Selective increase of AMPA binding to the AMPA/quisqualate receptor in the hippocampus in response to acute stress

Modulatory effect of glutamate GluR2 receptor on the caudal neurosecretory Dahlgren cells of the olive flounder, Paralichthys olivaceus

A neuromodulatory role for glutamate has been reported for magnocellular neuroendocrine cells in mammalian hypothalamus. We examined the potential role of glutamate as a local intercellular messenger in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) in the euryhaline flounder Paralichthys olivaceus. In pharmacological experiments in vitro, glutamate (Glu) caused an increase in electrical activity of Dahlgren cells, recruitment of previously silent cells, together with a greater proportion of cells showing phasic (irregular) activity. The glutamate substrate, glutamine (Gln), led to increased firing frequency, cell recruitment and enhanced bursting activity. The glutamate effect was not blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, or the GluR1/GluR3 (AMPA) receptor antagonist IEm1795-2HBr, but was blocked by the broad-spectrum α-amino-3-hydroxy- 5- methyl-4-isoxazo-lepropionic acid (AMPA) receptor antagonist ZK200775. Our transcriptome sequencing study revealed three AMPA receptor (GluR1, GluR2 and GluR3) in the olive flounder CNSS. Quantitative RT-PCR revealed that GluR2 receptor mRNA expression was significant increased following dose-dependent superfusion with glutamate in the CNSS. GluR1 and GluR3 receptor mRNA expression were decreased following superfusion with glutamate. L-type Ca²⁺ channel mRNA expression had a significant dose-dependent decrease following superfusion with glutamate, compared to the control. In the salinity challenge experiment, acute transfer from SW to FW, GluR2 receptor mRNA expression was significantly higher than the control at 2 h. These findings suggest that GluR2 is one of the mechanisms which can medicate glutamate action within the CNSS, enhancing electrical activity and hence secretory output.

Stress time-dependently influences the acquisition and retrieval of unrelated information by producing a memory of its own

Stress induces several temporally guided “waves” of psychobiological responses that differentially influence learning and memory. One way to understand how the temporal dynamics of stress influence these cognitive processes is to consider stress, itself, as a learning experience that influences additional learning and memory. Indeed, research has shown that stress results in electrophysiological and biochemical activity that is remarkably similar to the activity observed as a result of learning. In this review, we will present the idea that when a stressful episode immediately precedes or follows learning, such learning is enhanced because the learned information becomes a part of the stress context and is tagged by the emotional memory being formed. In contrast, when a stressful episode is temporally separated from learning or is experienced prior to retrieval, such learning or memory is impaired because the learning or memory is experienced outside the context of the stress episode or subsequent to a saturation of synaptic plasticity, which renders the retrieval of information improbable. The temporal dynamics of emotional memory formation, along with the neurobiological correlates of the stress response, are discussed to support these hypotheses.

A role for AMPA receptors in mood disorders

Major antidepressant agents increase synaptic levels of monoamines. Although the monoamine hypothesis of depression remains a cornerstone of our understanding of the pathophysiology of depression, emerging data has suggested that the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subtype of glutamate receptor may also play a pivotal role in depression. Positive allosteric modulators of AMPA receptors increase brain levels of brain-derived neurotrophic factor (BDNF) that impacts the viability and generation of neurons in key brain structures. AMPA receptor potentiators are active in rodent models predictive of antidepressant efficacy. The mechanisms by which AMPA receptor potentiators produce these biological effects, however, are uncertain. Current evidence points to an antidepressant mechanism that is independent of monoaminergic facilitation that is driven by neurogenesis, a process facilitated by increased BDNF expression. However, alternative hypotheses need to be considered given uncertainties in the relationship between BDNF increases and the effects of conventional antidepressant medications. Electrophysiological and protein conformational data indicate that structural variants of AMPA receptor potentiators can differentially modulate AMPA receptor-mediated currents, although the manner in which this impacts antidepressant efficacy is yet to be understood. Conventional antidepressants such as fluoxetine positively modulate AMPA receptors. This potentiation is engendered by specific phosphorylation pathways activated through the dopamine- and cAMP-regulated phosphoprotein of Mr 32,000 (DARPP-32). Other novel compounds with antidepressant-like effects in rodents may also produce their in vivo effects through potentiation of AMPA receptors. Thus, AMPA receptor potentiation might be a general mechanism through which the clinical outcome of antidepressant efficacy is achieved.

Provocation of kainic acid receptor mRNA changes in the rat paraventricular nucleus by insulin-induced hypoglycaemia

Hypoglycaemia induced by insulin injection is a powerful stimulus to the hypothalamic-pituitary-adrenal (HPA) axis and drives the secretion of corticotropin-releasing hormone and vasopressin from the neurones in the paraventricular nucleus (PVN), as well as the downstream hormones, adrenocorticotropic hormone and corticosterone. In some brain regions, hypoglycaemia also provokes increases in extracellular fluid concentrations of glutamate. Regulation of glutamatergic mechanisms could be involved in the control of the HPA axis during hypoglycaemic stress and one potential site of regulation might be at the receptors for glutamate, which are expressed in the PVN. Insulin (2.0 IU/kg, i.p.) or saline was administered to adult male Sprague-Dawley rats and the animals were sacrificed 30 min, 180 min and 24 h after injection. The amount of several kainic acid-preferring glutamate receptor mRNAs (i.e. KA2, GluR5 and GluR6) were assessed in the PVN by in situ hybridisation histochemistry. Injection of insulin induced a rapid fall in plasma glucose concentrations, which was mirrored by an increase in plasma corticosterone concentrations. KA2 and GluR5 mRNAs are highly expressed within the rat PVN, and responded to hypoglycaemia with robust increases in expression that endured beyond the period of hypoglycaemia itself. However, GluR6 mRNA is expressed in the areas adjacent to the PVN and hypoglycaemic stress failed to alter expression of this mRNA. These experiments suggest that kainic acid-preferring glutamate receptors are responsive to changes in plasma glucose concentrations and may participate in the activation of the PVN neurones during hypoglycaemic stress.

Stress generates emotional memories and retrograde amnesia by inducing an endogenous form of hippocampal LTP

Models of the neurobiology of memory have been based on the idea that information is stored as distributed patterns of altered synaptic weights in neuronal networks. Accordingly, studies have shown that post-training treatments that alter synaptic weights, such as the induction of long-term potentiation (LTP), can interfere with retrieval. In these studies, LTP induction has been relegated to the status of a methodological procedure that serves the sole purpose of disturbing synaptic activity in order to impair memory. This perspective has been expressed, for example, by Martin and Morris (2002: Hippocampus 12:609-636), who noted that post-training LTP impairs memory by adding "behaviorally meaningless" noise to hippocampal neural networks. However, if LTP truly is a memory storage mechanism, its induction should represent more than just a means with which to disrupt memory. Since LTP induction produces retrograde amnesia, the formation of a new memory should also produce retrograde amnesia. In the present report, we suggest that one type of learning experience, the storage of fear-related (i.e., stressful) memories, is consistent with this prediction. Studies have shown that stress produces potent effects on hippocampal physiology, generates long-lasting memories, and induces retrograde amnesia, all through mechanisms in common with LTP. Based on these findings, we have developed the hypothesis that a stressful experience generates an endogenous form of hippocampal LTP that substitutes a new memory representation for preexisting representations. In summary, our hypothesis implicates the induction of endogenous synaptic plasticity by stress in the formation of emotional memories and in retrograde amnesia.

