Facilitation Of Synaptic Transmission Research Articles

Оценка эффективности β2-адреноагониста формотерола в компенсации электрофизиологических проявлений стероидной миопатии в модельных экспериментах на животных

Цель исследования - изучение с помощью стимуляционной электромиографии в экспериментах на крысах эффективности β2-адреноагониста формотерола в компенсации электрофизиологических нарушений передней большеберцовой мышцы (m. tibialis anterior), вызванных длительным введением дексаметазона. Методика. Исследования выполнены на 40 половозрелых крысах-самках в возрасте 4-5 мес массой 180-200 г. Животные были разделены на 4 группы: контрольную (К - группа, без каких-либо воздействий, n=10) и 3 экспериментальных по 10 животных в каждой. Крысы 1-й группы на протяжении 30 сут получали дексаметазон, 2-й группы - дексаметазон в комплексе с формотеролом, 3-я - получала только формотерол. Дексаметазон вводили внутрибрюшинно (0,25 мг/кг) 1 раз в 2 сут. Формотерол (1,5 мкг/кг) вводили ежедневно под кожу. Через 30 сут на наркотизированных животных (тиопентал натрия 100мг/кг внутрибрюшинно) проводили острый опыт. Изучали электрофизиологические параметры передней большеберцовой мышцы с помощью экспериментальной двухканальной установки, оба канала были связаны с регистрирующим устройством - запоминающим цифровым осциллографом Tektronix (TDS2004C). Результаты. Установлено, что формотерол в комплексе с дексаметазоном, предотвращал уменьшение количества активируемых двигательных единиц мышцы и удлинение латентного периода М-ответов. А также обусловливал существенное повышение амплитуды М-ответов (на 115% в сравнении с контролем). Вместе с тем, формотерол в комплексе с дексаметазоном не предотвращал появление полифазных М-ответов, но компенсировал снижение их амплитуды. Введение дексаметазона с формотеролом уменьшало частоту случаев сниженной надежности синаптической передачи (до 30% против 70% в группе крыс, получавших дексаметазон изолированно). Вместе с тем, в случае комплексного введения дексаметазона с формотеролом наблюдалось удлинение латентного периода М-ответов после выполнения утомляющей работы. Заключение. Полученные данные указывают на высокую эффективность β2-адреноагониста формотерола в предотвращении электрофизиологических нарушений в мышце, вызванных длительным введением дексаметазона. Вместе с тем формотерол в комплексе с дексаметазоном, хотя и снижал частоту случаев низкой надежности синаптической передачи в группе (до 30% против 70% в группе, получавших дексаметазон изолированно), в полной мере не предотвращал данного эффекта стероидной миопатии. Aim. To study efficacy of β2-adrenergic agonist formoterol (F) in compensation of electrophysiological disorders in mixed-type skeletal muscles (m. tibial anterior) induced by long-term dexamethasone (D) treatment. Methods. Experiments were performed on sexually mature female rats (180-210 g) divided into four groups: 1) control, group C (intact rats, n=10); 2) first experimental group, 30D (30-day dexamethasone treatment, n=10); 3) second experimental group, 30D+F (30-day dexamethasone plus formoterol treatment, n=10); and third experimental group, 30F (30-day formoterol treatment, n=10). Dexamethasone (KRKA, Slovenia) was administered every two days, i.p., at a dose of 0.25 mg/kg, which was equivalent to the clinical therapeutic dose. Formoterol (Foradil, Novartis, Switzerland) was administered daily at a dose of 1.5 µ/kg, s.c. On day 30, rats were anesthetized with sodium thiopental (100 mg/kg), and stimulation electromyography was performed in an acute experiment. The anterior tibial muscle was stimulated with suprathreshold electrical current via the fibular nerve, and electrophysiological parameters of the muscle were recorded. Results. Formoterol in combination with dexamethasone prevented the decrease in the number of activated muscle motor units and the prolongation of M-response latency, which were typical for the 30D group. Also, formoterol not only corrected the decreased M-response amplitude but even significantly increased it in the 30D+F group by 115% compared to the control (p<0.05). At the same time, formoterol in combination with dexamethasone did not prevent the emergence of polyphasic M-responses (as observed in 50% of animals in the 30D+F group and 40% of animals in the 30D group) but compensated for the decrease in their amplitude. Administration of F+D reduced the incidence of less reliable synaptic transmission (up to 30% in the 30D+F group vs. 70% in the 30D group) but did not completely prevent it. Formoterol administered together with dexamethasone did not prevent marked facilitation of synaptic transmission in 50% of rats at the optimal neuromuscular stimulation rate (30 imp/s). However, this combination increased the amplitude of the first M-response in a series, which indicated the absence of blocked synapses in the 30D+F group. Formoterol in combination with dexamethasone prevented the typical for D-group disorder of M-response, which was more pronounced than in control, and the decrease in the number of activated muscle motor units after performing a fatigable work. At the same time, the D+F treatment increased the M-response latency after performing the fatigable work, which was typical for the 30D group but was not characteristic of the control. This fact confirmed a lower reliability of synaptic transmission both in 30D and 30D+F groups. Conclusion. The study demonstrated a high efficacy of the β2-adrenergic agonist formoterol for preventing electrophysiological disorders in the muscle induced by long-term administration of dexamethasone, which indicated myopathic alterations. At the same time, formoterol in combination with dexamethasone, although reduced the incidence of low-reliability synaptic transmission (up to 30% vs. 70% in the group receiving dexamethasone alone), did not completely prevent this manifestation of steroid myopathy.

