Regulation of GABA release via NMDA and 5-HT1A receptors in guinea pig dentate gyrus

Chronic memantine disrupts spatial memory and up-regulates Htr1a gene expression in the hippocampus of GPR39 (zinc-sensing receptor) KO male mice

GPR39 is a receptor involved in zincergic neurotransmission, and its role in regulating psychological functions is an active area of research. The purported roles of GPR39 at the cellular level include regulation of inflammatory and oxidative stress response, and modulation of GABAergic and endocannabinoid neurotransmission. GPR39 knock-out (KO) mice exhibit episodic-like and spatial memory (ELM and SM, respectively) deficits throughout their lifetime, and are similar in that respect to senescent wild-type (WT) conspecifics. Since a role for zinc has been postulated in neurodegenerative disorders, in this study we investigated the possibility of a pharmacological rescue of both types of declarative memory with memantine - a noncompetitive NMDAR antagonist used for slowing down dementia; or, a putative GPR39 agonist - TC-G 1008. First, we tested adult WT and GPR39KO male mice under acute 5 mg/kg memantine or vehicle treatment in an object recognition task designed to simultaneously probe the "what?", "where?" and "when?" components of ELM. Next, we investigated the impact of chronic memantine or TC-G 1008 on ELM and SM (Morris water maze, MWM) in both WT and GPR39KO mice. Following chronic experiments, we assessed with qRT-PCR hippocampal gene expression of targets previously associated with GPR39. We report: no effects of acute memantine on ELM; a tendency to improve the "where?" component of ELM in both WT and GPR39 KO mice following 12 days of memantine; and, a disruption of SM in GPR39KO mice after 24 days of memantine treatment. The latter result was associated with upregulation of Htr1a hippocampal expression.

Biased agonism in drug discovery: Is there a future for biased 5-HT1A receptor agonists in the treatment of neuropsychiatric diseases?

Serotonin (5-HT) is one of the fundamental neurotransmitters that contribute to the information essential for an organism's normal, physiological function. Serotonin acts centrally and systemically. The 5-HT_1A receptor is the most widespread serotonin receptor, and participates in many brain-related disorders, including anxiety, depression, and cognitive impairments. The 5-HT_1A receptor can activate several different biochemical pathways and signals through both G protein-dependent and G protein-independent pathways. Preclinical experiments indicate that distinct signaling pathways in specific brain regions may be crucial for antidepressant-like, anxiolytic-like, and procognitive responses. Therefore, the development of new ligands that selectively target a particular signaling pathway(s) could open new possibilities for more effective and safer pharmacotherapy. This review discusses the current state of preclinical studies focusing on the concept of functional selectivity (biased agonism) regarding the 5-HT_1A receptor and its role in antidepressant-like, anxiolytic-like, and procognitive regulation. Such work highlights not only the differential effects of targeted autoreceptors, vs. heteroreceptors, but also the importance of targeting specific downstream intracellular signaling processes, thereby enhancing favorable over unfavorable signaling activation.

Personality traits and polymorphisms of genes coding neurotransmitter receptors or transporters: review of single gene and genome-wide association studies

BACKGROUND

The most popular tool used for measuring personality traits is the Five-Factor Model (FFM). It includes neuroticism, extraversion, openness, agreeableness and conscientiousness. Many studies indicated the association of genes encoding neurotransmitter receptors/transporters with personality traits. The relationship connecting polymorphic DNA sequences and FFM features has been described in the case of genes encoding receptors of cannabinoid and dopaminergic systems. Moreover, dopaminergic system receives inputs from other neurotransmitters, like GABAergic or serotoninergic systems.

METHODS

We searched PubMed Central (PMC), Science Direct, Scopus, Cochrane Library, Web of Science and EBSCO databases from their inception to November 19, 2020, to identify original studies, as well as peer-reviewed studies examining the FFM and its association with gene polymorphisms affecting the neurotransmitter functions in central nervous system.

RESULTS

Serotonin neurons modulate dopamine function. In gene encoding serotonin transporter protein, SLC6A4, was found polymorphism, which was correlated with openness to experience (in Sweden population), and high scores of neuroticism and low levels of agreeableness (in Caucasian population). The genome-wide association studies (GWASs) found an association of 5q34-q35, 3p24, 3q13 regions with higher scores of neuroticism, extraversion and agreeableness. However, the results for chromosome 3 regions are inconsistent, which was shown in our review paper.

CONCLUSIONS

GWASs on polymorphisms are being continued in order to determine and further understand the relationship between the changes in DNA and personality traits.

Corticosterone in the ventral hippocampus differentially alters accumbal dopamine output in drug-naïve and amphetamine-withdrawn rats

Dysregulation in glucocorticoid stress and accumbal dopamine reward systems can alter reward salience to increase motivational drive in control conditions while contributing to relapse during drug withdrawal. Amphetamine withdrawal is associated with dysphoria and stress hypersensitivity that may be mediated, in part, by enhanced stress-induced corticosterone observed in the ventral hippocampus. Electrical stimulation of the ventral hippocampus enhances accumbal shell dopamine release, establishing a functional connection between these two regions. However, the effects of ventral hippocampal corticosterone on this system are unknown. To address this, a stress-relevant concentration of corticosterone (0.24ng/0.5 μL) or vehicle were infused into the ventral hippocampus of urethane-anesthetized adult male rats in control and amphetamine withdrawn conditions. Accumbal dopamine output was assessed with in vivo chronoamperometry. Corticosterone infused into the ventral hippocampus rapidly enhanced accumbal dopamine output in control conditions, but produced a biphasic reduction of accumbal dopamine output in amphetamine withdrawal. Selectively blocking glucocorticoid-, mineralocorticoid-, or cytosolic receptors prevented the effects of corticosterone. Overall, these results suggest that the ability of corticosterone to alter accumbal dopamine output requires cooperative activation of mineralocorticoid and glucocorticoid receptors in the cytosol, which is dysregulated during amphetamine withdrawal. These findings implicate ventral hippocampal corticosterone in playing an important role in driving neural systems involved in positive stress coping mechanisms in healthy conditions, whereas dysregulation of this system may contribute to relapse during withdrawal.

