Mutual Antagonism of κ‐Opiate and α2‐Adrenoceptor Agonist Effects on Intrasynaptosomal Free [Ca2+]i

Abstract

Synaptosomes prepared from rat cerebral cortices on Percoll discontinuous density gradients were loaded with the fluorescent EGTA analogue Quin 2 to allow measurement of intracellular free [Ca²⁺]_i. When either κ-opiate or α₂-adrenoceptor agonists were incubated with the synaptosomes, there was a highly significant (p < 0.004, p < 2.7 × 10⁻⁶, respectively) reduction in intrasynaptosomal free [Ca²⁺]_i relative to controls. As these synaptosomes are not depolarised, the data suggest that both α₂-adrenoceptor agonists and κ-opiate agonists inhibit neurotransmitter release, decreasing the availability of intraneuronal [Ca²⁺]_i rather than altering Ca²⁺ entry. However, when these two agonists were coincubated, there was a complete abolition of the effects of either agonist; in fact, there was an apparent increase in the intrasynaptosomal free [Ca²⁺]_i. Neither morphine nor [D-Ala²-D-Leu⁵]enkephalin, μ and δ opiate agonists respectively, had any significant effect on intrasynaptosomal free [Ca²⁺]_i. These results show that the individual effects of clonidine and dynorphin A_1–13 are in keeping with the role of these substances at autoreceptors controlling neurotransmitter release. The mutual antagonism of their effects on [Ca²⁺]_i is more difficult to explain but it may be a mechanism that prevents the occurrence of excessive inhibition of neuronal systems.