cAMP-dependent facilitation of glutamate release by beta-adrenergic receptors in cerebrocortical nerve terminals

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

Phosphodiesterase type 1 inhibition alters medial prefrontal cortical activity during goal-driven behaviour and partially reverses neurophysiological deficits in the rat phencyclidine model of schizophrenia

Positive modulation of cAMP signalling by phosphodiesterase (PDE) inhibitors has recently been explored as a potential target for the reversal of cognitive and behavioural deficits implicating the corticoaccumbal circuit. Previous studies show that PDE type 1 isoform B (PDE1B) inhibition may improve memory function in rodent models; however, the contribution of PDE1B inhibition to impulsivity, attentional and motivational functions as well as its neurophysiological effects have not been investigated. To address this, we recorded single unit activity in medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) in Lister Hooded rats treated with the PDE1B inhibitor Lu AF64386 and tested in the 5-choice serial reaction time task (5-CSRTT). We also asked whether PDE1B inhibition modulates neurophysiological deficits produced by subchronic phencyclidine (PCP) treatment, a rat pharmacological model of schizophrenia. Lu AF64386 significantly affected behavioural parameters consistent with a reduction in goal-directed behaviour, however without affecting accuracy. Additionally, it reduced mPFC neuronal activity. Pre-treatment with PCP did not affect behavioural parameters, however it significantly disrupted overall neuronal firing while increasing phasic responses to reward-predicting cues and disrupting mPFC-NAc cross-talk. The latter two effects were reversed by Lu AF64386. These findings suggest PDE1B inhibition may be beneficial in disorders implicating a dysfunction of the mPFC-NAc network.

β-Adrenergic Receptors/Epac Signaling Increases the Size of the Readily Releasable Pool of Synaptic Vesicles Required for Parallel Fiber LTP

The second messenger cAMP is an important determinant of synaptic plasticity that is associated with enhanced neurotransmitter release. Long-term potentiation (LTP) at parallel fiber (PF)-Purkinje cell (PC) synapses depends on a Ca²⁺-induced increase in presynaptic cAMP that is mediated by Ca²⁺-sensitive adenylyl cyclases. However, the upstream signaling and the downstream targets of cAMP involved in these events remain poorly understood. It is unclear whether cAMP generated by β-adrenergic receptors (βARs) is required for PF-PC LTP, although noradrenergic varicosities are apposed in PF-PC contacts. Guanine nucleotide exchange proteins directly activated by cAMP [Epac proteins (Epac 1-2)] are alternative cAMP targets to protein kinase A (PKA) and Epac2 is abundant in the cerebellum. However, whether Epac proteins participate in PF-PC LTP is not known. Immunoelectron microscopy demonstrated that βARs are expressed in PF boutons. Moreover, activation of these receptors through their agonist isoproterenol potentiated synaptic transmission in cerebellar slices from mice of either sex, an effect that was insensitive to the PKA inhibitors (H-89, KT270) but that was blocked by the Epac inhibitor ESI 05. Interestingly, prior activation of these βARs occluded PF-PC LTP, while the β1AR antagonist metoprolol blocked PF-PC LTP, which was also absent in Epac2 ^-/- mice. PF-PC LTP is associated with an increase in the size of the readily releasable pool (RRP) of synaptic vesicles, consistent with the isoproterenol-induced increase in vesicle docking in cerebellar slices. Thus, the βAR-mediated modulation of the release machinery and the subsequent increase in the size of the RRP contributes to PF-PC LTP.SIGNIFICANCE STATEMENT G-protein-coupled receptors modulate the release machinery, causing long-lasting changes in synaptic transmission that influence synaptic plasticity. Nevertheless, the mechanisms underlying synaptic responses to β-adrenergic receptor (βAR) activation remain poorly understood. An increase in the number of synaptic vesicles primed for exocytosis accounts for the potentiation of neurotransmitter release driven by βARs. This effect is not mediated by the canonical protein kinase A pathway but rather, through direct activation of the guanine nucleotide exchange protein Epac by cAMP. Interestingly, this βAR signaling via Epac is involved in long term potentiation at cerebellar granule cell-to-Purkinje cell synapses. Thus, the pharmacological activation of βARs modulates synaptic plasticity and opens therapeutic opportunities to control this phenomenon.

Nebivolol attenuates the anticonvulsant action of carbamazepine and phenobarbital against the maximal electroshock-induced seizures in mice

Background

Due to co-occurrence of seizures and cardiovascular disorders, nebivolol, a widely used selective β₁-blocker with vasodilatory properties, may be co-administered with antiepileptic drugs. Therefore, we wanted to assess interactions between nebivolol and four conventional antiepileptic drugs: carbamazepine, valproate, phenytoin and phenobarbital in the screening model of tonic–clonic convulsions.

Methods

Seizure experiments were conducted in the electroconvulsive threshold and maximal electroshock tests in mice. The chimney test served as a method of assessing motor coordination, whereas long-term memory was evaluated in the computerized step-through passive-avoidance task. To exclude or confirm pharmacokinetic interactions, we measured brain concentrations of antiepileptic drugs using the fluorescence polarization immunoassay.

Results

It was shown that nebivolol applied at doses 0.5–15 mg/kg did not raise the threshold for electroconvulsions. However, nebivolol at the dose of 15 mg/kg reduced the anti-electroshock properties of carbamazepine. The effect of valproate, phenytoin, and phenobarbital remained unchanged by combination with the β-blocker. Nebivolol significantly decreased the brain concentration of valproate, but did not affect concentrations of remaining antiepileptic drugs. Therefore, contribution of pharmacokinetic interactions to the final effect of the nebivolol/carbamazepine combination seems not probable. Nebivolol alone and in combinations with antiepileptic drugs did not impair motor performance in mice. Nebivolol alone did not affect long-term memory of animals, and did not potentiate memory impairment induced by valproate and carbamazepine.

Conclusions

This study indicates that nebivolol attenuated effectiveness of some antiepileptic drugs. In case the results are confirmed in clinical settings, this β-blocker should be used with caution in epileptic patients.

GPER-regulated lncRNA-Glu promotes glutamate secretion to enhance cellular invasion and metastasis in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a group of breast cancer with heterogeneity and poor prognosis and effective therapeutic targets are not available currently. TNBC has been recognized as estrogen-independent breast cancer, while the novel estrogen receptor, namely G protein-coupled estrogen receptor (GPER), was claimed to mediate estrogenic actions in TNBC tissues and cell lines. Through mRNA microarrays, lncRNA microarrays, and bioinformatics analysis, we found that GPER is activated by 17β-estradiol (E2) and GPER-specific agonist G1, which downregulates a novel lncRNA (termed as lncRNA-Glu). LncRNA-Glu can inhibit glutamate transport activity and transcriptional activity of its target gene VGLUT2 via specific binding. GPER-mediated reduction of lncRNA-Glu promotes glutamate transport activity and transcriptional activity of VGLUT2. Furthermore, GPER-mediated activation of cAMP-PKA signaling contributes to glutamate secretion. LncRNA-Glu-VGLUT2 signaling synergizes with cAMP-PKA signaling to increase autologous glutamate secretion in TNBC cells, which activates glutamate N-methyl-D-aspartate receptor (NMDAR) and its downstream CaMK and MEK-MAPK pathways, thus enhancing cellular invasion and metastasis in vitro and in vivo. Our data provide new insights into GPER-mediated glutamate secretion and its downstream signaling NMDAR-CaMK/MEK-MAPK during TNBC invasion. The mechanisms we discovered may provide new targets for clinical therapy of TNBC.

5-HT1B receptor agonist CGS12066 presynaptically inhibits glutamate release in rat hippocampus

CGS12066, a 5-hydroxytryptamine 1B (5-HT_1B) receptor agonist, has been reported to exhibit antidepressant activity. Considering that glutamatergic dysfunction is implicated in depression, the effect of CGS12066 on glutamate release in rat hippocampal nerve terminals and possible underlying mechanism were investigated. We observed that CGS12066 inhibited 4-aminopyridine (4-AP)-evoked glutamate release, and that a 5-HT_1B receptor antagonist blocked this inhibition. Western blot analysis and immunocytochemistry confirmed the presence of presynaptic 5-HT_1B receptor proteins. CGS12066-mediated inhibition of 4-AP-evoked glutamate release was completely abolished in the synaptosomes pretreated with inhibitors of Gi/Go-protein, adenylate cyclase (AC), and protein kinase A (PKA), namely pertussis toxin, MDL12330A, and H89, respectively. CGS12066 reduced the elevation of 4-AP-evoked intrasynaptosomal Ca²⁺ and cyclic AMP (cAMP) levels, but did not affect the synaptosomal membrane potential. Furthermore, in the presence of ω-conotoxin MVIIC, a N- and P/Q-type channel blocker, CGS12066-mediated inhibition of 4-AP-evoked glutamate release was markedly reduced; however, the intracellular Ca²⁺-release inhibitors dantrolene and CGP37157 did not affect the CGS12066 effect. Furthermore, CGS12066 reduced glutamatergic miniature excitatory postsynaptic current (mEPSC) frequency but did not affect mEPSC amplitude or glutamate-activated currents in hippocampal slices. Our data are the first to suggest that CGS12066 reduces AC/cAMP/PKA activation, through the activation of Gi/Go protein-coupled 5-HT_1B receptors present on hippocampal nerve terminals, subsequently reducing Ca²⁺ entry through voltage-dependent Ca²⁺ channels and reducing 4-AP-evoked glutamate release. This investigation into the role of 5-HT_1B receptors in glutamate release provides crucial information regarding the potential therapeutic role of 5-HT_1B receptors for treating depression.

