Different developmental schedules of dopaminergic and noradrenergic neurons in dissociation culture of fetal rat midbrain and hindbrain

The development of dopaminergic and noradrenergic neurons in dissociation cultures of mesencephalon and rhombencephalon obtained from 18-day-old rat fetuses was characterized by their capacity to take up and release catecholamines. In both types of cultures, uptake of [3H]dopamine and [3H]noradrenaline was obtained which could be inhibited by reserpine. Autoradiographic studies demonstrated an almost exclusive neuronal localization of the labeled catecholamines. The transmitters could be released by depolarization with K+ in a Ca2+-dependent manner during the entire cultivation period. In contrast, catecholamine uptake by cultures of neocortex was minimal, could not be inhibited by reserpine, and the accumulated radioactivity could not be released upon depolarization. These points provide evidence for an active accumulation of the exogenous transmitters and for the presence of stimulus-secretion coupling in a distinct population of neurons of both brain stem cultures. Striking differences between the two brain stem cultures concerned their sensitivity to desmethylimipramine and benztropine as well as the time course of the development of the uptake capacity. Desmethylimipramine inhibited the uptake of both catecholamines in rhombencephalic, but not in mesencephalic cultures. The reverse was true for benztropine. It is concluded that cultures of rhombencephalon contain predominantly noradrenergic, and those of mesencephalon dopaminergic cells. Comparison of the uptake behaviour suggested that noradrenergic neurons mature considerably later than dopaminergic neurons. The results show that dissociation cultures of mid- and hindbrain, inspite of their heterogeneous composition, can serve as valuable models for the study of development and function of dopaminergic and noradrenergic neurons, respectively.

Dopamine uptake blockers protect against the dopamine depleting effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the mouse striatum

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a recently described neurotoxin, produces a marked dopamine (DA) depletion in the mouse striatum. In this study, a series of DA uptake blockers was tested for their ability to prevent this effect of MPTP. The agents tested (amfonelic acid, benztropine, bupropion and mazindol) completely protected against DA depletion in the mouse striatum when given before DA-depleting doses of MPTP were administered, whereas atropine and trihexyphenidyl (which were employed for comparative purposes) did not. DA uptake blocking agents appear to represent a second general class of compounds, monoamine oxidase inhibitors being the first, which protect against the biologic effects of MPTP in the mouse.

Occurrence of serotonin-accumulating neurones in cultures of retina from the human foetus

Cultures were produced from retinas taken from human foetuses. Specific cells in culture have the capacity to take up exogenous serotonin, which was then visualized by immunofluorescence using an antisera to serotonin. The known serotonergic uptake blocker, chlorimipramine, inhibited the uptake of the amine while benztropine, a dopamine-uptake blocker, is ineffectual. These findings show that serotonin-accumulating cells are formed prenatally in man and demonstrate the usefulness of this preparation for future studies.

Amphetamine attenuates the stimulated release of dopamine in vivo

Carbon-fiber voltammetric electrodes have been used to measure the release of dopamine in the caudate nucleus of an anesthetized rat. Release is induced by electrical stimulation of the medial forebrain bundle. The amplitude of the observed release is attenuated by i.p. injection of amphetamine. A similar attenuation is induced by reserpine; however, at a slower rate. The combined regimen of amphetamine (1 or 10 mg/kg) and electrical stimulation does not deplete striatal dopamine levels and thus the decreased release of dopamine is not a result of depleted dopamine stores. Benztropine (25 mg kg-1) is able to cause a short term inhibition of the action of amphetamine (1 mg kg-1). The dopamine agonist pergolide (0.5 mg kg-1) does not affect the stimulated release. Haloperidol (1.0 mg kg-1) increases the amount of DA release, but is unable to attenuate the inhibition caused by amphetamine. Thus, it appears that the actions induced by amphetamine are a result of interaction with the neuronal uptake carrier and subsequent transport of dopamine from a functional to nonfunctional pool. In isolated striatal synaptic vesicles, amphetamine is found to block dopamine uptake and induce its release. This in vitro evidence provides a possible mechanism for the observed in vivo actions of amphetamine.

Effect of benztropine on the diurnal prolactin responses to haloperidol

Prolactin (PRL) responses to haloperidol were investigated at 0900 and 1800 h in six young healthy men under basal conditions and after benztropine mesylate administration. Haloperidol administration induced significantly higher PRL release during the evening compared to the morning. The anticholinergic drug, benztropine, potentiated the PRL responses to haloperidol both in the morning and in the evening. The possible mechanisms of these findings are discussed.

