Effects of Cholinergic Substances on Plasticity of Synapses in Olfactory Bulb of the Pike <Emphasis Type="Italic">Esox lucius</Emphasis>

An electrophysiological and morphometric study of effects of cholinergic substances on orthodromic potential (OP), induction of long-term post-tetanic potentiation (PTP), and potentiated OP in olfactory bulb (OB) of intact pike was carried out. The final effect of endogenous ACh on relay neurons of the OB was found to be inhibitory. Activation of M₁-like cholinoreceptors (ChR) was shown to play the key role in induction of long-term PTP. In response to tetanization of olfactory nerve (ON) after pirenzepine-induced inhibition of M₁-like ChR, no potentiation appears and the length of cross-section of axo-dendritic synapse active zone (AZ) does not change in the OB glomerular neuropil. Tetanization of ON after inhibition of M₂-like ChR by gallamine leads to the appearance of short-term PTP transformed later into long-term PTP accompanied by a significant decrease of length of cross-sections of axo-dendritic synapse AZ. Effects of both increase of endogenous ACh concentration by eserine (0.1 μM) and blockade of M₂-like ChR by gallamine (10 μM) on potentiated OP were manifested as a decrease of its amplitude to control level. The obtained data allow considering that endogenous ACh in the pike OB has a pronounced effect on induction, development, and stabilization of the long-term PTP. This holds equally true to both functional and morphological manifestations of plasticity of axo-dendritic synapses of the glomerular neuropil.     

Activity of Acetylcholinesterase in Olfactory Bulb of the Pike <Emphasis Type="Italic">Esox lucius</Emphasis> and Its Role in Development of Long-Term Posttetanic Potentiation

A complex electrophysiological, biochemical, and histochemical study is carried out for determination of activity and distribution of acetylcholinesterase (AChE) in olfactory bulb (OB) of the pike during long-term posttetanic potentiation (PTP). Between the 30th and 60th min after tetanus, a stable increase of enzymatic activity in parallel with a rise of potentiation is observed. Sixty min after tetanus, at the point of maximal development of long-term PTP (the potentiation value is 170%), the specific activity of AChE rises by 89%. This increase was found to be due to synthesis of the enzyme de novo, with involvement of the majority of mitral cells and a significant part of granular cells.     

Mechanism of developing "deprivation" potentiation of population responses of neurons in the CA1 field in hippocampal slices]

In rat hippocampal slices, synaptic transmission from Schaffer collaterals to CA1 neurons was studied by assessment of the effects of cessation of low-frequency testing stimulation to the population spike amplitude. A positive correlation between the duration of the stimulation interruption period (from 10 to 120 min) and the increment of the population spike amplitude (from 30 to 100% above baseline level). The "deprivational" potentiation (DP) offers a property of summation and is able to persist for long (testing was continued up to one hour). Prior induction of the long-term posttetanic potentiation (LPTP) that for 1-3 hours prevents the development of the late rather than the short-term LPTP phase, in our experiments also suppressed the DP induction after 60-min stimulation interruption interval. The same results were obtained in the presence of polymixin B (20 mcM), which is an inhibitor of protein kinase C (PKC) and PKC-dependent phase of the LPTP. This finding provides support for the earlier assumption about the similarity of the mechanisms responsible for the development of the DP and PKC-dependent LPTP phase related with protein phosphorylation.     

高低频强直刺激诱导大鼠视皮层长持续长时程增强后突触超微结构的变化

在发育期大鼠视皮层上以2与100 Hz强直刺激诱导长持续长时程增强(long-lasting long-term potentiation,L-LTP),然后观察突触超微结构的变化.在L-LTP形成后,运用电子显微镜及图像分析技术分析突触形态的变化.实验中观察到,突触界面曲率、突触数密度以及突触后致密物厚度在2与100 Hz组较对照组均显著增加,而突触间隙宽度减小.在100 Hz强直刺激诱导L-LTP组中,单位体积的活性区面积显著增加.100 Hz强直刺激诱导L-LTP组较2 Hz强直刺激诱导L-LTP组中单个突触活性区的面积大.以上结果表明100 Hz强直刺激诱导L-LTP组中新形成的突触较2 Hz强直刺激诱导L-LTP组中的突触大,提示100 Hz强直刺激引起的L-LTP可能伴随有突触后细胞骨架蛋白重组或合成的增加.     

