Organization of the septal region in the rat brain: cholinergic-GABAergic interconnections and the termination of hippocampo-septal fibers

Chemical identity of 5-HT2A receptor immunoreactive neurons of the rat septal complex and dorsal hippocampus

Brain 5-HT2A receptors have been implicated in various behavioural and physiological processes including hippocampus-dependent learning and memory. To clarify the cellular localization and chemical identity of 5-HT2A receptor-immunoreactive (-ir) neurons in the rat septal complex and dorsal hippocampus, an immunofluorescence histochemical study was performed using a monoclonal antibody to the 5-HT2A receptor. Pretreatment with colchicine increased the number of 5-HT2A receptor-ir cell bodies, indicating that the 5-HT2A receptor protein undergoes microtubule-dependent anterograde transport in axons and dendrites. 5-HT2A receptor immunoreactivity was detected in septal cholinergic neurons, identified with an antiserum to the vesicular acetylcholine transporter (VAChT), and in GABAergic cell bodies in the medial septum/diagonal band of Broca, identified with antisera to glutamic acid decarboxylase (GAD) and the calcium-binding protein parvalbumin. In the dorsal hippocampus, 5-HT2A receptor immunoreactivity was demonstrated in cells located in the pyramidal cell layer (CA1-3) throughout the Ammon's horn and in the granular cell layer of the dentate gyrus. Furthermore, 5-HT2A receptor immunoreactivity was present in most hippocampal interneurons identified by the presence of GAD65, parvalbumin, calbindin D-28k, somatostatin and neuropeptide Y. In contrast, 5-HT2A receptor immunoreactivity was present in only a few interneurons containing cholecystokinin and calretinin immunoreactivity. The results suggest that serotonin acting on 5-HT2A receptors can modulate hippocampal functions via direct actions on hippocampal glutamatergic principal cells and indirectly via actions on hippocampal interneurons with different phenotypes as well as GABAergic and cholinergic septohippocampal neurons.

Critically timed ethanol exposure reduces GABAAR function on septal neurons developing in vivo but not in vitro

Six-day 'binge' ethanol intoxication postnatal days (PD) 4-9 delays up-regulation of gamma-aminobutyric acid type A receptors (GABAARs) in developing rat septal neurons [Dev. Brain Res. 130 (2001) 25]. This distortion occurs during synaptogenesis and could contribute to cognitive dysfunction in fetal alcohol syndrome (FAS). Here, we asked two questions concerning requirements for vulnerability to GABAAR blunting by ethanol. First, we asked whether receptor blunting required PD 4-9 ethanol exposure in rat pups and found that just a brief 2-day exposure (PD 8-9) was as effective as all 6 days. However, 2-day exposure on PD 4-5 was ineffective, showing that 'binge' timing was important. We also asked whether 'binge' exposure directly inhibited intrinsic processes of septal neurons and could blunt GABAARs on cells maturing outside the brain. Embryonic septal neurons grown in serum-free dispersed culture developed extensive dendritic arborizations, spontaneous synaptic activity and robust whole-cell GABAAR function, but surprisingly, did not show developmental up-regulation of GABAARs like septal neurons maturing in vivo [Brain Res. 810 (1998) 100]. Furthermore, age-matched 6-day 'binge' ethanol exposure did not blunt GABAAR function in septal neurons in vitro. These results suggest developmental mechanisms driving up-regulation of GABAAR function in septal neurons in vivo briefly becomes vulnerable to ethanol insult in early postnatal life. While septal neurons express comparable functional GABAARs whether maturing in vivo or in vitro, vulnerability to ethanol-induced receptor blunting requires elements of an intact brain environment not replicated in culture.

Characterisation of hyperpolarization-activated currents (I(h)) in the medial septum/diagonal band complex in the mouse

Hyperpolarization-activated cyclic nucleotide gated (HCN) channel subunits are distributed widely, but selectively, in the central nervous system, and underlie hyperpolarization-activated currents (I(h)) that contribute to rhythmicity in a variety of neurons. This study investigates, using current and voltage-clamp techniques in brain slices from young mice, the properties of I(h) currents in medial septum/diagonal band (MS/DB) neurons. Subsets of neurons in this complex, including GABAergic and cholinergic neurons, innervate the hippocampal formation, and play a role in modulating hippocampal theta rhythm. In support of a potential role for I(h) in regulating MS/DB firing properties and consequently hippocampal neuron rhythmicity, I(h) currents were present in around 60% of midline MS/DB complex neurons. The I(h) currents were sensitive to the selective blocker ZD7288 (10 microM). The I(h) current had a time constant of activation of around 220 ms (at -130 mV), and tail current analysis revealed a half-activation voltage of -98 mV. Notably, the amplitude and kinetics of I(h) currents in MS/DB neurons were insensitive to the cAMP membrane permeable analogue 8-bromo-cAMP (1 mM), and application of muscarine (100 microM). Immunofluoresence using antibodies against HCN1, 2 and 4 channel subunits revealed that all three HCN subunits are expressed in neurons in the MS/DB, including neurons that express the calcium binding protein parvalbumin (marker of fast spiking GABAergic septo-hippocampal projection neurons). The results demonstrate, for the first time, that specific HCN channel subunits are likely to be coexpressed in subsets of MS/DB neurons, and that the resultant I(h) currents show both similarities, and differences, to previously described I(h) currents in other CNS neurons.

A parvalbumin-containing, axosomatic synaptic network in the rat medial septum: relevance to rhythmogenesis*

The medial septal diagonal band complex (MS/DB), made up of cholinergic and GABAergic neurons, plays an important role in the generation of the hippocampal theta rhythm. A GABAergic neuron type in the MS/DB that has fast spiking properties was shown previously to contain parvalbumin immunoreactivity and to form axosomatic connections with unidentified somata. The aim in the current study was to determine the neurochemical identities of these target neurons. In slices and in perfused-fixed brain, staining for parvalbumin immunoreactivity first of all revealed the presence of two types of parvalbumin-positive somata in the MS/DB: medially located neurons with parvalbumin-positive basket-like terminals on them, and more laterally located neurons with fewer parvalbumin-positive contacts on them. In MS/DB slices, the terminals of fast spiking neurons filled with biocytin correspondingly made either numerous contacts that surrounded the parvalbumin-positive cell body in basket-like formation, or 1-5 contacts on a localized patch of the soma. These contacts were shown by electron microscopy to form synaptic junctions. No terminals of biocytin-filled fast spiking neurons were observed on cholinergic neurons, and dual staining in perfused-fixed brain did not reveal the presence of parvalbumin-containing terminals on cholinergic somata. Our results suggest therefore that there are two subtypes of parvalbumin-containing neuron in the MS/DB, and that these are interconnected via axosomatic synapses. The contrasting topographical organization of the two types of parvalbumin-containing neuron suggests that they may receive different types of afferent input, but this will require substantiation in future studies. We propose that generation of rhythmic activity in the MS/DB is controlled by contrasting contributions from two types of parvalbumin-positive neuron, and that the role of the cholinergic neuron is modulatory.

Hippocampal theta rhythm is reduced by suppression of the H-current in septohippocampal GABAergic neurons

Hippocampal learning and memory tasks are tightly coupled to the hippocampal theta rhythm, which is critically dependent on the medial septum/diagonal band of Broca (MSDB) although the underlying mechanisms remain unclear. The MSDB sends both cholinergic and GABAergic projections to the hippocampus. Here we show that: (i) septo-hippocampal GABAergic but not cholinergic neurons have a pacemaking current, the H-current, and that its selective blockade by ZD7288 reduces their spontaneous firing in rat brain slices; and (ii), local infusions of ZD7288 into the MSDB reduce exploration and sensory evoked hippocampal theta bursts in behaving rats. Thus, the H-current in septohippocampal GABAergic neurons modulates the hippocampal theta rhythm.

Picrotoxin increased acetylcholine release from rat cultured embryonic septal neurons

GABA is a major inhibitory neurotransmitter in the mature mammalian brain. In the early stages of brain development, it has been reported that GABA(A) receptor stimulation and the associated increase in Cl(-) conductance lead to membrane depolarization. In this study, we tested the effects of picrotoxin, a GABA(A) receptor Cl(-) channel blocker, on spontaneously released acetylcholine (ACh) from cultured rat embryonic septal cells. Picrotoxin increased spontaneously released ACh. These results indicate that blockade of GABA-activated Cl(-) channel increases neuronal excitability even in an early stage of the development.

