Anxiogenic-like effect of glycine and D-serine microinjected into dorsal periaqueductal gray matter of rats

Datumetine exposure alters hippocampal neurotransmitters system in C57BL/6 mice

Our previous study showed that datumetine modulates NMDAR activity with long term exposure leading to memory deficit and altered NMDAR signaling. We aim to explore the neurotransmitters perturbations of acute datumetine-NMDAR interaction. Fifteen C57/BL6 mice were used for the study, they are divided into three groups of 5 animals each. Animals were administered DMSO (DMSO/Control), 0.25 mg/kg body weight of datumetine (0.25 Datumetine) and 1 mg/kg bodyweight of datumetine (1.0 Datumetine) intraperitoneally for 14 days. At the end of treatment, animals were euthanized in isofluorane chamber, perfused transcardially with 1XPBS followed by PFA. Immunofluorescence procedure was done to check the distribution of neurons, astrocytes, microglia and major neuronal subtypes in the hippocampus. Expansion and electron microscopy techniques were used to assess the condition of the synapses. Quantitative data were expressed as mean ± SEM and analyzed using ANOVA with Tukey post hoc using p < 0.05 as significant. Datumetine increased the expression of CD11b, GFAP, vGlut1, GABA, CHRNA7 and TH while expression of TrPH and NeuN were reduced in the hippocampus compared to control animals. Synaptic loss was evident in datumetine exposed animals with reduced synaptic vesicles accompanied by a thickness of postsynaptic density than that of control animals. This study concludes that acute datumetine exposure alters hippocampal neurotransmitter systems.

D-cycloserine injected into the dorsolateral periaqueductal gray induces anxiolytic-like effects in rats

D-cycloserine (DCS) is a partial agonist of the glycine site coupled to the NMDA receptor (NMDAR). As a consequence, depending on the doses used it can function as an agonist or antagonist at this site. In rodents, anxiolytic-like effects have been observed after the systemic administration of high doses of DCS. The brain sites of these effects have not been investigated. Direct brain injection of glycine site antagonists or agonists into the dorsolateral periaqueductal gray (dlPAG), a brain structure involved in the modulation of defensive-related behaviors, produces anxiolytic- or anxiogenic-like effects, respectively. The present study investigated if the dlPAG could be a brain site of the anxiolytic effects observed after DCS systemic administration. Male Wistar rats received intra-dlPAG injections of DCS (25, 50, 100 or 200 nmol) and were exposed to the elevated plus-maze (EPM) or to the light-dark box. DCS, at the dose of 200 nmol, increased open arm exploration and the time spent in the light compartment, respectively. Based on this result we tested the effects of intra-dlPAG DCS (200 nmol) administration in animals submitted to the Vogel conflict tests. Anxiolytic-like effect was also observed in this test indicated by the increase of punished responses. The drug did not change locomotor activity, discarding potential confounding factors. These results indicated that administration of DCS, a partial agonist of the NMDAR-associated glycine site, into the dlPAG induces anxiolytic-like effects in different models, pointing to a possible site of action of this compound.

Effects of low-dose D-serine on recognition and working memory in mice

RATIONALE

D -Serine is an endogenous co-agonist of the N-methyl-D: -aspartate (NMDA) receptor and has been suggested to improve cognitive deficits in schizophrenia.

OBJECTIVES

The present study investigates the effects of treatment with D -serine in mice on tasks that require recognition learning and working memory, two cognitive domains that are impaired in schizophrenia.

METHODS

We studied the effects of various regimens of systemic administration of D -serine (50 mg/kg/day) on BALB/c mice performing object recognition, T-maze alternation, and open-field exploration tasks. For the object recognition task, we also contrasted the effects of D -serine and D -cycloserine and investigated whether D -serine could reverse alterations induced by subchronic injections of the NMDA antagonist MK-801. D -Serine levels after injections were measured by high-performance liquid chromatography.

RESULTS

In the object recognition task, pre-training treatment with D -serine or D -cycloserine significantly enhanced recognition memory 24 h after training. A single administration of D -serine 30 min (but not 6 h) after training produced similar enhancement, suggesting an effect on memory consolidation. Daily treatment with D: -serine enhanced both object recognition and T-maze performance over multiple days and improved short-term memory in MK-801-treated mice. D -Serine treatment did not alter open-field exploration. Behavioral effects were accompanied by increased levels of D -serine in the hippocampus of treated animals.

CONCLUSIONS

Our results show that treatment with D -serine can improve performance in tasks related to recognition learning and working memory, suggesting that this agent can be useful for the treatment of disorders involving declines in these cognitive domains.

