Increase In Amplitude Of Excitatory Postsynaptic Potentials Research Articles

Different mechanisms of Ca2+ regulation that influence synaptic transmission: Comparison between crayfish and Drosophila neuromuscular junctions

A brief historical background on synaptic transmission in relation to Ca(2+) dynamics and short-term facilitation is described. This study focuses on the mechanisms responsible for the regulation of intracellular calcium concentration ([Ca(2+)](i)) in high output terminals of larval Drosophila compared to a low-output terminal of the crayfish neuromuscular junction (NMJ). Three processes; plasmalemmal Na(+)/Ca(2+) exchanger [NCX], Ca(2+)-ATPase (PMCA), and sarcoplasmic/endoplasmic Ca(2+)-ATPase (SERCA) are important in regulating the [Ca(2+)](i) are examined. When the NCX is compromised by reduced [Na(+)](o), no consistent effect occurred; but a NCX blocker KB-R7943 decreased the excitatory postsynaptic potential (EPSP) amplitudes. Compromising the PMCA with pH 8.8 resulted in an increase in EPSP amplitude but treatment with a PMCA specific inhibitor carboxyeosin produced opposite results. Thapsigargin exposure to block the SERCA generally decreases EPSP amplitude. Compromising the activity of the above Ca(2+) regulating proteins had no substantial effects on short-term depression. The Kum(170TS) strain (with dysfunctional SERCA), showed a decrease in EPSP amplitudes including the first EPSP within the train. Synaptic transmission is altered by reducing the function of the above three [Ca(2+)](i) regulators; but they are not consistent among different species as expected. Results in crayfish NMJ were more consistent with expected results as compared to the Drosophila NMJ. It is predicated that different mechanisms are used for regulating the [Ca(2+)](i) in high and low output synaptic terminals.

Quantifying the Magnitude of Changes in Synaptic Level Parameters With Long-Term Potentiation

Experimental evidence supports a number of mechanisms for the synaptic change that occurs with long-term potentiation (LTP) including insertion of AMPA receptors, an increase in AMPA receptor single channel conductance, unmasking silent synapses, and increases in vesicle release probability. Here we combine experimental and modeling studies to quantify the magnitude of the change needed at the synaptic level to explain LTP with these proposed mechanisms. Whole cell patch recordings were used to measure excitatory postsynaptic potential (EPSP) amplitude in response to near minimal afferent stimulation before and after LTP induction in CA1 pyramidal cells. Detailed neuron and synapse level models were constructed to estimate quantitatively the changes needed to explain the experimental results. For cells in normal artificial cerebrospinal fluid (ACSF), we found a 60% average increase in EPSP amplitude with LTP. This was explained in the models by a 63% increase in the number of activated synapses, a 64% increase in the AMPA receptor single channel conductance, or a 73% increase in the number of AMPA receptors per potentiated synapse. When the percentage LTP was above the average, the required increases through the proposed mechanisms became nonlinear, particularly for increases in the number of receptors. Given constraints from other experimental studies, our quantification suggests that neither unmasking silent synapses nor increasing the numbers of AMPA receptors at synapses is sufficient to explain the magnitude of LTP we observed, but increasing AMPA single channel conductance or vesicle release probability can be sufficient. Our results are most compatible with a combination of mechanisms producing LTP.

Sensitivity of transformed (phasic to tonic) motor neurons to the neuromodulator 5-HT

