Electrophysiological analysis of the dorsal hippocampal commissure projections to the entorhinal area

Synaptic effects evoked in the entorhinal area by dorsal hippocampal commissure (dorsal psalterium) projections were analysed in anesthetized adult guinea-pigs by means of a field potential analysis. Stimuli applied to the caudal part of the dorsal psalterium evoked a complex response in the dorsal third of the entorhinal area. The early part of the entorhinal response consisted of a slow wave interrupted by a spike potential. The electrophysiological characteristics and the laminar distribution of the slow wave and of the spike potential, together with the presence of time-locked unit activity, suggested that dorsal psalterium projections evoke monosynaptic excitatory postsynaptic potentials leading to cellular discharge in radially oriented neurons of layers II and III. The commissural fibers responsible for these effects originate in the contralateral presubiculum. The early part of the entorhinal response was followed by three waves in close temporal sequence. These waves were polysynaptically generated and associated with excitatory and inhibitory synaptic effects. Inhibition was demonstrated for the monosynaptically generated spike potential. Whether these effects were mediated by intracortical circuits and/or extrinsic projections cannot be stated from the present results. Causal relations were observed between the entorhinal monosynaptic response and that evoked by dorsal psalterium stimulation in the ipsilateral dentate gyrus, previously shown to be relayed by perforant path fibers. The results indicate that presubicular commissural projections to the entorhinal area monosynaptically activate neurons of the perforant pathway, whose discharge brings about activation of the ipsilateral dentate gyrus.     

Electrophysiological analysis of the hippocampal output to the presubiculum

The hippocampal output to the dorsal presubiculum was analysed in the guinea-pig by field potential analysis. Perforant path volleys, synaptically elicited by stimulation of the dorsal psalterium of one side, were used to activate the lamellar circuit of the same side and, through interhippocampal impulses, the hippocampal pyramidal neurons of the opposite side. The discharge of the pyramidal neurons of the hippocampus was followed by a negative fast wave with associated unit firing in lamina principalis interna and in the innermost part of lamina principalis externa of the presubicular cortex. These findings suggested the generation of excitatory synaptic effects in lamina principalis interna and innermost lamina principalis externa. The presubicular response was present in preparations with the fornix bilaterally sectioned. It was eliminated by interruption of the caudally-directed hippocampal projections, which suggested that it was mediated by these hippocampal efferents. The generation site of the presubicular response in deep cell laminae was not consistent with the terminal field of the hippocampus-presubiculum projections but was with that of the subiculum-presubiculum projections. The hippocampal discharge evoked in the dorsal subiculum a response with shorter latency and lower threshold than the presubicular response. These data suggest that the presubicular response might be evoked through hippocampus-subiculum-presubiculum connections. The results show that the hippocampal output evokes excitatory synaptic effects in the presubiculum and that these effects are segregated in deep cell laminae. The fact that these laminae give origin to an important projection to the anterior thalamic nuclei suggests that hippocampal impulses may be transferred to subcortical structures through the presubiculum.     

Contacts between medial and lateral perforant pathway fibers and parvalbumin expressing neurons in the subiculum of the rat

The entorhinal cortex (EC) projects via the perforant pathway to all subfields in the hippocampal formation. One can distinguish medial and lateral components in the pathway, originating in corresponding medial and lateral subdivisions of EC. We analyzed the innervation by medial and lateral perforant pathway fibers of parvalbumin-expressing neurons in the subiculum. A neuroanatomical tracer (biotinylated dextran amine, BDA) was stereotaxically injected in the medial or lateral entorhinal cortex, thus selectively labeling either perforant pathway component. Transport was allowed for 1 week. Transported BDA was detected with streptavidin-Alexa Fluor 488. Parvalbumin neurons were visualized via immunofluorescence histochemistry, using the fluorochrome Alexa Fluor 594. Via a random systematic sampling scheme using a two-channel, sequential-mode confocal laser scanning procedure, we obtained image series at high magnification from the molecular layer of the subiculum. Labeled entorhinal fibers and parvalbumin-expressing structures were three dimensionally (3D) reconstructed using computer software. Further computer analysis revealed that approximately 16% of the 3D objects ('boutons') of BDA-labeled fibers was engaged in contacts with parvalbumin-immunostained dendrites in the subiculum. Both medial and lateral perforant pathway fibers and their boutons formed such appositions. Contacts are suggestive for synapses. We found no significant differences between the medial and lateral components in the relative numbers of contacts. Thus, the medial and lateral subdivisions of the entorhinal cortex similarly tune the firing of principal neurons in the subiculum by way of parvalbumin positive interneurons in their respective terminal zones.     