Dopamine induces release of calcium from internal stores in layer II lateral entorhinal cortex fan cells.

The entorhinal cortex plays an important role in temporal lobe processes including learning and memory, object recognition, and contextual information processing. The alteration of the strength of synaptic inputs to the lateral entorhinal cortex may therefore contribute substantially to sensory and mnemonic functions. The neuromodulatory transmitter dopamine exerts powerful effects on excitatory glutamatergic synaptic transmission in the entorhinal cortex. Interestingly, inputs from midbrain dopamine neurons appear to specifically target clusters of excitatory cells located in the superficial layers of the entorhinal cortex. We have previously demonstrated that dopamine facilitates synaptic transmission through the activation of D1-like receptors. This facilitation of synaptic transmission is dependent on both activation of classical D1-like-receptors, and upon activation of dopamine receptors linked to increases in phospholipase C, inositol triphosphate (IP3), and intracellular calcium. In the present study we combined electrophysiological recordings of evoked excitatory postsynaptic currents with imaging of intracellular calcium using the fluorescent indicator fluo-4 to monitor calcium transients evoked by dopamine in electrophysiologically identified putative fan and pyramidal cells of the lateral entorhinal cortex. Bath application of dopamine (1 μM), or the phosphatidylinositol (PI)-linked D1-like-receptor agonist SKF83959 (5 μM), induced reliable and reversible increases in fluo-4 fluorescence and excitatory postsynaptic currents in fan cells, but not in pyramidal cells. In contrast, application of the classical D1-like-receptor agonist SKF38393 (10 μM) did not result in significant increases in fluorescence. Blocking release of calcium from internal stores by loading cells with the IP3 receptor blocker heparin (1 mM) or the ryanodine receptor blocker dantrolene (20 μM) abolished both the calcium transients and the facilitation of evoked synaptic currents induced by dopamine. Dopamine also induced calcium transients in fan cells when calcium was excluded from the extracellular medium, further indicating that the calcium transients are linked to release from internal stores. These results indicate that following D1-like-receptor binding, dopamine selectively induces transient elevations in intracellular calcium via activation of IP3 and ryanodine receptors, and that these elevations are linked to the facilitation of synaptic responses in putative layer II entorhinal cortex fan cells.

Transcranial magnetic stimulation (TMS) responses elicited in hindlimb muscles as an assessment of synaptic plasticity in spino-muscular circuitry after chronic spinal cord injury

Electromagnetic stimulation applied at the cranial level, i.e. transcranial magnetic stimulation (TMS), is a technique for stimulation and neuromodulation used for diagnostic and therapeutic applications in clinical and research settings. Although recordings of TMS elicited motor-evoked potentials (MEP) are an essential diagnostic tool for spinal cord injured (SCI) patients, they are reliably recorded from arm, and not leg muscles. Mid-thoracic contusion is a common SCI that results in locomotor impairments predominantly in legs. In this study, we used a chronic T10 contusion SCI rat model and examined whether (i) TMS-responses in hindlimb muscles can be used for evaluation of conduction deficits in cortico-spinal circuitry and (ii) if plastic changes at spinal levels will affect these responses. In this study, plastic changes of transmission in damaged spinal cord were achieved by repetitive electro-magnetic stimulation applied over the spinal level (rSEMS). Spinal electro-magnetic stimulation was previously shown to activate spinal nerves and is gaining large acceptance as a non-invasive alternative to direct current and/or epidural electric stimulation. Results demonstrate that TMS fails to induce measurable MEPs in hindlimbs of chronically SCI animals. After facilitation of synaptic transmission in damaged spinal cord was achieved with rSEMS, however, MEPs were recorded from hindlimb muscles in response to single pulse TMS stimulation. These results provide additional evidence demonstrating beneficial effects of TMS as a diagnostic technique for descending motor pathways in uninjured CNS and after SCI. This study confirms the ability of TMS to assess plastic changes of transmission occurring at the spinal level.

