Disappearance Of Dopamine Research Articles

Differential enhancement of ERK, PKA and Ca2+ signaling in direct and indirect striatal neurons of Parkinsonian mice

Parkinson's disease (PD) is characterized by severe locomotor deficits due to the disappearance of dopamine (DA) from the dorsal striatum. The development of PD symptoms and treatment-related complications such as dyskinesia have been proposed to result from complex alterations in intracellular signaling in both direct and indirect pathway striatal projection neurons (dSPNs and iSPNs, respectively) following loss of DA afferents. To identify cell-specific and dynamical modifications of signaling pathways associated with PD, we used a hemiparkinsonian mouse model with 6-hydroxydopamine (6-OHDA) lesion combined with two-photon fluorescence biosensors imaging in adult corticostriatal slices. After DA lesion, extracellular signal-regulated kinase (ERK) activation was increased in response to DA D1 receptor (D1R) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) stimulation. The cAMP-dependent protein kinase (PKA) pathway contributing to ERK activation displayed supersensitive responses to D1R stimulation after 6-OHDA lesion. This cAMP/PKA supersensitivity was specific of D1R-responding SPNs and resulted from Gαolf upregulation and deficient phosphodiesterase activity. In lesioned striatum, the number of D1R-SPNs with spontaneous Ca2+ transients augmented while Ca2+ response to AMPA receptor stimulation specifically increased in iSPNs. Our work reveals distinct cell type-specific signaling alterations in the striatum after DA denervation. It suggests that over-activation of ERK pathway, observed in PD striatum, known to contribute to dyskinesia, may be linked to the combined dysregulation of DA and glutamate signaling pathways in the two populations of SPNs. These findings bring new insights into the implication of these respective neuronal populations in PD motor symptoms and the occurrence of PD treatment complications.

Terminal effects of ethanol on dopamine dynamics in rat nucleus accumbens: an in vitro voltammetric study.

To assess the direct effects of acute ethanol on dopamine (DA) terminals, evoked DA release and uptake were measured in rat nucleus accumbens slices using fast-scan cyclic voltammetry. Low and moderate concentrations of ethanol (20, 45 and 100 mM) did not alter evoked DA release, while high concentrations (150 and 200 mM) significantly decreased DA release (18 and 36%, respectively) in a calcium-dependent manner. No significant difference was found between the rate of DA disappearance measured before and after the drug. These data indicate that uptake of DA through the dopamine transporter is unaffected by ethanol, even at high concentrations. Therefore, low to moderate concentrations of ethanol have no effect on DA dynamics at the level of the nerve terminal in the nucleus accumbens. This is consistent with the hypothesis that cell body regions of DA neurons are the primary target for the stimulating and reinforcing effects of ethanol. High concentrations of ethanol can locally depress DA release, and this may correlate with the sedative actions of the drug.

Characterization of the phencyclidine-induced increase in prefrontal cortical dopamine metabolism in the rat.

1. We have investigated the effects of a schizophrenomimetic drug phencyclidine (PCP) and N-methyl-D-aspartate (NMDA)-related agents alone or in combination on dopamine metabolism in the medial prefrontal cortex and striatum of the rats by measuring the tissue concentrations of dopamine and its metabolite, 3,4-dihyroxyphenylacetic acid (DOPAC), and the rate of dopamine disappearance (dopamine utilization) after its synthesis inhibition. 2. Systemic injection of PCP and selective, non-competitive, NMDA antagonists caused an increase of both tissue concentrations of DOPAC and dopamine utilization in the prefrontal cortex but not in the striatum. The PCP-induced augmentation of cortical dopamine metabolism was not influenced by selective lesion of ascending noradrenergic neurones. 3. Intra-prefrontal cortical infusion of PCP or selective competitive or non-competitive antagonists of the NMDA receptor mimicked the ability of systemic PCP injection to enhance DOPAC levels and dopamine utilization in the prefrontal cortex. However, an NMDA antagonist injected into the cell body area of the mesocortical dopaminergic neurones failed to affect dopamine metabolism in the prefrontal cortex. 4. The increasing effects of PCP and selective NMDA antagonists on cortical dopamine utilization were not additive, although a dopamine receptor antagonist, haloperidol, still accelerated the disappearance of dopamine, even in the presence of PCP. 5. Intra-cortical or intra-ventricular infusion of NMDA or D-alanine but not L-alanine, attenuated the ability of systemic PCP administration to facilitate prefrontal dopamine utilization. 6. These data suggest that PCP might activate prefrontal cortical dopaminergic neurones, at least in part, by blocking the NMDA receptor in the prefrontal cortex which participates in a tonic inhibitory control of the mesoprefrontal dopaminergic projections.

