Properties and dynamics of inhibitory synaptic communication within the CA3 microcircuits of pyramidal cells and interneurons expressing parvalbumin or cholecystokinin.

KEY POINTS: To understand how a network operates, its elements must be identified and characterized, and the interactions of the elements need to be studied in detail. In the present study, we describe quantitatively the connectivity of two classes of inhibitory neurons in the hippocampal CA3 area (parvalbumin-positive and cholecystokinin-positive interneurons), a key region for the generation of behaviourally relevant synchronous activity patterns. We describe how interactions among these inhibitory cells and their local excitatory target neurons evolve over the course of physiological and pathological activity patterns. The results of the present study enable the construction of precise neuronal network models that may help us understand how network dynamics is generated and how it can underlie information processing and pathological conditions in the brain. We show how inhibitory dynamics between parvalbumin-positive basket cells and pyramidal cells could contribute to sharp wave-ripple generation. ABSTRACT: Different hippocampal activity patterns are determined primarily by the interaction of excitatory cells and different types of interneurons. To understand the mechanisms underlying the generation of different network dynamics, the properties of synaptic transmission need to be uncovered. Perisomatic inhibition is critical for the generation of sharp wave-ripples, gamma oscillations and pathological epileptic activities. Therefore, we aimed to quantitatively and systematically characterize the temporal properties of the synaptic transmission between perisomatic inhibitory neurons and pyramidal cells in the CA3 area of mouse hippocampal slices, using action potential patterns recorded during physiological and pathological network states. Parvalbumin-positive (PV+) and cholecystokinin-positive (CCK+) interneurons showed distinct intrinsic physiological features. Interneurons of the same type formed reciprocally connected subnetworks, whereas the connectivity between interneuron classes was sparse. The characteristics of unitary interactions depended on the identity of both synaptic partners, whereas the short-term plasticity of synaptic transmission depended mainly on the presynaptic cell type. PV+ interneurons showed frequency-dependent depression, whereas more complex dynamics characterized the output of CCK+ interneurons. We quantitatively captured the dynamics of transmission at these different types of connection with simple mathematical models, and describe in detail the response to physiological and pathological discharge patterns. Our data suggest that the temporal propeties of PV+ interneuron transmission may contribute to sharp wave-ripple generation. These findings support the view that intrinsic and synaptic features of PV+ cells make them ideally suited for the generation of physiological network oscillations, whereas CCK+ cells implement a more subtle, graded control in the hippocampus.

How many subtypes of inhibitory cells in the hippocampus?

Hippocampal inhibitory cells are diverse. It is supposed that they fall into functionally distinct subsets defined by a similar morphology and physiology. Switching between functions could be accomplished by activating receptors for modulating transmitters expressed selectively by different subsets of interneurons. We tested this hypothesis by comparing morphology, physiology, and neurotransmitter receptor expression for CA1 hippocampal interneurons. We distinguished 16 distinct morphological phenotypes and 3 different modes of discharge. Subsets of inhibitory cells were excited or inhibited by agonists at receptors for noradrenaline, muscarine, serotonin, and mGluRs. Most cells responded to 2 or 3 agonists, and 25 different response combinations were detected. Subsets defined by morphology, physiology, and receptor expression did not coincide, suggesting that hippocampal interneurons cannot easily be segregated into a few well-defined groups.

Differences between somatic and dendritic inhibition in the hippocampus.

Hippocampal synaptic inhibition is mediated by distinct groups of inhibitory cells. Some contact pyramidal cells perisomatically, while others terminate exclusively on their dendrites. We examined perisomatic and dendritic inhibition by recording from CA3 inhibitory and pyramidal cells and injecting biocytin to visualize both cells in light and electron microscopy. Single perisomatic inhibitory cells made 2-6 terminals clustered around the soma and proximal pyramidal cell processes. Dendritic cells established 5-17 terminals, usually on different dendrites of a pyramidal cells. Perisomatic terminals were larger than those facing dendritic membrane. Perisomatic inhibitory cells initiated the majority of simultaneous IPSPs seen in nearby pyramidal cells. Single IPSPs initiated by perisomatic sodium-dependent action potentials. Activation of inhibitory fibers terminating on dendrites could suppress calcium-dependent spikes. Thus, distinct inhibitory cells may differentially control dendritic electrogenesis and axonal output of hippocampal pyramidal cells.

