Modulation by adrenal steroids of limbic function

The effects of various steroid hormones on the long-term potentiation (LTP) of the rat hippocampus were evaluated. LTP was elicited in the dentate gyrus of adrenalectomized animals with priming tetanic stimulation (200 Hz-0.03 cps) of its main afferent, the perforant pathway. Single pulse EPSP (excitatory post-synaptic potential) slope, and PS (population spike) amplitude values were compared before and after the i.v. injection of the hormones and subsequently after the priming stimulation every 15 min up to 1 h. 18-OH-deoxycorticosterone (18-OH-DOC) produced a significant decrease of the EPSP LTP and arrested the PS enhancement in comparison with vehicle at every time post-tetanic stimulation. Its 21-acetate derivative produced a moderate decrease of the EPSP and had no effect on the PS LTP in comparison with vehicle. Deoxycorticosterone (DOC) exhibited similar effects on the EPSP although less marked than with 18-OH-DOC while the PS only decreased in the first 30 min post-train. Corticosterone decreased both EPSP and PS for the first 15 and 30 min after priming stimulation, respectively, matching values with those of vehicle afterwards. Its 21-acetate produced an initial decrease of the EPSP and had no effect on the PS LTP. Allo-tetrahydro-DOC produced little, if any, initial enhancement of the PS LTP in comparison with vehicle. These results show that the adrenal steroids tested can modulate hippocampal LTP, a plastic phenomenon in the mammalian CNS which is known to be related to memory and learning processes. Moreover, adrenal steroids can independently modify the PS or EPSP components of the LTP, suggesting different loci of action at the neuronal level.     

Effects of adrenal steroids and their reduced metabolites on hippocampal long-term potentiation

We studied the effects of steroid hormones on the hippocampal long-term potentiation (LTP), a putative mechanism of neuronal plasticity and memory storage in the CNS. In vivo experiments were performed in rats under chloral hydrate anesthesia (0.4 mg/kg i.p.). All animals were adrenalectomized 48 h before recording. LTP was induced after priming tetanic stimulation at the perforant pathway (PP) and single pulse field potentials were obtained from the dentate gyrus (DG). The excitatory post-synaptic potential (EPSP) slope and population spike (PS) amplitude were analyzed before and after the i.v. injection of the steroids and after the induction of LTP, and followed up to 1 h. Results obtained with the hormones were compared with matched control animals injected with vehicle alone, Nutralipid 10%. Previous results from our laboratory showed that deoxycorticosterone (DOC) decreased the magnitude of the EPSP at all times after priming stimulation and the PS decreased during the first 30 min of the LTP. Corticosterone decreased the EPSP in the first 15 min and the PS during the first 30 min after priming stimuli. In these experiments the mineralocorticoids aldosterone and 18-OH-DOC elicited a decrease of the EPSP at all times post-train; and no significant difference against vehicle was observed in the PS. Post-injection values were not changed except for 18-OH-DOC at a dose of 1 mg, where a decrease of both the EPSP (P less than 0.01) and the PS (P less than 0.02) was observed against vehicle. ATH-progesterone at 0.1 mg/rat also decreased the EPSP values significantly after priming stimulation and no significant changes against vehicle were observed in the PS. These results show that adrenal steroids can modulate hippocampal LTP, that they can act at different neuronal loci and with different time courses in the development of the phenomena.     

Analysis of function of the glutamatergic and GABAergic mediator systems in the olfactory cortex of spontaneously hipertensive rats in vitro

The effect of persistent hypertension on neuronal activity and synaptic transmission has been studied on olfactory cortex slices of SHR rats. The profilies of focal potentials in hypertensive rats demonstrated a short duration of the 2-amino-3-(5-methyl-3-hydroxyisoxazol-4-yl)-propanoic acid (AMPA) component of excitatory postsynaptic potential (EPSP), a small amplitude and long duration of the N-methyl D-aspartate (NMDA) component of EPSP, and a large amplitude of the GABA_B-dependent slow inhibitory postsynaptic potentials. The sensitivity of glutamate receptors responsible for the generation of AMPA- and NMDA-mediated EPSPs was low after the exposure to 1 mM L-glutamate. The amplitudes of the AMPA- and NMDA-mediated EPSPs decreased. Tetanization of slices from hypertensive rats induced a short-term potentiation followed by a depression. The data obtained indicate that persistent hypertension has depressive effects on the basic glutamatergic and GABAergic parameters of synaptic activity of neurons as well as on learning and memory. Apparently, these processes were evoked by glutamate excitotoxicity in the brain of hypertensive rats.     