Analysis of Quantal Characteristics of GABA Release during Short-Term Depression and Facilitation of Synaptic Transmission

Analysis of Quantal Characteristics of GABA Release during Short-Term Depression and Facilitation of Synaptic Transmission

Аналіз квантових показників вивільнення ГАМК при короткотривалій депресії та полегшенні синаптичної передачі

Changes in amplitudes of evoked inhibitory postsynaptic currents (eIPSCs) from rat cultured hippocampal neurons were studied using whole-cell patch-clamp technique in postsynaptic neuron and local extracellular electrical paired pulse stimulation of single presynaptic axon. Paired pulse depression (PPD) and paired pulse facilitation (PPF) were observed in studied 43 neurons. According to coefficient of variance (CV) analysis was found that CV of second respond was significantly larger than CV first by 57 % (n 26) during depression and significanty smaller by 27 % (n = 17) during facilitation. Thus, only presynaptic mechanism underlies short-term depression and facilitation. We also estimated quantal parameters assuming that transmitter release is reasonably described by a binomial distribution. We found that under our experimental conditions there were no significant changes in GABA release probability during PPD and PPF. The values of mean quantal content and mean number of sites of transmitter release were the same for first stimulus in pair and were equal 9.7 in depression and 5 in facilitation of synaptic transmission. The values both of them for second stimulus were significant decreased by 28 and 26% during PPD and were significant increased by 14 and 11% during PPF, respectively. Thus, there is significant difference in quantal parameters characterizing GABA release during short-term depression and facilitation of synaptic transmission.

Long-term spinal cord stimulation modifies canine intrinsic cardiac neuronal properties and ganglionic transmission during high-frequency repetitive activation.

Long‐term spinal cord stimulation (SCS) applied to cranial thoracic SC segments exerts antiarrhythmic and cardioprotective actions in the canine heart in situ. We hypothesized that remodeling of intrinsic cardiac neuronal and synaptic properties occur in canines subjected to long‐term SCS, specifically that synaptic efficacy may be preferentially facilitated at high presynaptic nerve stimulation frequencies. Animals subjected to continuous SCS for 5–8 weeks (long‐term SCS: n = 17) or for 1 h (acute SCS: n = 4) were compared with corresponding control animals (long‐term: n = 15, acute: n = 4). At termination, animals were anesthetized, the heart was excised and neurones from the right atrial ganglionated plexus were identified and studied in vitro using standard intracellular microelectrode technique. Main findings were as follows: (1) a significant reduction in whole cell membrane input resistance and acceleration of the course of AHP decay identified among phasic neurones from long‐term SCS compared with controls, (2) significantly more robust synaptic transmission to rundown in long‐term SCS during high‐frequency (10–40 Hz) presynaptic nerve stimulation while recording from either phasic or accommodating postsynaptic neurones; this was associated with significantly greater posttrain excitatory postsynaptic potential (EPSP) numbers in long‐term SCS than control, and (3) synaptic efficacy was significantly decreased by atropine in both groups. Such changes did not occur in acute SCS. In conclusion, modification of intrinsic cardiac neuronal properties and facilitation of synaptic transmission at high stimulation frequency in long‐term SCS could improve physiologically modulated vagal inputs to the heart.

