[Show abstract][Hide abstract] ABSTRACT: NMDA receptors (NMDARs) mediate fast excitatory synaptic transmission and play a critical role in synaptic plasticity associated with learning and memory. NMDAR hypoactivity has been implicated in the pathophysiology of schizophrenia, and clinical studies reveal reduced negative symptoms of schizophrenia with a dose of pregnenolone that elevates serum levels of the neuroactive steroid pregnenolone sulfate (PregS). This report describes a novel process of delayed onset potentiation whereby PregS approximately doubles the cell's response to NMDA via a mechanism that is pharmacologically and kinetically distinct from rapid positive allosteric modulation by PregS. The number of functional cell surface NMDARs in cortical neurons increases 60 -100% within 10 min of exposure to PregS as shown by surface biotinylation and affinity purification. Delayed onset potentiation is reversible and selective for expressed receptors containing the NR2A or NR2B, but not NR2C or NR2D, subunits. Moreover, substitution of NR2B J-K helices and M4 domain with the corresponding region of NR2D ablates rapid allosteric potentiation of the NMDA response by PregS but not delayed onset potentiation. This demonstrates that the initial phase of rapid positive allosteric modulation is not a first step in NMDAR upregulation. Delayed onset potentiation by PregS occurs via a non- canonical, pertussis toxin sensitive G-protein coupled and Ca(2+) dependent mechanism that is independent of NMDAR ion channel activation. Further investigation into the sequelae for PregS stimulated trafficking of NMDARs to the neuronal cell surface may uncover a new target for the pharmacological treatment of disorders in which NMDAR hypofunction has been implicated.
[Show abstract][Hide abstract] ABSTRACT: NMDA receptor (NMDAR)-mediated excitatory synaptic transmission plays a critical role in synaptic plasticity and memory formation, whereas its dysfunction may underlie neuropsychiatric and neurodegenerative diseases. The neuroactive steroid pregnenolone sulfate (PS) acts as a cognitive enhancer in impaired animals, augments LTP in hippocampal slices by enhancing NMDAR activity, and may participate in the reduction of schizophrenia's negative symptoms by systemic pregnenolone. We report that the effects of PS on NMDAR function are diverse, varying with subunit composition and NR1 splice variant. While PS potentiates NR1-1a/NR2B receptors through a critical steroid modulatory domain in NR2B that also modulates tonic proton inhibition, potentiation of the NMDA response is not dependent upon relief of such inhibition, a finding that distinguishes it from spermine. In contrast, the presence of an NR2A subunit confers enhanced PS-potentiation at reduced pH, suggesting that it may indeed act like spermine does at NR2B-containing receptors. Additional tuning of the NMDAR response by PS comes via the N-terminal exon-5 splicing insert of NR1-1b, which regulates the magnitude of proton-dependent PS potentiation. For NR2C- and NR2D-containing receptors, negative modulation at NR2C receptors is pH-independent (like NR2B) while negative modulation at NR2D receptors is pH-dependent (like NR2A). Taken together, PS displays a rich modulatory repertoire that takes advantage of the structural diversity of NMDARs in the CNS. The differential pH sensitivity of NMDAR isoforms to PS modulation may be especially important given the emerging role of proton sensors to both learning and memory, as well as brain injury.
Journal of Neurochemistry 08/2011; 119(3):486-96. · 4.24 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Compounds targeting the benzodiazepine binding site of the GABAA-R are widely prescribed for the treatment of anxiety disorders, epilepsy, and insomnia as well as for pre-anesthetic sedation and muscle relaxation. It has been hypothesized that these various pharmacological effects are mediated by different GABAA-R subtypes. If this hypothesis is correct, then it may be possible to develop compounds targeting particular GABAA-R subtypes as, for example, selective anxiolytics with a diminished side effect profile. The pyrazolo[1,5-a]-pyrimidine ocinaplon is anxioselective in both preclinical studies and in patients with generalized anxiety disorder, but does not exhibit the selectivity between alpha1/alpha2-containing receptors for an anxioselective that is predicted by studies using transgenic mice.
We hypothesized that the pharmacological properties of ocinaplon in vivo might be influenced by an active biotransformation product with greater selectivity for the alpha2 subunit relative to alpha1. One hour after administration of ocinaplon, the plasma concentration of its primary biotransformation product, DOV 315,090, is 38% of the parent compound. The pharmacological properties of DOV 315,090 were assessed using radioligand binding studies and two-electrode voltage clamp electrophysiology. We report that DOV 315,090 possesses modulatory activity at GABAA-Rs, but that its selectivity profile is similar to that of ocinaplon.
