Inhibition Of Presynaptic Ca2 Research Articles

Sevoflurane inhibits presynaptic calcium influx without affecting presynaptic action potentials in hippocampal CA1 region.

Although diverse effects of volatile anesthetics have been investigated in various studies, the mechanisms of action of such anesthetics, especially sevoflurane, remain elusive. In contrast to their potent modulation of inhibitory synaptic transmission there is little information about their effects on excitatory transmission in the brain. In this study, we examined the effect of sevoflurane on the excitatory synaptic transmission at CA1 synapses in hippocampal slices of mice. Sevoflurane at 5% was mixed with 95% O2 and 5% CO2 and bubbled in artificial cerebral spinal fluid (0.69 mM). Extracellular recordings of the field excitatory postsynaptic potential (fEPSP) and presynaptic fiber volley (FV) were made at physiological temperature. In addition, fluorescent measurements of presynaptic Ca2+ transients were performed while simultaneously recording fEPSP. Application of sevoflurane reduced the amplitude of fEPSP (45 ± 8%, n = 5). This effect was accompanied by concurrent enhancement of the paired-pulse facilitation of fEPSP (127 ± 5%, n = 12), suggesting a possible presynaptic site of action of sevoflurane. The amplitude of FV was not significantly affected (102 ± 5%, n = 5). In contrast, fluorescent measurements revealed that presynaptic Ca2+ influx was suppressed by sevoflurane (69 ± 5%, n = 7), as was simultaneously recorded fEPSP (44 ± 5%, n = 7). Our results suggest that sevoflurane potently suppresses excitatory synaptic transmission via inhibition of presynaptic Ca2+ influx without affecting presynaptic action potentials.

Examining Cholinergic Synaptic Signaling in the Thalamic Reticular Nucleus.

This protocol describes how to obtain monosynaptic cholinergic responses in neurons of the thalamic reticular nucleus (TRN) by making use of extracellular stimulation techniques. These methods are easy to implement and allow for the study of various forms of cholinergic synaptic plasticity and modulation. For many synapses throughout the mammalian brain, short-term plasticity is mediated by endocannabinoids released from postsynaptic neurons that activate presynaptic type I cannabinoid receptors (CB1Rs), resulting in the inhibition of presynaptic Ca2+ channels and a reduction of release probability. Neurons in the TRN are known to liberate endocannabinoids that can control transmitter release at GABAergic terminals. However, expression of CB1Rs on cholinergic terminals contacting the TRN has not been demonstrated. Here we outline strategies aimed to record stable postsynaptic responses and to quantify changes in cholinergic synaptic strength, using presynaptic modulation of acetylcholine (ACh) release by a CB1R agonist as an illustrative example.

A new signalling pathway for parallel fibre presynaptic type 4 metabotropic glutamate receptors (mGluR4) in the rat cerebellar cortex

In the rodent cerebellum, pharmacological activation of mGluR4 acutely depresses excitatory synaptic transmission at parallel fibre–Purkinje cell synapses. This depression involves the inhibition of presynaptic calcium (Ca2+) influx that ultimately controls glutamate release. In this study, we investigate the molecular basis of mGluR4-mediated inhibition of presynaptic Ca2+ transients. Our results demonstrate that the mGluR4 effect does not depend on selective inhibition of a specific type of presynaptic voltage-gated Ca2+ channel, but rather involves modulation of all classes of Ca2+ channels present in the presynaptic terminals. In addition, this inhibitory effect does not involve the activation of G protein-activated inwardly rectifying potassium channels, TEA-sensitive potassium channels or two-pore-domain potassium channels. Furthermore, this inhibition does not require pertussis toxin-sensitive G proteins, and is independent of any effect on adenylyl cyclases, protein kinase A, mitogen-activated protein kinases or phosphoinositol-3 kinase activity. Interestingly we found that mGluR4 inhibition of presynaptic Ca2+ influx employs a newly defined signalling pathway, notably that involving the activation of phospholipase C and ultimately protein kinase C.

