Analysis of recordings of single-unit firing and population activity in the dorsal subiculum of unrestrained,freely moving rats

We examined neuronal activity in the dorsal subiculum of unrestrained, male adult Wistar rats, which were implanted with a moveable eight-electrode microdrive. The subiculum is the primary hippocampal formation output area and is comparatively uninvestigated neurophysiologically. We compared subicular unit activity and the subicular EEG while rats occupied a small, restricted environment and also correlated neuronal activity with the ongoing behavior of the animal. Units were separated using their electrophysiological characteristics into bursting units, regular spiking units, theta-modulated units, and fast spiking units. The bursting and regular spiking unit classes are similar to hippocampal CA1 units, whereas the fast spiking units appear to be interneurons. Bursting units were variable in their behavior: some units bursted regularly, and others bursted only occasionally. Theta-modulated units have not been described before; these were similar to regular spiking units in all respects except that they increased their firing significantly when theta oscillations were present in the simultaneous EEG record. Subicular EEG was similar to hippocampal EEG, with theta oscillations dominating "alert, moving" behaviors, while large amplitude irregular activity (LIA), which included sharp waves, predominated when theta oscillations were not present, mainly during "alert, still" and "quiet" behaviors. A relatively small proportion of subicular recordings (approximately 32%) were phase-locked to theta; this is a smaller proportion than in areas from which the subiculum takes major inputs. The relative lack of entrainment of subicular neurons by this important intrinsic rhythm is suggestive of a limit to which theta might be capable of affecting both subicular and hippocampal information processing more generally.     

Nucleus accumbens nitric oxide immunoreactive interneurons receive nitric oxide and ventral subicular afferents in rats

The nitric oxide generating neurons of the nucleus accumbens exert a powerful influence over striatal function, in addition, these nitrergic inputs are in a position to regulate the dopaminergic and glutamatergic inputs on striatal projection neurons. It was the aim of this study to establish the source of the glutamatergic drive to nitric oxide synthase interneurons of the nucleus accumbens. The nucleus accumbens nitric oxide-generating neurons receive asymmetrical, excitatory, presumably glutamatergic inputs. Possible sources of these inputs could be the limbic and cortical regions known to project to this area. To identify sources of the excitatory inputs to the nitric oxide synthase-containing interneurons of the nucleus accumbens in the rat we first examined the ultrastructural morphology of asymmetrical synaptic specializations contacting nitric oxide synthase-immunohistochemically labeled interneurons in the nucleus accumbens. Neurons were selected from different regions of the nucleus accumbens, drawn using camera lucida, processed for electron microscopic analysis, and the boutons contacting nitric oxide synthase-labeled dendrites were photographed and correlated to the drawings. Using vesicle size as the criterion the source was predicted to be either the prefrontal cortex or the ventral subiculum of the hippocampus. To examine this prediction, a further study used anterograde tracing from both the prefrontal cortex and the ventral subiculum, and nitric oxide synthase immunohistochemistry with correlated light and electron microscopy. Based on appositions by anterogradely labeled fibers, selected nitric oxide synthase-labeled neurons within the nucleus accumbens, were examined with electron microscopic analysis. With this technique we confirmed the prediction that subicular afferent boutons make synaptic contact with nitric oxide synthase interneurons, and demonstrated anatomically that nitric oxide synthase boutons make synaptic contact with the dendritic arbors of nitric oxide synthase interneurons. We suggest that the subicular input may excite the nitric oxide synthase neurons synaptically, while the nitric oxide synthase-nitric oxide synthase interactions underlie a nitric oxide signaling network which propagates hippocampal information, and expands the hippocampus's influence on 'gating' information flow across the nucleus accumbens.     