CB1 receptors down-regulate a cAMP/Epac2/PLC pathway to silence the nerve terminals of cerebellar granule cells

Cannabinoid receptors mediate short-term retrograde inhibition of neurotransmitter release, as well as long-term depression of synaptic transmission at excitatory synapses. The responses of individual nerve terminals in VGLUT1-pHluorin transfected cerebellar granule cells to cannabinoids have shown that prolonged activation of cannabinoid type 1 receptors (CB1Rs) silences a subpopulation of previously active synaptic boutons. Adopting a combined pharmacological and genetic approach to study the molecular mechanisms of CB1R-induced silencing, we found that adenylyl cyclase inhibition decreases cAMP levels while it increases the number of silent synaptic boutons and occludes the induction of further silencing by the cannabinoid agonist HU-210. Guanine nucleotide exchange proteins directly activated by cAMP (Epac proteins) mediate some of the presynaptic effects of cAMP in the potentiation of synaptic transmission. ESI05, a selective Epac2 inhibitor, and U-73122, the specific inhibitor of phospholipase C (PLC), both augment the number of silent synaptic boutons. Moreover, they abolish the capacity of the Epac activator, 8-(4-chlorophenylthio)-2'-O-methyladenosine 3',5'-cyclic monophosphate monosodium hydrate, to prevent HU-210-induced silencing consistent with PLC signaling lying downstream of Epac2 proteins. Furthermore, Rab3-interacting molecule (RIM)1α KO cells have many more basally silent synaptic boutons (12.9 ± 3.5%) than wild-type cells (1.1 ± 0.5%). HU-210 induced further silencing in these mutant cells, although 8-(4-chlorophenylthio)-2'-O-methyladenosine 3',5'-cyclic monophosphate monosodium hydrate only awoke the HU-210-induced silence and not the basally silent synaptic boutons. This behavior can be rescued by expressing RIM1α in RIM1α KO cells, these cells behaving very much like wild-type cells. These findings support the hypothesis that a cAMP/Epac/PLC signaling pathway targeting the release machinery appears to mediate cannabinoid-induced presynaptic silencing.

Adrenergic regulation of GABA release from presynaptic terminals in rat cerebral cortex

The α1-adrenoceptor agonist phenylephrine and the β-adrenoceptor agonist isoproterenol have opposite effects on evoked EPSPs (eEPSPs) in the cerebral cortex. The suppressive effects of phenylephrine on eEPSPs are mediated by modulation of postsynaptic glutamate receptors, whereas enhancement of eEPSPs by isoproterenol is due to facilitation of glutamate release from presynaptic terminals. The present study used whole-cell patch-clamp recordings from layer V pyramidal neurons in visuocortical slice preparations to assess the effects of phenylephrine and isoproterenol on the release probability of γ-aminobutyric acid (GABA). The present study recorded evoked inhibitory postsynaptic potentials (eIPSCs) by repetitive electrical stimulation (duration, 100 μs; 10 stimuli at 33 Hz) and miniature IPSCs (mIPSCs). The effects of phenylephrine (100 μM) depended on the amplitude of eIPSCs: phenylephrine decreased the paired-pulse ratios (PPRs) of eIPSCs with smaller amplitudes (<~600 pA) but increased PPRs of eIPSCs with larger amplitude. Phenylephrine also exhibited amplitude-dependent modulation of mIPSCs, i.e., an increase in the frequency of smaller mIPSC events (<~20 pA) and a decrease in the frequency of larger events. These findings suggest that α1-adrenoceptor activation facilitates GABA release from a subpopulation of GABAergic terminals that induce smaller-amplitude IPSCs in postsynaptic neurons. In contrast, isoproterenol (100 μM) consistently decreased the PPR of eIPSCs and increased the frequency of mIPSCs, suggesting that presynaptic β-adrenoceptors increase release probability from most GABAergic terminals. The complexity of adrenoceptor modulations in GABAergic synaptic transmission by α1-adrenoceptor and β-adrenoceptor activation may be due to the presence of pleiotropic subtypes of GABAergic interneurons in the cerebral cortex.

Cross-talk between metabotropic glutamate receptor 7 and beta adrenergic receptor signaling at cerebrocortical nerve terminals

The co-existence of presynaptic G protein coupled receptors, GPCRs, has received little attention, despite the fact that interplay between the signaling pathways activated by such receptors may affect the neurotransmitter release. Using immunocytochemistry and immuhistochemistry we show that mGlu7 and β-adrenergic receptors are co-expressed in a sub-population of cerebrocortical nerve terminals. mGlu7 receptors readily couple to pathways that inhibit glutamate release. We found that when mGlu7 receptors are also coupled to pathways that enhance glutamate release by prolonged exposure to agonist, and β-adrenergic receptors are also activated, a cross-talk between their signaling pathways occurs that affect the overall release response. This interaction is the result of mGlu7 receptors inhibiting the adenylyl cyclase activated by β adrenergic receptors. Thus, blocking Gi/o proteins with pertussis toxin provokes a further increase in release after receptor co-activation which is also observed after activating β-adrenergic receptor signaling pathways downstream of adenylyl cyclase with the cAMP analog Sp8Br or 8pCPT-2-OMe-cAMP (a specific activator of the guanine nucleotide exchange protein directly activated by cAMP, EPAC). Co-activation of mGlu7 and β-adrenergic receptors also enhances PLC-dependent accumulation of IP1 and the translocation of the active zone protein Munc13-1 to the membrane, indicating that release potentiation by these receptors involves the modulation of the release machinery.

Norepinephrine ignites local hotspots of neuronal excitation: How arousal amplifies selectivity in perception and memory

Emotional arousal enhances perception and memory of high-priority information but impairs processing of other information. Here, we propose that, under arousal, local glutamate levels signal the current strength of a representation and interact with norepinephrine (NE) to enhance high priority representations and out-compete or suppress lower priority representations. In our "glutamate amplifies noradrenergic effects" (GANE) model, high glutamate at the site of prioritized representations increases local NE release from the locus coeruleus (LC) to generate "NE hotspots." At these NE hotspots, local glutamate and NE release are mutually enhancing and amplify activation of prioritized representations. In contrast, arousal-induced LC activity inhibits less active representations via two mechanisms: 1) Where there are hotspots, lateral inhibition is amplified; 2) Where no hotspots emerge, NE levels are only high enough to activate low-threshold inhibitory adrenoreceptors. Thus, LC activation promotes a few hotspots of excitation in the context of widespread suppression, enhancing high priority representations while suppressing the rest. Hotspots also help synchronize oscillations across neural ensembles transmitting high-priority information. Furthermore, brain structures that detect stimulus priority interact with phasic NE release to preferentially route such information through large-scale functional brain networks. A surge of NE before, during, or after encoding enhances synaptic plasticity at NE hotspots, triggering local protein synthesis processes that enhance selective memory consolidation. Together, these noradrenergic mechanisms promote selective attention and memory under arousal. GANE not only reconciles apparently contradictory findings in the emotion-cognition literature but also extends previous influential theories of LC neuromodulation by proposing specific mechanisms for how LC-NE activity increases neural gain.

Cyclooxygenase 2 inhibitor celecoxib inhibits glutamate release by attenuating the PGE2/EP2 pathway in rat cerebral cortex endings

The excitotoxicity caused by excessive glutamate is a critical element in the neuropathology of acute and chronic brain disorders. Therefore, inhibition of glutamate release is a potentially valuable therapeutic strategy for treating these diseases. In this study, we investigated the effect of celecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor that reduces the level of prostaglandin E2 (PGE2), on endogenous glutamate release in rat cerebral cortex nerve terminals (synaptosomes). Celecoxib substantially inhibited the release of glutamate induced by the K(+) channel blocker 4-aminopyridine (4-AP), and this phenomenon was prevented by chelating the extracellular Ca(2+) ions and by the vesicular transporter inhibitor bafilomycin A1. Celecoxib inhibited a 4-AP-induced increase in cytosolic-free Ca(2+) concentration, and the celecoxib-mediated inhibition of glutamate release was prevented by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker ω-conotoxin MVIIC. However, celecoxib did not alter 4-AP-mediated depolarization and Na(+) influx. In addition, this glutamate release-inhibiting effect of celecoxib was mediated through the PGE2 subtype 2 receptor (EP2) because it was not observed in the presence of butaprost (an EP2 agonist) or PF04418948 [1-(4-fluorobenzoyl)-3-[[6-methoxy-2-naphthalenyl)methyl]-3-azetidinecarboxylic acid; an EP2 antagonist]. The celecoxib effect on 4-AP-induced glutamate release was prevented by the inhibition or activation of protein kinase A (PKA), and celecoxib decreased the 4-AP-induced phosphorylation of PKA. We also determined that COX-2 and the EP2 receptor are present in presynaptic terminals because they are colocalized with synaptophysin, a presynaptic marker. These results collectively indicate that celecoxib inhibits glutamate release from nerve terminals by reducing voltage-dependent Ca(2+) entry through a signaling cascade involving EP2 and PKA.