The localization of leucine-enkephalin immunoreactivity within the guinea pig hippocampus

Summary The distribution of enkephalin immunoreactive fibres has been studied in the hippocampus, subiculum and entorhinal cortex of the guinea pig. Two immunoreactive enkephalin fibre systems were found. One system corresponds to the mossy fibre system from the fascia dentata to CA3 and courses at the level of the mossy fibre end bulb in a longitudinal direction along the main axis of the hippocampus. Another system originates in the medial and lateral entorhinal cortex, traverses the subiculum, and then courses in the stratum molecu-lare/lacunosum to CA1 and CA3; part of these fibres crosses the hippocampal fissure and reaches the stratum moleculare of the fascia dentata. In the fascia dentata intense immunoreactivity was observed in the distal and middle one-third of the stratum moleculare at the side of the terminations of the lateral and medial perforant path fibres. Various types of immunoreactive cell bodies were found in the fascia dentata, CA3, CA1, subiculum and in the entorhinal cortex.Supported in part by the Dutch Organization of Pure Scientific Research, FUNGO/ZWO     

大鼠隔-下托投射定位关系的确定

目的：研究隔—下托投射定位关系。材料与方法：1％WGA—HRP与l0％HRP混合液用微量注射器注射人大民背侧下托，存活36h，取材。海马切片用H2O2—DAB明显色确定注射点位置，隔区切片用TMB显色显示逆行标记神经元。结果：HRP逆行标记细胞主要出现于同侧内侧隔核(MS)和斜带核(NDBV)，Bregma点0．70mm-0．48mm的平面上达到高峰。对侧MS和NDBV仅见少量逆行标记神经元。结论：下托接受同侧MS-NDBV大量神经元投射，其中背海马下托前部与MS—NDBV的中部存在明显定位关系。     

Entorhinal efferents reach the caudato-putamen

An entorhinal efferent system additional to the well known perforant path to the hippocampal formation is described. The projection originates in the deep layers V and VI of the entorhinal cortex and reaches the striatum. Based on these observations the entorhinal cortex is suggested as a major output structure for the hippocampal region.     

A projection from the deep layers of the entorhinal area to the hippocampal formation in the rat brain

The methods of retrograde fluorescent tracing and anterograde transport of the lectin Phaseolus vulgaris leucoagglutinin (PHA-L) were used to demonstrate the existence of projections from layers IV and VI of the entorhinal area to the hippocampal formation in the rat brain. These two layers of the medial and lateral entorhinal area innervate the molecular layer of Ammon's horn and the area dentata. In the area dentata the projection from layer IV follows that of the perforant path, while that from layer VI innervates the outer two-thirds of the molecular layer, the subgranular zone and the deep part of the hilus of the area dentata.     

Cortical afferents of the nucleus accumbens in the cat,studied with anterograde and retrograde transport techniques

The cortical afferentation of the nucleus accumbens in the cat was studied with the aid of retrograde tracing techniques. Retrograde experiments were carried out with horseradish peroxidase or one of the fluorescent tracers Bisbenzimid, Nuclear Yellow and Fast Blue. In the anterograde experiments [³H]leucine and [³⁵S]methionine were used as tracers.Following injections in the nucleus accumbens, retrogradely-labelled cells were found in the medial frontal cortex, the anterior olfactory nucleus, the posterior part of the insular cortex, the endopiriform nucleus, the amygdalo-hippocampal area, the entorhinal and perirhinal cortices and the subiculum of the hippocampal formation. In the medial frontal cortex most of the labelled cells were found in layers III and V of the prelimbic area (area 32 of Brodmann), but retrogradely-filled neurons were also present in the infralimbic area and in the caudoventral part of the lateral bank of the proreal gyrus. Retrogradely-labelled cells in the entorhinal and perirhinal cortices were located in the deep cellular layers. Following large injections in the nucleus accumbens, retrograde labelling in the subiculum extended from the most dorsal, septal pole to the most ventral, temporal pole.Injections of anterograde tracers were placed in the frontal cortex, the entorhinal and perirhinal cortices and the hippocampal formation. The prelimbic area was found to project via the internal capsule to mainly the rostral half of the nucleus accumbens, whereas in the caudal half of the nucleus only a lateral region receives frontal cortical fibres. Following injections in the infralimbic area only fibres passing through the nucleus accumbens were labelled. Afferents from the entorhinal and perirhinal cortices reach the nucleus accumbens by way of the external capsule and terminate mainly in a ventral zone of the nucleus accumbens.Afferents from the entorhinal area are distributed to the entire accumbens, whereas the termination field of the perirhinal afferents is largely restricted to the lateral part of the nucleus accumbens. Both the frontal cortex and the entorhinal and perirhinal cortices appear to project also to the nucleus caudatus and the tuberculum olfactorium. These cortical areas also project to the contralateral striatum.Both anterograde and retrograde tracing experiments demonstrated a topographical relationship between the subiculum and the nucleus accumbens. The ventral pole of the subiculum projects via the fornix to the medial part of the caudal half of the nucleus accumbens and to a small dorsomedial area in its rostral half. Successively more dorsal portions in the subiculum project to successively more ventrolateral parts in the rostral nucleus accumbens. The projection from the hippocampus was found to extend also to the tuberculum olfactorium. The results of the present study do not provide unambiguous criteria for the delimitation of the nucleus accumbens in the cat.     

