Atypical miniature end-plate currents in the normal rat neuromuscular synapse and after acetylcholinesterase inhibition and cooling

Leningrad State University, A. A. Ukhtomskii Leningrad Institute of Physiology. (Presented by Academician of the Academy of Medical Sciences of the USSR S. N. Golikov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 109, No. 6, pp. 523–525, June, 1990.     

gamma-Aminobutyrate sensitivity does not change during long-term potentiation in rat hippocampal slices

Long-term potentiation is a long-lasting enhancement of synaptic efficacy following brief, high-frequency, repetitive stimulation of a monosynaptic input. Intracellular recordings have shown that the inhibitory postsynaptic potential changes in amplitude during long-term potentiation. Yet how this may occur is unclear. To test for a possible alteration in postsynaptic sensitivity to the recurrent inhibitory transmitter gamma-aminobutyrate, we have examined the effect of gamma-aminobutyrate, focally applied to the hippocampal CA1 cell-body layer, on the extracellular recorded action potential (population spike). We found that the degree, duration, dose-dependence and time-course of inhibition produced by gamma-aminobutyrate are unchanged during long-term potentiation. This suggests that a change in sensitivity of CA1 pyramidal cells to the transmitter gamma-aminobutyrate is not the reason for the alteration in the inhibitory postsynaptic potential during long-term potentiation.     

Muscarinic acetylcholine receptor-dependent induction of persistent synaptic enhancement in rat hippocampus in vivo

Presynaptic terminal autoinhibitory muscarinic acetylcholine (ACh) receptors are predominantly of the M2/M4 subtypes and antagonists at these receptors may facilitate cognitive processes by increasing ACh release. The present study examined the ability of the M2/M4 muscarinic ACh receptor antagonist N,N'-bis [6-[[(2-methoxyphenyl)methyl]amino]hexyl]-1,8-octane diamine tetrahydrochloride (methoctramine) to induce and modulate synaptic plasticity in the CA1 area of the hippocampus in urethane-anesthetized rats. Both methoctramine and another M2/M4 antagonist, {11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one} (AF-DX 116), caused a rapid onset and persistent increase in baseline synaptic transmission after i.c.v. injection. Consistent with a requirement for activation of non-M2 receptors by endogenously released ACh, the M1/M3 receptor selective antagonists 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and 4,9-dihydro-3-methyl-4-[(4-methyl-1-piperazinyl)acetyl]-10H-thieno[3,4-b][1,5]benzodiazepin-10-one dihydrochloride (telenzepine) prevented the induction of the persistent synaptic enhancement by methoctramine. The requirement for cholinergic activation was transient and independent of nicotinic ACh receptor stimulation. The synaptic enhancement was inhibited by the prior induction of long-term potentiation (LTP) by high frequency stimulation but induction of the synaptic enhancement by methoctramine before high frequency stimulation did not inhibit LTP. Unlike high frequency stimulation-evoked LTP, the synaptic enhancement induced by methoctramine appeared to be NMDA receptor-independent. The present studies provide evidence for the rapid induction of a persistent potentiation at hippocampal glutamatergic synapses by endogenous ACh in vivo following disinhibition of inhibitory M2 muscarinic autoreceptors.     

Intracellular demonstration of an N-methyl-D-aspartate receptor mediated component of synaptic transmission in the rat hippocampus

Rat hippocampal CA1 pyramidal neurones were monosynaptically activated via stimulation of the Schaffer collateral-commissural pathway. On changing from a 1 mM Mg2+-containing to a Mg2+-free medium there was a pronounced prolongation of the intracellularly recorded excitatory postsynaptic potential. This effect was reversibly abolished by the selective N-methyl-D-aspartate (NMDA) antagonist, D-2-amino-5-phosphonovalerate (APV). We propose that Mg2+ normally prevents expression of NMDA receptor-mediated responses during low-frequency stimulation. During a period of tetanic stimulation, however, cells may depolarize sufficiently to allow a significant NMDA component of the response to be manifest. This could then initiate long-term potentiation.     