Genetic loss of D-amino acid oxidase activity reverses schizophrenia-like phenotypes in mice

Reduced function of the N-methyl-d-aspartate receptor (NMDAR) has been implicated in the pathophysiology of schizophrenia. The NMDAR contains a glycine binding site in its NR1 subunit that may be a useful target for the treatment of schizophrenia. In this study, we assessed the therapeutic potential of long-term increases in the brain levels of the endogenous NMDAR glycine site agonist D-serine, through the genetic inactivation of its catabolic enzyme D-amino acid oxidase (DAO) in mice. The effects of eliminating DAO function were investigated in mice that display schizophrenia-related behavioral deficits due to a mutation (Grin 1(D481N)) in the NR1 subunit that results in a reduction in NMDAR glycine affinity. Grin 1(D481N) mice show deficits in sociability, prolonged latent inhibition, enhanced startle reactivity and impaired spatial memory. The hypofunctional Dao 1(G181R) mutation elevated brain levels of D-serine, but alone it did not affect performance in the behavioral measures. Compared to animals with only the Grin 1(D481N) mutation, mice with both the Dao1(G181R) and Grin 1(D481N) mutations displayed an improvement in social approach and spatial memory retention, as well as a reversal of abnormally persistent latent inhibition and a partial normalization of startle responses. Thus, an increased level of D-serine resulting from decreased catalysis corrected the performance of mice with deficient NMDAR glycine site activation in behavioral tasks relevant to the negative and cognitive symptoms of schizophrenia. Diminished DAO activity and elevations in D-serine may serve as an effective therapeutic intervention for the treatment of psychiatric symptoms.

Role of glutamate NMDA receptors and nitric oxide located within the periaqueductal gray on defensive behaviors in mice confronted by predator

RATIONALE

The midbrain periaqueductal gray (PAG) is part of the brain system involved in active defense reactions to threatening stimuli. Glutamate N-methyl-D: -aspartate (NMDA) receptor activation within the dorsal column of the PAG (dPAG) leads to autonomic and behavioral responses characterized as the fear reaction. Nitric oxide (NO) has been proposed to be a mediator of the aversive action of glutamate, since the activation of NMDA receptors in the brain increases NO synthesis.

OBJECTIVES

We investigated the effects of intra-dPAG infusions of NMDA on defensive behaviors in mice pretreated with a neuronal nitric oxide synthase (nNOS) inhibitor [Nomega-propyl-L: -arginine (NPLA)], in the same midbrain site, during a confrontation with a predator in the rat exposure test (RET).

MATERIALS AND METHODS

Male Swiss mice received intra-dPAG injections of NPLA (0.1 or 0.4 nmol/0.1 microl), and 10 min later, they were infused with NMDA (0.04 nmol/0.1 microl) into the dPAG. After 10 min, each mouse was placed in the RET.

RESULTS

NMDA treatment enhanced avoidance behavior from the predator and markedly increased freezing behavior. These proaversive effects of NMDA were prevented by prior injection of NPLA. Furthermore, defensive behaviors (e.g., avoidance, risk assessment, freezing) were consistently reduced by the highest dose of NPLA alone, suggesting an intrinsic effect of nitric oxide on defensive behavior in mice exposed to the RET.

CONCLUSIONS

These findings suggest a potential role of glutamate NMDA receptors and NO in the dPAG in the regulation of defensive behaviors in mice during a confrontation with a predator in the RET.

Alterations in NMDA receptor subunit densities and ligand binding to glycine recognition sites are associated with chronic anxiety in Alzheimer's disease

Glutamatergic deficits are established neuropathological features of Alzheimer's disease (AD) and are known to correlate with cognitive impairments. In contrast, the role of glutamatergic alterations in behavioral and psychological symptoms of dementia (BPSD) is unclear. There is considerable preclinical evidence for the importance of glycine recognition sites (GlyRS) of N-methyl-D-aspartate (NMDA) receptors in the regulation of anxiety behaviors. This study aimed to correlate several glutamatergic measures with chronic anxiety in AD. Twenty-one AD patients assessed by the Neuropsychiatric Inventory (NPI) were divided into low anxiety (LA) and high anxiety (HA) subgroups. GlyRS and NMDA channel were measured by brain homogenate binding with [(3)H]MDL105,519 and [(3)H]MK-801, respectively. Densities of NMDA receptor NR2A, NR2B and alternate spliced NR1 subunits were quantified by immunoblotting. We found that the binding affinity to GlyRS was significantly higher in HA compared to LA, and this higher GlyRS affinity correlated with selective reduction of NR2A density as well as with elevated anxiety scores. Our observations suggest a novel mechanism whereby subunit specific changes in the NMDA receptor complex may be linked to chronic anxiety in AD via effects on GlyRS function. We propose that NR2A and GlyRS should be further assessed as novel targets of behavioral pharmacotherapy in AD.

Dose-related anxiogenic effect of glycine in the elevated plus maze and in electrodermal activity

We investigated effect of glycine on anxiety at different doses using electrodermal activity and an elevated-plus maze. A single dose of glycine was injected intraperitoneally into three different groups of mice at 250 mg/kg, 750 mg/kg, and 1250 mg/kg. The anxiety scores with the elevated-plus maze, consisting of two open arms and two enclosed arms, were measured 30 minutes after injection. Then skin conductance level was measured. Glycine significantly decreased the times spent on the open arms in middle-dose and high-dose groups compared with the control group. The skin conductance level was statistically lower in high dose group than control groups. Conclusion; glycine at a dose of 750 mg/kg induced a nearly maximal anxiogenic effect because a higher dose was not more effective.

