A whole image of the hippocampal pyramidal neuron revealed by intracellular pressure-injection of horseradish peroxidase

Amygdala-hippocampal interactions in synaptic plasticity and memory formation

Memories of emotionally arousing events tend to endure longer than other memories. This review compiles findings from several decades of research investigating the role of the amygdala in modulating memories of emotional experiences. Episodic memory is a kind of declarative memory that depends upon the hippocampus, and studies suggest that the basolateral complex of the amygdala (BLA) modulates episodic memory consolidation through interactions with the hippocampus. Although many studies in rodents and imaging studies in humans indicate that the amygdala modulates memory consolidation and plasticity processes in the hippocampus, the anatomical pathways through which the amygdala affects hippocampal regions that are important for episodic memories were unresolved until recent optogenetic advances made it possible to visualize and manipulate specific BLA efferent pathways during memory consolidation. Findings indicate that the BLA influences hippocampal-dependent memories, as well as synaptic plasticity, histone modifications, gene expression, and translation of synaptic plasticity associated proteins in the hippocampus. More recent findings from optogenetic studies suggest that the BLA modulates spatial memory via projections to the medial entorhinal cortex, and that the frequency of activity in this pathway is a critical element of this modulation.

Axonal Conduction Velocity in CA1 Area of Hippocampus is Reduced in Mouse Models of Alzheimer's Disease

The timing of action potentials arrival at synaptic terminals partially determines integration of synaptic inputs and is important for information processing in the CNS. Therefore, axonal conduction velocity (VC) is a salient parameter, influencing the timing of synaptic inputs. Even small changes in VC may disrupt information coding in networks requiring accurate timing. We recorded compound action potentials in hippocampal slices to measure VC in three mouse models of Alzheimer's disease. We report an age-dependent reduction in VC in area CA1 in two amyloid-β precursor protein transgenic mouse models, line 41 and APP/PS1, and in a tauopathy model, rTg4510.

Postmeal Optogenetic Inhibition of Dorsal or Ventral Hippocampal Pyramidal Neurons Increases Future Intake

Memory of a recently eaten meal can serve as a powerful mechanism for controlling future eating behavior because it provides a record of intake that likely outlasts most physiological signals generated by the meal. In support, impairing the encoding of a meal in humans increases the amount ingested at the next eating episode. However, the brain regions that mediate the inhibitory effects of memory on future intake are unknown. In the present study, we tested the hypothesis that dorsal hippocampal (dHC) and ventral hippocampal (vHC) glutamatergic pyramidal neurons play a critical role in the inhibition of energy intake during the postprandial period by optogenetically inhibiting these neurons at specific times relative to a meal. Male Sprague Dawley rats were given viral vectors containing CaMKIIα-eArchT3.0-eYFP or CaMKIIα-GFP and fiber optic probes into dHC of one hemisphere and vHC of the other. Compared to intake on a day in which illumination was not given, inhibition of dHC or vHC glutamatergic neurons after the end of a chow, sucrose, or saccharin meal accelerated the onset of the next meal and increased the amount consumed during that next meal when the neurons were no longer inhibited. Inhibition given during a meal did not affect the amount consumed during that meal or the next one but did hasten meal initiation. These data show that dHC and vHC glutamatergic neuronal activity during the postprandial period is critical for limiting subsequent ingestion and suggest that these neurons inhibit future intake by consolidating the memory of the preceding meal.

Cognitive control of meal onset and meal size: Role of dorsal hippocampal-dependent episodic memory

There is a large gap in our understanding of how top-down cognitive processes, such as memory, influence energy intake. Similarly, there is limited knowledge regarding how the brain controls the timing of meals and meal frequency. Understanding how cognition influences ingestive behavior and how the brain controls meal frequency will provide a more complete explanation of the neural mechanisms that regulate energy intake and may also increase our knowledge of the factors that contribute to diet-induced obesity. We hypothesize that dorsal hippocampal neurons, which are critical for memory of personal experiences (i.e., episodic memory), form a memory of a meal, inhibit meal onset during the period following a meal, and limit the amount ingested at the next meal. In support, we describe evidence from human research suggesting that episodic memory of a meal inhibits intake and review data from human and non-human animals showing that impaired hippocampal function is associated with increased intake. We then describe evidence from our laboratory showing that inactivation of dorsal hippocampal neurons decreases the interval between sucrose meals and increases intake at the next meal. We also describe our evidence suggesting that sweet orosensation is sufficient to induce synaptic plasticity in dorsal hippocampal neurons and raise the possibility that impaired dorsal hippocampal function and episodic memory deficits contribute to the development and/or maintenance of diet-induced obesity. Finally, we raise some critical questions that need to be addressed in future research.

