Does acute L-DOPA increase active release of dopamine from dopaminergic neurons?

Locomotor activity and resting local field potential oscillatory rhythms of 6-OHDA mouse model of Parkinson's disease in response to acute and repeated treatments with L-dopa

Phase-amplitude coupling (PAC) of local field potential (LFP) has been recognized as higher-order representation of brain states. Neuronal loss in the striatum leads to Parkinson's disease (PD) symptoms and modifies LFP oscillation. However, PAC in the striatum of PD mouse model induced by 6-hydroxydopamne (6-OHDA) remained to be investigated. Male Swiss albino ICR mice were implanted with intracranial electrode and injected with 6-OHDA to the left striatum. Levodopa (L-dopa) (10 mg/kg, oral) was used for treatment once a day from day 15-19. Locomotor activity and resting LFP signals were selectively analyzed on day 15 and 19. One-way ANOVA revealed significant decreases in travelled distance induced by 6-OHDA on both days (p ≤ 0.05). However, the decreased travelled distances were significantly reversed by L-dopa. On day 15, LFP powers of theta, alpha, beta and low gamma waves were significantly increased by 6-OHDA injection and the powers of beta and low gamma were significantly reversed to control level by treatment with L-dopa. On day 19, LFP powers of delta, theta, alpha, beta and low gamma waves were significantly increased by 6-OHDA injection and the powers of low gamma were significantly reversed to control level by treatment with L-dopa. Theta-gamma PAC analyses also confirmed significant increase in modulation index (MI) induced by 6-OHDA on day 19. However, L-dopa failed to significantly reverse the MI to control level. These findings indicated theta-gamma coupling in the striatum of PD mouse model. Taken together, change in striatal theta-gamma PAC might be one of biomarkers in addition to hypokinesia and increased LFP powers that reflect disrupted neural mechanisms in PD mouse model.

Local administration of L-DOPA in the chicken ventromedial hypothalamus increases dopamine release in a dose-dependent manner

L-DOPA induced extracellular dopamine (DA), norepinephrine (NE) and serotonin (5-HT) in the ventromedial hypothalamus (VMH) of chickens were measured by in vivo microdialysis. Several doses of 3,4-dihydroxy-l-phenylalanine (l-DOPA) were administered locally through the microdialysis probe into the VMH of chickens for 10 min. Local perfusion of l-DOPA increased the extracellular levels of DA. The increased DA was dose-related and was significantly higher compared to the baseline and control group. The maximal level of DA was 212% and 254%, respectively, of the baseline following administration of 1 and 2 μg/ml l-DOPA. There were no changes in NE and 5-HT levels from baseline after l-DOPA perfusion. l-DOPA (1 μg/ml) was mixed with Ca(2+)-free Ringer, tetrodotoxin (TTX) (2 μM) and high K(+) and was perfused for 30 min into the chicken VMH. TTX and Ca(2+)-free Ringer's solution inhibited the effectiveness of l-DOPA in increasing DA release. The NE and 5-HT levels were significantly lower than the baseline. After administration of K(+) a significant increase of DA, NE and 5-HT was observed. The microdialysis results are consistent with our objective that l-DOPA induced extracellular DA increases in the VMH in a dose-dependent manner and the released DA, NE and 5-HT within the dialysate were related to neuronal activity.

