Anatomic organization and physiology of the limbic cortex

On the origin of interictal activity in human temporal lobe epilepsy in vitro

The origin and mechanisms of human interictal epileptic discharges remain unclear. Here, we describe a spontaneous, rhythmic activity initiated in the subiculum of slices from patients with temporal lobe epilepsy. Synchronous events were similar to interictal discharges of patient electroencephalograms. They were suppressed by antagonists of either glutamatergic or gamma-aminobutyric acid (GABA)-ergic signaling. The network of neurons discharging during population events comprises both subicular interneurons and a subgroup of pyramidal cells. In these pyramidal cells, GABAergic synaptic events reversed at depolarized potentials. Depolarizing GABAergic responses in neurons downstream to the sclerotic CA1 region contribute to human interictal activity.

Network and pharmacological mechanisms leading to epileptiform synchronization in the limbic system in vitro

Seizures in patients presenting with mesial temporal lobe epilepsy result from the interaction among neuronal networks in limbic structures such as the hippocampus, amygdala and entorhinal cortex. Mesial temporal lobe epilepsy, one of the most common forms of partial epilepsy in adulthood, is generally accompanied by a pattern of brain damage known as mesial temporal sclerosis. Limbic seizures can be mimicked in vitro using preparations of combined hippocampus-entorhinal cortex slices perfused with artificial cerebrospinal fluid containing convulsants or nominally zero Mg(2+), in order to produce epileptiform synchronization. Here, we summarize experimental evidence obtained in such slices from rodents. These data indicate that in control animals: (i) prolonged, NMDA receptor-dependent epileptiform discharges, resembling electrographic limbic seizures, originate in the entorhinal cortex from where they propagate to the hippocampus via the perforant path-dentate gyrus route; (ii) the initiation and maintenance of these ictal discharges is paradoxically contributed by GABA (mainly type A) receptor-mediated mechanisms; and (iii) CA3 outputs, which relay a continuous pattern of interictal discharge at approximately 1Hz, control rather than sustain ictal discharge generation in entorhinal cortex. Recent work indicates that such a control is weakened in the pilocarpine model of epilepsy (presumably as a result of CA3 cell damage). In addition, in these experiments electrographic seizure activity spreads directly to the CA1-subiculum regions through the temporoammonic pathway. Studies reviewed here indicate that these changes in network interactions, along with other mechanisms of synaptic plasticity (e.g. axonal sprouting, decreased activation of interneurons, upregulation of bursting neurons) can confer to the epileptic, damaged limbic system, the ability to produce recurrent limbic seizures as seen in patients with mesial temporal lobe epilepsy.

The stimulatory effect of canrenoate, a mineralocorticoid antagonist, on the activity of the hypothalamus-pituitary-adrenal axis is abolished by alprazolam, a benzodiazepine, in humans

Mineralocorticoid receptors (MR) in the hippocampus play a major role in the control of the hypothalamus-pituitary-adrenal (HPA) axis, mediating the proactive feedback of glucocorticoids in the maintenance of basal activity. Intracerebroventricular and intrahippocampal MR blockade stimulates HPA axis in animals; the systemic administration of mineralocorticoid antagonists enhances spontaneous and CRH-stimulated ACTH and cortisol secretion in humans. Benzodiazepines, namely alprazolam, activate central gamma-aminobutyric acid (GABA)ergic receptors, which are mainly distributed in the hippocampus. Alprazolam has a inhibitory effect on HPA axis either in basal conditions or after central nervous system-mediated stimuli. In humans, alprazolam strongly reduces the corticotroph responsiveness to removal of glucocorticoid feedback by metyrapone. We studied the effect of alprazolam (0.02 mg/kg, orally) on the effect of canrenoate (CAN), an MR antagonist (200 mg as an iv bolus, followed by 200 mg infused in 250 ml saline) or placebo on ACTH, cortisol, and dehydroepiandrosterone (DHEA) secretion in six normal young women (aged 25-32 yr; body mass index, 19-23 kg/m(2)). During placebo, ACTH, cortisol, and DHEA secretion showed a progressive decrease (baseline vs. nadir, mean +/- SEM, from 1830-2400 h, 2.6 +/- 0.3 vs. 1.4 +/- 0.3 pmol/liter, 133.2 +/- 16.4 vs. 46.9 +/- 5.2 nmol/liter, and 22.6 +/- 2.3 vs. 18.6 +/- 2.3 nmol/liter, respectively), although statistical significance was obtained for ACTH and cortisol only (P < 0.05). During CAN treatment, ACTH, cortisol, and DHEA secretion showed a progressive rise, which began at approximately 2100 h and peaked between 2300 and 2400 h (2.9 +/- 0.3 pmol/liter, 172.6 +/- 27.9 nmol/liter, and 45.3 +/- 10.7 nmol/liter, respectively; P < 0.05). Alprazolam abolished the CAN-induced increases in ACTH, cortisol, and DHEA levels (1.8 +/- 0.1 pmol/liter, 59.7 +/- 8.6 nmol/liter, and 19.8 +/- 6.7 nmol/liter; P < 0.05), inducing hormonal peaks overlapping with those recorded after placebo in the absence of any treatment. In conclusion, our study demonstrates that the inhibitory effect of GABAergic activation by alprazolam overrides the stimulatory effect of mineralocorticoid blockade by canrenoate on the HPA axis in humans. These findings emphasize the role of GABA in the control of the HPA axis in humans.

Hyperthermia induces age-dependent changes in rat hippocampal excitability

The mechanisms underlying the generation of febrile seizures are poorly understood. This study investigated hyperthermia-induced changes in the hippocampus, a structure implicated in febrile seizures. It was hypothesized that neuronal excitability in the hippocampus changes with increasing temperature, and that this change is different in adult as compared with immature rats. Adult and immature (15-17 days postnatal) male rats were studied under urethane anesthesia during normothermia, moderate hyperthermia (38-39.5 degrees C), and severe hyperthermia (>39.5 degrees C). Paired-pulse inhibition of the orthodromically activated population spikes in the dentate gyrus and cornu ammonis 1 region of the hippocampus (CA1), two structures within the hippocampus, was measured after stimulation of the medial perforant path and Schaffer collaterals, respectively. In the adult rat, paired-pulse inhibition was increased in the dentate gyrus during moderate and severe hyperthermia but decreased in CA1 during severe hyperthermia (all p values < 0.05). In the immature rat, paired-pulse inhibition was unchanged in the dentate gyrus but decreased in CA1 during moderate hyperthermia (p < 0.05). We suggest that hyperthermia contributes to seizure susceptibility in the immature hippocampus by decreasing CA1 inhibition. In the adult rat, a decrease in CA1 inhibition requires a higher degree of hyperthermia, and hippocampal seizure generation is opposed by an increase in dentate gyrus inhibition.

Associative interactions within the superficial layers of the entorhinal cortex of the guinea pig

Associative fiber systems in the entorhinal cortex (EC) have been extensively studied in different mammals with tracing techniques. The largest contingent of intra-EC cortico-cortical fibers runs in the superficial layers and is distributed predominantly within longitudinal cortical bands. We studied the patterns of intrinsic EC connectivity in the in vitro isolated guinea pig brain preparation by performing current-source density analysis of field potential laminar profiles recorded with multi-channel silicon probes. The response pattern evoked by stimulation of the lateral olfactory tract was utilized to identify the lateral (l-EC) and medial (m-EC) entorhinal cortex. Stimulation of the deep layers did not evoke consistent responses. Local stimulation of the superficial layers in different portions of the EC induced an early, possibly direct response restricted to layer II-III in the close proximity to the stimulating electrode, followed by a late potential in the superficial layer I, that propagated at distance with a progressively increasing latency. The monosynaptic nature of the delayed response was verified by applying a pairing test. The results demonstrated that stimulation in the rostral-medial part of the EC generated activity restricted to the rostral pole of the l-EC, stimulation of the m-EC induced an associative activation that propagated rostrocaudally within the m-EC, stimulation of the caudal pole of the m-EC induced an additional response directed laterally, and stimulation of the lateral band of the EC determined a prominent longitudinal propagation of neuronal activity, but also induced associative potentials that propagated medially. The results are in partial agreement with the general picture derived from the anatomical studies performed in different species. Even though the largest associative interactions between superficial layers are restricted within either the m-EC or the l-EC, both rostral and caudal stimuli in the EC region close to the rhinal sulcus induced activity that propagated across the border between l- and m-EC.