Biphasic effects of cannabinoids on acetylcholine release in the hippocampus: site and mechanism of action

Cannabinoids have been shown to critically modulate cholinergic neurotransmission in the hippocampus, yet opposing effects of cannabinoid receptor 1 (CB1R) agonists on hippocampal synaptic acetylcholine (ACh) efflux have been reported. This study shows that administration of a synthetic CB1R agonist results in a biphasic, dose-dependent, effect on hippocampal ACh: a low (0.5 mg/kg, i.p.) and a high (5 mg/kg, i.p) dose of WIN55,212-2 induces a transient stimulation and a prolonged inhibition of hippocampal ACh efflux, respectively. Both effects of WIN55,212-2 are mediated through CB1 receptors coupled to Gi but involve different neuroanatomical sites. Thus, intrahippocampal infusion of the CB1R antagonist SR141716A or pertussis toxin blocked the inhibition of hippocampal ACh release induced by the high dose of WIN55,212-2, but was without effect on the stimulatory action of the low dose. In contrast, this latter effect was blocked by SR141716A or pertussis toxin infused, in dual microdialysis experiments, in the septum, in which the majority of cholinergic cell bodies projecting to the hippocampus reside. The stimulatory and inhibitory effects of WIN55,212-2 on hippocampal ACh involve dopamine D1 and D2 receptor activation, respectively, given that pretreatment with D1 and D2 receptor antagonists prevents the respective actions of WIN55,212-2. We propose that the in vivo observed biphasic effects of CB1R agonists on hippocampal ACh release result from a differential, functional association of anatomicaly distinct subpopulations of CB1-Gi coupled receptors to neurotransmitter systems that have opposing effects on ACh release. This concept could provide a theoretical framework to understand endocannabinoids as state-dependent modulators of neuronal activity.

Acetylcholinesterase inhibitors activate septohippocampal GABAergic neurons via muscarinic but not nicotinic receptors

Acetylcholinesterase (AChE) inhibitors, which increase synaptic levels of available acetylcholine (ACh) by preventing its degradation, are the most extensively prescribed drugs for the treatment of Alzheimer's disease. In animals, AChE inhibitors improve learning and memory, reverse scopolamine-induced amnesia, and produce hippocampal theta rhythm. The medial septum/diagonal band of Broca (MSDB), which maintains hippocampal theta rhythm and associated mnemonic functions via the septohippocampal pathway, is considered a critical locus for mediating the effects of AChE inhibitors. Using electrophysiological recordings and fluorescent labeling techniques to identify living septohippocampal neurons in rat brain slices, we report that AChE inhibitors, in the absence of exogenous ACh, produce a profound excitation in 94% of septohippocampal GABAergic neurons and an inhibition in 24% of septohippocampal cholinergic neurons. The inhibitory and excitatory effects of AChE inhibitors, presumably, occur due to accumulation of ACh that is released locally within the MSDB via axon collaterals of septohippocampal cholinergic neurons. The excitatory effects of AChE inhibitors on septohippocampal GABAergic neurons were blocked by muscarinic but not nicotinic receptor antagonists, especially by the M3 receptor antagonist, 4-diphenylacetoxy-N-methylpiperidine mustard, and not by M1 or M2/M4 muscarinic receptor antagonists. M3 muscarinic receptor mRNA colocalized with the calcium-binding protein, parvalbumin, a marker of septohippocampal GABAergic neurons. These findings may be useful in designing therapeutic strategies that do not depend on endogenous ACh and may therefore be effective in situations where AChE inhibitors cease to be effective, such as in progressive neurodegeneration.

Intraseptal infusion of oxotremorine impairs memory in a delayed-non-match-to-sample radial maze task

The medial septal nucleus is part of the forebrain circuitry that supports memory. This nucleus is rich in cholinergic receptors and is a putative target for the development of cholinomimetic cognitive-enhancing drugs. Septal neurons, primarily cholinergic and GABAergic, innervate the entire hippocampal formation and regulate hippocampal formation physiology and emergent function. Direct intraseptal drug infusions can produce amnestic or promnestic effects depending upon the type of drug administered. However, intraseptal infusion of the cholinomimetic oxotremorine has been reported to produce both promnestic and amnestic effects when administered prior to task performance. The present study examined whether post-acquisition intraseptal infusion of oxotremorine would be promnestic or amnestic in a delayed-non-match-to-sample radial maze task. In this task rats must remember information about spatial locations visited during a daily sample session and maintain that information over extended retention intervals (hours) in order to perform accurately on the daily test session. Treatments may then be administered during the retention interval. Alterations in maze performance during the test session an hour or more after treatment evidences effects on memory. In the present study, intraseptal infusion of oxotremorine (1.0-10.0 microg) produced a linear dose-related impairment of memory performance. Importantly, we also observed disrupted performance on the day after treatment. This persistent deficit was related only to memory over the retention interval and did not affect indices of short-term memory (ability to avoid repetitive or proactive errors during both the pre- and post-delay sessions). The persistent deficit contrasts with the acute amnestic effects of other intraseptally administered drugs including the cholinomimetics carbachol and tacrine. Thus, intraseptal oxotremorine produced a preferential disruption of memory consolidation as well as a persistent alteration of medial septal circuits. These findings are discussed with regards to multi-stage models of hippocampal-dependent memory formation and the further development of therapeutic strategies in the treatment of mild cognitive impairment as well as age-related decline and Alzheimer's dementia.

Glucose increases hippocampal extracellular acetylcholine levels upon activation of septal GABA receptors

Activation of septal GABA receptors impairs learning and memory and this effect likely involves an influence on the hippocampus. We found previously that the memory-impairing effects of septal infusions of the GABA agonist muscimol are reversed by hippocampal infusions of glucose and suggested that glucose reverses these deficits by increasing hippocampal acetylcholine (ACh). In this study, we report that septal infusions of muscimol produce dose-dependent decreases in ACh levels in hippocampal dialysates. Importantly, increasing glucose levels in the hippocampus elevates hippocampal extracellular ACh levels in rats given septal infusions of muscimol, but not in rats given vehicle. Thus, glucose increases hippocampal extracellular ACh levels when the ACh system is inhibited, an effect that likely contributes to the effects of glucose on memory.

Role of the medial septum diagonal band of Broca cholinergic neurons in oestrogen-induced spine synapse formation on hippocampal CA1 pyramidal cells of female rats

Oestrogen is known to influence pyramidal cell spine synapse plasticity in the CA1 subfield of the hippocampus. Apart from direct oestrogen action on the hippocampus, oestrogen effects mediated by subcortical structures are known to be important. The purpose of this study was to investigate whether the medial septum diagonal band of Broca (MSDB) takes part in mediating oestrogen effects to the hippocampus. Special attention was given to the role of cholinergic MSDB neurons that project to the hippocampus, as a rather large population of them contains oestrogen receptors and, consequently, may be sensitive to oestrogen signals. Adult female rats were ovariectomized. Oestradiol- and cholesterol-filled cannulae (control) were implanted into the MSDB. To selectively eliminate the cholinergic population of MSDB neurons of oestrogen-treated animals, a group of rats was injected with 192 IgG-saporin (SAP) into the lateral ventricle 1 week before the cannula implant. Immunostaining with anti-choline acetyltransferase and parvalbumin (PA) showed that cholinergic but not PA-containing GABAergic neurons were substantially reduced in the MSDB of SAP rats. Comparative electron microscopic unbiased stereological analysis on the spine synapse density of CA1 area pyramidal cells was performed between all animal groups. Rats that received oestradiol-filled cannulae showed a higher (30%) spine synapse density than control animals. Oestrogen-treated rats that had received SAP treatment showed no significant difference to controls. Thus, this observation indicates that septo-hippocampal cholinergic neurons are involved in mediating oestrogen effects to the hippocampus. The relevance of this observation to mnemonic functions and Alzheimer's disease is discussed.

Gonadal hormones act extrinsic to the hippocampus to influence the density of hippocampal astroglial processes

The important effects of estrogen on the morphology of hippocampal neurons are well established. The mechanisms leading to such changes, nevertheless, have proved confusingly complex, since interactions between glia and neurons, as well as neuronal influences from other brain fields, are involved. This study addresses the possibility that estrogen-sensitive projections from the medial septum/diagonal band of Broca induce astroglial reactions. Estrogen- and cholesterol-filled (controls) cannulae were implanted into the medial septum/diagonal band of Broca of adult ovariectomized rats. Comparative semiquantitative immunohistochemical analysis on the density of the glial fibrillary acidic protein-containing processes and cells were performed on hippocampal slices of locally estrogen-treated and control animals. Rats that received estrogen-filled cannulae showed a lower density of glial processes in the hippocampal CA1 and CA3 subfields than animals of the control group. These effects could not be observed in the dentate gyrus. Cell counts revealed no significant difference in the number of glial fibrillary acidic protein-positive cells in any of the examined areas. Two major conclusions can be drawn from these results. First, the data show that estrogen, in fact, has an indirect influence on hippocampal cells through septo-hippocampal projections. Furthermore, estradiol can have an indirect negative effect on hippocampal astrocytes, causing a reduction in the density of their processes.

