Presynaptic lonotropic receptors

A phospho-deficient α3 glycine receptor mutation alters synaptic glycine and GABA release in mouse spinal dorsal horn neurons

Glycine receptors (GlyRs), together with GABAA receptors, mediate postsynaptic inhibition in most spinal cord and hindbrain neurons. In several CNS regions, GlyRs are also expressed in presynaptic terminals. Here, we analysed the effects of a phospho-deficient mutation (S346A) in GlyR α3 subunits on inhibitory synaptic transmission in superficial spinal dorsal horn neurons, where this subunit is abundantly expressed. Unexpectedly, we found that not only were the amplitudes of evoked glycinergic inhibitory postsynaptic currents (IPSCs) significantly larger in GlyRα3(S346A) mice than in mice expressing wild-type α3GlyRs (GlyRα3(WT) mice), but so were those of GABAergic IPSCs. Decreased frequencies of spontaneously occurring glycinergic and GABAergic miniature IPSCs (mIPSCs) with no accompanying change in mIPSC amplitudes suggested a change in presynaptic transmitter release. Paired-pulse experiments on glycinergic IPSCs revealed an increased paired-pulse ratio and a smaller coefficient of variation in GlyRα3(S346A) mice, which together indicate a reduction in transmitter release probability and an increase in the number of releasable vesicles. Paired-pulse ratios of GABAergic IPSCs recorded in the presence of strychnine were not different between genotypes, while the coefficient of variation was smaller in GlyRα3(S346A) mice, demonstrating that the decrease in release probability was readily reversible by GlyR blockade, while the difference in the size of the pool of releasable vesicles remained. Taken together, our results suggest that presynaptic α3 GlyRs regulate synaptic glycine and GABA release in superficial dorsal horn neurons, and that this effect is potentially regulated by their phosphorylation status. KEY POINTS: A serine-to-alanine point mutation was introduced into the glycine receptor α3 subunit of mice. This point mutation renders α3 glycine receptors resistant to protein kinase A mediated phosphorylation but has otherwise only small effects on receptor function. Patch-clamp recordings from neurons in mouse spinal cord slices revealed an unexpected increase in the amplitudes of both glycinergic and GABAergic evoked inhibitory postsynaptic currents (IPSCs). Miniature IPSCs, paired-pulse ratios and synaptic variation analyses indicate a change in synaptic glycine and GABA release. The results strongly suggest that α3 subunit-containing glycine receptors are expressed on presynaptic terminals of inhibitory dorsal horn neurons where they regulate transmitter release.

Involvement of P2Y1, P2Y6, A1 and A2A Receptors in the Purinergic Inhibition of NMDA-Evoked Noradrenaline Release in the Rat Brain Cortex

In the cerebral cortex, glutamate activates NMDA receptors (NMDARs), localized in noradrenergic neurons, inducing noradrenaline release that may have a permissive effect on glutamatergic transmission, and therefore, on the modulation of long-term plasticity. ATP is co-released with noradrenaline, and with its metabolites (ADP and adenosine) is involved in the purinergic modulation of electrically-evoked noradrenaline release. However, it is not known if noradrenaline release evoked by activation of NMDARs is also under purinergic modulation. The present study aimed to investigate and to characterize the purinergic modulation of noradrenaline release evoked by NMDARs. Stimulation of rat cortical slices with 30 µM NMDA increased noradrenaline release, which was inhibited by ATP upon metabolization into ADP and adenosine and by the selective agonists of A1 and A2A receptors, CPA and CGS2680, respectively. It was also inhibited by UTP and UDP, which are mainly released under pathophysiological situations. Characterization of the effects mediated by these compounds indicated the involvement of P2Y1, P2Y6, A1 and A2A receptors. It is concluded that, in the rat brain cortex, NMDA-evoked noradrenaline release is modulated by several purinergic receptors that may represent a relevant mechanism to regulate the permissive effect of noradrenaline on NMDA-induced neuroplasticity.