Isoproterenol facilitates GABAergic autapses in fast-spiking cells of rat insular cortex

In the cerebral cortex, fast-spiking (FS) cells are the principal GABAergic interneurons and potently suppress neural activity in targeting neurons. Some FS neurons make synaptic contacts with themselves. Such synapses are called autapses and contribute to self-inhibition of FS neural activity. β-Adrenoceptors have a crucial role in regulating GABAergic synaptic inputs from FS cells to pyramidal (Pyr) cells; however, the β-adrenergic functions on FS autapses are unknown. To determine how the β-adrenoceptor agonist isoproterenol modulates inhibitory synaptic transmission in the autapses of FS cells, paired whole-cell patch-clamp recordings were obtained from FS and Pyr cells in layer V of rat insular cortex. Previous studies found that isoproterenol (100 μM) had pleiotropic effects on unitary inhibitory postsynaptic currents (uIPSCs) in FS→Pyr connections, whereas autapses in FS cells were always facilitated by isoproterenol. Facilitation of autapses by isoproterenol was accompanied by decreases in the paired-pulse ratio of second to first uIPSC amplitudes and the coefficient of variation of the uIPSC amplitude, which suggests that β-adrenergic facilitation is likely mediated by presynaptic mechanisms. The discrepancy between isoproterenol-induced modulation of uIPSCs in FS autapses and in FS→Pyr connections may reflect the presence of different presynaptic mechanisms of GABA release in each synapse.

β-Adrenergic receptors activate exchange protein directly activated by cAMP (Epac), translocate Munc13-1, and enhance the Rab3A-RIM1α interaction to potentiate glutamate release at cerebrocortical nerve terminals

The adenylyl cyclase activator forskolin facilitates synaptic transmission presynaptically via cAMP-dependent protein kinase (PKA). In addition, cAMP also increases glutamate release via PKA-independent mechanisms, although the downstream presynaptic targets remain largely unknown. Here, we describe the isolation of a PKA-independent component of glutamate release in cerebrocortical nerve terminals after blocking Na(+) channels with tetrodotoxin. We found that 8-pCPT-2'-O-Me-cAMP, a specific activator of the exchange protein directly activated by cAMP (Epac), mimicked and occluded forskolin-induced potentiation of glutamate release. This Epac-mediated increase in glutamate release was dependent on phospholipase C, and it increased the hydrolysis of phosphatidylinositol 4,5-bisphosphate. Moreover, the potentiation of glutamate release by Epac was independent of protein kinase C, although it was attenuated by the diacylglycerol-binding site antagonist calphostin C. Epac activation translocated the active zone protein Munc13-1 from soluble to particulate fractions; it increased the association between Rab3A and RIM1α and redistributed synaptic vesicles closer to the presynaptic membrane. Furthermore, these responses were mimicked by the β-adrenergic receptor (βAR) agonist isoproterenol, consistent with the immunoelectron microscopy and immunocytochemical data demonstrating presynaptic expression of βARs in a subset of glutamatergic synapses in the cerebral cortex. Based on these findings, we conclude that βARs couple to a cAMP/Epac/PLC/Munc13/Rab3/RIM-dependent pathway to enhance glutamate release at cerebrocortical nerve terminals.

Proteomic analysis of ERK1/2-mediated human sickle red blood cell membrane protein phosphorylation

BACKGROUND

In sickle cell disease (SCD), the mitogen-activated protein kinase (MAPK) ERK1/2 is constitutively active and can be inducible by agonist-stimulation only in sickle but not in normal human red blood cells (RBCs). ERK1/2 is involved in activation of ICAM-4-mediated sickle RBC adhesion to the endothelium. However, other effects of the ERK1/2 activation in sickle RBCs leading to the complex SCD pathophysiology, such as alteration of RBC hemorheology are unknown.

RESULTS

To further characterize global ERK1/2-induced changes in membrane protein phosphorylation within human RBCs, a label-free quantitative phosphoproteomic analysis was applied to sickle and normal RBC membrane ghosts pre-treated with U0126, a specific inhibitor of MEK1/2, the upstream kinase of ERK1/2, in the presence or absence of recombinant active ERK2. Across eight unique treatment groups, 375 phosphopeptides from 155 phosphoproteins were quantified with an average technical coefficient of variation in peak intensity of 19.8%. Sickle RBC treatment with U0126 decreased thirty-six phosphopeptides from twenty-one phosphoproteins involved in regulation of not only RBC shape, flexibility, cell morphology maintenance and adhesion, but also glucose and glutamate transport, cAMP production, degradation of misfolded proteins and receptor ubiquitination. Glycophorin A was the most affected protein in sickle RBCs by this ERK1/2 pathway, which contained 12 unique phosphorylated peptides, suggesting that in addition to its effect on sickle RBC adhesion, increased glycophorin A phosphorylation via the ERK1/2 pathway may also affect glycophorin A interactions with band 3, which could result in decreases in both anion transport by band 3 and band 3 trafficking. The abundance of twelve of the thirty-six phosphopeptides were subsequently increased in normal RBCs co-incubated with recombinant ERK2 and therefore represent specific MEK1/2 phospho-inhibitory targets mediated via ERK2.

CONCLUSIONS

These findings expand upon the current model for the involvement of ERK1/2 signaling in RBCs. These findings also identify additional protein targets of this pathway other than the RBC adhesion molecule ICAM-4 and enhance the understanding of the mechanism of small molecule inhibitors of MEK/1/2/ERK1/2, which could be effective in ameliorating RBC hemorheology and adhesion, the hallmarks of SCD.

Influence of nebivolol on anticonvulsant effect of lamotrigine

Objective:

The present study describes the effect of nebivolol (NBV) either alone or in combination with lamotrigine (LTG) using increasing current electroshock seizures (ICES) model in mice.

Materials and Methods:

Male albino mice of Swiss strain each weighing 18-30 g were used. Lamotrigine (Lamitor tablets, Torrent; 1.5 and 3.0 mg/kg) and NBV (Nebicard tablets, Torrent; 0.25 and 0.5 mg/kg) were suspended in 0.25% of carboxy methyl cellulose (CMC) in 0.9% saline and administered orally in volumes of 10 mg/kg. Control animals received an equivalent volume of 0.25% CMC in 0.9% saline suspension. The anticonvulsant effects of the drugs were measured using ICES model whereas cognitive behavior was measured by the spontaneous alternation behavior and grip-strength test. The biochemical estimation was done by measuring the lipid peroxidation and reduced glutathione (GSH).

Results:

Both NBV and LTG produced significantly enhanced seizure threshold (ST), decreased grip strength, inhibited lipid peroxidation, and increased brain GSH levels in acute and chronic dosages likened to control group, whereas there was no significant effect on alternation scores. The combination of NBV with LTG significantly potentiated the ST when compared to LTG.

Conclusion:

Nebivolol showed antiepileptic effects in addition to its reported antihypertensive effect, which could be attributed to action of the two drugs through different mechanisms or due to drug interaction that may be pharmacodynamic or pharmacokinetic needing elucidation.

Dark cell change of the cerebellar Purkinje cells induced by terbutaline under transient disruption of the blood-brain barrier in adult rats: morphological evaluation

This study aimed to establish a cerebellar degeneration animal model and to characterize the dark cell change of Purkinje cells. We hypothesized that terbutaline, a β2-adrenoceptor agonist, induces cerebellar degeneration not only in neonatal rats, but also in adult rats. Nine-week-old adult male Sprague-Dawley rats were anesthetized and infused with 25% mannitol via the left common carotid artery. Thirty seconds later, terbutaline was infused via the same artery. Dark-stained Purkinje cells were observed in the entire cerebellum on day 3. Prominent Bergmann glial cells accompanied by swelling of the glial processes were present, and were closely associated with the dark-stained Purkinje cells. These findings were found continuously throughout day 30. Ultrastructurally, dilated Golgi vesicles and/or endoplasmic reticulum and large lamella bodies were present in both severely changed and slightly changed Purkinje cells. Bergmann glial cells in the area of synaptic contacts of the severely changed Purkinje cells showed swelling. The Bergmann glial process in close contact with the slightly changed Purkinje cell dendrite in molecular layer showed slight swelling, and large lamella bodies in the dendrite were observed close to the dendritic spines. These findings may suggest that terbutaline induced a failure of Bergmann glial cell and resulted in dark cell degeneration of the Purkinje cells due to glutamate excitotoxicity.

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.

