The effect of reserpine treatment in vivo upon L-dopa and amphetamine evoked dopamine and DOPAC efflux in vitro from the corpus striatum of male rats

Summary In the present experiment we tested the effects of L-DOPA and amphetamine upon dopamine and DOPAC efflux in vitro from superfused corpus striatal tissue fragments of male rats who had been pretreated with reserpine. Male rats were treated with reserpine (5mg/kg) or its vehicle at 24 hours prior to sacrifice and superfusion of the corpus striatum. Two different modes of L-DOPA (5 M) and amphetamine (10 M) stimulation, a brief 10-minute and a continuous 60-minute infusion, were tested for their ability to evoke striatal dopamine and DOPAC efflux. Depletion of monoamine storage capacity as achieved with reserpine significantly reduced the amount of basal dopamine and DOPAC released from superfused striatal tissue fragments of male rats. Although basal release rates were significantly reduced, the amount of dopamine and DOPAC released in response to in vitro L-DOPA infusions (10 or 60 minute infusions) was equivalent between reserpine and vehicle treated animals. In contrast, amphetamine stimulated DA release was significantly reduced in male rats treated with reserpine. For both L-DOPA and amphetamine, significantly greater amounts of dopamine were obtained with the 60-versus 10-minute infusion modes. These results demonstrate that the capacity for L-DOPA, but not amphetamine, to evoke dopamine efflux is unaltered under conditions when monoamine storage ability is diminished.     

The effect of long-term treatment with deprenyl on basal and L-DOPA evoked dopamine release in vitro from the corpus striatum of aged rats

Summary In the present experiment, male rats (15–17 months) were injected with deprenyl (0.25 mg/kg) three times per week for six months. At 21–23 months of age the male rats were sacrificed, the corpus striatum removed and superfused in vitro. Basal and evoked dopamine and DOPAC levels, as obtained with either two infusions of L-DOPA (L-DOPA/L-DOPA) or L-DOPA followed by amphetamine (L-DOPA/AMPH), were measured from effluent superfusion samples and compared with values obtained from similarly aged animals treated identically with saline and from that obtained with young (2–4 months) animals. Treatment with deprenyl resulted in significantly greater basal dopamine and significantly lower basal DOPAC output compared with basal release levels from saline-treated aged rats and young animals. Responses to L-DOPA/LDOPA or L-DOPA/AMPH evoked dopamine and DOPAC release did not differ between deprenyl and saline-treated aged rats, however, both groups showed a significantly reduced response profile to these stimulations (L-DOPA/ L-DOPA or L-DOPA/AMPH) compared to that of young rats. These results indicate that the selective Type-B monoamine oxidase inhibitor, deprenyl, exerts a basic change in dopamine metabolism within the corpus striatum of aged rats resulting in an increase of endogenous dopamine and a decrease in endogenous DOPAC output.     

Intracerebral dialysis monitoring of striatal dopamine release and metabolism in response to L-DOPA

Summary We have used intracerebral dialysis to monitor the striatal extracellular fluid (ECF) in rats with unilateral lesions of the nigrostriatal dopamine (DA) pathway. Dialysis samples were collected before and after L-dihydroxyphenylalanine (L-DOPA) administration both in the presence and absence of carbidopa, an extracerebral DOPA decarboxylase (DDC) inhibitor. The baseline ECF levels of DA, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were always higher in the intact than in the lesioned striata. In the normal striata, dopamine (DA) concentrations increased following L-DOPA administration. Pretreatment with carbidopa prolonged the duration of the DA increase. In the lesioned striata, DA levels increased following L-DOPA administration only in animals pretreated with carbidopa. Following L-DOPA administration, striatal HVA and DOPAC levels increased considerably more in animals not pretreated with carbidopa than they did in pretreated animals. This increase was particularly marked in the lesioned striata and leads us to conclude that extracerebrally produced HVA and DOPAC can enter the brain extracellular space.     

