Pathways through cingulate, neo- and entorhinal cortices mediate atropine-resistant hippocampal rhythmical slow activity

Hippocampal Interneuronal α7 nAChRs Modulate Theta Oscillations in Freely Moving Mice

Muscarinic acetylcholine receptors (mAChRs) are critically involved in hippocampal theta generation, but much less is known about the role of nicotinic AChRs (nAChRs). Here we provide evidence that α7 nAChRs expressed on interneurons, particularly those in oriens lacunosum moleculare (OLM), also regulate hippocampal theta generation. Local hippocampal infusion of a selective α7 nAChR antagonist significantly reduces hippocampal theta power and impairs Y-maze spontaneous alternation performance in freely moving mice. By knocking out receptors in different neuronal subpopulations, we find that α7 nAChRs expressed in OLM interneurons regulate theta generation. Our in vitro slice studies indicate that α7 nAChR activation increases OLM neuron activity that, in turn, enhances pyramidal cell excitatory postsynaptic currents (EPSCs). Our study also suggests that mAChR activation promotes transient theta generation, while α7 nAChR activation facilitates future theta generation by similar stimulations, revealing a complex mechanism whereby cholinergic signaling modulates different aspects of hippocampal theta oscillations through different receptor subtypes.

Chaotic dynamics as a mechanism of rapid transition of hippocampal local field activity between theta and non-theta states

We propose a dynamical model of the local hippocampal circuit realizing the transition between the theta and non-theta states. We model the interaction between hippocampal local rhythm generators and the external periodic input from the medial septum and diagonal band of Broca (MS-DBB). With our model, bifurcation of the nonlinear dynamics serves as a mechanism that realizes two distinctive oscillations in the hippocampus, where the amplitude of the oscillatory input from the MS-DBB works as a bifurcation parameter. We model the network of the hippocampal interneurons with a network of simple class 1 neuron models connected mutually with gap junctions. The model neurons exhibit highly synchronous periodic oscillations under the existence of an external force from the MS-DBB, just as the real hippocampus shows theta oscillation under the rhythmic input from the MS-DBB. The model shows diffusion-induced chaotic dynamics under an aperiodic MS-DBB activity, just as the large amplitude irregular activity appears following the disappearance of the rhythmicity of the MS-DBB neurons in the real brain. The model is consistent with both previous experimental findings reporting the existence of local rhythm generators in the hippocampus and the executive role of the MS-DBB in synchronizing theta oscillation in vivo. Our model also replicates the traveling waves of theta oscillations in two-dimensionally coupled networks.

The nucleus reuniens of the thalamus sits at the nexus of a hippocampus and medial prefrontal cortex circuit enabling memory and behavior

The nucleus reuniens of the thalamus (RE) is a key component of an extensive network of hippocampal and cortical structures and is a fundamental substrate for cognition. A common misconception is that RE is a simple relay structure. Instead, a better conceptualization is that RE is a critical component of a canonical higher-order cortico-thalamo-cortical circuit that supports communication between the medial prefrontal cortex (mPFC) and the hippocampus (HC). RE dysfunction is implicated in several clinical disorders including, but not limited to Alzheimer's disease, schizophrenia, and epilepsy. Here, we review key anatomical and physiological features of the RE based primarily on studies in rodents. We present a conceptual model of RE circuitry within the mPFC-RE-HC system and speculate on the computations RE enables. We review the rapidly growing literature demonstrating that RE is critical to, and its neurons represent, aspects of behavioral tasks that place demands on memory focusing on its role in navigation, spatial working memory, the temporal organization of memory, and executive functions.

Hippocampus and Entorhinal Cortex Recruit Cholinergic and NMDA Receptors Separately to Generate Hippocampal Theta Oscillations

Although much progress has been made in understanding type II theta rhythm generation under urethane anesthesia, less is known about the mechanisms underlying type I theta generation during active exploration. To better understand the contributions of cholinergic and NMDA receptor activation to type I theta generation, we recorded hippocampal theta oscillations from freely moving mice with local infusion of cholinergic or NMDA receptor antagonists to either the hippocampus or the entorhinal cortex (EC). We found that cholinergic receptors in the hippocampus, but not the EC, and NMDA receptors in the EC, but not the hippocampus, are critical for open-field theta generation and Y-maze performance. We further found that muscarinic M1 receptors located on pyramidal neurons, but not interneurons, are critical for cholinergic modulation of hippocampal synapses, theta generation, and Y-maze performance. These results suggest that hippocampus and EC neurons recruit cholinergic-dependent and NMDA-receptor-dependent mechanisms, respectively, to generate theta oscillations to support behavioral performance.

Cholinergic blockade reduces theta-gamma phase amplitude coupling and speed modulation of theta frequency consistent with behavioral effects on encoding

Large-scale neural activation dynamics in the hippocampal-entorhinal circuit local field potential, observable as theta and gamma rhythms and coupling between these rhythms, is predictive of encoding success. Behavioral studies show that systemic administration of muscarinic acetylcholine receptor antagonists selectively impairs encoding, suggesting that they may also disrupt the coupling between the theta and gamma bands. Here, we tested the hypothesis that muscarinic antagonists selectively disrupt coupling between theta and gamma. Specifically, we characterized the effects of systemically administered scopolamine on movement-induced theta and gamma rhythms recorded in the superficial layers of the medial entorhinal cortex (MEC) of freely moving rats. We report the novel result that gamma power at the peak of theta was most reduced following muscarinic blockade, significantly shifting the phase of maximal gamma power to occur at later phases of theta. We also characterize the existence of multiple distinct gamma bands in the superficial layers of the MEC. Further, we observed that theta frequency was significantly less modulated by movement speed following muscarinic blockade. Finally, the slope relating speed to theta frequency, a correlate of familiarity with a testing enclosure, increased significantly less between the preinjection and recovery trials when scopolamine was administered during the intervening injection session than when saline was administered, suggesting that scopolamine reduced encoding of the testing enclosure. These data are consistent with computational models suggesting that encoding and retrieval occur during the peak and trough of theta, respectively, and support the theory that acetylcholine regulates the balance between encoding versus retrieval.

The reuniens and rhomboid nuclei: neuroanatomy, electrophysiological characteristics and behavioral implications

The reuniens and rhomboid nuclei, located in the ventral midline of the thalamus, have long been regarded as having non-specific effects on the cortex, while other evidence suggests that they influence behavior related to the photoperiod, hunger, stress or anxiety. We summarise the recent anatomical, electrophysiological and behavioral evidence that these nuclei also influence cognitive processes. The first part of this review describes the reciprocal connections of the reuniens and rhomboid nuclei with the medial prefrontal cortex and the hippocampus. The connectivity pattern among these structures is consistent with the idea that these ventral midline nuclei represent a nodal hub to influence prefrontal-hippocampal interactions. The second part describes the effects of a stimulation or blockade of the ventral midline thalamus on cortical and hippocampal electrophysiological activity. The final part summarizes recent literature supporting the emerging view that the reuniens and rhomboid nuclei may contribute to learning, memory consolidation and behavioral flexibility, in addition to general behavior and aspects of metabolism.