Involvement of dorsolateral periaqueductal gray N-methyl-D-aspartic acid glutamate receptors in the regulation of risk assessment and inhibitory avoidance behaviors in the rat elevated T-maze

The involvement of the dorsolateral periaqueductal gray in the regulation of fear-related behaviors such as escape and freezing is well established. It is still a matter of investigation, however, whether this midbrain area may have a relevant role in the modulation of more subtle defensive responses associated with anxiety such as risk assessment and inhibitory avoidance. By stimulating N-methyl-D-aspartic acid glutamate receptors located in the dorsolateral periaqueductal gray with its prototypical agonist N-methyl-D-aspartic acid (50 pmol), we report here an increase in both risk assessment and inhibitory avoidance behaviors of male Wistar rats tested in the elevated T-maze. These results are indicative of an anxiogenic-like effect. The selective N-methyl-D-aspartic acid receptor antagonist DL-2-amino-7-phosphonoheptanoic acid (2.0 and 4.0 nmol) had the opposite effect on both defensive tasks. Pretreatment with an ineffective dose of DL-2-amino-7-phosphonoheptanoic acid (1.0 nmol) prevented the N-methyl-D-aspartic acid anxiogenic-like effect. At the dose range of DL-2-amino-7-phosphonoheptanoic acid and/or N-methyl-D-aspartic acid tested, neither the escape response from one of the elevated T-maze open arms nor the general exploratory activity as assessed in the open-field test was affected. The present results suggest that the dorsolateral periaqueductal gray column is also involved in the regulation of defensive behaviors related to anxiety, and N-methyl-D-aspartic acid glutamate receptors are recruited for this action.

Elevated T-maze evaluation of anxiety and memory effects of NMDA/glycine-B site ligands injected into the dorsal periaqueductal gray matter and the superior colliculus of rats

Rat behaviors in the elevated T-maze (ETM) were evaluated following tectum microinjections of either glycine (GLY, 1, 10, 80 and 120 nmol) or d-serine (D-SER, 160 and 320 nmol), the putative endogenous agonists of GLY-B site at NMDA receptor, or the respective antagonist 7-chloro-kynurenic acid (7CK, 8 nmol). ETM performance was appraised by two validated scores of anxiety, i.e., the inhibitory avoidance duration (AD) and risk assessment behavior, and two scores derived from a newly developed approach to inhibitory avoidance learning curves, i.e., the learning median number of trials (T50) and avoidance variability (standard deviation of learning curve). Effects on aversive memory consolidation were assessed through changes in the AD measured 48 h after the full-acquisition of inhibitory avoidance. Drug effects were compared to those of vehicle. In most cases, microinjection of GLY-B site agonists into the dorsal periaqueductal gray (dPAG) produced increases in AD, which were compatible with an increase in anxiety. However, neither the intra-periaqueductal injection of 80 nmol GLY, nor that of 160 nmol D-SER, increased the AD. On the other hand, these microinjections invariably produced a parallel left shift in avoidance learning curves, thereby reducing the T50 but not the variability. Effects of 120 nmol GLY on AD and T50 were both antagonized by a previous microinjection of 7CK into the dPAG. The inverse relationship of AD and T50 suggests that increases in the anxiety level reduce the number of trials required for the acquisition of inhibitory avoidance. The above data also suggest the higher consistency and drug sensitivity of T50 as compared to the AD. In turn, whereas the microinjection of 120 nmol GLY into the superior colliculus (SC) did not affect the T50, it increased the AD. On the other hand, there was an increase in avoidance variability following the microinjection of either 120 nmol GLY into the SC or 8 nmol 7CK into the dPAG. Therefore, the GLY-B receptors within these structures seem to play opposite roles on avoidance variability. In contrast, neither of these treatments changed T50. Finally, whereas the risk assessment was solely decreased by the microinjection of GLY into the SC, the aversive memory was only impaired by the microinjection of 7CK into the dPAG. Overall, these data suggest that NMDA/GLY-B receptors of dPAG mediate both anxiety and aversive memory, while those in the SC are most likely involved with attention and visuomotor components of risk assessment behavior.

Lack of interaction between NMDA and cholecystokinin-2 receptor-mediated neurotransmission in the dorsolateral periaqueductal gray in the regulation of rat defensive behaviors

Several neurotransmitters, including GABA, serotonin, glutamate, and cholecystokinin, modulate defensive behaviors in the dorsolateral periaqueductal gray (dlPAG). Although both glutamate and cholecystokinin have been shown to facilitate these behaviors, a possible interaction between them remains to be examined. The present study investigates whether activation or antagonism of N-methyl-D-aspartic acid (NMDA) glutamate and cholecystokinin 2 (CCK(2)) receptors located in the dlPAG would interact in animals tested in the elevated T-maze. The effect of the NMDA (50 pmol) was evaluated in rats pretreated with the CCK(2) receptor antagonist LY225910 (0.05 nmol). In addition, the effect of the CCK(2) receptor agonist CCK-4 (0.08 nmol) was evaluated in rats pretreated with the NMDA receptor antagonist AP-7 (1.0 nmol). Intra-dlPAG injection of NMDA increased risk assessment and inhibitory avoidance behaviors. This NMDA anxiogenic-like effect was unaltered by the pretreatment with LY225910. Similarly, the shortening of escape latencies induced by CCK-4 was unaffected by AP-7. No drug changed the general exploratory activity as assessed in the open-field. These results, showing that the activation of dlPAG NMDA or CCK(2) receptors facilitate anxiety- and fear-related behaviors, further implicate glutamate and cholecystokinin-mediated neurotransmission in this midbrain area on modulation of defensive behaviors. However, the regulatory action of these two excitatory neurotransmitters seems to be exerted through independent mechanisms.

