Characterization of stress-induced suppression of long-term potentiation in the hippocampal CA1 field of freely moving rats

Ketamine induced synaptic plasticity operates independently of long-term potentiation

Synaptic plasticity occurs via multiple mechanisms to regulate synaptic efficacy. Homeostatic and Hebbian plasticity are two such mechanisms by which neuronal synapses can be altered. Although these two processes are mechanistically distinct, they converge on downstream regulation of AMPA receptor activity to modify glutamatergic neurotransmission. However, much remains to be explored regarding how these two prominent forms of plasticity interact. Ketamine, a rapidly acting antidepressant, increases glutamatergic transmission via pharmacologically-induced homeostatic plasticity. Here, we demonstrate that Hebbian plasticity mechanisms are still intact in synapses that have undergone homeostatic scaling by ketamine after either systemic injection or perfusion onto hippocampal brain slices. We also investigated this relationship in the context of stress induced by chronic exposure to corticosterone (CORT) to better model the circumstances under which ketamine may be used as an antidepressant. We found that CORT induced an anhedonia-like behavioral phenotype in mice but did not impair long-term potentiation (LTP) induction. Furthermore, corticosterone exposure does not impact the intersection of homeostatic and Hebbian plasticity mechanisms, as synapses from CORT-exposed mice also demonstrated intact ketamine-induced plasticity and LTP in succession. These results provide a mechanistic explanation for how ketamine used for the treatment of depression does not impair the integrity of learning and memory processes encoded by mechanisms such as LTP.

Comparing the effects of crocin at different doses on excitability and long-term potentiation in the CA1 area, as well as the electroencephalogram responses of rats under chronic stress

Stress adversely affects the cellular and electrophysiological mechanisms of memory; however, crocin has beneficial effects on brain functions. Nonetheless, the electrophysiological effects of using this active saffron component at different doses are not yet studied in rats under chronic restraint stress. Therefore, this study compared the impact of crocin at different doses on the excitability and long-term potentiation (LTP) in the CA1 area of rats, as well as their electroencephalogram (EEG) responses, hippocampal and frontal cortical glucose levels under chronic restraint stress (an emotional stress model). Forty rats were allocated into five groups of control, sham, restraint stress (6 h/day/21 days), and two stress groups receiving intraperitoneal injections of crocin (30, 60 mg/kg/day). Besides measuring the slope and amplitude of field excitatory postsynaptic potentials (fEPSPs) in the input-output and LTP curves, the EEG waves and hippocampal and frontal cortical glucose levels were assessed in all groups. Chronic restraint stress significantly decreased the fEPSP slope and amplitude in the input-output curves and after LTP induction. Both doses of crocin (60 and particularly 30 mg/kg) significantly improved fEPSP slope and amplitude in the stressed groups. Also, stress and crocin only at a dose of 30 mg/kg altered the EEG waves. Hippocampal and frontal cortical glucose levels displayed no significant differences in the experimental groups. Crocin at doses of 60 mg/kg/day and particularly 30 mg/kg/day reversed the harmful effects of chronic restraint stress on LTP as a cellular memory-related mechanism. However, only the lower dose of crocin affected the electrical brain activity in EEG.

Stress-Induced Enhanced Long-Term Potentiation and Reduced Threshold for N-Methyl-D-Aspartate Receptor- and β-Adrenergic Receptor-Mediated Synaptic Plasticity in Rodent Ventral Subiculum

Stress is a biologically relevant signal and can modulate hippocampal synaptic plasticity. The subiculum is the major output station of the hippocampus and serves as a critical hub in the stress response network. However, stress-associated synaptic plasticity in the ventral subiculum has not been adequately addressed. Therefore, we investigated the impact of a single exposure to an inherently stressful two-way active avoidance conditioning on the induction of long-term potentiation (LTP) at CA1-subiculum synapses in ventral hippocampal slices from young adult rats 1 day after stressor exposure. We found that acute stress enhanced LTP and lowered the induction threshold for a late-onset LTP at excitatory CA1 to subicular burst-spiking neuron synapses. This late-onset LTP was dependent on the activation of β-adrenergic and glutamatergic N-methyl-D-aspartate receptors and independent of D1/D5 dopamine receptor activation. Thereby, we present a cellular mechanism that might contribute to behavioral stress adaptation after acute stressor exposure.

