Interconnection and synchronization of neuronal populations in the mouse medial septum/diagonal band of Broca.

The medial septum/diagonal band of Broca (MS/DBB) is crucial for hippocampal theta rhythm generation (4-12 Hz). However, the mechanisms behind theta rhythmogenesis are still under debate. The MS/DBB consists, in its majority, of three neuronal populations that use acetylcholine, GABA, or glutamate as neurotransmitter. While the firing patterns of septal neurons enable the MS/DBB to generate rhythmic output critical for the generation of the hippocampal theta rhythm, the ability to synchronize these action potentials is dependent on the interconnectivity between the three major MS/DBB neuronal populations, yet little is known about intraseptal connections. Here we assessed the connectivity between pairs of MS/DBB neurons with paired patch-clamp recordings. We found that glutamatergic and GABAergic neurons provide intraseptal connections and produce sizable currents in MS/DBB postsynaptic cells. We also analyzed linear and nonlinear relationships between the action potentials fired by pairs of neurons belonging to various MS/DBB neuronal populations. Our results show that while the synchrony index for action potential firing was significantly higher in pairs of GABAergic neurons, coherence of action potential firing in the theta range was similarly low in all pairs analyzed. Recurrence analysis demonstrated that individual action potentials were more recurrent in cholinergic neurons than in other cell types. Implementing sparse connectivity in a computer model of the MS/DBB network reproduced our experimental data. We conclude that the interplay between the intrinsic membrane properties of different MS/DBB neuronal populations and the connectivity among these populations underlie the ability of the MS/DBB network to critically contribute to hippocampal theta rhythmogenesis.

Modulation of HCN channels in lateral septum by nicotine.

The effects of addictive drugs most commonly occur via interactions with target receptors. The same is true of nicotine and its multiple receptors in a variety of cell types. However, there are also side effects for given substances that can dramatically change cellular, tissue, organ, and organism functions. In this study, we present evidence that nicotine possesses such properties, and modulates neuronal excitability. We recorded whole-cell voltages and currents in neurons situated in the dorsal portion of the lateral septum in acute coronal brain slices of adult rats. Our experiments in the lateral septum revealed that nicotine directly affects HCN - hyperpolarization-activated cyclic nucleotide gated non-selective cation channels. We demonstrate that nicotine effects persist despite the concurrent application of nicotinic acetylcholine receptors' antagonists - mecamylamine, methyllycaconitine, and dihydro-ß-erythroidine. These results are novel in regard to HCN channels in the septum, in general, and in their sensitivity to nicotine, in particular.

Amyloid ß peptides modify the expression of antioxidant repair enzymes and a potassium channel in the septohippocampal system.

Alzheimer's disease (AD) is a progressive, neurodegenerative brain disorder characterized by extracellular accumulations of amyloid ß (Aß) peptides, intracellular accumulation of abnormal proteins, and early loss of basal forebrain neurons. Recent studies have indicated that the conformation of Aß is crucial for neuronal toxicity, with intermediate misfolded forms such as oligomers being more toxic than the final fibrillar forms. Our previous work shows that Aß blocks the potassium (K(+)) currents IM and IA in septal neurons, increasing firing rates, diminishing rhythmicity and firing coherence. Evidence also suggests that oxidative stress (OS) plays a role in AD pathogenesis. Thus we wished to determine the effect of oligomeric and fibrillar forms of Aß1₋42 on septohippocampal damage, oxidative damage, and dysfunction in AD. Oligomeric and fibrillar forms of Aß1₋42 were injected into the CA1 region of the hippocampus in live rats. The rats were sacrificed 24 hours and 1 month after Aß or sham injection to additionally evaluate the temporal effects. The expression levels of the K(+) voltage-gated channel, KQT-like subfamily, member 2 (KCNQ2) and the OS-related genes superoxide dismutase 1, 8-oxoguanine DNA glycosylase, and monamine oxidase A, were analyzed in the hippocampus, medial, and lateral septum. Our results show that both forms of Aß exhibit time-dependent differential modulation of OS and K(+) channel genes in the analyzed regions. Importantly, we demonstrate that Aß injected into the hippocampus triggered changes in gene expression in anatomical regions distant from the injection site. Thus the Aß effect was transmitted to anatomically separate sites, because of the functional coupling of the brain structures.

