Barbiturate-enhanced paired-pulse depression in neonatal rats

Entorhinal cortex stimulation modulates amygdala and piriform cortex responses to olfactory bulb inputs in the rat

The rodent olfactory bulb sends direct projections to the piriform cortex and to two structures intimately implicated in memory processes, the entorhinal cortex and the amygdala. The piriform cortex has monosynaptic projections with the amygdala and the piriform cortex and is therefore in a position to modulate olfactory input either directly in the piriform cortex, or via the amygdala. In order to investigate this hypothesis, field potential signals induced in anesthetized rats by electrical stimulation of the olfactory bulb or the entorhinal cortex were recorded simultaneously in the piriform cortex (anterior part and posterior part) and the amygdala (basolateral nucleus and cortical nucleus). Single-site paired-pulse stimulation was used to assess the time courses of short-term inhibition and facilitation in each recording site in response to electrical stimulation of the olfactory bulb and entorhinal cortex. Paired-pulse stimulation of the olfactory bulb induced homosynaptic inhibition for short interpulse interpulse intervals (20-30 ms) in all the recording sites, with a significantly lower degree of inhibition in the anterior piriform cortex than in the other structures. At longer intervals (40-80 ms), paired-pulse facilitation was observed in all the structures. Paired-pulse stimulation of the entorhinal cortex mainly resulted in inhibition for the shortest interval duration (20 ms) in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. Double-site paired-pulse stimulation was then applied to determine if stimulation of the entorhinal cortex can modulate responses to olfactory bulb stimulation. For short interpulse intervals (20 ms) heterosynaptic inhibition was observed in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. The level of inhibition was greater in the basolateral nucleus than in the other structures. Taken together these data suggest that the entorhinal cortex exerts a main inhibitory effect on the olfactory input via the amygdala basolateral nucleus and to a lesser extent the piriform cortex. The potential role of these effects on the processing of olfactory information is discussed.

Epileptogenic insult causes a shift in the form of long-term potentiation expression

The relationship between epilepsy, modeled here by pentylenetetrazol kindling, and learning deficits, modeled here by long-term potentiation (LTP), was studied. The field excitatory postsynaptic potentials and population spikes (PS) were recorded from strata radiatum and pyramidale, respectively, in urethane-anesthetized rat dorsal hippocampus CA1 area upon stimulation of Schaffer collaterals. To induce LTP, a 100 Hz primed-burst stimulation protocol was used. Experiments were carried out at approximately 30 days after the last pentylenetetrazol dose. The effects of voltage dependent calcium channel blocker verapamil and N-methyl-D-aspartate receptor antagonist MK-801 on LTP expression were examined. Tetanic stimulation elicited both field excitatory postsynaptic potential LTP and PS LTP in control animals, and LTP-induction of the PS in control animals was attenuated by MK-801, but not by verapamil. By contrast, kindled rats showed LTP of the PS only. MK-801 reduced the extent of potentiation of PS amplitude and verapamil inhibited the PS amplitude potentiation, completely. The results suggest that seizure induction modifies mechanisms underlying LTP induction and causes a shift in the form of LTP expression. The pentylenetetrazol-kindling-induced increase in PS LTP is sensitive to verapamil and not to MK-801 and therefore primarily dependent on activation of voltage dependent calcium channels rather N-methyl-D-aspartate receptors. Kindling may lead to a shift in synaptic plasticity thresholds much like the shift that occurs during aging, and such alterations may contribute to deficits in learning and memory.

Granule cell disinhibition in dentate gyrus of genetically seizure susceptible El mice

Paired-pulse inhibition was investigated electrophysiologically in the dentate gyrus using hippocampal slices from epileptic El mice. At short interpulse intervals (IPIs), the inhibition was 30% in the El, and 90% in the control ddY mice at the ages of 10 and 15 weeks. No difference in inhibition was observed at the age of 5 weeks. Bicuculline, a GABAA receptor antagonists, attenuated the inhibition during short IPIs n the ddY mice, while in the El mice, phenobarbital and flunitrazepam, which enhance GABAA receptor function, restored the inhibitory activity comparable to that of the ddY. The disinhibition progressed with growth, closely correlating with seizure development in El mice. These results suggest that decrease in the GABAergic inhibition occurs in the dentate gyrus of the El mice with growth. GABA concentration in the hippocampus was also quantified using HPLC. In El mice, GABA level was significantly lower than that in ddY mice at the ages of 5 and 15 weeks. Thus, the disinhibition observed in the El dentate gyrus at 15 weeks of age does not appear to be directly related to the content of GABA. GABAergic disinhibition suggests possible loss of unknown inhibition control factor(s) in the El dentate gyrus as growth progresses. The growth-dependent disinhibition in the granule cells may be prerequisite for epileptogenesis in El mice.

