The ontogeny of apomorphine-induced alterations of neostriatal dopamine release: Effects on potassium-evoked release

The effects of apomorphine (0.05, 0.1, and 1.0 mg/kg, s.c.) on K⁺-evoked dopamine release were studied through the use of in vivo microdialysis in the neostriatum of developing and adult rats. Fifteen-minute samples were collected from urethane-anesthetized rats 5, 10–11, 21–22, 35–36 days of age, and adults, and quantified by high performance liquid chromatography with electrochemical detection. Apomorphine attenuated K⁺-evoked dopamine release in all age groups, suggesting that the dopamine autoreceptor modulating release in the neostriatum is functional by 5 days of age. A dose-response effect of apomorphine was observed in all age groups except at 5 and 10 days of age. Absolute levels of extracellular dopamine were significantly lower at 5 and 10 days of age compared with the other ages, and the effectiveness of a high-K⁺ artificial cerebrospinal fluid to evoke dopamine release increased with age.     

The Ontogeny of Apomorphine-Induced Alterations of Neostriatal Dopamine Release: Effects on Spontaneous Release

The effects of apomorphine (0.05, 0.1, and 1.0 mg/kg, s.c.) on the extracellular levels of dopamine and the dopamine metabolite 3, 4-dihydroxyphenylacetic acid were studied through the use of in vivo microdialysis in the neostriatum of developing and adult rats. Fifteen-minute samples were collected from urethane-anesthetized rats 5, 10–11, 21–22, and 35–36 days old and adults and quantified by HPLC with electrochemical detection. Apomorphine attenuated extracellular levels of dopamine in all age groups, suggesting that the dopamine autoreceptor modulating release in the neostriatum is functional by 5 days of age. A dose-response effect of apomorphine on extracellular dopamine was observed in all age groups except at 10–11 days of age. Extracellular levels of 3, 4-dihydroxyphenylacetic acid were also significantly decreased in all age groups, consistent with the hypothesis that synthesis-modulating dopamine autoreceptors in the neostriatum are functional by 5 days of age. Apomorphine had a significantly greater effect on extracellular 3, 4-dihydrpxyphenylacetic acid levels at the 0.05 and 0.1 mg/kg doses in the 5- and 10–11-day-old age groups compared with the other ages. Absolute levels of extracellular dopamine were significantly attenuated at 5 days of age compared with the other ages, and absolute levels of extracellular 3, 4-dihydroxyphenylacetic acid monotonically increased with age.     

Effects of short- and long-acting dopamine agonists on sensitized dopaminergic neurotransmission in rats with unilateral 6-OHDA lesions

The effects of short and long-acting dopamine agonists on sensitized dopaminergic transmission in an animal model of Parkinson's disease were investigated. Rats with 6-hydroxydopamine (6-OHDA) lesions of the left nigrostriatal dopaminergic pathway were pre-exposed i.p. to 50 mg/kg methyl levodopa for 10 days. After a 7-day withdrawal period, these animals were treated with saline i.p., 0.05 mg/kg apomorphine s.c., or 0.5 mg/kg cabergoline i.p., once daily for 7 days. On the 8th day, rats in each treatment group received a challenge dose of 0.05 mg/kg apomorphine or saline s.c. The temporal changes in the number of rotations away from the 6-OHDA lesion side were evaluated after the challenge. The apomorphine challenge increased the number of rotations more markedly in the apomorphine pretreated rats than in the other pretreatment groups. In cabergoline pretreated rats, the number of rotations was significantly lower than that of saline-pretreated animals. Pretreatment with saline did not alter the apomorphine sensitivity of rotational behavior. These findings suggest that the repeated administration of long-acting dopamine agonists may reduce sensitized dopaminergic transmission in dopamine-depleted rats, whereas short-acting ones may further enhance sensitization of the transmission process.     