Tackling the elusive challenges relevant to conquering the 100-plus year old problem of Alzheimer's disease.

Despite over one hundred years of intense effort studying Alzheimer's disease (AD), we still do not understand its cause(s) and this adversely affects our ability to develop strongly effective treatments and means to prevent it. This is because our research efforts are not aligned to decipher this age-related disorder that, well after its discovery, has become a major cause of death throughout the world. We are therefore recommending a process to analyze some of the principal factors that hinder our progress in this field of research. Recognizing these barriers - and acting on such a recognition by seeking to resolve them experimentally and garnering societal support to do so - will constitute critical steps towards establishing strategies that will lead to a sorely needed paradigm shift in AD research, and ultimately to the prevention and the effective treatment of this devastating condition. This will probably also spur progress in many related neuropsychiatric disorders, and in that sense act as a seminal endeavor with far-reaching consequences. In order to accomplish this complex task, the biomedical community must acknowledge and come to a consensus about the factors that limit our progress, and then work together to generate new algorithms to tackle these fundamental issues rationally, effectively and deliberately.

Biological basis for cerebral dysfunction in schizophrenia in contrast with Alzheimer's disease.

Schizophrenia and Alzheimer's disease are two disorders that, while conceptualized as pathophysiologically and clinically distinct, cause substantial cognitive and behavioral impairment worldwide, and target apparently similar - or nearby - circuitry in regions such as the temporal and frontal lobes. We review the salient differences and similarities from selected historical, nosological, and putative mechanistic viewpoints, as a means to help both clinicians and researchers gain a better insight into these intriguing disorders, for which over a century of research and decades of translational development was needed to begin yielding treatments that are objectively effective, but still very far from entirely satisfactory. Ongoing comparison and "cross-pollination" among these approaches to disorders that produce similar deficits is likely to continue improving both our insight into the mechanisms at play, and the development of biotechnological approaches to tackle both conditions - and related disorders - more rapidly and efficaciously.

Probing and trapping a sensitive conformation: amyloid-ß fibrils, oligomers, and dimers.

Alzheimer's disease (AD) is a devastating neurodegenerative disease with pathological misfolding of amyloid-ß protein (Aß). The recent interest in Aß misfolding intermediates necessitates development of novel detection methods and ability to trap these intermediates. We speculated that two regions of Aß may allow for detection of specific Aß species: the N-terminal and 22-35, both likely important in oligomer interaction and formation. We determined via epitomics, proteomic assays, and electron microscopy that the Aß(42) species (wild type, ΔE22, and MetOx) predominantly formed fibrils, oligomers, or dimers, respectively. The 2H4 antibody to the N-terminal of Aß, in the presence of 2% SDS, primarily detected fibrils, and an antibody to the 22-35 region detected low molecular weight Aß species. Simulated molecular modeling provided insight into these SDS-induced structural changes. We next determined if these methods could be used to screen anti-Aß drugs as well as identify compounds that trap Aß in various conformations. Immunoblot assays determined that taurine, homotaurine (Tramiprosate), myoinositol, methylene blue, and curcumin did not prevent Aß aggregation. However, calmidazolium chloride trapped Aß at oligomers, and berberine reduced oligomer formation. Finally, pretreatment of AD brain tissues with SDS enhanced 2H4 antibody immunostaining of fibrillar Aß. Thus we identified and characterized Aßs that adopt specific predominant conformations (modified Aß or via interactions with compounds), developed a novel assay for aggregated Aß, and applied it to drug screening and immunohistochemistry. In summary, our novel approach facilitates drug screening, increases the probability of success of antibody therapeutics, and improves antibody-based detection and identification of different conformations of Aß.