The function of motor nerves innervating slow tonic skeletal muscle in hens with delayed neuropathy induced by tri-o-tolyl phosphate

The reduction of neurogenic posttetanic potentiation in the slow twitch, soleus muscle is an index of impaired motor nerve function in cats with organophosphate-induced neuropathy. We have applied the measurement of posttetanic potentiation to study the functional state of the slow, tonic, plantaris muscle and its motor innervation in adult White Leghorn hens with tri-o-tolyl phosphate (TOTP)-induced neuropathy. At suitable intervals following single oral doses of vehicle or TOTP (500 mg/kg), nerve conduction velocity and posttetanic potentiation were measured in anesthetized hens. Conduction in the sciatic nerve was not altered by TOTP. The plantaris muscle of birds treated with vehicle (peanut oil) either failed to contract or responded to nerve stimulation at 0.4 Hz with very small twitches. Following nerve stimulation at frequencies inducing tetanus (50-140 Hz), the muscles responded with large, slow twitches that gradually decayed in amplitude. The area under the curve formed by the amplitude of these twitches over time (posttetanic potentiation) was directly proportional to the frequency and duration of nerve stimulation. In hens at 1,2, and 4 weeks following treatment with TOTP, the average amount of posttetanic potentiation was reduced concomitantly with the development of ataxia, paralysis, and pathological changes in the peripheral nerves. This difference between vehicle- and TOTP-treated hens was not significant, owing to large interbird variations. Since TOTP-treated hens showed greater disturbances in gait following moderate exercise, the fatigue of posttetanic potentiation with periodic neuronal stimulation was measured.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activation of synaptic NMDA receptors induces membrane insertion of new AMPA receptors and LTP in cultured hippocampal neurons

Long-term potentiation (LTP) of excitatory transmission in the hippocampus likely contributes to learning and memory. The mechanisms underlying LTP at these synapses are not well understood, although phosphorylation and redistribution of AMPA receptors may be responsible for this form of synaptic plasticity. We show here that miniature excitatory postsynaptic currents (mEPSCs) in cultured hippocampal neurons reliably demonstrate LTP when postsynaptic NMDA receptors are briefly stimulated with glycine. LTP of these synapses is accompanied by a rapid insertion of native AMPA receptors and by increased clustering of AMPA receptors at the surface of dendritic membranes. Both LTP and glycine-facilitated AMPA receptor insertion are blocked by intracellular tetanus toxin (TeTx), providing evidence that AMPA receptors are inserted into excitatory synapses via a SNARE-dependent exocytosis during LTP.     

The effect of metabotropic glutamate receptors on longitude of posttetanic reaction in spinal motoneurons of frog

Effects of metabotropic glutamate receptors of the duration of posttetanic changes in monosynaptic excitatory postsynaptic potentials (mEPSP), evoked by afferent and reticulospinal input stimulation, were investigated in lumbar motoneurons of the frog isolated spinal cord. It was found that application of MAP4 (25 microM), a selective antagonist of group III of these receptors, prolonged posttetanic potentiation and depression of synaptic transmission, whereas activation of this group of metabotropic glutamate receptors by L-AP4 (1 mM), a selective agonist of these receptors, suppressed the amplitude of synaptic responses, but did not affect the dynamics of development of posttetanic changes. The NMDA receptor antagonist AP5 (50 microM), added to the perfusing solution, blocked completely the effects produced by MAP4. Neither selective antagonist MCCG (400 microM), nor agonist tACPD (50 microM) of group II metabotropic glutamate receptors affected the terms of mEPSP posttetanic potentiation and depression, although the latter, in contrast to the antagonist, in most cases increased the synaptic potential amplitude. The data obtained permit to suggest that group III metabotropic receptors may control the duration of posttetanic changes of synaptic transmission in the frog spinal motoneurons. The long-term changes in the investigated synapses seem to be mediated by activation of postsynaptic metabotropic glutamate receptors (most likely, of group I receptors), which is normally masked with activation of group III presynaptic autoreceptors. The mechanism of such an induction essentially depends on activation of NMDA type of inotropic glutamate receptors.     