Immunocytochemical characterization of hippocamposeptal projecting GABAergic nonprincipal neurons in the mouse brain: a retrograde labeling study

The neurochemical contents of hippocamposeptal projecting nonprincipal neurons were examined in the mouse brain by using retrograde labeling techniques. We used the immunofluorescent multiple labeling method with a confocal laser-scanning microscope. First of all, the hippocamposeptal projecting nonprincipal neurons were glutamic acid decarboxylase 67-immunoreactive (IR), i.e., these hippocamposeptal projecting nonprincipal neurons were immunocytochemically GABAergic in the mouse brain. Next, most (93.0%) of the hippocamposeptal projecting GABAergic neurons were somatostatin-like immunoreactive (SS-LIR). The SS-LIR hippocamposeptal projecting neurons were frequently found in the stratum oriens of the CA1 and CA3 regions, and were also occasionally found in the stratum radiatum, stratum lucidum, and stratum pyramidale of the CA3 region. They were also frequently found in the dentate hilus. On the other hand, at least 40.6% of SS-LIR neurons in the hippocampus projected to the medial septum. Next, 38.0% of hippocamposeptal projecting GABAergic neurons were calbindin D28K (CB)-IR. Although the distribution of the CB-IR hippocamposeptal projecting neurons was generally similar to that of the SS-LIR projecting neurons in Ammon's horn, they were never seen in the dentate hilus. At least 22.1% of CB-IR GABAergic neurons in the hippocampus projected to the medial septum. Furthermore, 5.8% of hippocamposeptal projecting GABAergic neurons were parvalbumin-IR, which were most always found in Ammon's horn. Finally, no hippocamposeptal projecting GABAergic neurons were neuronal nitric oxide synthase-IR nor calretinin-IR. These results indicate that the SS-LIR neurons play a crucial role in the hippocamposeptal projection of the mouse brain, and they are also assumed to be involved in the theta oscillation of the mouse hippocampus.

Immunohistochemistry of c-fos in Mouse Brain During Postnatal Development: Basal Levels and Changing Response to Metrazol and Kainate Injection

Levels and cellular distribution of FOS, the product of c-fos (onco)gene, were studied by immunohistochemistry during the development of mouse brain at rest and after the administration of convulsants. Basal FOS immunoreactivity became detectable only after postnatal day 20 (P20). Metrazol and kainate at the appropriate doses induced convulsions at all ages but, in both cases, FOS accumulated in limbic areas (particularly in the dentate gyrus) only after a certain age: P20 for kainate and P30 for Metrazol. Surprisingly, considering the different molecular targets of Metrazol and kainate, respectively, and the different type of convulsions elicited, the cell groups in the limbic areas in which FOS increased were the same in the two cases. These results suggest that both drugs produced FOS increase by finally activating the same circuit. During ontogeny, the ability to accumulate FOS, which appears after P20, could be the sign of the attained maturity of signal transduction mechanisms in the cells of the hippocampal formation; endogenous signals originating from the activity of the nervous system increase the basal FOS levels and exogenous signals (i.e. like those given, probably locally, by kainate) further increase these levels. Metrazol manifests its capability to induce FOS accumulation only at later ages. We suggest that this occurs because the Metrazol target is probably distant from the hippocampal region and thus the maturity of a nerve pathway(s) is also required for c-fos induction.

Social and individual recognition in rodents: Methodological aspects and neurobiological bases

What animals know about each other, and how they construct and use knowledge of their social world involves at least an ability to recognise different social categories. Although much evidence has accumulated that animals are able to identify and classify other individuals into different categories, few studies have definitively demonstrated true individual recognition, i.e. discrimination between individuals on the basis of their idiosyncratic characteristics. Furthermore, the neural structures and pathways involved in social and, a fortiori, individual recognition have as yet been poorly investigated. This paper discusses various methods and measures currently used to assess different forms of social categorisations in animals, with special reference to rodents. Recent progress concerning the neurobiological bases involved in social recognition is also discussed. Finally, integrative perspectives for studying individual recognition in the context of social cognition is underlined in relation to different approaches investigating rodents' ability to use learned olfactory information.

The effects of afferent stimulation on neurons in slices of the medial septal area and their modulation by biologically active substances in hibernating and awake ground squirrels

Evoked neuron activity in slices of the medial septal area and its modulation by neuropeptides and monoamines was studied in two groups of ground squirrels--hibernating and awake animals. Electrical stimulation of the medial forebrain bundle evoked predominantly inhibitory effects of different durations. In addition, responses were seen consisting of resetting of the phase of background volleys to the stimulus after initial inhibition: there were also small numbers of short-latency single-spike responses. All the neuropeptides tested. which had been identified from the brains of hibernating animals, induced differentiated reversible effects consisting of modulation of responses; changes in evoked activity were seen significantly more often than shifts in spontaneous activity. The effects depended on the state of the animal. Thus. peptide TSKYR increased the duration of inhibition in hibernating ground squirrels but shortened inhibition in awake animals. Peptide TSKY. which had little effect in hibernating animals, increased the duration of inhibition in awake animals. Dipeptide DY. which decreased the duration of inhibition and increased the amplitude of the activatory components of responses in hibernating ground squirrels. had little effect in awake animals. The effects of noradrenaline and serotonin correlated to a large extent with their effects on spontaneous activity. It is suggested that endogenous substances are involved in creating the conditions required for increasing the latent excitability and reactivity of septal neurons during hibernation. This allows the medial septal area to function as a "sentry post," allowing the receipt of signals and urgent arousal during hibernation.

Collateral projections from the median raphe nucleus to the medial septum and hippocampus

It has previously been shown that the median raphe nucleus (MR) is a source of pronounced projections to the septum and hippocampus. The present study examined collateral projections from MR to the medial septum (MS) and to various regions of the hippocampus. The fluorescent retrograde tracers, Fluororuby and Fluorogold, were injected into the septum and hippocampus, respectively, and the median raphe nucleus was examined for the presence of single- and double-labeled neurons. The dorsal raphe nucleus (DR) was also examined for the presence of single- and double-labeled cells and comparisons were made with the MR. The main findings were: (1) pronounced numbers of retrogradely labeled cells (approximately 50 cells/section) were present in MR with injections in the MS or in various regions of the hippocampus; (2) approximately 8-12% of MR cells were double-labeled following paired injections in the MS-CA1, MS-CA3, and MS-dentate gyrus of the dorsal hippocampus, the lateral MS-dentate gyrus, and the MS-ventral hippocampus; (3) single- and double-labeled cells were intermingled throughout MR and present in greater numbers in the rostral than caudal MR; and (4) significantly more single- and double-labeled cells were present in MR than in DR with all combinations of injections. These findings demonstrate that MR projects strongly to the MS and hippocampus, and that a significant population of MR neurons (8-12%) sends collateral projections to both sites. It is well established that the MR nucleus serves a direct role in the desynchronization of the electroencephalographic (EEG) activity of the hippocampus-or the blockade of the hippocampal theta rhythm. The MR neurons that we have identified with collateral projections to the septum and hippocampus may be critically involved in the modulation/control of the hippocampal EEG. A role for the MR in memory associated functions of the hippocampus is discussed.

A model of theta rhythm production in the septal-hippocampal system and its modulation by ascending brain stem pathways

Recent experimental observations have disclosed the existence of a septal-hippocampal feedback circuit, composed of medial septum diagonal band of Broca (ms-dbB) GABAergic projections to the inhibitory interneurons of the hippocampus, and hippocampal GABAergic projections to the ms-dbB, the major targets of which are the GABAergic septo-hippocampal projection cells. We propose that this feedback circuit provides the mechanism for the rhythmic suppression of interneuronal activity in the hippocampus, which is observed as low-level GABAergic-mediated theta activity. We also propose that this circuit may be the mechanism by which ascending brain stem pathways to the ms-dbB, in particular from the reticular formation, can influence hippocampal information processing in response to particular behavioral states, by exercising control over the level and frequency of theta activity in the hippocampus. In support of these proposals, we describe a minimal computational model of the feedback circuit which uses a set of four coupled differential equations describing the average dynamic activity of the populations of excitatory and inhibitory cells involved in the circuit. We demonstrate through simulations the inherently robust 4-6-Hz oscillatory dynamics of the circuit, and show that manipulation of internal connection strengths and external modulatory influences on this circuit changes the dynamics in a way which closely mimics corresponding manipulations in recent neurophysiological experiments investigating theta activity.

