rms80, thanks for the info. yeah, i looked into it and it seems that L-dopa is a dopamine agonist so, if anything, it should increase the number of receptors and make them more sensitive. for personal feedback, i didnt feel any kind of rebound effect with USPLabs powerfull either. it is still important to maintain a healthy balance so i agree with your statement regarding cycling.
by any chance do you know which of the five dopamine receptors this effects? all of them maybe?
Dopamine has 5 subtypes of receptors- D1, D2, D3, D4, and D5. There are also two families of receptors: D1-like family (D1 and D5), and the D2-like family (D2,D3, and D4).
Cocaine, methamphetamine, and other types of euphoric drugs primarily activate the D1.
I also found this:
D1/D2 DOPAMINE RECEPTOR STIMULATION BY L-DOPA
A[14C]-2-DEOXYGLUCOSE AUTORADIOGRAPHIC STUDY
JOEL. M. TRUGMAN, CHRISTINA L. JAMES and G. FREDERICK WOOTEN
In rats with unilateral 6-hydroxydopamine substantia nigra lesions, the effects of selective D1 and D2 dopamine receptor antagonists on L-DOPA-induced rotation and regional cerebral glucose utilization (RCGU) changes were examined. Contralateral rotation induced by L-DOPA(25mg/kg) was effectively blocked by D1(SCH 23390, 1.0mg/kg)and D2(eticlopride, 2.0 mg/kg) antagonists, in combination, but not by either antagonist alone. This suggests that in the dopamine-depleted rat, L-DOPA administration results in the stimulation of both D1and D2 receptor systems, each capable of independently eliciting a full motor response. L-DOPA altered RCGU in the following brain regions ipsilateral to the lesion: entopeduncular nucleus (EP, +105%), substantia nigra pars reticulata (SNr, +121%), subthalamic nucleus(STN, +32%), deep layers of the superior colliculus (DLSC, +35%), and laternal habenula nucleus (LHN, -52%). The effects in the EP and SNr were blocked completely by D1 antagonist pretreatment but only partially attenuated by D2 antagonist pretreatment, indicating the critical dependence of these changes on D1 stimulation. In contrast, combined D1 and D2 antagonist pretreatment, but neither drug alone, blocked the L-DOPA- induced increases in the STN and DLSC. The effects of L-DOPA in the LHN were attenuated by either SCH 23390 or eticlopride, and blocked completely by the antagonist combination.
These results provide evidence that dopamine formed following the decarboxylation of L-DOPA stimulates both D1 and D2 receptors in vivo and that stimulation of each receptor contributes uniquely to its physiological effects. Neural mechanisms of action of L-DOPA are discussed in the context of these findings.
This study states that D2 and D3 (D2-like family) receptors can down-regulate dopamine synthesis, but that D4 does not. So by process of elimination, D1, D2, and D3 definitely have a role in down regulation of some kind, but I have seen contradictory reports about D4.
Inhibition of dopamine synthesis by dopamine D2 and D3 but not D4 receptors
CM O'Hara, A Uhland-Smith, KL O'Malley and RD Todd
Department of Psychiatry, Washington University School of Medicine, St. Louis, Missouri, USA.
The goal of the current study was to determine which of the D2-like receptors (D2, D3 or D4) are involved in autoreceptor regulation of dopamine synthesis. We have derived a model system utilizing a mouse mesencephalic cell line, MN9D, which both synthesizes and releases dopamine, to characterize the modulation of tyrosine hydroxylase activity, the rate limiting enzyme in the conversion of tyrosine to dopamine, by the D2-like receptors. Previously, we have shown that stimulation of D2 and D3, but not D4, dopamine receptors transfected into MN9D cells inhibited the release of dopamine. In the current study, we show that quinpirole stimulation of transfected D2 and D3, but not D4, dopamine receptors inhibited K+-stimulated tyrosine hydroxylase activity in a pertussis toxin-sensitive manner, strongly suggesting G-protein coupling as a mechanistic pathway. The D2 receptor effect could be maintained for at least 60 min, whereas the D3 receptor effect desensitized. Treatment with 10 microM forskolin, which raises cyclic AMP levels or with 100 nM okadaic acid, a potent phosphatase inhibitor, had no effect on the D2-or D3-mediated inhibition, suggesting that these effects may be independent of both cyclic AMP- and okadaic acid-sensitive phosphatase activity.
Taken together, these data confirm the hypothesis that dopamine D2 and D3 receptors can perform dual roles in autoreceptor regulation.