Long-term adaptation resulting in a ‘tonic-like’ state can be induced in phasic motor neurons of the crayfish, Procambarus clarkii, by daily low-frequency stimulation [Lnenicka, G.A., Atwood, H.L., 1985b. Long-term facilitation and long-term adaptation at synapses of a crayfish phasic motoneuron. J. Neurobiol. 16, 97–110]. To test the hypothesis that motor neurons undergoing adaptation show increased responses to the neuromodulator serotonin (5-HT), phasic motor neurons innervating the deep abdominal extensor muscles of crayfish were stimulated at 2.5 Hz, 2 h/day, for 7 days. One day after cessation of conditioning, contralateral control and conditioned motor neurons of the same segment were stimulated at 1 Hz and the induced excitatory post-synaptic potentials (EPSPs) were recorded from DEL 1 muscle fibers innervated by each motor neuron type. Recordings were made in saline without and with 100 nM 5-HT. EPSP amplitudes were increased by 5-HT exposure in all cases. Conditioned muscles exposed to 5-HT showed a 2-fold higher percentage of increase in EPSP amplitude than did control muscles. Thus, the conditioned motor neurons behaved like intrinsically tonic motoneurons in their response to 5-HT. While these results show that long-term adaptation (LTA) extends to 5-HT neuromodulation, no phenotype switch could be detected in the postsynaptic muscle. Protein isoform profiles, including the myosin heavy chains, do not change after 1 week of conditioning their innervating motor neurons.

Regeneration of cerebral-buccal interneurons and recovery of ingestion buccal motor programs in Aplysia after CNS lesions.

In the sea slug Aplysia, rhythmic biting is eliminated after bilateral cerebral-buccal connective (CBC) crushes and recovers within 14 days postlesion (dpl). The ability of cerebral-buccal interneuron-2 (CBI-2) to elicit ingestion buccal motor programs (iBMPs; i.e., fictive rhythmic ingestion) and to regenerate synaptic connections with target buccal neurons was assessed with intracellular recordings and dye injections. Isolated central ganglia were obtained from control animals and from lesioned animals at selected times after bilateral CBC crushes. Within 3 wk postlesion, transected CBI-2 axons sprouted at least 10 fine neurites confined to the core of the CBC that projected across the crush site toward the buccal ganglia. When fired with depolarizing current steps, CBI-2 was not observed to elicit iBMPs in preparations until 14 dpl. Thereafter a progressive enhancement in CBI-2's ability to elicit iBMPs was observed with time postlesion. By 40 dpl, CBI-2-elicited iBMPs were indistinguishable from those of controls. CBI-2 regenerated monosynaptic connections with appropriate buccal premotor- and motorneurons by 14 dpl, and the strength of these connections increased with time postlesion. Dramatic frequency facilitation was exhibited by the regenerating CBI-2 buccal synapses; for instance, at early postlesion times, no observable excitatory postsynaptic potentials (EPSPs) were obtained with 1- Hz stimulation of CBI-2, while at 7 Hz, a dramatic increase in EPSP amplitude was obtained with successive spikes. The present study shows that the time course of axonal and synaptic regeneration by command-like interneuron CBI-2 is correlated with the recovery of ingestion buccal motor programs elicited by CBI-2. These results parallel our previous findings of functional neural regeneration in the feeding system and suggest that functional neural regeneration is at least in part mediated by regeneration of specific synaptic pathways.

Electrophysiological properties and synaptic responses of cells in the trigeminal principal sensory nucleus of postnatal rats.

