Presynaptic Effects Research Articles

The Mechanism for Prejunctional Enhancement of Neuromuscular Transmission by Ethanol in the Mouse

Ethanol has been shown to have both presynaptic and postsynaptic effects on synaptic transmission. However, the mechanisms by which ethanol affects evoked neurotransmitter release have not been studied at the mouse neuromuscular junction, a synapse at which binomial analysis of neurotransmitter release and measurements of prejunctional ionic currents can be made. Ethanol (400 mM) increased neurotransmitter release independently of both the cAMP and phorbol ester/Munc13 signaling pathways. Binomial analysis of neurotransmitter release revealed that ethanol increases the average probability of secretion without an effect on the immediately available store of the neurotransmitter. Application of ethanol also resulted in an inhibition of potassium currents in the motor nerve endings. These results suggest that the potentiating effects of ethanol on neurotransmitter release at the skeletal neuromuscular junction are mediated by an inhibition of the delayed rectifier potassium current, thus increasing both calcium entry into the nerve ending and the probability of neurotransmitter release. Identifying the mechanism through which ethanol enhances neurotransmitter release at the neuromuscular junction may be useful in determining the processes underlying the enhancement of neurotransmitter release at other synapses.

Memantine depresses glutamate release through inhibition of voltage-dependent Ca 2+ entry and protein kinase C in rat cerebral cortex nerve terminals: An NMDA receptor-independent mechanism

Memantine has been used to treat several neurological diseases, including those associated with excessive glutamate release. It has been believed that the neuroprotective effect of memantine results from its inhibitory effect on glutamate-induced neurotoxicity via postsynaptic N-methyl- d-aspartate receptor (NMDAR) antagonism. However, the presynaptic effect of memantine on glutamate release has never been examined. Therefore, the aim of this study was to investigate the effect of memantine on the release of glutamate from rat cerebral cortex nerve terminals (synaptosomes). Results showed that memantine inhibited the release of glutamate evoked by 4-aminopyridine (4-AP) in a concentration-dependent manner. The effect of memantine on the evoked glutamate release was insensitive to the NMDAR antagonist D-AP5, but prevented by the chelating intrasynaptosomal Ca 2+ ions, and by the vesicular transporter inhibitor bafilomycin A1. In addition, memantine reduced depolarization-induced increase in cytosolic Ca 2+ without any effect on synaptosomal excitability, and the reduction of glutamate release could be prevented by blocking the N and P/Q type Ca 2+ channels. Furthermore, the memantine-mediated inhibition on 4-AP-evoked glutamate release could be diminished by the protein kinase C (PKC) inhibitors, and memantine significantly reduced the depolarization-induced phosphorylation of PKC, and PKCα. Thus, the effect of memantine on evoked glutamate release is linked to a decrease in [Ca 2+] i contributed by Ca 2+ entry through presynaptic voltage-dependent Ca 2+ channels and to the subsequent suppression of the PKC signaling cascade.

Muscarinic Signaling in the Cochlea: Presynaptic and Postsynaptic Effects on Efferent Feedback and Afferent Excitability

Acetylcholine is the major neurotransmitter of the olivocochlear efferent system, which provides feedback to cochlear hair cells and sensory neurons. To study the role of cochlear muscarinic receptors, we studied receptor localization with immunohistochemistry and reverse transcription-PCR and measured olivocochlear function, cochlear responses, and histopathology in mice with targeted deletion of each of the five receptor subtypes. M2, M4, and M5 were detected in microdissected immature (postnatal days 10-13) inner hair cells and spiral ganglion cells but not outer hair cells. In the adult (6 weeks), the same transcripts were found in microdissected organ of Corti and spiral ganglion samples. M2 protein was found, by immunohistochemistry, in olivocochlear fibers in both outer and inner hair cell areas. M3 mRNA was amplified only from whole cochleas, and M1 message was never seen in wild-type ears. Auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) were unaffected by loss of Gq-coupled receptors (M1, M3, or M5), as were shock-evoked olivocochlear effects and vulnerability to acoustic injury. In contrast, loss of Gi-coupled receptors (M2 and/or M4) decreased neural responses without affecting DPOAEs (at low frequencies). This phenotype and the expression pattern are consistent with excitatory muscarinic signaling in cochlear sensory neurons. At high frequencies, both ABRs and DPOAEs were attenuated by loss of M2 and/or M4, and the vulnerability to acoustic injury was dramatically decreased. This aspect of the phenotype and the expression pattern are consistent with a presynaptic role for muscarinic autoreceptors in decreasing ACh release from olivocochlear terminals during high-level acoustic stimulation and suggest that muscarinic antagonists could enhance the resistance of the inner ear to noise-induced hearing loss.

