Activation of the CXCR4 Receptor by Chemokine CXCL12 Increases the Excitability of Neurons in the Rat Central Amygdala

Primarily regarded as immune proteins, chemokines are emerging as a family of molecules serving neuromodulatory functions in the developing and adult brain. Among them, CXCL12 is constitutively and widely expressed in the CNS, where it was shown to act on cellular, synaptic, network, and behavioral levels. Its receptor, CXCR4, is abundant in the amygdala, a brain structure involved in pathophysiology of anxiety disorders. Dysregulation of CXCL12/CXCR4 signaling has been implicated in anxiety-related behaviors. Here we demonstrate that exogenous CXCL12 at 2 nM but not at 5 nM increased neuronal excitability in the lateral division of the rat central amygdala (CeL) which was evident in the Late-Firing but not Regular-Spiking neurons. These effects were blocked by AMD3100, a CXCR4 antagonist. Moreover, CXCL12 increased the excitability of the neurons of the basolateral amygdala (BLA) that is known to project to the CeL. However, CXCL12 increased neither the spontaneous excitatory nor spontaneous inhibitory synaptic transmission in the CeL. In summary, the data reveal specific activation of Late-Firing CeL cells along with BLA neurons by CXCL12 and suggest that this chemokine may alter information processing by the amygdala that likely contributes to anxiety and fear conditioning.

Restorative effect of NitroSynapsin on synaptic plasticity in an animal model of depression

In the search for new options for the pharmacological treatment of major depressive disorder, compounds with a rapid onset of action and high efficacy but lacking a psychotomimetic effect are of particular interest. In the present study, we evaluated the antidepressant potential of NitroSynapsin (NS) at behavioural, structural, and functional levels. NS is a memantine derivative and a dual allosteric N-methyl-d-aspartate receptors (NMDAR) antagonist using targeted delivery by the aminoadamantane of a warhead nitro group to inhibitory redox sites on the NMDAR. In a chronic restraint stress (CRS) mouse model of depression, five doses of NS administered on three consecutive days evoked antidepressant-like activity in the chronically stressed male C57BL/6J mice, reversing CRS-induced behavioural disturbances in sucrose preference and tail suspension tests. CRS-induced changes in morphology and density of dendritic spines in cerebrocortical neurons in the medial prefrontal cortex (mPFC) were also reversed by NS. Moreover, CRS-induced reduction in long-term potentiation (LTP) in the mPFC was found to be prevented by NS based on the electrophysiological recordings. Our study showed that NS restores structural and functional synaptic plasticity and reduces depressive behaviour to the level found in naïve animals. These results preliminarily revealed an antidepressant-like potency of NS.

Posterior hypothalamic theta rhythm: Electrophysiological basis and involvement of glutamatergic receptors

The posterior hypothalamic area (PHa), including the supramammillary nucleus (SuM) and posterior hypothalamic nuclei, forms a crucial part of the ascending brainstem hippocampal synchronizing pathway, that is involved in the frequency programming and modulation of rhythmic theta activity generated in limbic structures. Recent investigations show that in addition to being a modulator of limbic theta activity, the PHa is capable of producing well-synchronized local theta field potentials by itself. The purpose of this study was to examine the ability of the PHa to generate theta field potentials and accompanying cell discharges in response to glutamatergic stimulation under both in vitro and in vivo conditions. The second objective was to examine the electrophysiological properties of neurons located in the SuM and posterior hypothalamic nuclei. Extracellular in vivo and in vitro as well as intracellular in vitro experiments revealed that glutamatergic stimulation of PHa with kainic acid induces well-synchronized local theta field oscillations in both the supramammillary and posterior hypothalamic nuclei. Furthermore, the glutamatergic PHa theta rhythm recorded extracellularly was accompanied by the activity of specific subtypes of theta-related neurons. We identify, for the first time, a subpopulation of supramammillary and posterior hypothalamic neurons that express clear subthreshold membrane potential oscillations in the theta frequency range.

Activation of the 5-HT7 receptor and MMP-9 signaling module in the hippocampal CA1 region is necessary for the development of depressive-like behavior

Major depressive disorder is a complex disease resulting from aberrant synaptic plasticity that may be caused by abnormal serotonergic signaling. Using a combination of behavioral, biochemical, and imaging methods, we analyze 5-HT7R/MMP-9 signaling and dendritic spine plasticity in the hippocampus in mice treated with the selective 5-HT7R agonist (LP-211) and in a model of chronic unpredictable stress (CUS)-induced depressive-like behavior. We show that acute 5-HT7R activation induces depressive-like behavior in mice in an MMP-9-dependent manner and that post mortem brain samples from human individuals with depression reveal increased MMP-9 enzymatic activity in the hippocampus. Both pharmacological activation of 5-HT7R and modulation of its downstream effectors as a result of CUS lead to dendritic spine elongation and decreased spine density in this region. Overall, the 5-HT7R/MMP-9 pathway is specifically activated in the CA1 subregion of the hippocampus during chronic stress and is crucial for inducing depressive-like behavior.

Evidence for the interaction of COX-2 with mGluR5 in the regulation of EAAT1 and EAAT3 protein levels in the mouse hippocampus. The influence of oxidative stress mechanisms

Since we found that inhibition of cyclooxygenase-2 (COX-2) with concomitant application of a metabotropic glutamate receptor subtype 5 (mGluR5) antagonist (MTEP) down-regulates mGluR7 in the hippocampus (HC) and changes behavior of mice, our team decided to investigate the mechanism responsible for the observed changes. The amino acid glutamate (Glu) is a major excitatory neurotransmitter in the brain. Glu uptake is regulated by excitatory amino acid transporters (EAAT). There are five transporters with documented expression in neurons and glia in the central nervous system (CNS). EAATs, maintain the correct transmission of the Glu signal and prevent its toxic accumulation by removing Glu from the synapse. It has been documented that the toxic level of Glu is one of the main causes of mental and cognitive abnormalities. Given the above mechanisms involved in the functioning of the Glu synapse, we hypothesized modification of Glu uptake, involving EAATs as the cause of the observed changes. This study investigated the level of selected EAATs in the HC after chronic treatment with mGluR5 antagonist MTEP, NS398, and their combination using Western blot. Concomitant MTEP treatment with NS398 or a single administration of the above causes changes in LTP and modulation of EAAT levels in mouse HC. As EAATs are cellular markers of oxidative stress mechanisms, the E. coli lipopolysaccharide (LPS) challenge was performed. The modified Barnes maze test (MBM) revealed alterations in the mouse spatial learning abilities. This study reports an interaction between the mGluR5 and COX-2 in the HC, with EAAT1 and EAAT3 involvement.

5-HT7 receptors enhance inhibitory synaptic input to principal neurons in the mouse basal amygdala

The basal amygdala (BA) has been implicated in encoding fear and its extinction. The level of serotonin (5-HT) in the BA increases due to arousal and stress related to aversive stimuli. The effects of 5-HT₇ receptor (5-HT₇R) activation and blockade on the activity of BA neurons have not yet been investigated. In the present study, a transgenic mouse line carrying green fluorescent protein (GFP) reporter gene was used to identify neurons that express the 5-HT₇R. GFP immunoreactivity was present mainly in cells that also expressed GAD67 or parvalbumin (PV), the phenotypic markers for GABAergic interneurons. Most cells showing GFP fluorescence demonstrated firing patterns characteristic of BA inhibitory interneurons. Activation of 5-HT₇Rs resulted in a depolarization and/or occurrence of spontaneous spiking activity of BA interneurons that was accompanied by an increase in the mean frequency and mean amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from BA principal neurons. These effects were blocked by a specific 5-HT₇R antagonist, SB269970 and were absent in slices from 5-HT₇R knockout mice. Activation of 5-HT₇Rs also decreased the mean frequency of spontaneous excitatory postsynaptic currents (sEPSCs) recorded from BA principal neurons, which was blocked by the GABA_A receptor antagonist picrotoxin. Neither inhibitory nor excitatory miniature postsynaptic currents (mIPSCs/mEPSCs) were affected by 5-HT₇R activation. These results show that in the BA 5-HT₇Rs stimulate an activity-dependent enhancement of inhibitory input from local interneurons to BA principal neurons and provide insights about the possible involvement of BA serotonergic receptors in neuronal mechanisms underlying fear memory.

