Activity-Dependent Global Downscaling of Evoked Neurotransmitter Release across Glutamatergic Inputs in Drosophila

Within mammalian brain circuits, activity-dependent synaptic adaptations, such as synaptic scaling, stabilize neuronal activity in the face of perturbations. Stability afforded through synaptic scaling involves uniform scaling of quantal amplitudes across all synaptic inputs formed on neurons, as well as on the postsynaptic side. It remains unclear whether activity-dependent uniform scaling also operates within peripheral circuits. We tested for such scaling in a Drosophila larval neuromuscular circuit, where the muscle receives synaptic inputs from different motoneurons. We used motoneuron-specific genetic manipulations to increase the activity of only one motoneuron and recordings of postsynaptic currents from inputs formed by the different motoneurons. We discovered an adaptation which caused uniform downscaling of evoked neurotransmitter release across all inputs through decreases in release probabilities. This "presynaptic downscaling" maintained the relative differences in neurotransmitter release across all inputs around a homeostatic set point, caused a compensatory decrease in synaptic drive to the muscle affording robust and stable muscle activity, and was induced within hours. Presynaptic downscaling was associated with an activity-dependent increase in Drosophila vesicular glutamate transporter expression. Activity-dependent uniform scaling can therefore manifest also on the presynaptic side to produce robust and stable circuit outputs. Within brain circuits, uniform downscaling on the postsynaptic side is implicated in sleep- and memory-related processes. Our results suggest that evaluation of such processes might be broadened to include uniform downscaling on the presynaptic side.SIGNIFICANCE STATEMENT To date, compensatory adaptations which stabilise target cell activity through activity-dependent global scaling have been observed only within central circuits, and on the postsynaptic side. Considering that maintenance of stable activity is imperative for the robust function of the nervous system as a whole, we tested whether activity-dependent global scaling could also manifest within peripheral circuits. We uncovered a compensatory adaptation which causes global scaling within a peripheral circuit and on the presynaptic side through uniform downscaling of evoked neurotransmitter release. Unlike in central circuits, uniform scaling maintains functionality over a wide, rather than a narrow, operational range, affording robust and stable activity. Activity-dependent global scaling therefore operates on both the presynaptic and postsynaptic sides to maintain target cell activity.

Synaptic Deficits at Neuromuscular Junctions in Two Mouse Models of Charcot-Marie-Tooth Type 2d

Patients with Charcot-Marie-Tooth Type 2D (CMT2D), caused by dominant mutations in Glycl tRNA synthetase (GARS), present with progressive weakness, consistently in the hands, but often in the feet also. Electromyography shows denervation, and patients often report that early symptoms include cramps brought on by cold or exertion. Based on reported clinical observations, and studies of mouse models of CMT2D, we sought to determine whether weakened synaptic transmission at the neuromuscular junction (NMJ) is an aspect of CMT2D. Quantal analysis of NMJs in two different mouse models of CMT2D (Gars(P278KY), Gars(C201R)), found synaptic deficits that correlated with disease severity and progressed with age. Results of voltage-clamp studies revealed presynaptic defects characterized by: (1) decreased frequency of spontaneous release without any change in quantal amplitude (miniature endplate current), (2) reduced amplitude of evoked release (endplate current) and quantal content, (3) age-dependent changes in the extent of depression in response to repetitive stimulation, and (4) release failures at some NMJs with high-frequency, long-duration stimulation. Drugs that modify synaptic efficacy were tested to see whether neuromuscular performance improved. The presynaptic action of 3,4 diaminopyridine was not beneficial, whereas postsynaptic-acting physostigmine did improve performance. Smaller mutant NMJs with correspondingly fewer vesicles and partial denervation that eliminates some release sites also contribute to the reduction of release at a proportion of mutant NMJs. Together, these voltage-clamp data suggest that a number of release processes, while essentially intact, likely operate suboptimally at most NMJs of CMT2D mice.

SIGNIFICANCE STATEMENT

We have uncovered a previously unrecognized aspect of axonal Charcot-Marie-Tooth disease in mouse models of CMT2D. Synaptic dysfunction contributes to impaired neuromuscular performance and disease progression. This suggests that drugs which improve synaptic efficacy at the NMJ could be considered in treating the pathophysiology of CMT2D patients.