Projections from the presubiculum and the parasubiculum to morphologically characterized entorhinal-hippocampal projection neurons in the rat

The relations between the inputs from the presubiculum and the parasubiculum and the cells in the entorhinal cortex that give rise to the perforant pathway have been studied in the rat at the light microscopical level. Projections from the presubiculum and the parasubiculum were labeled anterogradely, and, in the same animal, cells in the entorhinal cortex that project to the hippocampal formation were labeled by retrograde tracing and subsequent intracellular filling with Lucifer Yellow. The distribution and the number of appositions between the afferent fibers and hippocampal projection neurons in the various layers of the entorhinal cortex were analyzed. The results show that layers I–IV of the entorhinal cortex contain neurons that give rise to projections to the hippocampal formation. The morphology of these projection neurons is highly variable and afferents from the presubiculum and the parasubiculum do not show a preference for any specific morphological cell type. Both inputs preferentially innervate the dendrites of their target cells. However, presubicular and parasubicular projections differ with respect to the layer of entorhinal cortex they project to. The number of appositions of presubicular afferents with cells that have their cell bodies in layer III of the entorhinal cortex is 2–3 times higher than with cells in layer II. In contrast, afferents from the parasubiculum form at least 2–3 times as many synapses on the dendrites of cells located in layer II than on neurons that have their cell bodies in layer III. Cells in layers I and IV of the entorhinal cortex receive weak inputs from the presubiculum and parasubiculum. Not only is the presubiculum different from the parasubiculum with respect to the distribution of projections to the entorhinal cortex, they also differ in their afferent and efferent connections. In turn, cells in layer II of the entorhinal cortex differ in their electrophysiological characteristics from those in layer III. Moreover, layer II neurons give rise to the projections to the dentate gyrus and field CA3/CA2 of the hippocampus proper, and cells in layer III project to field CA1 and the subiculum. Therefore, we propose that the interactions of the entorhinal-hippocampal network with the presubiculum are different from those with the parasubiculum.     

Dissociations of the medial and lateral perforant path projections into dorsal DG, CA3, and CA1 for spatial and nonspatial (visual object) information processing

Medial perforant path plasticity can be attenuated by 2-amino-5-phosphonovaleric acid (APV) infusions, whereas lateral perforant path plasticity can be attenuated by naloxone infusions. The present experiment was designed to evaluate the role of each entorhinal efferent pathway into the dorsal hippocampus for detection of spatial and nonspatial (visual object) changes in the overall configuration of environmental stimuli. Dorsal dentate gyrus infusions of either APV or naloxone attenuated detection of a spatial change, whereas only naloxone infusions disrupted novel object detection. Either APV or naloxone infusions into dorsal CA3 disrupted both spatial and novel object detection. APV infusions into dorsal CA1 attenuated detection of a spatial change, whereas naloxone infusions into dorsal CA1 disrupted novel object detection. These data suggest that each dorsal hippocampal subregion processes spatial and nonspatial (visual object) information from perforant path efferents in a unique manner that is consistent with the intrinsic properties of each subregion.     