Induction of tardive dyskinesia in Cebus apella and Macaca speciosa monkeys: a review

Two different studies were performed in subhuman primates in an attempt to induce symptoms of tardive dyskinesia. The first study lasted for over 5 years. This involved elderly Macaca speciosa. The animals were given first 25 mg of fluphenazine decanoate and later the enanthate IM (3.2 mg/kg) every 2 weeks and on 5 days a week, haloperidol, first IM and later PO. Haloperidol was given first in doses of 1.0 mg/kg and ultimately after years of therapy, in doses of 6.4 mg/kg per day. Those animals who survived gained weight to over 10 kg. After neuroleptic withdrawal, tardive dyskinesia became evident in 1 month. The symptoms of tardive dyskinesia following cessation of medication lasted a maximum of 1 year. This animal model produced very impressive symptoms in one of the three animals treated who survived. This is not a very practical animal model from the aspects of economics (costly), time (5 years), and animal availability (rare and endangered species). However, the symptoms of tardive dyskinesia are very striking and identical with human tardive dyskinesia in a susceptible animal. A more practical experimental animal model involved Cebus apella. Depot fluphenazine (0.1 to 3.2 mg/kg) was given continuously every 2 weeks for 1 year. In this species the symptoms of tardive dyskinesia became progressively prolonged and intense with each course of fluphenazine therapy and withdrawal, suggesting that reversible tardive dyskinesia may turn into irreversible tardive dyskinesia. With each succeeding course of fluphenazine therapy (1 month) and withdrawal (1-3 months), the animals appeared to be sensitized to both the acute extrapyramidal and the tardive dyskinesia symptoms. These animals were also given various experimental drug treatments including biperiden lactate, benztropine mesylate, and d-amphetamine after they developed signs of tardive dyskinesia.

Effects of benztropine mesylate on haloperidol-induced prolactin secretion and serum haloperidol levels in rats

The effects of benztropine mesylate on haloperidol-induced prolactin secretion and serum haloperidol levels were investigated in 240 rats. Animals were pretreated with benztropine mesylate or saline 20 min prior to receiving haloperidol or saline. Serum prolactin and haloperidol levels were analyzed at six time periods over 150 min. There was no significant difference in prolactin levels of control animals, i.e., saline pretreated/saline treated rats compared to benztropine mesylate pretreated/saline treated rats. Haloperidol caused a significant rise (p less than 0.0001) in serum prolactin compared with controls. The prolactin concentration for the 30-150-min sampling period was significantly higher when the rats received benztropine mesylate prior to haloperidol (p less than 0.05). There was a significant correlation (r = 0.57, p less than 0.001) between serum haloperidol levels and serum prolactin levels in haloperidol-treated animals pretreated with either saline or benztropine mesylate. Additionally, serum haloperidol levels were not significantly different in animals pretreated with benztropine mesylate compared with those pretreated with saline. Thus, the enhancement of prolactin levels by benztropine mesylate was independent of any effect of haloperidol metabolism. This study appears to indicate that in the rat, cholinergic mechanisms exert a weak inhibitory effect on prolactin secretion under conditions of dopamine blockade.

Striatal dopamine uptake in the rat: in vivo analysis by fast cyclic voltammetry

Electrical stimulation of the median forebrain bundle of the chloral hydrate anaesthetised rat evoked dopamine release in the ipsilateral striatum, which was monitored with fast cyclic voltammetry. On cessation of stimulation, the extracellular concentration of dopamine fell to sub-detectable levels over a period of about 15 s. This fall appeared to be due to a saturable, low affinity uptake system that could be inhibited by nomifensine (20 mg/kg i.p.). These experiments constitute the first characterisation of a dopamine uptake mechanism obtained in the intact animal.

alpha-MSH and Pro-Leu-Gly-NH2 (PLG; MIF-1): influence on dopamine (DA) uptake in crude synaptosomal preparations from rat mediobasal hypothalamus (MBH) and caudate putamen (CP)

