Genetic activation of BK currents in vivo generates bidirectional effects on neuronal excitability

Phosphorylation of a constitutive serine inhibits BK channel variants containing the alternate exon "SRKR"

BK Ca²⁺-activated K⁺ currents exhibit diverse properties across tissues. The functional variation in voltage- and Ca²⁺-dependent gating underlying this diversity arises from multiple mechanisms, including alternate splicing of Kcnma1, the gene encoding the pore-forming (α) subunit of the BK channel, phosphorylation of α subunits, and inclusion of β subunits in channel complexes. To address the interplay of these mechanisms in the regulation of BK currents, two native splice variants, BK₀ and BK_SRKR, were cloned from a tissue that exhibits dynamic daily expression of BK channel, the central circadian pacemaker in the suprachiasmatic nucleus (SCN) of mouse hypothalamus. The BK₀ and BK_SRKR variants differed by the inclusion of a four–amino acid alternate exon at splice site 1 (SRKR), which showed increased expression during the day. The functional properties of the variants were investigated in HEK293 cells using standard voltage-clamp protocols. Compared with BK₀, BK_SRKR currents had a significantly right-shifted conductance–voltage (G-V) relationship across a range of Ca²⁺ concentrations, slower activation, and faster deactivation. These effects were dependent on the phosphorylation state of S642, a serine residue within the constitutive exon immediately preceding the SRKR insert. Coexpression of the neuronal β4 subunit slowed gating kinetics and shifted the G-V relationship in a Ca²⁺-dependent manner, enhancing the functional differences between the variants. Next, using native action potential (AP) command waveforms recorded from SCN to elicit BK currents, we found that these splice variant differences persist under dynamic activation conditions in physiological ionic concentrations. AP-induced currents from BK_SRKR channels were significantly reduced compared with BK₀, an effect that was maintained with coexpression of the β4 subunit but abolished by the mutation of S642. These results demonstrate a novel mechanism for reducing BK current activation under reconstituted physiological conditions, and further suggest that S642 is selectively phosphorylated in the presence of SRKR.

Myelinated Ah-type trigeminal ganglion neurons in female rats: neuroexcitability, chemosensitivity to histamine, and potential clinical impact

Migraine is a chronic neurological disorder characterized by recurrent moderate-to-severe headaches often associated with numerous autonomic nervous system symptoms, and it is more prevalent in women. To fully understand the underlying mechanism, standard electrophysiology was performed with trigeminal ganglion neurons (TGNs) isolated from adult rats of both genders using the whole-cell patch clamp technique to test the distribution, neuroexcitability, and chemosensitivity to histamine. In addition to traditionally classified A- and C-type TGNs, myelinated Ah-type TGNs were also observed in females. The electrophysiological features showed low firing threshold and the capability to fire repetitively upon stimulation. Ah-type neurons also functionally expressed persistent TTX-R Na(+) channels with more hyperpolarized activating voltage. Iberiotoxin and NS11021 significantly altered the discharge profiles of Ah-type TGNs. Finally, Ah-type TGNs showed a more potent reaction to histamine, with relatively larger inward currents and membrane depolarization compared with C-types. These data provide evidence of the gender-specific distribution of myelinated Ah-type TGNs in adult female rats, characterized by a low threshold and high frequency of firing that are at least partially attributable to persistent TTX-R Na(+) and BK-KCa channel expression and potent chemosensitivity to histamine, suggesting that Ah-type TGNs may play a key role in gender differences in migraine.

Properties of Slo1 K+ channels with and without the gating ring

High-conductance Ca(2+)- and voltage-activated K(+) (Slo1 or BK) channels (KCNMA1) play key roles in many physiological processes. The structure of the Slo1 channel has two functional domains, a core consisting of four voltage sensors controlling an ion-conducting pore, and a larger tail that forms an intracellular gating ring thought to confer Ca(2+) and Mg(2+) sensitivity as well as sensitivity to a host of other intracellular factors. Although the modular structure of the Slo1 channel is known, the functional properties of the core and the allosteric interactions between core and tail are poorly understood because it has not been possible to study the core in the absence of the gating ring. To address these questions, we developed constructs that allow functional cores of Slo1 channels to be expressed by replacing the 827-amino acid gating ring with short tails of either 74 or 11 amino acids. Recorded currents from these constructs reveals that the gating ring is not required for either expression or gating of the core. Voltage activation is retained after the gating ring is replaced, but all Ca(2+)- and Mg(2+)-dependent gating is lost. Replacing the gating ring also right-shifts the conductance-voltage relation, decreases mean open-channel and burst duration by about sixfold, and reduces apparent mean single-channel conductance by about 30%. These results show that the gating ring is not required for voltage activation but is required for Ca(2+) and Mg(2+) activation. They also suggest possible actions of the unliganded (passive) gating ring or added short tails on the core.

