More details on E-/R-type Ca2+ channel, rhythmicity, synchronization and epilepsy (Marco Weiergräber & Toni Schneider):

Voltage-gated calcium channels (VGCC) are key regulators of Ca2+-entry into neurons, and control a variety of Ca2+-dependent functions, including neurotransmitter release, gene expression, mRNA stability and neuronal excitability (Spafford and Zamponi, 2003;Dolmetsch, 2003;Spitzer et al., 2002). The mechanism of generating rhythmic activity within the CNS is based on such ion channels, which share similarity to channels of cardiac pacemaker centers. Synchronous rhythms in the thalamic reticular nucleus are even generated in small clusters of electrically coupled neurons (Long et al., 2004) and a novel interneuron population was identified in the neocortex which generates theta frequencies (Blatow et al., 2003). At both locations, GABAergic chemical transmission and electric coupling were described to be responsible for synchrony. Based on the expression pattern of the E-type Ca2+ channel, we assume that the release of GABA is triggered by E-type VGCC.
VGCC play a fundamental role in shaping the autonomous rhythmic activity of single neurons and the periodicity of network oscillations (for an overview see: (Destexhe and Sejnowski, 2001)). An extensively studied example is the generation of rhythmic bursts of action potentials in thalamic relay neurons (Huguenard, 1996;Destexhe et al., 1998;Futatsugi and Riviello, Jr., 1998;Kim et al., 2001;Zhang et al., 2002;Porcello et al., 2003). These neurons, embedded in the thalamocortical circuitry are a key component in the generation and regulation of cortical sleep as well as thalamocortical dysfunctions, resulting in dyssomnia and generalized non-convulsive seizures (Petit-Mal, absence-epilepsy).
A fundamental circuit diagram of the mammalian thalamus has been established as the result of morphological investigations allied with physiological studies in vivo and in vitro (Danober et al., 1998). The thalamic network cannot be viewed in isolation as being solely engaged in the transmission of sensory information from the periphery to the centers for perception in the cerebral cortex, because thalamic and cortical networks interact in a highly coherent manner during forebrain activities that underlie perception, cognition and the sleep-wake cycle. Besides the cortex and the thalamic relay neurons, the reticular thalamic nucleus (RTN) plays a key role in the thalamocortical-corticothalamic circuitry (Danober et al., 1998;Bazhenov et al., 1998;Destexhe and Sejnowski, 2001). It is well-known that the GABAergic cells of the RTN are innervated by collateral branches of thalamocortical and corticothalamic fibres. This bi-directional collateral input to the RTN forms the basis of the fundamental circuit diagram of the thalamus common to all mammals.
A number of voltage- and ligand-gated ion channels is involved in this circuits and they are similar to what we have earlier discussed for cardiac pacemaker cells. Two cell-types will be described in more detail in order to display rhythmogenesis in neuronal networks: RTN cells and thalamic relay neurons. Both cell types excert voltage-independent as well as various voltage-dependent membrane conductances based on the activity of voltage-dependent Na+ and K+ channels, hyperpolarisation and cyclic-nucleotide gated (HCN) ion channels as well as voltage-gated Ca2+-channels.
The low-threshold voltage-gated calcium channels and probably the Cav2.3 containing E-type channels (De Borman et al., 1999) are the most important voltage-dependent conductance for RTN and thalamic relay cells (Coulter et al., 1989;Coulter et al., 1990;tsakiridou et al., 1995;Talley et al., 2000). These conductances control two distinct response modes, the tonic or the burst mode, which are operative when the thalamic relay cell respond to afferent input. The tonic mode of firing occurs when the LVA Ca2+ and certain other associated conductances are inactivated. The relay cell then responds like the linear neuronal integrator described by cable modelling: its response to input is characterized by a steady stream of action potentials of a frequency and duration that corresponds fairly linearly to input strength and duration. During the burst mode, which occurs when the LVA channels are de-inactivated and thus able to be activated again, the neuronal response to a depolarizing input consists of brief bursts of action potentials separated by silent periods. We assume that Cav2.3 influences the duration of the silent periods (Fig. 1).


Fig. 1: Schematic view of ion current components which are made responsible for individual phases during bursting activity (modified according to McCormick & Pape, 1990).

