Allosteric potentiation by diazoxide of AMPA receptor currents and synaptic potentials

Potential Alzheimer's early biomarkers in a transgenic rat model and benefits of diazoxide/dibenzoylmethane co-treatment on spatial memory and AD-pathology

Alzheimer's disease (AD) is the major form of dementia prevalent in older adults and with a high incidence in females. Identification of early biomarkers is essential for preventive intervention to delay its progression. Furthermore, due to its multifactorial nature, a multi-target approach could be therapeutically beneficial. Our studies included 4- (pre-pathology) and 11-month (mild-pathology) TgF344-AD rats, a transgenic Alzheimer's model that exhibits age-dependent AD progression. We identified two potential early biomarker genes for AD, early growth response 2 (EGR2) and histone 1H2AA (HIST1H2AA), in the hippocampus of 4-month females. Out of 17,168 genes analyzed by RNA sequencing, expression of these two genes was significantly altered in 4-month TgF344-AD rats compared to wild-type littermates. We also evaluated co-treatment with diazoxide (DZ), a potassium channel activator, and dibenzoylmethane (DIB), which inhibits eIF2α-P activity, on TgF344-AD and wild-type rats. DZ/DIB-treatment mitigated spatial memory deficits and buildup of hippocampal Aβ plaques and tau PHF in 11-month TgF344-AD rats but had no effect on wild-type littermates. To our knowledge, this preclinical study is the first to report EGR2 and HIST1H2AA as potential AD biomarkers in females, and the benefits of DZ/DIB-treatment in AD. Evaluations across multiple AD-related models is warranted to corroborate our findings.

7-Chloro-5-(furan-3-yl)-3-methyl-4H-benzo[e][1,2,4]thiadiazine 1,1-Dioxide as Positive Allosteric Modulator of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptor. The End of the Unsaturated-Inactive Paradigm?

5-Arylbenzothiadiazine type compounds acting as positive allosteric modulators of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-PAMs) have received particular attention in the past decade for their nootropic activity and lack of the excitotoxic side effects of direct agonists. Recently, our research group has published the synthesis and biological activity of 7-chloro-5-(3-furanyl)-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine 1,1-dioxide (1), one of the most active benzothiadiazine-derived AMPA-PAMs in vitro to date. However, 1 exists as two stereolabile enantiomers, which rapidly racemize in physiological conditions, and only one isomer is responsible for the pharmacological activity. In the present work, experiments carried out with rat liver microsomes show that 1 is converted by hepatic cytochrome P450 to the corresponding unsaturated derivative 2 and to the corresponding pharmacologically inactive benzenesulfonamide 3. Surprisingly, patch-clamp experiments reveal that 2 displays an activity comparable to that of the parent compound. Molecular modeling studies were performed to rationalize these results. Furthermore, mice cerebral microdialysis studies suggest that 2 is able to cross the blood-brain barrier and increases acetylcholine and serotonin levels in the hippocampus. The experimental data disclose that the achiral hepatic metabolite 2 possesses the same pharmacological activity of its parent compound 1 but with an enhanced chemical and stereochemical stability, as well as an improved pharmacokinetic profile compared with 1.

Thermodynamics and structural analysis of positive allosteric modulation of the ionotropic glutamate receptor GluA2

Positive allosteric modulators of the ionotropic glutamate receptor-2 (GluA2) are promising compounds for the treatment of cognitive disorders, e.g. Alzheimer's disease. These modulators bind within the dimer interface of the LBD (ligand-binding domain) and stabilize the agonist-bound conformation slowing receptor desensitization and/or deactivation. In the present study, we employ isothermal titration calorimetry to determine binding affinities and thermodynamic details of binding of modulators of GluA2. A mutant of the LBD of GluA2 (LBD-L483Y-N754S) that forms a stable dimer in solution was used. The potent GluA2 modulator BPAM-97 was used as a reference compound. Evidence that BPAM-97 binds in the same pocket as the well-known GluA2 modulator cyclothiazide was obtained from X-ray structures. The LBD-L483Y-N754S:BPAM-97 complex has a Kd of 5.6 μM (ΔH=-4.9 kcal/mol, -TΔS=-2.3 kcal/mol; where 1 kcal≈4.187 kJ). BPAM-97 was used in a displacement assay to determine a Kd of 0.46 mM (ΔH=-1.2 kcal/mol, -TΔS=-3.3 kcal/mol) for the LBD-L483Y-N754S:IDRA-21 complex. The major structural factors increasing the potency of BPAM-97 over IDRA-21 are the increased van der Waals contacts to, primarily, Met496 in GluA2 imposed by the ethyl substituent of BPAM-97. These results add important information on binding affinities and thermodynamic details, and provide a new tool in the development of drugs against cognitive disorders.