Medial septal dysfunction by Aß-induced KCNQ channel-block in glutamatergic neurons.

Amyloid ß (Aß) peptides play a central role in the pathophysiology of Alzheimer's disease (AD). The cellular mechanisms underlying Aß toxicity, however, are poorly understood. Here we show that Aß(25-35) and Aß(1-40) acutely and differentially affect the characteristics of 3 classes of medial septum (MS) neurons in mice. In glutamatergic neurons Aß increases firing frequency and blocks the A- and the M-current (I(A) and I(M), respectively). While the I(A) block is similar in other MS neuron classes, the block of I(M) is specific to glutamatergic neurons. I(M) block and a simulated Aß block mimic the Aß-induced increase in spontaneous firing in glutamatergic neurons. Calcium imaging shows that under control conditions glutamatergic neurons rarely fire while nonglutamatergic neurons fire coherently at theta frequencies. Aß increases the firing rate of glutamatergic neurons while nonglutamatergic neurons lose theta firing coherence. Our results demonstrate that Aß-induced dysfunction of glutamatergic neurons via I(M) decrease diminishes MS rhythmicity, which may negatively affect hippocampal rhythmogenesis and underlie the memory loss observed in Alzheimer's disease.

Intrahippocampal amyloid-ß (1-40) injections injure medial septal neurons in rats.

Alzheimer's disease (AD) is a devastating disorder that leads to memory loss and dementia. Neurodegeneration of cholinergic neurons in the septum and other basal forebrain areas is evident in early stages of AD. Glutamatergic neurons are also affected early in AD. In these stages, amyloid-ß-peptide (Aß) plaques are present in the hippocampus and other cortices but not in the basal forebrain, which includes the septum. We postulate that early deposition of hippocampal Aß damages the axon terminals of cholinergic and glutamatergic septo-hippocampal neurons, leading to their degeneration. To determine the mechanisms underlying septal degeneration, fibrillar Aß1-40 was injected into the Cornu Ammonis (CA1) hippocampal region of rats. Controls were injected with reverse peptide Aß40-1. A 16% reduction in NeuN+ cells was observed around the injection sites when compared to controls (p < 0.05) one week after injections. Stereology was used to estimate the number of choline acetyl transferase (ChAT), glutamate and glutamic acid decarboxylase 67 (GAD67) immunoreactive septal neurons. Medial septal ChAT and glutamate immunoreactive neurons were reduced 38% and 26%, respectively by hippocampal injections of Aß1-40 peptide in relation to controls. In contrast, the number of GAD67 inmunoreactive neurons was not significantly reduced. Apoptotic cells were detected in the medial septal region of Aß1-40 treated animals but not in controls. These results indicate that limited Aß-induced hippocampal lesions lead to an overall damage of vulnerable septal neuronal populations, most likely by Aß interaction with septo-hippocampal axon terminals. Thus, axon terminals constitute an important target for novel therapeutics dedicated to control Aß-induced toxicity.

Differential expression of voltage-gated K+ currents in medial septum/diagonal band complex neurons exhibiting distinct firing phenotypes.