Parvalbumin-immunoreactive neurons in the entorhinal cortex of the rat: localization, morphology, connectivity and ultrastructure

Summary We studied the distribution, morphology, ultrastructure and connectivity of parvalbumin-immunoreactive neurons in the entorhinal cortex of the rat. Immunoreactive cell bodies were found in all layers of the entorhinal cortex except layer I. The highest numbers were observed in layers II and III of the dorsal division of the lateral entorhinal area whereas the lowest numbers occurred in the ventral division of the lateral entorhinal area, Most such neurons displayed multipolar configurations with smooth dendrites. We distinguished a type with long dendrites and a type with short dendrites. We also observed pyramidal immunoreactive neurons. A dense plexus of immunoreactive dendrites and axons was prominent in layers II and III of the dorsal division of the lateral entorhinal area and the medial entorhinal area. None of the parvalbuminimmunoreactive cells became retrogradely labelled after injection of horseradish peroxidase into the hippocampal formation. By electron microscopy, immunoreactivity was observed in cell bodies, dendrites, myelinated and unmyelinated axons and axon terminals. Immunoreactive dendrites and axons occurred in all cortical layers. We noted many myelinated immunoreactive axons. Immunoreactive axon terminals were medium sized, contained pleomorphic synaptic vesicles, and established symmetrical synapses. Both horseradish peroxidase labelled and unlabelled immunonegative cell bodies often received synapses from immunopositive axon terminals arranged in baskets. Synapses between immunoreactive axon terminals and unlabelled dendritic shafts and spines were abundant. Synapses with initial axon segments occurred less frequently. In addition, synaptic contacts were present between immunopositive axon terminals and cell bodies and dendrites. Thus, the several types of parvalbumin-containing neuron in the entorhinal cortex are interneurons, connected to one another and to immunonegative neurons through a network of synaptic contacts. Immunonegative cells projecting to the hippocampal formation receive axo-somatic basket synapses from immunopositive terminals. This connectivity may form the morphological substrate underlying the reported strong inhibition of cells in layers II and III of the entorhinal cortex projecting to the hippocampal formation.     

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

Cells of origin of the entorhinohippocampal pathway were retrogradely labeled by injection of Fast Blue into the ipsilateral hippocampus. The cells, which were located in layers I, II and III of the lateral entorhinal cortex, were then intracellularly injected with Lucifer Yellow to reveal their complete morphology. We could thus establish that among the hippocampally projecting entorhinal cells there are pyramidal and pyramid-like cells, spiny stellate cells of various shapes, sparsely spinous horizontal and multipolar cells. The involvement of horizontal and multipolar neurons in projections has not previously been recognized although all of these cell types have already been described in Golgi studies.

The results indicate that the organization of the perforant path is more complex than has been assumed. Finally, they are at variance with the classical concept which subdivides cortical neurons into projection neurons (pyramidal and spiny stellate) and interneurons (non-pyramidal, local circuit neurons).     

Excitation Waves Returning to the Hippocampus via the Entorhinal Cortex Can Reactivate Populations of “Trained” Field CA1 Neurons during Deep Sleep

It is suggested that information on new stimuli from the neocortex is transmitted to the hippocampus, where temporal traces persist in the form of mosaics of modified synapses. During sleep, populations of neurons storing these traces are reactivated and return the information required for consolidation of a permanent memory trace to the neocortex. A possible mechanism for the reactivation of “trained” hippocampal neurons during memory consolidation consists of the reverberation of excitation in the neuron circuits linking the hippocampus and entorhinal cortex. Our studies in rats included recording of responses in hippocampal field CA1 to stimulation of Schaffer collaterals with potentiated synapses during waking and sleep. During deep sleep, discharges of field CA1 neurons were followed by waves of excitation which passed through the entorhinal cortex and reached the hippocampus and dentate gyrus via fibers of the perforant path, evoking neuron discharges in the latter. Repeated neuron discharges in field CA1 occurred on interaction of the early excitation wave returning directly via perforant path fibers and the late wave returning via Schaffer collaterals, not via the trisynaptic path via the dentate gyrus and hippocampal field CA3 but probably via field CA2.     

Zinc-positive afferents to the rat septum originate from distinct subpopulations of zinc-containing neurons in the hippocampal areas and layers