Behavioral consequences of co-administration of MTEP and the COX-2 inhibitor NS398 in mice. Part 2

Our earlier study demonstrated, that antidepressant-like and also cognitive action of MTEP, a metabotropic glutamate receptor subtype 5 (mGluR5) antagonist, was influenced by cyclooxygenase-2 (COX-2) inhibition in mice. We detected a decrease in the mGluR7 protein level in the hippocampus (HC) of mice co-treated chronically with MTEP and NS398 (a COX-2 inhibitor). We found both antidepressant-like effects and cognitive to be associated with mGlu7 receptor-mediated mechanisms.

5-HT7 receptors increase the excitability of hippocampal CA1 pyramidal neurons by inhibiting the A-type potassium current

Accumulating evidence suggests a widespread role of serotonin 5-HT₇ receptors (5-HT₇Rs) in the physiology of cognitive and affective processing. However, we still lack insights into 5-HT₇R electrophysiology. Studies analyzing the 5-HT₇R-mediated changes in CA1 pyramidal neuron activity revealed that 5-HT₇R activation leads to the opening of hyperpolarization-activated cyclic nucleotide-gated cation channels (HCNs). However, our group and others have shown that CA1 pyramidal cells increase their excitability following 5-HT₇R activation, an effect which cannot be explained by HCN channel opening. This suggests a different ionic mechanism might be responsible. To investigate this, we performed whole-cell patch clamp recordings of CA1 pyramidal cells in rat brain slices. It was found that acute 5-HT₇R activation increased membrane excitability and decreased spiking latency. Both effects were blocked by a selective 5-HT₇R antagonist. Spike latency in CA1 pyramidal cells is known to be regulated by transient outward voltage-dependent A-type potassium channels. Subsequent voltage clamp recordings revealed that acute 5-HT₇R activation inhibited A-type potassium currents. Pharmacological block of Kv4.2/4.3 potassium channel subunits prevented the 5-HT₇R agonist-induced changes in excitability and spiking latency, whereas blocking HCN channels had no influence on these effects. Taken together, the results reveal an ionic mechanism previously not known to be associated with 5-HT₇R activation. Inhibition of A-type potassium channels can fully account for increased CA1 pyramidal cell excitability after 5-HT₇R activation. These results can help explain a number of behavioral and physiological findings and will hopefully lead to a better understanding of 5-HT₇ receptor signaling in health and disease.

Astrocytes determine conditioned response to morphine via glucocorticoid receptor-dependent regulation of lactate release

To date, neurons have been the primary focus of research on the role of glucocorticoids in the regulation of brain function and pathological behaviors, such as addiction. Astrocytes, which are also glucocorticoid-responsive, have been recently implicated in the development of drug abuse, albeit through as yet undefined mechanisms. Here, using a spectrum of tools (whole-transcriptome profiling, viral-mediated RNA interference in vitro and in vivo, behavioral pharmacology and electrophysiology), we demonstrate that astrocytes in the nucleus accumbens (NAc) are an important locus of glucocorticoid receptor (GR)-dependent transcriptional changes that regulate rewarding effects of morphine. Specifically, we show that targeted knockdown of the GR in the NAc astrocytes enhanced conditioned responses to morphine, with a concomitant inhibition of morphine-induced neuronal excitability and plasticity. Interestingly, GR knockdown did not influence sensitivity to cocaine. Further analyses revealed GR-dependent regulation of astroglial metabolism. Notably, GR knockdown inhibited induced by glucocorticoids lactate release in astrocytes. Finally, lactate administration outbalanced conditioned responses to morphine in astroglial GR knockdown mice. These findings demonstrate a role of GR-dependent regulation of astrocytic metabolism in the NAc and a key role of GR-expressing astrocytes in opioid reward processing.

Stress-induced changes in the activity of parvocellular neurosecretory cells in the paraventricular nucleus of the hypothalamus

This paper summarizes a series of studies aimed at characterizing the effects of stress-related changes in synaptic inputs to the hypothalamic paraventricular nucleus (PVN). This structure generates an integrated physiological stress response by activating the hypothalamus-pituitary-adrenal (HPA) axis. Corticotropin-releasing hormone (CRH)-synthesizing parvocellular neuroendocrine neurons of the PVN play a key role in this process. They receive extensive excitatory and inhibitory innervation conveying information about interoceptive and exteroceptive stressful stimuli from a variety of sources within the brain. These synaptic inputs modulate the activity of PVN neurons, which regulates the amount of CRH released into the portal circulation of the anterior pituitary. It has been demonstrated that with either single or repeated stress sessions, the efficacy of excitatory and inhibitory synapses on parvocellular neuroendocrine neurons changes considerably, which may be related to repeated stress-induced sensitization of the HPA axis. The nature of these changes depends on the type of stress and its duration. Changes in synaptic inputs and the excitability of parvocellular neuroendocrine neurons are thought to be responsible for dysfunctions of the HPA axis observed in affective disorders. Assessing how this controlling function of PVN neurons is modulated in response to stress is crucial to our understanding of the pathophysiology of affective disorders.

Tetrabromobisphenol A-induced depolarization of rat cerebellar granule cells: ex vivo and in vitro studies

The brominated flame retardant tetrabromobisphenol A (TBBPA) is toxic to cultured brain neurons, and glutamate receptors partially mediate this effect; consequently, the depolarizing effect of TBBPA on neurons is to be expected, but it is yet to be actually demonstrated. The aim of this study was to detect TBBPA-evoked depolarization and identify the underlying mechanisms. The plasma membrane potential of rat cerebellar granule cells (CGC) in cerebellar slices or in primary cultures was measured using whole-cell current clamp recordings, or the fluorescent probe oxonol VI, respectively. The contribution of NMDA and AMPA receptors, voltage-gated sodium channels and intracellular calcium mobilization was tested using their selective antagonists or inhibitors. Direct interactions of TBBPA with NMDARs were tested by measuring the specific binding of radiolabeled NMDAR ligands to isolated rat cortical membrane fraction. TBBPA (25 μM) strongly depolarized CGC in cerebellar slices, and at ≥ 7.5 μM concentration-dependently depolarized primary CGC cultures. Depolarization of the primary CGC by 25 μM TBBPA was partly reduced when MK-801 was applied alone or in combination with either TTX or CNQX, or where bastadin 12 was applied in combination with ryanodine, whereas depolarization was completely prevented when MK-801, CNQX and TTX where combined. TBBPA had no effect on the specific binding of NMDAR radio-ligands to isolated cortical membranes. These results demonstrate the depolarizing effect of TBBPA on CGC, which is mainly mediated by ionotropic glutamate receptors, while voltage-gated sodium channels are also involved. We found no evidence for the direct activation of NMDARs by TBBPA.

Hyperforin Potentiates Antidepressant-Like Activity of Lanicemine in Mice

N-methyl-D-aspartate receptor (NMDAR) modulators induce rapid and sustained antidepressant like-activity in rodents through a molecular mechanism of action that involves the activation of Ca²⁺ dependent signaling pathways. Moreover, ketamine, a global NMDAR antagonist is a potent, novel, and atypical drug that has been successfully used to treat major depressive disorder (MDD). However, because ketamine evokes unwanted side effects, alternative strategies have been developed for the treatment of depression. The objective of the present study was to determine the antidepressant effects of either a single dose of hyperforin or lanicemine vs. their combined effects in mice. Hyperforin modulates intracellular Ca²⁺ levels by activating Ca²⁺-conducting non-selective canonical transient receptor potential 6 channel (TRPC6) channels. Lanicemine, on the other hand, blocks NMDARs and regulates Ca²⁺ dependent processes. To evaluate the antidepressant-like activity of hyperforin and lanicemine, a set of in vivo (behavioral) and in vitro methods (western blotting, Ca²⁺ imaging studies, electrophysiological, and radioligand binding assays) was employed. Combined administration of hyperforin and lanicemine evoked long-lasting antidepressant-like effects in both naïve and chronic corticosterone-treated mice while also enhancing the expression of the synapsin I, GluA1 subunit, and brain derived neurotrophic factor (BDNF) proteins in the frontal cortex. In Ca²⁺ imaging studies, lanicemine enhanced Ca²⁺ influx induced by hyperforin. Moreover, compound such as MK-2206 (Akt kinase inhibitor) inhibited the antidepressant-like activity of hyperforin in the tail suspension test (TST). Hyperforin reversed disturbances induced by MK-801 in the novel object recognition (NOR) test and had no effects on NMDA currents and binding to NMDAR. Our results suggest that co-administration of hyperforin and lanicemine induces long-lasting antidepressant effects in mice and that both substances may have different molecular targets.

