Dopamine Terminals Research Articles

Loud noise enhances nigrostriatal dopamine toxicity induced by MDMA in mice.

The neurotoxicity of 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) has been intensely investigated due to the widespread abuse of this drug and its neurotoxic effects. In mice, MDMA neurotoxicity has been demonstrated for striatal dopamine (DA) terminals. However, the current literature has reported great variability in the effects induced by MDMA; this is partially due to changes in environmental conditions. For instance, elevated temperature and a crowded noisy environment markedly increase the neurotoxic effects induced by MDMA. The environmental factor loud noise is often present during ecstasy intake; however, only a few studies have analysed the consequence of a concomitant exposure to loud noise and ecstasy intake. In the present experimental work, we investigated whether nigrostriatal DA toxicity occurring after MDMA administration was potentiated in the presence of loud noise (100 dBA). We administered MDMA to C57/Black mice using a "binging" pattern for two durations of white noise exposure. We found a marked enhancement of MDMA toxicity (7.5 mg/Kg x4, 2 hours apart, i.p.) in the presence of white noise exposure lasting for at least 6 hours. The striatal damage was assessed by assaying DA levels as well as the loss of tyrosine hydroxylase (TH) and the increase in striatal glial fibrillary acidic protein (GFAP) immunohistochemistry. Since loud noise often accompanies ecstasy intake, the present findings call for more in-depth studies aimed at disclosing the fine mechanisms underlying this enhancement.

PCB-induced inhibition of the vesicular monoamine transporter predicts reductions in synaptosomal dopamine content.

Both Aroclor mixtures and individual non-coplanar polychlorinated biphenyl (PCB) congeners reduce dopamine (DA) concentrations in cells in culture and in the brains of developing and adult laboratory animals. These reductions may involve inhibition of the dopamine transporter (DAT) and the vesicular monoamine transporter (VMAT) responsible, respectively, for the uptake of extracellular DA and the packaging of nerve terminal cytosolic DA into synaptic vesicles. However, the relative contribution of each monoamine transporter to the PCB-induced reductions in tissue DA has not been determined. Accordingly, we exposed striatal synaptosomes from adult rats to individual PCB congeners, a commercial mixture of PCBs or known monoamine transporter inhibitors; measured synaptosomal DA; and related these changes to media DA and concentrations of 3,4-dihydroxyphenylacetic (DOPAC). PCB-induced elevations in media DA concentrations are not sufficient to explain the reductions in tissue DA because known DAT inhibitors elevate media DA to a much greater extent than PCBs and yet induce similar decreases in tissue DA concentrations. On the other hand, PCB-induced elevations in DOPAC, reflective of increases in nerve terminal cytosolic DA, are sufficient to explain the reductions in tissue DA, because a known VMAT inhibitor elevates DOPAC and reduces tissue DA to an extent similar to that seen with PCBs. Taken together, these results suggest that elevations in DOPAC, reflective of increases in nerve terminal cytosolic DA due to VMAT inhibition, rather than elevations in media DA due to DAT inhibition, are largely responsible for the observed decreases in tissue DA content.

Histological evidence supporting a role for the striatal neurokinin-1 receptor in methamphetamine-induced neurotoxicity in the mouse brain