D-amino acids in the central nervous system in health and disease

Recent evidence has shown that d-amino acids are present in animals and humans in high concentrations and fulfill specific biological functions. In the central nervous system, two d-amino acids, d-serine and d-aspartate, occur in considerable concentrations. d-Serine is synthesized and metabolized endogenously and the same might account for d-aspartate. d-Serine has been studied most extensively and was shown to play a role in excitatory amino acid metabolism, being a co-agonist of the N-methyl-d-aspartate (NMDA) receptor. Insight into d-serine metabolism is relevant for physiological NMDA receptor (NMDAr) activation and for all the disorders associated with an altered function of the NMDAr, such as schizophrenia, ischemia, epilepsy, and neurodegenerative disorders. d-Aspartate appears to play a role in development and endocrine function, but the precise function of d-aspartate and other d-amino acids in animals and humans requires further investigation. As d-amino acids play biological roles, alterations in the concentrations of d-amino acids might occur in some disorders and relate to the pathogenesis of these disorders. d-Amino acid concentrations may then not only help in the diagnostic process, but also provide novel therapeutic targets. Consequently, the presence and important roles of d-amino acids in higher organisms do not only challenge former theories on mammalian physiology, but also contribute to exciting new insights in human disease.

Involvement of nitric oxide-dependent pathways of dorsolateral periaqueductal gray in the effects of ethanol in rats submitted to the elevated plus-maze test

Our previous study showed the microinjection of drugs that influence the nitric oxide (NO)-mediated neurotransmission in the hippocampus impacts upon the anxiolytic-like effect of ethanol. In this study, we examined whether NO-dependent pathways of the dorsolateral periaqueductal gray (dlPAG) participate in the anxiolytic effect of ethanol in rats submitted to the elevated plus-maze test. We evaluated the impact on ethanol effects of the nitric oxide synthase (NOS) inhibitor 7-nitroindazole, the soluble guanylate cyclase inhibitor 1H-(1,2,4)-oxodiazolo (4,3-a) quinoxalin-1-one (ODQ), the cyclic guanylate monophosphate (cGMP) analogue 8-bromo-cGMP and the NO donor sodium nitroprusside. The results showed that ODQ and 7-nitroindazole increased the percentage of open arm entries and of time spent on open arms in the elevated plus maze in rats injected with ethanol at 1.0g/kg, a dose that did not produce anxiolysis per se. Conversely, 8-bromo-cGMP and sodium nitroprusside blocked the increased exploration of open arms exhibited by rats treated with a higher dose of ethanol (1.2g/kg). Taken together, the results suggest that the inhibition of NO-dependent pathways of the dlPAG enhances the anxiolytic effect of ethanol, whereas the activation of these pathways results in an opposite effect.

Transmissão pelo glutamato como alvo molecular na ansiedade

O glutamato (GLU) é o principal neurotransmissor excitatório do cérebro de mamíferos. Os receptores do GLU são classificados em ionotrópicos ou metabotrópicos. A interferência do GLU no desenvolvimento neural, na plasticidade sináptica, no aprendizado e na memória, na epilepsia, na isquemia neural, na tolerância e na dependência a drogas, na dor neuropática, na ansiedade e na depressão tem limitado o uso de compostos que agem nos receptores de GLU, quando existe a necessidade de ações mais seletivas dessas drogas. Dados pré-clínicos em roedores e humanos têm mostrado que compostos que reduzem a ativação do GLU, pelo bloqueio dos seus receptores ou através da redução da sua liberação dos terminais, produzem um perfil ansiolítico em modelos de ansiedade. A aplicação desses compostos em áreas específicas do cérebro, envolvidas na mediação do comportamento defensivo, tal como a substância cinzenta periaquedutal dorsal, também reproduzem o mesmo perfil ansiolítico de ação. O conhecimento crescente acerca da neurotransmissão pelo GLU e o desenvolvimento de compostos mais seletivos atuantes nesta neurotransmissão, renovaram a atenção para esse sistema neurotransmissor como alvo molecular possível para uma nova classe de drogas no tratamento de condições neuropsiquiátricas. Embora incompleta, esta revisão tenta atrair a atenção para a importância de estudos colaborativos entre clínicos e pesquisadores de ciências básicas na geração de idéias para alvos potenciais no desenvolvimento de novos compostos ansiolíticos. e desta maneira contribuir para a compreensão das bases biológicas da ansiedade.