Local generation of theta-frequency EEG activity in the parasubiculum

The parasubiculum is a major component of the hippocampal formation that receives inputs from the CA1 region, anterior thalamus, and medial septum and that projects primarily to layer II of the entorhinal cortex. Hippocampal theta-frequency (4-12 Hz) electroencephalographic (EEG) activity has been correlated with sensorimotor integration, spatial navigation, and memory functions. The present study was aimed at determining if theta is also generated locally within the parasubiculum versus volume conducted from adjacent structures. In urethan-anesthetized rats, the phase-reversal of theta activity between superficial and deep layers of the parasubiculum was demonstrated using differential recordings from movable bipolar electrodes that eliminate the influence of volume-conducted activity. Parasubicular theta was abolished by atropine, and was in phase with theta in stratum radiatum/lacunosum-moleculare of the CA1 region. Whole cell current-clamp recordings in brain slices were then used to determine if parasubicular theta may be generated in part by membrane potential oscillations in layer II neurons. Membrane potential oscillations occurred in most layer II neurons, including four putative interneurons, when cells were held at near-threshold voltages using current injection. The frequency of oscillations increased from 3.2 to 6.1 Hz when bath temperature was raised from 22 to 32 degrees C, and oscillations persisted in the presence of blockers of fast ionotropic glutamatergic and GABAergic synaptic transmission. Oscillations are therefore likely generated by intrinsic, voltage-dependent ionic conductances. These results indicate that theta field activity is generated locally within the parasubiculum and that intrinsic membrane potential oscillations, synchronized by local inhibitory inputs, may contribute to the generation of this activity.

Effects of systemic lupus erythematosus on spatial cognition and cerebral regional metabolic reactivity in BxSB lupus-prone mice

Brain-reactive auto-antibodies appear as key elements in the progressive CNS disturbances associated with systemic lupus erythematosus. The BxSB lupus prone mice are a model of this pathology, in which a gene located on the Y chromosome provokes a sex specific morbidity in males. This study was aimed to establish and characterize the relationships between behavioral disorders, neurological deficiencies and the aged-related immunological perturbations in this murine model. For this purpose, spatial and motor abilities were evaluated in male and female mice at six and 26 weeks of age. The results showed that the older males were greatly altered in their spatial abilities while the young ones and the females, whatever their age, were not. None of the animals had motor skill and motor learning disabilities. These spatial alterations were associated with modifications of basal neuronal activity measured by the cytochrome oxidase histochemical method in several areas directly or indirectly involved in spatial behavior, such as the hippocampus, the amygdala, the parietal and perirhinal cortex. Immunological study allowed us to correlate the behavioral abnormalities to the appearance of antibodies reactivities against cellular and nuclear components.

Involvement of GABAergic and cholinergic medial septal neurons in hippocampal theta rhythm

Hippocampal theta rhythm (HPCtheta) may be important for various phenomena, including attention and acquisition of sensory information. Two types of HPCtheta (types I and II) exist based on pharmacological, behavioral, and electrophysiological characteristics. Both types occur during locomotion, whereas only type II (atropine-sensitive) is present under urethane anesthesia. The circuit of HPCtheta synchronization includes the medial septum-diagonal band of Broca (MSDB), with cholinergic and gamma-aminobutyric acid (GABA)ergic neurons comprising the two main projections from MSDB to HPC. The primary aim of the present study was to assess the effects of GABAergic MSDB lesions on urethane- and locomotion-related HPCtheta, and compare these effects to those of cholinergic MSDB lesions. Saline, kainic acid (KA), or 192 IgG-saporin (SAP) was injected into MSDB before recording. KA preferentially destroys GABAergic MSDB neurons, whereas SAP selectively eliminates cholinergic MSDB neurons. A fixed recording electrode was placed in the dentate mid-molecular layer, and stimulating electrodes were placed in the posterior hypothalamus (PH), and medial perforant path (PP). Under urethane anesthesia, HPCtheta was induced by tail pinch, PH stimulation, and systemic physostigmine; none of the rats with KA or SAP showed HPCtheta in any of these conditions. During locomotion, HPCtheta was attenuated, but not eliminated, in rats with KA or SAP lesions. Intraseptal KA in combination with either intraseptal SAP or PP lesions reduced locomotion-related HPCtheta beyond that observed with each lesion alone, virtually eliminating HPCtheta. In contrast, intraseptal SAP combined with PP lesions did not reduce HPCtheta beyond the effect of each lesion alone. We conclude that both GABAergic and cholinergic MSDB neurons are necessary for HPCtheta under urethane, and that each of these septohippocampal projections contributes to HPCtheta during locomotion.

Complete sparing of spatial learning following posterior and posterior plus anterior cingulate cortex lesions at 10 days of age in the rat

Neonatal posterior cingulate cortex lesions spare the spatial deficits that characterize adult lesions. The present experiments examined the possibility that the anterior cingulate cortex mediates the spared spatial behavior. Rats were given bilateral lesions of the posterior cingulate cortex or anterior plus posterior cingulate cortex on postnatal days 4 (P4), 10 (P10), or in adulthood (P120). All groups were tested for spatial learning on the Morris place task in adulthood. Adult animals were impaired on place learning relative to controls whereas place learning was spared in all the neonatal groups and sparing was complete in the group receiving day 10 lesions. The results are discussed in relation to neural mechanisms, including fiber rerouting, synaptic changes and generation of new neurons, that may mediate spared spatial following neonatal posterior cingulate cortex lesions. Also discussed is evidence indicating that the neonatal brain, especially the day 10, has a special ability to compensate for injury.

Muscarinic blockade slows and degrades the location-specific firing of hippocampal pyramidal cells

The firing of rat hippocampal pyramidal cells is determined both by the animal's location and by the state of the hippocampal EEG. Because cholinergic transmission plays a role in EEG activity, we expected that its modification would alter place cell activity. We therefore investigated the effects on place cell activity of blocking muscarinic transmission with intracerebroventricular injections of scopolamine. Scopolamine reduced both the rate of place cell discharge inside firing fields and the spatial coherence of the fields; discharge outside of the fields also showed small increases. After injections, fields were shifted farther from their previous location than for saline controls, indicating reduced reproducibility after muscarinic blockade. Scopolamine increased the time rats were stationary, but changes in place cell activity persisted even after analysis was restricted to periods of walking, suggesting that the behavioral changes cannot account for the cell discharge changes. The scopolamine effects were dose dependent to an extent that varied between different measures. The firing rates of interneurons showed only a minor trend to decrease after scopolamine. Nevertheless, the spatial coherence of interneuron firing patterns was reduced, consistent with the recent demonstration that their positional firing is mediated by the location-specific firing of pyramids (Marshall et al., 2002). These results demonstrate that acetylcholine enhances positional firing patterns in the hippocampus. Muscarinic blockade weakens the positional firing of most place cells and therefore renders them less useful for precise representation of the environment. This effect may underlie the difficulties in spatial learning and problem solving caused by abnormalities of cholinergic transmission.

