Aporphines. 16. Action of aporphine alkaloids on locomotor activity in rats with 6-hydroxydopamine lesions of the nucleus accumbens

Activation of neurotensin receptor type 1 attenuates locomotor activity

Intracerebroventricular administration of neurotensin (NT) suppresses locomotor activity. However, the brain regions that mediate the locomotor depressant effect of NT and receptor subtype-specific mechanisms involved are unclear. Using a brain-penetrating, selective NT receptor type 1 (NTS1) agonist PD149163, we investigated the effect of systemic and brain region-specific NTS1 activation on locomotor activity. Systemic administration of PD149163 attenuated the locomotor activity of C57BL/6J mice both in a novel environment and in their homecage. However, mice developed tolerance to the hypolocomotor effect of PD149163 (0.1 mg/kg, i.p.). Since NTS1 is known to modulate dopaminergic signaling, we examined whether PD149163 blocks dopamine receptor-mediated hyperactivity. Pretreatment with PD149163 (0.1 or 0.05 mg/kg, i.p.) inhibited D2R agonist bromocriptine (8 mg/kg, i.p.)-mediated hyperactivity. D1R agonist SKF-81297 (8 mg/kg, i.p.)-induced hyperlocomotion was only inhibited by 0.1 mg/kg of PD149163. Since the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) have been implicated in the behavioral effects of NT, we examined whether microinjection of PD149163 into these regions reduces locomotion. Microinjection of PD149163 (2 pmol) into the NAc, but not the mPFC suppressed locomotor activity. In summary, our results indicate that systemic and intra-NAc activation of NTS1 is sufficient to reduce locomotion and NTS1 activation inhibits D2R-mediated hyperactivity. Our study will be helpful to identify pharmacological factors and a possible therapeutic window for NTS1-targeted therapies for movement disorders.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

Dopamine D(2) activity of R-(-)-apomorphine and selected analogs: a microdialysis study

In the present study, R-(-)-apomorphine and three of its analogs were studied for their potency in decreasing the release of dopamine in the striatum after subcutaneous administration and for their oral bioavailability using the microdialysis technique in freely moving rats. The analogs R-(-)-N-n-propylnorapomorphine and R-(-)-11-hydroxy-N-n-propylnoraporphine displayed a higher potency than R-(-)-apomorphine in decreasing the release of dopamine in the striatum. A high dose of R-(-)-11-hydroxyaporphine, a dopamine D(2) receptor partial agonist, had a small effect on the release of dopamine in the striatum. The catechols R-(-)-N-n-propylnorapomorphine and R-(-)-apomorphine displayed a comparable oral bioavailability (1%), while the mono-hydroxy analog R-(-)-11-hydroxy-N-n-propylnoraporphine displayed a slightly higher oral bioavailability (3%). In conclusion, R-(-)-N-n-propylnorapomorphine and R-(-)-11-hydroxy-N-n-propylnoraporphine did not show a substantial improvement in bioavailability. However, due to the clear difference in their efficacy in decreasing dopamine release, in spite of the similar agonist binding affinities to the dopamine D(2) receptor of the two analogs compared to R-(-)-apomorphine, they could be useful alternatives for apomorphine in the treatment of Parkinson's disease.

The response of apomorphine administered into the accumbens in rats with bilateral lesions of the nucleus accumbens, induced with 6-hydroxydopamine

Bilateral lesions of the nucleus accumbens, induced with 6-hydroxydopamine, reduced motor activity and produced a 20-35% depletion of the concentrations of dopamine (DA) and its main metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Small doses of apomorphine (1-10 ng), injected into the nucleus accumbens of sham-lesioned rats, decreased motor activity, while larger doses (1-10 micrograms) produced hyperactivity. In rats lesioned with 6-hydroxydopamine, apomorphine caused hyperactivity only, and this apomorphine-induced response was more pronounced than in sham-lesioned rats. Large doses of apomorphine decreased, only in sham-lesioned animals, the levels of DOPAC and HVA. These data suggest that the apomorphine-induced hypomotility is mediated by presynaptically located DA receptor systems in the nucleus accumbens, whereas the apomorphine-induced hypermotility is likely to be mediated by postsynaptically located DA receptor systems.