Neural Substrates of Eyeblink Conditioning: Acquisition and Retention

Classical conditioning of the eyeblink reflex to a neutral stimulus that predicts an aversive stimulus is a basic form of associative learning. Acquisition and retention of this learned response require the cerebellum and associated sensory and motor pathways and engage several other brain regions including the hippocampus, neocortex, neostriatum, septum, and amygdala. The cerebellum and its associated circuitry form the essential neural system for delay eyeblink conditioning. Trace eyeblink conditioning, a learning paradigm in which the conditioned and unconditioned stimuli are noncontiguous, requires both the cerebellum and the hippocampus and exhibits striking parallels to declarative memory formation in humans. Identification of the neural structures critical to the development and maintenance of the conditioned eyeblink response is an essential precursor to the investigation of the mechanisms responsible for the formation of these associative memories. In this review, we describe the evidence used to identify the neural substrates of classical eyeblink conditioning and potential mechanisms of memory formation in critical regions of the hippocampus and cerebellum. Addressing a central goal of behavioral neuroscience, exploitation of this simple yet robust model of learning and memory has yielded one of the most comprehensive descriptions to date of the physical basis of a learned behavior in mammals.

Regional specific increases of [3H]AMPA binding and mRNA expression of AMPA receptors in the brain of mu-opioid receptor knockout mice

Previous pharmacological studies have indicated the possible existence of functional interactions between opioidergic and glutamatergic neurons in the CNS. In the present study, [(3)H]AMPA binding and the expression of mRNAs encoding flip and flop variants of three subtypes of AMPA glutamate receptor GluR1-3 were examined by in situ hybridization technique in order to investigate whether there is a change in the AMPA receptor system of mice lacking the mu-opioid receptor. In the mu-opioid receptor knockout mice, [(3)H]AMPA binding was increased in the hippocampal CA1 and dentate gyrus, cortex, and caudate putamen compared with that of the wild-type animals. The expression of GluR1 flip mRNA was increased in the cortex and caudate putamen of mu-opioid receptor knockout mice. The expression of GluR1 flop mRNA was increased in the cortex, caudate putamen, and hippocampal CA1 layer of mu-opioid receptor knockout mice. The expression of GluR2 flip mRNA was decreased in the hippocampal dentate gyrus of mu-opioid receptor knockout mice. The expression of GluR2 flop was not altered in any regions studied. The expression of GluR3 flip was increased in the cortical area and caudate putamen of mu-opioid receptor knockout mice. The expression of GluR3 flop was increased in the cortical area, hippocampal CA3 area, and caudate putamen of mu-opioid receptor knockout mice. These results indicate that [(3)H]AMPA binding and the expression of GluR1-3 mRNA were increased in a region and subunit specific manner, and suggest that changes in the AMPA receptor system are accompanied by the absence of mu-opioid receptor gene.

Effect of chronic stress on synaptic currents in rat hippocampal dentate gyrus neurons

We investigated the effect of chronic stress on synaptic responses of rat dentate granule cells to perforant path stimulation. Rats were subjected for 3 wk to unpredictable stressors twice daily or to control handling. One day after the last stressor, hippocampal slices were prepared and synaptic responses were determined with whole-cell recording. At that time, adrenal weight was found to be increased and thymus weight as well as gain in body weight were decreased in the stressed versus control animals, indicative of corticosterone hypersecretion during the stress period. In slices from rats with basal corticosteroid levels (at the circadian trough, under rest), no effect of prior stress exposure was observed on synaptic responses. However, synaptic responses of dentate granule cells from chronically stressed and control rats were differently affected by in vitro activation of glucocorticoid receptors, i.e., 1-4 h after administration of 100 nM corticosterone for 20 min. Thus the maximal response to synaptic activation of dentate cells at holding potential of -70 mV [when N-methyl-D-aspartate (NMDA) receptors are blocked by magnesium] was significantly enhanced after corticosterone administration in chronically stressed but not in control animals. In accordance, the amplitude of alpha-amino-3-hydroxy-5-methylisolazole-4-propionic acid (AMPA) but not of NMDA receptor-mediated currents was increased by corticosterone in stressed rats, over the entire voltage range. Corticosterone treatment also decreased the time to peak of AMPA currents, but this effect did not depend on prior stress exposure. The data indicate that following chronic stress exposure synaptic excitation of dentate granule cells may be enhanced when corticosterone levels rise. This enhanced synaptic flow could contribute to enhanced excitation of projection areas of the dentate gyrus, most notably the CA3 hippocampal region.

Effects of single or repeated restraint stress on GluR1 and GluR2 flip and flop mRNA expression in the hippocampal formation

The mRNAs encoding the flip and flop isoforms of the glutamate receptor subunits GluR1 and 2 were detected and quantified by in situ hybridization in the hippocampal formation of rats following acute (6h) or chronic (6h daily for 21 days) restraint stress. The GluR1 flip mRNA was slightly reduced in CA1 after chronic stress and the GluR2 flip mRNA was increased in the dentate gyrus (DG), CA4, and CA3 after acute stress. There were no changes in the mRNA encoding the flop isoforms of either GluR1 or 2 in the hippocampus. In entorhinal cortex, the GluR1 flip mRNA was significantly increased after both acute and chronic stress, while the flop isoform increased only after chronic stress. The GluR2 flip mRNA was slightly increased after acute and chronic stress. However, no changes were found for the flop isoform of GluR2. These results suggest that different assembly of AMPA receptors subunits and isoforms may underlie, in a different way, the neuronal plastic changes induced by specific type and intensity of stressful stimuli.

Blockade by N-methyl-D-aspartate of elevation of activator protein-1 binding after stress in rat adrenal gland

Cold immobilization stress induced a marked elevation of expression of activator protein-1 (AP1) complex in rat hypothalamus, pituitary, adrenal, and gastric mucosa, but not in other discrete brain structures examined, when determined immediately after stress for 3 hr. Adrenal AP1 binding linearly increased with the duration of stress up to 6 hr, whereas the increase was seen in both adrenal cortex and medulla of rats stressed for 3 hr. In adrenals, the elevation exhibited decline profiles different from those of expression of cAMP response element binding protein. Western blotting revealed that stress for 3 hr induced significant increases in expression of the components of AP1 complex, c-Fos, c-Jun, and Jun-B proteins, in adrenals, without markedly affecting expression of Fos-B, Fra-2, and Jun-D proteins. The prior systemic administration of N-methyl-D-aspartate (NMDA) led to significant prevention of the elevation after stress for 3 hr in adrenals, whereas the NMDA antagonist dizocilpine alone induced a marked increase in adrenal AP1 binding, without altering the elevation by stress. These results suggest that stress may modulate de novo protein synthesis at the level of gene transcription by AP1 complex through a molecular mechanism associated with NMDA receptor channels in rat adrenal glands.

