Phorbol esters and D2-dopamine receptors

4-β-Phorbol-12,13-dibutyrate (PDBu), a powerful activator of protein kinase C (PKC), enhanced dopamine (DA) release evoked by electrical stimulation (1 Hz, 2 min) from the striatum and the prefrontal cortex of the rabbit. However, acetylcholine (ACh) release from the striatum (1 Hz, 2 min), was only enhanced slightly by PDBu. The increase in DA release induced by PDBu was reduced markedly at higher frequencies of stimulation. Sulpiride (10 μM) alone, a D2 DA-receptor antagonist, or combined with nomifensine (3 μM), a neuronal-uptake inhibitor, did not prevent PDBu-induced facilitation of DA release from prefrontal cortex or striatum. The D2 DA agonists (LY-171555, bromocriptine and apomorphine) inhibited in a concentration-dependent manner the stimulation-evoked overflow of DA and ACh from the striatum, and of DA from the prefrontal cortex. Pretreatment with PDBu antagonized the inhibitory effect of the three agonists on DA and ACh release. A reduction both in E(max) nd IC50 was observed in PDBu-treated slices. Removal of endogenous DA by pretreatment with reserpine and α-methyl-p-tyrosine, failed to prevent PDBu-induced antagonism of apomorphine effects on ACh release, indicating that the antagonism of agonist effects was not due to higher synaptic levels of endogenous DA. The inactive enantiomer of PDBu, 4-α-12,13-dibutyrate did not enhance DA release and failed to modify the effects of D2 agonists on DA release. In summary, if PDBu acts through activation of PKC, our results suggest that: 1) PKC can modulate the effects induced by activation of release-modulatory D2 DA receptors located on DA terminals and cholinergic neurons of the striatum and of D2 DA autoreceptors of the prefrontal cortex. A reduced number of membrane receptors or an uncoupling of these receptors could occur by PKC-mediated phosphorylation of D2 DA receptors; 2) activation of PKC enhances the evoked release of DA. This increase in release occurs independently of an effect on release-modulatory D2 DA receptors; 3) more rapid stimulation rates may lead to greater PKC activation reducing the facilitatory effect of PDBu on DA release; and 4) the functional status of a neuron may determine its PKC activity level which, in turn, determines the magnitude of PDBu effects. This may explain the magnitude of facilitation of release produced by PDBu: DA-prefrontal cortex > DA striatum > ACh striatum.