Synaptic Plasticity, Metaplasticity and Depression

The development of a persistent depressive affective state has for some time been thought to result from persistent alterations in neurotransmitter-mediated synaptic transmission. While the identity of those transmitters has changed over the years, the literature has lacked mechanistic connections between the neurophysiological mechanisms they regulate, and how these mechanisms alter neuronal function, and, hence, affective homeostasis. This review will examine recent work that suggests that both long-term activity-dependent changes in synaptic strength (“plasticity”), and shifting set points for the ease of induction of future long-term changes (“metaplasticity”), may be critical to establishing and reversing a depressive behavioral state. Activity-dependent long-term synaptic plasticity involves both strengthening and weakening of synaptic connections associated with a dizzying array of neurochemical alterations that include synaptic insertion and removal of a number of subtypes of AMPA, NMDA and metabotropic glutamate receptors, changes in presynaptic glutamate release, and structural changes in dendritic spines. Cellular mechanisms of metaplasticity are far less well understood. Here, we will review the growing evidence that long-term synaptic changes in glutamatergic transmission, in brain regions that regulate mood, are key determinants of affective homeostasis and therapeutic targets with immense potential for drug development.

Metaplastic regulation of the median raphe nucleus via serotonin 5-HT1A receptor on hippocampal synaptic plasticity is associated with gender-specific emotional expression in rats

Gender differences in psychiatric disorders are considered to be associated with the serotonergic (5-HTergic) system; however the underlying mechanisms have not been clearly elucidated. In this study, possible involvement of the median raphe nucleus (MRN)-hippocampus 5-HTergic system in gender-specific emotional regulation was investigated, focusing on synaptic plasticity in rats. A behavioral study using a contextual fear conditioning (CFC) paradigm showed that the females exhibited low anxiety-like behavior. Extracellular 5-HT levels in the hippocampus were increased by CFC only in the males. Long-term potentiation (LTP) in the hippocampal CA1 field was suppressed after CFC in the males, which was mimicked by the synaptic response to MRN electrical stimulation. In the MRN, 5-HT immunoreactive cells significantly increased in the females compared with those in the males. Pretreatment with the 5-HT1A receptor agonists tandospirone (10 mg/kg, i.p.) and 8-OH DPAT (3 mg/kg, i.p.) significantly suppressed LTP induction in the males. Synaptic responses to CFC and 5-HT1A receptor interventions were not observed in the females. These results suggest that the metaplastic 5-HTergic mechanism via 5-HT1A receptors in the MRN-hippocampus pathway is a key component for gender-specific emotional regulation and may be a cause of psychiatric disorders associated with vulnerability or resistance to emotional stress.

Study protocol for the randomised controlled trial: antiglucocorticoid augmentation of anti-Depressants in Depression (The ADD Study)

BACKGROUND

Some patients with depression do not respond to first and second line conventional antidepressants and are therefore characterised as suffering from treatment refractory depression (TRD). On-going psychosocial stress and dysfunction of the hypothalamic-pituitary-adrenal axis are both associated with an attenuated clinical response to antidepressants. Preclinical data shows that co-administration of corticosteroids leads to a reduction in the ability of selective serotonin reuptake inhibitors to increase forebrain 5-hydroxytryptamine, while co-administration of antiglucocorticoids has the opposite effect. A Cochrane review suggests that antiglucocorticoid augmentation of antidepressants may be effective in treating TRD and includes a pilot study of the cortisol synthesis inhibitor, metyrapone. The Antiglucocorticoid augmentation of anti-Depressants in Depression (The ADD Study) is a multicentre randomised placebo controlled trial of metyrapone augmentation of serotonergic antidepressants in a large population of patients with TRD in the UK National Health Service.

METHODS/DESIGN

Patients with moderate to severe treatment refractory Major Depression aged 18 to 65 will be randomised to metyrapone 500 mg twice daily or placebo for three weeks, in addition to on-going conventional serotonergic antidepressants. The primary outcome will be improvement in Montgomery-Åsberg Depression Rating Scale score five weeks after randomisation (i.e. two weeks after trial medication discontinuation). Secondary outcomes will include the degree of persistence of treatment effect for up to 6 months, improvements in quality of life and also safety and tolerability of metyrapone. The ADD Study will also include a range of sub-studies investigating the potential mechanism of action of metyrapone.