Several studies have documented the effect of methamphetamine (METH) on the toxicity of the dopamine (DA) terminals of the striatum but only a few studies have assessed the damaging effects of METH on striatal neurons postsynaptic to the nigrostriatal DA terminals. In the present study, we employed histological methods to study the effect of METH on DA terminals and striatal neurons. We also assessed the role of the striatal neurokinin-1 (NK-1) receptor on pre- and post-synaptic METH-induced damage. Male mice were treated with METH (10 mg/kg) four times at 2-h intervals and were sacrificed 3 days after the treatment. A number of animals received the non-peptide NK-1 receptor antagonist WIN-51,708 (10 mg/kg) 30 min before the first and fourth injections of METH. Immunocytochemical staining for tyrosine hydroxylase (TH) showed significant deficits throughout all aspects of the caudate-putamen in animals exposed to METH. Pretreatment with WIN-51,708 prevented the METH-induced loss of TH immunostaining. Sections from a separate set of mice were stained with Fluoro-Jade B (FJB), a fluorochrome that binds specifically to degenerating fibers and cell bodies of neurons. Treatment with METH shows Fluoro-Jade B positive cell bodies in the striatum and pretreatment with WIN-51,708 abolished Fluoro-Jade B staining. Moreover, double labeling with Fluoro-Jade B and glial fibrillary acidic protein (GFAP) shows reactive astrocytosis in the area adjacent to the Fluoro-Jade B-positive cells but no Fluoro-Jade B staining of the astrocytes. This observation suggests that the degenerating cells must be striatal neurons and not astrocytes. The data demonstrate that METH induces pre- and post-synaptic damage in the striatum and the damage can be prevented with pharmacological blockade of the NK-1 receptor. These findings represent a new direction in the study of the mechanism of toxicity to METH and could be useful in the treatment of some neurological disorders.

In vivo monitoring of dopamine overflow in the central nervous system by amperometric techniques combined with carbon fibre electrodes

Electrochemical techniques are extensively used to investigate in vivo and in vitro events associated with neurotransmission, particularly dopamine (DA) transmission. In vivo amperometric measurements only concern evoked extracellular neurochemical events over short time periods independently of changes in basal release. In this context, DA release is evoked either by brief and low electrical or by chemical stimulation, which, respectively, mimics or elicits the physiological discharge rates of DA neurones. The combination of electrochemically treated carbon fibre electrodes with differential pulse amperometry (DPA) has been extensively used to monitor DA overflow on a 1-s time scale. The more recent in vivo combination of untreated carbon fibre electrodes with continuous amperometry gives better temporal information allowing the precise description of the kinetic parameters of the mechanisms which regulate DA overflow. The results obtained in rats extended by the results obtained in mice lacking a protein involved in DA transmission demonstrate that DA autoregulation and DA uptake: (i) contributes to the operational properties of DA terminals in converting action potentials into DA release as a high pass filter which favours short bursts of action potentials and (ii) inhibits excessive DA release which might result from prolonged and large increases in the impulse flow.

Partial recovery of brain metabolism in methamphetamine abusers after protracted abstinence.

Methamphetamine is a highly addictive drug of abuse that is neurotoxic to dopamine terminals. The authors recently reported that decreases in dopamine transporters (used as markers of dopamine terminals) in the striatum of methamphetamine abusers recover with protracted abstinence and that relative to comparison subjects, recently detoxified methamphetamine abusers have lower metabolism in the striatum and thalamus. In this study, the authors assessed whether metabolism recovers with protracted abstinence. Brain glucose metabolism was measured with positron emission tomography and [18F]fluorodeoxyglucose in five methamphetamine abusers who were evaluated after both a short (<6 months) and protracted (12-17 months) abstinence interval, eight methamphetamine abusers tested only after protracted abstinence, and 11 comparison subjects who were not drug users. Significantly greater thalamic, but not striatal, metabolism was seen following protracted abstinence relative to metabolism assessed after a short abstinence interval, and this increase was associated with improved performance in motor and verbal memory tests. Relative to the comparison subjects, the methamphetamine abusers tested after protracted abstinence had lower metabolism in the striatum (most accentuated in the caudate and nucleus accumbens) but not in the thalamus. The persistent decreases in striatal metabolism in methamphetamine abusers could reflect long-lasting changes in dopamine cell activity, and decreases in the nucleus accumbens could account for the persistence of amotivation and anhedonia in detoxified methamphetamine abusers. The recovery of thalamic metabolism could reflect adaptation responses to compensate for the dopamine deficits, and the associated improvement in neuropsychological performance further indicates its functional significance. These results suggest that while protracted abstinence may reverse some of the methamphetamine-induced alterations in brain function, other deficits persist.

