A subset of glutamate receptors that are specifically sensitive to the glutamate analog N-methyl-D-aspartate (NMDA) are molecular coincidence detectors, necessary for activity-dependent processes of neurodevelopment and in sensory and cognitive functions. The activity of these receptors is modulated by the endogenous amino acid D-serine, but the extent to which D-serine is necessary for the normal development and function of the mammalian nervous system was previously unknown. Decreased signaling at NMDA receptors has been implicated in the pathophysiology of schizophrenia based on pharmacological evidence, and several human genes related to D-serine metabolism and glutamatergic neurotransmission have been implicated in the etiology of schizophrenia. Here we show that genetically modified mice lacking the ability to produce D-serine endogenously have profoundly altered glutamatergic neurotransmission, and relatively subtle but significant behavioral abnormalities that reflect hyperactivity and impaired spatial memory, and that are consistent with elevated anxiety.

The recent discovery of glycine transporters in both the central nervous system and the periphery suggests that glycine transport may be critical to N-methyl-D-aspartate receptor (NMDAR) function by controlling glycine concentration at the NMDAR modulatory glycine site. Data obtained from whole-cell patch-clamp recordings of hippocampal pyramidal neurons, in vitro, demonstrated that exogenous glycine and glycine transporter type 1 (GLYT1) antagonist selectively enhanced the amplitude of the NMDA component of a glutamatergic excitatory postsynaptic current. The effect was blocked by 2-amino-5-phosphonovaleric acid and 7-chloro-kynurenic acid but not by strychnine. Thus, the glycine-binding site was not saturated under the control conditions. Furthermore, GLYT1 antagonist enhanced NMDAR function during perfusion with medium containing 10 M glycine, a concentration similar to that in the cerebrospinal f luid in vivo, thereby supporting the hypothesis that the GLYT1 maintains subsaturating concentration of glycine at synaptically activated NMDAR. The enhancement of NMDAR function by specific GLYT1 antagonism may be a feasible target for therapeutic agents directed toward diseases related to hypofunction of NMDAR.N-methyl-D-aspartate receptors (NMDAR) play a crucial role in several aspects of fast excitatory neurotransmission including the gating of an excitatory conductance with partial permeability to calcium (1) and, in some regions, long-term synaptic plasticity (2). Modulation of the NMDAR function occurs at a number of sites that are distinct from the glutamatebinding site (3). One of these sites is the strychnine-insensitive binding site where glycine acts to allosterically facilitate NMDAR function (4-6).Glycine is a necessary coagonist (7), potentiating NMDAR function in a wide variety of preparations of cerebral cortex with an apparent dissociation constant in the range from about 100 to 300 nM (8). Glycine-dependent enhancement of iontophoretically applied NMDA and of the NMDA component of an evoked synaptic potential in cortical slices in vitro has been demonstrated (6); however, the effective concentration of the applied glycine could not be determined and its physiological role in NMDAR modulation remains unclear (9). This is in large part because the concentration of glycine in the cerebrospinal fluid is about 6 M (10), which ought to saturate the NMDAR glycine site.The recent molecular and biochemical characterization of a class of glycine transporter proteins (11-14) in brain suggests that the glycine concentration in microdomains may be regulated by these transporters. Of these, the glycine transporter type 1 (GLYT1) is expressed primarily in glia and neurons of the neocortex and archicortex in association with regions of high NMDA expression (15-17). We examined the role of glycine and GLYT1 on synaptic currents elicited by stimulation of the CA3-CA1 Schaffer collateral axons in hippocampal slices of the rat in vitro. METHODSWhole-cell recordings were obtained with a technique modified from...

The amygdala is involved in the associative processes for both appetitive and aversive emotions, and its function is modulated by stress hormones. The neuropeptide corticotrophin releasing factor (CRF) is released during stress and has been linked to many stress-related behavioral, autonomic, and endocrine responses. In the present study, nonanxiety-inducing doses of a potent CRF type 1 and 2 receptor agonist, urocortin (Ucn), was infused locally into the basolateral amygdala (BLA) of rats. After 5 daily injections of Ucn, the animals developed anxiety-like responses in behavioral tests. Intravenous administration of the anxiogenic agent sodium lactate elicited robust increases in blood pressure, respiratory rate, and heart rate. Furthermore, in the absence of any additional Ucn treatment, these behavioral and autonomic responses persisted for Ͼ30 d. Whole-cell patch-clamp recordings from BLA neurons of these hyper-reactive animals revealed a pronounced reduction in both spontaneous and stimulation-evoked IPSPs, leading to a hyperexcitability of the BLA network. This Ucn-induced plasticity appears to be dependent on NMDA receptor and subsequent calcium-calmodulin-dependent protein kinase II (CaMKII) activation, because it is blocked by pretreatment with NMDA receptor antagonists and by coadministration of CaMKII inhibitors. Our results show for the first time a stress peptide-induced behavioral syndrome that can be correlated with cellular mechanisms of neural plasticity, a novel mechanism that may explain the etiological role of stress in several chronic psychiatric and medical disorders.

