Stimulus parameters affecting paired-pulse depression of dentate granule cell field potentials. I. Stimulus intensity

NMDA receptor-dependent synaptic plasticity in dorsal and intermediate hippocampus exhibits distinct frequency-dependent profiles

The hippocampus may be functionally differentiated along its dorsoventral axis. In contrast to the wealth of data available on synaptic plasticity mechanisms in the dorsal hippocampus, little is known about synaptic plasticity processes in the intermediate hippocampus. Behavioral data suggest that this structure may play a distinct role in learning and memory. Here, we compared amplitudes, frequency-dependency and persistency of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal (DDG) and intermediate dentate gyrus (IDG). In freely moving rats, high-frequency stimulation (HFS) at 200 Hz (10 burst of 15 stimuli) elicited LTP of similar magnitude in both structures that persisted for over 24 h. The intermediate dentate gyrus is more likely to exhibit persistent LTP than its dorsal counterpart, however: HFS at 200 Hz (3 or 1 burst(s)) or 100 Hz elicited short-term potentiation (STP) in DDG, unlike in the IDG, where LTP could be recorded for at least 4 h. Whereas low frequency stimulation (LFS) at 1 Hz elicited long-lasting LTD (>24 h) in the DDG, it had no significant effect on fEPSP profile in the IDG. LFS at 2 Hz elicited short-term depression in DDG and had no effect in IDG. LTP in both IDG and DDG required activation of N-methyl-D-aspartate receptors. Paired-pulse and input-output responses differed in IDG and DDG. Our data suggest that afferent input from the entorhinal cortex generates a different response profile in the dorsal vs. intermediate DG, which may in turn relate to their postulated distinct roles in synaptic information processing and memory formation. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Impaired in vivo synaptic plasticity in dentate gyrus and spatial memory in juvenile rats induced by prenatal morphine exposure

Prenatal morphine exposure induces neurobiological changes, including deficits in learning and memory, in juvenile rat offspring. However the effects of this exposure on hippocampal plasticity, which is critical for learning and memory processes, are not well understood. The present study investigates the alterations of spatial memory and in vivo hippocampal synaptic plasticity in juvenile rats prenatally exposed to morphine. On gestation days 11-18, pregnant rats were randomly chosen to be injected twice daily with morphine or saline. Each juvenile offspring (postnatal day 22-31) performed one two-trial Y-maze task to evaluate spatial memory. Afterwards, the in vivo field excitatory postsynaptic potential (fEPSP) and population spike (PS) were recorded in the perforant path dentate gyrus (DG) pathway in the hippocampus. Prenatal morphine exposure reduced depotentiation (DP), but not long-term potentiation (LTP), of the EPSP slope. However, both LTP and DP of the EPSP slope were depressed in prenatal morphine-exposed juvenile offspring. The morphine group also showed poorer performance for the Y-maze task than the control group. Depressed PS LTP, but not EPSP LTP, in the morphine group suggested that prenatal morphine exposure changed GABAergic inhibition, which mediates EPSP-spike potentiation. Then a loss of GABA-containing neurons in the DG area of the morphine group was observed using immunohistochemistry. Taken together, our results suggest that prenatal morphine exposure impairs the juvenile offspring's dentate synaptic plasticity and spatial memory, and that decreased GABAergic inhibition may play a role in these effects. These findings might contribute to an explanation for the cognitive deficits in children whose mothers abuse opiates during pregnancy.

Developmental exposure to perchlorate alters synaptic transmission in hippocampus of the adult rat

BACKGROUND

Perchlorate is an environmental contaminant that blocks iodine uptake into the thyroid gland and reduces thyroid hormones. This action of perchlorate raises significant concern over its effects on brain development.

OBJECTIVES

The purpose of this study was to evaluate neurologic function in rats after developmental exposure to perchlorate.

METHODS

Pregnant rats were exposed to 0, 30, 300, or 1,000 ppm perchlorate in drinking water from gestational day 6 until weaning. Adult male offspring were evaluated on a series of behavioral tasks and neurophysiologic measures of synaptic function in the hippocampus.

