P. Andersen

Unit analysis of hippocampal population spikes

Summary1.The assumption that the negative extracellular population spikes recorded from the pyramidal or granular layers in the hippocampal formation in response to appropriate afferent volleys is due to the sum of individual unitary discharges was tested by recording unit activity and population spike with microelectrodes, using normal and ultrashort amplifier time constants.2.Unit spikes were correlated in time with the population spike.3.The size of the population spike was altered by varying the stimulus strength, or by using a conditioning-test technique. In either case the number of units recorded followed the change in the size of the population spike. For very large population spikes, the technique failed since unit spikes could then no longer be clearly distinguished.4.The theoretically expected shape of the population spike as a summation of individual unit discharges was derived, and discussed in terms of the observed results.5.We conclude that over a wide range the height of the population spike is an increasing function of the number of discharging cells and can thus be used as a measure of the extent to which an afferent volley discharges a cell population.

Two different responses of hippocampal pyramidal cells to application of gamma-amino butyric acid.

1. Extra‐ and intracellular recordings were made from CA1 cells in hippocampal slices in vitro. The effects of ionophoretically applied GABA on somatic and dendritic regions were studied. 2. Ionophoresis of GABA at dendritic sites gave a reciprocal effect by inhibiting the effect of excitatory synapses close to the dendritic application, while facilitating those lying further away. For example, GABA delivered to the mid‐radiatum dendritic region reduced the population spike generated by a radiatum volley, while facilitating the population spike evoked by oriens fibre stimulation. Similarly, when single cells were recorded from, mid‐apical dendritic delivery of GABA abolished the synaptically driven discharges evoked by fibres terminating at this part of the dendritic tree, but facilitated the responses to input from fibres terminating on the basal dendrites of the same cell. 3. With intracellular recording two effects were observed. Applied near the soma, GABA induced a hyperpolarization associated with an increased membrane conductance. When applied to dendrites, GABA caused a depolarization also associated with an increased membrane conductance. Both types of GABA applications could inhibit cell discharges, although in some cases the depolarizing response could facilitate other excitatory influences or cause cell firing by itself. 4. Both the hyperpolarizing and depolarizing GABA responses persisted after blockade of synaptic transmission by applying a low calcium high magnesium solution, indicating mediation via a direct effect upon the cell membrane. 5. The reversal potential for the hyperpolarizing GABA effect was similar to the equilibrium potential for the i.p.s.p. evoked from alveus or orthodromically, and was 10‐12 mV more negative than the resting potential. The size of the depolarizing response was also dependent upon the membrane potential. By extrapolation an estimated equilibrium potential was calculated as about ‐40 mV. 6. Our results support the idea that the hyperpolarizing basket cell inhibition at the soma is mediated by the release of GABA. This hyperpolarizing response causes a general inhibition of firing. The dendritic effects of GABA, however, seem to represent another type of inhibition, which by shunting synaptic currents makes possible a selective inhibitory influence on afferents synapsing locally while facilitating more remotely placed excitatory synapses. We propose the term discriminative inhibition for this postulated new type of control of pyramidal cell discharges.

Possible mechanisms for long‐lasting potentiation of synaptic transmission in hippocampal slices from guinea‐pigs.

1. Long‐lasting potentiation of synaptic transmission was studied in the CA1 region of guinea‐pig hippocampal slices maintained in vitro. 2. Stimulating pulses were delivered alternately to two independent afferent pathways, stratum radiatum and stratum oriens. The presynaptic volleys and field e.p.s.p.s. were recorded from the same two layers, while an electrode in the pyramidal cell body layer recorded the population spike or in other experiments the extra‐ or intracellular potentials from a single pyramidal cell. 3. A short tetanus to either of the two input pathways produced a long‐lasting enhancement of the field e.p.s.p. as well as an increased size and a reduced latency of the population spike. This long‐lasting potentiation was observed for up to 110 min after tetanization. Extracellular unit recordings showed that this potentiation is accompanied by an increased probability of firing and a reduced firing latency. Intracellular recordings showed an increased e.p.s.p., through the increase was smaller and less regular than for the extracellular field e.p.s.p. 4. No corresponding changes were seen in the field potential responses to stimulation of the untetanized input path, or in the intracellularly measured soma membrane potential, resistance, or excitability. The latter two properties were measured by intracellular injection of current pulses. It is concluded that long‐lasting potentiation is specific to the pathway which has received the tetanization. 5. Following tetanization there was also a short‐lasting (usually 2‐4 min) depression, most often seen for the control pathway but sometimes visible on the tetanized side as well, superimposed on the potentiation. It is concluded that the short‐lasting depression is not confined to any particular pathway but is a generalized (unspecific) phenomenon.

