The Ca _V 3.3 calcium channel is the major sleep spindle pacemaker in thalamus

Abstract

Low-threshold (T-type) Ca2+ channels encoded by the Ca V 3 genes endow neurons with oscillatory properties that underlie slow waves characteristic of the non-rapid eye movement (NREM) sleep EEG. Three Ca V 3 channel subtypes are expressed in the thalamocortical (TC) system, but their respective roles for the sleep EEG are unclear. Ca V 3.3 protein is expressed abundantly in the nucleus reticularis thalami (nRt), an essential oscillatory burst generator. We report the characterization of a transgenic Ca V 3.3 −/− mouse line and demonstrate that Ca V 3.3 channels are indispensable for nRt function and for sleep spindles, a hallmark of natural sleep. The absence of Ca V 3.3 channels prevented oscillatory bursting in the lowfrequency (4-10 Hz) range in nRt cells but spared tonic discharge. In contrast, adjacent TC neurons expressing Ca V 3.1 channels retained low-threshold bursts. Nevertheless, the generation of synchronized thalamic network oscillations underlying sleep-spindle waves was weakened markedly because of the reduced inhibition of TC neurons via nRt cells. T currents in Ca V 3.3 −/− mice were <30% compared with those in WT mice, and the remaining current, carried by Ca V 3.2 channels, generated dendritic [Ca 2+ ] i signals insufficient to provoke oscillatory bursting that arises from interplay with Ca 2+ -dependent small conductance-type 2 K + channels. Finally, naturally sleeping Ca V 3.3 −/− mice showed a selective reduction in the power density of the σ frequency band (10-12 Hz) at transitions from NREM to REM sleep, with other EEG waves remaining unaltered. Together, these data identify a central role for Ca V 3.3 channels in the rhythmogenic properties of the sleep-spindle generator and provide a molecular target to elucidate the roles of sleep spindles for brain function and development.2+ channels encoded by the Ca V 3 genes activate near resting membrane potentials and generate low-threshold Ca 2+ spikes leading to burst firing and low-frequency oscillatory discharge that are prominent in some thalamic, olivary, and cerebellar neurons (1). Among the low-threshold Ca 2+ currents carried by Ca V 3 channels, those mediated by Ca V 3.3 channels are unique in that they display the slowest time course, the fastest recovery from inactivation, and often the most depolarized activation voltages (2, 3). Moreover, Ca V 3.3 mRNA is expressed predominantly in brain and shows highest regional specificity (3-5). To date, identification of specific physiological roles for Ca V 3.3 channels has been hampered for several reasons. First, these channels typically are coexpressed with Ca V 3.1 and/or Ca V 3.2 channels (4, 5), and specific pharmacological tools are not available (1). Second, Ca V 3.3 channels often are found in distal dendrites, limiting accessibility for electrophysiological characterization (6, 7). Finally, Ca V 3.3 −/− mice have not been reported, whereas Ca V 3.1 −/− and Ca V 3.2 knockdown mice have helped address the roles of Ca V 3.1 and Ca V 3.2 channels in sleep and pain, respectively (8-10)...

Rebound burst firing in the reticular thalamus is not essential for pharmacological absence seizures in mice

“…1F). These results are consistent with those reported previously (22) and suggest that Ca V 3.3 is responsible for most T-type Ca 2+ currents in TRN neurons. The small amount of residual current observed in Ca V 3.3 −/− neurons at -40 mV is probably mediated by Ca V 3.2 and as observed previously (22).…”

“…The mAHPs prevent an excessive increase in axon fiber excitability and contribute to the gradual decrease in tonic firing rate during a continuous activity. In TRN neurons, the mAHP is largely dependent on Ca V 3.3 T-type channels that are the major T-type Ca 2+ channel in this region (22). In Ca V 3.3 −/− and Ca V 3.2 −/− /3.3 −/− mice, the mAHP is greatly reduced due to the lack of T-type channels.…”

The calcium-binding protein parvalbumin modulates the firing 1 properties of the reticular thalamic nucleus bursting neurons

“…Statistics are described in the text. (Astori et al 2011;Halassa et al 2011). The RTN is characterized by a rather homogenous population of GABAergic neurons, expressing high levels of the "slow Ca 2ϩ buffer" PV (Celio 1990;Schwaller 2009 (Talley et al 1999) are associated with the bursts of discharges that characterize most RTN cell activity.…”

“…Immunohistochemical analysis of a PV-EGFP reporter mouse line (Meyer et al 2002) showed that in the RTN a minority (6% Ϯ1) of the neurons were PV negative (data not shown). It has been recently proposed that the tonically firing cells are characterized by a lower responsiveness to low Ca 2ϩ currents, I T (Lee et al 2007), possibly due to a large difference in the distal vs. proximal distribution of the LVA channels Ca v 3.2 and Ca v 3.3, (Crandall et al 2010), or due to the lack of Ca v 3.3, which is essential for LTS (Astori et al 2011). On the whole it appears that this cell population has physiological properties that are distinct from the bursting types and are independent of the presence of PV.…”

One-cell model for inhibiting Cav3.3 mRNA expression by RNA interference

“…Cav3.3 T-type calcium channels are mainly expressed in the cerebrum, spinal cord, dorsal root ganglion (DRG) neurons, adrenal cortex and thyroid gland. It has been shown that Cav3.3 in nucleus reticularis thalami is involved with the sleep spindles, a hallmark of natural sleep [12]. To date, there is no specific blocking agent for T-type calcium channels.…”