Correlation between the discharge rate of non-dopamine neurons in substantia nigra and ventral tegmental area and the motor activity induced by apomorphine

The effects of systemic apomorphine on the discharge rates of non-dopamine neurons of the ventral tegmental area and the substantia nigra were investigated in the behaving rat to determine the relationship between the neural responses and the motor activity induced by the dopamine agonist. Apomorphine, 3.0 mg/kg, induced large increases in motor activity and in the rate of firing of non-dopamine neurons in both ventral tegmental area and substantia nigra. The effects were similar in both structures, but only a portion of the non-dopamine neurons sampled were sensitive to the dopamine agonist. The motor and unit responses were correlated for latencies, magnitude and duration. These effects were dose-responsive, 0.75 mg/kg and 1.5 mg/kg inducing smaller behavioral and neural responses than 3.0 mg/kg. Apomorphine, 3.0 mg/kg, given to rats pretreated with haloperidol, 1.5 mg/kg, 60 min before the recording session, induced smaller behavioral and neural responses than in controls. The dopamine agonist given to rats in which gross motor activity was prevented through light anesthesia with urethan, 600 mg/kg, led to a decrease in the magnitude of the unit response in ventral tegmental area, and to a potentiation of the response in substantia nigra. In rats with bilateral electrolytic lesions of nucleus accumbens given one week earlier, apomorphine induced a smaller behavioral response than in controls, and differential effects on the neural responses. In ventral tegmental area the response was the same as in controls, but in substantia nigra it was blocked. These results indicate the presence in substantia nigra and ventral tegmental area of subpopulations of non-dopamine neurons responding with excitation to experimental manipulations that activate dopamine receptors. The dissociation between the motor effects of apomorphine and the neural effects in the subjects prevented from expressing gross motor activity, and in the lesioned animals, indicates that the neural responses were not the result of behavioral feedback. And the differential effects of apomorphine in ventral tegmental area and substantia nigra in these two groups of subjects suggest that the dopamine motor influence, at this brain level, may be fractionated, different groups of non-dopamine neurons conveying different aspects of the dopamine influence on motor activity to premotor neurons. The results, taken together, support the notion that non-dopamine efferent neurons in ventral tegmental area and substantia nigra function as dopamine output neurons, their output being critical for the behavioral effects of dopamine agonists.

Behavioral effects of apomorphine isomers in the rat: selective locomotor-inhibitory effects of S(+)N-n-propylnorapomorphine

The optical isomers of apomorphine (APO) and N-n-propylnorapomorphine (NPA) were evaluated behaviorally in the rat. Both R(-) isomers induced motor-excitatory effects and strong stereotyped sniffing, licking, and gnawing, as has been reported previously. The S(+) isomers selectively inhibited locomotor activity and did not induce stereotypy or catalepsy. These actions of the S(+) aporphines were selective against locomotor activity stimulated by low doses of R(-) isomers. (+)NPA (ID50 = 0.2 mg/kg) was 20 times more potent than (+)APO (ID50 = 4 mg/kg) in antagonizing the locomotor arousal-inducing effects of (-)APO (at ED50 = 0.3 mg/kg). (+)NPA also inhibited spontaneous locomotor activity much more potently (ID50 = 3.0 mg/kg) than did (+)APO (ID50 greater than 50 mg/kg). Neither S(+) aporphine had a significant effect against stereotypy induced by the R(-) isomers, even at high doses (up to 30 mg/kg). Inhibition of the effects of (-)APO by (+)NPA appeared not to be due to altered uptake of (-)APO into brain. These results suggest that S(+)NPA or its congeners and analogs may have selective antidopaminergic actions in limbic rather than striatal areas of mammalian brain.

Rotational behaviour elicited by intracerebral injections of apomorphine and pergolide in 6-hydroxy-dopamine-lesioned rats. II: The striatum of the rat is heterogeneously organized for rotational behaviour

The present study compares the role of the nucleus accumbens and the striatum, as well as various regions of the striatum, in the ability of intracerebral injections of the dopamine agonists apomorphine and pergolide to elicit rotational behaviour in 6-hydroxy-dopamine-lesioned rats. We found that apomorphine and pergolide elicit rotational behaviour when injected into the denervated striatum, but not when injected into the ipsilateral nucleus accumbens. The striatum seems heterogeneously organized as regards rotational behaviour since maximal-apomorphine rotation was elicited from the corpus of the striatum as compared to the effects produced by injections into the head and the tail of the striatum. This topographical distribution is similar to the distribution of dopamine-stimulated cyclic AMP. The pergolide response is more evenly distributed in the striatum. It is suggested that the difference in the topographical distribution of the ability of apomorphine and pergolide to elicit rotational behaviour reflects a regional distribution of dopamine receptors in the striatum of the rat.

