Presynaptic inhibition in the hippocampus

Bipolar mania and epilepsy pathophysiology and treatment may converge in purine metabolism: A new perspective on available evidence

Decreased ATPergic signaling is an increasingly recognized pathophysiology in bipolar mania disease models. In parallel, adenosine deficit is increasingly recognized in epilepsy pathophysiology. Under-recognized ATP and/or adenosine-increasing mechanisms of several antimanic and antiseizure therapies including lithium, valproate, carbamazepine, and ECT suggest a fundamental pathogenic role of adenosine deficit in bipolar mania to match the established role of adenosine deficit in epilepsy. The depletion of adenosine-derivatives within the purine cycle is expected to result in a compensatory increase in oxopurines (uric acid precursors) and secondarily increased uric acid, observed in both bipolar mania and epilepsy. Cortisol-based inhibition of purine conversion to adenosine-derivatives may be reflected in observed uric acid increases and the well-established contribution of cortisol to both bipolar mania and epilepsy pathology. Cortisol-inhibited conversion from IMP to AMP as precursor of both ATP and adenosine may represent a mechanism for treatment resistance common in both bipolar mania and epilepsy. Anti-cortisol therapies may therefore augment other treatments both in bipolar mania and epilepsy. Evidence linking (i) adenosine deficit with a decreased need for sleep, (ii) IMP/cGMP excess with compulsive hypersexuality, and (iii) guanosine excess with grandiose delusions may converge to suggest a novel theory of bipolar mania as a condition characterized by disrupted purine metabolism. The potential for disease-modification and prevention related to adenosine-mediated epigenetic changes in epilepsy may be mirrored in mania. Evaluating the purinergic effects of existing agents and validating purine dysregulation may improve diagnosis and treatment in bipolar mania and epilepsy and provide specific targets for drug development.

Epilepsy in a mouse model of GNB1 encephalopathy arises from altered potassium (GIRK) channel signaling and is alleviated by a GIRK inhibitor

De novo mutations in GNB1, encoding the Gβ1 subunit of G proteins, cause a neurodevelopmental disorder with global developmental delay and epilepsy, GNB1 encephalopathy. Here, we show that mice carrying a pathogenic mutation, K78R, recapitulate aspects of the disorder, including developmental delay and generalized seizures. Cultured mutant cortical neurons also display aberrant bursting activity on multi-electrode arrays. Strikingly, the antiepileptic drug ethosuximide (ETX) restores normal neuronal network behavior in vitro and suppresses spike-and-wave discharges (SWD) in vivo. ETX is a known blocker of T-type voltage-gated Ca2+ channels and G protein-coupled potassium (GIRK) channels. Accordingly, we present evidence that K78R results in a gain-of-function (GoF) effect by increasing the activation of GIRK channels in cultured neurons and a heterologous model (Xenopus oocytes)-an effect we show can be potently inhibited by ETX. This work implicates a GoF mechanism for GIRK channels in epilepsy, identifies a new mechanism of action for ETX in preventing seizures, and establishes this mouse model as a pre-clinical tool for translational research with predicative value for GNB1 encephalopathy.

Neuroprotective Role of the B Vitamins in the Modulation of the Central Glutamatergic Neurotransmission

BACKGROUND

Regulation of glutamate release is crucial for maintaining normal brain function, but excess glutamate release is implicated in many neuropathological conditions. Therefore, the minimum glutamate release from presynaptic nerve terminals is an important neuroprotective mechanism.

OBJECTIVE

In this mini-review, we analyze the three B vitamins, namely vitamin B2 (riboflavin), vitamin B6 (pyridoxine), and vitamin B12 (cyanocobalamin), that affect the 4-aminopyridine (4- AP)-evoked glutamate release from presynaptic nerve terminal in rat and discuss their neuroprotective role.

METHODS

In this study, the measurements include glutamate release, DiSC3(5), and Fura-2.

RESULTS

The riboflavin, pyridoxine, and cyanocobalamin produced significant inhibitory effects on 4-aminopyridine-evoked glutamate release from rat cerebrocortical nerve terminals (synaptosomes) in a dose-dependent relationship. These presynaptic inhibitory actions of glutamate release are attributed to inhibition of physiologic Ca2+-dependent vesicular exocytosis but not Ca2+-independent nonvesicular release. These effects also did not affect membrane excitability, while diminished cytosolic (Ca2+)c through a reduction of direct Ca2+ influx via Cav2.2 (N-type) and Cav2.1 (P/Q-type) Ca2+ channels, rather than through indirect Ca2+induced Ca2+ release from ryanodine-sensitive intracellular stores. Furthermore, their effects were attenuated by GF109203X and Ro318220, two protein kinase C (PKC) inhibitors, suggesting suppression of PKC activity. Taken together, these results suggest that riboflavin, pyridoxine, and cyanocobalamin inhibit presynaptic vesicular glutamate release from rat cerebrocortical synaptosomes, through the depression Ca2+ influx via voltage- dependent Cav2.2 (N-type) and Cav2.1 (P/Q-type) Ca2+ channels, and PKC signaling cascade.

CONCLUSION

Therefore, these B vitamins may reduce the strength of glutamatergic synaptic transmission and is of considerable importance as potential targets for therapeutic agents in glutamate- induced excitation-related diseases.

Conditional knockout of CRMP2 in neurons, but not astrocytes, disrupts spinal nociceptive neurotransmission to control the initiation and maintenance of chronic neuropathic pain

ABSTRACT

Mechanistic studies principally focusing on primary afferent nociceptive neurons uncovered the upregulation of collapsin response mediator protein 2 (CRMP2)-a dual trafficking regulator of N-type voltage-gated calcium (Cav2.2) as well as Nav1.7 voltage-gated sodium channels-as a potential determinant of neuropathic pain. Whether CRMP2 contributes to aberrant excitatory synaptic transmission underlying neuropathic pain processing after peripheral nerve injury is unknown. Here, we interrogated CRMP2's role in synaptic transmission and in the initiation or maintenance of chronic pain. In rats, short-interfering RNA-mediated knockdown of CRMP2 in the spinal cord reduced the frequency and amplitude of spontaneous excitatory postsynaptic currents, but not spontaneous inhibitory postsynaptic currents, recorded from superficial dorsal horn neurons in acute spinal cord slices. No effect was observed on miniature excitatory postsynaptic currents and inhibitory postsynaptic currents. In a complementary targeted approach, conditional knockout of CRMP2 from mouse neurons using a calcium/calmodulin-dependent protein kinase II alpha promoter to drive Cre recombinase expression reduced the frequency and amplitude of spontaneous excitatory postsynaptic currents, but not miniature excitatory SCss. Conditional knockout of CRMP2 from mouse astrocytes using a glial fibrillary acidic protein promoter had no effect on synaptic transmission. Conditional knockout of CRMP2 in neurons reversed established mechanical allodynia induced by a spared nerve injury in both male and female mice. In addition, the development of spared nerve injury-induced allodynia was also prevented in these mice. Our data strongly suggest that CRMP2 is a key regulator of glutamatergic neurotransmission driving pain signaling and that it contributes to the transition of physiological pain into pathological pain.