Heterogeneity in the Dorsal Subiculum of the Rat. Distinct Neuronal Zones Project to Different Cortical and Subcortical Targets

The aim of the present study was to relate the distribution of efferents of the dorsal subiculum to their origin along the proximodistal axis of the subiculum. The distribution of subicular projections was studied in detail by means of the sensitive anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L), and the precise origin of these projections analysed with retrogradely transported fluorescent tracers, using double- and triple-labelling protocols. Injections of PHA-L in the proximal part of the dorsal subiculum, i.e. that part which borders field CA1, result in labelling of the infralimbic, entorhinal and perirhinal cortices, the nucleus accumbens and the lateral septal region, the interanteromedial nucleus of the thalamus, the core of the nucleus gelatinosus, and the mammillary nuclei, in particular in the rostral parts of the medial nucleus. In contrast, injections in the distal part of the dorsal subiculum, i.e. that part which borders the presubiculum, give rise to labelling in the retrosplenial and postrhinal cortices, the presubiculum, the anterior thalamic complex, the shell of the nucleus gelatinosus, and the mammillary nuclei, preferentially in the caudal part of the medial nucleus. The results of injections of different retrograde tracers, simultaneously placed in two or three targets of the subicular efferents, confirm the results of the anterograde tracing experiments. Moreover, they clearly demonstrate that the population of subicular neurons which, for example, projects to the nucleus accumbens and the interanteromedial nucleus of the thalamus is almost completely segregated from the population that projects to the retrosplenial cortex and the anterior complex of the thalamus. Thus within the dorsal subiculum, populations of neurons can be differentiated so that each population projects to a unique set of target structures. These cell populations are differentially positioned along the proximo-distal axis. In view of additional evidence indicating that some of the major afferents to the subiculum are organized along the same axis, we suggest that the heterogeneity of the dorsal subiculum along the proximo-distal axis reflects a general organizational characteristic of this hippocampal field.

Brain afferents to the medullary dorsal reticular nucleus: a retrograde and anterograde tracing study in the rat

The medullary dorsal reticular nucleus (DRt) was recently shown to belong to the supraspinal pain control system; neurons within this nucleus give origin to a descending projection that increases spinal nociceptive transmission and facilitates pain perception [Almeida et al. (1999), Eur. J. Neurosci., 11, 110-122]. In the present study, the areas of the brain that may modulate the activity of DRt neurons were investigated by using of tract-tracing techniques. Injection of a retrograde tracer into the DRt resulted in labelling in multiple areas of the brain. In the contralateral orbital, prelimbic, infralimbic, insular, motor and somatosensory cortices labelling was prominent, but a smaller ipsilateral projection from these same areas was also detected. Strong labelling was also noted in the central amygdaloid nucleus, bed nucleus of stria terminalis and substantia innominata. Labelled diencephalic areas were mainly confined to the hypothalamus, namely its lateral and posterior areas as well as the paraventricular nucleus. In the mesencephalon, the periaqueductal grey, red nucleus and deep mesencephalic nucleus were strongly labelled, whereas, in the brainstem, the parabrachial nuclei, rostroventromedial medulla, nucleus tractus solitarius, spinal trigeminal nucleus, and the parvocellular, dorsal, lateral and ventral reticular nuclei were the most densely labelled regions. All deep cerebellar nuclei were labelled bilaterally. These data suggest that the DRt integrates information from the somatosensory, antinociceptive, autonomic, limbic, pyramidal and extrapyramidal systems while triggering its descending facilitating action upon the spinal nociceptive transmission.

Memory impairment in temporal lobe epilepsy: the role of entorhinal lesions

Temporal lobe epilepsy (TLE) patients are frequently afflicted with deficits in spatial and other forms of declarative memory. This impairment is likely associated with the medial temporal lobe, which suffers widespread damage in the disease. Physiological and lesion studies, as well as examinations of the complex connectivity of the medial temporal lobe in animals and humans, have identified the entorhinal cortex (EC) as a key structure in the function and dysfunction of this brain region. Lesions in EC layer III, which normally provides monosynaptic input to area CA1 of the hippocampus, frequently occur in TLE and may be causally related to the memory impairments seen in the disease. Lesions that are initially largely restricted to EC layer III can be produced in rats by focal intra-entorhinal injections of 'indirect excitotoxins' such as aminooxyacetic acid or gamma-acetylenic GABA. These animals eventually show more extensive neurodegeneration in temporal lobe structures and, after a latent period, exhibit spontaneously recurring seizure activity. These progressive features, which may mimic events that occur in TLE, provide new opportunities to explore the role of the EC in memory deficits associated with TLE. These animals will also be useful for evaluating new treatment strategies that focus on the prevention of pathological events in the EC.

Role of the dopamine D3 receptor in reactivity to cocaine-associated cues in mice

Environmental stimuli previously associated with drug effects can acquire secondary reinforcing properties, able to maintain drug-seeking behaviour or induce relapse. We have used a classical Pavlovian conditioning procedure to assess the role of the dopamine D3 receptor (D3R) in the expression of drug-conditioned responses. Mice repeatedly receiving cocaine in a particular environment distinct from home-cages displayed hyperlocomotion after subsequent exposure to the drug-paired environment. Cocaine-conditioned hyperactivity was inhibited by BP 897 or SB-277011-A, D3R-selective partial agonist and antagonist, respectively. D3R gene-targeted mice showed a trend towards an increase in cocaine cue-conditioned hyperactivity. BP 897 had no effect on reactivity to neutral or aversive cues. Cocaine-conditioned mice had increased levels of D3R mRNA and binding in the nucleus accumbens (NAc), and transcripts of brain-derived neurotrophic factor (BDNF), a factor controlling D3R expression, in the ventral tegmental area (VTA). Cocaine had no effects on D3R or BDNF genes when administered in home-cages. Cocaine cue-conditioned c-fos expression was found in cortical areas, notably in the somatosensory cortex, where it was inhibited by BP 897, and in several regions belonging or linked to the limbic system. In conditioned mice, BP 897 inhibited c-fos expression in VTA and activated it in amygdala. These results demonstrate a modulation of reactivity to cocaine cues by the D3R, the expression of which is elevated in the NAc by the repeated association of drug effects with a particular context, through a BDNF-dependent mechanism. D3R-selective partial agonist or antagonist inhibit cocaine cue-conditioned activity possibly by normalizing exacerbated D3R function in the NAc, but our results also point to a possible participation of a pathway involving the VTA, amygdala and somatosensory cortex.

Theta oscillations in the hippocampus

Theta oscillations represent the "on-line" state of the hippocampus. The extracellular currents underlying theta waves are generated mainly by the entorhinal input, CA3 (Schaffer) collaterals, and voltage-dependent Ca(2+) currents in pyramidal cell dendrites. The rhythm is believed to be critical for temporal coding/decoding of active neuronal ensembles and the modification of synaptic weights. Nevertheless, numerous critical issues regarding both the generation of theta oscillations and their functional significance remain challenges for future research.