Selective COX-2 inhibition prevents progressive dopamine neuron degeneration in a rat model of Parkinson's disease

Several lines of evidence point to a significant role of neuroinflammation in Parkinson's disease (PD) and other neurodegenerative disorders. In the present study we examined the protective effect of celecoxib, a selective inhibitor of the inducible form of cyclooxygenase (COX-2), on dopamine (DA) cell loss in a rat model of PD. We used the intrastriatal administration of 6-hydroxydopamine (6-OHDA) that induces a retrograde neuronal damage and death, which progresses over weeks. Animals were randomized to receive celecoxib (20 mg/kg/day) or vehicle starting 1 hour before the intrastriatal administration of 6-OHDA. Evaluation was performed in vivo using micro PET and selective radiotracers for DA terminals and microglia. Post mortem analysis included stereological quantification of tyrosine hydroxylase, astrocytes and microglia. 12 days after the 6-OHDA lesion there were no differences in DA cell or fiber loss between groups, although the microglial cell density and activation was markedly reduced in animals receiving celecoxib (p < 0.01). COX-2 inhibition did not reduce the typical astroglial response in the striatum at any stage. Between 12 and 21 days, there was a significant progression of DA cell loss in the vehicle group (from 40 to 65%) that was prevented by celecoxib. Therefore, inhibition of COX-2 by celecoxib appears to be able, either directly or through inhibition of microglia activation to prevent or slow down DA cell degeneration.

Selective recovery of striatal 125I-α-conotoxinmii nicotinic receptors after nigrostriatal damage in monkeys

Evidence suggests that nicotinic receptors play a role in nigrostriatal function, a finding that may be relevant to Parkinson's disease. Knowledge of the conditions that regulate nicotinic receptor expression is therefore important. Previous studies showed that several different nicotinic receptors, including α-conotoxinMII (α-CtxMII)-sensitive receptors, are decreased after nigrostriatal damage. Nigrostriatal dopaminergic terminals also demonstrate a capacity for recovery after lesioning. The present experiments were therefore done to determine whether there were changes in striatal nicotinic receptors with recovery. To address this, we used two well-characterized animal models of nigrostriatal damage produced using the selective dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Studies in mice showed that striatal 125I-α-CtxMII, as well as 125I-epibatidine and 125I-A85380 binding sites significantly recovered 1 month after lesioning, suggesting that α6* and most likely α4* receptors are increased. Experiments were next done in monkeys since striatal 125I-α-CtxMII receptors constitute a large percentage of nicotinic receptors and are more vulnerable to nigrostriatal damage in this model that closely mirrors Parkinson's disease. In monkeys allowed to recover from the toxic effects of MPTP for a 1–2 year period, there was a significant improvement in the Parkinson disability score. There was also a reversal in lesion-induced declines in striatal α-CtxMII-sensitive receptors, but no significant change in 125I-epibatidine and 125I-A85380 receptors. These findings suggest that α3*/α6* sites are selectively increased in monkey striatum with recovery. The present data show that recovery of 125I-α-CtxMII receptors occurs in parallel with the dopamine transporter, indicating that these nicotinic receptors sites are localized to presynaptic dopamine terminals in both species.

Infralimbic D1 receptor agonist effects on spontaneous novelty exploration and anxiety-like defensive responding in CD-1 mice

Mesocortical dopamine (DA) terminals in the ventromedial prefrontal cortex (vmPFC) integrate cognitive/emotional processing functions underlying adaptive and appropriate behavioral responding to stressful environmental events. Results from several studies have also shown that stressor-enhanced prefrontal DA activation exerts detrimental effects on cognitive performance. However, questions have arisen as to whether stressor-enhanced vmPFC DA transmission exerts direct control over conditioned or unconditioned responses to threatening events, or whether enhanced prefrontal DA transmission gates cognitive processing to facilitate adaptive responding in threatening situations. We have previously shown that infralimbic (IL) vmPFC dopamine D2 agonist and antagonist drug infusions reduced anxiety-like responding in the elevated plus-maze (EPM) and disrupted spontaneous exploration in the Y-maze in CD-1 mice. In the present study, the effects of IL vmPFC infusions of the specific D1 receptor agonist, SKF-81297, in CD-1 mice were evaluated on spontaneous exploration in the Y-maze, anxiety-like responding in a 2-trial elevated plus-maze procedure, and anti-predator defensive responding in the Mouse Defense Test Battery (MDTB). SKF-81297 infusions disrupted spontaneous alternation performance along with potentiated repetitive 2-arm responding in the Y-maze. In the elevated plus-maze, pre-trial 1 IL SKF-81297 infusions reduced anxiety-like responding (enhanced open arm entries and time ratio, unprotected stretch attends and head dips), and reduced closed arm time ratio and protected risk assessment activity (protected stretch attends). In trial 2, 24 h later (no drug infusions), open arm entries, open time ratio, and unprotected head dips remained enhanced relative to trial 2 vehicle controls. In the MDTB, avoidance distance was enhanced in the approach test; risk assessment (approach) was enhanced in the closed alley test; and defensive threat (upright postures) was enhanced in the forced contact test. Results are discussed with respect to possible influences of IL vmPFC DA receptors on cognitively mediated responding to differing levels of threat in mice.

