Aminergic blockade modulates long-term potentiation in the dentate gyrus of freely moving rats

Long-term potentiation (LTP) was induced in the dentate gyrus of freely moving rats by tetanic stimulation of the medial entorhinal cortex under conditions of catecholamine depletion by 200 mg/kg alpha-Methyl-para-tyrosine (AMPT) or blockade of alleged dopamine receptors by 0.5 mg/kg haloperidol. Both substances did not change significantly the normal excitability of the glutamatergic perforant pathway, but affected the establishment of LTP. Whereas the potentiation effect on the EPSP component of the monosynaptic field potential was not changed by both substances when compared to the potentiation of controls, the potentiation of the population spike was prolonged and enhanced. These results point to an effect of catecholaminergic blockade on postsynaptic membranes of the target cells or on other components of the neuronal network but not to a specific influence on the homosynaptic mechanisms of LTP.     

The effects of kindling on GABA-Mediated inhibition in the dentate gyrus of the rat. I. Paired-pulse depression

Double-pulse stimulation of the perforant path input to the dentate gyrus was used in the following experiments to produce paired pulse depression in that site. This effect provided an estimate of GABA-mediated recurrent inhibition. The depression was enhanced by drugs that facilitate or mimic GABA action and attenuated by drugs which block GABA transmission. Paired-pulse depression was significantly increased following amygdala kindling and was further enhanced to near maximal levels by subsequent kindling in the dentate. In addition, kindling did not increase the rate at which inhibition failed under conditions of excessive activation. Trains of 5 Hz stimulation, applied to the perforant path, caused paired-pulse depression to disappear and elicited a brief AD. Following kindling, the latency to AD onset tended to be increased rather than shortened, suggesting an enhanced resistance to inhibitory failure. These results indicated that kindling increased, rather than reduced, inhibition in the dentate gyrus.     

Large long-lasting potentiation in the dentate gyrus in vitro during blockade of inhibition

The sequence of regeneration following intracranial optic nerve crush has been studied using electrophysiological visual mapping in the frog Hyla moorei. Compared to our earlier series with extracranial crush, the time course was slower and the intermediate projections more disorganized. It is suggested that the apparent discrepancy between the early patterns of regeneration in Xenopus and Rana compared to Hyla, fish and newt, is not due to species differences but to the location of the lesion site.     

Lonf-term potentiation in the dentate gyrus is preferentially induced at theta rhythm periodicity

In urethane-anesthetized rats, high frequency stimulation was applied to the medial perforant pathway at various time intervals (50, 100, 200, 350 and 500 ms) following stimulation of the same pathway by a single pulase of equal intensity. Recordings of dentate gyrus granule cell evoked responses were made to investigate the range of stimuli that are effective in inducing long-term potentiation (LTP). LTP was induced almost exclusively at the 200 ms interval, corresponding to the periodicity if the theta rhythm. Taken in conjunction with similar findings reported in the CA1 field of the hippocampal slice, these results suggest that the correlation between theta rhythm periodicity and LTP is a general phenomenon within the hippocampal formation and lends further support to the hypothesis that the naturally occuring theta rhythm may play a modulatory role in the induction of LTP.     

Involvement of nitric oxide in long-term potentiation in the dentate gyrus in vivo

Effects of NO synthase inhibitors on long-term potentiation (LTP) were investigated in the dentate gyrus of anesthetized rats. Administration ofN^G-nitro-L-arginine methyl ester (L-NAME; 50 nmol i.c.v.) greatly blocked the generation of LTP without affecting basal responses.N^G-nitro-L-arginine (50 nmol) orN^G-methyl-L-arginine (50 nmol) also showed similar effects. The LTP-blocking effect of L-NAME was reversed by coadministration ofL-arginine (500 nmol). These results suggest that NO participates in the generation of LTP at the dentate gyrus in vivo.     