Evidence for glutamate, in addition to acetylcholine and GABA, neurotransmitter synthesis in basal forebrain neurons projecting to the entorhinal cortex

Basal forebrain neurons play important parts in processes of cortical activation and memory that have been attributed to the cortically projecting, cholinergic neurons. Yet, non-cholinergic neurons also project to the cerebral cortex and also appear to participate in processes of cortical modulation and plasticity. GABAergic neurons compose a portion of the cortically projecting cell group, but do not fully account for the non-cholinergic cell contingent. In the present study in the rat, we investigated whether the non-cholinergic, non-GABAergic cell component might be composed of glutamatergic neurons. We examined afferents to the entorhinal cortex, which is known to be modulated by basal forebrain neurons and to be critically involved in memory. Dual immunofluorescent staining was performed for cholera toxin, as retrograde tracer, and phosphate-activated glutaminase, the synthetic enzyme for the neurotransmitter pool of glutamate. The retrogradely labeled cells were distributed across the basal forebrain through the medial septum, diagonal band, magnocellular preoptic area and substantia innominata. The major proportion (approximately 80%) of the retrogradely labeled cells was found to be immunopositive for phosphate-activated glutaminase. Equal minor proportions (approximately 40%) were immunopositive for choline acetyltransferase and glutamic acid decarboxylase. In other material dual-immunostained for neurotransmitter enzymes, approximately 95% of choline acetyltransferase- and approximately 60% of glutamic acid decarboxylase-immunopositive neurons were also immunopositive for phosphate-activated glutaminase. From these results it appears that a significant proportion of these cell groups, including their cortically projecting contingents, could synthesize glutamate together with acetylcholine or GABA as neurotransmitters and another proportion of cells could synthesize glutamate alone. Accordingly, as either co-transmitter or primary transmitter within basalocortical afferents, glutamate could have the capacity to modulate the entorhinal cortex and promote its role in memory.

Alterations induced by gestational stress in brain morphology and behaviour of the offspring

Retrospective studies in humans suggest that chronic maternal stress during pregnancy, associated with raised plasma levels of CRH, ACTH and cortisol may increase the likelihood of preterm birth, developmental delays and behavioural abnormalities in the children. In adulthood, it may contribute to the significant association between the incidence of schizophrenia, increased left or mixed handedness, reduction in cerebral asymmetry and anomalies in brain morphology. Our studies and others have shown that prenatal stress in rats can mimic these developmental and behavioural alterations. These rats show a reduced propensity for social interaction, increased anxiety in intimidating or novel situations and a reduction in cerebral asymmetry and dopamine turnover, consistent with those in schizophrenic humans. Prenatally-stressed (PS) rats also show behaviour consistent with depression, including a phase-shift in their circadian rhythm for corticosterone, sleep abnormalities, a hedonic deficit and greater acquisition of learned helplessness under appropriate conditions. These behavioural abnormalities are associated with impaired regulation of the hypothalamic-pituitary-adrenal axis response to stress and increased CRH activity. PS males may show demasculinisation and feminisation of their sexual behaviour. The developmental and behavioural abnormalities in PS offspring could occur through sensitisation of the foetal brain by maternal stress hormones to the action of glucocorticoid and CRH and to neurotransmitters affected by them. This may have long-lasting consequences and could explain the precipitation of depressive symptoms or schizophrenia by psychosocial stress in later life. The character of the behavioural abnormalities probably depends on the timing of the maternal stress in relation to development of the particular neuronal systems.

Anticonvulsant effect of bilateral injection of N6-cyclohexyladenosine into the CA1 region of the hippocampus in amygdala-kindled rats

In this study the role of adenosine A(1) receptors of CA1 region of the hippocampus on amygdala-kindled seizures was investigated in rats. Results obtained showed that in kindled animals, bilateral injection of N(6)-cyclohexyladenosine (CHA), an adenosine A(1) receptor agonist, at doses of 0.1, 1 and 10 microM into the CA1 region of the hippocampus significantly decreased the afterdischarge duration and stage 5 seizure duration and increased the latency to stage 4 seizure, but there were no changes in seizure stage. Also, bilateral injection of 1,3-dimethyl-8-cyclopenthylxanthine (CPT), an adenosine A(1) receptor antagonist, at doses of 0.5 and 1 microM into the CA1 region of the hippocampus could not produce any changes in the seizure parameters. Intrahippocampal pretreatment of CPT (1 microM) before CHA (0.1 and 1 microM), reduced the effects of CHA on seizure parameters significantly. Thus, it may be suggested that CA1 region of the hippocampus plays an important role in spreading seizure spikes from the amygdala to other brain regions and activation of adenosine A(1) receptors in this region, participates in anticonvulsant effects of adenosine agonists.

Muscimol injected into the medial prefrontal cortex of the rat alters ethanol self-administration

The role of the rodent prefrontal cortex in the regulation of ethanol self-administration has not been widely explored. Understanding the role of GABAergic transmission in this area in relation to ethanol self-administration is important, as the GABA system may be one of several targets for alcohol's actions in the brain. Rats were initiated to drink 10% ethanol from a dipper using a sucrose-substitution procedure. When baseline behavior was stable, bilateral microinjections of muscimol (a GABA(A) agonist) into the prefrontal cortex were tested at doses of 17.5, 30, 100 and 300 ng/microl. Ethanol self-administration was decreased by approximately 40% at the 30-ng dose and 30% at the 100-ng dose. No effects were observed at either the 17.5- or 300-ng dose. The effect on the pattern of self-administration was to shorten the size of the first run of drinking without affecting the rate of drinking. The hypothesis is put forward that the injections increased glutamatergic output to the nucleus accumbens (nAcc) that in turn increased accumbens output. This increased output is proposed as similar to the effects of dopaminergic (DA) manipulations within this system.

Hippocampal fields in the hedgehog tenrec. Their architecture and major intrinsic connections

The Madagascan lesser hedgehog tenrec was investigated to get insight into the areal evolution of the hippocampal formation in mammals with poorly differentiated brains. The hippocampal subdivisions were analyzed using cyto- and chemoarchitectural criteria; long associational and commissural connections were demonstrated with tracer techniques. The hedgehog tenrec shows a well differentiated dentate gyrus, CA3 and CA1. Their major intrinsic connections lie within the band of variations known from other species. The dentate hilar region shows calretinin-positive mossy cells with extensive projections to the molecular layer. The calbindin- and enkephalin-positive granule mossy fibers form a distinct endbulb and do not invade the CA1 as reported in the erinaceous hedgehog. Isolated granule cells with basal dendrites were also noted. A CA2 region is hard to identify architecturally; its presence is suggested due to its contralateral connections. Subicular and perisubicular regions are clearly present along the dorsal aspects of the hemisphere, but we failed to identify them unequivocally along the caudal and ventral tip of the hippocampus. A temporal portion of the subiculum, if present, differs in its chemoarchitecture from its dorsal counterpart. The perisubicular region, located medially adjacent to the dorsal subiculum may be equivalent to the rat's presubiculum; evidence for the presence of a parasubiculum was rather weak.

Cholinergic modulation of synaptic physiology in deep layer entorhinal cortex of the rat

We have recently shown that cholinergic effects on synaptic transmission and plasticity in the superficial (II/III) layers of the rat medial entorhinal cortex (EC) are similar, but not identical, to those in the hippocampus (Yun et al. [2000] Neuroscience 97:671-676). Because the superficial and deep layers of the EC preferentially convey afferent and efferent hippocampal projections, respectively, it is of interest to compare cholinergic effects between the two regions. We therefore investigated the physiological effects of cholinergic agents in the layer V of medial EC slices under experimental conditions identical to those in the previous study. Bath application of carbachol (0.5 microM) induced transient depression of field potential responses in all cases tested (30 of 30; 18.5% +/- 2.3%) and rarely induced long-lasting potentiation (only 3 of 30; 20.4% +/- 3.2% in successful cases). At 5 microM, carbachol induced transient depression only (20 of 20, 48.9% +/- 2.8%), which was blocked by atropine (10 microM). Paired-pulse facilitation was enhanced during carbachol-induced depression, suggesting presynaptic action of carbachol. Long-term potentiation (LTP) could be induced in the presence of 10 microM atropine by theta burst stimulation, but its magnitude was significantly lower (9.1% +/- 4.7%, n = 15) compared to LTP in control slices (22.4% +/- 3.9%, n = 20). These results, combined with our previous findings, demonstrate remarkably similar cholinergic modulation of synaptic transmission and plasticity across the superficial and deep layers of EC.