RESULTS

At the highest perchlorate dose, triiodothyronine (T(3)) and thyroxine (T(4)) were reduced in pups on postnatal day 21. T(4) in dams was reduced relative to controls by 16%, 28%, and 60% in the 30-, 300-, and 1,000-ppm dose groups, respectively. Reductions in T(4) were associated with increases in thyroid-stimulating hormone in the high-dose group. No changes were seen in serum T(3). Perchlorate did not impair motor activity, spatial learning, or fear conditioning. However, significant reductions in baseline synaptic transmission were observed in hippocampal field potentials at all dose levels. Reductions in inhibitory function were evident at 300 and 1,000 ppm, and augmentations in long-term potentiation were observed in the population spike measure at the highest dose.

CONCLUSIONS

Dose-dependent deficits in hippocampal synaptic function were detectable with relatively minor perturbations of the thyroid axis, indicative of an irreversible impairment in synaptic transmission in response to developmental exposure to perchlorate.

Modeling the nonlinear properties of the in vitro hippocampal perforant path-dentate system using multielectrode array technology

A modeling approach to characterize the nonlinear dynamic transformations of the dentate gyrus of the hippocampus is presented and experimentally validated. The dentate gyrus is the first region of the hippocampus which receives and integrates sensory information via the perforant path. The perforant path is composed of two distinct pathways: 1) the lateral path and 2) the medial perforant path. The proposed approach examines and captures the short-term dynamic characteristics of these two pathways using a nonparametric, third-order Poisson-Volterra model. The nonlinear characteristics of the two pathways are represented by Poisson-Volterra kernels, which are quantitative descriptors of the nonlinear dynamic transformations. The kernels were computed with experimental data from in vitro hippocampal slices. The electrophysiological activity was measured with custom-made multielectrode arrays, which allowed selective stimulation with random impulse trains and simultaneous recordings of extracellular field potential activity. The results demonstrate that this mathematically rigorous approach is suitable for the multipathway complexity of the hippocampus and yields interpretable models that have excellent predictive capabilities. The resulting models not only accurately predict previously reported electrophysiological descriptors, such as paired pulses, but more important, can be used to predict the electrophysiological activity of dentate granule cells to arbitrary stimulation patterns at the perforant path.

Modeling the nonlinear dynamic interactions of afferent pathways in the dentate gyrus of the hippocampus

The dentate gyrus is the first region of the hippocampus that receives and integrates sensory information (e.g., visual, auditory, and olfactory) via the perforant path, which is composed of two distinct neuronal pathways: the Lateral Perforant Path (LPP) and the Medial Perforant Path (MPP). This paper examines the nonlinear dynamic interactions among arbitrary stimulation patterns at these two afferent pathways and their combined effect on the resulting response of the granule cells at the dentate gyrus. We employ non-parametric Poisson-Volterra models that serve as canonical quantitative descriptors of the nonlinear dynamic transformations of the neuronal signals propagating through these two neuronal pathways. These Poisson-Volterra models are estimated in the so-called "reduced form" with experimental data from in vitro hippocampal slices and provide excellent predictions of the electrophysiological activity of the granule cells in response to arbitrary stimulation patterns. The data are acquired through a custom-made multi-electrode-array system, which stimulated simultaneously the two pathways with random impulse trains and recorded the neuronal postsynaptic activity at the granule cell layer. The results of this study show that significant nonlinear interactions exist between the LPP and the MPP that may be critical for the integration of sensory information performed by the dentate gyrus of the hippocampus.

Thyroid hormone insufficiency during brain development reduces parvalbumin immunoreactivity and inhibitory function in the hippocampus

Thyroid hormones are necessary for brain development. gamma-Amino-butyric acid (GABA)ergic interneurons comprise the bulk of local inhibitory circuitry in brain, many of which contain the calcium binding protein, parvalbumin (PV). A previous report indicated that severe postnatal hypothyroidism reduces PV immunoreactivity (IR) in rat neocortex. We examined PV-IR and GABA-mediated synaptic inhibition in the hippocampus of rats deprived of thyroid hormone from gestational d 6 until weaning on postnatal d 30. Pregnant dams were exposed to propylthiouracil (0, 3, 10 ppm) via the drinking water, which decreased maternal serum T(4) by approximately 50-75% and increased TSH. At weaning, T(4) was reduced by approximately 70% in offspring in the low-dose group and fell below detectable levels in high-dose animals. PV-IR was diminished in the hippocampus and neocortex of offspring killed on postnatal d 21, an effect that could be reversed by postnatal administration of T(4). Dose-dependent decreases in the density of PV-IR neurons were observed in neocortex and hippocampus, with the dentate gyrus showing the most severe reductions (50-75% below control counts). Altered staining persisted to adulthood despite the return of thyroid hormones to control levels. Developmental cross-fostering and adult-onset deprivation studies revealed that early postnatal hormone insufficiency was required for an alteration in PV-IR. Synaptic inhibition of the perforant path-dentate gyrus synapse evaluated in adult offspring, in vivo, revealed dose-dependent reductions in paired pulse depression indicative of a suppression of GABA-mediated inhibition. These data demonstrate that moderate degrees of thyroid hormone insufficiency during the early postnatal period permanently alters interneuron expression of PV and compromises inhibitory function in the hippocampus.

