Adrenergic and 5-HT 2 receptors on the same astroglial cell. A microspectrofluorimetric study on cytosolic Ca 2+ responses in single cells in primary culture

Abstract

Noradrenaline (NA) or 5-hydroxytryptamine (5-HT) evoked cytosolic Ca 2+ mobilization in single type 1 astrocytes in primary culture from the cerebral cortex of newborn rat. The Ca 2+ indicator dye fura-2/AM was used in a microspectrofluorimetric system to visualize fluctuations in the intracellular Ca 2+ concentration. Activation of the adrenergic receptors α 2, α 2 and β, or activation of the 5-HT 2 receptors elicited different responses of Ca 2+ mobilization with different types of Ca 2+ spikes or oscillations. Principally, 4 different types of Ca 2+ responses could be obtained: a sharp spike, which declined back to baseline; an initial sharp spike, which declined to a smaller but sustained Ca 2+ elevation; an initial sharp spike which declined and showed low amplitude oscillations; and a sharp spike which declined back to baseline with baseline oscillations. Applications of the a 2 adrenoceptor agonist clonidine to individual astroglial cells evoked Ca 2+ transients mostly in young cultures (cultivated for 7–10 days), while applications of the α 1 adrenoceptor agonist phenylephrine evoked Ca 2+ transients mostly in older cultures (17–21 days of cultivation). Applications of the β adrenoceptor agonist isoproterenol evoked Ca 2+ transients in both young and older cultures, however, more frequent in older cultures. The a 2 and β receptor responses were dependent on external Ca 2+ levels. The NA-evoked Ca 2+ responses were seen in cultivated cells at all ages, but were more frequent in older cultures. Approximately 50% of the astroglial cells in 8 day old cultures responded to 5-HT with a cytosolic Ca 2+ mobilization and 80% of the cells in 21 day old cultures responded. Calcium transients evoked by different receptor agonists on the same astrocyte were obtained. Most cells which responded to 5-HT 2 receptor stimulation also responded to NA. The responses were blocked by specific receptor antagonists whereafter the receptors could be restimulated. Single type 1 astrocytes responded to α 1, β and 5-HT 2 receptor stimulation. Other cells responded to α 1, β and 5-HT 2 receptor agonists. Some cells only responded to 5-HT 2 receptor stimulation while others only responded to α 2 or NA, per se. Three conclusions can be made. (1) Adrenergic and 5-HT 2 receptors, the activation of which elicited cytosolic Ca 2+ mobilization, are expressed on single type 1 astrocytes. (2) Adrenergic and 5-HT 2 receptors are expressed on the same type 1 astrocyte in primary culture. (3) There seems to be a prominent heterogeneity among cells concerning membrane receptor sets.