Spatial processing in the brain: the activity of hippocampal place cells

The startling discovery by O'Keefe & Dostrovsky (Brain Res. 1971; 34: 171-75) that hippocampal neurons fire selectively in different regions or "place fields" of an environment and the subsequent development of the comprehensive theory by O'Keefe & Nadel (The Hippocampus as a Cognitive Map. Oxford, UK: Clarendon, 1978) that the hippocampus serves as a cognitive map have stimulated a substantial body of literature on the characteristics of hippocampal "place cells" and their relevance for our understanding of the mechanisms by which the brain processes spatial information. This paper reviews the major dimensions of the empirical research on place-cell activity and the development of computational models to explain various characteristics of place fields.

Network and intrinsic contributions to carbachol-induced oscillations in the rat subiculum

Low-frequency network oscillations occur in several areas of the limbic system where they contribute to synaptic plasticity and mnemonic functions that are in turn modulated by cholinergic mechanisms. Here we used slices of the rat subiculum (a limbic area involved in cognitive functions) to establish how network and single neuron (intrinsic) membrane mechanisms participate to the rhythmic oscillations elicited by the cholinergic agent carbachol (CCh, 50-100 microM). We have found that CCh-induced network oscillations (intraoscillatory frequency = 5-16 Hz) are abolished by an antagonist of non-N-methyl-D-aspartate (NMDA) glutamatergic receptors (n = 6 slices) but persist during blockade of GABA receptors (n = 16). In addition, during application of glutamate and GABA receptor antagonists, single subicular cells generate burst oscillations at 2.1-6.8 Hz when depolarized with steady current injection. These intrinsic burst oscillations disappear during application of a Ca(2+) channel blocker (n = 6 cells), intracellular Ca(2+) chelation (n = 6), or replacement of extracellular Na(+) (n = 4) but persist in recordings made with electrodes containing a blocker of voltage-gated Na(+) channels (n = 7). These procedures cause similar effects on CCh-induced depolarizing plateau potentials that are contributed by a Ca(2+)-activated nonselective cationic conductance (I(CAN)). Network and intrinsic oscillations along with depolarizing plateau potentials were abolished by the muscarinic receptor antagonist atropine. In conclusion, our findings demonstrate that low-frequency oscillations in the rat subiculum rely on the muscarinic receptor-dependent activation of an intrinsic oscillatory mechanism that is presumably contributed by I(CAN) and are integrated within the network via non-NMDA receptor-mediated transmission.

Network activity evoked by neocortical stimulation in area 36 of the guinea pig perirhinal cortex

The perirhinal cortex is a key structure involved in memory consolidation and retrieval. In spite of the extensive anatomical studies that describe the intrinsic and extrinsic associative connections of the perirhinal cortex, the activity generated within such a network has been poorly investigated. We describe here the pattern of synaptic interactions that subtend the responses evoked in area 36 of the perirhinal cortex by neocortical and local stimulation. The experiments were carried out in the in vitro isolated guinea pig brain. The synaptic perirhinal circuit was reconstructed by integrating results obtained during intracellular recordings from layer II-III neurons with simultaneous current source density analysis of laminar profiles performed with 16-channel silicon probes. Both neocortical and local stimulation of area 36 determined a brief monosynaptic excitatory potential in layer II-III neurons, followed by a biphasic synaptic inhibitory potential possibly mediated by a feed-forward inhibitory circuit at sites close to the stimulation electrode and a late excitatory postsynaptic potential (EPSP) that propagated at distance within area 36 along the rhinal sulcus. During a paired-pulse stimulation test, the inhibitory postsynaptic potential (IPSP) and the late EPSP were abolished in the second conditioned response, suggesting that they are generated by poli-synaptic circuits. Current source density analysis of the field responses demonstrated that 1) the monosynaptic activity was generated in layers II-III and 2) the sink associated to the disynaptic responses was localized within the superficial layer of area 36. We conclude that the neocortical input induces a brief monosynaptic excitation in area 36 of the perirhinal cortex, that is curtailed by a prominent inhibition and generates a recurrent excitatory associative response that travels at distance within area 36 itself. The results suggest that the perirhinal cortex network has the potentials to integrate multimodal incoming neocortical information on its way to the hippocampus.

Action potential bursting in subicular pyramidal neurons is driven by a calcium tail current

Subiculum is the primary output area of the hippocampus and serves as a key relay center in the process of memory formation and retrieval. A majority of subicular pyramidal neurons communicate via bursts of action potentials, a mode of signaling that may enhance the fidelity of information transfer and synaptic plasticity or contribute to epilepsy when unchecked. In the present study, we show that a Ca(2+) tail current drives bursting in subicular pyramidal neurons. An action potential activates voltage-activated Ca(2+) channels, which deactivate slowly enough during action potential repolarization to produce an afterdepolarization that triggers subsequent action potentials in the burst. The Ca(2+) channels underlying bursting are located primarily near the soma, and the amplitude of Ca(2+) tail currents correlates with the strength of bursting across cells. Multiple channel subtypes contribute to Ca(2+) tail current, but the need for an action potential to produce the slow depolarization suggests a central role for high-voltage-activated Ca(2+) channels in subicular neuron bursting.

The subiculum: a review of form, physiology and function

We review the neuroanatomical, neurophysiological and functional properties of the mammalian subiculum in this paper. The subiculum is a pivotal structure positioned between the hippocampus proper and entorhinal and other cortices, as well as a range of subcortical structures. It is an under-investigated region that plays a key role in the mediation of hippocampal-cortical interaction. We argue that on neuroanatomical, physiological and functional grounds, the subiculum is properly part of the hippocampal formation, given its pivotal role in the hippocampal circuit. We suggest that the term "subicular complex" embraces a heterogenous range of distinct structures and this phrase does not connote a functionally or anatomically meaningful grouping of structures. The subiculum has a range of electrophysiological and functional properties which are quite distinct from its input areas; given the widespread set of cortical and subcortical areas with which it interacts, it is able to influence activity in quite disparate brain regions. The rules which govern the plasticity of synaptic transmission are not well-specified; it shares some properties in common with the hippocampus proper, but behaves quite differently in other respects. Equally, its functional properties are not well-understood, it plays an important but ill-defined role both in spatial navigation and in mnemonic processing. The important challenges for the future revolve around the theoretical specification of its unique contribution to hippocampal formation processing on the one hand, and the experimental investigation of the many open questions (anatomical, physiological, pharmacological, functional) regarding its properties, on the other.

Subicular lesions cause dendritic atrophy in CA1 and CA3 pyramidal neurons of the rat hippocampus

The subiculum is a major source of output projections from hippocampus to cortical and subcortical regions. Our previous studies have demonstrated the selective loss of CA1 pyramidal neurons of the hippocampus, and operant and spatial learning impairment in subicular lesioned rats [Govindaiah et al. (1997) Brain Res. 745, 121-126; Laxmi et al. (1999) Brain Res. 816, 245-148]. In the present study, the effect of ibotenate lesions of the subiculum on the dendritic morphology of CA1 and CA3 pyramidal neurons of the hippocampus was investigated in 30-day-old male Wistar rats. The ventral subiculum was lesioned bilaterally with multiple injections of ibotenic acid, stereotaxically. The dendritic branching points and intersections were studied in apical and basal dendrites up to 320 and 160 microm, respectively, in Golgi-impregnated CA1 and CA3 pyramidal neurons of the hippocampus. The results revealed a significant (P<0.001) decrease in the number of dendritic branching points, intersections and total number of dendrites in both apical and basal dendrites of CA1, as well as CA3 pyramidal neurons of the hippocampus. It is surprising that the subicular lesions caused dendritic atrophy of CA3 neurons without affecting the cell density. The results of the present study demonstrate the dendritic atrophy of hippocampal neurons following selective subicular lesions. This might be responsible for the impairments in operant and spatial learning tasks in these rats as observed in our earlier studies. In addition, hippocampal damage is also associated with an impairment in the process of the active monitoring of movements in space, rather than place learning per se [Whishaw (1998) Neurosci. biobeh. Rev. 22, 209-220]. Accordingly, further studies are required to correlate the differential effect of subicular lesions on impairments in learning and movement in space in rats.