Coincidence Detection and Stress Modulation of Spike Time-Dependent Long-Term Depression in the Hippocampus

Associative long-term depression (LTD) in the hippocampus is a form of spike time-dependent synaptic plasticity that is induced by the asynchronous pairing of postsynaptic action potentials and EPSPs. Although metabotropic glutamate receptors (mGluRs) and postsynaptic Ca(2+) signaling have been suggested to mediate associative LTD, mechanisms are unclear further downstream. Here we show that either mGluR1 or mGluR5 activation is necessary for LTD induction, which is therefore mediated by group I mGluRs. Inhibition of postsynaptic phospholipase C, inositol-1,4,5-trisphosphate, and PKC prevents associative LTD. Activation of PKC by a phorbol ester causes a presynaptic potentiation of synaptic responses and facilitates LTD induction by a postsynaptic mechanism. Lithium, an inhibitor of the PKC pathway, inhibits LTD and the presynaptic and postsynaptic effects of the phorbol ester. Furthermore, LTD is sensitive to the postsynaptic application of synthetic peptides that inhibit the interaction of AMPA receptors with PDZ domains, suggesting an involvement of protein interacting with C-kinase 1 (PICK1)-mediated receptor endocytosis. Finally, enhanced PKC phosphorylation, induced by behavioral stress, is associated with enhanced LTD. Both increased PKC phosphorylation and stress-induced LTD facilitation can be reversed by lithium, indicating that this clinically used mood stabilizer may act on synaptic depression via PKC modulation. These data suggest that PKC mediates the expression of associative LTD via the PICK1-dependent internalization of AMPA receptors. Moreover, modulation of the PKC activity adjusts the set point for LTD induction in a behavior-dependent manner.

Role of the central ascending neurotransmitter systems in the psychostimulant effects of caffeine.

Caffeine is the most consumed psychoactive drug in the world. It is a non-selective adenosine receptor antagonist that in the brain targets mainly adenosine A1 and A2A receptors. The same as classical psychostimulants, caffeine produces motor-activating, reinforcing and arousing effects. This depends on the ability of caffeine to counteract multiple effects of adenosine in the central ascending neurotransmitter systems. Motor and reinforcing effects depend on the ability of caffeine to release pre- and postsynaptic brakes that adenosine imposes on the ascending dopaminergic system. By targeting A1-A2A receptor heteromers in striatal glutamatergic terminals and A1 receptors in striatal dopaminergic terminals (presynaptic brake), caffeine induces glutamate-dependent and glutamate-independent release of dopamine. These presynaptic effects of caffeine are potentiated by the release of the postsynaptic brake imposed by antagonistic interactions in the striatal A2A-D2 and A1-D1 receptor heteromers. Arousing effects of caffeine depend on the blockade of multiple inhibitory mechanisms that adenosine, as an endogenous sleep-promoting substance, exerts on the multiply interconnected ascending arousal systems. Those mechanisms include a direct A1-receptor mediated modulation of the corticopetal basal forebrain system and an indirect A2A-receptor mediated modulation of the hypothalamic histaminergic and orexinergic systems.

Lead Exposure during Synaptogenesis Alters Vesicular Proteins and Impairs Vesicular Release: Potential Role of NMDA Receptor–Dependent BDNF Signaling

