Presynaptic effects of biogenic amines modulating synaptic transmission between identified sensory neurons and giant interneurons in the first instar cockroach

Intracellular recording was used to investigate the modulatory effects of serotonin and octopamine on the identified synapses between filiform hair sensory afferents and giant interneurons in the first instar cockroach, Periplaneta americana. Serotonin at 10(-4) mol l(-1) to 10(-3) mol l(-1) reduced the amplitude of the lateral axon-to-ipsilateral giant interneuron 3 excitatory postsynaptic potentials. and octopamine at 10(-4) mol l(-1) increased their amplitude. Similar effects were seen on excitatory postsynaptic potentials in dorsal giant interneuron 6. Several lines of evidence suggest that both substances modulate the amplitude of excitatory postsynaptic potentials by acting presynaptically, rather than on the postsynaptic neuron. The fitting of simple binomial distributions to the postsynaptic potential amplitude histograms suggested that, for both serotonin and octopamine, the number of synaptic release sites was being modulated. Secondly, the amplitudes of miniature excitatory postsynaptic potentials recorded in the presence of tetrodotoxin were unaffected by either modulator. Finally, recordings from contralateral giant interneuron 3, which has two identifiable populations of synaptic inputs, showed that each modulator had a more pronounced effect on excitatory postsynaptic potentials evoked by the lateral axon than on those evoked by the medial axon. Immunocytochemistry confirmed that neuropilar processes containing serotonin are present in close proximity to these synapses.     

Potassium channel blocker-induced presynaptic modulation of inhibitory synaptic transmission in hippocampal neurons of rats

The effects of blockers of voltage-gated potassium channels, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), on inhibitory postsynaptic currents (IPSC) evoked by local electrical stimulation of zones of unitary synaptic terminals on hippocampal neurons were studied using a voltage-clamp technique under conditions of low density cell culture. At activation of the transmitter release in the absence of action potentials (when the terminals are in a tetrodotoxin-containing medium), external application of 5 mM 4-AP reversibly increased the averaged IPSC amplitude by 90±30%, while a similar effect of 10 mM TEA reached only 20±7%. The amplitudes of individual evoked IPSC varied between 10 and more than 150 pA. Amplitude histograms of IPSC in all studied neurons (n=14) were of a polymodal nature and could not described by a Gaussian law. An increase in the averaged IPSC amplitude under the influence of potassium channel blockers cannot be described as resulting only from modification of the number of trials without transmitter release (blank events). The mechanism of potassium channel blocker-induced facilitation of IPSC evoked by single synaptic terminals is discussed.     

Calcium helps neurons identify synaptic targets during development

Correct wiring of brain circuitry during development involves the selective formation and retention of synaptic connections between neurons. In this issue of Neuron, Lohmann and Bonhoeffer show that dendritic filopodia can distinguish among prospective presynaptic axonal targets during development. Contact with the appropriate target triggers local calcium signals that stabilize the filopodia and tell them to form mature synapses.     

Purinergic receptors and synaptic transmission in enteric neurons

Purines such as ATP and adenosine participate in synaptic transmission in the enteric nervous system as neurotransmitters or neuromodulators. Purinergic receptors are localized on the cell bodies or nerve terminals of different functional classes of enteric neurons and, with other receptors, form unique receptor complements. Activation of purinergic receptors can regulate neuronal activity by depolarization, by regulating intracellular calcium, or by modulating second messenger pathways. Purinergic signaling between enteric neurons plays an important role in regulating specific enteric reflexes and overall gastrointestinal function. In the present article, we review evidence for purine receptors in the enteric nervous system, including P1 (adenosine) receptors and P2 (ATP) receptors. We will explore the role they play in mediating fast and slow synaptic transmission and in presynaptic inhibition of transmission. Finally, we will examine the molecular properties of the native receptors, their signaling mechanisms, and their role in gastrointestinal pathology.     