Quantitative ultrastructural differences between local and medial septal GABAergic axon terminals in the rat hippocampus.

Functionally distinct subsets of hippocampal inhibitory neurons exhibit large differences in the frequency, pattern and short-term plasticity of GABA release from their terminals. Heterogeneity is also evident in the ultrastructural features of GABAergic axon terminals examined in the electron microscope, but it is not known if or how this corresponds to interneuron subtypes. We investigated the feasibility of separating morphologically distinct clusters of terminal types, using the approach of measuring several ultrastructural parameters of GABAergic terminals in the CA1 area of the rat hippocampus. Septo-hippocampal axon terminals were anterogradely labeled by biotinylated dextran amine and visualized by pre-embedding immunogold staining to delineate one homogeneous terminal population. Long series (100-150) of ultrathin sections were cut from stratum oriens and stratum radiatum of the CA1 area, and GABAergic terminals were identified by post-embedding immunogold staining. Stereologically unbiased samples of the total GABAergic axon terminal population and a random sample of the septal axon terminals were reconstructed in 3D, and several of their parameters were measured (e.g. bouton volume, synapse surface, volume occupied by vesicles, mitochondria volume). Septal terminals demonstrated significantly larger mean values for most parameters than the total population of local GABAergic terminals. There was no significant difference between terminals reconstructed in the basal and apical dendritic regions of pyramidal cells, neither for the septal nor for the local population. Importantly, almost all parameters were highly correlated, precluding the possibility of clustering the local terminals into non-overlapping subsets. Factor and cluster analysis confirmed these findings. Our results suggest that similarly to excitatory terminals, inhibitory terminals follow an "ultrastructural size principle," and that the terminals of different interneuron subtypes cannot be distinguished by ultrastructure alone.

Total number and ratio of excitatory and inhibitory synapses converging onto single interneurons of different types in the CA1 area of the rat hippocampus.

The least known aspect of the functional architecture of hippocampal microcircuits is the quantitative distribution of synaptic inputs of identified cell classes. The complete dendritic trees of functionally distinct interneuron types containing parvalbumin (PV), calbindin D(28k) (CB), or calretinin (CR) were reconstructed at the light microscopic level to describe their geometry, total length, and laminar distribution. Serial electron microscopic reconstruction and postembedding GABA immunostaining was then used to determine the density of GABA-negative asymmetrical (excitatory) and GABA-positive symmetrical (inhibitory) synaptic inputs on their dendrites, somata, and axon initial segments. The total convergence and the distribution of excitatory and inhibitory inputs were then calculated using the light and electron microscopic data sets. The three populations showed characteristic differences in dendritic morphology and in the density and distribution of afferent synapses. PV cells possessed the most extensive dendritic tree (4300 microm) and the thickest dendrites. CR cells had the smallest dendritic tree (2500 microm) and the thinnest shafts. The density of inputs as well as the total number of excitatory plus inhibitory synapses was several times higher on PV cells (on average, 16,294) than on CB (3839) or CR (2186) cells. The ratio of GABAergic inputs was significantly higher on CB (29.4%) and CR (20.71%) cells than on PV cells (6.4%). The density of inhibitory terminals was higher in the perisomatic region than on the distal dendrites. These anatomical data are essential to understand the distinct behavior and role of these interneuron types during hippocampal activity patterns and represent fundamental information for modeling studies.

GABAergic interneurons containing calbindin D28K or somatostatin are major targets of GABAergic basal forebrain afferents in the rat neocortex.