Temporal contiguity requirements for long-term associative potentiation/depression in the hippocampus

A previous study utilizing the powerful ipsilateral and weak crossed projection from the entorhinal cortex to the dentate gyrus in the rat revealed that long-term potentiation is an associative process in these systems. If the weak crossed projection alone receives potentiating stimulation consisting of 8 high-frequency trains 17.5 ms in duration, it does not exhibit long-term potentiation. However, long-term potentiation can be induced in the crossed projection if it is activated concurrently with the converging ipsilateral system. The present study is designed to determine the degree of synchrony required for the associative interactions by varying the timing and order of the potentiating trains delivered to the two converging systems.The associative induction of long-term potentiation does not require perfectly synchronous activation of the converging systems. The order of the trains is crucial, however. Long-term potentiation of the crossed projection can be induced if activity in the ipsilateral system is concurrent with or follows activity in the crossed projection. Indeed, there can be as much as 20ms between the 17.5ms trains in the two systems, and long-term potentiation of the crossed projection is still induced. Activation of the ipsilateral system that precedes activation of the crossed system depresses the responses evoked by the crossed system. If potentiating stimulation of the ipsilateral system follows activation of the crossed projection by too long an interval (200 ms, for example), then the crossed projection is depressed rather than potentiated.These results are discussed with regard to the nature of the associative process permissive for the induction of long-term potentiation and lead us to the conclusion that perfect temporal contiguity is not a requirement of this prototypical elemental memory unit.     

Alterations in synaptic curvature in the dentate gyrus following induction of long-term potentiation,long-term depression,and treatment with the N-methyl-d-aspartate receptor antagonist CPP

Alterations in curvature of the post synaptic density (PSD) and apposition zone (AZ), are believed to play an important role in determining synaptic efficacy. In the present study we have examined curvature of PSDs and AZs 24 h following homosynaptic long-term potentiation (LTP), and heterosynaptic long-term depression (LTD) in vivo, in awake adult rats. High frequency stimulation (HFS) applied to the medial perforant path to the dentate gyrus induced LTP while HFS stimulation of the lateral perforant path induced LTD in the middle molecular layer of the dentate gyrus (DG). Curvature changes were analysed in this area using three dimensional (3-D) reconstructions of electron microscope images of ultrathin serial sections. Very large and significant changes in 3-D measurements of AZ and PSD curvature occurred 24 h following both LTP and LTD, with a flattening of the normal concavity of mushroom spine heads and a change to convexity for thin spines. An N-methyl-d-aspartate (NMDA) receptor antagonist CPP (3-[(R)-2-Carboxypiperazin-4-yl]-propyl-1-phosphonic acid) blocked the changes in curvature of mushroom and thin spine PSDs and apposition zones, actually increasing the concavity of mushroom spines as the spine engulfed the presynaptic bouton. In order to establish whether these changes resulted from the effect of the NMDA antagonist or from its coincidence with synaptic activation during testing we examined the effects of CPP alone on PSD and apposition zone curvature. It was found that CPP alone also caused a small decrease in curvature of both PSD and apposition zone of mushroom and thin spines.     

Changes of acetylcholinesterase activity in the brain following septal lesions in the rat

Acetylcholinesterase activity in different parts of the limbic system (hippocampus, hypothalamus and amygdala) was decreased following medial septal lesions. On the other hand, this activity in the thalamus, frontal cortex, bulbus olfactorius, nucleus ruber, substantia nigra and basal ganglia was unaffected.These results raise the possibility that there is a functional relationship of the medial septum and those parts of the limbic system that we have studied.     

Joint application of independent component analysis and non-stationary fluctuation analysis for studying the mechanisms of the early phase of long-term potentiation in the rat hippocampus

Interest in studies of synaptic plasticity have led to the development of specific methods for analyzing evoked postsynaptic currents: quantum analysis, component analysis, non-stationary fluctuation analysis, etc. However, the use and interpretation of these methods are not always consistent, which leads to the acquisition of contradictory results from similar experiments. In the present study, simulations were used to analyze the influences of the heterogeneity of the shapes of postsynaptic currents on the results obtained by non-stationary fluctuation analysis. Assessment of postsynaptic channel conductivity was found to depend on the number of synapses involved in generating the response. It is hypothesized that the increase in the assessment of AMPA channel conductivity reported in the literature in conditions of long-term potentiation may be related to changes in the synaptic composition of postsynaptic currents. The hypothesis was tested using a new method for independent component analysis. Studies using the simulation showed that the joint application of this method and non-stationary fluctuation analysis avoids errors of this type. The procedures developed here were applied to data obtained from physiological experiments; the results of this exercise provided general support for the hypothesis. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti imeni I. P. Pavlova, Vol. 55, No. 3, pp. 293–304, May–June, 2005.     