Hippocampal Long-Term Potentiation Attenuated by Lesions in the Marginal Division of Neostriatum

The marginal division (MrD) is a spindled-neurons consisted zone at the caudal border of the neostriatum in the mammalian brain and has been verified as contributing to associative learning and declarative memory in the rat and human with behavior and functional magnetic resonance imaging methods. It was proved to have functional connections with the limbic system. Whether the MrD has influence on the hippocampal long-term potentiation (LTP) was investigated in this study. LTP was induced from the dentate gyrus (DG) in the hippocampus by high-frequency stimulation (HFS) to the perforant path (PP). The amplitude of the population spike (PS) and the slope of the excitatory postsynaptic potential (EPSP) increased significantly to form LTP in the DG of the hippocampus after HFS of PP in normal and saline-injected control groups of rats. Lesions introduced in the MrD reduced significantly both the amplitude of PS and the slope of the EPSP following HFS of the PP. The results indicated that lesions in the MrD could attenuate LTP formation in the hippocampus. Our data suggest that the MrD might very possibly have excitatory functional influence on the hippocampus and therefore might influence the function of the hippocampus.     

Effects of low-frequency tetanic stimulation of the synaptic inputs on different forms of long-term potentiation in the rat hippocampus

In experiments on transversal slices of the dorsal hippocampus of rats, we found that low-frequency stimulation of the mossy fibers (MF) against the background of pre-settled long-term post-tetanic potentiation in the MF-CA3 pyramidal neuron (PN) dendrites synaptic system evoked depotentiation in all studied slices. Depotentiation was considerably decreased by a non-competitive blocker of the NMDA glutamate receptors, ketamine (100 μM), as well as by an inhibitor of calmodulin, trifluoroperazine (10 μM), and an inhibitor of calcineurin, cyclosporin A (250 μM). At the same time, depontentiation was not changed by 50 μM polymixin B, an inhibitor of protein kinase C. Long-term potentiation of synaptic transmission in the Schaffer collaterals (SchC)-CA1 PN dendrites system, which was evoked by 2.5-min-long anoxia/aglycemia episodes, resulted exclusively from enhancement of the NMDA component of population EPSP, while their AMPA component was not modified, i.e., in this case potentiation was of a postsynaptic nature. Under these conditions, low-frequency stimulation of SchC resulted in a further increase in the intensity of synaptic transmission due to increases in both the NMDA and AMPA components of population EPSP. The above form of potentiation could be suppressed by 100 μM ketamine, 10 μM trifuoroperazine, 250 μM cyclosporin A, or 10 μM N-nitro-L-arginine. Weak (near-threshold) high-frequency stimulation of SchC induced long-lasting potentiation of synaptic transmission due to an isolated increase in the AMPA component of population EPSP, i.e., this potentiation was of a postsynaptic nature. In the latter case, low-frequency SchC stimulation resulted in further facilitation of synaptic transmission. Intensive tetanic high-frequency stimulation of the above fibers induced long-term potentiation of a presynaptic nature, while their low-frequency stimulation depotentiated synaptic transmission.     

Effects of A1 and A2 Adenosine Receptor Antagonists on the Induction and Reversal of Long-Term Potentiation in Guinea Pig Hippocampal Slices of CA1 Neurons