The arborization pattern and postsynaptic targets of the GABAergic component of the basal forebrain projection to neo- and mesocortical areas have been studied by the combination of anterograde tracing and pre- and postembedding immunocytochemistry. Phaseolus vulgaris leucoagglutinin (PHAL) was iontophoretically delivered into the region of the diagonal band of Broca, with some spread of the tracer into the substantia innominata and ventral pallidum. A large number of anterogradely labelled varicose fibres were visualized in the cingulate and retrosplenial cortices, and a relatively sparse innervation was observed in frontal and occipital cortical areas. Most of the labelled axons were studded with large en passant varicosities (Type 1), whereas the others (Type 2) had smaller boutons often of the drumstick type. Type 1 axons were distributed in all layers of the mesocortex with slightly lower frequency in layers 1 and 4. In the neocortex, layer 4, and to a smaller extent upper layer 5 and layer 6 contained the largest number of labelled fibres, whereas only a few fibres were seen in the supragranular layers. Characteristic type 2 axons were very sparse but could be found in all layers. Most if not all boutons of PHAL-labelled type 1 axons were shown to be GABA-immunoreactive by immunogold staining for GABA. Altogether 73 boutons were serially sectioned and found to make symmetrical synaptic contacts mostly with dendritic shafts (66, 90% of total targets), cell bodies (6, 8.2% of total), and with one spine. All postsynaptic cell bodies, and the majority of the dendritic shafts (44, 60.3% of total targets) were immunoreactive for GABA. Thus at least 68.5% of the total targets were GABA-positive, but the majority of the dendrites not characterized immunocytochemically for technical reasons (15.1%) also showed the fine structural characteristics of nonpyramidal neurons. The target interneurons included some of the somatostatin- and calbindin-containing subpopulations, and a small number of parvalbumin-containing neurons, as shown by double immunostaining for PHAL and calcium-binding proteins or neuropeptides. We suggest that the innervation of inhibitory interneurons having extensive local axon arborizations may be a mechanism by which basal forebrain neurons-most notably those containing GABA--have a powerful global effect on the majority of principal cells in the entire cortical mantle.

Parvalbumin- and calbindin D28k-immunoreactive neurons in the hippocampal formation of the macaque monkey.

Calcium-binding proteins calbindin D28k (CaBP) and parvalbumin (PV) were localized in neurons of the monkey hippocampal formation. CaBP immunoreactivity is present in all granule cells and in a large proportion of CA1 and CA2 pyramidal neurons, as well as in a distinct population of local circuit neurons. In the dentate gyrus, CaBP-immunoreactive nongranule cells are present in the molecular layer and in the hilar region, but they do not include the pyramidal basket cells at the hilar border. In the Ammon's horn, CaBP-positive, nonpyramidal neurons are more frequent in the CA3 area than in any other parts of the hippocampal formation. They are concentrated in the strata oriens and pyramidale of areas CA1-3, whereas only a few small neurons were found in the strata lucidum and radiatum of CA3 and in the stratum moleculare of the CA1 area. PV is exclusively present in local circuit neurons both in the dentate gyrus and in Ammon's horn. In the dentate gyrus the presumed basket cells at the hilar border exhibit PV immunoreactivity. In the hilar region and molecular layer only a relatively small number of cells are immunoreactive for PV. Most of these PV-positive cell bodies are located in the inner half of the molecular layer, with occasional horizontal cells at the hippocampal fissure. In Ammon's horn, strata oriens and pyramidale of areas CA1-3 contain a large number of PV-positive cells. There are no PV-immunoreactive cells in the strata lucidum, radiatum, or lacunosum moleculare. The CaBP- and PV-containing neurons form different subpopulations of cells in the monkey hippocampal formation. With the exception of a basket cell type in the monkey dentate gyrus, the CaBP- and PV-positive cell types were found to be remarkably similar in rodents and primates.

Subpopulations of GABAergic neurons containing parvalbumin, calbindin D28k, and cholecystokinin in the rat hippocampus.

The possible coexistence of calbindin D28k with parvalbumin and of calbindin D28k with cholecystokinin was studied in nonpyramidal cells of the rat dorsal hippocampal formation. Neighbouring Vibratome sections were immunostained either for calbindin D28k and parvalbumin or for calbindin D28k and cholecystokinin. The cells, halved during sectioning, were identified in both sections immunostained for different antigens. The coexistence of calbindin D28k and parvalbumin in the same neuron was rare throughout the hippocampal formation with the exception of stratum oriens of the CA1 region, where 9.6% of the parvalbumin-immunoreactive cells also contained calbindin D28k. In stratum radiatum of the CA3 region, calbindin D28k and cholecystokinin coexisted in 12.5% and 21.2% of the calbindin D28k and cholecystokinin-immunoreactive cells, respectively. In other regions of the hippocampal formation, the two markers coexisted in less than 5% of the cells of either type. The present results demonstrate that calbindin D28k-, parvalbumin- and cholecystokinin-containing nonpyramidal cells represent largely nonoverlapping cell populations and may thus be involved in different inhibitory circuits.

