Pa Schwartzkroin

Synaptic connections of dentate granule cells and hilar neurons: Results of paired intracellular recordings and intracellular horseradish peroxidase injections

Simultaneous intracellular recordings were made in the dentate gyrus of rat hippocampal slices, from pairs of the following cell types: granule cells, interneurons located in the granule cell layer, hilar interneurons, and spiny hilar mossy cells. Granule cells were found to have strong excitatory effects on mossy cells and interneurons. Interneurons inhibited granule cells as well as other interneurons. No synaptic connections from mossy cells onto other cell types were found, within the confines of the slice, using intracellular recording methods. However, at the ultrastructural level, axon terminals of horseradish peroxidase-filled mossy cells were found making synaptic contacts in the hilus on dendrites of interneurons. These studies provide the first step towards determining the functional interactions of the various cell types in the fascia dentata.

Intracellular recording from hippocampal CA1 interneurons before and after development of long-term potentiation

Synaptic efficacy is modified following a brief train of high frequency stimulation (HFS) to a cells afferent fibers (long-term potentiation, LTP). Modulation of synaptic efficacy could be achieved through changes in excitability of interneurons. To test this hypothesis, we used the in vitro hippocampal slice to record intracellularly from CA1 basket cell interneurons before and following a period of HFS to fibers in stratum radiatum. Stimulation of stratum radiatum consistently produced action potential discharge in the basket cell interneuron prior to the discharge of a population spike in stratum pyramidale. This result is consistent with findings that basket cells may receive direct excitatory afferent input from neurons extrinsic to the CA1 region, and mediate a powerful feed-forward inhibition. Interneuron responses were recorded under 3 experimental conditions, in which stimulus intensity was varied. The level of basket cell excitability, as assessed by responses to orthodromic input, was not decreased following a train of HFS. Rather, the evoked EPSP in basket cells was often enhanced following high frequency stimulation to the cells afferent fibers. These findings support the notion that decrement in basket cell activity is not responsible for LTP in CA1 hippocampus.

Electrophysiological actions of somatostatin (SRIF) in hippocampus: an in vitro study.

Summary1.The electrophysiological actions of somatostatin (somatotropin release inhibiting factor; SRIF) were investigated in thein vitro hippocampal slice preparation. Intracellular recordings were obtained from pyramidal neurons in area CA1 in slices of hippocampus from guinea pigs and rabbits.2.Somatostatin, applied via micropressure ejection to CA1 pyramidal-cell somata, was primarily excitatory. The effects, however, were quite variable, with nearly all cells displaying pronounced tachyphylaxis. A majority of cells was depolarized by SRIF, but hyperpolarizations or biphasic depolarization/hyperpolarization responses were also recorded. Only minimal conductance changes were associated with the SRIF-induced voltage changes.3.Depletion of SRIF, by injection of the intact animal with cysteamine several hours before preparing slices, resulted in no obvious abnormalities in hippocampal slice electrophysiology.4.Our results obtained with application of exogenous SRIF are consistent with the concept that SRIF acts as an excitatory neurotransmitter/neuromodulator in hippocampus. However, our attempts to demonstrate endogenous SRIF action have thus far been unsuccessful.

Intracellular dyes mask immunoreactivity of hippocampal interneurons

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.

Further studies of the effects of somatostatin and related peptides in area CA1 of rabbit hippocampus.

Summary1.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.