In vivo and in vitro studies on putative interneurones in the rat hippocampus: possible mediators of feed-forward inhibition

Disentangling the role of TRPM4 in hippocampus-dependent plasticity and learning: an electrophysiological, behavioral and FMRI approach

Hippocampal long-term potentiation (LTP) has been extensively studied as a cellular model of learning and memory. Recently, we described a central function of the Transient Receptor Potential M4 (TRPM4) channel in hippocampal LTP in mice in vitro. Here, we used Trpm4 knock-out (Trpm4^-/-) rats to scrutinize TRPM4's role in the intact brain in vivo. After having confirmed the previous in vitro findings in mice, we studied hippocampal synaptic plasticity by chronic recordings in freely moving rats, hippocampus-dependent learning by a behavioral battery and hippocampal-cortical connectivity by fMRI. The electrophysiological investigation supports an involvement of TRPM4 in LTP depending on the induction protocol. Moreover, an exhaustive analysis of the LTP kinetics point to mechanistic changes in LTP by trpm4 deletion. General behavior as measured by open field test, light-dark box and elevated plus maze was inconspicuous in Trpm4^-/- rats. However, they showed a distinct deficit in spatial working and reference memory associated to the Barnes maze and T-maze test, respectively. In contrast, performance of the Trpm4^-/- in the Morris water maze was unaltered. Finally, fMRI investigation of the effects of a strong LTP induction manifested BOLD responses in the ipsilateral and contralateral hippocampus and the prefrontal cortex of both groups. Yet, the initial BOLD response in the stimulated hippocampal area of Trpm4^-/- was significantly enhanced compared to WT rats. Our findings at the cellular, behavioral and system level point to a relevant role for TRPM4 in specific types of hippocampal synaptic plasticity and learning but not in hippocampal-prefrontal interaction.

Inhibition shapes the organization of hippocampal representations

Hippocampal neurons become tuned to stimuli that predict behaviorally salient outcomes. This plasticity suggests that memory formation depends upon shifts in how different anatomical inputs can drive hippocampal activity. Here, I present evidence that inhibitory neurons can provide such a mechanism for learning-related changes in the tuning of pyramidal cells. Inhibitory currents arriving on the dendrites of pyramidal cells determine whether an excitatory input can drive action potential output. Specificity and plasticity of this dendritic modulation allows for precise, modifiable changes in how afferent inputs are integrated, a process that defines a neuron's receptive field. In addition, feedback inhibition plays a fundamental role in biasing which excitatory neurons may be co-active. By defining the rules of synchrony and the rules of input integration, interneurons likely play an important role in the organization of memory representation within the hippocampus.

GABAA receptor-mediated feedforward and feedback inhibition differentially modulate the gain and the neural code transformation in hippocampal CA1 pyramidal cells

Diverse variety of hippocampal interneurons exists in the CA1 area, which provides either feedforward (FF) or feedback (FB) inhibition to CA1 pyramidal cell (PC). However, how the two different inhibitory network architectures modulate the computational mode of CA1 PC is unknown. By investigating the CA3 PC rate-driven input-output function of CA1 PC using in vitro electrophysiology, in vitro-simulation of inhibitory network, and in silico computational modeling, we demonstrated for the first time that GABAA receptor-mediated FF and FB inhibition differentially modulate the gain, the spike precision, the neural code transformation and the information capacity of CA1 PC. Recruitment of FF inhibition buffered the CA1 PC spikes to theta-frequency regardless of the input frequency, abolishing the gain and making CA1 PC insensitive to its inputs. Instead, temporal variability of the CA1 PC spikes was increased, promoting the rate-to-temporal code transformation to enhance the information capacity of CA1 PC. In contrast, the recruitment of FB inhibition sub-linearly transformed the input rate to spike output rate with high gain and low spike temporal variability, promoting the rate-to-rate code transformation. These results suggest that GABAA receptor-mediated FF and FB inhibitory circuits could serve as network mechanisms for differentially modulating the gain of CA1 PC, allowing CA1 PC to switch between different computational modes using rate and temporal codes ad hoc. Such switch will allow CA1 PC to efficiently respond to spatio-temporally dynamic inputs and expand its computational capacity during different behavioral and neuromodulatory states in vivo.

Elevated levels of brain-pathologies associated with neurodegenerative diseases in the methionine sulfoxide reductase A knockout mouse

One of the posttranslational modifications to proteins is methionine oxidation, which is readily reversible by the methionine sulfoxide reductase (Msr) system. Thus, accumulation of faulty proteins due to a compromised Msr system may lead to the development of aging-associated diseases like neurodegenerative diseases. In particular, it was interesting to monitor the consequential effects of methionine oxidation in relation to markers that are associated with Alzheimer's disease as methionine oxidation was implied to play a role in beta-amyloid toxicity. In this study, a knockout mouse strain of the methionine sulfoxide reductase A gene (MsrA ( -/- )) caused an enhanced neurodegeneration in brain hippocampus relative to its wild-type control mouse brain. Additionally, a loss of astrocytes integrity, elevated levels of beta-amyloid deposition, and tau phosphorylation were dominant in various regions of the MsrA ( -/- ) hippocampus but not in the wild-type. Also, a comparison between cultured brain slices of the hippocampal region of both mouse strains showed more sensitivity of the MsrA ( -/- ) cultured cells to H(2)O(2) treatment. It is suggested that a deficiency in MsrA activity fosters oxidative-stress that is manifested by the accumulation of faulty proteins (via methionine oxidation), deposition of aggregated proteins, and premature brain cell death.

Stress-facilitated LTD induces output plasticity through synchronized-spikes and spontaneous unitary discharges in the CA1 region of the hippocampus

Long-term potentiation (LTP) and long-term depression (LTD) of the excitatory synaptic inputs plasticity in the hippocampus is believed to underlie certain types of learning and memory. Especially, stressful experiences, well known to produce long-lasting strong memories of the event themselves, enable LTD by low frequency stimulation (LFS, 3 Hz) but block LTP induction by high frequency stimulation (HFS, 200 Hz). However, it is unknown whether stress-affected synaptic plasticity has an impact on the output plasticity. Thus, we have simultaneously studied the effects of stress on synaptic plasticity and neuronal output in the hippocampal CA1 region of anesthetized Wistar rats. Our results revealed that stress increased basal power spectrum of the evoked synchronized-spikes and enabled LTD induction by LFS. The induction of stress-facilitated LTD but not LFS induced persistent decreases of the power spectrum of the synchronized-spikes and the frequency of the spontaneous unitary discharges; However, HFS induced LTP in non-stressed animals and increased the power spectrum of the synchronized-spikes, without affecting the frequency of the spontaneous unitary discharges, but HFS failed to induce LTP in stressed animals without affecting the power spectrum of the synchronized-spikes and the frequency of the spontaneous unitary discharges. These observations that stress-facilitated LTD induces the output plasticity through the synchronized-spikes and spontaneous unitary discharges suggest that these types of stress-related plasticity may play significant roles in distribution, amplification and integration of encoded information to other brain structures under stressful conditions.