Mathematical insights into the effects of levodopa

Parkinson’s disease has been traditionally thought of as a dopaminergic disease in which cells of the substantia nigra pars compacta (SNc) die. However, accumulating evidence implies an important role for the serotonergic system in Parkinson’s disease in general and in physiological responses to levodopa therapy, the first line of treatment. We use a mathematical model to investigate the consequences of levodopa therapy on the serotonergic system and on the pulsatile release of dopamine (DA) from dopaminergic and serotonergic terminals in the striatum. Levodopa competes with tyrosine and tryptophan at the blood-brain barrier and is taken up by serotonin neurons in which it competes for aromatic amino acid decarboxylase. The DA produced competes with serotonin (5HT) for packaging into vesicles. We predict the time courses of LD, cytosolic DA, and vesicular DA in 5HT neurons during an LD dose. We predict the time courses of DA and 5HT release from 5HT cell bodies and 5HT terminals as well as the changes in 5HT firing rate due to lower 5HT release. We compute the time course of DA release in the striatum from both 5HT and DA neurons and show how the time course changes as more and more SNc cells die. This enables us to explain the shortening of the therapeutic time window for the efficacy of levodopa as Parkinson’s disease progresses. Finally, we study the effects 5HT1a and 5HT1b autoreceptor agonists and explain why they have a synergistic effect and why they lengthen the therapeutic time window for LD therapy. Our results are consistent with and help explain results in the experimental literature and provide new predictions that can be tested experimentally.

l-dopa-induced reversal in striatal glutamate following partial depletion of nigrostriatal dopamine with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

We have reported that 1 month following acute (20mg/kg x 4) or subchronic (30 mg/kg/day x 7d) administration of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, there is an increase or decrease, respectively, in the extracellular level of striatal glutamate as determined by in vivo microdialysis [Robinson S, Freeman P, Moore C, Touchon JC, Krentz L, Meshul CK (2003) Acute and subchronic MPTP administration differentially affects striatal glutamate synaptic function. Exp Neurol 180:73-86]. The goal of this study was to determine the effects of treatment with l-dopa (15 mg/kg) for 21 days on striatal glutamate starting on day 8 after the first dose of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine was administered to mice. Following acute administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, the increase in extracellular striatal glutamate due to lesion of the nigrostriatal pathway was completely reversed to a level below that found in the vehicle-treated group after l-dopa treatment. Subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment resulted in a decrease in striatal extracellular glutamate that was reversed to the level close to that observed in the vehicle-treated group. There was no change in the density of nerve terminal glutamate immunolabeling associated with the synaptic vesicle pool, suggesting that the alterations in extracellular glutamate most likely originated from the calcium-independent pool. There was a similar decrease in the relative density of tyrosine hydroxylase immunolabeling, a marker for dopamine terminals, within the dorsolateral striatum in both the acute and subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated groups that had been administered l-dopa. There was a decrease in the relative density of immunolabeling within the dorsolateral striatum for the glutamate transporter, GLT-1, following acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment in the groups administered either vehicle or l-dopa. There was no change in GLT-1 immunolabeling following subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The results demonstrate that the reversal in the extracellular level of striatal glutamate following l-dopa treatment in both the acute and subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated groups is not due to changes in either striatal dopamine nerve terminals or in the density of the glutamate transporter, GLT-1.

L-3,4-Dihydroxyphenylalanine as a neurotransmitter candidate in the central nervous system

Historically, 3,4-dihydroxyphenylalanine (DOPA) has been believed to be an inert amino acid that alleviates the symptoms of Parkinson's disease by its conversion to dopamine via the enzyme aromatic L-amino acid decarboxylase. In contrast to this generally accepted idea, we propose that DOPA itself is a neurotransmitter and/or neuromodulator, in addition to being a precursor of dopamine. Several criteria, such as synthesis, metabolism, active transport, existence, physiological release, competitive antagonism, and physiological or pharmacological responses, must be satisfied before a compound is accepted as a neurotransmitter. Recent evidence suggests that DOPA fulfills these criteria in its involvement mainly in baroreflex neurotransmission in the lower brainstem and in delayed neuronal death by transient ischemia in the striatum and the hippocampal CA1 region of rats.

