A novel component of cannabis extract potentiates excitatory synaptic transmission in rat olfactory cortex in vitro

Cannabis is a potential treatment for epilepsy, although the few human studies supporting this use have proved inconclusive. Previously, we showed that a standardized cannabis extract (SCE), isolated Delta(9)-tetrahydrocannabinol (Delta(9)-THC), and even Delta(9)-THC-free SCE inhibited muscarinic agonist-induced epileptiform bursting in rat olfactory cortical brain slices, acting via CB1 receptors. The present work demonstrates that although Delta(9)-THC (1microM) significantly depressed evoked depolarizing postsynaptic potentials (PSPs) in rat olfactory cortex neurones, both SCE and Delta(9)-THC-free SCE significantly potentiated evoked PSPs (all results were fully reversed by the CB1 receptor antagonist SR141716A, 1microM); interestingly, the potentiation by Delta(9)-THC-free SCE was greater than that produced by SCE. On comparing the effects of Delta(9)-THC-free SCE upon evoked PSPs and artificial PSPs (aPSPs; evoked electrotonically following brief intracellular current injection), PSPs were enhanced, whereas aPSPs were unaffected, suggesting that the effect was not due to changes in background input resistance. Similar recordings made using CB1 receptor-deficient knockout mice (CB1(-/-)) and wild-type littermate controls revealed cannabinoid or extract-induced changes in membrane resistance, cell excitability and synaptic transmission in wild-type mice that were similar to those seen in rat neurones, but no effect on these properties were seen in CB1(-/-) cells. It appears that the unknown extract constituent(s) effects over-rode the suppressive effects of Delta(9)-THC on excitatory neurotransmitter release, which may explain some patients' preference for herbal cannabis rather than isolated Delta(9)-THC (due to attenuation of some of the central Delta(9)-THC side effects) and possibly account for the rare incidence of seizures in some individuals taking cannabis recreationally

SK channels regulate excitatory synaptic transmission and plasticity in the lateral amygdala

At glutamatergic synapses, calcium influx through NMDA receptors (NMDARs) is required for long-term potentiation (LTP); this is a proposed cellular mechanism underlying memory and learning. Here we show that in lateral amygdala pyramidal neurons, SK channels are also activated by calcium influx through synaptically activated NMDARs, resulting in depression of the synaptic potential. Thus, blockade of SK channels by apamin potentiates fast glutamatergic synaptic potentials. This potentiation is blocked by the NMDAR antagonist AP5 (D(-)-2-amino-5-phosphono-valeric acid) or by buffering cytosolic calcium with BAPTA. Blockade of SK channels greatly enhances LTP of cortical inputs to lateral amygdala pyramidal neurons. These results show that NMDARs and SK channels are colocalized at glutamatergic synapses in the lateral amygdala. Calcium influx through NMDARs activates SK channels and shunts the resultant excitatory postsynaptic potential. These results demonstrate a new role for SK channels as postsynaptic regulators of synaptic efficacy.

Propofol effects on excitatory synaptic efficacy in the CA1 region of the developing hippocarnpus

The anesthetic, propofol, effectively suppresses excitatory synaptic transmission and facilitates long-term depression (LTD) in the CA1 region of the hippocampus. Here, we have examined whether these effects are different in the developing hippocampus. We found that propofol in suppressing whole-cell excitatory postsynaptic currents (EPSC) was more effective in 21 day old rats than either in 7 day old rats or under the condition of high intracellular chloride concentration in 21 day old rats. Furthermore, the propofol concentration to facilitate the NMDA receptor-dependent LTD was lower at postnatal day 21 than at postnatal day 7. Interestingly, the decay time of EPSC was decreased during the development from postnatal day 7 to 21, but it was increased by the recording condition of high intracellular chloride concentration or by propofol administration. All these effects of propofol were dependent on the chloride channel opening. These observations suggest that propofol may induce differential anesthetic effects in the developing hippocampus, at least partially, depending on the intracellular chloride concentration. (c) 2005 Elsevier B.V. All rights reserved.