Muscarinic tone sustains impulse flow in the septohippocampal GABA but not cholinergic pathway: implications for learning and memory

Systemic infusions of the muscarinic cholinergic receptor antagonists atropine and scopolamine (atr/scop) produce an amnesic syndrome in humans, subhuman primates, and rodents. In humans, this syndrome may resemble early symptoms of Alzheimer's disease. Behavioral studies in rats have demonstrated that the medial septum/diagonal band of Broca (MSDB), which sends cholinergic and GABAergic projections to the hippocampus, is a critical locus in mediating the amnesic effects of atr/scop. The amnesic effects of atr/scop in the MSDB have been presumed but not proven to be caused by a decrease in hippocampal acetylcholine (ACh) release after blockade of a muscarinic tone in the MSDB. Using electrophysiological recordings and fluorescent-labeling techniques to identify living septohippocampal neurons in rat brain slices, we now report that, contrary to current belief, a blockade of the muscarinic tone in the MSDB does not decrease impulse flow in the septohippocampal cholinergic pathway; instead, it decreases impulse flow in the septohippocampal GABAergic pathway via M(3) muscarinic receptors. We also report that the muscarinic tone in the MSDB is maintained by ACh that is released locally, presumably via axon collaterals of septohippocampal cholinergic neurons. As such, cognitive deficits that occur in various neurodegenerative disorders that are associated with a loss or atrophy of septohippocampal cholinergic neurons cannot be attributed solely to a decrease in hippocampal acetylcholine release. An additional, possibly more important mechanism may be the concomitant decrease in septohippocampal GABA release and a subsequent disruption in disinhibitory mechanisms in the hippocampus. Restoration of impulse flow in the septohippocampal GABA pathway, possibly via M(3) receptor agonists, may, therefore, be critical for successful treatment of cognitive deficits associated with neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

Morphology of local axon collaterals of electrophysiologically characterised neurons in the rat medial septal/ diagonal band complex

Neurons in the medial septal/diagonal band complex (MS/DB) in vivo exhibit rhythmic burst-firing activity that is phase-locked with the hippocampal theta rhythm. The aim was to assess the morphology of local axon collaterals of electrophysiologically identified MS/DB neurons using intracellular recording and biocytin injection in vitro. Cells were classified according to previous criteria into slow-firing, fast-spiking, regular-spiking, and burst-firing neurons; previous work has suggested that the slow-firing neurons are cholinergic and that the other types are GABAergic. A novel finding was the existence of two types of burst-firing neuron. Type I burst-firing neurons had significantly longer duration after hyperpolarisation potentials when held at -60 mV, and at -75 mV, type I neurons exhibited a low-threshold spike with more rapid activation and inactivation kinetics than those of type II neurons. We have, also for the first time, described the main features of the local axon collaterals of the five neuron types. All filled neurons possessed a main axon that gave forth 1-12 local primary axon collaterals. All electrophysiological types, except for the type I burst-firing neuron, had a main axon that coursed toward the fornix. Myelination of the main axon was a prominent feature of all but the slow-firing neurons. Branching of the primary axon collaterals of the fast-spiking and type I burst-firing neurons was more extensive than that of the other cell types, with those of the slow-firing neurons exhibiting the least branching. All cell types possessed axon collaterals of the en passant type, and some in addition had twiglike or basketlike axon terminals. All cell types made synapses on distal dendrites; a proportion of the fast-spiking and burst-firing cells in addition had basketlike terminals that made synaptic contacts on proximal dendrites and on somata. Two morphological types of somata were postsynaptic to the basket cells: large (20-30-microm) oval cells with dark cytoplasm, and large oval cells with paler cytoplasm, often with an apical dendrite. The presence of lamellar bodies in the large dark neurons suggests that they may be cholinergic neurons, because previous work has localised these structures in some neurons that stain for choline acetyltransferase. Our work suggests therefore that there may be GABAergic neurons in the MS/DB that form basket synaptic contacts on at least two types of target cell, possibly cholinergic and GABAergic neurons, which means that the basket cells could play a key role in the generation of rhythmic activity in the MS/DB.

Neural correlates of sensory gating in the rat: decreased Fos induction in the lateral septum

In the P(50) gating or conditioning-testing paradigm in the rat, two identical click stimuli are presented with an inter-click interval of 500 ms. The reaction towards the second click, as measured with evoked potentials, is reduced in respect to that towards the first click; this phenomenon is called sensory gating. In the present experiments, the inter-click interval was varied systematically and auditory evoked potentials were measured. Sensory gating was found to occur only at intervals between 500 and 1000 ms, but not at longer intervals. Fos immunohistochemistry was then performed using two groups of rats exposed to double clicks: the inter-click interval was 500 ms in the experimental group and 2500 ms in the control group. Fos induction was analyzed in selected brain structures. In the auditory pathways, Fos-immunoreactive neurons were found in both groups of rats in the inferior colliculus and medial geniculate body. Fos-immunoreactive cells were also examined in the septum and hippocampus. In the ventral part of the lateral septal nucleus, the labeled neurons were significantly fewer in the experimental animals compared to the control group. Smaller and non-significant quantitative differences of Fos-positive neurons were documented in the medial septum and hippocampal CA1 region. These data point out a selective decrease in the lateral septum of Fos induced by auditory sensory gating, and suggest an involvement of this structure, and possibly of other parts of the septo-hippocampal system, in sensory gating mechanisms. The results might be relevant for theories on sensory gating deficits in schizophrenia.

Age-related changes in rhythmically bursting activity in the medial septum of rats

The effects of aging on the firing of septohippocampal neurons were estimated in unanesthetized, restrained young, old and very old rats (respectively 3, 23 and 30 months). Extracellular recordings were obtained during various states of arousal. The mean spontaneous activity for the overall neuronal population was not modified by aging. In contrast, the percentage of rhythmically bursting neurons was significantly lower in aged rats. During wakefulness, decrease of bursting activity was observed in old and very old rats (P<0.01 and P<0.001) whereas during rapid eye movement sleep it appeared only in the oldest group (P<0.01). The frequency of the bursts decreased in 30-month-old rats during wakefulness while it remained unchanged in both aged groups during rapid eye movement sleep. In old rats, at a time when the cholinergic septal neurons already deteriorated, a third of neurons recorded during rapid eye movement sleep exhibited a pattern of activity composed of long duration bursts with higher intraburst frequency than in young or very old rats. Our study shows that rhythmically bursting septal activity is impaired in aged rats and that the amplitude of the changes depends on advancing age and on states of arousal. Our findings suggest that age-induced loss and atrophy of cholinergic septal neurons contribute to the disorganization of the rhythmic activity but that functional alterations, influenced by the states of arousal, may also be considered.

Septo-hippocampal drug interactions in post-trial memory processing

To determine if serotonin and GABA regulate post-trial memory processing of the cholinergic projection from the septum to the hippocampus, mice were trained on footshock avoidance in a T-maze. Immediately after training, drugs were injected into the septum, hippocampus or both. Retention was tested 1 week after training and drug administration. Ketanserin, a serotonin type 2 receptor antagonist at a dose of 0.5 ng, had no measurable effect on retention, but it reduced the dose of bicuculline, in the septum, or arecoline in the hippocampus that was needed to improve retention. DOI, a serotonin type 2 receptor agonist at a dose of 2.5 ng, had the opposite effect of increasing the doses of bicuculline and arecoline needed to improve retention. Bicuculline, a GABA(A) receptor antagonist at a dose of 0.1 pg, did not affect retention when injected alone into the septum, but it reduced the dose of arecoline needed to improve retention in the hippocampus. Muscimol, a GABA(A) receptor agonist at a dose of 5 ng, injected into the septum, increased the dose of arecoline needed to improve retention. The results of this study are compatible with models that propose that serotonin innervation from the median raphe drives GABA interneurons in the medial septum that synapse on cholinergic neurons projecting to the hippocampus.