Simultaneous activation of muscarinic and GABAB receptors as a bidirectional target for novel antipsychotics

Recent preclinical studies point to muscarinic and GABA_B receptors as novel therapeutic targets for the treatment of schizophrenia. This study was aimed to assess the role of muscarinic and GABA_B receptor interactions in animal models of schizophrenia, using positive allosteric modulators (PAMs) of GABA_B receptor (GS39783), muscarinic M₄ (VU0152100) and M₅ (VU0238429) receptor, and partial allosteric agonist of M₁ receptor (VU0357017). DOI-induced head twitches, social interaction and novel object recognition tests were used as the models of schizophrenia. Analyses of DOI-induced increases in sEPSCs (spontaneous excitatory postsynaptic currents) were performed as complementary experiments to the DOI-induced head twitch studies. Haloperidol-induced catalepsy and the rotarod test were used to examine the adverse effects of the drugs. All three activators of muscarinic receptors were active in DOI-induced head twitches. When administered together with GS39783 in subeffective doses, only the co-administration of VU0152100 and GS39783 was effective. The combination also reduced the frequency but not the amplitude of DOI-induced sEPSCs. Neither VU0357017 nor VU0238429 were active in social interaction test when given alone, and also the combination of VU0152100 and GS39783 failed to reverse MK-801-induced deficits observed in this test. All muscarinic activators when administered alone or in combination with GS39783 reversed the MK-801-induced disruption of memory in the novel object recognition test, and their actions were blocked by specific antagonists. None of the tested compounds or their combinations influenced the motor coordination of the animals. The compounds had no effect on haloperidol-induced catalepsy and did not induce catalepsy when administered alone. Pharmacokinetic analysis confirmed lack of possible drug-drug interactions after combined administration of GS39783 with VU0357017 or VU0152100; however, when the drug was co-administered with VU0238429 its ability to pass the blood-brain barrier slightly decreased, suggesting potential drug-drug interactions. Our data show that modulation of cholinergic and GABAergic systems can potentially be beneficial in the treatment of the positive and cognitive symptoms of schizophrenia without inducing the adverse effects typical for presently used antipsychotics.

The 5-HT7 receptor antagonist SB 269970 ameliorates corticosterone-induced alterations in 5-HT7 receptor-mediated modulation of GABAergic transmission in the rat dorsal raphe nucleus

RATIONALE

Chronic stress and corticosterone have been shown to affect serotonin (5-HT) neurotransmission; however, the influence of stress on the activity of the dorsal raphe nucleus (DRN), the main source of 5-HT in the forebrain, is not well understood. In particular, it is unknown if and how stress modifies DRN 5-HT₇ receptors, which are involved in the modulation of the firing of local inhibitory interneurons responsible for regulating the activity of DRN projection cells.

OBJECTIVES

Our study aimed to investigate the effect of repeated corticosterone injections on the modulation of the inhibitory transmission within the DRN by 5-HT₇ receptors and whether it could be reversed by treatment with a 5-HT₇ receptor antagonist.

METHODS

Male Wistar rats received corticosterone injections repeated twice daily for 14 days. Spontaneous inhibitory postsynaptic currents (sIPSCs) were then recorded from DRN projection cells in ex vivo slice preparations obtained 24 h after the last injection.

RESULTS

Repeated corticosterone administration resulted in decreased frequency, but not amplitude, of sIPSCs in DRN projection cells. There were no changes in the excitability of these cells; however, corticosterone treatment suppressed the 5-HT₇ receptor-mediated increase in sIPSC frequency. Administration of the 5-HT₇ receptor antagonist SB 269970 for 7 days beginning on the eighth day of corticosterone treatment reversed the detrimental effects of corticosterone on 5-HT₇ receptor reactivity and GABAergic transmission in the DRN.

CONCLUSIONS

Elevated corticosterone level reduces DRN 5HT₇ receptor reactivity and decreases GABAergic transmission within the DRN, which can be reversed by the 5-HT₇ receptor antagonist SB 269970.

Nitric Oxide Synthase Inhibitor Attenuates the Effects of Repeated Restraint Stress on Synaptic Transmission in the Paraventricular Nucleus of the Rat Hypothalamus

Corticotropin-releasing hormone (CRH)-synthesizing parvocellular neuroendocrine cells (PNCs) of the hypothalamic paraventricular nucleus (PVN) play a key role in the activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Several studies have demonstrated that synaptic inputs to these cells may undergo stress-related enhancement but, on the other hand, it has been reported that exposition to the same stressor for prolonged time periods may induce a progressive reduction in the response of the HPA axis to homotypic stressors. In the present study rats were subjected to 10 min restraint sessions, repeated twice daily for 3 or 7 days. Miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) were then recorded from PNCs in ex vivo hypothalamic slice preparations obtained 24 h after the last restraint. Restraint stress repeated over 3 days resulted in increased mean frequency and decreased rise time and decay time constant of mEPSCs, accompanied by a decrease in the excitability of PNCs, however, no such changes were evident in slices obtained from rats subjected to restraint over 7 days. There were no changes in mIPSCs after repeated restraint. Administration of the unspecific nitric oxide synthase (NOS) blocker Nω-Nitro-L-arginine (L-NNA) before each restraint, repeated over 3 days, prevented the occurrence of an increase in mEPSC frequency. However, animals receiving L-NNA and subjected to repeated restraint had similar changes in PNCs membrane excitability and mEPSC kinetics as stressed rats not receiving L-NNA. Comparison of the effects of a single 10 min restraint session followed by either an immediate or delayed (24 h) decapitation revealed an increase in the mean mEPSC frequency and a decrease in the mean mIPSC frequency in slices prepared immediately after restraint, with no apparent effects when slice preparation was delayed by 24 h. These results demonstrate that restraint, lasting 10 min and repeated twice daily for 3 days, induces a selective and long-lasting enhancement of excitatory synaptic input onto PNCs, partially by a NOS-dependent mechanism, and reduces PNC excitability, whereas prolongation of repeated stress for up to 7 days results in an adaptation.

Neurochemical and behavioral studies on the 5-HT1A-dependent antipsychotic action of the mGlu4 receptor agonist LSP4-2022

LSP4-2022 is a novel, orthosteric agonist of mGlu₄ receptor that induces antipsychotic-like activity in animal studies. In the present study, the involvement of 5-HT_1A receptors in LSP4-2022-induced antipsychotic actions and the neurochemical background of that interaction were investigated. In several behavioral tests the actions of effective doses of the compound (0.5-2 mg/kg) were antagonized via the administration of the 5-HT_1A antagonist WAY100635 (0.1 mg/kg). The co-administration of sub-effective dose of the 5-HT_1A agonist (R)-(S)-8-OH-DPAT (0.01 mg/kg) intensified the activity of ineffective doses of LSP4-2022, having no influence on the efficacy of the active doses. The co-administration of effective doses of both compounds did not intensify each other's action. In the microdialysis in vivo tests, MK-801 (0.6 mg/kg) induced an enhancement of the release of dopamine, serotonin, glutamate and GABA in the prefrontal cortex. Administration of LSP4-2022 (2 mg/kg) abolished this MK-801-induced effect on neurotransmitter release. Co-administration with WAY100635 (0.1 mg/kg), a 5-HT_1A antagonist, completely (dopamine, serotonin) or partially (glutamate, GABA) counteracted this LSP4-2022-induced effect. Subsequently, the patch-clamp recordings of spontaneous EPSCs were performed. sEPSCs were evoked in slices from the mouse prefrontal cortex by DOI (10 μM). LSP4-2022 (2.5; 5 and 10 μm) reversed DOI-induced changes in both the frequency and amplitude of the sEPSCs, but the more robust effect on the frequency was observed. The administration of WAY100635 had no effect on the LSP4-2022-induced effects on sEPSCs, indicating that the mGlu₄-5-HT_1A interaction does not occur via single-neuron signaling but involves neuronal circuits that regulate neurotransmitter release. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

5-HT7 receptor modulates GABAergic transmission in the rat dorsal raphe nucleus and controls cortical release of serotonin

