Tetrahydroberberine suppresses dopamine-induced potassium current in acutely dissociated CA1 pyramidal neurons from rat hippocampus

Effects of tetrahydroberberine (THB) on dopamine (DA)-induced response have been investigated in single pyramidal neurons freshly dissociated from CA1 area of the hippocampus using the nystatin perforated patch-clamp, whole-cell recording technique under voltage-clamp configuration. At a holding potential (VH) of -20mV, DA-induced responses included a transient outward current, a slow inward current and a combination of the two. The outward current had a reversal potential of -83.5 +/- 8.0 mV which was close to K+ equilibrium potential and was sensitive to TEA, suggesting that this outward current was carried by K+. Application of THB reversibly suppressed three type responses induced by DA with different degrees of inhibitory ratio. THB inhibited the DA-induced outward current in a concentration-dependent manner with an IC50 of 13 microM. The maximal value of the concentration-response curve for DA-induced outward current was suppressed by THB, suggesting a non-competitive inhibition. The results support the hypothesis that THB acts as a novel dopaminergic system antagonist. Furthermore, THB inhibits the DA-induced K+ current non-competitively in single dissociated cells, implying that THB may exhibit other pharmacological action on the central nervous system.

[Effect of (+/-)12-chloroscoulerine on brain dopamine receptors]

AIM

To assess potencies of tetrahydroprotoberberines (THPB) and hydrobenzyltetrahydroisoquinolines (HBTI) on DA receptors.

METHODS

The receptor binding assay with calf striatum to D1 and D2 receptors, and the animal behavior tests were used.

RESULTS

(+/-) 12-Chloroscoulerine (CSL) was the most potent one among the THPB and HBTI. The affinities of CSL to D1 and D2 receptors were 13 and 51 nmol.L-1, respectively. In rats, CSL showed an antagonistic effect on the stereotypy and induced catalepsy. In the 6-OHDA lesioned rats, however, CSL exerted the agonistic effect to DA receptors.

CONCLUSION

CSL had dual actions to DA receptors and its effects were similar to that of (-)stepholidine.

Dopamine-induced ionic currents in acutely dissociated rat neurons of CNS

AIM

To determine whether or not the dopamine (DA) can induce ionic current in single neuron acutely dissociated from different central areas including striatum, ventral tegmental area (VTA), substantia nigra pars compacta (SNC) and hippocampal CA1 area.

METHODS

Using a new patch-clamp whole-cell recording technic, namely nystatin-perforated whole-cell configuration under voltage-clamp mode.

RESULTS

In 36 single neurons isolated from the striatum and VTA, DA 10-1000 mumol.L-1 was quite diverse to elicit ionic current responses. In 19 SNC neurons, 5 neurons (26%) response to 1 mmol.L-1 DA represented as a small outward current (11.3 +/- 2.4 pA) at a holding potential (V(H)) of -20 mV. In 25 of 69 (36%) examined hippocampal CA1 pyramidal neurons, however, application of DA induced 3 types of current responses: outward current (8 neurons) accompanied with an increase of membrane conductance, slow inward current (5 neurons) with an decrease of membrane conductance and outward-following inward current (12 neurons) at a V(H) of -20 mV. The concentration-response relationship of DA-induced currents showed the typical sigmoid shape with the threshold dose, being the maximum response dose are 3 mumol.L-1 and 1 mmol.L-1 respectively. The current-voltage (I-V) relationship of DA-induced responses did not show any voltage-dependent manner and the reversal potential (E(DA)) was close to the equilibrium potential of potassium (E(K)) calculated with the Nernst equation. TEA 5 mol.L-1 effectively inhibited the DA-induced response.

CONCLUSION

These results suggest that DA-induced outward current is carried by K+ in single hippocampal CA1 pyramidal neuron.

(-)-stepholidine vs 12-chloroscoulerine enantiomers on firing activity of substantia nigral dopamine neurons

AIM

To compare the potencies between (-)-stepholidine ((-)-SPD) and 12-chloroscoulerine (CSL) enantiomers on firing of substantia nigra (SN) dopamine (DA) neurons.