1. Using simultaneous recordings of the field EPSP and the population spike in the CA1 neurons of guinea pig hippocampal slices, we confirmed that delivery of a high-frequency stimulation (tetanus: 100 pulses at 100 Hz) produced robust long-term potentiation of synaptic efficacy (LTP) in two independent components, a synaptic component that increases field excitatory postsynaptic potentials (EPSPs) and a component that results in a larger population spike amplitude for a given EPSP size (E-S potentiation).2. In the same cells, reversal of LTP (depotentiation; DP) in the field EPSP and in the E-S component is achieved by delivering low-frequency afferent stimulation (LFS:1 Hz, 1000 pulses) 20 min after the tetanus.3. When the tetanus or LFS was applied to CA1 inputs in the presence of an adenosine A₁ receptor antagonist, 8-cyclopentyltheophylline (1 M), the field EPSP was enhances in LTP and attenuated in DP, while the E-S relationship was not significantly affected in either LTP or DP.4. When similar experiments were performed using an A₂ receptor antagonist, CP-66713 (10 M), the field EPSP was blocked in LTP but facilitated in DP, while E-S potentiation was enhanced during both LTP and DP.5. The results show that endogenous adenosine, acting via A₁ or A₂ receptors, modulates both the synaptic and the E-S components of the induction and reversal of LTP. Based on the results, we discuss the key issue of the contribution of these receptors to the dynamics of neuronal plasticity modification in hippocampal CA1 neurons.     

Long-Term Plasticity Is Proportional to Theta-Activity

Background

Theta rhythm in the hippocampal formation is a main feature of exploratory behaviour and is believed to enable the encoding of new spatial information and the modification of synaptic weights. Cyclic changes of dentate gyrus excitability during theta rhythm are related to its function, but whether theta epochs per se are able to alter network properties of dentate gyrus for long time-periods is still poorly understood.

Methodology/Principal Findings

We used low-frequency stimulation protocols that amplify the power of endogenous theta oscillations, in order to estimate the plasticity effect of endogenous theta oscillations on a population level. We found that stimulation-induced augmentation of the theta rhythm is linked to a subsequent increase of neuronal excitability and decrease of the synaptic response. This EPSP-to-Spike uncoupling is related to an increased postsynaptic spiking on the positive phases of theta frequency oscillations. Parallel increase of the field EPSP slope and the population spike occurs only after concurrent pre- and postsynaptic activation. Furthermore, we observed that long-term potentiation (>24 h) occurs in the dentate gyrus of freely behaving adult rats after phasic activity of entorhinal afferents in the theta-frequency range. This plasticity is proportional to the field bursting activity of granule cells during the stimulation, and may comprise a key step in spatial information transfer. Long-term potentiation of the synaptic component occurs only when the afferent stimulus precedes the evoked population burst, and is input-specific.

Conclusions/Significance

Our data confirm the role of the dentate gyrus in filtering information to the subsequent network during the activated state of the hippocampus.     

Long-term potentiation in the neurons of the edible snail

A brief high-frequency stimulation of the anal nerve of the isolated nerve ring of snail Helix induced a pronounced increase in the amplitude of EPSPs, evoked in identified neurons of left parietal and visceral ganglions by low frequency (once in 5 min) stimulation of the same nerve. The amplitude of EPSP returned to the control level 30-120 min after tetanization. We called this effect long-term potentiation. A brief application of serotonin (10 microM) in the majority of neurons also induced lasting either 15-30 min or more than 2 hours facilitation of EPSP, evoked by anal nerve stimulation. Intracellular cAMP injections, being without effect on EPSP amplitude in many neurons, in certain neurons caused an increase in EPSP amplitude, lasting up to 30 min. It is suggested that the 3 factors shown to increase synaptic efficiency in molluscan neurons may have common mechanisms of action.     

Double evoked responses to the electrical stimulation of Shaffer's collaterals in CA1 hippocampal field

Double evoked responses to single current pulses applied to Shaffer's collaterals were observed in CAI hippocampal field in freely moving rats. The second response was an irregular population EPSP with constant latency sometimes accompanied by a population spike. The effect was observed in 22 of 54 tested rats (40.74%). In 10 of these 22 animals (45.45%) the second response was evoked by a weak testing stimulus of at least of a single collateral, in the remaining 12 rats the second response appeared to stimulation with increased current or after potentiation of the stimulated pathways. The second responses were very sensitive to the functional state of an animal and were recorded at the state of rest or during sleep. The latencies of the second response measured from the beginning of the first response were very close in different animals and with low intraindividual variability indicating that the same circuits are involved in its generation. It is suggested that hippocampus can support dynamic processes such as reverberation of signals.     