Theta band oscillation and synchrony in the hippocampal formation and associated structures: the case for its role in sensorimotor integration

The current review advances the argument that it is naïve to ascribe a unitary function to the hippocampal formation (HPC). Rather, it is more productive to consider the hippocampal formation as consisting of a number of subsystems, each subsystem defined by its own particular neural circuitry. Among examples of neural circuitry appearing in current hippocampal literature are theta, beta and gamma oscillations, sharp waves, place cells and head orientation cells. Data are reviewed supporting the case that theta band oscillation and synchrony is involved in mechanisms underlying sensorimotor integration. Specifically, the neural circuitry underlying the production of oscillation and synchrony (theta) in limbic cortex and associated structures function in the capacity of providing voluntary motor systems with continually updated feedback on their performance relative to changing environmental (sensory) conditions. A crucial aspect of this performance is the intensity with which the motor programs are initiated and maintained. The ascending brainstem HPC synchronizing pathways make the primary contribution in this regard. These pathways originate in the rostral pontine region, ascend and synapse with caudal diencephalic nuclei, which in turn send projections to the medial septal region. The medial septum functions as the node in the ascending pathways, sending both cholinergic and GABA-ergic projections to the HPC. An updated version of the sensorimotor integration model including anatomical details is presented and discussed.

Age-related changes in rat hippocampal theta rhythms: a difference between type 1 and type 2 theta

The age-related changes in two types of theta rhythms recorded from the hippocampus in young (4 months-old), mature (12-13 months-old) and aged (22-25 months-old) rats were investigated. The type 1 theta rhythm was measured from hippocampal EEG recorded from walking rats and the type 2 theta was measured from the EEG induced by reticular pontin oralis nucleus (PON) stimulation in urethane anesthetized rats. The peak frequency and the peak power were detected from power spectra calculated on each theta sample by fast Fourier transformation (FFT). No age-related alteration was observed on the peak frequency of type 1 theta rhythm. However, on type 2 theta rhythm, the peak frequency was decreased in the aged rats compared with the young and the mature rats. The type 2 theta rhythm is cholinergic, and therefore this result suggests that age-related deterioration can be clearly observed in the cholinergic system including the hippocampus in rats.

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.

Nucleus reuniens thalami modulates activity in hippocampal field CA1 through excitatory and inhibitory mechanisms

The nucleus reuniens thalami (RE) originates dense projections to CA1, forming asymmetrical synapses on spines (50%) and dendrites (50%). The hypothesis that RE input modulates transmission in CA1 through excitation of both pyramidal cells and interneurons was tested using electrophysiological methods in the anesthetized rat. The RE-CA1 afferents were selectively stimulated at their origin; evoked field potentials and unit activity were recorded in CA1. RE-evoked depth profiles showed a prominent negative deflection in the stratum lacunosum-moleculare and a positive one in the stratum radiatum. The lacunosum-moleculare sink-radiatum source configuration is compatible with RE-elicited depolarization of apical dendrites of pyramidal cells. Despite a consistent and robust paired pulse facilitation of RE-evoked field potentials, population spikes in the stratum pyramidale were not detected at any tested condition. This indicates the inability of RE-CA1 input to discharge pyramidal cells. However, stimulation of RE-elicited spiking of extracellularly recorded units in strata oriens/alveus and distal radiatum, indicative of the activation of local interneurons. Thus, RE seems to modulate transmission in CA1 through a (subthreshold) depolarization of pyramidal cells and a suprathreshold excitation of putative inhibitory oriens/alveus and radiatum interneurons. RE-evoked monosynaptic or disynaptic field potentials were associated with stimulation of rostral or caudal RE, respectively. Anatomically, a projection from caudal to rostral RE was demonstrated that can account for the disynaptic RE-CA1 input. Because caudal RE receives input from the hippocampus via the subiculum, we propose the existence of a closed RE-hippocampal circuit that allows RE to modulate the activity in CA1, depending on hippocampal output.

Information flow between hippocampus and related structures during various types of rat's behavior

The relationships among the CA1 field of hippocampus, the entorhinal-piriform area, the subiculum and the lateral septum were studied in various behavioral states in the rat. The EEG signals recorded simultaneously from chronically implanted electrodes were analyzed by means of a multichannel autoregressive (AR) model. Power spectra, ordinary, multiple and partial coherences, and directed transfer functions were calculated. The method of analysis which took into account all signals simultaneously, not pair-wise, made it possible to estimate the spectral characteristics and the directions of the EEG flow between structures. The pattern of the EEG activity propagation depended on the type of behavior, difficulty of the task performed by the animal, and the phase of the trial. Our results not only confirmed the existence of connections between analyzed structures, but also showed that these connections may have different strengths during various types of behavior.

The entorhinal cortex entrains fast CA1 hippocampal oscillations in the anaesthetized guinea-pig: role of the monosynaptic component of the perforant path

Entorhinal inputs reach the hippocampal CA1 field through a trisynaptic circuit involving dentate granule cells and CA3 pyramidal neurons, as well as through a monosynaptic path ending on the distal apical dendrites of CA1 pyramidal cells. The influence of monosynaptic entorhinal inputs onto CA1 operations is poorly understood. In this study, we characterized the involvement of the monosynaptic pathway in the generation of the fast CA1 oscillation bursts (30-60 Hz) that occur in the dorsal hippocampus of anaesthetized guinea-pigs after partial cortex removal. Using multiple-site extracellular and intracellular recording, we found that in this particular preparation, devoid of theta rhythm, fast oscillations are temporally coherent over a large portion of the CA1 region along the hippocampal septotemporal axis. Current source density analysis revealed that fast CA1 oscillations involve two dipoles reflecting synchronous synaptic activities in the stratum lacunosum-moleculare of the hippocampus proper and in the stratum moleculare of the dentate gyrus. These layers constitute the two major termination zones of entorhinal afferents, suggesting that the entorhinal cortex entrains fast CA1 oscillations. This hypothesis was corroborated by the concomitant occurrence of fast oscillation bursts in the entorhinal cortex and CA1 region. Furthermore, fast CA1 oscillations were abolished by lidocaine or tetrodotoxin injections in the entorhinal cortex. Finally, acute interruption of the hippocampal trisynaptic loop did not affect the stratum lacunosum-moleculare dipole recorded extracellularly, but also intracellularly, as high-frequency postsynaptic potentials in CA1 pyramidal cells. These results indicate that the monosynaptic pathway is involved in the genesis of fast CA1 oscillations.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term potentiation and depression of synaptic transmission in rat posterior cingulate cortex