METHODS

Extracellular single unit recordings in paralyzed rats.

RESULTS

In rats, (-)-SPD, (-)-, ( +/- )-, and (+)-CSL attenuated i.v. apomorphine (Apo, 10 micrograms.kg-1)-induced inhibition on firing of DA cell, and their ED50 values were 15.1 (11.9-19.4), 7.8 (7.0-8.7), 12.6 (2.0-17.9) micrograms.kg-1, and 2.9 (2.6-3.3) mg.kg-1, respectively. Thus, (-)-CSL was 1 time more potent than (-)-SPD and 371 times more potent than (+)-CSL on D2 receptors. In reserpinized rats, (-)-SPD, (-)-, ( +/- )-, and (+)-CSL blocked the inhibition caused by i.v. 4 mg.kg-1 SKF-38393, with ED50 values of 0.53 (0.51-0.55), 0.51 (0.43-0.60), 1.2 (0.7-2.0), and 5.9 (4.9-7.1) mg.kg-1, respectively. The potency of (-)-CSL was similar to that of (-)-SPD on D1 receptors and 11 times higher than that of (+)-CSL.

CONCLUSION

CSL enantiomers are D1/D2 mixed antagonists as (-)-SPD. (-)-CSL is the most, while (+)-CSL is the least, potent one among them.

Effect of desipramine on spontaneous activity of hippocampal CA1 neuron after transient cerebral ischemia in rats

AIM

To study the spontaneous firing of CA1 neurons in rat hippocampus after transient cerebral ischemia and the effect of desipramine (Des) on the post-ischemic electric activity of CA1 neurons.

METHODS

Single-unit extracellular recordings were performed in rats on d 3 after 10 min of cerebral ischemia by occlusion of 4 arteries. Des and saline were injected into a tail vein. The histological changes of CA1 neurons was assessed by the neuronal density of the CA1 sector.

RESULTS

The spontaneous firing rate of CA1 neurons on d 3 after ischemia was enhanced in comparison with the control value. Des (0.2 and 0.4 mg.kg-1, i.v., n = 5 & 6, respectively) reduced dose-dependently the increase of firing rate with maximal inhibition by 6 min (58% & 85%) to 9 min (69% & 94%) (vs vehicle group, P < 0.01). About 50% cells in CA1 region showed necrotic changes.

CONCLUSION

Des antagonized the hyperexcitability of CA1 neurons after cerebral ischemia.

Molecular modeling of interactions between tetrahydroprotoberberines and dopamine receptors

AIM

To build up the structure models of dopamine receptors, then combined with the receptor models, to investigate the action mechanism of tetrahydroprotoberberines (THPB) on dopamine receptors at the molecular level.

METHODS

Using the three-dimensional structure of bacteriorhodopsin as a template, we have constructed dopamine D1 and D2 receptor models on computer. l-Stepholidine was selected as the leading compound of THPB and docked into D1 and D2 receptor active sites.

RESULTS

After manual adjustment and energy minimization, the ligand-receptor interaction models were achieved. Based on these models, the possible action mechanism of THPB on dopamine receptors was suggested that the protonated N atom of THPB form electrostatic interaction and hydrogen-bonding interaction with residue Asp in TM3 of the receptor, the two substituents in D ring of THPB form hydrogen-bonding interactions with two Ser residues in TM5 of the receptor, and the aryl groups form pi-pi interactions with some aryl residues of the receptor around ligand.

CONCLUSION

Our ligand-receptor interaction models should be helpful for rational design of more potent drugs.

[Effects of (-) SPD and (-) THP on the firing of noradrenergic neurons in locus coeruleus]

The effects of (-) SPD and (-) THP on the noradrenergic neurons in rat locus coeruleus were investigated by recording the in vivo spontaneous firing. It was found that (-) SPD could increase the firing frequency of the NE neurons via antagonistic action on alpha 2 autoreceptors in a dose-dependent manner, while alpha 2 receptor agonist clonidine exerted an inhibitory effect. (-) THP induced inhibitory firing of the neurons, however, irreversible.

