Loss of Striatonigral GABAergic Presynaptic Inhibition Enables Motor Sensitization in Parkinsonian Mice

SummaryDegeneration of dopamine (DA) neurons in Parkinson’s disease (PD) causes hypokinesia, but DA replacement therapy can elicit exaggerated voluntary and involuntary behaviors that have been attributed to enhanced DA receptor sensitivity in striatal projection neurons. Here we reveal that in hemiparkinsonian mice, striatal D1 receptor-expressing medium spiny neurons (MSNs) directly projecting to the substantia nigra reticulata (SNr) lose tonic presynaptic inhibition by GABAB receptors. The absence of presynaptic GABAB response potentiates evoked GABA release from MSN efferents to the SNr and drives motor sensitization. This alternative mechanism of sensitization suggests a synaptic target for PD pharmacotherapy

Antagonistic properties of caged GABA compounds used for activation of GABAA receptors in neocortical pyramidal neurons

Caged photolysable compounds have served to be pivotal to neuroscientific investigations; allowing the cognizing of molecular kinetics and properties of neuronal micro-machinery such as neurotransmitter receptors. Precision in terms of temporal and spatial resolution of neurotransmitter release endowed by photolysis has multitudinal applicabilities in the realm of GABAA receptors (GABAARs), their neuronal niche and effects on neuronal and network activity. Caged compounds, in their caged form, may display certain unideal traits such as undesired interactions with the system and antagonistic activity on the target receptor. This study aims to reevaluate the GABAAR antagonistic actions of caged Rubi-GABA, which was found to antagonize these receptors at significantly lower concentrations than those reported in the literature. Furthermore, this study electrophysiologically characterizes the possible antagonistic properties of a novel quinoline-derived UV-photolysable caged GABA compound, 8 DMAQ GABA, whose activity, in its caged form appears to have a much more favorable antagonism profile compared to the widely used RuBi-GABA. To assess the antagonistic effects of these compounds on GABAAR-mediated miniature inhibitory postsynaptic currents (mIPSCs) patch-clamp recordings were carried out in the whole-cell voltage clamp configuration on cortical layer 2/3 cortical pyramidal neurons in acute neocortical slices prepared from 16-18 day-old rat rats. The results of this study indicate a revised antagonism profile for caged Rubi-GABA, with marked GABAAR toxicity in the low micromolar range. The study also scrutinizes the photo-kinetic properties of both caged GABA compounds and reveals that the rate of GABA release from 8-DMAQ is slower than from RuBi-GABA

Astrocytic GABA transporter activity modulates excitatory neurotransmission

Astrocytes are ideally placed to detect and respond to network activity. They express ionotropic and metabotropic receptors, and can release gliotransmitters. Astrocytes also express transporters that regulate the extracellular concentration of neurotransmitters. Here we report a previously unrecognized role for the astrocytic GABA transporter, GAT-3. GAT-3 activity results in a rise in astrocytic Na(+) concentrations and a consequent increase in astrocytic Ca(2+) through Na(+)/Ca(2+) exchange. This leads to the release of ATP/adenosine by astrocytes, which then diffusely inhibits neuronal glutamate release via activation of presynaptic adenosine receptors. Through this mechanism, increases in astrocytic GAT-3 activity due to GABA released from interneurons contribute to 'diffuse' heterosynaptic depression. This provides a mechanism for homeostatic regulation of excitatory transmission in the hippocampus

‘Astrocytic cradle’ controls extracellular potassium and glutamate during synaptic transmission

It is widely recognised that astrocytes are able to shape synaptic transmission by restricting glutamate transients to the synaptic cleft. In this thesis, I demonstrate that during synaptic transmission K+ efflux through postsynaptic NMDA receptors depolarises the astrocytic membrane and thus slows down glial glutamate uptake. This effect involves the rectifying K+ channels (Kir4.1), predominantly located at perisynaptic astrocytic processes (PAPs). Genetic upregulation of this channel subtype in astrocytes does not affect glutamate transporters efficiency but curtails increase in presynaptic glutamate release probability during extracellular K+ rises. Thus, activity-dependent accumulation of extracellular K+ can boost glutamate release from the presynaptic site while decreasing astroglial glutamate uptake. Both factors occasion increased extrasynaptic glutamate escape and therefore inter-synaptic crosstalk in the hippocampus

Activity-dependent regulation of GABA release at immature mossy fibers-CA3 synapses: role of the Prion protein