Reversible Recruitment of a Homeostatic Reserve Pool of Synaptic Vesicles Underlies Rapid Homeostatic Plasticity of Quantal Content

Homeostatic regulation is essential for the maintenance of synaptic strength within the physiological range. The current study is the first to demonstrate that both induction and reversal of homeostatic upregulation of synaptic vesicle release can occur within seconds of blocking or unblocking acetylcholine receptors at the mouse neuromuscular junction. Our data suggest that the homeostatic upregulation of release is due to Ca(2+)-dependent increase in the size of the readily releasable pool (RRP). Blocking vesicle refilling prevented upregulation of quantal content (QC), while leaving baseline release relatively unaffected. This suggested that the upregulation of QC was due to mobilization of a distinct pool of vesicles that were rapidly recycled and thus were dependent on continued vesicle refilling. We term this pool the "homeostatic reserve pool." A detailed analysis of the time course of vesicle release triggered by a presynaptic action potential suggests that the homeostatic reserve pool of vesicles is normally released more slowly than other vesicles, but the rate of their release becomes similar to that of the major pool during homeostatic upregulation of QC. Remarkably, instead of finding a generalized increase in the recruitment of vesicles into RRP, we identified a distinct homeostatic reserve pool of vesicles that appear to only participate in synchronized release following homeostatic upregulation of QC. Once this small pool of vesicles is depleted by the block of vesicle refilling, homeostatic upregulation of QC is no longer observed. This is the first identification of the population of vesicles responsible for the blockade-induced upregulation of release previously described. Significance statement: The current study is the first to demonstrate that both the induction and reversal of homeostatic upregulation of synaptic vesicle release can occur within seconds. Our data suggest that homeostatic upregulation of release is due to Ca(2+)-dependent priming/docking of a small homeostatic reserve pool of vesicles that normally have slow-release kinetics. Following priming, the reserve pool of vesicles is released synchronously with the normal readily releasable pool of synaptic vesicles. This is the first description of this unique pool of synaptic vesicles.

The role of cAMP in synaptic homeostasis in response to environmental temperature challenges and hyperexcitability mutations

Homeostasis is the ability of physiological systems to regain functional balance following environment or experimental insults and synaptic homeostasis has been demonstrated in various species following genetic or pharmacological disruptions. Among environmental challenges, homeostatic responses to temperature extremes are critical to animal survival under natural conditions. We previously reported that axon terminal arborization in Drosophila larval neuromuscular junctions (NMJs) is enhanced at elevated temperatures; however, the amplitude of excitatory junctional potentials (EJPs) remains unaltered despite the increase in synaptic bouton numbers. Here we determine the cellular basis of this homeostatic adjustment in larvae reared at high temperature (HT, 29°C). We found that synaptic current focally recorded from individual synaptic boutons was unaffected by rearing temperature (<15°C to >30°C). However, HT rearing decreased the quantal size (amplitude of spontaneous miniature EJPs, or mEJPs), which compensates for the increased number of synaptic releasing sites to retain a normal EJP size. The quantal size decrease is accounted for by a decrease in input resistance of the postsynaptic muscle fiber, indicating an increase in membrane area that matches the synaptic growth at HT. Interestingly, a mutation in rutabaga (rut) encoding adenylyl cyclase (AC) exhibited no obvious changes in quantal size or input resistance of postsynaptic muscle cells after HT rearing, suggesting an important role for rut AC in temperature-induced synaptic homeostasis in Drosophila. This extends our previous finding of rut-dependent synaptic homeostasis in hyperexcitable mutants, e.g., slowpoke (slo). In slo larvae, the lack of BK channel function is partially ameliorated by upregulation of presynaptic Shaker (Sh) IA current to limit excessive transmitter release in addition to postsynaptic glutamate receptor recomposition that reduces the quantal size.

The mechanism of choline-mediated inhibition of acetylcholine release in mouse motor synapses

The mechanism of action of tonically applied choline, the agonist of α7 nicotinic acetylcholine receptors (nAChRs), to the spontaneous and evoked release of a neurotransmitter in mouse motor synapses in diaphragm neuromuscular preparations using intracellular microelectrode recordings of miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs) was studied. Exogenous choline was shown to exhibit a presynaptic inhibitory effect on the amplitude and quantal content of EPPs for the activity of neuromuscular junction evoked by single and rhythmic stimuli. This effect was inhibited either by antagonists of α7-nAChRs, such as methyllycaconitine and α-cobratoxin, or by blocking SK-type calcium-activated potassium (KCa) channels with apamin or blocking intraterminal ryanodine receptors with ryanodine. A hypothesis was put forward that choline in mouse motoneuron nerve terminals can activate presynaptic α7-nAChRs, followed by the release of the stored calcium through ryanodine receptors and activation of SK-type KCa channels, resulting in sustained decay of the quantal content of the evoked neurotransmitter release.

