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Low‐voltage‐activated or T‐type Ca2+ channels (T‐channels) are widely expressed, especially in the central nervous system where they contribute to pacemaker activities and are involved in the pathogenesis of epilepsy. Proper elucidation of their cellular functions has been hampered by the lack of selective pharmacology as well as the absence of generic endogenous regulations. We report here that both cloned (α1G, α1H and α1I subunits) and native T‐channels are blocked by the endogenous cannabinoid, anandamide. Anandamide, known to exert its physiological effects through cannabinoid receptors, inhibits T‐currents independently from the activation of CB1/CB2 receptors, G‐proteins, phospholipases and protein kinase pathways. Anandamide appears to be the first endogenous ligand acting directly on T‐channels at submicromolar concentrations. Block of anandamide membrane transport by AM404 prevents T‐current inhibition, suggesting that anandamide acts intracellularly. Anandamide preferentially binds and stabilizes T‐channels in the inactivated state and is responsible for a significant decrease of T‐currents associated with neuronal firing activities. Our data demonstrate that anandamide inhibition of T‐channels can regulate neuronal excitability and account for CB receptor‐independent effects of this signaling molecule.

In several types of neurons, firing is an intrinsic property produced by specific classes of ion channels. Low‐voltage‐activated T‐type calcium channels (T‐channels), which activate with small membrane depolarizations, can generate burst firing and pacemaker activity. Here we have investigated the specific contribution to neuronal excitability of cloned human T‐channel subunits. Using HEK‐293 cells transiently transfected with the human α_1G (Ca_V3.1), α_1H (Ca_V3.2) and α_1I (Ca_V3.3) subunits, we describe significant differences among these isotypes in their biophysical properties, which are highlighted in action potential clamp studies. Firing activities occurring in cerebellar Purkinje neurons and in thalamocortical relay neurons used as voltage clamp waveforms revealed that α_1G channels and, to a lesser extent, α_1H channels produced large and transient currents, while currents related to α_1I channels exhibited facilitation and produced a sustained calcium entry associated with the depolarizing after‐potential interval. Using simulations of reticular and relay thalamic neuron activities, we show that α_1I currents contributed to sustained electrical activities, while α_1G and α_1H currents generated short burst firing. Modelling experiments with the NEURON model further revealed that the α_1G channel and α_1I channel parameters best accounted for T‐channel activities described in thalamocortical relay neurons and in reticular neurons, respectively. Altogether, the data provide evidence for a role of α_1I channel in pacemaker activity and further demonstrate that each T‐channel pore‐forming subunit displays specific gating properties that account for its unique contribution to neuronal firing.

We describe here several novel properties of the human alpha(1G) subunit that forms T-type calcium channels. The partial intron/exon structure of the corresponding gene CACNA1G was defined and several alpha(1G) isoforms were identified, especially two isoforms that exhibit a distinct III-IV loop: alpha(1G-a) and alpha(1G-b). Northern blot and dot blot analyses indicated that alpha(1G) mRNA is predominantly expressed in the brain, especially in thalamus, cerebellum, and substantia nigra. Additional experiments have also provided evidence that alpha(1G) mRNA is expressed at a higher level during fetal life in nonneuronal tissues (i.e. kidney, heart, and lung). Functional expression in HEK 293 cells of a full-length cDNA encoding the shortest alpha(1G) isoform identified to date, alpha(1G-b), resulted in transient, low threshold activated Ca(2+) currents with the expected permeability ratio (I(Sr) > I(Ca) >/= I(Ba)) and channel conductance ( approximately 7 pS). These properties, together with slowly deactivating tail currents, are typical of those of native T-type Ca(2+) channels. This alpha(1G)-related current was inhibited by mibefradil (IC(50) = 2 microM) and weakly blocked by Ni(2+) ions (IC(50) = 148 microM) and amiloride (IC(50) > 1 mM). We showed that steady state activation and inactivation properties of this current can generate a "window current" in the range of -65 to -55 mV. Using neuronal action potential waveforms, we show that alpha(1G) channels produce a massive and sustained Ca(2+) influx due to their slow deactivation properties. These latter properties would account for the specificity of Ca(2+) influx via T-type channels that occurs in the range of physiological resting membrane potentials, differing considerably from the behavior of other Ca(2+) channels.

Ion channels are crucial components of cellular excitability and are involved in many neurological diseases. This review focuses on the sodium leak, G protein-coupled receptors (GPCRs)-activated NALCN channel that is predominantly expressed in neurons where it regulates the resting membrane potential and neuronal excitability. NALCN is part of a complex that includes not only GPCRs, but also UNC-79, UNC-80, NLF-1 and src family of Tyrosine kinases (SFKs). There is growing evidence that the NALCN channelosome critically regulates its ion conduction. Both in mammals and invertebrates, animal models revealed an involvement in many processes such as locomotor behaviours, sensitivity to volatile anesthetics, and respiratory rhythms. There is also evidence that alteration in this NALCN channelosome can cause a wide variety of diseases. Indeed, mutations in the NALCN gene were identified in Infantile Neuroaxonal Dystrophy (INAD) patients, as well as in patients with an Autosomal Recessive Syndrome with severe hypotonia, speech impairment, and cognitive delay. Deletions in NALCN gene were also reported in diseases such as 13q syndrome. In addition, genes encoding NALCN, NLF- 1, UNC-79 and UNC-80 proteins may be susceptibility loci for several diseases including bipolar disorder, schizophrenia, Alzheimer’s disease, autism, epilepsy, alcoholism, cardiac diseases and cancer. Although the physiological role of the NALCN channelosome is poorly understood, its involvement in human diseases should foster interest for drug development in the near future. Towards this goal, we review here the current knowledge on the NALCN channelosome in physiology and diseases.