Influence of age and of pre-treatment with D-cycloserine on the behavior of ethanol-treated rats tested in the elevated plus-maze apparatus

There is evidence that ethanol is able to influence central functions through the antagonism of the NMDA-receptor system. It has been shown that this system is also involved in the modulation of anxiety-related behavior in rats. Recently, we observed gender- and age-related behavioral influences in rats tested on the elevated plus-maze apparatus The present study was undertaken in order to investigate: (1) the effects of ethanol (0.8, 1.0 or 1.2 g/kg, i.p.) on the behavior of male and female rats tested on the elevated plus-maze at 2, 3, 4 or 5 months of age; (2) the effect of the pre-treatment with D-cycloserine (3.0 or 6.0 mg/kg), an agonist of the glutamate NMDA-receptor system, 30 min before the ethanol (1.2 g/kg) injections, in rats tested in the elevated plus-maze at 2 months or 4 months of age. The results demonstrated that ethanol did not affect the time spent and the frequency of entries on the open arms of the elevated plus-maze in rats tested at 2 months of age, but increased these parameters in older animals. Moreover, the results showed that D-cycloserine, at doses that did not affect the behavior of control animals, antagonized the increased frequency of entries and time spent on open arms produced by ethanol in rats tested at 4 months of age. Our results suggest an age-related influence on the anxiolytic action of ethanol in rats tested in the elevated plus-maze. Moreover, the results suggest that the NMDA-receptor system can be involved in this effect, and strengthens the evidence for the participation of the NMDA-receptor system in anxiety-related behavior.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

The Vogel conflict test: procedural aspects, gamma-aminobutyric acid, glutamate and monoamines

A multitude of mechanisms are involved in the control of emotion and in the response to stress. These incorporate mediators/targets as diverse as gamma-aminobutyric acid (GABA), excitatory amino acids, monoamines, hormones, neurotrophins and various neuropeptides. Behavioural models are indispensable for characterization of the neuronal substrates underlying their implication in the etiology of anxiety, and of their potential therapeutic pertinence to its management. Of considerable significance in this regard are conflict paradigms in which the influence of drugs upon conditioned (trained) behaviours is examined. For example, the Vogel conflict test, which was introduced some 30 years ago, measures the ability of drugs to release the drinking behaviour of water-deprived rats exposed to a mild aversive stimulus ("punishment"). This model, of which numerous procedural variants are discussed herein, has been widely used in the evaluation of potential anxiolytic agents. In particular, it has been exploited in the characterization of drugs interacting with GABAergic, glutamatergic and monoaminergic networks, the actions of which in the Vogel conflict test are summarized in this article. More recently, the effects of drugs acting at neuropeptide receptors have been examined with this model. It is concluded that the Vogel conflict test is of considerable utility for rapid exploration of the actions of anxiolytic (and anxiogenic) drugs. Indeed, in view of its clinical relevance, broader exploitation of the Vogel conflict test in the identification of novel classes of anxiolytic agents, and in the determination of their mechanisms of action, would prove instructive.

Modulation of defensive behavior by periaqueductal gray NMDA/glycine-B receptor

Glutamate (GLU) associated with glycine, act as co-transmitter at the N-methyl-D-aspartate/glycine-B (NMDA/GLY(B)) receptor. Dorsal periaqueductal gray (dPAG) neurons express NMDA/GLY(B) receptors suggesting a GLU physiological role in mediating the responses elicited by stimulation of this area. Immunohistochemical data provided evidence of a possible correlation among elevated plus-maze (EPM), fear-like defensive behavior, and dPAG activity. The present data show that whereas the NMDA/GLY(B) receptor agonists increased the open-arm avoidance responses in the EPM, the antagonists had the opposite effects. Microinjection of NMDA/GLY(B) receptor agonists within the dPAG during test sessions in the EPM resulted in an enduring learned fear response detected in the retest. Therefore, in addition to the proposed role for the dPAG in panic attacks (escape), these findings suggest that the dPAG can also participate in more subtle anxiety-like behaviors.

Vibrissal sense is not the main sensory modality in rat exploratory behavior in the elevated plus-maze

Four groups of male Wistar rats were submitted to acute bilateral removal of mystacial vibrissae at different lengths from the follicle. Each group was divided into two subgroups, tested under high (150 Lux) and low environmental illumination (2 Lux). All the subjects were allowed to freely explore an elevated plus-maze for 5 min. Results indicated that rats tested under low illumination tended to explore the open arms more frequently and longer then rats tested under high illumination. When tested under low illumination, rats in the group that suffered whole vibrissa removal stayed longer in the open arms than those in the other groups but did not differ in the number of entries. The average increase in the length of open arm entries, rather than a decrease in aversion to the open arms, may be due to the need of more time to obtain information about the environment since there is no light and the vibrissae were removed. This effect was not seen with rats tested under high illumination, possibly because vision could be used to obtain relevant information.