Two distinct populations of cholinergic neurons in the septum of raccoon (Procyon lotor): evidence for a separate subset in the lateral septum

The present study focused on cholinergic neurons in the lateral septal region of the raccoon detected by choline acetyltransferase (ChAT)-immunostaining. For comparison of the cholinergic neurons of the medial and lateral septal nuclei, soma sizes were measured, and several antibodies were applied that differentially characterize these cells in several species: low-affinity neurotrophin receptor p75 (p75(NTR)), calbindin-D(28k) (CALB), and constitutive nitric oxide synthase (cNOS). To compare the basic organization of the raccoon septum with that in other mammals, parvalbumin (PARV) immunocytochemistry and Wisteria floribunda-agglutinin (WFA) lectin histochemistry also were used in double-staining experiments. The ChAT-immunoreactive neurons of the rostral lateral septum are arranged in laminae. Accumulations of cholinergic varicosities, often clearly ensheathing noncholinergic neurons, occupy small territories of the rostral septum. Such regions become larger in the caudal septum. They are assumed to correspond to the septohippocampal and septofimbrial nuclei of the rat. In contrast to the large medial septal cholinergic neurons of the raccoon that contain p75(NTR), CALB, and cNOS, the cholinergic neurons of the lateral septum are smaller and do not express these markers. A further peculiarity is that the region of the lateral septum that contains cholinergic neurons corresponds to WFA-labelled extracellular matrix zones that contain chondroitin sulfate proteoglycans. In addition to clustered thread- or ring-like accumulations of the WFA, sparsely labelled perineuronal nets surround the lateral septal cholinergic neurons. Similar to other species that have been investigated, perineuronal nets are completely absent around cholinergic cells of the medial septum. The PARV-containing neurons of this region, however, are enwrapped by perineuronal nets as they are in the rat. Within the medial septum, the PARV-containing neurons are restricted to ventral bilateral territories that are devoid of cholinergic cells. In this respect, they differ from the more vertically arranged PARV-containing medial septal cells in rodents and primates. Apart from striking differences in numbers and distribution patterns, the raccoon lateral septal cholinergic neurons resemble those detected by Kimura et al. (Brain Res [1990] 533:165-170) in the ventrolateral septal region of rat and monkey. Their participation in the functions of the lateral septum remains to be elucidated.

Interactions between hippocampus and medial septum during sharp waves and theta oscillation in the behaving rat

The medial septal region and the hippocampus are connected reciprocally via GABAergic neurons, but the physiological role of this loop is still not well understood. In an attempt to reveal the physiological effects of the hippocamposeptal GABAergic projection, we cross-correlated hippocampal sharp wave (SPW) ripples or theta activity and extracellular units recorded in the medial septum and diagonal band of Broca (MSDB) in freely moving rats. The majority of single MSDB cells (60%) were significantly suppressed during SPWs. Most cells inhibited during SPW (80%) fired rhythmically and phase-locked to the negative peak of the CA1 pyramidal layer theta waves. Because both SPW and the negative peak of local theta waves correspond to the maximum discharge probability of CA1 pyramidal cells and interneuron classes, the findings indicate that the activity of medial septal neurons can be negatively (during SPW) or positively (during theta waves) correlated with the activity of hippocampal interneurons. We hypothesize that the functional coupling between medial septal neurons and hippocampal interneurons varies in a state-dependent manner.

Hippocampal theta rhythm in anesthetized rats: role of AMPA glutamate receptors

The present study describes the effects of intraseptal infusions of 1 nmol AMPA and 12 nmol NBQX on both frequency and amplitude of physostigmine-induced theta rhythm in urethane-anesthetized rats. Infusion of AMPA increased the theta frequency. This effect was blocked by a prior infusion of NBQX. Infusion of NBQX decreased the theta amplitude, and this effect was not altered by AMPA. These results suggest that the septal AMPA/glutamate receptors exert subtle modulatory influences on septohippocampal cells involved in theta rhythm generation.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Intrinsic oscillations in CA3 hippocampal pyramids: physiological relevance to theta rhythm generation

The theta rhythm is the most remarkable hippocampal activity correlated with various physiological and behavioral phenomena. Although analyzed by numerous investigators, during the last five decades, the mechanisms that lead to its generation still remain reason of debate. In the present report it is shown that hippocampal neurons, recorded from juvenile and adult slices, are endowed with intrinsic properties that allow the generation of a steady oscillatory activity. The frequency of this rhythmic ongoing process is highly sensitive to the level of the membrane potential, reaching values in the theta range, up to 6 Hz, for more depolarized values of membrane potential. Membrane potential oscillations are unmasked by loading pyramidal neurons with intracellular cesium, are sodium-independent, and are generated by the sequential activation of calcium and potassium conductances. Finally, like theta rhythm, regularly occurring membrane potential oscillations can be detected since postnatal day 10 and their frequency increases with age, i.e., during the following 2 weeks.

Immunolesion of the cholinergic basal forebrain: effects on functional properties of hippocampal and septal neurons

Deficits in cholinergic function have been documented in a variety of brain disorders including Alzheimer's Disease and, to a lesser extent, in normal ageing. In the present article, we have reviewed our recent findings on the effects of the loss of basal forebrain cholinergic neurons on the functional properties of the septohippocampal pathway. In vivo and ex vivo investigations were performed in rats following basal forebrain cholinergic lesion with the specific immunotoxin 192 IgG-saporin. Our results suggest a significant contribution of cholinergic neurons in the rhythmically bursting activity recorded within the medial septum. In addition, they give evidence that acetylcholine may tonically decrease the glutamatergic synaptic responses in the hippocampus whereas the GABAergic mediated inhibitory potentials are not affected. The possible contribution of these cholinergic mechanisms in the age-related functional alterations of the septohippocampal activity is discussed.

GABA receptors and benzodiazepine binding sites modulate hippocampal acetylcholine release in vivo

In the present study, the regulation of acetylcholine release from the ventral hippocampus by gamma-aminobutyric acid (GABA) was investigated in vivo. GABA receptor agonists and antagonists were administered locally in the medial septum and the adjacent vertical limb of the diagonal band of Broca, or in the hippocampus by retrograde dialysis. Acetylcholine release was measured in the ventral hippocampus. In addition, the modulation of acetylcholine release via septal benzodiazepine binding sites was assessed by intraseptal administration of an agonists and an antagonist at the benzodiazepine binding site. Intraseptal administration of the GABA(A) receptor agonist muscimol and the GABA(B) receptor agonist baclofen, but not the agonist of the benzodiazepine binding site midazolam, decreased acetylcholine release in the hippocampus. The GABA(A) receptor antagonist bicuculline and the antagonist of the benzodiazepine binding site flumazenil, but not the GABA(B) receptor antagonist 3-N-(3,4,-dichlorobenzyl) aminopropyl-P-diethoxymethyphosphinic acid (CGP 52432) increased acetylcholine release in the hippocampus upon intraseptal administration. The same GABA receptor ligands were administered in the ventral hippocampus. CGP 52432 induced a small increase in acetylcholine release, whereas baclofen, muscimol and bicuculline did not affect local acetylcholine release. Thus, endogenous GABA causes tonic inhibition of acetylcholine release in the ventral hippocampus via septal GABA(A) receptors and, to a lesser extent, via GABA(B) receptors in the medial septum and hippocampus. The GABAergic inhibition in the medial septum is reduced by antagonists of the benzodiazepine binding site.

Ultrastructural analysis of the progression of neurodegeneration in the septum following fimbria-fornix transection

The fimbria-fornix transection paradigm has been used as a model of retrograde neurodegeneration within the medial septal nucleus and anterograde degeneration of axon terminals within the lateral septal nucleus. Because the maintenance and survival of neurons may depend on the integrity of both efferents and afferents, the ultrastructure of neurons in the medial septal nucleus and dorsolateral septal nucleus was analysed at three, seven, 14, 30 days, and six months following unilateral transection of the fimbria-fornix in adult rats. Degeneration of axonal and somatodendritic compartments occurred in both nuclei on the side ipsilateral to fimbria-fornix transection. Degeneration of axons and terminals was present by three days and dissipated thereafter, although degenerating axodendritic and axosomatic terminals were still detected at 14-30 days postlesion. Dendrosomal alterations in both septal nuclei manifested as redistribution of organelles, dispersion and loss of rough endoplasmic reticulum, formation of membrane-bound vacuolar cisternae and membranous inclusions, loss of cytoplasmic matrix, and dispersion of chromatin throughout the nucleoplasmic matrix. These changes occurred in the absence of apparent ultrastructural damage to mitochondria and condensation of the nucleus. Dendritic pathology in both the medial and dorsolateral septal nuclei was most prominent at 14-30 days postlesion, but the neuropil recovered to control appearance by six months postlesion. In contrast, the cytoplasmic rarefaction and vacuolation of neuronal cell bodies were persistent in both the medial septal nucleus and the dorsolateral septal nucleus. We conclude that, following disconnection from the hippocampus, ultrastructural abnormalities occur within neurons in both the medial and lateral septal nuclei. The characteristics and time-course for these changes are similar in both nuclei. The neuropilar degeneration was transient, in contrast to the neuronal cell body injury which was persistent and was morphologically consistent with long-term neuronal atrophy.