Fast sodium channel dependency of the somatodendritic release of dopamine in the rat's brain

Somatodendritic release of dopamine (DA) was studied by microdialysis. The basal release of endogenous extracellular DA in the rat's substantia nigra was 1.0 ± 0.14 fmol/min (mean ± S.E.M.; n = 6). Compared with the basal extracellular level of DA found in the striatum (12.9 ± 0.94; n = 12), the former value was about 13 times smaller than the latter value. The addition to the Ringer solution of the fast sodium channel inhibitor, tetrodotoxin (TTX), at a concentration of 1 μM, produced the total disappearance of DA in the substantia nigra. When TTX was removed from the Ringer solution, the DA extracellular level came back slowly to the control value. This result suggests that the somatodendritic basal endogenous DA release measured by microdialysis is dependent on the sodium channel conductance.

Effects of ( d-amphetamine on dopaminergic neurotransmission; a comparison between the substantia nigra and the striatum

The effects of ( d-amphetamine (d-AMP) on dopaminergic neurotransmission in the cell body/dendritic region of the nigrostriatal pathway, the substantia nigra, have been investigated and compared to the effects obtained in the terminal region of the pathway, the striatum. The rate of synthesis of dopamine (DA) was quantified as accumulation of 3,4-dihydroxyphenylalanine (DOPA), after inhibition of aromatic l-amino acid decarboxylase with 3-hydroxybenzyl-hydrazine (NSD 1015). As measures of the metabolism of DA the concentrations of the metabolites of DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT) were determined. As indices of release of DA, the accumulation of 3-methoxytyramine (3-MT), after inhibition of monoamine oxidase with pargyline and the disappearance of DA, after inhibition of its synthesis with α-methyl- p-tyrosine were assessed. ( d-Amphetamine insignificantly increased the concentration of DA in the striatum but profoundly decreased it in the substantia nigra (to 60% of controls). Both in the striatum and in the substantia nigra treatment with d-AMP induced clearcut decreases of the concentrations of DOPAC. Also the concentration of HVA was profoundly decreased in the striatum but only marginal effects on HVA were observed in the substantia nigra. In both structures of the brain, d-AMP increased the concentration of 3-MT. Depending on the dose, d-AMP increased or had no effect on the accumulation of DOPA in the striatum but consistently decreased it in the substantia nigra. In the striatum, d-AMP increased the pargylineinduced accumulation of 3-MT, without affecting the concentration of DA. However, in the substantia nigra the concentration of DA was profoundly decreased (to 50% of controls) in combination with unaltered accumulation of 3-MT, unless the rats were pretreated with haloperidol. If so, the effects of d-AMP on the concentration of DA and accumulation of 3-MT were the same in the substantia nigra and in the striatum. The results indicate that d-AMP depleted stores of DA in the substantia nigra, due to its releasing action, in combination with its decreasing effect on the rate of synthesis of DA. The decrease in rate of synthesis of DA is suggested to be due to the d-AMP-induced decrease in the firing rate of dopaminergic neurones.

