Review summary (print version)Background Fast-spiking, parvalbumin-expressing interneurons (PV + interneurons) play a key role in several functions of the brain. They contribute to feedback and feedforward inhibition, and are critically involved in the generation of network oscillations. A hallmark property of these interneurons is speed. In essence, these cells convert an excitatory input signal into an inhibitory output signal within a millisecond. How these remarkable signaling properties are implemented at the molecular and cellular level has been unclear. Furthermore, how PV + interneurons shape complex network functions has remained an open question. Advances Outlook PV+ interneurons may also play a key role in numerous brain diseases. These include epilepsy, but also complex psychiatric diseases, such as schizophrenia. Thus, PV + interneurons may become important therapeutic targets in the future. However, much needs to be learned about the basic function of these interneurons before clinical neuroscientists will have a chance to successfully use PV + interneurons for therapeutic purposes. 3 Full article (online) AbstractThe success story of fast-spiking, parvalbumin-expressing (PV + ) GABAergic interneurons is amazing. In 1995, the properties of these interneurons were completely unknown. 20 years later, thanks to the massive use of subcellular patchclamp techniques, simultaneous multiple-cell recording, optogenetics, in vivo measurements, and computational approaches, our knowledge about PV + interneurons became more extensive than for several types of pyramidal neurons (Box 1). These findings have implications beyond the "small world" of basic research on GABAergic cells. For example, the results provide a first proof of principle that neuroscientists might be able to close the gaps between molecular, cellular, network, and behavioral level, which represents one of the main challenges at the present time. Furthermore, the results may form the basis for using PV + interneurons as therapeutical targets for brain diseases in the future. However, much needs to be learned about the basic function of these interneurons before clinical neuroscientists will be able to use PV + interneurons for therapeutic purposes.

Coherent network oscillations in the brain are correlated with different behavioural states. Intrinsic resonance properties of neurons provide a basis for such oscillations. In the hippocampus, CA1 pyramidal neurons show resonance at theta (θ) frequencies (2‐7 Hz). To study the mechanisms underlying θ‐resonance, we performed whole‐cell recordings from CA1 pyramidal cells (n= 73) in rat hippocampal slices. Oscillating current injections at different frequencies (ZAP protocol), revealed clear resonance with peak impedance at 2‐5 Hz at ≈33 °C (increasing to ≈7 Hz at ≈38 °C). The θ‐resonance showed a U‐shaped voltage dependence, being strong at subthreshold, depolarized (≈‐60 mV) and hyperpolarized (≈‐80 mV) potentials, but weaker near the resting potential (‐72 mV). Voltage clamp experiments revealed three non‐inactivating currents operating in the subthresold voltage range: (1) M‐current (IM), which activated positive to ‐65 mV and was blocked by the M/KCNQ channel blocker XE991 (10 μm); (2) h‐current (Ih), which activated negative to ‐65 mV and was blocked by the h/HCN channel blocker ZD7288 (10 μm); and (3) a persistent Na+ current (INaP), which activated positive to ‐65 mV and was blocked by tetrodotoxin (TTX, 1 μm). In current clamp, XE991 or TTX suppressed the resonance at depolarized, but not hyperpolarized membrane potentials, whereas ZD7288 abolished the resonance only at hyperpolarized potentials. We conclude that these cells show two forms of θ‐resonance: ‘M‐resonance’ generated by the M‐current and persistent Na+ current in depolarized cells, and ‘H‐resonance’ generated by the h‐current in hyperpolarized cells. Computer simulations supported this interpretation. These results suggest a novel function for M/KCNQ channels in the brain: to facilitate neuronal resonance and network oscillations in cortical neurons, thus providing a basis for an oscillation‐based neural code.