Effect of cyclo(Leu-Gly) on the supersensitivity of dopamine receptors in spontaneously hypertensive rats

Spontaneously hypertensive rats exhibited dopamine receptor supersensitivity as evidenced by a greater hypothermic response to apomorphine in comparision with normotensive Wistar-Kyoto rats. A single injection of cyclo(Leu-Gly) given prior to apomorphine administration did no alter apomorphine induced hypothermia in either the normotensive or the hypertensive rats. Chronic administration of cyclo(Leu-Gly) for 7 days did not affect apomorphine response in normotensive rats, but blocked the exaggerated response to apomorphine in the hypertensive rats. These studies suggest that cyclo(Leu-Gly) interacts with the dopamine receptors and that the central dopamine receptors may play a role in the pathophysiology of hypertension.     

Apomorphine Does Not Alter Amphetamine-Induced Dopamine Release Measured in Striatal Dialysates

Amphetamine facilitates the release of dopamine from nerve terminals, but the mechanisms underlying this effect have not been fully delineated. The present experiments were designed to test the extent to which amphetamine-induced dopamine release is dependent on impulse flow and autoreceptor function in dopaminergic neurons. Rats were pretreated with a low dose of apomorphine (0.05 mg/kg) to inhibit dopamine neuronal activity, and the striatal dopaminergic response to amphetamine (0.5 mg/kg) was assessed by in vivo dialysis in freely moving animals. Consistent with previous results, apomorphine alone substantially decreased, whereas amphetamine increased, striatal dialysate dopamine concentrations. However, whereas apomorphine pretreatment decreased the locomotor response to amphetamine, the amphetamine-induced increase in dialysate dopamine was unaffected. These results indicate that amphetamine-facilitated dopamine release is independent of neuronal firing and autoreceptor regulation, consistent with the putative accelerative exchange-diffusion mechanism of amphetamine-induced dopamine release. Other possible mechanisms underlying the inhibitory effects of apomorphine on amphetamine locomotor activation are discussed.     

Effects of the potential neuroleptic peptide des-tyrosine1-γ-endorphin and haloperidol on apomorphine-induced behavioural syndromes in rats and mice

The effects of haloperidol and Des-Tyr¹-γ-endorphin (DTγE) were studied on climbing induced in mice by high doses of apomorphine and on the yawning syndrome induced in rats by low doses of apomorphine. Haloperidol in a dose of 0.0046 mg/kg s.c. potentiated climbing whereas at higher doses climbing was inhibited (ED₅₀=0.03 mg/kg). DTγE had no effect on climbing under normal conditions in doses up to 2 mg/kg s.c.. After three days of handling and saline pre-injections DTγE potentiated climbing in doses from 0.1 to 1 mg/kg.Haloperidol inhibited yawning induced by low doses of apomorphine (ED₅₀=0.01 mg/kg). DTγE, on the other hand, potentiated yawning induced by low apomorphine at doses of 0.02 and 0.04 mg/kg s.c.. From the point of view that low doses of apomorphine predominantly activate presynaptic dopamine autoreceptors while higher doses predominantly activate postsynaptic dopamine receptors the following tentative conclusions are drawn. 1) Haloperidol blocks presynaptic dopamine autoreceptors at low doses and postsynaptic dopamine receptors at higher doses. 2) DTγE sensitizes presynaptic dopamine autoreceptors at low doses, thereby strengthening the local feedback mechanism at the dopaminergic nerve ending, and sensitizes postsynaptic dopamine receptors at higher doses.     

The effect of cyclo (Leu-Gly) on chemical denervation supersensitivity of dopamine receptors induced by intracerebroventricular injection of 6-hydroxydopamine in mice

Chemical denervation supersensitivity of postsynaptic dopamine receptors was induced in mice by intracerebroventricular injection of 6-hydroxydopamine. Fourteen days after the 6-hydroxydopamine injection, mice exhibited greater spontaneous locomotor activity and hypothermic response when challenged intraperitoneally with apomorphine. Whole brain levels of dopamine were reduced by 80%. Daily subcutaneous administration of cyclo (Leu-Gly) (50 μg/mouse/day) for 14 days inhibited the development of dopamine receptor supersensitivity induced by 6-hydroxydopamine as evidenced by the blockade of an apomorphine induced locomotor and hypothermic effect. Cyclo (Leu-Gly) did not alter the depletion of brain dopamine induced by 6-hydroxydopamine. These data suggest that cyclo (Leu-Gly) can block the development of dopamine receptor supersensitivity induced by 6-hydroxydopamine without protecting the neurons from dopamine depletion.     