Layer by layer analysis of synaptic organization in the optic tectum of the prog

The distribution of axo-axonal and axo-dendritic synapses, nerve endings, and bodies of neurons by depth in the optic tectum ofRana temporaria L. was investigated under normal conditions and 6–9, 60, and 134 days after removal of the contralateral eye. Counting was carried out on long oriented sections examined in the electron microscope. In outer plexiform layer 9 the density of synapses was greatest near the surface of the tectum and decreased in the direction away from it; no inner sublayers with differing density of synapses could be distinguished. In the outer zone of layer 9 (to a depth of about 30 ) many axo-axonal synpases were discovered. Endings of myelinated optic fibers of large diameter ("dark" terminal degeneration) were widely distributed in the same layer. The density of axo-dendritic synapses in deep plexiform layer 5 was similar to that in layer 9. Many nerve endings containing granular vesicles as well as pale synaptic vesicles were found in layer 5 and neighboring zones.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 11, No. 2, pp. 130–136, March–April, 1979.     

Posttetanic changes in the afferent and efferent activity of monosynaptic reflex in kittens

Both the afferent volleys from the dorsal root and the monosynaptic reflex discharges from the corresponding ventral root were recorded with hook electrodes during stimulation of the nerves innervating the triceps surae muscles. The effects of conditioning high frequency tetanus on the magnitudes of these afferents and reflex volleys were examined in kittens of postnatal age 1-90 days and in adult cats. In young kittens under barbiturate anaesthesia, large-amplitude monosynaptic reflex discharge can be evoked without prior conditioning. The amplitude of this reflex discharge decreased with increasing age of the animal. Application of conditioning tetanic stimuli to the muscle nerves resulted in posttetanic depression followed by posttetanic potentiation of the monosynaptic reflex. The magnitude of posttetanic depression was much higher than that of potentiation in the first postnatal week. As the age increased, the magnitude of depression decreased while the magnitude of potentiation increased. The afferent volley showed a considerable posttetanic potentiation in older kittens and cats. No significant potentiation or depression was observed in the younger animals. Possible mechanisms contributing to posttetanic depression and potentiation are discussed.     

Sarcomere length-dependence of activity-dependent twitch potentiation in mouse skeletal muscle

Background  

It has been reported that potentiation of a skeletal muscle twitch response is proportional to muscle length with a negative slope during staircase, and a positive slope during posttetanic potentiation. This study was done to directly compare staircase and posttetanic responses with measurement of sarcomere length to compare their length-dependence.     

Synaptic Competition Sculpts the Development of GABAergic Axo-Dendritic but Not Perisomatic Synapses

The neurotransmitter GABA regulates many aspects of inhibitory synapse development. We tested the hypothesis that GABA_A receptors (GABA_ARs) work together with the synaptic adhesion molecule neuroligin 2 (NL2) to regulate synapse formation in different subcellular compartments. We investigated mice (“γ2 knockdown mice”) with an engineered allele of the GABA_AR γ2 subunit gene which produced a mosaic expression of synaptic GABA_ARs in neighboring neurons, causing a strong imbalance in synaptic inhibition. Deletion of the γ2 subunit did not abolish synapse formation or the targeting of NL2 to distinct types of perisomatic and axo-dendritic contacts. Thus synaptic localization of NL2 does not require synaptic GABA_ARs. However, loss of the γ2 subunit caused a selective decrease in the number of axo-dendritic synapses on cerebellar Purkinje cells and cortical pyramidal neurons, whereas perisomatic synapses were not significantly affected. Notably, γ2-positive cells had increased axo-dendritic innervation compared with both γ2-negative and wild-type counterparts. Moreover heterologous synapses on spines, that are found after total deletion of GABA_ARs from all Purkinje cells, were rare in cerebella of γ2 knockdown mice. These findings reveal a selective role of γ2 subunit-containing GABA_ARs in regulating synapse development in distinct subcellular compartments, and support the hypothesis that the refinement of axo-dendritic synapses is regulated by activity-dependent competition between neighboring neurons.     

