Residual Dopamine Research Articles

A rationale for monoamine oxidase inhibition as neuroprotective therapy for Parkinson's disease.

Neurons in the substantia nigra may be vulnerable to oxidant stress because (a) the metabolism of dopamine generates peroxides, which, in the presence of iron, can lead to the formation of the highly reactive hydroxyl free radical; and (b) neuromelanin within nigral neurons can bind metals such as iron and aluminum and thereby promote the site-specific formation of free radicals. Postmortem studies show increased iron, decreased glutathione, and increased lipid peroxidation in the substantia nigra of patients with Parkinson's disease (PD). Recent studies also report iron and aluminum accumulation within neuromelanin granules of patients with PD. These findings suggest that the substantia nigra in the patient with PD is in a state of oxidant stress and that antioxidant therapy might protect residual dopamine neurons and slow the natural progression of PD. Selective inhibitors of monoamine oxidase type B (MAO-B) have been chosen for study because of their capacity to interfere with the oxidative metabolism of dopamine and so diminish the likelihood that free radicals will be formed. Initial studies demonstrate that the MAO-B inhibitor L-deprenyl (selegiline) delays the development of disability in otherwise untreated patients with early Parkinson's disease. Although the mechanism responsible for these observations remains unclear, these results are consistent with the possibility that L-deprenyl provides neuroprotective effects.

Ultrastructure of spared dopamine terminals in caudate-putamen nuclei of adult rats neonatally treated with intranigral 6-hydroxydopamine

Residual dopamine terminals in the dorsal striatum, caudate-putamen nuclei (CPN), of adult rats neonatally lesioned with 6-hydroxydopamine (6-OHDA) sustain a relatively high level of dopamine release. We examined whether there were morphological differences in the spared dopamine terminals that might correlate with this increased efficacy. Postnatal male rat pups from 50 litters were pretreated with desmethylimipramine (DMI) to protect from non-specific monoamine damage, then given unilateral intranigral injections of 6-OHDA or vehicle. Coronal sections through the CPN and substantia nigra of the surviving adult animals from each litter were co-processed for immunoautoradiographic or immunoperoxidase localization of the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). Quantitative ultrastructural analysis established that in animals showing maximal (greater than 90%) depletions in immunoautoradiographic labeling for TH, the number of TH-labeled axons in the CPN ipsilateral to the 6-OHDA injections was reduced to one third of the number seen in the contralateral, unlesioned hemisphere, or the CPN from vehicle-injected animals. The ultrastructural features of residual terminals ipsilateral to 6-OHDA lesions were morphologically similar to those of the contralateral side or in vehicle-injected animals. However, in comparison with controls, these TH-labeled terminals had significantly larger mean cross-sectional diameters. When subdivided into groups according to size, there were significantly fewer small (0.0–0.1 μm 2) and more large (0.41–0.50 μm 2) TH-immunoreactive profiles in lesioned versus unlesioned CPN. The remaining TH-labeled terminals ipsilateral to the 6-OHDA lesions also appeared to be more often in direct contact with unlabeled soma and proximal dendrites as opposed to dendritic spines in the unlesioned CPN. These results suggest that the enhanced activity of dopamine neurons innervating the CPN after nigral 6-OHDA lesions may contribute to changes in size and target of their terminals. Alternatively, the observed large size of remaining dopamine terminals may reflect selective vulnerability of smaller axons to 6-OHDA toxicity.

Partial dopamine depletions result in an enhanced sensitivity of residual dopamine neurons to apomorphine