The uptake of tritiated dopamine [3H] (DMI insensitive DA uptake) by synaptosomal fractions isolated from rat mediobasal hypothalamus (MBH) and caudate putamen (CP) was measured in the presence of different concentrations of alpha-melanocyte stimulating hormone (alpha-MSH) and Pro-Leu-Gly-NH2 (PLG; MIF-1) which is an inhibitor of alpha-MSH release. Compared to control, [3H]DA uptake increased significantly when the synaptosomal fraction of CP was incubated with 0.1 and 1 microM of alpha-MSH and also when the rat was previously injected with alpha-MSH. A simultaneous reduction of endogenous dopamine content was observed. Kinetic studies suggest that the enhanced uptake induced by alpha-MSH 1 microM is the consequence of the rise in Vmax, without changes in the apparent km. The uptake of [3H]DA in hypothalamic (MBH) preparations on the other hand, was not modified by the presence of alpha-MSH. PLG did not have any significant effect on [3H]DA uptake either in the CP or in the MBH. alpha-MSH may act as a modulator of the dopaminergic nigrostriatal system and the results obtained incubating CP synaptosomes in its presence demonstrate a possible direct modulator action by alpha-MSH on the terminal area of the substantia nigra neurons.

Effects of phencyclidine, amphetamine and related compounds on dopamine release from and uptake into striatal synaptosomes

The ability of phencyclidine (PCP), amphetamine and other substances to stimulate dopamine release from and inhibit dopamine uptake into rat striatal synaptosomes was examined in a continuous superfusion system. Inhibition of uptake was measured by determining inhibition of [3H]dopamine displacement by unlabeled dopamine ([1H]dopamine). The displacement of [3H]dopamine by 10(-7) M [1H]dopamine was temperature- and sodium-sensitive and calcium-independent. [1H]Dopamine was an order of magnitude more potent than serotonin or norepinephrine in displacing [3H]dopamine. The concentrations of reserpine required to inhibit [3H]dopamine uptake and [3H]dopamine displacement by [1H]dopamine were similar. Nomifensine, benztropine, PCP and amphetamine also inhibited the displacement of [3H]dopamine by [1H]dopamine at concentrations which have been shown previously to inhibit the uptake of [3H]dopamine, suggesting that the mechanism behind displacement and uptake are very similar. PCP, at 10(-7) to 10(-5) M, significantly inhibited [3H]dopamine displacement by 10(-7) M [1H]dopamine, PCP was less potent than nomifensine or benztropine in inhibiting [3H]-dopamine displacement by 10(-7) M [1H]dopamine, but was equipotent to amphetamine. Superfusion of the synaptosomes for 6 min with PCP, 10(-6)M, induced increases in the spontaneous release of dopamine. In this regard, PCP was less potent than amphetamine, reserpine, flupenthixol, or benztropine. Upon initial exposure of the synaptosomes to amphetamine at 10(-7) to 10(-5) M, a substantial calcium-dependent release of dopamine was induced. In contrast, PCP did not stimulate the early calcium-dependent release of dopamine. These results indicate that PCP is less potent than amphetamine at releasing dopamine and may affect dopamine metabolism in the striatum primarily by inhibiting the reuptake of this catecholamine.

Effect of apomorphine, a dopamine receptor agonist, on penile tumescence in normal subjects

Apomorphine HCl (Apo) (0.25, 0.5 or 0.75 mg sc), a dopamine (DA) receptor agonist, induced penile erections (PEs) (monitored by mercury strain gauges and continuous recording on paper strip charts) in 7 out of 9 normal subjects and placebo in 1 of these 9 (p less than 0.05). Apo-induced PEs recurred in each of the 6 subjects retested. Benztropine (2 mg iv) had no effect on Apo-induced penile tumescence (PT). These data suggest (a) DA mechanisms play a role in normal erectile function (b) DA-mediated PT is not modulated by cholinergic systems (c) evaluation of the erectile response to Apo may provide a simple ancillary test to the investigation of impotence and a way of identifying a subpopulation of impotent subjects with impaired DA function who may respond to long-acting DA agents (d) Apo-induced PT may provide a novel way of studying DA function in man.

Reversal of haloperidol induced deficits in self-stimulation by anti-Parkinsonian drugs

The effects of 0.1 mg/kg dose of haloperidol on self-stimulation were assessed in rats having 3 stable self-stimulation response rates of approximately 10, 50 and 100 responses per min generated by 3 levels of current intensity. Haloperidol reduced overall response rates at all current intensities and generated extinction-like response patterns. Concurrent anti-Parkinsonian (benztropine 4.0 mg/kg or diphenhydramine 4.0 mg/kg) eliminated the within session extinction performance pattern and at the highest current intensity benztropine restored self-stimulation to a non-drug performance level. These results suggested that haloperidol induced impairments in self-stimulation are secondary to a motoric rather than reinforcement dysfunction.