Interactions between recording technique and AMPA receptor modulators

Whole cell recording (EPSCs) and extracellular recording (field EPSPs) were compared in hippocampal field CA1 with regard to the effects of experimental treatments that increase AMPA receptor gated currents. Cyclothiazide, which maintains AMPA receptors in the sensitized state, caused a rapid and pronounced increase in EPSCs but only minor changes in field EPSPs. This difference was evident in recordings carried out at 22 and 32 degrees C and with different solutions in the clamp pipette. The larger effect of cyclothiazide on EPSCs was unaffected by blockade of GABA and NMDA receptors. Two-dimensional current source density analyses derived from 64 recording sites were used to provide extracellular estimates of AMPA receptor mediated synaptic currents. With this method, cyclothiazide again had much smaller effects than were obtained with whole cell clamp. Differences between whole cell and extracellular recordings were present, although not as pronounced, for the ampakines, a class of drugs that slow both deactivation and desensitization of AMPA receptors. Additionally, increases in synaptic responses produced by frequency facilitation, a manipulation that enhances the number of bound receptors, were not qualitatively different between recording techniques. These results support the conclusion that the whole cell clamp technique may alter AMPA receptors in such a way as to increase the relative importance of desensitization.

6-Chloro-3-alkylamino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide derivatives potently and selectively activate ATP sensitive potassium channels of pancreatic beta-cells

6-Chloro-3-alkylamino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide derivatives were synthesized and characterized as activators of adenosine 5'-triphosphate (ATP) sensitive potassium (K(ATP)) channels in the beta-cells by measuring effects on membrane potential and insulin release in vitro. The effects on vascular tissue in vitro were measured on rat aorta and small mesenteric vessels. Selected compounds were characterized as competitive inhibitors of [(3)H]glibenclamide binding to membranes of HEK293 cells expressing human SUR1/Kir6.2 and as potent inhibitors of insulin release in isolated rat islets. 6-Chloro-3-(1-methylcyclobutyl)amino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (54) was found to bind and activate the SUR1/Kir6.2 K(ATP) channels in the low nanomolar range and to be at least 1000 times more potent than the reference compound diazoxide with respect to inhibition of insulin release from rat islets. Several compounds, e.g., 3-propylamino- (30), 3-isopropylamino- (34), 3-(S)-sec-butylamino- (37), and 3-(1-methylcyclopropyl)amino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (53), which were found to be potent and beta-cell selective activators of K(ATP) channels in vitro, were found to inhibit insulin secretion in rats with minimal effects on blood pressure and to exhibit good oral pharmacokinetic properties.

Effects of the potent ampakine CX614 on hippocampal and recombinant AMPA receptors: interactions with cyclothiazide and GYKI 52466

R,S-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor up-modulators of the benzamide type ("ampakines") have previously been shown to enhance excitatory synaptic transmission in vivo and in vitro and AMPA receptor currents in excised patches. The present study analyzed the effects of an ampakine (CX614; 2H,3H, 6aH-pyrrolidino[2",1"-3',2']1,3-oxazino[6',5'-5,4]benz o[e]1, 4-dioxan-10-one) that belongs to a benzoxazine subgroup characterized by greater structural rigidity and higher potency. CX614 enhanced the size (amplitude and duration) of field excitatory postsynaptic potentials in hippocampal slices and autaptically evoked excitatory postsynaptic currents in neuronal cultures with EC(50) values of 20 to 40 microM. The compound blocked desensitization (EC(50) = 44 microM) and slowed deactivation of responses to glutamate by a factor of 8.4 in excised patches. Currents through homomeric, recombinant AMPA receptors were enhanced with EC(50) values that did not differ greatly across GluR1-3 flop subunits (19-37 microM) but revealed slightly lower potency at corresponding flip variants. Competition experiments using modulation of [(3)H]fluorowillardiine binding suggested that CX614 and cyclothiazide share a common binding site but cyclothiazide seems to bind to an additional site not recognized by the ampakine. CX614 did not reverse the effect of GYKI 52466 on responses to brief glutamate pulses, which indicates that they act through separate sites, a conclusion that was confirmed in binding experiments. In sum, these results extend prior evidence that ampakines are effective in enhancing synaptic responses, most likely by slowing deactivation, and that their effects are exerted through sites that are only in part shared with other modulators.