Facilitation of Synaptic Transmission in the Anterior Cingulate Cortex in Viscerally Hypersensitive Rats

Electrophysiological studies have shown the enhanced response of anterior cingulate cortex (ACC) to colorectal distension in viscerally hypersensitive (VH) rats, which can be observed up to 7 weeks following colonic anaphylaxis, independent of colon inflammation, suggesting a mechanism for learning and triggering of pain memories in the ACC neuronal circuitry. Activity-dependent plasticity in synaptic strength may serve as a key mechanism that reflects cortical plasticity. However, only a few reports have indicated the synaptic plasticity of ACC in vivo. In the present study, electrophysiological recording showed long-lasting potentiation of local field potential in the medial thalamus (MT)-ACC synapses in VH rats. Theta burst stimulation in the MT reliably induced long-term potentiation in the MT-ACC pathway in normal rats, but was occluded in the VH state. Further, repeated tetanization of MT increased ACC neuronal activity and visceral pain responses of normal rats, mimicking VH rats. In conclusion, we demonstrated for the first time that visceral hypersensitivity is associated with alterations of synaptic plasticity in the ACC. The ACC synaptic strengthening in chronic visceral pain may engage signal transduction pathways that are in common with those activated by electrical stimulation, and serves as an attractive cellular model of functional visceral pain.

Bidirectional plasticity in striatonigral synapses: A switch to balance direct and indirect basal ganglia pathways

There is no hypothesis to explain how direct and indirect basal ganglia (BG) pathways interact to reach a balance during the learning of motor procedures. Both pathways converge in the substantia nigra pars reticulata (SNr) carrying the result of striatal processing. Unfortunately, the mechanisms that regulate synaptic plasticity in striatonigral (direct pathway) synapses are not known. Here, we used electrophysiological techniques to describe dopamine D(1)-receptor-mediated facilitation in striatonigral synapses in the context of its interaction with glutamatergic inputs, probably coming from the subthalamic nucleus (STN) (indirect pathway) and describe a striatonigral cannabinoid-dependent long-term synaptic depression (LTD). It is shown that striatonigral afferents exhibit D(1)-receptor-mediated facilitation of synaptic transmission when NMDA receptors are inactive, a phenomenon that changes to cannabinoid-dependent LTD when NMDA receptors are active. This interaction makes SNr neurons become coincidence-detector switching ports: When inactive, NMDA receptors lead to a dopamine-dependent enhancement of direct pathway output, theoretically facilitating movement. When active, NMDA receptors result in LTD of the same synapses, thus decreasing movement. We propose that SNr neurons, working as logical gates, tune the motor system to establish a balance between both BG pathways, enabling the system to choose appropriate synergies for movement learning and postural support.

State-dependent modulation of stimulus-response relations in cortical networks in vitro

Variable responses of neuronal networks to repeated identical electrical or sensory stimuli reflect the interaction of the stimulus’ response with ongoing activity and its modulation by adaptive mechanisms such as cognitive context, network state, cellular excitability or synaptic transmission capability. To identify the rules that underlie the modulation of stimulus-response relations we set up a state-dependent stimulation paradigm in generic neuronal networks in vitro. Extracellular neuronal activity was recorded and stimulated from rat cortical cell cultures on microelectrode arrays at 60 sites. Spontaneous and evoked network activity was examined under control conditions, under blockage of GABAA-receptors as well as under overexpression of the synaptic protein DOC2B [1]. We were interested in the interactions that arise between spontaneous and stimulus-evoked activity dynamics and how these shape and modulate stimulus-response relations. Spontaneous network activity consisted of recurring periods of globally synchronized burst firing, so-called network bursts. The duration of intervals that preceded network bursts best predicted the length of the following network burst. This supported a process of network depression to a low threshold during bursts followed by subsequent recovery [2]. Facilitation of synaptic transmission by overexpressing DOC2B yielded ~30 % more spikes per network burst. The intervals between bursts increased by ~ 75 %, suggesting interdependence between resource activation and the time it needs for them to be recovered. Response length and delay depended on the timing of stimulation relative to preceding bursting. Response length increased exponentially and saturated with increasing duration of pre-stimulus inactivity t, y(t) = A(1-e-αt). Response delay, in turn, decreased exponentially and saturated at a low level, y(t) = Be-βt + C. The rate constant β describes the coupling between recovery from depression and response delay. We found activity-dependent recovery dynamics with longer spontaneous bursts yielding smaller β and vice versa. Stimulus-response modulation persisted under the blockage of inhibition, that introduced overall longer responses and shorter delays. Longer network bursts with more spikes occurred less frequently and recovery rates concomitantly decreased. The average firing rate was, however, unchanged, supporting a pool of available resources that is repeatedly used and replenished during and between network bursts, respectively.

Human models of hyperalgesia and pain (chilli pepper with your acid indigestion, Sir?)