Changes in striatal dopamine neurohistochemistry and biochemistry after incomplete transient cerebral ischemia in the rat

To evaluate the development of striatal ischemic cell damage in relation to alterations in dopamine (DA) transmission, one year old male Wistar rats underwent a 15 min incomplete cerebral ischemia (ICI) induced by occlusion of the common carotid arteries and by hypovolemic hypotension. The animals were divided into the following experimental groups: sham operated rats, rats with ICI without reperfusion, and rats with ICI followed by 60 min, 24 h, 72 h and 144 h of recirculation. The ischemia induced striatal lesions were investigated in serial coronal brain sections, stained with cresylviolet or immunostained for dopamine and cAMP regulated phosphoprotein (DARPP-32), for tyrosine hydroxylase (TH) and for glial fibrillary acidic protein (GFAP) immunoreactivities (IR). Measurements of striatal dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels were made on analogous experimental groups using HPLC methods. Signs of degeneration in small to medium sized neurons were already seen after 60 min of postischemic reperfusion together with slight decreases of DARPP-32 IR and increases of GFAP IR. The damage continued to increase up to 144 h, and after 24 h of recirculation there were clearly defined areas of reduced DARPP-32 IR, overlapping with increased TH IR and increased GFAP IR. The levels of DA, DOPAC and HVA increased sharply after 60 min (151%, 462% and 201%, respectively) remained high after 24 h and normalized after 72 h of recirculation. The DA metabolism was high after 60 min and had already normalized after 24 h of recirculation. The increased DA metabolism in striatal nerve terminals in response to ischemic injury may reflect an early degenerative change in the DA terminals. The long-lasting increase in TH IR may to some extent represent an adaptive change in response to the disappearance of DA receptor-containing nerve cells. Based on the present findings it is possible that an increased D1 transmission in neostriatum immediately following the ischemic injury may contribute to striatal nerve cell degeneration in which an enhancement of NMDA receptor transduction may be implicated.

The effects of GBR 12909, a dopamine re-uptake inhibitor, on monoaminergic neurotransmission in rat striatum, limbic forebrain, cortical hemispheres and substantia nigra.

In order to investigate the physiological importance of the membrane pump in eliminating released dopamine (DA) we have studied the effects of the putative selective dopamine re-uptake inhibitor, GBR 12909, on synthesis and metabolism of monoamines in the rat striatum, limbic forebrain, cortical hemispheres and substantia nigra (SN). The effects of the drug on the firing rate of catecholamine containing neurons in the SN and locus coeruleus (LC) were also investigated. For comparison we have investigated the effects of desipramine and maprotiline. As a measure of the synthesis of noradrenaline (NA), DA and 5-hydroxytryptamine (5-HT) we determined the 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) accumulation after inhibition of aromatic L-amino acid decarboxylase by 3-hydroxy-benzylhydrazine (NSD 1015). As indirect measurements of DA and NA release in vivo, we have assessed pargyline-induced 3-methoxytyramine (3-MT) and normetanephrine (NM) accumulation and disappearance rates of DA and NA after inhibition of their synthesis by alpha-methyl-p-tyrosine (alpha-MT). Administration of GBR 12909 (2.5, 5, 10, 20 or 40 mg/kg) decreased the NSD 1015-induced DOPA accumulation in the striatum and in the limbic forebrain. In contrast, only minor effects of the drug were seen on the DOPA accumulation in the cortical hemisphere and on the cerebral 5-HTP accumulation. GBR 12909 increased the 3-MT accumulation in the striatum, limbic forebrain and the cortical hemispheres, an effect that was even more pronounced in haloperidol-pretreated animals. However, GBR 12909 did not alter the 3-MT accumulation in the SN either when given alone or when given to haloperidol-pretreated rats. In haloperidol-pretreated rats GBR 12909 markedly enhanced the DA disappearance in the striatum and in the limbic forebrain, but not in the SN. Furthermore, GBR 12909 did not significantly affect the firing rate of dopaminergic neurons in the SN or that of noradrenergic neurons in the LC. Taken together, our results support the notion that GBR 12909 is a specific DA uptake inhibitor without a transmitter releasing action. In addition, our findings indicate that DA re-uptake is of physiological importance in the elimination of DA from the synaptic cleft in the striatum, limbic forebrain and cortical hemispheres, but not in the SN. Furthermore, a large part of the DA taken up by the dopaminergic terminals in the striatum and in the limbic forebrain seems to be re-incorporated into the storage vesicles.

