Anatomic organization and physiology of the limbic cortex

The electrocorticogram in relation to physiology and behavior: a new analysis

Recent research in animals indicates that the generalized regulation of cortical activity that is represented by activation of the electrocorticogram is dependent on ascending cholinergic and serotonergic projections. The effect of the activity of these systems is correlated with concurrent motor activity in a detailed and specific manner. Combined blockade of central cholinergic and serotonergic function results in impaired cerebral control of behavior, i.e., a dementia-like syndrome. Previous concepts, which held that electrocortical activation is mediated via a reticulo-thalamo-cortical pathway and that it is related primarily to arousal, vigilance, and the sleep-waking cycle appear to be incomplete or erroneous.

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.

Current source density of sustained potential shifts associated with electrographic seizures and with spreading depression in rat hippocampus

The membrane currents responsible for the sustained potential shifts associated with electrographic seizures and with spreading depression in hippocampus were studied in the anesthetized rat. Probes incorporating 16 sensors in a straight line, spaced at 150-microns distances, were recording the potential changes with DC-coupled amplifiers in CA1 and dentate gyrus (DG) of one hemisphere. Seizures and spreading depression were provoked by repetitive stimulation of different afferent pathways. Seizures always began in DG before CA1, regardless of the pathway stimulated. Tonic seizures were associated with a sustained negative potential shift that was largest in the cell body layers. Current source density was computed from these recordings and confirmed the presence of a current sink limited to the cell body layer throughout the duration of electrographic seizures. Spreading depression was associated with a very large sink located in the layer of apical dendrites, maximal among the proximal segment of dendrites, to which the cell body layer served as a source. We conclude that seizures are associated with an inward current in neuron cell bodies, probably flowing through membrane channels of as yet no know physiological function.

Control of neuronal excitability by corticosteroid hormones

The rat adrenal hormone corticosterone can cross the blood-brain barrier and bind to two intracellular receptor populations in the brain--the mineralocorticoid and glucocorticoid receptors. Recent studies have revealed that the corticosteroid hormones are able to restore changes in neuronal membrane properties induced by current or neurotransmitters, probably through a genomic action. In general, mineralocorticoid receptors mediate steroid actions that enhance cellular excitability, whereas activated glucocorticoid receptors can suppress temporarily raised neuronal activity. The steroid-mediated control of excitability and the implications for information processing in the brain are reviewed in this article.

Regional differences in the regulation of acetylcholine release upon D2 dopamine and N-methyl-D-aspartate receptor activation in rat nucleus accumbens and neostriatum

The effect of D2 dopamine receptor activation on either the electrically, or N-methyl-D-aspartate induced release of radiolabeled acetylcholine (ACh) was investigated in different areas of the nucleus accumbens and the neostriatum of rats, by using a superfusion technique. Sequential slices of 100 microns were chopped along either a rostrocaudal, mediolateral or dorsoventral axis. In every slice the effect of a supramaximal concentration of the selective D2 receptor agonist quinpirole on the release of ACh was measured. In the entire neostriatum the release of ACh was reduced by approximately 70% in the presence of quinpirole. By contrast, in the nucleus accumbens, a gradual decrease in the inhibitory effect of quinpirole on the release of ACh was observed along both the rostral-to-caudal and the lateral-to-medial axes. Whereas in the rostrolateral part a 50% inhibition could be observed, in the caudomedial part no significant inhibition could be detected. Also the N-methyl-D-aspartate induced release of ACh was smaller in the caudomedial part as compared to the rostrolateral part of the nucleus accumbens. It is concluded that the nucleus accumbens is a very heterogeneous structure with respect to the regulation of the release of ACh by D2 dopamine and N-methyl-D-aspartate receptor activation.