The perception of harmful stimuli (termed nociception from Sherrington’s terms ‘nociceptive’ and ‘nociceptor’) is closely related to, but distinct from, the sensation of pain. Polymodal nociceptors (PMNs) are peripheral sensory neurons that respond to noxious (i.e. potentially tissuedamaging) stimuli. They are mainly non-myelinated C fibres whose endings respond to intense thermal, mechanical and chemical stimuli (and hence are ‘polymodal’). PMN fibres terminate in the dorsal horn of the spinal cord, forming synaptic connections with transmission neurons running to the thalamus. Chemical stimuli acting on PMNs include bradykinin, protons, ATP and vanilloids (e.g. capsaicin, the component of chilli peppers responsible for their fiery flavour). PMNs are sensitized by prostaglandins, which explains the analgesic effect of antiinflammatory drugs, particularly in the presence of inflammation. The transient receptor potential vanilloid receptor 1 (TRPV1) receptor responds to noxious heat as well as capsaicin-like agonists. Pain (distinct from nociception) includes a strong emotional component; its intensity depends critically on several additional factors in conjunction with the noxious stimulus itself, notably the context in which tissue injury occurs (e.g. football pitch vs. dentist’s chair), its chronicity and any associated inflammation. All of these strongly influence the threshold at which a potentially harmful stimulus is perceived as painful.Reduction in this threshold defines the state of hyperalgesia (enhanced pain from even a mildly noxious stimulus), familiar to anyone who has suffered a sprained ankle or a scald, and which results from a combination of sensitization of peripheral nociceptive nerve terminals and facilitation of central pain pathways. Peripheral sensitization is due to mediators such as bradykinin and prostaglandins, while the central component reflects facilitation of synaptic transmission in the central pain pathways [1]. Central facilitation displays the phenomenon of ‘wind-up’ (synaptic potentials steadily increase in amplitude with each stimulus) and is prevented by antagonists of NMDA, and of substance P and by inhibitors of nitric oxide synthesis, although none of these antagonists or inhibitors has as yet led to new and therapeutically useful analgesic drugs. Substance P and calcitonin gene-related peptide (CGRP), released from the central connections of C-fibres in the dorsal horn of the spinal cord, are also released peripherally (as a consequence of an axon reflex), causing neurogenic inflammation, which amplifies and sustains the activation of nociceptive afferent fibres. Nerve growth factor (NGF), a cytokine-like mediator produced by inflamed tissue, acts on a kinaselinked receptor known as TrkA located on PMNs, and increased production of NGF may cause hyperalgesia [2]. Many analgesics, particularly opioids, disproportionately reduce the distress associated with pain compared with their effect on awareness of the noxious stimulus per se, and this is not explained by nonspecific sedative effects. The activity of opioid analgesics in animal models such as the mouse ‘tail-flick’ (a flick of the tail in response to heat), which mainly assess antinociceptive activity, is poorly correlated with clinical efficacy [1]. Can tests in humans do better? Symptoms of pain in diseases such as myocardial infarction, arthritis or pancreatitis are dramatically Sherrington also introduced the term ‘synapse’. British Journal of Clinical Pharmacology DOI:10.1111/j.1365-2125.2010.03725.x

Facilitation of synaptic transmission and pain responses by CGRP in the amygdala of normal rats.

Calcitonin gene-related peptide (CGRP) plays an important role in peripheral and central sensitization. CGRP also is a key molecule in the spino-parabrachial-amygdaloid pain pathway. Blockade of CGRP1 receptors in the spinal cord or in the amygdala has antinociceptive effects in different pain models. Here we studied the electrophysiological mechanisms of behavioral effects of CGRP in the amygdala in normal animals without tissue injury.Whole-cell patch-clamp recordings of neurons in the latero-capsular division of the central nucleus of the amygdala (CeLC) in rat brain slices showed that CGRP (100 nM) increased excitatory postsynaptic currents (EPSCs) at the parabrachio-amygdaloid (PB-CeLC) synapse, the exclusive source of CGRP in the amygdala. Consistent with a postsynaptic mechanism of action, CGRP increased amplitude, but not frequency, of miniature EPSCs and did not affect paired-pulse facilitation. CGRP also increased neuronal excitability. CGRP-induced synaptic facilitation was reversed by an NMDA receptor antagonist (AP5, 50 μM) or a PKA inhibitor (KT5720, 1 μM), but not by a PKC inhibitor (GF109203X, 1 μM). Stereotaxic administration of CGRP (10 μM, concentration in microdialysis probe) into the CeLC by microdialysis in awake rats increased audible and ultrasonic vocalizations and decreased hindlimb withdrawal thresholds. Behavioral effects of CGRP were largely blocked by KT5720 (100 μM) but not by GF109203X (100 μM).The results show that CGRP in the amygdala exacerbates nocifensive and affective behavioral responses in normal animals through PKA- and NMDA receptor-dependent postsynaptic facilitation. Thus, increased CGRP levels in the amygdala might trigger pain in the absence of tissue injury.

