Light-evoked recovery from wortmannin-induced inhibition of catecholamine secretion and synaptic transmission.

Wortmannin (WT) is known to inhibit catecholamine (CA) secretion in chromaffin cells. This effect was found to be sensitive to UV light in experiments designed to perform simultaneous monitoring of changes in [Ca2+]i and CA secretion in perfused rat adrenal medullas. When the change in [Ca2+]i was measured using calcium green-1 (490 nm excitation), a 35-min treatment with 10 microM WT caused a 69% inhibition of CA secretion evoked by excess (30 mM) extracellular K+ and a moderate inhibition of the [Ca2+]i response. In contrast, the same treatment of fura-2-loaded cells with WT caused only an 11% inhibition of the high-K+-evoked secretion and no significant attenuation of the [Ca2+]i response. However, during interruption of fluorometry with fura-2, the inhibitory effect of WT developed at a rate similar to that exhibited in calcium green-1-loaded cells. The WT-induced inhibition of high-K+- or bradykinin-evoked secretory responses, which was otherwise irreversible, was reversed by exposing WT-treated chromaffin cells to 380-nm light. When WT was reapplied to the cells of which the secretory ability had been restored by light irradiation, the secretory response was inhibited with a time course similar to that shown during the initial treatment with WT. The photosensitive effect of WT was also demonstrated using bullfrog sympathetic ganglia in which WT-induced inhibition of synaptic transmission was reversed by irradiation with 380-nm light. These results suggest that UV light removes the inhibitory effects of WT by disrupting the covalent bond formed between WT and a target molecule which remains to be determined, although myosin light chain kinase has been reported as the target molecule in both cases examined in this study.     

Phospholipid-sensitive calcium-dependent protein kinase: inhibition by antipsychotic drugs

Phospholipid-sensitive Ca²⁺-dependent protein kinases partially purified from the rat cerebral cortex, pig spleen, and bovine heart were shown to be inhibited, to varying degrees, by several antipsychotic drugs including trifluoperazine, chlorpromazine, fluphenazine, haloperidol, and chlorprothixene and by the local anesthetic dibucaine. None of these drugs were found to have any significant effect on cyclic AMP-dependent and cyclic GMP-dependent protein kinases. Kinetic analysis suggests that the primary effect of the drugs is mediated through a competitive inhibition of enzyme activation by interacting with phosphlipid.     

Use-dependent inhibition of Na+ currents by benzocaine homologs.

Most local anesthetics (LAs) elicit use-dependent inhibition of Na+ currents when excitable membranes are stimulated repetitively. One exception to this rule is benzocaine, a neutral LA that fails to produce appreciable use-dependent inhibition. In this study, we have examined the use-dependent phenomenon of three benzocaine homologs: ethyl 4-diethylaminobenzoate, ethyl 4-ethoxybenzoate, and ethyl 4-hydroxybenzoate. Ethyl 4-hydroxybenzoate at 1 mM, like benzocaine, elicited little use-dependent inhibition of Na+ currents, whereas ethyl 4-diethylaminobenzoate at 0.15 mM and ethyl 4-ethoxybenzoate at 0.5 mM elicited substantial use-dependent inhibition--up to 55% of peak Na+ currents were inhibited by repetitive depolarizations at 5 Hz. Each of these compounds produced significant tonic block of Na+ currents at rest and shifted the steady-state inactivation curve (h infinity) toward the hyperpolarizing direction. Kinetic analyses showed that the decaying phase of Na+ currents during a depolarizing pulse was significantly accelerated by all drugs, thus suggesting that these drugs also block the activated channel. The recovery time course for the use-dependent inhibition of Na+ currents was relatively slow, with time constants of 6.8 and 4.4 s for ethyl 4-diethylaminobenzoate and ethyl 4-ethoxybenzoate, respectively. We conclude that benzocaine and 4-hydroxybenzoate interact with the open and inactivated channels during repetitive pulses, but during the interpulse the complex dissociates too fast to accumulate sufficient use-dependent block of Na+ currents. In contrast, ethyl 4-diethylaminobenzoate and ethyl 4-ethoxybenzoate dissociate slowly from their binding site and consequently elicit significant use-dependent block. A common LA binding site suffices to explain the presence and absence of use-dependent block by benzocaine homologs during repetitive pulses.     