5-HT1A parital agonism and 5-HT7 antagonism restore episodic memory in subchronic phencyclidine-treated mice: role of brain glutamate, dopamine, acetylcholine and GABA

RATIONALE

The effect of atypical antipsychotic drugs (AAPDs), e.g., lurasidone, to improve cognitive impairment associated with schizophrenia (CIAS), has been suggested to be due, in part, to enhancing release of dopamine (DA), acetylcholine (ACh), and glutamate (Glu) in cortex and hippocampus.

RESULTS

The present study found acute lurasidone reversed the cognitive deficit in novel object recognition (NOR) in subchronic (sc) phencyclidine (PCP)-treated mice, an animal model for CIAS. This effect of lurasidone was blocked by pretreatment with the 5-HT_1AR antagonist, WAY-100635, or the 5-HT₇R agonist, AS 19. Lurasidone significantly increased medial prefrontal cortex (mPFC) ACh, DA, and Glu efflux, all of which were blocked by WAY-100635, with similar effects in the dorsal striatum (dSTR), except for the absence of an effect on Glu increase. AS 19 inhibited Glu, but not DA efflux, in the dSTR. The selective 5-HT₇R antagonist, SB-26970, increased mPFC DA, 5-HT, Glu, and, importantly, also GABA efflux and striatal DA, NE, 5-HT, and Glu efflux, indicating tonic inhibition of the release of these neurotransmitters by 5-HT₇R stimulation. These results provide new evidence that GABA release in the mPFC is tonically inhibited by 5-HT₇R stimulation and suggest that a selective 5-HT₇R antagonist might be clinically useful to enhance cortical GABAergic release. All SB-269970 effects were blocked by AS 19 or WAY-100635, suggesting 5-HT_1AR agonism is necessary for the release of these neurotransmitters by SB-269970. Lurasidone increased ACh, DA, and NE but not Glu efflux in mPFC and dSTR DA and Glu efflux in 5-HT₇ KO mice.

CONCLUSION

We conclude that lurasidone-induced Glu efflux in mPFC requires 5-HT₇R antagonism while its effects on cortical ACh and DA efflux are mainly due to 5-HT_1AR stimulation.

Neural mechanisms underlying GABAergic regulation of adult hippocampal neurogenesis

Within the dentate gyrus of the adult hippocampus is the subgranular zone, which contains a neurogenic niche for radial-glia like cells, the most primitive neural stem cells in the adult brain. The quiescence of neural stem cells is maintained by tonic gamma-aminobutyric acid (GABA) released from local interneurons. Once these cells differentiate into neural progenitor cells, GABA continues to regulate their development into mature granule cells, the principal cell type of the dentate gyrus. Here, we review the role of GABA circuits, signaling, and receptors in regulating development of adult-born cells, as well as the molecular players that modulate GABA signaling. Furthermore, we review recent findings linking dysregulation of adult hippocampal neurogenesis to the altered GABAergic circuitry and signaling under various pathological conditions.

Neuronal Circuitry Mechanisms Regulating Adult Mammalian Neurogenesis

The adult mammalian brain is a dynamic structure, capable of remodeling in response to various physiological and pathological stimuli. One dramatic example of brain plasticity is the birth and subsequent integration of newborn neurons into the existing circuitry. This process, termed adult neurogenesis, recapitulates neural developmental events in two specialized adult brain regions: the lateral ventricles of the forebrain. Recent studies have begun to delineate how the existing neuronal circuits influence the dynamic process of adult neurogenesis, from activation of quiescent neural stem cells (NSCs) to the integration and survival of newborn neurons. Here, we review recent progress toward understanding the circuit-based regulation of adult neurogenesis in the hippocampus and olfactory bulb.

Impact of actin filament stabilization on adult hippocampal and olfactory bulb neurogenesis

Rearrangement of the actin cytoskeleton is essential for dynamic cellular processes. Decreased actin turnover and rigidity of cytoskeletal structures have been associated with aging and cell death. Gelsolin is a Ca(2+)-activated actin-severing protein that is widely expressed throughout the adult mammalian brain. Here, we used gelsolin-deficient (Gsn(-/-)) mice as a model system for actin filament stabilization. In Gsn(-/-) mice, emigration of newly generated cells from the subventricular zone into the olfactory bulb was slowed. In vitro, gelsolin deficiency did not affect proliferation or neuronal differentiation of adult neural progenitors cells (NPCs) but resulted in retarded migration. Surprisingly, hippocampal neurogenesis was robustly induced by gelsolin deficiency. The ability of NPCs to intrinsically sense excitatory activity and thereby implement coupling between network activity and neurogenesis has recently been established. Depolarization-induced [Ca(2+)](i) increases and exocytotic neurotransmitter release were enhanced in Gsn(-/-) synaptosomes. Importantly, treatment of Gsn(-/-) synaptosomes with mycotoxin cytochalasin D, which, like gelsolin, produces actin disassembly, decreased enhanced Ca(2+) influx and subsequent exocytotic norepinephrine release to wild-type levels. Similarly, depolarization-induced glutamate release from Gsn(-/-) brain slices was increased. Furthermore, increased hippocampal neurogenesis in Gsn(-/-) mice was associated with a special microenvironment characterized by enhanced density of perfused vessels, increased regional cerebral blood flow, and increased endothelial nitric oxide synthase (NOS-III) expression in hippocampus. Together, reduced filamentous actin turnover in presynaptic terminals causes increased Ca(2+) influx and, subsequently, elevated exocytotic neurotransmitter release acting on neural progenitors. Increased neurogenesis in Gsn(-/-) hippocampus is associated with a special vascular niche for neurogenesis.

Augmentation effect of combination therapy of aripiprazole and antidepressants on forced swimming test in mice

RATIONALE

A deficiency in brain monoamine systems (serotonin, dopamine, and/or norepinephrine) have long been hypothesized for the pathogenesis of depression. Drugs enhancing neurotransmission of those monoamines have been proven to have antidepressant effects. We hypothesized that aripiprazole, a partial D(2) agonist, could increase the activity of various antidepressants in the mice forced swimming test (FST), an animal model of depression.