We used stimulation of corpus callosum (CAL) and the subiculo-cingulate tract (SCT), in an in vitro brain slice preparation, to study activity-dependent changes in synaptic efficacy in posterior cingulate cortex (PCC). SCT stimulation monosynaptically excites the apical dendrites of deep laminae (V-VI) pyramidal neurons, while CAL afferents drive these same cells via synapses on their basal dendrites. In contrast, most superficial laminae (II/III-IV) pyramids appear to be driven polysynaptically via ascending axonal collaterals of deep pyramids. In slices retaining these connectivities, we contrasted characteristics of synaptic plasticity in superficial vs deep laminae field and intracellular potentials evoked by conditioning stimuli given at frequencies of 100, 20, 8, 5 and 1 Hz. Tetanic stimulation (100 Hz) of SCT or CAL yielded homosynaptic long-term potentiation (LTP) of each pathway, while stimulus trains of 8-20 Hz did not. 1-5 Hz stimulation of SCT and CAL elicited homosynaptic long-term depression (LTD) of synaptic strength in each pathway. Associative LTD was induced by interleaving 5 Hz pulses to the SCT pathway with 100 Hz theta-burst stimulation of CAL, but was not induced when these stimulus loci were switched. Heterosynaptic non-associative LTD was also observed in the alternate pathway following tetanization of either SCT or CAL. In all cases, LTP and LTD were observed only in deep laminae recordings. In contrast, superficial records showed only paired-pulse facilitation and short-term post-tetanic potentiation. In in vivo experiments in anaesthetized rats, PCC responses to SCT stimulation were contrasted with responses to stimulation of anteroventral and anterodorsal thalamic nuclei (AV/AD). SCT-elicited field potentials closely resembled those evoked in the slice, with maximal amplitude tuned to the 4-8 Hz frequency band. AV/AD stimulation elicited field potentials which were not frequency tuned. Overall, these data suggest that the acute circuit properties of PCC superficial laminae, modulated by thalamic input and synaptic plasticity in deep laminae, can transform hippocampal synaptic inflow before relaying it to extracingulate targets.

Classification of theta-related cells in the entorhinal cortex: cell discharges are controlled by the ascending brainstem synchronizing pathway in parallel with hippocampal theta-related cells

Single-unit discharge patterns of entorhinal cortex (EC) cells were characterized in relation to simultaneously recorded hippocampal (HPC) field activity according to criteria used previously to classify cells in the hippocampal formation, medial septum, cingulate cortex, and caudal diencephalon. EC cells related to HPC theta field activity were classified as 1) phasic theta-on, if they discharged rhythmically, and in phase, with ongoing HPC theta, but nonrhythmically during large, irregular hippocampal field activity (LIA); 2) tonic theta-on, if they discharged nonrhythmically and increased their discharge rates during HPC theta relative to LIA; 3) phasic theta-off, if they discharged rhythmically, and in phase, with ongoing HPC theta, but increased their discharge rates during LIA; and 4) tonic theta-off, if they discharged nonrhythmically and decreased their discharge rates during HPC theta relative to LIA. Cells not meeting any of these criteria were classified as nonrelated. A total of 168 EC cells were recorded, and of these 56 (33%) were classified as theta related, with the remaining 112 (67%) classified as nonrelated. Of the 56 theta-related cells, 41 (73%) had significantly higher discharge rates during HPC theta than during LIA and were classified as theta-on cells (15 phasic theta-on cells and 26 tonic theta-on cells). Nine of the 26 tonic theta-on cells showed a phase relation of their arrhythmic discharges to simultaneously recorded HPC theta field activity. EC phasic theta-on cells did not discharge preferentially on any portion of the HPC theta field recorded from the region of the stratum moleculare of the dentate gyrus. In general, cells classified as phasic revealed a wide distribution of phase preferences. The remaining 15 (26.7%) cells were classified as theta-off cells and discharged at higher rates during HPC LIA than during HPC theta field activity (3 phasic theta-off cells and 12 tonic theta-off cells). Systemic administration of physostigmine significantly increased the discharge rate of tonic and phasic theta-on cells relative to LIA. Electrical stimulation in the posterior hypothalamic region (PH) significantly increased the discharge rate of EC theta-on cells and significantly decreased the discharge rate of EC theta-off cells relative to HPC LIA. The discharge rates of nonrelated EC cells were not influenced by electrical stimulation of the PH. Procaine microinfusion into the medical septum (MS) abolished spontaneously occurring HPC theta and theta induced with PH stimulation. In addition, 5 min after MS procaine, the ability of PH stimulation to modulate EC theta-on cell discharge was abolished. The modulation of cellular discharges produced by PH stimulation recovered by 60 min post-procaine infusion into the MS. The findings support two main conclusions: 1) theta-related cells in the EC are comprised of two main populations of cells, theta-on and theta-off, similar to other regions of limbic cortex and nuclei of the ascending brainstem synchronizing pathway; 2) the ascending brainstem synchronizing pathway exerts both similar and parallel effects on theta-related cells in entorhinal cortex and hippocampus.

Effects of perforant path procaine on hippocampal type 2 rhythmical slow-wave activity (theta) in the urethane-anesthetized rat

Previous research has suggested that the entorhinal cortex plays a major role in the production of type 1 rhythmical slow-wave activity (RSA) recorded in the hippocampus of the freely moving preparation. In the present experiment we investigated the contribution of the entorhinal cortex to the type 2 fields recorded under urethane anesthesia. Rats had stimulating electrodes and cannulae filled with procaine positioned in the perforant pathway of one or both hemispheres. Recording electrodes were positioned in the dorsal hippocampus of each hemisphere to record perforant path and commissural/associational evoked potentials and RSA fields. Following unilateral procaine blockade, a decrease in RSA amplitude was observed in the stratum oriens and fissure regions of both hemispheres. Concomitant with this change in RSA, there was a loss of perforant path evoked responses, although commissural/associational control potentials remained unaltered. A greater reduction in RSA amplitude was observed following bilateral procaine microinfusion. RSA phase reversal also occurred more dorsally in microelectrode depth profiles conducted through the hippocampus during perforant path inactivation. In current source density analyses performed under baseline conditions, large rhythmic sinks were observed in stratum oriens, in stratum radiatum, and in strata adjacent to the hippocampal fissure. A rhythmic source was often observed in stratum pyramidale. Following perforant path inactivation decreases in the magnitude of the phasic sinks located near the fissure and stratum radiatum were observed. In contrast to the reduction in RSA amplitude observed in the stratum oriens region, the sink in this region and the source in stratum pyramidale remained relatively unaltered. These results demonstrate that the entorhinal region contributes to the production of RSA observed under urethane anesthesia. Furthermore, the CSD and amplitude changes following perforant path inactivation suggest that a substantial portion of RSA recorded in stratum oriens may result from ventrally located RSA dipoles.

