Firing properties of anatomically identified neurons in the medial septum of anesthetized and unanesthetized restrained rats

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.

Modulation of hippocampal theta oscillations and spatial memory by relaxin-3 neurons of the nucleus incertus

Hippocampal theta rhythm is thought to underlie learning and memory, and it is well established that "pacemaker" neurons in medial septum (MS) modulate theta activity. Recent studies in the rat demonstrated that brainstem-generated theta rhythm occurs through a multisynaptic pathway via the nucleus incertus (NI), which is the primary source of the neuropeptide relaxin-3 (RLN3). Therefore, this study examined the possible contribution of RLN3 to MS activity, and associated hippocampal theta activity and spatial memory. In anesthetized and conscious rats, we identified the ability of intraseptal RLN3 signaling to modulate neuronal activity in the MS and hippocampus and promote hippocampal theta rhythm. Behavioral studies in a spontaneous alternation task indicated that endogenous RLN3 signaling within MS promoted spatial memory and exploratory activity significantly increased c-Fos immunoreactivity in RLN3-producing NI neurons. Anatomical studies demonstrated axons/terminals from NI/RLN3 neurons make close contact with septal GABAergic (and cholinergic) neurons, including those that project to the hippocampus. In summary, RLN3 neurons of the NI can modulate spatial memory and underlying hippocampal theta activity through axonal projections to pacemaker neurons of the MS. NI/RLN3 neurons are highly responsive to stress and express corticotropin-releasing factor type-1 receptors, suggesting that the effects observed could be an important component of memory processing associated with stress responses.

Discharge profiles of identified GABAergic in comparison to cholinergic and putative glutamatergic basal forebrain neurons across the sleep-wake cycle

Whereas basal forebrain (BF) cholinergic neurons are known to participate in processes of cortical activation during wake (W) and paradoxical sleep (PS or P, also called REM sleep), codistributed GABAergic neurons have been thought to participate in processes of cortical deactivation and slow-wave sleep (SWS or S). To learn the roles the GABAergic neurons might play, in relation to cholinergic and glutamatergic neurons, we juxtacellularly recorded and labeled neurons during natural sleep-wake states in head-fixed rats. Neurobiotin (Nb)-labeled cells were identified immunohistochemically as choline acetyltransferase (ChAT)+, glutamic acid decarboxylase (GAD)+, or ChAT-/GAD-. Of the latter, some were identified as glutamatergic by immunostaining of their terminals with the vesicular glutamate transporter (VGluT2). In contrast to ChAT+ neurons, which all discharged maximally during W and PS, GAD+ neurons comprised multiple sleep-wake subgroups. Some GABAergic neurons discharged maximally during W and PS, as WP-max active cells (36%), and in positive correlation with gamma electroencephalographic (EEG) activity. Some discharged maximally during SWS, as S-max active cells (28%), and in positive correlation with delta EEG activity. Others increased their discharge progressively during sleep to discharge maximally during PS, as P-max active cells (36%), and in negative association with electromyographic (EMG) activity. ChAT-/GAD- cells comprised WP-max (46%), S-max (17%), P-max (17%), and W-max active cells (14%), whose discharge was positively correlated with EMG activity. GABAergic neurons would thus play similar or reciprocal roles to other cholinergic and glutamatergic BF neurons in regulating cortical activity and muscle tone along with behavior across sleep-wake states.

GABAergic neurons of the medial septum lead the hippocampal network during theta activity

Information processing in the hippocampus critically relies on its reciprocal interaction with the medial septum (MS). Synchronization of the septo-hippocampal system was demonstrated during both major hippocampal activity states, the regular theta rhythm and the large amplitude irregular activity. Previous experimental and modeling data suggest that the MS provides rhythmic drive to the hippocampus, and hippocampo-septal feedback synchronizes septal pacemaker units. However, this view has recently been questioned based on the possibility of intrahippocampal theta genesis. Previously, we identified putative pacemaker neurons expressing parvalbumin (PV) and/or the pacemaker hyperpolarization-activated and cyclic nucleotide-gated nonselective cation channel (HCN) in the MS. In this study, by analyzing the temporal relationship of activity between the PV/HCN-containing medial septal neurons and hippocampal local field potential, we aimed to uncover whether the sequence of events during theta formation supports the classic view of septal drive or the challenging theory of hippocampal pacing of theta. Importantly, by implementing a circular statistical method, a temporal lead of these septal neurons over the hippocampus was observed on the course of theta synchronization. Moreover, the activity of putative hippocampal interneurons also preceded hippocampal local field theta, but by a shorter time period compared with PV/HCN-containing septal neurons. Using the concept of mutual information, the action potential series of PV/HCN-containing neurons shared higher amount of information with hippocampal field oscillation than PV/HCN-immunonegative cells. Thus, a pacemaker neuron population of the MS leads hippocampal activity, presumably via the synchronization of hippocampal interneurons.