Resting and active properties of pyramidal neurons in subiculum and CA1 of rat hippocampus

Action potentials are the end product of synaptic integration, a process influenced by resting and active neuronal membrane properties. Diversity in these properties contributes to specialized mechanisms of synaptic integration and action potential firing, which are likely to be of functional significance within neural circuits. In the hippocampus, the majority of subicular pyramidal neurons fire high-frequency bursts of action potentials, whereas CA1 pyramidal neurons exhibit regular spiking behavior when subjected to direct somatic current injection. Using patch-clamp recordings from morphologically identified neurons in hippocampal slices, we analyzed and compared the resting and active membrane properties of pyramidal neurons in the subiculum and CA1 regions of the hippocampus. In response to direct somatic current injection, three subicular firing types were identified (regular spiking, weak bursting, and strong bursting), while all CA1 neurons were regular spiking. Within subiculum strong bursting neurons were found preferentially further away from the CA1 subregion. Input resistance (R(N)), membrane time constant (tau(m)), and depolarizing "sag" in response to hyperpolarizing current pulses were similar in all subicular neurons, while R(N) and tau(m) were significantly larger in CA1 neurons. The first spike of all subicular neurons exhibited similar action potential properties; CA1 action potentials exhibited faster rising rates, greater amplitudes, and wider half-widths than subicular action potentials. Therefore both the resting and active properties of CA1 pyramidal neurons are distinct from those of subicular neurons, which form a related class of neurons, differing in their propensity to burst. We also found that both regular spiking subicular and CA1 neurons could be transformed into a burst firing mode by application of a low concentration of 4-aminopyridine, suggesting that in both hippocampal subfields, firing properties are regulated by a slowly inactivating, D-type potassium current. The ability of all subicular pyramidal neurons to burst strengthens the notion that they form a single neuronal class, sharing a burst generating mechanism that is stronger in some cells than others.     

Different levels of Ih determine distinct temporal integration in bursting and regular-spiking neurons in rat subiculum

Pyramidal neurons in the subiculum typically display either bursting or regular-spiking behaviour. Although this classification into two neuronal classes is well described, it is unknown how these two classes of neurons contribute to the integration of input to the subiculum. Here, we report that bursting neurons posses a hyperpolarization-activated cation current ( I _h) that is two-fold larger (conductance, 5.3 ± 0.5 nS) than in regular-spiking neurons (2.2 ± 0.6 nS), whereas I _h exhibits similar voltage-dependent and kinetic properties in both classes of neurons. Bursting and regular-spiking neurons display similar morphology. The difference in I _h between the two classes of neurons is not responsible for the distinct firing patterns, as neither pharmacological blockade of I _h nor enhancement of I _h using a dynamic clamp affects the qualitative firing patterns. Instead, the difference in I _h between bursting and regular-spiking neurons determines the temporal integration of evoked synaptic input from the CA1 area. In response to stimulation at 50 Hz, bursting neurons, with a large I _h, show ∼50% less temporal summation than regular-spiking neurons. The amount of temporal summation in both neuronal classes is equal after pharmacological blockade of I _h. A computer simulation model of a subicular neuron with the properties of either a bursting or a regular-spiking neuron confirmed the pivotal role of I _h in temporal integration of synaptic input. These data suggest that in the subicular network, bursting neurons are better suited to discriminate the content of high-frequency input, such as that occurring during gamma oscillations, than regular-spiking neurons.     

Control of bursting by local inhibition in the rat subiculum in vitro

The subiculum, which provides the major hippocampal output, contains different cell types including weak/strong bursting and regular-spiking cells, and fast-spiking interneurons. These cellular populations play different roles in the generation of physiological rhythms and epileptiform activity. However, their intrinsic connectivity and the synaptic regulation of their discharge patterns remain unknown. In the present study, the local synaptic responses of subicular cell types were examined in vitro . To this purpose, slices were prepared at a specific orientation that permitted the antidromic activation of projection cells as a tool to examine local circuits. Patch recordings in cell-attached and whole-cell configurations were combined with neurobiotin labelling to classify cell types. Strong (≈75 %), but not weak (≈22 %), bursting cells typically fired bursts in response to local synaptic excitation, whereas the majority of regular-spiking cells (≈87 %) remained silent. Local excitation evoked single spikes in more than 70 % of fast-spiking interneurons. This different responsiveness was determined by intrinsic membrane properties and not by the amplitude and pharmacology of synaptic currents. Inhibitory GABAergic responses were also detected in some cells, typically as a component of an excitatory/inhibitory sequence. A positive correlation between the latency of the excitatory and inhibitory responses, together with the glutamatergic control (via non-NMDA receptors) of inhibition, suggested a local mechanism. The effect of local inhibition on synaptically activated firing of different cell types was evaluated. It is shown that projection bursting cells of the subiculum are strongly controlled by local inhibitory circuits.     

