Magnitude and duration of hyperactivity following neonatal 6-hydroxydopamine is related to the extent of brain dopamine depletion

This experiment examined the relationship between the extent of brain dopamine (DA) neuron destruction in the neonatal rat and locomotor hyperactivity during subsequent development. Brain DA neurons were destroyed selectively in neonatal rats by intraventricular injections of 6-hydroxydopamine (6-OHDA) following desmethylimipramine (DMI) pretreatment of both days 3 and 6 of life. Groups of rats received total doses of 50, 70, 100 or 200 microgram of 6HDA or the vehicle solution. Each group of rats given 6-OHDA displayed 3- to 5-fold increases in locomotor activity relative to vehicle control rats on days 16 and 18 of life. Rats given 50 or 70 microgram of 6-OHDA displayed hyperactivity that diminished during days 18-32 of life, approaching the level of activity seen in vehicle-treated rats. It contrast, rats given 100 or 200 microgram of 6-OHDA displayed consistently high levels of locomotion during days 18-32 of life. When tested as adults (days 55-66 of life) only those rats given 200 micrograms of 6-OHDA as neonates continued to display locomotor hyperactivity. The extent of 6-OHDA-induced depletion of DA was proportional to the magnitude of locomotor hyperactivity seen during neonatal life. Brain DA was depleted to the greatest extent in rats which displayed permanent hyperactivity. Regardless of the extent of depletion of brain DA, adult rats given intraventricular injections of 125, 200 or 275 micrograms of 6-OHDA at 48 days of age (following pargyline and DMI pretreatment) displayed no significant change in locomotor activity. These results indicate that the magnitude and duration of locomotor hyperactivity seen following neonatal 6-OHDA injections are correlated with the extent of loss of central DA neurons and suggest that brain DA projections exert important influences on the ontogeny of normal locomotion.     

Reductions in spontaneous locomotor activity in aged male, but not female, rats in a model of early Parkinson's disease

The excessive loss of dopamine (DA) neurons that occurs with Parkinson's disease is usually confined to older individuals. While 6-hydroxydopamine (6-OHDA) is often used in animal models of DA neuron degeneration, there have been relatively few studies that have examined the effects of 6-OHDA in older animals. In the present study, we compared the effects of a bilateral, partial lesion with 6-OHDA in young (4 months), middle-aged (14 months), and aged (24 months) Fischer-344 rats of both sexes. Animals were given a single injection of vehicle or 100 mug 6-OHDA into the right lateral ventricle. Four weeks later, spontaneous locomotor activity was monitored. Microdialysis experiments were carried out 1 to 3 days later. The 6-OHDA treatments had no effect on horizontal activity or total distance traveled in young adults. However, with aged rats, there was a decrease in both measures in the vehicle-treated control rats compared to young adult controls, and a further decrease in the lesioned aged male rats. The 6-OHDA treatments led to significant decreases in both potassium- and amphetamine-evoked overflow of DA from the striatum in all groups. Thus, partial bilateral lesions of the nigrostriatal DA system led to decreases in evoked release of DA in the striatum of male and female rats of all three ages, but to changes in spontaneous activity only in the aged males. These results indicate that there are both age and sex differences in the brain's response to 6-OHDA, and imply that compensatory or neuroprotective mechanisms in the young brain and aged female brain are more efficient than in the aged male brain.     

Microdialysates of amines and metabolites from core nucleus accumbens of freely moving rats are altered by dizocilpine