In the rodent brain stem trigeminal complex, select sets of neurons form modular arrays or "barrelettes," that replicate the patterned distribution of whiskers and sinus hairs on the ipsilateral snout. These cells detect the patterned input from the trigeminal axons that innervate the whiskers and sinus hairs. Other brain stem trigeminal cells, interbarrelette neurons, do not form patterns and respond to multiple whiskers. We examined the membrane properties and synaptic responses of morphologically identified barrelette and interbarrelette neurons in the principal sensory nucleus (PrV) of the trigeminal nerve in early postnatal rats shortly after whisker-related patterns are established. Barrelette cell dendritic trees are confined to a single barrelette, whereas the dendrites of interbarrelette cells span wider territories. These two cell types are distinct from smaller GABAergic interneurons. Barrelette cells can be distinguished by a prominent transient A-type K(+) current (I(A)) and higher input resistance. On the other hand, interbarrelette cells display a prominent low-threshold T-type Ca(2+) current (I(T)) and lower input resistance. Both classes of neurons respond differently to electrical stimulation of the trigeminal tract. Barrelette cells show either a monosynaptic excitatory postsynaptic potential (EPSP) followed by a large disynaptic inhibitory postsynaptic potential (IPSP) or just simply a disynaptic IPSP. Increasing stimulus intensity produces little change in EPSP amplitude but leads to a stepwise increase in IPSP amplitude, suggesting that barrelette cells receive more inhibitory input than excitatory input. This pattern of excitation and inhibition indicates that barrelette cells receive both feed-forward and lateral inhibition. Interbarrelette cells show a large monosynaptic EPSP followed by a small disynaptic IPSP. Increasing stimulus intensity leads to a stepwise increase in EPSP amplitude and the appearance of polysynaptic EPSPs, suggesting that interbarrelette cells receive excitatory inputs from multiple sources. Taken together, these results indicate that barrelette and interbarrelette neurons can be identified by their morphological and functional attributes soon after whisker-related pattern formation in the PrV.

Passive electrotonic properties of rat hippocampal CA3 interneurones.

1. The linear membrane responses of CA3 interneurones were determined with the use of whole-cell patch recording methods. The mean input resistance (RN) for all cells in this study was 526 +/- 16 MOmega and the slowest membrane time constant (tau0) was 73 +/- 3 ms. 2. The three-dimensional morphology of 63 biocytin-labelled neurones was used to construct compartmental models. Specific membrane resistivity (Rm) and specific membrane capacitance (Cm) were estimated by fitting the linear membrane response. Acceptable fits were obtained for 24 CA3 interneurones. The mean Rm was 61.9 +/- 34.2 Omega cm2 and the mean Cm was 0.9 +/- 0.3 microF cm-2. Intracellular resistance (Ri) could not be resolved in this study. 3. Examination of voltage attenuation revealed a significantly low synaptic efficiency from most dendritic synaptic input locations to the soma. 4. Simulations of excitatory postsynaptic potentials (EPSPs) were analysed at both the site of synaptic input and at the soma. There was little variability in the depolarization at the soma from synaptic inputs placed at different locations along the dendritic tree. The EPSP amplitude at the site of synaptic input was progressively larger with distance from the soma, consistent with a progressive increase in input impedance. 5. The 'iso-efficiency' of spatially different synaptic inputs arose from two opposing factors: an increase in EPSP amplitude at the synapse with distance from the soma was opposed by a nearly equivalent increase in voltage attenuation. These simulations suggest that, in these particular neurones, the amplitude of EPSPs measured at the soma will not be significantly affected by the location of synaptic inputs.

EPSP amplitude modulation at the rat Ia-alpha motoneuron synapse: effects of GABAB receptor agonists and antagonists.

The object of this study was to examine the relationship between excitatory postsynaptic potential (EPSP) amplitude, posttetanic potentiation, and EPSP amplitude modulation at synapses made by group Ia afferents on motoneurons in the rat. These relationships were evaluated in cells in untreated rats and in cells in rats treated with the gamma-aminobutyric acid-B (GABAB) receptor agonist baclofen and antagonist CGP-35348, which were used to manipulate Ca2+ entry into presynaptic terminals and consequently probability of transmitter release from them. There was no evidence for postsynaptic action of these drugs from measurement of their effects on motoneuron properties. During high-frequency stimulation (32 shock bursts at 167 Hz), EPSP amplitude either decreased (negative modulation) or increased (positive modulation) in response to successive stimuli at different connections. In untreated rats this frequency-dependent amplitude modulation behavior was inversely but weakly correlated with EPSP amplitude measured at low frequency. Intravenous (iv) administration of the GABAB agonist, baclofen, produced a marked and progressive decrease in EPSP amplitude measured at low frequency coincident with a change in frequency-dependent EPSP amplitude modulation toward more positive values (synaptic facilitation). In contrast, an increase in EPSP amplitude occurred after iv administration of the GABAB antagonist CGP-35348 that was accompanied by a negative shift in EPSP amplitude modulation during high-frequency stimulation. The negative shift in EPSP amplitude modulation (synaptic depression) after CGP-35348 application was much smaller than the positive shift induced by baclofen when normalized to the change in EPSP amplitude. Posttetanic potentiation decreased after baclofen but did not increase after CGP-35348. The relationship between modulation and EPSP amplitude was much steeper after GABAB receptor manipulation in either direction than that observed in the population of motoneurons in untreated preparations. This suggests that in the rat differences in probability of release play at most a small role in determining EPSP amplitude across the motoneuron pool.