Anesthetized Long Evans rats show similar protein expression and long-term potentiation as Fischer 344 rats but reduced short-term potentiation in motor cortex

A number of studies describe strain-related differences in the motor behavior of rats. Inbred albino F344 rats are found to be impaired in procedural spatial learning, skilled reaching, and over ground locomotion in relation to pigmented out bred Long Evans (LE) rats. These deficits could be related to the functional differences in the motor cortex of the two strains, and the objective of the present study was to examine this hypothesis. Synaptic transmission was examined in the two rat strains, using long-term potentiation (LTP) and short-term potentiation (STP), two electrophysiological measures of neural function and learning. Field potentials were evoked in the motor cortex of anesthetized Long Evans and Fischer 344 (F344) rats in response to contralateral white matter stimulation. The main findings indicated that (1) baseline-evoked responses in the two strains was similar, indicating similar basal levels of synaptic strength, (2) LTP was induced in both strains of rats, suggesting similar synaptic efficacy in the two strains of rats, and (3) STP was enhanced in the Fischer 344 rats, suggesting differences in synaptic function. Protein expression also revealed that the two strains did not differ with respect to structural or synaptic protein expression. Thus, the two strains exhibit motor skill differences despite a great degree of physiological similarity in motor cortex. The results are discussed in relation to the greater utility of using the Long Evans rat for examining the neural basis of plasticity and models of disease, especially if motor tasks are evaluated.

Perinatal exposure to polychlorinated biphenyls alters excitatory synaptic transmission and short-term plasticity in the hippocampus of the adult rat

Developmental exposure to polychlorinated biphenyls (PCBs) has been associated with cognitive deficits in humans and laboratory animals. Previous work has demonstrated a reduced capacity to support long-term potentiation (LTP) in animals exposed to a PCB mixture, Aroclor 1254 (A1254) via the dam in utero and throughout the preweaning period [Brain Res. 850;1999:87-95; Toxicol. Sci. 57;2000:102-11]. Assessment of normalized input/output (I/O) functions collected prior to LTP induction failed to reveal consistent differences in baseline synaptic transmission between control and PCB-exposed groups. The present study was designed to systematically evaluate excitatory and inhibitory synaptic transmission using a more extensive I/O analysis and paired pulse functions to assess short-term plasticity. Pregnant Long-Evans rats were administered either corn oil (control) or 6 mg/kg per day of A1254 by gavage from gestational day (GD) 6 until pups were weaned on postnatal day (PND) 21. In adult male offspring (5-11 months of age), field potentials evoked by perforant path stimulation were recorded in the dentate gyrus under urethane anesthesia. Detailed I/O functions were assessed by averaging the responses evoked in the dentate gyrus to stimulus pulses delivered to the perforant path in an extensive ascending intensity series. Population spike (PS) and postsynaptic potential (PSP) amplitudes recorded in the dentate gyrus were significantly enhanced in PCB-exposed animals relative to controls at midrange intensities. No group differences were observed in EPSP slope amplitudes. Short-term plasticity was assessed by delivering pairs of stimulus pulses at interpulse intervals (IPIs) ranging from 10 to 70 ms. In the dentate gyrus this range of intervals activates both inhibitory and excitatory mechanisms leading to a pattern of depression at brief intervals (<30 ms) followed by facilitation as the interval between pulses is extended. Paired pulse depression was decreased at an intermediate IPI (30 ms) with submaximal stimulus intensities. These data augment previous work demonstrating persistent changes in hippocampal plasticity as a result of exposure to PCBs during development. Furthermore, as increases in field potential amplitudes were observed, these findings support previous conclusions that A1254-induced LTP deficits are not readily attributable to reductions in synaptic excitability. Thus, in addition to impairment in use-dependent synaptic plasticity reported previously, the present report reveals that basic components of information processing within the hippocampus are permanently altered as a result of perinatal exposure to PCBs.