Effect of the NMDA receptor antagonist, MK-801, on locomotor activity and on the metabolism of dopamine in various brain areas of mice

Various doses (0.1–0.5 mg/kg i.p.) of the N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, produced a dose-dependent increase in well-coordinated locomotor activity of NMRI mice. Higher doses (> 0.5 mg/kg) produced a typical motor syndrome characterized by head weaving, body rolling, ataxia and salivation. MK-801, 0.2 mg/kg i.p., a dose which produced marked locomotor stimulation, increased the rate of disappearance of dopamine in the striatum and in the limbic forebrain of the animals, whereas the rate of disappearance of noradrenaline remained unchanged in the limbic forebrain and in the hippocampus. MK-801 increased the rate of tyrostne hydroxylation (measured as the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after inhibition of DOPA decarboxylase) in the striatum with no change in DOPA formation in the limbic forebrain. The levels of 3,4-dihydroxyphenylacetic acid (DOPAC) remained unchanged both in the striatum and in the limbic forebrain following the administration of MK-801. It is concluded that MK-801 may facilitate the activation of dopaminergic mechanisms through an indirect (perhaps by reducing glutamatergic activity) rather than a direct effect on dopamine neurons.

Oxytocin attenuates the cocaine-induced exploratory hyperactivity in mice

The effects of pimozide, a dopamine-receptor blocker and oxytocin, a neurohypophyseal neuropeptide were investigated in mice on the cocaine-induced exploratory hyperactivity. The action of oxytocin on changes of dopaminergic neurotransmission induced by cocaine was also measured. Cocaine-induced exploratory hyperactivity could be blocked by pimozide (1 mg kg-1, s.c.). Oxytocin (0.05-1.0 micrograms) inhibited the cocaine-induced hyperactivity in an U-shaped dose-response manner. In the nucleus accumbens, oxytocin antagonized the increased dopamine disappearance, elicited by cocaine, but not in the nucleus caudatus. The data suggest that oxytocin may influence the behavioural effect of cocaine by modulating dopaminergic neurotransmission in mesolimbic dopaminergic terminal region of the brain.

Characterization of the in vivo release of dopamine as recorded by different types of intracerebral microdialysis probes

Microdialysis of dopamine was performed in the striatum with four different microdialysis cannulas: a trans-cerebral probe, a U-shaped probe, a I-shaped probe and a commercially available I-shaped probe (Carnegie). The commercial cannula was studied with as well as without a guide cannula. The effect of infusion of tetrodotoxin (TTX) or calcium free-Ringer solution on the dialysate content of dopamine released was determined 2 h after implantation (day 1) as well as 24 h after implantation (day 2). Two hours after implantation, all cannulas displayed a certain amount of TTX-independent overflow of dopamine. The best results were obtained with the I-shaped cannula: already 2 h after implantation of the probe, about 85% of the release of dopamine was TTX-dependent. In case of the trans-cerebral cannula and the Carnegie cannula (implanted without guide in anesthetized rats), 70-75% of the output of dopamine was TTX-dependent. When the Carnegie cannula was implanted with a guide cannula, only 45% of the dopamine output was TTX-dependent. The responses to calcium-free perfusion were much more variable. Dopamine output in I-shaped and the U-shaped probes was virtually insensitive to calcium-free perfusion in acute experiments. Twenty-four hours after implantation of the probes, the calcium- and TTX-dependency was much more pronounced. All types of cannulas studied now sampled dopamine that was completely TTX-dependent. Calcium-free perfusion caused a reproducible disappearance of dopamine from the dialysates to 30% of controls, in all cannulas studied. When the removable Carnegie cannula was re-implanted 24 h after its first implantation, 91% of the dopamine overflow was TTX-sensitive. These results confirm earlier conclusions that microdialysis experiments should be carried out at least 24 h after implantation of the probe.