Methamphetamine-stimulated striatal dopamine release declines rapidly over time following microdialysis probe insertion

To investigate changes in striatal dopamine release over a series of brief methamphetamine (METH) exposures, METH was pulsed three times at 2-h intervals, with the first exposure occurring 2 h after microdialysis probe insertion. Whether METH was administered directly into the striatum via the microdialysate (20 μM of METH for 10 min), or via peripheral intraperitoneal (i.p.) injection (1 mg/kg METH, i.p.), the dopamine (DA) peak elicited by the third METH exposure was only 50% as large as that elicited by the first exposure, 4 h earlier. This decline in the magnitude of METH-induced DA release probably continued over at least 24 h, since the magnitude of a single peak 26 h after probe implantation was only one-seventh of that at 2 h. This reduction in the response to METH was a function of time post-probe insertion, and not of prior METH exposure. Thus, peak size was the same at 6 h post-implantation in animals which received two prior METH pulses or no prior METH pulses, and in both cases this 6-h peak was substantially lower than that at 2 h post-implantation. Circadian influences were also excluded as a factor, because size of the initial METH-induced DA peak did not vary as a function of time of probe implantation. It is concluded that METH-stimulated striatal DA release declines rapidly over time post-probe insertion. When METH exposures occur repeatedly at short intervals, this decline can mimic, but is not caused by, desensitization or depletion in response to prior METH exposure.     

Sex differences in striatal dopamine release in healthy adults.

BACKGROUND: Sex differences in addictive disorders have been described. Preclinical studies have implicated the striatal dopamine system in these differences, but human studies have yet to substantiate these findings. METHODS: Using positron emission tomography (PET) scans with high-specific-activity [11C] raclopride and a reference tissue approach, we compared baseline striatal dopamine binding potential (BP) and dopamine release in men and women following amphetamine and placebo challenges. Subjective drug effects and plasma cortisol and growth hormone responses were also examined. RESULTS: Although there was no sex difference in baseline BP, men had markedly greater dopamine release than women in the ventral striatum. Secondary analyses indicated that men also had greater dopamine release in three of four additional striatal regions. Paralleling the PET findings, men's ratings of the positive effects of amphetamine were greater than women's. We found no sex difference in neuroendocrine hormone responses. CONCLUSIONS: We report for the first time a sex difference in dopamine release in humans. The robust dopamine release in men could account for increased vulnerability to stimulant use disorders and methamphetamine toxicity. Our findings indicate that future studies should control for sex and may have implications for the interpretation of sex differences in other illnesses involving the striatum.     

L-DOPA modulation of corpus striatal dopamine and dihydroxyphenylacetic acid output from intact and 6-OHDA lesioned rats

Summary In the present report we examined the differences in in vitro dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) efflux from the corpus striatum (CS) of intact versus 6-hydroxydopamine (6-OHDA) lesioned (in substantia nigra) male rats in response to different doses of two pulse infusions of L-dihydroxyphenylalanine (L-DOPA). In the first experiment, we tested the effects of two 20-min infusions of 5 uM L-DOPA. In the second experiment we repeated this protocol using 50 uM L-DOPA. There was an overall significantly greater output of DA for intact versus 6-OHDA lesioned rats for both doses. Moreover, in Experiment 1, the 5 uM L-DOPA produced a peak DA response to the second infusion which was significantly higher than that of the first infusion in the intact, but not lesioned rats. In Experiment 2, the 50 uM L-DOPA group showed no significant differences in DA output between the two infusions for both intact and lesioned rats. In contrast to DA responses, there were no overall significant differences in DOPAC output between intact and 6-OHDA lesioned rats for both doses. However, for both doses tested, the peak DOPAC output from the second infusion was significantly increased in lesioned, but not intact rats. These data demonstrate that L-DOPA evoked DA and DOPAC output are differentially modulated in intact and 6-OHDA lesioned striatum. The lesions of the striatal dopaminergic system may alter these responses through changes in intraneuronal storage and metabolism of DA following L-DOPA infusion.     