Antidepressant mechanisms: functional and molecular correlates of excitatory amino acid neurotransmission

Specific targeting of the serotonergic and noradrenergic systems for the development of antidepressant compounds has resulted in drugs with more favourable side-effect profiles but essentially no greater efficacy than those compounds discovered more than 40 years ago. Alternative targets are now being considered in the hope that they will have a faster onset of action and be useful for those patients currently unresponsive to conventional treatments. Excitatory amino acid neurotransmission has been attributed various roles in both normal and abnormal brain function. The N-methyl-D-aspartate receptor in particular has long been postulated to play a role in the formation of memories. Major depressive disorder is characterised by alterations in cognitive function, as well as affect. Although there is evidence that early adverse events and stress can have a causal influence on depression, the underlying neurobiology of the disorder is poorly understood. This review will document current evidence for the involvement of excitatory amino acid neurotransmission in the pathophysiology of the affective disorders. The preclinical literature suggests that both electroconvulsive stimulation and antidepressant drugs can affect hippocampal long-term potentiation and the expression of excitatory amino acid receptor subtypes. Exposing animals to stress, including the kind that produces learned helplessness, can also affect synaptic plasticity in the hippocampus. There is clinical evidence that patients with chronic depression have structural brain abnormalities, including hippocampal atrophy, and a preliminary study has shown that an N-methyl-D-aspartate receptor antagonist may have antidepressant efficacy.

Sex differences and opposite effects of stress on dendritic spine density in the male versus female hippocampus

Dendritic spines are postsynaptic sites of excitatory input in the mammalian nervous system. Despite much information about their structure, their functional significance remains unknown. It has been reported that females in proestrus, when estrogen levels are elevated, have a greater density of apical dendritic spines on pyramidal neurons in area CA1 of the hippocampus than females in other stages of estrous (Woolley et al., 1990). Here we replicate these findings and in addition, show that females in proestrus have a greater density of spines in area CA1 of the hippocampus than males. Moreover, this sex difference in spine density is affected in opposite directions by stressful experience. In response to one acute stressful event of intermittent tailshocks, spine density was enhanced in the male hippocampus but reduced in the female hippocampus. The decrease in the female was observed for those that were stressed during diestrus 2 and perfused 24 hr later during proestrus. The opposing effects of stress were not evident immediately after the stressor but rather occurred within 24 hr and were evident on apical and to a lesser extent on basal dendrites of pyramidal cells in area CA1. Neither sex nor stress affected spine density on pyramidal neurons in somatosensory cortex. Sex differences in hippocampal spine density correlated with sex hormones, estradiol and testosterone, whereas stress effects on spine density were not directly associated with differences in the stress hormones, glucocorticoids. In summary, males and females have different levels of dendritic spine density in the hippocampus under unstressed conditions, and their neuronal anatomy can respond in opposite directions to the same stressful event.

AMPA receptor binding in adult guinea pig brain stem auditory nuclei after unilateral cochlear ablation

This study determined if an asymmetric hearing loss, due to unilateral cochlear ablation, could induce the regulation of intracellular AMPA receptors in brain stem auditory nuclei. In young adult guinea pigs, the high-affinity specific binding of [(3)H]AMPA was measured in the cochlear nucleus (CN), the superior olivary complex (SOC), and the auditory midbrain at 2-147 postlesion days. After correction for tissue shrinkage, changes in specific binding relative to that in age-matched unlesioned controls were interpreted as altered numbers and/or activity of intracellular AMPA receptors. In the CN, transient elevations and/or deficits in binding were evident in most regions, which usually recovered by 147 days. However, persistently deficient binding was evident ipsilaterally in the anterior part of the anteroventral CN (AVCNa). In the SOC, transient elevations in binding were evident at 2 days in the medial limb of the lateral superior olive (LSOmed) and the medial superior olive. Between 7 and 147 days, most SOC nuclei exhibited transient, temporally synchronized postlesion deficits in binding. However, late in the survival period, deficits persisted ipsilaterally in the LSOmed and the lateral (LSOlat) limb of the lateral superior olive. In the midbrain, transient elevations and/or deficits in binding were evident in the dorsal nucleus of the lateral lemniscus as well as in the central and dorsal nucleus of the inferior colliculus. A persistent deficit was evident in the intermediate nucleus of the lateral lemniscus. The findings implied that auditory neurons contain regulatory mechanisms that control the numbers and/or activity of intracellular AMPA receptors. Regulation was induced by cochlear nerve destruction and probably by changes in the excitation of glutamatergic neurons. Many of the regulatory changes were transient, except in the ipsilateral AVCNa and LSO, where postlesion downregulations were persistent. The downregulation in the ipsilateral AVCNa was probably induced directly by the loss of cochlear nerve endings. However, other regulatory changes may have been induced by signals carried on pathways emerging from the ipsilateral CN and on centrifugal auditory pathways.

The N-methyl-D-aspartate receptor, synaptic plasticity, and depressive disorder. A critical review

The roles of the N-methyl-D-aspartate (NMDA) receptor and NMDA receptor-mediated synaptic plasticity are reviewed in the context of depressive disorder and its treatment. The mode of action of antidepressant treatment is poorly understood. Animal studies have suggested that many antidepressant drugs show activity at the NMDA receptor and that NMDA antagonists have antidepressant profiles in preclinical models of depression. A post-mortem study in humans has suggested that certain binding characteristics of the NMDA receptor may be down-regulated in the brains of suicide victims. "Depressogenic" stressors in animals and chronic administration of antidepressant agents perturb NMDA-dependent synaptic plasticity in the hippocampus.

Gene expression of two glutamate receptor subunits in response to repeated stress exposure in rat hippocampus

1. Glutamatergic mechanisms are thought to be involved in stress-induced alterations of brain function, especially in the hippocampus. We have hypothesized that repeated stress exposure may evoke changes of hippocampal glutamate receptors at the level of gene expression. 2. The study was designed to analyze the levels of mRNA coding for NMDAR1, the essential subunit of the N-methyl-D-aspartate (NMDA) receptor subtype, and for GluR1, an AMPA glutamate receptor subunit, after repeated immobilization stress in rat hippocampus. Toward this aim, we applied a competitive RT-PCR technique which allowed precise and reliable quantification of the transcripts. 3. We found that repeated immobilization stress for 7 days significantly increased GluR1 mRNA levels, by 27% (P<0.01), as measured 24 hr after the last stress exposure. Levels of mRNA coding for NMDAR1 were slightly elevated, but the difference failed to be significant. 4. These results demonstrate selective changes in the gene expression of glutamate receptor subunits, which are likely to take part in the mechanisms leading to enhanced excitability and vulnerability of hippocampal neurons and to potential damage during repeated or chronic stress exposure.

Acute stress does not alter 5-HT1A receptor density

Previous research suggests that 5-HT1A receptors are altered with exposure to chronic stress. No previous studies have examined the effect of acute stress on 5-HT1A. Using receptor autoradiography it was observed that there were no differences in [3H]-8-OH-DPAT binding between control rats and rats that received 20 minutes of restraint stress 2 hours prior to sacrifice. This study suggests that the changes in 5-HT1A receptor density associated with chronic stress develop over the course of repeated stress.