Malfunctions of potassium channels are increasingly implicated as causes of neurological disorders. However, the functional roles of the large-conductance voltage-and Ca 2؉ -activated K ؉ channel (BK channel), a unique calcium, and voltage-activated potassium channel type have remained elusive. Here we report that mice lacking BK channels (BK ؊/؊ ) show cerebellar dysfunction in the form of abnormal conditioned eye-blink reflex, abnormal locomotion and pronounced deficiency in motor coordination, which are likely consequences of cerebellar learning deficiency. At the cellular level, the BK ؊/؊ mice showed a dramatic reduction in spontaneous activity of the BK ؊/؊ cerebellar Purkinje neurons, which generate the sole output of the cerebellar cortex and, in addition, enhanced short-term depression at the only output synapses of the cerebellar cortex, in the deep cerebellar nuclei. The impairing cellular effects caused by the lack of postsynaptic BK channels were found to be due to depolarization-induced inactivation of the action potential mechanism. These results identify previously unknown roles of potassium channels in mammalian cerebellar function and motor control. In addition, they provide a previously undescribed animal model of cerebellar ataxia. P otassium channels are the largest and most diverse class of ion channels underlying electrical signaling in the brain (1). By causing highly regulated, time-dependent, and localized polarization of the cell membrane, the opening of K ϩ channels mediates feedback control of excitability in a variety of cell types and conditions (1). Consequently, K ϩ channel dysfunctions can cause a range of neurological disorders (2-6), and drugs that target K ϩ channels hold promise for a variety of clinical applications (7).Among the wide range of voltage-and calcium-gated K ϩ channel types, one stands out as unique: the large-conductance voltage-and Ca 2ϩ -activated K ϩ channel (BK channel, also termed Slo or Maxi-K) differs from all other K ϩ channels in that it can be activated by both intracellular Ca 2ϩ ions and membrane depolarization (8). These channels are widely expressed in central and peripheral neurons, as well as in other tissues (9), and are regarded as a promising drug target (10). However, the functions of the BK channels in vivo have not previously been directly tested in any vertebrate species. We therefore decided to examine the functions of these channels by inactivating the gene encoding the pore-forming channel protein. MethodsA complete description of the methods is given in Supporting Methods, which is published as supporting information on the PNAS web site.Generation of BK Channel ␣ Subunit-Deficient Mice. In the targeting vector (Fig. 5, which is published as supporting information on the PNAS web site), the pore exon was flanked by a single loxP site and a floxed neo͞tk cassette. Correctly targeted embryonic stem cells were injected into C57BL͞6 blastocysts and resulting chimeric mice mated with C57BL͞6. Homozygous BK-deficient mice (F 2 generation) ...

In humans, mutations in the KCNQ2 or KCNQ3 potassium-channel genes are associated with an inherited epilepsy syndrome. We have studied the contribution of KCNQ/M-channels to the control of neuronal excitability by using transgenic mice that conditionally express dominant-negative KCNQ2 subunits in brain. We show that suppression of the neuronal M current in mice is associated with spontaneous seizures, behavioral hyperactivity and morphological changes in the hippocampus. Restriction of transgene expression to defined developmental periods revealed that M-channel activity is critical to the development of normal hippocampal morphology during the first postnatal weeks. Suppression of the M current after this critical period resulted in mice with signs of increased neuronal excitability and deficits in hippocampus-dependent spatial memory. M-current-deficient hippocampal CA1 pyramidal neurons showed increased excitability, reduced spike-frequency adaptation, attenuated medium afterhyperpolarization and reduced intrinsic subthreshold theta resonance. M channels are thus critical determinants of cellular and neuronal network excitability, postnatal brain development and cognitive performance.

In hippocampal pyramidal cells, a single action potential (AP) or a burst of APs is followed by a medium afterhyperpolarization (mAHP, lasting ∼0.1 s). The currents underlying the mAHP are considered to regulate excitability and cause early spike frequency adaptation, thus dampening the response to sustained excitatory input relative to responses to abrupt excitation. The mAHP was originally suggested to be primarily caused by M-channels (at depolarized potentials) and h-channels (at more negative potentials), but not SK channels. In recent reports, however, the mAHP was suggested to be generated mainly by SK channels or only by h-channels. We have now re-examined the mechanisms underlying the mAHP and early spike frequency adaptation in CA1 pyramidal cells by using sharp electrode and whole-cell recording in rat hippocampal slices. The specific M-channel blocker XE991 (10 µM) suppressed the mAHP following 1-5 APs evoked by current injection at −60 mV. XE991 also enhanced the excitability of the cell, i.e. increased the number of APs evoked by a constant depolarizing current pulse, reduced their rate of adaptation, enhanced the afterdepolarization and promoted bursting. Conversely, the M-channel opener retigabine reduced excitability. The h-channel blocker ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride; 10 µM) fully suppressed the mAHP at −80 mV, but had little effect at −60 mV, whereas XE991 did not measurably affect the mAHP at −80 mV. Likewise, ZD7288 had little or no effect on excitability or adaptation during current pulses injected from −60 mV, but changed the initial discharge during depolarizing pulses injected from −80 mV. In contrast to previous reports, we found that blockade of Ca 2+ -activated K + channels of the SK/K Ca type by apamin (100-400 nM) failed to affect the mAHP or adaptation. A computational model of a CA1 pyramidal cell predicted that M-and h-channels will generate mAHPs in a voltage-dependent manner, as indicated by the experiments. We conclude that M-and h-channels generate the somatic mAHP in hippocampal pyramidal cells, with little or no net contribution from SK channels.