Behavioral activity and stereotypy in rats induced by L-DOPA metabolites: a possible role in the adverse effects of chronic L-DOPA treatment of Parkinson's disease

L-3,4-Dihydroxyphenylalanine (L-DOPA) is a common and effective treatment for Parkinson's disease, but dyskinesia continues to be a serious adverse effect with chronic use. Evidence suggests that L-DOPA induces increases in dopamine, which then binds to supersensitive dopamine receptors, resulting in dyskinesia. We have shown previously that L-DOPA directly causes stereotypy in rats, suggesting that chronic L-DOPA-induced dyskinesia is also caused by L-DOPA itself. This raises the possibility that other L-DOPA metabolites have a role in dyskinesia. We examined the behavioral effects of five L-DOPA metabolites (3-methoxytyramine, 3-MT; 3,4-dihydroxyphenylalanine, DOPAC; dopamine; homovanillic acid, and 3-o-methyl-DOPA) in rats. A unilateral, intracerebroventricular injection of 3-MT (10-200 microg, 40 microl) over 30 min, dose-dependently increased behavioral activity and stereotypy. This effect was suppressed by the dopamine D1/5-receptor antagonist SCH 23390, but not by the dopamine D2/3/4-receptor antagonist sulpiride. Dopamine denervation resulted in behavioral supersensitivity to 3-MT. Neither dopamine nor DOPAC levels increased in the striatum after 3-MT administration, as measured using in vivo voltammetry. The behavioral changes paralleled a rise in 3-MT in the contralateral striatum. DOPAC also caused behavioral changes and stereotypy, but to a smaller degree than 3-MT. Dopamine-denervated rats did not exhibit a supersensitive response to DOPAC, however. Other L-DOPA metabolites did not cause behavioral effects. These data suggest that 3-MT directly induced dopamine-D1/5-receptor-mediated behavioral changes in rats, and that 3-MT may have a role in dyskinesia due to chronic L-DOPA treatment in Parkinson's disease patients.

Dual effects of L-3,4-dihydroxyphenylalanine on aromatic L-amino acid decarboxylase, dopamine release and motor stimulation in the reserpine-treated rat: evidence that behaviour is dopamine independent

The comparative effects of L-3,4-dihydroxphenylalanine (L-DOPA) on dopamine synthesis, release and behaviour were studied in the reserpine-treated rat. Acute administration of L-DOPA (25-200 mg/kg) dose-dependently inhibited the activity of aromatic L-amino acid decarboxylase (AADC) in the substantia nigra and corpus striatum. The antiparkinsonian drugs budipine (10 mg/kg) and amantadine (40 mg/kg) enhanced AADC activity in these regions, and prevented or reversed AADC inhibition by L-DOPA. Dual probe dialysis revealed that low doses of L-DOPA (25-50 mg/kg) dose-dependently stimulated the release of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in nigra and striatum, whilst high doses of L-DOPA (100-200 mg/kg) completely suppressed the release of dopamine, but not DOPAC. Sulpiride (50 microM) administered via the probes antagonized dopamine release in response to 25 mg/kg L-DOPA, but greatly facilitated release by 200 mg/kg L-DOPA. Dopamine release was blocked by the centrally acting AADC inhibitor NSD 1015, but facilitated by the central AADC activator budipine. In behavioural tests L-DOPA (plus benserazide, 50 mg/kg) only reversed akinesia at 200 mg/kg, and not at 25-100 mg/kg. Pretreatment with either NSD 1015 (100 mg/kg) or budipine (10 mg/kg) markedly potentiated the motor stimulant action of a threshold dose of L-DOPA (100 mg/kg). A combination of NSD 1015 (100 mg/kg) and benserazide (50 mg/kg) potentiated L-DOPA behaviour more effectively than either inhibitor alone. NSD 1015-facilitated L-DOPA behaviour was antagonized by sulpiride (100 mg/kg) and not by SCH 23390 (1 mg/kg), whereas budipine-facilitated L-DOPA behaviour was fully antagonized by SCH 23390 and only partially by sulpiride. These results show that behaviourally active doses of L-DOPA in the reserpinized rat are not accompanied by significant increases in extracellular dopamine and are therefore probably not dopamine mediated. We propose that L-DOPA is capable of directly stimulating dopamine D2 and possibly non-dopamine receptors, thereby inhibiting dopamine efflux presynaptically and promoting motor activation postsynaptically. A stimulant action of L-DOPA on motor behaviour, preferentially mediated by D1 > D2 receptors, suggests that L-DOPA may also be capable of yielding a dopamine-like response in the absence of detectable dopamine release. These findings are incorporated into a new model of L-DOPA's actions in the reserpinized rat, and their possible implications for our understanding of L-DOPA in Parkinson's disease are discussed.