The medial septum/diagonal band complex (MSDB) controls hippocampal excitability, rhythms and plastic processes. Medial septal neuronal populations display heterogeneous firing patterns. In addition, some of these populations degenerate during age-related disorders (e.g. cholinergic neurons). Thus, it is particularly important to examine the intrinsic properties of theses neurons in order to create new agents that effectively modulate hippocampal excitability and enhance memory processes. Here, we have examined the properties of voltage-gated, K(+) currents in electrophysiologically-identified neurons. These neurons were taken from young rat brain slices containing the MS/DB complex. Whole-cell, patch recordings of outward currents were obtained from slow firing, fast-spiking, regular-firing and burst-firing neurons. Slow firing neurons showed depolarization-activated K(+) current peaks and densities larger than in other neuronal subtypes. Slow firing total current exhibited an inactivating A-type current component that activates at subthreshold depolarization and was reliably blocked by high concentrations of 4-AP. In addition, slow firing neurons expressed a low-threshold delayed rectifier K(+) current component with slow inactivation and intermediate sensitivity to tetraethylammonium. Fast-spiking neurons exhibited the smaller I(K) and I(A) current densities. Burst and regular firing neurons displayed an intermediate firing phenotype with I(K) and I(A) current densities that were larger than the ones observed in fast-spiking neurons but smaller than the ones observed in slow-firing neurons. In addition, the prevalence of each current differed among electrophysiological groups with slow firing and regular firing neurons expressing mostly I(A) and fast spiking and bursting neurons exhibiting mostly delayer rectifier K(+) currents with only minimal contributions of the I(A). The pharmacological or genetic modulations of these currents constitute an important target for the treatment of age-related disorders.

Septo-hippocampal networks in chronic epilepsy.

The medial septum inhibits the appearance of interictal spikes and seizures through theta rhythm generation. We have determined that medial septal neurons increase their firing rates during chronic epilepsy and that the GABAergic neurons from both medial and lateral septal regions are highly and selectively vulnerable to the epilepsy process. Since the lateral septal region receives a strong projection from the hippocampus and its neurons are vulnerable to epilepsy, their functional properties are probably altered by this disorder. Using the pilocarpine model of temporal lobe epilepsy we examined the pilocarpine-induced functional alterations of lateral septal neurons and provided additional observations on the pilocarpine-induced functional alterations of medial septal neurons. Simultaneous extracellular recordings of septal neurons and hippocampal field potentials were obtained from chronic epileptic rats under urethane anesthesia. Our results show that: (1) the firing rates of lateral septal neurons were chronically decreased by epilepsy, (2) a subset of lateral septal neurons increased their firing rates before and during hippocampal interictal spikes, (3) the discharges of those lateral septal neurons were well correlated to the hippocampal interictal spikes, (4) in contrast, the discharges of medial septal neurons were not correlated with the hippocampal interictal spikes. We conclude that epilepsy creates dysfunctional and uncoupled septo-hippocampal networks. The elucidation of the roles of altered septo-hippocampal neuronal populations and networks during temporal lobe epilepsy will help design new and effective interventions dedicated to reduce or suppress epileptic activity.

Medial septal beta-amyloid 1-40 injections alter septo-hippocampal anatomy and function.

Degeneration of septal neurons in Alzheimer's disease (AD) results in abnormal information processing at cortical circuits and consequent brain dysfunction. The septum modulates the activity of hippocampal and cortical circuits and is crucial to the initiation and occurrence of oscillatory activities such as the hippocampal theta rhythm. Previous studies suggest that amyloid beta peptide (Abeta) accumulation may trigger degeneration in AD. This study evaluates the effects of single injections of Abeta 1-40 into the medial septum. Immunohistochemistry revealed a decrease in septal cholinergic (57%) and glutamatergic (53%) neurons in Abeta 1-40 treated tissue. Additionally, glutamatergic terminals were significantly less in Abeta treated tissue. In contrast, septal GABAergic neurons were spared. Unitary recordings from septal neurons and hippocampal field potentials revealed an approximately 50% increase in firing rates of slow firing septal neurons during theta rhythm and large irregular amplitude (LIA) hippocampal activities and a significantly reduced hippocampal theta rhythm power (49%) in Abeta 1-40 treated tissue. Abeta also markedly reduced the proportion of slow firing septal neurons correlated to the hippocampal theta rhythm by 96%. These results confirm that Abeta alters the anatomy and physiology of the medial septum contributing to septo-hippocampal dysfunction. The Abeta induced injury of septal cholinergic and glutamatergic networks may contribute to an altered hippocampal theta rhythm which may underlie the memory loss typically observed in AD patients.

