Pharmacological characteristics of facilitation of hippocampal population spikes by cholinomimetics

In rats under urethane anaesthesia, various cholinomimetics, acetylcholine-antagonists and other agents were released iontophoretically in the pyramidal layer of area CA1. Like acetylcholine, a variety of cholinomimetics readily enhanced population spikes evoked by fimbrial-commissural stimulation. Judging by the equipotent iontophoretic currents, the strongest muscarinic agonist was muscarine. Other potent agonists included carbachol. methacholine, propionylcholine, bethanechol and the much slower-acting arecoline, pilocarpine and oxotremorine. Choline was about 5 times weaker than acetylcholine. Though not as effective as acetylcholine, some nicotinic agonists also consistently enhanced population spikes, particularly dimethylphenylpiperazinium and acetylthiocholine. Other nicotinic agents, such as butyrylcholine, nicotine and tetramethylammonium were much less active. Both scopolamine and atropine, given systematically in high doses (10–80 mg/kg), strongly depressed or abolished the action of muscarinic agonists, but to a lesser and more variable extent the action of ACh. They did not antagonize dimethylphenylpiperazinium. When applied iontophoretically. α-bungarotoxin, tubocurarine or mecamylamine did not block the action of any of the cholinomimetics. Indeed, in higher doses they tended to promote population spikes (a comparable enhancement was also seen with larger iontophoretic doses of atropine or scopolamine). On the other hand, gallamine and dihydro-β-erythroidine antagonized muscarine but not dimethylphenylpiperazinium; a less selective block of cholinomimetics was produced by suxamethonium.It was concluded that both muscarinic and nicotinic receptors (or receptors with mixed properties) appear to be involved in the facilitatory action of acetylcholine on population spikes evoked by fimbrialcommissural stimulation.     

Effects of bilobalide, a sesquiterpene in Ginkgo biloba leaves, on population spikes in rat hippocampal slices

The effects of bilobalide, a sesquiterpene isolated from the leaves of Ginkgo biloba L., were investigated in a rat hippocampal slice preparation. Bilobalide (10-500 microM) significantly increased the amplitude of population spikes evoked by electrical stimulation of Schaffer collateral/commissural fibers in a concentration-dependent manner. Paired-pulse inhibition at interpulse intervals of 10-50 ms was significantly reduced in the presence of bilobalide (50 microM). The inhibitory action of muscimol (1 microM) was attenuated by bilobalide (100 microM). These results suggest that bilobalide induces an enhancement of excitability of CA1 pyramidal neurons, which involves, at least in part, a reduction in GABAergic inhibition in rat hippocampus.     

Facilitation by acetylcholine of tetrodotoxin-resistant spikes in rat hippocampal pyramidal cells

The electrical activity of hippocampal pyramidal cells was studied in slice cultures during blockade of the regenerative Na currents. In the presence of tetrodotoxin, these neurones had a mean resting potential of -68 mV, a membrane input resistance of 87 M omega and displayed marked non-linearities in their current voltage relationship. In response to depolarizing stimuli, pyramidal cells generated action potentials of small amplitude, slow rise and long duration. These tetrodotoxin-resistant spikes were abolished by calcium conductance blockers such as cobalt and cadmium ions. Acetylcholine applied to the bath or by iontophoresis depolarized pyramidal cells, elicited spontaneous tetrodotoxin-resistant spikes and facilitated spiking evoked by depolarizing rectangular current pulses or a current ramp. The effects of acetylcholine were not only slow in onset, but also prolonged; they were completely reversible and sensitive to atropine and calcium-antagonists such as cadmium and cobalt ions which, respectively, reduced and abolished these effects. After hyperpolarizations following injection of depolarizing current pulses were suppressed by acetylcholine and often transformed into depolarizing afterpotentials. Acetylcholine had no effect on voltage-independent conductances as determined by application of hyperpolarizing current pulses. These results could be explained by inhibition of the voltage-dependent K+-current, i.e. the M current (blockade of the calcium current could remove any depolarizing influence resulting from M current inhibition) or by a direct activation of a voltage-dependent calcium current by muscarinic agonists.     