Neurobiology of L-DOPAergic systems

L-DOPA is proposed to be a neurotransmitter and/or neuromodulator in CNS. It is released probably from neurons, which may contain L-DOPA as an end-product, and/or from some compartment other than catecholamine-containing vesicles. The L-DOPA itself produces presynaptic and postsynaptic responses. All are stereoselective and most are antagonized by competitive antagonist. In striatum, L-DOPA is neuromodulator, mother of catecholamines, not only a precursor for dopamine but also a potentiator of children for presynaptic beta-adrenoceptors to facilitate dopamine release and postsynaptic D2 receptors, and ACh release inhibitor. All may cooperate for Parkinson's disease. Meanwhile, supersensitization of increase in L-glutamate release to nanomolar levodopa was seen in Parkinson's model rats, which may relate to dyskinesia or "on-off" during chronic therapy. In lower brainstem, L-DOPA tonically activates postsynaptic depressor sites of NTS and CVLM and pressor sites of RVLM. L-DOPA is probably a neurotransmitter of primary baroreceptor afferents terminating in NTS. GABA, the inhibitory neuromodulator for baroreflex in NTS, tonically functions to inhibit, via GABAA receptors, L-DOPA release and depressor responses to levodopa. Levodopa inversely releases GABA. L-DOPAergic monosynaptic relay from NTS to CVLM and from PHN to RVLM is suggested. Tonic L-DOPAergic baroreceptor-aortic nerve-NTS-CVLM relay seems to carry baroreflex information. Disturbance of neuronal activity to release L-DOPA in NTS, loss of the activity in CVLM, enhancement of the activity with decreased decarboxylation and increase in sensitivity to levodopa in RVLM may be involved in maintenance of hypertension in SHR. This is a story of "L-DOPAergic receptors" with extremely high affinity and low density.

Picomolar concentrations of L-dopa stereoselectively potentiate activities of presynaptic beta-adrenoceptors to facilitate the release of endogenous noradrenaline from rat hypothalamic slices

Interactions between (-)-isoproterenol and DOPA on the release of endogenous noradrenaline (NA) evoked by electrical field stimulation (2 Hz, alternative polarity) were studied in rat superfused hypothalamic slices in the presence of 3-hydroxybenzylhydrazine, an inhibitor of L-aromatic amino acid decarboxylase, and cocaine. Isoproterenol (0.3-3 nM) facilitated the NA release in a concentration-dependent manner, while 10 pM L-DOPA alone produced no effect. This facilitation at 0.3-3 nM was potentiated by 20-70% by simultaneously applied 10 pM L-DOPA but that at 3 nM was not modified by 10 pM D-DOPA. This potentiation of the isoproterenol (3 nM)-induced facilitation of the NA release was concentration-dependent at 1-10 pM of L-DOPA. L-DOPA methyl ester (1 nM) antagonized the L-DOPA (10 pM)-induced potentiation of the facilitation of the NA release by 3 nM isoproterenol to a level of the facilitation by isoproterenol alone, whereas 10 nM (-) propranolol antagonized both the facilitation by isoproterenol alone and its potentiation by L-DOPA to a control level. Picomolar concentrations of L-DOPA stereoselectively act on a recognition site for itself, and then potentiate activities of presynaptic beta-adrenoceptors to facilitate the NA release from rat hypothalamic slices.