In vivo microdialysis combined with high performance liquid chromatography (HPLC) with electrochemical detection, was used to study the effect of MK-801 (0.1 mg/kg i.p.) on extracellular concentrations of dopamine (DA) 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), norepinephrine (NE) and DOPAC/DA ratio in intact, 6-hydroxydopamine (6-OHDA)-lesioned, DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzyl-amine hydrochloride)-lesioned and reserpine-treated rats. The results revealed high basal DA (0.735+/-0.05 fmol/microl), DOPAC (195.93+/-20.18 fmol/microl) and NE (0.585+/-0.01 fmol/microl), low 5-HT (0.334+/-0.032 fmol/microl) and high DOPAC/DA ratio (265.11+/-20.73) in intact cACC. 6-OHDA alone (8 microg/2 microl) depleted DA (-66%), DOPAC (-65%), and NE (-62%). On the other hand, in desipramine (DMI)-pretreated rats, 6-OHDA induced a large depletion of DA (-94%), DOPAC (-97%) and reduced DOPAC/DA ratio (-73%), but increased NE to 142% of intact and 369% of 6-OHDA-lesioned rats. DSP4 (50 mg/kg) decreased NE (-97%), DOPAC (-75%) and DOPAC/DA ratio (-69%). Reserpine (5 mg/kg s.c.) significantly decreased DOPAC (-84%), DOPAC/DA ratio (-81%), 5-HT (-69%) and NE (-86%), but nonsignificantly increased DA. In the intact rats, MK-801 did not change DA, but increased DOPAC and DOPAC/DA ratio. In 6-OHDA-lesioned rats, MK-801 increased DA, whereas in 6-OHDA+DMI rats MK-801 additionally increased DOPAC and DOPAC/DA ratio. DSP4 and reserpine reduced the ability of MK-801 to increase DOPAC and DOPAC/DA ratio. MK-801 did not change NE concentration in dialysates collected from intact rats, but increased that from 6-OHDA+DMI-lesioned rats. In DSP4-lesioned and reserpine-treated rats, MK-801 increased NE but to a level lower than that observed in the intact rats. These results suggest that systemic administration of a low dose of MK-801, which induces profound locomotor stimulation without stereotypy, increases DOPAC and DOPAC/DA ratio in the cACC of intact rats, whereas it additionally increases the depleted DA and NE concentrations especially in 6-OHDA-lesioned rats pretreated with DMI.     

Variability among brain regions in the specificity of 6-hydroxydopamine (6-OHDA)-induced lesions

Summary 6-Hydroxydopamine (6-OHDA; 200 g, 150 g or 110 g) or vehicle was infused stereotaxically into the lateral ventricles of rats, usually following pretreatment with desmethylimipramine (DMI). Various brain regions were then assayed for dopamine (DA), serotonin (5-HT) and norepinephrine (NE). As expected, 6-OHDA depleted DA in all brain regions examined. Unexpectedly, however, the two highest doses of 6-OHDA significantly decreased 5-HT levels in the hippocampus and increased 5-HT levels in the striatum. In addition, despite pretreatment with doses of DMI commonly considered adequate to block 6-OHDA-induced depletion of NE, all doses of 6-OHDA tested significantly reduced NE levels in the hippocampus, hypothalamus and septum.We interpret our data as suggesting that some brain regions are susceptible to nonspecific toxic effects of 6-OHDA at doses commonly employed. Furthermore, these nonspecific effects may or may not occur, depending on seemingly minor variations in experimental technique.     

Hyperactivity and hypoactivity produced by lesions to the mesolimbic dopamine system

Spontaneous locomotor activity and the locomotor response to amphetamine and apomorphine were studied in rats subjected to either radiofrequency (RF), 6-hydroxydopamine (6-OHDA) or both RF and 6-OHDA lesions of the mesolimbic dopamine (DA) system. Large 6-OHDA lesions of the ventral tegmental area (VTA) or of the nucleus accumbens (N.Acc.) produced hypo-activity in the open field, a complete blockade of the locomotor stimulating effects of D-amphetamine and a profound supersensitive response to apomorphine as measured by a significant increase in locomotor activity as compared to sham-operated animals. In contrast, smaller 6-OHDA lesions of the VTA produced significant increases in spontaneous daytime and nocturnal activity with the biggest effect occurring at the lowest dose. RF lesions to the VTA produced even greater hyperactivity which was blocked by the addition of a 6-OHDA lesion to the N.Acc. The rats with RF lesions to VTA alone that were spontaneously hyperactive remained hyperactive after injection of amphetamine, whereas apomorphine produced a significant decrease in this hyperactivity. In contrast, the rats with the combined RF lesion and N.Acc. 6-OHDA lesion showed a blockade of the locomotor stimulating effects of D-amphetamine and a potentiated response to apomorphine identical to that observed with a N.Acc. lesion alone. All lesion groups revealed massive depletion of DA in the N.Acc. and anterior striatum with significantly greater depletions in those groups showing hypoactivity and hypo-responsiveness to amphetamine. All groups except the N.Acc. 6-OHDA alone group showed significant depletions of DA in the posterior striatum. Thus, limited destruction of the mesolimbic DA system can produce hyperactivity, but more extensive destruction of this system in the region of the N.Acc. and anterior striatum can reverse this hyperactivity and produce a hypo-responsiveness to the locomotor stimulating effects of amphetamine. These results suggest an essential role for dopamine in the expression of spontaneous and stimulant-induced activity. Furthermore, the much larger increase in spontaneous activity in the RF-VTA lesion group as compared to the VTA-6-OHDA groups suggests the presence of an, as yet unidentified, powerful inhibitory influence to the mesolimbic DA system within the midbrain tegmentum.     