Dopamine Autoreceptor Stimulation Increases Protein Carboxyl Methylation in Striatal Slices

We have investigated the possibility that protein carboxyl methylation is involved in coupling dopamine autoreceptor stimulation to intracellular events such as inhibition of dopamine synthesis or release. The dopamine agonists apomorphine and TL-99 were found to stimulate methyl ester formation in striatal slices preloaded with [3H]methionine. The stimulatory effects of apomorphine were dose-dependent, were not due to changes in [3H]methionine uptake or S-[3H]-adenosylmethionine formation, and were blocked by the stereospecific dopamine antagonist (+)-butaclamol. Stimulation of methyl ester formation by dopamine agonists is readily observed only when slices are prepared from rats pretreated with reserpine to deplete endogenous brain catecholamines. This suggests that in slices prepared from normal rats endogenous dopamine (DA) released during slice preparation and incubation masks the effects produced by exogenously administered dopamine agonists on protein carboxyl methylase (PCM) activity. Additional experiments suggested that the effects of apomorphine were mediated via an interaction with DA autoreceptors rather than with postsynaptic DA receptors. Destruction of monoamine neurons and their associated autoreceptors by injecting 6-hydroxydopamine into the area of the medial forebrain bundle abolished the stimulatory effects of apomorphine on methyl ester formation in striatal slices. Furthermore the putative selective DA autoreceptor agonist EMD 23 448 was also found to stimulate methyl ester formation in striatal slices. These findings, discussed in terms of calcium-dependent functions, support the hypothesis that PCM may be a key component in the biochemical transduction of DA autoreceptor stimulation.     

ONTOGENETIC DEVELOPMENT OF NEOSTRIATAL DOPAMINE RECEPTORS IN THE RAT 1

Abstract— The actions of dopamine and apomorphine on the kinetic properties of striatal adenylate cyclase were investigated during ontogenesis in the rat. The maximum stimulatory effect of dopamine (5 × 10^?5 M) was constant from birth to maturity (1 to 60 days of age). In contrast, the stimulatory effect elicited by apomorphine (5 × 10^?5 M) was almost the same as that of dopamine in 6-day-old rats, but it declined during maturation reaching 50% of the initial value at 60 days of age. The apparent K_m value for dopamine did not change during development, while the K_m for apomorphine was higher in the adult than in the newborn. Apomorphine appeared to have a greater affinity than dopamine for the striatal adenylate cyclase both in adult and newborn rats.     

The effect of chronic levodopa on haloperidol induced behavioral supersensitivity in the guinea pig

The effect of chronic levodopa-carbidopa administration (200 mg/kg for 21 days) on guinea pigs rendered behaviorally supersensitive by the prior administration of haloperidol (.5 mg/kg for 21 days) was examined. Animals who showed an increased behavioral response to apomorphine after chronic haloperidol administration were treated with levodopa-carbidopa and then apomorphine - induced stereotypy was reexamined. Although the chronic levodopa control groups and the chronic haloperidol control remained supersensitive to the behavioral effect of apomorphine, the haloperidol-levodopa group's behavioral response to apomorphine returned to normal. Both chronic dopaminergic antagonist and agonist administration have been demonstrated to induce heightened apomorphine-induced stereotypy and this has been interpreted as a reflection of altered striatal dopamine receptor site sensitivity. The finding that the serial administration of a chronic dopaminergic antagonist followed by a chronic dopaminergic agonist results in a return to normal of a striatal dopamine receptor-dependent behavior suggests that these chronic treatments affect dopamine receptor sites by different mechanisms of action. Since neuroleptic induced dopaminergic supersensitivity in animals is an accepted model of tardive dyskinesia, levodopa may also reverse dopaminergic supersensitivity in patients and might be a potential therapeutic agent in tardive dyskinesia.     