Modulation of synaptic transmission in hippocampal CA1 neurons by a novel neurotoxin (beta-pompilidotoxin) derived from wasp venom

We examined the effects of beta-pompilidotoxin (beta-PMTX), a neurotoxin derived from wasp venom. on synaptic transmission in the mammalian central nervous system (CNS). Using hippocampal slice preparations of rodents, we made both extracellular and intracellular recordings from the CA1 pyramidal neurons in response to stimulation of the Schaffer collateral/commissural fibers. Application of 5-10 muM beta-PMTX enhanced excitatory postsynaptic potentials (EPSPs) but suppressed the fast component of the inhibitory postsynaptic potentials (IPSPs). In the presence of 10 muM bicuculline, beta-PMTX potentiated EPSPs that were composed of both non-NMDA and NMDA receptor-mediated potentials. Potentiation of EPSPs was originated by repetitive firings of the prosynaptic axons, causing Summation of EPSPs. In the presence of 10 muM CNQX and 50 muM APV, beta-PMTX suppressed GABA(A) receptor-mediated fast IPSPs but retained GABA(B) receptor-mediated slow IPSPs. Our results suggest that beta-PMTX facilitates excitatory synaptic transmission by a presynaptic mechanism and that it causes overexcitation followed by block of the activity of some population of interneurons which regulate the activity of GABA(A) receptors. (C) 2001 Published by Elsevier B.V. Ireland Ltd and the Japan Neuroscience Society.

Corticosterone targets distinct steps of synaptic transmission via concentration specific activation of mineralocorticoid and glucocorticoid receptors

Hippocampal neurons are affected by chronic stress and have a high density of cytoplasmic mineralocorticoid and glucocorticoid receptors (MR and GR). Detailed studies on the genomic effects of the stress hormone corticosterone at physiologically relevant concentrations on different steps in synaptic transmission are limited. In this study, we tried to delineate how activation of MR and GR by different concentrations of corticosterone affects synaptic transmission at various levels. The effect of 3-h corticosterone (25, 50, and 100nM) treatment on depolarization-mediated calcium influx, vesicular release and properties of miniature excitatory post-synaptic currents (mEPSCs) were studied in cultured hippocampal neurons. Activation of MR with 25nM corticosterone treatment resulted in enhanced depolarization-mediated calcium influx via a transcription-dependent process and increased frequency of mEPSCs with larger amplitude. On the other hand, activation of GR upon 100nM corticosterone treatment resulted in increase in the rate of vesicular release via the genomic actions of GR. Furthermore, GR activation led to significant increase in the frequency of mEPSCs with larger amplitude and faster decay. Our studies indicate that differential activation of the dual receptor system of MR and GR by corticosterone targets the steps in synaptic transmission differently.

An allosteric potentiator of M4 mAChR modulates hippocampal synaptic transmission.

Muscarinic acetylcholine receptors (mAChRs) provide viable targets for the treatment of multiple central nervous system disorders. We have used cheminformatics and medicinal chemistry to develop new, highly selective M₄ allosteric potentiators. VU10010, the lead compound, potentiates the M₄ response to acetylcholine 47-fold while having no activity at other mAChR subtypes. This compound binds to an allosteric site on the receptor and increases affinity for acetylcholine and coupling to G proteins. Whole-cell patch clamp recordings revealed that selective potentiation of M₄ with VU10010 increases carbachol-induced depression of transmission at excitatory but not inhibitory synapses in the hippocampus. The effect was not mimicked by an inactive analog of VU10010 and was absent in M₄ knockout mice. Selective regulation of excitatory transmission by M₄ suggests that targeting of individual mAChR subtypes could be used to differentially regulate specific aspects of mAChR modulation of function in this important forebrain structure.

Glial cells modulate the synaptic transmission of NTS neurons sending projections to ventral medulla of Wistar rats

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Beta 1-integrins are required for hippocampal AMPA receptor-dependent synaptic transmission, synaptic plasticity, and working memory.