Temporal overlap of excitatory and inhibitory afferent input in guinea-pig CA1 pyramidal cells

1. The temporal interaction of evoked synaptic excitation and GABAA-mediated inhibition was examined in CA1 pyramidal cells. Single and paired intracellular recordings were carried out in pyramidal cell dendrites and somata, and interneurons of the guinea-pig hippocampal slice. Current-clamp, sharp electrode and whole-cell voltage-clamp recordings were made. 2. Kinetics of dendritic and somatic inhibitory responses were similar. Notably, kinetics of dendritic unitary IPSPs were as fast as kinetics of somatic unitary IPSPs. 3. GABAA-mediated influences were present throughout the orthodromic pyramidal cell EPSP/EPSC. Comparison of the kinetics of pharmacologically isolated monosynaptic IPSPs, IPSCs and inhibitory conductances (g GABAA), showed fastest kinetics for g GABAA. Close temporal overlap was observed between monosynaptic g GABAA and the rising phase of the evoked EPSP/EPSC. The onset of g GABAA coincided with or preceded onset of the EPSP/EPSC. 4. Onsets of feedforward IPSPs coincided with the rising phase of the pyramidal cell EPSP in > 80 % of paired recordings. Fastest feedforward inhibitory responses exerted near complete overlap with evoked excitation. 5. Onsets of recurrent IPSPs did not occur during the rising phase of the evoked EPSP, but > 3.0 ms after the peak of the pyramidal cell EPSP. 6. Orthodromically evoked interneuron spikes were observed at stimulation intensities that were below the threshold for eliciting EPSPs in concomitantly recorded pyramidal cells. The activation of feedforward inhibitory responses by weakest excitatory input, and the large temporal overlap between feedforward inhibition and evoked excitation, suggest that in situ any excitatory input in CA1 is effectively controlled by fast synaptic inhibition.

The synaptic basis of GABAA,slow

Although two kinetically distinct evoked GABAA responses (GABAA,fast and GABAA,slow) have been observed in CA1 pyramidal neurons, studies of spontaneous IPSCs (sIPSCs) in these neurons have reported only a single population of events that resemble GABAA,fast in their rise and decay kinetics. The absence of slow sIPSCs calls into question the synaptic basis of GABAA,slow. We present evidence here that both evoked responses are synaptic in origin, because two classes of minimally evoked, spontaneous and miniature IPSCs exist that correspond to GABAA,fast and GABAA,slow. Slow sIPSCs occur infrequently, suggesting that the cells underlying these events have a low spontaneous firing rate, unlike the cells giving rise to fast sIPSCs. Like evoked GABAA,fast and GABAA,slow, fast and slow sIPSCs are modulated differentially by furosemide, a subtype-specific GABAA antagonist. Furosemide blocks fast IPSCs by acting directly on the postsynaptic receptors, because it reduces the amplitude of both miniature IPSCs and the responses of excised patches to applied GABA. Thus, in the hippocampus, parallel inhibitory circuits are composed of separate populations of interneurons that contact anatomically segregated and pharmacologically distinct postsynaptic receptors.

Sharp, local synchrony among putative feed-forward inhibitory interneurons of rabbit somatosensory cortex

Many suspected inhibitory interneurons (SINs) of primary somatosensory cortex (S1) receive a potent monosynaptic thalamic input (thalamocortical SINs, SINstc). It has been proposed that nearly all such SINstc of a S1 barrel column (BC) receive excitatory synaptic input from each member of a subpopulation of neurons within the topographically aligned ventrobasal (VB) thalamic barreloid. Such a divergent and convergent network leads to several testable predictions: sharply synchronous activity should occur between SINstc of a BC, sharp synchrony should not occur between SINstc of neighboring BCs, and sharp synchrony should not occur between SINs or other neurons of the same BC that do not receive potent monosynaptic thalamic input. These predictions were tested by cross-correlating the activity of SINstc of the same and neighboring BCs. Correlations among descending corticofugal neurons of layer 5 (CF-5 neurons, identified by antidromic activation) and other neurons that receive little or no monosynaptic VB input also were examined. SINs were identified by a high-frequency (>600 Hz) burst of three or more spikes elicited by VB stimulation and had action potentials of short duration. SINstc were further differentiated by short synaptic latencies to electrical stimulation of VB thalamus (<1.7 ms) and to peripheral stimulation (<7.5 ms). The above predictions were confirmed fully. 1) Sharp synchrony (+/-1 ms) was seen between all SINstc recorded within the same BC (a mean of 4.26% of the spikes of each SINtc were synchronized sharply with the spikes of the paired SINtc). Sharp synchrony was not dependent on peripheral stimulation, was not oscillatory, and survived general anesthesia. Sharp synchrony was superimposed on a broader synchrony, with a time course of tens of milliseconds. 2) Little or no sharp synchrony was seen when CF-5 neurons were paired with SINstc or other neurons of the same BC. 3) Little or no sharp synchrony was seen when SINstc were paired with other SINstc located in neighboring BCs. Intracellular recordings obtained from three SINs in the fully awake state supported the assertion that SINs are GABAergic interneurons. Each of these cells met our extracellular criteria for identification as a SIN, each had a spike of short duration (0.4-0.5 ms), and each responded to a depolarizing current pulse with a nonadapting train of action potentials. These results support the proposed network linking VB barreloid neurons with SINstc within the topographically aligned BC. We suggest that sharp synchrony among SINstc results in highly synchronous inhibitory postsynpatic potentials (IPSPs)in the target neurons of these cells and that these summated IPSPs may be especially effective when excitatory drive to target cells is weak and asynchronous.

Synaptic effects of identified interneurons innervating both interneurons and pyramidal cells in the rat hippocampus

GABAergic interneurons sculpt the activity of principal cells and are themselves governed by GABAergic inputs. To determine directly some of the sources and mechanisms of this GABAergic innervation, we have used dual intracellular recordings with biocytin-filled microelectrodes and investigated synaptic interactions between pairs of interneurons in area CA1 of the adult rat hippocampus. Of four synaptically-coupled interneuron-to-interneuron cell pairs, three presynaptic cells were identified as basket cells, preferentially innervating somata and proximal dendrites of pyramidal cells, but one differing from the other two in the laminar distribution of its dendritic and axonal fields. The fourth presynaptic interneuron was located at the border between strata lacunosum moleculare and radiatum, with axon ramifying within stratum radiatum. Action potentials evoked in all four presynaptic interneurons were found to elicit fast hyperpolarizing inhibitory postsynaptic potentials (mean amplitude 0.35 +/- 0.10 mV at a membrane potential of -59 +/- 2.8 mV) in other simultaneously recorded interneurons (n=4). In addition, three of the presynaptic interneurons were also shown to produce similar postsynaptic responses in subsequently recorded pyramidal cells (n=4). Electron microscopic evaluation revealed one of the presynaptic basket cells to form 12 synaptic junctions with the perisomatic domain (seven somatic synapses and five synapses onto proximal dendritic shafts) of the postsynaptic interneuron in addition to innervating the same compartments of randomly-selected local pyramidal cells (50% somatic and 50% proximal dendritic synapses, n=12). In addition, light microscopic analysis also indicated autaptic self-innervation in basket (12 of 12) and bistratified cells (six of six). Electron microscopic investigation of one basket cell confirmed six autaptic junctions made by five of its boutons. Together, these data demonstrate that several distinct types of interneuron have divergent output to both principal cells and local interneurons of the same (basket cells) or different type. The fast synaptic effects, probably mediated by GABA in both postsynaptic interneurons and principal cells are similar. These additional sources of GABA identified here in the input to GABAergic cells could contribute to the differential temporal patterning of distinct GABAergic synaptic networks.