Matrix metalloproteinase inhibition alters functional and structural correlates of deafferentation-induced sprouting in the dentate gyrus

Molecules comprising the extracellular matrix (ECM), and the family of matrix metalloproteinases (MMPs) that regulate them, perform essential functions during neuroplasticity in both developing and adult nervous systems, including substrate guidance during neuritogenesis and the establishment of boundaries for axonal terminal fields. MMP proteolysis of ECM molecules may perform a permissive or inductive role in fiber remodeling and synaptogenesis initiated by deafferentation. This study examined functional and structural effects of MMP inhibition during the early phases of deafferentation-induced sprouting, characterizing components of the degeneration/proliferation cycle that may be dependent on MMP activity. Adult rats received unilateral lesions of the entorhinal cortex to induce collateral sprouting of the crossed temporodentate fiber pathway. This was followed by intraventricular infusion of the MMP inhibitor FN-439 (2.9 mg/kg) or saline vehicle. After 7 d postlesion, rats underwent in vivo electrophysiological recording or histological processing for electron microscopic analysis. Lesioned rats receiving vehicle exhibited normal sprouting and synaptogenesis, with the emergence of the capacity for long-term potentiation (LTP) within the sprouting pathway, and the successful clearance of degenerating terminals with subsequent synaptic proliferation. In contrast, lesioned rats receiving the MMP inhibitor failed to develop the capacity for LTP and showed persistent cellular debris. Current source density analysis also revealed an FN-439-induced disruption of the current sink, normally localized to the middle region of the granule cell dendrites, corresponding to the terminal field of the crossed temporodentate fibers. These results establish a role for MMP-dependent processes in the deafferentation/sprouting cycle.

Levetiracetam has no significant gamma-aminobutyric acid-related effect on paired-pulse interaction in the dentate gyrus of rats

Gamma-aminobutyric acid (GABA)ergic mechanisms of the novel antiepileptic drug, levetiracetam (Keppra), have been both favored and rejected. Since paired-pulse interaction is accepted in functionally assessing GABAergic mechanisms, we investigated whether levetiracetam affects the paired-pulse inhibition/facilitation of the field potentials, evoked in the dentate gyrus of urethane-anesthesized rats. This model revealed a strong paired-pulse inhibition at 20-ms interstimulus interval, a noteworthy paired-pulse facilitation at 80-ms interstimulus interval, and a moderate paired-pulse inhibition at 500-ms interstimulus interval. Bicuculline (3 mg/kg/h, i.v.) and baclofen (10 mg/kg, i.v.) markedly depressed paired-pulse inhibition at 20-ms interstimulus interval, while clonazepam (1 mg/kg, i.p.), diazepam (10 mg/kg, i.v.), and phenobarbital (40 mg/kg, i.v.) enhanced it. Bicuculline also depressed paired-pulse inhibition at 500-ms interstimulus interval. Bicuculline, baclofen, and diazepam reduced paired-pulse facilitation at 80-ms interstimulus interval. Distinct from these GABA(A) receptor- and GABA(B) receptor-related drugs, levetiracetam (17 and 540 mg/kg, i.v.) had no significant effect on either paired-pulse interaction in this model, a result not favoring any major role of GABAergic mechanisms in its antiseizure action.

Differential effects of short- and long-term potentiation on cell firing in the CA1 region of the hippocampus

Long-term potentiation (LTP) in the hippocampus enhances the ability of a stimulus to produce cell firing, not only by increasing the strength of the EPSPs, but also by increasing the efficiency of the input/output (I/O) function of pyramidal neurons. This means that EPSPs of a given size more easily elicit spikes after LTP, a process known as EPSP-spike (E-S) potentiation. In contrast to LTP, it is not known whether the synaptic strengthening produced by paired-pulse facilitation (PPF) also results in changes in the I/O function. We have addressed this question by examining E-S curves from rat hippocampal area CA1 in response to both PPF and LTP. We describe a novel form of I/O modulation in which PPF produces E-S depression; that is, the E-S curve is shifted to the right, indicating a decreased ability of EPSPs to elicit action potentials. Consistent with the notion that E-S potentiation observed with LTP is caused by long-term increases in the excitatory-inhibitory ratio, we show that PPF-induced E-S depression relies on short-term decreases in this ratio. These results indicate that different forms of synaptic plasticity that produce the same degree of EPSP potentiation can result in dramatically different effects on cell firing, because of the dynamic changes in the excitatory-inhibitory balance within local circuits.