Does long term potentiation in periacqueductal gray (PAG) mediate lasting changes in rodent anxiety-like behavior (ALB) produced by predator stress?--Effects of low frequency stimulation (LFS) of PAG on place preference and changes in ALB produced by predator stress

The effects on rodent behavior of low frequency bilateral stimulation (LFS, 900 pulses at 1 Hz) of periacqueducatal gray (PAG) was investigated. The first experiment examined aversive qualities of LFS in a place preference paradigm. There was no evidence of a place preference after 1 or 7 applications of LFS. After the first LFS, rats showed longer latencies to leave the conditioned chamber, suggesting a positively reinforcing effect of LFS. Latency differences were not accounted for by freezing or immobility prior to leaving. Rats with electrodes outside the PAG did not show these effects. After repeated LFS, stimulated rats did not differ from controls in place preference or in anxiety-like behavior (ALB). Experiment 2 studied the effects of predator stress in unimplanted rats on an extended battery of measures of ALB in hole board, plus maze and light/dark box tests of rodent anxiety. Effects of electrode damage in the PAG on ALB was also examined. In addition, the effect of 7 applications of bilateral LFS of PAG on ALB following a 5 min unprotected exposure of rats to a cat (predator stress) was examined. Predator stress lastingly changed a wide variety of behaviors in the plus maze, [Rodgers, Behav. Pharmacol. 8 (1997) 477] replicating and extending previous reports. A new finding is an increase in light avoidance in the light/dark box test. Moreover, factor analysis revealed open arm avoidance, risk assessment, light avoidance and cautious exploration loaded on independent factors, replicating and extending previous findings. Bilateral, but not unilateral, damage specific to PAG was also found to be anxiolytic in plus maze measures of ALB. Bilateral implants in the PAG seemed to prevent many of the effects of predator stress on ALB measured 8 days later. Nevertheless, predator stress did decrease head dips in the open arm and LFS reversed this effect. Light avoidance also increased following predator stress and LFS reversed this increase. These findings suggest the PAG occupies an important position in the final common path of substrate changes mediating effects of predator stress on a range of behaviors in the rodent. The fact that LFS in the PAG can reverse stress induced changes in behavior supports the idea that LTP in PAG mediates stress induced increases in anxiety in rodents, as it does in the cat [Adamec, Neurosci. Biobevav. Rev. 21(6) (1997) 755; Adamec, J. Psychopharmacol. 2000 (in press); Adamec, J. Psychopharmacol. 2000 (in press); Adamec, J. Psychopharmacol. 12(2) (1998) 129; Adamec, J. Psychopharmacol. 12(13) (1998) 227].

Expression and conditioned inhibition of fear-potentiated startle after stimulation and blockade of AMPA/Kainate and GABA(A) receptors in the dorsal periaqueductal gray

Previous work showed that the dorsal periaqueductal gray is involved in the inhibition of fear-potentiated startle. The present study investigated the effects of blockade and stimulation of Kainate/AMPA and GABA(A) receptors within the dorsal periaqueductal gray on expression and conditioned inhibition of fear-potentiated startle. Blockade of the Kainate/AMPA receptors enhanced whereas stimulation of the Kainate/AMPA receptors decreased expression of fear-potentiated startle. These effects do not reflect conditioned inhibition since this modulation was not changed by injections of Kainate/AMPA receptor agonists or antagonists into the dorsal periaqueductal gray. Stimulation and blockade of GABA(A) receptors within the dorsal periaqueductal gray neither affected expression of fear-potentiated startle nor its conditioned inhibition. The present results together with findings from the literature indicate that glutamate in the dorsal periaqueductal gray is a critical substrate for the expression and modulation of fear-related behaviours.

L-type calcium channels selectively control the defensive behaviors induced by electrical stimulation of dorsal periaqueductal gray and overlying collicular layers

The present study reports the involvement of L-type calcium channels in the control of defensive behaviors produced by electrical stimulation of dorsal periaqueductal gray and overlying collicular layers. Rats that had chemitrodes in the dorsal midbrain and which stimulation produced freezing or flight behaviors with less than 55 microA were selected for drug experiments. Stimulation was repeated the day after the screening session 20 min following the microinjection into the dorsal periaqueductal gray of 15 nmol of either verapamil, a selective L-type calcium channel antagonist, or cobalt chloride (CoCl(2)), a calcium-specific channel modulator. Post-drug sessions were performed 48 h after. Threshold functions were obtained by logistic fitting of accumulated response frequencies. Verapamil and CoCl(2) significantly attenuated the output of immobility, exophthalmus, running and jumping. Although to a lesser degree, verapamil also attenuated defecation. Because CoCl(2) had no effect on defecation, the attenuation of this response by verapamil suggests a non-specific action of this drug. Neither verapamil nor CoCl(2) changed the output of micturition. Finally, whereas there was a complete recovery of defensive thresholds following the microinjection of verapamil, the attenuating effects of CoCl(2) were still present 48 h after. These results support an important role of L-type calcium channels in the neurogenesis of dorsal periaqueductal gray-evoked immobility, exophthalmus, running and jumping, but not defecation and micturition responses.

