Influence of epidermal growth factor on long-term potentiation in the hippocampal slice

Rat hippocampal slices were perfused with epidermal growth factor (EGF) at a concentration of 10(-8) M. EGF is a well known mitogen which exhibits neurotrophic action on neurons of the CNS. During extra- and intracellular recordings in the pyramidal cell body layer of the CA1-region no influence of EGF on evoked potentials was seen if single-pulse or paired-pulse stimulation was used. Furthermore EGF has no influence on the resting membrane potential of the cells investigated. However after tetanic stimulation a significant increase in the magnitude of long-term potentiation was observed. Therefore it is concluded, that EGF might be involved in modulation of neuronal plasticity.     

Involvement of S-100 protein in anoxic long-term potentiation

In in vitro rat hippocampal slices a short period (2 min) of hypoxia resulted in lasting potentiation of the population spike transynaptically evoked in CA1 by stimulation of Schaffer collaterals (“anoxic LTP”). Pretreatment of slices with antiserum against S-100 protein fully prevented this anoxic LTP. Since also “classical” (i.e., induced by high-frequency electrical stimulation) long-term potentiation is prevented by anti S-100 serum, this represents one more important similarity between these events.     

The duration of long-term potentiation in the CA1 region of the hippocampal slice preparation

The duration of long-term potentiation (LTP) of the monosynaptic excitatory Schaffer collateral-commissural input to hippocampal neurons of the CA1 region was examined in the in vitro slice. Relatively stable evoked potentials were obtained under conventional perfusion conditions at least for 10 hours. Tetanic stimulation (100 Hz, 1 sec) increased the population spike (pop-spike) amplitude by about 150% and the slope of the field-EPSP by about 30% over the pre-LTP baseline, whereas the latency and peak latency of the pop-spike decreased. In comparison to control experiments (same number of stimuli at 0.2 Hz) the differences were statistically significant for 2 hr (field-EPSP) and for greater than or equal to 10 hr (pop-spike), respectively. Repeated tetanization (3 X 100 Hz/1 sec), however, substantially prolongs EPSP-LTP (greater than or equal to 10 hr) and doubles the approximated half-life of pop-spike LTP. The threshold current intensity to elicit pop-spike responses decreased after the induction of LTP. Furthermore, the smaller field-EPSP values necessary to evoke near-threshold pop-spikes demonstrate an E-S potentiation (left-shift) at least in the low-intensity range. While the total duration of potentiation of the different parameters has not been determined, all the above mentioned effects could be observed at least 10 hr following the repeated tetanization. It is proposed that the slice preparation is suitable for the investigation of mechanisms of a postulated late phase of LTP if appropriate conditions are used.     

Acute alcohol blocks neurosteroid modulation of synaptic transmission and long-term potentiation in the rat hippocampal slice

Effects of ethanol (22 mM) on the modulation of synaptic transmission and long-term potentiation (LTP) by the neurosteroid dehydroepiandrosterome sulfate (DHEAS; 10 μM) was examined in the in vitro rat hippocampal slice preparation. The synaptic responses were elicited by Schaffer collateral stimulation and recorded extracellularly in the somatic and dendritic regions of CA1 pyramidal neurons. LTP induction produced an increase ( 55% to 75%) in the amplitude of synaptic responses in ethanol and ethanol plus DHEAS (ethanol/DHEAS) treated slices. These increases were significantly smaller than the 130% increase observed previously in slices treated with DHEAS, but were not significantly different from the 82% increase observed in control slices. These results indicate that an ethanol/DHEAS interaction prevents the enhancement of LTP normally observed with DHEAS treatment of hippocampal slices. An ethanol/DHEAS interaction also altered DHEAS's effects on individual synaptic components of the synaptic response to Schaffer collateral stimulation. Ethanol applied before but not after DHEAS prevented DHEAS's enhancement of the NMDA receptor-mediated synaptic component. DHEAS's depression of the GABA_A receptor-mediated synaptic component was also blocked by ethanol. Ethanol or DHEAS individually had no effect on the AMPA receptor-mediated synaptic component, but application of ethanol after DHEAS resulted in a small enhancement of this synaptic component, an effect that was not observed if ethanol was applied before DHEAS. These results show that ethanol and DHEAS interact, altering DHEAS's effects on synaptic transmission and LTP in the hippocampas. Such an interaction may be involved in ethanol's actions on the CNS and raises the possibility that ethanol and DHEAS may act via a common site or pathway.     