OBJECTIVES

The scope of this study was to investigate the antidepressant-like effect of aripiprazole, when combined with conventional antidepressants drugs.

MATERIALS AND METHODS

This study assessed the effects of co-administration of aripiprazole with selective serotonin reuptake inhibitors (SSRIs; sertraline, paroxetine, and citalopram), selective serotonin-norepinephrine reuptake inhibitors (SNRIs; venlafaxine and minalcipran), selective norepinephrine reuptake inhibitor (NRI; desipramine), and the dual dopamine and norepinephrine reuptake inhibitor (bupropion), using the FST in mice. Subactive doses of aripiprazole and antidepressants sertraline, paroxetine, citalopram, venlafaxine, minalcipran, bupropion (4 and 8 mg/kg), and desipramine (2 and 4 mg/kg) were given i.p. 30 and 45 min, respectively, before the test.

RESULTS

Aripiprazole (0.03 and 0.06 mg/kg) combined with inactive doses of antidepressants, increased the activity of all antidepressants with the exception of bupropion and desipramine.

CONCLUSION

The augmentation effects of aripiprazole, in the present study, are in agreement with clinical evidence suggesting that aripiprazole may enhance the efficacy of therapeutic effect of SSRIs and SNRIs but not of NRI. These results suggest that augmentation effect of aripiprazole only appears when 5-HT system is activated and might implicate complex regulation between dopamine and 5-HT(1A) and 5-HT(2A) receptors.

Synaptic connections between GABAergic elements and serotonergic terminals or projecting neurons in the ventrolateral orbital cortex

The ventrolateral orbital cortex (VLO) is part of an endogenous analgesic system, consisting of the spinal cord-thalamic nucleus submedius-VLO periaqueductal gray (PAG)-spinal cord loop. The present study examined morphological connections of GABAergic (gamma-aminobutyric acidergic) neurons and serotonergic projection terminals from the dorsal raphe nucleus (DR), as well as the relationship between GABAergic terminals and VLO neurons projecting to the PAG, by using anterograde and retrograde tracing combined with immunofluorescence, immunohistochemistry, and electron microscopy methods. Results indicate that the majority (93%) of GABAergic neurons in the VLO also express the 5-HT(1A) (5-hydroxytryptamine 1A) receptor, and serotonergic terminals originating from the DR nucleus made symmetrical synapses with GABAergic neuronal cell bodies and dendrites within the VLO. GABAergic terminals also made symmetrical synapses with neurons expressing GABA(A) receptors and projecting to the PAG. These results suggest that a local neuronal circuit, consisting of 5-HTergic terminals, GABAergic interneurons, and projection neurons, exists in the VLO, and provides morphological evidence for the hypothesis that GABAergic modulation is involved in 5-HT(1A) receptor activation-evoked antinociception.

Electrophysiological effects of dizocilpine (MK-801) in adult rats exposed to ethanol during adolescence

BACKGROUND

Despite evidence showing persistent changes in N-methyl-D-aspartate (NMDA)-receptor function following ethanol (EtOH) exposure, the contribution of NMDA systems to the long-term neurophysiological consequences of adolescent EtOH exposure is unclear. The aims of this study were the following: (1) to determine whether adolescent EtOH exposure produces neurophysiological changes after a prolonged withdrawal period in adult rats and (2) to assess protracted alterations in neurophysiological responses to the NMDA antagonist MK-801 in adult rats exposed to EtOH during adolescence.

METHODS

Adolescent male Wistar rats were exposed to EtOH vapor for 12 h/d for 5 weeks. The effects of MK-801 (0.0 to 0.1 mg/kg, intraperitoneally) on the electroencephalogram (EEG) and auditory event-related potentials (ERPs) were assessed after 8 weeks of abstinence from EtOH.

RESULTS

Experiments in aim 1 revealed that adolescent EtOH exposure reduced EEG variability in the frontal cortex in the 4 to 6 Hz band but had no effect on cortical and hippocampal EEG power and ERPs. Experiments in aim 2 showed that MK-801 significantly reduced EEG power in the parietal cortex (4 to 6 Hz, 6 to 8 Hz, 8 to 16 Hz, 16 to 32 Hz) and hippocampus (16 to 32 Hz) and EEG variability in the parietal cortex (6 to 8 Hz, 16 to 32 Hz) following adolescent EtOH exposure. MK-801 produced a significant decrease in hippocampal EEG variability (4 to 6 Hz, 8 to 16 Hz, 16 to 32 Hz) in control, but not in EtOH-exposed rats. MK-801 reduced frontal P1 ERP amplitude and latency in response to the rare tone in EtOH-exposed rats compared to controls. In contrast, MK-801 significantly reduced P3 ERP amplitude and latency in control, but not in EtOH-exposed rats.

CONCLUSIONS

The effects of MK-801 on hippocampal EEG variability and P3 ERP amplitude and latency are significantly attenuated after a prolonged withdrawal period following adolescent EtOH exposure. However, the inhibitory effects of MK-801 on cortical and hippocampal EEG power were enhanced in rats exposed to EtOH during adolescence. Taken together, these data suggest protracted changes in NMDA systems following adolescent EtOH exposure.

GABAergic modulation is involved in the ventrolateral orbital cortex 5-HT 1A receptor activation-induced antinociception in the rat

The ventrolateral orbital cortex (VLO) is a component of an endogenous analgesic system consisting of an ascending pathway from the spinal cord to VLO via the thalamic nucleus submedius (Sm) and a descending pathway relaying in the periaqueductal gray matter (PAG). This study examines whether the activation of 5-HT 1A receptors in VLO produces antinociception and whether GABAergic modulation is involved in the VLO 5-HT 1A receptor activation-evoked antinociception. The radiant heat-evoked tail flick (TF) reflex was used as an index of nociceptive response in lightly anesthetized rats. Microinjection of the 5-HT 1A receptor agonist 8-OH-DPAT (1.0, 2.0, 5.0 microg) into VLO produced dose-dependent antinociception, which was reversed by the 5-HT 1A receptor antagonist (NAN-190, 20 mug). We also found that VLO application of the GABA A receptor antagonist bicuculline or picrotoxin (100 ng) enhanced the 8-OH-DPAT-induced inhibition of the TF reflex, whereas the GABA A receptor agonist muscimol (250 ng) or THIP (1.0 microg) significantly attenuated the 8-OH-DPAT-induced inhibition. These results suggest that 5-HT 1A receptors are involved in VLO-induced antinociception and that GABAergic disinhibitory mechanisms participate in the 5-HT 1A receptor-mediated effect. These findings provide support for the hypothesis that 5-HT 1A receptor activation may inhibit the inhibitory action of the GABAergic interneurons on the output neurons projecting to PAG leading to activation of the brainstem descending inhibitory system and depression of nociceptive inputs at the spinal cord level.