The "anxiety state" and its relation with rat models of memory and habituation

Rats selected as "anxious", "nonanxious," or normal according to their behavior in an elevated plus maze were submitted to memory tasks and the densities of central benzodiazepine receptors in the amygdala and the hippocampus were studied. Anxious rats exibited better retention scores in the inhibitory avoidance task while nonanxious rats exibited worse retention scores in inhibitory and two-way active avoidance tasks compared to normal rats. No significant differences were detected in the retention scores for habituation to an open field. Nonanxious rats presented a lower benzodiazepine receptor density in the hippocampus but not in the amygdala compared to the other groups. These data suggest that the benzodiazepine receptors are involved in the effect of "anxiety" or emotional states on memory storage processes.

Pharmacological evaluation of ricinine, a central nervous system stimulant isolated from Ricinus communis

The extract of the pericarp of castor bean (Ricinus communis) showed some typical central nervous system stimulant effects when administered to mice. The animals became exophthalmic, presented tremors and clonic seizures and died a few minutes after receiving larger doses of the extract. At lower doses the extract improved memory consolidation and showed some neuroleptic-like properties, such as a decrease in exploratory behavior and catalepsy. The memory-improving effect and the seizure-eliciting properties of the extract were also observed with the administration of ricinine, a neutral alkaloid isolated from the extract. However, the neuroleptic-like properties of the extract were not observed with ricinine. As the therapeutic index of ricinine is of the order of 200, the compound may be considered as a promising cognition-enhancing drug that may be used for the treatment of human amnesias.

Local connections between the columns of the periaqueductal gray matter: a case for intrinsic neuromodulation

Chemical stimulation of the lateral or ventrolateral columns of the midbrain periaqueductal gray matter (PAG) in conscious animals produces opposite responses (viz., defensive behavior and pressor responses from the lateral column vs. quiescence and depressor responses from the ventrolateral column), raising the possibility that the two columns are interconnected. To test this hypothesis, two types of anatomical experiments were performed in rats. First, the anterograde axonal marker Phaseolus vulgaris leuco-agglutinin (PHA-L) was injected into individual PAG columns or adjoining regions which included the Edinger-Westphal, dorsal raphe, and precommissural nuclei. The results shows that each column projects bilaterally to all of the other PAG columns, and also provides local connections within its own column. Furthermore, the Edinger-Westphal and precommissural nuclei project to all four PAG columns, while the dorsal raphe nucleus projects only to the ventrolateral and lateral columns. In a second experiment, we found that cardiovascular-related PAG projection neurons of both the lateral and ventrolateral columns receive an input from the reciprocal PAG column. This was demonstrated by a double tracer neuroanatomical study in which PHA-L was first iontophoretically ejected into either the lateral or ventrolateral PAG columns and then, several days later the retrograde transneuronal viral tracer, pseudorabies virus, was injected into the stellate sympathetic ganglion. Intra-PAG circuits were visualized by a dual immunohistochemical procedure. These results suggest that during the fight-or-flight response when the 'fight' program is activated, inhibition of the 'flight' PAG network may occur and the converse situation may occur during the flight response.

NMDA-coupled periaqueductal gray glycine receptors modulate anxioselective drug effects on plus-maze performance

The present study was carried out to investigate a possible interaction between the effects of anxiety modulating drugs which act at the GABA-A receptor complex and selective N-methyl-D-aspartic acid (NMDA) coupled glycine receptor (GLY-B receptor) ligands within the dorsal periaqueductal gray (DPAG). The plus-maze performance of rats pretreated with diazepam (0.37 and 0.75 mg/kg, i.p.) or pentylenetetrazole (15 and 30 mg/kg, i.p.), standard anxiolytic and anxiogenic drugs respectively, was assessed following intra-periaqueductal injections of either glycine (0.2 M, 0.4 microl/30 s, i.c.) or its competitive antagonist, 7-chlorokynurenic acid (7ClKYN, 0.02 M, 0.4 microl/30 s, i.c.). Whilst diazepam produced a typical anxiolytic effect in intracranially-injected CSF rats, increasing open arm exploration, pentylenetetrazole displayed an opposite anxiogenic profile. Either anxiogenic or anxiolytic effects were seen in peripherally-injected vehicle rats following intra-periaqueductal injections of glycine or 7ClKYN, respectively. Intra-periaqueductal injection of glycine markedly attenuated the anxiolytic effect of diazepam. Moreover, while the anxiogenic effects of pentylenetetrazole were barely changed by glycine, they were markedly attenuated by intra-periaqueductal injection of 7ClKYN. Interaction of diazepam and 7ClKYN produced non-selective sedative-like effects which masked any possible anxiolytic action. Accordingly, the present results suggest that the NMDA-coupled glycine receptors located in the DPAG interfere with anxioselective effects of GABA-A acting drugs on the elevated plus-maze. In spite of the prevailing notion that the NMDA coupled glycine receptor is saturated at in vivo brain concentrations of glycine, our results also suggest that either unoccupied or low-affinity GLY-B receptors are likely to be activated by glycine injection into DPAG.

