Chronic fluoxetine administration increases expression of the L-channel gene Cav1.2 in astrocytes from the brain of treated mice and in culture and augments K+-induced increase in [Ca2+

Abstract

We have recently shown that freshly isolated astrocytes from the mouse brain express mRNA for the L-channel gene Cav1.3 to at least the same degree (per mg mRNA) as corresponding neurons. The amount of extracellular Ca2+ actually entering cultured astrocytes by its opening is modest, but due to secondary Ca2+-mediated stimulation of the ryanodine receptor (RyR) the increase in free cytosolic Ca2+ [Ca2+]i is substantial. The other Cav1 subtype expressed in brain is Cav1.2, which is even expressed in higher density. Although the different primers used for the two genes preclude exact quantitative comparison, the present study suggests that this is also the case in the freshly isolated astrocytes and neurons, which express equal Cav1.2 densities. Again, most of the increase in [Ca2+]i occurred by RyR activity. In contrast to Cav1.3 the expression of Cav1.2 was greatly increased (doubled) after two weeks of treatment with fluoxetine hydrochloride (10mg/kg). Accordingly [Ca2+]i in cultured astrocytes exposed to the addition of 10–60mM KCl increased substantially in cultured astrocytes treated chronically with fluoxetine with the lag time until the effect was observed depending upon the fluoxetine concentration. This effect was inhibited by nifedipine or siRNA against Cav1.2. The increase in K+-induced rise in [Ca2+]i after fluoxetine treatment is directly opposite to a decrease in [Ca2+]i after treatment with any of the anti-bipolar drugs lithium, carbamazepine or valproic acid, due to reduced capacitative Ca2+ influx. We have previously shown a similar effect after fluoxetine treatment, but it becomes overridden by the Cav1.2 up-regulation.