Integrins comprise a large family of cell adhesion receptors that mediate diverse biological events through cell-cell and cell-extracellular matrix interactions. Recent studies have shown that several integrins are localized to synapses with suggested roles in synaptic plasticity and memory formation. We generated a postnatal forebrain and excitatory neuron-specific knock-out of beta1-integrin in the mouse. Electrophysiological studies demonstrated that these mutants have impaired synaptic transmission through AMPA receptors and diminished NMDA receptor-dependent long-term potentiation. Despite the impairment in hippocampal synaptic transmission, the mutants displayed normal hippocampal-dependent spatial and contextual memory but were impaired in a hippocampal-dependent, nonmatching-to-place working memory task. These phenotypes parallel those observed in animals carrying knock-outs of the GluR1 (glutamate receptor subunit 1) subunit of the AMPA receptor. These observations suggest a new function of beta1-integrins as regulators of synaptic glutamate receptor function and working memory.

Ablation of P/Q-type Ca2+ channel currents, altered synaptic transmission, and progressive ataxia in mice lacking the α1A-subunit

The Ca2+ channel α1A-subunit is a voltage-gated, pore-forming membrane protein positioned at the intersection of two important lines of research: one exploring the diversity of Ca2+ channels and their physiological roles, and the other pursuing mechanisms of ataxia, dystonia, epilepsy, and migraine. α1A-Subunits are thought to support both P- and Q-type Ca2+ channel currents, but the most direct test, a null mutant, has not been described, nor is it known which changes in neurotransmission might arise from elimination of the predominant Ca2+ delivery system at excitatory nerve terminals. We generated α1A-deficient mice (α1A−/−) and found that they developed a rapidly progressive neurological deficit with specific characteristics of ataxia and dystonia before dying ≈3–4 weeks after birth. P-type currents in Purkinje neurons and P- and Q-type currents in cerebellar granule cells were eliminated completely whereas other Ca2+ channel types, including those involved in triggering transmitter release, also underwent concomitant changes in density. Synaptic transmission in α1A−/− hippocampal slices persisted despite the lack of P/Q-type channels but showed enhanced reliance on N-type and R-type Ca2+ entry. The α1A−/− mice provide a starting point for unraveling neuropathological mechanisms of human diseases generated by mutations in α1A.

Src, a molecular switch governing gain control of synaptic transmission mediated by N-methyl-d-aspartate receptors

The N-methyl-d-aspartate (NMDA) receptor is a principal subtype of glutamate receptor mediating fast excitatory transmission at synapses in the dorsal horn of the spinal cord and other regions of the central nervous system. NMDA receptors are crucial for the lasting enhancement of synaptic transmission that occurs both physiologically and in pathological conditions such as chronic pain. Over the past several years, evidence has accumulated indicating that the activity of NMDA receptors is regulated by the protein tyrosine kinase, Src. Recently it has been discovered that, by means of up-regulating NMDA receptor function, activation of Src mediates the induction of the lasting enhancement of excitatory transmission known as long-term potentiation in the CA1 region of the hippocampus. Also, Src has been found to amplify the up-regulation of NMDA receptor function that is produced by raising the intracellular concentration of sodium. Sodium concentration increases in neuronal dendrites during high levels of firing activity, which is precisely when Src becomes activated. Therefore, we propose that the boost in NMDA receptor function produced by the coincidence of activating Src and raising intracellular sodium may be important in physiological and pathophysiological enhancement of excitatory transmission in the dorsal horn of the spinal cord and elsewhere in the central nervous system.

Mice deficient for prion protein exhibit normal neuronal excitability and synaptic transmission in the hippocampus.

We recorded in the CA1 region from hippocampal slices of prion protein (PrP) gene knockout mice to investigate whether the loss of the normal form of prion protein (PrPC) affects neuronal excitability as well as synaptic transmission in the central nervous system. No deficit in synaptic inhibition was found using field potential recordings because (i) responses induced by stimulation in stratum radiatum consisted of a single population spike in PrP gene knockout mice similar to that recorded from control mice and (ii) the plot of field excitatory postsynaptic potential slope versus the population spike amplitude showed no difference between the two groups of mice. Intracellular recordings also failed to detect any difference in cell excitability and the reversal potential for inhibitory postsynaptic potentials. Analysis of the kinetics of inhibitory postsynaptic current revealed no modification. Finally, we examined whether synaptic plasticity was altered and found no difference in long-term potentiation between control and PrP gene knockout mice. On the basis of our findings, we propose that the loss of the normal form of prion protein does not alter the physiology of the CA1 region of the hippocampus.