Chronic lead exposure accelerates decay of long-term potentiation in rat dentate gyrus in vivo

Long-term potentiation (LTP) is a model of synaptic plasticity believed to encompass the underlying neurobiological mechanisms that support memory function. Chronic developmental lead (Pb) exposure is known to be associated with cognitive dysfunction in children and animals. Disruption of the induction of long-term potentiation (LTP) has been reported in the hippocampus following chronic exposure to environmentally relevant levels of Pb in rats. Under urethane anesthesia, we have previously observed Pb-induced increases in the threshold for LTP induction. With higher train intensities, LTP was induced and no declines in the amplitude of responses within a 60-min posttrain period were evident. The present study was designed to assess the effects of Pb on the more enduring forms of LTP in the dentate gyrus of the conscious rat. Beginning in the late gestational period, rats were chronically exposed to 0.2% Pb²⁺-acetate through the drinking water of the pregnant dam, and directly through their own water supply at weaning. As adults, electrodes were permanently implanted in male offspring and field potentials evoked by perforant path stimulation were recorded from the dentate gyrus over several weeks. LTP was induced by delivering theta-burst patterned stimulation at a maximal stimulus intensity through the perforant path electrode, and input/output (I/O) functions were monitored for 1 month. Population spike (PS) amplitude was increased maximally 1 h after train delivery. The time constant of decay (τ) calculated from pooled data for each group yielded declines in PS amplitude by 63% in 17.4 days in controls and 13.4 days in Pb-exposed animals. Quantitative estimates of decay in individual animals were achieved in two ways: (1) by calculating difference scores in I/O functions from the maximal LTP at 1 h, and (2) by interpolating day to decay by 63% from declines from maximal LTP. The interpolated values were used to compare the incidence of animals showing decay of 63% within 1 week posttrain. Both analyses revealed a more accelerated rate of decay of LTP in animals developmentally exposed to Pb relative to controls. Endurance of potentiated responses for days to weeks is believed to be supported by structural modifications and synaptic growth. The reported effects of Pb on growth-related processes may thus contribute to a reduced persistence of LTP and the resulting cognitive deficits engendered by developmental Pb exposure.     

Anisomycin blocks the late phase of long-term potentiation in the dentate gyrus of freely moving rats

Freely moving rats, chronically implanted with stimulation electrodes in the medial entorhinal cortex and recording electrodes in the dentate gyrus, received two 400 micrograms intraventricular injections of anisomycin during a tetanization procedure that induced a long-lasting potentiation (72 hours) of the monosynaptic field potential. Inhibition of protein synthesis during the tetanization procedure did not immediately influence the induction of long-term potentiation (LTP). However, 3-4 hours after the beginning of tetanization the potentiation effect decayed progressively and was abolished totally during the remaining 7 day observation period. In control experiments anisomycin did not affect the slope of field EPSP's and produced a reversible depression of the population spike amplitude. These data indicate a relatively specific effect of the protein synthesis inhibitor on mechanisms involved in a late phase of LTP stabilization.     

Development of potentiation in the dentate gyrus of rat: Physiology and anatomy

DUFFY, C. J. AND T. J. TEYLER. Development of potentiation in the dentate gyrus of rat: physiology and anatomy. BRAIN RES. BULL. 3(5) 425–430, 1978.—The physiological development of potentiating processes in the rat dentate gyrus were compared to morphological development. Rapid Golgi techniques were coupled with in vitro studies of dentate granule cell frequency potentiation, post-tetanic potentiation and long-term potentiation. Frequency potentiation and long-term potentiation exhibited a developmental progression between 7 and 210 days postnatal. Posttetanic potentiation remained constant across this period. The relation of these findings to synaptogenesis and dendritic spine formation are discussed.     

Neuropeptide Y inhibits glutamate release and long-term potentiation in rat dentate gyrus

The effect of intracerebroventricular injection of neuropeptide Y (NPY) was assessed on LTP in dentate gyrus. We report that NPY attenuated LTP and inhibited KCl-induced glutamate release in synaptosomes prepared from dentate gyrus. Activity of the stress-activated kinase, c-Jun NH2-terminal kinase (JNK) in synaptosomes was increased by incubation with NPY or following intracerebroventricular injection. Activation of JNK might underlie the inhibitory effect of NPY on LTP.     