Distribution of hippocampal CA1 and subicular efferents in the prefrontal cortex of the rat studied by means of anterograde transport of Phaseolus vulgaris-leucoagglutinin

Projections of the hippocampal formation to the prefrontal cortex were visualized in the rat by means of the anterograde tracer Phaseolus vulgaris-leucoagglutinin. These projections distribute only to the prelimbic and the medial orbital cortices and arise exclusively from restricted portions of field CA1 of the Ammon's horn and the subiculum. The most dorsal portion of CA1 does not contribute fibers to this projection. In the subiculum, its origin is restricted to the proximal half, i.e., the portion that directly borders field CA1. Fibers from field CA1 and the subiculum have comparable distribution patterns in the prelimbic and medial orbital cortices. The density and distribution in the prefrontal cortex of the projections from the proximal portion of the subiculum depends on the location of the injections along the dorsoventral axis of the hippocampal formation: the intermediate portion of the subiculum projects more densely and diffusely than its dorsal and ventral portions. In the prelimbic cortex, labeled fibers are present in all layers, showing marked morphological differences in deep versus superficial layers. In layers V and VI, most of the fibers are vertically oriented, while in layers II and III they are short and oriented towards the pial surface. Although no clear differences in terminal distribution were observed along the rostrocaudal extent of the prelimbic cortex, its dorsal and ventral portions show different innervation patterns. In the ventral portion of the prelimbic cortex, varicose fibers and terminal arborizations were present in all cortical layers, deep (V and VI) as well as superficial (II and III). In its dorsal part, the innervation was less dense and mostly present in the deep layers (V and VI). The fiber and terminal distribution in the medial orbital cortex was diffuse in all layers with a slight preference for layers deep to layer II.

The electrophysiology of the olfactory-hippocampal circuit in the isolated and perfused adult mammalian brain in vitro

The viability and general electrophysiological properties of the limbic system in the adult mammalian brain isolated and maintained in vitro by arterial perfusion are described. The isolated brain preparation combines the advantages of intact synaptic connectivity and accessibility of different areas of the encephalic mass with those of the in vitro approach, i.e., stability and control of the ionic environment. Extracellular field potential as well as intracellular recordings were performed at different levels in the limbic system of isolated adult guinea pig brains. The results demonstrate that in the piriform, entorhinal, and hippocampal cortices, the intrinsic electrical properties of individual cells as well as the spontaneous and evoked electrical activity in the neuronal ensembles they comprise, were virtually identical to those observed in vivo. The properties of the limbic system loop were determined.

In vivo intrahippocampal microinfusion of carbachol and bicuculline induces theta-like oscillations in the septally deafferented hippocampus

In their laboratory the authors have previously demonstrated that hippocampal slices could be induced to generate trains of "theta-like" oscillations by whole-bath perfusions of carbachol. Until recently, it has not been possible to generate similar activity in the septally deafferented hippocampus of an otherwise intact brain by microinfusions of carbachol. This study presents a full report of the first demonstration of a theta-like oscillation in the in vivo, septally deafferented hippocampal formation. Rats were anesthetized with urethane and implanted with microinfusion cannulae in the region of the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB) and at single or multiple sites in the stratum moleculare of the fascia dentata. The MS/vDBB was microinfused with procaine hydrochloride to produce a reversible suppression lasting for approximately 20 minutes. Intrahippocampal microinfusions of carbachol or bicuculline alone (in the postprocaine condition of the MS/vDBB) failed to produce any theta-like oscillations. The combination of carbachol and bicuculline produced trains of theta-like oscillations during suppression of the MS/vDBB very similar to those seen in the slice preparations. The oscillations were blocked by intravenous administration of atropine sulfate, and they had the same depth profile as that of theta. Theta-on cells were shown to discharge in rhythmic bursts in synchrony with the oscillations. Thus, it would appear that the essential nature of the medial septal input to the hippocampal formation, for the generation of theta field activity in the intact brain, consists of a critical balance between cholinergic and GABAergic circuitry.

Neural mechanisms underlying brain waves: from neural membranes to networks

In this review, a number of experimental findings and theoretical concepts that have led to new insights into the mechanisms underlying brain waves are presented. At the cellular level, the new evidence that certain types of neuron have intrinsic oscillatory properties that may underlie rhythmic EEG activities is discussed. In particular, the question of whether spindle oscillations are autonomous or input-dependent is addressed. At the neural network level, the main circuits of the thalamus and cortex that are responsible for the occurrence and modulation of spindles and alpha activity are described. In addition, the properties of rhythmic activities outside the alpha band are considered, particularly in relation to the prominent beta activity of the visual cortex. At the theoretical level, the possibility that neural networks may behave as complex dynamic systems with the properties of deterministic chaos is discussed. Finally, the fact that brain rhythms may have functional implications for the working of neural networks is examined in relation to 2 cases: the possibility that oscillations may subserve a gating function, and that oscillations may play a role in the formation of assemblies of neurons that represent given stimulus patterns.