Possible involvement of serotonergic neurons in the reduction of locomotor hyperactivity caused by amphetamine in neonatal rats depleted of brain dopamine

This experiment attempted to determine the mechanism by which amphetamine reduces locomotor hyperactivity in neonatal rats given brain dopamine (DA)-depleting 6-hydroxydopamine (6-OHDA) injections. Brain DA neurons were destroyed selectively in neonatal rats by intraventricular (i.v.t.) injections of 6-OHDA following desmethylimipramine (DMI) pretreatment. Control rats received DMI and i.v.t. injections of the 6-OHDA vehicle solution. Rats given the 6-OHDA treatment displayed 7-fold increases in locomotor activity compared to controls during days 16–55 of life. Throughout this period, amphetamine (1 mg/kg) reduced locomotor hyperactivity in 6-OHDA-treated rats but increased locomotor activity in control rats. The reduction of hyperactivity caused by amphetamine (0.5–4 mg/kg) was dose-related and was not accompanied by stereotyped behavior. Like amphetamine, methylphenidate (4 mg/kg) reduced locomotor hyperactivity in rats given 6-OHDA. The DA antagonist, spiroperidol (50–200 μg/kg) failed to attenuate the hyperactivity-reducing effect of amphetamine in 6-OHDA-treated rats at doses which abolished the stimulant effect of amphetamine in control rats. However, the serotonin antagonist methysergide (0.5–4 mg/kg) produced dose-dependent antagonism of the effect of amphetamine in 6-OHDA-treated rats. Pretreatment with propranolol (5 mg/kg), phentolamine (5 mg/kg), atropine (0.5 mg/kg) or naloxone (10 mg/kg) failed to alter the reduction in locomotor hyperactivity caused by amphetamine. The serotonin releasing agent, fenfluramine (3 mg/kg), and the serotonin agonist, quipazine (0.5–4 mg/kg), both reduced locomotor hyperactivity in 6-OHDA-treated rats while not altering locomotion in control rats. These results confirm previous observations that amphetamine reduces locomotor hyperactivity caused by neonatal 6-OHDA administration and suggest that this effect is mediated by increased serotonergic neurotransmission.     

The effect of water-deprivation on locomotor activity in rats treated with 6-hydroxydopamine

Male albino rats treated with 6-hydroxydopamine (6-OHDA) became more hyperactive than did vehicle-treated controls when both groups were water-deprived. Rats were treated with vehicle, 150 or 250 micrograms of 6-OHDA intraventricularly, after pretreatment with desmethylimpramine (25 mg/kg) and pargyline (50 mg/kg). Eleven days after these treatments, under ad libitum water conditions, the 6-OHDA-treated rats were slightly hypoactive. After several days of water-deprivation all three groups showed increased mean locomotor activity levels, but rats treated with 6-OHDA showed a much greater increase than did vehicle-treated rats. Under subsequent ad libitum and deprivation conditions, locomotor activity decreased and increased, respectively, in all 3 groups. Again, the changes in activity levels of the 6-OHDA-treated groups were greater than those of the vehicle-treated group. In addition, rats treated with 250 micrograms 6-OHDA seemed to become sensitized to the novel environment of the stabilimeter rather than habituating to it. Dopamine (DA) levels were decreased as a result of the injections of 6-OHDA, and significant correlations were found between DA levels in the caudate-putamen and nucleus accumbens, and locomotor activity levels. These results, as well as those obtained by others, suggest that there is an interaction among DA levels, deprivational states, and locomotor activity levels in rats.     