Reduced inhibition in an animal model of cortical dysplasia

Cortical dysplasia has a strong association with epilepsy in humans, but the underlying mechanisms for this are poorly understood. In utero irradiation of rats produces diffuse cortical dysplasia and neuronal heterotopia in the neocortex and hippocampus. Using in vitro neocortical slices, whole-cell patch-clamp recordings were obtained from pyramidal neurons in dysplastic cortex and control neocortex. Spontaneous IPSCs were reduced in amplitude (35%) and frequency (70%) in pyramidal cells from dysplastic cortex. Miniature IPSCs were reduced in frequency (66%) in dysplastic cortex. Two additional measures of cortical inhibition, monosynaptic evoked IPSCs and paired pulse depression of evoked EPSCs, were also impaired in dysplastic cortex. Spontaneous EPSCs were increased in amplitude (42%) and frequency (77%) in dysplastic cortex, but miniature EPSCs were not different between the two groups. These data demonstrate significant physiological impairment in inhibitory synaptic transmission in experimental cortical dysplasia. This supports previous immunohistochemical findings in this model and observations in humans of a reduction in the density of inhibitory interneurons in dysplastic cortex.

Modulation of paired-pulse responses in the dentate gyrus: effects of prenatal protein malnutrition

Since our major hypothesis is that prenatal protein malnutrition significantly affects hippocampal neuroplasticity, this study examined the effects of prenatal protein malnutrition on the modulation of dentate granule cell excitability in freely moving rats at 15, 30 and 90 days of age across the vigilance states of quiet waking (QW), slow-wave sleep (SWS) and rapid eye movement (REM) sleep. Using paired-pulse stimulation, the paired-pulse index (PPI), a measure of the type and degree of modulation of dentate granule cell excitability elicited by stimulation of the medial perforant path, was obtained for each vigilance state at each stage of development. Four specific measures of granule cell excitability were computed, namely, PPI using both population spike amplitude (PSA) and EPSP slope measures, absolute values of PSA(1) and EPSP(1) slope. PPI values obtained at 15, 30 and 90 days of age, however, were altered during normal ontogenetic development, but not by vigilance state. At 15 days of age, the malnourished group exhibits greater early inhibition of the PPI using the PSA measure at IPIs between 20 and 30 ms regardless of vigilance state, while at 30 days of age, the malnourished group exhibits greater facilitation at IPIs between 50 and 70 ms during QW and SWS, but not during REM sleep. In the control adult (PND90) and juvenile (PND30) animal, PSA(1) values are significantly higher during SWS than in QW or REM sleep. However, for the younger malnourished animals (PND15 and PND30), PSA(1) values were found to be significantly greater during REM sleep rather than SWS. Therefore, as the animal matures, there appears to be a shift in vigilance state dependent synaptic transmission through the hippocampal trisynaptic circuit from REM sleep to SWS in both control and malnourished animals, with the change occurring later in malnourished animals when compared to control ones. Furthermore, our findings suggests that prenatal protein malnutrition significantly alters modulation of dentate granule cell excitability (i.e., PPI values using the PSA measure) during the earlier stages of development but not in adulthood.

Field potential recordings in dentate gyrus of anesthetized rats: stability of baseline

Urethane is a standard anesthetic utilized for in vivo recordings in the hippocampus. In studies of long-term potentiation (LTP), the measure of interest is the response amplitude minutes to hours following train delivery. In the absence of experimental treatment, we have consistently observed upward drift in the amplitude of the population spike (PS) and EPSP slope of the dentate gyrus (DG) evoked field response in acute surgical preparations performed in the urethanized rat. The present study systematically monitored PS amplitude and EPSP slope in the DG every 30 minutes for 6 hours following optimal positioning of Teflon-coated bipolar stainless steel electrodes under urethane anesthesia. At maximal stimulus intensities, large time-dependent increases in PS amplitude (70-80%) were observed over the first 2-4 hours, an effect that was exaggerated at lower stimulus intensities. Increases in the EPSP slope were smaller in magnitude (20-30%) and stabilized within a shorter period of time (1-2 hours). Animals were warmed on a heating pad and body and brain temperature remained constant over the recording session. Reducing stimulating electrode size and recording with glass micropipettes did not alleviate the upward drift in response amplitude. Similar increases were also seen under pentobarbital anesthesia. To dissociate anesthetic from surgical effects, recordings were obtained from animals previously prepared with indwelling electrodes and injected with urethane. Although slight declines (10-15%) in EPSP slope occurred over time, no significant alterations in PS amplitude were seen in the chronic preparation at high stimulus intensities. Low stimulus intensities yielded a more variable response pattern and, in direct contrast to the acute preparation, time-dependent declines, not increases, were noted in both parameters. These data suggest that generalized surgical trauma contributes to the upward drift in response amplitude and indicate that long stabilization periods are required in acute surgical preparations for accurate field potential recordings.