Augmentation of dopamine release by (-)-stepholidine from rabbit and rat caudate slices

AIM

To study the effect of (-)-stepholidine (SPD) on dopamine (DA) release evoked by electric stimulation on slices of rabbit caudate nucleus.

METHODS

Slices of rabbit caudate nucleus were preincubated with [3H]DA and then perfused and stimulated electrically.

RESULTS

Quinpirole (Qui), a selective DA D2 receptor agonist, reduced [3H]DA overflow elicited by 24 mA electric stimulation (IC50 = 0.12, 95% confidence limits 0.09-0.17 mumol . L-1). SPD markedly increased the potential of the stimulation-evoked [3H]DA overflow in a dose-dependent manner and reversed Qui (1 mumol . L-1)-induced attenuation of [3H]DA release. The pA2 value calculated from the data of [3H]DA overflow for SPD was 7.495. In the experiments with rat caudate nucleus slices, SPD (0.1 mmol . L-1) increased the [3H]DA outflow from 3.3 +/- 0.2% to 6.5 +/- 0.5% of the tissue [3H]DA content, which was further enhanced by the protein kinase C (PKC) activator phorbol 12,13-dibutyrate (PDB, 1 mumol . L-1) to 10.1 +/- 1.0% of the total [3H]DA in the tissue.

CONCLUSION

SPD is a presynaptic D2 autoreceptor antagonist and induces a synergic effect on [3H]DA release process with PKC.

[Effect of (-) stepholidine on dopamine turnover in various brain regions]

It has been shown before that DA antagonist (-) stepholidine [(-) SPD] changes the function of feedback regulation in the striatum. In order to compare the effect of the drug on other brain DA systems, the DA content and DOPAC level in brain areas or nuclei of rat were measured by HPLC-ECD. It was found that (-) SPD (10 mg/kg, ip) did not significantly affect the DA contents in the striatum, olfactory bulb, N. accumbens, substantia nigra, ventral tegmentum area, N. amygdala, hypothalamus, pituitary, piriform cortex, but did markedly increase the DOPAC levels in these brain areas or nuclei, and thus increase the DA turnover (DOPAC/DA). However, (-) SPD (10 mg/kg, ip) did not significantly affect either the DA contents or DOPAC levels in the prefrontal cortex and cingulate cortex, thus leaving the DA turnover in these areas unchanges. These results indicated that (-) SPD did affect the nigro-striatal, midbrain-limbic and hypothalamus-pituitary DA systems, but not the midbrain-cortex DA system. It is supposed that this difference may be due to the modulation by presynaptic DA autoreceptors in the major brain areas of DA nerve endings or neurosoma, but none in the cortex DA nerve endings.

Agonistic actions of pergolide on firing activity of dopamine neurons in substantia nigra compacta area

AIM

To study the potency of pergolide as a D2 receptor agonist on the firing activity of substantia nigra compacta (SNC) dopamine (DA) neurons compared with that of bromocriptine and to determine whether pergolide has the nature of D1 receptor agonist in vivo.

METHODS

Extracellular single unit recording techniques.

RESULTS

Both pergolide and bromocriptine decreased the spontaneously firing rate of "sensitive" and "insensitive" DA cells. In regard of ID50 values, pergolide (11.9, 95% fiducial limits, 5.7-25.1 micrograms kg-1) was more potent than bromocriptine (7.8, 95% fiducial limits, 3.3-18.5 mg kg-1). The discharge inhibition of pergolide was attenuated following the injection of selective D2 receptor antagonist spiperone 0.25 mg kg-1 or selective D1 receptor antagonist Sch-23390 1-2 mg kg-1. However, the inhibition caused by bromocriptine was not always attenuated by spiperone.

CONCLUSION

Pergolide is 650 times more potent than bromocriptine at D2 receptors, and possesses D1 receptor agonist characteristics in vivo.