Mechanisms of seizure propagation in 2-dimensional centre-surround recurrent networks

Understanding how seizures spread throughout the brain is an important problem in the treatment of epilepsy, especially for implantable devices that aim to avert focal seizures before they spread to, and overwhelm, the rest of the brain. This paper presents an analysis of the speed of propagation in a computational model of seizure-like activity in a 2-dimensional recurrent network of integrate-and-fire neurons containing both excitatory and inhibitory populations and having a difference of Gaussians connectivity structure, an approximation to that observed in cerebral cortex. In the same computational model network, alternative mechanisms are explored in order to simulate the range of seizure-like activity propagation speeds (0.1-100 mm/s) observed in two animal-slice-based models of epilepsy: (1) low extracellular [Formula: see text], which creates excess excitation and (2) introduction of gamma-aminobutyric acid (GABA) antagonists, which reduce inhibition. Moreover, two alternative connection topologies are considered: excitation broader than inhibition, and inhibition broader than excitation. It was found that the empirically observed range of propagation velocities can be obtained for both connection topologies. For the case of the GABA antagonist model simulation, consistent with other studies, it was found that there is an effective threshold in the degree of inhibition below which waves begin to propagate. For the case of the low extracellular [Formula: see text] model simulation, it was found that activity-dependent reductions in inhibition provide a potential explanation for the emergence of slowly propagating waves. This was simulated as a depression of inhibitory synapses, but it may also be achieved by other mechanisms. This work provides a localised network understanding of the propagation of seizures in 2-dimensional centre-surround networks that can be tested empirically.

Changes in the views of neuronal connectivity and communication after Cajal: examples from the hippocampus

Intracellular recordings with concurrent visualization of the neuron as well as immunocytochemical studies in the last couple of decades confirmed the selectivity, and revealed additional complexity, in the synaptic connections in hippocampal circuits described by Santiago Ramón y Cajal. Even minor anatomical details began to gain functional meaning via the state-of-the-art combined approaches. The revolution of molecular biology brought about the rapid development of anatomy aimed at the localization of the numerous receptor subunits, ion channels, transporters and other proteins at the regional, cellular and subcellular levels that are being cloned every day (e.g., see Nusser, 2000). These fine-grain immunocytochemical data appear to have an immense predictive power for physiological and pharmacological studies and continue to serve as the ultimate test of hypotheses drawn from functional studies. Knowledge of the precise anatomical distribution of extrasynaptic receptors is required to understand the functional roles of various nonsynaptic mediators and diffuse pathways in the brain, as well as to the design of selective drugs for pharmacotherapy. Cajal would be delighted to see the revitalization of functional neuroanatomy, particularly of molecular anatomy, among the modern disciplines in the neurosciences today.

Neurons in Golgi-stain-like images revealed by GFP-adenovirus infection in vivo

Neurons in the adult brain have a very complex morphology with many processes, including tremendously long axons. Since dendrites and axons play key roles in the input and output of neural information, respectively, the visualization of complete images of these processes is necessary to reveal the mechanism of neural information processing. Here we made a recombinant adenovirus vector which encodes green fluorescent protein (GFP) tagged with a palmitoylation site, a membrane-targeting signal, produced specific antibodies to GFP, and used them as probes for staining the nervous system. In the neocortex, after injection of the recombinant virus and immunoperoxidase staining with the antibodies, many different types of cells were labeled in a Golgi stain-like fashion. Although the number of labeled cells varied depending on the amount of virus injected, the recombinant virus was considered to be infectious to cortical neurons of all cell types without selectivity. In contrast, the viral infection in the cerebellar cortex and superior cervical ganglion showed some selectivity toward the cell type. It is expected that this recombinant virus will be a useful tool for the morphological analysis of neuronal connections, especially the analysis of microcircuitry in the cerebral cortex.

On the structure of ictal events in vitro

PURPOSE

To analyze the cellular and network mechanisms of sustained seizures, we reviewed the literature and present new data on in vitro epileptiform events. We considered single and recurring synchronized population bursts occurring on a time scale from tens of milliseconds to 1 min.

METHODS

We used intracellular and field potential recordings, together with computer network simulations, derived from three types of experimental epileptogenesis: gamma-aminobutyric-acidA (GABAA) blockade, low extracellular [Mg2+]o, and 4-aminopyridine (4-AP).

RESULTS

In all three models, sustained depolarizing synaptic currents developed, either through N-methyl-D-aspartate (NMDA) receptors, depolarizing GABAA receptors, or both. Ectopic action potentials (APs), probably originating in axonal structures, occurred in 4-AP and (as shown by other researchers) after tetanic stimulation; ectopic APs, occurring at sufficient frequency, should also depolarize dendrites, by synaptic excitation, enough to trigger bursts.

CONCLUSIONS

Ictal-like events appear to arise from two basic mechanisms. The first mechanism consists of sustained dendritic depolarization driving a series of dendritic bursts. The second mechanism consists of an increase in axonal and presynaptic terminal excitability driving a series of bursts analogous to interictal spikes.