Temporal pattern sensitivity of long-term potentiation in hippocampal CA1 neurons

High-frequency electrical stimulation in the hippocampus leads to an increase in synaptic efficacy that lasts for many hours. This long-term potentiation (LTP) of synaptic transmission is presumed to play a crucial role in learning and memory in the brain. However, the frequency of stimulation generally used to obtain LTP is beyond the normal physiological range of activity of hippocampal neurons. We found that LTP can be induced by an electrical stimulation whose frequency is comparable to that of the naturally occurring firing activity of hippocampal neurons if the stimulating pulseinterval train has a special time structure. In the present experiment, we compared the magnitude of LTP induced by the four types of stimuli which have the same pulse number and the same mean frequency but different time structure in interstimulus intervals. One type of stimuli has regular intervals, and this served as a control stimulus. In the other three types of stimuli, the adjacent interstimulus interval had the following statistical properties: in type 1, their correlations are positive; in type 2, negative; and in type 3, independent. The magnitude of LTP induced by these four types of stimuli showed clear order relationships: type 3/type 1 control > type 2. Detailed analysis of the evoked potential during a period of temporal pattern stimulation revealed that the amplitude of the population spikes of repetitive firing, especially of the second and third population spikes, had the same order relationship as the LTP. Because 2-amino-5-phosphonovalerate (APV) (50 M) selectively abolished the second and the third population spikes but not the first, and blocked the formation of LTP, the second and the third peaks which appeared as part of the late component of excitatory postsynaptic potentials (EPSP) must involve LTP formation through the activities of N-methy-D-aspartate (NMDA) channels. From the experimental data, a dynamic induction rule concerning LTP in specific neural networks was derived by which the temporal information of the input stimuli can be extracted and transformed into the weight space of synaptic connections in hippocampal networks (see Fig. 1. CA1).     

Melanin concentrating hormone increase hippocampal synaptic transmission in the rat

A retrograde facilitation has been demonstrated in the one trial step-down inhibitory avoidance of melanin-concentrating hormone (MCH), when it was infused into rat hippocampal formation. Considering the high density of specific binding sites for the MCH peptide on the hippocampus and the participation of this structure on learning and memory processes we have studied the effects of MCH on the hippocampal synaptic transmission. For this purpose, slices of rat hippocampus were perfused with different concentration of MCH. The main result of the present study was a long-lasting potentiation on the hippocampal evoked response on dentate gyrus induced by MCH (4-11 microM) at 30, 60 and 120 min with a maximum effect at 120 min. Previous perfusion of DL - 2- amino - 5 phosphonovaleric acid (APV, 20 microM) was unable to impair the increased hippocampal evoked response induced by MCH 4 microM. On the other hand, the channel blocker Dizocilpine (MK-801, 10 microM) completely impaired the increased hippocampal synaptic plasticity induced by MCH perfusion. We postulate the increased hippocampal synaptic efficacy induced by MCH as one of the mechanisms underlying the retrograde facilitation on the inhibitory avoidance paradigm, observed after MCH hippocampal microinjection. We cannot rule out other MCH neurochemical mechanism and other areas of the brain involved in the MCH effects.     

Effect of adrenalin and adrenal desympathization on brain-seizure activity

The effect of adrenalin and bilateral adrenal desympathization on brain-seizure activity evoked by electrical stimulation of the dorsal hippocampus was studied in adult cats. A few days after bilateral adrenal desympathization the threshold of epileptogenic hippocampal stimulation was lowered and the duration of the evoked seizure response increased. Intravenously injected adrenalin raised the threshold of epileptogenic hippocampal stimulation. After injection of small doses of adrenalin directly into the mesencephalic reticular formation the evoked seizure activity was inhibited: The threshold of epileptogenic hippocampal stimulation was raised and the total duration of the seizure discharges reduced. It is postulated that one of the important factors limiting brain-seizure activity is an increase in the circulating blood adrenalin level.     