Properties of interictal discharges induced by hippocampal kindling

The present experiment was designed to reveal the characteristics of interictal discharges (IIDs) induced by kindling of the rabbit hippocampus. Out of 21 animals, 13 developed stage 5 convulsions with a mean of 18 stimulations (Kindled (K) group), whereas the remaining eight animals did not (incomplete kindling (IK) group). A correlation between the duration of the afterdischarge and the behavioral stages was found in the K group. However, changes in frequency of total IIDs during kindling did not differ between the two groups. In the acute experiments performed after kindling, IIDs were classified into two types: simple and complex IIDs. The former was further classified into two subtypes (A and B) according to the laminar profile in the CA1 region. The A type of simple IIDs showed a negative polarity in the apical dendritic layer, while the B type showed a negative polarity in the basal dendritic layer. Complex IIDs basically consisted of two to three simple IIDs and were often followed by large irregular activity. Retrospective analysis was done, based on the classification of IIDs in the acute experiments (n = 12). Consequently, in the K group (n = 7), the frequency of complex IIDs rather than that of simple ones was closely related to the enhancement of behavioral responses during kindling. On the other hand, in the IK group (n = 5), simple IIDs occurred at a higher frequency, and did not parallel the changes in seizure behavior. It is concluded that complex IIDs play an important role in the propagation as well as the evolution of kindling effects.

Hippocampal interneurons express a novel form of synaptic plasticity

Individual GABAergic interneurons in hippocampus can powerfully inhibit more than a thousand excitatory pyramidal neurons. Therefore, control of interneuron excitability provides control over hippocampal networks. We have identified a novel mechanism in hippocampus that weakens excitatory synapses onto GABAergic interneurons. Following stimulation that elicits long-term potentiation at neighboring synapses onto excitatory cells, excitatory synapses onto inhibitory interneurons undergo a long-term synaptic depression (interneuron LTD; iLTD). Unlike most other forms of hippocampal synaptic plasticity, iLTD is not synapse specific: stimulation of an afferent pathway triggers depression not only of activated synapses but also of inactive excitatory synapses onto the same interneuron. These results suggest that high frequency afferent activity increases hippocampal excitability through a dual mechanism, simultaneously potentiating synapses onto excitatory neurons and depressing synapses onto inhibitory neurons.

Physiological properties of anatomically identified basket and bistratified cells in the CA1 area of the rat hippocampus in vitro

Basket and bistratified cells form two anatomically distinct classes of GABAergic local-circuit neurons in the CA1 region of the rat hippocampus. A physiological comparison was made of intracellularly recorded basket (n = 13) and bistratified neurons (n = 6), all of which had been anatomically defined by their efferent target profile (Halasy et al., 1996). Basket cells had an average resting membrane potential of -64.2 +/- 7.2 vs. -69.2 +/- 4.6 mV in bistratified cells. The latter had considerably higher mean input resistances (60.2 +/- 42.1 vs. 31.3 +/- 10.9 M Ohms) and longer membrane time constants (18.6 +/- 8.1 vs. 9.8 +/- 4.5 ms) than basket cells. Differences were also apparent in the duration of action potentials, those of basket cells being 364 +/- 77 and those of bistratified cells being 527 +/- 138 microseconds at half-amplitude. Action potentials were generally followed by prominent, fast after-hyperpolarizing potentials which in basket cells were 13.5 +/- 6.7 mV in amplitude vs. 10.5 +/- 5.1 in bistratified cells. The differences in membrane time constant, resting membrane potential, and action potential duration reached statistical significance (P < 0.05). Extracellular stimulation of Schaffer collateral/commissural afferents elicited short-latency excitatory postsynaptic potentials (EPSPs) in both cell types. The average 10-90% rise time and duration (at half-amplitude) of subthreshold EPSPs in basket cells were 1.9 +/- 0.5 and 10.7 +/- 5.6 ms, compared to 3.3 +/- 1.3 and 20.1 +/- 9.7 ms in bistratified cells, the difference in EPSP rise times being statistically significant. Basket and bistratified EPSPs were highly sensitive to a bath applied antagonist of non-N-methyl-D-aspartate (NMDA) receptors, whereas the remaining slow-rise EPSP could be abolished by an NMDA receptor antagonist. Increasing stimulation intensity elicited biphasic inhibitory postsynaptic potentials (IPSPs) in both basket and bistratified cells. In conclusion, basket and bistratified cells in the CA1 area show prominent differences in several of their membrane and firing properties. Both cell classes are activated by Schaffer collateral/commissural axons in a feedforward manner and receive inhibitory input from other, as yet unidentified, local-circuit neurons.

Stimulus parameters affecting paired-pulse depression of dentate granule cell field potentials. II. Low-frequency stimulation

Low frequency (1 Hz) stimulation of the perforant path produces a depression in the population spike (PS) of dentate granule cell field potentials and also may affect the strength of paired pulse depression. The effects of 1 Hz stimulation (30 s train) on paired pulse depression (20 and 200 ms interpulse intervals, IPI) were evaluated in the unanesthetized rat under two conditions: (i) when the stimulus intensity of both pulses was increased simultaneously (5-100%); and (ii) when the stimulus intensity of the first (conditioning) pulse was increased (5-100%), while the stimulus intensity of the second (test) pulse was held constant (50%). The test PS amplitude was predicted based upon either the conditioning PS amplitude at the end of the 1 Hz train or upon the additive effects of paired pulse depression and 1 Hz stimulation. These predicted values then were assessed for the best fit to observed values following 1 Hz trains. Under both stimulus conditions, the 1 Hz depression in the conditioning PS amplitude exhibited characteristics that were identical to late paired pulse depression recorded before the train. A decrease in the test PS amplitude also was observed following 1 Hz stimulation at the 20 and 200 ms IPIs. The best fit to observed values of the test PS at the end of 1 Hz trains was provided by estimates based upon the additive effects of 1 Hz stimulation and paired pulse depression. These results indicate that the strength of paired pulse depression in the unanesthetized rat is unchanged following 1 Hz stimulation, and further, that the 1 Hz depression in dentate granule cell field potentials most likely reflects the cumulative influence of late paired pulse depression.

Stimulus parameters affecting paired-pulse depression of dentate granule cell field potentials. I. Stimulus intensity

Paired pulse stimulation of the perforant path provides a measure of inhibition of dentate granule cell field potentials that is reflected in the depression of the second (test) population spike (PS) relative to the first (conditioning) PS. The assumption that the strength of paired pulse depression is dependent upon the amplitude of the conditioning PS was investigated by increasing the stimulus intensity of both pulses (5-100% of maximum, Experiment 1), or by increasing the stimulus intensity of the conditioning pulse (5-100%) while maintaining a constant stimulus intensity of the test pulse (50%, Experiment 2). In both experiments, the threshold for early paired pulse depression (20 ms interpulse interval, IPI) was reached with moderate stimulation (30-40% of maximum). Above threshold, the test PS was depressed to a relatively constant amplitude in Experiment 1, in contrast to a nearly linear decrease observed in Experiment 2 with increasing strength of early paired pulse, relative to the conditioning reflects the lower stimulus intensity of the test pulse, relative to the conditioning pulse, in the second study, thereby allowing the increasing strength of early paired pulse depression to be detected more easily. The threshold for late paired pulse depression was reached near (20%, Experiment 1) or below (5%, Experiment 2) the PS threshold of dentate granule cells, and a paradoxical decrease in late paired pulse depression was detected with maximal stimulation in both studies. Together, these results suggest that early paired pulse depression exhibits a strong dependence upon the amplitude of the conditioning PS, whereas late paired pulse depression is marginally affected by the conditioning PS amplitude and is influenced by additional processes at both extremes of the stimulus intensity continuum.