Alterations in striatal dopamine overflow during rotational behavior induced by amphetamine,phencyclidine, and MK-801

Rats lesioned unilaterally in the medial forebrain bundle with 6-OHDA rotated ipsilateral to the lesion following injections of amphetamine, phencyclidine (PCP), and MK-801. Concurrent measurement of striatal dopamine (DA) in the intact striatum with in vivo microdialysis revealed a dissociation between rotational behavior and alterations in DA overflow induced by the three drugs. Amphetamine produced robust ipsilateral rotational behavior and a substantial elevation in striatal DA (∼130% increase at asymptote). PCP produced comparable increases in rotational behavior, but only ∼30% increase in striatal DA. MK-801 also had a comparable behavioral effect but failed to alter DA overflow in the intact striatum. Since MK-801, a noncompetitive NMDA antagonist which does not enhance extracellular dopamine in the striatum, is able to produce ipsilateral rotational behavior in rats with unilateral nigrostriatal lesions, it is likely that the effects of PCP may also be determined predominantly through NMDA blockade in this model. Synapse 26:218–224, 1997. © 1997 Wiley-Liss Inc.     

D-Amphetamine reduces striatal substance P concentrations by presynaptic release of dopamine

Single, but not repeated, doses ofD-amphetamine sulfate cause dose-dependent reductions in substance P (SP) immunoreactive material within the rat striatum. A 10 mg/kg (i.p.) dose reduces SP levels from 1.37 to 0.97 pmol/10 mg tissue after 2 h. Similar reductions in striatal SP levels are observed after administration of methylphenidate (50 or 100 mg/kg). No changes in SP concentrations occur within the substantia nigra or hypothalamus afterD-amphetamine. The amphetamine-induced decline in striatal SP is blocked by pretreating rats with α-methyl tyrosine methyl ester (225 mg/kg i.p.), or haloperidol (1, 3 or 10 mg/kg i.p.), or after the dopaminergic nigrostriatal tract has been lesioned using 6-hydroxydopamine. These data indicate that the mechanism by whichD-amphetamine lowers striatal SP involves presynaptic release of dopamine from the terminals of nigrostriatal neurons.     

Activation of striatal dopamine receptors induces pain inhibition in rats

Summary In the rat, elevating dopamine content in corpus striatum with electrical stimulation of substantia nigra or direct administration of apomorphine (50–200g) into the lateral cerebral ventricle or apomorphine (2–10g) into the caudate-putamen complex decreased pain sensitivity (as shown by an increase in the latency to hind-paw lick in the hot plate test). Furthermore, the decreased pain sensitivity after the central administration of apomorphine was antagonized by pretreatment with haloperidol (a dopamine antagonist). On the other hand, lowering dopamine content in corpus striatum with electrolytic destruction of substantia nigra and 6-hydroxydopamine lesions to the substantia nigra, as well as direct injection of haloperidol into the lateral cerebral ventricle or caudate-putamen complex increased pain sensitivity. The data indicate that activation of striatal dopamine receptors in rat brain induces pain inhibition.     

Presynaptic inhibition of dopamine synthesis in rat striatum: effects of chronic dopamine depletion and receptor blockade

These studies assessed the effects of dopamine (DA) depletion and receptor blockade on presynaptic inhibition of DA synthesis in the rat striatum. Chronic reserpine administration significantly decreased striatal DA levels but did not affect in vivo tyrosine hydroxylase activity, as determined by following dihydroxyphenylalanine (DOPA) accumulations. Both reserpine and haloperidol increased the sensitivity of presynaptic striatal DA response as determined by the ability of apomorphine (APO) to inhibit DOPA accumulation in NSD-1015-treated rats. The effect of concurrent administration of reserpine plus haloperidol on presynaptic response was additive. Additivity occurred at doses or reserpine and haloperidol while induced maximum sensitivity when administered singularly. The data suggest that increases in sensitivity of presynaptic DA response following DA depletion and receptor blockade are mediated by separate regulatory mechanisms.     

A simple in vitro technique to measure the release of endogenous dopamine and dihydroxyphenylacetic acid from striatal tissue using high performance liquid chromatography with electrochemical detection

An easily constructed, inexpensive and simple system is described for the superfusion of neural tissue. To characterize the system, the dynamics of endogenous dopamine and dihydroxyphenylacetic acid release from striatal tissue were studied before and after exposure to potassium, amphetamine or cocaine.     