Lead (Pb(2+)) exposure is known to affect presynaptic neurotransmitter release in both in vivo and cell culture models. However, the precise mechanism by which Pb(2+) impairs neurotransmitter release remains unknown. In the current study, we show that Pb(2+) exposure during synaptogenesis in cultured hippocampal neurons produces the loss of synaptophysin (Syn) and synaptobrevin (Syb), two proteins involved in vesicular release. Pb(2+) exposure also increased the number of presynaptic contact sites. However, many of these putative presynaptic contact sites lack Soluble NSF attachment protein receptor complex proteins involved in vesicular exocytosis. Analysis of vesicular release using FM 1-43 dye confirmed that Pb(2+) exposure impaired vesicular release and reduced the number of fast-releasing sites. Because Pb(2+) is a potent N-methyl-D-aspartate receptor (NMDAR) antagonist, we tested the hypothesis that NMDAR inhibition may be producing the presynaptic effects. We show that NMDAR inhibition by aminophosphonovaleric acid mimics the presynaptic effects of Pb(2+) exposure. NMDAR activity has been linked to the signaling of the transsynaptic neurotrophin brain-derived neurotrophic factor (BDNF), and we observed that both the cellular expression of proBDNF and release of BDNF were decreased during the same period of Pb(2+) exposure. Furthermore, exogenous addition of BDNF rescued the presynaptic effects of Pb(2+). We suggest that the presynaptic deficits resulting from Pb(2+) exposure during synaptogenesis are mediated by disruption of NMDAR-dependent BDNF signaling.

The PLC/IP3R/PKC pathway is required for ethanol-enhanced GABA release

Research on the actions of ethanol at the GABAergic synapse has traditionally focused on postsynaptic mechanisms, but recent data demonstrate that ethanol also increases both evoked and spontaneous GABA release in many brain regions. Using whole-cell voltage-clamp recordings, we previously showed that ethanol increases spontaneous GABA release at the rat interneuron-Purkinje cell synapse. This presynaptic ethanol effect is dependent on calcium release from internal stores, possibly through activation of inositol 1,4,5-trisphosphate receptors (IP(3)Rs). After confirming that ethanol targets vesicular GABA release, in the present study we used electron microscopic immunohistochemistry to demonstrate that IP(3)Rs are located in presynaptic terminals of cerebellar interneurons. Activation of IP(3)Rs requires binding of IP(3), generated through activation of phospholipase C (PLC). We find that the PLC antagonist edelfosine prevents ethanol from increasing spontaneous GABA release. Diacylglycerol generated by PLC and calcium released by activation of the IP(3)R activate protein kinase C (PKC). Ethanol-enhanced GABA release was blocked by two PKC antagonists, chelerythrine and calphostin C. When a membrane impermeable PKC antagonist, PKC (19-36), was delivered intracellularly to the postsynaptic neuron, ethanol continued to increase spontaneous GABA release. Overall, these results suggest that activation of the PLC/IP(3)R/PKC pathway is necessary for ethanol to increase spontaneous GABA release from presynaptic terminals onto Purkinje cells.

Presynaptic inhibition of neural vasodilator pathways to submucosal arterioles by release of purines from sympathetic nerves

Capsaicin-sensitive extrinsic sensory nerves and submucosal vasodilator neurons provide important vasodilator input to submucosal arterioles, but relatively little is known about the signaling between these populations and the sympathetic vasoconstrictor innervation. This study examined whether release of sympathetic purines can modulate dilator nerves. In vitro submucosal preparations from guinea pig ileum were modified to leave the parent mesenteric artery intact so that perivascular sympathetic and extrinsic afferent nerves could be activated by a bipolar stimulating electrode placed on the parent artery, and submucosal vasodilator neurons were activated using focal electrodes placed on submucosal ganglia. The outside diameter of submucosal arterioles was monitored using videomicroscopy, and dilator responses were examined after preconstricting vessels 80-95% with prostaglandin F(2alpha) (400 nM). Mesenteric nerve stimulation evoked a frequency-dependent dilation, with suramin (100 microM) present throughout to inhibit P(2X) receptor-mediated vasoconstrictions. In the presence of guanethidine (10 microM) to inhibit sympathetic purine release, superfusion of ATP (200 nM-6 microM) caused a concentration-dependent inhibition of nerve-evoked dilations. Vasodilations to substance P (10 nM) were not inhibited by ATP in the presence of guanethidine, implicating a presynaptic effect of ATP on neurotransmitter release. The inhibitory effect of ATP was blocked by the adenosine receptor antagonist 8-phenyltheophylline (8-PT; 10 microM). In addition, 8-PT increased the amplitude of nerve-evoked dilations, suggesting a tonic inhibitory effect of adenosine receptors on vasodilator release. Dilations evoked by electrical stimulation of submucosal ganglia were also inhibited almost 50% by ATP (2 microM) and its nonhydrolyzable analog, alpha,beta-methylene-ATP (10 microM). These data suggest that sympathetic varicosities release ATP or a related purine that can act at presynaptic adenosine receptors on extrinsic sensory and submucosal vasodilator neurons to inhibit neurotransmitter release.