Reduction of GABA-mediated inhibitory postsynaptic potentials in hippocampal CA1 pyramidal neurons following oral flurazepam administration

Oral administration of the benzodiazepine, flurazepam, for one week results in tolerance in vivo and in vitro and in a reduction in recurrent and feedforward inhibition in vitro in the CA1 pyramidal cell region of hippocampus. In the present study CA1 pyramidal cells were examined intracellularly in vitro in rat hippocampal slices (500 microns) from rats sacrificed two or seven days after cessation of oral flurazepam treatment. Following drug treatment, the membrane characteristics of CA1 pyramidal cells were not significantly different from control neurons. GABAA-mediated, early inhibitory postsynaptic potentials were significantly reduced in amplitude (60%) in pyramidal neurons from rats killed two days, but not in those killed seven days, after the end of drug administration. The decrease in early inhibitory postsynaptic potential amplitude was observed using just-subthreshold, threshold and supramaximal orthodromic stimulation as well as following antidromic activation. The magnitude of the decrease in the early inhibitory postsynaptic potential amplitude was similar in the presence of the GABAB antagonist, CGP 35348, and could not be attributed to differences in the strength of afferent stimulation between flurazepam-treated and control groups. The size of the GABAB-mediated, late inhibitory postsynaptic potentials was also significantly decreased (45%) in comparison to control cells. Reversal potentials for both the early (-72 mV) and late (-92 mV) hyperpolarizations were not significantly different between groups. Following high intensity orthodromic stimulation, in the presence of an intracellular sodium channel blocker (QX-314) which also blocks the GABAB-mediated late hyperpolarization, a bicuculline-sensitive late depolarizing potential was unmasked in neurons from FZP-treated rats, but never from control cells. Excitatory postsynaptic potential amplitude was significantly increased in flurazepam-treated neurons and the threshold for the synaptically-evoked action potential was significantly increased. Following depolarizing current injection, the duration and frequency of pyramidal cell discharges and the action potential threshold were not altered by oral flurazepam treatment. The amplitude of the fast afterhyperpolarization was also not changed. Overall, the findings indicate an impairment of transmission at GABAergic synapses onto hippocampal CA1 pyramidal cell neurons after chronic benzodiazepine treatment at a time when rats are tolerant to the anticonvulsant effects of the benzodiazepines in vivo.

Bicuculline enhances the late GABAB receptor-mediated paired-pulse inhibition observed in rat hippocampal slices

The inhibition of CA1 pyramidal neurones in rat hippocampal slices was studied using extracellular recordings of population spike potential responses to paired orthodromic stimulation. Variation of the interpulse interval allowed the separation of an early phase of inhibition (interpulse interval 5-20 ms), blocked by the GABAA receptor antagonist bicuculline (1 microM; n = 11), and a late phase (interpulse interval 200-400 ms) blocked by the GABAB receptor antagonist phaclofen (1 mM; n = 5) but enhanced by bicuculline (n = 11). Similar enhancement was not observed when conditioning response amplitudes were increased by increasing the stimulus strength, rather than bicuculline. Orthodromic stimulation leads to synaptic excitation of both pyramidal neurones and inhibitory interneurones, and may also lead to activation of inhibitory inputs onto interneurones. Bicuculline could prevent inhibition of the interneurones, and hence enhance the late, GABAB receptor-mediated inhibition. Conversely, the therapeutic administration of benzodiazepines would be postulated to enhance the inhibition of inhibitory interneurones, leading to an iatrogenic decrease in GABAB receptor-mediated inhibition.

Electrophysiological diversity of pyramidal-shaped neurons at the granule cell layer/hilus border of the rat dentate gyrus recorded in vitro

In the rat dentate gyrus, pyramidal-shaped cells located on the border of the granule cell layer and the hilus are one of the most common types of gamma-aminobutyric acid (GABA)-immunoreactive neurons. This study describes their electrophysiological characteristics. Membrane properties, patterns of discharge, and synaptic responses were recorded intracellularly from these cells in hippocampal slices. Each cell was identified as pyramidal-shaped by injecting the marker Neurobiotin intracellularly (n = 17). In several respects the membrane properties of the sampled cells were similar to "fast-spiking" cells (putative inhibitory interneurons) that have been described in other areas of the hippocampus. For example, input resistance was high (mean 91.3 megohms), the membrane time constant was short (mean 7.7 ms), and there was a large afterhyperpolarization following a single action potential (mean 10.5 mV at resting potential). However, the action potentials of most pyramidal-shaped cells were not as brief (mean 1.2 ms total duration) as those of most previously described fast-spiking cells. Many pyramidal-shaped neurons had strong spike frequency adaptation relative to other fast-spiking cells. Although these latter two characteristics were apparent in the majority of the sampled cells, there were exceptional pyramidal-shaped neurons with fast action potentials and weak adaptation, demonstrating the electrophysiological variability of pyramidal-shaped cells. Responses to outer molecular layer stimulation were composed primarily of excitatory postsynaptic potentials (EPSPs) rather than inhibitory postsynaptic potentials (IPSPs), and were usually small (EPSPs evoked at threshold were often less than 2 mV), and brief (less than 30 ms). There was variability, because in a few cells EPSPs evoked at threshold were much larger. However, regardless of EPSP amplitude, suprathreshold stimulation (up to 4 times the threshold stimulus strength) rarely evoked more than one action potential in any cell. The results suggest that stimulation of perforant path axons produces limited excitatory synaptic responses in pyramidal-shaped neurons. This may be one of the reasons why they are relatively resistant to prolonged perforant path stimulation. The pyramidal-shaped neurons located at the base of the granule cell layer have been associated historically with a basket plexus around granule cell somata, and have been called pyramidal "basket" cells. However, basket-like endings were rare and axon collaterals outside the granule cell layer as the outer molecular layer and the central hilus, and antidromic action potentials could be recorded in some cells in response to weak stimulation of these areas. Taken together with the electrophysiological variability, the results indicate that these cells are physiologically heterogeneous.

Localization of mRNAs encoding two forms of glutamic acid decarboxylase in the rat hippocampal formation

The mRNAs for two forms of glutamic acid decarboxylase (GAD65 and GAD67) were localized in the rat hippocampal formation by nonradioactive in situ hybridization methods with digoxigenin-labeled cRNA probes. Some neurons in all layers of the hippocampus and dentate gyrus were readily labeled for each GAD mRNA, and the patterns of labeling for GAD65 and GAD67 mRNAs were very similar. All major groups of previously described GAD- and GABA-containing neurons appeared to be labeled for each GAD mRNA. Such findings suggest that most GABA neurons in the hippocampal formation contain both GAD mRNAs. When the labeling of neurons in the hippocampal formation and cerebral cortex was compared in the same sections, the intensity of neuronal labeling for GAD67 mRNA was generally similar in the two regions. However, the intensity of labeling for GAD65 mRNA was generally stronger for many neurons in the hippocampal formation than for most neurons in the cerebral cortex. Neurons in the hilus of the dentate gyrus were particularly well labeled for GAD65. The nonradioactive labeling for the GAD mRNAs was confined to the cytoplasm of neuronal cell bodies, and this allowed a clear visualization of the relative number and location of labeled neurons. Several distinct patterns of GAD mRNA-containing neurons were observed among different regions of the hippocampal formation. In the hilus of the dentate gyrus, GAD mRNA-containing neurons were numerous in the regions deep to the granule cell layer as well as in more central parts of the hilus. Within CA3, the densities (quantities) of labeled neurons varied among the regions. In the inner or hilar segment of CA3, the density of labeled neurons was often lower than that in the outer part of CA3 where numerous labeled neurons were distributed throughout all layers. In CA1, GAD mRNA-labeled neurons were distributed in a relatively laminar pattern with the highest density in stratum pyramidale and moderate densities in stratum oriens and at the interface between strata radiatum and lacunosum-moleculare. Lower densities were found within the latter two layers. The prominent localization of the two GAD mRNAs in the hippocampal formation suggests that a dual system for GABA synthesis is necessary for normal GABAergic function in this brain region. Most putative GABA neurons contain relatively high levels of GAD67 mRNA as might be expected if this GAD form is responsible for the synthesis of GABA for metabolic and baseline synaptic function.(ABSTRACT TRUNCATED AT 400 WORDS)