Calcium/calmodulin-dependent kinase II and long-term potentiation enhance synaptic transmission by the same mechanism.

Ca(2+)-sensitive kinases are thought to play a role in long-term potentiation (LTP). To test the involvement of Ca2+/calmodulin-dependent kinase II (CaM-K II), truncated, constitutively active form of this kinase was directly injected into CA1 hippocampal pyramidal cells. Inclusion of CaM-K II in the recording pipette resulted in a gradual increase in the size of excitatory postsynaptic currents (EPSCs). No change in evoked responses occurred when the pipette contained heat-inactivated kinase. The effects of CaM-K II mimicked several features of LTP in that it caused a decreased incidence of synaptic failures, an increase in the size of spontaneous EPSCs, and an increase in the amplitude of responses to iontophoretically applied alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. To determine whether the CaM-K II-induced enhancement and LTP share a common mechanism, occlusion experiments were carried out. The enhancing action of CaM-K II was greatly diminished by prior induction of LTP. In addition, following the increase in synaptic strength by CaM-K II, tetanic stimulation failed to evoke LTP. These findings indicate that CaM-K II alone is sufficient to augment synaptic strength and that this enhancement shares the same underlying mechanism as the enhancement observed with LTP.

Brain-derived neurotrophic factor rapidly enhances synaptic transmission in hippocampal neurons via postsynaptic tyrosine kinase receptors.

Although neurotrophins are primarily associated with long-term effects on neuronal survival and differentiation, recent studies have shown that acute changes in synaptic transmission can also be produced. In the hippocampus, an area critically involved in learning and memory, we have found that brain-derived neurotrophic factor (BDNF) rapidly enhanced synaptic efficacy through a previously unreported mechanism--increased postsynaptic responsiveness via a phosphorylation-dependent pathway. Within minutes of BDNF application to cultured hippocampal neurons, spontaneous firing rate was dramatically increased, as were the frequency and amplitude of excitatory postsynaptic currents. The increased frequency of postsynaptic currents resulted from the change in presynaptic firing. However, the increased amplitude was postsynaptic in origin because it was selectively blocked by intracellular injection of the tyrosine kinase receptor (Ntrk2/TrkB) inhibitor K-252a and potentiated by injection of the phosphatase inhibitor okadaic acid. These results suggest a role for BDNF in the modulation of synaptic transmission in the hippocampus.

BDNF impairs recovery of synaptic transmission after hypoxia even with blockage of adenosine A2A receptors

Trabalho Final do Curso de Mestrado Integrado em Medicina, Faculdade de Medicina, Universidade de Lisboa, 2014

Depression of Serotonin Synaptic Transmission by the Dopamine Precursor L-DOPA

Imbalance between the dopamine and serotonin (5-HT) neurotransmitter systems has been implicated in the comorbidity of Parkinson's disease (PD) and psychiatric disorders. L-DOPA, the leading treatment of PD, facilitates the production and release of dopamine. This study assessed the action of L-DOPA on monoamine synaptic transmission in mouse brain slices. Application of L-DOPA augmented the D2-receptor-mediated inhibitory postsynaptic current (IPSC) in dopamine neurons of the substantia nigra. This augmentation was largely due to dopamine release from 5-HT terminals. Selective optogenetic stimulation of 5-HT terminals evoked dopamine release, producing D2-receptor-mediated IPSCs following treatment with L-DOPA. In the dorsal raphe, L-DOPA produced a long-lasting depression of the 5-HT1A-receptor-mediated IPSC in 5-HT neurons. When D2 receptors were expressed in the dorsal raphe, application of L-DOPA resulted in a D2-receptor-mediated IPSC. Thus, treatment with L-DOPA caused ectopic dopamine release from 5-HT terminals and a loss of 5-HT-mediated synaptic transmission.