Presynaptic targets for acute ethanol sensitivity

Acute exposure to ethanol is known to modulate signalling within the nervous system. Physiologically these effects are both presynaptic and postsynaptic in origin; however, considerably more research has focused primarily on postsynaptic targets. Recent research using the model organism Caenorhabditis elegans has determined a role for specific proteins (Munc18-1 and Rab3) and processes (synaptic vesicle recruitment and fusion) in transducing the presynaptic effects of ethanol. In the present paper, we review these results, identifying the proteins and protein interactions involved in ethanol sensitivity and discuss their links with mammalian studies of alcohol abuse.

Not all partial dopamine D2 receptor agonists are the same in treating schizophrenia. Exploring the effects of bifeprunox and aripiprazole using a computer model of a primate striatal dopaminergic synapse

Species differences in physiology and unique active human metabolites contribute to the limited predictive value of preclinical rodent models for many central nervous system (CNS) drugs. In order to explore possible drivers for this translational disconnect, we developed a computer model of a dopaminergic synapse that simulates the competition among three agents and their binding to pre- and postsynaptic receptors, based on the affinities for their targets and their actual concentrations. The model includes presynaptic autoreceptor effects on neurotransmitter release and modulation by presynaptic firing frequency and is calibrated with actual experimental data on free dopamine levels in the striatum of the rodent and the primate. Using this model, we simulated the postsynaptic dopamine D2 receptor activation levels of bifeprunox and aripiprazole, two relatively similar dopamine D2 receptor agonists. The results indicate a substantial difference in dose–response for the two compounds when applying primate calibration parameters as opposed to rodent calibration parameters. In addition, when introducing the major human and rodent metabolites of aripiprazole with their specific pharmacological activities, the model predicts that while bifeprunox would result in a higher postsynaptic D2 receptor antagonism in the rodent, aripiprazole would result in a higher D2 receptor antagonism in the primate model. Furthermore, only the highest dose of aripiprazole, but not bifeprunox, reaches postsynaptic functional D2 receptor antagonism similar to 4 mg haloperidol in the primate model. The model further identifies a limited optimal window of functionality for dopamine D2 receptor partial agonists. These results suggest that computer modeling of key CNS processes, using well-validated calibration paradigms, can increase the predictive value in the clinical setting of preclinical animal model outcomes.

Effects of cholinergic neuromodulation in the rat dorsal cochlear nucleus: a calcium imaging study performed on brain slices.

Event Abstract Back to Event Effects of cholinergic neuromodulation in the rat dorsal cochlear nucleus: a calcium imaging study performed on brain slices. Aron Koszeghy1*, Balázs Pál1, Géza SzXucs1 and Zoltán Rusznák1 1 University of Debrecen, Institute of Physiology, Hungary In a previous study our laboratory showed that the cholinergic agonist, carbachol increased the spontaneous activity of the giant neurons of the dorsal cochlear nucleus (DCN) with electro physiological technics. The increased activity was partly due to a potassium conductance decrease mediated by M3 and M4 muscarinic cholinergic receptors situated directly on the giant cells. We also observed cholinergic modulational effect on the synaptic inputs of the giant cells. To further investigate this presynaptic effect proposed by the electrophysiological experiments, in this work we used “multy neuronal calcium imaging” on Oregon Green-AM loaded DCN slices. We studied the effect of carbachol on the neurons of DCN in the presence of a glutamatergic, glycinergic and GABA-ergic neurotransmission blocking “cocktail” (NBQX, AP5, strychnine, bicuculline). The effect observed on the neurons - parted from each other by this pharmacological manipulation - is presumably mediated through receptors situated directly on these cells themselves. With this method we observed significant calcium event frequency increase in a number of small cells in the DCN. The average diameter of these carbachol-responding-small-cells was 7.1 micron. It suggests that these cells might be the granule cells of the DCN as they have 6-8 micron soma diameter conventionally. Afterwards also in the presence of the glutamatergic, glycinergic and GABA-ergic neurotransmission blocking “cocktail” we examined whether the effect of carbachol can be prevented with atropine. In the case of eight of nine cells showing activity increase for carbachol, atropin prevented the effect of carbachol. Our results point out a significant cholinerg neuro modulatory effect on some of the small cells of the DCN. According to our experimental procedure this effect seems to be mediated by muscarinic receptors situated directly on these “carbachol-responding” small cells, which might be the granule cells of the DCN. Conference: IBRO International Workshop 2010, Pécs, Hungary, 21 Jan - 23 Jan, 2010. Presentation Type: Poster Presentation Topic: Sensory and motor systems Citation: Koszeghy A, Pál B, SzXucs G and Rusznák Z (2010). Effects of cholinergic neuromodulation in the rat dorsal cochlear nucleus: a calcium imaging study performed on brain slices.. Front. Neurosci. Conference Abstract: IBRO International Workshop 2010. doi: 10.3389/conf.fnins.2010.10.00239 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 05 May 2010; Published Online: 05 May 2010. * Correspondence: Aron Koszeghy, University of Debrecen, Institute of Physiology, Debrecen, Hungary, aron.koeszeghy@meduniwien.ac.at Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Aron Koszeghy Balázs Pál Géza SzXucs Zoltán Rusznák Google Aron Koszeghy Balázs Pál Géza SzXucs Zoltán Rusznák Google Scholar Aron Koszeghy Balázs Pál Géza SzXucs Zoltán Rusznák PubMed Aron Koszeghy Balázs Pál Géza SzXucs Zoltán Rusznák Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Presynaptic effects of grayanotoxin III on excitatory and inhibitory nerve terminals in rat ventromedial hypothalamic neurons