Presynaptic Interneuron Subtype- and Age-Dependent Modulation of GABAergic Synaptic Transmission by β-Adrenoceptors in Rat Insular Cortex

β-Adrenoceptors play a crucial role in the regulation of taste aversion learning in the insular cortex (IC). However, β-adrenergic effects on inhibitory synaptic transmission mediated by γ-aminobutyric acid (GABA) remain unknown. To elucidate the mechanisms of β-adrenergic modulation of inhibitory synaptic transmission, we performed paired whole cell patch-clamp recordings from layer V GABAergic interneurons and pyramidal cells of rat IC aged from postnatal day 17 (PD17) to PD46 and examined the effects of isoproterenol, a β-adrenoceptor agonist, on unitary inhibitory postsynaptic currents (uIPSCs). Isoproterenol (100 μM) induced facilitating effects on uIPSCs in 33.3% of cell pairs accompanied by decreases in coefficient of variation (CV) of the first uIPSC amplitude and paired-pulse ratio (PPR) of the second to first uIPSC amplitude, whereas 35.9% of pairs showed suppressive effects of isoproterenol on uIPSC amplitude obtained from fast spiking (FS) to pyramidal cell pairs. Facilitatory effects of isoproterenol were frequently observed in FS–pyramidal cell pairs at ≥PD24. On the other hand, isoproterenol suppressed uIPSC amplitude by 52.3 and 39.8% in low-threshold spike (LTS)–pyramidal and late spiking (LS)–pyramidal cell pairs, respectively, with increases in CV and PPR. The isoproterenol-induced suppressive effects were blocked by preapplication of 100 μM propranolol, a β-adrenoceptor antagonist. There was no significant correlation between age and changes of uIPSCs in LTS–/LS–pyramidal cell pairs. These results suggest the presence of differential mechanisms in presynaptic GABA release and/or postsynaptic GABAA receptor-related assemblies among interneuron subtypes. Age- and interneuron subtype-specific β-adrenergic modulation of IPSCs may contribute to experience-dependent plasticity in the IC.

Spreading depression: from serendipity to targeted therapy in migraine prophylaxis

Despite the relatively well-characterized headache mechanisms in migraine, upstream events triggering individual attacks are poorly understood. This lack of mechanistic insight has hampered a rational approach to prophylactic drug discovery. Unlike targeted abortive and analgesic interventions, mainstream migraine prophylaxis has been largely based on serendipitous observations (e.g. propranolol) and presumed class effects (e.g. anticonvulsants). Recent studies suggest that spreading depression is the final common pathophysiological target for several established or investigational migraine prophylactic drugs. Building on these observations, spreading depression can now be explored for its predictive utility as a preclinical drug screening paradigm in migraine prophylaxis.

Partial compensation for N-type Ca(2+) channel loss by P/Q-type Ca(2+) channels underlines the differential release properties supported by these channels at cerebrocortical nerve terminals

N-type and P/Q-type Ca(2+) channels support glutamate release at central synapses. To determine whether the glutamate release mediated by these channels exhibits distinct properties, we have isolated each release component in cerebrocortical nerve terminals from wild-type mice by specifically blocking N-type Ca(2+) channels with omega-conotoxin-GVIA and P/Q-type Ca(2+) channels with omega-agatoxin-IVA. In addition, we have determined the release properties at terminals from mice lacking the alpha(1B) subunit of N-type channels (Ca(v) 2.2) to test the possibility that P/Q-type channels can compensate for the loss of N-type Ca(2+) channels. We recently demonstrated that, while evoked glutamate release depends on P/Q- and N-type channels in wild-type nerve terminals, only P/Q-type channels participate in these knockout mice. Moreover, in nerve terminals expressing solely P/Q-type channels, metabotropic glutamate receptor 7 (mGluR7) fails to inhibit the evoked Ca(2+) influx and glutamate release. Here, we show that the failure of mGluR7 to modulate evoked glutamate release is not due to a lack of receptors, as nerve terminals from mice lacking N-type Ca(2+) channels express mGluR7. Indeed, we show that other receptor responses, such as the inhibition of forskolin-induced release, are preserved in these knockout mice. N-type channels are more loosely coupled to release than P/Q-type channels in nerve terminals from wild-type mice, as reflected by the tighter coupling of release in knockout nerve terminals. We conclude that the glutamate release supported by N- and P/Q-type channels exhibits distinct properties, and that P/Q-type channels cannot fully compensate for the loss of N-type channels.

Presynaptic and postsynaptic modulation of glutamatergic synaptic transmission by activation of alpha(1)- and beta-adrenoceptors in layer V pyramidal neurons of rat cerebral cortex

Adrenergic agonists have different modulatory effects on excitatory synaptic transmission depending on the receptor subtypes involved. The present study examined the loci of alpha(1)- and beta-adrenoceptor agonists, which have opposite effects on excitatory neural transmission, involved in modulation of glutamatergic transmission in layer V pyramidal cells of rat cerebral cortex. Phenylephrine, an alpha(1)-adrenoceptor agonist, suppressed the amplitude of AMPA receptor-mediated excitatory postsynaptic currents evoked by repetitive electrical stimulation (eEPSCs, 10 pulses at 33 Hz). The coefficient of variation (CV) of the 1st eEPSC amplitude and paired-pulse ratio (PPR), which were sensitive to extracellular Ca(2+) concentration, were not affected by phenylephrine. Phenylephrine suppressed miniature EPSC (mEPSC) amplitude without changing its frequency. In contrast, isoproterenol, a beta-adrenoceptor agonist, strongly increased the amplitude of the 1st eEPSC compared with that of the 2nd to 10th eEPSCs, which resulted in a decrease in PPR. Isoproterenol-induced enhancement of eEPSC amplitude was accompanied by a decrease in CV. Isoproterenol increased the frequency of mEPSCs without significant effect on amplitude. Phenylephrine suppressed inward currents evoked by puff application of glutamate, AMPA, or NMDA, whereas isoproterenol application was not accompanied by significant changes in these inward currents. These findings suggest that phenylephrine decreases eEPSCs through postsynaptic AMPA or NMDA receptors, while the effects of isoproterenol are mediated by facilitation of glutamate release from presynaptic terminals without effect on postsynaptic glutamate receptors. These two different mechanisms of modulation of excitatory synaptic transmission may improve the "signal-to-noise ratio" in cerebral cortex.

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.

Pre- and postsynaptic beta-adrenergic activation enhances excitatory synaptic transmission in layer V/VI pyramidal neurons of the medial prefrontal cortex of rats

Norepinephrine exerts an important influence on prefrontal cortical functions. The physiological effects of beta-adrenoceptors (beta-ARs) have been examined in other brain regions. However, little is known about beta-AR regulation of synaptic transmission in the prefrontal cortex (PFC). The present study investigated beta-AR modulation of glutamate synaptic transmission in layer V/VI pyramidal cells of the medial PFC (mPFC) of rats. Our results show that 1) isoproterenol (ISO), a selective beta-AR agonist, increased the frequency of spontaneous and miniature excitatory postsynaptic currents (EPSC's); 2) ISO enhancement of miniature EPSC's (mEPSC's) frequency no longer appeared in the presence of the voltage-gated Ca(2+) channel blocker cadmium; 3) ISO enhanced the evoked excitatory postsynaptic currents (eEPSC's) mediated by non-N-methyl-D-aspartic acid receptors (non-NMDA-Rs) and NMDA-Rs. The ISO facilitation of non-NMDA-R eEPSC was blocked by the membrane-permeable cyclic adenosine monophosphate (cAMP) inhibitor Rp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt (Rp-cAMPS); 4) ISO enhanced NMDA-induced current, with no effect on glutamate-induced non-NMDA-R current; 5) ISO enhancement of NMDA-R eEPSC and NMDA-induced current was blocked by intracellular application of Rp-cAMPS or the cAMP-dependent protein kinase (PKA) inhibitor PKI(5-24); and 6) ISO suppressed the paired-pulse facilitation of non-NMDA-R and NMDA-R eEPSC's. Taken together, these results provide the first electrophysiological demonstration that beta-AR activation facilitates excitatory synaptic transmission in mPFC pyramidal cells through pre- and postsynaptic mechanisms, probably via cAMP or cAMP/PKA signaling.

mGluR7 inhibits glutamate release through a PKC-independent decrease in the activity of P/Q-type Ca2+ channels and by diminishing cAMP in hippocampal nerve terminals