Electrophysiological analysis of the hippocampal projections to the entorhinal area

Responses evoked in the entorhinal area by impulse volleys originating in the ipsilateral hippocampus were analysed in the guinea-pig by means of field potential analysis. Perforant path volleys, synaptically elicited by stimulation of the dorsal psalterium of one side, were used to activate the hippocampal lamellar circuit of the same side and, through interhippocampal impulses, the hippocampal pyramidal neurons of the contralateral side. Discharge of the hippocampal pyramidal neurons was followed by a response, a fast negative deflection preceded and followed by slow waves, in the dorsal third of the ipsilateral entorhinal area. Laminar distribution of the fast negative deflection and of the time-locked unit activity suggested that excitatory synaptic effects followed by neuron discharge were generated in neurons of layers VI-II of the entorhinal area. The increasing latency of the fast negative deflection and of unit firing over the cortical depth suggested that these synaptic effects were generated in temporal sequence, going from layer VI to layer II. The entorhinal response disappeared after a lesion at the caudal border of the hippocampus interrupting the caudally-directed hippocampal efferents. The anatomy of the hippocampal and subicular projections to the entorhinal area in the guinea-pig, together with electrophysiological data obtained in recordings from the ipsilateral subiculum, suggested that the hippocampal impulses were relayed to layers VI-V of the entorhinal area by the subiculum. The delayed activation of layers IV-II was possibly mediated by intracortical connections. Double-shock experiments showed that impulses of hippocampal origin inhibited the response to dorsal psalterium volleys of entorhinal neurons giving origin to perforant path fibers. The data show that the hippocampal output activates the deep layers of the entorhinal area from which it is possibly relayed to numerous cortical and subcortical regions. Moreover, the inhibitory effects exerted on neurons originating perforant path fibers give evidence of a negative feedback control system operating in the hippocampal region.     

Neural correlates of social odor recognition and the representation of individual distinctive social odors within entorhinal cortex and ventral subiculum

Recognition of individual conspecifics is important for social behavior and requires the formation of memories for individually distinctive social signals. Individual recognition is often mediated by olfactory cues in mammals, especially nocturnal rodents such as golden hamsters. In hamsters, this form of recognition requires main olfactory system input to the lateral entorhinal cortex (LEnt). Here, we tested whether neurons in LEnt and the nearby ventral subiculum (VS) would show cellular correlates of this natural form of recognition memory. Two hundred ninety single neurons were recorded from both superficial (SE) and deep layers of LEnt (DE) and VS while male hamsters investigated volatile odorants from female vaginal secretions. Many neurons encoded differences between female's odors with many discriminating between odors from different individual females but not between different odor samples from the same female. Other neurons discriminated between odor samples from one female and generalized across collections from other females. LEnt and VS neurons showed enhanced or suppressed cellular activity during investigation of previously presented odors and in response to novel odors. A majority of SE neurons decreased firing to odor repetition and increased activity to novel odors. In contrast, DE neurons often showed suppressed activity in response to novel odors. Thus, neurons in LEnt and VS of male hamsters encode information that is critical for the identification and recognition of individual females by odor cues. This study reveals cellular mechanisms in LEnt and VS that may mediate a natural form of recognition memory in hamsters. These neuronal responses were similar to those observed in rats and monkeys during performance in standard recognition memory tasks. Consequently, the present data extend our understanding of the cellular basis for recognition memory and suggest that individual recognition requires similar neural mechanisms as those employed in laboratory tests of recognition memory.     

