A study of the transition from spindles to spike and wave discharge in feline generalized penicillin epilepsy: EEG features

Earlier work suggested that the epileptic bursts occurring in the form of spike and wave discharges in feline generalized penicillin epilepsy were closely related to spindles. The present study showed that after i.m. penicillin, spindles elicited by single-shock midline thalamic stimulation gradually change into spike and waves. Hybrid forms are often seen during the transition phase. The transformation of spindles into spike and waves initially involves an increase in amplitude and the development of positive phases of spindle waves. Furthermore, every second (or in cats with lesions of the midbrain reticular formation, every second and third) spindle wave is gradually eliminated and replaced by a slow wave. The remaining enhanced spindle wave becomes the spike of the spike and wave complex. In conformity with this development, spectral analysis shows that no gradual frequency shift occurs during this transformation, but that the intraburst frequency decreases by a half or a third in one step from that characteristic of spindles to that typical for spike and waves. Spindle and spike and wave frequencies vary from cat to cat, but the above ratios are constant across animals. The spike and waves of cats with lesions of the midbrain reticular formation resemble those of human generalized epilepsy more closely than those induced in intact animals. A continuous transition from spindles to spikes and wave is thus demonstrable suggesting that spike and wave bursts are elicited by the same thalamocortical volleys which normally induce spindles.     

Dopamine D2 receptor expression in hippocampus and parahippocampal cortex of rat,cat, and human in relation to tyrosine hydroxylase-immunoreactive fibers

A detailed study comparing the distribution of D2 receptors and tyrosine hydroxylase-immunoreactive fibers in the hippocampus and parahippocampal cortices of the rat, cat, and human was conducted. The distribution of [¹²⁵I]epidepride binding to D2 receptors along the transverse and longitudinal axes of the hippocampus and parahippocampus differed among the species. In rat hippocampus, the number of sites was highest in septal portions of lacunosum-moleculare of CA1 and stratum moleculare of the subiculum. Virtually no binding to D2 receptors existed in the temporal hippocamps. For the cat hippocampus, the highest binding existed in the inner one-third of the molecular layer of the dentate gyrus (DG). There were also significant numbers of D2 receptors in strata radiatum and oriens of the CA subfields, with almost undetectable levels in lacunosum moleculare and subiculum. The number of sites was higher in the septal than temporal hippocampus. In the human hippocampus, highest binding was observed in the molecular layer of DG and the subiculum, with lower levels in strata oriens and lacunosum-moleculare of CA3, and very low binding in CA1. The histochemical demonstration of the pattern of mossy fibers revealed an organization complementary to that of D2 receptors in cat and human. In none of the species was there significant expression of D2 receptors in the entorhinal cortex, except in the caudal extreme of this region in the rat. In that region a trilaminar pattern was exhibited that continued into the perirhinal cortex. A trilaminar pattern of D2 receptor expression was observed in the perirhinal cortex of all species, with the highest values in the external and deep laminae and low expression in the middle laminae. The organization of dopamine fibers was assessed by comparing the distribution of tyrosine hydroxylase-positive and dopamine β-hydroxylase-immunoreactive fibers in these same regions. It revealed consistent mismatches between the pattern of D2 receptor expression and dopaminergic innervation in all three species. The implications for this mismatch are discussed. It is hypothesized that the distribution of D2 receptors, and not of dopamine fibers, determines what neural systems dopamine influences in the hippocampal complex. © 1994 Wiley-Liss, Inc.     