Excitatory effects of muscarine on septohippocampal neurons: involvement of M3 receptors

Cholinergic mechanisms in the septohippocampal pathway contribute to several cognitive functions and impaired cholinergic transmission in this pathway may be related to the memory loss and dementia that accompanies normal aging and Alzheimer's disease and behavioral studies suggest that muscarinic mechanisms in the medial septum/diagonal band of Broca (MSDB) may contribute to these functions. The goal of the present study was to begin a characterization of the physiological and pharmacological effects of muscarine on antidromically identified septohippocampal neurons (SHNs). Muscarinic agonists produced a concentration-dependent excitation in >90% of SHNs tested using extracellular recordings in an in vitro rat brain slice preparation. The SHNs excited by muscarine had a broad range of conduction velocities (0.2 to 3.7 m/s; mean: 1.6+/-0.06 m/s; n=110), suggesting involvement of neurons with both slow (possibly cholinergic) and fast (possibly GABAergic) conducting fibers. The muscarine-induced excitations in SHNs were found not to be mediated via M1, M2 or M4 receptors, as they were not blocked by the M1-selective antagonists, pirenzepine or telenzepine or by the M2/M4-selective antagonist, methoctramine. In contrast, the M3-selective antagonist, 4-DAMP-mustard, blocked muscarinic excitations in a majority of SHNs, indicating the presence of M3 as well as non-M3-type responses. McN-A-343, an M1 and M5-selective agonist, excited 33% of neurons tested, confirming involvement of non-M3 receptors (possibly M5) and M3 receptors. Since the cholinergic and GABAergic MSDB neurons together innervate almost every type of hippocampal neuron, the effects of muscarine on SHNs would also have a profound effect on hippocampal circuitry.

GABAergic and cholinergic basal forebrain and preoptic-anterior hypothalamic projections to the mediodorsal nucleus of the thalamus in the cat

The present study examined projections of GABAergic and cholinergic neurons from the basal forebrain and preoptic-anterior hypothalamus to the "intermediate" part of the mediodorsal nucleus of the thalamus. Retrograde transport from this region of the mediodorsal nucleus was investigated using horseradish peroxidase-conjugated wheatgerm agglutinin in combination with peroxidase-antiperoxidase immunohistochemical staining for glutamic acid decarboxylase and choline acetyltransferase. A relatively large number of retrogradely-labelled glutamic acid decarboxylase-positive neurons are located in the basal forebrain, amounting to more than 7% of the total population of glutamic acid decarboxylase-positive cells in this region. Moreover, retrogradely-labelled choline acetyltransferase-positive cells are interspersed among glutamic acid decarboxylase-positive neurons, accounting for about 6% of the total choline acetyltransferase-positive cell population in the basal forebrain. The glutamic acid decarboxylase-positive and choline acetyltransferase-positive retrogradely-labelled neurons are distributed throughout several regions of the basal forebrain, including the medial septum, the diagonal band of Broca, the magnocellular preoptic nucleus, the substantia innominata pars anterior, the substantia innominata pars posterior, and the globus pallidus where only a few retrogradely-labelled neurons were seen. The choline acetyltransferase-positive mediodorsal-projecting neurons are morphologically different from the choline acetyltransferase-positive neurons in the basal forebrain, suggesting that those projecting to the mediodorsal nucleus are a small proportion of the cholinergic neuronal population in the basal forebrain. In the preoptic-anterior hypothalamus, many retrogradely-labelled glutamic acid decarboxylase-positive cells were found, amounting to more than 7% of the total population of glutamic acid decarboxylase-positive cells in this region. These retrogradely-labelled glutamic acid decarboxylase-positive neurons are distributed throughout the preoptic-anterior hypothalamus in a continuous line with those in the basal forebrain, including the lateral preoptic area, the medial preoptic area, the bed nucleus of the stria terminalis, and the anterior and dorsal hypothalamic areas. The highest percentage of mediodorsal-projecting GABAergic neurons is in the anterior lateral hypothalamus where more than 25% of the total population of glutamic acid decarboxylase-positive cells project to the mediodorsal nucleus of the thalamus. Overall, of the large population of retrogradely-labelled neurons in the basal forebrain and preoptic-anterior hypothalamus, a significant proportion are glutamic acid decarboxylase-positive neurons (> 60% in the basal forebrain and > 30% in the preoptic-anterior hypothalamus), while the choline acetyltransferase-positive neurons amount to a smaller percentage of the neurons projecting to the mediodorsal nucleus (< 13% in the basal forebrain and < 2% in the preoptic-anterior hypothalamus). These results provide anatomical evidence of direct GABAergic projections from the basal forebrain and preoptic-anterior hypothalamic regions to the "intermediate" part of the mediodorsal nucleus in the cat. This GABAergic projection field could be the direct pathway by which the basal forebrain directly modulates thalamic excitability and may also be involved in mechanisms modulating electroencephalographic synchronization and sleep through the "intermediate" mediodorsal nucleus.

Parvalbumin immunoreactive neurons in the rat septal complex have substantial glial coverage and receive few direct contacts from catecholaminergic terminals

Our previous studies have demonstrated that septohippocampal neurons in the rat septal complex have substantial glial coverage and have a number of synaptic associations with catecholaminergic terminals. While similar ultrastructural characteristics are observed for septal cholinergic neurons, the morphology and synaptic relations of catecholaminergic terminals with septal GABAergic neurons is largely unknown. Since the GABAergic septohippocampal neurons colocalize the calcium-binding protein, parvalbumin (PVA), the present study examined the ultrastructural relations of PVA neurons with catecholaminergic terminals in the septal complex. Single sections were dually labeled with antibodies to PVA and either tyrosine hydroxylase (TH) or dopamine-beta-hydroxylase (DBH). By light microscopy, processes with TH- and DBH- (TH/DBH) immunoreactivity were near PVA-labeled neurons. By electron microscopy, PVA-labeled perikarya had an average diameter of 14.9+/-6 microm and were ovoid or elongated. PVA-labeled perikarya (n = 124) had a large amount of astrocytic coverage (75+/-14%) and a low amount of terminal coverage (15+/-12%). PVA-labeled perikarya and dendrites mostly were contacted by terminals lacking immunoreactivity for either PVA or TH/DBH (82% of 1,663). Of the TH/DBH terminals or axons near PVA somata and dendrites, few (3% of 1,663) directly contacted them while the majority abutted adjacent glial or neuronal profiles. Some TH/DBH- and PVA-labeled terminals contacted the same dendrites; a few of these contained immunoreactivity for PVA. The results demonstrate that PVA-containing GABAergic septal neurons, like cholinergic neurons, are mostly surrounded by astrocytes and have very little terminal coverage. However, in contrast to cholinergic neurons, PVA-containing neurons are contacted primarily by non-catecholaminergic terminals suggesting that any functional interactions would be indirect. These findings further support the functional diversity of subpopulations of septohippocampal neurons.

The pharmacology of post-trial memory processing in septum

The septum is recognized as important in learning and memory, but relatively little is known about the role of specific neurotransmitter receptors in memory processing in the septum. We evaluated the role of the classical neurotransmitters in mice that were prepared for intraseptal microinfusion of drug solution after footshock avoidance training in T-maze. Retention for the footshock training was determined 1 week after training and drug administration. The results indicated that receptor agonists of dopamine, norepinephrine, glutamate and acetylcholine improved retention, while the antagonists impaired retention. Receptor agonists of serotonin, gamma-amino butyric acid (GABA) and opioids impaired retention, while antagonists improved retention.