The effects of repeated induction of long-term potentiation in the dentate gyrus

Previous experiments have shown that long-term potentiation (LTP) generally lasts for only a few days or weeks. The LTP phenomenon would be more attractive as a memory model if it were more enduring. The experiments reported in this paper were designed to test the effects of repeated induction of LTP on the duration of LTP. Three groups of animals received 5 LTP-inducing stimulation sessions. In one group of animals, the stimulation sessions were administered every 24 h. In the 2nd group, the sessions were administered after the LTP effects had decayed to 50% of the peak values. In the 3rd group, the sessions were administered only after the response amplitudes had completely returned to pre-LTP baseline levels. None of the LTP measures were altered, in any group, as a result of repeated induction of LTP. The thresholds, the asymptotic levels of potentiation, and the decay rates were the same after each session. Other treatments may alter the duration of LTP effects, but repeated induction of LTP does not appear to have any lasting effects.     

Long-term potentiation in the dentate gyrus of the anaesthetized rat is accompanied by an increase in protein efflux into push-pull cannula perfusates

Changes in protein content of push-pull cannula perfusates from the dentate gyrus of anaesthetized rats were analyzed before and after the induction of long-term potentiation (LTP). LTP, induced by either high-frequency stimulation of the perforant path or raising the extracellular calcium concentration, was associated with increases in the protein content of the perfusates. Tetanically induced LTP was accompanied with a large but delayed increase (apparent in the second hour after the stimulation) in protein efflux. In contrast, when LTP was induced by the elevation of extracellular calcium concentration, a smaller but more immediate increase in protein efflux was observed. When 5-D-aminophosphonovalerate was used to block the induction of LTP, no increase was observed in either case. These results indicate that LTP in the dentate gyrus is accompanied by an increase in the efflux of proteins into push-pull cannula perfusates. The possible origins of these proteins and their role in LTP are discussed.     

Blockade of norepinephrine-induced long-lasting potentiation in the hippocampal dentate gyrus by an inhibitor of protein synthesis

The mechanism of action of norepinephrine (NE)-induced potentiation of the population spike in the dentate gyrus of hippocampal slices was examined and compared with NE effects in field CA1. NE-induced potentiation was confined to the dentate gyrus, where slices perfused for 30 min with concentrations of NE as low as 5 microM exhibited potentiation of the perforant path evoked population spike. Potentiation began within 15 min, and lasted many hours after NE was washed out. Experiments where slices were pre-incubated with the protein synthesis inhibitor emetine indicated that there are two distinct phases to NE-induced potentiation. The initial short-term NE-induced potentiation (NEP) seen during NE application was not affected by a 30 min pre-incubation with emetine, whereas the long-lasting potentiation (NELLP) which persists after NE washout was completely blocked by emetine at a concentration which we have previously shown to be effective in blocking hippocampal long-term potentiation (LTP). Additional experiments indicated that both phases of NE-induced potentiation were completely blocked by the beta-antagonist propranolol and the beta 1-antagonist metoprolol. Furthermore, pre-incubation of slices with the direct-acting adenylate cyclase stimulant forskolin shifted the dose-response curves for both phases of NE-induced potentiation to the left. These results suggest that NE-induced potentiation is probably mediated by beta 1-receptor stimulation of adenylate cyclase. We have previously shown an importance for beta 1-receptor stimulation of adenylate cyclase in the production of LTP in the dentate. Thus, these results demonstrate a number of similarities between hippocampal LTP and NELLP in the dentate gyrus.     

Effects of pentylenetetrazol kindling on glutamate receptor genes expression in the rat hippocampus

It has been hypothesized that changes in the excitatory amino acid receptor biosynthesis may be involved in the mechanism of kindling—an animal model of epileptogenesis. In order to test this hypothesis, we investigated the effects of pentylenetetrazol kindling on the expression of genes coding for NMDAR1 and GluR2 in the rat hippocampal formation. Pentylenetetrazol kindling decreased the hippocampal NMDAR1 mRNA level after 3 and 24 h; lowered the GluR2 flip level and elevated the flop mRNA one in the CA1 field and dentate gyrus after 3 and 24 h, respectively. A receptor autoradiography showed an increase in the [³H]MK-801 binding density in the hippocampus following both acute and repeated pentylenetetrazol administration. We conclude that an early occurrence of downregulation of the glutamate receptor gene expression may be an adaptive response of glutamate receptors to an oversupply of excitatory amino acids during repeated seizures.     