Multisecond periodicities in basal ganglia firing rates correlate with theta bursts in transcortical and hippocampal EEG

Multisecond oscillations in firing rate with periods in the range of 2-60 s (mean, 20-35 s) are present in 50-90% of spike trains from basal ganglia neurons recorded from locally anesthetized, immobilized rats. To determine whether these periodic oscillations are associated with similar periodicities in cortical activity, transcortical electroencephalographic (EEG) activity was recorded in conjunction with single- or dual-unit neuronal activity in the subthalamic nucleus (STN) or the globus pallidus (GP), and the data were analyzed with spectral and wavelet analyses. Multisecond oscillations in firing rates of 31% of the STN neurons and 46% of the GP neurons with periodicities significantly correlated with bursts of theta (4-7 Hz) activity in transcortical EEG. Further recordings of localized field potentials in the hippocampus and frontal or parietal cortices simultaneously with GP unit activity showed field potentials from the hippocampus, but not from the frontal or parietal cortices, exhibited bursts of theta rhythm that were correlated with GP firing rate oscillations. These results demonstrate a functional connectivity between basal ganglia neuronal activity and theta band activity in the hippocampus.     

Contribution of single-unit spike waveform changes to temperature-induced alterations in hippocampal population spikes

Brain temperature changes accompany exploratory behavior and profoundly affect field potential amplitudes recorded in hippocampus. The waveform alterations in fascia dentata include a reduction in population spike area, which might be explained by fewer granule cells firing in response to a given stimulus or by an alteration in the size or shape of the individual action potentials. This study was designed to assess these alternate possibilities. In experiment 1, changes in the shape and firing rates of single cells recorded in the fascia dentata of awake rats were compared with changes in the population spike before and after a bout of activity. Single-unit amplitudes were significantly reduced following exploration, and there was a small (< 3%) change in unit spike-width. These changes, however, were insufficient to account, in a linear fashion, for the entire decline in the population spike. In experiment 2, radiant heat was used to manipulate brain temperature in anesthetized rats. As in the first experiment, the magnitude of change in the extracellular units was much smaller than the change in population spike amplitude. The spontaneous firing rates of the cells were also modified by brain temperature changes. In experiment 3, the polysynaptic, contralateral commissural response (which covaries with changes in the ipsilateral population spike at a fixed temperature) was measured as a function of either exploratory behavior or radiant heat. The relationship between the ipsilateral population spike and corresponding polysynaptic commissural response was altered following exploration and passive warming in a manner consistent with a reduction in net granule cell output, reduced transmission efficacy through the polysynaptic circuit, or a combination of these. Taken together these data suggest that at least two factors contribute to temperature-dependent changes in the perforant path-evoked population spikes recorded in the fascia dentata: changes in the size of individual action potentials and alterations in discharge of action potentials in response to a given stimulus.     

Models of synchronized hippocampal bursts in the presence of inhibition. I. Single population events

1. We constructed model networks with 520 or 1,020 cells intended to represent the CA3 region of the hippocampus. Model neurons were simulated in enough detail to reproduce intrinsic bursting and the electrotonic flow of currents along dendritic cables. Neurons exerted either excitatory or inhibitory postsynaptic actions on other cells. The network models were simulated with different levels of excitatory and inhibitory synaptic strengths in order to study epileptic and other interesting collective behaviors in the system. 2. Excitatory synapses between neurons in the network were powerful enough so that burst firing in a presynaptic neuron would evoke bursting in its connected cells. Since orthodromic or antidromic stimulation evokes both a fast and a slow phase of inhibition, two types of inhibitory cells were simulated. The properties of these inhibitory cells were modeled to resemble those of two types of inhibitory cells characterized by dual intracellular recordings in the slice preparation. 3. With fast inhibition totally blocked, a stimulus to a single cell lead to a synchronized population burst. Thus the principles of our epileptic synchronization model, developed earlier, apply even when slow inhibitory postsynaptic potentials (IPSPs) are present, as apparently occurs in the epileptic hippocampal slice. The model performs in this way because bursting can propagate through several generations in the network before slow inhibition builds up enough to block burst propagation. This can occur, however, only if connectivity is sufficiently large. With very low connection densities, slow IPSPs will prevent the development of full synchronization. 4. We performed multiple simulations in which the fast inhibitory conductance strength was kept fixed at various levels while the strength of the excitatory synapses was varied. In each simulation, we stimulated either one or four cells. For each level of inhibition, the peak number of cells bursting depended sensitively on excitatory synaptic strength, showing a sudden increase as this strength reached a critical level. The critical excitation, which depended on the level of inhibition, corresponded to the level at which bursting can propagate from cell to cell at the particular level of inhibition. 5. We performed an analogous series of simulations in which the strength of excitatory synapses was held constant while the strength of fast inhibitory synapses was varied, stimulating a single neuron in each case. These simulations correspond to experiments that have been done in the hippocampal slice as low doses of picrotoxin are washed into a slice, gradually abolishing fast inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)     