Presynaptic inhibition rapidly stabilises recurrent excitation in the face of plasticity

Hebbian plasticity, a mechanism believed to be the substrate of learning and memory, detects and further enhances correlated neural activity. Because this constitutes an unstable positive feedback loop, it requires additional homeostatic control. Computational work suggests that in recurrent networks, the homeostatic mechanisms observed in experiments are too slow to compensate instabilities arising from Hebbian plasticity and need to be complemented by rapid compensatory processes. We suggest presynaptic inhibition as a candidate that rapidly provides stability by compensating recurrent excitation induced by Hebbian changes. Presynaptic inhibition is mediated by presynaptic GABA receptors that effectively and reversibly attenuate transmitter release. Activation of these receptors can be triggered by excess network activity, hence providing a stabilising negative feedback loop that weakens recurrent interactions on sub-second timescales. We study the stabilising effect of presynaptic inhibition in recurrent networks, in which presynaptic inhibition is implemented as a multiplicative reduction of recurrent synaptic weights in response to increasing inhibitory activity. We show that networks with presynaptic inhibition display a gradual increase of firing rates with growing excitatory weights, in contrast to traditional excitatory-inhibitory networks. This alleviates the positive feedback loop between Hebbian plasticity and network activity and thereby allows homeostasis to act on timescales similar to those observed in experiments. Our results generalise to spiking networks with a biophysically more detailed implementation of the presynaptic inhibition mechanism. In conclusion, presynaptic inhibition provides a powerful compensatory mechanism that rapidly reduces effective recurrent interactions and thereby stabilises Hebbian learning.

Synapses between neurons change during learning and memory formation, a process termed synaptic plasticity. Established models of plasticity rely on strengthening synapses of co-active neurons. In recurrent networks, mutually connected neurons tend to be co-active. The emerging positive feedback loop is believed to be counteracted by homeostatic mechanisms that aim to keep neural activity at a given set point. However, theoretical work indicates that experimentally observed forms of homeostasis are too slow to maintain stable network activity. In this article, we suggest that presynaptic inhibition can alleviate this problem. Presynaptic inhibition is an inhibitory mechanism that weakens synapses rather than suppressing neural activity. Using mathematical analyses and computer simulations, we show that presynaptic inhibition can compensate the strengthening of recurrent connections and thus stabilises neural networks subject to synaptic plasticity, even if homeostasis acts on biologically plausible timescales.

Dorsal-Ventral Differences in Modulation of Synaptic Transmission in the Hippocampus

Functional diversification along the longitudinal axis of the hippocampus is a rapidly growing concept. Modulation of synaptic transmission by neurotransmitter receptors may importantly contribute to specialization of local intrinsic network function along the hippocampus. In the present study, using transverse slices from the dorsal and the ventral hippocampus of adult rats and recordings of evoked field postsynaptic excitatory potentials (fEPSPs) from the CA1 stratum radiatum, we aimed to compare modulation of synaptic transmission between the dorsal and the ventral hippocampus. We found that transient heterosynaptic depression (tHSD, <2 s), a physiologically relevant phenomenon of regulation of excitatory synaptic transmission induced by paired stimulation of two independent inputs to stratum radiatum of CA1 field, has an increased magnitude and duration in the ventral hippocampus, presumably contributing to increased input segregation in this segment of the hippocampus. GABA_B receptors, GABA_A receptors, adenosine A1 receptors and L-type voltage-gated calcium channels appear to contribute differently to tHSD in the two hippocampal segments; GABA_BRs play a predominant role in the ventral hippocampus while both GABA_BRs and A1Rs play important roles in the dorsal hippocampus. Activation of GABA_B receptors by an exogenous agonist, baclofen, robustly and reversibly modulated both the initial fast and the late slow components of excitatory synaptic transmission, expressed by the fEPSPslope and fEPSP decay time constant (fEPSP_τ), respectively. Specifically, baclofen suppressed fEPSP slope more in the ventral than in the dorsal hippocampus and enhanced fEPSP_τ more in the dorsal than in the ventral hippocampus. Also, baclofen enhanced paired-pulse facilitation in the two hippocampal segments similarly. Blockade of GABA_B receptors did not affect basal paired-pulse facilitation in either hippocampal segment. We propose that the revealed dorsal-ventral differences in modulation of synaptic transmission may provide a means for specialization of information processing in the local neuronal circuits, thereby significantly contributing to diversifying neuronal network functioning along the dorsal-ventral axis of hippocampus.

Low-frequency Stimulation Decreases Hyperexcitability Through Adenosine A1 Receptors in the Hippocampus of Kindled Rats

INTRODUCTION

In this study, the role of A1 adenosine receptors in improving the effect of Low-Frequency Electrical Stimulation (LFS) on seizure-induced hyperexcitability of hippocampal CA1 pyramidal neurons was investigated.

METHODS

A semi-rapid hippocampal kindling model was used to induce seizures in male Wistar rats. Examination of the electrophysiological properties of CA1 pyramidal neurons of the hippocampus using whole-cell patch-clamp recording 48 h after the last kindling stimulation revealed that the application of LFS as two packages of stimulations at a time interval of 6 h for two consecutive days could significantly restore the excitability CA1 pyramidal neurons evidenced by a decreased in the of the number of evoked action potentials and enhancement of amplitude, maximum rise slope and decay slope of the first evoked action potential, rheobase, utilization time, adaptation index, first-spike latency, and post-AHP amplitude. Selective locked of A1 receptors by the administration of 8-Cyclopentyl-1,3-dimethylxanthine (1 μM, 1 μl, i.c.v.) before applying each LFS package, significantly reduced LFS effectiveness in recovering these parameters.

RESULTS

On the other hand, selective activation of A1 receptors by an injection of N6-cyclohexyladenosine (10 μM, 1 μl, i.c.v.), instead of LFS application, could imitate LFS function in improving these parameters.

CONCLUSION

It is suggested that LFS exerts its efficacy on reducing the neuronal excitability, partially by activating the adenosine system and activating its A1 receptors.