The driving system for hippocampal theta in the brainstem: an examination by single neuron recording in urethane-anesthetized rats

The brainstem has been shown to be involved in generating hippocampal theta; however, which brainstem region plays the most important role in generating the rhythm has remained unclear. To reveal which brainstem region triggers the theta, the hippocampal local field potential was recorded simultaneously with single unit activity in the brainstem of urethane-anesthetized rat. The firing latencies before theta onset and offset were compared among recording sites (deep mesencephalic nucleus, DpMe; pedunculopontine tegmental nucleus, PPT; nucleus pontis oralis, PnO). We examined the activities of 59 cells; PPT showed the highest proportion of neurons changing their firing rates at theta onset (14/16, 87.5%). The proportion in the PnO was 14/22 (63.6%), but the neurons in the PnO showed the earliest changes in latencies (0.57s before theta onset). The change in the PPT was 0.96s after theta onset. Regarding the theta offset, the PPT showed the highest proportion of neurons changing their firing rates at theta offset (9/16, 56.3%; the proportion in the PnO was 5/22, 22.7%), but the difference in latent time was not significant among recorded regions. The neurons in the DpMe did not show any remarkable firing tendency at theta onset and offset. From these results, we propose a driving system of hippocampal theta, in which neurons in the PnO first trigger the theta onset and then those in the PPT maintain the theta by activating broadly the brainstem areas for the wave.

The presence of pacemaker HCN channels identifies theta rhythmic GABAergic neurons in the medial septum

The medial septum (MS) is an indispensable component of the subcortical network which synchronizes the hippocampus at theta frequency during specific stages of information processing. GABAergic neurons exhibiting highly regular firing coupled to the hippocampal theta rhythm are thought to form the core of the MS rhythm-generating network. In recent studies the hyperpolarization-activated, cyclic nucleotide-gated non-selective cation (HCN) channel was shown to participate in theta synchronization of the medial septum. Here, we tested the hypothesis that HCN channel expression correlates with theta modulated firing behaviour of MS neurons by a combined anatomical and electrophysiological approach. HCN-expressing neurons represented a subpopulation of GABAergic cells in the MS partly overlapping with parvalbumin (PV)-containing neurons. Rhythmic firing in the theta frequency range was characteristic of all HCN-expressing neurons. In contrast, only a minority of HCN-negative cells displayed theta related activity. All HCN cells had tight phase coupling to hippocampal theta waves. As a group, PV-expressing HCN neurons had a marked bimodal phase distribution, whereas PV-immunonegative HCN neurons did not show group-level phase preference despite significant individual phase coupling. Microiontophoretic blockade of HCN channels resulted in the reduction of discharge frequency, but theta rhythmic firing was perturbed only in a few cases. Our data imply that HCN-expressing GABAergic neurons provide rhythmic drive in all phases of the hippocampal theta activity. In most MS theta cells rhythm genesis is apparently determined by interactions at the level of the network rather than by the pacemaking property of HCN channels alone.

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.

The hippocamposeptal pathway generates rhythmic firing of GABAergic neurons in the medial septum and diagonal bands: an investigation using a complete septohippocampal preparation in vitro