Heterosynaptic potentiation of cholinergic excitatory postsynaptic responses of command neurons in the common snail

Short-term heterosynaptic potentiation of cholinergic excitatory postsynaptic currents and potentials in defensive behavior command neurons was found to be evoked by stimulation of the visceral sac in the common snail. It is suggested that a mechanism increasing the choline resistance of the postsynaptic zones of command neuron membranes is involved in potentiating the excitatory postsynaptic responses to sensory stimulation. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 57, No. 6, pp. 612–720, November–December, 2007.     

Long-term change in synaptic transmission in CA3 circuits followed by spontaneous rhythmic activity in rat hippocampal slices

The relevance of long-term potentiation (LTP) at excitatory synapses in CA3 circuits to generation of spontaneous epileptiform bursts in CA3 was investigated using rat hippocampal slices. CA3 pyramidal cells were antidromically stimulated through Schaffer collaterals. Evoked field potentials were extracellularly recorded from the stratum pyramidale and the stratum radiatum in CA3. Therefore, field potentials reflecting recurrent excitatory post-synaptic potentials (EPSPs) and inhibitory post-synaptic potentials (IPSPs) were positive at the stratum pyramidale and negative at the stratum radiatum. First, we tested how the amplitude of the evoked field potentials depends on a γ-aminobutyric acid (GABA_A) antagonist. Both of the positive and negative field potential peaks reduced in the medium containing penicillin (2 mM) or bicuculline (20 μM). This suggests that unmasked EPSPs due to suppression of IPSPs do not result in an increase in the evoked potentials. Second, CA3 pyramidal cells were antidromically stimulated by tetanic stimulation of Schaffer collaterals in order to induce LTP at synapses in CA3 circuits. Both of the positive and negative field potentials increased, suggesting that recurrent EPSPs were enhanced by tetanic stimulation. Induction of LTP at recurrent excitatory synapses was followed by spontaneous epileptiform bursts which persisted throughout experiments (1.5 h), while LTP of afferent synaptic potential evoked by hilar test stimulation was not induced. These results suggest that LTP at the afferent synapses is not necessary to spontaneous epileptiform bursts in CA3, but LTP at excitatory synapses between CA3 pyramidal cells contribute to spontaneous epileptiform bursts.     

Blockade of NMDA receptors unmasks a long-term depression in synaptic efficacy in rat prefrontal neurons in vitro

Summary All the experiments were carried out in slices of rat prefrontal cortex maintained in vitro. The effect of 2-amino-5-phosphonovalerate (APV) was tested on the postsynaptic potential (PSP) recorded in layer V pyramidal cells, in response to single or high frequency stimulation of the superficial layers I–II. Wash-out of Mg²⁺ increased the amplitude and duration of the PSPs. This effect resulted from activation of N-methyl-D-aspartate (NMDA) receptors since it was suppressed by bath application of APV. Furthermore, in every cell tested in Mg²⁺ containing medium (N=16), exposure to APV reversibly reduced both mono- and polysynaptic components of the PSPs, indicating that, even in the control solution, activation of NMDA-coupled channels contributed to these synaptic events. Finally, the anomalous voltage-dependence of the EPSP in the presence of Mg²⁺ and its sensitivity to APV suggests that at least a fraction of the NMDA receptors are postsynaptically located. Tetanization was applied to the afferents of cells bathed in control- or APV-medium. Long-term potentiation (LTP) or long-term depression (LTD) is defined as an increase or a decrease respectively, of the PSPs peak amplitude or initial slope, lasting 20 min. In the control medium, LTP in synaptic efficacy was observed in 34% of the cells and LTD in 48% (N=23). When exposed to APV, none of the cells tested (N=16) showed LTP of the response. In contrast, the tetanus induced a LTD of the PSP amplitude or slope in 14 out of these 16 cells. The percentage of cells showing LTD in synaptic efficacy (87%) when the NMDA receptors activation was blocked was significantly higher than that in control-medium.     

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 (= mean²/variance) were almost in proportion to the magnitude of LTP, and the ratio of 1/CV² 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.     