Frequency-dependent release of acetylcholine and dopamine from rabbit striatum: its modulation by dopaminergic receptors

The release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) was monitored from single slices of the rabbit striatum. In all cases, the evoked overflow of ACh showed a higher peak and was of shorter duration than that of 3H products. For ACh, the release per pulse showed a marked decline with increasing frequency of stimulation, whereas flat frequency-release curves were obtained for DA. At 0.1 and 1 Hz the evoked overflows of ACh were 15 and 7 times greater, respectively, than those of DA. Haloperidol (0.03 microM) and sulpiride (1 microM) produced large increases in the evoked overflow of DA and ACh at 3 and 10 Hz; little effect was observed at lower frequencies. These results indicate that the frequency-release curves for DA and ACh are different and that at high frequencies the slope of the curves is modified by activation of pre- and postsynaptic DA receptors. Apomorphine inhibited in a concentration-dependent fashion the evoked overflow of DA and ACh; greater inhibition was obtained at lower frequencies of stimulation. At 0.3 Hz the DA agonist was two times more potent in inhibiting DA than ACh overflow (IC50: 12.0 +/- 2.2 versus 22.0 +/- 2.8 nM; p less than 0.01). The greater sensitivity of pre- than postsynaptic sites to apomorphine was also seen at higher frequencies (3 Hz). Benztropine (1 microM) reduced the evoked overflow of ACh at 10 Hz, and enhanced that of 3H products at all rates of stimulation (0.3-10 Hz). These results suggest that the release of DA and ACh is regulated by dopaminergic receptors. They also indicate that the effects of DA agonists and antagonists and of uptake inhibitors on DA and ACh release are highly dependent on the frequency of stimulation used.

The effect of low doses of d-amphetamine on drug-induced hyperactivity in the mouse

The effect of low doses (7.5-250 micrograms/kg) of d-amphetamine was studied on benztropine- or gamma-butyrolactone-induced hyperactivity in the mouse. Amphetamine 30-125 micrograms/kg significantly attenuated the drug-induced hyperactivity. This effect was antagonised by both cinanserin and p-chlorophenylalanine. The results suggest that low doses of d-amphetamine release 5-hydroxytryptamine, which in turn has an inhibitory effect on locomotor activity. This effect was masked at higher doses of d-amphetamine due to a release of dopamine.

Effect of brain monoamines on the secretion of adrenocorticotrophic hormone

The role of brain monoamines (5-HT, NA and DA) in the secretion of adrenocorticotrophic hormone (ACTH) was studied in view of contradictory reports. Plasma corticosterone levels and the rate of synthesis of corticosterone in vitro by the adrenal gland were estimated in albino rats and have been taken as the index of ACTH activity. These estimations were done in unstressed and stressed, and in untreated and treated rats. Drugs were administered intracerebroventricularly to the rats to cause selective degeneration of tryptaminergic, noradrenergic or dopaminergic neurons. The results show that plasma corticosterone levels and the rate of synthesis of corticosterone were significantly decreased after selective degeneration of tryptaminergic neurons in unstressed rats. After selective degeneration of either tryptaminergic or noradrenergic neurons, the acute increase in the plasma corticosterone levels and rate of synthesis of corticosterone in vitro by adrenal glands in stressed rats were significantly inhibited. These results have been interpreted to suggest that the central tonic control on adrenal glands may be 5-HT mediated and that during stress ACTH secretion may be both 5-HT and NA mediated. DA does not seem to have significant role in the regulation of ACTH secretion.

A comparison of atropine, benztropine and diphenhydramine on the reversal of haloperidol induced suppression of self-stimulation

Acute haloperidol administration (0.25, 0.5 mg/kg) produced a severe reduction in locomotor activity and operant responding for intracranial self-stimulation in rats. If the rats were also given 10 mg/kg of either diphenhydramine or benztropine, this behavioral inhibition was substantially reversed. Atropine (10 mg/kg), however, did not significantly alter the haloperidol inhibition. In a second study, the rats were tested 30 and 120 minutes after the haloperidol treatment (0.25 mg/kg). Again, diphenhydramine and benztropine but not atropine substantially reversed the haloperidol suppression after 30 minutes. When retested after 120 minutes, however, atropine and benztropine but not diphenhydramine partially reversed the haloperidol suppression. Thus, the present study provides support that diphenhydramine can be effective in reversing haloperidol induced suppression of self-stimulation but for a shorter duration than benztropine.