Role of EGR1 in hippocampal synaptic enhancement induced by tetanic stimulation and amputation

Hippocampal neurons fire spikes when an animal is at a particular location or performs certain behaviors in a particular place, providing a cellular basis for hippocampal involvement in spatial learning and memory. In a natural environment, spatial memory is often associated with potentially dangerous sensory experiences such as noxious or painful stimuli. The central sites for such pain-associated memory or plasticity have not been identified. Here we present evidence that excitatory glutamatergic synapses within the CA1 region of the hippocampus may play a role in storing pain-related information. Peripheral noxious stimulation induced excitatory postsynaptic potentials (EPSPs) in CA1 pyramidal cells in anesthetized animals. Tissue/nerve injury caused a rapid increase in the level of the immediate-early gene product Egr1 (also called NGFI-A, Krox24, or zif/268) in hippocampal CA1 neurons. In parallel, synaptic potentiation induced by a single tetanic stimulation (100 Hz for 1 s) was enhanced after the injury. This enhancement of synaptic potentiation was absent in mice lacking Egr1. Our data suggest that Egr1 may act as an important regulator of pain-related synaptic plasticity within the hippocampus.     

Modulation of Cyclic AMP Metabolism by Glucocorticoids in PC18 Cells

Abstract: Glucocorticoids modulate signal transduction mechanisms in a number of cell systems. As the adrenal medulla is exposed to relatively high levels of adrenal cortical glucocorticoids in vivo, particularly during periods of stress, the aim of the present study was to determine whether glucocorticoids modulate cyclic AMP (cAMP) metabolism in an in vitro model of this system, the PC18 cell line. Dexamethasone significantly potentiated cAMP accumulation in response to the adenosine analogue N ⁶- R -phenylisopropyl adenosine (PIA), and in response to forskolin. This effect was both time- and concentration-dependent. Maximal potentiation was observed after 48 h of exposure to 1 µ M dexamethasone. Corticosterone and to a lesser extent aldosterone also significantly potentiated PIA-dependent cAMP accumulation. In contrast, estradiol, testosterone, and triiodothyronine had no potentiative effect. Potentiation could be eliminated by coincubation with the protein synthesis inhibitor cycloheximide. In the presence of Ro 20-1724, a cAMP-phosphodiesterase inhibitor, the degree of potentiation of both PIA- and forskolin-dependent cAMP accumulation was significantly decreased by 50–60%. These data suggested that altered cAMP-phosphodiesterase activity may be involved in this response. However, cytosolic and membrane-bound low K _m cAMP-phosphodiesterase activity was unchanged in dexamethasone-treated cells compared with controls. Similarly, there were no significant differences in basal, PIA-, forskolin-, or GTPγS-stimulated adenylate cyclase activities between groups. These studies indicate that glucocorticoids can potentiate cAMP accumulation in intact PC18 cells. The mechanism underlying this potentiation is likely to be multifactorial, but may be due in part to decreased cAMP catabolism.     

Ephaptic feedback in identified synapses of terrestrial snails

A hypothesis for the existence of the intrasynaptic ephaptic feedback (EFB) in the invertebrate central nervous sytem was tested. Excitatory postsynaptic potentials (EPSPs) and currents (EPSCs) evoked by the activation of the recently described monosynaptic connection between the identified snail neurons were recorded intracellularly. In case of the EFB presence, the postsynaptic tetanization with hyperpolarization pulses could activate presynaptic Ca2+ channels and enhance the EPSP amplitude, whereas a steady postsynaptic hyperpolarization should induce a "supralinear" increase in EPSC amplitudes as it has been found in the rat hippocampus. In the first series of the experiments, 10 trains of hyperpolarizing pulses (40-50 mV, 1 Hz, pulse duration 0.5 s, train duration 45 s) were delivered postsynaptically. No significant changes in EPSP amplitudes were found. In the second series of the experiments, the EPSC amplitudes were measured during varying postsynaptic hyperpolarization. At the membrane potential 100 mV, the EPSP amplitude was significantly higher than theoretically predicted from the classical linear dependence. Such a "supralinear" effect of postsynaptic depolarization can be explained by the presence of the EFB. This finding is the first evidence for the EFB existence in the invertebrate central nervous system.     