Diurnal modulation of long-term potentiation in the hamster hippocampal slice

Long-term potentiation (LTP) was examined in hippocampal slices from Syrian hamsters entrained to a LD 14:10 cycle. Population spike (PS) amplitudes from CA1 pyramidal cells were measured before (control) and after tetanizing the Schaffer/collateral commissural pathway. Slices from animals sacrificed during the day, between zeitgeber time (ZT) 0430 and 0530, were incubated, and then tetanized between ZT 1340 and 1930, where ZT=0 denotes lights on. Slices from animals sacrificed during the night, between ZT 1830 and 1930, were incubated, and tetanized between ZT 0030 and 0410. LTP, a sustained increase in PS amplitude following tetanus, was evoked in both groups. PS amplitude increased by 102.7+/-20.3% in animals sacrificed during the day and by 48.0+/-7.5% in animals sacrificed during the night (p<0.05). Thus hamster slices prepared during the day show more robust LTP (a doubling of PS amplitude), a difference persisting in slices incubated for several hours.     

Caffeine inhibits post-tetanic potentiation but does not alter long-term potentiation in the rat hippocampal slice

The effects of caffeine were investigated on the extracellular excitatory postsynaptic potentials (EPSPs) recorded in the stratum radiatum of CA₁ of the rat hippocampal slice in response to stimulation of the Schaffer collaterals. Caffeine in concentrations from 500 μM to 10 mM caused an increase in the amplitude of the EPSP, which reached a peak after 5–10 min perfusion. This increase was antagonized by pretreatment with 20 μM trifluoperazine. Paired-pulse facilitation, augmentation and potentiation were strongly inhibited by caffeine. Augmentation was most sensitive to caffeine, being abolished by 500 μM caffeine. Long-term potentiation (LTP) induced by high-frequency stimulation was not significantly inhibited by caffeine either by 5–10 min or by 60 min perfusion with 10 mM caffeine. Moreover, the caffeine-induced increase in the low-frequency EPSP could be reversed by 30 min washout, demonstrating that it was not an LTP type increase.     

Transient heterosynaptic depression in the hippocampal slice

Two independent, excitatory, monosynaptic afferent fiber systems projecting onto basal and apical dendritic layers of the CA1 sub-field of the hippocampus were tested for interactive effects. Long-term potentiation of either of the pathways was accompanied by a transient depression in the other pathway. Similar transient depression could be elicited by antidromic stimulation of the CA1 cells. No long-term heterosynaptic effects were observed.     

Effect of intermittent hypoxia on long-term potentiation in rat hippocampal slices

Intermittent hypoxia (IH) during sleep has been shown to induce apoptosis in a time-dependent manner and spatial learning deficits in adult rats. Recently, we have demonstrated that IH induced significant decreases in Ser-133-phosphorylated cAMP-response element-binding protein (pCREB) without changes in total CREB. The expression of cleaved caspase 3 in the hippocampal CA1, a marker of apoptosis, peaked at 3 days of IH and returned to normoxic values at 14 days of IH. In addition, biphasic changes in spatial task learning were correlated with the CREB phosphorylation time course. In the present study, the rat hippocampal slice preparation was used to evaluate the ability to induce and maintain a CA1 population spike long-term potentiation (PS-LTP) in room air (RA)-maintained and IH-exposed rats. A significant decrease in the ability to sustain PS-LTP for 15 min in slices prepared from IH-exposed rats for either 3 days (34% of total) or 7 days (51% of total) as compared to slices prepared from RA-maintained rats (76% of total) was observed. These results suggest that the diminishment in the ability of neuronal tissue to express and sustain PS-LTP is correlated with previously reported biphasic changes in CREB phosphorylation and programmed cell death.     

Involvement of postsynaptic G proteins in hippocampal long-term potentiation

The possible involvement of postsynaptic guanosine 5-triphosphate (GTP)-binding proteins (G proteins) in long-term potentiation (LTP) was studied in rat hippocampal slices, using whole-cell recording techniques. The inclusion of guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) or guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) in the recording pipette significantly reduced or abolished the baclofen-induced hyperpolarization of pyramidal neurons, which indicates uncoupling of the signal transduction from G protein-coupled receptors by these compounds. Both GDP beta S and GTP gamma S significantly attenuated the magnitude of LTP in the fimbria-CA3 synapses, but not in the mossy fiber-CA3 synapses. GTP gamma S did not attenuate LTP in the Schaffer-CA1 synapses. The effects of guanine nucleotide analogs on fimbrial LTP were reversed by postsynaptic depolarization during high frequency stimulation. These results suggest that postsynaptic G proteins may be involved in the generation of LTP in the fimbrial synapses, possibly by affecting membrane depolarization during high frequency afferent activation.     

A hypoxic injury potential in the hippocampal slice

In rat hippocampal slices, neurons in the stratum pyramidale of the CA1 were stimulated orthodromically and antidromically while the resultant extracellular population spikes were monitored. Hypoxic conditions were then induced. After disappearance of the orthodromic population spike, a second orthodromic population spike appeared. We have titled this the hypoxic injury potential since it reflects the onset of permanent injury to neurons in area CA1 of the hippocampus.     