The medial septum diagonal band area (MS/DB) projects to the hippocampus through the fornix/fimbria pathway and is implicated in generating hippocampal theta oscillations. The hippocampus also projects back to the MS/DB, but very little is known functionally about this input. Here, we investigated the physiological role of hippocamposeptal feedback to the MS/DB in a complete in vitro septohippocampal preparation containing the intact interconnecting fornix/fimbria pathway. We demonstrated that carbachol-induced rhythmic theta-like hippocampal oscillations recorded extracellularly were synchronized with powerful rhythmic IPSPs in whole-cell recorded MS/DB neurons. Interestingly, we found that these IPSPs evoked rebound spiking in GABAergic MS/DB neurons. In contrast, putative cholinergic and glutamatergic MS/DB neurons responded only weakly with rebound spiking and, as a result, were mostly silent during theta-like oscillations. We next determined the mechanism underlying the rebound spiking that followed the IPSPs in MS/DB GABAergic neurons using phasic electrical stimulation of the fornix/fimbria pathway. We demonstrate that the increased rebound spiking was attributable to the activation of I(h) current, because it was significantly reduced by low concentrations of the I(h) antagonist ZD7288 [4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride]. Together, these results suggest that rhythmical activity in hippocampus is transferred to the MS/DB and can preferentially phase the spiking of GABAergic MS/DB neurons because of their significant expression of I(h) currents. Our data demonstrate that hippocamposeptal inhibition facilitates theta rhythmic discharges in MS/DB GABAergic neurons while favoring the inhibition of most ACh and glutamate neurons.

Dendritic nicotinic receptors modulate backpropagating action potentials and long-term plasticity of interneurons

Stratum radiatum interneurons, unlike pyramidal cells, are rich in nicotinic acetylcholine receptors (nAChRs); however, the role of these receptors in plasticity has remained elusive. As opposed to previous physiological studies, we found that functional alpha7-subunit-containing nAChRs (alpha7-nAChRs) are abundant on interneuron dendrites of rats. Moreover, dendritic Ca2+ transients induced by activation of alpha7-nAChRs increase as a function of distance from soma. The activation of these extrasynaptic alpha7-nAChRs by cholinergic agonists either facilitated or depressed backpropagating action potentials, depending on the timing of alpha7-nAChR activation. We have previously shown that dendritic alpha7-nAChRs are involved in the regulation of synaptic transmission, suggesting that alpha7-nAChRs may play an important role in the regulation of the spike timing-dependent plasticity. Here we provide evidence that long-term potentiation is indeed boosted by stimulation of dendritic alpha7-nAChRs. Our results suggest a new mechanism for a cholinergic switch in memory encoding and retrieval.

Quantitative ultrastructural differences between local and medial septal GABAergic axon terminals in the rat hippocampus

Functionally distinct subsets of hippocampal inhibitory neurons exhibit large differences in the frequency, pattern and short-term plasticity of GABA release from their terminals. Heterogeneity is also evident in the ultrastructural features of GABAergic axon terminals examined in the electron microscope, but it is not known if or how this corresponds to interneuron subtypes. We investigated the feasibility of separating morphologically distinct clusters of terminal types, using the approach of measuring several ultrastructural parameters of GABAergic terminals in the CA1 area of the rat hippocampus. Septo-hippocampal axon terminals were anterogradely labeled by biotinylated dextran amine and visualized by pre-embedding immunogold staining to delineate one homogeneous terminal population. Long series (100-150) of ultrathin sections were cut from stratum oriens and stratum radiatum of the CA1 area, and GABAergic terminals were identified by post-embedding immunogold staining. Stereologically unbiased samples of the total GABAergic axon terminal population and a random sample of the septal axon terminals were reconstructed in 3D, and several of their parameters were measured (e.g. bouton volume, synapse surface, volume occupied by vesicles, mitochondria volume). Septal terminals demonstrated significantly larger mean values for most parameters than the total population of local GABAergic terminals. There was no significant difference between terminals reconstructed in the basal and apical dendritic regions of pyramidal cells, neither for the septal nor for the local population. Importantly, almost all parameters were highly correlated, precluding the possibility of clustering the local terminals into non-overlapping subsets. Factor and cluster analysis confirmed these findings. Our results suggest that similarly to excitatory terminals, inhibitory terminals follow an "ultrastructural size principle," and that the terminals of different interneuron subtypes cannot be distinguished by ultrastructure alone.

Glutamatergic synaptic depression by synthetic amyloid beta-peptide in the medial septum

