Computational experiments support a competitive function in the the CA3 region of the hippocampus

The comprehension of activities and functions of complex brain structures requires, among other things, information on simultaneous activities in several regions. Results reported in the literature using multi(micro/macro)electrode recordings or imaging techniques provide incomplete information due either to the small size and/or small number of investigated regions or to the poor spatiotemporal resolution, respectively. This is particularly true for the hippocampus and its subfields, and mathematical modeling and computer simulation have been used with the aim of obtaining information when this is lacking. Global activities in the CA3 field of the hippocampus, and in particular the genesis of theta rhythm and sharp waves, have been investigated here by a mathematical model formulated within the frame of a kinetic theory of neural systems. The model has taken into account data of experimental results both on different PSPs recorded in hippocampal neurons and on recurrent pyramidal collateral geometries. The computational ‘experiments’ to which the model was subjected suggest that the sharp waves arise through a selective and short block of the fast inhibitory neurons of CA3, produced by a medial septum inhibitory input, whereas the theta activity is produced by a durable, continuous inhibition of the slow inhibitory neurons. Information obtained also suggests that the recurrent pyramidal collaterals subserve a competitive, rather than a cooperative, organization. Based on these results a hypothesis on the possible functional organization of the CA3 field and of the entire hippocampus has been formulated. According to this hypothesis, the CA3 imposes a serial order on the flow of activity arriving at the hippocampus from the entorhinal cortex and from its connected polymodal cortical regions. This ordering permits cortical activities, arriving at CA3 on appropriate time intervals, to produce effects in regions of brain to which the CA3 projects. The competing cortical activities are lost.     

Hypoxia and development of interneurones of the rat hippocampus

Interneuronal connections are an important component of the integration circuits of the hippocampus. They are formed mostly postnatally, i.e. in a phase when their development can be strongly influenced by external environmental factors. In model postnatal hypoxia, a morphometric analysis showed reduction of the number of dendritic segments (chiefly 3rd order segments) in 18-day-old rats. The total length of the dendrites was unaltered, but mean segment length increased. Interneuron density in the experimental animals was higher, especially in the stratum radiatum, but pyramidal cell density was lower than in the controls. These findings are evidence that hypoxia has a complex effect on the microstructure of the hippocampus and that, in the long run, it can lead to impairment of the balance of the elementary processes of excitation and inhibition.     

Calretinin immunoreactivity in cholinergic motor neurones,interneurones and vasomotor neurones in the guinea-pig small intestine

Summary Immunoreactivity for calretinin, a calcium-binding protein, was studied in neurones in the guinea-pig small intestine. 26±1% of myenteric neurones and 12±3% of submucous neurones were immunoreactive for calretinin. All calretinin-immunoreactive neurones were also immunoreactive for choline acetyltransferase and hence are likely to be cholinergic. In the myenteric plexus, two subtypes of Dogiel type-I calretinin-immunoreactive neurones could be distinguished from their projections and neurochemical coding. Some calretinin-immunoreactive myenteric neurones had short projections to the tertiary plexus, and hence are likely to be cholinergic motor neurones to the longitudinal muscle. Some of these cells were also immunoreactive for substance P. The remaining myenteric neurones, immunoreactive for calretinin, enkephalin, neurofilament protein triplet and substance P, are likely to be orad-projecting, cholinergic interneurones. Calretinin immunoreactivity was also found in cholinergic neurones in the submucosa, which project to the submucosal vasculature and mucosal glands, and which are likely to mediate vasodilation. Thus, calretinin immunoreactivity in the guinea-pig small intestine is confined to three functional classes of cholinergic neurones. It is possible, for the first time, to distinguish these classes of cells from other enteric neurones.     