Behavioral activity of some novel aporphines in rats with 6-hydroxydopamine lesions of caudate or nucleus accumbens

The behavioral actions of some novel aporphines have been examined in rats with selective unilateral 6-hydroxydopamine (6OHDA)-induced destruction of nigrostriatal dopaminergic neurons, and in rats with bilateral 6OHDA-induced destruction of mesolimbic dopaminergic neurons. Dopaminomimetics such as apomorphine (APO) in these animal models elicit circling behavior and locomotor activity respectively. In animals with unilateral nigrostriatal lesions (-)-2,10,11-trihydroxy-N-n-propylnoraporphine (TNPA) and (-)-10,11-methylenedioxy-N-n-propylnoraporphine (MDO-NPA) elicited weak, but prolonged, contraversive circling, whereas (-)-2,10,11-trihydroxyaporphine (2-OH.APO) was inactive. In animals with bilateral destruction of mesolimbic dopaminergic neurons TNPA and MDO-NPA elicited a strong stimulation of locomotor activity, while 2-OH.APO was inactive. The results suggest that TNPA and MDO-NPA, but not 2-OH.APO, exert central dopaminomimetic effects in vivo. The results are also consistent with previous data indicating that N-propyl substitution of aporphines causes a relative enhancement of activity in animal models which emphasise effects at mesolimbic rather than striatal dopaminergic receptors.

Neurochemical studies of the mesolimbic dopaminergic pathway: [3H]spiperone labels two bindings sites in homogenates of the nucleus accumbens of rat brain

The binding of [3H]spiperone to membranes of the nucleus accumbens of the rat brain was studied in vitro and found to be of high affinity, rapid, saturable, reversible and stereospecific. Dissociation and saturation experiments indicated the presence of two specific binding sites with apparent dissociation constants of 70 pM and greater than 1 nM. Specific binding with 25 pM [3H]spiperone represented greater than 90% of total binding and was displaced by dopaminergic agonists, neuroleptic drugs and ergot derivatives. The rank order of potency for the ergot derivatives was bromocryptine greater than pergolide greater than lergotrile, and that for D-2 antagonists was domperidone greater than sulpiride greater than molindone greater than metoclopramide. Noradrenergic, histaminergic and serotonergic components of the binding were not detected. [3H]Spiperone binds to high-affinity sites in homogenates of nucleus accumbens, which are likely to be D-2 receptors.

N-n-propyl-norapomorphine: an extremely potent stimulant of dopamine autoreceptors

N-n-propyl-norapomorphine (NPA) is 10-20 times more potent than apomorphine in producing hypomotility and inhibiting both striatal and limbic dopamine (DA) synthesis and the firing rate of nigral dopaminergic cells in rats. The threshold subcutaneous doses of NPA and apomorphine to significantly inhibit motor activity or DA synthesis are 1.25 and 24 microgram/kg, respectively. The intravenous ED 50 of NPA to inhibit dopaminergic firing is 0.36 microgram/kg and that for apomorphine is 9.1 microgram/kg. The above effects of NPA, as those of apomorphine, are antagonized by haloperidol and, stereospecifically by (--) sulpiride.

Apparent tolerance to some aspects of amphetamine stereotypy with long-term treatment

Previous reports have demonstrated that long-term amphetamine treatment results in a progressive augmentation of locomotion and focused stereotypy in the rat. A series of experiments were conducted to determine whether an increase in dopamine receptor sensitivity is the primary mechanism underlying the behavioral alterations associated with multiple amphetamine injections. Detailed observations of the focused stereotyped behaviors produced by amphetamine revealed that although some components were enhanced with long-term treatment, others were reduced. Thus, whereas repeated administration of 2.5 mg/kg d-amphetamine produced a progressive increase in repetitive head and limb movements, long-term treatment with 5.5 mg/kg d-amphetamine resulted in a reduction of licking and biting behaviors (oral stereotypies). These results, which suggest that different mechanisms mediate the various components of focused stereotypy, argue against the supersensitivity hypothesis. In fact, the apparent tolerance that develops to oral stereotypies may reflect a decrease in dopamine receptor sensitivity since repeated amphetamine administration also reduces the oral stereotypies produced by 0.5 or 2.0 mg/kg apomorphine, a direct acting dopamine agonist. Thus, the behavioral alterations produced by repeated amphetamine injections cannot be explained solely by an increase in receptor sensitivity.