A kinetic study of the rate of formation of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain of the rat: Implications for the origin of dopac

The rates of disappearance of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain of rats treated with the tyrosine hydroxylase inhibitor α-methyl- p-tyrosine were measured. Levels of dopamine, DOPAC and HVA in tissue were quantified in the striatum, nucleus accumbens and frontal cortex of the rat by means of high pressure liquid chromatography with electrochemical detection. Rats were sacrificed 1, 2 or 3 hr after the administration of α-methyl- p-tyrosine (200 mg/kg, i.p.). Levels of dopamine, DOPAC and HVA in tissue were logarithmically transformed, plotted against the duration of inhibition of tyrosine hydroxylase and the rate constant of the decline of dopamine and of its metabolites calculated. The rate constant of decline of DOPAC in each area of brain studied was found to be greater than that for dopamine. The rate constant for the decline of dopamine was found to be greater in areas of the brain presenting the largest DOPAC/dopamine tissue ratios (frontal cortex > nucleus accumbens > striatum). The present results suggest that a substantial amount of DOPAC in brain derives from a newly formed pool of dopamine.

Comparison of the Effects of the Novel Antipsychotic Agent Remoxipride on Dopamine and Noradrenaline Turnover in the Rat Brain

The ex vivo effects of the antipsychotic dopamine receptor antagonist remoxipride on dopamine and noradrenaline turnover in the rat brain have been compared. Remoxipride (5-125 mumol/kg intraperitoneally) produced large increases in striatal DOPAC and HVA concentrations with only maginal effects on striatal, hippocampal and hypothalamic MHPG concentrations. Both remoxipride (5.6 mumol/kg intraperitoneally) and haloperidol (0.23 mumol/kg intraperitoneally) increased the rate of striatal dopamine disappearance following inhibition of tyrosine hydroxylase by H 44/68 without corresponding effect on hypothalamic and frontal cortical noradrenaline disappearance. It is concluded that at doses of remoxipride producing compensatory increases in dopamine turnover, there is little or no effect on noradrenaline turnover, confirming the in vitro dopamine:noradrenaline selectivity of this compound.

Synthesis and Release of Dopamine in Rat Brain: Comparison Between Substantia Nigra Pars Compacta, Pars Reticulata, and Striatum

Dopamine (DA) is synthesized and released not only from the terminals of the nigrostriatal dopaminergic neuronal pathway, but also from the dendrites in the substantia nigra. We have investigated the regulation of the DA turnover, the DA synthesis rate, and the DA release in the substantia nigra pars compacts (SNpc) and pars reticulata (SNpr) in vivo. As a measure of DA turnover, we have assessed the concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid. As a measure of the DA synthesis rate, we have determined the 3,4-dihydroxyphenylalanine accumulation after inhibition of aromatic L-amino acid decarboxylase by 3-hydroxybenzylhydrazine. As a measure of DA release, we have investigated the disappearance rate of DA after inhibition of its synthesis by alpha-methyl-p-tyrosine and the 3-methoxytyramine accumulation following monoamine oxidase inhibition by pargyline. Both the DA turnover and the DA synthesis rate increased following treatment with the DA receptor antagonist haloperidol and decreased following treatment with the DA receptor agonist apomorphine in the SNpc and in the SNpr, but the effects of the drugs were less pronounced than in the striatum. gamma-Butyrolactone treatment, which suppresses the firing of the dopaminergic neurons, increased the DA synthesis rate in the striatum (165%), but had no such effect in the SNpc or SNpr. Haloperidol, apomorphine, and gamma-butyrolactone increased, decreased, and abolished, respectively, the DA release in the striatum, but the drugs had no or only slight effects on the alpha-methyl-p-tyrosine-induced DA disappearance and on the pargyline-induced 3-methoxytyramine accumulation in the SNpc or SNpr. Taken together, these results indicate that the DA synthesis rate, but not the DA release, are influenced by DA receptor activity and neuronal firing in the SNpc and SNpr. This is in contrast to the situation in the striatum, where both the DA synthesis rate and the DA release are under such control.