Development and reduction of synaptic potentiation induced by perforant path kindling

The effects of perforant path kindling on perforant path-dentate synaptic transmission and on granule cell excitabilities were examined by the field potential recording technique in freely moving rats. Both population EPSPs and population spikes continued to increase 1 h after the first kindling stimulation and thereafter, and the potentiation of the population EPSP attained 149% of the prekindling control value. Cumulative effects of the kindling stimulations on the population EPSP, however, were rarely observed and the population EPSP, potentiated by the first few kindling stimulations, decreased gradually in the later stage of kindling. In some animals, the amplitude of the population spike sustained a highly potentiated value in the later kindling period in spite of the decrease in the population EPSP. Even in animals that showed a reduction in both population EPSPs and population spikes in the later kindling period, an enormous population spike, which we refer to as the “outbreak of the giant population spike”, was sometimes evoked suddenly without corresponding changes in the population EPSP. These results indicate that an abnormal hyperexcitability of the granule cell was produced by the kindling stimulations. Furthermore, the polysynaptic potential presumably produced through the recurrent excitatory hippocampal-entorhinal circuit was observed to increase after kindling. The significance to kindling, and the possible mechanisms of the abnormal hyperexcitability of postsynaptic neurons may be related to a change in brain norepinephrine concentration and its influence on the membrane potential.     

The action of norepinephrine in the dentate gyrus: Beta-mediated facilitation of evoked potentials in vitro

The effects of superfusion of norepinephrine (NE) on perforant path (PP) evoked potentials in the dentate gyrus were evaluated in the rat hippocampal slice preparation. Superfusion of NE (10 microM) produced a facilitation of the PP evoked responses. Facilitation of the synaptically-evoked responses was expressed in the field potential as an increase in extracellular excitatory postsynaptic potential (EPSP) (117% of control), a decrease in population spike onset latency (94% of control) and an increase in population spike amplitude (131% of control). In 24% of the slices the facilitation of the population spike amplitude lasted longer than 30 min. Isoproterenol, a beta-agonist, mimicked NE effects while timolol, a beta-antagonist, blocked them. Facilitation of the population spike amplitude by NE could not be accounted for solely by the increase in EPSP slope also produced by NE. Superfusion of NE did not produce facilitation of the antidromically evoked field potentials, but in 4 of 8 slices produced a small decrease. NE effects were activity-independent, since the subsequently evoked PP responses were facilitated even when the PP was not concurrently stimulated during superfusion with NE.     

Lateral entorhinal cortical kindling can be established without potentiation of the entorhinal-granule cell synapse

Kindling is an animal model of epilepsy which involves a permanently enhanced neuronal response to an electrical stimulus. It has been proposed that long-term potentiation (LTP) of excitatory synaptic transmission is the cellular basis of kindling. Therefore, LTP was examined in the monosynaptic projections from the lateral entorhinal cortex (LEC) to dentate granule cells (DG) in unrestrained, unanesthetized rats kindled via the LEC. Population excitatory postsynaptic potentials (pEPSPs) were recorded from the granule cells before, during, and after kindling of the LEC. Controls were unkindled rats recorded during the same time period as the experimental rats. No consistent changes were found in plateau pEPSP amplitudes or initial slopes although kindling via the LEC proceeded through the typical stages. There was also no significant change in the stimulus intensity needed to elicit a 50% maximal or "plateau" pEPSP. Thus, whereas kindling was indeed established by stimulation of the LEC, there was no evidence of LTP detected in the granule cell response either during the development or after completion of kindling. Either LTP does not underlie the mechanism of kindling via this pathway or it occurs in different brain regions receiving LEC input.     

Long-lasting effects of partial hippocampal kindling on hippocampal physiology and function