Hippocampal theta activity following selective lesion of the septal cholinergic system

The characteristic electroencephalographic patterns within the hippocampus are theta and sharp waves. Septal neurons are believed to play an essential role in the rhythm generation of the theta pattern. The present study examined the physiological consequences of complete and selective damage of septohippocampal cholinergic neurons on hippocampal theta activity in rats. A selective immunotoxin against nerve growth factor receptor bearing cholinergic neurons (192 immunoglobulin G-saporin), [Wiley R. G. et al. (1991) Brain Res. 562, 149-153] was infused into the medial septal area (0.11-0.42 microgram). Hippocampal electrical activity was monitored during trained wheel running, drinking and the paradoxical phase of sleep, as well as following cholinomimetic treatment. A moderate dose of toxin (0.21 microgram) eliminated the septohippocampal cholinergic projection, as evidenced by a near total absence of choline acetyltransferase-immunoreactive neurons in the medial septum and the vertical limb of the diagonal band, and by the absence of acetylcholinesterase-positive fibers in the dorsal hippocampus. In the same rats, parvalbumin immunoreactivity, a reliable marker for septohippocampal GABAergic neurons, [Freund T. F. (1989) Brain Res. 478, 375-381], remained unaltered. In addition, retrograde transport of the tracer fluorogold demonstrated that the parvalbumin cell population preserved its axonal projection to the hippocampus. Following toxin treatment, the power of hippocampal theta, but not its frequency, decreased in a dose-dependent manner. Reduction of theta power occurred between three and seven days after the toxin treatment and remained unaltered thereafter up to eight weeks. A dose which eliminated all septohippocampal cholinergic neurons (0.21 microgram) left a small but significant theta peak in the power spectra during wheel running, paradoxical phase of sleep and intraseptal infusion of carbachol (5 micrograms). Peripheral administration of physostigmine (1 mg/kg) induced only slow (1.5-2.0 Hz) rhythmic waves. No changes were observed in the gamma (50-100 Hz) band. These findings indicate that the integrity of the septohippocampal GABAergic projection is sufficient to maintain some hippocampal theta activity. We hypothesize that cholinergic neurons serve to increase the population phase-locking of septal cells and thereby regulate the magnitude of hippocampal theta.

Differential behavioral effects of supracallosal and infracallosal lesions of the septohippocampal pathways: no ameliorative effects of oxotremorine or pilocarpine on radial-maze performance

We examined the effects in young adult female Long-Evans rats of single or combined lesions of the infracallosal and supracallosal septohippocampal pathways on a battery of behavioral tasks over two postoperative periods (14-65 and 75-150 days, respectively). During the first period, rats with lesions of the infracallosal pathways, whether given alone or in combination with lesions of the supracallosal pathways, were more active in the open field and in their home cage, and showed increased reactivity to novel extracage stimuli. Behavioral results during the second postoperative period were similar to those of the first except that rats with lesions of the infracallosal pathways (either alone or in combination with lesions of the supracallosal pathways) were no longer hyperactive in their home cage and rats with the infracallosal lesion alone were no longer hyperactive in the open field. We also observed in rats with lesions of the infracallosal pathways impaired performance in the radial-arm maze task, whether conducted under an uninterrupted protocol (first and second postoperative periods) or with a 1-min intratrial interruption (second postoperative period). Thus, behavioral deficits were observed only in rats with a lesion to the infracallosal component of the septohippocampal pathways, the behavior of rats with the combined lesions being similar to that of rats with single lesions of the infracallosal pathways in most measures. The behavior of rats with lesions of the supracallosal pathways did not differ from that of sham-operated controls in any measure at either postoperative period. Acute, systemic injections of oxotremorine (0.03 or 0.1 mg/kg, ip) or pilocarpine (0.32 or 1.0 mg/kg, ip), two muscarinic agonists, did not affect radial-arm maze performance under either the uninterrupted or interrupted protocol. The use of nonspecific muscarinic agonists does not appear to be sufficient to enhance radial-arm maze performance in rats with infracallosal septohippocampal lesions which, in contrast to supracallosal lesions, were shown to induce a deficit in this task.

Stimulation-induced RSA-like field activity in region CA1 of the hippocampal slice: amplitude maxima and topography

In the present experiment RSA-patterned stimulation was applied to afferents in the CA1 region of the hippocampal slice preparation and depth profiles of the resultant field activity were performed. RSA-patterned stimulation applied to the stratum lacunosum-moleculare region resulted in field activity through the CA1 region with similar morphology, phase, and amplitude profiles to the type 1 RSA profile found in vivo. In contrast, RSA-patterned stimulation applied to the stratum oriens and midstratum radiatum regions did not result in field profiles characteristic of the RSA recorded in vivo. The results of the present study confirm predictions made by previous modelling experiments and CSD analyses performed in the freely moving animal, which suggest that the type 1 RSA profile is primarily the result of distal excitation onto CA1 pyramidal cells.

Theta rhythms in the rat medial entorhinal cortex in vitro: evidence for involvement of muscarinic receptors

Slice preparation obtained from the rat were used to study cholinergically induced field potentials in the medial entorhinal cortex. Perfusion of slices containing medial entorhinal cortex with acetylcholine, muscarine and eserine induced theta-like activity in a frequency range of 3-10 Hz and an amplitude of 200-300 microV. Nicotine, in contrast, did not produce any rhythmical slow waveforms. The cholinergically induced theta-like oscillations were abolished by perfusion of the muscarinic antagonists atropine sulphate and scopolamine but were unaffected by the nicotine blockers hexamethonium and mecamylamine.

Fast (beta) rhythms in the hippocampus: a review

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

Carbachol-induced theta-like activity in entorhinal cortex slices

The present study was conducted for two purposes: the first was to evaluate whether activation of cholinergic receptors of the entorhinal cortex in vitro (complete deafferentation) with carbachol (100 microM) was capable of producing theta (theta)-like slow activity. The second purpose was to determine whether carbachol-induced slow waveforms were mediated by muscarinic or nicotinic receptors. We demonstrated that carbachol was capable of producing theta-like slow activity. This activity was not altered by nicotinic antagonists, (+)-tubocurarine and hexametonium. Atropine and scopolamine, in contrast, completely blocked in vitro induced slow waves, indicating entorhinal muscarinic receptors to be actively involved in the mechanism generating cholinergic theta rhythm.

