No abstract

Hyperpolarization-activated cation (HCN) channels are believed to be involved in the generation of cardiac pacemaker depolarizations as well as in the control of neuronal excitability and plasticity. The contributions of the four individual HCN channel isoforms (HCN1±4) to these diverse functions are not known. Here we show that HCN2-de®cient mice exhibit spontaneous absence seizures. The thalamocortical relay neurons of these mice displayed a near complete loss of the HCN current, resulting in a pronounced hyperpolarizing shift of the resting membrane potential, an altered response to depolarizing inputs and an increased susceptibility for oscillations. HCN2-null mice also displayed cardiac sinus dysrhythmia, a reduction of the sinoatrial HCN current and a shift of the maximum diastolic potential to hyperpolarized values. Mice with cardiomyocytespeci®c deletion of HCN2 displayed the same dysrhythmia as mice lacking HCN2 globally, indicating that the dysrhythmia is indeed caused by sinoatrial dysfunction. Our results de®ne the physiological role of the HCN2 subunit as a major determinant of membrane resting potential that is required for regular cardiac and neuronal rhythmicity.

Dendritic spines compartmentalize calcium, and this could be their main function. We review experimental work on spine calcium dynamics. Calcium influx into spines is mediated by calcium channels and by NMDA and AMPA receptors and is followed by fast diffusional equilibration within the spine head. Calcium decay kinetics are controlled by slower diffusion through the spine neck and by spine calcium pumps. Calcium release occurs in spines, although its role is controversial. Finally, the endogenous calcium buffers in spines remain unknown. Thus, spines are calcium compartments because of their morphologies and local influx and extrusion mechanisms. These studies highlight the richness and heterogeneity of pathways that regulate calcium accumulations in spines and the close relationship between the morphology and function of the spine.

A large body of evidence from in vitro studies suggests that GABA is depolarizing during early postnatal development. However, the mode of GABA action in the intact developing brain is unknown. Here we examine the in vivo effects of GABA in cells of the upper cortical plate using a combination of electrophysiological and Ca 2 þ -imaging techniques. We report that at postnatal days (P) 3-4, GABA depolarizes the majority of immature neurons in the occipital cortex of anaesthetized mice. At the same time, GABA does not efficiently activate voltage-gated Ca 2 þ channels and fails to induce action potential firing. Blocking GABA A receptors disinhibits spontaneous network activity, whereas allosteric activation of GABA A receptors has the opposite effect. In summary, our data provide evidence that in vivo GABA acts as a depolarizing neurotransmitter imposing an inhibitory control on network activity in the neonatal (P3-4) neocortex.

In the last decade, the measurement of activity-dependent intrinsic optical signals (IOSs) in excitable tissues has become a useful tool for collecting data about spatial patterns of information processing in mammalian brain and spread of excitation. Although the extent of the IOS correlates well with the extent of electrical excitation, its time course is much slower, suggesting that it does not directly monitor the electrical activity. The aim of this study was to investigate the mechanisms responsible for generation of IOSs. Coronal neocortical brain slices of juvenile rats were electrically stimulated at the border of layer VI and the white matter. The induced columnar-shaped IOSs were recorded using dark-field video microscopy. At corresponding locations, alterations in extracellular K+ concentration and extracellular space (ECS) volume were registered using ion-selective microelectrodes. After stimulation, a transient increase of extracellular K+ concentration up to 10 mM and a transient decrease of ECS volume by approximately 4% could be observed. The comparison of the time courses of these parameters yielded considerable differences between extracellular K+ concentration increase and IOS, but obvious similarities between alterations in ECS volume and IOS. To test the hypothesis that changes in IOS reflect changes in ECS, but not extracellular K+ concentration, we recorded under conditions that are known to prevent activity-induced changes in ECS, i.e., in low Cl- solutions and in the presence of furosemide. Both treatments similarly decreased stimulation-induced IOSs and alterations of ECS. However, the effect of these treatments on changes of extracellular K+ was different and did not correspond to the changes of IOS. We conclude that activity-dependent IOSs in rat neocortical slices measured by near-infrared video microscopy reveal changes in ECS. Furthermore, the pharmacological and ion substitutional experiments make it likely that activity-induced IOSs are attributable to cell swelling via a net KCI uptake and a concomitant water influx.