Behavior-dependent paired-pulse responses in the hippocampal CA1 region

Early life stress impairs fear memory and synaptic plasticity; a potential role for GluN2B

Programming of the brain by early life stress has been associated with alterations in structure and function of the dorsal hippocampus. Yet, the underlying molecular mechanisms remain largely elusive. In this study, we examined the effects of early life stress (ELS) - by housing mouse dams with limited nesting and bedding material from postnatal days 2-9 and examined in 6 month old offspring; 1) auditory fear conditioning, 2) expression of the hippocampal N-methyl-d-aspartate receptor (NMDA-R) subunits 2A and 2B (GluN2A, GluN2B), and expression of PSD-95 and synaptophysin, and 3) short- and long-term (LTP) synaptic plasticity. Given its critical role in NMDA receptor function and synaptic plasticity, we further examined the role of GluN2B in effects of ELS on synaptic plasticity and fear memory formation. We demonstrate that ELS impaired fear memory in 6 month old mice and decreased hippocampal LTP as well as the paired-pulse ratio (PPR). ELS also reduced hippocampal GluN2B expression. Interestingly, pharmacological blockade of GluN2B with the selective antagonist Ro25 6981 was less effective to reduce synaptic plasticity in ELS mice, and was also ineffective to impair memory retrieval in ELS mice. These studies suggest that ELS reduces hippocampal synaptic plasticity and fear memory formation and hampers GluN2B receptor function. As such, GluN2B may provide an important target for future strategies to prevent lasting ELS effects on cognition.

Role of brain glycogen in the response to hypoxia and in susceptibility to epilepsy

Although glycogen is the only carbohydrate reserve of the brain, its overall contribution to brain functions remains unclear. It has been proposed that glycogen participates in the preservation of such functions during hypoxia. Several reports also describe a relationship between brain glycogen and susceptibility to epilepsy. To address these issues, we used our brain-specific Glycogen Synthase knockout (GYS1^Nestin-KO) mouse to study the functional consequences of glycogen depletion in the brain under hypoxic conditions and susceptibility to epilepsy. GYS1^Nestin-KO mice presented significantly different power spectra of hippocampal local field potentials (LFPs) than controls under hypoxic conditions. In addition, they showed greater excitability than controls for paired-pulse facilitation evoked at the hippocampal CA3–CA1 synapse during experimentally induced hypoxia, thereby suggesting a compensatory switch to presynaptic mechanisms. Furthermore, GYS1^Nestin-KO mice showed greater susceptibility to hippocampal seizures and myoclonus following the administration of kainate and/or a brief train stimulation of Schaffer collaterals. We conclude that brain glycogen could play a protective role both in hypoxic situations and in the prevention of brain seizures.

Acute stress disrupts paired pulse facilitation and long-term potentiation in rat dorsal hippocampus through activation of glucocorticoid receptors

Cognitive functions such as learning and memory are widely believed to depend on patterns of short- and long-term synaptic plasticity. Factors, such as acute stress, which affect learning and memory, may do so by altering patterns of synaptic plasticity in distinct neural circuits. Numerous studies have examined the effects of acute stress on long-term synaptic plasticity; however, few have examined its influence on short-term plasticity. The present experiments directly assessed the effects of acute stress on short-term synaptic plasticity as measured by paired pulse facilitation (PPF) of excitatory postsynaptic potentials recorded from rat dorsal hippocampus (dHip) in vivo. Long-term potentiation (LTP) was also examined. Acute stress induced by exposure to an elevated platform impaired PPF and LTP in the dHip. Pretreatment of rats exposed to stress with mifepristone (RU38486; 10 mg kg⁻¹) blocked the stress-induced impairment of both PPF and LTP. These data demonstrate that activation of glucocorticoid receptors during acute stress disrupts normal patterns of both PPF and LTP in the dHip.