Inhibition by bromoestrogens of the effects of estradiol on apomorphine-induced climbing behavior

The chronic administration of estrogens to mice or rats will result in antidopaminergic effects. Apomorphine-induced climbing behavior in mice, the result of direct stimulation of dopamine receptors in the striatal and mesolimbic regions, is a simple animal model for examining these antidopaminergic effects of estrogens. Bromoestrogens, inhibitors of catechol estrogen formation, have been utilized in order to examine the role of estrogen metabolism in dopaminergic antagonism. Mice were pretreated for 3 days with 2-bromoestradiol, 4-bromoestradiol, or 2,4-dibromoestradiol dibenzoates alone or in combination with estradiol benzoate prior to apomorphine administration. The haloestrogens did not alter the climbing-induced responses elicited by apomorphine, whereas estradiol benzoate clearly attentuated the actions of apomorphine. Furthermore, the bromoestradiol dibenzoates were effective in reversing the effects of estradiol benzoate when the two steroids (estradiol benzoate and a bromoestrogen dibenzoate) were administered simultaneously during pretreatment. Thus, the bromoestrogens are able to inhibit the antidopaminergic effects of estradiol exhibited in the apomorphine-induced mouse climbing model.     

Chronic treatment with antidepressants prevents the inhibitory effect of small doses of apomorphine on dopamine synthesis and motor activity.

In control rats small doses of apomorphine (25 to 100 μg/kg) decreased motor activity and reduced DOPAC content in the caudate nucleus. A larger dose (500 μg/kg) increased motor activity and elicited stereotypy. Chronic treatment with imipramine, amitryptiline and mianserine (10, 10 and 2.5 mg/kg twice daily for 10 days respectively) counteracted or reversed the effect of small doses of apomorphine on motor activity, left DOPAC content unchanged and potentiated the central stimulant response to the larger dose of apomorphine. Changes in apomorphine responses were observed after ten but not after two days of imipramine treatment and persisted unaltered up to 4 days after imipramine withdrawal. It is suggested that chronic treatment with antidepressants induces persistent subsensitivity in presynaptic dopamine receptors. The relevance of the findings in the therapeutic effect of these drugs is discussed.     

The effects of apomorphine and haldol on PCP induced behavioral and motor abnormalities in the rat

Behavioral abnormalities and motor dysfunctions induced by the administration of PCP to rats is antagonized by both the dopamine agonist apomorphite and the dopamine antagonist haloperidol. In the test situation used, however, the antagonistic effects of apomorphine were much more rapid than those of haloperidol. These results suggest that PCP may be acting on specific presynaptic dopamine receptors and that this reaction is effectively antagonized by low doses of apomorphine in a manner similar to its effects in reducing schizophrenic symptoms and choreiform movement disorders.     

Dopamine stimulated increase of GTP levels in the rat retina

Dopamine stimulates a 7-10-fold increase of GTP concentration in whole rat retina maintained in vitro. Half-maximal stimulation of GTP levels were obtained with 10(-6) M dopamine, and significant increases in GTP levels were seen with 10(-7) M dopamine. Intracellular GTP levels were significantly increased within 4 min after exposure to dopamine and maximal effects were reached within 30 min. Dopamine agonists, apomorphine and bromocriptine, also stimulate a 7-10-fold increase in GTP concentration, whereas other catecholamines (norepinephrine, epinephrine, and isoproterenol) were less potent. Several other neurotransmitters present in rat retina (gamma-aminobutyric acid, glycine, glutamine, and taurine) had no effect on GTP levels. Although dopamine also stimulates increases in cyclic AMP levels in the retina, dibutyryl cyclic AMP and 8-bromo-cyclic AMP had no effect on GTP levels, indicating that the dopamine-stimulated increase of GTP is independent of the catalytic production of cyclic AMP by adenylate cyclase. Since dopamine-stimulated adenylate cyclase activity requires GTP, the dopamine-stimulated increase in GTP concentration described in this report may serve to facilitate dopamine stimulation of adenylate cyclase activity.     

