Voltage‐gated calcium channels play crucial roles in the glutamate‐induced phase shifts of the rat suprachiasmatic circadian clock

Abstract

The resetting of the circadian clock based on photic cues delivered by the glutamatergic retinohypothalamic tract is an important process helping mammals to function adaptively to the daily light-dark cycle. To see if the photic resetting relies on voltage-gated Ca(2+) channels (VGCCs), we examined the effects of VGCC blockers on the glutamate-induced phase shifts of circadian firing activity rhythms of suprachiasmatic nucleus (SCN) neurons in hypothalamic slices. First, we found that a cocktail of amiloride, nimodipine and omega-conotoxin MVIIC (T-, L- and NPQ-type VGCC antagonists, respectively) completely blocked both phase delays and advances, which were, respectively, induced by glutamate application in early and late night. Next, we discovered that: (i) amiloride and another T-type VGCC antagonist, mibefradil, completely obstructed the delays without affecting the advances; (ii) nimodipine completely blocked the advances while having less impact on delays; and (iii) omega-conotoxin MVIIC blocked largely, if not entirely, both delays and advances. Subsequent whole-cell recordings revealed that T-type Ca(2+) currents in neurons in the ventrolateral, not dorsomedial, region of the SCN were larger during early than late night, whereas L-type Ca(2+) currents did not differ from early to late night in both regions. These results indicate that VGCCs play important roles in glutamate-induced phase shifts, T-type being more important for phase delays and L-type being so for phase advances. Moreover, the results point to the possibility that a nocturnal modulation of T-type Ca(2+) current in retinorecipient neurons is related to the differential involvement of T-type VGCC in phase delays and advances.

Excitatory Actions of GABA in the Suprachiasmatic Nucleus

“…Brain slices were prepared as described previously (Kim et al, 2001(Kim et al, , 2005. Animals were anesthetized with sodium pentobarbitone (100 mg/kg, i.p.)…”

“…Extracellular recordings from SCN slices were conducted as described previously (Kim et al, 2001(Kim et al, , 2005. The slices were superfused continuously (1-1.5 ml/min) with warm physiological saline (35°C, pH 7.4) aerated with 95% O 2 /5% CO 2.…”

“…Current-clamp recordings were obtained from neurons in SCN slices equilibrated for Ͼ1 h in the recording chamber (Kim et al, 2005). Micropipettes (tip diameter, 1.0 -1.5 m; 3-5 M⍀) pulled from borosilicate tubing (Sutter Instrument) and filled with gramicidin (50 g/ml)-containing internal solution [containing (in mM) 143 K-gluconate, 2 KCl, 10 HEPES, and 0.5 EGTA, pH 7.2-7.3] were used for recording.…”

Tetraethylammonium (TEA) increases the inactivation time constant of the transient K+ current in suprachiasmatic nucleus neurons

“…The mechanisms underlying the circadian regulation of cellular electrical activity remain largely unknown. Several ionic currents including a persistent Na + current, delayed rectifier K + channels (I DR ), large conductance Ca 2+ -activated K + channels (BK), transient K + current (A-type, I A ), and voltage-dependent L-and T-type Ca 2+ channels regulate action potential firing or show significant day-night differences in activity or both (Itri et al, 2004;Jackson et al, 2004;Kim et al, 2005;Kononenko et al, 2004;Kuhlman and McMahon, 2004;Meredith et al, 2006;Pennartz et al, 2002;Pitts et al, 2006). Identification of the contribution that each ion channel makes to the firing of SCN neurons requires selective chemical tools with defined mechanisms of action.…”

Role of Na+/Ca2+exchanger in Ca2+homeostasis in rat suprachiasmatic nucleus neurons

“…Indeed, transmembrane Ca 2ϩ influx appears to be required for maintaining rhythmicity of the clock gene per1 in the rat SCN and of both per1 gene and PER2 protein in the mouse SCN (Lundkvist et al 2005). Furthermore, glutamate-mediated phase shifts during the night critically depend on Ca 2ϩ influx via voltage-dependent Ca 2ϩ channels (Irwin and Allen 2007;Kim et al 2005) as well as ryanodine receptor-mediated Ca 2ϩ signaling (Ding et al 1998;Gillette and Mitchell 2002). These findings indicate a critical role of intracellular Ca 2ϩ in maintaining circadian rhythmicity and mediating photic entrainment in the SCN.…”