Surgical management of abdominal manifestations of type 1 neurofibromatosis: experience of a single center

Neurofibromatosis type 1 (NF1) is a genetic disease characterized by neoplastic and nonneoplastic disorders involving tissues of neuroectodermal and mesenchymal origin. The mainly involved districts are skin, the central nervous system, and eye and there is a wide range of severity of clinical presentations. Abdominal manifestations of NF1 include five kinds of tumors: neurogenic tumors (neurofibromas, malignant peripheral nerve sheath tumors [MPNSTs], and ganglioneuromas); neuroendocrine tumors (pheochromocytomas and carcinoids); nonneurogenic gastrointestinal stromal tumors (GISTs); embryonal tumors; and miscellaneous. The present experience depends on the participation in the National Project for Diagnosis and Treatment of Rare Diseases. In the group of patients with a diagnosis of von Recklinghausen disease, 10 patients underwent surgical treatment for gastrointestinal and retroperitoneal tumors associated with NF1. Three patients underwent adrenalectomy for pheochromocytoma (in one case associated with jejunal wall neurofibroma); two patients were found to be affected by MPNST (recurrent and unresectable in one case). One patient was affected by giant gastric GIST and jejunal neurofibroma; two patients were affected by extraperitoneal neurofibroma (pararenal and pararectal position); one patient was affected by giant colic neurofibroma and one patient was affected by retroperitoneal bilateral plexiform neurofibromas. Early diagnosis of these abdominal manifestations is very important because of the risk of malignancy, organic complications (such as pheochromocytoma), or hemorrhagic-obstructive complications such as in case of tumors of the gastrointestinal tract (GISTs and neurofibromas). The importance of an annual clinical evaluation on the part of a multidisciplinary pool of clinicians in highly specialized centers allows early detection of complications and of neoplastic transformation. Genetic screening allows preclinical diagnosis with a sensibility of 95 per cent. Further studies are necessary to detect predictive factors of malignant tumor development of severe clinical conditions.

Synaptic plasticity along the sleep-wake cycle: implications for epilepsy

Activity-dependent changes in synaptic efficacy (i.e., synaptic plasticity) can alter the way neurons communicate and process information as a result of experience. Synaptic plasticity mechanisms involve both molecular and structural modifications that affect synaptic functioning, either enhancing or depressing neuronal transmission. They include redistribution of postsynaptic receptors, activation of intracellular signaling cascades, and formation/retraction of dendritic spines, among others. During the sleep-wake cycle, as the result of particular neurochemical and neuronal firing modes, distinct oscillatory patterns organize the activity of neuronal populations, modulating synaptic plasticity. Such modulation, for example, has been shown in the visual cortex following sleep deprivation and in the ability to induce hippocampal long-term potentiation during sleep. In epilepsy, synchronized behavioral states tend to contribute to the initiation of paroxystic discharges and are considered more epileptogenic than desynchronized states. Here, we review some of the current understandings of synaptic plasticity changes in wake and sleep states and how sleep may affect epileptic seizures.

Reticular stimulation evokes suppression of CA1 synaptic responses and generation of theta through separate mechanisms

The induction of hippocampal theta by reticular stimulation involves a relay to the hippocampus via the posterior hypothalamic-supramammillary region and then the medial septum. Interestingly, sensory- or behaviour-induced theta is accompanied by suppression of hippocampal field CA1 synaptic responses. In the present study, performed on anaesthetized rats, we observed that reticular stimulation also induced a suppression of the CA1 pyramidal cell population spike and the corresponding dendritic field excitatory postsynaptic potential evoked by field CA3 stimulation. This suppression was observed at stimulation intensity below the threshold for generation of CA1 theta and was maximal at stimulation intensities at the threshold for theta. The frequency and amplitude of theta waves, by contrast, increased further with increasing reticular stimulation voltage. Neural inactivation by microinjection of the local anaesthetic procaine (20% w/v, 0.1-0.2 microL) or the inhibitory ligand gamma aminobutyric acid (0.8 m, 0.5 micro L) in the posterior hypothalamic regions, especially the ipsilateral medial supramammillary region, or the medial septum attenuated both the suppression of CA1 pyramidal cell synaptic excitability and theta generation. However, the effects of microinjection on suppression and theta were not always in parallel. Furthermore, the effect of microinjection of gamma aminobutyric acid on reticularly elicited suppression was observed from relatively fewer sites in the posterior hypothalamus as compared with that on theta activation. These results suggest that reticular stimulation evokes an inhibition of CA1 pyramidal cell excitability that (i) is mediated, at least in part, via medial supramammillary and septal regions, but (ii) involves a separate neural pathway from theta generation.