Interdependence of PKC-Dependent and PKC-Independent Pathways for Presynaptic Plasticity

Diacylglycerol (DAG) is a prominent endogenous modulator of synaptic transmission. Recent studies proposed two apparently incompatible pathways, via protein kinase C (PKC) and via Munc13. Here we show how these two pathways converge. First, we confirm that DAG analogs indeed continue to potentiate transmission after PKC inhibition (the Munc13 pathway), but only in neurons that previously experienced DAG analogs, before PKC inhibition started. Second, we identify an essential PKC pathway by expressing a PKC-insensitive Munc18-1 mutant in munc18-1 null mutant neurons. This mutant supported basic transmission, but not DAG-induced potentiation and vesicle redistribution. Moreover, synaptic depression was increased, but not Ca2+-independent release evoked by hypertonic solutions. These data show that activation of both PKC-dependent and -independent pathways (via Munc13) are required for DAG-induced potentiation. Munc18-1 is an essential downstream target in the PKC pathway. This pathway is of general importance for presynaptic plasticity.

The effect of estrogen and progesterone on spreading depression in rat neocortical tissues

Although gender differences in the incidence of migraine with aura appear to be related to high circulating levels of ovarian hormones, the underlying mechanisms are not yet fully understood. Several studies have suggested a major role for spreading depression (SD) in the pathogenesis and symptomatology of migraine with aura. To investigate a possible role of SD in the association of high female hormones and attacks of migraine with aura, the effects of beta-estradiol and progesterone on SD were studied in rat neocortical tissues. Application of both hormones enhanced the repetition rate as well as the amplitude of SD in neocortical slices treated with hypotonic artificial cerebrospinal fluid. beta-Estradiol and progesterone also dose dependently increased the amplitude of SD induced by KCl microinjection. Both hormones exhibited a pronounced, persisting, and significant enhancement of long-term potentiation of the field excitatory postsynaptic potential in the neocortical tissues. The changes in SD characteristics in the presence of estrogen and progesterone may responsible for increased migraine with aura attacks associated by high female hormones. These hormones may exert their effects on SD via facilitation of synaptic transmission.

Mechanism of β-bungarotoxin in facilitating spontaneous transmitter release at neuromuscular synapse

The mechanism of the action of β-bungarotoxin (β-BuTx) in the facilitation of spontaneous transmitter release at neuromuscular synapse was investigated in Xenopus cell culture using whole-cell patch clamp recording. Exposure of the culture to β-BuTx dose-dependently enhances the frequency of spontaneous synaptic currents (SSCs). Buffering the rise of intracellular Ca 2+ with BAPTA-AM hampered the facilitation of SSC frequency induced by β-BuTx. The β-BuTx-enhanced SSC frequency was reduced when the pharmacological Ca 2+-ATPase inhibitor thapsigargin was used to deplete intracellular Ca 2+ store. Application of membrane-permeable inhibitors of inositol 1,4,5-trisphosphate (IP 3) but not ryanodine receptors effectively occluded the increase of SSC frequency elicited by β-BuTx. Treating cells with either wortmannin or LY294002, two structurally different inhibitors of phosphatidylinositol 3-kinase (PI3K) and with phospholipase C (PLC) inhibitor U73122, abolished the β-BuTx-induced facilitation of synaptic transmission. The β-BuTx-induced synaptic facilitation was completely abolished while there was presynaptic loading of the motoneuron with GDP βS, a non-hydrolyzable GDP analogue and inhibitor of G protein. Taken collectively, these results suggest that β-BuTx elicits Ca 2+ release from the IP 3 sensitive intracellular Ca 2+ stores of the presynaptic nerve terminal. This is done via PI3K/PLC signaling cascades and G protein activation, leading to an enhancement of spontaneous transmitter release.