Modulation of striatal enkephalinergic neurons by antipsychotic drugs

In this paper we review the detailed mechanisms underlying the modulation of enkephalinergic neurons by dopaminergic neurons in rat striatum. Several lines of evidence, which showed that striatal levels of [Met5]enkephalin (ME) increase after the nigrostriatal dopaminergic pathway was interrupted by hemitransection or direct administration of 6-hydroxydopamine to the substantia nigra, or after repeated injections of either reserpine or haloperidol, suggest that dopamine (DA) plays an important role in regulating the metabolism of ME-containing neurons in the striatum. The increase in ME content after repeated injections of haloperidol was found in areas heavily innervated by DA neurons such as striatum or nucleus accumbens but not in hypothalamus, brain stem, and hippocampus. Further studies suggest that striatal cholinergic interneurons may partially mediate the action of haloperidol on enkephalinergic neurons. Several studies have been carried out to determine whether the elevation of striatal ME content after haloperidol treatment was caused by an increase in the synthesis or by a decrease in the utilization of ME. The rate of decline of striatal ME content in haloperidol-treated rats was steeper than that of controls after intraventricular injection of cycloheximide, which indicated that haloperidol accelerates the turnover of ME. This hypothesis was confirmed by our recent findings that the level of mRNA coding for preproenkephalin A, determined by cell-free translation and blot hybridization with cDNA clones, is increased after repeated injections of haloperidol.     

Use-dependent inhibition of C-axon multimodal units of cat skin by lidocaine and N-propyl-ajmaline]

Subcutaneous application of local anesthetic drug lidocaine and cardiac antiarrhythmic n-propyl-ajmaline produced the reversible use-dependent inhibition of feline polymodal mechano-heat C-fiber cutaneous sensory units (CMH-units) excited by moderate noxious mechanical stimulus. The discharge rate as well as the number of evoked spikes of polymodal sensory units treated with the drugs decreased below the values observed under noxious chemical excitation of CMH-units. The repeated mechano-stimulation with 5 to 30 sec interval between stimuli produced complete though a reversible block of the treated units. Quaternary amine n-propyl-ajmaline induced use-dependent inhibition of CMH-units in lower concentrations than tertiary amine lidocaine. The use-dependent inhibition of CMH-units is discussed in connection with nociception and local analgesia.     

Crossover inhibition in the retina: circuitry that compensates for nonlinear rectifying synaptic transmission

In the mammalian retina, complementary ON and OFF visual streams are formed at the bipolar cell dendrites, then carried to amacrine and ganglion cells via nonlinear excitatory synapses from bipolar cells. Bipolar, amacrine and ganglion cells also receive a nonlinear inhibitory input from amacrine cells. The most common form of such inhibition crosses over from the opposite visual stream: Amacrine cells carry ON inhibition to the OFF cells and carry OFF inhibition to the ON cells (”crossover inhibition”). Although these synapses are predominantly nonlinear, linear signal processing is required for computing many properties of the visual world such as average intensity across a receptive field. Linear signaling is also necessary for maintaining the distinction between brightness and contrast. It has long been known that a subset of retinal outputs provide exactly this sort of linear representation of the world; we show here that rectifying (nonlinear) synaptic currents, when combined thorough crossover inhibition can generate this linear signaling. Using simple mathematical models we show that for a large set of cases, repeated rounds of synaptic rectification without crossover inhibition can destroy information carried by those synapses. A similar circuit motif is employed in the electronics industry to compensate for transistor nonlinearities in analog circuits.     

