Dopamine turnover in the mediobasal hypothalamus in rat fetuses

In this study, the dopamine turnover in the mediobasal hypothalamus, the key compartment of the neuroendocrine regulation of reproduction, was evaluated in fetal male and female rats. High-performance liquid chromatography with electrochemical detection was used to measure 3,4-dihydroxyphenylalanine, dopamine and 3,4-dihydroxyphenylacetic acid in the mediobasal hypothalamus of fetuses on the 21st day of intrauterine development and in primary cell culture (cell extracts and culture medium) of the same brain region, explanted at the 17th fetal day and maintained for seven days. The same technique was applied to determine dopamine release from fetal neurons of the mediobasal hypothalamus in response to an excess of K+ in the perifusion system or in culture. L-3,4-Dihydroxyphenylalanine, dopamine and 3,4-dihydroxyphenylacetic acid were detected both ex vivo and in culture. The ratios of the concentrations of L-3,4-dihydroxyphenylalanine/dopamine and 3,4-dihydroxyphenylacetic acid/dopamine were significantly higher in vitro than ex vivo, showing a lower rate of dopamine production and a higher rate of its degradation in the experiments in vitro. Moreover, it has been demonstrated that an excess of K+, i.e. a membrane depolarization, resulted in a highly increased release of dopamine in the perifusion system and in culture. The dopaminergic activity in the developing mediobasal hypothalamus showed sexual dimorphism that was manifested in a greater concentration of 3,4-dihydroxyphenylalanine and dopamine, at least in cell extracts of cultures, as well as in a higher rate of dopamine release, both in the perifusion system and in culture in males compared to females. Thus, dopamine is synthesized and released in response to a membrane depolarization in the mediobasal hypothalamus of rats as early as the end of intrauterine development, suggesting its contribution to the inhibitory control of pituitary prolactin secretion.     

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.     

Formation of catecholaminergic structures in the mediobasal hypothalamus of the rat

Laboratory of Hormonal Regulation, N. K. Kol'tsov Institute of Developmental Biology, Academy of Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR A. P. Avtsyn.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 107, No. 4, pp. 490–493, April, 1989.     

Regulation of dopamine release by impulse flow and by autoreceptors as studied by in vivo voltammetry in the rat striatum

Extracellular dopamine concentration has been monitored in the striatum of pargyline treated, anaesthetized rats using differential normal pulse voltammetry. The catechol oxidation current recorded with electrochemically treated carbon fiber electrodes disappeared when the dopaminergic terminals were selectively destroyed by 6-hydroxydopamine. Calibration of the basal oxidation current revealed that the extracellular dopamine concentration was 26 nM. Brief and moderate electrical stimulation of the nigrostriatal pathway at the level of the medial forebrain bundle induced a large increase in the dopamine current. The observed elevation in the dopamine signal lasted as long as the stimulation. It varied with the frequency (0-25 Hz) of the pulses in an exponential manner. Stimulation pulses distributed in a bursted pattern were twice as potent as an equivalent number of pulses regularly spaced. High frequency stimulations (50 Hz) were also investigated in anaesthetized rats (without pargyline) with untreated carbon fiber electrodes; they induced a very large increase in the dopamine extracellular concentration (up to 8-15 microM). Interruption of the dopaminergic impulse flow either by an electrolytic lesion or by a low dose of apomorphine (0.05 mg/kg) caused an immediate decrease of the dopamine current. The time courses and amplitudes (-70%) of these effects were identical. Subsequent injection of haloperidol (0.5 mg/kg) reversed the apomorphine effect up to +360% of the control basal value. Administration of dopaminergic antagonists such as haloperidol (0.05 and 0.5 mg/kg) or metoclopramide (2 mg/kg) significantly increased the dopamine current up to 317, 340 and 215% of the respective control values. Nomifensine (4 mg/kg) produced a big increase (+417%) of the extracellular dopamine levels. The effect of electrical stimulation of the dopaminergic pathway was potentiated by drugs such as amphetamine (2 mg/kg), nomifensine (4 mg/kg) or haloperidol (0.05 and 0.5 mg/kg) but was not altered by apomorphine (0.05 mg/kg). The study by in vivo voltammetry of the variations in the striatal extracellular dopamine concentrations shows that the release of dopamine is under the influence of both the frequency of impulse flow and of dopaminergic striatal autoreceptors.     

The effect of unilateral cortical lesions on the circadian changes in rat striatal ascorbate and homovanillic acid levels measured in vivo using voltammetry

Linear sweep voltammograms were recorded in the striata of rats with a unilateral cortical lesion. The height of the ascorbate peak was 55% smaller on the lesioned side compared with the intact side, the homovanillic acid peak showed no significant change. The nocturnal increase in the release of striatal ascorbate was reduced by 80% on the lesioned side whilst the circadian variation in homovanillic acid was unaffected. These results support the hypothesis that there is a link between glutamate release and extracellular ascorbate concentrations in the brain.     

