A self-activating intrinsic brake on bursting in CA3 neurons

Altered intrinsic neuronal excitability and reduced Na+ currents in a mouse model of Alzheimer's disease

Transgenic mice that overproduce beta-amyloid (Aβ) peptides can exhibit central nervous system network hyperactivity. Patch clamp measurements from CA1 pyramidal cells of PSAPP and wild type mice were employed to investigate if altered intrinsic excitability could contribute to such network hyperfunction. At approximately 10 months, when PSAPP mice have a substantial central nervous system Aβ load, resting potential and input resistance were genotype-independent. However, PSAPP mice exhibited a substantially more prominent action potential (AP) burst close to the onset of weak depolarizing current stimuli. The spike afterdepolarization (ADP) was also larger in PSAPP mice. The rate of rise, width and height of APs were reduced in PSAPP animals; AP threshold was unaltered. Voltage-clamp recordings from nucleated macropatches revealed that somatic Na(+) current density was depressed by approximately 50% in PSAPP mice. K(+) current density was unaltered. All genotype-related differences were absent in PSAPP mice aged 5-7 weeks which lack a substantial Aβ load. We conclude that intrinsic neuronal hyperexcitability and changes to AP waveforms may contribute to neurophysiological deficits that arise as a consequence of Aβ accumulation.

Synaptic activation of mGluR1 generates persistent depression of a fast after-depolarizing potential in CA3 pyramidal neurons

Burst firing is an important property of hippocampal pyramidal neurons. Group I metabotropic glutamate receptors (mGluRs) produce a multitude of effects on both the synaptic and intrinsic properties of neurons. We investigated whether brief activation of these receptors results in persistent modifications to the intrinsic excitability of rat hippocampal CA3 pyramidal cells (CA3-PCs). In whole-cell current-clamp recordings, current stimuli consisting of filtered, pseudo-random noise produced action potential firing with a mean frequency of ∼1.5-2 Hz. Analysis of spike intervals revealed that this firing included a substantial component (∼20%) of high-frequency (∼100 Hz) bursting activity. Activation of group I mGluRs with (S)-3,5-dihydroxyphenylglycine [(S)-DHPG] selectively eliminated the high-frequency bursts, an effect that persisted > 30 min after (S)-DHPG washout. The fast after-depolarizing potential (ADP) of CA3-PCs is known to be important for generating high-frequency action potential bursting. This ADP was persistently depressed following a short application of (S)-DHPG. This effect was blocked by the mGluR1 antagonist, (S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385). In contrast, the depression was resistant to the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate, N-methyl-D-aspartate (NMDA) and γ-aminobutyric acid (GABA)(A) antagonists. Unlike other manipulations that generate persistent depression of the ADP in CA3-PCs, DHPG-mediated ADP depression was insensitive to the Kv7 channel inhibitor 10,10-bis(4-Pyridinylmethyl)-9(10H)-anthracenone dihydrochloride (XE991) and strong intracellular Ca(2+) buffering by 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Synaptic activation of mGluRs in the associational-commissural pathway also resulted in persistent depression of the ADP in postsynaptic CA3-PCs, which was blocked by LY367385. These data represent the first evidence that synaptic activation of mGluR1 can modulate the intrinsic excitability properties of hippocampal neurons.