Cerebrospinal dopamine metabolites in rats after intrastriatal administration of 6-hydroxydopamine or 1-methyl-4-phenylpyridinium ion

Dopamine (DA) and its main cerebral metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured in striatum and cerebrospinal fluid (CSF) from cisterna magna in rats bilaterally lesioned by intrastriatal administration of 6-hydroxydopamine (6-OHDA) or 1-methyl-4-phenylpyridinium ion (MPP⁺). 6-OHDA caused a progressive lesion in striatum that is only moderately reflected in the decrease in dopamine metabolite concentration in CSF. MPP⁺ caused an acute but less selective lesion in the dopamine striatal system, as indicated by a significant reduction in striatal GABA content, followed by a slow recovery in dopamine striatal metabolism and content. The locomotor activity was dramatically reduced in both groups 48 hours after the treatment but remained significantly decreased after two months only in 6-OHDA lesioned animals. A positive correlation was found between HVA CSF concentration and striatal DA content in MPP⁺ lesioned rats, but not in 6-OHDA lesioned rats. It is concluded that the concentration of dopamine metabolites in CSF can be altered only after a severe striatal lesion: reduction of striatal dopamine content below 50% of normal values and involvement of neuronal or non-neuronal elements other than the dopaminergic system, similarly to the lesions caused by MPP⁺. These results may partly explain why CSF dopamine metabolites concentrations were significantly decreased both in advanced stages of parkinsonism and in other neurodegenerative disorders.     

Central noradrenergic involvement in yohimbine excitation of acoustic startle: Effects of DSP4 and 6-OHDA

It was previously shown that i.p. administration of the alpha 2-adrenergic antagonist yohimbine increased the magnitude of the acoustic startle response in rats. The purpose of the present study was to determine possible central noradrenergic involvement in yohimbine's effect on startle. Pretreatment with N-(2-chloroethyl)-N-ethyl-2-bromo-benzylamine (DSP4; 50 mg/kg, i.p.; 1-2 days before testing) completely blocked the excitatory effect of yohimbine on startle. DSP4 reduced forebrain and spinal cord NE levels by 47% and 56%, respectively, without affecting forebrain or spinal serotonin (5-HT), or forebrain dopamine (DA). Pretreatment with the NE reuptake blocker desmethylimipramine (DMI; 20 mg/kg, i.p.; 30 min before DSP4) prevented the ability of DSP4 to block the yohimbine effect. DMI partially reversed the NE-depleting effects of DSP4. Neither bilateral adrenalectomy nor intravenously administered 6-hydroxydopamine (6-OHDA; 20 mg/kg; 1-2 days before testing) altered the excitatory effect of yohimbine, indicating that peripheral NE is not involved. 6-OHDA (2 X 200 micrograms) injected into the lateral ventricles blocked yohimbine's effect, and depleted NE by 95% (spinal cord) and 86% (forebrain), without affecting 5-HT in either region. 6-OHDA also depleted forebrain DA levels by 49%. Finally, intrathecal administration of 6-OHDA (20 micrograms; 14 days before testing) into the subarachnoid space of the lumbar spinal cord blocked the excitatory effect of yohimbine, and produced an extensive (94%) depletion of spinal cord NE. Intrathecal 6-OHDA did not alter spinal levels of 5-HT or forebrain levels of NE, 5-HT or DA. In summary, these data indicate that central descending NE neurons are necessary for yohimbine's excitatory effect on startle.     