(S)-2,3-dihydro-[3,4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide: (S18986-1) a positive modulator of AMPA receptors enhances (S)-AMPA-mediated [3H]noradrenaline release from rat hippocampal and frontal cortex slices

The present study describes the effect of (S)-2,3-dihydro-[3, 4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide (S18986-1), a positive allosteric modulator of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors with cognitive-enhancing effects, on (S)-AMPA-induced [3H]noradrenaline release in rat hippocampal and frontal cortex slices. (S)-AMPA significantly increased [3H]noradrenaline release in rat hippocampus and frontal cortex slices, whereas S18986-1 (3-1000 microM) alone, was inactive. However, S18986-1 between 30 and 1000 microM potently enhanced (+200%) (S)-AMPA-mediated [3H]noradrenaline release in both hippocampal and frontal cortex slices. The capacity of S18986-1 to potentiate [3H]noradrenaline release was specific for AMPA receptors as S18986-1 failed to potentiate either kainate and N-methyl-D-aspartate (NMDA)-mediated release of [3H]noradrenaline in rat hippocampal slices. Moreover, 1, 2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX) and 1-(4-aminophenyl)-3-methylcarbamoyl-4-methyl-3, 4-dihydro-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI-53655) but not (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine ((+)-MK-801), inhibited (S)-AMPA and S18986-induced stimulation of (S)-AMPA-mediated [3H]noradrenaline release. In addition, S18986-1-induced stimulation of (S)-AMPA-evoked [3H]noradrenaline release was markedly attenuated in the presence of tetrodotoxin (1 microM) and in Ca(2+)-free buffer. S18986-1 enhanced (S)-AMPA-mediated [3H]noradrenaline release to a greater extent than its corresponding (R)-enantiomer S19024-1 and racemic mixture S17951-1. However, positive allosteric modulators of AMPA receptors such as aniracetam failed to potentiate AMPA-mediated noradrenaline release in hippocampal slices, whereas cyclothiazide potently enhanced (S)-AMPA-mediated [3H]noradrenaline release. These results suggest that the capacity of S18986-1 to enhance AMPA receptor-mediated release of noradrenaline in rat hippocampus and frontal cortex, could contribute to the cognition enhancing mechanisms of S18986-1.

The waveform of synaptic transmission at hippocampal synapses is not determined by AMPA receptor desensitization

Relationships between the kinetic properties of AMPA receptors and the decay phase of fast excitatory transmission were investigated using modulatory drugs. The benzothiadiazide compound cyclothiazide blocked receptor desensitization in patches excised from hippocampus but had only a weak influence on receptor deactivation, i.e., on the decay of responses produced by a 1-ms pulse of glutamate. The ampakine drug CX516 (BDP-12) produced an opposite pattern of effects: a fourfold slowing of deactivation with little change in desensitization. A structurally related drug (CX554 or BDP-20) had prominent effects on both desensitization and deactivation. The halfwidth of field EPSPs measured in the CA1 region of hippocampal slices increased 50-100% in the presence of CX516 or CX554 but by less than 15% at concentrations of cyclothiazide that fully blocked desensitization in patch experiments. These results indicate that receptor deactivation plays a substantially greater role than receptor desensitization in determining the duration of synaptic responses.

Cyclothiazide and AMPA receptor desensitization: analyses from studies of AMPA-induced release of [3H]-noradrenaline from hippocampal slices

1. Responses in brain produced by the activation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) subtype of ionotropic receptor for L-glutamate are often rapidly desensitizing. AMPA-induced desensitization and its characteristics, and the potentiating effect of cyclothiazide were investigated in vitro by analysing AMPA-induced release of [3H]-noradrenaline from prisms of rat hippocampus. 2. AMPA (1-1000 microM) stimulated the release of [3H]-noradrenaline in a concentration-dependent manner that was both calcium-dependent and tetrodotoxin-sensitive, and attenuated by the AMPA-selective antagonists, NBQX (1 and 10 microM), LY 293558 (1 and 10 microM) and GYKI 52466 (10 and 30 microM). 3. By use of an experimental procedure with consecutive applications of AMPA (100 microM, 28 min apart), the second response was reduced, indicative of receptor desensitization, and was reversed by cyclothiazide in a concentration-dependent manner (1-300 microM). The concentration-response curve for AMPA-induced release of [3H]-noradrenaline was shifted leftwards, but the reversal by cyclothiazide of the desensitized response was partial and failed to reach the maximal response of the first stimulus. 4. Observations made with various schedules of cyclothiazide application indicated that the initial AMPA-evoked response was already partially desensitized (150% potentiation by 100 microM cyclothiazide) and that the desensitization was not likely to be due to a time-dependent diminution and was longlasting (second application of cyclothiazide was ineffective). 5. Co-application of a number of drugs with actions on second messenger systems, in association with the second AMPA stimulus, revealed significant potentiation of the AMPA-induced release of [3H]-noradrenaline: forskolin (10 microM, +78%), Rp-cAMPS (100 microM, +65%), Ro 31-8220 (10 microM, +163%) and thapsigargin (100 pM, + 161%). 6. The AMPA receptor-mediated response regulating the release of [3H]-noradrenaline from rat hippocampal slices was desensitized and cyclothiazide acted to reverse partially the desensitization in a concentration-dependent manner. Since the time-course of desensitization was longer lasting than that noted in previous electrophysiological studies, multiple events may be involved in the down-regulation of AMPA receptor activity including receptor phosphorylation and depletion of intracellular Ca2+ stores.