The serotonergic modulation of synaptic plasticity in the rat hippocampo-medial prefrontal cortex pathway

The ability of the serotonergic (5-HTergic) system to affect the hippocampo-medial prefrontal cortex (mPFC) synaptic properties was examined in rats with lesions of 5-HTergic neurons. Intracerebroventricular injections of 5,7-dihydroxytryptamine (5,7-DHT) resulted in selective depletion of 5-HT and 5-hydroxyindoleacetic acid in the cerebral cortex, hippocampus and raphe regions. The 5,7-DHT-lesioned rat had no changes in basal synaptic transmission in the hippocampo-mPFC pathway. Conversely, we observed the augmentation of short-term synaptic plasticity, i.e. paired-pulse facilitation, when compared with sham-operated rats in this pathway. The magnitude of long-term potentiation (LTP) was significantly augmented in 5,7-DHT-lesioned rats. This augmentation of hippocampo-mPFC LTP had a significant correlation with the degree of cortical 5-HT levels. These results suggest that the 5-HTergic system may modulate plastic properties at the hippocampal-mPFC synapses in vivo.

Differences in paired-pulse facilitation and long-term potentiation between dorsal and ventral CA1 regions in anesthetized rats

To clarify hippocampal regional differences in synaptic plasticity, paired-pulse facilitation (PPF, a form of short-term plasticity), long-term potentiation (LTP, a form of long-term plasticity), and their interactions were studied in the dorsal and ventral hippocampal CA1 regions of anesthetized rats. Baseline PPF and post-LTP PPF experiments were conducted at interstimulus intervals (ISIs) of 20-320 ms. A general protocol (100 Hz, 1 s) and a stronger protocol (250-Hz pulse series) were applied for LTP induction. PPF were observed in both regions; however, the degree was lower and the range of ISIs was narrower in the ventral region compared with the dorsal region. The degree of ventral LTP was lower than that of the dorsal LTP. The interaction between PPF and LTP was observed in both regions (PPF change correlated inversely with degree of baseline PPF). However, this was also different in each region. Dorsal PPF increased or decreased; in contrast, ventral PPF of short ISIs after LTP only decreased. These regional differences in short-term and long-term synaptic plasticity may explain a consequence of different afferent inputs and information processing.

Local properties of CA1 region in hippocampo-prefrontal synaptic plasticity in rats

We studied paired-pulse facilitation and long-term potentiation/depression in anesthetized rats to determine whether the hippocampal CA1 region inhibits local differences in short-term and long-term synaptic plasticity in its projections to the prefrontal cortex. We compared projections with the PFC from the posterior dorsal and ventral hippocampal CA1 regions (pdCA1 and vCA1 respectively). The two pathways displayed similar properties. However, the PPF properties of the pdCA1, projections differed dramatically from those of the pdCA1 projections. The pdCA1 projections showed the opposite of facilitation (i.e. suppression) at 25-50 ms intervals and more pronounced facilitation at 100-400 ms intervals. These results suggest that there are functional differences between these pathways.

Enhanced but fragile inhibition in the dentate gyrus in vivo in the kainic acid model of temporal lobe epilepsy: a study using current source density analysis

