Hippocampal field potentials evoked by stimulation of multiple limbic structures in freely moving rats

Coupling of theta oscillations between anterior and posterior midline cortex and with the hippocampus in freely behaving rats

Theta oscillations in the hippocampus support cognitive processing. Theta-range rhythmicity has also been reported in frontal and posterior cortical areas--where it tends to show consistent phase-relations with hippocampal rhythmicity. Theta-range rhythmicity may, then, be important for cortico-cortical and/or cortico-hippocampal interactions. Here, we surveyed the rat frontal and posterior midline cortices for theta-related oscillations and examined their relationships with hippocampal activity in freely moving rats. Variation in electroencephalography across 4 general classes of spontaneous behavior demonstrated different profiles of theta-like activities through the rat midline cortices. Analysis of cortico-cortical and cortico-hippocampal coherences showed distinct, behavior-dependent, couplings of theta and delta oscillations. Increased theta coherence between structures was most obvious during nonautomatic behaviors and least during immobility or grooming. Extensive coupling of theta oscillations throughout the rat midline cortices and hippocampus occurred during rearing, and exploratory behavior. Such increases in coherence could reflect binding of cortico-hippocampal pathways into temporary functional units by behavioral demands. Extensive coupling of frontal delta, which lacked coherence with posterior areas (including the hippocampus), suggests that different frequencies of rhythmicity may act to bind groups of structures into different functional circuits on different occasions.

Elicited hippocampal theta rhythm: a screen for anxiolytic and procognitive drugs through changes in hippocampal function?

Hippocampal damage produces cognitive deficits similar to dementia and changes in emotional and motivated reactions similar to anxiolytic drugs. The gross electrical activity of the hippocampus contains a marked 'theta rhythm'. This is a relatively high voltage sinusoidal waveform, resulting from synchronous phasic firing of cells, variation in which correlates with behavioural state. Like the hippocampus, theta has been linked to both cognitive and emotional functions. Critically, it has recently been shown that restoration of theta-like rhythmicity can restore lost cognitive function. We review the effects of systemic administration of drugs on hippocampal theta elicited by stimulation of the reticular formation. We conclude that reductions in the frequency of reticular-elicited theta provide what is currently the best in-vivo means of detecting antianxiety drugs. We also suggest that increases in the power of reticular-elicited theta could detect drugs useful in the treatment of disorders, such as dementia, that involve memory loss. We argue that these functionally distinct effects should be seen as indirect and that each results from a change in a single form of cognitive-emotional processing that particularly involves the hippocampus.

Responses of hippocampal neurons at different stages of acquisition of conditioned reflex avoidance in rats

The electrophysiological properties of hippocampal slices from trained and untrained rats presented with identical numbers of combinations during acquisition of a conditioned two-sided avoidance reaction were compared. Associative learning (presentation of combinations of conditioned and unconditioned stimuli) was accompanied by gradual increases in the amplitude of the population spike in hippocampal field CA1. However, achievement of the learning criterion was accompanied by a decrease in the amplitude of the population spike. The same type of difference between groups was seen in field CA1 using low-intensity stimulation of Schaffer collaterals, while there was no change in the amplitude at maximum-intensity stimulation. After the rats achieved the learning criterion, the level of facilitation in conditions of paired stimuli was significantly greater than in untrained animals and also as compared with the passive and active controls. These data provide evidence supporting the hypothesis that changes in the efficiency of synaptic transmission occur during the learning process. However, the direction of changes depends on the extent of training.