Modulation of normal and altered hippocampal excitability states by septal networks.

The septal region of the basal forebrain plays a dual role: 1) It modulates hippocampal excitability, facilitating synaptic plasticity within hippocampal circuits. Through this mechanism, the septum facilitates diverse cognitive processes that involve hippocampal circuits. 2) Additionally, the septum maintains the hippocampal networks working within normal ranges, decreasing the probability of abnormal excitability states. Through this second mechanism, the septum prevents the occurrence of epileptic discharges. Thus, septal alterations may lead to both decreased cognitive functions and epilepsy, as observed in elderly patients affected with Alzheimer's disease.

Septo-hippocampal networks in chronically epileptic rats: potential antiepileptic effects of theta rhythm generation.

A series of experiments was carried out testing the hypothesis that the septal region decreases the hippocampal susceptibility to hyperexcitability states through theta rhythm generation. Medial septal neurons were simultaneously recorded with hippocampal field potentials to investigate the septo-hippocampal function in the pilocarpine model of chronic epilepsy. The theta rhythm from chronically epileptic rats had lower amplitude (20% less) and higher frequency than controls (from 3.38 to 4.25 Hz), suggesting that both generator and pacemaker structures are affected during the epileptic process. At the cellular level, the group of rhythmically bursting firing medial septal neurons, in the epileptic animals, significantly and chronically increased their firing rates in relation to controls (from 13.86 to 29.14 spikes/s). Peristimulus histograms performed around hippocampal sharp waves showed that all high-frequency firing neurons, including rhythmically bursting neurons and most slow firing neurons, decreased firing rates immediately after hippocampal epileptic discharges. Thus inhibitory hippocampo-septal influences prevail during hippocampal epileptic discharges. The occurrence of epileptic discharges was reduced 86-97% of the number observed during spontaneous theta and theta induced by sensory (tail pinch) or chemical stimulation (carbachol), suggesting that the presence of the theta state regardless of how it was produced was responsible for the reduction in epileptic discharge frequency. The understanding of the theta rhythm "anti-epileptic" effect at the cellular and molecular levels may result in novel therapeutic approaches dedicated to protect the brain against abnormal excitability states.

Septal networks: relevance to theta rhythm, epilepsy and Alzheimer's disease.

Information processing and storing by brain networks requires a highly coordinated operation of multiple neuronal groups. The function of septal neurons is to modulate the activity of archicortical (e.g. hippocampal) and neocortical circuits. This modulation is necessary for the development and normal occurrence of rhythmical cortical activities that control the processing of sensory information and memory functions. Damage or degeneration of septal neurons results in abnormal information processing in cortical circuits and consequent brain dysfunction. Septal neurons not only provide the optimal levels of excitatory background to cortical structures, but they may also inhibit the occurrence of abnormal excitability states.

Characterization of medial septal glutamatergic neurons and their projection to the hippocampus.

The two neuronal populations that have been typically investigated in the septum use acetylcholine and GABA as neurotransmitters. The existence of noncholinergic, non-GABAergic, most likely glutamatergic septal neurons has recently been reported. However, their morphological characteristics, numbers, distribution, and connectivity have not been determined. Furthermore, the projection of septal glutamatergic neurons to the hippocampus has not been characterized. To address these issues, subpopulations of cholinergic and GABAergic neurons were identified by immunohistochemistry. In addition, the retrograde tracer fluorogold was injected into the hippocampus to determine the characteristics of a glutamatergic septo-hippocampal projection. Our work revealed that although glutamatergic neurons are found throughout the septum, they concentrate in medial septal regions. Using stereological probes, approximately 16,000 glutamatergic neurons were estimated in the medial septal region. Triple immunostaining showed that most glutamatergic neurons do not immunoreact with cholinergic or GABAergic neuronal markers (anti-ChAT or anti-GAD67 antibodies, respectively). Fluorogold injections into CA1, CA3, and dentate gyrus of the hippocampus showed that septal glutamatergic neurons project to each of these hippocampal regions, forming approximately 23% of the septo-hippocampal projection. Most cell bodies of septo-hippocampal glutamatergic neurons were located in the medial septum. The remaining cell bodies were found in the diagonal band. This data shows that glutamatergic neurons constitute a significant neuronal population in the septum and that a subpopulation of these neurons projects to hippocampal regions. Thus, the septo-hippocampal projection needs to be reconsidered as a three neurotransmitter pathway.