Septal unit activity and hippocampal EEG during the sleep-wakefulness cycle of the rat

The functional relationship between septal unit activity and hippocampal EEG was studied during the sleep-wakefulness cycle in chronically implanted unrestrained rats. Results were statistically analyzed in order to obtain the time interval and auto and crosscorrelation histograms. During REM sleep and restless wakefulness the hippocampal EEG showed a theta rhythm of 4–8 cps and the septal unit activity was characterized by the presence of rhythmic bursts with a frequency in every single instance similar to that of the hippocampus. The septal units showed a bimodal time interval histogram. Their autocorrelation histogram exhibited a sinusoidal pattern of decreasing amplitude. The crosscorrelation histogram of septal units and the hippocampal EEG also showed a sinusoidal pattern suggestive of the functional relationship between both neural structures. During slow wave sleep and quiet wakefulness there was a disappearance of the hippocampal theta rhythm and rhythmic septal unit bursts. The second mode was missing in the time interval histogram and the auto and crosscorrelation histograms showed an irregular pattern. It is concluded that medial septal nucleus and hippocampal temporal patterns of activity are similar during REM sleep and restless wakefulness.     

Models of synchronized hippocampal bursts in the presence of inhibition. II. Ongoing spontaneous population events

1. We extended our computer model of the CA3 region of the hippocampal slice in order to study spontaneous activity occurring in the presence and absence of synaptic inhibition. This was done by providing a steady inward current to the excitatory neurons, whose value was randomly chosen for each cell. With the parameters used, many of the excitatory cells would, if synaptically isolated, remain quiescent, whereas others would burst periodically with periods as brief as 750 ms. Simulations were run for as long as 10 s of neural activity. 2. In the presence of synaptic inhibition, neural activity became organized into recurring, partially synchronized events: clusters of neurons (6% to 12% of the population) would discharge together, with a period averaging 340 ms, shorter than the burst period of any individual neuron. A consequence of periodic clusters of cellular bursts was the widespread occurrence of periodic synchronized synaptic potentials, as have been observed in hippocampal slices and human temporal neocortical slices. The periods between these synaptic potentials are similar in the model to those observed experimentally. 3. The period could be slowed by either increasing the time constant of the slow inhibitory postsynaptic potential (IPSP), or by making the excitatory synapses more powerful. The period seems to be generated in part as follows. Consider those cells with rapid spontaneous discharge rates. An upper bound for the period corresponds to the interval between 1) such a cell's becoming responsive enough to an excitatory synaptic input to burst, and 2) such a cell's bursting spontaneously (i.e., in response to its own intrinsic inward current). For cells with rapid spontaneous discharge rates, the interval defined in this way is approximately 350 ms. 4. Different cells participated in each cluster. A given cluster was initiated by one cell or by two cells bursting together, and spread via excitatory synapses. Excitatory synaptic paths could be traced from the initiating cell(s), directly or through other participants, to all cells participating in a cluster. Spread of activity was limited by two mechanisms, so that not all cells synaptically excited by a participating cell would themselves participate. First, cells might be refractory from having participated in a recent cluster (since the intercluster period was less than the refractory time from a cellular burst to its responsiveness to a synaptic stimulus). Second, some cells might be synaptically inhibited. Synaptic inhibition in this model did not act rapidly enough to suppress the cluster totally.(ABSTRACT TRUNCATED AT 400 WORDS)     