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

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

Nitric oxide-containing pyramidal neurons of the subiculum innervate the CA1 area

Neurons and axon terminals containing neuron-specific nitric oxide synthase (nNOS) were examined in the rat subiculum and CA1 area of Ammon's horn. In the subiculum, a large subpopulation of the pyramidal neurons and non-pyramidal cells are immunoreactive for nNOS, whereas in the neighbouring CA1 area of Ammon's horn only non-pyramidal neurons are labelled with the antibody against nNOS. In the pyramidal layer of the subiculum, nNOS-positive axon terminals form both asymmetric and symmetric synapses. In the adjacent CA1 area the nNOS-positive terminals that form symmetric synapses are found in all layers, whereas those terminals that form asymmetric synapses are only in strata radiatum and oriens, but not in stratum lacunosum-moleculare. In both the subiculum and CA1 area, labelled terminals make symmetric synapses only on dendritic shafts, whereas asymmetric synapses are exclusively on dendritic spines. Previous observations demonstrated that all nNOS-positive non-pyramidal cells are GABAergic local circuit neurons, which form exclusively symmetric synapses. We suggest that nNOS-immunoreactive pyramidal cells of the subiculum may innervate neighbouring subicular pyramidal cells and, to a smaller extent, pyramidal cells of the adjacent CA1 area, forming a backward projection between the subicular and hippocampal principal neurons. Electronic Publication     

Electrophysiological properties of neurons in the rat subiculum in vitro

The present study determined the membrane and synaptic properties of neurons in the rat subiculum. Using the in vitro hippocampal slice preparation, intracellular recordings were obtained from 91 subicular neurons. Membrane properties and morphological characteristics were similar to those reported for hippocampal pyramidal neurons. Two categories of subicular neurons were distinguished based on their response to a depolarizing current pulse. One type of neuron showed bursting behavior and the second type was characterized as regular firing. Analysis of the charging functions during hyperpolarizing current pulses yielded a mean ₀ and ₁ for subicular neurons of about 13 ms and 0.60 ms, respectively. Using the model of an equivalent cylinder, the mean dendrite-to-soma conductance ratio () was estimated at 6.0 and electrotonic length constant (L) at 0.7. There was no difference in these values between bursting and regular firing neurons. Tetrodotoxin-resistant potentials (presumed calcium hump/spike) were evoked from bursting subicular neurons at lower current intensities than CA1 pyramidal neurons. Calcium humps could only be evoked from about half the regular firing subicular neurons. Subicular cells showed an excitatory/inhibitory postsynaptic potential (EPSP/IPSP) sequence in response to electrical stimulation in different layers of the CA1 area. An EPSP could also be evoked from stimulation of the superficial or deep layers of the presubiculum and was attributed to activation of entorhinal fibers of passage. At high stimulation intensity, an antidromic spike was often evoked following stimulation in the presubicular area or CA1 alveus. The evoked EPSPs were blocked by addition of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to the bathing medium. In magnesium-free, CN-QX bathing solution, a longer lasting depolarization was recorded; this response was blocked by application of a N-methyl-d-aspartate (NMDA) receptor antagonist (AP5). Iontophoretic application of glutamate or quisqualate (10 mM) along the soma-dendritic axis of subicular neurons leads to either a short-latency depolarization or a burst of action potentials. Application of 10 mM GABA near the recording site usually produced a hyperpolarization, which, at times, was mixed with a depolarization. Mixed hyperpolarizing/depolarizing responses were observed when GABA was applied to the basal or apical dendritic areas. There were no significant differences in the synaptic properties or responses to drug application between bursting and regular firing neurons. These results indicate that subicular neurons (1) are composed of a heterogeneous population of cell types, (2) have similar electrical properties to other hippocampal principal neurons, (3) receive glutaminergic synapses from CA1 and entorhinal cortical neurons, (4) project to the presubicular area and fornix (via the alveus), (5) are inhibited by local circuit neurons, and (6) display complex responses to GABA.     