Enhanced survival of cultured dopamine neurons by treatment with soluble extracts from chemically deafferentiated striatum of adult rat brain

A soluble fraction was extracted from a chemically deafferentiated striatum of adult Wistar rats after unilateral lesioning of the nigrostriatal pathway by 6-hydroxydopamine (6-OHDA) injection. The soluble extract from the lesioned side enhanced the survival of cultured mesencephalic dopamine (DA) neurons of 14-day-old rat embryos as evidenced by quantitative counting of tyrosine hydroxylase-like immunoreactive cells. The neurotrophic activity of this striatal extract for DA neurons was highest 14 days after 6-OHDA injection and became negligible in 28 days. The extract showed no promoting effects on cultured γ-aminobutyric acid (GABA)-containing mesencephalic neurons. These observations indicate that the striatum of adult rats may initiate de novo synthesis of trophic substance(s) for DA neurons but not for GABA neurons when subjected to nigral dopaminergic deafferentiation.     

Partial protection by desmethylimipramine of the mesocortical dopamine neurones from the neurotoxic effect of 6-hydroxydopamine injected in ventral mesencephalic tegmentum. The role of noradrenergic innervation

As shown in the rat by estimation of dopamine (DA) and noradrenaline (NA) levels, bilateral 6-hydroxydopamine (6-OHDA, 4 micrograms/microliter) lesions made in the ventral mesencephalic tegmentum (VMT) destroy both ascending DA and NA neurones. Pretreatment of rats with desmethylimipramine (DMI, 30 mg/kg, i.p.), 30 min before microinjection of 6-OHDA into the VMT partially prevented the destruction of the DA neurones innervating the prefrontal and cingulate cortices but not those innervating subcortical structures (nucleus accumbens, olfactory tubercles, septum). Results obtained from the prefrontal cortex of rats with extensive lesions of the ascending NA neurones performed 15 days prior to the 6-OHDA lesions of the VMT in the presence of DMI, imply that NA innervation of the VMT seems to be required for DMI to protect the cortical DA neurons from the neurotoxic effect of 6-OHDA.     

Amphetamine and apomorphine responses in the rat following 6-OHDA lesions of the nucleus accumbens septi and corpus striatum.

Eight mug of 6-hydroxydopamine (6-OHDA) injected bilaterally into the nucleus accumbens septi (NAS) or the caudate nucleus of the rat resulted in 79% and 50% depletion of endogenous dopamine (DA) at these respective sites. Fourteen days after the injection a low dose of amphetamine failed to induce the characteristic locomotor response in the NAS-lesioned rats but did so in the caudate-lesioned animals. By contrast the caudate lesion, but not the NAS lesions, abolished intense forms of stereotyped behaviour induced by higher doses of amphetamine. Both lesioned groups exhibited supersensitivity to the dopamine agonist, apomorphine; the NAS group showed enhanced locomotor activity and the caudate group enhanced stereotyped behaviour. The block of amphetamine locomotion and the enhanced response to apomorphine were maximal around 14 days after the operation and gradually attenuated up to 90 days. Theer is evidence that remaining DA levels in the NAS are greater at 90 than at 14 days postoperatively. Thus recovery of behavioural effects correlated with an increase in the remaining levels of DA in the NAS.     

Behavioural and biochemical effects of dopamine and noradrenaline depletion within the medial prefrontal cortex of the rat