Design, Synthesis, and Pharmacological Evaluation of Novel β2/3 Subunit-Selective γ-Aminobutyric Acid Type A (GABAA) Receptor Modulators

Subunit-selective modulation of γ-aminobutyric acid type A receptors (GABA_AR) is considered to exert fewer side effects compared to unselective clinically used drugs. Here, the β2/3 subunit-selective GABA_AR modulators valerenic acid (VA) and loreclezole (LOR) guided the synthesis of novel subunit-selective ligands with simplified structures. We studied their effects on GABA_ARs expressed in Xenopus laevis oocytes using two-microelectrode voltage clamp technique. Five compounds showed significantly more efficacious modulation of GABA-evoked currents than VA and LOR with retained potency and selectivity. Compound 18 [( E)-2-Cyano-3-(2,4-dichlorophenyl)but-2-enamide] induced the highest maximal modulation of GABA-induced chloride currents ( E_max: 3114 ± 242%), while 12 [( Z)-3-(2,4-dichlorophenyl)but-2-enenitrile] displayed the highest potency (EC₅₀: 13 ± 2 μM). Furthermore, in hippocampal neurons 12 facilitated phasic and tonic GABAergic inhibition, and in vivo studies revealed significantly more potent protection against pentylenetetrazole (PTZ)-induced seizures compared to VA and LOR. Collectively, compound 12 constitutes a novel, simplified, and subunit-selective GABA_AR modulator with low-dose anticonvulsant activity.

Is There a Canonical Cortical Circuit for the Cholinergic System? Anatomical Differences Across Common Model Systems

Acetylcholine (ACh) is believed to act as a neuromodulator in cortical circuits that support cognition, specifically in processes including learning, memory consolidation, vigilance, arousal and attention. The cholinergic modulation of cortical processes is studied in many model systems including rodents, cats and primates. Further, these studies are performed in cortical areas ranging from the primary visual cortex to the prefrontal cortex and using diverse methodologies. The results of these studies have been combined into singular models of function-a practice based on an implicit assumption that the various model systems are equivalent and interchangeable. However, comparative anatomy both within and across species reveals important differences in the structure of the cholinergic system. Here, we will review anatomical data including innervation patterns, receptor expression, synthesis and release compared across species and cortical area with a focus on rodents and primates. We argue that these data suggest no canonical cortical model system exists for the cholinergic system. Further, we will argue that as a result, care must be taken both in combining data from studies across cortical areas and species, and in choosing the best model systems to improve our understanding and support of human health.

Enzymatic conversion of ATP to adenosine contributes to ATP-induced inhibition of glutamate release in rat medullary dorsal horn neurons

Purine nucleotides, such as ATP and ADP, activate ionotropic P2X and metabotropic P2Y receptors to regulate neurotransmitter release in the peripheral as well as central nervous system. Here we report another type of ATP-induced presynaptic modulation of glutamate release in rat medullary dorsal horn neurons. Glutamatergic excitatory postsynaptic currents (EPSCs) induced by electrical stimulation of trigeminal tract were recorded from horizontal brain stem slices using a whole-cell patch clamp technique. ATP decreased the amplitude of glutamatergic EPSCs in a reversible and concentration dependent manner and increased the paired-pulse ratio. In addition, ATP reduced the frequency of miniature EPSCs without affecting the current amplitude, suggesting that ATP acts presynaptically to reduce the probability of glutamate release. The ATP-induced decrease in glutamatergic EPSCs was not affected by P2X and P2Y receptor antagonists, but was completely blocked by DPCPX, a selective adenosine A1 receptor antagonist. The ATP-induced decrease in glutamatergic EPSCs was also inhibited by an inhibitor of tissue nonspecific alkaline phosphatase but not by inhibitors of other enzymes such as ecto-nucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidases. The results suggest that exogenously applied purine nucleotides are rapidly converted to adenosine by specific enzymes, and subsequently act on presynaptic A1 receptors to inhibit glutamate release from primary afferent terminals. This type of modulation mediated by purine nucleotides may play an important role in regulating nociceptive transmission from orofacial tissues.

Nicotinic α7 receptor activation selectively potentiates the function of NMDA receptors in glutamatergic terminals of the nucleus accumbens

We here provide functional and immunocytochemical evidence supporting the co-localization and functional interaction between nicotinic acetylcholine receptors (nAChRs) and N-methyl-D-aspartic acid receptors (NMDARs) in glutamatergic terminals of the nucleus accumbens (NAc). Immunocytochemical studies showed that a significant percentage of NAc terminals were glutamatergic and possessed GluN1 and α7-containing nAChR. A short-term pre-exposure of synaptosomes to nicotine (30 µM) or choline (1 mM) caused a significant potentiation of the 100 µM NMDA-evoked [³H]D-aspartate ([³H]D-Asp) outflow, which was prevented by α-bungarotoxin (100 nM). The pre-exposure to nicotine (100 µM) or choline (1 mM) also enhanced the NMDA-induced cytosolic free calcium levels, as measured by FURA-2 fluorescence imaging in individual NAc terminals, an effect also prevented by α-bungarotoxin. Pre-exposure to the α4-nAChR agonists 5IA85380 (10 nM) or RJR2429 (1 µM) did not modify NMDA-evoked ([³H]D-Asp) outflow and calcium transients. The NMDA-evoked ([³H]D-Asp) overflow was partially antagonized by the NMDAR antagonists MK801, D-AP5, 5,7-DCKA and R(-)CPP and unaffected by the GluN2B-NMDAR antagonists Ro256981 and ifenprodil. Notably, pre-treatment with choline increased GluN2A biotin-tagged proteins. In conclusion, our results show that the GluN2A-NMDA receptor function can be positively regulated in NAc terminals in response to a brief incubation with α7 but not α4 nAChRs agonists. This might be a general feature in different brain areas since a similar nAChR-mediated bolstering of NMDA-induced ([³H]D-Asp) overflow was also observed in hippocampal synaptosomes.