Increased ability of apomorphine to reduce serum concentrations of prolactin in rats treated chronically with alpha-methyltyrosine

Serum concentrations of prolactin were significantly increased in rats for up to 9 hours after a single i.p. injection of α-methyltyrosine (50 mg/kg); apomorphine caused a dose-dependent reduction of the elevated prolactin concentrations. Doses of apomorphine required to reduce serum prolactin concentrations.were lower in animals previously injected with α-methyltyrosine three times a day for 10 days than in animals which received a single injection of α-methyltyrosine. These results suggest that chronic disruption of the normal release of dopamine from tuberoinfundibular neurons leads to the development of increased sensitivity of dopamine receptors involved with the inhibition of prolactin release from the anterior pituitary.     

Dopaminergic modulation of visual responses in toads I. Apomorphine-induced effects on visually directed appetitive and consummatory prey-catching behavior

This study confirms for a phylogenetically basal terrestrial vertebrate that dopaminergic modulations interfere with the visually directed appetitive and consummatory feeding behaviors orienting and snapping, respectively. (1) In common toads Bufo bufo, intralymphatic administration of the dopamine D₂/D₁-receptor agonist apomorphine led to a dose-dependent facilitation of prey-snapping in response to moving objects. The snapping activity reached a maximum 15–35 min after apomorphine injection. (2) To changes in configurational stimulus features, the basic pattern of discrimination was maintained; however, the acuity of discrimination was reduced due to the high snapping response level. (3) The apomorphine-induced facilitation of snapping was accompanied by a suppression of prey-oriented lunging and turning. Toads snapped only if prey occurred frontally in the visual field at a relatively short distance. The snapping behavior was fixed in its form and stereotyped regarding its immediate release. (4) About 90 min after apomorphine administration, prey-oriented turning behavior was restored and displayed a facilitatory rebound. (5) In comparative experiments with the species B. marinus, both prey-oriented turning and snapping responses were suppressed by apomorphine in a dose-dependent manner. (6) After pre-treatment with the dopamine antagonist haloperidol, apomorphine showed no measurable effect on the visual release of prey orienting or snapping. (7) The results contribute to the sensorimotor and the motivation hypothesis of dopamine function proposed for higher vertebrates and stimulate a comparative discussion of anatomic homologies and functional analogies. Accepted: 10 July 1996     

Effects of chronic antidepressant treatment on nigrostriatal and mesolimbic dopamine autoreceptors in the rat

Dopamine autoreceptors were studied by determining the effects of chronic antidepressant treatment on the ability of several doses of apomorphine to decrease 3,4-dihydroxyphenylalanine accumulation (an index of dopamine synthesis in vivo) after saline or γ-hydroxybutyric acid lactone (γ-butyrolactone). 3,4-Dihydroxyphenylalanine accumulation was measured in nigrostriatal [nucleus caudatus putamen] and mesolimbic [nucleus accumbens and tuberculum olfactorium] nerve terminals. Apomorphine decreased 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen, tuberculum olfactorium and nucleus accumbens in a dose-related manner. Chronic imipramine (10 days) treatment attenuated the low and high dose apomorphine-induced decrease in 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen to a greater extent than the tuberculum olfactorium or nucleus accumbens. In γ-butyrolactone-treated animals chronic treatment with imipramine, amitriptyline or bupropion (10 days) attenuated the low dose apomorphine effect in the nucleus caudatus putamen, but not the tuberculum olfactorium or nucleus accumbens. Only 2 days of imipramine treatment had no effect on the apomorphine-induced decrease in 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen with or without γ-butyrolactone treatment. These data suggest that chronic treatment with three antidepressants produces dopamine autoreceptor subsensitivity in nigrostriatal neurons more than mesolimbic neurons and that this effect is not seen with short-term imipramine treatment.     