Temporal lobe epilepsy is related to many structural and physiological changes in the brain. We used kainic acid in rats as an animal model of temporal lobe epilepsy, and studied the neural interactions of the dentate gyrus in urethane-anesthetized rats in vivo. Our initial hypothesis was that sprouting of mossy fibers, the axons of the granule cells, increases proximal dendritic excitatory currents in the inner molecular layer of the dentate gyrus. Extracellular currents were detected in vivo using current source density analysis. Backfiring the mossy fibers in CA3 or orthodromic excitation of the granule cells through the medial perforant path induced a current sink at the inner molecular layer. However, the sink or inferred excitation at the inner molecular layer was not increased in kainic acid-treated rats and the sink actually correlated negatively with the degree of mossy fiber sprouting. It is inferred that the latter sink was mediated mainly by association fibers and not by recurrent mossy fibers. After kainic acid treatment, paired-pulse inhibition of the population spikes in the dentate gyrus was increased. In contrast, reverberant activity that involved looping around an entorhinal-hippocampal circuit was increased in kainic acid-treated rats, compared to control rats. The increase of inhibition in kainic acid-treated rats was readily blocked by a small dose of GABA(A) receptor antagonist bicuculline. The latter dose of bicuculline induced paroxsymal spike bursts in kainic acid-treated but not control rats, demonstrating that the increased inhibition in dentate gyrus was fragile. In conclusion, after kainic acid induced seizures, the dentate gyrus in vivo showed an increase in inhibition that appeared to be fragile. The hypothesized increase in proximal dendritic excitation due to mossy fiber sprouting was not detected. However, the fragile inhibition could explain the seizure susceptibility in patients with temporal lobe epilepsy.

Dorsal-ventral differentiation of short-term synaptic plasticity in rat CA1 hippocampal region

Two forms of short-term synaptic plasticity (STP), paired-pulse facilitation (PPF) and frequency potentiation (FP) of CA1 field excitatory postsynaptic potentials (EPSP) to afferent stimulation were compared in slices taken from the dorsal and ventral parts of rat hippocampus. While dorsal slices showed significant PPF at all interpulse intervals (20-1400 ms, 80% at 40 ms), PPF in ventral slices was substantially weaker at intervals shorter than 100 ms (19% at 40 ms) and nil at longer intervals. While dorsal slices showed substantial FP at frequencies 1-40 Hz and frequency depression at 50-100 Hz, ventral slices showed only a much smaller potentiation at 1 Hz and substantial depression at 20-100 Hz. Decreasing [Ca(2+)](o) from 2 to 1 and 0.5 mM substantially reduced the baseline EPSPs in both groups of slices but its effect on PPF was greater in ventral slices. On the contrary when [Ca(2+)](o) was increased to 5 mM only dorsal slices showed an enhancement of baseline EPSP. It is concluded that ventral hippocampus CA1 area has a specific deficit in STP, which is related to the important presynaptic role of calcium and is consistent with a relatively higher transmitter release probability.

Factors affecting paired-pulse facilitation in hippocampal CA1 neurons in vitro

Factors underlying paired-pulse facilitation (PPF) were studied by intracellular and field recordings of CA1 neurons in the hippocampal slice in vitro, following stimulation of the Schaffer collaterals apical dendritic afferents. Similar magnitudes of PPF were found using the slopes or peaks of the excitatory postsynaptic potentials (EPSPs) recorded intracellularly or extracellularly at the soma or dendrites. The paired-pulse EPSP facilitation index (EPI), defined as the ratio of EPSP slope evoked by the second pulse (E2) to that evoked by the first pulse (E1), had a broad peak at 30-60 ms interpulse interval (IPI). EPI was largest at small E1 and decreased with an E1 increase. Spiking excitability was enhanced after the second as compared to the first pulse as evidenced by (1) a decreased latency to fire and (2) an increased tendency to fire double or multiple spikes. The PPF of spiking resulted partly from an increased E2 and partly from a diminished inhibition evoked by the second pulse. Whether the first pulse elicited a spike or not had no significant effect on the EPI. Multiple spiking evoked by the second pulse was partly blocked by the GABAB antagonist CGP35348 (1 mM). The PPF of the EPSP slopes, however, was not significantly affected by GABAB antagonists, GABAA antagonist bicuculline or NMDA antagonist 2-aminophosphonovalerate. In conclusion, PPF may serve as a means of amplification of synaptic transmission such that reliable spike output may result from a given set of synapses.