Septohippocampal disinhibition

Intracellular recordings from CA1 and CA2/3 neurons in rats under urethane anesthesia revealed the following effects of medial septal stimulation (10 pulse trains at approximately 100 Hz): (1) in most cases only minimal signs of any synaptic potential; (2) a marked and prolonged (200-500 ms) depression of on-going inhibitory postsynaptic potentials (IPSPs), particularly evident when IPSPs were reversed by Cl- injection; (3) a corresponding increase in input resistance: (4) depolarization when recording with non-Cl(-)-containing electrodes; (5) a predominant hyperpolarization when recording with Cl(-)-containing electrodes; and (6) a marked reduction of the variability of resistance and voltage data. These observations indicate that septal stimulation can strongly depress tonic inhibition in the hippocampus. Septal trains also tended to weaken IPSPs evoked in pyramidal cells by fimbrial stimulation, reducing conductance increase during IPSPs by an average of 42% (S.D. +/- 24.3). Septal inputs to the hippocampal CA1 and CA2/3 regions appear to have a major disinhibitory function.

Sequential dependencies regulate sensory evoked responses of single units in the rat hippocampus

Unit activity from 3 major types of hippocampal cells were recorded from the CA1, CA3 and dentate granule cell layers of the hippocampus in awake freely moving rats. Units were classified as complex spike (ComSp), theta and dentate granule cells (G-cell) based on their spontaneous and stimulus-evoked firing characteristics. Single trial records for each cell type were collected during criterion performance of a two-tone discrimination task. Results showed that: tone-evoked discharges of theta- and G-cells, but not ComSp cells, were influenced by the reward status of the tone presented; the firing tendency of all cell types was significantly influenced by the preceding trial sequence; different types of preceding trial sequences including single and double alternation and runs of positive or negative trials significantly affected the firing tendency of all 3 cell types; the pattern of theta- and G-cell discharges differed with respect to latency to peak discharge and the duration of the discharge following long runs of similar trials. These effects were similar to previously described sequential influence on synaptically identified sensory evoked potentials in the dentate gyrus.

Enhanced long-term potentiation induced in rat dentate gyrus by coactivation of septal and entorhinal inputs: temporal constraints

High-frequency activation of the entorhinal cortical (perforant path) inputs to the rat dentate gyrus can produce a long-term potentiation (LTP) of perforant path-dentate evoked responses. In this paper we examined the enhanced LTP effects produced by coactivation of septal and entorhinal inputs to the dentate gyrus. Trains of electrical stimulation applied to the two inputs were found to increase the magnitude of LTP to a level above that produced by trains applied to the perforant path alone. The largest LTP increments were observed when the septal trains were applied less than 100 ms prior to the perforant path trains. If the septal trains followed the perforant path trains there was no additional increment in LTP magnitude, regardless of the intertrain interval. The relationship of this cooperativity effect to mechanisms of associative learning is discussed.

Hippocampal theta rhythm and the firing of neurons in walking and urethane anesthetized rats

Recordings were taken from single neurons in the hippocampus and dentate gyrus of rats during walking and urethane anesthesia. Firing histograms for these cells were constructed as a function of the phase of the concurrent extracellularly recorded hippocampal slow wave theta rhythm. Care was taken to be sure of the site of recording of the theta rhythm and its phase with respect to a reliable reference, so that comparisons of the phases of firing could be made across animals. The firing of most of these neurons is deeply modulated as a function of the phase of the theta rhythm. This is true whether the theta rhythm occurs during walking or during urethane anesthesia, but for some types of cells the mean phases of firing are different in the two types of theta rhythm. During walking, pyramidal cells and interneurons in all hippocampal subregions and dentate granule cells have a maximum probability of firing near the positive peak of the theta rhythm recorded in the outer molecular layer of the dentate (dentate theta). During urethane anesthesia, the maximum firing probability for interneurons in CA1 and for dentate granule cells occurs near the negative peak of the dentate theta, while the phases of maximum firing for pyramidal cells and interneurons in CA3 and CA4 become widely distributed. The phases of maximum firing of pyramidal cells in CA1 are, if anything, more narrowly distributed around the positive peak of the dentate theta during urethane anesthesia than during walking. These differences in the firing of hippocampal cells during walking and urethane anesthesia represent some of the differences in cellular mechanisms distinguishing two kinds of hippocampal theta rhythm.