In adulthood, mossy fibers (MFs), the axons of granule cells of the dentate gyrus (DG), release glutamate onto CA3 principal cells and interneurons. In contrast, during the first week of postnatal life MFs release -aminobutyric acid (GABA), which, at this early developmental stage exerts a depolarizing and excitatory action on targeted cells. The depolarizing action of GABA opens voltage-dependent calcium channels and NMDA receptors leading to calcium entry and activation of intracellular signaling pathways involved in several developmental processes, thus contributing to the refinement of neuronal connections and to the establishment of adult neuronal circuits. The release of GABA has been shown to be down regulated by several neurotransmitter receptors which would limit the enhanced excitability caused by the excitatory action of GABA. It is worth noting that the immature hippocampus exhibits spontaneous correlated activity, the so called giant depolarizing potentials or GDPs that act as coincident detector signals for enhancing synaptic activity, thus contributing to several developmental processes including synaptogenesis. GDPs render the immature hippocampus more prone to seizures. Here, I explored the molecular mechanisms underlying synaptic transmission and activity-dependent synaptic plasticity processes at immature GABAergic MF-CA3 synapses in wild-type rodents and in mice lacking the prion protein (Prnp0/0 mice). In the first paper, I studied the functional role of kainate receptors (KARs) in regulating GABA release from MF terminals. Presynaptic KARs regulate synaptic transmission in several brain areas and play a central role in modulating glutamate release at adult MF-CA3 synapses. I found that functional presynaptic GluK1 receptors are present on MF terminals where they down regulate GABA release. Thus, application of DNQX or UBP 302, a selective antagonist for GluK1 receptors, strongly increased the amplitude of MF-GABAA-mediated postsynaptic currents (GPSCs). This effect was associated with a decrease in failure rate and increase in PPR, indicating a presynaptic type of action. GluK1 receptors were found to be tonically activated by glutamate present in the extracellular space, since decreasing the extracellular concentration of glutamate with a glutamate scavenger system prevented their activation and mimicked the effects of KAR antagonists. The depressant effect of GluK1 on GABA release was dependent on pertussis toxin (PTx)-sensitive G protein-coupled kainate receptors since it was prevented when hippocampal slices were incubated in the presence of a solution containing PTx. This effect was presynaptic since application of UBP 302 to cells patched with an intracellular solution containing GDP S still potentiated synaptic responses. In addition, the depressant effect of GluK1 on GABA release was prevented by U73122, which selectively inhibits phospholipase C, downstream to G protein activation. Interestingly, U73122, enhanced the probability of GABA release, thus unveiling the ionotropic type of action of kainate receptors. In line with this, we found that GluK1 receptors enhanced MF excitability by directly depolarizing MF terminals via calcium-permeable cation channels. We also explored the possible involvement of GluK1 in spike time-dependent (STD) plasticity and we found that GluK1 dynamically regulate the direction of STD-plasticity, since the pharmacological block of this receptor shifted spike-time dependent potentiation into depression. The mechanisms underlying STD-LTD at immature MF-CA3 synapses have been investigated in detail in the second paper. STD-plasticity is a Hebbian form of learning which consists in bi-directional modifications of synaptic strength according to the temporal order of pre and postsynaptic spiking. Interestingly, we found that at immature mossy fibers (MF)-CA3 synapses, STD-LTD occurs regardless of the temporal order of stimulation (pre versus post or viceversa). However, as already mentioned, while STD-LTD induced by positive pairing (pre before post) could be shifted into STD-LTP after blocking presynaptic GluK1 receptors, STD-LTD induced by negative pairing (post before pre) relied on the activation of CB1 receptors. At P3 but not at P21, endocannabinoids released by the postsynaptic cell during spiking-induced membrane depolarization retrogradely activated CB1 receptors, probably expressed on MF terminals and persistently depressed GABA release in the rat hippocampus. Thus, bath application of selective CB1 receptor antagonists prevented STD-LTD. Pharmacological tools allow identifying anandamide as the endogenous ligand responsible of activity-dependent depressant effect. To further assess whether STD-LTD is dependent on the activation of CB1 receptors, similar experiments were performed on WT-littermates and CB1-KO mice. While in WT mice the pairing protocol produced a persistent depression of MF-GPSCs as in rats, in CB1-KO mice failed to induce LTD. Consistent with these data, in situ hybridization experiments revealed detectable levels of CB1 mRNA in the granule cell layer of P3 but not of P21mice. These experiments strongly suggest that at immature MF-CA3 synapses STD-LTD is mediated by CB1 receptors, probably transiently expressed, during a critical time window, on MF terminals. In the third paper, I studied synaptic transmission and activity dependent synaptic plasticity at immature MF-CA3 synapses in mice devoid of the prion protein (Prnp0/0). The prion protein (PrPC) is a conserved glycoprotein widely expressed in the brain and involved in several neuronal processes including neurotransmission. If converted to a conformationally altered form, PrPSc can cause neurodegenerative diseases, such as Creutzfeldt-Jakob disease in humans. Previous studies aimed at characterizing Prnp0/0 mice have revealed only mild behavioral changes, including an impaired spatial learning, accompanied by electrophysiological and biochemical alterations. Interestingly, PrPC is developmentally regulated and in the hippocampus its expression parallels the maturation of MF. Here, we tested the hypothesis that at immature (P3-P7) MF-CA3 synapses, PrPC interferes with synaptic plasticity processes. To this aim, the rising phase of Giant Depolarizing Potentials (GDPs), a hallmark of developmental networks, was used to stimulate granule cells in the dentate gyrus in such a way that GDPs were coincident with afferent inputs. In WT animals, the pairing procedure induced a persistent increase in amplitude of MF-GPSCs. In contrast, in Prnp0/0 mice, the same protocol produced a long-term depression (LTD). LTP was postsynaptic in origin and required the activation of cAMP-dependent PKA signaling while LTD was presynaptic and was reliant on G protein-coupled GluK1 receptor and protein lipase C downstream to G protein activation. In addition, at emerging CA3-CA1 synapses of PrPC-deficient mice, stimulation of Schaffer collateral failed to induce LTP, known to be PKA-dependent. Finally, we also found that LTD in Prnp0/0 mice was mediated by GluK1 receptors, since UBP 302 blocked its induction. These data suggest that in the immature hippocampus PrPC controls the direction of synaptic plasticity