Inflammatory mediators of opioid tolerance: Implications for dependency and addiction

Each year, over 50 million Americans suffer from persistent pain, including debilitating headaches, joint pain, and severe back pain. Although morphine is amongst the most effective analgesics available for the management of severe pain, prolonged morphine treatment results in decreased analgesic efficacy (i.e., tolerance). Despite significant headway in the field, the mechanisms underlying the development of morphine tolerance are not well understood. The midbrain ventrolateral periaqueductal gray (vlPAG) is a primary neural substrate for the analgesic effects of morphine, as well as for the development of morphine tolerance. A growing body of literature indicates that activated glia (i.e., microglia and astrocytes) facilitate pain transmission and oppose morphine analgesia, making these cells important potential targets in the treatment of chronic pain. Morphine affects glia by binding to the innate immune receptor toll-like receptor 4 (TLR4), leading to the release of proinflammatory cytokines and opposition of morphine analgesia. Despite the established role of the vlPAG as an integral locus for the development of morphine tolerance, most studies have examined the role of glia activation within the spinal cord. Additionally, the role of TLR4 in the development of tolerance has not been elucidated. This review attempts to summarize what is known regarding the role of vlPAG glia and TLR4 in the development of morphine tolerance. These data, together, provide information about the mechanism by which central nervous system glia regulate morphine tolerance, and identify a potential therapeutic target for the enhancement of analgesic efficacy in the clinical treatment of chronic pain.

Inactivation of GIRK channels weakens the pre- and postsynaptic inhibitory activity in dorsal raphe neurons

The serotonergic tone of the dorsal raphe (DR) is regulated by 5-HT_1A receptors, which negatively control serotonergic activity via the activation of G protein-coupled inwardly rectifying K⁺ (GIRK) channels. In addition, DR activity is modulated by local GABAergic transmission, which is believed to play a key role in the development of mood-related disorders. Here, we sought to characterize the role of GIRK2 subunit-containing channels on the basal electrophysiological properties of DR neurons and to investigate whether the presynaptic and postsynaptic activities of 5-HT_1A, GABA_B, and GABA_A receptors are affected by Girk2 gene deletion. Whole-cell patch-clamp recordings in brain slices from GIRK2 knockout mice revealed that the GIRK2 subunit contributes to maintenance of the resting membrane potential and to the membrane input resistance of DR neurons. 5-HT_1A and GABA_B receptor-mediated postsynaptic currents were almost absent in the mutant mice. Spontaneous and evoked GABA_A receptor-mediated transmissions were markedly reduced in GIRK2 KO mice, as the frequency and amplitude of spontaneous IPSCs were reduced, the paired-pulse ratio was increased and GABA-induced whole-cell currents were decreased. Similarly, the pharmacological blockade of GIRK channels with tertiapin-Q prevented the 5-HT_1A and GABA_B receptor-mediated postsynaptic currents and increased the paired-pulse ratio. Finally, deletion of the Girk2 gene also limited the presynaptic inhibition of GABA release exerted by 5-HT_1A and GABA_B receptors. These results indicate that the properties and inhibitory activity of DR neurons are highly regulated by GIRK2 subunit-containing channels, introducing GIRK channels as potential candidates for studying the pathophysiology and treatment of affective disorders.

Prefrontal Cortical GABAergic Dysfunction Contributes to Aberrant UP-State Duration in APP Knockout Mice

Genetic and biochemical studies have focused on the role of amyloid β protein in the pathogenesis of Alzheimer's disease. In comparison, the physiological roles of its precursor protein, amyloid precursor protein (APP), in synaptic and network activity is less well studied. Using an APP knockout (APP-/-) mouse model, we show that the duration of UP state, which is a key feature of cortical synaptic integration occurring predominantly during slow-wave sleep, is significantly increased in the prefrontal cortex (PFC) in the absence of APP. This was accompanied by a specific reduction in the glutamine synthetase and tissue GABA content and sequential upregulation in the levels of GABABR expression. Pharmacological reinforcement of GABA signaling by application of either a GABA uptake inhibitor or an agonist of GABABR rescued the abnormality of UP-state duration and the former rescues altered GABABR expression as well. In addition to revealing an essential role of APP in the regulation of PFC network function, this study evidences the viability of GABA signaling pathway and its receptors, especially GABABRs, as a target for the treatment of aberrant neural network activity and thus information processing.

Presynaptic localization of GluK5 in rod photoreceptors suggests a novel function of high affinity glutamate receptors in the mammalian retina

Kainate receptors mediate glutamatergic signaling through both pre- and presynaptic receptors. Here, we studied the expression of the high affinity kainate receptor GluK5 in the mouse retina. Double-immunofluoresence labeling and electron microscopic analysis revealed a presynaptic localization of GluK5 in the outer plexiform layer. Unexpectedly, we found GluK5 almost exclusively localized to the presynaptic ribbon of photoreceptor terminals. Moreover, in GluK5-deficient mutant mice the structural integrity of synaptic ribbons was severely altered pointing to a novel function of GluK5 in organizing synaptic ribbons in the presynaptic terminals of rod photoreceptors.

Mechanisms of Neuronal Silencing After Cortical Spreading Depression

Cortical spreading depression (CSD) is associated with migraine, stroke, and traumatic brain injury, but its mechanisms remain poorly understood. One of the major features of CSD is an hour-long silencing of neuronal activity. Though this silencing has clear ramifications for CSD-associated disease, it has not been fully explained. We used in vivo whole-cell recordings to examine the effects of CSD on layer 2/3 pyramidal neurons in mouse somatosensory cortex and used in vitro recordings to examine their mechanism. We found that CSD caused a reduction in spontaneous synaptic activity and action potential (AP) firing that lasted over an hour. Both pre- and postsynaptic mechanisms contributed to this silencing. Reductions in frequency of postsynaptic potentials were due to a reduction in presynaptic transmitter release probability as well as reduced AP activity. Decreases in postsynaptic potential amplitude were due to an inhibitory shift in the ratio of excitatory and inhibitory postsynaptic currents. This inhibitory shift in turn contributed to the reduced frequency of APs. Thus, distinct but complementary mechanisms generate the long neuronal silence that follows CSD. These cellular changes could contribute to wider network dysfunction in CSD-associated disease, while the pre- and postsynaptic mechanisms offer separate targets for therapy.

Modulation of excitation on parvalbumin interneurons by neuroligin-3 regulates the hippocampal network

Hippocampal network activity is generated by a complex interplay between excitatory pyramidal cells and inhibitory interneurons. Although much is known about the molecular properties of excitatory synapses on pyramidal cells, comparatively little is known about excitatory synapses on interneurons. Here we show that conditional deletion of the postsynaptic cell adhesion molecule neuroligin-3 in parvalbumin interneurons causes a decrease in NMDA-receptor-mediated postsynaptic currents and an increase in presynaptic glutamate release probability by selectively impairing the inhibition of glutamate release by presynaptic Group III metabotropic glutamate receptors. As a result, the neuroligin-3 deletion altered network activity by reducing gamma oscillations and sharp wave ripples, changes associated with a decrease in extinction of contextual fear memories. These results demonstrate that neuroligin-3 specifies the properties of excitatory synapses on parvalbumin-containing interneurons by a retrograde trans-synaptic mechanism and suggest a molecular pathway whereby neuroligin-3 mutations contribute to neuropsychiatric disorders.

