Abstract
Intrasynaptosomal free calcium concentrations measured in rat cortical synaptosomes using the calcium sensitive fluorescent dye quin 2, were found to be 0.107–0.122 μM (n = 33). Stimulation of α 2-adrenoceptors or κ-opiate receptors with clonidine (0.1–100 μM) or dynorphin A fragment (1–13) (0.2–5.0 μM) respectively, produced a dose dependent reduction in the intrasynaptosomal free calcium concentration. The α 2-adrenoceptor mediated reduction produced by clonidine (100 μM) was antagonised by the selective α 2-antagonist idazoxan (RX781094) (200 μM). Dynorphin A-(1–13) (1 μM) caused a reduction which was antagonised by a high (20 μM) but not low (2.0 μM) concentration of naloxone. We conclude that unlike the μ- and δ-opiate receptor, stimulation of α 2-adrenoceptors or κ-opiate receptors elicits changes in intrasynaptosomal free calcium concentration which are independent of the voltage sensitive calcium channel. This does not preclude the possibility that the α 2-adrenoceptor and κ-opiate receptor also have an effect at the voltage sensitive calcium channel.
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