Effects of mineralocorticoid and glucocorticoid receptors on long-term potentiation in the CA3 hippocampal field

We have previously shown that the two types of adrenal steroid receptors, mineralocorticoid MR. and glucocorticoid GR. produce opposite effects on long-term potentiation LTP. in the dentate gyrus in vivo. and CA1 hippocampal field in vitro. More specifically, MR activation enhanced and prolonged LTP, whereas GR activation suppressed LTP in these areas and also produced a long-term depression LTD. of the synaptic response. In the present experiment we investigated acute effects of MR and GR activation on LTP induction in the mossy fiber and commissural associational input to the CA3 hippocampal field, since the mechanisms underlying LTP induction in these two pathways differ, the former being N-methyl-D-aspartate receptor NMDAR. independent while the latter being NMDAR-dependent. Rats were either adrenalectomized ADX or adrenally intact. ADX animals were acutely injected with either the specific MR agonist, aldosterone, the specific GR agonist RU 28362 or vehicle. One hour following the injection, the animals were prepared for electrophysiological recording stimulation. Field potential recordings were performed in the radiatum or laconosum moleculare layers of the CA3 field, with stimulation of either the mossy fibers or the commissural associational input from the contralateral hemisphere. We also replicated our previous findings by recording in the dentate gyrus with stimulation of the medial perforant pathway, in the same animals. As observed in our previous study in the dentate gyrus, we found an enhancement and a suppression of LTP with MR and GR activation, respectively. Similarly, for the commissural associational input to CA3, MR activation enhanced LTP, while GR activation reduced it. In contrast, for the mossy fiber input to CA3, neither MR nor GR activation significantly affected LTP induction. These results indicate that adrenal steroids may modulate LTP induction in the hippocampus via an interaction with glutamatergic NMDAR.

Characterization of ionotropic glutamate receptors in rat hypothalamus, pituitary and immortalized gonadotropin-releasing hormone (GnRH) neurons (GT1-7 cells)

Evidence from various sources suggested that the Gonadotropin-Releasing Hormone (GnRH) neuron does not contain glutamate receptors. Northern analysis of the hypothalamus showed the presence of NMDAR1, GluR1, GluR4 and GluR6 mRNA, while the pituitary showed the presence of NMDAR1, GluR1 and GluR6 mRNA. Western blot analysis also showed the presence of NMDAR1 and GluR1 protein. Since there are relatively few GnRH neurons in the hypothalamus, and GT1-7 cells have been considered to be a GnRH neuronal cell line, GT1-7 cells were studied in detail. GT1-7 cells contained NMDAR1 mRNA levels as shown by Northern analysis but did not contain GluR1, GluR4, or GluR6 mRNA. They did not show the presence of NMDAR1 and GluR1 protein by Western analysis. In addition, GT1-7 cells showed no NMDA receptor binding using the competitive inhibitor CGP-39563 and the noncompetitive inhibitor MK-801. Likewise, no binding was detected for kainate receptors. However, a small amount of binding for AMPA receptors was found in GT1-7 cells. GT1-7 cells did not exhibit glutamate toxicity and NMDA failed to elicit inward currents using patch-clamp techniques, although GABA did induce currents in the cells. As a whole, these studies suggest that GT1-7 cells lack or possess only low levels of ionotropic glutamate receptors.

Rapid effects of kainate administration on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor properties in rat hippocampus

We investigated changes in AMPA receptor properties in rat hippocampus 5 h after systemic kainate administration. Quantitative [3H]AMPA autoradiography and Western blot analysis of receptor subunits GluR1-3 in different subcellular fractions were used to evaluate possible alterations in binding characteristics and immunological properties of the receptors in synaptic and nonsynaptic fractions. Both ligand-binding and Western blots revealed significant changes in binding and immunological properties of nonsynaptic receptors but relatively smaller changes in synaptic receptors 5 h after kainate administration. GluR2/3 showed a greater relative change in the synaptic receptor population compared to GluR1, suggesting either a shift in subunit composition of AMPA receptors or the formation of a synaptic subpopulation of AMPA receptors with truncated C-terminal domain of GluR1 subunits. The effects of kainic acid were blocked by cycloheximide treatment indicating that the changes were due at least in part to increased synthesis of AMPA receptor subunits. The results indicate that excessive synaptic activity produces rapid changes in both synaptic and nonsynaptic AMPA receptor properties.

NMDA Receptor Antagonism in the Lateral/Basolateral but Not Central Nucleus of the Amygdala Prevents the Induction of Facilitated Learning in Response to Stress

Exposure to an acute stressful event facilitates classical eye-blink conditioning in the male rat. The facilitation persists for days after the stressor and its induction is prevented by antagonism of the N-methyl-d-aspartate (NMDA) type of glutamate receptor. To determine whether NMDA receptor antagonists prevent the facilitated conditioning by activity in the amygdala, a competitive antagonist, AP5, was injected bilaterally into the lateral/basolateral versus central nuclei of the amygdala. Local injection of d,l-2-amino-5-phosphonovaleric acid (AP5) into the lateral/basolateral nucleus before stressor exposure prevented the facilitated learning 24 hr later, whereas antagonism in the central nucleus before stressor exposure did not. To determine when the necessary activation occurs, AP5 was injected into the lateral/basolateral nucleus before versus after exposure to the acute stressful event. Blockade of NMDA receptors before but not after stressor exposure prevented the facilitated acquisition of the conditioning in response to stress. These results suggest that exposure to a stressful event transiently activates NMDA receptors in basolateral/lateral nuclei of the amygdala and thereby induces a persistent enhancement of associative learning.

Transient and persistent consequences of acute stress on long-term potentiation (LTP), synaptic efficacy, theta rhythms and bursts in area CA1 of the hippocampus

Previous studies reported that exposure to an acute stressor of restraint and intermittent tailshock impairs long-term potentiation (LTP) in area CA1 of the rat hippocampus. In the first experiment, the longevity of the stress-induced impairment of LTP was determined. LTP of the excitatory postsynaptic potential (EPSP) was impaired 2 but not 4 days after stressor cessation. Exposure to the stressor also persistently enhanced the synaptic response to the tetanic stimulation patterned after theta rhythm activity (10, 100 Hz bursts delivered at 5 Hz). In a second experiment, we tested the hypothesis that exposure to the stressor enhanced synaptic efficacy itself. EPSPs were recorded from freely moving rats before, during and after stressor exposure. The synaptic response was not enhanced during stressor exposure. Instead, cessation of the stressor (and perhaps movement associated with release from restraint) induced a transient (< 2 min) decrease in synaptic efficacy. To determine whether exposure to the stressor enhances endogenous theta rhythms in area CA1, electroencephalographic (EEG) recordings were obtained from freely moving rats before, during and after exposure to the stressor. The power of theta (4-8 Hz) and low frequency (0.1-3.9 Hz) activity was enhanced in response to the tailshock aspect of the stressor. Together, the results indicate that exposure to an acute stressful event increases theta activity and its cessation transiently decreases synaptic efficacy. These transient effects are succeeded by a persistently sensitized response to theta burst stimulation and impaired LTP.

Chronic corticosterone treatment induces parallel changes in N-methyl-D-aspartate receptor subunit messenger RNA levels and antagonist binding sites in the hippocampus

Some of the effects of glucocorticoids on the function and neuronal plasticity of the hippocampus are mediated by N-methyl-D-aspartate receptor activation. We tested the hypothesis that chronic corticosterone administration increases N-methyl-D-aspartate receptor expression in the hippocampus of the rat. We used in situ hybridization histochemistry to measure the messenger RNA levels for the NR1, NR2A and NR2B subunits of the N-methyl-D-aspartate receptor and [3H]dizocilpine maleate (a non-competitive antagonist) binding to measure N-methyl-D-aspartate receptor density. Since corticosterone depresses circulating testosterone levels, we also examined whether the effects of corticosterone are mediated by or interact with the effects of testosterone. In the intact animal, corticosterone increased messenger RNA levels for NR2A and NR2B but not NR1 subunits of the N-methyl-D-aspartate receptor in all regions of the hippocampus. Testosterone had no significant effect on messenger RNA levels of any of the subunits. The subunit composition determines the functional and pharmacological properties of the N-methyl-D-aspartate receptor. We used ifenprodil inhibition of [3H]dizocilpine maleate binding, which has been used to distinguish NR2A- from NR2B-containing receptors, to determine whether corticosterone altered the proportion of high- and low-affinity sites for ifenprodil in parallel with the changes in subunit messenger RNA levels. Corticosterone increased the density of [3H]dizocilpine maleate binding sites without changing the dissociation constant for [3H]dizocilpine maleate or the proportion of high- and low-affinity sites for ifenprodil. These data suggest that the effects of corticosterone on hippocampal function are mediated, in part, by parallel increases in NR2A and NR2B subunit levels and the number of receptor channel binding sites.

