Catecholamine biosynthesis in specific brain areas of the rat as determined by liquid chromatography and amperometric detection

Occupancy of dopamine D2/3 receptors in rat brain by endogenous dopamine measured with the agonist positron emission tomography radioligand [11C]MNPA

Estimates of dopamine D(2/3) receptor occupancy by endogenous dopamine using positron emission tomography (PET) in animals have varied almost threefold. This variability may have been caused by incomplete depletion of dopamine or by the use of antagonist radioligands, which appear less sensitive than agonist radioligands to changes in endogenous dopamine. PET scans were performed in rats with the agonist PET radioligand [(11)C]MNPA ([O-methyl-(11)C]2-methoxy-N-propylnorapomorphine). [(11)C]MNPA was injected as a bolus plus constant infusion to achieve steady-state concentration in the body and equilibrium receptor binding in the brain. Radioligand binding was compared at baseline and after treatment with reserpine plus alpha-methyl-para-tyrosine, which cause approximately 95% depletion of endogenous dopamine. Depletion of dopamine increased radioligand binding in striatum but had little effect in cerebellum. Striatal [(11)C]MNPA binding potential was 0.93 +/- 0.12 at baseline and increased to 1.99 +/- 0.25 after dopamine depletion. Occupancy of D(2/3) receptors by endogenous dopamine at baseline was calculated to be approximately 53%. Striatal binding was displaceable with raclopride, but not with BP 897 (a selective D(3) compound), thus confirming the D(2) receptor specificity of [(11)C]MNPA binding. Radioactivity extracted from rat brain contained only 8-10% radiometabolites and was insignificantly altered by administration of reserpine plus alpha-methyl-para-tyrosine. Hence, dopamine depletion did not increase the PET measurements via an effect on radiotracer metabolism. Our in vivo estimate of dopamine's occupancy of D(2/3) receptors at baseline is higher than that previously reported using antagonist radioligands and PET, but is similar to that reported using agonist radioligands and ex vivo measurements.

An immunochemical and biochemical study of catecholaminergic activity in dissociated hypothalamic cultures

Dopamine (DA)-containing cells of the medial basal hypothalamus (MBH) were dissociated and maintained in culture for up to 9 days. Cultures were evaluated both biochemically and immunochemically for DA activity. DA biosynthesis was determined using incorporation of [3H]tyrosine and was analyzed by HPLC with electrochemical detection. Immunochemical studies were performed to identify tyrosine hydroxylase (TH)-positive and neuron-specific enolase (NSE)-positive cells. Morphometric analyses determined the cell size, density, process length and the percent of neurons which were catecholaminergic. TH-positive neurons ranged from 6 to 8% of the total neuronal population when examined over days 3-9 of culture and the length of TH-positive neurites was significantly greater than that of NSE-positive cells. There was incorporation of [3H]tyrosine into DA as evidenced by the presence of [3H]DA in both the media and tissue and the inhibition of synthesis with alpha-methyl-p-tyrosine. There was a greater amount of labeled DA in the media than in the tissue at every time point examined. On the other hand, biosynthesis of DA in fresh brain tissue revealed approximately equal levels of DA in the media and tissue. These studies indicate that DA continues to be synthesized in dissociated cultures of MBH as evidenced by both the biochemical and immunochemical analyses and that there appears to be some alteration in the ability of these neurons to store the newly synthesized amine.

Mobilization of storage pool dopamine and late ipsilateral augmentation of striatal dopamine synthesis in the trained circling rat

Rats were infused intraventricularly with [3H]tyrosine over a 20-min period during various times while circling. 3,4-Dihydroxyphenylethylamine (dopamine) and dihydroxyphenylacetic acid (DOPAC) levels were measured using HPLC with electrochemical detection and fractions were collected for tritium monitoring. During the first 20 min of circling, the specific activity of dopamine was increased by 290% in striatum contralateral to the circling direction whereas DOPAC specific activity was increased 50% on the same side. This differential change in relative specific activity suggests that unlabeled storage pool dopamine was mobilized to DOPAC during circling. Synthesis of dopamine and DOPAC in contralateral striatum returned to baseline levels as turning slowed (50-70 min). When turning ceased, there was an increase in ipsilateral striatal dopamine synthesis during the 20-min period following circling. We hypothesize that this ipsilateral increase represents either a "stop" signal following circling or a release of inhibition of ipsilateral nigral neurons.