The modulation of calcium channels by metabotropic glutamate receptors (mGluRs) is a key event in the fine-tuning of neurotransmitter release. Here we report that, in hippocampal nerve terminals from adult rats, the inhibition of glutamate release by the group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) is largely mediated by mGluR7. In this preparation, P/Q-type Ca(2+) channels support the major component of glutamate release while the remaining release is supported by N-type Ca(2+) channels. The release associated with P/Q channels was modulated by mGluR7, either in the presence of omega-conotoxin-GVIA or after decreasing the extracellular Ca(2+) concentration [Ca(2+)](o) to abolish the contribution of N-type Ca(2+) channels. Under these conditions, L-AP4 (1 mm) reduced the evoked glutamate release by 35 +/- 2%. This inhibition was largely prevented by pertussis toxin, but it was insensitive to inhibitors of protein kinase C (bisindolylmaleimide) and protein kinase A (H-89). Furthermore, this inhibition was associated with a reduction in the Ca(2+) influx mediated by P/Q channels in the absence of any detectable change in cAMP levels. However, L-AP4 decreased the levels of cAMP in the presence of forskolin. The activation of this additional signalling pathway was very efficient in counteracting the facilitation of glutamate release induced by forskolin. Thus, mGluR7 mediates the inhibition of glutamate release at hippocampal nerve terminals primarily by inhibiting P/Q-type Ca(2+) channels, although augmenting the levels of cAMP reveals the ability of the receptor to decrease cAMP.

beta(2)-Adrenoceptor-mediated facilitation of glutamatergic transmission in rat ventromedial hypothalamic neurons

Adrenergic modulation of glutamatergic spontaneous miniature excitatory postsynaptic currents (mEPSCs) was investigated in mechanically dissociated rat ventromedial hypothalamic (VMH) neurons using a conventional whole-cell patch clamp technique. Noradrenaline (NA) reversibly increased mEPSC frequency without affecting the current amplitude in a concentration-dependent manner, indicating that NA acts presynaptically to facilitate the probability of spontaneous glutamate release. NA (10 microM) action on glutamatergic mEPSC frequency was completely blocked by 1 microM ICI-188551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methyl-ethyl)amino]-2-butanol], a selective beta(2)-adrenoceptor antagonist, and mimicked by 1 microM formoterol, a selective beta(2)-adrenoceptor agonist. Neither alpha-adrenoceptor nor beta(1)-adrenoceptor blockers affected the NA-induced increase in mEPSC frequency. NA action on glutamatergic mEPSC frequency was completely occluded in the presence of either 10 microM forskolin, an adenylyl cyclase (AC) activator, or blocked by 1 microM SQ22536 [9-(tetrahydro-2-furanyl)-9H-purin-6-amine], a selective AC inhibitor. Furthermore, the NA-induced increase in mEPSC frequency was completely attenuated by either 1 muM KT5720 or 1 microM H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide), specific PKA inhibitors. However, NA still could increase mEPSC frequency either in the Ca(2+)-free external solution or in the presence of 1 microM thapsigargin. The results suggest that activation of presynaptic beta(2)-adrenoceptors facilitates spontaneous glutamate release to VMH neurons via cAMP/PKA signal transduction pathway. beta(2)-Adrenoceptor-mediated presynaptic modulation of excitatory glutamatergic transmission would therefore be expected to play a pivotal role in the regulation of a variety of behavioral functions, which are mediated by the VMH.

Modulation of adenylyl cyclase activity in young and adult rat brain cortex. Identification of suramin as a direct inhibitor of adenylyl cyclase

Adenylyl cyclase (AC) in brain cortex from young (12-day-old) rats exhibits markedly higher activity than in adult (90-day-old) animals. In order to find some possibly different regulatory features of AC in these two age groups, here we modulated AC activity by dithiothreitol (DTT), Fe(2+), ascorbic acid and suramin. We did not detect any substantial difference between the effects of all these tested agents on AC activity in cerebrocortical membranes from young and adult rats, and the enzyme activity was always about two-fold higher in the former preparations. Nevertheless, several interesting findings have come out of these investigations. Whereas forskolin- and Mn(2+)-stimulated AC activity was significantly enhanced by the addition of DTT, increased concentrations of Fe(2+) ions or ascorbic acid substantially suppressed the enzyme activity. Lipid peroxidation induced by suitable combinations of DTT/Fe(2+) or by ascorbic acid did not influence AC activity. We have also observed that PKC- or protein tyrosine kinase-mediated phosphorylation apparently does not play any significant role in different activity of AC determined in cerebrocortical preparations from young and adult rats. Our experiments analysing the presumed modulatory role of suramin revealed that this pharmacologically important drug may act as a direct inhibitor of AC. The enzyme activity was diminished to the same extent by suramin in membranes from both tested age groups. Our present data show that AC is regulated similarly in brain cortex from both young and adult rats, but its overall activity is much lower in adulthood.

Alpha-ketoisocaproic acid increases phosphorylation of intermediate filament proteins from rat cerebral cortex by mechanisms involving Ca2+ and cAMP

We have previously described that alpha-ketoisocaproic acid (KIC), the main metabolite accumulating in maple syrup urine disease (MSUD), increased the in vitro phosphorylation of cytoskeletal proteins in cerebral cortex of 17- and 21-day-old rats through NMDA glutamatergic receptors. In the present study we investigated the protein kinases involved in the effects of KIC on the phosphorylating system associated with the cytoskeletal fraction and provided an insight on the mechanisms involved in such effects. Results showed that 1 mM KIC increased the in vitro incorporation of 32P into intermediate filament (IF) proteins in slices of 21-day-old rats at shorter incubation times (5 min) than previously reported. Furthermore, this effect was prevented by 10 microM KN-93 and 10 microM H-89, indicating that KIC treatment increased Ca2+/calmodulin- (PKCaMII) and cAMP- (PKA) dependent protein kinases activities, respectively. Nifedipine (100 microM), a blocker of voltage-dependent calcium channels (VDCC), DL-AP5 (100 microM), a NMDA glutamate receptor antagonist and BAPTA-AM (50 microM), a potent intracellular Ca2+ chelator, were also able to prevent KIC-induced increase of in vitro phosphorylation of IF proteins. In addition, KIC treatment was able to significantly increase the intracellular cAMP levels. This data support the view that KIC increased the activity of the second messenger-dependent protein kinases PKCaMII and PKA through intracellular Ca2+ levels. Considering that hyperphosphorylation of cytoskeletal proteins is related to neurodegeneration it is presumed that the Ca2+-dependent hyperphosphorylation of IF proteins caused by KIC may be involved to the neuropathology of MSUD patients.

Presynaptic mechanism underlying cAMP-induced synaptic potentiation in medial prefrontal cortex pyramidal neurons

cAMP, a classic second messenger, has been proposed recently to participate in regulating prefrontal cortical cognitive functions, yet little is known about how it does so. In this study, we used forskolin, an adenylyl cyclase activator, to examine the effects of cAMP on excitatory synaptic transmission in the medial prefrontal cortex (mPFC) using whole-cell patch-clamp recordings from visually identified layer II-III or V pyramidal cells in vitro. We found that bath application of forskolin significantly increased the amplitude of excitatory postsynaptic currents (EPSCs) in a concentration- and age-dependent manner. This enhancement was completely abolished by coapplication of cAMP-dependent protein kinase (PKA) inhibitor and p42/p44 mitogen-activated protein kinase (MAPK) kinase inhibitor, but not application of either drug alone. The membrane-permeable cAMP analog adenosine 3',5'-cyclic monophosphorothioate, Sp-isomer, triethylammonium salt, or activation of beta-adrenergic receptor by isoproterenol mimicked the effect of forskolin to potentiate EPSCs. However, neither exchange protein activated by cAMP (Epac) inhibitor brefeldin A nor hyperpolarization and cyclic nucleotide-activated channel blocker 4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride (ZD7288) affected forskolin response. The augmentation of EPSCs by forskolin was accompanied by a reduction of the synaptic failure rate, coefficient of variation and paired-pulse ratio of EPSCs, and an increase in release probability and number of releasable synaptic vesicles. Forskolin also significantly increased the frequency of miniature EPSCs without altering their amplitude distribution. These results indicate that cAMP acts presynaptically to elicit a synaptic potentiation on the layer V pyramidal neurons of mPFC through converging activation of PKA and p42/p44 MAPK signaling pathways.

Localization of beta1-adrenergic receptors in the cochlea and the vestibular labyrinth

Sympathetic activation in a "fight or flight reaction" may put the sensory systems for hearing and balance into a state of heightened alert via beta1-adrenergic receptors (beta1-AR). The aim of the present study was to localize beta1-AR in the gerbil inner ear by confocal immunocytochemistry, to characterize beta1-AR by Western immunoblots, and to identify beta1-AR pharmacologically by measurements of cAMP production. Staining for beta1-AR was found in strial marginal cells, inner and outer hair cells, outer sulcus, and spiral ganglia cells of the cochlea, as well as in dark, transitional and supporting cells of the vestibular labyrinth. Receptors were characterized in microdissected inner ear tissue fractions as 55 kDa non-glycosylated species and as 160 kDa high-mannose-glycosylated complexes. Pharmacological studies using isoproterenol, ICI-118551 and CGP-20712A demonstrated beta1-AR as the predominant adrenergic receptor in stria vascularis and organ of Corti. In conclusion, beta1-AR are present and functional in inner ear epithelial cells that are involved in K+ cycling and auditory transduction, as well as in neuronal cells that are involved in auditory transmission.