Modulation of synaptic plasticity by stress hormone associates with plastic alteration of synaptic NMDA receptor in the adult hippocampus

Stress exerts a profound impact on learning and memory, in part, through the actions of adrenal corticosterone (CORT) on synaptic plasticity, a cellular model of learning and memory. Increasing findings suggest that CORT exerts its impact on synaptic plasticity by altering the functional properties of glutamate receptors, which include changes in the motility and function of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid subtype of glutamate receptor (AMPAR) that are responsible for the expression of synaptic plasticity. Here we provide evidence that CORT could also regulate synaptic plasticity by modulating the function of synaptic N-methyl-D-aspartate receptors (NMDARs), which mediate the induction of synaptic plasticity. We found that stress level CORT applied to adult rat hippocampal slices potentiated evoked NMDAR-mediated synaptic responses within 30 min. Surprisingly, following this fast-onset change, we observed a slow-onset (>1 hour after termination of CORT exposure) increase in synaptic expression of GluN2A-containing NMDARs. To investigate the consequences of the distinct fast- and slow-onset modulation of NMDARs for synaptic plasticity, we examined the formation of long-term potentiation (LTP) and long-term depression (LTD) within relevant time windows. Paralleling the increased NMDAR function, both LTP and LTD were facilitated during CORT treatment. However, 1-2 hours after CORT treatment when synaptic expression of GluN2A-containing NMDARs is increased, bidirectional plasticity was no longer facilitated. Our findings reveal the remarkable plasticity of NMDARs in the adult hippocampus in response to CORT. CORT-mediated slow-onset increase in GluN2A in hippocampal synapses could be a homeostatic mechanism to normalize synaptic plasticity following fast-onset stress-induced facilitation.     

Intrasynaptic ephaptic feedback in central synapses]

A review is given of experiments performed in the author's laboratory on slices from the rat visual cortex and hippocampus. The aim was to test the existence of the positive feedback in central synapses according to a mechanism of electrical (ephatic) linking proposed by A. L. Byzow. The hypothesis predicts that, in a subset of central synapses, artificial postsynaptic membrane potential (MP) hyperpolarization should increase the amplitude of the excitatory postsynaptic current (EPSC) and potential (EPSP) not only due to a deviation from the equilibrium potential but also due to increased presynaptic transmitter release. In a part of the experiments, we found changes in several traditional parameters of transmitter release during hyperpolarization: number of response failures, coefficient of variation of response amplitude and quantal content of minimal EPSC/EPSP. The effects were especially prominent for the giant mossy fibre-CA3 synapses. For them, "supralinear" amplitude-voltage relations at hyperpolarized membrane potentials and voltage--dependent paired--pulse facilitation ratios were found. All these "non-classical" effects disappeared when composite, rather than minimal, EPSCs were evoked. These data were consistent with simulation experiments performed on the Byzov's synaptic model with the ephaptic feedback and therefore they strengthen the hypothesis. Independent of their interpretation, the data reveal a novel feedback mechanism. The mechanism provides a possibility for the central postsynaptic neurone to control the efficacy of a subset of synapses via postsynaptic MP modifications. The mechanism can essentially increase the efficacy of large ("perforated") synapses. It explains the significance of the increased number of such synapses following experimental challenges such as leading to induction of the long-term potentiation or to behavioural conditioning.     

灭多威对美洲大蠊腹六神经节突触传递的阻断作用

用电生理学方法研究了灭多威对美洲大蠊Periplanetaamerwana腹六神经节(A6节)突触传递的影响。用灭多威溶液浸泡A6节,电刺激尾须神经粗支,用甘露醇间隙法记录兴奋性突触后电位(EPSP)和突触后动作电位。给予弱刺激只记录到EPSP时,灭多威作用初期EPSP幅度增加、时程延长,能诱发突触后动作电位,随后EPSP逐渐减小至消失,冲洗可恢复,突触前反应保持不变。增加电刺激强度记录到突触后动作电位时,灭多威可阻断A6节的突触传递,阻断时间是浓度依赖性的,阻断是可逆的,但冲洗30 min仍保留一定的后作用。对美洲大蠊雄性成虫腹腔注射灭多威测定致死中量(LD₅₀)为(3.56±0.01) μg/g体重。根据灭多威的作用机理对其阻断A6节突触传递的特点以及对虫体的毒杀机制进行了讨论。