Serotonin 5-hT1A receptor activation prevents phosphorylation of NMDA receptor NR1 subunit in cerebral ischemia

The mechanisms involved in the neuroprotective effect of serotonin 5-HT1A receptor agonists on brain damage induced by ischemia remain to be fully elucidated. Given that serotonergic drugs may regulate N-methyl-D-aspartate (NMDA) receptor function, which is implicated in events leading to ischemia-induced neuronal cell death, this study sought to determine the effects of the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), on the levels of NMDA receptor NR1 subunit in gerbil hippocampus after transient global cerebral ischemia. Pretreatment with 8-OH-DPAT (1 mg/kg) prevented the neuronal loss in CA1 subfield 72 h after ischemia. NMDA receptor NR1 levels in whole hippocampus were not affected 24 h after ischemia, but the levels of the subunit phosphorylated at the protein kinase A (PKA) site, pNR1(Ser897), were significantly increased, and this increase was prevented by the same 8-OH-DPAT dose, a probable consequence of the increased phosphatase 1 (PP1) enzyme activity found in ischemic gerbils pretreated with the 5-HT1A receptor agonist. The results suggest that NR1 subunit phosphorylation plays a role in the neuroprotective effect of 8-OH-DPAT on cell damage induced by global cerebral ischemia in the gerbil hippocampus and support the potential interest of 5-HT1A receptor activation in the search for neuroprotective strategies.

Neuroprotective effects of serotonin 5-HT 1A receptor activation against ischemic cell damage in gerbil hippocampus: Involvement of NMDA receptor NR1 subunit and BDNF

It is known that the activation of 5-hydroxytryptamine receptor type 1A (5HT(1A) receptor) may protect against brain damage induced by transient global ischemia. The biochemical mechanisms that underlie this neuroprotective effect remain however to be fully elucidated. Given that serotonergic drugs may regulate N-methyl-d-aspartate (NMDA) receptor function, which is implicated in events leading to ischemia-induced neuronal cell death, and also stimulate the expression of brain-derived neurotrophic factor (BDNF), which is down-regulated in cerebral ischemia, we sought to determine the effects of the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), on the levels of NMDA receptor NR1 subunit and BDNF in gerbil hippocampus after transient global cerebral ischemia. Pretreatment with 8-OH-DPAT (1 mg/kg) prevented the neuronal loss in CA1 subfield 72 h after ischemia and also the dramatic decrease in BDNF immunoreactivity observed in this area at an earlier time. NMDA receptor NR1 levels in whole hippocampus were not affected 24 h after ischemia, but the levels of the subunit phosphorylated at the protein kinase A (PKA) site, pNR1(Ser897), were significantly increased, and this increase was prevented by the same 8-OH-DPAT dose, a probable consequence of the increased phosphatase 1 (PP1) enzyme activity found in ischemic gerbils pretreated with the 5-HT(1A) receptor agonist. The results indicate that both NR1 subunit phosphorylation and the neurotrophin BDNF account, at least in part, for the neuroprotective effect of 8-OH-DPAT on cell damage induced by global ischemia in the gerbil hippocampus and support the potential interest of 5-HT1A receptor activation in the search for neuroprotective strategies.

Regulators of adult neurogenesis in the healthy and diseased brain

1. In recent decades evidence has accumulated demonstrating the birth and functional integration of new neurons in specific regions of the adult mammalian brain, including the dentate gyrus of the hippocampus and the subventricular zone. 2. Studies in a variety of models have revealed genetic, environmental and pharmacological factors that regulate adult neurogenesis. The present review examines some of the molecular and cellular mechanisms that could be mediating these regulatory effects in both the normal and dysfunctional brain. 3. The dysregulation of adult neurogenesis may contribute to the pathogenesis of neurodegenerative disorders, such as Huntington's, Alzheimer's and Parkinson's disease, as well as psychiatric disorders such as depression. Recent evidence supports this idea and, furthermore, also indicates that factors promoting neurogenesis can modify the onset and progression of specific brain disorders, including Huntington's disease and depression.

Functional degradation of visual cortical cells in aged rats

Functional degradation of mammalian visual cortex is associated with aging. It has been hypothesized that much of the decline might be mediated by a degradation of cortical inhibitory system during senescence. In the present work, we compared the properties of adaptation, onset latency and signal-to-noise ratio in primary visual cortex of young and old rats using extracellular single-unit techniques. The short-term synaptic plasticity of young and old rats was also studied using field potential recording techniques. We found significant increased adaptation, prolonged onset latency, lower signal-to-noise ratio and decreased short-term synaptic plasticity in aged rats. The results are in accordance with previously reported functional declines in old monkeys and old cats, indicating a universal mechanism of degradation in cortical function that accompanies old age in different mammalian species.

Functional degradation of visual cortical cells in old cats

Visual function declines with age. Using extracellular single-unit in vivo recordings, we compared the function of primary visual cortical (area 17) cells in young and old paralyzed, anesthetized cats. The results reveal that cortical neurons in old cats exhibit higher visually evoked responses, higher spontaneous activities, lower signal-to-noise ratios, and weaker orientation and direction selectivity than do cells in young adult cats. These findings are consistent with previously reported age related declines in cortical function in senescent macaque monkeys. Thus, similar declines in cortical function accompany old age in different mammalian species with well developed cortices.