Sildenafil induces hyperalgesia via activation of the NO-cGMP pathway in the rat neuropathic pain model

Persistent stimulation of nociceptors and C-fibers by tissue injury causes hyperalgesia and allodynia by sensitization of nociceptors and facilitation of synaptic transmission in the spinal cord. The important participant in the inflammatory response of injured peripheral nerve may be nitric oxide (NO). The aim of the present study was to test the sensitivity of PDE5 inhibitor sildenafil in chronic constriction injury (CCI) model a rat model of neuropathic pain. Sciatic nerve injury is associated with development of hyperalgesia 14 days after the nerve ligation. Sildenafil (100 and 200 microg/rat, i.t.) produced a significant decrease in pain threshold, which in lower dose did not alter the nociceptive threshold. The hyperalgesic effect of sildenafil was blocked by L-NAME and methylene blue (MB), which on per se treatment showed antinociceptive effect in nerve ligated rats. The results from the present study indicated that the major activation of NO-cGMP pathway in the chronic constriction injury model of neuropathic pain. The aggravation of hyperalgesic response might be due to the increased cGMP levels resulting in PKG-I activation and its upregulation.

Mechanism of β‐bungarotoxin in the facilitation of spontaneous transmitter release at developing neuromuscular synapse

The mechanism of action of β-bungarotoxin (β-BuTx) in the facilitation of spontaneous transmitter release at neuromuscular synapse was investigated in Xenopus cell culture using whole-cell patch clamp recording. Exposure of the culture to β-BuTx dose-dependently enhances the frequency of spontaneous synaptic currents (SSCs). Buffering the rise of intracellular Ca2+ with BAPTA-AM hampered the facilitation of SSC frequency induced by β-BuTx. The β-BuTx-enhanced SSC frequency was abolished when pharmacological Ca2+-ATPase inhibitor thapsigargin was used to deplete intracellular Ca2+ store. Application of membrane-permeable inhibitors of inositol 1,4,5-trisphosphate (IP3) but not ryanodine receptors effectively occluded the increase of SSC frequency elicited by β-BuTx. Treating cells with either wortmannin or LY294002, two structurally different inhibitors of phosphatidylinositol 3-kinase (PI3K) and with phospholipase C (PLC) inhibitor U73122, abolished β-BuTx-induced facilitation of synaptic transmission. The β-BuTx-induced synaptic facilitation was completely abolished while there was presynaptic loading of the motoneuron with GDPβS, a non-hydrolyzable GDP analogue and inhibitor of G protein. Taken collectively, these results suggest that β-BuTx elicits Ca2+ release from IP3 sensitive intracellular Ca2+ stores of the presynaptic nerve terminal. This is done via PI3Kγ/PLC signaling cascades and G protein activation, leading to an enhancement of spontaneous transmitter release.

Inactivation of C/EBP Transcription Factors Specifically Affects the Synaptic Plasticity of a Common Snail Neuron

Our previous studies showed that the acquisition of nociceptive sensitization in common snails is accompanied by long-term facilitation of synaptic transmission in defensive behavior command neuron LP11, this being dependent on translation and transcription processes. The characteristics of the neurochemical mechanisms of plasticity in the different sensory inputs of this nerve cell were identified. The mechanisms of induction of synaptic facilitation of the responses of neuron LP11 to chemical sensory stimulation of the snail's head involved NMDA glutamate receptors, serotonin receptors, cAMP, and serotonin-modulated transcription-regulating protein SMP-69. The mechanisms of induction of synaptic facilitation of another sensory input of the neuron--from tactile receptors on the head - involved protein kinase C. The present study addresses the involvement of C/EBP transcription factors (CCAAT-enhancer-binding protein) in the processes of synapse-specific plasticity of neuron LP11 during the acquisition of sensitization in snails. C/EBP was inactivated using oligonucleotides specifically binding to these proteins. The results showed that acquisition of sensitization during intracellular administration of oligonucleotides led to the selective suppression of synaptic facilitation in the responses of neuron LP11 to chemical sensory stimulation of the snail's head. Synaptic facilitation in the responses to tactile stimulation of the head or foot developed as in neurons in control sensitized snails. It is suggested that C/EBP transcription factor is selectively involved in the mechanisms of synapse-specific plasticity of the sensory input of neuron LP11 from chemoreceptors on the head during the acquisition of sensitization in snails.