Effects of central and peripheral administration of kynurenic acid on hippocampal evoked responses in vivo and in vitro

Kynurenic acid is an excitatory amino acid antagonist with preferential activity at the N-methyl-D-aspartate subtype of glutamate receptors. It is produced endogenously in the brain, but is synthesized more effectively in the periphery. The influence of peripheral kynurenic acid on brain function is unclear because kynurenic acid is likely to penetrate the blood-brain barrier poorly. To determine the potential central effects of peripheral kynurenic acid, we compared its effects in the hippocampus after peripheral or direct administration. The hippocampus of the rat was chosen as a test system because this region receives glutamatergic inputs, and because responses to stimulation of these inputs can be compared after peripheral drug administration in vivo, and after direct administration of drugs in vitro. Peripherally-administered kynurenic acid was injected via a catheter in the jugular vein. Bath-application to hippocampal slices was used to test effects of direct administration. Area CA1 pyramidal cells and dentate gyrus granule cells were examined by extracellular recording and stimulation of area CA3 or the perforant path, respectively. Pairs of identical stimuli were used to assess paired-pulse inhibition and paired-pulse facilitation. Kynurenic acid decreased evoked responses in area CA1 and the dentate gyrus, both in vivo and in vitro. Effective concentrations were in the low micromolar range, and therefore were likely to be mediated by antagonism of N-methyl-D-aspartate receptors. In both preparations, area CA1 was more sensitive than the dentate gyrus, and paired-pulse facilitation was affected, but not paired-pulse inhibition. Control solutions had no effect. We conclude that kynurenic acid can enter the brain after peripheral administration, and that peripheral and direct effects in the hippocampus are qualitatively similar. Therefore, we predict that effects of endogenous kynurenic acid that was synthesized peripherally or centrally would be similar. Furthermore, the results suggest that modulation of the glycine site of the N-methyl-D-aspartate receptor, for example by kynurenic acid, may vary considerably among different brain areas.

Selective loss of early suppression in the dentate gyrus precedes kainic acid induced electrographic seizures

The role of inhibitory and facilitatory processes in the induction of seizures was studied in a kainic acid (KA) model of epilepsy. The dentate gyrus (DG) response to paired-pulse stimulation of the perforant path (PP) was monitored prior to and immediately following the initial KA induced afterdischarge (AD) in rats chronically prepared with stimulation recording electrodes. The subjects received a 1-h program of stimulation consisting of repeated sequences of pulse pairs at a short (20-30 ms), intermediate (45-90 ms), and long (200-300 ms) interpulse interval (IPIs). The stimulation program was administered both under control conditions and immediately following systemic injection of KA. During the control condition, stable suppression of population spike measures was obtained at the short (early phase) and long (late phase) IPIs, while facilitation was observed at the intermediate IPI. Administration of KA resulted in a progressive loss of suppression prior to the initial AD at the short IPI; neither facilitation nor the late phase of suppression were significantly affected. The early phase decreased further following the initial discharge. Since the early phase most likely reflects recurrent inhibition, these results provide evidence that inhibitory loss precedes the occurrence of KA induced AD, and that this inhibitory loss is increased further following the initial evoked AD. A use-dependent disinhibition is one possible explanation for the change in responsiveness that precedes the AD. This disinhibition could result from a depressed response at GABA-A receptors, an increased responsiveness at GABA-B receptors or possibly both.

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% Pb(2+)-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 (tau) 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.