The effects of 6-hydroxydopamine (6-OHDA) lesions of catecholamine terminals within the medial prefrontal cortex on spontaneous motor activity, dopamine (DA)-dependent stereotyped behaviour and subcortical dopamine turnover were investigated in the rat. Two types of lesions were examined, bilateral injection of 6-OHDA into the medial prefrontal cortex of untreated rats (6-OHDA alone), and bilateral injection of 6-OHDA into the medial prefrontal cortex of animals pretreated with the noradrenaline (NA) uptake blocking agent desmethylimipramine (6-OHDA/-DMI). Ten days after surgery the 6-OHDA lesions produced no significant change in spontaneous motor activity and had no overall effects on stereotyped behaviour induced by apomorphine or (+)-amphetamine. This lesion caused gross depletion of NA within the medial prefrontal cortex and curiously, elevated DA concentrations within this site. No changes in DA concentration were recorded within subcortical sites, although concentrations of DA metabolites within striatum and nucleus accumbens were reduced. In contrast, the 6-OHDA/DMI lesion of the medial prefrontal cortex significantly enhanced spontaneous motor activity and amphetamine-induced stereotyped behaviour. Apomorphine-induced stereotypy, on the other hand, was significantly reduced. Biochemically the lesion caused a large depletion of DA with relatively little loss of NA within the medial prefrontal cortex. In addition, from this and another study (ref. 33), increases in DA and its metabolite concentrations were measured in striatum and nucleus accumbens, together with an apparent increased in DA turnover within these subcortical sites. It is thus apparent that in the absence of a substantial portion of the DA innervation of the medial prefrontal cortex, with a largely intact NA innvervation, there is an increase in motor activity and amphetamine-induced stereotypy which may be related to functional changes in DA activity within subcortical telecephalic structures. Such a finding might suggest that DA within the frontal cortex has a behaviourally inhibitory role in the rat, although further work is required to substantiate this.     

Assessment of the effects of neonatal subcutaneous 6-hydroxydopamine on noradrenergic and dopaminergic innervation of the cerebral cortex

Female rats, treated at birth with 6-hydroxydopamine (3 x 100 mg/kg s.c. at 24 h intervals) or vehicle, were subjected at 112 days of age to unilateral electrolytic lesions of the locus coeruleus. Two weeks later regions of the telencephalon, both ipsi- and contralateral to the lesion, were simultaneously assayed for norepinephrine (NE) and dopamine (DA) content, and for tyrosine hydroxylase (TOH) and dopamine-beta-hydroxylase (DBH) activities. In the vehicle-treated rats the lesion resulted in at least an 80% reduction of NE and DBH on the ipsilateral side, relative to the contralateral side. TOH was reduced to a similar extent only in the parietal cortex and hippocampus. In the prefrontal cortex and cingulate gyrus TOH was decreased by only 31% and 64% respectively; the remainder was interpreted to be associated with projections of the mesocortical dopamine system. From this data it was possible to calculate that the ratio of TOH to DA in dopaminergic terminals is about 10-fold greater than the ratio of TOH to NE in noradrenergic terminals. Neonatal 6-hydroxydopamine treatment resulted in practically total elimination of noradrenergic terminals throughout the telencephalon, and the locus coeruleus lesion had no additional effect. The drug treatment produced no significant change in DA content or in the TOH to DA ratio in the prefrontal cortex and cingulate gyrus, indicating complete sparing of the mesocortical DA projections.     

Intracerebral administration of 2,4-diclorophenoxyacetic acid induces behavioral and neurochemical alterations in the rat brain

Although, the mechanism of 2,4-dichlorophenoxyacetic acid (2,4-D) neurotoxicity remains unknown, the monoaminergic system appears to mediate some of its effects in rats as we previously reported. In this study; we examined the 2,4-D effects on locomotor activity, circling behavior and monoamine levels after the injection into the basal ganglia of male adult rats. These effects were compared with those induced after selective lesions of dopaminergic neurons with 6-hydroxydopamine (6-OHDA). 2,4-D-injected into one striatum (100 microg/rat) produced a marked depression in locomotor activity and elicited a moderate circling towards the ipsilateral side at 6 and 24 h postinjection. These behavioral changes were accompanied by a decrease and an increase of serotonin (5-HT) and homovanillic acid (HVA) levels, respectively. 2,4-D administration (100 microg/rat) into the nucleus accumbens, induced similar behavioral and neurochemical patterns to the intrastriatal 2,4-D injection, although rats did not present notorious turning. When 2,4-D was injected into one medial forebrain bundle (MFB, 50 microg/rat), animals presented ipsilateral circling, while locomotor activity was unchanged at 3 and 7 days post-injection. These last rats also exhibited diminished levels of striatal 5-HT, dopamine (DA) and their metabolites without changes in the substantia nigra (SN). Animals sacrificed 3 and 7 days after a 6-OHDA injection into one of the MFB, presented progressive depletion of dopamine in striatum and SN. 2,4-D as well as 6-OHDA-treated rats into one of the MFB were challenged with low dose (0.05 mg/kg s.c.) of apomorphine (only at 7 days post-injection) to evaluate a possible DA-receptor supersensitivity. Only 6-OHDA treated rats showing a vigorous contralateral rotation activity. These results indicate that 2,4-D induced a regionally-specific neurotoxicity in the basal ganglia of rats. The neurotoxic effects of 2,4-D on basal ganglia by interacting with the monoaminergic system depended not only on the exact location of the 2,4-D injection, but also on the dose and time period of post-injection. Toxicity produced by 2,4-D appears to be different in monoaminergic terminals, axonal fibers, and cell bodies.     