The 5-HT₇ receptor is one of the several serotonin (5-HT) receptor subtypes that are expressed in the dorsal raphe nucleus (DRN). Some earlier findings suggested that 5-HT₇ receptors in the DRN were localized on GABAergic interneurons modulating the activity of 5-HT projection neurons. The aim of the present study was to find out how the 5-HT₇ receptor modulates the GABAergic synaptic input to putative 5-HT DRN neurons, and whether blockade of the 5-HT₇ receptor would affect the release of 5-HT in the target structure. Male Wistar rats with microdialysis probes implanted in the prefrontal cortex (PFC) received injections of the 5-HT₇ receptor antagonist (2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine hydrochloride (SB 269970), which induced an increase in the levels of 5-HT and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA) in the PFC. In another set of experiments whole-cell recordings from presumed projection neurons were carried out using DRN slices. SB 269970 application resulted in depolarization and in an increase in the firing frequency of the cells. In order to activate 5-HT₇ receptors, 5-carboxamidotryptamine (5-CT) was applied in the presence of N-[2-[4-(2-methoxyphenyl)-1piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635). Hyperpolarization of cells and a decrease in the firing frequency were observed after activation of the 5-HT₇ receptor. Blockade of 5-HT₇ receptors caused a decrease in the mean frequency of spontaneous inhibitory postsynaptic currents (sIPSCs), while its activation induced an increase. The mechanism of these effects appears to involve tonically-active 5-HT₇ receptors modulating firing and/or GABA release from inhibitory interneurons which regulate the activity of DRN serotonergic projection neurons.

Impaired effect of activation of rat hippocampal 5-HT7 receptors, induced by treatment with the 5-HT7 receptor antagonist SB 269970

Effects of the 5-HT(7) receptor antagonist SB 269970, administered for 14 days (1.25 mg/kg), were studied in ex vivo slices of rat hippocampus. To activate the 5-HT(7) receptor, 5-carboxamidotryptamine (5-CT, 200 nM) was applied in the presence of WAY 100635 (2 μM), a 5-HT(1A) receptor antagonist. In contrast to control preparations, no 5-HT(7) receptor-mediated increase in excitability nor depolarization and an increase in the input resistance of CA1 and CA3 pyramidal neurons were present in slices prepared from rats treated with SB 269970. The treatment also abolished the stimulatory effect of 5-HT(7) receptor activation on spontaneous excitatory postsynaptic currents recorded from CA1 stratum radiatum/lacunosum-moleculare interneurons. These data demonstrate that repeated administration of SB 269970 impairs the reactivity of the CA1 hippocampal neuronal network to 5-HT(7) receptor activation.

Isomer-nonspecific action of dichlorodiphenyltrichloroethane on aryl hydrocarbon receptor and G-protein-coupled receptor 30 intracellular signaling in apoptotic neuronal cells

Extended residual persistence of the pesticide dichlorodiphenyltrichloroethane (DDT) raises concerns about its long-term neurotoxic effects. Little is known, however, about DDT toxicity during the early stages of neural development. This study demonstrated that DDT-induced apoptosis of mouse embryonic neuronal cells is a caspase-9-, caspase-3-, and GSK-3β-dependent process, which involves p,p'-DDT-specific impairment of classical ERs. It also provided evidence for DDT-isomer-nonspecific alterations of AhR- and GPR30-mediated intracellular signaling, including changes in the levels of the receptor and receptor-regulated mRNAs, and also changes in the protein levels of the receptors. DDT-induced stimulation of AhR-signaling and reduction of GPR30-signaling were verified using selective ligands and specific siRNAs. Co-localization of the receptors was demonstrated with confocal microscopy, and the presence of functional GPR30 was detected by electrophysiology. This study demonstrates that stimulation of AhR-signaling and impairment of GPR30-signaling play important roles in the propagation of DDT-induced apoptosis during the early stages of neural development.

The antipsychotic-like effects of positive allosteric modulators of metabotropic glutamate mGlu4 receptors in rodents

BACKGROUND AND PURPOSE

Because agonists at metabotropic glutamate receptors exert beneficial effects in schizophrenia, we have assessed the actions of Lu AF21934 and Lu AF32615, two chemically distinct, selective and brain-penetrant positive allosteric modulators (PAMs) of the mGlu4 receptor, in several tests reflecting positive, negative and cognitive symptoms of schizophrenia in rodents.

EXPERIMENTAL APPROACH

Hyperactivity induced by MK-801 or amphetamine and head twitches induced by 2,5-dimethoxy-4-iodoamphetamine (DOI) in mice were used as models for positive symptoms. Disruption of social interaction and spatial delayed alternation tests induced by MK-801 in rats were used as models for negative and cognitive symptoms of schizophrenia, respectively.

KEY RESULTS

Lu AF21934 (0.1-5 mg·kg(-1) ) and Lu AF32615 (2-10 mg·kg(-1) ) dose-dependently inhibited hyperactivity induced by MK-801 or amphetamine. They also antagonized head twitches and increased frequency of spontaneous excitatory postsynaptic currents (EPSCs) in brain slices, induced by DOI. In mice lacking the mGlu4 receptor (mGlu4 (-/-) ) mice, Lu AF21934 did not antagonize DOI-induced head twitches. MK-801-induced disruption in the social interaction test was decreased by Lu AF21934 at 0.5 mg·kg(-1) and by Lu AF32615 at 10 mg·kg(-1) . In the delayed spatial alternation test, Lu AF21934 was active at 1 and 2 mg·kg(-1) , while Lu AF32615 was active at 10 mg·kg(-1) .

CONCLUSIONS AND IMPLICATIONS

We propose that activation by PAMs of the mGlu4 receptor is a promising approach to the discovery of novel antipsychotic drugs.

Repeated restraint stress enhances glutamatergic transmission in the paraventricular nucleus of the rat hypothalamus

Corticotropin-releasing hormone (CRH)-synthesizing parvocellular neuroendocrine neurons of the hypothalamic paraventricular nucleus (PVN) play a key role in the activation of the hypothalamic-pituitary-adrenal axis (HPA). It is well known that excitatory and inhibitory inputs that regulate the activity of these neurons may undergo stress-related modifications; however, the effect of repeated restraint stress on the function of glutamatergic and GABAergic synapses on PVN parvocellular neuroendocrine neurons has not been fully understood so far. Adolescent male Wistar rats were subjected to restraint lasting 10 min and repeated twice daily for 3 days. Brain slices were prepared 24 hours after the last restraint session and were studied ex vivo. Whole-cell patch-clamping was used to record spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) from parvocellular neuroendocrine neurons of the PVN. Repeated restraint stress resulted in an increase in the mean frequency of sEPSCs and in a decrease in the rise time and the decay time constant of sEPSCs. There was no change in the mean amplitude of sEPSCs. The parameters characterizing sIPSCs also remained unaltered. In addition, the injected current vs. spiking rate ratio of parvocellular neurons was decreased. In conclusion, restraint stress, repeated for 3 days, selectively enhances excitatory synaptic inputs to parvocellular neurons of the PVN, these modifications being accompanied with a decrease in the intrinsic excitability of PVN neuroendocrine parvocellular neurons.

Stress- and antidepressant treatment-induced modifications of 5-HT₇ receptor functions in the rat brain

This paper summarizes a series of electrophysiological studies aimed at finding the effects of the activation of 5-HT(7) receptors on neuronal excitability as well as on excitatory and inhibitory synaptic transmission in the hippocampus and in the frontal cortex of the rat. These studies demonstrated that 5-HT(7) receptors play an important role in the modulation of the activity of the hippocampal network by regulating the excitability of pyramidal cells of the CA1 area, as well as via their effect on GABA and glutamatergic transmission. The reactivity of 5-HT(7) receptors in the hippocampus is decreased by repeated administration of antidepressant drugs and increased by a prolonged high level of corticosterone. More importantly, administration of antidepressant drug, imipramine, prevents the occurrence of corticosterone-induced changes in the function of hippocampal 5-HT(7) receptors. It has also been found that the blockade of 5-HT(7) receptors by the selective antagonist SB 269970, lasting for a few days, causes similar changes to those observed after long-term administration of antidepressants. Thus, it seems that the pharmacological blockade of 5-HT(7) receptors produces faster effects compared to classic antidepressant drugs. A similarity between the changes in the glutamatergic transmission induced by the blockade of 5 HT7 receptors and those caused by repeated administration of the antidepressant drug, imipramine, has also been found in the frontal cortex. It has also been shown that the changes in glutamatergic transmission and the impairment of long-term synaptic plasticity in the frontal cortex of animals subjected to repeated restraint stress are reversed by the blockade of 5-HT(7) receptors. Overall, these studies, together with the data provided by other investigators, support the hypothesis that 5-HT(7) receptor antagonists may become a prototype of a new class of antidepressant drugs. Such compounds will not function by blocking 5-HT reuptake, as many of the currently used drugs, but through a direct interaction with the 5-HT(7) receptor. This type of action is highly selective and usually does not require the occurrence of adaptive changes in neuronal functions, thus allowing for a much quicker therapeutic effect.