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Local interneurons and information processing in the olfactory glomeruli of the moth Manduca sexta

Intracellular recordings were made from the major neurites of local interneurons in the moth antennal lobe. Antennal nerve stimulation evoked 3 patterns of postsynaptic activity: (i) a short-latency compound excitatory postsynaptic potential that, based on electrical stimulation of the antennal nerve and stimulation of the antenna with odors, represents a monosynaptic input from olfactory afferent axons (71 out of 86 neurons), (ii) a delayed activation of firing in response to both electrical- and odor-driven input (11 neurons), and (iii) a delayed membrane hyperpolarization in response to antennal nerve input (4 neurons). Simultaneous intracellular recordings from a local interneuron with short-latency responses and a projection (output) neuron revealed unidirectional synaptic interactions between these two cell types. In 20% of the 30 pairs studied, spontaneous and current-induced spiking activity in a local interneuron correlated with hyperpolarization and suppression of firing in a projection neuron. No evidence for recurrent or feedback inhibition of projection neurons was found. Furthermore, suppression of firing in an inhibitory local interneuron led to an increase in firing in the normally quiescent projection neuron, suggesting that a disinhibitory pathway may mediate excitation in projection neurons. This is the first direct evidence of an inhibitory role for local interneurons in olfactory information processing in insects. Through different types of multisynaptic interactions with projection neurons, local interneurons help to generate and shape the output from olfactory glomeruli in the antennal lobe.

Theta modulation of neurons of the hippocampus of the rabbit and its interrelationship with other parameters of spontaneous and evoked activity

The reliability of the existing functional criteria of the differentiation of pyramidal ("neurons with complex spikes") and inhibitory ("theta neurons") cells of the hippocampus is examined on the basis of a statistical analysis of the spontaneous and evoked activity of neurons of the hippocampus of the awake rabbit. The analysis shows that the parameters of average frequency, the presence of theta modulation of activity, the behavior of the neurons in situations evoking theta rhythm in the EEG of the hippocampus (inhibition or activation during the effect of sensory stimuli), and the character of the influences of stimulation of the medial septal region of the internal connections of the hippocampus do not permit the reliable identification of different types of neurons of the hippocampus in the awake rabbit. The available data on the functional classification of neurons of the hippocampus are discussed in connection with notions regarding their state in situations associated with the generation of theta rhythm.

Decreased heterosynaptic and homosynaptic paired pulse inhibition in the rat hippocampus as a chronic sequela to limbic status epilepticus

We studied a rat model of chronic epilepsy that shares key features with certain patients with temporal lobe epilepsy. This model relies on a previous period of limbic system status epilepticus established by focal stimulation to one hippocampus. Animals were examined 1 month after recovery from such status epilepticus and compared to unstimulated controls and to animals that received stimulation but did not develop status epilepticus. Two experimental procedures were employed to study changes in paired pulse inhibition of population spike (PS) discharges elicited in CA1 pyramidal cells. One procedure (homosynaptic) delivered two identical stimuli to the CA3 region contralateral to the recording site; the other procedure (heterosynaptic) delivered a conditioning stimulus to the ipsilateral angular bundle and a separate test stimulus to the contralateral CA3. For both procedures, influences of stimulus intensities and of interpulse intervals on the potency of paired pulse inhibition were determined. Based on the results, standardized protocols that assayed the maximal amount of paired pulse inhibition were developed. With the homosynaptic protocol, there was one period of inhibition (interpulse intervals up to 300 ms). Animals that previously experienced limbic status epilepticus had markedly less paired pulse inhibition under these conditions than did controls. The stimulated, non-status epilepticus animals were not different from controls. For the heterosynaptic protocol, there were 2 phases of paired pulse inhibition, early (< 50 ms) and late (> 300 ms), separated by a period of paired pulse facilitation. After status epilepticus there were, compared to controls, decreases in both early and late phases of inhibition. The stimulated, non-status epilepticus animals were not different from controls. For the paired pulse facilitation, there was no difference between the animals that experienced status epilepticus and controls. These findings indicate a profound and enduring disturbance of GABA-mediated inhibition in this model. The heterosynaptic paired pulse protocol deals with a number of confounding issues associated with the homosynaptic protocol in this regard. Furthermore, the results suggest the inhibitory disturbance is diffuse, affecting various inhibitory circuits in the hippocampus.

Dormancy of inhibitory interneurons in a model of temporal lobe epilepsy

In humans temporal lobe epilepsy (TLE) is characterized by recurrent seizures, neuronal hyperexcitability, and selective loss of certain neuronal populations in the hippocampus. Animal models of the condition indicate that a diminution of inhibition mediated by gamma-aminobutyric acid (GABA) accounts for the altered function, and it has been hypothesized that the diminution arises because GABAergic basket interneurons are "dormant" as a result of their being disconnected from excitatory inputs. In hippocampal slices, inhibitory postsynaptic potentials (IPSPs) were elicited in CA1 pyramidal cells by activation of basket cells; responses from an animal model of TLE were compared to those from control tissue. IPSPs evoked indirectly by activation of terminals that then excited basket cells were reduced in the epileptic tissue, whereas IPSPs evoked by direct activation of basket cells, when excitatory neurotransmission was blocked, were not different from controls. These results provide support for the "dormant basket cell" hypothesis and have implications for the pathophysiology and treatment of human TLE.

Convergence of projections from the rat hippocampal formation, medial geniculate and basal forebrain onto single amygdaloid neurons: an in vivo extra- and intracellular electrophysiological study

We recorded extra- and intracellular responses from rat amygdaloid neurons in vivo after electrical stimulation of the hippocampal formation (dentate gyrus, hippocampal fields CA3 and CA4, entorhinal cortex, subicular complex); medial geniculate; and basal forebrain (diagonal band, ventral pallidum, olfactory tubercle, nucleus accumbens, bed nucleus of stria terminalis, lateral preoptic area, substantia innominata). Stimulation of hippocampal formation structures evoked IPSPs or EPSP-IPSP sequences in which the IPSP had a lower threshold than the EPSP. Recordings from candidate inhibitory neurons in the amygdala indicated that excitatory afferents from the hippocampal formation contact both amygdaloid inhibitory and principal neurons (feedforward inhibition), and that the inhibitory neurons have a lower threshold of activation. Medial geniculate stimulation also evoked EPSP-IPSP sequences. In marked contrast to these results, stimulation of basal forebrain structures evoked short latency IPSPs in amygdaloid neurons. This provides the first physiological evidence for direct inhibition of the amygdala by the basal forebrain. Basal forebrain stimulation also evoked EPSP-IPSP sequences in amygdaloid neurons. Individual amygdaloid neurons could show responses to stimulation of the hippocampal formation, basal forebrain, and medial geniculate, indicating that synaptic input from these areas converges onto single amygdaloid cells. The findings provide further information about the synaptic organization of afferents to the amygdala, and indicate that single amygdaloid neurons play a role in the synaptic integration of input from these diverse sources.

Chronic ethanol treatment reduces inhibition in CA1 of the rat hippocampus

The effect of chronic ethanol exposure on inhibition in the rat hippocampal slice was investigated using paired-pulse stimulation techniques with stimulation in stratum radiatum or stratum oriens of CA1. Experimental animals were fed ethanol in a liquid diet for 20 weeks and were withdrawn for at least 8 weeks prior to electrophysiological recording. Prior ethanol treatment had no effect on basic input-output relationships for the extracellular population spike. Ethanol treatment significantly reduced the recurrent inhibition produced by antidromic stimulation in a manner dependent upon stimulus intensity. In addition, with orthodromic paired-pulse stimulation of either stratum radiatum or oriens, a trend toward an augmentation of the facilitation of population spike amplitude was observed, suggesting that feedforward inhibition may also be reduced. These results are similar to those found with treatments that reduce inhibition. Therefore, we conclude that chronic ethanol exposure produces an enduring disruption of inhibitory neuronal function in the rat hippocampus.

