The role of voltage‐gated calcium channels in neurotransmitter phenotype specification: Coexpression and functional analysis in Xenopus laevis

Abstract

Calcium activity has been implicated in many neurodevelopmental events, including the specification of neurotransmitter phenotypes. Higher levels of calcium activity lead to an increased number of inhibitory neural phenotypes, while lower levels of calcium activity lead to excitatory neural phenotypes. Voltage-gated calcium channels (VGCCs) allow for rapid calcium entry and are expressed during early neural stages, making them likely regulators of activity-dependent neurotransmitter phenotype specification. To test this hypothesis, multiplex fluorescent in situ hybridization was used to characterize the coexpression of eight VGCC α1 subunits with the excitatory and inhibitory neural markers, xVGlut1 and xVIAAT, in Xenopus laevis embryos. VGCC coexpression was higher with xVGlut1 than xVIAAT, especially in the hindbrain, spinal cord, and cranial nerves. Calcium activity was also analyzed on a single-cell level and spike frequency was correlated with the expression of VGCC α1 subunits in cell culture. Cells expressing Cav2.1 and Cav2.2 displayed increased calcium spiking compared to cells not expressing this marker. VGCC antagonist diltiazem and agonist (−)BayK 8644 were used to manipulate calcium activity. Diltiazem exposure increased the number of glutamatergic cells and decreased the number of GABAergic cells, while (−)BayK 8644 exposure decreased the number of glutamatergic cells without having an effect on the number of GABAergic cells. Given that the expression and functional manipulation of VGCCs is correlated with neurotransmitter phenotype in some, but not all, experiments, VGCCs likely act in combination with a variety of other signaling factors to determine neuronal phenotype specification.

Methylmercury exposure during early Xenopus laevis development affects cell proliferation and death but not neural progenitor specification

“…Investigation of the role of calcium during these events could provide additional insight due to observed antagonism of calcium channels by MeHg (Yuan et al 2005), and the importance of calcium in cell migration and neural phenotype specification. Recently, frequent calcium spiking in neural progenitors, suggested to be inhibited by MeHg (Fahrion et al 2012; Kong et al 2013), has been implicated to lead to a GABAergic phenotype (Lewis et al 2014; Marek et al 2010), providing an additional possible mechanism for the enhanced downregulation of inhibitory neuronal populations by MeHg and the resulting neurological sensory and motor defects.…”

Genetic disruption of voltage-gated calcium channels in psychiatric and neurological disorders

“…Furthermore, it has also been observed that calcium spike activity mediates dopaminergic and GABAergic specification within the ventral suprachiasmatic nucleus of Xenopus laevis. This specification is seen to be activity-dependent and is prevented by inhibiting calcium transients ( Marek et al, 2010 , Lewis et al, 2014 ). Thus, it is not surprising that subtle alterations in calcium channel function can alter neuronal connectivity, influencing cognitive functions later in life.…”

Neurotransmitter Switching? No Surprise

“…The lower expression of GABA and xGAD67 transcripts was rescued in the absence of extracellular calcium by reimposition of intracellular calcium transients at the frequency normally generated in cultured neurons (Figure 5) (Gu and Spitzer, 1995; Watt et al, 2000). In more recent work, blockade of calcium spikes increased the number of neurons expressing xVGluT1 and decreased the number of neurons expressing xGAD67 transcripts; enhancing calcium spiking reduced the number of neurons expressing xVGluT1 without detectable effect on the number of neurons expressing xGAD67 (Lewis et al, 2014). …”