(-)-Stepholidine acts as a D1 partial agonist on firing activity of substantia nigra pars reticulata neurons in 6-hydroxydopamine-lesioned rats

(-)-Stepholidine blocks expression, but not development, of cocaine conditioned place preference in rats

The purpose of this study was to investigate the effects of (-)-stepholidine (SPD), a compound with dopamine D1 partial agonist and D2/D3 antagonist properties, on the development and expression of cocaine conditioned place preference (CPP). Subjects (N = 65; male Long Evans rats) were tested using a CPP procedure consisting of 3 phases: (1) a 15-min pre-exposure session where animals could explore each compartment freely, (2) eight 30-min conditioning sessions where animals were restricted to one side or the other with cocaine (10 mg/kg) or saline, respectively, on alternating days and (3) a 15-minute preference test session where animals could explore each compartment freely. To test the effects of SPD on expression of cocaine CPP, rats were administered vehicle (distilled water with 20 % DMSO), 10, 15 or 20 mg/kg SPD (intraperitoneally) 30 min prior to the test session. We found that 20 mg/kg of SPD significantly blocked the expression of cocaine CPP. To test the effects of SPD on the development of CPP, 0 (vehicle), 10, 15 or 20mg/kg SPD were administered 30 min prior to each cocaine conditioning session and vehicle before each saline conditioning session; no treatment was given prior to the test session. A preference test showed that each SPD group maintained a CPP similar to the vehicle group. These data indicate that SPD can block the expression of a cocaine CPP but has no effect on its development, suggesting that it inhibits the effects of cocaine cues on cocaine incentive motivated behavior. These results suggest that SPD may be a potential treatment for cue-driven aspects of cocaine use disorder.

(-)-Stepholidine is a potent pan-dopamine receptor antagonist of both G protein- and β-arrestin-mediated signaling

RATIONALE

(-)-Stepholidine is a tetrahydroberberine alkaloid that is known to interact with dopamine receptors and has also been proposed as a novel antipsychotic agent. Its suggested novelty lies in the fact that it has been proposed to have D1-like receptor agonist and D2-like receptor antagonist properties. Thus, it might be effective in treating both positive and negative (cognition) symptoms of schizophrenia. However, its activity on specific dopamine receptor subtypes has not been clarified, especially with respect to its ability to activate D1-like receptors.

OBJECTIVES

We wished to examine the affinity and functional activity of (-)-stepholidine at each of the human dopamine receptor subtypes expressed in a defined cellular environment.

METHODS

D1-D5 dopamine receptors were stably expressed in cell lines and their interactions with (-)-stepholidine were examined using radioligand binding and various functional signaling assays. Radioligand binding assays were also performed using bovine striatal membranes.

RESULTS

(-)-Stepholidine exhibited high (nM) affinity for D1 and D5 receptors, somewhat lower (two- to four-fold) affinity for D2 and D3 receptors, and low micromolar affinity for D4 receptors. Functionally, (-)-stepholidine was ineffective in activating G protein-mediated signaling of D1-like and D2 receptors and was also ineffective in stimulating β-arrestin recruitment to any dopamine receptor subtype. It did, however, antagonize all of these responses. It also antagonized D1-D2 heteromer-mediated Ca(2+) mobilization. Radioligand binding assays of D1-like receptors in brain membranes also indicated that (-)-stepholidine binds to the D1 receptor with antagonist-like properties.

CONCLUSIONS

(-)-Stepholidine is a pan-dopamine receptor antagonist and its in vivo effects are largely mediated through dopamine receptor blockade with potential cross-talk to other receptors or signaling proteins.