Hypothesis: Hughlings Jackson and presynaptic inhibition: is there a big picture?

Presynaptic inhibition is a very powerful inhibitory mechanism and, despite many detailed studies, its purpose is still only partially understood. One accepted function is that, by reducing afferent inflow to the spinal cord and brainstem, the tonic level of presynaptic inhibition prevents sensory systems from being overloaded. A corollary of this function is that much of the incoming sensory data from peripheral receptors must be redundant, and this conclusion is reinforced by observations on patients with sensory neuropathies or congenital obstetric palsy in whom normal sensation may be preserved despite loss of sensory fibers. The modulation of incoming signals by presynaptic inhibition has a further function in operating a "gate" in the dorsal horn, thereby determining whether peripheral stimuli are likely to be perceived as painful. On the motor side, the finding that even minimal voluntary movement of a single toe is associated with widespread inhibition in the lumbosacral cord points to another function for presynaptic inhibition: to prevent reflex perturbations from interfering with motor commands. This last function, together with the normal suppression of muscle and cutaneous reflex activity at rest, is consistent with Hughlings Jackson's concept of evolving neural hierarchies, with each level inhibiting the one below it.

LRRK2 functions in synaptic vesicle endocytosis through a kinase-dependent mechanism

Mutations in leucine-rich repeat kinase 2 (LRRK2) are associated with Parkinson’s disease, but the precise physiological function of the protein remains ill-defined. Recently, our group proposed a model in which LRRK2 kinase activity is part of an EndoA phosphorylation cycle that facilitates efficient vesicle formation at synapses in the Drosophila melanogaster neuromuscular junctions.Flies harbor only one Lrrk gene, which might encompass the functions of both mammalian LRRK1 and LRRK2. We therefore studied the role of LRRK2 in mammalian synaptic function and provide evidence that knockout or pharmacological inhibition of LRRK2 results in defects in synaptic vesicle endocytosis, altered synaptic morphology and impairments in neurotransmission. In addition, our data indicate that mammalian endophilin A1 (EndoA1,also known as SH3GL2) is phosphorylated by LRRK2 in vitro at T73 and S75, two residues in the BAR domain. Hence, our results indicate that LRRK2 kinase activity has an important role in the regulation of clathrin-mediated endocytosis of synaptic vesicles and subsequent neurotransmission at the synapse.

Activation of presynaptic GABA(B(1a,2)) receptors inhibits synaptic transmission at mammalian inhibitory cholinergic olivocochlear-hair cell synapses

The synapse between olivocochlear (OC) neurons and cochlear mechanosensory hair cells is cholinergic, fast, and inhibitory. The inhibitory sign of this cholinergic synapse is accounted for by the activation of Ca(2+)-permeable postsynaptic α9α10 nicotinic receptors coupled to the opening of hyperpolarizing Ca(2+)-activated small-conductance type 2 (SK2)K(+) channels. Acetylcholine (ACh) release at this synapse is supported by both P/Q- and N-type voltage-gated calcium channels (VGCCs). Although the OC synapse is cholinergic, an abundant OC GABA innervation is present along the mammalian cochlea. The role of this neurotransmitter at the OC efferent innervation, however, is for the most part unknown. We show that GABA fails to evoke fast postsynaptic inhibitory currents in apical developing inner and outer hair cells. However, electrical stimulation of OC efferent fibers activates presynaptic GABA(B(1a,2)) receptors [GABA(B(1a,2))Rs] that downregulate the amount of ACh released at the OC-hair cell synapse, by inhibiting P/Q-type VGCCs. We confirmed the expression of GABA(B)Rs at OC terminals contacting the hair cells by coimmunostaining for GFP and synaptophysin in transgenic mice expressing GABA(B1)-GFP fusion proteins. Moreover, coimmunostaining with antibodies against the GABA synthetic enzyme glutamic acid decarboxylase and synaptophysin support the idea that GABA is directly synthesized at OC terminals contacting the hair cells during development. Thus, we demonstrate for the first time a physiological role for GABA in cochlear synaptic function. In addition, our data suggest that the GABA(B1a) isoform selectively inhibits release at efferent cholinergic synapses.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

Ferulic acid suppresses glutamate release through inhibition of voltage-dependent calcium entry in rat cerebrocortical nerve terminals

This study investigated the effects and possible mechanism of ferulic acid, a naturally occurring phenolic compound, on endogenous glutamate release in the nerve terminals of the cerebral cortex in rats. Results show that ferulic acid inhibited the release of glutamate evoked by the K⁺ channel blocker 4-aminopyridine (4-AP). The effect of ferulic acid on the evoked glutamate release was prevented by chelating the extracellular Ca²⁺ ions, but was insensitive to the glutamate transporter inhibitor DL-threo-beta-benzyl-oxyaspartate. Ferulic acid suppressed the depolarization-induced increase in a cytosolic-free Ca²⁺ concentration, but did not alter 4-AP-mediated depolarization. Furthermore, the effect of ferulic acid on evoked glutamate release was abolished by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking ryanodine receptors or mitochondrial Na⁺/Ca²⁺ exchange. These results show that ferulic acid inhibits glutamate release from cortical synaptosomes in rats through the suppression of presynaptic voltage-dependent Ca²⁺ entry.

The influence of cold temperature on cellular excitability of hippocampal networks

The hippocampus plays an important role in short term memory, learning and spatial navigation. A characteristic feature of the hippocampal region is its expression of different electrical population rhythms and activities during different brain states. Physiological fluctuations in brain temperature affect the activity patterns in hippocampus, but the underlying cellular mechanisms are poorly understood. In this work, we investigated the thermal modulation of hippocampal activity at the cellular network level. Primary cell cultures of mouse E17 hippocampus displayed robust network activation upon light cooling of the extracellular solution from baseline physiological temperatures. The activity generated was dependent on action potential firing and excitatory glutamatergic synaptic transmission. Involvement of thermosensitive channels from the transient receptor potential (TRP) family in network activation by temperature changes was ruled out, whereas pharmacological and immunochemical experiments strongly pointed towards the involvement of temperature-sensitive two-pore-domain potassium channels (K_2P), TREK/TRAAK family. In hippocampal slices we could show an increase in evoked and spontaneous synaptic activity produced by mild cooling in the physiological range that was prevented by chloroform, a K_2P channel opener. We propose that cold-induced closure of background TREK/TRAAK family channels increases the excitability of some hippocampal neurons, acting as a temperature-sensitive gate of network activation. Our findings in the hippocampus open the possibility that small temperature variations in the brain in vivo, associated with metabolism or blood flow oscillations, act as a switch mechanism of neuronal activity and determination of firing patterns through regulation of thermosensitive background potassium channel activity.