DISCUSSION

Strengths of the ADD study include broad inclusion criteria meaning that the sample will be representative of patients with TRD treated within the UK National Health Service, longer follow up, which to our knowledge is longer than any previous study of antiglucocorticoid treatments in depression, and the range of mechanistic investigations being carried out. The data set acquired will be a rich resource for a range of research questions relating to both refractory depression and the use of antiglucocorticoid treatments.

TRIAL REGISTRATION

Current Controlled Trials: ISRCTN45338259; EudraCT Number: 2009-015165-31.

Distinct neurochemical and functional properties of GAD67-containing 5-HT neurons in the rat dorsal raphe nucleus

The serotonergic (5-HTergic) system arising from the dorsal raphe nucleus (DRN) is implicated in various physiological and behavioral processes, including stress responses. The DRN is comprised of several subnuclei, serving specific functions with distinct afferent and efferent connections. Furthermore, subsets of 5-HTergic neurons are known to coexpress other transmitters, including GABA, glutamate, or neuropeptides, thereby generating further heterogeneity. However, despite the growing evidence for functional variations among DRN subnuclei, relatively little is known about how they map onto neurochemical diversity of 5-HTergic neurons. In the present study, we characterized functional properties of GAD67-expressing 5-HTergic neurons (5-HT/GAD67 neurons) in the rat DRN, and compared with those of neurons expressing 5-HTergic molecules (5-HT neurons) or GAD67 alone. While 5-HT/GAD67 neurons were absent in the dorsomedial (DRD) or ventromedial (DRV) parts of the DRN, they were selectively distributed in the lateral wing of the DRN (DRL), constituting 12% of the total DRL neurons. They expressed plasmalemmal GABA transporter 1, but lacked vesicular inhibitory amino acid transporter. By using whole-cell patch-clamp recording, we found that 5-HT/GAD67 neurons had lower input resistance and firing frequency than 5-HT neurons. As revealed by c-Fos immunohistochemistry, neurons in the DRL, particularly 5-HT/GAD67 neurons, showed higher responsiveness to exposure to an open field arena than those in the DRD and DRV. By contrast, exposure to contextual fear conditioning stress showed no such regional differences. These findings indicate that 5-HT/GAD67 neurons constitute a unique neuronal population with distinctive neurochemical and electrophysiological properties and high responsiveness to innocuous stressor.

Unraveling the time domains of corticosteroid hormone influences on brain activity: rapid, slow, and chronic modes

Brain cells are continuously exposed to corticosteroid hormones, although the levels vary (e.g., after stress). Corticosteroids alter neural activity via two receptor types, mineralocorticoid (MR) and glucocorticoid receptors (GR). These receptors regulate gene transcription but also, as we now know, act nongenomically. Via nongenomic pathways, MRs enhance and GRs suppress neural activity. In the hypothalamus, inhibitory GR effects contribute to negative feedback regulation of the stress axis. Nongenomic MR actions are also important extrahypothalamically and help organisms to immediately select an appropriate response strategy. Via genomic mechanisms, corticosteroid actions in the basolateral amygdala and ventral-most part of the cornu ammonis 1 hippocampal area are generally excitatory, providing an extended window for encoding of emotional aspects of a stressful event. GRs in hippocampal and prefrontal pyramidal cells increase surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and strengthen glutamatergic signaling through pathways partly overlapping with those involved in long-term potentiation. This raises the threshold for subsequent induction of synaptic potentiation and promotes long-term depression. Synapses activated during stress are thus presumably strengthened but protected against excitatory inputs reaching the cells later. This restores higher cognitive control and promotes, for example, consolidation of stress-related contextual information. When an organism experiences stress early in life or repeatedly in adulthood, the ability to induce synaptic potentiation is strongly reduced and the likelihood to induce depression enhanced, even under rest. Treatment with antiglucocorticoids can ameliorate cellular effects after chronic stress and thus provide an interesting lead for treatment of stress-related disorders.