Acute and repeated treatment with the 5-HT7 receptor antagonist SB 269970 induces functional desensitization of 5-HT7 receptors in rat hippocampus

BACKGROUND

SB 269970, a 5-HT(7) receptor antagonist may produce a faster antidepressant-like effect in animal models, than do antidepressant drugs, e.g., imipramine. The present work was aimed at examining the effect of single and repeated (14 days) administration of SB 269970 on the 5-HT(7) receptor in the hippocampus.

METHODS

The reactivity of 5-HT(7) receptors was determined using 5-carboxamidotryptamine (5-CT), which increased the bursting frequency of spontaneous epileptiform activity in hippocampal slices. Additionally, the effects of SB 269970 administration on the affinity and density of 5-HT(7) receptors were investigated using [(3)H]-SB 269970 and the influence of SB 269970 and imipramine on mRNA expression levels of Gα(s) and Gα(12) mRNA were studied using RT-qPCR.

RESULTS

Acute and repeated treatment with SB 269970 led to attenuation of the excitatory effects of activation of 5-HT(7) receptors. Neither single nor repeated administration of SB 269970 changed the mean affinity of 5-HT(7) receptors for [(3)H]-SB 269970. Repeated, but not single, administration of SB 269970 decreased the maximum density of [(3)H]-SB 269970 binding sites. While administration of imipramine did not change the expression of mRNAs for Gα(s) and Gα(12) proteins after both single and repeated administration of SB 269970, a reduction in Gα(s) and Gα(12) mRNA expression levels was evident.

CONCLUSIONS

These findings indicate that even single administration of SB269970 induces functional desensitization of the 5-HT(7) receptor system, which precedes changes in the receptor density. This mechanism may be responsible for the rapid antidepressant-like effect of the 5-HT(7) antagonist in animal models.

Repeated blockade of 5-HT₇ receptors depresses glutamatergic transmission in the rat frontal cortex

The effects of the intraperitoneal administration of the 5-HT₇ receptor antagonist SB 269970 were studied in the rat frontal cortex. In ex vivo slices prepared from rats receiving 14 daily doses of the drug (1.25 mg/kg) the mean frequency and the mean amplitude of glutamate-mediated, spontaneous excitatory postsynaptic currents (sEPSCs) recorded from layer II/III pyramidal neurons, were decreased. In contrast, single administration of SB 269970 affected neither the frequency nor the amplitude of sEPSCs. Treatment with SB 269970 did not affect membrane excitability of pyramidal cells. These data indicate that repeated, but not single, treatment with SB 269970 results in an attenuation of glutamatergic transmission in the frontal cortex, most likely due to a combination of pre- and postsynaptic mechanisms.

The 5-HT(7) receptor antagonist SB 269970 counteracts restraint stress-induced attenuation of long-term potentiation in rat frontal cortex

The effects of restraint lasting for 10 min, repeated twice daily for 3 days, were studied ex vivo in rat frontal cortex slices prepared 24 h after the last stress session. In slices originating from stressed animals, the amplitude of extracellular field potentials recorded in cortical layer II/III was increased. Stress also resulted in a reduced magnitude of long-term potentiation (LTP) of field potentials. In a separate experimental group, rats were subjected to restraint lasting for 10 min, twice daily for 3 days but, additionally, animals received injections of 5-HT(7) receptor antagonist SB 269970 (1.25 mg/kg) before each restraint stress session. In this group, the amplitude of field potentials and the magnitude of LTP were not different from the control, indicating that stress-induced modifications of basal glutamatergic transmission and synaptic plasticity were prevented by the 5-HT(7) receptor antagonist.

The GABA B receptor agonist CGP44532 and the positive modulator GS39783 reverse some behavioural changes related to positive syndromes of psychosis in mice

BACKGROUND AND PURPOSE

An important role of GABAergic neurotransmission in schizophrenia was proposed a long time ago, but there is limited data to support this hypothesis. In the present study we decided to investigate GABA(B) receptor ligands in animal models predictive for the antipsychotic activity of drugs. The GABA(B) receptor antagonists CGP51176 and CGP36742, agonist CGP44532 and positive allosteric modulator GS39783 were studied.

EXPERIMENTAL APPROACH

The effects of all ligands were investigated in MK-801- and amphetamine-induced hyperactivity tests. The anti-hallucinogenic-like effect of the compounds was screened in the model of head twitches induced by (±)1-(2.5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Furthermore, the effect of GS39783 and CGP44532 on DOI-induced frequency of spontaneous excitatory postsynaptic currents (EPSCs) in slices from mouse brain frontal cortices was investigated. The anti-cataleptic properties of the compounds were also assessed.

KEY RESULTS

The GABA(B) receptor activators CGP44532 and GS39783 exhibited antipsychotic-like effects both in the MK-801- and amphetamine-induced hyperactivity tests, as well as in the head-twitch model in mice. Such effects were not observed for the GABA(B) receptor antagonists. DOI-induced increased frequency of spontaneous EPSCs was also decreased by the compounds. Moreover, CGP44532 and GS39783 inhibited haloperidol-induced catalepsy and EPSCs.

CONCLUSION AND IMPLICATIONS

These data suggest that selective GABA(B) receptor activators may be useful in the treatment of psychosis.

5-HT7 receptors modulate GABAergic transmission in rat hippocampal CA1 area

The effects of the activation of serotonin-7 (5-HT(7)) receptors were investigated in the CA1 area pyramidal cells and stratum radiatum fast spiking GABAergic interneurons of rat hippocampal slices. To activate 5-HT(7) receptors, 5-carboxamidotryptamine (5-CT), a nonselective 5-HT(1A)/5-HT(7) agonist, was applied in the presence of N-[2-[4-(2-methoxyphenyl)-1piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY 100635), a selective 5-HT(1A) receptor antagonist. The activation of 5-HT(7) receptors resulted in a dose-dependent increase in the mean frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from pyramidal neurons while the mean amplitude of sIPSCs remained unaltered. A nonselective glutamate receptor antagonist, kynurenic acid, and voltage-gated sodium channel blocker, tetrodotoxin (TTX), attenuated but did not prevent the 5-HT(7) receptor-mediated increase of sIPSCs frequency in pyramidal cells. 5-CT application did not influence the excitability of stratum radiatum interneurons but it dose-dependently increased the mean frequency of spontaneous excitatory postsynaptic currents (sEPSCs) recorded from interneurons while the mean amplitude of sEPSCs remained unaltered. These data suggest that the activation of 5-HT(7) receptors results in an enhancement of the GABAergic transmission in the hippocampal CA1 area via two mechanisms. The first one involves an enhancement of excitatory glutamatergic input to GABAergic interneurons and is likely to be mediated by presynaptic 5-HT(7) receptors. The second effect, most likely related to the activation of 5-HT(7) receptors located on interneurons, results in an enhancement of the release of GABA.

Imipramine counteracts corticosterone-induced alterations in the effects of the activation of 5-HT(7) receptors in rat hippocampus

Using extracellular recording we studied changes in the reactivity of rat hippocampal slices to an agonist of the 5-HT(7) receptor, 5-carboxamidotryptamine (5-CT; 0.025-1 microM), induced by an earlier treatment of animals with corticosterone. Spontaneous bursting activity was recorded in ex vivo slices incubated in the presence of 2-[4-(2-methoxyphenyl)-1piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY 100635; 2 microM), an antagonist of the 5-HT(1A) receptor, in the medium devoid of Mg2+ ions. Saturation binding assays were performed using [(3)H]-(2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine hydrochloride (SB 269970), a specific antagonist of the 5-HT(7) receptor. Repetitive, but not single, corticosterone administration lasting 7 and 21 days, resulted in an enhancement of the effect related to the 5-HT(7) receptor activation without changes in its binding properties. In a separate set of experiments rats were treated with corticosterone for 3 weeks and additionally with imipramine, beginning on the eighth day of corticosterone administration. In the corticosterone plus imipramine group the excitatory effect of 5-CT was weaker than in the corticosterone group, indicating that corticosterone-induced functional modifications in the reactivity of the 5-HT(7) receptor were reversed and further weakened by imipramine treatment. This effect was accompanied by a reduction in the density of [3H]-SB 269970 binding sites. Thus, imipramine treatment counteracts the corticosterone-induced increase in the reactivity of the hippocampal circuitry to the activation of the 5-HT(7) receptor.