Enhanced but fragile inhibition in the dentate gyrus in vivo in the kainic acid model of temporal lobe epilepsy: a study using current source density analysis

Temporal lobe epilepsy is related to many structural and physiological changes in the brain. We used kainic acid in rats as an animal model of temporal lobe epilepsy, and studied the neural interactions of the dentate gyrus in urethane-anesthetized rats in vivo. Our initial hypothesis was that sprouting of mossy fibers, the axons of the granule cells, increases proximal dendritic excitatory currents in the inner molecular layer of the dentate gyrus. Extracellular currents were detected in vivo using current source density analysis. Backfiring the mossy fibers in CA3 or orthodromic excitation of the granule cells through the medial perforant path induced a current sink at the inner molecular layer. However, the sink or inferred excitation at the inner molecular layer was not increased in kainic acid-treated rats and the sink actually correlated negatively with the degree of mossy fiber sprouting. It is inferred that the latter sink was mediated mainly by association fibers and not by recurrent mossy fibers. After kainic acid treatment, paired-pulse inhibition of the population spikes in the dentate gyrus was increased. In contrast, reverberant activity that involved looping around an entorhinal-hippocampal circuit was increased in kainic acid-treated rats, compared to control rats. The increase of inhibition in kainic acid-treated rats was readily blocked by a small dose of GABA(A) receptor antagonist bicuculline. The latter dose of bicuculline induced paroxsymal spike bursts in kainic acid-treated but not control rats, demonstrating that the increased inhibition in dentate gyrus was fragile. In conclusion, after kainic acid induced seizures, the dentate gyrus in vivo showed an increase in inhibition that appeared to be fragile. The hypothesized increase in proximal dendritic excitation due to mossy fiber sprouting was not detected. However, the fragile inhibition could explain the seizure susceptibility in patients with temporal lobe epilepsy.

Enhanced excitability induced by ionizing radiation in the kindled rat

Evidence derived from both clinical and experimental investigations has suggested an influence of ionizing radiation on focal epileptogenicity. To better characterize this influence we applied focal ionizing radiation to a kindled epileptic focus in the rat amygdala. The right and left basolateral amygdala and right frontal cortex were implanted with concentric bipolar electrodes. Rats were kindled through a minimum of 10 stage 5 seizures by afterdischarge-threshold electrostimulation of the left amygdala, after which generalized seizure thresholds were determined prior to irradiation. The left amygdala was exposed to single-fraction central-axis doses of either 18 or 25 Gy using a beam-collimated (60)Co source (1.25 MeV). Generalized seizure thresholds were then redetermined at weekly intervals for 10 weeks and at monthly intervals for an additional 3 months. We observed no significant changes in seizure threshold during the postirradiation interval; however, we did observe persistent changes in seizure dynamics manifesting within the first week postirradiation. These consisted of an increased tendency for seizure activity to propagate into brain stem circuits during the primary ictus (i.e., "running fits") and an increased tendency for secondary convulsions to emerge postictally. These effects involving seizure dynamics have not been reported previously and appear to represent a radiation-induced disinhibition of one or more neural circuits. The disparity between these effects and earlier reports of seizure-suppressive effects resulting from analogous radiation exposures is discussed in relation to kindling and status epilepticus-induced pathogenesis within the hippocampus.

Pharmacological analysis of the subunit composition of the AMPA receptor in hippocampal neurons

Experiments were performed on isolated neurons from hippocampal field CA1 and the dentate fascia to identify the subunit composition and distribution of splicing variants of AMPA receptor subunits. Currents evoked by the application of kainate were recorded using a whole-cell patch clamping method. The presence of GluR2 subunits in receptors was associated with a sharp reduction in the activity of the selective channel blocker IEM-1460. The composition of flip versions of subunits was assessed using cyclothiazide. AMPA receptors in the major cell types (pyramidal and granule cells) had low sensitivity to IEM-1460, while AMPA receptors of other cells (interneurons) had high or intermediate sensitivity. Cyclothiazide had strong potentiating effects on the main cell types in both structures as compared with interneurons. Thus, there is a correlation between the sensitivities of hippocampal neurons to IEM-1460 and cyclothiazide. The main cell types in both structures expressed large quantities of the GluR2 subunit in their AMPA receptors, with high levels of flip subunits, as compared with the other cell types, in which GluR2 subunits were virtually absent and the flop version predominated. This appears to reflect the functional features of different types of neurons.

Synaptic plasticity in the hippocampal area CA1-subiculum projection: implications for theories of memory

This paper reviews investigations of synaptic plasticity in the major, and underexplored, pathway from hippocampal area CA1 to the subiculum. This brain area is the major synaptic relay for the majority of hippocampal area CA1 neurons, making the subiculum the last relay of the hippocampal formation prior to the cortex. The subiculum thus has a very major role in mediating hippocampal-cortical interactions. We demonstrate that the projection from hippocampal area CA1 to the subiculum sustains plasticity on a number of levels. We show that this pathway is capable of undergoing both long-term potentiation (LTP) and paired-pulse facilitation (PPF, a short-term plastic effect). Although we failed to induce long-term depression (LTD) of this pathway with low-frequency stimulation (LFS) and two-pulse stimulation (TPS), both protocols can induce a "late-developing" potentiation of synaptic transmission. We further demonstrate that baseline synaptic transmission can be dissociated from paired-pulse stimulation of the same pathway; we also show that it is possible, using appropriate protocols, to change PPF to paired-pulse depression, thus revealing subtle and previously undescribed mechanisms which regulate short-term synaptic plasticity. Finally, we successfully recorded from individual subicular units in the freely-moving animal, and provide a description of the characteristics of such neurons in a pellet-chasing task. We discuss the implications of these findings in relation to theories of the biological consolidation of memory.

Endogenous guanidino compounds as uremic neurotoxins

Epileptic and cognitive symptomatologies are among the most typical manifestations of uremic encephalopathy. Several guanidino compounds (GCs) may play an important role in the etiology of uremic encephalopathy. Four GCs appeared to be highly increased as well in serum, cerebrospinal fluid, and brain of uremic patients, whereas the levels of other metabolically relevant GCs were not or only moderately increased and others were even decreased. These highly increased compounds or "uremic" GCs are creatinine (CTN), guanidine (G), guanidinosuccinic acid (GSA), and methylguanidine (MG). All four compounds were shown to be experimental convulsants in brain concentrations similar to those found in uremic brain. We have described a possible mechanism for the contribution of GCs to uremic hyperexcitability, referring to the in vitro effects of uremic GCs on inhibitory and excitatory amino acid receptors. The excitatory effects of uremic GCs on the central nervous system may be explained by the activation of N-methyl-D-aspartate (NMDA) receptors by GSA, concomitant inhibition of GABA(A) receptors by uremic GCs, and other depolarizing effects. These effects might also indicate the putative contribution of uremic GCs to the etiology of uremic encephalopathy.