Location and identification of excitatory synapses on hippoeampal pyramidal cells

Summary1.In rabbits and cats anaesthetized by urethane-chloralose or pentobarbital sodium, stimulation of the commissural afferent pathway produced a negative field potential with maximal amplitude in the CA3 basal dendritic layer, and with a latency indicative of monosynaptic activation of excitatory synapses on the basal dendrites.2.Mossy fibre stimulation resulted in a similar field potential restricted to the mossy fibre layer. Comparable negative field potentials were found in the layer of apical dendrites in CA1 in response to commissural and Schaffer collateral stimulation, suggesting a dendritic location of these synapses.3.All negative field potentials grew in amplitude on tetanic stimulation, to produce large extracellular spikes, indicating their association with excitatory synaptic activity.4.Usually, all pathways employed failed to produce EPSPs on single shock stimulation, in spite of their capability of discharging the cells, suggesting that the synaptic depolarization takes place at some distance from the soma.5.Electron microscopy of degenerated commissural afferent fibres showed them to make contact with spines or the smooth surface of thin dendrites. The indentification of the postsynaptic element as pyramidal cell dendrite was ascertained. The mossy fibres end on ramified dendritic spines in CA3.6.By comparison with normal electron micrographs, all the pathways, shown physiologically to be excitatory, terminate on thin dendrites, the contacts being of type 1.

Two generators of hippocampal theta activity in rabbits

Spontaneous and hypothalamically induced theta activity was studied in rabbits lightly anaesthetized with urethane or urethane-chloralose. Well-developed theta activity was found over a large area of the dorsal part of the hippocampal formation, roughly corresponding to the CA1 field. Cross-correlation analysis between a stationary and a moving electrode showed that a large sheet of tissue oscillated in remarkable synchrony. This region was at least 8 mm along the longitudinal axis of the hippocampus and 6 mm in a plane transverse to this axis, thus comprising the whole of the rostro-caudal extension of the CA1 region. For technical reasons the temporal half was not explored. Depth recordings showed two foci of theta activity, one in the basal part of CA1 (stratum oriens) and a second, separate region with considerably larger amplitudes in the dentate region, having its maximum in the molecular layer. Due to the folded nature of the dentate fascia, an electrode often recorded two maxima corresponding to its upper and lower blades. Wave form analysis showed that the dentrate and CA1 rhythmic activity was roughly 180 degrees out of phase. The dentate theta activity remained in conditions where the CA1 theta activity was absent, either spontaneously or due to experimental interference. Systematic micro-electrode tracking showed absence of theta activity in the CA3 region. Nor was convincing theta activity found in the subiculum, parasubiculum, presubiculum or entohinal areas.

Organization of the hippocampal output

Summary1.The spatial organization of the efferent projections of CA1 and CA3 hippocampal pyramids has been studied using recordings of fibre volleys, orthodromic and antidromic population spikes and synaptic field potentials, following microelectrode stimulation of the fimbria, CA1 alveus, or subiculum.2.Only CA3 pyramidal cells were found to send their axons into the fimbria. In the septal two thirds of the hippocampus the CA1 pyramidal cells project in a caudal direction to the pyramidal part of the subiculum. The temporal third was not explored for technical reasons.3.Fimbrial fibres are arranged in a strictly parallel fashion, the rostro-medial CA3 cells distributing their axons near to the hippocampus, while those located at the temporal extreme distribute their axons to the outer edge of the fimbria. The organization of the Schaffer collaterals and the projections of the CA1 cells consisted of parallel lamellae, oriented nearly transversely to the longitudinal axis of the hippocampus in rabbits (more obliquely in cats). The findings indicate that CA3 cell discharge via the Schaffer collaterals represents a major input driving the CA1 cells.4.The dichotomy with regard to hippocampal output suggests that the CA3 and CA1 regions of the hippocampus may subserve different functions, thus probably participating differentially in various behavioural situations.5.This organization makes it possible to study the behaviour of animals with selective and regional de-efferentation of the CA3 or of the CA1 regions by making discrete lesions in the fimbria and alveus, respectively. Alternatively, recording the fibre volley from the fimbria may provide a useful monitor of the output of the CA3 region during different behaviours.

Functional characteristics of unmyelinated fibres in the hippocampal cortex.