Chronic intoxication with 3-nitropropionic acid in rats induces the loss of striatal dopamine terminals without affecting nigral cell viability

3-Nitropropionic acid (3NP) is a succinate dehydrogenase inhibitor allowing the generation of animal models of Huntington's disease. In the present study, we found that a 5-day continuous chronic infusion of 3NP produces loss of [ 3H]mazindol binding and tyrosine hydroxylase (TH) immunoreactivity in the striatal area of degeneration. This loss of dopamine terminals was not due to a loss of nigral neurons since the expression of TH as well as the number of TH-expressing neurons remained unaltered in the substantia nigra of rats treated by 3NP. This suggests that the 3NP-induced dopamine terminal loss is secondarily related to the striatal degeneration andlor to a direct effect of 3NP on striatal terminals and not to a primary effect on nigral cells.

Effect of amphetamine, cocaine and depolarization by high potassium on extracellular dopamine in the nucleus accumbens shell of SHR rats. An in vivo microdyalisis study

Spontaneously hypertensive rats (SHR) exhibit behavioural abnormalities (hyperactivity and hyper reactivity to stress) that resemble the behavioural abnormalities of human attention-deficit with hyperactivity disorder (ADHD). Because dopamine has been implicated in ADHD we studied by in vivo microdialysis the dopamine output in the nucleus accumbens (NAc) shell of 6 week-old (pre-hypertensive stage) SHR rats and in their normotensive age matched Wistar Kyoto controls (WKY). We observed that SHR rats had significant higher basal dialysate dopamine concentrations (about 20%) than WKY. Systemic administration of amphetamine (0.25 and 0.5 mg/kg s.c.), and methylphenidate (1 and 2 mg/kg i.p.) produced an higher increase in dialysate dopamine in the NAc shell of SHR rats as compared with WKY rats, although only after the administration of the lowest dose of amphetamine and methylphenidate this difference was found to be significant. In contrast when the microdialysis fiber was perfused by 30 or 60 mM K +, a lower increase of dialysate dopamine was observed in SHR rats as compared with WKY rats. These apparently contradictory results can be explained by postulating that SHR rats have a higher tone of NAc shell dopamine transmission and synthesis associated with a lower storage capacity of vesicles in dopamine terminals of the same area.

Nicotinic receptor modulation of dopamine transporter function in rat striatum and medial prefrontal cortex.