N-methyl-D-aspartate receptor (NMDAR) activation requires both the binding of glutamate to its recognition site and occupancy of the strychnine insensitive glycine modulatory site (GMS). Pharmacological studies suggest that the glycine transporter, GlyT1, maintains subsaturating concentrations of glycine at synaptic NMDARs. To characterize further the role of GlyT1, we generated mice in which the gene encoding GlyT1 was inactivated by homologous recombination through insertion of a PGK-Neo cassette in place of exons 2 and 3. Real-time quantitative PCR revealed no transcripts in newborn homozygous [GlyT1(؊͞؊)] mice and a 50% reduction in heterozygous (HZ) [GlyT1(؉͞؊)] mice as compared with WT littermates. The activity of Na ؉ -dependent glycine transport in forebrain homogenates was similarly affected. Homozygous mice died within 12 h of birth. In acute hippocampal slices, exogenous glycine or D-serine (10 M) enhanced NMDAR currents with Schaffer collateral stimulation in WT mice but not HZ mice, suggesting that the GMS was more occupied in the latter. The NMDAR͞␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor ratio of the excitatory postsynaptic currents was significantly increased in the HZ mice. In the water maze, the HZ mice exhibited better spatial retention. Furthermore, HZ mice were less sensitive to an amphetamine disruption of prepulse inhibition than WT mice but were more sensitive to the effects of MK-801. Thus, reduced expression of GlyT1 enhances hippocampal NMDAR function and memory retention and protects against an amphetamine disruption of sensory gating, suggesting that drugs which inhibit GlyT1 might have both cognitive enhancing and antipsychotic effects.glutamate ͉ glial cell ͉ memory ͉ prepulse inhibition

We have shown previously that low doses of selective sigma (cr)-receptor ligands potentiate the excitatory response of pyramidal neurons to NMDA in the CA, region of the dorsal hippocampus in the rat. Because progesterone competitively displaces the binding of the ligand N-[3H]allyl-normetazocine (SKF-10,047), the present studies were undertaken to determine in viva the effect of neuroactive steroids on NMDAinduced excitation of rat CA, pyramidal neurons. Low doses of dehydroepiandrosterone (DHEA) potentiated the NMDA response selectively and dose-dependently.The effect of DHEA was reversed by the selective v antagonist N-dipropyl-2-(4-methoxy-3-(2-phenylethoxy)phenyl)-ethylamine monohydrochloride (NE-100) and by haloperidol, but not by spiperone. Progesterone had no effect by itself but reversed, at low doses, the potentiation of the NMDA response induced by DHEA as well as those induced by nonsteroidal u ligands. Neither pregnenolone nor pregnenolone sulfate had any effect on the NMDA response-nor did they antagonize the potentiation of the NMDA response induced by DHEA and by nonsteroidal IJ ligands. A pet-tussis toxin pretreatment, which inactivates G,,,-proteins, abolished the potentiating effects of DHEA. Ovariectomy enhanced the potentiation of the NMDA response by the nonsteroidal CT ligand di(2-tolyl)guanidine (DTG). There was a reciprocal occlusion of the effects of DHEA and DTG; DTG did not potentiate the NMDA response further after DHEA, and DHEA did not do so after DTG. These results suggest that some neuroactive steroids modulate the NMDA response via g receptors.

The combination treatments were as well tolerated as fluoxetine monotherapy and more clinically effective. The study results, which add to a growing body of evidence, suggest that use of antidepressant combinations from treatment initiation may double the likelihood of remission compared with use of a single medication.

At endogenous brain concentrations, the astrocyte-derived metabolite kynurenic acid (KYNA) antagonizes the a7 nicotinic acetylcholine receptor and, possibly, the glycine co-agonist site of the NMDA receptor. The functions of these two receptors, which are intimately involved in synaptic plasticity and cognitive processes, may, therefore, be enhanced by reductions in brain KYNA levels. This concept was tested in mice with a targeted deletion of kynurenine aminotransferase II (KAT II), a major biosynthetic enzyme of brain KYNA. At 21 days of age, KAT II knock-out mice had reduced hippocampal KYNA levels (À71%) and showed significantly increased performance in three cognitive paradigms that rely in part on the integrity of hippocampal function, namely object exploration and recognition, passive avoidance, and spatial discrimination. Moreover, compared with wild-type controls, hippocampal slices from KAT II-deficient mice showed a significant increase in the amplitude of long-term potentiation in vitro. These functional changes were accompanied by reduced extracellular KYNA (À66%) and increased extracellular glutamate ( + 51%) concentrations, measured by hippocampal microdialysis in vivo. Taken together, a picture emerges in which a reduction in the astrocytic formation of KYNA increases glutamatergic tone in the hippocampus and enhances cognitive abilities and synaptic plasticity. Our studies raise the prospect that interventions aimed specifically at reducing KYNA formation in the brain may constitute a promising molecular strategy for cognitive improvement in health and disease.

On the basis of a previous report suggesting the effectiveness of the beta-adrenoceptor/5-HT1A antagonist pindolol to accelerate the antidepressant effect of a selective serotonin reuptake inhibitor (SSRI) and to produce a therapeutic effect in drug-resistant depressed patients, these open studies were undertaken to further explore the safety and efficacy of this strategy. In a first study, nine untreated unipolar depressed patients were given the SSRI paroxetine (20 mg/day) together with pindolol (2.5 mg thrice daily). One patient stopped taking pindolol because of increased irritability after 3 days. One week later, seven patients had their Hamilton Rating Scale for Depression Score decreased by more than 50%. In a second study, 19 drug-resistant unipolar depressed patients (9 on paroxetine, 5 on sertraline, 3 on fluoxetine, and 2 on moclobemide) were also given pindolol at the same regimen and were assessed weekly. Two patients (one on sertraline, one on moclobemide) stopped taking pindolol also because of increased irritability after 2 and 3 days, respectively. After 1 week of pindolol addition, 10 patients had a more than 50% decrease of their depression score. By day 14, all of the patients had a score of 10 or less, with the exception of those on sertraline. Improvement was maintained in all patients for at least 28 days on this combination. The results of these studies indicate that pindolol is safe when used in combination with an SSRI or moclobemide. Given the positive results obtained in this second open trial in 28 patients, this treatment strategy should be tested under double-blind conditions to establish its efficacy.