Effects of L-dopa on extracellular dopamine in striatum of normal and 6-hydroxydopamine-treated rats

In vivo microdialysis was used to examine the effect of L-3,4-dihydroxyphenylalanine (L-DOPA) administration upon dopamine (DA) in extracellular fluid both in intact striatum and in striatum of rats treated with the catecholaminergic neurotoxin 6-hydroxydopamine (6-HDA). Basal extracellular levels of DA were not significantly altered by 6-HDA unless the DA content of striatal tissue was reduced to less than 20% of control. Peripheral aromatic amino acid decarboxylase (AADC) inhibition (RO4-4602, 50 mg/kg i.p.) followed by L-DOPA treatment (100 mg/kg i.p.) elevated extracellular DA in striatum of control rats from 37 +/- 5 to 68 +/- 11 pg/sample (n = 7; values corrected for recovery of the dialysis probe). In animals with severe bilateral depletions of DA in striatal tissue (mean depletion 87%; n = 6), L-DOPA increased extracellular DA in striatum from 8 +/- 3 to 266 +/- 60 pg/sample. In animals with large unilateral depletions of DA in striatal tissue (mean depletion 96%; n = 6), the increase in extracellular DA in striatum after L-DOPA was greater on the lesion side (from 7 +/- 4 to 245 +/- 67 pg/sample) than on the intact side (from 28 +/- 11 to 61 +/- 8 pg/sample). Animals with unilateral DA depletions showed contralateral circling behavior after L-DOPA. Increases in extracellular DA approaching the magnitude of those occurring in DA-depleted striata were observed when intact animals were treated with nomifensine (5 mg/kg i.p.; n = 5), an inhibitor of high-affinity DA uptake, in addition to L-DOPA.

Responses of rat substantia nigra compacta neurones to L-DOPA

1. The electrophysiological actions of L-DOPA were studied on substantia nigra dopamine-containing neurones by use of intracellular recordings, current and voltage clamp, in vitro. 2. L-DOPA (3-300 microM), applied by superfusion, decreased the spontaneous firing activity of the neurones, hyperpolarized the membrane potential and produced an outward current. 3. These effects of L-DOPA were graded and were antagonized by sulpiride, a D2-receptor antagonist. 4. The L-DOPA activated membrane hyperpolarization persisted in slices exposed to tetrodotoxin while it was markedly reduced or abolished in solutions with no calcium plus cobalt (2 mM). 5. In slices treated with carbidopa, an inhibitor of the aromatic amino acid decarboxylase enzyme, the actions of L-DOPA were greatly reduced. 6. We propose that L-DOPA inhibits the firing of substantia nigra zona compacta neurones by being converted to dopamine which is then released by these cells.

The effect of L-dopa upon in vitro dopamine release from the corpus striatum of young and old male rats

In the present experiments, we examined the effects of superfusion of L-DOPA upon in vitro dopamine (DA) release from the corpus striatum (CS) in young (2-4 months) and old (24 months) male rats. In Experiment I, responses to two successive increasing doses of L-DOPA (1.0 and 10 microM) indicated a clear age difference with DA release from CS of young rats significantly greater than that of old rats in response to the 10 microM dose of L-DOPA. Interestingly, CS fragments of young and old male rats responded marginally to the 1.0 microM L-DOPA infusion, and no significant differences between these two age groups were obtained. Since we have previously observed that the addition of naloxone to the superfusion medium restored the potassium stimulated DA release of old rats to that of young rats, in Experiment II, responses of CS tissue fragments of young and old rats superfused in medium with or without naloxone (100 microM) were tested with a single 10 microM L-DOPA infusion. The results of Experiment II indicated that the addition of naloxone to the superfusion medium did not alter the responses of CS tissue to L-DOPA in either young or old rats; however, the overall response of the young rats remained significantly greater than that of the old rats. Taken together, these results demonstrate a significant age-related decrement in the capacity of 10 microM L-DOPA to stimulate DA release from CS fragments.(ABSTRACT TRUNCATED AT 250 WORDS)