Inhibitory control of sensory gating in a computer model of the CA3 region of the hippocampus

 A model of the CA3 region of the hippocampus was used to simulate the P50 auditory-evoked potential response to repeated stimuli in order to study the neuronal circuits involved in a sensory-processing deficit associated with schizophrenia. Normal subjects have a reduced P50 auditory-evoked potential amplitude in response to the second of two paired auditory click stimuli spaced 0.5 s apart. However, schizophrenic patients do not gate or reduce their response to the second click. They have equal auditory-evoked response amplitudes to both clicks. When schizophrenic patients were medicated with traditional neuroleptics, the evoked potential amplitude to both clicks increased, but gating of the second response was not restored or improved. Animal studies suggest a role for septohippocampal cholinergic activity in sensory gating. We used a computational model of this system in order to study the relative contributions of local processing and afferent activity in sensory gating. We first compared the effect of information representation as average firing rate to information representation as cell assemblies in order to evaluate the best method to represent the response of hippocampal neurons to the auditory click. We then studied the effects of nicotinic cholinergic input on the response of the network and the effect of GABA_B receptor activation on the ability of the local network to suppress the test response. The results of our model showed that nicotinic cholinergic input from the septum to the hippocampus can control the flow of sensory information from the cortex into the hippocampus. In addition, postsynaptic GABA_B receptor activation was not sufficient to suppress the test response when the interstimulus interval was 500 ms. However, presynaptic GABA_B receptor activity may be responsible for the suppression of the test response at this interstimulus interval. Received: 3 December 2001 / Accepted: 23 October 2002 / Published online: 28 February 2003 Correspondence to: K. A. Moxon (e-mail: karen.moxon@drexel.edu, Tel.:+1-215-8951959, Fax: +1-215-8954983) Supported by USPHS, MH01245, MH58414, MH-50787, MH-01121, and research grants from the Department of Veterans Affairs and the National Alliance for Research on Schizophrenia and Depression.     

Fast and slow excitation of inhibitory cells in the CA3 region of the hippocampus

Pyramidal cells form excitatory synaptic connections with local inhibitory neurons in the hippocampus. This recurrent synapse plays a crucial stabilizing role in the control of hippocampal activity, since it transforms pyramidal cell population. Using a combination of dual recording from presynaptic and postsynaptic cells and anatomical techniques, we show that these synaptic connections often comprise a single site for liberation of excitatory transmitter. The resulting excitatory postsynaptic potentials (EPSCs) have a fast time course and a similar amplitude to miniature EPSCs recorded in tetrodotoxin and cobalt. In contrast, activation of metabotropic glutamate receptors (mGluRs) by transmitter liberated during repetitive activation of these synapses produces an excitation with a much slower time course. In addition to somatodendritic mGluRs, which excite inhibitory cells, a different species of mGluR is present on inhibitory cell terminals. This mGluR is activated by higher concentrations of the agonist t-1-amino-cyclopentyl–1,3-decarboxylate and acts to reduce γ-aminobutyric acid release. mGluRs, thus, have a dual action to enhance and to depress synaptic inhibition in the hippocampus. © 1995 John Wiley & Sons, Inc.     

Caspase-6 activity in the CA1 region of the hippocampus induces age-dependent memory impairment

Active Caspase-6 is abundant in the neuropil threads, neuritic plaques and neurofibrillary tangles of Alzheimer disease brains. However, its contribution to the pathophysiology of Alzheimer disease is unclear. Here, we show that higher levels of Caspase-6 activity in the CA1 region of aged human hippocampi correlate with lower cognitive performance. To determine whether Caspase-6 activity, in the absence of plaques and tangles, is sufficient to cause memory deficits, we generated a transgenic knock-in mouse that expresses a self-activated form of human Caspase-6 in the CA1. This Caspase-6 mouse develops age-dependent spatial and episodic memory impairment. Caspase-6 induces neuronal degeneration and inflammation. We conclude that Caspase-6 activation in mouse CA1 neurons is sufficient to induce neuronal degeneration and age-dependent memory impairment. These results indicate that Caspase-6 activity in CA1 could be responsible for the lower cognitive performance of aged humans. Consequently, preventing or inhibiting Caspase-6 activity in the aged may provide an efficient novel therapeutic approach against Alzheimer disease.     