Mad honey poisoning caused by grayanotoxin (GTX) is associated with autonomic nervous system symptoms, such as excessive perspiration, hypersalivation, vomiting, and bradycardia. Neurons in the ventromedial hypothalamus (VMH) play an important role in body homeostasis and in the activity of the autonomic nervous system. Among the 18 isoforms of GTX found in mad honey, GTX I–IV are a unique class of toxic diterpenoids; GTX III is the principal toxic isomer. In the present study, we determined the effects of GTX III on synaptic transmission in VMH neurons. Both spontaneous and evoked GABA-ergic and glutamate-ergic postsynaptic currents were measured using patch clamp recordings in single VMH neurons which had been mechanically dissociated. GTX III increased the frequency of spontaneous GABA-ergic and glutamate-ergic postsynaptic currents (sIPSCs and sEPSCs, respectively) in a dose-dependent manner without affecting their amplitude, demonstrating that GTX III enhances transmitter release from both inhibitory and excitatory nerve terminals synapsing onto VMH neurons. GTX III significantly enhanced the amplitude and the success rate (Rsuc) of both evoked inhibitory and excitatory postsynaptic currents (eIPSCs and eEPSCs, respectively), suggesting that GTX III increases the probability of transmitter release from these terminals, and also the amount of transmitter released from a single nerve terminal. The action of GTX III on sIPSC frequency was absent in a Na +-free solution and in the presence of tetrodotoxin (TTX; 300 nM) or cadmium (Cd 2+; 100 μM). The present study indicates that GTX increases Ca 2+ influx through voltage-dependent Ca 2+ channels secondary to activation of voltage-dependent Na + channels in inhibitory and excitatory nerve terminals synapsing on VMH neurons, and the subsequent increased release of GABA and glutamate from these terminals may be responsible for the autonomic symptoms of GTX intoxication.

Plasticity between Neuronal Pairs in Layer 4 of Visual Cortex Varies with Synapse State

In neocortex, the induction and expression of long-term potentiation (LTP) and long-term depression (LTD) vary depending on cortical area and laminae of presynaptic and postsynaptic neurons. Layer 4 (L4) is the initial site of sensory afference in barrel cortex and primary visual cortex (V1) in which excitatory inputs from thalamus, L6, and neighboring L4 cells are integrated. However, little is known about plasticity within L4. We studied plasticity at excitatory synaptic connections between pairs and triplets of interconnected L4 neurons in guinea pig V1 using a fixed delay pairing protocol. Plasticity outcomes were heterogeneous, with some connections undergoing LTP (n = 7 of 42), some LTD (n = 19 of 42), and some not changing (n = 16 of 42). Although quantal analysis revealed both presynaptic and postsynaptic plasticity expression components, reduction in quantal size (a postsynaptic property) contributing to LTD was ubiquitous, whereas in some cell pairs, this change was overridden by an increase in the probability of neurotransmitter release (a presynaptic property) resulting in LTP. These changes depended on the initial reliability of the connections: highly reliable connections depressed with contributions from presynaptic and postsynaptic effects, and unreliable connections potentiated as a result of the predominance of presynaptic enhancement. Interestingly, very strong, reliable pairs of connected cells showed little plasticity. Pairs of connected cells with a common presynaptic or postsynaptic L4 cell behaved independently, undergoing plasticity of different or opposite signs. Release probability of a connection with initial 100% failure rate was enhanced after pairing, potentially avoiding silencing of the presynaptic terminal and maintaining L4-L4 synapses in a broader dynamic range.