Loss of rhythmically bursting neurons in rat medial septum following selective lesion of septohippocampal cholinergic system

The medial septum contains cholinergic and GABAergic neurons that project to the hippocampal formation. A significant proportion of the septohippocampal neurons (SHN) exhibit a rhythmically bursting (RB) activity that is involved in the generation of the hippocampal theta rhythm. The neurochemical nature of septal RB neurons is not firmly established. To address this question, the septal unit activity has been recorded in rats after selective destruction of the cholinergic septal neurons by the immunotoxin 192 IgG-saporin. Experiments have been performed in urethan-anesthetized and unanesthetized rats, 14-21 days after lesion. Acetylcholinesterase (AChE) histochemistry revealed a near-complete loss of cholinergic septal neurons and of cholinergic fibers in the hippocampus. The recorded neurons were located in the medial septum-diagonal band of Broca area. A number of these neurons were identified as projecting to the hippocampus (SHN) by their antidromic response to the electrical stimulation of the fimbria-fornix. In urethan-anesthetized lesioned rats, the percentage of RB neurons decreased significantly as compared with controls (17 vs. 41% for SHNs and 5 vs. 19% for unidentified septal neurons). The axonal conduction velocity and the burst frequency of the SHNs that retained a RB activity were higher in lesioned as compared with control rats. The number of spikes per burst was lower and the burst duration was shorter in lesioned rats as compared with controls. The urethan-resistant hippocampal theta was altered both in terms of frequency and amplitude. In unanesthetized lesioned rats, no RB septal neurons were found during arousal, as compared with 25% in controls. Their number was also markedly reduced during paradoxical sleep (9.7 vs. 38.5%). Histochemistry in 192 IgG-saporin-treated rats showed that RB neurons were found in areas devoid of AChE-positive neurons but containing parvalbumine-positive (presumably GABAergic) neurons. These data show that RB activity is considerably reduced after selective lesion of the cholinergic medial septal neurons. They suggested that the large majority of the RB septal neurons are cholinergic and that the few neurons that display RB activity in lesioned rats are GABAergic.

Neurons in the guinea-pig lateral septum generate rhythmic bursting activities, in vitro, following application of carbachol

We used intracellular recordings in a guinea pig brain slice preparation to characterize the actions of carbachol in neurons of the mediolateral part of the lateral septum (LSml). Bath application of carbachol elicited depolarization and rhythmic bursting activities in a majority of neurons (61.5%) through muscarinic receptors. Within the LSml, three types (I, II and III) of rhythmic bursting neurons could be distinguished on the basis of the properties (waveform, duration and amplitude) of their bursts induced by carbachol. In the three types of neurons, the frequency of the bursts depended on the membrane potential whereas the addition of tetrodotoxin to the bath did not suppress carbachol-induced rhythmic bursting activities. Therefore, these events were not driven by synaptic mechanisms but originated from endogenous properties of LSml neurons activated by carbachol. Thus, LSml neurons can be considered as conditional bursters.

Cortical input to the basal forebrain

The arborization pattern and postsynaptic targets of corticofugal axons in basal forebrain areas have been studied by the combination of anatomical tract-tracing and pre- and postembedding immunocytochemistry. The anterograde neuronal tracer Phaseolus vulgaris leucoagglutinin was iontophoretically delivered into different neocortical (frontal, parietal, occipital), allocortical (piriform) and mesocortical (insular, prefrontal) areas in rats. To identify the transmitter phenotype in pre- or postsynaptic elements, the tracer staining was combined with immunolabeling for either glutamate or GABA, or with immunolabeling for choline acetyltransferase or parvalbumin. Tracer injections into medial and ventral prefrontal areas gave rise to dense terminal arborizations in extended basal forebrain areas, particularly in the horizontal limb of the diagonal band and the region ventral to it. Terminals were also found to a lesser extent in the ventral part of the substantia innominata and in ventral pallidal areas adjoining ventral striatal territories. Similarly, labeled fibers from the piriform and insular cortices were found to reach lateral and ventral parts of the substantia innominata, where terminal varicosities were evident. In contrast, descending fibers from neocortical areas were smooth, devoid of terminal varicosities, and restricted to the myelinated fascicles of the internal capsule en route to more caudal targets. Ultrastructural studies obtained indicated that corticofugal axon terminals in the basal forebrain areas form synaptic contact primarily with dendritic spines or small dendritic branches (89%); the remaining axon terminals established synapses with dendritic shafts. All tracer labeled axon terminals were immunonegative for GABA, and in the cases investigated, were found to contain glutamate immunoreactivity. In material stained for the anterograde tracer and choline acetyltransferase, a total of 63 Phaseolus vulgaris leucoagglutinin varicosities closely associated with cholinergic profiles were selected for electron microscopic analysis. From this material, 37 varicosities were identified as establishing asymmetric synaptic contacts with neurons that were immunonegative for choline acetyltransferase, including spines and small dendrites (87%) or dendritic shafts (13%). Unequivocal evidence for synaptic interactions between tracer labeled terminals and cholinergic profiles could not be obtained in the remaining cases. From material stained for the anterograde tracer and parvalbumin, 40% of the labeled terminals investigated were found to establish synapses with parvalbumin-positive elements; these contacts were on dendritic shafts and were of the asymmetrical type. The present data suggest that corticofugal axons innervate forebrain neurons that are primarily inhibitory and non-cholinergic; local forebrain axonal arborizations of these cells may represent a mechanism by which prefrontal cortical areas control basal forebrain cholinergic neurons outside the traditional boundaries of pallidal areas.

Glutamatergic modulation of cortical acetylcholine release in the rat: a combined in vivo microdialysis, retrograde tracing and immunohistochemical study

The microdialysis technique with one or two probes was used to investigate the modulation of cortically projecting cholinergic neurons by glutamatergic input in the rat in vivo. Male albino Wistar rats (250-300 g) were used. Under chloral hydrate anaesthesia microdialysis membranes were positioned in the parietal cortex, nucleus basalis magnocellularis (NBM) or medial septum. Acetylcholine was assayed using high-performance liquid chromatography (HPLC) with electrochemical detection while GABA was detected using HPLC with fluorimetric detection after derivatization of the amino acid with o-phthalaldehyde. Septo-cortical neurons were retrogradely labelled with fluoro-gold. Double labelling with choline acetyltransferase (ChAT) immunoreactivity was performed to identify these neurons. Our main findings were that: (i) i.c.v. administration of the NMDA antagonist 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 1-5 nmol) increased cortical acetylcholine outflow; (ii) local administration of CPP (100 microM) to the cortex had no effect on cortical acetylcholine outflow; (iii) local administration of CPP (100 microM) to the NBM decreased cortical acetylcholine outflow; (iv) local administration of CPP (100-200 microM) to the septum increased cortical GABA and acetylcholine outflow; (v) administration of muscimol to the septum prevented the effect of CPP on cortical acetylcholine outflow; (vi) retrograde tracing with fluoro-gold labelled cell bodies in the medial septum; (vii) septal fluoro-gold-positive neurons were not ChAT-immunoreactive. Our in vivo neurochemical results, in combination with retrograde tracing and immunohistochemistry, indicate that the cortically projecting cholinergic system is indirectly regulated by a glutamatergic input via a polysynaptic GABAergic circuitry located in the septum.

Galanin-immunoreactive synaptic terminals on basal forebrain cholinergic neurons in the rat

Previous studies have indicated that galanin is one of the most abundant peptides in the basal forebrain and that it has a significant modulatory influence on cholinergic transmission. The aim of the present study was to use a light electron microscopic correlation technique to determine whether galanin-immunoreactive terminals form synaptic contacts with basal forebrain cholinergic cells of the rat. Sections from fixed-perfused brains were stained at the light and electron microscopic levels for galanin and choline acetyltransferase immunoreactivity in the same section by using a dual-colour immunohistochemical method. The results showed that galanin-immunoreactive axonal terminals are unevenly distributed in the medial septal nucleus, the diagonal band, and the nucleus basalis. Galanin-positive synapses were most prominent on choline acetyltransferase-positive neurons in the lateral parts of the nucleus of the diagonal band and in the posterior half of the nucleus basalis, which is where there was the greatest overlap between the distribution of galanin-immunoreactive terminals and choline acetyltransferase-positive neurons. The origins of these galanin-positive terminals are not known, but the results confirm that the basal forebrain galaninergic system has a synaptic influence on basal forebrain cholinergic neurons in the rat.