Modeling of evoked field potentials in hippocampal CA1 area describes their dependence on NMDA and GABA receptors

A computer model of the hippocampal CA1 area, which receives synaptic inputs from CA3 neurons via the Schaffer collaterals, was constructed. Pyramidal cells (PC) and two types of interneurons were represented by compartmental models, and mechanisms of feed-forward inhibition (FFI) and recurrent inhibition were incorporated. Four types of receptor mediated synaptic conductances were used in the model: those of AMPA, GABA(A), GABA(B) and N-methyl-D-aspartate (NMDA). The output of the model, i.e. the field potential calculated at various points in space, was able to qualitatively reproduce the main features of field potentials, which were recorded in hippocampal slices maintained in vitro for both subthreshold and suprathreshold stimulation. In both the experiments and the model, the influence of NMDA and GABA synaptic currents affected mostly the late, decaying phase of evoked field potentials. The modeled interaction of NMDA and GABA components could explain the enhancement of the field potential late phase, which was observed experimentally during paired-pulse stimulation.

The selectively distributed theta system: functions

The present paper provides three interwoven or interrelated approaches: (1) the dependence of frontal theta response from the spontaneous activity will be pointed out. This helps in understanding that 'frontal theta' is a major oscillation of the human frontal cortex and has a response-controlling function; (2) it will be shown that complex stimulations such as bimodal stimulation enhances the theta response; and by bringing together the results outlined in a number of previous reviews the so-called 'selectively distributed theta system of the brain' is described.

Responses of rat subicular neurons to convergent stimulation of lateral entorhinal cortex and CA1 in vivo

There has been little electrophysiological examination of the afferent projection from lateral entorhinal cortex to dorsal subiculum. Here we provide evidence that synaptic inputs from lateral entorhinal cortex and CA1 converge onto single dorsal subicular neurons in vivo. Subicular responses to CA1 stimulation consisted of excitation and/or long-duration inhibition. Neurons excited by CA1 activation usually showed inhibition to entorhinal stimulation. The latter inhibition was usually of short duration, however, long duration inhibition was seen in a significant proportion of responses. Entorhinal stimulation produced excitatory responses in four bursting cells and it was these cells that also tended to show the longest inhibition. Only bursting cells could be driven antidromically by entorhinal stimulation. Biocytin-filled multipolar and pyramidal cells displayed excitation-inhibition sequences to CA1 and inhibition to entorhinal stimulation. These data strongly suggest that subicular inhibitory neurons receive excitatory input from CA1 and display mutual inhibition. The source of entorhinal-evoked inhibition is less clear. The relative sparseness of observed entorhinal-evoked responses suggests that the input to dorsal subiculum from any one part of lateral entorhinal cortex is spatially restricted. These data show that excitation-inhibition sequences can be seen in subicular pyramidal and multipolar cells and that single subicular neurons receive convergent inputs from CA1 and entorhinal cortex. We show for the first time that bursting cells can be driven both orthodromically and antidromically by direct entorhinal stimulation. These data support the existence of a reciprocal excitatory connection between lateral entorhinal cortex and dorsal subiculum and suggest further that this connection may involve only bursting subicular neurons.

Differential learning-stage dependent patterns of c-Fos protein expression in brain regions during the acquisition and memory consolidation of an operant task in mice

The present study analysed the effects of the stage of learning of an appetitive operant conditioning task on the spatial and temporal patterns of c-Fos protein levels in the brain of BALB/c mice. c-Fos levels were assessed by immunohistochemistry at either 60, 120 or 180 min after either the first, the second or the fifth daily training session and compared to sham animals. The results show an increase of c-Fos-positive nuclei in several subcortical and cortical brain regions, 60-min post-acquisition. Because these activations were a function of task mastery, the data indicate that they were specifically related to learning. Following the first acquisition session, significant increases in c-Fos-positive neurons were observed in the dorsal hippocampus (CA3), anterior cingulate, occipital and parietal cortices. Following the second daily training session, c-Fos was highly expressed in some subcortical regions, the hippocampus, the subiculum, the entorhinal, and posterior cingulate areas. Moreover, a significant correlation was found between the progression of performance from day 1 to day 2 and c-Fos expression on the hippocampal CA1 subfield. Following complete acquisition, no further task-dependent increases in c-Fos-labelled nuclei was observed in any brain region sampled, suggesting that the intervention of c-Fos-induced mechanisms in the consolidation process were terminated. The training stage-dependent changes in regional post-training c-Fos expression in the hippocampus and the connected limbic regions suggest that this neuronal network is actively engaged in memory consolidation processes.

Sexual dimorphism in the subiculum of the rat hippocampal formation

Data accumulated over the last years demonstrate that the hippocampal formation of rodents is sexually dimorphic with respect to its functional attributes. Neuroanatomical substrates that might contribute to explain these gender-related differences have been described in the dentate gyrus, and in the CA3 and CA1 hippocampal fields. However, the subiculum, the source of the major efferent projection of the hippocampal formation, has not been searched for the presence of sex-related differences. To address this issue, we have used stereological methods applied to adult rats of both sexes to estimate the volume of the subiculum, the total number of subicular neurons, and the total number and size of the synapses established by subicular neurons. The apical dendritic trees of Golgi-impregnated subicular neurons were also quantitatively analyzed. We have found that the volume of the subiculum and of its neuronal layer, and the total number of subicular neurons were greater in males than in females. Conversely, the total dendritic length of the apical arborization of the subicular neurons, and the number of dendritic spines and axospinous synapses were higher in females than in males. However, the size of the postsynaptic densities of the individual synapses was smaller in female than in male rats and, as a result, the surface area of the total active synaptic zones did not differ between the sexes. These findings provide an additional morphological clue for the comprehension of the sex dimorphisms within the hippocampal circuitries and, consequently, for a better understanding of the functional sex differences ascribed to the hippocampal formation.

Behaviors induced or disrupted by complex partial seizures

We reviewed the neural mechanisms underlying some postictal behaviors that are induced or disrupted by temporal lobe seizures in humans and animals. It is proposed that the psychomotor behaviors and automatisms induced by temporal lobe seizures are mediated by the nucleus accumbens. A non-convulsive hippocampal afterdischarge in rats induced an increase in locomotor activity, which was suppressed by the injection of dopamine D(2) receptor antagonist in the nucleus accumbens, and blocked by inactivation of the medial septum. In contrast, a convulsive hippocampal or amygdala seizure induced behavioral hypoactivity, perhaps by the spread of the seizure into the frontal cortex and opiate-mediated postictal depression. Mechanisms underlying postictal psychosis, memory disruption and other long-term behavioral alterations after temporal lobe seizures, are discussed. In conclusion, many of the changes of postictal behaviors observed after temporal lobe seizures in humans may be found in animals, and the basis of the behavioral change may be explained as a change in neural processing in the temporal lobe and the connecting subcortical structures.

[Bradycardia as an epileptic manifestation in temporal epilepsy: report of a case]

We describe a patient who had cardiac arrhythmia as epileptic manifestation. In a 34-year-old woman who had many episodes of loss of consciousness, the simultaneous ECG and video-EEG monitoring recorded bradycardia with a short episode of asystole (4 seconds) and left temporal rhythmic theta activity on EEG. MRI showed a small mass lesion in the left parahippocampal gyrus. Alterations in cardiac rhythm have been reported in epileptic seizures and tachycardia is the most common finding associated with them; bradyarrhythmia during seizures was uncommon. Many interconnections among insular cortex, limbic system and hypothalamus, may be responsible for vegetative manifestations in temporal lobe epilepsy.

Prominence of direct entorhinal-CA1 pathway activation in sensorimotor and cognitive tasks revealed by 2-DG functional mapping in nonhuman primate

The trisynaptic pathway from entorhinal cortex to the hippocampus has long been regarded as the major route of information transfer underlying memory consolidation. Most physiological studies of this pathway involve recording from hippocampal slices. We have used both single- and double-label 2-deoxyglucose autoradiographic methods to image the pattern of activation in the hippocampal formation of 14 rhesus monkeys performing cognitive tasks, varying in content (spatial or nonspatial), process (working memory or associative memory), and mode of response (oculomotor or manual). These studies revealed a highly differentiated pattern of metabolic activation throughout the rostrocaudal extent of the hippocampal formation that was common to all behavioral conditions examined. This pattern consisted of intense activation of the stratum lacunosum-moleculare of CA1 and the subiculum, contrasting with barely detectable activity in CA3 and modest activation in the dentate gyrus, which did not include its molecular layer. These findings indicate a remarkable invariance in hippocampal activation under conditions of varied content, varied process, and varied mode of response and an heretofore-unappreciated preferential engagement of the direct rather than the trisynaptic pathway during performance of a wide range of behavioral tasks.