Firing relations of medial entorhinal neurons to the hippocampal theta rhythm in urethane anesthetized and walking rats

The firing of neurons from layers II and III of medial entorhinal cortex (MEC) was examined in relation to the hippocampal theta rhythm in urethane anesthetized and walking rats. 1) MEC neurons showed a significant phase relation to the hippocampal theta rhythm in both walking and urethane anesthetized rats, suggesting that this region contributes to the generation of both atropine-resistant and atropine-sensitive theta rhythm components. 2) The proportion of phase-locked cells was three times greater in walking rats (22/23 cells) as compared to anesthetized rats (8/23 cells), indicating that MEC cells made a greater contribution during walking theta rhythm. This difference was also manifest in the greater mean vector length for the group of phase-locked MEC cells during walking: 0.39 +/- 0.13 versus 0.21 +/- 0.08. Firing rate differences between walking and urethane conditions were not significant. 3) In walking rats, MEC cells fired on the positive peak of the dentate theta rhythm (group mean phase = 5 degrees; 0 degrees = positive peak at the hippocampal fissure). This is close to the reported phases for dentate granule and hippocampal pyramidal cells. The distribution of MEC cell phases in urethane anesthetized rats was broader (group mean phase = 90 degrees), consistent with the phase data reported for hippocampal projection cells. These findings suggest that medial entorhinal neurons are the principal determinant of theta-related firing of hippocampal neurons and that their robust rhythmicity in walking as compared to urethane anesthesia accounts for EEG differences across the two conditions.

Colchicine-induced deafferentation of the hippocampus selectively disrupts cholinergic rhythmical slow wave activity

It has been proposed that hippocampal rhythmical slow wave activity (RSA or theta-rhythm) induced by sensory stimulation (atropine-sensitive theta) is generated by the cholinergic septo-hippocampal system. Although ablations of the septum or its projections to the hippocampus disrupt hippocampal RSA, such non-selective lesions damage both cholinergic and non-cholinergic septo-hippocampal inputs. The present study assesses the effects of a selective septal neurotoxic lesion on hippocampal electrical activity. Colchicine, which has been reported to be selectively toxic to cholinergic neurons in the medial septum, was injected into the right lateral ventricle, and electrodes were implanted bilaterally into the dorsal hippocampus of female Sprague-Dawley rats. Hippocampal electrical activity was recorded 10-14 days later from the ipsilateral (colchicine-treated) and contralateral (control) hemispheres during locomotor activity or immobility. RSA ranging from 6.3 to 8.7 Hz was evoked in both hippocampi during mobility. Following i.p. administration of an anesthetic dose of urethane, hippocampal RSA at a frequency of 4 Hz could be elicited in the control hemisphere (n = 12) of all animals by pinching the tail. RSA was absent in 6 of 9 animals in the colchicine-treated hemisphere. RSA from control and treated hemispheres persisting after urethane administration was abolished by 5 mg/kg of scopolamine, thus verifying its cholinergic nature. A decrease in the number of choline acetyltransferase (ChAT)-immunoreactive neurons in the medial septum and a depletion of acetylcholinesterase (AChE)-staining in the hippocampus were evident in the hemisphere ipsilateral to colchicine administration. These data support the septal pacemaker hypothesis of hippocampal theta-rhythm and further demonstrate the neurotoxic effect of colchicine on septo-hippocampal cholinergic neurons by the induction of a functional alteration. The selective disruption of cholinergic neurons in the medial septum by colchicine provides a means to dissociate the contribution of septal cholinergic and non-cholinergic components to hippocampal electrical activity.

Relay stations and neurotransmitters between the pallidal region and the hippocampus

The effects of internal pallidum and lateral habenula stimulation on epileptiform activity of cat's hippocampus were studied. A steady interictal activity was induced by locally applied sodium penicillin (PCN) solution. Both pallidal and habenular electrical stimulation caused an increase in spike frequency and amplitude. Intraperitoneally injected atropine sulphate failed to modify pallidal and habenular influences. Intraperitoneal methysergide bimaleate (5-HT antagonist) suppressed the effects of habenular stimulation. In contrast to the effects of pallidal and habenular stimulation, raphe electrical stimulation inhibited hippocampal spiking and intra-raphal muscimol (a GABA receptor agonist) enhanced hippocampal-based epilepsy. After muscimol, raphe stimulation at the same threshold parameters failed to affect hippocampal activity. In cats with habenular lesions hippocampal spike frequency and amplitude were reduced and intra-raphal muscimol did not affect the hippocampus. The results are discussed in the light of a complex interrelationship between basal ganglia and hippocampus. The role of the lateral habenula and of the medial raphe as relay stations between the two regions is emphasized.

Ultrastructure and aspects of functional organization of pyramidal and nonpyramidal entorhinal projection neurons contributing to the perforant path

Identified entorhino-hippocampal projection neurons were investigated for their ultrastructure. Spinous projection neurons (pyramidal and spiny stellate cells) display common features such as symmetric axosomatic terminals on their somata, asymmetric synapses on the spines, and both types of synapses on the dendritic shafts. Their axons descend towards the white matter, branching occasionally via collaterals which establish contact with local spines and rarely on dendritic shafts and somata. The sparsely spinous projection neurons (multipolar and horizontal-bipolar) typically show deep nuclear infolds and symmetric and asymmetric synapses on their somata and dendritic shafts. Axons also collateralize in the soma vicinity and form local synapses. It is concluded that the entorhino-hippocampal projection neurons (both spiny and sparsely spinous) act locally and distally thus performing simultaneously as local-circuit and as projection neurons. In accordance with other morphological and electrophysiological reports it appears likely that the generation, modulation, and suppression of entorhinal excitation waves is mediated by these neurons through direct excitation, feed-forward and feed-back inhibition, and disinhibition.

Basal forebrain cell loss following fimbria/fornix transection

Following fimbria/fornix transection, cells in the medial septum appear to undergo retrograde degeneration as shown by Nissl and acetylcholine esterase (AChE) staining. Recent studies using immunocytochemical techniques have also demonstrated loss of choline acetyltransferase (ChAT) and nerve growth factor receptor (NGFr) labeling of neurons in this region. Whether the apparent loss of ChAT- and NGFr-positive neurons is the result of the actual death of these neurons, or is instead a loss of ChAT enzyme or NGFr expression below levels detectable by immunocytochemical methods, remains an unresolved issue. In order to address this question, rhodamine-labeled fluorescent latex microspheres were injected into the hippocampus where they retrogradely transported to the cell bodies of the medial septum. Five days later these animals received either unilateral or bilateral fimbria/fornix lesions and were allowed to survive an additional 4 weeks. Compared to unlesioned control animals, unilaterally lesioned animals showed a 91% loss of fluorescently labeled cells and bilaterally lesioned animals showed a 93% loss. The inability to detect the fluorescent microspheres in the medial septum suggests that the majority of medial septal cells die after fimbria/fornix transection. ChAT and NGFr immunohistochemical staining were also performed. Cells stained for ChAT were reduced in number by 92% in animals with unilateral lesions and by 75% in animals with bilateral lesions, while NGFr-stained cells were reduced in number by 75% in animals with unilateral lesions and by 68% in animals with bilateral lesions.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve growth factor infusions combined with fetal hippocampal grafts enhance reconstruction of the lesioned septohippocampal projection