Distribution of neuropeptide Y receptors in the rat hippocampal region

The distribution of binding sites for neuropeptide Y (NPY) was studied in the rat hippocampal region by using [3H]NPY together with quantitative in vitro receptor autoradiography. The highest density of specifically bound [3H]NPY was found in regio superior and regio inferior of Ammon's horn. Within these fields, stratum oriens, stratum pyramidale and stratum radiatum harboured the highest densities of [3H]NPY binding while stratum moleculare was relatively poor in [3H]NPY binding sites. In area dentata, the highest density of [3H]NPY binding was found in the inner one third of the molecular layer. In the presubiculum and in the entorhinal area, the outer two layers were slightly more enriched in [3H]NPY binding sites than were the deep layers. In all hippocampal subfields a clear gradient of increased [3H]NPY binding was found at successively more ventral levels.     

The substance P innervation of the rat hippocampal region

Summary The distribution of substance P (SP) immunoreactive nerve cell bodies and preterminal processes was studied in the rat brain by using several anti-SP-antibodies in combination with immunohistochemical techniques. In normal rats and in rats pretreated with colchicine, SP immunoreactive preterminal processes were found in the hippocampal region, but SP positive cellbodies could be detected only after colchicine pretreatment. Medium-sized to large, multipolar cells immunoreactive for SP were found in stratum oriens of the hippocampal subfield CA3 and in the hilus of the area dentata. Medium-sized to small, round or fusiform cells were detected in the pyramidal layer of the ventral subiculum and in layers III–VI of the ventral entorhinal area. The SP stained preterminal processes were of two types. Numerous fine, varicose axons were stained in different parts of Ammon's horn, while in the retrohippocampal structures, the SP immunoreactivity was present in small distinctly stained puncta. These frequently formed pericellular arrangements around unstained cells, indicative of axosomatic contacts between SP terminals and cells in the hipocampus. In Ammon's horn, the densest SP innervation was found in strata oriens, radiatum and moleculare of subfields CA3a and CA2. Scattered fibers were also present in the stratum oriens of CA3a-c and in the hilus, in particular at ventral levels. In retrohippocampal structures, the SP innervation predominated in the deep pyramidal layer of the subiculum, the second layer of the presubiculum and in layers VI and IV of the medial and lateral entorhinal area. Many of these terminals may arise from local interneurons as well as from sources outside the hippocampal region.Taken together, these studies demonstrate a far more extensive innervation by SP, or a closely related peptide, of the rat hippocampal region than was previously recognized. This suggests that SP may play an important role in neurotransmission within the hippocampal region.Stephen Davies was supported by Travel grants from the Wellcome Trust and the Gurantors of Brain.     

Endogenous and network bursts induced by N-methyl-D-aspartate and magnesium free medium in the CA3 region of the hippocampal slice

The epileptogenic properties of N-methyl-D-aspartate and magnesium-free medium were investigated in the CA3 region of the hippocampal slice preparation in the rat. Bath application of N-methyl-D-aspartate (5-10 microM) or magnesium-free medium induced both spontaneous and stimulus-evoked bursts. Both endogenous and network bursts were generated, the former always preceding the latter. The paroxysmal depolarizing shift underlying the network bursts generated by N-methyl-D-aspartate and magnesium-free medium resembled a giant excitatory postsynaptic potential with a reversal potential near 0 mV and a synaptic input in the apical dendrites above the mossy fibre zone. In the presence of N-methyl-D-aspartate or magnesium-free medium, population bursts were synchronized by activating single CA3 neurons. N-methyl-D-aspartate receptor antagonists prevented the development of N-methyl-D-aspartate-induced spontaneous and stimulus-evoked bursts. However, the only N-methyl-D-aspartate receptor antagonist effective in preventing such bursts in magnesium-free medium was DL-3-[(+/-)-2-carboxypiperazin-4-yl-]-propyl-1-phosphonic acid. Endogenous bursting in the CA3 region has not been observed with other convulsants and thus may reflect the novel voltage dependence of the N-methyl-D-aspartate receptor gated ionic channel. N-methyl-D-aspartate receptors may also partially contribute to the excitatory interaction between CA3 neurons and thereby account for the synchronization of the population observed when activating single CA3 neurons.     