When low voltage activated channels (LVA) activate, it produces a spike-like, triangular depolarization of roughly 20-30 mV and lasting about 50 ms: this is the low-threshold calcium spike. The depolarization associated with this low-threshold spike produces, at its peak, a high frequency burst of 2-10 conventional action potentials (lasting 1-2 ms, 80 mV amplitude, > 400 Hz). In this way, IT and the resultant low-threshold spike provide an amplification that permits a hyperpolarized cell to generate action potentials in response to moderate EPSP, and the resulting firing pattern is bursty. Similar to what has been described for cardiac pacemakers one also finds hyperpolarisation and cyclic nucleotide gated (HCN) channels (Moosmang et al., 1999;Moosmang et al., 2001;Ludwig et al., 2003). The HCN channels probably carry Ih. The combination of IT and Ih lead to rhythmic bursting and shows characteristics similar to slow diastolic depolarization in cardiac pacemaker cells (Stieber et al., 2004). Appropriate membrane voltage shifts can thus effectively switch the cell between rhythmic bursting and tonic firing.
The distribution of neuronal LVA channels has been well characterized by in situ hybridization methods (Talley et al., 1999;Talley et al., 2000). Cav3.1 was predominantly expressed in thalamic relay neurons whereas Cav3.2 and Cav3.3 were detected in the reticular thalamic nucleus. Interestingly, the Cav2.3 (1E) voltage-gated calcium channel was also found to be expressed in the reticular thalamic nucleus (De Borman et al., 1999), but its functional role still remains unresolved.
Due to the expression of Cav2.3 within the reticular thalamic nucleus and gabaergic cortical interneurons, 1E might play an essential role in this thalamocortical-corticothalamic circuitry and therefore also be involved in thalamocortical dysfunction resulting in sleep disorders and absence epilepsy. It has been shown that T-type voltage-gated calcium channels play a crucial role in determination of absence epilepsy susceptibility in GAERs as well as in Cav3.1 defiecient mice (Talley et al., 2000;Kim et al., 2001). In GAERS it was clearly shown that not Cav3.1 but Cav2.3 displayed decreased transcription levels in cerebellum as well as in brain stem (De Borman et al., 1999). This is of high relevance as both the cerebellum and brainstem nuclei project onto the thalamocortical-corticothalamic circuitry.
Furthermore, the Cav2.3 voltage-gated calcium channel is expressed in gabergic interneurons of the cortex and in the temporal lobe (Timmermann et al., 1999;Timmermann et al., 2002) suggesting a possible role for absence epilepsy, and for temporal lobe epilepsy (TLE).

Recent results related to the E-/R-type voltage-gated Ca2+ channel.
Structure and function of E-type voltage-gated Ca2+ channels were elucidated during the last years in great detail for those organs in which this channel type is highly expressed as in the endocrine system (Vajna et al., 2001;Pereverzev et al., 2002c;Pereverzev et al., 2002b) and the central nervous system (Sochivko et al., 2002;Dietrich et al., 2003). However, in those tissues, where it is expressed at a very low density as in heart and kidney (Weiergräber et al., 2000), its function is only partially known (Lu et al., 2004).

1. Structure of the E-type Ca2+ channel. The exact composition of the heteromeric E-type Ca2+ channel is not yet identified for the specific tissues were this channel was detected and related to specific functions (e.g. brain, heart, endocrine organs). However, the modulation of the ion conducting subunit by several auxiliary subunits has been investigated in heterologous systems as X. laevis oocytes (Parent et al., 1997) and stably transfected HEK-293 cells (Nakashima et al., 1998) showing that most of the known auxiliary subunits interact with and modulate the Cav2.3 subunit. Thus, the real composition of the E-type channel complex must be determined by purification of the channel from the tissue of interest and by identification of the associated subunits. Further, the Cav2.3 subunit itself is alternatively spliced. Two major splice variants were identified (Pereverzev et al., 1998;Vajna et al., 1998) and found to be expressed for Cav2.3e either in heart, endocrine tissues, and cerebellum or for Cav2.3c in the remaining central nervous system except cerebellum (Grabsch et al., 1999;Schramm et al., 1999).

2. Kinetic properties and modulation of the E-type Ca2+ channel. After establishing stable cell lines expressing native and mutated Cav2.3 splice variants, its modulation by receptors (Mehrke et al., 1997) and a new region related to the modulation of the Cav2.3 channel by incoming Ca2+ was investigated (Pereverzev et al., 2002a;Leroy et al., 2003). This positive feedback modulation by the native charge carrier itself could be of great importance for the in vivo regulation of synaptic plasticity in hippocampal neurons (Dietrich et al., 2003) which will be investigated in the future.

3. Expression of E-type Ca2+ channels. The tissues-specific expression of Cav2.3 splice variants as mentioned (Grabsch et al., 1999;Schramm et al., 1999) is not yet completely understood. Especially in heart, where its expression was deduced from immunohistochemical studies (Weiergräber et al., 2000), Cav2.3 splice variants are expressed in a coordinated manner. Besides the major Cav2.3e variant, also the neuronal Cav2.3c variant is deteced during later prenatal development (Lu et al., 2004). The functional meaning of the successive expression is not yet understood.

4. Function of E-type Ca2+ channels in organs and tissues. Cav2.3 triggers the glucagon release (Pereverzev et al., 2004, unpublished results) as well as the insulin release from the islets of Langerhans (Pereverzev et al., 2002b) and from INS-1 cells (Vajna et al., 2001;Pereverzev et al., 2002c) which was confirmed by other groups in an independent pharmacological approach (Schulla et al., 2003). Further, E-type channels share responsibility for the synaptic plasticity in the mossy fiber CA3 neurons with other non-L-type voltage gated Ca2+ channels (Sochivko et al., 2002;Dietrich et al., 2003). This finding is very important for understanding the basic mechanisms involved in memory formation and modulation of long term potentiation in a model in which Cav2.3 is located distant from the neurotransmitter release site (Brenowitz and Regehr, 2003), but is “recruited” by incoming Ca2+ (Dietrich et al., 2003). We assume that this kind of synaptic plasticity is mediated through a new mechanism including Ca2+-sensitive interactions at the loop between domain II and III of the Cav2.3 subunit, detected by us (Leroy et al., 2003). Recent findings point to Ca2+-sensitive protein kinase C as an indirect partner for Cav2.3 (Klöckner et al., 2004).

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