Effects of pargyline and pyrogallol on the methamphetamine-induced dopamine depletion

The formation of 6-hydroxydopamine (6-OHDA) from dopamine (DA) was investigated in the striatum of male Sprague-Dawley rats following a single administration of methamphetamine hydrochloride (100 mg/kg, sc). Rats were sacrificed 30, 60, and 90 min, and 1 wk after injection, and striatal 6-OHDA, DA, and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured by HPLC with electrochemical detection. Methamphetamine decreased striatal DA and DOPAC levels (to 65 and 50% at 90 min, respectively) in the time-course study and also resulted in a long-lasting dopamine depletion (34%) 1 wk after its administration. However, endogenous 6-OHDA formation proved difficult to detect after administration of the methamphetamine alone. Pretreatment with the monoamine oxidase (MAO) inhibitor pargyline (100 mg/kg, ip) and the catechol-O-methyltransferase (COMT) inhibitor pyrogallol (25 mg/kg, ip) resulted in the HPLC detection of a 6-OHDA-like substance 30 min after methamphetamine administration when the oxidizing potential was set at 0.5 V, but not when it was set at 0.2 V. Moreover, pargyline (25 mg/kg, ip) alone or in combination with pyrogallol exacerbated the long-lasting dopamine depletion induced by methamphetamine (50 mg/kg, sc). These results indicate that simultaneous inhibition of MAO and COMT provides a cellular environment that encourages the autoxidation of dopamine to a 6-OHDA-like substance.     

The locus coeruleus and dopaminergic function in rat brain: implications to parkinsonism.

Clinical and experimental evidence suggests that degeneration of the locus coeruleus (LC) may be responsible for certain symptoms of Parkinson's disease (PD). We have, therefore, studied the effects of LC lesion on dopamine (DA) metabolism in the rat striatum. Unilateral depletion of norepinephrine (NE) was obtained by stereotaxic injection of 6-hydroxydopamine (6-OHDA) into the dorsal NE bundle (DNEB). Rats were sacrificed 1 or 3 weeks after lesioning. 6-OHDA induced approximately 50% depletion of NE in the ipsilateral hippocampus at 1 week postinjection, and over 75% depletion after 3 weeks. DNEB lesions had no effect on DA or DOPAC levels in the ipsi- or contralateral striatum at either time point. Lesions also failed to affect DA synthesis or utilization in either striatum. The metabolism of exogenous levodopa in the striatum was also unaffected. It is suggested that any possible effect of the LC on DA transmission in the striatum is not mediated by the DNEB.     

Left and right 6-hydroxydopamine lesions of the medial prefrontal cortex differentially affect voluntary ethanol consumption