The medial septum/diagonal band region, which participates in learning and memory processes via its cholinergic and GABAergic projection to the hippocampus, is one of the structures affected by beta amyloid (betaA) deposition in Alzheimer's disease (AD). The acute effects of betaA (25-35 and 1-40) on action potential generation and glutamatergic synaptic transmission in slices of the medial septal area of the rat brain were studied using current and patch-clamp techniques. The betaA mechanism of action through M1 muscarinic receptors and voltage-dependent calcium channels was also addressed. Excitatory evoked responses decreased (30-60%) in amplitude after betaA (2 microM) perfusion in 70% of recorded cells. However, the firing properties were unaltered at the same concentration. This depression was irreversible in most cases, and was not prevented or reversed by nicotine (5 microM). In addition, the results obtained using a paired-pulse protocol support pre- and postsynaptic actions of the peptide. The betaA effect was blocked by calcicludine (50 nM), a selective antagonist of L-type calcium channels, and also by blocking muscarinic receptors with atropine (5 muM) or pirenzepine (1 microM), a more specific M1-receptor blocker. We show that in the medial septal area this oligomeric peptide acts through calcium channels and muscarinic receptors. As blocking any of these pathways blocks the betaA effects, we propose a joint action through both mechanisms. These results may contribute to a better understanding of the pathophysiology at the onset of AD. This understanding will be required for the development of new therapeutic agents.

Noncholinergic lesions of the medial septum impair sequential learning of different spatial locations

The medial septum and diagonal band of Broca (MSDB) are major afferents to the hippocampus and are important for learning, memory, and hippocampal theta rhythm. In the present study, we assessed the effect of cholinergic or noncholinergic MSDB lesions on the sequential learning of different goal locations in the same environment, a type of task that is proposed to require hippocampal theta rhythm. Rats were administered saline, 192-IgG saporin (SAP), or kainic acid (KA) into the MSDB and then behaviorally tested. On any day, a single arm of a radial maze was rewarded with food, but the location of this rewarded arm changed between days. As in previous studies, intraseptal SAP reduced the number of cholinergic neurons although sparing GABAergic septohippocampal neurons. KA had the reverse effect, reducing GABAergic septohippocampal neurons and sparing cholinergic neurons. KA, but not SAP, impaired performance on the repeated acquisition task. Saline and SAP rats showed rapid within-session learning, whereas KA rats were much slower to learn the goal location. Performance on a 30 min retention trial was also impaired, although this may be attributable to incomplete acquisition. These findings provide evidence that noncholinergic, but not cholinergic, MSDB neurons are important in helping the animal deal with high loads of memory interference, and provides partial support for the idea that hippocampal theta rhythm is involved.

Septohippocampal properties ofN-methyl-D-aspartate-induced theta-band oscillation and synchrony

Microinfusion of N-methyl-D-aspartate (NMDA) into apical dendrites of hippocampal CA1 pyramidal cells of urethane-anesthetized rats resulted in long lasting (20-30 min) induction of hippocampal synchrony at the field and cellular level. Power but not frequency of NMDA-induced theta was significantly greater than tail pinch-induced theta activity. This effect was antagonized by intrahippocampal infusion of AP5, but unaffected by i.v. atropine sulfate. During AP5 blockade tail pinch theta frequency and power were significantly reduced. Microinfusion of NMDA into the medial septum also resulted in long lasting induction of hippocampal theta field activity. Contrary to the results of hippocampal NMDA microinfusions, frequency but not power of NMDA-induced theta was significantly greater than tail pinch- induced theta activity. Microinfusion of AP5 into the medial septum significantly lowered power of tail pinch-induced theta but did not affect frequency. Wheel running behavior of rats induced by low levels of electrical stimulation of the posterior hypothalamic nucleus (PH) was completely abolished by microinfusion of AP5 into the medial septum, accompanied by a significant reduction in theta power and frequency. Wheel running and theta were maintained at control levels with high intensity PH stimulation. We propose that: (1) the glutamatergic septohippocampal projection represents a third pathway capable of generating hippocampal field and cellular synchrony, independent of that generated by the septohippocampal cholinergic and GABAergic projections, and (2) the septohippocampal glutamatergic projection serves to function as an interface between cholinergic and GABAergic modulated sensory processing Type 2 theta and movement related Type 1 theta.

A distinct group of non-cholinergic neurons along the mid-line of the septum and within the rat medial septal nucleus

The septum is a critical and integral component of the limbic brain that serves as a link between diverse brain structures while being necessary for human cognition and emotionality. A major anatomical component of the septum is designated as the medial septum/diagonal band of Broca complex (MS/DB). A primary focus of much research has been to investigate cholinergic neurons within the MS/DB, as these are the rodent brain's main source of acetylcholine to the cortex and hippocampus. On the other hand, we have chosen to investigate a specific group of neurons that lie on the midline of the MS/DB in an area distinguished anatomically as the medial septal nucleus (MSN). Based on somatic morphology and electrophysiological characteristics we conclude that these neurons, characterized into three different types, are non-cholinergic.