Complex autonomous firing patterns of striatal low-threshold spike interneurons

During sensorimotor learning, tonically active neurons (TANs) in the striatum acquire bursts and pauses in their firing based on the salience of the stimulus. Striatal cholinergic interneurons display tonic intrinsic firing, even in the absence of synaptic input, that resembles TAN activity seen in vivo. However, whether there are other striatal neurons among the group identified as TANs is unknown. We used transgenic mice expressing green fluorescent protein under control of neuronal nitric oxide synthase or neuropeptide-Y promoters to aid in identifying low-threshold spike (LTS) interneurons in brain slices. We found that these neurons exhibit autonomous firing consisting of spontaneous transitions between regular, irregular, and burst firing, similar to cholinergic interneurons. As in cholinergic interneurons, these firing patterns arise from interactions between multiple intrinsic oscillatory mechanisms, but the mechanisms responsible differ. Both neurons maintain tonic firing because of persistent sodium currents, but the mechanisms of the subthreshold oscillations responsible for irregular firing are different. In LTS interneurons they rely on depolarization-activated noninactivating calcium currents, whereas those in cholinergic interneurons arise from a hyperpolarization-activated potassium conductance. Sustained membrane hyperpolarizations induce a bursting pattern in LTS interneurons, probably by recruiting a low-threshold, inactivating calcium conductance and by moving the membrane potential out of the activation range of the oscillatory mechanisms responsible for single spiking, in contrast to the bursting driven by noninactivating currents in cholinergic interneurons. The complex intrinsic firing patterns of LTS interneurons may subserve differential release of classic and peptide neurotransmitters as well as nitric oxide.     

Heterogeneous electrotonic coupling and synchronization of rhythmic bursting activity in mouse Hb9 interneurons

The neurons and mechanisms involved in mammalian spinal cord networks that produce rhythmic locomotor activity remain largely undefined. Hb9 interneurons, a small population of discretely localized interneurons in the mouse spinal cord, are conditionally bursting neurons. Here we applied potassium channel blockers with the aim of increasing neuronal excitability and observed that under these conditions, postnatal Hb9 interneurons exhibited bursts of action potentials with underlying voltage-independent spikelets. The bursts were insensitive to antagonists to fast chemical synaptic transmission, and the bursting and spikelets were blocked by tetrodotoxin. Calcium imaging studies using 2-photon excitation in spinal cord slices revealed that clustered Hb9 interneurons exhibited synchronous and occasional asynchronous, calcium transients that were also insensitive to fast synaptic transmission blockade. All transients were blocked by the gap junction blocker carbenoxolone. Paired whole cell patch-clamp recordings of Hb9 interneurons in the late postnatal mouse revealed common chemical synaptic inputs but no evidence of current transfer (i.e., electrotonic coupling) between the neurons. However, Hb9 and a previously defined population of non-Hb9 interneurons were electrotonically coupled. In the absence of fast chemical transmission in the whole spinal cord preparation, 2-photon excitation calcium imaging revealed bursting activity of Hb9 interneurons synchronous with rhythmic ventral root output. Thus Hb9 interneurons are both endogenous bursters and rhythmically active within a heterogeneous electrotonically coupled network. A network with these properties could produce the wide range of stable rhythms necessary for locomotor activity.     

Long-term effects of brain-derived neurotrophic factor on the frequency of inhibitory synaptic events in the rat superficial dorsal horn

Chronic constriction injury (CCI) of rat sciatic nerve produces a specific pattern of electrophysiological changes in the superficial dorsal horn that lead to central sensitization that is associated with neuropathic pain. These changes can be recapitulated in spinal cord organotypic cultures by long term (5–6 days) exposure to brain-derived neurotrophic factor (BDNF) (200 ng/ml). Certain lines of evidence suggest that both CCI and BDNF increase excitatory synaptic drive to putative excitatory neurons while reducing that to putative inhibitory interneurons. Because BDNF slows the rate of discharge of synaptically-driven action potentials in inhibitory neurons, it should also decrease the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) throughout the superficial dorsal horn. To test this possibility, we characterized superficial dorsal horn neurons in organotypic cultures according to five electrophysiological phenotypes that included tonic, delay and irregular firing neurons. Five to 6 days of treatment with 200 ng/ml BDNF decreased sIPSC frequency in tonic and irregular neurons as might be expected if BDNF selectively decreases excitatory synaptic drive to inhibitory interneurons. The frequency of sIPSCs in delay neurons was however increased. Further analysis of the action of BDNF on tetrodotoxin-resistant miniature inhibitory postsynaptic currents (mIPSC) showed that the frequency was increased in delay neurons, unchanged in tonic neurons and decreased in irregular neurons. BDNF may thus reduce action potential frequency in those inhibitory interneurons that project to tonic and irregular neurons but not in those that project to delay neurons.     