The objective of this project was to study the behavioral and physiological effects at 6–9 weeks after evoking 15 afterdischarges (ADs) in hippocampal CA1 (partial hippocampal kindling). Rats were trained on the open radial arm maze (RAM) with all eight arms baited, kindled, and then tested again on the RAM, followed by in vitro recordings at 8–9 weeks after kindling. Partial kindling was manifested by an increase in hippocampal AD duration. Enhancement of the commissural basal dendritic excitatory postsynaptic potential (EPSP) was observed for at least 1 day after the ADs kindled rats performed worse than control rats during the 1st but not during the 7th or 8th week after kindling. Rats that were slow in acquiring the RAM showed more RAM errors after kindling than those that showed fast acquisition. At 8–9 weeks after kindling, as shown by field potential recording in the hippocampal slice in vitro, kindled rats showed an increase in paired-pulse facilitation (PPF) of the EPSP in CA1 but a decreased PPF of the perforant path to dentate gyrus EPSP; no change in the PPF of the population spike was found in CA1 or DG. In a second group of rats that were not run on the RAM, at 6 weeks after kindling, PPF of the population EPSP and population spike were enhanced in the kindled rats compared to the control rats in CA1, but not in DG or CA3 in vitro (at 1.5, 2, or 4 times threshold intensity). In conclusion, partial hippocampal kindling induced persistent physiological effects for up to 8–9 weeks, and it is suggested that the normalization of the paired-pulse population spike response in CA1 and DG at more than 6 weeks after kindling may be accompanied by a recovery of RAM performance. © 1994 Wiley-Liss, Inc.     

Focal penicillin epilepsy in an isolated cerebral hemisphere.

The cortex of an entire cerebral hemisphere in the cat was chronically isolated to allow assessment of the role of intrinsic association pathways in the regulation of focal epileptic activity induced by topical application of penicillin. EEG recordings indicated that interictal spike activity in the normal hemisphere was maximal at the focus and markedly reduced in amplitude at distances of 5 to 7 mm. Activity in the isolated hemisphere was similar in morphology to that of the normal cortex, but there were largeamplitude epileptiform discharges 10 to 15 mm from the focus. These distant penicillin spikes appeared early in the development of the focus and were not attributable to diffusion of penicillin. Intracellular recordings at the center of the focus and at remote sites showed the characteristic paroxysmal depolarizing shifts (PDSs) previously described for intact cortex, i.e., large-amplitude membrane depolarizations with superimposed high-frequency spike activity and subsequent spike inactivation. Hyperpolarizing potentials followed termination of PDSs in most cells and were occasionally seen as the sole accompaniment of the focal penicillin discharges. We suggest that the similarity of intracellular findings in normal and isolated cortices could have resulted from a direct effect of penicillin on neurons, whereas the spread of interictal spikes in the isolated hemisphere apparently involved abnormal utilization of existing intracortical association pathways.     

A study of the transition from spindles to spike and wave discharge in feline generalized penicillin epilepsy: Microphysiological features

The microphysiological features underlying the evolution of spike and wave discharge from spindles in feline generalized penicillin epilepsy were investigated using extracellular microelectrode recordings. Action potentials generated by single cortical neurons were related to the EEG features of the transition from spindles to spike and waves using statistical methods of analysis on a computer. The probability of an action potential being discharged by a spindle wave was weak. It progressively increased after penicillin during the transformation of the spindle wave into the spike of the spike and wave complex. As this occurred, periods of decreased firing probability coinciding with the slow wave of the spike and wave complex developed immediately after each period of enhanced firing probability. The spike and wave pattern was thus characterized by a remarkable oscillation between periods of increased excitation of cortical neurons corresponding to the spike, and periods of markedly decreased firing probability corresponding to the wave of the spike and wave complex. This was associated with increased synchronization of discharge of neighboring cortical neurons. We propose that the transformation of spindles to spike and waves is the consequence of a single feature: increased excitability of cortical neurons to spindle-inducing thalamocortical volleys. Under those conditions cortical cells discharge action potentials more consistently with each thalamocortical volley. Because of this the high threshold intracortical recurrent inhibitory pathway is recruited into the discharging process and induces recurrent periods of powerful inhibition of cortical neurons.     