Long-lasting enhancement of synaptic excitability of CA1/subiculum neurons of the rat ventral hippocampus by vasopressin and vasopressin(4–8)

Vasopressin (VP) is axonally distributed in many brain structures, including the ventral hippocampus. Picogram quantities of VP injected into the hippocampus improve the passive avoidance response of rats, presumably by enhancing memory processes. Vasopressin is metabolized by the brain tissue into shorter peptides, such as [pGlu 4r,Cyt 6]VP(4–9[ and [pGIu 4,Cyt 6,]VP(4–8), which preserve the behavioral activity but lose the peripheral activities of the parent hormone. Using brain slices, we investigated whether VP or VP(4–8) affects excitatory postsynaptic potentials (EPSPs) and/or membrane responses to depolarization in neurons of the CA 1 /subiculum of the ventral hippocampus. The EPSPs were evoked by stimulating the stratum radiatum of the CAI field; the membrane responses were elicited by current injections. Exposure of slices for 15 min to 0.1 nM solution of these peptides resulted in an increase in the amplitude and slope of the EPSPs in 21 neurons (67%) tested. No consistent change in either the resting membrane potential or the input resistance of the neurons was observed. The peptide-induced increase in EPSPs reached a maximum 30–45 min after peptide application. In 14 of these neurons (66%), the peptide-induced increase in EPSPs remained throughout the entire 60–120 min washout period. In the remaining 7 neurons (33%), the initial increase in EPSPs amplitude was followed by a gradual decline to the pre-administration level. The increase in EPSP amplitude was often. but not always, associated with a decrease in the threshold and increase in the number of action potentials in response to depolarizing current injection. Suppression of GABA A receptor-mediated inhibition and N-methyl- d-aspartate (NMDA) receptor-mediated excitation did not prevent the effects of VP and VP(4–8[ on the EPSP amplitude or the threshold for action potentials. The results demonstrate that 0.1 nM concentrations of these neuropeptides can elicit a long-lasting enhancement of the excitability of CA1/subiculum neurons of the ventral hippocampus to excitatory, glutamatergic synaptic input. This novel action of VP and its metabolite in the ventral hippocampus may be the physiological action, mediating the memory-enhancing effect of these peptides.

Modulation of transmitter release from the terminals of the locust wing stretch receptor neuron by muscarinic antagonists

The forewing stretch receptor (SR) neuron makes monosynaptic connections with wing depressor motoneurons; in this article the pharmacology of its output onto the first basalar motoneuron (BA1) has been investigated. The SR, like other insect afferents that have been studied so far, appears to be cholinergic; transmission was suppressed reversibly by the nicotinic antagonist gallamine (10(-4) M) and irreversibly by alpha-bungarotoxin (10(-6) M). The choline reuptake blocker hemicholinium-3 (10(-4) M) also caused a reversible reduction in the amplitude of SR excitatory postsynaptic potentials (EPSPs) recorded in BA1. The receptor subtype nonselective muscarinic antagonists atropine (10(-4) M), scopolamine (10(-4) M), and quinuclidinyl benzilate (10(-5) M), unlike nicotinic antagonists, caused an augmentation in EPSP amplitude. This effect does not appear to be caused by an increase in sensitivity of the motoneuron to acetylcholine (ACh), since atropine produced a marked reduction rather than an increase in the amplitude of responses to ACh pressure applied to the soma of BA1. Scopolamine only caused a modest reduction in the amplitude of ACh somatic responses. The simplest explanation for these observations is that muscarinic antagonists bring about an increase in EPSP amplitude by blockade of presynaptic autoreceptors that normally down-regulate the release of ACh from SR terminals. The effects of muscarinic receptor subtype-selective antagonists indicate that presynaptic receptors in this preparation may have a pharmacological profile more similar to that of vertebrate M2 receptors than to that of M1 or M3 subtypes. The functional significance of autoreceptors in this preparation are discussed.