Peripheral administration of group III mGlu receptor agonist ACPT-I exerts potential antipsychotic effects in rodents

Several lines of evidence implicate dysfunction of glutamatergic neurotransmission in the pathophysiology of schizophrenia. Previous behavioral studies have indicated that metabotropic glutamate (mGlu) receptors may be useful targets for the treatment of psychosis. It has been shown that agonists and positive allosteric modulators of group II mGlu receptors produce potential antipsychotic effects in behavioral models of schizophrenia in rodents. Group III mGlu receptors seem to be also promising targets for a variety of neuropsychiatric and neurodegenerative disorders. However, despite encouraging data in animal models, most ligands of group III mGlu receptors still suffer from weak affinities, incapacity to cross the blood-brain barrier or absence of full pharmacological characterization. These limitations slow down the validation process of group III mGlu receptors as therapeutic targets. In this work, we choose to study an agonist of group III mGlu receptors (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid (ACPT-I) using intraperitoneal administration in three animal behavioral models predictive of psychosis or hallucinations. The results of the present study show that ACPT-I, given at doses of 10 or 30mg/kg, decreased MK-801-induced hyperlocomotion and at a dose of 100mg/kg decreased amphetamine-induced hyperlocomotion in rats. Furthermore, ACPT-I dose-dependently decreased DOI-induced head twitches in mice and suppresses DOI-induced frequency and amplitude of spontaneous EPSPs in slices from mouse brain frontal cortices. These data demonstrate that ACPT-I is a brain-penetrating compound and illustrates its promising therapeutic role for the treatment of schizophrenia.

Repeated administration of imipramine attenuates glutamatergic transmission in rat frontal cortex

The effects of repeated administration of a tricyclic antidepressant, imipramine, lasting 14 days (10 mg/kg p.o., twice daily), were studied ex vivo in rat frontal cortex slices prepared 48 h after last dose of the drug. In slices prepared from imipramine-treated animals the mean frequency, and to a lesser degree the mean amplitude, of spontaneous excitatory postsynaptic currents recorded from layer II/III pyramidal neurons, were decreased. These effects were accompanied by a reduction of the initial slope ratio of pharmacologically isolated N-methyl-D-aspartate to AMPA/kainate receptor-mediated stimulation-evoked excitatory postsynaptic currents. Imipramine treatment also resulted in a decrease of extracellular field potentials evoked in layer II/III by stimulation of underlying sites in layer V. These results indicate that chronic treatment with imipramine results in an attenuation of the release of glutamate and an alteration in the postsynaptic reactivity of ionotropic glutamate receptors in rat cerebral cortex.

Effects of repetitive administration of tianeptine, zinc hydroaspartate and electroconvulsive shock on the reactivity of 5-HT(7) receptors in rat hippocampus

The influence of repeated administration of tianeptine, an atypical antidepressant, which was administered twice daily (10 mg/kg) for 14 days and zinc hydroaspartate, a compound exhibiting antidepressant-like activity, which was administered twice daily (65 mg/kg) for 14 days, and the effects of electroconvulsive shocks (ECS) delivered once daily for 10 days, were investigated ex vivo in rat hippocampal slices. Slices were prepared 2 days after the last session of treatment of animals, and spontaneous epileptiform bursts were recorded extracellularly from the CA3 area. 5-HT(7) receptor-mediated increase in bursting frequency was induced by bath application of of 5-carboxamidotryptamine (5-CT; 0.025-1 microM) in the presence of N-[2-[4-(2-methoxyphenyl)-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY 100635; 2 microM), an antagonist of the 5-HT(1A) receptor. The data indicate an enhancement of the excitatory effect of the activation of 5-HT(7) receptors after ECS repeated ten times, but not by a single ECS. Neither tianeptine nor zinc, administered for 14 days, altered the reactivity of 5-HT(7) receptors.

Sensory learning-induced enhancement of inhibitory synaptic transmission in the barrel cortex of the mouse

In adult mice, repetitive pairing of stimulation of mystacial vibrissae with an electrical shock to the tail induces expansion of the cortical representation of stimulated vibrissae accompanied by elevation of the GABAergic markers. Here, we show that this associative learning paradigm results in a selective increase in the frequency of spontaneous inhibitory postsynaptic currents in layer IV excitatory neurons located within the barrel representing stimulated vibrissae, evident 24 h after the end of training. The mean amplitude of spontaneous inhibitory postsynaptic potentials recorded from excitatory neurons was unchanged. Recordings from layer IV excitatory and fast spiking neurons showed that the training induced changes neither in the mean frequency nor it the mean amplitude of spontaneous excitatory postsynaptic currents. On the other hand, the mean amplitude of field potentials evoked by the stimulation of layer VI and recorded in layer IV was significantly reduced. These data indicate that aversive training results in a selective and long-lasting enhancement of GABAergic transmission within the cortical representation of stimulated vibrissae, which may result in a decrease in layer VI-evoked field responses.

Imipramine treatment ameliorates corticosterone-induced alterations in the effects of 5-HT1A and 5-HT4 receptor activation in the CA1 area of rat hippocampus

This study tested whether imipramine reverses adaptive modifications in the function of hippocampal 5-HT1A and 5-HT4 receptors induced by repetitive administration of corticosterone. Rats received corticosterone for 1 or 3 weeks or imipramine for 2 weeks. The fourth experimental group was treated with corticosterone for 3 weeks and additionally with imipramine, beginning on the eighth day of corticosterone administration. Hippocampal slices were prepared 48 h after the last drug administration. 5-HT1A and 5-HT4 receptor-mediated effects on CA1 population spike amplitude were measured. While repeated corticosterone attenuated the inhibitory effect of 5-HT1A receptor activation by 8-OH-DPAT and enhanced the excitatory effect of 5-HT4 receptor activation by zacopride, imipramine treatment of naïve rats resulted in opposite changes. In the corticosterone plus imipramine group, the effect of 8-OH-DPAT and zacopride were not different from control, indicating that corticosterone-induced adaptive changes in the reactivity of 5-HT1A and 5-HT4 receptors were reversed by imipramine treatment.

Repeated administration of citalopram and imipramine alters the responsiveness of rat hippocampal circuitry to the activation of 5-HT7 receptors

The effects of a selective serotonin reuptake inhibitor, citalopram, and a tricyclic antidepressant drug, imipramine, administered repetitively for 14 days, were investigated ex vivo in rat hippocampal slices. Spontaneous epileptiform bursts were recorded from the CA3 area in nominally Mg(2+)-free incubation conditions. 5-carboxamidotryptamine (5-CT) dose-dependently increased bursting frequency in the presence of N-[2-[4-(2-methoxyphenyl)-1 piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY 100635). This effect could be dose-dependently blocked by (2R)-1-[(3-Hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine hydrochloride (SB 269970), thus implicating the involvement of 5-HT(7) receptors. Repeated treatment with citalopram or imipramine resulted in an attenuation of the excitatory effects of the activation of hippocampal 5-HT(7) receptor.