Extracellular recordings from identified dopamine neurons were used to assess the effect of 6-hydroxydopamine-induced partial lesions of the nigrostriatal dopamine system on the sensitivity of the residual dopamine neurons to the dopamine agonist apomorphine. This was done by testing the response of identified nigral dopamine neurons in control and lesioned rats to systemic apomorphine administration at two time points: 1) 6-10 days post-lesion, when the loss of dopamine cells is nearly complete, and 2) 4-8 weeks post-lesion, which should be sufficient time for changes in dopamine receptor density to occur. As reported previously, dopamine neurons in control rats were inhibited by systemic administration of apomorphine, with their sensitivity being inversely related to their initial firing rate. The sensitivity of the residual dopamine neurons to apomorphine was unaltered in rats tested 6-10 days after depletions of at least 60% of striatal dopamine. However, by 4-8 weeks post-lesion, there was a significant increase in the sensitivity to apomorphine; furthermore, sensitivity was no longer related to baseline firing rate, but instead was uniformly high in all dopamine neurons tested at this time. This enhanced sensitivity was not altered by hemisection of the striatonigral projection, suggesting that the increased sensitivity to apomorphine was most likely a result of a time-dependent up-regulation of somatodendritic autoreceptors on the residual dopamine neurons.

Differential Mechanisms of Dopamine and Somatostatin Inhibition of Prolactin Secretion from Anterior Pituitary Cells

Abstract The relative potencies of dopamine and somatostatin to inhibit prolactin secretion by pituitary cells in primary culture were compared. Hormone secretion was evaluated under basal conditions as well as after challenging it with thyrotropin-releasing hormone, vasoactive intestinal peptide or with drugs affecting either the activity of adenylate cyclase (forskolin), and protein kinase C (phorbol 12 myristate 13 acetate), or eliciting Ca(2) (+) fluxes in the cell by various ways (A23187, a Ca(2+) ionophore, the dihydropyridine agonist BAY-K-8644, or K(+) depolarization which activates voltage-sensitive Ca(2+) channels). In order to test whether all effects of dopamine and somatostatin were mediated by inhibitory guanosine triphosphate binding proteins, the experiments were systematically carried out in the presence or absence of pertussis toxin, an agent which selectively uncouples given subsets of G proteins from corresponding receptors. Dopamine markedly inhibited basal as well as thyrotropin-releasing hormone-, vasoactive intestinal peptide-, forskolin- and BAY-K-8644-stimulated release of prolactin. In contrast, dopamine was only able to induce partial inhibition of hormone release when secretion was triggered by tumour-promoting activator, A23187 or K(+) depolarization. Under all conditions tested, inhibition by somatostatin was significant, but of limited amplitude. Pertussis toxin completely reversed the effects of somatostatin. In contrast, complete reversal of dopamine effects by pertussis toxin was only achieved after hormone stimulation by tumour-promoting activator, alone or with A23187. Under all other conditions a residual dopamine inhibition was maintained in the presence of the toxin. The amplitude of this residual toxin resistant inhibition was comparable in all other cases to that observed for unstimulated (basal) prolactin release. It is concluded that: 1) As expected, dopamine is a potent inhibitor of secretory processes involving cyclic AMP accumulation or voltage-sensitive Ca(2+) channel activation. In contrast, the amine is only a partial inhibitor of exocytosis resulting from non-voltage-sensitive Ca(2+) channel-gated increase in Ca(2+) or direct activation of protein kinase C. 2) Somatostatin is a partial inhibitor of prolactin under all conditions tested. Dopamine and all somatostatin effects are mediated by pertussis toxin-sensitive G proteins. However, a small, but significant, proportion of dopamine inhibition is resistant to pertussis toxin and can thus be assumed to involve a distinct mode of action. This alternate mechanism of dopamine inhibition operates under all conditions except after treatment with tumour-promoting activator, suggesting that it can be inactivated by protein kinase C stimulation.

D 1 and D 2 receptor contributions to ingestive and locomotor behavior are altered after dopamine depletions in neonatal rats

The effects of selective D 1 and D 2 antagonists on ingestive and locomotor behavior were studied in adult rats depleted of dopamine (DA) as neonates and their vehicle-treated controls. Blockade of either D 1 and D 2 receptors inhibited food intake and produced akinesia in control animals. Simultaneous administration of subthreshold doses of these antagonists produced similar behavioral deficits, indicating a synergism between the two receptor subtypes. In contrast, adults depleted of DA as neonates were unaffected by either D 1 or D 2 blockade. However, combined administration of these antagonists produced behavioral deficits comparable to those seen in controls. These data demonstrate that while activity within residual DA neurons remains critical for the expression of ingestive and locomotor behavior in rats depleted of DA as neonates, the precise nature of the interactions between the D 1 and D 2 receptor subtypes is altered after the depletion.