Feeding-induced increase in the extracellular concentration of histamine in rat hypothalamus as measured by in vivo microdialysis

The extracellular concentration of histamine (HA) in the hypothalamus of conscious and freely moving rats was measured by in vivo microdialysis and the effects of fasting and feeding on the HA concentration were examined. In non-fasted rats, the basal HA concentration was almost constant from 11.00 to 17.00 h on the day following implantation of the dialysis probe, the mean value being 11.1 pg/30 min. No significant change in the HA concentration was observed in rats deprived of food for 24 h. In 24-h fasted rats, feeding for 15 min produced a transient and significant increase in the HA concentration. These results suggest that histaminergic activity in the rat hypothalamus increases during feeding.     

Effects of neurotensin on dopamine release and metabolism in the rat striatum and nucleus accumbens: cross-validation using in vivo voltammetry and microdialysis

The effects of the neuropeptide neurotensin on dopamine release and metabolism in the posteromedial nucleus accumbens and anterior dorsomedial striatum of the anesthetized rat were investigated using in vivo chronoamperometry and intracerebral microdialysis techniques. A dose-dependent augmentation of dopamine efflux as evidenced by increases in the chronoamperometric signal was observed in the nucleus accumbens following intracerebroventricular injections of neurotensin. However, neurotensin failed to alter extracellular concentrations of dopamine in the striatum. The selective effects of neurotensin on mesolimbic dopamine neurons were confirmed using in vivo microdialysis. These results demonstrate that neurotensin can selectively enhance the release and metabolism of dopamine in neurons projecting from the ventral tegmental area to the nucleus accumbens.     

Measurement of stimulated dopamine release in the rat by in vivo voltammetry: the influence of stimulus duration on drug responses

High-speed cyclic voltammetry at carbon fibre microelectrodes was used to monitor stimulated striatal dopamine (DA) release in the anaesthetised rat. The effects of metoclopramide, chlorpromazine, benztropine and dexamphetamine on DA release after 1 s and 10 s stimulations were investigated. The antagonists enhanced DA release most clearly with 1 s stimuli. Benztropine had no effect on DA release with the 10 s stimulations, but elevated release after 1 s trains. Dexamphetamine reduced release with 10 s stimuli but enhanced release after 1 s trains. The results are discussed in terms of neuronal DA uptake and releasable DA pool size.     

Phencyclidine increases extracellular dopamine metabolites in rat medial frontal cortex as measured by in vivo dialysis

An acute intraperitoneal injection of phencyclidine (PCP) caused a tetrodotoxin-reversible increase in extracellular release of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the dialysates from the medial frontal cortex of the rat. Moreover, there was an increase in the tissue content of DOPAC and HVA with acceleration of dopamine (DA), but not noradrenaline, utilization in the cortical area after systemic administration of PCP. These results suggest that PCP facilitates DA metabolism in the medial frontal cortex by increasing impulse flow in the DA neurons projecting to the prefrontal region.     

Effect of naloxone on ethanol-induced membrane-bound enkephalin convertase activation in the rat mesencephalon and hypothalamus

All-Union Center for Drug Addiction, Ministry of Health of Russia, Moscow. Institute of Psychiatry and Neurology, Warsaw, Poland. (Presented by Academician of the Russian Academy of Medical Sciences P. Ashmarin.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 114, No. 8, pp. 174–175, August, 1992.     

The effect of dopamine on neurohypophysial hormone release in vivo and from the rat neural lobe and hypothalamus in vitro.

1. The rat hypothalamus (containing the supra-optic nuclei, paraventricular nuclei, median eminence and proximal pituitary stalk) has been incubated in vitro and shown to be capable of releasing the neurohypophysial hormones, oxytocin and arginine vasopressin, at a steady basal rate about one twentieth that of the rat neural lobe superfused in vitro. 2. The hypothalamus and neural lobe in vitro released both hormones in a similar arginine vasopressin/oxytocin ratio of about 1-2:1. However, when release was expressed relative to tissue hormone content, the hypothalamus was shown to release about three times as much arginine vasopressin and six times as much oxytocin as the neural lobe. 3. Dopamine in a concentration range of 10(-3)-10(-9)M caused graded increases in hormone release from the hypothalamus in vitro to a maximum fivefold increase over preceding basal levels. The demonstration that apomorphine also stimulated hormone release whereas noradrenaline was relatively ineffective suggested that a specific dopamine receptor was involved. A separate cholinergic component in the release process was indicated by the finding that acetylcholine stimulated release to a maximum fivefold increase in concentrations of 10(-3)-10(-9)M. 4. The fact that the isolated hypothalamus can be stimulated by dopamine and acetylcholine to release increased amount of oxytocin and arginine vasopressin raises the question of the origin and fate of the hormones released in this way. The possibility that they could be released into the hypophysial portal circulation from median eminence to affect the anterior lobe of the pituitary is discussed. 5. In similar doses, both dopamine and noradrenaline injected into the lateral cerebral ventricles of the brain of the anaesthetized, hydrated, lactating rat caused the release of arginine vasopressin and oxytocin. Apomorphine release both hormones but at a higher dose level and to less effect than the catecholamines. 6. The hormone release induced in vivo by dopamine could be prevented by the prior administration of haloperidol or phentolamine and these antagonists were equally effective in blocking the hormone release due to noradrenaline. The involvement of a specific dopamine receptor was more clearly implicated by the use of pimozide which completely inhibited the hormone release due to dopamine and apomorphine but not that due to noradrenaline. 7. It is suggested that the release of neurohypophysial hormones can be stimulated via a dopaminergic nervous pathway in addition to a cholinergic one. The possibility that the osmoreceptor mechanism for the release of antidiuretic hormone from the neural lobe of the pituitary may involve such a dopaminergic pathway is discussed.     