Early stress exposure impairs synaptic potentiation in the rat medial prefrontal cortex underlying contextual fear extinction

Traumatic events during early life may affect the neural systems associated with memory function, including extinction, and lead to altered sensitivity to stress later in life. We recently reported that changes in prefrontal synaptic efficacy in response to extinction trials did not occur in adult rats exposed to early postnatal stress (i.e. footshock [FS] stress during postnatal day 21–25 [3W-FS group]). However, identifying neurocircuitry and neural mechanisms responsible for extinction retrieval after extinction training have not been precisely determined. The present study explored whether synaptic transmission in the hippocampal-medial prefrontal cortex (mPFC) neural pathway is altered by extinction retrieval on the day after extinction trials using electrophysiological approaches combined with behavioral analysis. We also elucidated the effects of early postnatal stress on the synaptic response in this neural circuit underlying extinction retrieval. Evoked potential in the mPFC was enhanced following extinction retrieval, accompanied by reduced freezing behavior. This synaptic facilitation (i.e. a long-term potentiation [LTP]-like response) did not occur; rather synaptic inhibition was observed in the 3W-FS group, accompanied by sustained freezing. The behavioral deficit and synaptic inhibition observed in the 3W-FS group were time-dependently ameliorated by the partial N-methyl-d-aspartate (NMDA) receptor agonist d-cycloserine (15 mg/kg, i.p.). These findings suggest that the LTP-like response in the hippocampal-mPFC pathway is associated with extinction retrieval of context-dependent fear memory. Early postnatal stress appears to induce neurodevelopmental dysfunction of this neural circuit and lead to impaired fear extinction later in life. The present data indicate that psychotherapy accompanied by pharmacological interventions that accelerate and strengthen extinction, such as d-cycloserine treatment, may have therapeutic potential for the treatment of anxiety disorders, including posttraumatic stress disorder.     

Altered synaptic and electrical properties of lumbar motoneurons in the neurological glial mutant<Emphasis Type="Italic"> taiep</Emphasis> rat

Maturation and differentiation of electrical properties of neurons and synaptic transmission are modulated by neuronal interaction. In vitro experiments have shown that these processes also seem to be regulated by signals from non-neuronal elements such as glial cells. It is not known, however, whether glial alterations in intact neural networks may also affect the maturation of electrical properties and synaptic transmission during development. We used the taiep rat, a neurological mutant with a progressive demyelination and astrogliosis, as an experimental model to study the postnatal development of motoneurons in an altered glial environment. Using the patch-clamp technique, we made intracellular recording from motoneurons of Rexeds lamina IX in spinal cord slices of neonatal rats (postnatal day P4–P10). The electrical properties of normal motoneurons changed significantly with age, showing decreasing input resistance (R_in) and increasing membrane capacity (C_m). The rheobase increased with age, accompanied by an increase of the amplitude and a decrease of the duration of action potentials (APs). In contrast, mutant neurons showed no age-dependent changes of R_in, C_m, or AP characteristics. After blocking inhibitory transmission, intralaminar bipolar stimulation elicited, in both control and taiep motoneurons, fast glutamatergic excitatory postsynaptic potentials (EPSPs). Two types of taiep motoneurons were identified according to the temporal patterns of synaptic responses; (1) taiep _SYN neurons, which showed no significant differences to control motoneurons, and (2) taiep _ASYN neurons, in which the initial EPSP was followed by a variable number of delayed, asynchronous EPSP responses (for up to 300 ms). All these electrophysiological findings suggest that the mutation in taiep rats interfered with the development of the electrical properties of neurons and with the maturation of synaptic transmission, probably due to alterations in the neuron-glia interactions.This work was presented in part at the Fifth European Meeting on Glial Cell Function in Health and Disease (Euroglia 2002, Rome, Italy)     

The activity-dependent potentiation of the slow Ca2+-activated K+ current regulates synaptic efficacy in rat CA1 pyramidal neurons