Compensatory changes in striatal dopamine neurons following recovery from injury induced by 6-OHDA or methamphetamine: A review of evidence from microdialysis studies.

This paper presents evidence from microdialysis experiments that three different behavioural outcomes of depletion of striatal dopamine (DA) by either 6-hydroxydopamine or methamphetamine (sparing of function, recovery of function, and loss of function) may be related to differences in the ability of residual DA neurons to maintain extracellular concentrations of DA. It is shown that when up to 80% of the mesostriatal DA system is destroyed, the remaining DA terminals maintain normal extracellular dopamine concentrations. Following a lesion in the 80-95% range, most animals maintain a relatively normal extracellular concentration of DA, but the ability to respond to increased demand is reduced in some animals. When over 95% of the normal DA input to the striatum is destroyed, there is a sharp drop in the extracellular concentration of DA and a nearly complete loss in the ability to increase DA release upon demand. Thus, this new method of sampling extracellular DA and its metabolites in freely moving animals provides insights into the behavioural capacities of animals and into the neural mechanisms that underlie recovery of function following brain damage.

Food intake of rats depleted of dopamine as neonates is impaired by inhibition of catecholamine biosynthesis

The food intake of rats depleted of brain dopamine (DA) as neonates was markedly reduced by doses of the catecholamine synthesis inhibitor, α-methyl- p-tyrosine (AMT), that had little effect on control animals. In contrast, rats depleted of DA as neonates were subsensitive to doses of the DA antagonist, haloperidol, that greatly inhibited food intake in controls. Unexpectedly, low doses of haloperidol potentiated the effects of AMT in both control and DA-depleted animals. These results suggest that activity within residual DA neurons may remain important for the expression of ingestive behavior in rats depleted of DA as neonates although the precise mechanisms may differ from those operating in normal animals

Tyrosine Hydroxylase Content of Residual Striatal Dopamine Nerve Terminals Following 6‐Hydroxydopamine Administration: A Flow Cytometric Study

Fluorescence-activated cell sorting based on immunolabeling with a monoclonal antibody to tyrosine hydroxylase and a fluorescein-conjugated secondary antibody was used to identify striatal synaptosomes derived from nigrostriatal dopamine nerve terminals. The amount of tyrosine hydroxylase immunoreactivity in dopaminergic striatal synaptosomes prepared from control rats was compared to the amount in dopaminergic synaptosomes prepared from rats that had received intraventricular injections of 6-hydroxydopamine. Although the absolute number of dopaminergic synaptosomes was decreased in lesioned animals, those residual dopamine terminals present contained more tyrosine hydroxylase than did dopamine terminals from control rats. Both the decrease in the absolute number of dopamine terminals and the increase in tyrosine hydroxylase immunoreactivity in residual terminals were proportional to the extent of the lesion, as determined by measurement of striatal dopamine levels. These results suggest that an increase in the amount of tyrosine hydroxylase protein in residual terminals may represent one compensatory mechanism by which residual dopamine neurons maintain normal striatal function after partial destruction of the nigrostriatal dopamine projection.

Compensatory responses to nigrostriatal bundle injury

Intracerebral injections of the neurotoxin 6-hydroxydopamine (6-HDA) can produce selective, near-total destruction of the dopamine (DA)-containing neurons of the nigrostriatal bundle. The dysfunctions in animals with these lesions show many parallels with those present in Parkinsonian patients. Among these are the extensive loss of DA neurons in the basal ganglia, neurological impairments including akinesia, paradoxical kinesia in response to activating conditions, and improved sensory-motor function after the administration of DOPA. Moreover, as with patients with preclinical Parkinsonism, 6-HDA-treated rats with less extensive lesions show few or no behavioral dysfunctions, but are unusually sensitive to the akinesia-inducing effects of stress and dopaminergic antagonists. In this review, we summarize the behavioral effects of 6-HDA-induced depletion of striatal DA in the rat and then focus on the compensatory changes that may underlie the preclinical stage of the disorder. These compensations appear to include an increase in the number of active DA neurons, an increase in the release of DA per impulse from residual terminals, and a decrease in the amount of DA inactivated by high affinity uptake. Collectively, these alterations permit a few residual DA neurons to maintain a normal level of control over cellular activity in the striatum.