Unit activity, evoked potentials and slow waves in the rat hippocampus and olfactory bulb recorded with a 24-channel microelectrode

Activity from a number of neighboring neurons can be recorded simultaneously with multichannel microelectrodes. A new version of a 24-channel microelectrode system has been fabricated and used to record different types of neurophysiological data in the rat brain. The system called PRONG (Parallel Recording Of Neural Groups) includes a microelectrode, a lightweight reusable connector, a 24-channel FET-hybrid preamplifier, a 3-band 24-channel amplifier, a 24-channel spike monitor, high-speed digital and analog interfaces and a computer. The electrode-recording locations are arranged in 2 arrays of 12 sites. The arrays are spaced 100 micron apart along either edge of the recording section and the sites within each array are spaced 120 micron apart. The electrodes are fabricated using photolithography in patterned layers totaling 17 micron thick and 114 micron wide in the recording section. The recording sites are 20 micron2 and are plated with platinum black. Performance of the PRONG was compared with that of conventional single microelectrodes and with results in the literature on three kinds of extracellular activity in the rat hippocampal formation and olfactory bulb: action potentials, evoked field potentials and slow-wave activity. The selectivity and sensitivity of the PRONG compared favorably with characteristics of conventional electrodes. Background noise averaged 15 microV and no signal cross talk was observed between neighboring channels. Discriminable action potentials (signal-to-noise ratios of 2:1 to 15:1) were observed at 37-95% of the viable recording sites with a maximum of 19 units in one recording. Units were observed in waking animals for up to 4 days. The waveforms, firing repertoires and laminar distribution of units were the same as those recorded with conventional microelectrodes. This indicates that penetration by the PRONG spares tissue from functional damage. "Instant" laminar profiles were created for commissural and perforant path evoked potentials in the hippocampal formation. These profiles were nearly identical with those created by successive recordings made with conventional microelectrodes. Laminar profiles and behavioral activity appeared to be "normal" as collected with this electrode. These results set the foundation for use of the PRONG as a tool for the study of local neural interactions.

Effects of physostigmine on septo-hippocampal averaged evoked field potentials

Field potentials were elicited in the CA3 field of freely-moving animals by paired-pulse stimulation of the ventral portion of the medial septum before and after exposure to physotigmine. Physostigmine had no effect on response components elicited by a single stimulus or by the first stimulus of a pulse pair. However, the amplitude of a late positive component elicited by the second stimulus of a pair was increased significantly following physostigmine. These results suggest that activity in a cholinergic portion of the septo-hippocampal pathway may serve to modulate the responsiveness of neurons in the CA3 field to subsequent stimulation.

Electrophysiological characteristics of hippocampal complex-spike cells and theta cells

Stimulating electrodes were chronically implanted in the ventral hippocampal commissure and the entorhinal cortex or angular bundle of rats. Moveable metal microelectrodes which could be passed through the hippocampus were implanted. All hippocampal units were classified s complex-spike cells or theta cells on the basis of the form of their action potentials and their rates of firing in various behaviors. Field potentials and unit firing evoked from the stimulating electrodes were recorded during slow wave sleep. Complex-spike cells (1) could often be antidromically activated in CA3 (it was not attempted in CA1); (2) could only be induced to fire one or two action potentials in response to a single stimulus; (3) had action potentials at the same time as the local population-spike and, in condition-test studies, were depressed when the population-spike was depressed. (The population-spike is presumably the summed synchronous action potentials of pyramidal cells.) Theta cells: (1) were antidromically activated in only one out of 25 cases; (2) usually could fire long bursts of action potentials in response to a sufficiently intense single stimulus; (3) this firing occurred before, during, and after the local orthodromic population-spike. Most complex-spike cells in Ammon's horn must be pyramidal cells (projection cells), and vice versa. The case for theta cells is more difficult. Some are non-pyramidal cells with locally ramifying axons, but at least some are projection cells. The data is consistent with most of them being inhibitory interneurons, but this is not established.