Parallel processing of short-term memory for sensitization in Aplysia

How is the short-term memory for a single form of learning distributed among the various elements of a neuronal circuit? To answer this question, we examined the short-term memory for sensitization, using the siphon component of the defensive gill- and siphon-withdrawal reflex. We found that the memory for short-term sensitization is represented by at least four sites of circuit modification, each involving a different type of plasticity. These include (1) presynaptic facilitation of the sensory neuron connections onto both interneurons and motorneurons; (2) presynaptic inhibition at the connections of the L30 inhibitory neurons onto the excitatory interneuron L29; (3) posttetanic potentiation of the excitatory connections made by L29 onto a specific subclass of siphon motorneurons, the LFS cells; and (4) an increase in the tonic firing rate of the LFS siphon motor neurons, resulting in neuromuscular facilitation. Each of the heterosynaptic changes seems to involve a common modulatory transmitter and to utilize a common second messenger system. Moreover, each of these sites seems capable of encoding a different component of the short-term memory. Facilitation of the connections of sensory neurons should contribute to the increase in amplitude of the response; the disinhibition of the L29 interneurons and the posttetanic potentiation at L29 synapses should contribute to an increase in the duration of the response; and the increase in tonic firing of the LFS subclass of siphon motor neurons seems capable of contributing both to an increase in response amplitude and to changes in response topography.     

Synaptic plasticity at archicortical and neocortical levels

Research carried out by the author and his collaborators, devoted to analysis of the properties and neurophysiological mechanisms of long-term (for several hours) potentiation, is surveyed. Long-term potentiation of focal potentials and unitary responses of strictly hippocampal structures (areas CA₁ and CA₃) in the unanesthetized rabbit is described. Enhancement of excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials was found after tetanization. No corresponding changes of sensitivity to acetylcholine or acetylcholinesterase activity were found by microiontophoretic and histochemical methods during long-term potentiation. Statistical analysis of EPSPs evoked by microstimulation, based on the quantal hypothesis of synaptic transmission, showed an increase in the number of quanta of transmitter release during potentiation. Long-term potentiation of focal potentials during stimulation of the subcortical white matter in surviving neocortical slices and also long-term potentiation of focal and unitary responses of the sensomotor cortex of the unanesthetized rabbit are described. Potentiation of the "indirect" component of the global response of the pyramidal tract was found. The data suggest the presence of long-term potentiation of monosynaptic neocortical responses. It is concluded that the main mechanism of both hippocampal and neocortical long-term potentiation is increased efficiency of excitatory synapses. It is postulated that synapses modified in this way are used in the formation of memory traces.Brain Institute, All-Union Mental Health Research Center, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 5, pp. 651–665, September–October, 1984.     

Post-tetanic modification of the efficacy of excitatory transmission in neuronal networks with interhemispheric connections

The modifiable reciprocal transcallosal monosynaptic excitatory connections were for the first time detected in vivo experiments in rat motor cortex using multiunit recording and crosscorrelation analysis, It was shown that high-frequency microstimulation (MCS) of a small group of cortical cells of one hemisphere produces long-term changes in the efficacy of transcallosal excitatory connections, and also ipsilateral connections in both hemispheres. The posttetanic changes appear as long-term potentiation (LTP) and long-term depression (LTD). The bursting neurons were found to have more favorable conditions for the induction of LTP of most converging inputs (in contrast to cells with other discharge patterns). Both LTP and LTD could be simultaneously induced in synapses formed by axon collaterals of a callosal cell on several neurons. LTP and LTD could be simultaneously obtained at diverse synapses of the same cell. The number of spontaneously active callosal neurons as well as the number and efficacy of transcallosal connections increased after the MCS, whereas the number and efficacy of ipsilateral connections decreased. Basing on these data we assume that the ipsilateral inhibition is more effective than the transcallosal inhibition. MCS results in the modification of the pattern of initially existing connections between numerous neurons of an ensemble including cells of both hemispheres.     