Modulation by diadenosine polyphosphates of synaptic transmission in the hippocampus

The influence of diadenosine polyphosphates (diadenosine tetraphosphate and diadenosine pentaphosphate) on synaptic transmission in theCA3-CA1 region was studied in rat hippocampal slices. We used combined recording of excitatory postsynaptic current, EPSC (byin situ whole-cell voltage clamp), and population action potential, PAP (in the hippocampalCA1 region). Diadenosine polyphosphates were shown to suppress both EPSC and PAP. This effect could be blocked by A₁ adenosine receptor antagonist, and differed qualitatively from that produced by adenosine itself. As distinct from adenosine, prolonged application of diadenosine polyphosphates caused fas inhibition of PAP followed, by its slow partial recovery which could be removed by preincubation with protein kinase C inhibitor (staurosporine or sphingosine).Neirofiziologiya/Neurophysiology, Vol. 26, No. 6, pp. 423–426, November–December, 1994.     

Long-lasting synaptic potentials and the modulation of synaptic transmission.

Long-lasting postsynaptic potentials (PSPs) generated by decreases in membrane conductance (permeability) have been reported in many types of neurons. We investigated the possible role of such long-lasting decreases in membrane conductance in the modulation of synaptic transmission in the sympathetic ganglion of the bullfrog. The molecular basis by which such conductance-decrease PSPs are generated was also investigated. Synaptic activation of muscarinic cholinergic receptors on these sympathetic neurons results in the generation of a slow EPSP (excitatory postsynaptic potential), which is accompanied by a decrease in membrane conductance. We found that the conventional "fast" EPSPs were increased in amplitude and duration during the iontophoretic application of methacholine, which activates the muscarinic postsynaptic receptors. A similar result was obtained when a noncholinergic conductance-decrease PSP--the late-slow EPSP--was elicited by stimulation of a separate synaptic pathway. The enhancement of fast EPSP amplitude increased the probability of postsynaptic action potential generation, thus increasing the efficacy of impulse transmission across the synapse. Stimulation of one synaptic pathway is therefore capable of increasing the efficacy of synaptic transmission in a second synaptic pathway by a postsynaptic mechanism. Furthermore, this enhancement of synaptic efficacy is long-lasting by virtue of the long duration of the slow PSP. Biochemical and electrophysiological techniques were used to investigate whether cyclic nucleotides are intracellular second messengers mediating the membrane permeability changes underlying slow-PSP generation. Stimulation of the synaptic inputs, which lead to the generation of the slow-PSPs, increased the ganglionic content of both cyclic AMP and cyclic GMP. However, electrophysiological analysis of the actions of these cyclic nucleotides and the actions of agents that affect their metabolism does not provide support for such a second messenger role for either cyclic nucleotide.     

Activation of Rho GTPases by synaptic transmission in the hippocampus

Ras-related GTPases of the Rho family, such as RhoA and RhoB, are well-characterised mediators of morphological change in peripheral tissues via their effects on the actin cytoskeleton. We tested the hypothesis that Rho family GTPases are involved in synaptic transmission in the CA1 region of the hippocampus. We show that GTPases are activated by synaptic transmission. RhoA and RhoB were activated by low frequency stimulation, while the induction of long-term potentiation (LTP) by high frequency stimulation was associated with specific activation of RhoB via NMDA receptor stimulation. This illustrates that these GTPases are potential mediators of synaptic transmission in the hippocampus, and raises the possibility that RhoB may play a role in plasticity at hippocampal synapses during LTP.     

Mechanisms of inhibition of synaptic transmission in the sympathetic ganglia of rats with alloxan diabetes]

In experiments on the isolated superior cervical sympathetic ganglia of rats with alloxan diabetes rhythmic stimulation of preganglionic nerves was effected; summation presynaptic spikes and EPSPs of ganglionic neurons were registered. In rats with moderately severe alloxan diabetes progressive depression of rhythmic ganglion potentials was connected with suppression of the mediator emission to the impulse due to rapid exhaustion of its operational fraction. Rats with severe diabetes displayed also postsynaptic suppression of the ganglionic neurons. Dynamic characteristics of the transmitter turnover assessed on the basis of consideration of the successive patterns of posttetanic potentiation showed insignificant changes in the mediator output and a significant (by 38%) suppression of the mediator reserve per sec in comparison with control.     

