models for UNC-18 function during vesicle exocytosis are under consideration. We have tested these models by characterizing unc-18 mutants of the nematode Caenorhabditis elegans. In the absence of UNC-18, the size of the readily releasable pool is severely reduced. Our results show that the near absence of fusion-competent vesicles is not caused by a reduction in syntaxin levels, by a mislocalization of syntaxin, by a defect in fusion or by a failure to open syntaxin during priming. Rather, we found a reduction of docked vesicles at the active zone in unc-18 mutants, suggesting that UNC-18 functions, directly or indirectly, as a facilitator of vesicle docking.

Presynaptic regulation of dopaminergic neurotransmission

The development of electrochemical recordings with small carbon-fiber electrodes has significantly advanced the understanding of the regulation of catecholamine transmission in various brain areas. Recordings in vivo or in slice preparations monitor diffusion of catecholamine following stimulated synaptic release into the surrounding tissue. This synaptic 'overflow' is defined by the amount of release, by the activity of reuptake, and by the diffusion parameters in brain tissue. Such studies have elucidated the complex regulation of catecholamine release and uptake, and how psychostimulants and anti-psychotic drugs interfere with it. Moreover, recordings with carbon-fiber electrodes from cultured neurons have provided analysis of catecholamine release and its plasticity at the quantal level.

Synapse number and synaptic efficacy are regulated by presynaptic cAMP and protein kinase A

The mechanisms by which neurons regulate the number and strength of synapses during development and synaptic plasticity have not yet been defined fully. This lack of fundamental knowledge in the fields of neurodevelopment and synaptic plasticity can be attributed, in part, to compensatory mechanisms by which neurons accommodate for the loss of function in their synaptic partners. This is generally achieved either by scaling up neuronal transmitter release capabilities or by enhancing the postsynaptic responsiveness. Here, we demonstrate that regulation of synaptic strength and number between identified Lymnaea neurons visceral dorsal 4 (VD4, the presynaptic cell) and left pedal dorsal 1 (LPeD1, the postsynaptic cell) requires presynaptic activation of a cAMP-PKA-dependent signal. Experimental activation of the cAMP-PKA pathway resulted in reduced synaptic efficacy, whereas inhibition of the cAMP-PKA cascade permitted hyperinnervation and an overall enhancement of synaptic strength. Because synaptic transmission between VD4 and LPeD1 does not require a cAMP-PKA pathway, our data show that these messengers may play a novel role in regulating the synaptic efficacy during early synaptogenesis and plasticity.