Drinking behavior and motor function in rat pups depleted of brain dopamine during development.

The ability of rat pups depleted of brain dopamine (DA) at either 3 or 15 days of age to increase fluid intake in response to intracellular dehydration was studied. Despite near-total depletions of striatal DA, animals depleted at either age ingested normal amounts of fluid even as soon as 3-7 days after incurring the brain damage. The effects of DA receptor blockade on ingestion and motor function were also studied. Haloperidol had markedly different effects as a function of the age at which the depletions were sustained. Weanlings that were depleted of DA at 3 days of age failed to exhibit the drug-induced adipsia, akinesia, and catalepsy seen in controls and animals that were depleted at 15 days of age. These findings suggest that the DA depletions were performed at different periods of brain organization. The resultant neural controls of ingestion and motor function are qualitatively different in the two groups of depleted animals. Rats depleted at 3 days of age utilize nondopaminergic mechanisms, whereas animals lesioned at 15 days of age continue to rely upon residual DA neurons for both behaviors.

Increased dopamine release from striata of rats after unilateral nigrostriatal bundle damage

Dopaminergic control of striatal neurons is retained in rats sustaining lesions of the nigrostriatal bundle (NSB) as long as 10% of the projection remains, suggesting that enhanced efficiency of dopamine (DA) transmission may compensate for the denervation of the striatum. To examine this hypothesis we have studied the extracellular concentration of striatal DA using brain dialysis. In control rats, haloperidol (1 mg/kg, i.p.) or depolarization of striatal tissue with 25 mM KCl increased, and γ-butyrolactone (500 mg/kg, i.p.) decreased DA and homovanillic acid (HVA) levels in striatal dialysates. Three weeks after unilateral injection of 6-hydroxydopamine (6-OHDA) to substantia nigra, DA content in the ipsilateral was decreased by 60–98%. Nevertheless, extracellular DA concentration in the lesioned striata remained unchanged in rats with 60–90% DA depletions. More extensive lesions (96% DA depletion) were accompanied by 60% reduction in DA release. In contrast, extracellular HVA levels in the lesioned striata decreased proportionally to the depletion of tissue DA, indicating decreased inactivation of extracellular DA. We propose that the capacity of the residual DA terminals to maintain normal levels of extracellular DA after 60–90% NSB lesions may serve to compensate for the partial denervation of the striatal tissue. Disruption of striatal DA functions and postsynaptic supersensitivity after more extensive lesions may be associated with the failure of the NSB to fully compensate for loss of DA terminals. In striata contralateral to the 6-OHDA lesion, increased DA release was also observed. In addition, 60–90% ipsilateral DA depletions were accompanied by 32% and 42% increases in DA and HVA content in contralateral tissue, respectively. The possibility of the contralateral sprouting of DA terminals is discussed.

The chemical development of selective and specific serotonin S2‐antagonists

AbstractFour types of chemical intermediates were prepared to synthesize novel piperidine derivatives inspired by the butyrophenones benperidol and lenperone and by the diphenylbutyl neuroleptic pimozide. The fist type, comprising benzimidazolone alkyl intermediates, yielded declenperone and milenperone and also the “symmetrical” compound domperidone, which was pharmacologically unusual by its specific peripheral dopamine antagonism. Declenperone was the most potent serotonin antagonist of that series, and replacement of the benzimidazolinonepropyl by the quinazolinedioneethyl moiety led to ketanserin, which was devoid of residual dopamine antagonism. Very potent serotonin S2‐antagonists were also obtained by using intermediates prepared from 2‐aminoaza‐heterocycles. Pirenperone and setoperone had a complex pharmacological profile, including significant dopamine and norepinephrine antagonism. These activity components were absent from the profile of R 56413 and ritanserin, which were obtained by using the fourth intermediate, benzhydrylenepiperidine. R 56413 and ritanserin are the most specific of the presently known serotonin S2‐antagonists.

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.