“Self” versus “Non-Self” Connectivity Dictates Properties of Synaptic Transmission and Plasticity

Autapses are connections between a neuron and itself. These connections are morphologically similar to “normal” synapses between two different neurons, and thus were long thought to have similar properties of synaptic transmission. However, this has not been directly tested. Here, using a micro-island culture assay in which we can define the number of interconnected cells, we directly compared synaptic transmission in excitatory autapses and in two-neuron micronetworks consisting of two excitatory neurons, in which a neuron is connected to one other neuron and to itself. We discovered that autaptic synapses are optimized for maximal transmission, and exhibited enhanced EPSC amplitude, charge, and RRP size compared to interneuronal synapses. However, autapses are deficient in several aspects of synaptic plasticity. Short-term potentiation only became apparent when a neuron was connected to another neuron. This acquisition of plasticity only required reciprocal innervation with one other neuron; micronetworks consisting of just two interconnected neurons exhibited enhanced short-term plasticity in terms of paired pulse ratio (PPR) and release probability (Pr), compared to autapses. Interestingly, when a neuron was connected to another neuron, not only interneuronal synapses, but also the autaptic synapses on itself exhibited a trend toward enhanced short-term plasticity in terms of PPR and Pr. Thus neurons can distinguish whether they are connected via “self” or “non-self” synapses and have the ability to adjust their plasticity parameters when connected to other neurons.     

Ultrastructural characteristics of synaptogenesis in monolayer cultures of spinal cord

Stages of formation of different types of synapse between cells of the dissociated spinal cord and spinal ganglia of 12–14 day mouse embryos, in monolayer cultures, were studied electron-microscopically. The participation of cones of growth in the formation of different junctions between structures of the monolayer was traced. It was shown that the appearance of synaptic vesicles in the growing axon precedes the onset of membrane specialization in the region of contact between axon and target cell. Ultrastructural characteristics of axo-dendritic, axo-somatic, and axo-axonal synapses formed during growth are given. On the 24th day of culture structural complexes of axonal glomerulus type, incorporating axo-axonal and axo-dendritic synapses, were discovered. It is suggested that desmosomes participate in the formation of both chemical synapses and synapses of gap junction type.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 336–343, May–June, 1984.     

外周输入依赖的嗅球颗粒细胞的突触结构可塑性

活动依赖的突触结构可塑性是学习和记忆的基础.哺乳动物,尤其是啮齿类动物,具有高度发达的嗅觉系统和惊人的气味学习和记忆能力.本研究以CNGA2敲除而导致外周输入缺失的小鼠为模型,研究嗅球内活动依赖的突触结构可塑性.利用特异性的突触前和突触后标记物,发现外周输入缺失减少了突触标记蛋白突触素(synaptophysin)和抑制性突触标记蛋白桥蛋白(gephyrin)在嗅球外网状层和颗粒细胞层中的表达;兴奋性突触标记蛋白囊泡谷氨酸转运蛋白1(VGluT1)的表达水平只在外网状层中有显著下降,而在颗粒细胞层中没有明显变化.进一步通过活体质粒电转标记嗅球颗粒细胞后发现,CNGA2敲除小鼠颗粒细胞上位于外网状层中的远端树突棘密度显著减小,而位于颗粒细胞层中的近端树突棘密度没有明显变化.这些结果表明颗粒细胞上的树-树突触具有对外周活动依赖的结构可塑性,而轴-树突触则无.     