The modulation of Ca2+ and K+ channels but not changes in cAMP signaling contribute to the inhibition of glutamate release by cannabinoid receptors in cerebrocortical nerve terminals

While cannabinoid receptors activate multiple signaling pathways in the brain, it remains unclear what influence the inhibition of adenylylcyclase has on the inhibition of glutamate release. In cerebrocortical nerve terminals, the cannabinoid receptor agonist WIN55,212-2 reduced KCl-evoked glutamate release through a mechanism that restricted the rise of cytoplasmic free Ca2+, but not the changes in plasma membrane depolarization. These effects were consistent with the inhibition of Ca2+ channels. Furthermore, WIN55,212-2 reduced 4-aminopyridine (4AP) evoked glutamate release to a larger extent by modulating the behavior of both Ca2+ and K(+)-channels. The inhibition of 4AP-evoked release was associated with a decrease in cytoplasmic free Ca2+ and in plasma membrane depolarization that was reverted by the potassium channel blocker, tetraethylammonium. Interestingly, the reduction of KCl- and 4AP-evoked release by WIN55,212-2 was independent of adenylylcyclase activity and did not affect cAMP. Forskolin and the beta-adrenergic receptor increase intrasynaptosomal cAMP and promote a PKA-dependent tetrodotoxin (TTX)-sensitive increase in the spontaneous release of glutamate. These two responses were reduced by WIN55,212-2. However, the glutamate release induced by Sp-8-Br-cAMPS, which directly activated PKA without affecting cAMP, was also similarly reduced by WIN55,212-2. Hence, we conclude that the inhibition of glutamate release by WIN55,212-2 is unrelated to changes in cAMP and that the inhibition of release that a decrease in cAMP might produce is occluded by the activation of additional pathways such as the inhibition of Ca2+ channels and/or the activation of K(+)-channels that strongly depress glutamate release.

Propionic and methylmalonic acids increase cAMP levels in slices of cerebral cortex of young rats via adrenergic and glutamatergic mechanisms

We have previously described that propionic (PA) and methylmalonic (MMA) acids increased the in vitro phosphorylation of cytoskeletal proteins through cAMP-dependent protein kinase and glutamate. In the present study we investigated the in vitro effects of 1 mM glutamate, 2.5 mM MMA and 2.5 mM PA on cAMP levels in the slices of cerebral cortex of young rats. Results showed that PA, MMA and glutamate increased cAMP levels after 30 min of incubation, while the beta-adrenergic agonist epinephrine elicited a similar effect only at a shorter incubation time. Then effects were prevented by the beta-adrenergic antagonist propranolol, rather than by glutamate antagonists (AP5, CNQX and MCPG), suggesting that they were mediated by beta-adrenergic receptors. In addition, glutamate antagonists per se induced increased cAMP levels; however propranolol prevented only the effect elicited by the metabotropic glutamate antagonist MCPG. Taken together, it is feasible that PA and MMA increase cAMP synthesis via a beta-adrenergic/G protein coupled pathway, in a glutamate-dependent manner. Although additional studies will be necessary to evaluate the importance of these observations for the neuropathology of propionic and methylmalonic acidemias, it is possible that high brain cAMP levels may contribute to a certain extent to the neurological dysfunction of the affected individuals.

Elevation of intracellular cAMP evokes activity-dependent release of adenosine in cultured rat forebrain neurons

Adenosine is an important regulator of neuronal excitability. Zaprinast is a cyclic nucleotide phosphodiesterase inhibitor, and has been shown in the hippocampal slice to suppress excitation. This action can be blocked by an adenosine receptor antagonist, and therefore is presumably due to adenosine release stimulated by exposure to zaprinast. To explore the mechanism of this phenomenon further, we examined the effect of zaprinast on adenosine release itself in cultured rat forebrain neurons. Zaprinast significantly stimulated extracellular adenosine accumulation. The effect of zaprinast on adenosine appeared to be mediated by increasing intracellular cyclic adenosine monophosphate (cAMP) and activation of protein kinase A (PKA): (i) zaprinast stimulated intracellular cAMP accumulation; (ii) a cAMP antagonist (Rp-8-Br-cAMP) significantly reduced the zaprinast effect on adenosine; (iii) an inhibitor of phosphodiesterase (PDE)1 (vinpocetine) and an activator of adenylate cyclase (forskolin) mimicked the effect of zaprinast on adenosine. We also found that zaprinast had no effect on adenosine in astrocyte cultures, and tetrodotoxin completely blocked zaprinast-evoked adenosine accumulation in neuronal cultures, suggesting that neuronal activity was likely to be involved. Consistent with a dependence on neuronal activity, NMDA receptor antagonists (MK-801 and D-APV) and removal of extracellular glutamate by glutamate-pyruvate transaminase blocked the effect of zaprinast. In addition, zaprinast was shown to stimulate glutamate release. Thus, our data suggest that zaprinast-evoked adenosine accumulation is likely to be mediated by stimulation of glutamate release by a cAMP- and PKA-dependent mechanism, most likely by inhibition of PDE1 in neurons. Furthermore, regulation of cAMP, either by inhibiting cAMP-PDE activity or by stimulating adenylate cyclase activity, may play an important role in modulating neuronal excitability. These data suggest the existence of a homeostatic negative feedback loop in which increases in neuronal activity are damped by release of adenosine following activation of glutamate receptors.

Co-activation of PKA and PKC in cerebrocortical nerve terminals synergistically facilitates glutamate release

Protein kinase A and protein kinase C are involved in processes that enhance glutamate release at glutamatergic nerve terminals. However, it is not known whether these two kinases co-exist within the same nerve terminal, nor is it clear what impact their simultaneous activation may have on neurotransmitter release. In cerebrocortical nerve terminals, co-application of forskolin, which increases cAMP levels and activates protein kinase A, and 4beta-phorbol dibutyrate, a direct activator of protein kinase C, synergistically enhanced the spontaneous release of glutamate. This enhancement exhibited both tetrodotoxin-sensitive and tetrodotoxin-resistant components. Interestingly, the tetrodotoxin-resistant component of release was not observed when cyclic AMP-dependent protein kinase (PKA) and calcium- and phospholipid-dependent protein kinase (PKC) were activated separately, but developed slowly after the co-activation of the two kinases, accounting for 50% of the facilitated release. This release component was dependent on voltage-dependent Ca2+ channels that opened spontaneously after PKA and PKC activation and occurred in the absence of Na+ channel firing. These data provide functional evidence for the co-existence of PKA- and PKC-signalling pathways in a subpopulation of glutamatergic nerve terminals.

Enhanced adenosine A2A receptor facilitation of synaptic transmission in the hippocampus of aged rats

Adenosine either inhibits or facilitates synaptic transmission through A1 or A2A receptors, respectively. Since A2A receptor density increases in the limbic cortex of aged (24 mo) compared with young adult rats (2 mo), we tested if A2A receptor modulation of synaptic transmission was also increased in aged rats. The A2A receptor agonist, CGS21680 (10 nM), caused a larger facilitation of the field excitatory postsynaptic potential (fEPSP) slope in hippocampal slices of aged (38%) than in young rats (19%), an effect prevented by the A2A receptor antagonist, ZM241385 (20 nM). In contrast to young rats, where CGS21680 facilitation of fEPSPs is prevented by the protein kinase C inhibitor, chelerythrine (6 microM), but not by the protein kinase A inhibitor, H-89 (1 microM), the CGS21680-induced facilitation of fEPSP slope in aged rats was prevented by H-89 (1 microM) but not by chelerythrine (6 microM). Also, in contrast to the beta-receptor agonist, isoproterenol (30 microM), CGS21680 (100-1,000 nM) enhanced cAMP levels in hippocampal nerve terminals of aged but not young rats. Finally, we observed a significant increase of both the binding density of [3H]CGS 21680 and the [3H]ZM241385 as well as of the anti-A2A receptor immunoreactivity in hippocampal nerve terminal membranes from aged compared with young rats. This shows that A2A receptor-mediated facilitation of hippocampal synaptic transmission is larger in aged than young rats due to increased A2A receptor density in nerve terminals and to the modified transducing system operated by A2A receptors, from a protein kinase C mediated control of A1 receptors into a direct protein kinase A dependent facilitation of synaptic transmission.

Opposing facilitatory and inhibitory modulation of glutamate release elicited by cAMP production in cerebrocortical nerve terminals (synaptosomes)

Activation of cAMP-protein kinase A (PKA) is widely reported to facilitate synaptic transmission. Here, we examined the presynaptic loci of PKA action using isolated nerve terminals (synaptosoms). The adenylyl cyclase (AC) activator, forskolin, failed to have any effect on 4-aminopyridine (4-AP)-evoked glutamate release, when added alone. However, in the presence of the alkylxanthine, IBMX, forskolin strongly facilitated glutamate release. This potentiation of release was blocked by the PKA inhibitors Rp-cAMPS and H7. Given that IBMX has dual activity, antagonizing adenosine receptors as well as inhibiting cAMP phosphodiesterase, we examined the effect of a selective adenosine A(1) receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and RO20-1724, a specific phosphodiesterase inhibitor. Both unmasked the forskolin-mediated modulation of glutamate release. Conversely, the adenosine analogue, N(6)-cyclohexyladenosine (CHA), reversed the facilitation induced by forskolin+RO20-1724. Adenosine A(1) receptor activation, therefore, appears to curtail cAMP/PKA-induced potentiation of glutamate release. Looking at the targets for cAMP/PKA-mediated potentiation of glutamate release, while synaptosomal excitability was only marginally increased, basal and 4-AP-evoked-increases in [Ca(2+)](c) were substantially enhanced by forskolin+IBMX. Moreover, glutamate release elicited by Ca(2+)-ionophore (ionomycin)-induced Ca(2+)-entry was facilitated by forskolin+IBMX. cAMP/PKA-mediated facilitation of glutamate release may therefore involve modulation of Ca(2+)-entry, as well as downstream events controlling synaptic vesicle recruitment and exocytosis.