Presynaptic glycine receptors increase GABAergic neurotransmission in rat periaqueductal gray neurons

The periaqueductal gray (PAG) is involved in the central regulation of nociceptive transmission by affecting the descending inhibitory pathway. In the present study, we have addressed the functional role of presynaptic glycine receptors in spontaneous glutamatergic transmission. Spontaneous EPSCs (sEPSCs) were recorded in mechanically dissociated rat PAG neurons using a conventional whole-cell patch recording technique under voltage-clamp conditions. The application of glycine (100 µM) significantly increased the frequency of sEPSCs, without affecting the amplitude of sEPSCs. The glycine-induced increase in sEPSC frequency was blocked by 1 µM strychnine, a specific glycine receptor antagonist. The results suggest that glycine acts on presynaptic glycine receptors to increase the probability of glutamate release from excitatory nerve terminals. The glycine-induced increase in sEPSC frequency completely disappeared either in the presence of tetrodotoxin or Cd²⁺, voltage-gated Na⁺, or Ca²⁺ channel blockers, suggesting that the activation of presynaptic glycine receptors might depolarize excitatory nerve terminals. The present results suggest that presynaptic glycine receptors can regulate the excitability of PAG neurons by enhancing glutamatergic transmission and therefore play an important role in the regulation of various physiological functions mediated by the PAG.

GABAA receptors facilitate spontaneous glutamate release in rat periaqueductal gray neurons

The functional role of presynaptic γ-aminobutyric acid (GABA)(A) receptors in excitatory glutamatergic transmission was examined in rat periaqueductal gray neurons recorded using a conventional whole-cell patch clamp technique. Muscimol, a GABA(A) receptor agonist, significantly increased the frequency of spontaneous excitatory postsynaptic currents without affecting their amplitude, and this effect was completely blocked by the selective GABA(A) receptor antagonist. The muscimol-induced facilitation of spontaneous excitatory postsynaptic current frequency disappeared either in the presence of tetrodotoxin or Cd. The results suggest that the activation of presynaptic GABA(A) receptors directly depolarizes glutamatergic terminals resulting in the facilitation of spontaneous glutamate release, and that presynaptic GABA(A) receptors play an important role in the regulation of various physiological functions mediated by the periaqueductal gray.

Facilitation of transmitter release from rat sympathetic neurons via presynaptic P2Y(1) receptors

BACKGROUND AND PURPOSE

P2Y(1) , P2Y(2) , P2Y(4) , P2Y(12) and P2Y(13) receptors for nucleotides have been reported to mediate presynaptic inhibition, but unequivocal evidence for facilitatory presynaptic P2Y receptors is not available. The search for such receptors was the purpose of this study.

EXPERIMENTAL APPROACH

In primary cultures of rat superior cervical ganglion neurons and in PC12 cell cultures, currents were recorded via the perforated patch clamp technique, and the release of [(3) H]-noradrenaline was determined.

KEY RESULTS

ADP, 2-methylthio-ATP and ATP enhanced stimulation-evoked (3) H overflow from superior cervical ganglion neurons, treated with pertussis toxin to prevent the signalling of inhibitory G proteins. This effect was abolished by P2Y(1) antagonists and by inhibition of phospholipase C, but not by inhibition of protein kinase C or depletion of intracellular Ca(2+) stores. ADP and a specific P2Y(1) agonist caused inhibition of Kv7 channels, and this was prevented by a respective antagonist. In neurons not treated with pertussis toxin, (3) H overflow was also enhanced by a specific P2Y(1) agonist and by ADP, but only when the P2Y(12) receptors were blocked. ADP also enhanced K(+) -evoked (3) H overflow from PC12 cells treated with pertussis toxin, but only in a clone expressing recombinant P2Y(1) receptors.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that presynaptic P2Y(1) receptors mediate facilitation of transmitter release from sympathetic neurons most likely through inhibition of Kv7 channels.