The neuropharmacology of (-)-stepholidine and its potential applications

(-)-Stepholidine (SPD), a natural product isolated from the Chinese herb Stephania, possesses dopamine (DA) D1 partial agonistic and D2 antagonistic properties in the nigrostriatal and mesocorticolimbic DAergic pathways. These unique dual effects have suggested that SPD can effectively restore previously imbalanced functional linkage between D1 and D2 receptors under schizophrenic conditions, in which, SPD improves both the negative and positive symptoms of schizophrenia. SPD also relieves the motor symptoms of Parkinson's disease (PD) when co-administered with Levodopa. Furthermore, SPD exhibits neuroprotective effects through an antioxidative mechanism and slows down the progression of neuronal degeneration in the substantia nigra (SN) of PD patients and/or animal models. Therefore, SPD is a novel, natural compound with potentially therapeutic roles in the treatment of schizophrenia and/or PD.

l-Stepholidine increases the frequency of sEPSC via the activation of D1 dopamine signaling pathway in rat prelimbic cortical neurons

AIM

To investigate the effect of l-stepholidine (SPD) on the frequency of spontaneous excitatory postsynaptic currents (sEPSC) in the pyramidal cells between layers V and VI in the prelimbic cortex (PL).

METHODS

A whole-cell patch clamp in rat brain slices was used.

RESULTS

SPD significantly increased the frequency of sEPSC in a concentration-dependent manner. A selective D1 dopamine receptor antagonist SCH23390 blocked SPD-mediated effects, whereas the D1 agonist SKF38393, but not the D2/3 antagonist sulpiride, mimicked SPD-mediated increase in the frequency of sEPSC. Moreover, both protein kinase A (PKA) inhibitor N-(2- [p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide hydrochloride and protein kinase C (PKC) inhibitor chelerythrine attenuated the effect of SPD on sEPSC.

CONCLUSION

SPD elicits its effect on the frequency of sEPSC on the PL pyramidal cells via presynaptic D1 receptors, and is dependent on PKA and PKC signaling pathways.

A possible mechanism for the dopamine-evoked synergistic disinhibition of thalamic neurons via the "direct" and "indirect" pathways in the basal ganglia

The mechanism of synaptic plasticity which we have previously proposed for striatal spiny neurons, along with published data on the predominance of dopamine-sensitive D1/D2 receptors on strionigral/striopallidal neurons, was used as the basis to propose the hypothesis that the induction of long-term potentiation/depression of the efficiency of the cortical inputs to these cells may result from the excitatory/inhibitory actions of dopamine on the activity of the neurons originating the "direct" and "indirect" pathways through the basal ganglia. Thus, the action of dopamine increases disinhibition of thalamic neurons via the "direct" pathway and decreases their inhibition via the "indirect" pathway. Both effects lead to increases in the activity of thalamic cells and in the activity of the efferent neocortical neurons which they excite. The actions of dopamine on striosomal neurons, which mainly have D1 receptors, may also be to induce long-term potentiation of cortical inputs. This effect should lead to increased inhibition of dopaminergic cells and decreases in their dopamine release, which may promote the maintenance of a stable dopamine concentration in the cortex-basal ganglia-thalamus-cortex neural network.

Comparison of (-)-stepholidine and D1 or D2 agonists on unit firing of globus pallidus in 6-hydroxydopamine-lesioned rats

(-)-Stepholidine(SPD), isolated from the Chinese herb Stephania, is demonstrated to be a DA antagonist, but it also shows D1 agonistic action on rotational behavior in unilateral 6-hydroxydopamine (6-OHDA) lesioned rats. The present study further ascertains its D1 agonistic property on firing activity of globus pallidus (GP) in control and 6-OHDA-lesioned rats. In the control rats, the firing activities of the GP neurons elicited by DA agonists (i.v.), such as apomorphine (D1/D2), SKF38393 (D1), and LY171555 (D2), were readily reversed by SPD (4 mg/kg, i.v.); but SPD, per se, induced variable alterations. In the 6-OHDA-lesioned rats, apomorphine, SKF38393 and LY171555 displayed the marked inhibition as well as excitation on the unit firing. The individual firing variations (87.1+/-17.8, 55.1+/-15.7 and 62.1+/-16.7%, respectively) were much larger than those in the control group, and were completely abolished by SPD (2 mg/kg). However, SPD also showed D1 partial agonistic action on the GP neuron firing. Moreover, the pre-blockade of D2 receptors with spiperone (0.5 mg/kg, i.v.), SPD exhibited the D1 agonist action which was reversed by the D1 antagonist SCH23390. These results suggest that SPD has a dual action on the GP neuron firing in the 6-OHDA-lesioned rats, i.e., antagonist to D2 DA receptors and partial agonist to D1 receptors.