Tyramine-immunoreactive neuronal structures in the rat brain: abundance in the median eminence of the mediobasal hypothalamus

Immunoreactivity to p-tyramine, one of the natural trace amines, was studied in the rat brain by an anti-p-tyramine antibody. Immunoreactivity to this amine is very weak in the nigrostriatal dopaminergic neurons and terminals, and weak in the locus coeruleus noradrenergic ones. It was intensified in these structures after monoamine oxidase inhibition. On the other hand, this amine was highly concentrated in the median eminence of the mediobasal hypothalamus, in which its physiological function on prolactin release has been demonstrated.     

Functional in vivo interaction between growth hormone and dopamine systems are correlated to changes in striatal somatostatin levels as detected by voltammetry

The effects of growth hormone (GH) and somatomedin C (SmC), as well as those of apomorphine, dopamine (DA) agonist, or haloperidol (DA antagonist), upon the size of striatal voltammetric peaks 2 and 5 were investigated. Local intrastriatal injections of GH or SmC were followed by an increase in the height of both peak 2 (corresponding to the oxidation of extracellular dihydro-phenylacetic acid, DOPAC, a metabolite of DA) and peak 5 (which may represent the oxidation of striatal extracellular somatostatin, SRIF). Treatment with haloperidol also increased the size of the striatal catechol peak but was responsible for a reduction of the neuropeptidergic signal. By contrast, apomorphine determined a decrease in striatal peak 2 (DOPAC) while increasing the levels of peak 5 (SRIF). The data further support the chemical identification of peak 5 at +800 mV as related to the in vivo oxidation of SRIF; in addition they indicate the presence of a functional relationship between this neuropeptide and the GH and DA systems in the striatum of anaesthetised rats.     

Negative feedback control of serotonin release in vivo: comparison of 5-hydroxyindolacetic acid levels measured by voltammetry in conscious rats and by biochemical techniques

All evidence that serotonin release from central neurones is controlled by a negative feedback mechanism comes from in vitro studies. To study this problem in vivo we performed differential pulse voltammetry in conscious rats, in which carbon fibre electrodes had been implanted 2-15 weeks previously. The effects of monoamine oxidase inhibition (which decreases the amount of 5-hydroxyindoleacetic acid), as well as that of probenecid (which increases 5-hydroxyindoleacetic acid), suggests that 5-hydroxyindoleacetic acid rather than serotonin is measured. Blockade of the presynaptic serotoninergic autoreceptors by methiothepin, metergoline or quipazine led to an increase in differential current of the peak attributed to 5-hydroxyindoleacetic acid in hippocampus, hypothalamus and striatum. Stimulation of these receptors by m-chlorophenylpiperazine, MK-212 or LSD decreased the signal attributed to 5-hydroxyindoleacetic acid. A decrease in the signal was also seen with cinanserin. Stimulation of presynaptic alpha 2-adrenoreceptors by clonidine decreased the signal. Metergoline, quipazine and cinanserin showed biphasic effects, and no effect was observed with methysergide. In general, a reasonable agreement with the results of Baumann & Waldmeier obtained in vitro with electrically stimulated [3H]serotonin prelabelled cortex slices was achieved with differential pulse voltammetry. Only partial agreement with the results of voltammetry was obtained if 5-hydroxyindoleacetic acid was determined biochemically under comparable conditions. Qualitatively, the effects observed with methiothepin, m-chlorophenylpiperazine, clonidine and LSD were in good agreement with those measured with voltammetry as well as with the in vitro effects obtained in electrically stimulated cortex slices. No, or only partial correlation with the results obtained with voltammetry was found with MK 212, cinanserin, metergoline and quipazine. It is concluded that voltammetry preferentially measures extraneuronal 5-hydroxyindoleacetic acid rather than overall changes of this metabolite.     

Effect of clonidine on the release of serotonin from the rat hippocampus as measured by microdialysis.

The purpose of the present study is to clarify the effect of clonidine on the release of serotonin from the rat hippocampus in vivo. For this purpose, endogenous serotonin release was measured by brain microdialysis. Potassium-evoked serotonin release from the hippocampus of freely moving rats was significantly inhibited when clonidine (10(-5) M) was added to the perfusion solution, while the 5-hydroxyindoleacetic acid output remained unchanged. In catecholaminergically denervated rats, clonidine (10(-5) M) also inhibited the potassium-evoked serotonin release from the hippocampus and the 5-hydroxyindoleacetic acid output was unaffected by clonidine. These results suggest that the inhibitory effect of clonidine on serotonin release from the hippocampus might reflect the activation of alpha 2-adrenoceptors which are localized on the serotonergic nerve terminals.