GABAergic Excitation Promotes Neuronal Differentiation in Adult Hippocampal Progenitor Cells

Hippocampal activity influences neurogenesis in the adult dentate gyrus; however, little is known about the involvement of the hippocampal circuitry in this process. In the subgranular zone of the adult dentate gyrus, neurogenesis involves a series of differentiation steps from radial glia-like stem/progenitor (type-1) cells, to transiently amplifying neuronal progenitor (type-2) cells, to postmitotic neurons. In this study, we conducted GFP-targeted recordings of progenitor cells in fresh hippocampal slices from nestin-GFP mice and found that neuronal progenitor (type-2) cells receive active direct neural inputs from the hippocampal circuitry. This input was GABAergic but not glutamatergic. The GABAergic inputs depolarized type-2 cells because of their elevated [Cl(-)](i). This excitation initiated an increase of [Ca(2+)](i) and the expression of NeuroD. A BrdU-pulse labeling study with GABA(A)-R agonists demonstrated the promotion of neuronal differentiation via this GABAergic excitation. Thus, it appears that GABAergic inputs to hippocampal progenitor cells promote activity-dependent neuronal differentiation.

Antipsychotic and antidepressive effects of second generation antipsychotics: two different pharmacological mechanisms?

Second generation antipsychotics display antidepressive effects in schizophrenic patients that are more pronounced than those of traditional neuroleptics and that go beyond antidepressive effects secondary to the reduction of positive symptoms. The antidepressive potential of second generation antipsychotics is presumably related to their pharmacological mechanisms, which differ from those of traditional neuroleptics. Among others, 5-HT(2A) antagonism is of special relevance for most of the new antipsychotics in this respect. But also special interactions with the dopaminergic system, as is the case with amisulpride and aripiprazole, or noradrenalin- and/or serotonin-reuptake-inhibition, as with ziprasidone and zotepine, should be considered. It can be summarised that the antipsychotic and antidepressive effects of second generation antipsychotics are mostly based on different pharmacological mechanisms. This might be especially true for direct antidepressive effects, i. e. antidepressive effects that are not mediated by the reduction of positive symptoms.

Thalamic nucleus submedius receives GABAergic projection from thalamic reticular nucleus in the rat

GABAergic projection from thalamic reticular nucleus to thalamic nucleus submedius in the medial thalamus of the rat was studied by using immunohistochemistry for GABA, retrograde labeling with Fluoro-Gold combined with immunohistochemistry for GABA, and anterograde labeling with biotinylated dextranamine. Immunohistochemistry displayed that only GABA immunoreactive terminals were observed in the thalamic nucleus submedius, while GABA immunoreactive neuronal cell bodies were located in the thalamic reticular nucleus and lateral geniculate nucleus. Injection of Fluoro-Gold into the thalamic nucleus submedius resulted in massive retrogradely labeled neuronal cell bodies in the rostroventral portion of the ipsilateral thalamic reticular nucleus and a few in the contralateral thalamic reticular nucleus, and most of these cell bodies showed GABA immunopositive staining. Many biotinylated dextranamine anterogradely labeled fibers and terminals in the thalamic nucleus submedius were observed after injection of biotinylated dextranamine into the thalamic reticular nucleus. The present results provide a morphological evidence for a hypothesis that a disinhibitory effect on output neurons elicited by opioid or 5-hydroxytryptamine inhibiting a GABAergic terminal in the thalamic nucleus submedius may lead to activation of the descending inhibitory system and depression of the nociceptive inputs at the spinal cord level.

Serotonin receptors: their key role in drugs to treat schizophrenia

Serotonin (5-HT)-receptor-based mechanisms have been postulated to play a critical role in the action of the new generation of antipsychotic drugs (APDs) that are usually referred to as atypical APDs because of their ability to achieve an antipsychotic effect with lower rates of extrapyramidal side effects (EPS) compared to first-generation APDs such as haloperidol. Specifically, it has been proposed by Meltzer et al. [J. Pharmacol. Exp. Ther. 251 (1989) 238] that potent 5-HT2A receptor antagonism together with weak dopamine (DA) D2 receptor antagonism are the principal pharmacologic features that differentiate clozapine and other apparent atypical APDs from first-generation typical APD. This hypothesis is consistent with the atypical features of quetiapine, olanzapine, risperidone, and ziprasidone, which are the most common treatments for schizophrenia in the United States and many other countries, as well as a large number of compounds in various stages of development. Subsequent research showed that 5-HT1A agonism may be an important consequence of 5-HT2A antagonism and that substitution of 5-HT1A agonism for 5-HT2A antagonism may also produce an atypical APD drug when coupled with weak D2 antagonism. Aripiprazole, the most recently introduced atypical APD, and a D2 receptor partial agonist, may also owe some of its atypical properties to its net effect of weak D2 antagonism, 5-HT2A antagonism and 5-HT1A agonism [Eur. J. Pharmacol. 441 (2002) 137]. By contrast, the alternative "fast-off" hypothesis of Kapur and Seeman [Am. J. Psychiatry 158 (2001) 360] applies only to clozapine and quetiapine and is inconsistent with the "slow" off rate of most atypical APDs, including olanzapine, risperidone and ziprasidone. 5-HT2A and 5-HT1A receptors located on glutamatergic pyramidal neurons in the cortex and hippocampus, 5-HT2A receptors on the cell bodies of DA neurons in the ventral tegmentum and substantia nigra and GABAergic interneurons in the cortex and hippocampus, and 5-HT1A receptors in the raphe nuclei are likely to be important sites of action of the atypical APDs. At the same time, evidence has accumulated for the important modulatory role of 5-HT2C and 5-HT6 receptors for some of the effects of some of the current APDs. Thus, 5-HT has joined DA as a critical target for developing effective APDs and led to the search for novel drugs with complex pharmacology, ending the exclusive search for single-receptor targets, e.g., the D3 or D4 receptor, and drugs that are selective for them.