(1) In transverse hippocampal slices (350 micrometer thick), taken from guinea pigs initially anaesthetized with ether, intracortical afferent fibres were activated by small current pulses delivered through tungsten microelectrodes. Extracellular potentials were recorded from the zone of activated fibres in dendritic layers while intracellular recordings were made from the soma of CA1 pyramidal cells. (2) When recording was made from the same level as the stimulating cathode, the extracellular potential consisted of a diphasic deflection followed by a larger negative wave with a superimposed population spike. The negative wave corresponded to an intracellularly recorded EPSP, and is called an extracellular EPSP, whereas the initial diphasic deflection had no intracellular counterpart. (3) The initial diphasic deflection was linearly related to the size of both the intracellular and extracellular EPSP. It was not changed by removal of calcium ions from the bathing fluid, whereas all postsynaptic activity disappeared. The diphasic deflection was propagated along fibres lying parallel to the pyramidal layer with a velocity of 0.3 m/sec. It could follow short bursts of stimulation at 300 Hz. The absolute refractory period was 2.0 msec. (4) The initial diphasic deflection is interpreted as the compound action potential of the largely unmyelinated afferent fibres to the CA1 neurones.

Automated analysis of rhythmicity of physiologically identified hippocampal formation neurons

SummaryThe participation of physiologically identified hippocampal neurons in spontaneous and hypothalamically induced theta activity was studied in rabbits lightly anaesthetized with urethane. Dentate granule cells were identified by their orthodromic response to perforant path stimulation, CA1 and CA3 pyramids by antidromic activation from the alveus and Schaffer collaterals, respectively, and basket cells by their response to increasing orthodromic activation.The discharges of many hippocampal cells were grossly correlated to the pattern of slow wave activity. Few cells were spontaneously active during irregular slow wave activity. With the appearance of rhythmical slow wave activity of 4–6 Hz, the unit discharges also increased in frequency. Dentate granule cells had the lowest threshold for activation and also a longer duration of the increased discharge frequency, compared to other cell types.There was a characteristic pattern of transition for dentate granule cells and CA1 pyramidal cells from a silent to an active state. The cell discharges paralleled the changes in amplitude, regularity, and frequency of theta slow waves. Large-amplitude, high-frequency theta was correlated with rhythmic burst discharges of up to 2–3 spikes per burst. As theta amplitude and frequency decreased, the number of spikes per burst reduced until only regular single spikes occurred. When theta activity was replaced by irregular slow wave activity, the cell discharges became irregular and sometimes ceased entirely. At high levels of activation, CA1 pyramids often showed clusters of high-frequency discharges with declining amplitude (complex spikes).For each cell a cycle histogram was constructed, placing the discharges in one of 20 bins according to their time relation to the simultaneously recorded slow theta waves. In addition, by Fourier transformation of the cycle histograms, the technique allowed a quantitative description of the degree and type of rhythmicity.The analysis indicated that virtually all dentate granule cells and CA1 pyramidal cells were phaselocked to the negative portion of the theta waves recorded from the corresponding region.The cells differed in their degree of coupling, as expressed by the modulation index of their cyclic histograms. Dentate granule cells had higher modulation indices than the CA1 pyramids. There was a suggestion that basket cells and CA3 cells had smaller modulation indices, but the low number of cells recorded mitigate against any strong conclusions.The results are interpreted as corroborating earlier findings that the dentate granule region and the CA1 pyramidal region are the main generators of hippocampal theta activity.A “size principle” was proposed to explain the role of synaptic depolarizing pressure in the rhythmic activation of hippocampal neurons and the fact that small neurons (dentate granules and CA1 pyramids) were better driven than larger neurons.

A comparison of distal and proximal dendritic synapses on CA1 pyramids in guinea‐pig hippocampal slices in vitro

1. In vitro slices of guinea‐pig hippocampus have been employed to compare excitatory synapses located distally and proximally on the dendritic tree of CA1 pyramidal cells.

Mode of activation of hippocampal pyramidal cells by excitatory synapses on dendrites

SummaryFollowing selective activation of four afferent paths that terminate exclusively on dendrites, only a small proportion of pyramidal cells in the hippocampal fields CA1 and CA3 discharged impulses. Following a single afferent volley, an EPSP was never observed even in cells synaptically excited. On tetanic stimulation (about 10/sec), a large EPSP developed, but this was not a prerequisite for an action potential.Studies of the extracellular field potentials corresponding to the EPSP and the population spike potential, indicated that the EPSP was generated across the dendritic membrane and that the spike was initiated in the neighbouring part of the dendritic tree, propagating from there along the thicker dendrites towards the soma. This conduction had an average velocity of 0.4m/sec, and, presumably, a relatively low safety factor.In certain cases, the intrasomatic electrode recorded small all-or-nothing spikes which presumably were generated in the dendritic tree. These small spikes (D-spikes) invaded the soma only if assisted by some additional depolarization, for example by frequency potentiation of excitatory synapses.The results indicate two functional types of pyramidal dendrites, the conducting and the synaptic type.