The influence of repeated administration of clozapine and haloperidol on the effects of the activation of 5-HT(1A), 5-HT(2) and 5-HT(4) receptors in rat frontal cortex

The effects of a repeated treatment with antipsychotic drugs, clozapine and haloperidol, on the modulation of network activity ex vivo by 5-HT receptors were examined in rat frontal cortical slices using extracellular recording. Rats were treated for 21 days with clozapine (30 mg/kg p.o.), or haloperidol (1 mg/kg p.o.). Spontaneous bursting activity was induced in slices prepared 3 days after the last drug administration by perfusion with a medium devoid of Mg(2+) ions and with added picrotoxin (30 mM). The application of 2-3 microM 8-OH-DPAT, acting through 5-HT(1A) receptors, resulted in a reversible decrease of bursting frequency. In the presence of 1 microM DOI, the 5-HT(2) agonist, or 5 microM zacopride, the 5-HT(4) agonist, bursting frequency increased. Chronic clozapine treatment resulted in an attenuation of the effect of the activation of 5-HT(2) receptors, while the effects related to 5-HT(1A) and 5-HT(4) receptor activation were unchanged. Treatment with haloperiol did not influence the reactivity to the activation of any of the three 5-HT receptor subtypes. These data are consistent with earlier findings demonstrating a selective downregulation of 5-HT(2A) receptors by clozapine and indicate that chronic clozapine selectively attenuates the 5-HT-mediated excitation in neuronal circuitry of the frontal cortex while leaving the 5-HT-mediated inhibition intact.

Repeated administration of antidepressants decreases field potentials in rat frontal cortex

The effects of repeated administration of a tricyclic antidepressant, imipramine, and a selective serotonin reuptake blocker, citalopram, for 14 days (10 mg/kg p.o., twice daily), were studied ex vivo in rat frontal cortex slices prepared 48 h after last dose of the drug. Treatment with both antidepressants resulted in a decrease in the amplitude of field potentials evoked in layer II/III by stimulation of underlying sites in layer V. The amplitude ratio of pharmacologically isolated N-methyl-D-aspartic acid (NMDA) to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor-mediated components of the field potential was reduced. These results indicate that chronic treatment with imipramine or citalopram results in an attenuation of glutamatergic synaptic transmission in the cerebral cortex.

5-HT7 receptors increase the excitability of rat hippocampal CA1 pyramidal neurons

In the CA1 area of rat hippocampal slices, a combined application of 5-CT, a potent 5-HT(1A) and 5-HT(7) receptor agonist, and WAY 100635, a selective 5-HT(1A) receptor antagonist, resulted in a reversible increase of the CA1 extracellular population spike amplitude. In whole-cell recording from identified pyramidal neurons, the effects of 5-CT applied in the presence of WAY 100635 involved a reduction of the slow afterhyperpolarization (sAHP) and the frequency adaptation of action potential firing, which could be blocked by a specific 5-HT(7) receptor antagonist SB 269970. The results indicate that the activation of 5-HT(7) receptors increases the excitability of hippocampal CA1 pyramidal cells.

Adaptive changes in the reactivity of 5-HT1A and 5-HT2 receptors induced in rat frontal cortex by repeated imipramine and citalopram

Using extracellular ex vivo recording we studied changes in the reactivity of rat frontal cortical neurons to the 5-HT(1A), 5-HT(2) and 5-HT(4) receptor agonists (+/-)-2-dipropyloamino-8-hydroxy-1,2,3,4-tetrahydronaphtalene hydrobromide (8-OH-DPAT), (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) and zacopride, respectively, induced by a repeated treatment with imipramine or citalopram. Rats were treated with imipramine or citalopram for 14 days (10 mg/kg p.o.) twice daily. Frontal cortical slices were prepared 2 days after the last drug administration. Spontaneous epileptiform discharges were induced in slices by perfusion with a medium devoid of Mg(2+) ions and with added picrotoxin (30 microM). While the application of 2 microM 8-OH-DPAT resulted in a reversible decrease of the discharge frequency, in the presence of DOI (1 microM) or zacopride (5 microM), the discharge frequency was increased. Both repeated imipramine and citalopram enhanced the effect of the activation of 5-HT(1A) receptor and attenuated the effect related to 5-HT(2) receptor activation, while the effect of the activation of 5-HT(4) receptor remained unchanged. Moreover, imipramine, but not citalopram, induced a reduction of epileptiform discharge frequency and an increase of the time of occurrence of epileptiform activity. These data indicate that antidepressants enhance the 5-HT-mediated inhibition in neuronal circuitry of the frontal cortex.

Comparison of the effects of 5-HT1A and 5-HT4 receptor activation on field potentials and epileptiform activity in rat hippocampus

The effects of serotonin (5-HT) as well as 5-HT(1A) and 5-HT(4) receptor agonists, (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT) and zacopride, respectively, on population spikes evoked by electrical stimulation and on spontaneous epileptiform activity were investigated in CA1 area of hippocampal slices. Spontaneous epileptiform activity was recorded from slice in a nominally Mg(2+)-free medium. While 5-HT application resulted in a decrease of population spikes evoked in standard incubation conditions, in accordance with earlier studies, it exerted two opposite effects on epileptiform activity. The early inhibitory effect was mimicked by 8-OH-DPAT while the later, excitatory, by zacopride. The application of 8-OH-DPAT decreased, and that of zacopride increased, the amplitude of population spikes. A comparison of the dose-dependence of the excitatory and inhibitory effects of serotonergic agonists on the amplitude of the population spike and on the frequency of epileptiform discharges indicated that the latter is a more sensitive measure of the activation of 5-HT(1A) and 5-HT(4) receptors than the former. Thus, spontaneous epileptiform activity recorded in a nominally Mg(2+)-free slice medium represents a convenient model for investigation of hippocampal neuronal reactivity to the activation of various 5-HT receptor subtypes.

[Use of models for spontaneous paroxysmal activity in studies of adaptive changes in serotonin receptors]

Certain antidepressant medications exert effects on the serotonergic (5-HT) system but their mechanisms are still not well understood. Using extracellular ex vivo recording of spontaneous, epileptiform activity of neurons we determined changes in the reactivity of cortical and hippocampal slices to agonists of main subtypes 5-HT receptors, induced by repeated administration of the antidepressants. Repeated treatment with antidepressants enhances the 5-HT-mediated inhibition in both frontal cortex and hippocampal regions. Drugs administration induce opposite adaptive changes within 5-HT2 and 5-HT1A receptors in frontal cortex as well as in 5-HT1A and 5-HT4 receptors in the hippocampus. These effects may contribute to therapeutic effects of antidepressants administration.

Prolonged corticosterone treatment alters the responsiveness of 5-HT1A receptors to 8-OH-DPAT in rat CA1 hippocampal neurons

Hippocampal 5-HT(1A) receptors have been shown to be suppressed by glucocorticoids in a variety of animal studies, however the molecular mechanism and the functional meaning of this effect are still not well understood. The present study was designed to investigate the impact of repeated administration of corticosterone (10 mg/kg s.c. twice daily for 7 days) on the functional consequences of 5-HT(1A) receptor stimulation measured electrophysiologically in hippocampal slices. Additionally, the effects of corticosterone on 5-HT(1A) receptor binding and on receptor mRNA levels in the hippocampus were studied. Prolonged, but not acute treatment with corticosterone attenuated (+/-)-8-hydroxy-2-di- N-propylamino)tetralin hydrobromide (8-OH-DPAT)-induced inhibition of population spikes, and 8-OH-DPAT-induced hyperpolarization in rat CA1 hippocampal neurons. Chronic, but not acute treatment with corticosterone also decreased 5-HT(1A) receptor binding in the CA1 region (in the ventral part only) and the dentate gyrus. A single dose of corticosterone increased [(3)H]8-OHDPAT binding in the dentate gyrus and in the CA3 and CA4 hippocampal regions. Only acute, but not prolonged treatment with corticosterone decreased the level of 5-HT(1A) receptor mRNA in the CA1 region and dentate gyrus of the hippocampus. 5-HT turnover in the hippocampus was not influenced by chronic corticosterone. It is concluded that a chronically elevated level of corticosterone can induce functional desensitization of 5-HT(1A) receptors in the CA1 area of the hippocampus, although this effect is not always followed consequently by decreases in 5-HT(1A) receptor synthesis in this or other areas of the hippocampus.

Group II mGlu receptor agonists inhibit behavioural and electrophysiological effects of DOI in mice

It has been suggested that metabotropic glutamate (mGlu) receptor agonists selective for Group II mGlu receptors may have antipsychotic action. Therefore, we studied whether the effects, which could be related to psychotomimetic action of hallucinogenic drugs, are inhibited by Group II mGlu receptor agonists. The selective mGlu2/3 agonists LY354740 and LY379268 inhibited (+/-)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced head twitches in mice in a dose-dependent manner. Furthermore, LY379268 suppressed an increase in the frequency of spontaneous excitatory synaptic potentials induced by bath-applied DOI in layer V pyramidal cells recorded in the murine medial frontal cortex. The data indicate that Group II mGlu receptor agonists may counteract the effects of hallucinogenic drugs.