Parallel effects of joint receptors on motor neurones and intersegmental interneurones in the locust

Summary At the distal end of a mesothoracic tibia of the locust,Schistocerca gregaria, is a chordotonal organ which monitors the position and movement of the tarsus relative to the tibia. It contains approximately 35 receptors that variously encode different spatial and temporal parameters (position, velocity and direction of movement). Some excite intersegmental interneurones that respond phasically or tonically, with directional sensitivity to active or imposed movements of the tarsus. Some of these interneurones are also excited by intrinsic movements of the tarsal segments. Others, besides being excited by tarsal proprioceptive inputs, are also excited by exteroreceptors on the tarsus.When stimulated mechanically or electrically, chordotonal afferents evoke excitatory postsynaptic potentials with a central latency of between 0.9 and 1.4 ms simultaneously in the intersegmental interneurones and in tarsal motor neurones. The central arborizations of the afferents, the intersegmental interneurones and the tarsal motor neurones overlap in certain neuropilar regions of the mesothoracic ganglion. Other afferents cause an inhibition of the motor neurones, with a longer and non-consistent latency suggesting the involvement of other intercalated interneurones.These results indicate that proprioceptive inputs from the tarsal joint receptors are transmitted in parallel and monosynaptically to tarsal motor neurones and to the intersegmental interneurones.     

Sustained saturating level of glycine induces changes in NR2B-containing-NMDA receptor localization in the CA1 region of the hippocampus

Post-synaptic actions of glycine are terminated by specialized transporters. There are two genes encoding glycine transporters, GlyT1 and GlyT2. Glycine acts as a co-agonist at N -methyl- d -aspartate glutamatergic receptors (NMDARs). Blockage of GlyT1 enhances NMDAR function by controlling ambient glycine concentrations. Using whole-cell patch-clamp recordings of acute hippocampal slices, we investigated NMDAR kinetics of CA1 pyramidal neurons of mice expressing 50% of GlyT1 (GlyT1+/−). In this study, we report that the glycine modulatory site of the NMDAR at CA1 synapses is saturated in GlyT1+/− but not in wild-type (WT) mice. We also found that the effect of ifenprodil, a highly selective NR2B-containing-NMDAR antagonist, is significantly reduced at CA1 synapses in GlyT1+/− compared to WT mice while immunoblotting experiments do not show significant differences for NR1, NR2A-B-C-D subunits in both types of mice, suggesting alteration in NR2B-containing-NMDAR localization under a state of chronic saturating level of endogenous glycine. Using a pharmacological approach with MK-801 and DL-TBOA, we discriminated synaptic vis-à-vis extra-synaptic NMDARs. We found that NR2B-containing-NMDARs are expressed at a higher level in the extra-synaptic area of CA1 pyramidal neurons from GlyT1+/− compared to WT mice. Our results demonstrate that chronic saturating level of glycine induces significant changes in NMDAR localization and kinetic. Therefore, results from our study should help to gain a better understanding of the role of glycine in pathological conditions.     

Bifemelane induces translocation of protein kinase C in the CA3, but not the CA1, region of guinea-pig hippocampus

We made use of the [3H]phorbol 12,13-dibutyrate binding assay to investigate the effects of bifemelane on the subcellular distribution of protein kinase C in the CA3 and CA1 regions of guinea-pig hippocampal slices. Bifemelane, a drug that augments the long-term potentiation in the CA3 region, significantly induced the translocation of [3H]phorbol 12,13-dibutyrate binding activity from the cytosol to the membrane in a dose-dependent manner (10(-8) to 10(-6) M) and with no effects on total binding activity in the CA3 region. Bifemelane, at a concentration of 10(-6) M, was without effect on the subcellular distribution of [3H]phorbol 12,13-dibutyrate binding activity in the CA1 region. These observations suggest that bifemelane acts directly on the hippocampus to induce translocation of protein kinase C in the CA3 region. Such an effect may be associated with the bifemelane-induced augmentation of the long-term potentiation in this region of the brain.     

Phase resetting analysis of high potassium epileptiform activity in CA3 region of the rat hippocampus

The theory of phase resetting can reveal important information about the dynamic behavior of a periodic system when a single brief stimulus is applied to that system at the appropriate time. Phase resetting studies have revealed the existence in some biological systems of a vulnerable stimulus window generating desynchronization and suppression of the activity. The objective of this study was to test the hypothesis that a "singular" stimulus could annihilate this activity. Perfusion with the high-K solution produced synchronous, quasi-periodic population bursts with inter-burst interval of ~0.8-1.5 seconds. A single 0.1 ms duration anodic pulse of programmable delay and magnitude was applied to the somatic layer of the CA3 pyramidal cells. Three types of phase-resetting behavior were observed: (1) Weak resetting with little or no effect on the timing of the subsequent burst, (2) Strong resetting where the applied current pulse delayed the next event by one time period, (3) Singular behavior where the applied pulse partially or completely suppressed the subsequent bursting. The singular stimulus parameter window, however, was very narrow making it difficult to generate the singular behavior reliably. Nevertheless, the results indicate that singularities exist for high potassium neural activity and that a well timed pulse applied with the right amplitude can suppress this activity. This study suggests that phase resetting of a population of neurons is possible for quasi-periodic interictal activity and similar techniques could be applied to the control of epileptic seizures.     