Age differences in hippocampal subfield volumes from childhood to late adulthood

The hippocampus is composed of distinct subfields: the four cornu ammonis areas (CA1‐CA4), dentate gyrus (DG), and subiculum. The few in vivo studies of human hippocampal subfields suggest that the extent of age differences in volume varies across subfields during healthy childhood development and aging. However, the associations between age and subfield volumes across the entire lifespan are unknown. Here, we used a high‐resolution imaging technique and manually measured hippocampal subfield and entorhinal cortex volumes in a healthy lifespan sample (N = 202), ages 8–82 yrs. The magnitude of age differences in volume varied among the regions. Combined CA1‐2 volume evidenced a negative linear association with age. In contrast, the associations between age and volumes of CA3‐DG and the entorhinal cortex were negative in mid‐childhood and attenuated in later adulthood. Volume of the subiculum was unrelated to age. The different magnitudes and patterns of age differences in subfield volumes may reflect dynamic microstructural factors and have implications for cognitive functions across the lifespan. © 2015 Wiley Periodicals, Inc.     

Interactions between septal and entorhinal inputs to the rat dentate gyrus: Facilitation effects

We examined facilitation effects between the medial septum and perforant path inputs to the dentate gyrus for the four possible combinations of paired-pulse activation. Facilitation effects occurred in all cases. The largest facilitation effects occurred when the septal pulse served as the conditioning pulse for the population spike subsequently evoked by a perforant path pulse. Using 3 pulses, we also examined the influence of septal activation on paired-pulse facilitation of the perforant path-granule cell population spike. A septal stimulation pulse, applied 6-10 ms prior to the onset of the population spike evoked by a perforant path conditioning pulse, did not affect the perforant path-dentate test response at any interpulse interval. If the septal pulse occurred immediately prior to population spike onset, however, there was a significantly greater depression of the test response from 70-3000 ms, but no effect at early intervals (20-50 ms). The effect of the septal pulse appears more consistent with a direct action of the septal terminals on granule cells than with an indirect action via the recurrent inhibitory interneurons.     

Monosynaptic and disynaptic activation of pyriform cortex neurons by synchronous lateral olfactory tract volleys in the rabbit

To elucidate the organization of synaptic inputs to pyriform cortex neurons, intracellular and extracellular responses of single units were analyzed in urethane-anesthetized rabbits. The lateral olfactory tract (LOT) or the olfactory bulb (OB) was electrically stimulated. Intracellular recordings revealed two types of cells (type I and type II cells), according to the types of EPSP evoked by the LOT or OB shock. The EPSP in the type I cells had shorter latencies (0.0 to 0.9 ms) from the onset of the component 2 (C2) wave of the field potential (which signals the onset of the synaptic depolarization of the apical dendrites of the pyramidal cells in the PC), and that in the type II cells had longer latencies (1.0 to 6.0 ms). A conditioning LOT or OB shock did not suppress the testing EPSP in the type I cells, whereas the conditioning stimulation greatly suppressed the testing EPSP in most of the type II cells. Extracellular recordings from units responding synaptically to the LOT or OB shock revealed a group of units which had short latencies (0.7 to 1.9 ms) of spike discharges. Those units, which were likely to be the same cells as the type I cells, are believed to mediate excitatory synaptic inputs to the type II cells. On the basis of these results, we concluded that type I cells are monosynaptically activated by LOT volleys, whereas type II cells are activated di- or polysynaptically by way of a relay from type I cells. The type I cells were recorded in both the superficial and the deep parts of the pyriform cortex, although they were recorded more frequently in the superficial part. On the other hand, most of the type II cells were recorded in the deep part of the PC. These results support and extend the previous model, in which the monosynaptically activated superficial pyramidal cells give rise to excitatory inputs to other pyramidal cells and neurons in deep layers.     

Long-term potentiation in thin hippocampal sections studied by intracellular and extracellular recordings.

To study the mechanism for long-term potentiation, extracellular and intracellular potential changes were examined in thin hippocampal sections in vitro. The field potential elicited by mossy fiber stimulation in the CA3 region (primary response) was augmented for observation periods of 15–60 min after conditioning tetanic stimulation. Tetanization of a group of mossy fibers potentiated responses induced by another group of presynaptic fibers. The augmented primary response was sometimes followed by a train of after-discharges. When long-lasting potentiation of the primary response was observed, an increase in excitatory postsynaptic potential amplitude or a depression of inhibitory postsynaptic potentials was found intracellularly. The afterdischarge train was associated with a large intracellular depolarization of long durations. These results suggest that long-term potentiation of the primary response is due to an enhanced excitatory synaptic transmission and a depression of the inhibitory circuit, and that the depression of inhibitory postsynaptic potentials may be related partly to paroxysmal activities of neurons.     