Distribution, morphological features, and synaptic connections of parvalbumin- and calbindin D28k-immunoreactive neurons in the human hippocampal formation.

Calcium binding proteins calbindin D28k (CaBP) and parvalbumin (PV) are known to form distinct subpopulations of gamma-aminobutyric acid (GABA)ergic neurons in the rodent hippocampal formation. Light and electron microscopic morphology and connections of these protein-containing neurons are only partly known in the primate hippocampus. In this study, CaBP and PV were localized in neurons of the human hippocampal formation including the subicular complex (prosubiculum, subiculum, and presubiculum) in order to explore to what extent these subpopulations of hippocampal neurons differ in phylogenetically distant species. CaBP immunoreactivity was present in virtually all granule cells of the dentate gyrus and population of in a proportion of pyramidal neurons in the CA1 and CA2 regions. A distinct population of CaBP-positive local circuit neurons was found in all layers of the dentate gyrus and Ammon's horn. Most frequently they were located in the molecular layer of the dentate gyrus and the pyramidal layer of Ammon's horn. In the subicular complex pyramidal neurons were not immunoreactive for CaBP. In the prosubiculum and subiculum immunoreactive nonpyramidal neurons were equally distributed in all layers, whereas in the presubiculum they occurred mainly in the superficial layers. Electron microscopy showed typical somatic and dendritic features of the granule, pyramidal, and local circuit neurons. CaBP-positive mossy fiber terminals in the hilus of the dentate gyrus and terminals of presumed pyramidal neurons of Ammon's horn formed asymmetric synapses with dendrites and spines. CaBP-positive terminals of nonprincipal neurons formed symmetric synapses with dendrites and dendritic spines, but never with somata or axon initial segments. PV was exclusively present in local circuit neurons in both the hippocampal formation and subicular complex. Most of the PV-positive cell bodies were located among or close to the principal cell layers. However, large numbers of immunoreactive neurons were also found in the molecular layer of the dentate gyrus and in strata oriens of Ammon's horn. PV-positive cells were equally distributed in all layers of the subicular complex. Electron microscopy showed the characteristic somatic and dendritic features of local circuit neurons. PV-positive axon terminals formed exclusively symmetric synapses with somata, axon initial segments and dendritic shafts, and in a few cases with dendritic spines. The CaBP- and PV-containing neurons formed similar subpopulations in rodents, monkeys, and humans, although the human hippocampus displayed the largest variability of these immunoreactive neurons in their morphology and location. Calcium binding protein-containing neurons frequently occurred in the molecular layer of the human dentate gyrus and in the stratum lacunosum-moleculare of Ammon's horn.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunostaining for substance P receptor labels GABAergic cells with distinct termination patterns in the hippocampus.

A specific antiserum against substance P receptor (SPR) labels nonprincipal neurons in the cerebral cortex of the rat (T. Kaneko et al. [1994], Neuroscience 60:199-211; Y. Nakaya et al. [1994], J. Comp. Neurol. 347:249-274). In the present study, we aimed to identify the types of SPR-immunoreactive neurons in the hippocampus according to their content of neurochemical markers, which label interneuron populations with distinct termination patterns. Markers for perisomatic inhibitory cells, parvalbumin and cholecystokinin (CCK), colocalized with SPR in pyramidallike basket cells in the dentate gyrus and in large multipolar or bitufted cells within all hippocampal subfields respectively. A dense meshwork of SPR-immunoreactive spiny dendrites in the hilus and stratum lucidum of the CA3 region belonged largely to inhibitory cells terminating in the distal dendritic region of granule cells, as indicated by the somatostatin and neuropeptide Y (NPY) content. In addition, SPR and NPY were colocalized in numerous multipolar interneurons with dendrites branching close to the soma. Twenty-five percent of the SPR-immunoreactive cells overlapped with calretinin-positive neurons in all hippocampal subfields, showing that interneurons specialized to contact other gamma-aminobutyric acid-ergic cells may also contain SPR. On the basis of the known termination pattern of the colocalized markers, we conclude that SPR-positive interneurons are functionally heterogeneous and participate in different inhibitory processes: (1) perisomatic inhibition of principal cells (CCK-containing cells, and parvalbumin-positive cells in the dentate gyrus), (2) feedback dendritic inhibition in the entorhinal termination zone (somatostatin and NPY-containing cells), and (3) innervation of other interneurons (calretinin-containing cells).