Involvement of dopamine D1 and D2 receptors in Fos immunoreactivity induced by stepholidine in both intact and denervated striatum of lesioned rats

Stepholidine (SPD), a natural product, has been demonstrated in previous studies as a D1 agonist and D2 antagonist. In this work SPD-induced Fos immunoreactivity was examined. In the normal rats, Fos was induced in the striatum by SPD (1-20 mg/kg, i.p.) dose-dependently. The distribution of Fos-positive cells induced by SPD showed a rostral-caudal decline, matching the distribution of D2 dopamine receptors. The Fos-positive cells were mainly found in striatal neurons retrogradely labeled with horseradish peroxidase (HRP) from GP but not from SN, and could be abolished by the pretreatment of a D2 agonist LY171555 (2 mg/kg, i.p.), suggesting that the Fos expression in normal rats was due to the D2 antagonistic action of SPD. In the unilateral 6-hydroxydopamine-lesioned rats, SPD (4 mg/kg, i.p.) induced Fos expression in intact and denervated side of the striatum with different characteristics. Similar to that of normal rats, the Fos expression in intact side possessed the rostral-caudal gradient and could be abolished by the pretreatment of LY171555. However, in the denervated side, the Fos positive cells were widely distributed, and mainly found in striatal neurons retrogradely labeled from SN but not from GP. Furthermore, this expression was prevented by the pretreatment of SCH23390 (0.2 mg/kg, i.p.) but not LY171555, suggesting that the Fos expression in denervated side was due to the D1 agonistic action of SPD. Therefore, we concluded that the Fos expression induced by SPD in intact and denervated striatum was mediated via D2 and D1 receptor respectively, supporting the previous standpoint that SPD possesses the dual action, i.e antagonist to D2 and agonist to D1 receptors. Furthermore, it is suggested that the contralateral turning behavior induced by SPD may result from the D1-mediated excitation of striatonigral neurons of the denervated side of the lesioned rats.

Involvement of receptor reserve in D1 agonistic action of (-)-stepholidine in lesioned rats

(-)-Stepholidine (SPD) is a natural product. Previous studies had demonstrated that SPD displayed D1 agonism in unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats and D1 antagonism in reserpinized rats and normal rats. The aim of the present study was to explain this peculiar pharmacological action based on behavioral and biochemical experiments. In the unilaterally 6-OHDA-lesioned rats, SPD (4 mg/kg, s.c.) induced contralateral rotation as did apomorphine (APO), but the rotation response to SPD was 60% lower than that to APO (0.5 mg/kg, i.p.). Coadministration with APO (0.5 mg/kg, i.p.) and SPD (0.5 to 10 mg/kg, s.c.) produced a biphasic action curve. At low doses (0.5 or 1 mg/kg), SPD potentiated APO action; at high doses (4 or 10 mg/kg), however, SPD suppressed APO. In striatal homogenate of the unilaterally lesioned rats, SPD stimulated cyclic AMP (cAMP) formation and produced a maximal response comparable to that of dopamine (DA) in the denervated striatum, but 70% lower than that of DA in the intact striatum. Coadministration of 10 microM DA with various concentrations of SPD yielded different results, with a biphasic response in the intact side and a synergistic effect in the denervated side. Furthermore, based on the determination of receptor-mediated cAMP formation, the D1 receptor reserve was analyzed in both denervated and intact striatum by using the DA receptor inactivator N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The results showed that following EEDQ administration, the receptor density [revealed by [3H]R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-be nzazepine ([3H]SCH-23390) binding] and the agonist-stimulated adenylate cyclase (AC) activity (revealed by cAMP formation) were reduced concurrently. In the intact striatum, the reduction in SPD-stimulated AC activity paralleled the receptor loss, indicating the absence of receptor reserve, while in the denervated striatum the reduction in AC activity was less than the receptor loss, indicating a significant level of receptor reserve (estimated 16.4%). By comparison, receptor reserve for DA was 45.7 and 25.3% in the denervated and intact striatum, respectively, representing an 80% increase of receptor reserve. In conclusion, SPD is a D1 partial agonist, and receptor reserve permits SPD to display its D1 agonistic action in the unilaterally 6-OHDA-lesioned rats.