Physiology of the entorhinal and perirhinal projections to the hippocampus studied by current source density analysis

Evoked field potentials and current-source-density analysis were used to study the olfactory, entorhinal, and perirhinal projections to the hippocampus. In urethane-anesthetized rats, various structures were electrically stimulated, and evoked potentials were mapped using glass micropipettes or multichannel silicon probes. Stimulation of the olfactory bulb, lateral olfactory tract, piriform cortex, amygdala-entorhinal transition, lateral entorhinal cortex, or lateral perforant path (LPP) evoked an outer molecular layer sink (inferred distal dendritic excitation) in the dentate gyrus, with progressively decreasing onset latency. Medial perforant path (MPP) stimulation evoked a middle molecular layer sink (mid-dendritic excitation) in the dentate gyrus. LPP and MPP were also inferred to monosynaptically excite the distal dendrites of CA3, often resulting in a population spike in CA3. CA3 spiking, in turn, was often followed by excitation at the inner molecular layer of the dentate gyrus. LPP and MPP evoked distal dendritic sinks but no population spikes in CA1. Stimulation of the perirhinal cortex activated a sink in the subiculum/CA1 border without activating the dentate gyrus. In addition, reverberatory activity through a hippocampal-entorhinal-hippocampal pathway may be activated by MPP or CA3 stimulation. It is suggested that the parallel projections of the entorhinal and perirhinal inputs to the distal dendrites of hippocampal principal neurons enhance local and distributed processing as characterized by CA3 to dentate gyrus feedback, and hippocampal-entorhinal reverberation.

Epileptogenesis in the parahippocampal region. Parallels with the dentate gyrus

Limbic seizures have often been attributed to pathology in the hippocampus, such as the well described condition termed Ammon's Horn sclerosis, in which many of the hippocampal principal cells have degenerated. However, several studies in both the clinical and basic literature indicate that the parahippocampal region may also play an important role. This region sustains a characteristic pattern of damage in most animal models of epilepsy that is similar to that identified in humans with intractable temporal lobe epilepsy. Perhaps the most striking aspect of parahippocampal pathology is the marked loss of neurons in layer III of the entorhinal cortex. The similarity of cell loss in layer III and cell loss in the hilus of the dentate gyrus is compared, as is the characteristic resistance of layer II neurons and dentate granule cells. Cellular electrophysiological results are used as a basis for the hypothesis that synaptic inhibition plays a role in the relative vulnerability of these neurons. Studies of neurogenesis in both areas is also discussed. It is proposed that this may be an additional factor that influences vulnerability in these areas.

Two-phase computational model training long-term memories in the entorhinal-hippocampal region

The computational model described here is driven by the hypothesis that a major function of the entorhinal cortex (EC)-hippocampal system is to alter synaptic connections in the neocortex. It is based on the following postulates: (1) The EC compares the difference between neocortical representations (primary input) and feedback information conveyed by the hippocampus (the "reconstructed input"). The difference between the primary input and the reconstructed input (termed "error") initiates plastic changes in the hippocampal networks (error compensation). (2) Comparison of the primary input and reconstructed input requires that these representations are available simultaneously in the EC network. We suggest that compensation of time delays is achieved by predictive structures, such as the CA3 recurrent network and EC-CA1 connections. (3) Alteration of intrahippocampal connections gives rise to a new hippocampal output. The hippocampus generates separated (independent) outputs, which, in turn, train long-term memory traces in the EC (independent components, IC). The ICs of the long-term memory trace are generated in a two-step manner, the operations of which we attribute to the activities of the CA3 (whitening) and CA1 (separation) fields. (4) The different hippocampal fields can perform both nonlinear and linear operations, albeit at different times (theta and sharp phases). We suggest that long-term memory is represented in a distributed and hierarchical reconstruction network, which is under the supervision of the hippocampal output. Several of these model predictions can be tested experimentally.

Oscillatory activity in entorhinal neurons and circuits. Mechanisms and function

Layers II and V of the entorhinal cortex (EC) occupy a privileged anatomical position in the temporal lobe memory system that allows them to gate the main flow of information in and out of the hippocampus, respectively. In vivo studies have shown that layer II of the EC is a robust generator of theta as well as gamma activity. Theta may also be present in layer V, but the layer V network is particularly prone to genesis of short-lasting high-frequency oscillations ("ripples"). Interestingly, in vitro studies have shown that EC layers II and V, but not layer III, have the potential to act as independent pacemakers of population oscillatory activity. Moreover, it has also been shown that subgroups of principal neurons both within layers II and V, but not layer III, are endowed with autorhythmic properties. These are characterized by subthreshold oscillations where the depolarizing phase is driven by the activation of "persistent" Na+ channels. We propose that the oscillatory properties of layer II and V neurons and local circuits are responsible for setting up the proper temporal dynamics for the coordination of the multiple sensory inputs that converge onto EC and thus help to generate sensory representations and memory encoding.

Olfactory inputs activate the medial entorhinal cortex via the hippocampus

The lateral and medial regions of the entorhinal cortex differ substantially in terms of connectivity and pattern of activation. With regard to olfactory input, a detailed and extensive physiological map of the olfactory projection to the entorhinal cortex is missing, even if anatomic studies suggest that the olfactory afferents are confined to the lateral and rostral entorhinal region. We studied the contribution of the medial and lateral entorhinal areas to olfactory processing by analyzing the responses induced by lateral olfactory tract stimulation in different entorhinal subfields of the in vitro isolated guinea pig brain. The pattern of synaptic activation of the medial and lateral entorhinal regions was reconstructed either by performing simultaneous multisite recordings or by applying current source density analysis on field potential laminar profiles obtained with 16-channel silicon probes. Current source density analysis demonstrated the existence of a direct monosynaptic olfactory input into the superficial 300 microm of the most rostral part of the lateral entorhinal cortex exclusively, whereas disynaptic sinks mediated by associative fibers arising from the piriform cortex were observed at 100-350 microm depth in the entire lateral aspect of the cortex. No local field responses were recorded in the medial entorhinal region unless a large population spike was generated in the hippocampus (dentate gyrus and CA1 region) by a stimulus 3-5x the intensity necessary to obtain a maximal monosynaptic response in the piriform cortex. In these conditions, a late sink was recorded at a depth of 600-1000 microm in the medial entorhinal area (layers III-V) 10.6 +/- 0.9 (SD) msec after a population spike was simultaneously recorded in CA1. Diffuse activation of the medial entorhinal region was also obtained by repetitive low-intensity stimulation of the lateral olfactory tract at 2-8 Hz. Higher or lower stimulation frequencies did not induce hippocampal-medial entorhinal cortex activation. These results suggest that the medial and the lateral entorhinal regions have substantially different roles in processing olfactory sensory inputs.

Maternal morphine alters parvalbumin immunoreactivity patterns in neonatal mouse brain

The influence of chronic maternal morphine on the parvalbumin immunoreactive patterns in developing mouse brain was studied. Female Swiss mice were administered daily saline or morphine (30 or 60 mg/kg) for a period of 7 days before mating, gestation, and 21 days postpartum. Their pups were sacrificed on postnatal day 18 and the brains were examined histologically and immunohistochemically for parvalbumin-positive neurons. Histological observations revealed no significant changes in the cell number of the morphine-exposed neonatal forebrain, whereas the number of parvalbumin-positive neurons increased in layers II-IV of the parietal cortex I. Moreover, the number of parvalbumin-positive dendrites increased remarkably in the cingulate and parietal I cortices of the morphine-exposed neonates, indicating the region-specific increase in the PV immunoreactive profiles. These results are consistent with the key roles played by the above brain regions in the altered behavioral patterns of the maternally addicted neonates, such as impaired somatosensory and cognitive performances. The mechanism of morphine action on parvalbumin expression in neonatal mouse brain is not evident, but alterations in the expression patterns of parvalbumin in specific regions of the developing brain might be one of the cellular mechanisms by which addictive drugs modify the functional aspects of the developing CNS.