Chloride distribution in the CA1 region of newborn and adult hippocampus by light microscopic histochemistry

GABA, the main inhibitory neurotransmitter in the central nervous system, exerts its effect by rendering the postsynaptic GABAA receptors permeable to chloride ions. Thus, depolarizing or excitatory effects of GABA, experienced in early postnatal life or in certain regions and/or conditions of the adult brain, is thought to be associated with a reversed transmembrane chloride gradient. However, there is only limited direct information about the correlation of the actual excitatory versus inhibitory effects of GABA and the local chloride distribution. Precipitation of chloride with silver is a potential way to immobilize and visualize chloride ions in biological tissue. We examined the applicability of light microscopic histochemistry, based on trapping tissue chloride with silver ions during freeze-substitution or aldehyde fixation, to visualize the chloride distribution in hippocampal slices. The freeze-substitution procedure yielded better chloride retention while with aldehyde fixation tissue preservation was more appropriate. Both methods were qualitative only, had limited applicability to the superficial 20-30 microns of slices, but were able to demonstrate a reduced extracellular-to-intracellular chloride gradient in the CA1 pyramidal neurons of the newborn hippocampus as compared to adult animals. In the 4-aminopyridine model of epilepsy, redistribution of chloride from extracellular to intracellular space could also be demonstrated.     

Auditory neurones in the brain of the cricket Gryllus bimaculatus (De Geer): Ascending interneurones

Two types of auditory interneurone which ascend from the prothoracic ganglion to the brain in the cricket Gryllus bimaculatus (De Geer) are described. Intracellular recordings were made from the axons of the neurones in the brain under closed-field stimulus conditions and the recorded cells then stained with either cobalt or Lucifer Yellow. Both neurone types—the Plurisegmental ascending low frequency neurone 1 (PALF1), and the Plurisegmental ascending high frequency neurone 1 (PAHF1)—show response characteristics which would make them well suited to encoding the conspecific calling and courtship songs respectively. Further, the projection areas of both neurone types in the brain overlap those of previously identified intraganglionic interneurones, particularly in the anterior-ventral protocerebrum, and it is suggested that an auditory neuropile may exist in this region.     

Calretinin-containing interneurons innervate both principal cells and interneurons in the CA1 region of the human hippocampus

Hippocampal interneurons consist of functionally diverse cell types, most of them target the dendrites or perisomatic region of pyramidal cells with a few exceptions, like the calretinin-containing cells in the rat: they selectively innervate other interneurons. However, no electron microscopic data are available about the synaptic connections of calretinin-immunoreactive neurons in the human hippocampus. We aimed to provide these data to establish whether interneuron-selective interneurons indeed represent an essential feature of hippocampal circuits across distant species. Two types of calretinin-immunostained terminals were found in the CA1 region: one of them presumably derived from the thalamic reuniens nucleus, and established asymmetric synapses on dendrites and spines. The other type originating from local interneurons formed symmetric synapses on both pyramidal and interneuron dendrites. Distribution of postsynaptic targets showed that 26.8% of the targets were CR-positive interneuron dendrites, and 25.2% proved to be proximal pyramidal dendrites. CR-negative interneuron dendrites were also contacted (12.4%). Small caliber postsynaptic dendrites were not classified (28%). Somata were rarely contacted (7.6%). The present data suggest that calretinin-positive boutons do show a preference for other interneurons, but a considerable proportion of the targets are pyramidal cells. We propose that interneuron-selective inhibitory cells exist in the human Ammon's horn, and boutons innervating pyramidal cells derive from another cell type that might not exist in rodents.     