Regulation of glutamatergic neurotransmission in the striatum by presynaptic adenylyl cyclase-dependent processes

The aim here was to examine the possible roles of adenylyl cyclase- and protein kinase A (PKA)-dependent processes in ionotropic glutamate receptor (iGluR)-mediated neurotransmission using superfused mouse striatal slices and a non-metabolized L-glutamate analogue, D-[3H]aspartate. The direct and indirect presynaptic modulation of glutamate release and its susceptibility to changes in the intracellular levels of cyclic AMP (cAMP), Ca(2+) and calmodulin (CaM) and in protein phosphorylation was characterized by pharmacological manipulations. The agonists of iGluRs, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate, stimulated the basal release of D-[3H]aspartate, while N-methyl-D-aspartate (NMDA) was without effect. Both the AMPA- and kainate-mediated responses were accentuated by the beta-adrenoceptor agonist isoproterenol. These facilitatory effects were mimicked by the permeable cAMP analogue dibutyryl-cAMP. The beta-adrenoceptor antagonist propranolol, the adenylyl cyclase inhibitor MDL12,330A, the inhibitor of PKA and PKC, H-7, and the PKA inhibitor H-89 abolished the isoproterenol effect on the kainate-evoked release. The dibutyryl-cAMP-induced potentiation was also attenuated by H-7. Isoproterenol, propranolol and MDL12,330A failed to affect the basal release of D-[3H]aspartate, but dibutyryl-cAMP was inhibitory and MDL12,330A activatory. In Ca(2+)-free medium, the kainate-evoked release was enhanced, being further accentuated by the CaM antagonists calmidazolium and trifluoperazine, though these inhibited the basal release. The potentiating effect of calmidazolium on the kainate-stimulated release was counteracted by both MDL12,330A and H-7. We conclude that AMPA- and kainate-evoked glutamate release from striatal glutamatergic terminals is potentiated by beta-adrenergic receptor-mediated adenylyl cyclase activation and cAMP accumulation. Glutamate release is enhanced if the Ca(2+)- and CaM-dependent, kainate-evoked processes do not prevent the excessive accumulation of intracellular cAMP.

Presynaptic cross-talk of beta-adrenoreceptor and 5-hydroxytryptamine receptor signalling in the modulation of glutamate release from cerebrocortical nerve terminals

1. The presynaptic interactions between facilitatory beta-adrenoreceptors and inhibitory 5-hydroxytryptamine (5-HT) receptors modulating glutamate release from cerebrocortical nerve terminals were examined. 2. 4-aminopyridine (4-AP, 1 mM)-evoked glutamate release was facilitated by the membrane permeant cyclic-3',5'-adenosine monophosphate (cAMP) analogue, 8-bromo-cAMP (8-Br-cAMP), used to directly activate cAMP-dependent protein kinase (PKA). 3. The beta-adrenoreceptor agonist, isoprenaline (ISO), effected a concentration-dependent potentiation of 4-AP-evoked glutamate release which was abolished by the beta-adrenoreceptor antagonist, propranolol, and the PKA inhibitor, Rp-cyclic-3',5'-adenosine-monophosphothioate (Rp-cAMPS). 4. 5-HT receptor activation by 100 microM 5-HT produced an inhibition of 4-AP-evoked glutamate release in nerve terminals. The inhibitory effect of 5-HT could be mimicked by the selective 5-HT(1A) receptor agonist, 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and antagonized by 1-(2-methoxyphenyl)-4-(4-phthalimidobutyl)piperazine (NAN-190). 5. When 5-HT (or 8-OH-DPAT) was used in conjunction with ISO or 8-Br-cAMP, the beta-adrenoreceptor- and PKA-mediated potentiation of glutamate release was abrogated. 6. The inhibitory crosstalk of 5-HT(1A) receptors to beta-adrenoceptor-mediated facilitation of glutamate release was abolished in the presence of NAN-190. 7. Examination of voltage-dependent Ca(2+) influx revealed that, while ISO and 5-HT alone caused a respective potentiation and diminution of the 4-AP-evoked increase in [Ca(2+)](c), the co-presence of 5-HT abolished the ISO mediated potentiation of Ca(2+) influx. 8. Together, these results suggest that beta-adrenoreceptors and 5-HT(1A) receptors coexist on the cerebrocortical nerve terminals and that the cross-talk between the two receptor signalling pathways occurs at a locus downstream from cAMP production, possibly at the level of voltage-dependent Ca(2+) influx.

Adenosine A(2A) receptor facilitation of hippocampal synaptic transmission is dependent on tonic A(1) receptor inhibition

Adenosine tonically inhibits synaptic transmission through actions at A(1) receptors. It also facilitates synaptic transmission, but it is unclear if this facilitation results from pre- and/or postsynaptic A(2A) receptor activation or from indirect control of inhibitory GABAergic transmission. The A(2A) receptor agonist, CGS 21680 (10 nM), facilitated synaptic transmission in the CA1 area of rat hippocampal slices (by 14%), independent of whether or not GABAergic transmission was blocked by the GABA(A) and GABA(B) receptor antagonists, picrotoxin (50 microM) and CGP 55845 (1 microM), respectively. CGS 21680 (10 nM) also inhibited paired-pulse facilitation by 12%, an effect prevented by the A(2A) receptor antagonist, ZM 241385 (20 nM). These effects of CGS 21680 (10 nM) were occluded by adenosine deaminase (2 U/ml) and were made to reappear upon direct activation of A(1) receptors with N(6)-cyclopentyladenosine (CPA, 6 nM). CGS 21680 (10 nM) only facilitated (by 17%) the K(+)-evoked release of glutamate from superfused hippocampal synaptosomes in the presence of 100 nM CPA. This effect of CGS 21680 (10 nM), in contrast to the isoproterenol (30 microM) facilitation of glutamate release, was prevented by the protein kinase C inhibitors, chelerythrine (6 microM) and bisindolylmaleimide (1 microM), but not by the protein kinase A inhibitor, H-89 (1 microM). Isoproterenol (30 microM), but not CGS 21680 (10-300 nM), enhanced synaptosomal cAMP levels, indicating that the CGS 21680-induced facilitation of glutamate release involves a cAMP-independent protein kinase C activation. To discard any direct effect of CGS 21680 on adenosine A(1) receptor, we also show that in autoradiography experiments CGS 21680 only displaced the adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentyladenosine ([(3)H]DPCPX, 0.5 nM) with an EC(50) of 1 microM in all brain areas studied and CGS 21680 (30 nM) failed to change the ability of CPA to displace DPCPX (1 nM) binding to CHO cells stably transfected with A(1) receptors. Our results suggest that A(2A) receptor agonists facilitate hippocampal synaptic transmission by attenuating the tonic effect of inhibitory presynaptic A(1) receptors located in glutamatergic nerve terminals. This might be a fine-tuning role for adenosine A(2A) receptors to allow frequency-dependent plasticity phenomena without compromising the A(1) receptor-mediated neuroprotective role of adenosine.

The inhibition of glutamate release by metabotropic glutamate receptor 7 affects both [Ca2+]c and cAMP: evidence for a strong reduction of Ca2+ entry in single nerve terminals

Metabotropic glutamate receptors (mGluRs) from group III reduce glutamate release. Because these receptors reduce cAMP levels, we explored whether this signaling pathway contributes to release inhibition caused by mGluRs with low affinity for L-2-amino-4-phosphonobutyrate (L-AP4). In biochemical experiments with the population of cerebrocortical nerve terminals we find that L-AP4 (1 mm) inhibited the Ca(2+)-dependent-evoked release of glutamate by 25%. This inhibitory effect was largely prevented by the pertussis toxin but was insensitive to inhibitors of protein kinase C bisindolylmaleimide and protein kinase A H-89. Furthermore, this inhibition was associated with reduction in N-type Ca(2+) channel activity in the absence of any detectable change in cAMP levels. In the presence of forskolin, however, L-AP4 decreased the levels of cAMP. The activation of this additional signaling pathway was very efficient in counteracting the facilitation of glutamate release induced either by forskolin or the beta-adrenergic receptor agonist isoproterenol. Imaging experiments to measure Ca(2+) dynamics in single nerve terminals showed that L-AP4 strongly reduced the Ca(2+) response in 28% of the nerve terminals. Moreover, immunochemical experiments showed that 25-35% of the nerve terminals that were immunopositive to synaptophysin were also immunoreactive to the low affinity L-AP4-sensitive mGluR7. Then, mGluR7 mediates the inhibition of glutamate release caused by 1 mm L-AP4, primarily by a strong inhibition of Ca(2+) channels, although high cAMP uncovers the receptor ability to decrease cAMP.