Release of endogenous dopamine and cholecystokinin from rat striatal slices: effects of amphetamine and dopamine antagonists

The release of immunoreactive cholecystokinin (CCK) and dopamine was monitored simultaneously from superfused rat striatal slices. Exposure of the tissue to medium containing elevated of dopamine (10⁻⁷ and 10⁻⁶)M), the dopamine agonist pergolide (10⁻⁷, M), the D₂-antagonist sulpride (1 μM) or the D₁-antagonist (SCH 23390) had no significant effect on basal overflow or on evoked release of CCK. On the other hand, preincubation of striatal slices withd-amphetamine (10⁻⁵ M) enhanced basal and veratrine-stimulated dopamine release but markedly suppressed evoked CCK release. Sulpiride blocked this action of amphetamine whereas SCH 23390 was ineffective. The data suggests that whereas it is difficult to observe any effects of exogenous dopamine agonists or antagonists on evoked CCK release, endogenously released dopamine appears to interact with D₂-receptors to suppress evoked CCK release from rat striatal slices.     

Interactions between handling and acrylamide on the striatal dopamine receptor

Rats were either handled for one week or were left undisturbed in their home cages. Twenty-four hours after exposure to 100 mg/kg acrylamide, rats were sacrificed and spiroperidol binding investigated in the striatum. In handling-gentled rats, spiroperidol binding was reduced by acrylamide. In non-handled animals, no overall drug effects were seen, but the effect of acrylamide varied with the time of sacrifice. These results demonstrate the importance of environmental factors in the response to neurotoxicants.     

L-DOPA modulates striatal dopaminergic function in vivo: Evidence from PET investigations in nonhuman primates

Significant increases in striatal L-[¹¹C]DOPA retention were observed in adult female rhesus monkeys with positron emission tomography (PET) following administration of drugs that increase cerebral L-DOPA concentrations. The monkeys were scanned twice: at baseline (using 10–50 μg of tracer substance) and during continuous administration of L-DOPA (3 or 15 mg/kg/h) and 6-R-Erythro-4,5,6,7-tetrahydrobiopterin (6R-BH₄) (5 mg/kg/h) and during combined administration of both drugs. PET scans of L-[¹¹C]DOPA distribution were obtained in GE2048-15B or GE4096-15WB Plus positron tomographs. In all studies the specific striatal L-[¹¹C]DOPA influx rate increased by an average of 17–20%. These increases were significantly higher than the retest variability obtained with saline infusions under identical experimental conditions. In individual monkeys the magnitude of increase in the striatal L-[¹¹C]DOPA influx rate varied from no effect of the drug infusion to a 45% increase. Taken together, the results of this study demonstrate that L-DOPA in itself can affect dopaminergic neurotransmission in vivo and also adds further evidence that the neuromodulatory effects of the amino acid are predominantly autoreceptor antagonist-like. The findings most likely have importance for the further understanding of the dopaminergic system in neurodegenerative and psychiatric disorders. Synapse 25:56–61, 1997. © 1997 Wiley-Liss, Inc.     

Electroconvulsive shock blocks the opioid-mediated inhibition of dopamine release in rat striatal slices

Summary The fractional release technique was applied to investigate the effects of single electroshock (ECS) on the opioid-mediated inhibition of dopamine release in rat striatal slices. Animals were submitted to ECS 24h before the experiments. The results demonstrate that pre-treatment with ECS suppresses the inhibition of dopamine release mediated by opioid receptors. These data suggest that single ECS treatment modifies the sensitivity of the opioid receptors located on the presynatic dopamine terminals in the rat striatum.     

Neurotensin facilitates dopamine release in vitro from rat striatal slices

Neurotensin, (0.1-10 microM) stimulated the release of [3H]dopamine from rat striatal slices in a calcium-dependent manner, and potentiated the K+-evoked release of [3H]dopamine and endogenous dopamine. This effect was dose-dependent, (1 nM-1 microM) with an EC50 of approximately 10 nM, and was mediated by means of a receptor of similar structure-activity profile to those described in other tissues.     

Regulation of dopamine synthesis in striatal synaptosomes during depolarization

Incubation of striatal synaptosomes under depolarizing conditions (veratridine or KCl) increased the release of dopamine (DA) and produced a small increase in DA synthesis. This was not accompanied by an activation of either soluble or membrane-bound tyrosine hydroxylase, but was associated with a large depletion of synaptosomal DA. Thus, depolarization may increase DA synthesis by reducing the inhibition of tyrosine hydroxylase by end-product.