The N-methyl-d-aspartate receptor antagonist CPP alters synapse and spine structure and impairs long-term potentiation and long-term depression induced morphological plasticity in dentate gyrus of the awake rat

Long-term morphological synaptic changes associated with homosynaptic long-term potentiation (LTP) and heterosynaptic long-term depression (LTD) in vivo, in awake adult rats were analyzed using three-dimensional (3-D) reconstructions of electron microscope images of ultrathin serial sections from the molecular layer of the dentate gyrus. For the first time in morphological studies, the specificity of the effects of LTP and LTD on both spine and synapse ultrastructure was determined using an N-methyl-d-aspartate (NMDA) receptor antagonist CPP (3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid). There were no differences in synaptic density 24 h after LTP or LTD induction, and CPP alone had no effect on synaptic density. LTP increased significantly the proportion of mushroom spines, whereas LTD increased the proportion of thin spines, and both LTP and LTD decreased stubby spine number. Both LTP and LTD increased significantly spine head evaginations (spinules) into synaptic boutons and CPP blocked these changes. Synaptic boutons were smaller after LTD, indicating a pre-synaptic effect. Interestingly, CPP alone decreased bouton and mushroom spine volumes, as well as post-synaptic density (PSD) volume of mushroom spines.These data show similarities, but also some clear differences, between the effects of LTP and LTD on spine and synaptic morphology. Although CPP blocks both LTP and LTD, and impairs most morphological changes in spines and synapses, CPP alone was shown to exert effects on aspects of spine and synaptic structure.

Late Cortical Disinhibition in Human Motor Cortex: A Triple-Pulse Transcranial Magnetic Stimulation Study

In human motor cortex transcranial magnetic stimulation (TMS) has been used to identify short-interval intracortical inhibition (SICI) corresponding to gamma-aminobutyric acid type A (GABA(A)) effects and long-interval intracortical inhibition (LICI) and the cortical silent period (SP) corresponding to postsynaptic GABA(B) effects. Presynaptic GABA(B) effects, corresponding to disinhibition, can also be identified with TMS and have been shown to be acting during LICI by measuring SICI after a suprathreshold priming stimulus (PS). The duration of disinhibition is not certain and, guided by studies in experimental preparations, we hypothesized that it may be longer-lasting than postsynaptic inhibition, leading to a period of late cortical disinhibition and consequently a net increase in corticospinal excitability. We tested this first by measuring the motor-evoked potential (MEP) to a test stimulus (TS), delivered after a PS at interpulse intervals (IPIs) < or =300 ms that encompassed the period of PS-induced LICI and its aftermath. MEP amplitude was initially decreased, but then increased at IPIs of 190-210 ms, reaching 160 +/- 17% of baseline 200 ms after PS (P < 0.05). SP duration was 181 +/- 5 ms. A second experiment established that the onset of the later period of increased excitability correlated with PS intensity (r(2) = 0.99) and with the duration of the SP (r(2) = 0.99). The third and main experiment demonstrated that SICI was significantly reduced in strength at all IPIs < or =220 ms after PS. We conclude that TMS-induced LICI is associated with a period of disinhibition that is at first masked by LICI, but that outlasts LICI and gives rise to a period during which disinhibition predominates and net excitability is raised. Identification of this late period of disinhibition in human motor cortex may provide an opportunity to explore or modulate the behavior of excitatory networks at a time when inhibitory effects are restrained.