Effect of thapsigargin on inhibitory synaptic transmission between cultured neurons of the rat hippocampus

We carried out electrophysiological experiments on cultured neurons of the rat hippocampus. The voltage-clamp technique and extracellular stimulation of single presynaptic axons were used for measurements of the evoked inhibitory postsynaptic currents (eIPSCs). It was found that 1 μM thapsigargin is capable of modulating inhibitory synaptic transmission, and the effects were ambivalent. Among 21 examined cells, eIPSCs decreased in 15 neurons and were augmented in 6 units; the kinetic parameters of these currents underwent no changes. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 374–376, July–October, 2007.     

Wiener analysis of spike transmission by identified molluscan neurons

A mathematical model of spike train transmission by identified molluscan neurons was obtained by Wiener analysis. Poisson-distributed sequences of near-threshold stimuli were used as input trains for model construction and testing. Assuming that the error of describing responses of the synapse-neuron system purely by mean outflow frequency is 100%, addition of a linear component to the equation of the model reduces this error to 25%, and addition of a term allowing for nonlinear properties of the system reduces it to 16%. Comparison of the standard error of predicted responses of the model to testing stimulus trains and of responses of a real neuron to these same trains showed that the prediction error with allowance for nonlinear properties does not exceed 21%. Choice of adequate criteria for comparing model and experimental results is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 49–54, January–February, 1984.     

Histochemical diagnosis of the neurons of cholinergic synaptic transmission

The authors studied neurons of the medulla oblongata of 5 human fetuses (22-27 weeks of development). Cholinacetyltransferase (CAT) activity was examined by the Berth method. Three neuronal types were diagnosed in the nuclei of the medulla oblongata with regard to CAT localization in the cytoplasm and synapses: (a) cholinergic-cholinoceptive neurons having CAT in the cytoplasm and in the innervating afferent fibers; (b) cholinergic-noncholinoceptive neurons with high CAT content, innervated with noncholinergic afferent fibers; (c) noncholinergic-cholinoceptive neurons carrying cholinergic synapses.     

Pronase modifies synaptic transmission and activity of identifiedLymnaea neurons

Proteolytic enzymes can have significant effects on the physiological properties of neurons. Although several actions of proteolytic enzymes on the physiology of single neurons have been described, the effects of these enzymes on network properties in the central nervous system (CNS) have received less attention. The effects of bath-applied pronase (0.05%) on synaptic connections and spontaneous activity in theLymnaea CNS were examined. Brief application (i.e. 2–3 min) of pronase modified some, but not all, synapses in the CNS. For example, the chemical synapse between two interneurons, RPeD11 and RPeD1, and between the interneuron, RPeD1, and RPA motoneurons were examined. Both these synapses were either biphasic or monophasic (depolarizing) under control conditions. Pronase exposure eliminated the depolarizing phase of the RPeD11→RPeD1 synapse, but had no effect on the connection between RPeD1 and RPA neurons. In addition, the effects of pronase on electrical-coupling between two peptidergic neurons, VD1 and RPD2, in the CNS were investigated. Pronase decreased the total network input resistance and cell input resistances as well as the steady-state coupling ratio. Furthermore, exposure to pronase induced various changes (i.e. depolarization, hyperpolarization, bursting patterns and afterdischarges) in the activity pattern of different identified neurons in the CNS. Collectively, these data show that even brief exposure to a low concentration of pronase can acutely modify both synapses and neuronal activity.     

Calcium and endocannabinoids in the modulation of inhibitory synaptic transmission

Synapses in the central nervous system can be highly plastic devices, being able to modify their efficacy in relaying information in response to several factors. Calcium ions are often fundamental in triggering synaptic plasticity. Here, we will shortly review the effects induced by postsynaptic increases of calcium concentration at GABAergic and glycinergic synapses. Both postsynaptic and presynaptic mechanisms mediating changes in synaptic strength will be examined. Particular attention will be devoted to phenomena of retrograde signaling and, specifically, to the recently discovered role, played by the endocannabinoid system in retrograde synaptic modulation.     