Level of arousal during the small irregular activity state in the rat hippocampal EEG

The sleeping rat cycles between two well-characterized hippocampal physiological states, large irregular activity (LIA) during slow-wave sleep (SWS) and theta activity during rapid-eye-movement sleep (REM). A third, less well-characterized electroencephalographic (EEG) state, termed "small irregular activity" (SIA), has been reported to occur when an animal is startled out of sleep without moving and during active waking when it abruptly freezes. We recently found that the hippocampal population activity of a spontaneous sleep state whose EEG resembles SIA reflects the rat's current location in space, suggesting that it is also a state of heightened arousal. To test whether this spontaneous SIA state corresponds to the SIA state reported in the literature and to compare the level of arousal during SIA to the other well-characterized physiological states, we recorded unit activity from ensembles of hippocampal CA1 pyramidal cells, EEG from the hippocampus and the neocortex, and electromyography (EMG) from the dorsal neck musculature in rats presented with auditory stimuli while foraging for randomly scattered food pellets and while sleeping. Auditory stimuli presented during sleep reliably induced SIA episodes very similar to spontaneous SIA in hippocampal and neocortical EEG amplitudes and power spectra, EMG amplitude, and CA1 population activity. Both spontaneous and elicited SIA exhibited neocortical desynchronization, and both had EMG amplitude comparable to that of waking LIA. We conclude based on this and other evidence that spontaneous SIA and elicited SIA correspond to a single state and that the level of arousal in SIA is higher than in the well-characterized sleep states but lower than the active theta state.     

Model of synchronized epileptiform bursts induced by high potassium in CA3 region of rat hippocampal slice. Role of spontaneous EPSPs in initiation

1. We constructed a computer model of the in vitro CA3 region of the rat hippocampal slice bathed in a high-potassium medium. Our aim was to understand better the mechanisms of initiation of synchronized bursts and the processes that regulate the interburst interval in the experimental system. 2. Our model began with a previously published model of the longitudinal CA3 hippocampal slice. The model contains three interconnected cell populations: 9,000 (excitatory) pyramidal cells; 450 inhibitory cells whose postsynaptic action is somatic and decays quickly, corresponding to chloride-dependent inhibition mediated by gamma-aminobutyric acid (GABA)A channels, and 450 inhibitory cells whose postsynaptic action is dendritic, of delayed onset and long lasting, that corresponds to K-dependent inhibition mediated by GABAB channels. 3. The model was then modified to account for specific features of the high-K experimental system: 1) the pyramidal cells do not generate intrinsic bursts; 2) EIPSP(CI) and EK are both shifted in a depolarizing direction; 3) spontaneous (i.e., not caused by presynaptic firing) excitatory postsynaptic potentials (EPSP)s were included; and 4) a steady current was injected into the pyramidal cells to depolarize them. 4. This model generates synchronized population bursts with interburst intervals of approximately 1.0-1.5 s. Bursts in individual pyramidal cells are preceded by barrages of EPSPs. These results agree with experiment. 5. Our model agrees with the following additional experiments: 1) synchronized bursts are abolished by partial blockade of excitatory synapses; 2) burst frequency is increased by partial blockade of a slow-intrinsic-K conductance; and 3) blockade of chloride-dependent inhibition leads to bursts of longer duration with longer interburst intervals. 6. The basic structural features of this model are similar to, but not identical to, the model of the disinhibited hippocampal slice. Spontaneous EPSPs appear to be critical in the high-K system for initiating, but not for synchronizing, population bursts. The experimental data and simulation results raise interesting questions about the role of spontaneous EPSPs in initiating synchronized discharges in other epileptic systems and on the possible role of spontaneous EPSPs in the normal brain.     

Striatal and septal influence on hippocampal theta and spikes in the cat

The experiments studied the modulation exerted by the septum and the caudate nucleus on hippocampal activity in the cat. Injections (i.v.) of sodium penicillin were performed in order to obtain a steady interictal epileptic activity. Hippocampal slow rhythmic activity showed a marked decrease either in duration or in frequency following penicillin activation. Both septal and caudate electrical stimulation inhibited spike frequency through a theta eliciting mechanism. Caudate stimulation failed to determine any sort of effect after medial septum lesions. The importance of the septum as modulation station between basal ganglia and hippocampus is emphasized.