New developments in the molecular pharmacology of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate and kainate receptors

Separation of non-N-methyl-D-aspartate subtypes of glutamate receptors, known as alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate receptors, is traced through conventional pharmacology to molecular biology. The physiology and pharmacology of recombinant receptor subtypes (GluR1-7 and KA1-2) are described. Competitive antagonists, e.g., the quinoxalinedione, 2,3-dihyroxy-6-nitro-7-sulphamoyl-benz(F)quinoxaline, and the decahydroisoquinoline, 3S,4aR,6R, 8aR-6-[2-(1(2)H-tetrazol-5-yl)ethyl]-decahydroisoquinolin e-3-carboxylate, have a broad antagonist spectrum, except that the latter is inactive on GluR6. The 2,3-benzodiazepines noncompetitively antagonise the AMPA receptor GluR1-4. Desensitisation of AMPA (GluR1-4) and kainate (GluR5-7 and KA1-2) receptors is blocked by cyclothiazide and concanavalin A, respectively. Polyamine toxins block AMPA receptors not containing GluR2 and unedited kainate receptors (GluR5-6). These data correlate well with results on native neurons characterised by techniques such as in situ hybridisation.

Effects of AMPA receptor modulators on the production of arachidonic acid from striatal neurons

The abilities of different compounds acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors to modulate the overflow of [3H]arachidonic acid from rat striatal neurons were examined. The combination of AMPA (0.1 mM) and carbachol (1mM) stimulated [3H]arachidonic acid production, this effect could be dose-dependently enhanced by the newly discovered allosteric modulator of AMPA receptors: cyclothiazide. Competitive (6-cyano-7-nitroquinoxaline-2,3-dione [CNQX] and 6-(1-imidazolyl)-7-nitroquinazoline-2,3-dione [YM 900]) and non-competitive antagonists, like 1-(amino)-phenyl)-4-methyl -7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), antagonized the responses induced by either AMPA + carbachol or AMPA + carbachol + cyclothiazide. In order to appreciate the respective part of AMPA-versus kainate-preferring receptors experiments were performed with kainic acid (0.1 mM) and the more specific kainate agonist domoic acid (0.1 mM). Kainic acid behaves like AMPA, but the response induced by the combination domoic acid + carbachol could not be potentiated by cyclothiazide. On the contrary, concanavalin A potentiated the responses evoked by kainic acid or domoic acid (in combination with carbachol) but did not enhance the AMPA-evoked response. It could be concluded that both AMPA- and kainate-preferring receptors are present in cultured rat striatal neurons and that these two types of receptors were involved together with muscarinic receptors in the overflow of [3H]arachidonic acid.

Actions of diazoxide on CA1 neurons in hippocampal slices from rats

Membrane effects of diazoxide (DZX) were examined in CA1 pyramidal neurons, mainly by whole-cell recording in slices kept at 33 degrees C (from Sprague-Dawley rats). Bath applications of DZX (0.65 mM) did not significantly change the resting input conductance; but instantaneous inward rectification was reduced by 47 +/- 14% (near -110 mV). There was a similar depression of a large, sustained voltage-dependent outward current (by 44 +/- 11% near 0 mV). A nearly identical reduction of the outward current recorded in a Ca current suppressing medium (but not in 30 mM tetraethylammonium) indicated that the DZX-sensitive current includes the delayed rectifier. In Mn, low-Ca medium containing tetraethylammonium and carbachol, DZX potentiated (by 43 +/- 12%) the D-type slowly decaying outward current seen after hyperpolarizing pulses at a holding potential of approximately -50 mV. DZX abolished or depressed slow inward currents, such as the tetrodotoxin-sensitive persistent Na current, high voltage activated Ca currents (IC50 = 0.47 mM), and the Q current. In 6 of 13 cells recorded with electrodes containing either guanosine or adenosine diphosphate, DZX potentiated the voltage-dependent outward current, but input conductances were reduced. In conclusion, although there was little indication that it activates classical KATP channels in CA1 neurons, DZX strongly depresses several voltage-dependent, slowly inactivating outward and inward currents, which are important modulators of cell excitability.