Calcium-induced long-term potentiation in the hippocampal slice: Characterization of the time course and conditions

A transient increase in extracellular calcium concentration causes a long-lasting enhancement of radiatum fibers evoked excitatory postsynaptic potential and population spike responses of CA1 pyramidal neurons which resembles long-term potentiation (LTP). The duration of this potentiation is much longer than described previously and is probably limited by the survival of the preparation itself (greater than 8 hr). Therefore, Ca-induced LTP can be used for the investigation of a postulated late phase of LTP. Ca effects were activity-independent, since the subsequently evoked responses were facilitated even when the presynaptic fibers were not concurrently stimulated during or immediately after superfusion with the high Ca medium. In contrast, if too frequent testing of the synaptic input was done during the high Ca pulse, a short lasting depression instead of potentiation was observed. A lower extracellular magnesium concentration in the standard medium (1.3 instead of 2.0 mM MgSO4) prevents the potentiation of the EPSP at least for the first few hours. Presumably, both tetanus- and Ca-induced LTP share some common mechanisms, since an additional tetanization after Ca induction was not followed by an additional LTP. Compared to the potentiation following tetanization, the Ca-induced LTP was, however, not accompanied by a potentiation of the EPSP/spike ratio within the range of the population spike threshold intensity.     

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.     

Potassium-induced long-term potentiation in rat hippocampal slices

We observed that a transient increase in extracellular potassium concentration (50 mM for 40 s) was sufficient to induce long-term potentiation (LTP) of synaptic transmission in area CA1 of the hippocampal slice. Potassium-induced potentiation of the Schaffer collateral/commissural synapses demonstrated several features characteristic of tetanus-induced LTP: (1) population excitatory post-synaptic potential (EPSP) amplitudes were enhanced to a similar magnitude (on average 70% above baseline) which (2) lasted for more than 20 min; (3) induction was blocked by bath application of the specific N-methyl-d-aspartate (NMDA) receptor antagonistd-2-amino-5-phosphonovalerate (d-APV), and (4) was attenuated by reduction of the concentration of calcium in the extracellular medium. Induction of either potassium-induced LTP or tetanus-induced LTP occluded the subsequent expression of the other. Finally, exposure to high potassium in the absence of electrical stimulation was sufficient to induce LTP. Taken together, these data indicate that brief depolarizing stimuli other than tetanus can induce LTP. Because potassium-induced LTP is not restricted to the subset of afferents examined electrophysiologically, such a method could facilitate analyses of the biochemical events underlying both the induction and expression of LTP.     

Impairment of long-term potentiation and paired-pulse facilitation in rat hippocampal dentate gyrus following developmental lead exposure in vivo

Neonatal rats were exposed to lead from parturition to weaning via the milk of dams drinking 0.2% lead acetate solution. The alterations of long-term potentiation (LTP) and paired-pulse facilitation (PPF) of hippocampal dentate gyrus in adult rats (90–115 days) following developmental lead exposure were studied in vivo. Input/output (I/O) function, paired-pulse facilitation (PPF), excitatory postsynaptic potential (EPSP) and population spike (PS) amplitude were measured in the dentate gyrus (DG) in response to stimulation applied to the lateral perforant path. The results showed that LTP was induced in control rats with an average PS potentiation of 321.1±50.0% (n=18), which was significantly greater than the increase in PS potentiation (173.5±30.0%, n=17, p<0.001) in lead-exposed rats after tetanizing stimulation. The mean EPSP potentiation increased to 172.4±27.0% (n=18) in control and 138.8±21.4% (n=17) in lead-exposed rats after tetanizing stimulation. The lead-induced impairment of LTP of PS potentiation was more serious than that of EPSP potentiation. Following pairs stimulation of perforant fiber at 250 μA and an interpulse interval (IPI) of 10–1000 ms, the average peak facilitation of PS was 211.3±25.0% (n=13) in control and 187.7±23.0% (n=11) in lead-exposed rats. The average facilitation period duration of PS was 243.0±35.8 ms (n=13) in control and 138.0±24.4 ms (n=11) in lead-exposed rats. These results suggested that developmental lead exposure in neonatal rats caused impairments in LTP and PPF of hippocampal dentate gyrus.     