Total number and distribution of inhibitory and excitatory synapses on hippocampal CA1 pyramidal cells.

The integrative properties of neurons depend strongly on the number, proportions and distribution of excitatory and inhibitory synaptic inputs they receive. In this study the three-dimensional geometry of dendritic trees and the density of symmetrical and asymmetrical synapses on different cellular compartments of rat hippocampal CA1 area pyramidal cells was measured to calculate the total number and distribution of excitatory and inhibitory inputs on a single cell.A single pyramidal cell has approximately 12,000 microm dendrites and receives around 30,000 excitatory and 1700 inhibitory inputs, of which 40 % are concentrated in the perisomatic region and 20 % on dendrites in the stratum lacunosum-moleculare. The pre- and post-synaptic features suggest that CA1 pyramidal cell dendrites are heterogeneous. Strata radiatum and oriens dendrites are similar and differ from stratum lacunosum-moleculare dendrites. Proximal apical and basal strata radiatum and oriens dendrites are spine-free or sparsely spiny. Distal strata radiatum and oriens dendrites (forming 68.5 % of the pyramidal cells' dendritic tree) are densely spiny; their excitatory inputs terminate exclusively on dendritic spines, while inhibitory inputs target only dendritic shafts. The proportion of inhibitory inputs on distal spiny strata radiatum and oriens dendrites is low ( approximately 3 %). In contrast, proximal dendritic segments receive mostly (70-100 %) inhibitory inputs. Only inhibitory inputs innervate the somata (77-103 per cell) and axon initial segments. Dendrites in the stratum lacunosum-moleculare possess moderate to small amounts of spines. Excitatory synapses on stratum lacunosum-moleculare dendrites are larger than the synapses in other layers, are frequently perforated ( approximately 40 %) and can be located on dendritic shafts. Inhibitory inputs, whose percentage is relatively high ( approximately 14-17 %), also terminate on dendritic spines. Our results indicate that: (i) the highly convergent excitation arriving onto the distal dendrites of pyramidal cells is primarily controlled by proximally located inhibition; (ii) the organization of excitatory and inhibitory inputs in layers receiving Schaffer collateral input (radiatum/oriens) versus perforant path input (lacunosum-moleculare) is significantly different.

Innervation of different peptide-containing neurons in the hippocampus by GABAergic septal afferents.

The termination pattern of septohippocampal axons visualized by anterograde transport of Phaseolus vulgaris leucoagglutinin was studied in the hippocampal formation in the rat, with special reference to the innervation of neurons immunoreactive for the neuroactive peptides cholecystokinin, somatostatin or vasoactive intestinal polypeptide. The type I, GABAergic, septohippocampal afferents were shown to terminate on neurons immunoreactive for each of the three peptides. The cholecystokinin-like immunoreactive neurons in all regions, and the somatostatin-immunoreactive cells in stratum oriens of CA1 region were the most preferred targets. Cholecystokinin-immunoreactive cells, especially those in the granule cell layer of the dentate gyrus, were often seen to be contacted by type II (presumed cholinergic) axons as well. The somatostatin-immunoreactive cells in the hilus were also innervated by type I septohippocampal axons, although less frequently than those in stratum oriens of the CA1 subfield. Each type of peptidergic neuron received multiple symmetrical synaptic input from the Phaseolus vulgaris leucoagglutinin-labelled septal afferents, as confirmed by correlated electron microscopy. The majority of these neuropeptide-containing cells are known to be GABAergic, and to have distinct input and output relationships. Thus, the present results demonstrate that the GABAergic septohippocampal pathway can control a wide range of putative inhibitory circuits, and thereby influence the pattern of electrical activity in the hippocampal formation.