Role of the synaptic microenvironment in functional modification of synaptic transmission

Experiments on hippocampal slices showed that perfusion with a dextran solution more effectively facilitates AMPA-mediated transmission in structurally complex synapses of mossy fibers of Shaffer collaterals. Estimates for changes in the extracellular Ca²⁺ concentration in the close vicinity of a reconstructed synapse during the action potential development are obtained. The results together with data about the rather small (0.5 μm) characteristics distance between neighboring synapses showed that the probability of mutual intersynaptic influence via the microenvironment is high. A probable functional role of such influences is discussed.     

Changes in synaptic transmission produced by hydrogen peroxide

The effect of hydrogen peroxide (H₂O₂) on excitatory and inhibitory synaptic transmission was studied at the lobster neuromuscular junction. H₂O₂ produced a dose dependent decrease in the amplitude of the junction potential (V_ejp). This decrease was due to changes in both presynaptic transmitter release and the postsynaptic response to the neurotransmitter. Observed presynaptic changes due to exposure to H₂O₂ were a decrease in the amount of transmitter released, that is, quantal content, as well as a decrease in the fast facilitation, that is, the amplitude increase of successive excitatory junction potentials at a rate of 3 Hz. To discern postsynaptic changes, glutamate, the putative excitatory neurotransmitter for this preparation was applied directly to the bathing medium in order to bypass the presynaptic release process. H₂O₂ produced a decreased response of the glutamate receptor/ ionophore. The action of H₂O₂ was not selective to excitatory (glutamate-mediated) transmission because inhibitory (GABA-mediated) transmission was also depressed by H₂O₂. This effect was primarily presynaptic since H₂O₂ produced no change in the postsynaptic response to applied GABA.     

Comparative research on synaptic transmission in the sympathetic ganglia of rabbits and frogs during acetylcholinesterase inhibition

Organophosphorus inhibitor of acetylcholinesterase (AChE) armin (1 x 10(-6) M) induced a variety of pre- and postsynaptic effects resulting from the AChE inhibition and subsequent accumulation of acetylcholine (ACh) in the synaptic cleft. The intensity of postsynaptic effects (level of neuron depolarization, degree of action potential depression) was shown to be different in the ganglia of frog and rabbit. This could be explained by differences in the total amount of ACh released in response to nerve stimulation as well as at rest. Both muscarinic and nicotinic cholinoreceptors were involved in the process of sustained depolarization of the neurons in the rabbit superior cervical ganglion after AChE inhibition. In frog ganglion neurons the nicotinic receptors did not participate in depolarization evidently due to their fast desensitization. The activation of presynaptic muscarinic receptors resulted in decrease of ACh released by nerve stimulation seems to weaken depolarization and blockade of synaptic transmission in sympathetic ganglia treated by AChE inhibitors.     

Control of excitatory synaptic transmission by C-terminal Src kinase

The induction of long-term potentiation at CA3-CA1 synapses is caused by an N-methyl-d-aspartate (NMDA) receptordependent accumulation of intracellular Ca(2+), followed by Src family kinase activation and a positive feedback enhancement of NMDA receptors (NMDARs). Nevertheless, the amplitude of baseline transmission remains remarkably constant even though low frequency stimulation is also associated with an NMDAR-dependent influx of Ca(2+) into dendritic spines. We show here that an interaction between C-terminal Src kinase (Csk) and NMDARs controls the Src-dependent regulation of NMDAR activity. Csk associates with the NMDAR signaling complex in the adult brain, inhibiting the Src-dependent potentiation of NMDARs in CA1 neurons and attenuating the Src-dependent induction of long-term potentiation. Csk associates directly with Src-phosphorylated NR2 subunits in vitro. An inhibitory antibody for Csk disrupts this physical association, potentiates NMDAR mediated excitatory postsynaptic currents, and induces long-term potentiation at CA3-CA1 synapses. Thus, Csk serves to maintain the constancy of baseline excitatory synaptic transmission by inhibiting Src kinase-dependent synaptic plasticity in the hippocampus.     