 Activity-dependent modifications of neuronal excitability are of key functional importance because they accomplish general postsynaptic control of the flow of synaptic signals. We tested the modifications of synaptic efficacy evoked in rat CA1 hippocampal pyramidal neurons during the short-term activity-dependent reduction in excitability termed ”response depression”. The in vitro slice technique and recordings with sharp electrodes in the current- and voltage-clamp modes were used. Depression was induced by repeatedly stimulating the Schaffer collateral and stratum oriens. Repeated synaptic stimuli also depressed subsequent responses evoked by transmembrane current pulse injection and vice versa. Depression was characterised by a marked decrease in synaptic efficacy that outlasted stimuli for several minutes and was generalized to all pyramidal cells. The action potential frequency adaptation, the slow after-hyperpolarization and the underlying slow Ca²⁺-dependent K⁺ current (I _AHP) were potentiated during depression. The potentiated I _AHP caused depression by acting as a cumulative negative feedback that reduced synaptic efficacy by increasing the membrane conductance and hyperpolarizing the neurone. This depression may act as a homeostatic negative feedback mechanism to limit the rise in intracellular Ca²⁺ concentration and stabilize the membrane potential following intense synaptic activation. Received: 2 June 1998 / Accepted: 17 September 1998     

Activity-dependent release of adenosine inhibits the glutamatergic synaptic transmission and plasticity in the hypothalamic hypocretin/orexin neurons

The hypocretin (orexin) neurons in the lateral hypothalamus play a crucial role in the promotion of arousal. Adenosine, an endogenous sleep-promoting factor, modulates both neuronal excitatory and synaptic transmission in the CNS. In this study, the involvement of endogenous adenosine in the regulation of excitatory glutamatergic synaptic transmission to hypocretin neurons was investigated in the hypothalamic slices from transgenic mice by using different frequencies of stimulation. A train of low-frequency stimulation (0.033, 1 Hz) had no effect on the amplitude of evoked excitatory postsynaptic currents (evEPSCs) in hypocretin neurons. Blockade of adenosine A1 receptors with selective A1 receptor antagonist 8-cyclopentyltheophylline (CPT), the amplitude of evEPSCs did not change during 0.033 and 1 Hz stimuli. When the frequency of stimulation was increased upto 2 Hz, a time-dependent depression of amplitude was recorded in hypocretin neurons. Administration of CPT caused no significant change in depressed synaptic response induced by 2 Hz stimulus. While depression induced by 10 and 100 Hz stimuli was partially inhibited by the CPT but not by the selective A2 receptor antagonist 3,7-dimethyl-1-(2-propynyl)xanthine. Further findings have demonstrated that high-frequency stimulation could induce long-term potentiation (LTP) of glutamatergic synaptic transmission to hypocretin neurons in acute hypothalamic slices. The experiments with CPT suggested that A1 receptor antagonist could facilitate the induction of LTP, indicating that endogenous adenosine, acting through A1 receptors, may suppress the induction of LTP of excitatory synaptic transmission to hypocretin neurons. These results suggest that in the hypothalamus, endogenous adenosine will be released into extracellular space in an activity-dependent manner inhibiting both basal excitatory synaptic transmission and LTP in hypocretin neurons via A1 receptors. Our data provide further support for the notion that hypocretin neurons in the lateral hypothalamus may be another important target involved in the endogenous adenosine modulating the sleep and wakefulness cycle in the mammalian CNS.     

Intracellular studies of the interaction between paired-pulse facilitation and the delayed phase of long-term potentiation in the hippocampus

The mechanisms of the early (up to 1 h) and late (up to 3 h) phases of long-term potentiation were studied by analyzing the interaction between long-term potentiation and presynaptic paired-pulse facilitation. “Minimal” excitatory postsynaptic potentials were measured in pyramidal neurons in field CA1 of rat hippocampal slices in conditions of paired-pulse stimulation of the radial layer. In most neurons, paired-pulse facilitation decreased after induction of long-term potentiation, and this reduction lasted throughout the recording period (up to 3.5 h). Changes in paired-pulse facilitation correlated with the extent of long-term facilitation and with the initial level of paired-pulse facilitation, and the extent of facilitation depended on the initial level of paired-pulse facilitation. This latter relationship was different for the early and late phases of long-term potentiation and increased with time. Overall, the data obtained here demonstrate a significant role for presynaptic mechanisms in maintaining both the early and late phases of long-term potentiation. It is suggested that the basic mechanism of the early phase of potentiation is an increase in the probability that transmitter will be released, which also leads to an increase in the number of effective release sites, due to transformation of “presynaptically quiet” synapses into effective synapses. It is proposed that the development of the late phase is based on simultaneous pre- and postsynaptic structural transformations which increase the number of synaptically active zones. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 84, No. 5-6, pp. 426–435, May–June, 1998.