Nicotine activates nicotinic acetylcholine receptors (nAChRs) on dopamine (DA) terminals to evoke DA release, which subsequently is taken back up into the terminal via the DA transporter (DAT). nAChRs may modulate DAT function thereby contributing to the regulation of synaptic DA concentrations. The present study determined the dose-response for nicotine (0.1-0.8 mg/kg, s.c.) to modulate DA clearance in striatum and medial prefrontal cortex (mPFC) using in vivo voltammetry in urethane anesthetized rats and determined if this effect was mediated by nAChRs. Exogenous DA (200 microM) was pressure-ejected at 5-min intervals until reproducible baseline signals were obtained. Subsequently, nicotine or saline was administered, and DA pressure ejection continued at 5-min intervals for 60 min. In both striatum and mPFC, signal amplitude decreased by approximately 20% across the 60-min session in saline-injected rats. A monophasic dose-response curve was found in striatum, with a maximal 50% decrease in signal amplitude after 0.8 mg/kg. In contrast, a U-shaped dose-response curve was found in mPFC, with a maximal 50% decrease in signal amplitude after 0.4 mg/kg. Onset of nicotine response occurred 10 to 15 min after injection in both brain regions; however, the amount of time before maximal response was 45 and 30 min in striatum and mPFC, respectively. Mecamylamine (1.5 mg/kg) completely inhibited the nicotine-induced (0.8 and 0.4 mg/kg) decrease in signal amplitude in striatum and mPFC, respectively, indicating mediation by nAChRs. Thus, nicotine enhances DA clearance in striatum and mPFC in a mecamylamine-sensitive manner, indicating that nAChRs modulate DAT function in these brain regions.

Ultrastructural localization of the norepinephrine transporter in superficial and deep layers of the rat prelimbic prefrontal cortex and its spatial relationship to probable dopamine terminals.

The prefrontal cortex (PFC) is a likely site of action for the therapeutic efficacy of antidepressants that inhibit norepinephrine (NE) reuptake. Moreover, drugs that block the NE transporter (NET) increase extracellular levels of both NE and dopamine (DA), an interaction that may contribute to their therapeutic properties. To examine the subcellular localization of NET and to investigate the spatial relationships between presumed NE and DA axons within the rat prelimbic PFC, we combined immunogold-silver localization of NET with immunoperoxidase staining for the catecholamine synthetic enzyme tyrosine hydroxylase (TH). An additional aim was to quantify the proportion of profiles dually labeled for NET and TH to test the common observation that TH immunolabeling is relatively selective for DA axons. NET-immunoreactive (NET-ir) axonal profiles were typically unmyelinated and occasionally were observed to form symmetric axodendritic synapses. The majority of immunogold NET labeling was unexpectedly observed in the cytoplasm rather than on the plasma membrane. Furthermore, in tissue dually labeled for both NET and TH, only 8-10% of profiles contained both markers. Unlike observations for singly labeled profiles, gold-silver particles for NET in dually labeled axons were localized primarily to the plasmalemma. A systematic survey of terminals labeled only for TH revealed that they were typically separated by at least 1.2 mum from NET-ir varicosities, and the two profile types were not seen to contact common targets. These results suggest that, in the rat PFC, NE axons (1) contain predominantly cytoplasmic NET, (2) infrequently contain TH immunolabeling, and (3) may interact with probable DA afferents by means of extrasynaptic mechanisms.

Chronic ethanol exposure alters presynaptic dopamine function in the striatum of monkeys: a preliminary study.