Correlation of rates of calcium entry and release of endogenous norepinephrine in rat brain region synaptosomes

Voltage-dependent 45Ca2+ uptake and endogenous norepinephrine (NE) release were measured simultaneously in synaptosomes isolated from rat hypothalamus, brainstem, and cerebellum at 1, 3, 5, 15, and 30 s. In synaptosomes depolarized by 125 mM KCl, 45Ca2+ uptake and NE release exhibited fast and slow components. Rates of NE release and 45Ca2+ uptake were fastest from 0 to 1 s. NE release and 45Ca2+ uptake rates from 1 to 5 s were less than 15% of 0-1 s rates. Both resting (5 mM KCl) and depolarization-induced (125 mM KCl) NE release paralleled 45Ca2+ uptake from 1 to 30 s. Voltage-dependent NE release was approximately 1% and 2% of total synaptosomal NE content at 1- and 30-s measurement intervals, respectively, and did not differ between the three brain regions studied. Calcium and potassium dependence studies showed that NE release was stimulated by increased potassium and that depolarization-induced NE release was dependent on the presence of external calcium. These results show that calcium-dependent NE release from synaptosomes is correlated with calcium entry. Both processes exhibit fast and slow temporal components.

Study of dopamine turnover by monitoring the decline of dopamine metabolites in rat CSF after alpha-methyl-p-tyrosine

CSF was continuously withdrawn from the third ventricle of anesthetized rats. CSF 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid concentrations were determined every 15 min by liquid chromatography coupled with electrochemical detection. Acute tyrosine hydroxylase inhibition [with alpha-methyl-p-tyrosine (alpha-MPT)] induced an exponential decline in levels of DOPAC and HVA in CSF. The decline in DOPAC and HVA concentrations was identical in CSF and forebrain but was much slower in the striatum, suggesting that CSF metabolites of 3,4-dihydroxyphenylethylamine (dopamine) reflect whole forebrain metabolites. The decay in CSF DOPAC and HVA levels after dopamine synthesis inhibition was also used as an in vivo index of forebrain dopamine turnover after various pharmacological treatments. Haloperidol pretreatment accelerated this decay, confirming the increase in brain dopamine turnover induced by neuroleptics. After reserpine pretreatment (15 h before), alpha-MPT produced a very sharp decay in levels of DOPAC and HVA. This result indicates that the residual dopamine that cannot be stored after reserpine treatment is very rapidly renewed and metabolized. Nomifensine strongly diminished the slope of DOPAC and HVA level decreases after alpha-MPT, a result which can be explained either by a slower dopamine turnover or by the involvement of storage dopamine pools. These results exemplify the use of monitoring the decay of dopamine metabolites after alpha-MPT administration in the study of the pharmacological action of drugs on the central nervous system of the rat.

Catecholamine biosynthesis and vasopressin and oxytocin secretion in the spontaneously hypertensive rat: an in vitro study of localized brain regions

The in vitro synthesis of catecholamines and the secretion of vasopressin (AVP) and oxytocin (OT) was measured in localized regions of the hypothalamo-neurohypophyseal system in the spontaneously hypertensive rat (SHR). The posterior pituitary (PP), median eminence (ME) and supraoptic (SON) and paraventricular (PVN) nuclear regions were incubated in vitro in media containing 3H-tyrosine. Media and tissue levels of AVP and OT were measured as well as norepinephrine and dopamine content and biosynthesis. There were no differences in peptide release in either the PP, ME or SON. However, there was a marked increase in peptide release from the PVN of the SHR. Media AVP levels were 0.3 pg/ml/micrograms protein in the WKY as compared to 2.1 pg/ml/micrograms protein in the SHR. OT release was increased 2 fold, from 0.85 to 1.7 pg/ml/micrograms protein. PVN content of both AVP and OT was significantly lower in the SHR. ME and SON peptide levels were not changed, while neurohypophyseal AVP levels were increased in the SHR. With regard to the catecholamines appreciable norepinephrine synthesis was measured in the PVN and SON while there was little 3H-norepinephrine in the ME or PP. In the hypertensive rat, there was an increase in norepinephrine synthesis in the PVN with no change in the SON. These results provide further support for fundamental changes in the catecholaminergic and peptidergic systems of the hypothalamo-neurohypophyseal axis of the SHR.