Adenosine augmentation ameliorates psychotic and cognitive endophenotypes of schizophrenia

An emerging theory of schizophrenia postulates that hypofunction of adenosine signaling may contribute to its pathophysiology. This study was designed to test the "adenosine hypothesis" of schizophrenia and to evaluate focal adenosine-based strategies for therapy. We found that augmentation of adenosine by pharmacologic inhibition of adenosine kinase (ADK), the key enzyme of adenosine clearance, exerted antipsychotic-like activity in mice. Further, overexpression of ADK in transgenic mice was associated with attentional impairments linked to schizophrenia. We observed that the striatal adenosine A2A receptor links adenosine tone and psychomotor response to amphetamine, an indicator of dopaminergic signaling. Finally, intrastriatal implants of engineered adenosine-releasing cells restored the locomotor response to amphetamine in mice overexpressing ADK, whereas the same grafts placed proximal to the hippocampus of transgenic mice reversed their working memory deficit. This functional double dissociation between striatal and hippocampal adenosine demonstrated in Adk transgenic mice highlights the independent contributions of these two interconnected brain regions in the pathophysiology of schizophrenia and thus provides the rationale for developing local adenosine augmentation therapies for the treatment of schizophrenia.

Adenosine release during seizures attenuates GABAA receptor-mediated depolarization

Seizure-induced release of the neuromodulator adenosine is a potent endogenous anticonvulsant mechanism, which limits the extension of seizures and mediates seizure arrest. For this reason several adenosine-based therapies for epilepsy are currently under development. However, it is not known how adenosine modulates GABAergic transmission in the context of seizure activity. This may be particularly relevant as strong activation of GABAergic inputs during epileptiform activity can switch GABA(A) receptor (GABA(A)R) signaling from inhibitory to excitatory, which is a process that plays a significant role in intractable epilepsies. We used gramicidin-perforated patch-clamp recordings to investigate the role of seizure-induced adenosine release in the modulation of postsynaptic GABA(A)R signaling in pyramidal neurons of rat hippocampus. Consistent with previous reports, GABA(A)R responses during seizure activity transiently switched from hyperpolarizing to depolarizing and excitatory. We found that adenosine released during the seizure significantly attenuated the depolarizing GABA(A)R responses and also reduced the extent of the after-discharge phase of the seizure. These effects were mimicked by exogenous adenosine administration and could not be explained by a change in chloride homeostasis mechanisms that set the reversal potential for GABA(A)Rs, or by a change in the conductance of GABA(A)Rs. Rather, A(1)R-dependent activation of potassium channels increased the cell's membrane conductance and thus had a shunting effect on GABA(A)R currents. As depolarizing GABA(A)R signaling has been implicated in seizure initiation and progression, the adenosine-induced attenuation of depolarizing GABA(A)R signaling may represent an important mechanism by which adenosine can limit seizure activity.

Presynaptic modulation of early olfactory processing in Drosophila

Most animals are endowed with an olfactory system that is essential for finding foods, avoiding predators, and locating mating partners. The olfactory system must encode the identity and intensity of behaviorally relevant stimuli in a dynamic environmental landscape. How is olfactory information represented? How is a large dynamic range of odor concentrations represented in the olfactory system? How is this representation modulated to meet the demands of different internal physiological states? Recent studies have found that sensory terminals are important targets for neuromodulation. The emerging evidence suggests that presynaptic inhibition scales with sensory input and thus provides a mechanism to increase dynamic range of odor representation. In addition, presynaptic facilitation could be a mechanism to alter behavioral responses in hungry animals. This review will focus on the GABA(B) (gamma-aminobutyric acid) receptor-mediated presynaptic inhibition, and neuropeptide-mediated presynaptic modulation in Drosophila.

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Curcumin inhibits glutamate release in nerve terminals from rat prefrontal cortex: possible relevance to its antidepressant mechanism

There is abundant evidence suggesting the relevance of glutamate to depression and antidepressant mechanisms. Curcumin, a major active compound of Curcuma longa, has been reported to have the biological function of antidepressant. The aim of the present study was to investigate the effect of curcumin on endogenous glutamate release in nerve terminals of rat prefrontal cortex and the underlying mechanisms. The results showed that curcumin inhibited the release of glutamate that was evoked by exposing synaptosomes to the K(+) channel blocker 4-aminopyridine (4-AP). This phenomenon was blocked by the chelating the extracellular Ca(2+) ions, and by the vesicular transporter inhibitor bafilomycin A1, but was insensitive to the glutamate transporter inhibitor DL-threo-β-benzyl-oxyaspartate (DL-TBOA). Further experiments demonstrated that curcumin decreased depolarization-induced increase in [Ca(2+)](C), whereas it did not alter the resting membrane potential or 4-AP-mediated depolarization. Furthermore, the inhibitory effect of curcumin on evoked glutamate release was prevented by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking intracellular Ca(2+) release or Na(+)/Ca(2+) exchange. These results suggest that curcumin inhibits evoked glutamate release from rat prefrontocortical synaptosomes by the suppression of presynaptic Ca(v)2.2 and Ca(v)2.1 channels. Additionally, we also found that the inhibitory effect of curcumin on 4-AP-evoked glutamate release was completely abolished by the clinically effective antidepressant fluoxetine. This suggests that curcumin and fluoxetine use a common intracellular mechanism to inhibit glutamate release from rat prefrontal cortex nerve terminals.

Luteolin inhibits the release of glutamate in rat cerebrocortical nerve terminals

The present study investigated the effect and possible mechanism of luteolin, a food-derived flavonoid, on endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes). Luteolin inhibited the release of glutamate evoked by the K(+) channel blocker 4-aminopyridine (4-AP), and this phenomenon was concentration-dependent. The effect of luteolin on the evoked glutamate release was prevented by the chelation of the extracellular Ca(2+) ions and by the vesicular transporter inhibitor, but was insensitive to the glutamate transporter inhibitor. Luteolin decreased the 4-AP-induced increase in [Ca(2+)](C), whereas it did not alter 4-AP-mediated depolarization. Furthermore, the effect of luteolin on evoked glutamate release was abolished by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking the ryanodine receptors or the mitochondrial Na(+)/Ca(2+) exchange. In addition, the inhibitory effect of luteolin on evoked glutamate release was prevented by the mitogen-activated/extracellular signal-regulated kinase (MEK) inhibitors. Western blot analyses showed that luteolin decreased the 4-AP-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and synapsin I, the main presynaptic target of ERK. Thus, it was concluded that luteolin inhibits glutamate release from rat cortical synaptosomes through the suppression of presynaptic voltage-dependent Ca(2+) entry and MEK/ERK signaling cascade.