The purpose of the present study was to examine whether zinc-positive and zinc-negative hippocampal neurons in rats differed with respect to their projections to the septum. By combining retrograde axonal transport of the fluorescent tracer Fluoro-Gold with histochemical demonstration of zinc selenide complexes in zinc-containing neurons after intraperitoneal injection of sodium selenite, we were able to visualize the distribution of retrogradely Fluoro-Gold labeled neurons and zinc-containing neurons in the same sections. After unilateral injection of Fluoro-Gold into the rat septum a few retrogradely labeled cells were observed in layer IV of the ipsilateral medial entorhinal area, and numerous labeled cells were observed mainly in the superficial layers of the ipsilateral subicular areas and throughout the CA1 and CA3 pyramidal cell layers, as well as in the contralateral CA3 pyramidal cell layer. Zinc-containing neurons were observed in layers IV–VI of the medial entorhinal area, layers II and III of the parasubiculum, layers II, III and V of presubiculum, and in the superficial CA1 and deep CA3 pyramidal cell layers. Cells double-labeled with Fluoro-Gold and zinc selenide complexes were primarily located in distal (relative to the area dentata) parts of the superficial CA1 pyramidal cell layer and distal parts of the deep CA3 pyramidal cell layer and in layers II and III of presubiculum. Only a very few double-labeled cells were seen in the contralateral CA3. The result demonstrates that the hippocampo-septal projection of rats is a mixture of zinc-positive and zinc-negative fibers. Where-as zinc-negative fibers originate from neurons throughout the hippocampal and retrohippocampal areas, zinc-positive fibers originate from distinct subgroups of zinc-containing cells in different areas and layers.     

Effect of early isolation on signal transfer in the entorhinal cortex-dentate-hippocampal system

Deprivation of socio-sensory interactions during early life impairs brain function in adulthood. In previous investigations we showed that early isolation severely affects neuron development in several structures of the hippocampal region, including the entorhinal cortex. In the present study we investigated the effects of early isolation on signal processing along the entorhinal cortex-dentate-CA3-CA1 system, a major memory circuit of the hippocampal region. Male and female guinea-pigs were assigned at 6-7 days of age to either a social or an isolated environment. At 90-100 days of age the animals were anesthetized and field potentials were recorded from the entorhinal cortex-dentate-CA3-CA1 circuit, driven by dorsal psalterium commissural volleys. Analysis of the input-output function in the different structures showed that in isolated males there was a small reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in layer II of the entorhinal cortex. No changes occurred in isolated females. In isolated males and females there was a reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in the dentate gyrus, CA3 and CA1, but this effect was larger in males. In isolated males, but not in females, the population spike/population excitatory postsynaptic potential ratio was reduced in all investigated structures, indicating that in males the size of the discharged neuron population was reduced more than due to the decreased input. Results show that isolation reduces the synaptic function in the whole entorhinal cortex-dentate gyrus-CA3-CA1 system. While the entorhinal cortex was moderately impaired, the dentate-hippocampal system was more severely affected. The impairment in the signal transfer along the entorhinal cortex-dentate gyrus-CA3-CA1 system was heavier in males, confirming the larger susceptibility of this sex to early experience. This work provides evidence that malfunctioning of a major hippocampal network may underlie the learning deficits induced by impoverished surroundings during early life.     

Evolution of cerebral cortex involvement in the acquisition of associative learning

The presence of the c-Fos protein has been evidenced in the piriform cortex, subiculum, entorhinal and perirhinal cortices, and parietal and occipital cortices at different stages (Sessions 2, 4, and 6) in the acquisition of a trace conditioning in behaving rabbits. c-Fos immunostaining was also measured after a reminder (7th) session. c-Fos immunoreactivity increased significantly across conditioning on the contralateral side of the piriform, entorhinal, perirhinal, and parietal cortices as compared with the ipsilateral side of conditioned animals and the contralateral side of pseudo-conditioned ones. No difference in c-Fos immunostaining was observed between contra- and ipsilateral sides in the subiculum of conditioned animals. c-Fos production decreased significantly across conditioning but presented a noticeable bilateral increase after the reminder session in the piriform, entorhinal, perirhinal, and parietal cortices, but not in the subiculum. Peak production of c-Fos was observed after the 2nd and 7th (reminder) conditioning sessions for the piriform, entorhinal, perirhinal, and parietal cortices, and after the 4th session for the subiculum. It is proposed that different cortical areas process associative learning with different strengths and side dominances.     

Entorhinal projections terminate onto principal neurons and interneurons in the subiculum: a quantitative electron microscopical analysis in the rat