Development of responses of globus pallidus and entopeduncular nucleus neurons to stimulation of the caudate nucleus and precruciate cortex

Extracellular recordings were made from neurons in the globus pallidus and entopeduncular nucleus (GP-ENTO) of anesthetized kittens of 2 to 177 days of age and from four adult cats. Stimulation of the striatum and the precruciate cortex produced responses in GP-ENTO neurons of the youngest kittens tested (2 days of age). In kittens of 1 to 10 days, about 70% of the GP-ENTO neurons responded to either caudate or cortical stimulation with a purely excitatory response (i.e., an evoked action potential). With increasing age the frequency of occurrence of this type of response decreased and the occurrence of inhibitory responses or of sequences of excitation followed by inhibition increased. In addition to these changes in the form of the evoked responses, other response parameters exhibited age-dependent alterations. Latency to response decreased with age and the ability of GP-ENTO neurons to follow repetitive stimuli increased as the kittens became older. These findings suggest that although GP-ENTO neurons are functional as early as 2 days postnatally in the kitten, subsequent maturation of the responsiveness of these neurons continues for several postnatal months.     

Neurophysiology of limbic system pathways in the rat: Projections from the amygdala to the entorhinal cortex

We studied the responses of rat entorhinal neurons to electrical stimulation of the amygdala. Four main results were obtained: (1) excitatory postsynaptic potentials were recorded in entorhinal neurons in response to electrical stimulation of the amygdala. Cells in layers II, III and V of the entorhinal cortex were responsive. (2) Excitatory responses were followed by inhibitory postsynaptic potentials. (3) Frequency potentiation of both excitatory and inhibitory responses was observed when 10/s stimulation was used. (4) Three amygdala neurons were antidromically activated by entorhinal stimulation; and two layer II entorhinal cells that were excited by amygdala stimulation were also antidromically activated by dentate gyrus stimulation. These results provide evidence for a monosynaptic, excitatory projection from the amygdala to the entorhinal cortex. In addition, the data indicate that amygdala neurons are only one synapse removed from the excitation of dentate gyrus granule cells.     

Projection of the entorhinal layer II neurons in the rat as revealed by intracellular pressure-injection of neurobiotin

A component of the perforant path, projection of the entorhinal layer II neurons, was investigated by recovering intracellularly labeled layer II neurons from the medial or intermediate entorhinal cortex in rats. The labeled layer II spiny stellate neurons had axon collaterals in layers I, II, and III of the entorhinal cortex as well as some axon collaterals in the subiculum. The stem axons gave rise to terminal axon branches that covered the entire extent (suprapyramidal blade, crest, and infrapyramidal blade) of the dentate gyrus and the CA2–3 fields in the transverse plane, forming a sheet-like formation. The axon arbor in the hippocampal formation spread up to 2 mm wide in a septotemporal direction. The sheet-like formation of the axon arbors was a narrow layer in the suprapyramidal blade and in the stratum lacunosum-moleculare of the CA2–3 fields. The layer became wider in the crest and infrapyramidal blade of the dentate gyrus. This study shows that the entorhinohippocampal circuit is not a simple circle from single cells level.     

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.     