Propranolol and metoprolol enhance the anticonvulsant action of valproate and diazepam against maximal electroshock

The anticonvulsive potential of classical antiepileptics co-administered with beta-adrenergic receptor antagonists against generalized tonic-clonic seizures was evaluated in the model of maximal electroshock (MES)-induced convulsions. Propranolol, acebutolol, metoprolol and atenolol were tested in the doses not affecting the electroconvulsive threshold. Propranolol and metoprolol lowered the ED(50) of valproate and diazepam. Acebutolol reduced valproate's but not diazepam's ED(50) value. In contrast, hydrophilic atenolol, not penetrating via blood-brain barrier, affected neither the action of valproate nor diazepam. None of the studied drugs changed the protective activity of carbamazepine and phenytoin against MES. beta-blockers per se did not alter the motor performance of mice. Moreover, propranolol and metoprolol did not influence diazepam-evoked impairment of locomotor activity. The free plasma and brain levels of antiepileptic drugs were not affected by beta-blockers. In conclusion, the use of certain beta-adrenoceptor antagonists, such as propranolol and metoprolol, might improve the antiepileptic potential of valproate and diazepam.

beta-Adrenoceptor blockade enhances the anticonvulsant effect of glutamate receptor antagonists against maximal electroshock

In this study, we evaluated whether beta-adrenoceptor antagonists may modify the protective efficacy of dizocilpine (MK-801), a NMDA receptor antagonist, and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), a non-NMDA (AMPA/kainate) receptor antagonist, against maximal electroshock-induced seizures in mice. Propranolol, acebutolol, metoprolol and atenolol were used in doses that did not alter the electroconvulsive threshold. Propranolol potentiated the anticonvulsant activity of MK-801 and GYKI 52466, significantly lowering their ED(50) values from 0.38 and 15.0 to 0.15 (P<0.001) and 8.4 mg/kg (P<0.001), respectively. Similarly, metoprolol lowered the ED(50) of MK-801 and GYKI 52466 from 0.38 and 15.0 to 0.17 (P<0.05) and 11.2 mg/kg (P<0.05). Acebutolol enhanced the protective action of GYKI 52466, lowering its ED(50) value from 15.0 to 12.2 mg/kg (P<0.05), but not that of MK-801. Atenolol, not penetrating the blood-brain barrier, did not affect the anticonvulsive efficacy of MK-801 and GYKI 52466. In conclusion, beta-adrenoceptor antagonists may act synergistically with excitatory amino acid receptor antagonists to inhibit generalised tonic-clonic seizures.

Evidence that the anti-spasmogenic effect of the beta-adrenoceptor agonist, isoprenaline, on guinea-pig trachealis is not mediated by cyclic AMP-dependent protein kinase

1. The spasmolytic and anti-spasmogenic activity of beta-adrenoceptor agonists on airways smooth muscle is thought to involve activation of the cyclic AMP/cyclic AMP-dependent protein kinase (PKA) cascade. Here we have tested the hypothesis that PKA mediates the anti-spasmogenic activity of isoprenaline and other cyclic AMP-elevating agents in guinea-pig isolated trachea by utilizing a number of cell permeant cyclic AMP analogues that act as competitive 'antagonists' of PKA. 2. Anion-exchange chromatography of guinea-pig tracheae resolved two peaks of PKA activity that corresponded to the type I ( approximately 5%) and type II ( approximately 93%) isoenzymes. 3. Pre-treatment of tracheae with zardaverine (30 microM), vasoactive intestinal peptide (VIP) (1 microM) and the non-selective activator of PKA, Sp-8-CPT-cAMPS (10 microM), produced a non-parallel rightwards shift in the concentration-response curves that described acetylcholine (ACh)-induced tension generation. The type II-selective PKA inhibitor, Rp-8-CPT-cAMPS (300 microM), abolished this effect. 4. Pre-treatment of tracheae with Sp-8-Br-PET-cGMPS (30 microM) produced a non-parallel rightwards shift of the concentration-response curves that described ACh-induced tension generation. The selective cyclic GMP-dependent protein kinase (PKG) inhibitor, Rp-8-pCPT-cGMPS (300 microM), abolished this effect. 5. Pre-treatment of tracheae with isoprenaline (1 microM) produced a 10 fold shift to the right of the ACh concentration-response curve by a mechanism that was unaffected by Rp-8-Br-cAMPS (300 microM, selective inhibitor of type I PKA), Rp-8-CPT-cAMPS (300 microM) and Rp-8-pCPT-cGMPS (300 microM). 6. We conclude that the anti-spasmogenic activity of Sp-8-CPT-cAMPS, zardaverine and VIP in guinea-pig trachea is attributable to activation of the cyclic AMP/PKA cascade whereas isoprenaline suppresses ACh-induced contractions by a mechanism(s) that is independent of PKA and PKG.

beta-Adrenoceptor and nNOS-derived NO interactions modulate hypoglycemic pial arteriolar dilation in rats

We examined the relative contributions from nitric oxide (NO) and catecholaminergic pathways in promoting cerebral arteriolar dilation during hypoglycemia (plasma glucose congruent with 1.4 mM). To that end, we monitored the effects of beta-adrenoceptor (beta-AR) blockade with propranolol (Pro, 1.5 mg/kg iv), neuronal nitric oxide synthase (nNOS) inhibition with 7-nitroindazole (7-NI, 40 mg/kg ip) or ARR-17477 (300 microM, via topical application), or combined intravenous Pro + 7-NI or ARR-17477 on pial arteriolar diameter changes in anesthetized rats subjected to insulin-induced hypoglycemia. Additional experiments, employing topically applied TTX (1 microM), addressed the possibility that the pial arteriolar response to hypoglycemia required neuronal transmission. Separately, Pro and 7-NI elicited modest but statistically insignificant 10-20% reductions in the normal ~40% increase in arteriolar diameter accompanying hypoglycemia. However, combined Pro-7-NI was accompanied by a >80% reduction in the hypoglycemia-induced dilation. On the other hand, the combination of intravenous Pro and topical ARR-17477 did not affect the hypoglycemia response. In the presence of TTX, the pial arteriolar response to hypoglycemia was lost completely. These results suggest that 1) beta-ARs and nNOS-derived NO interact in contributing to hypoglycemia-induced pial arteriolar dilation; 2) the interaction does not occur in the vicinity of the arteriole; and 3) the vasodilating signal is transmitted via a neuronal pathway.

Presynaptic signalling mediated by mono- and dinucleotides in the central nervous system

Synaptosomal preparations from rat midbrain exhibit specific responses to both ATP and Ap(5)A, which elicit a Ca(2+) entrance to the presynaptic terminals. Studies of isolated single terminals showed that not all the terminals contain ionotropic receptors for nucleotides, in fact only 46% of them do. Of these, 12% responded only to the dinucleotide Ap(5)A, and 20% to the mononucleotide ATP. At the presynaptic level, diinosine pentaphosphate, Ip(5)I, is a good tool to specifically block dinucleotide responses, which are inhibited at low nM concentration, versus the high microM concentrations required to block ATP responses. There is evidence for a presynaptic control of mononucleotide and dinucleotide responses, mediated by metabotropic and ionotropic receptors. Stimulation of adenosine A1 receptors increases the affinity of dinucleotide receptors by five orders of magnitude, from 30 microM to 680 pM for control and in the presence of A1 agonist, respectively.

Reciprocal autoreceptor and heteroreceptor control of serotonergic, dopaminergic and noradrenergic transmission in the frontal cortex: relevance to the actions of antidepressant agents

The frontal cortex (FCX) plays a key role in processes that control mood, cognition and motor behaviour, functions which are compromised in depression, schizophrenia and other psychiatric disorders. In this regard, there is considerable evidence that a perturbation of monoaminergic input to the FCX is involved in the pathogenesis of these states. Correspondingly, the modulation of monoaminergic transmission in the FCX and other corticolimbic structures plays an important role in the actions of antipsychotic and antidepressant agents. In order to further understand the significance of monoaminergic systems in psychiatric disorders and their treatment, it is essential to characterize mechanisms underlying their modulation. Within this framework, the present commentary focuses on our electrophysiological and dialysis analyses of the complex and reciprocal pattern of auto- and heteroreceptor mediated control of dopaminergic, noradrenergic and serotonergic transmission in the FCX. The delineation of such interactions provides a framework for an interpretation of the influence of diverse classes of antidepressant agent upon extracellular levels of dopamine, noradrenaline and serotonin in FCX. Moreover, it also generates important insights into strategies for the potential improvement in the therapeutic profiles of antidepressant agents.