A combination of intracerebral grafting and intraventricular infusion of nerve growth factor was used to attempt to reconstruct the cholinergic component of the septohippocampal pathway following fimbria-fornix lesions in the rat. Four groups were tested: lesion only, lesion plus fetal hippocampal graft, lesion plus nerve growth factor, and lesion plus graft plus nerve growth factor. Choline acetyltransferase immunoreactivity, acetylcholinesterase fiber staining and behavior-dependent theta activity on electroencephalogram were used to assess the extent of pathway reconstruction. Nerve growth factor was infused for the first two weeks following the fimbria-fornix lesion, while electrophysiological measurements and histological analysis were conducted six to eight months later. The lesion plus graft plus nerve growth factor infusion group had long-term savings of choline acetyltransferase-immunoreactive cells as compared to the lesion only or lesion plus graft groups. In addition the lesion plus graft plus nerve growth factor infusion group had more extensive reinnervation of the hippocampus compared to all other groups. Behavioral-dependent theta activity on electroencephalogram was observed in some animals of both lesion plus graft and lesion plus graft plus nerve growth factor infusion groups, but not in other groups; however, unlike intact animals, the restored theta could be blocked completely by scopalamine. These results demonstrate that a combination of short-term intraventricular nerve growth factor infusion and fetal hippocampal grafts enhances reconstruction of the damaged septohippocampal circuit.

The role of serotonin in the control of cerebral activity: studies with intracerebral 5,7-dihydroxytryptamine

Intact rats treated with centrally acting antimuscarinic (atropinic) drugs display large amplitude irregular slow waves in both the neocortex and hippocampus during behavioral immobility and some stereotyped automatic behaviors (Type 2 behavior). However, rhythmical slow activity in the hippocampus and low voltage fast activity in the neocortex occur in close correlation with spontaneous changes in posture, head movement, walking, rearing, swimming or struggling when held (Type 1 behavior). It has previously been proposed that these waveforms, jointly referred to as atropine-resistant cerebral activation (ARCA) are dependent on ascending serotonergic projections. As a further test of this hypothesis, we have studied rats in which forebrain levels of serotonin and 5-hydroxyindoleacetic acid were reduced to 3-10% of control levels as a result of multiple intrabrainstem injections of 5,7-dihydroxytryptamine. This treatment strongly reduced or abolished ARCA in most cases but did not reduce atropine-sensitive cerebral activation which appears to be dependent on ascending cholinergic projections from the basal forebrain to the cerebral cortex. Therefore, ARCA appears to be dependent on ascending serotonergic inputs to the forebrain.

The effects of unilateral destruction of fimbria-fornix and supracallosal pathways in the rat

Rats with unilateral lesions of either the supracallosal regions (including the dorsal cingulate cortex) and the fimbria-fornix either on the same (S) or the opposite (O) sides of the brain were studied in a 16-hole open field without pharmacologic intervention and, subsequently, after 0.1 and after 1.0 mg/kg scopolamine HBr. Their performances were compared with those of unoperated control animals subjected to the same testing regime. Certain of their behaviors were compared with those of a larger number of animals with bilateral hippocampal destruction (and their control groups) from prior studies. Unilateral lesions of fimbria-fornix and supracallosal afferents to the hippocampal formation produced a decrease in hole poking activity relative to control animals. A further decrease in hole-poking behavior, coupled with increased locomotion, was observed in rats with fimbria-fornix and cingulate cortex lesions on opposite sides of the brain (group O). The smaller dose of scopolamine accentuated these effects. Indeed, the behavior of group O after scopolamine treatment was similar to animals with large bilateral hippocampal lesions. The large dose of scopolamine induced stereotyped rearing or hole poking in the brain-damaged animals but not in the control group. These findings suggest that both the fimbria-fornix and the supracallosal pathway is necessary for normal hippocampal function and that the behavioral deficit is greater when these structures are damaged on the opposite sides of the brain.

Morphology of identified entorhinal neurons projecting to the hippocampus. A light microscopical study combining retrograde tracing and intracellular injection

Cells of origin of the entorhinohippocampal pathway were retrogradely labeled by injection of Fast Blue into the ipsilateral hippocampus. The cells, which were located in layers I, II and III of the lateral entorhinal cortex, were then intracellularly injected with Lucifer Yellow to reveal their complete morphology. We could thus establish that among the hippocampally projecting entorhinal cells there are pyramidal and pyramid-like cells, spiny stellate cells of various shapes, sparsely spinous horizontal and multipolar cells. The involvement of horizontal and multipolar neurons in projections has not previously been recognized although all of these cell types have already been described in Golgi studies. The results indicate that the organization of the perforant path is more complex than has been assumed. Finally, they are at variance with the classical concept which subdivides cortical neurons into projection neurons (pyramidal and spiny stellate) and interneurons (non-pyramidal, local circuit neurons).

Hippocampal interictal spikes induced by kindling: relations to behavior and EEG

Hippocampal spontaneous interictal spikes (SISs) were recorded during the course of daily tetanization (kindling) of afferent fibers to the hippocampal CA1 region. SISs were detected after 3-10 tetanizations. A clear variation of SIS rate with behavior was observed. SIS rate was high during slow-wave sleep (SWS), waking immobility, face-washing and chewing and low during rapid-eye-movement sleep (REMS), walking and rearing. Scopolamine hydrochloride (2.5-5 mg/kg i.p.) increased the SIS rate during walking. Despite the negative correlation of SIS occurrence with the theta rhythm in normal rats, abolishing the theta rhythm by medial septal lesions did not affect the suppression of SISs during REMS as compared to SWS. When interictal or postictal spikes were seen together with the theta rhythm, the spikes tended to occur at a phase of about 240 degrees after the positive peak of the alvear surface rhythm.

Differential distribution of beta and theta EEG activity in the entorhinal cortex of the cat

The basic characteristics of the electroencephalograms (EEGs) of the entorhinal cortex (EC) were compared to those of the olfactory bulb (OB) and prepiriform cortex (PPC) in the awake cat, using methods that take into account linear (coherence functions) and non-linear relationships (mutual information). The frequency spectra of EC signals differ from those of EEG signals recorded from OB or PPC, but in the beta frequency band (35-40 Hz) coherent activity between these latter two areas and the EC could be put in evidence. Taking the values of coherence as a measure of linear coupling between populations of neurons, this coupling is strong between PPC and the rostral part of the EC, and becomes weaker towards more lateral, caudal and medial areas. This decrease in coupling is confirmed by non-linear analysis methods. On the basis of intracortical depth profiles it was shown that the beta activity is locally generated in layer II of the EC. Another rhythmic activity was also recorded in EC in the theta frequency range. Theta rhythm is generated mainly in the caudal part of EC. This area has the strongest coupling with the hippocampal formation. These analyses lead to the conclusion that in the EC olfactory and limbic domains have different topographical distributions but with a certain degree of overlap.