HTDP-2, a new synthetic compound, inhibits glutamate release through reduction of voltage-dependent Ca²⁺ influx in rat cerebral cortex nerve terminals

AIM

The present study was aimed at investigating the effect of trans-6-(4-chlorobutyl)-5-hydroxy-4-(phenylthio)-1-tosyl-5,6-dihydropyridine-2(1H)-one (HTDP-2), a novel synthetic compound, on the release of endogenous glutamate in rat cerebrocortical nerve terminals (synaptosomes) and exploring the possible mechanism.

METHODS

The release of glutamate was evoked by the K⁺ channel blocker 4-aminopyridine (4-AP) and measured by an on-line enzyme-coupled fluorimetric assay. We also used a membrane potential-sensitive dye to assay nerve terminal excitability and depolarization, and a Ca²⁺ indicator, Fura-2-acetoxymethyl ester, to monitor cytosolic Ca²⁺ concentrations ([Ca²⁺](c)).

RESULTS

HTDP-2 inhibited the release of glutamate evoked by 4-AP in a concentration-dependent manner. Inhibition of glutamate release by HTDP-2 was prevented by the chelating intraterminal Ca²⁺ ions, and by the vesicular transporter inhibitor bafilomycin A1, but was insensitive to the glutamate transporter inhibitor DL-threo-β-benzyloxyaspartate. HTDP-2 did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization whereas it decreased the 4-AP-induced increase in [Ca²⁺](c). Furthermore, the inhibitory effect of HTDP-2 on the evoked glutamate release was abolished by the N-, and P/Q-type Ca²⁺ channel blocker ω-conotoxin MVIIC, but not by the ryanodine receptor blocker dantrolene, or the mitochondrial Na⁺/Ca²⁺ exchanger blocker CGP37157.

CONCLUSION

Based on these results, we suggest that, in rat cerebrocortical nerve terminals, HTDP-2 decreases voltage-dependent Ca²⁺ channel activity and, in so doing, inhibits the evoked glutamate release.

Idebenone inhibition of glutamate release from rat cerebral cortex nerve endings by suppression of voltage-dependent calcium influx and protein kinase A

The present study was aimed at investigating the effect and the possible mechanism of idebenone on endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes). Idebenone inhibited the release of glutamate that was evoked by exposing synaptosomes to the K(+) channel blocker 4-aminopyridine (4-AP), and this phenomenon was concentration dependent. Inhibition of glutamate release by idebenone was prevented by chelating extracellular Ca(2+), or by the vesicular transporter inhibitor bafilomycin A1, but was insensitive to DL-threo-beta-benzyl-oxyaspartate, a glutamate transporter inhibitor. Idebenone decreased the depolarization-induced increase in the cytosolic free Ca(2+) concentration ([Ca(2+)](C)),whereas it did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization. The inhibitory effect of idebenone on evoked glutamate release was prevented by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking intracellular Ca(2+) release or Na(+)/Ca(2+) exchange. Furthermore, the idebenone effect on 4-AP-evoked Ca(2+) influx and glutamate release was completely abolished by the protein kinase A (PKA) inhibitors, H89 and KT5720. On the basis of these results, it was concluded that idebenone inhibits glutamate release from rat cortical synaptosomes and this effect is linked to a decrease in [Ca(2+)](C) contributed by Ca(2+) entry through presynaptic voltage-dependent Ca(2+) channels and to the suppression of PKA signaling cascade.

GABA transporter GAT1: a crucial determinant of GABAB receptor activation in cortical circuits?

The GABA transporter 1 (GAT1), the main plasma membrane GABA transporter in brain tissue, mediates translocation of GABA from the extracellular to the intracellular space. Whereas GAT1-mediated uptake could generally terminate the synaptic effects of GABA, recent studies suggest a more complex physiological role. This chapter reviews evidence suggesting that in hippocampal and neocortical circuits, GAT1-mediated GABA transport regulates the electrophysiological effects of GABA(B) receptor (GABA(B)R) activation by synaptically-released GABA. Contrasting with synaptic GABA(A) receptors, GABA(B)Rs display high GABA binding affinity, slow G protein-coupled mediated signaling, and a predominantly extrasynaptic localization. Such GABA(B)R properties determine production of slow inhibitory postsynaptic potentials (IPSPs) and slow presynaptic effects. Such effects possibly require diffusion of GABA far away from the release sites, and consequently both GABA(B)R-mediated IPSPs and presynaptic effects are strongly enhanced when GAT1-mediated uptake is blocked. Studies are reviewed here which indicate that GABA(B)R-mediated IPSPs seem to be produced by dendrite-targeting GABA neurons including specifically, although perhaps not exclusively, the neurogliaform cell class. In contrast, the GABA interneuron subtypes that synapse onto the perisomatic membrane of pyramidal cells mostly signal via synaptic GABA(A)Rs. This chapter reviews data suggesting that neurogliaform cells produce electrophysiological effects onto other neurons in the cortical cell network via GABA(B)R-mediated volume transmission that is highly regulated by GAT1 activity. Therefore, the role of GAT1 in controlling GABA(B)R-mediated signaling is markedly different from its regulation of GABA(A)R-mediated fast synaptic transmission.

Pre-synaptic GABA receptors inhibit glutamate release through GIRK channels in rat cerebral cortex

Neuronal G protein-gated inwardly rectifying potassium (GIRK) channels mediate the slow inhibitory effects of many neurotransmitters post-synaptically. However, no evidence exists that supports that GIRK channels play any role in the inhibition of glutamate release by GABA(B) receptors. In this study, we show for the first time that GABA(B) receptors operate through two mechanisms in nerve terminals from the cerebral cortex. As shown previously, GABA(B) receptors reduces glutamate release and the Ca(2+) influx mediated by N-type Ca(2+) channels in a mode insensitive to the GIRK channel blocker tertiapin-Q and consistent with direct inhibition of this voltage-gated Ca(2+) channel. However, by means of weak stimulation protocols, we reveal that GABA(B) receptors also reduce glutamate release mediated by P/Q-type Ca(2+) channels, and that these responses are reversed by the GIRK channel blocker tertiapin-Q. Consistent with the functional interaction between GABA(B) receptors and GIRK channels at nerve terminals we demonstrate by immunogold electron immunohistochemistry that pre-synaptic boutons of asymmetric synapses co-express GABA(B) receptors and GIRK channels, thus suggesting that the functional interaction of these two proteins, found at the post-synaptic level, also occurs at glutamatergic nerve terminals.