Three-dimensional trajectories of the hippocampal rhythmic slow activity in the behaving rat

Movement-concurrent rhythmic slow activity (RSA) was recorded by means of 3 pairs of bipolar electrodes which were arranged so that they orthogonally encompassed the dorsal hippocampus in the freely moving rat. The 3-channel data were combined by synchronously plotting them on independent voltage axes and the resultant 3-channel Lissajous' trajectory (3-CLT) was depicted in 3-dimensional microcomputer processing. A type of 3-CLT which traversed the surface of an ellipsoid with the long axis nearly perpendicular to the medial CA1 pyramidal layer, appeared most abundantly during walking. A 3-CLT depicting curves on a spherical surface was frequently produced during sitting. A linear 3-CLT of short duration was occasionally produced during rearing. These findings suggest that alteration of voltage axis specific to a behavior is possibly generated associating with the hippocampal RSA.

Plasticity in Alzheimer's disease: too much or not enough?

The goal of optimizing restorative sprouting in Alzheimer's disease is based on the premise that sprouting is beneficial and is deficient in AD. The beneficial aspects of neuronal plasticity have been questioned, however, and other studies suggest that some aspects of sprouting may be exaggerated in AD and contribute to the formation of plaques, tangles, and other neuropathological hallmarks of this disorder. Manipulation of the sprouting response may represent a promising treatment strategy in AD, but whether the goal is to augment or impede sprouting may depend upon the extent of the damage and the severity of the disease state.

Characterization of the release of cholecystokinin-8 from isolated nerve terminals and comparison with exocytosis of classical transmitters

In the present study, the release of the neuropeptide cholecystokinin-8 (CCK) from purified nerve terminals (synaptosomes) of the rat hippocampus was characterized with respect to the subcellular distribution, the release upon addition of various agents, the release kinetics, the Ca2+ and ATP dependence of release, and the relationship between CCK release and elevations of intraterminal free Ca2+ concentration ([Ca]i). These characteristics were compared with those for the release of classical transmitters in similar preparations. CCK-like immunoreactivity (CCK-LI) is enriched in the purified synaptosomal fraction of hippocampus homogenates and released in a strictly Ca2(+)-dependent manner upon chemical depolarization, addition of 4-aminopyridine, or stimulation with the Ca2+ ionophore ionomycin. The presence of Ca2+ in the medium significantly stimulates the basal efflux of CCK-LI from synaptosomes. The release upon stimulation develops gradually in time with no significant release in the first 10 s and levels off after 3 min of depolarization. At this time, a large amount of CCK-LI is still present inside the synaptosomes. A correlation exists between the release of CCK-LI and the elevations of [Ca]i. The release of CCK-LI is decreased, but not blocked, upon ATP depletion. These characteristics markedly differ from those for classical transmitters, which show a fast component of Ca2(+)-dependent (exocytotic) release, an absolute dependence on cellular ATP, and no marked stimulation of basal efflux in the presence of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential release of amino acids, neuropeptides, and catecholamines from isolated nerve terminals

We have investigated transmitter release from small and large dense-core vesicles in nerve terminals isolated from guinea pig hippocampus. Small vesicles are found in clusters near the active zone, and large dense-core vesicles are located at ectopic sites. The abilities of Ca2+ channel activation and uniform elevation of Ca2+ concentration (with ionophores) to evoke secretion of representative amino acids, catecholamines, and neuropeptides were compared. For a given increase in Ca2+ concentration, ionophore was less effective than Ca2+ channel activation in releasing amino acids, but not in releasing cholecystokinin-8. Titration of the average Ca2+ concentration showed that the Ca2+ affinity for cholecystokinin-8 secretion was higher than that for amino acids. Catecholamine release showed intermediate behavior. It is concluded that neuropeptide release is triggered by small elevations in the Ca2+ concentration in the bulk cytoplasm, whereas secretion of amino acids requires higher elevations, as produced in the vicinity of Ca2+ channels.