Analysis of fluctuations of "minimal" excitatory postsynaptic potentials during long-term potentiation in guinea pig hippocampal slices.

In previous studies, quantal analysis assuming a simple binomial model has shown that long-term potentiation (LTP) is accompanied by an increase in both mean quantal content (m) and quantal size (v), whereby the increase in m predominates. In the present study, "compound" binomial distributions with variable probabilities were convolved with Gaussian distributions in computer experiments to simulate amplitude histograms of intracellular excitatory postsynaptic potentials (EPSPs). A deconvolution procedure assuming equal "quantal" separation (v) between discrete components, but without assuming binomial statistics, was applied to the simulated distributions to determine v. It was found that with a small ratio of standard deviation of noise to v (Sn/v less than 0.4), a reliable estimate of v can be obtained even for small samples (N = 100). When Sn/v was larger (0.4-0.6), approximate v estimates (within +/- 10-20% of the simulated v) could be obtained by averaging estimates from about 10 small samples (N = 100). "Minimal" EPSPs were recorded in area CA1 of guinea pig hippocampal slices. 37 EPSP amplitude samples of 9 neurones were measured before and up to 55 min after 10 tetanizations of stratum radiatum. In accordance with the previous data, the increase in v accounted for only about 10% of the average post-tetanic increase in EPSP amplitude and was not correlated with the latter. However, for an EPSP subset with small LTP magnitude, the increase in v accounted for an essential part of the LTP magnitude while the increase in m did not correlate with it. The results are in agreement with previous data obtained in the context of the simple binomial model and are interpreted as indicating primarily a presynaptic mechanism of LTP maintenance. The results suggest two types of synaptic mechanism of LTP maintenance related to the increases in m and v, respectively. The latter mechanism is saturated at about 10 to 30% increase in post-tetanic amplitude above the pre-tetanic EPSP amplitude.

Persistent enhancement of transmitter release accompanying long-term potentiation in the guinea pig hippocampus

In order to examine temporal changes in enhancement of transmitter release during long-term potentiation (LTP), we examined amplitude fluctuation of excitatory postsynaptic potentials (EPSPs) for longer periods than 2 h after tetanic stimulation (up to 4 h in the longest observation). The relative magnitude of excitatory postsynaptic potentiation (EPSP) fluctuation (coefficient of variation, CV) reduced throughout the observation periods in association with an increase in EPSP amplitude after tetanic stimulation. The reciprocals of squared CVs (= mean 2 variance ) were almost in proportion to the magnitude of LTP, and the ratio of 1 CV 2 and the LTP magnitude did not change significantly for up to 4 h. These findings suggest that a prolonged enhancement of transmitter release from presynaptic terminals underlies LTP, and the relative contribution of this presynaptic enhancement does not change significantly for 2 h (maybe up to 4 h, or longer) after tetanic stimulation.