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 alpha2C-adrenoceptor modulates GABA release in mouse striatum

The alpha(2C)-adrenoceptor occurs in high density in the striatum relative to other brain regions, but its biological role in striatal physiology is perplexing because of the paucity of noradrenergic terminals in this region. In this study, mice with a targeted inactivation of the alpha(2C)-adrenoceptor gene (alpha(2C)-KO mice), and genetically related mice (WT mice), were used to study the potential role of the striatal alpha(2C)-adrenoceptor in modulating GABA release. Perfused brain slices were pre-loaded with [(3)H]GABA and were stimulated electrically. In WT mice, the alpha(2)-adrenoceptor agonist, UK14304 (brimonidine), significantly enhanced [(3)H]GABA release from striatal slices, while the alpha(2)-adrenoceptor antagonist, RX821002, alone evoked a significant decrease in [(3)H]GABA release. In alpha(2C)-KO mice, the effect of RX821002 was absent, while UK14304 retained its ability to enhance [(3)H]GABA release. Pharmacological depletion of monoamines in WT mice also abolished the effect of RX821002 on [(3)H]GABA release. In hippocampal slices, RX821002-induced reduction in [(3)H]GABA release was present in WT and alpha(2C)-KO mice. In the presence of tetrodotoxin, RX821002 increased [(3)H]GABA release in striatal slices from both WT and alpha(2C)-KO mice. Together, these data imply that alpha(2A)- and alpha(2C)-adrenoceptors are located on different neurons in the striatum, that alpha(2C)-adrenoceptor-mediated effects on striatal GABA release are mediated by an endogenous catecholamine that could be dopamine, and that the alpha(2C)-adrenoceptor effect of RX821002 does not occur at the GABAergic terminal.

5-hydroxytryptamine 1A receptor activation protects against N-methyl-D-aspartate-induced apoptotic cell death in striatal and mesencephalic cultures

Apoptosis and glutamate-mediated excitotoxicity may play a role in the pathogenesis of many neurodegenerative disorders, including Parkinson's disease (PD). In the present study, we investigated whether stimulation of the 5-hydroxytryptamine 1A (5-HT1A) receptor attenuates N-methyl-D-aspartate- (NMDA) and 1-methyl-4-phenylpyridinium (MPP(+))-induced apoptotic cell death in cell culture models. A brief exposure (20 min) of M213-2O striatal cells to NMDA and glutamate produced a delayed increase in caspase-3 activity and DNA fragmentation in a dose- and time-dependent manner. NMDA-induced caspase-3 activity and DNA fragmentation were almost completely blocked by the 5-HT1A agonists 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) and (R)-5-fluoro-8 hydroxy-2-(dipropylamino)-tetralin (R-UH-301). Additionally, the protective effects of 8-OH-DPAT and R-UH-301 on NMDA-induced caspase-3 activation and apoptosis were reversed by pretreatment with the 5-HT1A antagonists N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635) and S-UH-301, respectively. Similarly, dose- and time-dependent increases in caspase-3 activity and DNA fragmentation were observed in rat primary mesencephalic neurons after a brief exposure to NMDA and glutamate. Caspase-3 activation and DNA fragmentation in primary mesencephalic neurons were almost completely inhibited by 8-OH-DPAT. This neuroprotective effect of 8-OH-DPAT was reversed by WAY 100635. Additionally, 8-OH-DPAT blocked tyrosine hydroxylase (TH)-positive cell death after NMDA exposure and also almost completely attenuated the NMDA-induced Ca(2+) influx in primary mesencephalic cultures. Furthermore, 8-OH-DPAT and R-UH-301 blocked apoptotic cell death in the primary mesencephalic neurons that were exposed to the Parkinsonian toxin MPP(+). Together, these results suggest that 5-HT1A receptor stimulation may be a promising pharmacological approach in the development of neuroprotective agents for PD.

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.

17beta-estradiol effect on the extracellular concentration of amino acids in the glutamate excitotoxicity model in the rat

Estrogen has demonstrated a neuroprotective role in a rat model of glutamate excitotoxicity and other neurodegenerative disorders. We studied the effect of 17beta-estradiol on glutamate-induced increases in amino acids levels (aspartate, histidine, taurine and GABA) in the rat cortex. Local perfusion of glutamate produced a transient increase of aspartate, histidine, taurine and GABA in the extracellular fluid. Pretreatment with 17beta-estradiol significantly reduced the increases of taurine and moderately attenuated that of histidine, whereas aspartate and GABA releases were not modified. The effect of 17beta-estradiol on histidine release was reversed by the antiestrogen tamoxifen, suggesting a receptor-dependent mechanism. Good correlations between the volumes of the glutamate-induced lesions and the extracellular concentrations of taurine and aspartate were observed. These findings suggest that the attenuation of the glutamate-induced release of taurine by 17beta-estradiol may participate in the neuroprotective effects of 17beta-estradiol and that increased levels of aspartate and taurine are markers for the severity of the glutamate-induced cortical lesions.

Activation of ventral tegmental area cells by the bed nucleus of the stria terminalis: a novel excitatory amino acid input to midbrain dopamine neurons

We examined the role of excitatory amino acids (EAAs) in the activation of midbrain dopaminergic (DA) neurons evoked by stimulation of the ventromedial and ventrolateral (subcommissural) bed nucleus of the stria terminalis (vBNST). Using anesthetized rats and extracellular recording techniques, we found that 84.8% of ventral tegmental area (VTA) DA neurons were activated synaptically by single-pulse electrical stimulation of the vBNST. In contrast, similar stimulation did not affect the activity of presumed GABA neurons in the VTA. Three characteristic responses were observed in VTA DA neurons: short latency activation (<25 msec; 55.1% of cells), long latency activation (>65 msec; 56% of cells), and inhibition (61.8% of cells, usually followed by long latency excitation). Microinfusion of antagonists of EAA receptors (3 mm kynurenic acid, 100 microm AP-5, or 50 microm CNQX) from a micropipette adjacent to the recording electrode significantly reduced both short and long latency activations evoked in DA neurons by vBNST stimulation. Specific responses were attenuated similarly by AP-5 alone, CNQX alone, or a cocktail of AP-5+CNQX, indicating that joint activation of NMDA plus non-NMDA receptors was required. Stimulation of the vBNST by local microinfusion of glutamate increased the firing and bursting activity of VTA DA neurons. Similar microinfusion of GABA decreased bursting of VTA DA neurons without altering their firing rate. Retrograde and anterograde labeling and antidromic activation of vBNST neurons by VTA stimulation confirmed a direct projection from the vBNST to the VTA. These results reveal that inputs from the vBNST exert a strong excitatory influence on VTA DA neurons mediated by both NMDA and non-NMDA receptors.