Effects of cholinergic agonists on two non-pyramidal cell types in rat hippocampal slices

In the hippocampus, pyramidal cells (PCs) are not the only cell type sensitive to cholinergic stimulation. Two non-pyramidal cell types from animals as young as 8 days demonstrated clear, direct responses to application of cholinergic agonists. These cholinergic actions are excitatory, mostly blocked by muscarinic antagonists, and persist under conditions which block synaptic transmission (TTX, low Ca2+/high Mg2+). Cholinergic agonists may affect different conductances in interneurons than in PCs, sometimes resulting in rapid depolarization. Demonstration of direct excitatory cholinergic effects on inhibitory interneurons supports the view that cholinergically-evoked hyperpolarizations in PCs are due to local circuit interactions.

Potentiation of inhibition with perforant path kindling: an NMDA-receptor dependent process

Kindling produces a long-lasting enhancement of excitatory and inhibitory neurotransmission. Both long-term potentiation and kindling-induced potentiation of hippocampal excitatory neurotransmission are suppressed by N-methyl-D-aspartate (NMDA) receptor antagonists. These antagonists also greatly retard the development of electrical kindling. We have previously reported prolonged afterdischarges (AD) in animals stimulated in the perforant path and treated with the NMDA antagonist, dizocilpine maleate (MK-801), despite a retardation in the development of kindling. In the present study the potentiation of excitation and inhibition was assessed during perforant path kindling when NMDA channels were blocked with MK-801. Paired pulse inhibition at 8 interpulse intervals (IPI 20-1000 ms) was monitored before and during kindling development. MK-801 (1 mg/kg, i.p.) delivered 30 min prior to perforant path stimulation increased AD thresholds and delayed kindling development. Potentiation of the excitatory postsynaptic potential (EPSP) and of paired pulse inhibition measured 20-24 h after each drug administration/stimulation were suppressed in MK-801-treated animals. Paradoxically, AD durations were prolonged by MK-801. Longer AD durations could be accounted for by a higher incidence of secondary AD bouts in MK-801 relative to control animals. Development of potentiation of the early phase of paired pulse inhibition (IPI 20-30 ms) was delayed and the potentiation of the late phase of inhibition (IPIs of 200-1000 ms) was completely blocked by MK-801. Thus, some of the enhancement of inhibition seen with kindling is dependent upon NMDA neurotransmission. Suppression of this potentiated inhibition may account for prolonged focal ADs in the perforant path and dentate gyrus.

Chronic benzodiazepine treatment of rats induces reduction of paired-pulse inhibition in CA1 region of in vitro hippocampus

Paired-pulse inhibition was studied extracellularly in in vitro hippocampal slices from rats sacrificed 48 h or 7 days after 1 week flurazepam (FZP) treatment. Population spikes and field excitatory postsynaptic potentials (EPSPs) were recorded with NaCl-containing glass micropipettes in the stratum pyramidale and stratum radiatum, respectively, of the CA1 region. Conditioning pulses were delivered by stimulating Shaffer collaterals (orthodromic) or the alveus (antidromic). Orthodromic test pulses were delivered with interpulse intervals of 10-200 ms. There was a significant reduction in paired-pulse inhibition in slices from treated vs control rats in both the orthodromic-orthodromic and antidromic-orthodromic paradigms. Reduced inhibition was evident 48 h, but not 7 days, after the end of FZP treatment. Furthermore, there was a significant prolongation of the half decay time of the field EPSP, without a significant change in the initial slope or maximum amplitude. The results may suggest an impairment of endogenous gamma-aminobutyric acid function in the hippocampus after chronic benzodiazepine (BZ) treatment and may provide a basis for a mechanism of BZ tolerance.

In vivo intrahippocampal microinfusion of carbachol and bicuculline induces theta-like oscillations in the septally deafferented hippocampus

In their laboratory the authors have previously demonstrated that hippocampal slices could be induced to generate trains of "theta-like" oscillations by whole-bath perfusions of carbachol. Until recently, it has not been possible to generate similar activity in the septally deafferented hippocampus of an otherwise intact brain by microinfusions of carbachol. This study presents a full report of the first demonstration of a theta-like oscillation in the in vivo, septally deafferented hippocampal formation. Rats were anesthetized with urethane and implanted with microinfusion cannulae in the region of the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB) and at single or multiple sites in the stratum moleculare of the fascia dentata. The MS/vDBB was microinfused with procaine hydrochloride to produce a reversible suppression lasting for approximately 20 minutes. Intrahippocampal microinfusions of carbachol or bicuculline alone (in the postprocaine condition of the MS/vDBB) failed to produce any theta-like oscillations. The combination of carbachol and bicuculline produced trains of theta-like oscillations during suppression of the MS/vDBB very similar to those seen in the slice preparations. The oscillations were blocked by intravenous administration of atropine sulfate, and they had the same depth profile as that of theta. Theta-on cells were shown to discharge in rhythmic bursts in synchrony with the oscillations. Thus, it would appear that the essential nature of the medial septal input to the hippocampal formation, for the generation of theta field activity in the intact brain, consists of a critical balance between cholinergic and GABAergic circuitry.

Commissural responses of rat retrohippocampal neurons

Synaptic responses of commissurally activated rat subicular and entorhinal neurons were studied intracellularly in vivo by stimulating the contralateral dentate gyrus. The most prominent synaptic responses in both subicular and entorhinal neurons were inhibitory postsynaptic potentials (IPSPs). IPSPs were generated in combination with antidromic spikes and/or excitatory postsynaptic potentials (EPSPs) and orthodromic spikes. No dependency between any two response types were found. Commissurally projecting subicular neurons (identified by the presence of antidromic spikes evoked by contralateral stimulation) were found, extending previous anatomical studies. Commissurally projecting entorhinal neurons were found in layer II, confirming previous anatomical studies. Positive correlations between antidromic spike latency and depth of recording sites supported the interpretation that axons projected along the fiber bundles of the hippocampal commissures and angular bundle to distribute to their targets. Possible circuits that could have mediated the excitatory and inhibitory responses of these retrohippocampal neurons are considered.

Local circuit synaptic interactions between CA1 pyramidal cells and interneurons in the kainate-lesioned hyperexcitable hippocampus

Following kainate (KA)-induced lesions of subfield CA3--a lesion relevant to human temporal lobe epilepsy--remaining pyramidal cells in CA1 display synchronous hyperexcitability associated with a loss of synaptic inhibition. Despite this loss, inhibitory interneurons in CA1 remain viable, and the density and function of GABAergic receptors on the CA1 pyramidal cells are maintained at approximately normal levels. To further evaluate inhibition in this system, the authors examined interactions between pyramidal cells and inhibitory interneurons in paired intracellular recordings. Recordings were carried out in rat hippocampal slices 2-4 weeks following bilateral intraventricular KA injections. The frequency of synaptic interactions between CA1 basket cells and pyramidal cells was lower in hyperexcitable slices than in controls; both synapses in the recurrent inhibitory circuit appeared to be involved. No recurrent excitatory interactions were seen between pyramidal cell pairs in lesioned or normal slices. The weakened interconnections between pyramidal cells and interneurons are consistent with the decreased inhibition previously found in this model. Unexpectedly, strong stimulation, which may directly activate local inhibitory circuitry, was effective in reducing hyperexcitability in KA-lesioned slices. These data suggest that development of recurrent excitatory connections among CA1 hippocampal pyramidal cells contribute little to tissue excitability, and support the hypothesis that a functional uncoupling between inhibitory interneurons and CA1 pyramidal cells is responsible for the seizure-like activity typical of KA-lesioned hippocampus. The data are also consistent with the hypothesis that in the KA model, the structural circuitry needed for inhibition in CA1 is maintained, and can be functionally activated by appropriate stimuli.