These findings imply that DOV 315,090 could contribute to the action of ocinaplon in vivo, but that the anxioselective properties of ocinaplon cannot be readily explained by a subtype selective effect/action of DOV 315,090. Further inquiry is required to identify the extent to which different subtypes are involved in the anxiolytic and other pharmacological effects of GABAA-R modulators.
[Show abstract][Hide abstract] ABSTRACT: Studies using mice with point mutations of GABA(A) receptor alpha subunits suggest that the sedative and anxiolytic properties of 1,4-benzodiazepines are mediated, respectively, by GABA(A) receptors bearing the alpha(1) and alpha(2) subunits. This hypothesis predicts that a compound with high efficacy at GABA(A) receptors containing the alpha(1) subunit would produce sedation, whereas an agonist acting at alpha(2) subunit-containing receptors (with low or null efficacy at alpha(1)-containing receptors) would be anxioselective. Electrophysiological studies using recombinant GABA(A) receptors expressed in Xenopus oocytes indicate that maximal potentiation of GABA-stimulated currents by the pyrazolo-[1,5-a]-pyrimidine, DOV 51892, at alpha(1)beta(2)gamma(2S) constructs of the GABA(A) receptor was significantly higher (148%) than diazepam. In contrast, DOV 51892 was considerably less efficacious and/or potent than diazepam in enhancing GABA-stimulated currents mediated by constructs containing alpha(2), alpha(3), or alpha(5) subunits. In vivo, DOV 51892 increased punished responding in the Vogel conflict test, an effect blocked by flumazenil, and increased the percentage of time spent in the open arms of the elevated plus-maze. However, DOV 51892 had no consistent effects on motor function or muscle relaxation at doses more than 1 order of magnitude greater than the minimal effective anxiolytic dose. Although the mutant mouse data predict that the high-efficacy potentiation of GABA(A1a) receptor-mediated currents by DOV 51892 would be sedating, behavioral studies demonstrate that DOV 51892 is anxioselective, indicating that GABA potentiation mediated by alpha(1) subunit-containing GABA(A) receptors may be neither the sole mechanism nor highly predictive of the sedative properties of benzodiazepine recognition site modulators.
Journal of Pharmacology and Experimental Therapeutics 01/2007; 319(3):1244-52. · 3.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Benzodiazepines remain widely used for the treatment of anxiety disorders despite prominent, often limiting side effects including sedation, muscle relaxation, and ataxia. A compound producing a robust anxiolytic action comparable to benzodiazepines, but lacking these limiting side effects at therapeutic doses (an anxioselective agent), would represent an important advance in the treatment of generalized anxiety disorder, and perhaps other anxiety disorders. Here we report that the pyrazolo[1,5-a]-pyrimidine, ocinaplon, exhibits an anxioselective profile in both preclinical procedures and in patients with generalized anxiety disorder, the most common of the anxiety disorders. In rats, ocinaplon produces significant muscle relaxation, ataxia, and sedation only at doses >25-fold higher than the minimum effective dose (3.1 mg/kg) in the Vogel "conflict" test. This anticonflict effect is blocked by flumazenil (Ro 15-1788), indicating that like benzodiazepines, ocinaplon produces an anxiolytic action through allosteric modulation of GABA(A) receptors. Nonetheless, in eight recombinant GABA(A) receptor isoforms expressed in Xenopus oocytes, the potency and efficacy of ocinaplon to potentiate GABA responses varied with subunit composition not only in an absolute sense, but also relative to the prototypical benzodiazepine, diazepam. In a double blind, placebo controlled clinical trial, a 2-week regimen of ocinaplon (total daily dose of 180-240 mg) produced statistically significant reductions in the Hamilton rating scale for anxiety scores. In this study, the incidence of benzodiazepine-like side effects (e.g., sedation, dizziness) in ocinaplon-treated patients did not differ from placebo. These findings indicate that ocinaplon represents a unique approach both for the treatment and understanding of anxiety disorders.
Proceedings of the National Academy of Sciences 05/2005; 102(20):7380-5. · 9.81 Impact Factor