Studies of dentate granule cell modulation: paired-pulse responses in freely moving rats at three ages

Dentate granule cell population responses to paired-pulse stimulations applied to the perforant pathway across a range of interpulse intervals (IPI) were examined in freely moving rats at 15, 30, and 90 days of age. The profile of the paired-pulse index (PPI), a measure of the type and degree of modulation of dentate granule cell excitability, was shown to change significantly as a function of age.

The effect of urethane anesthesia on evoked potentials in dentate gyrus

We examined the effect of urethane (1000 mg/kg, followed by 50 mg/kg per h, i.v.), an anesthetic commonly used by physiologists, on evoked potentials recorded in dentate gyrus in adult Wistar rats by stimulating the ipsilateral perforant path, via chronically implanted electrodes. Urethane decreased paired-pulse inhibition. Under urethane, with paired-pulse stimulation, the ratio of the second population spike amplitude to the first increased by 11.1-20.7% at 25-60 ms interstimulus interval (n = 18, P < 0.05). At 25 ms, the proportion was 3.6 +/- 1.6 while awake, and 14.7 +/- 3.5 under urethane. Urethane depressed granule cell excitability and strength of synaptic responses. Under urethane, the ratio of the population spike amplitude obtained at 250 microA stimulation to the maximal response in the same input/output response examination decreased by 20%, and the ratio of the excitatory postsynaptic response slopes fell by 10%. These results indicate that urethane affects neurotransmission in the hippocampus, and suggest that its effect may be exerted in part on excitatory neurotransmission.

Effects of pentobarbital on entorhinal tetanic responses and the progression of afterdischarges during the early course of amygdala kindling in rats

We investigated the relationship between the progress of afterdischarges (AD) and the development of facilitated entorhinal tetanic responses by amygdala kindling stimulations in conscious and pentobarbital (PB)-treated rats. The entorhinal responses consisted of deep negative components and the following shallow positive components. The negative potential (mean +/- SE) reflecting excitatory synaptic activation in the test response evoked by a single stimulation (600 microA) before kindling stimulations was greater in PB-treated rats (1.3 +/- 0.21 mV, n = 6) than in conscious rats (0.5 +/- 0.08 mV, n = 9). The positive potential reflecting inhibitory synaptic activation in the test response was also greater in PB-treated rats (0.6 +/- 0.14 mV, n = 6) than in conscious rats (0.2 +/- 0.04 mV, n = 9). The magnitude of the tetanic response was estimated as the area between the excitatory negative potential and the baseline in the averaged tetanic response during each kindling stimulation (10 Hz, 100 pulses). The magnitude of the tetanic response was significantly enhanced in association with the prolongation of AD duration in the conscious rats. In the PB-treated (50 mg/kg intraperitoneally, i.p.) rats, enhancement of tetanic response was very slight and the progress of AD duration was prevented. There was a linear correlation (r = 0.9) between the magnitude of tetanic response and AD duration. These findings indicate that PB suppresses kindling-induced enhancement of excitatory synaptic activation in tetanic responses and that this enhancement is intimately related to the development of AD.

Development of inhibitory and excitatory synaptic transmission in the rat dentate gyrus