Comparison of the dopaminergic effects of apomorphine and (-)-N-n-propylnorapomorphine

(-)-N-n-propylnorapomorphine (NPA) was found to be 2.3 times more active than apomorphine in producing stereotypy in novice mice. This potency ratio was not changed by reserpine pretreatment (4 h prior). However, when mice pretreated with reserpine 24 h earlier were used, NPA was found to be 6.5 times more active in producing locomotor stimulation and 8.7 times more active in producing stereotypy than apomorphine. In these mice, a second dose of reserpine or alpha-methyl-p-tyrosine (alphaMT) given 4 h prior to NPA administration considerably reduced the locomotor effects of NPA. Such treatments did not modify the effects of apomorphine. Phenoxybenzamine failed to alter the responses of both these drugs. It was concluded that, while apomorphine possesses direct dopamine (DA) receptor stimulant effect, that of NPA is partly direct and partly indirect in nature. In novice mice, NPA was 91 times more active than apomorphine in inhibiting the alphaMT-induced depletion of brain DA. The question is raised why the powerful DA receptor agonistic effect of NPA did not produce equivalent behavioral responses in mice. The likely explanation would be that, in addition to its effect on the striatonigral DA system from which the stereotypic response originates, NPA also exerts a predominant effect on the mesolimbic areas. The combined effect of NPA on these two components of the DA system is reflected in the biochemical results. The overall dopaminergic effect of NPA is several times greater than that of apomorphine.

Different behavioural patterns induced by apomorphine: evidence that the method of administration determines the behavioural response to the drug

The behavioural effects of s.c. injected apomorphine was studied on habituated rats in a test-box designed to measure 8 different components of behaviour. Apomorphine, 1 mg/kg, induced two different behaviours: The "G-type" of behaviour characterized by compulsive gnawing and the "LS-type" of behaviour characterized by increased locomotion, sniffing and repetitive head and limb movements. G-type behaviour was induced when apomorphine, dissolved by heating, was injected s.c. into the flank of the animal. LS-type behaviour was induced both when apomorphine, dissolved by heating, was injected s.c. into the neck and when it was dissolved by heating together with a high concentration of ascorbic acid (1 mg/ml) and injected s.c. into the flank. G-type behaviour could not be elicited by changing the dose which induced LS-type behaviour or vice versa. We therefore conclude that these different behavioural effects of apomorphine were not dose--response effects but were elicited by at least two different synaptic mechanisms in the brain. Experimentally induced changes from one of these apomorphine-induced behaviours to another can therefore not merely be interpreted as a change in the intensity of the behavioural response as is done in e.g. commonly used stereotypy rating scales.

Selective 6OHDA-induced destruction of mesolimbic dopamine neurons: abolition of psychostimulant-induced locomotor activity in rats

Selective large scale destruction of mesolimbic dopamine-containing terminals is produced by bilateral injection of 8 mug of 6-hydroxydopamine (6OHDA) into the nucleus accumbens septi (NAS) of rats pretreated with pargyline and desipramine (DMI). The DMI prevents the destruction of the noradrenergic innervation of the forebrain normally produced by the NAS 6OHDA lesion, without affecting the destruction of dopamine-containing neurons. The locomotor stimulation produced by the psychostimulants d-amphetamine (1.5 mg/kg) and cocaine (20 mg/kg) is blocked in rats with selective destruction of the mesolimbic dopamine system. In contrast the locomotor stimulation produced by the directly acting dopamine agonist apomorphine (1.0 mg/kg) is enhanced, which may indicate supersensitivity of the denervated dopamine receptors. These results lend further support to the view that psychostimulant-induced locomotr stimulation in rats results from effects on mesolimbic dopamine neurons. In addition, the protection by DMI of noradrenergic neurons from the toxic effects of 6OHDA is evidence that 6OHDA, as used here, destroys catecholamine neurons mainly by an uptake-dependent specific mechanism.

Aporphines. 15. Action of aporphine alkaloids on dopaminergic mechanisms in rat brain

Of eleven aporphine analogues tested on striatal adenylate cyclase only (-)-apomorphine and (+/-)-N-n-propyl-norapomorphine (+/-(NPA)) were effective in stimulating the cyclase from rat brain. Inactive compounds included (+/-)-isoapomorphine, (-)-1,2-dihydroxyaporphine and (+/-)-10-hydroxy-N-n-propylnoraporphine. (+)-Bulbocapnine was an effective antagonist of the stimulating effects of dopamine or (-)-apomorphine on striatal adenylate cyclase. Injection of (-)-apomorphine into the lateral ventricle of rats with unilateral 6-hydroxydopamine-induced lesions of the nigro-striatal pathway caused the animals to rotate away from the side of the lesion. Intraventricular injection of 25 mug (+/-)-10-hydroxy-N-n-propylnorapomorphine was ineffective in producing rotation. The results are discussed in relation to the structural requirements for CNS dopamine receptor agonists.