Pericardial peptides enhance synaptic transmission and tension in phasic extensor muscles of crayfish

Two identified peptides, which are structurally related to FMRF-NH 2 and are known to be associated with lobster pericardial organs, increase nerve-evoked tension and excitatory postsynaptic potentials (EPSPs) recorded from crayfish deep abdominal extensor muscles. At low stimulus frequencies, which produce marked depression of muscle twitches with successive stimuli, the peptides quickly and reversibly restore tension. Increased quantal content of transmitter release, rather than changes in postsynaptic input resistance, accounted for most of the increase in EPSP amplitude. The results support earlier suggestions that these two peptides may act as circulating neurohormones and provide the first evidence for peptidergic modulation of a phasic neuromuscular system in a crustacean.

Conjoint action of phosphatidylinositol and adenylate cyclase systems in serotonin-induced facilitation at the crayfish neuromuscular junction

1. Pulsatile application of serotonin (5-HT) leads to facilitation of excitatory postsynaptic potentials (EPSPs) in crayfish "opener" neuromuscular preparations. The facilitation resulting from a single application of serotonin shows two phases: an early, rapidly decaying phase, and a less intense, long-lasting phase of 1- to 2-h duration. A previous study implicated the phosphatidylinositol system as an essential component in serotonin-induced facilitation, especially the early phase. The present study was conducted to determine the roles of the adenylate cyclase and phosphatidylinositol systems in both phases of serotonin-induced facilitation. 2. Relatively brief applications of agents known to affect the intracellular concentration of cAMP (forskolin, 1 microM; and IBMX, 100 microM) cause an increase in EPSP amplitude, which persists for 1-2 h. 3. The duration of the less intense, long-lasting phase of serotonin-induced facilitation is prolonged in the presence of 1 microM IBMX. This concentration of IBMX does not affect EPSP amplitude by itself. A membrane-permeant analog of cAMP (applied in concentrations less than or equal to 1 mM) is also not effective in altering EPSP amplitude. However, when dibutyryl cAMP is applied in the presence of 1 microM IBMX, EPSP amplitude is increased (60-80%). 4. Localized presynaptic injection of the "Walsh Inhibitor" (PKI), which inhibits cAMP-activated protein kinase, blocks the less intense, long-lasting phase of serotonin-induced facilitation at synapses near the site of injection. Normal facilitation develops at synapses within the same preparation remote from the site of injection. Distribution of the injected inhibitor within the axon can be visualized by tagging PKI with a fluorescent marker.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic plasticity in the molluscan peripheral nervous system: physiology and role for peptides

The plasticity of a synapse in the molluscan peripheral nervous system was examined under a variety of experimental, physiological, and pharmacological conditions. These studies employed the isolated salivary glands and attached buccal ganglia of the freshwater snail Helisoma. Action potentials evoked in buccal neuron 4 normally evoke a large excitatory postsynaptic potential (EPSP) which drives an action potential in gland secretory cells. In order to measure modulation of the EPSP, action potential generation in gland cells was prevented by bathing the preparation in low calcium, high magnesium salines. The relationship between the gland EPSP amplitude and specific physiological properties of neuron 4 was analyzed. In common with some central molluscan synapses, the EPSP was found to be strongly influenced by the membrane potential of neuron 4. Specifically, its amplitude was reduced by hyperpolarization of the neuron 4 soma. The relationship between EPSP amplitude and somatic potential of neuron 4 was linear in the range from resting potential (-47 +/- 6mV) to -100 mV. Furthermore, the EPSP amplitude was directly proportional to the action potential half-width of neuron 4. In order to evaluate the possible physiological role of this action potential/EPSP relationship, we examined whether gland EPSPs are modulated during the spike broadening that occurs in both spontaneous burst activity and imposed impulse trains. The preceding action potential/EPSP relationship was maintained under both of these conditions, i.e., EPSP magnitude increased as spikes broadened during bursts or trains. The peptidergic modulation of neuroglandular transmission was also examined. The molluscan peptide SCPB was found to depolarize neuron 4 and an increase in EPSP amplitude was concomitantly observed.(ABSTRACT TRUNCATED AT 250 WORDS)