Are emotion and motivation localized in the limbic system and nucleus accumbens?

Groups of control rats without surgery or groups of rats prepared with surgical lesions of the septal nuclei, amygdala, cingulate cortex, accumbens nucleus, section of the fornix, or sham surgical operations, were tested in a simple autonomic conditioning situation (conditioned defecation). Control rats and rats with lesions of the accumbens nucleus were also tested in a straight alley maze under varying conditions of water deprivation. In general, the lesions had little effect except that amygdaloid damage impaired acquisition of conditioned defecation. The theory that 'emotion' and 'motivation' can be localized in the 'limbic system' and nucleus accumbens is discussed critically.

Hippocampal rhythmical slow activity following ibotenic acid lesions of the septal region. I. Relations to behavior and effects of atropine and urethane

The effects of intraseptal injections of various concentrations of ibotenic acid on hippocampal electrical activity were studied in freely moving and urethane-anesthetized rats. Ibotenic acid selectively abolished the atropine-sensitive form of hippocampal rhythmical slow activity (RSA) normally seen during urethane anesthesia. Large amplitude irregular activity (LIA) and RSA in the waking state were somewhat depressed as well. Despite this, clear RSA persisted in the waking state in association with locomotion or struggling (Type 1 behavior). As in normal rats, such RSA was resistant to systemic administration of atropine. Analysis of brain sections stained with gallocyanin or for acetylcholinesterase showed that ibotenic acid produced cell loss in the dorsal lateral septal nucleus and the septohippocampal nucleus. Cells in the medial septal and diagonal band nuclei were resistant to ibotenic acid. The results suggest that intrinsic septal circuitry is critically involved in the generation of the atropine-sensitive (presumably cholinergic) form of RSA. The mechanisms by which LIA and the two forms of RSA are generated in the hippocampus is discussed.

Electrical activity of the cingulate cortex. II. Cholinergic modulation

The role of the cholinergic innervation in the modulation of cingulate electrical activity was studied by means of pharmacological manipulations and brain lesions. In the normal rat, an irregular slow activity (ISA) accompanied with EEG-spikes was recorded in the cingulate cortex during immobility as compared to walking. Atropine sulfate, but not atropine methyl nitrate, increased ISA and the frequency of cingulate EEG-spikes. Pilocarpine suppressed ISA and EEG-spikes during immobility, and induced a slow (4-7 Hz) theta rhythm. Unilateral or bilateral lesions of the substantia innominata and ventral globus pallidus area using kainic acid did not significantly change the cingulate EEG or its relation to behavior. Large electrolytic lesions of the medial septal nuclei and vertical limbs of the diagonal band generally decreased or abolished all theta activity in the cingulate cortex and the hippocampus. However, in 5 rats the cingulate theta rhythm increased while the hippocampal theta disappeared after a medial septal lesion. The large, postlesion cingulate theta, accompanied by sharp EEG-spikes during its negative phase, is an unequivocal demonstration of the existence of a theta rhythm in the cingulate cortex, independent of the hippocampal rhythm. Cholinergic afferents from the medial septum and diagonal band nuclei are inferred to be responsible for the behavioral suppression of cingulate EEG-spikes and ISA, and partially for the generation of a local cingulate theta rhythm. However, an atropine-resistant pathway and a theta-suppressing pathway, possibly coming from the medial septum or the hippocampus, may also be important in cingulate theta generation.

Restoration of rhythmic slow activity (theta) in the subcortically denervated hippocampus by fetal CNS transplants

Solid pieces of the fetal septal region (SG) or hippocampus (HPC) were implanted in a cavity formed by aspiration of the fimbria fornix (FF) and the overlying cingulate cortex on one side in adult rats. In other lesioned animals cell suspensions obtained from the fetal septal area (SS) or the locus coeruleus region of the brainstem (LC) were injected stereotaxically into the deafferented host hippocampus. Six to 9 months after transplantation the animals had chronic recording electrodes implanted into both hippocampi. EEG and unit activity were monitored during running in a wheel, drinking and immobility. Unilateral fimbria-fornix lesions abolished rhythmic slow activity (RSA or theta, theta) in the ipsilateral hippocampus and no recovery was seen up to 9 months later in either the control FF-lesioned animals or in the rats with LC suspension grafts. Recovery of RSA, however, was observed in all animals with solid septal grafts and in some rats with solid HPC grafts. Similar to normal rats, RSA was present only during running and absent during drinking and sitting still. Coherence measurements of RSA between the transplanted and intact hemispheres resulted in high values (0.70-0.95). Concurrent with RSA, interneurons and granule cells in the host hippocampus fired rhythmically at RSA frequency (6-9 Hz). The depth profile and the antero-posterior distribution of the power of RSA correlated with the amount and distribution of the graft-induced acetylcholinesterase-positive reinnervation of the host hippocampus. In contrast to the animals with solid septal grafts, placed within the FF lesion cavity, the rats with intrahippocampal septal suspension grafts displayed only short duration bursts of RSA, and mainly during immobility. Based on these findings it is suggested that at least a proportion of the RSA 'pacemaker' cells of the host septum survives the transection of the fimbria-fornix fibers and that a graft of fetal septal or hippocampal tissue implanted into the lesion cavity may be capable of relaying this pacemaker activity to the host hippocampus. This effect may be due to the ability of the grafted tissue to promote the regeneration of new, direct or indirect, septo-hippocampal connections across the lesion cavity.

Near-total loss of 'learning' and 'memory' as a result of combined cholinergic and serotonergic blockade in the rat

Previous work has indicated that activation of the cerebral cortex (i.e. elicitation of low-voltage fast activity in the neocortex and rhythmical slow activity in the hippocampus) is dependent on corticipetal cholinergic and serotonergic projections. Treatment with a combination of p-chlorophenylalanine (an inhibitor of the synthesis of serotonin) plus atropine or scopolamine (muscarinic cholinergic antagonists) can suppress all cerebral activation. In this paper, the behavioral effects of single or combined blockade of cholinergic and serotonergic neurotransmission were studied using a shock avoidance test, an open field test, a swim-to-platform test, a hypothalamic self-stimulation test and a test of grooming behavior. The results show that blockade of cerebral activation produces a condition analogous to global dementia but does not produce sleep or coma. The hypothesis that cholinergic and serotonergic neurotransmission provides a basis for learning and memory is discussed critically.