Morphology of physiologically identified retinal X and Y axons in the cat's thalamus and midbrain as revealed by intraaxonal injection of biocytin

Prior morphological studies of individual retinal X and Y axon arbors based on intraaxonal labeling with horseradish peroxidase have been limited by restricted diffusion or transport of the label. We used biocytin instead as the intraaxonal label, and this completely delineated each of our six X and 14 Y axons, including both thalamic and midbrain arbors. Arbors in the lateral geniculate nucleus appeared generally as has been well documented previously. Interestingly, all of the labeled axons projected a branch beyond thalamus to the midbrain. Each X axon formed a terminal arbor in the pretectum, but none continued to the superior colliculus. In contrast, 11 of 14 Y axons innervated both the pretectum and the superior colliculus, one innervated only the pretectum, and two innervated only the superior colliculus. Two of the Y axons were quite unusual in that their receptive fields were located well into the hemifield ipsilateral with respect to the hemisphere into which they were injected. These axons exhibited remarkable arbors in the lateral geniculate nucleus, diffusely innervating the C-laminae and medial interlaminar nucleus, but, unlike all other X and Y arbors, they did not innervate the A-laminae at all. In addition to these qualitative observations, we analyzed a number of quantitative features of these axons in terms of numbers and distributions of terminal boutons. We found that Y arbors contained more boutons than did X arbors in both thalamus and midbrain. Also, for axons with receptive fields in the contralateral hemifield (all X and all but two Y axons), 90-95% of their boutons terminated in the lateral geniculate nucleus; the other two Y axons had more of their arbors located in midbrain.

Preservation of topography in the connections between the subiculum, field CA1, and the entorhinal cortex in rats

In order to examine whether the entorhinal-hippocampal-entorhinal circuit is reciprocal and topographic, the connections between the subiculum, the CA1 field, and the entorhinal cortex were studied with the carbocyanine dye (Dil), which moves in both retrograde and anterograde directions. We investigated the organization of reciprocal connections revealed by injections of Dil in the entorhinal cortex along the rhinal sulcus. Anterograde fluorescent labeling showed the same pattern reported in previous studies of the dorsal hippocampus. When the injection site of DiI extended into the deep layers (IV-VI) of the same cortical column, the anterograde labeling of the perforant path was accompanied by retrograde labeling of the subicular neurons and the CA1 neurons. The distribution of labeled cells overlapped the distribution of labeled fibers, and the distribution of labeled cells paralleled that of the labeled fibers in the CA1 field. DiI injection into the medial entorhinal cortex revealed fewer retrogradely labeled subicular neurons than injection into the lateral entorhinal cortex, whereas the number of labeled CA1 neurons was not dependent on the injection site. The number of labeled CA1 neurons was always several times greater than the number of subicular neurons. Thus, the amount of information conveyed by the CA1 projection might be higher than that conveyed by the subicular projection. These results indicate that the entorhinal cortex, CA1, and the subiculum are connected reciprocally and topographically. We believe that the framework of the major hippocampal circuit proposed in previous studies should be reconsidered. We propose that the CA1 projection, rather than the subicular projection, is the main projection that feeds back information from the hippocampus to the entorhinal cortex.

Trigeminal sensory innervation on perforators of the circle of Willis in rabbits by wheat germ agglutinin-conjugated horseradish peroxidase anterograde tracing

Distribution patterns of sensory innervation from the trigeminal ganglion to the perforators of the circle of Willis in rabbits were investigated by wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) anterograde tracing. Twenty Japanese white rabbits were anesthetized by inhaling 1% halothane. Using a microsurgical technique, 4 microliters of 2% WGA-HRP in 1 M KCl solution, colored with brilliant blue, was micro-injected into the medial part of the left trigeminal ganglion in 14 animals with a pressure injection system. Another six served as controls to exclude the possibility of labeling non-trigeminal axons. Forty-eight hours later, the perforators in the cisternal and intracerebral segments along with their parent arteries were dissected from the brain according to Dacey's dissecting technique after transcardial perfusion, reacted with the 3,3',5,5'-tetramethyl benzidine method of Mesulam. The results revealed that sensory nerves on the perforators of the circle of Willis were less densely innervated than those on their parent arteries due to the difference in diameter. The posteromedial perforating arteries arising from the P1 segment of the posterior cerebral artery to the tegmentum, posteroventral thalamus and posterior hypothalamus were more prominently and consistently innervated than other perforators. The sensory fibers were seen on the cisternal segment of the perforating arteries. A parallel or twisted pattern was found in the perforators less than 100 microns in diameter, while a meshwork pattern was visualized in the proximal part of some bigger ones. Fine sensory fibers could be traced on the perforators as small as 40 microns in diameter.(ABSTRACT TRUNCATED AT 250 WORDS)

The trajectory of the sympathetic nerve fibers to the rat cochlea as revealed by anterograde and retrograde WGA-HRP tracing

Wheat germ agglutinin-horseradish peroxidase conjugate was injected in the unilateral superior cervical ganglion (SCG), and the projection pathways of postganglionic sympathetic nerve fibers innervating the cochlea were traced in the rat. The labeled axons advanced along the internal carotid artery (ICA), and a few advanced caudally in the major petrosal nerve (MPN) and entered the facial nerve, while the majority ran rostral to the pterygopalatine ganglion at the point where they crossed the MPN in the carotid canal. The rest of the labeled fibers remained on the surface of the ICA and advanced to the cranial cavity. Most of the labeled fibers along the facial nerve joined the cochlear nerve and finally reached the osseous spiral lamina through the spiral ganglion. Some of the labeled fibers ran along the anterior inferior cerebellar artery from the basilar artery which was previously thought to have been the only pathway. We could not find any labeled fiber on the modiolar artery from anterior inferior cerebellar artery in the cochlea. These observations are consistent with our hypothesis that the sympathetic fibers innervating the neural tissues or related structures follow nerve fibers and meninges as matrices of projection pathways rather than arteries.