Glutamic acid decarboxylase isoforms are differentially distributed in the septal region of the rat.

The septal region of the brain consists of a heterogeneous population of GABAergic neurons that play an important role in the generation of hippocampal theta rhythms. While GABAergic neurons employ two isoforms of the enzyme glutamic acid decarboxylase (GAD) for the synthesis of GABA, distribution of GAD isoforms has not been investigated in the septum. Immunohistochemical techniques were used to investigate the expression of GAD enzymes in medial and lateral septum. GAD65 and GAD67 immunohistochemistry revealed dense fibers and punctuated immunoreactivity in septal regions. While few GAD65-positive neuronal somas were detected in medial septum, a significantly higher number of immunoreactive neurons were detected in lateral septum. GAD65- and GAD67-positive neurons in the lateral septum exhibit higher complexity of dendritic arborizations than in the medial septum where staining was mainly restricted to the soma. Presumptive axon terminals (puncta) showed abundant immunoreactivity predominantly for GAD65 isoforms in all septal regions. This suggests that septal GABAergic neurons differentially express GAD enzymes thereby potentially reflecting functional differences. Differences found between medial and lateral septal GABAergic neuronal populations are in agreement with the concept that medial and lateral septum are brain structures with highly different connectivity and function despite anatomical proximity.

Effect of halothane on type 2 immobility-related hippocampal theta field activity and theta-on/theta-off cell discharges.

Rats were studied in acute and chronic (freely moving) recording conditions during exposure to different levels of the volatile anesthetic halothane, in order to assess effects on hippocampal theta field activity in the chronic condition and on theta-related cellular discharges in the acute condition. Previous work has shown that the generation of hippocampal type 2 theta depends on the coactivation of cholinergic and GABAergic inputs from the medial septum. Based on these data and recent findings that halothane acts on interneuron GABA(A) receptors, we predicted that exposure of rats to subanesthetic levels would result in the induction of type 2 theta field activity. In the chronic condition, exposure to subanesthetic levels of halothane (0.5-1.0 vol %) was found to induce theta field activity during periods of immobility (type 2 theta) with a mean increase of 39% in amplitude (mV) compared to control levels during movement. The total percentage of signal power (V2) associated with peak theta frequencies (80% compared to control levels of 47%) was also increased by halothane. Over the whole range of administered halothane concentrations, theta field frequency progressively declined from a mean peak frequency of 6.5 +/- 0.8 Hz at 0.5 vol % halothane to a mean peak frequency of 4.0 +/- 1.8 Hz at 2.0 vol % halothane. Subsequent administration of a muscarinic cholinergic antagonist, atropine sulfate, selectively abolished all type 2 immobility-related theta field activity, while type 1 movement-related theta was still intact. At anesthetic levels (1.5-2.0 vol %) in acute experiments, hippocampal field activity spontaneously cycled between theta and large-amplitude irregular activity. Analysis of depth profiles in four experiments revealed they were identical to those previously described for rats under urethane anesthesia conditions. In addition, the discharge properties of 31 theta-related cells, classified as tonic and phasic theta-on and tonic and phasic theta-off cells, did not differ significantly from those described previously in rats anesthetized with urethane. These data provide further support for an involvement of GABA(A) receptors in the generation of hippocampal theta.