The role of the posterior hypothalamus in controlling the paradoxical phase of sleep

Chronic experiments were performed on seven cats to study the effects of high-frequency electrical stimulation of the posterior hypothalamic area on the characteristics of paradoxical sleep; the excitability of this structure at different stages of paradoxical sleep was determined. These studies showed that at the stage showing ECoG desynchronization and phasic events (stage 1), the response threshold for behavioral arousal resulting from stimulation of posterior hypothalamus was 20-30% higher than at the stage characterized by alpha-like activity in the ECoG and the absence of phasic phenomena (stage 2). Transient stimulation of the posterior hypothalamus at stage 1, at a level which was subthreshold for arousal from this state, led to a transition to stage 2 or a reduction in phasic events in paradoxical sleep without altering the qualitative characteristics of phase 1. Stimulation of the posterior hypothalamus at a level subthreshold for arousal from stage 2 and applied continuously during paradoxical sleep led to a reduction in the duration of this stage by 25-50% and to an increase in the proportion of stage 2 in the structure of paradoxical sleep. These results provide evidence that the posterior hypothalamus is involved in the inhibitory control of the 'executive' mechanisms of paradoxical sleep responsible for the ECoG desynchronization and the phasic manifestations of this state. It is suggested that the functional activity of the posterior hypothalamus at stage 1 also increases at stage 2, thus evidently fulfilling a 'guard' function.

Analysis of the connectional organization of neural systems associated with the hippocampus in rats

The hippocampus of the rat enjoys a central significance for researchers interested in the neural mechanisms of memory and spatial information processing. Many of the theoretical models advanced to explain function in this system, however, do not reflect the wealth of information on the connectivity of these structures, and employ greatly simplified treatments of its complex connectivity. We were interested in whether a more analytical approach, which begins with analysis of the connectivity of the system, might provide insights complementary to those derived by synthetic models. Accordingly, we collated detailed neuroanatomical information about the connectivity of the hippocampal system in the rat, and analysed the resulting data. Analyses of connectivity based on a variety of different analytical techniques have recently been used to elucidate the global organization of other systems in the macaque and cat, and have given rise to successful predictions. We applied non-metric multidimensional scaling and non-parametric cluster analysis to our summary matrix of connection data. The analyses produced organizational schemes that were consistent with known physiological properties and provided the basis for making tentative predictions of the further structures that may contain 'place' and 'head-direction' cells, which structures we identify. The consistency between the analyses of connectivity and the distribution of physiological properties across the system suggests that functional relationships are constrained by the organization of the connectivity of the system, and so that structure and function are linked at the systems level.

Expression of neurotrophin-3 in the mouse forebrain: insights from a targeted LacZ reporter

To obtain insights into the expression of neurotrophin-3 (NT-3) in the mouse, we have utilized mice in which the Escherichia coli lacZ gene is integrated into the neurotrophin-3 locus (NT-3(lacZneo), Fariñas et al. [1994] Nature 369:658-661). In this mouse strain, beta-galactosidase production is under control of the NT-3 promoter in its normal chromosomal environment, and histochemical measurement of beta-galactosidase provides a simple, sensitive method to determine which cells express NT-3. Our data correlate well with previous in situ mRNA and immunocytochemical studies reporting the localization of NT-3. For example, in adult NT-3(lacZneo)/+ mice, beta-galactosidase is expressed in high amounts in limbic areas of the cortex (cingulate, retrosplenial, piriform, and entorhinal), in the visual cortex, in the hippocampal formation (dentate granule cells, CA2 cells, fasciola cinereum, induseum griseum, tenia tecta, presubiculum, and parasubiculum), and in the septum (septohippocampal nucleus and lateral dorsal septum). In contrast with other studies, our results suggest more extensive expression of NT-3 in adult and aged mouse brains with cortical expression apparent at 4.5 months. To further define the cell populations expressing NT-3 in adult mice, we have combined immunocytochemistry with histochemical staining and found that beta-galactosidase is coexpressed with a neuronal marker (NeuN) and with parvalbumin and neuropeptides, markers for GABAergic interneurons. Our studies of embryonic beta-galactosidase expression in NT-3(lacZneo)/+ mice suggest sites of NT-3 expression not previously described, including embryonic piriform cortical cells and dentate granule cell precursors.

Patterned activity in stratum lacunosum moleculare inhibits CA1 pyramidal neuron firing

CA1 pyramidal cells are the primary output neurons of the hippocampus, carrying information about the result of hippocampal network processing to the subiculum and entorhinal cortex (EC) and thence out to the rest of the brain. The primary excitatory drive to the CA1 pyramidal cells comes via the Schaffer collateral (SC) projection from area CA3. There is also a direct projection from EC to stratum lacunosum-moleculare (SLM) of CA1, an input well positioned to modulate information flow through the hippocampus. High-frequency stimulation in SLM evokes an inhibition sufficiently strong to prevent CA1 pyramidal cells from spiking in response to SC input, a phenomenon we refer to as spike-blocking. We characterized the spike-blocking efficacy of burst stimulation (10 stimuli at 100 Hz) in SLM and found that it is greatest at approximately 300-600 ms after the burst, consistent with the time course of the slow GABA(B) signaling pathway. Spike-blocking efficacy increases in potency with the number of SLM stimuli in a burst, but also decreases with repeated presentations of SLM bursts. Spike-blocking was eliminated in the presence of GABA(B) antagonists. We have identified a candidate population of interneurons in SLM and distal stratum radiatum (SR) that may mediate this spike-blocking effect. We conclude that the output of CA1 pyramidal cells, and hence the hippocampus, is modulated in an input pattern-dependent manner by activation of the direct pathway from EC.

Expression of the cannabinoid receptor CB1 in distinct neuronal subpopulations in the adult mouse forebrain

Cannabinoids can modulate motor behaviour, learning and memory, cognition and pain perception. These effects correlate with the expression of the cannabinoid receptor 1 (CB1) and with the presence of endogenous cannabinoids in the brain. In trying to obtain further insights into the mechanisms underlying the modulatory effects of cannabinoids, CB1-positive neurons were determined in the murine forebrain at a single cell resolution. We performed a double in situ hybridization study to detect mRNA of CB1 in combination with mRNA of glutamic acid decarboxylase 65k, neuropeptide cholecystokinin (CCK), parvalbumin, calretinin and calbindin D28k, respectively. Our results revealed that CB1-expressing cells can be divided into distinct neuronal subpopulations. There is a clear distinction between neurons containing CB1 mRNA either at high levels or low levels. The majority of high CB1-expressing cells are GABAergic (gamma-aminobutyric acid) neurons belonging mainly to the cholecystokinin-positive and parvalbumin-negative type of interneurons (basket cells) and, to a lower extent, to the calbindin D28k-positive mid-proximal dendritic inhibitory interneurons. Only a fraction of low CB1-expressing cells is GABAergic. In the hippocampus, amygdala and entorhinal cortex area, CB1 mRNA is present at low but significant levels in many non-GABAergic cells that can be considered as projecting principal neurons. Thus, a complex mechanism appears to underlie the modulatory effects of cannabinoids. They might act on principal glutamatergic circuits as well as modulate local GABAergic inhibitory circuits. CB1 is very highly coexpressed with CCK. It is known that cannabinoids and CCK often have opposite effects on behaviour and physiology. Therefore, we suggest that a putative cross-talk between cannabinoids and CCK might exist and will be relevant to better understanding of physiology and pharmacology of the cannabinoid system.