Baroreflexes of the rat. V. Tetanus-induced potentiation of ADN A-fiber responses at the NTS

In a long-term neuromuscular blocked (NMB) rat preparation, tetanic stimulation of the aortic depressor nerve (ADN) enhanced the A-fiber evoked responses (ERs) in the cardiovascular region, the nucleus of the solitary tract (dmNTS). The potentiation persisted for at least several hours and may be a mechanism for adaptive adjustment of the gain of the baroreflex, with functional implications for blood pressure regulation. Using a capacitance electrode, we selectively stimulated A-fibers and acquired a stable 10-h "A-fiber only" ER baseline at the dmNTS. Following baseline, an A+C-fiber activating tetanus was applied to the ADN. The tetanus consisted of 1,000 "high current" pulses (10 trains; 300 mus, 100 Hz, 1 s), with intertrain interval of 9 s. A 10-h A-fiber only posttetanic test phase repeated the stimulus pattern of the baseline. Fourteen tetanus experiments were done in 12 rats. Compared with the baseline before tetanus, the A-fiber ER magnitudes of posttetanus hours were larger [F(13, 247) = 3.407, P < .001]; additionally, the 10-h posttetanus magnitude slopes were more positive than during 10 h before tetanus (df = 13; t = -3.47; P < 0.005); thus, an ADN A+C fiber-activating tetanus produced increases in the magnitude of the A-fiber ERs in the dmNTS that persisted for several hours. In an additional rat, application of an NMDA receptor antagonist, prior to the tetanus, blocked the potentiation effect. The stimulus protocols, magnitude and duration of the effect, and pharmacology resemble associative long-term potentiation (LTP).     

Moderate AMPA receptor clustering on the nanoscale can efficiently potentiate synaptic current

The prevailing view at present is that postsynaptic expression of the classical NMDA receptor-dependent long-term potentiation relies on an increase in the numbers of local AMPA receptors (AMPARs). This is thought to parallel an expansion of postsynaptic cell specializations, for instance dendritic spine heads, which accommodate synaptic receptor proteins. However, glutamate released into the synaptic cleft can normally activate only a hotspot of low-affinity AMPARs that occur in the vicinity of the release site. How the enlargement of the AMPAR pool is causally related to the potentiated AMPAR current remains therefore poorly understood. To understand possible scenarios of postsynaptic potentiation, here we explore a detailed Monte Carlo model of the typical small excitatory synapse. Simulations suggest that approximately 50% increase in the synaptic AMPAR current could be provided by expanding the existing AMPAR pool at the expense of 100–200% new AMPARs added at the same packing density. Alternatively, reducing the inter-receptor distances by only 30–35% could achieve a similar level of current potentiation without any changes in the receptor numbers. The NMDA receptor current also appears sensitive to the NMDA receptor crowding. Our observations provide a quantitative framework for understanding the ‘resource-efficient’ ways to enact use-dependent changes in the architecture of central synapses.     

Temperature dependence of synaptic responses in guinea pig hippocampal CA1 neurons in vitro

1. Temperature-dependent properties of synaptic transmission were studied by recording orthodromic responses of the population spike and excitatory postsynaptic potential in CA1 pyramidal neurons of guinea pig hippocampal slices.2. Increasing the temperature of the perfusing medium from 30 to 43°C resulted in a decrease in the amplitude of the population spike (A-PS) and a reduced slope of the excitatory postsynaptic potential (S-EPSP). Bath application of the -aminobutyric acid receptor antagonist, picrotoxin, or a change in the calcium concentration of the perfusate did not affect the A-PS during heating.3. Increasing the strength of the synaptic input to that eliciting a PS with an amplitude 50, 75, or 100% of maximal at 30°C resulted in a significant increase in the A-PS during the middle phase of hyperthermia (35–39°C).4. The long-term potentiation (LTP) induced at either 30 or 37°C showed the same percentage increase in both the amplitude of the population spike and the S-EPSP after delivery of a tetanus (100 Hz, 100 pulses) to CA1 synapses.5. The results of the present study, therefore, indicate that the decrease in CA1 field potential was linearly related to the temperature of the slice preparation, while LTP was induced in these responses during heating from 30 to 37°C.