mu-Opioid agonists inhibit the enhanced intracellular Ca(2+) responses in inflammatory activated astrocytes co-cultured with brain endothelial cells.

In order to imitate the in vivo situation with constituents from the blood-brain barrier, astrocytes from newborn rat cerebral cortex were co-cultured with adult rat brain microvascular endothelial cells. These astrocytes exhibited a morphologically differentiated appearance with long processes. 5-HT, synthetic mu-, delta- or kappa-opioid agonists, and the endogenous opioids endomorphin-1, beta-endorphin, and dynorphin induced higher Ca(2+) amplitudes and/or more Ca(2+) transients in these cells than in astrocytes in monoculture, as a sign of more developed signal transduction systems. Furthermore, stimulation of the co-cultured astrocytes with 5-HT generated a pronounced increase in intracellular Ca(2+) release in the presence of the inflammatory or pain mediating activators substance P, calcitonin gene-related peptide (CGRP), lipopolysaccharide (LPS), or leptin. These Ca(2+) responses were restored by opioids so that the delta- and kappa-opioid receptor agonists reduced the number of Ca(2+) transients elicited after incubation in substance P+CGRP or leptin, while the mu- and delta-opioid receptor agonists attenuated the Ca(2+) amplitudes elicited in the presence of LPS or leptin. In LPS treated co-cultured astrocytes the mu-opioid receptor antagonist naloxone attenuated not only the endomorphin-1, but also the 5-HT evoked Ca(2+) transients. These results suggest that opioids, especially mu-opioid agonists, play a role in the control of neuroinflammatory activity in astrocytes and that naloxone, in addition to its interaction with mu-opioid receptors, also may act through some binding site on astrocytes, other than the classical opioid receptor.