Calcium channel blockade attenuates abnormal synaptic transmission in the dentate gyrus elicited by entorhinal amyloidopathy

Entorhinal‐hippocampal network is one of the earliest circuits which is affected by Alzheimer's disease (AD). There are numerous data providing the evidence of synaptic deficit in the dentate gyrus (DG) of AD animal model. However, there is little known about how entorhinal cortex (EC) amyloidophaty affects each excitatory and/or inhibitory transmission in the early stage of AD. On the other hand, it is believed that calcium dyshomeostasis has a critical role in the etiology of AD. Here, the effect of the EC amyloid pathogenesis on excitatory or inhibitory post synaptic currents (EPSC and IPSC, respectively) in the DG granule cells and then the possible neuroprotective action of L‐type calcium channel blockers (CCBs), nimodipine and isradipine, were examined. The amyloid beta (Aβ) 1–42 was injected bilaterally into the EC of male rats and one week later, synaptic currents in the DG granule cells were assessed by whole cell patch clamp. EPSCs were evoked by stimulating the perforant pathway. Voltage clamp recording showed profound decrease of evoked EPSC amplitude and paired pulse facilitation in the DG granule cells of Aβ treated rats. Furthermore, AMPA/NMDA ratio was significantly decreased in the Aβ treated animals. On the other hand, amplitude of IPSC currents was significantly increased in the DG granule cells of these animals. These modifications of synaptic currents were partially reversed by daily intracerebroventricular administration of isradipine or nimodipine. In conclusion, our results suggest that Aβ in the EC triggers decreased excitatory transmission in the DG with substantial decrement in AMPA currents, leading to a prominent activity of inhibitory circuits and increased inhibition of granule cells which may contribute to the development of AD‐related neurological deficits in AD and treatment by CCBs could preserve normal synaptic transmission against Aβ toxicity.     

Effects of somatosensory deafferentation on spectral characteristics of the sensorimotor EEG in the adult cat

A quantitative assessment of the sensorimotor EEG before and after transection of the dorsal columns at either a high (C1 to C3) or low (C5 to T1) cervical level was undertaken in unrestrained, adult cats. Electroencephalographic signals recorded unilaterally from postcruciate cortex (A:23) at medial (L:2 to 5) and lateral (L: 10 to 12) sites were subjected to bandpass frequency analysis. The incidence of 12- to 15-Hz sleep spindles and sensorimotor rhythm (SMR) activity was evaluated in comparable pre/postlesion EEG segments. Frequency analyses focused on the distribution of voltage in four bands (4 to 7, 8 to 11, 12 to 15, and 18 to 23 Hz). The findings showed that dorsal column transections markedly altered EEG spectral distributions. Most consistently affected was 8- to 15-Hz activity which increased significantly over sites corresponding to peripheral receptive fields below the level of the lesions. Observed increases in sleep spindles and SMR activity contributed to this finding. In prelesion recordings, voltage in all bands increased progressively over the course of slow-wave sleep to REM onset. Abrupt peaks in 12- to 15-Hz and 18- to 23-Hz activity preceded the REM stage. Dorsal column transections eliminated this sequence of frequency/voltage changes. These findings were interpreted in terms of the release of intrinsic rhythmic discharge patterns over ventrobasal thalamocortical projection pathways.     

Physiological correlates of responses to gamma-aminobutyric acid (GABA) recorded from rat visual cortical neurons in vitro

Responses to focal application of gamma-aminobutyric acid (GABA) were compared to synaptic potentials elicited by afferent stimulation of rat visual cortical neurons, using a slice preparation and conventional intracellular recording techniques. GABA produced three types of responses: a brief hyperpolarization (mean reversal potential, -72 mV), brief depolarization (mean reversal potential, -50 mV), or a prolonged hyperpolarization (mean reversal potential, -80 mV). Synaptic potentials included simple or complex EPSPs and EPSPs followed by mono- or biphasic IPSPs. A comparison of the characteristics of the GABA responses and synaptic potentials indicated that GABA may mediate both phases of the IPSP in these cells. Our results suggest that despite differences in the circuitry of the visual cortex as opposed to other neocortical and allocortical (hippocampal) areas (Mountcastle and Poggio, 1968; Colonnier and Rossignol, 1969; Creutzfeldt, 1978; Kuhlenbeck, 1978), the inhibitory control of cortical pyramidal and nonpyramidal neurons by GABA is quite similar.