Current evidence suggests that dopamine (DA) neurotransmission plays a crucial role in the positive reinforcing and stimulating actions of ethanol (Grace, 2000; Weiss and Porrino, 2002; Diana et al., 2003). Acutely administered ethanol excites DA neurons in the ventral tegmental area (Brodie et al., 1999) and in the substantia nigra pars compacta (Mereu et al., 1984), thereby elevating DA release in terminal fields like the nucleus accumbens (Imperato and Di Chiara, 1986). While these effects on cell bodies are well documented, presynaptic DA terminals do not appear to be influenced directly by acutely applied reinforcing doses of ethanol (Yim and Gonzales, 2000; Budygin et al., 2001a,b). Although the acute actions of ethanol on DA neuronal activity appear clear, many questions remain about the consequences of long-term voluntary ethanol ingestion on the dynamics of DA release and reuptake (Weiss and Porrino, 2002; Diana et al., 2003). One of the important factors contributing to the uncertainty concerning DA system changes in response to chronic ethanol exposure is the limitation of animal models of alcoholism. Oral ethanol self-administration of high doses over a substantial period of time can be obtained in mice, rats, and monkeys (Grant, 2000). However, macaque monkeys will voluntarily self-administer ethanol for years and are similar to humans in their physiology, neurobiology, and genetic code. Thus, monkey models of ethanol self-administration can provide a more homologous approach to study alcoholism (Vivian et al., 2001). Recently, we developed a novel monkey model of voluntary excessive ethanol drinking (Vivian et al., 2001) that has allowed us to characterize the effects of long-term ethanol exposure on striatal DA dynamics using fast-scan cyclic voltammetry (FSCV) in vitro. Using this model (Vivian et al., 2001), six monkeys were allowed continuous access to ethanol (4% w/v) and water for 22 h/day for 18 months following an induction procedure. Monkeys in this protocol consumed varying amounts of ethanol. Average daily intakes were 1.3, 2.2, 2.8, 3.1, 3.2, and 3.5 g/kg/day, and the group daily average intake was 2.7 g/kg/day. The mean blood ethanol concentration (BEC) of the drinking monkeys over the 12 months prior to sacrifice (total of 344 measurements) was 95.6 mg% with a minimum of 0 and a maximum of 485 mg%. BECs were taken 8 h following the session start ( 6 PM) every fifth day. Control animals were ethanol-naive monkeys previously used as controls in studies of high fat diets and atherosclerosis (n 3) or behavioral studies (n 2). Since the data obtained from the animals exposed to previous experimental manipulations were not different from results obtained in naive monkeys, all of these data were analyzed together. The control and alcohol-drinking monkeys were matched for age, weight, and sex. Necropsies took place during the normal 2-h time-out period just after the monkeys had 22 h of continuous access to ethanol. The animals were initially anesthetized with ketamine (10 mg/kg, i.m.) and maintained at a deep surgical plane of anesthesia with pentobarbital, according to the Animal Care and Use guidelines of Wake Forest University Health Sciences. They were then transcardially perfused with cold modified Kreb’s buffer containing (in mM): NaCl 126, KCl 3, MgCl2 1.5, CaCl2 2.4, NaH2PO4 1.2, glucose 11, NaHCO3 25.9, and saturated with 95% O2 / 5% CO2. Brains were rapidly

Withdrawal from chronic cocaine up-regulates 5-HT 1B receptors in the rat brain

In the present study we examined the effect of prolonged treatment with cocaine (a sensitization and discrimination paradigm) on the expression of serotonin (5-HT) 1B receptors in rat brain structures using a quantitative autoradiographic analysis. To estimate the distribution of 5-HT 1B receptors in several brain coronal sections, we used [ N-methyl- 3H]GR 125743, a 5-HT 1B/1D receptor antagonist, in the presence of ketanserin (a drug used to block 5-HT 1D receptors). The binding of [ N-methyl- 3H]GR 125743 in the areas containing dopamine cell bodies (the ventral tegmental area, the substantia nigra) and terminals (the nucleus accumbens shell and core, but not in the caudate-putamen) and in the subiculum of the hippocampus was increased after withdrawal from repeated cocaine in both the discrimination and the sensitization paradigms, either being effective as confirmed by behavioral experiments. Neither acute cocaine injection nor the psychostimulant challenge following its repeated administration affected the binding of [ N-methyl- 3H]GR 125743 in the above brain areas. Our results indicate that withdrawal from chronic cocaine induces up-regulation of 5-HT 1B receptors in a number of rat brain structures.

Increased amphetamine-induced locomotion during inactivation of the basolateral amygdala

At low doses, amphetamine has been shown to produce reliable increases in locomotor activity through its actions on the mesolimbic dopamine (DA) terminals in the nucleus accumbens (NAC). The basolateral amygdala (BLA) has recently been reported to have anatomical projections to the NAC, suggesting that it might serve to alter or modulate the function of the NAC. To test this hypothesis, the current experiment produced lidocaine-reversible lesions of the BLA and assessed changes in NAC function by examining alterations in locomotor activity in response to s.c. amphetamine (2 mg/kg). While BLA inactivation alone was found to have no effect on spontaneous or basal locomotor activity, it produced a significant potentiation of amphetamine-induced hyperactivity. These results suggest that BLA inactivation removes a system that inhibits the locomotor response to amphetamine. The data are, therefore, consistent with the view that the BLA may serve to modulate NAC function.