Calretinin is present in non-pyramidal cells of the rat hippocampus--I. A new type of neuron specifically associated with the mossy fibre system.

Calretinin-containing cells were visualized with immunocytochemistry in the rat dorsal hippocampal formation. Calretinin immunoreactivity was present exclusively in non-pyramidal cells in all layers of the dentate gyrus and the CA1-3 areas. Calretinin-positive neurons and processes were most abundant in the hilus of the dentate gyrus and in the stratum lucidum of the CA3 region. Several calretinin-immunoreactive cells were located within the hippocampal fissure. A distinct band of calretinin-immunoreactive fibres occupied the superficial part of the granule cell layer and the lowest part of the molecular layer. Closer examination of the calretinin-positive cells revealed that they formed two distinct cell groups. One group of cells, found exclusively in the stratum lucidum of the CA3 area and in the hilus of the dentate gyrus, was covered with numerous spines. Their somata and dendrites were restricted to stratum lucidum and to the hilus. Cells of the other group had smooth, often varicose, radially running dendrites, and were present in all areas and layers of the hippocampal formation. Two to three thick primary dendrites arose from the irregularly shaped cell body of spiny cells and emitted fine secondary branches only distally (70-100 microns) from the soma, where they formed a profuse network. The extensive dendritic tree of the cells spread horizontally within stratum lucidum and span a distance of 400-600 microns both in the septotemporal and in the transverse directions. The layer-specific location of these cells and their processes suggested that the majority of their input may derive from mossy fibres. This presumption has been confirmed by electron microscopic examination. A large number of asymmetrical synapses were found to cover the soma, the dendritic shafts and the spines (four to six synapses/spine) of the cells. A large proportion of the synapses were formed by boutons, which showed the distinctive features of mossy fibre terminals. Three to six primary dendrites arose from the multipolar, bipolar or pyramidal-shaped somata of spine-free cells, which were smaller than the somata of spiny cells. The smooth and frequently varicose dendrites branched proximally and ran primarily radially. Dendrites ascended or descended through several layers and received both asymmetrical and symmetrical synapses. In the CA1 subfield, the vertically running dendrites frequently contacted other calretinin-immunoreactive spine-free dendrites or cell bodies. Two or three calretinin-immunoreactive dendrites were often seen to be attached for over 100 or, occasionally, 200 microns and several puncta adherentia were observed between them using the electron microscope.(ABSTRACT TRUNCATED AT 400 WORDS)

Septal GABAergic neurons innervate inhibitory interneurons in the hippocampus of the macaque monkey.

The septohippocampal projection was visualized in three Macaca mulatta monkeys by anterograde transport of Phaseolus vulgaris leucoagglutinin. Following injections of the lectin into the medial septal nucleus, P. vulgaris leucoagglutinin-labelled fibres were found in the hippocampal complex, mainly in stratum oriens of the CA1 subfield, throughout the CA3 subfield, and in the hilus and stratum moleculare of the dentate gyrus. The majority of labelled axons were varicose, and formed multiple contacts with cell bodies and dendrites of calbindin D28k- and parvalbumin-immunoreactive non-pyramidal cells. GABA immunoreactivity of P. vulgaris leucoagglutinin-labelled axons and their postsynaptic targets was investigated by sectioning varicose axon segments for correlated light and electron microscopy, and processing alternate ultrathin sections for postembedding immunogold staining for GABA. All P. vulgaris leucoagglutinin-labelled boutons examined were GABA-immunoreactive and the majority of them formed symmetrical synapses with GABA-immunoreactive cell bodies and dendrites. The results demonstrate that a GABAergic septohippocampal pathway exists in the monkey, and, similar to the rat, terminates on different types of GABAergic neurons, including the parvalbumin- and calbindin D28k-containing non-pyramidal cells.

Electrotonic profile and passive propagation of synaptic potentials in three subpopulations of hippocampal CA1 interneurons.