Real-time characterization of dopamine overflow and uptake in the rat striatum

The rate of overflow and disappearance of dopamine from the extracellular fluid of the rat striatum has been measured during neuronal stimulation. Overflow of dopamine was induced by electrical stimulation of the medial forebrain bundle with biphasic pulse trains. The instantaneous concentration of dopamine was measured with a Nafion-coated, carbon fiber microelectrode implanted in the brain. The measurement technique, fast-scan cyclic voltammetry, samples the concentration of dopamine in < 10 ms at 100 ms intervals. Identification of dopamine is made with cyclic voltammetry. Stimulated overflow was measured as a function of electrode position, stimulation duration, stimulation frequency, and after administration of l-DOPA and nomifensine. The observed concentration during a 2-s, 60-Hz stimulation was found to alter with position of the carbon fiber electrode. For stimuli of 3 s or less the amount of overflow was found to be a linear function of stimulus duration at a fixed electrode position. The observed overflow was found to be steady-state at a frequency of 30 Hz. suggesting a balance between uptake and synaptic overflow under these conditions. The experimental data was found to be successfully modelled when the balance of uptake and stimulated overflow was considered. It was assumed that each stimulus pulse releases a constant amount of dopamine (125 nM), and that uptake follows a Michaelis Menten model for a single uptake site with K m = 200nM and V max= 5μM/s. The increase in stimulated overflow observed after l-DOPA (250 mg/kg) could be modelled by a 1.6-fold increase in the amount of dopamine release with no alteration of the uptake parameters. The increase in stimulated overflow after nomifensine (2 or 20 mg/kg), a competititve inhibitor of uptake, could be modelled by an increase in K m In addition, the fit of the modelled data to the experimental data was improved when diffusion from the release and uptake sites was considered.

Microinjection of anti-vasopressin serum into limbic structures of the rat brain: effects on passive avoidance responding and on local catecholamine utilization

Rats which had received bilateral microinjections of 1:50 diluted anti-vasopressin serum into the dorsal or ventral hippocampus, immediately after the learning trial of a one-trial passive avoidance test, showed a reduction in avoidance latency scores during subsequent retention tests 24 and 48 h later. Postlearning microinjection of anti-vasopressin serum into either the dorsalateral septum or the caudate nucleus was without effect on the retention of passive avoidance behavior. Microinjection of anti-vasopressin serum 1 h before the 24-h retention session into either the dorsal hippocampus, the ventral hippocampus or the dorsolateral septum attenuated avoidance responding during both the 24-h and 48-h retention sessions, whereas preretention microinjection of the serum into the caudate nucleus was not effective. Intracerebroventricular administration of the anti-vasopressin serum in amounts similar to those used in the microinjection experiments did not affect retention scores when given either immediately after the learning trial or before the first retention session. One week after the behavioral experiments, a repeated microinjection of anti-vasopressin serum decreased the local α-methyl-p-tyrosine methylester (α-MPT)-induced disappearance of noradrenaline in the ventral hippocampus and the dorsal hippocampus respectively. Microinjection of the antiserum in the dorsolateral septum enhanced noradrenaline disappearance in this brain region. No effect was found on α-MPT-induced dopamine disappearance in the caudate nucleus following local microinjection of anti-vasopressin serum. These data show that endogenous vasopressin in both the dorsal and the ventral hippocampus is functionally involved in consolidation processes as well as in retrieval processes related to passive avoidance behavior, while that in the dorsolateral septum seems to be involved in retrieval processes only. They also show that noradrenergic mechanisms in these 3 brain regions respond to the local reduction of the amount of bioavailable vasopressin in a direction opposite to that observed after local microinjection of vasopressin, which suggests that vasopressin might act by modulating noradrenergic neurotransmission.