Glycinergic synaptic transmission in the cochlear nucleus of mice with normal hearing and age-related hearing loss

The principal inhibitory neurotransmitter in the mammalian cochlear nucleus (CN) is glycine. During age-related hearing loss (AHL), glycinergic inhibition becomes weaker in CN. However, it is unclear what aspects of glycinergic transmission are responsible for weaker inhibition with AHL. We examined glycinergic transmission onto bushy cells of the anteroventral CN in normal-hearing CBA/CaJ mice and in DBA/2J mice, a strain that exhibits an early onset AHL. Glycinergic synaptic transmission was examined in brain slices of mice at 10-15 postnatal days old, 20-35 days old, and at 6-7 mo old. Spontaneous inhibitory postsynaptic current (sIPSC) event frequency and amplitude were the same among all three ages in both strains of mice. However, the amplitudes of IPSCs evoked (eIPSC) from stimulating the dorsal CN were smaller, and the failure rate was higher, with increasing age due to decreased quantal content in both mouse strains, independent of hearing status. The coefficient of variation of the eIPSC amplitude also increased with age. The decay time constant (τ) of sIPSCs and eIPSCs were constant in CBA/CaJ mice at all ages, but were significantly slower in DBA/2J mice at postnatal days 20-35, following the onset of AHL, and not at earlier or later ages. Our results suggest that glycinergic inhibition at the synapses onto bushy cells becomes weaker and less reliable with age through changes in release. However, the hearing loss in DBA/2J mice is accompanied by a transiently enhanced inhibition, which could disrupt the balance of excitation and inhibition.

Redox-sensitive synchronizing action of adenosine on transmitter release at the neuromuscular junction

The kinetics of neurotransmitter release was recognized recently as an important contributor to synaptic efficiency. Since adenosine is the ubiquitous modulator of presynaptic release in peripheral and central synapses, in the current project we studied the action of this purine on the timing of acetylcholine quantal release from motor nerve terminals in the skeletal muscle. Using extracellular recording from frog neuromuscular junction we tested the action of adenosine on the latencies of single quantal events in the pro-oxidant and antioxidant conditions. We found that adenosine, in addition to previously known inhibitory action on release probability, also synchronized release by removing quantal events with long latencies. This action of adenosine on release timing was abolished by oxidants whereas in the presence of the antioxidant the synchronizing action of adenosine was further enhanced. Interestingly, unlike the timing of release, the inhibitory action of adenosine on release probability was redox-independent. Modulation of release timing by adenosine was mediated by purinergic A1 receptors as it was eliminated by the specific A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and mimicked by the specific A1 agonist N(6)-cyclopentyl-adenosine. Consistent with data obtained from dispersion of single quantal events, adenosine also reduced the rise-time of multiquantal synaptic currents. The latter effect was reproduced in the model based on synchronizing effect of adenosine on release timing. Thus, adenosine which is generated at the neuromuscular junction from the breakdown of the co-transmitter ATP induces the synchronization of quantal events. The effect of adenosine on release timing should preserve the fidelity of synaptic transmission via "cost-effective" use of less transmitter quanta. Our findings also revealed important crosstalk between purinergic and redox modulation of synaptic processes which could take place in the elderly or in neuromuscular diseases associated with oxidative stress like lateral amyotrophic sclerosis.

The role of muscle-specific tyrosine kinase (MuSK) and mystery of MuSK myasthenia gravis

MuSK myasthenia gravis is a rare, severe autoimmune disease of the neuromuscular junction, only identified in 2001, with unclear pathogenic mechanisms. In this review we describe the clinical aspects that distinguish MuSK MG from AChR MG, review what is known about the role of MuSK in the development and function of the neuromuscular junction, and discuss the data that address how the antibodies to MuSK lead to neuromuscular transmission failure.

Inverse relationship between release probability and readily releasable vesicles in depressing and facilitating synapses

We tested the hypothesis that the probability of vesicular exocytosis at synapses is positively correlated with the pools of readily releasable synaptic vesicles, as shown for mammalian neurons grown in tissue culture. We compared synapses of two identified glutamatergic neurons: phasic (high-output, depressing) and tonic (low-output, facilitating) crustacean motor neurons, which differ 100- to 1000-fold in quantal content. Estimates of vesicles available for exocytosis were made from depletion during forced release and from electron microscopic observation of vesicles docked at synaptic membranes near active zones. Both measurements showed a significantly larger pool of readily releasable vesicles in facilitating synapses, despite their much lower quantal output during stimulation. Thus, the probability for release of docked vesicles is very much lower at facilitating synapses, and the presence of more docked vesicles does not predict higher synaptic release probability in these paired excitatory neurons.