Imipramine but not 5-HT(1A) receptor agonists or neuroleptics induces adaptive changes in hippocampal 5-HT(1A) and 5-HT(4) receptors

It has been reported that the treatment with a tricyclic antidepressant imipramine induces an increase in the sensitivity of 5-HT(1A) receptors and a decrease in the sensitivity of 5-HT(4) receptors in the rat hippocampus. 5-HT(1A) receptor agonists and neuroleptics also affect 5-HT(1A) receptors in different brain areas; therefore, it was of interest to compare their effects on hippocampal 5-HT receptors with the influence of the well-established antidepressant imipramine. We studied the effects of repeated treatment with imipramine, the 5-HT(1A) receptor agonists 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) and buspirone, and the neuroleptics haloperidol and clozapine on the sensitivity of rat hippocampal CA1 neurons to 5-HT(1A)- and 5-HT(4) receptor activation. Imipramine was administered for 21 days (10 mg/kg p.o., twice daily), 8-OH-DPAT for 7 days (1 mg/kg s.c., twice daily) and buspirone for 21 days (5 mg/kg s.c., twice daily). The rats received haloperidol (1 mg/kg) and clozapine (30 mg/kg) for 6 weeks in drinking water. Hippocampal slices were prepared 2 days after the last treatment with imipramine, 8-OH-DPAT or buspirone, and 5 days after the last treatment with the neuroleptics. Using an extracellular in vitro recording, we studied changes in the amplitude of stimulation-evoked population spikes, induced by 5-HT, 8-OH-DPAT and the 5-HT(4) receptor agonist zacopride. Activation of 5-HT(1A) receptors decreased, while activation of 5-HT(4) receptors increased the amplitude of population spikes. Imipramine significantly enhanced the inhibitory effects of 5-HT and 8-OH-DPAT, and attenuated the excitatory effect of zacopride. No other treatment used in the present study changed the sensitivity of hippocampal CA1 neurons to 5-HT(1A) and 5-HT(4) receptors activation. These findings indicate that adaptive changes in the sensitivity of hippocampal neurons to 5-HT(1A) and 5-HT(4) receptors agonists are specific to imipramine and may thus-at least partly-mediate its effects.

Repeated imipramine administration enhances the effects of NMDA receptor ligands on synchronous activity in rat frontal cortex in vitro

This study assessed the effects of repeated administration (14 days) of imipramine on the function of NMDA receptors by measuring the frequency of spontaneous epileptiform discharges which develop in rat frontal cortical slices incubated in Mg2+-free conditions. Imipramine significantly enhanced both the excitatory effect of NMDA and the inhibitory effect of the competitive NMDA receptor antagonist CGP 37849 on the frequency of discharges. These results are consistent with studies indicating that chronic administration of antidepressant drugs induces adaptive changes in NMDA receptor/ channel complex in the cerebral cortex.

Opposite effects of antidepressants and corticosterone on the sensitivity of hippocampal CA1 neurons to 5-HT1A and 5-HT4 receptor activation

Using extracellular and intracellular ex vivo recording techniques we studied changes in the reactivity of hippocampal pyramidal CA1 neurons to serotonin (5-HT) and to the 5-HT1A- and 5-HT4 receptor agonists (+/-)-2-dipropylamino-8-hydroxy- 1,2,3 ,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT) and zacopride, respectively, evoked by repeated electroconvulsive shock (ECS), imipramine and corticosterone treatments. Rats were subjected to ECS for 1 or 10 days, treated with imipramine for 1, 7, 14 or 21 days (10 mg/kg p.o., twice daily) and with corticosterone for 7 days (10 mg/kg s.c., twice daily). Hippocampal slices were prepared 2 days after the last treatment. Activation of 5-HT1A receptors decreased the amplitude of population spikes evoked by stimulation of the Schaffer/collateral-commissural pathway and hyperpolarized CA1 cells. Activation of 5-HT4 receptors increased the population spike amplitude and decreased the amplitude of slow afterhyperpolarization. Both repeated ECS and imipramine enhanced the effects related to 5-HT1A receptor activation and attenuated the effects of 5-HT4 receptor activation. The action of imipramine was significant after a 7-day treatment and reached a maximum after 14 daily applications, remaining at the same level in a group of animals treated for 21 days. Repeated corticosterone attenuated the inhibitory effect of 5-HT and 8-OH-DPAT on the population spike amplitude and enhanced the increase in population spike amplitude induced by zacopride. These findings indicate that antidepressant treatments and repeated corticosterone have opposite effects on hippocampal responsiveness to 5-HT1A and 5-HT4 receptor activation. In consequence, antidepressants enhance, whereas corticosterone reduces the 5-HT-mediated inhibition of hippocampal CA1 cells, which may be relevant to the antidepressant and pro-depressant effects of either treatment, respectively.

[Electrophysiologic tests for testing the effects of antidepressant drugs and corticosterone on reactivity of serotonin receptors in the hippocampus]

Disturbances in the serotonin (5-HT) system and the limbic-hypothalamo-pituitary-adrenal axis (LHPA) have been implicated in the pathophysiology of depression. It is well established that hippocampus is a central component of limbic circuitry that participates in the modulation of cognition, mood and behavior, and is involved in the control of the LHPA axis. Therefore, the hippocampus provides a unique environment to study the interplay between serotonergic system, antidepressants and corticosteroids. Activity of hippocampal cells can be modulated by 5-HT via inhibitory 5-HT1A and excitatory 5-HT4 receptors. Repeated treatment with antidepressants increases the responsiveness of hippocampal pyramidal neurons to the 5-HT1A and attenuates the responsiveness to the 5-HT4 receptor agonists, with a time course which correlates with the delayed onset of the therapeutic effect of antidepressants in humans. Moreover, repeated corticosterone, which may constitute a model of a prolonged nonadaptable stress, has opposite effect on hippocampal responsiveness to the 5-HT1A and 5-HT4 receptor activation. Such an action results in an enhancement of the 5-HT-mediated inhibition by antidepressants and a reduction in the inhibitory effect of 5-HT by corticosterone which may be relevant to antidepressant/antiaxiety and proaxiety effects, respectively, of both treatments.

Repeated corticosterone administration increases excitatory effect of 5-HT4 receptor agonist in the rat hippocampus

The objective of the present study was to investigate whether repeated exposure of rats to high level of corticosterone affects responses of CA1 hippocampal cells to the 5-HT4 receptor agonist zacopride. To assess responsiveness of CA1 neurons to zacopride we used extracellular recording of population spikes evoked in CA1 cells by the stimulation of the Schaffer/collateral-commissural pathway in hippocampal slices. Rats were treated with corticosterone for 7 days (10 mg/kg sc, twice daily), slices were prepared two days after the last treatment. Zacopride induced an increase in the amplitude of population spike and repeated corticosterone treatment enhanced this excitatory effect. It is concluded that repeated treatment with corticosterone increases the responsiveness of hippocampal CA1 neurons to the 5-HT4 receptor activation.

Antidepressant treatment influences group I of glutamate metabotropic receptors in slices from hippocampal CA1 region

We investigated the effects of repeated electroconvulsive shock or imipramine treatment on inositol phosphate accumulation and on the reactivity of neurons to metabotropic glutamate (mGlu) receptor agonists in rat hippocampal slices. (1S,3R)-1-carboxycyclopentane-3-acetic acid (1S,3R-ACPD), a nonselective mGlu receptor agonist, caused a concentration-dependent increase in inositol phosphate in slices from the CA1 region of the hippocampus, an effect that was not modified by imipramine or electroconvulsive shock treatment. 1S,3R-ACPD or the selective agonist of the I group of mGlu receptor, (R,S)-3,5-dihydroxyphenylglycine ((R,S)-3,5-DHPG), produced a concentration-dependent increase of the population spike recorded in the CA1 cell layer. This effect of 1S,3R-ACPD was markedly attenuated by both repeated imipramine and electroconvulsive shock treatment, and the action of (R,S)-3,5-DHPG was markedly attenuated by prolonged imipramine treatment (electroconvulsive shock was not tested). Our results indicate that antidepressant treatment may induce a subsensitivity of group I mGlu receptors when assessed by electrophysiological but not biochemical measures.