To elucidate the role of dendritic morphology in signal transfer, the passive propagation of somatic and dendritic potentials was compared in multi-compartment models of three interneuron subpopulations in the CA1 region. Nine calbindin-, 15 calretinin- and 10 parvalbumin-containing cells were modelled incorporating the detailed geometry, the currents of the action potentials in the soma, and the AMPA, N-methyl-D-aspartate and GABA-B receptor-mediated postsynaptic currents in the dendrites. The cable properties show characteristic differences among the subpopulations. The morphotonic length of calbindin and calretinin cell dendrites is larger than of parvalbumin cells. Thus parvalbumin cells are more compact than calbindin or calretinin cells unless the ratio of their axial and membrane resistivities exceeds the ratios of the other two cell types by more than 33%. In calbindin cells, the distal parts of the extremely long dendrites that invade the alveus are virtually isolated from the soma for passively propagating signals. The synaptic potentials evoked at a given morphotonic distance from the soma show larger differences locally on the dendrites than on the soma in all subpopulations. Both the somatic and dendritic amplitude ratios are the smallest in PV cells. In calbindin cells the somatic amplitude of synaptic potentials evoked at the same morphotonic distance from the soma is similar regardless of the number of branchpoints along their path. In calretinin and parvalbumin cells, from dendrites with long primary segments synaptic potentials reach the soma with larger amplitude than from dendrites that are branching close to the soma. The dendrites with the larger impact on somatic membrane potential are usually the dendrites that enter the stratum lacunosum-moleculare. These results indicate that dendritic morphology plays a role in changing the effectiveness of synaptic potentials evoked at different dendritic locations, and in this way is likely to be an important factor in determining the integrative properties of the different neuron populations.

Calretinin is present in non-pyramidal cells of the rat hippocampus--II. Co-existence with other calcium binding proteins and GABA.

The possible co-existence of calretinin with other calcium binding proteins, parvalbumin and calbindin D28k, and with GABA, was studied in non-pyramidal cells of the rat dorsal hippocampal formation, using the mirror technique. The majority of the calretinin-containing neurons (83%) were found to be immunoreactive for GABA (79% in the dentate gyrus, 84% in the CA2-3, and 88% in the CA1 subfield). Most of the GABA-negative calretinin-immunoreactive neurons were located in the hilus of the dentate gyrus and in stratum lucidum of the CA3 subfield. Detailed analysis of the calretinin-immunoreactive cells of these subfields revealed that the two morphologically distinct types of calretinin neurons, i.e. the spiny and the spine-free cells, differ in their immunoreactivity for GABA. The overwhelming majority (92%) of the spine-free neurons were GABA-positive, whereas the immunoreactivity of spiny cells was ambiguous. At the sensitivity threshold of the immunocytochemical techniques used in the present study, most of the spiny cells (89%) had to be considered as GABA-negative, although the staining intensity in their cell bodies was somewhat above background level. Colchicine treatment resulted in a degeneration of calretinin-immunoreactive neurons; therefore, its effect on the GABA content of spiny neurons could not be evaluated. Nevertheless, the observations suggest that calretinin-containing neurons are heterogeneous both morphologically and neurochemically. Examination of the co-existence of calcium binding proteins revealed that none of the hippocampal cells contained both calretinin and parvalbumin in any regions of the hippocampal formation. Some overlap was detected between the calretinin- and the calbindin D28k-containing cell populations, 5.1% of the former and 6.2% of the latter were immunoreactive for both calcium binding proteins. This may be due to a small degree of cross-reactivity of the calbindin D28k antiserum with calretinin. Thus, our results demonstrate that the majority of calretinin-immunoreactive neurons are GABAergic and represent a subpopulation of non-pyramidal cells with no or only a negligible overlap with the subpopulations containing the other calcium binding proteins, parvalbumin and calbindin.

Structural basis of the cholinergic and serotonergic modulation of GABAergic neurons in the hippocampus.