Influence of r-type (Ca v2.3) and t-type (Ca v3.1–3.3) antagonists on nigral somatodendritic dopamine release measured by microdialysis

The release of dopamine from soma and dendrites of dopaminergic neurons in substantia nigra has been reported to be calcium-dependent, but it remains to be determined which calcium channels mediate this effect. We have used in vivo microdialysis in rat substantia nigra and striatum to investigate the effect of Ca v3.1–3.3 (T-type) and Ca v2.3 (R-type) calcium channel antagonists on somatodendritic and terminal dopamine release. Local reverse dialysis administration of 0.1–10 μM of the Ca v2.3 inhibitor SNX-482, or 100 μM of mibefradil, decreased the concentrations of dopamine and its metabolites in dialysate from substantia nigra, whereas 1 μM mibefradil or 40–80 μM nickel(II) induced an increase in nigral dialysate dopamine concentrations. Dopamine concentrations in striatal dialysates were decreased only by 10 μM of SNX-482 or 100 μM of mibefradil. Nickel(II) induced an increase in striatal dialysate dopamine concentration similar to that in substantia nigra. The results indicate a role for Ca v2.3 (R-type) voltage sensitive calcium channels in the calcium dependency of somatodendritic dopamine release, but argue against a calcium dependency mediated substantially by Ca v3.1–3.3 (T-type) channels.

Changes in function and ultrastructure of striatal dopaminergic terminals that regenerate following partial lesions of the SNpc.

Following partial substantia nigra lesions, remaining dopaminergic neurones sprout, returning terminal density in the dorsal striatum to normal by 16 weeks. This suggests regeneration and maintenance of terminal density is regulated to release appropriate levels of dopamine. This study examined the structure and function of these reinnervated terminals, defining characteristics of dopamine uptake and release, density and affinity of the dopamine transporter (DAT) and ultrastructural morphology of dopamine terminals in the reinnervated dorsal striatum. Finally, rotational behaviour of animals in response to amphetamine was examined 4 and 16 weeks after substantia nigra pars compacta (SNpc) lesions. Dopamine transport was markedly reduced 16 weeks after lesioning along with reduced density and affinity of DAT. Rate of dopamine release and peak concentration, measured electrochemically, was similar in lesioned and control animals, while clearance was prolonged after lesioning. Ultrastructurally, terminals after lesioning were morphologically distinct, having increased bouton size, vesicle number and mitochondria, and more proximal contacts on post-synaptic cells. After 4 weeks, tendency to rotate in response to amphetamine was proportional to lesion size. By 16 weeks, rotational behaviour returned to near normal in animals where lesions were less than 70%, although some animals demonstrated unusual rotational patterns at the beginning and end of the amphetamine effect. Together, these changes indicate that sprouted terminals are well compensated for dopamine release but that transport mechanisms are functionally impaired. We discuss these results in terms of implications for dyskinesia and other behavioural states.

Positron‐labeled DOPA analogs to image dopamine terminals

AbstractAltered dopamine function has been implicated in disorders such as Parkinson's disease (PD) and other motor disorders, schizophrenia, substance abuse, attention deficit disorder, and other neuropsychiatric diseases. A useful approach to the development of PET imaging agents for dopamine terminals is to target the enzymes involved in dopamine biosynthesis. Tyrosine hydroxylase (TH) converts dietary L‐p‐tyrosine to L‐3,4‐dihydroxyphenylalanine (L‐DOPA), which is the first and rate‐limiting step in dopamine synthesis. Thus, a TH‐targeted imaging agent would be the ideal PET tracer of dopamine neuronal activity. However, efforts to develop TH‐based tracers have so far been unsuccessful. The next enzyme in dopamine synthesis is aromatic L‐amino acid decarboxylase (AAAD), which converts L‐DOPA to dopamine. AAAD was the first target in the development of imaging agents for dopamine neurons. The first useful AAAD PET tracer was 6‐[18F]Fluoro‐L‐DOPA (FDOPA). Later, a practicable synthesis of L‐[β‐11C]DOPA was developed. However, the adherence of these tracers to all the catabolic pathways of L‐DOPA has been a disadvantage because of the contributions of labeled metabolites to PET images. Thus, alternative tracers with simpler metabolism such as fluoro‐L‐m‐tyrosine (FMT) have been developed. PET studies in nonhuman primates have suggested that FMT is the PET imaging agent of choice for AAAD‐containing neurons. Initial human studies show that this marker is useful in the assessment of AAAD‐related neuropsychiatric disorders. This article discusses the origins and validation of dopamine PET tracers and delineates the similarities and differences between therapeutic drug development and radiopharmaceutical development as exemplified by studies on the dopamine system. Drug Dev. Res. 59:249–260, 2003. © 2003 Wiley‐Liss, Inc.