Mannitol induces selective astroglial death in the CA1 region of the rat hippocampus following status epilepticus

In the present study, we addressed the question of whether treatment with mannitol, an osmotic diuretic, affects astrogliovascular responses to status epilepticus (SE). In saline-treated animals, astrocytes exhibited reactive astrogliosis in the CA1-3 regions 2-4 days after SE. In the mannitol-treated animals, a large astroglial empty zone was observed in the CA1 region 2 days after SE. This astroglial loss was unrelated to vasogenic edema formation. There was no difference in SE-induced neuronal loss between saline- and mannitol-treated animals. Furthermore, mannitol treatment did not affect astroglial loss and vasogenic edema formation in the dentate gyrus and the piriform cortex. These findings suggest that mannitol treatment induces selective astroglial loss in the CA1 region independent of vasogenic edema formation following SE. These findings support the hypothesis that the susceptibility of astrocytes to SE is most likely due to the distinctive heterogeneity of astrocytes independent of hemodynamics. [BMB Reports 2015; 48(9): 507-512]     

Function of synapses in the CA1 region of the hippocampus: their contribution to the generation or control of epileptiform activity

1. In the kainic acid lesioned hippocampus there is a loss of functional inhibition that is associated with reduction of the IPSPs recorded intracellularly from the surviving CA1 pyramidal cells. The possible pre- or postsynaptic origin of this change has been investigated. 2. Iontophoretic application of GABA to the soma and dendrites of CA1 pyramidal cells indicated that there had been no change in the efficacy of the postsynaptic GABA receptors on these cells. 3. Although a pre-synaptic mechanism is implicated, at one week post lesion we were unable to find any difference in the Ca+ dependent K+ evoked release of endogenous GABA. However, at survival times greater than 1 week immunohistological studies showed a decrease in the number of somatostatin positive non-pyramidal cells in the stratum oriens of the CA1 area. 4. In addition to the reduction of functional inhibition, changes in excitatory neurotransmitter mechanisms were also found to contribute to the epileptiform burst discharge. A slow component of the epileptiform EPSP recorded from CA1 pyramidal cells has been recorded and was found to be antagonized by the NMDA-receptor antagonist D-APV. 5. Methods of controlling epileptiform activity in the kainic acid lesioned hippocampus have been tested. Stimulation of the substantia nigra and ventral tegmental areas produced profound inhibition of pyramidal cell activity in control hippocampi; however, they, were found to be ineffective in controlling the epileptiform burst. 6. A second method involved the use of hippocampal suspension grafts. Whilst this approach has yielded some encouraging data, further studies are necessary before the mechanism of the improvement in inhibitory synaptic function can be explained.     

Novel control by the CA3 region of the hippocampus on neurogenesis in the dentate gyrus of the adult rat

The dentate gyrus is a site of continued neurogenesis in the adult brain. The CA3 region of the hippocampus is the major projection area from the dentate gyrus. CA3 sends reciprocal projections back to the dentate gyrus. Does this imply that CA3 exerts some control over neurogenesis? We studied the effects of lesions of CA3 on neurogenesis in the dentate gyrus, and on the ability of fluoxetine to stimulate mitotic activity in the progenitor cells. Unilateral ibotenic-acid generated lesions were made in CA3. Four days later there was no change on the number of either BrdU or Ki67-positive progenitor cells in the dentate gyrus. However, after 15 or 28 days, there was a marked reduction in surviving BrdU-labelled cells on the lesioned side (but no change in Ki-67+ cells). pCREB or Wnt3a did not co-localise with Ki-67 but with NeuN, a marker of mature neurons. Lesions had no effect on the basal expression of either pCREB or Wnt3a. Subcutaneous fluoxetine (10 mg/kg/day) for 14 days increased the number of Ki67+ cells as expected on the control (non-lesioned) side but not on that with a CA3 lesion. Nevertheless, the expected increase in BDNF, pCREB and Wnt3a still occurred on the lesioned side following fluoxetine treatment. Fluoxetine has been reported to decrease the number of “mature” calbindin-positive cells in the dentate gyrus; we found this still occurred on the side of a CA3 lesion. We then showed that the expression GAP-43 was reduced in the dentate gyrus on the lesioned side, confirming the existence of a synaptic connection between CA3 and the dentate gyrus. These results show that CA3 has a hitherto unsuspected role in regulating neurogenesis in the dentate gyrus of the adult rat.