Excitotoxic median raphe lesions aggravate working memory storage performance deficits caused by scopolamine infusion into the dentate gyrus of the hippocampus in the inhibitory avoidance task in rats

The interactions between the median raphe nucleus (MRN) serotonergic system and the septohippocampal muscarinic cholinergic system in the modulation of immediate working memory storage performance were investigated. Rats with sham or ibotenic acid lesions of the MRN were bilaterally implanted with cannulae in the dentate gyrus of the hippocampus and tested in a light/dark step-through inhibitory avoidance task in which response latency to enter the dark compartment immediately after the shock served as a measure of immediate working memory storage. MRN lesion per se did not alter response latency. Post-training intrahippocampal scopolamine infusion (2 and 4 microg/side) produced a more marked reduction in response latencies in the lesioned animals compared to the sham-lesioned rats. Results suggest that the immediate working memory storage performance is modulated by synergistic interactions between serotonergic projections of the MRN and the muscarinic cholinergic system of the hippocampus.

Effects of excitotoxic median raphe lesion on working memory deficits produced by the dorsal hippocampal muscarinic receptor blockade in the inhibitory avoidance in rats

The experiments investigated the interactions between median raphe nucleus (MRN) serotonergic and septo-hippocampal muscarinic cholinergic systems in the modulation of forming and storing performances of working memory. Rats with ibotenic acid-induced MRN-lesion bilaterally received scopolamine (2-4 microg/each side) infusion into the dentate gyrus of the dorsal hippocampus and were tested in a single trial step-through inhibitory avoidance. Initial preference to the dark compartment (escape latency) was taken as the measure of non-mnemonic behaviours and response latency to enter the dark compartment immediately after the foot-shock was used to measure working memory. The high-dose scopolamine infusion 10 min before the training decreased escape latencies in the sham-lesioned rats, whereas had no effect in the MRN-lesioned rats. Although MRN lesion per se did not alter response latency, it alleviated pre-training scopolamine-induced decrease, but aggravated post-training scopolamine-induced reduction in this parameter. These results suggest that the antagonistic interactive processes between MRN-serotonergic and hippocampal cholinergic systems modulate non-mnemonic component of working memory formation, whereas the storing performance of working memory is modulated by the synergistic interactions between these systems in the hippocampus, mainly in the dentate gyrus.

Attenuation of ischemic efflux of endogenous amino acids by the novel 5-HT(1A)/5-HT(2) receptor ligand adatanserin

Using sodium azide (NaN3)-induced anoxia plus aglycaemia as a model of chemically-induced ischemia in the hippocampal slice, we have evaluated the effects of the novel 5-HT(1A) partial agonist/5-HT(2) receptor antagonist adatanserin and the 5-HT(1A) receptor agonist BAYx3702 on the efflux of endogenous glutamate, aspartate and GABA. BAYx3702 (10-1000 nM) produced a significant (P<0.05) dose-related attenuation of ischemic efflux of both glutamate and GABA with maximum decrease being observed at 100 nM (73 and 69%, respectively). This attenuation was completely reversed by the addition of the 5-HT(1A) antagonist, WAY-100635 (100 nM). Similarly, adatanserin (10-1000 nM) produced a significant (P<0.05) dose-related attenuation in glutamate and GABA efflux with a maximum of 72 and 81% at 100 nM, respectively. This effect was completely reversed by the 5-HT(2A/C) receptor agonist, DOI but unaffected by WAY-100635. The 5-HT(2A) receptor antagonist MDL-100907 produced a comparable attenuation of glutamate when compared to adatanserin, while the 5-HT(2C) receptor antagonist, SB-206553, had no effect on ischemic efflux. None of these compounds significantly altered aspartate efflux from this preparation. In conclusion, the 5-HT(1A) receptor partial agonist 5-HT(2) receptor antagonist, adatanserin is able to attenuate ischemic amino acid efflux in a comparable manner to the full 5-HT(1A) agonist BAYx3702. However, in contrast to BAYx3702, adatanserin appears to produce it effects via blockade of the 5-HT(2A) receptor. This suggests that adatanserin may be an effective neuroprotectant, as has been previously demonstrated for full 5-HT(1A) receptor agonists such as BAYx3702.

Regulation of sensorimotor gating in rats by hippocampal NMDA: anatomical localization

Prepulse inhibition (PPI) of the startle reflex is a measure of sensorimotor gating that is reduced in humans with certain neuropsychiatric disorders, including schizophrenia, and in rats after manipulations of limbic cortico-striato-pallido-pontine circuitry. We have reported that PPI is reduced after specific manipulations of the hippocampal complex (HPC) in rats, but the mechanisms for these effects remain poorly understood. For example, dopaminergic substrates clearly regulate PPI, but the PPI-disruptive effects of intra-HPC carbachol or NMDA are not reversed by D2 receptor antagonists. This study examined the anatomical specificity within the hippocampal complex of the PPI-disruptive effects of NMDA infusion. Startle magnitude and PPI were assessed after acute bilateral infusion of NMDA (0, 0.4 or 0.8 microg) into the dorsal subiculum (DS), region CA1, the ventral subiculum (VS), the rostral entorhinal cortex (ECr) and the caudal entorhinal cortex (ECc). A dorsal-ventral gradient for NMDA effects was observed, with a dose-dependent disruption of PPI after NMDA infusion into the VS or EC, but not the DS, and with intermediate level effects observed after NMDA infusion into CA1. A second set of studies confirmed that the failure of NMDA effects in the DS did not reflect site-related differences in startle magnitude or baseline levels of PPI. These findings demonstrate the importance of the ventral, but not the dorsal HPC, in the glutamatergic regulation of PPI.

