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, 156 (1-2), 61-6

Activation of Dentate Hilar Neurons by Stimulation of the Fimbria in Rat Hippocampal Slices

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Activation of Dentate Hilar Neurons by Stimulation of the Fimbria in Rat Hippocampal Slices

H E Scharfman. Neurosci Lett.

Abstract

It is has been shown that the major afferent input to the dentate gyrus, the perforant path, excites dentate hilar neurons. However, little is known about the other inputs to hilar cells. Therefore, we examined the responses of hilar neurons to stimulation of the fimbria. We positioned our stimulating electrodes so that granule cells were not excited antidromically by fimbria stimulation, although action potentials were easily triggered in area CA3b and CA3c pyramidal cells by such stimulation. In these experiments, fimbria stimulation evoked responses from every hilar cell tested, including examples of both of the major cell types, the spiny hilar 'mossy' cells (n = 15) and the relatively aspiny, 'fast-spiking' cells (putative interneurons, n = 5). Hilar cell responses consisted primarily of EPSPs that could trigger action potentials, but small IPSPs were also evoked in some cases, particularly in the fast-spiking cells. Excitation was blocked by an antagonist of the AMPA/kainate receptor subtype of excitatory amino acid receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 microM, n = 5), whereas the cholinergic antagonist atropine (10 microM) had no effect (n = 4). When sequential intracellular recordings were made from hilar cells and area CA3 pyramidal cells in the same slice, hilar cell EPSPs began after action potentials of CA3b pyramidal cells, and stimulus strengths required to evoke hilar cell EPSPs were above threshold for area CA3b pyramidal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Figures

Fig. 1
Fig. 1
Representative responses of hilar cells and granule cells to fimbria stimulation. A: responses of spiny hilar ‘mossy’ cells. 1,2: responses to fimbria stimulation (at the dots) are superimposed on hyperpolarizing and depolarizing intracellular current steps (arrows point to the start and end of the current injection). Current amplitudes were +0.3 nA, −0.3 nA in part 1 and +0.4 nA, −0.3 nA for part 2. Membrane potentials, −65 mV (1), −67 mV (2). 3: responses to depolarizing and hyperpolarizing current injection (± 0.3 nA) are shown in the absence of stimulation to illustrate the electrophysiological characteristics of the spiny hilar cell type (for review, see ref. 25). Note the irregular firing behavior and large spontaneous potentials (arrowheads). Membrane potential, −62 mV. B: responses of ‘fast-spiking’ hilar cells (possible interneurons). 1: excitatory responses of two different cells to fimbria stimulation. Membrane potentials, −60 mV (left) and −58 mV (right). Calibration for 1 and 2, 15 ms; for all other parts of this figure, calibration, 50 ms. 2: inhibitory response of a different cell to fimbria stimulation, superimposed on responses to ± 0.3 n A current steps. Membrane potential, −68 mV. 3: responses to current injection (± 0.3 nA) are shown in the absence of stimulation. Same cell as in B2. Note the lack of spike frequency adaptation and the large afterhyperpolarization after each action potential. Membrane potential, −64 mV. C: responses of a granule cell. 1,2: the responses of the same granule cell are shown following fimbria stimulation (1) and molecular layer stimulation (2). In part 2, the response is shown in the absence of current injection as well as in the presence of +0.2 nA and −0.5 nA current steps. Membrane potential for both 1 and 2, −70 mV. 3: the responses of the same granule cell as in parts 1 and 2 are shown to current injection (±0.3 nA) without synaptic stimulation. Membrane potential, −77 mV.
Fig. 2
Fig. 2
Blockade of hilar cell responses to fimbria stimulation by the AMPA/kainate receptor antagonist CNQX but not by the muscarinic antagonist atropine. A: the response of a spiny hilar cell to fimbria stimulation is shown before (left) and 15 min following (center) perfusion of the slice with 5 μM CNQX. The trace at right shows a response to the same stimulus 75 min after perfusion with drug-free buffer was resumed. Dots indicate the stimulus artifact. Membrane potential, −65 mV. B: two responses to the same fimbria stimulus are shown for a different spiny hilar cell. At left is a response elicited before 10 μM atropine was added to the bathing medium, and at right is a response elicited 30 min after addition of atropine. Membrane potential, −64 mV.

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