HDT-1, a new synthetic compound, inhibits glutamate release in rat cerebral cortex nerve terminals (synaptosomes)

AIM

Excessive glutamate release has been proposed to be involved in the pathogenesis of several neurological diseases. In this study, we investigated the effect of HDT-1 (3, 4, 4a, 5, 8, 8a-hexahydro-6,7-dimethyl-4a-(phenylsulfonyl)- 2-tosylisoquinolin-1(2H)-one), a novel synthetic compound, on glutamate release in rat cerebrocortical nerve terminals and explored the possible mechanism.

METHODS

The release of glutamate was evoked by the K+ channel blocker 4-aminopyridine (4-AP) or the high external [K+] and measured by one-line enzyme-coupled fluorometric assay. We also determined the loci at which HDT-1 impinges on cerebrocortical nerve terminals by using membrane potentialsensitive dye to assay nerve terminal excitability and depolarization, and Ca2+ indicator Fura-2 to monitor Ca2+ influx.

RESULTS

HDT-1 inhibited the release of glutamate evoked by 4-AP and KCl in a concentration-dependent manner. HDT-1 did not alter the resting synaptosomal membrane potential or 4-APevoked depolarization. Examination of the effect of HDT-1 on cytosolic [Ca2+] revealed that the diminution of glutamate release could be attributed to reduction in voltage-dependent Ca2+ influx. Consistent with this, the HDT-1-mediated inhibition of glutamate release was significantly prevented in synaptosomes pretreated with the N- and P/Q-type Ca2+ channel blocker omega-conotoxin MVIIC.

CONCLUSION

In rat cerebrocortical nerve terminals, HDT-1 inhibits glutamate release through a reduction of voltage-dependent Ca2+ channel activity and subsequent decrease of Ca2+ influx into nerve terminals, rather than any upstream effect on nerve terminal excitability.

GABA(B) receptor modulation of feedforward inhibition through hippocampal neurogliaform cells

Feedforward inhibition of neurons is a fundamental component of information flow control in the brain. We studied the roles played by neurogliaform cells (NGFCs) of stratum lacunosum moleculare of the hippocampus in providing feedforward inhibition to CA1 pyramidal cells. We recorded from synaptically coupled pairs of anatomically identified NGFCs and CA1 pyramidal cells and found that, strikingly, a single presynaptic action potential evoked a biphasic unitary IPSC (uIPSC), consisting of two distinct components mediated by GABA(A) and GABA(B) receptors. A GABA(B) receptor-mediated unitary response has not previously been observed in hippocampal excitatory neurons. The decay of the GABA(A) receptor-mediated response was slow (time constant = 50 ms), and was tightly regulated by presynaptic GABA(B) receptors. Surprisingly, the GABA(B) receptor ligands baclofen and (2S)-3-{[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl}(phenylmethyl)phosphinic acid (CGP55845), while affecting the NGFC-mediated uIPSCs, had no effect on action potential-evoked presynaptic Ca2+ signals monitored in individual axonal boutons of NGFCs with two-photon microscopy. In contrast, baclofen clearly depressed presynaptic Ca2+ transients in non-NGF interneurons. Changes in extracellular Ca2+ concentration that mimicked the effects of baclofen or CGP55845 on uIPSCs significantly altered presynaptic Ca2+ transients. Electrophysiological data suggest that GABA(B) receptors expressed by NGFCs contribute to the dynamic control of the excitatory input to CA1 pyramidal neurons from the temporoammonic path. The NGFC-CA1 pyramidal cell connection therefore provides a unique and subtle mechanism to shape the integration time domain for signals arriving via a major excitatory input to CA1 pyramidal cells.

Corequirement of PICK1 binding and PKC phosphorylation for stable surface expression of the metabotropic glutamate receptor mGluR7

The presynaptic metabotropic glutamate receptor (mGluR) mGluR7 modulates excitatory neurotransmission by regulating neurotransmitter release and plays a critical role in certain forms of synaptic plasticity. Although the dynamic regulation of mGluR7 surface expression governs a form of metaplasticity in the hippocampus, little is known about the molecular mechanisms regulating mGluR7 trafficking. We now show that mGluR7 surface expression is stabilized by both PKC phosphorylation and by receptor binding to the PDZ domain-containing protein PICK1. Phosphorylation of mGluR7 on serine 862 (S862) inhibits CaM binding, thereby increasing mGluR7 surface expression and receptor binding to PICK1. Furthermore, in mice lacking PICK1, PKC-dependent increases in mGluR7 phosphorylation and surface expression are diminished, and mGluR7-dependent plasticity at mossy fiber-interneuron hippocampal synapses is impaired. These data support a model in which PICK1 binding and PKC phosphorylation act together to stabilize mGluR7 on the cell surface in vivo.

Noradrenaline modulates transmission at a central synapse by a presynaptic mechanism

The lateral division of the central amygdala (CeAL) is the target of ascending fibers from the pain-responsive and stress-responsive nuclei in the brainstem. We show that single fiber inputs from the nociceptive pontine parabrachial nucleus onto CeAL neurons form suprathreshold glutamatergic synapses with multiple release sites. Noradrenaline, acting at presynaptic alpha2 receptors, potently inhibits this synapse. This inhibition results from a decrease in the number of active release sites with no change in release probability. Introduction of a presynaptic scavenger of Gbetagamma subunits blocked the effects of noradrenaline, and botulinum toxin A reduced its effects, showing a direct action of betagamma subunits on the release machinery. These data illustrate a mechanism of presynaptic modulation where the output of a large multiple-release-site synapse is potently regulated by endogenously released noradrenaline and suggests that the CeA may be a target for the central nociceptive actions of noradrenaline.