GTP regulation of (-)-stepholidine binding to R(H) of D1 dopamine receptors in calf striatum

(-)-Stepholidine (SPD) exhibits antagonist effects on normosensitive dopamine (DA) receptors, but it has an agonist action on rotation in unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. The present work endeavors to further elucidate the mechanism of its agonist action on D1 receptors. [3H]R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-be nzazepine ([3H]SCH-23390) and [3H]spiperone were used, respectively, as radioligands in D1 and D2 DA receptor binding assays in calf striatal membranes. Experimental data were analyzed by a non-linear regression computer program, GraphPAD InPlot 3.15. The competition curves were fitted first by a single-site equation and then by a two-site equation. The results showed that both apomorphine (APO) and SPD competitively inhibited [3H]SCH-23390 binding. Their competition curves fitted best to the two-site equation (P < 0.05) with a high-affinity site (R(H)) and a low-affinity site (R(L)) to DA receptors. The K(H) and K(L) values (nM) were 2.7 +/- 0.45 and 378 +/- 62 for APO, and 3.9 +/- 2.2 and 126 +/- 25 for SPD, respectively. In contrast, the competition curve of SCH-23390, a selective D1 DA receptor antagonist, fitted best to a single-site model with a Ki value of 1.7 +/- 0.5 nM. The R(H) of APO or SPD could be decreased by the addition of 450 microM GTP. In the [3H]spiperone binding test, the APO curve was modeled best by the two-site equation, while the SPD curve fitted best to a single-site model. In the rotational behavior test, APO induced 441 +/- 20 turns/30 min in the 6-OHDA-lesioned rats, and SPD induced 310 +/- 42 turns/30 min, while SCH-23390 antagonized the SPD-induced rotation but did not induce rotational behavior. These results suggest that SPD possesses agonist actions on D1 but antagonist effects on D2 DA receptors.

Dual actions of (-)-stepholidine on the dopamine receptor-mediated adenylate cyclase activity in rat corpus striatum

(-)-Stepholidine (SPD) is an antagonist of normosensitive dopamine (DA) receptors, but it exhibits D1 agonistic action on rotational behaviour in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNC). In the present study, agonistic and antagonistic effects of SPD on the DA receptor-mediated synaptosomal adenylate cyclase (AC) activity in rat striatum were investigated. After blockade of D2 receptors, SPD augmented AC activity dose-dependently. The EC50 value was 41.1 +/- 8.6 micromol/L. At the concentration of 10 micromol/L, SPD increased cAMP formation from a basal level (50.8 +/- 10.3 pmol/mg protein/min) to 133.7 +/- 31.8 pmol/mg protein/min. The SPD-induced stimulation of AC activity was almost completely reversed by 10 micromol/L Sch23390. These results indicate that SPD possesses an agonistic action on the D1 receptor. Forskolin-stimulated adenylate cyclase (FSAC) activity was used as a model to elucidate the effect of SPD on D2 receptors. The results indicate that DA inhibited FSAC activity dose-dependently, while SPD partially restored FSAC activity. Taken together, these results support the conclusion that SPD has dual actions on DA receptors that mediate AC activity, i.e., an agonistic action on D1 receptors and an antagonistic action on D2 receptors.