Neuroimmunomodulation via limbic structures--the neuroanatomy of psychoimmunology

During the last 20 years, mutual communications between the immune, the endocrine and the nervous systems have been defined on the basis of physiological, cellular, and molecular data. Nevertheless, a major problem in the new discipline "Psychoneuroimmunology" is that controversial data and differences in the interpretation of the results make it difficult to obtain a comprehensive overview of the implications of immunoneuroendocrine interactions in the maintenance of physiological homeostasis, as well as in the initiation and the course of pathological conditions within these systems. In this article, we will first discuss the afferent pathways by which immune cells may affect CNS functions and, conversely, how neural tissues can influence the peripheral immune response. We will then review recent data, which emphasize the (patho)physiological roles of hippocampal-amygdala structures and the nucleus accumbens in neuroimmunomodulation. Neuronal activity within the hippocampal formation, the amygdaloid body, and the ventral parts of the basal ganglia has been examined most thoroughly in studies on neuroendocrine, autonomic and cognitive functions, or at the level of emotional and psychomotor behaviors. The interplay of these limbic structures with components of the immune system and vice versa, however, is still less defined. We will attempt to review and discuss this area of research taking into account recent evidences for neuroendocrine immunoregulation via limbic neuronal systems, as well as the influence of cytokines on synaptic transmission, neuronal growth and survival in these brain regions. Finally, the role of limbic structures in stress responses and conditioning of immune reactivity will be commented. Based on these data, we propose new directions of future research.

Effect of N omega-nitro-L-arginine methyl ester, a nitric oxide synthesis inhibitor, on stress- and morphine-induced prolactin release in male rats

1. The effect of the nitric oxide synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) was investigated on stress- and morphine-induced prolactin (PRL) secretion in vivo in male rats, by use of a stress-free blood sampling and drug administration method by means of a permanent indwelling catheter in the right jugular vein. 2. Three doses of L-NAME were tested (1, 10 and 30 mg kg-1) and were given intraperitoneally one hour before blood sampling; control rats received saline. After the first blood sample, rats received an initial intravenous injection of morphine (3, 6 or 12 mg kg-1) or were subjected to immobilization stress. In the case of a morphine administration, rats received a second dose of morphine (3, 6 or 6 mg kg-1, respectively) 90 min later, when tolerance had developed, while rats subjected to immobilization stress received 6 mg kg-1 morphine 90 min after onset of stress. 3. L-NAME had no effect on basal plasma PRL concentration. However, it potentiated acute morphine-induced PRL secretion and attenuated the subsequent tolerance in a dose-dependent way. Immobilization stress-induced PRL secretion was inhibited dose-dependently by L-NAME, as was its subsequent tolerance to morphine; however, in this case, in a reversed dose-dependent way. 4. When the highest dose of morphine (12 mg kg-1) was combined with the highest dose of L-NAME pretreatment (30 mg kg-1), all rats showed a dramatic potentiation of the morphine-induced PRL rise compared to controls. Moreover, all of these rats died within 90 min after the administration of morphine. 5. These results show that NO plays a role in the acute opioid action on PRL release during stress as well as in the development of tolerance to the opioid effect, and some possible mechanisms are discussed.

Involvement of NMDA receptor in the regulation of plasma interleukin-6 levels in mice

MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclopepten-5,10-imine maleate), a non-competitive NMDA receptor antagonist (0.01-1 micrograms), injected intracerebroventricularly (i.c.v.) dose dependently increased the baseline levels of plasma interleukin-6 in mice. In the 1-h immobilization-stressed animals, MK-801 (1 micrograms) administered i.c.v. produced an additive increase of plasma interleukin-6. NMDA (N-methyl-D-aspartate) (3, 10 ng) administered i.c.v. attenuated dose dependently the 1-h immobilization stress-induced rise in plasma interleukin-6 level. Neither 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) (0.01-0.5 micrograms) nor alpha-methyl-4-carboxyphenylglycine (MCPG) (1-20 micrograms), antagonists of non-NMDA and metabotropic glutamate receptors, respectively, i.c.v. administered, affected the basal and stress-induced plasma interleukin-6 levels. These data indicate that NMDA receptors may be involved in the suppressive regulation of the plasma interleukin-6 levels.

Mechanism of altered synaptic strength due to experience: relation to long-term potentiation

An increase in medial perforant synaptic strength can be observed for hippocampal slices from rats exposed to environmental enrichment. The expression of enhanced synaptic strength exhibits properties similar to long-term potentiation (LTP), a physiological model of memory storage. Similarities include an increase in strength of the synaptic response in the absence of an altered paired-pulse ratio and an increase in the binding of the glutamate agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. Furthermore, environmental enrichment interacts with the mechanisms responsible for the induction of LTP by inhibiting further increases in synaptic strength following LTP-inducing stimulation. The results provide evidence for experience-mediated influences on postsynaptic mechanisms regulating medial perforant path synaptic strength.

Stress, Glucocorticoids, and Damage to the Nervous System: The Current State of Confusion

An extensive literature demonstrates that glucocorticoids (GCs), the adrenal steroids secreted during stress, can have a broad range of deleterious effects in the brain. The actions occur predominately, but not exclusively, in the hippocampus, a structure rich in corticosteroid receptors and particularly sensitive to GCs. The first half of this review considers three types of GC effects: a) GC-induced atrophy, in which a few weeks' exposure to high GC concentrations or to stress causes reversible atrophy of dendritic processes in the hippocampus; b) GC neurotoxicity where, over the course of months, GC exposure kills hippocampal neurons; c) GC neuroendangerment, in which elevated GC concentrations at the time of a neurological insult such as a stroke or seizure impairs the ability of neurons to survive the insult. The second half considers the rather confusing literature as to the possible mechanisms underlying these deleterious GC actions. Five broad themes are discerned: a) that GCs induce a metabolic vulnerability in neurons due to inhibition of glucose uptake; b) that GCs exacerbate various steps in a damaging cascade of glutamate excess, calcium mobilization and oxygen radical generation. In a review a number of years ago, I concluded that these two components accounted for the deleterious GC effects. Specifically, the energetic vulnerability induced by GCs left neurons metabolically compromised, and less able to carry out the costly task of containing glutamate, calcium and oxygen radicals. More recent work has shown this conclusion to be simplistic, and GC actions are shown to probably involve at least three additional components: c) that GCs impair a variety of neuronal defenses against neurologic insults; d) that GCs disrupt the mobilization of neurotrophins; e) that GCs have a variety of electrophysiological effects which can damage neurons. The relevance of each of those mechanisms to GC-induced atrophy, neurotoxicity and neuroendangerment is considered, as are the likely interactions among them.