Overeating after midbrain 6-hydroxydopamine: prevention by central injection of selective catecholamine reuptake blockers

Earlier research from this laboratory showed that 6-hydroxydopamine (6-OHDA) injected into the ventral midbrain of rats causes overeating and the gradual development of obesity in association with extensive depletion of forebrain norepinephrine. It was proposed that depletion of norepinephrine or epinephrine was the cause of the behavioral abnormality. This was questioned on the basis of experiments showing that 6-OHDA can cause non-selective lesions and also that non-selective neurotoxins can cause hyperphagia. To help resolve this question, the present experiments used 191 rats in 28 groups to test 3 doses of 6-OHDA and 3 different pretreatments with reuptake blockers, amphetamine, desmethylimipramine and benztropine. The result of midbrain 6-OHDA alone was hyperphagia that was highly correlated with the dose. Pretreatment with intraperitoneal desmethylimipramine (DMI) caused gastric distress, and systemic amphetamine caused delayed overeating, but these unwanted side effects were avoided by injecting the drugs directly into the midbrain 15 min before the 6-OHDA. DMI and amphetamine partially prevented catecholamine depletion in the forebrain and totally prevented subsequent hyperphagia and obesity. Benztropine prevented dopamine depletion, but had no effect on hyperphagia. We conclude that some of the neurons protected by local injection of amphetamine and DMI are probably norepinephrine or epinephrine fibers that ascend through the ventral midbrain and have as one of their functions the inhibition of food intake. These neurons could also play a role in the control of body weight.

Benztropine-induced prolongation of responses to vasodilator nerve stimulation in the canine paw pad

After blockade of noradrenergic transmission with guanethidine in anaesthetized dogs, stimulation of the peripheral end of the cut tibial nerve (1 Hz, 10 pulses) produces a vasodilator response which is localized to the circulation of the paw pads. The time course of the response is considerably prolonged after systemic administration of the dopamine-uptake blocking drug, benztropine (1 mg/kg). This effect is not due to inhibition of Uptake 1, as benztropine, unlike desmethylimipramine, does not prolong responses to noradrenergic vasoconstrictor nerve stimulation. The results support previous evidence suggesting that the dilator response to tibial nerve stimulation involves neural release of dopamine.

Comparison of the release of [3H]dopamine from isolated corpus striatum by amphetamine, fenfluramine and unlabelled dopamine

Amphetamine-induced release of previously accumulated [3H]dopamine ([3H]DA) was compared to the release of [3H]DA produced by unlabelled DA and fenfluramine. Like unlabelled DA, amphetamine was more potent than fenfluramine in releasing [3H]DA in all tissue preparations (untreated, pargyline-treated, and pargyline- and reserpine-treated corpus striatal slices). In tissue treated with both reserpine and pargyline, benztropine greatly reduced the efflux of [3H]DA produced by amphetamine and unlabelled DA but had only a slight effect on fenfluramine-induced release of [3H]DA. In the same tissue preparation, Q10 values for the release of [3H]DA produced by 3 X 10(-7) M amphetamine (1.8) and 3 X 10(-6) M unlabelled DA (1.7) were similar to that for the spontaneous release of [3H]DA (1.7). However, when the concentrations of amphetamine and unlabelled DA were increased to 10(-4) M, the Q10 values for the release of [3H]DA were diminished at the lower temperatures. These results suggest that amphetamine may release [3H]DA by two mechanisms: (1) by accelerated exchange diffusion due to its use of the DA uptake carrier to enter into neurons (this would predominate at low concentrations of amphetamine), and (2) by passive entrance into neurons and displacement of [3H]DA from binding sites (this would predominate at high concentrations of amphetamine).

Monoamine oxidase-catalyzed metabolism of 3,4-dihydroxyphenylethylamine in the dopaminergic synaptosomes from rat corpus striatum

This communication describes conditions under which the monoamine oxidase-catalyzed metabolism of 3,4-dihydroxyphenylethylamine can be assayed in dopaminergic synaptosomes from the rat striatum. In contrast to the activity of the isolated enzyme or that of free mitochondria, the synaptosomal reaction exhibits sigmoidal kinetics with respect to substrate concentration. This is consistent with a kinetic mechanism in which intrasynaptosomal substate partitions between reaction and saturable storage in synaptic vesicles. The reaction is inhibited at moderately decreased oxygen tension, where catecholamine uptake is unaffected. The specificity of this effect suggests that it reflects limited availability of oxygen as an enzyme substrate.