The synaptic organization of projections to the subiculum from superficial layers of the lateral and medial entorhinal cortex was analyzed in the rat, using anterograde neuroanatomical tracing followed by electron microscopical quantification. Our aim was to assess the synaptic organization and whether the two projection components (lateral, medial) within the perforant pathway are qualitatively and quantitatively similar with respect to the types of synapses formed and with respect to the postsynaptic targets of these entorhinal projections. The tracer biotinylated dextran amine (BDA) was injected into the lateral and medial entorhinal cortex, respectively, and resulting anterograde labeling in the subiculum was studied. For each of the two projection components, we analyzed in four animals (2 x 2) a total of 100 synapses/animal with respect to features of the synapse type, i.e. asymmetrical or symmetrical, as well as regarding their postsynaptic target, i.e. dendritic shaft or spine. No clear differences were observed between the two pathways. The majority of the synapses were of the asymmetrical type, making contact with spines (78%) or with dendritic shafts (14%). A low percentage of symmetrical synapses targeted dendritic shafts (4.2%) or spines (1.3%). About 2.5% of the synapses remained undetermined. The findings indicate that the majority of entorhinal fibers reaching the subiculum exert an excitatory influence primarily onto principal neurons, with a much smaller feed forward inhibitory component. Only a small percentage of entorhinal fibers in the subiculum appears to be inhibitory, largely influencing interneurons.     

Neurons and terminals in the retrohippocampal region in the rat''s brain identified by anti-gamma-aminobutyric acid and anti-glutamic acid decarboxylase immunocytochemistry

Summary The distribution of gamma-aminobutyric acid (GABA) containing nerve cells and terminals was studied at the light and electron microscopic levels in the retrohippocampal region of the rat by using anti-glutamic acid decarboxylase (GAD) and anti-GABA antibodies in immunocytochemistry. Large numbers of GAD and GABA stained cells were found in all retrohippocampal structures. At the ultrastructural level, the immunoreactivity against GABA and against the synthesizing enzyme GAD was localized to cytoplasmic structures, including loose clumps of rough endoplasmic reticulum, ribosomal arrays, outer mitochondrial surfaces and in axonal boutons.The GAD- and GABA-immunorective(-i) cells were found in all subfields of the retrohippocampal region (e.g., the subicular complex, the entorhinal area). Within the entorhinal area a slightly larger number of immunoreactive cells could be detected in layers II and III than in the other layers. In the subiculum, pre- and parasubiculum the GAD and GABA-i cells were present in relatively large numbers in all layers, except the molecular layer, which contained only a small number of GABA cells. Within the entorhinal area, GAD and GABA stained cells ranged in size from small (13 m in diameter) to large (22 m in diameter). A large number of different morphological classes of cells were found, except pyramidal and stellate cells. In the pre- and parasubiculum, on the other hand, the GABA cells were generally small to medium in size and morphologically more homogeneous than in the subiculum and entorhinal area.The entire retrohippocampal region was densely innervated by GABA preterminal processes, with little variation in the regional density of innervation. Within the entorhinal area, presubiculum and subiculum, a clear difference was found in the laminar pattern of innervation. In all three subfields the densest innervation was in layer II. In the entorhinal area both GAD- and GABA-i axons form palisades of fibers around the somata of neurons, which are tightly packed together in this layer. In the electron microscope both GAD-i and GABA-i were demonstrated in these axons. Axosomatic synaptic contacts were common between axons and the stellate neurons and other cells of this layer. Layers IV and VI appeared less dense in GAD-i terminals but appeared more densely innervated than layers III and V. The lamina dessicans was relatively poor in GAD-i. In the subiculum and presubiculum, as well as all other subfields of the hippocampal region, the innervation is dominated by axo-somatic innervation of layer II cells. The outer third of the molecular layer was more densely innervated than the inner part. Taken together, the present study has shown that the retrohippocampal region is rich in GABAergic neurons as well as axon terminals, some of which form numerous synapic contacts with cells of the region. GABAergic neurotransmission is an important mechanism in retrohippocampal circuits not only for the resident interneuronal population but in the surround as well.     

Commissural and perforant path interactions in the rat hippocampus

Summary The interaction of the commissural and perforant path systems was studied by recording extracellular field potentials and single unit activity in the dentate gyrus in urethane-anesthetized rats. Conditioning commissural volleys suppressed extracellular synaptic potentials, population spikes and single unit discharges evoked by perforant path stimulation. Commissural stimulation (single or repetitive) failed to induce a population spike, however strong the stimulation. About half of the cells fired monosynaptically to perforant path volleys and 20% to commissural volleys. Half of the commissurally driven units fired before or coincided with field potential onset. The antidromic mechanism of these short latency unitary spikes was shown by the collision test. Commisural activation reduced spontaneous cell firing without previous excitation in 25% of the neurons. Less than 6% of the cells responded to stimulation of both inputs, indicating little convergence between the two pathways. We contend that a simple form of recurrent inhibition fails to explain the above findings, and the possibility of feed-forward inhibition by commissural activation has been raised.