5-HT and GABA Modulate Intrinsic Excitability of Type I Interneurons in Hermissenda

The sensory neurons (photoreceptors) in the visual system of Hermissenda are one site of plasticity produced by Pavlovian conditioning. A second site of plasticity produced by conditioning is the type I interneurons in the cerebropleural ganglia. Both photoreceptors and statocyst hair cells of the graviceptive system form monosynaptic connections with identified type I interneurons. Two proposed neurotransmitters in the graviceptive system, serotonin (5-HT) and gamma-aminobutyric acid (GABA), have been shown to modify synaptic strength and intrinsic neuronal excitability in identified photoreceptors. However, the potential role of 5-HT and GABA in plasticity of type I interneurons has not been investigated. Here we show that 5-HT increased the peak amplitude of light-evoked complex excitatory postsynaptic potentials (EPSPs), enhanced intrinsic excitability, and increased spike activity of identified type I(e(A)) interneurons. In contrast, 5-HT decreased spike activity and intrinsic excitability of type I(e(B)) interneurons. The classification of two categories of type I(e) interneurons was also supported by the observation that 5-HT produced opposite effects on whole cell steady-state outward currents in type I(e) interneurons. Serotonin produced a reduction in the amplitude of light-evoked complex inhibitory PSPs (IPSPs), increased spontaneous spike activity, decreased intrinsic excitability, and depolarized the resting membrane potential of identified type I(i) interneurons. In contrast to the effects of 5-HT, GABA produced inhibition in both types of I(e) interneurons and type I(i) interneurons. These results show that 5-HT and GABA can modulate the intrinsic excitability of type I interneurons independent of the presynaptic effects of the same transmitters on excitability and synaptic efficacy of photoreceptors.

Puberty, steroids and GABA A receptor plasticity

GABA(A) receptors (GABAR) mediate most inhibition in the CNS and are also a target for neuroactive steroids such as 3alpha,5[alpha]beta-THP (3alphaOH-5[alpha]beta-OH-pregnan-20-one or [allo]pregnanolone). Although these steroids robustly enhance current gated by alpha1beta2delta GABAR, we have shown that 3alpha,5[alpha]beta-THP effects at recombinant alpha4beta2delta GABAR depend on the direction of Cl(-) flux, where the steroid increases outward flux, but decreases inward flux through the receptor. This polarity-dependent inhibition of alpha4beta2delta GABAR resulted from an increase in the rate and extent of rapid desensitization of the receptor, recorded from recombinant receptors expressed in HEK-293 cells with whole cell voltage clamp techniques. This inhibitory effect of 3alpha,5[alpha]beta-THP was not observed at other receptor subtypes, suggesting it was selective for alpha4beta2delta GABAR. Furthermore, it was prevented by a selective mutation of basic residue arginine 353 in the intracellular loop of the receptor, suggesting that this might be a putative chloride modulatory site. Expression of alpha4betadelta GABAR increases markedly at extrasynaptic sites at the onset of puberty in female mice. At this time, 3alpha,5[alpha]beta-THP decreased the inhibitory tonic current, recorded with perforated patch techniques to maintain the physiological Cl(-) gradient. By decreasing this shunting inhibition, 3alpha,5[alpha]beta-THP increased the excitability of CA1 hippocampal pyramidal cells at puberty. These effects of the steroid were opposite to those observed before puberty when 3alpha,5[alpha]beta-THP reduced neuronal excitability as a pre-synaptic effect. Behaviorally, the excitatory effect of 3alpha,5[alpha]beta-THP was reflected as an increase in anxiety at the onset of puberty in female mice. Taken together, these findings suggest that the emergence of alpha4beta2delta GABAR at the onset of puberty reverses the effect of a stress steroid. These findings may be relevant for the mood swings and increased response to stressful events reported in adolescence.

Chronic Cocaine Enhances Corticotropin-Releasing Factor-Dependent Potentiation of Excitatory Transmission in Ventral Tegmental Area Dopamine Neurons

Current concepts suggest that stress-induced release of neuromodulators such as corticotropin-releasing factor (CRF) can drive drug-dependent behaviors. Although previous drug exposure can enhance behavioral and neurochemical responses to stress, it is unclear how such drug exposure alters the CRF modulation of excitatory synapses onto ventral tegmental area (VTA) dopamine neurons, a key locus of drug- and stress-induced neuroadaptation. Here, we demonstrate that, after repeated cocaine exposure, the magnitude and duration of the CRF-induced potentiation of NMDA receptor (NMDAR)-mediated neurotransmission was significantly increased compared with naive and saline-treated mice. Furthermore, CRF enhanced AMPA receptor (AMPAR)-mediated transmission only in mice that were exposed to cocaine. Increased frequency of AMPAR-mediated spontaneous miniature EPSCs and the intracellular blockade of CRF potentiation of AMPAR-mediated transmission suggest both presynaptic and postsynaptic effects of CRF. Importantly, pharmacological experiments revealed that CRF receptor 1 and protein kinase A pathways were newly recruited after repeated cocaine for the enhancement of CRF-induced NMDAR potentiation and the appearance of AMPAR potentiation. Thus, enhanced CRF-induced potentiation of excitatory synaptic transmission onto VTA dopamine neurons after cocaine preexposure is likely to produce an abnormal increase in dopamine release during stressful events and could augment activation of addictive behaviors in response to stress.