Theta-frequency membrane resonance and its ionic mechanisms in rat subicular pyramidal neurons

The neuron population of the hippocampal formation exhibits oscillatory activity within the theta (theta) frequency band (4-10 Hz), and the intrinsic resonance properties of individual hippocampal neurons contribute to this network oscillation. The subiculum is the pivotal output region of the hippocampal formation and it is involved in many of the physiological and pathological functions of the limbic system. To study the characteristics and underlying mechanisms of resonance activity in subicular pyramidal neurons, we performed whole-cell patch-clamp recordings from these neurons in rat horizontal brain slices. We applied sinusoidal currents with constant amplitudes and linearly increasing frequencies to measure the resonance frequency of subicular pyramidal neurons. We found that the resonance frequency of subicular pyramidal neurons was about 2 Hz at room temperature and 4-6 Hz at 32-35 degrees C. The resonance frequency increased at hyperpolarized membrane potentials and decreased at depolarized membrane potentials. We also investigated three sub-threshold currents involved in the resonance: a slow hyperpolarization-activated cation current; an instantaneously activating, inwardly rectifying potassium current; and an inwardly persistent sodium current. The application of ZD7288 abolished the resonance hump, indicating that hyperpolarization-activated cation current generated resonance. The application of Ba(2+) enlarged the resonance hump at hyperpolarized potentials below -80 mV, indicating that inwardly rectifying potassium current attenuated resonance. The application of TTX suppressed the resonance at depolarized potentials, indicating that persistent sodium current amplified resonance when neurons were depolarized. Thus, there is a theta-frequency resonance mediated by hyperpolarization-activated cation current in subicular pyramidal neurons. This theta-frequency resonance of individual subicular pyramidal neurons may participate in the population's theta oscillation and contribute to the functions of the subiculum.     

Performance on spatial working memory tasks after dorsal or ventral hippocampal lesions and adjacent damage to the subiculum

Rats with excitotoxic lesions of the dorsal or ventral hippocampus and control rats were trained on 2 spatial working memory tasks: the standard version of the radial maze with 8 baited arms and the non-matching-to-place procedure in the T maze. Dorsal lesions produced deficits in both tasks, whereas ventral lesions did not affect learning in either of them. A volumetric analysis of subicular damage showed that dorsal hippocampal lesions caused a deficit in the non-matching-to-place only when accompanied by damage to the dorsal subiculum; on the other hand, lesions to the dorsal hippocampus impaired performance in the radial-arm maze regardless of the extent of subicular damage.     

The subiculum: what it does, what it might do, and what neuroanatomy has yet to tell us

The subiculum is a pivotal but under-investigated structure positioned between the hippocampus proper and entorhinal and other cortices, as well as a range of subcortical structures. The subiculum has a range of electrophysiological and functional properties which are quite distinct from its input areas; given the widespread set of cortical and subcortical areas with which it interacts, it is able to influence activity in quite disparate brain regions. The rules governing plasticity of synaptic transmission in the hippocampal-subicular axis are poorly understood; this axis appears to share some properties in common with the hippocampus proper, but behaves quite differently in other respects. Equally, its functional properties are not well understood; it plays an important but ill-defined role in spatial navigation, mnemonic processing and control of the response to stress. Here, I review investigations of synaptic plasticity in the hippocampal-subicular pathway, recordings of subicular neurons in the freely moving behaving animal, the effects of behavioural and other stressors on subicular synaptic plasticity, and anatomical data on the dorso-ventral organization of the subiculum in relation to the hypothalamic-pituitary-adrenal (HPA) axis. I argue that there is a dorso-ventral segregation of function within the subiculum: the dorsal component appears principally concerned with the processing of information about space, movement and memory, whereas the ventral component appears to play a major regulatory role in the inhibition of the HPA axis.     