Dopaminergic projections to the medial prefrontal cortex (mPFC) were unilaterally lesioned with 6-hydroxydopamine (6-OHDA) to examine how dopamine (DA) asymmetry in the mPFC influences voluntary ethanol consumption. Differences in nucleus accumbens (NAS) DA neurotransmission have been related to individual differences in locomotor activity and in the rewarding efficacy of ethanol. Therefore, differences in locomotor activity were used to further characterize the effects of unilateral mPFC 6-OHDA lesions on ethanol consumption. Male Long Evans rats were assessed for high versus low levels of spontaneous locomotor activity. DA terminals in the left or right mPFC were unilaterally lesioned with 6-OHDA, resulting in an average DA depletion of 54% and 50%, respectively. After a minimum seven-day recovery period, preference for a 10% ethanol solution vs. water was determined in a 24-h 2-bottle home-cage free-choice paradigm. Left mPFC 6-OHDA lesions increased and right lesions decreased ethanol consumption. These differential effects of left and right lesions were primarily attributable to rats exhibiting low locomotor activity prior to surgery. The present data suggest that right greater than left cortical DA asymmetry in combination with low endogenous NAS DA (predicted by low locomotor activity levels) may increase the vulnerability to abuse ethanol.     

An ontogenic study on the effect of catecholamine receptor-stimulating agents on the turnover of noradrenaline and dopamine in the brain

The ontogeny of catecholamine-containing neurons and receptors in the brain sensitive to catecholamines was studied by examining (1) the decrease of dopamine (DA) and noradrenaline (NA) following synthesis inhibition, and (2) the ability of catecholamine receptor-stimulating agents to attenuate the decrease. Analysis was performed in rats at 4, 14 and 28 days postnatal age. NA and DA synthesis was inhibited by α-methyl-p-tyrosine methyl ester (MT), and apomorphine and clonidine were utilized as DA and NA receptor-stimulating agents respectively, and were injected 30 min prior to administration of MT. The brains were removed 2 h after MT and dissected into 4 regions: cortex, neostriatum, diencephalon, and midbrain-brain stem. The data obtained suggested that functional NA-containing neurons are present in the cortex and brain stem by 4 days of age, with feedback control over turnover of NA from effective postsynaptic receptors developing concurrently. Functional NA-containing neurons and receptors were not apparent in the diencephalon, according to criteria used in the present study, until 28 days. Functional DA-containing neurons appeared present in the striatum and diencephalon by 4 days of age, but ontogeny of the DA receptors and/or feedback control from effective postsynaptic receptors appeared to vary. These results were interpreted to suggest that the numerous mechanisms controlling the release of DA may be developing at different times. Additionally, clonidine, an NA receptor-stimulating agent, markedly reduced the rate of decrease of DA following synthesis inhibition in the striatum and diencephalon, particularly at 28 days of age. The possibility of an inhibitory input from NA onto DA neurons was considered.     

Striatal trophic activity is reduced in the aged rat brain

Our previous studies demonstrated that the survival of a mesencephalic graft was reduced in aged animals suggesting an age-related decline in target-derived neurotrophic activity. We tested this hypothesis by examining dopamine (DA) and trophic activities from the striatum of intact or unilateral 6-hydroxydopamine (6-OHDA) lesioned rats of increasing age. Fisher 344 rats were 4, 12, 18, and 23 months old (m.o.) at sacrifice. Half the animals had received unilateral 6-OHDA lesions of the mesostriatal DA pathway 8 weeks earlier. Striatal tissue punches were analyzed for DA, homovanillic acid (HVA), and DA activity (HVA/DA) using HPLC. The remainder of the striatal tissue was homogenized to generate tissue extracts which were added to E14.5 ventral mesencephalic cultures to test trophic activity. In the non-lesioned animals, striatal DA was reduced and striatal DA activity was increased in the 18 and 23 m.o. animals relative to the 4 and 12 m.o. animals. Striatal trophic activity was inversely related to age. In the lesioned animals, striatal DA ipsilateral to 6-OHDA infusion was below detection limits while the contralateral striatum exhibited age-related changes in DA similar to those seen in the non-lesioned animals. In 4 m.o. lesioned rats, striatal trophic activity ipsilateral to 6-OHDA infusion was elevated by 26% relative to the contralateral side. The ipsi/contra-lateral differences in striatal trophic activity were reduced in 12 m.o. animals and absent in the 18 and 23 m.o. groups. These data suggest that advancing age is associated with a reduction in striatal DA as well as trophic activity. Moreover, the aged striatum loses its ability to biochemically and trophically compensate for DA reduction and therefore may represent a more challenging environment for the survival, growth, and function of a fetal graft.