(-)-Stepholidine antagonizes the inhibition by D2 receptor agonists on synaptosomal tyrosine hydroylase in rat corpus striatum

AIM

To evaluate the action of (-)-stepholidine (SPD), a novel dopamine (DA) receptor antagonist, on inhibition by presynaptic D2 receptor agonists on the activity of synaptosomal tyrosine hydroxylase (TH) in the striatum.

METHOD

The TH activity was measured by HPLC-ECD.

RESULTS

The mixed DA receptor agonist apomorphine (Apo 10.4 nmol L-1) and selective D2 receptor agonists N-0437 (0.4 pmol L-1) and quinpirole (0.8 mumol L-1) inhibited the activity of TH, while selective D1 receptor agonist SKF 38393 (0.001-10 mumol L-1) and SPD (1-100 mumol L-1) failed to inhibit the TH activity. The inhibition of N-0437 and quinpirole were antagonized by D2 receptor antagonist spiperone and SPD.

CONCLUSION

The negative feedback regulation of presynaptic DA receptors is mediated via D2 receptors and SPD is an antagonist on presynaptic D2 DA receptors.

[On the central inhibition action of tetrahydroberberine without relevance to GABA receptors]

Using an earthworm (Eisenia foetida) dorsal muscle preparation, it was shown that tetrahydroberberine (THB, 10(-7)-10(-4) mol/L) did not affect both GABA and ACh receptors. DA receptor antagonist haloperidol (HAL) also exerted no effect. Owing to the blocking action of isonicotinyl hydrazine (INH) and thiosemicarbazide (TSC) on biosynthesis of GABA, and of picrotoxin (PT) and bicuculline (Bic) on the GABA-BZ receptor complex mediated transmission, all these agents could induce convulsion in mice. This action could be antagonized by amino- oxyacetic acid (AOAA) and benzodiazepine (BZ), but not by DA receptor antagonists THB and HAL. All the above observations indicate that the GABA inhibition is not involved in the central action of THB.

[Anti-inflammatory effect of Caragana microphylla Lam]

Caragana microphylla can antagonize the inflammation induced by carrageenin, hot water and croton oil. It can also inhibit the proliferation of granuloma, blood capillary permeability, phagocytic function of mononuclear phagocyte system, and synthesis or release of PGE2 at the inflamed part.

Effects of (-)-stepholidine and tetrahydroberberine on high potassium-evoked contraction and calcium influx in rat artery

The relaxant effects of (-)-stepholidine ((-)-SPD) and tetrahydroberberine (THB) on rat aorta were studied in vitro. (-)-SPD IC50 18.1 (95% confidence limits 11.1-29.5) mumol.L-1 and THB IC50 18.6 (95% confidence limits 9.2-37.9) mumol.L-1 inhibited the contractions caused by KCl (100 mmol.L-1) concentration-dependently. Both (-)-SPD and THB markedly inhibited the 160 mmol.L-1 KCl-stimulated 45Ca influx. The inhibitions by (-)-SPD 10 mumol.L-1 and 100 mumol.L-1 were 18 +/- 13% (P > 0.05) and 47.0 +/- 2.8% (P < 0.01), respectively. The inhibitions by THB 10 mumol.L-1 and 100 mumol.L-1 were 36 +/- 9% (P < 0.01) and 43 +/- 8% (P < 0.05), respectively. The results showed that the effective concentrations of the 2 drugs inhibiting high KCl-induced contraction and 45Ca transmembrane influx in rat thoracic aorta were at a similar level, and that they were nearly 1/100 and 1/10 of those of verapamil respectively, indicating that (-)-SPD and THB had similar calcium channel blocking effect on rat artery, but were weaker than verapamil.