The dopamine D1 receptor-rich main and paracapsular intercalated nerve cell groups of the rat amygdala: relationship to the dopamine innervation

The intercalated cell masses are GABAergic neurons interposed between the major input and output structures of the amygdala. Dopaminergic projections to the main and paracapsular intercalated islands were examined by determining the relationship of the dopamine nerve-terminal networks to the D1-receptor immunoreactive staining of cells within the intercalated islands, using double-fluorescence immunolabelling procedures in combination with confocal laser microscopy. The relationship of terminals positive for both tyrosine hydroxylase and dopamine β-hydroxylase (noradrenaline and/or adrenaline) to terminals positive for tyrosine hydroxylase but negative for dopamine β-hydroxylase (dopamine terminals) was studied in relation to the D1-receptor immunoreactivity in adjacent sections at various rostrocaudal levels. The microscopy and image analysis revealed that there was only a minor dopaminergic innervation of the D1 receptor-immunoreactive cells in the rostromedial and caudal component of the main intercalated island, suggesting volume transmission as the main communication mode for dopamine in these regions. In contrast, the D1 receptor-immunoreactive areas in the rostrolateral part of the main island and also the paracapsular intercalated islands showed a high degree of dopaminergic innervation, indicating that synaptic and perisynaptic dopamine transmission plays a dominant role in these regions. It is known that amygdala neurons are involved in the elicitation and learning of fear-related behaviors. We suggest that slow dopaminergic volume transmission in the rostromedial and caudal parts of the main intercalated island may have a role in tonic excitatory modulation in these parts of the main island, allowing GABAergic activity to develop in the central amygdaloid nucleus and thereby contributing to inhibition of fear-related behavioral and autonomic responses. In contrast, a faster synaptic and perisynaptic dopaminergic transmission in the rostrolateral part of the main intercalated island and in the paracapsular intercalated islands may have a role in allowing a more rapid elicitation of fear-related behaviors.

GABA Transmission in the Nucleus Accumbens Is Altered after Withdrawal from Repeated Cocaine

Repeated cocaine causes enduring changes in dopamine and glutamate transmission in the nucleus accumbens, and dopamine and glutamate terminals synapse on GABAergic accumbens neurons. The present study demonstrates that there are changes in GABA transmission in the accumbens at 3 weeks after discontinuing daily cocaine injections. No-net flux microdialysis revealed a significant increase in the basal levels of extracellular GABA in the accumbens of cocaine-treated rats. The elevated extracellular GABA was normalized by blocking voltage-dependent Na+ channels and provided increased tone on GABA(B) presynaptic autoreceptors and heteroreceptors because blocking GABA(B) receptors produced a greater elevation in extracellular GABA, dopamine, and glutamate in cocaine-treated compared with control subjects. For many G-protein-coupled receptors, increased agonist can cause receptor desensitization. Consistent with GABA(B) receptor desensitization, baclofen-stimulated GTPgammaS binding was reduced, and the reduction in G-protein coupling was accompanied by reduced Ser phosphorylation of the GABA(B2) receptor subunit. No effect by repeated cocaine was found in the levels of total GABA(B1) or GABA(B2) protein. Together, these data demonstrate that withdrawal from repeated cocaine treatment produces an increase in the basal levels of extracellular GABA in the accumbens that depends on neuronal activity. The increase may be mediated in part by functional desensitization of GABA(B) receptors, likely the result of diminished Ser phosphorylation of the GABA(B2) receptor.