γ-Hydroxybutyrate (GHB) induces GABA B receptor independent intracellular Ca 2+ transients in astrocytes, but has no effect on GHB or GABA B receptors of medium spiny neurons in the nucleus accumbens

We report on cellular actions of the illicit recreational drug γ-hydroxybutyrate (GHB) in the brain reward area nucleus accumbens. First, we compared the effects of GHB and the GABA B receptor agonist baclofen. Neither of them affected the membrane currents of medium spiny neurons in rat nucleus accumbens slices. GABAergic and glutamatergic synaptic potentials of medium spiny neurons, however, were reduced by baclofen but not GHB. These results indicate the lack of GHB as well as postsynaptic GABA B receptors, and the presence of GHB insensitive presynaptic GABA B receptors in medium spiny neurons. In astrocytes GHB induced intracellular Ca 2+ transients, preserved in slices from GABA B receptor type 1 subunit knockout mice. The effects of tetrodotoxin, zero added Ca 2+ with/without intracellular Ca 2+ store depletor cyclopiazonic acid or vacuolar H-ATPase inhibitor bafilomycin A1 indicate that GHB-evoked Ca 2+ transients depend on external Ca 2+ and intracellular Ca 2+ stores, but not on vesicular transmitter release. GHB-induced astrocytic Ca 2+ transients were not affected by the GHB receptor-specific antagonist NCS-382, suggesting the presence of a novel NCS-382-insensitive target for GHB in astrocytes. The activation of astrocytes by GHB implies their involvement in physiological actions of GHB. Our findings disclose a novel profile of GHB action in the nucleus accumbens. Here, unlike in other brain areas, GHB does not act on GABA B receptors, but activates an NCS-382 insensitive GHB-specific target in a subpopulation of astrocytes. The lack of either post- or presynaptic effects on medium spiny neurons in the nucleus accumbens distinguishes GHB from many drugs and natural rewards with addictive properties and might explain why GHB has only a weak reinforcing capacity.

Proteinase-Activated Receptor-1 Activation Presynaptically Enhances Spontaneous Glutamatergic Excitatory Transmission in Adult Rat Substantia Gelatinosa Neurons

Proteinase-activated receptors (PARs) have a unique activation mechanism in that a proteolytically exposed N-terminal region acts as a tethered ligand. A potential impact of PAR on sensory processing has not been fully examined yet. Here we report that synthetic peptides with sequences corresponding to PAR ligands enhance glutamatergic excitatory transmission in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole cell patch-clamp technique. The frequency of spontaneous excitatory postsynaptic current (EPSC) was increased by PAR-1 agonist SFLLRN-NH2 (by 47% at 1 microM) with small increases by PAR-2 and -4 agonists (SLIGKV-NH2 and GYPGQV-OH, respectively; at >3 microM); there was no change in its amplitude or in holding current at -70 mV. The PAR-1 peptide action was inhibited by PAR-1 antagonist YFLLRNP-OH. TFLLR-NH2, an agonist which is more selective to PAR-1 than SFLLRN-NH2, dose-dependently increased spontaneous EPSC frequency (EC50=0.32 microM). A similar presynaptic effect was produced by PAR-1 activating proteinase thrombin in a manner sensitive to YFLLRNP-OH. The PAR-1 peptide action was resistant to tetrodotoxin and inhibited in Ca2+-free solution. Primary-afferent monosynaptically evoked EPSC amplitudes were unaffected by PAR-1 agonist. These results indicate that PAR-1 activation increases the spontaneous release of L-glutamate onto SG neurons from nerve terminals in a manner dependent on extracellular Ca2+. Considering that sensory processing within the SG plays a pivotal role in regulating nociceptive transmission to the spinal dorsal horn, the PAR-1-mediated glutamatergic transmission enhancement could be involved in a positive modulation of nociceptive transmission.