Parvalbumin-immunoreactive neurons in the hippocampal formation of Alzheimer''s diseased brain

The number and topographic distribution of immunocytochemically stained parvalbumin interneurons was determined in the hippocampal formation of control and Alzheimer's diseased brain. In control hippocampus, parvalbumin interneurons were aspiny and pleomorphic, with extensive dendritic arbors. In dentate gyrus, parvalbumin cells, as well as a dense plexus of fibers and puncta, were associated with the granule cell layer. A few cells also occupied the molecular layer. In strata oriens and pyramidale of CA1–CA3 subfields, parvalbumin neurons gave rise to dendrites that extended into adjacent strata. Densely stained puncta and beaded fibers occupied stratum pyramidale, with less dense staining in adjacent strata oriens and radiatum. Virtually no parvalbumin profiles were observed in stratum lacunosum-moleculare or the alveus. Numerous polymorphic parvalbumin neurons and a dense plexus of fibers and puncta characterized the deep layer of the subiculum and the lamina principalis externa of the presubiculum. In Alzheimer's diseased hippocampus, there was an approximate 60% decrease in the number of parvalbumin interneurons in the dentate gyrus/CA4 subfield (P<0.01) and subfields CA1–CA2 (P<0.01). In contrast, parvalbumin neurons did not statistically decline in subfields CA3, subiculum or presubiculum in Alzheimer's diseased brains relative to controls. Concurrent staining with Thioflavin-S histochemistry did not reveal degenerative changes within parvalbumin-stained profiles. These findings reveal that parvalbumin interneurons within specific hippocampal subfields are selectively vulnerable in Alzheimer's disease. This vulnerability may be related to their differential connectivity, e.g., those regions connectionally related to the cerebral cortex (dentate gyrus and CA1) are more vulnerable than those regions connectionally related to subcortical loci (subiculum and presubiculum).     

Electrophysiological evidence using focal flash photolysis of caged glutamate that CA1 pyramidal cells receive excitatory synaptic input from the subiculum

The hippocampus sends efferent fibers to the subiculum, which projects to the entorinal cortex. Previous studies suggest that the hippocampal CA1 area may receive a projection back from the subiculum. This hypothesis was tested using whole cell recording from CA1 pyramidal cells while subicular neurons were selectively stimulated with focal flash photolysis of caged glutamate, which avoids stimulation of fibers of passage. Control experiments showed that focal flash stimulations caused direct glutamate-mediated depolarizations and bursts of action potentials in the recorded CA1 pyramidal cells, but only when the stimulation targeted the somatodendritic regions of a neuron, not the axons. To block GABA(A)-mediated inhibition and isolate local excitatory circuits, bicuculline was applied to minislices containing only the isolated CA1 area and the subiculum. Of 24 CA1 pyramidal cells, 25% (6 of 24) consistently generated repetitive excitatory postsynaptic currents (EPSCs) in response to flash stimulation in the subiculum. The responsive neurons were located 200-500 microm from the distal end of CA1 and 400-1,100 microm from the stimulation sites in subiculum, suggesting excitatory synaptic projections from the subicular neurons to CA1 pyramidal cells. This study provides new electrophysiological evidence that CA1 pyramidal cells receive excitatory synaptic input from the subiculum. Thus a reciprocal excitatory synaptic circuit connects the subiculum and the CA1 area in the normal adult rat.     