Repeated reserpine treatment alters firing pattern and responses of substantia nigral dopamine neurons

Repeated reserpine treatment (1 mg/kg x 6 days) increased the number of spontaneously active substantia nigra pars compacta (SNC) dopamine (DA) cells and altered the firing pattern to a more irregular one in locally anesthetized rats. The selective DA D1 receptor agonist, SKF 38393, although having little effect on SNC DA cells in normal rats, profoundly inhibited the firing rate of SNC DA cells in reserpinized rats. On the contrary, the DA D2 receptor agonist, N-0437, significantly inhibited the firing rate of SNC DA cells in control rats, however, the inhibition was not significantly altered by reserpine. The inhibitory effect of the mixed DA receptor agonist, apomorphine, was significantly enhanced after reserpine treatment. In addition, the inhibition of SNC DA cell firing produced by D1 and D2 receptor agonists in reserpinized rats was reversed only by their own subtype antagonists. These results suggest that repeated reserpine treatment renders SNC DA cells responsive to D1 receptor stimulation, and that D1 receptors play a more important role than D2 receptors in the supersensitivity of SNC DA cells induced by repeated reserpine treatment. The results also indicate that D1 and D2 receptor agonists inhibit SNC DA cell firing separately and synergically in reserpinized rats.

Characteristics of (-)-stepholidine on the firing activity of substantia nigral dopamine neurons after repeated reserpine treatment

Although (-)-stepholidine [(-)-SPD] exhibits antagonistic effects to normosensitive dopamine (DA) receptors, it shows agonistic effect (probably D1) on rotational behavior in rats with unilateral 6-OHDA lesions of substantia nigra pars compacta (SNC). In this study, another supersensitive model, reserpinized rats (1 mg/kg x 6 days, s.c.), were used to investigate the properties of (-)-SPD. In reserpinized rats, (-)-SPD reversed and/or significantly attenuated the firing inhibition caused by the mixed DA receptor agonist apomorphine (APO). After reserpinization, the selective D1 and D2 receptor agonists separately inhibited the firing rate of SNC DA neurons, and (-)-SPD reversed both D2 receptor agonist N-0437- and D1 receptor agonist SKF 38393-induced inhibition of the SNC DA cell firing. These results suggest that (-)-SPD shows D1 and D2 receptor antagonistic action and does not possess the same DA receptor agonistic effect on SNC DA cell firing activity in reserpinized supersensitive rats as it does in unilateral 6-OHDA-lesioned rotational behavior. Also, large doses of (-)-SPD inhibited the firing rate of SNC DA cells in reserpinized rats, but the inhibition was not reversed by D1 receptor antagonist SCH 23390 but was reversed by N-0437 or APO. This inhibition, thus, could be interpreted as depolarization inactivation (DI) due to blockade of DA receptors. Interestingly, in control (nonreserpinized) rats, (-)-SPD did not produce DI of SNC DA cells as it did on ventral tegmental area DA cells. These results imply that reserpinization modulates the onset of DI of SNC DA cells and that (-)-SPD may serve as a leading compound for exploring new types of atypical neuroleptics.

Effects of (-)-stepholidine on firing activity of dopamine neurons in ventral tegmental area of rats

Extracellular single-unit recording techniques were used to evaluate the effects of (-)-stepholidine (SPD) on the firing activity of ventral tegmental area (VTA) dopamine (DA) neurons. SPD reversed the DA agonist apomorphine (Apo)-induced inhibition of VTA DA cell firing rate (ED50 = 4.9, 4.5-5.3 micrograms.kg-1), and the reversal was more rapid than that of a classic DA antagonist haloperidol (Hal) (ED50 = 11.2, 9.1-13.8 micrograms.kg-1). Pretreatment with SPD or Hal 0.5 mg.kg-1 attenuated Apo-induced inhibition, and SPD rendered the VTA DA cells less sensitive to larger doses of Apo (1024-4096 micrograms.kg-1) than Hal did. Pharmacological analysis indicated that the effects of SPD were mainly mediated through D2 subtype receptors. In addition, SPD increased the firing rate of VTA DA cells, while higher doses (1.4, 0.6-3.3 mg.kg-1) of SPD dramatically inactivated 4/6 of the VTA DA cells sampled. This inhibition was considered to be due to depolarization inactivation. These results suggest that SPD is a DA receptor antagonist and can effectively block the D2 autoreceptors located in the VTA DA cells.