Knocking out P2X receptors reduces transmitter secretion in taste buds

In response to gustatory stimulation, taste bud cells release a transmitter, ATP, that activates P2X2 and P2X3 receptors on gustatory afferent fibers. Taste behavior and gustatory neural responses are largely abolished in mice lacking P2X2 and P2X3 receptors [P2X2 and P2X3 double knock-out (DKO) mice]. The assumption has been that eliminating P2X2 and P2X3 receptors only removes postsynaptic targets but that transmitter secretion in mice is normal. Using functional imaging, ATP biosensor cells, and a cell-free assay for ATP, we tested this assumption. Surprisingly, although gustatory stimulation mobilizes Ca(2+) in taste Receptor (Type II) cells from DKO mice, as from wild-type (WT) mice, taste cells from DKO mice fail to release ATP when stimulated with tastants. ATP release could be elicited by depolarizing DKO Receptor cells with KCl, suggesting that ATP-release machinery remains functional in DKO taste buds. To explore the difference in ATP release across genotypes, we used reverse transcriptase (RT)-PCR, immunostaining, and histochemistry for key proteins underlying ATP secretion and degradation: Pannexin1, TRPM5, and NTPDase2 (ecto-ATPase) are indistinguishable between WT and DKO mice. The ultrastructure of contacts between taste cells and nerve fibers is also normal in the DKO mice. Finally, quantitative RT-PCR show that P2X4 and P2X7, potential modulators of ATP secretion, are similarly expressed in taste buds in WT and DKO taste buds. Importantly, we find that P2X2 is expressed in WT taste buds and appears to function as an autocrine, positive feedback signal to amplify taste-evoked ATP secretion.

Increased efficiency of the GABAA and GABAB receptor-mediated neurotransmission in the Ts65Dn mouse model of Down syndrome

Cognitive impairment in Down syndrome (DS) involves the hippocampus. In the Ts65Dn mouse model of DS, deficits in hippocampus-dependent learning and synaptic plasticity were linked to enhanced inhibition. However, the mechanistic basis of changes in inhibitory efficiency remains largely unexplored, and efficiency of the GABAergic synaptic neurotransmission has not yet been investigated in direct electrophysiological experiments. To investigate this important feature of neurobiology of DS, we examined synaptic and molecular properties of the GABAergic system in the dentate gyrus (DG) of adult Ts65Dn mice. Both GABAA and GABAB receptor-mediated components of evoked inhibitory postsynaptic currents (IPSCs) were significantly increased in Ts65Dn vs. control (2N) DG granule cells. These changes were unaccompanied by alterations in hippocampal levels of GABAA (α1, α2, α3, α5 and γ2) or GABAB (Gbr1a and Gbr1b) receptor subunits. Immunoreactivity for GAD65, a marker for GABAergic terminals, was also unchanged. In contrast, there was a marked change in functional parameters of GABAergic synapses. Paired stimulations showed reduced paired-pulse ratios of both GABAA and GABAB receptor-mediated IPSC components (IPSC2/IPSC1), suggesting an increase in presynaptic release of GABA. Consistent with increased gene dose, the level of the Kir3.2 subunit of potassium channels, effectors for postsynaptic GABAB receptors, was increased. This change was associated with enhanced postsynaptic GABAB/Kir3.2 signaling following application of the GABAB receptor agonist baclofen. Thus, both GABAA and GABAB receptor-mediated synaptic efficiency is increased in the Ts65Dn DG, thus likely contributing to deficient synaptic plasticity and poor learning in DS.

Presynaptic ionotropic receptors controlling and modulating the rules for spike timing-dependent plasticity

Throughout life, activity-dependent changes in neuronal connection strength enable the brain to refine neural circuits and learn based on experience. In line with predictions made by Hebb, synapse strength can be modified depending on the millisecond timing of action potential firing (STDP). The sign of synaptic plasticity depends on the spike order of presynaptic and postsynaptic neurons. Ionotropic neurotransmitter receptors, such as NMDA receptors and nicotinic acetylcholine receptors, are intimately involved in setting the rules for synaptic strengthening and weakening. In addition, timing rules for STDP within synapses are not fixed. They can be altered by activation of ionotropic receptors located at, or close to, synapses. Here, we will highlight studies that uncovered how network actions control and modulate timing rules for STDP by activating presynaptic ionotropic receptors. Furthermore, we will discuss how interaction between different types of ionotropic receptors may create "timing" windows during which particular timing rules lead to synaptic changes.