Taurine rescues hippocampal long-term potentiation from ammonia-induced impairment

Hyperammonemia, a major pathophysiological factor in hepatic encephalopathy, impairs long-term potentiation (LTP) of synaptic transmission, a cellular model of learning and memory, in the hippocampus. We have now studied the protective action of taurine on this paradigm by analyzing LTP characteristics in mouse hippocampal slices treated with ammonium chloride (1 mM) in the presence of taurine (1 mM), an ubiquitous osmolyte, antioxidant, and neuromodulator, as well as other substances with such properties. Ammonia-treated slices displayed a significant impairment of LTP maintenance. Taurine and the mitochondrial enhancer l-carnitine, but not the antioxidants (ascorbate, carnosine, and the novel compound GVS-111) or the osmolyte betaine prevented this impairment. The protective effect of taurine was preserved under the blockade of inhibitory GABA_A and glycine receptors. It is suggested that taurine may rescue the mechanisms of hippocampal synaptic plasticity by improving mitochondrial function under hyperammonemic conditions.     

Thermal effects on long-term potentiation in the hamster hippocampus

Extracellular CA1 pyramidal cell activity was measured at different temperatures in hippocampal slices from the Syrian hamster (Mesocricetus auratus), a hibernator. Control records taken before and after tetanic stimulation of Schaffer collateral/commissural pathways were compared to determine if long-term potentiation (LTP) was established. LTP (an enhancement of the population spike amplitude or population synaptic response following tetanus) was elicited in slices at temperatures above 22 °C, but not in slices at temperatures of 20 °C. When LTP was established at temperatures above 24 °C, however, lowering the temperature to 20 °C did not abolish the LTP. Furthermore, when a tetanus was delivered at 20 °C and the bath temperature was then raised above 22 °C, LTP was established. These results for step changes in temperature suggest that the sequence of cellular mechanisms leading to LTP is activated, but then arrested in slices maintained at a constant temperature of 20 °C. Assuming this type of activity in the slice parallels in vivo hippocampal activity, it follows that the ability to elicit LTP in CA1 hippocampal pyramidal cells is lost when the core temperature of an animal entering hibernation falls to 20 °C.     

Alterations in calmodulin content in fractions of rat hippocampal slices during tetanic- and calcium-induced long-term potentiation

The content of cytosolic and membrane-bound calmodulin was radioimmunologically determined in fractions of rat hippocampal slices 5 min to 7 hours after long-term potentiation (LTP) had been induced by tetanization or exposure of slices to 4 mM Ca++. In light of concepts presuming multistage dynamics in LTP development as reflecting different cellular mechanisms, similar patterns of calmodulin alterations were observed with both models: The alterations in calmodulin content occurred during the early phase(s) of LTP development and continued for two and one hours during tetanic- and calcium-induced LTP, respectively. Thus, 5-30 min after LTP elicitation, membrane-bound calmodulin increased while cytosolic calmodulin diminished and, inversely, 30 min later an increase in cytosolic and decrease in membrane-bound calmodulin were observed. Consequently, the present results indicate that calmodulin was involved in the early phases(s) of LTP development in terms of a two-step translocation sequence. Hence, calmodulin translocation within both intracellular compartments may reflect the involvement of Ca++-calmodulin-dependent intraneuronal metabolic processes which might induce and/or temporarily maintain neuronal functional changes occurring immediately after repeated or intense stimulation of synaptic functions.     

Presynaptic ultrastructural correlates of long-term potentiation in the CA1 subfield of the hippocampus

To determine if long-term potentiation (LTP) is accompanied by changes in the ultrastructural distribution of calcium within presynaptic terminals, calcium was localized at the electron microscopic level using an oxalate/pyroantimonate histochemical technique. Following the induction of LTP at the Schaffer collateral/commissural synapses in the CA1 subfield of the rat hippocampal slice, there was a significant decrease (30%) in the percentage of synaptic vesicles containing calcium deposits. This effect could be accounted for by both a significant reduction in the average number of calcium deposit-bearing vesicles and a significant increase in the average number of synaptic vesicles per terminal profile in slices that displayed LTP. These changes persisted for at least one hour following the induction of LTP and were not observed in slices that received high-frequency stimulation in the presence of the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid (APV, 50 microM), which blocked LTP. These data suggest that LTP may be accompanied by long-term changes in intraterminal calcium homeostasis and the number of synaptic vesicles. These effects may be related to the reported increase in transmitter release following the induction of LTP.     

Pharmacological treatment dose-relatedly improves learning and hippocampal long-term potentiation

In the CA1 region of hippocampal slices prepared from guinea-pigs, which had been orally treated with sabeluzole, long-term potentiation was significantly enhanced in those slices obtained from animals given 2.5 or 10.0 mg/kg. In an active avoidance task oral treatment of guinea-pigs with 2.5 or 10.0 mg/kg of sabeluzole significantly increased learning. The dose of 0.63 mg/kg was inactive in both paradigms. These results provide new evidence supporting the involvement of LTP in learning.