Paired tone facilitation in dorsal cochlear nucleus neurons: a short-term potentiation model testable in vivo

It has been suggested that the dorsal cochlear nucleus (DCN) is involved in coding stimulus history or prior auditory activity [Manis (1989) J. Neurophys., 61, 149-161; Manis (1990) J. Neurosci., 10, 2338-2351]. The major output neurons of the DCN are the fusiform (pyramidal) cells which are thought to receive excitatory inputs from the descending branch of the acoustic nerve onto their basal dendrites and significant inhibitory glycinergic and GABAergic inputs to the soma and dendrites. The apical dendrites of these neurons lie within the molecular layer of the DCN and encounter parallel fibers which are thought to utilize the excitatory amino acid neurotransmitter glutamate. In this study of anesthetized chinchillas, we found that, in contrast to the responses of acoustic nerve fibers and most cochlear nucleus neurons which are masked by an appropriate preceding signal, many DCN principal cells are facilitated during the second of two identical stimuli. Facilitated DCN responses often have a reduced interspike interval and a more chopper-like temporal response pattern to the second characteristic frequency tone. This paired tone facilitation in the chinchilla DCN provides as in vivo model of short-term potentiation elicited by sensory stimulation similar to the paired-pulse facilitation observed with electrical stimulation in other models.

Hippocampal interneuron activity in unanesthetized rats: relationship to the sleep-wake cycle

Evoked population spikes and interneuronal discharges were recorded throughout the sleep-wake cycle in hippocampal regions CA1 and dentate gyrus (DG) of ten chronically implanted rats. During quiet wakefulness (QW) and slow-wave-sleep (SWS) (non-theta rhythm states), the primary shock of paired stimuli evoked in CA1 both high amplitude population spikes and multiple interneuron discharges when compared to active wakefulness (AW) and rapid-eye-movement (REM) sleep (theta rhythm states). A second shock was delivered to CA1 afferents 60 ms after the first shock. This second shock evoked a small population spikes during non-theta states, whereas it evoked higher amplitude population spikes in theta states. The second shock also evoked unit interneuron discharges in non-theta states but not in theta states. In the dentate gyrus, identical primary afferent stimulation evoked similar interneuron activity and uniform amplitude population spikes throughout the sleep-wake cycle. In contrast, the secondary shocks evoked a striking potentiation of the field population spike during sleep, SWS and REM sleep compared to AW and OW. Evoked DG interneuron spikes following the second population spike were greater in number during SWS compared to the other stages. Our findings suggest that hippocampal field potentials and interneuron activity recorded in vivo are regionally regulated, have unique state-dependent expression and are strongly influenced by inhibitory feed-forward mechanisms.

Effect of atropine and PCPA on the behavioral modulation of paired-pulse response in the hippocampal CA1 region

This study investigated the effect of cholinergic and serotonergic inputs on the paired-pulse response in hippocampal CA1 region of behaving rats. Paired-pulses were delivered to the Schaffer collaterals, at an interpulse interval (IPI) of 30 ms, and field responses were recorded at the proximal CA1 apical dendritic layer. Previously, paired-pulse facilitation of the CA1 population spike was shown to be significantly larger during walking than immobility. The muscarinic cholinergic antagonist, atropine sulfate (50 mg/kg i.p.), strongly attenuated this paired-pulse facilitation during walking at given amplitude of the population spike evoked by the first pulse. Parachlorophenylalanine (PCPA; 100 mg/kg i.p. for 3 days), an inhibitor of serotonin synthesis, alone did not change paired-pulse facilitation and the combined action of PCPA and atropine resembled that of atropine alone. The excitatory postsynaptic potentials did not change significantly with behavior or after drugs. Thus, we have shown that the behaviorally dependent modulation of paired-pulse response in the CA1 region is mediated primarily by a cholinergic and not a serotonergic input.

Fast (beta) rhythms in the hippocampus: a review

Spontaneous or evoked brain activity in the hippocampus showed a 20-70 Hz beta rhythm under some conditions, typically during behavioral activation and accompanied by a theta rhythm. Beta rhythms are generated locally, perhaps by a recurrent feedback loop involving pyramidal cells and inhibitory interneurons. Modulation of the local circuit and rhythm by cholinergic inputs has also been demonstrated. Under some behavioral states, neural impulses modulated at the beta frequency may transmit preferentially through the trisynaptic circuit in the hippocampus. It is suggested that the beta rhythm may serve to establish transient physiological connections, reflected in coherence at the beta frequency, among neurons in the hippocampus and related structures. Thus, the beta rhythm may play an essential role in hippocampal function.