Enhancement of paired-pulse depression in the dentate gyrus in vivo by the NMDA antagonist, MK-801, and electrical kindling

We have previously demonstrated that late paired-pulse depression of dentate granule cell field potentials decreases when stimulus intensity is increased from moderate to high levels. Voltage-dependent N-methyl-D-aspartate (NMDA) currents are increasingly activated within this stimulus range, and are enhanced following the development of kindled seizures. The NMDA antagonist, MK-801 (0.25 and 1.0 mg/kg, i.p.), was used in the present experiments to evaluate the contribution of NMDA currents to the loss of late paired-pulse depression at high stimulus intensities in naive and kindled rats. Paired-pulse stimulus intensity functions were obtained from animals prepared with chronic electrodes in the perforant path and dentate gyrus. MK-801 administration had no effect on the stimulus intensity function for early paired-pulse depression (20-30 ms interpulse intervals, IPI) in either preparation. Late paired-pulse depression (150-500 ms IPI) was significantly enhanced in naive rats by MK-801. In contrast, MK-801 had no effect on the potentiation of late paired-pulse depression recorded from kindled animals. These findings suggest that the ability of NMDA currents to reduce the strength of late paired-pulse depression in naive animals is altered following the development of kindled seizures. A decrease in late paired-pulse depression was observed at high stimulus intensities under all experimental conditions. The latter findings indicate that the processes responsible for the reduction in late paired-pulse depression at high stimulus intensities are unaffected by either NMDA or kindling-induced modulation of late paired-pulse depression.

Changes in dentate granule cell field potentials during afterdischarge initiation triggered by 5 Hz perforant path stimulation

A failure of early paired pulse depression often precedes the onset of intermittent spontaneous seizures in animal models of status epilepticus. In the present study, changes in the strength of early and late paired pulse depression of dentate granule cell field potentials were compared in the unanesthetized rat during the initiation of a single afterdischarge (AD) evoked by perforant path stimulation (0.1 ms pulse duration, 5 Hz, 12-18 s duration, 50-1000 microA). Late paired pulse depression was measured by sequential changes in the population spike (PS) amplitude during 5 Hz stimulation (200 ms interpulse interpulse interval, IPI). When 5 Hz stimulation triggered an AD, the population spike (PS) was initially depressed and then increased to above pre-train values, indicating a loss of late paired pulse depression by the middle of the train. Early paired pulse depression was measured by inserting paired pulses (20 ms IPI) at spaced intervals throughout the 5 Hz train. In contrast to late paired pulse depression, early paired pulse depression remained at maximum strength until an abrupt failure was detected coincident with AD initiation. Two experimental treatments shown to increase the strength of late paired pulse depression, administration of the N-methyl-D-aspartate antagonist, MK-801 (0.25 mg/kg, i.p.), and the development of kindled seizures, produced an increase in AD thresholds and in the initial depression in the PS amplitude during 5 Hz stimulation. Together, these results suggest that a failure of late paired pulse depression may be a precipitating event in AD initiation triggered by 5 Hz stimulation in the unanesthetized rat.

Stimulus parameters affecting paired-pulse depression of dentate granule cell field potentials. II. Low-frequency stimulation

Low frequency (1 Hz) stimulation of the perforant path produces a depression in the population spike (PS) of dentate granule cell field potentials and also may affect the strength of paired pulse depression. The effects of 1 Hz stimulation (30 s train) on paired pulse depression (20 and 200 ms interpulse intervals, IPI) were evaluated in the unanesthetized rat under two conditions: (i) when the stimulus intensity of both pulses was increased simultaneously (5-100%); and (ii) when the stimulus intensity of the first (conditioning) pulse was increased (5-100%), while the stimulus intensity of the second (test) pulse was held constant (50%). The test PS amplitude was predicted based upon either the conditioning PS amplitude at the end of the 1 Hz train or upon the additive effects of paired pulse depression and 1 Hz stimulation. These predicted values then were assessed for the best fit to observed values following 1 Hz trains. Under both stimulus conditions, the 1 Hz depression in the conditioning PS amplitude exhibited characteristics that were identical to late paired pulse depression recorded before the train. A decrease in the test PS amplitude also was observed following 1 Hz stimulation at the 20 and 200 ms IPIs. The best fit to observed values of the test PS at the end of 1 Hz trains was provided by estimates based upon the additive effects of 1 Hz stimulation and paired pulse depression. These results indicate that the strength of paired pulse depression in the unanesthetized rat is unchanged following 1 Hz stimulation, and further, that the 1 Hz depression in dentate granule cell field potentials most likely reflects the cumulative influence of late paired pulse depression.