Progesterone enhances L-dopa-stimulated dopamine release from the caudate nucleus of freely behaving ovariectomized-estrogen-primed rats

In the present experiment we examined the effect of progesterone upon dopamine (DA) release induced by a direct infusion of unlabeled L-dihydroxyphenylalanine (L-DOPA) into the caudate nucleus of freely behaving rats. Ovariectomized rats were implanted with a push-pull cannula directed at the caudate nucleus and subjected to perfusion under 3 different hormonal conditions: (1) following 4 days of treatment with estradiol benzoate (EB), (2) following 4 days of treatment with estradiol benzoate plus progesterone at 4-6 h prior to perfusion (EB + P-4-6 h) and (3) following 4 days of treatment with estradiol benzoate plus progesterone at 28 h prior to perfusion (EB + P-28 h). During each perfusion session and under each of the 3 hormonal treatment conditions, L-DOPA was infused through the push side of the cannula. Three increasing doses of L-DOPA (10(-6), 10(-5) and 10(-4) M) were infused with a 45-75 min interval between infusions. Regardless of hormonal treatment condition, a clear dose-response increase in DA and 3,4-dihydroxyphenylacetic acid (DOPAC), but not 5-HIAA, output was observed in response to the increasing doses of L-DOPA infusion. For each of the 3 doses of L-DOPA, maximal DA output was observed for animals tested under the EB + P-4-6 h hormonal condition, with statistically significant differences in the areas under the L-DOPA-stimulated DA response curves obtained following the 10(-6) and 10(-5) M doses of L-DOPA infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Real-time characterization of dopamine overflow and uptake in the rat striatum

The rate of overflow and disappearance of dopamine from the extracellular fluid of the rat striatum has been measured during neuronal stimulation. Overflow of dopamine was induced by electrical stimulation of the medial forebrain bundle with biphasic pulse trains. The instantaneous concentration of dopamine was measured with a Nafion-coated, carbon fiber microelectrode implanted in the brain. The measurement technique, fast-scan cyclic voltammetry, samples the concentration of dopamine in less than 10 ms at 100 ms intervals. Identification of dopamine is made with cyclic voltammetry. Stimulated overflow was measured as a function of electrode position, stimulation duration, stimulation frequency, and after administration of L-DOPA and nomifensine. The observed concentration during a 2-s, 60-Hz stimulation was found to alter with position of the carbon fiber electrode. For stimuli of 3 s or less the amount of overflow was found to be a linear function of stimulus duration at a fixed electrode position. The observed overflow was found to be steady-state at a frequency of 30 Hz, suggesting a balance between uptake and synaptic overflow under these conditions. The experimental data was found to be successfully modelled when the balance of uptake and stimulated overflow was considered. It was assumed that each stimulus pulse releases a constant amount of dopamine (125 nM), and that uptake follows a Michaelis-Menten model for a single uptake site with Km = 200 nM and Vmax = 5 microM/s. The increase in stimulated overflow observed after L-DOPA (250 mg/kg) could be modelled by a 1.6-fold increase in the amount of dopamine release with no alteration of the uptake parameters. The increase in modelled by an increase in Km. In addition, the fit of the modelled data to the experimental data was improved when diffusion from the release and uptake sites was considered.

