Presynaptic CaMKIIα modulates dopamine D3 receptor activation in striatonigral terminals of the rat brain in a Ca2+ dependent manner

• Coactivation of dopamine D3 receptors with D1 receptors potentiates cAMP formation and GABA release in SNr.
• The potentiating effects of activating dopamine D3 receptors are antagonized by procedures that increase cytoplasmic Ca2+.
• The effects of increasing cytoplasmic Ca2+ are reversed by CAMKII antagonist KN-62.
• Dopamine D3 receptor activity is inhibited by binding to CAMKII.

CaMKIIα is expressed at high density in the nucleus accumbens where it binds to postsynaptic D3 receptors inhibiting their effects. In striatonigral projections, activation of presynaptic D3 receptors potentiates D1 receptor-induced stimulation of cAMP production and GABA release. In this study we examined whether the presynaptic effects of D3 receptor stimulation in the substantia nigra reticulata (SNr) are modulated by Ca2+ activation of CaMKIIα. In SNr synaptosomes two procedures that increase cytoplasmic Ca2+, ionomycin and K+-depolarization, blocked the additional stimulation of cAMP accumulation produced by coactivating D3 and D1 dopamine receptors. The selective CaMKIIα inhibitor KN-62 reversed the blockade produced by ionomycin and K+-depolarization. Incubation in either Ca2 -free solutions or with the selective Ca2+ blocker nifedipine, also reversed the blocking effects of K+-depolarization. Immunoblot studies showed that K+-depolarization increased CaMKIIα phosphorylation in a KN-62 sensitive manner and promoted CaMKIIα binding to D3 receptors. In K+-depolarized tissues, D3 receptors potentiated D1 receptor-induced stimulation of [3H]GABA release only when CaMKIIα was blocked with KN-62. In the presence of this inhibitor, the selective D3 agonist PD 128,907 reduced the ED50 for the D1 agonist SKF 38393 from 56 to 4 nM. KN-62 also enhanced the effects of dopamine on depolarization induced [3H]GABA release. KN-62 changed ED50 for dopamine from 584 to 56 nM. KN-62 did not affect D1 and D4 receptor responses. These experiments show that in striatonigral projections, CaMKIIα inhibits the action of D3 receptors in a Ca2+ dependent manner blocking their modulatory effects on GABA release. These findings suggest a mechanism through which the frequency of action potential discharge in presynaptic terminals regulates dopamine effects.