Behavioral stress modifies hippocampal plasticity through N-methyl-D-aspartate receptor activation

Behavioral stress has detrimental effects on subsequent cognitive performance in many species, including humans. For example, humans exposed to stressful situations typically exhibit marked deficits in various learning and memory tasks. However, the underlying neural mechanisms by which stress exerts its effects on learning and memory are unknown. We now report that in adult male rats, stress (i.e., restraint plus tailshock) impairs long-term potentiation (LTP) but enhances long-term depression (LTD) in the CA1 area of the hippocampus, a structure implicated in learning and memory processes. These effects on LTP and LTD are prevented when the animals were given CGP39551 (the carboxyethylester of CGP 37849; DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, before experiencing stress. In contrast, the anxiolytic drug diazepam did not block the stress effects on hippocampal plasticity. Thus, the effects of stress on subsequent LTP and LTD appear to be mediated through the activation of the NMDA subtype of glutamate receptors. Such modifications in hippocampal plasticity may contribute to learning and memory impairments associated with stress.

Modulating effect of the nootropic drug, piracetam on stress- and subsequent morphine-induced prolactin secretion in male rats

1. The effect of the nootropic drug, piracetam on stress- and subsequent morphine-induced prolactin (PRL) secretion was investigated in vivo in male rats, by use of a stress-free blood sampling and drug administration method by means of a permanent indwelling catheter in the right jugular vein. 2. Four doses of piracetam were tested (20, 100, 200 and 400 mg kg-1), being given intraperitoneally 1 h before blood sampling; control rats received saline instead. After a first blood sample, rats were subjected to immobilization stress and received morphine, 6 mg kg-1, 90 min later. 3. Piracetam had no effect on basal plasma PRL concentration. 4. While in the non-piracetam-treated rats, stress produced a significant rise in plasma PRL concentration, in the piracetam-pretreated rats PRL peaks were attenuated, especially in the group given 100 mg kg-1 piracetam, where plasma PRL concentration was not significantly different from basal values. The dose-response relationship showed a U-shaped curve; the smallest dose had a minor inhibitory effect and the highest dose had no further effect on the PRL rise. 5. In unrestrained rats, morphine led to a significant elevation of plasma PRL concentration. After the application of immobilization stress it lost its ability to raise plasma PRL concentration in the control rats, but not in the piracetam-treated rats. This tolerance was overcome by piracetam in a significant manner but with a reversed dose-response curve; i.e. the smaller the dose of piracetam, the higher the subsequent morphine-induced PRL peak. 6. There is no simple explanation for the mechanism by which piracetam induces these contradictory effects. Interference with the excitatory amino acid system, which is also involved in opiate action, is proposed speculatively as a possible mediator of the effects of piracetam.

Stress-induced atrophy of apical dendrites of hippocampal CA3c neurons: involvement of glucocorticoid secretion and excitatory amino acid receptors

Repeated restraint stress of rats for 21 days causes atrophy of apical dendrites of hippocampal CA3c pyramidal neurons. This effect is mimicked by daily corticosterone treatment for 21 days and is prevented y the anti-epileptic drug, phenytoin, known to interfere with excitatory amino acid release and action. The present study was designed to investigate the involvement of endogenous corticosterone secretion and excitatory amino acid receptors in the stress-induced hippocampal dendritic atrophy. Treatment of chronically stressed rats with the steroid synthesis blocker cyanoketone prevented stress-induced dendritic atrophy. Cyanoketone-treated animals showed an impaired corticosterone secretion in response to the stressor, while basal levels were maintained. Besides the involvement of endogenous corticosterone secretion, N-methyl-D-aspartate receptors also play a role, since the competitive receptor antagonist, CGP 43487, blocked stress-induced dendritic atrophy. In contrast, NBQX, a competitive inhibitor of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors, was ineffective at a dose that blocks ischemic damage. These results indicate that the reversible atrophy induced by 21 days of daily restraint stress requires corticosterone secretion and that excitatory mechanisms involving N-methyl-D-aspartate receptors play a major role in driving the atrophy.

Effects of adrenal steroid manipulations and repeated restraint stress on dynorphin mRNA levels and excitatory amino acid receptor binding in hippocampus

Adrenal steroid and stress effects were determined in hippocampus on levels of dynorphin (DYN) mRNA, expressed in dentate gyrus, and excitatory amino acid receptors, measured in Ammon's horn and dentate gyrus. Adrenalectomy (ADX) decreased DYN mRNA levels in dentate gyrus and replacement with aldosterone (ALDO), a specific type I adrenal steroid receptor agonist, prevented the decrease. Ru28362, a specific type II receptor agonist, had no effect. Likewise, kainate receptor binding to the stratum lucidum and hilus region of dorsal hippocampus was decreased after ADX and this decrease was prevented by ALDO but not by Ru28362 treatment. Similar though smaller effects were found for CNQX binding to AMPA receptors but only in the dentate gyrus molecular or infra- and supragranular layers. Although corticosterone (CORT) treatment of intact rats (40 mg/kg for 3 weeks) elevated DYN mRNA levels in dentate gyrus, up to 14 days of daily restraint stress (1 or 6 h/day) had no significant effect. Neither CORT treatment nor repeated restraint stress altered NMDA and non-NMDA glutamate receptors in hippocampus. The results of this study showing ADX-induced decreases of DYN mRNA and CNQX binding in dentate gyrus and decreased kainate binding in mossy fiber terminal regions are consistent with morphological evidence showing that adrenal steroids maintain normal integrity and structure of dentate gyrus neurons and do so via type I adrenal steroid receptors. These same parameters are apparently not sensitive to chronic restraint stress although the effects of other stressors must be examined.

Stress and cognitive function

Stress affects cognition in a number of ways, acting rapidly via catecholamines and more slowly via glucocorticoids. Catecholamine actions involve beta adrenergic receptors and also availability of glucose, whereas glucocorticoids biphasically modulate synaptic plasticity over hours and also produce longer-term changes in dendritic structure that last for weeks. Prolonged exposure to stress leads to loss of neurons, particularly in the hippocampus. Recent evidence suggests that the glucocorticoid- and stress-related cognitive impairments involving declarative memory are probably related to the changes they effect in the hippocampus, whereas the stress-induced catecholamine effects on emotionally laden memories are postulated to involve structures such as the amgydala.

Effect of stress and long-term potentiation (LTP) on subsequent LTP and the theta burst response in the dentate gyrus

Exposure to an aversive and stressful event is reported to have similar effects on hippocampal plasticity and behavior as does exposure to high-frequency stimulation of the hippocampus. Here we directly compared the effects of exposure to a stressor vs. a previous induction of LTP on a subsequent induction of LTP and the extracellular response to a tetanus patterned after endogenous theta rhythms. Stimulating the dentate gyrus via the perforant path, Sprague-Dawley rats (n = 65) were tetanized 2 h after exposure to a stressor consisting of restraint and 60, 1, s, 1 mA tail shocks. Unstressed controls were tetanized once and then again 2 h later. Exposure to the stressor impaired LTP of the EPSP 2 h later, as did a previous induction of LTP. In addition, exposure to the stressor altered the extracellular response to subsequent theta burst stimulation (10, 40 ms bursts at 100 Hz, each separated by 200 ms), as did a previous induction of LTP. Whereas unstressed rats exposed to the first tetanus exhibited a marked decline in the amplitude across successive bursts, stressed rats exhibited no such decline, a response pattern similar to that observed in unstressed rats exposed to a second tetanus. The similarity between the effects of stress and tetanic stimulation on hippocampal plasticity support the hypothesis that stress and LTP are converging on similar neuronal mechanisms.