Signaling through Gsα is required for the growth and function of neuromuscular synapses in Drosophila

Although synapses are assembled in a highly regulated fashion, synapses once formed are not static structures but continue to expand and retract throughout the life of an organism. One second messenger that has been demonstrated to play a critical role in synaptic growth and function is cAMP. Here, we have tested the idea that signaling through the heterotrimeric G protein, Gs, plays a coincident role with increases in intracellular Ca +2 in the regulation of adenylyl cyclases (ACs) during synaptic growth and in the function of synapses. In larvae containing a hypomorphic mutation in the dgs gene encoding the Drosophila Gsα protein, there is a significant decrease in the number of synaptic boutons and extent of synaptic arborization, as well as defects in the facilitation of synaptic transmission. Microscopic analysis confirmed that Gsα is localized at synapses both pre- and postsynaptically. Restricted expression of wild-type Gsα either pre- or postsynaptically rescued the mutational defects in bouton formation and defects in the facilitation of synaptic transmission, indicating that pathways activated by Gsα are likely to be involved in the reciprocal interactions between pre- and postsynaptic cells required for the development of mature synapses. In addition, this Gsα mutation interacted with fasII, dnc, and hyperexcitability mutants in a manner that revealed a coincident role for Gsα in the regulation of cAMP and FASII levels required during growth of these synapses. Our results demonstrate that Gsα-dependent signaling plays a role in the dynamic cellular reorganization that underlies synaptic growth.

Effect of I.C.V. injection of AT 4 receptor ligands, NLE 1-angiotensin IV and LVV-hemorphin 7, on spatial learning in rats

Central administration of angiotensin IV (Ang IV) or its analogues enhance performance of rats in passive avoidance and spatial memory paradigms. The purpose of this study was to examine the effect of a single bolus injection of two distinct AT 4 ligands, Nle 1-Ang IV or LVV-haemorphin-7, on spatial learning in the Barnes circular maze. Mean number of days for rats treated with either Nle 1-Ang IV or LVV-haemorphin-7 to achieve learner criterion is significantly reduced compared with controls ( P<0.001 and P<0.05 respectively). This is due to enhanced ability of the peptide-treated rats to adopt a spatial strategy for finding the escape hatch. In all three measures of learning performance, (1) the number of errors made, (2) the distance travelled and (3) the latency in finding the escape hatch, rats treated with either 100 pmol or 1 nmol of Nle 1-Ang IV or 100 pmol LVV-haemorphin-7 performed significantly better than the control groups. As early as the first day of testing, the rats treated with the lower dose of Nle 1-Ang IV or LVV-haemorphin-7 made fewer errors ( P<0.01 and P<0.05 respectively) and travelled shorter distances ( P<0.05 for both groups) than the control animals. The enhanced spatial learning induced by Nle 1-Ang IV (100 pmol) was attenuated by the co-administration of the AT 4 receptor antagonist, divalinal-Ang IV (10 nmol). Thus, administration of AT 4 ligands results in an immediate potentiation of learning, which may be associated with facilitation of synaptic transmission and/or enhancement of acetylcholine release.

Differences in multiple forms of short-term plasticity between excitatory and inhibitory hippocampal neurons in culture.

Synaptic transmission is highly dynamic, especially during periods of repetitive activity. This short-term synaptic plasticity, elicited by either pairs or short trains of action potentials at moderate frequencies (1-10 Hz), may give rise to either depression or facilitation of synaptic transmission. We analyzed these processes in isolated, synaptically coupled pairs of inhibitory or excitatory neurons grown in low-density cultures of hippocampal neurons. Most inhibitory and excitatory synapses in these cultures displayed paired pulse depression, although the responses of excitatory synapses were more variable and occasionally facilitation was seen. With tetanic stimuli, inhibitory synapses showed depression, but excitatory synapses showed a much richer repertoire of behaviors, including depression and facilitation. While many inhibitory synapses showed posttetanic depression following short trains of action potentials, excitatory synapses instead showed posttetanic facilitation. This facilitation is accompanied by an increase in paired pulse ratio, suggesting that it is the result of presynaptic mechanisms. Finally, excitatory synapses often displayed paired pulse and tetanic facilitation of asynchronous release, a process not seen in inhibitory synapses in these cultures. These similarities and differences in short-term plasticity exhibited by inhibitory and excitatory cells are likely to be critical for information processing and the control of neuronal excitability, under both normal and pathological conditions, such as epilepsy.