Turnover of dopamine and dopamine metabolites in rat brain: comparison between striatum and substantia nigra.

Measurements of the turnover of dopamine (DA) and DA metabolites have been performed in the striatum and substantia nigra (SN) of the rat. Turnover rates of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid have been assessed from the disappearance rates after blocking their formation by inhibition of monoamine oxidase by pargyline and of catechol-O-methyltransferase by tropolone. DA turnover has been measured as 3-methoxytyramine (3-MT) plus DA accumulation rate after MAO inhibition by pargyline and as accumulation rate of 3,4-dihydroxyphenylalanine (DOPA) after inhibition of aromatic amino acid decarboxylase by NSD 1015 or NSD 1034. These measures of DA turnover have been compared with alpha-methyl-p-tyrosine (alpha-MT)-induced DA disappearance rate. In SN all the different measures of DA turnover are in the same range (55-62 nmol/g protein/h) whereas in striatum DOPA accumulation rate after NSD 1015 and alpha-MT-induced DA disappearance rate (16-23 nmol/g/h) are much lower than DOPAC disappearance rate after pargyline, 3-MT plus DA accumulation rate after pargyline, and DOPA accumulation rate after NSD 1034 (39-46 nmol/g/h). The data confirm our previous findings indicating that the fractional turnover rate of DA is more rapid in SN than in striatum and that O-methylation of DA is relatively more important in SN. In striatum at least two pools of DA with different turnover rates appear to exist, whereas in SN, DA behaves as if located in a single compartment.

Brain monoamines and sexual behavior in Japanese quail: Effects of castration and steroid replacement therapy

In the male Japanese quail, testosterone is required for the activation of sexual behavior. This steroid dependent process may rely heavily on mediation via monoaminergic neurons. These experiments were conducted to study the relationship between reproductive state (hormonal and behavioral components) and levels of monoamines in selected areas of the brain in Japanese quail. In Experiment 1, monoamine levels in a number of brain areas were compared in castrates, testosterone-implanted castrates, and intact males. Monoamine levels were comparable to those previously measured in Japanese quail, and there were no significant differences due to treatment. Plasma luteinizing hormone (LH) levels and recovery of cloacal gland area in implanted castrates confirmed the afficacy of treatments. In Experiment 2, the disappearance of dopamine (DA) and norepinephrine (NE) following administration of a-methyl-para-tyrosine (aMPT) was used as an indicator of turnover rate. Male and female quail were gonadectomized at 3 weeks of age. At the age of five weeks, some gonadectomized males and females were given implants containing testosterone. Only intact males and testosterone-implanted castrated males showed reproductive behavior. Plasma gonadotropin levels were elevated in gonadectomized birds and reduced in steroid-implanted gonadectomized birds. The aMPT treatment significantly reduced the levels of DA and NE in the telecephalon and the level of DA in the hypothalamus. After aMPT treatment, the disappearance of NE in the telecephalon and of DA in the hypothalamus were significantly different according to the sex or treatment of the birds or both. Significant interactions between these two factors were observed. Disappearance rate of NE in the telecephalon was decreased by castration of males and increased by ovariectomy of females. Both effects were counteracted by testosterone. Reverse effects were observed for DA disappearance in the hypothalamus (increase with castration in males and decrease with ovariectomy in females). These results give evidence for altered aminergic function in specific areas of the brain relative to altered reproductive state.

Effect of MPTP and its pyridinium metabolites on monoamine uptake and on central catecholamine neurons in mice.