Are there unifying principles underlying the generation of epileptic afterdischarges in vitro?

To find general principles in the cellular mechanisms of epileptogenesis, one must analyze experimental epilepsy models and determine what exists in common between them. We consider here afterdischarges in hippocampal slices induced using either (1) GABAA blockade (e.g. with bicuculline), (2) a bathing solution lacking Mg2+ ions (low Mg-induced epilepsy), or (3) 4-aminopyridine (4AP). By 'afterdischarge' we mean an event that lasts hundreds of milliseconds or more, involving the synchronous firing of all the neurons in a population, shaped into a long initial burst and a series of one or more secondary bursts, and terminating in a prolonged afterhyperpolarization (AHP). We propose that the following features exists in common between these three experimental epilepsies: (1) recurrent excitatory synaptic connections; (2) sustained dendritic synaptic excitation, mediated by either AMPA or NMDA receptors, or both; (3) an intrinsic cellular response to sustained excitation, consisting of rhythmical dendritic bursts, primarily mediated by Ca spikes. In conclusion, if the picture outlined here proves correct, then the stereotypic appearance of epileptic afterdischarges--consisting of synchronized population bursts in series, whatever the network alteration leading to seizures--does indeed reflect a common set of mechanisms. The mechanisms cannot, apparently, be formulated in simple terms of this receptor or that receptor. Rather, we suggest, the recurrent excitatory synapses are able, under diverse circumstances, collectively to produce sustained dendritic conductances in neuronal populations. Pyramidal neurons, by virtue of their normal intrinsic membrane properties, respond to such sustained conductances with rhythmical bursts. The recurrent synapses, in a dual role, serve to maintain the synchrony of these bursts, and so shape the activity into a synchronized oscillation.

Hippocampal pyramidal cells excite inhibitory neurons through a single release site

Morphologically a synapse consists of a presynaptic release site containing vesicles, a postsynaptic element with membrane specialization, and a synaptic cleft between them. The number of release sites shapes the properties of synaptic transmission between neurons. Although excitatory interactions between cortical neurons have been examined, the number of release sites remains unknown. We have now recorded excitatory postsynaptic potentials evoked by single pyramidal cells in hippocampal interneurons and visualized both cells using biocytin injections. Light and electron microscopy showed that excitatory postsynaptic potentials were mediated by a single synapse. We also reconstructed the entire axon arborization of single pyramidal cells, filled in vivo, in sections counterstained for parvalbumin, which selectively marks basket and axo-axonic cells. Single synaptic contacts between pyramidal cells and parvalbumin-containing neurons were dominant (> 80%), providing evidence for high convergence and divergence in hippocampal networks.

Analysis of the propagation of disinhibition-induced after-discharges along the guinea-pig hippocampal slice in vitro

1. A model has been proposed of picrotoxin-induced hippocampal in vitro after-discharges; it depends critically upon alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors in the recurrent excitatory connections between pyramidal neurones, and upon the ability of pyramidal neurones to generate bursts at about 10 Hz when their dendrites are sufficiently depolarized. 2. We study here the question of whether this model can account for spatial--as well as temporal--aspects of after-discharges in guinea-pig hippocampal slices. For example, can the model explain the propagation along a transverse slice of the initial burst and the secondary bursts at about the same velocity, approximately 0.10-0.20 m s-1? Under what conditions might the secondary bursts exhibit a different spatial pattern to the initial burst, as we now show can occur in longitudinal slices? To examine these questions, we increased the number of cells in our model from 100 to 8000 (in a 20 x 400 array), arranging the excitatory synaptic connections in a spatially restricted fashion, with an average extent of 1.0 mm (as suggested experimentally). 3. Our model suggests that both AMPA and NMDA receptors contribute to the propagation pattern and velocity of the initial and the secondary bursts in an after-discharge. 4. When unitary AMPA and NMDA conductances are in the range where the primary burst lasts for 100-200 ms, and there are three or four secondary bursts, then both primary and secondary bursts propagate near to the experimentally observed velocity for transverse slices. In the model, however, secondary bursts propagate at somewhat slower velocities than the initial burst. 5. The mechanisms of propagation are different for the initial and for the secondary bursts: propagation of the primary burst depends upon the initiation of electrogenesis in 'resting' dendrites by AMPA and NMDA inputs that are rapidly increasing in time. Propagation of secondary bursts depends upon the timing of calcium spikes in depolarized dendrites with slowly varying NMDA inputs; the timing of calcium spikes can be influenced by a 'wave' of AMPA input, but calcium spikes--we predict--should occur even without the AMPA input, once the after-discharge has been initiated. The blockade of firing in an intermediate region of the disinhibited slice is predicted to have different effects on the primary burst and on secondary bursts distal to the region of blockade.(ABSTRACT TRUNCATED AT 400 WORDS)

Complete axon arborization of a single CA3 pyramidal cell in the rat hippocampus, and its relationship with postsynaptic parvalbumin-containing interneurons