Apical dendritic depolarizations and field interactions evoked by stimulation of afferent inputs to rat hippocampal CA1 pyramidal cells

The relationship between orthodromic extracellular field potentials and intradendritic depolarizations in apical dendrites of CA1 pyramidal neurons was investigated using the in vitro slice preparation of rat hippocampus. Orthodromic synaptic field potentials evoked by stimulation of afferent inputs in stratum radiatum or stratum oriens were used to measure extracellular voltage gradients generated over the pyramidal cell axis. Extracellular gradients were of opposite polarity over the region of pyramidal cell apical dendrites in stratum radiatum. The stratum radiatum-evoked gradient was negative towards the apical dendrites and the stratum oriens-evoked gradient negative towards the cell body layer, with gradients reaching values of up to 50 mV/mm over the apical dendritic axis. Intradendritic recordings obtained greater than 150 microns from stratum pyramidale directly measured the subthreshold apical dendritic excitatory postsynaptic potentials evoked by stratum radiatum or stratum oriens stimulation. These ground-referenced recordings were then compared to the transmembrane potential calculated by subtraction of the corresponding extradendritic field potential. Both stratum radiatum and stratum oriens stimulation evoked graded excitatory postsynaptic potentials that could be recorded in apical dendritic impalements up to 265 microns from stratum pyramidale. The calculated transmembrane potential of the stratum radiatum-evoked excitatory postsynaptic potential had a significantly greater rate of rise, peak amplitude, and rate of decay than that of the ground-referenced excitatory postsynaptic potential. In contrast, the rates of rise and decay of the transmembrane potential of the stratum oriens-evoked excitatory postsynaptic potential were reduced with respect to the ground-referenced recording. The peak amplitude of the stratum oriens-evoked transmembrane potential, however, varied according to the polarity of the corresponding extradendritic population spike response recorded in stratum radiatum. These data reveal that synaptic activation of either basal or apical dendrites of CA1 pyramidal cells evokes a depolarization that can be recorded over a substantial region of the apical dendritic arbor. Furthermore, extradendritic field potentials evoked by stimulation of these inputs produce opposite effects on the transmembrane potential of apical dendrites. The magnitude of the accompanying extracellular voltage gradients suggest that these shifts in transmembrane potential reflect ephaptic interactions at the apical dendritic level of pyramidal cells.

Functional reciprocal connections of the rat entorhinal cortex and subicular complex with the medial frontal cortex: an in vivo intracellular study

We used in vivo intracellular recording techniques in the rat in order to determine the properties of projections from the medial frontal cortex to the entorhinal cortex and subicular complex. Three main results were obtained. (1) A high proportion (65%) of neurons within the medial frontal cortex were antidromically activated at short latency (0.4-1.9 ms) by electrical stimulation of the entorhinal cortex or subicular complex. This provided physiological evidence for fast direct projections from the medial frontal cortex to the entorhinal cortex and subicular complex. (2) Clear excitatory postsynaptic potentials (EPSPs) were evoked in 8% of the cells within the entorhinal cortex, subicular complex, or adjacent cortices after electrical stimulation of the medial frontal cortex. (3) The most salient synaptic response was inhibition, as shown by the presence of inhibitory postsynaptic potentials (IPSPs) in 50% of the cells sampled. Similar results were obtained for the reciprocal pathway: 56% of the sampled cells in the entorhinal cortex or subicular complex responded with antidromic spikes to stimulation of the medial cortex; 4% of medial frontal neurons responded to stimulation of the entorhinal cortex or subicular complex with clear EPSPs, and 48% with IPSPs. The latencies of most synaptic responses, 15-25 ms, were inconsistent with monosynaptic activation. This suggests that oligosynaptic relays amplified the signal within or en route to their targets, and/or that cells with more slowly propagating axons were also present but not sampled by the intracellular electrodes. Finally, responsive fast-spiking cells (candidate inhibitory neurons) were encountered within target structures. The results provide evidence that these distant cortical regions are functionally connected in a reciprocal manner, and that both principal and inhibitory neurons are excited by this projection system.

Distribution of quinolinic acid phosphoribosyltransferase in the human hippocampal formation and parahippocampal gyrus

The morphological distribution of quinolinic acid phosphoribosyltransferase (QPRT), the degradative enzyme of the endogenous excitotoxin quinolinic acid, was studied in the human hippocampal formation and parahippocampal gyrus by immunohistochemical techniques. In seven neurologically normal human brains obtained at autopsy, QPRT-immunoreactivity (QPRT-i) was found in both glial cells and neurons. Glial cells exhibiting QPRT-immunoreactivity morphological features of astrocytes, were observed in all hippocampal subfields. The polymorphic layer of the dentate gyrus contained the highest density of QPRT-i glial cells. Numerous QPRT-i glial cells were also found along both sides of the fused hippocampal fissure and in the white matter including the alveus of Ammon's horn, whereas only a few were observed in the granule cell layer and the stratum pyramidale. Neurons containing QPRT-i were found mainly in the subiculum and in the strata oriens and pyramidale of CA1. They were mostly small and polymorphic or fusiform, thus indicating that they may belong to a subpopulation of interneurons. Moderate numbers of QPRT-i glial cells and neurons were also observed throughout layers II-VI of parahippocampal cortex. The localization of QPRT-i in selected glial cells and neurons suggests that in the regions examined these cellular elements might play specific roles in the regulation of quinolinic acid function.

Feed-forward inhibitory potentials and excitatory interactions in guinea-pig hippocampal pyramidal cells

1. The patterns of inhibition in the CA1 region of the rat hippocampus were evaluated by focal proximal and distal stratum radiatum stimulation, during intracellular recording. The characteristics of isolated inhibitory responses and the interactions of excitatory and inhibitory potentials were analysed. 2. The amplitude of minimal inhibitory postsynaptic potentials (IPSPs) evoked by both proximal and distal stimulation averaged -0.51 +/- 0.24 mV (mean +/- S.D.; n = 32). These responses demonstrated little variability from trial to trial and showed no net trends in amplitude at a stimulation rate of 1 Hz. 3. Minimal IPSPs demonstrated a short latency to onset (2.90 +/- 1.58 ms for proximal and 3.64 +/- 1.39 ms for distal) at stimulation levels which were insufficient to evoke an extracellular field potential. Thus, minimal IPSPs were recruited through feed-forward circuitry, based on the rapid onset and the lack of activation of recurrent collaterals. 4. The minimal IPSPs showed a similar 10-90% rise time for proximal and distal responses. However, the half-width and decay time constant (from the peak) were more prolonged for the distal stimulation, indicating that a late IPSP component was evoked primarily by the distal stimulation. This late component was not observed in isolation at low stimulation intensities. 5. The conductance transient underlying the IPSPs was calculated using a neurone cable model. The proximal IPSP responses were simulated by an alpha input function (at the soma) with a peak conductance in the range of 2.5-45 nS (alpha = 1.75; reversal potential -1.0 to -10 mV). The distal IPSP shape was only partially reproduced by a longer single transient (alpha = 1.25), suggesting the presence of a second, slower component. However, insufficient data on this slower component precluded a more exact simulation of the distal IPSP response. 6. Analysis of interactions between minimal excitatory postsynaptic potentials (EPSPs) and IPSPs showed that often the rising as well as the falling phase of the EPSP could be affected by the IPSP. At small stimulation levels, minimal EPSPs and IPSPs were closely overlapping, but the IPSPs were of significantly longer duration than EPSP responses. 7. The composite EPSP waveform shape became progressively truncated with increasing afferent stimulation. Feed-forward inhibition limited the time course of excitation to a narrow window, approximately 3-5 ms wide. Feed-forward postsynaptic inhibition significantly limited both the duration and the overall efficacy of small EPSPs in CA1 pyramidal neurones.