Opposite Regulation of Serotonin-S2 and Dopamine-D2 Receptors in Rat Brain Following Chronic Receptor Blockade

Abstract

Rats were chronically treated with setoperone, a mixed serotonin and dopamine antagonist. Alterations in serotonin-S₂ and dopamine-D₂ receptors in the brain and changes in behavioural responses to tryptamine and apomorphine were studied along with duration of treatment and drug withdrawal. As with neuroleptics, behavioural supersensitivity to apomorphine and increase in the number of striatal dopamine-D₂ receptor sites were apparent after 2 days setoperone treatment, both effects were maximal with 14 days treatment and were maintained over more than 20 days drug withdrawal. In contrast to the changes in the dopaminergic system, the rats showed a decreased response to tryptamine and serotonin-S₂ receptor sites in the frontal cortex were significantly reduced in numbers. Both effects developed in parallel over 14 days treatment and extinguished over 10 days drug withdrawal. K_D-values of radioligand binding to dopamine-D₂ and serotonin-S₂ receptor sites were unchanged by the setoperone treatment. The concomitant development and extinction of the in vivo and in vitro effects suggests a causal relationship between them. Chronic treatment with a selective histamine-H₁ antagonist (levocabastine) or the tranquilizer diazepam did not affect dopamine-D₂ or serotonin-S₂ receptor sites. These observations demonstrate that in contrast to the receptor regulation theory, serotonin-S₂ receptors are down regulated following persistent receptor blockade. Implications for the clinical use of serotonin antagonists and possible molecular mechanisms involved in the receptor regulation have been discussed.     

Effect of dietary proteins and carbohydrates on urinary and sympathoadrenal catecholamines

We previously observed that administration of tyrosine to rats or humans elevated urinary dopamine, norepinephrine and epinephrine levels. The present studies examine the effects on these urinary catecholamines of varying the ratio of protein to carbohydrate in the diets.Rats consumed diets containing 0, 18 or 40% protein (76, 58 and 36% carbohydrate respectively) for 8 days. The stress of consuming the protein-free food was associated with a 16% weight reduction, and with significantly lower serum, heart and brain tyrosine levels than those noted in rats eating the 18 or 40% protein diets. Absence of protein from the diet also decreased urinary levels of dopamine and DOPA but increased urinary norepinephrine and epinephrine, probably by increasing sympathoadrenal discharge; it also increased the excretion of DOPA in animals pretreated with carbidopa, a DOPA decarboxylase inhibitor. Carbidopa administration decreased urinary dopamine, norepinephrine and epinephrine as expected; however, among carbidopa-treated rats urinary norepinephrine and epinephrine concentrations were highest for animals consuming the protein-free diet, again suggesting enhanced release of stored catecholamines from sympathoadrenal cells. The changes in urinary catecholamines observed in animals eating the protein-free diet were similar to those seen in rats fasted for 5 days: dopamine levels fell sharply while norepinephrine and epinephrine increased.These data indicate that the effects of varying dietary protein and carbohydrate contents on dopamine secretion from peripheral structures differ from its effects on structures secreting the other two catecholamines. Protein consumption increases dopamine synthesis and release probably by making more of its precursor, tyrosine, available to peripheral dopamine-producing cells; it decreases urinary norepinephrine and epinephrine compared with that seen in protein-deprived animals, probably by diminishing the firing of sympathetic neurons and adrenal chromaffin cells.     

Apomorphine induced biting and fighting behaviour in reserpinized rats and an approach to the mechanism of action

Various patterns of aggressive behaviour have been induced by drugs. In the present study, the biting and the fighting responses were induced in rats by apomorphine alone, and reserpine plus apomorphine combination respectively, and these could be blocked completely by a dopamine receptor blocking agent. Dopamine, norepinephrine and clonidine given intraperitoneally or intraventricularly failed to induce these responses. Chemical agents known to increase the concentration of dopamine in the brain, induced the biting, but not the fighting response, whereas these behavioural patterns were more intense due to apomorphine in the rats pretreated with reserpine and dopamine or α-methyltyrosine and reserpine combinations. In amphetamine pretreated rats, apomorphine induced intense biting after 10 min and a few bouts of fighting after 30 min. It is suggested that (i) the receptors on which apomorphine acts may be called ‘Apomorphine Receptor’ rather than ‘Dopamine Receptor’, (ii) dopamine incompletely activates these receptors which are sensitised in the absence of catecholamines and induce a higher degree of stereotyped behaviour i.e. fighting, due to apomorphine.