Ascending subcortical pathways effectively modulate hippocampal information processing. Two components, the cholinergic and serotonergic pathways have been demonstrated to play an important role in the generation of behaviour-dependent hippocampal EEG patterns. Several findings suggest that the above projections influence the activity of hippocampal interneurons. Here we review the available data from physiological, pharmacological and receptor localization experiments, drawing attention to the crucial role of interneurons in the transfer and amplification of subcortical effects on cortical information processing. We hypothesize that, by exerting diverse actions on different subsets of interneurons, the cholinergic and serotonergic systems might change the balance of somatic and dendritic inhibition, and consequently change the integrative properties of hippocampal principal cells.

Pyramidal neurons are immunoreactive for calbindin D28k in the CA1 subfield of the human hippocampus.

The calcium binding protein calbindin D28k (CaBP) is localized in the granule cells of the dentate gyrus, in pyramidal neurons of the CA1 and CA2 subfields of the Ammon's horn as well as in distinct groups of local circuit neurons in both parts of the human hippocampal formation. Immunostaining was performed on human brain perfused 2 h after death. The localization of CaBP in the human was found to be similar to that in the monkey hippocampus suggesting that there is no species difference in the cellular localization of CaBP among different primates.

Inhibitory control of GABAergic interneurons in the hippocampus.

Hippocampal GABAergic interneurons are responsible for controlling the output and efficacy of synaptic input of large principal cell populations and, thereby, determine the oscillatory discharge patterns and synaptic plasticity in the hippocampus. Single interneurons are able to prevent repetitive firing of postsynaptic pyramidal cells (R. Miles, K. Tóth, A.I. Gulyás, N. Hájos, and T.F. Freund. Neuron, 16: 815-823, 1996), whereas on occasion a single pyramidal cell may be able to activate an interneuron under in vitro conditions (A.I. Gulyás, R. Miles, A. Sik, K. Tóth, N. Tamamaki, and T.F. Freund. Nature (London), 366: 683-687, 1993). Inhibition is therefore extremely powerful. Transient suppression of interneuronal activity allows the precise timing and synchronization of inhibitory postsynaptic potentials arriving at principal cells. A rhythmic suppression or modulation of interneuron discharge may be brought about by input from at least two major sources: (i) from other local interneurons or (ii) from subcortical centers. Of the possible local sources, in the present review particular attention will be paid to GABAergic neurons specialized to innervate other interneurons. Subcortical pathways known to modulate specific inhibitory functions in the hippocampus, i.e., the GABAergic and cholinergic septohippocampal and the serotonergic raphe hippocampal pathways, will also be reviewed. Roles of these control mechanisms may include the generation of theta and higher frequency oscillations and the selective removal of inhibition from the termination zone of specific excitatory afferents, thereby increasing their efficacy and (or) plasticity.

Pyramidal cell dendrites are the primary targets of calbindin D28k-immunoreactive interneurons in the hippocampus.

The axonal arborization and postsynaptic targets of calbindin D28k (CB)-immunoreactive nonprincipal neurons have been studied in the rat dorsal hippocampus. Two types of neurons were distinguished on the basis of soma location, the characteristics of the dendritic free, and the axon arborisation pattern. Type I cells were located in stratum radiatum of the CA1 and CA3 regions and occasionally in strata pyramidale and oriens. These cells had multipolar or bitufted dendritic trees primarily located in stratum radiatum. Their axons could be followed for a considerable distance, arborised within stratum radiatum, and were covered with regularly spaced small boutons. As demonstrated with postembedding immunogold staining, their axon terminals were gamma-aminobutyric acid (GABA) immunoreactive, and formed symmetrical synapses predominantly on proximal and distal dendrites of pyramidal cells (28% and 58%, respectively), and occasionally on spines (9%) or on GABA-positive dendrites (5%). Type II cells were found exclusively in stratum oriens of the CA1 and CA3 regions and possessed large, fusiform cell bodies and long, horizontally oriented dendrites. Their axon initial segments turned towards the alveus and disappeared in a myelin sheet, which was often possible to follow into the white matter. We conclude that type I CB-immunoreactive cells are likely to represent a major source of inhibitory synapses in the dendritic region of pyramidal cells, which are responsible for the control of dendritic electrogenesis. The distribution of local collaterals of type II cells-if they have any-remains unknown, but their main axon is likely to project to the medial septum.