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.

Membrane properties of interneurons in stratum oriens-alveus of the CA1 region of rat hippocampus in vitro

The membrane properties of interneurons situated near the border of stratum oriens and the alveus of the CA1 region were examined with intracellular recording and staining in rat hippocampal slices in vitro. Cellular staining with Lucifer Yellow indicated that the somata of these interneurons were multipolar and their dendrites projected horizontally along the alveus and vertically toward stratum lacunosum-moleculare. Intrinsic properties (input resistance, action potential amplitude, time constant) and spike after-potentials were typical of non-pyramidal cells. Action potential duration, however, was of relatively medium duration (1.15 ms) and slow afterhyperpolarizations followed depolarization-induced trains of action potentials. Spontaneous activity of interneurons was prominent and of either of two types: single action potentials or high frequency bursts of action potentials. Interneurons displayed marked, voltage- and time-dependent inward rectification and anodal break excitation. Analysis of the slope of the charging function of hyperpolarizing transients, suggested that these interneurons were electrically compact (dendrite to soma conductance ratio, p approximately 2.7; and electrotonic length constant, L approximately 1.1). Characteristically, interneurons sustained high frequency repetitive firing during long depolarizing pulses. The slope of the frequency-current relation was 442 Hz/nA for the first interspike interval and 117 Hz/nA for later intervals (no. 60), suggesting the presence of spike frequency adaptation. Physiologically, these interneurons resembled more closely basket cells of stratum pyramidale than stellate cells of stratum lacunosum-moleculare.

Evidence for a chronic loss of inhibition in the hippocampus after kindling: electrophysiological studies

Rats were kindled with either of 2 protocols: (1) a rapidly recurring hippocampal seizure (RRHS) paradigm in which 10 sec stimulus trains were delivered every 5 min through hippocampal electrodes; and (2) a traditional approach in which 1 sec stimulus trains were given to the amygdala once daily. Three groups of kindled rats were prepared: (1) one of amygdala-kindled rats that had experienced 9-15 seizures; (2) one of RRHS-kindled rats that had experienced 96 seizures; and (3) one of RRHS-overkindled rats that had experienced 144-336 seizures. After a 1 month seizure-free period, the animals were anesthetized with urethane and measurements were made on the potency of paired pulse inhibition in the CA1 region of the hippocampus. All groups of kindled animals were found to have significantly less paired pulse inhibition than control rats. This decrement was confined to interpulse intervals less than or equal to 70 msec. The amount of inhibition lost correlated with the number of seizure that had occurred. The GABAergic agonist muscimol restored paired pulse inhibition in kindled animals for interpulse intervals less than or equal to 70 msec towards normal values. These results indicate that not only RRHS, but also other modes of kindling, reduced GABAergic inhibition in the CA1 region of the hippocampus and that this diminution was long-lasting, if not permanent.

Function of synapses in the CA1 region of the hippocampus: their contribution to the generation or control of epileptiform activity

1. In the kainic acid lesioned hippocampus there is a loss of functional inhibition that is associated with reduction of the IPSPs recorded intracellularly from the surviving CA1 pyramidal cells. The possible pre- or postsynaptic origin of this change has been investigated. 2. Iontophoretic application of GABA to the soma and dendrites of CA1 pyramidal cells indicated that there had been no change in the efficacy of the postsynaptic GABA receptors on these cells. 3. Although a pre-synaptic mechanism is implicated, at one week post lesion we were unable to find any difference in the Ca+ dependent K+ evoked release of endogenous GABA. However, at survival times greater than 1 week immunohistological studies showed a decrease in the number of somatostatin positive non-pyramidal cells in the stratum oriens of the CA1 area. 4. In addition to the reduction of functional inhibition, changes in excitatory neurotransmitter mechanisms were also found to contribute to the epileptiform burst discharge. A slow component of the epileptiform EPSP recorded from CA1 pyramidal cells has been recorded and was found to be antagonized by the NMDA-receptor antagonist D-APV. 5. Methods of controlling epileptiform activity in the kainic acid lesioned hippocampus have been tested. Stimulation of the substantia nigra and ventral tegmental areas produced profound inhibition of pyramidal cell activity in control hippocampi; however, they, were found to be ineffective in controlling the epileptiform burst. 6. A second method involved the use of hippocampal suspension grafts. Whilst this approach has yielded some encouraging data, further studies are necessary before the mechanism of the improvement in inhibitory synaptic function can be explained.

Decrease of recurrent and feed-forward inhibitions under high pressure of helium in rat hippocampal slices

The effect of high helium pressure on inhibitory synaptic transmission was studied in rat hippocampal slices with extracellular recordings. Both feed-forward and recurrent GABAergic inhibition were tested in the CA1 region with paired-pulse stimulation paradigms. The efficiency of both types of inhibition decreased under high pressure (80 atm). However, the depression of synaptic and antidromic field potentials induced by perfusion of GABA or muscimol were not significantly affected by pressure. High pressure induced hyperexcitability of CA1 pyramidal cells. This effect was reduced by the application of 2-aminophosphonovalerate or GABA. The present results suggest that: (1) high pressure reduces the efficiency of the GABAergic inhibitory transmission but does not affect the sensitivity of GABAA receptors; (2) two different processes (reduction of GABAergic inhibition and facilitation of N-methyl-D-aspartate-mediated excitation) might be a direct consequence of the change in the voltage-sensitive ion channels under high pressure and might be involved in the development of the pressure-induced hyperexcitability of CA1 pyramidal cells.

Ultrastructure of stratum lacunosum-moleculare interneurons of hippocampal CA1 region

Intracellular recordings were obtained from nonpyramidal neurons (interneurons) in stratum lacunosum-moleculare (L-M) of the CA1 region of guinea pig hippocampal slices. These interneurons had response characteristics that distinguish them from pyramidal cells and other interneuron types: the L-M neurons had relatively broad action potentials with large spike afterhyperpolarizations, high input resistance and little spike-firing adaptation, and low spontaneous activity. Lucifer Yellow (LY) and horseradish peroxidase (HRP) were injected intracellularly into physiologically identified L-M interneurons, and the cells were characterized morphologically using light and electron microscopy. L-M somata were fusiform-shaped (15 x 25 micron), had multiple processes, and were located at the border between stratum (str.) lacunosum-moleculare and str. radiatum. L-M dendrites coursed through str. lacunosum-moleculare and projected into str. radiatum. L-M axons made axodendritic synaptic contacts primarily in str. lacunosum-moleculare and str. radiatum, but also in str. moleculare of the dentate gyrus. These axodendritic synaptic contacts were made onto spiny dendritic processes (presumably pyramidal cell and granule cell dendrites) and onto aspinous dendrites (presumably interneuron dendrites), and appeared to be of the symmetric type (type 2), characteristic of inhibitory synapses. In separate groups of animals, selective lesions were made of afferents to the CA1 and dentate regions of hippocampus, and subsequent degeneration of contacts and L-M interneuron somata and dendrites was examined at the ultrastructural level. Fibers originating from contralateral and ipsilateral CA3 region, and from ipsilateral entorhinal cortex, were found to make synaptic contact onto presumed L-M interneurons. Degenerating terminals appeared to be of the asymmetric type (type 1), characteristic of excitatory synapses. These morphological data are consistent with electrophysiological results showing that L-M interneurons can mediate feedforward inhibition of CA1 pyramidal cells.