Opposite modulation of astroglial proliferation by adenosine 5'-O-(2-thio)-diphosphate and 2-methylthioadenosine-5'-diphosphate: mechanisms involved

The contribution of P2Y(12,13) receptors to astroglial proliferation was investigated by testing the effects of two agonists with high affinity for these receptors, adenosine 5'-O-(2-thio)-diphosphate (ADPβS) and 2-methylthioadenosine-5'-diphosphate (2-MeSADP), in the incorporation of [(3)H]-thymidine. The effect of ATP, an endogenous inducer of astroglial proliferation, was also investigated. ADPβS and ATP (0.01-1 mM) increased astroglial proliferation up to 282%, an effect inhibited by the P2Y(1) receptor antagonist MRS 2179 (30 μM). The P2Y(12) receptor antagonists MRS 2395 (10 μM) and AR-C 66096 (10 μM) also reduced ADPβS proliferative effect, whereas the effect of ATP was attenuated by the A(2A) and A(2B) receptor antagonists SCH 58261 (30 nM) and MRS 1706 (10 nM), respectively. Studies of the signalling pathway activated showed that ADPβS effect was attenuated by pertussis toxin and by inhibition of phopholipase C (PLC), protein kinase C (PKC) and extracellular signal-regulated kinase1/2 (ERK1/2). The effect of ATP was also attenuated by inhibition of protein kinase A (PKA). The agonist 2-MeSADP (0.001-10 μM) had no effect in astroglial proliferation, but at higher concentrations (0.1-1 mM) it inhibited up to 63%, by mechanisms independent of P2Y(1,12,13) receptors activation. It was metabolised into 2-methylthioadenosine (2-MeSADO), the metabolite responsible for inhibition of astroglial proliferation. The effect of 2-MeSADO (0.1 mM) was attenuated by the A(3) receptors antagonist MRS 1523 (10 μM) and by the inhibitor of nucleoside transporters uridine (0.3 mM). 2-MeSADO did not induce apoptosis but increased lactate dehydrogenase release, an indicator of necrotic cell death. Astroglial proliferation induced by ADPβS was mediated by P2Y(1) and P2Y(12) receptors, leading to activation of PLC-PKC-ERK1/2 signalling pathway. The ATP proliferative effect was also mediated by PKA, supporting the contribution of the A(2) receptors. 2-MeSADP inhibition of astroglial proliferation depended on its conversion into 2-MeSADO, which activated A(3) receptors, blocked [(3)H]-thymidine uptake by astrocytes and led to cell death.

ATP-gated P2X3 receptors constitute a positive autocrine signal for insulin release in the human pancreatic beta cell

Extracellular ATP has been proposed as a paracrine signal in rodent islets, but it is unclear what role ATP plays in human islets. We now show the presence of an ATP signaling pathway that enhances the human beta cell's sensitivity and responsiveness to glucose fluctuations. By using in situ hybridization, RT-PCR, immunohistochemistry, and Western blotting as well as recordings of cytoplasmic-free Ca(2+) concentration, [Ca(2+)](i), and hormone release in vitro, we show that human beta cells express ionotropic ATP receptors of the P2X(3) type and that activation of these receptors by ATP coreleased with insulin amplifies glucose-induced insulin secretion. Released ATP activates P2X(3) receptors in the beta-cell plasma membrane, resulting in increased [Ca(2+)](i) and enhanced insulin secretion. Therefore, in human islets, released ATP forms a positive autocrine feedback loop that sensitizes the beta cell's secretory machinery. This may explain how the human pancreatic beta cell can respond so effectively to relatively modest changes in glucose concentration under physiological conditions in vivo.

Tunable calcium current through TRPV1 receptor channels

TRPV1 receptors are polymodal cation channels that open in response to diverse stimuli including noxious heat, capsaicin, and protons. Because Ca2+ is vital for TRPV1 signaling, we sought to precisely measure its contribution to TRPV1 responses and discovered that the Ca2+ current was tuned by the mode of activation. Using patch clamp photometry, we found that the fraction of the total current carried by Ca2+ (called the Pf%) was significantly smaller for TRPV1 currents evoked by protons than for those evoked by capsaicin. Using site-directed mutagenesis, we discovered that the smaller Pf% was due to protonation of three acidic amino acids (Asp646, Glu648, and Glu651) that are located in the mouth of the pore. Thus, in keeping with recent reports of time-dependent changes in the ionic permeability of some ligand-gated ion channels, we now show for the first time that the physiologically important Ca2+ current of the TRPV1 receptor is also dynamic and depends on the mode of activation. This current is significantly smaller when the receptor is activated by a change in pH, owing to atomic scale interactions of H+ and Ca2+ with the fixed negative charge of side chains in the pore.