5-HT(1A) and 5-HT(2A) serotonin receptor turnover in adult rat offspring prenatally exposed to cocaine

This study investigated the effects of prenatal exposure to cocaine on the intracellular kinetics (i.e. rate constant of receptor production and degradation) that govern the maintenance and regulation of cortical 5-HT(1A) and 5-HT(2A) receptor densities in offspring. Adult male rat offspring, prenatally exposed to saline or (-) cocaine (15 mg/kg, s.c., b.i.d, from gestational day 13 through 20), were injected with either vehicle or the irreversible receptor antagonist, EEDQ (10 mg/kg, s.c.), and sacrificed at various post-injection times to monitor the recovery of receptor densities in cerebral cortex. In both saline and cocaine exposed offspring, initial EEDQ-induced reductions (>80%) in 5-HT(1A) and 5-HT(2A) receptor densities were followed by a time-dependent repopulation that reached steady state ([B(max)](ss)) densities comparable to non-EEDQ treated controls by day 10 post-treatment. Calculation of 5-HT(1A) receptor kinetic parameters indicated that prenatal exposure to cocaine did not significantly alter: (1) the receptor production rate (saline: 0.809 fmol/mg protein/h; cocaine: 0.724 fmol/mg protein/h), (2) the receptor degradation rate constant (saline: 0.0063 h(-1); cocaine: 0.0062 h(-1)) or (3) the half-life (t(1/2)) of receptor repopulation (saline: 109.2 h; cocaine: 111.5 h). Similarly, 5-HT(2A) receptor rate constants for production (1. 550 fmol/mg protein/h) and degradation (0.0061 h(-1)) and consequently, t(1/2) (113.2 h), were not significantly altered by prenatal exposure to cocaine. These data suggest that within homogenates of cerebral cortex, prenatal exposure to cocaine did not alter the overall intracellular processes that underlie receptor production or degradation and determine steady state densities of 5-HT(1A) or 5-HT(2A) receptors.

Presynaptic NMDA receptor subunit immunoreactivity in GABAergic terminals in rat brain

N-methyl-D-aspartate (NMDA) receptors are commonly found post-synaptically; they mediate fast excitatory neurotransmission in the central nervous system. In this study, we provide immunocytochemical data supporting the existence of presynaptic NMDA receptors in GABAergic terminals using polyclonal antisera raised against the C-terminus of the NMDAR1 subunit. At the light microscope level, rich plexuses of NMDAR1-positive varicose fibers were found in various nuclei in the basal forebrain (bed nucleus of stria terminalis, septum, parastrial nucleus, vascular organ of the lamina terminalis), thalamus (paraventricular nucleus, midline nuclei), and hypothalamus (parvocellular paraventricular nucleus, arcuate nucleus, preoptic nucleus, suprachiasmatic nucleus). In the brainstem, labeled fibers were much less abundant and were confined to the ventral tegmental area, periaqueductal gray, parabrachial nucleus, and locus coeruleus. At the electron microscope level, NMDAR1-immunoreactive terminals examined in the bed nucleus of stria terminalis, parvocellular paraventricular hypothalamic nucleus, and arcuate nucleus formed symmetric synapses, contained darkly stained large dense-core vesicles, and displayed gamma-aminobutyric acid (GABA) immunoreactivity. Terminals with similar ultrastructural features were found in the paraventricular thalamic nucleus. These findings demonstrate the existence of NMDAR1 subunit immunoreactivity in subsets of GABAergic terminals, which raises questions about the potential roles and mechanisms of activation of presynaptic NMDA heteroreceptors in the rat central nervous system. The pattern of distribution and ultrastructural features of these boutons suggest that they may arise from local GABAergic projections interconnecting a group of brain structures mediating stress responses and/or other endocrine, autonomic, and limbic functions.

Median raphe nucleus mediates forming long-term but not short-term contextual fear conditioning in rats

The brain serotonin is involved in mediation of emotional behaviour including anxiety and related fear conditioning. It is known that the median raphe nucleus (MRN) is the origin of a serotonergic pathway and mainly innervates septo-hippocampal formation which plays an important role in emotional cognition. However, its regulatory role in different types of fear conditioning is still unclear. In the present study, the animals underwent ibotenic acid or sham lesions of the median raphe nucleus and the effects of MRN lesions on immediate and delayed fear conditioning to multiple contextual cues were studied. Freezing behaviour served as a measure of contextual fear. Sham-lesioned animals showed reliable conditional freezing when observed immediately following foot-shock (1.0 mA) for 3-min test and 48 h after the shock for 12-min test. Rats with MRN lesions displayed robust freezing behaviour immediately after the shock, even though they showed a marked deficit in freezing 48 h following the shock. These findings indicate that the MRN-serotonergic septo-hippocampal pathway is involved in the regulation of anxiety related to fear conditioning triggered by contextual cues, suggesting that short-term contextual fear is independent on the MRN while long-term contextual fear depends on the MRN.

Effects of some GABA and NMDA antagonists on a model of presynaptic hippocampal paired pulse inhibition

1. The effects of some NMDA antagonists (7-chlorokynurenic acid and CGS 19755) and of the GABA antagonist penicillin were tested in a model of presynaptic short-term paired-pulse inhibition elicited in rat hippocampal slice with high (+ 2 mM) calcium solutions subjected to paired (15 ms)-pulse stimulation paradigm. 2. In control condition a 15 ms paired-pulse stimulation delivered at the level of stratum radiatum, as revealed by the ratio between amplitudes of the conditioned and unconditioned CA1 population spikes (R2/R1), ranging from 1.27 to 2.57, a clear paired-pulse facilitation occurred. Slice perfusion with high (+ 2 mM) calcium shifted, within 30 min, as revealed by a significant (P<0.01) decrease in R2/R1 ratio, paired-pulse facilitation into inhibition. Further perfusion together to high (+ 2 mM) calcium with 0.5 mM penicillin or with 50 microM CGS 19755, but not with 50 microM 7-chlorokynurenic acid significantly decreases the degree of paired-pulse inhibition as revealed by a significative increase in the R2/R1 ratio. 3. The data, demonstrating an inhibitory influence of specific NMDA antagonists in a model a presynaptic paired-pulse inhibition, were discussed in relation with the specific psychodysleptic effects elicited by the drugs in animals and humans.