The expression of oestrogen-inducible nuclear progesterone receptors has been demonstrated in the PNS (Labombarda et al. 2000) and in primary glial cell cultures of the CNS (Jung-Testas et al. 1992) and PNS (Do Thi et al. 1998). The addition of progesterone to oestrogen-primed neurons causes an increase in calcium currents compared with no effect in non-oestrogen-treated neurons (Joels & Karst, 1995). Interestingly such an increase did not occur until 2 h after administration of progesterone. Consistent with mammalian studies, previous work in our laboratory has demonstrated oestrogen-inducible progesterone receptors in primary cultures of glial cells from the ring dove (Streptopelia risoria).

The purpose of this study was to examine the effects of progesterone on calcium mobilisation in oestrogen-primed and untreated glial cells. Primary cultures were obtained from neonatal ring dove squabs (1-2 days old) following cervical dislocation. Tissue was mechanically dissociated and the resulting supernatant was added to poly-L-lysine-coated coverslips. Cultures were grown for 10 days in Dulbecco’s modified Eagle’s medium. One group of cultures was then grown for a further 10-15 days in the same medium supplemented with 50 nM 17β-oestradiol whilst the control group were cultured for the same period in normal medium. After transferring the coverslips into a bath containing a modified Krebs-Ringer buffer, a calcium-sensitive dye, fura-2AM, and a microfluorimetry system (Cairns Optivision) incorporating imaging software (Axon Imaging Workbench) was used to monitor the intracellular calcium levels of individual cells. The addition of progesterone (100 µM) caused a significant increase in intracellular calcium after a period of 2330 s (unpaired t test, P = < 0.01, n = 5 for all groups). No effect was seen in non-oestrogen-treated cells. When similar studies were conducted in a calcium-free buffer no increase was seen in either group.

We believe that this is the first time that the addition of progesterone to oestrogen-primed glial cells has been shown to cause a modulation of intracellular calcium and that this effect is apparent within 40 min. The results further suggest that the underlying mechanism is one of influx of calcium across the cell membrane possibly via a membrane-bound receptor rather than release from intracellular stores.

This work was supported by the Royal Preston Hospital Neurosciences Directorate.