Differential subcellular localization of two populations of glutamate/AMPA receptors in the rat telencephalon

The distribution of glutamate AMPA receptors in the synaptosomal and microsomal fractions of neonatal and adult rat telencephalon was studied by determining the saturation kinetics at equilibrium of 3H-AMPA and 3H-CNQX binding. At both ages, synaptosomal preparations exhibited two populations of 3H-AMPA binding sites with a small number of high affinity sites and a large number of low affinity sites. 3H-AMPA binding to microsomal preparations from both neonatal and adult rat telencephalon exhibited a much higher proportion of high affinity relative to low affinity sites. 3H-CNQX binding to the same fractions did not parallel 3H-AMPA binding, but was correlated with the low affinity 3H-AMPA binding and with a marker of plasma membranes. The results suggest that nonsynaptic glutamate/AMPA receptors have a high affinity for agonist and become low affinity when inserted into postsynaptic membranes and that 3H-CNQX binds synaptic but not nonsynaptic glutamate/AMPA receptors with high affinity.

Sex differences in acute swim stress induced changes in the binding of AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) and kainate to glutamate receptors in mouse forebrain

Sex differences were found in the binding of [3H]AMPA and [3H]kainate to glutamate receptors in synaptosomal membranes prepared from mouse forebrain. The number of low affinity [3H]AMPA binding and the affinity of [3H]kainate binding was higher in membranes prepared from male mice than from females. Acute swim stress (3 min at 32 degrees C) decreased the number of low affinity [3H]AMPA binding sites and the affinity of [3H]kainate binding in membranes prepared from male mouse forebrain, but not in those prepared from female mice forebrain. As kainate is known to interact with low affinity AMPA binding sites, these observed changes may be associated with binding sites common to AMPA and kainate. They may represent a functional down-regulation of AMPA/kainate binding sites. These sex differences in binding to non-NMDA subclasses of glutamate receptors are similar to than those found in the binding of MK-801 to the NMDA subclass of glutamate receptors, in that the effects of acute swim stress were more pronounced in membranes prepared from male than from female mice. The number of low affinity [3H]AMPA binding sites were decreased by acute swim stress in membranes from male mice, whereas the number of low affinity [3H]MK-801 binding sites increased following acute swim stress.

Psychological stress repeatedly blocks hippocampal primed burst potentiation in behaving rats

Primed burst (PB) potentiation is a long-term increase in CA1 population spike amplitude produced by brief physiologically patterned electrical stimulation of the hippocampal commissure. Exposure of rats to a novel environment resulted in a blockade of short-term (Post-tetanic potentiation, PTP) and long-term (PB potentiation) plasticity in all cases (n = 6). When the animals had extensive exposure to the environment (14 consecutive days), PTP and PB potentiation occurred. With placement of the rats in a second novel environment, once again, neither PTP nor PB potentiation was observed. Placement of rats in each of the two novel environments produced a significant increase in serum corticosterone levels, while placement in the familiar environment produced no increase in response. These findings support the hypothesis that hippocampal plasticity is repeatedly susceptible to modulation by the stress of forced exposure to a novel environment.

Mineralocorticoid and glucocorticoid receptors in the brain. Implications for ion permeability and transmitter systems

In this review we have argued that corticosteroid hormones represent an endocrine signal that can influence neuronal communication. The steroids bind to intracellular receptors in the brain, resulting in slow effects that involve gene transcription, but they may also evoke rapid effects via membrane receptors. The signal carried by the corticosteroids is therefore divergent with respect to the dimension of space and time. Within the rat brain, at least two intracellular receptor subtypes, i.e. MRs and GRs, bind corticosterone. The affinity, density and localization of the MRs is different from the GRs, although the actual properties may vary somewhat depending on the condition of the animal. In general, due to the difference in affinity, low corticosteroid levels result in a predominant MR occupation, while higher steroid levels additionally occupy GRs. Recent studies indicate that predominant MR occupation is important for the maintenance of ongoing transmission in certain brain regions and for neuroprotection. By contrast, additional GR occupation (for a limited period of time) results in an attenuation of local excitability; yet, prolonged exposure to high steroid levels may become an endangering condition for neurons. Since predominant MR occupation on the one hand and additional GR occupation on the other hand induce different cellular actions, the ratio of MR/GR occupation is an important factor determining the net effect of corticosteroid hormones in the brain. How coordinated MR- and GR-mediated effects control neuronal communication under various physiological and pathological conditions will be a challenge for future research.

Excitotoxicity and neurological disorders: involvement of membrane phospholipids

Excitatory amino acids and their receptors play an important role in membrane phospholipid metabolism. Persistent stimulation of excitatory amino acid receptors by glutamate may be involved in neurodegenerative diseases and brain and spinal cord trauma. The molecular mechanism of neurodegeneration induced by excitatory amino acids is, however, not known. Excitotoxin-induced calcium entry causes the stimulation of phospholipases and lipases. These enzymes act on neural membrane phospholipids and their stimulation results in accumulation of free fatty acids, diacylglycerols, eicosanoids, and lipid peroxides in neurodegenerative diseases and brain and spinal cord trauma. Other enzymes, such as protein kinase C and calcium-dependent proteases, may also contribute to the neuronal injury. Excitotoxin-induced alterations in membrane phospholipid metabolism in neurodegenerative diseases and neural trauma can be studied in animal and cell culture models. These models can be used to study the molecular mechanisms of the neurodegenerative processes and to screen the efficacy of therapeutic drugs.

A single social stress-experience alters glutamate receptor-binding in rat hippocampal CA3 area

The distribution of glutamate receptors in the rat hippocampus was investigated 24 h after the social stress of confrontation with a dominant opponent. AMPA-type glutamate receptors were labeled with the antagonist [3H]CNQX, and NMDA-type receptors were labeled with the competitive antagonist [3H]CGP39653. Increased [3H]CGP39653 labeling was exclusively found in the CA3 stratum radiatum and a decreased [3H]CNQX labeling was found in several hippocampal areas. Consequently, the ratio NMDA/AMPA binding was significantly increased in CA3 stratum oriens and CA3 stratum radiatum. These results suggest that a single unescapable social stress of defeat alters the impact of the excitatory neurotransmitter glutamate, which is restricted to hippocampal CA3 neurons. Possible consequences of the present findings are discussed.

Chronic effects of trimipramine, an antidepressant, on hippocampal synaptic plasticity

The effects of trimipramine (TRIM), an antidepressant agent, on both the induction and the maintenance of long-term potentiation (LTP) was investigated in area CA1 of hippocampal slice preparations. Chronic administration (7-9 days) of TRIM in rat caused a large reduction in the magnitude of LTP induced by a theta burst stimulation (TBS) paradigm. Results indicate that the reduction of LTP produced by trimipramine does not seem to result from major changes in the physiological properties of the slice preparations. First, paired-pulse facilitation was not impaired following the drug administration suggesting that transmitter release was not modified in TRIM-treated slices. Second, the burst responses evoked by high-frequency stimulation exhibited the typical buildup of depolarization, which is due to both a reduction of IPSPs and the activation of NMDA receptors. Finally, the treatment did not change the amount of short-term potentiation induced by TBS nor did it modify the component of excitatory postsynaptic potentials (EPSPs) mediated by the activation of NMDA receptors, suggesting that the NMDA receptor functions remained intact in TRIM-treated slices. Taken together the present data suggest that the loss of LTP maintenance in TRIM-treated animals is more likely the result of the disruption by trimipramine of cellular processes that follow LTP induction. In addition, the results provide evidence for a possible correlation between the reduction in LTP expression and learning deficits produced by chronic administration of trimipramine.