Bi-directional modulation of fast inhibitory synaptic transmission by leptin

The hormone leptin has widespread actions in the CNS. Indeed, leptin markedly influences hippocampal excitatory synaptic transmission and synaptic plasticity. However, the effects of leptin on fast inhibitory synaptic transmission in the hippocampus have not been evaluated. Here, we show that leptin modulates GABA_A receptor-mediated synaptic transmission onto hippocampal CA1 pyramidal cells. Leptin promotes a rapid and reversible increase in the amplitude of evoked GABA_A receptor-mediated inhibitory synaptic currents (IPSCs); an effect that was paralleled by increases in the frequency and amplitude of miniature IPSCs, but with no change in paired pulse ratio or coefficient of variation, suggesting a post-synaptic expression mechanism. Following washout of leptin, a persistent depression (inhibitory long-lasting depression) of evoked IPSCs was observed. Whole-cell dialysis or bath application of inhibitors of phosphoinositide 3 (PI 3)-kinase or Akt prevented leptin-induced enhancement of IPSCs indicating involvement of a post-synaptic PI 3-kinase/Akt-dependent pathway. In contrast, blockade of PI 3-kinase or Akt activity failed to alter the ability of leptin to induce inhibitory long-lasting depression, suggesting that this process is independent of PI 3-kinase/Akt. In conclusion these data indicate that the hormone leptin bi-directionally modulates GABA_A receptor-mediated synaptic transmission in the hippocampus. These findings have important implications for the role of this hormone in regulating hippocampal pyramidal neuron excitability.     

Control of neural information transmission by synaptic dynamics.

The computational processing of a neural system is strongly influenced by the dynamical characteristics of the information transmission between neurons. In this work, the control of neural information transmission by synaptic dynamics is investigated by means of a master-equation-based stochastic model of pre-synaptic release of neurotransmitter-containing vesicles. The model incorporates facilitation of vesicle fusion with the pre-synaptic membrane due to intracellular calcium ions and depletion of readily releasable vesicles. The message to be transmitted is coded by the pre-synaptic firing sequence, and the received signal corresponds to the post-synaptic membrane potential response. At the sending end, the stochastic character of the vesicle release contributes to the entropy of the probability distribution of the number of vesicles released and represents noise with respect to information transmission. At the receiving end, the generation of post-synaptic membrane potentials is influenced by the temporal behaviour of ionic currents and membrane charging and is determined by means of a low-dimensional model. The rate and temporal types of neural coding are compatible with limiting cases of the synaptic information transmission as a function of initial vesicle release probability and pre-synaptic firing rate. The effects of the nonlinear dependencies of the vesicle release probability on intracellular calcium concentration and number of available vesicles are analysed. The model is compared with phenomenological and reduced models, a principal advantage being the capability of also determining fluctuations of dynamic variables Copyright 2002 Academic Press.     

Depression of glutamate-mediated synaptic transmission by benzyl alcohol.

The data obtained from this study suggest that the nonionizable anesthetic benzyl alcohol has two prominent actions on GABA- and glutamate-mediated synaptic transmission at the lobster neuromuscular junction. They are as follows: (1) depression of the excitatory end-plate potential and the postsynaptic membrane response to applied glutamate, and (2) a hyperpolarization of the postsynaptic resting membrane potential associated with a decrease in effective membrane resistance. No change in amplitude of the inhibitory end-plate potential or inhibitory reversal potential was seen. Excitatory miniature end-plate potential frequency was also unaffected. The depression of excitatory synaptic transmission appears to be due to a decreased responsiveness of the postsynaptic receptor-ionophore complex.     

Astrocytes optimize the synaptic transmission of information

Chemical synapses transmit information via the release of neurotransmitter-filled vesicles from the presynaptic terminal. Using computational modeling, we predict that the limited availability of neurotransmitter resources in combination with the spontaneous release of vesicles limits the maximum degree of enhancement of synaptic transmission. This gives rise to an optimal tuning that depends on the number of active zones. There is strong experimental evidence that astrocytes that enwrap synapses can modulate the probabilities of vesicle release through bidirectional signaling and hence regulate synaptic transmission. For low-fidelity hippocampal synapses, which typically have only one or two active zones, the predicted optimal values lie close to those determined by experimentally measured astrocytic feedback, suggesting that astrocytes optimize synaptic transmission of information.