We studied the ontogeny of inhibitory and excitatory processes in the rat dentate gyrus by examining paired-pulse plasticity in the hippocampal slice preparation. The mature dentate gyrus produces characteristic paired-pulse responses across a wide range of interpulse intervals (IPI). Paired-pulse effects on population excitatory postsynaptic potential (EPSP) slope and population spike (PS) amplitude were analyzed at postnatal day 6 (PN6), PN7/8, PN9/10, PN15/16, and PN > 60. The synaptic paired-pulse profile (10-5,000 ms IPI) matured by PN7/8. The triphasic pattern of short-latency depression, a relative facilitation at intermediate intervals, and long-latency depression was present at all ages tested. Paired-pulse effects on granule cell discharge indicated the presence of weak short-latency (20 ms IPI) inhibition at PN6, the earliest day that a population spike could be evoked. By PN7/8, short-latency inhibition was statistically equivalent to the mature dentate gyrus. Long-latency (500-2,000 ms IPI) PS inhibition was present, and equal to the mature dentate gyrus by PN6. The most consistent difference between the mature and developing dentate gyrus occurred at intermediate IPIs (40-120 ms) where spike facilitation was significantly depressed in the development groups. The studies indicate that short-term plasticity matures rapidly in the dentate gyrus and suggest that the inhibitory circuitry can function at a surprisingly early age.

Post-activation potentiation and depression in the neocortex of the rat: II. Chronic preparations

Although long-term potentiation (LTP) has been demonstrated in a number of subcortical sites in chronic preparations, there have been no demonstrations of LTP in the neocortex of chronic preparations. Even neocortical slice and acute preparations often require a drug-induced suppression of inhibition before LTP effects can be reliably induced. We have attempted to induce LTP in neocortical sites in 7 different experiments using chronically prepared adult rats. We were unable to obtain any evidence, even a trend, for the induction of LTP. The following manipulations were tested: (1) standard stimulation train parameters that have been shown to be highly effective in subcortical and hippocampal sites; (2) a 10-fold increase in the intra-train pulse durations; (3) variations in train pulse frequency (1 Hz to 300 Hz) and train duration (100 ms to 15 min); (4) co-activation of multiple inputs by stimulation of combinations of cortical sites or cortical and thalamic sites; (5) reduction of inhibition by administration of picrotoxin; 5) Housing of animals in an enriched environment; (6) utilization of the neocortical stimulation trains as a cue in a learning task; (7) application of pilocarpine to co-activate cholinergic systems. Although none of these manipulations produced LTP, the application of pilocarpine did facilitate the induction of a long-lasting depression effect. These findings contrast with the results obtained from anesthetized rats and from studies using brain slices, where LTP can be reliably induced. These results are discussed in light of other recent findings with respect to LTP and LTD effects.

Changes of neuronal transmission in the hippocampus after transient ischemia in spontaneously hypertensive rats and the protective effects of MK-801

BACKGROUND AND PURPOSE

I studied the mechanism of postischemic neuronal degeneration in the hippocampus by an electrophysiological method.

METHODS

Sequential changes of field potentials evoked by perforant path stimulation in the dentate gyrus and the CA1 region of the hippocampus were evaluated in spontaneously hypertensive rats up to 7 days after transient global ischemia induced by bilateral occlusion of the carotid arteries for 20 minutes after electrocauterization of the vertebral arteries. Animals were treated with vehicle or the excitotoxin antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10 amine (MK-801, 2 mg/kg or 5 mg/kg) intraperitoneally 30 minutes before ischemia.

RESULTS

Complete recovery of the population spike was observed in the dentate gyrus within 24 hours after recirculation, followed by a gradual reduction of population spike amplitude. In contrast, population spike in the CA1 region showed partial recovery 24 hours after recirculation, and an abrupt reduction of population spike amplitude occurred on day 2. There was no significant enhancement of population spike amplitude in either region throughout the experiment. Interneuronal recurrent inhibition in the dentate gyrus was enhanced on day 4, and ischemic changes were apparent in the CA1 pyramidal cells on day 7. Pretreatment with 5 mg/kg MK-801 prevented field potential and pathological changes completely in the dentate gyrus and partially in the CA1 region.

CONCLUSIONS

My results indicate that pathological changes of the CA1 pyramidal neurons after transient ischemia may not be the result of postischemic overstimulation. However, neuronal transmission in the CA1 region may be persistently impaired during or after transient ischemia.