Effects of clozapine and N-desmethylclozapine on synaptic transmission at hippocampal inhibitory and excitatory synapses

Clozapine is the first atypical antipsychotic, and improves positive and negative symptoms of many patients with schizophrenia resistant to treatment with other antipsychotic agents. Clozapine induces minimal extrapyramidal side effects, but is more often associated with seizures. A large number of studies have been conducted to elucidate pharmacological profiles of clozapine and its major active metabolite, N-desmethylclozapine (NDMC). However, there are only a limited number of electrophysiological studies examining their effects on synaptic transmission. In this study, we examined effects of clozapine and NDMC on synaptic transmission by measuring inhibitory and excitatory postsynaptic currents in rat cultured hippocampal neurons. We found that clozapine and NDMC have qualitatively similar actions. They depressed the inhibitory transmission at 1–30μM, and the excitatory transmission at 30μM, the former being much more sensitive. The depression of IPSCs by 30μM of these drugs was associated with an increase in the paired-pulse ratio. The GABA-induced currents were suppressed by these drugs, but less sensitive than IPSCs. The AMPA-induced currents were slightly potentiated by these drugs at 30μM. At 30μM, clozapine and NDMC slightly suppressed Ca2+ and Na+ channels. These results strongly suggest that clozapine and NMDC depress the inhibitory synaptic transmission mainly by antagonizing postsynaptic GABAA receptors, but at higher concentrations additionally by acting on presynaptic site, possibly in part through inhibition of presynaptic Ca2+ and Na+ channels. Preferential depression of inhibitory synaptic transmission by clozapine and NDMC might contribute to therapeutic actions and/or side-effects of clozapine.

71-Year-Old Man With Chronic Kidney Failure and Sudden Change of Mental Status

71-year-old man was admitted to the hospital for al- tered mental status. His wife reported that the patient had become progressively confused and had been having visual hallucinations, seeing people and animals that were not really there. These symptoms started about 2 weeks before admission as intermittent episodes. She could not identify any triggering events or temporal pattern for these episodes. In the past 2 days, the patient's symptoms had become more persistent. She added that he had become in- creasingly forgetful in the past year but had had no prob- lem performing simple tasks or taking care of himself. This type of confusion had never occurred previously. The patient had a 12-year history of type 2 diabetes, had undergone coronary artery bypass 3 years previously, had symptomatic peripheral vascular insufficiency, and had stage V chronic kidney disease primarily due to diabetic nephropathy. For the past 10 months, he had been undergo- ing hemodialysis 3 times weekly for 3.5 hours each time via a left arm arteriovenous fistula. He had last undergone dialysis 3 days previously and was due for his scheduled dialysis on the day of admission. Two months before admission, the patient developed critical lower limb ischemia and a nonhealing left foot ul- cer. Multiple vascular interventions failed to restore suffi- cient limb blood flow; a month later, the patient underwent a transmetatarsal amputation. Postoperatively, he devel- oped surgical wound infection, necessitating repeated de- bridement and intravenous vancomycin administration. The patient's medications included stable doses of as- pirin, atenolol, atorvastatin, calcium acetate, insulin, gaba-

Endocannabinoids in basal ganglia circuits

Endocannabinoids are endogenous lipid messengers that bind the cannabinoid (CB) receptors. These receptors mediate the multiple behavioral effects of l -Δ9 tetrahydrocannabinol, the main psychoactive component of marijuana (cannabis). The endocannabinoid system has been implicated in a large variety of functions, including regulation of appetite and energy metabolism, pain and inflammation, neuroprotection, and modulation of basal ganglia circuits.1 Not surprisingly, the endocannabinoid system has raised great interest as a potential pharmacologic target from management of disorders such as obesity, pain, nausea, and neurologic conditions such as multiple sclerosis and movement disorders. The aim of this article is to briefly review some basic concepts on the biochemistry, pharmacology, and synaptic modulatory role of the endocannabinoid system with focus on the basal ganglia circuits, and the potential implications for Parkinson disease (PD). These subjects have been covered in detail in several reviews.1–6 The family of endocannabinoids includes at least five derivatives of arachidonic acid; the two best characterized are arachydonoyl ethanolamide (named anandamide after the Sanskrit word for bliss, ananda) and 2-arachydonoil glycerol (2AG). These endocannabinoids are synthesized in postsynaptic targets on demand in response to increase in intracellular calcium (Ca2+), triggered by either depolarization or activation of metabotropic glutamate receptors1–3 (figure 1). Figure 1 Schematic representation of endocannabinoid signaling, as illustrated in this case for anandamide Influx of calcium (Ca2+) in the postsynaptic cell activates phospholipase D (PLD), which acts on N-arachydonoyl phosphatidylethanolamine (NAPE) to produce anandamide. Anandamide is released into the synaptic cleft and acts via presynaptic cannabinoid 1 (CB1) receptors. These receptors are guanine nucleotide binding (G) protein-coupled receptors coupled to Gi/o transduction pathways, and their activation leads to inhibition of adenylate cyclase (AC), opening of presynaptic potassium (K+) channels, and inhibition of presynaptic Ca2+ channels. These effects …