An in vivo steady-state method for the determination of catecholamine biosynthesis in the rat brain using high-performance liquid chromatography with electrochemical detection

A technique is described for the measurement of steady-state catecholamine (CA) synthesis in the rat brain in vivo, using [3H]tyrosine incorporation with high-performance liquid chromatography (HPLC) and electrochemical detection. Adult male rats chronically implanted with lateral intracerebroventricular (i.c.v.) cannulas, were injected i.c.v. with [3H]tyrosine. CA and tyrosine content and specific activity were measured in mediobasal hypothalamus, anterior hypothalamus and striatum. A time-dependent increase in CA synthesis occurred in all tissues over 20 min post-i.c.v. injection. The technique described may prove to be useful in the assessment of central neurotransmitter turnover in various physiological and pharmacological settings.

Hypothalamic catecholamine biosynthesis and pituitary gonadotropin secretion in vitro: effect of hyperprolactinemia

The effect of hyperprolactinemia on central catecholamine biosynthesis and anterior pituitary hormone release was studied using an in vitro methodology. The incorporation of [3H]tyrosine into hypothalamic and neurohypophyseal catecholamines was determined using a new method which combines high performance liquid chromatography (HPLC) with amperometric detection (LCEC). Elevated plasma prolactin levels, induced by pituitary transplants, resulted in increased in vitro biosynthesis of medial basal hypothalamic (MBH) dopamine (DA), but not norepinephrine (NE). Neurohypophyseal DA biosynthesis (including the intermediate lobe) was not affected. Plasma LH levels were depressed by hyperprolactinemia although the content of hypothalamic luteinizing hormone-releasing hormone (LHRH) was not changed. In parallel studies, the anterior pituitaries from these animals were incubated in vitro using a paired-half technique and LH and PRL release measured. While the basal release of prolactin was not altered by hyperprolactinemia, LH release was significantly decreased. Gonadotroph responsiveness to LHRH was significantly increased, while the inhibition of prolactin by dopamine was not altered. There was a decrease in pituitary prolactin content with normal LH levels. These experiments confirm several in vivo reports which show that hypothalamic dopaminergic but not noradrenergic activity is increased by prolactin. This action is specifically localized in the tuberoinfundibular dopaminergic neurons. Furthermore, these experiments suggest that these central changes result in alterations in both gonadotroph and mammotroph function.

Hypothalamic catecholamine biosynthesis and neuropeptides

In conclusion, both steady state and non-steady state methods for investigating the turnover of central catecholamines can provide valuable information regarding the central control of neuroendocrine function. The system described here, utilizing HPLC with electrochemical detection, offers a relatively easy and reliable method for determining steady state catecholamine biosynthesis in the hypothalamus and other brain areas.

Determination of dopamine and its acidic metabolites in brain tissue by HPLC with electrochemical detection in a single run after minimal sample pretreatment

A method, based on reverse-phase liquid-liquid chromatography, has been developed for the determination, in a single run, of dopamine (DA) and its acidic metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), combined with electrochemical detection (ECD). If applied to brain tissue, sample pretreatment can be reduced to centrifugation, filtration and adjustment of pH and perchlorate concentration prior to introduction into the liquid chromatograph. The relation between the perchlorate (counterion) concentration of the mobile phase and the retention (k') of the amines is linear, as is the relation between the H+ concentration of the mobile phase and the retention of the acidic metabolites. This flexible phase system, combined with a simple and therefore reproducible sample pretreatment, warrants a high throughput of samples. The procedure offers good possibilities for routine analysis of catecholamines and their acidic metabolites in the picogram range. Some typical examples of the behaviour of this phase system and the electrochemical detector are presented and discussed.

Neurohypophyseal dopamine biosynthesis in the spontaneously hypertensive rat

A study was performed to investigate the neurohypophyseal dopaminergic axis in terms of its biosynthetic activity and possible changes associated with spontaneous hypertension (SHR). An in vitro system was used in which isolated neuro-intermediate lobes were incubated with the catecholamine precursor, 3H-tyrosine. Dopamine (DA) content and 3H-DA were monitored using electrochemical detection coupled with high pressure liquid chromatographic separation. A time course study showed that there was significant incorporation of 3H-tyrosine into 3H-DA. In the SHR, both neurohypophyseal DA content and biosynthetic activity were reduced. Tissue levels of 3H-DA decreased from 651 to 297 dpm/posterior pituitary. A test of the effect of dehydration on neurohypophyseal dopaminergic activity revealed that water deprivation (48 hrs) caused an increase in DA biosynthesis in the hypertensive, but not the normotensive animal. This may have been due to a greater stimulation of the neurohypophyseal axis in the SHR since these animals showed significantly higher plasma vasopressin levels and hematocrits in response to dehydration. These results demonstrate that the neurohypophysis contains an active dopaminergic system which is altered in genetic hypertension.