Disparity in neurotransmitter release probability among competing inputs during neuromuscular synapse elimination

Competition among the several motor axons transiently innervating neonatal muscle fibers results in an increasing disparity in the quantal content and synaptic territory of each competitor, culminating in the permanent loss of all but one axon from neuromuscular junctions. We asked whether differences in the probability of neurotransmitter release also contribute to the increasing disparity in quantal content among competing inputs, and when in the process of competition changes in release probability become apparent. To address these questions, intracellular recordings were made from dually innervated neonatal mouse soleus muscle fibers, and quantal content and paired-pulse facilitation were evaluated for each input. At short interpulse intervals, paired-pulse facilitation was significantly higher for the weaker input with the smaller quantal content than the stronger input with the larger quantal content. Because neurotransmitter release probability across all release sites is inversely related to the extent of facilitation observed after paired-pulse stimulation, this result suggests that release probability is lower for weak compared with strong inputs innervating the same junction. A disparity in the probability of neurotransmitter release thus contributes to the disparity in quantal content that occurs during synaptic competition. Together, this work suggests that an input incapable of sustaining a high release probability may be at a competitive disadvantage for synaptic maintenance.

Glutamate receptor expression regulates quantal size and quantal content at the Drosophila neuromuscular junction

At the Drosophila glutamatergic neuromuscular junction, the postsynaptic cell can regulate synaptic strength by both changing its sensitivity to neurotransmitter and generating a retrograde signal that regulates presynaptic transmitter release. To investigate the molecular mechanisms underlying these forms of plasticity, we have undertaken a genetic analysis of two postsynaptic glutamate receptors that are expressed at this synapse. Deletion of both genes results in embryonic lethality that can be rescued by transgenic expression of either receptor. Although these receptors are redundant for viability, they have important differences. By transgenically rescuing the double mutant, we have investigated the relationship of receptor gene dosage and composition to synaptic function. We find that the receptor subunit composition regulates quantal size, Argiotoxin sensitivity, and receptor desensitization kinetics. Finally, we show that the activity of the receptor can regulate the retrograde signal functioning at this synapse. Thus, the diversity of receptors expressed at this synapse provides the cell with mechanisms for generating synaptic plasticity.

Synaptic vesicle dynamics in rat fast and slow motor nerve terminals

We have investigated whether rat motor nerve terminals with different in vivo activity patterns also have different vesicle trafficking characteristics. To do this, we monitored, using combined optical and electrical techniques, the rate of exocytosis (during different frequencies and patterns of activity), the releasable pool size, and the recycle time of synaptic vesicles in terminals on soleus (slow-twitch) and extensor digitorum longus [(EDL); fast-twitch] muscle fibers. EDL terminals had a higher initial quantal content (QC) than soleus, but during tonic or phasic stimulation at 20-80 Hz, EDL QC ran down to a greater extent than soleus QC. By recording loss of fluorescence from exocytosing vesicles labeled with the dye FM1-43, EDL terminals were found to destain faster than those in soleus. Simultaneous intracellular recording of end plate potentials, to count the number of vesicles released, permitted estimation of the total vesicle pool (VP) size and the recycle time by combining the optical and electrophysiological data. Soleus vesicle pool was larger than EDL, but recycle time was not significantly different. These terminals, therefore, are adapted to their in vivo activity patterns by alterations in QC and VP size but not recycle time.

The electrostatic basis of Mg++ inhibition of transmitter release

The inhibition by Mg(++) of stimulus-evoked transmitter release is attributed to a decrease in surface potential, Psi(0), on the outer surface of the presynaptic terminal and hence a lower surface calcium concentration, [Ca(++)](0). Data on the frog neuromuscular junction are quantitatively fit by assuming that there is a negative charge density, sigma, on the outer surface of the presynaptic terminal of 6.5 x 10(13) charges per cm(2) and that simple diffuse double layer theory is applicable. No specific binding of Mg(++) or Ca(++) is required. Without any additional assumptions, the inhibitory effect of univalent cations is also quantitatively predicted.