Faculty Bibliography

Recent studies have shown that prolonged stimulation of afferents of the rat fascia dentata in vivo leads to the development of chronic epileptiform activity of the dentate granule cell region, and degeneration of certain cell types in the adjacent hilus. To investigate the development of dentate hyperexcitability and the selective vulnerability of hilar cells, the hippocampal slice preparation offers an in vitro model in which cellular mechanisms can be examined. We have recorded intracellularly from granule cells and hilar cells in tissue slices from rat before, during, and following sustained stimulation of the major afferent input to the dentate gyrus, the perforant path. Results from intracellular studies in slices were consistent with in vivo studies. Hilar cells were far more sensitive to short-term or prolonged perforant path stimulation than granule cells. At a time when the granule cell population response was not affected by prolonged stimulation, simultaneous recordings from hilar cells and some granule layer interneurons showed that these cells were already depolarized, had very low input resistance, and showed other electrophysiological changes indicative of deterioration. In contrast, granule cells generally hyperpolarized during stimulation and their input resistance increased; no signs of injury were evident in granule cells. Some stimulus-induced changes in the physiological characteristics of granule cells, such as decreased spike frequency adaptation and reduced inhibitory postsynaptic potentials, may contribute to the development of dentate hyperexcitability

Following prolonged stimulation of the perforant path input to the dentate gyrus, long-lasting changes occur in the synaptic responses and cell properties of cells in the fascia dentata. The present study describes the effects of sustained stimulation on the major population of cells innervated by the dentate granule cells: are CA3 pyramidal cells of hippocampus. In 46% of slices from rat, sustained stimulation of perforant path was followed by spontaneous, synchronized, rhythmic bursting activity in area CA3 pyramidal cells that was evident for several hours. These bursts could be recorded extracellularly in the pyramidal cell layer, throughout the hilar region, and even in the granule cell layer. With intracellular recording, all of the cells of the fascia dentata were found to be affected by the pyramidal cell bursts. Hyperpolarizing, inhibitory postsynaptic potential (IPSP)-like events occurred in all granule cells tested during the CA3 pyramidal cell burst. In contrast, spiny hilar 'mossy' cells discharged synchronously with the pyramidal cells, as did some of the 'fast spiking' interneurons. However, most interneurons only depolarized a few millivolts during the pyramidal cell burst. These results show that sustained stimulation of the perforant path is followed by a period of hyperexcitability in area CA3 of the hippocampus, and that hyperexcitability in area CA3 influences the activity of the cells in the fascia dentata

Prolonged afferent stimulation of the rat dentate gyrus in vivo leads to degeneration only of those cells that lack immunoreactivity for the calcium binding proteins parvalbumin and calbindin. In order to test the hypothesis that calcium binding proteins protect against the effects of prolonged stimulation, intracellular recordings were made in hippocampal slices from cells that lack immunoreactivity for calcium binding proteins. Calcium binding protein-negative cells showed electrophysiological signs of deterioration during prolonged stimulation; cells containing calcium binding protein did not. When neurons without calcium binding proteins were impaled with microelectrodes containing the calcium chelator BAPTA, and BAPTA was allowed to diffuse into the cells, these cells showed no deterioration. These results indicate that, in a complex tissue of the central nervous system, an activity-induced increase in intracellular calcium can trigger processes leading to cell deterioration, and that increasing the calcium binding capacity of a cell decreases its vulnerability to damage

In area CA1 of hippocampus, a subpopulation of gamma-aminobutyric acid (GABA)-containing interneurons that make synaptic contacts on pyramidal cells also contains the neuropeptide, somatostatin. The effects of GABA and somatostatin on hippocampal pyramidal cells have been investigated separately, but it is not known whether an interaction occurs between these co-localized substances. We demonstrate that somatostatin has a potent inhibitory effect on GABA-mediated synaptic potentials which hyperpolarize pyramidal cells. This effect may be relevant to the well-documented epileptogenicity of the hippocampus, as well as the phenomenon of long-term potentiation, which is a well-studied example of synaptic plasticity

The results of several studies have suggested that local circuit neurons, or interneurons, of area CA1 of hippocampus use gamma-aminobutyric acid (GABA) as their neurotransmitter. However, when these cells were labelled by intracellular dye injection, and examined immunocytochemically with antisera raised against GABA, none of the interneurons were immunoreactive. Numerous non-injected interneurons in the same tissue section were clearly immunoreactive. These results suggest that intracellular dyes interfere with immunocytochemical staining of hippocampal interneurons