Stimulation of dopamine D-2 but not D-1 receptors reduces immobility time of rats in the forced swimming test: implication for antidepressant activity

The involvement of dopamine D-1 and D-2 receptor mechanisms was investigated in the forced swimming test with rats. d,1-Sulpiride, a D-2 receptor antagonist, reported to reduce desipramine-induced anti-immobility, did not alter the brain levels of desipramine. In addition, the anti-immobility effect of desipramine was not antagonized by SCH 23390, a D-1 receptor antagonist. Amineptine (20 mg/kg i.p., 60 min before testing), a dopamine uptake blocker, and LY171555 (0.2 mg/kg i.p., 60 min before testing), a dopaminergic D-2 stimulant reduced immobility time in the forced swimming test, but benserazide + 1-DOPA (200 mg/kg p.o., 45 min before testing), which increases dopamine release, or SKF 38393A (20 mg/kg s.c., 60 min before testing), a D-1 agent, did not. The anti-immobility effect but not the stereotypy was increased following chronic (21 days) LY171555 (0.1 and 0.2 mg/kg i.p.) treatment. The effect of acute or repeated (7 days) LY171555 (0.2 mg/kg i.p.) treatment was antagonized by 1-sulpiride (50 mg/kg i.p., 90 min before testing), a D-2 receptor antagonist. Neither SKF 38393A (20 mg/kg s.c., 60 min before testing) nor SCH 23390 (0.05 mg/kg s.c., 30 min before testing) modified the acute anti-immobility effect of LY171555 (0.2 mg/kg i.p.) SCH 23390 (0.025 and 0.05 mg/kg) increased the immobility time at doses which decreased motor activity. The increase in immobility time brought about by SCH 23390 was not antagonized by SKF 38393A (20 mg/kg). The findings indicate that activation of dopamine D-2 receptors could reduce immobility time.

The effect of selective D1 and D2 dopaminergic agents on sexual receptivity in the female rat

A number of dopaminergic drugs was tested for their effect on female sexual receptivity in ovariectomised plus adrenalectomised rats primed with oestradiol benzoate. The selective D1 agonist (SKF 38393 5-40 mg/kg) and D1 antagonist (SCH 23390 0.1-10 mg/kg) had no significant effect on sexual behaviour. The results of administration of mixed dopaminergic agents (DOPA, 10-50 mg/kg with benserazide or 100-200 mg/kg alone and haloperidol 0.01-1.0 mg/kg) and selective D2 dopaminergic agents (LY 171555 2.5-800 micrograms/kg, BHT 920 0.01-1.0 mg/kg and sulpiride 5-80 mg/kg) indicate that a central dopaminergic system has an inhibitory control on female sexual activity exerted through D2 receptors. Any stimulatory effects of the exogenous agonists were probably due to an action on presynaptic D2 receptors. The predominance of the D2 pre- and post-synaptic receptor activity appears to be influenced by the sexual receptivity of the animal.

In vivo release of dopamine and its metabolites following a direct infusion of L-dopa into the caudate nucleus of awake, freely behaving rats using a push-pull cannula

In vivo release of dopamine (DA) and its metabolites were determined following a direct infusion of 3,4-dihydroxyphenylalanine (L-DOPA) through a push-pull cannula in the caudate nucleus of unanesthetized, freely behaving rats. L-DOPA infusions increased the release of DA and dihydroxyphenylacetic acid (DOPAC) beginning with 10(-5) M L-DOPA, while homovanillic acid (HVA) was released consistently only following 10(-3) M L-DOPA. Maximal release of DA preceded that of DOPAC which preceded that of HVA. No salient changes in 5-hydroxyindoleacetic acid or behavior were observed following any L-DOPA dose.

Biphasic actions of L-DOPA on the release of endogenous dopamine via presynaptic receptors in rat striatal slices

In rat striatal slices, 30 nM of L-DOPA increased the impulse (5 Hz)-evoked release of dopamine (DA), without increasing the spontaneous release and tissue content of DA. The minimum dose required to increase spontaneous DA release was 0.1 microM and the dose which led to an accumulation of DA was 100 microM. In the presence of NSD-1055, a DOPA-decarboxylase inhibitor, L-DOPA-induced increases in spontaneous DA release were prevented and L-DOPA produced dual actions on the evoked release of DA, a stereoselective propranolol-sensitive increase at 30 nM and a stereoselective sulpiride-sensitive decrease at 1 microM. L-DOPA produces dual presynaptic regulatory actions on DA release, via facilitatory beta-adrenoceptors at 30 nM and inhibitory DA receptors at 1 microM. The primary action of L-DOPA appears to be the facilitation of release of DA rather than the conversion to DA.