Inverted-U relationship between the level of peripheral corticosterone and the magnitude of hippocampal primed burst potentiation

Studies have shown that peripheral levels of corticosterone correlate with the magnitudes of two well-described physiological models of memory, long-term potentiation (LTP) and primed burst (PB) potentiation. In the present experiments, the authors investigated the effects of experimenter-controlled manipulations of the levels of corticosterone on the magnitude of hippocampal PB potentiation in urethane-anesthetized rats. Primed burst potentiation is a long-lasting (at least 30 minutes) increase in the amplitude of the CA1 population spike and EPSP slope in response to physiologically patterned stimulation of the hippocampal commissure. The levels of serum corticosterone were controlled by implanting corticosterone pellets in adrenalectomized rats (ADX/PELLET). In the first experiment, a significant negative linear correlation between elevated (stress) levels of serum corticosterone (greater than 20 micrograms/dL) and the magnitude of PB potentiation in ADX/PELLET subjects (r = 0.60, P < .05) was found. In the second experiment, the shape of the corticosterone-PB potentiation function was different at low and intermediate levels of corticosterone than it was at high levels of corticosterone: There was a positive correlation at low levels (0-10 micrograms/dL), a peak response at intermediate levels (11-20 micrograms/dL), and a negative correlation at high levels (21-93 micrograms/dL) of corticosterone. Thus, the overall relationship between corticosterone and PB potentiation is an inverted-U function. These findings provide strong support for the hypothesis that corticosterone exerts a concentration-dependent biphasic influence on the expression of hippocampal plasticity.

Effect of Temperature and Calcium on the Binding Properties of the AMPA Receptor in Frozen Rat Brain Sections

The elucidation of the mechanisms regulating the properties of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) subtype of glutamate receptors is important for understanding glutamatergic transmission. Here we report that qualitative as well as quantitative analysis of tritiated ligand binding to the AMPA receptor on thin frozen rat brain tissue sections reveals the existence of several mechanisms regulating the binding properties of AMPA receptors. Preincubation of tissue sections at 35 degrees C results in a decreased amount of [3H]AMPA binding as compared to that measured following preincubation at 0 degrees C. The decrease in binding appears to be mainly localized to cell bodies as evaluated by autoradiography, and could be due to proteolysis. Preincubation with calcium at 35 degrees C produces increased levels of [3H]AMPA binding. The effect of calcium is mimicked by manganese and to a lesser extent by magnesium; it is concentration-dependent with a 50% effective concentration for calcium of approximately 150 microM, time-dependent and temperature-dependent. The calcium-induced increase in [3H]AMPA binding is different among various brain structures, being larger in area CA1 of the hippocampus and in the superficial layers of the cerebral cortex. The effect of calcium is partly reduced by preincubation with the calpain inhibitor leupeptin and potentiated by preincubation with purified calpain II. The calcium-induced increase in [3H]AMPA binding is associated with a decrease in the binding of an antagonist of AMPA receptors, [3H]6-nitro-7-cyanoquinoxaline-2,3-dione. The results indicate that the binding properties of the AMPA receptor are rapidly regulated by calcium-dependent processes, and possibly by calcium-dependent proteases. They suggest that modulation of the binding properties involves changes in the configuration of the receptor, producing opposite changes in the affinities of the receptor for agonists and antagonists. Finally, these results strengthen the hypothesis that changes in the properties of AMPA receptors might underlie various forms of synaptic plasticity.

Stress-induced facilitation of classical conditioning

Stress has been shown to impair subsequent learning. To determine whether stress would impair classical conditioning, rats were exposed to inescapable, low-intensity tail shock and subsequently classically conditioned under freely moving conditions with a brief periorbital shock unconditioned stimulus and a white noise conditioned stimulus. Unexpectedly stressed rats exhibited significantly more conditioned eyeblink responses and the magnitude of their individual responses was also enhanced. These results stand in contrast to the learning deficits typically observed and suggest that stress can enhance the acquisition of discrete conditioned responses.

Acute stress impairs (or induces) synaptic long-term potentiation (LTP) but does not affect paired-pulse facilitation in the stratum radiatum of rat hippocampus

Rats were exposed to restraint coupled with 60, 1-sec, 1-mA, 60-Hz tail shocks. One hippocampus was immediately dissected for in vitro measurement of paired-pulse facilitation and LTP of the excitatory postsynaptic potential (EPSP) recording from the stratum radiatum of field CA1. There was no change in paired-pulse facilitation, suggesting that acute exposure to the stressor does not result in a decrease in presynaptic neurotransmitter release. There was, however, a significant decrease in the percent LTP produced by theta burst stimulation relative to naive controls. These results are consistent with the hypothesis that the stress-induced impairment of LTP is a result of changes in the postsynaptic glutamate receptors, specifically the AMPA type.

Adrenal hormone effects on hippocampal excitatory amino acid binding

The influence of short-term adrenalectomy or corticosterone treatment on the binding of glutamate receptor subtypes in the rat hippocampus was explored using the technique of in vitro autoradiography. Analysis of NMDA, kainate and AMPA binding in the hippocampus was conducted on the brains of control, adrenalectomized, and adrenalectomized animals given corticosterone treatment. In addition, serum corticosterone levels were determined by RIA. No striking effects of acute adrenalectomy on the distribution or density of any glutamate receptor subtype were observed in the hippocampus. Adrenalectomy had a small but significant effect on kainate binding in the stratum lucidum and stratum radiatum of CA3 in the first experiment, but no effect in follow-up experiments. Short-term treatment with stress levels of corticosterone had no effect on the binding of NMDA or kainate in any hippocampal subfield. However, a small effect of high doses of corticosterone (CORT) was observed on AMPA binding in one subregion. Although the hippocampus is a target for glucocorticoids and uses excitatory amino acids as a primary neurotransmitter, transient manipulation of adrenal hormone levels did not directly modulate excitatory amino acid receptor binding.

Potassium-Induced Depolarization of Rat Telencephalic Synaptoneurosomes Increases [3H]Amino-3-Hydroxy-5-Methylisoxazole-4-Propionate Receptor Binding

Potassium-induced depolarization of synaptoneurosomes prepared from rat telencephalon was found to increase [3H]amino-3-hydroxy-5-methylisoxazole-4-propionate ([3H]AMPA) binding to the AMPA receptor. The effect required the presence of calcium because it was blocked by the calcium chelator EGTA but was not blocked by an antagonist of the N-methyl-D-aspartate receptor, aminophosphonopentanoate. The depolarization-induced increase in [3H]AMPA binding was markedly reduced by a blocker of voltage-dependent calcium channels, verapamil. Saturation kinetic experiments revealed that the increase in [3H]AMPA binding produced by potassium depolarization was due to an increase in the affinity of the AMPA receptor. These results provide additional support for a critical role of calcium in the regulation of the AMPA receptors. The synaptoneurosome preparation might represent an interesting tool to determine the role of different calcium-dependent enzymes involved in the regulation of the AMPA receptor.