1. In slice studies of mature and immature CA1 hippocampal pyramidal cells from rabbit, somatostatin 14 (SS14), the related peptide somatostatin 28(1-12) [SS(1-12)], and the synthetic analogue of somatostatin 14, SMS-201995 (SMS), had similar effects. When pressure-ejected onto cell somata, these peptides elicited depolarizations, often accompanied by action potential discharge. When applied to dendrites, the peptides produced depolarizations or hyperpolarizations. 2. When a large amount of one of the three somatostatin-related (SS) peptides was applied to the slice at some distance from the impaled cell, hyperpolarizations were observed that were not always blocked by tetrodotoxin (TTX) or low Ca2+. Since SS peptides were also found to depolarize interneurons in area CA1, it seems likely that the hyperpolarizations that were blocked by TTX or low Ca2+ were mediated via excitation of interneurons that in turn hyperpolarized pyramidal cells. 3. All SS peptides also had long-lasting effects on CA1 pyramidal cells that led to spontaneous firing of action potentials and an increase in the number of action potentials discharged in response to a given depolarizing current pulse; the spontaneous discharge effect was blocked by TTX or low Ca2+ plus Mn2+ and, thus, appeared to have a presynaptic mechanism. However, the increase in discharge in response to a constant depolarizing current pulse was not dependent on intact synaptic transmission and, therefore, was attributable to a direct postsynaptic effect of the SS peptides

A specific population of cells located in the hilus of the hippocampal fascia dentata was studied in guinea pig hippocampal slices using standard intracellular recording techniques. Twenty-one such cells were characterized using electrophysiological techniques and were identified morphologically as mossy cells following intracellular injection of the fluorescent dye Lucifer yellow. These cells had a resting membrane potential (mean, -64.6 mV), action potential amplitude (mean, 78.6 mV), action potential duration (mean, 2.2 msec), and time constant (mean, 24.2 msec) similar to those of hippocampal pyramidal cells of area CA3. Rectification seen in their I-V curves, and their ability to fire action potentials in accommodating trains or bursts in response to injected current pulses, were also similar to those of area CA3 pyramidal cells. However, these cells could be distinguished from area CA3 pyramidal cells by their higher input resistance (mean, 97.4 M omega) and higher level of spontaneous activity. The synaptic responses of mossy cells were also different from those of CA3 pyramidal cells. First, mossy cells responded to low levels of stimulation in all areas of the hippocampal slice that were tested, even areas as remote as area CA1. Second, the responses of mossy cells to stimulation consisted primarily of EPSPs. Hyperpolarizing IPSP-like events followed EPSPs in some cells, but the hyperpolarizations were small and monophasic, even after the cell was depolarized with current injection. This response contrasts with the smaller EPSP and the prominent, biphasic IPSP elicited by afferent stimulation of area CA3 pyramidal cells. The physiological and morphological characteristics of these cells suggest that they could play an important role in the integration of electrical activity in the hippocampus

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

Responses to GABA were recorded from 87 neurons of rat visual cortical slices. Pyramidal and nonpyramidal cells were identified by intracellular dye injection, and their responses were compared. All identified pyramidal and nonpyramidal cells, as well as unstained cells, responded to GABA ejected from a pipette that was positioned within 300 micron of the soma. Their responses were similar, regardless of their morphology. In addition, GABA responses of visual cortical neurons could not be distinguished from those of other areas of the neocortex, or pyramidal cells in area CA1 of hippocampus. Depending on the site of application, there appeared to be two types of GABA responses that were present in all cells. The first was generated by application of GABA to the soma (GABAs response; mean reversal potential = -71.7 mV). The second occurred when GABA was applied on dendrites (GABAD response; mean reversal potential = -49.3 mV). When GABA was ejected on proximal dendritic regions, both responses could be observed in combination. Both GABAs and GABAD responses were accompanied by extremely large increases in conductance. In some cells, a third type of GABA response was elicited following somatic or dendritic GABA application. This response was a relatively small-amplitude, long-lasting hyperpolarization which followed a GABAS and GABAD response (late hyperpolarization, mean reversal potential = -79.8 mV)

The effects of pressure-applied gamma-aminobutyric acid (GABA) on the soma and dendrites of pyramidal and non-pyramidal neurons of rat visual cortical slices were recorded intracellularly. When applied close to the soma, GABA produced hyperpolarizations and depolarizations, but when GABA was applied more than 250 microns from the soma only depolarizations were recorded. The results suggest that most visual cortical cells respond to GABA and that the responses of pyramidal and non-pyramidal cells to GABA are similar