Biphasic actions of L-DOPA on the release of endogenous noradrenaline and dopamine from rat hypothalamic slices

Effects of L-DOPA on the release of endogenous noradrenaline and dopamine from rat hypothalamic slices evoked by electrical field stimulation at 5 Hz were investigated in the absence and presence of p-bromobenzyloxyamine (NSD-1055), a DOPA-decarboxylase inhibitor. In the absence of NSD-1055, L-DOPA produced a facilitation of impulse-evoked release of noradrenaline at 0.1 microM but not at 1 and 10 microM, and had no effect on the spontaneous release. On the other hand, L-DOPA 0.1 to 10 microM dose-dependently increased the spontaneous release of dopamine and the highest concentration only increased the evoked release and tissue content of dopamine. In the presence of NSD-1055 10 microM, the increase in the spontaneous release of dopamine was prevented and L-DOPA produced biphasic regulatory effects on the evoked release of noradrenaline and dopamine, a facilitation at 0.1 microM and an inhibition at 1 microM. The facilitation was antagonized by (-)-propranolol 0.1 microM, but not by the (+)-isomer, whereas the inhibition was antagonized by S-sulpiride 1 nM, but not by the R-isomer. In conclusion, L-DOPA appears to produce biphasic actions on the release of endogenous noradrenaline and dopamine from rat hypothalamic slices, not through its conversion to dopamine but through presynaptic regulatory mechanisms, an inhibition via dopamine receptors at a micromolar concentration and a facilitation via beta-adrenoceptors at the lower concentration.

Suppression of MPTP-induced dopaminergic neurotoxicity in mice by nomifensine and L-DOPA

To examine effects of various pharmacological manipulations of dopamine (DA) metabolism on DA neurotoxicity of N-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP), C57 black mice were injected with MPTP (30 mg/kg s.c., once daily for two days) alone or in combination with apomorphine, bromocriptine, haloperidol, L-DOPA or nomifensine. MPTP markedly decreased neostriatal DA concentrations at 2, 10, 20 and 30 days post-treatment indicating persistent degeneration of nigrostriatal DA neurons. Suppression or acceleration of DA turnover rates by the DA agonists apomorphine and bromocriptine or by the DA antagonist haloperidol, respectively, did not affect MPTP toxicity. MPTP-induced neostriatal DA depletions were markedly suppressed by nomifensine, a DA reuptake inhibitor, and attenuated by exogenous L-DOPA. MPTP may be a substrate for the DA reuptake system and its specific transport into nigrostriatal terminals may be an important factor for its selective neurotoxicity.

In vivo and in vitro evidence of dopaminergic system down regulation induced by chronic L-DOPA

The biochemical modifications which occur in the dopaminergic system after chronic administration of L-DOPA are investigated. Levels of DA and of its metabolite 3-methoxytyramine (3-MT), an expression of the amount of DA released, were raised to the same extent in controls given a single dose of 1-DOPA and in chronically treated rats given 100 mg/kg of 1-DOPA plus 25 mg/kg of benserazide twice a day for 24 days. However, the reduction in neuronal function expressed by the decrease in 3-MT which follows treatment with DA agonists such as piribedil and apomorphine was less pronounced in the chronically L-DOPA treated rats. This suggests that such treatment causes a down regulation of DA receptors. These in vivo results were confirmed by in vitro analysis of DA receptor activity after chronic L-DOPA. Under these conditions there was a significant reduction in the number of [3H]-spiperone and [3H]-ADTN binding sites with no changes in their affinity. The in vivo and in vitro findings both suggest the involvement of a subsensitive compensatory mechanism or down regulation of dopaminergic neurons after chronic treatment with L-DOPA.