Activity-dependent recruitment of endogenous glutamate for exocytosis

The Ca(2+)-dependent release of the neurotransmitter glutamate from purified nerve terminals (synaptosomes) of the rat hippocampus was studied in a rapid perfusion apparatus. The response of the terminals was investigated with respect to the kinetics and duration of the release of endogenous glutamate upon brief and sustained stimulation and upon repetitive stimulation. The synaptosomes were stimulated by sustained chemical depolarization (0.5-3 min 30 mM K+). The cellular levels of glutamate, free Ca2+ and ATP in the nerve terminals were measured. The Ca(2+)-dependent release of glutamate showed an immediate elevation upon K(+)-depolarization. When the stimulation was maintained, a prolonged phase of glutamate release was observed. After 3 min, the Ca(2+)-dependent release stopped, although K(+)-depolarization was still effective. When synaptosomes were stimulated again after a relatively short stimulation period (30 s), the second response was similar to the previous one. After a longer stimulation period, maintained until termination of release, the second response did not show the immediate initial elevation of Ca(2+)-dependent glutamate release. Only 30 s after stimulation the release developed with a time profile comparable to the first response. This initial lack of response was not due to low cytosolic levels of glutamate or ATP or to changes in cellular Ca(2+)-buffering. It can be concluded that the capacity to release glutamate after brief depolarizations is fully restored during the repolarization period. However, if stimulation periods are of long duration (until termination of release), this capacity is no longer fully restored, especially with respect to a fast component of release. New glutamate is recruited only during the subsequent depolarization and with a delay.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of hippocampal commissures in the interhemispheric transfer of epileptiform afterdischarges in the rat: a study using linear and non-linear regression analysis

The role of the forebrain commissures and the septal area in the interhemispheric transfer of hippocampal afterdischarges (ADs) was investigated in the rat under halothane anesthesia. Electrical seizures were elicited from the dorsal hippocampus before and after commissurotomy. The degree of relatedness between EEG signals recorded from homologous sites of both hippocampi was quantified using two approaches: (i) a time domain analysis considering an AD as a succession of discrete bursts; the onset times of such bursts were measured and used to estimate interhemispheric onset delays; (ii) using signal analysis the linear (r2) and non-linear (h2) regression coefficients between pairs of EEG signals were computed as a function of time shift between the two signals. In this way the values of association (linear and non-linear) and the corresponding time delays were measured. In general a tetanus applied unilaterally to the dorsal CA3 field resulted in bilaterally synchronous ADs. The estimated interhemispheric time delay was in most cases zero. This bilateral synchrony disappeared after section of a specific part of the ventral hippocampal commissure (VHC), the dorso-caudal third, but was not affected by section of other commissural fibers or by a lesion of the septal area. This study also allowed evaluation of different methods of quantification of the association between EEG signals, namely the linear (r2) and the non-linear (h2) regression coefficients. The latter was shown to be a more robust measure than the former and to yield values of association even in cases in which r2 was at noise level. The experimental findings allow the conclusion that ADs elicited from an epileptogenic focus spread to homologous sites in the contralateral hemisphere following commissural systems that may be strong enough to ensure the forming of one bilateral oscillating system.

Projection from the nucleus reuniens thalami to the hippocampal region: light and electron microscopic tracing study in the rat with the anterograde tracer Phaseolus vulgaris-leucoagglutinin

In order to study the morphological substrate of possible thalamic influence on the cells of origin and area of termination of the projection from the entorhinal cortex to the hippocampal formation, we examined the pathways, terminal distribution, and ultrastructure of the innervation of the hippocampal formation and parahippocampal region by the nucleus reuniens of the thalamus (NRT). We employed anterograde tracing with Phaseolus vulgaris-leucoagglutinin (PHA-L). Injections of PHA-L in the NRT produce fiber and terminal labeling in the stratum lacunosum-moleculare of field CA1 of the hippocampus, the molecular layer of the subiculum, layers I and III/IV of the dorsal subdivision of the lateral entorhinal area (DLEA), and layers I and III-VI of the ventral lateral (VLEA) and medial (MEA) divisions of the entorhinal cortex. Terminal labeling is most dense in the stratum lacunosum-moleculare of field CA1, the molecular layer of the ventral part of the subiculum, MEA, and layer I of the perirhinal cortex. In layer I of the caudal part of DLEA and in MEA, terminal labeling is present in clusters. Injections in the rostral half of the NRT produce the same distribution in the hippocampal region as those in the caudal half of the NRT, although the projections from the rostral half of the NRT are much stronger. A topographical organization is present in the projections from the head of the NRT, so that the dorsal part projects predominantly to dorsal parts of field CA1 and the subiculum and to lateral parts of the entorhinal cortex, whereas the ventral part projects in greatest volume to ventral parts of field CA1 and the subiculum and to medial parts of the entorhinal cortex. The distribution of the reuniens fibers coursing in the cingulate bundle was determined by comparing cases with and without transections of this bundle. The fibers carried by the cingulate bundle exclusively innervate field CA1 of the hippocampus, the dorsal part of the subiculum, and the presubiculum and parasubiculum. They participate in the innervation of the ventral part of the subiculum and MEA. Electron microscopy was used to visualize the axon terminals of PHA-L-labeled reuniens fibers. These terminals possess spherical synaptic vesicles and form asymmetric synaptic contacts with dendritic spines or with thin shafts of spinous dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Paired-pulse facilitation in the nucleus accumbens following stimulation of subicular inputs in the rat