The complete axon arborization of a single CA3 pyramidal cell has been reconstructed from 32 (60 microns thick) sections from the rat hippocampus following in vivo intracellular injection of neurobiotin. The same sections were double-immunostained for parvalbumin--a calcium-binding protein selectively present in two types of GABAergic interneurons, the basket and chandelier cells--in order to map boutons of the pyramidal cell in contact with dendrites and somata of these specific subsets of interneurons visualized in a Golgi-like manner. The axon of the pyramidal cell formed 15,295 boutons, 63.8% of which were in stratum oriens, 15.4% in stratum pyramidale and 20.8% in stratum radiatum. Only 2.1% of the axon terminals contacted parvalbumin-positive neurons. Most of these were single contacts (84.7%), but double or triple contacts (15.3%) were also found. The majority of the boutons terminated on dendrites (84.1%) of parvalbumin-positive cells, less frequently on cell bodies (15.9%). In order to estimate the proportion of contacts representing synapses, 16 light microscopically identified contacts between boutons of the filled pyramidal cell axon and the parvalbumin-positive targets were examined by correlated electron microscopy. Thirteen of them were found to be asymmetrical synapses, and in the remaining three cases synapses between the labelled profiles could not be confirmed. We conclude that the physiologically effective excitatory connections between single pyramidal cells and postsynaptic inhibitory neurons are mediated by a small number of contacts, mostly by a single synapse. This results in a high degree of convergence and divergence in hippocampal networks.

Projection of the entorhinal layer II neurons in the rat as revealed by intracellular pressure-injection of neurobiotin

A component of the perforant path, projection of the entorhinal layer II neurons, was investigated by recovering intracellularly labeled layer II neurons from the medial or intermediate entorhinal cortex in rats. The labeled layer II spiny stellate neurons had axon collaterals in layers I, II, and III of the entorhinal cortex as well as some axon collaterals in the subiculum. The stem axons gave rise to terminal axon branches that covered the entire extent (suprapyramidal blade, crest, and infrapyramidal blade) of the dentate gyrus and the CA2-3 fields in the transverse plane, forming a sheet-like formation. The axon arbor in the hippocampal formation spread up to 2 mm wide in a septotemporal direction. The sheet-like formation of the axon arbors was a narrow layer in the suprapyramidal blade and in the stratum lacunosum-moleculare of the CA2-3 fields. The layer became wider in the crest and infrapyramidal blade of the dentate gyrus. This study shows that the entorhinohippocampal circuit is not a simple circle from single cells level.

Distribution patterns of sensory innervation from the trigeminal ganglion to cerebral arteries in rabbits studied by wheat germ agglutinin-conjugated horseradish peroxidase anterograde tracing

Distribution patterns of sensory nerves from the trigeminal ganglion to cerebral arteries in rabbits were studied by the wheat germ agglutinin-conjugated horseradish peroxidase anterograde tracing technique along with the 3,3',5,5'-teramethylbenzidine method. Labeled sensory nerves were densely distributed in whole-mount specimens of cerebral arteries after the injection of wheat germ agglutinin-conjugated horseradish peroxidase into the trigeminal ganglion. The characteristics of the innervation in rabbits included: 1) cerebrovascular sensory nerves were more dense in the ipsilateral side than in the contralateral side; 2) the anterior cerebral artery was less densely innervated than the posterior cerebral artery; 3) labeled nerves on the proximal segment of arteries were more prominent than those on the distal segment. The smallest pial branches of the middle cerebral, posterior cerebral, and anterior inferior cerebellar arteries overlying the fine sensory nerves were 50, 75, and 80 microns in diameter, respectively. Two patterns of the sensory innervation were seen. A meshwork pattern was mainly observed in the circle of Willis and the proximal segments of its main branches, as well as in the upper two thirds of the basilar artery; a parallel or slightly twisted pattern was shown in the small pial arterioles. Our results in this study may be useful to understand better the trigeminocerebrovascular system.

Crossing fiber arrays in the rat hippocampus as demonstrated by three-dimensional reconstruction

The hippocampus is a neural substrate playing a key role in short-term memory. In order to achieve a better understanding of how the hippocampus functions in "learning and memory," we conducted an intracellular horseradish peroxidase (HRP) study of the CA3 pyramidal neurons and the granule cells of the fascia dentata. The axon of the CA3 pyramidal neurons has two components, the longitudinal association system and the Schaffer collateral system. The latter component is organized in a lamellar fashion and follows the alvear fiber stream. An electron microscopic analysis of myelinated fibers suggested that most myelinated fibers in the hippocampus are organized parallel to the alvear fibers. The mossy fibers of the granule cells, however, do not follow the alvear fiber stream. We propose a new model of the organization of the intrinsic excitatory circuitry of the rat hippocampus in which the distinct lamellar organization of the pyramidal and granule cells creates a crossing neural network.