Differential effects of pentylenetetrazol-kindling on long-term potentiation of population excitatory postsynaptic potentials and population spikes in the CA1 region of rat hippocampus

The effects of pentylenetetrazol-kindling on synaptic transmission and the effectiveness of θ pattern primed-bursts (PBs) for the induction of long-term potentiation (LTP) of population excitatory postsynaptic potentials and population spikes were investigated in hippocampal CA1 of pentylenetetrazol-kindled rats. Experiments were carried out in the control and kindled animals at two post-kindling periods, i.e., 48-144 h (early phase) and 30-33 days (long lasting phase). Field potentials (population excitatory postsynaptic potentials, pEPSPs; and population spikes, PSs) were recorded at the stratum radiatum and the stratum pyramidale following stimulation of the stratum fibers, respectively. PBs were delivered to stratum fibers and PB potentiation was assessed. The results showed that 48-144 h after kindling there was no significant difference for pEPSP slope and PS amplitude between two groups. But at 30-33 days after kindling, the pEPSP slope in the stratum radiatum of kindled animals decreased, whereas the amplitude of PSs increased compared to those of controls. Shortly after kindling, control animals had normal LTP of pEPSP slope and PS amplitude in response to PBs, but kindled rats lack LTP of pEPSP slope and PBs induced LTP of PS amplitude in most of kindled animals. In 30-33 days after kindling, PB potentiation was not observed in the stratum radiatum of kindled animals but PBs induced LTP of PS amplitude, which was significantly greater than that of control animals. The effect is compatible with the hypothesis, which postulates kindling-associated functional deficit in hippocampus, especially CA1, as an explanation for the behavioral deficits seen with the kindling model of epilepsy.     

Cysteamine potentiates entorhinal activation of dentate gyrus granule cells in rats

A dense plexus of somatostatin-positive fibers and varicosities is observed in the outer two-thirds of the dentate gyrus molecular layer where the glutamatergic perforant path afferents from the entorhinal cortex terminate. To test for a functional interaction between these two pathways, we examined the effects of Cysteamine, which enhances somatostatin release for a few hours after administration but produces subsequent depletion of somatostatin lasting several days, on perforant path evoked potentials recorded in the dentate gyrus. Cysteamine (50–400 mg/kg, IP) increased the population spike dose-dependently both in anesthetized and in awake rats, but the slope of the population excitatory postsynaptic potential (EPSP) was left unchanged or even decreased. The antidromic population spike evoked by mossy fiber stimulation was not changed by cysteamine. The change is thought to be due to the increase in slope of the EPSP-spike relationship. In the hippocampal slice preparation, a similar effect of the drug (1–5 mM) on dentate evoked potentials was observed, suggesting that cysteamine acts through its effects on somatostatin in the hippocampus itself. In chronically implanted awake animals, the perforant path population spike was increased 1 h after cysteamine but returned to the predrug level by 24 h when somatostatin seemed to be depleted. These results suggest that hippocampal somatostatin released by cysteamine potentiates the response of dentate granule cells to perforant path input, without directly affecting synaptic transmission or general cell excitability.     

Quantitative analysis of long-term potentiation in the hippocampal dentate gyrus of the freely-moving 15-day-old rat

The magnitude and duration of long-term potentiation (LTP) of perforant path/dentate granule cell synapses was examined in freely moving rats beginning at 15 days of age. Measures of dentate granule cell population EPSP slope and population spike amplitude (PSA) obtained before and after tetanization were used to evaluate the level of LTP. Tetanization resulted insignificant enhancement of both the population EPSP slope (≈ +75%) and PSA (≈ +40%) measures. This enhancement was maintained without significant change for 18 h, after which both measures began a steady and continuous rise. Daily input/output response measures from age-matched nontetanized animals were used to factor out enhancement related to normal development. Under this schema, tetanization-induced enhancement of both EPSP slope and PSA measures decayed slowly, beginning 18–24 h after tetanization, returning to base-line 5 days after tetanization. Enhancement obtained from 90-day-old animals decayed to baseline 24 h after tetanization. The longer duration of LTP obtained from preweanlings is discussed with regard to the development of inhibitory systems modulating granule cell excitability.     