Blurring the boundaries: developmental and activity-dependent determinants of neural circuits

The human brain comprises approximately 100 billion neurons that express a diverse, and often subtype-specific, set of neurotransmitters and voltage-gated ion channels. Given this enormous complexity, a fundamental question is how is this achieved? The acquisition of neurotransmitter phenotype was viewed as being set by developmental programs 'hard wired' into the genome. By contrast, the expression of neuron-specific ion channels was considered to be highly dynamic (i.e., 'soft wired') and shaped largely by activity-dependent mechanisms. Recent evidence blurs this distinction by showing that neurotransmitter phenotype can be altered by activity and that neuron type-specific ion channel expression can be set, and perhaps limited by, developmental programs. Better understanding of these early regulatory mechanisms may offer new avenues to avert the behavioral changes that are characteristic of many mental illnesses.

Circadian dysfunction may be a key component of the non-motor symptoms of Parkinson's disease: insights from a transgenic mouse model

Sleep disorders are nearly ubiquitous among patients with Parkinson's disease (PD), and they manifest early in the disease process. While there are a number of possible mechanisms underlying these sleep disturbances, a primary dysfunction of the circadian system should be considered as a contributing factor. Our laboratory's behavioral phenotyping of a well-validated transgenic mouse model of PD reveals that the electrical activity of neurons within the master pacemaker of the circadian system, the suprachiasmatic nuclei (SCN), is already disrupted at the onset of motor symptoms, although the core features of the intrinsic molecular oscillations in the SCN remain functional. Our observations suggest that the fundamental circadian deficit in these mice lies in the signaling output from the SCN, which may be caused by known mechanisms in PD etiology: oxidative stress and mitochondrial disruption. Disruption of the circadian system is expected to have pervasive effects throughout the body and may itself lead to neurological and cardiovascular disorders. In fact, there is much overlap in the non-motor symptoms experienced by PD patients and in the consequences of circadian disruption. This raises the possibility that the sleep and circadian dysfunction experienced by PD patients may not merely be a subsidiary of the motor symptoms, but an integral part of the disease. Furthermore, we speculate that circadian dysfunction can even accelerate the pathology underlying PD. If these hypotheses are correct, more aggressive treatment of the circadian misalignment and sleep disruptions in PD patients early in the pathogenesis of the disease may be powerful positive modulators of disease progression and patient quality of life.

Quantitative localization of Cav2.1 (P/Q-type) voltage-dependent calcium channels in Purkinje cells: somatodendritic gradient and distinct somatic coclustering with calcium-activated potassium channels

P/Q-type voltage-dependent calcium channels play key roles in transmitter release, integration of dendritic signals, generation of dendritic spikes, and gene expression. High intracellular calcium concentration transient produced by these channels is restricted to tens to hundreds of nanometers from the channels. Therefore, precise localization of these channels along the plasma membrane was long sought to decipher how each neuronal cell function is controlled. Here, we analyzed the distribution of Ca(v)2.1 subunit of the P/Q-type channel using highly sensitive SDS-digested freeze-fracture replica labeling in the rat cerebellar Purkinje cells. The labeling efficiency was such that the number of immunogold particles in each parallel fiber active zone was comparable to that of functional channels calculated from previous reports. Two distinct patterns of Ca(v)2.1 distribution, scattered and clustered, were found in Purkinje cells. The scattered Ca(v)2.1 had a somatodendritic gradient with the density of immunogold particles increasing 2.5-fold from soma to distal dendrites. The other population with 74-fold higher density than the scattered particles was found within clusters of intramembrane particles on the P-face of soma and primary dendrites. Both populations of Ca(v)2.1 were found as early as P3 and increased in the second postnatal week to a mature level. Using double immunogold labeling, we found that virtually all of the Ca(v)2.1 clusters were colocalized with two types of calcium-activated potassium channels, BK and SK2, with the nearest neighbor distance of ∼40 nm. Calcium nanodomain created by the opening of Ca(v)2.1 channels likely activates the two channels that limit the extent of depolarization.

Mis-expression of the BK K(+) channel disrupts suprachiasmatic nucleus circuit rhythmicity and alters clock-controlled behavior

In mammals, almost all aspects of circadian rhythmicity are attributed to activity in a discrete neural circuit of the hypothalamus, the suprachiasmatic nucleus (SCN). A 24-h rhythm in spontaneous firing is the fundamental neural intermediary to circadian behavior, but the ionic mechanisms that pattern circuit rhythmicity, and the integrated impact on behavior, are not well studied. Here, we demonstrate that daily modulation of a major component of the nighttime-phased suppressive K(+) current, encoded by the BK Ca(2+)-activated K(+) current channel (K(Ca)1.1 or Kcnma1), is a critical arbiter of circadian rhythmicity in the SCN circuit. Aberrant induction of BK current during the day in transgenic mice using a Per1 promoter (Tg-BK(R207Q)) reduced SCN firing or silenced neurons, decreasing the circadian amplitude of the ensemble circuit rhythm. Changes in cellular and circuit excitability in Tg-BK(R207Q) SCNs were correlated with elongated behavioral active periods and enhanced responses to phase-shifting stimuli. Unexpectedly, despite the severe reduction in circuit amplitude, circadian behavioral amplitudes in Tg-BK(R207Q) mice were relatively normal. These data demonstrate that downregulation of the BK current during the day is essential for the high amplitude neural activity pattern in the SCN that restricts locomotor activity to the appropriate phase and maintains the clock's robustness against perturbation. However, a residually rhythmic subset prevails over the ensemble circuit to drive the fundamental circadian behavioral rhythm.