Appearance of retrogradely labeled neurons in the rat superior cervical ganglion after injection of wheat-germ agglutinin-horseradish peroxidase conjugate into the contralateral ganglion

Injection of wheat-germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the superior cervical ganglion (SCG) of the rat results in accumulation of WGA-HRP in sympathetic postganglionic neurons in the contralateral SCG. The sympathetic pathways involved and the mechanism underlying the labeling were investigated. The labeling in neurons in the contralateral SCG was apparent 6 h after injection and increased in intensity with longer survival times. The number of labeled neurons reached 1300 at 72 h after the injection. Transection of the external (ECN) or internal carotid nerves (ICN) resulted in considerable reduction in the number of labeled neurons. Combined transection of both ECN and ICN virtually eliminated labeling in the contralateral SCG. This provides strong evidence that these two nerves are the major pathways for WGA-HRP transport out of the SCG. No labeling was observed in the contralateral SCG following injection of horseradish peroxidase (HRP). Therefore, it seems unlikely that a direct nerve connection exists between the bilateral ganglia. Instead, the labeling of contralateral SCG neurons appears to depend on the transneuronal transport capacity of WGA-HRP, which conveys the marker in an anterograde direction along the postganglionic fibers to terminals in sympathetic target organs, and then delivers it transneuronally to contralateral SCG neurons. We suggest that the sympathetic nerve fibers originating in the bilateral SCGs run intermingled and are in close contact in their peripheral target organs.

Disposition of the slab-like modules formed by axon branches originating from single CA1 pyramidal neurons in the rat hippocampus

The hippocampus is thought to be an area where the neuronal circuits for short-term memory or the cognitive map may reside. In order to advance theoretical studies and neuronal model simulations of such circuits, the projection of the CA1 pyramidal neurons in the rat dorsal hippocampus, especially in the subiculum, was studied by means of intracellular and extracellular HRP injection. The CA1 pyramidal neurons project principally to the subiculum where each forms a slab-like axonal field 2 mm long along the septotemporal axis, which may be regarded as a module for columnar organization, at a specific rostrocaudal level of the subiculum. The modules of the CA1a pyramidal neurons are disposed in the rostral part of the subiculum, those of the CA1c pyramidal neurons in the caudal part, and those of the CA1b pyramidal neurons in the middle part of the subiculum. The CA1 pyramidal neurons also participate in the construction of the lamellar organization in the hippocampus in that their axon branches run rostrocaudally following the stream of the alvear fibers. The CA1 pyramidal neurons in the dorsal rat hippocampus transfer the topographic map from field CA1 to the subiculum with reversed order in the lamellar direction. The topographical relationship is composed of partially shifted, overlapping slab-like modules. As a result, information conveyed through a lamella will diverge into the subiculum approximately 2 mm wide, and information through a group of lamellae 2 mm wide will converge upon single subicular neurons.

Model of the origin of rhythmic population oscillations in the hippocampal slice

One goal of mammalian neurobiology is to understand the generation of neuronal activity in large networks. Conceptual schemes have been based on either the properties of single cells or of individual synapses. For instance, the intrinsic oscillatory properties of individual thalamic neurons are thought to underlie thalamic spindle rhythms. This issue has been pursued with a computer model of the CA3 region of the hippocampus that is based on known cellular and synaptic properties. Over a wide range of parameters, this model generates a rhythmic activity at a frequency faster than the firing of individual cells. During each rhythmic event, a few cells fire while most other cells receive synchronous synaptic inputs. This activity resembles the hippocampal theta rhythm as well as synchronized synaptic events observed in vitro. The amplitude and frequency of this emergent rhythmic activity depend on intrinsic cellular properties and the connectivity and strength of both excitatory and inhibitory synapses.

A versatile means of intracellular labeling: injection of biocytin and its detection with avidin conjugates

Biocytin is a biotin-lysine complex of low molecular weight containing about 65% biotin, which retains a high affinity for avidin. Since the latter molecule has been conjugated to several histochemical markers, the use of biocytin as an intracellular marker was investigated. Electrodes were filled with a solution of 4-6% biocytin dissolved in 0.5 M KCl and 0.05 M Tris buffer, pH 7-7.6. Neurons were recorded intracellularly in the supraoptic nucleus of an explant preparation of the rat supraoptico-neurohypophysial system and injected for 1-20 min with either hyperpolarizing or depolarizing current. Following variable recovery times, the explants were fixed in either 10% formalin or 4% paraformaldehyde overnight, sectioned on a vibratome, and incubated with the avidin-biotin complex (ABC) or avidin which had been conjugated to fluorescein, rhodamine, Texas Red or horseradish peroxidase and containing 1% Triton-X 100. A high percentage of injected neurons were recovered using each of the labels with about equal success. Both negative or positive current injection could be used with little electrode clogging. Labeling with fluorescent conjugates was qualitatively similar to that of Lucifer Yellow, whereas labeling with avidin coupled to horseradish peroxidase or with ABC was qualitatively similar to filling neurons directly with horseradish peroxidase. The advantages of this technique are the ease of injection of biocytin and the versatility in allowing the investigator to choose among light-emitting and light-absorbing images.