The effect of MPTP and its pyridinium metabolites MPDP+ and MPP+ on the in vitro [3H]monoamine uptake in synaptosomal preparations from mouse striatum and cerebral cortex was investigated. All compounds inhibited [3H]monoamine uptake in a dose-dependent manner in both regions analysed. MPP+ had the highest affinity to dopamine and noradrenaline uptake sites, while MPTP had the highest affinity to serotonin uptake sites. The results indicate that the affinity of MPP+ to different monoamine uptake sites appears to be better correlated to MPTP neurotoxicity as expressed in vivo than MPTP and MPDP+. Intracerebral injection of MPP+ into substantia nigra produced an almost complete disappearance of dopamine in striatum and noradrenaline in cerebral cortex, while injection of MPTP or MPDP+ had no or only moderate catecholamine-depleting effects. The MPP+-induced catecholamine depletion could be partially reversed by pretreatment with the catecholamine uptake blocker nomifensine. Histological analysis disclosed that MPP+ was a potent generally cytotoxic agent, while MPDP+ less and MPTP least so. The present results are compatible with the view that an interaction with the catecholamine uptake mechanism, probably through an uptake and accumulation of extraneuronally formed MPP+, is most likely the explanation for neuron-specific neurotoxic action on catecholamine neurons following MPTP administration.

Dose related effects of 6-OHDA on rabbit retinal dopamine concentrations and ERG B-wave amplitudes

The purpose of this study was to examine the dose related effects of 6-hydroxydopamine (6-OHDA) on the electroretinographic (ERG) B-wave amplitude and on retinal concentrations of dopamine (DA) and its main metabolites homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC). Doses of 6-OHDA (60, 300 or 1200 μg) were dissolved in a 0.9 percent NaCl and ascorbic acid solution and administered intravitreally in 300 μl volume in one eye of adult pigmented rabbits. The fellow eye received a similar volume of the drug vehicle. With the smallest dose of 6-OHDA (60 μg) no changes in ERG B-wave amplitude or retinal concentrations of DA, DOPAC or HVA were found. With the largest dose of 6-OHDA (1200 μg) complete extinction of the ERG and almost total disappearance of DA, DOPAC and HVA were observed. With the intermediate dose of 300 μg 6-OHDA significant increases in B-wave amplitudes and decreases in retinal DA, DOPAC and HVA concentrations were obtained. This latter dose did not affect retinal concentrations of serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and norepinephrine (NE). These results demonstrate that selective reductions of retinal dopamine and its metabolites can be obtained with a single intravitreal injection of 300 μg 6-OHDA in rabbits. The observed concomitant increase in B-wave amplitudes lends support to the hypothesis that DA acts as a mediator of lateral inhibition in the retina.

The effect of diet and chronic obesity on brain catecholamine turnover in the rat

Male Sprague-Dawley rats were fed a high calorie, high fat diet for 3 months to produce chronic diet-induced obesity (DIO) in which they gained 70% more weight than chow-fed controls. Thirty-six percent of the rats fed the DIO diet resisted the development of obesity (DR), gaining no more weight than chow-fed controls but serving as a comparison for the effects of the diet alone on the metabolism of brain catecholamines. The major influence of dietary composition was upon norepinephrine (NE) metabolism. Both DIO and DR rats had increased turnover of NE (107–217%) and/or shorter NE half-lives (42–67%) than controls in the hypothalamic paraventricular (PVN) and dorsomedial (DMN) nuclei and the median eminence (ME); dopamine (DA) turnover was similarly accelerated in the PVN. The DR rats alone exhibited decreased NE levels with increased disappearance of NE in frontal cortex and increased disappearance of DA in the ventromedial hypothalamic nucleus (VMH). The major effect of chronic obesity alone was a 31–33% decrease in DMN DA turnover and an 80% decrement in ME DA turnover associated with a 61% decrease in DA levels as compared to chow-fed controls. Therefore, the major effect of a high calorie, high fat diet was a diffuse acceleration of brain NE and DA turnover while chronic obesity led to decreased DA turnover in the DMN and ME.