[Feed-back regulation of presynaptic D2 receptors blockaded by l-stepholidine and l-tetrahydropalmatine]

The activity of tyrosine hydroxylase by accumulation of L-dopa content in the striatum under decarboxylase inhibitor (benserazide, 400 mg.kg-1) was measured by HPLC-ECD to ascertain whether l-SPD possesses an agonistic or antagonistic effect on D2 receptor subtype. The accumulated L-dopa content was increased with 1,4-butyrolactone (BL, 750 mg.kg-1) to 1.8 +/- s 0.6 micrograms.g-1 in rat striatum. After the rats were ip l-SPD 5 mg.kg-1 or l-tetrahydropalmatine (l-THP) 10-20 mg.kg-1, the increment of L-dopa level was much more than that in the BL group (P < 0.01) in a dose related manner. On the contrary, apomorphine (ip 5 mg.kg-1) abolished the accumulation of L-dopa induced by BL. Furthermore, l-SPD and l-THP not only reversed the negative feed-back regulation by apomorphine on the accumulated level of L-dopa, but also increased this level over the accumulation by BL. These results showed that l-SPD and l-THP do possess the antagonistic effect on presynaptic DA receptor (D2) without agonistic effect.

Comparison of effects of tetrahydropalmatine enantiomers on firing activity of dopamine neurons in substantia nigra pars compacta

Extracellular single unit recording techniques were used to elucidate the effects of enantiomers of tetrahydropalmatine (THP) on the firing activity of dopamine (DA) neurons in substantia nigra pars compacta (SNC). (-)-THP rapidly reversed the apomorphine (Apo)-induced inhibition of the SNC DA cell firing activity (ED50 = 0.77, 0.52-1.14, mg.kg-1), while much larger doses of (+)-THP were required to reverse the Apo-induced inhibition (ED50 = 23, 15.2-34.7, mg.kg-1) and the maximal reversal caused by (+)-THP was 79 +/- 9% of the basal firing rate. In paralyzed rats, (-)-THP (0.5-16 mg.kg-1) significantly increased the spontaneous firing rate of SNC DA neurons dose-dependently, while (+)-THP did not until the dose reached 16 mg.kg-1. Pretreatment with (-)-THP 4 mg.kg-1 attenuated Apo-induced inhibition of SNC DA cell firing rate, while (+)-THP 32 mg.kg-1 revealed a similar potency to block the Apo-induced inhibition. In addition, (+)-THP did not potentiate the effect caused by d-amphetamine (Amp) as some behavioral experiments have shown, but large dose of (+)-THP (32 mg.kg-1) blocked the Amp-induced inhibition of SNC DA cell firing activity as (-)-THP (4 mg.kg-1) did. These results suggest that the interaction between D2 receptors and THP enantiomers has stereoselectivity and that (-)-THP is a D2 antagonist while (+)-THP seems to be not.

Effects of l-stepholidine on tyrosine hydroxylase activity in rat corpus striatum

l-Stepholidine (SPD) 2.5 and haloperidol (Hal) 1.0 mg.kg-1 ip increased rat striatal L-3,4-dihydroxyphenylalanine (DOPA) and 3,4-dihydroxyphenylacetic acid (DOPAC) accumulation induced by NSD 1015 (50 mg.kg-1, ip), a decarboxylase inhibitor. SPD (2.5 mg.kg-1, ip) did not alter but apomorphine (2.0 mg.kg-1, ip) decreased the dopamine (DA) content elevated by gamma-butyrolactone (GBL, 750 mg.kg-1, ip) in the rat striatum. Ip injection of either SPD 5.0 or Hal 2.5 mg.kg-1 after NSD 1015 50 mg.kg-1 or NSD 1015 plus GBL 750 mg.kg-1 also augmented tyrosine hydroxylase (TH) activity in the rat striatum. These results suggest that SPD produces an antagonistic effect on presynaptic DA receptors.