Data transmission between neurons

Summary By modulating sinusoidally the impulse frequency of the inhibitory input to the slowly adapting stretch receptor of Crustacea, the impulse frequency of the sensory neuron becomes sinusoidally modulated. The systems involved are linear for amplitudes of modulation up to 70% of the steady state frequency. The transfer function is given and some of its implications are discussed.     

Rare and spatially segregated release sites mediate a synaptic interaction between two identified network neurons.

Laser-scanning confocal microscopy (LSCM), electron microcopy (EM), and cellular electrophysiology were used in combination to study the structural basis of an inhibitory synapse between two identified neurons of the same network. To achieve this, we examined the chemical inhibitory synapse between identified neurons belonging to the lobster (Homarus gammarus) pyloric network: the pyloric dilator (PD) and the lateral pyloric (LP) neurons. In order to visualize simultaneously these two neurons, we used intrasomatic injection of Lucifer Yellow (LY) in one and rhodamine/horseradish peroxydase (HRP) in the other. Under LSCM, we found only two zones of close apposition in a restricted part of the neuritic tree of the two network neurons. Then, within these two zones, the synaptic release sites were searched using EM. To this end, photoconversion of LY with immunogold and development of HRP with DAB were performed on the previously observed preparations. Structural evidence was found for only one release site per zone. To confirm this result, and because the zones of contact were always segregated in a restricted part of the dendrites, we used laser photoablation to selectively delete, either pre- or postsynaptically, the branches on which the release sites were located. In both cases, such restrictive ablation completely abolished the functional interaction between these neurons. Our results therefore demonstrate that an inhibitory synapse that is essential for the operation of a neural network relies on only very few sites of contact localized in a highly restricted part of each neuron's dendritic arbor.     

Glucagon-like peptide-1 modulates synaptic transmission to identified pancreas-projecting vagal motoneurons

Using a brainstem slice preparation, we aimed to study the pre- and postsynaptic effects of glucagon-like peptide-1 (GLP-1) on synaptic transmission to identified pancreas-projecting vagal motoneurons. Following blockade of GABAergic mediated currents with bicuculline, perfusion with 100 nM GLP-1 increased both amplitude and frequency of excitatory postsynaptic currents (EPSCs) in 21 of 52 neurons. Perfusion with the GLP-1 selective agonist exendin-4 (100 nM), also increased the frequency of spontaneous EPSCs, while pretreatment with the GLP-1 selective antagonist, exendin 9-39, prevented the effects of GLP-1. In the presence of kynurenic acid to block ionotropic glutamatergic currents, perfusion with GLP-1 increased the frequency of inhibitory postsynaptic currents (IPSCs) in 28 of 74 neurons; in 14 of these responsive neurons, GLP-1 also increased IPSC amplitude, indicating actions at both pre- and postsynaptic sites. Perfusion with exendin-4 increased the frequency of spontaneous IPSCs, while pretreatment with exendin 9-39 prevented the effects of GLP-1. These results suggest that GLP-1 modulates both excitatory and inhibitory synaptic inputs to pancreas-projecting vagal motoneurons.     

Major structural determinants of transmembrane proteins identified by principal component analysis

We identify amino acid characteristics important in determining the secondary structures of transmembrane proteins, and compare them with characteristics important for cytoplasmic proteins. Using information derived from multiple sequence alignments, we perform a principal component analysis (PCA) to identify the directions in the 20-dimensional amino acid frequency space that comprise the most variance within each protein secondary structure. These vectors represent the important position-specific properties of the amino acids for coils, turns, beta sheets, and alpha helices. As expected, the most important axis for most of the datasets was hydrophobicity. Additional axes, distinct from hydrophobicity, are surprising, especially in the case of transmembrane alpha helices, where the effects of aromaticity and beta-branching are the next two most significant characteristics. The axis representing beta-branching also has equal importance in cytoplasmic and transmembrane helices, a finding that contrasts with some experimental results in membrane-like environments. In a further analysis, we examine trends for some of the PCA axes over averaged transmembrane alpha helices, and find interesting results for aromaticity.     