Field potential evidence for long-term potentiation of feed-forward inhibition in the rat dentate gyrus

Trains of high-frequency stimulation to the perforant path cause (i) long-term potentiation (LTP) of the population excitatory post-synaptic potential (EPSP), (ii) a lasting increase in the population spike, and (iii) a lasting alteration of the relationship between the EPSP and population spike (E-S relationship), consisting of a decreased x-intercept and decreased slope of the linear regression. To compare the thresholds of these changes, we applied a series of trains, increasing in duration from below LTP threshold. The EPSP potentiated with about the same low threshold as the reduction in E-S slope, whereas the reduction in E-S x-intercept required longer trains. In the second experiment, LTP of the EPSP was reduced by concurrent high-frequency stimulation of the commissural input and a lasting reduction of the population spike height was observed. In a third experiment, picrotoxin, an antagonist of gamma-aminobutyric acid (GABA)-mediated inhibition, blocked the decrease in slope of the E-S relationship which normally accompanies LTP. These results imply that perforant path/granule cell LTP is normally accompanied by long-term potentiation of a feed-forward inhibitory pathway which may involve interneurones.     

Medial amygdala stimulation produces a long-lasting excitatory postsynaptic potential/spike dissociation in the dentate gyrus in vivo

We have previously found that high-frequency stimulation of the medial amygdala (MeA) induces long-term potentiation (LTP) of the population spike in the perforant path-dentate granule cell synapses of anesthetized rats. In the present study, we investigated the influence of MeA stimulation on the relationship between the population excitatory postsynaptic potential (pEPSP) and population spike in the dentate gyrus. High-frequency stimulation of the MeA produced a leftward shift of the E–S curve, in which population spike amplitude was plotted against pEPSP slope at various stimulus intensities. MeA-induced population spike LTP was also observed under blockade of GABAergic inhibition with picrotoxin. These results suggests that MeA stimulation leads to a long-lasting change in the internal firing characteristics of the dentate granule cells.     

The influence of long-term potentiation on the spatial relationship between astrocyte processes and potentiated synapses in the dentate gyrus neuropil of rat brain

The influence of long-term potentiation (induced by repeated high-frequency stimulation of the perforant pathway) on the distribution pattern of astrocyte processes in the neuropil of the hippocampal dentate area containing the potentiated synapses was investigated by quantitative electronmicroscopy. It has been found that significant changes occurred in the ramification of astrocyte processes as well as in their topographic relation to synaptic complexes. When comparing the results obtained in LTP animals with active control or sham-operated animals, we found significant higher numerical density, but smaller volume, higher surface density and closer apposition of astrocyte processes to the synaptic clefts, boutons terminaux or spines in the potentiated synapses containing neuropil. The glial reaction to synaptic activation has been seen most pronounced 8 h after the LTP induction. The results are pointing to a participation of the glia cells in the maintenance of the LTP effect as well as to a metabolic coupling between synaptic transmission and glia function for equilibrating the homeostasis by clearing the extracellular space next to the transmission zones.     

Facilitation of kindling by prior induction of long-term potentiation in the perforant path

Previous studies have revealed that a form of synaptic potentiation resembling long-term potentiation (LTP) occurs at various sites as a result of stimulation that leads to kindling. The present study evaluates what role this synaptic potentiation plays in the development of kindling following periodic stimulation of the entorhinal cortex of the rat. LTP was repetitively induced in the pathway from the entorhinal cortex (EC) to the dentate gyrus (DG) by daily stimulation with high frequency trains that led to LTP, but did not evoke afterdischarge (AD). Subsequently, animals received stimulation designed to induce kindling (that led to AD), and this stimulation was delivered once per day until kindled seizures were induced. While repetitive induction of LTP was not sufficient to produce kindling, prior induction of LTP significantly increased the rate of subsequent kindling as evidenced by a decrease in the number of kindling stimulations required to induce the kindled state. As a group, animals that had received stimulation designed to induce LTP developed kindled seizures after an average of 10 AD's, whereas a control group that had received non-potentiating stimulation required 25 AD's. These results indicate that LTP at EC-DG synapses cannot represent the mechanism of kindling following EC stimulation. However, synaptic potentiation at this site can facilitate the development of epileptogenesis in response to subsequent activation of the perforant path.