Effects of l-stepholidine on synaptosomal Ca(2+)-ATPase and subcellular calmodulin in rat striatum

Our results showed that l-stepholidine (l-SPD) inhibited basal Ca(2+)-ATPase activity in rat striatal synaptosomes with an IC50 of 31.5 mumol.L-1, suggesting its interaction with Ca2+ transport. l-SPD inhibited also calmodulin (CaM)-activated basal Ca(2+)-ATPase in a concentration-dependent manner. A complete reversal of CaM activation of Ca(2+)-ATPase was observed with l-SPD 10 mumol.L-1. The activity of synaptosomal Ca(2+)-ATPase and membrane-bound CaM level were decreased in haloperidol (1 mg.kg-1.d-1, ip) and l-SPD (5, 10, and 30 mg.kg-1.d-1, ip) treated rats for 7 and 14 d, respectively. But the activity of Ca(2+)-ATPase and membrane CaM level were increased after treatment with the same of doses haloperidol and l-SPD for 21 d. During the treatments with haloperidol and l-SPD cytosolic and nuclear CaM levels were not altered. These results suggest that l-SPD may modulate the release and synthesis of dopamine (DA) and the negative feedback regulation of presynaptic DA receptors by altering Ca2+ and CaM regulating processes in the central dopaminergic nervous system.

The antagonistic effects of tetrahydroprotoberberines on dopamine receptors: electrophysiological studies

Tetrahydroprotoberberines (THPBs), including (-)-stepholidine ((-)-SPD), (-)-tetrahydropalmatine ((-)-THP) and tetrahydroberberine (THB), have been demonstrated to be a new class of DA antagonists in biochemical and neuropharmacological studies. In this paper, the antagonistic action of THPBs was examined by means of single unit recording from nigral DA neuron in chloral hydrate-anesthetized and gallamine-paralyzed rats. Intravenous injection of these compounds could promptly and completely reverse the inhibition of the spontaneous firing induced by DA agonist apomorphine (APO) in a dose-dependent way. Pretreatment with (-)-SPD, (-)-THP or THB could significantly reduce the inhibitory effect of APO and shift the dose-action curve to the right. Besides, the compounds could increase the spontaneous firing of DA neurons. The above results not only strongly support the conclusion that (-)-SPD, (-)-THP and THB are DA antagonists, but also demonstrate that one of their blocking sites is at somatodendritic DA autoreceptors (D-2 receptors). In other words, (-)-SPD did not exhibit any DA agonistic action in this acute electrophysiological study, although its DA agonistic action can be demonstrated in rotational behavior of 6-OHDA-lesioned rats. The dual actions of (-)-SPD, dependent upon different experimental conditions, are discussed.

Dual actions of (−)-stepholidine on dopamine receptor subtypes after substantia nigra lesion

1. It was found that the contralateral rotation challenged by (-)-SPD (4 mg/kg, ip) in 6-OHDA-lesioned rats had a gradually progressive process with long latent period and a maximal response on 63 days after lesion. This steady contralateral rotation was preferably antagonized by D-1 antagonist SCH23390 than D-2 antagonists. During its latent period (-)-SPD exhibited the antagonistic effect to APO, while during its period of full response (-)-SPD could potentiate the APO-induced rotation. 2. In the rats lesioned with kainic acid plus 6-OHDA to destroy the SNC and SNR, (-)-SPD and SKF-38393 challenged neither contralateral nor ipsilateral rotation, while APO still induced the rotation but towards ipsilateral side, just opposite to that in 6-OHDA-lesioned rats. In this case, (-)-SPD antagonized the response to APO as did SCH 23390. 3. These evidences suggest that the agonistic action of (-)-SPD is resulted from D-1 receptor subtype at the SNR under supersensitive functional state. The fact that SNR lesion could completely eliminate the agonistic action of (-)-SPD further indicate that the D-1 receptors in the ipsilateral SNR may be the sites of action of (-)-SPD. The dual actions of (-)-SPD are dependent upon the supersensitivity of D-1 receptor subtypes which render the antagonistic action to convert into the agonistic.