Glutamate-induced suppression of inhibitory synaptic transmission in cultivated hippocampal neurons

In a dissociated culture of rat hippocampal neurons (14 to 24 daysin vitro), modulation effects of glutamate on GABA_A-ergic inhibitory transmission were studied with the use of simultaneous patch-clamp whole-cell recording from monosynaptically connected neuron pairs. In all experiments (n=49), 1.5-min-long or longer extracellular application of 0.5 to 100 μM glutamate suppressed evoked inhibitory postsynaptic currents (IPSC). This suppression usually included fast (seconds) and slow (τ=1.3 min) phases. In 83.7% of the cases studied, IPSC did not return to the control values during the entire subsequent recording period (from 10 to 64 min). When glutamate was applied in the presence of blockers of glutamate ionotropic receptors, DL-APV or CNQX, the fast phase of the effect was removed, while some suppression of inhibitory neuronal responses, although weaker, was preserved (n=19); in most cases (73.3%) this residual suppression was slow and long-lasting. It is concluded that both types of glutamate receptors, ionotropic and metabotropic, are involved in modulation of GABA_A-ergic synaptic transmission. The first above receptor type provides fast and reversible suppression, while the effect provided by the second type is slow and long-lasting.     

Evidence that synaptic transmission between giant interneurons and identified thoracic interneurons in the cockroach is cholinergic.

In the cockroach, a population of thoracic interneurons (TIs) receives direct inputs from a population of ventral giant interneurons (vGIs). Synaptic potentials in type-A TIs (TIAs) follow vGI action potentials with constant, short latencies at frequencies up to 200 Hz. These connections are important in the integration of directional wind information involved in determining an oriented escape response. The physiological and biochemical properties of these connections that underlie this decision-making process were examined. Injection of hyperpolarizing or depolarizing current into the postsynaptic TIAs resulted in alterations in the amplitude of the post-synaptic potential (PSP) appropriate for a chemical connection. In addition, bathing cells in zero-calcium, high-magnesium saline resulted in a gradual decrement of the PSP, and ultimately blocked synaptic transmission, reversibly. Single-cell choline acetyltransferase (ChAT) assays of vGI somata were performed. These assays indicated that the vGIs can synthesize acetylcholine. Furthermore, the pharmacological specificity of transmission at the vGI to TIA connections was similar to that previously reported for nicotinic, cholinergic synapses in insects, suggesting that the transmitter released by vGIs at these synapses is acetylcholine.     

Intrinsic and extrinsic determinants of ion channel localization in neurons

Neurons are an extremely diverse group of excitable cells with a wide variety of morphologies including complex dendritic trees and very long axons. The electrical properties of neurons depend not only on the types of ion channels and receptors expressed, but also on where these channels are located in the cell. Two extreme examples that illustrate the subcellular polarized nature of neurons and the tight regulation of ion channel localization can be seen at the axon initial segment and the node of Ranvier. The axon initial segment is important for initiation of action potentials in the axon, whereas the node of Ranvier is required for the rapid, faithful and efficient propagation of action potentials along the axon. Given the similarity of their functions it is not surprising that nearly every protein component of the axon initial segment is also found at the node. However, there is one very important difference between these two sites: nodes require extrinsic, glial-derived factors in order to form, whereas the axon initial segment is intrinsically determined by the neuron. This mini-review discusses recent results that have begun to clarify the intrinsic and extrinsic mechanisms underlying formation of nodes and axon initial segments, and poses several important unanswered questions regarding their unique mechanisms of formation.