The projection from the striatum to the nucleus basalis in the rat: an electron microscopic study

Previous studies have shown that the striatum provides synaptic inputs to the globus pallidus and entopeduncular nucleus in which GABA is co-localized with the peptides enkephalin and substance P. The aim of this study in the rat was to determine whether the striatal projections also make synaptic contact with the cholinergic neurons of the nucleus basalis, which lie near to the pallidal areas in the rat brain. The anterograde tracer biocytin was injected into different parts of the striatum, and brain sections were stained for biocytin and choline acetyltransferase immunoreactivity by using a dual colour method. Terminals labelled with biocytin by anterograde transport and which made synaptic contact with choline acetyltransferase-positive soma and dendrites were identified by light-electron microscopic correlation methods. In the cases where the biocytin injections had been made in the dorsal or lateral striatum, biocytin-labelled terminals made synaptic contact with cholinergic cells in the region between the main termination zones in the globus pallidus and the entopeduncular nucleus. In the cases where the injections had been made in the ventromedial and posterior striatum, there was greater overlap between choline acetyltransferase-positive structures and biocytin-labelled terminals in the main termination zones in the globus pallidus or entopeduncular nucleus, but relatively few of these terminals made synaptic contacts on to the cholinergic neurons. The results therefore indicate that the cholinergic nucleus basalis cells receive a relatively sparse synaptic input from all parts of the striatum. It has recently been shown that the cholinergic cells of the nucleus basalis selectively express high levels of substance P and opioid receptor messenger RNAs, while the non-cholinergic pallidal cells have much higher levels of GABA(A) receptor subunit messenger RNAs. It is concluded that the cholinergic neurons of the nucleus basalis in the rat may be selectively responsive to the peptidergic components of the striatal outputs, and that they are most likely to be influenced by both the limbic and sensorimotor parts of the striatum.

Modulation of cognitive processes by transsynaptic activation of the basal forebrain

Each of the neurotransmitter-specific afferents to the basal forebrain (BF) carry different types of information which converge to regulate the activity of cholinergic projections to telencephalic areas. Brainstem monoaminergic and cholinergic inputs are critical for context-dependent arousal. GABAergic afferents are gated by a variety of ascending and descending systems, and in addition provide an intrinsic control of BF output excitability. Corticofugal glutamatergic inputs represent reciprocal connections from sites to which BF afferents project, and carry information about the current level of cortical processing intensity and capacity. Peptidergic inputs arise from hypothalamic sources and locally modulate BF output as a function of motivational and homeostatic processes. The significance of these afferent systems can be studied by examining the behavioral consequences of infusion into the BF of drugs that act on the specific receptor systems. Although traditional analyses suggest that the BF has many behavioral functions that can be subdivided regionally, an analysis of studies employing transsynaptic approaches lead to the conceptualization of the BF as having a uniform function, that of maximizing cortical processing efficiency. The BF is conditionally active during specific episodes of acquisition and processing of behaviorally significant, externally-derived information, and drives cortical targets into a state of readiness by reducing interference and amplifying the processing of relevant stimuli and associations, thus allowing for more efficient processing. This paper describes the transsynaptic approach to studying BF function, reviews the neurobiological and behavioral consequences of altering neurotransmitter-specific inputs to the BF, and explores the functional significance of the BF.

Amphetamine sensitization enhances regional c-fos expression produced by conditioned fear

Chronically administered amphetamine can result in a paranoid psychosis that can be re-induced in former amphetamine abusers by psychological stressors. In an attempt to investigate the neurobiological correlates of this phenomenon, the present study examined the effects of prior D-amphetamine sensitization on regional c-fos expression induced by a psychological stressor. Rats received intermittent footshock in a distinctive environment for 30 min/day for three days. Three days after the last fear conditioning session, the animals received injections of saline or D-amphetamine (4 mg/kg, i.p.) once every second day for 16 days (eight injections in total). After a 14-day drug abstinent period, the animals were placed in the fear conditioning apparatus but without footshock. The amphetamine sensitization procedure significantly enhanced the effects of conditioned fear on c-fos expression in several brain regions. These included the cingulate cortex area 3, agranular insular cortex (layers 2 and 3), claustrum, piriform cortex, the shell region of the nucleus accumbens, medial striatum, ventral lateral septum, and CA3 and polymorphic layer of the hippocampal formation. These results indicate that D-amphetamine sensitization can have long-lasting effects on the neural circuitries activated by conditioned stressors.

A population of supramammillary area calretinin neurons terminating on medial septal area cholinergic and lateral septal area calbindin-containing cells are aspartate/glutamatergic

The excitatory amino acid, aspartate/glutamate content of septal complex calretinin (CR)-, choline acetyltransferase plus substance P-, and Leu-enkephalin (Leu-enk)-containing extrinsic afferents was examined. Experiments were carried out using the transmitter-specific [3H]-D-aspartate retrograde tracer technique in combination with immunostaining for CR, choline acetyltransferase, and Leu-enk. The extrinsic and intrinsic CR innervation of the same brain areas were elucidated on control rats and on animals in which the septum was surgically separated from its ventral afferents. Correlated light and electron microscopic double-immunostaining experiments were used to determine the synaptic connections between CR axon terminals and lateral septal area calbindin (CB)- and medial septal area choline acetyltransferase-immunoreactive neurons. Furthermore, to determine the synaptic power of supramammilloseptal aspartate/glutamatergic neurons on the septal complex, semiquantitative analyses were performed in the supramammillary area on retrogradely (1) [3H]-D-aspartate-radiolabeled and (2) HRP-labeled material. The results demonstrated that a population of the extrinsic CR axons originating in the supramammillary area are aspartate/glutamatergic. These fibers forming asymmetric synaptic contacts terminate on both CB and cholinergic neurons. Intraseptal CR neurons, which establish symmetric synapses, innervate only lateral septal area neurons, including the CB-containing cells. These observations, together with other published data, raise the possibility of a hippocampus-lateral septal (GABAergic CB-containing neurons)-supramammillary area (aspartate/glutamatergic cells)-medial septal (cholinergic neurons)-hippocampus signal loop, which might be involved in the generation and regulation of hippocampal theta rhythm activity.

Supramammillary neural discharge patterns and hippocampal EEG

Rhythmic multiunit activity, leading the negative peak of concurrently recorded dentate rhythmical slow-wave activity (RSA, theta) by a constant degree over a range of RSA frequencies, was observed in the supramammillary nucleus (SuM) after carbachol infusion (5%, 0.25-0.5 microliter) into the nucleus pontis oralis (RPO). As the rhythmic SuM activity survived bilateral transection anterior to SuM, its occurrence is not dependent on descending influences from the septum or hippocampus. Infusion of additional carbachol into the RPO resulted in epileptiform "sharp-waves" in the hippocampal EEG, and correlated nonrhythmic cell discharge in the SuM. Bilateral transection rostral to SuM abolished the sharp-wave related SuM activity (and restored SuM rhythmical activity). Thus, in contrast to RSA related rhythmical SuM discharge, sharp-wave related SuM discharge seems dependent on descending projections from the hippocampus.

Parvalbumin-containing cells of the angular portion of the vertical limb terminate on calbindin-immunoreactive neurons located at the border between the lateral and medial septum of the rat

In the septal complex, both parvalbumin and calbindin neurons cocontain GABA. In the same area, a large number of GABA-GABA synaptic connections can be observed. In order to further characterize their neurochemical nature, as well as the extrinsic and/or intrinsic origin of these GABA terminals, the following experiments were performed: (1) correlated light- and electron-microscopic double immunostaining for calbindin and parvalbumin on septal sections of control rats: (2) light microscopic parvalbumin immunostaining of septal sections after surgical isolation (5 days) of the septum from its telencephalic or (3) hypothalamic afferents; and (4) parvalbumin immunostaining of sections prepared from the entire brain 2 days following horseradish peroxidase injection into the border between the lateral and medial septum. The results demonstrated that: (1) in a well-circumscribed, vertically longitudinal area located between the lateral and medial septum, 0.1-0.6 mm anterior to the bregma, a group of calbindin-containing, nonsomatospiny neurons are surrounded by parvalbumin-immunoreactive baskets; (2) these basket-forming axon terminals establish symmetric synaptic contacts with their targets; and (3) their cells of origin are not in the medial septum, but in the angular portion of the vertical limb. These observations indicate that a portion of the septal complex GABA-GABA synaptic connections represent functional interaction between two different types of GABAergic neurons. The presynaptic GABAergic neurons contain parvalbumin, and the postsynaptic GABAergic cells are immunoreactive for calbindin. Furthermore, a population of the medial septum/diagonal band parvalbumin neurons project only to the hippocampus, while others, which may also send axons to the hippocampus, terminate on lateral septum calbindin cells as well.