Extrasynaptic signalling and plasticity mediated by N-Methyl-D-aspartate receptors

Synaptic N-Methyl-D-aspartate receptors (NMDARs) are crucial for neural coding and plasticity. However, little is known about the adaptive function of extrasynaptic NMDARs located on the dendritic shaft. Here we find that in CA1 pyramidal neurons backpropagating action potentials (bAPs) recruit shaft NMDARs exposed to ambient glutamate of non-vesicular origin. In contrast, spine NMDARs are "protected" under baseline conditions from such glutamate by perisynaptic transporters: bAP-evoked Ca2+ entry through these receptors can be detected upon synaptic glutamate release or local glutamate uncaging. During theta-burst firing, NMDAR-dependent Ca2+ entry either upregulates or downregulates an h-channel conductance (Gh) of the cell depending on whether synaptic glutamate release is intact or blocked. Gh plasticity in turn regulates dendritic input probed by local glutamate uncaging. Thus, the balance between activation of synaptic and extrasynaptic NMDARs can determine the sign of Gh-dependent plasticity. These results uncover a novel meta-plasticity mechanism potentially important for neural coding and memory formation

Presynaptic NMDA Receptors Mediate IPSC Potentiation at GABAergic Synapses in Developing Rat Neocortex

NMDA receptors are traditionally viewed as being located postsynaptically, at both synaptic and extrasynaptic locations. However, both anatomical and physiological studies have indicated the presence of NMDA receptors located presynaptically. Physiological studies of presynaptic NMDA receptors on neocortical GABAergic terminals and their possible role in synaptic plasticity are lacking.We report here that presynaptic NMDA receptors are present on GABAergic terminals in developing (postnatal day (PND) 12-15) but not older (PND21-25) rat frontal cortex. Using MK-801 in the recording pipette to block postsynaptic NMDA receptors, evoked and miniature IPSCs were recorded in layer II/III pyramidal cells in the presence of AMPA/KA receptor antagonists. Bath application of NMDA or NMDA receptor antagonists produced increases and decreases in mIPSC frequency, respectively. Physiologically patterned stimulation (10 bursts of 10 stimuli at 25 Hz delivered at 1.25 Hz) induced potentiation at inhibitory synapses in PND12-15 animals. This consisted of an initial rapid, large increase in IPSC amplitude followed by a significant but smaller persistent increase. Similar changes were not observed in PND21-25 animals. When 20 mM BAPTA was included in the recording pipette, potentiation was still observed in the PND12-15 group indicating that postsynaptic increases in calcium were not required. Potentiation was not observed when patterned stimulation was given in the presence of D-APV or the NR2B subunit antagonist Ro25-6981.The present results indicate that presynaptic NMDA receptors modulate GABA release onto neocortical pyramidal cells. Presynaptic NR2B subunit containing NMDA receptors are also involved in potentiation at developing GABAergic synapses in rat frontal cortex. Modulation of inhibitory GABAergic synapses by presynaptic NMDA receptors may be important for proper functioning of local cortical networks during development

Cannabinoid receptors contribute to astroglial Ca2+-signalling and control of synaptic plasticity in the neocortex

Communication between neuronal and glial cells is thought to be very important for many brain functions. Acting via release of gliotransmitters, astrocytes can modulate synaptic strength. The mechanisms underlying ATP release from astrocytes remain uncertain with exocytosis being the most intriguing and debated pathway. We have demonstrated that ATP and D-serine can be released from cortical astrocytes in situ by a SNARE-complex-dependent mechanism. Exocytosis of ATP from astrocytes can activate post-synaptic P2X receptors in the adjacent neurons, causing a downregulation of synaptic and extrasynaptic GABA receptors in cortical pyramidal neurons. We showed that release of gliotransmitters is important for the NMDA receptor-dependent synaptic plasticity in the neocortex. Firstly, induction of long-term potentiation (LTP) by five episodes of theta-burst stimulation (TBS) was impaired in the neocortex of dominant-negative (dn)-SNARE mice. The LTP was rescued in the dn-SNARE mice by application of exogenous non-hydrolysable ATP analogues. Secondly, we observed that weak sub-threshold stimulation (two TBS episodes) became able to induce LTP when astrocytes were additionally activated via CB-1 receptors. This facilitation was dependent on activity of ATP receptors and was abolished in the dn-SNARE mice. Our results strongly support the physiological relevance of glial exocytosis for glia–neuron communications and brain function