Anxiolytic activity of glycine-B antagonists and partial agonists--no relation to intrinsic activity in the patch clamp

On the basis of animal models, anxiety was one of the first suggested clinical applications of partial agonists of the glycineB site coupled to the NMDA receptor. It is not certain, however, whether these findings can be extended to full glycineB antagonists and what is the relation between intrinsic activity (degree of NMDA receptor antagonism) and anxiolytic effect. In the present study several NMDA receptor antagonists, including several glycineB antagonists/partial agonists, were tested for anxiolytic activity in the Vogel conflict test and the elevated plus-maze. Additionally, the intrinsic activities of the glycineB partial agonists used [ACPC, (R,+)-HA-966 and D-cycloserine] were compared in patch-clamp experiments in cultured neurones. In the plus-maze the most striking increase in the time spent in open arms (index of anxiolytic effect) was seen after diazepam and D-cycloserine (at doses that did not change locomotion). Also reliable (dose-dependent), although weaker, anxiolytic activity was produced by the uncompetitive NMDA receptor antagonist (+)MK-801 and the competitive antagonist CGP 39551. Modest anxiolytic-like effect in the plus-maze was also observed after the glycineB antagonist L-701,324 and the partial agonist (+,R)-HA-966. Uncompetitive antagonists memantine and amantadine, the glycineB partial agonist ACPC (up to 600 mg/kg) or the full antagonists MRZ 2/570, MRZ 2/571 and MRZ 2/576 had no effect. In the Vogel conflict test neither memantine, nor any of the full glycineB antagonists tested (L-701,324 and MRZ 2/576), showed anxiolytic activity. Patch-clamp studies revealed that the intrinsic activity of (+,R)-HA-966, D-cycloserine and ACPC was 13, 57 and 92%, respectively, as compared to that of glycine itself (100%). In conclusion, for the agents tested there is no clear relation between the levels of intrinsic activity, i.e. degree of NMDA receptor inhibition, and anxiolytic activity. Moreover, L-701,324 and MRZ-type glycineB full antagonists do not exchibit anxiolytic activity in the elevated plus-maze and Vogel conflict test.

Free D-aspartate and D-serine in the mammalian brain and periphery

It has long been assumed that L-forms of amino acids exclusively constitute free amino acid pools in mammals. However, a variety of studies in the last decade has demonstrated that free D-aspartate and D-serine occur in mammals and may have important physiological function in mammals. Free D-serine is confined predominantly to the forebrain structure, and the distribution and development of D-serine correspond well with those of the N-methyl-D-aspartate (NMDA)-type excitatory amino acid receptor. As D-serine acts as a potent and selective agonist for the strychnine-insensitive glycine site of the NMDA receptor, it is proposed that D-serine is a potential candidate for an NMDA receptor-related glycine site agonist in mammalian brain. In contrast, widespread and transient emergence of a high concentration of free D-aspartate is observed in the brain and periphery. Since the periods of maximal emergence of D-aspartate in the brain and periphery occur during critical periods of morphological and functional maturation of the organs, D-aspartate could participate in the regulation of these regulation of these developmental processes of the organs. This review deals with the recent advances in the studies of presence of free D-aspartate and D-serine and their metabolic systems in mammals. Since D-aspartate and D-serine have been shown to potentiate NMDA receptor-mediated transmission through the glutamate binding site and the strychnine-insensitive glycine binding site, respectively, and have been utilized extensively as potent and selective tools to study the excitatory amino acid system in the brain, we shall discuss also the NMDA receptor and uptake system of D-amino acids.

D-serine as a neuromodulator: regional and developmental localizations in rat brain glia resemble NMDA receptors

D-Serine is localized in mammalian brain to a discrete population of glial cells near NMDA receptors, suggesting that D-serine is an endogenous agonist of the receptor-associated glycine site. To explore this possibility, we have compared the immunohistochemical localizations of D-serine, glycine, and NMDA receptors in rat brain. In the telencephalon, D-serine is concentrated in protoplasmic astrocytes, which are abundant in neuropil in close vicinity to NMDA receptor 2A/B subunits. Ultrastructural examination of the CA1 region of hippocampus reveals D-serine in the cytosolic matrix of astrocytes that ensheath neurons and blood vessels, whereas NR2A/B is concentrated in dendritic spines. By contrast, glycine immunoreactivity in telencephalon is the lowest in brain. During postnatal week 2, D-serine levels in cerebellum are comparable to those in adult cerebral cortex but fall to undetectable levels by day 26. During week 2, we observe parallel ontogeny of D-serine in Bergmann glia and NR2A/B in Purkinje cells, suggesting a role for astrocytic D-serine in NMDA receptor-mediated synaptogenesis. D-Serine in the radial processes of Bergmann glia is also well positioned to regulate NMDA receptor-dependent granule cell migration. In the inner granule layer, D-serine is found transiently in protoplasmic astrocytes surrounding glomeruli, where it could regulate development of the mossy fiber/granule cell synapse. D-Serine seems to be the endogenous ligand of glycine sites in the telencephalon and developing cerebellum, whereas glycine predominates in the adult cerebellum, olfactory bulb, and hindbrain.