Three-dimensional analysis of the whole axonal arbors originating from single CA2 pyramidal neurons in the rat hippocampus with the aid of a computer graphic technique

The axonal arborization of single pyramidal neurons in field CA2 and the rostral adjacent area of the rat hippocampus was studied with intracellular staining following the pressure microinjection of horseradish peroxidase (HRP) in combination with the immunoperoxidase technique, and was analyzed three-dimensionally with the aid of a computer system. The axonal arbors were composed of two types of axon branches, which were distinguished as the primary and secondary axon branches on the basis of morphological criteria. The axon branches in the ipsilateral hippocampus exhibited almost the contour of the dorsal hippocampus. The large amount of axon branches labeled with HRP in the stratum (str.) oriens of field CA1 was comparable to that in the str. radiatum of the field. The labeled axon branches in the dorsal hippocampus were not distributed uniformly in terminal regions but were focused on the caudolateral CA1a-b subfields. Most primary axon branches ran to a focus along the alvear fibers. The lamellar organization in the CA2 pyramidal neurons may be composed of axon branches running caudally and terminal branches forming a focus. The dense association fibers along the septotemporal axis may connect the lamellar organized circuits to each other. Axon branches in the septal nuclei of each hemisphere formed a rather flat plane. The commissural fibers of the CA2 pyramidal neurons seemed to form a symmetrical projection field in the contralateral side against the median plane. The axonal arbors and dendritic expansion of the pyramidal neurons shown in this study appeared to reveal the whole image of the single CA2 pyramidal neuron.

A method for intracellular injection of horseradish peroxidase by pressure

A technique for intracellular injection of horseradish peroxidase and other tracers into neurones is described. The method utilises gas pressure to force the tracer solution out of glass micropipettes and allows electrophysiological recordings to be made simultaneously with the injection process. Constructional details of the simple and inexpensive equipment are given. The method has the advantages of being equally suitable for charged and uncharged tracer molecules, and of providing reliable indication of the success of the injection at the time of the injection attempt.

Inhibitory control of local excitatory circuits in the guinea-pig hippocampus

1. Exposure to the gamma-aminobutyric acid antagonist, picrotoxin, causes the discharge of hippocampal pyramidal cells to become synchronized. Synaptic mechanisms underlying the development of synchrony were investigated by recording from pairs of cells in the CA3 region of guinea-pig hippocampal slices. 2. Picrotoxin suppressed unitary inhibitory synaptic events. It appeared not to affect monosynaptic excitatory connections. Picrotoxin revealed latent excitatory connections in seven out of twenty-one dual recordings from burst-firing cells. 3. Post-synaptic events revealed by picrotoxin were elicited rarely by single action potentials. They were evoked with mean latencies of at least 8 ms and with more than 30% failures of transmission by bursts of three or more action potentials. They were suppressed by increasing extracellular Ca2+. They were considered to be mediated by polysynaptic excitatory pathways. 4. Polysynaptic excitatory post-synaptic potentials (e.p.s.p.s) had a smooth rising phase with time-to-peak of 15-40 ms and a falling phase of similar duration. Their amplitude was 2-3 mV at membrane potentials close to -70 mV. This shape was similar to that of summed e.p.s.p.s evoked by a burst of three to six action potentials at monosynaptic connections between CA3 cells. 5. One cell could evoke excitatory synaptic events in more than one follower cell, suggesting that axon collaterals mediating recurrent excitation were divergent. More than one polysynaptic excitatory pathway could exist between two cells. 6. We examined the role of recurrent excitatory synapses in the development of synchrony. As inhibition was suppressed by picrotoxin, simultaneous excitatory synaptic events appeared in recordings from pairs of cells. They occurred rhythmically at intervals of 0.5-3 s and grew in amplitude with time. Synchronous neuronal discharges were observed when the threshold for action potential generation was exceeded. 7. Firing induced in one cell could sometimes evoke a sequence of post-synaptic events in another cell as inhibition was suppressed. Initially, no connection was detected. On adding picrotoxin, a polysynaptic e.p.s.p. was revealed and with time longer-latency components were recruited to the synaptic event. The amplitude of later components grew until firing threshold was reached. 8. We suggest that synchronous firing develops due to the loss of inhibitory control over the spread of firing between CA3 pyramidal cells via divergent, polysynaptic, recurrent excitatory pathways.

Columnar organization in the subiculum formed by axon branches originating from single CA1 pyramidal neurons in the rat hippocampus

An intracellular horseradish peroxidase study combined with immunoperoxidase techniques was carried out on hippocampal CA1 pyramidal neurons in the rat. Most axon branches originating from a single CA1 pyramidal neuron ran caudally and terminated in the subiculum. The individual axon branches of the single pyramidal neurons bifurcated repeatedly in the subiculum and finally formed a slab-like or columnar terminal arborization (250-300 microns wide, 500-550 microns high and 1.8-2.2 mm long). The present results suggest, in association with other data, that the CA1 pyramidal neurons receive afferent information through lamellar organized connections and they send efferent information to the subiculum through columnar organized connections.