Activation of the NMDA receptor: a correlate in the dentate gyrus field potential and its relationship to long-term potentiation and kindling

Stimulation trains, but not stimulation pulses, are capable of inducing long-term potentiation (LTP). In this paper we report experiments designed to examine, in chronic preparations, the characteristics of a component unique to the train-evoked response. Stimulation trains applied to the perforant path evoked population EPSP's and population spikes in the dentate gyrus that were nearly identical to those evoked by single pulses of comparable intensity. The trains also triggered a prolonged potential, negative at the dendritic pole of our electrodes, which far outlasted the pulse-evoked response. We substracted pulse-evoked responses from these train-evoked responses which left us with a waveform that peaked at about 15 ms and lasted for about 50-70 ms. The GABA agonists, diazepam and sodium pentobarbital, had no significant effect on this component, but the NMDA antagonists, ketamine and MK-801, both depressed it by over 30%. The late component had a very low threshold, which might account for the frequent observation of LTP induction at very low thresholds. Also, the late component is reliably seen in all animals showing LTP, even in the occasional animals that show no population spikes. The late component did not appear to be affected by the induction of LTP, and was either not affected or was depressed following the completion of kindling. When the 'NMDA-component' of the train-evoked response was monitored, along with LTP, in an ascending intensity train series, it was found that both the NMDA-component and the LTP increased smoothly. There was no sudden appearance of the NMDA-component at the LTP threshold. The presence of an NMDA component in the field potential of the chronic preparation allows the monitoring of the levels of NMDA activation over prolonged periods.

Decreases in excitatory synaptic transmission and increases in recurrent inhibition in the rat dentate gyrus after transient cerebral ischemia

Excitatory transmission along the perforant path from the entorhinal cortex to the granule cells of the dentate gyrus was evaluated two days after 10 min of transient cerebral ischemia in the rat. The amplitude of the population spike, and the amplitude and the initial slope of the population excitatory postsynaptic potential (EPSP) evoked by the perforant path stimulation were measured across a range of stimulus intensities, and were compared with control values. Inhibitory interactions were evaluated using the paradigm of paired pulse stimulation, comparing the amplitude of the population spike evoked by the second pulse of a pair to the initial spike. The maximal values of the initial slope of the population EPSP and the population spike were reduced in the ischemic group. Also, the extent of paired pulse inhibition was greater in the ischemic group. These results suggest that: (1) excitatory synaptic transmission along the perforant path is impaired in the postischemic period, (2) inhibition of the dentate granule cells is enhanced in this period. These results are not consistent with the hypothesis that there is a hyperactivation of the tri-synaptic circuit in the chronic postischemic period that accounts for the excitotoxic death of CA1 neurons.

Noradrenergic modulation of olfactory bulb excitability in the postnatal rat

Noradrenergic centrifugal inputs to the rat olfactory bulb mainly terminate on granule cells, which are inhibitory interneurons. In the mature bulb, norepinephrine suppresses granule cell activity, thus increasing the excitability of the primary output neurons of the bulb. However, since the majority of granule cells develop postnatally, the effectiveness of noradrenergic inputs to the bulb during development is unclear. The present report describes the postnatal development of noradrenergic modulation of olfactory bulb function by examining the effects of noradrenergic beta-receptor agonists and antagonists on paired-pulse inhibition at the granule cell/mitral cell reciprocal synapse. The results demonstrate that noradrenergic modulation of olfactory bulb excitability emerges during the first postnatal week in the rat. These results suggest that noradrenergic centrifugal control of olfactory bulb activity appears early during postnatal development, and thus is capable of playing an important role in pup responses to olfactory cues early in life.

Abrupt decrease in synaptic inhibition in the postnatal rat olfactory bulb

Olfactory bulb responses to paired-pulse stimulation of the lateral olfactory tract were examined in urethane-anesthetized rats, aged 5 days to adult. Brief inter-pulse intervals resulted in a depression of test responses at all ages. The magnitude of this depression decreased dramatically between postnatal days 19 and 20 to approach adult levels. Longer inter-pulse intervals resulted in a facilitation of test response amplitude in adult animals. This facilitation was evident at adult levels by postnatal day 10. These results suggest that both inhibitory and facilitatory synaptic mechanisms appear early in the course of rat olfactory bulb development. Furthermore, presumed granule cell-mediated inhibition is present at unusually high levels in the developing bulb, decreasing sharply between days 19 and 20.

Early appearance of inhibition in the neonatal rat olfactory bulb

The functional development of inhibition in the rat olfactory bulb was examined in the present study. Inhibition of presumed mitral cell spontaneous activity following stimulation of the lateral olfactory tract was present by postnatal day 5, the youngest age tested. The duration of this inhibition was greatest in young animals, decreasing after postnatal day 15. Possible mechanisms of this enhanced inhibition in neonates were discussed.