Synaptic plasticity: multiple forms, functions, and mechanisms

Experiences, whether they be learning in a classroom, a stressful event, or ingestion of a psychoactive substance, impact the brain by modifying the activity and organization of specific neural circuitry. A major mechanism by which the neural activity generated by an experience modifies brain function is via modifications of synaptic transmission; that is, synaptic plasticity. Here, we review current understanding of the mechanisms of the major forms of synaptic plasticity at excitatory synapses in the mammalian brain. We also provide examples of the possible developmental and behavioral functions of synaptic plasticity and how maladaptive synaptic plasticity may contribute to neuropsychiatric disorders.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Serotonergic modulation of GABAergic and glutamatergic synaptic transmission in mechanically isolated rat medial preoptic area neurons

The medial preoptic area (MPOA) of the hypothalamus is critically involved in the regulation of male sexual behavior and has been implicated in several homeostatic processes. Serotonin (5-hydroxytryptamine, 5-HT) inhibits sexual behavior via effects in the MPOA, where there are high densities of 5-HT(1A) and 5-HT(1B) receptor subtypes. We used whole-cell recordings under voltage-clamp conditions to investigate the serotonergic modulation of gamma-aminobutyric acid (GABA)ergic and glutamatergic synaptic transmission in mechanically dissociated rat MPOA neurons with native presynaptic nerve endings. Spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in the MPOA were completely blocked by bicuculline. Serotonin reversibly reduced the GABAergic mIPSC frequency without affecting the mean current amplitude. Serotonergic inhibition of mIPSC frequency was mimicked by (+/-)-8-hydroxy-2-dipropylaminotetralin hydrobromide, a specific 5-HT(1A) receptor agonist, and blocked by 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine hydrobromide, a specific 5-HT(1A) receptor antagonist. 6-Cyano-7-nitroquinoxaline-2,3-dione completely blocked spontaneous glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in the MPOA. Serotonin reversibly decreased the glutamatergic mEPSC frequency without affecting the mean current amplitude. Serotonergic inhibition of mEPSC frequency was mimicked by CGS 12066B, a specific 5-HT(1B) receptor agonist, and blocked by SB 216641, a specific 5-HT(1B) receptor antagonist. Stimulation of adenylyl cyclase with forskolin increased the frequencies of GABAergic mIPSCs and glutamatergic mEPSCs, and blocked the inhibitory effects of 5-HT. H-89, a selective protein kinase A (PKA) inhibitor, decreased the frequencies of GABAergic mIPSCs and glutamatergic mEPSCs, and blocked their reduction by 5-HT. These findings suggest that 5-HT reduces the frequency of GABAergic mIPSCs and glutamatergic mEPSCs through 5-HT(1A) and 5-HT(1B) receptor-mediated inhibition, respectively, of the PKA-dependent pathway in the presynaptic nerve terminals of MPOA neurons.

Reduced spike-timing reliability correlates with the emergence of fast ripples in the rat epileptic hippocampus

Ripples are sharp-wave-associated field oscillations (100-300 Hz) recorded in the hippocampus during behavioral immobility and slow-wave sleep. In epileptic rats and humans, a different and faster oscillation (200-600 Hz), termed fast ripples, has been described. However, the basic mechanisms are unknown. Here, we propose that fast ripples emerge from a disorganized ripple pattern caused by unreliable firing in the epileptic hippocampus. Enhanced synaptic activity is responsible for the irregular bursting of CA3 pyramidal cells due to large membrane potential fluctuations. Lower field interactions and a reduced spike-timing reliability concur with decreased spatial synchronization and the emergence of fast ripples. Reducing synaptically driven membrane potential fluctuations improves both spike-timing reliability and spatial synchronization and restores ripples in the epileptic hippocampus. Conversely, a lower spike-timing reliability, with reduced potassium currents, is associated with ripple shuffling in normal hippocampus. Therefore, fast ripples may reflect a pathological desynchronization of the normal ripple pattern.

Adenosine modulates transmission at the hippocampal mossy fibre synapse via direct inhibition of presynaptic calcium channels

The modulation of synaptic transmission by presynaptic ionotropic and metabotropic receptors is an important means to control and dynamically adjust synaptic strength. Even though synaptic transmission and plasticity at the hippocampal mossy fibre synapse are tightly controlled by presynaptic receptors, little is known about the downstream signalling mechanisms and targets of the different receptor systems. In the present study, we identified the cellular signalling cascade by which adenosine modulates mossy fibre synaptic transmission. By means of electrophysiological and optical recording techniques, we found that adenosine activates presynaptic A1 receptors and reduces Ca2+ influx into mossy fibre terminals. Ca2+ currents are directly modulated via a membrane-delimited pathway and the reduction of presynaptic Ca2+ influx can explain the inhibition of synaptic transmission. Specifically, we found that adenosine modulates both P/Q- and N-type presynaptic voltage-dependent Ca2+ channels and thereby controls transmitter release at the mossy fibre synapse.

GABA immunoreactivity in auditory and song control brain areas of zebra finches

Inhibitory transmission is critical to sensory and motor processing and is believed to play a role in experience-dependent plasticity. The main inhibitory neurotransmitter in vertebrates, GABA, has been implicated in both sensory and motor aspects of vocalizations in songbirds. To understand the role of GABAergic mechanisms in vocal communication, GABAergic elements must be characterized fully. Hence, we investigated GABA immunohistochemistry in the zebra finch brain, emphasizing auditory areas and song control nuclei. Several nuclei of the ascending auditory pathway showed a moderate to high density of GABAergic neurons including the cochlear nuclei, nucleus laminaris, superior olivary nucleus, mesencephalic nucleus lateralis pars dorsalis, and nucleus ovoidalis. Telencephalic auditory areas, including field L subfields L1, L2a and L3, as well as the caudomedial nidopallium (NCM) and mesopallium (CMM), contained GABAergic cells at particularly high densities. Considerable GABA labeling was also seen in the shelf area of caudodorsal nidopallium, and the cup area in the arcopallium, as well as in area X, the lateral magnocellular nucleus of the anterior nidopallium, the robust nucleus of the arcopallium and nidopallial nucleus HVC. GABAergic cells were typically small, most likely local inhibitory interneurons, although large GABA-positive cells that were sparsely distributed were also identified. GABA-positive neurites and puncta were identified in most nuclei of the ascending auditory pathway and in song control nuclei. Our data are in accordance with a prominent role of GABAergic mechanisms in regulating the neural circuits involved in song perceptual processing, motor production, and vocal learning in songbirds.

Generalization of amygdala LTP and conditioned fear in the absence of presynaptic inhibition

Pavlovian fear conditioning, a simple form of associative learning, is thought to involve the induction of associative, NMDA receptor-dependent long-term potentiation (LTP) in the lateral amygdala. Using a combined genetic and electrophysiological approach, we show here that lack of a specific GABA(B) receptor subtype, GABA(B(1a,2)), unmasks a nonassociative, NMDA receptor-independent form of presynaptic LTP at cortico-amygdala afferents. Moreover, the level of presynaptic GABA(B(1a,2)) receptor activation, and hence the balance between associative and nonassociative forms of LTP, can be dynamically modulated by local inhibitory activity. At the behavioral level, genetic loss of GABA(B(1a)) results in a generalization of conditioned fear to nonconditioned stimuli. Our findings indicate that presynaptic inhibition through GABA(B(1a,2)) receptors serves as an activity-dependent constraint on the induction of homosynaptic plasticity, which may be important to prevent the generalization of conditioned fear.