Neuropeptide Y(5) receptors reduce synaptic excitation in proximal subiculum, but not epileptiform activity in rat hippocampal slices

Neuropeptide Y (NPY) potently inhibits excitatory synaptic transmission in the hippocampus, acting predominantly via a presynaptic Y(2) receptor. Recent reports that the Y(5) receptor may mediate the anticonvulsant actions of NPY in vivo prompted us to test the hypothesis that Y(5) receptors inhibit synaptic excitation in the hippocampal slice and, furthermore, that they are effective in an in vitro model of anticonvulsant action. Two putative Y(5) receptor-preferring agonists inhibited excitatory postsynaptic currents (EPSCs) evoked by stimulation of stratum radiatum in pyramidal cells. We recorded initially from area CA1 pyramidal cells, but subsequently switched to cells from the subiculum, where a much greater frequency of response was observed to Y(5) agonist application. Both D-Trp(32)NPY (1 microM) and [ahx(8-20)]Pro(34)NPY (3 microM), a centrally truncated, Y(1)/Y(5) agonist we synthesized, inhibited stimulus-evoked EPSCs in subicular pyramidal cells by 44.0 +/- 5.7% and 51.3 +/- 3.5% (mean +/- SE), in 37 and 58% of cells, respectively. By contrast, the less selective centrally truncated agonist, [ahx(8-20)] NPY (1 microM), was more potent (66.4 +/- 4.1% inhibition) and more widely effective, suppressing the EPSC in 86% of subicular neurons. The site of action of all NPY agonists tested was most probably presynaptic, because agonist application caused no changes in postsynaptic membrane properties. The selective Y(1) antagonist, BIBP3226 (1 microM), did not reduce the effect of either more selective agonist, indicating that they activated presynaptic Y(5) receptors. Y(5) receptor-mediated synaptic inhibition was more frequently observed in slices from younger animals, whereas the nonselective agonist appeared equally effective at all ages tested. Because of the similarity with the previously reported actions of Y(2) receptors, we tested the ability of Y(5) receptor agonists to suppress stimulus train-induced bursting (STIB), an in vitro model of ictaform activity, in both area CA3 and the subiculum. Neither [ahx(8-20)]Pro(34)NPY nor D-Trp(32)NPY were significantly effective in suppressing or shortening STIB-induced afterdischarge, with <20% of slices responding to these agonists in recordings from CA3 and none in subiculum. By contrast, 1 microM each of [ahx(8-20)]NPY, the Y(2) agonist, [ahx(5-24)]NPY, and particularly NPY itself suppressed the afterdischarge in area CA3 and the subiculum, as reported earlier. We conclude that Y(5) receptors appear to regulate excitability to some degree in the subiculum of young rats, but their contribution is relatively small compared with those of Y(2) receptors, declines with age, and is insufficient to block or significantly attenuate STIB-induced afterdischarges.     

Genesis and role of coordinated firing in a feedforward network: a model study of the enteric nervous system.

The enteric nervous system can generate complex motor patterns independently of the central nervous system. The ascending enteric reflex pathway consists of sensory neurons, long chains of a single class of orally directed interneuron and excitatory motor neurons. Because of the importance of this pathway in peristalsis, it was modelled from the firing of sensory neurons through to muscle membrane activation. The model was anatomically realistic in the number of neurons simulated and in the patterns of connections between neurons. The model was also realistic in the simulation of ligand-gated currents in neuron and muscle membrane, current flow in the muscle syncytium and voltage-dependent currents in muscle. Sensory neurons were activated in a manner consistent with a brief mechanical stimulus. Transmission between sensory neurons and first-order interneurons was by slow excitatory transmission, which caused interneurons to fire continuously for several hundred milliseconds. Interneurons then transmitted to higher order interneurons by fast excitatory postsynaptic potentials, each lasting for around 40 ms. As the activity propagated along the pathway, random firing became progressively more synchronized between neurons, until the network as a whole was firing in a coordinated manner. The coordinated firing was a robust phenomenon over a wide range of network and neuron parameters. It is therefore possible that this is a general property of feedforward networks that receive high levels of sustained input. The smooth muscle model indicated that bursting input to the muscle may increase the likelihood of muscle cells firing action potentials when compared with uniform input. In addition, the syncytium model explains how the predicted muscle excitation might be related to current experimental observations.