Switching off LTP: mGlu and NMDA receptor-dependent novelty exploration-induced depotentiation in the rat hippocampus

Both electrically induced synaptic long-term potentiation (LTP) and long-term depression have been extensively studied as models of the cellular basis of learning and memory mechanisms. Recently, considerable interest has been generated by the possibility that the activity-dependent persistent reversal of previously established synaptic LTP (depotentiation) may play a role in the time- and state-dependent erasure of memory. Here, we examined the requirement for glutamate receptor activation in experience-induced reversal of previously established LTP in the CA1 area of the hippocampus of freely behaving rats. Continuous exploration of non-aversive novelty for ~30 min, which was associated with hippocampal activation as measured by increased theta power in the electroencephalogram, triggered a rapid and persistent reversal of high frequency stimulation-induced LTP both at apical and basal synapses. Blockade of metabotropic glutamate (mGlu) receptors with mGlu5 subtype-selective antagonists, or N-methyl-D-aspartate (NMDA) receptors with GluN2B subunit-selective antagonists, prevented novelty-induced depotentiation. These findings strongly indicate that activation of both mGlu5 receptors and GluN2B-containing NMDA receptors is required for experience-triggered induction of depotentiation at CA3-CA1 synapses. The mechanistic concordance of the present and previous studies of experience-induced and electrically induced synaptic depotentiation helps to integrate our understanding of the neurophysiological underpinnings of learning and memory.

Estradiol rapidly rescues synaptic transmission from corticosterone-induced suppression via synaptic/extranuclear steroid receptors in the hippocampus

We investigated rapid protection effect by estradiol on corticosterone (CORT)-induced suppression of synaptic transmission. Rapid suppression by 1 μM CORT of long-term potentiation (LTP) at CA3-CA1 synapses was abolished via coperfusion of 1 nM estradiol. N-methyl-D-aspartate (NMDA) receptor-derived field excitatory postsynaptic potential (NMDA-R-fEPSP) was used to analyze the mechanisms of these events. Estradiol abolished CORT-induced suppression of NMDA-R-fEPSP slope. This CORT-induced suppression was abolished by calcineurin inhibitor, and the rescue effect by estradiol on the CORT-induced suppression was inhibited by mitogen-activated protein (MAP) kinase inhibitor. The CORT-induced suppressions of LTP and NMDA-R-fEPSP slope were abolished by glucocorticoid receptor (GR) antagonist, and the restorative effects by estradiol on these processes were mimicked by estrogen receptor α (ERα) and ERβ agonists. Taken together, estradiol rapidly rescued LTP and NMDA-R-fEPSP slope from CORT-induced suppressions. A GR→calcineurin pathway is involved in these suppressive effects. The rescue effects by estradiol are driven via ERα or ERβ→MAP kinase pathway. Synaptic/extranuclear GR, ERα, and ERβ probably participate in these rapid events. Mass-spectrometric analysis determined that acute hippocampal slices used for electrophysiological measurements contained 0.48 nM estradiol less than exogenously applied 1 nM. In vivo physiological level of 8 nM estradiol could protect the intact hippocampus against acute stress-induced neural suppression.

Persistent changes in ability to express long-term potentiation/depression in the rat hippocampus after juvenile/adult stress

BACKGROUND

The ventral hippocampus (VH) was recently shown to express lower magnitude long-term potentiation (LTP) compared with the dorsal hippocampus (DH). Exposure to acute stress reversed this difference, and VH slices from stressed rats expressed larger LTP than that produced in the DH, which was reduced by stress. Stressful experience in adolescence has been shown to produce long-lasting effects on animal behavior and on ability to express LTP/long-term depression (LTD) of reactivity to afferent stimulation in the adult. We are interested in possible interactions between juvenile and adult stress in their effects of adult plasticity.

METHODS

We studied the effects of a composite juvenile (28-30 days) stress, followed by a reminder stressful experience in the young adult (60 days) rat, on the ability to produce LTP and LTD in CA1 region of slices of the VH and DH.

RESULTS

Juvenile or adult stress produced a transient decrease in ability to express LTP in DH and a parallel increase in LTP in VH. Stress in the young adult after juvenile stress produced a striking prolongation of the DH/VH disparity with respect to the ability to express both LTP and LTD into the adulthood of the rat.

CONCLUSIONS

These results have important implications for the impact of juvenile stress on adult neuronal plasticity and on the understanding the functions of the different sectors of the hippocampus.