Anatomical tracing studies indicate that the nucleus accumbens receives inputs from limbic structures, and projects to the ventral pallidum. In order to get more fundamental insight into how information from the limbic areas is relayed via the nucleus accumbens, electrophysiological experiments were carried out in rats under halothane anaesthesia. Inputs originating in the subiculum were activated by electrical stimulation of the fornix fibres, and both field potentials and extracellular unit activity were recorded from the medial and lateral aspects of the nucleus accumbens. Evoked potentials consisted of two positive peaks (P1 at 10 ms and P2 at 25-30 ms). In between a negative-going wave (N1) was present. These initial components were followed by a complex negative wave (N2) with variable duration of 30-100 ms. The P2 and N2 components showed a conspicuous paired-pulse facilitation at stimulus intervals between 80 and at least 200 ms. When responses were recorded at increasing stimulus intensity, the second response emerged at lower threshold than the first response. The mechanisms underlying these phenomena were investigated by analysing the extracellularly recorded unit activity. Primarily, excitatory responses were found. Onset-latencies could be divided roughly into two clusters, one around 10 ms, representing monosynaptic inputs, and a second around 24-26 ms. Inhibitory responses were also found. Stimulation of the ventral pallidum was carried out in order to test whether the cells that could be driven by stimulation of the subicular inputs were projection cells. Latencies of antidromic action potentials ranged from 9 to 13 ms. A minority of the identified projection cells were activated by limbic inputs. The projection cells were found in the core region of the nucleus accumbens. Units that were inhibited by stimulation of the limbic inputs were found in the shell only, whereas excitatory responses were measured in both subdivisions of the nucleus accumbens. For the latter responses a significant enhancement, by a factor of four, was found using double pulse stimulation of the fornix at intervals of 100 ms. The basic electrophysiological properties are compared with those described in the literature, and speculations about the possible mechanisms responsible for the paired-pulse facilitation phenomena are put forward.

The subiculum: cytoarchitectonically a simple structure, but hodologically complex

The subiculum gives rise to the majority of hippocampal projections to various telencephalic and diencephalic structures. Previously, these projections have been described using anterograde tracing with radioactively labeled amino acids. As part of an ongoing detailed analysis of the connectivity of the hippocampal region in the rat, we studied the projections of the subiculum by means of the recently introduced sensitive anterograde tracer Phaseolus vulgaris-leukoagglutinin (PHA-L) and double-labeling protocols with retrogradely transported fluorescent tracers. Within the subiculum, populations of neurons can be differentiated that each give rise to projections to a unique set of target structures. These populations of neurons, characterized according to common efferent connectivity, are differentially positioned along the transverse axis of the subiculum. Thus, subicular cells near the border with the CA1 field project to targets different from those reached by projections from subicular cells situated close to the border with the presubiculum. We further observed that major afferents of the subiculum, i.e. those arising from field CA1 and from the entorhinal cortex, are also organized along the transverse axis of the subiculum. We suggest that within the subiculum, that appears homogeneous with respect to both cytoarchitectonic and chemoarchitectonic characteristics, a differentiation can be made with respect to its major connections. Whether this differentiation takes the form of a "columnar organization" as known for the neocortex, or a "compartmentation" as shown for the striatum is not yet clear.