Regulating Synaptic Strength

Regulator of G protein signaling (RGS) proteins stimulate the intrinsic guanosine triphosphatase activity of heterotrimeric guanine nucleotide-binding proteins (G proteins) that terminates G protein signaling. RGS2, which is abundant in brain, inhibits signaling through both G i/o and G q pathways and influences anxiety and aggression in mice. Han et al. recorded excitatory postsynaptic currents (EPSCs) from cultured hippocampal neurons making autaptic synapses and determined the relative amplitudes of EPSCs elicited 50 msec apart. Exogenous expression of RGS2 in rat or mouse neurons reduced the ratio of the amplitude of the second EPSC relative to that of the first (paired-pulse ratio, PPR), yielding paired-pulse depression. In contrast, neurons from mice lacking RGS2 ( RGS2 –/– mice) showed an increase in the relative amplitude of the second EPSC, yielding substantial paired-pulse facilitation. Pharmacological analysis indicated that the effects of RGS2 on PPR were mediated through G i/o but not G q , a conclusion substantiated by experiments involving an RGS2 mutant that suppressed G q signaling but acted as a dominant-negative in the G i/o pathway. Changes in PPR may result from differences in vesicle release probability and, indeed, release probability was reduced in RGS2 –/– mice compared with wild type. Analysis of Ca 2+ currents indicated a greater degree of basal Ca 2+ channel inhibition in RGS2 –/– neurons (likely due to increased abundance of free Gβγ). The authors conclude that RGS2 regulates Ca 2+ influx through presynaptic Ca 2+ channels, and thereby synaptic strength, by decreasing basal activity of G i/o and that regulation of RGS2 abundance could thus provide a mechanism for modulating synaptic efficacy. J. Han, M. D. Mark, X. Li, M. Xie, S. Waka, J. Rettig, S. Herlitze, RGS2 determines short-term synaptic plasticity in hippocampal neurons by regulating G i/o -mediated inhibition of presynaptic Ca 2+ channels. Neuron 51 , 575-586 (2006). [PubMed]

Inhibition of voltage-gated Ca2+ channels by antazoline.

We showed recently that imidazolines exert neuroprotection against hypoxia and NMDA toxicity in cerebellar and striatal neuronal cultures, through a voltage-dependent blockade of glutamatergic NMDA receptors. Here, we report that in striatal neuronal cultures from mouse embryos the imidazoline compound, antazoline, inhibits voltage-gated Ca2+ channels by acting at a phencyclidine-like site. This effect was fast, fully reversible, voltage-dependent and predominant on P/Q- and N-type Ca2+ channels. Taken together, these results suggest that imidazolines may elicit neuroprotective effects also by decreasing the release of glutamate through inhibition of presynaptic Ca2+ channels.

GABAB receptor-mediated modulation of presynaptic currents and excitatory transmission at a fast central synapse.

Large nerve terminals (calyces of Held) in the medial nucleus of the trapezoid body (MNTB) offer a unique opportunity to explore the modulation of presynaptic channels at a mammalian central synapse. In this study I examined gamma-aminobutyric acid-B (GABAB)-mediated presynaptic inhibition at the calyx of Held in slices of the rat auditory brain stem. The selective GABAB agonist baclofen caused a potent inhibition of synaptic transmission and presynaptic Ca2+ current. The inhibition of presynaptic Ca2+ channels was associated with a slowing of the activation kinetics of the underlying current, and the inhibition was relieved by strong depolarization. The inhibition of both synaptic transmission and presynaptic Ca2+ current was abolished by N-ethylmaleimide, a sulfhydryl alkylating agent that uncouples the G(o)/Gi class of G proteins from receptors. Baclofen does not activate a potassium conductance in the presynaptic terminal. Taken together, these results suggest that GABAB receptors inhibit synaptic transmission via G protein-mediated modulation of presynaptic Ca2+ channels at this large central synapse. Furthermore, these findings demonstrate that basic mechanisms of G protein-mediated inhibition of Ca2+ channels, proposed from recordings of neuron cell bodies, are well conserved at nerve endings in the mammalian brain.