The initiation and spread of epileptiform bursts in the in vitro hippocampal slice

We recorded spontaneous synchronized epileptiform bursts from hippocampal slices from guinea pig using an array of 16 extracellular electrodes placed over the stratum pyramidale of CA2 and CA3. The slices were made epileptogenic with the GABA antagonist picrotoxin (or occasionally penicillin). We found that spontaneous bursts always originate at a discrete focus at or near CA2. These bursts spread smoothly and uniformly across CA3 at an average velocity of 0.13 m/s. This velocity is slower than the conduction velocity of the Schaffer collaterals or mossy fibers. Picrotoxin produced afterdischarges following the initial primary burst, and these afterdischarges were found to originate and spread in a fashion nearly identical to the primary burst. These results indicate that CA2 is a unique region which must possess unusual cellular and/or synaptic connectivity properties which result in a decreased threshold for initiation of epileptiform activity. We consider several hypothetical patterns of local synaptic connectivity in the light of these results, and we discuss the possible role of residual inhibition in limiting the spread of synchronized discharges.

Models of the cellular mechanism underlying propagation of epileptiform activity in the CA2-CA3 region of the hippocampal slice

We have shown experimentally in the previous paper that spontaneous epileptiform activity, as recorded by extracellular field potentials, propagates smoothly across the CA2-CA3 region of the convulsant-treated hippocampal slice of the guinea pig at velocities of about 0.1 m/s. In the present paper, we used computer simulations of either 500 or 1000 cell arrays of model neurons to examine possible mechanisms underlying this propagation. We show that propagation of epileptiform field potentials can be explained plausibly by slow conduction along axons interconnecting CA2-CA3 neurons, provided that there are sufficiently many interconnections. This propagation can take place even if the interconnections occur randomly. The number of interconnections required decreases as the number of synchronously activated cells initiating a population burst increases. Axonal propagation at 0.1 m/s appears to be a plausible assumption, since conduction velocities along Schaffer collaterals have been experimentally estimated to be as slow as 0.2 m/s, and small recurrent collaterals are likely to conduct more slowly than the main axonal branches. If spontaneous synchronized population bursts are initiated by activity in four or fewer cells, then our model requires, for smooth field potential propagation, more interconnections than are believed to occur on the basis of dual intracellular recording.

The fibers which leave the Probst's longitudinal bundle seen in the brain of an acallosal mouse: a study with the horseradish peroxidase technique

The congenital absence of the corpus callosum, a brain anomaly frequently noted in humans, has been recently found to occur in some mice of the ddN strain in our laboratory. In the brains of these mice, the Probst's longitudinal bundle is always present on both cerebral hemispheres and gives rise to some aberrant fibers toward the midline. In this research, the neuroanatomical features of these fibers were studied by iontophoretical injections of horseradish peroxidase (HRP) into the neocortex of acallosal mouse brains. The results revealed that the fibers which leave the Probst's longitudinal bundle are, at least, of 3 kinds: namely, the fibers that run out from the anterior portion of the bundle and take a U-turn ipsilaterally without crossing the midline through the septal tissue to go back again into the longitudinal bundle at the level where they have left it; the commissural fibers that leave the bundle from its middle portion and cross through a tiny bridge of tissue associated with the ventral hippocampal commissure to the opposite hemisphere; and the fibers that arise from the posterior portion of the bundle and accumulate as an anomalous fascicle below the cingulum. The observation that no labeled fibers were seen within the anterior commissure in the present HRP materials suggests that the axons from neocortex which are prevented from crossing the midline in mice with congenital absence of the corpus callosum cannot find an alternative pathway via the anterior commissure.

Various types of non-pyramidal hippocampal neurons project to the septum and contralateral hippocampus

The morphology was studied of hippocampal neurons which had their somata in the hilus of the area dentata, and in stratum radiatum or stratum oriens of Ammon's horn, and which sent projections to the septum and contralateral hippocampus, respectively. The fluorescent marker Fast Blue was injected into the septum or contralateral hippocampus. Somata were then identified by their fluorescent label in slices of perfused brains. After intracellular injection of these somata with Lucifer Yellow, it was found that contralaterally projecting neurons were pyramidal cells, inverted fusiform and multipolar cells in CA3c, and stellate, fusiform and multipolar cells in the hilus. After septal injections, we identified two groups of aspiny stellate cells in the hilus; pyramidal basket cells, polygonal basket cells, horizontal basket cells in stratum oriens; and stellate cells in stratum radiatum of CA1 and CA3, as well as pyramid-like aspiny cells in stratum radiatum of CA1. These cells also had short locally arborizing axons, thus probably contributing to local circuits. Such cells may constitute a third class of hippocampal neurons combining the properties of principal cells and interneurons. These results support the opinion that the simple concept of separating hippocampal cells into projection neurons and local-circuit neurons needs reconsideration.

A simple device for filtering small volumes of micropipette filling solutions

An inexpensive device for filtering small volumes of liquids is described. It is versatile, simple to use and results in very little wastage. The device is particularly suitable for filtering micropipette filling solutions for intra- and extracellular injection.

Heterogeneity of neurons in the crustacean X-organ as revealed by intracellular recording and injection of horseradish peroxidase

The intracellular injection technique of HRP with simultaneous recordings of intracellular potential was applied to the crab X-organ-sinus gland peptidergic neurosecretory neurons. At least two classes of neurons were discriminated from the usual type of neurosecretory neurons morphologically as well as electrophysiologically. Possible roles of those neurons were suggested as the modulation and coordination of activities of the neurosecretory system.